The Influence Of HZSM-5 Textural And Acid Properties On Its Catalytic Performance In MTP Reaction

Among the factors that affect the MTP process,the zeolite plays a particularly important role.The hierarchical ZSM-5 zeolite not only maintains the characteristics and advantages of the microporous ZSM-5 structure,but also has the synergistic effect of the coupling of the hierarchical pore structure.So the hierarchical ZSM-5 zeolite has excellent advantages such as shape selectivity,diffusion property and carbon deposition resistance.In this paper,the crystallinity,surface acidity,acid distribution and textural properties of ZSM-5 were studied.Moreover,the relationships between these properties and its catalytic performance in MTP reaction and carbon deposition were discussed.The Silicalite-1@ZSM-5 and SiO2@ZSM-5 core-shell molecular sieves were also studied.The effects of crystallization conditions on the preparation and physicochemical properties of core-shell molecular sieves were investigated.And the effect of two structures of core-shell structure on catalytic performance in MTP reaction was studied.The highest relative crystallinity of the samples obtained by treating the ZSM-5 zeolite with NaOH solution was 97.84%.The maximum mesopore volume was 0.314 m2g-1(mesoporous porosity was 66.67%),and the minimum was 0.09 m2g-1,the mesopore porosity was 38.63%).The optimum conditions for increasing the porosity of the mesoporous porosity were 0.8 M,the temperature was 65?,the treatment time was 0.5h.In the MTP reaction,the selectivity of the product propylene and lower olefins increased from the original 20.68%and 49.59%to 31.89%and 64.12%,respectively.The optimum conditions for the selectivity of propylene and light olefins were as follows:alkali concentration was 0.4mol/L,treatment temperature was 75?,treatment time was 4h.The factors affecting the selectivity of propylene and light olefins followed the order:treatment time>treatment temperature>alkali concentration.That for the selectivity of aromatics selectivity followed by treatment time>alkali concentration>treatment temperature.The B acid of hierarchical HZSM-5 has a great influence on the distribution of MTP products,and the effect of L acid is relatively small.The selectivity of aromatics and C5+ hydrocarbons showed an upward trend with the increase of the amount of B and the strong B acid in the pores.Strong B acid is favorable for aromatics cycling,olefin methylation and cracking of high hydrocarbons,which promotes the secondary hydrogen transfer reaction and reduces the selectivity of olefins.The addition of strong B acid in the pores increases the selectivity of propylene and lower olefins.The aromatics and C5+ hydrocarbon selectivity showed a decreasing trend,indicating that the strong B acid in the pores inhibited aromatics cycling,olefin methylation and cracking of high hydrocarbons,hindering the secondary hydrogen transfer reaction and improving the selectivity of olefins.The carbon of the hierarchical HZSM-5 molecular sieves are mainly concentrated in inside pore after the MTP catalytic reaction,which is different from the the microporous HZSM-5zeolite.The relative crystallinity of the core-shell ZSM-5@SiO2 zeolite sample was 50.41%?76.79%and the BET specific surface area was 228.4?297.8 m2g-1,which was significantly lower than that of the industrial raw powder(324.2 m2g-1).Total pore volume were 0.255?0.336 m2g-1,which is higher than that of industrial raw powder(0.240 cm3g-1).In the MTP catalytic reaction,the low hydrocarbon selectivity and propylene selectivity increased from 50%and 20%to 53%and 22%,respectively.The relative crystallinity of the core-shell ZSM-5@Silicalite-1 zeolite prepared by surface crystallization was 111.81%?123.13%,and the BET specific surface area increased from 324.2 m2g-1 to 363.0 m2g-1,which was significantly higher than industrial raw powder.All samples have MTP reaction catalytic activity.The selectivity of low hydrocarbons and propylene are higher than industrial ZSM-5,the selectivity of propylene is increased from 20%to 27%,and the selectivity of low olefins is increased from 50%to 60%.The amount of carbon from 13.73%of the commercial HZSM-5 down to 2.43%.