| Titanosilicate TS-1 is the first typical titanosilicate zeolite catalyst,which has 10-membered oxygen ring(10MR)micropores,MFI framework and isolated Ti ions tetrahedrally coordinated in the framework.TS-1 has attracted great research interest in the past decades because of its important and unique catalytic redox properties,making it widely used for selective oxidations with hydrogen peroxide as the oxidant,such as epoxidation of propylene,ammoximation of cyclohexanone and hydroxylation of phenol and benzene.However,the 10 MR micropores strongly limit the catalytic applications of TS-1 with bulky substrates and organic hydroperoxides involved,due to the easy pore bolocking by heavy by-products.One successful attempt to overcome the diffusion problem is to fabricate the TS-1 zeolite with a hierarchical porosity.As a new type titanosilicate TS-1,the effects of structure properties,such as micropore diffusion length,external surface area,hierarchical porosity and skeletal Ti distribution,on the catalytic epoxidation performance of hierarchical TS-1 are still unclear.Herein,a series of TS-1/ hierarchical TS-1 zeolites with designed micropore diffusion lengths and external surface areas were synthesized,following with the fundamental study of the structure-property relationship of the obtained TS-1 catalyst in the epoxidation of various bulky alkenes.(1)Synthetic design: Pure microporous titanosilicate TS-1 with particle sizes of 2 μm and 200 nm were prepared.By using steam-assisted crystallization,three-dimensionally ordered mesoporous-imprinted titanosilicate(3DOm-i TS-1)was prepared,and its micropore diffusion length was 20 nm;The TS-1 titanosilicate(SPP TS-1)with hierarchical nanosheets self-pillared by using tetrabutyl hydroxide as the structure-directing agent,its micropore diffusion length was 2 nm.A series of TS-1/ hierarchical TS-1 zeolites catalysts with a gradient increase in micropore transmission path from 2 nm to 2 μm and an increase in external surface area from 10 % to 40 % were obtained.(2)Structure regulation: 3DOm-i TS-1 was prepared with 10,20 and 40 nm to realize the fine control of the micro/mesoporous transfer distance of the hierarchical TS-1 zeolites,which has the same structure,and the micropore diffusion lengths can be in the range of 10-40 nm,and three-dimensionally ordered mesoporous can be controlled in the range of 5-10 nm.SPP TS-1 zeolites with Si/Ti ratios of 25,50,75,and 100 were prepared to achieve same multi-level structure,the Si/Ti ratios of the hierarchical TS-1 zeolites are finely controlled.(3)Evaluation of selective oxidation reaction: Bulky olefins(1-hexene,cyclohexene and cyclooctene)were used as probe molecules to systematically evaluate micronsized,nanosized,three-dimensionally ordered mesoporous-imprinted titanosilicate and hierarchical nanosheets self-pillared TS-1 which participates catalytic performance in selective epoxidation of bulky olefins.The results showed that with the sequential decrease of the micropore diffusion lengths,the increasing proportion of the external surface area,the catalytic activity(conversion and selectivity)of the bulky olefin selective oxidation increased in turn.The regulation of the Si/Ti ratios and reaction evaluation showed that STP TS-1 with Si/Ti ratio of 50 had the best catalytic performance.(4)Construction of the structure-property relationship: In this paper,the influences of the micropore diffusion length,ratio of internal/external surface,hierarchical porosity and the distribution of Ti active sites on the bulk/external surface of selective oxidation of bulky olefins are studied and designed.The quantitative structure-property relationship between the porous structure of the titanosilicate zeolites and its ability to participate in the selective oxidation of bulky olefins was systematically established. |
