English

• 1. INTRODUCTION

  Metal-organic frameworks (MOFs) are a well-known class of crystalline porous materials constructed by joining metal ions with organic ligands^{[1, 2]}. Because of their unique properties, MOFs have been considered as promising materials in the fields of adsorption, separation, catalysis, gas storage, sensing, and so on^[3-9]. Recently, the modification of structures and components of MOFs has attracted special attention. For example, through incorporating various functional guests into the vacant cages of MOFs, the materials with excellent performances have been produced^[10].

  Zeolitic imidazole frameworks (ZIFs) are a famous category of MOFs. Among them, ZIF-8, i.e. Zn(MeIM)₂ (Fig. S1, HMeIM = 2-methyllimidazole), constructed through the self-assembly of Zn²⁺ and HMeIM, has attracted much attention because of its advantages such as high porosity and excellent stability in water^{[11, 12]}. The network of ZIF-8 exhibits a zeolitic sodalite topology (4²6⁴). The truncated octahedral cages (the diameter of about 14.7 Å) are connected with each other through the hexagonal and square windows (the length of side of about 6.0 Å, Fig. S1). The solvent-accessible volume is 47% of the total unit-cell volume. At present, ZIF-8 has been regarded as one of the optimal platforms or components for designing and preparing the functional materials^{[13, 14]}. For example, various functional guests, such as drugs^[15], metal nanoparticles^[16], and oxides^[17], have been incorporated into ZIF-8 for obtaining the materials with desired properties.

  Polyoxometalates (POMs) are nanosized transition metal oxygen anion clusters. POMs can be divided into heteropolyoxometalates, such as [XM₁₂O₄₀]^n- (X = P, Si, etc.; M = W, Mo, etc.)^{[18, 19]}, and isopolyoxometalates like [M_xO_y]^n- (M = Mo, W, V, etc.)^{[20, 21]}. POMs could exhibit high stability in water and various organic solvents. So far, POMs have been widely used for the design and preparation of various functional materials, because of their unique acidic, redox, electric, catalytic, and magnetic properties^[22-28]. For example, the POMs@MOFs materials with POMs encapsulated in the cages of MOFs could exhibit enhanced adsorption capacities toward cationic dyes, and the anion clusters of POMs could act as the adsorption active sites due to the electrostatic interaction with dye cations^[29-31]. Moreover, POMs have been introduced into porous MOFs for obtaining the materials with enhanced proton conductivities, and they could act as the proton carriers^{[32, 33]}.

  At present, some efforts have been devoted to the encapsulation of heteropolyoxometalates into the cages of ZIF-8. For example, several Keggin-type heteropolyoxometalates, including PW₁₂O₄₀^3- (PW₁₂), SiW₁₂O₄₀^4- (SiW₁₂), and PMo₁₂O₄₀^3- (PMo₁₂), have been incorporated into the cages of ZIF-8, through ball milling zinc oxide, HMeIM, and POMs in the stainless-steel milling pot. The produced materials exhibited high uptake capacity for dye molecules^[29]. CoW₁₂O₄₀^6- (CoW₁₂) was introduced into ZIF-8 through mixing the aqueous solution of CoW₁₂ with the methanolic solutions of zinc nitrate and HMeIM at room temperature. The CoW₁₂@ZIF-8 was used as an efficient and robust electrocatalyst for water oxidation^[34]. Moreover, the ultrathin ZIF-8 film containing PMo₁₀V₂O₄₀^5- (PMo₁₀V₂) was fabricated on the surface of ZnO nanorodes, through adding PMo₁₀V₂ aqueous solution into the suspension containing HMeIM and ZnO nanorods. The product was used as an enhancer for selective formaldehyde sensing^[35].

  Herein, Mo₇O₂₄^6- (Mo₇, Fig. S2), a representative isopolyoxometalate^{[20, 21]}, was incorporated into ZIF-8 for designing and preparing materials with the properties of dye separation and proton conduction. As shown in Scheme 1, on the basis of the same synthesis strategy, the powdered crystal of Mo₇@-ZIF-8 was synthesized, and the membrane of Mo₇@ZIF-8 covered on the surface of Al₂O₃ support was fabricated. The Mo₇@ZIF-8 crystals were used for adsorbing malachite green (MG, Fig. S3) from the aqueous solution and the mixed dye solution of MG and methyl orange (MO, Fig. S3). In addition, the proton conduction property of Mo₇@ZIF-8 membrane was preliminarily evaluated.

  Scheme 1

  

  Scheme 1.  Illustration of the synthesis processes of Mo₇@ZIF-8 crystals and membrane covered on the Al₂O₃ ceramic support

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  2. EXPERIMENTAL

  2.1 Materials and instruments

  All chemicals were of AR grade and used as received without additional purification. Fourier transform infrared (IR) spectra were measured on a Bruker IFS-66 V/S FT-IR spectrometer using the KBr pellets. Powder X-ray diffraction (PXRD) patterns were collected in the 2θ range of 4~40^o on a Rigaku D/Max 2550 automated diffractometer using CuKα radiation (λ = 1.5418 Å) at room temperature. The morphologies of the samples were examined using a JEOL JSM-6700F scanning electron microscope (SEM) at 5 kV. Nitrogen adsorption-desorption analysis was carried out at 77 K on a Micromeritics ASAP 2020 surface area and porosity analyser. Elemental mapping was performed using a Zeiss GerminiSEM 500 SEM with energy-dispersive X-ray spectroscopy (EDS) to investigate the components of sample surfaces. The contents of Mo and Zn were determined using an OPTIMA 3300DV ICP atomic emission spectrometer. Thermogravimetry (TG) analysis was carried out using a Setaram Labsys Evo 1600 thermoanalyzer under N₂ atmosphere at a heating rate of 10 ℃·min^-1. The concentrations of dye solutions were monitored using a Hitachi UV-2450 spectrophotometer. The electrochemical impedance measurements were performed on a Princeton Applied Research VersaSTAT3 potentiostat/galvanostat. In order to characterize the Mo₇@ZIF-8 membrane by IR, XRD, and elemental analysis, some samples of membrane were exfoliated from the support using a knife.

  2.2 Synthesis of Mo₇@ZIF-8 crystals

  The powdered crystals of Mo₇@ZIF-8 were synthesized based on the synthesis methods of CoW₁₂@ZIF-8^[34] and ZIF-8 membrane^[36]. Zinc chloride (0.55 g, 4.04 mmol) was dissolved in methanol (25.00 mL). HMeIM (2.60 g, 31.67 mmol) was dissolved in methanol (25.00 mL). (NH₄)₆Mo₇O₂₄·4H₂O (0.50 g, 0.40 mmol) was dissolved in water (3.00 mL). These solutions were added into an autoclave and stirred for 20 min. The autoclave was sealed and heated in an oven at 120 ℃ for 4 h. After the autoclave was cooled down to room temperature, the solid was filtrated and washed by methanol and water for several times. Then, the sample was immersed sequentially in DMF (250 mL, 24 h), methanol (250 mL, 24 h), and water (250 ml, 24 h) for removing the unreacted reactants from the surfaces and pores of sample. Finally, the product was dried in air at 60 ℃.

  2.3 Synthesis of Mo₇@ZIF-8 membrane

  The Mo₇@ZIF-8 membrane was synthesized through combining the synthesis methods of CoW₁₂@ZIF-8^[34] and ZIF-8 membrane^[36]. Zinc chloride (0.55 g, 4.04 mmol) and sodium formate (1.43 g, 21.03 mmol) were dissolved in methanol (25.00 mL). HMeIM (2.60 g, 31.67 mmol) was dissolved in methanol (25.00 mL) and (NH₄)₆Mo₇O₂₄·4H₂O (0.50 g, 0.40 mmol) was dissolved in water (3.00 mL). These solutions were added into an autoclave and stirred for 20 min. An α-alumina ceramic support (3.0cm × 2.0cm × 0.1cm) was placed almost vertically into the autoclave using a Teflon holder. The autoclave was sealed and heated in an oven at 120 ℃ for 4 h. After the autoclave was cooled down to room temperature, the Al₂O₃ support with Mo₇@ZIF-8 membrane on its surface was washed by methanol and water. Then, the sample was immersed sequentially in DMF (100 mL, 24 h), methanol (100 mL, 24 h) and water (100 mL, 24 h), respectively. Finally, the product was dried in air at room temperature.

  3. RESULTS AND DISCUSSION

  3.1 Synthesis methods

  There are several strategies which have been used for incorporating POMs into MOFs. Firstly, POMs@MOFs could be prepared through the solution-immersion processes. For example, through immersing MIL-101 crystals in the aqueous solutions of POMs, POMs can diffuse into the vacant cages of MIL-101 through the open nanosized windows^[37]. Secondly, POMs could be added into the precursor solutions containing metal ions and organic ligands, and the encapsulation of POMs could be achieved during the synthesis processes of MOFs. For example, PW₁₂ had been encapsulated into MIL-101, through previously adding PW₁₂ in the aqueous solution containing Cr³⁺ and terephthalic acid^[38]. This strategy could be mentioned as building the MOF bottles around the POM ships^[39]. Finally, some special and effective synthesis strategies were developed. For example, Keggin-type POMs could be encapsulated in the cages of ZIF-8 through ball milling zinc oxide, HMeIM, and Keggin-type POMs^[29].

  In this work, we aimed to incorporate Mo₇ into the powdered crystals and crystalline membrane of ZIF-8, and the strategy of building MOF bottles around the POM ships was selected because of the following reasons. Firstly, it was found that Mo₇ clusters could hardly diffuse into the cages of ZIF-8 through the solution-immersion process due to the Mo₇ cluster (about 9.3Å × 7.6Å × 5.2Å, Fig. S2) is bigger than the open windows (Fig. S1) of ZIF-8. Secondly, though the milling method is highly effective for producing the powdered crystals of POMs@ZIF-8, the crystalline membrane of POMs@ZIF-8 can not be fabricated through this method. Finally, CoW₁₂@ZIF-8 and PMo₁₀V₂@ZIF-8 have been successfully synthesized through adding the aqueous solutions of POMs into the organic phases containing zinc resources and HMeIM^{[34, 35]}. On the basis of above reasons, in this work, we synthesized Mo₇@ZIF-8 through applying the strategy of adding Mo₇ into the solution of Zn²⁺ and HMeIM, and the corresponding powdered crystals and crystalline membrane were obtained. During the synthesis processes, the addition of aqueous solution of POM into the methanolic solution of Zn²⁺ and HMeIM is based on the synthesis methods of CoW₁₂@ZIF-8^[34] and PMo₁₀V₂@ZIF-8^[35]. In the synthesis process of Mo₇@ZIF-8 membrane, sodium format was added for availing the growth of Mo₇@ZIF-8 membrane on the Al₂O₃ support. This method has been successfully used for the fabrication of ZIF-8 membrane on Al₂O₃ support^[36].

  3.2 IR and elemental analyses

  As shown in Fig. 1, the IR spectrum of Mo₇ displayed the characteristic adsorption peaks in the range of 800~1000 cm^-1. They could be assigned to the Mo–O band vibrations. In the IR spectra of Mo₇@ZIF-8 crystals and membrane, these characteristic adsorption peaks can be observed. In contrast, in the IR pattern of ZIF-8, there are no obvious adsorption peaks in these regions. Therefore, it could be concluded that Mo₇ is included in the Mo₇@ZIF-8 samples, which is in agreement with the result of element analysis. Element analysis confirmed the presence of Mo elements in the Mo₇@ZIF-8 samples, and the component of Mo₇@ZIF-8 was confirmed to be (C₈H₁₀N₄Zn)(Mo₇O₂₄)_0.03. The molecular ratio of POM and C₈H₁₀N₄Zn is slightly higher than that of CoW₁₂@ZIF-8 ((C₈H₁₀N₄Zn)(CoW₁₂O₄₀)_0.02)^[34]. CoW₁₂@-ZIF-8 was synthesized under ambient condition, and Mo₇@ZIF-8 products were synthesized under solvothermal condition. Thus, the higher POM loading might be due to the higher solubility of POM at higher temperature and pressure. However, this ratio is much lower than that of PW₁₂@ZIF-8 ((C₈H₁₀N₄Zn)(PW₁₂O₄₀)_0.125)^[29]. It implies that, when we want to obtain the Mo₇@ZIF-8 powdered crystals with higher loading of Mo₇, we should apply the method of milling zinc oxide, HMeIM, and Mo₇ together. The related experiments will be carried out in our future work.

  Figure 1

  

  Figure 1.  IR spectra of Mo₇, ZIF-8, Mo₇@ZIF-8 crystals and Mo₇@ZIF-8 membrane

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  3.3 PXRD, SEM, N₂ adsoropion, and TG

  The Mo₇@ZIF-8 crystals and membrane were characterized by PXRD. As shown in Fig. 2, the PXRD patterns of Mo₇@ZIF-8 samples matched well with that of ZIF-8, which indicates that ZIF-8 was synthesized during the synthesis processes. No peaks belonging to the crystalline Mo₇ were observed, which demonstrates that Mo₇ clusters were homogeneously dispersed in the cages of ZIF-8 rather than formed crystalline accumulations on the surface of the ZIF-8 samples. This is in agreement with the result of SEM. As shown in Figs. 3 and S4, no impurity was observed in the samples. The result of EDS analysis (Fig. S5) indicates that Mo and O elements were homogeneously dispersed in the sample. Mo₇ is composed of Mo and O elements. Thus, it could be concluded that Mo₇ clusters were homogeneously dispersed in Mo₇@ZIF-8. The N₂ adsorption-desorption isotherm of Mo₇@ZIF-8 crystals was measured at 77 K (Fig. 4). The BET surface area of Mo₇@ZIF-8 is about 727 m·²g^-1, which indicates that the material exhibits high porosity.

  Figure 2

  

  Figure 2.  PXRD patterns of ZIF-8 (simulated), Mo₇@ZIF-8 crystals, and Mo₇@ZIF-8 membrane

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  Figure 3

  

  Figure 3.  (a) SEM image of Mo₇@ZIF-8 crystals and (b) cross-section SEM image of Mo₇@ZIF-8 membrane covered on the Al₂O₃ ceramic support

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  Figure 4

  

  Figure 4.  N₂ adsorption-desorption isotherm of Mo₇@ZIF-8 crystals

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  Thermal analysis of Mo₇@ZIF-8 crystals was carried out under nitrogen atmosphere (Fig. S6a). The sample showed gradual weight loss in the temperature range of 30~700 ℃. For accurately evaluating the thermal stability of Mo₇@ZIF-8, the TG curves (Figs. S6b and S6c) of Mo₇ and ZIF-8 were used for comparison. As shown in the TG curve of Mo₇, from 30 to 400 ℃, Mo₇ gradually lost H₂O and NH₄⁺ in multiple steps^[40]. No weight loss was observed from 400 to 700 ℃. Moreover, it was reported that the crystal structure of ZIF-8 was maintained up to 500 ℃^[12]. Therefore, it could be concluded that Mo₇@ZIF-8 is stable up to 500 ℃. In the temperature range of 30~500 ℃, Mo₇@ZIF-8 sample gradually lost its initial weight due to the loss of volatile guests, including H₂O, NH₄⁺, and unreacted HMeIM, from the porous framework.

  3.4 Dye adsorption

  Usually, after POMs were incorporated into the cages of MOFs, the resulting Mo₇@ZIF-8 materials could exhibit high adsorption capacities toward cationic dyes, and exhibit poor adsorption capacities toward anionic dyes^{[29, 30, 41]}. This may be due to the electrostatic interactions between the POMs anions and dyes.

  MG is a toxic cationic dye, and the direct release of MG wastewater will seriously pollute the environment. Therefore, many materials have been developed for adsorbing MG from water. Herein, the adsorption performance of Mo₇@ZIF-8 for the removal of MG from aqueous phase was evaluated (Fig. 5). The adsorption kinetics of MG on Mo₇@ZIF-8 crystals was investigated with the pseudo-first-order and pseudo-second-order equations^[41]. When the pseudo-first-order equation was used, the calculated adsorption capacities were much lower than the experimental ones (Table S1). Moreover, the relatively poor linear relationships between time and lg(q_e-q_t) were observed (Fig. 5c). In contrast, when the pseudo-second-order equation was used to fit the adsorption data, the excellent linear relationships between time and t/q_t can be seen (Fig. 5d). Therefore, the pseudo-sencond-order kinetic model is more suitable than the pseudo-first-order model for describing the adsorption processes of Mo₇@ZIF-8 crystals toward MG.

  Figure 5

  

  Figure 5.  Adsorption of MG over Mo₇@ZIF-8 crystals. (a) Effect of time and initial MG concentrations on theadsorption capacities. (b) Adsorption isotherm. Plots of (c) pseudo-first-order and (d) pseudo-second-order kinetics models. Plots of (e) Langmuir and (f) Freundlich isotherm models (Temperature, 25 ℃; volume of dye solution, 100.00 mL; adsorbent, 0.100 g·L^-1)

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  In addition, the data of adsorption experiment performed at 298 K were analyzed using two typical isotherm models (Langmuir and Freundlich models)^[42]. When the Langmuir mode was utilized, a better linear relationship was observed (Figs. 5e and 5f). Therefore, the adsorption behavior of Mo₇@ZIF-8 toward MG should be described using the Langmuir model rather than the Freundlich mode. On the basis of the Langmuir model, it could be calculated that the maximum adsorption capacity was 1963 mg·g^-1. This adsorption capacity is lower than that of the composite based on ZIF-8 and graphene oxide (GO) (ZIF-8@GO, 3300 mg·g^-1), and is similar with that of the composite based on ZIF-8 and carbon nanotubes (CNTs) (ZIF-8@CNTs, 2034 mg·g^-1)^[42]. Considering Mo₇ is much cheaper than GO and CNTs, the Mo₇@ZIF-8 is competitive when compared with ZIF-8@GO and ZIF-8@CNTs in the practical applications. The adsorption capacity of Mo₇@ZIF-8 is higher than most adsorbents (Table S2). Thus, Mo₇@ZIF-8 could be considered as a cheap and effective adsorbent toward MG. It should be noted that the comparison of different materials based on their adsorption capacities should be cautious, because these values were not obtained under the same experimental conditions.

  In our experiments, it was found that Mo₇@ZIF-8 exhibited poor adsorption property toward MO, a representative anion dye. This might mean that Mo₇@ZIF-8 could be used for the separation or purification of dyes. As shown in Fig. 6, after the Mo₇@ZIF-8 crystals were added into a mixed dye solution containing MG and MO, the characteristic absorption peaks of MG decreased obviously, and the absorption peaks of MO have no obvious change.

  Figure 6

  

  Figure 6.  (a) UV-Vis spectra of MG and MO aqueous solutions. (b) Selective adsorption of Mo₇@ZIF-8 crystals toward the mixed solution of MG and MO (Temperature, 25 ℃; initial concentration, 10 mg·L^-1; volume of dye solution, 100.00 ml; adsorbent, 1.500 g·L^-1)

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  3.5 Proton conductivity

  In recent years, the utilization of MOFs as the proton-conducting materials has attracted much attention. In the structures of MOFs, the vacant cages are regularly aligned and connected through the open windows. Proton transport can be achieved inside the nanochannels of MOFs^[44]. For achieving enhanced proton conduction properties, various functional guests, such as imidazole, NH₄⁺, H₂SO₄, H₃PO₄, and toluenesulfonic acid, have been introduced in the cages of MOFs^{[45, 46]}. These functional guests could act as the proton carriers in the porous host frameworks to facilitate the proton conduction. This strategy has been widely accepted for developing the high proton conductive MOFs-based materials^[44-47].

  At present, there are only a few reports on the proton conduction studies of MOF membranes, though the membrane could exhibit charming properties^{[14, 48]}. Recently, a DNA-threaded ZIF-8 membrane (DNA@ZIF-8) was fabricated. The proton conductivity reached 0.17 S·cm^-1 at 75 ℃ under 97% relative humidity, which results from the presence of DNA and water molecules inside the cages of ZIF-8^[14]. Herein, the proton conductivity of Mo₇@ZIF-8 membrane was preliminarily evaluated. On the basis of the previous researches, it could be found that the ZIF-8-based materials should exhibit higher conductivity at higher temperature under higher relative humidity^{[14, 49]}. Thus, the proton conduction property of Mo₇@ZIF-8 membrane was evaluated under 98% relative humidity. The Nyquist and Arrhenius plots of the Mo₇@ZIF-8 membrane are shown in Fig. 7. The activation energy (E_a) is 0.217 eV. The proton conductivity of Mo₇@ZIF-8 membrane reached 9.5×10^-2 S·cm^-1 at 75 ℃. The NH₄⁺ and Mo₇ in the cages of ZIF-8 membrane might act as carriers for conducting proton, and the presence of these guests might avail the adsorption of ZIF-8 membrane toward water molecules at high temperature under high relative humidity condition. The conductivity of Mo₇@ZIF-8 membrane is lower than that of the DNA@ZIF-8 membrane. However, Mo₇ could be more easily obtained than DNA. Moreover, it could be expected that better proton conduction properties might be achieved when we incorporate more functional guests, such as imidazole, triazole, and H₃PO₄, into the Mo₇@ZIF-8 membrane.

  Figure 7

  

  Figure 7.  (a) Nyquist plots of Mo₇@ZIF-8 membrane obtained at different temperatures under 98% relative humidity. (b) Arrhenius plots of the membrane

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  4. CONCLUSION

  In sum, a facile approach was developed to incorporate Mo₇, a representative isopolyoxometalate, into ZIF-8. Through the strategy of building MOF bottles around the POM ships, the powdered crystals and crystalline membrane of Mo₇@ZIF-8 were synthesized. The Mo₇@ZIF-8 crystals could be used for dye adsorption. The maximum adsorption capacity of Mo₇@ZIF-8 crystals toward MG reached 1963 mg·g^-1. Through utilizing Mo₇@ZIF-8 as the adsorbent, MG could be effectively adsorbed from the mixed dye solution of MG and MO. In addition, the Mo₇@ZIF-8 membrane exhibited proton conduction property, and the proton conductivities of 9.5×10^-2 S·cm^-1 was reached at 75 ℃ under 98% relative humidity. In the following work, we will devote our efforts to further evaluating the proton conduction property of Mo₇@ZIF-8 membrane under different temperature and relative humidity conditions. Moreover, it could be expected that the proton conductivities of POMs@ZIF-8 membranes could be adjusted through encapsulating different POMs in the cages of ZIF-8 membrane.

  
  
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  Scheme 1  Illustration of the synthesis processes of Mo₇@ZIF-8 crystals and membrane covered on the Al₂O₃ ceramic support

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  Figure 1  IR spectra of Mo₇, ZIF-8, Mo₇@ZIF-8 crystals and Mo₇@ZIF-8 membrane

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  Figure 2  PXRD patterns of ZIF-8 (simulated), Mo₇@ZIF-8 crystals, and Mo₇@ZIF-8 membrane

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  Figure 3  (a) SEM image of Mo₇@ZIF-8 crystals and (b) cross-section SEM image of Mo₇@ZIF-8 membrane covered on the Al₂O₃ ceramic support

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  Figure 4  N₂ adsorption-desorption isotherm of Mo₇@ZIF-8 crystals

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  Figure 5  Adsorption of MG over Mo₇@ZIF-8 crystals. (a) Effect of time and initial MG concentrations on theadsorption capacities. (b) Adsorption isotherm. Plots of (c) pseudo-first-order and (d) pseudo-second-order kinetics models. Plots of (e) Langmuir and (f) Freundlich isotherm models (Temperature, 25 ℃; volume of dye solution, 100.00 mL; adsorbent, 0.100 g·L^-1)

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  Figure 6  (a) UV-Vis spectra of MG and MO aqueous solutions. (b) Selective adsorption of Mo₇@ZIF-8 crystals toward the mixed solution of MG and MO (Temperature, 25 ℃; initial concentration, 10 mg·L^-1; volume of dye solution, 100.00 ml; adsorbent, 1.500 g·L^-1)

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  Figure 7  (a) Nyquist plots of Mo₇@ZIF-8 membrane obtained at different temperatures under 98% relative humidity. (b) Arrhenius plots of the membrane

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