Efficient photocatalytic degradation of 24 imidazolium ionic liquids in an N-ZnO/simulated sunlight irradiation system and its mechanism

An efficient nitrogen (N)-ZnO photocatalytic composite was synthesized and well characterized to demonstrate its superior photocatalytic performance as well as the degradation kinetics of 24 ILs in an N-ZnO/Xe lamp irradiation system, as well as the pathways and mechanisms of the photocatalytic degradation (PCD) of typical ionic liquids [OMIm]Br was systematically studied. To investigate the catalytic effect, nitrogen was synthesized from various inorganic and organic nitrogen compounds to ZnO. The NH 4 NO 3 -ZnO composite calcined at 300 °C with a molar ratio of NH 4 NO 3 to Zn(NO 3 ) 2 ·6H 2 O of 10:1 exhibited superior catalytic performance as assessed by SEM, TEM, FT-IR, UVDRS, BET surface area, XPS, and XRD characterizations. After 24 h of exposure to Xe lamp irradiation, all 24 ILs were significantly degraded by the introduction of 0.125 g/L of catalyst, whereas simulated sunlight alone resulted in minimal degradation. The pseudo-first-order rate coefficients ( k ) for PCD were closely related to the carbon chain length of the alkyl substituents and the type of anion. Multiple linear regression analysis indicated no linear correlation between the parameters E , H°, E th , and C v° from the Gaussian calculation and k . Conversely, a positive correlation was observed for E HOMO (B=0.742, β = 1.282, P = 0.011), q H+ (B=1.040, β = 3.284, P = 0.006), and G° (B=0.001, β = 5.400, P = 0.007) with k . Mass spectrometry analysis revealed that the primary PCD pathway for [OMIm]Br involved C-N and/or C-C bond breaking, mono-, di-, and tri-hydroxylation, single and double oxygen transfer, and bromo-substitution reactions.