Journal Pre-proof Adsorption Of Methane And Nitrogen On Basolite Mofs: Equilibrium And Kinetic Studies David Ursueguía, Eva Díaz, Salvador Ordóñez PII:
To appear in:
Microporous and Mesoporous Materials
Received Date: 20 November 2019 Revised Date:
13 January 2020
Accepted Date: 23 January 2020
Please cite this article as: D. Ursueguía, E. Díaz, S. Ordóñez, Adsorption Of Methane And Nitrogen On Basolite Mofs: Equilibrium And Kinetic Studies, Microporous and Mesoporous Materials, https:// doi.org/10.1016/j.micromeso.2020.110048. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Inc. All rights reserved.
Highlights • • • • •
The adsorption of CH4 and N2 was studied on three Metal Organic Frameworks. The CH4/N2 ratio is maximum at low temperatures. Langmuir and fractional order kinetic models fit the best both for CH4 and N2. Freundlich and Sips isotherms obtained the best equilibrium fittings. Adsorption both in mesoporous and micropores are the faster steps.
ADSORPTION OF METHANE AND NITROGEN ON BASOLITE MOFs: EQUILIBRIUM AND KINETIC STUDIES
David Ursueguía, Eva Díaz, Salvador Ordóñeza
Catalysis, Reactors and Control Research Group (CRC), Department of Chemical and
Environmental Engineering, University of Oviedo, 33006, Spain e-mail: [email protected]
, Tel: +34 985 103 437; Fax: + 34 985 103 434
The adsorption/desorption behaviour of methane and nitrogen on three different Metal Organic Frameworks (MOFs) was studied in this work. The objective was to obtain new insights into the adsorption process of both molecules, since the similarity in the sizes of methane and nitrogen hinders the methane recovery from lean methane-containing emissions using adsorption technologies. In that way, the capacity of adsorption of methane and nitrogen was measured on Basolite C300, Basolite F300 and Basolite A100, being remarkable the Cu interaction of the Basolite C300 with the methane, as it was deduced from both the capacity and the heat of adsorption. Evaluation of the kinetic models of adsorption leads to observe the best fitting of the Langmuir and the fractional order models. For both adsorbates, the adsorption is easier than desorption on the surface of the three materials. The main differences observed among the adsorbents were the faster adsorption of both gases on Basolite A100, as well as the larger dependence on the occupied adsorption sites. Likewise, different adsorption isotherms were used for modelling the equilibrium results obtained, Freundlich and Sips isotherms providing the best results. These facts highlight the key role of certain surface heterogeneities on the adsorption capacity. Furthermore, parameters of both the adsorption isotherms and the kinetic models could explain the differences among the adsorbents in the adsorption and desorption performance, highlighting the utility of the proposed models.
Keywords: equilibrium; adsorption capacity; adsorption kinetics; greenhouse gases abatement; adsorption regeneration
Ventilation Air Methane (VAM) from underground coal mines are fugitive gas emissions that constitute a significant contribution to greenhouse gases, due mainly to the methane emissions in concentrations between 0.1-1 %, diluted in other gases such as air or carbon dioxide . Concentration of these emissions is fixed by safety constraints, since the explosive range of methane in air is between 5-15 %. Although this hazard has generally been minimized by combustion, this stream is, in fact, an energy source or even a chemical feedstock that could be exploited [2-5]. Therefore, the upgrade of these emissions would be of great interest. However, two intrinsic features of these streams: the dilution in large flows and the great variation of concentration [6-9], make compulsory a concentration step . Adsorption is one of the main technologies that is promising for increasing methane concentration in exhaust ventilation conducts, in fact, this technique achieved enrichments up to 51.3 % mol of methane from streams containing initially 2.4 % mol of methane . Considering that VAM stream consists mainly of N2, O2 and CH4, add to impurities, with molecular dynamic radius of 3.64, 3.46 and 3.82 Å, respectively, it is observed that the molecular diameter of methane is slightly larger. Special attention should be paid to nitrogen, as it is the major component in the mixture and its molecular size is the most similar to methane. Both N2 and CH4 present null dipole moment, but polarizability is greater for methane; therefore the theoretical strength of interaction of the methane molecule on a nonpolar adsorbent surface should be higher than for nitrogen . Among the possible adsorbents to be used, both activated carbons and zeolites [13,14] are suitable adsorbent media. However, in both cases, the CH4/N2 separation factor, as well as with other gases essayed as O2, are quite poor [15-18]. Metal Organic Frameworks (MOFs), highly porous materials with outstanding properties in the methane storage, are in an advantageous position as adsorbents in order to achieve methane concentration from aforementioned diluted streams. MOFs constitute a family of porous materials formed by two major components, a metal ion or metal oxide and an organic linker. These structures present high surface area and pore volume, and offer the possibility of obtaining many different combinations of its components. Likewise, the great variety of possible organic molecules –which can be functionalized- linking the metallic ions, allow tailoring the material to a specific separation . In fact, this family of porous materials has been used in recent years for different applications related to gas storage, mainly in order to solve various challenges in relation with energy, environment and health care sectors . It is 3
for this application, which research in MOFs materials has advanced greatly in order to develop materials for high-capacity methane storage , with the objective fixed by the US Department of Energy (DOE) of 350 cm3 (STP) cm-3 of methane storage capacity, based on the crystallographic density of MOF materials . Although this approach has overestimated the actual storage capacity of MOFs, these materials could be competitive adsorbents for methane concentration from dilute mixtures. To achieve that, it is critical to obtain detailed information about the required adsorption process. Most of the available manuscripts deal with the capacity storage determination and, in the great majority of the cases, at elevated pressures. Furthermore, the most commonly employed method is the determination of adsorption isotherms and the isosteric heat of CH4 adsorption. However, it does not focus on other parameters that can provide very valuable information to understand the process, such as adsorption isotherms, kinetic models, studying the material regenerability or even making a comparative among different reproducible (commercial) MOFs. In this context, the present research is focused on the role of the surface morphology and chemistry of three commercial MOFs on the methane and nitrogen adsorption capacity of these materials. Special attention was paid to the strength and the specificity of the interaction, recording adsorption data at total pressures close to the atmospheric, which is more usual in the methane adsorption from lean mixtures. These interactions were studied from the point of view of the adsorption isotherms fitting and kinetic models, as well as the regenerability in order to evaluate their potential for practical use. The three MOFs are commercialized by BASF: Basolite C300, F300 and A100. Basolite structures C300 and F300 are constituted by 1,3,5-benzene tricarboxylic acid (1,3,5-BTC) as ligand coordinated to Cu2+ and Fe3+, respectively. The former shows the HKUST-1 structure, whereas the second MOF, Fe-BTC, contains 21.2 % of iron, Fe(III) as nodal metal, and tripodal BTC as ligand. In the case of Basolite F300, its composition is similar to MIL-100(Fe) MOF, although the commercial product corresponds to a distorted structure of the crystalline MIL-100(Fe) . Concerning Basolite A100, it presents a MIL-53 structure, with the benzene-1,4-dicarboxylate (bdc) as ligand coordinated to Al+ . The studied behaviour of both methane and nitrogen interaction on the MOFs surface has a direct practical implication for the development of MOFs for enhanced methane separation from diluted mixtures.
EXPERIMENTAL SECTION Materials Basolite C300 [Cu3(C9H3O6)2], Basolite F300 [C9H3FeO6] and Basolite A100 [C8H5AlO5] were supplied by BASF (96 %; mass basis purity). All three materials are in the form of powder (1030 µm), and were stored in a desiccator to avoid its contact with the ambient air. Methane, nitrogen and helium, with a purity > 99.995 % mol, were supplied by Air Liquide.
Methods The CH4 and N2 adsorption-desorption amount on the adsorbents as a function of time was measured by a thermal gravimetric analyzer (Setaram, Sensys) in order to obtain information on the equilibrium and kinetics [25, 26]. Samples (10 mg) were pretreated in situ at 498 K and 0.1 MPa in pure He flowing at 20 mL/min for 1.5 h before measurement of either CH4 or N2 adsorption at 0.1 MPa and temperatures from 498 to 298 K with total flowrates of 20 mL/min. It should be noted that the maximum temperature is, in all cases, under decomposition temperature of the materials, as it was checked by thermogravimetry. The gas adsorption is measured by the weight increase during the experiment. Once the equilibrium is reached, the adsorption temperature is decreased stepwise until the ambient temperature. In order to obtain accurate values of methane and nitrogen uptake, the buoyancy effect was taken into consideration
adsorption/desorption data were corrected using a quartz blank calibration. Differential scanning calorimetry (DSC) measurements of total adsorption heats were made simultaneously in the same apparatus. Adsorption isotherms were obtained in an AutoChem II 2920 apparatus by flowing a methane/nitrogen mixture at a total pressure of 0.1 MPa. Experiments were performed at 298 K and 50 mL/min (s.t.p) total flow rate. The evolution of CH4 and N2 desorption signals were followed in a Pfeiffer Vacuum Omnistar Prisma mass spectrometer at 298 K, in order to obtain the final uptake for each gas on each material through a previous calibration of the device. The regenerability of the adsorbents was essayed by six successive experiences of adsorption and desorption for each gas. In this case, once the surface of the adsorbent was saturated by flowing the pure gas (0.1 MPa, 298 K and 50 mL/min) for 2 h, its desorption was measured at increasing temperature until 463 K with a temperature ramp of 5 K min-1 and pure helium flowing at 0.1 MPa and 50 mL/min.
RESULTS AND DISCUSSION
1. Pure components adsorption capacity Thermogravimetry at several temperatures between 498 and 298 K (Fig. 1) determined the adsorption capacities of pure components stepwise. After observing null retention at 498 K for both methane and nitrogen, the temperature was decreased and kept at each step until adsorption equilibrium was reached. It should be noted that the equilibrium was reached faster at the highest temperatures (about one hour) and required a longer period at 298 K (over 30 hours). Below 330 K, the uptake of both CH4 and N2 is higher for Basolite C300, whereas over this temperature, Basolite F300 presents a slightly higher retention. At the entire interval, the Basolite A100 presents the lowest adsorption capacity. The highest adsorption capacity of Basolite C300 below 330 K can be easily justified by the morphological features of the materials. Its surface area is 1514 m2/g, with a microporous volume of 0.56 cm3/g, whereas Basolite F300 presents a BET surface of 962 m2/g with a microporous volume of 0.30 cm3/g . Concerning the Basolite A100, the BET surface area is much lower, 662 m2/g , with a microporous volume of 0.35 cm3/g . Furthermore, the plot of the CH4 and N2 uptake versus the surface area of the three adsorbents suggest that the adsorption capacity of both gases at 298 K is mainly related with the surface area, Fig. 2. Likewise, although the uptake is most favorable for methane, it is remarkable the similar slope for both molecules, attributed to the similar diameter of both: 3.65 Å for N2 and 3.82 Å for CH4 [29, 30]. Capacities of adsorption here shown are higher than the 12 mg/g reported for CH4 uptake on a zeolite 5A at atmospheric pressure and 298 K  or a MOF-MWCNT nanocomposite (12 mg/g) ; and similar, in the case of Basolite C300, to NaX zeolite. Above 330 K, the adsorption capacity of both gases decreases drastically, especially for Basolite C300. The lower adsorption capacity value of Basolite C300 than for the Basolite F300, could be attributed to the higher accessibility of active sites in the case of Basolite F300, because of its lower fraction of microporous surface area (645 m2/g, 67 %) versus 1268 m2/g for the Basolite C300 (83 %) . This explanation is confirmed by the Basolite A100, with a 66 % of microporous surface area (437 m2/g) . Therefore, above 330 K, the adsorption capacity follows the same order as the mesoporous surface area: F300 (317 m2/g) > C300 (246 m2/g) > A100 (225 m2/g).
Figure 1. Evolution of CH4 (a) and N2 (b) adsorption with the temperature in the interval 298498 K for the three MOFs considered: F300 (▲), C300 (•) and A100 (x). Experimental conditions: 0.1 MPa and 20 mL/min.
Figure 2. Relation between surface area of the three adsorbents essayed and CH4 (▲) and N2 (•) pure gas adsorption capacity at 298 K, 0.1 MPa and 20 mL/min of total flow.
Basolite C300 presents a HKUST-1 structure formed by the link of benzene 1,3,5-tricarboxylate (BTC) and copper ions with a pore size of 10 Å [33, 34], but it also presents small cages of about 4, 10 and 11 Å diameter ; whereas Basolite F300 presents a structure similar to MIL100(Fe), constituted by Fe atoms and BTC linker, with pore sizes of 25 and 29 Å . Concerning Basolite A100, it has a MIL-53(Al) metal organic framework made up of Al and oxygen nodes with 1,4-benzodicarboxylic acid struts between the nodes, with an average pore size of 8.5 Å . Furthermore, although MIL-53 structure is characterized by its enormous flexibility and the occurrence of an oscillation (or breathing) during adsorption between two distinct conformations, called the large-pore phase (lp) and the narrow-pore phase (np), its capacity of adsorption is in all cases reduced . Observation of the CH4/N2 adsorption ratio with temperature (Fig. 3) – calculated as the methane and nitrogen molar adsorption ratio, at 0.1 MPa, and for each temperature evidences the increasing separation factor between both gases at the lowest temperature for Basolite F300 and A100, whereas in the case of Basolite C300 it is observed a maximum in the CH4/N2 ratio at 313-333 K. This behavior suggests a methane physical interaction with both Basolite F300 and A100, which in the last case is more pronounced with the absence of specific interaction with the hydroxyl groups of the structure. Contrary, for Basolite C300, the chemical specificity of the open Cu site could dominate the methane interactions. In fact, although both 8
molecules (methane and nitrogen) present null dipole moment, the polarizability of the methane molecule is slightly higher (26·10-25 and 17.6·10-25 cm3, respectively) . Long Xue et al.  reported adsorption isotherms for both CH4 and N2 on zeolite X/activated carbon composites. They observed a maximum methane adsorption capacities of 16 mg/g at 273 K, and a CH4/N2 ratio of the same order of magnitude as for Basolites, but at 20 K lower, thus, CH4/N2 separation on MOFs is more favoured. On the other hand, Chanajaree et al.  based on Monte Carlo simulations, obtained for the ZIF-78 MOF a CH4/N2 selectivity that doubles that obtained by the Basolites.
Figure 3. CH4/N2 molar adsorption capacity ratio for pure gas uptake for the three MOFs: F300 (▲), C300 (•) and A100 (x). Adsorption capacities were obtained at 0.1 MPa of total pressure.
2. Adsorption enthalpy for pure components adsorption Enthalpy changes derived of adsorption measurements were determined directly by DSC. The variation in the peaks in the interval 298-333 K was practically negligible. The average heat of adsorption at this interval was determined for methane, decreasing in the following order: 26.56 kJ/mol (Basolite C300) > 26.02 kJ/mol (Basolite A100) > 25.28 kJ/mol (Basolite F300). In the case of nitrogen, the order is coincident: 15.68 kJ/mol (Basolite C300) > 14.86 kJ/mol (Basolite A100) > 14.72 kJ/mol (Basolite F300). Therefore, the interaction of methane with the
adsorbents is stronger than in the case of nitrogen, although there are not important differences among the three different Basolites. The value presented for methane heat of adsorption for Basolite C300 is similar to the 26.8 kJ/mol experimentally observed at zero-coverage for ATC-CU material, which is characterized by oppositely adjacent open metal sites interaction to be separated from nitrogen . Thus, it could be inferred that the Cu available sites to be responsible of this type of interactions. Likewise, for the HKUST-1 structure, values of 21.1 kJ/mol were reported at zero coverage ; whereas in the case of MIL-53(Al) structure, 17 kJ/mol were reported at low pressures . It should be taken into account that the contribution of high-energy sites will be significantly outweighed at zero coverage, thus, it could be reasonable to obtain larger enthalpies at the lowest adsorbate concentration. However, add to this, the increase in the interaction potential in micropores, and also, in the case of MIL-53(Al) structure, the oscillation of the structure could influence. Thus, the Basolite A100, with the lowest percentage of microporosity, is the adsorbent with the lowest enthalpy of adsorption at low coverage.
3. Kinetic models for methane/nitrogen adsorption onto MOFs The development and study of a kinetic model is useful in order to explore the adsorption mechanism and for designing or optimizing the adsorption process. In this way, adsorption uptake curves of both methane and nitrogen at 298 K (which is the temperature of the largest capacity and hence, of higher interest), and 0.1 MPa flowing at 20 mL/min were fitted to several kinetic models. In general, the adsorption process is constituted by three consecutive steps: external mass transfer of the adsorbate from the bulk to the external surface of the adsorbent, internal diffusion of the adsorbate to the sorption sites and, finally, the adsorption itself. In order to analyse the adsorption kinetics of both methane and nitrogen, correlation between adsorbed amounts and time were employed. Data were fitted to adsorption models, where the adsorption step is considered the slowest stage: pseudo-first order, pseudo-second order, Langmuir model and fractional-order kinetic one; as well as the intra-particle diffusion model, where its rate depends on the rate at which components diffuse towards one another. The pseudo-first order kinetic equation : =
Where qe and qt are the adsorption capacities on the adsorbents at saturation time and at a given time, t, respectively; kf is the kinetic constant (s-1). 10
The pseudo-second order kinetic equation: =
Where ks is the kinetic constant (g/mg·s). The Langmuir model: =
Where kads is the reaction constant of adsorption terms, s-1, and kdes is the reaction constant of desorption terms, s-1. The fractional order kinetic equation: =
( − 1) !
Where k, s-1, m and n are the constants of the model. And, finally, the intraparticle diffusion model equation : =
Where ki is the rate constant, mg·(g·min0.5)-1, and C is related to the boundary layer thickness.
Figure 4. Experimental adsorption capacity and adsorption kinetic models for Basolite C300 (a, b), Basolite F300 (c, d) and Basolite A100 (e, f) for the adsorption of methane and nitrogen. Experimental (●) and models: Pseudo-First (Blue), Pseudo-Second (Green), Langmuir (Yellow) and Fractional order (Purple). Curves were obtained by thermogravimetry (298 K, 0.1 MPa and 20 mL/min of pure gas).
Table 1 summarizes the calculated constants and the accuracy of each model, whereas experimental adsorption capacities and adsorption kinetic models are shown in Fig. 4. Pseudofirst and pseudo-second order models are the most commonly used adsorption rate models. The first one assumes that the adsorption rate is proportional to the number of free adsorption sites, whereas the second one assumes that the adsorption capacity is proportional to the amount of active sites on the adsorbent. As it is observed, although pseudo-first order kinetic model can be fitted with the trend for the adsorption kinetic curves, there are some differences from the actual adsorption process, consisting mainly on the faster adsorption rate than the model prediction. It is especially remarkable the bad of the fitting for Basolite A100. In case of the pseudo-second order kinetic model, it is observed for Basolite C300 and F300 that, although the fitting of the model at the lowest times is accurate, the predicted equilibrium adsorption capacities is in all cases lower than the actual one. It is different the behavior of the model in the case of Basolite A100, which is quite good for the methane adsorption, although in the case of nitrogen, it predicts a faster process and larger adsorption capacity. According to the assumption of the chemical adsorption, from the pseudo-second order model, it can be inferred some type of preferential adsorption between the methane and the Basolite A100, by contraposition to nitrogen adsorption. This could be somehow related to the breathing effect of MIL-53(Al) structure, since it was reported that at 1 bar and high methane concentration, it is expected a structure in the lp state . Concerning the Langmuir model, it considers that the rate of adsorption is proportional to the percentage of unoccupied active sites and the concentration of the gas –which remains constant for the three adsorbents-, and the rate of desorption is proportional to the percentage of the covered surface. This model fits to the experimental data for Basolite F300 and C300, although some discrepancies are observed in the case of the Basolite A100. Considering just Basolite C300 and Basolite F300, with curves of adsorption more similar, the Basolite C300 shows higher values of the adsorption constant, which is congruent with the higher value of adsorption heat in comparison to Basolite F300. In the case of Basolite A100, the values of kads and kdes calculated are one order of magnitude larger than for Basolite C300 and Basolite F300. These larger values are congruent with the lower number of active sites available for the gas adsorption and then, a faster saturation.
Table 1. Kinetics of adsorption for the removal of methane and nitrogen Material
Nitrogen Pseudo-first order model
C300 F300 A100
k (s )
7.00·10-5 6.00·10-5 8.00·10-5
0.990 0.988 0.589
1.00·10-4 6.00·10-5 4.00·10-4
0.996 0.903 0.782
Pseudo-second order model
C300 F300 A100
1.92·10-6 2.73·10-6 3.92·10-5
0.788 0.800 0.954
3.59·10-6 4.97·10-6 4.40·10-4
0.797 0.836 0.112
C300 F300 A100
7.16·10-5 5.87·10-5 2.82·10-4
8.12·10-7 5.30·10-7 7.94·10-6
0.991 0.990 0.969
1.05·10-4 8.60·10-5 3.28·10-4
0 2.32·10-6 1.85·10-5
0.997 0.995 0.936
Fractional-order kinetic model
C300 F300 A100
0.002 0.002 0.001
0.631 0.620 0.787
0.730 0.696 0.998
0.993 0.999 0.963
0.001 0.002 0.001
0.749 0.691 0.900
0.768 0.730 1.120
0.990 0.992 0.791
Finally, the fractional order kinetic model is a semiempirical kinetic equation  that assumes that the adsorption rate is proportional to the nth power of the driving force and mth power of the adsorption time. That is, this model describes the adsorption rate by: k, an overall parameter that may couple various adsorption related factors; m, refers to diffusion resistance and n, reflects the effect of the driving force (number of unoccupied sites) which was related to the adsorption apparatus and filling mode of the adsorbent . However, this constant is not actually the kinetic constant the adsorption itself, but a complex combination of different parameters such as the kinetic constant for desorption, the surface coverage at equilibrium and the change of adsorbate concentration during the process . From Fig. 4 it is observed that equilibrium is first reached on Basolite A100, being especially relevant the largest n value for both gases in comparison to the other adsorbents; although the fitting of this model on this adsorbent, especially for nitrogen, is really poor. From the n parameter, lower than one in all 14
cases (except for nitrogen on Basolite A100), and really near to one for methane adsorption on Basolite A100, it is deduced than the rate of adsorption is not so dependent on the driving force for Basolite C300 and F300. On the other hand, the dependence of Basolite A100 to the percentage of the covered surface matches with the results obtained for Langmuir model. It is remarkable the good fitting to this kinetic model of experimental data with the only exception of the adsorption of nitrogen on Basolite A100. This leads to the conclusion that both the completely unspecific interaction of nitrogen molecule (null dipole moment and very low polarizability), and the dual configuration of the MIL-53(Al) structure, could avoid this fitting. The kinetic data were further fitted to the intraparticle diffusion model, for separating different diffusion stages of the adsorption process. Generally, if the so-called Weber-Morris plot of qt vs t0.5 gives a straight line, the sorption process is controlled by intra-particle diffusion only. In the case the data exhibit multi-linear plots, then two or more steps influence the adsorption process . Fig. 5 shows the plots of both methane and nitrogen uptake versus t0.5 for the three adsorbents. The curves obtained for both molecules exhibit multi-linearity, which confirms that adsorption takes places in different regimes. Furthermore, for Basolite C300 and Basolite F300, it is possible to distinguish even four differenced zones, which could be attributed to the film diffusion, gradual adsorption in mesopores, gradual adsorption in micropores, and finally, the equilibrium stage. The rate constant is obtained from the slope of the plot, thus adsorption both in mesoporous and micropores are the faster steps. In the case of Basolite A100, three areas can be observed, and the equilibrium step is reached at times scarcely over 100 min. For this material, a clear differenced behaviour is observed, since the adsorption stage is much shorter and uniform, probably due to its flexibility during the adsorption process.
Figure 5. Intraparticle diffusion model plots for (a) methane and (b) nitrogen adsorption for the three adsorbents: Basolite C300 (blue), Basolite F300 (green) and Basolite A100 (yellow). The continues lines split the areas for Basolite C300 and Basolite F300. Discontinuous line refers to Basolite A100.
4. Methane/Nitrogen adsorption isotherms The experimental data of adsorption isotherms at 298 K for both CH4 and N2 and the corresponding isotherms models fitting curves are illustrated in Fig. 6. This is useful in order to explore the adsorption mechanism, pointing out the possible differences among the adsorbents. In this work, a one-parameter isotherm, Henry, and two and three parameter ones were fitted to the experimental data, being summarized the parameters of each one in Table 2. Henry isotherm is the simplest one, and in this case, the amount of adsorbed adsorbate is proportional to the partial pressure of the adsorptive gas : =
Where qe is the amount of adsorbate at the equilibrium, ce, the equilibrium concentration, and kH, the Henry’s constant. Although the fit is quite good for the three adsorbents and the two gases, it can be observed how the fitting is more appropriate at relative pressures lower than 0.4, as expected with this simplified equilibrium model. Likewise, the main deviations were observed for nitrogen adsorption, although in the case of Basolite F300 methane R2 is just of 0.97. The other equilibrium models tested in this work are of two or three parameters. Hill-Deboer isotherm describes a situation in which there is mobile adsorption as well as lateral interaction among adsorbed molecules : . /
- (1 − θ) 0−
Where θ is the fractional coverage, K1 is the Hill-Deboer constant, and K2 is the energy constant of the interaction between adsorbed molecules. As can be observed, both in Fig. 6 as in Table 2, the fitting is really poor for this isotherm in all cases, thus, the initial hypothesis of mobile adsorption among the adsorption sites and the lateral interaction among either the adsorbed methane or nitrogen can be discarded. Fowler-Guggenheim isotherm takes into consideration the lateral interaction of the adsorbed molecules : - (1 − θ) . / 0 = −. (
34 ) +
Where KFG is the Fowler-Guggenheim equilibrium constant, and w is the interaction energy between adsorbed molecules. The fitting quality is better than in the previous case, although this fit is markedly worst for N2 adsorption. As it is observed in Table 2, the values of w are in all cases negative, which, according to the isotherm, is congruent with repulsive forces among adsorbed molecules. This fact is consistent with the observance based on the previous adsorption isotherm.
Figure 6. Comparison of different isotherm models with experimental data of Basolite C300 (a and b for methane, c and d for nitrogen), F300 (e and f for methane, g and h for nitrogen) and A100 (i and j for methane, k and l for nitrogen). Experimental (●) and models (dotted lines). Left plots: Henry (purple), Hill-Deboer (orange), Fowler-Guggenheim (black) and Langmuir (yellow). Right plots: Freundlich (red), Temkin (green) and Sips (blue). Curves were obtained from a AutoChem II 2920 (298 K, 0.1 MPa and 50 mL/min of total flow).
Langmuir isotherm assumes that the adsorption sites on the adsorbent surface have the same adsorption probability and that there is no interaction between adsorbed molecules : -
Where kL is Langmuir constant related to adsorption capacity and qm is referred to the monolayer adsorption capacity. Once again, the adjustment is better in the case of methane than nitrogen, resulting remarkable in the case of Basolite F300 (which presents the best fitting) the equal value of KL for nitrogen than for methane, although in all cases it is observed that the monolayer is higher for methane. Freundlich equation is applicable to adsorption processes that occur on heterogeneous surfaces. This isotherm in the linear form is as follows : .89
1 = log =3 + log -
Where kF is the Freundlich constant related to adsorption capacity and n refers to the adsorption intensity. According to Fig. 6, it is observed how the predicted adsorption isotherms present the same trend as the experimental data in all cases, with very good correlation coefficients. Only in the case of the nitrogen adsorption on Basolite A100 it is observed a worse regression, but even in this case, the trend data is appropriate. Thus, it is especially relevant the information obtained from the Freundlich isotherm. Hence, add to the heterogeneity of the surface, something intrinsic to the MOFs nature, it is observed higher values of the kF for methane than nitrogen over all the adsorbents, in agreement with the adsorption capacities of the three adsorbents, Fig. 1. Likewise, the decreasing order of kF, C300 > A100 > F300, is the same as the slope of mesopores zone in the intraparticle diffusion model equation, which corresponds to the adsorption rate in this part. Concerning the n parameter, presents in all cases values less than one, compatible with the increasing trend of the adsorption isotherms , Fig. 6. The Temkin isotherm takes into account the effects of indirect adsorbate/adsorbent interactions on the adsorption process. The isotherm is as follows: =
12 12 ln [email protected]
+ ln > >
Where b is the Temkin constant, which is related to the heat of adsorption, and kT is the Temkin isotherm constant. This model, in all cases predicts nearly absence of adsorption at very low relative pressures followed by a type-I curve trend, which does not correspond to the experimental data.
Table 2. CH4 and N2 adsorption parameters for the Basolite C300, F300 and A100 Material
Hill-Deboer K1 (L·mg-1)
Fowler-Guggenheim KFG (L·g-1)
Langmuir kL (mg·g-1)
Freundlich kF (L·g-1)
Temkin kT (L·g-1)
Sips c0 (mg·g-1)
Sips, also called Langmuir-Freundlich, isotherm is a semiempirical model that follows the following equation in the linearized form : . A
1 B = ln C + ln D#/" -' − -
Where c0 is iteratively found to obtain a linear relation between ln p and ln (c/(c0-c)), next n is determined by the slope and then a by the intersect point. The parameter a in the Sips model is the affinity constant and n is the heterogeneity coefficient of the adsorbate-adsorbent system. The larger the deviation of n from 1, the stronger the non-uniformity of the surface in the adsorption system . At this point, remark that for n equal to 1, this equation is the Langmuir one. In fact, from Table 2 is evidenced the proximate values of n parameter to the unity for Basolite A100, observing the largest differences for Basolite F300. This can be attributed even to the surface morphology, since in this material the microporosity is about 67 % of the whole surface, whereas for the other two adsorbents a higher uniformity was observed. Similarly, Shang et al. found values of n lower than 1 in the methane and nitrogen adsorption on molecular sieves with CHA-type structure . Fig 6 evidences the goodness of the fitting, especially in the case of methane adsorption, with the only exception of the nitrogen adsorption on Basolite A100. This model is a combined form of Langmuir and Freundlich expressions used for predicting the heterogeneous adsorption system and overcoming the drawback associated with Freundlich isotherm model of continuing increase in the adsorbed amount with an increase in concentration. Sips equation is similar to the Freundlich equation, but it has a finite limit when the concentration is sufficiently high. In fact, comparing value of R2 of all the isotherms, the Freundlich and Sips models have the highest 22
values of R2, and the isotherm with a better fitting for both molecules on the three adsorbents is the Sips one, pointing out the heterogeneous nature of the adsorbents. Moreover, the worst adjustment in all cases is for Basolite A100, and especially for nitrogen could be due to the adsorption-induced structural transition (breathing) characteristic of this structure. This phenomenon was already observed by the hysteresis loops in the adsorption and desorption of xenon and methane on MIL-53(Al) structure .
5. Regenerability of the adsorbent The stability of the adsorbent for the methane and nitrogen adsorption was studied by adsorption-desorption cycles. Fig. 7 (a) shows the maximum adsorption capacity of each gas in the first cycle and Fig. 7 (b) the adsorption index (adsorption capacity divided by the maximum adsorption capacity) of the three Basolites. Both methane and nitrogen adsorption experiments were carried out at 298 K, and then, its desorption was performed in pure helium at 423 K. After six cycles, the adsorption capacity of methane is over 80 % for Basolite F300 and A100, whereas decreases until 64 % for Basolite C300. In the case of nitrogen, the capacity in the last cycle is always over 80 %, reaching 92 % for Basolite A100. Therefore, it can be seen that the regenerability of nitrogen is easier than in the case of methane, in agreement with both the molecular properties of the adsorbates, and also the lower capacity and heat of adsorption. At a first insight, the regenerability seems not be related to the kinetic constants deduced from the Langmuir model, from which the Basolite C300 presented higher values of the adsorption constant than Basolite F300. What is more, Basolite A100 exhibited the largest values of both adsorption and desorption constants of methane and nitrogen, not hindering this fact the good regenerability of the material, probably due to the breathing effect. However, regenerability is carried out in a helium flow, fact that marks the difference since no adsorbate pressure is exerted.
Adsorption index (%)
100 90 80 70 60 50 0
Number of cycle Figure 7. Adsorption capacity of the first cycle of both methane and nitrogen for different MOFs: C300 (Blue), F300 (Orange) and A100 (Grey) (a); adsorption index performance for the different MOFs: C300 (•), F300 (■) and A100 (▲). Filled symbols correspond to methane and empty symbols to nitrogen.
CONCLUSIONS The work presented a thermodynamic and kinetic analysis of the methane and nitrogen adsorption on three different commercial Metal Organic Frameworks: Basolite C300, Basolite F300 and Basolite A100. The capacity of adsorption of both pure gases were analysed under isothermal conditions in the interval 298 – 498 K, observing higher adsorption capacity at the lowest temperature, and being remarkable the Cu interaction of the Basolite C300 with the methane, as it was deduced both from the capacity and the heat of adsorption. The curves of adsorption of both gases versus time at 298 K were fitted to several kinetic models. Among them, the Langmuir and fractional order models are the most suitable to describe the adsorption of both gases on the three adsorbents. Likewise, from the intraparticle diffusion model, it is observed a different behaviour between Basolite A100 and the other two adsorbents, reaching faster the equilibrium stage for the former. For both adsorbates, the adsorption is easier than desorption on the surface of the three materials. The main differences observed among the adsorbents were the faster adsorption of both gases on Basolite A100, as well as the larger dependence on the occupied adsorption sites. Equilibrium of the adsorption was fitted to several adsorption isotherms of one, two or three parameters, being the Freundlich and Sips isotherms which obtained the best adjustment due to the assumptions of certain surface heterogeneity and the increasing adsorption capacity with the increasing pressure (limited in the case of the Sips one). Concerning the regenerability of the Basolites, after six cycles, the regenerability of nitrogen is easier than in the case of methane, in agreement with both the molecular properties of the adsorbates, and also the lower capacity and heat of adsorption. As for the ease of regeneration, the Basolite A100 stands out, probably due to its breathing effect.
ACKNOWLEDGEMENTS This work was supported by the Research Fund for Coal and Steel of the European Union (contract UE-17-RFCS216-METHENERGY PLUS). D. Ursueguía acknowledges the Spanish Government for the FPU fellowship (FPU18/01448).
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CRediT author statement David Ursueguia: Investigation, formal analysis, writing original draft, Eva Diaz: Data curation, methodology, writing (review and editing), Salvador Ordóñez: Conceptualization, supervision, writing (review and editing)
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: