CHARACTERIZATION OF MO-P-ALUMINA SOL-GEL CATALYSTS BY SOLID-STATE P-31 AND AL-27 MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE AND 2-DIMENSIONAL AL-27 MULTIPLE-QUANTUM MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE

Conventional solid-state Al-27 and P-31 magic-angle spinning nuclear m agnetic resonance (MAS NMR) and an advanced two-dimensional Al-27 mult iple-quantum MAS NMR (2D Al-27 MQMAS NMR) have been applied to charact erize MoO3-P2O5-Al2O3 hydrotreating catalysts. These catalysts were pr epared by a sol-gel method with high Mo loadings (with Mo expected amo unts of similar to 20 and similar to 30 wt %) and a wide range of P co ntent (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 pr eparation (H3PO4 Or P2O5) and on the amount of P and Mo. From the Al-2 7 MAS NMR measurements on dried P-containing samples, formation of oct ahedral aluminum (Al-octa) and of AlPO4 (Al-tetra-O-P surface species) in small amounts was observed, After calcination at 500 degrees C, th e formation of tetrahedral aluminum (Al-tetra) and pentacoordinated al uminum (Al-penta) was also observed. The presence of molybdenum favore d 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 meas urements revealed the formation of monomeric or polymeric P oxocompoun ds after drying and polymeric P oxocompounds or AlPO4 after calcinatio n? respectively. The use of P2O5 as a P precursor gave rise to the for mation of polymerized P oxocompounds rather than of AlPO4 after calcin ation in the absence of Mo. 2D Al-27 MQMAS NMR gave an improved resolu tion 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 al uminum 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. T he presence of Mo introduced a large distortion in the alumina framewo rk, since the amount of Al-penta significantly increased in the presen ce of Mo. Al-tetra-O-P sites were also detected in P-containing alumin a catalysts, but the Al-octa-O-P sites disappeared after calcination.