Study Of Composite Catalyst System And Technology For Oxidative Dehydrogenation Of Butenes

This dissertation is dedicated in the exploration for a composite catalyst system for the better yield of 1,3-butadiene （BD） via oxidative dehydrogenation （ODH） of butenes, based on traditional metal oxides and novel carbon materials. On one hand, ODH reactions of butene isomers and mixed-butene were conducted over ZnFe₂O₄ and Co₉Fe₃Bi₁Mo₁₂O₅₁ and the composite catalyst system formed by them, different catalytic activities were compared and the mechanism for the synergistic effect between them was illustrated. Additionally, the mechanism for the temperature-induced ZnFe₂O₄ catalyst deactivation found during the optimization of reaction temperature was also explained in detail. On the other hand, ODH of 1-butene was taken as the model reaction of butene ODH reactions in the catalyst characterizations. The effect of phenols in the in-situ modified mesoporous carbon materials was studied for the possible application of this material as the intermediate of the composite catalyst for the ODH reaction. Finally, a preliminary study over a series of composite catalyst comprising of mesoporous carbon materials and metal oxide was conducted for their catalytic performance in 1-butene ODH reaction.1. Dual bed catalyst system:the synergistic effect and its optimization1) ZnFe₂O₄ and Co₉Fe₃Bi₁Mo₁₂O₅₁ were prepared via sol-gel method and co-precipitation respectively. The ODH reactions of butene isomers and mixed-butene were conducted on both individual catalyst and the dual bed catalyst system formed by packing both sequentially. The reaction results indicated a better catalytic performance of ZnFe₂O₄ with 2-butenes while that with 1-butene was observed on Co₉Fe₃Bi₁Mo₁₂O₅₁. Meanwhile, the dual bed catalyst system revealed an overall better activity on both individual isomers and mixed butenes, comparing with individual catalysts.2) Based on the conclusions drawn from the comparison of the above results, a mechanism of the dual bed catalyst system for its synergistic effect was proposed and proved then with a series of experiments. In the synergistic effect, the advantages of both catalyst toward different butene isomers were expanded resulted from the redistribution of butene isomer concentration in mixed butene. Furthermore, the existence of all butene isomers would lead to a lower proportion of isomerization reaction, thus increased the overall mixed butene conversion and BD yield.3) To obtain the best catalytic performance, the packing volume optimization of the catalysts was conducted. The best packing ratio of ZnFe₂O₄ and Co₉Fe₃Bi₁Mo₁₂O₅₁was determined within 4:6 and 6:4, which also proved the existing synergistic mechanism.2. Mechanism of temperature induced partial deactivation of ZnFe₂O₄1) An irregular decrease of conversion butene with the increasing temperature was observed in the optimization of ODH reaction conditions on ZnFe₂O₄ prepared in Chapter.3. A more obvious decrease as temperature was elevated above 400℃ indicated a partial deactivation of the catalyst.2) 1-butene pulse adsorption characterization experiments were designed and conducted. The high reaction temperature could increase the proportion of isomerization, and more importantly forcing 1-butene in transforming into residues on the active sites of the catalyst. The mass transfer and the oxygen adsorption would thus be hindered, which ultimately lowered the catalytic activity.3) 1-butene saturated adsorption and temperature programmed oxidation experiments were designed and conducted over ZnFe₂O₄ and its model composites. The results indicated the possible new crystal structure formed with Fe²⁺and Zn²⁺on the surface of the catalyst, replacing the original Zn²⁺and Fe³⁺in the spinel structure. This changed structure might result in the decreased oxygen mobility and promising active sites, which finally lead to this partial deactivation.4) XPS analysis was conduct on the surface of ZnFe₂O₄ saturated adsorbed with 1-butene under different temperatures. The results of elemental compositions and their valences confirmed the changed surface crystal structure and possible structure of the resulted residues. Further XPS analysis on ZnFe₂O₄ saturated adsorbed with 1-butene under 400℃ with different profiling depth revealed the increasing Fe²⁺and the decreasing Fe³⁺from near surface to the bulk. The Zn/Fe ratio also changed from 1:1 to 1:2 as profiling process continued. Thus the partial deactivation mechanism of ZnFe₂O₄ based on the temperature induced residue formation and crystal structure change was proposed and proved with the above results.3. In-situ modification of mesoporous carbon catalyst:controlling of phenol species1) Applying F127 as the template and organic-organic self-assembly as the synthetic route, the phenol species and amount added to phenol resins were carefully controlled, thus a series of in-situ modified carbon catalysts were prepared and named as MC-X. BET and TEM results revealed their mesoporous structures with large specific surface area （> 300 m2/g）, among which ordered mesoporous structures observed on SMC series could provide excellent channels for mass transfer. The results of Raman spectra and TGA analysis both indicated the SMC series with relatively better-constructed carbon frame and thermochemical stability （decomposition temperature> 673 K）.2) TPD results under He atmosphere indicated the successful tuning of catalytic active groups on MC-X by controlling the phenol species and amount in the phenol resin precursors. Meanwhile, MC-3 revealed an overall larger amount of active groups. The ODH of 1-butene over MC-X revealed the positive correlation between A（C=O）/A（C-O） and the BD selectivity, as well as the positive correlation between the total amount of C=O and BD yield. The relatively better BD yield （9.4%） also proved the good catalytic activity of MC-3.4) Preliminary study of composite catalyst comprising mesoporous carbon and bismuth molybdate oxide1) A series of composite catalysts （MRF） comprising different proportions of mesoporous carbon and Co₉Fe₃Bi₁Mo₁2O₅1 was prepare with controlled ratio. The ICP results indicated a well-preserved composition of Co₉Fe₃Bi₁Mo₁₂O₅₁ corresponding to the experimental designs. BET and TEM results revealed obvious mesoporous structures of MRFs, while ordered mesoporous structure was observed on MRF-3. Results of Raman spectra and TGA analysis indicated a promising thermochemical stability （decomposition temperature> 673 K）, among which MRF-3 exhibited a better-constructed carbon frame providing a good reaction condition.2) TPD results under He atmosphere indicated the successful tuning of catalytic active groups on MRF by controlling the composition ratio of the components. ODH results of 1-butene revealed better 1-butene conversion （94.9%） and BD yield on MRF-3 （80.5%）. Eliminating the impact of space velocity, a positive correlation between the active groups on the catalyst with its catalytic activity was established, which proved a successful controlled synthesis. This composite catalyst MRF-3 could provide not only the channels for the mass transfer but also more active sites, providing a promising candidate for the ODH of butenes.