Insights into Functionality-Specific Adsorption Dynamics and Stable Reaction Intermediates Using Fast Field Cycling NMR

Abstract

Fast field cycling, FFC, NMR relaxometry experiments are reported in the frequency range of 10 kHz to 40 MHz to characterize the molecular dynamics of a series of protic (methanol and water) and aprotic (dimethyl sulfoxide, acetone, cyclohexane, and n-heptane) adsorbates on a γ-alumina surface of catalytic interest. By analyzing the data in the T1 domain, two distinct peaks were observed for both methanol and acetone. In the case of methanol, the two peaks have been shown to originate from the two chemical environments of methanol, which at low field strengths differentiate the O1H and alkyl 1H interactions with the surface. In contrast, the second environment of adsorbed acetone is assigned to a stable reaction intermediate formed during an aldol reaction, which strongly influences other molecular adsorption processes at the surface. Inversion of the FFC-NMR data into the T1-domain enables the combination of the ability of low field NMR to characterize relaxation modes directly associated with adsorption with T1 measurements specific to functional groups and reaction intermediates, thereby avoiding misinterpretation of molecular adsorption characteristics and giving greater insight into adsorption and catalytic behavior than is possible from the overall relaxation dispersion profiles alone.