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

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 o r 30 wt% bismuth molybdenum oxide (Mo/Bi=1:1), and had BET surface areas of 32-67 m(2) g(-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 prepar ation conditions. The use of BiCl3 and MoOCl4, instead of (NH3)(6)Mo7O240. 4H(2)O and Bi(NO3)(3). 5H(2)O, as bismuth and molybdenum precursors had a p ositive influence on the catalytic performance of the sol-gel materials. Th e as-prepared xerogel (10BiMoTiO-XClP) produced furan with 37-48% selectivi ty at 10-25% butadiene conversion. Pulse thermal analytical studies of the 10BiMoTiO-XClP xerogel indicated the rapid oxidation of butadiene by the la ttice oxygen (Mars-Van Krevelen mechanism) and the complete reoxidation of the solid by molecular oxygen above ca. 700 K. Below this temperature, deco mposition of butadiene was the major process. Oxidation of butadiene by lat tice oxygen was strongly inhibited by the presence of carbonaceous deposits even above 800 K. XPS analysis of a used catalyst showed the presence of B i-0 and Bi3+, but the formation of Mo5+ was negligible. Significant restruc turing of the 10BiMoTiO-XClP xerogel during butadiene oxidation was demonst rated by XRD and HRTEM measurements. During this restructuring 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 conventio nal titania-supported bismuth molybdate reference material. It seems that a pplication of aerogels and xerogels in redox reactions is limited to modera te temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.