By Brian Pattengale* and Anthony D. Sabatelli** —

Lowering carbon dioxide (CO₂) levels in our atmosphere is one of the most important challenges to be overcome in the next century.  While fossil-fuel fired plants produce a visible smoke consisting almost entirely of water vapor, the invisible CO₂ released along with it is the real concern.  In a previous article, we highlighted the IP aspects of carbon capture technologies and the chemistry that is involved in the various processes (see "Rising Carbon Dioxide Levels: Capture Methods & Patent Trends").  Industrially, CO₂ is most predominantly used as an inert gas in various chemical and manufacturing processes, as a propellant, or to make dry ice which is the solid form of CO₂.¹ All of these applications simply end up releasing CO₂ into the atmosphere, negating the effects of capture.  In order to achieve the goal of lowering CO₂ emissions, captured CO₂ must be either stored permanently or chemically converted into another substance.  These two distinct processes, known as sequestration and conversion, respectively, are both potentially viable, and even profitable processes.  Entrepreneurial ventures are already bringing carbon-conscious products to market that consume CO₂ during their manufacturing.  The developments and intellectual property associated with CO₂ sequestration and conversion are the focus herein.

Carbon Sequestration

CO₂ sequestration, broadly defined, is the process of permanently storing CO₂.  One of the first industrial examples of CO₂ sequestration is the Sleipner gas field off the coast of Norway.  This is a natural gas field with a high CO₂ content of 9%.  This high level of CO₂ is nearly four times the Norwegian government emission limit of 2.5%, which if exceeded, would trigger large carbon emission fee penalties for the operator, Statoil Petroluem.  To comply with this limit, the CO₂ was pumped into the Utsira formation, which is a saline aquifer in the North Sea that was identified as a possible reservoir.² From its inauguration in 1996 to 2008, the CO₂ sequestration efforts at Sleipner resulted in the storage of 10 million tons of CO₂.  The operation is still ongoing.³  Statoil Petroleum holds patents related to methods for storing carbon dioxide in geologic formations (US9586759B2 & US9815626B2).

While storing CO₂ in geologic formations is evidently a viable option for CO₂ sequestration, scientists have been exploring other methods.  These technologies rely on storing CO₂ in a chemical form that is stable, easy to store, and can support a high CO₂ density.  A recent patent application (US20170274318A1) describes a process that uses ammonia to capture gaseous CO₂ to form an aqueous ammonium carbonate solution.  This aqueous carbonate is then subjected to an ion exchange reaction with a cation, such as sodium or potassium under conditions that form a solid carbonate.  Following the exchange reaction, the CO₂ sequestering carbonate is recovered, and the ammonia is regenerated to be reused for the capture cycle.  The sodium carbonate, potassium carbonate, or other carbonates are then stored.  Notably, these carbonates are also industrially useful for making glass, water softening, and other various uses.

Another patent application to ExxonMobil Research and Engineering Co (US20180229178A1) describes a process for sequestering CO₂ as a clathrate.  A clathrate is a crystal host that traps a molecular guest inside of its structure — sort of a "molecular cage".  In the described process, the CO₂ is trapped under pressure in a network of water molecules.  The application also describes various chemical additives or "promoter molecules" to facilitate forming and stabilizing the clathrate.  It is then encapsulated in a molecular barrier that prevents CO₂ escape.  The barrier-encapsulated clathrate containing the captive CO₂ can then be deposited on the sea floor at depths of over 100 meters, where the pressure ensures the stability of the clathrate.  This method could provide a feasible option in offshore locations where geologic reservoirs are unavailable for sequestration.

Carbon Conversion

In the above sequestration examples, CO₂ is stored permanently in either a reservoir or converted to a different chemical form from which CO₂ can be released, if desired.  In contrast, carbon conversion can be delineated as converting it into a new product by decreasing the oxidation state of carbon successively until a desired stable product is reached.  This process if often referred to as CO₂ reduction.  Products of CO₂ reduction include formaldehyde, methanol, methane, ethylene, and ethanol, amongst others.  In addition to consuming CO₂, the products produced are value-added products that are of industrial, pharmaceutical, or other uses.  Reduction of CO₂ is often achieved using electrolysis, which is a widely used process in industry that produces products such as aluminum, sodium hydroxide, or electroplated layers.  Electrolysis requires electricity, which is currently produced primarily by fossil fuel combustion.  There is, however, opportunity to develop solar-powered CO₂ electrolysis systems that would not re-contribute to carbon pollution.

CO₂ electrolysis is not currently prevalent in industry because it is difficult to perform, and it is challenging to control which reduction product is obtained (i.e., the reaction is not usually selective to a single product).  In addition, a common byproduct of CO₂ electrolysis is hydrogen gas, particularly when non-selective catalysts are used.  Scientists are therefore working to develop new catalysts that reduce CO₂ efficiently and achieve selectivity, which is defined as the ability to form one of the specific products listed above without forming the others or byproducts such as hydrogen gas.  In the following examples, there is a focus on electrocatalytic systems of different design/catalyst composition and factors that affect selectivity.

The least challenging reaction is the reduction of CO₂ to CO.  Toyota Motor Corp. patented a CO₂ reduction system employing an anodized silver electrode as the catalyst (US9435042B2).  It was found that anodization time greatly affected the CO selectivity, where longer time is reported to give a more selective electrode, reaching approximately 70% CO.  Toshiba Corp has filed an application for an analogous system that employs a molecular catalyst species known as a metalloporphyrin as the electrocatalyst material (US20180066370A1), reaching 60% CO selectivity.  Another system was reported by Toshiba consisting of gold particles synthesized in a specific manner that reaches >90% selectivity for CO (US20170073825A1).

Catalysts that form further products typically form a mixture of products containing CO and other products due to the complexities of the reactions involved, however any appreciable efficiency and selectivity for a complicated product is of interest in the field.  Panasonic reports a system composed of crystalline copper phthalocyanine, a molecular species, that produces predominantly ethylene (C₂H₄) at 40% efficiency (US20180142365A1).  Researchers at the California Institute of Technology discovered that a selectivity-determining layer composed primarily of organic molecules can be added to electrodes to tune the selectivity, where 41% selectivity for ethylene was reached (US20180291515A1).

Some additional systems aim to utilize the machinery within cells and microorganisms to perform CO₂ transformation, requiring careful interfacing of gas and reactant feedstocks with the bioreactor system.  An example (US10131924B2) requires the addition of an organic intermediate compound that is combined with CO₂ to form more complex products.  Perhaps the most impressive system to date in this category splits water using H₂ and O₂, and then H₂ is combined with CO₂ by a bacterial species that, all together, exceeds the efficiency of natural photosynthesis (US20180265898A1).

Finally, it is worth noting that CO₂ transformation has become more than a research venture, with entrepreneurs seeking to bring some exciting technologies to market.  Novomer developed a process to combine CO₂ with epoxides to form polymers, useful plastic materials (US8247520B2).  Another company, Liquid Light, holds many patents related to CO₂ reduction technologies and is backed by investors such as BP Ventures and The Coca-Cola Company, the latter of which is interested in a precursor for plant-based plastic bottles.⁴ _­ Catalytic Innovations, a startup focused on CO₂ transformation and oxidation catalysis, has recently applied to patent a process for efficiently converting CO₂ to alcohols such as ethanol (WO2018071818A9).  The ethanol produced will be consumer-grade and will be used in products such as perfume and spirits.⁵

The table below highlights patents and published applications related to CO₂ sequestration and conversion technologies.

* Brian Pattengale is a Postdoctoral Associate in the Energy Sciences Institute at Yale University, where he is investigating the photodynamic properties of emerging materials and their catalytic/photocatalytic applications to reactions such as water splitting or carbon dioxide reduction.  Prior to his position at Yale, Brian obtained his Ph.D. in Physical/Materials Chemistry at Marquette University, where he published numerous papers using ultrafast transient absorption and synchrotron X-ray absorption spectroscopies to study functional light absorbing and photocatalytic materials.
** Dr. Sabatelli is Patent Counsel with Wiggin and Dana LLP

¹ http://www.uigi.com/carbondioxide.html

² Schepper, S.D.; Mangerud, Gunn. Norwegian Journal of Geology. 2017, 97, 305.

³ Akervoll, I; Lindeberg, E; Lackner A. Energy Procedia. 2009, 1, 2557-2564.

⁴ https://llchemical.com/news/liquid-light-signs-agreement-further-advance-its-cosub2sub-chemicals-technology/

⁵ https://www.americaninno.com/rhodeisland/rhode-island-startup/catalytic-innovations-aims-to-create-carbon-negative-consumer-products/