Ag-Embedded Microfibrous Structured Packings For Low-temperature Gas-phase Selective Oxidation Of Alcohols

Oxidation of alcohols to carbonyl compounds is a pivotal process in chemical industry, but exigently needs a fundamental shift to the catalytic aerobic oxidation from the polluting methods based on toxic and expensive stoichiometric inorganic oxidants. Gas-phase selective oxidation of volatile alcohols can proceed in continuous packed-bed reactors with higher product yielding rate, being more attractive from the technology and efficiency viewpoints. The development of high-performance catalysts with a unique combination of excellent low-temperature activity/selectivity and high thermal conductivity remains challenging.A promising thin-sheet microfibrous silver based catalyst system has been demonstrated for low temperature gas-phase selective oxidation alcohols. The voidage tunable Ni-fiber (10-99 vol%) microfibrous structure was successfully prepared using regular wet layup papermaking/sintering method. An additional pre-wetting step with water and overnight drying was performed in order to make the Ni-fiber surface easily wettable. The catalyst that resulted in the best reactivity/selectivity was obtained by loading 10 wt% silver onto Ni-fiber followed by calcination in air at 600℃for 4 h. It was found that enhanced wettability of the Ni-fiber support was crucial for the incorporation of more Ag cations and selectively active oxygen species of the corresponding Ag/Ni-fiber-M catalyst than the Ag/Ni-fiber catalyst using untreated support. As a result, the Ag/Ni-fiber-M catalyst exhibited much better low-temperature activity/selectivity than the Ag/Ni-fiber catalyst. In addition, the microfibrous-structured silver catalyst was shown to provide significant promotion of steady-state volumetric reaction rate compared to the electrolytic silver, attributable to the large voidage and high heat/mass transfer. Deactivation of the catalyst was attributed to the formation of NixC compounds that likely cut off O2 activation/transportation between the support and silver particles. Spent catalyst was successfully regenerated through an oxidative treatment at 400℃.In order to avoid the formation of metal cabides, the copper-microfiber was selected as the support to prepare copper-microfibrous-structured silver catalyst, because the copper-microfiber not only has poor ability to dissolve carbon, but also exhibits some catalytic reactivity for the alcohol oxidation. The highest conversion of 95.5% was obtained with target product selectivity of 96.7% over the catalyst 10%Ag/Cu-fiber at 330℃, while the Cu-fiber also exhibited an excellent reactivity with the conversion of 81.2% and the selectivity of 97% under the identical conditions. This result suggested that copper is the active component and silver plays the role as additive. O2-TPD and XRD results showed that silver can increase and maintain the number of the active site Cu+. H2-TPR indicated that the introduction of silver can enhance the catalytic activity of Cu+. During longer-term test, the aggregation of the Ag and Cu2O as well as the formation of deposited carbon caused catalyst deactivation.