1
CARBON CAPTURE IN METAL-ORGANIC FRAMEWORKS

Mehrdad Asgari and Wendy L. Queen

Laboratory of Functional Inorganic Materials,Ecole Polytechnique Federale de Lausanne,, Sion, Switzerland

1.1 INTRODUCTION

1.1.1 The Importance of Carbon Dioxide Capture

Carbon dioxide, an important chemical gas found in the atmosphere, is critical for the continuation of life on earth. This molecule is required for photosynthesis that fuels plants, which serve as the main source of food for all humans and animals and further produce oxygen that is essential for human respiration [1]. Studies have shown that a small accumulation of CO2 in the atmosphere is necessary to warm earth to a level where glaciation is inhibited, producing an environment where plant and animal life can thrive [2]. However, there is recent evidence that human activity related to energy production is generating an abundance of CO2 in the atmosphere that can no longer be balanced by earth's natural cycles, an act that is expected to confront mankind with serious environmental problems in the future. Since CO2 is the most abundantly produced greenhouse gas (Figure 1.1) [3], it is directly implemented in global warming. It is predicted that if the negligent release of CO2 persists, it could have detrimental effects on our environment that include melting ice caps, rising sea levels, strong changes in weather patterns, ocean acidification, ozone layer depletion, poor air quality, and desertification; all of these things could lead to the potential demise of the human, plant, and animal life, making CO2 mitigation an urgent need [4, 5].

Figure 1.1 The contribution of different constituent in the greenhouse gas emission. Source: Victor et al. 2014 [3].

Eighty percent of the world's energy is currently supplied by the combustion of carbon-based fossil fuels [6], an anthropogenic activity that has led to steady increase in atmospheric CO2 levels. Since the beginning of the industrial revolution in the 1750s, atmospheric CO2 concentration has increased from 280 ppm [7] to above 400 ppm in March 2015 [8, 9]. While the best remediation method is to transition from traditional carbon-based fuels to clean energy sources, like wind and solar, energy transitions are historically slow [9]. As such, it is projected that the use of fossil fuels will continue for years to come, requiring the development of materials that can remediate the effects of CO2 through direct carbon capture and sequestration (CCS) and/or conversion of this greenhouse gas into value-added chemicals and fuels. While CO2 capture directly from air is considered to be an unfeasible task, carbon capture from large point sources, such as coal- or gas-fired power plants, could be realized. Currently, 42% of the world's CO2 emissions come from production of electricity and heat [10] and it is anticipated that approximately 80-90% of these emissions could be eliminated with the implementation of adequate CCS technology [11]. CCS is a multi-step process that includes the capture of CO2 and its transport to sites where it is subsequently stored. While the processes of storage and transport are well-developed technologies, the actual implementation of capture process on a global scale is still constrained by the development of an adequate gas separation technology. Thus, the discovery of new materials with high separation ability is a pertinent obstacle that must be overcome.

1.1.2 Conventional Industrial Process of Carbon Capture and Limitations: Liquid Amines

The most mature capture technology, which has been around since the 1930s, includes aqueous alkanolamine-based scrubbers [12]. These chemical absorbents feature an amine functionality that undergoes a nucleophilic attack on the carbon of the CO2 molecule (Figure 1.2) to form either a carbamate (in the case of primary or secondary amines) or a bicarbonate species (in the case of tertiary amines) [13]. While amine scrubbers are highly selective in the capture of CO2 relative to other components in a gas stream, operate well at low partial pressures, and can be readily included into existing infrastructure at power plants, they have several limitations that inhibit their implementation on scales large enough for post-combustion carbon capture [14]. The materials are quite corrosive to sources of containment requiring their dilution with water to concentrations ranging from 20 to 40 wt% of the amine [15]. The high heat capacities of the aqueous amine solutions combined with high adsorption enthalpies of CO2, approaching -100 kJ mol-1, creates a large parasitic energy cost for the subsequent release of CO2. While the strength of CO2 binding can be tuned to some degree with amine substitution (1° > 2° > 3°, i.e., monoethanolamine, diethanolamine, or triethanolamine) [13], the regeneration process typically requires temperatures that range from 120°C to 150°C [16-18]. The instability of the materials at these temperatures leads to a slow decomposition and hence a decrease in the materials' performance with subsequent absorption cycles. Given all of these problems, this technology, which has already been employed in hundreds of plants worldwide for CO2 removal from natural gas, hydrogen, and other gases, requires that approximately 30% of the energy produced from a power plant be put back into the carbon-capture process [12]. It is projected that solid adsorbent materials with lower heat capacities might cut the energy consumption assumed from the current carbon-capture technology considerably [19]. For this to be realized, much further work is required to design porous solid adsorbents that show (i) high stability in the presence of various components in the gas stream, particularly water, (ii) high selectivity and adsorption capacity, (iii) low cost, (iv) reversibility, and (v) scale ability [20]. To date, there are several classes of porous adsorbents studied for applications related to carbon capture including zeolites, activated carbons, and covalent organic frameworks; however, all of these materials suffer quite significantly from a minimal adsorption capacity and/or low selectivity [19, 21-25].

Figure 1.2 Reaction scheme for carbon dioxide with a (a) primary, secondary, or (b) tertiary amine.

1.1.3 Metal-Organic Frameworks and Their Synthesis

One materials solution to the aforementioned carbon-capture problem is a relatively new class of porous adsorbents known as metal-organic frameworks (MOFs), which are constructed by metal ions or metal-ion clusters linked together by organic ligands (Figure 1.3) [26, 27]. Since the discovery in the late 1990s that these materials can exhibit permanent porosity [28], they have rapidly moved to the forefront of materials research. Looking at publications related to carbon dioxide adsorption in MOFs, one can see a significant increase in the number since 2005, with over 500 publications in 2015 alone [29]. This is in part due to their unprecedented internal surface areas, up to 7000 m2 g-1 [30], which allows the adsorption of significant amount of guest species. Further, the molecular nature of the predefined organic linkers offers a modular approach to their design (Figure 1.3). Through judicious selection of the building blocks, MOF structures can be chemically tuned for a variety of environmentally relevant applications such as gas storage and separation, sensing, and catalysis [31-39]. MOFs have become particularly attractive due to recent reports of materials with high capacities and selectivities for the adsorption of various guest molecules [40, 41]. Currently, MOFs hold several world records related to small molecule adsorption that include (i) surface area [30], (ii and iii) room-temperature hydrogen [42] and methane storage [43], and (iv) carbon dioxide storage capacity [44]. The facile chemical tunability of MOFs is their primary advantage relative to other more traditional porous adsorbents such as activated carbons and zeolites. Further, their highly crystalline nature combined with a non-homogenous van der Waals potential energy landscape on the internal MOF surface dictates that incoming guest molecules bind in well-defined positions and orientations; this allows diffraction techniques to be used to readily unveil their site-specific binding properties. Understanding the structure function relationship allows one to tune the properties of existing materials or rationally design new materials with specified function.

Figure 1.3 (Left) Ball-and-stick model of an MOF, MOF-5 or Zn4O(1,4-benezendicarboxylate)3 [27], showing the modular nature of the frameworks, which can be used to tune MOF properties for the selective binding of gas molecules, making the materials of particular interest in applications related to carbon capture. Source: Li et al. 1999 [29]. Reproduced with permission of Nature Publishing Group. (Right) The number of publications related to "CO2 adsorption" and "metal-organic frameworks" increased significantly from 2005 to 2015. Source: Gallagher et al. 2016 [29]. Reproduced with permission of Royal Society of Chemistry.

MOFs are typically synthesized using a combination of metal salts and ligands via standard hydrothermal or solvothermal methods; reactions are usually...