Linking Zeolite structures to Reactivity in the Carbonylation of Dimethyl Ether

Abstract

Carbonylation of dimethyl ether (DME) to methyl acetate (MA) is one of the key reactions for converting syngas into valuable chemicals and renewable energy sources. The reaction is predominantly catalyzed at Brønsted acid sites (BASs) on the 8-membered ring (8-MR) of zeolite, which stabilizes the transient compound [Z–CH₃–CO]^‡ of the rate-determining step (RDS). Although many studies focus on local descriptors, such as the RDS activation energy and enthalpy or BAS site geometry, these models often fail to fully describe the complex interplay among BAS distribution, pore connectivity, and the three-dimensional frameworks. To develop a more globalized approach for clarifying the structure-reactivity correlation, we synthesized various 8-MR-possessing zeolites, including CHA, FER, LTA, ERI, AEI, and STI with different Si/Al ratios and evaluated their activity normalized per acid site in DME carbonylation reaction. Characterizations of PXRD, SEM(EDS), and BET confirmed successful synthesis of each zeolites, and NH₃-TPD and Pyridine-IR analyses quantified their acid properties. Density functional theory (DFT) calculations show that the activation barrier of the RDS and the stability of the most abundant surface intermediate vary depending on zeolite topology and active site environment. In this work, it was confirmed that "pocket-like" structures connecting two 8-MRs consecutively which make 8-MR channels have the most influential effect on catalytic behavior. Moreover, an adsorption isovolume-based descriptor, namely D_F, which captures the 3-dimensional distribution of BAS and the accessible reaction space, is introduced based on thorough geometry analyses and correlates specifically with the transition state stabilization in the RDS. It is noteworthy that MOR and FER exhibited the highest turnover frequency, correlated with their unique BAS arrangement and favorable channel geometry for transition state stabilization, as predicted by the isovolume-based descriptor proposed in this work. Consequently, extracting 3-dimensional structural descriptors that correlate with RDS is essential for accurate prediction of structure-reactivity relationships. This contribution provides an effective design principle for untested zeolite topologies and may guide the rational design of advanced catalysts for DME carbonylation and related shape-selective processes.