Direct Mechanism of the First Carbon–Carbon Bond Formation in the Methanol‐to‐Hydrocarbons Process

Abstract

In the past two decades, the reaction mechanism of C-C bond formation from either methanol or dimethyl ether (DME) in the methanol-to-hydrocarbons (MTH) process has been a highly controversial issue. Described here is the first observation of a surface methyleneoxy analogue, originating from the surface-activated DME, by in situ solid-state NMR spectroscopy, a species crucial to the first C-C bond formation in the MTH process. New insights into the first C-C bond formation were provided, thus suggesting DME/methanol activation and direct C-C bond formation by an interesting synergetic mechanism, involving C-H bond breakage and C-C bond coupling during the initial methanol reaction within the chemical environment of the zeolite catalyst.

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

“…By using the continuous‐flow NMR method, the methanol conversion over zeolite H‐ZSM‐5 was studied by Liu et al. under real MTH reaction conditions . As shown in Figure , at early stage, the surface methoxy species (59.5 ppm) and trimethyloxonium ion (80 ppm) were observed.…”

Bridging the Gap between the Direct and Hydrocarbon Pool Mechanisms of the Methanol‐to‐Hydrocarbons Process

“…[6] Surprisingly,t he exact route for the formation of adirect carbon À carbon (C À C) bond from methanol has very recently been recognized. [7][8][9][10][11][12][13][14][15][16][17] Previously,t he presence of trace impurities (for example,int he methanol, catalyst, and/ or carrier gas) were thought to be responsible for the formation of the initial CÀCb onds by the direct mechanism. [18,19] This assumption resulted from al ack of concrete experimental support, as well as theoretical calculations that predicted the direct mechanism was unrealistic (that is,higher activation barriers and/or unstable intermediates).…”

“…[13] Subsequently,d ifferent research groups (for example,K ondo,F an, CopØret, Lercher, Liu, and Studt), including our own, have delivered both experimental and theoretical evidence in support of the existence of ad irect mechanism during the early stages of the MTH process. [7][8][9][10][11][12][13][14][15][16][17] Among several direct coupling proposals,t he Koch-type carbonylation mechanism is now the most widely acknowledged route to form direct C À Cb onds in the MTH process (Scheme 1). [7,10,16] Lercher and coworkers proposed that methyl acetate (CH 3 CO 2 CH 3 derived by carbonylation of methanol) is the very first CÀCb ond-containing species during the MTH reaction over zeolite H-ZSM-5.…”

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

“…More EFAL species was generated by steam dealumination so as to highlight its influence.T oavoid the dealumination of H-ZSM-5-Nd in the reaction, the reactions were conducted at alower temperature of 275 8 8Cf or 1min. At lower temperatures (Figure 1a,b), apart from methanol (49.7 ppm), dimethyl ether (DME, 59.7 ppm), and the trimethyloxonium ion [14] (TMO + , 80 ppm), as ignal at 52.4 ppm is observable on H-ZSM-5-De but absent on H-ZSM-5-Nd (Figure 1g;Supporting Information, Figure S5), which probably corresponds to the formation of surface methoxy species bound to EFAL (SMS-EFAL). Importantly,amuch higher yield of ethene is also observable on H-ZSM-5-De,i ndicating that the hydrogen transfer favors the initial C À Cb ond formation in the initial period of methanol conversion.…”