Reductive catalytic depolymerization of industrial lignin and hemicellulose: process development and intensification

Modern biorefinery aims at utilizing renewable feedstocks to make a wide range of products such as chemicals, biofuels, and biomaterials while leaving as little residue as possible. Lignocellulosic biomass is a promising feedstock due to its availability, renewable nature, widespread application, and low competition with food. The fractionation of lignocellulose to its three main polymeric constituents and depolymerization of especially lignin and hemicellulose into monomers and oligomers before conversion into value-added chemicals is a key step in biorefineries. Despite a large potential, massive quantities of industrially produced hemicellulose and lignin are regularly simply burned for energy, which significantly hinders the realization of the sustainable bioeconomy. The current work is devoted to enhancing the depolymerization of lignin and hemicellulose fractions obtained from Finnish silver birch with the help of a novel semi-industrial method.
The lignin was depolymerized in different organic solvents or solvent mixtures under a hydrogen atmosphere in the presence of a heterogeneous catalyst. The goal was to acquire small aromatic compounds for further valorization. The influences of different parameters, including lignin solubility, reaction time, hydrogen pressure, reaction temperature, basic additives, type and loading of catalyst, as well as type and composition of organic/aqueous solvent on the kinetics was investigated. Selective and efficient depolymerization to monomers and dimers was achieved by process intensification.
The research on depolymerization of industrial hemicellulose was performed in two stages. The acidic hydrolysis of xylan to xylose was first studied in both batch and continuous reactors. Several commercial heterogeneous catalysts were screened, and the reaction parameters were optimized to find a compromise between the reaction kinetics of the hydrolysis and the undesired degradation of monosaccharide products to achieve the highest xylose yield. Moreover, the reaction kinetics was modelled successfully.
One flow through hydrolysis and hydrogenation of hemicellulose was investigated in a continuous reactor equipped with two catalyst beds. The xylose produced by hydrolysis was subsequently converted to xylitol in the second bed. The reaction temperature, hydrogen pressure ivand residence time were varied to study the kinetics. A high yield (c. 90%) of xylitol was achieved and the kinetics was modelled obtaining a good fit to the experimental data.
The process of biomass valorization is typically highly temperature sensitive and it is not self-evident that isothermal processing conditions are optimal. The heat capacity of the reactor system is crucial when the process is performed under dynamic conditions and the heat capacity of catalyst support is not very well known and it was here studied in order to simulate and perform secure experimental operation in the future. The specific heat capacities of typical catalytic supports used in biorefinery applications were characterized with a Tian-Calvet micro-calorimeter. The temperature dependence was investigated for each catalytic material and polynomial expressions were successfully applied for simulating the experimental data. Significant differences were observed between the supports and the results contribute significantly to future development in valorization of biomass in dynamic conditions.
The current work contributes to the development and intensification of novel and sustainable processes for valorizing forest biomass according to the principles of green chemistry and process technology.