Preparation Of Highly Dispersed Palladium Nanocatalyst And Its Performance In Oxidative Carbonylation Of Phenol

Diphenyl carbonate?DPC?is an important intermediate for the preparation of engineering thermal plastic polycarbonate?PC?.The main synthetic methods of DPC include phosgene,transesterification and phenol oxidative carbonylation.Among them,the raw material of phosgene method is highly toxic,which is harmful to human body and environment;the low conversion rate,the high energy consumption and the complex technique of the trasesterification method,lead to the inapplicability of the production;there are many advantages of oxidative carbonylation,including the cheap raw materials and pollution-free;therefore,it has a more promising future.For the phenol oxidative carbonylation process,it is essential to develop catalysts with high activity,long life and high selectivity.In generally,the preparation method of supported noble metal catalyst is simple;also,it is easy to separate and recover from the reaction system,so as to facilitate reuse,reduced the cost greatly as well and conforms to the development trend of green chemistry.For noble metal catalysts with high loads,only a few active components play a catalytic role in the process of catalytic reaction;therefore,the size of active component has an important influence on the catalytic performance.Generally speaking,the catalytic performance of supported catalyst increases with the decrease of size,which is called the noble metal nanocatalysts,and its good activity is attributed to the highly dispersed of noble metal,so that the catalytically active sites can be fully utilized.At present,the main methods,such as co-precipitation,chemical vapor deposition,impregnation,microwave-assisted synthesis and photo-deposition methods have been already used to synthesize nano-catalysts on metal oxides.Among them,microwave-assisted and photo-deposition methods,as efficient green environmental protection new technologies,which have been widely used in the preparation of catalysts.In this paper,the catalytic performance of catalysts prepared by microwave-assisted and photo-deposition methods were discussed,and the catalysts were characterized by XRD,TEM,XPS,and H₂-TPR.The main contents of this paper are as follows:?1?The manganese oxide octahedral molecular sieve?OMS-2?was used,and a series of supported Pd/OMS-2 catalysts were prepared by photo-deposition method?Pd/OMS-2-PD?,compared with the catalysts prepared by traditional precipitation?Pd/OMS-2-Pre?and impregnation methods?Pd/OMS-2-IM?,their properties in the oxidative carbonylation of phenol were investigated.The catalyst prepared by photo-deposition method showed a better catalytic performance than other two catalysts,which were prepared by the traditional methods.Among them,the yield of DPC could reach 20.4%at 24 hours by using Pd/OMS-2-PD,and there was no obvious decay of activity.The possible mechanism of photo-deposition was as follows:OMS-2 nanorods become excited states under ultraviolet irradiation,electrons?e^-?-hole pairs?h⁺?were produced,in which holes?h⁺?acted as oxidants,converted part of ethylene glycol into ethylene glycol free radical-OCH₂CH₂O·and promotes the removal of Cl^-in Pd Cl.The generated electrons?e^-?was captured by Pd²⁺,and then,Pd²⁺reduced to Pd⁰.The remained ethylene glycol acted as a dispersant to prevent the agglomeration of Pd particles.More Pd-O bonds were formed after Pd combined with OMS-2,which makes the active component Pd particles remain highly dispersity,even after calcination,and the residual glycol on the catalyst surface was removed after calcination at high temperature.?2?The performance of the catalysts prepared by photo-deposition method under different loadings and different p H condition were investigated.The results showed that the active component Pd particles were highly dispersed,no matter the loading amount is2.5 wt%or 5.0 wt%.,the particle size of Pd gradually increases when the load exceeded2.5 wt%,at 5.0 wt%,the particle diameter of Pd particles reached 3.48 nm and the catalytic activity also slightly decreased.The DPC yield reached 18.1%when the p H value was at 5.?3?A series of Pd/OMS-2 catalysts were prepared by microwave-assisted reduction?MW-Pd/OMS-2?and liquid-phase deposition reduction?LR-Pd/OMS-2?methods,with manganese oxide octahedral molecular sieve OMS-2 as support and ethylene glycol as reductant and their catalytic performance in phenol oxidative carbonylation were investigated.The heat transfer cannot be out simultaneously and not uniform when the catalyst was prepared by liquid phase deposition.The heat energy is directly converted by electromagnetic energy,to transfer the heat evenly when the catalyst was prepared by the microwave-assisted method and the Pd particle size was small and highly dispersity.?4?The catalytic performance prepared by microwave-assisted method was optimized by adjusting the p H of the precursor solution.With the increased of p H value,the concentration of OH^-gradually increased,and the Pd Cl was reduced and stabilized by the interaction of OH^-and acetaldehyde.The catalytic performance is optimal at p H=6,and the main particle size of Pd is 2.56 nm,the yield of DPC reached14.5%after catalyzing phenol oxidative carbonylation.