Hydrothermal conversion of hemicellulose sugars for the production of furfural

In the context of climate change and international efforts to achieve climate neutrality, the chemical industry is increasingly turning to renewable carbon sources. Existing renewable options such as recyclable plastics or CO2 capture from the atmosphere are not sufficient or are too energy-intensive, giving biogenic resources a key role in the future supply of raw materials. Due to the global decline in agricultural land per capita and the desire not to compete with food production, biogenic residues from agriculture and forestry are becoming increasingly important. Lignocellulose biorefineries make it possible to efficiently break down biomass into its main components (cellulose, lignin, and hemicellulose). However, the use of hemicellulose, which can account for up to 35% of the mass of biomass, remains commercially underrepresented. The conversion of hemicellulose, primarily composed of pentoses, into furfural, a versatile platform chemical, holds significant potential. There are over 80 known applications in various industries based on furfural. However, furfural production presents several challenges. A recurring issue is the formation of humins, which not only reduce yield but can also lead to process-related problems such as blockages. For this reason, the aim of this work was to carry out reaction kinetic investigations using model substances and an organosolv-based hemicellulose hydrolysate and then to investigate and optimize new process approaches for furfural production. The reaction kinetic studies were carried out in a continuous tubular reactor at temperatures of 160 to 200 °C, based on three reaction models. In addition to considering furfural degradation to include self-polymerization, the results were compared with existing literature. The models revealed differences in D-xylose conversion and furfural formation, with model 3, which includes a xylose intermediate, agreeing best with the experimental data. It has been shown that short-chain alcohols such as ethanol can effectively reduce the formation of humins during furfural production. The study of acid-catalyzed conversion of xylose in water/ethanol mixtures showed that ethanol can significantly increase the furfural yield from 36-52% to up to 90%, while humin formation decreases by an average of 60%. However, when Organosolv hemicellulose was used instead of xylose, no increase in furfural yield was observed, and an ethanol content of 50% even led to a lower yield, which can be attributed to the reduced hydrolysis capacity due to the high ethanol content. Nevertheless, ethanol also contributed to the reduction of the humins formed. Another method investigated in this work is hydrothermal reactive distillation (HRD). In this process, furfural is continuously removed via the vapor phase to prevent its degradation and the associated polymerization. Two biomasses and their hydrolysates were treated at temperatures of 170 to 210 °C using a steam releasing batch reactor. Analysis of the condensate and sump samples for reactants, products and by-products showed an outstanding efficiency of the HRD in terms of furfural yield (approx. 83 mol%) and purity, with no detection of residual sugars or humins in the condensate. The main by-products in the condensate were acetic acid and formic acid, while non-volatile substances such as levulinic acid and 5-hydroxymethylfurfural were enriched in the sump with a yield of up to 23 mol%.