Comments on the mechanism of MTG/HZSM-5 conversion

Abstract

A comparative F.t.i.r./g.c.-m.s. study of MTG conversion on a series of progressively dealuminated HZSM-5 and H-mordenite surfaces reveals the role of zeolite dealumination in modifying the zeolite pore void (i.e. catalytic volume) and therefore the shape-selectivity. This is illustrated by a different distribution of the primary alkenes formed under similar experimental conditions. G.c.-m.s. analysis of a reaction at the early stages of dimethylether conversion reveals that ethene, which forms in trace quantities on HZSM-5, is dominant on H-mordenite. The propene/butenes ratio is high (ca. 1.7) for H-mordenite, but is < 1.0 and decreases with increased dealumination for HZSM-5. Isobutene dominates for increasingly dealuminated HZSM-5. Methanol reformation proceeds concurrently with alkene formation, suggesting a route involving either propagation and/or decomposition to alkenes. The mechanistic implications of the interdependence of zeolite dealumination, modified pore void and identity of alkenes are analysed.

IR Spectral Investigation of the Multistage Single Catalytic Process of CH4 Conversion into Higher Hydrocarbons over HZSM-5 Structurally Modified with an Oxidative Element

“…Methanol would chemisorb strongly to form a methoxy species (2942 cm -1 ) showing a downward shift of 30 cm -1 from the dominant υ C-H of gaseous methanol at 2972 cm -1 . 8 Neither the gaseous methanol nor the surface methoxy species is evident in the present study. In effect, gaseous methanol shows other fundamental modes of υ O-H (3675 cm -1 ), υ C-H (2842 cm -1 ), δ C-H (1458 cm -1 ), δ O-H (1350 cm -1 ), and the very strongly absorbing υ C-O (1033 cm -1 ), 8 which are not apparent in the present spectra.…”

“…8 Neither the gaseous methanol nor the surface methoxy species is evident in the present study. In effect, gaseous methanol shows other fundamental modes of υ O-H (3675 cm -1 ), υ C-H (2842 cm -1 ), δ C-H (1458 cm -1 ), δ O-H (1350 cm -1 ), and the very strongly absorbing υ C-O (1033 cm -1 ), 8 which are not apparent in the present spectra. Also, the primary intermediate of CH 3 OH reactions over HZSM-5 surfaces is (CH 3 ) 2 O that absorbs very strongly at 1178 cm -1 for the υ C-O , which is not shown either.…”

“…In the first process, methane is partially oxidized over oxidative catalysts [1][2][3][4][5][6] to CH 3 OH. In Mobil's plant, CH 3 OH is dehydrated 7 into CH 3 OCH 3 that propagates 8 to higher ethers, viz., CH 3 OC 2 H 5 , CH 3 OC 3 H 7 , and CH 3 OC 4 H 9 over HZSM-5. The higher ethers then decompose to the corresponding primary olefin intermediates C 2 H 4 , C 3 H 6 , and C 4 H 8 , respectively, which are eventually alkylated by methanol regenerated in the previous step, 8 gasoline forms with high efficiency and octane number.…”

Benzene co-reaction with methanol and dimethyl ether over zeolite and zeotype catalysts: Evidence of parallel reaction paths to toluene and diphenylmethane

“…Analogously to the previous section, where the reactivity of methanol and DME towards benzene was studied as a representative molecule of the arene cycle, this section reflects the reactivity of methanol and DME towards isobutene, as a representative molecule of the alkene cycle in the MTH reaction. It is worth mentioning that this hydrocarbon is the most thermodynamically favored C4 alkene isomer under the typical MTH conditions, and it is therefore an abundant effluent product during the conversion of methanol and DME to hydrocarbons over different zeolites [125,141,[262][263][264].…”

Hydrogen Transfer versus Methylation: On the Genesis of Aromatics Formation in the Methanol-To-Hydrocarbons Reaction over H-ZSM-5

“…Isobutene was selected as probe molecule because (1) it is an abundant effluent product in the MTH reaction over many catalysts and (2) previous studies suggest that it may adsorb as stable tertiary carbocation under operating conditions, possibly favoring direct hydrogen transfer compared to linear alkenes. ,,− Importantly, H-ZSM-5 nanosheets were selected as catalyst for two reasons: (1) H-ZSM-5 is the most industrially relevant MTG catalyst, wherein alkene and arene cycles are clearly observed, , and (2) the nanosheet morphology enables shortening of the diffusion path of reactants and products, thereby focusing on the kinetic origin of the propagation of alkene- and arene-forming reactions. Further insight into the relative rates of methylation and hydrogen-transfer reactions were provided by isotopic labeling experiments and a set of static and dynamic molecular simulations.…”