Primary conversion of lignocellulosic biomass for the production of furfural and levulinic acid

The production of energy and chemicals from renewable resources has gained significant attention as a means to support the transition from fossil fuels towards clean and sustainable technologies. Due to its availability and rich carbohydrate composition, lignocellulosic biomass represents a valuable starting material and requires primary processes to unlock its components. The main focus of this research was to study and develop further knowledge on the primary steps of conversion of lignocellulosic materials, i.e. biomass fractionation, cellulose/hemicellulose hydrolysis, and the production of furanic compounds (furfural, 5-hydroxymethylfurfural) and levulinic acid. In an initial stage, the fractionation of Miscanthus × giganteus and sugarcane bagasse was investigated using hydrogen peroxide in formic acid solutions. This treatment removed more than 70% of the lignin in the feedstocks after 13 h at room temperature and after 15 min at temperatures over 150 C. The use of hydrogen peroxide in formic acid under transient temperature facilitated the separation of >90% of the lignin and 80-98% of hemicellulose sugars, resulting in nearly pure cellulosic pulps. Likewise, lignin was recovered from the liquor and presented aromaticity properties proper of Organosolv lignins. The conversion of lignocellulosic biomass to levulinic acid and furfural via acid hydrolysis was undertaken using sulphuric acid as catalyst. Biomass feedstocks, including agricultural, municipal wastes (paper) and rotational crops from Brazil and Ireland, led to levulinic acid yields between 150 and 400 kg/ton dry biomass. Additionally, the kinetics of acid hydrolysis of Miscanthus to produce levulinic acid and furfural were studied at mild temperatures (150-200 C) and high acid concentrations (0.10-0.53 M H2SO4). A two-stage process was found to maximise furfural (27.3 mol%) in a first reactor operated at 185 C with 0.5 M H2SO4. A second stage leads to levulinic acid yields between 58-72 mol% at temperatures of 160-200 C. A kinetic modelling study of the dehydration reactions of xylose, arabinose and glucose was carried out using formic acid as catalyst. Experimental data was obtained from the conversion of xylose, arabinose, glucose and furfural at 130-170 C and high formic acid concentrations (10-64 wt%). High temperatures (>160 C) favoured the formation of furfural (62-68 mol%) from xylose and arabinose; however, fundamental differences in the reaction mechanisms followed by the pentoses were found. Formic acid was not an effective catalyst of the conversion of glucose at temperatures under 150 C. Temperatures above 200 C favoured the formation of 5-hydroxymethylfurfural (20 mol%), while levulinic acid was maximised (40 mol%) at milder temperatures (170-200 C). Experimental evidence was obtained in relation to the side reactions affecting the conversion of xylose in liquors from the fractionation of Miscanthus. The conversion of xylose, arabinose, and glucose and the formation of 5-hydroxymethylfurfural and levulinic acid in the liquors were described satisfactorily by the kinetic models. However, when lignin was not separated prior to the reaction, the furfural concentrations observed were significantly 40% lower than those predicted by the models. By inducing the lignin separation from the liquor, no losses in the selectivity of furfural were observed. The effect of lignin model compounds and polymeric soluble lignin on the selectivities of the dehydration of xylose and glucose was investigated at 150 and 170 C and acid concentrations of 60-62 wt% HCOOH. The results confirmed that the oligomeric soluble lignin led to detrimental effects on the selectivity of the furfural formation from xylose.