The montmorillonite sample supplied by PT. Tunas Inti Makmur, Indonesia was firstly activated with 0.5 M H2SO4 solution under reflux for 6 h. The solid was filtered and washed with double distilled water until the filtrate pH was 7. The chemicals used, zinc chloride (ZnCl2·2H2O), NaOH, H2SO4, cetyl trimethyl ammonium chloride (CTMACl), methylene blue and H2O2 were supplied by Merck. Before metal dispersion, a clay suspension was prepared by diluting 5 wt.% montmorillonite powder in aquadest with the addition of CTMACl drop by drop at 2.5 mmol CTMACl/g-montmorillonite stirred at room temperature overnight. A pillarization agent of zinc chloride was prepared by mixing zinc chloride and NaOH at 1:1 M concentration in H2O:isopropanol (50:50) solution with vigorous stirring for 4 h. The Zn2+ precursor solution was then dropped slowly into the montmorillonite suspension to achieve a 5.0 wt.% Zn loading. The mixture was stirred overnight and then was filtered and washed using deionized water until the filtrate had pH 7 and was free from Cl− ions (AgNO3 test). The solids were dried in an oven for overnight and calcined at 500 °C for 4 h under N2 flow in order to remove carbonaceous material which might have been deposited in pores and to further remove impurities from material. The obtained sample was ground to 200-mesh and then was encoded as ZnO/montmorillonite. X-ray diffraction (XRD) patterns of powder samples were obtained with a Shimadzu X6000 diffractometer using Ni-filtered Cu Kα radiation (λ = 1.54 Å) and scanning speed 5°/min DRUV–Vis spectra of the samples were recorded on a JASCO V-670 spectrophotometer equipped with a diffuse reflectance accessory; the % R values were transformed via the Kulbelka–Munk formula. A gas sorption analyzer (NOVA 1200e) was used to determine the specific surface area and adsorption–desorption profile using N2 sorbate gas at 196 °C. Elemental analysis was performed with a Seico EDX. Photo-oxidation was investigated, with a UV–Visible spectrophotometer (HITACHI U-2080) and Shimadzu HPLC.
2.1. Photocatalytic activity
Photooxidation reaction of methylene blue (MB) was carried out in a 500 mL glass beaker in a thermo-controlled water bath. The beaker was charged with 250 mL aqueous solution of MB at varying concentrations and 0.2 g/L catalyst. The reactor was equipped with an ultraviolet lamp at and was held in a closed box. The radiation source was a Philips 20 W UV-B lamp of placed on top at a distance of 40 cm from the glass beaker. Water from the thermostatic bath was circulated through a reactor jacket to ensure a constant temperature of 25 ± 0.5 °C inside the reactor. In order to carry out the experiments under aerobic conditions, the solution was saturated by bubbling air at atmospheric pressure. H2O2 solution was added into the reactor at a molar ratio of 1:5 (H2O2:MB) before stirring and photon exposure. The concentration of MB was determined using a spectrophotometer (UV–Visible HITACHI U-2010) at 663 nm. HPLC analysis was also employed using a Shimadzu HPLC with acetonitrile:H2O (50:50) as a mobile flow.