Study On Low-loaded Palladium-Based Carbide Catalyst For Selective Hydrogenation Of Acetylene

Ethylene is the core and basis of the petrochemical industry,while ethylene production in China is heavily dependent on crude oil.Because our country has an energy distribution with plentiful and cost-effective coal reserves but a shortage of crude oil and natural gas,the development of ethylene production with coal as raw material has important strategic significance.At present,with the application of new regenerative calcium carbide production process,low-cost acetylene can be obtained in industry.Therefore,the research and development of efficient and stable catalysts for selective hydrogenation of acetylene has become the key to perfectly commercialize the coal-acetylene-ethylene route.In this paper,low-loaded Pd-based catalysts are studied,and the performance of metal carbides in the selective hydrogenation of acetylene is mainly explored.The specific research contents are as follows:Firstly,supported Pd C catalyst was prepared with N-containing silane coupling agent AAS as modifier.It is found that the as-prepared Pd C particles are more stable and exhibit higher ethylene selectivity than the in situ generated palladium carbide species during hydrogenation reaction,with ethylene selectivity being 15%higher even than that of Pd Ag alloy at 240°C.The catalyst showed good stability within 200 h.Penetration of C atoms can inhibit the migration of active bulk H to the surface of catalyst particles on one hand,and weaken the adsorption of ethylene on the other hand,thus suppressed further hydrogenation of ethylene.At the same time,there is a higher energy barrier on Pd C for polymerization,which makes the catalyst performance with lower C4 and green oil selectivity,thereby hindered carbon deposition on the catalyst.Secondly,various S-containing silane coupling agents with different coordinating ability were employed as regulators to prepare Pd C particles with variable particle sizes,and the effect of Pd C particle size on acetylene hydrogenation was studied.It was found that smaller Pd C particle size is beneficial to inhibit the generation of ethane,however the smaller Pd C particle size is prone to increase the generation of C4 and green oil to a certain extent.The maximum amount of polymerization product was generated as the particle size was reduced to around 2.0 nm,and the formation of C4 and green oil was inhibited if the size was larger or smaller.At the same time,DFT calculations reveals from the molecular level that ethylene is adsorbed weakly on Pd C particles with small particle size,and the energy barrier of hydrogenation to ethane is higher than that of larger particles.As the particle size is reduced to around 2.0 nm,it has the smallest C4generation energy barrier.Finally,the non-precious metal Fe carbide Fe₅C₂ was prepared,and its performance as carrier for Pd in acetylene hydrogenation was studied.It was found that Fe₅C₂ has an extremely strong inhibitory effect on the formation of ethane,and the ethylene selectivity is 30%-55%higher than that of the traditional alumina-supported Pd catalyst under the same reaction conditions.DFT calculation shows that the energy barrier of ethylene hydrogenation to ethane on Fe₅C₂ surface is extremely high,and the surface still maintains this amazing property after loading a small amount of metal Pd（0.0005～0.003wt%）,thus effectively inhibited the formation of ethane during hydrogenation.At the same time,after loading Pd,the energy barrier to the formation of polymerization products on the catalyst surface is increased.It would decrease the generation of C4 and green oil and delay carbon deposition,which is beneficial to improve the stability of the catalyst.