Study On Preparation Of Isobutane Dehydrogenation Catalyst By Wet Chemical Method

In recent years,with the development of integration of large-scale refining and chemical industry and MTO process,the production of C4 will be increasing.Reasonable development and utilization of C4 resources is of great significance for China’s petrochemical enterprises to enhance their competitiveness.Dehydrogenation of low-carbon alkanes is a research hotspot in the field of petrochemical application.Dehydrogenation of isobutane to isobutene is one of the main ways to utilize isobutane resources.The main low-carbon alkane dehydrogenation catalysts used in industry are Cr2O3/Al2O3,Pt/Al2O3 and Pt/ZnAl2O4.In this paper,three kinds of catalysts were prepared by using industrial alumina with wet chemical method,and the effects of preparation method and reaction conditions on isobutane dehydrogenation were investigated.The experimental results are as follows.Firstly,the chromium-based catalytic system supported on alumina is studied.The best catalytic performance is acquired when the content of Cr2O3 is 15 wt.%.After that,the introduction of 1.5 wt.%K2O into the catalyst can reduce the weak and medium acidic sites and improve the selectivity and stability of the catalyst.Afterwords,the optimum reaction conditions were selected at atmospheric pressure:H2/i-C4H10=1:2,600 ℃,the dehydrogenation activity of the catalyst reached the maximum:the conversion of isobutane was 52.9%,and the selectivity of isobutylene was 89.5%(TOS=8 h).Proper second promoter and pretreatment modification of Al2O3 support can improve the performance of the catalyst.Afterwards,the Pt-based catalytic system supported on alumina is studied.Based on Pt content of 0.5 wt.%,the addition of Sn can improve the performance of catalyst from both geometric and electronic effects,the catalyst has the best performance when the molar ratio of Pt/Sn is 1:1.After that,adding alkali metal K to the catalyst can neutralize medium and strong acid sites,resulting in the improvement of isobutene selectivity and catalyst stability.The optimum reaction conditions were H2/i-C4H10=1:2,600 ℃.Under these conditions,the conversion of isobutane was 39.6%,and the selectivity of isobutylene was 78.9%(TOS=12 h).Finally,the Pt-based catalytic system supported on Zinc modified ZnAl2O4-Al2O3 composite support is studied.Alumina was modified by impregnation method to prepare carrier 5-ZnAl2O4-Al2O3-1000(the content of Zn is 5 wt.%).And as the carrier to prepare Pt based catalysts.Compared with alumina,5-ZnAl2O4-Al2O3-1000 has smaller specific surface area,but the stability and selectivity of the catalyst have also been greatly improved.Compared with ZnAl2O4 supported catalyst prepared by co-precipitation method,it has higher dehydrogenation activity.Because the calcination temperature reaches 1000 ℃,the crystal phase of the catalyst changes from γ-Al2O3 toα-Al2O3,and formes ZnAl2O4.The introduction of zinc promotes the transformation ofθ-Al2O3 to α-Al2O3.Pt-Sn/5-ZnAl2O4-Al2O3-1000 has the lowest number of strong acid sites and the highest selectivity for isobutylene.At atmospheric pressure,H2/i-C4H10=1:2,600 ℃,the conversion of isobutane is 29.0%,and the selectivity for isobutylene is 94.4%(TOS=12 h).Three catalysts all use industrial alumina ball as support material,and adopt the impregnation method of wet chemical method,the process of industrial production is simple.The initial activity of K-Cr2O3/Al2O3 and Pt-Sn-K/Al2O3 catalysts has basically reached the reported industrial level.ZnAl2O4-Al2O3 composite support was formed by calcination of zinc-modified alumina support at high temperature.It was applied to Pt-Sn catalytic system with high stability and selectivity.Further optimization of pore structure and specific surface area of alumina spheres and study of pretreatment methods could realize long-term operation of isobutane dehydrogenation catalyst.