Pyrolysis reaction of squaric acid: A one-step method for producing expanded foam of mesoporous carbon
Résumé
A template-free approach is described for the synthesis of expanded foams of mesoporous carbon exhibiting high surface areas ranging from 550 to 1100 m2/g. The procedure is based on the exceptional carbonization reaction that occurs with squaric acid (H2C4O4), a strained four-membered carbocycle belonging to the oxocarbon acids. Indeed the pyrolysis reaction proceeds just above 300°C through an amazing one-step and sharp exothermic phenomenon coupled with a weight loss of 90%, thereby promoting a porous structure. This massive gas release behaves also as a "fluid" template during the carbon production, which explains the formation of expanded foams. This particular thermal behaviour seems related to the phase transition that occurs in H2C4O4 crystals at Tc=121°C. Below Tc the planar squaric acid molecules exhibit a fully ordered structure in a monoclinic system whereas for T>Tc the structure undergoes a disordered tetragonal structure where all C-O bonds of squaric acid become statistically equivalent in a perfect square, making a discrete thermal decomposition reaction possible.