Direct Correlation between Adsorption Energetics and Nuclear Spin Relaxation in a Liquid‐saturated Catalyst Material

Abstract

The ratio of NMR relaxation time constants <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mn>1</mml:mn></mml:msub> <mml:mo>/</mml:mo> <mml:msub><mml:mi>T</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> provides a non-destructive indication of the relative surface affinities exhibited by adsorbates within liquid-saturated mesoporous catalysts. In the present work we provide supporting evidence for the existence of a quantitative relationship between such measurements and adsorption energetics. As a prototypical example with relevance to green chemical processes we examine and contrast the relaxation characteristics of primary alcohols and cyclohexane within an industrial silica catalyst support. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mi>T</mml:mi> <mml:mn>1</mml:mn></mml:msub> <mml:mo>/</mml:mo> <mml:msub><mml:mi>T</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> values obtained at intermediate magnetic field strength are in good agreement with DFT adsorption energy calculations performed on single molecules interacting with an idealised silica surface. Our results demonstrate the remarkable ability of this metric to quantify surface affinities within systems of relevance to liquid-phase heterogeneous catalysis, and highlight NMR relaxation as a powerful method for the determination of adsorption phenomena within mesoporous solids.

NMR Investigation into the Influence of Surface Interactions on Liquid Diffusion in a Mesoporous Catalyst Support

“…The observed increase in away from this value suggest that adsorption interactions at the solid-liquid interface have a non-negligible effect on the self-diffusion of our primary alcohols within this material. Furthermore, there is a clear increase in with increasing alcohol carbon chain length; notably, this trend is similar to that previously observed when considering the nuclear spin relaxation characteristics ( T 1 ∕T 2 ratio) of these same adsorbate/adsorbent systems at intermediate magnetic field [17]. This observation is of particular interest as we have shown that the T 1 ∕T 2 ratio obtained under such conditions may be directly related to adsorption energetics [16].…”

“…Solid lines in a represent a fit to Eqs. (16) and (17) while solid lines in b represent a fit to Eqs. (18) and (19).…”

“…NMR relaxation studies have been particularly successful in determining adsorption phenomena within liquid-saturated catalyst materials; such measurements probe the longitudinal ( T 1 ) and/or transverse ( T 2 ) relaxation time constants of the confined liquid. Notably, the ratio of these time constants T 1 ∕T 2 is now well-established as a measure of relative surface affinity exhibited by liquids and liquid mixtures at the catalyst pore surface [13][14][15], and we have recently extended such analysis to demonstrate that this ratio may be interpreted as a quantitative indicator of adsorption energetics [16,17].…”

In situ high-pressure 13C/1H NMR reaction studies of benzyl alcohol oxidation over a Pd/Al2O3 catalyst

“…Such results strongly suggest that both transport and adsorption play an important role in determining catalyst performances. It is interesting to note as a similar trend in T 1 /T 2 for primary alcohols in porous oxide catalysts has been reported by Robinson et al 16 in a study involving both NMR and DFT calculations.…”

Monitoring real‐time curing of epoxies in situ using single‐sided NMR

“…It is also important to consider that T 2 offers only an indirect measure of molecular motion within a sample. Using other NMR measurement methods appropriate to single‐sided devices (e.g., T 1 relaxation, dipolar coupling) may offer supplementary means to track curing. Nonetheless, the method presented here offers a useful and important tool in nondestructively studying the real‐time curing of polymers on inorganic substrates.…”