Study On Transformation Of LPG In The Presence Of CO₂ To Olefins On Modified HZSM-5 Catalysts

Converting liquefied petroleum gas(LPG)to ethylene and propylene can raise its utilization value.On traditional HZSM-5 catalyst and at 600-650℃,LPG can not be converted completely and produce more CH4 and C5+ hydrocarbons.In this work modified HZSM-5 was prepared using several procedures.Effects of the acidity and pore structure adjustment on the LPG transformation in the presence of CO2 to light olefins were studied.Improved catalytic activity and C2H4+ C3H6 selectivity were achieved.For hierarchical HZSM-5 synthesized by recrystallization,as the crystallization time extended,the acid strength increased gradually,the acid amount firstly decreased and then increased.Applied to the transformation of LPG at 600℃,the conversion of LPG rose whereas the selectivity of C2H4+C3H6 firstly increased and then decreased.HZSM-5-20(Si/Al2=37)crystallized at 140℃ for 20 h showed the best catalytic performance.The conversion of LPG was 45.17%,the selectivity of C2H4+C3H6 68.30%,the methane selectivity 5.28%and C5+ selectivity of 11.18%.CO was found in the products.It was inferred that the crystallization was sufficient at this condition.For hierarchical B/HZSM-5 synthesized by recrystallization of Al-SBA-15 and H3BO3 at 140℃ for 44 h,the crystallinity lowered,the acid strength decreased and the weak acid amount increased compared with HZSM-5-44 by the same crystallization condition.B/HZSM-5(20)with Si/B=20 showed the best catalytic performance.The selectivity of C2H4+C3H6(63.74%)was increased by 6.49%compared with that of HZSM-5-44(56.25%).The total conversion of LPG decreased from 55.78%to 49.35%,C5+decreased by 10%and the CO selectivity decreased from 0.9%to 0.5%.For commercial HZSM-5(Si/Al2=81)with impregnation of 0.5-2%P,the surface area and the pore volume decreased,and the strong acid decreased.As the P content increased,the total conversion of LPG decreased from 70.54%to 13.28%.1P/HZSM-5 showed better catalytic performance for LPG transformation in the presence of CO2.The total conversion of LPG was 41.02%and the conversion of CO2 was 10.19%.The selectivity of C2H4+C3H6 was 68.30%,higher by 24.58%than that on HZSM-5.For boron or P modified HZSM-5,increase in weak acid increased the selectivity of C2H4+C3H6 whereas decrease in strong acid strength and amount decreased the transformation activity.To raise the transformation activity,Fe was introduced into B/HZSM-5 and P/HZSM-5 by impregnation.For Fe modified B/HZSM-5(20),the conversion of LPG was increased slightly,the selectivity of C2H4+C3H6 was not increased.0.1Fe-B/HZSM-5 showed better catalytic performance.The total conversion of LPG,the selectivity of C3H6+C2H4,the methane selectivity and the C5+selectivity were 51.62%,62.93%,6.44%and 14.66%,respectively.The CO2 conversion was 11.68%and the CO selectivity was 0.54%.For Fe modified P/HZSM-5,interaction between Fe with P resulted in the transforming of the octahedral Al to the tetrahedral framework Al.This interaction recovered partial B acid sites neutralized by phosphorus modification;meanwhile,L acid sites were produced,evidently increasing the acid amount and strength.Introduction of Fe increased the LPG conversion,the ethylene and propylene selectivity were increased by 2.0%and 3.0%,respectively.0.5Fe-P/HZMS-5 showed the best catalytic performance.The total conversion of LPG(49.53%)was 8.51%higher than that on 1P/HZSM-5(41.02%).The conversion of CO2 increased by 2.9%(13.09%vs 10.19%).The selectivity of C3H6+ C2H4,methane and C5+ was 68.25%,5.62%and 12.34%,respectively.Introduction of Fe affected the structure and acidity of 1P/HZ-5.Appropriate Fe enhanced the catalytic activity without changing the selectivity of C3H6+C2H4.Compared with transformation alone,the transformation of LPG in the presence of CO2 exhibited higher conversion.The conversion of LPG increased by 1.95%on B/HZSM-5(20)and increased by 0.5%on 0.5Fe-P/HZSM-5.With reaction as time-on-steam for 12.5 h,the conversion of propane clearly increased.The CO selectivity was 0.2-0.7%.The C2H4+C3H6 selectivity increased by 3.0-7.5%,and the C5+ selectivity reduced by 1.0-7.0%.The carbon deposition lowered by 2.0%.The catalytic activity stability was improved to some extent.