Furfural from pyrolysis of agroforestry waste: Critical factors for utilisation of C5 and C6 sugars

Furfural, a promising platform biochemical, plays an important role in the seamless transition of the current fossil fuel-centric economy to a carbon-neutral future. This paper reviews the state-of-the-art advances of furfural production from biomass with a particular focus on C6 sugars and the emerging pyrolysis method. To date, conversion of C6 to furfural has yet to be successful, while C5 sugars were primarily targeted in science and industry communities. Firstly, the effects of operating conditions and the roles of different catalysts have been reviewed. The optimum temperature varies from 300 to 600 °C for different feedstocks, with the residence time falling in the order of seconds. Water plays an essential role in the conversion of C6 sugars. The water-rich systems favour the production of 5-hydroxymethyl-furfural, while the water-lean systems facilitate furfural production. Secondly, the structure-activity correlations between furfural yield/selectivity and catalyst properties and the reaction pathways of C5 and C6 sugars were reviewed extensively. While the C5 only require a dehydration reaction to produce furfural, the C6 need dehydration and selective removal of the formaldehyde side group. Therefore, a specifically designed bifunctional catalyst is essential. In contrast to homogeneous mineral acids, solid heterogeneous catalysts are preferred for producing furfural due to their easiness of post-separation and environmental friendliness. More specifically, the sulphates and sulfonated catalysts are the most active due to their stronger acidity and thermal stability. Finally, the knowledge gaps and future perspectives are proposed, including unlocking C6 sugars, advancing pyrolysis technology and synthesis of advanced solid acid catalysts.