Reaction pathway analysis for the acid catalysed transformation of hexose carbohydrates to advantaged biofuel components

The valorisation of readily available and abundant lignocellulosic derived hexose carbohydrates by reaction with ethanol can provide an alternative to petroleum derived fossil fuels. The main barrier in effectively doing so is the lack of mechanistic knowledge regarding the reaction pathways linking the hexose sugar to the intended fuel component. In pursuit of the chemical mechanisms responsible for the hydrolysis of; D-fructose, D-galactose, D-glucose, D-mannose to levulinic acid in aqueous systems using 2.5 wt% H2SO4 at 423 K is deciphered upon. The mechanistic comprehension gained is used as constraints to evaluate the more complex ethanolysis (ethanol/H2SO4/hexose) system. It is also comprehensively shown that formic and levulinic acids are not formed stoichiometrically from lignocellulosic derived hexoses, as is widely believed in the literature. At steady-state conversions of the reactant, the formic and levulinic acid ratio for D-fructose, D-glucose, D-mannose and D-galactose is shown to be 1.08 ±0.04, 1.15 ±0.05, 1.20 ±0.10 and 1.19 ±0.04 respectively. Next, the ethanolysis process is introduced as a superior alternative to the aqueous hydrolysis systems. Two advantaged fuel components 5-ethoxymethylfurfural and ethyl levulinate are identified and numerical modelling is utilised to test the feasibility of carefully designed mechanistic propositions at 351 K catalysed by hydrogen cations. It is shown that the hydrogen cation is consumed by reaction with ethanol and that the overall system is ‘‘pseudo’’ catalytic. Condensed phase conditions (331-351 K) are deemed more suitable for the formation of 5-ethoxymethylfurfural, whilst biphasic conditions (>353 K) favours the formation of ethyl levulinate. A kinetic model is developed for the ethanolysis of D-fructose to 5-ethoxymethylfurfural (331-351 K), which includes kinetic contraints derived from conducting reactions with all key chemical intermediates in the system. The reaction mechanism for the ethanolysis of D-glucose to ethyl levulinate (351-423 K) catalysed by H2SO4 (0.015-0.075) mol/L is deciphered upon, with no significant amounts of ethyl levulinate formed below 393 K. Significantly the main reaction flux for ethyl levulinate formation from D-glucose does not advance through any furan intermediates as it does in the D-fructose ethanolysis mechanism, which is not the proposed pathway in the literature. Finally, the fuel properties of the range of fuel molecules produced in ethanolysis systems are presented. It is shown that by integrating mechanistic understanding, kinetic parameters and fuel properties of the synthesized molecules, drop-in tailor-made fuel additives can be synthesized in a highly flexible “one-pot” process designed for specific purposes be it for ‘‘diesel’’ or ‘‘gasoline’’ fuels.