© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 3, JANUARY 16, 1997
LETTERS Generation of Carbenes during Methanol Conversion over Bro1 nsted Acidic Aluminosilicates. A Computational Study P. E. Sinclair* and C. R. A. Catlow The DaVy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W1X 4BS, U.K. ReceiVed: September 23, 1996; In Final Form: NoVember 14, 1996X
Density functional theory calculations have been carried out in order to study the formation and deprotonation of surface methyl oxonium ions, -OCH3, at aluminosilicate Bro¨nsted acid sites. We have shown that acidcatalyzed deprotonation of methyl groups during the methanol to gasoline (MTG) conversion can occur to produce surface-stabilized carbenes with a calculated activation barrier of 215-232 kJ mol-1. The calculated barriers are in reasonable agreement with recent experimental findings indicating that methyl group deprotonation may be the rate-determining step during methanol conversion. Participation of gas phase carbenes in the MTG process is shown to be unlikely. Other mechanistic details are discussed.
The ability to convert methanol into hydrocarbons in the gasoline range has stimulated considerable interest since the discovery, in 1976, of the process in medium pore-sized acidic zeolites.1 This so-called “Mobil process” involves the microporous zeolite ZSM-5, which, due to its unique combination of pore architecture and Bro¨nsted acidity, is selective for formation of gasoline range hydrocarbons from methanol and/ or dimethyl ether without excess production of methane. However, many other zeolites, aluminophosphates, silicon aluminophosphates, and mesoporous framework materials are active for methanol conversion provided that sufficient Bro¨nsted acidity can be generated by, for example, substitution of Si in an all silicate framework by a trivalent ion such as Al3+. The resulting charge imbalance can then be removed by adding H+ ions giving rise to the well-studied [AlO(H)Si] species.2-4 It is this Bro¨nsted acid site that is known to be responsible for the catalytic activity of many microporous and mesoporous framework materials. Despite the continuing interest in the methanol to gasoline (MTG) process, there is little consensus on the reaction X
Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5647(96)02920-3 CCC: $14.00
mechanism. For example, those pathways that have received some measure of experimental support include the free carbene mechanism of Chang and Silvestri,5 the radical mechanisms of Clark et al.,6 the ketene mechanism of Jackson and Bertsch,7 the trimethyl oxonium ion mechanisms of van den Berg8 and Olah,9 and the methyl oxonium ion mechanism of Hutchings and Hunter.10 However, with the exception of the ketene and radical mechanisms, all of the reaction pathways listed above rely on the existence of sites within the zeolite that are basic enough to deprotonate methyl groups. The formation and subsequent deprotonation of surface methyl oxonium ions, or surface-stabilized methyl groups, is the subject of the present study. In particular, we have used the cluster approximation and spin-restricted, gradient-corrected density functional theory (DFT) as implemented in the code DGauss11 (implementation details are summarized below in ref 11) to study the formation and deprotonation of surface methyl oxonium ions, ZOCH3, by the process illustrated in Figure 1. We note that by “surface methyl oxonium ion”, we mean a Bro¨nsted acid site, [AlO(H)Si], in which the acidic proton has been replaced by a methyl © 1997 American Chemical Society
296 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Letters
Figure 1. DFT-optimized reaction path for the formation and deprotonation of a model surface methyl oxonium ion, ZOCH3 ) H3SiO(CH3)Al(OH)2OSiH3. The ∆Es are energy changes for the various steps and are tabulated in Table 2. The structures shown were optimized without geometry constraints. The deprotonation reaction was also studied on clusters with fixed Si-Si distances (see text). These structures are qualitatively similar to those above and are not shown. Further details can be found in Tables 1 and 2.
group, CH+ 3 . In Figure 1, ∆E(1)-∆E(4) refer to ZOCH3 formation from two methanol molecules with ∆E(1) representing the adsorption of two methanol molecules at the Bro¨nsted acid site, in a conformation directly preceeding the transition state along the reaction coordinate, while ∆E(2) represents the forward activation barrier. Likewise, ∆E(5)-∆E(7) refer to ZOCH3 deprotonation with ∆E(5) representing the forward activation barrier for this step. It has been suggested that the ZOCH3 ions are active intermediates in the MTG process,10,12,13 and they have been observed with FTIR12,19 and NMR spectroscopic techniques.13,20 Although formation of ZOCH3 from two methanol molecules Via the mechanism shown in Figure 1 has been studied previously by quantum mechanical techniques,16 we have returned to this problem in order to obtain more accurate energies; i.e., the present work uses a better quality computational method, and ones that are consistent with those calculated for ZOCH3 deprotonation; note the present work uses the same sized cluster throughout. Conclusions from the current calculations concerning this step are, however, identical to those reported in ref 16, and we will not discuss them further except in reference to the energy changes discussed subsequently and listed in Table 2. Other theoretical studies of the formation of ZOCH3 from one methanol molecule14,15,18 and from dimethyl ether17,18 at zeolitic Bro¨nsted acid sites indicate that the SN2 process shown in Figure 1 is energetically favored. As seen from Figure 1, the basic site used in the deprotonation of ZOCH3 is the conjugate base oxygen of the methylated acid site. All geometry-optimized structures have been characterized by their harmonic vibrational spectra. Small residual imaginary
frequencies (excluding the imaginary frequency corresponding to the reaction path at the transition state) with negligible intensities remained at some optimized geometries. These were always due to motion of the cluster-terminating groups, SiH3, and were found to contribute little to the absolute or relative energies. We emphasize that all energies reported in the present work are internal energies resulting from static electronic structure calculations at stationary points on the adiabatic potential energy surface. Thus, as with all such calculations, thermal and entropic effects are neglected. Figure 1 shows the deprotonation reaction occurring on a model cluster which was completely unconstrained throughout the geometry optimizations. In order to probe the effect of different zeolite structures on the important deprotonation step, this part of the reaction has also been studied on clusters in which the Si-Si distances were constrained to be 5.55, 5.98, and 6.51 Å. The constrained structures are qualitatively similar to those in Figure 1 and are not shown; specific details of each are given in Tables 1 and 2. Figure 1 and Tables 1 and 2 indicate that allowing the cluster complete freedom during the geometry searches leads to an unrealistic expansion of the active site as a result of ZOCH3 deprotonation: a Si-Si distance of 6.2 Å in the reactant, ZOCH3, results in a product (pr-II) Si-Si distance of 7.2 Å. Constraining this distance during deprotonation leads to a more reasonable distortion of the active site and, surprisingly, has little effect on the activation barrier for deprotonation; i.e., the process is expected to occur with a similar activation barrier at Bro¨nsted acid sites in different topological positions within a given zeolite framework provided that other effects, e.g., steric
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J. Phys. Chem. B, Vol. 101, No. 3, 1997 297
TABLE 1: DFT-Calculated Properties of the Optimized Structures along the Reaction Coordinate for ZOCH3 Deprotonation Shown in Figure 1a constraint none
r(Si-Si) ) 6.51
r(Si-Si) ) 5.98
r(Si-Si) ) 5.55
property µ q(CH3) q(CH2) q(H) r(Si-Si) µ q(CH3) q(CH2) q(H) µ q(CH3) q(CH2) q(H) µ q(CH3) q(CH2) q(H)
reactant ZOCH3
transition state (ts-II)
4.07 0.32 6.15 3.39 0.31 3.73 0.30 4.39 0.31
product (pr-II)
1.71
3.76
-0.14 0.43 6.52 1.61
0.00 0.45 7.17 3.85
-0.14 0.44 2.91
-0.01 0.46 4.57
-0.17 0.44 2.90
0.00 0.48 3.92
-0.21 0.44
0.00 0.47
a All dipole moments (µ) are in D, all distances (r) are in Å, and all Mulliken net atomic charges (q) are in au.
TABLE 2: DFT Calculated Energy Changes for the Reactions Shown in Figure 1a constraint none r(Si-Si) ) 6.51 r(Si-Si) ) 5.98 r(Si-Si) ) 5.55
∆E1
∆E2
∆E3
∆E4
∆E5
∆E6
-75
127
-99
50
221 215 222 232
-55 -41 -55 -57
∆E7 234 220
a
The labels refer to those in Figure 1. The entire process ZOH + 2CH3OH(g) f ZOH + CH2(g) + CH3OH(g) was modeled only on the zeolite fragment with no geometry constraints. ZOCH3 deprotonation was studied with all models. ∆E(2) is the activation barrier for ZOCH3 formation, and ∆E(5) is the activation barrier for ZOCH3 deprotonation. All energies are in kJ mol-1 and are corrected for zeropoint energies within the harmonic approximation.
effects are not significant. In all cases, deprotonation of ZOCH3 leads to the “carbene” product being partially inserted into the cluster framework with generation of a neighboring Bro¨nsted acid sitesa crucial step if the aluminosilicate is to be truly catalytic for methanol conversion. The effective atomic charge information in Table 1 indicates that the product (pr-II) should probably be thought of as a surface-stabilized carbene with an effective net charge of about zero. The reactive nature of the product (pr-II) is illustrated by the overall energy change for its formation from ZOCH3 of 165-175 kJ mol-1. Figure 1 and Table 2 also show that desorption of the carbene (singlet) is energetically expensive, i.e., of similar magnitude to formation of the carbene itself. Thus, gas phase carbene production is unlikely given the energetic cost of formation and desorption, and it is therefore probable that these species play little role in the MTG process. This result is in accord with the experimental work of Hunter and Hutchings23 and clearly indicates that formation of ethene Via carbene polymerization is unlikely. It is interesting to note that as the Si-Si distance decreases, the transition state for deprotonation (ts-II) becomes more product-like with the [C‚‚‚H] distance of the fragmenting bond increasing from 1.72 to 1.81 Å. This mechanistic change is also evident in the values of the Mulliken charges of the CH2 and H fragments shown in Table 1. These values, together with the character of the normal modes corresponding to the imaginary frequency at the transition states (not shown), indicate that the barrier for deprotonation is dominated by movement of the positively charged methyl proton to give a negatively charged CH2 fragment in the transition state. Presumably,
relaxation of the system to give a neutral CH2 fragment occurs as the reaction coordinate proceeds toward the product. It is clear that the “degree of deprotonation” in the transition state increases with decreasing Si-Si distance; i.e., q(CH2) becomes more negative, and it might be expected therefore that a better description of the environment would be more important as the Si-Si distance decreases. The deprotonation activation barriers for the different models vary by only 17 kJ mol-1. This variation reflects the different stabilities of the ZOCH3, which change unsystematically, and the energies of the transition states (ts-II), which increase with decreasing Si-Si distance. The energies shown in Table 2, however, lack any contribution from the environment of the extended aluminosilicate. This contribution can be qualitatively assessed by using the dipole moments, µ, as a measure of the degree of charge separation in the active site. Table 1 indicates that in proceeding along the reaction coordinate from ZOCH3 to the transition state (ts-II), there is always a decrease in µ; i.e., a polar environment would stabilize ZOCH3 more than the transition states (ts-II), and the activation barriers for deprotonation shown in Table 1 should be taken as lower bounds. We now consider a central question concerning the mechanism of the MTG process: do ZOCH3 species play a crucial role in hydrocarbon formation, or are they simply a side product? Jayamurty and Vasudevan,24 as a result of a temperatureprogrammed surface reaction (TPSR) study, concluded that the MTG process is made up of two distinct steps: dehydration of methanol to form dimethyl ether and conversion of dimethyl ether to hydrocarbons. Further, as a result of pre- and postadsorption of methanol and cofeeding of methanol and dimethyl ether during the reaction, they concluded that methanol was not involved in the second step of the process. They reported activation barriers of 108 and 195 kJ mol-1 for methanol dehydration and dimethyl ether conversion, respectively. Jayamurty and Vasudevan’s experimental activation barrier for dimethyl ether formation was based on the observation of water evolution during the TPSR run as it was shown that desorption, not activation, was the rate-determining factor for the observation of dimethyl ether. Their analysis did not, however, include the possibility that observation of water could be due to the formation of ZOCH3 (see, for example, Figure 1) as well as due to the formation of dimethyl ether. The theoretical activation barriers for the formation of dimethyl ether17 and ZOCH3 (present work and ref 18) from two methanol molecules at aluminosilicate Bro¨nsted sites are 89 and 127 kJ mol-1, respectively. Assuming that both reactions follow a simple Arrhenius-type rate equation
( )
k ) A exp
-Ea RT
where k is the rate, A is a preexponential factor related to entropy changes between the reactant and transition state, and Ea is the activation energy, then also assuming that A is approximately constant for dimethyl ether and ZOCH3 formation (they both have similar reactants and SN2 transition states), we estimate the relative rates of dimethyl ether:ZOCH3 formation to be ∼1000 at 650 K, suggesting that dimethyl ether, not ZOCH3, is formed from methanol during the initial stages of methanol conversion. However, if we note that conversion of dimethyl ether to methanol in the presence of water has a calculated activation barrier of 119 kJ mol-1,17 that dimethyl ether, once formed, can readily desorb from the acid site, and that ZOCH3 could also be formed from dimethyl ether itself, then the theoretical studies suggest that during the initial stages of the MTG reaction, using either methanol or dimethyl ether as the
298 J. Phys. Chem. B, Vol. 101, No. 3, 1997 feed stock, a mixture of both is formed with dimethyl ether probably being in abundance. Slowly, ZOCH3 is formed at accessible, reactive Bro¨nsted acid sites and if enough energy is present will react further, by, for example, the pathway in Figure 1. We note, however, that the calculated activation barriers between methanol, dimethyl ether, and ZOCH3 are similar given the uncertainties in the methods used to calculate them, especially if they are compared to the activation barrier for carbene formation. It is likely, therefore, that there will be several interconversions between these species before further reaction occurs. This explanation is in accord with the results of the pre-, post-, and cofeeding experiments of Jayamurty and Vasudevan. These experiments had little effect on the hydrocarbon product spectrum because if we assume that hydrocarbons are formed from the activation of ZOCH3, then since ZOCH3 can be formed from either methanol or dimethyl ether (the latter possibly Via the former), then the methanol:dimethyl ether ratio in the pores of the solid acid will not affect hydrocarbon formation greatly. The explanation can also account for the fact that in the initial stages of the MTG process from methanol, dimethyl ether is rapidly observed but there is a “lag time” between dimethyl ether formation and the detection of hydrocarbons.5 The agreement between the present calculated activation barriers for ZOCH3 deprotonation (215-232 kJ mol-1) and the experimental value for hydrocarbon formation (195 kJ mol-1) suggests that MTG conversion could indeed progress Via the process shown in Figure 1, i.e., essentially by the methyl oxonium ion pathway suggested by Hunter and Hutchings. In summary, our DFT calculations indicate that deprotonation of surface methyl oxonium ions during methanol conversion in acidic aluminosilicates can lead to a reactive, surface-stabilized carbene with regeneration of a Bro¨nsted acid site; i.e., basic sites exist that can deprotonate methyl groups. The process is likely to be the rate-determining step of the MTG process with an activation barrier of ∼200 kJ mol-1, which is in reasonably good agreement with experiment and lends support to the methyl oxonium ion pathway for MTG conversion suggested by Hunter and Hutchings. Although changes in the Si-Si distance of the cluster used to model the reaction caused noticeable changes to the position of the transition state along the reaction coordinate, it had little effect on the activation barriers and could not explain the variation in activity of different zeolites toward MTG conversion.
Letters Acknowledgment. We are grateful to Molecular Simulations Inc. for use of their Catalysis and Sorption software suite. We also thank EPSRC for a studentship for P.E.S. and for computer resources at the Atlas Centre, Rutherford, U.K. We are grateful to Prof. R. A. van Santen for helpful discussions. References and Notes (1) Meisel, S. L.; McCullogh, J. P.; Lechthaler, J. P.; Weisz, C. H. Chem. Technol. 1976, 6, 86. (2) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. ReV. 1994, 94, 2095. (3) Brand, H. V.; Curtis, L. A.; Iton, L. E. J. Phys. Chem. 1992, 96, 7725. Brand, H. V.; Curtis, L. A.; Iton, L. E. J. Phys. Chem. 1993, 97, 12773. (4) Shah, R.; Gale, J. D.; Payne, M. C. J. Phys. Chem. 1996, 100, 11688. (5) Chang, C. D.; Silvestri, A. J. J. Catal. 1977, 47, 249. (6) Clarke, J. K. A.; Darcy, R.; Hegarty, B. F.; O’Donoghue, E.; Ebrahimi, V.; Rooney, J. J. J. Chem. Soc., Chem. Commun. 1986, 425. (7) Jackson, J. E.; Bertsch, F. M. J. Am. Chem. Soc. 1990, 112, 9085. (8) van den Berg, J. P.; Wolthuizen, J. P.; van Hoof, J. H. C. Proceedings of the 5th International Zeolite Conference; L. V. C., Rees, Ed.; Heydon: London, 1981; p 649. (9) Olah, G. A. Pure Appl. Chem. 1981, 53, 201. (10) Hutchings, G. J.; Hunter, R. Catal. Today 1990, 6, 279. Review article; see also references therein. (11) DGauss version 3.0 DGauss is a pure DFT code developed by Cray Research Inc. It employs the local correlation functional of Vosko et al. (Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200) and the local exchange functional of Dirac (Dirac, P. A. M.; Proc. Cambridge Philos. Soc. 1930, 26, 376). Nonlocal corrections were included Via the BLYP recipe: Becke, A. D. Phys. ReV. 1988, A38, 3098. Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785. A DZVP basis set was used throughout in conjunction with the “A1” auxiliary basis set: Godbout, N.; Andzelm, J.; Wimmer, E.; Salahub, D. R. Can. J. Chem. 1992, 70, 560. (12) Forester, T. R.; Howe, R. F. J. Am. Chem. Soc. 1986, 108, 7154. (13) Bosacek, V. J. Phys. Chem. 1993, 97, 10732. (14) Zicovich-Wilson, C. M.; Viruela, P.; Corma, A. J. Phys. Chem. 1995, 99, 13224. (15) Blaskowski, S. R.; van Santen, R. J. Phys. Chem. 1996, 100, 3463. (16) Sinclair, P. E.; Catlow, C. R. A. J. Chem. Soc., Faraday Trans. 1996, 92, 2099. (17) Blaskowski, S. R.; van Santen, R. J. Am. Chem. Soc. 1996, 118, 5152. (18) Sinclair, P. E.; Catlow, C. R. A. J. Chem. Soc., Faraday Trans., in press. (19) Kubelova, L.; Novakova, J.; Nedomova, K. J. Catal. 1990, 124, 441. (20) Derouane, E. G.; Gilson, J. P.; Nagy, J. B. Zeolites 1982, 2, 42. (21) Messow, U.; Quitzsch, K.; Herden, H. Zeolites 1984, 4, 255. (22) Haase, F.; Sauer, J. J. Am. Chem. Soc. 1995, 117, 3780. (23) Hunter, R.; Hutchings, G. J. J. Chem. Soc., Chem. Commun. 1987, 377. (24) Jayamurthy, M.; Vasudevan, S. Catal. Lett. 1996, 36, 111.
