Catalytic Cracking Of Toluene As A Model Compound Of Tar Over Nano-minerals Supported Fe And Ni Catalysts

Biomass tar is one of the obstacles hindering the development of biomass gasification technology,thus efficiently converting biomass tar into clean fuels has become a key point.Catalytic cracking of biomass tar is considered as the most promissing biomass conversion technology.To date,the research mainly focuses on the development of the catalysts with low-cost,high catalytic activity and easy recycling properties.From the perspective of catalytic reactivity and economic feasibility,one of the hot spot is the utilizaiton of the abundant natural minerals with special nano-structure and physicochemical properties for conversion of biomass tar.Limonite and palygorskite are natural nano-mineral materials with nano-structure,large specific surface area and excellent thermal stability.In this dissertation,the Fe-Ni supported catalysts were synthesized based on these two minerals.The structural and physicochemical variations of the catalysts,as well as the interaction between the additives and carrier surface during the catalytic cracking of toluene(as a model compound of biomass tar)were investigated.And their effects on catalytic reactivity of the catalysts for toluene conversion were discussed.In addition,the catalytic steam and carbon dioxide reforming of toluene were conducted over Fe-Ni supported catalysts.The deactivation mechanism of the catalysts was analyzed using various methods and their regeneration performances were also evaluated.The main conclusions of this work are as follows:(1)A hematite catalyst with high catalytic activity for toluene conversion was synthesized using limonite as a precursor.The efficient reaction between the lattice oxygen in hematite structure and carbon deposition was able to maintain the high catalytic activity and stability of the catalyst.In the process of catalytic reactions,the hematite was gradually reduced,while its structure phase stepwisely changed from hematite to magnetite,then to wustite and finally to iron.Therefore,the spent catalyst with high magnetism could be easily recollected by the method of magnetic reclaim.Moreover,the catalyst regeneration can be achieved by calcining the spent catalyst in air atmosphere to remove carbon deposition and to compensate the lattice oxygen.(2)The hematite supported nickel catalyst was synthesized,and the interaction between hematite and nickel as well as their phase changes were revealed.It is found that the iron in hematite structure strongly intereacted with nickel at high reaction temperature,resulting in the formation of Fe-Ni alloy.However,excessive nickel loading would cause the agglomeration of Ni O and the decrease of specific surface area,and thus,reducing catalytic activity of the catalyst.The deactivation mechanism was elucidated through the analysis of both gas products and the spent catalysts.The results indicated that the consumption of lattice oxygen in the structure of hematite and the formation of graphitic type carbon deposition were the main reasons for the deactivation of the catalysts.(3)The variations of structure and physicochemical properties of the palygorskite supported Fe-Ni catalysts prepared under different calcining conditions,including calcination termperature,calcination time and calcination atmosphere were systematically investigated.And the correlation between these varations and the catalytic activities for toluene conversion of the catalysts were also discussed.It was found that the calcination termperature is more likely to cause the structure change of the catalysts than the calcination time,especially the influence on particle size and specific surface area.The catalyst prepared by reducing at 700 oC for 2 h has the best catalytic activity of toluene conversion.Both high calcination termperature and long calcination time would lead to sintering of the catalysts,thus affecting their catalytic performance.The results of different calcination atmosphere indicated that Fe-Ni alloy was the active component for catalytic cracking of toluene,and the catalytic active sequence of the supported components was Ni Fe2O4< Fe0< Ni0< Fe-Ni alloy.(4)Based on the understanding of structure and physicochemical property changes of the catalysts in different calcination conditions,systermatic studies were carried out to explore the catalytic steam and dry reforming of toluene over the palygorskite(Pal)supported Fe-Ni catalysts.Compared with the single metal supported catalysts(Nix/Pal and Fex/Pal)and pure palygorsktie,the bimetal system(Fe3Ni8/Pal)exhibited superior catalytic activity and stability for catalytic steam and dry reforming of toluene,and the syngas mainly consisted of H2 and CO was obtained.This result,on the one hand,should be attributed to the formation of Fe-Ni alloy with small particle size and high dispersity on the surface of palygorskite due to the strong interaction between iron and nickel.On the other hand,the strong oxygen affinity of iron could further enhance the adsorption capacity of H2 O and CO2,thus improving the resistance of carbon deposition and sintering of Fe3Ni8/Pal.