17981
2007, 111, 17981-17984 Published on Web 11/08/2007
Conversion of Methanol to Alkenes over Medium- and Large-Pore Acidic Zeolites: Steric Manipulation of the Reaction Intermediates Governs the Ethene/Propene Product Selectivity Stian Svelle,† Unni Olsbye,† Finn Joensen,‡ and Morten Bjørgen*,£ Centre for Materials Science and Nanotechnology, Department of Chemistry, UniVersity of Oslo, N-0315 Oslo, Norway, Haldor Topsøe A/S, DK-2800 Lyngby, Denmark, and Department of Chemistry, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway ReceiVed: September 12, 2007; In Final Form: October 19, 2007
Methanol conversion over H-beta and H-ZSM-5 zeolite catalysts is compared at identical reaction conditions (temperature ) 350 °C, WHSV ) 7.0 gg-1 h-1), and it is found that H-ZSM-5 yields seven times more ethene relative to propene than H-beta yields. By switching from a 12C methanol feed to 13C methanol, the transient incorporation of 13C atoms from methanol into the gas-phase products and reaction intermediates located within the zeolite voids is followed. For H-beta, penta- and hexamethylbenzene are involved in a hydrocarbon-pool-type mechanism, leading predominantly to propene and the higher alkenes, whereas for H-ZSM-5, the lower methylbenzenes are the intermediates and represent the only route for ethene formation. This work outlines how the zeolite topology determines the type of intermediate participating in the catalytic cycle, thereby controlling the ethene/propene product selectivity in the methanol to olefins reaction.
Introduction Methanol can be produced via syngas from natural gas or, in principle, any gasifiable carbonaceous material. In a subsequent step, methanol may be converted into hydrocarbons such as polymer-grade alkenes or gasoline over acidic zeolites. Such processes are generically known as the methanol to hydrocarbons (MTH) reaction. As in most heterogeneously catalyzed processes, selectivty control is a major issue also in the MTH case. For a methanol to olefins (MTO) or a methanol to propene (MTP) application, the ethene/propene ratio in the product will affect the process profitability, as the market demand favors propene. The ethene/propene yield in the methanol conversion is the focus of this report, which outlines how the zeolite topology may determine the selectivity by sterically controlling the size of the intermediates and thus the involved mechanism. The understanding of the MTH reaction mechanism has been greatly advanced in the past decade.1 From being a question of C-C bond formation directly from methanol, the attention is now turned toward the indirect hydrocarbon pool mechanism.2,3 Rather than being based on direct coupling of methanol to, for example, ethene,4 this mechanism involves stepwise methanol addition to the hydrocarbon pool, which subsequently rearranges and splits off alkenes. Several studies have shown that multiply methylated benzenes (“polymethylbenzenes”) or their protonated counterparts constitute the hydrocarbon pool.5 For example, in H-SAPO-34, hexaMB may split off alkenes under a concomitant formation of the corresponding lower polymethylbenzene.5a-c * To whom correspondence should be addressed. Fax: +47 73 55 08 77. Tel: +47 73 59 48 46. E-mail:
[email protected]. † University of Oslo. ‡ Haldor Topsøe A/S. £ Norwegian University of Science and Technology.
10.1021/jp077331j CCC: $37.00
HexaMB also shows a very high reactivity in H-beta.5d-h The exact mechanism by which the polymethylbenzene derivatives form alkenes in combination with methanol/dimethylether is still debated, but two main reaction paths have been proposed, the exocyclic methylation route5l and the paring reaction.5f Both proposals involve a sequential growth of alkyl side chains, which eventually are eliminated as the corresponding alkenes from the cyclic species. The mechanisms described for some catalysts, that is, H-beta (three-dimensional, large pores 7.6 × 6.4 Å) and H-SAPO-34 (7 × 10 Å cages connected by 3.8 × 3.8 Å windows), involve bulky intermediates (hexaMB) and may not be applicable for zeolites with smaller pores. Indeed, recent research has outlined major mechanistic differences from one catalyst topology to another. For H-ZSM-5 (three-dimensional, medium pore 5.4 × 5.6 Å), hexaMB is an insignificant intermediate compared to the lower polymethylbenzenes, that is, trimethylbenzene (triMB).5i,j A simplified reaction cycle for ethene formation over HZSM-5 is outlined in Scheme 1. According to this proposal, triMB splits off ethene to form toluene, which, in turn, is methylated twice to regenerate triMB. In addition to this aromatic-based route, which also yields some propene analogously to the ethene, another catalytic cycle, involving alkene methylations and cracking reactions, accounts for the major part of propene and higher alkenes formed over H-ZSM-5. The alkene cycle is illustrated in Scheme 2. The larger alkene, hexene in Scheme 2, is formed from the lower alkenes through sequential methylations. Hexene may crack into two propene molecules, and three methanol molecules have thus been converted into one molecule of propene. The methylations may well proceed beyond hexene, and cracking of the resulting alkene can give other alkenes than propene. Importantly, ethene © 2007 American Chemical Society
17982 J. Phys. Chem. C, Vol. 111, No. 49, 2007 SCHEME 1: Simplified Cycle for Ethene Formation Involving the Lower Methylbenzenes as Hydrocarbon Pool Species over H-ZSM-5; This Cycle Allows the Formation of Ethene from Two Methanol Molecules without the Direct Coupling of C1 Units
SCHEME 2: Cycle for C3+ Alkene Formation over H-ZSM-5. A Lower Alkene, as Exemplified by Propene, May Be Methylated Repeatedly, Thereby Forming a Larger Alkene Susceptible to Cracking; The Cracking Products May Then Desorb from the Catalyst and Enter the Gas Phase or Partake in a New Cycle Turnover. It is Important to Realize That Ethene is Not Involved in This Alkene-Based Cycle
is not a part of this cycle because ethene is an unlikely cracking product under the relevant conditions; in the cracking of octene over zeolite Y at 350 °C, the yields of propene and butenes are 14 and 33 times higher than the yield of ethene.6 Further, the methylation of ethene proceeds 17 and 50 times slower than the methylation of propene and butene.7 Thus, the partaking of ethene in the alkene cycle is insignificant, and the ethene is all dominantly formed from the aromatics route (Scheme 1). According to this dual cycle description for H-ZSM-5, ethene formation is mechanistically separated from the formation of propene and the higher alkenes. Manipulation of these two reaction cycles might influence the ethene/propene ratio in the products.5i,j Moreover, Haw and co-workers8 have shown that for H-SAPO-34, a hydrocarbon pool mechanism based on the lower polymethylbenzenes yields predominantly ethene, whereas the higher homologues (penta- and hexaMB) favor propene. These two insights may have far-reaching impact on MTO selectivity control. We have expanded our studies to cover the 12 ring zeolite beta, in addition to the 10 ring structure ZSM-5, in order to assess the effects of (i) steric manipulation of the aromatic reaction intermediates and (ii) the relative importance of the alkene cycle versus the aromatics cycle Experimental Section Methanol was converted over H-ZSM-5 (Si/Al ) 140) and dealuminated9 H-beta (Si/Al ) 120) at 350 °C in a fixed bed reactor using 60 mg catalyst. Methanol (130 hPa) was fed by saturating the carrier gas (He, 35 N mL/min), resulting in a WHSV of 7.0 gg-1 h-1. By using two separate feed lines, it was possible to switch from 12C- to 13C-methanol without disrupting the reaction conditions. After 18 min of 12C-methanol reaction, we switched to 13C-methanol, and the isotopic compositions of effluent compounds were determined at in-
Letters TABLE 1: Methanol Conversions (%) and Product Selectivities (C%) Measured for H-beta and H-ZSM-5 after 15 min on Stream at 350 °C product selectivities (C%) catalyst
conversion (%)
C2
C3
C4)
C4-
C5
C6+
H-beta H-ZSM-5
54.9 73.0
1.1 9.9
22.7 30.4
26.4 14.3
10.5 5.2
11.7 11.7
27.6 28.4
creasing 13C-methanol reaction times (i.e., time after the 12C-/ 13C-methanol switch) by GC-MS. The organic material confined within the zeolite pores after the reaction was analyzed by thermally quenching the reaction 2 min after the 12C-/13Cmethanol switch, dissolving the catalyst in HF, extracting the organics with CH2Cl2, and finally, analyzing the extract (GC-MS). Experimental details have been given previously.5i,j,10 Results and Discussion Table 1 presents the methanol conversion levels and product selectivities after 15 min of reaction at 350 °C. H-ZSM-5 is more active than H-beta, and more extensive measurements have shown that this difference cannot be accounted for only by deactivation, as this catalyst loses merely 5% activity after 15 min on stream at 400 °C and identical feed rate,7 meaning that H-ZSM-5 converts more methanol per acidic site than H-beta. The selectivities for the heaviest product fractions (C5 and C6+) are very similar for both catalysts. However, for the C4 fractions, H-beta has about twice the selectivity for both butenes and butanes compared to that of H-ZSM-5. Both the C2 (mostly ethene) and C3 (mostly propene) selectivities are lower for H-beta, particularly for ethene. The propene/ethene ratio obtained over H-beta is 21 compared to 3 for H-ZSM-5. Propene is the desired product in a MTP application, and our scope is to mechanistically understand this pronounced difference in propene/ethene selectivity. As mentioned in the Introduction, polymethylbenzenes have previously been shown to be intermediates in the MTH reaction. In order to evaluate their individual roles in H-beta and H-ZSM5, the reactivity of the polymethylbenzenes was monitored by the transient 13C incorporation after switching from 12C- to 13Cmethanol at 18 min on stream. After 2 min of 13C methanol reaction, the confined organics were analyzed as described above. For both catalysts, methylbenzenes are the dominant species in the zeolite pores. Their total 13C contents are presented in Figure 1. Clearly, for the beta zeolite (Figure 1a), hexaMB has the highest 13C content (almost 90% 13C after 2 min of 13C methanol reaction) and thus the fastest carbon incorporation from methanol. Consequently, and in compliance with previous studies, hexaMB stands out as a highly reactive species in H-beta. Furthermore, the incorporation of 13C is evidently slower as the number of methyl substituents decreases, and xylene shows the lowest reactivity. This trend in reactivity is reversed for the polymethylbenzenes confined in H-ZSM-5 (Figure 1b), for which the rate of carbon incorporation consistently decreases with increasing number of methyl substituents. Hence, in H-ZSM-5, hexaMB has the lowest reactivity toward methanol. These clear differences in reactivity among the polymethylbenzenes in H-beta and H-ZSM-5 imply that the key reaction intermediates are different and, consequently, that the mechanism for alkene formation from the aromatics differs for the catalysts. This statement is further supported by isotopic analysis of the gas-phase products discussed in the following section. Figure 2 shows the time evolutions in total 13C content for the C2-C6 alkenes and the polymethylbenzenes present in the effluent from H-beta (a) and H-ZSM-5 (b) after switching from
Letters
J. Phys. Chem. C, Vol. 111, No. 49, 2007 17983 route is demonstrated by their fast 13C incorporation seen in Figures 1 and 2. Second, it is hard to imagine that all of the larger C5+ alkenes are formed directly from an aromatics route. More likely, these C5+ alkenes are the products of an alkenebased route similar to that previously described for H-ZSM-5 (Scheme 2). Conclusions
Figure 1. Total 13C contents in confined organics after 18 min of 12Cmethanol reaction followed by 2 min of 13C-methanol reaction at 350 °C over H-beta (a) and H-ZSM-5 (b). The organics were made available for GC-MS analysis by dissolving the zeolite in HF. For H-beta, the higher methylbenzenes, that is, penta- and hexaMB, display the fastest incorporation of 13C from the methanol feed, whereas the opposite is seen for the H-ZSM-5 catalyst.
Figure 2. Time evolution of 13C content in effluent hydrocarbons from H-beta (a) and H-ZSM-5 (b) after the 12C-/13C-methanol switch at 350 °C. For H-ZSM-5, two distinct groups are seen, the C3+ alkenes and the aromatics plus ethene. No such grouping is seen for H-beta. 12C-
to 13C-methanol. While being absent in the product stream from H-ZSM-5, penta- and hexaMB were the only polymethylbenzenes present at appreciable concentrations in the effluent from the large-pore H-beta. Clearly, for H-beta, all effluent components have quite similar 13C content, even at the first analysis carried out 0.5 min after the switch. This similarity in 13C content among the gas-phase products is not seen for H-ZSM-5, where the products are separated in two groups at the first analysis; one group is constituted by ethene and the aromatics, whereas the C3-C6 alkenes, being significantly richer in 13C, form the other group. As stated above, the similar 13C contents in ethene and the lower polymethylbenzenes seen for H-ZSM-5 are the manifestations of a mechanistic link between these species (Scheme 1). The significantly higher 13C content in the C3-C6 alkenes is caused by incorporation of additional 13C by repeated methylations (by 13C-methanol) followed by cracking steps (Scheme 2). Although all compounds formed over H-beta have a very similar development in 13C content, the current data can only be satisfactorily rationalized by assuming that we also have the coexistence of an aromatics-based cycle and an alkene-based cycle in this zeolite. First, the partaking of penta- and hexaMB as intermediates in an aromatics-based
When methanol is converted over H-ZSM-5 and H-beta catalysts with comparable acid site densities at identical reaction conditions, H-ZSM-5 yields seven times more ethene relative to propene than H-beta yields. On the basis of isotopic labeling data, we are now in a position to draw a conclusion about the factors that give rise to the C3/C2 selectivity differences observed for these two catalysts. Both alkene- and aromatics-based reaction cycles operate over both catalysts, but the major difference between the reaction mechanism in H-ZSM-5 and H-beta is the active aromatic hydrocarbon pool species. In agreement with the conclusion drawn earlier by Haw and coworkers for H-SAPO-34, it can be stated that the hydrocarbon pool mechanism operating in H-ZSM-5, which is based on the lower polymethylbenzenes, yields predominantly ethene, whereas for H-beta, the higher homologues (penta- and hexaMB) are active, and propene is favored. This accounts for the very low ethene yield over H-beta compared to that over H-ZSM-5 under the current set of conditions. The very minor ethene formation seen for H-beta may take place from the aromatics route, the alkene route, or both. Two specific routes to pursue for increasing the yield of propene in a MTH application become apparent; operating at process conditions or using a catalyst which disfavors a methylbenzene cycle altogether would lead to negligible ethene yields since this cycle is virtually the only source to the undesired ethene. On the other hand, by increasing the zeolite channel dimensions by moving from H-ZSM-5 to a more spacious zeolite (in the present case, H-beta) as the catalyst, the hydrocarbon pool cycle is allowed to proceed via larger aromatic intermediates, which, in turn, also gives rise to a much lower ethene selectivity, as demonstrated in this contribution. References and Notes (1) (a) Olsbye, U.; Bjørgen, M.; Svelle, S.; Lillerud, K.-P.; Kolboe, S. Catal. Today 2005, 106, 108-111. (b) Sto¨cker, M. Microporous Mesoporous Mater. 1999, 29, 3-48. (c) Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. Acc. Chem. Res. 2003, 36, 317-326. (d) Haw, J. F. Phys. Chem. Chem. Phys. 2002, 4, 5431-5441. (2) (a) Dahl, I. M.; Kolboe, S. Catal. Lett. 1993, 20, 329-336. (b) Dahl, I. M.; Kolboe, S. J. Catal. Lett. 1994, 149, 458-464. (3) (a) Goguen, P. W.; Xu, T.; Barich, D. H.; Skloss, T. W.; Song, W.; Wang, Z.; Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 1998, 120, 2650-2651. (b) Haw, J. F.; Nicholas, J. B.; Song, W.; Deng, F.; Wang, Z.; Xu, T.; Heneghan, C. S. J. Am. Chem. Soc. 2000, 122, 4763-4775. (4) (a) Song, W.; Marcus, D. M.; Fu, H.; Ehresmann, J. O.; Haw, J. F. J. Am. Chem. Soc. 2002, 124, 3844-3845. (b) Marcus, D. M.; McLachlan, K. A.; Wildman, M. A.; Ehresmann, J. O.; Kletnieks, P. W.; Haw, J. F. Angew. Chem., Int. Ed. 2006, 45, 3133-3136. (c) Lesthaeghe, D.; van Speybroeck, V.; Marin, G. B.; Waroquier, M. Angew. Chem., Int. Ed. 2006, 45, 1-10. (d) Lesthaeghe, D.; van Speybroeck, V.; Marin, G. B.; Waroquier, M. Chem. Phys. Lett. 2006, 417, 309-315. (5) (a) Arstad, B.; Kolboe, S. Catal. Lett. 2001, 71, 209-212. (b) Arstad, B.; Kolboe, S. J. Am. Chem. Soc. 2001, 123, 8137-8138. (c) Song, W.; Haw, J. F.; Nicholas, J. B.; Heneghan, C. S. J. Am. Chem. Soc. 2000, 122, 10726-10727. (d) Bjørgen, M.; Bonino, F.; Kolboe, S.; Lillerud, K.P.; Zecchina, A.; Bordiga, S. J. Am. Chem. Soc. 2003, 125, 15863-15868. (e) Bjørgen, M.; Olsbye, U.; Svelle, S.; Kolboe, S. Catal. Lett. 2004, 93, 37-40. (f) Bjørgen, M.; Olsbye, U.; Petersen, D.; Kolboe, S. J. Catal. 2004, 221, 1-10. (g) Bjørgen, M.; Olsbye, U.; Kolboe, S. J. Catal. 2003, 215, 30-44. (h) Sassi, A.; Wildman, M. A.; Ahn, H. J.; Prasad, P.; Nicholas, J. B.; Haw, J. F. J. Phys. Chem. B 2002, 106, 2294-2303. (i) Bjørgen, M.; Svelle, S.; Joensen, F.; Nerlov, J.; Kolboe, S.; Bonino, F.; Palumbo, L.;
17984 J. Phys. Chem. C, Vol. 111, No. 49, 2007 Bordiga, S.; Olsbye, U. J. Catal. 2007, 249, 193-205. (j) Svelle, S.; Joensen, F.; Nerlov, J.; Olsbye, U.; Lillerud, K.-P.; Kolboe, S.; Bjørgen, M. J. Am. Chem. Soc. 2006, 128, 14770-14771. (k) Svelle, S.; Bjørgen, M.; Kolboe, S.; Kuck, D.; Letzel, M.; Olsbye, U.; Sekiguchi, O.; Uggerud, E. Catal. Lett. 2006, 109, 25-35. (l) Arstad, B.; Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 2004, 126, 2991-3001. (m) Bjørgen, M.; Bonino, F.; Arstad, B.; Kolboe, S.; Lillerud, K.-P.; Zecchina, A.; Bordiga, S. ChemPhysChem 2005, 6, 232-235. (n) Mole, T.; Bett, G.; Seddon, D. J. Catal. 1983, 84, 435-445. (o) Xu, T.; White, J. L. U. S. Patent 6,734,330, 2004.
Letters (6) Kissin, Y. V. Catal. ReV.sSci. Eng. 2001, 43, 85-146. (7) (a) Svelle, S.; Rønning, P. O.; Kolboe, S. J. Catal. 2004, 224, 115123. (b) Svelle, S.; Rønning, P. O.; Olsbye, U.; Kolboe, S. J. Catal. 2005, 234, 385-400. (8) Song, W.; Fu, H.; Haw, J. F. J. Am. Chem. Soc. 2001, 123, 47494754. (9) Bjørgen, M.; Kolboe, S. Appl. Catal., A 2001, 225, 285-290. (10) Svelle, S.; Olsbye, U.; Lillerud, K.-P.; Kolboe, S.; Bjørgen, M. J. Am. Chem. Soc. 2006, 128, 5618-5619.
