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.
Highlights
IntroductionAs one type of common active complexes in nucleophilic additions of organic synthesis,[1,2] can act as a key intermediate with the simplest formula (CH2 C O) during zeolite-catalyzed C1 chemistry,[3] such as methanol to olefins (MTO),[4,5] dimethyl ether (DME) carbonylation to methyl acatate (MA),[6,7] carbon dioxide to hydrocarbons,[8] syngas conversion,[9,10] and etc.[11,12] (Scheme 1) For example, during the multicatalyst relay catalysis of oxide-zeolite for selective conversion of syngas to light olefins, ketene is produced from syngas over a metal oxide catalyst (e.g., ZnCrOx), which diffuses into silicoaluminophosphate (SAPO) or MOR and further reacts with Brønsted acidic sites (BAS) to attain the desirable light olefins (C2=− C4=) selectivity under the shape selectivity of zeolites.[9,10] Similar chemistry has been exploited for direct hydrogenation of CO2 to hydrocarbons[8] by the bifunctional catalyst system comprising potassium superoxide doped iron oxide and acidic zeolites (e.g., ZSM-5 or MOR)
It was found that direct C−C bond couplings were associated with CSMS−OZeo bond ruptures and CSMS−CCO bond formations reflected by their coordination numbers (CN)
The free energy barriers observed for MOR-12MR, MOR8MR, and SSZ-13 were found to be 155.7, 128.6, and 143.0 kJ/mol, and these results are basically consistent with the static DFT calculations and pervious theoretical studies, as summarized in Tables S1 and S2.7,20−22,37 Notably, it is incontrovertible that MOR-8MR has a significantly higher activity on C−C bond coupling than do MOR-12MR and SSZ13, but the order of activity between MOR-12MR and SSZ-13 is indistinguishable by static DFT calculations because both MOR-12MR and SSZ-13 are loosely confined by the reaction process
Summary
As one type of common active complexes in nucleophilic additions of organic synthesis,[1,2] can act as a key intermediate with the simplest formula (CH2 C O) during zeolite-catalyzed C1 chemistry,[3] such as methanol to olefins (MTO),[4,5] dimethyl ether (DME) carbonylation to methyl acatate (MA),[6,7] carbon dioxide to hydrocarbons,[8] syngas conversion,[9,10] and etc.[11,12] (Scheme 1) For example, during the multicatalyst relay catalysis of oxide-zeolite for selective conversion of syngas to light olefins, ketene is produced from syngas over a metal oxide catalyst (e.g., ZnCrOx), which diffuses into silicoaluminophosphate (SAPO) or MOR and further reacts with Brønsted acidic sites (BAS) to attain the desirable light olefins (C2=− C4=) selectivity under the shape selectivity of zeolites.[9,10] Similar chemistry has been exploited for direct hydrogenation of CO2 to hydrocarbons[8] by the bifunctional catalyst system comprising potassium superoxide doped iron oxide and acidic zeolites (e.g., ZSM-5 or MOR). Zeolites (including metal-modified zeolites) and heteropolyacids are two types of solid acids dedicated to catalyze DME/methanol carbonylation.[15,16] Among these catalysts, acidic zeolites have been confirmed the high selectivity and conversion toward methyl acetate at relatively low temperatures because of their Brønsted acidity and pore architecture (Scheme S1)
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