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

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DOI

  • Julian Holzinger
  • ,
  • Malte Nielsen, Universitetet i Oslo, Haldor Topsøe A/S
  • ,
  • Pablo Beato, Haldor Topsøe A/S
  • ,
  • Rasmus Yding Brogaard, Haldor Topsøe A/S
  • ,
  • Carlo Buono, Universitetet i Oslo
  • ,
  • Michael Dyballa, Universitetet i Oslo
  • ,
  • Hanne Falsig, Haldor Topsøe A/S
  • ,
  • Jørgen Skibsted
  • Stian Svelle, Universitetet i Oslo

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 27 Al NMR spectroscopy. 27 Al 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 27 Al 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 27 Al 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.

Original languageEnglish
JournalJournal of Physical Chemistry C
Volume123
Issue13
Pages (from-to)7831-7844
Number of pages14
ISSN1932-7447
DOIs
Publication statusPublished - 2019

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