METAL ORGANIC FRAMEWORKS (MOF): A VIABLE OPTION FOR CARBON (IV) OXIDE (CO2) SEQUESTRATION

METAL ORGANIC FRAMEWORKS (MOF): A VIABLE OPTION FOR CARBON (IV) OXIDE (CO₂) SEQUESTRATION

Since the Industrial Revolution of the 19^th century, there has been continuous increase in the demand for energy. A significant portion of the energy sources are fossil fuels which include coal, crude oil, natural gas and petroleum. The exploration, processing and usage of these fossil fuels over the years released substantial amount of greenhouse gases (GHGs) into the atmosphere. So, despite the economic boom and improved standard of living that these energy sources have provided to mankind, the climate has been negatively affected.

NNPC data reveals that from the year 1996-2010, in Nigeria, 12,602,480.25 million ft³ of natural gas was flared ^[1]. Of course this must have released a substantial amount of GHGs into the atmosphere. United States Environmental Protection Agency (EPA) states that oil and gas production resulted in the release of 6.2 million metric tonnes of combustion gases (GHG) in 2013^[2]. BP statistical bulletin (2018) also equivalently states that 33444.0 million tonnes of CO₂ was emitted in 2017^[3] all over the world. These data all substantiate the claim that carbon emission is increasing and posing a serious threat to the climate. Emissions of GHGs (CO₂, methane, nitrous oxide, fluorinated gases and ozone) have steadily increased over the years with CO₂ consisting a significant portion. U.S. EPA records that CO₂ consists 81% of the GHGs in the US ^[4] in 2016 as shown in Fig. 2. 

Fig. 1: GHG emissions in 2016 by U.S. EPA

Apart from gas flaring earlier mentioned, there are other processes in the oil and gas industry that contribute to carbon emission. They include exhaust flue gases from engines and fired heaters, well testing and CO₂ use for enhanced oil recovery (EOR) operations. Due to the increased amount of GHGs in the atmosphere, climate change has therefore been a front burner issue all over the world. This is because of the negativities associated with it in the form desertification, flooding, drought, global warming, polar ice cap melting, acid rains, etc.

CO₂ sequestration from diverse source points is therefore a ready solution to the multi-faced problem of climate change. Metal Organic Framework (MOF), a mesoporous and microporous coordinated compound of metal clusters linked by organic linkers to form 3-D structure presents itself as a scientific solution that can be used to capture/adsorb CO₂. Different synthetic approaches and conditions are used in MOF synthesis; they include electrochemical, mechanochemical, sonochemical, solvothermal, electrochemical, flow chemistry and spray drying. There is also a growing field of computational prediction in MOF synthesis through Crystal Structure Prediction (CSP). The basic synthetic method of MOF is shown in Fig. 2.

Fig. 2: Basic MOF synthetic method       

In the network structure are pores (measured in Angstrom-Å) shown in Fig. 3 that can adsorb and trap different compounds such as gases (GHGs), chemical compounds, drugs, catalysts, proteins etc. MOFs are one of the most researched compounds in the field of reticular chemistry and material science with a numerous number of it already synthesized in the last decade. MOFs find application in diverse fields. The oil and gas industry is one of such fields. CO₂ sequestrations from gas flare stacks, natural gas during processing, automobiles, other heavy industries and even dry air using MOFs offer a significant means through which climate change can be controlled, therefore upholding the 2015 Paris Agreement and ensuring Sustainable Development Goals (SDG) 7 and 13 are actualized.

Fig. 3: MOF structure showing pore space shown with yellow sphere, Source: Furukawa et al., (2013) ^[5]

METAL ORGANIC FRAMEWORKS (MOFs)

Different MOFs have been synthesized since the inception of reticular chemistry. Some have shown great prospects for CO₂ capture. Notably among them are MOF-5, MOF-177, ZIF-8 (Zinc Imidazole Framework), HKUST-1 (Hong Kong University of Science and Technology), MIL-101 (Material of the Institute of Lavoisier) and M-MOF-74 (M=Zn, Mg, Mn, Fe, Co, Ni, or Cu). The list of MOF for CO₂ capture is “endless” as there continue to be more structures synthesized, modified and functionalized.

As earlier mentioned, metal clusters (also known as Secondary Building Blocks – SBU) and organic linkers are the basic building blocks of the structure. Fig. 4 shows examples of metal clusters such as Zn₄O(CO₂)₆, Cr₃OF(CO₂)₆, Cu₂(CO₂)₄, Zr₆O₈(CO₂), Ni₄(C₃H₃N₂)₈ etc. Likewise, examples of organic linkers which include fumaric acid, oxalic acid, H₂BDC, H₃BTC, H₃BTP, TIPA, H₄DH11PhDC etc. are also shown.

Fig. 4: Examples of metal clusters and organic linkers

**BDC=1,4-benzenedicarboxylicacid, BTC= means benzene, 1,3,5-tricarboxylic acid

It is worthy to note that since MOFs are mesoporous and microporous compounds, they exist in powdery form to the human eye. MOF production companies such as BASF have also synthesized MOF in the form of granules. 

Characterization (SEM, TGA, XRD and BET) of MOFs reveal that they are suitable for practical applications though there are still challenges faced in their deployment for commercial and practical uses. With an extensive BET surface area ranging up to 14,000m²/g in certain MOFs, the amount of CO₂ that can be adsorbed in its porous structure is enormous. The surface area of MOFs is so extensive that if for example, 1g of MOF-5 of surface area 3800 m²/g is spread on a flat surface, its surface area will cover a space of an entire football field. These are the available parking spaces that act as sites for adsorption of CO₂ in these porous coordinated networks (PCN).

Using adsorption isotherms and isosteric heat of adsorption to determine the uptake selectivity of specific gases in a mixture of gases by MOFs has shown that certain MOFs have the ability to adsorb selected gases over others at certain conditions. Several researchers have demonstrated through post synthetic modification (PSM) and functionalization of MOFs using amine compounds, that the selective uptake of CO₂ in the presence of nitrogen and water at low pressures, which are critical conditions of exhaust flue gases can be enhanced for its use in practical applications. Fig. 5 shows an uptake selectivity of CO₂ in a gaseous mixture containing methane (CH₄), hydrogen (H₂), nitrogen (N₂), and argon (Ar).

Fig. 5: Uptake selectivity of CO2 over other gases by a MOF compound

**MOF structure depicted by a 3-D green network structure

As the field of MOF synthesis and modification for practical application continuously develops, there seem to be a light at the end of the tunnel of GHGs (CO₂) posing serious challenge to atmosphere. With breakthroughs been recorded in the field of molecular chemistry and material science in the modification of MOFs for CO₂ adsorption, it is evident that in the nearest future, we will be soon see several source points of CO₂ emission fitted with equipments impregnated with metal organic frameworks. Such scientific breakthroughs will definitely obliterate the emission of CO₂ and eat away at the amount of GHGs present in the atmosphere. This will therefore, provide a significant solution to the multi-faced problem of climate change in the Nigeria and the world as a whole.

REFERENCES

1.      Aregbe, A.G. (2017). Natural Gas Flaring – Alternative Solutions. World Journal of Engineering and Technology, 5(1) 139-153.

2.      United States Environmental Protection Agency. Greenhouse Gas Reporting Program. http://www.epa.gov/ghgreporting/ghgdata/reported/petroleum.html

3.      United States Environmental Protection Agency. Overview of Greenhouse Gases. http://www.epa.gov./ghgemissions/overview-greenhouse-gases

4.      BP (2018). BP Statistical Review of World Review, 67^th Edition.

5.      Furukawa, H., Cordova, K.E., O’Keeffe, M & Yaghi, O.M. (2013). The Chemistry and Applications of Metal-Organic Frameworks. Science, 341, 1230444.