Catalytic fast pyrolysis of agricultural residues and dedicated energy crops for the production of high energy density transportation biofuels. Part II: Catalytic research

Catalytic fast pyrolysis (CFP) is an advanced controlled process of biomass thermal decomposition designed to produce high volumes of ready-to-use transportation biofuels or to provide improved quality bio-oil for subsequent refining or upgrading. The present work is the second part of a thorough review on the CFP of lignocellulosic biomass, emphasizing on the exploitation of agricultural residues and dedicated energy crops. Due to the increased interest on the production of high energy density transportation biofuels, such as green gasoline, green jet fuel and green Diesel, CFP has been studied intensely during the last years, using various biomass feedstocks in both in-situ and ex-situ catalytic studies. The scope of the present review is to illustrate and critically discuss the known science and the latest research outcomes of the CFP of agricultural residues and herbaceous or woody dedicated energy crops as it has been tested experimentally over the wide spectrum of available catalysts in the groups of zeolites, mesoporous catalysts, metal oxides, inorganic salts and carbon-based materials. The selection of the appropriate catalyst has a paramount effect on the features of the produced bio-oil, such as composition and properties of the obtained liquid end-product of the CFP process. This is because it influences the overall chemistry/pathways of the CFP process through the selective promotion of certain dehydration, decarboxylation, decarbonylation, hydrodeoxygenation, catalytic cracking and condensation (ketonization and Aldol condensation) reactions, and also, determines the stability and quality of the obtained bio-oil by decreasing its oxygen content and by improving its final properties for use as a fuel or for subsequent further upgrading. As it is discussed in this review, many existing catalysts are able to greatly enhance bio-oil quality by promoting the production of useful aromatic hydrocarbons, phenolics or alkanes. However, further research is needed towards the optimization of the catalytic performances in terms of activity, product selectivity and resistivity against deactivation.