544256 The Catalytic Performance of the Mn-Na-W/SiO2 Catalyst in Oxidative Coupling of Methane: Surface Structure and Further Additives

Monday, June 3, 2019: 5:36 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Naseem S. Hayek and Oz M. Gazit, Chemical Engineering, Israel Institute of Technology - Technion, Haifa, Israel

The Catalytic Performance of the Mn-Na-W/SiO2 Catalyst in Oxidative Coupling of Methane: Surface Structure and Further Additives

 

Naseem S. Hayek* and Oz M. Gazit

Wolfson Faculty of Chemical Engineering, Technion – Israel Institute of Technology, 3200003, Israel

 

E-mail: naseemha@technion.ac.il; Fax: +97248292850

 

Oxidative coupling of methane (OCM) has attracted much attention since 1982.1 This is because OCM is a direct and exothermic route for producing ethylene (Eq. 1), which is a vital building block in the chemical industry. Unfortunately, this process is still industrially inapplicable due to the lack of active and stable catalyst, which overcomes the formation of COx (Eq. 2), the thermodynamically favorable products.

1.           CH4 + 1/2O2 → 1/2C2H4 + H2O       

                               

2.           CH4 + 2O2 → CO2 + 2H2O              

Mn-Na-W/SiO2 is one of the most studied catalysts for OCM.2 Its high stability (>500 h) and C2 selectivity (>70%) make it a good candidate for practical applications. However, the highest C2 yield reached so far in a single-pass tubular reactor is less than 26%,3 restricting an economically feasible process.

In the preparation of the Mn-Na-W/SiO2 catalyst, a phase transition of the impregnated silica from amorphous to 𝛼-cristobalite occurs during calcination. This phase transition was shown to be critical for obtaining an active catalyst, which is thought to be due to better dispersion and stabilization of the active species. However, despite intensive research on this catalyst, the exact form of the active site and the surface properties that makes this catalyst active and selective are still not completely clear. Also, only a few studies were focused on the addition of more components into the Mn-Na-W/SiO2 catalyst, but no systematic studies were performed before.

The work here covers two main topics: 1) The effect of the Mn-Na-W/SiO2 surface parameters on its catalytic performance. 2) The effect of further additives on the catalytic performance of the Mn-Na-W/SiO2 catalyst. In the first part, we synthesized a set of catalysts with different properties using different silica precursors (i.e. dealuminated β-zeolite, mesoporous SBA-15, and nonporous fumed silica as silica precursors), together with two different Mn precursors that behave differently (i.e. Mn acetate and Mn nitrate).4,5 Characterizing the catalysts by inductively coupled plasma-optical emission spectroscopy, N2-physisorption, X-ray diffraction, high-resolution scanning electron microscopy-energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and catalytic testing enabled us to identify critical surface parameters that govern the activity and C2 selectivity of the catalyst. Although the current paradigm views the phase transition of silica to α-cristobalite as the critical step in obtaining dispersed and stable metal oxide sites, we show that the choice of precursors is equally or even more important with respect to tailoring the right surface properties. Moreover, surface porosity, metal dispersion, and Na2WO4 particle size were shown to be key factors in determining the activity and selectivity of the Mn-Na-W/SiO2 catalyst. Finally, combining our results with prior studies lead us to single out that the active center is composed of two adjacent Mn2O3 and Na2WO4 species.

In the second part, set of Mn-Na-W/SiO2 catalysts were synthesized with different additives (i.e. Ti, Sn, Zr, Ge, Ce, Nb, Fe, Co, and Cu), characterized using the previously mentioned techniques and tested under OCM conditions. Part of the additives promoted the catalytic performance of the conventional catalyst; however, they suffered from severe deactivation, making it questionable whether it is worth adding more components to the conventional catalyst as has been done before or not. In the presentation, interesting results from synthesis, characterization and catalysis will be discussed.

         

References

(1)      Keller G, E.; Bhasin, M. M. Synthesis of Ethylene via Oxidative Coupling of Methane. J. Catal. 1982, 73 (1), 9–19.

(2)      Arndt, S.; Otremba, T.; Simon, U.; Yildiz, M.; Schubert, H.; Schomäcker, R. Mn–Na2WO4/SiO2 as Catalyst for the Oxidative Coupling of Methane. What Is Really Known? Appl. Catal. A Gen. 2012, 425426, 53–61.

(3)      Zavyalova, U.; Holena, M.; Schlögl, R.; Baerns, M. Statistical Analysis of Past Catalytic Data on Oxidative Methane Coupling for New Insights into the Composition of High-Performance Catalysts. ChemCatChem 2011, 3 (12), 1935–1947.

(4)      Hayek, N. S.; Lucas, N. S.; Warwar Damouny, C.; Gazit, O. M. A Comparative Study of Precursor Effect on Manganese Post-Synthetic Incorporation into the T-Sites of Dealuminated Beta-Zeolite. Microporous Mesoporous Mater. 2017, 244, 31–36.

(5)      Hayek, N. S.; Lucas, N. S.; Warwar Damouny, C.; Gazit, O. M. Critical Surface Parameters for the Oxidative Coupling of Methane over the Mn–Na–W/SiO 2 Catalyst. ACS Appl. Mater. Interfaces 2017, 9 (46), 40404–40411.

 


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