545548 Investigation of the Active Phases in Alkali Metal Promoted Molybdenum Sulfide Catalysts for the Synthesis of Methanethiol from Syngas and Hydrogen Sulfide

Wednesday, June 5, 2019: 3:09 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Miao Yu1, Nikolay Kosinov1, Emiel Hensen2 and Wei Chen2, (1)Eindhoven University of Technology, Eindhoven, Netherlands, (2)Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry,, Eindhoven University of Technology, Eindhoven, Netherlands

1.      Introduction

Methanethiol (CH3SH) is an important industrial raw material widely used in the synthesis of valuable organosulfur compounds such as pesticides, pharmaceuticals and petrochemicals [1]. A state-of-art production method for CH3SH is the thiolation of methanol (CH3OH) over transition metal sulfide catalysts [2]. Even if the production of CH3SH is an efficient process involving high CH3OH conversion and CH3SH yield, this route needs a multi-step pathway for the synthesis of CH3SH, which results in high production cost. Considering that CH3OH is generated from syngas (CO and H2), Olin et al. [3] proposed to synthesize CH3SH directly from H2S-containing syngas. The benchmark catalysts for this reaction are alkali metal promoted molybdenum sulfide (MoS2) supported on silica or alumina [4].

Most of the earlier works have been focused on the optimization of the reaction conditions, while the active phase itself started to attract attention only recently. Cordova et al. proposed that the active phase related to the direct synthesis of methanethiol from CO/H2/H2S using K2MoO4/Al2O3 and K2MoS4/Al2O3 catalysts precursors, consists of layered 1T-MoS2 crystallites in which potassium ions are intercalated between the layers [4]. This KxMoS2 phase was characterized by X-ray photoelectron spectroscopy (XPS) and the amount of this phase was correlated with the CH3SH productivity.

It should be noted, however, that 1T-MoS2 has been reported to be unstable compared to 2H-MoS2 phase: it easily transforms to the 2H phase at high temperature [5]. The correlation between the amount of 1T phase and CH3SH productivity was solely based on XPS result before the reaction. Therefore, aim of this work was to study the stability of 1T-MoS2 under reaction conditions to examine whether the 1T- KxMoS2 phase is the actual active component for CH3SH synthesis.

2.      Experimental

Potassium-promoted molybdenum sulfide catalysts were prepared by co-impregnation of a 75-125 μm sieve fraction of crushed γ-Al2O3 extrudates (Vp=0.6 cm3/g) with an aqueous solution of suitable metal salts, ammonium heptamolybdate ((NH4)6Mo7O24·4H2O), Sigma Aldrich) and potassium nitrate (KNO3, Sigma Aldrich). After impregnation, the catalyst precursors were dried at 110oC for 24 h and subsequently calcined at 450oC for 2 h in flowing air for 2 h. The molybdenum loading was 8 wt.%, whereas the loading of potassium was varied to achieve the atomic ratio of K to Mo as 0.5, 1 and 2, which was confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The catalysts are denoted as KxMo/Al2O3, in which x is the atomic ratio of K to Mo. Besides, the catalysts were also characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and operando extended X-ray adsorption fine structure (EXAFS).

The activation of catalyst precursors and CH3SH synthesis were performed in a fixed-bed reactor. Prior to catalytic test, the precursors were in-situ sulfided with 10 vol% H2S/H2 under atmospheric pressure and 350oC for 2 h. The same procedure was also applied for “quasi” in situ XPS and TEM measurements. After activation, the feed gas was changed to mixed CO/H2/H2S (molar ratio equals to 1/2/1) with a total flow rate of 60 mL/min and 3 mL/min N2 as internal standard. The catalytic performance was measured under 10 bar from 280oC to 320oC, the outlet product composition was analyzed by an online compact gas chromatograph (GC).

3.      Results

The XPS spectra of Mo 3d core level of K0Mo/Al2O3 and K2Mo/Al2O3, which were in-situ sulfided at atmospheric pressure, are shown in Fig. 1a,. These spectra can be deconvoluted into four doublets: MoVI, MoV, MoIV and K-MoIV. The doublet of MoIV at 228.9 eV and 232.0 eV corresponds to Mo in pristine 2H-MoS2, while the doublet of K-MoIV at 228.0 eV and 231.1 eV corresponds to Mo in 1T-MoS2 [4]. Obviously, K-MoIV exists only in the spectrum of K2Mo/Al2O3, which means that the addition of K in MoS2 can induce its phase conversion from 2H to 1T. By comparing the area of doublet MoVI and KMoVI, the ratio of 1T to 2H phase in K2Mo/Al2O3 is about 1.2.

The sulfided samples were also analyzed by EXAFS at the Mo K-edge, and the Fourier transforms of K0Mo/Al2O3 and K2Mo/Al2O3 are shown in Fig. 1b. In the spectrum of K0Mo/Al2O3, there are two main peaks corresponding to the nearest Mo-S (2.40 Å) and Mo-Mo (3.17 Å) bonds, respectively. On the other hand, there is one extra scatterer at 2.77 Å, which is assigned to a distorted shorter Mo-Mo bond. Previous studies showed that the structure of MoS2 can be distorted when it changes from 2H-phase to 1T-phase [6], so the shorter Mo-Mo bond peak in EXAFS spectrum can be assigned to the 1T-phase in K2Mo/Al2O3.

Fig. 1. (a) Mo 3d XP spectra of K0Mo/Al2O3 and K2Mo/Al2O3 (b) Fourier transform of Mo K-edge EXAFS spectra of K0Mo/Al2O3 and K2Mo/Al2O3

The catalytic performance of all four catalysts at 320oC is shown in Fig. 2a. It is clear that the addition of K can both increase the CO conversion and CH3SH selectivity. Meanwhile, the XPS spectra of the K2Mo/Al2O3 after 2 h, 5 h, 10 h and 20 h under reaction conditions (10 bar and 350oC) were also measured, and the ratios of 1T to 2H phase were calculated and shown together with CO conversion and CH3SH selectivity in Fig. 2b.

Fig. 2. (a) CO conversion and product selectivity of all four catalysts under 320oC (b) Catalytic performance and 1T/2H ratio of K2Mo/Al2O3 in 20h

4.      Discussion

The results from XPS and EXAFS clearly show that the addition of K in MoS2 drives the conversion of 2H phase to 1T phase. Additionally, the ratios of 1T/2H calculated from XPS and EXAFS (by coordination numbers for different scattering paths) are very similar, which makes XPS a reliable method to identify the amount of 1T-MoS2 in this system. The 1T/2H ratio values (Fig. 2b) show that 1T phase converts to 2H phase during the reaction very quickly. Since the reaction temperature is the same as that for the sulfidation, high pressure should be responsible for the 1T to 2H transition. However, even though the ratio of 1T/2H decreases dramatically, the catalytic performance was quite stable for 20 h, suggesting that the structure of 1T-MoS2 is not an all-important aspect of K-promoted MoS2 for CH3SH synthesis.

5.      Conclusions

In our work, K-promoted MoS2 catalysts were prepared. The existence of 1T-MoS2 was confirmed by XPS and EXAFS. However, 1T-MoS2 quickly converted to 2H-MoS2 during the CH3SH synthesis reaction because of its instability at high pressure. Therefore, we argue that under CH3SH reaction conditions the 2H-MoS2 phase is the real active phase.

6.      References

[1] J. Sauer, W. Boeck, L. von Hippel, W. Burkhardt, S. Rautenberg, D. Arntz, W. Hofen, US patent 5852219, 1998.

[2] Mashkin, V.; Kudenkov, V.; Mashkina, A. Ind. Eng. Chem. Res. 1995, 34, 2964–2970.

[3] J. Olin, B. Buchholz, B. Loev, R. Goshorn, US patent 3070632, 1962.

[4] Cordova A, Blanchard P, Lancelot C, et al. ACS Catalysis, 2015, 5(5): 2966-2981.

[5] Eda G, Yamaguchi H, Voiry D, et al. Nano letters, 2011, 11(12): 5111-5116.

[6] Liu Q, Li X, He Q, et al. Small, 2015, 11(41): 5556-5564.


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