Kinetics and mechanism of tetrahydrofuran synthesis via 1,4-butanediol dehydration in high-temperature water.

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.

Kinetics and mechanism of p-isopropenylphenol synthesis via hydrothermal cleavage of bisphenol A.

Although bishydroxyarylalkanes are known to be reactive in high-temperature (T > 200 degrees C) liquid water (HTW), no mechanistic insight has been given to explain the reactivity of methylene bridge-containing diaryls under hydrothermal conditions. We examined the kinetics and mechanism of p-isopropenylphenol (IPP) synthesis via bisphenol A (BPA) cleavage in HTW. The cleavage reaction is first order in BPA. Cleavage of BPA in HTW occurs by specific acid catalysis, by specific base catalysis, and by general water catalysis. Under neutral conditions, the dominant mechanism is general base catalysis with water serving as the proton acceptor. We generated a detailed chemical kinetics model for the decomposition reaction based on a base-catalyzed mechanism in the literature. This three-parameter model fit the experimental data for BPA disappearance and formation of IPP and phenol and accurately predicted the yield of the IPP hydrolysis product acetone. Using acid- and base-catalyzed mechanisms, we explain the reactivity in HTW reported for other diaryl groups linked by methylene bridges and propose criteria for assessing the reactivity of methylene bridges under hydrothermal conditions.