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Abstract
This work developed synthesis methods for sustainable solid acid and base carbon catalysts from wood-based activated carbon in granular (GAC) and monolith (ACM) forms and applications in continuous esterification (acid-catalyzed esters) and condensation (base-catalyzed aviation fuels). Solid acid carbon catalysts were synthesized by modified hydrothermal or low-temperature hydrogen and water vapor plasma techniques. The catalytic activity of the hydrothermal and plasma sulfonated carbons was compared to commercial Amberlyst-15 in batch esterification reactions of 2-hydroxyisobutyric acid (2-HIBA) with ethanol. The hydrothermally sulfonated GAC (sGAC) and ACM (sACM) displayed reaction rates, and ester yields comparable to Amberlyst-15 (150 ˚C, 1h). Plasma methods successfully sulfonated the carbon materials at much lower temperatures and treatment times yet generated low ester yields due to low acid site density. In the continuous esterification of 2-HIBA, the sACM displayed a higher space-time-yield of 96 g Lcat-1 h-1 and selectivity of 76 % compared to the sGAC (87 g Lcat-1 h-1, 60 %) at 150 ˚C, 300 psig, and 3 mL min-1. Time-on-stream studies suggested greater catalytic stability of the sulfonated carbons over Amberlyst-15 and significantly lower production of unwanted by-products. A kinetic model was developed for 2-HIBA esterification over the sulfonated carbons for use in scale-up and reactor design. Both Eley-Rideal and Langmuir-Hinshelwood models agreed with the experimental data. The catalytic activity of sGAC and sACM for pyruvic acid esterification (a possible fermentation-derived biochemical) was also compared to Amberlyst-15. The sulfonated carbons displayed similar pyruvic acid conversion and selectivity, significantly higher turnover frequencies, lower water inhibition, yet lower ester yield than Amberlyst-15. Fermentation residuals, including water and salts (phosphates and sulfates) reduced catalytic activity, suggesting side reactions and accelerated ester hydrolysis. Solid base ACM supported hydrotalcite catalysts (HT/ACM) were synthesized using traditional calcination, rehydration, and sustainable air plasma techniques and demonstrated for continuous cyclopentanone self-condensation. The HT/ACM activated by air plasma (100 W,1 min) displayed higher catalytic efficiency than the calcined (500 ˚C, 4 h) and rehydrated ACM. The solid acid and base carbons synthesized in this work are promising catalysts for the catalytic upgrading of bio-based organic acids and cyclic and linear ketones.