Solid-state NMR studies of acidic catalysts.

摘要

The research presented in this dissertation involves the use of probe molecules and solid-state NMR techniques to investigate the local structure of acid catalysts.; The catalytic activity of bifunctional catalysts can depend strongly on the proximity between the catalytic centers. This distance information is generally difficult to obtain from any existing analytical method. We used 31P MAS NMR spectroscopy, combined with Ph2P(CH2) nPPh2 probe molecules, to investigate both acidities and distances between Bronsted acid sites in zeolite HY. The latter was measured by varying the chain length of the phosphine molecule. 31 P 2-D double-quantum NMR spectroscopy was used to confirm the spectral assignments. 31P one-pulse MAS NMR spectroscopy on samples with different loading levels was used to quantify the number of nearby sites. Extended studies on zeolite HY with different Si/Al ratios show Bronsted acid sites are not clustered. The method may help to understand the effect of spatial proximity between catalytic sites on reactivity and in the rational design of new industrial catalysts.; Rich information can be obtained from 17O NMR spectroscopy of zeolites, thus it has attracted much attention. We reported the first observation of the 17O NMR resonance due to oxygen atoms at Bronsted acid sites in zeolite HY, by using high magnetic field strengths. 17O/1H double resonance NMR experiments were used to prove unambiguously that the 17O signal arises from O nearby H atoms. The NMR parameters extracted were found to be sensitive upon gas adsorption. A 2-D heteronuclear correlation NMR technique was used to separate signals from different oxygen sites. The O-H distance was measured by using rotational echo double resonance NMR spectroscopy. This approach was extended to zeolite HZSM-5. 17O multiple-quantum (MQ) MAS NMR was applied to monitor both the normal framework and Bronsted acid oxygen sites in zeolite HY. The results demonstrate that 17O/ 1H double resonance NMR provides a powerful and sensitive method to investigate the local structure of different Bronsted acid sites and to probe their changes upon adsorption. The work suggests that catalytic process involving Bronsted acidity can be tracked in the same manner.