Catalytic Conversion of Lignin for the Production of Aromatics

Abstract

With the depletion of fossil fuels and increasing environmental awareness, there is much interest in the use of biomass as a more sustainable alternative feedstock for the production of renewable fuels and chemicals. Non-edible lignocellulosic biomass is the major and most sustainable source of biomass and consists mainly of three components: the polysaccharides cellulose and hemicellulose and an aromatic polymeric fraction called lignin. While commercial processes have already been developed for the cellulose and hemicellulose fractions, lignin has received less attention and only a few examples of successful lignin valorization processes are available. The aim of the work described in this thesis was 1) to develop a two-step catalytic conversion route for the production of aromatic bulk chemicals such as phenolics or benzene, toluene and xylenes (BTX) from lignin and 2) to gain more insight into the properties required of a good catalyst in terms of activity, selectivity and stability in lignin conversion processes. The two-step route studied here comprises a catalytic lignin depolymerisation reaction as the first step, followed by a hydrodeoxygenation step to reduce the oxygen content of the mixture and limit the amount of final products. With this aim, different reactions were developed and tested on various real lignin samples. Part I of this thesis is concerned with studies of two different routes for the catalytic depolymerization of lignin. In a variation on the well-known aqueous-phase reforming process, Pt/Al2O3 is used in combination with an acid or base as co-catalyst for the catalytic depolymerization of lignin in an ethanol/water solvent mixture to monomeric aromatic molecules in a process coined liquid-phase reforming (LPR). Higher yields and very little solids formation were observed compared to the aqueous-phase reforming of lignin, which takes place in water alone. The stability of the support and the metal phase of the Pt/γ-Al2O3 catalyst were studied under the reaction conditions used for the liquid phase reforming reaction. An alternative catalytic lignin depolymerization method was also studied, i.e. a bleaching method that was originally developed for the removal of residual lignin from cellulose pulps was applied for the oxidative depolymerization of lignin itself. Part II is dedicated to the second step of the two-step conversion route of lignin to aromatics, i.e. a study of two types of hydrodeoxygenation (HDO) catalysts with lignin model compounds. The HDO of a library of model compounds over a commercial sulfided CoMo/Al2O3 catalyst and the HDO of guaiacol over carbon nanofiber-supported tungsten and molybdenum carbides are reported. It was shown that the conversion of guaiacol to phenol over the carbides takes place via a direct demethoxylation pathway rather than by sequential demethylation and hydrodeoxygenation steps as is the case for the CoMo/Al2O3-catalyzed reactions. In Part III of this thesis the first depolymerization step is combined with the second hydrodeoxygenation step. Lignin is depolymerized via a LPR reaction with NaOH as a co-catalyst and subsequently hydrodeoxygenated with the CoMo/Al2O3 and Mo2C/CNF catalysts that were studied in Part II. This resulted in the first example of BTX production from actual lignin.