Dynamic Evolution of Aluminum Coordination Environments in Mordenite Zeolite and Their Role in the Dimethyl Ether (DME) Carbonylation Reaction.

Part of tetrahedral framework aluminum in a protonic mordenite (HMOR) will convert geometry to distorted tetrahedral and octahedral coordination. High-field 27 Al NMR data show that more framework Al atoms at T3 and T4 sites change geometry to nonframework structures than others. These nonframework Al species preferentially reside in the side pockets, which will decrease the accessibility of acid sites in the 8-membered ring (MR) channel, impairing the dimethyl ether (DME) carbonylation reaction. The arisen octahedrally coordinated Al species are framework-associated, which can be reverted into the zeolite framework. Herein, we find that a facile treatment with pyridine could force the octahedral coordination Al back into a tetrahedral environment, which could increase the number of available active sites and enhance the diffusion of DME, thus improving the reactivity (4 times) of the DME carbonylation reaction and prolonging the lifetime of catalysts.

Quantitatively Mapping the Distribution of Intrinsic Acid Sites in Mordenite Zeolite by High-Field 23Na Solid-State Nuclear Magnetic Resonance.

It is of great significance to accurately quantify the Brønsted acid sites (BASs) at different positions of mordenite (MOR) zeolite. However, H-MOR obtained from Na-MOR can hardly avoid dealumination under hydrothermal conditions, which causes difficulty in the acid characterization. Herein, 23Na-27Al D-HMQC was performed combined with high-field 23Na MQ MAS NMR and DFT calculation, which provided an unambiguous attribution of the 23Na chemical shifts and further helped to improve the resolution of 27Al MAS NMR. By fitting the 23Na and 1H MAS NMR spectra of Na/H-MOR, the intrinsic BAS contents in different T-sites were measured by characterizing the location and content of sodium ions. These Na/H-MOR zeolites with various acid distributions were used for DME carbonylation and showed that the amount of BASs in the T3 site was proportional to the activity of carbonylation. This study provides a new method for investigating the intrinsic acid properties of zeolites.

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

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 27 Al 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.

Capture and identification of coke precursors to elucidate the deactivation route of the methanol-to-olefin process over H-SAPO-34.

The evolution of retained species during the whole methanol-to-olefins process was revealed with the aid of GC-MS, thermogravimetric analysis (TG) and density functional theory (DFT) calculations. Precise routes for the transformation of retained methylbenzenes to methylnaphthalenes were proposed, based on the direct capture of three possible organic intermediates, to explain the catalyst deactivation procedure.