Before the sulfonation process, synthesized carbons were sieved in
order to isolate particle size between 0.149 and 0.210 nm. A series of
PCx without sieving was also sulfonated for the sake of comparison.
The sulfonation process was carried out as follow: 0.2 g of carbon was
added to a concentrated H2SO4 solution (10mL). The solutionwas heated
for 8 h at a constant temperature (80 °C) under reflux and then
cooled to room temperature. Finally, the samples were washed with
distilled water and centrifuged in order to isolate the black precipitate
from the reaction mixture. [33,34]. The sulfonated carbons were dried
at 100 °C for 12 h. The catalysts obtained by this way were labeled
PCxS-SO3H.