The Preparation And Performance Of The FeMnK@SAPO-34 Core-shell Catalyst For Fischer-Tropsch Synthesis

Light hydrocarbons are one of the most important basic feedstocks in modern chemical industry,and they are produced from naphtha steam cracking currently.However,it is urgent to develop alternative technology to produce light hydrocarbons from coal,natural gas and biomass.Then the Fischer-Tropsch synthesis which convert syngas?hydrogen and carbon monoxide?to light hydrocarbons is considered as the most promising technology route.But the major products in the industrial Fischer-Tropsch synthesis process are gasoline,diesel and lubricating oil with the limitation by the product distribution.In order to raise the light hydrocarbons selectivity,the reasearchers put forward several solutions from the process and catalysts respectively.For example the process is connected the Fischer-Tropsch reactor and hydrocarbons cracking reactor in series.Otherwise the catalysts are prepared by mixing the Fischer-Tropsch catalyst and zeolite physically and the bifunctional catalysts are synthesised by impregnating the Fischer-Tropsch active components to the zeolite.Whereas there are some drawbacks in the above methods like that firstly the process is long and the cost of investment is high,secondly the heavy hydrocarbons from Fischer-Tropsch synthesis can not be cracked completely over the zeolite,thirdly the activity of bifunctional catalysts is low because of the strong combination between the Fischer-Tropsch active components and the zeolite acid sites.In this regards,the catalyst structure is designed to be the core-shell structure,which is using the Fishcer-Tropsch catalyst as core and the zeolite as shell.The catalyst is expected to modify the light hydrocarbon selectivity by the core-shell structure that the formed heavey hydrocarbons will be cracked by the protonic acid over the shell zeolite inevitably when transferring outside the core Fischer-Tropsch catalyst.Thus we propose to fabricate a new core-shell structured catalyst for Fischer-Tropsch synthesis in this thesis.Firstly,the conventional FeMnK catalyst were prepared by co-precipitation method,and then the mixture of the silica sol and SAPO-34 powder is sprayed on the surface of FeMnK catalyst.After drying and calcination,the FeMnK@SAPO-34 core-shell catalyst is prepared completely.The core-shell catalyst is characterized by N2 adsorption-desorption test,X-ray diffraction,Scanning Electron Microscopy and H₂-TPR and investigated in a fixed-bed micro reactor.The results indicate that the specific surface area and pore volume of the core-shell catalyst increase significantly,and activity of the core FeMnK catalyst is not affected by the shell zeolite.It means that the Fischer-Tropsch synthesis and the hydrocarbons cracking were coupled in the FeMnK@SAPO-34 catalyst and the selectivity of the heavy hydrocarobns was suppressed obviously.The optimized reaction performance of the FeMnK@SAPO-34 with two-time coatings is the best that the conversion of CO was 74.8%and the selectivity of the light olefins and light hydrocarbons was 42.4%and 51.5%respectively?the selectivity of the light olefins and light hydrocarbons was 57%and 69.2%excluding CO₂?.