| The selective oxidation of alcohols to the corresponding carbonyl compounds is one of the most important chemical transformations in organic chemistry. Silver-based catalysts are widely used in the selective oxidation of alcohols.In this paper,a series of novel monolithic catalysts of Ag-based catalysts were used in the gas-phase selective oxidation of alcohols on a fixed-bed reactor under atmospheric pressure with molecular oxygen as the oxidant.The research included the following aspects:Firstly,by using papermaking/sintering process,sinter-locked three-dimensional microfibrous networks consisting of～3vo%8μm(dia.) nickel microfibers were utilized to entrap～25vol%100-200μm dia.supported silver catalysts(Ag/α-Al2O3, Ag/γ-Al2O3 and Ag/SiO2) particulates.The reaction of gas-phase selective oxidation for benzyl alcohol was carried out over the monolithic catalysts.It was found that the nickel microfibrous structure with supported silver catalysts exhibited much higher activity and selectivity than the corresponding supported silver catalysts.The excellent catalytic activity of the monolithic catalysts had connection with the novel structure of the metal microfibers.This nickel microfibrous media provides unique three dimensional web structures,large void volume,entirely open structure,large surface-to-volume ratio,high permeability,and high thermal conductivity.Due to these advantages,it can intensify interlayer heat/mass transfer between the supported silver catalysts particulates and then make the reaction bed temperature isothermal to maintain the high activity of the monolithic catalysts in the gas-phase selective oxidation of benzyl alcohol.Secondly,by using the regular papermaking/sintering processes,metal micro fibers entrapped support particulates(α-Al2O3,γ-Al2O3 and SiO2) were prepared. Active Ag component was then deposited onto the surface of the microfibrous structure withα-Al2O3,γ-Al2O3 and SiO2 support particulates by incipient wetness impregnation method,in order to obtain the new-type monolithic catalysts of Ag/α-Al2O3-Ni-fiber,Ag/γ-Al2O3-Ni-fiber,Ag/SiO2-Ni-fiber,Ag/α-Al2O3-Steel-fiber, Ag/γ-Al2O3-Steel-fiber and Ag/SiO2-Steel-fiber.The reactions of gas-phase selective oxidation for butanol,benzyl alcohol and 1,2-propylene glycol were carried out over these catalysts.It was found that Ag/α-Al2O3-Ni-fiber and Ag/α-Al2O3-Steel-fiber exhibited much higher activity than the other catalysts.In the process of gas-phase selective oxidation for benzyl alcohol,the Ag/α-Al2O3-Ni-fiber catalyst exhibited higher activity/selectivity compared to sandwiched bed and the packed bed with Ag/α-Al2O3 particulates.We obtained 25%increase in conversion and 10%increase in selectivity to benzaldehyde.Thirdly,a new-type microfibrous-structure-supported silver catalysts was developed and demonstrated for high efficiency gas-phase oxidation of various alcohols to their corresponding carbonyl products.Sinter-locked three-dimensional microfibrous networks consisting of 3-4 vol%8μm(dia.) Ni fibers or 12μm(dia.) SS-316L fibers and 96-97 vol%void volume were built up by the regular papermaking/sintering processes.Active Ag component was then deposited onto the surface of the sinter-locked fibers by incipient wetness impregnation method to obtain the monolithic catalysts of Ag/Ni-fiber and Ag/Steel-fiber.The gas-phase selective oxidation of alcohols to their corresponding carbonyl products were carried out over the monolithic catalysts.It was found that the optimal Ag loading was 10%and the optimal calcination temperature was 600℃in air.At relatively low reaction temperatures(e.g.,380℃),as-made microfibrous structured silver catalysts provided quite higher activity/selectivity for the oxidation of mono-/aromatic-/di-alcohols compared to the traditional electrolytic silver and Ag/α-Al2O3 catalysts.Finally,catalytically relevant physiochemical properties of the electrolytic silver, Ag/Ni-fiber,and Ag/Steel-fiber catalysts were explored by XRD,XPS,SEM,H2-TPR, O2-TPO,UV-visible diffuse reflectance spectroscopy,and chemisorption techniques. The remarkably high activity and selectivity of the monolithic catalysts were partially attributed to certain amount of Ag+ ions and Agnδ+ clusters.In addition,the microfibrous structure provided a unique combination of large void volume,entirely open structure,excellent heat/mass transfer and high permeability good thermal stability,and therefore led to a significant increase in the steady-state volumetric reaction rate,compared to electrolytic silver and Ag/α-Al2O3 catalysts.Especially,the Ag/Ni-fiber catalysts offered much better low-temperature activity than the Ag/steel-fiber catalysts,mostly due to the synergistic effect between Ag particles and Ni-fiber.The interaction at Ag particles and Ni-fiber interface not only visibly increased the amount of active/selective sites of Ag+ ions and Agnδ+ clusters but also significantly promoted their activity as evidenced by the low-temperature reduction feature.
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