Study Of Condensed Phase Hydrogenation Of Dimethyl Adipate To Hexanediol On Copper-based Catalysts

Hydrogenation of diesters to diols is one of the hot topics currently,mainly represented by the hydrogenation of dimethyl oxalate and dimethyl adipate.However,the extensively studied gas-phase hydrogenation strategy requires much higher ratio of hydrogen to ester,which is unfavorable for the industrial production due to the larger energy consumption.In this work,the catalyst Cu/SiO₂ generally used in gas-phase ester hydrogenation was applied in the condensed phase hydrogenation of dimethyl adipate.The Cu/SiO₂ exhibited a remarkably high diol selectivity of 100%and space-time yield of 0.81 g _HDO·g^-1_cat·h^-1 at the conversion of 95%,which was much higher than those reported in the literatures.Due to the complexity of the gas-liquid-solid catalytic system,the rate-determining step was studied firstly by combining the mass-transfer experiments,model analysis and the diffusion criterions（Carberry number and Wheeler-Weisz group）.The surface reaction was proved to be the rate-controlling step in the condensed phase hydrogenation of dimethyl adipate under the experimental conditions.Further kinetics studies showed that the reaction was one order with respect to H₂ but zero order to dimethyl adipate,which revealed the crucial important roles of hydrogen activation.In addition,to obtain further insight into the catalytic mechanism on basis of the structure-activity relationship of the copper-based catalysts,five kinds of Cu/SiO₂catalysts with different Cu⁰ and Cu⁺surface areas were prepared and evaluated in this study.The characterization results demonstrated that the series of catalysts had similar pore structure and copper nanoparticle size.And the space-time yield of hexanediol was found to be linear with the change of Cu⁰ surface areas,indicating that the Cu⁰ site was the key active site for the reaction.In fact,Cu⁰ is generally considered to be responsible for the adsorption and activation of hydrogen.These results are also confirmed by the above kinetic studies,further demonstrating that the activation of hydrogen by Cu⁰ sites is the controlling step of the surface reaction.This conclusion is totally different from that achieved in gas-phase hydrogenation of esters,in which the adsorption of esters on Cu⁺sites is always the rate-determining step.This work will provide important theoretical basis for the design and development of the highly efficient catalyst for the condensed hydrogenation of dimethyl adipate.The stability of Cu/SiO₂ in hydrogenation of condensed dimethyl adipate was significantly improved by carbon modification.In the process of the hydrogenation reaction,Cu/SiO₂ showed a bad stability with the conversion decreasing from 98%to52%during the 54 h reaction.However,it is interestingly found that the carbon modification could prolong the lifetime of the catalyst and suppress the deactivation effectively.The catalyst Cu-HP-2C with 2 wt.%carbon exhibited the best stability,whose conversion only decreased from 93%to 80%during a continuous reaction of100 h.It implied that the surface modification of Cu/SiO₂catalyst with a small amount carbon could inhibit the migration of copper nanoparticles on catalyst surface including those in bulk phase,which significantly prevented the aggregation of copper nanoparticles and improved the catalytic stability.