Separation and Purification Technology 141 (2015) 150–159

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Separation and Purification Technology

journal homepage: www.elsevier .com/locate /seppur

Adsorption equilibrium of carbon dioxide and nitrogenon the MIL-53(Al) metal organic framework

http://dx.doi.org/10.1016/j.seppur.2014.11.0401383-5866/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. Tel.: +351 212948385; fax: +351 212948550(R.P.P.L. Ribeiro). Tel.: +351 2948300; fax: +351 212948550 (I.A.A.C. Esteves).

E-mail addresses: rpp.ribeiro@fct.unl.pt (R.P.P.L. Ribeiro), iaesteves@fct.unl.pt(I.A.A.C. Esteves).

Bárbara C.R. Camacho, Rui P.P.L. Ribeiro ⇑, Isabel A.A.C. Esteves ⇑, José P.B. MotaRequimte/CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

a r t i c l e i n f o

Article history:Received 16 June 2014Received in revised form 1 September 2014Accepted 24 November 2014Available online 9 December 2014

Keywords:Adsorption equilibriumMetal organic frameworks (MOFs)MIL-53(Al)Carbon dioxideAdsorption potential theory

a b s t r a c t

The single-component adsorption equilibria of carbon dioxide (CO2) and nitrogen (N2) on a commercialsample of MIL-53(Al) metal organic framework were measured over a pressure range of 0–34 bar at303 K, 323 K, and 353 K, using a magnetic suspension microbalance. The adsorption equilibria of bothgases are characterized by type I isotherms that do not exhibit the guest-induced transition betweenMIL-53(Al)’s narrow-pore (np) and large-pore (lp) structures that has been observed on someMIL-53(Al) samples upon CO2 adsorption at the temperatures of this study. The observed CO2 loadingsat high pressure are consistent with a np-stabilized MIL-53(Al) form that possesses no visible breathingbehavior. The adsorption measurements show that CO2 is preferentially adsorbed over N2, indicating thatMIL-53(Al) can be potentially employed in adsorption-based separation processes for environmentalapplications, such as carbon capture from flue gases emitted by fossil-fueled power stations. The Sipsand Toth isotherm models were successfully fitted to the experimental adsorption data and the corre-sponding heats of adsorption determined from the isotherm models. The adsorption potential theorywas also employed to correlate the CO2 and N2 adsorption data, as well as previously determinedmethane adsorption data on the same adsorbent. This approach successfully collapses the adsorptionequilibrium data into a single temperature-independent characteristic curve.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The huge atmospheric concentration of carbon dioxide (CO2) iscurrently a major environmental concern. Most of the emitted CO2

is anthropogenic and energy production is among the top contrib-uting activities for these emissions. A combination of strategies,including the use of greener and renewable energy sources (e.g.,biogas), has been proposed to tackle the large amounts of emittedCO2. Carbon Capture and Storage (CCS) is also of vital importancesince it is the best route to avoid CO2 emissions, while still usingfossil fuels until a sustainable energy system based on renewablesis fully implemented. CCS can be implemented through three dif-ferent routes: pre-combustion, oxyfuel, and post-combustion.Post-combustion capture is extremely important since it allowsretrofitting the CO2 capture unit in already existing power stations[1].

Adsorption-based separation processes are being considered aspotential alternatives for CO2 capture from flue gases [1]. These

include both mature and emerging processes such as PressureSwing Adsorption (PSA) and Electric Swing Adsorption, respec-tively. The successful design of these separation processes is neces-sarily dependent on the proper choice of an adsorbent, which iswhy a wide spectrum of adsorbent materials is being evaluatedfor CO2 capture.

Among the myriad of available adsorbents, state-of-the-artmetal organic frameworks (MOFs) are being considered as suitableadsorbents for CCS applications. MOFs are a group of microporousmaterials that consist in metal ion clusters connected by organiclinkers. Many MOF materials have large surface area and pore vol-ume, are thermally stable, and selectively adsorb small molecules.Many MOFs properties can be tailored by changing the nature ofthe metal center and/or the organic ligand [2].

The MIL-53 family, which consists in trivalent metal ion (Al, Cr,Fe, Sc, etc.) terephthalates, is an interesting class of MOFs. Thesematerials have structural flexibility, or ‘‘breathing behavior’’ result-ing on the transition between two stable conformations—anarrow-pore (np) and a large-pore (lp) structure [3,4]—whose unitcell volume can change up to 40% [5]. The conformation change canbe triggered by the adsorption of specific guest molecules, such asH2O [6] or CO2 [3], by temperature changes [7], or by the action ofmechanical pressure [7]. In the former case, the breathing behavior

B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159 151

occurs due to strong interactions between the skeleton OH groupsand the carbon in CO2 or the oxygen in water, which may influencethe selective adsorption of polar molecules [8].

The MIL-53 materials offer exceptional adsorption capacity forseveral gases and diverse studies on CO2/CH4 separation have beenreported [5,9–14]. Amine modified MIL-53 samples have shownenhanced adsorption properties for CO2 capture and CO2/CH4 sep-aration in biogas upgrading applications [15].

The MIL-53’s aluminum version is constituted by corner-sharing AlO4(OH)2 octahedra connected by 1,4-benzenedicarboxyl-ate (BDC) ligands. This framework has diamond-shaped, one-dimensional channels of around 0.85 nm into which smallmolecules can be adsorbed [4,6]. MIL-53(Al) is extremely stableat high temperatures (up to 773 K), which is an uncommon prop-erty compared with its analogue materials.

The present study focuses on the adsorption equilibrium prop-erties of MIL-53(Al) for CO2 and nitrogen (N2), which are the maincomponents of flue gas. The adsorption equilibria of these twogases, at temperatures between 303 K and 353 K and pressures inthe range of 0–34 bar, are reported. The Sips and Toth isothermmodels were successfully fitted to the experimental data. Theadsorption potential theory (APT) was also employed to correlatethe measured equilibrium data, along with previously measuredmethane (CH4) adsorption isotherms on MIL-53(Al) [13]. Methaneis the main constituent of biogas. By applying the APT to theadsorption isotherms of the three adsorbates, it was possible tocollapse the adsorption equilibria of the main constituents of fluegas and biogas into a single temperature-independent characteris-tic curve.

2. Experimental

2.1. Materials

The sample of MIL-53(Al) powder employed in this study wassynthesized by BASF (Ludwigshafen, Germany) under the trade-mark Basolite A100� and purchased from Sigma–Aldrich (product

Fig. 1. Schematic diagram of the experimental setup employed in the adsorption equilibdata acquisition; T, Pt100 temperature sensor; PT, Omegadyne pressure transducers; M

no. 688738-10G). Detailed characterization of the MIL-53(Al) crys-tals, including mercury porosimetry, thermogravimetric analysis,X-ray powder diffraction, solid state nuclear magnetic resonance,and Fourier transform infrared spectroscopy, is reported elsewhere[13]. The particle size distribution of the crystals is well fitted by alog-normal distribution with mean diameter dp = 30 lm and stan-dard deviation rdp = 1.7 lm, which is in fairly good agreementwith the mean value dp = 32 lm reported by the manufacturer.

The sample was pre-treated prior to each sorption experiment inorder to remove any moisture and adsorbed impurities. The samplewas activated at 473 K in a muffle (Nabertherm B170 GmbH,Germany) for approximately 4 h, after which it was transferred athigh temperature and sealed inside the measurement cell in aninert atmosphere of helium. Finally, it was degassed in situ undervacuum at 353 K for at least 8 h. All gases were provided by AirLiquide and Praxair (Portugal): N2 N45, CO2 N48, and He N50.

2.2. Adsorption equilibrium experiments

The single-component adsorption/desorption isotherms weremeasured by the standard static gravimetric method [13] in anISOSORP 2000 high-pressure magnetic-suspension microbalance(Rubotherm GmbH, Bochum, Germany). In this apparatus the mea-surement cell is coupled to a suspension magnet instead of hang-ing directly at the balance. Using this free suspension coupling,the measuring force is transmitted contactlessly from the closedmeasurement cell to a Sartorius microbalance, located outsideunder ambient atmosphere. The microbalance has a resolution of10�5 g, uncertainty 60.002%, and reproducibility 63 � 10�5 g fora maximum load of 25 g.

Fig. 1 shows a schematic of the experimental setup employed inthe adsorption measurements. The temperature is measured usinga four-wire Pt100 probe (RS Amidata, Spain) controlled within 0.1 Kof the set-point value using a thermostatic bath F32 HL (JulaboGmbH, Germany). The pressure is measured by several pressuretransducers with different ranges, in order to ensure a good mea-surement accuracy at all pressures: Baratron model 627D (MKSInstruments GmbH, Germany) for 0–1 bar, accurate to 0.12% of

rium measurements (MSB, magnetic suspension microbalance; PCI, PC interface forKS, MKS Baratron pressure transducer).

152 B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159

the measured value; Omegadyne Inc. (Sunbury, OH, USA) modelsPX01C1-150A5T, PX01C1-500A5T, and PX03C1-3KA5T for0–10 bar (accuracy of 0.05% of Full Scale, FS), 0–35 bar (accuracyof 0.05% of FS), 0–69 bar (accuracy of 0.1% of FS), respectively. Thepressure and sample weight were monitored and recorded onlineusing in-house developed software.

The experimental methodology consists of sequential steps ofgas addition into the cell containing the sample, followed by sub-sequent equilibration under isothermal conditions. Pressure andweight changes are monitored until equilibration, which isassumed to occur when the rate change of both measured variablesapproaches zero. This procedure is repeated as many times as thenumber of points desired to obtain the adsorption isotherm. Afterreaching the maximum pressure, a similar procedure is repeated,but this time by stepwise depressurization of the measurementcell to probe the existence of hysteresis effects and to double checkthe previously measured data. The range of thermodynamicconditions spanned in the experiments performed is 0–34 barand 303–353 K.

3. Theoretical

3.1. Net, excess, and total adsorption

Different approaches can be adopted to report adsorption equi-librium measurements. One which has been recently proposed isthe concept of net adsorption [16]. Net adsorption, qnet, representsthe difference between the amounts of adsorbate present in thesame measurement cell with and without the adsorbent, at thesame temperature and pressure conditions. The net amountadsorbed in a gravimetric experiment, expressed per unit mass ofadsorbent, is given by

qnet ¼ ðw�ms �mh þ VhqgÞ=ms; ð1Þ

where w is the mass weighed in the balance, ms is the mass of solidin the measurement cell (measured in vacuum after degassing andthermal pre-treatment), mh and Vh are the total mass and volume ofall parts in the measurement cell that contribute to buoyancyeffects, and qg is the gas density at the equilibrium pressure andtemperature of the experiment. Reporting net adsorption data hasthe advantage of eliminating problems related with the use of probemolecules to determine the reference state, since the determinationof mh and Vh does not depend on the solid–fluid system.

However, adsorption measurements are often reported asexcess amount adsorbed, qex, which is the amount of adsorptivein the measurement cell in excess of the amount that would bepresent in the same system at the equilibrium density of the bulkgas; that is, qex gives the number of molecules in the nanopores ofthe adsorbent in excess of the amount that would be present in thepore volume at the equilibrium density of the bulk gas. The volumeof the solid matrix of the adsorbent is usually determined experi-mentally from helium (He) expansion, under the assumption thatHe is a probe molecule not adsorbed by the solid. However, He pic-nometry can be ambiguous since many studies show that in facthelium does adsorb, though in a small extent [17].

Alternatively, adsorption equilibrium can be expressed in termsof absolute adsorption, q. These three different adsorption quanti-ties—absolute, excess, and net—are related as follows:

q ¼ qex þ vpqg ¼ qnet þ ðvp þ v sÞqg ; ð2Þ

where vp is the specific pore volume of the adsorbent and vs is thespecific volume of the adsorbent impenetrable to the adsorbatemolecules (vs = 1/qs, where qs is the skeletal density of the adsor-bent). In the case of crystalline porous materials, (vp + vs) is equalto the specific volume of the unit cell of the crystal lattice.

3.2. Sips and Toth isotherm models

The semi-empirical Sips and Toth adsorption isotherm modelswere fitted to the measured adsorption equilibrium data. The Sipsisotherm model can be expressed as

q ¼ qsðbPÞ1=n

1þ ðbPÞ1=n ; ð3Þ

b ¼ b0 expQ

RT0

T0

T� 1

� �� �; ð4Þ

1n¼ 1

n0þ a 1� T0

T

� �; ð5Þ

where q is the concentration of the adsorbed phase (total amount ofadsorbate in equilibrium per amount of adsorbent), qs is themaximum adsorbed amount, b0 is the adsorption constant at thereference temperature T0, Q is the isosteric heat of adsorption at halfloading, and n is a parameter that characterizes the solid–fluidinteraction. The magnitude of n increases with the system heteroge-neity, and when n = 1 the Sips isotherm reduces to the Langmuirisotherm. The parameters n0 and a describe the temperature depen-dence of the heterogeneity parameter n.

By applying the Clausius–Clapeyron equation to the Sips iso-therm model, the isosteric heat of adsorption, Qst, can be derived;the result is

Q st ¼ Q � an2RT0 lnh

1� h

� �; ð6Þ

where h = q/qs, is the fractional loading and Q equals Qst whenh = 0.5.

The Toth isotherm model is given by

q ¼ qsbP

½1þ ðbPÞt�1=t ; ð7Þ

where

t ¼ t0 þ a 1� T0

T

� �; ð8Þ

and b has the same definition as for the Sips isotherm model; 1/t hassimilar physical meaning as the n parameter of Sips model.

The isosteric heat of adsorption for the temperature-dependentform of the Toth isotherm model is given by

Q st ¼ Q � aRT0

tln

h

ð1� hÞ1=t �ln h

1� ht

" #; ð9Þ

where Q equals Qst when the fractional loading is zero.

3.3. Adsorption potential theory

The adsorption equilibrium in microporous adsorbents can becorrelated using the Adsorption potential theory (APT), whichwas developed by Dubinin and co-workers from the original workof Polanyi. The APT has been well reviewed by Tien [18]. The APTassumes liquid-like behavior of the adsorbate confined in themicropores, although its properties may differ from the propertiesof liquid bulk at the same temperature due to the influence of theadsorbent force field. The difference in free energy between theadsorbed phase and the saturated liquid adsorbate at the sametemperature can be obtained from the ratio between the equilib-rium pressure and the saturation vapor pressure. This is referredto as the adsorption potential:

/ ¼ RT lnðPs=PÞ; ð10Þ

B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159 153

where R is the ideal gas constant, T is the system temperature, Ps isthe saturation vapor pressure of the adsorbate at temperature T,and P is the equilibrium pressure. It should be noted that to accountfor nonideal gas behavior at high pressure, P and Ps should bereplaced by the corresponding fugacities, f and fs. According toATP, for a given gas–solid system the volume of the adsorbed phase(W) is a function of / only:

W � qVm ¼Wð/Þ; ð11Þ

where Vm is the molar volume of the adsorbed phase; W(/) is gen-erally referred to as the characteristic curve of the solid–fluidsystem.

The APT is extremely useful to predict single-component adsorp-tion equilibrium from a limited set of experimental measurements.Since the characteristic curve is temperature independent, onlyadsorption equilibrium measurements at one temperature areneeded to obtain the characteristic curve, and this should be suffi-cient to extrapolate the amount adsorbed to other temperaturesfor the same gas–solid system by means of Eq. (11).

A generalization of the original APT [19] introduces an affinitycoefficient, b, to collapse the characteristic curves of various gaseson the same adsorbent into a single characteristic curve. In thisgeneralization of the theory, Eq. (11) is replaced by

W � qVm ¼Wð~/Þ; ð12Þ

where

~/ � /=b; ð13Þ

The affinity coefficient is, however, difficult to estimate a priori.The best correlation to date is based on the molecular parachor andis given by [20]:

b ¼ 8:27� 10�3 ðparachorÞ0:90 ð14Þ

Below the adsorbate critical temperature, Tc, Vm is assumed tobe equal to the molar volume of the saturated liquid at the systemtemperature. In the present work, one of the CO2 adsorption iso-therms was measured at a temperature slightly below Tc. Underthese conditions, Vm was estimated from the modified Rackettequation [21]:

Vm ¼RTc

PcZRa 1þ 1� T

Tc

� �� �2=7

; ð15Þ

where Pc is the critical pressure of the adsorbate and ZRa is the Rack-ett compressibility factor. The Wagner equation [22] was used toestimate the saturated vapor pressure under subcritical conditions:

lnPs

Pc

� �¼ Axþ Bx1:5 þ Cx3 þ Dx6

1� x; ð16Þ

where

x ¼ 1� TTc; ð17Þ

and A, B, C, and D are specific constants of the adsorptive [22].Above the adsorbate critical temperature, the definition of the

adsorbed phase is not straightforward, and in this study the recom-mendations of Agarwal and Schwarz [23] were followed. The satu-rated vapor pressure was estimated as

Ps ¼ ðT=TcÞ2Pc; ð18Þ

and Vm was obtained by

Vm ¼ Vb exp½XðT � TbÞ�; ð19Þ

where Tb and Vb are, respectively, the temperature and molar vol-ume of the liquid adsorbate at the normal boiling point, and X is

the thermal expansion coefficient of the adsorbate in a superheatedliquid state [24], which was estimated as

X ¼ lnb=Vb

Tc � Tb; ð20Þ

where b is the van der Waals volume.In order to obtain an analytical form of Wð~/Þ for future applica-

tion in process modeling, the characteristic curve was fitted to theDubinin–Astakhov (D–A) equation [18],

W ¼Ws expð�c~/nÞ; ð21Þwhere W/Ws is the fractional filling of the specific pore volume (Ws)of the adsorbent accessible to the adsorbate and c and n are param-eters related to the characteristic energy of the solid–fluid system.These parameters were obtained from the linear fitting

ln½lnðWs=WÞ� ¼ ln cþ n ln /; ð22Þ

Alternatively, ln(Ws/W) can be expressed as a polynomialexpansion in /; a third-order polynomial expansion usuallysuffices:

lnðWs=WÞ ¼ c1/þ c2/2 þ c3/

3: ð23Þ

4. Results and discussion

4.1. Net, excess, and total adsorption

The experimental adsorption equilibrium data were firstlyinterpreted in terms of net adsorption, since the determination ofthis thermodynamic property avoids the problems associated withthe use of probe molecules for the establishment of a referencestate. Therefore, CO2 and N2 net adsorption values were first calcu-lated from the measured data; then, the excess and absoluteadsorption values were calculated using Eq. (2). These quantitieswere calculated using estimates of vs and vp previously determinedby molecular simulation [13]. The skeletal density of the adsorbent(qs) was experimentally determined by He picnometry at 353 K;the estimated value, qs = 2.2 g/cm3, is in good agreement withthe one calculated by molecular simulation (2.13 g/cm3). The net,excess, and absolute adsorption values are listed in Tables 1 and 2.

Fig. 2 compares the net, excess, and absolute adsorption iso-therms for N2 at 303.22 K, to illustrate the typical trend obtainedfor all isotherms. The bottom graph displays the isotherms overthe full pressure region on a log–log scale. At sufficiently low pres-sure (i.e., below 2 bar), the differences between the three adsorp-tion descriptors are negligible since the bulk density of theadsorptive is significantly lower than its pore density. As the pres-sure is increased, the effect of the bulk density of the adsorptivebecomes increasingly more important, and the three types of iso-therm start to diverge from each other. Similar results wereobtained for the all isotherms measured.

All the absolute adsorption isotherms measured in this work areof type I according to the IUPAC classification; they have convexshape with no inflexion points, the slope of which continuouslydecreases with pressure. It is worth noting that the previouslyreported MIL-53(Al) breathing behavior upon CO2 adsorption[12,25] was not observed in the present work. The MIL-53(Al)employed in our experiments is a sample of commercial BasoliteA100�powder synthesized by BASF SE. The absence of the so-calledbreathing effect when CO2 is adsorbed on commercial BasoliteA100� samples was also reported by other groups [9,11].

It has been recently shown that stabilized MIL-53 metal–organic frameworks can be prepared via solvothermal synthesisstrategies that possess no or only slight breathing behavior com-pared to MIL-53 prepared by traditional methods, and that theselected synthesis solvent plays a critical role on the breathing

Table 1Experimental CO2 adsorption equilibrium data at 303, 323 and 353 K. The reference state corrections used are vp = 0.564 cm3/g and vs = 0.470 cm3/g.

303.16 K 323.18 K 353.37 K

P qnet qex q P qnet qex q P qnet qex q(bar) (mol/kg) (mol/kg) (mol/kg) (bar) (mol/kg) (mol/kg) (mol/kg) (bar) (mol/kg) (mol/kg) (mol/kg)

0.133 0.456 0.459 0.462 0.013 0.085 0.085 0.085 0.534 0.313 0.322 0.3320.586 1.170 1.181 1.194 3.193 2.054 2.111 2.178 0.976 0.522 0.538 0.5570.974 1.545 1.563 1.585 4.558 2.331 2.412 2.509 4.182 1.439 1.507 1.5883.523 2.897 2.964 3.043 6.573 2.667 2.785 2.926 7.166 1.946 2.063 2.2035.101 3.361 3.458 3.575 8.526 2.928 3.082 3.266 10.242 2.296 2.465 2.6676.454 3.651 3.776 3.924 9.764 3.082 3.259 3.471 13.126 2.534 2.752 3.0138.066 3.905 4.062 4.249 11.976 3.303 3.522 3.785 16.012 2.727 2.995 3.3169.572 4.111 4.298 4.522 15.052 3.524 3.804 4.139 19.281 2.872 3.199 3.590

12.074 4.355 4.595 4.881 17.492 3.661 3.990 4.383 25.205 3.055 3.490 4.01114.903 4.571 4.870 5.229 19.981 3.803 4.183 4.637 33.140 3.192 3.779 4.48217.596 4.706 5.065 5.495 26.086 3.978 4.488 5.098 23.055 3.025 3.421 3.89421.459 4.864 5.313 5.850 32.926 4.081 4.747 5.544 13.155 2.510 2.729 2.99025.505 5.059 5.606 6.261 21.400 3.941 4.350 4.840 4.946 1.631 1.711 1.80730.518 5.082 5.760 6.571 13.496 3.591 3.840 4.138 3.511 1.332 1.389 1.45719.286 4.973 5.371 5.848 1.976 1.613 1.648 1.689 1.468 0.728 0.752 0.78016.266 4.870 5.200 5.595 0.685 0.849 0.861 0.876 0.129 0.098 0.100 0.10213.133 4.689 4.951 5.264 0.041 0.189 0.190 0.19110.365 4.481 4.684 4.927

7.079 4.071 4.208 4.3714.162 3.410 3.490 3.5842.209 2.543 2.584 2.6341.412 1.971 1.998 2.0300.777 1.340 1.355 1.3720.348 0.804 0.810 0.818

Table 2Experimental N2 adsorption equilibrium data at 303, 323 and 353 K. The reference state corrections used are vp = 0.564 cm3/g and vs = 0.470 cm3/g.

303.22 K 323.19 K 353.14 K

P qnet qex q P qnet qex q P qnet qex q(bar) (mol/kg) (mol/kg) (mol/kg) (bar) (mol/kg) (mol/kg) (mol/kg) (bar) (mol/kg) (mol/kg) (mol/kg)

0.005 0.003 0.003 0.003 0.005 0.002 0.002 0.002 0.106 0.014 0.016 0.0180.025 0.008 0.009 0.009 0.018 0.004 0.004 0.005 0.716 0.037 0.048 0.0620.095 0.019 0.021 0.023 0.101 0.016 0.018 0.020 1.055 0.066 0.083 0.1030.309 0.050 0.056 0.063 0.754 0.080 0.094 0.109 2.998 0.134 0.182 0.2390.764 0.171 0.185 0.202 1.017 0.097 0.115 0.136 7.136 0.227 0.341 0.4781.028 0.193 0.212 0.235 3.015 0.200 0.252 0.315 12.057 0.321 0.514 0.7443.054 0.362 0.419 0.487 5.564 0.307 0.405 0.521 17.117 0.374 0.648 0.9748.096 0.620 0.771 0.952 10.001 0.461 0.636 0.845 22.287 0.415 0.770 1.196

13.075 0.779 1.023 1.315 15.017 0.569 0.832 1.146 28.180 0.402 0.850 1.38718.129 0.868 1.206 1.611 20.401 0.630 0.987 1.413 33.076 0.392 0.918 1.54822.961 0.913 1.342 1.855 24.989 0.663 1.100 1.622 30.384 0.400 0.883 1.46127.997 0.927 1.450 2.075 30.145 0.673 1.200 1.829 25.217 0.394 0.796 1.27731.807 0.928 1.523 2.234 34.137 0.681 1.277 1.990 19.708 0.393 0.707 1.08325.598 0.935 1.413 1.985 25.984 0.680 1.134 1.677 14.553 0.346 0.579 0.85715.377 0.867 1.155 1.498 17.460 0.636 0.941 1.306 9.345 0.265 0.414 0.59310.448 0.743 0.938 1.171 12.634 0.543 0.764 1.028 5.877 0.202 0.296 0.408

4.017 0.429 0.504 0.593 7.497 0.425 0.556 0.712 4.088 0.166 0.231 0.3092.058 0.324 0.362 0.408 3.999 0.302 0.371 0.455 1.916 0.105 0.136 0.173

2.016 0.192 0.228 0.270 0.399 0.043 0.050 0.057

154 B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159

behavior of the synthesized material [26]; this seems to be theapproach followed by BASF SE in the synthesis of their BasoliteA100� commercial samples.

The CO2 adsorption capacity of the Basolite A100� sample usedin the present study is slightly lower than those previouslyreported by other authors [9,11]. On the other hand, our measuredCO2 adsorption equilibrium data are in good agreement with thevalues reported by Bourrelly et al. [12] up to 6 bar. At higher pres-sures, the MIL-53(Al) sample synthesized by these authors under-went a large structural transition from the np-form to the lp-formand, therefore, the measured loadings at higher pressures are lar-ger than those reported in the present work. The results suggestthat the behavior of our Basolite A100�sample upon CO2 adsorptionis consistent with a np-stabilized form that possesses no visiblebreathing behavior.

4.2. Sips and Toth isotherm models

The semi-empirical Sips and Toth models were fitted to theexperimental absolute adsorption data using TableCurve 3D™ soft-ware (Systat Software Inc.). This was a straightforward task as theMIL-53(Al) sample did not show any visible breathing behavior; ifthis were not the case, it would not have been possible to fit aninflected isotherm with one of the aforementioned models. Figs. 3and 4 show, respectively, the CO2 and N2 experimental isotherms(in terms of absolute amount adsorbed) on MIL-53(Al) at 303 K,323 K, and 353 K, up to 33 bar.

Figs. 3 and 4 present the global fittings of the experimental databy the Sips and Toth models; Table 3 lists the values of the respec-tive fitted parameters. The average relative error (ARE) of the glo-bal fitting of the experimental adsorption values is defined as

0.0

0.5

1.0

1.5

2.0

2.5

0 5 10 15 20 25 30 35

Am

ount

Ads

orbe

d (m

ol/k

g)

P (bar)

N2

qnetqexq

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.1 1.0 10.0 100.0

Am

ount

Ads

orbe

d (m

ol/k

g)

P (bar)

N2

qnetqexq

Fig. 2. Absolute ( ), excess ( ), and net ( ) adsorption isotherms of N2 on MIL-53(Al) at 303.22 K. Closed symbols denote adsorption data and open symbolsdenote desorption data. The reference state corrections used are vp = 0.564 cm3/gand vs = 0.470 cm3/g.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0 10 20 30 40

q (m

ol/k

g)

P (bar)

CO2

Fig. 3. Absolute adsorption isotherms of CO2 on MIL-53(Al) at 303.16 K ( ),323.18 K ( ), and 353.37 K ( ). Closed symbols denote adsorption data and opensymbols denote desorption data. The solid line represents the fitting with the Sipsmodel and the dashed lines represent the fitting with the Toth model. The globalaverage relative error (ARE) is 5.7% and 8.7% for the Sips and Toth models,respectively.

0.0

0.5

1.0

1.5

2.0

2.5

0 10 20 30 40

q (m

ol/k

g)

P (bar)

N2

Fig. 4. Absolute adsorption isotherms of N2 on MIL-53(Al) at 303.22 K ( ), 323.19 K( ), and 353.14 K ( ). Closed symbols denote adsorption data and open symbolsdenote desorption data. The solid line represents the fitting with the Sips model andthe dashed lines represent the fitting with the Toth model. The global averagerelative error (ARE) is 5.5% and 9.7% for the Sips and Toth models, respectively.

B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159 155

AREð%Þ ¼ 100Nexp

X jqfit � qexpjqexp

; ð24Þ

where qfit and qexp are the fitted and experimental values,respectively, and Nexp is the number of experimental points,encompassing the set of experimental isotherms.

Figs. 3 and 4 show that there is good agreement between thesingle-component surfaces obtained from the global fittings ofboth models and the experimental data. Moreover, the estimatedamounts adsorbed at saturation (qs) and the isosteric heats ofadsorption (Q) determined from both isotherm models are in closeagreement. Although both models can correlate the data satisfacto-rily, the Sips model provides a better fitting, which can be con-firmed by the ARE errors given in Table 3. The ARE errorsobtained from the fitting with the Sips model are lower than 6%,which is a smaller value than the best ARE error for the Toth fitting(8.7%). The low individual and overall ARE error values obtainedtestify the good quality of the fittings. The only deviation is inthe low-pressure region, where the obtained errors are slightlyhigher. Despite this fact, it can be concluded that the fittingsobtained with Sips and Toth models are accurate and describe

the adsorption equilibria data within the pressure and temperatureranges studied.

The potential use of MIL-53(Al) as a capture material of CO2

from flue gases can be firstly evaluated by analysis of the capacityof this adsorbent for both CO2 and N2 adsorption. For comparisonpurposes, the isotherms of both gases at 303 K are plotted inFig. 5, showing that MIL-53(Al) adsorbs a significantly higheramount of CO2 than N2.

MIL-53(Al) has also been considered as a suitable material forbiogas purification [9,10]. Fig. 5 includes a plot of the CH4 adsorp-tion isotherm, which was previously measured by our group [13]at 303 K. Again, CO2 is preferentially adsorbed on MIL-53(Al) rela-tive to CH4, although in this case the difference in adsorption

Table 3Sips and Toth model fitting parameters for N2 and CO2 on MIL-53(Al).

Parameter N2 CO2

Sips Toth Sips Toth

qs (mol/kg) 5.931 6.173 9.039 9.285b0 (bar�1) 0.017 0.043 0.113 0.294a 0.199 0.100 0.142 0.010n0 or t0 1.207 0.567 1.437 0.617Q (kJ/mol) 12.04 14.19 24.99 24.94T0 (K) 303.22 303.22 303.16 303.16ARE (%) 5.5 9.7 5.7 8.7

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35

q (m

ol/k

g)

P (bar)

CO2 (this work)N2 (this work)CH4 (Lyubchyik et al)

Fig. 5. Absolute adsorption isotherms of CO2 ( ), N2 ( ), and CH4 ( ) on MIL-53(Al) at 303 K. Closed symbols denote adsorption data and open symbols denotedesorption data. The solid line represents the fitting with the Sips model. The CH4

data was previously reported by the group [13].

1

4

7

10

13

16

19

0 10 20 30 40

Sele

ctiv

ity

P (bar)

CO2/CH4

CO2/N2

Fig. 6. Equilibrium selectivity factor (a) as a function of pressure, at 303 K, for CO2/N2 (dashed line) and CO2/CH4 (solid line) in MIL-53(Al).

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7

Qst

(kJ/

mol

)

q (mol/kg)

(a)

CO2

N2

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7

Qst

(kJ/

mol

)

q (mol/kg)

(b)

CO2

N2

Fig. 7. Isosteric heats of adsorption predicted by the (a) Sips and (b) Toth models asa function of surface coverage for CO2 and N2 at 303 K ( ), 323 K ( ), and 353 K ( ).The black symbols ( ) represent the isosteric heat determined from the isostericplots of the experimental adsorption data.

156 B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159

capacity is much smaller than in the CO2/N2 case. This is confirmedby the equilibrium selectivity factor, a, which is shown in Fig. 6 as afunction of temperature. The selectivity factor is given by

ai=j ¼ qi=qj; ð25Þ

where qi and qj are the adsorbed amounts of the most retained spe-cies (i) and of the least adsorbed species (j), respectively. Fig. 6shows that aij for CO2/N2 is much higher than for CO2/CH4 and inboth cases the selectivity decreases with the increase of pressure.Although aij does not account for the contribution of adsorptionkinetics, it is a good parameter to make a first evaluation of thepotential application of an adsorbent for a specific separationproblem.

The isosteric heat of adsorption, Qst, was determined from eachisotherm model using Eqs. (6) and (9). Fig. 7a and b shows the heatof adsorption for CO2 and N2, as a function of the absolute amountadsorbed, q, as determined from the Sips and Toth models, respec-tively. Qst was also calculated independently from the isothermmodel by employing the integrated form of the Clausius–Clapeyronequation

ðln PÞq ¼ constant� Q st=RT: ð26Þ

According to Eq. (26), the isosteric heat can be obtained fromthe slope of the plot of ln(P) versus 1/T at a constant loading q.The Qst values determined using this procedure are representedby the black solid circles in Fig. 7. The results obtained show that,for the temperature range under study, the isosteric heat can beconsidered temperature independent. Also, the results obtainedshow that the Qst values determined from the isosteric plots are

0.0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10 12

W (c

m3 /g

)

Φ (105 J/mol)

(a)

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0 1 2 3 4

ln W

(cm

3 /g)

Φ (105 J/mol)

(b)

Fig. 8. (a) Characteristic curve obtained by collapsing the experimental data of CH4

( ), CO2 ( ) and N2 ( ) into a single curve; the solid line represents the fitting withthe D–A isotherm model. (b) Plot of the characteristic curve in log–log scale.

0.20

0.30

q (g

/g)

CO2

B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159 157

in fairly good agreement with the heats of adsorption derived fromthe fitted isotherm models (especially the Sips model). The isoster-ic heats determined for both adsorbates show a decreasing trendwith the adsorbate loading, suggesting that the MOF materialexhibits some heterogeneity. The Qst values obtained from the Sipsand Toth models (Table 3) for CO2 (�25 kJ/mol) and N2 (�13 kJ/mol) are in agreement with previously published data [25,27].

4.3. Adsorption potential theory

The adsorption potential theory (APT) was applied to theadsorption equilibrium data of CO2 and N2, and also to the CH4 datapreviously measured by Lyubchyk et al. [13]. The molar volume Vm

above the critical temperature of the adsorbate was calculatedusing Eq. (19). For this purpose a value for the thermal expansioncoefficient, X, had to be assumed. The first calculations were per-formed employing the values reported by Ozawa et al. [24], andthen these values were subsequently fine-tuned to enhance thequality of the temperature independence of the characteristiccurve for each adsorbate. The final X values are listed in Table 4.

After obtaining the characteristic curve for each adsorbate, thegeneralized version of the APT was employed to collapse all char-acteristic curves into a single master curve. For this purpose, theadsorption potential for each adsorbate was scaled by the respec-tive affinity coefficient, b. The b values employed as a first guesswere determined from Eq. (14). These initial estimates were subse-quently fine-tuned in order to obtain a better superposing of theadsorption data for the different gases into a single characteristiccurve. The b values calculated and employed are listed in Table 4.

The characteristic curve obtained with the APT is plotted inFig. 8, showing that a single curve was successfully obtained forthe three adsorbates. The solid lines represented in Fig. 8 corre-spond to the fitting of the Dubinin–Astakhov (D–A) model. TheD–A isotherm parameters obtained from the curve fitting are:

Ws ¼ 0:451 cm3=g ðr2 ¼ 0:985Þ;

c ¼ 1:434� 10�7 ðJ=molÞ�n;

n ¼ 1:295 ðr2 ¼ 0:981Þ:

The value of Ws (0.451 cm3/g) obtained is in satisfactory agree-ment with the pore volume obtained by molecular simulation(0.562 cm3/g) [13]. The differences between the obtained valuesmust be carefully interpreted, since it must be noticed that thesimulation pore volume was obtained considering an adsorbentwithout defects, which is unpractical under experimental applica-tions. Heymans et al. [9] reported a total pore volume of 0.549 cm3/g for a powdered sample of Basolite A100� and Möllmer et al. [27]reported a micropore volume of 0.463 cm3/g. Therefore, the micro-pore volume obtained from our APT analysis shows a satisfactoryagreement both with the published work and the molecular simu-lation work previously developed by our group.

Table 4Affinity coefficients (b) and thermal expansion coefficients (X) for CH4, CO2 and N2 onMIL-53(Al).

CH4 CO2 N2

Parachora 73.2 91.2 71.1bcalc

a 0.394 0.480 0.280bexp

b 0.470 0.420 0.280Xc (K�1) 0.0025Xexp

b (K�1) 0.0030 0.0055 0.0038

a From Wood’s work [20].b bexp and Xexp denote the parameters employed to minimize the data scattering

in the characteristic curve.c From Ozawa et al. work [24].

0.00

0.10

0 10 20 30 40

P (bar)

Fig. 9. Absolute adsorption isotherms of CO2 on MIL-53(Al) at 303.16 K ( ),323.18 K ( ), and 353.37 K ( ). The solid line represents the predictions with theD–A isotherm model. The average relative errors (ARE) obtained were 2.9% (303 K),2.9% (323 K) and 4.7% (353 K).

0.00

0.03

0.05

0.08

0.10

0 10 20 30 40 50

q (g

/g)

P (bar)

CH4

Fig. 10. Absolute adsorption isotherms of CH4 on MIL-53(Al) at 303.14 K ( ),323.15 K ( ), and 353.09 K ( ). The solid line represents the predictions with theD–A isotherm model. The average relative errors (ARE) obtained were 5.8% (303 K),11.3% (323 K) and 5.9% (353 K).

0.00

0.02

0.04

0.06

0.08

0 10 20 30 40

q (g

/g)

P (bar)

N2

Fig. 11. Absolute adsorption isotherms of N2 on MIL-53(Al) at 303.25 K ( ),323.20 K ( ), and 353.22 K ( ). The solid line represents the predictions with theD–A isotherm model. The average relative errors (ARE) obtained were 11.6%(303 K), 7.3% (323 K) and 13.7% (353 K).

158 B.C.R. Camacho et al. / Separation and Purification Technology 141 (2015) 150–159

Finally, Figs. 9–11 show the comparison between the experi-mental adsorption equilibrium data for CO2, N2, and CH4 and theD–A isotherm model predictions at 303, 323 and 353 K. Overall,the results obtained show good agreement between the D–A modelpredictions and the experimental data. The ARE errors obtainedwere always lower than 13.7% showing a satisfactory agreementbetween the experimental data and the data predicted with theD–A model.

5. Conclusions

The adsorption equilibria of CO2 and N2 were measured on asample of MIL-53(Al) (Basolite A100�, BASF SE) at 303 K, 323 Kand 353 K over the pressure range of 0–34 bar. The results obtainedshow preferential adsorption of CO2 over N2. The MIL-53(Al)

breathing effect previously reported by some authors upon CO2

adsorption was not observed in the commercial Basolite A100�

sample employed in this work. Its behavior upon CO2 adsorptionis consistent with a np-stabilized form that possesses no visiblebreathing behavior. The Sips and Toth isotherm models wereemployed to fit the experimental data and the heats of adsorptionwere determined.

The CO2 and N2 adsorption equilibrium data, along withpreviously obtained CH4 data, were correlated by the AdsorptionPotential Theory. The adsorption data of the three adsorbates(CO2, N2, and CH4) were successfully collapsed into a single tem-perature-independent characteristic curve, which was reasonablywell fitted by the D–A model. This approach allows the extrapola-tion of adsorption equilibrium data from a limited amount ofexperimental data, which can be useful in the design of adsorp-tion-based cyclic processes such as pressure-swing adsorption orgas-phase simulated moving-bed.

Acknowledgements

Financial support from FCT/MCTES (Portugal) through projectsPEst-C/EQB/LA0006/2013, EXCL/QEQ-PRS/0308/2012, PTDC/AAC-AMB/108849/2008 and PTDC/CTM/104782/2008 is gratefullyacknowledged.

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