Carbon-/sodium-modification Of Monolithic Pd/AlOOH/Al-Fiber Catalysts For The DMC Synthesis Via Carbonylation Of Methyl Nitrite

The dimethyl carbonate?DMC?synthesis via carbonylation of methyl nitrite?MN?has little pollution and high atomic utilization efficiency,but the commonly used Pd-based catalysts have several problems such as poor repeatability,poor stability and low activity.In case of the strongly exothermic carbonylation of MN to DMC,the above-mentioned Pd catalysts usually suffer from the poor intraparticle/interbed mass/heat transfer.As a result,generation of high-temperature“hot-spots”is inevitable.Structured catalysts,especially metal-fiber structured ones,have been proved to effectively avoid generating high-temperature“hot-spots”.Therefore,it is of great research significance and application value to develop structured catalysts with excellent catalytic performance and enhanced heat/mass transfer for DMC synthesis via the carbonylation of MN by loading active components and auxiliary agents on the surface of metallic fiber/foam substrates.In this paper,a typical three-dimensional network structure of the metal aluminum fiber?Al-fiber?was treated in steam for endogenous growth of a uniform honeycomb-like shell of two dimensional?2D?AlOOH nanosheets?ns-AlOOH?,with the aid of the fundamental oxidation reaction:2Al+4H₂O?2AlOOH+3H₂.Al-fiber structured Pd catalyst with good catalytic performance for the MN carbonylation to DMC was developed by impregnation with Pd and then carbon/sodium modification.We systematically investigated the effects of preparation conditions,modification of carbon/nitrogen additives and reaction conditions on catalytic performance of the catalysts.By means of CO₂-TPD,O₂-TPO,XPS,CO-FTIR and other characterization techniques,the nature of carbon-/sodium-modification of monolithic Pd/AlOOH/Al-fiber catalysts was investigated.The main results are as follows:?1?Monolithic Pd@C/AlOOH/Al-fiber for DMC synthesis via carbonylation of MN.We systematically investigated the effects of the preparation conditions and reaction conditions on the catalytic performance of monolithic Pd@C/AlOOH/Al-fiber catalysts for the MN carbonylation to DMC.After optimization,the Pd loading of the catalysts was 0.24wt%,the carbonation temperature with ethylene as carbon source and the calcination temperature in the air were 300? and 250?,respectively.Such catalyst can achieve⁹9.9%/⁷0.6%MN/CO conversion with MN-based DMC selectivity of 39.1%or CO-based DMC selectivity of 39.6%and is stable for at least 100 h without deactivation sign at 220? and a high GHSV of 25000 L kg^-1 h^-1 for a feed gas of CO/MN/N₂?7/10/83,vol/vol?mixture.According to the characterization and analysis of Pd@C/AlOOH/Al-fiber catalysts,it is found that the dissolved carbon can combine with Pd species to form PdC_x species.And the PdC_x species show ability to improve the linear CO adsorption on the surface of Pd species,which is important for the enhanced performance in the carbonylation of MN to DMC.?2?Na-modified monolithic Pd@C/AlOOH/Al-fiber for DMC synthesis via carbonylation of MNWe also investigated the effects of the preparation conditions and reaction conditions on the performance of Na-modified monolithic Pd@C/AlOOH/Al-fiber for the MN carbonylation to DMC.After optimization,the Na/Pd molar ratio was 10.8,the suitable precursor of sodium was sodium nitrate?NaNO₃?,the carbonation temperature with ethylene as carbon source was 350?,and the calcination temperature in the air was 250? when the Pd loading of the catalysts was 0.24 wt%.Such catalyst can achieve⁴9.7%/³9.7%MN/CO conversion with MN-based DMC selectivity of 54.3%or CO-based DMC selectivity of 46.8%and is stable for at least 100 h without deactivation sign at 200? and a high GHSV of 25000L kg^-1 h^-1 for a feed gas of CO/MN/N₂?7/10/83,vol/vol?mixture.According to the characterization and analysis of Na-modified monolithic Pd@C/AlOOH/Al-fiber catalysts,the fundamental reason for the high DMC selectivity obtained by Na-modified catalysts lies in the electronic modification of Pd by Na,which enhances the interaction between Pd and CO molecules.And the strong interaction between Pd and CO molecules can promote the activation of linear adsorption CO,which is responsible for the improved selectivity to DMC.