Study On Catalytic Performance Of Composite HZSM-5/SAPO-34 For The Transformation Of LPG To Propylene And Ethylene

Methanol to propylene(MTP)and coal-to-liquid processes produce considerable liquefied petroleum gas(LPG).For its transformation toward propylene and ethylene,the following advancement was achieved.Promotion of composite HZSM-5/SAPO-34 zeolite catalystComposite catalyst of HZSM-5/SAPO-34(CZS)was synthesized by presetting HZSM-5 in the synthetic gel of SAPO-34.Reaction at 600℃ was more advantageous to transform LPG to C3=+C2=.With increase in silica sol,the total acid amount and the strong acid/total acid ratio of the samples were reduced,the conversion of LPG was decreased while the selectivity of propylene and ethylene increased.The mass ratio of HZSM-5 to SAPO-34 could be adjusted by altering the crystallization time.As the crystallization time was extended,the conversion of LPG was gradually lowered because the content of HZSM-5 was reduced and the cracking activity became low.On the 72 h crystallization sample,all components of LPG were simultaneously converted.The total conversion of LPG of 72.80%and selectivity of 65.43%for C3=+C2=were achieved,which was higher than those on the physical mixed sample.Compared with that on HZSM-5 alone,the selectivity of C3=+C2=was increased by 21.13%while the selectivity of CH4 and C5-decreased by 3.51%and 16.80%,respectively.The composite catalyst was stable with time-on-stream of 250 h.Transformation in presence of CO2nFe/Mo2C was prepared by in-situ carbonization,and mechanically mixed with CZS then to obtain composite catalyst of nFe/Mo2C+CZS.When n(LPG):n(CO2):n(Ar)=1/1/1.5,the conversion of LPG was 64.24%with the highest selectivity of C2=+C3=(70.30%),the lowest selectivity of C5+(15.17%),and P/E of 1.48.Compared with the sample Mo2C+CZS,the total conversion of LPG went up by 3.64%and the selectivity of C3=+C2=kept almost unchanged.The added Fe increased the alkalinity of the catalyst,which favors the desorption of ethylene and propylene whereas the adsorption and activation of CO2,secondary reactions were suppressed.The coupling transformation of dehydrogenation and reverse water gas reaction would be accelerated,which promotes the transformation of LPG and increases the selectivity of the target product.As the mass ratio of 5%Fe/Mo2C to CZS was increased,the conversion of LPG increased first and then decreased.But the selectivity of propylene increased all the way.Compared with CZS,the sample with the ratio of 5:5 remarkably increased the P/E(2.28),while the conversion of LPG was decreased by 9.32%.There existed the competitive adsorption between CO2 and LPG on the catalyst surface,which is unfavorable for the transformation of LPG although the transformation in the presence of CO2 raised the selectivity of C2=+C3= and decreased CH4 and C5-.