Preparation And Catalytic Evaluation Of Mercury-Free Low-Gold-Content Catalysts For Acetylene Hydrochlorination

Highly toxic mercury catalysts used for acetylene hydrochlorination to produce vinyl chloride pollute the environment seriously and it is hazardous to human health. Therefore, the development of environmentally friendly mercury-free catalysts becomes the most important issue in the entire vinyl chloride industry.In this paper, HAuCl4·4H2O and CuCl2·2H2O were the precursor of active ingredients, supported catalysts were prepared by ultrasonic-assisted impregnation method for acetylene hydrochlorination to VCM. Influences of the supports (activated carbon, Al2O3 and TiO2), loading amount and synergistic effect of the active components were investigated. Besides, we investigated the effects of the loading order of the active ingredients and TiO2 additive on the existing form of active ingredients and catalytic activity of catalysts. We have obtained the following results:(1) We prepared Au-Cu/C (Al2O3 and TiO2) catalysts by ultrasonic-assisted impregnation method. XRD analysis shows that Au active component supported on activated carbon exists in the form of Au3+and on Al2O3 and TiO2 exists in the form of Au0. BET results indicated that Au active component supported on activated carbon exhibits larger specific surface area (562 m2/g) than that of Al2O3 (113 m2/g) and TiO2 (8 m2/g), which improves the dispersion of Au active component. After the picking of the best support activated carbon, further to explore the effect of loading amount on the existing form of active ingredients and catalytic performance of catalysts. With Au/Cu mass ratio of 0.25/4, the Au(0.25%)Cu(4%)/C catalyst exhibits good catalytic performance.(2) The interaction of the active ingredients Au and Cu were investigated. TPR results indicated that the reduction activation energy of Au3+â†'Au0 in Au-Cu/C (45.5 kJ/mol) is much higher than that of Au/C (35.6 kJ/mol), which indicates that the Au3+ species in Au-Cu/C catalyst is more difficult to be reduced due to the doped CuCl2. Au-Cu/C possesses more Au3+ species (28%) than Au/C (16%) determined by XPS. Furthermore, we have deduced the catalytic mechanism of Au-Cu/C catalyst.(3) The effect of Au and Cu loading order on the existing form of active ingredients and catalytic performance of catalysts were discussed. The results showed that Au-Cu/C catalyst exhibits better catalytic performance than Cu/Au-C and Au/Cu-C catalysts. Loading Au and Cu at the same time conductive to the interaction between Au and Cu and improves the dispersion of Au active component, so more Au3+species exist on the catalyst surface. The best catalytic performance was obtained over Au(0.25%)Cu(4%)/C catalyst which achieved 97.4% acetylene conversion after 3 h and the selectivity to VCM maintained above 99.0% in 150 h.(4) The effects of TiO2 on the catalytic activity of Au-Cu/C catalysts were investigated. The experimental results showed that Au(0.45%)Cu(4%)TiO2(9%)/C catalyst with 9% TiO2 exhibits the best catalytic performance. TPR results showed that interaction among Au, Cu and TiO2 makes the Au3+ species increasing and more difficult to be reduced. XPS results demonstrated that TiO2 provide electron for Au catalysts and improved the adsorption capacity of HCl reactant, so exhibits better catalytic performance and high selectivity to VCM.