(166a) Understanding Formation of Nitrogen Heterocycles during Catalytic Hydrothermal Liquefaction
Hydrothermal liquefaction (HTL) is a promising technology for conversion of wet wastes into fuel precursor oils. In this work, we studied catalytic-HTL of food waste using a series of acid and base catalysts. While base catalysts are generally more effective at increasing oil yield and decreasing its oxygen content, acidic catalysts are especially effective at removing nitrogen content. Removing nitrogen from the bio-oil is an important challenge for the industrial application of the process. Heterocyclic nitrogen compounds are especially problematic, as they are difficult to remove using hydrodenitrogenation. Moreover, in the context of fixing nitrogen, HTL has potential for re-utilization of fixed nitrogen, reducing the need for costly and energy intensive nitrogen reduction processes. Unfortunately, the complex reaction mixture resulting from catalytic HTL of food waste and other realistic feed streams is too complicated to understand mechanisms. Computational studies can overcome this challenge, providing a guide for targeted experimental work and forming the basis of science-based reactor models. To further simplify, we studied homogeneous formation of heterocyclic under HTL conditions. In this work, a reaction network that involves interactions between reducing sugars, amino acids, and dienes is chosen as a basis for the formation of undesirable nitrogen heterocycles. Specific attention is given to aza-Diels-Alder cycloadditions and Maillard reactions, as both of these are potential pathways to formation of organo-nitrogen compounds. Quantum chemical calculations are carried in Gaussian 16 on reactants, intermediates, and products using the CBS-QB3 method. Conformers of each structure are identified by scanning the potential energy surface. Transition states of each pathway are determined and validated using intrinsic reaction coordinate calculations. These results are used in CanTherm to calculate thermochemistry and rate coefficients while treating hindered rotors. These results suggest new mechanisms for formation of undesirable nitrogen heterocycles that can guide selection of HTL processes and conditions to minimize their formation.