(27e) Revisiting Liquid Phase Oxidation: Addressing Age-Old Mysteries With Computational Chemistry and Kinetic Modeling

Revisiting Liquid Phase Oxidation: Addressing Age-Old Mysteries with Computational Chemistry and Kinetic Modeling

Amrit Jalan and William H. Green

Department of Chemical Engineering, 77 Massachusetts Avenue, Room E18-566A, Cambridge, MA 02139, USA E-mail: whgreen@mit.edu

Liquid phase oxidation of hydrocarbons exposed to the air is a ubiquitous and technologically important process, so it has been heavily studied for more than 75 years.¹ However, textbook explanations of the reaction paths do not really hold up under scrutiny. For example, 30 years ago Korcek and co-workers² showed that the major oxidation products of alkanes are carboxylic acids and methyl ketones, but the known reactions producing these products are much too slow to explain the observed yields. Almost 50 years ago Ingold³ observed that alkyl aromatics exposed to air oxidize faster in polar solvents than in nonpolar solvents, but there has been no convincing explanation why. In many textbooks, the main radical-generating reaction in air oxidations is stated to be the unimolecular scission of hydroperoxides ROOH = RO• + •OH, but this reaction is too slow to explain observed oxidation rates. To resolve these discrepancies, existing kinetic models often use fitted kinetic parameters to reproduce experimental observations but have low predictive capability.

Here we report how a combination of high-level quantum chemistry calculations and computer-aided construction of detailed kinetic models for oxidation are being used to resolve these long-standing mysteries. More specifically, we discuss the following: 

1. New pathways for carboxylic acid/methyl ketone formation discovered using quantum chemistry tools, confirming Korcek’s 30-year old hypothesis.
2. First-principles predictions of product yields measured by Korcek et al. using computer-generated kinetic models for liquid phase oxidation.
3. Flux and sensitivity analysis to provide mechanistic insight and relate oxidation rate with elementary rate coefficients. This includes investigations into the role of peroxy chemistry and bimolecular ROOH scission reactions (e.g. ROOH + RH = RO• + R• + H₂O) as well as acceleration of oxidation rates in polar solvents.⁴

References

(1)        Denisov, E. T.; Afanas'ev, I. B. Oxidation and Antioxidants in Organic Chemistry and Biology; CRC Press, 2005.

(2)        (a) Jensen, R. K.; Korcek, S.; Mahoney, L. R.; Zinbo, M. J Am Chem Soc 1979, 101, 7574            (b) Hamilton, E. J.; Korcek, S.; Mahoney, L. R.; Zinbo, M. International Journal of Chemical Kinetics 1980, 12, 577  (c) Jensen, R. K.; Korcek, S.; Mahoney, L. R.; Zinbo, M. J Am Chem Soc 1981, 103, 1742    (d) Jensen, R. K.; Zinbo, M.; Korcek, S. Journal of Chromatographic Science 1983, 21, 394            (e) Jensen, R. K.; Korcek, S.; Zinbo, M.; Johnson, M. D. International Journal of Chemical Kinetics 1990, 22, 1095.

(3)        Howard, J. A.; Ingold, K. U. Canadian Journal of Chemistry 1964, 42, 1250.

(4)        Jalan, A.; West, R. H.; Green, W. H. The Journal of Physical Chemistry B 2013, 117, 2955.