Esterification of aqueous lactic acid solutions with ethanol using carbon solid acid catalysts: Amberlyst 15, sulfonated pyrolyzed wood and graphene oxide

Lactic acid esterification over three carbon based solid acid catalysts were compared: sulfonated carbonised wood, graphene oxide (GO) and Amberlyst 15(A15) with the aim to address their water tolerance related to their acidity and hydrophobicity. The acidic properties of the carbon materials were measured by calorimetry of NH3 adsorption and their hydrophobicity was evaluated by water vapor adsorption isotherms. Here, we disclose the exceptional strong acidity of GO with differential heat of NH3 adsorption (Q(diff)NH(3)) higher than 200 kJ mol(-1) while A15 and sulfonated carbon have significant lower acid strength with Q(diff)NH(3) lower than 150 kJ mol(-1). The super-acidity of GO is tentatively ascribed to the presence of "free" protons explained by the delocalization of the negative charges of counter anions within the graphene sheet. In presence of the minimum water content (3 mol L-1), GO is the most active catalyst with a TOF of 52 h(-1) and its activity is also less inhibited upon water addition compared to A15. These results are rationalized considering that TOF depends intrinsically on the strength of Bronsted active sites and on the active site accessibility more or less controlled by the its micro environment hydrophobicity seen to be equivalent for A15, sulfonated carbon and GO.