Studies On Catalytic Reaction And In-situ DRIFTS Of Selective Oxidation Of Isobutene

Methyl methacrylate (MMA) is an important precursor in the preparation of acrylic resins. Acetone, highly toxic hydrocyanic acid and sulphuric acid are used as raw materials in traditional method to produce MMA. The seriously poisonous substances and the strong acid not only results in the obvious environment pollution, but also results in the numerous ammonium bisulfate byproduct. Therefore, the selective oxidation of isobutene to methacrolein (MAL) which is a critical step for clean production of MMA from isobutene has been extensively studied for a long time worldwide. So the studies on catalysts and catalytic reaction process for selective oxidation of isobutene to methacrolein are important for both fundamental understanding and industrial application. In the paper, we synthesized series catalysts of V_x-UVM-7 and the V-UVM-7 modified with molybdenum (Mo) element and tested for the oxidation of isobutene. Then, we used in-situ DRIFTS technique to probe the mechanism of selective oxidation of isobutene. The technique is always applied to study surface reaction mechanism recently. Specific experiments as follows:Based on the main existing mechanism of selective oxidation of isobutene, we substituted the silicon (Si) with vanadium (V) in the bimodal mesoporous molecular sieve UVM-7 which possesse high specific surface and specific pore volume and explored the catalytical performance of the mesoporous system V_x-UVM-7 and the V-UVM-7 modified with molybdenum (Mo) element in the oxidation of isobutene to MAL. We have prepared V_x-UVM-7 series catalysts and Mo-V-UVM-7 catalysts in which the Mo element also substituted Si element or supported on the mesoporous materials with atrane route method and impregnation method. The oxidizing reactions were carried out in the fixed bed reactor. Simultaneously, we have studied some factors which can influence the conversion of isobutene and the selectivity of the methacrolein. These factors include the temperature of the reaction, the concentration of the isobutene, the flowing rate of raw material gas, the different content of V element and the different methods of incorporating Mo element into the mesoporous materials. We have synthesized many kinds of new catalysts and selected some of them with higher activity and selectivity. Furthermore, we have characterized these catalysts by means of XRD, FT-IR, TEM, ICP, N₂ adsorption/desorption, UV-vis and TG technologies. Results indicated that under the optimal reaction conditions, conversion of isobutene and selectivity of MAL reaches 82.6% and 35.6%, respectively.Some parameters such as the resolution, scanning time, catalyst particle size and gas velocity are fixed at best values of self-building in-situ DRIFTS experimental installation, and then it is systemically investigated for the isobutene selective oxidation over Mo-V-UVM-7 catalyst, including surface reaction and gas reaction behavior. Then the catalytic reaction mechanism, which can provide theoretical foundation for improving the catalyst, were discussed with the results of in-situ DRIFTS experiment. The selective oxidation of isobutene over catalyst surface effectively controls deep oxidation products of isobutene such as carbon dioxide and carbon monoxide. The centers of active sites of selective oxidation are surface lattice oxygens. The nature of adsorbed intermediates detected by in-situ DRIFTS are: ( i) a p-allylic intermediate; (ii) an enolic type compound formed in the presence of Bronsted acid sites; (iii) a p-bonedisobutene species interacting with Lewis acid site.