The Studies On The Gas-solid Synthesis Of Ti-ZSM-5 With B-ZSM-5 As The Precursor

The gas-solid isomorphous substitution provides one operated easily and well repeated method to synthesize this titanium silicalite zeolite with the MFI structure. However, the catalytic activities of Ti-ZSM-5 synthesized from the gas-solid synthesis method are usually much lower than those of TS-1 obtained from hydrothermal synthesis. In these years, the improvement on the synthesis of the precursor B-ZSM-5 by adjusting the aging temperature can make up this deficiency. The catalytic activity of Ti-ZSM-5 using B-ZSM-5 as precursor, which is synthesized by aging treatment, and then crystallization, is similar to that of TS-1 obtained from the hydrothermal synthesis. But the reasons for showing such a good catalytic performance of Ti-ZSM-5, the incorporation process into the zeolite framework and existing forms of Ti species are still not clear.In this paper, Ti-ZSM-5 is synthesized by the gas-solid isomorphous substitution using B-ZSM-5 as precursor, which is prepared by aging treatment, and then crystallization. Proceeding with the precursor synthesis and the synthesis of Ti-ZSM-5 by the substitution reaction with TiCl4, the reasons for showing the high catalytic activity, the incorporation of Ti atoms into the zeolite framework and the main location of active sites are studied in detail.1. It is found that the precursors B-ZSM-5 synthesized by aging treatment, and then crystallization, consist of cuboid-like crystals with different sizes, which are agglomerated into different sizes of sphere-like particles. The good catalytic performance and recyclability of Ti-ZSM-5, using B-ZSM-5 as the precursor synthesized by aging treatment, and then crystallization, are mainly decided by the size of cuboid-like crystals.2. Adjusting the aging temperature, the optimized aging temperature should be controlled in the range of 333 K to 353 K. The obtained B-ZSM-5 has the smaller cuboid-like particles, the larger pore volume and surface area. Ti-ZSM-5 sample using this B-ZSM-5 as the precursor has a better catalytic performance for phenol hydroxylation.3. In the synthesis process of the precursors B-ZSM-5, adding the organic alcohol can modify the existing states of B-ZSM-5 that the sphere-like particles agglomerated by cuboid-like crystals with different sizes. But B atoms are strongly hindered to be incorporated into the zeolite framework; both the framework titanium content and catalytic performance of Ti-ZSM-5 become lower. In addition, adding glycerin, the crystal shape of B-ZSM-5 is also changed into the cross-like single crystals. The needed shortest aging crystallization time at 438 K is only 4 h.4. In the synthesis of Ti-ZSM-5 substituted with TiCl4, the effect of different substitution temperatures, times and heat treatment temperatures on the catalytic performance of Ti-ZSM-5 has been well investigated. It is found that there mainly exist isolated framework Ti4+ species and non-perfect framework titanium species. A low substitution temperature favors the formation of non-perfect framework titanium species, which can be converted into tetrahedrally coordinated Ti species and polymerized Ti species (Ti-O-Ti), after heat treatment. In contrast, at high substitution temperature, isolated tetrahedrally coordinated titanium species are easy to be obtained. Based on these results, a conjecture of the incorporation of Ti atoms into the lattice is proposed. In addition, it is also found that adjusting the substitution temperature can effectively control the location of Ti species in Ti-ZSM-5. At low substitution temperature, the active Ti (â…£) species are mainly located on the external surface of Ti-ZSM-5; at high substitution temperature, the location of the active Ti (â…£) species in Ti-ZSM-5 is inside the pore structures.