The significant amount of nitrogen in biofuel produced from hydrothermal liquefaction (HTL) of microalgae is one of the most important challenges for the industrial application of the process. Heterocyclic nitrogen compounds are especially problematic, as they are difficult to remove using hydrodenitrogenation methods. Moreover, nitrogen recycling is critical to result in processes with greenhouse gas emissions less than those generated by production of petroleum fuels. Yield, composition, and energy content of algae bio-oil are all complex functions of algae type and HTL conditions. Thus, process optimization is necessary to achieve the best compromise between qualitative and quantitative properties of HTL biofuel. However, the development of a comprehensive optimization model is limited due to the lack of understanding of reaction kinetics, especially those involving nitrogen. Moreover, experimental measurements of individual reaction rates is extremely challenging due to the large number of parallel reaction pathways, the relative rates of which depend on reaction conditions and many of which yield products which are difficult to track on the molecular level. Computational studies can overcome some of these challenges, providing a guide for targeted experimental work and forming the basis of science-based reactor models. 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 known 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. Analysis of the data allows an understanding of the effect of temperature on the rate of these reactions and provides the lacking quantitative information required to optimize the quality of HTL biofuel.

Investigating reaction pathways for the formation of nitrogen heterocycles during hydrothermal liquefaction of microalgae

256th National Meeting (2018)

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