Many solid acid catalysts, such as cation-exchange resins [76], carbonaceous solid acids [79], and solid HPA catalysts [32], have shown high activity and selectivity for cellulose hydrolysis. Up to now, amorphous carbon bearing — SO3H showed the best recyclability among the reported results, and no decrease in activity was observed even after 25 cycles [42]. However, all these solid acid catalysts still suffered deactivation after a period of time, so the operation life of the catalysts still cannot meet the requirement for industrial applications. Catalyst deactivation is a problem that must be solved in the hydrolysis reaction. Fundamental understanding of the deactivation mechanisms during cellulose hydrolysis is the key to extending catalyst lifetime. The main deactivation mechanisms of solid acid catalysts are: (1) leaching of surface acid sites, (2) carbon deposition on the catalyst and poisoning by toxic substance, and (3) surface reconstruction.

Leaching of acid sites, especially sulfonic or sulfuric acid species, limits the reusability of these families of catalysts. It has been reported that all the reactants and products could cause leaching of such active species (especially H2O) even at low temperatures (e. g., 100 °C) [90]. The leaching seems unavoidable under these oper­ation conditions. Therefore, such kinds of solid acid catalysts are not proposed to be used in reactions in aqueous solutions. Moreover, it is rather complicated to regener­ate them by a pickling technique. It is well known that Al, Fe, Ni, and Pt are effective promoters to increase the stability and activity of sulfated metal oxide catalysts [91]. The promoters improve sulfate contents and acidities of solid acid catalysts.

Carbon deposition represents a significant effect on the recyclability of a viable catalyst for cellulose hydrolysis. Shi et al. [92] prepared a series of Al-promoted SO42-/ZrO2/SBA-15 catalysts and investigated the deactivation and regeneration capacities of the catalysts during the dehydration of xylose. It was found that when the catalysts were reused without regeneration, the yield of furfural decreased from

52.7 % to 19.1 %. Based on the characterization of the catalysts, the accumulation of byproducts was the main reason for the deactivation. Regeneration with H2O2 can completely recover the catalytic activity of the deactivated catalysts. After first regen­eration, the catalytic activity recovered completely. The corresponding xylose con­version rate and furfural selectivity were 98.6 % and 53.5 %, respectively, very close to those with the fresh catalysts (98.7 % and 53.4 %, respectively). During the hydrol­ysis of cellulose, the produced glucose was polymerized into carbonaceous polymers under certain hydrothermal conditions [93]. The carbonaceous polymers were further deposited on the surface of solid acids, which decreased the catalytic efficiency. The deactivated catalysts can be easily regenerated by calcination to remove the deposited coke. However, some catalysts (e. g., carbonaceous solid acid catalysts) cannot be treated by calcination at high temperature because both deposits and catalysts them­selves were combusted. As mentioned in Sect. 15.3.4.4, recovery of carbonaceous solid catalysts by incorporating paramagnetic compounds may be feasible.

Carbon deposition not only covers the active sites, but also binds to catalyst surface inducing a surface reconstruction and affecting activity. Especially for metal oxides, the diffusion of carbon into metal results in the formation of bulk metal carbide, causing the loss in both activity and selectivity [94]. The oxidative treatment was usually applied to regenerate used catalysts by removing carbonaceous phases. Saib et al. [95] studied the deactivation and regeneration of cobalt Fischer-Tropsch syn­thesis catalysts. The spent catalysts recovered their activity completely by oxidative regeneration.