Identification of Distinct Framework Aluminum Sites in Zeolite ZSM-23: A Combined Computational and Experimental 27Al NMR Study

Abstract

ZSM-23 (MTT) is a silicon-rich zeolite with one-dimensional, 10-membered ring channels, which has recently attracted interest as a promising catalyst in aromatic-free methanol-to-hydrocarbons conversion. To obtain a better understanding of the catalytic activity and ultimately to design a better catalyst, it is crucial to locate the active sites in the zeolite framework. This work investigates the tetrahedral aluminum framework sites in two zeolite H-ZSM-23 samples by experimental and computational 27Al NMR spectroscopy. 27Al MQMAS NMR experiments at six different magnetic fields (4.7–22.3 T) were utilized to resolve distinct Al sites. The detected tetrahedral framework Al sites were assigned to the specific tetrahedral sites in the crystal structure by DFT calculations of the 27Al chemical shieldings. A comprehensive investigation of the structural model, basis set, and exchange–correlation potential used in the DFT calculations was performed. Two avenues were pursued for calculating the 27Al isotropic c...