Although some desired properties can be obtained by means of adjusting the particle size, recycling of nanoparticle materials can be seen as an application barrier due to adsorption and viscous effects of reaction mixture. Carbonaceous solid acids ground to nanosize (10-100 nm) had high catalytic activity for the cellulose that was similarly grounded with product selectivity higher than 90 % [120]. It is interesting to note that in some cases, the recycled catalysts exhibited no deactivation in the catalytic hydrolysis of fresh non-treated cellulose. In our research, the mixed metal oxide, Zn — Ca-Fe oxide, exhibited moderately good catalytic activity for hydrolyzing crystalline cellulose [121]. Cellulose conversion rate and glucose selectivity was 42.6 % and

69.2 %, respectively.

By introducing magnetic nano-components into nano-catalysts, it may be possible to separate the catalysts with an external magnetic field [122]. Magnetic supported nano-catalysts are highly efficient and environment-friendly. They have double func­tions of magnetism and acidity. They are obtained according to the following steps: (1) preparation of magnetic nanoparticles (magnetic core); (2) coupling anchor points on the surface of the nanoparticles; and (3) coordination of active sites to the anchor points [123]. By additional magnetic field, magnetic nano-catalysts disperse into liquid phase equably, avoiding the aggregation of nanoparticles and increasing the contact area between reactants and catalysts. In our work, hydrotalcite nanoparticles were synthesized and activated, and used for catalytic cellulose hydrolysis. Cellu­lose conversion rate and glucose selectivity of 46.6 % and 85.3 %, respectively, were obtained and remained stable for four cycles [27].

However, hetero-junctions between different active oxide and magnetic core lead to the decrease of activity as compared with single phase active oxide. Beydoun et al. [124] synthesized magnetic photocatalysts by coating TiO2 particles on Fe3O4 magnetic nanoparticles. It was found that the photo-activity of titania-coated magnetite decreased. Moreover, nanometer-scale apertures are not large enough to allow the transport of sub-micron scale cellulose. Increasing the aperture in nanoparticles would facilitate mass-transfer of oligosaccharides to catalytic sites. Techniques of increasing aperture include change of synthesis conditions, addition of pore-formers and change of templates. The external surface hydrolysis reactions result in a non-shape-selective reaction as well as deposition of un-hydrolyzed cellulose, leading to short lifetime for the catalysts. These issues can be avoided by adopting other treatment methods such as ultrasound. To use the internal acid sites effectively, cellulose or its constituents can be dissolved in organic solvents or ILs for further reactions. It is expected that acid-functionalized magnetic nano-catalysts are promising materials for the hydrolysis of biomass.

15.4 Summary

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 chemicals, and can replace many conventional liquid acids for hydrolysis and pretreatment. Besides specific surface area, pore size, and pore volume, the active site concentration and acidic type are important factors for solid acid performance. Solid acid catalysts being considered for biomass hydrolysis should have a large number of B acid sites, a good affinity for the reactant substrates, and good thermal stability. Catalyst composition, porosity, and stability in the presence of water are other important properties for solid acids in biomass hydrolysis. A good solid catalyst with sufficient catalytic activity combined with appropriate reactor design should make it possible to realize biomass hydrolysis on a practical scale. The development of highly acidic solid catalysts with nanometer size that have special characteristics (e. g., paramagnetic properties) is an interesting area of research for developing practical systems for biomass hydrolysis. In the near future, through the combination of green solvents, nanoparticle techniques, and functional solid acid catalysts, it can be expected that chemical processes based on the catalysis of biomass will begin to replace petroleum — based processes to reach a sustainable economy.