Fabrication Of Hierarchical TS-1 Zeolite Monoliths For Enhanced Benzene Hydroxylation

Phenol is an important industrial commodity that has been widely used for production of resins,detergents,herbicides and pharmaceuticals.Currently,phenol has been mainly produced from benzene via cumene to cumene hydroperoxide(the so-called cumene process),leading to a low phenol yield with high energy consumption.The one-step hydroxylation of benzene to phenol is an attractive route due to its energy saving and environmentally-friendly characters.Three oxidants could be utilized in the direct hydroxylation of benzene to phenol,including molecular oxygen,nitrous oxide,and hydrogen peroxide.The use of hydrogen peroxide possesses a prominent advantage that the reaction can be carried out in a liquid phase under mild conditions,and water is the only by-product.Zeolites,supported metal,organic complexes can be used as catalysts.TS-1 zeolite exhibits superior catalytic performances for one-step benzene hydroxylation.However,the intracrystalline diffusion inhibits the access of reacting molecules to most active sites within zeolitic pores and the removal of product molecules from TS-1 catalysts,keeping both reaction rate and selectivity low.Hierarchically porous zeolites with auxiliary porosity are favourable to overcome the dilemma of low-level conversion and selectivity.Commercial catalysts are shaped into mechanically stable pellets using binder,which usually has negative effect to catalytic performance.Taking the above into consideration,we came up with a novel method to synthesize TS-1 monolithic matrices with a hierarchical pore structure by using the ice-templating of TS-1 crystal grains followed with the steam-assisted crystallization of the formed monolithic bulk.And two different monoliths with different structures were prepared by two processes.The ice-templating of porous structures by the solidification of water is an environmentally-friendly way to build hierarchically-structured bulks,the engineering of pore morphologies and their connections could be achieved by changing some variables e.g.freezing rate and solute concentration.The wealth of pore morphologies and the directional macropores together with easily accessible meso-and micro-pores make these ice-templated materials promising in heterogeneous catalysis.The catalytic performance of the TS-1 monolithic matrixes in benzene hydroxylation has been checked using a loop flow reactor.The results showed that both two hierarchical TS-1 monoliths exhibited better catalytic performance compared with TS-1 crystal grains.As for the reaction under 80�C for 4 h,93.5% selectivity and 24.6% yield to phenol were achieved in the loop flow reactor equipped with the TS-1 monolith constructed by the second process,accounting for the 18% and 10% increases over those in the traditional slurry reactor with TS-1 crystal grains.