Monolithic Metal Fiber-/Foam-Structured Pd-/Ni-Based Catalysts For Selective Hydrogenation Of Acetylene To Ethylene

Selective hydrogenation of acetylene is essential in the purification of ethylene,as the elimination of acetylene,an undesired by-product in the ethylene from thermal cracking of naphtha,is necessary for the downstream industry,especially for the production of polyethylene.Palladium?Pd?based catalysts are the most effective catalysts for this reaction nowadays,but suffer from the high cost.In addition,hydrogenation of acetylene is strongly exothermic and traditional particulate catalysts are weak in the removal of reaction heat,which is harmful to the selectivity and stability of catalysts.Therefore,it is particularly desirable to develop catalysts with high catalytic performance and thermal conductivity for the selective hydrogenation of acetylene.Recently,the structured catalysts have been developed rapidly because of their enhanced mass and heat transfer performance.Notably,the interaction between the active sites and the metal substrates can influence the structure of active sites and further improve the catalytic performance.Therefore,a series of metal substrates are applied in the catalyst design for the selective hydrogenation of acetylene to achieve the unique combination of high catalytic performance and enhanced heat/mass transfer.The main contents are summarized as follows:?1?Structured Pd/AlOOH/Al-fiber catalyst and its catalytic performance for selective hydrogenation of acetyleneIn our previous work,the AlOOH/Al-fiber substrates?60?m dia.Al-fiber?were synthesized by endogenous growth method and effectively promoted the activity of the as-supported Pd catalysts for the CO coupling to dimethyl oxalate reaction.Therefore,the structured Pd/AlOOH/Al-fiber catalysts are prepared by incipient impregnation method over the substrates and applied in the back-end selective hydrogenation of acetylene.The activity and stability of catalysts show interesting substrate annealing temperature dependence,which varies the amount of surface hydroxyl.Low substrate annealing temperature?100 ??enables the catalysts with hydroxyl-rich surfaces that provides more and stronger acid sites.This feature in turn strengthens the hydroxyl-Pd interaction to make the surface Pd species relatively electron-deficient,markedly enhancing the activation of hydrogen.Moreover,the hydroxyl promotes the adsorption of acetylene.As a result,the catalyst with the AlOOH/Al-fiber annealed at 100 ? exhibits two times turnover frequency(TOF,0.0167 s^-1)as that(0.0083 s^-1)of the catalyst with substrates annealed at 600 ?,and achieves full acetylene conversion under the back-end conditions at 70 ? with only 0.045 wt%Pd loading.In addition,the surface hydroxyl obviously suppresses the polymerization of acetylene,significantly improving the catalytic stability.?2?Structured Pd/Ni-foam catalysts and the modification effect of Mg/Zn on the selective hydrogenation of acetyleneThe Ni-foam?110 PPI?with high permeability and high thermal conductivity is employed to tailor the structured Pd/Ni-foam catalysts via simple incipient impregnation method.The Pd/Ni-foam catalyst presents high ethylene selectivity but low activity in the back-end process of the selective hydrogenation of acetylene.To improve the activity of the catalysts,Mg and/or Zn are introduced via co-impregnation method with Pd.The modification of Mg/Zn promotes the dispersion of Pd nanoparticles,improving the activity of catalysts.Moreover,the strong interaction between MgO and Ni-foam decreases electronic density of the surface Ni atoms,which promotes the electron transfer from Pd nanoparticles to the substrate.The electron deficiency of active sites results in further increase of catalytic activity but significant decrease in the ethylene selectivity.And meanwhile,the addition of Zn not only leads to the formation of PdZn alloy,but also weakens the MgO-Ni interactions,both of which improve the ethylene selectivity.The Pd-Mg-Zn/Ni-foam catalysts achieve high catalytic activity and ensure the high ethylene selectivity at high reaction temperature.In addition,the ethylene selectivity of Pd-Mg/Ni-foam catalyst increases with the reaction time due to the carbonaceous deposit on the catalyst surface.?3?Structured Pd/brass-fiber catalysts for selective hydrogenation of acetylene:Effect of calcination-assisted endogenous growth of ZnO-CuO_x on brass-fiberFor the selective hydrogenation of acetylene under front-end configurations,high ethylene selectivity of the catalyst is particularly desirable to avoid the hydrogenation of ethylene in the feed gas because of the high H₂ concentration.The modification of Cu and Zn proves to be effective in improving the selectivity of Pd-based catalysts.Hence the brass fibers?80?m fiber dia.?composed of Cu and Zn are employed to tailor Pd-based structured catalysts,aiming to achieve combination of high catalytic performance with enhanced heat/mass transfer.The Pd/brass-fiber catalysts are prepared by incipient impregnation method over the pre-calcined brass fibers with an endogenously grown ZnO-CuO_x composite layer.The Pd-substrate interaction leads to the Pd-Cu-Zn ternary structure of active sites during the reduction process,which is strongly dependent on the brass fiber calcination temperature as well as the catalyst reduction temperature.High brass fiber calcination temperature results in enrichment of ZnO over the surface of catalysts,which prevents the Pd-Cu interaction and further decreases the Cu content in the active sites.This favors the catalyst activity but decreases the ethylene selectivity.And meanwhile,high catalyst reduction temperature promotes the alloy degree of the PdCuZn ensembles,increasing both the Zn and Cu content in the active sites,which is favorable for the selectivity but not the activity.Over the preferred catalyst with the brass fiber calcined at 500 ? and reduction in H₂at 250 ?,higher than 90%selectivity is achieved at nearly full consumption of acetylene under front-end conditions.Moreover,the selectivity of Pd/brass-fiber catalysts tends to increase with the reaction temperature due to the weakened ethylene hydrogenation ability over the PdCuZn ensembles.?4?Brass-fiber structured Ni-based catalyst and its catalytic performance for selective hydrogenation of acetyleneTo seek more economical catalysts for the selective hydrogenation of acetylene,non-precious structured Ni/brass-fiber?80?m fiber dia.?catalysts are obtained via facile impregnation method using the pre-calcined brass fibers as support.Similar to the Pd/brass-fiber catalysts,the catalytic performance of Ni/brass-fiber catalysts is also dependent on the brass-fiber calcination temperature and the catalyst reduction temperature.When reduced at 400 ? in hydrogen,the strong Ni-substrate interaction promotes the formation of CuNi alloy,which presents high ethylene selectivity but low activity.However,high brass-fiber calcination temperature leads to the enrichment of surface ZnO,which weakens the Cu-Ni interaction and suppresses the reduction of NiO.The Ni-ZnO interaction results in the electron deficiency of active sites,which enhances the activity but concaminantly lowers the ethylene selectivity.While the reduction temperature rises up to 500 ?,ZnO can be reduced and alloyed with Ni to form NiZn alloy,thereby leading to significant increase in the alloying degree of the Ni active sites.As a result,the catalytic activity decreases while the ethylene selectivity increases.The preferred catalyst with brass-fiber calcined at 600 ? and reduction in H₂ at 400 ? achieves nearly 100%acetylene and 75%ethylene selectivity at 220 ? under front-end conditions.In addition,the stability of Ni/brass-fiber catalysts can be effectively promoted after dilution by adding calcined brass fibers into the catalyst bed.