Substitution of fossil-derived fuels by pyrolysis bio-oils is a potentially economic solution. However, the high acidity of pyrolysis bio-oils caused by carboxylic acids constitutes a big challenge to existing petroleum refining facilities. Catalysts are crucial for the successful deoxygenation of carboxylic acids to products that are less corrosive to reactors. This review addresses recent advances in the deoxygenation of carboxylic acids from pyrolysis bio-oils over many different types of catalysts, focusing on the required catalytic properties for partial or complete deoxygenation. Recent achievements suggest that deoxygenation of carboxylic acids can be fulfilled when the following catalytic properties are present: hydrogenation/hydrogenolysis sites such as metal sites or Bronsted acid sites to deliver hydrogen, acid sites for alcohol dehydration, and proper metal-oxygen bond strength in the oxide support for oxygen vacancy creation and regeneration. A synergic effect of these sites determines the final deoxygenation: partial or complete. Modification to the metal sites and acid sites can be achieved in a simple manner by proper promoter addition, which enables to tune the selectivity of the deoxygenation products to the desired ones.
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