**1. Introduction**

The Earth's global climate system is continually facing devastating changes due to various human-made and natural factors. Smithson [1] mentioned that the increase in greenhouse gas concentrations in the atmosphere directly impacts the global climate system, which is known as global warming [1]. These greenhouse gases trap the sun's radiation in the Earth's atmosphere; this phenomenon is known as the greenhouse effect, causing global warming. Carbon dioxide gas is blamed for being the main factor that causes the greenhouse effect because it is the most important anthropogenic greenhouse gas (Intergovernmental Panel on Climate Change (IPCC), 2007) [2]. Rochelle [3] stated that more than 85% of the world's energy demand is based on burning fossil fuels; this will result in massive emissions of CO2 into the atmosphere [3]. There are natural and anthropogenic sources for carbon dioxide gas emissions into the atmosphere. Chemical engineering industries are considered one of the primary anthropogenic sources of CO2 into the atmosphere in which natural gas and fossil fuels are burned for various purposes. As a result of the industrial revolution and the rapid increase in the population growth rate, more fossil fuels are burnt to satisfy the population's needs and demands. Hence, more carbon dioxide is emitted into the atmosphere. Consequently, the separation and capture of CO2 became a necessity.

CO2 can be separated and captured using five leading technologies, including absorption, adsorption, cryogenics, membrane, and microbial or algae [4]. In the meantime, the research trend has been focusing on three main types of technology for carbon capture, namely oxy-fuel combustion, pre-combustion, and post-combustion. Omoregbe (2020) investigated those main types of technology by using publications retrieved from the Web of Science database from the year 1998 to 2018. The results of the authors' investigations presented that from the year 1998 to 2007 there was almost no research output on carbon capture, until the year 2008, in which climate change abatement was first introduced, and the industrial and public awareness of clean, greener fossil energy options grew. The authors also stated that among the commonly studied carbon capture technologies, the post-combustion capture technology was the most referenced technology for carbon capture, with approximately 80.9% of total publications retrieved. On the other hand, the technology with the lowest number of publications is oxy-fuel combustion, with approximately 3.4% of total publications retrieved [5]. Several porous materials can be incorporated into these carbon capture technologies to allow and enhance the separation or the capture of CO2. One of the best-used porous materials in carbon capture technologies are metal-organic frameworks (MOFs).

During the last two decades, a new crystalline porous materials class has emerged [6]. This class of materials is known as MOFs. As a result of the MOFs' unique properties, this class has gained remarkable attention across the globe. The main limitation of the application of MOFs in the carbon capture processes is the high cost. The synthesis process of MOFs is very costly, which makes them economically unviable. This review paper investigates MOFs' applications in the CO2 capture, adsorption, separation, conversion, and reduction processes. It aims to draw and provide general guidelines and conclusions for the MOFs' importance as a porous material for carbon dioxide gas-related process.

#### **2. Fundamentals of Metal-Organic Frameworks (MOFs)**

MOFs are made-up of metal-containing nodes linked by organic ligand bridges and assembled primarily by strong coordination bonds; this can be shown in Figure 1 below. MOFs have well defined crystallographic and geometric three-dimensional (3D) microporous structures [7]. These structures are sturdy and durable, allowing the removal of the included gues<sup>t</sup> species, which results in permanent porosity.

MOFs can be easily designed, synthesized, and tuned. By comparing MOFs to other porous materials like zeolites and activated carbons, MOFs allows a facile optimization of the structures of their pores, surface functions, and other properties, making them applicable for several specific and precise applications as porous materials. MOFs can be categorized into two classes flexible/dynamic [8] and rigid. Flexible MOFs hold a dynamic and soft framework with a fast response to external stimuli, for example, gues<sup>t</sup> molecules, temperature, and pressure. This extraordinary and superb sensitivity to external stimuli allows the MOFs to possess special properties such as temperature/pressure-dependent molecular sieving, which puts them ahead of the traditional adsorbents, including activated carbons and zeolites.

On the other hand, rigid MOFs possess a stable and strong porous framework with an enduring porosity that is similar to zeolites. In the meantime, rigid MOFs have been used extensively for the selective gas adsorption processes. The selective adsorption mechanism in rigid MOFs is quite similar to zeolites; hence selective adsorption can be achieved based on the molecular sieving e ffect. Also, it is probably achieved according to the strength of the di fferent interactions between adsorbate–adsorbate and adsorbate–adsorbent. Li, et al. [9] mentioned that selective adsorption in rigid MOFs depends on three main factors: adsorbate–surface interactions, size/shape exclusion, and simultaneous corporation of both factors.

**MOF-177** 

**Figure 1.** Typical synthesis approach for metal-organic framework MOF-177.

Based on the literature, many MOFs are used to selectively adsorb different gases relying on the molecular sieving effect [10–13]. This implies that only the molecules with appropriate pore kinetic diameters can pass through the MOF pores (Figure 2). Table 1 below shows the kinetic diameters of several gases.

**Figure 2.** Schematic diagram of selective gas adsorption in Cu-BTC MOF (copper benzene-1,3,5 -tricarboxylate) based on molecular sieving effect.


**Table 1.** Kinetic diameters of several gases [14].

MOFs can be synthesized with an exceptionally high porosity under mild conditions via self-assembly reaction between organic linkers and several metal ions. Table 2 below provides chemical formulas and chemical structures of some organic ligands in some frequently used MOFs.

**Table 2.** Chemical formulas and chemical structures of some organic ligands in some frequently used MOFs [15].


In the field of porous materials, MOFs have excelled and surpassed the traditional porous materials in the following properties: they possess very high CO2 [16] and methane storage [17], uptake of hydrogen-based on physical adsorption [18], and surface area [19]. Hence, MOFs are extensively used in the carbon capture processes. Table 3 below shows a comparison between the strengths and weaknesses of carbon capture materials. It can be seen from Table 3 that MOFs have the highest working capacity among all other carbon capture materials.


**Table 3.** Strengths and weaknesses comparison of selected carbon-capture materials [20].

An extensive number of review articles have been published recently to highlight the extreme developments in the synthesis, design, and application of MOFs in the carbon dioxide capture and storage (CCS) field [21–23]. Simmons, et al. [24] stated that MOFs had been displayed as excellent materials for carbon dioxide storage, and also they are useful in the removal of carbon dioxide from flue gas stacks [24]. MOFs can be consequently considered ideal membrane-filling materials and adsorbents for CO2 gas storage, separation, and capture due to their pore surface controllable properties, adjustable pore sizes, and large surface area [25]. In synthetic MOFs, the pore size and channels can be trivial, reaching nanometers and angstrom. At high pressure, these small pores and channels of the MOFs can store CO2 gas up to 10 to 12 times greater than an empty container [26,27]. It is crucial to choose MOFs with high CO2 uptake at low pressures to facilitate an effective CO2 capture process for enclosed localities. Thus, MOFs provide a solid platform for many carbon capture processes. Table 4 below shows the list of abbreviations and acronyms used in this article.

> **Table 4.** List of the abbreviations and acronyms used in this review article.



**Table 4.** *Cont.*

#### **3. Carbon Dioxide (CO2) Capture Using Metal-Organic Frameworks**

Carbon dioxide capture and storage are now being vigorously investigated to simultaneously combat global climate change while producing more sustainable synthetic fuels to use for several purposes [28–33]. The key approaches for capturing CO2 using MOFs for the mitigation of the CO2 emissions resulting from fuel combustion power plants are pre-combustion capture, post-combustion capture, and oxy-fuel combustion capture. The CO2 capture selection method is mostly based on its advantages, disadvantages, and CO2 feed input conditions such as partial pressure and concentration of CO2 in the flue gas. Table 5 below shows a comparison between oxy-fuel combustion capture, pre-combustion capture, and post-combustion capture based on the advantages and disadvantages of each method.

Wang, et al. [34] mentioned that for newly-built power plants, oxy-fuel combustion could be adopted, whereas, for gasification plants, pre-combustion capture is the most appropriate carbon capture approach [34]. Moreover, post-combustion carbon capture is frequently favored for retrofitting power plants, since direct fuel combustion occurs in the boiler of all coal-fired power generation. Zou and Zhu [35] mentioned that the recognition ability of a CO2 porous adsorbent is usually evaluated by two main key factors: the adsorption capacity of CO2 and the selectivity of the material. The authors also mentioned that the ideal MOF materials with the high capturing ability of CO2 are expected to exhibit both high adsorption and high uptake for CO2 gas over other gases, like CH4 and N2 [35]. The effective CO2 capture ability of MOF materials is owing to their distinguishable chemical and structural features. These unique features include pore size, unsaturated or open metal sites, function control, polar functional groups into the pore channels, and alkylamine incorporation. Kang, et al. [36] stated that the gas separation process is one of the most challenging and critical steps for the industrial processes, and MOFs are potential candidates for this separation application [36]. Table 6 below shows a summary of MOF-based materials for CO2 capture.

**Table 5.** Advantages and disadvantages of oxy-fuel combustion capture, pre-combustion capture, and post-combustion capture.




Table 6 represents a summary of MOF based materials for CO2 capture and it is clearly seen that as the temperature increases, the adsorption capacity of CO2 decreases. Chen, Jin and Chen [62] mentioned in their study that the adsorption capacity and saturated adsorption capacity decreases

with the increase in adsorption temperature. They also added that an exponential function is the best function that describes the relationship between saturated adsorption capacity and temperature. Hence, to obtain a high CO2 adsorption capacity, the adsorption temperature should be low enough.

#### *3.1. Oxy-Fuel Combustion CO2 Capture*

Oxy-fuel combustion is the process at which hydrocarbon fuel is combusted in a nearly pure oxygen environment, as opposed to air. For controlling the temperature, oxygen is diluted in a portion of the flue gas rather than dilution in nitrogen. In a coal-fired power plant, the oxy-fuel combustion aims to produce flue gas that is enriched with CO2 and water vapor. This allows the separation or the capture of CO2 from the flue gas by using low-temperature desulfurization and dehydration processes [63]. Figure 3 below shows a block flow diagram of an oxy-combustion carbon capture system.

**Figure 3.** Block flow diagram of an oxy-fuel combustion carbon capture system.

Sumida, et al. [64] stated that oxy-fuel combustion refers to the combustion process in a nearly pure O2 environment of pulverized coal or other carbonaceous fuel. The significant merit of the oxy-fuel combustion process is based on the fact that the flue gas is almost entirely CO2. This eases the capture step, and also most of the existing power plants can be easily retrofitted with an oxy-fuel combustion system. Even though there are no full-scale plants that currently adopt oxy-fuel combustion, theoretical studies in combination with pilot-scale and laboratory studies have mentioned some operational issues and important design parameters that aid in studying the oxy-fuel combustion process [64]. The current carbon capture technologies include membranes and MOF-based adsorbents. Hu, et al. [65] mentioned that MOF, activated carbon, and Zeolite adsorbents are all physisorption-based and can be applied to oxyfuel, pre-combustion, post-combustion, and CO2 capture. The authors also mentioned that as a result of the weak interactions with CO2, activated carbon, and zeolite adsorbents are unlikely to be used for direct air capture.

On the other hand, MOFs can be tuned to undertake a strong interaction with CO2 making it suitable for direct air capture processes [65]. The nitrogen-free combustion atmosphere of the oxy-fuel combustion entails a flue gas with a high concentration of CO2 and water vapors for easier separation. The capture of CO2 is not required in oxy-fuel combustion since purification can be easily achieved by water vapors condensation. The MOFs' selectivity for other molecules is based on the polarizabilities, diversity of the competing molecules, and the differences between the quadruple moments. Consequently, MOFs have a poor selectivity for O2/N2 as a result of the similarity in their molecular nature. Thus, the application of MOFs in the oxy-fuel combustion process is restricted in the carbon capture processes [66].

#### *3.2. Pre-Combustion CO2 Capture*

Pre-combustion carbon capture technology involves capturing the carbon from the fuel before completing the combustion process [67]. A pre-treatment stage is conducted for the fuel, such as natural gas steam reforming, biomass, and coal gasification, before the actual combustion stage [68,69]. Syngas, which is a mixture of CO and H2, is produced in this pretreatment stage. By using the water gas shift (WGS) reaction, CO in the syngas is then reacted with steam to produce additional CO2 and H2 [63]. The separation of H2 and CO2 can then be achieved by various technologies. The pre-combustion technology has the merit of lower energy requirements; however, the efficiency and the temperature associated with the H2-rich gas turbine fuel is considered a big problem. Figure 4 below illustrates a block flow diagram of a pre-combustion carbon capture system.

**Figure 4.** Block flow diagram of pre-combustion carbon capture system.

Lea-Langton and Andrews [70] mentioned that the pre-combustion technology offers a lower-cost CCS since the combined-cycle gas turbine is the base power generator, which has an approximately 62% thermal efficiency in the latest technology plants compared to approximately 50% efficiency in the latest steam-cycle technology [70]. The addition of the MOFs to the pre-combustion processes aids in enhancing the separation of CO2 from the CO2/H2 mixture. Zhang, et al. [71] mentioned that MOFs are better than zeolites as adsorbents for the pre-combustion CO2/H2 separation because MOFs possess higher porosities and result in vast uptakes of CO2 at moderately high pressures [71]. Chung, et al. [72] discovered new MOFs adsorbent material that has a high CO2 working capacity, which might help in the reduction of CO2 emissions from the newly commissioned power plants that use the pre-combustion carbon capture technique. The authors have reported the in-silico discovery of high-performance adsorbents for the CO2 pre-combustion capture. A genetic algorithm was used by the authors to efficiently search for a large database of top candidates of MOFs. The MOFs with the highest performance that were identified from the in-silico search were then synthesized and activated. These MOFs in the study have shown a high CO2/H2 selectivity and a CO2 working capacity. The authors' also mentioned that one of the MOFs that they synthesized had shown the highest CO2 working capacity compared to all the MOFs reported in the literature under the same operating conditions [72]. Nandi, et al. [73] synthesized MOFs with high CO2/H2 selectivity that are suitable for the pre-combustion capture. The authors reported an ultra-microporous (3.5 and 4.8 Å pores) Ni-(4-pyridylcarboxylate) with a working capacity of (3.95 mmol/g), which makes it applicable for hydrogen purification under normal CO2 pre-combustion capture conditions. The MOFs reported in this study exhibited facile adsorption-desorption CO2 cycling and have a CO2 self-diffusivity of approximately 3 × 10−<sup>9</sup> m<sup>2</sup>/s, which is approximately double zeolite 13X [73]. Ding, et al. [74] stated that the usage of MOFs in the CO2 conversion and capture applications went through three main development stages. First, the CO2 selectivity and adsorption capacity of MOFs were tuned. Based on that, a large number of strategies were used and investigated for the enhancement of the MOF-CO2

interactions. The authors also mentioned that with time, many MOFs with strong interactions with CO2 were developed. Many researchers focused their attention on the applications of pre-combustion carbon capture for CO2 separation. The authors of the study mentioned that the MOF (Ni-4PyC) is considered as an ideal candidate for pre-combustion of CO2 capture since, at a very high pressure reaching 35 bar it adsorbed almost no H2 [74]. Herm, et al. [75] studied some selected MOFs with high structural flexibility, high surface area, or with open metal cation sites, for the utility in CO2 separation from H2 using pressure swing adsorption. The authors measured the single-component H2 and CO2 adsorption isotherms at a temperature of 313 K and pressures reaching 40 bar. For the pre-combustion CO2 capture and H2 purification, ideal adsorbed solution theory was employed by the authors in order to have a realistic estimation of the isotherms for the 80:20 and 60:40 H2/CO2 gas mixtures. The results of the authors' study have shown that the MOFs with high concentrations of the exposed metal cation sites, Cu-BTTri and Mg2 (dobdc), had significant improvements over the traditional commonly used adsorbents. Thus, those MOFs have promising applications in the CO2/H2 separations [75]. Asgari and Queen [76] stated that by considering the limited ability to tune the pore shape, pore size, and surface functionality of the zeolites and activated carbons, only fractional improvements in their separation efficiency can be achieved. On the other hand, the MOFs o ffer a record-breaking CO2 adsorption capacity in the pre-combustion CO2 capture pressure regime. In the meantime, NU-11 has the highest pressure CO2 adsorption with an absolute uptake of 856 cm<sup>3</sup> per gram of MOF at 25 ◦C and 30 bar. In addition, as mentioned by the authors, the MOFs facile structural tunability can allow significant improvements in the CO2 binding strength and hence increases the CO2 selectivity over H2. Also, the MOFs' surpassing internal surface area is considered a very important factor for high-pressure separations improvements [76]. Sumida, Rogow, Mason, McDonald, Bloch, Herm, Bae and Long [64] mentioned that MOF-based membranes are a promising strategy for pre-combustion CO2 capture. This is mainly because the pre-combustion gas mixture high pressure is an outstanding driving force for the membrane separation of H2 and CO2 [64].

#### *3.3. Post-Combustion CO2 Capture*

The post-combustion carbon capture (PCC) technology is from the most commonly used technologies in the carbon capture field, but there are certain drawbacks and limitations that govern its usage [77–81]. Zamarripa, Eslick, Matuszewski and Miller [79] mentioned that PCC is an expensive energy-intensive process that is subjected to a considerable number of researches using various types of technologies, including sorbents, solvents, and membranes [79]. In the post-combustion carbon capture technology, the carbon-based fuel first undergoes combustion before the separation of the CO2 from the flue gas is carried out [80]. A pre-treatment of the flue gas has to be conducted for the removal of all the corrosive substances and impurities. Since the flue gas temperature from the combustion units is more likely to be high, ranging from 120 ◦C to 180 ◦C, energy-intensive cooling systems are required before pretreatment [82]. Also, because of the high flue gas volume and low CO2 partial pressure, large size equipment is needed. Both aspects increase the capture cost considerably. Figure 5 below shows a block flow diagram of a post-combustion carbon capture system.

Hu, et al. [83] stated that the post-combustion capture of CO2 from the flue gas of a coal-fired power plants is a very important and critical approach because the CO2 emitted from fossil fuel combustion contributed to approximately 60% of total CO2 worldwide emission in 2004 [2]. Figueroa, et al. [84] mentioned that the flue gas emitted from post-combustion consists of approximately 75% N2 and 15% CO2, balanced by other impurities and moisture at ambient pressures (1 bar) and temperatures (30 ◦C) and pressures (1 bar) [84]. Consequently, an economically viable post-combustion CO2 capture approach should e fficiently separate the 15% CO2 from the 75% N2 with the lowest cost at ambient conditions without being a ffected by the moisture [83].

**Figure 5.** Block flow diagram of post-combustion carbon capture system.

Several studies have been conducted to investigate the efficiency of MOFs in the CO2 capture under post-combustion conditions. Martínez, et al. [85] investigated three distinct commercial amino-containing MOFs (MIL-53(Al), HKUST-1, and ZIF-8) in the adsorption of CO2 under post-combustion conditions. The authors' have modified these MOFs by wetness impregnation of tetraethylenepentamine (TEPA) molecules. The results of their study have shown that the amino-impregnated ZIF-8 samples have exhibited higher adsorption capacities by combining the chemical and physical adsorption of CO2 compared to the TEPA-impregnated microporous framework. Under CO2 post-combustion capture conditions, the TEPA-impregnated ZIF-8 samples went through a significant increase in the CO2 uptake reaching 104 mg CO2/gads, as a result of the moisture present, which governs the CO2 capture efficiency increase of amino groups [85]. Pai, et al. [86] studied five distinct diamine-appended MOFs that exhibit an S-shaped CO2 isotherm using a vacuum swing adsorption process in the post-combustion CO2 capture from dry flue gas. The authors' algorithm to maximize the CO2 recovery and purity has shown a linkage between the evacuation pressure, feed temperature, and the performance of the process. The MOFs that have achieved a target CO2 recovery ≥90% and purity ≥95%, namely, mmen-Mg2 (dobpdc), and mmen-Mn2 (dobpdc), were optimized by the authors to increase productivity and reduce parasitic energy. The authors also mentioned that the low affinity of N2 and the distinct shape of the CO2 isotherm were the main reasons for the lower energy consumption [86].

Hedin, et al. [87] stated that MOFs had been intensively studied as an efficient class of adsorbents for CO2 capture. Several studies mentioned by the authors have shown that certain members of this class of solid adsorbents can adsorb large quantities of CO2 while having a higher selectivity of CO2 over N2 for the post-combustion capture of CO2 from flue gas [87]. Furthermore, Samanta, et al. [88] stated that MOFs as a sorbent for the post-combustion capture of CO2 are expected to have a remarkable and significant adsorption capacity; however, they require substantial, intensive research efforts to be applicable under flue gas conditions. Also, they mentioned that the CO2/N2 selectivity further limits the usage of these sorbents for CO2 adsorption [88]. Maurya and Singh [89] comparatively studied some water-stable microporous adsorbents for post-combustion CO2 capture. They investigated three metal-organic frameworks (MOFs), a single-wall carbon nanotube (SWCNT) and two covalent organic frameworks (COFs), and a single-wall carbon nanotube (SWCNT) under equal flue gas conditions. The results of the simulation made by the authors depicted that the pure component CO2 adsorption capacity followed this descending order SWCNT > InOF-1 > COF-300 > UiO-66 > COF-108 > ZIF-8 under post-combustion conditions [89]. Babarao and Jiang [90] reported a computational study for the characterization of the cation and the capture of CO2 in the Li<sup>+</sup>-exchanged metal−organic frameworks (Li+-MOFs). The authors' have adopted a density functional theory for the cation locations

optimization and the evaluation of the atomic charges, and the molecular simulation used to investigate the separation of CO2/H2 and CO2/N2 gas mixtures for the pre-combustion and post-combustion CO2 capture. The results of the authors' study show that at ambient conditions, the selectivity is around 60 for CO2/N2 mixture and 550 for CO2/H2 mixture, higher than the selectivities in other nano-porous adsorbents and the non-ionic MOFs. They also mentioned that the charge of the cations and framework have a remarkable impact on the selectivity, that was found to decrease by a magnitude of 1 order by switching off the charges. Also, the Li<sup>+</sup>-MOF cations' hydration leads to a reduction in the free volume, leading to a lower adsorption extent [90]. Further studies have focused their research on the rule of photoresponsive MOFs in the CO2 capture under post-combustion conditions. Park, et al. [91] proposed a new photoresponsive MOF, namely Mg-IRMOF-74-III structure with azopyridine molecules bonded to its unsaturated metal sites for CO2 capture. The authors' computational simulations showed that the photochemical MOF had induced the trans-to-cis transition of the material leading to a remarkable alteration in the capacity of CO2. Their work aimed to provide a blueprint for the computational design of the new photoresponsive MOF before the actual experimental synthesis [91]. Wang, et al. [92] also mentioned that MOFs, with their distinguishable characteristics and their fine-tunable structures, are exceptional porous solid materials that can provide many powerful and efficient platforms for the exploration of high-performance adsorbents for the CO2 post-combustion capture [92].

Marti [93] mentioned that the post-combustion CO2 capture technology is currently the most utilized method for power production. The CCS targets for the post-combustion CO2 capture are to achieve 90% CO2 capture with less than 20% increase in the electricity cost. This financially translates to a CO2 separation and compression cost of \$30–50 per ton of CO2 [93]. Moreover, a very remarkable study has synthesized MOFs with no N2 adsorption. In this study, Hu, et al. [94] post-synthetically tethered distinct alkylamine molecules to the unsaturated Cr (III) centers in the MOF MIL-101 for post-combustion CO2 capture. The resulting MOFs of their study showed almost no N2 adsorption with a remarkably increased CO2 capture as a result of the interaction between CO2 molecules and amine groups under ambient conditions. The authors stated that MIL-101-diethylenetriamine extraordinary CO2 uptake, exceptional stability, very high CO2/N2 selectivity, and the mild regeneration energy, makes it very promising for CO2/N2 separation and post-combustion CO2 capture [94].

#### *3.4. MOFs as Filler in Mixed-Matrix Membranes for CO2 Separation*

Membrane separation is one of the most efficient and commonly used techniques in the CCS field. However, it is governed by certain limitations, including selectivity, permeability, pore size, fouling, and high cost of the membranes [95,96]. Most of the time, selectivity and permeability of the membranes are the main drawbacks to the efficiency of the membranes. Certain porous materials like MOFs, zeolites [97–99], and activated carbons are incorporated in the membranes to increase its CO2 separation efficiency. Figure 6 below describes how a MOF-based mixed matrix membrane (MMM) is used for CO2 capture. Figure 6 shows that the feed gas stream containing CO2 and CH4 moves across a MOF-based mixed matrix membrane surface that has a selective permeability to the CO2 gas over CH4 gas. The CO2 gas molecules diffuse through the membrane's pores via the solution-diffusion mechanism forming the permeate. Moreover, the CH4 gas molecules that did not diffuse through the membrane pores form the retentate. The MOF filler enhances the selectivity and permeability of the membrane, allowing more CO2 molecules to diffuse through the pores of the membrane.

**Figure 6.** MOF-based mixed matrix membrane (MMM) for CO2 capture.

With regard to the incorporation of MOFs into the membranes for carbon capture, reference [100] described a thin film composite (TFC) membrane that incorporates MOF nanoparticles and a polymer of intrinsic microporosity (PIM-1) for post-combustion CO2 capture. The novel TFC membrane design used by the author consists of three layers; the first layer is a CO2 selective layer synthesized of the mixed matrix PIM-1@MOF. The second layer is an ultra-permeable PDMxS gutter layer that is doped with MOF nanosheets. Moreover, the third layer has a porous polymeric substrate. The results of the study show that the PDMS@MOF gutter layer incorporated with amorphous nanosheets provides a 10,000–11,000 gas permeance units (GPU) permeance of CO2 permeance. The authors' owes the high CO2 permeance to the less gas transport resistance compared to the pristine PDMS gutter layers. Furthermore, TFC membrane assembly resulting from a nanosized MOF particles blend (NH2-UiO-66 and MOF-74-Ni) into PIM-1 improved the permeation of CO2 4660–7460 GPU and the selectivity of CO2/N2 of 26–33, in comparison to the pristine PIM-1 counterpart with aCO2 permeance of 4320 GPU and a CO2/N2 selectivity of 19. This enhancement in the CO2 separation after the incorporation of MOFs strongly suggests that MOFs' incorporation into the mixed matrix membranes improves CO2 separation [100]. Sun, et al. [101] introduced a novel metal-organic framework MOF-801 nanocrystal into a polyether-block-amide (PEBA) polymer in order to synthesize a new mixed-matrix material for the separation of CO2. The author found that the uniform incorporation of the MOF-801 microporous with preferential adsorption of CO2 provided selective and fast transport channels for CO2 over N2, leading to an increase in both the CO2/N2 mixed-gas selectivity and the CO2 permeance compared with the pure PEBA membrane. Based on the study, the MOF-801/PEBA optimized mixed-matrix composite membrane has shown a highly stabilized separation performance with CO2/N2 selectivity of 66 and CO2 permeance of 22.4 GPU under a mixed-gas permeation test. This shows grea<sup>t</sup> potential for CO2 separation and capture [101]. Chen, et al. [102] studied the CO2 separation performance of a new type of MMM with a microporous filler of MOF-801 and a polymer matrix of PIM-1. The author's experimental results show that the CO2-philic MOF-801 filler uniform dispersion provided a channel for a fast, selective CO2 transport; hence, the MOF-801/PIM-1 MMMs show a greater CO2 permeability and CO2/N2 ideal selectivity over the pure polymer membrane [102]. Majumdar, et al. [103] synthesized Mg-MOF-74 crystals for the preparation of a polymer/Mg-MOF-74 MMMs for the separation of CO2/CH4 gas mixture. Activation temperature and time of the Mg-MOF-74 crystals were determined to

enhance the performance of the polymer/Mg-MOF-74 mixed matrix. The authors used a solvent-casting method to incorporate the Mg-MOF-74 crystals into the polyvinyl acetate (PVAc) matrix to form a dense MMM. The results showed that the MMM's mixed gas permeability measurements improved in both the permeability and the selectivity of CO2 with the increase in the amount of MOF. This suggests a strong CO2 adsorption selectivity of Mg-MOF-74. Also, the authors' have found that the incorporation of Mg-MOF-74 has reduced the e ffect of plasticization [103]. Ahmad, et al. [104] investigated the gas separation properties of the MMMs 6FDA-DAM with three types of zirconium-based MOFs nanoparticles (MOF NPs, ca. 40 nm) up to 20 bar. The authors investigated the separation of CO2/CH4 at high feed pressure with di fferent CO2 concentrations in the feed in a temperature range of 35–55 ◦C. The results of the study show that incorporating Zr-MOFs in the 6FDA-DAM MMMs increased both the CO2 permeability and the CO2/CH4 selectivity of this polymer. This study suggests that 6FDA-DAM Zr-MOF MMMs have grea<sup>t</sup> potential in the carbon capture process [104].

The MMMs that are derived from MOF nanocrystals represent a promising alternative for overcoming the trade-o ff between selectivity and permeability of the pristine polymeric membrane. Chen, et al. [105] incorporated CO2-philic KAUST-nanocrystals into 6FDA-durene polyimide membrane. This incorporation made by the author has increased the permeability and the selectivity of the MMMs. The developed MMMs in this study has a promising application in the CO2 capture from natural gas and biogas [105]. Jiamjirangkul, et al. [106] mentioned that for the development of metal-organic frameworks (MOFs) nanofibrous membranes, chitosan nanofibers are a very promising template because of their high surface area with the presence of functional groups for the cationic/anionic binding. In their study, Cu-BTC-integrated chitosan/PVA nanofibrous membrane (Cu-BTC/CNFs) hybrids were synthesized. The CNFs/Cu/BTC-3 synthesized by the authors show an adsorption capacity of CO2/N2 over 14 times. Hence, the membrane has a grea<sup>t</sup> potential for selective capture and filtration of CO2 [106]. Lee, et al. [107] prepared a Ni-MOF-74 continuous and defect-free membrane on α-alumina support by using the technique of a layer-by-layer seeding followed by a secondary growth crystallization. The gas permeation properties of the membranes were investigated by the authors for small gases, including CO2, CH4, H2, and N2. The results of their study showed that the Ni-MOF-74 membrane exhibited a stronger adsorption a ffinity to CO2 compared to the other gases, and hence the Ni-MOF-74 membrane CO2 permeation was dominated by the surface di ffusion [107].

There is increasing attention nowadays to MMMs comprised of inorganic fillers scattered in an organic matrix for the separation of gas mixtures due to the membrane enhancement in the material robustness, separation selectivity, and throughput. Anastasiou, et al. [108] developed ZIF-8/graphene oxide (GO) hybrid nanofillers and ZIF-8 MOFs and incorporated them into a polysulfone (PSF) matrix. The authors then tested the membranes for their selectivity and permeation properties for CO2, CH4, and N2. The results highlighted that the PSF+ (ZIF-8/GO) MMMs showed an enhancement in the CO2 permeability (up to 87% increase) and the selectivity of the CO2/CH4 pair (up to 61% increase), compared to the pristine PSF membrane. Also, the selectivity of the PSF+ (ZIF-8/GO) MMM was increased up to 7-fold compared to the PSF + ZIF-8 MMM selectivity. Based on the results, the composite fillers that combine MOFs and the GO functionality have a grea<sup>t</sup> potential in boosting and tuning the performance of the polymeric membranes for CO2 separation from flue gas and natural gas [108]. Inorganic fillers can mainly define the optimal performance of the MMMs. Hence, the development and identification of new inorganic fillers are critical for optimizing the MMMs' performance. MMMs incorporated with MOF fillers are extensively investigated. However, MOF fillers with high performance remain scarce and in high demand. Yin, et al. [109] combined for the first time, an emerging MOF that has an exceptional physicochemical property, namely MUF-15 (Massey University Framework-15), into a MMM with PIM-1. As mentioned by the authors, based on the MUF-15 intrinsic ability to discriminate distinct gues<sup>t</sup> molecules, MUF-15 is considered an impressive filler that delivers the MMMs with excellent CO2 separation property. Hence, MUF-15 can be proposed as a strategy to further increase the MMM performance [109].

The enhancement of MOF membranes' CO2 separation performance is attracting the attention of several researchers. Wu, et al. [110] reported a versatile post-modification strategy based on polydopamine (PDA) grafting for the improvement of the MOF membranes' CO2 separation performance. The PDA was deposited by the authors' on the UiO-66 membrane via a simple and mild process. The results of their study show that the modified PDA/UiO-66 membrane exhibited an enhancement in the CO2/CH4 and CO2/N2 and selectivities of 28.9 and 51.6, respectively. These selectivity results were 2 to 3 times greater than the MOF membranes that are reported with similar permeance. Moreover, under moist conditions and in the 36 h measurement period, the PDA/UiO-66 membrane prepared in their study exhibited superb long-term stability for the capture of CO2 [110].

The search for e ffective carbon-capture materials has allowed the disclosure and institution of nanoporous fluorinated MOFs with a contracted pore system as a CO2-selective benchmark adsorbent. Chernikova, et al. [111] transplanted/integrated SIFSIX-3-M (M = Cu, Zn, and Ni) MOF adsorbent that encompasses a fluorine moieties with the periodic arrangemen<sup>t</sup> in a one-dimensional confined channel, showing a remarkable CO2 adsorption-based selectivity over H2 and CH4 and several other industrially related gas mixtures for carbon capture. The single and mixed-gas permeation tests made by the authors showed that the nanoporous MOF membrane is a highly CO2-selective membrane that shows a greater CO2-selectivity over H2, and CH4 is limited by the selective adsorption of CO2 in the SIFSIX-3-M functional and contracted channels [111]. The fabrication and design of novel MMMs with a simultaneously enhanced gas selectivity and permeability are greatly demanded by the industries as membrane technology for large-scale CO2 capture and storage. Traditional fillers consisting of isotropic bulky particles often limit the interfacial compatibility leading to a grea<sup>t</sup> loss in the MMMs' selectivity. Cheng, et al. [112] incorporated chemically stable MOF nanosheets into a highly permeable polymer matrix to synthesize defect-free MMMs. The authors homogeneously dispersed the MOF nanosheets within the polymer matrix, owing to their high aspect ratios that enhances the integration of the polymer-filler. The MMMs prepared by the authors showed a high selective separation performance for CO2, good antiaging, and anti-pressure abilities, thereby o ffering a new strategy in the development of advanced membranes for the industrial gas separation applications [112].

The e fficient separation of CO2 from CO2/CH4 mixtures with membranes has economic, environmental, and industrial importance. Membrane technologies are currently dominated by polymers due to their processing abilities and low manufacturing costs. However, polymeric membranes su ffer from either low gas permeabilities or low selectivities. MOFs are suggested as potential membrane candidates that o ffer both high selectivity and permeability for CO2/CH4 separation. Experimental testing of every single synthesized MOF material as a membrane is not practical due to the availability of thousands of di fferent MOF materials. Altintas and Keskin [113] used a multilevel, high-throughput computational screening methodology to examine the MOF database for membrane-based CO2/CH4 separation. MOF membranes o ffering the best combination of CO2 permeability (>10<sup>6</sup> Barrer) and CO2/CH4 selectivity (>80) were identified by combining grand canonical Monte Carlo and molecular dynamics simulations. The results revealed that the best MOF membranes are located above the Robeson's upper bound, indicating that they outperform polymeric membranes for CO2/CH4 separation. The impact of framework flexibility on the membrane properties of the selected top MOFs was studied by comparing the results of rigid and flexible molecular simulations.

The relationship between the structure of the MOFs and the performance was also investigated to provide atomic-level insights into the design of novel MOFs, which will be useful for CO2/CH4 separation processes. Prediction of permeabilities and selectivities of the MMM found the best MOF candidates to incorporate as filler particles into polymers, and it was found that MOF-based MMMs have significantly higher CO2 permeabilities and moderately higher selectivities than pure polymers [113]. Light-responsive metal-organic frameworks are attracting special attention for their use as a filler in MMMs for CO2 capture. Prasetya and Ladewig [114] synthesized a new generation-2 light-responsive MOF by using Zn as the metal source and both 1,4-diazabicyclo [2.2.2] octane (DABCO) and 2-phenyldiazenyl terephthalic acid as the ligands. The results showed that Zn-azo-dabco MOF

(Azo-DMOF-1) have exhibited photoresponsive adsorption of CO2 in both a static and dynamic conditions; this is owed to the abundance of azobenzene functionalities from the ligand. The authors' have also incorporated the MOF as a filler in a mixed matrix membrane with PIM-1 as the polymer matrix and evaluated MMM separation performance for CO2/N2 gas mixture. The results of their study showed that azo-DMOF-1 might increase the pristine polymer permeability and selectivity of CO2. Also, the azo-DMOF-1–PIM-1 composite membranes have a good performance, which has surpassed the 2008 Robeson Upper Bound [114]. Benzaqui, et al. [115] used a microporous Al trimesate-based MOF, namely MIL-96-(Al), as a porous hybrid filler in MMMs for the post-combustion separation of CO2/N2. The homogeneous and defect-free MMMs with a high MOF loading (up to 25 wt%) synthesized by the authors have super passed the pure polymer membranes for CO2/N2 separation [115]. Maina, et al. [116] reported a new route for the synthesis of hybrid membranes containing inorganic nanoparticles and MOFs with potential applications in several areas, including catalysis, separation, electrochemical, and sensing applications was reported [116]. Zhao, et al. [117] studied at di fferent temperatures, feed compositions and feed pressures, the separation and permeation properties of CO2/N2 and CO2/H2 mixtures for thin high-quality MOF-5 membranes synthesized by the secondary growth method. The MOF-5 membranes synthesized by the authors under the experimental conditions were studied to o ffer a selective permeation for CO2 over N2 and H2 in CO2/N2 and CO2/H2 feed mixture. The results showed that the MOF-5 membranes exhibit high permeance and separation for CO2. It was mentioned by the authors that the sharp increase in the MOF-5 membranes separation factor with the increase in the feed pressure is an unobserved phenomenon for other inorganic microporous membranes [117]. Hu, et al. [118] mentioned that as a result of the MOFs' functionality, easily tunable porosity, and morphology, they are regarded as an ideal filler for MMMs. Fan, et al. [119] incorporated two isomorphous MOFs Ni2(l-asp)2pz) and (Ni2(l-asp)2bipy with di fferent pore sizes into a poly(ether-block-amide) (Pebax-1657) to synthesize MMMs with gas permeation properties for CO2, N2, H2, and CH4. The results of their study show that the two series of MMMs showed an enhanced CO2/H2 selectivity and CO2 permeability compared to the pure polymer membrane. Ni2 (l-asp)2bipy@Pebax-20 have shown the highest CO2 permeation property in the study of 120.2 barrers with an enhanced CO2/H2 selectivity of 32.88 compared to the pure polymer membrane of, respectively, 55.85 barrers and 1.729. This study shows that the synthesized MMMs with MOF fillers are remarkable candidates for the future applications in CO2 capturing [119].

#### *3.5. MOFs in Photo-Catalytic Conversion of CO2*

In the road toward a sustainable low-carbon environment, aside from the physical capture and the underground injection and geologic sequestration of the anthropogenic CO2 emitted from the power plants or industrial processes, catalytic chemical conversion of CO2 into less harmful valuable chemicals is a very e fficient way to undertake the carbon capture process [120–122]. Moreover, CO2 molecules are very stable as a result of the C=O interactions; multistep reduction via photochemical or electrochemical methods are more imperative than water splitting reactions and have various technical implications. The CO2 reduction reaction can occur in several distinct pathways that yield a varied range of reduction products, including carbon monoxide (CO) [123–127], methanol (CH3OH) [128–135], methane (CH4) [136], ethylene (C2H4) [137,138], formic acid (HCOOH) [139], and others [140,141]. Hence, the target product needed is the factor that governs the overall design process for the CO2 reduction reaction. MOFs have grea<sup>t</sup> application as catalysts in the catalytic conversion reactions of CO2,includingconversionofCO2tofuels,hydrogenation,cycloaddition,andphoto-reductionofCO2.

The photo-reduction of CO2 occurs in the presence of ultraviolet (UV) and visible light irradiation. Depending on the reduction potentials, the CO2 reduction reaction products can include HCHO, CO, CH3OH, and CH4. Table 7 below provides the photo-reduction potentials for the CO2 reduction reaction.


**Table 7.** [142]: Standard photo-reduction potentials for the CO2 reduction reaction.

The photocatalytic CO2 reduction process comprises a series of reactions including adsorption of CO2, charge carrier separation and transportation, electron-hole pair photo-generation, and chemical reactions between the charge carriers and surface species [143–145]. However, certain photo-induced electrons on the surface of the catalyst are specifically utilized for CO2 reduction. Consequently, catalysts with high redox potential and relatively low bandgap value are favorable. The photo-induced activation of CO2 on the surface of MOFs includes some main steps. The catalytic material first adsorbs a photon leading to electron-hole pair separation. This separation excites a negative electron (e−) from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), forming a positive hole (H+) on the HOMO [146]. The CO2 molecules are then absorbed on the MOFs' catalytic center and accept electrons forming different products such as CH4, CO, and HCOOH. The mechanism of the CO2 photo-reduction process on MOFs is shown in Figure 7. Nevertheless, not all MOFs exhibit photocatalytic activity, and their electronic properties are identical. This may be determined based on the HOMO and LUMO of MOF materials.

**Figure 7.** CO2 photo-reduction process mechanism on MOFs.

The construction of MOF-based photo- and electro-catalysts is very promising as a result of their considerable flexible structures and active sites. Reddy, et al. [147] mentioned that MOFs are an emerging new class of functional materials with a highly porous structure, exceptional specific surface areas, and tunable surface chemistry; hence, they hold grea<sup>t</sup> potential as photocatalysts [147]. Wang, et al. [148] stated that the photocatalytic reduction of CO2 for valuable chemicals is an attractive way to create a better overall environment. The authors have proposed two distinct conversion processes. In the first process, CO2 is split into CO, and in the second process, CO2 is converted into organic chemicals (like CH3OH, CH4, and HCOOH) [148]. It is clear that MOFs have a bright future, prospect, and applications in the field of CO2 photocatalytic reduction. Table 8 below shows a summary of MOF-based catalysts for photocatalytic CO2 reduction.


**Table 8.** Summary of MOF-based catalysts for photocatalytic CO2 reduction.

a Average after 4-h operation. b (5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin. c Production in (ppm/gcat) after

1-h operation. d Production in (mmol/molcat) after 10 h operation. e Production in (mmol/L g) after 6-h operation. f Production in (mmol after) 0.5-h operation.

By looking at Table 8 above, which summarizes MOF-based catalysts for photocatalytic CO2 reduction, the highest and lowest yields for CO are produced by CdS/UiO-bpy/Co and TiO2, yielding 235 and 2.25 μ mol/g h, respectively. In addition, the highest and lowest yields for CH3OH are produced by TEOA and Zn2GeO4/ZIF-8 yielding 262.6, and 0.22 μ mol/g h, respectively. Furthermore, the highest and lowest yield for HCOO is produced by TEOA and NH2-UiO-66(Zr), yielding 685.6 and 3.4 μ mol/g h, respectively. These results prove that MOFs are successfully capable of photo catalytically, reducing CO2 into other useful products.

Photocatalytic reduction of CO2 into highly valuable added chemicals via clean, renewable solar energy sources is a remarkable pathway to address the environmental and energy issues. In recent days, MOFs have been exploited intensively as catalysts for the photocatalytic reduction of CO2 owing to their extinguishable CO2 capture abilities, photochemical and structural properties. Li, et al. [168] studied the recent progress made in the MOF-based photocatalysts for the reduction of CO2 on the basis of the products reduced, including the CO2 photocatalytic conversion into CO and other organic chemicals (methanol, formic acid, and methane). The authors' have also mentioned several modification techniques for the relevant improvements in the photocatalytic performance and the structural activity-corresponding relationships. The authors have mainly focused on the CO2 capture capacity role for the CO2 photocatalytic reduction performance over the MOF-based materials [168]. Li, et al. [169] constructed a MOF that incorporates unsaturated metal sites and accessible nitrogen-rich groups by using a solvothermal assembly of an acylamide-containing Cu(II) ions and tetracarboxylate ligand. The results showed that the MOFs synthesized had a high CO2-adsorbing capability, and high porosity exposed Lewis acid metal sites. The inherent structural features of the MOFs synthesized in the study makes them very promising candidates as heterogeneous catalysts for the chemical conversion of CO2; this was confirmed by their highly e fficient CO2 cycloaddition with the small-sized epoxides in the study.

The high e fficiency and remarkable size selectivity on the CO2 catalytic conversion allows the synthesized MOFs in the study to act as an advanced heterogeneous catalyst for the carbon fixation process [169]. Ding, et al. [170] presented an in-situ-growth strategy and a facile double solvent to integrate CdS NPs with MIL-101(Cr) to synthesize CdS/MIL-101(Cr) composite photocatalyst. The results show that under visible light irradiation, the CdS/MIL-101(Cr) exhibited a remarkable activity enhancement for the conversion of CO2 to CO [170]. Mu, Zhu, Li, Zhang, Su, Lian, Qi, Deng, Zhang, Wang, Zhu and Peng [144] assembled a composite thin film comprised of 2D/2D MOF/rGO heterostructures by using a Coulomb interaction for the use as a co-catalyst for the photocatalytic reduction of CO2 for the first time. The study results showed that the best thin-film catalyst with an optimal MOF/rGO ratio exhibited a high evolution rate of CO 3.8 × 10<sup>4</sup> μmol h−<sup>1</sup> gfilm<sup>−</sup><sup>1</sup> (0.46 min−<sup>1</sup> in TOF) and an acceptable selectivity of 91.74% [144]. Li and Zhu [171] focused their work on the MOF-based materials active sites that achieve e fficient charge separation for conductivity in the electrocatalytic CO2 reduction and visible-light absorption for photocatalytic CO2 reduction. The authors showed the distinguishable characteristics of the MOF-based material for catalytic CO2 reduction, the recent progress and development in the MOF-based material for the CO2 catalytic reduction, and the challenges and future perspectives for the development of MOF-based materials for CO2 reduction [171].

One of the most e ffective techniques in the preparation of the catalysts for the CO2 photocatalytic reduction into high value-added chemicals is by the use of metalloporphyrin as a light-harvesting mixed ligand to optimize the MOF. This method is valuable because it can improve the dispersibility of the prophyrin, thus inhibiting its potential agglomeration. Wang, et al. [172] incorporated through coordination mode a one-pot synthetic strategy to immobilize chemically Cu (II) tetra (4-carboxylphenyl) porphyrin (CuTCPP) into a UiO-66 MOF structure. Also, in-situ growth of the TiO2 nanoparticles onto the MOF was actualized with the composite's generation of the CuTCPP ⊂UiO-66/TiO2 (CTU/TiO2). The results showed that the catalytic results represented an optimal value of 31.32 μmol g<sup>−</sup><sup>1</sup> h−<sup>1</sup> CO evolution amount, which was approximately 7 times greater than that of the pure TiO2 obtained using photo-catalysis under Xe lamp irradiation (λ > 300 nm) Wang, Jin, Duan, She, Huang and Wang [172]. Dong, et al. [173] improved the CO2 conversion activity of a synthesized MOF by the regulation of the metal species in the MOFs metal-cluster nodes, and the MOF's best photocatalytic activity for CO2 conversion was obtained. The authors' synthesized a stable MOF, PCN-250-Fe3 with open metal sites and Fe2IIIFeII metal-cluster nodes, and they further improved the CO2 reduction activity by tuning the MII metal ions species in the cluster. The results of their study showed that all bi-metallic PCN-250-Fe2M (M = Mn, Ni, Zn, Co) exhibited higher catalytic activity and selectivity for stable reduction of CO2 into CO, compared to the mono-metallic PCN-250-Fe3. Further investigations revealed that introducing a second MII metal ions can enhance the migration of the photogenerated

electrons to the active sites and enhance the CO2 activation and adsorption by favoring the rout of CO2 reduction and limiting the production of hydrogen as an intermediate [173].

CO2 conversion into clean energy by using photocatalysts with a porous hollow structure that has a superb activity is of worldwide interest. Chen, et al. [174] prepared a porous hollow spheres ZnO/NiO with sheet-like subunits by calcination of Ni–Zn bimetallic organic frameworks. The prepared ZnO/NiO composites showed an improved photocatalytic activity for the CO2 recondition and an outstanding photocatalytic CO2 reduction performance [174]. Ye, Gao, Cao, Chen, Yao, Hou and Sun [167] synthesized and used an ultrathin two-dimensional Zn porphyrin-based metal-organic framework (Zn-MOF nanosheets) in the photoreduction of CO2 to CO. The two novelty noble-metal-free hybrid photocatalytic systems displayed outstanding selectivity and photocatalytic activity for CO emissions under mild photocatalytic reaction conditions. These studies highlight that the development of noble-metal-free photocatalytic systems and MOF-based materials for photocatalytic applications are promising [167]. Crake, et al. [175] effectively coupled under ultraviolet–visible (UV–vis) light irradiation TiO2 nanosheets and metal-organic framework (NH2-UiO-66) using an *in-situ* growth strategy for the formation of bifunctional materials for the combined photocatalytic reduction and capture of CO2. The results of their study showed that the nanocomposites were durable and dramatically more efficient in the reduction of CO2 to CO than their single components. Furthermore, the photocatalytic activity was significantly altered by the composition of the nanocomposites with the optimal TiO2 content doubling the evolution rate of CO compared to the pure TiO2 [175].

#### *3.6. MOF-Based Materials for Electrochemical and Electrocatalytic Conversion of CO2*

Electrochemical and electrocatalytic reduction of CO2 into hydrocarbons and value-added chemicals is a remarkable and clean way to mitigate greenhouse gas emissions as a result of our over-dependence on fossil fuels. The electrocatalytic CO2 reduction reaction consists of two half-reactions that can occur by two to fourteen-electron exchange process. These reactions are shown in Table 9, along with various standard electrode potentials vs. the standard hydrogen electrode (SHE).


**Table 9.** Standard electrochemical potentials for CO2 reduction [142].

Ma, et al. [176] employed Zn–Ni bimetal MOFs as precursors for the synthesis of Ni-N-doped porous interconnected carbon (NiNPIC) catalysts to enhance CO2RR electrocatalytic activity and selectivity. The interconnected porous structures and the high surface area of the catalyst have provided a convenient channel for mass diffusion and highly accessible Ni-N sites that lead to a higher electron transfer, less interface resistance, and greater electrolyte/gas transport in the CO2RR. The results demonstrated that the synthesized catalyst has a high conversion efficiency of CO2 into CO and excellent electrochemical stability at a moderate over-potential [176]. Table 10 below shows a summary of MOF-based catalysts for electrocatalytic CO2 reduction.


**Table 10.** Summary of MOF-based catalysts for electrocatalytic CO2 reduction.

1 Reversible Hydrogen Electrode.

By looking at Table 10 above, CO2 is mostly electro-catalytically reduced into CO. Furthermore, ZIF-CNT-FA-p shows the greatest FE (%) with a value of 100 for CO. On the other hand, the lowest FE (%) for CO is given by ZIF-8, with a value of 65.

The Cu-based catalysts exhibit distinguishable superiorities; however, achieving high selectivity for hydrocarbon is still a grea<sup>t</sup> challenge. Tan, et al. [187] reported a multifunction-coupled Cu–MOF tailor-made electrocatalyst by using time-resolved controllable restructuration from Cu2O to Cu2O@Cu-MOF. The restructured electrocatalyst from their study had a high CO2 adsorption capacity and electrocatalytic activity. The results showed that their synthesized MOF exhibited high performance towards hydrocarbons, with a hydrocarbon Faradaic efficiency (FE) of 79.4% [187]. Li, et al. [188] reported a remarkable 2D bismuth metal–organic framework (Bi-MOF) that exhibits an accessible permanent porosity for a highly efficient CO2 electrochemical reduction (ECR) to HCOOH. The results of the author's study show that the 2D Bi-MOF open-framework structure synthesized shows excellent Faradaic efficiency for the formation of HCOOH over a large potential window, reaching 92.2% at approximately −0.9 V [188].

In sustainable energy research, it is well known that metallic copper acts as an electrocatalyst for the CO2 reduction to multicarbon products like hydrocarbons and alcohols. However, a grea<sup>t</sup> challenge remains in the development of a selective, cost-effective, and stable catalyst/electrode material for this reduction reaction. Rayer, et al. [189] studied the potentials of copper carbonized MOF-derived electrocatalysts as catalytic materials for CO2 electrochemical reduction. The author used two copper-decorated commercial MOFs, PCN-62, and HKUST-1, pyrolyzed at a variable temperature range (400 to 800 ◦C), were coated on both copper and metallic nickel supports as inks. The authors' study shows that the MOF-derived coatings produce electrodes with higher selectivity and current density towards isopropanol compared with the uncoated copper electrodes. Also, the best-performing electrocatalyst in their study exhibits an isopropanol Faradaic efficiency (FE) of over 72% [189]. Zhang, et al. [190] synthesized a novel mixed-metallic MOF [Ag4Co2 (pyz)PDC4][Ag2Co(pyz)2PDC2] and transformed it into an Ag-doped Co3O4 catalyst that exhibits excellent electrocatalytic performance for CO2 reduction in water to syngas (H2 + CO). The as-prepared Ag/Co3O4 material showed a high CO selectivity in a 0.1 M KHCO3 aqueous solution (CO2 saturated) with an approximately 55.6% corresponding Faradaic efficiency. The results showed that the presence of Ag can increase the efficiency of CO greatly, hence inhibiting the H2 production [190]. Cao, et al. [191] used a nitrogen-rich Cu–BTT MOF as a catalyst for the electrochemical reduction of CO2. The results showed that the high-temperature pyrolysis product of Cu–N–C1100 has the best catalytic activity for production of CO and HCOOH [191]. Sun, et al. [192] synthesized nitrogen-doped mesoporous carbon nanoparticles that have atomically dispersed iron sites (namely mesoNC-Fe) using high-temperature pyrolysis of a Fe that contains ZIF-8 MOF. The hydrolysis of tetramethyl orthosilicate (TMOS) in the MOF framework made by the author prior to pyrolysis has a fundamental role in the maintenance of a high surface area in the formation phase of the carbon structure, impeding the iron (oxide) nanoparticles' formation.

The results showed that the combination of such a distinguishable coordination environment that has a high surface area in the mesoNC-Fe carbon structure makes more accessible active sites during catalysis and promotes CO2 electro-reduction [192].

Transforming CO2 into a broad range of chemicals, including methanol, is a high priority field of study owing to the direct link between CO2 emissions and global warming. There is an environmental and industrial need for substituting non-renewable energy fuels with renewable and sustainable energy sources. Electrochemical reduction acts as a superb approach in the conversion of CO2 to methanol by the employment of alternative energy sources at which an electrocatalyst plays a fundamental role. Many e fforts are being made by several researchers to understand and increase the catalytic efficiency of electrocatalysts. MOFs, composite materials, and metal oxide are employed for CO2 electrochemical reduction to methanol. However, MOFs catch most of the researchers' attention in CO2 conversion as a result of their high surface area, simplicity, and exquisite structural features. In recent decades, there have been significant applications of MOFs and their derivatives in CO2 reduction. Al-Rowaili, et al. [193] focused their work on the electro-reduction of CO2 to methanol by coalescing MOFs' vantages, and their composite materials. The authors highlighted the challenges in achieving CO2 electro-reduction with high e fficiency and selectivity [193]. Dong, et al. [194] introduced a highly stable 3D porphyrin-based MOF of PCN-222(Fe) into a heterogeneous catalysis by using a simple dip-coating method. Their study shows that their composite catalyst PCN-222(Fe)/C exhibited a high catalytic performance for the electrochemical CO2 conversion to CO with an overpotential of 494 mV and a maximum of 91% FECO in a CO2-saturated aqueous solution of 0.5 M KHCO3 [194].

Hod, et al. [195] demonstrated that MOF material thin-film electrophoretic deposition is an effective method for immobilizing the required quantity of a catalyst. The authors used in their study for electrocatalytic CO2 reduction a material that consists of functionalized Fe-porphyrins as catalytically competent, redox-conductive linkers. Their method yielded an electrochemically addressable catalytic site with a highly e ffective surface coverage. The chemical products of their reduction contain mixtures of CO and H2. These results show that the MOFs are very promising as catalysts for electrochemical reactions [195]. Wang, et al. [196] synthesized a nitrogen-doped carbon via the pyrolysis of a well-known MOF, namely ZIF-8, for the use as a catalyst in the electrochemical reduction of CO2, and a subsequent acid treatment was then applied. Their study's resulting electrode exhibited a Faradaic e fficiency to CO of approximately 78%, with hydrogen the only byproduct [196].

#### **4. Conclusions and Future Perspective**

MOFs' applications in the CO2 capture, adsorption, membrane separation, catalytic conversion, and electrochemical reduction processes were analyzed thoroughly in this paper. As an emerging new class of crystalline porous materials, MOFs have attracted grea<sup>t</sup> attention over recent decades. The high surface area, high porosity, well-defined structures, and spectacular CO2 adsorption of MOFs are in grea<sup>t</sup> demand for CO2 capture, separation, conversion, and reduction processes.

There are three primary approaches in carbon dioxide capture using MOFs: pre-combustion capture, post-combustion capture, and oxy-fuel combustion capture. The post-combustion CO2 capture is the most adopted technology in the carbon capture field. However, a large number of aspects should be considered before implementing MOFs in these technologies. These aspects include MOFs' adsorption capacity, thermal stability, selectivity, and life cycle in several operational conditions. Also, for large-scale processes, further details should be considered; an economic analysis must be conducted, the supply chain of the raw materials, including the MOFs, should be analyzed, and environmental impact analysis should be produced. In addition, the thermal stability for the MOFs should be increased for large-scale processes by regenerating the MOFs. This increase in thermal stability will allow the MOFs to be used under high operational conditions. Optimistically, MOFs' properties are continually improving and developing over time, which allows them to serve as the next generation material class for CO2 capture.

In the membrane separation process of CO2, MOFs act as a filler in mixed matrix membranes, enhancing the membranes' separation e fficiency. However, the membrane separation process is usually hindered by several factors, including the cost, permeability, and selectivity of the membrane. The greatest challenge in the membrane separation with MOFs is the cost. Unfortunately, membranes are usually expensive to develop and maintain; with MOFs' incorporation, they will be even more expensive. Hence, economic MOFs should be chosen as a filler for the mixed matrix membranes for an economically viable process.

In the reduction and conversion processes of CO2, MOFs acts as a catalyst for the processes. The applications of MOFs as a catalyst for these chemical processes are extensively overviewed in this article. Various parameters should be considered before choosing the MOFs, such as photocatalytic activity, electrical conductivity, and stability. The cost of the MOFs is also the major issue that faces their practical application.

Based on the above studies, MOFs are very promising materials for CO2 capture, separation, adsorption, and chemical processes. However, their application is mainly suppressed by their high cost. MOFs have a high cost as a result of the costly synthesis of their raw materials. The choice of the MOFs' synthesis approach plays a major role in the economic aspect of the process. The high cost of the MOFs' raw materials is mainly owing to the lack of industrial-scale manufacturing facilities [197]. The MOFs' synthesis process involves sophisticated and time-consuming batch operations with complicated separation techniques and costly organic solvents. Various studies have been conducted to find an e fficient approach for reducing the cost of the MOFs. DeSantis, et al. [198] performed a techno-economic analysis to identify the primary factors of MOF adsorbents' high production cost (Mg-MOF-74, Ni-MOF-74, HKAUST-1 and MOF-5) and find approaches for cost reduction. The authors found the cost of the solvent used in the MOFs' synthesis to be the main factor for the high cost. The authors also mentioned that by changing from solvothermal synthesis to liquid assisted grinding and aqueous synthesis it is estimated to decrease the cost by up to 83% [198]. Hence, a detailed economic analysis should be performed before choosing the MOF for any process. With their remarkable properties, MOFs are predicted to be further developed for more economical and e fficient applications.

**Funding:** This research was funded by Qatar Foundation NPRP10-0107-170119.

**Acknowledgments:** The work was made possible by a gran<sup>t</sup> from the Qatar National Research Fund under the National Priorities Research Program award number NPRP10-0107-170119. Its content is solely the responsibility of the authors and does not necessarily represent the o fficial views of QNRF.

**Conflicts of Interest:** The authors declare no conflict of interest.
