Functionalising hydrothermal carbons for catalysis – investigating solid acids in esterification reactions

Abstract

A simple and controllable acid functionalised catalyst synthesis is presented based on the hydrothermal carbonisation (HTC) of glucose in the presence of the structure directing agent sodium borate. This synthetic strategy in combination with a post-thermal carbonisation step allows direction of porosity/specific surface area, and HTC xerogel material functionality. All these parameters influence the introduction of S-containing functional groups (e.g. acidity) to the xerogel. The prepared acid functionalised HTC materials were characterised via N₂ sorption, back titration, elemental analysis, XPS, ATR FT-IR, and SEM, with their applicability as solid acids assessed through model esterification reactions of different organic acids (e.g. short alkyl chain and aromatic systems). An S-functionalised HTC catalyst described in this study with a high specific surface area and porosity (502 m² g⁻¹; 0.42 cm³ g⁻¹), and a loading of 1.1 mmol g⁻¹ SO₃H/SO₄²⁻ acid sites (from a 2.7 mmol g⁻¹ of total acid groups) was found to have comparable catalytic activities as commercial Amberlyst-36® catalyst in all the investigated esterification reactions. Catalyst re-usability under the applied batch conditions was improved by heating the catalyst at higher temperatures in order to remove deposited organic acids and their derivatives. The concept presented provides a basis for further development and optimisation of HTC supported catalysts in acid and other catalysis.