Research Highlight: The ‘Alchemy’ behind the Conversion of Syngas to Bioethanol

Editor’s Note: NESSE Member Pamela Carrillo writes to explain to us the motivation behind her graduate school research in the Chemistry Department at Stony Brook University and Brookhaven National Laboratory, and why she sees herself as a millennial alchemist.  

In the medieval times, alchemy aimed to transform common metals into precious gold.  Now, in grad school, I like to call myself “a millennial alchemist”. Why is this? In the next few paragraphs I will try to give you a little insight into the kind of chemical alchemy I chose for my graduate work.

For my Ph.D. dissertation, I aim to develop a new way to transform waste from biomass/combustion (carbon monoxide/dioxide) and hydrogen into a highly priced commodity, fuel.  Specifically, ethanol.  As of 2014, ~40% of the world’s energy was oil-based of which ~64% corresponds to the transportation system.1 The future scenery for energy demand is of continuous growth due to economic development and increasing world population, hence, the need for a cost-effective and benign alternative fuel.2 Fossil fuel combustion does not only lead to high CO2 emissions but also negatively impacts the environment due to petroleum extraction and refining processes. This may involve pollution, deforestation and social injustice.3

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Figure 1. Carbon-Neutral Cycle of Ethanol Production from Biomass-Derived Syngas

Ethanol from biomass-synthetic gas (syngas) conversion is a carbon-neutral and commercially viable solution (Figure 1) since it can be synthesized using existing oil refineries.4, 5 Another advantage is that it does not require large areas for crops compared to biochemically-derived ethanol (not compromising the land needed for food production).  In order to make this possible, heterogeneous catalysts are needed.  The heterogeneous catalysts focused on in this post are composed of metal nanoparticles deposited on high surface area metal oxides, called supports.  These nanocatalysts are composed of metals like rhodium, molybdenum, or copper, which “lay” on top of titanium, silicon dioxide, etc.

The metallic surfaces of the nanocatalysts have the special ability to attract CO/CO2 and H2 (gas molecules) in diverse ways in order to break their bonds and add them one to the other.6 (Figure 2) Due to the rather intricate reaction pathways this can result in the synthesis of ethanol from CO/CO2 hydrogenation, which generates methane, water, other alcohols, hydrocarbons and aldehydes. The gas/solid interactions that occur during CO/CO2 hydrogenation depend on the size of the metallic nanoparticles, their electronic state, physical conditions (pressure and temperature), and composition of the metal/support.7 The product distribution relies on which product is favored energetically. In most cases, methane and hydrocarbons are the preferred products.  Therefore, in order to kinetically favor the production of ethanol, addition of other metals such as iron, molybdenum, , alkali or the modification of the support can be performed.

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Figure 2. Schematic Representation of the Intermediates in the Reaction fromm CO Hydrogenation on a Rh (111) Metallic Surface (adapted from reference 6)

Finding the “elixir of life” or in this case the “perfect catalyst” is rather complex since this is a multicomponent system that can exhibit different characteristics than its single components.  Furthermore, when the catalysts go under CO/CO2 and H2 flow, temperature and/or pressure increase.  Additionally, their structure, morphology and electronic properties go under modification. In order to map these changes, an array of advanced techniques is required to probe these catalysts in “action”, in other words, under reaction conditions.  Due to their very small size, elemental composition and evolving behavior, synchrotron (high energy) techniques like X-ray diffraction (Figure 3), spectroscopy, as well as electron microscopy can be used to probe the structural, morphological and electronic requirements for ethanol production. Once this characterization has been performed,  we will be able to begin to design the roadmap for that “perfect” catalyst: one that can convert large quantities of CO/CO2 into high yields of ethanol with very small amounts of other undesirable by-products.

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Figure 3. Setting Up an in-situ X-Ray Diffraction Synchrotron Experiment at the Advance Photon Source at Argonne National Laboratory

So far, the most promising catalysts contain noble metals (e.g. rhodium), which are the most rare in the earth’s crust. They have abundances of 10-3-10-6 atoms of element per 106 atoms of silicon.8 Thus, the challenge does not only reside in understanding the guidelines behind the construction of the ideal catalysts but also building ones with earth-abundant elements which can work under low temperatures and pressures to minimize energy input.

Still, this is not the only aspect we should take into account. The conversion of a gaseous stream into a liquid does not only require a multi-component system but also a multidisciplinary team able to optimize the conditions so that this research can be translated into an efficient industrial process.  Even though the science behind catalysis is exciting, it is also rather complex and it plays a pivotal role in the transition from a fossil fuel-based energetic system into a renewable and more sustainable one – the latter being the one that motivated me to follow this path for my graduate project! I never thought that my love for chemistry would drive me to contribute to the protection of the Earth and it is one of the reasons why I passionately pursue it. Even though my “love for ethanol” is big, I know that the use of this alcohol as a sustainable alternative is only a small part of the solution to drive economy and society into a sustainable development model. The sustainable development goal can only truly be attained if several other ways of producing energy efficiently from renewable sources are developed.

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References:

1. Agency, I. E., Key World Energy Statistics. 2015.

2. (EIA), U. S. E. I. A., International Energy Outlook 2016.

3. O’Rourke, D.; Connolly, S. Annual Review of Environment and Resources 2003, 28, (1), 587-617.

4. Alonso, D. M.; Bond, J. Q.; Dumesic, J. A. Green Chem. 2010, 12, (9), 1493-1513.

5. Spivey, J. J.; Egbebi, A. Chem. Soc. Rev. 2007, 36, (9), 1514-1528.

6. Choi, Y.; Liu, P. Journal of the American Chemical Society 2009, 131, (36), 13054-13061.

7. Cuenya, B. R. Thin Solid Films 2010, 518, (12), 3127-3150.

8. Bullock, R. M. Science 2013, 342, (6162), 1054-1055.