Synthesis,Morphology Control And Catalytic Performance Evaluation Of W-substituted Pentasil Zeolite

Epoxide compounds are important chemical intermediates which have been widely used in fine chemical industry,petrochemical industry and organic synthesis.The main production pathway is from the selective oxidation of olefins and cycloolefins,and hydrogen peroxide could be considered as an ideal oxidant in this process due to its mild,eco-friendly and economical property.Besides,high-performance heterogeneous catalysts with high selectivity and excellent regenerability are highly desired,and Ti-substituted zeolites have been proven as catalyst candidates in previous study.Among hundreds of zeolite frameworks,Pentasil zeolites,which possess particular fivemembered ring in the framework,show considerably high performance in the selective oxidation of olefins due to their high crystallinity,excellent stability and flexibility of framework leading to easy introduction of framework Ti active sites.And the most widely used oxidation catalysts of Pentasil zeolites are TS-1(Titanium Silicalite-1,MFI framework)and TS-2(Titanium Silicalite-2,MEL framework).However,the conventional TS-1 and TS-2 zeolites show very limited activity in the selective oxidation with bulky substrates,such as cycloolefins,due to most of the active sites distributed in their micropores(pore diameter of ?0.55 nm)are inaccessible to the bulky substrates.On the other hand,considerable amount of silanol groups are distributed on the surfaces of TS-1 and TS-2 after substitution of titanium,resulting in more hydrophilic surfaces.Most of the active sites could be occupied by water molecules when using hydrogen peroxide as oxidant,leading to insufficient utilization of Ti active sites.In addition,undesired hydrolysis of epoxide products could happen due to the excessively strong acidity of Ti-substituted Pentasil zeolites.In view of this,the modification of Pentasil zeolites and fabrication of new-type epoxidation catalysts with hierarchical porosity,strong hydrophobicity and feasible oxidizability and acidity are highly desired.The object of this study is to extend the one-pot direct synthesis of W-substituted hierarchical Pentasil zeolites,and the hydrophobicity and catalytic property will be evaluated by selective oxidation of cycloolefin(cyclopentene,cyclohexene and cyclooctene).1.W-substituted Pentasil zeolite of MEL framework(W-MEL)was synthesized.The characterization results showed that the obtained W-MEL were with high crystallinity and pure phase of MEL topology,and the particles were in almond shape.W-MEL showed more hydrophobic surfaces than siliceous MEL and TS-2.Benefitted by the structure hydrophobicity and proper oxidizability and acidity,W-MEL much higher conversion of cycloolefins and considerably increased selectivity of epoxides comparing with Ti-substituted MEL catalysts(micron-sized and nano-sized TS-2).2.W-substituted self-pillared Pentasil nanosheets(W-SPP)by epitaxial growth of MEL and MFI were first synthesized.W-SPP showed a typical hierarchical porous morphology and the self-pillared nanosheets constituted a unique house-of-cards structure.The synthesis optimization were performed by inspecting the kinetics of crystallization,composition of the synthesis gel,and silicon and tungsten sources.The obtained W-SPP were used as catalyst for the selective oxidation of cycloolefins,and showed improved performances than Ti-substituted MFI(micron-sized and nano-sized TS-2)due to its feasible oxidability and enhanced hydrophobicity by effective inhibition of silanol groups.Moreover,the passivated W-SPP showed limited activity in the epoxidation of cyclooctene indicated that most of W active sites were distributed on the external surfaces of W-SPP.From the perspective of synthetic design,heteroatom substitution was achieved for the structural modification of Pentasil zeolites,and two types W-substituted Pentasil zeolites of high crystallinity,strong hydrophobicity as well as controllable morphology were successfully synthesized.Furthermore,the synthesized W-substituted Pentasil zeolites were used as catalyst for the selective oxidation of cycloolefins by using hydrogen peroxide as oxidant,and exhibited improved catalytic performances than the corresponding Ti-substituted Pentasil catalysts.It can be expected the demonstrated W-substituted zeolite materials will provided a basis for quantitatively studying the surface hydrophobicity and mass-transfer limitation on the catalytic epoxidation performance of zeolite catalysts.