Interpretation of NMR Relaxation as a Tool for Characterising the Adsorption Strength of Liquids inside Porous Materials. Issue 40 (21st August 2014)

• Record Type: Journal Article
• Authors: D'Agostino, Carmine; Mitchell, Jonathan; Mantle, Michael D.; Gladden, Lynn F.
• Is Part Of: Chemistry. Volume 20:Issue 40(2014)
• Page Start: 13009
• Page End: 13015
• Publication Date: 2014-08-21
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201403139
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>Nuclear magnetic resonance (NMR) relaxation times are shown to provide a unique probe of adsorbate–adsorbent interactions in liquid‐saturated porous materials. A short theoretical analysis is presented, which shows that the ratio of the longitudinal to transverse relaxation times (<italic>T</italic><sub>1</sub>/<italic>T</italic><sub>2</sub>) is related to an adsorbate–adsorbent interaction energy, and we introduce a quantitative metric <italic>e</italic><sub>surf</sub> (based on the relaxation time ratio) characterising the strength of this surface interaction. We then consider the interaction of water with a range of oxide surfaces (TiO<sub>2</sub> anatase, TiO<sub>2</sub> rutile, γ‐Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub>, θ‐Al<sub>2</sub>O<sub>3</sub> and ZrO<sub>2</sub>) and show that <italic>e</italic><sub>surf</sub> correlates with the strongest adsorption sites present, as determined by temperature programmed desorption (TPD). Thus we demonstrate that NMR relaxation measurements have a direct physical interpretation in terms of the characterisation of activation energy of desorption from the surface. Further, for a series of chemically similar solid materials, in this case a range of oxide materials, for which at least two calibration values are obtainable by TPD, the <italic>e</italic><sub>surf</sub> parameter yields a direct estimate of the maximum activation energy of desorption from the surface. The results suggest that <italic>T</italic><sub>1</sub>/<italic>T</italic><sub>2</sub> measurements may become a useful addition to the methods available to characterise liquid‐phase adsorption in porous materials. The particular motivation for this work is to characterise adsorbate–surface interactions in liquid‐phase catalysis.</p> </abstract>