Preparation Of Pd Monolithic Catalyst By Microemulsion Impregnation-Demulsifying Technology And It's Catalytic Performance On Hydrogenation Reaction

The multiphase selective hydrogenation is widely used in petrochemical and fine chemical industries. An important aim is to achieve high selectivity of intermediate product with an accepteable conversion rate. The monolithic catalyst shows great potential on the improvement of selectivity of series reactions campared with tranditional pellet catalyst in view of the structured support, high efficiency of intra-phase mass transfer and short diffusion path. Based on the feature of monolithic catalyst, the Pd immobilization technique with Pd nano particles synthesized by W/O microemulsion as precursor was investigated. The direct immobilization of Pd nano particles in porousα-Al₂O₃ ceramic tube was realized and a new preparation technique of Pd monolithic catalyst was developed in this study. In addition, the selective hydrogenation of 1,5-cyclooctadiene to cyclooctene was performed over the Pd monolithic catalyst.(1) Microemulsion systems and the synthesis of Pd nano particles therein. The effects on the W/O phase region of HLB value, content of surfactants and temperature as well as the stability of Tween80-Span80 and O13/80 microemulsion syetems were investigated. Then the Pd nano particles were synthesized in the stable W/O microemulsion. The results show that the stable W/O microemulsion can be obtained with the HLB value 13 and content 0.20 of surfactant, the mole ratio of water to surfactant 3 to 8 and 3 hours after completely mixing. The spherical or near spherical, with size 3-10 nm, well dispersed Pd nano particles can be synthesized at 30℃in the two types of microemulsion. To make reaction proceed rapidly and completely, the reductant-contaning microemulsion is adopted for O13/80 microemulsion. whereas the reductant solution must be adopted for Tween80-Span80 microemulsion.(2) Deposition of Pd nano particles in microemulsion by chemical-demulsifying. The chemical-demulsifying processes of Tween80-Span80 and O13/80 microemulsion systems contaning Pd nano particles were investigated. The results show that such agents as methanol, ethanol, THF and acetone can demulsify microemulsion and make Pd nano particles deposite because of their short chain structure and high hydrophilicity. As for Tween80-Span80 microemulsion system, the above demulsifying agents can greatly decrease the cohesion energy ratio(R) of microemulsion and lead to the transition to co-continuous structure in a sequence of methanol>ethanol>THF>acetone. As for O13/80 microemulsion system, the mixture of microemulsion and demulsifying agent remains homogeneous with the increase of the amount of demulsifying agents. The sequence of deposition rate of Pd nano particles from microemulsion is: THF>acetone>methanol>ethanol. The amphilicity of O13/80 microemulsion decreases gradually and is transformed into disordered mixture under the action of demulsifying agents.Compared with Tween80-Span80 microemulsion, during the process of Pd synthesis, it is more beneficial to maintain the stability of microemulsion in view of the reducing pattern. Meanwhile, no obvious phase transition is observed during the demulsifying process and it is a gradual process from highly ordered microemulsion to disordered mixture in O13/80 microemulsion Therefore it is easy to control the deposition rate of Pd particles, and more beneficial to the follwing Pd immobilization step on the support.(3) Immobilization of Pd nano particles in porousα-Al₂O₃ ceramic tube. To prepare Pd monolithic catalyst, the Pd nano particles were immobilized in porousα-Al₂O₃ ceramic tube by cycle-impregnating and chemical-demulsifying simultaneously. The decrease and recover of support flux during impregnation-demulsifying process as well as the effects on the phase, distribution and Pd loading amount of demulsifying pattern, and calcination temperature were investigated. The results show that the support flux can be recoved 95% through washing by water and ethanol alternatively and calcination at temperature 400℃. Combined with the results of TEM and XPS of Pd onα-Al₂O₃ powder and the results of SEM, XRD and AAS of Pd onα-Al₂O₃ monolithic support, it is demonstrated that, on the monolithic support, Pd nano particles form clusters which is composed of nano particles with original size in microemulsion. And Pd particles are well distributed in Pd crystalline phase in the pore structure of support. The Pd loading amount of monolithic catalyst can reach 0.312% and the square difference of ten times loading amount is lower than 0.06844.(4) Hydrogenation reaction of 1,5-COD over Pd monolithic catalyst. Firstly the optimized reaction conditions were obtained in SR(slurry reactor): n-heptane as solvent, agitating rate 1600min^-1, reaction pressure 1.0MPa, reaction temperature 47℃and initial concentration 0.4 kmol.m^-3. The effects on activity and selectivity of preparation parameters and operating parameters were investigated. The results show that Pd monolithic catalyst exhibites the highest activity and selectivity with Pd loading amount 0.151%, dripping ethanol slowly at the same time cycle impregnation, calcination temperature 400℃, support with pore size 1.9μm and flow rate 420 ml/min.Comparing the catalytic performance in MR(monolithic reactor), SR and FBR(fixed bed reactor) shows that the selectivity for COE in MR(94.08%) is near to that in SR(95.10%) and much higher than that in FBR(70.04%). The theoretical analysis shows that the improvement of selectivity can be contributed to the decrease of staying time and the local concentration of intermediate product in active layer. Therefore the ratio of two step apparent reaction constants k_1,(eff)/k_2,(eff) is fairly increased.(5) Kinetics of hydrogenation reaction of 1,5-COD. The model of reaction rate equation including the intra- and inter-porous efficiencies was presented based on the experimental data in SR and MR and using the indirect method from Santacersaria E for reference. The model(?) can fitthe expenmental data well, The parameters in model were evaluated byLandscape adaptive particle swarm optimization(LAPSO) method k₁₀=14729.05; k₂₀=84.248; E₁=33.7 kJ.mol^-1;E₂=31.438 kJ.mol^-1.