Increasing the Number of Aluminum Atoms in T₃ Sites of a Mordenite Zeolite by Low‐Pressure SiCl₄ Treatment to Catalyze Dimethyl Ether Carbonylation

Abstract

Controlling the location of aluminum atoms in a zeolite framework is critical for understanding structure–performance relationships of catalytic reaction systems and tailoring catalyst design. Herein, we report a strategy to preferentially relocate mordenite (MOR) framework Al atoms into the desired T3 sites by low‐pressure SiCl4 treatment (LPST). High‐field 27Al NMR was used to identify the exact location of framework Al for the MOR samples. The results indicate that 73 % of the framework Al atoms were at the T3 sites after LPST under optimal conditions, which leads to controllably generating and intensifying active sites in MOR zeolite for the dimethyl ether (DME) carbonylation reaction with higher methyl acetate (MA) selectivity and much longer lifetime (25 times). Further research reveals that the Al relocation mechanism involves simultaneous extraction, migration, and reinsertion of Al atoms from and into the parent MOR framework. This unique method is potentially applicable to other zeolites to control Al location.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

“…Besides the accessibility, the location of Brønsted acid sites in zeolites is another key factor influencing the confinement effects on intermediates and transition states and therefore determining their catalytic performance in Brønsted acid-catalyzed reactions. − For instance, by taking advantage of the fact that boron atom is preferentially located at the channel intersections and Al has no clear preference for any particular T site, ZSM-5 zeolite enriched with framework Al atoms in the 10-MR channels were successfully prepared by removing the boron atoms from the presynthesized B–Al-ZSM-5 zeolite samples in a postsynthesis treatment. Compared to conventional ZSM-5 zeolite containing randomly located Al atoms (Z5-50), the as-prepared ZSM-5 zeolite enriched with Al in the 10-MR channels (Z5–50) showed higher propene selectivity (86.2 vs 78.3) and lower C 4  /C 3  ratio (0.36 vs 0.43) in 1-hexene cracking reaction as well as longer catalytic lifetime, higher propene yield, and lower yield of alkanes and aromatics in methanol to propylene reaction .…”

“…In contrast, ZSM-11 zeolite with enriched Al sites in the straight channel was prepared in the absence of alkali metal ions, which led to an enhanced alkene-based cycle toward producing more propylene and butylene products . More recently, Liu and co-workers developed a low-pressure SiCl 4 treatment method to preferentially relocate the framework Al atoms into desired T 3 sites in MOR -type zeolite . Because of the molecular size limitation, SiCl 4 can only access the 12-MR channel and then replace framework Al by Si with the extraction of AlCl 3 .…”

“…453 More recently, Liu and co-workers developed a low-pressure SiCl 4 treatment method to preferentially relocate the framework Al atoms into desired T 3 sites in MOR-type zeolite. 455 Because of the molecular size limitation, SiCl 4 can only access the 12-MR channel and then replace framework Al by Si with the extraction of AlCl 3 . These AlCl 3 species migrate into the 8-MR channels and further react with silanol defects, leading to the reinsertion of Al into the framework T 3 sites.…”

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

“…Indeed, an inhibition effect by the methyl acetate reaction products was observed experimentally for high pressures of product only, likely in a regime where it interacts strongly with active sites. All in all, several factors make the 8MR structure desirable for carbonylation reactions that currently lead to synthesis and post-treatment efforts to remove 12MR sites (in mordenite) and locate most Brønsted acid sites at relevant positions to improve DME conversion. ,− Recently, surface barrier effects were invoked to explain the enhanced reactivity of mordenite after surface etching, even if an effect of the possible variation in distribution of surface OH groups cannot be excluded. Unfortunately, 1 H NMR and FTIR spectra of the etched samples were not compared to that of the initial sample.…”

Spatial Proximities between Brønsted Acid Sites, AlOH Groups, and Residual NH4+ Cations in Zeolites Mordenite and Ferrierite

“…The length of the 8-ring bypass channels connecting two 10-ring pores of ferrierite is about 8–9 Å. Confinement is considered an important factor in the catalytic function of the side pockets in mordenite. , The carbonylation of dimethylether and the conversion of syngas to ethylene via a ketene intermediate are favorably catalyzed by acid sites in the 8-ring pockets. More recently, the capture of CO 2 in the 8-ring pockets is an exciting example of confinement effects in mordenite .…”