Study On Novel Catalysts For Catalytic Dehydrogenation Of Isobutane To Isobutene

With the increasing demand for down-stream products of isobutene,a higher requirement has been raised for processes of isobutene production.Dehydrogenation of alkanes,as an important route for the utilization of abundant light alkanes and the production of high-valued alkenes,is particularly concerned.Although satisfactory performances can be achieved over commercially applied platinum and chromia-based catalysts,high cost and environmental issues are inevitable with their application.Therefore,the development of a novel class of economically efficient and environmental-friendly catalyst,is the key to widespread application of dehydrogenation processes.In this thesis,systematic research has been carried out for the development of novel dehydrogenation catalysts,including Mo-based and Mo S2-based catalysts,as well as some other supported metal sulfide catalysts.Not only the dehydrogenation performance,stability and regeneration behaviors of these catalysts have been evaluated,bot also the structure-activity relationship has been analyzed through detailed characterizations.Meanwhile,the active phase and reaction mechanism have been further explored.Dehydrogenation performance of isobutane over Mo-based catalyst was investigated initially.Mg Al₂O₄ supported catalyst with 5 wt% Mo loading exhibited better performance?with isobutene yield of 33 wt% at 560 oC?compared with Al₂O₃,Si O2,Zr O2,and Mg O supported catalysts,resulting from the well dispersion and appropriate redox and acidity behaviors of Mo species.Mo4+ species are speculated to be the active phase.Moreover,continuous reaction-regeneration test indicated a good stability of the catalyst,and isobutene yield in a continuously operated circulating fluidized bed reactor was maintained around 35 wt% within 200 h test.To further improve the dehydrogenation performance,sulfur was introduced into Mo-based catalyst.Results indicated that an isobutene yield of 56.3 wt% was achieved over 5Mo/Mg Al₂O₄-S catalyst.However,an induction period existed for the transformation of high valence Mo species to active Mo S2,the formation of which required an atmosphere of appropriate reducing gases?e.g.,CO?.Under the effect of H2,molybdate species were reduced to Mo O2,leading to detrimental interaction between Mo S2-O-Mo O2 and inhibiting the formation of Mo S2.Fortunately,the induction period disappeared when 5Mo/Mg Al₂O₄ was presulfided by H2 S,while isobutane conversion decreased gradually with TOS,and another sulfidation treatment was effective to recover deactivated catalyst.Sulfur introduction can also improve the dehydrogenation performance of Ni/Mg Al₂O₄ catalyst.Ni/Mg Al₂O₄ catalyst with low Ni O loading exhibited low activity,and when Ni O loading increased,cracking reactions aggravated,leading to the formation of a large amount of methane and coke.However,sulfur addition effectively inhibited the aggregation and reduction of Ni species,suppressed undesired cracking reactions,and remarkably increased the selectivity to isobutene.Besides,Ni S species constituted the active phase for isobutane dehydrogenation.It is worth noting that the optimum performance can be obtained at the beginning of the reaction over Ni/Mg Al₂O₄ catalyst after sulfidation.The optimum sulfidation conditions were at 560 oC for 3 h,with isobutane conversion of 64 wt%.The dehydrogenation performance of a series of silica supported metal sulfide catalysts was evaluated,and for most of them,isobutene yield was higher than 50 wt% at 560 oC.The two aspects of sulfur introduction were revealed: one is geometric effect diluting aggregated metallic species and reducing cracking activity,and the other is electronic effect facilitating desorption of olefin and increasing its selectivity.Moreover,the interaction model between isobutane molecules and catalysts before and after sulfidation was speculated,and main reasons of catalyst deactivation were also determined.