Structural Environments for Boron and Aluminum in Alumina−Boria Catalysts and Their Precursors from11B and27Al Single- and Double-Resonance MAS NMR Experiments

Abstract

Alumina−boria catalysts, obtained by calcination (550−950 °C) of pseudo-boehmite precursors impregnated with 0.25−3.4 wt % boron, have been characterized by 11B and 27Al MAS NMR of the central and satellite transitions, MQMAS, 11B{27Al} REDOR, and TRAPDOR NMR, and by homonuclear dipolar recoupling MAS NMR experiments. The 27Al MAS NMR spectrum of the satellite transitions for boehmite (γ-AlOOH) has allowed the first precise determination of the 27Al quadrupole coupling parameters for this important aluminate. The 27Al MAS NMR spectra of the alumina−boria catalysts show that the fraction of tetrahedral AlO4 sites decreases with increasing boron loading. This finding and the results from 11B MAS NMR suggest that a small fraction of tetrahedrally coordinated boron is incorporated within the first molecular layers of the γ-Al2O3 support. The observation of slightly stronger 11B−27Al dipolar couplings for BO4 relative to the trigonal BO3 sites from 11B{27Al} REDOR NMR supports this finding. Information on the dispersion of BO4 and BO3 environments have been obtained from 11B MAS NMR spectra of the satellite transitions and from 11B MQMAS NMR experiments. The presence of BO4 and BO3 species on the surface of the γ-Al2O3 support is demonstrated by 11B{27Al} REDOR and TRAPDOR NMR, while 11B homonuclear correlation experiments indicate BO4−BO3 connectivities and thereby a network on the surface for these units. For the alumina−boria catalyst corresponding to the highest boron content and calcination temperature, an additional BO3 resonance is observed in the 11B MQMAS spectrum, which is assigned to a separate B2O3 phase exhibiting a high degree of local order.