Study Of Supported Nano-Pd Catalysts With High Dispersion For Catalytic Hydrogenation

Supported Pd catalysts are widely used in selective hydrogenation of aromatics and reduction of nitroaromatic compounds due to their excellent catalytic activity and seletivity. The traditional impreganation process for preparation of supported Pd catalysts usually resulted in aggregation of Pd nanoparticles and low dispersion.Comparing with the conventional process,chemical reduction method with protection agents can prepare smaller nanosize Pd particles with high dispersion.At present,due to the limitation of the categories of protecting agentst,the supported catalysts prepared from this method still exist some problems such as short catalytic life,instability,complicated procedures and so on.In this work,we have developed a new technical process to prepare supported nano-Pd catalysts by using mixture of laurylether(Brij-35) and sorbitanmonolaurate(Tween-20) as binary stabilizing surfactants.Pd nanoparticles were obtained by chemical reduction of chloropalladium acid(H₂PdCl₄) with sodium borohydride(NaBH₄) in aqueous solution without any organic solvent.According to the properties of different supports,the preparation procedure was typically designed by two routes.We prepared four series of supported nano-Pd catalysts on different carriers including C,Al₂O₃,TiO₂ and SiO₂.The microstructure of supported Pd catalysts was characterized by means of TEM,XRD,ICP and SEM.The catalytic activity of supported nano-Pd catalyst was tested in several typical probe reactions including selective hydrogenations of bicyclic and nitro-aromatic compounds.Pd/C catalysts with different Pd loading amount 4wt%,5wt%and 10wt%were prepared from Route No.1 first reduction then support using Brij-35 and Tween-20 as protection agents. Well-dispersed Pd particles measuring 2-4nm were successfully prepared without aggregation. Both Tween-20 and Brij-35 were proved to be suitable for our purposes.Sponge-like nano-Pd crystals were highly dispersed on the surface of active carbon.Simultaneously,the spong-like Pd nanocrystals consist of several nano-Pd particles in the nanometric size rang 2-4nm.The amorphous Pd₂B compound may be formed with increasing Pd loading amount of Pd/C catalyst.5%Pd/Al₂O₃ and 5wt%Pd/SiO₂ were prepared from Route No.2 first support then reduction and 2.5wt%Pd/TiO₂ from both routes.Pd nanoparticles with the average size of 2-4nm in above three catalysts were highly dispersed on the surface and inside pores of the support without the formation of sponge-like nano-Pd crystals. The hydrogenation of p-phenylphenol(p-PP) can be regioselectively performed to produce p-cyclohexylphenol(p-CP) by using 10wt%Pd/C catalyst.The selectivity of p-CP can be as high as 92.3%at 413 K,3.5 MPa when the conversion of p-PP is 100%.THF is an effective solvent for the reaction which can successfully replace the corrosive acetic acid as reported in previous patent,p-CP can be separated and purified by using aqueous NaOH solution and toluene extraction,giving rise to 95%yield of separation and 99.9%high purity p-CP.2.5wt%Pd/TiO₂ was examined for the partial hydrogenation of 1-naphthol(NP-OH) in EtOH solvent.The hydrogenation products consist of tetrahydronaphthols(THNols),tetralone and tetralin(THN) without the formation of decaline.Kinetic study proved that the reaction was a complicated one containing multi-step parallel and consecutive hydrogenations. Tetralone was an intermediate for the hydrogenation of NP-OH to 1,2,3,4-THNol.Further hydrogenation of THNol to THN occur faster from 1,2,3,4-THNol than from 5,6,7,8-THNol. The 1,2,3,4-THNol selectivity can be up to 46.3%under 140℃,2.6MPa with 100% conversion of NP-OH.It is very important to optimize the pressure,reaction temperature, reaction time as well as catalyst in order to reach high 1,2,3,4-THNol selectivity.Selective hydrogenation of quinoline over 5wt%Pd/C catalyst in liquid phase gave rise mainly to 1,2,3,4-tetrahydroquinoline(py-THQ).High regioselectivity of py-THQ was up to 99.7%under 125℃,3.0MPa,3h when the conversion was 100%.The reaction intermediates were responsible for the catalyst deactivation in the second run.However,catalytic activity of the catalyst could be almost recovered through washing it repeatedly with organic solvent. Partial hydrogenation of 8-quinolinol(8-Qol) to 1,2,3,4-tetrahydroquinolinol(1,2,3,4-Qol) was performed over 4wt%Pd/C catalyst under mild conditions.Selectivity of 1,2,3,4-Qol 98.5%was obtained under optimized conditions of 100-120℃,2.0-2.5MPa,concentration of raw material 3-4g/30ml MeOH within reaction time 1-2h with 100%conversion.Hydrogenation of 4-nitrophthalonitrile(4-NPN) was studied under mild conditions over supported Pd catalysts.The influence of various reaction parameters and the effect of different support were investigated.The hydrogenation rate was dramatically affected by the property of supports.The hydrogenation rate increases in the order:Pd/Al₂O₃＞Pd/TiO₂＞Pd/C and the catalytic life increases in the order:Pd/Al₂O₃＞Pd/C＞Pd/TiO₂.Pd/Al₂O₃ catalyst exhibited the best activity and longest lifetime.Under optimized conditions of 80℃,0.6 MPa and concentration of raw material 0.64 mol/L,45min,the yield of 4- aminophthalonitrile (4-APN) was up to 99.7%with 100%conversion of 4-NPN.The Pd/Al₂O₃ was repeatedly used for 12 times and kept 100%conversion of 4-NPN and high yield of 4-APN,indicating excellent stability. 5wt%Pd/C catalyst was applied in selective hydrogenation of chloronitrobenzenes(CNBs) with no addition of dechlorine inhibitors.An induction period of the catalyst performance was observed.The hydrogenation rates of different CNBs over Pd/C were dramatically affected by the position of substituent -Cl group.The reaction rates increased in the order of p-＞m-＞o-Effects of various reaction parameters were investigated after the induction period.When the reaction pressure was set to 2.5MPa and reaction time of 3h,the conversion of o-,m- and p-CNB were all up to 100%at different temperature of 403K,393K,383K,and selectivity to the corresponding CANs were 100%.The catalyst can be repeatedly used for 40 runs with 100%conversion of the CNBs.