Recently, novel synthesis methods for nano-catalysts and nanostructures have been widely reported. The synthesis of silica-based particles focuses on nanostructures of nano-ropes, nano-tubes, and paintbrushes [96]. Hydrothermal treatment with acid can be used to improve the order and stability of the nanostructures after synthesized with an aqueous ammonia solution. By sol-gel synthesis, silica nano-tubes can be bundled to form nanostructures known as paintbrushes. The sol-gel processing of nanoparticles is commonly performed by a so-called semi-alkoxide route in which inorganic compounds like hydroxides, acetates, carbonates, and chlorides, are used as sources to alkaline earth ions [97]. Nanosized transition metal components are usually synthesized using alkoxide and semi-alkoxide routes.

Hybrid nanocomposites will find applications in catalysts. Hybrid (inorganic — organic) nano objects and higher level nanostructured networks were obtained by novel equilibrium and non-equilibrium self-assembly approaches [98]. By control­ling over the morphology of hybrid materials, controllable particle size and size distribution were achieved. Dong et al. [99] proposed a feasible and effective self­assembly method to synthesize different scale coordination polymers in highly dilutes solution. The nano and microscale particles gave better catalytic conversion rate (73 %) and selectivity in the hydroxylation of phenols than the bulk crystals.

Reversed micelle technique has been reported on micellular HPA [84]. The syn­thetic method was based on the reaction between the components dissolved in the lyophilic media and the reversed micelles. Cellulose was hydrolyzed for three con­tinuous repeated runs under the same reaction conditions, and complete hydrolysis was achieved. The highest glucose yield reached with the micellular HPA was 60 % with 85 % selectivity for the three continuous runs. The catalyst was separated from the reaction mixtures by centrifugation.

Supercritical anti-solvent precipitation synthesis was recently used for synthesiz­ing MnOx-CeO2 hollow spheres. As reported by Jiang et al. [100], a mixed solution of manganese acetylacetonate, and cerium acetylacetonate in methanol was injected into the precipitator. As the solution droplets contacting with supercritical carbon dioxide, the nanoparticles were precipitated. Then, the supercritical CO2 was allowed to flow to remove the residual methanol. The system was further depressurized to atmospheric pressure, and the generated nanoparticles were collected. Finally, they were calcined in a muffle furnace, and the MnOx-CeO2 hollow nano-spheres with an average diameter of about 50 nm and a wall thickness of 10-20 nm were obtained.

Magnetic nano-catalysts with ordered or disordered array of nano-crystallites have attracted great attention recently because of their wide application poten­tials. However, it is still a big challenge to tune the magnetic nano-crystallites into three-dimensional regular aggregates with varied nanostructures. By ultrasonic — chemical precipitation synthesis, magnetite particles with 15 nm average diameter were obtained [101]. Under ultrasonic agitation, Fe3O4 precipitates were produced immediately by adding sodium hydroxide into mixture of FeSO4 and FeCl3 with Fe3+ and Fe2+ molar ratio of 1.5:1. Moreover, C12H25OSO3Na could be added as surface active agent, assisting to obtain Fe3O4 nanoparticles with homogenous size and shape distribution.

The development of highly acidic nano-solid catalysts that have special charac­teristics (e. g., paramagnetic properties) is an interesting area for developing practical systems for biomass hydrolysis. Through the combination of novel nano-catalysts preparation techniques, it is expected that chemical processes based on the hydrolysis of cellulose will be developed rapidly.