pressure. teniperature, CoiicenLra tion, and cunrdct
KIIIX.
Such techniques as photocxcltation a n d radidrrion horribardmetit may be helpf~rlbur are of litnircd applicability for processing on a largc scale Bcvond, &cse four basic \ ariahles, the most colntlloli an us are the use of a catal) SI and the ni
---.
*
.
Successful development of new catalytic processes takes place when scientists and engineers in industry, government, and academic fields investigate established and “obsolete” methods as well as seemingly unrelated contributions from many sources to discover new reactions and uses
tant as the nature of the catalyst in the results one obtains. Generally, if an excellent catalyst is found for a reaction, engineers will find some way of incorporating this into a process. However, we are ultimately concerned with optimizing the combination of catalyst and a process in which it will be used, and many fine catalysts in the laboratory are difficult to apply in practice. An excellent laboratory catalyst may not fluidize well or may be poisoned easily by industrial feedstreams. The intimate interaction between the nature of the catalyst and the nature of the contacting must be recognized not only by the engineer concerned with industrial problems but also by the experimentalist a t the laboratory bench who does the initial work. The rate of a chemical reaction may be limited not by the intrinsic characteristics of the catalyst being used, but instead by the rate of absorption from a gaseous reactant being bubbled into a flask. What comes out of a reactor is profoundly affected by the residence time distribution of the molecules in the system and the environment in which
reactions and new catalysts. Discipline-oriented investigators are primarily concerned with the manner in which a reaction goes-in technical jargon “mechanism” -whereas the mission-oriented investigators are primarily concerned with the efect of the catalyst. Impacts from other fields will be discussed below. Selected Major Catalytic Processes
Detailed attention devoted to understanding an important and profitable industrial catalytic process in the past frequently has developed a useful concept in catalysis or opened up a field of investigation leading to a new process of high value. The imaginative carry-over of ideas developed from one process, catalytic or noncatalytic, to another area of catalysis has, in several instances, paid large dividends, although the exploitation of the idea has frequently been by a different kind of organization and in a substantially different direction. Examples are readily apparent by examining some of the significant catalytic Processes as listed below :
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Conversion of SO2 to SO8 on a supported platinum catalyst was the first large-scale heterogeneous catalytic process. The reaction is simple and there are no byproducts. At low temperatures, the forward reaction is not fast, and the gas must be heated to a “kindling temperature” before appreciable rates can be achieved. At high temperatures the maximum conversion is limited not by the rate of reaction but by thermodynamics. Some intermediate temperature range is optimum. The problem of how to steer between this Scylla and Charybdis has led to many useful ideas in reactor design broadly applicable for carrying out exothermic reactions of this type. The platinum catalyst in the converter was found to be highly susceptible to poisons, such as arsenic, so that it was replaced by catalysts incorporating vanadium oxide as the active ingredient. The development of these catalysts led, in turn, to their use for the catalytic oxidation of organic compounds as well as SO2 and are used today in largescale manufacture of intermediates such as phthalic anhydride and maleic anhydride. Fixation of nitrogen was a problem of central concern to chemists prior to World War I. The name most frequently associated with the successful synthesis of ammonia from nitrogen and hydrogen is that of Haber, who developed the process; but the industrial catalyst was developed by Mittasch, who, with typical Germanic thoroughness, examined over 20,000 catalyst compositions in his search for the optimum material. The resulting catalyst, essentially iron containing small amounts of alumina and potassium oxide, is still, with minor modifications, the catalyst used in industrial ammonia converters. One result of this detailed search was the concept of a textural promoter, an inert substance added in small amounts to the active catalyst ingredient, whose role is to maintain catalyst stability by preventing graingrowth of the primary ingredient, and hence loss of surface area and loss in activity. Textural promoters are now widely used, as in the incorporation of chromium oxide in zinc in the usual industrial catalyst for synthesis of methyl alcohol at high pressures. T o make nitric acid, a mixture of air and ammonia vapor is passed over multiple layers of platinum gauze which converts the ammonia to nitric oxide. The success of this process, in turn, led Andrussow to develop an industrial method of synthesizing hydrocyanic acid using essentially the same catalyst and contacting method, but with the addition of methane to the ammonia and air mixture. The general approach of trying to cause two or more reactions to occur simultaneously rather than separately has led to the invention of useful and profitable processes. 6
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Ln
c
8.0
=3 z
10
100 1000 DISTANCE FROM SURFACE O F CATALYST, MICRONS
4000
Electron microprobe analysis of a catalyst comprising I w t yo palladium impregnated on a spherical alumina support reveals marked nonuniformity of distribution of the metal. The peak at about 2000 p from the exterior surface suggests the existence of a jine-pore inner shell in the carrier [SatterJield, C. N., PelossoJ, A . A , , andSherwood, T . K.! A.Z.Ch.E. J . , 15, 226 (7969)]
The first significant use of solid catalysts in the petroleum industry was that of acid-treated clay and later, synthetic silica-alumina, to cause the cracking of petroleum feedstocks and thereby increase the yield and octane number of the gasoline produced. These solids are acidic and the intense effort to understand the mechanism of acid catalysis, stemming largely from this industrial development, as well as catalysis caused by liquid mineral acids, led to much work on carbonium ion chemistry and the application of acid catalysts in other hydrocarbon reactions such as hydrocracking and isomerization. Catalytic reforming, using a catalyst of platinum supported on an acidified alumina, produces an improvement in octane number of gasoline with little change in molecular weight and is also a source of most of the aromatic chemicals used as feedstocks in further petrochemical operations to manufacture polymers and synthetic fibers. This was the first “dual-function catalyst” used industrially. Both acidic sites and platinum sites are required on the catalyst to achieve the desired overall reaction. The pursuit of the concept of multifunctional catalysis has led to fruitful ideas about the proper balance of strength and number of metal and acid sites and has led to the design of improved catalysts for other petroleum processes. The reexamination of
catalysts used for other reactions in light of this concept also shows, for example, that poor selectivity of a catalyst may be caused not by the inherent characteristics of an active catalyst ingredient but rather by the incorporation, completely unwittingly, of an undesired function in the catalyst. For example, alumina is one of the most useful and ubiquitous catalyst carriers and is usually thought of as being inert. But some methods of preparation may lead to a material having acid sites. I n a recent industrial process using an alumina-supported catalyst for a hydrogenation reaction, the selectivity was much better with an impure feed material than with pure laboratory-grade reagent. Nitrogen bases in the impure material were poisoning acid sites on the alumina, thus preventing undesired cracking reactions. Stereospecific polymerization by so-called ZieglerNatta catalysts has made possible the production of polyethylene by low-pressure processes. I t has also led to much useful thinking about the relationship between the detailed geometry of adsorbed intermediates and the resulting structure of a polymer, the role of coordinate bonding in catalysis and the use of controlled polymerization to manufacture relatively small molecules, such as those comprising three or four monomers. The manufacture of acrylonitrile, CHz=CHCN, from propylene, air, and ammonia is another example of combining reactions to achieve a desired product in one step. Acrolein may be formed by the catalytic partial oxidation of propylene with air, but, by the addition of ammonia to the mixture, acrylonitrile, a more useful chemical, is formed. These processes are noteworthy also in that bismuth phosphomolybdate was the first heteropolyacid catalyst used industrially. Recently it has been replaced by one based on uranium and antimony, the first industrial use of uranium as a catalyst. Others could be added to the above list of processes, such as the use of controlled catalyst poisoning in the partial oxidation of ethylene to ethylene oxide on a silver catalyst. Clearly the detailed examination of any effective novelty in a catalytic process can lead to many fruitful ideas frequently applicable to a situation far removed from the original process.
Trends in Chemical' Processing
Examination of the characteristics of new processes being introduced and those being superseded shows several interesting developments. Mild reaction conditions. The trend is toward lower pressures and temperatures; less use of a sledge hammer,
more use of a feather. We see, for example, the manufacture of acetaldehyde from ethylene and oxygen in an aqueous catalyst system by the Wacker process which operates at the boiling point of water at pressures below 100 lb/sq in. Heat evolution and the reaction temperature can be well controlled by the pressure a t which the water is allowed to vaporize. This replaces the older vapor-phase oxidation of ethyl alcohol at temperatures of 375 to 500" C. New hydrochlorination processes take place in the liquid phase rather than the vapor phase. Hydrocracking in the petroleum industry is being carried out at substantially lower pressures than were called for a few years ago, achievable largely by the development of more active catalysts. High selectivity. Mild processing conditions, of themselves, generally allow high selectivity by making possible improved temperature control in contacting. Controlled poisoning of catalysis likewise helps bring about improved selectivity. Multifunctional catalyst system. This is a fruitful approach to understanding the behavior of present catalysts and to devising new compositions. One industrial group appears to have developed a family of multifunctional catalysts for a variety of hydrocarbon processes which involve different balances of number and kind of acid and metal sites. Homogeneous catalysis. Some homogeneous catalysts show remarkable activity and selectivity under very mild processing conditions. Only the tip of a n iceberg is visible, but it is probable that all major chemical and petroleum companies have, or should have, an active program investigating the impact of homogeneous catalysis on their products and processes. Fewer processing steps. Every processing step, be it reaction or separation, adds to costs. Use of a reactant which contains elements not existing in the final product represents additional processing steps and a by-product to be disposed of. For example, a large group of industrial reactions require oxidation. Here we see increasing emphasis on use of air or oxygen directly in place of more expensive oxidants. Direct oxidation and direct hydrogenation processes may well be of increasing importance in the future. Less reactive feedstocks. The trend in manufacture of chemical intermediates is toward less reactive and less expmsive starting materials. Fifteen years ago acetylene was a major raw material in chemical processing, useful because of its high reactivity. Gradually it has been replaced by ethylene or other olefinic compounds which are less reactive and less costly but where the availability of superior catalysts and improved processing techniques compensate for the lower reactivity. The obvious next VOL. 6 1
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step is to replace olefins by paraffins. The limitation here is not so much the lower reactivity of paraffins, but rather, as molecular weight is increased, the number of possible hydrocarbons of similar size and physical properties increases exponentially, causing a corresponding increase in the cost of isolating a single relatively pure compound. Even if this is surmounted, the quantity of the desired compound in the feed stream may be insufficient for an economical operation. Also, with increased molecular weight, the paraffin molecule may be readily attacked at more than one location, causing poor selectivity. In this respect the olefins have an obvious advantage. Future Impacts on Catalytic Processing
Modern instrumental methods. Most solid catalysts are complex and ill-dpfined materials. M’e‘need to know what we have before we can improve it, or before we can understand the reasons for various changes we observe in catalyst activity. Modern instrumental techniques make it possible to identify sources of catalyst activity in ways that would have been impossible a few years ago. The rate of reaction is proportional to catalyst area, and the determination of area by the wellestablished BET (Brunauer-Emmett-Teller) technique is routine, as is also gas chromatography- for quantitative analysis of products. I t is always helpful to see what one is working with and the eye and the optical microscope have been supplemented by the electron microscope, in which continuing improvement makes it possible to resolve smaller and smaller dimensions. The recent development of the scanning electron microscope makes it possible to obtain a three-dimensiondl picture of a surface which is frequently far more meaningful than that obtainable by the conventional electron microscope. By electron microprobe analysis a section of a catalyst pellet can be scanned, and the nature and concentration of metal as distributed throughout a catalyst pellet can be determined down to dimensions on the order of a micron. One can thus learn simply and rapidly how effectively one is able to distribute a catalyst ingredient throughout a carrier, or how active catalyst compounds may have migrated through the catalyst bed or through a catalyst pellet during processing in an industrial reactor. This technique also provides a nice way of determining the amounts and manner of deposition of metallic poisons. Crystallite size is a measure of the effectiveness with which a n expensive ingredient, such as platinum, is distributed on a carrier and this may be determined b y chemisorption and x-ray diffraction. Microcrystalline size in general and crystal habit are frequently important to know. Chemisorption of hydrogen or carbon monoxide may be used to distinguish between total surface area and the surface area of an active supported metal on a carrier. Instrumental methods are generally introduced as research tools and then become increasingly important in solving practical catalyst problems; for example, electron spin resonance (esr) and nuclear magnetic resonance 8
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
(nmr) may make it possible to tell which valence form of a n element is the active species for reaction-whether in a chromium catalyst the reactive form is chromium 111 or VI. M’ith little exaggeration, we may say that almost any new instrument that becomes available has a good probability of having some impact on our understanding of catalysis. Coordination chemistry and homogeneous catalysis. Transition metals can form complexes with a wide variety of neutral molecules-for example, with substituted phosphines and carbon monoxide. The metal atoms in these complexes may be in a low-positive, zero, or low-negative oxidation state. Some of these structures have remarkable activity and selectivity as homogeneous catalysts. Those that do so have one or more coordinate bonds, with the originally neutral molecule entering into this coordinate bond known as a ligand. The property of greatest importance in catalysis is the availability of a vacant coordination site in the molecular structure. These molecules are active a t low temperatures for such reactions as hydrogenation and for “insertion reactions”-the entry of an unsaturated molccule into a metal-ligand bond such as
0 They also cause addition reactions of unsaturated molecules such as the dimerization of olefins. An example of such a hydrogenation catalyst is the rhodium chloride complex shown below. It may be readily made H
CI
P
=
IPPhl3
S
=
SOLVENT
by mixing rhodium 111 chloride hydrate and excess triphenyl phosphine in boiling ethanol. The structure above, an octahedral complex, is the probable form in which the molecule exists as a hydrogenation catalyst. When we consider hydrogenation of a n olcfin as a n example, we note one of the sites is normally occupied by a solvent molecule. This is displaced by a n olefin molecule, the t ~ 7 0bound hydrogen atoms are then transferred to the olefin, and the saturated product diffuscs away from the transfer site, leaving the complex ready to dissociate another h) drogen molecule and to absorb another olefin molecule. Recently certain organometallic complexes have bcen found to fix atmospheric nitrogen and permit it to be reduced to ammonia, a t ambient temperature and pressure. I n utilizing homogenous catalysts in industrial process-
ing, one must consider possible poisoning effects, problems of how to isolate or separate the homogeneous catalyst from the product, and the fact that some of these catalysts may undergo changes into noncatalytic forms, a process that may be difficult to reverse. Another form of homogeneous catalysis that may be effective in hydrogenation or oxidation reactions is the use of compounds that are known to be easily and reversibly oxidized and reduced without the use of a heterogeneous catalyst. One example is decalin. Decalin (decahydronaphthalene) will readily hydrogenate other molecules, being simultaneously converted to tetralin in which only one of the two rings is completely saturated. The tetralin, in turn, is readily hydrogenated in sztu back to decalin by hydrogen. The decalintetralin pair can be thought of as a homogeneous catalyst. Similar pairs exist in oxidation reactions, and an inventive approach to new processes may proceed by considering the possibility of carrying out reactions using various other oxidizing or reducing agents which are converted back to their original forms i n situ by oxygen or hydrogen, respectively. For example, SO2 is a selective oxidation agent, being converted to H2S. Is there a way of oxidizing the H2S back to SO2 in situ with oxygen without forming elemental sulfur or other byproducts? Structural inorganic chemistry. I n catalysis by inorganic zeolites, crystalline aluminosilicates, otherwise known as molecular sieves, improved activity in hightemperature processing results if rare earths are incorporated in the manufacture of the catalyst. Structural inorganic chemistry reveals that during the activation procedure, the rare earths migrate into a portion of the structure in a way which probably contributes to structural stability during high-temperature regeneration. This suggests that the role of the rare earths may be predominantly that of a physical strengthening of structure rather than a chemical contribution. Many active catalysts are nonstoichiometric compounds, and a knowledge of structural inorganic chemistry suggests that there is a correlation between catalyst activity for oxidation reactions and ease of movement of monatomic oxygcn through the lattice of oxide catalysts. Surface chemistry. Nucleation and crystal growth are involved in the manufacture of heterogeneous catalysts. High activity occasionally is associated with a particular crystal habit. I n a recent manufacturing problem, the key to the successful preparation of an active catalyst was the use of a suitable nucleating agent to cause the desired crystalline form to precipitate.
AUTHOR Charles N . Satterfield is Professor of Chemical Engineering at the Massachusetts Institute of Technology, Cambridge, Mass. 02139. T h i s article is based on a recent talk at M.I. T . before a group of research managers. Professor Satterfield is also the author of a book, “ M a s s Transfer in Heterogeneous Catalysis,” to be published by the M.I. T . Press i n 1969.
Biological processes and enzymes. All life depends on a myriad of biological catalysts known as enzymes. These are proteins which may or may not have attached a second relatively small molecule known as a coenzyme or prosthetic group. Either the prosthetic group or the protein itself can be the principal source of catalytic action. Their capability of causing fast reaction at low temperature and their high selectivity have long intrigued biochemists, and the similarity of the structure of some prosthetic groups to that of organometallic compounds that can be synthesized in a laboratory has stimulated much interest by chemists. Catalase, a particularly efficient enzyme, sets a high standard of activity for the chemist to attempt to match. If not limited by diffusion processes, one molecule of catalase can reportedly cause the decomposition of a million molecules of hydrogen peroxide per second at room temperature. I n comparison, on each site of an industrial silicaalumina catalyst as used for catalytic cracking, one molecule of hydrocarbon reacts about every 4 sec, requiring a temperature of about 500” C to achieve this rate. The fastest commercial catalytic reaction is the oxidation of ammonia vapor on platinum gauze. This reaction goes so fast that it is limited by the rate at which molecules can reach the catalyst surface and amounts to about 100,000 molecules of ammonia oxidized per second per surface atom of platinum, a rate comparable to that of hydrogen peroxide decomposition on catalase, but achievable only at the temperature of red heat. One can anticipate a rapid increase in the use of enzymes and bacterial action to control chemical reactions. Single-cell bacteria produce protein when fed petroleum plus nutrients and ammonia. Control of pollution in the processing of sewage depends on bacterial action. Enzymes have long been used in leather processing, and recently detergent mixtures have appeared containing enzymes for enhancing cleaning ability. Earlier this year the first complete chemical synthesis of an enzyme-ribonuclease-was announced. Investigators in homogeneous catalysis and those seeking the cause of enzyme activity are clearly on converging paths. New materials. New materials are constantly being produced, in many cases for uses far removed from catalysis, and their potential application frequently needs to be assessed in catalysis. Synthetic molecular sieves were developed by one industrial organization who conceived their being useful primarily for separation processes, but a second industrial group, recognizing the chemical similarity between these crystalline niaterials and the amorphous alumina-silicates that are effective in catalytic cracking, developed the molecular sieves into a new group of catalysts that have had a revolutionary effect in the petroleum industry. If one is engaged in an empirical screening program, any new kind of compound, particularly if it seems to involve strong interatomic fields (i.e., ionic or metallic bonding rather than covalent bonding) may be interesting to consider-e.g., new nonstoichiometric compounds, new types of organometallic structures, intermetallic VOL. 6 1
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compounds, and compounds having unusual bonding. Xew processes that make finely divided materials, highly porous materials, open foamed structures, or new kinds of catalyst supports, may be worth investigation. From the viewpoint of the manufacturer or developer of these materials or processes the probability that any one of these would be useful in catalysis is fairly small, but from the viewpoint of the user, the large number of new possibilities that appear every year requires some sort of monitoring. Catalytic mechanisms. The detailed examination of how reaction occurs at a catalytic site may lead to practical suggestions on ways of controlling a reaction. For example, if it is desirable to minimize the concentration of absorbed oxygen on a catalyst surface to avoid overoxidation of a desired product, we might accomplish it by partially poisoning the catalyst, by lowering oxygen mobility in the bulk catalyst structure, by lowering the oxygen concentration in the gas phase, or by limiting access of oxygen to the catalyst by mass transport. These are not all exactly equivalent, but they operate in the same direction. Isotopic exchange studies reveal that oxygen has substantial mobility through the bulk of most catalysts that are active for oxidation reactions, such as those incorporating vanadium pentoxide, tungsten oxide, or molybdenum oxide; therefore, compounds that deviate from stoichiometry are interesting materials to consider as catalysts. Mechanism studies may reveal whether reaction is inhibited by product removal from the catalyst or by adsorption of reactant, which suggests how the reaction will change as pressure, temperature, and other variables are manipulated. Selectivity is determined in part by the difference in reactivities of the various molecules present and the competition between reactions. Kinetic studies, aimed a t the formulation of a mathematical expression to represent how the rate of reaction is affected by concentration of reactants and products, are pursued extensively, but kinetic studies are a weak eliminator of mechanisms and must be used very cautiously for this purpose. However, kinetic concepts are a source of fruitful ideas. If the per cent conversion is limited by equilibrium, yield may be improved by removing a product. I n a dehydrogenation reaction, we may simultaneously carry out a n oxidation to remove the hydrogen formed and improve the rate and conversion. Coupling between mass transfer and intrinsic kinetics. The key to a successful catalytic process frequently does not lie in new chemistry or a new catalyst as such, but rather in factors involving the physical properties of catalysts or reactor engineering. For example, the pore size distribution in a porous catalyst pellet can of itself have a remarkable effect on the nature and distribution of products. I n a rapid gas-phase reaction, as in the oxidation of ethylene to form ethylene oxide or oxidation of naphthalene to form phthalic anhydride, it is imperative that the catalyst pellets contain large pores only to permit the desired product to diffuse out of the catalyst readily and not be over-oxidized. In 10
INDUSTRIAL A N D ENGINEERING CHEMISTRY
many organic reactions, hydrogen pressure is utilized to prevent the accumulation of precursors leading to the formation of polymeric substances on the catalyst that would cause a gradual loss in activity. If the hydrogen does not have full access to internal sections of the catalyst, these polymerization and condensation processes can occur in the central portions of catalyst pellets or in fine pores and can even cause physical degradation of catalyst pellets. The interaction between the nature of contacting between reactants, products, and catalyst as affected for example by agitation, and the intrinsic kinetics, may lead to unusual effects not readily interpreted by many observers. T h e hydrogenation of edible oils in batches on a nickel catalyst is a n old process, but only over the last 10 or 15 years has it been realized that the optimum reaction conditions, developed empirically over many decades of slow improvements, correspond to operation with a hydrogen-deficient catalyst-one in which the concentration of adsorbed hydrogen on the catalyst surface is relatively low. This promotes the rate of isomerization over that of hydrogenation and leads to the product’s having the desired softening characteristics. The low hydrogen concentration on the catalyst surface can be achieved by high catalyst-to-oil ratio, mild agitation, low hydrogen pressure, somewhat elevated temperatures, or various combinations thereof. T o some extent, these are more or less interchangeablc in the results that they produce, since it is the local hydrogen concentration on the catalyst surface which is significant and not how it is achieved. In most hydrogenation reactions, however, the opposite effect is desired-to saturate the catalyst with hydrogen, which may indeed require operation a t high hydrogen pressures to prevent the accumulation of inactivating deposits. The residence time distribution of elements of fluid as they pass through the reactor and the extent to which they mix with one another and contact a catalyst can markedly affect the efficiency of the process. I n hydrocracking of petroleum fractions, it is vital to reduce the content of sulfur and nitrogen compounds-especially certain nitrogen-containing compounds-to low levels by a catalytic pretreatment before they are contacted with the active hydrocracking catalyst. Ideally, one desires nearly plug-type flow through the reactor, and bypassing in the pretreating catalyst bed must be reduced to a n absolute minimum to achieve these objectives. A trickle-bed reactor contains a solid bed of catalyst over which a liquid trickles, usually with cocurrent or countercurrent flow of gas. High capacity and good performance are associated with high holdup of liquid in the system; again a residence time distribution as close as possible to plug flow is desired. We see that catalysis is interdisciplinary and that impacts on catalyst improvement and catalytic processing may come from many directions. In this, as in many other fields, innovation and significant breakthroughs are most likely to come from individuals or groups who are able to synthesize disparate elements and contributions from many sources.
