CF4 Capture and Separation of CF4-SF6 and CF4-N2 Fluid Mixtures Using Selected Carbon Nanoporous Materials and Metal-Organic Frameworks: A Computational Study.

The adsorption of pure fluid carbon tetrafluoride and the separation of CF4-SF6 and CF4-N2 fluid mixtures using representative nanoporous materials have been investigated by employing Monte Carlo and molecular dynamics simulation techniques. The selected materials under study were the three-dimensional carbon nanotube networks, pillared graphene using carbon nanotube pillars, and the SIFSIX-2-Cu metal-organic framework. The selection of these materials was based on their previously reported efficiency to separate fluid SF6-N2 mixtures. The pressure dependence of the thermodynamic and kinetic separation selectivity for the CF4-SF6 and CF4-N2 fluid mixtures has therefore been investigated, to provide deeper insights into the molecular scale phenomena taking place in the investigated nanoporous materials. The results obtained have revealed that under near-ambient pressure conditions, the carbon-based nanoporous materials exhibit a higher gravimetric fluid uptake and thermodynamic separation selectivity. The SIFSIX-2-Cu material exhibits a slightly higher kinetic selectivity at ambient and high pressures.

Introduction

One of the main aims of international environmental treaties and agreements, such as the Kyoto Protocol and the Paris Agreement within the United Nations Framework Convention on Climate Change, is to reduce greenhouse gas emissions in the atmosphere in order to prevent dangerous anthropogenic interference with the climate system.[1,2] For this reason, six categories of greenhouse gases, sometimes also distinguished as CO2[3] and non-CO2 ones,[4] have been classified among the most potent ones. These greenhouse gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons, perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

Among the non-CO2 greenhouse gases, tetrafluoromethane (CF4), the simplest PFC, has a very long atmospheric lifetime (50,000 years) and a 100 year global warming potential of 6630–7390.[5] Although its concentration in the atmosphere is low (around 74 ppt), CF4 contributes significantly to the greenhouse effect.[6] Therefore, its capture or recovery from CF4/N2 mixtures from industrial emission sources,[7,8] mainly associated with the aluminum production and semiconductor fabrication processes, is considered important for the reduction of global warming. Moreover, the recycle of CF4 from CF4/N2 mixtures is also important in a wide range of applications involving a positron trap[9] since these mixtures are used as buffer gases for the cooling of positrons.[10]

The recovery and recycle of CF4 are also highly important in mixtures with SF6 having significant technological applications. Mixtures of CF4 with SF6 are used as insulating and arc-quenching media[11−18] at very low temperatures, down to −50 °C, especially in circuit breakers operating at these low temperatures.[11] In these mixtures, CF4 is added in order to avoid liquefaction of SF6 gas at low temperatures. These mixed gas circuit breakers are installed in converting stations, where alternating current is converted to direct current. The direct current is subsequently transmitted over long distances on a high-voltage direct current transmission line. According to the recent climate negotiation in Paris, a speedy implementation of these networks is a main path to introduce renewable energy and to harmonize power markets.

To achieve an efficient CF4 capture, several technologies based on thermal decomposition, plasma treatment, absorption, adsorption, cryogenic recovery, and membrane separation have been developed. Among them, physical adsorption using efficient nanoporous adsorbents[6,7,19−33] is considered as the most competitive technology for capturing CF4 due to its low energy consumption, low cost, and easy management. However, only some limited studies in the literature are devoted to CF4 capture using nanoporous adsorbents. In this respect, we decided to further explore the CF4 capture and the separation of SF6–CF4 and CF4–N2 fluid mixtures using some selected carbon-based nanoporous materials and metal–organic frameworks (MOFs), which have been found in our previous studies[34−36] to be very efficient for SF6 capture and SF6–N2 fluid mixture separation. These previous studies[34,36] had revealed that three-dimensional interconnected single-wall carbon nanotube networks and pillared graphene nanostructures, consisting of parallel graphene layers stabilized by carbon nanotubes placed vertically to the graphene planes, exhibit a significant SF6 uptake and high adsorption selectivity for SF6 over N2, in comparison with the best performing materials in the literature. Our studies[35] also revealed that the SIFSIX-2-Cu MOF exhibits high thermodynamic and kinetic adsorption selectivity for SF6 over N2.

Based on these findings, the aim of the present study was to further explore the efficiency of these different types of nanoporous materials in capturing CF4 and separating SF6–CF4 and CF4–N2 fluid mixtures, using a combination of force field-based grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. This paper is organized as follows: the employed computational methodology is presented in Section , the results and corresponding discussion in Section , and finally, the concluding remarks are summarized in Section .

Computational Methods

As mentioned in the introduction, two types of carbon-based nanoporous materials were investigated in the framework of the present study. The first type is a porous nanotube network (PNN),[37] composed of (8,8) single-walled carbon nanotubes which are connected through junctions, forming a three-dimensional cubic carbon nanotube network with an intertube distance of 11 Å and a corresponding three-dimensional porous network. The second type is the PILS pillared graphene that[34,38] consisted of parallel graphene layers stabilized by carbon nanotubes placed vertically to the graphene planes. The pillared graphene nanostructure investigated in the present study is composed of (6,6) single-walled carbon nanotubes, with a nanotube diameter of 8.142 Å, which act as pillars between adjacent graphene layers. In this specific nanostructure, the interlayer distance is 11.2 Å and the intertube one is 20.9 Å. The supercells used in our simulations comprised 1 × 1 × 1 unit cells in the case of PNN and 1 × 1 × 2 unit cells in the case of PILS. The PNN supercell is cubic with a 41 Å dimension, whereas the PILS supercell is orthorhombic, with dimensions 42.60 Å × 39.35 Å × 44.84 Å.

The third nanoporous material taken into account in the present investigation is the [Cu(4,4′-dipyridylacetylene)2(SiF6)] MOF, named SIFSIX-2-Cu, resembling pillared square grids with a pore dimension of 13.05 Å. This particular MOF was synthesized by pillaring two-dimensional nets of organic ligands and metal nodes with hexafluorosilicate (SiF62–) anions to form three-dimensional networks with primitive cubic topology.[39] The SIFSIX-2-Cu supercell was constituted by a 3 × 3 × 5 replica of its unit cell, which was geometry optimized in our previous studies at the periodic density-functional theory level.[35] As mentioned in our previous studies, the SIFSIX-2-Cu supercell was saturated by adding terminal −NH2 groups, and the charges of the terminal atoms were adjusted in order to give a total zero charge to the supercell.[35] The supercell dimensions are 41.25 Å × 41.25 Å × 42.01 Å. Further details about the special characteristics of all the investigated nanomaterials can be found in our previous studies.[34−36]

A combination of force field-based GCMC and MD simulations was employed to explore the adsorption of pure fluid CF4 in PNN, PILS, and SIFSIX-2-Cu and the separation of the CF4/SF6 (two bulk molar compositions CF4/SF6: 1:1 and 9:1) and CF4/N2 binary mixtures (bulk molar composition CF4/N2: 1:9). These simulations were performed at 303 K and in the pressure range 0.1–20 bar. The well-established all-atom rigid potential model for SF6 developed by Dellis and Samios[40] and the TraPPE potential model for N2[41] have been selected to be employed in our simulation studies. According to the literature,[40,41] these models provide very accurate description of fluid properties of SF6 and N2 under a wide range of thermodynamic conditions and the vapor–liquid coexistence curve and critical points of the pure systems. The intramolecular geometries, charges, and Lennard-Jones 12–6 potential parameters of all the interaction sites in the potential model of SF6 can also be found in a previous study of one of the authors in the literature,[35] whereas the corresponding ones for N2 can be found in the previous study of Potoff and Siepmann.[41] The OPLS rigid potential model[42] has been employed for CF4. The intramolecular geometries, charges, and Lennard-Jones 12–6 potential parameters of all the interaction sites in the employed potential models of CF4, SF6, and N2 are presented in Table . The carbon atoms in the PNN and PILS materials are not charged and interact via a Lennard-Jones 12–6 potential having εCC = 28.2 K and σCC = 3.4 Å. The partial charges and Lennard-Jones parameters corresponding to each type of atom in the SIFSIX-2-Cu material are also presented in our previous studies.[35]

Table 1

Partial Charges and Lennard-Jones 12–6 Parameters Corresponding to Each Type of Interaction Site in the Employed Potential Models of CF4, SF6, and N2a

interaction site	q (|e|)	σ (Å)	ε (K)
CF4
C	0.48	3.50	48.8126
F	–0.12	2.95	26.6708
SF6
S	0.66	3.228	165.14
F	–0.11	2.947	27.02
N2
N	–0.482	3.31	36.0
COM	0.964

C–F bond length: 1.332 Å, S–F bond length: 1.564 Å, and N–N bond length: 1.1 Å.

The reproducibility of the vapor–liquid equilibrium (VLE) curve is of foremost importance during adsorption studies. Thus, in order to further validate the selected force field for CF4 and SF6, the VLE T–ρ curves have been calculated using NVT Gibbs ensemble Monte Carlo (GEMC) simulations[43] and compared to available experimental data.

The RASPA Monte Carlo simulation code[44] was used in order to perform the GCMC and GEMC simulations. The fugacities of the adsorbed species, which were used as inputs for the GCMC simulations, were calculated by employing the Peng–Robinson equation of state.[45] For each simulated thermodynamic condition, 105 Monte Carlo cycles were performed for the equilibration and production runs. In these Monte Carlo cycles, different types of trial moves were attempted, including creation or deletion of molecules, translation or rotation, and exchange of the molecular identity. Note that the calculated error bars in the calculated fluid uptakes in the case of the GCMC simulations were in the range of about 1%. The corresponding error bars in the calculated gas and liquid densities in the cases of the GEMC simulations were in the range 0.4–2.0%, with the largest values observed under thermodynamic conditions approaching the critical point. NVT-MD simulations were subsequently carried out to calculate the self-diffusion coefficients of the components of the investigated adsorbed mixtures at the ambient pressure of 1 bar and the high pressure of 20 bar. The MD simulations were performed for all the adsorbed mixtures corresponding to each one of the investigated nanoporous materials. The integration of the equations of motion was achieved by employing the well-established leapfrog-type Verlet algorithm[46] and using an integration time step of 1 fs. The intramolecular geometry of SF6, CF4, and N2 molecules was constrained using the quaternion formalism.[46] A Nose–Hoover thermostat with a temperature relaxation time of 0.5 ps was used to constrain the temperature during the simulations.[47] A 12 Å cutoff was used to treat the short-range interactions, and long-range corrections for the Lennard-Jones interactions were also taken into account in all MD and MC simulations. Test simulation runs, employing higher cutoffs (15 and 18 Å), have shown that when using the long-range corrections for the Lennard-Jones interactions, the results obtained converge for all the investigated cutoffs. The long-range electrostatic interactions were also treated using the standard Ewald summation method.[46] All MD simulations were run for 5 ns, after a 1 ns equilibration period, using the DL_POLY simulation code.[48] During the simulations, the intramolecular geometries of the adsorbents were kept rigid.

Results and Discussion

VLE of CF4 and SF6

The NVT-GEMC simulations for the calculation of the VLE T–ρ curves were performed at eight experimentally available coexistence points in the temperature range 130–200 K for pure CF4 and in the range 230–300 K for pure SF6. The simulated systems consisted of 320 CF4 and SF6 molecules, equally distributed to the two simulated systems. The molecules in the initial configurations in each subsystem were randomly placed. The density of the two subsystems was set to (ρl + ρv)/2, where ρl and ρv are the experimental values of the density of the liquid and vapor phase at each investigated temperature, respectively. The calculated T–ρ vapor–liquid coexistence curves for pure CF4 and SF6 are presented in Figure , along with the experimental ones.

Figure 1

Calculated, from GEMC simulations, T–ρ vapor–liquid coexistence curves of pure CF4 and SF6, plotted together with the experimental ones. The estimated critical points are also presented in comparison with the experiments.

Apparently, the good agreement between the calculated and experimental data can be clearly observed. Moreover, the critical density and temperature of CF4 and SF6 were estimated by using the well-known critical scaling relation and the law of rectilinear diameters[49]

The estimated critical values (Tc, ρc) for CF4 were Tc = 239.3 K and ρc = 0.6078 g/cm3, which are in reasonable agreement with the experimental ones (227.5 K, 0.6257 g/cm3) and much closer to the experiment compared to the values predicted by other force fields of CF4, presented in previous studies in the literature.[50] Similarly, the estimated critical values (Tc, ρc) for SF6 were Tc = 314.7 K and ρc = 0.757 g/cm3, which are also in good agreement with the experimental ones (318.7 K, 0.743 g/cm3).[40] Note also that the critical exponent β in eq has been estimated to be 0.3105 and 0.3119 in the cases of CF4 and SF6, respectively. Consequently, the employed force fields of CF4 and SF6 provide realistic descriptions of the VLE, which according to the literature,[51] is of paramount importance in adsorption studies.

Pure CF4 Adsorption

The gravimetric adsorption isotherms of pure CF4 at 303.15 K and in the pressure range 0.1–20 bar for all the investigated materials are illustrated in Figure . Apparently, the CF4 gravimetric uptake is higher in the case of PNN at the low-pressure range, up to about 5 bar. However, at higher pressures, the gravimetric uptake in the case of PILS and SIFSIX-2-Cu is higher due to the larger free volume of these materials. For instance, at ambient pressure (1 bar), the calculated CF4 uptake in the PNN, PILS, and SIFSIX-2-Cu materials is 4.89, 3.33, and 1.49 mmol/g, respectively. These values are among the highest reported regarding the uptake of CF4 at ambient pressure, when compared to previous studies for several nanoporous materials in the literature.[32] At 20 bar, the corresponding values for PNN, PILS, and SIFSIX-2-Cu are 9.48, 12.12, and 13.34 mmol/g, respectively. Representative snapshots, depicting the adsorbed CF4 molecules in the investigated materials at 1 and 20 bar, are presented in Figure . Within the pressure range studied, it can be deduced that the saturation has not been achieved for any of the adsorbents considered here. To evaluate the interaction strength between the adsorbents and the adsorbates, the isosteric heat of adsorption (Qst) was calculated at low coverage, corresponding to the pressure of 0.1 bar, which is also presented in Figure . The calculated isosteric heat of adsorption of pure CF4, obtained in the framework of this study, is presented together with the calculated values for pure SF6 and N2, which were obtained in our previous studies.[35,36] Regarding pure CF4 adsorption, the highest predicted isosteric heat of adsorption at low coverage is observed in the case of PNN (24.0 kJ/mol), followed by PILS (22.6 kJ/mol), whereas the lowest value of Qst corresponds to SIFSIX-2-Cu (15.8 kJ/mol). The higher uptake of CF4 at low pressures in the case of PNN can be explained in terms of the calculated Qst of pure CF4 at low coverage. The trends observed for the values of Qst corresponding to each one of the investigated materials are consistent with the corresponding trends regarding the CF4 adsorption at low pressures, further confirming that the CF4 adsorption at low pressures is thermodynamically driven. The same trends have also been observed in the case of the pure SF6 and N2 adsorption. Nevertheless, in the case of SF6, the calculated Qst values are higher in comparison with CF4. The corresponding Qst values for PNN, PILS, and SIFSIX-2-Cu in the case of pure SF6 adsorption are 32.9, 27.7, and 22.1 kJ/mol, respectively. In the case of pure N2 adsorption, the corresponding Qst values are significantly lower. The calculated Qst values for PNN, PILS, and SIFSIX-2-Cu in the latter case are 13.0, 12.2, and 9.1 kJ/mol, respectively.

Figure 2

Gravimetric adsorption isotherms of pure CF4 at 303.15 K up to 20 bar (top) and isosteric heat of adsorption in the low coverage for CF4, SF6, and N2 (bottom).

Figure 3

Representative snapshots, depicting the adsorbed CF4 molecules in the investigated PNN, PILS, and SIFSIX-2-Cu materials at 303.15 K and P = 1, 20 bar.

Separation of CF4/SF6 Fluid Mixtures

Subsequently, as mentioned in Section , GCMC simulations were performed in order to calculate the co-adsorption isotherms of CF4/SF6 fluid mixtures. Two bulk molar compositions (CF4/SF6: 1:1 and 9:1) were taken into account in our calculations, and the results obtained for the PNN, PILS, and SIFSIX-2-Cu materials are presented in Figure .

Figure 4

Calculated co-adsorption isotherms of CF4/SF6 fluid mixtures ().

Apparently, the molar fraction of the binary mixture influences the co-adsorption behavior of the adsorbents. In the case of the equimolar mixture, the SF6 co-adsorption is much more favored at low pressures, especially in the cases of PNN and PILS. This finding is consistent with the fact that the isosteric heat of adsorption of pure SF6 at low coverage is higher in comparison with CF4. As the pressure increases, we observe an increase in the loading of CF4 that keeps the loading of SF6 almost constant for PNN and PILS. In contrast, the sulfur hexachloride loading of SIFSIX-2-Cu exhibits a continuous increase and surpasses the corresponding loading of the carbon-based adsorbents. This reveals that the thermodynamically driven favored adsorption of SF6 becomes less pronounced in higher pressures, as attributed to two reasons. The first one is related to the larger size and kinetic diameter of SF6 with respect to CF4. Additionally, size- and shape-dependent packing effects start to play an important role. The same reasons have a dominant role in the case of the binary mixture containing 10% sulfur hexafluoride. For this mixture composition, increased loading for SF6 is observed in the pressure range below 2 bar, which is more pronounced for PNN and PILS. At higher pressures, there is a clear enhancement of the CF4 loading, exceeding the loading of SF6 for the adsorbents under study. This in an indication of the high competitiveness of both molecules to occupy the available free volume, leading to the displacement of SF6 at high pressures. Thus, the selectivity will be reduced at high pressures.

The trends observed in the calculated co-adsorption isotherms coupled with the values obtained for Qst are more clearly reflected on the calculated values of the thermodynamic adsorption selectivity where xi and yi are the molar fractions of each component i (i = SF6, CF4) in the adsorbed and bulk phases, respectively. The calculated thermodynamic adsorption selectivity for the investigated fluid mixture as a function of pressure at 303.15 K is presented in Figure . The thermodynamic adsorption selectivity decreases with the increase in the pressure in the cases of PNN and PILS for both mole fractions of CF4 (0.5 and 0.9). The selectivity values observed in both cases are quite similar, with the ones corresponding to PNN being slightly higher. Interestingly, in the case of SIFSIX-2-Cu, the selectivity reaches a maximum value, which is observed at 2 and 5 bar in the cases where the bulk mole fraction of CF4 is 0.5 and 0.9, respectively. A similar pressure dependence of the selectivity for the SF6/N2 separation was also observed for the SIFSIX-2-Cu nanoporous material[35] and the FAU-ZTC zeolite,[52] which has a similar pore diameter with SIFSIX-2-Cu. The appearance of such a maximum, which has been observed for both the investigated bulk mixture compositions, can be interpreted in terms of competitive adsorption phenomena at high pressures. At these pressures, the adsorption of CF4 in the nanopores is facilitated due to its smaller kinetic diameter in comparison with SF6. On the other hand, at lower pressures, the separation mechanism is mainly a thermodynamically driven one.[35,52] The shift of the selectivity maximum at higher pressures observed for the CF4/SF6 mixture with a 9:1 bulk molar composition can also be attributed to the lower bulk composition of SF6. This lower bulk composition of SF6 leads to a more pronounced increase in the slope of the uptake of CF4 with pressure at higher pressures in comparison with SF6, as it can also be observed in Figure , resulting in a lower selectivity due to packing effects.

Figure 5

Pressure dependence of the calculated thermodynamic adsorption selectivity for the investigated fluid mixture compositions at 303.15 K.

MD simulations were further performed in the canonical NVT ensemble, using the calculated SF6 and CF4 uptakes corresponding to 1 and 20 bar, respectively, and the bulk mixture composition with , to explore the diffusivity of the guest molecules in the PNN, PILS, and SIFSIX-2-Cu materials. The self-diffusion coefficients of SF6 and CF4 were calculated using the well-known Einstein relation applied to the mean-square displacements for both guests averaged over all the MD trajectories and using a multi-time step origin. The calculated self-diffusion coefficients are presented in Table .

Table 2

Calculated Self-Diffusion Coefficients of the Adsorbed SF6 and CF4 Molecules in the Investigated Nanomaterials, Corresponding to the Bulk Fluid Mixture with and Pressures of 1 and 20 bar, Respectively

material	D(SF6) (10–9 m2/s)	D(CF4) (10–9 m2/s)	D(CF4)/D(SF6)
P = 1 bar,
PILS	7.55	12.44	1.65
PNN	2.46	2.91	1.18
SIFSIX-2-Cu	2.30	6.81	2.96
P = 20 bar,
PILS	1.91	2.85	1.49
PNN	0.98	1.37	1.52
SIFSIX-2-Cu	2.12	3.85	1.82

From this table, we can see that the relative percentage decrease in the self-diffusion of the adsorbed SF6 and CF4 molecules upon the increase in the pressure is more pronounced in the case of the PNN and PILS materials. However, the ratio D(CF4)/D(SF6) is higher in the case of SIFSIX-2-Cu, signifying that the kinetic separation of the adsorbed mixture components is slightly more favored in SIFSIX-2-Cu.

Separation of CF4/N2 Fluid Mixtures

The co-adsorption isotherms of a CF4/N2 fluid mixture (with bulk molar composition CF4/N2: 1:9) were also investigated in the present treatment, and the results obtained for the PNN, PILS, and SIFSIX-2-Cu materials are presented in Figure . From this figure, we can see that the actual gravimetric uptakes of each one of the mixture components are quite low at ambient pressures, and only at the high-pressure region, they exhibit values in the range 2–5 mmol/g.

Figure 6

Calculated co-adsorption isotherms of CF4/N2 fluid mixtures ().

The calculated values of the thermodynamic adsorption selectivity are also presented in Figure . From these figures, we can see that the thermodynamic selectivity exhibits the highest values in the case of the carbon-based nanoporous materials, particularly for the PNN. This finding is also consistent with our previous studies,[34] which had revealed that the thermodynamic separation selectivity of SF6/N2 mixtures is significantly enhanced in the case of the PNN and PILS materials. However, the selectivity values observed in the case of the CF4/N2 mixture are significantly lower in comparison with the ones obtained for the SF6/N2 mixture with the same molar composition. This finding is also consistent with the lower isosteric heat of adsorption values Qst for CF4 in comparison with SF6. From Figure , it can also be observed that for both the PNN and PILS materials, the selectivity values decrease with the increase in the pressure. On the other hand, in the case of SIFSIX-2-Cu, the selectivity remains almost constant along the investigated pressure range, exhibiting a value around five. The much lower selectivity value in the case of SIFSIX-2-Cu can also be attributed to the fact that the actual uptake value of CF4 along the investigated pressure range is lower in comparison with the one corresponding to N2, whereas the opposite behavior is observed in the case of PNN and PILS. All these findings clearly indicate that the thermodynamic adsorption selectivity in the case of the CF4/N2 fluid mixture is more pronounced for the carbon-based nanoporous materials, particularly in the case of PNN.

Figure 7

Pressure dependence of the calculated thermodynamic adsorption selectivity for the investigated fluid mixture composition at 303.15 K.

Due to the very low uptake of the mixture components at 1 bar, MD simulations were performed in the canonical NVT ensemble using the calculated CF4 and N2 uptakes corresponding to 20 bar and the bulk mixture composition with , to explore the diffusivity of the guest molecules in the PNN, PILS, and SIFSIX-2-Cu materials. The calculated self-diffusion coefficients of CF4 and N2 are presented in Table .

Table 3

Calculated Self-Diffusion Coefficients of the Adsorbed N2 and CF4 Molecules in the Investigated Nanomaterials, Corresponding to the Bulk Fluid Mixture with and Pressures of 20 bar

P = 20 bar,
material	D(N2) (10–9 m2/s)	D(CF4) (10–9 m2/s)	D(N2)/D(CF4)
PILS	25.92	13.89	1.87
PNN	9.19	5.58	1.65
SIFSIX-2-Cu	27.65	7.48	3.70

From this table, it can be clearly observed that the ratio D(N2)/D(CF4) is higher in the case of SIFSIX-2-Cu, indicating that the kinetic separation of the adsorbed mixture components is more favored in SIFSIX-2-Cu, as in the case of the CF4–SF6 mixture.

Conclusions

The adsorption of pure fluid carbon tetrafluoride and the separation of CF4–SF6 and CF4–N2 fluid mixtures, using three-dimensional carbon nanotube networks (PNN), pillared graphene with carbon nanotube pillars (PILS), and the SIFSIX-2-Cu MOF, were investigated by employing a combination of Monte Carlo and MD simulation techniques. These particular nanoporous materials were selected based upon their very satisfactory performance for SF6 capture and SF6–N2 fluid mixture separation, which was revealed in our previous studies.

The results obtained regarding pure CF4 adsorption have revealed that the highest predicted isosteric heat of adsorption at low coverage is observed in the case of PNN (24.0 kJ/mol), followed by PILS (22.6 kJ/mol), whereas the lowest value of Qst corresponds to SIFSIX-2-Cu (15.8 kJ/mol). These trends are consistent with the corresponding trends regarding the gravimetric uptake of pure CF4 adsorption at low pressures, further confirming that the CF4 adsorption at low pressures is thermodynamically driven. However, at higher pressures, the gravimetric uptake in the case of PILS and SIFSIX-2-Cu is higher due to the larger pore dimensions of these systems.

The results obtained have also revealed that in the case of the CF4–SF6 fluid mixtures, under near-ambient pressure conditions, the carbon-based nanoporous materials exhibit a higher gravimetric fluid uptake and thermodynamic separation selectivity. On the other hand, the SIFSIX-2-Cu material exhibits a higher kinetic selectivity at both ambient and high pressures. Regarding the separation of the CF4–N2 mixtures, the carbon-based nanoporous materials exhibit a higher thermodynamic separation selectivity in comparison with the SIFSIX-2-Cu MOF but significantly lower in comparison with the values obtained in our previous studies for the SF6–N2 mixtures. However, as for the SF6–CF4 fluid mixture, the SIFSIX-2-Cu material also exhibits a higher kinetic selectivity at high pressures, in the range of 20 bar, in the case of the CF4–N2 mixture.