J. Phys. Chem. 1981, 85, 937-939
937
Mechanism of Methanol Formation Atsushi Takeuchl and James R. Katzer' Center for Catalytic Science and Technology, Depatfment of Chemical Englneering, University of Delaware, Newark, Delaware 197 1 1 (Received: October 27, 1980; In Final Form: February 9, 1981)
The mechanism of methanol synthesisfrom CO + H2catalyzed by Rh/Ti02 was studied by use of a 50-50 mixture of 13C1s0and lzC1sO. The major products formed were 13CH3160Hand 12CH31sOHindicating that methanol synthesis occurs by a nondissociative mechanism.
Introduction The Fischer-Tropsch synthesis is an old, well-known reaction for producing organic compounds with more than one carbon atom from CO plus HP. Only recently has there been renewed interest in this reaction, and, although much new knowledge has accumulated, there are still many questions about the reaction mechanism. Two general mechanisms have been proposed for the Fischer-Tropsch synthesis. One is the nondissociative mechanism in which adsorbed CO does not undergo dissociation but instead undergoes stepwise hydrogenation as a molecule and may involve an insertion reaction. The most frequently proposed version of this mechanism involves an enol intermediate and condensation reactions as originally proposed by Storch, Golumbic, and Anderson1 and followed by Emmett and co-workers2and V a n n i ~ e . The ~ other, the dissociative mechanism, involves adsorbed CO which undergoes dissociation to give adsorbed carbon and adsorbed oxygen species on the catalyst surface. The adsorbed carbon atom is hydrogenated stepwise and gives rise to products or acts as a monomer in chain growth giving higher molecular weight products. The typical version of this involves surface carbide as an intermediate. This mechanism was originally proposed by Fischer and Tropsch4 and has recently received strong support from several experimental directions.58 Both mechanisms are reviewed in detail from the viewpoint of metal cluster chemistry by M ~ e t t e r t i e s . ~ X-ray and ultraviolet photoelectron spectroscopy, and infrared spectroscopy have clearly shown that CO can undergo dissociation on some metal surfaces at relatively low temperatures.1° However, it is very difficult by use of spectroscopic methods alone to demonstrate whether the surface carbon species formed by CO dissociation is a true reaction intermediate or only an adsorbed species. Isotopic tracer studies are a powerful tool in determining whether certain appropriately labeled species participate directly in the reaction. Emmett et a1.2 using radioactive 14C-labeledcompounds showed that alcohols, aldehydes, (1) H. H. Storch, N. Golumbic, and R. B. Anderson, "The FischerTropsch and Related Synthesis", Wiley, New York, 1951. (2) J. T. Kummer and P. H. Emmett, J . Am. Chem. SOC.,75, 5177 (1953); W. K. Hall, R. J. Kokes, and P. H. Emmett, ibid., 82,1027 (1960); J. T. Kummer, H. H. Podgurski, W. B. Spencer, and P. H. Emmett, ibid., 73, 564 (1951). (3) M. A. Vannice, J . Catal., 37, 462 (1975). (4) F. Fischer and H. Tropsch, Brennst. Chem., 7, 97 (1926). (5) R. W. Joyner, J . Catal., 50, 176 (1977). (6) A. Jones and B. D. McNicol, J. Catal., 47, 384 (1977). (7) M. Araki and V. Ponec, J . Catal., 44, 439 (1976).
(8) P. Biloen, J. N. Helle, and W. M. H. Sachtler, J. Catal., 58, 95 (1979). (9) E. L. Muetterties and J. Stein, Chem. Reo., 79, 479 (1979). Faraday Trans. (10) R. W. Joyner and M. W. Roberts, J. Chem. SOC., 1,70,1819 (1974); K. Kishi and M. W. Roberts, ibid., 71,1715 (1975); P. Ramamoorthy and R. D. Gonzalez, J. Catal., 59, 130 (1979).
TABLE I: Isotopic Composition of Carbon Monoxide before and after Reaction with H, Catalyzed by Rh/TiOZa composition, mol % mol wt
28 29 30 31
isotopic species
for 0% COconvrn
2.3 c l*C160 13C160 45.1 c 1zC180 51.5 k 13C180 1.1c
0.2 0.2 0.4 0.2
for 17.8% COconvrn
for 27.8% COconvrn
10.8 i 0.4 44.1 i 2.0 40.1 c 2.0 5.0 c 0.3
11.6 0.8 40.9 c 0.8 42.3 i 0.5 5.2 c 0.2
a Reaction conditions: batch reactor; initial reactant pressure of P C O = 25 torr, P,, = 610 torr; temperature = 150 "C.
and ethylene were chain initiators in the Fischer-Tropsch synthesis. More recently Araki and Ponec' and Wise and co-workers'l have shown that surface carbon species formed by CO dissociation produce CH4as rapidly as does gas-phase CO and Biloen et a1.8 have shown that these surface carbon species are statistically incorporated into C2 to C4 hydrocarbons. On the other hand, alcohols have been considered to be the precursors of hydrocarbons at high pressure in the presence of a cobalt catalyst.lZ Methanol has been considered the precursor of the oxygenated products formed in the Fischer-Tropsch synthesis because the product distribution is very similar to the distribution obtained in methanol homologation where methanol reacts with synthesis gas (H2/C0 = 1) at 180 "C in the presence of dicobalt 0ctacarbony1.l~ In addition to the potential importance of methanol as an intermediate in the Fischer-Tropsch synthesis, it is a key chemical in commerce, being synthesized in massive quantities from CO plus H,l4 and may become a major intermediate in the production of gas01ine.l~ On oxide catalysts the methanol synthesis mechanism has usually been assumed to be nondissociative, but methanol can also be a main product when highly dispersed supported metals including Rh, Pt, Pd, and Ir are used.16J7 Direct evidence of the mechanism of methanol formation does not exist nor is there any direct evidence of the role of CO dissociation on oxide or metal catalysts. In this Letter we present the first isotopic evidence that methanol synthesis involves a nondissociative reaction mechanism and begin to lay the basis for clarifying the mechanism of alcohol synthesis more generally. ~~
~~
(11)J. G. McCarty and H. Wise, J . Catal., 57, 406 (1979). (12) D. Gall, E. J. Gibson, and C. C. Hall, J . Appl. Chem., 2, 371 (1952). (13) I. Wender, R. A. Friedel, and M. Orchin, Science, 113, 206 (1951). (14) A. B Stiles, AZChE J.,23, 362 (1977). (15) S. L. Meisel, J. P. McCullough, C. H. Lechthaler, and P. B. Weisz, CHEMTECH, 6, 86 (1976). (16) M. Ichikawa, Bull. Chem. SOC.Jpn., 51,2268 (1978); M. Ichikawa, ibid., 51, 2273 (1978). ' (17) M. L. Poutsma, L. F. Elek, P. A. Ibarbia, A. P. Risch, and J. A. Rabo, J . Catal., 52, 157 (1978).
0022-3654/81/2085-0937$01.25/00 1981 American Chemical Society
938
Letters
The Journal of Physical Chemistty, Vol. 85, No. 8, 1981
Experimental Section An all-glass internal-recycle reactor operated in the batch mode was used for the reaction studies; the reactor volume was 1100 cm3.18 An isotopic mixture of CO was made from about equal quantities of 13C160and l2Cl80; the mass spectrometer analysis of the mixture is given in Table I. The CO mixture was introduced into the reactor to a pressure of 25 torr, and then Hz was added to produce a total pressure of 635 torr. The reactor containing prereduced Rh/Ti02 catalyst was preheated to and held at 150 "C. The temperature and pressures were selected to produce the most favorable rate and thermodynamic equilibrium conditions for methanol s y n t h e s i ~ . ~The .~~ catalyst had been used repeatedly for the hydrogenation reaction prior to the isotopic study and had shown stable activity behavior. Analysis of the reaction products was carried out by gas chromatography using an Alz03column for hydrocarbon separation, a THEED column for the alcohol separation, and a molecular sieve column for CO analysis. Isotopic species in the reactant and products were analyzed with a mass spectrometer (Hewlett-Packard 5930A) at an ionization voltage of 70 V. The gaseous isotopic species were injected into the mass spectrometer through a heated expansion volume connected to sample U-tubes filled at several reaction times and then cooled to liquid nitrogen temperature. Product methanol, ethanol, and higher alcohols were collected in a cold trap and were analyzed with a gas chromatograph-mass spectrometer equipped with a THEED column. 13C160and lZCl8Owere obtained from Stohler Isotope Chemicals. A Rh/TiOz catalyst containing 3.0 wt % Rh was used. Ti02 was obtained by hydrolysis of titanium 2-propoxide, drying the washed filter cake, and calcining in air at 400 "C for 5 h. Rh was incorporated onto the catalyst by ion exchange from dilute Rh(N03)3solution to achieve an extremely high degree of Rh dispersion. The catalyst was washed, dried, and calcined at 300 "C in flowing 02 for 2 h. One-tenth gram was placed in the reactor; it was reduced for 15 h in 40 torr of H2 at 200 "C and evacuated. This latter procedure was repeated before each run. Results Several runs with CO + H2 showed that CH4 was the predominant product, and its concentration increased almost linearly with time. The concentration of saturated hydrocarbons increased almost linearly with time, and the concentration of hydrocarbons decreased sharply with increasing carbon number. For very short times the olefin concentration increased linearly and rapidly with time but quickly became time independent and then decreased somewhat indicating a two-step reaction involving olefin formation followed by olefin hydrogenation. The alcohol concentrations grew steadily with time, and the ratio of methanol to ethanol decreased with time suggesting that methanol may be an intermediate in the reaction network. Concentrations of alcohols with more than two carbon atoms were negligibly small. In one experiment the reacting gas mixture containing isotopic CO was sampled after 25.4 h, at which time the carbon monoxide conversion was 17.8%. The product distribution (in mole percent) of hydrocarbons and alcohols was as follows: methane, 74.3; ethane, 2.53; ethylene, 0.05; propane, 2.71; propylene, 1.38; n-butane, 0.57; butenes, 1.3; (18) W. D. Fitzharris and J. R. Katzer, Ind. Eng. Chem. Fundam., 17, 130 (1978). (19) G. Natta, "Catalysis",Vol. 111, P. H. Emmett, Ed., Reinhold, New York, 1955, Chapter 8.
TABLE 11: Isotopic Composition of Methane from CO Hydrogenation over Rh/TiO, Using a 46% I3C0-54%I2CO Mixturea composition, mol % for 17.8% CO conwn
for 27.8% CO conwn 45* 1 53 * 1 16 "CH, 55 ?: 1 47 t 1 17 WH, a Reaction conditions: batch reactor; initial reactant pressures of Pco = 25 torr, PH,= 610 torr; temperature = 150 "C. mol wt
methane
TABLE 111: Isotopic Composition of Methanol from CO Hydrogenation over Rh/TiO, Using a 45% 13C160-52% 12C1s0 Mixturea composition, mol %
mol wt
methanol
for 17.8%
for 27.8%
convrnb
convrn
co
co
12 38 47 1 3 ~ ~ ~ 1 8 0 3 a Reaction conditions: batch reactor; initial reactant pressures of Pc0 = 25 torr, PH = 610 torr; temperature = The change of methane fragmentation may 150 "C. influence the composition. 32 33 34 36
1ZCH,'60H WH,160H 12CHj180H
