(624c) Reactions and Chemical Kinetics of Amino Acids in Hot Compressed Water

James D. Sheehan and Phillip E. Savage

Subcritical water is a promising, “green” liquid-phase reaction medium for valorizing biomass such as cultivated “energy” crops (e.g., microalgae) and organic wastes into value-added chemicals and biofuels. The thermochemical conversion of biomass by hot, compressed, water is facilitated by its properties as a solvent, reactant, and catalyst. As liquid water approaches its critical point (374 °C, 220 bar), its properties deviate markedly from those at ambient conditions. For instance, the static dielectric constant of water decreases to values comparable to polar organic solvents, leading to greater solubility of small organic molecules. Further, the ion product of water may increase up to three orders of magnitude, leading to higher concentrations of hydronium and hydroxide ions that promote acid and base catalyzed reactions.¹

The application of hydrothermal media for processing biomass feedstocks that include microalgae,² municipal sewage sludge,³ and lignocellulosic biomass⁴ into energy dense biocrude-oils, nutrient-enriched aqueous-soluble products, and gaseous products has been explored extensively over the last twenty years. In addition, there have been numerous studies investigating the kinetics of model compounds (i.e., representative of biomass) in hydrothermal media and the effects of operating conditions (e.g., temperature, time) on the formation of molecular products.^5–8 The fate of proteins within biomass undergoing hydrothermal treatment has garnered interest due to proteins being a potential source of valuable products like amino acids, organic acids, and nutrients, but also of N- and O-containing heterocyclic compounds that are undesirable in biocrude-oils and detrimental to their fuel quality.

This presentation will introduce experimental findings on the hydrothermal reactions of two amino acids, tryptophan and glutamic acid. The hydrothermal treatment of both tryptophan and glutamic acid is of interest due to their production of the heterocyclic compounds indole and 2-pyrrolidone, respectively, which are documented to partition into biocrude-oils.^9,10 The reaction pathways and mechanisms of both amino acids in hydrothermal media will be discussed and the effects of experimental conditions such as temperature, time, and pH, will be elucidated by chemical kinetic models.

References

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(2) López Barreiro, D.; Prins, W.; Ronsse, F.; Brilman, W. Hydrothermal Liquefaction (HTL) of Microalgae for Biofuel Production: State of the Art Review and Future Prospects. Biomass and Bioenergy 2013, 53, 113–127, DOI:10.1016/j.biombioe.2012.12.029.

(3) Qian, L.; Wang, S.; Xu, D.; Guo, Y.; Tang, X.; Wang, L. Treatment of Municipal Sewage Sludge in Supercritical Water : A Review. Water Res. 2016, 89, 118–131, DOI:10.1016/j.watres.2015.11.047.

(4) Akhtar, J.; Aishah, N.; Amin, S. A Review on Process Conditions for Optimum Bio-Oil Yield in Hydrothermal Liquefaction of Biomass. Renew. Sustain. Energy Rev. 2011, 15, 1615–1624, DOI:10.1016/j.rser.2010.11.054.

(5) Li, J.; Brill, T. B. Spectroscopy of Hydrothermal Reactions 25: Kinetics of the Decarboxylation of Protein Amino Acids and the Effect of Side Chains on Hydrothermal Stability. J. Phys. Chem. A 2003, 107, 5987–5992, DOI:10.1021/jp0224766.

(6) Rogalinski, T.; Liu, K.; Albrecht, T.; Brunner, G. Hydrolysis Kinetics of Biopolymers in Subcritical Water. J. Supercrit. Fluids 2008, 46, 335–341, DOI:10.1016/j.supflu.2007.09.037.

(7) Rogalinski, T.; Ingram, T.; Brunner, G. Hydrolysis of Lignocellulosic Biomass in Water under Elevated Temperatures and Pressures. J. Supercrit. Fluids 2008, 47, 54–63, DOI:10.1016/j.supflu.2008.05.003.

(8) Changi, S.; Faeth, J. L.; Mo, N.; Savage, P. E. Hydrothermal Reactions of Biomolecules Relevent for Microalgae Liquefaction. Ind. Eng. Chem. Res. 2015, 54, 11733–11758.

(9) Dote, Y.; Inoue, S.; Ogi, T.; Yokoyama, S. Distribution of Nitrogen to Oil Products from Liquefaction of Amino Acids. Bioresour. Technol. 1998, 64, 157–160.

(10) Sheehan, J. D.; Savage, P. E. Molecular and Lumped Products from Hydrothermal Liquefaction of Bovine Serum Albumin. ACS Sustain. Chem. Eng. 2017, DOI:10.1021/acssuschemeng.7b02854.