Hydrothermal carbons produced from tannin by modification of the reaction medium: Addition of H+ and Ag+

The present study deals with the preparation of carbonaceous materials from flavonoid tannin by a two-step process: hydrothermal carbonisation (HTC) at 180 degrees C in aqueous solution, followed by pyrolysis at 900 degrees C. Whereas such process and the resultant materials have been described elsewhere (Braghiroli et al., 2014), two modifications of the HTC step were introduced and investigated for the first time: 1) pH reduction, by para-toluenesulphonic acid addition, as catalyst of tannin rearrangement reactions; 2) silver nitrate addition as nucleating agent by silver metal nanoparticles formation. Carbon from tannin simply pyrolysed at 900 degrees C (i.e., without HTC step) was also prepared and used as a reference material. The impacts of these modifications on yield, composition, porosity and nanotexture of the resultant carbon materials were investigated in detail. The carbons produced at low pH were monolitic and had te highest surface areas, up to 800 m(2) g(-1), with the same microporous size distribution as the material prepared at non-modified pH. The presence of silver produced a decrease and a homogenisation of the microsphere particle size with an increase of the yield. Adding H+ and Ag+ both produced more crystalline carbons, although HTC itself at non-modified pH produced less crystalline carbon compared to the reference material. (C) 2015 Elsevier B.V. All rights reserved.