Synthesis of ZSM-5 zeolite from rice husk ash: Characterization and implications for photocatalytic degradation catalysts

Silica synthesized from raw rise husks (white particles) through submission to consecutive chemical treatment using NaOH and HCl solutions was well incorporated with other components forming zeolites; under hydrothermal conditions, to synthesize two series of ZSM-5 (Z) of different Si/Al ratios, namely Z40 and Z80. Transition metal oxides (V, Co) were incorporated with Z samples by in situ synthesis and by impregnation. Various techniques were employed for samples characterization including XRD, FTIR, SEM, N2 adsorption and pyridine-FTIR. The photocatalytic activity of V and Co incorporated Z zeolites in addition to neat ZSM-5 samples towards degradation of acid green (AG) dye were well investigated and compared under similar conditions. The characterization results of the catalysts indicated that Z80 measured higher surface area (616 m2/g) and crystallites size (44.4 nm) than Z40 (536 m2/g and 35.8 nm) besides, Z40 signifies the presence of distinct phase of silica capable of decreasing the pore radius and volume of this particular sample (22.9 Å, 0.49 cm3/g) when compared with the former (25.3 Å, 0.623 cm3/g). The degradation rate of Z40 was 16 times higher than Z80 in presence of UV irradiation reflecting the importance of absorbability of silica phase present on Z40 as well as acidity and decreased crystallites size. V and Co impregnated Z (VZimp, CoZimp) samples were found to exhibit higher and unique photocatalytic activities for AG degradation than in situ incorporated ones. CoZ80imp exhibited the highest activity (0.14 min−1) among all samples exceeding CoZ40imp (0.11 min−1) and VZ40imp (0.043 min−1). The enhanced photocatalytic activity of CoZ80imp is attributed to charge transfer excited complex between Co in Z along with AG ligand in addition to increasing the percentages of Co3O4 and cobalt silicate moieties over those in CoZ40imp and to the higher surface area of CoZ80imp, comparatively. More informations on local structures of metal oxides inside zeolites and their photocatalytic activities towards AG were deduced, correlated and discussed.