Elsevier

Applied Catalysis A: General

Volume 366, Issue 1, 15 September 2009, Pages 29-43
Applied Catalysis A: General

Effects of H2S and process conditions in the synthesis of mixed alcohols from syngas over alkali promoted cobalt-molybdenum sulfide

https://doi.org/10.1016/j.apcata.2009.06.034Get rights and content

Abstract

The present work is an investigation of how the process conditions influence the synthesis of mixed alcohols from syngas over a K2CO3/Co/MoS2/C catalyst. The emphasis in the investigations is upon the effects of H2S in the syngas feed. However the effects of the temperature and of the partial pressures of H2 and CO are also investigated. With or without H2S in the feed the pre-sulfided catalyst requires an initiation period to reach a stabilized behavior, but the duration of this period depends upon the H2S level. Operation with a feed containing more than 103 ppmv H2S leads to a fairly rapid stabilization of the product distribution and ensures that higher alcohols are the dominant reaction products. With less than 57 ppmv H2S in the feed the stabilization of the product distribution is much slower, and methanol is the dominant product. An investigation of the reaction kinetics indicates a high CO coverage and low hydrogen coverage. Hydrogen sulfide in the syngas feed generally promotes chain growth for both alcohols and hydrocarbons, but lowers the alcohol selectivity by enhancing the hydrocarbon formation. The highest alcohol productivity reached in these investigations was 0.276 g/g cat./h, and this was achieved at 350 °C, 100 bar, GHSV = 5244 h−1, Feed: 49.9 vol% H2, 50.1 vol% CO. Finally it is found that sulfur fed to the reactor as H2S is incorporated into the condensed alcohol product, and the incorporation of sulfur species into the product continues for some time after H2S has been removed from the feed. When the catalyst is operated with an S-free syngas feed, the amount of sulfur in the condensed liquid product decreases over time, but after 35 h of operation with an S-free syngas the alcohol product still contains 340 ppmw of sulfur. Thiols appear to be the dominant sulfur compounds in the product.

Graphical abstract

The work is an investigation of how the process conditions influence the synthesis of mixed alcohols from syngas over a K2CO3/Co/MoS2/C catalyst. The presence of more than 103 ppmv H2S in the feed stabilizes a high fraction of higher alcohols in the product (see figure), but the presence of H2S in the feed also leads to the incorporation of sulfur species into the alcohol product.

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Introduction

In the recent years there have been violent fluctuations in the price of oil. Furthermore there is now a widespread consensus that the anthropogenic emission of CO2 to a significant extent is contributing to the increase in global temperature. A solution that potentially addresses both these issues could be an increased use of biofuels in the transportation sector. An interesting route to biofuels is the gasification of biomass to form syngas with a subsequent conversion of the syngas to fuel chemicals. Due to i.a. their high octane numbers alcohols are interesting as additives to or substitutes for oil derived gasoline. Smaller alcohols, and particularly methanol, do however suffer from a limited miscibility with gasoline, which hampers their introduction as fuel additives [1], [2]. These miscibility problems can be reduced significantly by including higher alcohols in the alcohol/gasoline mixture, as these higher species act as co-solvents to methanol and stabilize the alcohol/gasoline blend [2], [3], [4]. Such a mixture of methanol and higher alcohols can be produced directly from syngas over various modified methanol [5], [6], [7], [8] or Fischer–Tropsch synthesis [9], [10], [11] catalysts. An interesting catalytic system, which is the subject of the present investigation, is alkali promoted molybdenum sulfide [12] – often additionally promoted with cobalt [12], [13], [14] or nickel [12], [15], [16], [17]. This catalytic system was discovered by the Dow Chemical Company [18], [19], [20], [21] and the Union Carbide Corporation [22], [23], [24] in the 1980s and has the potential to yield a substantial fraction of higher alcohols. The main product is linear, primary alcohols, while the predominant by-product is short-chained hydrocarbons. In addition to these products a small amount of oxygenated by-products, particularly esters, is also formed [25].

Although the ability of this system to produce alcohols is well established, there are still a number of unresolved issues related to the properties of the Alkali/MoS2 catalyst. One such issue is the stability of the catalyst in a syngas atmosphere, and a question here is how the presence of H2S in the syngas affects the stability. While some authors [26] mention that the catalytic properties essentially are unaltered by extended exposure to sulfur-free syngas, other investigations [27] have shown that the structure and properties of the catalyst change over time, when sulfur-free syngas is being fed to the synthesis reactor. It has also been reported that the catalyst requires the presence of 50–100 ppmv H2S in the syngas feed [10].

Another unresolved issue related to the use of the sulfide catalyst is the possible incorporation of sulfur species into the liquid alcohol product. Early research by the Dow Chemical Company showed that small amounts of low boiling, sulfur-containing compounds are incorporated into the alcohol product [20], [25], [28]. To solve this problem the Dow process included a stabilizer column, which reportedly would be able to reduce the sulfur level in the alcohol product to 10 ppm with a modest vapor boil-up [25]. The environmental regulations concerning the sulfur level in automotive fuels are becoming increasingly strict. In the US the EPA Tier II regulations have since 2006 limited the average sulfur concentration in gasoline to 30 ppmw, while the limit on sulfur in highway diesel is 15 ppmw [29]. Within the EU an upper sulfur limit of 10 ppmw in gasoline and diesel must be completely phased in at the beginning of 2009 [30]. A substantial concentration of sulfur in the alcohol product would therefore be highly undesirable, if the product is to be used as an automotive fuel or a fuel additive. However despite the importance of such a potential incorporation of sulfur species into the reaction product, this subject has received very little attention in the open literature. It is therefore not entirely clear, to what extent sulfur is incorporated into the alcohol product, and which sulfur compounds might be present in the condensed product.

Both the stability of the catalyst and the product sulfur content are likely to be affected by the presence of sulfur sources in the syngas feed. Here H2S is the most important source, since it is the dominant sulfur compound in a typical syngas obtained from gasification of carbonaceous feedstocks [31].

Isotopic labeling experiments by Santiesteban et al. [32], [33] indicated that the chain growth mechanism is based upon a CO insertion into an alkyl group as illustrated in Fig. 1. Furthermore experiments with co-feeding of methanol and ethanol along with the syngas have shown that these alcohols can be readsorbed and converted into higher species over the catalyst [12], [32], [34]. While these elements of the reaction mechanism have been resolved, investigations of the reaction kinetics over molybdenum sulfide catalysts are relatively scarce [35], [36], [37] and are typically conducted without H2S in the syngas feed. The reaction kinetics could also be affected by the presence of hydrogen sulfide in the syngas feed, and this influence of H2S should be elucidated.

This work investigates the effects of the process conditions upon the alcohol synthesis over a K2CO3/Co/MoS2/C catalyst. A significant emphasis is on the effects of H2S in the syngas feed. This includes effects upon the reaction kinetics, upon the stability of the catalyst and upon the sulfur concentration in the condensed reaction product. The effects of other operating parameters such as temperature and H2/CO pressures are however also investigated.

Section snippets

Experimental

The experimental work is conducted using a high-pressure flow reactor setup, which is illustrated in Fig. 2. The reactor consists of a quartz tube (i.d. 8 mm; o.d. 10 mm; length 1545 mm), which contains the 250 mm long bed of catalyst particles (dp,average = 0.9 mm). The quartz tube is placed inside a TP347 stainless steel pressure shell. As the interior of the quartz tube is pressurized, nitrogen is dosed to the pressure shell to ensure that no pressure gradient exists across the quartz tube wall.

Stability

The first part of this investigation concerns the stability of the catalyst in the syngas atmosphere and the influence of co-fed H2S upon the stability. This investigation is conducted using the catalyst KCoMo-1. Fig. 3 shows the CO conversion and the CO2-free alcohol selectivity as functions of time on stream in an H2S-free syngas. In parallel Fig. 4 shows the CO conversion and the alcohol selectivity as functions of time on stream with a syngas feed that contains 218 ppmv H2S.

From Fig. 3, Fig.

Conclusion

The preceding text has covered various effects of the process conditions upon the synthesis of higher alcohols over a K2CO3/Co/MoS2/C catalyst, and several conclusions can be drawn on the basis of the presented results.

Irrespective of the presence of H2S in the syngas feed the pre-sulfided catalyst requires an initiation period, before a steady state is achieved, but the duration of this initiation depends upon the H2S concentration in the feed. With H2S levels from 0 to 57 ppmv the fraction of

Acknowledgements

This work is part of the CHEC (Combustion and Harmful Emission Control) Research Center. The present work is financed by The Technical University of Denmark, Haldor Topsøe A/S and the Danish Research Council for Technology and Production under project 274-07-0445. We thank Haldor Topsøe A/S for providing the catalyst used in the experiments, and for assisting in the analysis of the liquid reaction product.

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