Characterization of Mo-P-alumina sol-gel catalysts by solid-state 31P and 27Al magic-angle spinning nuclear magnetic resonance and two-dimensional 27Al multiple-quantum magic-angle spinning nuclear magnetic resonance

Conventional solid-state Al-27 and P-31 magic-angle spinning nuclear magnetic resonance (MAS NMR) and an advanced two-dimensional Al-27 multiple-quantum MAS NMR (2D Al-27 MQMAS NMR) have been applied to characterize MoO3-P2O5-Al2O3 hydrotreating catalysts. These catalysts were prepared by a sol-gel method with high Mo loadings (with Mo expected amounts of similar to 20 and similar to 30 wt %) and a wide range of P content (from 0 to 13 wt % of P), The chemical environments of Al and P strongly depend on the nature of the P precursor used in the sample preparation (H3PO4 Or P2O5) and on the amount of P and Mo. From the Al-27 MAS NMR measurements on dried P-containing samples, formation of octahedral aluminum (Al-octa) and of AlPO4 (Al-tetra-O-P surface species) in small amounts was observed, After calcination at 500 degrees C, the formation of tetrahedral aluminum (Al-tetra) and pentacoordinated aluminum (Al-penta) was also observed. The presence of molybdenum favored the interaction of phosphorus with the alumina framework, leading to significant amounts of Al-tetra-O-P. At higher P and Mo loadings, Al-2(MoO4)(3) was also detected in the calcined sample, P-31 MAS NMR measurements revealed the formation of monomeric or polymeric P oxocompounds after drying and polymeric P oxocompounds or AlPO4 after calcination? respectively. The use of P2O5 as a P precursor gave rise to the formation of polymerized P oxocompounds rather than of AlPO4 after calcination in the absence of Mo. 2D Al-27 MQMAS NMR gave an improved resolution compared with conventional Al-27 MAS NMR. In dried samples, purl alumina and MoO3-alumina showed a single distorted Al-octa site while P-containing catalysts showed additional octahedral and tetrahedral aluminum sites interacting with phosphorus (Al-octa-O-P and Al-tetra-O-P, respectively). After calcination, mainly Al-octa and Al-tetra sites with a small amount of Al-penta sites were observed in pure alumina. The presence of Mo introduced a large distortion in the alumina framework, since the amount of Al-penta significantly increased in the presence of Mo. Al-tetra-O-P sites were also detected in P-containing alumina catalysts, but the Al-octa-O-P sites disappeared after calcination.