Study On Cu-Co Catalysts For Lower Alcohol Synthesis

Mixed lower carbon alcohols have been identified as potential fuel additives for its high octane rating and low pollution,which make a huge application value in the field of alternative energy and fuel cells.Researches have shown that catalytic conversion of syngas,derived from biomass,coal,natural gas or shale gas,is one of the most promising pathways of industrialization for lower alcohol synthesis.In recent years,with excellent activity and high selectivity to C₂₊alcohols at mild reactions,modified F-T synthesis catalysts,typically Cu-Co catalysts have been considered as one of the most promising catalysts.This paper mainly investigated the influences of activation conditions,preparation conditions and preparation methods on the performance of Cu-Co catalyst for the selective conversion of syngas into C₂₊alcohols.Characterizied by BET,H₂-TPR,XRD,TG-DTA and SEM-EDS techniques,the relationship between catalyst structure and reaction performance has been analyzed.The results are as follows:1.A series of Cu-Co catalysts with different calcination temperature were prepared by co-precipitation method and used as catalysts for the selective conversion of syngas into C₂₊alcohols.The performance results showed the selectivity of alcohols presented a volcano-shaped curve with the calcination temperature increase.At a calcination temperature of 350^oC the selectivity of alcohols was maximum（33.5%）.The CO conversion and product chain-lengthening capability increased following increase the calcination temperature.The XRD patterns of Cu-Co catalyst precursors were constituted by the peaks of characteristic of Cu₂CO₃（OH）₂ and Co₂CO₃（OH）₂.The H₂-TPR-MS revealed that there exsited numerous undecomposed basic carbonates in catalyst precursors when the calcination temperature was 300^oC.When the calcination temperature exceeded 350^oC,the reduction temperature of Cu-Co catalysts increased when the calcination temperature increased,following decrease the selectivity of alcohols.2.The influences of reduction temperature and atmosphere had been investigated on the performance of Cu-Co catalyst prepared by co-precipitation method.The performance results showed that the CO conversion,the selectivity and space-time yield（STY）of alcohols were significantly increased when the catalysts were reduced in low temperature,low concentrations of hydrogen for a long time.3.The co-precipitation and citrate complex method were compared for preparing Cu-Co catalysts and these two catalysts were used for the selective conversion of syngas into C₂₊alcohols.Compared with the co-precipitation method,the CO conversion increased from 11.2%to 33.9%when using citrate complex method.More importantly,the stability of the catalyst prepared by citrate complex method had been markedly improved.SEM-EDS revealed a relatively homogenous element distribution at the catalyst surface,The Co species particles serve as spacers between the Cu particles,inhibit segregation and sintering phenomena of the Cu species.4.A series of Cu-Co catalysts with different Cu/（Cu+Co）molar ratio were prepared by co-precipitation method and used as catalysts for the selective conversion of syngas into C₂₊alcohols.The results showed the selectivity of alcohols presented a volcano-shaped curve with the molar ratio of Cu/（Cu+Co）.At a Cu/（Cu+Co）molar ratio of 0.5 the selectivity of alcohols was maximum（33.5%）.The CO conversion decreased following increase the Cu/（Cu+Co）molar ratio.5.A series of Cu-Co catalysts with different Cu/（Cu+Co）molar ratio were prepared by citrate complex method and used as catalysts for the selective conversion of syngas into C₂₊alcohols.The results showed dramatic modifications of the CO conversion,selectivity and STY of alcohols in the hydrogenation of CO on Cu-Co catalysts with different molar ratios of Cu to Co.The STY of alcohols was highest at a Cu/（Cu+Co）molar ratio of 0.35 with177.31g·kg^-1_cat·h^-1.H₂-TPR results showed the presence of Cu could remarkably facilitate the reduction of catalysts and the selectivity of C₂₊alcohols increased with the decrease in reduction temperature of Cu-Co catalysts.Moreover,the reduction temperature was lowest at a Cu/（Cu+Co）ratio of 0.35.It indicated the synergy effect existed between cobalt and copper and this synergy effect was significant with a Cu/（Cu+Co）ratio of 0.35.SEM-EDS revealed a relatively uniform element distribution at the catalyst surface and the enrichment of copper on surface was not observed during thermal treatment.The XRD patterns displayed the characteristic peaks of CuO and Co₃O₄ of the fresh Cu-Co bimetallic catalysts and the presence of copper or cobalt species could significantly inhibit sintering of the counterpart.Furthermore,both alcohols and hydrocarbon products follows the classical Anderson–Schulz–Flory（ASF）plot and its chain-growth capability dropped with increase in Cu/（Cu+Co）molar ratio.