Fabrication Of Fe₃O₄/?-Al₂O₃ Catalysts For Direct Conversion Of Syngas To Lower Olefins

Lower olefins?ethylene,propylene,and butylene?are key building blocks of chemical industry for production of basic chemical products.Traditionally,lower olefins are produced by crude oil cracking.Fischer-Tropsch to lower olefins?FTO?is an effective non-petroleum route for the preparation of basic chemical materials.A novel two-step method was applied to synthesize iron-based supported catalyst,which is to prepare nano-Fe₃O₄ first by thermal decomposition method and sequentially load them on?-Al₂O₃ by impregnation.TEM and XRD results manifested that the controllable,uniform Fe₃O₄ nanoparticles are monodispersed on the surface of?-Al₂O₃.H₂-TPR demonstrated that the reduction of Fe species was facilitated because of the weak interaction between Fe species and the support compared with the catalyst prepared by directly impregnating ammonium iron citrate on?-Al₂O₃.These superior properties contribute to an enhanced CO conversion?13.5%?and high lower olefins selectivity?53.5%?.Effect of promoters was investigated at the same Fe loading and nanoparticle size.The appropriate addition of K could enhance catalytic activity and suppress secondary hydrogenation.On the contrary,S has a negative impact on CO conversion and greatly decreases C₅₊selectivity.Particularly,the combination of K and S could obtain more pronounced CO conversion and high lower olefins selectivities than single promoter.DFT calculations demonstrated that K promotes CO adsorption while S inhibits,and the coexistence of K and S is more able to promote CO adsorption than a single promoter.Size effects were explored by precisely tailoring the iron oxide particle size,keeping the Fe loading constant.It is found that 12.0 nm nano-Fe₃O₄ on?-Al₂O₃ with or without K plus S promoters showed the best catalytic activity among the catalysts with different particle size.For unpromoted catalysts,Turnover Frequency?TOF?varied almost identically to CO conversion.It indicates that shape-features such as terrace,edge,dominate catalytic performance.It is interesting that for the catalysts with K plus S promoters,TOF increases proportionately to particles size,unlike the CO conversion varition.Thus,synergetic promoting effect of K and S results in an remarkable enhancement of activity,which compensate or even mask the influence of particle size.The effect of Zn,added by forming a Zn_xFe_3-xO₄-type solid solution,was investigated for FTO.The addition of an appropriate amount of Zn could enhance the conversion of CO.In the absence of K plus S promoters,Zn could promote water gas shift?WGS?reation.Additionally,the addition of Zn could inhibit the secondary hydrogenation of olefins,thereby increasing the O/P ratio.In the presence K plus S promoters,the suitable amount of Zn increased CO conversion?⁵0%?and high lower olefins selectivities?⁴0%?,and the effect of Zn on product distribution is nil.