Probing the effect of Mn dispersion on the catalytic performance of Mn-Na-W/SiO2 in the oxidative coupling of methane

Oxidative coupling of methane (OCM) has received much attention since the fundamental work of Hinsen et al.1 and Keller et al.2 (Eq. 1). Since 1981, hundreds of materials have been tested as catalysts.3 A good catalyst for this reaction has to be thermally stable and highly selective towards C2 products over the total oxidation of methane, which is by far the most thermodynamically favored reaction route under these conditions (Eq. 2). (1) CH₄ + 1/2 O₂-C₂H₆ or C₂H₄ + H₂O â0₂₉₈ = â140 kJ/mol (2) CH₄  + 2 O₂ ÂCO₂ + 2H₂O ↠0₂₉₈ = â'890 kJ/mol Mn-Na2WO4/SiO2 catalyst has been identified as one of the more promising catalysts having remarkable thermal and catalytic stability at the reaction conditions. Moreover, this catalyst obtained performance of 33% CH4 conversion at 80% C2 selectivity giving a total C2 yield of 25%.6 However, the economic threshold for practical applications requires the catalyst to demonstrate enhanced properties in order to achieve above 30% yield. The performance of this catalyst was found to be dependent on a complicated synergy between the three components (Na, Mn, W) on silica, where the absence of either one of them leads to inferior catalytic performance.5 A key step in catalyst preparation is the Na induced phase transition, from amorphous silica to α-cristobalite. This step is deemed as crucial for the dispersion of the active metal sites which in turn are leads to the good catalytic performance. In this study, we investigate the effect of pre-phase transition dispersion of the Mn on the catalytic performance of Mn-Na-W/SiO2 catalyst. We do this using the post-synthetic modification of Î-zeolite (i.e. dealumination) to form vacant T- sites, in which the Mn sites are incorporated into the zeolite framework. Only after this initial dispersion, the phase transition to 1/4-cristoballite is induced by Na impregnation and calcination. In this talk the effect of Mn precursor, either Mn acetate or Mn nitrate, is evaluated for dispersion and the formation of active phases. The various catalysts are then tested in a tubular fixed-bed reactor at 750 °C, ~1 bar, 4:1 CH4:O2 feed molar ratio, 20000 and 60000 ml g-1h-1 GHSV. The tested catalysts are compared to control catalyst that is prepared using conventional fumed silica as the support. Materials are characterized using ICP-OES, N2 physisorption, DRIFT, XRD, HR-SEM and TPR. All our results point towards a significant influence of the Mn precursor on the dispersion of Mn in the dealuminated Î-zeolite framework, which is consistent with catalytic results following the phase transition. XRD results show that Mn nitrate forms mainly bulk MnO2, while Mn acetate does not show any distinctive peaks related to MnxOy. This is supported by DRIFT measurements showing that when Mn acetate is used the available T-sites are filled while when Mn nitrate is used the T sites remain open. N2 physisorption measurements show pore contraction in the case of Mn acetate where in the case of Mn nitrate the pore volume remains largely unchanged. HR- SEM, indicates the formation of large crystals of MnO2 when Mn nitrate is used, see Figure 1a, while no apparent separate phase is seen when Mn acetate is used, see Figure 1b. These results are further corroborated by TPR experiments, in which Mn nitrate exhibits peaks characteristic of bulk MnO2 while Mn acetate material exhibit a wide peak characteristic to enhanced Mn-surface interactions. Following the impregnation with Na2WO4 and calcination, all the materials have the α-cristobalite structure with almost complete surface area loss (450-650 to <5 m2g-1) and with Mn2O3, Na2WO4 and MnWO4 as main phases detected in XRD. The results for the OCM reaction show that the incorporation of Mn into the zeolitic framework, followed by phase transition, provide superior performance in terms of CH4 conversion and C2 selectivity. Backscattering HRSEM images of Mn incorporated in the Î-zeolite. (a) Mn Nitrate and (b) Mn Acetate.