Characterization of Acidity of Zeolites by NMR Probe Molecules：A Theoretical Investigation

Abstract

Abstract:

In this contribution, trimethylphosphine oxide (TMPO), pyridine and trimethylphosphine (TMP) were selected as NMR probe molecules to study the acid strength of several industrially used zeolites (H-Y, H-ZSM5 and H-MOR) theoretically. The relationship between chemical shifts of these probe molecules and acid strength of zeolites was revealed. In the meanwhile, the applicability of probe molecules on the characterization of Bronsted acidity was discussed. Since the ¹H, ¹⁵N and ³¹P chemical shift ranges of adsorbed D5-pyridine and TMP on the Bronsted acid sites in zeolites were relatively very smaller (i.e., δ 18.2 ~ 18.7, δ -191.0~-186.9, δ -8.7~-3.5 for ¹H, ¹⁵N and ³¹P, respectively), they were only used to detect the existence of acidic protons in the zeolites. However, the ³¹P chemical shift range of adsorbed TMPO ranges from δ 62.8 to δ 90.5, and it may be expected that the adsorbed TMPO were very sensitive to measure solid acid strengths of the zeolites catalysts, compared with the ³¹P NMR of adsorbed TMPO.

Key words: NMR, chemical shifts, probe molecule, quantum chemical calculation, zeolite, adsorption energy

CLC Number:

O641. 12

WU Zhong-qin1,2, ZHENG An-min1, YANG Jun1, DENG Feng1*. Characterization of Acidity of Zeolites by NMR Probe Molecules：A Theoretical Investigation[J]. Chinese Journal of Magnetic Resonance, 2007, 24(4): 501-509.

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