Effect Of Ce Promoter On The Structure And Catalytic Performance Of Cu/AC Catalyst

Dimethyl carbonate(DMC),a nontoxic or slightly toxic green chemical,has attracted wide attention owing to its unique molecular structure consisting of a variety of functional groups and the excellently active chemical property.The copper-based catalyst containning chloride is generally used as the main catalyst of direct vapor-phase oxidative carbonylation of methanol.In the carbonylation process,HCl uauslly appears because of the loss of Cl ions,resulting in the serious problems of catalyst deactivation and equipment corrosion,which restricts the industrial actual applications.Ion-exchange type low chloride catalyst exhibits better catalytic activity and stability by removing most of Cl ions using the solid state ion-exchange method,but it is difficult to completely remove the residual chlorine due to the lower ion-exchange rate.However,although the chloride-free copper-based catalyst,which was preparated using chlorine-free copper salt as raw material,avoids the equipment erosion problem caused by HCl,but the catalyst activity is still unsatisfactory.In recent years,abundant researches indicated that the catalyst containing chlorine such as the supported CuCl2 catalysts and Wacker-type catalysts showed the excellent advantages in high yield of DMC and the puzzled question of easy deactivation.To further improved catalytic activity and stability of these catalysts,Introducing promoter have been performed for them,but there still exists the avoidless problems of catalyst deactivation and equipment corrosion caused by loss of Cl ions.As a result,our research group prepared the Cu0/AC,Cu2O/AC and CuO/AC catalysts using chlorine free copper source as precursor by modification of AC support,the thermal decomposition of copper acetate,.the reduction of copper nitrate by hydrazine hydrate and polyol,the basic copper nitrate pyrolysis and so on.These obtained catalysts have better catalytic performance of methanol oxidative carbonylation.Based on our previous results,in this paper,the promoter Ce have been introduced into Cu/AC catalyst by adopting the different impregnation sequences,calcination temperatures,percentage of promoter and process conditions.The as-prepared samples were characterized by powder X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),H2-Temperature programmed reduction(H2-TPR),Atomic absorption spectroscopy(AAS)so as to further investigate their catalytic activity,surface properties and the variation of active species on the catalyst surface.The research conclusions have been drawn as follows:(1)The distribution of active species on the catalyst surface was improved by adding the promoter Ce to the Cu/AC catalyst.The obtained catalysts by first impregnating Ce and then impregnating Cu make Cu species uniformly dispersed,and consquencely,the activity and selectivity of the catalysts were improved due to the appearance of Cu2O species by the reduction of partial Cu species with the interaction between Ce and Cu.(2)For as-prepared catalysts by first impregnating Ce and then Cu,it can be found that the high-valent Cu species were gradually reduced to the low-valence Cu species at the calcination temperature in the range of 150?450?;the existence of Ce could weaken the reduction of Cu+ species,so there are still a small amount of Cu20 active species at 300 ? and above.The catalyst under the calcination temperature of 300 ? exhibited the best catalyst activity because of the higher content of Cu20 and elementary substance Cu on the catalyst surface.(3)There existed the dispersed CuO species and crystalline Cu20 and Cu species on the catalyst surface without adding Ce(with 0%Ce).While there are Cu2O and Cu0 coexist in the catalysts with 1?3%Ce.For the latter,with the increase of Ce content,the Cu20 content gradually decreased and the Cu0 content increased.As a results,the introduction of promoter Ce improved the catalytic performance of catalyst owing to lower of Cu0 electron binding energy by affecting Cu active center electronic environment.(4)In order to optimize the reaction process conditions,we sift out and investigated the best catalyst with the highest catalytic performance prepared under the following conditions:first impregnate Ce and then Cu,the calcination temperature of 300 ? and the percentage of 2%Ce.The study findings indicated that the optimal reaction process conditons are the reaction temperature of 130 ?,the system pressure of 0.5 Mpa,CO/O2 air flow ratio of 11/1 and space velocity of 3092 h-1.Under these conditions,the selectivity and space time yield of DMC were 88.87%and 155.12 mg g-1·h-1,respectively,and the conversion rate of methanol reached 4.56%.