Synthesis and Aerobic Oxidation Catalysis of Mesoporous Todorokite-Type Manganese Oxide Nanoparticles by Crystallization of Precursors

Abstract

The pursuit of a high surface area while maintaining high catalytic performance remains a challenge due to a trade-off relationship between these two features in some cases. In this study, mesoporous todorokite-type manganese oxide (OMS-1) nanoparticles with high specific surface areas were synthesized in one step by a new synthesis approach involving crystallization (i.e., solid-state transformation) of a precursor produced by a redox reaction between MnO4 – and Mn2+ reagents. The use of a low-crystallinity precursor with small particles is essential to achieve this solid-state transformation into OMS-1 nanoparticles. The specific surface area reached up to ca. 250 m2 g–1, which is much larger than those (13–185 m2 g–1) for Mg-OMS-1 synthesized by previously reported methods including multistep synthesis or dissolution/precipitation processes. Despite ultrasmall nanoparticles, a linear correlation between the catalytic reaction rates of OMS-1 and the surface areas was observed without a trade-off relationship between particle size and catalytic performance. These OMS-1 nanoparticles exhibited the highest catalytic activity among the Mn-based catalysts tested for the oxidation of benzyl alcohol and thioanisole with molecular oxygen (O2) as the sole oxidant, including highly active β-MnO2 nanoparticles. The present OMS-1 nanomaterial could also act as a recyclable heterogeneous catalyst for the aerobic oxidation of various aromatic alcohols and sulfides under mild reaction conditions. The mechanistic studies showed that alcohol oxidation proceeds with oxygen species caused by the solid, and the high surface area of OMS-1 significantly contributes to an enhancement of the catalytic activity for aerobic oxidation.

β-MnO2 nanoparticles as heterogenous catalysts for aerobic oxidative transformation of alcohols to carbonyl compounds, nitriles, and amides

“…The reaction rate with β-MnO 2 -1 for the aerobic oxidation of 1a was approximately three times larger than that with OMS-2. 56 On the basis of mechanistic studies, 53,54,56,68 we propose that the present MnO 2 -catalyzed system likely includes the Mars-van Krevelen mechanism where substrate oxidation with oxygen supplied from the solid, and the catalytic oxidation activity well agrees with the reactivity of surface lattice oxygen species estimated from TPR analysis. Therefore, this reactivity difference between β-MnO 2 -1 and OMS-2 can be explained by the reactivity of the oxygen atoms in manganese dioxides: 54 (i) the reduction rate per surface area estimated from the H 2 -TPR profiles of β-MnO 2 was approximately twice that of α-MnO 2 and (ii) β-MnO 2 consists of only planar oxygen sites with lower vacancy formation energy than α-MnO 2 (Fig.…”

“…We very recently reported that the catalytic activity of OMS-1 ultra-small nanoparticles (249 m 2 g −1 ) was higher than that of β-MnO 2 -1 (106 m 2 g −1 ); however, the intrinsic activity per surface area of β-MnO 2 -1 (3.3 × 10 −2 mmol m −2 h −1 ) was higher than that of the OMS-1 nanoparticles (2.1 × 10 −2 mmol m −2 h −1 ). 68 Other Mn-containing oxides (Mg 6 MnO 8 , 59 Mn 2 O 3 , Mn 3 O 4 , and MnO) and Fe-, Co-, Ni-, and Cu-based oxides were almost inactive. The oxidation of 1a proceeded almost quantitatively at 80 °C for 4 h, and there was no significant difference in the yield of 2a (97–98%) among β-MnO 2 -1 , -3 , and -4 (entry 1, Table 2).…”

Bismuth phosphate nanoparticle catalyst for direct oxidation of methane into formaldehyde

“…X-ray diffraction (XRD), energy dispersive X-ray fluorescence spectroscopy (ED-XRF), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry-differential thermal analysis (TG-DTA), H 2 temperature-programmed reduction (H 2 -TPR), nitrogen adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were performed using previously described instruments. 44,[59][60][61] The details are described in the ESI. † 2.2.…”

The tail of imidazole regulated the assembly of two robust sandwich-type polyoxotungstate-based open frameworks with efficient visible-white-light-driven catalytic oxidation of sulfides

“…A more serious concern is that most of these heterogeneous catalysts are amorphous or semicrystalline, posing great difficulties in understanding the relationship between the structure and functionality. [19][20][21][22] Hence, seeking recyclable, crystalline surrogate photocatalysts is an urgent need.…”