Controllable HDO of lignin model compounds via aqueous phase reforming

The depolymerisation of lignin into its monomeric constituents is a promising route to the production of aromatic bulk chemicals from lignocellulosic biomass and lignin-containing waste streams (e.g. from the paper industry). In order to obtain an industrial product, further downstream processing of the monomeric mixture is required. Therefore, the selective removal of methoxy (-OMe) and hydroxy (-OH) groups from mixtures of monomeric lignin model compounds over Pt-based catalysts in the aqueous phase was investigated. This hydrogenolysis / reforming approach promisingly yielded a narrow range of products, the composition of which, can be controlled via the system hydrogen concentration. Optimisation of the hydrogen supply during the reaction is crucial to direct the reaction towards phenol while simultaneously preventing ring hydrogenation. It was found that at temperatures of ca. 250 °C, using a Pt-based catalyst,-OMe groups were converted to -OH groups, in turn reacting with H2O to produce CH3OH (MeOH). This MeOH is then instantly reformed over the Pt catalyst to produce 3H2 and CO2, thus providing an in-situ supply of hydrogen directly at the active sites of the catalyst, facilitating -OMe and -OH abstraction. Therefore, the amount of -Ome groups in turn limits the supply of hydrogen. It is important to note that aromatic ring hydrogenation does not occur as the hydrogen is produced in-situ and consumed simulataneously at the catalytic site. It was found, as expected, that the addition of small quantities of MeOH at the beginning of the reaction accelerates the conversion, although nevertheless the hydrogenation of phenol seems to be the slowest reaction in the reaction pathway, making it a promising product of the process. Pt nanoparticle catalysts supported on traditional support media, including g-Al2O3, ZrO2, TiO2, and activated carbon (C), were investigated along with a more cost effective / sustainable Ni/C alternative. Pt/ZrO2 showed the best results regarding conversion followed by Pt/C and Pt/g-Al2O3. Pt/TiO2 and Ni/C catalysts showed no significant conversion toward the desired products. Reactions were performed over 10 h, with the product distribution analysed via liquid phase sampling at hourly intervals. The main process problems encountered in these investigations concerned the formation of coke and polymerisation, whilst adoption of a continuous process provided insight into the severity of these side reactions.