Photocatalytic Oxidation of Aqueous 1,2-Dichlorobenzene by Polyoxometalates Supported on the NaY Zeolite

In this work, we report the effectiveness of the NaY zeolite as a solid support for several photocatalytically active polyoxometalate (POM) salts (H2NaPW12O40, H4SiW12O40, or H3PMo12O40) as indicated by the photodegradation kinetics of the probe compound 1,2-dichlorobenzene (DCB) in water. The photooxidation of DCB was carried out in illuminated (254−370 nm), oxygenated solution at pH 1.0. NaY zeolite-supported POMs exhibited photocatalytic activity over the wavelength range 254−350 nm. Photocatalytic activity did not drop off significantly until λ > 350 nm. At 350 nm, k obs values at an optimum NaY zeolite/POM combination were 1.02 × 10-2 , 6.5 × 10-3 , and 5.1 × 10-3 min-1 for PW12O403 -, SiW12O404 -, and PMo12O40 3-, respectively. Unsupported POMs exhibited no detectable photocatalytic activity at λ > 340 nm. At all wavelengths, photooxidation of DCB in the presence of 0.1% NaY zeolite with each of three different 0.5 mM polyoxometalates (POMs) was first order in DCB. The optimum POM/NaY zeolite ratios at 254 nm for DCB oxidation were 0.5 mM PW12O40 3-/0.2 wt % NaY zeolite (k obs = 1.69 × 10-2 min-1), 0.75 mM SiW12O40 4-/0.1 wt % NaY zeolite (k obs = 8.1 × 10-3 min-1), and 0.5 mM PMo12O40 3-/0.1 wt % NaY zeolite (k obs= 6.8 × 10-3 min-1). The k obs values are a factor of 4−8 times higher than those with PW12O40 3-, SiW12O40 4-, and PMo12O40 3- alone, respectively. Although the system is clearly complex, with oxidation rates dependent on [POM], [DCB]o, zeolite loading, and λ, the results indicate a promising approach for enhancing polyoxometalate photochemistry and for developing new photocatalytic materials.