Sol-gel bismuth-molybdenum-titanium mixed oxides II. Oxidation of butadiene to furan

摘要

Sol-gel bismuth-molybdenum-titanium mixed oxides (xerogels and aerogel) and conventional titania-supported bismuth molybdenum oxides were compared in the selective oxidation of butadiene to furan. The catalysts contained 10 or 30 wt% bismuth molybdenum oxide (Mo/Bi=1:1), and had BET surface areas of 32-67 m~2g~(-1). In contrast to the constant bulk atomic ratio Mo/Bi=1:1, the Mo/Bi surface ratio varied over a broad range, depending on the preparation conditions. The use of BiCl_3 and MoOCl_4 instead of (NH_3)_6Mo_7O_(24).4H_2O and Bi(NO_3)_3.5H_2O, as bismuth and molybdenum precursors had a positive influence on the catalytic performance of the sol-gel materials. The as-prepared xerogel(10BiMoTiO-XCIP) produced furan with 37-48% selectivity at 10-25 butadiene conversion. Pulse thermal analytical studies of the 10BiMoTiO-XCIP xerogel indicated the rapid oxidation of butadiene by the lattice oxygen (Mars-Van Krevelen mechanism) and the complete reoxidation of the solid by molecular oxygen above ca. 700 K. Below this temperature, decomposition of butadiene was the major process. Oxidation of butadiene by lattice oxygen was strongly inhitited by the presence of carbonaceous deposits even above 800 K. XPS analysis of a used catalyst showed the presence of Bi~0 and Bi~(3+), but the formation of Mo~(5+) was negligible. Significant restructuring of the 10BiMoTiO-XCIP xerogel during butadiene oxidation was demonstrated by XRD and HRTEM measurements. During this resstructuring the unusual redox properties of the sol-gel Bi-Mo-Ti mixed oxides were partly lost and their performance in butadiene oxidation was similar to that of a conventional titania-supported bismuth molybdate reference material. It seems that application of aerogels and xerogels in redox reactions is limited to moderate temperatures.