Structural environments for boron and aluminum in alumina-boria catalysts and their precursors from 11B and 27Al single- and double-resonance MAS NMR experiments

Alumina-boria catalysts, obtained by calcination (550-950 degrees C of pseudo-boehmite precursors impregnated with 0.25-3.4 wt % boron, have been characterized by B-11 and Al-27 MAS NMR of the central and satellite transitions, MQMAS, B-11{Al-27} REDOR, and TRAPDOR NMR, and by homonuclear dipolar recoupling MAS NMR experiments. The Al-27 MAS NMR spectrum of the satellite transitions for boehmite (gamma-AlOOH) has allowed the first precise determination of the Al-27 quadrupole coupling parameters for this important aluminate. The Al-27 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 B-11 MAS NMR suggest that a small fraction of tetrahedrally coordinated boron is incorporated within the first molecular layers of the gamma-Al2O3 support. The observation of slightly stronger B-11-Al-27 dipolar couplings for BO4 relative to the trigonal BO3 sites from B-11 {Al-27} REDOR NMR supports this finding. Information on the dispersion of BO4 and BO3 environments have been obtained from B-11 MAS NMR spectra of the satellite transitions and from B-11 MQMAS NMR experiments. The presence of BO4 and BO3 species on the surface of the gamma-Al2O3 support is demonstrated by B-11{Al-27} REDOR and TRAPDOR NMR, while B-11 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 B-11 MQMAS spectrum, which is assigned to a separate B2O3 phase exhibiting a high degree of local order.