Selective Hydrogenation Of Cyclohexyl Acetate Over Cu-based Catalysts

Cyclohexanol is widely used in textiles,medicine,dyes,coatings and other industries,and it is also a high value-added chemical intermediate,acting as an important chemical raw material for the synthesis of hexamethylenediamine and caprolactam.Three traditional methods for the synthesis of cyclohexanol in industry are as follows:phenol hydrogenation,cyclohexane oxidation and cyclohexene hydration.However,all of them have some drawbacks.A new synthesis route to produce cyclohexyl acetate via the esterification of cyclohexene and acetic acid,followed by further hydrogenation to co-produce ethanol and cyclohexanol,not only provides a new idea for the production of cyclohexanol,but also alleviates the current excess production capacity of acetic acid.Due to the advantages of high activity in hydrogenation and high selectivity to C-C bond cleavage,Cu-based catalysts have been widely used in the hydrogenation of ester compounds.In this thesis,different Cu-based catalysts after modified with Zn or Zr were prepared and investigated in the hydrogenation of cyclohexyl acetate.In the first part,a series of Zn-modified Cu₂Zn_x/Al₂O₃ catalysts were prepared by deposition and precipitation method for the hydrogenation of cyclohexyl acetate to co-produce ethanol and cyclohexanol.Compared with Cu/Al₂O₃ catalyst,addition of a small amount of Zn to Cu-based catalyst could significantly improve the catalytic activity.As a result,a Cu₂Zn_1.25/Al₂O₃ catalyst showed the remarkable hydrogenation catalytic performance in the hydrogenation of cyclohexyl acetate.At 250°C and under3 MPa of H₂,the Cu₂Zn_1.25/Al₂O₃ catalyst could furnish 93.9%conversion of cyclohexyl acetate and 97.2%selectivity to ethanol along with 97.1%selectivity to cyclohexanol in a batch reactor.In addition,the catalyst showed excellent stability,and there was no obvious deactivation after five cycles.It was found that the addition of an appropriate amount of Zn species could promote the dispersion of copper nanoparticles and adjust the distribution of acid strength and acidity on the catalyst surface.The IR spectroscopic studies using pyridine as probes showed that there were Lewis acid sites on the Cu₂Zn_x/Al₂O₃ catalyst surface,and the Cu₂Zn_1.25/Al₂O₃ catalyst surface had abundant weak acid sites,which plays an important role in the adsorption and activation of carbon-oxygen bonds.Moreover,the Cu₂Zn_1.25/Al₂O₃ catalyst had the highest Cu⁺/（Cu~0+Cu⁺）molar ratio,which may be the main reason for its exceedimgly catalytic performance.In the second part,a series of Cu-Zr binary catalysts were prepared by sol-gel method（SG）,co-precipitation method（CP）and impregnation method（IM）and applied for the hydrogenation of cyclohexyl acetate to co-produce ethanol and cyclohexanol.Among them,the Cu₃Zr₇-SG catalyst showed excellent catalytic performance for the target reaction.Under the optimized preparation and reaction conditions,the Cu₃Zr₇-SG catalyst afforded a 97.4%conversion of cyclohexyl acetate along with 96.3%and95.5%selectivity to ethanol and cyclohexanol,respectively.Furthermore,it showed good stability during 72 h time-one-stream test in a fixed-bed reactor.Compared with the catalysts prepared by the other two methods,the Cu₃Zr₇-SG catalyst had weaker interaction between Cu and Zr species so that the Cu nanoparticles could be more easily reduced.Additionally,the Cu₃Zr₇-SG catalyst surface had more weak acid sites and moderate basic sites.The synergistic effect of acid-base sites was helpful for the adsorption and activation of carbon-oxygen bonds.More importantly,the Cu₃Zr₇-SG catalyst had the highest Cu⁺/（Cu~0+Cu⁺）and O_ads/(O_ads+O_lat)molar ratios and the most Cu⁺species and abundant oxygen vacancies,which may be the reasons for its excellent catalytic performance.