ABSTRACT
In the oxidation reaction of volatile organic compounds catalyzed by metal oxides, distinguishing the role of active metal sites and oxygen mobility at specific preferentially exposed crystal planes and diverse temperatures is challenging. Herein, Co3O4 catalysts with four different preferentially exposed crystal planes [(220), (222), (311), and (422)] and oxygen vacancy formation energies were synthesized and evaluated in styrene complete oxidation. It is demonstrated that the Co3O4 sheet (Co3O4-I) presents the highest C8H8 catalytic oxidation activity (R250 °C = 8.26 µmol g-1 s-1 and WHSV = 120,000 mL h-1 g-1). Density functional theory studies reveal that it is difficult for the (311) and (222) crystal planes to form oxygen vacancies, but the (222) crystal plane is the most favorable for C8H8 adsorption regardless of the presence of oxygen vacancies. The combined analysis of temperature-programmed desorption and temperature-programmed surface reaction of C8H8 proves that Co3O4-I possesses the best C8H8 oxidation ability. It is proposed that specific surface area is vital at low temperature (below 250 °C) because it is related to the amount of surface-adsorbed oxygen species and low-temperature reducibility, while the ratio of surface Co3+/Co2+ plays a decisive role at higher temperature because of facile lattice oxygen mobility. In situ diffuse reflectance infrared Fourier spectroscopy and the 18O2 isotope experiment demonstrate that C8H8 oxidation over Co3O4-I, Co3O4-S, Co3O4-C, and Co3O4-F is mainly dominated by the Mars-van Krevelen mechanism. Furthermore, Co3O4-I shows superior thermal stability (57 h) and water resistance (1, 3, and 5 vol % H2O), which has the potential to be conducted in the actual industrial application.
ABSTRACT
Toluene methylation with methanol to produce xylene has been widely investigated. A simultaneous side reaction of methanol-to-olefin over zeolites is hard to avoid, resulting in an unsatisfactory methylation efficiency. Here, CO2 and H2 replace methanol in toluene methylation over a class of ZnZrO x -ZSM-5 (ZZO-Z5) dual-functional catalysts. Results demonstrate that the reactive methylation species (H3CO*; * represents a surface species) are generated more easily by CO2 hydrogenation than by methanol dehydrogenation. Catalytic performance tests on a fixed-bed reactor show that 92.4% xylene selectivity in CO-free products and 70.8% para-xylene selectivity in xylene are obtained on each optimized catalyst. Isotope effects of H2/D2 and CO2/13CO2 indicate that xylene product is substantially generated from toluene methylation rather than disproportionation. A mechanism involving generation of reactive methylation species on ZZO by CO2 hydrogenation and migration of the methylation species to Z5 pore for the toluene methylation to form xylene is proposed.
ABSTRACT
Mn-Co mixed metal oxide is considered as efficient catalyst for the total oxidation of volatile organic compounds. In this study, nanocube-like metal-organic frameworks (Mn3[Co(CN)6]2·nH2O) are adopted as the precursor to generate Mn-Co oxides with different Mn/Co molar ratios, which affect little on phase structure and textural properties. The obtained MOF-Mn1Co1 with uniform metal dispersion is rich in high valence of surface Mn4+ and Co3+ species, leading to high low-temperature catalytic activity of total toluene oxidation. The results of toluene-TPD followed by TPO and toluene-TPSR match well with the catalytic performances of MOF-Mn1Co1, MOF-Mn1Co2, and MOF-Mn2Co1, and in situ FITR proves that the benzoate route exists over MOF-Mn1Co1. It is found that a moderate ratio of Mn/Co (1:1) favors good low-temperature reducibility and high Oads/Olatt, resulting in superior oxidation performance. However, the stability in the existence of water for MOF-Mn1Co1 is not satisfied, which should be overcome in the future.
