New insights into the strength and accessibility of acid sites of sulfonated hydrothermal carbon

Sulfonation of hydrothermal carbon leads to a very active acid catalyst, in spite of the very low surface area determined by N2 physisorption and the even distribution of sulfonic groups, as demonstrated by scanning transmission electron microscopy analysis. However, CO2 adsorption at 273 K indicates the presence of ultramicropores, not detected with N2 at 77 K. Acidity was characterized by different procedures, but the most relevant was 31P solid state NMR of adsorbed triethylphosphine oxide. The solvent used for the adsorption is crucial, as the solid seems not to be acidic in hexane, but it shows acidic groups comparable to arylsulfonic ones when the adsorption takes place in methanol. This is probably due to the breakage of the network of hydrogen bonds that forms the ultramicropores, leading to an improved accessibility to the sites. This result is in agreement with the hydrophilicity of the solid, demonstrated by water adsorption.