CO Hydrogenation To Methanol Over Cu/Zn/Al/Zr Catalysts Prepared Via Oriented Isomorphous Substitution Method

Zinc malachite?Cu,Zn?₂CO₃?OH?₂ phase was highly effective precursor of Cu/ZnO catalysts for methanol synthesis from syngas.During the thermal decomposition of zinc malachite,CuO/ZnO solid solution was formed and the Cu species obtained through reduction of the solid solution were evenly embedded into ZnO lattice structure.The strong interaction between copper and zinc can markedly enhance the activity and stability of the catalysts.Hence,the key to the preparation of stable,highly active Cu/ZnO catalysts lie in the formation of phase-pure precursors,viz.,zinc malachite.However,with the traditional co-precipitation process,it is difficult to control the direction of substitution due to the coexistence of Zn²⁺,Cu²⁺ions competing for precipitation.Therefore,the prepared precursor mostly forms mixed phase,which is not favorable to the improvement of catalyst performance.In the early stage,we have prepared phase-pure?Cu,Zn?₂CO₃?OH?₂ through the oriented isomorphous substitution,resulting in Cu/ZnO/Al₂O₃ catalysts with large specific surface area by doping with Al metal.However,the phase of Al doped precursor was no longer pure,the?Cu,Zn?₂CO₃?OH?₂ and Cu₃Zn₃Al₂?OH?₁₆CO₃·4H₂Owerealldetected.SincetheAl-rich Cu₃Zn₃Al₂?OH?₁₆CO₃·4H₂O phase contained a large number amount of Zn-Al oxide matrix structure,which trapped CuO,so that decreased the Cu dispersion.Thus concluded that hydrotalcite phase was a target activity unfavorable phase.In this thesis,Zr doped?Cu,Zn?₂CO₃?OH?₂ precursors were prepared by oriented isomorphous substitution method.Further,the precipitation-reduction method was applied to preparation of catalysts.The effect of Zr doping content on the precursor phase composition and catalytic performance was explored,the proportional Cu⁺/Cu⁰ were controlled by changing the contents of reducing agent.The main conclusions observed were as follows:?1?The phase-pure zinc malachite?Cu,Zn?₂CO₃?OH?₂ with atomically uniform distribution was prepared by oriented isomorphous substitution method.When,the higher Al content was doped,part zinc malachite transformed to hydrotalcite phase Cu₃Zn₃Al₂?OH?₁₆CO₃·4H₂O.However,the hydrotalcite phase was inhibited after Zr doping to promote the formation of pure-phase zinc malachite precursors.?2?There were more positive ion vacancies after Zr⁴⁺doping to keep the system in balance,which was favorable to fix the Cu⁺into ZnO lattice structure,thus forming the Cu⁰-Cu⁺-O-Zn²⁺/Al₂O₃-ZrO₂ active sites.The doping of Zr⁴⁺not only increased the specific surface area of the catalysts.The formation of Cu⁰-Cu⁺-O-Zn²⁺/Al₂O₃-ZrO₂ active sites significantly enhanced the synergistic effect between copper and zinc species,and thus improve the reactivity of the catalysts.The highest methanol time space yield of 476.0 g?kg_Cu^-1?h^-1 was observed for the catalysts with Zr/Al=30/100,which was about two-fold higher than the catalysts prepared by traditional co-precipitation method.?3?The STY of CH₃OH was closely related to the valence state of the active copper species,i.e.Cu⁺/Cu⁰ ratio,which can be optimized by varying the content of reducing agent NaBH₄.The Cu²⁺mainly reduced to Cu⁺while the reducing agent content lower than 100/100.At the same time,the Cu²⁺mainly reduced to Cu⁰ for the catalysts,which obtained with the reducing agent content higher than100/100.A maximum CH₃OH space-time yield of 289.5 g?kg_Cu^-1?h^-1 was obtained with NaBH₄/Cu=100/100,in which the content of Cu⁺in the ZnO lattice structure was highest.?4?The zinc malachite was converted to metal oxides during the liquid reduction process,the metal oxides tend to agglomerated and thus lead to decrease in the specific surface area of catalysts.In addition,the effective copper surface area S_Cu decreased with the increasing of NaBH₄ content,and the synergistic effect between Cu and Zn species weakened,Therefore,the activity of catalysts reduced.