Preparation of morphology-tuned γ-MnO2 and catalytic performance for the liquid-phase oxidation of toluene

Abstract

Introducing surfactants including hexadecyltrimethylammonium bromide (CTAB), macrogol 6000 (PEG6000), and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) triblock copolymer (P123) into the refluxing aqueous crystal nucleus slurry yielded morphology-tuned microcrystalline γ-MnO2. γ-MnO2 and the influence of surfactant modification were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption (BET), thermogravimetry analysis (TGA), O2 temperature programmed desorption (O2-TPD), and temperature programmed H2 reduction (H2-TPR). Surfactants led to differences in γ-MnO2 morphology, surface area, oxygen desorption behavior and reducibility. The effect of reflux time on catalyst morphology is discussed. The catalytic performance of γ-MnO2 during the solvent-free atmospheric oxidation of toluene was evaluated. PEG6000 modified γ-MnO2 exhibited the highest catalytic activity judging by surface area because of a greater mixed valency and more anion vacancies. The greatest mass specific activity was obtained for P123 modified γ-MnO2 with the largest surface area. Optimized reaction conditions yielded an 18.1% toluene conversion, and 87.4 and 73.2% total selectivity and selectivity for benzoic acid, respectively. © Editorial office of Acta Physico-Chimica Sinica.