Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations

Two-dimensional nuclear magnetic resonance (NMR) relaxation time correlation measurements have been used to observe the behavior of liquids inside porous catalyst pellets; in particular, liquids of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO2). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T-1/T-2, for the liquids confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liquid. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement experiments, in which one liquid replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liquids in an alumina-supported palladium catalyst (Pd/Al2O3) was also studied. The variation in the strengths of surface interactions was more pronounced in the Pd/Al2O3 catalyst than in the Ru/SiO2 catalyst. This work demonstrates the applicability of NMR relaxation time correlation experiments to real catalytic systems containing metallic components. From these measurements, information on the access of reactants to surface adsorption sites can be inferred.