Theory and technology of catalytic petrochemical productions: educational manual 9786010432499

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Theory and technology of catalytic petrochemical productions: educational manual
 9786010432499

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AL-FARABI KAZAKH NATIONAL UNIVERSITY

L. R. Sassykova

THEORY AND TECHNOLOGY OF CATALYTIC PETROCHEMICAL PRODUCTIONS Educational manual

Almaty «Qazaq University» 2018

1

UDC 665.71 (075.8) LBC 35.514 я 73 S 23 Recommended for publication by the decision of the Faculty of Chemistry and Chemical Technology Academic Council, RISO of the Kazakh National University named after al-Farabi (Protocol №4 dated 29.12.2017); Еducational-methodical section of chemical-technological specialties and specialties of professional education, the arts and services of Republican Educational Methodical Council (REMC) of higher and postgraduate education of MES RK at M. Auezov SKSU (Protocol №3 dated 25.10.2017) Reviewers: Doctor of Chemistry, Ass. Professor S.A. Tungatarova Doctor of Chemistry, Professor B.S. Selenova Doctor of Chemistry, Professor S.M. Tazhibayeva

S 23

Sassykova L.R. Theory and technology of catalytic petrochemical productions: educational manual. – Almaty: Qazaq University, 2018. – P. 296. ISBN 978-601-04-3249-9 The educational manual is constructed in accordance with the requirements of the credit technology program for bachelors enrolled in the specialty «Chemical Technology of Organic Substances». The course is designed to study the basic concepts: theoretical bases of action of catalysts in petrochemistry and oil refining, the active centers of catalysts, types of the catalysts, the concepts of chemistry of catalytic refining processes, current trends of development of petrochemistry and oil refining. The educational manual also has a glossary that can help in mastering the basic terms used in the field of catalytic petrochemical industries. For better assimilation of educational material the questions to self-checking were added to each chapter. The educational manual is intended for students, bachelors, masters and doctoral students specializing in the field of chemical technology of organic substances and chemicals, petrochemicals, catalysis and oil and gas business. Published in authorial release.

UDC 665.71 (075.8) LBC 35.514 я 73 © Sassykova L.R., 2018 © Al-Farabi KazNU, 2018

ISBN 978-601-04-3249-9

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The science of catalysis develops on the border of a number of related sciences – physical and organic chemistry, chemistry of complex compounds, chemistry and solid state physics. Up to 90% of the processes of the modern chemical industry and almost all biochemical processes are areas of application of catalysis. The further development of the main branches of the chemical and oil refining industry will be based on the increasing use and improvement of catalytic methods, and the state of scientific research in the field of catalysis will largely determine the technical level of the most important industries. The aim of this educational manual is to get acquainted with the theoretical principles of obtaining catalysts for petrochemical processes, the technology of their production processes; сonsideration of modern theoretical concepts of catalytic petrochemical production, study of the main scientific achievements in the field of theory and technology of catalytic petrochemical industries. This manual has been prepared to assist students in the mastering of the discipline «Theory and technology of catalytic petrochemical productions» and preparing for exams. In the manual the materials published in the modern domestic and foreign sources in accordance with the curriculum are collected and systematized. The manual presents the concepts of the basic doctrines of catalysis, gives a modern definition of catalysis and formulates the basic concepts and principles that underlie this phenomenon, the main factors determining the catalytic action are considered, examples of catalytic cycles and mechanisms of catalytic reactions are given, the role and importance of catalysis in the development of modern industry, the solution of questions of energy and ecology, modern trends in the development of catalytic science are shown. In the manual for the best learning of the educational material, questions for self-checking have been added to each chapter. The manual contains a glossary and a list of recommended literature. 3

The publication of this educational manual is timely and urgent as according to education reform in RK and need of multilingual education for the republic with obligatory opening of groups of training at English extreme shortage of manuals in English is observed. The skill to estimate activity and selectivity of catalysts by the kinetic results, to use the modern achievements of catalysis, the general laws of catalytic petrochemical processes, basic reaction processes and reactors of catalytic petrochemical industries and the main scientific achievements in the field of theory and technology of catalytic petrochemical industries are need for obtaining the necessary amount of knowledge for the production of highly qualified specialists for the oil-producing and oil-refining industry in Kazakhstan. This manual is due to its simplicity, logicality of presentation and accessibility of the represented material will allow the bachelors and masters with the english learning system for one semester to familiarize with the content of the discipline «Theory and technology of catalytic petrochemical productions». This educational manual is may be useful to students studying on the specialty «Chemical technology of organic substances», «Petrochemicals», «Catalysis», «Oil and gas business», doctoral students and university teachers to prepare for lectures, seminars and practical classes.

4

The share of catalytic processes in oil refining and petrochemical industry is 80-90% of all processes of oil and gas processing. It should be taken into account that the deepening of oil, gas and waste products processing to increase the yield of the target products is achieved mainly through the use of various catalysts. The main catalytic processes in oil refining are catalytic cracking, catalytic reforming, hydrocracking, hydrotreatment, alkylation, isomerization, dehydrogenation, polymerization.

1.1. Catalysis Concept. Catalysis Preconditions. Catalysis is a phenomenon at which chemical reactions are acelerated by small quantities of foreign substances, called catalysts. Catalysis is a well-established scientific discipline, dealing not only with fundamental principles or mechanisms of catalytic reactions but also with preparation, properties, and applications of various catalysts. A number of academic and industrial institutes and laboratories focus on the study of catalysis and catalytic processes as well as on the improvement of existing and development of new catalysts. The phenomenon of catalysis was first recognized by swedish chemist Berzelius in 1835 when he published his classical work in which he offered a new term catalysis (from greek – «to decom5

pose»). Approximately at the same time Mitcherlikh coined the term «contact action». However, some catalytic reactions such as the production of alcoholic beverages by fermentation or the manufacture of vinegar by ethanol oxidation were practiced long before. Already Aristotle (384322 BC) wrote about active agents (catalysts) and passive substances (reagents). Production of soap by fat hydrolysis and diethyl ether by dehydration of ethanol belong to the catalytic reactions that were performed in the 16th and 17th centuries. The word «catalysis» is probably introduced in the 16th century by the chemist A. Libavius in his textbook «Alchemy» for the first time and had a value of «decomposition» or «destruction». In fact, even the «non-biological» i.e non-enzymatic catalysis has been known long before Libavius and especially Berzelius. The first known example of a non-biological catalytic process is the synthesis of ether from alcohol, with the participation of sulfuric acid (VIII-th century, Jabir ibn Khayyam): C2H5OH + HO-C2H5 → H2O + C2H5-O-C2H5

(1)

Secondarily, this reaction was discovered in 1540 by Valery Kordus and was received the technological design in the works of Frobenius. B. Ostwald wrote that «catalyst is a compound that speeds up a chemical reaction without affecting the position of equilibrium». P. Sabatier noted: «catalyst» is a substance or system that changes the rate of the reaction, by participating in the sequence of steps, but without turning into products. The most general definition of catalysis was given by academician A.A. Balandin (1898-1967): «Catalysis is the effect of a substance on a reaction that selectively changes its kinetics, but retains its stoichiometric and thermodynamic conditions; this effect consists in replacing some elementary processes by other, cyclic, in which the active substance participates. The introduced substance is called a catalyst, it does not change quantitatively as a result of the reaction and does not shift the equilibrium». So catalysis is a multistage physical and chemical process of selective change of the mechanism and speed of thermodynamic pos6

sible chemical reactions by the substance – catalyst forming the intermediate chemical combinations with participants of reactions. Today catalysis can be defined as the change of the rate of chemical reactions under the action of certain substances-catalysts which repeatedly enter intermediate chemical interactions with the reactants and restore their chemical composition after each cycle to intermediate interactions. Catalysis is the universal and very diverse phenomenon, widespread in nature and used by mankind for thousands of years before the awareness of the essence of catalytic processes. The best example is the enzymatic catalysis. People use biological catalysts – enzymes – for thousands of years in the fermentation process (for the preparation of, for example, lactic acid products). There is the positive catalysis, i.e. increase in reaction rate under the influence of the catalyst, and there is the negative catalysis, leading to decrease of speed of chemical transfomation. Positive catalysis: the intermediate interaction of reactants with the catalyst opens new, energetically more favorable (i.e. with the smaller height of an energy barrier), in comparison with a thermolysis, a reactionary path (route). Negative catalysis: on the contrary, the fast and energetically lighter stage of chemical interaction is suppressed (inhibited). It should be noted that the term «Catalysis» means catalysis predominantly only positive. The catalyst is included in the intermediate compounds, but does not figure in the main reaction stoichiometric equation and therefore does not affect the balance of the main reaction and is not consumed in it. The catalyst through participation in formation of intermediates provides course of reaction on other way having lower observed activation energy and higher speed of transformation of reagents into products. Thus a very significant feature of catalysis is the fact that the catalyst preserves its composition throughout intermediate chemical interactions with the reactants. The catalyst is not wasted in the course of catalysis. This means that the catalysis is not associated with changes in the free energy of the catalyst and hence the catalyst cannot influence the thermodynamic equilibrium of chemical reactions. Near balan7

ced state the catalyst equally accelerates both direct, and inverse reactions. During removal from balanced state this condition cannot be carried out. It often turns out that the activation energy of a catalytic reaction is less than the activation energy of a non-catalytic reaction (tab.1). In the course of catalytic reactions a catalyst does not undergo any transformations. In many cases these occurs changes in the catalyst structure, sometimes in its compositions as a result of interaction with the admixture, and even with the main components of the reaction mixture. In this regard in the industrial catalytic runs operations of replacement, periodic or continuous reactivation of the catalyst are provided. For certain transformations and reactions the catalysts with the certain properties are used. Of course the catalyst composition and chemical structure are extremely varied. Industrial catalysts on the whole contain different combinations of almost all elements. Most catalysts contain several elements. They can be in their elemental form: for example numerous metallic catalysts and activated carbon; or in the form of different compounds, both comparatively simple, like oxides, sulfides, halides; or as complex like metal complexes with organic ligands or polyatomic compounds of protein nature such as enzymes. Table 1 Comparative values of activation energies for non-catalytic reactions and reactions with catalysts No.

1 1 2 3 4 5

Reaction

2 Decomposition of acetaldehyde Decomposition of acetaldehyde Decomposition of nitrous oxide Decomposition of nitrous oxide Decomposition of nitrous oxide

Catalyst

Temperature of reaction, °C

3 -

4 450

Energy of activation, kcal/mol 5 46-50

Iodine vapors

320

32.5

-

900

58.5

Iodine vapors

900

49.0

Surface of Pt

700

32.5

8

1 6 7

8

9

10

11

12 13 14 15 16 17 18 19 20 21 22

2 Decomposition of nitrous oxide Decomposition of hydrogen peroxide in aqueous solutions Decomposition of hydrogen peroxide in aqueous solutions Decomposition of hydrogen peroxide in aqueous solutions Decomposition of hydrogen peroxide in aqueous solutions Decomposition of hydrogen peroxide in aqueous solutions H2+D2 ↔ 2HD H2+D2 ↔2HD H2+D2 ↔ 2HD H2+D2 ↔2HD Ortho– H2 ↔ para-H2 Ortho– H2 ↔ para-H2 Ortho-H2 ↔ para-H2 Ortho– H2 ↔ para-H2 Ortho-H2 ↔para-H2 Saccharose inversion Saccharose inversion

3 Surface of Au

4 900

5 29.0

-

0-50

17-18

Iodine ion

0-50

13-14

Ion of Fe3+

0-50

10

Surface of Pt

0-50

11-12

Catalase enzyme

0-50

1-2

Copper foil Silver foil Gold foil Gold foil Gold foil Copper Palladium Proton Sucrose (malt)

600-750 310-350 400-460 330-750 700-800 ˃300 50-230 100-300 60 25 25

60 23 16 14 60 17.5 5.2 10-12 4 25 13

1.2. From the history of the development of catalysis History of catalysis and technology of catalytic processes cannot be represented as if the catalysis has been created by a handful of researchers. The similar view is unfair to many scientists, process engineers thanks to whom creation of catalytic productions has become possible. Opening of the phenomenon of catalysis can be referred to one of the greatest achievements of the chemical science which has served to development of all modern chemical technology. According to some scientists, the phenomenon of catalysis could have been of 9

decisive importance in the process of the origin of life. The first reports on the synthesis of sulfur ether and ethylene from ethanol with the use of acid catalysts date back to the 16th-17th centuries, however, the 19th century can be considered as a starting point in the history of catalysis. It is enough to tell that almost all outstanding physicists and chemists of the 19th century were engaged in an issue of catalysis. French chemists N. Clemann and S. Desorme, developing the process of obtaining sulfuric acid during the combustion of sulfur, had found out that the formed dioxide of sulfur was oxidized only in the presence of saltpetre with the heating of which nitrogen oxides were released. At the same time, nitrogen oxides are not consumed in chemical reactions. Clemann and Desorme had shown that the process can be accelerated without the use of saltpetre, adding only gaseous nitrogen dioxide to a mixture of sulfur dioxide, air and water vapor. Clemann and Desorme published their results in 1806. To substantiate the discovery, they advanced the theory that oxygen in nitrogen dioxide is in a more active form for the oxidation of sulfuric acid than air oxygen. In accordance with this, nitric oxide was assigned the role of an oxygen carrier. In Russia in 19th century the main center of scientific research was the Imperial Academy of Sciences in St.Petersburg. Exactly here in 1811 the Russian chemist of the German origin K. Kirchhoff represents three samples of sugar and the sugar syrups obtained from potato starch. He discovered the ability of dilute sulfuric acid to cause the transformation of starch into sucrose and, further, into glucose. Three years later, he found that this reaction can proceed in the presence of barley malt. Kirchhoff studied in detail the effect of acid concentration and temperature on the rate of hydrolysis of starch, determined the optimal reaction parameters. The significance of the results of his research of that time was great not only in the development of chemical technology, but also in relation to the state tasks of that time. In Russia the problem of obtaining food sugar on the basis of domestic raw materials was particularly acute. Other scientists, for example, the academician T. Lovits (Kirchhoff's teacher) and the teacher of the Moscow university I.Ya. Bindgeym also worked on this problem. During the researches at hydrolysis of wheat flour Kirchhoff for the first time carried out enzymatic 10

transformation of starch into sugar. Later results of this work formed the basis of works not only production workers and Russian scientists (A.A. Kolli and N.A. Bunge), but also of Payen and Perso who isolated in 1833 enzyme amylase. The undoubted merit of Kirchhoff is the statement of the special role of sulfuric acid in the hydrolysis reaction. He found that its amount remained unchanged after the transformation of the starch. At the same time, he put forward the idea of the effect of sulfuric acid on the process of hydrolysis, similar to the effect of heat on a number of reactions. L. Tenar carried out a series of experiments on the decomposition of ammonia in the presence of various metals. Having discovered hydrogen peroxide in 1818 and studying its properties for several years, he discovered that many solids cause the decomposition of hydrogen peroxide. In 1817, E. Devi prepared a finely divided platinum – «platinum black», in the presence of which vapors of alcohol and ether were oxidized by oxygen (flameless combustion). These experiments were subsequently continued by I.V. Debereiner (Germany), he oxidized the alcohol to acetic acid in the presence of platinum. In 1823, he discovered another peculiar property of platinum – in its presence hydrogen was spontaneously ignited. Thus, the ability of spongy platinum to catalyze chemical reactions was discovered. Parallel to researches of influence of metals on changes of organic compounds M. Faraday (Great Britain) investigated influence of impurity on catalytic functions of platinum. From this point the phenomenon of a catalysis became an object of science and a basis of catalytic methods of carrying out chemical reactions. In the next years also other examples of sharp acceleration of chemical reactions in the presence of some substances had been found (Y.Ya. Bertselius, Yu. Libikh, E. Mitcherlikh, C.L. Bertholet, K. Kyulman, etc.). By scientists it was already noted that a surface of solid bodies, or the solid body, can be the reaction activator. The most part of the first hypotheses of catalysis limited catalytic influence of the agent to a framework only of physical impact on the course of reactions. These hypotheses emphasized that any chemical change of reagents proceeds under laws of a stoichiometry. However uniform idea 11

of a catalysis didn’t exist up to the 30th of the 19th century in spite of the fact that search of the reasons of nonparticipation of masses of the catalyst in the stoichiometric equations was an ultimate goal of all these explanations. The first generalizations of the facts of catalytic action had been made by L. Mitcherlikh and Y.Ya. Bertselius in 1834-1835. Mitcherlikh has for the first time united catalytic processes under the name of contact reactions in 1834: when initial substances aren’t exposed to changes in the chemical nature, but in the presence of small amounts of the contact material introduced therein, they undergo a chemical transformation. In 1835, Berzelius, summarizing the accumulated experience, proposed a different way of considering catalytic reactions. It was he who proposed the term «catalysis» (from the Greek « katalisis» – destruction) to describe the phenomena of non-stoichiometric interference of «third bodies», catalysts, into chemical reactions. Influence of the small additive which isn’t participating in reaction on the course of this reaction was unclear to chemists of the 19th century. According to Bertselius, «catalytic ability» (catalytic activity in modern understanding) of many both simple, and complex bodies in a solid type and a form of solution, is one of manifestations of the electrochemical relations of matter. A great merit of Bertselius for development of prerequisites of emergence of kinetics and for its formation was that he has managed to generalize the researches performed earlier out of stoichiometric influence of substances on various reactions and to unite all these processes by a community of the reason of catalytic force, Bertselius called catalytic force as «the reason of chemical action» and had put forward the thesis about «thousands of catalytic reactions in an organism». Although Bertselius couldn’t find a uniform explanation of mechanisms of «catalytic processes», the generalizations (restriction of catalytic processes, and also display of prevalence and specifics of catalytic transformations) which are contained in his work, promoted more active restriction of mechanisms of catalytic reactions. Otherwise, than Bertselius, the German chemist Yu. Libikh argued. In 1839 with theoretical conclusions in the field of a catalysis Yu. Libikh had acted and offered an explanation of the nature of this 12

phenomenon connected with gradual change of affinity of the reacting substances. In his work, he pointed out that «[the cause of catalysis] is the ability possessed by a body that is in a state of decomposition or connection, i.e. in the state of chemical action, in the ability to cause in the other body in contact with it the same chemical activity, or to make it capable of undergoing the same change that it itself experiences». Thus, Libikh connected a deviation from a stoichiometry with a continuity of the chemical interaction begun under the influence of these reasons, considering that the catalyst at the same time remains chemically invariable. Libikh’s theory was a generalization of formal physical (operation of chemically unchangeable catalyst) and chemical (stoichiometric) ideas of the mechanism of catalytic transformations. Scientific research in the field of a heterogeneous catalysis has begun at the end of 19 – beginning of 20-centuries with the works on dehydration of alcohols on clays and on decomposition of ammonia and hydrogen peroxide on various solid bodies. In the 19th century a large number of heterogeneous catalytic processes had been discovered, and since the 20th century an active study of the mechanism of heterogeneous catalysis had begun. This was determined by the needs of the development of chemical technology, and, most of all, the processes of obtaining mineral acids and ammonia, and subsequently – the enormous needs of the implementation of the processes of oil refining and petroleum synthesis. Awareness and understanding of the nature of heterogeneous catalysis was facilitated by the possibility of using a complex of physical and kinetic methods for studying heterogeneous-catalytic systems. The most fruitful for creation of modern ideas of a heterogeneous catalysis was D. Mendeleyev's idea which has been developed later by N.D. Zelinsky, about joint influence of physical and chemical properties of a surface of the catalyst on the converted molecules. In the middle of 19 century A.I. Khodnev had put forward idea of formation of intermediate superficial compounds which role was most consistently considered at the end of 19 – beginning of 20 centuries by P. Sabatye. The development of catalysis and the technology of catalytic processes was not strictly progressive. There were a lot of mistakes and errors on this path. Most of them are associated with a lack of 13

understanding of the physical and mathematical foundations of catalysis and the technology of catalytic processes. At the turn of the 19– and 20-centuries research in the field of catalysis and chemical technology was carried out by Professor of the Petersburg Technological Institute A. Krupsky. He belonged to that category of the Russian scientists who aspired as it is possible to connect the theory with practice more closely. His merits in the field of the production technology of soda and sulfuric acid are great. He began for the first time to consider chemistry and physical chemistry theoretical bases of chemical technology. The state of the catalytic processes of organic substances in the pre-revolutionary Russia did not correspond to the development of Russian chemistry, which for its time was one of the most advanced in the world. N.N. Zinin discovered the reaction of obtaining aniline by reduction of nitrobenzene. A.M. Butlerov developed methods for hydrating olefins into alcohols and other processes of organic catalysis, which later became the basis for important industrial technologies. Professor of St.Petersburg Forest Institute M.G. Kucherov discovered the reaction of hydration of acetylene under the influence of mercury salts. This reaction has for many years been used for the industrial production of acetic aldehyde and acetic acid from it. The academician V.N. Ipatyev is considered the founder of a heterogeneous catalysis of organic reactions. To him this area wasn’t studied systematically. Researches went in a wide interval of pressure (to 1,000 atm) and at the same time at rather high temperatures (to 500°C). The State Institute of High Pressures (SIHP) had been organized in 1928 by V.N. Ipatyev. He had a fundamental influence on the formation in the USSR of a school on organic industrial catalysis. In the United States this scientist created many industrial catalytic petrochemical processes and became the founder of petrochemical industries. The enormous role in the field of mutual transformations of hydrocarbons of various classes was played by extensive researches of the academician N.D. Zelinsky. The catalytic processes of oil refining developed by him were widely adopted in the industry. The nickel catalyst offered by him is used in industrial processes of hydrogenation and dehydrogenation. Catalytic synthesis of diene hydrocarbons was of great importance. In 1928 the academician S.V. Lebedev 14

had developed a way of obtaining butadiene on the two-component catalyst allowing to combine dehydrogenation with ethanol dehydration. In 1932 thanks to works of professor N.F. Yushkevich at Chemical Institute of technology of D.I. Mendeleyev the industrial vanadic catalyst for oxidation of SO2 in SO3 in production of sulfuric acid had been created. In the same year production of sulfur from sulfur dioxide with use of a number of catalytic processes had begun. In 1941-1943 in Germany two plants on production of sulfur had been constructed. N.F. Yushkevich has developed also catalysts for the nitric industry. In 1934 on the Novomoskovsk chemical plant two units of synthesis of methanol with a productivity of 4 – 5 thousand tons of methanol a year had been launched. In 1936 N.D. Zelinsky, B.A. Kazansky and A.F. Plate had opened dehydrocyclization of alkanes and their aromatization on Pt or Cr2O3 into toluene and other alkylaromatic compounds. The major petrochemical catalytic process, reforming, bases on these researches. After 1937 the catalytic ways of oil refining with a set of various chemical processes were included into oil industry: splitting carbon – carbon compounds and isomerization of primary products of splitting (cracking); dehydrogenation and isomerization of hydrocarbons with formation of branched and aromatic molecules; hydrogenation of nonsaturated hydrocarbons with removal of sulfur and nitrogen in the form of hydrogen sulfide and ammonia (hydrotreating); introduction of hydrocarbonic fragments to a benzene ring of aromatic compounds (alkylation). Until 1937 the cracking of oil was carried out exclusively by a thermal method: the oil fractions were treated at a temperature of about 500°C and a pressure of 50-60 atm. Catalytic cracking is carried out at ~ 50-500°C and atmospheric pressure in the presence of bentonite clays or artificially prepared aluminosilicates. This process produces a higher octane fuel and aromatic hydrocarbons that can be used for further chemical processing. A major event in the production of polymers was the discovery of stereospecific polymerization of unsaturated compounds in the presence of mixed Ziegler-Natta catalysts (1952). An example of this type of catalyst is a mixture of triethylaluminum and titanium tetrachloride. The use of these catalysts made it possible to obtain the macromolecules with a certain spatial configuration of the mono15

meric units. Products made of such polymers have excellent performance properties. It is worth mentioning the developed by Morton (1947) exceptionally active catalytic system, known by the code name «alfine» and representing a mixture of allyl sodium, sodium isopropylate and sodium chloride. In the presence of an alfine, butadiene is polymerrized in a few minutes to form chains containing tens and hundreds of thousands of monomeric units. In Kazakhstan D.V. Sokolsky is the founder of school of sciences of a catalysis. D.V. Sokolsky is one of the very important organizer of a science, the Hero of Socialist Work. D.V. Sokolsky scientific interests have been directed on working out of the theory and practice of many areas of catalysis. D.V. Sokolsky had made a big contribution to works on catalytic purification of gases which had been for the first time begun in the former Union. The school of sciences in field of catalysis created by D.V. Sokolsky successfully continues researches in the field of development of new catalytic and electrochemical technologies for processes of petrochemical, inorganic and organic synthesis, oil refining and gas. Academician D.V. Sokolsky made an invaluable contribution to the formation of the chemical faculty of the al-Farabi KazNU, he was the founder of the department of catalysis and petrochemistry (in 1945-1970 – the department of catalysis and technical chemistry). He worked as an assistant professor, head of the department, pro-rector of KazNU (former S.M. Kirov KazSU). He was Deputy Director of the Institute of Chemical Sciences, the Scientific Secretary of the Presidium of the Academy of Sciences of the Kazakh SSR and Vicepresident of the Academy of Sciences of the Kazakh SSR. In 19691987 years he was a Director of the Institute of Organic Catalysis and Electrochemistry (from 2015 the JSC «D.V. Sokolsky Institute of Fuel, Catalysis and Electrochemistry») of the Academy of Sciences of the Kazakh SSR, which was founded on December 1, 1969 (see also p.3.9 in this manual). Data on history of a homogeneous and heterogeneous catalysis are briefly presented in the tab.2. 16

Table 2 Brief history of catalysis Years Authors Process 1 2 3 Homogeneous catalysis 1746 J. Robek SO2 + 0.5 O2 → SO3 1782 K. Scheele RCOOH + R’OH → RCOOR’ + H2O 1878 A.M. Butlerov m CnH2n → -(CnH2n)-m 1881 M.G. С2Н2 + Н2О → СН3СНО Kucherov 1928- Yu. Newland 2 С2Н2 → СН2=СНС≡СН 1929 1938 O. Roelen CnH2n + СО + Н2 → CnH2n+1СНО 1939- V. Reppe 1. С2Н2 + СО + НХ→ 1945 СН2=СНСОХ 2. С2Н4 + СО + НX → СН3СН2СОХ (X – OH, OR, NR2, SR) 3. 4 С2Н2 → cyclooctatetraene 4. С2Н2 + 2 CH2O → HOCH2C≡CCH2OH 1953- K. Ziegler, m α-CnH2n → -(CnH2n)-m 1955 D. Natta

1959

J. Smidt, I.I. Moiseyev I.I. Moiseyev

С2Н4 + 0.5 О2 → СН3СНО

С2Н4 + CH3COOH + 0.5 О2 → СН2=СНОOCCH3 + H2O 1960 The company CH3OH + CO → СН3СОOH BASF AG 1970 F.E. Paulik, CH3OH + CO → СН3СОOH D.F. Roz, the company «Monsanto» 1972 The company The conjugate process for the «Halkon» production of styrene and propylene oxide Heterogeneous catalysis 1778 J. Priestley СnH2n+1OH → СnH2n + H2O 1796 M.VanС2Н5ОН → СН3СНО + Н2 Marum 1960

17

Catalyst 4 NO2 Mineral acids H2SO4 Hg2+ Cu(I) Со2(СО)8 1. Ni(CO)4 2. Ni(0), Co(0) 3. Ni(CN)2 4. Cu2C2

TiCl3-AlR3, TiCl4-AlR3, homogeneous and heterogeneous catalysts for the polymerization of 1alkenes and dienes PdCl2-CuCl2 PdCl2-CuCl2-CH3COONa CoI2 Rh(I)-CH3I

Clay (aluminosilicate) metals: Ag,Cu, Ce, Fe, Ni, Pb, Sn, Mn

1 1831 1844 1863 1867 1867

2 P. Phillips M. Faraday G. Debus G. Deacon A. Hoffmann

1877

M.M. Zaitsev

1890

?

1900s

P. Sabatye

1903

V. Ostwald

19081914

Mattiash, Haber, Bosch (BASF) Bosch (BASF) СО + 2 Н2 → СH3OН

1923 1930

1931 1930s 1940s 1955

1960

1967

3 SO2 + 0.5 O2 → SO3 С2Н4 + Н2 → С2Н6 HCN + Н2 → СH3NН2 4 HCl + O2 → 2 Сl2 + 2 H2O CH3OH + 0.5 O2 → СH2O + H2O Hydrogenation of organic compounds in the liquid phase СН3ОН+0.5 O2 → СH2O + H2O ( Н2) Hydrogenation and oxidation of organic compounds 2 NH3 + 3 O2 → NO + NO2 + 3 H2O 3 H2 + N2 = 2 NH3

1. nСО + (2n+1)Н2 → СnH2n+2 + nН2O 2. 2nСО + (n+1)Н2 → СnH2n+2 + nCO2/ products contain alkenes and oxygen-containing compounds T.E. Lefort С2Н4 + 0.5 О2 → (СН2СН2)О Catalytic cracking, reforming Catalytic hydrocracking The company С6Н6 + 4.5 О2 → maleic «Halkon» anhydride + 2 Н2О + 2 СО2 The company C3H6 + 1,5 О2 + NH3 → «Sohio» CH2=CHCN + 3 Н2О + 2 СО2 The company С2Н4 + CH3COOH + 0,5 О2 «Bayer» → СН2=СНОOCCH3 + H2O E. Fisher, H. Tropsch

4 Pt Pt Pt Cu Pt Pd, Pt metals: Ag,Cu Ni Pt FeO+Al2O3+Ca+K+SiO2 ZnO∙Cr2O3, ZnO∙CuO∙Cr2O3 1. Co 2. Fe

Ag/carrier MoO3∙Al2O3, Pt/ Al2O3 V2O5 + promotors MoO3∙Bi2O3∙P2O5+ additives Pd(OAc)2-CH3COOK/ SiO2

1.3. Classification of catalysis and catalytic reactions On state of aggregation of the reactants and catalyst distinguish homogeneous catalysis where reactants and catalyst are in the same phase and heterogeneous catalysis, where the catalyst system com18

prises several phases. In oil processing heterogeneous catalysis, especially with the solid catalyst, is widespread much more, than homogeneous. By the nature of the intermediate chemical interaction of reactants and the catalysts catalysis can be subdivided into the following three classes: 1) a hemolitic catalysis when chemical interaction proceeds on the homolytic mechanism; 2) a heterolytic catalysis – in case of the heterolytic nature of the intermediate interaction; 3) the bifunctional (composite) catalysis including both types of chemical interaction. The nature of the intermediate chemical interaction, but not aggregate state of a reaction system defines properties which the active catalyst should to possess. By homolytic, mainly so-called electronic catalysis reactions of redox type (such catalysis therefore often call redox or oxidationreduction) are performed the following reactions: ‒ hydrogenation, ‒ dehydrogenation, ‒ hydrogenolysis of heteroorganic compounds of oil, ‒ oxidation and reduction in the production of elemental sulfur, – steam conversion of hydrocarbons in the production of hydrogen, – the hydrogenation of carbon monoxide to methane, and others. The transition metals (with unfilled d– or f-shell) of the first subgroup (Cu, Ag) and group VIII (Fe, Ni, Co, Pt, Pd) of the periodic system of Mendeleyev, their oxides and sulfides, mixtures thereof (nickel molybdates, cobalt, vanadates, tungstates, chromates), and metal carbonyls and others have catalytic activity with respect to such reactions. Heterolytic or so-called ionic catalysis, takes place in the reactions of; ‒ catalytic cracking, ‒ isomerization, ‒ cyclization, ‒ alkylation, ‒ dealkylation, 19

‒ ‒ ‒ ‒

the polymerization of hydrocarbons, alcohol dehydration, olefin hydration, hydrolysis and many other chemical and petrochemical processes. Ionic catalysts for reactions include both liquid and solid acids and bases (on this sign a heterolytic catalysis is often called as acidbase) to catalysts of ionic reactions: H2SO4, HF, HCl, Н3РО4, HNO3, СН3СООН, AlCl3, BF3, SbF3, alumina, zirconia, aluminosilicates, zeolites, ion exchange resins, alkali and others. In technical catalysis (for example by catalytic reforming and hydrocracking) are widely used the bifunctional catalysts consisting of the carrier of acid type (the alumina, aluminosilicates promoted by haloids, zeolite, etc.) with the metal deposited on it – the catalyst of hemolitic reactions (Pt, Pd, Co, Ni, Mo). Catalysis phenomena is not linked with the change of the free energy of catalyst. This makes catalytic reactions radically different from so-called induced reactions. In most of technical catalytic processes a small amount of catalyst promotes the transformation of rather significant amounts of reactants. Thus, one mass part of catalyst causes transformations in sulfuric acid production of ten thousand (104), in naphthalene oxidation into phthalic anhydride of thousand, and in nitric acid production – by ammonia oxidation of one million reactant mass parts. The biological catalysts generally named enzymes are complex compounds of protein nature. Some of enzymes increase the rate of chemical reactions by the order 1010-1012. Very important and characteristic property of the enzymes as catalysts is specificity of their action. The microheterogeneous catalysis takes the intermediate place between homogeneous and heterogeneous in which as the catalyst use big polymeric molecules. For small molecules interacting on them, they are similar to heterogeneous particles, but form one physical phase with the reagents. This group includes enzymatic reactions in which the catalyst are large proteinaceous molecules of complex structure and composition (enzymes). Therefore microheterogeneous catalysis is also called enzymatic. 20

Homogeneous and microheterogeneous catalysis are single-phase processes, and the regularity characteristic of homogeneous and gas-liquid chemical processes are applicable to them. The microheterogeneous catalysis is a catalysis on macromolecules or colloidal particles (10-5 – 10-7 cm) which have a huge specific surface with a large number of the active centers, than is provided exclusively high activity of such catalysts. For example, colloidal Pt and Pd solutions show very high activity at the lowered temperatures (promote vigorous decomposition of peroxide of hydrogen at concentration of the catalyst of 10-8 g/l). Colloidal particles differ in most developed surface and at the same time the maximum power supersaturation. Their state is most removed from steady crystalline state. This leads to the fact that colloids are the most vigorous catalysts. Thus, colloidal platinum, unlike metallic, decomposes concentrated solutions of peroxide with an explosion. Solid palladium black absorbs up to 873 volumes of hydrogen per 1 g of palladium, palladium hydrosol – up to 2,952 volumes per 1 g. Under the influence of platinum black carbon monoxide and oxygen do not react. The sols of platinum, palladium, iridium and osmium oxidize carbon monoxide to carbon dioxide. When hydrogenating colloidal metals of aromatic compounds, as well as hydroaromatic ketones and oximes, stereoisomers can be obtained. In ordinary heterogeneous catalysis this cannot be done. When hydrogenating aromatic compounds in an acidic solution, the formation of cyclohexane derivatives proceeds easier, in which the substituents occupy the cis-position; in the neutral solution, first of all, the formation of trans-derivatives takes place. This rule can be used at catalytic definition of a structure of organic compounds when it is difficult to determine by purely chemical way position of substituents as, for example, at stereoisomers of derivatives of cyclohexane. The rate of hydrogenation of various substances in the presence of the dispersed phase of platinum sol often exceeds the rate of hydrogenation of platinum black in the 30-40 times. Ultra-thin powders – nickel organosols in organic liquids, for example, oils, are successfully used for the hydrogenation of fats, oils and other unsaturated compounds in the liquid phase. 21

It is perspective the use of organosols of molybdenum in benzene for catalytic desulfurization of liquid motor fuels. Also apply ultra-thin metal powders of iron and other metals for catalytic oxidation processes – combustion of fuels in rocket engines. The same catalyst, depending on the method of preparation, can be either active or nonactivated, as is observed in hydrogenation reactions. The preparation of metal hydrosols by reduction of hydrosols of their oxides in the presence of protective colloid portalbinic – or lysulbinic-acid sodium gives stable, reversibly-soluble but not hydrogenated ketone group catalysts. The colloidal metals prepared in the presence of gum arabic well hydrogenate the ketone group to the hydroxy group. Hydrosols of basic metals are very sensitive to reactants – acids and oxidizers. Besides, it is difficult to prepare them. For this reason in the colloidal catalysis sols of noble metals of palladium and platinum are applied, as a rule, and more rarely – gold, silver and copper. Palladium and platinum sols are good catalysts for hydrogenation reactions. High activity in the oxidation of oxygen has colloid osmium. The specifics of the mechanism of microheterogeneous catalysis are not yet fully understood. Microheterogeneous catalysis refers to the least studied region of catalysis, although colloidal catalysts by activity are many times higher than heterogeneous catalysts and are active already at room temperature. This is due to the difficulty of obtaining stable colloidal solutions and to the lability of colloidal particles, which change in size with time and are sensitive to traces of impurities. It is necessary to add the following two groups to them: – the heterogenized catalysis, when a catalytically active fragment is attached to a surface or a molecule, for example, the ionized liquid of the second phase. The typical example is the catalysis on the basis of acid which is carried out by the anionically-modified polymers; – an electrocatalysis, when some mechanisms which promote the speed of half-cell reactions on surfaces of electrodes are used. Although catalytic phenomena are quite common in nature and man was faced with them long ago, large scale utilization of catalysis in industry began only in the last centure. A major landmark in 22

the development of industrial catalysis was the elaboration of the contact process of sulphuric acid production based on oxidation of sulfur dioxide obtained in pyrites annealing or in sulfur burning in atmospheric oxygen in the presence of different platinum supports. This process of concentrated sulfuric acid obtaining played an important part in the development of synthetic dye industry. Of great importance for the development of chemical industry was the solution on the basis of catalysis of the problem of fixing of atmospheric nitrogen. It was only with the help of catalysts that the chemical inertness of nitrogen was overcome and ammonia was synthesized from atmospheric nitrogen and hydrogen. Ammonia oxidetion is an example of selective catalysis widely used in industry where the catalyst not only increases the rate of chemical transformation, but also directs it towards the formation of one particular product out of a number of possible products. In most cases in the presence of the given catalyst besides the basic reaction proceeds some more parallel and consecutive reactions and the initial substances turn to a mixture of various products. A content of the reacted initial substances, transformed at presence of given catalyst to desirable products, characterises its selectivity. Selectivity of the catalyst is changed with the change of reactions conditions. Catalysts selectivity depends also on a thermodynamic equilibrium. Sometimes selectivity conditionally express in oil processing as the relation of yields of target and by-products, for example such as gasoline/gas, gasoline/coke or gasoline/gas + coke. Selectivity of catalyst is meant ability of catalyst selectively realize the primary one among another thermodynamically possible ways of reactions. This ability of catalyst is very important for organic catalysis. For example in fig.1 the possible transformations of ethanol at presence of different catalysts is presented. By the way from ethanol it is possible to obtain about 40 different products depending out the composite of catalyst. Very important feature of catalysis is specificity of operation of the catalyst. It is impossible to consider catalytic activity as the universal property of the catalyst. Many catalysts are active only with respect to one particular reaction or a narrow group of reactions. Especially specific is the reaction of biological catalysts i.e. enzy23

mes. In a majority of cases enzymes catalyze the transformations of only a few chemical compounds from among the great number of compounds with similar structure, or even only one. Some catalysts not enzymes are active with respect to rather wide groups of reactions. For example, catalysts with acid nature are active with respect to a large number of reactions of isomerization, hydrolysis, alcohol dehydration, alkylation and many others. Also metallic nickel catalysts are very active in the different reactions of hydrogenation. The catalytic activity is defined by the specific speed of this catalytic reaction, i.e. quantity of the product which is formed in the unit of time per unit of volume of the catalyst or reactor.

Initial material

Processes

Catalysts

2 C2H4+2H2O

Al2O3, H2SO4 (oleum)

2 CH3CHO+2H2O

Cu (reduced)

C4H9OH+H2O

Na metallic and alkaline promotors

C4H8+H2+H2O

Zn·Al2O3

CH3COOC2H5+2H2

Cu-catalysts with additives

2C2H5OH

Figure 1. Some conversions of ethyl alcohol in the presence of various catalysts

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Stability is one of the most important indexes of quality of the catalyst, characterizes its ability to keep the activity in time. It affects on: – the stability of the systems, – the duration of their time between repairs, – process design, catalyst consumption, – material and economic costs, – environmental issues, – the technical and economic parameters of the process. Questions for self-checking: 1. Describe catalysis as the universal and very diverse phenomenon. 2. Explain the role, features and importance of catalysis in industry and petrochemical productions. 3. Describe the catalyst concepts, catalysis preconditions. 4. Tell about the first known example of a non-biological catalytic process. 5. Describe the main ideas of prominent scientists about nature of catalytic action. 6. Explain the features of biological catalysts. Give the examples. 7. Explain the features of reactions in the presence of catalysts. What is «catalytic activity»? 8. Explain the concept «selectivity of a catalyst». Give the examples of processes and catalysts. 9. Explain the concept «spesificity of operation of the catalyst». Give the examples of reactions and catalysts. 10. Tell about classification of catalysis and catalytic reactions. 11. Explain the features of heterogeneous and homogeneous catalysis. 12. Explain the concepts: «hemolytic catalysis», «heterolytic catalysis», «bifunctional catalysis», «ionic catalysis». 13. Explain the features of microheterogeneous catalysis. 14. Explain the essence of the heterogenized catalysis. Give an example. 15. Tell about an electrocatalysis. Give an example. 16. Briefly describe the history of catalysis. 17. Tell about works of Kirchhoff, Y.Ya. Bertselius, Mitcherlikh and Yu. Libikh. 18. Explain the role of works of N.N. Zinin, A.M. Butlerov, V.N. Ipatyev in development of catalysis. 19. Tell about D. Mendeleyev’s idea which had been developed later by N.D. Zelinsky. 20. Briefly describe the history of technology of catalytic processes, from N. Clemann and S. Desorme to nowday.

25

21. Tell about comparative values of activation energies for non-catalytic reactions and reactions with catalysts. 22. Explain the concepts «positive catalysis», «negative catalysis». 23. Who was the founder of school of sciences of a catalysis in Kazakhstan? 24. Tell about D.V. Sokolsky and his scientific school. 25. Tell about the most famous scientists in the field of catalysis in Kazakhstan.

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2.1. Stages of heterogeneous catalysis Catalytic process represents set of catalytic reactions on the surfaces of the catalyst with processes of a supply of reagents into a reaction zone, and removal of products of reaction. The catalysis process on solid porous catalysts consists of the following elementary steps: 1. Diffusion of reactants from the center of the flow to the surface of the catalyst grains; 2. Diffusion of the reagents in the pores of the catalyst grain; 3. Activated adsorption (chemisorption) on the surface of the catalyst with the formation of surface chemical compounds – activated complexes «reagents-catalyst»; 4. Rearrangement of atoms with the formation of surface complexes «product-catalyst»; 5. Desorption of the product from the surface; 6. Diffusion of the product in the pores of the catalyst grain; 7. Diffusion of the product from the surface of the catalyst grain to the center of the flow. A large number of catalysts are granules consisting of powders, or porous bodies. In this case it is important to deliver reagents to a surface. 27

Distinguish: – kinetic area of course of a catalysis in which the speed of reaction is defined by directly chemical transformations on surfaces; – the area of external diffusion, when the reaction is limited by the supply of reagents from the gas or liquid to the outer surface or their withdrawal from the outer surface of the catalysts; – area of internal diffusion, when the rate-limiting step is the transfer of a substance in the pores within the catalyst grains. The overall speed of a heterogeneous catalytic process is determined by the relative rates of individual stages and can be limited by the slowest of them. Sometimes the slowest stage is one of the chemical interactions on the surface of the catalyst, and sometimes diffusion processes. One of the essential stages of heterogeneous catalytic reactions is the transfer of the reactants to the active surface of the porous catalyst. If the reaction proceeds quickly enough, the rate of the process can be limited by the supply of reagents from the center of the flow to the outer surface of the particle, and also by diffusion of the reagents in the pores of the catalyst grain. The real kinetic regularities of a heterogeneous catalytic process are determined both by the actual reaction kinetics on the active surface and by the conditions of mass and heat transfer. In the presence of a catalyst of a certain composition and structure, the temperature regime of the catalytic processes is of greatest practical importance. The place of the heterogeneous catalytic reaction is the surface of the solid catalyst. To increase it, porous catalysts are used, the inner surface area of which reaches tens and hundreds of square meters in one cubic centimeter. The outer surface of such a body is less than 10-3 m2, i.e. it is in 103-105 times smaller than the internal one, and therefore its contribution to the overall transformation rate can be ignored. First, the reagents diffuse out of the gas volume through the boundary layer to the outer surface of the catalyst particle (stage I), then penetrate into its pores (stage II), in which at the movement on their surface reaction (stage III) proceeds. The products are removed in the opposite way. 28

2.2. Adsorption in heterogeneous catalysis. 2.2.1. The general regularities and definitions In heterogeneous catalysis on a solid catalyst, the chemical reaction of the reactants with the catalyst takes place only on its reactive substances accessible to molecules of the so-called reaction surface through adsorption. The specific reaction surface of a heterogeneous catalyst is determined by its porous structure, i.e. the amount, size and nature of the pore distribution. Adsorption of substances precedes the heterogeneous catalytic reactions on the surface. Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid adsorbent, forming a molecular or atomic film (the adsorbate). It is different from absorption, in which a substance diffuses into a liquid to form a solution. Adsorption is the bonding of molecules or particles to a surface. On the other hand, absorption is the filling of pores in a solid. The bonding to the surface is usually (but not always) weak and reversible. Adsorption is increasing the concentration of molecules, atoms or ions near the surface of the solid. The adsorbed substance is called adsorbate and a solid body on which the adsorption takes place, is an adsorbent. The adsorption of reactants and desorption, respectively, are necessary steps in the majority of catalytic processes. Adsorption is operative in most natural physical, biological, and chemical systems, and is widely used in industrial applications such as activated charcoal, synthetic resins and water purification. At physical adsorption atoms or molecules of an adsorbate keep their identity, and forces responsible for adsorption are similar to van-der-vaalsov forces in real gases. At chemical adsorption, the adsorbed molecule forms a chemical compound with the solid, usually with the surface atoms. This gives rise to a covalent, ionic or coordination bond. Criteria for differences in physical and chemical adsorption are not always clear. 29

1. The heat of adsorption Small heat (2 – 6 kcal/mole for simple molecules, 10 – 20 kcal/mol for complex molecules) correspond to physical adsorption, large heat (20 – 100 kcal/mol) – to chemical adsorption. This best criterion difficult to apply when heats are found at the boundary (10 – 20 kcal/mol). Chemical adsorption heat can be small, if the adsorption molecules are dissociated. 2. The rate of adsorption It is believed that the physical adsorption occurs quickly, without the activation energy, and chemical adsorption – slow, with the activation energy. However it is known that the chemical adsorption of oxygen and hydrogen on the pure metals occurs at liquid nitrogen temperatures almost instantly. However, physical adsorption on the porous bodies may be slow due to the slow diffusion into the pores. 3. Temperature adsorption interval Most often, physical adsorption takes place near the boiling point of the adsorbate, the adsorption and the chemical – at higher temperatures. However, this criterion is difficult to apply to highly porous bodies. Capillary condensation in pores occurs at a much higher temperature than the physical adsorption on a plane surface. 4. Reversibility of physical and nonreversibility of chemical adsorption At high temperatures, chemical adsorption is reversible too. 5. Preservation at physical adsorption by molecules of an adsorbate of their identity unlike chemical adsorption. Existence of chemical adsorption can be seen by effect of charging of a surface at adsorption and by spectra of adsorbed molecules. Obviously, only the use of multiple criteria allows to attribute to physical adsorption or chemical. Fundamentals of thermodynamics of adsorption were created by J. Gibbs at the end of the 19th century. According to J. Gibbs, near a phase boundary in equilibrium two-phase system local properties of phases change. Change of the free energy of J. Gibbs of two-phase system can be written down in the form of equation (2): dGα=SαdT+σds+Σiμα, 30

(2)

where: S is the entropy of the system; α is an index of the surface phase; T is temperature; σ is un interphase surface tension; s is the surface area of the partition; μi and ni are respectively the chemical potential and the number of moles of the i-th component. In case of one-component gas phase which can be considered as ideal gas at a constant temperature J. Gibbs's equation has an appearance (3) dσ = αdμ =RTlnp. (3) From this it follows that at physical adsorption on an inert surface an adsorption is defined by adsorbate pressure (or concentration in solution). The assumption about inertness of a surface does not allow to apply J. Gibbs's equation to chemical adsorption, a catalysis, a swelling adsorbate. The magnitude of adsorption of the gaseous phase is measured by volume (volumetric) or weight (gravimetric) method. The size of vapor-phase adsorption is measured by a volume (volumetric) or weight (gravimetric) method. In the first case, in any way, it is possible to determine the changed concentration (pressure) of the i-th component (adsorbate) in the volume of gas associated with the adsorbent or liquid. In the second case the amount of adsorbate is found directly at the interface with the adsorbent. For a long time for the measurement of adsorption on powdered adsorbents the quartz Mc Bain’ balances are used. Now various automated microbalance allowing to measure adsorption even on sides of monocrystals are developed for this purpose. At very small fillings of a surface adsorption is proportional to pressure (concentration) of an adsorbate and the adsorption isotherm is described by the equation W. Henry: a = KHp (4) or a = KHc,

(5)

where KH is W. Henry coefficient. The W. Henry equation not difficult to deduce from the J. Gibbs equation (2). For larger fillings the W. Henry equation is impracticable. In 1906, G. Freundlich proposed an isotherm: 31

a = Kpn,

(6)

where K and п are constants (G. Friendlich’s isotherm). The theory of adsorption isotherms was created by I. Lengmyur in 1914-1918 y.y. On I.Lengmyur’s model all atoms of a surface have energetically identical adsorption centers, that is the homogeneous surface is considered. On one center one molecule of an adsorbate is adsorbed and when filling all centers one monolayer is formed. Neglect interaction between the next adsorbed molecules in the theory. At the adsorption equilibrium the speed of adsorption is equal to desorption speed. Speed of adsorption is proportional to fractional pressure р (or concentration с) of the adsorbed substance and a share of the free surface (1 – θ), and the speed of a desorption is proportional only to a share (from a monolayer) fillings of a surface θ. At equilibrium these speeds are equal: kap(1 – θ) = kdθ,

(7)

where кА and kd are kinetic constants of adsorption and a desorption respectively; θ = a/am (Аm – the maximal adsorption). From here I. Lengmyur's equation turns out: A=am . bp/1+bp.

(8)

At an adsorption isotherm conclusion I. Lengmyur did not divide physical and chemical adsorption. Actually the isotherm was applied more likely to chemical adsorption. In actual researches the polymolecular physical adsorption was often observed and even when fillings, smaller monolayer, at first is formed not a monolayer, but clusters on a part of a surface. If for the chemical adsorption note individual cases of endothermic adsorption, so the physical adsorption is always an exothermic. Entropy adsorbate due to reduced mobility of molecules during adsorption is always less than the entropy of the initial adsorbed compound. An important characteristic of the process is the adsorption heat of adsorption, which is usually attributed to the number of moles of 32

adsorbate and expressed in kJ/mol or kcal/mol. There are an integral and differential heats of adsorption. The integral heat of absorption of Q is equal to decrease of an enthalpy (at V = to const) at change of adsorption from 1, (or from zero) to 2: Q = – ∆H = – (H2 – H1) (9) The differential heat of absorption of q is equal to dH enthalpy decrease at change of adsorption a on da: Q =-dH/da

(10)

Physical adsorption proceeds very quickly if it is not complicated by the diffusion phenomena. The maximal speed of adsorption of gas on a solid surface can be calculated on the basis of the molecular kinetic theory. Chemical adsorption (chemisorption) is a chemical reaction of a molecule (or atom) with a solid surface, resulting in formation of surface chemical compound. The modern quantum chemistry considers the covalent and ionic bonds, and bonds with the multicenter delocalized orbitals, coordination and hydrogen bonds. The adsorbed molecule and solid should be considered as uniform quantum and chemical system in which it is necessary to consider also changes in an electronic condition of a molecule in the course of a chemosorption, and corresponding changes in a solid. At the description of a chemosorption there is one more factor which is almost not considered at the description of physical adsorption – adsorption time. If physical adsorption proceeds very quickly, then for completion of a chemosorption, as well as any chemical reaction, time determined by activation energy is required. Desorption is a gap (evaporation) of the particles from the surface into the gas phase or in solution. In many catalytic reactions desorption leads to the formation of the final product. At a monomolecular desorption the molecule A is desorpted immediately from the active site of M (M-A, M+A). At bimolecular desorption there is a recombination of two adsorbed particles; for example, the molecule H2 is formed from two adsorbed hydrogen atoms at a desorption: 33

М–Н + М–Н → 2М + Н2

(11)

Desorption is always endothermic, because the desorption activation energy is the sum of the activation energy ЕА and the adsorption heat of adsorption Qa. Ed=Ea+Qa

(12)

In case of not activated adsorbtion Ed=Qa

(13)

Precise method of definition Ed or Qa is the temperature programmed desorption (TPD) or in abbreviated form a thermal desorption.

2.2.2. Adsorption isotherms 1. Adsorption isobar characterizes the change in the volume of adsorbed substances as a function of temperature at constant pressure: V = f(T)p (14). 2. Isostere of adsorption defines value of pressure in the normal mode depending on temperature at the constant volume: P  f (T)v (15). 3. The isotherm of adsorption transfers change of volume of the adsorbed substance in system at a constant temperature: Vf(T)p (16). The dependence of one type can easily be converted into another one according to the Mendeleev-Clapeyron equation: pVm  RT

(17)

Therefore, it will suffice to stop only on the adsorption isotherm. Three types of adsorption isotherms are of great practical importance: 1. I. Langmuir, 2. G. Freundlich, 3. The logarithmic isotherm of Temkin-Frumkin-Shlygin. 34

Let us consider the different adsorption isotherms in more detail. I. Langmuir isotherm (fig. 2). The Langmuir adsorption equation is based on the following assumptions: 1. The surface of adsorbent is uniform, i.e. all atoms of a surface have energetically identical adsorptive centers. 2. Interaction between the adjacent adsorbed molecules is absent. 3. On one center one molecule of an adsorbate is adsorbed and when filling all centers one monolayer is formed. 4. At achievement of the adsorptive balance the speed of adsorption will be equal to desorption speed. We consider two limiting cases of the Langmuir isotherm: 1. The substance is slightly adsorbed (low pressure) Then bp > 1, hence θ → 1, which corresponds to the formation of a monolayer, and the adsorption value does not depend on the pressure (this case in the graph will correspond to the saturation state on the curve in fig. 2). Thus, only for the average degree of filling of the sorbent surface the Langmuir isotherm is applicable.

Figure 2. Isotherm of I. Langmuir

G. Freundlich isotherm In wider interval of change of pressure the best results are yielded by a sedate isotherm of G. Friendlich: 35

V=Cp1/n.

(18)

For mass quantities of an adsorbed substance, the equation will have the form x/m= Cp1/n, (19) where x is the amount of adsorbed matter absorbed by m adsorbent moles at a pressure p. The Freundlich isotherm (fig. 3) is not applicable at high pressures, since according to these equations the amount of adsorbed matter grows to infinity as the pressure increases. But this can not be, since saturation sets in, after which the pressure does not affect the amount of adsorption.

Figure 3. Comparison of the adsorption isotherms of Langmuir and Freundlich: 1 – Langmuir isotherm; 2 – Freundlich isotherm

Freundlich isotherm is applied in the field of average fillings of a surface and average equilibrium pressure. Along with the adsorption isotherms W. Henry, I. Langmuir and G. Freundlich, to describe both physical and chemical adsorption, also are used the logarithmic isotherm of M.I. Temkin, A.N. Frumkin and A.I. Shlygin, who first used it in the study of electrochemical adsorption of hydrogen on metals a=a’ln cp, where a’ and c are the constants. 36

(20)

Sometimes it is also called as an isotherm of M.I. Temkin, explained isotherm by repulsion of molecules during chemisorption or surface heterogeneity. In chemisorption processes logarithmic isotherm is applicable over wide ranges of filling. For heterogeneous catalysis, proceeding on the surface of solid catalysts are of importance all forms of adsorption, but crucial role in heterogeneous catalysis belongs to chemisorption: all heterogeneous catalytic processes start from chemisorption and end with almost chemodesorption. Physical adsorption, though does not play a crucial role in heterogeneous catalysis, nevertheless it is useful as means to a research of the porous structure of solids. It is convenient to determine the specific surface area, pore size and shape, the presence of closed pores and other porous geometrical structure components of catalysts and carriers, especially in combination with electron microscopy and mercury porosimetry. According to modern physical and chemical ideas of the catalysis nature the catalyst and reagents should be considered as uniform catalytic reactionary system in which chemical reactions test not only reagents under the influence of the catalyst, but also the catalyst at reaction with reagents. As a result of such mutual interaction in the reaction system the stationary composition of the catalyst surface, determining its catalytic activity is established. It follows that the catalyst is not just a place for carrying out the reaction, but a direct participant in the chemical interaction, and its catalytic activity is determined by the chemical nature of the catalyst and its chemical affinity for the reactants.

2.2.3. Adsorbents Adsorbent is a substance on the surface of which adsorption takes place. Compounds that contain functional groups are, very often, strongly adsorbed on activated carbon. Activated carbon, silica gel and zeolites are used as the adsorbents. The adsorbents are used usually in the form of spherical pellets, rods, moldings or monoliths with hydrodynamic diameter between 0.5 and 10 mm. They must 37

have high abrasion resistance, high thermal stability and small micropore diameter, which results in higher exposed surface area and hence high capacity of adsorption. The adsorbents must also have a distinct macropore structure which enables fast transport of the gaseous vapours. Here we very briefly consider examples of the most commonly used adsorbents. – Silica gel is a chemically inert, nontoxic, polar and dimensionally stable amorphous form of SiO2. It is prepared by the reaction between sodium silicate and sulphuric acid, which is followed by a series of after-treatment processes such as aging, pickling, etc. – Zeolites are natural or synthetic aluminum silicates which form a regular crystal lattice and release water at high temperature. Zeolites are polar in nature. They are manufactured by hydrothermal synthesis of sodium aluminosilicate. Non-polar zeolites are synthesized by dealumination of polar zeolites. Non-polar zeolites are mostly used in non-polar organics removal. Zeolites, as well as silica gels and active aluminum oxide, are significantly sorbed in relation to vapors water. In addition, the zeolites are distinguished by maintaining a sufficiently high activity on the relevant target components with respect to high (up to 150 – 250°C) temperatures. However, compared with other types of industrial adsorbents, they have a relatively small volume of adsorption cavities, as a result of which with very small limiting values of adsorption (see more information about zeolites in Chapters 4 and 7). – Activated carbon can be manufactured from carbonaceous material, including coal, peat, wood, or nutshells. The manufacturing process consists of two phases, carbonization and activation. The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons, from the raw material. The carbonized particles are «activated» by exposing them to an oxidizing agent, usually steam or carbon dioxide, at high temperature. The size of the pores developed during activation is a function of the time that they are treated in this stage. Longer exposure times result in larger pore sizes.

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Questions for self-checking: 1. List the stages of heterogeneous catalysis. Give the brief description of the stages. 2. Explain the concepts «kinetic area», area of «external diffusion», area of «internal diffusion». 3. Explain the concept «adsorption» as the stage proceeding before the chemical reaction in heterogeneous catalysis. 4. List the main features for the classification of adsorption types. 5. Explain the features of physical adsorption. 6. Explain the features of chemical adsorption. 7. Explain the concepts «integral and differential heats of adsorption». 8. Tell about theory of adsorption isotherms created by I. Lengmyur. 9. Explain the concepts: isostere of adsorption, adsorption isobar, adsorption isotherm. 10. Explain the features of different types of adsorption isotherms which are of great practical importance: I. Langmuir, G. Freundlich and the logarithmic isotherm of Temkin-Frumkin-Shlygin. 11. Compare the different adsorption isotherms. 12. Why does the adsorption isotherm of Langmuir inaccurately describe the dependence of the amount of adsorbed matter on pressure during physical adsorption? 13. What is desorption? Tell about desorption in conditions of catalytic reaction. 14. Explain the concept «adsorbent». 15. Give examples of adsorbents.

39

3.1. Ideas of G. Taylor, P. Sabatye, M. Faradey It is necessary to divide clearly the theory of catalysis and the theory of preparation of catalysts. The theory of catalysis speaks about what catalyst is necessary (chemical composition, dispersion, porosity, acid-base properties). The theory of preparation of catalysts considers the methods of preparation of catalysts with the given properties. At the same time it is necessary to consider a possibility of capture of cations, anions and gases (H2, O2, N2, etc.) and the direction of genesis of catalysts at reaction. It is hard to say, what is more difficult: to predict the catalyst or to prepare it. The general theory of catalysis is not created yet. However now there is a number of the reasonable hypotheses which are concern to separate problems of a catalysis. The active centers of the catalyst are, first of all, centers or the zones of a surface of the catalyst different from its other surface by chemical composition, or the sites adjoining volume zones of unusual structure. The concept about the active centers of catalysts was entered into science by the English scientist G. Taylor in 1924 when he suggested that the catalytically active is not all surface of the heterogeneous catalyst, but only some of its sites. For example, ledges or hollows on a surface. Confirmation of the concept of the active centers is the experimental data obtained by G. Teylor at studying poisoning of the Pt-catalyst in reaction of hydrogenation of olefins in a vacuum system (10-7 mm hg). He found that (СН3)2S is a poison for 40

platinum, i.e. blocks its surface. It turned out that it is enough to block 2% of the surface atoms of platinum, and the catalyst ceases to work. Thus, on the surface of platinum there are sites, which are active for catalysis, their share is small in relation to the general surface and if to block them, reaction does not go. In case of silica-alumina catalyst it is enough to block only 4% of a surface by catalytic poison that its activity in reaction of cracking of hydrocarbons decreased to zero. The theory of volume intermediate compounds (P. Sabatye) is one of the first theories of both a homogeneous, and heterogeneous catalysis. The theory does not distinguish a homo – and heterogeneous catalysts which, forming various volume intermediate compounds, are reduced after each cycle of transformations. Acceleration of reaction is reached because each separate stage of process occurs faster, than all not catalytic process. P. Sabatye considered that in the course of catalysis the unstable phase intermediate products are formed and broken up: hydrides in reactions of hydrogenation and oxides in oxidizing reaction. The advantage of theory It is based on ideas of the chemical nature of interaction between the catalyst and reactants and leads to particular instructions for selection of the catalyst, it needs to be looked for among those substances which react with reagents. Idea of the optimum durability of the intermediate compounds follows from this theory: compounds should be formed easily, but not be too durable. Such approach in certain cases resulted in success, for example, at selection of the catalyst of synthesis of ammonia it is necessary to begin with drawing up a number of metals capable to give nitrides on decrease of durability of the last. The best catalyst of synthesis of ammonia is the iron catalyst promoted by some additives. This theory offers approximate approach for selection of catalysts. Such catalyst will be the best for which changes of the free energy of stages will be approximately equal. Theory shortcomings Formation of the intermediate compounds in the form of a separate phase theoretically not reasonably and also contradicts test data, ignoring of physical condition of the catalyst and some other. 41

In parallel with the theory of volume intermediate compounds the adsorption theories of a catalysis were developed. These theories belong to physical theories of catalysis. M. Faradey considered that the catalytic reaction proceeds in the adsorption layer on the surface of the catalyst and acceleration is reached as a result of concentrating of reactants in this layer. For water and acetone such increase in concentration makes 1,244 and 305 times, respectively, that is due to such concentrating of substance the reaction rate is many orders lower, than observed on experience. The reason for the significant increase in the reaction rate in the presence of the catalyst is explained in modern versions of the theory by the disintegrating action of the catalyst on the bonds in the reagents. M. Polyani offered a hypothesis according to which the activeted molecules with the loosened bonds are adsorbed easier and possess higher heat of absorption, than molecules of the initial and the forming substances, that is the catalyst decreases the activation energy and by that accelerates a catalytic reaction. The theory of the surface intermediate compounds is one of the theories of heterogeneous catalysis most widespread now. A.A. Balandin, G.K. Boreskov considered that during catalysis not phase, but the surface intermediate compounds in which atoms of the catalyst retain link with a crystal lattice are formed. In the oxidation of carbon monoxide by molecular oxygen on CuO instead of the formation of a bulk Cu2O phase upon reduction of CuO by carbon monoxide, it is assumed only the removal of a certain amount of surface oxygen not accompanied by the formation of a new phase. According to the chain theory of a catalysis (N.N. Semenov and V.V. Voyevodsky) the chain mechanism of reaction which arises on the surface of the catalyst thanks to its radical character is the reason of acceleration of catalytic processes, at the same time the catalyst is considered as the polyradical giving rise to chain transformations. For the reaction А + В→ С, occurs on the catalyst in two stages, we have: 42

(21)

K° + A →A° + K,

(22)

A° + B + K→ C + K°,

(23)

where K° – catalyst-polyradical. When interacting of the molecule A with the catalyst radical A° which easily enter into the further transformations is obtained. 3.2. A.A. Balandin’ «Multiplet theory of catalysis» 3.2.1. The principles of structural correspondence in catalysis Currently there is no general theory of heterogeneous catalysis. And as it became clear its creation is hardly possible. But among the proposed theories is one that allows you to assess the catalytic effects on the semi-quantitative level, and even predict them for reactions of hydro– and dehydrogenation of hydrocarbons. It is the Balandin' multiplet theory of catalysis. None of the ideas put forward in catalytic chemistry, has not been met with such enthusiasm and none of them has caused such a raging discussion, as the idea of A.A. Balandin about the need to structural correspondence between the molecules of the reactants and the structure of the active centers. In 1927 A.A. Balandin, on the example of reactions of catalytic specific dehydrogenation of naphthenes and heteroatomic cyclic compounds, established the specificity of the action of catalysts. So, for example, on the catalysts of the platinum series at 250 °C dehydrogenation of cyclohexane to benzene takes place: Ni, 2500C + 3H2

(24) Cyclohexane

Benzene

Dehydrogenation of polymethylene cycles is carried out to form alkyl derivatives of hydrocarbons (N.D. Zelinsky, M.B. Turova): 43

CH3 Pt+Pd 0

>300°СC >300 Cycloheptane

(25) Methylcyclohexane

Heterocycles are dehydrogenated on similar catalysts and on five and six-membered rings (N.D. Zelinsky, Yu.K. Yuryev):

-2H2 NH

NH Pyrrolidine

(26)

Pyrrole

The determined specific patterns of catalytic action have induced A.A. Balandin to find out the reason of the mechanism of intermediate interaction at the interface between the phases of the reacting component – the catalyst, and activity manifestations by the catalyst. A young employee of the Moscow University A.A. Balandin (1928) put forward the hypothesis about detailed structure of the active centers of metal catalysts. In accordance with this hypothesis, which was subsequently called «The multiplet theory of catalysis», the active centers of heterogeneous catalysts have a complex structure. The catalyst will be active in that case when geometrical parameters of the active site «are convenient» for a reagent chemosorption i.e. when there is a structural correspondence between the active center and a molecule of reagent. Balandin claimed that on the catalyst surface it is possible to allocate adsorption complex «multiplet» – the groups of atoms contacting with the reacting molecules. There are no clear ideas of the nature of the intermediate multiplet complex. The group of the reacting atoms in a molecule between which redistribution of connections is carried out and which adjoin to catalyst atoms is called an index group. 44

The essence of the principle of structural correspondence comes down to the following postulates: 1. The active centers represent fragments of crystal lattices and are composed of several atoms, as a rule, of pairs, triples, and sextets (multiplets). 2. If two atoms of reagent are chemisorbed on one atom which is a part of the active center, then between them there is a chemical bond; if on two atoms – that the link which is available between them is broken (fig. 4). 3. Hydrogenation of the double bond C = C olefin is chemisorbed on two metal atoms.

Figure 4. The scheme of catalytic action on the doublet active center: 1 – dehydrogenation of alcohol RCH2OH; 2 ‒ dehydrogenation of the alkane RCH2CH3

The most active for catalyzing the hydro-dehydrogenating of a carbon-carbon bond are metals whose angle MCC in the formed complex will be close to the tetrahedral, i.e. to 109° (fig. 5). Using the example of the catalytic hydrogenation reaction of ethylene (fig. 5, a), it was possible to calculate the value of the angle, which turned out to be equal to 109°. 45

a

b

Figure 5. Preservation of lengths of bonds and valence angles at formation of a multiplet complex: a) on the example of catalytic hydrogenation of ethylene, b) the angle for the hydrogenation of the double bond

On the basis of these postulates for the first time in history in catalysis became possible to predict the presence or absence of activity in a variety of metals. Really (fig. 5b), assuming that the distance C-C in the activated complex is the average between the lengths of single (0.154 nm) and a double (0.134 nm) bonds, the MCC angle is 109o and bond length C-M is equal to the length bond of the C-H in alkane, that the interatomic distances MM can be found (as base of the trapezium MCCM), which should be optimal for catalyzing of hydro- and dehydrogenation of carbon-carbon bond. Interatomic distances of M-M for metals change over a wide range – from 0.23 nm for beryllium to 0.53 nm for cesium. Calculation on fig.5 shows that in the considered process metals should have high catalytic activity with interatomic distance from 0.25 to 0.28 nm. Really, if the value of the loosened bond C-C in ethylene – c (1.54 Å), binding energy carbon – the active center of the catalyst – b (1.82 Å), a valent angle –  is known (109°), then it is possible to calculate interatomic distance a in a doublet on the catalyst according to a formula (27): (b-a)/2=c·cos (180º-θ) (27) From this it follows that the doublet with interatomic distance 2.74 Å can be the optimum catalyst for the considered reaction that corresponds to the catalysts on the basis of Pt and Pd having in a crystal lattice parameters corresponding 2.74-2.77 Å. It is experi46

mentally shown that for this process Rh at which parameters of a lattice correspond to distance 2.69 Å is optimum. Thus, knowing lattice parameters, it is possible to calculate deviations of a valent angle. Catalysts for the dehydrogenation of cycloalkanes are: Pt-2.77 Å, Ni-2.48 Å, Ir-2.70 Å, Rh-2.69 Å, Cu-2.56 Å – they are characterized by the face-centered and hexagonal crystal lattices with a triangular arrangement of atoms (facets in the form of octahedra and basopinacoid). A.A. Balandin offered the mechanism of formation of a multiplet complex and formulated the theory of selectivity of the action of catalysts on the example of doublet interaction on reaction (28) and fig. 6: (28)

Figure 6. Mechanism of formation of a multiplet complex (doublet interaction)

Where at the atoms A, B, C, D there may be substituents; the bonds A-B and C-D can be simple, double, triple; atoms can belong to different molecules. A.A. Balandin postulated that reaction (29): С6Н12 ↔ С6Н6 + 3Н2

(29)

occurs on «sextet» of atoms (fig. 7a, b), at the same time the hydrocarbonic cycle is chemisorbed plainly. The catalyst centers a, b, c are responsible for the cleavage of hydrogen molecules; d, e, f – for the formation of double bonds.

47

a

b Figure 7. The scheme of an arrangement of a molecule of cyclohexane at dehydrogeneration on the sextet

The corresponding sextets are available on sides of two types of crystal lattices – cubic face-centered (A-1) and hexagonal (A-3) (tab. 3). Thus, the circle of the catalytically active metals was even more narrowed. In tab. 4 the metals having «convenient» interatomic distances are shown. Nevertheless, the metals having a cubic body centered lattice (Fe, Cr, Mo), show activity in reaction of dehydrogeneration of cyclohexane. This example A.A. Balandin explained with the fact that on such metals not plane, but costal orientation of a hydrocarbonic cycle (fig. 8) is implemented. At dehydrogeneration the hydrocarbonic cycle is rolled on the active site as a wheel. Thus, cyclohexane is dehydrogenated on a doublet of atoms sequentially in three stages: С6Н12 → С6Н10 → С6Н8 → С6Н6

(30)

According to submissions of the multiplet theory reaction of dehydrogenation of cyclohexane on the metals having a hexagonal and 48

cubic face-centered lattice should happen in one stage with the low activation energy, and on metals with a body centered cubic lattice – through stages and with the significantly higher activation energy. These predictions come true. In particular, on platinum (the face-centered cubic lattice) cyclohexane is dehydrogenated selectively with an activation energy of about 20 kcal/mol, whereas on iron (the body-centered lattice) in the reaction products revealed significant amounts of cyclohexene, and the activation energy is significantly higher (up to 50 kcal /mol). Table 3 Metals, on the faces which there are sextets atoms and interatomic distances in them (nm) Cubic face-centered lattice (А-1) Pt 0.277 Pd 0.275 Ir 0.271 Rh 0.269 Cu 0.255 Co 0.251 Ni 0.249

Hexagonal crystal lattice (А-3) Re Tc Os Zn Ru

0.247 0.270 0.267 0.266 0.265

0.276 0.274 0.274 0.291 0.270

Table 4 A prediction of catalytic activity of metals in hydrogenation of the multiple bonds on the basis of a structural factor of the multiplet theory of a catalysis

Figure 8. Costal orientation of cyclohexane on a doublet

49

Thus, the first time in the study appeared catalysis theoretical concepts, though based only on the geometrical characteristics of the catalyst, but allowing to predict the catalytic activity of metals. This has caused great interest of chemists around the world and has given rise to acute discussions. In favor of structural correspondence of the theory of Balandin many facts showed: 1. Predictions about high activity of metals which were not studied earlier (iridium, osmium, ruthenium, rhenium) were confirmed. 2. The experiments with monocrystals showed that in reaction of a dehydrogeneration of cyclohexane the side {111} is 103 times more active than a side {100} (on which there are no sextets). 3. By the English physical chemist Beek the experimental dependence of catalytic activity of metals in the hydrogenation of the olefins on the interatomic distance was obtained, which is determined by the position of the metal in the periodic table (fig.9). This dependence is fully consistent with structural correspondence of Balandin multiplet theory. 4. In reaction of dehydrocyclization of n-alkanes the metals predicted by the theory were active. However, other experiments which implementation was stimulated the multiplet theory contradicted the principle of structural correspondence: – According to the theory, amorphous metals shouldn’t have essential catalytic activity whereas actually it not so. – Amorphous metals can show catalytic activity in the reactions considered above, on a surface of amorphous particles there can always be sets of atoms with necessary distance. Professor of MSU O.M. Poltorak et al. prepared a set of the put (deposited) platinum catalysts in which particle size changed from 0.6 to 5.0 nm. Concentration of the sextets on a surface of particles of platinum varied from zero (in case of small particles) up to very high size (at larger crystals). However, this does not affect the value of the specific catalytic activity in the model reactions of hydrogenation of unsaturated compounds. Besides, it turned out that hydrides of a titanium, zirconium, chromes which are similar to metal have no suitable interatomic distances, however their activity in hydrogenation is rather high. 50

Figure 9. Catalytic activity of the transitional metals in ethylene hydrogenation (Curve of Beek)

Australian Professor Jack Garnett (1968) found that the specific rates of deuterium-hydrogen exchange in benzene are the same for heterogeneous platinum catalyst and a homogeneous platinum complex, which could not form sextets. There were also purely theoretical objections to the principle of structural conformity. So, Beek curve (fig.9) cannot be interpreted unambiguously: in addition to the interatomic distances also all other parameters – the strength of the lattice, the heat of adsorption, the structure of the outer electron shell, etc are changing. Important theoretical objection consisted in the fact that the originally proposed multiplet systems for the direct and inverse reactions by A.A. Balandin are different, for example, in the case of hydrogenation and dehydrogenation reactions (fig.10).

Figure 10. Multiplet complexes for reactions: a) dehydrogeneration, b) hydrogenation

Doublets for direct and inverse reactions are different, and it contradicts the principle of a microreversibility (i.e. detailed 51

equilibrium) according to which direct and inverse reactions pass through the same transient states. Let’s notice that the author of the multiplet theory in response to this remark was forced to enter new type of the active sites – a quadruplet (fig.11). At the same time two atoms (black) conduct reaction of dehydrogeneration, and two others (gray) – of hydrogenation.

Figure 11. A quadruplet on which there is a reversible hydrogenation – an olefin dehydrogeneration

However, the application of this approach to reactions sextet automatically leads to the need to postulate the 12-atom active center, which is hardly realistic.

3.2.2. The principles of energy correspondence in catalysis Geometrically regular arrangement of atoms, which is available on the faces of the crystal, is only a necessary but insufficient condition for the successful implementation of the catalytic reaction. And in the 1930s A.A. Balandin formulated the second principle of the multiplet theory – energy correspondence principle, which takes into account the interaction energy of the reactants with the catalyst. If the catalyst strongly interacts with a reagent, then between them there will be a strong chemical bond and the catalyst will fail (or as tell catalytics, «will get poisoned»). If reagent interacts with the catalyst weakly – at the level of energy of a hydrogen bond, – there will be no reagent activation. For example, crystals of copper and/or gold do not practically interact with olefins and, therefore, do not catalyze their transformation. This means that for the catalyst activity it is required some kind of medium-sized energy of interaction between the reagent and catalyst. And if it is necessary to predict the activity of a particular catalyst it is necessary to be able to calculate energy of the active center binding with the reagent molecule. 52

Multiplet theory suggests the following method for estimating this energy. Chemical transformation of AB + CD into AC + BD can be represented through a complex multiplet as follows (fig. 12). The multiplet activated complex may be written as [АКВКСКDК]. Energy of its formation will characterize the adsorption potential of the catalyst q compensating a difference in energy of gap and bonding in a substratum and in a multiplet complex. When the adsorption potential q is the negative, occurs easy formation of a complex, but at the same time the products are allocated difficultly. When a value q is positive, binding is weak and a complex is formed difficultly. Follows from the principle of energy correspondence that both too weak, and too strong binding of the reacting atoms with the catalyst is non-efficient for catalysis. The weak interaction does not provide weakening of initial bonds B and CD. In catalysis is looked for a compromise – a good activation at bad sorption or bad activation at strong adsorption.

Figure 12. Scheme of the reaction through the formation of the complex multiplet

Let’s compare the adsorption potentials of two catalysts: in the first (unmixed) catalyst the active centers are formed by atoms of one type, and in the second (mixed) – atoms of a different type. On these catalysts multiplet complexes will be the following view (fig. 13).

a

b

Figure 13. To calculation of the adsorption potential q: a – unmixed catalyst, b – the mixed catalyst

53

Let/s assume that formation heats of bonds of atoms B and C with Ki atoms in the active centers will be in the following ratios: QBK(2) > QBK(1) and QСK(2) > QСK(1)

(31)

Then the adsorption potentials of these catalysts can be written down as: q1 = QАK(1) + QBK(1) + QСK(1) + QDK(1)

(32)

q2 = QАK(1) + QBK(2) + QСK(2) + QDK(1)

(33)

and q2 > q1 As it was written above, there should be a correspondence on energy between the active center and a substratum. How it is possible to use it? A.A. Balandin designated energies of bonds in reagents as E(AB+CD) = Ep, and in products of E(AD+BC) = En and showed that it is possible to determine the adsorption potential of the optimum catalyst (fig.14). On the fig.14 the following designations are shown: the Ep line – energy of formation of a multiplet complex, line En – energy of transformation of a multiplet complex with formation of products and regeneration of the catalyst. As appears from fig.14, change of activation energy Ea depending on the magnitude of the adsorption potential q is described by curves with the maxima corresponding to a heat of formation of the intermediate compounds, which is equal to a half of heat effect of reaction. Thus, the principle of energy correspondence allows to involve thermodynamic characteristics to selection of catalysts. It may be spread (as some approximation) and to other types of catalytic reactions. For use of this principle it is necessary to know, at least in general, composition and structure of the active complex and energy of bonds of the reacting atoms in a molecule and with the catalyst atoms. Thus, according to A.A. Balandin, reaction of «doublet’ type – the most widespread. For performing of doublet reaction (33) it is 54

necessary that the reacting atoms A, B, C and D adjoined to atoms of a surface of the catalyst: АВ + CD→ АС + BD

(34)

Figure 14. Change of activation energy (E) depending on the value of the adsorption potential (q): a – thermally neutral reaction, – endothermic reaction, b – the exothermic reactions (volcano curves according to A.A. Balandin; the bold polygonal line is designated a path of reaction)

Such contact requires correspondence between atomic distances in the reacting molecules and in the catalyst. At great or smaller values of parameter of a lattice reaction will not go. Thus, the adsorption potential of the optimum catalyst is equal to a half-sum of the energies of the bonds which are breaking off and formed during reaction. The multiplet theory is not deprived of shortcomings. Thus, according to this theory, reaction rate is uniquely determinated by activation energy value. At the same time the great influence on speed is exerted by the value of entropy of activation which can differ considerably for different reactions. It is necessary to explain the high rate of catalytic reaction while observing the strict ordering of the active complex, which would correspond to high negative entropy values of activation. Conclusion of a ratio of energy correspondence more or less logical only for endothermic reactions. In case of the exothermic reactions this conclusion results in uncertain results. Thus, the multiplet theory should be considered as more or less approximate scheme of the mechanism of some types of catalytic reactions. 55

3.2.3. Some kinetic regularities. Factors that may affect the nature of the process Conclusions from the multiplet theory: 1. The catalysis occurs in a monolayer on the surface of the catalyst. 2. Beyond-index parts of molecules at catalysis remain invariable. This statement is right if substituents do not strongly displace electrons in index group. 3. To different index groups there should to correspond different catalysts. 4. The multiplet theory is capable to explain gradual increase in a catalytic activity at a promoter adding, and then, after achievement of the maximal activity, – decreasing of activity or poisoning of catalyst. It follows from the principle of structural correspondence. Really, if to enter sulfur atoms in a lattice of metal nickel, then sulfur deforms a lattice, reducing its parameter which can be more favorable for a catalysis of some reactions. However, if into the catalyst is introduced a lot of sulfur, it screens the part of the nickel surface, and activity is reduced. 5. Since the surface of the catalyst may be non-uniform, its various regions must have different adsorption and catalytic activity. 6. The most important consequences from the multiplet theory are the principles of structural and energy correspondence between reactants and the catalyst. These principles allow to receive more concrete results. Balandin proceeds from the notion that the rate of catalytic reaction is determined by the properties of some intermediate state. Its structure is complex, and five main catalytic layers can be distinguished in it: V diffusion layer Gas IV layer of substituents III layer of reacting atoms (index group) II layer of active centers Catalyst I the inner layer of the catalyst (carrier and impurities) The layers I and II refer to a solid catalyst, the remaining layers to a gas (or liquid) phase. Between the layers of II and III there is an interphase border. 56

The main for the reaction are the layers II and III – the active centers of the catalyst and the reacting atoms of the molecules in contact with the catalyst. Geometric and energy factors in the multiplet theory belong to these layers. Atoms of layer IV – substituents – do not participate in the reaction, but influence, and sometimes very strongly, on the reaction rate in layer III. This influence affects the binding energy, sometimes introducing only a small correction, and sometimes greatly changing its significance. The analogous effect of substituents is the influence of the atoms of the catalyst in layer I, which do not enter the active center. A layer I is complex. It includes both the deep atoms of the lattice of the crystalline catalyst, and the atoms adjacent to the active center; atoms of the main substance of the catalyst of different degree of unsaturation, caused by different position on the surface, different number of neighbors, and atoms of additives forming a solid solution or a separate phase. It is possible to refer to them also influence of carriers. All these neighboring atoms owing to deformation influence the active center and on its binding energy with atoms of the catalyzed molecules. The effect of the atoms of layer I on the value of the adsorption potential indicates that the binding energy inside the crystal is influenced by the influence of neighboring atoms. The sum of all binding energies in the reaction index, atoms A, B, C, D with the catalyst is the normal adsorption potential qo. Influence of neighboring atoms is connected with the position of the active center on an inhomogeneous surface. Thus, q = qo + λ (35), where λ is the sublimation energy, taking into account the degree of unsaturation of the active centers; q0 depends only on the chemical nature of the catalyst; λ depends on the method of preparation of the catalyst, therefore, the value q will also depend on the method of preparation of the catalyst. As the adsorption potential of the catalyst increases from q1 to q2 (q1 < q2), the reaction rate changes as a result of a change in the method of its preparation. For catalysts of the same phase composition, the method of preparation can vary λ by 81 kJ/ mol. 57

No less famous than the creation of the structural and energy principles of the multiplet theory, acquired the fundamental research of Balandin in the field of studies of the kinetics of catalytic reactions. Using ideas about hydrogen and hydrocarbonic parts of a surface of the catalyst on which adsorption obeys to Langmuir’s isotherm, Balandin has defined the general equation of catalytic hydrogenation (35) which made it possible to cover a huge experimental material on the kinetics of hydrogenation reactions, made it possible to explain the zero order of reaction observed at the beginning of the reaction, and the first order of reaction at the end of process: (36) The dependence of the rate and order of the hydrogenation reactions on pressure and temperature were explained. For the quantitative characteristic of reactions of selective hydrogenations Balandin entered a concept «an indicator of selectivity» and pointed out its relationship with the change in free energy in the course of the reaction. Of particular importance is Balandin’s theory for explaining the regularities obtained for the hydrogenation of mixtures of two substances. With the simplest assumptions about the ratio of the adsorption coefficients of the reacting substances, the theory makes it possible to derive equations that determine the degree of selectivity and its dependence on the process conditions and the nature of the catalyst. For establishment of an indicator of selectivity at hydrogenation of mixture of two substances in comparison with this equation it is necessary to consider existence in system of the second substance and a product of its hydrogenation and, besides, it is necessary to express reaction speed to some moment of time τ. The selectivity indicator α is defined by a ratio α=k2b'2/k1b'1 58

(37),

where k2/k1 is a ratio of constants of speeds of reaction; b'2/b'1 is a ratio of the adsorptive coefficients of the hydrogenated substances on a hydrocarbonic surface. According to the theory, it can be expected that hydrogenation takes place on different catalysts with different adsorption capacity and at different rates, ie, α depends on the nature of the catalyst. None of the previously proposed theories of catalytic hydrogenation claimed and could not claim such a full coverage of the most diverse and, at first glance, disparate facts. In the theory of hydrogenation of Balandin it is assumed that various unsaturated compounds are hydrogenated on the same active centers. This assumption is hardly valid for compounds with differrent bonds. Also it is not enough to take into account in Balandin’s theory the possible influence of the nature of the solvent under conditions of liquid-phase hydrogenation. When considering the mechanism of hydrogenation processes in the liquid phase, it must be taken into account that the presence of a solvent complicates the kinetics of the process. In the multiplet theory, the influence of the solvent is taken into account in connection with its adsorption capacity. It is necessary to note a number of other factors that may affect the nature of the process. The first group of these factors, determined by the possible interaction between the solvent and the reacting substances or the intermediate complex, is taken into account by the theory of absolute reaction rates. The most important factor connecting the influence of the solvent with the reaction rate is the solvation of the reacting substances or the activated complex or the formation of the hydrogen bond. The maximum of a potential barrier at a solvatation of the activated complex is reduced by ΔH – the heat of a solvatation. The activation energy of E2 for a solvated complex is below E1, the activation energy in the absence of solvation. If there is no effect of other factors, the solvation of the activated complex should lead to an increase in the reaction rate. The opposite effect on the reaction rate should be provided by the solvation of the starting materials. In this case, the potential energy of the initial substances is lowered by the amount of heat of solvation ΔH, and the activation energy increases. Solvation or the formation of a hydrogen bond can also affect the selectivity of hydrogenation. For nonpolar organic solvents, one 59

can assume the same mechanism of the process and close hydrogen activation rates. In this case, α is sufficiently constant. For acetic acid the association with cinnamon acid through hydrogen bond is possible that it significantly changes both the speed of reaction, and selectivity of hydrogenation. The considered positions, which are quite obvious in chemical kinetics, receive a very peculiar explanation in heterogeneous catalysis, in particular, in hydrogenation reactions. Here the effect of solvation will be manifested, first of all, in connection with the adsorption capacity of hydrogenated substances. For weakly adsorbed compounds (olefinic hydrocarbons), whose concentration on the surface is small, their more stable retention in solution due to solvation will further reduce the adsorption density on the surface and reduce the reaction rate. On the contrary, for such strongly adsorbed compounds as nitro compounds, acetylene hydrocarbons, etc., displacing hydrogen from the surface, the solvation effect will reduce their surface concentration, improve the activation of hydrogen and, as a result, increase the reaction rate in complete contradiction with the conclusions of the theory of absolute reaction rates. From the point of view of potential reaction profiles (fig. 15), the solvation of reaction products should not change the height of the energy barrier and, consequently, the speed of the process. Meanwhile, the blocking of the surface by reaction products is one of the main reasons for the change in the reaction rate and its order. Solvation of the reaction products, which facilitates their desorption from the surface, can greatly affect the speed of the process. The second group of factors affecting the speed and direction of the process is associated with the role of hydrodynamic factors – the rate of mixing, the viscosity of the solvent, etc., which determines the possibility of the process in the diffusion or kinetic areas. In the diffusion region, the reaction rate depends on the rate of stirring of the reaction medium. In the kinetic region, the adsorption or chemical steps of the reaction become the limiting stages. Accordingly, the reaction rate does not depend on the intensity of the stirring and another dependence is observed on the amount of catalyst and temperature, and so on. 60

Differentiation of diffusive and kinetic areas and the theoretical analysis of processes of hydrogenation is of great importance for industrial processes.

Figure 15. Change of potential energy of system for a solvatation of the activated reaction complex: 1 – energy barrier of the reaction in the absence of solvation of the activated complex; 2 – energy barrier during solvation of the activated complex

From the point of view of the modern science the principle of energy correspondence in a general view remains fair though its concrete application strongly changed. As for geometrical correspondence, the actual geometry of the surface interaction appeared absolutely another.

3.3. Bases of the theory of the active ensembles of N.I. Kobozev The theory of the active ensembles of N.I. Kobozev is a theory of heterogeneous catalysis. In certain respects, this theory is close to the multiplet theory, but a number of points it is contrary to the latter. The theory of the active ensembles was formulated by N.I. Kobozev in 1938. In it the issue was first raised that the composition and properties of the active sites of metal catalysts can be defined not only the consideration of the elements of the crystal structure, but also the study of the catalytic activity of individual atoms or 61

groups deposited on the surface of the catalytically inert carrier and non-crystal lattice. In the theory of the active ensembles the active site (center) is considered as the pre-crystal formation from several atoms, the natomic «ensemble» fixed on the surface of the carrier by the adsorption forces. Kobozev suggested to stabilize atoms of the active phase of the catalyst, unstable to association, on an adsorbent surface, i.e. to obtain on the not catalytically active carrier a metal layer in atomic and dispersible, but not in a crystalline state. Such catalysts have been called as adsorptive. Physical and chemical and catalytic properties of the pre-crystall systems are of interest as they represent the systems, transitional from molecular to a crystalline state, and they are a peculiar transition from homogeneous and enzymatic to a heterogeneous catalysis. For this type of catalyst it is necessary to use very dilute solutions of metal salts and maintain their preparation under mild conditions of drying and reduction. The study of the catalytic activity of adsorptive catalysts, depending on the degree of coverage of the surface by the active substance α has opened a number of unique regularities. General A and specific a=A/α activity passes through a maximum at a very small degree of filling, although one would expect the continuous growth of activity with increasing amounts of deposited metal. Any real surface of the carrier is characterized by block, mosaic structure, with the result that on the surface may cause isolated areas of migration (separated from each other by energy or geometric barriers (fig. 16, 17). They can be: 1. Real cells (for example, sides of the partial crystals). 2. The areas of the carrier adjacent to the centers of adsorption with the increased adsorption potential, the characteristic of the energetically nonuniform surface. 3. The cracks and other surface disturbances. 4. Crystal defects. 5. Infringement of stoichiometric composition. Thus, these places can be a cause of the area of migration. For the formation of the active centers in the form of clusters of n-atoms 62

does not matter what the origin and nature of these inhomogeneities, or mosaic.

Figure 16. Energy (I) and geometric (II) barriers on the surface of the carrier

It is important that during the formation of the metal layer a surface of the carrier allows the free migration of particles only in limited spaces – areas of migration. These surface violations are an obstacle for the free movement of atoms of the catalyst on the carrier, forming potential wells where the applied atoms which got to one migratory cell should be accumulated.

a

b

Figure 17. The distribution of atoms in the isolated areas of migration by Kobozev theory. Geometrical and energy barriers (a) separating migration areas (b): S – geometric barrier (protruding areas of the carrier surface); U – energy barrier (potential well); • – active atom (ensemble of 1-2-3 atoms); U – is the specular reflection of S

63

The nonuniform surface of the carrier can be considered as a set of the areas limited by energy barriers. Atoms of ensembles can freely move in each such area (in the field of migration), but transition out of its limits is complicated by existence of a potential barrier on border. At temperature increase mobility of atoms increases, the crystallization leading to sintering of catalyst and decrease of its catalytic activity begins. On the massive catalysts, according to N.I. Kobozev, there are also fields of migration in which there are amorphous atoms giving the catalytically active ensembles. Thus, the surface of the carrier is a set of the closed fields of migration. Confirmation of these representations were the experimental data on a sintering of the adsorptive catalysts. Speed of deactivation of catalysts submits to the equation of first order on concentration of the active ensembles on a surface. And it means that the active ensembles on a surface do not depend from each other and do not interact among themselves. The size of fields of migration, especially on porous carriers, considerably exceeds a radius of action of molecular forces (in tens of times). Therefore it is quite admissible that entering of separate atoms of the catalyst in the same field of migration are independent events, distribution of the active phase on the surface of the carrier subjects to the law of a chance. If the active phase is rather evenly applied on the surface of the carrier, then amount of the atoms concentrated in the field migrations will change with change of degree of fullness. Therefore on a surface there will be the fields of migration containing various amounts of atoms of the active phase. The atoms of the active phase which are trapped in one area of migration as a result of surface movement within the migratory cell and relatively high Me-Me bond energy are associated to one n-atomic ensemble. The carrier of catalytic activity is the active center of catalytic process for each this reaction – the "ensemble" consisting of particular number of atoms. Kobozev came to a conclusion that the amount of atoms necessary for creation of the active ensemble depends on the mechanism of process and on type of the turned bond. 64

The theory was first applied to supported (deposited) catalysts containing a very small amount of active component supported on carbon, a pumice, silica gel. It was assumed that the deposited material atoms do not form a crystal lattice, and are grouped into the amorphous catalytically active ensembles. The theory of the active ensembles can be formulated by three main positions: 1. The support of catalytic activity is an atomic (pre-crystal) phase of the catalyst. A surface of the carrier performs usually role of inert substrate. 2. For each process the active center is the group (ensemble) of a particular number of n-atoms of the catalyst. 3. The catalyst atoms which trapped under the law of randomness into one field of migration roll down in a potential hole and are associated there into n-atomic ensemble. Theory of active ensembles does not reject the influence of the carrier on a specific activity of the catalyst. By increasing the concentration of the active phase on the surface of the carrier is increased the probability of formation of polyatomic assemblies and reduces the number of the areas of migration containing one atom. Therefore, for the reactions that take place on a single ensemble, the specific activity of the catalysts is maximal at the lowest possible degree of filling and sharply decreases with increasing degree of filling. If the specific activity with increasing α passes through a maximum, the ensemble consists of two, three or more atoms. As at larger fillings the specific activity decreases, the active ensemble does not may contain a large number of atoms. Thus, the deposited catalyst has the maximal specific activity in that case when under the law of randomness on a surface the greatest number of the active ensembles of the necessary structure is formed. Dependence of catalytic activity on degree of fullness can be classified by two main types (fig. 18). 1 type (fig. 18a). The curve of the total activity A has one maximum. The curve of a specific activity α exponential coincides with degree of fullness, asymptotically approaching an axis of activity (ordinate). 65

The ensemble from one atom of the catalyst is active. Curves of this kind were observed for processes of oxidation, a catalysis of detonating gas and decomposition of hydrogen dioxide. Curves of this kind are especially characteristic of drawing on a surface of ions, each of which can be the active center. 2 type (fig. 18b). Curves of the total and specific activity have maxima, and the maximum of a specific activity lies in more diluted layers, than a maximum of the general activity. The specific activity passes through a maximum at some degree of fullness. This type dependence describes processes of hydrogenation of olefinic, acetylene and aromatic hydrocarbons and a dehydrogeneration of cycloalkanes on various adsorptive catalysts. Maxima of curves of the general and specific activities lie in the field of the strong dilutions ~10-3-10-2 of monoatomic catalyst layers on the carrier and only in this area of fillings the subsequent conclusions of the theory of the active ensembles are fully applicable. N.I. Kobozev was not limited to qualitative aspect of the issue, and on the basis of application of a mathematical apparatus of the law of randomness he showed a possibility of the quantitative calculation of a number of atoms in the active ensemble, sizes of field of migration and activity of simple ensemble, proceeding from dependence of a specific activity on degree of fullness. N.I. Kobozev suggested the quantitative calculation of the migration area (p), the number of atoms in the ensemble (n), the total number of migration cells (z0), the activity of a single ensemble (rn).

ab Figure 18. Dependence of specific (αspec.) and total activity (Atotal) on the degree of filling the surface of the catalyst ()

66

The area of the migration region, expressed in the atomic areas of the catalyst is equal to: p= ∆/σ, (38) where ∆ is the area of the migration area in cm2, σ is the area occupied by one atom of the catalyst. The total number of migration cells on the catalyst surface is: z0=s/∆,

(39)

s – where the surface of the catalyst per 1 g or 1 cm3 of a carrier. The probability of entering of n-atoms in one cell ( wn ) is:

 n  wn   e , n!

(40)

The average number of atoms in the ensemble at an average surface concentration of the catalyst (catoms /cm2) is equal to

  c ,

(41)

As α=c·σ,

(42)

so  

   p , 

(43)

hence follows:

wn 

p n   n  e  p . n!

(44)

The total number of ensembles (zn) is:

z n  z  wn  z 0

0

 p   n  e  p n!

.

(45)

The total catalytic activity of an ensemble is determined by equality: 67

A  z n  rn  z 0  rn

 p   n  e  p , n!

(46)

where rn – a catalytic activity of one active site with n atoms. The specific catalytic activity is:

a

A  n1  p  rn  z 0  p n e .  n!

(47)

Accordingly, the total and specific activities reach a maximum at the following values of α: A  max  n / p;

a  max  n  1 / p .

(48) (49)

Thus, by degree of filling of a surface of the catalyst with the active centers at maximum activity it is possible to determine the migration area and number of atoms in ensemble. For a monoatomic ensemble (n = 1), the equation of specific activity is:

s a  r   e  p , 

(50)

which corresponds to the graphical dependence of aspecific on the curves with one maximum (fig.18, a).

3.4. General regularities of crystalline and pre-crystalline catalysts. Aggravation effects in catalysis. Valence and energy mechanisms of activation in catalysis The theory of active ensembles of Kobozev was repeatedly considered as a kind of transition from heterogeneous to homogeneous 68

catalysis, especially when an individual metal atom or an ion on the surface of a carrier functions as an active ensemble. In this direction, attention has also been increased to the theory of Kobozev in connection with the use of so-called immobilized homogeneous catalysts or catalysts containing surface fixed complexes of transition metals. Complex homogeneous catalysts in solutions show high activity and specificity. An important advantage of homogeneous catalysts is the possibility of carrying out catalytic processes at low temperatures. However, homogeneous catalysts are difficult to apply in industrial processes: they are practically inseparable from the reaction products and the solvent and have a low thermal stability. To increase the resistance of homogeneous catalysts, increasing their activity and thermostability a fixing of homogeneous complex catalysts on the surface of inorganic solid or gel-like carriers is widely used. A similar technique is applied in enzymatic catalysis. It is useful to apply the theory of ensembles to exchange zeolites in which sodium or hydrogen ions are replaced in the process of exchange adsorption by a strictly dosed amount of ions of transition elements. Heterogeneous catalysts include as a necessary component any surface – either own crystal phase, or the carrier. The theory of active ensembles of Kobozev has allowed to understand in what degree and what processes are sensitive to operation of the carrier. By comparison of inorganic and enzymatic catalysts Kobozev had allocated group of ionic and organic catalysts, having entered for them the concept of «fermentoids» (enzyme-like). These catalysts in itself show very low activity in solutions and can’t be in any comparison with ordinary heterogeneous catalysts. However deposition of similar catalysts on the carrier can both destroy and increase activity in tens, hundreds and thousands of times. The carrier is not only a fixator for ensembles, but also an essential participant in the creation of an active structure. For such systems, the influence of the nature and structure of the carrier on the properties of catalysts is no longer an exception, but a rule. The basis of these processes is a general phenomenon «weighting» or «complexity» of the active element by attaching chemical groups or complex absorbent surfaces. This phenomenon is called aggravation. High activity of complex catalysts is reached, as a rule, 69

due to decrease in energy of activation of process, and the effect of aggravation has the energetic nature. The catalyst, participating in the act of chemical transformation, can apprehend a part of energy of reaction and if process of thermal deactivation proceeds slowly, then the catalyst will be already energetically excited in the subsequent act due to which the activation energy of the process can be reduced. The second feature of complex structure catalysts is that the excitation energy, obtained in one way or another by one part of the active structure, is not transmitted uniformly over all atomic circuits. There are special chains of atoms that can easily transmit (translate) the energy of excitation. By such translational paths, energy can be transferred from the carrier to the active group and contribute to overcoming the activation barrier. The use of the energy of one reaction to excite the other is the most important catalytic mechanism. In enzymatic catalysis, the enzyme is considered as an ionicorganic complex consisting of two parts: agon (protogon) and ferron (a protein molecule). Agon includes various functional groups and metal ions and is responsible for the selectivity. Ferron is responsible for the activity of the catalyst. The effect of aggravation is the weighting of the protein part of the enzyme molecule, leading to an increase in its activity during catalysis. The physical meaning of the aggravation effect is that complex organic molecules have a nonspecific feature to retain or capture energy obtained in one way or another, and then transmit it to the active center.The aggravator serves as an energy trap. This activetion mechanism is called energy. Unlike enzymatic catalysis, the mechanism of activation in heterogeneous catalysis is called valence, since the activation is due to the transfer of electrons through chemisorption. Kobozev’s position on the effect of aggravation has acquired independent significance, and it complements the interpretation of catalytic acts from the standpoint of any other theory, including Balandin’s multiplet theory. In terms of their physico-chemical characteristics, enzymes do not differ much from other catalysts, and for a long time they were sharply distinguished by only one property – high chemical specifi70

city, adjustment for a definite transformation of certain substrates. But as the technique of obtaining enzymes in pure form was perfected, the nature of their active groups was clarified, it became possible to determine the true activity of enzymes in the form of the number of substrate molecules, converted under certain conditions by one active group of the enzyme per unit time. Enzymes differ from inorganic catalysts by colossal activity, which together with chemical specificity is the main feature of enzymatic catalysis. The absolute activity of the enzymes reaches enormous values, which exceed by several orders even the most productive inorganic catalysts. High thermodynamic affinity for the substrate is essential, but not the defining property of the enzyme. The establishment of the mechanism of the decomposition of hydrogen peroxide made it possible to find the true values of the activation energy E and the pre-exponential factor k0 (see p.1.1, tab. 1) for different catalysts. Huge activity of a catalase and other enzymes is entirely caused by large decrease in energy of activation in comparison with other types of catalysts: none of the inorganic catalysts is capable of decomposing H2O2 with an activation barrier below 46 kJ (Pt), while catalase conducts it at half the height of the energy barrier 23 kJ. The question of the difference and similarity of heterogeneous inorganic and heterogeneous biological catalysts has a fundamental meaning, as it is most typically expressed here, with one side, the usual "valence", and on the other – a special energy form of catalysis. The energy nature of activation is manifested in the dependence of the absolute activity of the catalysts, i.e., the number of transforming substrate molecules per active group in 1 sec., on the thermal effect of the reaction Qreact (fig. 19). Ultrahigh activity of enzymes is connected with extraordinary easy passing of a substratum in contact with an enzyme molecule through an activation barrier. The significant role is played also by their secondary structure peculiar to difficult molecules of the proteinaceous nature. Very important is the mobility of the elements of the enzyme during the adsorption and subsequent transformation of the elements. Active sites of enzymes and reactants form chains or cycles («chains of redistribution of bonds»), through which, as a result of the motion of protons and electrons, the multiplicity of 71

bonds synchronously changes, which causes a high compensation of the energy of breaking existing (former) bonds and a sharp decrease in the activation energy of the reaction. The enzyme strictly orientates the reagent molecules along the reaction coordinate, which increases the number of effective collisions by approximately 1, 000 times. Molecules of reacting substances under the action of enzymes pass into the most reactive forms, most often ionic, which increases the reaction rate 1, 000 times more. That the reacting substance has passed into the most reactive state, the additional reserve of energy is necessary. One of sources of this additional energy is multipoint adsorption of the reacting molecule on enzyme with use of a part of energy of adsorption on reorganization of a molecule. The second possible way to increase the energy intensity of the system is indicated by Kobozev – this is the realization in catalysis of the energy activation mechanism. Kobozev emphasized that catalysis is regarded as the exchange of bonds or electrons, taking place under conditions of statistical and energy balance with the external environment. This «valence form» of catalysis is considered so universal that it is usually not even a question of the existence of some other form of it. And yet this other form of catalysis exists and is very widely represented in the form of biological enzymatic catalysis, which encompasses a huge area of catalytic transformations in living matter. The valence mechanism of catalytic action cannot be considered quite general and there must exist another, very powerful form of catalytic activation, which is realized in biocatalysis. The question of distinction and similarity of heterogeneous inorganic and heterogeneous biological catalysts has basic value as exactly here it is most typically expressed, on the one hand, usual «valent», and with another – a special energy form of a catalysis. The energy nature of the activation manifests itself in the dependence of the absolute activity of the catalysts, i.e., the number of transforming substrate molecules per active group in 1 sec, on the thermal effect of the reaction Qreaction. Unlike enzymes for metals there is a certain minimum value of thermal effect q, at which the energy activation of the center by the crystal lattice does not occur. The quantitative approach of a complex enzyme catalyst to an elemental inorganic catalyst with a change in the energy parameters 72

of the reaction leads directly to the question: is the described energetic, i.e., non-valent, activation of enzymes an exceptional feature or is it inherent in simple catalytic systems – atomic ensembles and inorganic crystal catalysts?

Figure 19. Dependence of absolute activity on the thermal effect of the reaction: I – enzymes; II – metal catalysts; III – adsorption catalysts on carriers

If we turn to the activity of ensembles on indifferent carriers, then the dependence of activity on the thermal effect disappears. The change in activity for catalysts on such carriers as BaSO4, CaSO4, MgO, SiO2, asbestos, Al2O3, coal, lies within the same order. Among this type of carriers it is difficult even to indicate any active ensemble, which is distinguished by its action. However, the picture changes abruptly with the transition to metallic carriers, by which is meant the crystal lattice of the catalyst itself. The combination of the active ensemble with the crystal lattice of the catalyst sharply increases the activity of the ensemble. The activation of the ensemble by its own lattice increases with the thermal reaction effect by the same exponential law as the activity of the enzymes, and reaches high values for reactions with large thermal effects. Thus, even for the simplest inorganic catalysts taken in the crystalline form, there is not only a valence mechanism, but a mixed valence-energy mechanism. This significantly expands the initial view of the indifferent attitude of atomic metal ensembles to the carriers on which they are adsorbed. 73

Representations of the theory of active ensembles remain completely valid for inert carriers of the type of oxides, coals, but not for the intrinsic crystal lattice. For the crystal lattice of the catalyst itself, energy catalysis or auto activation is converted into a pronounced catalytic effect capable at rather exothermic reactions to increase efficiency of the active centers in tens and hundreds of times. Even if we assume that all the atoms of the surface of platinum black are active during the decomposition of H2O2, the calculated activity is 20-40 times less than the experimental one. The main role is played by increasing the productivity of each active center due to the evolving energy of the reaction. At the same time, it is possible that some loosely bound atoms of a lattice themselves receive a catalytic activity as a result of energetic excitation. Feature of enzymes which distinguishes them from other catalysts and gives them so high activity is almost full recuperation of energy of reaction, i.e. ability almost completely to transfer energy of reaction through an aggravator and active group. Therefore, a small thermal effect is enough to start the energy auto-activation of the enzyme. For metal catalysts, due to the auto-activation threshold (q = 80 kJ), the energy capture is 0-0.6 and, therefore, at best, up to 40% of the energy is lost useless. Although the heterogeneous catalyst and enzyme operate in a common valence-energy mechanism, the enzyme structure is a much more advanced apparatus for energy recuperation than the crystal lattice. Absorption of energy of reaction by enzyme, its movement on bonds, return to the external environment and partial return to an active group – all these stages of a fermental catalysis depend not only on complexity of a proteinaceous molecule that is the general condition, but also from its structure. In this respect, the structural problem of biocatalysis merges with the energy problem. Like any catalyst, an atomic ensemble consisting of 2-3 atoms constantly receives and gives up the energy of the reaction, but unlike enzymes, it does not have a molecular device to hold any appreciable amount – it instantly spreads over many degrees of freedom of the carrier and is lost for catalysis. Therefore, the active centers of inorganic catalysts work mainly due to self-energy associated with their valence unsaturation. Active groups of enzymes, although in their energy level, are much lower than these centers (these are 74

ordinary chemical molecules), but are constantly «fueled» by the energy of the reaction captured by the aggravator. This activation method has a huge efficiency, unattainable for conventional valence-active catalysts. The situation changes with the transition to active centers associated with its own crystal lattice, primarily metals. The active center associated with the crystal lattice carries out the primary reaction act of the transformation of the substrate by the usual valence mechanism. At the same time, the reaction energy is released on the active center, which transforms this center into an increased active state. Due to the increase in its productivity, more and more energy begins to be released on it. The excess of this energy is captured by the lattice, exciting nearby atoms. This excited state of the lattice begins relaying, for example, in the form of excitons, from atom to atom and reaches the nearest active center, which is excited to them and, in turn, further enhances the auto-activation process. At a very high intensity of the process, it can lead to disordering of the surface atoms of the lattice and to the appearance of new active centers. As a result, an «energy avalanche» or «energy chain» occurs on the surface of the catalyst, the breakdown of which is the transfer of energy into the environment through thermal lattice vibrations or electron shocks. Due to the complete energy recuperation by enzymes, the length of the «energy chain» in them, that is, the productivity of one center, is high, reaching about 50,000 acts, while for the most active platinum catalysts this chain is only about 70 acts. Thus, there may be a probability of the existence in catalysis of a general activation mechanism for certain classes of catalysts associated with at least partial energy recovery in the system and with the possibility of a noticeable increase in the activity of heterogeneous catalysts as a result of a decrease in the energy release of activation. Catalysts using the mechanism of recuperation and transfer of energy of the reaction act already as system catalysts, for which the carrier is a non-indifferent substrate, but enters the general catalytically acting system through the energy exchange function. The problem is to create, on the basis of active centers and sufficiently thermostable macromolecules, for example corresponding polymers, a system with a high degree of energy recuperation and its recoil. 75

This effect should be shown especially by comparison of activity of the polycrystalline catalyst, for example, of platinum black, and the diluted adsorptive catalysts at which active ensembles are so disconnected by the carrier (distances of the order of several tens of Å) that the transfer of energy from one center to another is practically excluded, if only the carrier has no especially high energy conductivity and small energy dissipation. On the other hand, at the disposal of adsorption catalysts there is a large surface of a carrier with a significant number of defects, so that the energy capture of such catalysts can exceed this ability in platinum black. Therefore, the combination of capture and energy transfer on adsorption catalysts is particularly effective with the sufficient energy release. For processes with small thermal effects, the activity of platinum black is 3-5 times higher than that of adsorption catalysts. During the oxidation of ethyl alcohol, a very sharp increase in activity (140 times) of one platinum atom is observed in the transition from dilute platinum layers on silica gel to platinum black. This shows that the spatial separation of ensembles on carriers strongly hinders the transfer of energy during the processes of its recuperation. The effect of energy transfer must depend on the nature of the carrier. Thus, when using metallic cadmium as a carrier, the activity of an ensemble of platinum during the decomposition of hydrogen peroxide is approximately 10 times greater than its activity on other carriers. Thus, selection of carriers with enhanced recuperation and energy conduction by an exciton or other mechanism is possible. It should be considered with at least two mechanisms of capture and transmission of energy through a crystalline and protein carrier – with an exciton mechanism and a resonant one. Thus, catalysts and enzymes work along the same valence-energy mechanism. Atoms of the active phase, penetrating into the carrier, excite neighboring atoms of the carrier and create new adsorption centers. As a result, it is possible to consider the active site as a set of many atoms, the functions of which are delimited to a certain degree. If one of these atoms directly carries out the act of reaction, then a role of other atoms included in the active center comes down to preliminary adsorption and partial activation of the reacting subs76

tances with their subsequent migration on the atoms which are carrying out reaction. The introduction of an «inert» carrier (alumina, silica gel) into a system containing a catalyst (platinum, palladium, nickel) to form a mechanical mixture can cause a significant increase in the hydrogenation rate for well adsorbed unsaturated compounds and on catalysts containing adsorbed hydrogen. For palladium and compounds poorly adsorbed (hexene), the addition of the carrier does not affect the speed. This confirms the separation of functions of individual sets of the surface and the significant influence of bulk catalyst layers on the surface state. For a carrier of a given chemical nature, the porosity of the carrier should have a large effect on the activity. It is essential that the activity of the catalyst per unit surface area (specific activity) in the reactions of hydrogenation of benzene, dehydrogenation of cyclohexane and dehydrocyclization of n-hexane strongly depends on the porosity of the support. The selectivity of the action of the catalysts varies with a change in the nature of the carrier in such wide ranges that this practically excludes the idea of inertness of the carrier in catalytic processes. It should be noted that until now the active ensembles theory did not give practical tips on the selection of catalysts, or defined ideas about the processes proceeding on the industrial catalysts. Electron microscopy didn’t confirm the amorphy of supported catalysts.

3.5. Chemical theory of the active surface of S.Z. Roginsky Concept «the active surface». Poisoning and blocking of a surface of the catalyst Now in a catalysis the issue of the nature of the active surface is not resolved. The experimental confirmation of inhomogeneity of a surface of catalysts at the same time proves presence of sites of various activity on the surface. From this point of view, emphasized Roginsky, the concept of a standard chemically surface appears to be as abstract as the concept of the ideal solid-state or an ideal gas. Roginsky considered that any kind of physical heterogeneity of the surface is unstable in catalysis conditions: physical disturbance of the 77

crystalline lattice are unstable over time and particularly under the influence of temperature. Instability of physical inhomogeneity is especially shown on films of pure metals, which were condensed from vacuum on a surface and cooled up to the temperature of liquid air. Such films are characterized by high dispersion, their physical properties indicate significant disordering of structure. At the same time pure and highly dispersive films with strongly broken lattice structure are very little stable when heating: already at a temperature of liquid air their recrystallization begins, and at room temperature the film supported on a suitable crystal face quickly becomes monocrystal. In contrast, the chemical stability of disorders is shown, for example, that the total outgassing of metals can be achieved only in a high vacuum as a result of continuous pumping at temperatures close to the melting temperature of the crystalline body. Even more difficult the removal of non-volatile impurities, for which a heat pumping is not applicable. The positive effect has a transition from pure single-component catalyst to multicomponent. Mixed and promoted catalysts for a wide variety of contact groups and for different processes are more perfect than the simple contacts. Features of the complex contacts are often associated with the formation of the interfaces between the solid phases with the different chemical composition and with the introduction of impurities into the surface layer of the solid body in the place of the phases contact. Idea about the considerable stability of chemical disturbances in good agreement with the fact that the technical catalysts are capable to work for a long time at high temperatures. Roginsky made the assumption that at genesis of prime contacts there can be an activation of the catalyst by impurity, similar to activation which is artificially carried out in the complex systems by the entered additives. Different crystals and different parts of a surface can have various chemical micro-composition and various activity, the nonuniform surfaces as a result turn out. In catalysis the crystals or sites which incidentally received the optimal content of impurities will show themselves. Impurity can poison the catalyst, and the catalytic reaction may be a source of the promoting and poisoning impurity. 78

Chemical theory of the active surface is the foundation of Roginsky views on the nature of the active surface. The pure specifically degassed metals (including metal thin films deposited on glass) completely catalytically inactive to hydrogenation of ethylene in the presence of nickel, copper, iron, platinum, palladium, and tungsten. The films of the same metal with a precisely dosed amount of gas impurities originally change the catalytic activity: depending on the content of the captured impurity the films activity passes through a sharp maximum. Activity is demonstrated by the metal films taking hydrogen, oxygen and nitrogen during the time of condensation. Lack of capture of gas and effect of a promotion for argon indicates an essential role of the chemical forces binding the additive with a lattice. At a high absolute and specific activity the layers promoted by gases differ in small stability. For most of them decrease of the activity at repeated carrying out reaction is observed. This decrease is partially caused by poisoning with products of reaction. Along with it deactivation, associated to reorganization of a layer is observed. In particular, the tungsten layers promoted by nitrogen are ireversible lose activity already at short-term heating in vacuum. For technical contacts only those impurity which contents decrease continuously is replenished with reaction, or the impurity which are strongly held by a solid body can have activating action. Thus, Roginsky concluded that absolutely pure catalysts cannot be. In any even the purest reagents used during the preparation of catalysts, may be impurities of other substances. In the precipitation and reactivation of the catalyst, such an impurity remains to a varying degree. Further capture of micro additives from solutions at preparation owing to a high adsorption capacity of a surface of the catalyst is possible. This capture of micro additives, inevitable at genesis, is capable to change catalytic properties of a solid body strongly. From the point of view of the theory of chemical inhomogeneity the strongest effect on the catalyst surface the chemically active substances, and first of all the contact poisons most of which effectively chemically interact with a catalyst lattice should have. There must be optimal promotional effect with the small amounts of impurities. The same additives depending on their concentration in the catalyst can increase its activity (promoter), or decrease (contact poisons). 79

Blocking of a part of a surface of the catalyst by poisons is the base of poisoning. As a result the blocked part of a surface doesn’t participate in process. At the same time on the surface does not create any new areas, and part of the surface, remained without poisoning, working with the same parameters as before the poisoning. From this perspective, it is easy to formulate the characteristics that must be shown in case of poisoning of a homogeneous (uniform) surface. A. Poisoning by blocking of a homogeneous (uniform) surface Poisoning by a uniform surface blocking in the kinetic region is equivalent to a simple reduction in the absolute value of the surface and reduce of the catalyst amount. 1. The mechanism of the reaction and the ratio between the velocities of parallel and consecutive stages from which the total catalytic process is obtained, must not be changed. The poisoning should not cause changes in the composition of the reaction products. 2. The main kinetic characteristics of the process should not be changed. 3. The activation energy Ec of process, defined at a constant surface coverage by a poison, should coincide with a similar value for the process that takes place on the catalyst which wasn’t poisoned. At poisoning with blocking the changing magnitude is the general catalytic activity or a specific reaction rate. Proportional decrease of a catalytic activity with concentration of poison is a sign of uniformity of a surface and it is difficult explainable for the nonuniform surfaces. B. The poisoning by blocking of non-uniform surface At generality of poisoning mechanisms on the homogeneous and heterogeneous surfaces the new for the non-uniform surfaces is: 1) A possibility of non-equivalence of sites, blocked and free from poison. Disappears a mandatory for a homogeneous surface unambiguous relationship between the amount of blocking poison and the catalyst activity. As essential parameter there is a depen80

dence of activity on the nature of placement of poison on the surface of the catalyst. Being located differently on the surface, the same amount of poison can have a different impact on the activity. The poison is distributed on a surface irrespective of activation energy of sites. In the analysis of processes of poisoning on non-uniform surfaces Roginsky proceeds from function of distribution of active sites р(Е) on reaction activation energy E (fig. 20). Reaction is carried out on sites with the smallest energy of activation (double hatching in fig. 20). The nature of distribution of poison to surfaces is reflected the controlling strip of mn defining a share of the poisoned sites. Reaction is carried out on sites with the least activation energy. At the arbitrary distribution of poison on a surface the amount of the active sites is not enough, and the amount of the poison which is got on these sites is also small. On the contrary, there is a lot of inactive sites, respectively on these sites there is a large amount of poison. Then the quantity of the active sites participating in reaction changes in proportion to amount of the applied poison. Decrease in number of sites of any type without change of activation energy, order of reaction and other features of process will be the single result of poisoning. Thereof proportional decrease of activity with concentration of poison will be observed and, thus, the nonuniform surfaces will imitate the homogeneous.

a

b

Figure 20. Poisoning by blocking the catalyst with an inhomogeneous surface: a – the poison is evenly distributed over all types of areas (the area of poisoning is shaded); b – change in the total activity of the catalyst from the concentration of poison

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Proportional decrease of activity from concentration of poison is possible also upon slice poisoning of catalyst. Decrease of number of the working catalyst without change of all other characteristics of process will be result of similar poisoning. Therefore, irrespective of what nature of distribution of sites of the nonuniform surface on activity, poisoning of catalyst will be similar to poisoning of the homogeneous surfaces. 2) Microscopic blocking begins with the least active sites while reaction is carried out on the most active sites with the least activetion energy. The control band mn in fig. 21 moves from right to left. At the same time, the activity of the catalyst to a certain extent will not depend on the concentration of the poison, and only after reaching a certain critical concentration of the poison will there be a sharp poisoning of the catalyst and, accordingly, a change in the activation energy. To do this, it is necessary that the heats of adsorption of poison and reactants are different for the same site.

a

b

Figure 21. Poisoning by blocking an inhomogeneous surface: a– the poison concentrates on the active sites in the least active areas with Emax., b-change in total activity from the poison concentration

3) In case of poisoning poison fills the surface, starting with the most active sites. The control band mn in fig. 22 moves from left to right. The first portion of the poison act most strongly. Change of activity from amount of poison is defined by a distribution function of the active 82

sites on surfaces. Activation energy gradually changes with degree of fullness of a surface. At poisoning of the nonuniform surfaces activation energy of reaction changes and depends on the nature of distribution of poison on a surface.

a

b

Figure 22. Poisoning by blocking an inhomogeneous surface: a-poisoning begins with the most active sites at Emin, b – change of activity with concentration of poison

3.6. Theory of supersaturation Theory of supersaturation does not concretize the nature and structure of the active surface, and, proceeding from thermodynamic concepts, allocates as typical carriers of special properties of the active surface the thermodynamically unstable states possessing an excess free energy. From this point of view the theory covers all types of special active structures, peaks, holes, ensembles, weak points of a lattice, etc. and considers at the same time a chemical inhomogeneity as one of many types of the non-equilibriums having a kinetic growth origin. A measure of the excess free energy of a surface can be its supersaturation – the change in free energy upon transition from a given state system to a stable, equilibrium under given conditions. The excess free energy of the solid body isn’t directly used, and does not appear in catalysis. It serves as a prerequisite of the formation of certain structures having desirable properties, including the catalytic. 83

Being thermodynamic unstable under one conditions, these structures under other conditions should become stable, but it cannot affect their catalytic properties because the behavior of each separate atom of the catalyst is not defined by such integral category as chemical potential. The value of supersaturation of a surface must depend on a method or a way of its preparation, speed of its formation and on some other conditions in which there is the surface formation. According to it possible types of supersaturations are classified in the theory as follows: 1) dispersion supersaturations, associated to emergence of large specific surface areas; 2) the phase supersaturations caused by obtaining of unusual unstable phases, presence of incomplete phase changes; 3) the chemical supersaturations arising in the conditions of producing bodies with unusual chemical composition that is most often associated to capture of impurity. Conditions of preparation of catalysts allow to change supersaturation magnitude over a wide range. At preparation of oxide-coated or metal catalysts supersaturation may be regulated by changing pressure of the emitted gas or pressure of gas-reducer and also temperature. For obtaining the active supersaturated structures preparation of catalysts needs to be carried out in the conditions far from equilibrium. Removal from equilibrium not only does possible emergence of excess energy in the products of reaction, but also provides primary emergence of the corresponding forms. Regulating the environment composition, it is possible to influence in the desirable direction composition of the additives remaining in the catalyst. It, in turn, determines durability of capture and stability of ultimate structures. Concentration and a form in which there is an impurity are important. There are optimum on the content of impurity, and excess of impurity, as a rule, reduces activity. Therefore there is a conclusion about probability of an optimum on concentration of impurity and an optimum on supersaturation. These two values must be linked with each other: the higher the supersaturation, the smaller the optimal concentration of impurities and fewer requirements to chemical 84

active form from which it is introduced. On the contrary, adding more active materials into the system, it is possible to lower requirements to supersaturation and to pass to the substances capable to modify ready contacts. The theory of supersaturation allows to prove need of keeping of a number of the conditions important for obtaining the active catalysts: 1. It is expedient to prepare the catalyst quickly. The quicker there is a process, the higher probability of formation of disequilibriums, the more supersaturation of a surface. 2. At reduction of catalysts in dynamic conditions it is necessary to increase feed rate of gas-reducer for the maximal removal of the reaction products and creation of the maximal supersaturation. 3. At endothermic processes of preparation of catalysts it is favorable to work at high temperatures (to field of a sintering of catalysts) with fast rise in temperature. 4. If the catalyst is prepared by a multistep process (sedimentation, drying, decomposition, reduction), it is possible to collect and summarize the supersaturation in stages; i.e. necessary to carry out each stage so that the supersaturation is maximized. Thus, in the theory of supersaturation the quantitative interpretation of relation of the free energy of a surface with its activity is for the first time was given. This bound in certain cases may be characterized quantitatively, by calculating the supersaturation magnitude at preparation of catalysts. In the organic catalysis the catalytic activity in many cases is defined not only the free energy, but also structural correspondence, and it is quite possible that the catalyst with smaller supersaturation, but with more suitable lattice parameters will be more active. On the base on the principle of conformity of the energy it’s need to wait for a certain optimum supersaturation. The higher the free energy of the surface, the stronger it binds with the reacting substances, that should change the limiting step of the process. Stability of supersaturated systems is often small. During the catalyzed process some stationary state of the catalyst determined by the nature of the carried-out reaction and process conditions is set. A stationary state the catalyst may achieve from both high, and low supersaturations. 85

3.7. The main ideas of Boreskov. Boreskov’s rule Boreskov emphasized a defining role of the intermediate chemical interaction in heterogeneous catalysis: 1. Changes in rates of chemical reactions in heterogeneous catalysis is caused by intermediate surface chemical interaction of the reactants with the catalyst. The activity of the solid catalyst in the given reaction is determined primarily by its chemical properties. 2. Catalytic activity is inherent to the normal surface of crystalline solids and isn't associated with a special condition or special structural blocks of their surface. 3. Specific catalytic activity (activity of unit of a surface) of catalysts of constant composition is approximately identical. The major factor defining specific catalytic activity is chemical composition and chemical structure of the catalyst. 4. Increase in activity of the unit volume characterizing the industrial value of the catalyst is reached by increase in the working surface. It may be provided with increase in an internal surface and creation of the optimum pore structure of the catalyst providing high degree of use of its internal surface. At simultaneous course of several reactions change of pore structure allows to regulate in particular limits also selectivity of operation of catalysts. Boreskov believed that for creation the theory of catalysis must first resolve the issue of the nature of the interaction between the catalyst and the reactants. The nature of this interaction is shown in correspondence of chemical properties of the catalyst with properties of reactants and in specificity of action of the majority of catalysts. Underestimation of a role of chemical interaction in heterogeneous catalysis, according to Boreskov, is shown in interest in searches of special catalytic active structures (active centers) at absolutely poor studying of dependence of catalytic activity on chemical composition of the catalyst and properties of the surface intermediate compounds. As a result the unity of the catalytic process as a process comprising reacting of the reactants with the solid catalyst, is broken. The catalyst is considered only as the geometrical place of course of reactions which is characterized by a particular potential; the 86

impact of reactants on the catalyst leading to change of its properties is ignored. For the development of the theory of preparation of catalysts, it is very important to know to what extent the specific catalytic activity of a constant-composition catalyst can vary, ie, to what extent does it depend on the state of the substance, on the methods of preparing catalysts. It was previously considered that, depending on preparation conditions, heat treatment and other factors the catalytic specific activity of the catalysts of constant composition can be changed widely. The total activity of the catalysts as noted Boreskov, is proportional to the area of their working surface and can be expressed as the product of the area of the surface on the specific catalytic activity. The share of the working surface of the catalyst depends on the specific activity, pore structure, grain size of the catalyst, temperature, composition of the reactionary environment and other factors defining reaction rate. It is difficult to determine the area of the working surface of the catalyst. But even under these conditions use of a specific activity as specific characteristic of catalysts of the given chemical composition represents great importance for the theory of catalysis. If the single factor determining the value of a specific activity is chemical composition of the catalyst, then particular dependence of catalytic activity on the position of the elements in a periodic system of D.I. Mendeleyev thereby is found. Independence of a specific activity of a method of preparation of the catalyst, structure of a surface and degree of dispersion of metal is confirmed in oxidizing reactions of hydrogen and SO2 on platinum catalysts. At change of a surface area and activity on 1 g of the catalyst on 5 orders the specific activity on unit of a surface changes no more than by 2-3 times, corresponding to a condition of approximate constancy of a specific activity. So, at hydrogenation of benzene on nickel catalysts which surface areas differed on two orders and the degree of dispersion changed from 28·10-16 to 30·10-16-35·10–8 cm, the specific activity for all samples defined at 100 °C was almost constant and changed by 1.4 times. Boreskov’s provisions have been deepened, specified and developed further in works of both G. K. Boreskov, and his pupils; despite 87

criticism from adherents of structural approaches to understanding of the phenomenon of a catalysis, they have played very important role in formation of modern ideas of science of a catalysis and have defined the directions of researches for several decades ahead. Beginning with the moment when Boreskov first expressed his views, research on catalysis began to develop in three main directions: 1. Creation of the theory prediction of catalytic action: – a detailed study of the nature of the intermediate compounds, the mechanism of reactions: – disclosure of the dependence of the specific catalytic activity on the chemical composition and structure of the catalyst at the atomic-molecular level. 2. Creation of scientific bases of preparation and technology of catalysts The purpose of this direction is a development of theoretical and experimental approaches to purposeful synthesis of catalysts with the set properties: ‒ chemical composition, ‒ phase composition, ‒ structure of an active component (or active center), ‒ textural characteristics. It is necessary to emphasize especially that scientific bases of preparation of catalysts include the solution of two tasks: – management of process of synthesis of chemical and phase composition of the catalyst, – management of formation of texture of the catalyst. 3. Development of scientific foundations of technology of catalytic processes: – study of the kinetics of catalytic reactions, – mathematical modeling of catalytic processes: determination of the type of catalytic reactor, optimal process conditions, technological scheme. Boreskov’s rule Specific catalytic activity depends on a condition of a surface a little and is defined by generally chemical composition of the catalyst 88

and its chemical structure. However this rule is limited to the following provisions: 1. Various activity of sides of crystals. 2. Essential changes of the catalyst composition happen under the influence of reactants. The method of preparation of catalysts has significant effect on their activity because from a method of preparation the value of the area of the working surface, degree of its purity and depth of interaction between catalyst components depend. Application of the EPR method showed that cation with a particular charge can act as the active site. Thus, only a part of a surface area is active (sometimes to 40 – 50%) and the active sites differ from other part of a surface in chemical composition or a charge. For the catalyst working in the stationary conditions the number of the active areas depends only on chemical composition and the nature of reaction. 3.8. Electronic effects in catalysis. Catalysis by metals and alloys. The band theory. Ideas of L.V. Pisarzhevskii, D. Dauden. The valence theory of Pauling Heterogeneous catalytic reactions, like any chemical reaction at all, are based on the electronic mechanism, since the transformation caused in the reacting molecule by catalytic reaction, are determined by the movement of valence electrons. However, feature of interaction in heterogeneous catalysis is manifested in the fact that the exchange by electrons between the reacting bonds is through the catalyst with participation of catalyst electrons. Electronic interaction on a surface defines energy and the nature of the arising bond. These factors influence the speed and the direction of the catalyzed reaction. On the other hand, radiuses of atoms and ions are defined by a structure of electronic shells. Therefore the geometry of a crystal and its surface is defined by also electronic factors. Metal catalysts are used in hemolytic reactions such as the hydrogenation and dehydrogenation processes, platforming, oxidation. Most commonly are used metallic catalysts on carriers. 89

The main characteristic feature of metals that distinguishes them from semiconductors and dielectrics, is the presence of mobile electrons, providing high electrical conductivity of the metal. Metal consists of systems of cations and free electrons. Activity of metal catalysts in different reactions is various: 1) For hydrogenation reactions Pt, Pd, Ni and other metals have high activity; 2) For ammonia synthesis metals may be arranged on decrease of the activity: Ru > Fe > Co > Ni > Rh > Re > Pt; 3) For hydrogenolysis reactions: Rh > W > Ni > Fe > Pt > Co; 4) In oxidation reactions platinum has maximum activity and silver has the maximum selectivity. So, the transitional metals, especially such as, Pt, Pd, Rh, Co, Fe, Ni, have high catalytic activity. For the first time the question of the relationship of electronic properties of a solid body with its catalytic ability was raised by L.V. Pisarzhevskii. The presence in a crystal lattice of metal of the equilibrium process leading to ionization defines dependence of catalytic activity on electronic structure of a solid body. In the catalysis by solid bodies (metals, alloys, etc.) the big role is played by incomplete d-electronic orbitals. Direct participation of d-electrons or holes in a d-zone in this catalysis is allowed and supposed that value of d-electrons, first of all, is connected with formation of the metal phases having stability in the conditions of catalysis. The problem of electronic structure of the transitional metals is far from the complete decision and now at the description of properties of metals it is impossible to do without application of both a method of the molecular orbitals (MO), and a method of the valence linkages (VL) promoting clarification of a structure of metals in several different aspects. The electronic condition of the transitional metals is defined by their physical properties (melting points and boilings, interatomic distances, durability or hardness of a crystal lattice, etc.). For metals of the 4-th period the lattice strength increases from K, Ca, and Ti, V, reaches a maximum for chromium and then decreases for Mn and again somewhat growing in the row Fe → Co → Ni. When taking 90

into account the distribution of d– and s-electrons in transition metals in addition to these physical properties of great importance is attached to the magnetic properties. From a modern point of view the magnetic properties of metals are determined by d-electrons with unpaired spins. In the transitional metals d-electrons participate in formation of a metallic binding. Therefore the quantity of d-electrons on one atom in a crystal sharply changes in comparison with an isolated atom. The main difficulty of establishment of dependence of catalytic activity of the transitional metals on an electronic structure consists in it. Depending on distribution of d-electrons in a crystal its electronic characteristics change. For an explanation of catalytic activity of metals two theories are used: 1) Band theory of metals; 2) Valence theory of metals of L. Pauling. The greatest success in the learning of an electronic structure of metals is achieved with application of a band theory. Special properties of the transitional metals are explained by presence of d-electrons. The main characteristical difference of metals is the overlap of the top zones, for example, for the fourth period the overlap of 3р-, 3d– and 4S-zones takes place. As a result in metal there is no forbidden band dividing the top empty and filled with electrons lower zones. The border between the filled and vacant levels of zones is called E. Fermi’s level, that is an energy level and probability of filling with electrons at the temperature above -273.15ºC which is equal to 50%. As for d-electrons, they occupy more narrow orbitals. From them 3d-level has more localized character, and 5d-electrons – more collective zonal character. The model of s– and d-exchange which allows presence of localized system of d-electrons and collective system of conduction electrons at the same time is most widespread. So, in oxides of the transitional metals d-electrons occupy five d-orbitals on the background of wide s– or p-zone. At adsorption of substance on metals the charged particles are formed. If interaction energies with a solid body are small or among metals remain almost constant, then the heat of absorption will depend generally on an electron work function. 91

However, experimental data indicate that the work function of the electrons (υ) does not play a decisive role in catalysis on metals. For example, the change of the electron work function in the metals row υCo