Conversion of lignocellulose to biofuels and chemicals via sugar platform: An updated review on chemistry and mechanisms of acid hydrolysis of lignocellulose

Lignocellulosic biomass has been considered as one of the most promising feedstocks for production of the second-generation biofuels and biochemicals without food and fuel debate. Sugar platform is one of the most important pathways for lignocellulose conversion. In this paper, acid hydrolysis of lignocellulose for sugar release were reviewed and discussed, focusing on the chemistry and kinetics. The hierarchical and multi-layered structure of lignocellulose cell wall is the major barrier for efficient hydrolysis of cellulose by limiting the mass transfer of catalysts. Concentrated acids can hydrolyze lignocellulose under mild condition with high sugar yields, but the difficulty in separation of acid and sugar with relatively high loss of the acids has become the major obstacle for commercialization. Novel strategy should be developed to efficiently recover acid catalysts. Dilute acid hydrolysis consumes much less acids, but requires high temperature to achieve a high degree of cellulose hydrolysis; however, decomposition of the monosaccharides, especially xylose, becomes more serious. Increasing the sugar yield and reducing the sugar degradation should be primarily considered for dilute acid hydrolysis. Efforts can be made by kinetic control and integration of the process to reduce energy consumption and extra cost of post-treatment of the hydrolysate. Though solid acid hydrolysis has advantages such as ease in catalyst reuse, there is still a long way to go for practical application.