Photochemical behaviour of phenylbenzoquinone at the surface of the clays: Kaolinite, bentonite and montmorillonite

The kinetics of direct photochemical transformation of phenylbenzoquinone (PhQ) was studied at the surface of clays, light absorbing and scattering supports. When compared to the spectrum in solution, the UV absorption of PhQ on clays shows a hyperchromic as well as a solvatochromic effects. In kaolinite, bentonite and montmorillonite, the shift was about Δλ = 30 nm and the molar absorption coefficient was roughly three times higher. For various layers of clays, the disappearance of PhQ, followed a pseudo-first order kinetics. The diffusion of the compound from the lower part to the illuminate one was found to be low toward photolysis within the range 15–70 μm. In these conditions, the first order rate constant at the surface of kaolinite, k0, was found equal 2.3 × 10−2 min−1 in aerated conditions. This was roughly constant for the two other tested clays, bentonite and montmorillonite, where k0 was estimated to be 2.1 × 10−2 min−1 and 1.8 × 10−2 min−1, respectively. The photocyclization was found to be oxygen concentration dependent: k vacuum z / k air z = Φ vacuum / Φ air ≈ 3.5 . Diffuse-reflectance experiments clearly show the implication of the triplet state excited state in the photolysis of PhQ.