I
EMERSON H. LEE Plastics Division, Monsanto Chemical
Selection und Evaluation
Co.,Texas City,
Tex.
of. . .
Contact Catalysts Need a new or better catalyst for a gas phase process? systematic approach to catalyst development research
IT
was reported
by Balandin that
20,000 catalist formulations were tried by German chemists for ammonia synthesis (7). This reflects the fact that choice and development of catalysts have been a n art rather than a science. However, such extensive screening programs are expensive and time consuming, and a system is desirable for determining what properties control catalyst activity, selectivity, and stability, and what physical limitations are involved in using the catalysts. Usually in commercial processes, catalysts must have a high activity. Activity as used here is defined as moles of product per unit time per unit of catalyst weight or surface a t a given temperature and concentration of reactants. Thus a highly active catalyst will give a high production rate of products for a given capital investment in reactor equipment. In addition, it is desirable that the catalyst be highly selective. Here per cent selectivity is defined as moles of a given product obtained, divided by total moles possible from a reactant converted, multiplied by 300. The attainment of high selectivity for particular products may be highly important from an economic standpoint. A third requirement for a good catalyst is stability. The catalyst should not be subject to mechanical separation or disintegration, and should maintain a high level of activity and selectivity through chemical stability and resistance to poisoning. In many cases the cost of the catalyst and its ability to be regenerated are important economic factors. Preliminary Work
In considering a new reaction, it is of value to know the thermodynamics of the possible reactions. If the reaction is
reversible, it is important to know the equilibrium concentration of products, since this is the thermodynamic limit of conversion, with or without a catalyst. It is important to remember that the net forward reaction rate approaches zero near equilibrium. The catalyst may rapidly convert the reactants to a metastable equilibrium state which is not the true equilibrium from the standpoint of free energy; the catalyst may thus appear to alter the thermodynamics of a system. For a new reaction it is best to try to classify the important step or steps of the reaction as hydration, hydrogenation, oxidation, separation of carbon-carbon bonds, and others. One may then try known catalysts to verify the hypothesized step or steps (dd, 4.5).Unfortunately, the important step of a reaction is not always easy to guess. A dual catalyst may be required to complete the reaction successfully (78). Dual catalysts may sometimes be mechanically mixed, or more often are components of a n intimate mixture on the same support. The support may be a cocatalyst as in the case of platinum on alumina for hydrocarbon reforming processes. Tests should be made for noncatalyzed, gas phase reactions under the conditions of operation. This can be conveniently done by passing the reactants through the empty reactor (76) or that packed with an inert material. In some cases to evaluate catalyst performance it is necessary to correct over-all conversions for gas phase reactions that occur simultaneously. If the catalyst is supported, the catalytic activity of the bare support should be determined in separate tests. Testing Methods
Rapid screening of catalysts may be accomplished by usc of microreactors
Here is a
(39) or rising temperature reactors (8). A bank of test reactors can be used for direct comparison of catalysts, using the same feed stream and operating conditions. In doing rapid screening, one should be very careful about cleaning the reactor between runs, because residue from one catalyst in the reactor ma); contaminate the next catalyst and affect its performance. In a differential flow reactor, a relatively small amount of feed materials are converted to products. If a small amount of catalyst is maintained in a region of relatively constant temperature in a differential reactor, then the measured conversion rate can be associated with an average temperature and concentration of reactants and products (87). Such a flow reactor may be either a fixed bed or fluidized bed. Multiplication of mass flow rate of feed by weight fraction of product formed in the process will give weight of product formed per unit of time. Division of this number by catalyst weight thus gives weight of product (or equivalent moles) per unit time per unit weight of catalyst, or catalyst activity, a t the assigned temperature and concentration of reactants and products. In such a reactor, differences between catalyst performance are quickly and quantitatively determined. Variation of temperature of operation gives data for an Arrhenius plot xvhich can be used to determine the apparent activation energy of the reaction; in practice it is often convenient to plot log (moles per second per gram catalyst) us. 1 / T , which usually gives a linear relation. T h e apparent reaction orders can be determined by varying concentration of the reactants and products at a fixed temperature. There is often a wide variation of specific surface areas of catalysts, such VOL. 53, NO. 3
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that activities in moles per second per unit of surface may be necessary for comparing the inherent catalytic properties of the solids (54,74, 87). However, this applies only when the internal surface is readily available to reactants. In such materials as activated carbons or zeolites, many pores may be too small to be accessible (70, 79). Also involved is the question of rate of transport of gases within catalyst pores. If the products of a fixed or fluidized bed differential reactor are recycled, then differential reaction rates at progressively higher conversion levels may be measured by intermittant sampling of the product stream. The same information may also be obtained by using a preconverter with sample taps before and after the differential reactor section. A wide range of conversion levels may be obtained by variation of feed rate; however, because of possible mass transfer limitations a t the lower rates, it is generally better to vary both feed rate and catalyst weight. In a fixed bed integral reactor the measured conversion is that integrated over a relatively wide range of temperatures and/or concentrations of the reactants within the catalyst bed. In this case, differences in catalyst properties are not so directly determined from total conversions. However, a fixed bed isothermal reactor with a manifold of sampling taps along the reactor length can be used to measure differential reaction rates between taps, such that differential rates can be calculated at all conversion levels. In the process involving several consecutive reactions between initial and final states, data from a one-step differential reactor may be inconclusive or misleading. In this case? the integral reactor with multiple sample taps will give the desired information. One should use a reactor diameter larger than the catalyst pellet diameter by a factor of ten or more, to prevent bypassing near the walls of the reactor. The samples must be withdrawn at a relatively slow rate, in order that the flow pattern in the reactor is not disturbed. The resulting data are very useful for evolving a kinetic model in terms of temperature, pressure, and concentration of reactants and products (77,43, 52, 67), or to derive an empirical mathematical model from designed experiments (74, 7 5 ) . If one desires to scale up the reactor, there is the possibility of using reactor and catalyst dimensions and other pertinent data to determine scaling factors through dimensional analysis ( 73). Rate Controlling Steps in a Process
The over-all conversion rate of reactants to products may be controlled by: A . Bulk mass transfer of gases near the vicinity of the catalyst pellets.
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B. Mass transport of gases within the pores of the catalyst. C. Adsorption, dissociation, migration, reaction, desorption at the catalyst surface. In a fixed bed reactor one may test for A by varying catalyst weight and flow rate by the methods described in the literature. Variation of catalyst weight or flow rate should not affect the measured catalyst activity unless bulk mass transfer is rate controlling. The same values of conversion should be obtained when the ratio, T.l-lF, is the same for different catalyst weights in the reactor, where ?I’ is catalyst weight and F is flow rate. Since higher velocities improve mass transfer, A may usually be eliminated as a rate controlling step by using sufficiently high flow velocities. Limitations of mass transfer outside the catalyst pellet are not as often found as that within the pores of the pellet. This is seen by the fact that the apparent activity of a catalyst often increases with smaller pellet sizes; a constant activity is observed for all pellets below a certain diameter, showing that B has been eliminated as the rate controlling step. In such a case the activity, selectivity, apparent activation energy, and apparent reaction order may vary with pellet size (74, 87). True values for the surface reaction constants are observed only when pellets of sufficiently small diameter are used. Thus, in gas phase reactions catalyzed by porous solids, comparisons between catalysts may be inconclusive unless tests have been made for limitations of mass transfer in the gas phase by varying catalyst weight, flow rate? and catalyst pellet size. The rate controlling steps at rhe surface of the catalyst may sometimes be determined by kinetic models (40, 43), tracer studies (72, 32, 49), adsorption studies (63)>or through various perturbations of the system such as the effect of radiation on the solid (24, 64, 77, 75). Relating Catalyst Properties and Their Performance
When limitations of bulk mass transfer and mass transport within catalyst pores can be removed in an isothermal fixed bed reactor, then measured catalyst activity and selectivity may be directly related to the physical and chemical properties of the catalyst. Since specific surface areas of various catalysts vary widely, it is usually necessary to compare catalyst activities in terms of moles reacted per unit time per unit of surface at fixed temperature and feed concentration. When there are no limitations of mass transport in the gas phase, such a quantity can be described as the intrinsic
INDUSTRIAL AND ENGINEERING CHEMISTRY
catalyst activity. That is the velocity at which a unit of surface converts reactant to product at a given temperature, when the surface is in contact with a given concentration of reactant, and is a function only of the chemical and physical character of the solid. Such a correlation may be difficult to determine with solid catalysts with very fine pores, where the pellet size would have to be very small, or where the smaller pores are not accessible to the reacting gases. The surface of the catalyst is modified by the chemical environment of reactants and products. Measurement of certain catalyst properties should thus be made under reaction conditions or on catalyst samples quenched under reaction conditions if possible (7, 30. 66, 69. 80). The first effluent from the catalyst reactor should be carefully analyzed and compared with later conversions. Such a comparison may give evidence of changes in catalyst properties caused by the reaction stream or show poisoning of the catalyst by the feed (34. 36, 56). Unfortunately, a catalyst may be poisoned by compounds in concentrations too small to detect except by special analytical procedures (76. 62). Modern theories of catalysis indicate that chemisorption of one or more reactants on the catalyst surface is an important step of the catalyzed reaction. Furthermore, chemisorption involves an electron transfer or sharing, such that sorbed species are ions, radicals, or covalent-type complexes (7, 3. 7, 77, 26, 27, 40, 47, 57, 69, 77). In the reaction process the catalyst mUSt readily adsorb one or more reactants, and desorb the products at an appreciable rate. Since the solid donates or accepts electrons from the adsorbed molecules, electron transfer between the solid and these adsorbed molecules may be the rate controlling step of the catalyzed reaction. In this case the important properties of the solid are-the free electron density of the solid; the potential barrier for electron emission a t the surface of the catalyst; and the density-potenrial gradient of the electrons in the solid ( 3 ) . It follows then that relations should sometimes be found between electrical conductivity. electronic work function (40, 47. 77), thermoelectric power (57), and catalytic activity of solids. A relation between semiconductor type and catalytic activity and selectivity is sometimes observed (3, 25, 37, 57, 65). The relative affinity of the solid and adsorbed molecules for electrons is an important factor in determining the type of chemisorption bond formed. Thus the difference of ionization energy of the molecule and electronic work function of the catalyst would be expected to be a important characteristic of the system (40, 47, 77). For comparing
CONTACT CATALYSTS the electronic character of solid catalysts, one may use an acid-base scheme (5, 58) or an electronegativity scale [35) as in the table. A solid with a small electronic work function is a strong electron donor and can be thought of as a Lewis base. A solid with a higher electronic work function would more resemble a Lewis acid.
Electronic Character of Solid Catalysts Elecatronic Work ElectroFunc- nega- Lewis tiori tivity Acidity
Small Weak Large
Strong
Basic Acidic
Applications to Chemical Processes T h e following program is suggested for choice a n d evaluation of catalysts for an industrial gas phase process. Estimate thermodynamic possibilities of the reaction, and if the reaction is reversible, estimate equilibrium conversion levels of the desired product a t various conditions. Check for noncatalytic gas phase reactions, and if necessary, correct overall data for this conversion. Classify the main reaction and type of catalyst required.
Electronic Character Electron donor Electron acceptor
This relative basicity or acidity compared to an adsorbed hvdrocarbon, for example, affects the direction of electron transfer and the type of adsorbed complex formed ( 9 , 20, 30. 48, 55, 58); thus one might find a regular trend of intrinsic catalyst activities and selectivities as the electronic work function or basicity of the catalyst is varied. A maximum activity or selectivity may occur a t some intermediate electronic property of the solid, when the proper balance between adsorption rate of reactants and desorption rate of products has been reached. Danforth and Martin found that the poisoning power of alkalies on acidic cracking catalysts increased \sith basicity of the alkali ion (27). Conversely, with a given catalyst, a regular trend of measured activities may be found as the basicity of a hydrocarbon reactant varies ( 2 ) . Another important property of catalysts, particularly metals, is the number of d or f orbital vacancies in the electronic structure (3, 6, 26, 28, 37, 66). This undoubtedly changes the nature of chemisorption of reactants and products among other things (26). T h e properties of semiconductor catalvsts sometimes depend on whether they are p or n type (23. 26, 65). Since p types conduct by electron holes. they may be compared to Lewis acids or electron acceptors. Under experimental conditions the electrical character of silica-alumina cracking catalyst (a strong acid) has been found to be a p-type semiconductor (80). Such electron acceptors containing hydrogen may give rise to protons and become proton donors, or Bronsted acids Semiconductors of n-type conduct by electrons, tend to chemisorb hydrogen. and thus should be better hydrogenation catalysts than p-type semiconductors (25. 37). A catalyst can often be obtained by using a metal salt of one of rht principal anions taking part in the reaction, for
Test for mass transfer limitations by variation of flow rate, catalyst weight, and pellet size, using a differential reactor or integral reactor with multiple sample taps. Carefully analyze first effluent from the reactor to test for poisoning of the catalyst. V a r y the properties of the catalyst in known directions, and correlate with catalyst activity and selectivity, after mass transfer limitations are removed. Attention must be paid to effects on catalyst properties caused by variations in methods of preparing a catalyst of a given composition; if the catalyst is supported, check the bare support for catalytic activity. Extend catalyst testing over a wide range of conversion levels, using a differential reactor with recycle or with a preconverter, or by using an integral reactor with multiple sample taps, after the desired catalyst properties are found. A bank of reactors is useful for accurate comparison of catalysts. Develop a kinetic or other mathematical expression for the reaction in terms of temperature, pressure, and concentrations of the reactants and products. Compare physical and chemical properties (surface area, pore size distribution, and X-ray diffraction) of new and used catalysts to determine reasons for changes in activity, selectivity, and stability. Analyze the used catalyst for by-product residues (evacuate and trap). Test methods of regeneration of the catalyst. Test potential poisons in the feed stream.
example, supported silver chloride for a chlorination catalyst. Such a system apparently provides a n intermediate source of very reactive atomic chlorine a t the catalyst surface through chemical exchange that occurs between solid and gas phases. The method of preparation of catalysts is often very critical; important factors are method of precipitation, temperature of activation, and prereduction (78, 28, 42, 44, 59, 72). It may be necessary to measure catalyst properties as a function of method of preparation, as a separate phase of work. Variations may be found in surface area, pore size distribution, crystal structure, phase distribution, grain size, and chemical combination (4, 79, 22, 38, 54, 60, 70, 73). These may affect activity, selectivity, and stability of the catalyst. Thus one may need to use gas adsorption (32, 67, 68),mercury injection (29). differential thermal analysis (53),
X-ray analysis (46, 47), and magnetic susceptibility (66) to determine such changes. If a catalyst is supported, it may be necessary to know the separate surface areas of the support and the catalyst (62, 681, or if the catalyst is in solid solution, a separate phase, or chemically combined with the support (33, 73, 82). The study of differences of adsorption of gases on various catalyst modifications may give insight on the changes of surface states (20, 30, 32, 48, 50), which is of prime importance in characterizing the catalyst. These ideas may be tested for any given reaction system by first finding a solid that catalyzes the reaction, as observed with a differential reactor. Proper flow rates and pellet sizes should then be determined to avoid the limiting effects of mass transport within the pores or outside the catalyst pellets. Then the chemical or physical character of a series of samples may be modified in VOL. 53, NO. 3
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known directions and changes of catalytic activity and selectivity observed. For example, the acidity (basicity, electronegativity, electronic work function) can be varied to give a set of catalysts with varying electron donating tendencies. Or the semiconductor character may be gradually changed in a set of samples. If the catalysts are metals, a set of alloys or metalswith varying numbers of empty d orf orbital vacancies can be used.
Acknowledgment Many of the ideas were acquired through work with other members of this laboratory. Thanks are given to C. G. Hatfield and K. V. Wise for suggestions and review of this article. Literature Cited (1) Balandin, ‘4. A., Bull. Acad. Sci. C.S.S.R., Diu. Chem. Sci.S.S. R. 1955,. p. _ 557 (Engl. transl.) , (2) Balandin, A . A,, Khidekel, M. L., Proc. Acad. Sci. U.S.S.R. 123,I 83 (1958) . I (Engl. transl). (3) Baker, M. McD., Jenkins, G. I., in “Advances in Catalysis” (W. G. Frankenburg, V. I. Komarewsky, E. K . Rideal, ed.), Vol. VII, p. 1, Academic Press, New York, 1955. (4) Barrett, W.T., Sanchez, M. G., Smith, J. G., in “Advances in Catalysis,” (A. Farkas, ed.), Vol. IX, p. 551, Academic Press, New York, 1957. (5) Benesi, H. A,, J . Am. Chem. Soc. 78, 5490 (1956). (6) Bond, G. C., Mann, R. S., J . Chem. SGC. 1959, p. 3566. (7) Boudart, M., J . Am. Chem. SGC.74, 1531 (1952). (8) Bridges, Joanne M., Houghton, G., Ibid., 81, 1334 (1959). (9) Brodd, R. J., J . Phys. Chem. 62, 54 (1958). (10) Broussard, L., Shoemaker, D. P., J . Am. Chem. Soc. 82, 1041 (1960). (11) Brun, Pierre, Comfit. rend. 248, 2993 (1959). (12) Burrell, R. L., Tuxworth, R. H., J . Phys. Chem. 60, 1043 (1956). (13) Boucher, D. F., .4lves, G. E., Chem. Eng. Progr. 5 5 , 55 (1959). (14) Box, G. E. P., Biometrics 10, 16 (1954). (15) Box, G. E. P.: Wilson, K. B., J . Roy. Statirtical Soc. Ser. B. XIII, 1 (1951). (16) Campbell, K. C., Thompson, S. J., Trans. Faraday SOC.5 5 , 985 (1959). (17) Carr, N. L., IKD.EKG. CHEII. 52, 391 (1960). (18) Ciapetta, F. G., Plank, C. J., in “Catalysis” (P. H. Emmett, ed.), Vol. I, p. 315, Reinhold, New York, 1954. (19) Cimino, A., J . Phys. Chem. 61, 1676 (1956). (20) Cimino, A,, Cipollini, E., Molinari, E., Liuti, G., Manes, L., G u n . chim. ital. 90, 91 (1960). (21) Danforth, J. D., Martin, D. F., J . Phys. Chem. 60, 422 (1956). (22) Davis, R . J., Griffith, R . H., Marsh, J. D. F., “Advances in Catalysis” (.4.Farkas, ed.), Vol. IX, p. 155, Academic Press, New York, 1957.
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(23) Dixon, J. K . , Longfield, J. E., in “Catalysis,” (P. H. Emmett, ed.), Vol. VII, p. 347. Reinhold, New York, 1960. (24) Dousmanis, G. C., Phyr. Reo. 112, 369 (1958). (25) Dowden, D. A,, Bull. soc. chnm. Beiges 67. 439 11958). (26) ’Dowdeni D. A , , in “Chemisorption” (C. Tarner, ed.), .4cademic Press, New York, 1957. (27). _ Dowden, D. A , , J . Chem. SOC.1950, p. 242.
(28) p w d e n , D. A.. Bridger, G. LV.; in Advances in Catalysis” (A. Farkas, ed.), Vol. IX, p. 669. Academic Press, New York, 1957. (29) Drake, L. E., Ritter, H . L.: IND. END.CHEW.37, 787 (1945). (30) Eischens. R. P., Plisken, b‘, A , in “Advances in Catalysis“ (D. D. Eley, i V . G. Frankenburg, V. I. Komarewsky, ed.), Vol. X, p. 2, Academic Press, New York, 1958. (31) Eley, D. D.! Shooter. D., Proc. Chem. Soc. 1959, p. 315. (32) Emmett, P. H., in “Advances in Catalysis’’ (A. Farkas, ed.), Vol. IX, p. 645, Academic Press, New York. 1957. (33) Gault, F. G., Comfit. rend. 245, 1620 (1937). (34) GLrland, C. W.,J. Phjir. Chem. 63, 1423 (1959). (35) Gordy, W.,Orville-Thomas, I%*.J., J . Chem. Phys. 24,439 (1956). (36) Gossett,’E. C., Pet;o/. R&nu 39, 177 (1960). (37) Gray, T. J., McCain, C. C., Masse, N. G., J . Phys. Chem. 63, 472 (19j9). (38) Gwathmay, A. T.! Cunningham, R. E., in “Advances in Catalysis” (D. D. Eley, 1%’. G. Frankenburg, V. I. Komarewsky, ed.), Vol. X, p. 57. Academic Press, New York, 1958. (39) Hall, W.K., MacIver, D. S.,Weber. H. P., IND.ENG.CHEM5 2 , 421 (1960). (40) Hauffe, K., in “Advances in Catalysis” (W. G. Frankenburg, V. I. Komarewsky, E.K. Rideal, ed.), Vol. VII, p. 213, Academic Press, New York, 1955. (41) Hauffe, K., in “Advances in Catalysis” (A. Farkas, ed.), Vol. IX, p. 187, Academic Press, New York, 1957. (42) Hindin, S. G., M’eller, S. W., in “Advances in Catalysis” (.4.Farkas. ed.), Vol. IX, p. 70, .4cademic Press, Nett, York, 1957. (43) Horiuti, J., Toyoshima, I., J . Inst. Cat. Hokkaido Unio. 6,146 (1958). (44) Innes, W. B., in “Catalysis” (P. H . Emmett, ed.), Vol. I, p. 245, Reinhold, New York, 1954. (45) Innes, W. B., in “Catalysis” (P. H. Emmett, ed.), Vol. 11, p. 1, Reinhold, New York, 1955. (46) Jellinek, M. H., Fankuchen, I.: “Advances in Catalysis” (it‘.G. Frankenburg, V. I. Komarewsky, E. K. Rideal, ed.), Vol. I, p. 257, Academic Press, New York, 1948. (47) Keeling, R. O., Jr., J . Chem. Phqs. 31, 279 (1959). (48) Keier, N. P., Kutseva, L. N., Bull. Acad. Sci. U.S.S.R., Diu. Chem. Sci. S.S.R. 1959, p. 774 (Engl. transl.). (49) Kemball, Charles, Bull. SGC. chim. Belges 67, 373 (1958). (50) Kokes, R. J., Emmett, P. H., J . A m . Chem. SOC.82, 1037 (1960). (51) Krause, A , , IND.ENG.CHEM.51, 1358 (1959).
INDUSTRIAL AND ENGINEERING CHEMISTRY
(52) Kwan, Takao, J . Phys. Chem. 60, 1033 (1956). (53) Lodding, William, Hammell, Lawrence, Anal. Chrm. 32, 657 (1960). (54) Maatman, R . W., Prater, J . Phys. Chern. 63, 1312 (1959). (55) Matsen, F. A,, Makrides, A. C., Hackerman, N.,J. Chem. Phys. 22, 1800 (1954). (56) Maxted, E. B., in “Advances in Catalysis” (1%’. G. Frankenburg, V. I. Komarewsky. E. K. Rideal, ed.), Vol. 111, p. 129, Academic Press, New York, 1951. (57) Parravano, G., Domenicalli, C. .4., J . Chpm. Phys. 26, 359 (1957). (58) Pines, H., Haag, W. O., J . Am. Chem. Soc. 82, 2471 (1960). (59) Purri, €3. R.! Singh, D. P., Natti, J., Sharma, L. R., IND. ENG. CHEM.50, 1071 (1958). (60) Rienacher, G., Chem. Tech. (Berlin) 11, 264 (1959) ; C A 35, 21103e (1959). (61) Roth, J. F., Ellwood, R. J., Anal. Chem. 31, 1739 (1959). (62) Rubjn, S., Passell, T. O., Bailey, E., Ibid., 29, 736 (1957). (63) Scholten, J. J. F., Konvalinka, J. A,, Zwietering, P., Trans. Faraday SGC.56, 262 (1960). (64) Schwab, G. hf., in “Advances in Catalysis” (A., Farkas, ed.), Vol. IX, p. 229. Academic Press, New York, 1957. (65) Schwab, G. M., Block, J.: Angew. Chem. 71, 101 (1956). (66) Selwood, P. W., in “Advances in Catalysis” (A. Farkas, ed.), Vol. IX, p. 93, Academic Press, New York, 1957. (6’) Smith, R . B.. Chem. Eng. Progr. 5 5 , 76 (1959). (68) Spaendel, L., Boudart, M., J . Phys. Chem. 64, 204 (1960). (69) Suhrman, R., in “Advances in Catalysis” (\$’. G. Frankenburg, V. I. Komarewsky, E. K. Rideal, ed.), Vol. VII, p. 303. Academic Press, New York, 1955. (70) Szabo, Z. G., Solymosi, F., Batta, I., 2.fihysz.k. Chem. 23,56 (1960). (71) Taylor, E. H., Kohn, H. Lt’., J . Am. Chem. Soc. 79, 252 (1957). (72) Turkevich, J., Laroche, J., Z. fihystk. Chem. (Frankfurt) 15, 399 (1958). (73) IVaters, R. F.: Peri, J. B., John, G. S., Seelig, H. G., IND.ENG.CHEM.52, 415 (1960). (74) Wheeler, A , , in “Advances in Catalysis’’ (LV. G. Frankenburg, V. I. Komarewsky, E. K. Rideal, ed.), Vol. 111, p. 250, Academic Press, New York, 1950. (75) Il’iener. H. B., Young, P. W., J . A$$. Chem. (London) 8,336‘!1958). (76) Weissberger, -4,, ed., Technique of Organic Chemistry,” 2nd ed., Vol. 11, p. 1, Interscience, New York, 1956. (77) Weisz, P. B., J . Chem. Phys. 21, 1531 (1953). (78) Weisz, P. B.: Science 127, 887 (1956). (79) Weisz, P. B., Frilette, V. J., J . P h p . Chem. 64, 382 (1960). (80) Weisz, P. B., Prater, C. D., J . Chem. Phys. 21, 2236 (1953). (81) Weisz, P. B., Prater, C. D., in “Advances in Catalysis” (W. G. Frankenburg, V. I. Komarewsky, E. K. Rideal, ed.), Vol. VI, p. 144, Academic Press, New York, 1954. (82) Yamaguchi, S., Takeuchi, T , , J . Colloid Sci. 12, 263 (1957). RECEIVED for review July 29, 1960 ACCEPTED December 2, 1960
