Mechanistic Basis for Effects of High-Pressure H₂ Cofeeds on Methanol-to-Hydrocarbons Catalysis over Zeolites

Abstract

Cofeeding high-pressure (16 bar) H2 with methanol (0.005 bar) during methanol-to-hydrocarbons conversion over acidic zeolites with varying topologies (CHA, AEI, FER, and BEA) results in a ∼2× to >15× enhancement in catalyst lifetime compared to He cofeeds, as determined by the cumulative turnovers attained per proton before the final methanol conversion level drops below 15%C. These beneficial effects of prolonged catalyst lifetime are observed without any impact on the carbon backbone of effluent hydrocarbon products characteristic of the particular zeolite topology. The olefins-to-paraffins ratio of C2+ hydrocarbons, however, decreases due to enhanced paraffins production, and the magnitude of this decrement depends on the specific zeolite topology. The observations of marked lifetime improvements and topology-dictated variations in the paraffin make of MTH effluent with H2 cofeeds can be interpreted based on the different proclivities of zeolitic protons confined in varying topological environments for catalyzing hydrogenation of hydrocarbons that are predominantly formed via formaldehyde-based alkylation routes (e.g., 1,3-butadiene) or methanol-based alkylation routes (e.g., ethene and propene). Independent kinetic studies reveal that measured hydrogenation rates per H+ of 1,3-butadiene are at least 1 order of magnitude (∼7× to ∼320×) higher than that of ethene or propene, which provides an explanation for the observed lifetime improvements in MTH with H2 cofeeds. Further, trends in the reactivities of ethene and propene with H2 over the different zeolites help explicate the topology-dependent variations in the paraffin content of the effluent hydrocarbons during MTH with H2 cofeeds.

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

“…8b and e) are essentially identical propene and butene hydrogenation barriers (ΔG҂ of 161-167 kJ mol −1 , Fig. 4), demonstrating the importance of the α,δ-attack to explain experimentally observed differences 43 in alkene and diene rate constants. The direct formation of 2-butene from butadiene, however, cannot be experimentally verified as n-butene double bond isomerization and skeletal isomerization are facile resulting in an equilibrated mixture of isobutene, 1-butene, and 2-butene at MTO and hydrogenation conditions.…”

“…2) indicate that there is no significant thermodynamic preference to hydrogenate species involved in polyaromatic formation (aromatics, dienes, and formaldehyde) compared to alkenes, and that C=C bond stability increases with C-atom substitution. This indicates that the tendency for dienes to be hydrogenated over alkenes-as shown experimentally 43 -arises from a kinetic preference, likely because of the increased stability of allylic carbocations caused by resonance. A preference for formaldehyde hydrogenation over alkene hydrogenation has not been directly observed, but has been predicted by DFT calculations contrasting formaldehyde and ethene hydrogenation.…”

“…The increases in catalyst lifetime observed by the previously discussed kinetic studies 37,38,43 have demonstrated that hydrogenation reactions play an important role in increasing catalyst lifetime in zeolites of varying topologies. No theoretical study, however, has investigated and compared hydrogenation mechanisms across multiple alkenes, dienes, aldehydes, and arenes.…”

“…Measured second order rate constants of butadiene hydrogenation in H-CHA (H-SSZ-13), H-SSZ-39, H-FER, and H-BEA are 7-318 times higher than rate constants of ethene and propene hydrogenation, suggesting that butadiene is selectively hydrogenated regardless of zeolite topology. 43 This selectivity, however, is not because of a thermodynamic preference as reaction free energies for diene hydrogenation (−33 to −50 kJ mol −1 ) are similar to those of alkene hydrogenation (−33 to −63 kJ mol −1 ) (Fig. 2).…”

“…Additionally, rate constants of ethene hydrogenation in H-CHA (H-SSZ-13), H-SSZ-39, H-FER, and H-BEA suggest that rate constants of ethene hydrogenation are 1.5-16× lower than rate constants of propene hydrogenation. 43 Unlike ethene, which involves a single unique hydrogenation scheme, there are two unique hydrogenation schemes for propene: formation of a primary carbocation by protonation the secondary (β) carbon (Fig. 5a) or formation of a secondary carbocation by protonation of the primary (α) carbon (Fig.…”