High added-value products from the hydrothermal carbonisation of olive stones

Olive stones (OS) were submitted to hydrothermal carbonisation (HTC) in order to evaluate the possibility of producing high added-value products, mainly furfural (FU) and 5-hydroxymethylfurfural (5-HMF) on one hand and hydrochars and carbons on the other hand. Temperature (160-240 °C), residence time (1-8 h), initial pH (1-5.5) and liquid/solid ratio (4-48 w/w) were systematically varied in order to study the main products and to optimise FU production. FU production yield up to 19.9 %, based on the hemicellulose content, was obtained. Other minor, but valuable, compounds such as 5-methylfurfural (5-MF) and some phenolic compounds were also produced. The hydrochar was carbonised at 900 °C, and the resultant carbon material was highly ultramicroporous with a peak of pore size distribution centred on 0.5 nm and a surface area as high as 1065 m² g^-1, typical of most carbon molecular sieves.