The Theoretical Optimization And Applied Research On Palladium-based Nanocatalysts For Selective Hydrogenation Of Dienes

Supported noble metal nanocatalysts play an important role in the chemical industry,energy,and environment.Due to their unique characteristics such as volume effect,surface effect,and quantum size effect,the supported noble metal nanocatalysts have become research hotspots in the field of catalysis.However,the design and development of new catalysts are mainly based on“repeated tests”at present.Therefore,how to rationally design highly active and stable supported noble metal nanocatalysts is still one of the challenges in the field of catalysis.The key to this work is to deeply understand the relationship between catalyst structure and performance.This paper focuses on the relationship between structure and performance of palladium-based catalysts supported on alumina and their application in selective hydrogenation of pyrolysis gasoline and butadiene.The effects of active components morphologies and alloy additives on the activity and selectivity of palladium-based catalysts were mainly studied.New efficient catalysts were designed and prepared for selective hydrogenation of pyrolysis gasoline and butadiene.It is expected to deeply understand the effect of additives and active component particle morphology on the activity and selectivity of supported noble metal Pd catalysts,and further realize the application in the selective hydrogenation of butadiene and pyrolysis gasoline.The main results are as follows:（1）For the hydrogenation of styrene,the equilibrium structures of Pd（100）,Pd（110）,and Pd（111）surface models were calculated by density functional theory（DFT）.And different adsorption sites and forms were discussed.The adsorption and diffusion of reactants（styrene and hydrogen）and the activation energy of styrene hydrogenation were obtained.The adsorption strength of styrene and hydrogen decreased in the order of Pd（110）>Pd（111）>Pd（100）.The order of activation energy was Pd（111）>Pd（100）>Pd（110）,and the diffusion energy barrier of styrene or H is less than that of hydrogenation of styrene,indicating that the real rate-limiting step is the hydrogenation process rather than diffusion process.The electronic structures（PDOS and d-band center）on the surface of three kinds of Pd crystal planes are the origin for the difference between adsorption energy and activation energy.Therefore,the hydrogenation activity of styrene follows the order of Pd（111）>Pd（100）>Pd（110）,and hydrogenation is most likely to occur on the Pd（111）surface.（2）The trends of the adsorption properties of styrene and hydrogen over Pd-based alloys doped by eight transition metal atoms around Pd were obtained by DFT calculation.Except for Pd-Cu alloy catalyst,the adsorption energies of other Pd-based alloy catalysts for styrene were increased,which promoted the further hydrogenation of styrene.In addition,the phase transition temperatures of styrene to ethylbenzene on the surfaces of Pd Ni,Pd Pt,Pd Ir,Pd,Pd Au,Pd Cu,Pd Ag,Pd Rh,and Pd Co decrease to 412k,410k,399k,387k,382k,372k,367k,368k,and 322k respectively.The lower phase transition temperature of styrene to ethylbenzene means a more rigorous reaction temperature range.Therefore,from the thermodynamic point of view,it is considered that Ni doping is more conducive to the selective hydrogenation of styrene.（3）The effect of the surface layer components of Pd-Ag bimetal on the catalytic performance of selective hydrogenation of 1,3-butadiene to 1-butene was studied by the first-principles calculation simulation method.The adsorption of reactants decreases with the increase of the surface layer Ag doping concentration.The adsorption energy（e V）order of 1,3-butadiene is Pd（111）（1.59）<Pd₂Ag₁/Pd（111）（1.18）<Pd₁Ag₂/Pd（111）（0.46）<Ag/Pd（111）（0.02）,and the adsorption energy（e V）order of 1-butene is Pd（111）（0.61）<Pd₂Ag₁/Pd（111）（0.50）<Pd₁Ag₂/Pd（111）（0.28）<Ag/Pd（111）（0.14）.The decrease of adsorption is mainly due to the decrease of d-band center from Fermi level and the increase of filling of metal antibonding state of adsorbate.At the same time,with the increase of Ag content,the activity and selectivity of selective hydrogenation of 1,3-butadiene to 1-butene increased,and the binding strength of all adsorbents decreased.In addition,ab initio atomic thermodynamic calculation shows that the introduction of Ag on Pd surface can promote the formation of 1-butene in a wide range of temperature and hydrogen partial pressure.（4）Quantitative simulation calculations for the selective hydrogenation process of1,3-butadiene were carried out on 28 kinds of Pd₃M₁（111）alloy surface models,and the adsorption energy of reaction intermediate species and the elementary reaction energy barrier were obtained.The adsorption energies of 1,3-butadiene on different Pd₃M₁（111）alloy surfaces were distributed in a“concave”shape in each cycle.The transition metals on both sides of each cycle as the second component would weaken the adsorption strength of Pd-based alloys.In the middle of each cycle,the transition metal as the second component would enhance the adsorption strength of Pd-based alloys.The adsorption strength of butadiene was used as the descriptor of 1,3-butadiene conversion rate,and the desorption energy of 1-butene was used as the descriptor of selectivity.By screening Pd-based alloys with weaker adsorption of 1-butene than pure Pd and stronger adsorption of1,3-butadiene than pure Pd,a complete set of theoretical methods to evaluate catalysis performance of different Pd-based alloys for the selective hydrogenation of 1,3-butadiene was been established.It was found that Pd₃Ni₁ balanced the 1,3-butadiene conversion rate and 1-butene selectivity to a certain extent.（5）Based on the theoretical design results,the supported Pd₃Ni₁ bimetallic alloy catalyst was prepared.The results of TPR and XPS showed that Pd-Ni alloy was formed,and the electron cloud density of Pd center in Pd₃Ni₁/Al₂O₃ catalyst increased,which indicated that the adsorption-dissociation ability of Pd site for 1-butene decreased,which was beneficial to improve the selectivity of butadiene hydrogenation.Under the optimized process conditions(reaction temperature 40-60,reaction pressure 1.0 MPa-1.5 MPa,feed volume space velocity 5-7 h^-1,molar ratio of hydrogen to butadiene 2-4),the long-term stability of the catalyst was evaluated for 1000 h.The average conversion of butadiene was 97.21%,the average monoolefin yield was 98.30%,and the stability of the catalyst was excellent.The catalyst developed based on Pd₃Ni₁/Al₂O₃ catalyst was first used in a new 200000 t/a alkylation unit of a petrochemical company.At present,it has been in continuous operation for 33 months,and the butadiene content of hydrogenation product is less than 50 g/g.The catalyst shows excellent hydrogenation activity and operation flexibility.（6）The macroscopic kinetics of first-stage selective hydrogenation of pyrolysis gasoline on Pd/Al₂O₃ catalyst was studied by the power function equation under experimental conditions similar to industrial conditions.The predicted results were in good agreement with the experimental data.The apparent activation energies of dienes and monoenes hydrogenation on Pd/Al₂O₃ catalysts were 29.72 k J/mol and 41.18 k J/mol respectively,which were consistent with the intrinsic activation energy reported in the relevant literature.The concentration orders of dienes and monoenes in the rate equation on Pd catalyst were 1.69 and 3.26,respectively.The concentration orders of dienes and monoenes were larger than that of hydrogen,indicating that the concentration changes of dienes and monoenes had a great influence on the reaction rate.The F-test results of kinetics model of dienes and monoenes indicated that the kinetic model was adequate.The kinetic model for the selective hydrogenation of pyrolysis gasoline was reliable and laid a solid foundation for the analysis and design of the first-stage reactor of pyrolysis gasoline hydrogenation.