Hydrolytic cleavage of the non-terminal α-1,4-glycosidic bonds in α-, β-, and γ-cyclodextrins and the anomeric-terminal one in d-maltose was investigated to examine how the cleavage rate for α-, β-, and γ-cyclodextrins is slower than that for d-maltose. Effects of water and temperature were studied by applying in situ 13C NMR spectroscopy and using a dimethyl sulfoxide (DMSO)–water mixture over a wide range of water mole fraction, xw = 0.004–1, at temperatures of 120–180 °C. The cleavage rate constant for the non-anomeric glycosidic bond was smaller by a factor of 6–10 than that of the anomeric-terminal one. The glycosidic-bond cleavage is significantly accelerated through the keto–enol tautomerization of the anomeric-terminal d-glucose unit into the d-fructose one. The smaller the size of the cyclodextrin, the easier the bond cleavage due to the ring strain. The remarkable enhancement in the cleavage rate with decreasing water content was observed for the cyclodextrins and d-maltose as well as d-cellobiose. This shows the important effect of the solitary water whose hydrogen bonding to other water molecules is prohibited by the presence of the organic dipolar aprotic solvent, DMSO, and which has more naked partial charges and higher reactivity. A high 5-hydroxymethyl-2-furaldehyde (5-HMF) yield of 64% was attained in a non-catalytic conversion by tuning the water content to xw = 0.30, at which the undesired polymerization by-paths can be most effectively suppressed. This study provides a step toward designing a new optimal, earth-benign generation process of 5-HMF starting from biomass.

The effect of rotations on the line shape of the bending vibrational spectrum for supercritical water was analyzed using classical molecular dynamics simulation for the flexible point-charge SPC/Fw model. The experimental infrared spectrum of the bending mode at the low densities of 0.01–0.04 g·cm−3 and at 400 °C was essentially reproduced without any other assumptions. The spectrum line shape at low densities consists of two broad rotational bands due to the rotational couplings, as in the case of the O–H stretch mode. This is due to the time-scale separation breakdown but is not due to the presence of any definite clusters. The rotational couplings become more significant at higher temperatures. The separations between the bending band center and the rotational broad side-bands are found to be linearly correlated with the inverse of the total moment of inertia of the water isotopic species, which is clear molecular-level evidence for the rotational couplings.

Noncatalytic conversion of d-cellobiose (at 0.5 M) into 5-hydroxymethyl-2-furaldehyde (5-HMF), a platform chemical for fuels and synthetic materials, was analyzed at 120–200 °C over a wide range of water mole fraction, xw = 0.007–1 in a binary dimethyl sulfoxide (DMSO)–water mixture by means of the in situ 13C NMR spectroscopy. Effects of the water content were revealed as follows: (i) The tautomerization of the anomeric residue of d-cellobiose from d-glucose to d-fructose type was not initially observed at a lower water content, in contrast to the significant tautomerization into the d-fructose type in a higher water content and pure water. (ii) The lower the water content, the faster the glycosidic-bond cleavage by hydrolysis, because of the high reactivity of solitary water molecules with the large partial charges more naked as in supercritical water clusters due to the isolation by the organic solvent DMSO. (iii) The amount of d-fructose as the intermediate product was larger at the higher xw; despite the increase of d-fructose, the production of 5-HMF from d-fructose was slowed down. (iv) A high 5-HMF yield of 71% was reached at xw = 0.20–0.30 that was 6–10 times the initial d-cellobiose concentration. The best yield of 5-HMF was attained in the low xw region when the polymerization paths into NMR-undetectable species via 5-HMF and anhydromonosaccharides were effectively suppressed. This study provides a new framework to design optimal, noncatalytic reaction process to produce 5-HMF from cellulosic biomass by tuning the water content as well as the temperature and the reaction time.

Self-diffusion coefficients D for water, benzene, and cyclohexane were determined in high-temperature conditions along the liquid branch of the coexistence curve and in supercritical conditions including an extremely low density region. The diffusion data available in literature were compared and evaluated. A fifth-order polynomial for ln D with the single variable T−1 (ln(D/10−9 m2·s−1) = a0+ a1x + a2x2 + a3x3 + a4x4 + a5x5 with x = 1000/(T/K)) was found to provide good fitting along the liquid branch of the coexistence curve. A single polynomial function for the scaled quantity ρD/T1/2 with the two variables, density ρ and temperature T−1 (third-order polynomial of ρ and T−1 and the cross terms), can universally represent the diffusion data over a wider range including both the gas−liquid coexistence and the extremely low density conditions. The function gives a reliable and reasonable behavior of D in the medium-density supercritical states in which the experimental uncertainty is rather large due to the severe conditions. The temperature and density differentials thus obtained were used to shed light on the effect of hydrogen bonding that makes water different from nonpolar organic liquids. The temperature dependence of the self-diffusion coefficient for water is larger than those for organic liquids, due to the large contribution of the attractive hydrogen-bonding interaction. The density dependence is larger for organic liquids than that for water.