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

Abstract

Conventional solid-state 27Al and 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) and an advanced two-dimensional 27Al multiple-quantum MAS NMR (2D 27Al 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 ∼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 (H3PO4 or P2O5) and on the amount of P and Mo. From the 27Al MAS NMR measurements on dried P-containing samples, formation of octahedral aluminum (Alocta) and of AlPO4 (Altetra−O−P surface species) in small amounts was observed. After calcination at 500 °C, the formation of tetrahedral aluminum (Altetra) and pentacoordinated aluminum (Alpenta) was also observed. The presence of molybdenum favored the interaction of phosphorus with the alumina framework, leading to significant amounts of Altetra−O−P. At higher P and Mo loadings, Al2(MoO4)3 was also detected in the calcined sample. 31P 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 27Al MQMAS NMR gave an improved resolution compared with conventional 27Al MAS NMR. In dried samples, pure alumina and MoO3−alumina showed a single distorted Alocta site while P-containing catalysts showed additional octahedral and tetrahedral aluminum sites interacting with phosphorus (Alocta−O−P and Altetra−O−P, respectively). After calcination, mainly Alocta and Altetra sites with a small amount of Alpenta sites were observed in pure alumina. The presence of Mo introduced a large distortion in the alumina framework, since the amount of Alpenta significantly increased in the presence of Mo. Altetra−O−P sites were also detected in P-containing alumina catalysts, but the Alocta−O−P sites disappeared after calcination.