Microporous and Mesoporous Materials xxx (2009) xxx–xxx

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Microporous and Mesoporous Materials

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

Insights on Adsorption Characterization of Metal-Organic Frameworks:A Benchmark Study on the Novel soc-MOF

J. Moellmer a, E.B. Celer c, R. Luebke b, A.J. Cairns b, R. Staudt a,*, M. Eddaoudi b,*, M. Thommes c,*

a Institut für Nichtklassische Chemie e.V., Permoserstr. 15, D-04318 Leipzig, Germanyb Department of Chemistry, University of South Florida 4202 E. Fowler Avenue, Tampa, FL, USAc Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, FL, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 May 2009Accepted 3 June 2009Available online xxxx

Keywords:AdsorptionCharacterizationGas separationHigh pressureMOF

1387-1811/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.micromeso.2009.06.014

* Corresponding authors.E-mail addresses: staudt@inc.uni-leipzig.de (R. St

(M. Eddaoudi), Matthias.Thommes@quantachrome.co

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In order to explore the potential of novel open metal organic frameworks (MOFs) with soc topology forgas storage applications, we performed a systematic physical adsorption study with Hydrogen, Methaneand Carbon dioxide as adsorptives over a wide range of temperatures (77–323 K) and pressures (0–5 MPa) by using a volumetric low pressure adsorption analyzer equipped with a cryostat, and a high pres-sure gravimetric system. The advanced interpretation of our systematic adsorption data in combinationwith the results from a comprehensive structural and surface characterization allows one assessing thepotential of these novel MOFs for gas storage and separation applications.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

Hybrid porous solids such as metal organic framework materi-als (MOFs) [1–4] are potential candidates for applications in catal-ysis, drug delivery, separation and gas storage. An accurate texturalcharacterization (with regard to surface area, pore size and poros-ity) as well as the determination of adsorption properties, are cru-cial in order to optimize performance of such materials in practicalapplications.

In this paper, we focus in particular on certain important as-pects and challenges associated with the textural characterizationof MOF materials by means of physical adsorption, as well as oninterpretation of adsorption properties. Despite the fact that MOFsare crystalline samples, they may show deviations from perfectcrystalline structure due to various factors. For instance, theymay exhibit reduced pore volumes due to nonvolatile reactantsin the pores, partial collapse, and/or other activation related prob-lems, which can be detected during physical adsorptioncharacterization.

In order to discuss some important aspects of physical adsorp-tion on MOFs, we selected a stable, novel MOF material (an indiumbased MOF with soc topology; i.e. socMOF) which is of potentialinterest for gas storage as well as gas separation applications[5,6]. This novel MOF has a highly ionic framework and relatively

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audt), eddaoudi@cas.usf.edum (M. Thommes).

t al., Micropor. Mesopor. Mate

narrow channels (<1 nm). Details of the synthesis and an extensivecharacterization by using a variety of methods (crystallographicanalysis, elemental microanalysis, single-crystal X-ray diffraction,powder X-ray diffraction, inelastic neutron scattering, Nitrogenadsorption), as well as Hydrogen adsorption properties at 77 and87 K have been very recently reported by Eddaoudi and co-workers[5], followed by molecular simulation study addressing the mech-anism of Hydrogen adsorption in this socMOF [6]. However, todate, the material had not been subject to a comprehensive adsorp-tion study. Hence, in order to explore the potential of this MOFwith soc topology for gas storage applications, we performed a sys-tematic physical adsorption study of Methane and Carbon dioxideas adsorptives over a wide range of temperatures (from 77 to323 K) and pressures (0–5 MPa) by using a volumetric low pres-sure adsorption analyzer equipped with a cryostat, and a high pres-sure gravimetric system. The advanced interpretation of resultingadsorption data in combination with the comprehensive structuraland surface characterization (based on Argon adsorption) allowsone to discuss aspects of prospective applications of these novelMOFs for gas storage and/or separation.

2. Materials and experimental

2.1. Synthesis of soc MOF

A new batch of socMOF (socMOF Me080) was prepared accord-ing to the protocol described in Ref. [5]. The reaction between in-dium nitrate and 3,30,5,50-azobenzenetetracarboxylic acid, as the

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organic linker in the presence of piperazine yielded orange polyhe-dral crystals formulated as [In3O(C16N2O8H6)1.5(H2O)3](H2O)3(NO3)(hereafter denoted as 1) by elemental microanalysis and single-crystal X-ray diffraction studies. The trimeric units each containthree [InO5(H2O)] octahedra sharing one central l3 oxo-anion, witha terminal water molecule in the apical position of the octahedron.The units are linked by six separate organic linkers to produce anovel 3-D periodic structure. To better understand the frameworktopology, the network of 1 can be simplified as 4-connected rect-angular-planar nodes (organic linker) and 6-connected nodes (in-dium trimeric building units). The assembly of these nodesresults in the generation of a 3-D network having an unprece-dented ‘‘soc” topology.

In the crystal structure of 1 (see Fig. 1), each indium atom is tri-valent, yielding an overall cationic framework (+1 per formulaunit). This charge is balanced by highly disordered ½NO3�� anionsthat are presented only in the cavity and occupy statistically twopositions on the threefold axis with equal probability. Therefore,a total of four ½NO3�� ions reside in each nanometer-scale carcer-and-like capsule of 1, and are unable to escape owing to steric hin-drance (window dimensions: 7.651 Å � 5.946 Å, point to point andnot including van der Waals radii). Interesting structural featuresof 1 include its two types of infinite channels. The first channel ishydrophilic because the water molecules which coordinated tothe In metal centers are pointed inside these channels. The guestwater molecules occupy the remaining free volume in these chan-nels and form H-bonds with coordinated water molecules. Thechannels of the second type with a diameter <1 nm are guest freein the as-synthesized 1.

2.2. Experimental – adsorption studies

Systematic physical adsorption studies of Argon, Hydrogen,Methane and Carbon dioxide as adsorptives were performed overa wide range of temperatures (77–323 K) and pressures (0–5 MPa) by using a volumetric low pressure adsorption analyzerequipped with a cryostat, and a high pressure gravimetric system.

2.2.1. Low pressure high resolution adsorption isothermsHigh resolution adsorption isotherms on socMOF were obtained

with Hydrogen, Methane, Carbon dioxide, Argon and Nitrogen from

Fig. 1. X-ray crystal structure of indium soc-MOF: (a) ball-and-stick view of oxygen cent6-connected node having trigonal-prismatic geometry, (b) the organic linker, 3,3’5,5’-arectangular-planar geometry, (c) ball-and-stick, and (d) polyhedral representations ofindium = dark green). Hydrogen atoms and water molecules, and are omitted for clarity.representation of the framework viewed along the y-direction. (For interpretation of thethis article.)

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0 to 0.1 MPa over a wide range of temperatures by using an Aut-sorb1-MP (Quantachrome Instruments, USA) coupled with a novelcryostat (Quantachrome Instr./Oxford Instr.) or a recirculating de-war/bath. The analysis station of the volumetric adsorption appa-ratus is equipped – in addition to the standard pressuretransducers in the dosing volume (manifold) of the apparatus –with high precision pressure transducers (Baratron MKS) dedicatedto read the pressure in the sample cell itself. Hence, the sample cellis isolated during equilibration, which ensures a very small effec-tive dead volume and therefore a highly accurate determinationof the adsorbed amount.

Prior to the analysis the sample was outgassed at room temper-ature for 6 h, and then at 408 K for 12 h under turbomolecularpump vacuum.

2.2.2. High pressure gravimetric adsorption measurementsHigh pressure adsorption isotherms were measured within a

temperature range from 273 to 323 K by a gravimetric method.The adsorption measurements were performed on a magnetic sus-pension balance (Rubotherm GmbH, Germany) that can be oper-ated up to 50 MPa. Highly accurate pressure transducers(Newport Omega, Germany) were used in a range from vacuumup to 5 MPa (Accuracy 0.05%).

In a typically way, a stainless steel sample holder was loadedwith 22 mg of 1 and the system was evacuated for 6 h at roomtemperature. Subsequently, the sample was heated up to 408 Kovernight at 2.6 � 10�3 Torr until constant mass. This procedureensured removal of preadsorbed gases and/or solvents. Duringadsorption measurements, the gas was dosed into the balance toelevated pressures. Equilibrium weights were achieved in 30 minfor each gas. For calculating the Gibbs-Excess-data, buoyancy mea-surements were required with Helium at room temperature(298 K) [7].

3. Results and discussion

3.1. Aspects of physical adsorption characterization

3.1.1. GeneralMOFs are crystalline, and hence surface area, pore structure and

pore size distribution can be calculated geometrically from corre-

ered indium-carboxylate trimer, TMBB, In3O(CO2)6(H3O)3, which can be viewed as azobenzenetetracarboxylic acid, which can be viewed as a 4-connected node with

the cuboidal cage of 1 (color scheme: carbon = gray, oxygen = red, nitrogen = blue,Nitrate anions are depicted without disorder in space filling format. (e) space fillingreferences in colour in this figure legend, the reader is referred to the web version of

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sponding crystallographic structures. However, as already men-tioned before, experimental MOF samples may show deviationsfrom perfect crystalline structure which can be detected by physi-cal adsorption. Nitrogen adsorption at 77 K is considered to be thestandard adsorptive for micro-and mesopore size analysis, but it ismeanwhile generally accepted that Nitrogen adsorption is not sat-isfactory with regard to a quantitative assessment of the micropo-rosity, especially in the range of ultramicropores (pore widths<0.7 nm). Consequently, alternative probe molecules have beensuggested, e.g. Argon and Carbon dioxide. Although the kineticdiameters of Nitrogen, Argon and Carbon dioxide are similar,(0.36, 0.34 and 0.33 nm, respectively [8]) the adsorption behaviorof these three adsorptives is quite different. For many microporoussystems (in particular zeolites) the use of Argon as adsorptive at itsboiling temperature (87.3 K) appears to be very useful. Due to thelack of a quadrupole moment Argon does not give rise to specificinteractions with most surface functional groups and exposed ions.As a consequence, for instance in case of zeolites, Argon fillsmicropores of dimensions 0.5–1 nm at much higher relative pres-sures (i.e. 10�5 < P/P0 < 10�3) than Nitrogen (i.e. 10�7 < P/P0 < 10�5), which leads to accelerated diffusion and equilibrationprocesses, and allows to obtain accurate high resolution adsorptionisotherms within a reasonable time frame [12–14]. Differentranges of micropore filling for Argon and Nitrogen (at 87 and77 K, respectively) are also observed in MOF materials – particu-larly if they possess a highly ionic framework, as in the case of soc-MOF studied. Shown in Fig. 2 are Nitrogen and Argon adsorptionisotherms (starting at a relative pressure of 10�6) plotted in asemi-logarithmic scale, and the different pore filling ranges of Ar-gon and Nitrogen adsorption are clearly demonstrated. Further-more, in case of monatomic Argon adsorption, the application ofadvanced theoretical approaches (based on statistical mechanics)is facilitated.

The use of Argon as adsorptive has not only advantages forobtaining a reliable pore size analysis, but also for surface areaanalysis. It is known that the quadrupole moment of the N2 mole-cule leads for instance to specific interactions with polar hydroxylsurface groups, causing an orientating effect on the adsorbed Nitro-gen molecules [15,16]. Consequently, on polar surfaces the effec-tive cross-sectional area of adsorbed Nitrogen is smaller than thecustomary value of 0.162 nm2. For a completely hydroxylated sur-face cross-sectional area of 0.135 nm2 was proposed, based onmeasurements of the N2 volume adsorbed on silica spheres ofknown diameter [17]. Hence, since Argon molecule is monatomicand much less reactive than the diatomic Nitrogen molecule, Argonadsorption (at 87 K) may seem to be an alternative adsorptive forsurface area determination. Due to the absence of a quadrupolemoment and the higher boiling temperature, the cross-sectional

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area of Argon (0.142 nm2 at 87.3 K) is less sensitive to differencesin the structure of the adsorbent surface [12–14].

3.1.2. Comments to the application of the BET methodGenerally, surface area values are of relative nature (also within

the context of fractal analysis) and should always be related to themethod, conditions and probe molecules used in the experimentalwork [8–14]. Here we want to discuss some important aspects ofthe application of the widely accepted BET (Brunauer, Emmettand Teller) method [18] for surface area analysis of microporousmaterials such as MOFs. Usually, two stages are involved in theevaluation of the BET area. First, it is necessary to transform aphysisorption isotherm into the ‘BET plot’ and from there to derivethe value of the BET monolayer capacity, nm. The second stage isthe calculation of the specific surface area, S, which requiresknowledge of the molecular cross-sectional area. The monolayercapacity nm is calculated from the adsorption isotherm using theBET equation 1/[n((P0/P) � 1)] = (1/nmC) + [(C � 1)/nmC] (P/P0),where n is the adsorbed amount, nm is the monolayer capacityand C is an empirical constant which gives an indication of the or-der of magnitude of the attractive adsorbent–adsorbate interac-tions. In the original work of Brunauer, Emmett and Teller it wasfound that type II Nitrogen isotherms (according to the IUPAC clas-sification [10]) on various nonporous adsorbents gave linear BETplots over the approximate range P/P0 = 0.05–0.3. Thereafter, thespecific surface area S can be obtained from the monolayer capac-ity nm by the application of the simple equation: S = Nm Lr, where Lis the Avogadro constant and r is the so-called cross-sectional area(the average area occupied by each molecule in a completedmonolayer).

In addition to problems arising from the chemical and geomet-rical heterogeneity of the surface, the porosity (i.e. existence of mi-cro- or mesopores) plays an important role for the applicability ofthe BET equation. The BET equation is applicable for surface areaanalysis of nonporous- and mesoporous materials consisting ofpores of wide pore diameter, but is in a strict sense not applicablein case of microporous adsorbents (a critical appraisal of the BETmethod is given in Refs. [8–13]). A major problem is that it is dif-ficult to separate the processes of mono-multilayer adsorptionfrom micropore filling – usually completed at relative pressures(P/P0) below 0.1. Another problem is associated with the size andshape of adsorptive molecule, i.e. the effective yardstick used to as-sess the surface area. Therefore, the surface area obtained byapplying the BET method on adsorption isotherms from micropo-rous solids does not reflect the true internal surface area, butshould be considered as a kind of ‘‘apparent” or ‘‘equivalent BETarea” [11,12]. Recently, a molecular simulation study was per-formed to test the applicability of the BET method for determiningsurface areas of microporous MOFs [19,20]. The BET surface areas(calculated from the simulated isotherms) agreed very well withthe accessible surface areas calculated directly from the crystalstructures as well as experimental surface areas reported in the lit-erature. Even though the adsorption in so-called submicropores(pore width <0.7 nm) gives rise to micropore filling (as indicatedabove), which invalidates one of the BET assumptions, the agree-ment between BET and accessible surface areas was quite goodacross the whole series of materials [8–10].

A problem directly related to the discussion concerning theapplicability of the BET method for assessing the surface areas ofmicroporous materials is the determination of the proper relativepressure range for applying the BET method. If one applies theBET equation within it is classical range (relative pressure range0.05–0.3) on adsorption data obtained on microporous materials,one does not find a linear range, the C constant maybe negative(which is unphysical) and the obtained BET area depends on theselected data points. This is demonstrated in Fig. 3a, which shows

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a BET plot in the relative pressure range from 0.05 to 0.3 derivedfrom the Argon 87 K isotherm on socMOF (which is shown inFig. 2). As it can be clearly seen, the experimental data give not riseto a linear range, and the negative intercept indicates a negative Cconstant (the BET surface area obtained from this plot is 892 m2/g).Hence, the question is how to find the linear range of the BET plotfor microporous materials in a way that it reduces any subjectivityin the assessment of the monolayer capacity. A proper procedurewas recently suggested [21], which requires that not only thequantity of C must be positive (i.e. any negative intercept on theordinate of the BET plot is an indication that one is working outsidethe valid range of the BET equation), but the application of the BETequation should be limited to the pressure range where the termn (P � P0) or alternatively n (1 � P/P0) continuously increases withP/P0 (n is the adsorbed amount). Here we also use the Argon 87 Kdata obtained on socMOF (from Fig. 1) to illustrate the applicationof this procedure to find the linear BET range (see Fig. 3). FromFig. 3b it is clearly visible that based on this criterion all data pointsabove a relative pressure of 0.04 have to be eliminated for applica-tion of the BET equation. The resulting BET plot (Fig. 3c) clearlyshows that the BET equation is applied for relative pressures below0.04, and a linear plot with positive C constant is obtained. Pleasenote also, that the resulting Argon BET specific area is with1148.3 m2/g ca. 13% higher than the surface area obtained if theclassical BET range is being used.

3.1.3. Comments to pore size analysisIn absence of mesoporosity, the physisorption isotherm on a

MOF is of Type I with a plateau which is virtually horizontal onecan correlate the adsorbed amount in the plateau region directlywith the micropore volume. To convert the adsorbed amount intothe micropore volume Vp, the so-called Gurvich rule [9–13] has

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been often applied. Here it is assumed that the pores are filled witha liquid adsorptive of bulk-like properties, an assumption which ismade also in other so-called macroscopic, thermodynamic meth-ods used for pore size and porosity analysis (e.g. Dubinin-Radush-kevitch, methods based on the Kelvin equation, and even theHorvath-Kawazoe, Saito-Foley approaches [8–10]). However, thesemethods not allow for the fact that the degree of molecular packingin small pores is dependent on both pore size and pore shape. Thisproblem has been addressed in various modern approaches basedon statistical mechanics, such as the Density Functional Theory(DFT) or methods of molecular simulation (Monte Carlo Simulation(MC), Molecular Dynamics (MD)) [22,23]. Furthermore, these mod-ern approaches take correctly into account that the shape of sorp-tion isotherms does not depend only on the texture of the porousmaterial but also on the state of the pore fluid. Pore size analysisdata for micro- and mesoporous molecular sieves obtained withthese novel methods agree very well with the results obtainedfrom independent methods (based on XRD, TEM etc.). It has beendemonstrated that the application of these novel theoretical andmolecular simulation based methods lead to: (i) a much moreaccurate pore size analysis, and (ii) allows performing pore sizeanalysis over the complete micro/mesopore size range.

MOF materials have very specific surface properties, whichhampers the development of characteristic DFT or molecular sim-ulation based methods which could be applied for the pore sizeanalysis of a certain MOF sample. However, as also shown in a re-cent paper [24] the application of modern microscopic methodsbased on statistical mechanics such as NLDFT may still generatemore accurate pore size distributions as compared to classicalmethods, if the pore geometry assumed in the DFT methodmatches the pore geometry in the experimental sample, eventhough there is not a perfect agreement between the strength of

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assumed adsorptive/adsorbent interactions and the real/experi-mental system.

The socMOF material studied in this work can be considered asa nanoporous material with an ionic framework, main channels(theoretical pore diameter can be considered an atom to atomdiameter, i.e. distance between the nuclei of the outer ‘‘wall atoms”of these main channels is ca. 1 nm) and nanometer scale carcerandcapsules. A schematic of the structure of this socMOF is shown inFig. 1, and from this it follows that the assumption of a cylindricalpore model seem to be realistic with regard to the main pore chan-nels. Hence, assuming a cylindrical pore model, we now apply twodifferent NLDFT kernels which reflect the adsorption potential ofArgon (at 87.3 K) with carbonaceous materials or oxidic/zeolitematerials, respectively. Fig. 4a shows a comparison of the experi-mental Argon (87.3 K) adsorption data with the theoretical iso-therm calculated from the application of the NLDFT method.From this comparison it follows clearly that the assumption of anoxidic surface leads to a theoretical isotherm which fits the exper-imental adsorption isotherm much better than the theoretical iso-therm obtained by assuming a carbon surface. Indeed, the internalpore diameter obtained from a cylindrical NLDFT pore model(0.61 nm) assuming an oxidic (zeolitic) surface (see Fig. 4b) agreesreasonably well with the accessible pore diameter (i.e. the internaldiameter) of the cylindrical-like (main) channels of this socMOF.The internal pore diameter Din is defined as Din = D–rss, which isthe diameter of the cylindrical later formed by the centers of thepore wall (adsorption sites) atoms, D, less the effective diameterof a wall atom (e.g. rss = 0.276 nm for oxygen [23]). In contrast tothis pore size result obtained by using the oxidic pore model, thepore diameter obtained from the carbon model, i.e. ca. (1.2 nm) ap-pears to be unrealistic because it is larger than the theoretical(atom to-atom) pore diameter of ca. 1 nm.

3.1.4. Comments to the determination of isosteric heatsThe determination of the isosteric heat of adsorption has be-

come an important aspect of characterizing the potential of metalorganic frameworks for Hydrogen or other gas storage applica-tions. The isosteric heat is a direct indicator of the strength andheterogeneity of the interaction between the adsorptive and theadsorbent. There are various methods available to obtain the isos-teric heat of adsorption, but the application of the Clausius–Cla-peyron expression [7,9,11,25,26] is considered to be quiteaccurate and is frequently used. However, an accurate applicationof this method requires at least three closely spaced (in tempera-ture) adsorption isotherms [26]. Hence, it is therefore problematicthat in many reports/publications the isosteric heat is often onlyderived from two experimental adsorption isotherms, which maylead to errors in the determination of the heat of adsorption. With-

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in this context, we studied Hydrogen adsorption on socMOF at var-ious temperatures (87–107 K), and some of these adsorptionisotherm data obtained are shown in Fig. 5a. Corresponding isos-teric heat of adsorption data are shown in Fig. 5b, which also re-veals how the number of adsorption isotherms used in thecalculation affects the isosteric heat values particularly in the re-gion of low loading. As already discussed in Ref. [6] the heat ofadsorption of 6.5 kJ/mol for H2 on socMOF is higher than for typicalactivated carbon materials, and higher than isosteric heat valuesfound for H2 adsorption on some other MOF materials (e.g. MOF177; 4.5 kJ/mol [4]). The constancy of the heat of adsorption isindicative of the homogeneity of the H2, adsorption sites, whichis a desirable property for H2 storage materials [4] and highlightsthe importance of open metal sites for H2 adsorption [6]).

3.2. Adsorption properties of socMOF

MOF materials are considered as promising candidates forHydrogen and other gas storage applications [27–35]. Within thiscontext we investigated the adsorption properties of this novelsocMOF in particular for CH4 and CO2 adsorption at sub- and super-critical conditions over a wide temperature and pressure range (weare here not focusing on details of the Hydrogen adsorption behav-ior in socMOF because this has already been discussed in detail inRefs. [5,6]).

An interesting example which demonstrates how the shape ofan adsorption isotherm can be affected by the states of pore andbulk fluid phases is shown in Fig. 6, in which the adsorption iso-therms of Hydrogen and Methane at 107.4 K in socMOF are com-pared. The adsorption isotherms exhibit totally different shapes,which indeed is mainly due to the fact that at this temperatureMethane is subcritical (T/Tc = 0.56), whereas Hydrogen is supercrit-ical (T/Tc = 3.23). Contrary to Hydrogen, Methane completely fillsthe pore space of this MOF with a liquid-like state, indicated bythe observed plateau in the adsorption isotherm (i.e. a perfect typeI adsorption isotherm is observed).

From the CH4 adsorption isotherm it is further possible to ob-tain surface area and porosity information. Methane, similarly asArgon, has no dipole or quadrupole moment, and should serve asreliable probe molecule textural characterization.

In particular with regard to assessing the CH4 or CO2 gas storagecapacities of this socMOF it is of interest to determine whetherthere is any advantage to determine the pore volume by usingMethane (below the critical temperature, Tc) instead of using porespace/volumes obtained from other probe molecules (e.g. such asArgon and Nitrogen). Hence, we used the Methane adsorption dataat 107.4 K to calculate the pore volume by applying the Gurvitchmethod (at a relative pressure of 0.95). The obtained specific pore

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Hydrogen 107 KHydrogen 107 KMethane 107 KMethane 107 K

a b

Fig. 6. Adsorption of Methane and Hydrogen at 107.4 K on socMOF. At this temperature Methane is subcritical (T/Tc = 0.56), whereas Hydrogen is supercritical (T/Tc = 3.23).(a) Linear display of adsorption isotherms. (b) Semi-logarithmic isotherm plot which resolves the low pressure adsorption behavior.

6 J. Moellmer et al. / Microporous and Mesoporous Materials xxx (2009) xxx–xxx

ARTICLE IN PRESS

volume of 0.45 cm3/g (calculated from Methane adsorption at107.4 K) is in good agreement with the pore volume from the Ar-gon 87.3 K isotherm (0.47 cm3/g).

CO2 adsorption was studied over a temperature range from sub-critical to supercritical temperatures (i.e. 273–323 K, Tc(CO2) =304 K). Contrary to Methane adsorption at 107.4 K, where thecomplete adsorption isotherm (up to the rel. pressure range of 1)can be obtained at low pressures (P0 � 525 Torr), the saturationpressure of Carbon dioxide for instance at 273 K is around26,150 Torr (=3.4851 MPa). Hence, in order to obtain the CO2 iso-therm at 273 K (T/Tc = 0.89) on socMOF in a rel. pressure rangefrom 10�5 to 1 (see Fig. 7a), the lower pressure region from relativepressures 10�5–10�2 was measured with a high resolution volu-metric apparatus, whereas data points at high pressures were ob-tained with a magnetic suspension balance. It appears that CO2

fills the socMOF pores – compared for instance to Ar at 87.3 Kand CH4 at 107.4 K – at much higher relative pressure range (i.e.from 5 � 10�3 to 5 � 10�2), in contrast to Argon 87.3 K andMethane 107.4 K adsorption. The shift in pore filling range isexpected because CO2 (at 273 K) is by far closer to the critical tem-perature as compared to Argon adsorption at 87.3 K (T/Tc = 0.58)and Methane at 107.4 K (T/Tc = 0.56). Similar as in case of Methaneadsorption at 107 K, one can in principle obtain pore volume/poros-ity information from CO2 adsorption at 273 K. However, contrary tothe CH4/107 K isotherm, where the measured surface excess datacorrespond to the absolute amount adsorbed (see discussionbelow), this is not the case for CO2. There is a need to distinguishbetween the absolute adsorption and the surface excess adsorptionin case of high pressure adsorption conditions. Excess and absoluteadsorption isotherms are related by the expression

Please cite this article in press as: J. Moellmer et al., Micropor. Mesopor. Mate

Nex ¼ Nads � qbulkVads; ð1Þ

where qbulk is the density of the gas phase and Vads is the volume ofthe adsorbed phase.

Experimentally, one can only determine the surface excessamount adsorbed, and at sufficiently low pressures and tempera-tures, i.e. low bulk gas densities (i.e. typically Nitrogen and Argonadsorption at their boiling temperatures) the surface excess andabsolute adsorbed amount correspond to each other. In a low-tem-perature region the adsorption of gases on surfaces can be indeedanalyzed in terms of a two-phase model, in which an adsorbedphase coexists with the bulk phase [7,11,13,36]. However, at high-er temperatures the model of an adsorbed phase becomes progres-sively unrealistic because (i) the tendency of molecules toaccumulate near the surface of the adsorbent becomes less pro-nounced and (ii) due to the weaker physisorption at elevated tem-peratures higher pressures have to be applied in order to reachsignificant surface coverage. As a consequence the density of thebulk gas phase is no longer negligible relative to the density nearthe surface and a clear separation between adsorbed phase andbulk gas phase is not possible, i.e. the profile of the local densityq (z) exhibits a smooth transition from the surface into the bulkgas. For this situation the definition of the adsorbed amount, i.e.the determination of the volume of the adsorbed phase becomesmore problematic.

For sufficiently narrow pores the volume Vads can be associatedwith the porous volume of the sample (which can be for instanceobtained by Nitrogen or Argon adsorption at 77 or 87 K,respectively).

r. (2009), doi:10.1016/j.micromeso.2009.06.014

Pgσ

A Vρnn +=

⎟⎟⎠

⎞⎜⎜⎝

⎛−

=

liq

g

σ

A

ρρ

1

nn

Pgσ

A Vρnn +=

⎟⎟⎠

⎞⎜⎜⎝

⎛−

=

liq

g

σ

A

ρρ

1

nn

0

2

4

6

8

10

0 1 2 3

Pressure [MPa]

n A[m

mol

g-1

]a

b

c

Fig. 7. Adsorption of CO2 on socMOF: (a) CO2 adsorption on socMOF at relativepressures 10�5–0.8. The low pressure adsorption up to relative pressures of 10�2

was obtained with a volumetric high resolution adsorption analyzer. Data points athigher relative pressures were obtained with a magnetic suspension balance(gravimetric method). (b) Calculation of absolute adsorbed amount of CO2 onsocMOF at 273 K – good agreement between two different approaches. (c) Surfaceexcess and absolute adsorbed amounts for CO2 at temperatures from 273–323 K.

0

2

4

6

8

0 1 2 3

Pressure [MPa]

n[m

mol

g-1

]

n CH (273K) Ads soc-MOFn CH (298K) Ads soc-MOFn CH (323K) Ads soc-MOFn CH (273K) Ads soc-MOFn CH (298K) Ads soc-MOFn CH (323K) Ads soc-MOF

Fig. 8. CH4 adsorption (surface excess and absolute adsorbed amount) on socMOFover a temperature range from 273 to 323 K.

J. Moellmer et al. / Microporous and Mesoporous Materials xxx (2009) xxx–xxx 7

ARTICLE IN PRESS

On the other hand if one assumes that Vads varies according toVads = Nads/qads, where qads is the density of the adsorbed phasethen Eq. (1) becomes

Nex ¼ Nadsð1� qbulk=qadsÞ ð2Þ

The internal pore diameter of socMOF is with ca. 0.65 nm quite nar-row, hence we applied both approaches (Eqs. (1) and (2)) discussedabove to determine the absolute adsorbed amount for the CO2. Asclearly demonstrated in Fig. 7b, good agreement was found be-tween the two methods (by using a pore volume of ca. 0.5 cm3/g ob-tained from Nitrogen adsorption[5] and assuming that adsorbedCO2 phase has density equal liquid density of CO2 at 273 K). Theabsolute adsorption isotherm data obtained in this way for CO2 at273 K can now also be used for obtaining porosity informationand in fact the pore volume Vp obtained by the Gurvitch method

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agrees with 0.44 cm3/g quite well with the appropriate pore volumevalues of 0.47 and 0.45 cm3/g obtained from Argon (87.3 K) adsorp-tion and Methane (107.4 K) adsorption, respectively. This goodagreement in the pore volumes obtained from subcritical CH4,CO2 and Ar adsorption, however indicates that using Argon adsorp-tion is sufficient for obtaining meaningful pore volume/porosityinformation also in the context of CH4 adsorption/storage.

Contrary to Methane, Carbon dioxide has a quadrupole momentwhich will give rise to specific interactions with the ionic frame-work of socMOF, and consequently one expects that CO2 shouldhave a higher adsorption affinity than CH4 with regard to socMOF.This follows from Figs. 7c, 8 and in particular from Fig. 9.

Fig. 7c shows the surface excess and absolute adsorbed amountsfor CO2 at temperatures from 273 to 323 K. Adsorption isothermsfor CH4 (surface excess and absolute adsorbed amount) over thesame temperature range are shown in Fig. 8. The uptakes of bothCO2 and CH4 are larger than for instance for some zeolites and car-bons [27], but smaller than values reported for other MOFs (e.g. IR-MOF-1 or Cu-BTC [32]).

Fig. 9 compares the adsorption (i.e. surface excess) of CO2 withCH4 at three different temperatures.

Consistent with the adsorption isotherms CO2 has a higher isos-teric heat than CH4 (see Fig. 10, i.e. for CH4 = 18.8 kJ/mol; and forCO2 = 28.5 kJ/mol) indicative of the stronger affinity of CO2 forthe MOF adsorption sites, as compared to CH4. The increase inthe heat of adsorption at higher loadings (higher pressures) indi-cates that adsorbate–adsorbate interactions become increasinglyimportant, while the interactions between adsorbate and adsor-bent remains unchanged.

As already indicated, the significant difference in adsorbedamounts between CO2 and CH4 is also due to the fact that CO2

has a significant quadrupole moment, whereas CH4 is nonpolar.However, as already discussed in connection with Fig. 6, the ther-modynamic state of the adsorptives/adsorbate, can also contributeto the shape of the adsorption isotherm and the adsorbed amounts.At 273 K, the CO2 bulk phase is subcritical (T=TcCO2

= 0.89), whereasCH4 (T=Tc;CH4 = 1.43) is far above it’s bulk critical temperature.Hence, contrary to CH4, CO2 may indeed form at this temperaturea more liquid-like phase in the pores. Therefore, one would expectthat the loading ratio (or ratio of adsorbed amounts) will be af-fected by temperature, i.e. the ratio of CO2/CH4 adsorption shouldbe the highest at 273 K (when CO2 is subcritical, and CH4 is super-critical), and the smallest at 323 K, a temperature where bothadsorptives are supercritical.

Accordingly we plotted the ratio of CO2/CH4 uptake (loadingratio) as a function of pressure for the three experimentaltemperatures. Fig. 11a shows the loading ratios for each of thethree temperatures over a pressure range up to 3 MPa, whereas

r. (2009), doi:10.1016/j.micromeso.2009.06.014

Fig. 10. Isosteric heat of adsorption for CO2 and CH4 on socMOF. Isosteric heat ofadsorption: CH4 = 18.8 kJ/mol; CO2 = 28.5 kJ/mol. The increase in the heat ofadsorption at higher loadings (higher pressures) indicates that adsorbate–adsorbateinteractions become increasingly important.

Fig. 9. Comparison of CO2 and CH4 adsorption at various temperatures on socMOFas a function of pressure. Interestingly, the difference in surface excess amountsbetween CH4 and CO2 adsorption decreases with increasing measurement temper-ature, i.e. the difference in surface excess is the smallest when both fluids are abovetheir bulk critical temperature (323 K, T/Tc(CO2) = 1.06; and T/Tc(CH4) = 1.69), and isthe largest at 273 K (T/Tc(CH4) = 1.43, T/Tc(CO2) = 0.89).

8 J. Moellmer et al. / Microporous and Mesoporous Materials xxx (2009) xxx–xxx

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Fig. 11b enlarges the region up to 0.3 MPa. The loading ratiosbetween CO2/CH4 decreases as expected with increasing pressure,and are the highest in the pressure region below 0.3 MPa for alltemperature.

However, in this pressure region (which also represents thepressure region in which the MOF pores fill with CO2) the loadingratio is at 273 K significantly higher as compared to 323 K; thisillustrates how difference in thermodynamic states of adsorbed

0

2

4

6

8

0 0.1 0.2 0.3

Pressure [MPa]

load

ing

rati

o nσ C

O2:

nσ CH

4[-

]

ratio nσσCO2:nσσ

CH4 (273K) Ads soc-MOFratio nσσ

CO2:nσσCH4 (298K) Ads soc-MOF

ratio nσσCO2:nσσ

CH4 (323K) Ads soc-MOF

a

Fig. 11. (a) Loading ratio (or ratio of adsorption capacities) of CO2/CH4 calculated fromcalculated from the data shown in Fig. 10 up to pressures of 3 MPa.

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phases contribute to different adsorbed amounts of CO2 and CH4

at the same pressure. This would also suggest that in the pressurerange <0.3 MPa (please note that as stated in Ref. [33], almost allindustrial applications of new materials for gas separation pro-cesses involve CO2 partial pressures of <0.5 MPa) the socMOFmay be more efficient for CO2/CH4 separation at 273 K (wherethe bulk CO2 phase is still in a subcritical state) as compared to323 K (where CO2 is supercritical). However, at a pressure around0.3 MPa the loading ratio curves cross, leading to the observationthat at higher pressures the CO2/CH4 loading ratio is more favor-able at 323 K (a temperature where both fluids are above their bulkcritical temperatures). The crossing of the loading curves indicatesthat at higher pressures the isothermal compressibility of theadsorbate phases becomes important, i.e. the supercritical CO2

phase at 323 K for instance appears to be more compressible athigher pressures as compared to the more liquid-like CO2 adsor-bate phase at 273 K. Hence, whereas the state of the bulk and poreCO2 phase changes, the state of CH4 remains unchanged (i.e. super-critical) for all experimental temperatures. These data suggest thatCO2/CH4 separation should be more efficient at high pressures(>3 MPa) at temperatures above the bulk critical temperature forboth fluids, whereas at pressure <3 MPa, CO2/CH4 separation ismore favorable at a temperature sufficiently below the bulk criticaltemperature of CO2. The ratio of CO2/CH4 adsorption capacities wehave found for socMOF are comparable to the results of [34] for Cu-MOF extrudates, which were found to be higher than for activatedcarbons and other MOFs (e.g. MOF 5).

4. Conclusions

We have addressed some important aspects of physicaladsorption characterization and interpretation of adsorption dataon MOF materials in general but here specifically on a MOF withsoc topology. We studied the temperature dependency of Hydro-gen, Carbon dioxide and Methane adsorption in a series of sys-tematic physisorption experiments conducted over a widerange of temperatures and pressures. Our data show that it isimportant to take into account that the shape of the adsorptionisotherms reflects details of the pore structure and surface chem-istry, but is also strongly affected by the difference in thermody-namic states of pore and bulk fluid phases. This has to be takeninto account in particular if one compares differences in adsorp-tion behavior (incl. adsorption kinetics) and adsorption capacityof fluids in MOF material, which is for instance important forthe CO2/CH4 separation problem. Furthermore, our results sug-gest that similarly as in the case of zeolites, Argon adsorptionat 87 K is a very suitable probe molecule for physical adsorptioncharacterization.

0

2

4

6

8

3210

Pressure [MPa]

load

ing

rati

o nσ C

O2:

nσ CH

4[-

]

ratio nσCO2:nσ

CH4 (273K) Ads soc-MOFratio nσ

CO2:nσCH4 (298K) Ads soc-MOF

ratio nσCO2:nσ

CH4 (323K) Ads soc-MOF

b

the data shown in Fig. 10a up to pressures of 0.3 MPa. (b) Loading ratio of CO2/CH4

r. (2009), doi:10.1016/j.micromeso.2009.06.014

J. Moellmer et al. / Microporous and Mesoporous Materials xxx (2009) xxx–xxx 9

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Acknowledgements

We would like to thank the DFG for financial support (DFG-Pro-jekt SPP 1362 MOF STA428/17-1) M. Eddaoudi gratefully acknowl-edge the financial support by National Science Foundation (DMR0548117) Department of Energy DOE-BES (DE0FG02-07ER4670).

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