Characterization of Mo-P-Alumina Sol-Gel Catalysts by Solid-State ↑(31)p and↑(27)A1 Magic-Angle Spinning Nuclear Magnetic Resonance and Two-Dimensional↑(27)A1 Multiple-Quantum Magic-Angle Spinning Nuclear Magnetic Resonance

摘要

Conventional solid-state 27A1 and 31p magic-angle spinning nuclear magnetic resonance (MAS NMR) and an advanced two-dimensional 27A1 multiple-quantum MAS NMR (2D↑(27)A1 MQMAS NMR) have been applied to characterize MoO3-P↓(2)O↓(5)-A1↓(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 ℃, 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, A1↓(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 A1PO4 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 A1PO4 after calcination in the absence of Mo. 2D ↑(27)A1 MQMAS NMR gave an improved resolution compared with conventional ↑(27)A1 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.