Determining the Surface Structure of Silicated AluminaCatalysts via Isotopic Enrichment and Dynamic Nuclear PolarizationSurface-Enhanced NMR Spectroscopy

摘要

Isotopic enrichment of ²⁹Si and DNP-enhanced NMR spectroscopy are combined to determine the detailed surface structure of a silicated alumina catalyst. The significant sensitivity enhancement provided by DNP is vital to the acquisition of multinuclear and multidimensional experiments that provide information on the atomic-level structure of the species present at the surface. Isotopic enrichment not only facilitates spectral acquisition, particularly given the low (1.5 wt %) Si loading, but also enables spectra with higher resolution than those acquired using DNP to be obtained. The unexpected similarity of conventional, CP, and DNP NMR spectra is attributed to the presence of adventitious surface water that forms a sufficiently dense ¹H network at the silica surface so as to mediate efficient polarization transfer to all Si species regardless of their chemical nature. Spectra reveal the presence of Si–O–Si linkages at the surface (identified as Q⁴(3Al)–Q⁴(3Al)) and confirm that the anchoring of the surface overlayer with the alumina occurs through Al^IV and Al^V species only. This suggests the presence of Q³/Q⁴ Siat the surface affects the neighboring Al species, modifying the surfacestructure and making it less likely Al^VI environments arein close spatial proximity. In contrast, Q¹/Q² species, bonded to the surface by fewer covalent bonds, have lessof an effect on the surface, and more Al^VI species areconsequently found nearby. The combination of isotropic enrichmentand DNP provides a definitive and fully quantitative description ofthe Si-modified alumina surface, and we demonstrate that almost one-thirdof the silicon at the surface is connected to another Si species,even at the low level of coverage used, lowering the propensity forthe formation of Brønsted acid sites. This suggests that a variationin the synthetic procedure might be required to obtain a more evencoverage for optimum performance. The work here will allow for morerigorous future investigations of structure–function relationshipsin these complex materials.