Quantifying rate enhancements for acid catalysis in CO₂-enriched high-temperature water
Hunter, Shawn E. | Savage, Phillip E.
Thermodynamic calculations revealed that 10 to 100-fold increases in reaction rate are obtainable with added CO₂ (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 CO₂ 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, CO₂ 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 CO₂ 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 CO₂. This behavior is consistent with previous results for these reactions wherein mineral acid, rather than CO₂, was added to lower the pH. A summary of experimental results reported for reactions in CO₂-enriched HTW revealed that product yields of some reactions can be increased by a factor of 23 with added CO₂. Taken collectively, these results suggest that CO₂ 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, 2008Show more [+] Less [-]