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

Abstract

The versatile methanol‐to‐hydrocarbon (MTH) process on acidic zeolites allows for a production of olefins, aromatics and gasoline from a wide range of non‐fossil resources. Elucidation of the MTH reaction mechanism is helpful to improve product selectivity as well as develop high performance catalysts. The application of solid‐state NMR in the investigation of MTH reaction has significantly contributed to get insight into the MTH chemistry. In this review, we summarize mechanistic insight that has been gained into the MTH reaction by using solid‐state NMR spectroscopy. Special emphasis is given to the observation and determination of C1 species and active intermediates particularly cyclic carbocations to uncover the exact reaction route for the initial C−C bond formation and the hydrocarbon pool mechanism. The detection of host‐guest interactions involved in the MTH reaction with the advanced 13C‐27Al double‐resonance NMR is discussed for a better understanding of the active sites and the reactivity of hydrocarbon pool species.

Selective methanethiol-to-olefins conversion over HSSZ-13 zeolite

“…The 13 C NMR spectra feature two main groups of signals at (i) 10 -30 ppm, corresponding to sp 3 carbon atoms and (ii) 120 -140 ppm, representing sp 2 carbon atoms in arenes and olefins. [22][23][24] These features are also observed in the 2D HETCOR spectra of the used HSSZ-13 sample. The 13 C signal at 129 ppm mainly correlates with the 1 H signal at 8.6 ppm, which indicates arenes to be the main sp 2 carbon species.…”

“…25 Considering the intensity differences, the used HSSZ-13 zeolite contained much more aromatic species than the used HZSM-5 sample, which is consistent with the known preference for the aromatic cycle on 8MR zeolites. [22][23][24] Furthermore, the NMR spectrum of used HZSM-5 presents a higher relative intensity of sp 3 carbon species (signal ~21 ppm), implying a higher concentration of saturated hydrocarbons than typically reported during CH 3 OH conversion over HZSM-5. 26 We hypothesize that this is due to the formation and deposition of non-volatile thiols, thioethers and oligosulfides on HZSM-5.…”

New Spectroscopic Insight into the Deactivation of a ZSM‐5 Methanol‐to‐Hydrocarbons Catalyst

“…NMR spectroscopy is highly successful at providing insight in the initial and steady state stages. However, it has difficulties identifying deactivating species at longer reaction times due to the reduced hydrogen content and preponderance of spinning side bands [28–36] …”

Dual Active Sites on Molybdenum/ZSM‐5 Catalyst for Methane Dehydroaromatization: Insights from Solid‐State NMR Spectroscopy

“…Although the exact structure of the active Mo species is still in debate, previous reports underlined the importance of the bifunctionality of Mo/ZMS‐5 for the MDA reaction: [22–25] dehydrogenation and oligomerization of methane occur on active Mo sites forming C 2 intermediates such as ethene and acetylene, followed by cyclization producing aromatics and naphthalene on the Brønsted acid sites (BAS) in zeolite. The subsequent transformation of hydrocarbons can be understood by the hydrocarbon pool mechanism that resembles the well‐established organocatalytic process in methanol‐to‐hydrocarbon reaction [26–28] . The hydrocarbon pool can be simply considered as a catalytic scaffold in zeolite, constituted by the organics trapped in zeolite channels or cages, to which the reactant is added and products are removed in a catalytic cycle.…”