MoO₃/γ-Al₂O₃ Catalysts For The Dehydrogenation Of N-Butane To Butenes

At this moment five dehydrogenation technologies are available for licensing, all of them relying on either platinum-stannum or chromium oxide catalysts. The platinum-stannum catalysts are expensive, very complicated and require the severe conditions of commercial operations. Chromium oxide catalysts deactivate rapidly and require frequent regeneration. Moreover, after regeneration a part of the chromium is in the oxidation state Cr6+, which is carcinogenic. It has been an important task to look for a feasible catalyst to replace Pt and Cr2O3 for the dehydrogenation of light paraffins. MoO₃ catalyst is a type of important catalyst for the dehydrogenation of light paraffins and would be a feasible candidate to replace Pt and Cr2O3.MoO₃/γ-Al₂O₃ catalysts of different loadings were prepared by the impregnation method. The surface composition of catalysts was characterized by XRD, FT-IR and Raman techniques. Their catalytic activity measurements have been carried out in the dehydrogenation of n-butane to n-butene at 550℃. The results indicated that the surface molybdenum oxide species onγ-Al₂O₃ are primarily isolated, tetrahedrally coordinated at low loadings , tend toward polymerized, octahedrally coordinated at high loadings and tend toward crystalline MoO₃ and Al2(MoO4)3 at exceeding monolayer coverage. The highest MoO₃ proportion of monolayer coverage was about 20wt.%. The n-butene selectivity of the isolated MoO4 species was higher than that of the polymerized MoO6 and crystalline MoO₃ and Al2(MoO4)3 species. The highest n-butene selectivity attained by the 5wt.% catalyst was 62.54%. The catalysts of the 15wt.%-35wt.% loadings attained the preferable yield of n-butene that was 11.0-11.7%. The catalytic activity of the catalysts with multilayer coverage (35wt.% loadings) didn't reduce significatively owing to the polymerization of the molybdenum oxide species comparing with the catalysts with monolayer coverage (20wt.% loadings) and the n-butene yield was only lower 0.08% than that of the latter. the reaction conditions effect on the dehydrogenation of n-butane were investigated. The results showed that when the reaction conditions were reaction temperature during 530 to 570℃, the ratio of H2 to n-C4H10 during 0.5 to 2 and the WHSV during 3000 to 6000 ml/h/gcat, the catalyst could get better n-butane conversion, butene selectivity and butene yield.The distribution of 1-butene, cis-2-butene and trans-2-butene was affected by reaction temperature mainly, and hardly influenced by the other reaction conditions.The metals and nonmetals were droped to the catalyst of the 20wt.% loading for improving the catalytic performance. The metals effect on the surface configuration of the catalysts. The XRD and Raman showed that the quantity of crystalline Al2(MoO4)3, crystalline MoO₃ and MoO6 species when the metals droped to the catalysts, and the molybdates started to form on the surface. In the range of 0.25 to 1 of the molar ratio of M/Mo, the quantity of magnesium molybdate and zinc molybdate increased with the ratio, but that of iron molybdate increased first and decreased afterwards.Mg and Zn improved greatly the selectivity of the products, but decreased the activity obviously, so the yields of the objective products were not satisfactory. Fe decreased the selectivity of C1- C3 and isobutylene, and increased that of n-butene.Moreever, the activity of the Fe-droped catalysts did not decrease obviously. Cu did not improve the selectivity of the products obviously, but it improved the catalystic activity, so it attained to the better yield of n-butene. The yield of n-butene increased first and decreased afterwards with the molar ratio of metal to Mo. The Mg, Zn, Cu-droped catalysts attained to the best yield(12.96%, 12.87% and15.09%) of n-butene in the 0.25 ratio. The Fe-droped catalysts attained to the best yield(14.77%) of n-butene in the 0.75 ratio. The order of the improving is Cu, Fe, Mg and Zn according to the yield of n-butene.P improved the selectivity of the products in a certain extent, and did not decrease the conversion of n-butane, so the yield of n-butene increased in a certain extent. B improved greatly the selectivity of the products, but decreased the activity a little. The fitting B can increase the yield of n-butene. The yield of n-butene increased first and decreased afterwards with the molar ratio of nonmetal to Mo. The P, B-droped catalysts attained to the best yield(14.30%, 14.42%) of n-butene in the 0.25 and 0.5 ratio. The effect of P on improving of performance corresponded to that of B according to the yield of n-butene.