OXIDATIVE DEHYDROGENATION OF BUTENES OVER TIN-PHOSPHORUS-OXYGEN AND LITHIUM-TIN-PHOSPHORUS - OXYGEN CATALYSTS

[摘要] The mechanism of catalytic oxidative dehydrogenation (OXD) of butenes to 1,3-butadiene over Sn-P-O and Li-Sn-P-O catalysts was investigated in batch recirculation and microcatalytic pulse reactors. Mechanistic features of the reaction were examined using deuterium labeled butene (isotopic tracer technique) and $sp{18}$O-labeled carbon dioxide (oxygen isotope exchange) experiments. Solid state changes in the catalyst were examined through BET surface area, electron microscopy, x-ray powder diffraction, and x-ray photoelectron spectroscopy. Reaction products include 1,3-butadiene, carbon dioxide, water, and butene isomers.Both Sn-P-O and Li-Sn-P-O catalysts have activity of about 11% conversion and about 98% initial selectivity for butadiene after 15 minutes reaction time at 300$spcirc$C. Catalyst deactivation is caused by the formation of coke, which decreases the catalyst surface area. The Sn-P-O catalyst forms coke more readily than the Li-Sn-P-O catalyst. The activity of the aged catalysts can be partially recovered with treatment in 150 torr oxygen at 500$spcirc$C. The rate of formation of butadiene is zero order in both oxygen and butene. The rate of formation of carbon dioxide is zero order in butene and about 0.5 order in oxygen. The OXD reaction is inhibited by product butadiene, where low conversion data can be modeled by a modified Langmuir-Hinshelwood type rate expression. But the rate expression does not fit CO$sb 2$ well. The activation energy for butadiene formation is about 19 kcal/mole over the Li-Sn-P-O catalyst. Microcatalytic pulse experiments carried out in the absence of gas phase oxygen indicated that the reactions probably proceed by consuming catalyst surface oxygen. Two consecutive sets of $sp{18}$O labeled CO$sb 2$ pulse experiments demonstrate that only surface oxygen is relatively mobile, and that bulk diffusion of oxygen to the surface may not play a very important role in the OXD mechanisms. Perdeuterated butene is less reactive than non-deuterated butene. Comparison of the rates of formation and analysis of isotopic compositions of the products revealed a significant kinetic isotopic effect for the OXD reaction. Therefore, carbon-hydrogen bond cleavage is considered rate limiting. Isomerization may occur via a concurrent non-oxidative reaction over weak acid sites. Experimental data are consistent with an oxidation-reduction cycle involving a Sn$sp{+4}$ cation active center.

[发布机构] Rice University