Next Generation Liquid Phase Biorefinery Catalysts

Oil refineries generate low volume speciality chemicals and high volume fuel and basic chemicals at the same time using heterogenous catalysts specifically designed for these purposes. This is a successful strategy, allowing all of the value of the feedstock to be extracted but at the same time producing vast quantities of CO2. Biorefineries processing lignocellulose waste from agriculture (e.g logging), woody crops (e.g eucalyptus) and biomass residues (straw, bagasse, risk husk) for fuels would reduce greenhouse emissions by more than 60%. Lignocellulose is a biopolymer comprised of cellulose and hemicellulose cross-linked together with lignin via ester and ether linkages. This rigid structure, useful for plants in keeping them upright, hampers conversion leading to pre-treatment steps such as fermentation, gasification or pyrolysis. Any pre-treatment process offsets the benefits of using the biofuel due to waste generated and chemical resources expended e.g. acid/base treatment.
Compared to petroleum refining (vapour phase, >400C), the reaction conditions for biostock are very different. Extensively taking place in the liquid phase, lower temperature reactions such as hydrolysis, aldol condensation and oxidation reactions are typical. Therefore the catalysts used in oil refineries do not lend themselves well to the biorefinery as they are focussed on reduction at high temperature not oxidation at low temperature. Low concentrations from fermentation liquors (10%) in aqueous solutions containing impurity species, means the catalyst needs to be water tolerant, transform organic molecules directly and retain reactivity in the presence of the broth impurities
To tackle these challenges, high surface area supports with enhanced reactant accessibility to access active acid/base groups are required with tuneable hydrophobicity, hydrothermal stablily over a wide pH range that are resistant to in situ leaching. Catalyst porosity is crucial to enable diffusion of bulky, viscous reactants in these liquors to active sites; support materials with larger pores (large pore zeolites, mesoporous materials) are one solution but there is always uncertainty about where the crucial reactions are taking place - using sites in the walls of the material or on the surface of the materials. The source of this problem lies in the way these materials are made; normally placing materials in the reaction mixture within the synthesis step and/or not controlling how the catalysis is laid down on a surface and built up into the 3D active system. This means it is impossible to identify the structural-property relationships which are so important to solving materials connundra in these systems. This CDT takes a very different approach to build catalysts from the base up using lessons learnt from the lower dimensional systems to build into this system controlling the positioning, growth and interaction of the actives sites on/in the catalyst to separate and transform organic molecules in the liquor.