Study On The Preparation Of Novel Photocatalysts And Their Synergistic Catalysis-adsorption Effect With Molecular Sieves

The products of cyclohexane oxidation,cyclohexanol and cyclohexanone,are raw materials to produce monomers for nylon polymers production.However,the conventional thermocatalytic oxidation system adopts strongly high toxic and corrosive oxidants,and generally induces high energy consumption and serious environmental pollution.Comparison with that process,the photocatalytic oxidation of cyclohexane is a new green chemical engineering which can proceed at ambient temperature and pressure using oxygen or air as oxidant.Therefore,the photocatalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone has been one of the research hotspots in phototcatalytic organic synthesis field.At present,the research on photocatalytic cyclohexane oxidation focuses on the design and modification of photocatalyst.The oxidizing capacity of photocatalyst for the partial cyclohexane oxidation is limited in order to avoid over oxidation,which has an adverse impact on the product yield.So it has important and practical significance to further study on the increase of target products without changing the activity of photocatalyst.With the aim at improving the product yield of photocatalytic cyclohexane oxidation,a novel visible-light-induced photocatalyst has been prepared and its synthetic and photocatalytic mechanism has been explored.In addition,molecular sieve powder as an partner for photocatalyst is added in the system of cyclohexane oxidation,which plays a great role in increasing the yield of target products.On the one hand,the participation of molecular sieve can improve the concentration of oxygen in the liquid phase.On the other hand,the products with higher polarity relative to cyclohexane can adsorb preferentially on the molecular sieve and be restored,which results in promoting the product yield.Based on the synergistic effect,photocatalyst and molecular sieve were integrated to improve their recycle ability.The main research described above has been summarized as follows:Firstly,the nonmetal doped Cr₂O₃ photocatalysts with visible-light catalytic activity for cyclohexane oxidation were first synthesized from a chromium-containing MOF material MIL-101?Cr?.The novel C-N dual-doped Cr₂O₃ photocatalyst was synthesized using a two-step method,including premier carbonization in nitrogen atmosphere and succeeding calcination in air.The C-doped Cr₂O₃ was obtained only by calcination.Nitrogen doping occurs in the carbonization process with nitrogen gas acting as nitrogen source,while carbon doping happens during the two steps with organic ligands acting as carbon source.In the process of synthesis,carbonization temperature is a crucial factor,which determines the doping amounts of two elements and morphologies of the samples.Both C-doped Cr₂O₃ and C-N dual-doped Cr₂O₃ have photocatalytic activity for cyclohexane oxidation with molecular oxygen under visible light irradiation.And photocatalytic activity increases with the increase of doping amounts.On the one hand,the doped carbon and nitrogen substituted for the oxygen sites in Cr₂O₃ form a new isolated narrow band above the valence band which is responsible for the visible light sensitivity.On the other hand,the oxygen molecular can be transformed by oxygen vacancy to be superoxide radical which has strong oxidizing capacity to oxidize cyclohexane.The increase of nonmetal doping amounts can significantly increase the concentration of oxygen vacancy,thus improving photocatalytic activitySecondly,four kinds of molecular sieves were synthesized,namely NaX,NaY,Beta60 and Beta400 molecular sieve.The coexist of as-synthesized molecular sieve with the novel visible-light-induced photocatalyst for the partial oxidation system of cyclohexane enhanced the product yield.Molecular sieves possess high specific surface area and adjustable adsorption properties.Oxygen molecules are adsorbed and restored by molecular sieves participarted in reaction with the photocatalyst,which increases oxygen concentration in the reactor and improves oxygen transformation more efficiently.Moreover,the product with higher polarity relative to cyclohexane can adsorb preferentially on the molecular sieve.Therefore,chemical equilibrium can be tuned with the participant of molecular sieve in the photocatalytic system.The higher specific surface area and lower aluminium-silicon ratio of molecular sieve are the excellent properties for involving in the photocatalytic system.Results show that NaY and Beta60 molecular sieves are the most excellent participants for the photocatalytic oxidation of cyclohexane.Finally,the photocatalyst and molecular sieve were integrated into core-shell and layer-by-layer structures respectively with enhanced recycle ability and catalytic activity.The catalyst-adsorbent with core-shell structure was composed of molecular sieve as core and C-doped Cr₂O₃ as shell.The spherical molecular sieve was synthesized using resin as template,then MIL-101?Cr?was adhered to its outer surface using starch as organic binder.After calcination,C-doped Cr₂O₃ shell layer was formed with both organic ligand and binder acted as carbon source.The layer-by-layer catalyst-adsorbent was supported on the stainless steel net.The first molecular sieve layer was synthesized by secondary growth method and the second C-doped Cr₂O₃ layer was supported on its upper surface using the same process mentioned above.The integral whole of photocatalyst and adsorbent improved photocatalytic performance due to the promotion of the separation and storage of reactant and product in comparison with the cases of cyclohexane oxidation using photocatalyst and molecular sieve powder.The cyclohexanol and cyclohexanone yields are raised about 1.19 times with the core-shell system,while those of the layer-by-layer system are increased to about 1.02 times.