The Research Of Propionic Acid Ketonization Reaction Over Ceria,Zirconium Based Catalysts

With the depleting of fossil fuels and the growing concerns on envrionment,thebiomass as a alternative energy sources has been gained significant attention to replace fossil fuels.The bio-oil is a complex mixture of oxygen-containing compounds,which is derived from the decomposition of the biomass.There is 17wt%carboxylic acid in bio-oil.Due to the acidity and instability,the conversion of carboxylic acid is important in the upgrading of bio-oil.Ketonization is an important approach for carboxylic acid upgrading to produce ketone.Therefore,it is of great importance for the study of ketonization of carboxylic acid over catalysts.The reaction activity of propionic acid ketonization on CeO₂,ZrO₂ and their modified catalysts was studied.The effect of Ga on the structures of the Ce-Ga mixed oxides prepared by co-precipitation method was investigated using X-ray diffraction,Raman,high-resolution transmission electron microscopy and solid-state NMR.The results showed that Ga with the content up to 25 mol.%was incorporated into the CeO₂ unit cell with the formation of Ce-Ga mixed oxides,which was acoompanied of the increase of surface area and oxygen vacancies.The characteration of X-ray photoelectron spectroscopy suggested that some oxygen atoms were removed,and Ce⁴⁺was reduced into Ce³⁺.The reducibility and acidity-basicity were all changed.Diffuse reflectance infrared Fourier transform spectroscopy(DFIFTS)of propionic acid adsorption showed that the amount of adsorbed propionic acid was maximized at Ce_0.75Ga_0.25O_2-x.Additionly,the more monodentate propionate and less bidentate propionate were present on the Ce-Ga mixed oxides(Ga/Ce?1/3),indicating the adsorption configurations were related to the structure varation.The conversion of propionic acid was investigated at 290-350 ^oC on the Ce-Ga mixed oxides.The rate was maximized on Ce_0.75Ga_0.25O_2-x.Kinetic study showed that ketonization of propionic acid on CeO₂ and Ce_0.75Ga_0.25O_2-x followed the same mechanism,but the adsorption strength of propionic acid on Ce_0.75Ga_0.25O_2-x was stronger than that on CeO₂.The ketonization rate on Ce-Ga mixed oxides(Ga/Ce?1/3)was enhanced due to the higher concentration of monodentate propionate present on the surface.Ketonization of propionic acid was investigated at 270-350 ^oC over Ce_xZr_1-xO₂mixed oxides with varying Ce/Zr ratios prepared by co-precipitation method.The structure of Cex Zr1-x O2 mixed oxides transformed from fluorite cubic structure to tetragona phase with increasing Zr content.The phase segregations took place in the Ce_0.5Zr_0.5O₂ sample.With increasing Zr content,the acidity of the Ce_xZr_1-xO₂increased,and the density of the strong base sites reduced.The DRIFTS of propionic acid adsorption indicated that increasing Zr content in Ce_xZr_1-xO₂ favored monodentate adsorption over bidentate adsorption.The rate was maximized at a medium-strength acid/base ratio of?1.07 on Ce_0.1Zr_0.9O₂.This value is close to 1,indicating the balanced medium-strength acid-base sites played a crucial role in the ketonization.Vapor-phase ketonization of propionic acid was studied at 300-375 ^oC over ZrO₂.The tetragonal ZrO₂(t-ZrO₂)were more active than monoclinic ZrO₂(m-ZrO₂).There were more medium-strength Lewis acid-base sites with lower coordination exposed on m-ZrO₂ relative to t-ZrO₂.The in situ DRIFTS of adsorbed propionic acid under ketonization reaction revealed that the monodentate propionates were more active than bidentate propionates.In comparison with m-ZrO₂,the t-ZrO₂ surface favored monodentate adsorption over bidentate adsorption.Additionally,the adsorption strength of monodentate propionate was weaker on t-ZrO₂.These differences in adsorption configuration and adsorption strength of propionic acid were affected by the zirconia structure.The higher surface concentration of monodentate propionates contributed to the higher ketonization rate in the steady state.