Design Of Ionic Liquid Supported Palladium-Based Catalyst And Study On Catalytic Performance Of Acetylene Hydrochlorination

It is well known that vinyl chloride is mainly produced by acetylene method in China.In industry,the synthesis of vinyl chloride is mainly based on mercury chloride supported on activated carbon as catalyst.However,the volatility of mercuric chloride leads to the decrease of catalyst activity and environmental pollution.Therefore,the development of mercury-free catalysts is imminent.Palladium-based catalysts have high initial activity in hydrochlorination of acetylene,which has attracted extensive attention of researchers.However,the stability of palladium-based catalysts is not good,which shows that the activity of the catalysts will be reduced or even deactivated after several hours of reaction.The reasons for deactivation can be summarized as follows:on the one hand,palladium easily forms sublimated complexes with acetylene,which leads to the loss of active components;on the other hand,the active species Pd²⁺is easily reduced to Pd⁰without catalytic activity;finally,carbon deposition on the surface of catalyst carrier is also an important reason for catalyst deactivation.Therefore,improving the stability of palladium-based catalysts is the focus of research.In this paper,many studies have been done to improve the stability of palladium-based catalysts.Pd-IL/AC catalysts prepared by adding ionic liquids can effectively improve the activity and stability of palladium catalysts.The optimum loading was found by screening:0.5 wt%for palladium and 10 wt%for IL.At the same time,the most suitable reaction conditions were selected as follows:reaction temperature is 160°C,reaction space velocity is 740 h^-1,V?HCl?/V?C₂H₂?=1.2.Under these conditions,the maximum activity of 0.5Pd-10IL/AC catalyst was 98.6%,55%higher than that of 0.5Pd/AC catalyst.Within 10 h reaction time,acetylene conversion decreased by only 3.4%,while the activity of 0.5Pd/AC catalyst decreased from 43.6%to 10%,and had been completely deactivated.Through the analysis of TEM,XPS,BET,chemical adsorption of CO and other characterization techniques,the addition of IL mainly improves the dispersion of Pd loaded on activated carbon carrier,inhibits the agglomeration of Pd in the reaction process,and protects Pd²⁺species from reduction.At the same time,it was found that the introduction of IL made the highly dispersed palladium species more likely to enter the pore of activated carbon,while the palladium species in the Pd/AC catalyst without IL were mainly distributed on the surface of activated carbon.Through the analysis of TG characterization technology,IL began to decompose at 280°C,indicating that IL was stable at the reaction temperature.ICP results show that the introduction of IL can inhibit the loss of Pd.Combining with the results of long-term evaluation,we conclude that the introduction of IL greatly improves the stability of palladium-based catalysts.In order to investigate the performance and reaction mechanism of palladium-based and gold-based catalysts with IL as well,we evaluated the two catalysts at 180°C with space velocity of 740 h^-11 and V?HCl?/V?C₂H₂?=1.2.The acetylene conversion of 0.55Pd-10IL/AC catalyst was 99%,which was 9%higher than that of 1.0Au-10IL/AC catalyst.When the temperature is 140°C,the same result can be obtained.That is,the activity of palladium-based catalysts is higher than that of gold-based catalysts under the same other conditions with introducing IL.XPS and TEM were used to investigate the reason why the activity of palladium-based catalysts was higher than that of gold-based catalysts with the same number of active centers under the same conditions.It was found that after the introduction of ionic liquids,both palladium-based catalysts and gold-based catalysts were highly dispersed,and the number of active centers was sufficient,so the reason could not be analyzed from the valence and dispersion.Through kinetic study,we found that the HCl reaction order of Pd-based catalysts varied with the feed gas ratio.When V?HCl?/V?C₂H₂?<15,the HCl reaction order of palladium-based catalysts is0.77.When V?HCl?/V?C₂H₂?>15,the HCl reaction order of palladium-based catalysts is 1.54,while the HCl reaction order of gold-based catalysts does not change much,which is 0.80 and 0.73,respectively.That is to say,palladium-based catalysts have more catalytic potential when the HCl concentration is high.Through TPD characterization,we found that Pd-IL/AC catalyst has the same capacity of adsorbing C₂H₂ as Au-IL/AC,but the capacity of Pd-IL/AC catalyst to adsorb HCl is much higher than Au-IL/AC.Therefore,it may be that HCl has a greater positive effect on the adsorption sites of palladium-based catalysts,resulting in higher activity of palladium-based catalysts than gold-based catalysts,and the enhanced adsorption capacity of HCl will also improve the conversion of acetylene.