Structure-Performance Relationship Of Co-based Catalysts For Lower Carbon Alcohol Synthesis Directly From Syngas

A reliable, cheap and clean energy supply is important for the sustainable development of economics. The coal chemical engineering is limited by its high energy consumption, heavy pollution etc., an efficient and cleaner coal conversion technology is highly desired. Recently, with the development of gasification of coal and biomass, syngas chemistry is becoming a mainstay of the utilization of renewable energy. Lower carbon alcohols, refered to C1-C5 straight chain alcohol mixtures, have rather high octane numbers and can be used as fuel additives, even as alternative fuels in the future. Therefore, direct synthesis of lower carbon alcohols from syngas is a potential approach to realize efficient and clean utilization of fossil resources as well as sustainable development of energy, with broad prospects towards industrialization. However, due to the low activity and selectivity of catalysts, the process is not commercialized yet. The establishment of structure-performance relationship is important for the catalyst R&D. The mechanism of CO hydrogenation is complex, including C-O bond cleavage, growth of carbon chain, insertion of non-dissociated CO, hydrogenation and so on. Combination of dissociated and non-dissociated CO via controlling of CO activation process is a general strategy for lower carbon alcohols synthesis, which can be achieved using dual-site catalysts.The aim of this dissertation is to establish structure-performance relationship of Co-based catalysts for lower carbon alcohols synthesis directly from syngas. Two kinds of dual-site catalysts were prepared in a controllable way. One is CuCo/SiO2 bimetallic catalyst, and the other is Co/CeO2 catalyst, which represent for bimetallic sites and Co-CoOx interface sites, respectively. Based on a comprehensive analysis of bulk and surface structure of calcinated, reduced and reacted catalysts and intrinsic kinetics, the life of active sites and structure-performance relationship were investigated. Main conclusions are described as follows:1. A series of SiO2-supported CuCo catalysts with different Cu/Co ratios were prepared by a wetness impregnation method. The performance shows that the interaction between Cu and Co could strongly affect the activity and selectivity. The addition of small amounts of Cu sharply reduces the CO conversion, and enhances the selectivity of alcohols by a factor of 2. In calcinated CuCo catalysts, Cu prefers to enter into the crystal lattice of Co3O4, with the formation of Cu2+xCo2+1-xCo3+2O4 mixed spinel structure. The reaction temperature of CuCo bimetallic catalysts is significantly lower than that of monometallic catalysts. And there’re no formation of Cu and Co metastable oxides during reduction. An alloy-like CuCo bimetallic phase was observed after reduction. Large amounts of Cu separate from the CuCo bulk phase and cover on the surface. Meanwhile, only Co sites with low coordination are exposed without the formation of continuous Co ensembles. After reaction, the alloy-like bimetallic phase still exists.2. After elimination of internal and external diffusion, the intrinsic reaction kinetics of Co5/SiO2 and Co5Cu2.5/SiO2 were explored, and the parameters were obtained via fitting with the power-law function. The activation energies of hydrocarbons are higher than those of alcohols over Co-based catalysts, which reflects that the rise in temperature benefits the formation of hydrocarbons. CO hydrogenation over Co-based catalysts follows the Langmuir-Hinshelwood model, indicating that the catalyst surface is covered by a large amounts of adsorbed CO and carbon species, which inhibit the adsorption and activation of H2. Compared with Co alone catalysts, the addition of Cu weakens CO adsorption and HCO species dissociation and decreases the number of CHx species, and then enhances the insertion of HCO, resulting in the rise of alcohols selectivity and the drop in CO conversion. With a decrease in continuous Co sites, the formation of C3 products is inhibited. Furthermore, alcohols are prone to hydrogenation over CuCo catalysts with an increase in temperature.3. The selectivity of C1-C5 alcohols over Co/CeO2 catalysts goes down with an increase of Co loading amounts. And it increases at first and then decreases along with the increasing of time on stream. In calcinated Co/CeO2 catalysts, Co exists on the surface in a form of dispersive CoOx. Interaction between CoOx and CeO2 inhibited the reduction of Co oxides. But after reaction, most of Co species were reduced to metallic state. With the increasing of Co loading amounts, metallicity of Co was enhanced. In the initial stage of CO hydrogenation reaction, Co was partial oxidized by O atoms formed from CO dissociation and became Coδ+ species, which were stabilized by CeO2. Interface of Co0-Coδ+ is the active site of alcohols formation.