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.,(β-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 (Brφnsted 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 β-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 α-H.Finally,the importance of surface structure and properties of metal oxides on the carboxylic acids ketonization reaction is explained.%从可再生的木质纤维素生物质制备液体燃料受到越来越多的关注,有机羧酸是生物质解聚生物油的重要成分,使得生物油具有酸性、腐蚀性和不稳定性.因而,羧酸的去除十分关键.酮基化反应将两分子羧酸转化为酮、二氧化碳和水,不使用氢气的情况下高效脱氧且增加碳链长度.此外,生成的酮为重要化学品.目前酮基化反应的机理和活性位的研究还存在争论.因酮基化反应过程生成的中间产物不同(如β-酮酸、酮烯、羧化物、酰基碳正离子等),研究者们提出了不同的反应机理,如β-酮酸机理和酮烯机理.酮基化反应属于结构敏感性反应,因此金属氧化物表面结构的不同会导致酮基化反应活性不同.酸碱位协同作用在酮基化反应过程中是必不可少的,同时氧空位可以提高酮基化反应的活性.本综述重点从酮基化反应机理、金属氧化物的表面结构、酸碱性及氧化还原性方面对酮基化反应进行了评述,并对其进行了展望.
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