The Hydrogenation Of CO₂-Derived Ethylene Carbonate To Methanol And Ethylene Glycol Over Cu-Based Catalysts

In order to deal with environmental and energy problems,utilization and conversion of carbon dioxide has attracted much attention in industry and academia.Indirect hydrogenation of carbon dioxide to methanol,via the cycloaddition of CO₂with ethylene oxide from the well-established‘‘Omega process”to produce ethylene carbonate(EC)and the following hydrogenation of EC,is an effective route to convert carbon dioxide to value-added chemicals.The hydrogenation of EC can afford higher methanol yields under mild conditions.More importantly,this indirect CO₂ conversion process can be considered as an example of a green carbon cycle that generates no carbon waste with overall 100%atom economy.With regards to the hydrogenation of EC,Cu-based catalysts have been widely studied in that Cu~0/Cu⁺active species on Cu-based catalysts can furnish excellent catalytic performance for the target reaction.Nevertheless,there are still some problems to be solved.In particular,the respective contribution and proper ratio of Cu~0/Cu⁺species are still a matter of debate.More importantly,it still sufferes from excess hydrogen/ester ratio,unsatisfactory methanol selectivity and catalyst stability.Therefore,development of Cu-based catalysts with high activity and high stability is of significance for the industry and academia.In this thesis,the preparation methods for Cu/SiO₂ catalysts were first investigated to adjust the molar ratios of Cu⁺/(Cu~0+Cu⁺),surface acidity on the catalyst surface and the hydrogenation performance towards the EC hydrogenation.Then,modification of Cu/SiO₂ catalysts will be adopted by addition of MoOx or sorbiol on the basis of the optimized preparation procedures.The specific contributions,optimal ratios of Cu~0 or Cu⁺species,surface acidity and Cu-promoters interface will be studied in detail.Moreover,the adsorption and activation of EC on Cu~0/Cu⁺species will be characterized in combination with various techniques.Furthermore,the relationship between the adsorption?activation process of EC and the reaction performance will be established.The results are summarized as follows:(1)Efficient synthesis of methanol and ethylene glycol via the hydrogenation of EC on Cu/SiO₂ catalysts with balanced Cu⁺-Cu~0sitesWe prepared a 29.1 wt%Cu/SiO₂-MHT catalyst by a modified hydrothermal method and applied to the hydrogenation of EC successfully.In comparison with its three analogues with similar Cu loading and comparable Cu particle size but prepared by different methods,we found that the catalytic performance of Cu/SiO₂ catalysts including the activity and the selectivity to Me OH could be tuned by adjusting the Cu⁺/(Cu~0+Cu⁺)ratio via different preparation methods.As a result,the Cu/SiO₂-MHT catalyst furnished 89%Me OH yield and 37.6 h^-1 turnover frequency,which are one of the highest values among all the heterogeneous Cu catalysts up to now to our best knowledge.The results reveal that the cooperative effect exists between Cu~0 and Cu⁺.When Cu~0 species are sufficient to activate H₂,the more Cu⁺species and the higher Cu⁺surface areas,the more favorable formation of Me OH.Furthermore,more Lewis acid amount on the Cu/SiO₂-MHT catalyst is also beneficial for Me OH formation.(2)Continuous hydrogenation of EC to methanol and ethylene glycol at Cu-MoO_x interface with a low H₂/ester ratioA series of MoO_x-promoted Cu/SiO₂ catalysts,which were prepared via a one-pot modified hydrothermal method and derived from copper phyllosilicate precursors.The Cu-0.02MoO_x/SiO₂catalyst exhibited superior catalytic performance(almost full EC conversion with 89%Me OH selectivity and 99%EG selectivity)and good stability(>150 h)at a H₂/EC ratio(20).Remarkably,the turnover frequency(TOF)and space time yield of Me OH(STY_{Me OH})reached up to 17.5 h^-1and 0.207g×g^-1_cat×h^-1,respectively.The characterization results and kinetic experiments revealed that decoration of Cu species by MoO_x obviously altered the electronic properties of the catalysts,enhanced the Cu⁺/(Cu~0+Cu⁺)ratios and reduced the activation energy.Such decoration may resulted in possible formation of Cu-O-Mo bond between Cu species and MoO_x species located on the periphery of Cu-MoO_x interface,which might strengthen the adsorption and activation of EC,stabilize the Cu nanoparticles,and thus enhnace the selectivity to Me OH and catalyst stability.(3)Ultrastable auto-reduced Cu@C/SiO₂ catalyst prepared via a sacrificial coating induced encapsulation strategy for the continuous hydrogenation of ECWe applied a novel approach for preparation of an ultrastable auto-reduced Cu@C/SiO₂ nanocatalyst for the hydrogenation of CO₂-derived EC,in which Cu nanoparticles are encapsulated by graphitized carbon overlayers.Sorbitol-coated Cu nanoparticles,Cu@sorbitol/SiO₂,were prepared by a modified ammonia evaporation method.Then,one-step calcination of Cu@sorbitol/SiO₂ under N₂ atmosphere leads to decomposition of sorbitol to CO and CO₂,not only directly realizing reduction of Cu²⁺to Cu~0/Cu⁺without further using other reducing agents,but also forming carbon overlayers on Cu surface via the CO disproportionation.To our delight,at an extremely low H₂/EC molar ratio of 20,the Cu@C/SiO₂ catalyst exhibited superior catalytic performance(91%Me OH yield and a turnover frequency of 43.6 h^-1),which notably surpasses most of the previously reported Cu-based catalysts.Of particular note is that the Cu@C/SiO₂ catalyst didn't show any deactivation during 736 h time-on-stream test.The characterization and kinetic studies revealed that the carbon overlayers on the outer surface of Cu nanoparticles functioned synergistically with surface Cu~0/Cu⁺sites to promote the EC hydrogenation rather than block the surface reaction.Moreover,the carbon overlayers suppress the sintering of Cu nanoparticles and stabilize the ratio of Cu~0/Cu⁺.