Preparation And Properties Of Vinyl Acetate Catalyst For Ethylene Vapor Phase Oxidation

Supported palladium-gold catalysts are catalytic material developed in the 1970s. It has a great many applications in the field of vinyl acetate synthesis catalyst with its good selectivity and activity. The supported noble metal catalysts are simple for post-processing, easy separating and recycling, and are consistent with the trend of the green chemistry development.In order to exert supported palladium-gold catalysts performance in the process of vinyl acetate synthesis from ethylene gas phase oxidation, it is significant to choose appropriate carrier for the preparation and application of catalyst. In this paper, the volumetric immersion method was adopted to produce supported palladium-gold catalysts with different carriers, the load rates of palladium-gold for different catalysts were inspected,the influences upon the load rate palladium-gold for supported palladium-gold catalysts by different carriers, the precipitation agent and precipitation pH value were analyzed. Therefore, it defines the catalyst carrier, the precipitation agent and the proper pH value after precipitation.To evaluate the catalytic activity of vinyl acetate synthesis from ethylene gas phase oxidation, a set of fixed bed reactor was installed according to the requirements of experiment, influence on the performance of the supported palladium- gold catalyst by different reaction conditions was focused in the research. The catalyst was characterized by SEM and XRD, and good effect was achieved. The experimental results were summarized as follows:1. The impact of the weight ratio of gold to palladium was significant on space time yield and selectivity of catalyst. The space time yield and selectivity of Pd-Au/4A molecular sieve catalysts were 680g/（L·h） and 92.5% for best value, when the weight ratio of gold to palladium was 0.86. The space time yield of Pd-Au/5A molecular sieve catalysts was 702g/（L·h） for optimal value when the weight ratio of gold to palladium equaled 0.87. The selectivity of Pd-Au/5A molecular sieve catalysts was 92.8% for best values when the weight ratio of gold to palladium equaled 1.13. As the weight ratio of gold to palladium increasing, the conversion of oxygen are rising firstly and then declining. The space time yield of Pd-Au/Al₂O₃ catalysts was 655g/（L·h） for optimal value when the weight ratio of gold to palladium equaled 0.85. The selectivity of Pd-Au/Al₂O₃ catalysts was 91.6% for best value when the weight ratio of gold to palladium was 1.14. With the weight ratio of gold to palladium increasing, the by-products of reaction were increasing and the conversion of oxygen was reducing.2. The influence of reaction temperature on the catalytic activity has been studied in this work. Pd-Au/4A molecular sieve catalysts had the best space time yield of 707g/（L·h） under the condition of 155℃and the best selectivity of 92.3% at 150℃. The space time yield and selectivity of Pd-Au/4A molecular sieve catalysts were 736g/（L·h） and 92.4% for best values under the condition of 155℃. The space time yield of Pd-Au/Al₂O₃ catalysts reached the best value 665g/（L·h） at 140℃and the optimal value of selectivity was up to 90.3% under the condition of 135℃.3. The space time yield of catalyst was impacted by the concentration of oxygen enhanced. When the concentration of oxygen increased, the space time yield improved significantly, so the concentration of oxygen is a control means to enhance the catalytic activity. Compared with Pd-Au/Al₂O₃ catalysts, the reaction selectivity of Pd-Au/molecular sieve based catalysts is better.4. The significant impact of reaction pressure on the catalytic activity was discussed. Pd-Au/4A molecular sieve catalysts had the best space time yield of 709g/（L·h） at 0.72MPa. Pd-Au/5A molecular sieve catalysts had the best space time yield of 723g/（L·h） at 0.70MPa. Pd-Au/Al₂O₃ catalysts had the best space time yield of 672g/（L·h） at 0.60MPa. In a word, the influence of reaction pressure was obvious on the catalytic space time yield, but the experimental results showed that the selectivity of catalysts were less affected.5. It is helpful to improve the space velocity for the activity of the Pd-Au/4A molecular sieve catalysts, Pd-Au/5A molecular sieve catalysts and Pd-Au/Al₂O₃ catalysts. The Pd-Au/5A molecular sieve catalysts are the best catalytic activity within this experiment. The space time yield and selectivity of Pd-Au/5A molecular sieve catalysts are up to 736g/（L·h） and 92.4% respectively at 155℃, 0.70MPa.6. The analysis of SEM and XRD spectra shows that the metal group of catalysts grow in condensation after running for some time, making the dispersion decrease, which is the main reason for domesticated period at the initial stage of catalyst application.