Initial Carbon–Carbon Bond Formation during the Early Stages of the Methanol‐to‐Olefin Process Proven by Zeolite‐Trapped Acetate and Methyl Acetate

Abstract

Methanol‐to‐olefin (MTO) catalysis is a very active field of research because there is a wide variety of sometimes conflicting mechanistic proposals. An example is the ongoing discussion on the initial C−C bond formation from methanol during the induction period of the MTO process. By employing a combination of solid‐state NMR spectroscopy with UV/Vis diffuse reflectance spectroscopy and mass spectrometry on an active H‐SAPO‐34 catalyst, we provide spectroscopic evidence for the formation of surface acetate and methyl acetate, as well as dimethoxymethane during the MTO process. As a consequence, new insights in the formation of the first C−C bond are provided, suggesting a direct mechanism may be operative, at least in the early stages of the MTO reaction.

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

“…They are dominated by methanol (49.7 ppm), DME (59.7 ppm), aromatics (134.2 ppm) with traces of dimethoxymethane (CH 3 OCH 2 OCH 3 ,D MM, 100.2 ppm), methanediol (89.7 ppm) and formate (172.0 ppm) (Figure 3a-c). [10] It is noteworthy that some weak signals appear at 72.4, 177.9, 186.3, and 226.2 ppm along with high-field signals at 10-30 ppm, which are absent for pure methanol feeding (Figure 1). [10] It is noteworthy that some weak signals appear at 72.4, 177.9, 186.3, and 226.2 ppm along with high-field signals at 10-30 ppm, which are absent for pure methanol feeding (Figure 1).…”

“…[8] As suggested by Lercher, [9] the C À Cc oupling could occur easily on H-ZSM-5 zeolite by the reaction of electrophilic SMS with nucleophilic CO,f orming methyl acetate,w hich was also observed on H-SAPO-34. [10] It is generally accepted that ethene is the initial alkene in the MTOr eaction. How the initial CÀCb ond is formed in consistent with the production of ethene in the early stage of MTOi ss till unknown.…”

Electrophilic aromatic substitution over zeolites generates Wheland-type reaction intermediates

“…2a). The observed bands at ~295, 334 and 415 nm are attributed to neutral alkylated benzenes, carbenium ions with alkyl side chain carbocation and aromatic ring protonation, respectively, whereas the band at 596 nm presumably develops due to the existence of π-complexes between the zeolite and aromatic species (vide infra for NMR results) [31][32][33][34][35][36][37][38][39] . The on-line MS data reveal the existence of diethyl ether, lower olefins (ethylene and propylene) and ethylbenzene ( Fig.…”

“…3, Supplementary Figs. 3-6 and Supplementary Methods II) [31][32][33][34][35][36][37][38] . Figure 3a-d reflects a redshift of the absorption maxima of a σ-complex (Wheland species), either upon ethylation of the zeolite or in the presence of an electron-donating group attached to the aromatic ring.…”

“…13, the surface propoxy species (SPS) is formed due to the interaction between zeolite and propylene, which later undergoes the S E Ar pathway with benzene to form cumene. From a different perspective, benzene is efficiently acting as a trapping agent to various electrophiles (either olefin or SES) during the EB reaction, in order to prevent the ethanol-derived homologation/chain growth reaction to form higher aromatics, which is an established phenomenon during zeolite catalysed alcohol conversion processes 26,34 . The other zeolite-trapped minor products, like xylene or tetramethylbenzene, are formed as a result of isomerization of ethylbenzene and diethylbenzene, respectively, under the experimental conditions of the EB reaction ( Supplementary Fig.…”