Synthesis and catalytic properties of a new titanosilicate molecular sieve with the structure analogous to MWW-type lamellar precursor

A new titanosilicate molecular sieve structurally analogous to MWW-type lamellar precursors (designated as Ti-YNU-1) was postsynthesized by adjusting the titanium content in the synthesis gel and washing the as-synthesized material with acid under refluxing conditions. The samples were characterized with XRD, ICP, TG/DTA, FTIR, UV–vis, and 29Si MAS NMR techniques as well as N2, Ar, and H2O adsorption experiments. It was indicated that compared with Ti-MWW, Ti-YUN-1 had more silanols in the structure and that its interlayer spacing was expanded by nearly 2.5 Å. In addition, the local environment of T1 sites of the proposed structure of Ti-YNU-1 was also different from that of Ti-MWW. This led to a great increase in the pore openings connected to supercages. The formation of Ti-YNU-1 was highly dependent on the Si/Ti ratio in the synthesis gel and also might be related to the removal of about 80% templating molecules by acid treatment before calcination. Extensive evaluation of the catalytic performance of various titanosilicates for the oxidation of different cycloalkenes showed that Ti-YNU-1 behaved like a 12-membered ring (MR) zeolite. This is further confirmed by the results that a strong peak around 6.7 Å was present in the pore size distribution curve obtained from Ar adsorption for the Ti-YNU-1 sample but absent in that for the Ti-MWW sample, and that cyclohexene conversion increased but 1-hexene conversion decreased with increasing Si/Ti ratio. Compared with the other titanosilicate materials, Ti-YNU-1 showed high activity, selectivity, and stability in the liquid-phase epoxidation of bulky cycloalkenes with H2O2 as an oxidant. An investigation of the effect of several protic and aprotic solvents on the catalytic property of Ti-YNU-1 in the epoxidation of cyclohexene revealed that acetonitrile should be the solvent of choice considering both conversion and selectivity.