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Effect of CO2 in the synthesis of mixed alcohols from syngas over a K/Ni/MoS2 catalyst
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
2013 (English)In: Fuel, ISSN 0016-2361, E-ISSN 1873-7153, Vol. 107, 715-723 p.Article in journal (Refereed) Published
Abstract [en]

An unsupported K-Ni-MoS2 catalyst for higher alcohol synthesis from syngas (H-2/CO) has been studied during 360 h on stream. It shows a gradual increase in activity with time on stream and some possible reasons for this are discussed in the paper. The main focus of this paper was to study the on the effect of CO2-containing syngas, relative CO2-free syngas under identical reaction conditions and identical inlet H-2 and CO partial pressures (340 degrees C, 100 bar, GHSV = 6920 ml/(g(cat) h)). The effect of increased partial pressure of H-2 and CO was also studied, and to a minor extent also the effect of changed gas hourly space velocity (GHSV). Under the studied conditions, addition of CO2 was found to greatly decrease total product yield, while the selectivities to alcohol and hydrocarbons (C%, CO2-free), respectively, were unchanged. CO2 addition, however, led to a great change in the distribution within the alcohol and hydrocarbon groups. With CO2 added the methanol selectivity increased much while selectivity to longer alcohols decreased. For hydrocarbons the effect is the same, the selectivity to methane is increased while the selectivity to longer hydrocarbons is decreased. It has earlier been shown that product selectivities are greatly affected by syngas conversion level (correlated to outlet concentration of organic products, i.e. alcohols, hydrocarbons etc.) which can be altered by changes in space velocity or temperature. This means that alcohol selectivity is decreased in favor of increased hydrocarbon selectivity and longer alcohol-to-methanol ratio when syngas conversion is increased. At first it might be thought that the selectivity changes occurring when CO2 is present in the feed, just correlate to a decreased organic product concentration in the reactor and that the selectivities with CO2-containing and CO2-free syngas would be identical under constant concentration of organic products in the reactor. However, CO2-addition studies where space velocity was varied showed that significantly lower alcohol selectivity (mainly ethanol selectivity) and increased hydrocarbon selectivity (mainly methane) were found at similar organic outlet concentrations as when CO2-free syngas was feed. Comparing addition of extra H-2 or extra CO, it was found that a high H-2/CO ratio (H-2/CO = 1.52 tested in our case) favors maximum product yield, especially methanol formation, while a lower H-2/CO ratio (H-2/CO = 0.66 tested in our case) leads to higher yield of higher alcohols simultaneously minimizing hydrocarbon and methanol formation.

Place, publisher, year, edition, pages
2013. Vol. 107, 715-723 p.
Keyword [en]
Syngas, Higher alcohols, MoS2, CO2-containing syngas, Synthetic fuels
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-121106DOI: 10.1016/j.fuel.2012.11.044ISI: 000316214200078Scopus ID: 2-s2.0-84878360614OAI: oai:DiVA.org:kth-121106DiVA: diva2:617087
Note

QC 20130422

Available from: 2013-04-22 Created: 2013-04-19 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Catalytic conversion of syngas to higher alcohols over MoS2-based catalysts
Open this publication in new window or tab >>Catalytic conversion of syngas to higher alcohols over MoS2-based catalysts
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis concerns catalytic conversion of syngas (H2+ CO) into a blend of methanol and higher alcohols, an attractive way of producing fuels and chemicals. This route has the potential to reduce the oil dependence in the transport sector and, with the use of biomass for the syngas generation, produce CO2-neutral fuels.

Alkali promoted MoS2-based catalysts show a high selectivity to higher alcohols, while at the same time being coke resistant, sulfur tolerant and displaying high water-gas shift activity. This makes this type of catalyst especially suitable for being used with syngas derived from biomass or coal which typically has a low H2/CO-ratio.

This thesis discusses various important aspects of higher alcohol synthesis using MoS2-based catalysts and is a summary of four scientific papers. The first part of the thesis gives an introduction to how syngas can be produced and converted into different fuels and chemicals. It is followed by an overview of higher alcohol synthesis and a description of MoS2-based catalysts. The topic alcohol for use in internal combustion engines ends the first part of the thesis.

In the second part, the experimental part, the preparation of the MoS2-based catalysts and the characterization of them are handled. After describing the high-pressure alcohol reactor setup, the development of an on-line gas chromatographic system for higher alcohol synthesis with MoS2 catalysts is covered (Paper I). This method makes activity and selectivity studies of higher alcohol synthesis catalysts more accurate and detailed but also faster and easier. Virtually all products are very well separated and the established carbon material balance over the reactor closed well under all tested conditions. The method of trace level sulfur analysis is additionally described.

Then the effect of operating conditions, space velocity and temperature on product distribution is highlighted (Paper II). It is shown that product selectivity is closely correlated with the CO conversion level and why it is difficult to combine both a high single pass conversion and high alcohol selectivity over this catalyst type. Correlations between formed products and formation pathways are additionally described and discussed. The CO2 pressure in the reactor increases as the CO conversion increases, however, CO2 influence on formation rates and product distribution is to a great extent unclear. By using a CO2-containing syngas feed the effect of CO2 was studied (Paper III).

An often emphasized asset of MoS2-based catalysts is their sulfur tolerance. However, the use of sulfur-containing feed and/or catalyst potentially can lead to incorporation of unwanted organic sulfur compounds in the product. The last topic in this thesis covers the sulfur compounds produced and how their quantity is changed when the feed syngas contains H2S (Paper IV). The effect on catalyst activity and selectivity in the presence of H2S in the feed is also covered.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. ix, 106 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2015:2
Keyword
conversion, higher alcohols, mixed alcohols, MoS2, syngas
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-158549 (URN)978-91-7595-392-2 (ISBN)
Public defence
2015-02-06, D2, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150115

Available from: 2015-01-15 Created: 2015-01-09 Last updated: 2015-01-15Bibliographically approved

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