Article pubs.acs.org/Langmuir
Tuning the Adsorption Properties of UiO-66 via Ligand Functionalization Gregory E. Cmarik,† Min Kim,‡ Seth M. Cohen,‡ and Krista S. Walton*,† †
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States ‡ Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States S Supporting Information *
ABSTRACT: UiO-66 is one of the few known water-stable MOFs that are readily amenable to direct ligand substitution. In this work, UiO-66 has been synthesized with amino-, nitro-, methoxy-, and naphthyl-substituted ligands to impart polar, basic, and hydrophobic characteristics. Pure-component CO2, CH4, N2, and water vapor adsorption isotherms were measured in the materials to study the effect of the functional group on the adsorption behavior. Heats of adsorption were calculated for each pure gas on each material. The results indicate that the amino-functionalized material possesses the best adsorption properties for each pure gas due to a combination of polarity and small functional group size. The naphthyl-functionalized material exhibits a good combination of inhibited water vapor adsorption and high selectivity for CO2 over CH4 and N2. well as two functionalized variants,19 and a computational study has predicted a number of additional functionalized variants.18 Until now, no experimental study of the impact on the gas and water vapor adsorption properties caused by modulation of the functionalities has been done for the promising UiO-66 family of materials. Previous studies on the impact of functional groups on adsorption properties for other materials have been published. Two comparable series of MOFs have been synthesized with a number of modified linkers in a similar manner to the UiO-66 materials studied here: IRMOF (IRMOF = isoreticular MOF)20,21 and DMOF (DMOF = dabco MOF).22−24 The published results indicate the most promising adsorption enhancements occur for certain polar functional groups, particularly amino, alkoxy, and nitro functional groups.20 The results also indicate improved hydrophobic character with addition of nonpolar functional groups such as naphthyl functionalities.25,26 However, neither the IRMOF or the DMOF series of materials are suitable for use in humid systems due to framework degradation,11 whereas UiO-66 is stable.13 In this work, we report the CO2, CH4, N2, and water vapor adsorption properties of UiO-66 and four functionalized variants of UiO-66 including a new material, UiO-66−2,5(OMe)2. Additionally, isosteric heats of adsorption are calculated for each of the pure-component gases. Finally,
1. INTRODUCTION Adsorption separations play an important role in numerous industrial applications. Two major applications are natural gas sweetening and carbon dioxide capture from flue gas.1 Solid adsorbents offer one of the most promising routes to energy efficient separations in these processes. Several specific attributes are required when selecting an adsorbent for a separation process. The adsorbent must be stable and reusable, possess high capacity and selectivity for the target molecule, and require minimal energy to regenerate. Hybrid organic−inorganic adsorbents, such as metal−organic frameworks (MOFs), have been well studied for their adsorption properties with regard to CH4 and H2 storage as well as CO2 capture.1−4 Many MOFs have exceptional properties including high adsorption capacity,5 selectivity,6 surface areas,7 and stability.8,9 However, a number of concerns still exist surrounding MOF stability, and this has been the focus of several recent studies.10−12 An important feature of MOFs which offers a route to satisfying the adsorbent selection criteria is the modular nature of the organic linker. One MOF that has been successfully synthesized with many different functional groups, exhibits exceptional stability, and has good adsorption properties is UiO-66 (UiO = University of Oslo).13 The UiO-66 family of microporous materials is based on a 3D structure of zirconium-oxo clusters. UiO-66 is synthesized with benzene dicarboxylate (bdc−2) and many modified dicarboxylate linkers; even tetracarboxylate linkers have also been successfully used to synthesize functionalized versions of UiO-66.14,15 To date, experimental studies have reported the adsorption properties of the parent UiO-66 material16−18 as © 2012 American Chemical Society
Received: September 1, 2012 Revised: October 10, 2012 Published: October 11, 2012 15606
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the samples. The results are presented in Table 1. The BET surface areas for UiO-66, UiO-66−NH2, and UiO-66−NO2 are
ideal selectivity was calculated for each material by three common methods: ideal selectivity as a ratio of Henry’s constants, a ratio of uptake of two pure component gases at relevant pressures, and Ideal Adsorbed Solution Theory (IAST). Amino (−NH2), nitro (−NO2), methoxy (−OMe), and naphthyl (−Naphyl) functional groups were chosen to be representative of polar, basic, and hydrophobic functionalities. The results indicate that the polar functional groups improve CO2/CH4 and CO2/N2 selectivity, with the amino group showing the best performance. All of the materials are shown to be stable after water vapor exposure and adsorb significant amounts of water regardless of the nature of the organic linker, although the naphthyl variant adsorbs 25% less than the other materials at saturation.
Table 1. BET Surface Area Comparison of Functionalized UiO-66 Materials sample UiO-66 UiO-66−NH2 UiO-66−NO2 UiO-66−1,4-Naphyl UiO-66−2,5-(OMe)2 a
pore volume (cm3/g)b
BET surface area (m2/g)a 1105 1123 792 757 868
± ± ± ± ±
15 64 27 9 31
Results are an average of two experiments. calculated at p/p0 = 0.8.
0.55 0.52 0.40 0.42 0.38 b
± ± ± ± ±
0.02 0.02 0.03 0.02 0.02
Average of values
identical to, or slightly higher than, previously reported values.14 The BET surface area for UiO-66−1,4-Naphyl is greater than the previously reported value of 615 m2/g.14 UiO66−2,5-(OMe)2 would be expected to have similar or lower surface area than the −NO2 and −Naphyl functionalized materials based on additional framework mass and linker size, but instead the opposite result is observed. This result may be attributed to the flexibility of the methoxy groups allowing additional nitrogen to be adsorbed in the monolayer. As expected, the pore volumes for all functionalized materials are lower than the parent UiO-66. 3.2. Adsorption Isotherms. As shown in Figure 1, the two major regimes of CO2 adsorption are plainly evident within the
2. EXPERIMENTAL METHODS 2.1. Chemicals. Zirconium(IV) chloride, terephthalic acid, 2amino-terephthalic acid, 2-nitro-terephthalic acid, 1,4-naphthalenedicarboxylic acid, glacial acetic acid, methanol, and N,N-dimethylformamide (DMF) were procured from commercial sources (Fisher and SigmaAldrich). N,N-Dimethylformamide was stored over 3 Å molecular sieves prior to use, and all other chemicals were used without modification. 2,5-Dimethoxy-terephthalic acid was synthesized via Williamson etherification as previously reported.27 2.2. Materials Synthesis. Materials were synthesized following procedures previously reported.14 A modification of the procedure was used to produce UiO-66−2,5-(OMe)2; specifically, 600 μL of glacial acetic acid was added to the reaction solution, and the oven temperature was reduced to 110 °C. See Table S2 in the Supporting Information for more specific details. 2.3. Materials Activation. After synthesis was complete, the samples were transferred to centrifuge tubes. The sample powders were recovered by centrifugation and decanting. Washing of the powders consisted of soaking for 1 h in 10 mL of fresh DMF followed by centrifugation three times. Washed samples were solvent exchanged by soaking for 1 day in 10 mL of methanol followed by centrifugation three times. Solvent-exchanged samples were dried under vacuum at 105 °C. 2.4. Experimental Methods and Procedures. Nitrogen adsorption isotherms were obtained at 77 K using a multiport volumetric apparatus (Quadrasorb, Quantachrome). Pure-component isotherms were measured in a gravimetric adsorption apparatus (IGA1, Hiden Isochema) from 0 to 20 bar at 25 °C and 0 to 3 bar at 25, 35 and 45 °C. Activation of the samples was conducted in situ at 110 °C and under vacuum. The sample density of UiO-66−NH2 was experimentally determined to be 3.5 g/cm3 via helium displacement. This density was used for each of the UiO-66 materials in the buoyancy correction of the adsorption isotherm. A density of 3.5 g/ cm3 is significantly higher (∼200%) than the estimated sample density of 1.3 g/cm3 based on the volume and weight of an ideal unit cell of UiO-66−NH2.18 The impact of this seemingly large difference on the resulting adsorption isotherm is that the higher density will show a reduced adsorption value by less than 2% at pressures up to 5 bar, but the reduction in loading reaches close to 8% at 20 bar. Water vapor isotherms were measured using a gravimetric adsorption apparatus (IGA-3, Hiden Isochema) in flowing mode at 1 bar and 25 °C with dry air as a carrier gas. Brunauer−Emmett−Teller (BET) surface area calculations were conducted over a pressure range of 0.007 < p/p0 < 0.035, which produces results consistent with the two criteria for accurate surface area determination.28 All gases used in the adsorption experiments (carbon dioxide, methane, nitrogen, helium, and dry air) were Ultra-High Purity or Bone Dry grade (Airgas).
Figure 1. CO2 adsorption isotherms for UiO-66 and functionalized versions at 298 K.
measured range of pressures for the entire series of UiO-66 materials. At low pressures, the polar functionalities show a significant increase in adsorption over the nonpolar groups. Specifically, UiO-66−NH2 shows the highest adsorption loadings, followed by nearly identical low pressure loadings for the −NO2 and −OMe variants. The lower CO2 adsorption in UiO-66−NO2 and UiO-66−2,5-(OMe)2 are attributed to reduced pore volume and surface area induced by the bulkier functional groups. UiO-66 and UiO-66−1,4-Naphyl exhibit lower loadings at low pressures ( UiO-66−NH2 ≈ UiO-66−1,4-Naphyl > UiO-66, which is close to what is seen in Figure 5. The nitrogen adsorption isotherms were also fit to the Langmuir model. The same method employed previously was used to calculate the heat of adsorption as a function of CH4 loading. The results of the calculations are shown in Figure 6. Following the trends observed for CO2 and CH4, the isosteric heat of adsorption for UiO-66 is lowest (14 kJ/mol), and the functionalized materials are clustered higher at around 17 kJ/ mol. This increase can be attributed to the reduced pore size of the functionalized materials. Beyond the initial observation that functionalizing the framework leads to an increase in the heat of adsorption, the trends are difficult to discern due to the low adsorption and thus small range of loadings over which the calculations are valid. 3.4. Water Vapor Adsorption. Figure 7 shows water adsorption isotherms at 298 K for the functionalized MOFs. UiO-66 and UiO-66−NH2 have been previously studied,12 15609
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−NH2 and −NO2 versions. However, somewhat surprisingly, this MOF displays the highest water adsorption saturation loadings in spite of having the smallest pore volume (Table 1). It could be that the functional groups act as a directing agent for water in the pores, which would allow for more efficient packing compared to the other MOFs. This is in contrast to the naphthyl functionalized material, which exhibits the lowest adsorption capacity of all the MOFs. The low adsorption is due to the hydrophobicity of the naphthyl group, whereas the methoxy group is electron donating in addition to providing steric effects. Previous studies on the effect of functional groups on the water vapor adsorption of MOFs are limited. Akiyama et al. measured the effect of nitro, amino and sulfonic acid functionalization on the water vapor adsorption characteristics of the mesoporous material MIL-101.31 The results indicate that the amino and sulfonic acid groups show greater uptake at low relative humidity, which is attributed to the hydrogen bonding capabilities of these functional groups. The unfunctionalized and nitro functionalized materials show step changes at nearly the same point. The nitro functional group has a different effect on these two materials which can be attributed to the pore size difference. The onset of pore filling in MIL-101−NO2 is not altered by the polar nitro group due to the large pore diameter, whereas the small pores of UiO-66− NO2 lead to an enhancement of water adsorption at low relative humidity. 3.5. Structural Stability. Each of the MOFs was examined for structural stability upon water exposure by comparing PXRD patterns of the samples before and after the adsorption measurements. As shown in Figure 8, all samples present unchanged PXRD patterns after water exposure. In each case, the peak positions and the ratios of peak heights remain the same indicating no loss of crystallinity. The crystalline structure of the frameworks clearly remain intact, although recent studies on this material show that ligand exchange reactions can occur in the presence of water.32 3.6. Adsorption Selectivity. Selectivity is a fundamentally important issue for gas separation, and one method for determining the ideal selectivity is calculating the ratio of the Henry’s constants for two gases.33 Since Toth and Langmuir models were used to parametrize the measured adsorption isotherms for each material and each gas at 298, 308, and 318 K, calculation of the Henry’s constant is simply the product of adsorption affinity, b, and saturation capacity, qsat, given in eq 5.
Figure 6. Isosteric heat of adsorption as a function of N2 loading.
Figure 7. Water vapor adsorption as a function of relative humidity at 298 K and 1 bar. UiO-66 provided for comparison.12
while the three additional materials are presented for comparison. Compared to UiO-66, all functionalized versions show enhanced interactions with water at relative humidities below 25%. With the exception of UiO-66−1,4-Naphyl, all of these materials exhibit a slight hysteresis upon desorption that is indicative of pore filling (Figure S3, Supporting Information). UiO-66−NH2 and UiO-66−NO2 present the most hydrophilic pore environments and show similar adsorption isotherms with significant amounts of water loading occurring below 20% RH. UiO-66−1,4-Naphyl shows the most hydrophobic nature among the five materials but, unlike other naphthyl functionalized materials,25,26 it adsorbs significant amounts of water. One step change at 15% is evident, which is similar to the other functionalized materials and may be attributed the narrower pore windows compared to UiO-66, which increases the adsorption potential. This material shows a very small hysteresis (Figure S3, Supporting Information), which indicates that pore filling is limited. The unfunctionalized UiO-66 material shows similar water adsorption at low water vapor concentrations, but finally adsorbs large amounts of water above 20% RH at the onset of pore filling. This shows that the naphthyl ring inhibits pore filling of water but does not prevent water adsorption entirely. The water vapor isotherm for UiO-66−2,5-(OMe) 2 approaches Type I isotherm behavior, which is similar to the
Hi = qisatbi
(5)
Table 2 shows the ratio of the Henry’s constants for each binary gas mixture at three temperatures. The −NH2 and −OMe versions have significantly higher ideal selectivities for CO2 over methane compared to the parent MOF. The −Naphyl version is the only variation that has a lower value than UiO-66. This difference is most likely due to the enhanced interaction of the naphthyl group with methane; thus the competition with CO2 is more significant. For CO2/N2 ideal selectivity, we see similar trends as the methane mixtures, but the selectivity is at least a factor of 3 higher for this mixture. The same is true for the parent MOF; it already exhibits a greater affinity for CO2 over N2, so any functionalization (except −Naphyl) will enhance the CO2 interaction. UiO-66− NH2 and UiO-66−2,5-(OMe)2 have particularly high selectivities that are >60 at room temperature. However, it should also 15610
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Figure 8. PXRD patterns before and after exposure to water vapor and subsequent reactivation.
Table 2. Ideal Selectivity at Various Temperatures As Determined from a Ratio of Henry’s Constants T (K)
UiO-66
−1,4-Naphyl
−NH2
−NO2
−2,5-(OMe)2
CO2/CH4
298 308 318
10.1 7.5 6.4
6.5 7.3 5.7
17.0 18.3 13.0
12.3 10.3 8.2
18.0 14.2 13.1
CO2/N2
298 308 318
37.5 27.7 21.9
30.0 33.1 23.9
66.5 61.5 43.0
51.4 42.2 31.7
62.2 52.3 39.7
CH4/N2
298 308 318
3.7 3.7 3.4
4.6 4.6 4.2
3.9 3.4 3.3
4.2 4.1 3.9
3.5 3.7 3.0
variant, known as UTSA-25a, shows a selectivity of 12.5.30 In this case, the effect of addition of a small, polar functional group leads to a 3.5× increase in zero-loading selectivity, whereas for the UiO-66 materials an increase of 1.7× is observed. One major difference between the two families of materials lies in the metal cluster of UiO-66. The hydroxylated metal cluster perhaps plays a significant role in CO2 adsorption which leads to a smaller selectivity improvement upon functionalization of the organic linker in UiO-66. The recent review of Sumida et al. details the CO2/N2 selectivities for a large number of MOFs.3 The selectivity calculation is reflective of flue gas composition on which the review is focused and is different from the method used above. The formula used is
be pointed out that these MOFs also adsorb the largest quantities of water. Thus, it could be preferable to balance an adequate CO2 selectivity with low water affinity, such as the behavior exhibited by UiO-66−1,4-Naphyl. The selectivity of CH4 over N2 is revealing in terms of the preferred interactions and can shed light on the CO2 results. The selectivity for methane over nitrogen ranges from 3 to 4.6, which indicates a slight preference for methane. This mixture is also the only gas system for which the −Naphyl functional group enhances the selectivity compared to the parent MOF. Thus, it should be expected that the CO2 selectivity will be lower for this MOF. A comparison of the effect of functional groups on adsorption can be found in the isostructural family of DMOF-1, where the unfunctionalized materials show a CO2/ CH4 selectivity of 3.7,30 and the hydroxyl-functionalized 15611
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α=
Article
nCO2PN2 n N2PCO2
properties as a result of the different functional groups. UiO66 and the −NH2, −NO2, −1,4-naphthyl, and −2,5-dimethoxyfunctionalized versions of UiO-66 were studied. The results show that CO2, CH4, and N2 adsorption are improved with addition of small, polar functional groups that do not significantly reduce surface area and pore volume. The polar −NH2, −NO2, and −OMe modifications show selectivity improvements for CO2/CH4 and CO2/N2 over UiO-66, but the affinity for water is also enhanced. The isosteric heat of adsorption for CO2 at low loading varies significantly between the five materials, but each of the four functionalized materials converge to approximately 27 kJ/mol at higher loadings. The isosteric heat of adsorption for UiO-66−NH2 is roughly 28 kJ/ mol at low loadings and decreases slightly as a function of loading, whereas the −NO2 and −OMe functionalized materials have heats of adsorption around 32 kJ/mol at low loadings with a rapid decrease as loading increases. A smaller change in heat of adsorption as a function of loading is desirable in vacuum swing adsorption applications, which indicates that UiO-66−NH2 is the most promising material studied here for dry applications. The stability of each material on exposure to water vapor was confirmed with PXRD. Water vapor adsorption studies revealed that the −NH2 and −NO2 functionalized MOFs readily adsorb water and saturate near 20% relative humidity. The −OMe functionalized material also readily adsorbs water, and exhibits the highest water adsorption loadings at saturation. The −Naphyl version is not strictly hydrophobic, but water adsorption was depressed compared to the other MOFs. This property, along with reasonable CO2 selectivity, indicates that UiO-66−1,4-Naphyl could be an interesting candidate for further study in humid gas separations.
(6)
where α is the selectivity, ni is the amount of gas adsorbed at predefined pressures, and Pi is the pressure of the pure gas. The pressures used are PN2 = 0.75 bar and PCO2 = 0.15 bar. Table 3 Table 3. CO2/N2 Selectivity for UiO-66−X Compared to Reference Materials at 298 K material
α(CO2/N2)
CO2 loading, 0.15 bar (wt%)
UiO-66 UiO-66−NH2 UiO-66−1,4-Naphyl UiO-66−NO2 UiO-66−2,5-(OMe)2 bio-MOF-11[3] Fe-BTT[3] ZIF-78[3] Cu-BTTri[3] Zn2(BTetB)[3]
22.8 32.3 22.2 25.5 29.8 65 18 30 19 19
2.37 4.91 1.80 2.57 3.91 5.4 5.3 3.3 2.3 1.8
shows the result of this selectivity calculation for the materials in this work at 298 K compared to several reference MOFs. The results indicate the UiO-66 materials with polar functionalities slightly outperform other MOFs with polar functional groups and similar pore sizes such as ZIF-78. Also, UiO-66 and UiO-66−1,4-Naphyl outperform other MOFs with unfunctionalized organic linkers. Additionally, Ideal Adsorbed Solution Theory (IAST)34 was applied to the modeled isotherms to determine the selectivity for CO2 over N2. Due to the constraint of a binary gas mixture, the composition of the gas phase was chosen to be 15% CO2 and 85% N2. Figure 9 shows the results of the calculation and
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Additional figures and tables of adsorption data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. Present Address
Min Kim: Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS MOF characterization and adsorption measurements by G.C. were supported by the National Science Foundation CAREER Award 0969261 (K.S.W.) and the Army Research Office Award W911NF-10-1-0079 (K.S.W.). The synthesis of MOFs and initial surface area measurements by M.K. were supported by a grant from the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-08ER46519 (S.M.C.).
Figure 9. Selectivity by applying IAST for a 15:85 CO2/N2 gas mixture at 298 K.
reaffirms the previous observation that UiO-66−NH2 is the best performing material of the five studied. Also, the nitro functionalized material does not show a significantly improved selectivity for CO2 over N2 versus UiO-66 unlike the two other polar functionalities.
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REFERENCES
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4. CONCLUSIONS UiO-66 can be synthesized using a variety of organic linkers and is shown to exhibit a wide variation in adsorption 15612
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dx.doi.org/10.1021/la3035352 | Langmuir 2012, 28, 15606−15613
