The Effect Of Zinc Modification On The Structure And Performance Of Alumina-supported Cobalt Catalysts For Fischer-Tropsch Synthesis

Fischer–Tropsch synthesis （FTS） is an attractive alternative to petroleum for theproduction of clean, sustainable and renewable automotive fuels, which use syngasas reactants, generating from natural gas, cheap coal, or renewable biomassresources. Cobalt-based catalysts are one of the most important candidates for FTSbecause of their acceptable cost, high activity, low water-gas shift activity and highselectivity to long chain paraffins.In this work, three series of supports were prepared: Zinc was doped by bothco-precipitation and impregnation methods to obtain a series of alumina supportswith zinc modification; ZnAl₂O₄supports with different nanostructured morphologyhave been successfully synthesized using cheap raw material; A well-designedzeolite capsule catalyst with a Core （Co/Al₂O₃）-Shell （SAPO-34） structure wassuccessfully synthesized by zeolite shell growing via in situ hydrothermal method.In addition, cobalt was loaded on the supports by incipient wetness impregnationmethod. Various characterization methods as well as Fischer-Tropsch synthesis testswere employed to provide the in-depth information about the structure and reactiveperformance of catalysts. The main results are as follows:（1） Zinc was mostly in the form of zinc aluminate after higher temperaturecalcination whether it was introduced by co-precipitation, or by incipientwetness impregnation. However, zinc aluminate was homogeneously dispersedthroughout the framework of alumina when zinc was introduced byco-precipitation; while in the support when zinc was added by impregnation,zinc aluminate was aggregated on the surface of alumina. Moreover, zincmodification preserved the cobalt species on the support surface. The activity ofFischer-Tropsch reaction was a function of both cobalt dispersion and cobaltreducibility, and the deactivation was mainly due to the sintering of cobaltparticles. The cobalt catalyst supported on PZnAl1, in which zinc wasintroduced by co-precipitation, showed the highest catalytic activity in FTSwith good stability in this study.（2） The nanostructured ZnAl₂O₄with controlled morphologies such as particles,rods, and sheets have been prepared using Zn（NO3）2·6H2O reagent as zincsources and Al（NO3）3·9H2O reagent as alumina sources.（3） Nanostructured ZnAl₂O₄with different morphologies possessed different specific surface area and porous structure, which had strong impacts on the sizeof cobalt species, on the cobalt-support interaction, and then, on FTS catalyticactivity and stability. The catalyst supported on alumina nanosheets, owning thehighest specific surface area, showed the highest cobalt dispersion and anenhanced FTS activity. The nanorod ZnAl₂O₄with larger pore size had morestable pore structure during catalyst preparation, the presence of the stable largerpores in nanorod ZnAl₂O₄supported catalyst facilitated the diffusion of reactantsand products, and protected the metallic active cobalt phase against sinteringduring reaction（4） The core-shell structure of the catalyst provided a tailor-made confined reactionenvironment with spatially confined effect, shape selectivity of channels andacidity. Thus the distribution of products from FTS was altered, the formation oflong-chain hydrocarbons was significantly suppressed, and the selectivity oflight alkenes was improved obviously.