(693e) Elucidation of the Active Phase of K Promoted MoS2 for the Direct Production of Higher Alcohols From Synthesis Gas: An Operando Infrared Study

Elucidation of the active phase of K promoted MoS₂ for the direct production of higher alcohols from synthesis gas: an operando infrared study

Vera P. Santos¹, Bart van der Linden¹, Adam Chojecki², Gerolamo Budroni², Steven Corthals², Hirokazu Shibata², Garry R Meima², Freek Kapteijn¹, Michiel Makkee¹,
and Jorge Gascon¹*

¹Catalysis Engineering/ChemEng., Technical University of Delft, Julianalaan 136-2628 BL Delft, The Netherlands

²Dow Benelux B.V., P.O. Box 48-4530 AA Terneuzen, The Netherlands

e-mail: j.gascon@tudelft.nl

Introduction

The increasing demand for renewable octane booster has drawn considerable interest towards the synthesis of mixed alcohols. Although bio-ethanol and to some extent methanol are currently used as gasoline additives with a high octane number (> 100), higher alcohols are preferred because of their lower volatility, higher solubility in hydrocarbons and higher calorific value [1]. Coal, biomass, and natural gas can be converted into higher alcohols via synthesis gas (Fischer-Tropsch synthesis) over several transition metals [2]. Among them, MoS₂ is one of the most promising catalysts, since it shows high resistance to sulphur poisoning and deactivation by coking [3]. In contrast to un-promoted MoS2, that produces mainly hydrocarbons (C1-C6), catalyst based on alkaline promoted MoS2 display a high selectivity towards higher alcohols. Despite extensive investigations on the synthesis of hydrocarbons and alcohols the mechanism over MoS₂ based systems, there is still no agreement on the nature of the active sites and the role of alkali promoters.

Here, we apply operando infrared spectroscopy in combination with high throughput (HTP) screening measurements to elucidate the role of potassium on the activation of carbon monoxide on the MoS2 surface and stabilization of reaction intermediates.

Materials and Methods

Unsupported MoS2 was prepared by thermal decomposition of (NH4)2MoS4 in nitrogen at 520 °C. The promotion was carried out by physical mixture of MoS2 with potassium carbonate (6% wt K).

Syngas synthesis (1:1 H2/CO ratio) was performed on a fixed-bed reactor, operated at 90 bar, 310 °C and using a gas hourly space velocity of 10,000 l/l/h..

Operando DRIFTS was carried out at 350 ^oC, 30 bar, a space velocity of 7000 l/l/h., and with a ratio CO/H2 of 1, using an in-situ high pressure cell and undiluted catalyst samples (no KBr). It was established that the breaking period for the reactor setup at 310 °C and 90 bars will give the same results as that of 350 °C and 30 bar in the DRIFT set-up.

Results and Discussion

The conversion profile and alcohol selectivity as a function of time on stream for some catalysts are shown in Figure 1a. The conditioning of molybdenum sulfide catalysts is characterized by long transient periods (break-in), accompanied by a continuous decrease in activity due to the permanent reconstruction of the active phase. However, the major significant differences in the evolution of the conversion of carbon monoxide and in the product distribution occur during the first 120 hours. For this reason, the operando study was focused on this very onset of the break-in period. As demonstrated by the screening studies, the presence of potassium on MoS2 shifts the selectivity towards alcohols (C1-C4 alcohols, 45% selectivity) and decreases the deactivation rate of MoS2, showing that potassium acts simultaneously as a chemical and structural promoter. Figure 1b shows the evolution of hydrocarbons and alcohols as function of time on stream for the promoted MoS2 (K-MoS2) obtained from the operando DRIFTS study. A complete match between the HTP experiments and operando DRIFTS was observed. The time resolved operando IR spectra illustrate the formation of hydrocarbons and CO2 already from the very beginning of the break in stage. Subsequent analyses of the adsorbed species by infrared spectroscopy reveal that alkoxy species are an important intermediate in the syngas to alcohol route, and that potassium plays a strong role on the stabilization of these species.

The electronic properties of MoS2 also change upon promotion, thereby allowing for a relatively enhanced activation of the CO molecule in respect to the hydrogen-assisted CO activation route over MoS2.

Conclusions

Several infrared in-situ works have been reported on alkali promoted MoS2 catalysts for the synthesis of higher alcohols, but most of them deal with supported catalysts. The few IR studies on bulk MoS2 use KBr as diluent, something surprising if the promotion effect of K is to be unraveled. In this work we make a crucial step in resolving the role of alkali promotion on CO activation, stabilization of intermediates, and intrinsic properties of MoS2 by combining operando spectroscopy and high-throughput experimentation. This insight is crucial for the further development of a syngas process for the production of higher alcohols.

 References

[1] R. G. Herman, Catal. Today 55, 233 (2000).

[2] Jacob A. Moulijn, Michiel Makkee, and Annelies van Diepen in: Chemical Process Technology, John Wiley & Sons Ltd, England, 2001.


[3] S. Zaman, K. J. Smith, Catal. Rev. 54, 41 (2012).