Microporous and Mesoporous Materials 175 (2013) 85–91

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Microporous and Mesoporous Materials

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Carbon dioxide adsorption in microwave-synthesized metal organic frameworkCPM-5: Equilibrium and kinetics study

Rana Sabouni, Hossein Kazemian, Sohrab Rohani ⇑Department of Chemical and Biochemical Engineering, Western University, London, ON, Canada N6A 5B9

a r t i c l e i n f o

Article history:Received 9 November 2012Received in revised form 25 February 2013Accepted 22 March 2013Available online 29 March 2013

Keywords:CPM-5Carbon dioxideAdsorption equilibriumKineticsSelectivity

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⇑ Corresponding author. Tel.: +1 519 661 4116; faxE-mail address: srohani@uwo.ca (S. Rohani).

a b s t r a c t

The adsorption equilibrium and diffusion of CO2 in CPM-5 (crystalline porous materials) were experimen-tally studied using a volumetric approach at three different temperatures 273, 298, and 318 K and gaspressures up to 105 kPa. The Freundlich adsorption equilibrium model was applied to correlate theadsorption isotherms, and the classical microspore diffusion model was applied to obtain the adsorptionkinetic curves and diffusivity of CO2 in CPM-5. The selectivity of CO2 over N2 was estimated from the sin-gle component isotherms at conditions relevant to post combustion applications (0.15 bar PCO2 and0.75 bar pN2). It was found that at pressures up to 105 kPa, the CPM-5 adsorbent has CO2 adsorption capa-city of 3 mmol/g, 2.3 mmol/g, at 273 K and 298 K, respectively, which is significantly higher than those ofMOF-5 under the same conditions. The selectivity factor was 14.2 and 16.1 at 273 and 298 K, respectively.The CO2 diffusivity in CPM-5, estimated from the adsorption kinetic data measured at low pressures, are1.86 � 10�12 m2/s, 7.04 � 10�12 m2/s, and 7.87 � 10�12 m2/s at 273 K, 298 K and 318 K, respectively. Theinitial isosteric heat of adsorption of CO2 on the CPM-5 is 36.1 kJ/mol. CPM-5 shows attractive adsorptionproperties as an adsorbent for separation of CO2 from flue gas.

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1. Introduction are subject to degradation in the presence of O , SO , NO and

Carbon dioxide (CO2) is one of the major greenhouse gases re-sponsible for global warming. As the excessive discharge of CO2

in the atmosphere keeps increasing, mainly due to the combustionof huge amounts of fossil fuels, more serious concerns are raisedwith respect to its impact on the global warming and environmen-tal damages. Carbon dioxide has risen by more than a thirdsince the industrial revolution from 280 ppm by volume to368 ppm in 2000 [1], and 388 ppm in 2010 [1]. According to theIntergovernmental Panel on Climate Change (IPCC) [2], the atmo-sphere may contain up to 570 ppm of carbon dioxide in 2100 caus-ing a rise in the mean global temperature of around 1.9 �C and anincrease in the mean sea level of 3.8 m [3]. Accordingly, tremen-dous efforts have been invested to develop effective methods forcapturing CO2 from post-combustion flue gas. Aqueous alkanola-mine absorbents is the most studied method to date and have beenknown for several decades, however it is still considered asstate-of-the-art [4]. The most well known alkanolamine for CO2

capture is monoethanolamine (MEA). However this technique hasseveral significant drawbacks: The process in general requireslarge equipment size and intensive energy input, low carbon diox-ide loading capacity, high equipment corrosion rate, and amines

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2 2 2

HCl which adds additional requirements for solvent make-up andwaste steam disposal. In addition, the main disadvantage of alka-nolamines solutions is the high heat capacity which can reachthe heat capacity of pure water for low concentration of the alka-nolamine (e.g. 20 and 40 mol% MEA solutions) [4,5]. According toLi et al. [6] no single technology has been identified to meet the re-quirements set by the DOE/NETL: 90% CO2 capture at less than a35% increase in the cost electricity. Therefore, there is a crucialneed for developing an alternative capture technology that canlower the operation cost and have significant advantages for en-ergy efficiency. Accordingly, adsorption processes using solid phy-sical adsorbents possess many potential advantages compared tothe other capture technologies (i.e. chemical and physical absorp-tion processes). These include less regeneration energy require-ment, greater capacity, higher selectivity, ease of handling,minimal environmental impact and low costs. Adsorption and sto-rage of CO2 in nano-porous adsorbents is a viable route comparedto other technologies, because of their high CO2 adsorption capaci-ties, low energy requirement [7,8], low heat capacities which pre-sent a promising way to reduce the regeneration energy penalty[4]. Recently, metal organic frameworks (MOFs) have emerged asa new class of zeolitic like porous materials that have potentialindustrial applications including gas separation, adsorption andstorage processes [9], heterogeneous catalysis [10], and pharma-ceutical manufacturing processes and drug delivery carriers [11].Metal organic frameworks have gained significant interests as

Symbols used

CPM-5 crystalline porous materialsMW microwave irradiationEIA energy information administrationGHG greenhouse gasIPCC intergovernmental panel on climate changeMOFs metal organic frameworksBTC 1, 2, 3-benzenetricaboxylateDMF N,N-dimethylformamideq the adsorbed amount (mmol/g)pi the equilibrium pressure (kPa)k constant for a given adsorbent and adsorbate at

particular temperature

n constant for a given adsorbent and adsorbate at particu-lar temperature

Dc diffusivity constant (m2/s)t time in (s)rc particles radius (m)mt/m1 fractional uptake of adsorbentQst isosteric heat of adsorption (kJ/mol)P pressure (kPa)T temperature (K)R the gas constantna the adsorbed amount (mmol/g)

86 R. Sabouni et al. / Microporous and Mesoporous Materials 175 (2013) 85–91

promising adsorbents, due to their outstanding properties includ-ing high specific surface areas and pore volume as well as highlydiverse structural chemistry [12], and tuneable pore size from mi-croporous (i.e. <2 nm) to mesoporous (i.e. 2–50 nm) scale [11]. Ac-cording to the literature, MOF-210 and MOF-200 are the mosteffective MOFs for CO2 adsorption exhibiting CO2 adsorption capa-city of 54.5 mmol/g (74.2 wt.%) at 298 K and 5000 kPa [13] withextremely high BET(and Langmuir) surface areas of 4530 (10400),6240 (10400) m2/g for MOF-200 and MOF-210, respectively. BothMOF-200 and MOF-210 exceed the CO2 uptake of the top pre-viously reported MOFs such as MOF-177 and MIL-101 (Cr)(33.5 mmol/g (60.8 wt.%) and 40 mmol/g (56.9 wt.%), respectively)[14,15]. NU-100 is another example of high CO2 uptake MOFswhich showed a CO2 adsorption capacity of 52.6 mmol/g(69.8 wt.%) at 298 K and 4000 kPa and BET surface area of6143 m2/g [16]. In addition to the high CO2 adsorption uptake ofthe previous top MOFs, suitable adsorbents for CO2 capture fromflue gas should satisfy several other important criteria to competewith the present technologies, including: (1) high adsorption capa-city: the CO2 equilibrium adsorption capacity represented by itsadsorption isotherm is the most important criterion to evaluatenew adsorbents in terms of the capital cost of the capture system.With the knowledge of the adsorption equilibrium capacity, theamount of the adsorbent required can be calculated, consequentlythe volume of the adsorber vessel. The suitable adsorbent for CO2

capture from flue gas should exhibit a minimum CO2 adsorptioncapacity of 2–4 mmol/g [17]; (2) high selectivity for CO2: the ad-sorption selectivity is defined as the ratio of CO2 capacity to otherbulk gas components (i.e. N2 and O2). The selectivity is one of themain properties of the adsorbent materials, because it has a directimpact on the purity of the CO2 captured and consequently on theeconomics of the separation process [17]; (3) adequate adsorption/desorption kinetics: good adsorbent should exhibit fast adsorption/desorption kinetics under the operating conditions and high rate ofadsorption; (4) stability during repeated adsorption/desorption cy-cling: Stability is a crucial property of an adsorbent, because it de-termines the lifetime of the adsorbents and the frequency of theirreplacement; (5) mechanical strength: suitable adsorbents shoulddemonstrate stable microstructure and morphology under severaloperating conditions, such as high volumetric flow rate of flue gas,vibration, and temperature. In addition, good adsorbents shouldtolerate presence of moisture and other impurities in the feed(i.e. water vapor, O2 and SO2). Otherwise, the CO2 adsorption pro-cess will require large sorbent makeup rate. As a result, mechanicalstrength of adsorbents has direct impact on the overall economicsof the CO2 separation process; (6) low cost: the cost of an adsor-bent is one of the most important characteristics. According to astudy performed by Tarka et al. [18] on the sensitivity analysis of

adsorbents for economic performance, a cost of $5/kg of adsorbentoffers a very good picture and $15/kg of adsorbent is not economic-al. Thus, $10/kg of adsorbent is considered economical for CO2 cap-ture process. Finally, a key challenge in the CO2 adsorption processis to develop adsorbents with high adsorption capacity for CO2 atlower partial pressures. Zongbi Bao et al. [19] have reported theCO2 adsorption capacity of 8.61 mmol/g (27.5 wt.%) at 298 K andlow pressure of 100 kPa on Mg-MOF-74. Furthermore, at roomtemperature and pressure of 100 kPa, MOF-5 and MOF-177 haveexhibited CO2 adsorption capacity of 1.2 mmol/g (4.5 wt.%) and1.7 mmol/g (7 wt.%), respectively [20,21]. Recently, CPM-5 synthe-sized under microwave irradiation has proven to be a promisingcandidate for CO2 adsorption due to its high CO2 adsorption capa-city of 2.55 mmol/g at room temperature and low pressure of100 kPa [22].

The objective of the present work is to determine the adsorp-tion equilibrium isotherms, kinetic and diffusion mechanisms,isosteric heat of adsorption and diffusivity of CO2 in CPM-5 at threedifferent temperatures of 273 K, 298 K, and 318 K. All the CO2 ad-sorption experiments in this work were carried out at low pres-sures (i.e. up to 105 kPa) to evaluate the potential of CPM-5 forapplication as adsorbent for CO2 capturing from flue gas at lowpressure.

2. Experimental

2.1. Synthesis of CPM-5

All chemicals including indium (II) nitrate hydrate(In(NO3)3�xH2O; 99.5%, J.T Baker; USA), 1,2,3-benzenetricaboxylicacid (H3BTC; 99%, Aldrich, USA), and N,N-dimethylformamide(DMF; 99.9%, Aldrich, USA), were used as purchased withoutfurther purification. The CPM-5 was synthesized following ourpreviously reported procedure [22] using microwave irradiationas heating source (Discovery system model of CEM LaboratoryMicrowave, USA). The detailed synthesis procedure can be foundelsewhere in [22]. The purity of the sample was confirmed by thepowder X-ray diffraction.

2.2. Characterization of CPM-5

Crystallinity of the products was examined by X-ray powderdiffraction (XRPD) technique using a Rigaku–MiniFlex powder dif-fractometer (Japan; CuKa = 1.54059 Å) over 2h range of 5�–40�with step width of 0.02�. Surface area of the adsorbents was mea-sured by BET technique (Micromeritics ASAP 2010, USA). Furthercharacterization of the produced CPM-5 sample including,

R. Sabouni et al. / Microporous and Mesoporous Materials 175 (2013) 85–91 87

morpholopgy thermogravimetric analysis (TGA) curve, N2 adsorp-tion/desorption isotherms, and Fourier Transform Infrared (FTIR)spectra can be found in the authors’ previous article [22].

2.3. CO2 Adsorption study

Adsorption equilibrium of CO2 and N2 in CPM-5 was measuredvolumetrically using a BET instrument (Micromeritics ASAP 2010,USA) at CO2 pressure up to 105 kPa and three temperatures of273, 298, and 318 K for CO2 and 273, 298 K for N2. Ultra high pureCO2 and N2 (Praxair Canada Inc.) were used as received for the ad-sorption measurements and backfill gas, respectively. The tem-perature was controlled using a circulating jacket(ThermoNeslab; Newington, NH, USA). The rate of adsorption(ROA) of CO2 on CPM-5 was also measured using the ASAP 2010system equipped with the ‘‘Rate of Adsorption’’ software at thesame time when the adsorption equilibrium data were collected.All the diffusion data were measured at stepped pressure incre-ments, from 0 to 105 kPa. For each increment pressure the ROAsoftware reports the amount adsorbed as a function of time. Theamount adsorbed is converted into transient adsorption uptakesto generate the adsorption kinetics. The adsorption equilibriumamount is considered as the final adsorption amount at the term-inal pressure and temperature. To do the measurement, knownamounts of samples (e.g. 90–100 mg) were loaded into the BETsample tube and degassed under vacuum (10�6 kPa) at 423 K over-night. Ultra pure helium gas (Praxair Canada Inc.) was used to mea-sure the free space. To study the effect of re-generation on the usedadsorbent, the spent sample was degassed again for the second cy-cle at 423 K under vacuum (10�6 kPa) for 5 h.

The Freundlich isotherm model was used to correlate the ad-sorption isotherms at different tested temperatures. The Freun-dlich isotherm equation fitted well the experimental data withregression coefficients greater than 0.99. The Freundlich isothermcan be written as following:

q ¼ Kp1=ni ð1Þ

where q is the adsorbed amount (mmol/g), pi is the equilibriumpressure (kPa), and K, n are constants for a given adsorbent and ad-sorbate at particular temperature. Estimates of k and n can be ob-tained from the experimental data of CO2 adsorption isothermsusing the intercept and slope of a linear Freundlich plot of ln(q) ver-sus ln(pi). The adsorption isotherm parameters for Freundlich equa-tion are listed in Table 1.

Working capacity or delta loading, Dq is an important parameterthat determines the economics of Pressure Swing Adsorption pro-cess (PSA). To increase the CO2 working capacity the adsorber vo-lume must be smaller. Therefore, the capital and capture cost willdecrease. Working capacity is defined as the difference in theamount of component (CO2) that needs to be adsorbed in molesper kilogram of adsorbent material, at the adsorption pressure(qCO2 adsorption) and the amount adsorbed at the desorption/eva-cuation pressure (qCO2 desorption) (Eq. (2)) [17].The adsorptionpressure could range between 0.1 and 10 MPa, and the desorptionpressure could range from 0.01 to 0.1 MPa [23],

Dqco2¼ Dqco2

adsorption� Dqco2desorption ð2Þ

Table 1Freundlich isotherm equation parameters for CO2 adsorption on CPM-5.

Temperature (K) K n R2

273 0.143 1.552 0.9986298 0.049 1.214 0.9992318 0.008 0.984 0.9982

3. Results and discussion

3.1. Characterization of the synthesized CPM-5 samples

CPM-5 is a highly porous indium based metal organic frame-work with a surface area of 2187 m2/g [22]. CPM-5 consists ofIn3O clusters as metal centres connected by 1, 2, 3-benzenetrica-boxylic acid (H3BTC) as a linear organic linker. In addition, CPM-5 has three unique cage-within-cage based porous structureswhich contribute to a high CO2 uptake capacity. Fig. 1 presentsthe structure of the CPM-5 particles [24]. To the best of the authors’knowledge, CPM-5 has not been studied in details for CO2 adsorp-tion including adsorption equilibrium isotherm at different tem-peratures (i.e. 273 K, 298 K and 318 K), adsorption kinetics,isosteric heat of adsorption, and diffusivity of CO2 in CPM-5.

One of the main drawbacks for industrial scale applications ofmost of MOF compounds is low physico-chemical stability of thesematerials when exposed to air and water vapour. Thus this in-stability may limit their industrial applications. Many zinc-basedMOFs such as MOF-5 and MOF-177 decompose very easily uponexposure to air and moisture; therefore they have to be stored un-der vacuum or controlled atmosphere. The stability of MOF-5 uponexposure to air was studied by Yang et al. [25] and Kaye et al. [26].It was shown that decomposition of MOF-5 structure was completeafter one week of exposure and qualitatively a different XRDpattern was obtained. Fig. 1 demonstrates the XRPD patterns ofthe CPM-5 samples over 4 weeks of exposure to the environment

Fig. 1. Structures of In12@In24 cage CPM-5. The green solid lines represent BTCs.[24]. (For interpretation of the references to colour in this figure, the reader isreferred to the web version of this article.)

88 R. Sabouni et al. / Microporous and Mesoporous Materials 175 (2013) 85–91

humidity at room temperature. According to the XRPD pattern,however, the synthesized CPM-5 was shown to have a very stablestructure after exposure to ambient conditions for several weeks.

It can be seen that the main peaks of the samples well matchedwith previously published XRPD pattern for CPM-5 prepared undermicrowave irradiation [22] (Fig. 2). It is found that the XRPD pat-tern remains unchanged for the aged samples. This reveals thatthe CPM-5 structure is stable under laboratory condition (296 Kand relative humidity of 62%) for several weeks. Furthermore, shelflife of the CPM-5 aged for several weeks was studied by measuringthe effective surface area by BET. The surface area of the fresh CPM-5, 2 weeks old and 4 weeks old samples were 2187 cm2/g,2085 cm2/g and 2050 cm2/g, respectively, confirming the durabil-ity of samples in the normal lab environment. Accordingly, CPM-5 can be considered as a durable and attractive MOF candidatewith long shelf life for practical industrial application.

Fig. 3. (a) CO2 adsorption isotherms at different adsorption temperatures, 273 K,298 K and 318 K Experimental data; (empty symbols). Frenundlich isotherm modeldata (slid line), desorption isotherm data (filled symbols), (b) N2 adsorptionisotherms at different adsorption temperatures, 273 K, and 298 K Experimentaldata; (empty symbols), desorption isotherm data (filled symbols), and pressures upto 105 kPa.

3.2. Adsorption equilibrium

Experimental and modelled CO2 adsorption isotherms at threedifferent temperatures of 273 K, 298 K, and 318 K under CO2 pres-sure of 0–105 kPa are plotted in Fig. 3a. the experimental N2 ad-sorption isotherms are plotted in Fig. 3b at 273 and 298 K underN2 pressure of 0–105 kPa. In general, it was observed that theamount of adsorbed gases decrease with increasing temperaturefor both gases as a result of higher thermal energy of the moleculesat higher temperatures. Moreover, the CO2 adsorption capacity ishigher than that for N2 at all temperatures due to the greater quad-rupole moment and polarization of CO2 (13.4 � 10�40 cm2 and26.3 � 10�25 cm3, respectively) compared to N2 (4.7 � 10�40 cm2

and 17.7 � 10�25 cm3, respectively) [27] result in higher affinityof the surface of the material for CO2. According to the isothermaldata, CO2 uptake at 273 K, 298 K and 318 K under 105 kPa of feedpressure were 3 mmol/g (13.2 wt.%), 2.3 mmol/g (10.1 wt.%), and1 mmol/g (4.3 wt.%), respectively. However the N2 uptake at273 K and 298 K and pressure of 105 kPa were 0.86 and0.4 mmol/g, respectively.

The CO2 adsorption uptake of CPM-5 at 298 K under 105 kPa is2.3 mmol/g (10.1 wt.%) higher than those of the published resultsat similar conditions of MOF-5 (i.e. 1.2 mmol/g (4.5 wt.%)) and

Fig. 2. XRD patterns of the microwave-synthesized CPM-5 prepared in this study.

MOF-177 (i.e. 1.7 mmol/g (7 wt.%)), respectively, [20,21] and inthe same range of MIL-53(Al) (i.e. 10.6 wt.%), UMCM-150 (i.e.10.2 wt.%), and Ni-STA-12 (i.e. 9.9 wt.%) [28–30]. However, CPM-5 showed a CO2 adsorption capacity lower than functionalizedand open metal sites MOFs such as Mg-MOF-74 8.61 mmol/g (i.e.27.5 wt.%) [19], HKUST-1 6.8 mmol/g (i.e. 27 wt.%) [31], NH2 MIL-53(Al) (i.e. 12 wt.%) [28]. This comparison can be explained in moredetails based on the framework structure of the CPM-5, the CPM-5material composed of negatively charged single metal bindingblocks {In(CO2)4} and positively charged trimeric clusters {In3O}lead to three unique cage within cage based porous materials inwhich a large Archimedean cage donated as the In24 encapsulatesa small Archimedean cage donated as the In12 cage (see Fig.1(a,b)). The overall 3D structure shown in Fig. 1b can be explainedas bod-centered cubic packing of the larger In24 cages each ofwhich contains one In12 at the center. This unique core–shell typeIn12@In24 3D structure partition the pore size into small charge se-parated domains that can enhance the adsorption of small gas mo-lecules such as CO2 through better size match and stronger chargedinduced forces. Therefore CPM-5 shows higher CO2 adsorptionthan MOF-5 and MOF-177. However, still some functionalizedMOFs as mentioned before show higher CO2. For example: Mg-MOF-74 features high density of exposed Mg2+ cation sites. Thosestrong adsorption sites have high affinity to CO2 uptake at lowpressures [32]. Mason et al. [32] showed that Mg-MOF-74 CO2 ad-sorption isotherm exhibited steep increase in the CO2 uptake atlow pressures due to the presence of coordinatively unsaturatedMg2+ sites on the surface of the metal. As temperature increasedthe isotherm data become nearly linear beyond 120 �C. The aminefunctional group in NH2 MIL-53(Al) exhibits strong CO2 bindingsites especially at low pressure relevant to flue gas separation. Asa result, the open metal sites and functionalized frameworks showenhanced CO2 adsorption selectivity compared to CPM-5 (see Sec-tion 3.3).

R. Sabouni et al. / Microporous and Mesoporous Materials 175 (2013) 85–91 89

The working capacity of CPM-5 at pressure range from adsorp-tion pressure of 1 kPa and desorption pressure of 10 kPa is 2.4, 2and 0.91 mmol/g at 273, 298, and 318 K, respectively.

Fig. 3 shows a gradual increase in the adsorbed amount of CO2

with increasing the CO2 pressure without reaching a plateau in theadsorption isotherm. Therefore, CPM-5 can adsorb more CO2 athigher pressures. In addition, it is observed that the adsorptionand desorption isotherms match each other, indicating that the ad-sorption process is reversible [21] as shown in Fig. 3.

3.2.1. Adsorption cyclesIn order to investigate the regeneration process of the CPM-5

adsorbents and its efficiency, the CPM-5 was tested under four re-peated adsorption–desorption cycles at 273 K and 298 K as indi-cated in Fig. 4. The CPM-5 samples were degassed overnightunder vacuum (10�6 kPa) at 423 K after each adsorption test to re-generate the CPM-5 for the next CO2 adsorption cycle. The De-crease in the CO2 adsorption capacity is negligible even afterthree repeated adsorption cycles at both tested temperatures of273 and 298 K revealing that CPM-5 can be considered as a promis-ing candidate for CO2 adsorption. The reduction in the maximumCO2 adsorption capacity from first cycle to the fourth cycle rangesfrom 0.36 wt.% to 1.6 wt.% at 273 K and from 0.31 wt.% to 0.62 wt.%at 298 K.

3.3. Selectivity

The selectivity factor, Sads was evaluated using the most basicmethod based on the experimental single component gas adsorp-tion isotherms [4]. The selectivity factor is defined as molar ratioof the adsorbed amount at the relevant partial pressure of thegases for post combustion CO2 capture (i.e. 0.15 bar PCO2 and0.75 bar N2). The selectivity should be normalized to the composi-tion of the gas mixture as given by the following equation:

Sads ¼q1=q2

p1=p2ð3Þ

Fig. 4. CO2 adsorption isotherms on CPM-5 using multiple cycles at (a) and (b) 273 K, (band (c) and (d) 298 K, (d) represents the adsorbed amount of CO2 as a function of num

where Sads is the selectivity factor, qi represents the quantity ad-sorbed of component i, and pi represents the partial pressure ofcomponent i.

The CPM-5 has selectivity factor of 14.2 and 16.1 at 273 and298 K, respectively. Comparing the CPM-5 selectivity factor valuewith the literature, it was found that CPM-5 exhibited higher selec-tivity factor than MOF-177 (i.e. Sads = 4) and MOF-253 (i.e. Sads = 9)[4] at the same conditions. On the other hand CPM-5 showed lowerselectivity compared to the top Mg-MOF-74 featuring open metalsites in literature (i.e. Sads = 44) [4]. The better selectivity of Mg-MOF-74 compare to CPM-5 can be related to the presence ofMg2+ sites which exhibits a high affinity to CO2 as discussed in de-tails previously (see Section 3.2).

3.4. CO2 Adsorption kinetics

Kinetics of CO2 adsorption on the CPM-5 samples were studiedvolumetrically in the pressure range of 5–105 kPa at three differenttemperatures of 273, 298, and 318 K by means of a MicromerticsBET instrument (ASAP 2010) using the ROA software. The diffusiv-ity of CO2 in CPM-5 was calculated experimentally by correlatingthe diffusion time with the fractional adsorption uptake (mt/m1)based on the classical micropore diffusion model as shown in Eq.(4) with only 99% > (mt/m1)>70% [33].

1� mt

m1¼ 6

p2 exp�p2Dct

r2c

� �ð4Þ

The diffusivity (Dc, m2/s) can be calculated from the slope of lin-ear plot of ln (1 – mt/m1) versus time (t) at a given pressure.

The adsorption kinetics is one important factor in evaluating thesuitability of the adsorbent for gas adsorption applications, be-cause it controls the cycle time of a fixed bed adsorption systemand has an impact on the amount of adsorbent required and thesize of the adsorber column. Fig. 5 shows the fractional CO2 adsorp-tion curves on CPM-5 at 273 K and 298 K and pressure of 105 kPa.The kinetic plots at other pressures have similar shape. From Fig. 5

) represents the adsorbed amount of CO2 as a function of number of cycles at 273 Kber of cycles at 298 K and pressures up to 105 kPa.

Fig. 5. CO2 adsorption kinetics at (a) 298 K and (b) 273 K on CPM-5(1); solid line,Modeling data filled squared,Experimetnal data, and pressures up to 105 kPa.

Table 3Comparison of experimental CO2 diffusivity with the previously reported diffusivitydata in literature.

Adsorbent Temperature (K) DCO2 (m2/s) Reference

CPM-5 298 7.04 � 10�12 This workMOF-5 296 7.9 � 10�13 [36]MOF-177 298 2.3 � 10�9 [21]

Fig. 6. Heat of adsorption of CO2 on CPM-5.

90 R. Sabouni et al. / Microporous and Mesoporous Materials 175 (2013) 85–91

it can be found that the CPM-5 reaches the CO2 saturation capacitywithin 3–10 s depending on the applied temperatures and pres-sures. CO2 diffusivity in CPM-5 obtained by regressing the uptakecurves with Eq. (4) at three temperatures and pressure of105 kPa are summarized in Table 2. According to the data, theCO2 diffusivity in CPM-5 increases with increasing temperatureand it ranges from 1.86 to 7.87 � 10�12 (m2/s). In Table 3, CO2 dif-fusion data in CPM-5 are compared with those of other adsorbentsreported in literature under similar adsorption conditions. As it isshown in Table 3, the CPM-5 sample developed in this work exhi-bits a modest adsorption for CO2 compared to other adsorbents.While the CO2 diffusivity in CPM-5 is higher than MOF-5; however,MOF-177 exhibited higher diffusivity for CO2.

3.5. Isosteric heat of adsorption

Isosteric heat of adsorption is an important parameter for de-sign of practical gas separation processes such as pressure swingand thermal swing adsorption. It determines the extent of adsor-bent temperature during the adsorption (exothermic) process.The isosteric heat of adsorption at a given adsorption can be ob-tained from the Clausius–Clapeyron equation (Eq. (5)) as follows:

Table 2CO2 diffusivity in CPM-5 at three different temperatures andpressures up to 105 kPa.

Temperature (K) DCO2(10�12 m2/s)

273 1.86298 7.04318 7.87

Q st

�RT2 ¼@lnP@T

� �jna

ð5Þ

where Qst is isosteric heat of adsorption (kJ/mol), P is the pressure(kPa), T is the temperature (K), R is the universal gas constant, na

is the adsorbed amount (mmol/g). Integrating Eq. (5) gives the fol-lowing equation (Eq. (6)):

lnP ¼ Q st

RTþ C ð6Þ

The heat of adsorption can be calculated from the slop of linearplot of ln P versus 1/T at a given adsorption amount [32]. The CO2

adsorption isotherm at 273, 298, and 318 K were used to measurethe heat of adsorption. Fig. 6 illustrates the variation of Qst on CPM-5 as a function of adsorbed amount of CO2. It is observed from Fig.6 that, isosteric heat of adsorption decreases with loading indicat-ing strong interaction between the quadrupole momentums of car-bon dioxide with the adsorbent surface. The initial Qst of CO2 forCPM-5 is 36.1 kJ/mol, which is higher than HKUST-1 metal organicframework (24.2 kJ/mol) [34] and in the same range of 13X zeoliteand MOF-5 (33–34.1 kJ/mol), however it is lower than other MOFsmaterials featuring amine functionality or open metal organic fra-meworks such as MIL-100(Cr) (i.e. 62 kJ/mol) [35], NH2-MIL-53(Al)(i.e. 50 kJ/mol) [28].

4. Conclusion

This study has revealed that CPM-5 synthesized under micro-wave irradiation is an attractive candidate for CO2 adsorptiondue to its high CO2 adsorption capacity of 3 mmol/g (or 13 wt.%)at 273 K and pressure of 105 kPa), stability under ambient condi-tions for several weeks, and high surface area of 2187 m2/g. The in-itial heat of adsorption was estimated to be 36.1 kJ/mol from thecorresponding isotherms at 273 K, 298 K, and 318 K. The Freun-dlich adsorption equation fits well the isotherm data. The adsorp-tion kinetics curves were obtained experimentally. The CO2

diffusivity in CPM-5 estimated from the adsorption kinetic dataat CO2 pressures up to 105 kPa are 1.86 � 10�12, 7.04 � 10�12,and 7.86 � 10�12 m2/s at 273 K, 298 K and 318 K.

R. Sabouni et al. / Microporous and Mesoporous Materials 175 (2013) 85–91 91

Acknowledgements

This work was funded by the NSERC (Natural Sciences and En-gineering Research Council of Canada).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in theonline version, at http://dx.doi.org/10.1016/j.micromeso.2013.03.024.

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