Identificador: TDX:305
Autors: González Sánchez, Guillermo
Resum:
In this manuscript wee study and describe the hydrothermolysis reactions of lignin model compounds in sub and supercritical water, through the proposal of kinetical models. We selected vanillic acid and dibenzyl ether as model compounds for exploring the hydrothermal reactivity of some functional groups appearing in lignin structure. The utilization of renewable materials, in particular biomass, for producing fuels and chemical derivatives represents a promissory field from an economical point of view. Moreover, the development of chemical processes that substitute harmful, expensive solvents with cheap and innocuous ones is becoming mandatory for diminishing operation costs whilst obeying stringent environmental regulation.<br/><br/>The hydrothermal reactions of vanillic acid in water near the critical point have been investigated in a continuous tubular reactor at temperatures between 280 and 500 ºC and pressures between 225 and 300 bar. Formation of 2-methoxy-phenol (guaiacol) as primary product through decarboxylation of vanillic acid is the main decomposition reaction. Decarboxylation kinetics is first order on vanillic acid concentration. The decreases with pressure at supercritical temperatures. The effect of pressure on reaction rates above the critical temperature is discussed on the basis of a reaction mechanism that involves an activated state with a positive volume of activation, which corresponds to an unimolecular dissociation. The secondary products formed through competitive hydrolytic and pyrolytic reactions of 2-methoxy-phenol are also considered.<br/><br/>We propose a reaction pathways scheme for establishing the mass balances for all the reaction species. The kinetic model was solved by numerical integration to compute the rate constant values. Good agreement is found between the experimental values and the predicted ones by the model for the fractional yield profile with time of the various reaction species.<br/><br/>We have also studied the hydrothermal conversion of dibenzyl ether in sub- and supercritical water and the selectivity towards the main reaction products. The experimental study has been performed in a continuous flow reactor covering pressures from 225 to 362 bar, temeperatures form 325 to 390 ºC and short reaction time to minimize secondary reactions of the primary products. The molar ratio between water and the organic compound, Ow, was fixed at 100 modH2OmolDBE0-1. The main reaction products have been identified and quantified by means of HPLC/UV analysis. Two competitive reaction pathways for dibenzyl ether are identified: hydrolysis, which account for benzyl alcohol formation, and pyrolysis accounting for toluene and benzaldehyde production. A reaction pathway has been proposed for developing a kinetic model. The rate constants have been estimated from the experimental data. The effect of pressure on the reaction rates is negligible for the subcritical temperatures, while the rate constants for the different reactions decrease with pressure in the temperature region. The activation volume for the transition state of these reaction pathways is positive, in agreement with reported values for free-radical pathways and with a SN1 mechanism for ether hydrolysis. A activation volumes have a change in magnitude. These changes are explained in terms of strong electrostatic interactions between solvent, reactants and activated complex for the hydrolysis reaction.<br/><br/>Overall, our results show that it is possible to tune the solvent physico-chemical properties needed for lignin depolymerization, through the optimal choose of operation parameters near the critical point of water. They also claim for complementary research using spectrometry techniques and molecular simulation modelling in order to reveal important aspects about electrostatic interactions between all the reaction species, as well to predict phase behaviour in reaction solution.
Repositori URV