Article pubs.acs.org/IC
Hydrophobic Cobalt-Ethylimidazolate Frameworks: Phase-Pure Syntheses and Possible Application in Cleaning of Contaminated Water Biswa Nath Bhadra, Pill Won Seo, Nazmul Abedin Khan, and Sung Hwa Jhung* Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Republic of Korea S Supporting Information *
ABSTRACT: Two highly porous Co-based metal−azolate frameworks (MAFs), MAF-5(Co) and MAF-6(Co), which are isostructural with MAF-5(Zn) and MAF-6(Zn), respectively, were first synthesized in high yield and purity at room temperature. The syntheses compared two mixing methods, slow and fast, using cobalt acetate as the metal ion (Co2+) source and 2-ethylimidazole as the ligand. Triethylamine was applied as an additive/promoter in aqueous/ethanol solutions, and benzene and cyclohexane were used as hydrophobic templates. Phase-pure MAF-5(Co) and MAF-6(Co) were obtained in high yield by optimizing the mixing speed, reactant composition, and solvent/template ratio. It was found that fast mixing of the reactant mixtures was effective for synthesizing MAF(Co) materials. MAF-5(Co) and MAF-6(Co) were found to be very hydrophobic, similar to the MAFs composed of Zn, suggesting possible applications in water purification. MAF-5(Co) and MAF-6(Co) were then applied to adsorb n-octane as a model oil and nonpolar adsorbate from water, and the obtained results were compared to those of related materials, i.e., MAF4(Co and Zn), MAF-5(Zn), and MAF-6(Zn), as well as with Cu-BTC (Cu-benzenetricarboxylate) and a conventional adsorbent, activated carbon. Surprisingly, despite having low porosity, MAF-5(Co) showed remarkable competitiveness among the typical porous materials for n-octane removal. The results suggest that the framework structure such as cavity and aperture sizes rather than surface area plays a significant role in n-octane removal. Moreover, MAF-5(Co) can easily be regenerated by simple evacuation and reused, and thus it was found to be a potential adsorbent for the removal of spilled oil from water. Additionally, MAFs were applied in the adsorption of diclofenac sodium from water, showing the competitiveness of MAFs in water purification probably because of hydrophobicity.
1. INTRODUCTION Since the exploration of metal−organic frameworks (MOFs) as a class of zeolite-analogous materials, they have attracted much attention because of their tunable structural diversity and high and regular porosity, as well as their usefulness in a huge number of applications.1−9 Metal−azolate frameworks (MAFs) are one of the emerging classes of MOFs that are promising for crystallographers and materials scientists because of the strong and directional coordination ability between azolate ligands and metal ions.10 For example, tetrazolates,11 pyrazolates,12−14 triazolates,14−16 and imidazolates16−18 can suitably coordinate to various metal ions, and thus several MAF structures having numerous potential applications can be obtained. MAFs can be easily designed and prepared; especially, imidazolate derivativebased MAFs are noteworthy for their high porosity and exceptional stability, among others.16−21 Furthermore, imidazolate-derived MAFs generally provide relatively larger pore sizes16,19 because of their relatively longer bridging length as compared to that of oxygen atoms in inorganic prototypes.22 The tunable pores/cavities, as well as the hydrophobic inner/ © XXXX American Chemical Society
outer surface(s), are fascinating for adsorption/separation of bulky organic molecules. Although imidazolates have simple controllable coordination behavior toward tetrahedral metal ions (Zn2+, Co2+, Cu2+, etc.), it has been found that the substituent on the 2-position of imidazole plays a key role in the formation of different threedimensional (3D) frameworks. Simple imidazole (Im) leads to tightly packed metal imidazolates {[M(Im)2]∞} (M = Zn, Co, Cu) having zeolite-like,23 3D polymeric,24 and zigzag chainlike25 microporous structures. On the other hand, very wellknown zeolitic imidazolate frameworks ZIF-8 and ZIF-67 having SOD-[Zn(Mim)2] or [Co(Mim)2] structures have been obtained from 2-methylimidazole (Mim) and Zn2+ and Co2+ metal ions, respectively.18,19,26 However, Zn2+ metal ions and 2ethylimidazole (Eim) with a slightly longer or more flexible ethyl group in the 2-position of the imidazole ligand provided three isomeric MAFs, viz. MAF-5 {ANA-[Zn(Eim)2]}, MAF-6 Received: August 7, 2016
A
DOI: 10.1021/acs.inorgchem.6b01882 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
ethanol solvents. Moreover, n-octane, the major constituent of gasoline, was adsorbed from water onto several MAFs that were synthesized from low-priced and less harmful building blocks. The obtained results were compared to those of two other typical adsorbents, Cu-BTC and activated carbon (AC), to understand the relative efficiency, as well as the adsorption properties. Adsorption of diclofenac sodium (DCF), an emerging contaminant, was also done, together with the possible removal of spilled oil, to check the competence of MAFs in water purification.
{RHO-[Zn(Eim)2]}, and MAF-32 {QTZ-[Zn(Eim)2]}, depending on their synthesis conditions.27 Despite being produced from the same metal ion (Zn2+) and 2-alkylimidazoles, MAF-4(Zn), MAF-5(Zn), MAF-6(Zn), and MAF32(Zn) provided different inner/outer surface properties, as well as different pore/aperture sizes, which thus resulted in interesting performance in terms of the adsorption/separation of many molecules.18,27−30 Therefore, it is very important to develop new MAF materials having suitable cavities with special surface properties based on the needs of different applications. A huge number of analogous MOFs (such as isomorphous, isostructural, and isoreticular MOFs) have been developed and their usefulness based on their different metal centers or organic ligands has been reported.31 Typical analogous MOFs include (i) those composed of different metal ions, (ii) those with different linkers, and (iii) those with tagged functional groups. ZIF-8 and ZIF-67 (which are also named as MAF-4) are typical analogous MOFs composed of different metal centers; therefore, they have different properties originating from the metal ions. As mentioned earlier, Zn2+-derived MAF5(Zn), MAF-6(Zn), and MAF-32(Zn) isomeric structures27,28 are available; they are attractive for their properties, as well as for their numerous applications.17,28,30 However, to the best of our knowledge, the synthesis of isostructural MAFs based on a Co-metal ion and Eim-ligand and their possible applications have not yet been reported. Adsorption is a useful technique for removing environmental pollutants because of its ease/simplicity, mild operation conditions, and cost effectiveness. However, it is very essential to choose/develop materials/adsorbents that are capable of removing chemical pollutants from the environment, especially from water. For example, oil spillage cleaning has attracted much attention recently because of frequent ship-wrecks and huge uses of oil worldwide. Several materials have been developed and have shown their efficiency in oil removal from water.32−42 In addition, MOF-based porous adsorbents, including copper(II) benzenetricarboxylate (Cu-BTC), silver(I) 3,5-bis(trifluoromethyl)-1,2,4-triazolate (FMOFs), and MAF4(Zn) have also shown their efficacy in oil removal.43−45 Again, high temperature (500−600 °C) annealing of a precursor mixture [terephthalic acid, Fe(II) salt, and LiOH]46 and an MOF [Fe(II)-terepthalate]47 provided magnetic porous carbon (containing metal oxides),46,47 which also showed moderate or good performance in oil removal capability. Most of the adsorbents mentioned above might be practically useful; however, they have a few drawbacks, such as high energy consumption in production, limited adsorption capability, operational difficulties, poor reusability, high cost, and poor eco-friendliness. Various advanced materials,48−58 including MOF-based materials, have been reported, showing their potentiality in environmental remediation as well as adsorption technologies. Therefore, MOFs/MAFs, particularly hydrophobic ones, are attractive for their role in oil capturing because most reported sorbents for oil removal are hydrophobic, with the rare exception of hydrophilic Cu-BTC.43,54 In this report, we introduce a phase-selective synthetic method for MAF-5(Co) and MAF-6(Co), which are isostructural with MAF-5(Zn) and MAF-6(Zn), respectively. The selective syntheses were monitored based on the absence or presence of the additive triethylamine (TEA), without or with hydrophobic organic templates of benzene (BZ) or cyclohexane (CY), slow or fast mixing methods, the reactant compositions of Co(OAc)2:Eim:TEA, and the effect of water or
2. EXPERIMENTAL SECTION 2.1. Typical Synthesis Methods. The syntheses of MAF-5(Co) and MAF-6(Co) were carried out by the reactions between Eim (ligand) and Co(OAC)2·4H2O at room temperature. Different synthetic approaches, as shown in Scheme 1, were accomplished to
Scheme 1. Schematic Diagram of the Syntheses of Co(II)MAFs
obtain phase-pure MAF-5(Co) and MAF-6(Co) with good porosity. For example, we compared the dropwise addition (“slow addition,” 1 mL/min) of a solution consisting of metal ions into the ligand solution with the addition of the metal ion solution into the ligand solution all at once (“fast addition”) with or without CY or BZ templates and with or without the TEA additive. In the dropwise addition method, Co(OAC)2 was first dissolved in either water or ethanol as the solvent and then added to the previously prepared Eim solution with or without a specific composition of BZ or CY and with a definite molar amount of TEA. The mixture was stirred for 30 min (unless stated otherwise) at room temperature. The produced crystals were collected by filtration upon washing with ethanol and dried in an electric oven at 100 °C to give a violet powder as the product. For syntheses via the fast addition method, solutions prepared in very similar ways were mixed rapidly all at once and stirred for 30 min. MAF-4(Zn and Co),59,60 MAF-5(Zn), MAF-6(Zn),27 and Cu-BTC30 were also synthesized by following reported methods. Details of the synthesis methods are described in the Supporting Information (SI). 2.2. Characterization Methods. An X-ray diffractometer (D2 Phaser, equipped with Cu Kα radiation, Bruker, Germany) was used to analyze the crystal phases of the synthesized materials. The surface areas and pore volumes of the synthesized MAFs were measured at −196 °C using a porosity analyzer (Tristar II 3020, Micromeritics, B
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Inorganic Chemistry Table 1. Synthesis Conditions and Obtained Results Including Crystal-Phases, Yield, and Surface Areas reaction conditionsa
reaction results organic templates (%)
entry
molar composition (Co(OAC)2/Eim/TEA)
1 2 3 4 5 6 7 8 9 10 11
1:8:8 1:8:8 1:8:8 1:8:8 1:8:0 1:8:8 1:8:8 1:8:8 1:8:8 1:16:16 1:4:4
solvent water
ethanol
benzene
cyclohexane
obtained phase
0 5 10 20 10 0 10 0 0 0 0
0 0 0 0 0 0 0 5 10 0 0
no crystals MAF-5 + MAF-32 MAF-5 MAF-6 MAF-5 MAF-5 MAF-6d MAF-6 MAF-6 MAF-6 MAF-5 + MAF-32
yieldb (%)
BET surface areac (m2/g)
90 55 35 40 48 28 70 58
576 875 590 1100 925 940 1181 1050
a
All reaction were carried out by fast addition method for a reaction time of 30 min. bYield calculated based on Co(II). cSurface area measured by BET method. dWith a trace of other phases. USA). All of the studied materials were degassed at 150 °C for up to 12 h before nitrogen adsorption. The Brunauer−Emmett−Teller (BET) equation and t-plot were applied to estimate the surface areas and micropore volumes, respectively. The total pore volumes of the synthesized materials were determined from the adsorbed amounts of nitrogen (at P/P0 = 0.99). The morphology of the selected MAF crystals was observed with field emission scanning electron microscopy (FE-SEM, SU-8220, Hitachi). Thermogravimetric analyses (TGA) of the selected materials were done at a ramping speed of 10 °C/min under N2 flow using a Perkin Elmer TGA 4000 system. The adsorbed amounts of water and n-octane over some selected materials were determined in the vapor phase (at P/P0 = 0.5) with the TGA apparatus at 30−100 °C up to an adsorption time of 60 min. Detailed descriptions of the water adsorption experiments are in the SI. The Fourier transform infrared (FT-IR) spectra for selected virgin, noctane-adsorbed, and regenerated MAF-5(Co) samples were analyzed with a Jasco FT-IR-4100 system in the attenuated total reflection mode at a maximum resolution of 0.9 cm−1. 2.3. Adsorption Experiments. To check the oil removal efficiency of the studied adsorbents, solutions for batch adsorption experiments with the desired concentrations (0.1−1.5 g of n-octane/ L) were prepared by dispersion of n-octane into deionized water by ultrasound irradiation (max. 750 W, 20% energy of full power, VCX 750, Sonics & materials, Inc.). Before applying these solutions to noctane adsorption, all of the adsorbents were evacuated in an oven under vacuum at 100 °C overnight and preserved in a desiccator. For each adsorption test, fixed quantities of the adsorbents (15 mg) and the solutions (25 mL) were maintained and the suspensions were mixed well with a shaker (Lab Companion: Incubator Shaker) at a constant speed of 250 rpm at 25 °C for a set time. After finishing the adsorption, the solutions were separated from the solids with a syringe filter (polytetrafluoroethylene, hydrophobic, 0.5 μm). The remaining n-octane in the aqueous solutions was separated by the solvent extraction method using CHCl3 (5 mL × 3). The n-octane solutions in CHCl3 were then analyzed using a gas chromatograph (IGC 7200, DS Science) equipped with a flame ionization detector. Dichlorobenzene was used as the external standard. All the adsorption experiments were done three times and the average values are shown and discussed in this paper. The n-octane-adsorbed MAF-5(Co) was collected by filtration and washed with ethanol; the material was then dried overnight at 130 °C using a vacuum oven for complete removal of adsorbed n-octane. The adsorption of n-decane and n-dodecane was also done similar to the way described above for n-octane adsorption. The adsorption of DCF was carried out for 12 h at 25 °C using a conventional batch method30,58 from a 25 mL solution (50 mg/L) with 5 mg of adsorbent. The remaining DCF concentrations in the solutions were estimated using a UV spectrometer at 276 nm (UV1800, Shimadzu, Japan). Details of the methods used to calculate the
adsorptive performance of the studied adsorbents for selected adsorbates are summarized in the SI.
3. RESULTS AND DISCUSSION 3.1. Synthesis of MAF(Co)s. MAF-4(Co, or Zn) or ZIF8(Zn)/-67(Co) crystals have been obtained using Mim from diverse methods,61 such as the solvothermal method (in DMF),62 steam-assisted synthesis,63 room-temperature synthesis in water with TEA64 or in methanol without TEA,59,60,65 microwave,66 sonochemical irradiation,67 and the solvent-free method.68 Similarly, reactions between Eim (ligand) with Zn2+ metal ions [ZnO/Zn(OH)2] have provided MAF-5(Zn), MAF6(Zn), and MAF-32(Zn) having different topologies based on their synthesis methods/conditions. Therefore, similar methods were applied to obtain isostructural MAF(Co)s from Eim; however, the trials were unfortunately ineffective. Therefore, it was important as well as interesting to develop an effective method to synthesize Co-based MAF-5 or MAF-6 structures. Modified methods were therefore applied to obtain MAF5(Co) and −6(Co) from Eim with Co(OAC)2. The studied reaction conditions with their respective results are summarized in Table 1. Initially, a reactant composition of Co(OAC)2/ Eim/TEA = 1:8:8 in mixed solvents (H2O and BZ with different ratios) were undertaken, where TEA and BZ were used as an additive 64 and a hydrophobic template,27 respectively. The XRD patterns of the powders obtained by the slow mixing method from the solvents with different compositions showed the formation of different crystal phases (as illustrated in Figure S1). The MAF-5(Co) and MAF32(Co) mixtures and MAF-5(Co) as the dominant phase with a trace of MAF-32(Co) were formed with the addition of 5% BZ and 10% BZ, respectively. An MAF-6(Co) phase was obtained when the amount of BZ was increased up to 20% (Figure S1). Because synthesis by the slow mixing method in 10% BZ with water showed the formation of relatively pure MAF-5(Co) (Figure S1), the fast mixing method was applied for the same reactant composition, and the XRD pattern of the obtained material was compared to that obtained from the slow mixing method (Figure 1). Surprisingly, the fast mixing method provided pure MAF-5(Co) with a very good yield (90%) in 30 min (Figure 1b). Then, the effect of reaction time was evaluated upon applying the fast addition method. The crystal phases of the obtained products at different times (Figure 2) revealed that C
DOI: 10.1021/acs.inorgchem.6b01882 Inorg. Chem. XXXX, XXX, XXX−XXX
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To check the effect of the solvent, as well as the change in template from BZ to CY (more hydrophobic one), syntheses using the same reactant composition [Co(OAC)2/Eim/TEA = 1:8:8] were carried out in ethanol, and the obtained results are shown in Figure S3. The figure reveals that MAF-5(Co) with low crystallinity (or a low yield of 40%) was obtained from the 100% ethanol solvent (Table 1, entry 6); however, MAF-6(Co) was obtained when 5−10% of CY was added to the ethanol. In contrast, the use of 10% BZ in place of 10% CY in ethanol produced MAF-6(Co) with a trace of MAF-5(Co). On the other hand, MAF-5(Co) and MAF-6(Co) were selectively synthesized with slightly lower yields by changing the reactant composition in ethanol without an organic template. For example, increasing the concentration of Eim and TEA with a fixed Co(OAC)2 concentration (Table 1, entry 10) provided MAF-6(Co), whereas a mixture of MAF-5(Co) and MAF32(Co) was obtained from the opposite composition (Table 1, entry 11), as shown in Figure S4. The results obtained for Co2+ with Eim clearly indicate the formation of isostructural frameworks in suitable conditions with those reported for Zn2+ with Eim.18,27 In addition, the selective formation of Co-based MAF crystals was also dependent on the solvent, as well as on the reactant composition. More importantly, phase-pure and highly porous MAF-5(Co) (yield 90%) was obtained from reactants with a composition of Co(OAC)2/Eim/TEA = 1:8:8 in water with 10% BZ under fast mixing conditions. Also, the best-quality MAF-6(Co) was obtained in moderate yield (70%) from the same reactant composition in ethanol with 10% CY. 3.2. Property and Stability of MAF-5(Co) and -6(Co). The phase purities, as well as the successful syntheses of MAFs (Zn and Co) and Cu-BTC, were confirmed from the exact matching of the measured XRD patterns of the synthesized materials with their corresponding simulated patterns (as shown in Figures 1b, S3, and S5). Their textural properties such as surface area, micropore volume, and total pore volume, which were evaluated from N2-adsorption isotherms (Figure S6), are summarized in Table 2. The obtained porosity data
Figure 1. XRD patterns of obtained materials from the reactant composition (molar) of Co-salt/Eim/TEA = 1:8:8 (solvent: water with 10% benzene) by (a) slow and (b) rapid mixing methods.
Table 2. Textural Properties of Applied Adsorbents
Figure 2. Effect of reaction time on MAF-5(Co) crystallization from the reactant composition of Co-salt/Eim/TEA = 1:8:8 in water with 10% benzene.
pure MAF-5(Co) was formed in the studied reaction time range (15−240 min). From these observations, it can be said that 30 min is sufficient for the crystallization, even though longer times provided better crystallinity (or a slightly higher yield). Moreover, it was also observed that by the rapid addition method pure MAF-5(Co) was obtained even in the absence of the TEA additive with 10% BZ in water, as judged by the XRD pattern shown in Figure S2. However, the huge reduction in yield in the absence of TEA (from 90% to 35%; entries 5 and 3, respectively, of Table 1) suggests the positive effect of the TEA additive on the MAF-5(Co) yield.
entry
adsorbents
SABET (m2/g)
PVtot (cm3/g)
PVmic (cm3/g)
1 2 3 4 5 6 7 8
MAF-4(Co) MAF-5(Co) MAF-6(Co) MAF-4(Zn) MAF-5(Zn) MAF-6(Zn) Cu-BTC AC
1259 576 1181 1335 467 1317 1219 1016
0.65 0.41 1.15 0.64 0.27 0.71 0.69 0.56
0.53 0.27 0.47 0.58 0.16 0.47 0.49 0.29
showed that MAF-5(Co) had a lower porosity (SABET = 576 m2/g) than MAF-6(Co) (SABET = 1181 m2/g), similar to the tendency seen in their Zn analogs [MAF-5(Zn): SABET = 467 m2/g and MAF-6(Zn): SABET = 1317 m2/g].28 The newly synthesized MAF-5(Co) and MAF-6(Co) samples were analyzed further with FE-SEM. The images shown in Figure 3 revealed that MAF-5(Co) and MAF-6(Co) did not contain other phases, which is in accordance with their XRD patterns, even though nonhomogeneous particle size and somewhat aggregated crystals were observed for MAF-5(Co) and MAF6(Co), respectively. D
DOI: 10.1021/acs.inorgchem.6b01882 Inorg. Chem. XXXX, XXX, XXX−XXX
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The stability of any material is fairly essential for their practical application. The thermal stability of MAF-5(Co) and MAF-6(Co) was analyzed by using TGA. The TGA patterns shown in Figure S10 suggest that MAF-5(Co) and MAF-6(Co) were thermally stable up to 400 °C. Consequently, these MAFs can be easily activated by the usual thermal evacuation method under temperatures of up to 400 °C. The chemical stability of MAF-5(Co) was determined by analyzing the XRD patterns of the material after room-temperature (RT) or solvothermal (ST) treatment in typical solvents (water, methanol, and BZ). The XRD analyses of MAF-5(Co) after RT/ST treatment in typical solvents with diverse polarities are shown in Figure S11. It was confirmed that the material was stable in all of the tested solvents at room temperature and also in boiling methanol and benzene for at least 5 days (Figures S11a and S11b). However, the stability of MAF-5(Co) was somewhat poor in boiling water (Figure S11c). The surface areas of methanol, benzene, and water-treated MAF-5(Co) were calculated from N2adsorption isotherms (Figure S12). There was no loss of the porosity of MAF-5(Co) in hot methanol and benzene; however, the porosity was slightly reduced after water treatments at room temperature (Table S2). Moreover, the porosity was reduced nearly to the half of pristine MOF in boiling water which is in accordance with their XRD patterns (Figure S11) that were shown earlier. 3.3. Performance of MAFs in Adsorption of n-Octane. The relative adsorptive performance of MAF-4(Co), MAF5(Co), MAF-6(Co), and Cu-BTC in n-octane removal was tested visually using solutions of n-octane dispersed in water (10 g/L). The images taken after 30 min shaking, as shown in Figure S13, showed that the original milky white water solution became transparent in the order of MAF-5(Co) > MAF-4(Co) ≥ MAF-6(Co) ≫ Cu-BTC based on visualization of text or a logo through the solution. Adsorption tests were carried out for the MAF(Co)s and their Zn analogs [MAF-4(Zn), MAF5(Zn), and MAF-6(Zn)], along with AC, for 60 min. The quantified results are compared all together in Figure 5, which
Figure 3. SEM images of (a) MAF-5(Co) and (b) MAF-6(Co).
The reported Zn MAFs, even with different topologies (SOD, ANA, RHO, and QTZ), had hydrophobic surfaces (either inner or outer) because all the N atoms of the ligands were engaged in coordination and the alkyl groups were aligned to their surfaces.16,29 In order to check the hydrophobicity of the synthesized MAF-5(Co) and MAF-6(Co) samples, the adsorbed amounts of water were measured at 30 °C in the vapor phase. The obtained results shown in Figure 4 illustrate
Figure 4. Adsorbed amounts of water over Cu-BTC, MAF-5(Co), and MAF-6(Co) at 30 °C in vapor phase (relative pressure, P/P0 = 0.5) up to 60 min.
that a very minute amount of water was captured by MAF5(Co) and MAF-6(Co), showing their hydrophobicity; however, considerable water uptake was observed over CuBTC because of its well-known hydrophilicity.54,69 The results regarding the hydrophobicity/hydrophilicity of the three materials did not change with adsorption temperatures up to 100 °C, as shown in Figure S7. In addition, the adsorbed quantities of n-octane vapor over Cu-BTC, MAF-5(Co), and MAF-6-(Co) were measured at 30 °C up to 60 min, and the ratios of the amounts of adsorbed n-octane and water were calculated in order to quantify the hydrophobicities of the tested materials. The higher n-octane/water ratios for MAF5(Co) and MAF-6(Co) than Cu-BTC, as shown in Figure S8, again confirmed that the synthesized materials are hydrophobic. Moreover, morphologies of water droplet over MAF-5(Co), MAF-6(Co), and Cu-BTC were also compared. As illustrated in Figure S9, the size of water droplet over the MAF-5(Co) or MAF-6(Co) is smaller than that over Cu-BTC, confirming again the hydrophobicity of the MAFs. Therefore, it can be concluded that MAF-5(Co) and MAF-6(Co), similar to the MAF(Zn)s, are highly hydrophobic, and therefore, the surface hydrophobicity of these MAFs might play an important role in water purification via adsorption.58
Figure 5. Amounts of adsorbed n-octane over MAF-4, -5, and -6(Co); MAF-4, -5, and -6(Zn); Cu-BTC; and AC. Adsorption condition: ∼0.015 g adsorbents in 25.0 mL solution (10 g/L), 60 min.
shows that the adsorbed quantities (qt, from the 10 g/L solution) follow an order similar to that of their corresponding images (Figure S13) in the case of the MAF(Co)s and CuBTC. The results obtained for the sorbent Cu-BTC (3.4 g/g) and MAF-4(Zn) (6.5 g/g) were comparable to the adsorption results of soybean oil over Cu-BTC (4 g/g)43 and MAF-4(Zn) (6.6 g/g).45 These relatively small amounts of n-octane adsorption over the two adsorbents might be due to the E
DOI: 10.1021/acs.inorgchem.6b01882 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry different physicochemical properties of n-octane and soybean oil. However, interestingly, the surface areas or pore volumes (Table 2) of the adsorbents played no role in the adsorption. In other words, the obtained adsorption capacity of MAF-5(Co) for n-octane (10.4 g/g), even with its low porosity, was notable among the tested porous adsorbents, including other MOFs. Therefore, while continuing to highlight MAF-5(Co), MAF4(Co), and MAF-6(Co) were taken into consideration to understand the adsorption phenomena. To better understand the relative performance, the effect of contact time on the adsorption of n-octane from water over MAF-4(Co), MAF-5(Co), and MAF-6(Co) was also estimated over a time range from 5 to 60 min at 25 °C, and the obtained results were compared to those over AC, as shown in Figure 6.
Figure 7. (a) Adsorption isotherms (the solid lines are guides to the eye) and (b) Freundlich plots of the isotherms for n-octane adsorption over MAF-5(Co) and MAF-6(Co).
Figure 6. Effect of contact time on adsorption of n-octane over AC, MAF-4(Co), MAF-5(Co), and MAF-6(Co). The solid lines are guides to the eye.
As shown in Table S1, MAFs with different framework structures/topologies provided different pore architectures with diverse apertures and cavity sizes.27,28 Depending upon the above characteristics, the MAFs/MOFs would show different adsorption characteristics in the separation or selective adsorption of hydrocarbons.27,72−74 Considering the isostructures of MAF(Zn)s and MAF(Co)s, MAF-5(Co) had apertures that were smaller and larger than MAF-4(Co) and MAF-6(Co), respectively, with different “distorted” cavity shapes. Therefore, the structural/surface properties of the adsorbents might be considered the reason for the high n-octane uptake over MAF5(Co), i.e., higher qt and qe than MAF-4(Co) and MAF-6(Co), as shown in Figures 6 and 7a, even with its low porosity. For a clearer understanding of the high n-octane uptake by MAF-5(Co), the adsorption of longer n-alkanes such as ndecane and n-dodecane was done over MAF-5(Co) and MAF6(Co). The tested adsorbates, n-octane, n-decane, and ndodecane, have similar kinetic diameters of 3.9 Å and lengths of 13.46, 15.98, and 18.51 Å, respectively, as measured by the modified Tanford method.30 The results for the adsorption of n-octane, n-decane, and n-dodecane illustrated in Figure 8 show that the quantity adsorbed onto MAF-5(Co) decreased with increasing n-alkane length; however, the reverse trend was observed for MAF-6(Co). MAF-5 and MAF-6 can easily accommodate the alkane molecules considering the aperture sizes (Table S1) and kinetic diameters. MAF-5(Co), whose cavity diameter is comparable (or slightly smaller size) to the n-octane length, might effectively accommodate the flexible n-octane molecules based on length selectivity.30 Therefore, MAF-5(Co) had the highest adsorption
The adsorption results indicated that n-octane adsorption over MAF-5(Co) was the most favorable among the selected adsorbents across the whole adsorption time (5−60 min) to follow the order: MAF-5(Co) > MAF-6(Co) > MAF-4(Co) > AC. The adsorption reached equilibrium within 20 min over MAF-5(Co), whereas more than 30 min was needed for the others. The highest adsorption capacity with the MAF-5(Co) adsorbent, even though it had the lowest surface area among the three MAFs(Co), suggests the presence of a special dominating interaction between MAF-5(Co) and the adsorbate, considering the general importance of porosity in adsorption when there is no special mechanism excluding van der Waals interactions.70 The adsorption over MAF-5(Co) and MAF-6(Co) was investigated using n-octane dispersed in water in wide concentrations (1−15 g/L) for an extended period of up to 60 min considering the equilibration time (within 30 min, as shown in Figure 6). The isotherms illustrated in Figure 7a show the multilayer adsorption of n-octane over both MAF-5(Co) and MAF-6(Co), based on type-V adsorption isotherms. The high correlation factor (RF2) evaluated from the Freundlich plots (Figure 7b) again confirms the multilayer adsorption, probably because of hydrophobic interactions [considering hydrophobic n-octane and MAF(Co)s, Figures 4 and S7] of noctane over the MAF(Co)s in accordance with the adsorption of soybean oil over MAF-4(Zn).45,71 Therefore, there might be little difference in the n-octane adsorption mechanism over MAF-5(Co) and MAF-6(Co). F
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Figure 8. Adsorbed amounts (after 30 min) of n-octane (C8), ndecane (C10), and n-dodecane (C12) over MAF-5(Co) and MAF6(Co).
for n-octane, even with its low porosity. Conversely, the MAF6(Co) structure had a suitable cavity for n-dodecane,30 and thus it tended to adsorb higher amounts of longer n-alkanes, especially n-dodecane (13.0 g/g). These results indicate that the cavity size plays a dominant role in the adsorption of nalkanes over the studied adsorbents. In addition, the relatively lower adsorption efficiency over MAF-4(Co) (Figures 5 and 6), even with its higher surface area (Table 2), might be due to the fact that its aperture size (Table S1) is smaller than the kinetic diameter of n-octane, considering the well-known shape selectivity. Therefore, it can be suggested that the aperture and cavity sizes of MAFs play crucial roles in the adsorption of n-alkanes. Reusability is an important parameter for efficient adsorbents considering cost-effectiveness, as well as prospective commercialization. The reusability of MAF-5(Co) for the adsorption of n-octane was checked not only by FT-IR but also by adsorption experiments because MAF-5(Co) was the highest adsorbing adsorbent for n-octane among the studied MAFs. The FT-IR analysis of virgin MAF-5(Co), n-octane-adsorbed MAF-5(Co), and regenerated MAF-5(Co) are shown in Figure 9a. The adsorption could be confirmed with the bands of methyl rocking (at 1379 cm−1) and C−H stretching (at 2854 and 2962 cm−1) of n-octane for the n-octane-adsorbed MAF-5(Co). Moreover, bands of C−H scissoring (at 1440 cm−1) and methyl rocking (at 734 cm−1) also supported the adsorption. In contrast, the absence of such bands in the recycled MAF-5(Co) suggests the ready desorption of n-octane, as well as regenerable sorbency by simple vacuum drying. The adsorbent, after regeneration, was used further for up to four cycles, and the obtained results are summarized in Figure 9b. There was negligible loss in performance in removing n-octane from water over MAF-5(Co), which thus leads us to emphasize that MAF5(Co) is a reusable adsorbent for n-octane removal from water. Similar results might be expected for n-dodecane adsorption and reuse over MAF-6(Co). Therefore, MAF-5(Co) and MAF6(Co) can be suggested as relatively economical, recyclable, and competent adsorbents for the removal of hydrocarbons such as spilled oil from water. 3.4. Possible Application of MAFs in Adsorption of Organics from Water. To check other possible applications of MAF-5(Co) and MAF-6(Co) in the adsorption of organic micropollutants from aqueous environments, DCF75 was selected as a model compound. DCF is a well-known pharmaceutical and personal care product (PPCP)76,77 that
Figure 9. (a) FTIR spectra of virgin, n-octane-adsorbed and regenerated MAF-5(Co) and (b) recyclability of MAF-5(Co) for adsorption of n-octane from water.
should be removed from the environment, especially river or seawater. The nonsteroidal drug DCF is widely used to reduce pain and inflammation. The adsorbed quantity of DCF over MAF-5(Co) and MAF-6(Co) demonstrated that MAFs are quite competitive against AC (Figure S14). The higher qt over MAF-6(Co) than over MAF-5(Co) might be because of the higher surface area of MAF-6(Co). The comparable adsorbed quantities of DCF over MAF-6(Co) and UiO-6678 also suggest the possible application of MAFs in water purification.
4. CONCLUSIONS A series of Co(II)−azolate frameworks such as MAF-5(Co) and MAF-6(Co), isostructural with their Zn(II)−azolate analogs, were first synthesized with high phase-purity at room temperature. Fast mixing rather than the slow addition method was more efficient and resulted in phase-pure MAF-5(Co) and MAF-6(Co) crystals. MAF-5(Co) was effectively synthesized in water and BZ; however, MAF-6(Co) was preferentially obtained in ethanol and CY. The TEA additive played an important role in crystallization, as well as in boosting the yield. Based on the present research, hydrophobic, phase-pure MAF5(Co) and MAF-6(Co) can be obtained in high yield. The synthesized MAFs were found to be efficient sorbents for the adsorption of n-alkanes from water, and the obtained results might be explained by the hydrophobic interaction followed by the multilayer adsorption mechanism. More importantly, the highest uptake of n-octane and n-dodecane by MAF-5(Co), G
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even with its lower surface area, and MAF-6(Co), respectively, might be explained by the matching between the cavity sizes and the lengths of the adsorbates and by the fact that MAF5(Co) and MAF-6(Co) had suitable aperture sizes for their respective adsorbates. Readily synthesized MAF-5(Co) and MAF-6(Co) are interesting sorbents for water cleaning considering their noticeable removal efficiency for model oils (n-octane and n-dodecane) among the studied porous adsorbents such as MAFs/MOFs/AC and easy recyclability. Moreover, MAF-6(Co) showed competitive adsorption of DCF from water. Therefore, the MAF-5(Co) and MAF-6(Co) might be useful in the removal of hazardous organics from water.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01882. Experimental details, additional tables, figures including XRD, N2 adsorption isotherms, water adsorption results, TGA patterns, pictograms of n-octane adsorption, and DCF adsorption results. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 82-53-950-5341. Fax: 82-53-950-6330. E-mail: sung@ knu.ac.kr. Funding
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and future Planning (Grant No. 2015R1A2A1A15055291). Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b01882 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01882 Inorg. Chem. XXXX, XXX, XXX−XXX
