Iron-Based Catalysts For Higher Alcohols Synthesis From Syngas:Preparation And The Structure-Activity Relationship

Higher alcohols(HA),defined as alcohols possessing two or more carbons,can be widely used as alternative fuels,gasoline additives,as well as the intermediates for fine chemicals production.Based on the energy structure of China,in which coal is more abundant than natural gas and crude oil,it is promising to develop the technology of higher alcohols synthesis(HAS)directly from syngas.Fe-based catalysts have been widely used in HAS,but the phase separation of CuFe dual sites during the reaction results in the low space-time yield(STY)of HA and the poor stability.This work mainly focuses on the fabrication of new dual sites in Fe-based catalysts to improve the performance for HAS.And we also investigated the evolution of the dual-sites and the reaction mechanism,which may provide new ideas for further design of highperformance catalysts for HAS.A series of Au-Fe3O4 nanoparticles with different Fe/Au molar ratios were prepared and dispersed on α-Al2O3,which could evolve into Au-Fe2.2C catalysts after reduction and carburization.The optimal catalyst with a Fe/Au molar ratio of 10 achieved the most Au-Fe2.2C Janus nanoparticles and exhibited the highest ROH selectivity of 52.5%and the highest STY of HA of 0.195 g gcat-1 h-1.It is demonstrated that Au facilitates CO nondissociative adsorption and insertion,and ε’-Fe2.2C enhances the CO dissociation and carbon chain growth.The Janus structure enhanced the synergy of Au-Fe2.2C dual sites,and the interaction between Au and Fe promoted the formation and stability of ε’-Fe2.2C,which resulted in excellent stability during the 200 h reaction.To further improve the proximity between dual sites,the catalysts containing ε’(CoxFe1-x)2.2C alloy carbide were obtained from the Co/α-Fe2O3 nanorods precursors after reduction and carburization.The catalysts reduced above 300℃ could ensure an adequate interdiffusion between Co and Fe species to form CoFe alloy and then achieved a high fraction(ca.32%)of ε’-(CoxFe1-x)2.2C species during HAS reaction.It is found that the STY of HA increased linearly with the increasing fraction of ε’-(CoxFe1x)2.2C alloy carbide,which acting as uniformly atomic neighboring CoxC-FexC dual sites,facilitated the synergy between CO nondissociative adsorption and CO dissociation,thus boosting the selective production of higher alcohols.We further investigated the impact of Na promoter on the formation of ε’-(CoxFe1x)2.2C and the catalytic performance.It is demonstrated that Na promoter could improve the fraction of ε’-(CoxFe1-x)2.2C,thus facilitating the synergy between CO dissociation and CO insertion.On the other hand,the incorporation of Na promoter could weaken the H2 activation,resulting in a weakened the hydrogenation of CHO*and CHx*into methanol and CH4.Na could also enhance the CO adsorption and facilitate the C-C coupling,leading to an improved the fraction of C2+products(HA and HC2+).The STY of HA showed a volcano-like trend as the increasing content of Na and reached the maximum of 0.243 g gcat-1 h-1 at 0.25 wt.%Na loading.When the Na content is further increased,the excessively weakened hydrogenation could significantly reduce the catalytic activity and inhibit the hydrogenation of oxygenate intermediates such as CHxCO*and CHxCHO*to form HA,resulting in the decrease of HA selectivity and STY.To further improve the content and dispersion of ε’-(CoxFe1-x)2.2C sites,the monodisperse CoxFe3-xO4 spinel nanoparticles were used as precursors.It is found that the Co/Fe molar ratio significantly influenced the evolution of active sites during HAS reaction.The optimal Co1Fe2 catalyst mainly composed of ε’-(CoxFe1-x)2.2C alloy carbide,while χ-Fe5C2 and ε’-(CoxFe1-x)2.2C coexisted in the Co1Fe3 catalyst,and a large amount of CoFe alloy existed in Co1Fe1.5.ε’-(CoxFe1-x)2.2C showed medium CO bonding strength,which facilitated the formation of oxygenates.Consequently,Co1Fe2 achieved the best selectivity and STY of ROH as high as 51.2%and 0.206 g gcat-1 h-1 respectively and it exhibited an outstanding stability over 300 h.Individual ε’-Fe2.2C,CoFe alloy,and χ-Fe5C2 all showed strong CO adsorption capability,which favored CO dissociation and thus hydrocarbons were the main products.These understanding may provide guidance for the controllable preparation of alloy carbide catalysts.