Composition of impregnation solutions and wet impregnated Mo-P/gamma-Al2O3 catalysts as investigated by P-31 and Mo-95 NMR

Impregnation solutions and wet impregnated Mo-P/gamma-Al2O3 catalysts containing the impregnation liquid in the pores of the support were investigated by NMR spectroscopy. In phosphorus and molybdenum-containing solutions used for impregnation of gamma-Al2O3 mainly HxP2Mo5O23(6-x-) and HyPO4(3-y-) ions were found. Great differences existed between the states of phosphorus during the last impregnation step of Mo-P/gamma-Al2O3 catalysts which were coimpregnated or sequentially impregnated with or without intermediate calcination. In coimpregnated catalysts chemisorbed mono- and polyphosphates, physisorbed phosphates, as well as HxP2Mo5O23(6-x-) and HyPO4(3-y-) ions remaining in solution were observed. In sequentially impregnated catalysts which were calcined after the impregnation of P only chemisorbed phosphates and some amorphous AlPO4 could be detected after the second impregnation with a Mo solution, Sequential impregnation without calcination after phosphorus impregnation is an intermediate case, especially since NH4H2PO4 was used as phosphorus source which interacts less strongly with the alumina surface than H3PO4. During the second impregnation of these catalysts with a Mo solution, a loosening of the interaction of some phosphate ions with the surface and even desorption into the pore-filling liquid were observed. (C) 1996 Academic Press, Inc.