Studies On Liquid Phase Ketonization Of Carboxylic Acids Over Composite Metal Oxides In Near-critical Water

With the gradual depletion of non-renewable fossil resources such as coal,petrol and natural gas,it’s imperative to find an alternative renewable resource.Biomass is the only renewable resource that contains carbon,from which it’s the future trend to prepare chemicals.Bio-based carboxylic acids are the easiest to prepare on a large scale in biomass.Due to the high oxygen content,short carbon chains and low added-value of bio-based carboxylic acids,it’s of great importance to convert bio-based carboxylic acids to high added-value products with low oxygen content and long carbon chains.Liquid phase ketonization can convert organic carboxylic acids to long carbon chain ketones with higher added-value without vaporization and achieve high value utilization of organic carboxylic acids with low energy cost at lower temperature subsequently.With acetic acid,propanoic acid and butyric acid as model substrates,the paper carried out the studies on liquid phase ketonization of carboxylic acids to prepare long carbon chain ketones in near-critical water systematically,including the selections among composite metal oxides with different molar ratios,catalyst characterization and catalytic activity evaluation,the optimization of technical conditions and the kinetics fitting.A new technique to prepare long carbon chain ketones from direct liquid phase ketonization of carboxylic acids in near-critical water was established.The main work of this paper was summarized as follows:(1)Ce-Ni,Ce-Zr-Mn,Ce-Zr and Ce-Mn composite metal oxides catalysts with different molar ratios were prepared by co-precipitation and their catalytic activities in liquid phase ketonization of propanoic acid were studied and compared.The results show:The ketonization activities of Ce-Ni composite oxides are low and the doping of Ni does not improve the ketonization activity apparently;Among Ce-Zr-Mn composite metal oxides,the ketonization activities of single component metal oxides are the worst while the ketonization activities of three-component metal oxides are not superior to those of two-component metal oxides apparently;Among Ce-Zr and Ce-Mn composite oxides catalysts,the ketonization activities of both Ce-Zr and Ce-Mn composite metal oxides increase first and then decrease with the increasing amount of Zr or Mn.Ce2ZrOx and Ce0.6Mn0.4Ox are the catalysts with the highest ketonization activities in Ce-Zr and Ce-Mn composite metal oxides respectively.However,the hydrothermal stability of Ce0.6Mn0.4Ox is bad.Ce2ZrOx catalyst has high ketonization activity and good hydrothermal stability at the same time.Consequently,Ce2ZrOx catalyst is chosen for further research.Part of Ce-Zr oxides catalysts were characterized and the reasons for the high ketonization activity of Ce2ZrOx catalyst were explained from the perspectives of catalyst’s structures and properties:the introduction of Zr forms the Ce-O-Zr solid solution structure in Ce2ZrOx catalyst,which increases the oxygen vacancy content and basic sites on the catalyst’s surface and makes the specific surface area of the catalyst larger.These structures and properties make the catalyst possess more active sites and absorb more propanoic acid molecules,which can promote ketonization.(2)With propanoic acid as substrate and Ce2ZrOx as catalyst,part of technical conditions for liquid phase ketonization of propanoic acid were optimized and the reaction kinetics was also fitted.The optimization of technical conditions show:The appropriate catalyst loading for liquid phase ketonization of propanoic acid is 20 mg and the appropriate propanoic acid concentration is 5wt%.The conversion of propanoic acid and the yield of 3-pentanone reach 85%and 81.6%respectively under the reaction condition of 340℃ and 30 h.Through kinetics fitting,the activation energy for liquid phase ketonization of propanoic acid is 98.9 kJ/mol,the formation activation energy for 3-pentanone is 90.0 kJ/mol and the formation activation energy for other side products is 105.7 kJ/mol.(3)The reaction substrates for liquid phase ketonization of carboxylic acids were expanded and liquid phase ketonization of acetic acid and butyric acid was studied respectively.What’s more,the kinetics data for liquid phase ketonization of carboxylic acids with different carbon chain lengths was compared under the same condition.The results show:Apart from propanoic acid,Ce2ZrOx catalyst can also catalyze liquid phase ketonization of acetic acid and butyric acid efficiently.Under the same reaction condition,the order of liquid phase ketonization activities of acetic acid,propanoic acid and butyric acid over Ce2ZrOx is acetic acid>propanoic acid>butyric acid,illustrating the point that ketonization of carboxylic acids is harder to occur with the increasing lengths of their carbon chains.The reaction techniques and kinetics for liquid phase ketonization of acetic acid were studied heavily.The results show:The appropriate catalyst loading for Ce2ZrOx is 20 mg.The conversion of acetic acid and the yield of acetone reach 96.8%and 82.8%respectively under the reaction condition of 340℃ and 25 h.Through kinetics fitting,the activation energy for liquid phase ketonization of acetic acid is 82.8 kJ/mol,the formation activation energy for acetone is 79.3 kJ/mol and the formation activation energy for other side products is 84.1 kJ/mol.The activation energy for liquid phase ketonization of acetic acid is lower than that of propanoic acid(98.9 kJ/mol),illustrating the point that ketonization of acetic acid is easier to occur than ketonization of propanoic acid.The result above explains the reason that ketonization activity of acetic acid is apparently higher than that of propanoic acid from the perspective of kinetics.The research work in this paper can provide a new way for liquid phase ketonization of organic carboxylic acids to prepare high added-value chemicals with low energy cost.It can also provide a reference for the development of efficient and water resistant ketonization catalysts.