2.5 Characterization techniques
N2 physisorption was conducted at 77 K on a Micromeritics Tristar 3000 apparatus. Prior to the measurements, the acid resin catalyst was outgassed at 343 K for 8 h. The bulk composition of the catalyst was determined by the inductively coupled plasma-atomic emission spectroscopy (ICP–AES; Thermo Elemental IRIS Intrepid). Exactly 200 mg of the catalyst was added to 20 mL of 25 wt% HNO3 solution and heated to boiling. Then, 20 mL of 40% of HF was added and stirred vigorously. The solution was diluted to 200 mL with distilled water for analysis. The XRD pattern was acquired on a Philips X’Pert Pro powder X-ray diffractometer using Cu Kα radiation (λ = 0.15418 nm). The tube voltage was 40 kV, and the current was 40 mA. The 2θ angles were scanned from 10 to 70o at 10o min−1. TEM characterization and elemental mapping were conducted on an FEI TECNAI F20 G2 electron microscope operated at 200 kV, to which a Gatan GIF 2001 spectrometer was attached. The energy filter images were recorded under the EFTEM mode. The width of the energy filter slit was 10 eV, the GIF incidence aperture was 0.6 mm, and the 3-window method was applied during elemental mapping collection.