| The development of renewable liquid biofuel is a crucial route to increase the energy supply and reduce the pollutant emission.Liquid fuels will be used in the aircraft for quite a long time due to the large weight of other energy devices,especially for the jet aircraft which consumes vast quantities of fuels.Different methods can be used to convert biomass into bio-jet fuel,among which the two-step hydrogenation technique has the most potential for industrial application because of the mature technology,low cost and simple process.Oil is first converted through hydrodeoxygenation to C15–C18 long-chain alkanes,which are then converted to normal and isomer C9–C15 alkanes,i.e.,jet fuel,by hydrocracking and hydroisomerization.The selection and design of catalyst is important to the two-step hydrogenationprocessespeciallythesecondstepof hydroisomerization/hydrocracking process.Microporous molecular sieves are widely used as the catalyst support in hydrocracking processs.However,their pore size is smaller than 2 nm,which hinders the diffusion of long-chain alkanes in catalyst channel resulting in low conversion and selectivity to C9–C15 alkanes.In order to solve this issue,mesoporous molecular sieve MCM-41 with pore size of 2–10 nm and large surface area was used as the catalyst support in this paper.Bifunctional catalyst Pt/Al-MCM-41 was developed for hydrocracking of bio-alkanes for bio-jet fuel production.Fist,the effects of synthesis conditions of MCM-41 catalyst support,including molar ratio of Si to Al,gelation pH,aging time,aging temperature and template type,on catalyst activity and selectivity were examined in the paper.The optimal MCM-41preparation conditions were:Si to Al in a molar ratio of 10,pH of 10.5,crystallization time of 48h,crystallization temperature of 150°C,and template of CTAB.Under these conditions,the BET surface area,pore volume,and the most probable pore of the MCM-41 were 726.1 m2/g?1.40 cm3/g,and 2.55 nm,respectively.The ratio of metal sites to acid sites(CPt/CA)on Pt/Al-MCM-41 bifunctional catalyst is important to both the catalyst activity and product selectivity.Catalysts with different metal/acid site ratios were prepared on Al-MCM-41-10 support using various platinum loadings.Both the hydrocracking activity and selectivity to kerosene increased with the increase in CPt/CA ratio in the range of 0.182–0.338.However,too high metal/acid site ratio could cause the agglomeration of platinum particles,resulting in the waste of catalyst and reduction of catalyst activity and selectivity to kerosene.The optimal metal/acid site ratio was about 0.203.The hydrocarcking processes on Pt/Al-MCM-41 catalysts with different CPt/CA ratios were analyzed and compared by comparison of cracking and isomer products.It is widely accepted that normal alkanes are first converted to isomer alkanes with the same carbon number,and then cracked into normal and isomer alkanes with less carbon numbers.The strength of acid site is relatively high when CPt/CA is lower than0.203,and the isomer alkanes with the same carbon number as the feedstock are more likely to be cracked resulting in higher yield of cracking products.The strength of metal site is relatively high when CPt/CA is higher than 0.203,resulting in higher yield of isomer alkanes with the same carbon number as the feedstock.The effects of reaction conditions,including temperature,pressure,weight hour space velocity?WHSV?and H2/n-paraffin volume ratio,on hydrocracking activity and selectivity to kerosene were investigated.The optimal temperature,pressure,WHSV and H2/n-paraffin volume ratio were 330°C,2 MPa,1 h-1 and 2200,respectively,under which the bio-alkanes conversion of 98.7%,kerosene yield of47.0%and kerosene/gasoline ratio of 1.12 were obtained on 0.7%Pt/Al-MCM-41-10catalyst at the Si/Al ratio of 10 and CPt/CA ratio of 0.203.The quality of bio-jet fuel produced from the hydrocracking of bio-alkanes over Pt/Al-MCM-41 bifunctional catalyst was analyzed.It is determined by the Test Center of Civil Aviation Administration of China that all the properties of bio-jet fuel product reached the ASTM D7566 standard.It is noted that the freezing point reached up to-73.8°C,indicating superior low temperature fluidity.Finally,the stability and deactivation behavior of 0.7%Pt/Al-MCM-41-10catalyst were determined.Stable catalyst performance was observed within continuous reaction for 120 hours.The catalyst activity was reduced by about 20%after 72–120 hours.The main reason for catalyst deactivation was the agglomeration of platinum particles on the surface of catalyst,resulting in the blocking of some channels and reduction of active sites. |
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