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Solid acids as heterogeneous catalysts for cellulose hydrolysis have drawn increasing attention; however, current solid acids face challenges such as high catalyst loading (low catalytic activity), poor catalyst-substrate interaction, deficient hydrothermal stability, and unsatisfactory recyclability. This review critically discussed the recent efforts and progress in overcoming the issues of solid acids and developing high-performance solid acids for hydrolyzing cellulose. The key structural features of solid acids and their effects on the interactions with cellulose and cellulose hydrolysis were addressed in detail. Strategies and perspectives to enhance performance, hydrothermal stability and recyclability of solid acids were provided. 

Zeolites (ZSM-5 and Beta) with different SiO2/Al2O3 ratios were synthesized as solid acids for hydrolyzing cellulose in an inorganic ionic liquid system (lithium bromide trihydrate solution, LBTH) under mild conditions. The results indicated that the texture properties of zeolite had little effect on catalytic activity, while acidity of zeolite was crucial to the cellulose hydrolysis. In the LBTH system, H-form zeolites released H+ into the solution from their acid sites via ion-exchange with Li+, which hydrolyzed the cellulose already dissolved. This unique homogeneous hydrolysis mechanism was the primary reason for the excellent performance of the zeolites in catalyzing cellulose hydrolysis in the LBTH system. It was found cellulose could be completely hydrolyzed to glucose and oligoglucan by 2% (w/w on cellulose) zeolite at 140 °C within 3 h with a single-pass glucose yield 61%. The zeolites could be recovered with 50% initial catalytic activity after regeneration and reused with stable catalytic activity. 

Lignin was depolymerized in the solid state and separated from lignocellulosic biomass in high-purity and in a less condensed form for valorization.