Preparation of nano-structured cryptomelane materials for catalytic oxidation reactions

Abstract

Manganese-based octahedral molecular sieves of the type K-OMS-2 (cryptomelane structure) were prepared via reflux method by synproportionation of KMnO 4 and Mn 2? in acidic aqueous suspension. For this method, different manganese anions (sulphate, chloride and acetate) were used, which exert a strong influence on the prepared materials. Several techniques such as X-ray diffraction, Fourier transformer infrared, Raman spectroscopy, transmission electron microscopy, differential and gravimetric thermal analysis and H 2 -temperature programmed reduction were used to characterize the prepared samples. The results revealed that the prepared samples were mainly pure mono-phase cryptomelane materials. The obtained K-OMS-2 material on using the sulphate anion has more available lattice oxygen as compared to that prepared by the other anions. The catalytic activity of the prepared samples was tested towards the oxidative dehydrogenation of cyclohexane. Over the K-OMS-2RS sample, cyclohexane conversion was significantly higher than the other prepared samples.

Elucidation of Unexpectedly Weak Catalytic Effect of Doping with Cobalt of the Cryptomelane and Birnessite Systems Active in Soot Combustion

“…3B) the set of characteristic bands was observed in the Raman spectra at ca. 185 cm −1 and 385 cm −1 (attributed to Mn-O-Mn bending vibrations within the MnO 2 octahedral lattice), 575 cm −1 (displacement of oxygen anions neighboring to manganese cations along the direction of octahedral chains) and 635 cm −1 (Mn-O stretching modes within tetrahedral sites), which remains in agreement with previously published literature data [12,15,33]. Based on the analysis of Raman active vibrational modes, it can be confirmed, that neither cobalt presence nor its concentration did not significantly influence the structure of both cryptomelane and birnessite.…”

“…700 and 1610 cm −1 visible in Fig. 4B can be ascribed to the vibrations of the MnO 6 octahedral framework, characteristic of the cryptomelane structure [33]. Some adsorbed carbonate species gave rise to the bands observed in the range 1150-1725 cm −1 in the case of both cobalt-impregnated cryptomelane samples (Fig.…”

Chemical Surface Analysis on Post-Thermal Treatment of the K-OMS-2 Catalysts and Catalytic Oxidation Efficiency at Low Temperature

“…Secondly, the bending vibration of the hydroxy groups in tunnel structure represented the water molecules and OH groups that observed at ~1600 cm -1 . Thirdly, the broadband detected at approximately 3300 cm -1 was assigned to the stretching vibration of the hydroxy groups in the lattice of the K-OMS-2 structure (Ousmane et al 2014;Said et al 2018Said et al , 2016Zhang et al 2012). The FTIR spectra of calcined K-OMS-2 samples with and without drying process (105ºC for overnight) in the range of 600-4000 cm -1 is displayed in Figure 7(a) and 7(b), respectively.…”

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“…The layered manganese oxides are classical type of inorganic host layer as birnessite [5], lithiophorite [6], cryptomelane [7,8], romanechite, hollandite [9], and todorokite [10]. Natural δ-MnO2 (birnessite) is a highly disordered and occurred in a finely dispersed state, commonly mixed with other phases such as phyllosilicates and Fe oxy-hydroxides.…”