Adsorption of CO2, CO, H2, and N2 on Zeolites, Activated Carbons, and Metal-Organic Frameworks with Different Surface Nonuniformities

Abstract

The single-component adsorption of CO₂, CO, N₂, and H₂ at 25 and 35 °C was studied using microporous faujasite-framework zeolites (NaY and NaX), activated carbons (GCN and MSP), and metal–organic frameworks (A100 and Z1200) as starting points for the separation of CO₂ from syngases produced by gasifying biomass-based solid wastes. The indicated adsorption isotherms and uptake of the adsorbates strongly depended on the adsorbates themselves as well as on the adsorbents because of significant differences in the surface features, such as surface nonuniformity, and in the molecular properties. The selectivity of CO₂ to the other gases also varied with the adsorbents due to the distinctive energetic characteristics. The surfaces of the zeolites were the most energetically heterogeneous ones, yielding higher CO₂ uptake at low pressures, while the two activated carbons and A100 had moderate surface heterogeneities, and MSP showed the highest CO₂ uptake at high pressures, such as 6 bar, at which the micropore volume and surface area are important. Z1200, which has highly homogeneous surfaces and no high-affinity-binding sites, exhibited the lowest CO₂ adsorption capacity regardless of equilibrated pressure. The surface nonuniformities of the six sorbents were consistent with the calculated isosteric heats of CO₂ adsorption. CO₂ could be reversibly adsorbed on NaY and MSP but not on GCN, with some metal impurities, although all these adsorbents showed a fully reversible process for CO adsorption. The estimated working capacity for CO₂ adsorption at 25 °C was 0.78–6.50 mmol/g, depending on the sorbents used. The highest value was disclosed for MSP, the surface energetic heterogeneity of which was between that of zeolites and Z1200. Such a high working capacity bodes well for use in our later applications.

1. Introduction

Gasification of sustainable resources, such as biomass (e.g., wood and wood processing wastes and forest and agricultural residues), can yield syngases that consist of H₂, CO, CO₂, and N₂, with other gaseous components in relatively small quantities, and many kinds of sustainable biofuels and chemicals can be driven from such syngases [1,2]. For this, it is necessary to separate undesirable gases, such as CO₂ and N₂, that exist with H₂ and CO from the main stream using appropriate processes [3,4]. Numerous studies have been conducted on adsorptive CO₂ separation that combine a mechanism to preferentially adsorb the guest in a mixture with other gases on the surfaces of porous sorbents with a way to recover their bare surfaces, such as pressure-swing adsorption (PSA), thereby allowing for the periodic adsorption–regeneration operation [4,5,6,7]. To date, numerous porous solids have been proposed as promising CO₂ sorbents [6,8,9,10,11,12,13].

Zeolites are favored for the adsorption of gaseous adsorbates because of the presence of cationic sites [4,5]. Among them, low-silica zeolites, such as Y, 13X, and A, are preferred for separating CO₂ from gas mixtures with CO, N₂, H₂, CH₄, and O₂, owing to their high affinity toward the target gas [6,14], although such low-SAR (Si-to-Al ratio) zeolites often have an issue that complete regeneration using a pressure swing technique is difficult [15]. Amorphous activated carbons (ACs) with high micro- and mesoporosity have been frequently studied as representative carbon-based materials for CO₂ separation [4,7,16], while molecular-sieve-type carbons reported for CO₂/CH₄ separation from landfill gases [17] are relatively more expensive than ACs, making them inappropriate for widespread use in the gas separation industry. Such ACs contain many types of surface functional groups (e.g., carbonyl, phenol, carboxyl, lactone, and ether) that have an acidic property but that can be altered to basic centers after thermal treatment, making them good binding sites for CO₂ [18,19]. Not only do these surface properties depend dramatically on the carbon sources and activation processes, but the impurity metals present in ACs also have a substantial influence on the adsorption of adsorbates [4,19,20]; thus, it may not be easy to develop AC sorbents with on-demand physical and surface properties for CO₂ separation, such as a well-defined pore size and distribution, channel system, and surface functionality, unlike zeolites. Since the first synthesis of CoC₆H₃(COOH_1/3)₃ with a three-dimensional network of channel and uniform pores by Yaghi et al. [21], the use of other sources for metal-based organic building blocks and organic linkers has led to tremendous crystalline metal–organic frameworks (MOFs), including zeolitic imidazolate frameworks (ZIFs) [9,10,12,13,22,23,24,25], enabling their framework structures and surface properties to be rationally designed and tuned, which makes them different from the benchmark porous solids, such as zeolites and ACs. Some MOFs possess not only high BET surface areas (S_BET) compared to zeolites and ACs (e.g., 4230 for MIL-101 [26], 4500 for MOF-177 [27], 5100 for PCN-68 [28], and 6240 m²/g for MOF-210 [29]) but also extremely high-accessible pore volumes (e.g., MOF-200 (S_BET = 4530 m²/g) and MOF-210 with ca. 3.59 cm³/g) [29]. One of the most intriguing properties of many MOFs is that the metal centers after quest removal can become accommodation sites for adsorbates, similar to zeolites. Due to such textural and surface features, MOFs are extensively studied for the adsorptive separation of desired gases, such as CO₂, from gas mixtures containing other light gases [12,13,25,29,30].

Most of the early studies on adsorptive gas separation using zeolites, ACs, and MOFs have predominantly focused on CO₂/CH₄ systems [31,32,33] because of the substantial market need to purify natural gas, which contains 80–95% CH₄ and 5–10% CO₂, with N₂ and heavier hydrocarbons making up the remainder of the gas [20]. The suitability of such porous solid sorbents may depend on their potential applications. Exceptionally high CO₂ uptake (30.4–54.5 mmol/g) has been reported on recently spotlighted MOFs, such as MOF-200 and MOF-210, MIL-101, PCN-68, and MOF-177 [28,29,30,34], at 25 or 31 °C. However, such CO₂ adsorption has been possible at pressures near 35–50 bar (1 bar = 10⁵ Pa), which is too high because many industrial PSA systems commonly allow adsorption at 4–6 bar [14,20]. To the best of our knowledge, few early studies were able to directly compare the performances in CO₂ adsorption between the traditional benchmarks and the new class of materials at the typical PSA pressure. Indeed, Garces et al. [35] established a comparison of CO₂ adsorption on such solid adsorbents, but they allowed the use of only up to 1.2 bar pressure, which is almost the pressure for desorption (regeneration) rather than adsorption in typical PSA processes. We are, of course, aware of the adsorption of CO₂ and other gases, such as CH₄, N₂, H₂, CO, and N₂O, on two different types of porous sorbents, i.e., zeolites (13X, 4A) and ACs [36,37,38], and zeolites (13X, 5A) and MOFs (Cu-BTC, MOF-5, MOF-177) [39,40]. However, all these studies were conducted at adsorption pressures that were too low (slightly over the atmospheric pressure).

With the goal of producing sustainable syngases from biomass-based industrial wastes, their gasification under well-controlled conditions is under investigation in another part of this research program. As per our preliminary results, the gas streams produced were composed of approximately 20% CO₂, 38% CO, 4% N₂, and 38% H₂, with gases such as HCN, NH₃, HCl, and H₂S at tens to hundreds of ppm levels, varying with raw biomass wastes fed to the gasifier and its operation conditions. Thus, it is essential to primarily separate the 20% CO₂ from the mixture using cost-effective, energy-efficient separation technologies, such as PSA. For this, we studied the pure-component adsorption of CO₂, CO, N₂, and H₂ on zeolites, ACs, and MOFs up to a maximum pressure of 7.5 bar and at temperatures of 25 and 35 °C, conditions that are similar to typical operating conditions (T = 15–40 °C; P = 4–6 bar) for many industrial PSA systems [14,20,41]. The porous adsorbents chosen here have a significant difference in surface energetic heterogeneity, and the adsorbates possess distinct molecular properties, such as polarizability (α) and quadrupole moment (q_m). Though our ultimate goal was to separate CO₂ from the gas mixture, it should be emphasized that this study was conducted using only a single component of each gas in the specified range of temperature and pressure to examine the influence of surface nonuniformity on the molecular adsorption of light gases.

2. Materials and Methods

2.1. Adsorbents

NaY (CBV 100, Zeolyst, Conshohocken, PA, USA) and NaX (Z10-08EP, Zeochem, Houston, TX, USA) were used as representatives of zeolites. Two kinds of ACs were used for this study: a Norit GCN612 produced from coconut shells using a steam activation process, hereafter designated as “GCN”, and an MSP-20 phenolic-resin-based AC (Kansai Coke and Chemicals, Amagasaki, Japan), denoted as “MSP”. MOFs chosen here were Basolite A100 and Basolite Z1200 (Aldrich, St. Louis, MO, USA). The former is an aluminum-based MOF isostructural to MIL-53(Al) [9], and the latter is a Zn(II)-based ZIF with a zeolite-like topology that is an isostructural form of ZIF-8 synthesized by Yaghi and coworkers [10]. These powder-type MOF analogues are, respectively, referred to as “A100” and “Z1200”.

2.2. Characterization of Microporous Adsorbents

Liquid N₂ isotherms of the adsorbents used were collected using a Micromeritics ASAP 2020. A small amount of each sample (ca. 40 mg) was loaded into a factory-made adsorption cell in the apparatus and evacuated for 1 h at 30 °C, for 1 h at 90 °C, and finally overnight at 200 °C in vacuum, before N₂ was introduced into the sample cell at −196 °C. S_BET values were determined according to the BET multipoint technique. The size of the micropores present in the evacuated samples was calculated from the N₂ sorption data using the original Horvath–Kawazoe model, assuming slit-type micropores for ACs, and the Saito–Foley cylindrical pore model for the others [42,43], while porosity data regarding mesopores were obtained using the Barrett-Joyner-Halenda (BJH) pore model. Finally, quantities of Al and Si in zeolite samples were determined using inductively coupled plasma (ICP) measurements, and metal impurities existing in AC sorbents were qualitatively acquired using a Rigaku Model ZSX100e wavelength-dispersive X-ray florescence (XRF) spectrometer.

2.3. Volumetric Adsorption Measurements

The adsorption of CO₂, CO, N₂, and H₂ on the adsorbents chosen was carried out using a stainless steel high-vacuum system with a dynamic vacuum below 10⁻⁸ Torr (1 Torr = 133.3 Pa), equipped with an Oerlikon Leybold Vacuum Model TURBOVAC 151 turbomolecular pump (TMP) with a Turbo.Drive TD 20 Classic controller backed with a Kodivac GHP-340K mechanical pump. Dynamic vacuum levels were monitored with a Pfeiffer Model TPG 261 single-gauge controller. During the measurements, changes in gas pressure were measured using an absolute Honeywell Model Super TJE ultra-precision pressure transducer (Type AP112) with a full range of 200 psi (1 psi = 6.896 × 10³ Pa) connected to a Sensys Model 1300 pressure indicator, and the temperature of a manifold of the system was monitored with a Hanyoung Nux Model BK-6M Digicator. A schematic of such an adsorption system is shown in Figure S1, and details of a similar volumetric system have been also provided elsewhere [44,45,46].

An appropriate amount of each adsorbent (ca. 0.5 g) was placed over a 1 μm stainless steel filter welded inside a stainless steel adsorption cell in a cylindrical electric furnace with a Misung S&I Model TC500P temperature controller, heated to 300 °C using a heating rate of 3 °C/min under relatively low vacuum (10⁻³ Torr) with a rotary vane pump and evacuated overnight, cooled to room temperature, and further treated under high vacuum (typically 10⁻⁷ Torr) with the help of the TMP. After this pretreatment, the electric furnace was replaced by a Peltier-type thermoelectric heating/cooling device coupled with an Omega Model CN 7500 temperature controller so the adsorption cell could be constantly controlled at either 25 or 35 °C, at which adsorption measurements were allowed. The time to be equilibrated at every dose did not exceed 2 min, irrespective of the adsorbent and the adsorption temperature and pressure. Temperatures of the adsorption cell during sample pretreatment and gas adsorption were monitored using an Omega Model 410B Digicator with a K-type thermocouple that was inserted just above the powder sample in the cell through a feedthrough. Before CO₂ (99.999%), CO (99.998%), N₂ (99.9999%), and H₂ (99.9999%, Linde, Danbury, CT, USA) were admitted into the adsorption cell, all the adsorbates were further purified by allowing them to flow through Alltech moisture traps and Oxytraps.

3. Results

N₂ adsorption–desorption measurements with NaY, NaX, GCN, MSP, A100, and Z1200 were conducted using the standard BET technique (Figure S2, see Supplementary Materials). All the samples basically showed the standard International Union for Pure and Applied Chemistry (IUPAC) type I isotherm [47], which is a typical characteristic of microporous framework substances. The N₂ sorption data were subjected to the standard BET porosity analysis protocol with the help of appropriate pore models for collecting their textural properties, such as S_BET, micropore opening (d_mc), micropore volume (V_mc), mesopore volume (V_ms), and total pore volume (V_t). All these results, including the SAR values of the two zeolites, are listed in Table 1.

S_BET values were, respectively, 847 and 724 m²/g for NaY and NaX samples. Both these zeolites with d_mc = 7.71 ± 0.13 Ǻ had a V_mc of 0.292 ± 0.021 and a V_t of 0.345 ± 0.011 cm³/g, which was in reasonable agreement with earlier reports on such faujasite-framework zeolites [48,49]. GCN had an S_BET of 1132 m²/g, while MSP had an S_BET of 2508 m²/g, which was the largest among all porous sorbents used. The micropore sizes of these two carbonaceous substances (respectively, 5.02 and 5.53 Ǻ) could be considered almost the same; however, each corresponding pore volume was significantly different (Table 1). The textural properties of A100 could be characterized to be S_BET = 838 m²/g, d_mc = 10.96 Ǻ, V_mc = 0.361 cm³/g, and V_t = 1.109 cm³/g. The indicated S_BET and V_mc values were in excellent accordance with those (830 m²/g and 0.358 cm³/g) reported for commercial A100 in the form of a powder [50]. However, the d_mc and V_t values in our measurements differed from those in the literature. Heymans et al. [50] reported a V_t of 0.549 cm³/g, which is comparable or close to the values (respectively, 0.508 and 0.581 cm³/g) for laboratory-synthesized MIL-53(Al) [51] and another commercial A100 sample [52], while Fierro and coworkers [53] reported a d_mc of 5.3 Ǻ and a BJH pore volume of 5.5 cm³/g (thus accounting for only V_ms) for a commercial powder sample of A100. Such differences might be associated with the well-known large breathing phenomenon [54]. Pores having d_mc = 12.27 Ǻ, V_mc = 0.553 cm³/g, and V_t = 0.739 cm³/g existed in Z1200, with S_BET = 1301 m²/g, in reasonable agreement with the values (d_mc = 11 Ǻ, V_mc = 0.67 cm³/g, V_t = 0.77 cm³/g, and S_BET = 1417 m²/g) reported for a sample of Basolite Z1200 [55]. Finally, the well-known t-plot method gave V_mc values consistent with those compiled in Table 1 (not included here).

The adsorption of CO₂, CO, N₂, and H₂ on the six adsorbents at 25 °C was measured as a function of equilibrated pressure up to ca. 7.5 bar. Figure 1 and Figure 2 show the isotherms for the adsorption of each adsorbate on the respective NaY and NaX. NaY had much greater CO₂ uptake at all pressures covered compared to the other gases. At 6 bar, the extent of the adsorption of the gases was CO₂ (6.70) > CO (1.95) > N₂ (1.08) > H₂ (0.13 mmol/g). The type I isotherm was visibly shown for CO₂ but not for CO, N₂, and H₂. The adsorption of H₂ on NaY obeyed Henry’s law at all pressures because its uptake passed through zero when the isotherm was extrapolated to zero pressure, while the isotherms for CO₂, CO, and N₂ obeyed the law only below low or medium pressure, depending on the adsorbates. At high pressures (>3.5 bar), NaX gave a CO₂ uptake of nearly 5.4 mmol/g (Figure 2), which is lower, by 1.3 mmol/g, than that measured for NaY. When CO and N₂ were adsorbed on NaX, their uptake was higher than that obtained for NaY. There was no difference in the extent of H₂ adsorption between both zeolites at all given pressures.

Isotherms for the adsorption of CO₂, CO, N₂, and H₂ at 25 °C on the two kinds of ACs, GCN and MSP, are shown in Figure 3 and Figure 4, respectively. Both carbonaceous adsorbents showed not only a similar shape of each corresponding gas isotherm but also a comparable capacity to adsorb CO, N₂, and H₂. They, however, had a large difference in the CO₂ uptake at high pressures; for example, at 6 bar, 6.13 mmol CO₂/g could be adsorbed on GCN, while 10.41 mmol CO₂/g was adsorbed on MSP, which is almost 2-fold greater. However, the isotherms of CO, N₂, and H₂ with the AC sorbents were similar to those for the zeolites, whereas the CO₂ isotherm differed from that on the zeolites. Such a difference in CO₂ adsorption between the ACs and zeolites could be related to surface nonuniformity with high-affinity binding sites for CO₂ [12,17].

The extent of the adsorption of CO₂, CO, N₂, and H₂ at 25 °C on A100 and Z1200 after evacuation at 300 °C is shown in Figure 5 and Figure 6, respectively. At all pressures, on both adsorbents, the adsorption of CO₂ was the largest, followed by the adsorption of CO, N₂, and H₂, in that order, which was the same as what appeared for the zeolites and ACs. A100 consistently yielded a higher CO₂ adsorption capacity than that obtained with Z1200 at all pressures, suggesting that the former’s surfaces could be somewhat more electrophilic [50,53,56]. It is noteworthy that the CO₂ adsorption isotherm of A100 was similar to that of the AC adsorbents. On Z1200 surfaces, even CO₂ adsorption was seen at whole pressures that was almost governed by Henry’s law. This suggests that all the adsorbates interacted weakly with this sorbent.

Key parameters that must be assessed in gas adsorption studies for separation are the selectivity factor and the working capacity for a target gas, primarily CO₂ here, and these can be estimated using the adsorption data shown before. For multicomponent gas mixtures, the selectivity factor is usually calculated using the correlation (n_i/p_i)/(n_j/p_j), where n_i and n_j are the adsorbed amounts of components i and j, respectively, on the adsorbent surface and p_i and p_j are the mole fractions of components i and j, respectively, in the bulk gas phase [4,7,20,57]. However, since isotherms of only a single component were collected in this study, we calculated the selectivity factor using a simple correlation (n_i/n_j) proposed by Corma and coworkers [14], where n_i and n_j represent, respectively, the molar uptake of gasses i and j at a given pressure in the corresponding pure-component isotherms. For this, we compiled the molar uptake values of CO₂, CO, N₂, and H₂ adsorbed on each adsorbent from the isotherms in Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6, as partly listed in Table 2, in which such uptake values at 1 and 6 bar were collected because of a common operating pressure window for adsorption–desorption cycles in most PSA systems [20]. Figure 7 shows the CO₂/CO, CO₂/N₂, and CO₂/H₂ selectivities of all the adsorbents with respect to equilibrated pressure. All the estimated selectivities decreased with equilibrated pressure, regardless of the adsorbents used, which is in excellent agreement with previous reports [14,37].

The only exception was Z1200, in which all the selectivities somewhat increased with pressure up to ca. 2.5 bar and then were almost constant, although such pressure dependency on selectivity varied somewhat with adsorbates. To allow a ready comparison, the selectivities for the six porous materials at 1 and 6 bar are listed in Table 2. Finally, the CO₂ working capacity is defined as the difference between specific equilibrium molar uptake values at high and low pressures that were, respectively, chosen to be 6 and 1 bar here because of common adsorption–desorption cycles for the PSA processes [14,20]. The estimated CO₂ working capacities for each adsorbent are also collated in Table 2.

It is imperative to explore whether the adsorbents used in this adsorption study can be easily regenerated using appropriate desorption techniques. Three repeated measurements for CO₂ adsorption on NaY, GCN, and MSP following each evacuation at 25 °C for 2 h are provided in Figure 8, in which the first adsorption isotherms were measured on the clean surface of each adsorbent. These consecutive CO₂ adsorptions on NaY and MSP surfaces were completely in line with the first measurement isotherm corresponding to each sorbent, as shown in Figure 8a,c, indicating that CO₂ adsorption could be fully reversible and their surfaces could be easily recovered after evacuation, in good agreement with results reported for CO₂ adsorption on a zeolite 13X and commercial G-32 H activated carbon [37]. Obviously, there is a limit to these measurements, owing to the use of evacuation under a 10⁻⁸-Torr dynamic vacuum, in addition to a pressure swing as a driving force to desorb the adsorbed CO₂. Furthermore, there are reports that complete regeneration of zeolites after CO₂ adsorption is improbable [11,37]. To resolve this point, we desorbed CO₂ from NaY that had undergone CO₂ adsorption for the third time using a fast pressure swing from nearly 7.5 bar to 1 bar within 1 min following CO₂ adsorption, as shown in Figure 8a. The CO₂ isotherm measured from 1 bar was identical to isotherms indicated for the evacuated surfaces, once again showing that CO₂ adsorption on the zeolite NaY is a fully reversible process. Meanwhile, after the first exposure to CO₂ at 25 °C and up to 7.5 bar, there was irreversible CO₂ uptake on GCN at pressure > 2.5 bar, as shown in Figure 8b. However, at whole pressures, fully reversible CO₂ adsorption occurred on a carbon surface with small CO₂ residues, as revealed by the last isotherm. As for the NaY, both ACs were completely regenerated under the fast pressure swing to 1 bar after each third isotherm measurement (not shown here).

We were interested in metal impurities existing in GCN activated carbon to find out why small amounts of CO₂ were irreversibly accommodated on a clean surface of this sorbent in a high-pressure region. The wavelength-dispersive XRF spectrum of an as-received GCN sample was collected to identify such metal impurities and compared to that measured for MSP that had shown no irreversible CO₂ adsorption, and these XRF spectra are shown in Figure 9. The original intensity of a peak at 2θ = 136.55° due to K present in GCN reduced by 90% for a ready comparison with peaks of other elements, as indicated by an asterisk in Figure 9a. The most striking difference between the two AC samples was the 136.55° peak by K, the intensity of which was much greater in the XRF spectrum for GCN. Both ACs indicated the presence of transition metals, such as Zn, Cu, Ni, and Fe, but all these impurities were relatively at low levels. Peaks by Mg, Na, Si, Ca, and Al were observed with GCN but not or weakly observed with MSP; however, the effect of such impurities on irreversible CO₂ adsorption might be negligible compared to the effect of K.

Isotherms for CO adsorbed repeatedly on NaY, GCN, and MSP are provided in Figure 10. All samples showed that such repeated CO adsorption after every evacuation was on a single isotherm, which implies that there is no irreversible CO adsorption. This indicates that unlike the previous repeated CO₂ adsorption on a sample of GCN, the metal impurities, particularly K, in this adsorbent played no appreciable role in the CO adsorption. To confirm that CO adsorption is fully reversible even under a simulated PSA condition, after the third CO adsorption, NaY was pressure-swung to 1 bar in a way similar to that described for CO₂ adsorption, and then CO was again introduced into the adsorption cell. This result is shown in Figure 10a, as indicated by the open symbol, and we found that fully reversible CO adsorption occurred on NaY surfaces. After a similar pressure swing, all the ACs also underwent fully reversible CO adsorption (not shown here). It is clear that all these adsorbents are easily regenerated for CO adsorption even when such a pressure swing technique is used.

Isosteric heats of adsorption of CO₂ on the six porous materials used were calculated by applying the Clausius–Clapeyron equation,

(1)$\Delta H={ RT}^2{\left(\frac{\partial \mathrm{l}\mathrm{n}\mathrm{P}}{\partial \mathrm{T}}\right)}_n=R{\left\{-\frac{\partial \mathrm{l}\mathrm{n}\mathrm{P}}{\partial \left(\frac{1}{T}\right)}\right\}}_n,$

where ΔH is the isosteric heat of adsorption (kJ/mol), R is the universal gas constant, T is the adsorption temperature (K), P is the adsorption pressure (bar), and n is the adsorbed CO₂ amount (mmol/g). To do this, high-resolution CO₂ isotherms on NaY and NaX at the three temperatures were collected with a narrow pressure interval because of steep changes in CO₂ adsorption with these sorbents at low pressures, while for the other adsorbents, CO₂ isotherms at 35 and 40 °C were measured additionally to make corresponding sets with the 25 °C adsorption data in Figure 3, Figure 4, Figure 5 and Figure 6. Details of the fits and parameters of the widely used Toth model [58] and the ΔH calculations are provided in Figure S3 and Table S1. Changes in ΔH were plotted as a function of CO₂ uptake, as shown in Figure 11. At coverage near zero, the ΔH values on NaY and NaX were 40.8 and 53.8 kJ/mol, respectively (Figure 11), which are similar to those reported in the literature [59,60], and these values decreased up to loadings of ca. 6.1 and 3.9 mmol/g, respectively, after which, both values increased. Samples of GCN and MSP had almost the same ΔH values (25.4 and 23.3 kJ/mol) at zero loading, which are similar to the adsorption energies reported for other kinds of ACs [16,38]. However, these two ACs exhibited different behaviors in the decrease in ΔH with the CO₂ amount adsorbed; that is, the ΔH values rapidly decreased on GCN but not on MSP, suggesting that the former surfaces are much more heterogeneous, perhaps due to significant levels of impurity metals, such as K, as disclosed by the XRF measurements in Figure 9. A zero-coverage ΔH of 17.4 kJ/mol was indicated for Z1200, and this value remained constant. The indicated value is similar to that (15.9 kJ/mol) computed for CO₂ adsorption on ZIF-8 using a force field optimization simulation technique [61]. The value of ΔH with A100 was around 26.3 kJ/mol, comparable to that (20.1 kJ/mol) reported for MIL-53(Al) [62] but significantly lower than that (63 kJ/mol) determined calorimetrically for CO₂ adsorption on MIL-100 at 30 °C [34].

4. Discussion

4.1. Effect of the Textural Properties of Adsorbents on the Adsorption of Adsorbates

The range of the indicated micropore volume fraction, f_mc, was ca. 0.75–0.94, except for A100, in which V_ms was dominant. The smallest mouth size of micropores was 5.02 Ǻ, which was obtained for GCN. The kinetic molecular diameters of CO₂, CO, N₂, and H₂ used as adsorbates are, respectively, 3.3, 3.69, 3.64–3.80, and 2.83–2.89 Ǻ [12,63]. Thus, it is beyond doubt that no molecular sieving effect is anticipated in the adsorption of the gases even on NaX, because its d_mc value is much larger than that of the biggest gas molecule (N₂). It is noteworthy that all the isotherms were collected not only by adsorbing only a single gas component on each sorbent but also by allowing an adequate equilibration time upon every measurement; therefore, rate-dependent adsorption, known as the kinetic effect [4,5], is also improbable. In such an adsorption situation, the equilibrium-dependent mechanism would be predominant.

There was a visible difference in S_BET values and microporosity between the six adsorbents, as shown in Table 1. The question is whether such differences could contribute to the distinctive uptake of each adsorbate, particularly CO₂, among the sorbents. The S_BET values were in the following order: MSP >> Z1200 > GCN > NaY ≈ A100 > NaX. The order of CO₂ adsorption was NaY (5.60) > NaX (4.60) > MSP (3.25) > GCN (2.75) > A100 (2.20) >> Z1200 (0.66 mmol/g) at 1 bar and MSP (9.75) >> NaY (6.70) > GCN (5.87) > NaX (5.38) > A100 (5.18) > Z1200 (3.95 mmol/g) at 6 bar (see Table 2). Thus, we could find no consistency between S_BET and the CO₂ adsorption capacity at either pressure. The V_mc calculations were in the order: MSP >> Z1200 > GCN > A100 > NaY > NaX (Table 1), basically similar to that of the S_BET values. However, Z1200 with relatively high S_BET and V_mc values yielded the lowest CO₂ uptake at both pressures. Consequently, textural data, such as S_BET and V_mc, could not be successfully correlated with the indicated uptake of the adsorbates.

4.2. Molecular Properties of Adsorbates and Their Adsorption Behaviors

The adsorbates chosen possess different molecular properties, such as α and multipole moments [4,5,17]. The α value of CO₂ is 26.5–29.11 × 10⁻²⁵ cm³, which is the largest among the adsorbates, and CO, N₂, and H₂ have α values of 19.5 × 10⁻²⁵ cm³, 17.403–17.6 × 10⁻²⁵ cm³, and 8.0–8.042 × 10⁻²⁵ cm³, respectively [12,17]. The dipole moment of CO is 0.1098–0.112 × 10⁻¹⁸ esu·cm, where esu is the electrostatic unit (1 esu = 3.336 × 10⁻¹⁰ C); however, all the others are non-polar molecules and have no permanent dipole moment, thereby having no electrostatic dipole–adsorbent interactions [4,12,17]. The q_m values for CO₂, CO, N₂, and H₂ are 43.0, 25.0, 15.2, and 6.62 × 10²⁵ esu·cm², respectively [12,17]. Thus, CO₂ also has the largest q_m value, which is greater, by 1.5–6.5 times, than the q_m values of the other gases. A combination of such molecular properties of adsorbates and textural and surface features of adsorbents shows a general trend—the higher the S_BET of porous adsorbents, the larger the adsorption of gas molecules with a high α value, while an adsorbent surface with a high electric field gradient would prefer adsorbates with a high q_m value [4,5].

Keeping the previous discussion in mind, the capability of the three categorized sorbents (zeolite, AC, and MOF) to accommodate the adsorbates at the conditions chosen, i.e., 25 or 35 °C and up to 7.5 bar, was of particular interest because of the potential candidate to selectively separate CO₂ and N₂, primarily the former component, at this stage, after the gasification of biomass solid wastes. At all the pressures, the measured uptake of the adsorbates gave consistent results with their molecular properties. The six adsorbents all displayed equilibrium adsorption amounts along the series CO₂ > CO > N₂ > H₂ (see Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6 and Table 2), which is equal to the order of the α and q_m values, although their absolute uptake depends on the sorbents. This implies that the larger the α and q_m values of a gas molecule, the greater the extent of its adsorption on porous solid sorbents. However, the q_m value of CO₂ might not contribute much to its high adsorption on MSP-20 at 6 bar. The reason is the almost negligible electrostatic adsorbate–adsorbent interactions, since charges existing on such ACs are weak or close to each other and it is difficult for them to exert a significant electric field or field gradient on the surface [4]. Hence, the van der Waals interactions via dispersion and repulsion forces, whose equations are a linear function of α but not of q_m, play a dominant role in the adsorption of adsorbates on ACs. Based on the previous discussion of the textural and surface properties of the adsorbents and the electronic properties of the adsorbates, it is clear that both α and q_m properties are strongly associated with the extent of these adsorptions on those materials, except for AC sorbents, on which the highest CO₂ uptake mainly results from the largest S_BET value, indicating that porous solids with a high electric field gradient, thereby creating strong quadrupole–adsorbent interactions, may be better for the adsorption of CO₂ with a high q_m value. Such interactions take place on adsorbents with a variety of sites that can have a wide range of adsorption affinities [4,12,36,37] and influence not only the shape of the isotherms of adsorbates, particularly CO₂, but also the selectivity of CO₂ to other gases, which will be thoroughly discussed next.

4.3. Surface Nonuniformity of Adsorbents and Their Selectivity of CO₂ Adsorption

We can obtain the difference in surface properties among the six adsorbents from the shape of the adsorption isotherms for CO₂, CO, N₂, and H₂ at 25 °C. In the CO₂ adsorption data shown in Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6, two zeolite samples, i.e., NaY and NaX, exhibited an initial steep rise in its uptake within 1 bar, which differs from the isothermal behavior of the other sorbents. This is characteristic of porous solid substances with high-affinity sites on which CO₂ is strongly adsorbed [36,37]. In contrast, CO₂ uptake on Z1200 linearly increased with equilibrated pressure (Figure 6), suggesting that this sorbent predominantly consists of CO₂ adsorption sites with an affinity weaker than that of even MSP-20 and GCN. These ACs and A100 showed a CO₂ isotherm that could be characterized by an intermediate behavior between zeolites and Z1200 (Figure 3, Figure 4 and Figure 5). A similar trend was, to a much lesser extent, observed in the isotherms for CO and N₂, although it was not appreciable for H₂ because of the lowest α and q_m values.

The two zeolites possess extra-framework sites (Na⁺) well known to be occupied at positions with four- and six-membered Al–O–Si rings [49,64]. The surfaces even with only a single metal species are energetically nonuniform because of such different coordination environments, and the cations on the surface would create an electric field and field gradient that facilitate the adsorption of gas molecules with high multipole moments due to the extra-framework charge–multipole interactions [65]. CO₂ has the greatest q_m value (but no dipole), as discussed previously, and would undergo strong charge–quadrupole interactions; thus, the zeolites yield the steepest increase in CO₂ adsorption at low pressures (Figure 1 and Figure 2). However, ZIF-8 has been known to contain no coordinatively unsaturated sites (CUSs) interacting with adsorbates [61], and Z1200 isostructural to ZIF-8 exhibits isotherms with surface characteristics of the weakest affinity to even CO₂. In the case of GCN, MSP, and A100, the intermediate interaction between their surfaces and gas molecules is due to the presence of moderate-strength affinity sites. Appropriate thermal pretreatments of carbonaceous materials with various oxygen-containing functional groups result in their conversion to heterogeneous surfaces with a highly basic nature [66], which is preferable to CO₂ adsorption. However, even after the ACs studied here were pretreated at 300 °C in high-dynamic vacuum overnight, their electrostatic affinity to CO₂ seemed much weaker than that of the zeolites but not weaker than that of the sorbent without CUSs, i.e., Z1200. A100 as an isostructural form of MIL-53(Al) consists of chains of corner-sharing AlO₄(OH)₂ octahedra interconnected by benzenedicarboxylates [9]. The OH groups, denoted as μ₂-OH [56], are not a CUS (open metal sites); regardless, it has been frequently proposed that they can somewhat strongly interact with gas molecules with high α and q_m properties, such as CO₂ [32,56]. Consequently, the shape of the isotherms measured for the adsorbates, particularly CO₂, is a good indicator of high-affinity sites in the sorbents, and the indicated order of the surface energetic heterogeneity is zeolites >> ACs ≈ A100 > Z1200.

The previous discussion of the surface nonuniformity of the adsorbents is readily supported by changes in the ΔH value calculated as a function of CO₂ coverage. At almost zero loading, the order of the ΔH value was NaX (53.8) > NaY (40.8) >> A100 (26.3) ≈ GCN (25.4) ≈ MSP (23.3) > Z1200 (17.5 kJ/mol), as shown in Figure 11. The zeolites showed the steepest decrease in ΔH values with CO₂ loadings, indicating the highest surface heterogeneity with a wide range of adsorption sites with different activation energies [4,8,14,37,56,67]. However, the ΔH value on Z1200 was constant in the whole range of loadings allowed, indicating that this sorbent is energetically homogeneous [4,8,17,56]. All ACs and A100 displayed an intermediate behavior. All NaY, NaX, and A100 showed ΔH values that increased after a certain CO₂ coverage, which is due to the lateral interactions between the adsorbed molecules [17]. NaX has ΔH higher than that of NaY by 13 kJ/mol. This would result from the lower SAR of that zeolite (Table 1), as in the case of CO₂ adsorption on Na-SSZ-13 with different SARs shown by Lobo and coworkers [67]. A significant dependence of ΔH for CO₂, on the cage and window sizes of zeolite, has been reported [67]; however, such an effect is not anticipated, because the framework of NaY and NaX is the same. When CO₂ is adsorbed on A100, an increase in ΔH occurs from a much lower loading (ca. 2 mmol/g) compared to the two zeolites. This may be associated with a structural shrinkage of A100 due to the interaction between CO₂ and μ₂-OH sites [11,32,56], thereby facilitating such adsorbate–adsorbate interactions. Consequently, the estimated ΔH values for CO₂ adsorption again show the same order of the surface energetic heterogeneity of the adsorbents as that based on the shape of the measured isotherms.

Not only did the adsorbents used give significantly different selectivities of CO₂ to CO, N₂, and H₂, but the selectivity factors also varied with the equilibrium pressures allowed (see Figure 7 and Table 2). The estimated selectivity factor is CO₂/H₂ > CO₂/N₂ > CO₂/CO, and this order is maintained irrespective of the porous sorbent and chosen pressure, although the absolute values of each selectivity depend on the adsorbates and adsorbents. Such an order can be readily correlated to the difference (∆q_m) in q_m between CO₂ and other gases, which means that the ∆q_m value is the largest in a combination of CO₂–H₂ and the smallest in CO₂–CO. Of course, the difference (∆α) in α between CO₂ and the remaining adsorbates is apparently the same as the conclusion reached from the ∆q_m values, but this ∆α effect is minor [4,5,68]. This clearly implies that the larger the ∆q_m value, the greater the selectivity factor. Unfortunately, among the adsorbates used, a pair consisting of CO₂ and CO—which are of particular interest in the efficient separation of CO₂ from a gas mixture containing high concentrations of CO (near 38%)—as stated previously, gives the smallest ∆q_m value, unlike the CO₂–CH₄ systems (∆q_m = q_m for CO₂ because q_m = 0 for CH₄) that were the focus of many earlier studies [17,31,32,33].

Except for Z1200, all the adsorbents showed a decrease in the CO₂/CO, CO₂/N₂, and CO₂/H₂ selectivity factors with equilibrated pressures, although the extent of such a decrease depended strongly on the sorbents (Figure 7 and Table 2), indicating energetically heterogeneous surfaces [4,69,70]. The Z1200, for which the selectivity factors increased at loadings below 2.5 bar and then remained constant, is characterized by energetically homogeneous surfaces [4,69,70]. This exceptional case is consistent not only with the isotherms in Figure 6 that are a linear function of pressure even for CO₂ with the greatest α and q_m values but also with the constant ΔH value at whole loadings of CO₂ (Figure 11), as discussed earlier. Even the sorbents with energetically heterogeneous surfaces showed a significant difference in the extent of heterogeneity. For example, changes in the CO₂/H₂ selectivity factor, which would readily provide us with such a difference because of the largest ∆q_m between the two molecules, with respect to equilibrated pressure, appeared in the order: NaY ≈ NaX >> A100 > GCN > MSP (Figure 7c). This shows that the surface energetic heterogeneity is the highest in the zeolites but the weakest in the ACs. However, the steep decrease in ΔH for CO₂ gave the surface nonuniformity in the order: NaX > NaY ≈ GCN > MSP > A100 (Figure 11). Such a difference between the CO₂/H₂ selectivity factor- and ΔH-based surface heterogeneity orders may be due to higher H₂ adsorption on ACs than on A100 (Figure 3, Figure 4 and Figure 5).

Of particular concern to us was the CO₂/CO selectivity of the adsorbents used, because of the need for a PSA sorbent to preferentially separate CO₂ from both CO₂- and CO-rich gas streams in which such a selectivity may be critical. At 1 bar, the CO₂/CO selectivity of A100 was 8.2, which is higher than those of the other adsorbents (Table 2). However, it is necessary to mention again that the suitability of porous solids as PSA adsorbents should be considered based on selectivity data at higher pressures but not exceeding 10 bar, such as 4–6 bar [14,20]. Z1200 has a selectivity factor of 6.2 at 6 bar, which is higher than that (2.6–4.3) shown for the conventional benchmarks, such as zeolites and ACs, and another MOF, i.e., A100 (Table 2). Despite this, MOFs have some limitations to their widespread use as adsorbents in PSA systems for CO₂ separation from gas mixtures containing other light gases [63]. Representatively, MOF analogues are much less cost-effective because they are relatively expensive [11,63] and, to the best of our knowledge, have not yet been commercially proven in industrial PSA processes. Therefore, we think that NaY and MSP, which allow somewhat greater CO₂/CO selectivity among the benchmarks, may be a good option. The common pitfall of ACs is a relatively low CO₂/N₂ selectivity [71], also indicated in our study, where MSP yields a CO₂/N₂ selectivity of 4.5 at 6 bar, which is lower than that for the NaY. However, such a disadvantage may be resolved in the case of a CO₂-rich mixture stream, such as one with 38% CO₂/4% N₂, with both CO and H₂ making up the remainder of the stream.

4.4. Working Capacity of CO₂ Adsorption on the Adsorbents

One of the other parameters that should be assessed in adsorptive gas separation studies is the working capacity of adsorption of a target gas since the effectiveness of any microporous solid substance as a PSA sorbent is critically dependent on the extent of reloadings of the adsorbate in subsequent PSA cycles. We found that the estimated CO₂ working capacity at 25 and 35 °C greatly varies with the adsorbents used (see Table 2 and Table S2), which is associated with their surface energetic heterogeneity and the population of high-affinity sites for CO₂. MSP had the highest working capacity for CO₂ adsorption at 25 °C, ca. 6.50 mmol/g. This value is much higher than the 2.2 mmol CO₂/g working capacity reported for an LTA framework zeolite Rho showing the highest CO₂/CH₄ selectivity among zeolites studied to date [14] and, furthermore, is somewhat larger than or comparable to that (ca. 4–6.5 mmol/g) calculated at the chosen working pressure (1 and 6 bar) using CO₂ isotherms reported for MOF-200 and MOF-210, MIL-101, PCN-68, and MOF-177 [28,29,30,34]. Even at 35 °C, MSP had a high CO₂ adsorption working capacity (5.97 mmol/g), the largest among the six sorbents listed in Table S2. Thus, this AC is probably the most promising adsorbent because the working capacity determines the amount of CO₂ processed in every adsorption–desorption cycle in PSA systems, although the selectivity factors are comparable to or less than those of the zeolites, A100, and Z1200, at 6 bar. In addition, the working capacity for CO₂ adsorption on the MOFs at 25 and 35 °C was greater than that on NaY and NaX but was similar to that obtained for GCN (Table 2 and Table S2). Consequently, it is thought that among the porous sorbents, MSP may be a better choice for our later application in CO₂ separation from biomass-gasification-based syngases even at the small sacrifice of CO₂/N₂ and CO₂/H₂ selectivities.

4.5. Regenerability of Adsorbents

We studied whether the successful regeneration of porous solid adsorbents should be an essential consideration for industrial PSA applications. We also studied CO₂ and CO adsorption–desorption cycles with the zeolites and ACs using appropriate regeneration techniques, such as evacuation and fast pressure swing. Despite some earlier studies that have claimed difficulty in regenerating zeolite sorbents after CO₂ adsorption under common PSA operating conditions [7,11,37], we could not see such an irreversibility in CO₂ adsorption at 25 °C on faujasite-framework zeolites (NaY). CO adsorption on this material is also a fully reversible process (Figure 8a and Figure 10a).

The regenerability of the two ACs used is distinctive between themselves as well as between CO₂ and CO adsorption. MSP showed fully reversible CO₂ and CO adsorption regardless of the regeneration technique, whereas on GCN, such adsorption was possible for only CO (Figure 8b,c and Figure 10b,c). The irreversible behavior in the repeated CO₂ adsorption on GCN, which occurred at pressure > 2.5 bar, may be due to the inorganic elements indigenous to the carbon source, particularly K, whose XRF intensity is much greater than that of peaks not only of the other metal impurities in GCN but also of the K existing in MSP (Figure 9). Commercial ACs include substantial amounts of mineral substances as impurity metals, such as Na, K, Si, Al, Fe, and Ca, depending on the original carbon-rich sources, typically wood, coal, lignin, coconut shell, and chemical resins, and the activation processes [19,72]. Even though such indigenous inorganic matters can be remarkably reduced using appropriate acid washing treatments with usually hot HCl and HF solutions, it is difficult to completely remove them, especially from micropores in ACs. Only a small amount of K (ca. 123 ppm) with traces of Fe, Ni, and Cr (all less than 25 ppm) was reported to exist in another MSP sample [73]. However, such metal impurity levels seem to have no appreciable effect on even CO₂ adsorption, as observed in our measurements. It is proposed that inorganic metal impurities in carbonaceous sorbents should be identified and that their levels should be determined, if necessary, because of the significant dependence of such impurities on the original carbon sources and factory manufacturing processes.

5. Conclusions

Equilibrium adsorption of CO₂, CO, N₂, and H₂ at 25 °C (including 35 °C for CO₂) on zeolites, ACs, and MOFs was strongly associated not only with their physicochemical and surface features, but also with their molecular properties. All the adsorbents were microporous, and their V_mc, S_BET, and surface polarity could influence the adsorption of the adsorbates but not directly correlate to them. For a given sorbent, the q_m and α properties of the adsorbates played a critical role in determining their adsorption isotherm shapes, uptake, and selectivity factors, while with a chosen adsorbate, this adsorption strongly depended on the surface energetic heterogeneity of the sorbents. All the adsorbents, except for Z1200, were energetically heterogeneous but had a noticeable difference in terms of surface nonuniformity. The ΔH values calculated for CO₂ adsorption on the six adsorbents and the changes with its coverage were in excellent agreement with the surface heterogeneity. Zeolites, NaY, and NaX exhibited relatively high CO₂ adsorption at low pressures (near 1 bar), which was mainly because of strong extra-framework charge–quadrupole interactions between the Na cations and CO₂. As adsorption pressure increases, the S_BET of the adsorbents rather than their surface energetic heterogeneity may be more critical, as MSP shows the highest CO₂ uptake at 6 bar. Neither of the adsorbents studied had a problem in regeneration, even when using fast pressure swing desorption, except for GCN, on which CO₂ was irreversibly adsorbed at high pressures due to a significant quantity of metal impurities. The highest working capacity for CO₂ adsorption (6.50 and 5.97 mmol/g at 25 and 35 °C, respectively) was obtained for MSP. This adsorbent may be a good candidate for separating CO₂ from biomass-waste-driven syngases.

Author Contributions

Conceptualization, M.H.K.; methodology, K.H.K. and M.H.K.; validation, K.H.K. and M.H.K.; visualization, K.H.K.; investigation, K.H.K. and M.H.K.; resources, K.H.K. and M.H.K.; data curation, K.H.K. and M.H.K.; writing—draft, K.H.K.; writing—review, editing, and modification, M.H.K.; supervision, M.H.K.; project administration, M.H.K.; funding acquisition, M.H.K. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data reported here are available upon request.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Adsorption isotherms of CO2, CO, N2, and H2 on NaY at 25 °C. Solid lines are just to guide the eyes.

Figure 2. Adsorption isotherms of CO2, CO, N2, and H2 on NaX at 25 °C. Solid lines are just to guide the eyes.

Figure 3. Adsorption isotherms of CO2, CO, N2, and H2 on GCN at 25 °C. Solid lines are just to guide the eyes.

Figure 4. Adsorption isotherms of CO2, CO, N2, and H2 on MSP at 25 °C. Solid lines are just to guide the eyes.

Figure 5. Adsorption isotherms of CO2, CO, N2, and H2 on A100 at 25 °C. Solid lines are just to guide the eyes.

Figure 6. Adsorption isotherms of CO2, CO, N2, and H2 on Z1200 at 25 °C. Solid lines are just to guide the eyes.

Figure 7. Selectivity of CO2 to CO, N2, and H2 for microporous adsorbents with respect to pressure: (a) CO2/CO, (b) CO2/N2, and (c) CO2/H2.

Figure 8. Repeated measurements of CO2 adsorption at 25 °C and pressures of up to ca. 7.5 bar with selected sorbents: (a) NaY, (b) GCN, and (c) MSP. An open symbol in (a) indicates CO2 adsorption on a surface subjected to a pressure swing to 1 from ca. 7.5 bar after the third adsorption.

Figure 9. Wavelength-dispersive XRF spectra for fresh samples of the activated carbon sorbents used: (a) GCN, and (b) MSP. The original intensity of a peak at 2θ = 136.55° is marked with an asterisk due to K, as an impurity metal, being reduced by 90% for a ready comparison with other peaks.

Figure 10. Repeated measurements of CO adsorption at 25 °C and pressures of up to ca. 7.5 bar with selected sorbents: (a) NaY, (b) GCN, and (c) MSP. An open symbol in (a) indicates CO adsorption on a surface subjected to a pressure swing from ca. 7.3 bar to 1 bar after the third adsorption.

Figure 11. Isosteric heats of CO2 adsorption on NaY, NaX, GCN, MSP, A100, and Z1200.

Table 1

Textural properties of the zeolites, ACs, and MOFs used.

Adsorbent	Category	SAR a	SBET	dmc b (Å)	Pore Volume (cm3/g)			f mc c
			(m2/g)		Vmc	Vms	Vt
NaY	Zeolite	2.3	847	7.85	0.313	0.028	0.334	0.94
NaX d	Zeolite	1.3	724	7.58	0.272	0.097	0.357	0.76
GCN	AC	-	1132	5.02 e	0.439	0.076	0.479	0.92
MSP	AC	-	2508	5.53 e	0.991	0.299	1.146	0.86
A100 f	MOF	-	838	10.96	0.361	0.899	1.109	0.33
Z1200 g	MOF	-	1301	12.27	0.553	0.125	0.739	0.75

AC: activated carbon; MOF: metal–organic framework; SAR: Si-to-Al ratio; S_BET: BET surface area; d_mc: micropore diameter; f_mc: micropore fraction; V_mc: micropore volume; V_ms: mesopore volume; V_t: total pore volume; “-”: not applicable. ^a Using inductively coupled plasma (ICP) measurements. ^b Using the Saito–Foley cylindrical pore model. ^c Defined as f_mc = V_mc/V_t. ^d Commonly called “zeolite 13X”. ^e Using the original Horvath–Kawazoe slit pore model. ^f A commercial form isostructural to MIL-53(Al) in Ref. [9]. ^g A commercial form isostructural to ZIF-8 in Ref. [10].

Table 2

Uptake of CO₂, CO, N₂, and H₂ on microporous adsorbents at 25 °C and 6 bar and their selectivity factors and working capacity for CO₂ adsorption.

Adsorbent	Gas Uptake (mmol/g) a					Selectivity Factor a,b			WorkingCapacity (mmol/g) c
	CO2	CO	N2	H2		CO2/CO	CO2/N2	CO2/H2
NaY	6.70 (5.60)	1.95 (1.10)	1.08 (0.23)	0.13 (0.02)		3.4 (5.1)	6.2 (24.4)	51.5 (280)	1.10
NaX	5.38 (4.60)	2.08 (1.12)	1.43 (0.65)	0.12 (0.02)		2.6 (4.1)	3.8 (4.1)	44.8 (230)	0.78
GCN	5.87 (2.75)	1.94 (0.57)	1.48 (0.40)	0.19 (0.04)		3.0 (4.8)	4.0 (6.9)	30.9 (68.7)	3.12
MSP	9.75 (3.25)	2.72 (0.68)	2.19 (0.50)	0.31 (0.06)		3.6 (4.8)	4.5 (6.5)	31.4 (54.1)	6.50
A100	5.18 (2.20)	1.22 (0.27)	0.89 (0.19)	0.11 (0.02)		4.3 (8.2)	5.8 (11.6)	47.1 (110)	2.98
Z1200	3.95 (0.66)	0.64 (0.12)	0.52 (0.09)	0.10 (0.02)		6.2 (5.5)	7.6 (7.3)	39.5 (33)	3.29

^a Values in parenthesis are at 1 bar. ^b Based on each equilibrium uptake on the corresponding pure-gas isotherms. ^c Difference between CO₂ uptake values at 1 and 6 bar.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su151511574/s1: Figure S1: Schematic of an adsorption system. Figure S2: N₂ sorption isotherms of (a) NaY, (b) NaX, (c) GCN, (d) MSP, (e) A100, and (f) Z1200. Figure S3: Toth model fits to CO₂ adsorption on the six adsorbents at 25, 35, and 40 °C. Table S1: Toth model parameters for CO₂ adsorption isotherms on zeolites, ACs, and MOFs used. Table S2: Uptake of CO₂ on microporous adsorbents at 25 and 35 °C and 6 bar and their working capacity.

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