A 27Al MQMAS and Off-Resonance Nutation NMR Investigation of Mo−P/γ-Al2O3 Hydrotreating Catalyst Precursors

Abstract

γ-Al2O3 and γ-Al2O3 impregnated with phosphorus and/or molybdenum, amorphous AlPO4, and Al2(MoO4)3 have been studied by the recently introduced multiple-quantum magic angle spinning (MQMAS) NMR and off-resonance nutation NMR. Average quadrupolar coupling parameters of the resonances in the compounds were determined with 27Al off-resonance nutation spectroscopy. The MQMAS NMR experiment was used to increase the resolution of the spectra in order to gain insight in the distribution of the quadrupole parameters and to resolve overlapping lines from surface species on γ-Al2O3 supports. This combined use of advanced NMR techniques provided information about the bulk and surface structure of γ-Al2O3- and γ-Al2O3-supported catalyst precursors. When phosphorus and molybdenum loadings below “monolayer” coverage are employed, the single pulse spectra did not reveal the formation of new aluminum-containing compounds. At higher phosphorus loading the formation of a new phase was observed that from the MQMAS experiment could be clearly assigned to amorphous AlPO4. In a calcined sample containing both molybdenum and phosphorus, 27Al NMR showed that Al2(MoO4)3 and some AlPO4 had been formed, which could not be detected by XRD and hardly by single-pulse-excitation (SPE) MAS NMR. The fact that the observed distribution of aluminum atoms over octahedral and tetrahedral positions did not change with the loading of the support (except when AlPO4 was formed) led to the conclusion that aluminum atoms with both types of coordination are found not only in the bulk of γ-Al2O3 but also on its surface in about the same ratio.