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

摘要

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,TCP,TG/DTA,FTIR,UV-vis,and ~(29)Si MAS NMR techniques as well as N_2,Ar,and H_2O 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 A.In addition,the local environment of T_1 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 A 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 H_2O_2 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.