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Conventional solid-state 27 Al and 31 P magic-angle spinning nuclear magnetic resonance (MAS NMR) and an advanced two-dimensional 27 Al multiple-quantum MAS NMR (2D 27 Al MQMAS NMR) have been applied to characterize MoO 3 -P 2 O 5 -Al 2 O 3 hydrotreating catalysts. These catalysts were prepared by a sol-gel method with high Mo loadings (with Mo expected amounts of ∼20 and ∼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 (H 3 PO 4 or P 2 O 5 ) and on the amount of P and Mo. From the 27 Al MAS NMR measurements on dried P-containing samples, formation of octahedral aluminum (Al octa ) and of AlPO 4 (Al tetra -O-P surface species) in small amounts was observed. After calcination at 500 °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 (MoO 4 ) 3 was also detected in the calcined sample. 31 P MAS NMR measurements revealed the formation of monomeric or polymeric P oxocompounds after drying and polymeric P oxocompounds or AlPO 4 after calcination, respectively. The use of P 2 O 5 as a P precursor gave rise to the formation of polymerized P oxocompounds rather than of AlPO 4 after calcination in the absence of Mo. 2D 27 Al MQMAS NMR gave an improved resolution compared with conventional 27 Al MAS NMR. In dried samples, pure alumina and MoO 3 -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.

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