Extra‐Framework Aluminum‐Assisted Initial C−C Bond Formation in Methanol‐to‐Olefins Conversion on Zeolite H‐ZSM‐5

Abstract

Surface methoxy species bound to an extra-framework Al (SMS-EFAL) was unambiguously identified by advanced C-{ Al} double-resonance solid-state NMR technique in the methanol-to-olefins reaction on H-ZSM-5 zeolite. The high reactivity of the SMS-EFAL leads to the formation of surface ethoxy species and ethanol as the key intermediates for ethene generation in the early reaction stage. A direct route for the initial C-C bond formation in ethene was proposed and corroborated by density functional theory calculations.

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…A formaldehyde analogue of [Al−OCH 2 ] + was identified to be a crucial intermediate for the first C−C bond formation (Scheme ). This work puts forward an important proposal that the synergy of Brønsted acid and Lewis acid in zeolite catalysts could facilitates the catalytic methanol to olefin reaction, which has been supported by NMR experiments …”

Mechanism of Methanol‐to‐hydrocarbon Reaction over Zeolites: A solid‐state NMR Perspective

“…Since the MTH reactions are usually performed at high temperature, zeolites are prone to be dealuminated, generating extra‐framework Al (EFAL) species which can serve as Lewis acid sites . In a recent report, Xu, Deng and coworkers unambiguously identified a surface methoxy species bound to EFAL (SMS‐EFAL) on dealuminated ZSM‐5 in MTO reaction . As shown in Figure , in addition to the traditional methoxy group (SMS, 58.2 ppm), DME (59.7 ppm, 62.8 ppm) and adsorbed methanol (49.7 ppm, 51.3 ppm), a 13 C signal at 52.4 ppm is observable on dealuminated H‐ZSM‐5 (H‐ZSM‐5‐De), but absent on non‐dealuminated ZSM‐5 (H‐ZSM‐5‐Nd), which corresponds to the formation of SMS‐EFAL species.…”

“…Formaldehyde is a key species for the first C−C bond formation in the methane‐formaldehyde mechanism (Scheme ) or acts as the precursor to the first C−C bond species in the CO mechanism (Scheme ) . In a recent work, Xu, Deng and coworkers found that when formaldehyde was reacted with surface methoxy species bound to EFAL (SMS‐EFAL) over ZSM‐5, the induction period was significantly shortened with highly selective formation of ethene. The analysis of the adsorbed species over ZSM‐5 using 13 C NMR showed that in addition to methyl acetate and acetate, acetaldehyde (226.2, 20.1 ppm) and ethoxy species (72.4, 13.2 ppm) were also observable (Figure ).…”

Insight into Three‐Coordinate Aluminum Species on Ethanol‐to‐Olefin Conversion over ZSM‐5 Zeolites

“…The generated Si‐OH‐Al can either act as BAS for ethanol dehydration (Supporting Information, Scheme S1, Pathway A) or react with neighboring AlOH back to EFAl‐Al 3+ species and Si‐O‐Al sites after dehydration. The latter is confirmed by Deng and co‐workers in methanol conversion at 250–300 °C . In that work, EFAl species on dealuminated zeolite H‐Y are found to promote the facile generation of methoxy groups through 13 C NMR experiments and theoretical calculations, which is unfavorable on zeolite H‐Y without EFAl species under the same conditions.…”

“…For instance, alcohol can donate an OH electron pair to LAS, and consequently, the very active CH 3 CH 2 + carbene ion is generated, which can be stabilized on negative charged framework oxygen O − to form the ethoxy group as a key intermediate for the ethylene production (Supporting Information, Scheme S2). On the Lewis acidic Al defect, a methoxy group was directly observed on dealuminated H‐Y zeolite after abstracting the electron‐pair OH − from methanol at Lewis acidic EFAl sites . Introducing EFAl‐Al 3+ species into zeolites, therefore, would promote the CH 3 CH 2 + carbene ion production and the catalytic performance for the ethanol dehydration.…”