Evidence for a “Carbene‐like” Intermediate during the Reaction of Methoxy Species with Light Alkenes on H‐ZSM‐5

Abstract

Secrets revealed: IR spectroscopy shows that the recovered acidic hydroxy groups after the reaction of [D3]methoxy groups with light alkenes on zeolites are all deuterated (see scheme). The results indicate that the reaction occurs through methylene rather than methyl units.

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

“…As ar esult, the first C À Cbond formation takes place through the typical insertion reaction of carbene/ylide from SMS into the sp 3 C À Hbond of methanol/DME during MTO( species A/A' ' in path a, Scheme 1a nd Figure 2c). [14,[38][39][40] In addition to our ssNMR observation (Figure 2a-c), the existence of such neighboring oxygen group-assisted SMS-type species has been characterized by Hunger et al [14,15] and Kondo et al [16,40] Species A' ' eventually led to surface-ethanolic species (B in path a, Scheme 1), which undergoes dehydration (through protonation of alcohol/ether oxygen atoms by Brønsted acid sites of the zeolite) to form ethylene and regenerates ZeOH. [17] In principle,t he presence of two adjacent methoxy groups (Figure 2a-c) could also be consistent with the existence of the oxonium ylide mechanism (Scheme S2).…”

“…[16,38,39] As ar esult, SMS of ac arbene/ylide nature are likely to be formed, as it was previously characterized by Kondo et al and Hunger et al by IR and NMR spectroscopy,r espectively. [15,40] The existence of such carbene-like SMS was experimentally confirmed as well, by atrapping experiment with cyclohexane as ap robe molecule at ! 493 K, where methylcyclohexane was formed through an insertion reaction of carbene/ylide into the sp 3 CÀHb ond of cyclohexane.…”

Kinetics of Butene Isomer Methylation with Dimethyl Ether over Zeolite Catalysts

“…Surface methoxides have been shown to be thermally stable species as they are able to persist in vacuum at 673 K, monitored via in situ IR spectroscopy, before surfacebound C−H stretches diminish during coke formation. 32 This enhanced stability arises from the absence of mechanisms for surface C 1 species to desorb unlike their higher homologues, which can generate an olefin via β-elimination. 34,48 These species have also been directly observed using solid-state 13 C MAS NMR under vacuum 33 and continuous flow 34 conditions on MFI at 473 K. In this report, the temperature of a 200 mg BEA sample was increased to 423 K in 1.67 cm 3 s −1 He after steady state cis-2-butene methylation.…”

Mechanism of Decomposition of Surface Ethoxy Species to Ethene and Acidic OH Groups on H-ZSM-5

“…9−11 Contrary to the prediction, monomeric alkoxy species corresponding to products of protonation of light olefins (ethene, propene, and butenes) have not been identified experimentally as isolated stable intermediates 12 either by infrared (IR) spectroscopy or nuclear magnetic resonance (NMR). On the other hand, unimolecular alkoxy species were observed from the dehydration of methanol and ethanol to methoxy and ethoxy species, respectively, over acidic OH groups by IR spectroscopy [13][14][15][16] These alkoxy species are regarded as the initial intermediate for the conversion of methanol 21−24 and ethanol 25−27 to olefins or ethers over zeolites with shape selectivity. 28 The dehydration mechanism of ethanol has also been studied experimentally 29−32 and by theoretical approaches.…”