Nuclear spin relaxation as a probe of zeolite acidity: a combined NMR and TPD investigation of pyridine in HZSM-5.

Neil Robinson,Carmine D'Agostino,Pierre Bräuer,Andrew P E York

doi:10.1039/d1cp01515j

Abstract

The relative surface affinities of pyridine within microporous HZSM-5 zeolites are explored using two-dimensional 1H nuclear magnetic resonance (NMR) relaxation time measurements. The dimensionless ratio of longitudinal-to-transverse nuclear spin relaxation times T1/T2 is shown to exhibit strong sensitivity to the silica/alumina ratio (SAR) of these zeolites, which is indicative of material acidity. This trend is interpreted in terms of increased pyridine surface affinity with decreasing SAR. Temperature programmed desorption (TPD) analysis corroborates this observation, revealing a distinct increase in the heat of desorption associated with adsorbed pyridine as a function of decreasing SAR. A direct correlation between NMR and TPD data suggests NMR relaxation time analysis can be a valuable tool for the non-invasive characterisation of adsorption phenomena in microporous solids.

Highlights

Microporous solids such as zeolites and metal organic frameworks have potential applications across a variety of processes including chemical conversion, storage, sensing and separations.[1,2] In the field of heterogeneous catalysis zeolites are regularly applied to facilitate a range of reactions such as cracking,[3,4] alkylation[5] and dehydration.[6,7,8] A key feature regarding the activity of such materials is that of surface acidity, characterised by the presence of Brønsted and/or Lewis acid sites within the micropore network, and across the external material surface
Correlation peaks away from this diagonal are characterised by T1/T2 4 1; as suggested by eqn (8), the position of these peaks is expected to be dictated by the relative surface affinities of pyridine within these structures
The T1/T2 values obtained from the logarithmic mean of these correlation peaks are summarised in Table 1 and discussed further below

Summary

Introduction

Microporous solids (exhibiting pore diameters o2 nm) such as zeolites and metal organic frameworks have potential applications across a variety of processes including chemical conversion, storage, sensing and separations.[1,2] In the field of heterogeneous catalysis zeolites are regularly applied to facilitate a range of reactions such as cracking,[3,4] alkylation[5] and dehydration.[6,7,8] A key feature regarding the activity of such materials is that of surface acidity, characterised by the presence of Brønsted (proton donating) and/or Lewis (electron accepting) acid sites within the micropore network, and across the external material surface. PAPER Carmine D'Agostino et al Nuclear spin relaxation as a probe of zeolite acidity: a combined NMR and TPD investigation of pyridine in HZSM-5

Results

Conclusion