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

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journal contribution
posted on 2018-11-05, 00:00 authored by Julian Holzinger, Malte Nielsen, Pablo Beato, Rasmus Yding Brogaard, Carlo Buono, Michael Dyballa, Hanne Falsig, Jørgen Skibsted, Stian Svelle
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 chemical shifts: the isolated-sites approach where clusters are extracted from large supercells with high Si/Al ratios and an approach targeting lower Si/Al ratios with a fully periodic model. It is found that for the ZSM-23 zeolites with Si/Al = 24 and 37 investigated here, the latter approach gives the best agreement with experiment.