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

Abstract

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 NH4NO3-ZnO composite calcined at 300 °C with a molar ratio of NH4NO3 to Zn(NO3)2·6H2O 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°, Eth, and Cv° from the Gaussian calculation and k. Conversely, a positive correlation was observed for EHOMO (B=0.742, β = 1.282, P = 0.011), qH+ (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.