Study On Preparation Of Mg-Fe Catalysts And Process For The Direct Synthesis Of Light Olefins From CO Hydrogenation

Direct synthesis of light olefin from CO hydrogenation has been considered as a possible solution and practical interest to meet the great demands for chemical feedstocks such as ethylene,propylene and butylene,which may provide a more economical way than the conventional methods such as catalytic cracking of naphtha and methanol to olefins（MTO,MTP）process.The conversion of syngas to light olefins（STO）via Fischer-Tropsch synthesis（FTS）is one of the most promising processes for non-petroleum based supply.However,the low selectivity of the desired products ruled by the Anderson-Schulz-Flory distribution function and the high selectivity of CH4 and CO₂ by-products have restricted the application of this process into an industrial scale.At the present time,the yield and selectivity of light olefins are far from optimal,due to the limitation of Anderson-Schulz-Flory（A-S-F）distribution in Fischer-Tropsch synthesis and secondary reactions such as hydrogenation and polymerization of primary olefin products.Therefore,catalysts with high selectivity and yields to produce light olefins as the desired products still need to be further investigated,and enhancing the olefin selectivity and controlling the product distribution are still the main problems.Production of light olefins and CO₂ inhibition calls for the development of effective catalysts to achieve high CO conversion,desired hydrocarbon distribution and favorable olefin selectivity.In this paper,Mg-Fe hydrotalcite-like compounds（MgFe-HTLcs）with layered structure and surface basicity are used as catalysts to study the catalytic behavior on light olefin synthesis by potassium promotion.The catalytic performance with different Mg/Fe molar ratios（Fe/K=10/1）were studied and compared with the catalyst prepared by microwave hydrothermal procedure.Meanwhile,the conversion of CO₂ to light olefins by hydrogenation over a series of K/Fe-Zn（Fe/K=10/1;Fe/Zn mole ratio=3/1,1/1,1/3,1/5）catalysts was also studied to reduce the CO₂ selectivity during the process of STO.The textural properties,bulk structures,and reduction and adsorption behavior of the catalysts were characterized by XRD、CO₂-TPD、H₂-TPR、SEM、XPS and BET methods,in order to provide scientific basis for design of new catalysts on selective production of light olefins from CO hydrogenation.The results showed that the samples K/MgFe-HTLcs had larger surface areas and smaller pore size compared with the catalysts prepared by microwave-hydrothermal method.Fe₅C₂ and MgCO₃ phases were observed after reaction in all the samples.In CO hydrogenation,all catalyst samples exhibited high activity and light olefin selectivity.The CO conversion,light olefin selectivity reached 97%and 42.74 wt%in overall hydrocarbon distribution were obtained on K/MgFe-HTLcs catalysts,respectively.Also,lower CO₂ and C₅⁺ selectivity were found over K/MgFe-HTLcs,suggesting that the structure of K/MgFe-HTLcs could inhibit the growth of carbon chain and improve the hydrocarbon distribution.The process of CO₂ hydrogenation to light olefins could inrecase the total olefin yields,and reduces the CO₂ selectivity in CO Hydrogenaiton.At set reaction conditions of H₂/CO₂ of 3,GHSV of 1000 h-1,320℃,and 0.5 MPa,the 1Fe-1Zn-K catalyst with H₂/CO reduction showed the best performance with the CO₂ conversion of 51.03%.The selectivity of C2-C4 olefins in overall hydrocarbons and the ratio of olefin to paraffin in the C2-C4 fraction reached 53.58%and 6.86,respectively.Also,the catalysts exhibited good stablitiy.