100 mL xylene were prepared in a 250 mL 3-necked flask and heated to 130 °C (reflux). 194 mg of solid acid powder was crushed with mortar and pestle. 16 mg samples of the solid acid powder were prepared on a glass disk. The PP cylinders were dipped with a holder into the boiling solvent for 5 s intervals. Directly after that, the cylinders were rolled through the prepared amount of solid acid on a glass disk until all solid acid powder was attached to the cylinder. The coated cylinders were washed with EtOH and dried at 100 °C for 2 h.