Solvent-free Synthesis Of ZSM-5 Zeolite And The Studies Of Its Catalytic Reactions

ZSM-5 zeolite has been extensively used in the petrochemical industries due to its unique pore structure, large specific surface area, and superior catalytic activity. In this work, the synthesis method of ZSM-5 zeolite, its catalytic reaction mechanism, the effects of the acid strength and the pore confinement on the activity of catalytic reactions and the reaction mechanism have been studied, and some meaningful results have been obtained.(1)The conventional hydrothermal synthesis route requires a large amount of water and organic structure directing agents, which usually produces the industrial pollutants. Therefore, the development of green routes for zeolite synthesis is of great importance for the sustainability of the world. In this work, solvent-free method with NH4 F as the mineral agent has been employed for the synthesis of pure silicon and aluminum-silicon ZSM-5 zeolite. XRD, SEM and MAS NMR were used to character the structure information of samples. Under the F- synthesis conditions the introduction of the hetero-atoms such as aluminum and germanium is easier. Our work expends the application of the solvent-free synthesis method.(2)The acid strength and the pore confinement effect are the two factors that affect the catalytic activity. The pore confinement is mainly due to the weak interaction which actives the reactions and stabilizes reaction intermediates and transition states. These result in the improvement of the catalytic activity. The pore confinement effect of the ZSM-5 zeolite has been systematically investigated by theoretical methods. The molecular adsorption energy, HOMO energy and the bond gap between the HOMO and LUMO orbital have been considered. It is found that the stability of the adsorption molecular mainly depends on the matching of their size. Due to the effect of the pore confinement of the ZSM-5 zeolite, the HOMO energy and the alkaline of the adsorption molecular is increased, which leads to the improvement of the catalytic activity. The conjugative effect of benzene, naphthalene and anthracene may also influence the bond gap of the HOMO-LUMO orbital. When they are introduced to the pores, the electron jumps from π bond to π* bond more easily, so as to improve the reactivity of the adsorption species.(3)The activation of olefin plays important role in the petroleum chemical industry. The understanding on the reaction path should be very important for studying the catalytic reaction mechanisms,selecting and designing more efficient catalysts. The H/D exchange reaction of propene over zeolite involves C-H bond dissociation and generation, which is a model reaction of olefin conversion. In this work, the mechanism of H/D exchange reactions of propene over acidic zeolite catalysts has been systematically investigated by theoretical methods. Basing on the calculated activation barriers, the propene and the acid proton of ZSM-5 zeolite passed through five times of protonation and deprotonation and finally generated the propene-D5, and the H/D exchange of the last hydrogen atom is crucial. By studying the three possibly routes, we found that when the amount of propene adsorbed on the ZSM-5 zeolite was low, the n-propoxy route was preperferred; and as the amount of it increasing, the dimerization route has the most advantage. Meanwhile, the stronger acid strength of Al-ZSM-5 could make the activation energy barriers lower and the H/D exchange reactions easier. Moreover, the confinement effect can improve the catalytic activity effectively.