Study On Preparation And Properties Of Pd/Al₂O₃ Catalyst For CO-MN Coupling Of Dimethy Oxalate

The effect of Pd crystal morphology,size,loading and loading methods on the performance of Pd-based catalysts(Pd/Al2O3)was studied systematically by coupling of CO and dimethyl nitrite(MN)to dimethyl oxalate.The main work and conclusions are as follows.Based on the liquid phase synthesis method,cubic Pd grains with different sizes were synthesized by adjusting the amount of KBr.And then used these cubic grains as seeds,octahedral Pd grains with different sizes were obtained through liquid phase synthesis.A series of supported Pd/Al2O3 catalysts were prepared by volume impregnation method using cubic and octahedral Pd particles as active species.The evaluation results of the prepared catalysts show that the catalyst performance based on octahedral Pd crystallites is significantly better than that based on cubic Pd crystallites,ie,higher MN conversion and DMO selection under same condition.In the catalyst prepared with Pd particles of the same morphology,the conversion of MN and the selectivity of DMO increased with the decrease of Pd grain size.TEM characterization results show that the octahedral Pd grains expose more(111)crystal faces.In-situ IR adsorption of CO shows that CO was more strongly adsorbed on the(111)crystal surface of the octahedral Pd than on the(100)crystal surface of the cube.It is known from the literature that the bridge adsorption of CO on Pd is the main form involved in the formation of intermediate products.Therefore,the(111)crystal plane on octahedral Pd is more conducive to CO coupling.By controlling the Pd precursor solution injection rate and the synthesis temperature,Pd grains with different sizes and morphologies were prepared.The evaluation results of the prepared catalysts show that the Pd crystals prepared at 80? with precursor injection using a variable speed injection method have better coupling catalytic performance.The TEM results show that the Pd grains synthesized under this condition have a minimum size of 2 to 3 nm and exposed(111)crystal surface at most.The in-situ IR adsorption characterization of CO shows that the Pd crystallites prepared under this condition have the strongest CO bridge adsorption.The effect of Pd loading(0.1%?1.0%(wt.))and ultrasonic time on the performance of the catalyst was investigated by single factor experiments.The results show that the MN conversion rate increases with the increase of Pd loading,while the selectivity of DMO decreases with the increase of Pd loading in the investigated range.The appropriate Pd loading determined by the comprehensive conversion rate and selectivity is 0.3%to 0.5%(wt.%).The conversion rate of MN increased first and then stabilized with the increase of ultrasound time.The selectivity of DMO is not influenced by the ultrasound time.The results of TEM and ICP analysis indicate that ultrasonication is beneficial to improve the dispersion of Pd nanocrystallites and the actual loading of Pd.The suitable ultrasonic time is 20min.Based on the optimized preparation of the catalyst(2?3 nm Pd grains,ultrasonic time 20 min,loading 0.3%),the effect of temperature,batching ratio,and space velocity on the performance of the catalyst is investigated by single factor experiments.The results showed that the conversion of MN increases with increasing temperature,but the selectivity of DMO did not change much.The conversion rate of MN shows a tendency of rising first and then decreasing with the increase of CO:MN ingredients ratio.The selectivity of DMO increases monotonously with the increase of CO:MN ingredients ratio.The change of airspeed has little effect on the selectivity of DMO,but the MN conversion rate decreases monotonously with the increase of airspeed.The relative optimum process conditions a determined as follows:temperature 130?,ratio of reactants CO:MN=1.25:1,and space velocity 4000 ml·h-1·g-1.