Effect of potassium addition method on MoS2 performance for the syngas to alcohol reaction

Applied Catalysis A: General 462–463 (2013) 302–309

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Effect of potassium addition method on MoS2 performance for the syngas to alcohol reaction D. Ferrari a,∗ , G. Budroni b , L. Bisson b,1 , N.J. Rane b,2 , B.D. Dickie c , J.H. Kang d , S.J. Rozeveld d a

Hydrocarbons R&D, The Dow Chemical Company, 2301N Brazosport Blvd, Freeport, TX 77541, USA Hydrocarbons R&D, Dow Benelux B.V., Terneuzen, Herbert H. Dowweg 5, Hoek 4542 NM, The Netherlands Analytical Sciences, Dow Benelux B.V., Terneuzen, Herbert H. Dowweg 5, Hoek 4542 NM, The Netherlands d Core R&D, The Dow Chemical Company, Midland, MI 48667, USA b c

a r t i c l e

i n f o

Article history: Received 3 January 2013 Received in revised form 26 April 2013 Accepted 5 May 2013 Available online 13 May 2013 Keywords: Molybdenum sulfide Syngas to alcohols Electron microscopy Catalysis Alkali doping

a b s t r a c t Molybdenum disulfide promoted with alkali metals is one of the most studied catalysts for the conversion of synthesis gas to alcohols. In this work, MoS2 was promoted by adding potassium precursors by physical mixing (PM) and incipient wetness impregnation (IWI) and tested in a small scale reactor. The catalysts prepared by impregnation showed poorer performance compared to the sample prepared by physical mixing of K2 CO3 and MoS2 . The catalysts were characterized by means of X-ray diffraction, TGA–MS, SEM–EDS, TEM and XPS. The characterization results obtained by SEM–EDS and XRD analyses do not show major differences between the catalysts studied here. In contrast, the TGA results indicated that physical mixing leads to catalysts that lose sulfur at temperatures above 330 ◦ C and TEM images showed that randomly oriented small MoS2 crystallites are obtained from PM preparation while IWI preparation gave long MoS2 crystallites. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Due to recent oil price volatility, the chemical industry has shown renewed interest in alternative feedstocks such as natural gas, coal and biomass for the production of energy and chemicals. Synthesis gas (syngas) derived from these alternative feedstocks is a mixture of hydrogen and carbon monoxide that can be converted to hydrocarbons via the so-called Fischer–Tropsch process but also to alcohols according to a different reaction pathway [1]. Higher alcohols such as ethanol or propanol can be used as intermediates for the production of olefins or as fuel additives. Globally, the research on the conversion of syngas to alcohols has been ongoing in the past 6 decades both industrially and at academic institutions [1–3]. The conversion of syngas to alcohols has been studied at Dow since the 1980s [4–7] showing that molybdenum sulfide (MoS2 ) based materials are among the most promising catalysts for this

∗ Corresponding author. Tel.: +1 9792382074; fax: +1 9792380028. E-mail addresses: [email protected] (D. Ferrari), [email protected] (G. Budroni), [email protected] (B.D. Dickie), [email protected] (J.H. Kang), [email protected] (S.J. Rozeveld). 1 Present address: Solvay Rhodia, CRTA, 52 rue de la Haie Coq, 93308 Aubervilliers Cedex, France. 2 Present address: UOP, A Honeywell Company, 8400 Joliet Road, McCook, IL 60525, USA. 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.05.006

reaction. Historically, the best selectivity to alcohols has been obtained by promoting MoS2 with additional transition metals such as cobalt or nickel as well as alkali metals such as potassium. Typically the potassium precursor can be added by incipient wetness or physical mixing [6]. However, no direct comparison between impregnation and physical mixing and their impact on the catalyst performance has been reported. The role played by the alkali metal has been a topic of study for a number of groups. Lee and Woo [8,9] investigated the promotion of MoS2 with different potassium precursors. It was shown that depending on the counter ion (i.e. OH− , CO3 2− , S2− , versus Cl− , or SO4 2− ), potassium promoted molybdenum sulfide catalysts yield different selectivities to alcohols. In other words, in order to be able to promote selectivity toward alcohols, potassium needs to be able to separate from its counter ion and spread on the catalyst surface. Furthermore, Woo et al. also hypothesize that K interacts with the oxygen of CO favoring the formation of an acyl intermediate that is subsequently hydrogenated to alcohols. Iranmahboob et al. [10,11] analyzed potassium promoted cobalt molybdenum sulfide with surface techniques and was able to demonstrate that the catalyst surface was enriched in potassium after exposure to reaction conditions. In a recent paper by Menart et al. [12] the effect of adding potassium to cobalt molybdenum sulfide catalysts was investigated. Although cobalt may play a role in the interaction of potassium with molybdenum, the authors show that adding potassium by

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Table 1 Catalyst composition. Sample

Alkali addition method

Precursor

K loading on MoS2

Sample weight (mg)

MoS2 MoS2 MoS2 MoS2

Physical mixing with mortar and pestle Physical mixing with mortar and pestle Incipient wetness impregnation Incipient wetness impregnation

K2 CO3 K2 SO4 KOH KOH

6 wt% 6 wt% 6 wt% 12 wt%

62.6 64.6 88.4 108.9

PM 6 K2 CO3 PM 6 K2 SO4 IWI 6 KOH IWI 12 KOH

K loadings do not include the counter ion.

washing the catalyst precursor with a solution of potassium carbonate before thermal decomposition at high temperature yields catalysts with comparable performance to catalysts prepared by physical mixture of potassium carbonate with the finished cobalt molybdenum catalysts. In this paper, the effect on the molybdenum sulfide catalyst performance of two different alkali addition methods in combination with different precursors was investigated. MoS2 was promoted by adding potassium precursors by physical mixing (PM) and incipient wetness impregnation (IWI) and tested in a small scale reactor. The catalysts were characterized by means of X-ray diffraction, TGA–MS, SEM–EDS, TEM and XPS. 2. Experimental 2.1. Catalyst preparation The unsupported molybdenum sulfide catalysts were prepared starting from ammonium tetrathiomolybdate, (NH4 )2 MoS4 (ATTM) purchased from Aldrich. MoS2 was prepared by heating ATTM under nitrogen flow (300 ml/min) according to the following temperature program: 300 ◦ C (2.5 ◦ C/min), hold for 2 h; 450 ◦ C (2.5 ◦ C/min), hold for 0.5 h; 500 ◦ C (2.5 ◦ C/min), hold for 1 h. Potassium carbonate and potassium sulfate were respectively added to MoS2 by physical mixing the powders in air using a mortar and pestle. An amount of potassium precursor was added to MoS2 in order to obtain 6 wt% potassium in the final compound. Potassium hydroxide was used to impregnate MoS2 samples to obtain potassium loadings of 6 and 12 wt% of the final compound. Incipient wetness impregnation was performed by adding 0.32 ␮l of potassium hydroxide solution (27.2 and 47.8 wt%) per mg of MoS2 . Both preparation methods were used to prepare samples promoted with potassium and cesium carbonate to yield samples containing 12 wt% potassium or 7 wt% cesium of the final compound respectively. The handling of both impregnated and physically mixed catalysts was performed in air. However, the impregnated catalysts were dried in nitrogen atmosphere at 60 ◦ C for 1 h. The same thermal treatment was applied to MoS2 before mixing with potassium carbonate or sulfate. All samples were stored in sealed vials flushed with argon before reaction. The catalyst compositions studied in this work are summarized in Table 1. 2.2. Catalyst testing The catalytic tests were performed at Avantium Chemicals B V. A catalyst volume of 100 ␮l was introduced in reactor tubes and kept in place by glass wool (the corresponding catalyst weight is reported in Table 1). The catalytic testing was performed with an H2 /CO ratio of 1 with the addition of 50 ppm H2 S. Initially, the catalysts were stabilized for 70 h at 280 ◦ C and 4000 h−1 , then data were collected at 3000 h−1 , at 280, 310 and 330 ◦ C. The reactor pressure was held at 90 bar throughout the whole experiment. After completion of the catalytic test, the catalysts were cooled to room temperature under nitrogen. No passivation step was applied.

The reactor effluent was analyzed by gas chromatography by using both a 490 Micro GC equipped with a thermal conductivity detector and a TRACE GC Ultra Gas Chromatograph with a flame ionization detector. The catalyst performance is will be expressed as selectivity to products of interest as a function of CO conversion. All selectivities are carbon based and include CO2 as a product. Only data with carbon balance between 97 and 100% are reported in this article. The reported data are averages of 3 points at each condition. 2.3. Characterization Selected samples were analyzed by X-ray diffraction (XRD) using a Bruker AXS D8 discover high throughput diffractometer with Cu K␣ radiation and a Vantec detector. Diffraction patterns were analyzed using the Jade software package. The samples were ground in a mortar with a pestle in air. The thermogravimetric analyses coupled with mass spectrometry (TGA–MS) were performed with a TA Instruments model Q5000 TGA coupled to a Balzer Thermostar GSD 300 MS. Helium was used as purge gas. The scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDS) were performed using a FEI Nova 600 SEM. The SEM was equipped with Bruker AXS XFlash 4030 silicon drift detectors for elemental identification and quantification. The transmission electron microscopy (TEM) imaging was performed using a JEOL 2010F field emission gun transmission electron microscope. The TEM samples were prepared by microtoming the powders which were first embedded in an acrylic epoxy resin (LR White hard grade). The TEM sections were cut to a thickness of ∼50 nm and were then transferred to a Cu TEM grid with a lacey carbon support (Ted Pella). The X-ray photoelectron spectra were measured on an AXIS HSi (Kratos Analytical) instrument with monochromatic Al K␣ X-ray (1486.6 eV) and a hemispherical analyzer at 90◦ take-off angle. All binding energies were calibrated based on the C 1s peak of adventitious carbon at 285.0 eV. The spectra were analyzed using Casa XPS software to measure the peak areas and to curve-fit the spectra. 3. Results and discussion The work presented by Lee at al. [8] was used as reference for the choice of potassium precursors in this study. Among the salts studied by Lee et al. potassium hydroxide was reported to yield the highest amount of higher alcohols. On the contrary, potassium sulfate was among the promoters producing mainly hydrocarbons. Since these two precursors represented the extreme in performance and thus in ability to promote MoS2 , they were chosen for the characterization study. In this work potassium was added to MoS2 by physical mixture of K2 CO3 and K2 SO4 , or incipient wetness impregnation of KOH and these catalysts (Table 1) were tested for the conversion of syngas to alcohols. Typically, the reaction proceeds through the formation of methanol which is in turn converted to ethanol [13]. This is one of the contributing factors to the decreasing trend in methanol selectivity with conversion (Fig. 1)

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Table 2 Catalytic data obtained with K2 CO3 promoted MoS2 . All catalysts contain 12 wt% K. PM = physical mixing; IWI = incipient wetness impregnation. All data points were obtained at 90 bar, H2 /CO molar ratio of 1, 50 ppm H2 S and GHSV 4000 h−1 . Temperature (◦ C)

Addition method

IWI IWI IWI PM PM PM

CO conversion (%)

310 330 330 310 330 330

12.9 19.6 19.4 14.5 20.4 20.4

Selectivity (%) CO2

MeOH

EtOH

PrOH

CH4

C2 + HC

31.4 34.1 33.9 23.9 30.0 29.9

24.9 17.9 17.7 40.7 25.4 25.5

16.2 15.9 15.6 16.9 20.6 20.9

4.5 5.7 6.1 1.7 2.5 2.5

16.3 18.1 17.8 13.5 17.3 17.1

4.5 5.3 5.4 1.6 1.8 1.8

60

Alcohol Selectivity (%)

50 40 30 20 10 0 0

5

10

15

20

25

30

CO Conversion (%)

Fig. 1. Alcohol selectivity as a function of CO conversion. The experimental conditions are 90 bar, GHSV of 3000 h−1 , H2 /CO molar ratio of 1 and temperatures 280, 310 and 330 ◦ C. MeOH = methanol; EtOH = ethanol.

and the explanation for the increase in ethanol selectivity for some of the catalysts. When comparing catalysts containing the same potassium loading (Fig. 1) at 12% CO conversion, it can be observed that 27% ethanol selectivity is achieved by MoS2 PM 6 K2 CO3 , while MoS2 IWI 6 KOH only gives 15%. On the other hand, both catalysts prepared by impregnation, MoS2 IWI 6 KOH and MoS2 IWI 12 KOH, achieve higher CO conversion than MoS2 PM 6 K2 CO3 at a given reaction condition. Furthermore, the data indicate that a higher potassium concentration (MoS2 IWI 12 KOH) results in a higher ethanol selectivity than MoS2 IWI 6 KOH but still lower than MoS2 PM 6 K2 CO3 . These results seem to indicate that the potassium addition method has a greater impact on the catalyst performance than potassium concentration itself. Separate experiments in a different reactor system with samples promoted with K2 CO3 by impregnation in comparison with samples physically mixed showed the same trends as reported above (Table 2). The samples promoted by physical mixture yielded higher selectivity to alcohols while the impregnated samples produced higher amounts of C2+ hydrocarbons. The same trend was also observed with cesium promoted

MoS2 . Table 3 reports the catalytic performance of four catalysts that were prepared using Cs2 CO3 and CsNO3 as precursors. The Cs precursors were added to MoS2 (7 wt% Cs) by impregnation and physical mixing respectively. The results reveal that at a given conversion, higher alcohol selectivities can be achieved when the same Cs precursor is added by physical mixing instead of impregnation regardless of the Cs counter ion. Both data sets reported in Tables 2 and 3 suggest that when comparing the same counter ion (carbonate or nitrate), the alkali metal (K or Cs) addition method has an effect on the catalyst performance. Hence, this may imply that exposure to water during impregnation could have a deteriorating effect on the catalyst or that it may prevent the alkali metals from effectively promoting MoS2 . For example, water could facilitate the catalyst oxidation or could interact with MoS2 hindering access for the alkali metal. In line with what was observed by Lee and Woo [8,9], the data in Fig. 1 also show that physical mixing of K2 SO4 results in a poor catalyst performance characterized by decreasing alcohol production with conversion. Woo et al. attributed the different performance between the catalysts promoted with different potassium precursors to the ability of potassium to spread on the catalyst surface. Moreover, they tried to draw a correlation between performance and the pKa of the acids corresponding to the counter ions. The hydrocarbon selectivity distribution (Fig. 2) reflects the alcohol selectivity. Catalyst MoS2 PM 6 K2 CO3 shows the lowest methane selectivity as well as the lowest higher hydrocarbon selectivity (C2 + HC). Both impregnated samples show higher methane selectivity than MoS2 PM 6 K2 CO3 at all studied CO conversions, however, high potassium concentration (12%) leads to lower C2 + HC selectivity. Surprisingly, catalyst MoS2 PM 6 K2 SO4 shows similar methane selectivity to MoS2 PM 6 K2 CO3 , but much higher C2 + HC selectivity. In summary, we have observed that the catalyst performance is strongly affected by the alkali addition method. The addition of potassium by physical mixing (MoS2 PM 6 K2 CO3 ) yields higher selectivity to ethanol while the methanol selectivity follows the same trend as for the impregnated samples. Moreover, potassium

Table 3 Catalytic data obtained with Cs promoted MoS2 . All catalysts contain 7 wt% Cs. PM = physical mixing; IWI = incipient wetness impregnation. All data points were obtained at GHSV 3000 h−1 . Precursor

Addition method

Temperature (◦ C)

CO conversion (%)

Cs2 CO3 Cs2 CO3 Cs2 CO3 CsNO3 CsNO3 CsNO3 Cs2 CO3 Cs2 CO3 Cs2 CO3 CsNO3 CsNO3 CsNO3

PM PM PM PM PM PM IWI IWI IWI IWI IWI IWI

280 310 330 280 310 330 280 310 330 280 310 330

2.1 4.3 8.3 3.0 6.1 10.1 4.1 9.4 15.5 5.4 13.3 21.3

Selectivity (%) CO2

MeOH

EtOH

PrOH

CH4

C2 + HC

23.3 25.2 29.9 23.1 26.0 30.6 24.3 29.3 34.6 28.3 34.4 39.0

42.7 35.5 24.3 43.0 34.0 21.9 41.9 29.0 16.2 34.2 20.2 11.1

18.3 22.6 23.5 16.9 21.9 24.3 16.1 20.4 21.1 14.9 17.2 15.5

3.6 4.3 6.2 2.6 3.9 6.2 2.3 3.4 5.6 3.0 4.4 6.3

8.5 7.7 9.6 10.4 9.8 10.6 11.8 13.4 15.2 14.7 17.3 18.6

2.4 2.8 3.8 2.8 2.5 3.8 2.6 3.1 4.9 3.8 4.9 7.4

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Hydrocarbon Selectivity (%)

30 25 20 15 10 5 0 0

5

10

15

20

25

30

CO Conversion (%)

Fig. 2. Hydrocarbon selectivity as a function of CO conversion. The experimental conditions are 90 bar, GHSV of 3000 h−1 , H2 /CO molar ratio of 1 and temperatures 280, 310 and 330 ◦ C. CH4 = methane; C2 + HC = hydrocarbons other than methane.

addition by impregnation results in high methane selectivity and only higher potassium amounts (12%) decrease the production of higher hydrocarbons to levels comparable to the physically mixed sample. Possibly water hinders the interaction between potassium and MoS2 thus higher amounts of potassium are required to obtain the same level of promotion as with physically mixed K2 CO3 . Initial characterization studies aimed at identifying differences within the bulk structure of the catalyst. Fig. 3 shows the XRD patterns of the catalysts before (top) and after (bottom) reaction. The XRD patterns revealed that all catalysts were amorphous to a large extent. Nevertheless, these results indicated that K2 SO4 is formed during catalyst preparation at room temperature in air. After the reaction, the intensity of the K2 SO4 peaks is higher still and appears to be proportional to the potassium concentration in

Fig. 3. XRD patterns of the catalysts before (top) and after (bottom) reaction.

305

the sample. Before reaction, K2 SO4 is possibly formed by the reaction of K2 CO3 with MoS2 in air, which might be giving rise to the active phase for the alcohol synthesis along with K2 SO4 . During the reaction more K2 SO4 could form due to the presence of H2 S and oxygen carried by CO. It must be stressed that K2 SO4 is not considered to be the active species involved in the catalysis toward alcohols as confirmed by the catalytic data. K2 SO4 could simply be the byproduct of a more intimate reaction between the potassium precursor and MoS2 . Since after the reaction the samples were removed from the reactor and exposed to air, it could be possible that potassium sulfate was formed by oxidation during sample handling. However, no molybdenum oxide was observed in the XRD patterns. Considering the catalytic performance, one could expect that the two addition methods result in a different dispersion of potassium on the MoS2 surface. Our initial hypothesis was that potassium does not spread homogeneously when added by impregnation and therefore it does not promote alcohol formation. In contrast, the scanning electron microscopy analysis of the catalysts before reaction (Fig. 4) showed that potassium is well spread when added by impregnation for both MoS2 IWI 6 KOH and MoS2 IWI 12 KOH. However, the impregnation of high potassium concentrations (MoS2 IWI 12 KOH) results in a thick layer of potassium containing material on the outer surface of the catalyst, as indicated by the yellow areas around the particles (see Figs. 4 and 5). The extent of surface coverage by potassium is less after reaction. The images in Fig. 4 also illustrate that potassium is well spread across the catalyst particle before reaction for MoS2 PM 6 K2 CO3 , although some loose lumps containing potassium are observed. Although it is not possible to quantitatively correlate color intensities with potassium concentration, it is evident that the potassium spreading is enhanced during the reaction (Fig. 5). This finding is in agreement with what has been reported by Woo [9], Christensen [13] and Xiao [14]. Finally, in line with the findings reported by Lee et al. [8] potassium did not spread before (Fig. 4) or after (Fig. 5) reaction for MoS2 PM 6 K2 SO4 . We believe that at room temperature the hygroscopicity of K2 CO3 plays a crucial role in the ability of potassium to spread. In fact, K2 CO3 is a deliquescent salt whereas K2 SO4 does not absorb much moisture from the environment and hence cannot become mobile. On the contrary, when too much water is present it may hinder the interaction between potassium and MoS2 or produce changes to the MoS2 resulting in poor performance. The SEM images (Fig. 5) also indicate that despite the different preparation method, the particle size of the catalysts is comparable. The XPS analysis was performed on the four samples and confirmed that potassium spreads on all samples during reaction (Table 4). The data support the observation from the SEM analysis that the potassium amount at the surface decreases for MoS2 IWI 12 KOH after reaction. This is probably due to the fact that potassium is more concentrated on the surface of the particle before reaction and it diffuses into the bulk of the catalyst under reaction conditions. However, an increase in surface potassium levels after the reaction is observed for all samples containing 6 wt% potassium (i.e. MoS2 IWI 6 KOH, MoS2 PM 6 K2 SO4 and MoS2 PM 6 K2 CO3 ). This latter observation could be coming from potassium segregation prior to reaction so that it is less detectable on the surface. The data so far could indicate that potassium spreads during the reaction, but the nature of the initial interaction between K and MoS2 , with perhaps water playing a key role, determines the overall catalyst performance. In statistical terms, these two observations are not contradictory since both lead to the conclusion that K spreads on the catalyst surface and the immediate underlying layers. XPS is a local technique and will reveal differences in potassium concentration

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Fig. 4. SEM–EDS elemental maps of the catalysts before reaction. Blue = molybdenum; yellow = potassium.

Fig. 5. SEM–EDS elemental maps of the catalysts after reaction. Blue = molybdenum; yellow = potassium; red = silicon.

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Table 4 XPS analysis of the catalysts before and after reaction with syngas. The results are given in atom %. 12K Imp KOH

K Mo Mo6+ Mo5+ Mo4+ Mox+ /Mo4+ O S Sulfate Sulfide Sulfate/sulfide

6K Imp KOH

6K PM K2 CO3

6K PM K2 SO4

Fresh

Spent

Fresh

Spent

Fresh

Spent

Fresh

Spent

23.2 6.3 3.2 0.7 2.4 1.65 53.9 16.5 10.9 5.6 1.94

15.7 5.0 2.7 0.7 1.7 2.03 64.8 14.4 11.3 3.1 3.67

11.8 16.6 1.9 1.4 13.3 0.25 36.6 35.0 8.4 26.6 0.31

12.8 10.5 3.6 0.7 6.2 0.70 57.8 19.0 7.4 11.6 0.63

11.0 15.0 2.8 1.3 11.0 0.37 42.4 31.5 9.7 21.8 0.44

13.6 11.7 2.6 1.1 8.0 0.46 49.8 24.9 9.1 15.9 0.57

1.9 20.2 2.0 1.7 16.5 0.22 34.7 43.2 7.2 36.1 0.20

6.0 16.0 2.6 1.5 11.9 0.34 46.2 31.8 7.5 24.3 0.31

depending on the analyzed areas. Along the same lines Andersen et al. presented DFT calculations showing that K is energetically more favorable on the surface than intercalated into the MoS2 layers [15]. From the XPS results MoS2 IWI 12 KOH appears to be more oxidized than any of the other samples showing a very high sulfate/sulfide ratio even before reaction. The MoS2 PM 6 K2 SO4 material has the lowest sulfate/sulfide ratio, probably due to the fact that potassium sulfate has little interaction with MoS2 . For MoS2 IWI 6 KOH and MoS2 PM 6 K2 CO3 the observed potassium sulfate (Fig. 3) originates from red-ox reactions involving potassium carbonate, air, water and sulfur. The direct comparison of MoS2 IWI 6 KOH and MoS2 PM 6 K2 CO3 indicates that the latter undergoes oxidation to a lesser extent during reaction. This is illustrated by both the sulfate/sulfide and the Mox+ /Mo4+ ratios. The latter ratio is that between higher oxidation Mo species (5+ and 6+) and Mo4+ . The thermal stability of the catalysts was investigated by thermogravimetric analysis in order to test our hypothesis that potassium reacts differently with MoS2 depending on the addition method. The TGA–MS results are shown only for samples of interest in Fig. 6 while Fig. 7 shows the derivative weight loss curves to better visualize the difference in thermal stability between the three samples. The results indicated that both impregnated samples (MoS2 IWI 6 KOH and MoS2 IWI 12 KOH) lose little weight at low temperature, but undergo a major weight loss at about 290 ◦ C. This latter weight loss is within the catalytic testing temperature range as indicated by the pink band in Fig. 6.

In contrast, MoS2 PM 6 K2 CO3 loses a significant amount of weight at low temperature. This weight loss is mainly coming from water and CO2 indicating the influence of K2 CO3 . In addition, the sample undergoes a major weight loss at about 350 ◦ C, which is above the highest temperature (330 ◦ C) reached during testing. The mass spectroscopy data showed that the weight change at temperatures above 250 ◦ C was coming from the loss of sulfur species for all analyzed samples. This is evidence for better catalyst stability against temperature when potassium is added by physical mixing. This could imply that once the catalysts have reached reaction temperature, different catalysts with different sulfur contents have been generated and could explain why higher alcohol selectivity is achieved with MoS2 PM 6 K2 CO3 . In order to further investigate the influence of the potassium addition method on catalyst morphology, samples MoS2 PM 6 K2 CO3 and MoS2 IWI 6 KOH after reaction were imaged by transmission electron microscopy (Fig. 8). The samples look very different at relatively high magnification (left) with randomly oriented small MoS2 crystalline domains for MoS2 PM 6 K2 CO3 while MoS2 IWI 6 KOH has long MoS2 crystallites. The crystallite difference may be a key feature to explain the difference in selectivity of the catalysts. It cannot be excluded that the counter ion may play a role in the final morphology. However, the selectivity trends on same counter ion confirm that the addition method is a crucial parameter for the promotion of MoS2 . Given the fact that the samples were prepared from the same MoS2 batch, it is possible that the crystal structure of MoS2 changes under reaction conditions and that these changes appear to be correlated to the way potassium is added. It may be concluded that the potassium addition by

Fig. 6. Thermogravimetric analysis of samples before reaction. Percentage weight loss as a function of temperature. The pink band highlights the reaction temperature range.

Fig. 7. Thermogravimetric analysis of samples before reaction. Derivative weight curves as a function of temperature. The pink band highlights the reaction temperature range.

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Fig. 8. TEM micrographs at two magnifications of the catalysts after reaction. (a) and (b) show MoS2 PM 6 K2 CO3 while (c) and (d) show MoS2 IWI 6 KOH.

physical mixing leads to a larger amount of defects in the catalyst structure upon exposure to syngas. A few authors [16,17] describe the intercalation of alkalis between the MoS2 planes, showing that the d-spacing correspond˚ The ing to the (0 0 0 2) planes can increase from 6.2 to up to 8.4 A. authors also show that the level of intercalation changes with time. For both samples represented in Fig. 8 an attempt was made to measure the d-spacing corresponding to the (0 0 0 2) planes. The measured d-spacing for both samples was 0.61 nm so there is no clear evidence for potassium intercalation here. However, it seems that when potassium is added by physical mixing the formation of small MoS2 crystallites occurs, suggesting that potassium may have a more intimate contact with MoS2 . These results indicate that the presence of large amounts of water during impregnation leads to large MoS2 crystallites and hence poor performance. In contrast, a hygroscopic salt such as K2 CO3 is able to absorb enough water in order to spread on the catalyst surface whilst interacting with MoS2 . This phenomenon results in the formation of small crystallites and higher catalyst stability leading to better alcohol selectivity. On the other hand, the catalyst promoted with K2 SO4 shows poor performance due to the fact that potassium did not spread on the surface, possibly caused by the low hygroscopicity of K2 SO4 . This means that the catalyst is essentially unpromoted MoS2 and therefore more active toward hydrocarbon formation.

4. Conclusions Depending on the addition method employed to promote MoS2 with potassium, the catalyst performance in the conversion of syngas to alcohols changes dramatically, especially with respect to the product distribution. The catalyst prepared by physical mixing of K2 CO3 and MoS2 showed superior performance compared to the samples prepared by impregnation. Similar behavior was observed also with Cs promoted MoS2 and different counter ions. The results of SEM–EDS, XRD and XPS analyses on selected samples could not explain the observed difference in catalytic behavior. SEM–EDS analysis showed good potassium dispersion independent of the addition method, the XRD patterns appeared to be similar for all investigated samples and the XPS analysis shows the formation of sulfate species on the surface of all samples. However, the TGA showed that physical mixing of K2 CO3 gives a catalyst that releases sulfur at temperatures above reaction conditions while TEM images showed that randomly oriented small MoS2 crystallites are obtained in contrast to long crystallites for KOH impregnated samples. Our hypothesis is that the addition of potassium by physical mixing leads to better performance owing to more intimate interaction between potassium and MoS2 that results in a higher catalyst stability with respect to temperature and in a more efficient potassium promotion. Furthermore, we postulate that the hygroscopic

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nature of K2 CO3 may allow it to spread and react with MoS2 even at room temperature and may play a role in the final promotion effect of potassium leading to the formation of a more selective catalyst toward C2+ alcohols.

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[8]

Acknowledgements [9]

Dr. Mark Kaminsky is acknowledged for scientific discussion. Dr. Chris John, Dr. Niels Luchters and Dr. Andreea Gluhoi from Avantium Chemicals BV are acknowledged for data collection, data analysis and technical support. References

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