Microporous and Mesoporous Materials 180 (2013) 114–122

Contents lists available at SciVerse ScienceDirect

Microporous and Mesoporous Materials

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

Microwave synthesis and characterization of MOF-74 (M = Ni, Mg)for gas separation

1387-1811/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.micromeso.2013.06.023

⇑ Corresponding author at: Chemical Engineering Department, New Mexico StateUniversity, Las Cruces, NM 88003, USA. Tel.: +1 575 646 4346; fax: +1 575 6467706.

E-mail address: sdeng@nmsu.edu (S. Deng).

Xiaofei Wu a, Zongbi Bao b, Bin Yuan a, Jun Wang a, Yingqiang Sun a, Hongmei Luo a, Shuguang Deng a,b,⇑a Chemical Engineering Department, New Mexico State University, Las Cruces, NM 88003, USAb Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

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

Article history:Received 4 May 2013Received in revised form 10 June 2013Accepted 12 June 2013Available online 25 June 2013

Keywords:MOF-74Microwave-assisted synthesisAdsorptionSelectivityGas separation

Isostructural MOF-74 (M = Ni, Mg) were successfully synthesized with both hydrothermal method (1 and3) and microwave-assisted method (2 and 4). These MOF-74 samples were characterized with scanningelectron microscopy for crystal structure, powder X-ray diffraction for phase structure, and nitrogenadsorption for pore textural properties. The experimental results showed that MOF-74 samples synthe-sized by the microwave-assisted method had a smaller particle size with relatively more uniform particlesize distribution. The microwave effects also helped to produce a larger specific surface area and micro-pore volume, with a similar median pore diameter. Adsorption equilibrium and kinetics of various gases(CO2, CH4, N2, C2H4, C2H6, C3H6 and C3H8) on these MOF-74 samples were determined at 298 K and gaspressures up to 1 bar. Adsorption equilibrium selectivity (a), combined equilibrium and kinetic selectiv-ity (b), and adsorbent selection parameter for pressure swing adsorption processes (S) were estimated.The relatively high values of adsorption selectivity indicates the potential to separate CO2/CH4, CO2/N2,C2H4/C2H6, C3H6/C3H8 and C3H6/C2H4 pairs in a vacuum swing adsorption process using the MOF-74 asadsorbent. The microwave-assisted method was found to improve MOF-74 with a larger adsorptioncapacity and somewhat higher selectivity for gas separation.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

As a new class of microporous/mesoporous materials, metal–or-ganic frameworks (MOFs) are currently investigated as potentialcandidates for gas storage and separation, [1–8] due to their supe-rior properties such as exceptionally high specific surface area,well-characterized crystalline architectures, controlled pore size,and suitable chemical functionalities [9–13]. Among the hundredsMOFs being synthesized and characterized in the past decades, agroup of isostructural MOF compounds with open metal ions, M/DOBDC (DOBDC4� = 2,5-dihydroxyterephthalic acid, also donatedas M-MOF-74, M2(dhtp), or CPO-27-M, where M = Zn, [14] Ni,[15] Co, [16] Mn, [17] Mg, [18] and Fe [19]), have been found manypromising applications, especially in hydrogen storage, [15,20] car-bon dioxide capture, [21–26] and separations of CO2/CH4/H2, [27]CO2/CH4, [28,29] and hydrocarbon, [30–32] both in computationaland experimental studies. The MOF-74 structure is based on coor-dinated carboxyl and hydroxy groups. A 3-D hexagonal packing ofhelical O5M chains is connected with 2,5-dihydroxyterephthaltelinkers, showing a 1-D arrangement of parallel hexagonal channels

of dimensions 10.3 � 5.5 Å2 [14,33]. The coordinated solvent mol-ecule (H2O or DMF) can be easily removed by a thermal treatmentunder a vacuum, resulting in an activated stable framework struc-ture with a high concentration of coordinatively unsaturated metalcations. These unsaturated metal centers (UMCs), also known asopen metal sites, can offer extra binding sites to the guest gas mol-ecules (like H2 [2] and CO2 [34]). Strong interactions between theUMCs and various guest gas molecules (CO2, H2, NO, etc.) are ex-pected and observed. Thus, a significant amount of gas could be ad-sorbed even at a low pressure, leading to high adsorptioncapacities and possibly a high selectivity as well.

The traditional solvothermal method for synthesizing MOF-74is time-consuming with a reaction time from 20 h to 3 days.[14–19] Actually, the relative long synthesis time is always a problemfor the solvothermal method. In the past few years, several newsynthesis techniques including ultrasound (US), [35] surfactant-as-sisted method, [36] and microwave-assisted method [37–41] havebeen applied in the MOFs synthesis process. Compared to the con-ventional solvothermal method, the microwave-assisted methodhas attracted a great attention because it sharply reduces the over-all processing time, increases the product yield and improves thequality of the product. The successful applications of microwave-assisted method in both organic and inorganic synthesis have wellbeen documented [42,43]. The effect of microwave irradiation onorganic synthesis could be attributed to a combination of thermal

X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122 115

effects, arising from the heating rate, superheating or ‘‘hot spots’’and the selective absorption of radiation by polar substances[43]. Yet microwave-assisted method is still in the early stages ofdevelopment in the synthesis of porous hybrid materials, includingMOFs. MIL-101 was successfully synthesized using microwaveirradiation in less than 1 h by different groups, [39,41,44] leadingto more uniform crystal sizes with smaller dimensions(40–100 nm). Also, a slightly larger surface area and pore volumewere obtained, compared to the samples synthesized by thesolvothermal method. The XRD pattern showed that within1 min of synthesis, small nanoparticles began to form. A mixed-li-gand metal–organic framework (MOF) Zn2(NDC)2(DPNI)[NDC = 2,6-naphthalenedicarboxylate, DPNI = N,N0-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydiimide] was synthesized withboth conventional heating (2 days) and microwave heating (1 h)[37]. Although the evacuated microwave sample has a much lowerBET surface area and micropore volume than those of the conven-tional sample, it has much higher CO2/CH4 selectivity (�30, basedupon the IAST analysis of the single-component isotherms), whichis among the highest selectivities reported for CO2/CH4separation.By using microwave synthesis, Cu-BTC could be obtained in a muchshorter synthesis time with improved yield and physical properties[45]. A quantitative investigation of the acceleration in the synthe-sis of Cu-BTC under microwave irradiation was also carried out byKhan and co-workers [40]. Their results showed that the acceler-ated synthesis was mainly due to the rapid nucleation rather thanaccelerated crystal growth. Moreover, the increased rates bymicrowave heating for both nucleation and crystal growth wereattributed to the increased pre-exponential factor of the Arrheniusequation rather than activation energy (Ea) because a high activa-tion energy causes a slow reaction.

Jhung and co-workers [38] synthesized CPO-27-M (M = Zn, Niand Co) with ultrasound, microwave and conventional electricheating from the very similar reactant mixtures to understandthe effect of the synthesis methods and metal ions on the synthesisrates and on the physicochemical properties of the obtained MOFs.The acceleration by ultrasound or microwave was observed in bothnucleation and crystal growth stages. Co-MOF-74 was also synthe-sized by microwave heating somewhere else [46]. Product with vir-tually identical textural properties to those synthesized by thesolvothermal method was obtained. In order to better understandthe influence of the microwave-assisted method on these isostruc-tural MOFs, including pore textual properties, gas adsorptioncapacity and mixture gas selectivities, we present a systemicallycomparison between MOF-74 (M = Ni, Mg) synthesized by bothconventional hydrothermal method and microwave-assistedmethod. Pure gas adsorption equilibria and intracrystalline diffu-sivities of CO2, CH4, N2, C2H4, C2H6, C3H6 and C3H8 on MOF-74 sam-ples were measured using a volumetric analyzer at a roomtemperature (298 K).

2. Experimental

2.1. Materials synthesis

The Mg-MOF-74 and Ni-MOF-74 were successfully synthesizedby both conventional hydrothermal method and microwave-as-sisted method following the procedures given elsewhere [15,18].All chemicals, magnesium nitrate hydrate (Mg(NO3)2�6H2O, +99%,from Fluka), nickel nitrate hydrate (Ni(NO3)2�6H2O, +99%, from Flu-ka), 2,5-dihydroxyterephthalic acid (DOT) (98%, from Aldrich), eth-anol (+99.5%, from Sigma–Aldrich), N,N-dimethylformamide(DMF) (99%, Aldrich), methanol (CH3OH, 99%, Aldrich), were usedas received without further purification.

2.1.1. Ni-MOF-74The solution for Ni-MOF-74 was prepared by dissolving a mix-

ture of Ni(NO3)2�6H2O (1.902 g, 6.54 mmol) and DOT (0.382 g,1.92 mmol) under sonication in a 1:1:1 (v/v/v) mixture of DMF(53.3 mL), ethanol (53.3 mL), and water (53.3 mL).

For the conventional hydrothermal method synthesizing Ni-MOF-74 (1), a homogeneous solution was evenly transferred totwo 125-mL Teflon lined stainless-steel autoclaves, about 80 mLeach. The autoclaves were capped tightly and heated to 373 K witha heating rate of 5 K/min in an oven. After the reaction under theautogenous pressure for 24 h, the sample was then removed fromthe oven and allowed to cool to the room temperature. The motherliquor was then carefully decanted from the product and replacedwith methanol. Fresh methanol was used to exchange the DMF for3 days at room temperature. The final product was then isolated byfiltration and washed thoroughly with methanol.

For microwave-assisted method synthesizing Ni-MOF-74 (2),the prepared solution was evenly transferred to four 80-mL reac-tion vessels, about 40 mL each. The reaction vessels were cappedtightly and kept in the microwave reaction system (Multiwave3000/Synthos 3000, Anton Paar). Synthesis was carried out at373 K, with a heating rate of 5 K/min and reaction time of90 min. After the reaction, the extracting processes were the same.

2.1.2. Mg-MOF-74The solution for Mg-MOF-74 was prepared by dissolving the

mixture of Mg(NO3)2�6H2O (1.439 g, 5.62 mmol) and DOT(0.338 g, 1.7 mmol) under sonication in a 15:1:1 (v/v/v) mixtureof DMF (130 mL), ethanol (9 mL), and water (9 mL). Mg-MOF-74has been successfully synthesized by a hydrothermal method (3)from previous work in our lab, [28,31] so we directly cite the datahere as a comparison. For microwave-assisted method synthesiz-ing Mg-MOF-74 (4), the procedure was exactly the same with Ni-MOF-74, except the temperature is 398 K.

The guest molecules incorporated in the crystals were removedunder a dynamic vacuum at 523 K for 12 h.

2.2. Material characterization

To characterize and analyze produced samples, powder X-raydiffraction (PXRD), scanning electron microscopy (SEM) images,and thermal gravimetric analysis (TGA) methods were employed.The XRD pattern was recorded using a Rigaku Miniflex-II X-ray dif-fractometer with Cu Ka (k = 1.5406 Å) radiation, 30 kV/15 mA cur-rent, and kb-filter. A step scan with an increment of 0.02� in 2h anda scan rate of 1�/min was employed to obtain the high-resolutionpatterns. Samples for SEM analysis were coated with a thin layerof gold using as putter coater. A thermogravimetric analyzer (Per-kin Eimer, Pyris 1) was used to get the TGA curve with samplesheld in platinum pans in a continuous flow nitrogen atmosphere.Heating rate was 10 K/min during the measurements from roomtemperature (298 K) up to 773 K. N2 adsorption and desorptionisotherm at 77 K was employed to determine pore textural proper-ties including the specific Brunauer–Emmett–Teller (BET) surfacearea, micropore volume and pore size distribution by using aMicromeritics ASAP 2020 adsorption porosimeter.

2.3. Adsorption measurements

The adsorption isotherms of CO2, CH4, N2, C2H4, C2H6, C3H6 andC3H8 at room temperatures (298 K) and gas pressure up to800 mmHg were measured volumetrically in the MicromeriticsASAP 2020 adsorption apparatus. Temperature was achieved byusing a Dewar with a circulating jacket connected to a thermostaticbath with a precision of ±0.1 K. About 0.1 g of adsorbent samplewas used for the gas adsorption studies. The initial degassing

116 X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122

process was carried out at 523 K for 12 h under a 0.0001 mmHgvacuum pressure. The helium gas was used to determine the freespace of the system. The degas procedure was repeated on thesame sample between measurements at 523 K for 6 h. Ultrahighpurity grade CO2, CH4, He, N2, C2H4, C2H6, C3H6 and C3H8 fromMatheson Co. were used as received.

When the adsorption equilibrium data were collected, the ki-netic adsorption characteristics were measured at the same timeby a previously described method [28,31,47–49]. At a given dose,the changes in gas pressure and adsorption volume as a functionof time were recorded and converted into the uptake profiles, gen-erating the adsorption kinetics, and the final adsorption amount atthe terminal pressure determined the adsorption equilibriumamount at a given pressure.

Fig. 1. Scanning electronic micrograph of the as-synthesized MOF-74 samples.

3. Results and discussion

3.1. Physical properties

Microwave-assisted method dramatically reduces the reactiontime for MOF-74, from 1 day to 100 min. It has been shown thatthe microwave irradiation accelerated not only nucleation but alsocrystal growth [40,50]. Factors including rapid and more uniformheating, microwave superheating, creation of ‘‘Hot Spots’’, and in-creased rate of gel dissolution may explain the microwave effecton the synthesis process of nanoporous materials [51].

Fig. 1 displays the scanning electron microscopy (SEM) imagesof the MOF-74 sample prepared in this work. The crystals of Ni-MOF-74 (1 and 2) are quite similar in shape, small polyhedralaggregates with each other, just like flowers. The particle size of1 is in the range of 1�3 lm; while for 2, the particle size is moreuniform, with an average size about 1 lm. The morphology for 4is totally different, which is of column-like with length 3–5 lm.Our previous work [28] showed that crystals of 3 were shuttle-likewith the particle size in the range of 5–25 lm with an average sizeof 20 lm. So from the comparison, it is obvious that the MOF-74samples synthesized by microwave-assisted method (2 and 4)have a smaller particle size with a relatively more uniform particlesize distribution.

The crystal size is determined by a combination effect of thenucleation rate and the crystal growth rate. i.e., small particlesare obtained when the nucleation rate is higher than the crystalgrowth rate [50,52]. So in the microwave-assisted method, themore significant acceleration effect on nucleation step may bethe main reason for the decreased crystal size.

The powder X-ray diffraction patterns shown in Fig. 2 are usedto confirm the phase structure of the MOF-74 samples prepared inthis work. The XRD pattern of 4 perfectly matches with the calcu-lated pattern from the established crystal structure data, indicatingthe as-synthesized sample 4 has the exactly correct structure withgood crystallinity. The XRD patterns of Ni-MOF-74 (1 and 2) doshow the two main peaks (6.7� and 11.7�, 2h), with the remainingsmall peaks not so sharp and clear. So we still could say MOF-74samples were successfully produced in this work by both hydro-thermal method and microwave-assisted method.

In order to determine pore textural properties including thespecific Brunauer–Emmett–Teller (BET) surface area, Langmuirsurface area, micropore volume and pore size distribution, N2

adsorption and desorption isotherms on these MOF-74 samplesat 77 K (Fig. 3) were measured in an ASAP-2020 adsorption appa-ratus (Micromeritics). All nitrogen isotherms are of a typical typeI isotherm by IUPAC definition, with a very sharp uptake at P/P0

from 10�5 to 10�1, showing a signature characteristic of micropo-rous materials. Isotherms of Ni-MOF-74 (1 and 2) almost reachtheir saturations at P/P0 = 0.1, while a steady increase of adsorbed

nitrogen is observed on isotherms of Mg-MOF-74 (3 and 4), maysuggest that the micropores inside the Mg-MOF-74 structure can-not be easily approached and have not been fully occupied bynitrogen.

Table 1 summaries the textural properties of MOF-74 samples,it’s obvious that in both cases, MOF-74 samples synthesized bythe microwave-assisted method (2 and 4) have larger surface area,corresponding to larger micropore volume, which is expected andobserved in many cases. The increase in surface area and micro-pore volume of MOF samples synthesized by the microwave-as-sisted method is probably due to the small crystal size of the

5 10 15 20 25 30 35 40 45 50

4

2

Inte

nsity

calculated

2θ

1

Fig. 2. Comparison of the experimental XRD patterns of as-synthesized MOF-74samples (1, magenta; 2, blue; 4, black) along with the simulated pattern (red,bottom) using the single X-ray crystal structure data. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

1 2 3 4

N2 Q

uant

ity A

dsor

bed

(cm

³/g S

TP)

Relative Pressure (P/Po)

Fig. 3. Nitrogen adsorption and desorption isotherms at 77 K on MOF-74 samples(1, magenta; 2, blue; 3, red; 4, black; filled symbols, adsorption; open symbols,desorption). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

Table 1Textural properties of MOF-74 samples.

Adsorbent BET surfacearea (m2/g)

Langmuirsurface area(m2/g)

HK-medianpore diameter(Å)

Microporevolume(cm3/g)

1 882 1302 10.3 0.3852 1252 1841 10.0 0.5643 1174 1733 10.2 0.5564 1416 2085 10.0 0.682

8 10 12 140

2

4

6

8

10

12

14

16

Pore Width (Å)

Diff

eren

tial P

ore

Volu

me

(cm

3 /g)

12

a

8 10 12 140

2

4

6

8

10

12

14

16

18

20

Pore Width (Å)

Diff

eren

tial P

ore

Volu

me

(cm

³/g)34

b

Fig. 4. DFT pore size distributions of Ni-MOF-74 (a; 1, magenta; 2, blue) and Mg-MOF-74 (b; 3, red; 4, black). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Table 2Textural properties of MOF-74 samples from literatures.

Adsorbent BET surfacearea (m2/g)

Langmuirsurface area(m2/g)

Microporevolume(cm3/g)

Porevolume(cm3/g)

Ref.

Mg-MOF-74 877 1030 0.37 [18]Mg-MOF-74 1495 1905 [22]Mg-MOF-74 1206 [30]Mg-MOF-74 1800 2060 0.5727 [23]Mg-MOF-74 1406 [56]Ni-MOF-74 922 [56]Ni-MOF-74 639 927 0.317 0.32 [25]Ni-MOF-74 1083 0.41 [15]Ni-MOF-74 1423 0.54 [57]Ni-MOF-74 1200 1315 0.47 [58]

X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122 117

MOFs because small crystals may have high external surface[38,53]. The median pore diameters calculated by Horvath-Kawa-zoe model are quite close, around 10 Å, which agree well withthe theoretical value (�11 Å). The pore size distributions of theseMOF-74 samples are also correlated by DFT (Density Function The-ory) model, as shown in Fig. 4. DFT was first used as the basis of apractical method for determining pore size distribution in therange from micropore to mesopore on porous carbon, [54] andthen found it applicable to other adsorbents, including silica, alu-mina, and MOFs. In both cases, pore size distribution of MOF-74samples synthesized by the microwave-assisted method (2 and4) is transferred to a narrower region, suggesting microwave effect

helps to get highly microporous MOF samples (higher microporevolume).

As a comparison, Table 2 lists the textural properties of MOF-74samples from literatures, which are synthesized by the conven-tional hydrothermal method. The values for both MOFs varywidely. Different pre-treatments of MOF-74 samples, e.g. by grind-ing or repeated washing with water or prior exchange with analternate solvent like methanol, will influence the results a lot[18]. The correct activation process of the MOF-74 sample also ap-pears to be crucial, in order to completely remove the coordinatedsolvent molecule (like water) from the structure and access its fullsurface area. The very close TGA curves of as-synthesized MOF-74

0 20 40 60 80 100 1200.0

0.4

0.8

1.2

1.6

1 2 3 4

Pressure (kPa)

CH

4 upt

ake

(mm

ol/g

)

a

0 20 40 60 80 100 1200

2

4

6

8

10

1 2 3 4

Pressure (kPa)

CO

2 upt

ake

(mm

ol/g

)

b

0 20 40 60 80 100 1200

2

4

6

8

10

1 2 3 4

Pressure (kPa)

C2H

4 upt

ake

(mm

ol/g

)

c

0 20 40 60 80 100 1200

2

4

6

8

1 2 3 4

Pressure (kPa)

C2H

6 upt

ake

(mm

ol/g

)

d

0 20 40 60 80 100 1200

2

4

6

8

10

1 2 3 4

Pressure (kPa)

C3H

6 upt

ake

(mm

ol/g

)

e

0 20 40 60 80 100 1200

2

4

6

8

10

1 2 3 4

Pressure (kPa)

C3H

8 upt

ake

(mm

ol/g

)

f

0 20 40 60 80 100 1200.0

0.4

0.8

1.2

1.6

1 2 3 4

Pressure (kPa)

N2 u

ptak

e (m

mol

/g)

g

Fig. 5. Adsorption isotherms of CH4 (a), CO2 (b), C2H4 (c), C2H6(d), C3H6 (e), C3H8(f) and N2 (g) at 298 K (1, magenta; 2, blue; 3, red; 4, black; dash line, Langmuir equation;solid line, dual-site Sips equation; dot line, single-site Sips equation). (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

118 X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122

Table 3Equation parameters for the Langmuir equation (a); dual-site Sips equation (b); and the single-site Sips equation (c).

Adsorbent CH4 R2 N2 R2

qm (mmol/g) b (kPa�1) qm (mmol/g) b (kPa�1)

(a)1 2.69 0.00255 >0.999 6.86 0.00148 >0.9992 4.62 0.0033 >0.999 10.58 0.00131 >0.9994 16.42 0.00083 >0.999 10.84 0.0013 >0.999

Adsorbent Adsorbate qm,A (mmol/g) bA (kPa�1) nA qm,B (mmol/g) bB (kPa�1) nB R2

(b)4 CO2 7.77 0.55 1 10.77 0.0028 0.94 >0.999

C2H6 7.53 0.02 0.97 2.74 0.04 0.42 >0.999C2H4 5.81 0.28 0.79 20.5 0.0002 2.24 >0.999C3H8 4.73 0.57 0.45 4.45 0.12 1.38 >0.999C3H6 3.21 0.637 0.33 11.58 0.043 3.85 0.998

Adsorbent Adsorbate qm (mmol/g) b (kPa�1) n R2

(c)1 CO2 6.18 0.019 1.52 0.999

C2H6 2.71 0.316 1.24 >0.999C2H4 7.34 0.004 2.71 >0.999C3H8 4 0.016 2.55 0.996C3H6 7.88 0.0016 3.7 0.995

2 CO2 7.54 0.0345 1.36 0.998C2H6 5.59 0.0425 0.95 >0.999C2H4 9.34 0.04 2.29 0.996C3H8 6.03 0.29 1.49 0.99C3H6 7.86 0.481 1.99 0.98

Table 4Summary of the Henry’s constants.

Adsorbent K (mmol g�1 kPa�1)

CH4 CO2 C2H4 C2H6 C3H6 C3H8 N2

1 0.0069 0.377 1.077 0.144 3.083 1.1 0.01022 0.0152 0.529 3.131 0.213 10.72 3.2 0.01384 0.0136 4.061 1.775 0.16 8.877 2.936 0.0141

X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122 119

samples (see supporting material) indicates that almost sameamount of the coordinated solvent molecule could be removedduring the activation process in this work. Moreover, the differentpressure ranges (P/P0) used to calculate in BET or Langmuir modelwill result in different results. In our work, the BET model was ap-plied to the isotherm for P/P0 between 0.06 and 0.30, with a corre-lation coefficient >0.995. Again, systematic error from differentinstruments should be considered. Even so, the surface areas ofthe MOF-74 samples synthesized by the microwave-assisted meth-od (2 and 4) in this work are among the highest values. In our lab,we keep all the experimental conditions identical as much as pos-sible (between 1 and 2, 3 and 4), so the increase in surface area andmicropore volume of MOF-74 samples synthesized by the micro-wave-assisted method is significant.

3.2. Adsorption equilibrium

Fig. 5(a–g) gives the single-component adsorption isotherms ofCO2, CH4, N2, C2H4, C2H6, C3H6 and C3H8 on MOF-74 samples at298 K. All the adsorption isotherms are reversible, which were con-firmed by measuring the desorption braches. As shown in the plots,MOF-74 samples synthesized by the microwave-assisted method(2 and 4) do adsorb significant larger amount of gases. It’s expectedand easy to understand, considering the higher surface areas andmicropore volumes. CO2 uptake as high as 9.95 mmol/g(43.78 wt.%) on 4 is obtained, at 1 bar, 298 K, which is definitelyamong the very best sorbents for CO2 capture (As a comparison,the adsorption amount of CO2 on Cu3(BTC)2, MOF 177, and

MOF-505 is found to be 4.1, 0.8 and 3.3 mmol/g, respectively[55]. The CO2 uptake on zeolite 5A and MOF-5 is about 20.8 and4 wt.%, respectively, [48] at 1 bar and 298 K). The adsorption capac-ities of C2H4, C2H6, C3H6 and C3H8 are also very high, especially onMg-MOF-74 samples (3 and 4). On the contradictory, the adsorp-tion amounts of CH4 and N2 are low (�1 mmol/g, at 1 bar, 298 K).

The isotherms of CH4 and N2 are quite linear, which can be wellfitted with Langmuir isotherm model, indicating weak interactionbetween guest gas molecules and adsorbents. While the other iso-therms all show sharp uptake at a low pressure region, showingvery strong adsorbate–adsorbent interactions due to the unsatu-rated metal centers. Because of the steepness of equilibrium data,the classical Langmuir model cannot adequately describe theadsorption isotherms throughout all the pressures examined.Therefore, single-site (Eq. (1a)) and dual-site (Eq. (1b)) Sip’s equa-tions, also called Langmuir–Freundlich equations, are employed tocorrelate these isotherms on Ni-MOF-74 (1 and 2) and Mg-MOF-74(3 and 4), respectively:

q ¼ qmðbpÞ1=n

1þ ðbpÞ1=n ð1aÞ

q ¼ qm;AðbAPÞ1=nA

1þ ðbAPÞ1=nA þ qm;BðbBPÞ1=nB

1þ ðbBPÞ1=nB

ð1bÞ

where q is the amount adsorbed of the pure component in mole perunit mass (mmol/g), p is the pressure of the bulk gas at equilibrium(kPa), qm,A and qm,B (mmol/g) are the maximum loading capacities atadsorption sites A and B of the adsorbent, bA and bB (kPa�1) are theaffinity parameters for sites A and B, nA and nB are solid heterogene-ity parameters for sites A and B.

The dash lines, solid lines, and dot lines in Fig. 5 represent theLangmuir, dual-site Sips, and single-site Sips model using theequation parameters listed in Table 3(a–c). From Fig. 5 and thehigh R2, we can see all the models fit the isotherms quite well.

The values of Henry’s constants (K) help us understand theinteraction between adsorbate and adsorbent. Isotherms of CH4

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

Time (s)

Frac

tiona

l upt

ake

N2

CH4

CO2

C2H4

C2H6

C3H6

C3H8

a

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

Time (s)

Frac

tiona

l upt

ake

N2

CH4

CO2

C2H4

C2H6

C3H6

C3H8

b

0 100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

Time (s)

Frac

tiona

l upt

ake

N2

CH4

CO2

C2H4

C2H6

C3H6

C3H8

c

Fig. 6. Fractional adsorption uptakes of N2 (black), CH4 (red), CO2 (blue), C2H4 (magenta), C2H6 (olive), C3H6 (navy), and C3H8 (violet) on 1 (a), 2 (b) and 4 (c) at 298 K,�10 mmHg. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 5Summary of the diffusion time constants.

Adsorbent Dc/r2c (s�1)

CH4 CO2 C2H4 C2H6 C3H6 C3H8 N2

1 1.68 � 10�2 6.58 � 10�3 5.87 � 10�3 9.14 � 10�3 2.49 � 10�3 6.37 � 10�3 1.63 � 10�2

2 1.48 � 10�2 6.32 � 10�3 7.95 � 10�3 1.29 � 10�2 1.01 � 10�3 6.3 � 10�3 1.76 � 10�2

3 4.05 � 10�2 8.11 � 10�3 7.12 � 10�3 1.39 � 10�2 3.14 � 10�3 3.26 � 10�3 1.97 � 10�2

4 2.72 � 10�2 7 � 10�3 7.85 � 10�3 1.29 � 10�2 1.77 � 10�3 7.5 � 10�3 1.57 � 10�2

120 X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122

and N2 could be well correlated with Langmuir adsorption iso-therm model, so the product of the two Langmuir constants (qm

and b) generates the Henry’s constant directly. An alternativemethod [31] was chosen to calculate the Henry’s constant for theremaining gases. An isotherm model in Virial form is given by

P ¼ qk

expðA1qþ A2q2 þ � � �Þ ð2Þ

where P is pressure (kPa), q is adsorption amount (mmol/g), A1 andA2 are Virial coefficient, and K is the Henry’s constant (mmol/g kPa).When q or P approaches zero, Eq. (2) could be developed as

lnpq

� �¼ A1q� ln K ð3Þ

So by plotting ln (P/q) vs. q, we can get the Henry’s constant. Ta-ble 4 gives us the summary of the Henry’s constants in this work.

3.3. Adsorption kinetics

To explore the diffusion rate difference between all the gases indifferent samples, the kinetic adsorption characteristics were mea-sured at the same time when the adsorption equilibrium data werecollected by a previously described method [28,31,47–49]. Theuptake curves at 298 K and 10 mmHg are plotted in Fig. 6. CH4

and N2 diffuse fast on all the samples due to the relatively small ki-netic diameter, while it takes long time for large gas molecules(C3H6 and C3H8) to reach the adsorption equilibrium.

In order to better understand the kinetic behavior, the followingmicropore diffusion model [28,31,47] was used to fit the fractionaladsorption uptake curves and to extract the diffusion time con-stant (Dc/r2

c , s�1):

mt

m1� 6ffiffiffiffi

pp

ffiffiffiffiffiffiDt

c

r2c

s� Dt

c

3r2cðmt=m1 < 0:85Þ ð4Þ

Table 6Estimated separation selectivities for gas mixture pairs at 298 K.

Adsorbent Gas mixture ai,j = Ki/Kj bi,j = (Ki/Kj)(Dc,i/Dc,j)0.5

Si,j = (Ki/Kj)(Dqi/Dqj)*

1 CO2/CH4 54.6 34.2 228CO2/N2 40 23 85.3C2H4/C2H6 7.5 6 4.1C3H6/C3H8 2.8 1.8 2.5C3H6/C2H4 2.9 1.9 3.3

2 CO2/CH4 34.8 22.7 105CO2/N2 38.3 22.9 96C2H4/C2H6 14.7 11.5 10.4C3H6/C3H8 3.4 1.3 2.5C3H6/C2H4 3.4 1.2 3

3 CO2/CH4 283 127 834CO2/N2 276 177 777C2H4/C2H6 15.3 10.9 13.9C3H6/C3H8 18.7 18.3 8.9C3H6/C2H4 10.1 6.7 4.7

4 CO2/CH4 298 151 779CO2/N2 288 192 759C2H4/C2H6 11.1 8.7 9.8C3H6/C3H8 3 1.5 1.5C3H6/C2H4 5 2.4 2.5

* For CO2/CH4 and CO2/N2, the adsorption and desorption pressures are assumed tobe 1 and 0.1 bar; for others, the adsorption and desorption pressures are assumed tobe 1 and 0.01 bar.

X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122 121

where mt=m1 is the fractional adsorption uptake, Dc (m2/s) is theintracrystalline diffusivity of gas molecules in porous media, rc

(m) is the crystal radius, and t (s) is the time. This model assumesthe mass transfer resistance for gas adsorption is dominated bythe intracrystalline diffusion and the adsorbent crystals can be re-garded as an approximately spherical object, which are reasonableon occasions where the kinetic diameter of gas is comparable withapertures. The results are summarized in Table 5. Generally speak-ing, the diffusion behaviors of all gases do not show much differ-ence on different MOF-74 samples because of the very similarpore size distributions. Besides, the small difference between thediffusivities of gas mixture pairs indicates that a kinetic-based sep-aration is difficult.

3.4. Estimation of the separation selectivity

In order to evaluate the ability of MOF-74 for separating gasmixtures, several methods were developed and applied for esti-mating the separation selectivity of gas mixtures from pure com-ponent adsorption equilibrium and kinetic data [28,31,47–49].

The ratio of Henry’s constants yields the intrinsic thermody-namic selectivity a:

ai;j ¼ Ki=Kj ð5Þ

If both adsorption equilibrium and kinetics are considered,combined separation selectivity (bi,j) can be defined as:

bi;j ¼ ai;j

ffiffiffiffiffiffiffiDc;i

Dc;j

sð6Þ

where Dc,i and Dc,j are the diffusion time constants for component iand j.

The adsorbent selection parameter S defined in the followingequation is found to be more useful in adsorbent evaluation andselection due to its combination of the ratio of adsorption capacitydifference of components i and j for pressure swing adsorptionprocess:

S ¼ Dqi

Dqjai;j ð7Þ

where Dq1 and Dq2 are the working capacity that is calculated asthe adsorption equilibrium capacity difference at adsorption pres-sure and desorption pressure for components i and j, respectively.

Table 6 summarizes the selectivities for several gas mixturepairs at 298 K. All MOF-74 samples show excellent equilibriumselectivity (high a value) for separating CO2/CH4 and CO2/N2, espe-cially for Mg-MOF-74 (3 and 4). The combined equilibrium and ki-netic selectivity reduce about 30–40%, while the impressive highadsorbent selection parameters indicate promising applicationsin pressure swing adsorption processes. These adsorbents alsodemonstrate decent selectivities for separating C2H4/C2H6, andmodest selectivities for separating C3H6/C3H8 and C3H6/C2H4.

The influence of microwave irradiation on selectivities of differ-ent MOF-74 samples seems to be different. 2 shows better selectiv-ities of C2H4/C2H6, C3H6/C3H8 and C3H6/C2H4, compared to 1; while4 shows better selectivities of CO2/CH4 and CO2/N2, compared to 3.

4. Conclusion

Isostructural MOF-74 (M = Ni, Mg) were successfully synthe-sized with both hydrothermal method (1 and 3) and microwave-assisted method (2 and 4). The microwave-assisted methodsharply reduced the reaction time (from 1 day to 100 min), andthe products have a smaller particle size with a relatively moreuniform particle size distribution. Better pore textural properties(larger surface area and micropore volume, with a similar medianpore diameter) have also been obtained. The BET surface areas ofNi-MOF-74 and Mg-MOF-74 have been increased by 370 and242 m2/g, respectively. Adsorption equilibria, Henry’s constants,and intracrystalline diffusivities for various gases (CO2, CH4, N2,C2H4, C2H6, C3H6 and C3H8) on these MOF-74 samples were deter-mined experimentally. Langmuir model, single-site and dual-siteSips model fit well the adsorption equilibrium data. Intracrystal-line diffusivities were obtained by fitting uptake profiles to a sim-plified micropore diffusion model. Adsorption equilibriumselectivity (a), combined equilibrium and kinetic selectivity (b),and adsorbent selection parameter for pressure swing adsorptionprocesses (S) were estimated, indicating the potential to separateCO2/CH4, CO2/N2, C2H4/C2H6, C3H6/C3H8 and C3H6/C2H4 pairs in avacuum swing adsorption process using MOF-74 as adsorbents.The microwave-assisted method was also found to improve thegas adsorption capacity (CO2 adsorption amount on 4 is as highas 9.95 mmol/g, i.e. 43.78 wt.%, at 1 bar, 298 K) and adsorptionselectivity. The microwave-assisted method appears to be veryefficient for synthesizing MOF materials with reduced synthesistime and improved MOF productivity and quality.

Acknowledgments

This project was partially supported by U.S. Air Force ResearchLaboratory (FA8650-11-C-2127), U.S. Department of Energy (DE-EE0003046), U.S. National Science Foundation (EEC 1028968),New Mexico State University Office of Vice President for Research(GREG award for X. Wu), and the Hengyi Fund of Zhejiang Univer-sity (S. Deng). S. Deng is grateful for the U.S. Department of Statefor the Fulbright award (Distinguished Chair in Energy Conserva-tion) and his host institute (NUST, MISiS) in Moscow, Russia.

References

[1] Y.S. Bae, R.Q. Snurr, Angew. Chem. Int. Ed. 50 (2011) 11586–11596.[2] D.J. Collins, H.C. Zhou, J. Mater. Chem. 17 (2007) 3154–3160.[3] R.J. Kuppler, D.J. Timmons, Q.R. Fang, J.R. Li, T.A. Makal, M.D. Young, D.Q. Yuan,

D. Zhao, W.J. Zhuang, H.C. Zhou, Coord. Chem. Rev. 253 (2009) 3042–3066.[4] J.R. Li, R.J. Kuppler, H.C. Zhou, Chem. Soc. Rev. 38 (2009) 1477–1504.[5] J. Liu, P.K. Thallapally, B.P. McGrail, D.R. Brown, J. Liu, Chem. Soc. Rev. 41 (2012)

2308–2322.

122 X. Wu et al. / Microporous and Mesoporous Materials 180 (2013) 114–122

[6] S.Q. Ma, H.C. Zhou, Chem. Commun. 46 (2010) 44–53.[7] L.J. Murray, M. Dinca, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294–1314.[8] A.W.C. van den Berg, C.O. Arean, Chem. Commun. (2008) 668–681.[9] R. Banerjee, H. Furukawa, D. Britt, C. Knobler, M. O’Keeffe, O.M. Yaghi, J. Am.

Chem. Soc. 131 (2009) 3875.[10] H.K. Chae, D.Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi, A.J. Matzger, M.

O’Keeffe, O.M. Yaghi, Nature 427 (2004) 523–527.[11] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O.M. Yaghi,

Science 295 (2002) 469–472.[12] G. Ferey, Chem. Soc. Rev. 37 (2008) 191–214.[13] G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I.

Margiolaki, Science 309 (2005) 2040–2042.[14] N.L. Rosi, J. Kim, M. Eddaoudi, B.L. Chen, M. O’Keeffe, O.M. Yaghi, J. Am. Chem.

Soc. 127 (2005) 1504–1518.[15] P.D.C. Dietzel, B. Panella, M. Hirscher, R. Blom, H. Fjellvag, Chem. Commun.

(2006) 959–961.[16] P.D.C. Dietzel, Y. Morita, R. Blom, H. Fjellvag, Angew. Chem. Int. Ed. 44 (2005)

6354–6358.[17] W. Zhou, H. Wu, T. Yildirim, J. Am. Chem. Soc. 130 (2008) 15268.[18] P.D.C. Dietzel, R. Blom, H. Fjellvag, Eur. J. Inorg. Chem. (2008) 3624–3632.[19] E.D. Bloch, L.J. Murray, W.L. Queen, S. Chavan, S.N. Maximoff, J.P. Bigi, R.

Krishna, V.K. Peterson, F. Grandjean, G.J. Long, B. Smit, S. Bordiga, C.M. Brown,J.R. Long, J. Am. Chem. Soc. 133 (2011) 14814–14822.

[20] C.O. Arean, S. Chavan, C.P. Cabello, E. Garrone, G.T. Palomino, ChemPhysChem11 (2010) 3237–3242.

[21] D. Britt, H. Furukawa, B. Wang, T.G. Glover, O.M. Yaghi, Proc. Natl. Acad. Sci.USA 106 (2009) 20637–20640.

[22] S.R. Caskey, A.G. Wong-Foy, A.J. Matzger, J. Am. Chem. Soc. 130 (2008) 10870–10871.

[23] Z.R. Herm, J.A. Swisher, B. Smit, R. Krishna, J.R. Long, J. Am. Chem. Soc. 133(2011) 5664–5667.

[24] A.C. Kizzie, A.G. Wong-Foy, A.J. Matzger, Langmuir 27 (2011) 6368–6373.[25] J. Liu, Y. Wang, A.I. Benin, P. Jakubczak, R.R. Willis, M.D. LeVan, Langmuir 26

(2010) 14301–14307.[26] J.A. Mason, K. Sumida, Z.R. Herm, R. Krishna, J.R. Long, Energy Environ. Sci. 4

(2011) 3030–3040.[27] Z.R. Herm, R. Krishna, J.R. Long, Micropor. Mesopor. Mater. 151 (2012) 481–

487.[28] Z.B. Bao, L.A. Yu, Q.L. Ren, X.Y. Lu, S.G. Deng, J. Colloid Interface Sci. 353 (2011)

549–556.[29] P.D.C. Dietzel, V. Besikiotis, R. Blom, J. Mater. Chem. 19 (2009) 7362–7370.[30] Y.S. Bae, C.Y. Lee, K.C. Kim, O.K. Farha, P. Nickias, J.T. Hupp, S.T. Nguyen, R.Q.

Snurr, Angew. Chem. Int. Ed. 51 (2012) 1857–1860.[31] Z.B. Bao, S. Alnemrat, L. Yu, I. Vasiliev, Q.L. Ren, X.Y. Lu, S.G. Deng, Langmuir 27

(2011) 13554–13562.

[32] E.D. Bloch, W.L. Queen, R. Krishna, J.M. Zadrozny, C.M. Brown, J.R. Long, Science335 (2012) 1606–1610.

[33] L. Valenzano, B. Civalleri, S. Chavan, G.T. Palomino, C.O. Arean, S. Bordiga, J.Phys. Chem. C 114 (2010) 11185–11191.

[34] A.O. Yazaydin, R.Q. Snurr, T.H. Park, K. Koh, J. Liu, M.D. LeVan, A.I. Benin, P.Jakubczak, M. Lanuza, D.B. Galloway, J.J. Low, R.R. Willis, J. Am. Chem. Soc. 131(2009) 18198.

[35] N.A. Khan, M.M. Haque, S.H. Jhung, Eur. J. Inorg. Chem. (2010) 4975–4981.[36] Y.J. Zhao, J.L. Zhang, B.X. Han, J.L. Song, J.S. Li, Q.A. Wang, Angew. Chem. Int. Ed.

50 (2011) 636–639.[37] Y.S. Bae, K.L. Mulfort, H. Frost, P. Ryan, S. Punnathanam, L.J. Broadbelt, J.T.

Hupp, R.Q. Snurr, Langmuir 24 (2008) 8592–8598.[38] E. Haque, S.H. Jhung, Chem. Eng. J. 173 (2011) 866–872.[39] S.H. Jhung, J.H. Lee, J.W. Yoon, C. Serre, G. Ferey, J.S. Chang, Adv Mater, 19

(2007) 121-+.[40] N.A. Khan, E. Haque, S.H. Jhung, Phys. Chem. Chem. Phys. 12 (2010) 2625–

2631.[41] Z.X. Zhao, X.M. Li, S.S. Huang, Q.B. Xia, Z. Li, Ind. Eng. Chem. Res. 50 (2011)

2254–2261.[42] M. Baghbanzadeh, L. Carbone, P.D. Cozzoli, C.O. Kappe, Angew. Chem. Int. Ed.

50 (2011) 11312–11359.[43] A. de la Hoz, A. Diaz-Ortiz, A. Moreno, Chem. Soc. Rev. 34 (2005) 164–178.[44] D.Y. Hong, Y.K. Hwang, C. Serre, G. Ferey, J.S. Chang, Adv. Funct. Mater. 19

(2009) 1537–1552.[45] Y.K. Seo, G. Hundal, I.T. Jang, Y.K. Hwang, C.H. Jun, J.S. Chang, Micropor.

Mesopor. Mater. 119 (2009) 331–337.[46] H.Y. Cho, D.A. Yang, J. Kim, S.Y. Jeong, W.S. Ahn, Catal. Today 185 (2012) 35–40.[47] Z.B. Bao, S. Alnemrat, L.A. Yu, I. Vasiliev, Q.L. Ren, X.Y. Lu, S.G. Deng, J. Colloid

Interface Sci. 357 (2011) 504–509.[48] D. Saha, Z.B. Bao, F. Jia, S.G. Deng, Environ. Sci. Technol. 44 (2010) 1820–1826.[49] D. Saha, S.G. Deng, Langmuir 25 (2009) 12550–12560.[50] S.H. Jhung, T.H. Jin, Y.K. Hwang, J.S. Chang, Chem. Eur. J. 13 (2007) 4410–4417.[51] G.A. Tompsett, W.C. Conner, K.S. Yngvesson, ChemPhysChem 7 (2006) 296–

319.[52] T.O. Drews, M. Tsapatsis, Curr. Opin. Colloid Interface 10 (2005) 233–238.[53] S.L. Cresswell, J.R. Parsonage, P.G. Riby, M.J.K. Thomas, J. Chem. Soc. Dalton

(1995) 2315–2316.[54] N.A. Seaton, J.P.R.B. Walton, N. Quirke, Carbon 27 (1989) 853–861.[55] A.R. Millward, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005) 17998–17999.[56] J. Liu, A.I. Benin, A.M.B. Furtado, P. Jakubczak, R.R. Willis, M.D. LeVan, Langmuir

27 (2011) 11451–11456.[57] D. Peralta, G. Chaplais, A. Simon-Masseron, K. Barthelet, G.D. Pirngruber,

Energy Fuel 26 (2012) 4953–4960.[58] J.G. Vitillo, L. Regli, S. Chavan, G. Ricchiardi, G. Spoto, P.D.C. Dietzel, S. Bordiga,

A. Zecchina, J. Am. Chem. Soc. 130 (2008) 8386–8396.