Front. Chem. Eng. China 2010, 4(2): 236–239 DOI 10.1007/s11705-009-0239-9

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Developing a molecular platform for potential carbon dioxide fixing Mette MIKKELSEN, Mikkel JøRGENSEN, Frederik C. KREBS (✉) Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark

© Higher Education Press and Springer-Verlag 2009

Abstract This paper presents an attempt to develop a new system for fixing carbon dioxide from the atmosphere. The proposed molecular system has been designed to have the capacity to spontaneously bind CO2 from the atmosphere with high affinity. The molecular system is furthermore designed to have the ability to liberate CO2 at a later stage in the process, i.e., in a separate compartment. The liberated CO2 presents a carbon neutral way of obtaining pure CO2. The proposed molecular system is based on a small stable organic molecule that potentially have two forms: one without bound CO2 and one with bound CO2. One class of molecules that undergo a reaction compatible with our purposal is the merocyanine dyes that exhibit photochromic properties. Based on this structural class of molecules, a system for the potential fixing of CO2 has been developed. Keywords CO2 fixing agent, merocyanine dyes, CO2 liberation

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Introduction

Global warming due to excessive emissions of the green house gas carbon dioxide is recognized as one of the major environmental problems facing humanity in the near future [1]. One possible measure to counter this effect is to capture CO2 from point-continuous sources such as coalor oil-based power plants. One of the most widespread technologies to selectively remove and separate CO2 from power plant exhaust gases is amine-solution-based CO2 absorption/desorption systems using one of the liquids monoethanolamine (MEA), diethanolamine (DEA), or methyldiethanolamine (MDEA). The concentrated CO2 can then be stored in underground reservoirs, such as Received June 2, 2009; accepted August 3, 2009 E-mail: [email protected]

depleted oil and gas wells, aquifers, or on the ocean floor; furthermore, it may be chemically fixed into solid substances, e.g., inorganic carbonates [2]. Large-scale geological storage has already demonstrated feasibility, while other technologies like ocean and carbonate CO2 storage is still in the research phase [2]. Sequestration of CO2 is however only one way of dealing with the increasing amount of CO2 in the atmosphere. Other research areas focus on converting CO2 into a liquid energy carrier, e.g., methanol or ethanol. Current technologies for fixation and conversion of CO2 can be divided into the following areas: 1) chemical transformations of CO2 (such as production of carbonates, carbamates, methanol, and ethanol) [3–5], 2) photochemical transformations (such as production of carbon monoxide, formic acid, and methane) [6–10], 3) electrochemical transformations (CO2 reduction under electrolytic conditions to give carbon monoxide, formic acid, and methanol) [6,11,12], 4) biological transformations (such as formation of sugar, ethanol, and acetic acid) [13], 5) reforming CO2 (production of synthesis gas (carbon monoxide and hydrogen) by reforming natural gas with CO2) [14,15], and 6) inorganic transformation (formation of carbonates) [3,16]. Common for some of these transformations are that CO2 binds irreversibly, and it is therefore difficult to liberate CO2 from the product. A good example is CaCO3 that needs to be heated to high temperatures to liberate CO2. It can also be released by subjecting the system to extreme values of pH (acidic). One of the larger challenges with CO2 reduction to a liquid energy carrier is to get a pure source of CO2 that is free from CO2 emission, and it is ironic that the reduction itself does not pose as great a challenge. This point can be illustrated by looking at the energy efficiency in the separation and capture of CO2, which contributes with 75 percent of the overall carbon capture and storage (CCS) cost. The overall CCS

Mette MIKKELSEN et al. Developing a molecular platform for potential carbon dioxide fixing

process increases the electricity production cost by up to 40 percent. Hence, there is a need for further development in CO2 separation and capture to reduce the overall energy cost [17]. The need for an energy source, which does not depend on the diminishing fossil fuel resources, is an eminent problem also in lieu of the emerging climate changes, which researchers all over the world are trying to solve. Renewable energies that have been successful to date in reducing CO2 emissions are windmills and solar cells. They provide a cheap and clean source of energy, when required, but are inherently transient in their mode of operation, since the energy source is only available under certain conditions (i.e., wind or sunlight, respectively). There is a growing need for a supplementary energy source. This source could come from the reduction of CO2 to give a liquid carbonaceous fuel, methanol for example, which is attractive because of its high energy density and convenience of use in an already well-established infrastructure (cars, petrol stations, ships, planes, public transport, etc.). While hydrogen has been identified as the fuel of the future, there are still many technological problems in terms of handling and storage that have not been overcome. If it is possible to develop a route to obtain a pure source of CO2, without using large amounts of energy during the capture and desorption process, this route would be of immediate interest. The main objective of this paper is therefore to devise a molecular system that will bind CO2 at room temperature followed by a CO2 liberation that will be prompted by a controlled stimulus, i.e., light. The pure CO2 can then be reduced with a catalyst to extract fuel. The reduction of CO2 is extensively reviewed in the literature [5,18–21].

2 Proposal for the development of a CO2 fixing system Since the largest potential source of CO2 is atmospheric, the molecular system has been designed to extract CO2 from the atmosphere and obtain it in a pure form. Therefore, it must be possible to separate the fixation and liberation processes in different compartments. Conceptually, this can be envisaged by having two separate compartments, where one compartment is in chemical contact with the atmosphere, and the other compartment is

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in contact with a container, where liberated CO2 is stored. By cycling the CO2 fixing reagent (S) between the two, it is conceptually possible to devise a system that efficiently achieves the process outlined in Fig. 1.

Fig. 1 The proposed CO2 fixation–light stimulated CO2 liberation cycle. The CO2 source can be in a dilute form mixed with other gases (i.e., nitrogen and oxygen). The CO2 product should be obtained in a pure form and free from other gases

This paper proposes the development of a small molecule that is stable and has two forms: one without bound CO2 and one with bound CO2. The binding of CO2 should be spontaneous under atmospheric CO2 concentrations and insensitive to other atmospheric components. Furthermore, the product with bound CO2 should be stable. The liberation of CO2 should be triggered by a controllable stimulus (i.e., light). The molecule should have the ability to bind CO2 with high affinity and preferably with the ability to liberate CO2 at a later stage in the process followed by a reduction to get a liquid energy carrier. One class of molecules that undergo a reaction compatible with our purpose is the merocyanine dyes that exhibit photochromic properties. Photochromism is a phenomenon associated with light-induced reversible isomerization of a molecule species between two isomeric forms having different absorption properties [22]. An example of a merocyanine dye is shown in Scheme 1. Upon irradiation with UV light, the bond between the sp3-hybridized spiro-carbon and the oxygen atom breaks, and the ring opens to give the merocyanine form. The spiro-carbon thereby achieves sp2 hybridization and becomes planar, the aromatic group rotates, and it aligns its π-orbitals with the rest of the molecule; thus, a conjugated system forms with the ability to absorb photons of visible light and therefore appears colored. When the UV source is removed, the molecules gradually relax to their ground state, the carbon-oxygen bond reforms, the spiro-carbon becomes sp3 hybridized again, and the

Scheme 1 Photochromic cycle for the merocyanine dyes where illumination at 380 nm forms the colored zwitter ion (merocyanine form) that reverts to the neutral form (spiropyran) in time or by illumination with light at 570 nm [23]

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Front. Chem. Eng. China 2010, 4(2): 236–239

Scheme 2 Illustration of the idea behind the zwitter ionic CO2 fixing reagent 1 and its reaction with CO2 to form 2

molecule returns to its colorless state [24]. The reversion of the merocyanine form, which is thermally unstable, back to the spiropyran may be a fast process under thermal conditions or by visible light absorption [23,25]. Since the discovery of the photochromic reactions of spiropyrans in 1952, their molecular property changes have been applied to various applications [26,27]. Spiropyrans are used in filters, displays, eye-protective laser goggles, and selfdeveloping photography [22]. The proposed molecular system should be designed such that the first step in the fixing cycle is based on the equilibrium seen in Scheme 1. The second step in the cycle is the formation of a stable molecule [28], which is formed, when the carbonate anion makes an intramolecular ring closure. The intramolecular ring closure should be facilitated by the opposite charges and the energy gain in the formation of the five- or six-membered ring. A potential target molecule based on phenolate as the anion is shown in Scheme 2. The molecule encompasses the ideas behind the CO2 fixing cycle. The molecule is zwitter ionic and should have an anion, which is nucleophilic enough to react with gaseous CO2 and lead to a neutral compound upon subsequent intramolecular ring closure. The intension is to complete the cycle of releasing CO2 from 2 by irradiation with light at a suitable wavelength. The anion could potentially be exchanged with a stronger nucleophile, e.g., nitrogen, since aromatic and aliphatic amines, as shown in Scheme 3, are capable of binding CO2.

Scheme 3 Equilibrium between an amine and gaseous CO2 and an alkyl ammonium alkyl carbamate

CO2 combines rapidly with amines at ordinary temperatures and pressures to form carbamates [29]. The reaction between CO2 and amines is essentially an equilibrium, which is controlled by pH. Two molecules of an amine react with gaseous CO2 to form a carbamic acid salt. Carbamates are thermally unstable and release CO2 upon heating [29].

By replacing the phenolate moiety with an aniline moiety, the proposed liberation with light may still be possible. We hope this proposed CO2 fixing cycle will inspire the research community to pursue this idea further in an effort to obtain emission free methods for separating and capturing CO2 from flue gases. The acquired pure CO2 holds great potential for future use in the production of synthetic carbonaceous fuels. Acknowledgements This work was supported by the Danish Research Council for Technology and Production Sciences (FTP 274-05-0356).

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