Research Progress in Ketonization of Biomass-derived Carboxylic Acids over Metal Oxides

With the increasing needs for transportable fuels and the growing concerns on environmental pollution, significant attention has been paid to the conversion of renewable lignocellulosic biomass to liquid fuels. As a major component of bio-oil from biomass depolymerization, organic carboxylic acids make the bio-oil acidic, corrosive and unstable, which are harmful for storage, transportation, and upgrading of bio-oil. Therefore, the removal of carboxylic acids is very important. Ketonization reaction, also called ketonic decarboxylation, converts two moles carboxylic acids to ketone (symmetrical or asymmetrical ketones), carbon dioxide and water, which removes oxygen efficiently and increases the carbon chain length without using hydrogen. In addition, ketones are important chemicals and have been widely used in chemical industry as organic solvent. The mechanism and active site for ketonization are still under debate. Various mechanisms have been proposed for the ketonization, based on different reaction intermediates evolved (i.e., beta-keto-acids, ketene, carboxylates and acyl carbonium ions). Ketonization reaction is a surface-structure-sensitive reaction, thus reaction activity depends on surface-structure of the metal oxides (such as crystal surfaces and particle size). The concerted function of oxygen anions (Bronsted bases) and unsaturated metal cations (Lewis acids) is crucial for ketonization. The amphoteric oxides show better catalytic activity than pure acidic or basic oxides. Oxygen vacancy formed on the surface of metal oxides is a key factor for high ketonization activity, which can stabilize the reaction product and reduce the activation energy. This paper reviews the progress in ketonization from the aspects of reaction mechanism, and the effects of surface structure, acidity and basicity, and reducibility of metal oxides on ketonization. The beta-keto-acids based mechanism and ketene based mechanism will be discussed in detail to understand how does the C-C coupling happen and the fundamental role of a-H. Finally, the importance of surface structure and properties of metal oxides on the carboxylic acids ketonization reaction is explained.