Roles of water for chemical reactions in high-temperature water

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 lowe...

Dissolution and Hydrolysis of Cellulose in Subcritical and Supercritical Water

Hot compressed water (HCW, here water above 200 °C) possesses very interesting properties This series of articles gives an overview of the state of the art as regards the understanding of reactions in HCW In the first part the macroscopic and microscopic properties of HCW will be described, followed by a summary of synthesis reactions published The impact of the unique properties shall be discussed in part II, with the thermal degradation of tert-butylbenzene and the oxidation of methanol being used as examples The studies reported will show that the microscopic properties are of more importance to understanding reactions in HCW than assumed in the past

Hot compressed water as reaction medium and reactant properties and synthesis reactions

In this paper we propose a new method to hydrolyze cellulose rapidly in supercritical water (SCW) to recover glucose, fructose and oligomers (cellobiose, cellotriose, cellotetraose, etc.). Cellulose decomposition experiments were conducted with a flow type reactor in the range of temperature from 290 to 400°C at 25 MPa. A high pressure slurry feeder was developed to feed the cellulose–water slurries. Hydrolysis product yields (around 75%) in supercritical water were much higher than those in subcritical water. At a low temperature region, the glucose or oligomer conversion rate was much faster than the hydrolysis rate of cellulose. Thus, even if the hydrolysis products, such as glucose or oligomers, are formed, their further decomposition rapidly takes place and thus high yields of hydrolysis products cannot be obtained. However, around the critical point, the hydrolysis rate jumps to more than an order of magnitude higher level and becomes faster than the glucose or oligomer decomposition rate. This is the reason why we obtained a high yield of hydrolysis products in supercritical water.

Cellulose hydrolysis in subcritical and supercritical water

Experiments on the role of water in petroleum formation

Solvent effects during reactions in supercritical water

Reaction of guaiacol (o-methoxyphenol) in supercritical water and with added salts showed its hydrolysis to catechol (o-hydroxyphenol) and methanol to be through a polar transition state. Correlation of the reaction kinetics following a modified Herbrandson analysis provided a quantitative summary of the greater polarity of the hydrolysis transition state relative to the hydrolysis reactants. Salt addition further shifted the transition-state equilibrium toward the transition-state species.

Hydrolysis in supercritical water: identification and implications of a polar transition state

Dibenzyl ether as a probe into the supercritical fluid solvent extraction of volatiles from coal with water

The extreme pressure dependence of the solvent properties—density, dielectric constant, and solubility parameter—has been exploited in the scrutiny of the mechanism of hydrolysis in supercritical water. Molecules containing the structural moiety of a saturated carbon attached to a heteroatom-containing leaving group undergo parallel pyrolysis and hydrolysis reactions in supercritical water. Herein, kinetics, labelling studies, and salt effects are used to assess likely mechanisms. Under isothermal conditions, the selectivity to the hydrolysis pathway increased with water density. Moreover, the rate constant for hydrolysis also increased with water density and with the addition of salts. An SN2 mechanism with H2O as the nucleophile appears to be the most likely candidate.

Susan H. Townsend

Papers

Solvent effects during reactions in supercritical water

Hydrolysis in supercritical water: identification and implications of a polar transition state

Dibenzyl ether as a probe into the supercritical fluid solvent extraction of volatiles from coal with water

Hydrolysis in supercritical water: Solvent effects as a probe of the reaction mechanism