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Fuel Synthesis from Syngas

Introduction Nowadays the society we are living in is experiencing high oil prices and global warming threats, which are mainly caused by extensive use of traditional fossil fuels such as coal and crude oil. In the future, renewable and environmental friendly sources of energy would be more and more attractive. Synthesis gas (also called as syngas, which is a mixture of mainly CO and H2) could potentially play a significant role in addressing the above mentioned problems. Basically it can be derived from any carbon containing raw material: coal, biomass such as straw and wood or even wastes. Provided that a sustainable and effective way of electrochemical conversion is found, even CO2 and H2O could serve as initial materials for syngas production. (Electrochemical fixation of CO2, CASE project). Then, syngas can be converted to various liquid fuels such as methanol, Fisher-Tropsch oil (FT oil, a mixture of hydrocarbons) and higher alcohols (Figure 1). Alcohols are widely used in our society as solvents, pharmaceuticals, and starting materials for synthesis of various chemicals, including fuels or fuel additives.1,2,3

. The scope of this project is to develop catalysts for the direct conversion of syngas to alcohols, either methanol (low pressure synthesis) or higher alcohols.

Opportunities for syngas production and conversion of syngas to various liquid fuels and chemicals.

Methanol Synthesis

Introduction Synthesis of methanol from syngas (a mixture of carbon monoxide and hydrogen with small amounts of carbon dioxide) in an industrial scale is typically carried out at elevated temperatures and pressures (up to 2500C and 100 bar respectively 4

Synthesis of methanol from synthesis gas at lower temperature and pressure are desirable if sustainable fuel methanol is to be synthesized in decentralized units following biomass gasification or synthesis gas

), which requires high operational and investment costs. The only commercially used catalyst for methanol production is Cu/ZnO/Al2O3, which is optimized to operate under conditions mentioned above.

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production by electrolysis. The «Dream Carbon Cycle» is schematically illustrated in the picture below: carbon dioxide and water are electrochemically converted to syngas (a mixture of CO and H2), which is then converted into methanol at low pressure and temperature over a catalyst that needs to be developed. Finally, methanol can be used as a fuel.

Why decrease temperature and pressure?

High pressure processes not only contribute significantly to the total process cost of chemical engineering plant, but also add up certain degree of complexity to the equipment used, thus increasing the investment costs. Consequently, low pressure process would make methanol a more sustainable fuel, both economically and environmentally.

From thermodynamic point of view, one has to go down in temperature in order to produce methanol from syngas with reasonable conversion at low pressures, as it can be seen from the figure below (assuming stoichiometric amounts of CO and H2 and no water-gas shift activity; the same trends apply for CO2 containing mixtures):

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It is clear that at lower temperatures thermodynamic yield of methanol is sufficiently high even at low pressures. However, at present no catalyst is known to activate the process at such low temperatures.

Research and development Based on DFT (Density Functional Theory) calculations performed by the theoretical group at DTU Fysik and Stanford University (Dep. of Chemical Engineering at Stanford University), it has been proposed that certain metal alloys should be catalytically active towards methanol synthesis from syngas at lower temperatures (150-2000C) and pressures close to atmospheric A limited amount of work has been done in the field of using alloyed catalysts for methanol synthesis. In the early 80s it has been shown that certain copper-thorium and copper cerium based alloys are active towards methanol synthesis at industrial conditions 5,6

The main objective of the research is to validate DFT calculations by synthesizing, testing and characterization of several most interesting alloyed catalysts. Depending on the progress of the research, one of the catalysts should be optimized

. However, nothing economically more viable than copper based catalysts has been proposed yet

Higher alcohols synthesis

Introduction Higher alcohols (C2+OH) are very attractive as fuels or fuel additives because they have a low vapor pressure, high water tolerance, good solubility in hydrocarbons, high heating value and high commercial value.7

Currently, higher alcohols such as ethanol are mainly produced according to two processes. The first process is essentially fermentation of sugars that are derived from sugarcane or corn; 2,8 the second one is hydration of ethylene produced from petroleum. However, Crutzen et al. 9

Syngas can be converted into alcohols in a two step process: first a mixture of CO and H2 is thermochemically produced by gasification of biomass, and then syngas is converted into higher alcohols catalytically.

found that corn-ethanol production process can emit nitrous oxide that contributes to global warming as well. Moreover, food shortage problems might come up if materials such as sugarcane and corn are utilized for fuel production. Therefore, an alternative way to produce higher alcohols from syngas is favorable.

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The catalysts and process for the higher alcohol synthesis (HAS) can be divided into four catagories 10,11,12

1. Modified methanol synthesis catalysts: alkali-doped ZnO/Cr2O3 (high temperature methanol synthesis catalyst) and alkali-doped Cu/ZnO and Cu/Zn/Al2O3 (low temperature methanol synthesis catalyst).

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2. Modified FT catalysts: alkali-doped Co/Fe/Ni-Cu-Al2O3; 3. Modified Mo-based catalysts: Carbides and Sulfides; 4. Noble metal catalysts, Rh, Pd, Pt, Re: heterogeneous oxide supported metals and homogeneous

metal complexes;

Table 1 illustrates that the higher alcohol synthesis typically takes place at 250-325 °C, 50-200 bar, H2/CO = 1 mol/mol and GHSV = 2000-10000 h-1. In comparison the dedicated methanol synthesis operates at a lower temperature and pressure (225 °C - 275 °C, <100 bar) with a more hydrogen rich feed (often 80-90 vol% H2), and typically also with a higher space velocity 13,14,15

Table 1: Typical operating conditions for various higher alcohol synthesis processes reported in the literature

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Catalyst

. T

[°C]

P

[bar] H2/CO [mol/mol]

Feed CO2

[vol%]

GHSV

[h-1]

CO conv.

[mol%]

K/ZnO/Cr 350-425 90-200 2-3 0-3 3000-15000 14

K/Cu/Zn/Al 270-300 60-100 0.5-2 0-1 400-6000 20-60

K/Cu/Co/Al 260-340 60-200 1-2 2-4 3000-6000 12-15

Cs/Cu/ZnO 275-325 50-100 0.45-1 0 3000-7000 10-20

K/MoS2 or K/Co/MoS2 255-325 30-200 1 0 2000-10000 10-20

Rh-based 210-300 1-50 1-2 0 3000-13000 2-10

Modified methanol synthesis catalysts exhibit the highest selectivity and activity for alcohols in term of CO conversion; however, methanol is the predominant alcohol product. The selectivity for the formation of C2+ alcohols is limited by the kinetics of chain growth. The C1 (methanol) to C2 (ethanol) step is the rate determining step for HAS. Noble metal catalysts such as Rh showed highest higher alcohols selectivity; however, they are limited in availability, higher cost and lower activity, which make them unattractive for commercial application.

Modified Mo-based (MoS2) catalysts exhibit relatively high selectivity and activity for higher alcohols, which has been studied by Christensen et al. 17,18

Modified F-T synthesis catalysts are one of the most promising candidates for HAS via syngas. It has moderate alcohol selectivity and activity for alcohols, but relatively high activity for higher alcohols. Wu et al. (CASE Process Lab) is working on the Cu/Ni and Cu/Co catalyst systems. The current work is focused on changing the catalytic selectivity of the catalysts towards ethanol and higher alcohols by trying different supports and modifying basicity of the catalysts with alkali and other promoters.

at CASE Process Lab.

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Challenges • Catalysts with high selectivity towards higher alcohols.

• Sufficiently active catalysts.

• Cheap catalysts, i.e. based on abundant elements.

• Chemical stability in synthesis gas.

The main problem lies in that the catalyst need to:

• Be efficient in breaking the C-O bond to produce long chain intermediates CxHy*, but retain a

large coverage of intact CO molecules to form CxHyCO*.

• Be efficient in hydrogenating CxHyCO* intermediates into alcohols, but not too good, as high

hydrogenation activity leads to termination as CxHy.

Approaches • Design of catalysts using both density functional theory (DFT) and literature-based prediction.

• Synthesis of catalysts by different methods such as co-precipitation and flame spray pyrolysis.

• High pressure testing of catalysts for activity and selectivity.

• Characterization techniques: BET (DTU KT), H2-TPR (DTU KT), XPS (DTU Fysik), ICP (Haldor Topsøe A/S), and XRD (DTU Fysik).

• In-situ characterization like TEM (DTU Cen), XAS (ANKA-KIT, Inst. for chem. technol. and polymer chem. at Univ. Karlsruhe), and XRD (DTU Fysik).

Characterization of new catalysts The characterization of the catalyst aims at getting to know the traits of a material in many different aspects. Important for a catalytic material is the performance to convert the Syngas into the desired product, so a high turnover rate. But also the selectivity, the ability to convert only into the desired product or with little byproducts is important. Both can be tested at DTU in test facilities at industrial conditions and in microreactors with a greater control over the gas flow and heat distribution. To get, however, a deeper insight into the basic function of a catalyst we also use methods to study changes in the shape and the structure of the catalyst on the atomic level. This is done with x-ray diffraction (XRD) as well as electron microscopy. What makes our characterization abilities special is that we are able to study the catalysts with those methods closer to their working conditions than normal x-ray diffractometers and electron microscopes. With that, we aim for getting a deeper understanding of the basic catalytic process on the atomic scale 19

These are some of the devices we have available:

. These information can be used by the CASE participants who predict and synthesize the catalysts and therefore create a feedback loop to improve the catalysts systematically.

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Titan ETEM This machine is an Environmental Transmission Electron Microscope (ETEM). It features the full capability

of a monochromated, image Cs aberration corrected TEM with the possibility to study specimens in a gas atmosphere and at elevated temperatures. It is still possible to get atomic resolution at these conditions 20. Within CASE, this microscope is mainly used for dynamic studies of the catalysts focusing on the crystal structure, particle sizes and other effects on the nm to Å scale. The electron microscopes are located at the Center for Electron Nanoscopy (DTU Cen).

An example of a catalytic study carried out in the ETEM is CuSn:

During the catalytic process the catalyst undergoes structural changes. In this case it is CuSn catalyst particles that sinter and get deactivated during the process.

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In-situ XRD This is an X-ray diffractometer equipped with a gas handling system and an in-situ cell to study specimens in gas atmosphere and at elevated temperatures. The XRD reveals the crystal structure and changes thereof during the exposure to the gas and heat. Libraries allow identifying different phases in a mixture and during changes. In the future it will be possible to treat and investigate the same sample in the XRD and transfer it over to the TEM for further investigation This XRD system is also part of DTU Cen.

Equipment for catalyst testing at low pressures at DTU Fysik. Products are analyzed by Gas Chromatography.

High pressure fixed-bed continuous-flow reactor (maximum 110 bar and 600 °C) at Department of Chemical Engineering (DTU KT)

Diagram illustrating the catalyst testing setup. BFM: Bubble Flow Meter; MF(P)C: Mass Flow (Pressure) Controller; P: Manometer. Heat traced tubing is indicated by sinusoidal curves.

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BET-Autosorb-iQ2 (Quantachrome Instruments) at Department of Chemical Engineering (DTU KT)

H2-TPR on NETZSCH STA-449 at Department of Chemical Engineering (DTU KT)

Institute for Synchrotron Radiation (ISS – ANKA, Karlsruher Institut für Technologie , Germany)

List of participants Name Affiliation Anker Degn Jensen DTU KT Qiongxiao Wu DTU KT Jakob Munkholt Christensen DTU KT Jakob Birkedal Wagner DTU Cen Christian Danvad Damsgaard DTU Cen Linus Daniel Leonhard Duchstein DTU Cen Søren Dahl DTU Fysik Irek Sharafutdinov DTU Fysik Christian Fink Elkjær DTU Fysik Jens Kehlet Nørskov Dep. of Chemical Engineering at Stanford University Jan-Dierk Grunwaldt Inst. for chem. technol. and polymer chem. at Univ. Karlsruhe Burcin Temel Haldor Topsøe A/S

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