Molecular Understanding of the Catalytic Consequence of Ketene Intermediates under Confinement

Abstract

Neutral ketene is a crucial intermediate during zeolite carbonylation reactions. In this work, the roles of ketene and its derivates (viz., acylium ion and surface acetyl) associated with direct C−C bond coupling during the carbonylation reaction have been theoretically investigated under realistic reaction conditions and further validated by synchrotron radiation X-ray diffraction (SR-XRD) and Fourier transformed infrared (FT-IR) studies. It has been demonstrated that the zeolite confinement effect has significant influence on the formation, stability, and further transformation of ketene. Thus, the evolution and the role of reactive and inhibitive intermediates depend strongly on the framework structure and pore architecture of the zeolite catalysts. Inside side pockets of mordenite (MOR), rapid protonation of ketene occurs to form a metastable acylium ion exclusively, which is favorable toward methyl acetate (MA) and acetic acid (AcOH) formation. By contrast, in 12MR channels of MOR, a relatively longer lifetime was observed for ketene, which tends to accelerate deactivation of zeolite due to coke formation by the dimerization of ketene and further dissociation to diene and alkyne. Thus, we resolve, for the first time, a long-standing debate regarding the genuine role of ketene in zeolite catalysis. It is a paradigm to demonstrate the confinement effect on the formation, fate, and catalytic consequence of the active intermediates in zeolite catalysis.

Light olefin synthesis from a diversity of renewable and fossil feedstocks: state-of the-art and outlook

“…Larger 12MR channels facilitate longer contact time of ketene and, hence, formation of a broader range of products and coking, while the 8MR confinement leads to protonation of ketene and to the formation of stable acylium ions, which further participate in catalytic reactions. 424 Recently, Wang et al showed that the application of H-RUB-13 zeolite instead of SAPO-34 provided a shift in the product distribution to propylene–butylene fraction due to the changes in hydrocarbon pool mechanism from the aromatic-mediated to alkene-based cycle. 415…”

Resolving the Intricate Mechanism and Selectivity of Syngas Conversion on Reduced ZnCr2Ox: A Quantitative Study from DFT and Microkinetic Simulations

“…A decade ago, de Lasa and co-workers reported that CH 3 OH is the intermediate converting syngas to hydrocarbons on the ZnO-Cr 2 O 3 + ZSM-5 bifunctional catalyst. In addition to FTS, , ketene was also considered as a critical intermediate in other processes of zeolite-catalyzed C1 chemistry, such as methanol to olefins (MTO), , dimethyl ether carbonylation to methyl acetate, − and carbon dioxide to hydrocarbons . Therefore, whether the reaction on OX-ZEO catalysts is a combination of methanol synthesis and MTO deserves further investigation.…”

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

“…Although AIMD simulations have been widely used in the dynamic simulation of zeolite-catalyzed reactions with the output of a variety of structural and electronic information under the reaction conditions, [97][98][99][100][101][102] the accuracy of the DFT functional in AIMD mainly remains on Rung 2 of GGA, such as PBE-D3 and revPBE-D3. This is because the increasing computational cost on one SCF convergence would be significantly magnified to 10 5 -10 7 times that in 10 5 -10 7 time steps.…”

“…Carbocations are also important intermediates in other zeolitecatalyzed reactions, such as poly-ethylated benzenium ions in the ethanol to olefins (ETO) reaction as the ''sister-reaction'' of MTO, 42,46,75,149,314 cyclic carbenium ions in 1-ethylcyclohexene isomerization, 213 alkylated naphthyl carbenium ions in the conversion of ethene to propene, 315 acylium ion in DME/ methanol carbonylation 102 and Friedel-Crafts acylation, 316,317 and alkylated phenyl carbenium ions in alkylaromatic transalkylation. 117,276,[318][319][320][321][322] Chowdhury et al 149 employed spectroscopic approaches to validate the reaction and deactivation mechanism of the ETO process in H-ZSM-5, where alkyl side chain carbenium ions, alkylated phenyl carbenium ions, and poly-arenium ions were detected as the intermediates to produce alkenes by operando UV-Vis diffuse reflectance spectroscopy.…”

“…Recently, we found that the acylium ion can be well-stabilized by the strongly confined side pocket of mordenite as the other active intermediate in DME/MeOH carbonylation, and the ketene would rapidly protonate to the acylium ion under the promotion of zeolite confinement. 102 Besides the stabilization of the small acylium ion by mordenite, the large carboxonium intermediates can be stabilized by zeolite confinement in the Nazarov cyclization and the tert -butylation of alcohols. This finally lead to the highly efficient conversion of the substrate under relatively mild conditions.…”