Quantifying rate enhancements for acid catalysis in CO 2 ‐enriched high‐temperature water

Thermodynamic calculations revealed that 10 to 100‐fold increases in reaction rate are obtainable with added CO 2 (0.1–1 MPa) for an acid‐catalyzed reaction in high‐temperature liquid water (HTW) that is first order in H + concentration. These calculations suggest that CO 2 is most effective as a rate‐enhancing additive in HTW at lower temperatures (150–200°C). When compared with increased temperature as a competitive option for accelerating acid‐catalyzed reactions in HTW, CO 2 addition generally carries a lower pressure penalty (and no temperature penalty) for the model acid‐catalyzed reaction with activation energies of up to 35 kcal/mol. An experimental survey revealed that CO 2 addition is effective for achieving increased reaction rates for dibenzyl ether hydrolysis in HTW, but that bisphenol A cleavage, methyl benzoate hydrolysis, and o ‐phthalic acid decarboxylation were not significantly impacted by added CO 2 . This behavior is consistent with previous results for these reactions wherein mineral acid, rather than CO 2 , was added to lower the pH. A summary of experimental results reported for reactions in CO 2 ‐enriched HTW revealed that product yields of some reactions can be increased by a factor of 23 with added CO 2 . Taken collectively, these results suggest that CO 2 addition may be a practical technique for making HTW more attractive as a reaction medium for acid‐catalyzed organic synthesis. © 2007 American Institute of Chemical Engineers AIChE J, 2008