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Polystyrene Sulfonate threaded in MIL-101Cr(III): a Cationic Polyelectrolyte Synthesized Directly into a Metal-Organic Framework Liang Gao, Chi-Ying Vanessa Li, and Kwong-Yu Chan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504623r • Publication Date (Web): 30 Apr 2015 Downloaded from http://pubs.acs.org on May 4, 2015
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Chemistry of Materials
Polystyrene Sulfonate threaded in MIL-101Cr(III): a Cationic Polyelectrolyte Synthesized Directly into a Metal-Organic Framework Liang Gao, Chi-Ying Vanessa Li*, and Kwong-Yu Chan* Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong ABSTRACT: Incorporation of ion-exchange polymer in a metal-organic framework (MOF) is an attractive strategy to achieve fast ion exchange by increasing surface area and porosity of the material. Synthesis of a cationic polyelectrolyte in a MOF is reported here for the first time. Sodium poly(4-styrene sulfonate) threaded in MIL-101 (NaPSS~MIL-101) is synthesized directly with polymerization in situ of the MOF. NaPSS~MIL-101 exhibits superior exchange kinetics, high selectivity with co-ion rejection, reversibility, and durability. The polyelectrolyte threaded in MOF has a larger specific volume compared to its bulk state and possesses advantageous properties. The fixed charges of the polyelectrolyte are exposed for full interaction with solvated ions and solvent, without the need of swelling or restructuring the porous framework.
1. INTRODUCTION Separation of ions and charged molecules is an important process in a wide range of applications, such as water purification, protein separation, precious metals recovery, and remediation of radioactive contaminations.1-3 An effective means is solid-phase extraction by ion exchange.1 The conventional ion-exchange polymeric resins, however, have very low surface area, and charge sites are hidden due to strong binding and interaction of polymer chains.4-6 In most cases, timeconsuming pre-wetting or swelling is needed to facilitate ion exchange. In addition, structural changes due to swelling are macroscopic and non-uniform, leading to osmotic shock and polymer leaching from the beads. Efforts to increase the contact efficiency between ion-exchange sites and guest species has been demonstrated to be an effective strategy to improve the overall performance of conventional ion-exchange beads. Economy and Domongurez5 proposed a class of ion-exchange nanofibers, which yields orders of magnitude increase in ionexchange rate because of higher surface-to-volume ratio compared to the bead-type ion-exchanger. Harmer et al.6 prepared a high-surface area silica-Nafion composite, whose catalytic activities were hundreds of times higher than low-surface area pure Nafion sphere. Similarly, Choi et al. 7 also reported that extending ion-exchange polymer within high-surface mesoporous silica can remarkably improve its catalytic activity. Metal-organic frameworks (MOFs), alternatively named porous coordination polymers (PCPs),8-16 is a new class of nanoporous materials generally possessing high surface area and high porosity. MOFs have been applied in a wide range of fields, such as catalysis,17 gas storage and separation,18 conducting materials,19 sensing and delivery of bioactive materials.20 They have only recently, been explored for ionexchange applications. Ion-exchange functionality can be incorporated into MOFs in three ways: (1) charge-balancing species attached to the metal sites of MOF. These species are almost exclusively anionic and can be formed by introducing charged secondary building units 21-22 or anion stripping postsynthesis modifications23; (2) counter ions of charged groups covalently bonded to the ligands of MOFs. These charged organic ligands are generated by self-assembly24 or post-
synthesis modifications25; and (3) ions of guest inorganic salts residing in pores of MOFs and introduced by impregnation26 or one-pot synthesis27. Although these approaches yield MOFs incorporated with ion exchange functionality, there are still limitations in their applications. For instance, the cationic MOFs typically have slow ion-exchange rate and MOF encapsulating inorganic Keggin acid27 or H2SO426 are inevitably vulnerable to leaching. Incorporating an ion-exchange polymer into a MOF has been demonstrated recently with the polymer threading through the cavities of the framework.28 Enhanced ion exchange kinetics and durability was displayed and attributed to the full exposure and dispersion of non-crosslinked polymers in the highly porous MOF matrix. The synthesis was based on the pioneering works of Kitagawa, Uemura and co-workers’ approach of polymerization inside MOFs,29-35 but with a further amination step to introduce ion-exchange function. The open porous structure of MOF allows the physically trapped polymers to contact solvent and exchange ions efficiency. The MOF framework provides durable regenerative ion exchange. Furthermore, the non-crosslinked linearity of polymers, a feature of synthesis in situ of the MOF framework, enhances contact efficiency and full utilization of ion-exchange sites. In principle, this strategy of forming an ion exchange polymer within a MOF is applicable to a variety of MOFs and ion exchange polymers. ZIF-8 was chosen in the previous work for its high stability in alkaline condition and threaded with polyvinyl benzyl trimethyl ammonium hydroxide (PVBTAH) for a compatible synthesis and its anion exchange function.28 The hydrophobic ZIF-8 can only be impregnated with hydrophobic species. Hence, vinylbenzyl chloride monomers were impregnated into ZIF-8 MOF and polymerized in situ. Charges were introduced by animation of the PVBC~ZIF-8 matrix which subsequently turned hydrophilic. It is important to extend the strategy of threading ionexchange polymer in MOF with an example of cation exchange polymer. Furthermore, the high temperature and high pH requirements of the amination step precludes many MOFs from being selected, thus limits the range of structural properties of the final polyelectrolyte~MOF composite. It is desired
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to simplify the multi-step synthesis of polyelectrolyte in MOF with a one-step in situ polymerization process. We report here for the first time a direct synthesis of polyelectrolyte threaded in MOF with a one-step in situ polymerization. Report here is the first cation exchange polymer in MOF, viz. sodium poly(4styrene sulfonate) (NaPSS) synthesized directly in MIL-101.In addition to being hydrophilic, the inertness of Cr(III) prevents structure modification of MIL-101 upon electrolyte contact.36 Although the redox active Cr(III) of MIL-101 can potentially affect radical polymerizations due to quenching of propagation radicals, there are recent reports37 of successful radical polymerization within MIL-101Cr(III). This suggests that the quenching process did not hinder polymerization. Most likely, radicals generation far exceeds quenching and facilitate polymerization. X-ray photoelectron spectroscopy (XPS) analysis of the synthesized NaPSS~MIL-101Cr(III) confirms the absence of Cr(II) which would have appeared if significant quenching has taken place (see Fig. S1 in ESI). Successful exchange with mono and multi-valent metal cations, as well as exclusion of anion, are demonstrated. Compared to the neat MIL-101(Cr) and commercialized cation exchange resin Amberlite IR-120, a gel type resin of sulfonated styrene-divinylbenzene, NaPSS~MIL-101 exhibits significantly improved adsorption capacity, charge selectivity, cationic dye removal and release kinetics, making it a promising candidate for the practical applications of solid-phase extraction.
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unreacted monomers. The synthesized NaPSS is interlocked in the porous structure of MIL-101Cr(III). The synthesis procedure is simple and has fewer steps than the previous synthesis of ion exchange polymer in MOF, viz. PVBTAH~ZIF-8.28 Fig. 2(a) shows the scanning electron microscopy (SEM) image of the synthesized NaPSS~MIL-101 powder which has the same octahedral shape as a neat MIL-101 powder. Powder X-ray diffraction (XRD) confirms that NaPSS~MIL-101 has the same XRD pattern to that of MIL-101 (Fig. 2(b)). The decrease in relative diffraction intensity between 2θ=5 to 6o can be explained by the change in electron density of MIL-101 after filling with NaPSS. The corresponding transmission electron microscopy (TEM) image in Fig. 2(c) confirms the highly porous structure of NaPSS~MIL-101 with cavities ordered in the same pattern as MIL-101. Chemical confirmation of NaPSS confined in MIL-101 was given by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR), and energy-dispersive Xray fluorescence spectroscopy attached to a transmission electron microscope (TEM- EDX). Fig. S2(a) shows the FTIR spectra of NaPSS~MIL-101 and neat MIL-101. Two additional peaks appeared at 1215 cm-1 and 1044 cm-1 in the NaPSS~MIL-101 spectrum, corresponding to the symmetric O=S=O vibration and S-O stretching.39 Since both MIL-101 and NaPSS polymer have an aromatic ring, the corresponding FTIR peaks cannot be used to argue for a presence of polystyrene backbone in NaPSS~MIL-101. To provide additional chemical characterization, NaPSS polymer within NaPSS~MIL-101 was isolated from its MIL-101 host (procedure detailed in ESI) for 1H NMR spectroscopy. As shown in Fig. S2(b), locations of the 1H NMR peaks of the NaPSS isolated from NaPSS~MIL-101 match well with those of a bulk NaPSS separately prepared with a standard procedure in solution. The peak at 9 ppm is assigned to the H of terephthalic acid in MIL-101 fragments that remain after digestion. The peaks between 0 to 3 ppm are assigned to aliphatic H, while two peaks between 6 and 8 ppm belong to H of the aromatic ring. All the peaks of NaPSS~MIL-101 are broader than those of bulk NaPSS. This broadening is likely to be caused by the contamination with paramagnetic Cr(III) species which is present in the isolated NaPSS as confirmed by EDX (Fig. S2(c))
Figure 1. Schematic of NaPSS~MIL-101 synthesis and structures of MIL-101 and NaPSS~MIL-101.
2. RESULTS AND DISCUSSION 2.1 Synthesis and characterization of NaPSS~MIL-101 The MIL-101 Cr(III) framework was synthesized following procedure developed by Férey et al.38 Its structure and the synthesis of NaPSS~MIL-101 is shown schematically in Fig. 1. Sodium-4-styrene sulfonate monomer was dissolved in a mixed DMF/H2O solvent and impregnated into MIL-101. In situ radical polymerization was initiated by AIBN and allowed to proceed at 80oC for 5 days. The product was washed thoroughly to remove polymers formed exterior of the MOF and
Figure 2. (a) SEM image of NaPSS~MIL-101 particles (scale bar=2µm); (b) XRD patterns for NaPSS~MIL-101 and MIL-101; 2
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Chemistry of Materials
(c) TEM images for NaPSS~MIL-101 from different directions (scale bar=50nm).
This can be quantified by estimating the mass density of NaPSS threaded in MIL-101, much lower than that of bulk NaPSS. The linear chain structure and non-crosslinked tacticity also suggest the larger free volume. This free volume of NaPSS allows full contact with solvent and solvated ions leading to fast and effective ion exchange. The residing NaPSS is not chemically bonded to the host MIL-101, nor strongly adsorbed onto the pore wall of MIL101. Undulating forces of the NaPSS with ~55 monomer units is suggested and contributed to entropy that is localized within the MIL-101 framework. This localized entropy is an important feature for effective ion-exchange. Without the MOF framework, the NaPSS may entangled into a bulk material, losing its porosity and ion-exchange sites. On the other hand, the absence of binding between MIL-101 and NaPSS may suggest a freely mobile NaPSS which can eventually leave the MIL-101 framework with the solvent. This concern of leaching is dispelled since there is no observable changes in NaPSS~MIL-101 soaked in water over a month (~700 h). The S/Cr atom ratio remains constant (Fig. S6) and there is no observable NaPSS absorbance peak of the filtrate of NaPSS~MIL-101 in ultraviolet–visible (UV-Vis) spectrum.
The EDX mapping via TEM in Fig. S3, indicates uniform distributions of the elements S, Na and Cr. A uniform distribution of NaPSS into the MIL-101 framework is therefore suggested. The loading of NaPSS is estimated to be ca. 15.4 wt.% by XPS (Fig. S4) and ca. 14 wt.% by CHNS element analysis. By digesting MIL-101, NaPSS polymer in the cavities was released and characterized by gel permeation chromatography (GPC) to have a molecular weight of around Mn=11300 (Fig. S5). This molecular weight is equivalent to ca. 55 sodium-4sytrene sulfonate monomers. 2.2 Porous Structure and Stability of NaPSS~MISL-101 The porosity and structure of NaPSS~MIL-101 was analysed by N2 sorption experiments and compared to identical characterizations performed for neat MIL-101. Fig. 3 shows the adsorption isotherms and corresponding pore size distributions of MIL-101 and NaPSS~MIL-101. As expected, a decrease in Brunauer–Emmett–Teller (BET) surface area from 3024 m2/g for neat MIL-101 to 1850 m2/g for NaPSS~101 was observed. This large decrease is significantly more than what is caused by an increase in mass when MIL-101 is loaded with 15%(m/m) NaPSS. Similarly, a decrease of pore volume from 1.42 mL/g for MIL-101 to 0.85 mL/g for NaPSS~MIL-101 is significantly more than the factor 1/(1.15) due to mass increase. Therefore, NaPSS must be present inside the cavities of MIL-101, leading to decrease in pore volume and surface area accessible by nitrogen. From the increase of mass and decease of pore volume, the specific volume of NaPSS in MIL-101 is estimated to be 2.17 mL/g and the theoretical maximum loading of NaPSS in MIL-101 is 61% mNaPSS /mMIL-101. Furthermore, the pore size distribution calculated by Barrett-Joyner-Halenda (BJH) method suggests narrowing of pore from 2.4 nm for MIL-101 to 2.1 nm upon filling of NaPSS (Fig. 3b). In addition to decrease of mean pore size of NaPSS~MIL-101, the broadening of the 2.1 nm indicates perturbation in the cavity and window dimensions due to varying NaPSS occupation of MIL-101 cavities. These N2 sorption results support the representation of NaPSS~MIL-101 in Fig. 1 with NaPSS polymer threading through cavities of MIL-101 The 2.4 nm peak in the pore size distribution of MIL101Cr(III) is the average representation of several characteristic dimensions of the MIL-101Cr(III) porous framework. The MOF has two types of cavities interconnected by three types of windows.37 The largest internal diameters of the cavities are 2.9 nm and 3.4 nm, correspond to internal volumes 12,700 Å3 and 20,600 Å3, respectively. The number of small cavities are twice the amount of the large cavities. Inscribed diameters are 1.6 nm and 1.2 nm for the hexagonal and pentagonal, respectively. Effective volume of a sodium styrene sulfonate molecule in MIL-101 can be estimated from the specific volume of NaPSS in the MOF, which is 2.17 mL/g. Therefore, small and large cavities can contain a maximum of 17 and 28 monomer units, respectively. The molecular weight of extracted NaPSS is ~55 monomer units. Therefore NaPSS must interpenetrate through at least three large cages or four small cages, or a combination of them in MIL-101 as shown in the 3D structure of Fig. 1. Similar to PVBTAH threaded in ZIF-8,28 the NaPSS in MIL-101 has a larger free volume compared to bulk NaPSS.
Figure 3. (a) N2 isothermal (77K); and (b) BJH pore size distribution calculated based on adsorption branch for MIL-101 and NaPSS~MIL-101.
2.3 Metal Cations Exchange The ion-exchange property of NaPSS~MIL-101 is evaluated for metal cations. Exchange of Na+ with K+, Cu2+, Fe3+ , and Co2+ is performed by soaking NaPSS~MIL-101 in separate DMF solutions containing individual metal nitrates. After equilibration, the metal ion exchanged NaPSS~MIL-101 material is rinsed with DMF and characterized by EDX with spectra of Na+, K+, Cu2+, Fe3+ shown in Fig. S7. Successful metal cation exchange is demonstrated by disappearance of Na peaks and appearance of the peaks of exchanged metal species. In the control experiments of metal nitrates interacted with neat MIL-101, no changes were observed in EDX metal peaks. 3
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These results confirm that cation exchange functionality is only contributed by the NaPSS polymer. To investigate the kinetics of ion exchange and compared quantitatively with a commercial material, Co(II) exchange is conducted for it can be monitored by UV-Vis (Fig. S8). Concentration profiles of Co(II) after contact with NaPSS~MIL101, neat MIL-101, and a commercial cation-exchange resin IR-120, are shown in Fig. 4a. NaPSS~MIL-101 demonstrated significantly higher Co(II) adsorption and at a much faster rate than that of IR-120 and neat MIL-101. In the first 10 min, over 70% cobalt in solution was adsorbed by NaPSS~MIL-101, while only 15% and 25% Co(II) were adsorbed by IR-120 and MIL-101, respectively. After 40 min, the total adsorption amount for NaPSS~MIL-101 is more than four times higher than that of IR-120.
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accuracy and sensitivity can be obtained by using DMF when Uv-vis was used to monitor the progress of cobalt exchange. Co2+ ion exchange in water is studied using ICP-MS instead to avoid baseline drifting and the poor UV-Vis signal/noise ratio of Co2+. Since Co2+ is much better solvated by water than DMF, adsorption(removal) by NaPSS~MIL-101 is not as high as removal from DMF. However, as shown in Fig. S10(b), there is still high cobalt ion removal of 60% by NaPSS~MIL101 compared to only 28% by IR-120 for the cobalt ion removal in water. The observed difference of Co adsorption between DMF and water is shown Fig. S10(c). There is enhanced ion exchange than that of DMF, perhaps due to better swelling of IR-120 resin in water. Although cobalt(II) ion exchange is clearly indicated, it is necessary to investigate whether this ion-exchange occurs inside the cavities of NaPSS~MIL-101 or just on the external surface. To clarify this issue, cobalt form of NaPSS~MIL-101 is cut by ultra-microtome to expose the internal surface of the nanoparticles and subject to spatial elemental characterization. The TEM-EDX mapping of the cross-section of cobalt exchanged NaPSS~MIL-101 (Fig. 4b) clearly shows the highly uniform distribution of both Co and S elements in the MIL101 matrix. This confirms ion exchange occurred within the cavities of NaPSS~MIL-101. Since the ion exchange capacity of NaPSS~MIL-101 is higher than the total amount of Co2+ in the solutions, there are sodium form of PSS remaining in the material. 2.4 Anion Rejection In addition to fast and high capacity exchange, charge selectivity is a critical performance indicator of a good cation exchange material. According to Donnan’s theory,40 the presence of fixed negative charge inside NaPSS~MIL-101 can counteract concentration gradient of anions, and thus reject anions entering into its pores, provided that the charge concentration is sufficiently high. For NaPSS~MIL-101, in each nanocage, the fixed negative charge density is estimated to be ca. 1.4×108 C/m3 (compares to 9.6×107 C/m3 at 1 M concentration). However, it is also noted that neat MIL-101 displayed preferential adsorption of anions, likely due to its metal anchored groups and inherent charges. Furthermore, van der Waals forces may compete with electrostatic repulsion and provide adsorption insensitive to charges. It is therefore important to demonstrate the anion exclusion property of NaPSS~MIL-101. We observed that NaPSS has a dramatic influence on the anion adsorption property of MIL-101. The anion exclusion property of NaPSS~MIL-101 is demonstrated by monitoring the uptake of ferricyanide (Fe(CN)63-) ions by UV-Vis spectroscopy. The Fe(CN)63- ion is sufficiently small (hydrated diameter: ~0.94 nm) with steric hindrance to its transport or adsorption. Shown in Fig. 5, NaPSS~MIL-101 exhibits 100% exclusion of ferricyanide ion which is present at 0.4 mM concentration. There is no change in the Fe(CN)63- peak in the UV-Vis adsorption spectra for 120 min, as shown in Fig. 5(b). On the other hand, neat MIL-101 adsorbs ferricyanide steadily, as observed from the UV-Vis spectra in Fig. 5(a). After 120 min, 95% of the 0.4 mM Fe(CN)63- is adsorbed. This anion adsorption of MIL-101 is likely due to anion exchange between ferricyanide and OH-/F- ions attached on the Cr metal site of MIL-101. These results clearly confirm the effective-
Figure 4. (a) Cobalt (II) ion removal by 0.1 g each of NaPSS~MIL-101, MIL-101 and a commercial cation exchange resin, IR-120. The volume of cobalt nitrate DMF solution is 2mL and the initial concentration of Co2+ is 6.25 mM. The UV-vis absorbance spectra of the respective ion exchange material can be found in Fig. S8. Inset photography: the light red cobalt nitrate solution (at time = 0) turned colorless after contacting with NaPSS~MIL-101 for 40 min. (b)TEM-EDX mapping measured on the ultra-microtomed slice of Co(II) exchanged NaPSS~MIL101. Cross-sections of cobalt stained NaPSS~MIL-101 was exposed after the particles were cut into slices of 50~80 nm thick. The pink square in the top STEM image is where TEM-EDX mapping was performed.
As the actual ion exchange sites in IR-120 is much more than NaPSS~MIL-101, it is reasonable to speculate that the highly porous and open network structure of NaPSS~MIL-101 significantly enhance the accessibility to the internal active sites. Access to hidden sites of IR-120 is a much slower process, likely due to the limited swelling in DMF. While neat MIL-101 also adsorbs cobalt, as shown in Fig. 4a and Fig. S8. This is due to physical adsorption rather than columbic interaction. The cobalt adsorbed on neat MIL-101 can be completely removed by DMF washing, with corresponding peaks in the EDX spectra disappearing (Fig. S9(b)) after washing. On the other hand, as shown in EDX of Fig. S9(a), the Co(II) ions in NaPSS~MIL-101 cannot be washed away by DMF due to the electrostatic interaction between Co(II) and the sulfonate group on polystyrene within MIL-101. DMF was used as a solvent since low concentration cobalt ions has a more pronounced Uv-vis absorption peak in DMF compared to that of cobalt ions in water, as shown in Fig. S10(a). Thus, better 4
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Chemistry of Materials in (b); (c) Ferricyanide adsorption kinetic comparison between MIL-101 and NaPSS~MIL-101.
ness of sulfonate group on MIL-101 for excluding co-ions in the pores of MIL-101. From the decrease of pore volume shown in the sorption isotherms of Fig. 3a, NaPSS only partially filled the pores of MIL-101. There is, however, only one single peak centering at 2.1 nm in the pore size distribution. The absence of any peaks of shoulders around 2.4 nm which is the pore size of neat MIL-101, suggests there are very few empty cavities in MIL101. Should there be many unoccupied cavities, there should be two peaks in the pore size distribution, at 2.1 nm and 2.4 nm, corresponding to filled and unfilled cavities. This is also supported by the TEM-EDX mapping analysis, which shows the uniform distribution of NaPSS within MIL-101 in microscopic scale (Fig. S3). Even though each pore is only partially filled, electrostatic repulsion will exclude ferricyanide anions effectively. In the negatively charged environment of a NaPSS filled MIL-101 cavity, there should be exclusion of ferricynaide anions. In pristine MIL-101, van der Waals’ (vdw) attraction plays a major role in adsorption of ferricyanide ions, in addition to possible electrostatic interaction. However, the presence of NaPSS in partially filled MIL-101 pore has sufficient electrostatic repulsion to dominate over the vdw interaction. The Debye length of a 6.25 mM ferricyanide anions is 24 nm (S-2 in ESI). This value is about ten times higher than that of the pore width of NaPSS~MIL-101. Therefore, the anionic PSS- can effectively repel the ferricyanide anions within a MIL-101 cavity. While the PSS- repulsion may not extended directly to a neighboring cavity, it may perturb the charge distribution of the neighboring framework, hence indirectly excluding ferricyanide ions in a neighboring cavity should it be empty without PSS- . The term rejection coefficient (R) can be used to estimate the degree of such kind of depletion of co-ions. The rejection coefficient (R) is estimated to be larger than 99% for 6.25mM ferricyanide anions. (S-2 in ESI). We emphasize that after hot water, EtOH and DMF purification process, additional NH4F treatment procedure 41 can effectively suppressed the adsorption of ferricyanide on MIL-101.
2.5 Selective and Regenerative Adsorption of Organic Dye Lastly, we demonstrate NaPSS~MIL-101 provides fast, selective, and regenerative removal of a cationic dye Rhodamine B (RhB), with properties listed in Table S1, is a common pollutant in the effluents of textile plants.42 RhB is a more bulky cation (MW=479) than the inorganic ions tested above. After contacting 10 mL 10-5 M RhB with 15 mg NaPSS~MIL-101 for 3 min, almost 100% RhB was removed. This compares favourably to 40% and 10% removal by MIL-101 and IR-120, respectively (Fig. 6). The maximum adsorption capacity Q0 can be estimated by fitting the adsorption isotherms to a Langmuir model. The Q0 of NaPSS~MIL-101 is ~35 mg/g, which is an order of magnitude higher than those of MIL-101 (~1.3 mg/g) and IR-120 (~1.7 mg/g) (Fig. S11). The adsorption rate for NaPSS~MIL-101 can be estimated from the initial linear region of the Langmuir plot to be 0.179 g/(mg×s). This value is two orders of magnitude higher than that of IR-120 (0.001 g/(mg×s)), and 2.5 times higher than MIL-101 (0.068 g/(mg×s)) (Fig. S11).
Figure 6. (a) Dye adsorbed vs time; (b) Rhodamine B removal normalized to fixed initial [RhB] but with increasing amounts of absorbent of NaPSS~MIL-101, MIL-101 and IR-120. The corresponding Langmuir fitting and determination of adsorption rates are shown in Fig. S11 in ESI.
The economic feasibility of using NaPSS~MIL-101 for dye removal in wastewater relies on its reuse and ease of regeneration. RhB dye desorption from the dye loaded NaPSS~MIL101 was tested in a batch system. We observed that 20% of the loaded RhB dye can be released in a 50 v/v% H2O/EtOH solvent. On the other hand, pure water cannot elude loaded RhB. The result suggests that there is some hydrophobic interaction between RhB and NaPSS~MIL-101. Since ion-exchange plays a role in RhB capture, the RhB elusion process is highly sensitive to ionic strength of the eluding agent. Indeed, variations in the NaCl concentration of the elusion solution produce large differences in the desorption efficiency. Optimized elusion is given by 1 M NaCl in 50% EtOH solution. (Fig. S12) Benefiting from the high porosity of NaPSS~MIL- 101, the RhB dye desorption is fast and 80% RhB desorbed in 2 hr. IR-120 exhibits slower and ineffective RhB dye release with only ~15%
Figure 5. UV-Vis spectra of 0.4mM Fe(CN)63- aqueous solution as a function of contacting time with (a) MIL-101 and (b) NaPSS~MIL-101. Given the concentration of ferricyanide is relatively low, Donnan exclusion effect can be pronounced, and thus ferricyanide was strictly rejected by NaPSS~MIL-101 as we observed for the constant ferricyanide concentration UV-vis spectra 5
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RhB desorbed in 2 hr (Fig. S13). Reuse and regeneration of NaPSS~MIL-101 is demonstrated in four repeated cycles of adsorption and desorption without significant loss in performance, according to Fig. S14. No change in the MOF structure is observed from the XRD pattern of NaPSS~MIL-101 subjected to repeated cycles of RhB adsorption and desorption (Fig. S15). Selectivity of the NaPSS~MIL-101 is further tested with an anionic dye of Acid Blue 9 (AB9), which can be partially adsorbed due to significant van der Waals attraction due to its size, despite facing charge repulsion. The properties of AB9 are listed in Table S1 and compared to those of RhB. UV-Vis spectroscopy monitored adsorption experiments are performed in three separate solutions of pure RhB, pure AB9, and an equal molar mixture of RhB and AB9. In Fig. S16(a), excellent RhB cation exchange can be observed with a rapid drop in the RhB intensity @ 550 nm in the first 20 seconds. The RhB concentration normalized to initial concentration is shown in Fig.S16(c) indicating a ~90% removal in 50 seconds. On the other hand, the AB9 intensity @ 630 nm, as shown in Fig.S16(b) remains the same after 30 min, indicating 100% rejection by NaPSS~MIL-101for the negatively charged AB9 dye.
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anions by neat MIL-101 is similarly indicated in Fig.5(a) for ferricyanide ion. 2.6 Superior Ion Exchange offered by the Unique Properties of Polymers in MOF The outstanding ion exchange performance of NaPSS~MIL101 is in many ways parallel to that of PVBTAH~ZIF-8, the anionic polymer in MOF. In NaPSS~MIL-101 there is a linear polymer threaded through a lattice of open cells of the MOF. For a polymer chain in the bulk form of dry state, van der Waals forces bind the chains tightly together and any pores created due to random binding are macroscopic, non-uniform, and un-coordinated. The apparent specific volume of NaPSS in MIL-101, 2.17 mL/g, estimated by decrease of MOF pore volume and added mass of polymer, is significantly higher than the bulk equivalent of 1.23 mL/g. The high free volume can also offer high diffusion coefficients to enhance the molecular traffic.43 Since the charge sites are hidden due to strong binding and interaction of polymer chains in conventional resins, significant swelling is needed to facilitate ion exchange with the increase in pore volume. Swelling takes time as the solvent slowly impregnates into the resin which is initially nonporous. The structural changes due to swelling are macroscopic, non-uniform, and not reversible. This limits the efficiency in re-utilization and repeated use of ion-exchange resins. On the other hand, polymers are uniformly organized in the porous network of MOF whose structure does not change upon contact with solvent. As illustrated in Fig. 8(a), the polymers in the dry state are attached to the pore wall and conform to the outline of the MOF. There is strong interaction between the framework and NaPSS, contributed separately by hydrophobic interaction and electrostatic interaction between -SO3group and the Cr(III) site. Upon immersion into a good (aqueous or polar) solvent, the MOF structure remains unchanged. But it is reasonable to expect the solvent to significantly reduce the interaction between the -SO3- group and the framework. Hydrophobic interaction may not be affected by the solvent, hence the NaPSS only partially detached from the framework and it reconfigures with the -SO3- groups solvated off the framework, as shown in Fig. 8(b). Thus, the ion exchange function can be enhanced by the more exposed –SO3sites. In addition to fast and high capacity exchange of counterions, the polymers in MOF demonstrate excellent co-ions exclusion. This high selectivity is difficult to achieve in conventional resin type ion exchange materials where channelling can occur through macro-pores and through regions of lower charge density. It should be noted that the loadings of polymer achieved are below 50% of the maximum. Higher IEC can be achieved by increased loading. At the same time, increased loading can also increase co-ion exclusion, though there may be a trade-off in exchange kinetics due to the decease in porosity with a higher packing of polymers in the MOF.
Figure 7. Charge selectivity and ion-exchange kinetics of NaPSS~MIL-101: (a) percentage of cationic (RhB) and anionic dyes remaining in solution monitored with time; (b) Uv-vis spectra in the presence of NaPSS~MIL-101 monitored with time.
In the mixed solution with equal molar RhB and AB9, the UV–Vis spectrum (Fig.7) displays two peaks at 550 nm and 630 nm, corresponding to the two respective species. When immersed with NaPSS~MIL-101, the RhB peak at 550 nm steadily decreases in intensity, indicating effective cation exchange (Fig. 7). The AB9 peak at 630 nm, however, remains unchanged, indicating effective anion exclusion by NaPSS~MIL-101. The initial purple colour of the mixed solution gradually changes to blue, the colour of pure AB9 solution. The change in UV peak intensity is converted to concentration change in Fig. 7(a), showing high selectivity of NaPSS~MIL-101 towards charge of the dyes. In the control experiments of adsorption by neat MIL-101, both UV-vis peaks at 550 nm and 630 nm, decreased, indicating indiscriminative adsorption, as shown in Fig. S17. However, the decrease in AB9 peak intensity is larger, indicating a preferential adsorption of the anionic dye. The color of the control experiment remains purple, but slightly pinkish. The adsorption of 6
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Chemistry of Materials nations of MOFs and polymers can fine tune to a specific application with the desired chemical compatibility, stability, ion exchange capacity, selectivity, and kinetics. Experimental section Synthesis For NaPSS~MIL-101 synthesis, typically, 0.3g sodium-4styrene sulfonate (SS) was dissolved by 1.5 mL DMF and water mixture (DMF: water=10:2 v/v). Then, 3 wt.% AIBN (based on SS) was introduced as initiator. The obtained SSAIBN in DMF/H2O solution was mixed with 0.7g MIL-101 in a mortar. The polymerization was conducted at 80 oC for 5 days, or longer to increase the polymerization yield. The obtained powder was soaked in HPLC grade DMF for 3 days to completely remove the unreacted monomer and loosely attached polymer between MIL-101 particles. DMF was then removed by acetone soaking. The final product NaPSS~MIL101 was obtained after drying at 80oC overnight. Perhaps due to the charge rejection effects, the second refilling of SS monomer into NaPSS~MIL-101 cannot further increase the loading percentage of NaPSS polymer. Metal ions exchange and ferricyanide rejection For the metal ions exchange test, 0.05g NaPSS~MIL-101 was soaked in 0.1 M metal nitrate (KNO3, Cu(NO3)2, Fe(NO3)3) DMF solution for 12 h. The solid NaPSS~MIL-101 was filtrated out and soaked in DMF for another 12 h to completely remove physically trapped nitrates. Finally, the metal loaded NaPSS~MIL-101 was repeatedly washed with acetone to remove DMF and dried at 80oC overnight. For ferricyanide rejection test, 0.03g NaPSS~MIL-101 was soaked in 0.4mM potassium ferricyanide DMF solution, and UV-vis was used to monitor the concentration. To confirm the ion exchange and rejection performance were related to the cation exchange polymer NaPSS, rather than the physical trapping of MIL-101, neat MIL-101 is used as control experiment under identical treatments. Dye adsorption and desorption For kinetics test, 15mg NaPSS~MIL-101 was mixed with 10mL 10-5 M Rhodamine B (RhB) aqueous solution. The suspensions were then filtered by PTFE filter (pore size: 450nm) and analyzed for the concentration of residual RhB at 5s, 10s, 20s, 40s, 1min, 2min, 5min, 10min. The syringe filter had no effect on the dye concentration. The maximum absorbance was chosen to determine the dye content, and the absorbance value for the original solution was normalized as 100%. The absorbance value at 750 nm was used as a baseline value to correct the baseline drift. For comparison, the dye adsorption kinetics of 200mg IR-120 and 15mg MIL-101 were also tested under identical conditions. The batch dye desorption was performed by soaking dye-loaded NaPSS~MIL-101 in 1M NaCl EtOH/H2O solution. After filtering, the RhB concentration in filtrate was monitored by Uv-vis. The complete cycle process can be divided into 4 steps, and is described in Section S-3.
Figure 8. (a) In the dry state, ion-exchange polymer is attached to the pore wall of the MOF and counter ions are tightly paired. (b) The MOF structure does not alter when immersed in a solvent. The polymer partially detaches from the pore wall and become more extended after contacting solvent.
While NaPSS~MIL-101 and PVBTAH~ZIF-828 have similar characteristics of polymers~MOF and advantages over conventional resins, they show a few differences that can be further exploited and developed for other specific applications. Synthesis of NaPSS~MIL-101 has fewer steps since it is possible to impregnate the hydrophilic MIL-101 directly with an ionic monomer. On the other hand, the alkaline resistance required for anion exchange in high pH favors the choice of ZIF-8 which is hydrophobic. Therefore, a non-ionic monomer, VBC was impregnated and amination was carried out only after in situ polymerization. Fortunately, ZIF-8 has good thermal and chemical stability and can resist the high pH and high temperature conditions. The variety of ion exchange applications and operation conditions calls for different choices of MOFs, polymers and synthesis paths. The success of NaPSS~MIL-101 and PVBTAH~ZIF-8 is encouraging for more polymer/MOF combinations to be developed. 3. CONCLUSIONS In conclusion, we prepared a cation-exchange polymer~MOF composite, NaPSS~MIL-101, by in-situ polymerizing sodium4-styrene sulfonate monomer within the nanocavities of MIL101Cr(III). The functional cation-exchange polymer threading through MIL-101 matrix shows robustness to water soaking for a long period (>30 days). Comparing to conventional ion exchange resin, the high surface area (>1800 m2/g) and high pore volume (0.85 mL/g) offers efficient contacting of NaPSS~MIL-101 with guest species. This structural feature translated into orders of magnitude increase in both ionexchange and regeneration rates and high ion adsorption capacity, Due to the presence of the negatively charged sulfonate group inside MIL-101, only the cationic guest can enter NaPSS~MIL-101 and the access of anionic guest is denied. Based on these ion-exchange properties demonstrated here, this composite can be potentially useful to several applications ranging from separation, purification and ion-conduction, and therefore present promising features for solid-phase extraction. With this example of cation exchange type and the earlier anion exchange type, we have demonstrated effectively the concept of polymer in MOF and how excellence in performance can be correlated to the unique structural properties of the material. In particular, the excess free volume and exposed charged sites of the polymer facilitate full interaction with solvent and ions without structural changes of the MOF framework during solvent entry. Further variations in combi-
AUTHOR INFORMATION Corresponding Authors * Kwong-Yu Chan,
[email protected]. 7
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* Chi-Ying Vanessa Li,
[email protected].
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Funding Sources The authors acknowledge financial supports from Strategic Research Theme (SRT) on Clean Energy, and University Development Fund (UDF) for Initiative for Clean Energy and Environment (ICEE).
ACKNOWLEDGMENTS The authors thank Mr. Frankie Chan of Electron Microscopy Unit at The University of Hong Kong for assistance in materials characterizations. G. Liang gratefully acknowledges Waters Corporation in Shanghai for GPC testing. He also deeply appreciates Dr. Xiaowu Dong at Zhejiang University and Prof. Gérard Férey at Institute Lavoisier for their knowledge sharing and suggestions.
ASSOCIATED CONTENT Supporting Information Available: General characterization, chemicals, data fitting theory, Figure S1 to S17 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.
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