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 lambda > 350 nm. At 350 nm, k(obs) values at an optimum NaY zeolite/POM combination were 1.02 x 10(-2), 6.5 x 10(-3), and 5.1 x 10(-3) min(-1) for PW12O403-, SiW12O404-, and PMo12O40, respectively. Unsupported POMs exhibited no detectable photocatalytic activity at lambda > 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 PW12O403-/0.2 wt % NaY zeolite (k(obs) = 1.69 x 10(-2) min(-1)), 0.75 mM SiW12O404-/0.1 wt % NaY zeolite (k(obs) = 8-1 x 10(-3) min(-1)), and 0.5 mM PMo12O403-/0.1 wt % NaY zeolite (k(obs)= 6.8 x 10(-3) min(-1)). The k(obs) values are a factor of 4-8 times higher than those with PW12O403-, SiW12O404-, and PMo12O403- alone, respectively. Although the system is clearly complex, with oxidation rates dependent on [POM], [DCB](o), zeolite loading, and A, the results indicate a promising approach for enhancing polyoxometalate photochemistry and for developing new photocatalytic materials.