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Decomposition experiments of microcrystalline cellulose were conducted in subcritical and supercritical water (25 MPa, 320−400 °C, and 0.05−10.0 s). At 400 °C hydrolysis products were mainly obtained, while in 320−350 °C water, aqueous decomposition products of glucose were the main products. Kinetic studies of cellulose, cellobiose, and glucose at these conditions showed that below 350 °C the cellulose decomposition rate was slower than the glucose and cellobiose decomposition rates, while above 350 °C, the cellulose hydrolysis rate drastically increased and became higher than the glucose and cellobiose decomposition rates. Direct observation of the cellulose reaction in high-temperature water at high-pressure conditions by using a diamond anvil cell (DAC) showed that, below 280 °C, cellulose particles became gradually smaller with increasing reaction time but, at high temperatures (300−320 °C), cellulose particles disappeared with increasing transparency and much more rapidly than expected from the lower temperature results. These results suggest that cellulose hydrolysis at high temperature takes place with dissolution in water. This is probably because of the cleavage of intra- and intermolecular hydrogen linkages in the cellulose crystal. Thus, a homogeneous atmosphere is formed in supercritical water, and this results in the drastic increase of the cellulose decomposition rate above 350 °C.

Experiments were performed on the products of glucose decomposition at short residence times to elucidate the reaction pathways and evaluate kinetics of glucose and fructose decomposition in sub- and supercritical water. The conditions were a temperature of 300−400 °C and pressure of 25−40 MPa for extremely short residence times between 0.02 and 2 s. The products of glucose decomposition were fructose, a product of isomerization, 1,6-anhydroglucose, a product of dehydration, and erythrose and glyceraldehyde, products of C−C bond cleavage. Fructose underwent reactions similar to glucose except that it did not form 1,6-anhydroglucose and isomerization to glucose is negligible. The mechanism for the products formed from C−C bond cleavage could be explained by reverse aldol condensation and the double-bond rule of the respective enediols formed during the Lobry de Bruyn Alberda van Ekenstein transformation. The differential equations resulting from the proposed pathways were fit to experimental results to obtain the kinetic rate constants.

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Hydrolysis of 10 metal salt aqueous solutions of 6 metal oxides was conducted in supercritical water. Continuous and rapid production of metal oxide fine particles was achieved by mixing a metal salt aqueous solution with preheated water fed from another line. The reaction time required was less than 2 min. Particle size, morphology, and crystal structure of the obtained metal (hydrous) oxides were examined. Particle size (20 to 600 nm) was different among the systems but the size range was relatively narrow in all the cases. [

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The synthesis of colloidal metal oxide nanocrystals with controlled shape is of fundamental and technological interest because in this way it is possible to tune their shape-dependent physical properties and thus consolidate their promising applications in optics, catalysis, biosensing, and data storage. Recently, the organic-solution phase [1][2][3] and liquid-solid-solution phase synthetic transfer routes [4] have been demonstrated to be versatile pathways toward such shape-controlled metal oxide nanocrystals. In all of these methods, organic surfactants play a key role in determining the growth and stability of nanocrystals. Combining this concept and the properties of supercritical water (SCW) will lead to a novel approach for the synthesis of metal oxide nanocrystals. For example, SCW is chemically stable and processing with it is environmentally benign; [5] it acts as a unique medium to aid in the spontaneous nucleation and crystallization of metal oxide nanoparticles; [6] and by using organic ligand molecules that are miscible with SCW, crystal growth can be limited and agglomeration can be inhibited in favor of small, well-dispersed particles. [7,8] As a well-known metal oxide, and because of its novel properties, ceria (CeO 2 ) has been extensively applied in catalysis, electrochemistry, and optics. For example, ceria nanocrystals have high oxygen-storage capability and act as an important component in three-way catalytic converters to clean up automotive exhausts.[9] To elevate catalytic activity, it is desirable to prepare ceria samples with a high surface area. So far, ceria nanocrystals with spherical, wire, rod, and tadpole shapes have been synthesized. [10] Along with this research, there is another recent trend aimed at tuning the ceria-crystal shape in order to expose reactive crystal planes for high reactivity.[11]Sayle et al. predicted in their theoretical study that the (100) surface is more reactive than (110) or (111) for the CeO 2 / YSZ(110) system (where YSZ is yttria-stabilized zirconia).[11a]Yan and co-workers synthesized ceria nanoparticles with various shapes using a hydrothermal method, and experimentally observed that cubic particles (ca. 36 nm) with exposed {100} crystal planes showed the highest oxygen-storage capacity.[11d]Despite these recent advances, it is still a great challenge to synthesize ceria nanocrystals of high quality in terms of uniform size, well-defined crystal shape, and ease of fabrication.Here, we report a simple and rapid approach for producing colloidal ceria nanocrystals on scales of less than 10 nm, with tailor-made ceria crystal planes, using organic-ligand-assisted supercritical water as the medium. The synthetic strategy is depicted in Figure 1. This strategy depends on: 1) sub-decananometer single-crystal formation in a supercritical hydrothermal process; [6] 2) the miscibility of the organic ligand molecules with high-temperature water, which is due to the lower dielectric constant of the water; [12] and 3) controlled nanocrystal growth from the ...

Glucose decomposition kinetics in subcritical and supercritical water were studied for the temperatures 573, 623, and 673 K, pressures between 25 and 40 MPa, and residence times between 0.02 and 2 s. Glucose decomposition products were fructose, saccharinic acids, erythrose, glyceraldehyde, 1,6-anhydroglucose, dihydroxyacetone, pyruvaldehyde, and small amounts of 5-hydroxymethylfurfural. Fructose was also studied and found to decompose to products similar to those of glucose, except that its epimerization to glucose was negligibly low and no formation of 1,6-anhydroglucose was detected. We concluded that only the forward epimerization of glucose to fructose was important. The glucose decomposition pathway could be described in terms of a forward epimerization rate, r gf , a fructose to decomposition products rate, r f , and a glucose to decomposition products rate, r g . A kinetic model based on this pathway gave good correlation of the experimental data. In the subcritical region, r g , r f , and r gf showed only small changes with pressure at a given temperature. In the supercritical region, the rate of glucose decomposition decreased with pressure at a given temperature. The reason for this decrease was mainly due to the decrease in r gf . The pressure effect in the supercritical region shows that there is a shift among the kinetic rates, which can lead to higher selectivity for glucose when decomposing cellulosic materials.