Study On Structure And Catalytic Performance Of Cu/AC Catalysts After Modification Of Activated Carbon

Dimethyl carbonate (DMC) is an environmentlly benign green-chemical product with a wide range of applications, as well as versatile chemical properties by the presence of a carbonyl group and two methoxy groups in its molecule. The synthesis and application of DMC is regarded as advanced topics which has recently attracted much attention of scholars. Several approaches have been known for DMC production so far. Among them, the direct oxidative carbonylation of methanol with CO and O2 has shown to be the most promising processes for the production of DMC, which is consider of a simple synthetic process, non-corrosive to equipment and easy to separation. Our group has got some results in the chlorine-free catalyst research:The supported catalysts with different valent states of copper prepared by Cu(CH3COO)2 were dispersed uniformly on the surface of AC. The catalytic activities of Cu2O supported via pyrolysis of Cu(CH3COO)2 were studied on oxidative carbonylation of methanol to DMC, and it has high catalytic activity and stability. It also avoids the problem of residual chlorine in the CuCl2 catalyst and Wacker catalyst. Many studies have shown that although the chlorine catalyst has higher catalytic activity, it is limited in industrial application beacause of the quick deactivation and heavy corrosion coused by losing Cl. In this thesis, the effect of modification of AC, heat-treatment temperature and the formation of surface copper species on the catalyst were investigated. Also the adsorption capacity of CO, O2, CH3OH and desorption activation energy of catalyst were studied. XRD, H2-TPR, XPS, TG, FT-IR and TPD were used to characterize these catalysts. After comparing the results of evaluation and characterization, the following conclusions were obtained:(1) The effects of modification of AC was investigated on the catalytic activity. The results showed that the physical properties (such as specific area, pore size distribution) and chemical surface properties which affect the distribution of surface functional groups and content were changed. It may cause different hydrophilic properties of AC and different dispersion of Cu(CH3COO)2 on the surface of carrier. The results of the characterization indicated that Cu(0)/Cu(Ⅰ)/Cu(Ⅱ) formation, distribution, morphology and content during the calcination were affected by AC modification. The unsaturated functional groups (C=N,C=C) increased and hydrophobic nature enhanced after AC modification of ammonia. The space time yield of DMC and conversion based on methanol was reached 152.81mg·g-1·h-1 and 7.42% showing high activity while Cu(0):Cu(Ⅰ):Cu(Ⅱ)=1:2.55:4.58. (2) Different valence states of copper species spreading on the surface of carrier shown catalytic. With the heat-treatment temperature increased gradually, the valence states of copper species on the catalyst surface decreased and Cu(CH3COO)2 was reduced in a deeper level. The results indicated that catalyst heat treated at 405℃exhibite high catalytic activity which contains crystalline Cu0 and Cu1 and highly dispersed CuⅡ. The copper species in the fresh catalyst transformed into its oxidation state gradually in the process of oxidative carbonylation of methanol.(3) The pulse adsorption of CO, O2 and CH3OH were measured on the catalysts with pure Cu/Cu2O/CuO loading. The results showed that the type of active species and adsorption capacity were related to the catalytic activity. The degree of the effect of adsorbate on the catalytic activity is CH3OH>CO>O2.(4) According to CO-TPD, O2-TPD and CH3OH-TPD patterns of the catalyst heat treated at 405℃, the amount of CH3OH adsorption is far more than CO and O2. The desorption activation energy of CH3OH based on peak temperature value with different temperature programmed rate is E=44.8±1.3kJ/mol.