Characterization of Mo-P-Alumina Sol-Gel Catalysts by Solid-State ~(31)P and ~(27)Al Magic-Angle Spinning Nuclear Magnetic Resonance and Two-Dimensional ~(27)Al Multiple-Quantum Magic-Angle Spinning Nuclear Magnetic Resonance

摘要

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_2O_5-Al_2O_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 10 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_3PO_4 or P_2O_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 ℃, 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 amount 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_2O_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.