Studies On Cerium Based Catalysts For Ketonization Of Propionic Acid

In order to mitigate the adverse impacts of fossil energy on climate and the environment,the conversion of renewable biomass into valuable chemicals and fuels has received increasing attention.Lignocellulosic and animal/vegetable grease are two major biomass sources of carboxylic acids,which are expected to be converted into bio-based fuels and chemicals through ketonization reactions.In this C-C coupling reaction,two carboxylic acids react to form a ketone molecule and produce water and CO₂ as by-products.Metal oxides are highly active and selective catalysts for ketonization reaction,among which cerium oxide（CeO₂）is one of the ketonization catalysts with best activity.However,CeO₂ also has some problems such as low reactivity and poor reaction stability,and the mechanism of ketonization reaction on CeO₂ is not clear,and the specific reaction intermediates is still controversial.Therefore,it is necessary to study the design strategy and reaction mechanism of CeO₂ catalyst to achieve stable and efficient ketonization.In this paper,the effects of CeO₂ catalysts on vapor phase ketonization were studied.Firstly,CeO₂ rod,otahedron and cube were synthesized and exposed to（110）,（111）and（100）surfaces respectively.The ketonization of propionic acid catalyzed by CeO₂rod,octahedron and cube was studied.The ketonization reaction rates of CeO₂ rod,octahedron and cube were 54.3,40.4 and 25.1 mmol·m^-2·h^-1 at 350℃ respectively,indicating that（110）surface was the most active surface for ketonization.The reaction was tracked by in situ infrared and transient mass spectrometry.The results show that monodentate propionate,as a few surface species,played a decisive role in the reactivity.The density functional theory（DFT）study shows that the relative concentration of monodentate propionate under high acid coverage is closely related to the surface geometry.The results show that the surface geometry of CeO₂ determines the abundance of monodentate propionate,and the abundance of monodentate propionate determines the ketonization activity of CeO₂.Secondly,improving the stability of ketonization over metal oxide catalysts remains an important challenge.In this work,a series of Ce MnO_x nanorods catalysts were prepared,and their application in ketonization was tested.These catalysts exhibit rod-like morphology and expose specific（110）surfaces.,the turnover frequencies（TOFs）of ketonization of propionic acid on Ce Mn_0.1O_x and CeO₂-R catalysts are 7.86and 4.12 s^-1 respectively.In addition,Ce Mn_0.1O_x catalyst also shows excellent catalytic stability compared with the results in literatures.The propionic acid conversion on Ce Mn_0.1O_x remains at 94.5%of the initial conversion after 25 h(reaction conditions:350℃,W/F=0.05 h,P_acid=4.0 k Pa).However,the conversion of propionic acid on CeO₂-R can only maintain 70.2%of the initial conversion.In situ infrared results show that the bidentate propionate adsorbed on Ce Mn_0.1O_x catalyst is easier to desorption than CeO₂-R catalyst,thus improving the catalytic stability of ketonization.Thirdly,on the basis of the solvothermal method,the CeO₂ catalysts were design by MOF strategy and were applied to propionic acid ketonization.A series of CeO₂-UiO℃ tahedral catalysts were synthesized using cerium-organic framework Ce-UiO-66 as raw material,and was used for the vapor phase ketonization of propionic acid.The TEM and BET results show that CeO₂-UiO octahedron is composed of mesoporous nano-CeO₂ crystals.XPS and Raman results show that CeO₂-UiO-450 catalyst forms more oxygen vacancies than CeO₂-P catalyst prepared by precipitation method.Compared with CeO₂-P,the intrinsic reaction rate of CeO₂-UiO-450 was increased.The TOF of propionic acid on CeO₂-UiO-450(7.45 s^-1)is 1.38 times higher than that of CeO₂-P(5.41 s^-1)at 350℃.The IR results show that monodentate propionate is reactive adsorption configuration and its reaction rate on CeO₂-UiO-450 was faster than that of CeO₂-P.There is a linear correlation between the vacancy concentration and the intrinsic rate,suggesting that oxygen vacancy promoted ketonization on CeO₂.Finally,on the basis of improving the activity of the design strategy,Na-CeO₂catalysts were prepared by improving the traditional precipitation method to improve the stability of ketonization.A series of CeO₂ with different Na content doping were successfully synthesized by impregnation method,and were used for catalytic ketonization of propionic acid.The TEM results show that the prepared catalysts are Na doped CeO₂ polyhedron particles.Compared with pure CeO₂-P,Na doped CeO₂catalysts significantly enhance the ketonization stability.The initial conversion of propionic acid on Na_0.1Ce_0.9O_x was 9.1%at 350℃ with W/F=0.05 h,and almost unchanged（8.8%）after 8.5 h.Meanwhile,conversion on CeO₂-P decreased from 10.6%to 2.8%at 8.5 h.Therefore,alkali metal doped CeO₂ nanoparticles can be used for long-term ketonization of carboxylic acid.The carbon deposition on Na_0.1Ce_0.9Ox is 1.8%,which is obviously lower than that on CeO₂-R（3.6%）.The enhancement of surface basic O caused by Na doping is the main reason for the improvement of CeO₂ stability.