NMR spectroscopic characterization of flame-made amorphous silica-alumina for cyclohexanol and glyceraldehyde conversion

Zichun Wang, Yijiao Jiang, Catherine Stampfl, Alfons Baiker, Michael Hunger, Jun Huang*

*Corresponding author for this work

Research output: Contribution to journalArticle

Abstract

Amorphous silica-aluminas (ASAs), possessing both Brønsted acid sites (BAS) and Lewis acid sites (LAS), are important bifunctional catalysts in various industrial applications. Solid-state NMR spectroscopy has been widely used for characterizing the local structure and the surface sites of ASAs with probe molecules. In this work, four-, five- and six-coordinated Al species have been observed on the flame-made ASAs by 27Al MQ MAS NMR experiment. 1H/27Al TRAPDOR MAS NMR experiments confirmed that surface Al species contribute to the formation of BAS and protonate ammonia probe molecules. The adsorption of ammonia on Lewis acidic Al sites (δ1H=3.0 ppm) was evidenced by various 1H MAS NMR experiments on samples dehydrated at different temperatures, allowing the distinction from ammonium ions (δ1H=6.7 ppm) formed at BAS. The signal of ammonia adsorbed on LAS increased with increasing Al content in the ASAs. These properties together with the absence of pore diffusional constraints render the flame-made ASAs excellent catalysts for cyclohexanol dehydration and the conversion of glyceraldehyde in ethanol to ethyl lactate, outperforming the performance of other ASAs or zeolites.

Original languageEnglish
Pages (from-to)287-293
Number of pages7
JournalChemCatChem
Volume12
Issue number1
Early online date16 Oct 2019
DOIs
Publication statusPublished - 8 Jan 2020

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Keywords

  • silica-alumina
  • acidity
  • solid-state NMR
  • cyclohexane dehydration
  • glyceraldehyde conversion

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