Synthesis And Kinetics Study Of Cu-based Catalysts For Dehydrogenation Of Alcohols

Cu-based catalysts play pivotal roles in methanol synthesis,selective catalytic reduction of NO_x,water-gas shift reaction,electroreduction of CO₂,selective hydrogenation of alkynes,dehydrogenation of alcohols and hydrogenolysis of glycerol.However,the main drawback of traditional Cu-based catalysts lies in the difficulty to control precisely the composition of catalysts,low dispersion of Cu and the weak interaction between Cu and support when the content of Cu is high.Moreover,the application of Cu-based catalysts under high temperature remains a challenging problem due to the low Tammann temperature of Cu.Therefore,it is of great importance to synthesize stable Cu-based catalysts with controlled structure and morphology from the precursors of well-defined materials,such as layered double hydroxide and metal-organic framework materials.At the same time,the dehydrogenation of alcohols to aldehydes and ketones over Cu-based catalysts is a promising approach to produce high-valued chemicals,because the only by-product in this process is hydrogen and the formed products are easy to separate and refine.Herein,the dehydrogenation of 1,4-butanediol toγ-butyrolactone,the dehydrogenation of ethanol to acetaldehyde and the dehydrogenation of cyclohexanol to cyclohexanone were selected as target reactions,and several Cu-based catalysts are prepared and tested in above reactions.The textural properties,morphology,surface properties of these catalysts were characterized in details.Besides,the relationship between the structure and the performance of these catalysts,and the kinetics study of these reactions were discussed.The main contents are as follows:1.Cu₂/Zn₂Mg₂Al₂O₇ catalyst was prepared via the controlled calcination and reduction of hydrotalcite-like structured Cu₂Zn₂Mg₂Al₂（OH）₁₆CO₃ precursor,and used in the dehydrogenation of 1,4-butanediol.Characterization results found that the fine skeleton of hydrotalcite could enhance the dispersion of Cu,strengthen the interaction between Cu and support and improve the utilization efficiency of Cu.As a result,Cu₂/Zn₂Mg₂Al₂O₇ was highly active,selective and stable for the dehydrogenation of1,4-butanediol toγ-butyrolactone.The selectivity and space-time yield ofγ-butyrolactone reached 99.5%and 7.1 g/g-cat/h with a 99.2%conversion of 1,4-butanediol(240 ℃,WHSV=7.7 h^-1).The kinetics of this reaction was consistent with the first order.Besides,Cu₂/Zn₂Mg₂Al₂O₇ was also effective for the dehydrogenation of 1,5-pentanediol and 1,4-pentanediol.2.A series of Cu_1.4/Zn_xMg_4.6-xAl₂O_7.6 catalysts with varied acidity and basicity were prepared by adjusting the molar ratio of Zn/Mg in the hydrotalcite-like Cu_1.4Zn_xMg_4.6-xAl₂（OH）₁₆CO₃ precursors,and used in the dehydrogenation of ethanol.Compared with the traditional impregnation method,Cu_1.4/Zn_3.2Mg_1.4Al₂O_7.6 from hydrotalcite-like precursor possesses plenty of unsaturated Cu sites,large interface between Cu and support,strong metal-support interaction,and showed excellent performance for the dehydrogenation of ethanol.The selectivity and space-time yield of acetaldehyde reached 99.1%and 7.0 g/g-cat/h,respectively,at 280 ℃ and WHSV=11.9 h^-1.And kinetic experiments disclosed that the adsorption of ethanol over Cu_1.4/Zn_3.2Mg_1.4Al₂O_7.6 might be the rate determining step in the dehydrogenation of ethanol.3.An ultra-stable Cu/ZnO@SiO₂ catalyst was synthesized,in which silicon was introduced into CuZn-BTC precursor via a steam-assisted hydrolysis method before calcination and reduction,and used in the dehydrogenation of ethanol.It was found that Cu particles were immobilized by the octahedral shaped SiO₂ hollow and fine ZnO nanoparticles in Cu/ZnO@SiO₂,which improved the stability of the catalyst.Cu/ZnO@SiO₂ could maintain its activity within 200 h on stream in a severe condition（350 ℃）.Meanwhile,SiO₂ hollow could keep the framework of CuZn-BTC,increase the specific surface area and the exposed active sites of Cu/ZnO@SiO₂.As a result,it exhibited prominent activity for the dehydrogenation of ethanol.The selectivity and STY of acetaldehyde reached 98.5%and 9.0 g/g-cat/h,respectively(350 ℃,WHSV=3.2 h^-1).4.A series of CM-x catalysts were prepared via the calcination and reduction of Cu-BTC/MgO-x precursors using MgO as a core.These catalysts possess high basicity,strong interaction between Cu and MgO,and well-distributed Cu nanoparticles,which are beneficial to enhance the accessibility between cyclohexanol and Cu nanoparticles,inhibit the dehydration of cyclohexanol,and increase the selectivity of cyclohexanone.The selectivity and space-time yield of cyclohexanone reached 99.5%and 6.2 g/g-cat/h,respectively,over CM-3 at 270 ℃ and WHSV=9.7 h^-1.Furthermore,the adsorption of cyclohexanol might be the rate determining step over CM-3,and the calculated apparent activation energy was 63.1 k J/mol.It was also found that those hydrogen acceptor solvents could enhance the conversion of cyclohexanol obviously.In summary,a series of Cu-based catalysts for dehydrogenation of alcohols were prepared and characterized,the relationship between their structure and performance,and the kinetics of these reactions were discussed in this dissertation.We hope our research could provide reference for the application of Cu-based catalysts in the heterogeneous catalysis.