Gas-Solid Synthesis, Characterization And Catalytic Properties Of Ti-ZSM-5 Zeolite

Titanium silicalite-1 (TS-1) is a titanium-containing zeolite with the MFI structure that was first synthesized in 1983. Because of the unique catalytic properties in organic reactions with aqueous hydrogen peroxide (30%) as the oxidant under mild conditions, TS-1 has received much attention of the researchers. There are two methods for the synthesis of TS-1, hydrothermal synthesis and secondary synthesis or post synthesis. In the past decades, the hydrothermal synthesis, especially the low cost system, developed fast. At the same time, the secondary synthesis developed slowly. In this paper, different ZSM-5 precursors were used to prepare Ti-ZSM-5 through gas-solid method. The effect of precursors on the chemical-physics properties of Ti-ZSM-5 was investigated to find a kind of precursor more suitable for the gas-solid synthesis of Ti-ZSM-5. Through modifying the synthesis conditions of the B-ZSM-5 precursor to control the amount of hydroxyl nests and the crystal size, and optimizing the gas-solid synthesis conditions, Ti-ZSM-5 with better catalytic properties was successfully prepared. The mechanism of the gas-solid synthesis of Ti-ZSM-5 is highlighted through the work mentioned above.The main contents of this thesis are as follows:1. Ti-ZSM-5 samples were successfully synthesized by using B-ZSM-5 with a molar ratio of SiO₂ to B₂O₃ 5 in the gel and Al-ZSM-5 zeolites with a molar ratio of SiO₂ to Al₂O₃ 30, 239, 600, respectively, as the precursors. The gas-solid synthesis included two steps which were pretreatment in HCl solution or HNO₃ solution and titanation with gaseous TiCl₄. The results show that: The chemical-physics properties of the precursors play a very important role in theincorporation of titanium into the zeolite framework. With B-ZSM-5 as the precursor, the synthesized Ti-containing samples have much more framework titanium species and less extra-framework titanium species which result in a higher activity in the oxidation reactions. At the same time, the acid sites from the residual boron in the Ti-containing samples lead to a low slectivity to PO. As to the Al-ZSM-5 with a molar ratio of SiO₂ to Al₂O₃ 30, much Al atoms remain in the framework after the acid treatment. Ti-ZSM-5 with it as precursor has much acid sites which changed the catalytic properties. Ti-ZSM-5 starting with the silicon-rich ZSM-5 with a molar ratio of SiO₂ to Al₂O₃ 239 has more framework titanium and better catalytic activity than that starting with the silicon-rich ZSM-5 with a molar ratio of SiO2 to Al₂O₃ 600 does. The later has much anatase TiO₂ and rutile TiO₂, while the former and Ti-ZSM-5 starting with Al-ZSM-5 with a molar ratio of SiO₂ to Al₂O₃ 30 contain only anatase TiO₂. The de-[B]-Ti-ZSM-5 contains trace of anatase TiO₂. Either for the reaction of propylene epoxidation or for the reaction of phenol hydroxylation, anatase TiO₂ or rutile TiO₂ does not accelerate the decomposing of H₂O₂. For the reaction of propylene epoxidation, the amount of framework titanium dominates the conversion of hydrogen peroxide. For the hydroxylation of phenol, the crystal size, as well as the amount of framework titanium, plays a very important role in the conversion of phenol. Phenol protonation caused by strong acid sites hindered the electrophilic reaction of phenol hydroxylation.Titanium can be incorporated into the zeolite framework through both the reaction of "hydroxyl nests" with TiCl₄ and the replacement of framework Al or B by Ti during the gas-solid reaction.2. Ti-ZSM-5 samples were successfully synthesized by using B-ZSM-5 with different molar ratio of SiO₂ to B₂O₃ as the precursors. The effect of (n)SiO₂/(n)B₂O₃ on the incorporation of titanium into the framework was investigated. Propylene epoxidation and phenol hydroxylation properties were also investigated. The results show that (n)SiO₂/(n)TiO₂ in the samples increased with an increase in the (n)SiO₂/(n)B₂O₃ in the precursors. The conversion of H₂O₂ increased with the decreasing of (n)SiO₂/(n)B₂O₃ or (n)SiO₂/(n)TiO₂, and...