We conducted an experimental investigation into the kinetics and mechanism of tetrahydrofuran synthesis from 1,4-butanediol via dehydration in high-temperature liquid water (HTW) without added catalyst at 200-350 degrees C. The reaction was reversible, with tetrahydrofuran being produced at an equilibrium yield of 84% (at 200 degrees C) to 94% (at 350 degrees C). The addition of CO2 to the reaction mixture increased the reaction rate by a factor of 1.9-2.9, because of the increase in acidity resulting from the formation and dissociation of carbonic acid. This increase was much less than that expected (factor of 37-60) from a previously suggested acid-catalyzed mechanism. This disagreement prompted experiments with added acid (HCl) and base (NaOH) to investigate the influence of pH on the reaction rate. These experiments revealed three distinct regions of pH dependence. At high and low pH, the dehydration rate increased with increasing acidity. At near-neutral pH, however, the rate was essentially insensitive to changes in pH. This behavior is consistent with a mechanism where H2O, in addition to H+, serves as a proton donor. This work indicates that the relatively high native concentration of + (large KW), which has commonly been thought to lead to the occurrence of acid-catalyzed reactions in HTW without added catalyst, does not explain the dehydration of 1,4-butanediol in HTW without catalyst. Rather, H2O serves directly as the proton donor for the reaction.

We report, for the first time, the acceleration of acid-catalyzed reactions in high-temperature liquid water by the addition of carbon dioxide to the reaction medium. The reactions examined are the dehydration of cyclohexanol to form cyclohexene and the alkylation of p-cresol with tertbutyl alcohol to form 2-tert-butyl-4-methylphenol. In some cases, the mean product yield more than doubled when carbon dioxide was added to the aqueous medium. The basis of this rate enhancement is reaction between carbon dioxide and water to yield carbonic acid, which subsequently dissociates to increase the hydronium ion concentration above that of water alone. We also use the thermodynamics of the CO 2 -H 2 O system to estimate the pH of carbon dioxideenriched high-temperature water. This analysis demonstrates that it is possible to lower the pH of high-temperature water by several units through the addition of carbon dioxide to the reaction medium. These results demonstrate the feasibility of using CO 2 -H 2 O mixtures as an environmentally benign reaction medium for acid catalysis.

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We examined the hydrolysis of dibenzyl ether, benzyl t-butyl ether, methyl t-butyl ether, methylbenzoate, and diphenylcarbonate in high-temperature liquid water, both with and without added acid or base. The apparent reaction order for H+ did not exceed 0.2 for any of the compounds investigated. This result indicates that hydrolysis of these compounds in high-temperature water (HTW) does not follow the kinetics expected for specific acid catalysis (H+ reaction order = 1.0), as does the hydrolysis at ambient temperatures. Rather, the greater thermal energy in the HTW system allows protonation by water molecules to become faster than protonation by H+ at near-neutral conditions. Because the water-catalyzed path is faster, the occurrence of these acid-catalyzed reactions in HTW with no added acid is not due to the elevated value of K w, the ion product. This finding contradicts the conventional wisdom in this field.

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