Control of zeolite microenvironment for propene synthesis from methanol

Abstract

Optimising the balance between propene selectivity, propene/ethene ratio and catalytic stability and unravelling the explicit mechanism on formation of the first carbon–carbon bond are challenging goals of great importance in state-of-the-art methanol-to-olefin (MTO) research. We report a strategy to finely control the nature of active sites within the pores of commercial MFI-zeolites by incorporating tantalum(V) and aluminium(III) centres into the framework. The resultant TaAlS-1 zeolite exhibits simultaneously remarkable propene selectivity (51%), propene/ethene ratio (8.3) and catalytic stability (>50 h) at full methanol conversion. In situ synchrotron X-ray powder diffraction, X-ray absorption spectroscopy and inelastic neutron scattering coupled with DFT calculations reveal that the first carbon–carbon bond is formed between an activated methanol molecule and a trimethyloxonium intermediate. The unprecedented cooperativity between tantalum(V) and Brønsted acid sites creates an optimal microenvironment for efficient conversion of methanol and thus greatly promotes the application of zeolites in the sustainable manufacturing of light olefins.

How Many Molecules Can Fit in a Zeolite Pore? Implications for the Hydrocarbon Pool Mechanism of the Methanol-to-Hydrocarbons Process

“…We recognize that they are not the only relevant factors. For zeolites, the Si:Al ratio [7,69], and hence the number of Brønsted sites, the presence of heteroatoms [70] (if any) and even the morphological characteristics of the material [71] all play key roles in the chemistry. However, while these aspects affect the type and the distribution of the products, for H-ZSM-5, the structure is largely unchanged by variations in Si:Al ratio or the introduction of small numbers of heteroatoms [69,70,72].…”

“…For zeolites, the Si:Al ratio [7,69], and hence the number of Brønsted sites, the presence of heteroatoms [70] (if any) and even the morphological characteristics of the material [71] all play key roles in the chemistry. However, while these aspects affect the type and the distribution of the products, for H-ZSM-5, the structure is largely unchanged by variations in Si:Al ratio or the introduction of small numbers of heteroatoms [69,70,72]. This strongly suggests that the consequences of the available volume are independent of these factors and that the results presented here are generally applicable.…”

Net Zero and Catalysis: How Neutrons Can Help

“…The balance between the type of product can be adjusted by the choice of catalyst and reaction conditions, so this process provides platform chemicals and fuel [68,69]. The reaction over H-ZSM-5 has been extensively studied by INS [70][71][72][73][74]. The advantage of INS for studies of microporous (zeolites, aluminophosphates (ALPOs), siliconaluminophosphates (SAPOs)) and mesoporous (MCM-41 type) materials is that they are almost completely transparent across the entire 0-4000 cm −1 range (the incoherent scattering cross section's of Al, Si, P, O are all <5 barn).…”

QM/MM study of the reactivity of zeolite bound methoxy and carbene groups

“…6 The more favourable oxonium ylide mechanism starts with the formation of trimethyl oxonium (TMO) via the reaction of dimethyl ether with a dimethyl oxonium ion (protonated dimethyl ether); subsequently, TMO is deprotonated by a basic site to form dimethyl oxonium methyl ylide (DOMY) as shown in Scheme 1A that can undergo a Stevens rearrangement to form methylethyl ether shown in Scheme 1B or an intermolecular methylation (Scheme 1C), resulting in the formation of ethylmethyl oxonium ion; however, the inability of the zeolite framework to deprotonate the TMO and stabilise the DOMY made this routes seem infeasible. 11,12,13 Scheme 1. Illustration of oxonium ylide mechanism via TMO to ethene.…”