Solid acid mediated hydrolysis of biomass for producing biofuels

Solid acid catalysts, which have favorable characteristics such as efficient activity, high selectivity, long catalyst life and ease in recovery and reuse, have great potential for efficiently transforming lignocellulosic biomass into biofuels and can replace many conventional liquid acids for hydrolysis and pretreatment. This work briefly introduces conventional biomass pretreatment and hydrolysis techniques, and reviews in detail the characteristics of biomass hydrolysis for five types of solid acid catalysts grouped as H-form zeolites, transition-metal oxides, cation-exchange resins, supported solid acids and heteropoly compounds. Carbonaceous solid acid (CSA) catalysts are considered as the most promising catalyst for cellulose hydrolysis, since they provide good access of reactants to the acidic sites of SO3H groups. High glucose yields of up to 75% with 80% selectivity have been achieved at 150 °C for 24 h with CSA. However, separation of CSA from un-hydrolyzed cellulose residues after hydrolysis needs further research since these catalysts have similar physical and chemical properties to the residues. Use of functionalized CSA catalysts that contain paramagnetic groups is one method to improve CSA separation and reuse. Suggestions are given for promoting catalytic efficiency for each kind of solid acid catalysts. Methods to promote reactions or increase selectivities such as microwave, ultrasonication and nanotechnology are introduced. Finally, we highlight a recent strategy that exploits acid-functionalized paramagnetic nanoparticles suitable for cellulose hydrolysis, and address new opportunities for the use of solid acid catalysts.