*3.2.1. Variations of hydrothermal carbonization (HTC) process*

In parallel with the studies centered in the activation of hydrochars, in the last few years, the scientific community also started to explore new synthesis routes to obtain a porous structure during the HTC step, being prepared porous carbons with BET surface areas up to 700m<sup>2</sup> g−1. The methodologies proposed avoid the need of further thermal or chemical activation, may enable the synthesis of heteroatom-doped solids [90] and can also allow the synthesis of hierarchical materials [91].

Fechler et al. reported the synthesis of porous carbon materials with BET areas between 425 and 672 m<sup>2</sup> g−1 through HTC (180°C overnight) of glucose mixed with several eutectic salt mixtures (i.e. LiCl/ZnCl<sup>2</sup> , NaCl/ZnCl<sup>2</sup> , and KCl/ZnCl<sup>2</sup> ) in the presence of a small amount of water [90]. The authors proved that both the amount of water added and the salt composition are determinant for the successful synthesis of materials, which are formed by very small particles aggregation, identical to aerogels. The use of the eutectic mixture LiCl/ZnCl<sup>2</sup> originated the carbon materials with the highest surface area. When 2-pyrrol-carboxyaldehyde was added as co-reagent of the mixture glucose and ZnCl<sup>2</sup> , a material with 3% of nitrogen and BET area of 576 m<sup>2</sup> g−1 was obtained. Fellinger et al. used glucose as carbon source and borax (Na<sup>2</sup> B4 O7 ) as both catalyst and structure-directing agent to prepare hierarchical structured carbogels under HTC conditions [91]. Further carbonization under nitrogen at 550 or 1000°C allowed to obtain a carbon material with a BET area of 614 m<sup>2</sup> g−1 and 70% of mesopore volume.
