Study On Catalyst And Reaction Mechanism For Selective Conversion Of LPG To Light Olefins

To enable liquefied petroleum gas（LPG）to convert to propylene and ethylene,and on traditional microporous micron HZSM-5 LPG tends to change into heavier C5⁺ and CH₄,modified HZSM-5 by different methods was designed and fabricated,the catalytic activity of which for light olefins production was improved.The structure-activity relationship and reaction mechanism were analyzed by means of characterization of phase,pore structure,acidity and catalytic performance for LPG,which provided clues for the utilization of mixed light hydrocarbons（HCs）.Effect of interconnected mesopores and the acidityHZSM-5 with different crystallite aggregation,mesopore property and acidity was synthesized by the recrystallization of Al-SBA-15.The mesopore-connected,micropore-shortened and weakly acidic HZSM-5 suppressed the adverse side reactions and enhanced remarkably the formation of light olefins from LPG.At reaction temperature of 600℃ and contact time（W/F）of 15 g·h/mol,the selectivity of C₃H₆+C₂H₄ reached 68.3%at the total conversion of LPG of 58.8%.Products of C5+ HCs and CH₄ were reduced.Secondary reactions of primary olefins were reduced.Effect of tuning the framework or surface acidity of HZSM-5As the acidity of the MFI zeolite framework decreased,the selectivity of the C₃H₆+C₂H₄ products were increased.Considering both the conversion of LPG components and the selectivity of target products,the catalytic activity of FeHZ-5 for LPG transformation was relatively better than those of BHZ-5 and GaHZ-5.When n（Fe/Al）=0.3 in the MFI framework,the total conversion of LPG on 0.3FeHZ-5 was 76.15%,and the selectivity of C₃H₆+C₂H₄ was 62.26%.Compared with HZSM-5,the selectivity of C₃H₆+C₂H₄ was increased by 5.79%whereas the selectivity of C5⁺ was decreased by 5.77%.When the HZSM-5 surface was loaded by different amount of Fe,the trace of Fe reduced the surface acidity of HZSM-5 and increased the selectivity of C₃H₆+C₂H₄.With the increase in the amount of Fe,the acidity of samples increased,and the catalytic activity for LPG reaction increased,but the selectivity of C₃H₆+C₂H₄ were decreased.Effect of acidity,pore shape selectivity and continuous diffusion of composite zeoliteBy comparison of the physicochemical properties and catalytic properties for LPG transformation of SAPO-18（OSi）,SAPO-18（0.4Si）and HZSM-5,HZSM-5/SAPO-18 composite zeolite was prepared.As the amount of SiO₂ in the SAPO-18 crystallization solution was decreased,the selectivity of C-₃H₆ was increased due to the decrease in the acidity of SAPO-18 in the HZ5/SA18 composite sample.When SiO₂ was not added,a small amount of SiO₂ produced by the dissolution of HZSM-5 was used to synthesize HZ5/SA18 sample with the lowest acid of SAPO-18.Based on the properties of HZ5/SA18 samples and mechanically mixed HZ5+SA18（0Si）samples with different weight ratios of HZSM-5/SAPO-18,it can be seen that reducing the continuous diffusion of the reaction molecules in the HZSM-5 channel restrained side reactions such as hydrogen transfer and aromatization,and promoted the selectivity of C-₃H₆.Compared with the same weight ratio of mechanically mixed HZ5+SA18（0Si）samples,the C-₃H₆ selectivity was increased by 5%on HZSM-5/SAPO-18,indicating that the less acidity of SAPO-18 in the composite sample improves the shape selectivity of C-₃H₆.HZ5（6.75）/SA18（0Si）showed the best catalytic activity.The conversion of LPG was 61.70%,the selectivity of product C₃H₆+C₂H₄ was 72.80%,and the selectivity of C5⁺ was significantly decreased（8.61%）.Reaction behavior of feed on HZ5（6.75）/SA18（0Si）As reaction temperature was decreased from 600℃ to 500℃,the conversion of C₃ and C₄ alkanes decreased significantly whereas the C₄ olefins remained high conversion（>85%）.Under the same conditions,the conversion of single component increased in order:C₃H₈<n-C₄H₁₀<n-C₄H₈.The conversion of butylene and primary products of C-₃H₆ and C₂H₄ was quite different,which was 97.8%,69.5%and 23.1%,respectively.