Catalytic Behaviors Of Activated Carbon Supported Copper Catalyst For Oxidative Carbonylation Of Methanol

Dimethyl carbonate（DMC） is an environmentally benign chemical which is used widely in industry in the near future. Activated carbon（AC） has many desirable properties, such as high surface area, abundant pore structure, and possibility to modify the surface functionalities. AC supported Cu catalysts have shown high catalytic activity and selectivity in the synthesis of DMC by oxidative carbonylation of methanol. These chloride-free catalysts can avoid the equipment corrosion caused by chloride elution and have been studied extensively. The recent study prove that the valence distribution and dispersion of Cu species have greatly affected the catalytic activity of Cu/AC catalyst for oxidative carbonylation of methanol to DMC. The low chemical valence of Cu species including Cu2 O and Cu have been reported as the active centers. The Cu²⁺ was autoreduced to Cu2 O and Cu as the active centers during the calcination process of Cu/AC catalyst, which is conducive to the improvement of catalytic activity. Generally, the higher calcination temperature is used, the lower valence of Cu species is formed on AC. However, calcination at high temperature would results in the aggregation of Cu species, and thus the dispersion of Cu species as well as the catalytic activity of Cu/AC catalyst decrease.On the one hand, Cu²⁺ can be reduced by some reductive agent at low temperature, which can avoid the aggregation of Cu species caused by calcination at high temperature. On the other hand, the surface oxygenated groups and nitrogen-containing groups influence not only the dispersion of actvie species but also the interaction between precursor and support resulting in the change of autoreduction temperature for metal oxide. By adjusting the amounts and types of AC surface functional groups can also lower the reduction temperature and avoid the aggregation of Cu species.In this thesis, the Cu/AC catalyst is prepared by polyol reduction process. The influence of reduction temperature, the amount of ethylene glycol and copper loading on the structure and catalytic activity of Cu/AC catalyst is studied. Moreover, HNO₃ oxidation treatment, thermal treatment and N-doped at high temperature are employed to modify the type and amount of surface functional groups on AC. The relationship between the surface functional groups and the structure as well as catalytic activity of Cu/AC catalyst are investigated. The obtained main conclusions are listed as following.（1） The reduction temperature, amount of ethylene glycol and Cu loading have significantly affected the valence distribution and dispersion of Cu species during the polyol reduction process. As the reduction temperature is 160oC, the dominated Cu species is Cu2 O with small particle size. The corresponding catalyst shows the optimal catalytic activity. With the reduction temperature elevated to 200oC, the dominated Cu species is Cu⁰ with bigger particle size and the catalytic activity decreases. The optimal amount of ethylene glycol used to prepare Cu₂O/AC catalyst is 100 mL and the mean crystalline size of Cu2 O is 30.2 nm. The optimal amount of ethylene glycol used to prepare Cu⁰/AC catalyst is 120 mL and the mean crystalline size of Cu⁰ is 33.8 nm. Both the Cu₂O/AC and Cu⁰/AC catalysts show high catalytic activity due to the similar crystalline size of Cu2 O and Cu⁰.（2） The copper loading mainly influenced the particle size of Cu species. As the copper loading is lower than 2.8%, Cu species is highly dispersed on AC support, but the catalysts offer few active Cu species and thus show lower catalytic activity. The high copper loadings（above 2.8%） results in Cu species aggregation and consequent reduction of catalytic activity. The optimal copper loading is 2.8%.（3） The amount of surface oxygenated groups increases with increasing concentration of HNO₃. On the one hand, the increased surface oxygenated groups promote the reduction of Cu²⁺ to Cu⁺ and Cu⁰ as avtive centers. On the other hand, the dispersion of Cu species first increases and then decreases with the increase of surface oxygenated groups on AC. The maintenance dispersion effect of stable groups and the surface mobility effect of Cu species originating from the decomposition of unstable groups determine the dispersion of Cu species. When AC is treated by 4 M HNO₃ solution, the average particle size is 11.8 nm. The dispersion of Cu species and the space time yield of DMC for the corresponding catalyst have attained the highest.（4） The dispersion of Cu species is effectively improved by the increase of surface oxygenated groups on AC. But the formation rate of DMC on unit active surface area of（Cu⁺+Cu⁰） for acid treated catalysts is（1.4-1.6）×10^-7 mol?m^-2s^-1,which is much less than that(2.4×10^-7 mol?m^-2s^-1) of the original Cu/AC catalyst. The deactivation of Cu/AC catalysts is attributed to the agglomeration and oxidation of active Cu species during the reaction process.（5） The thermal treatment temperature influences the specific surface area and graphitization degree of AC. When the thermal treatment temperature is in the range of 400oC-1200oC, no apparent change of the surface area of AC is observed. As the temperature has elevated to 1400oC and 1600oC, the surface area of AC decreases significantly with the enhanced graphitization degree.（6） The selective removal of unstable carboxylic acid and anhydrides groups can effectively improve the dispersion of Cu species over AC support. However, further removal of lactones, phenols, carbonyl/quinones and pyrones is detrimental for the dispersion of Cu species. Meanwhile, the removal of surface oxygenated groups by thermal treatment is conducive to the formation of more π-sites, and thus promote the reduction of Cu²⁺ to Cu⁺ and Cu⁰ as the active centers.（7） The surface area of（Cu⁺+Cu⁰） is improved by thermal treatment of AC, but the formation rate of DMC on unit active surface area of（Cu⁺+Cu⁰） is（0.7-1.5）×10^-7 mol?m^-2s^-1,which is much less than that(2.4×10^-7 mol?m^-2s^-1) of the original catalyst. AC support transforms from hydrophily to hydrophobicity due to the removal of surface oxygenated groups, which is detrimental for the adsorption of CH3 OH resulting in the decreased local concentration of CH3 OH on active Cu species.（8） Thermal treatment of AC under ammonia atmosphere results in that more surface oxygenated groups are removed compared with nitrogen atmosphere. But surface nitrogen groups including pyridinic-N（N-6）, pyrrole nitrogen（pyrrolic-N） and quaternary nitrogen（N-Q） are introduced at the same time, which is conducive to the improvement of Cu species dispersion and methanol absorption capacity of catalyst. Besides, the increased surface nitrogen groups has enhanced the interaction of precursor–support and suppressed the reduction of Cu²⁺.