Modification of acid sites in ZSM-5 by ion-exchange: An in-situ FTIR study

Weiqiang Wu, Eric Weitz*

*Corresponding author for this work

Research output: Contribution to journalArticle

36 Scopus citations

Abstract

A study of the acid sites in M n+ -ZSM-5 zeolites (M n+ = H + , Al 3+ , Ca 2+ , and Ba 2+ ), synthesized by ionexchange from the NH4 + form, has been carried out using X-ray diffraction, 27 Al MAS NMR, and in-situ FTIR spectroscopy. X-ray diffraction data indicate that the crystalline structure of ZSM-5 is maintained on ion-exchange. 27 Al MAS NMR spectroscopy confirms that Lewis acid sites in all of the M n+ -ZSM-5 zeolites are mainly located on the tetrahedral aluminum atoms in the zeolite framework. However, octahedral extra-framework aluminum is another source of Lewis acid sites in H + -ZSM-5 and Al 3+ -ZSM-5. Brønsted acid sites are identified as originating from the hydroxyl groups that bridge Al and Si atoms. The acid sites in M n+ -ZSM-5 have been characterized by in-situ FTIR spectroscopy with pyridine as the probe molecule. FTIR spectroscopy demonstrated that the number of accessible acid sites and ratio of Lewis to Brønsted acid sites in M n+ -ZSM-5 can be effectively modified by ion-exchange. The number of acid sites in M n+ -ZSM-5 increases in the same order as the acidity of cations with Ba 2+ < Ca 2+ < NH 4 + < Al 3+ < H + . Though the strength of both Lewis and Brønsted acid sites is virtually identical for all M n+ -ZSM-5 zeolites, the ratio of Brønsted to Lewis acid sites varies as a result of the loaded cation. In addition, modification of acid sites in M n+ -ZMS-5 by loading different cations is discussed in terms of the accessibility of the surface of the zeolite channels and the reactivity of the cations with Brønsted acid hydroxyls. We conclude that larger cations can effectively constrict the zeolite channels and impede the ability of pyridine to access acid sites on the surface of the channels.

Original languageEnglish (US)
Pages (from-to)405-415
Number of pages11
JournalApplied Surface Science
Volume316
Issue number1
DOIs
StatePublished - Jan 1 2014

Keywords

  • Acidity
  • FTIR
  • Zeolite

ASJC Scopus subject areas

  • Chemistry(all)
  • Condensed Matter Physics
  • Physics and Astronomy(all)
  • Surfaces and Interfaces
  • Surfaces, Coatings and Films

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