Materials Design for N2O/CO2 Capture and Separation: Comparison
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Please note this is a comparison between Version 2 by Dean Liu and Version 1 by Daniel Ballesteros.

The adsorption of greenhouse gases (GHG) as a method to reduce their emissions into the atmosphere is an alternative that is easier to implement industrially and cheaper than other existing technologies, such as chemical capture, cryogenic separation, or membrane separation. The vast majority of works found in the literature have focused their efforts on capturing CO2 as it is the largest GHG.

  • N2O adsorption
  • N2O/CO2 selectivity
  • porous materials

1. Kinetic and Thermodynamic Considerations

Since silica-based adsorbent materials do not have much N2O adsorption capacity, activated carbons, zeolites, and MOFs have been more extensively studied in N2O adsorption processes. These materials usually have large surface areas and high microporosity content as well as the advantage of being able to modify the pore network and even functionalize them with ligands capable of improving selectivity in CO2–N2O separation processes. In any case, all of these materials behave in the same way against pressure and temperature; the higher the pressure and the lower the temperature, the greater the adsorption [31,32][1][2]. In general, the influence of the temperature is greater than that of the pressure. However, it is important to know the mechanism of N2O adsorption is not entirely clear. For instance, it is known that the greater the surface area and pore volume, the greater the adsorption capacity, as shown in the study carried out by D. Park et al. [8][3]. In these cases, the diffusion through the micropores dominates the adsorption kinetics over the diffusion of the meso and/or macropores. At low pressures, the interaction between N2O and the surface of the micropores is stronger and, therefore, the adsorption kinetic increases, however, at high pressures the interaction of the N2O molecules with the surface in larger pores is favored [8][3]. Y. Peng et al. [31][1] studied the behavior of various activated carbons with different pore sizes in the N2O adsorption and found that in carbons with a higher content of micropores (Kureha [14.7 mmol g−1] and Ovcls [12.1 mmol g−1]) the adsorbent–adsorbate interactions were stronger; however, in carbons with a higher content of total pore volume (micro and mesopores) (Vruf [19.2 mmol g−1]) the highest adsorption capacity was achieved at 193 K, that is, materials with a higher adsorption capacity will be those containing a greater total pore volume and materials with a greater micropore volume, the adsorption kinetics will be faster.
In relation to the adsorption mechanism, it is also important to take into account the kinetic diameter (to design a suitable pore structure), which refers to the free path of a molecule and indicates the size of the molecule [33][4]. Therefore, the smaller the kinetic diameter of the gas molecule (note that for N2O and CO2 it is the same [3.3 Å]), the easier it enters the pore, as long as the pore is larger than the kinetic diameter. However, there are some discrepancies in the literature. For instance, D. Park et al. [8][3] found that although N2O has a smaller kinetic diameter than other molecules such as O2 (3.46 Å) or N2 (3.64 Å), it is expected that it enters the pores of activated carbon and carbon molecular sieves in an easier way. However, the adsorption rate of N2O was lower than that O2, suggesting that factors other than the kinetic diameter influence the adsorption mechanism. So, the adsorption capacity also depends on other factors [30,31[1][5][6],34], which cause the mechanism of N2O adsorption to be not as clear as expected considering the kinetic diameter and textural properties. Therefore, apart from these properties, it is important to take into account the thermodynamics of the process, that is, the affinity between adsorbent and adsorbate. The parameters such as diffusivity, adsorption enthalpy (−ΔH0), or Henry constant (KH) help to highlight the interaction between adsorbate and adsorbent and other parameters such as gate-opening pressure (or threshold pressure) can help to establish the optimal adsorption pressure in the adsorption process of a gas, alone or in a mixture of gases.
The adsorption enthalpy (−ΔH0) helps to understand the interactions established between the adsorbate and the adsorbent on the surface of the latter, providing useful information to design a more efficient adsorption process. At the beginning of the adsorption process, N2O will be adsorbed on the sites of the adsorbent with the highest energy level (microporous), interacting with the walls of the adsorbent and causing the release of energy in the form of heat as a consequence of the adsorbent–adsorbate interaction, and faster adsorption kinetics. After the higher energy sites are occupied, the occupation of the lower energetic sites begins (greater porous) and then the amount of adsorbed N2O begins to increase at the same time that the adsorption enthalpy decreases [8,31,34][1][3][6]. However, diffusivity studies indicate that the diffusion time constant decreases considerably at high pressures in N2O adsorption, due to partial blockage of the pores [35[7][8],36], which leads to slower adsorption kinetics. This fact is even more accentuated in those materials that have a narrower pore size distribution such as zeolite 4A [34][6] and zeolite 5A [35][7]. In contrast, it is interesting to note, in view of the selective separation of N2O and CO2 that although N2O and CO2 have very similar characteristics [37][9], something completely different happens during CO2 adsorption. At high pressure, the CO2 concentration on the surface of the adsorbent increases and surface diffusion (slip) of CO2 molecules across the surface of the adsorbent occurs, increasing the amount of CO2 adsorbed [35][7] without producing the pore blockage.
Henry’s constant (KH) can be applied to quantify the degree of adsorption of a given material if working at low pressures (<100 kPa) [17][10]. It is a value that is a function of the properties of the adsorption process and of the adsorbent. Thus, KH decreases with increasing temperature. This was highlighted by J.C. Groen et al. [38][11] when studying the adsorption capacity of N2O in the silicalite-1 zeolite. It was determined that the KH value changed substantially with a slight change in adsorption temperature, from 273 K (q = 2.4 mmol g−1) to 298 K (q = 1.7 mmol g−1), KH was 13.09 and 5.03 mol kg−1 kPa−1, respectively. Therefore, a higher adsorption capacity implies a higher KH value.
MOFs have been studied most extensively in N2O capture processes. They have shown a high potential for capturing N2O and for gas separation, due to their high ordering and narrow microporosity. The main drawback lies on the high cost of their synthesis, which could cause it to be difficult to use on a large scale. Nonetheless, much of the research on N2O capture and separation has focused on adsorbents based on Metallic Organic Frameworks, given their adaptability to certain processes [39][12].
The first studies based on MOFs in N2O adsorption were zinc-based MOFs materials (MOF-5 and MOF-177), which are considered good adsorbents for H2 [43[13][14],44], given their high surface areas (900 and >3500 m2 g−1, respectively) and size porosity around 1 nm. However, they have not proven to be such good N2O adsorbents, showing adsorption capacities of less than 1 mmol g−1 in the best case (MOF-5). Contrary, they proved to have more affinity for gases such as CO2 and CH4 working at high pressures (1000 kPa, 298 K) [35][7].

2. Separation Efficiency

The potential adsorption capacity of MOFs is related to their ability to modify internal structural stresses, which allows MOFs to modify the “opening” of their pores as a function of the temperature. This is known as threshold pressure or gate-opening pressure. Thus, in theory, the lower the gas pressure threshold, the greater the preference for being adsorbed [12,42][15][16]. Previous investigations on nickel-based MOFs [45,46][17][18] reveal that the incorporation of nickel allows a structural change of the material that leads to an increase in adsorption capacity. Based on this fact, K. L. Kauffmann et al. [47][19] studied a nickel-based MOF (NiDBM-Bpy) against a gas mixture (N2, O2, CH4, N2O, and CO2) as a function of threshold pressure. Above the threshold pressure it is known that the adsorption of a gas increases rapidly. However, in this regard, there are two currents of thought: (a) the one that thinks that the threshold pressure would serve to selectively adsorb one gas over another and (b) the one that thinks that there is a cooperation between the gases so that everyone can access to the adsorption sites. In any case, both CO2 and N2O compete strongly (given their similar physical properties) for the adsorption sites, so the thermodynamic conditions of adsorption are a determining factor in achieving the effective separation of both gases. The study on the gate-opening pressure of some MOFs against CO2 and N2O mixtures has been carried out, and varies depending on the MOF and the adsorption temperature and pressure used, as deduced, for example, from the investigations carried out by D.L. Chen et al. [12][15] with the MOF ZIF-7 (zeolitic imidazolate framework) and by L. Wang et al. [42][16] with the MOFs ELM-11, ELM-12, and MIL-53Al. Based on the gate-opening pressure at 298 K, ZIF-7 shows a lower pressure for N2O (0.35 bar−1) than for CO2 (0.50 bar−1), that is, for N2O it will be easier to interact with the adsorbent inside the pores. With ELM-11 (N2O: 1.0 bar−1, CO2: 0.8 bar−1) and ELM-12 (N2O: 6.0 bar−1, CO2: 4.0 bar−1) the opposite occurs. Therefore, the greater the gate-opening pressure difference between gases, the greater the selectivity toward the gas with lower pressure is expected when the applied adsorption pressure is between the minimum and maximum gate-opening pressures. For example, for MIL-53Al, the gate opening pressures for N2O and CO2 are 2.7 and 5.0 bar−1, respectively. Then, with adsorption pressure (2.7 < p < 5.0), the adsorbed amount of N2O is maximum (8 mmol g−1) and that of CO2 minimum (3 mmol g−1). With p < 2.7 bar and p > 5.0 bar, the adsorbed quantities of N2O and CO2 are equal, 3 and 8 mmol g−1, respectively [42][16].
More recently and continuing with the research on nickel-based MOFs, X. Zhang et al. [11][20] have developed the solvothermal synthesis of a nickel acetate functionalized MOF (Ni-MOF) to obtain a polar microporous (Ø 1 nm) structure. After activation of Ni-MOF (323 K/12 h under vacuum), the material is capable of separating CO2 (q = 3.24 mmol g−1, 100 kPa, 298 K) and N2O (q = 2.81 mmol g−1, 100 kPa, 298 K) of other gases such as CH4, N2, O2, mainly due to the fact that N2O, and CO2 have a smaller kinetic diameter and both molecules strongly interact with the pore surface. Strong competition for adsorption sites is observed, which prevents the selective separation of both gases (CO2, N2O).
Delving into the study of the adsorption mechanism and the competition for the adsorption sites of a gaseous mixture (CO2, N2O, CH4, and N2), F.A. Kloutse et al. [17][10] carried out monocomponent and multicomponent adsorption studies on an MOF functionalized with Cu (CuBTC). This adsorbent is non-polar, has unsaturated Cu sites, and two interconnected pore sizes (0.9 and 0.5 nm) that result in the creation of two preferential adsorption sites. The monocomponent data revealed a high adsorption capacity for N2O and CO2 on CuBTC at the unsaturated copper sites (direct interaction with the adsorbent surface), which increased considerably with pressure (q(N2O) = 5.4 and 10.9 mmol g−1; q(CO2) = 4.5 and 10.2 mmol g−1, at 120 and 900 kPa, respectively). As can be seen, CuBTC is capable of capturing very similar amounts of CO2 and N2O, under the same pressure and temperature conditions, and the small difference between N2O and CO2 capacity could be due to the small permanent dipole moment of N2O and its polarizability. Additionally, the smaller kinetic diameters of CO2 and N2O could be the main cause of the greater adsorption capacity of both gases compared to CH4 and N2 (with a larger kinetic diameter). Unfortunately, the multicomponent study reveals that the adsorption capacity of each gas decreases considerably (q(N2O) = 3.7 mmol g−1 and q(CO2) = 3.1 mmol g−1, at 112 kPa and 297 K) because molecular competition between CO2 and N2O for adsorption sites. CO2 and N2O begin to dominate the adsorption to the detriment of CH4 and N2 at pressures above 250 kPa, however, CuBTC is not capable of selectively separating CO2 and N2O.
One of the drawbacks of MOFs is that they need to be activated, usually under conditions of temperature and vacuum, to remove the solvent from the material structure. The control of the activation temperature is essential to remove the maximum amount of solvent from the structure without collapsing and to maximize the surface area, in addition to allowing more metallic active sites to be exposed to bind to the adsorbate molecules during the adsorption process, as shown by J. Yang et al. [40][21] when investigating the influence of activation temperature (using high vacuum, 10−10 bar) in the metal–organic-framework MIL-100Cr in the adsorption/separation of N2O and N2 process. Researchers reveals the preference of unsaturated Cr3+ metal sites for N2O versus N2 and that the adsorption of N2O is maximum when the adsorbent is activated at 523 K (1.95 mmol g−1 activation at 423 K; 5.78 mmol g−1 activation at 523 K). At 523 K, and for this material, the solvent molecules more strongly anchored to the Cr3+ sites are removed, so the surface area increases considerably and more Cr3+ sites remain free to preferentially bind N2O, improving adsorption capacity and separation of N2. At more than 523 K, the structure collapses.
Following the MIL-100 metal–organic framework, L. Wang et al. [41][22] replaced Cr with Fe, thus generating two different adsorption sites: on the one hand Fe3+ sites that interact with CO2 (−ΔH0 = 12 kJ mol−1) and on the other hand unsaturated Fe2+ sites that interact with N2O (−ΔH0 = 36 kJ mol−1). During the activation of the adsorbent at 573 K the presence of Fe2+ sites predominate, so a small difference is observed in the adsorbed amounts of CO2 and N2O.
Given the similarities between CO2 and N2O that cause their separation to be notably difficult, it seems that taking advantage of the acidic properties of both gases (CO2 is acidic and N2O is not) could help achieve this. The functionalization of MOFs with alkaline or acid molecules seems to have sufficient potential to achieve a higher selectivity of MOF-based adsorbents, as has been shown, for example, with the functionalization of the MIL-100Cr MOF with ethylenediamine [37][9], in order to generate alkaline holes in the structure capable of selectively adsorbing CO2 (acid). The amount of N2O adsorbed after modifying the MOF with ethylenediamine is greatly reduced relative to the unmodified MOF. However, the yield of adsorbed CO2 also decreases considerably because after incorporating ethylenediamine, the volume of pores decreases greatly. The porosity control after functionalization is, therefore, a fundamental aspect to take into account in the adsorbents design, in order to improve adsorption performance.

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