Dependence of Dye Molecules Adsorption Behaviors on Pore

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Surfaces, Interfaces, and Applications

Dependence of dye molecules adsorption behaviors on pore characteristics of mesostructured MOFs fabricated by surfactant template Xiaowei Zhang, Bangyun Xiong, Jingjing Li, Libing Qian, Lei Liu, Zhe Liu, Pengfei Fang, and Chunqing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06517 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 3, 2019

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ACS Applied Materials & Interfaces

Dependence of dye molecules adsorption behaviors on pore characteristics of mesostructured MOFs fabricated by surfactant template Xiaowei Zhang,a Bangyun Xiong,b Jingjing Li,b Libing Qian,a Lei Liu,a Zhe Liu,a Pengfei Fanga and Chunqing He a* aKey

Laboratory of Nuclear Solid State Physics Hubei Province, School of Physics and

Technology, Wuhan University, Wuhan 430072, China, bSchool

of Materials Science and Energy Engineering, Foshan University, Foshan 528000, China.

Key words: positron annihilation, dye, Langmuir adsorption, electrostatic attraction, MOFs, micropore and mesopores, template

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Abstract In this work, mesostructured metal organic frameworks (MOFs) of MIL-101-Crs with different specific surface areas were synthesized successfully under solvothermal conditions using cationic surfactant cetyltrimethyl ammonium bromide (CTAB) as a structural template. It was found that crystallinity degrees, specific surface areas and pore size distributions strongly depended on the loading of CTAB. Nitrogen adsorption and positron annihilation lifetime spectroscopy (PALS) results showed that the mean mesopore size increased with loading more CTAB due to the formation of larger templated mesopores. Although Langmuir adsorption of both methylene blue (MB) and methyl orange (MO) was confirmed in MIL-101-Crs, the experimental results showed different adsorption behaviors for them depending on dye molecular size, the pore structure and charge properties of dye molecules / MOFs in solution. The MB molecules were found to be mainly adsorbed in the interspaces between grains and the templated mesopores, whereas the MO molecules were adsorbed in the inherent pores as well as the templated ones in MOFs due to the unsaturated metal sites’ electrostatic attraction on them. Remarkably, MO adsorption capacity was observed to be proportional to the specific surface area, which allowed one to get a good linear fitting of experimental data. Interestingly, the good consistence between the fitting experimental parameter, i.e. the number of adsorbed MO-s per unit specific surface area, and the calculated one according to our rough estimation strongly suggests that MO-s are electrostatically attracted and rotating around the unsaturated metal sites on MOFs’ inner surfaces, which exclude other MO-s to be adsorbed around due to the “hindering effect” of the rotating motion.

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1.Introduction Nowadays, water pollution, especially dye and heavy metal pollution, has become a worldwide problem. In order to solve this problem, many methods such as chemical coagulation photodegradation, active sludge, trickling filters, adsorption and other ways have been developed to remove dyes from wasted streams.1-6 Among these methods, adsorption is regarded as the most effective technique due to its simple operation and low cost. Metal organic frameworks (MOFs), which are composed of metal ions and organic ligands connected by chemical bonds, are currently attracting considerable attentions for their potential applications in many fields.7-9 They can be applied in gas adsorption and gas separation,10-12 heterogeneous catalysis,13-15 molecular sensor,16-18 drug delivery,19-21 hydrogen storage22-25 and so on, due to their large specific surface areas, controlled regular structures and high free volumes. Recently, the application of MOFs in environmental governance has drawn much attention. Luo’ group synthesized La-doping UiO-66 and applied it in phosphate capture, and the phosphate adsorption capacity is as high as 348.43 mg/g.26 However, the unique structure of MOFs may restrict the entry of bulky dye molecules.27 On the other hand, the existence of small entrance slows down the diffusion of guest molecules in MOFs despite they can get inside.28 Hence, a well-interconnected system of MOFs is necessary for the applications involving bulky molecules and heterogeneous catalysis. In order to obtain mesoporous MOFs accessible for guest molecules, a way to elongate organic ligands has got some progresses. For instance, MOF-74 with regular pore size around 9.8 nm has been synthesized.29 Unfortunately, the instability is an inevitable challenge after the removal of the solvent from the newly developed MOFs. The successful way of template methods30 to synthesize mesostructured MOFs is almost available and feasible.31-41 This achievement leads to a great inspiration and results in further applications of MOFs. 3



Traditional characterization methods like transmission electron microscope (TEM) and gas adsorption-desorption are usually applied to investigate pore properties of MOFs. Recently, positron annihilation lifetime spectroscopy (PALS), with high sensitivity to defects and pores and non-destruction to samples, has been accepted as an effective probe to characterize pores in porous materials.42-47 Positronium (Ps) consists of a positron and an electron, i.e. a hydrogen-like atom with two states, which are spin-antiparallel para-positronium (p-Ps) and spin-parallel orthopositronium (o-Ps). The ratio of the number for p-Ps to o-Ps is 1:3. The intrinsic lifetimes for pPs and o-Ps in vacuum are 0.125 ns and 142 ns, respectively. However, the lifetime of o-Ps will be shortened after being localized in pores of condensed matters, and it is often determined by the pore size. Furthermore, o-Ps atoms can easily diffuse from small pores to large ones if they are interconnected due to the lower zeropoint energy of Ps atoms in large pores,48 and finally annihilate in the larger pores. This technique can provide information on the pore sizes and population of particular pores, even if the pores are isolated and/or inaccessible for gas molecules due to the small sizes and/or pore windows. Hence, PALS is an ideal technology to characterize porous materials, and o-Ps lifetime in them can be reliably correlated to the pore size using the Tao−Eldrup model (for pores smaller than 2 nm) and its extended model (for pores larger than 2 nm).49-51 Nevertheless, newly developed MOFs with various pores were seldom studied using PALS and only a few literatures can be found. 49-50, 52-56

MIL-101-Cr is an important member of MOF family with three types of inherent pores with ideal sizes of 1.26 nm, 2.9 nm and 3.4 nm.57 In this work, mesostructured MOFs of MIL-101-Cr with different specific surface areas were synthesized under solvothermal condition. The crystallinity, stability, potential of particle surface, specific surface area and pore structure were 4


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studied by X-ray diffraction (XRD), thermogravimetric analysis, zeta potential analyzer, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption and desorption at 77 K. PALS was applied to study the pore properties of various MIL-101-Crs and those adsorbed with dyes, respectively. Dependences of dye molecules adsorption in various MIL-101-Crs on the pore characteristics, pH of solutions, charge properties of dye molecules/MOFs in solution were investigated. Interesting results showed that Langmuir adsorption of methylene blue (MB) and methyl orange (MO) (the molecular formulas are shown in Scheme 1) occurred in MIL-101-Crs in spite of the different adsorption behaviors of them. A linear correlation between MO adsorption capacities in MIL-101-Crs and their specific surface areas strongly suggested a rotating motion of MO-s around the unsaturated metal sites on the pore surfaces of MIL-101-Crs.

2.Experimental 2.1 Synthesis of mesostructured MIL-101-Crs The precursor solutions were prepared as follows: 5 mmol Cr (NO3)3·9H2O (99.99%), 5 mmol 1,4-benzene dicarboxylic acid (H2BDC, 99.99%) and 0.2 ml hydrofluoric acid (wt%, 40%) were mixed in 24 ml deionized water and stirred for 5 minutes. After being stirred thoroughly, 0.5 mmol, 1 mmol, 1.5 mmol and 2 mmol cetyltrimethyl ammonium bromide (CTAB) were added in the solution, respectively. Then, the solutions were kept in Teflon-lined stainless-steel autoclaves at 220 ℃ for 8 hours. Green solids were obtained after cooling the solutions to room temperature. To remove the CTAB template, the resulting green solids were washed by DMF and EtOH for 3 times separately. After that, the products were dried at 150 ℃ in vacuum. The

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mesostructured MIL-101-Crs prepared with increasing the ratios of CTAB were named as sample A0 (prepared without CTAB), A1, A2, A3 and A4, respectively.

2.2 Characterization The crystallinities of all samples were evaluated by X-ray diffraction (XRD) (Bruker AXS D8FOCUS) with a CuKα source. Fourier transform infrared spectroscopy (FTIR5700) was applied in order to make sure that CTAB template was removed from the samples completely. The stabilities were studied by thermogravimetric analysis in nitrogen atmosphere. Scanning electron microscopy (SEM) was used to observe the particle morphology. Transmission electron microscopy (TEM, JEOL2010) was used to investigate the morphology of the mesostructured MIL-101-Crs. N2 adsorption and desorption measurements at 77 K were carried out using a JWBK122W static N2 adsorption instrument for the samples. The zeta potentials were measured by a nano ZS90 Zeta potential analyzer.

The MOF samples of MIL-101-Crs and those adsorbed with different dyes were measured in vacuum around 25 ℃ by PALS, which consists of a conventional fast-fast coincidence system. The time resolution (full width at half-maximum) is approximately 300 ps. The start and stop signals for positron annihilation are recorded by two gamma detectors. The total count of positron annihilation lifetime spectrum for each MOF sample was 106. The powder of MOF samples was pressed into slices by a tablet machine at pressure of 8 MPa and the thickness of each slice was around 1.5 mm. Two slices of the same MOF sample sandwiched a 20 µCi positron source (22NaCl), which was sealed with two thin Kapton films of 7 µm. The positron lifetime spectrum of single crystal Ni was used as a reference in order to subtract the source 6


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components of positron annihilation in Kapton films and 22NaCl. The positron lifetime spectra of all MOF samples and those adsorbed with different dyes were well decomposed into five or three components by LT routine58 after the source correction.

Ultraviolet and visible spectrophotometer was used to study MB and MO adsorption properties in the MOF samples. In the dye adsorption experiments, each sample (50 mg) was put in 50 mL aqueous solution of MB or MO at different initial concentrations. The MB or MO adsorption capacity qe of MOFs in a water solution can be expressed as,

qe 

(Ci  Ce )V m

(1)

where Ci is the initial MB or MO concentration, Ce is the concentration of residual MB or MO in the solution after their adsorption in MOFs, V is the volume of the MB or MO solution and m is the mass of selected MOF samples.

The adsorption capacities of MB and MO in MOFs were discussed according to either Langmuir or Freundlich isotherm adsorption models. Generally, Langmuir and Freundlich adsorption behaviors for an adsorbent fulfill the following equations respectively, Ce Ce 1   qe qm K L qm

lnqe 

lnCe  lnK F n

, (Langmuir)

(2)

, (Freundlich)

(3)

where Ce (mg/L) is the equilibrium concentration, qe (mg/g) is the equilibrium adsorption capacity for some concentration of MB or MO, qm (mg/g) is the maximum adsorption capacity,

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KL and KF are the equilibrium constants of Langmuir and Freundlich adsorption, respectively. In Eq. (3), n is the constant indicating the Freundlich adsorption intensity.

3.Results and discussions 3.1 Removal of CTAB and formation of mesostructured MOFs The FT-IR spectra and X-ray diffraction patterns for the synthesized MIL-101-Cr samples are shown in Figure 1. In Figure 1 (a), in comparison to CTAB, -CH stretching peaks could not be found in the range of 2750-3000 cm-1 in the FT-IR spectra for the MOF samples despite that the data was expanded 10 times for all samples, which confirms that CTAB was cleared out from the samples completely. In addition, a broad peak of –OH is observed for all samples around 3500 cm-1, which demonstrates that it is easy to adsorb water molecules for the MOF samples even if the samples were heated before testing. In Figure 1 (b), the characteristic peaks can been found at 5.45°, 6.16°, 8.72° and 9.35° for A0, and these diffraction peaks are absolutely in agreement with the standard values of the simulated PXRD patterns of MIL-101-Cr without other excrescent impurities.59-60 However, the intensities of the characteristic peaks for the mesostructured MOFs become weaker and weaker with loading more CTAB. It is difficult to distinguish the characteristic peaks for A4 and their intensities are rather low, which means worse crystallinity in A4. On the other hand, broadening of the peak width is noticed for the mesostructured MIL-101Crs in comparison with A0 prepared without CTAB, reflecting the presence of particles with smaller sizes. This result is verified by the SEM characterization. As shown in Figure 2, it can be found that the size of MOF particles becomes smaller and smaller with increasing the CTAB ratio. Regular octahedrons are observed with the size around 500 nm for A0, which are in good agreement with previous reports.60-61 For the SEM image of A4 as shown in Figure 2 (c), 8


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numerous nanoparticles with the sizes of various curvatures smaller than 100 nm aggregate into irregular shaped nanoparticles with rough exterior surfaces. Hence, the addition of CTAB in the precursor solution influences the aggregation of nanoparticles and leads to worse crystallinity of MIL-101-Cr. In the TEM images, highly organized and textured mesoporous structures with size around a few nanometers present in A2 and A4 as shown in Figure 2 (e) and (f). This confirms the successful synthesis of mesoporous MOFs with ordered larger mesopores, and the highly ordered structure for templated mesopores is found for the samples prepared with more CTAB porogen. The formation of templated mesopores destroyed the inherent crystal structure of MIL-101-Cr, which resulted in the worse crystallinity for all mesostructured MIL-101-Crs synthesized with CTAB. In order to study the stability of various MIL-101-Crs, thermogravimetric analysis was performed as shown in Figure 1 (c). It can be found the weight loss is around 30 % for all samples mainly caused by the desorption of H2O. The decomposition temperatures for A0, A1, A2, A3 and A4 are 357 ℃, 347 ℃, 341 ℃, 345 ℃ and 333 ℃, respectively. The decrement of decomposition temperature with the addition of CTAB suggests that the stability of MIL-101-Crs became slightly worse due to the destruction of MOFs’ inherent structure with the formation of templated pores by the introduction of CTAB porogen.

3.2 Specific surface areas, pore sizes and pore size distributions of MOFs Figure 3 displays the N2 adsorption-desorption isotherms at 77 K and the pore size distributions calculated from N2 adsorption for MOF samples. The N2 isotherms of MOF samples correspond to the type Ⅰ (Langmuir isotherm). Hysteresis loops are found around the highest relative pressure (P/P0=1) and gradually expand for MIL-101-Crs synthesized with loading more CTAB, due to N2 capillary condensation in open spaces between small particles as shown in Figure 2 (b) 9



and (c). As listed in Table 1, the BET specific surface area reaches 2891 m2/g with a pore volume of 1.45 cm3/g for A0. However, the BET specific surface area decreases obviously from 2891 m2/g to 1108 m2/g accompanied by the decrement of t-plot specific micropore surface area from 2590 m2/g to 953 m2/g with loading more CTAB. It is obvious that the total mesopores volume (Vmeso) of MOFs increases from 0.26 cm3/g to 0.41 cm3/g as well as the mean pore size D increases from 1.99 nm to 3.13 nm with the loading of CTAB, due to the formation of templated mesopores as shown in Figure 2 (e) and (f). However, one should keep in mind that the specific surface areas decreased because of the less inherent pores and more templated mesopores in them as shown in Figure 3 (b). It is noticed that there exist three types of inherent pores for all samples. Pore size distributions of synthesized MOF display the sizes of different inherent pores centering at 1.5 nm, 2.0 nm and 2.5 nm as shown in Figure 3 (b), which seem to be a little different from the theoretical values (1.3 nm, 2.9 nm and 3.4 nm) for MIL-101-Cr crystalline structure. The concentrations for the three kinds of inherent pores turn lower and lower for mesostructured MIL-101-Crs prepared with loading more CTAB. Remarkably, additional mesopores in the range from 4.0 nm to 6.5 nm for A1, A3 and A4 are found and their concentrations increase with loading more CTAB, indicative of the formation of larger templated mesopores.

In order to further investigate the pore properties, PALS experiments for the samples were performed. Table 2 lists the results of positron lifetime spectra calculated by LT routine and typical positron lifetime spectra for A0 and A4 are shown in Figure S1 (see the Supporting Information). The lifetime spectra for MOF samples were well resolved into five lifetimes with fitting variances between 0.95 and 1.08, including three long-lived o-Ps components. The first 10


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component τ1 fixed at 0.125 ns means p-Ps lifetime and the second one τ2 around 0.4 ns is free positron lifetime in MOFs. The long-lived lifetime τ3 around 2 ns represents o-Ps annihilation in the lattice defects of MOFs, and τ4, τ5 are attributed to o-Ps annihilation in micropores and mesopores/interspaces between grains, respectively. For A0, the pore diameters calculated from τ4 and τ5 are respectively 1.26 nm and 3.20 nm, which are entirely consistent with the inherent micropore size (1.26 nm) and inherent mesopore sizes (between 2.9 nm and 3.4 nm).57 This result indicates that PALS is a very useful probe for characterizing porous MOFs. The inherent micropores in size of 1.26 nm are expected to remain unchanged upon introducing the CTAB porogen during synthesization. Accordingly, τ4 is fixed at 10.37 ns for all mesostructured MIL101-Crs, representing the positron annihilation in the inherent micro pores of 1.26 nm. From τ5, the mean mesopore sizes calculated by Tao-Eldrup extended model for the mesostructured MIL101-Crs are 3.14 nm, 3.34 nm, 3.72 nm and 4.15 nm, respectively, which correspond to the mean sizes of the mesopores. Further, it can be found that the o-Ps intensities are in the order I5＞I4＞ I3, which indicates the interconnectivity of pores for all MOFs is well because the higher interconnectivity can facilitate o-Ps diffusion from smaller pores to larger pores. 48, 51, 62-63

3.3 Dependence of dye adsorption on pore properties and adsorbents in MIL-101-Crs Figure S2 (see the Supporting Information) displays the UV spectra of the parent MB solution and the solution after MB adsorption in MIL-101-Crs. The absorbance at 664 nm of the solution with the residual MB for A0 is a little smaller than that of the parent MB solution, suggesting that sample A0 could only absorb a very small quantity of MB. Then, the absorbance of the MB solution with MIL-101-Crs becomes smaller and smaller, indicating relatively more MB molecules could be adsorbed in MIL-101-Crs prepared with loading more CTAB. Figure 4 (a) 11



shows the residual MB concentrations after its adsorption in A0 and A4 at different pH. The residual concentration of MB maintains stable around 28 mg/L for A0 and 10 mg/L for A4, respectively. Compared with the initial MB concentration of 30 mg/L, a low quantity of MB can be adsorbed by A0 in the interspaces between grains. The ionizations of MB are complex in a water solution, as shown in Scheme 2. In a strong acid aqueous solution, MB+ and (MB+)n clusters will react with H+s by forming mono protonated dye cation MBH2+. Meanwhile, MBH2+ will react with OH- at pH over 7, thus it is favorable for the presence of more MB+s and aggregates of (MB+)n clusters in a water solution.64-66 Figure 4 (c) displays the zeta potentials at different pH values for A0 and A4. It can be found that the particle surfaces of A0 are positively charged, which causes the repulsion between the species related MB (MBH2+, MB+ and (MB+)n clusters) and the MOF particle surfaces, and stops the access of them to the inherent pores. Nevertheless, the zeta potential of A0 decreases with the increment of pH over 7, which indicates that the charge repulsion between the MOF particle surfaces and the species related to MB, i.e. MB+, (MB+)n and MBH2+, would become weaker and weaker. Hence, one might expect that it was easier for MB to get into the inherent pores through the pore windows. However, the existence of steric –CH3 groups of MB hinders MB+s and MBH2+s to diffuse in the pores because the sizes of pore windows for MIL-101-Cr are only 1.2 nm and 1.6 nm in diameter.57 In particular, it can be noticed that the MB concentration in a water solution with A0 remains unchanged at pH over 7. According to the equilibrium equation of MB, in a water solution with pH over 7, more aggregates of (MB+)n clusters would be formed and it is difficult for them to access to the inherent pores through the small pore windows because of their larger sizes. For A4, it can be found the zeta potential of A4 shows a similar tendency as a function of pH values but smaller than that of A0, and the reduction of the repulsion between the MOF surfaces and the MB 12


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species could result in some physical adsorption of them in the templated mesopores and the interspaces between grains in A4.

The UV spectra of parent MO solution and the solution after MO adsorption in MIL-101-Crs are shown in Figure S3 (see the Supporting Information). The absorbance is essentially 0 at 462 nm for the residual MO solution with sample A0, signifying that a large quantity of MO was adsorbed in A0. However, the absorbance becomes higher and higher for MIL-101-Crs prepared with the addition of CTAB, indicating that less and less MO molecules were adsorbed in them with decreasing specific surface areas. The residual MO concentration after its adsorption in A0 and A4 at different pH is displayed in Figure 4 (b). For pH＜7, the residual MO concentrations remain stable for both A0 and A4. The residual MO concentration for A0 is almost 0 mg/L, while it is 7.5 mg/L for A4. MIL-101-Cr contains numerous potential unsaturated chromium sites in trimeric chromium(III) octahedral clusters, which can be considered as Lewis acid sites in the structure.67 The charged MO-s would share electron pairs with the unsaturated chromium sites under electrostatic attraction in a water solution. Hence, the decrement of MO adsorption performance for A4 is probably due to the significant reduction of its specific surface area. Since more unsaturated chromium sites on inner surfaces of A0 are exposed to MO-s in solution, more MO-s can be adsorbed. In a water solution, MO can be ionized to MO- and Na+. Thus, MO- can react with H+ and form a MOH, which can be described by the equilibrium equation, as shown in Scheme 3. Accordingly, the distribution curves of MOH and MO- as a function of pH values were calculated from the above equilibrium equation with pKa=3.4,68 and are shown in Figure 4 (d). It can be found that the percent of MOHs is much higher than that of MO-s at a low pH value. Accompanied by the adsorption of MO-s on the pore surfaces, MOH would be ionized 13



into MO-s and H+s due to the ionization equilibrium. For pH＞7, the residual MO concentration increases for both A0 and A4 with increasing the pH of the MO solution. Considering the fact that MO-s are negatively charged in a water solution, the presence of OH-s would compete with MO-s to occupy the sites relevant to the unsaturated chromium on the inner surfaces of pores, which weakens the MO adsorption performance of A0 and A4. Actually, with the addition of pH, the decrement in positive potential of the particle surfaces for both A0 and A4 is due to OH-s’ occupation of some positive unsaturated chromium sites.

In order to elucidate the dyes adsorption behaviors in MIL-101-Crs, PALS experiments were further performed for those MIL-101-Crs adsorbed with MB and MO, respectively. The positron lifetime spectra of the MIL-101-Crs and those adsorbed with different dyes are shown in Figure S4 and S5 (see the Supporting Information), and the analyzed results were listed in Table 2. The lifetime spectra for MIL-101-Crs adsorbed with MO can be decomposed into three lifetimes with only one long-lived o-Ps component of 2.6 ns. The disappearance of τ4 and τ5 obviously demonstrated that MO molecules were adsorbed in both the inherent pores and the templated mesopores/interspaces between grains. Nevertheless, the lifetime spectra for MIL-101-Crs (A0 and A4) adsorbed with MB are resolved into five lifetimes with three long-lived o-Ps components, for which τ3 is around 2 ns for the free volumes, τ4 is 10.37 ns for the inherent micropores, and τ5 is o-Ps annihilation lifetime in mesopores as mentioned above. It can be found the intensities (I5) of o-Ps annihilation in the mesopores decreased significantly from 11.11 % and 8.49 % to 2.78 % and 2.95 % in A0 and A4 with adsorption of MB, respectively. Correspondingly, the mean mesopore sizes for A0 is essentially unchanged after MB adsorption, while that of A4 with more templated mesopores decreased from 4.15 nm to 2.67 nm. These results strongly suggest that 14


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only the interspaces between grains and the templated mesopores were occupied by MB. Thus, dye adsorption performance of MIL-101-Crs depends not only on dye molecular size, but also on the pore structure, specific surface area and charge properties of MOFs/dye molecules in a water solution.

3.4 Langmuir adsorption of dye molecules in MOFs In order to study MOFs’ adsorption performance, Langmuir and Freundlich isotherm adsorption models were utilized to fit the adsorption data based on the formulas (2) and (3), as shown in Figure 5. Table 3 and 4 list the fitting parameters according to Langmuir and Freundlich isotherms. For the isotherms fitting of MB adsorption, it is noticed that the fitting variance R2 for Freundlich adsorption isotherm ranges from 0.3802 to 0.9359, and R2 for Langmuir adsorption isotherm is close to 1. Simultaneously, for the isotherms fitting of MO adsorption, R2 for Freundlich adsorption isotherms varies from 0.8762 to 0.943, and R2 for Langmuir adsorption isotherms is also close to 1. The fitting results strongly imply that monolayer adsorption of MB and MO molecules occurred in the present MOFs, i.e. the adsorption of them in MIL-101-Crs is Langmuir type. Accordingly, the adsorption capacities of MB and MO were calculated according to Langmuir adsorption. The adsorption capacity qm of MB increases from 2.05 mg/g to 21.83 mg/g, and that of MO decreases from 196.08 mg/g to 64.10 mg/g with increasing the loading of CTAB for fabricating MIL-101-Crs. Figure 6 displays the relation between the specific surface area SBET and the MB adsorption capacity qm for MIL-101-Crs. Despite the fact that the specific surface area is large for A0, qm of MB is rather low because it is hard for MBH2+s/MB+s/(MB+)n clusters to get in the inherent pores due to the charge repulsion between the positive particle surfaces and them. As shown in the inset of Figure 6, qm of MB increases in MOFs prepared with 15



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more CTAB, which is attributed to the adsorption of MB in templated mesopores and/or interspaces between grains. However, as shown in Figure 7, the MO adsorption capacity in the MOFs was found to increase with the specific surface area, indicating that MO adsorption occurred mainly in either inherent pores and/or templated pores. Particularly, qm of MO in MIL101-Cr prepared without CTAB (A0) is the highest because of its abundant micropores.

According to the above results of PALS and dye adsorption, the adsorption of MB and MO in different pores of the MIL-101-Crs are summarized in Table 5. MB molecules can be only physically adsorbed in the interspaces between grains and the templated pores, not only because of the repulsion between the adsorbate and the positive charged particle surfaces of MOFs, but also because of the larger bulk sizes of MBH2+s/MB+s/(MB+)n clusters relative to the pore windows. On the contrary, MO-s can be easily adsorbed in different types of pores, particularly in the inherent pores.

3.5 Evidence of MO-s rotating around the unsaturated metal sites on surfaces As mentioned above, the MO adsorption capacity qm increases linearly with the specific surface area SBET of MIL-101-Crs. The data between SBET and qm can be well fitted with a line extrapolated to the origin of coordinate, suggesting qm is absolutely proportional to SBET. The linear fitting is expressed as in the following equation,

qm  k  S BET

(4)

where k=0.0619 mg/m2 is the coefficient of the linear function, which actually provides us the amount of adsorbed MO-s per unit specific surface area. Considering the molar mass of MO 16


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(M=327.33 g/mol) and Avogadro's constant NA, we can estimate the number of adsorbed MO-s per unit specific surface area is n0 

k N A  1.14 1017 / m 2 . M

On one hand, MO-s are electrostatically attracted on the unsaturated metal sites on pore surfaces of the MOFs in a water solution. On the other hand, the adsorption of MO is confirmed to be Langmuir type as discussed above. Given the MO-s are well arranged on the pore surfaces, the number of adsorbed MO-s per unit specific surface area is estimated69 to be 7.44×1018/m2, which is far beyond the experimentally fitted parameter 1.14×1017/m2. Further, considering the Langmuir adsorption of MO in MIL-101-Crs and the fact that MO-s are electrostatically adsorbed around the unsaturated metal sites on the pore surfaces of MOFs, adsorbed MO-s may move around the unsaturated metal sites on the inner surface of MOFs in a water solution. Thus, it’s reasonable to assume that the adsorbed MO-s rotate around the unsaturated metal sites freely as shown in Figure 8, and no other MO-s are likely adsorbed on the others in the “sweeping” areas because of the “hindering effect” of rotating MO-s. Hence, the number of adsorbed MO-s per unit surface area may be roughly calculated as follows,

n0 

1 d2

(5)

where d is the size of MO (1.54 nm).69 Accordingly, the number of adsorbed MO-s per unit surface area is calculated to be 1.34×1017/m2, which is interestingly in good agreement with that from the fitting parameter of the adsorption data. Therefore, the result strongly suggests that MOmolecules adsorbed in MIL-101-Crs in a water solution are electrostatically attracted and rotating around the unsaturated metal sites on the MOFs’ pore surfaces as depicted in the 17



schematic graph of Figure 8, and the “hindering effect” of rotating motion excludes other MO-s to be adsorbed on the unsaturated metal sites in the “sweeping” area.

4.Conclusion In summary, mesostructured MIL-101-Crs were synthesized via a solvothermal method using amphiphilic surfactant CTAB as a structure directing template. Accompanied by the decrement in the crystallinity degree, the stability, the BET specific surface areas and the total pore volumes of the MOFs were found to decrease in the MIL-101-Crs synthesized with more CTAB. The total mesopore volume and the average mesopore size increased obviously with loading more porogen, due to the formation of larger highly ordered mesopores as demonstrated by N2 adsorption and positron annihilation lifetime spectroscopy (PALS). Dependence of dye molecules (i.e. methylene blue and methyl orange) adsorption on the pore characteristics, charge properties of MOFs in solutions were investigated. The adsorptions of MB and MO molecules in MIL-101-Crs were Langmuir type. The MB adsorption in MOFs were found to occur in the templated pores/interspaces between grains, and be independent to the pH of solution. While, MO molecules were adsorbed in both the inherent pores and the templated pores of MOFs, and the adsorption capacity depended on the pH of the solution because of the weakened attractive interaction between the unsaturated metal sites and MO-s by OH-s’ occupation on them. Remarkably, it was found that MO adsorption capacity was proportional to the specific surface area and a good linear fitting of experimental data was obtained. Interestingly, the fitting experimental parameter (i.e. the number of adsorbed MO-s per unit specific surface area) had a good consistence with our rough estimation. This strongly suggests that MO-s are electrostatically attracted and rotating around the unsaturated metal sites on MOFs’ inner 18


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surfaces, and other MO-s cannot be adsorbed on the others nearby due to the “hindering effect” of the rotating motions of MO-s.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11875209, 11575130 and 11705029), and partly by the Fundamental Research Funds for the Central Universities under grant No.2042018kf0239. Thanks to Center for Electron Microscopy Wuhan University.

Supporting Information Available The following files are available free of charge. The contents of Supporting Information including: images of positron annihilation spectra for samples and those adsorbed with dyes, UV spectra of dye solution after adsorption in MOFs, dye adsorption capacities in different initial concentration and details of experiments.

19



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Figures

Figure 1 (a) FT-IR spectra for the samples and CTAB. The data of FT-IR spectra for the samples from 2750 cm-1 to 3000 cm-1 was expanded 10 times. Test condition: samples were heated by xenon lamp before testing; (b) Powder X-ray diffraction patterns for the samples. (c) Thermogravimetric analysis for the samples. 30


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Figure 2 SEM images of (a) A0, (b) A2 and (c) A4 and TEM images of (d) A0, (e) A2 and (f) A4.

31



Figure 3 (a) N2 adsorption and desorption isotherms at 77 K for the samples. Filled circles and open circles represent adsorption and desorption, respectively. (b) Pore size distributions from N2 adsorption for the samples calculated by DFT. The data of A1, A3 and A4 from 4.0 nm to 6.5 nm was increased by tenfold.

32


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Figure 4 The residual MB (a) and MO (b) concentrations at different pH after their adsorption in A0 and A4 with an elapsed time of 30 minutes. The initial concentration for MB or MO is 30 mg/L. (c) Zeta potentials at different pH values for sample A0 and A4, respectively. (d) The distribution curves of MOH and MO- in a water solution with different pH values. The lines are guided for eyes.

33



Figure 5 The fitting results of (a) Freundlich isotherms for MB, (b) Langmuir isotherms for MB, (c) Freundlich isotherms for MO and (d) Langmuir isotherms for MO.

34


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Figure 6 The relation between the specific surface area SBET and MB adsorption capacity qm. The inset is the relation between CTAB content and MB adsorption capacity qm. The dashed lines are guided for eyes.

Figure 7 The relation between the specific surface area SBET and MO maximum adsorption capacity qm. The solid line is a linear fitting of the experimental data. 35



Figure 8 Rotations of MO-s in MOFs in a water solution.

36


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Tables Table 1. The BET Specific Surface Area SBET, Specific Micropore Area Smicro, Adsorption Volume Vads, Mesopore Volume Vmeso and Mean Pore Size D for MOF Samples.

aS

micro

Sample

SBET (m2/g)

Smicro (m2/g) a

Vads (cm3/g) b

Vmeso (cm3/g) c

D (nm)

A0

2891

2590

1.45

0.26

1.99

A1

2624

2361

1.43

0.31

2.18

A2

1797

1669

1.05

0.26

2.33

A3

1576

1396

1.06

0.40

2.70

A4

1108

953

0.87

0.41

3.13

is the t-plot specific micro pore surface area.

bV ads

is determined from the adsorption

branch of the N2 isotherms at P/P0=0.99. cVmeso is calculated by subtracting Vmicro from total specific pore volume.

Table 2. The o-Ps Lifetimes and Intensities in Different MIL-101-Crs and Those Adsorbed with Dyes. Sample

τ3 (ns)

τ4 (ns)

τ5 (ns)

I3 (%)

I4 (%)

I5 (%)

D (nm)

A0

1.95±0.14

10.37±0.30

39.03±1.11

2.39±0.09

4.03±0.26

11.11±0.15

3.20

A1

2.51±0.22

10.37(Fixed)

38.00±0.34

0.94±0.06

2.90±0.06

11.07±0.17

3.14

A2

2.40±0.34

10.37(Fixed)

41.43±0.70

1.36±0.10

4.61±0.22

16.27±0.17

3.34

A3

2.40±0.18

10.37(Fixed)

47.20±2.40

2.22±0.09

3.44±0.19

5.77±0.08

3.72

A4

2.17±0.11

10.37(Fixed)

52.90±1.00

1.86±0.11

2.85±0.08

8.49±0.12

4.15

A0-MB

1.76±0.05

10.37(Fixed)

37.56±0.95

2.78±0.07

1.45±0.05

2.78±0.05

3.11

A4-MB

1.93±0.25

10.37(Fixed)

29.30±1.50

0.73±0.08

1.14±0.12

2.95±0.07

2.67

A0-MO

2.64±0.05

/

/

2.06±0.03

/

/

/

A4-MO

2.58±0.05

/

/

2.74±0.05

/

/

/

37



Page 38 of 40

Table 3. Parameters of Freundlich and Langmuir Isotherms for MB Adsorption in MIL101-Crs. Freundlich isotherms

Langmuir isotherms

kF (mg/g)

n

R2

qm (mg/g)

KL (L/mg)

R2

A0

1.02

7.44

0.9359

2.05±0.06

0.160

0.9971

A1

3.09

4.54

0.6205

7.58±0.32

0.551

0.9982

A2

5.08

4.63

0.3802

11.35±0.35

0.578

0.9962

A3

5.91

3.62

0.6428

16.56±0.36

0.963

0.9977

A4

8.34

4.08

0.8105

21.83±0.41

0.576

0.9986

Table 4. Parameters of Freundlich and Langmuir Isotherms for MO Adsorption in MIL101-Crs. Freundlich isotherms

Langmuir isotherms

kF (mg/g)

n

R2

qm (mg/g)

KL (L/mg)

R2

A0

35.0

2.60

0.8762

196.08±2.75

0.244

0.9988

A1

31.7

3.01

0.9021

153.85±1.29

0.293

0.9996

A2

21.4

3.25

0.9123

104.17±1.19

0.177

0.9992

A3

17.6

3.41

0.9430

92.59±2.62

0.071

0.9952

A4

10.4

2.87

0.9187

64.10±0.59

0.101

0.9995

Table 5. The Comparison of MB and MO Adsorption in A0 and A4 Mesopores

Samples

Inherent micropores

Inherent mesopores

Interspaces /Templated mesopores

A0-MB

No

No

Yes

A4-MB

No

No

Yes

A0-MO

Yes

Yes

Yes

A4-MO

Yes

Yes

Yes

38


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Schemes Scheme 1. The Molecular Formulas of MO and MB.

Scheme 2. The Equilibrium Equations between MB+, MBH2+ and (MB+)n in A Water Solution, Where n = 2 for A Dimer, n = 3 for A Trimer Etc.

Scheme 3. The Equilibrium Equation between MOH and MO- in A Water Solution, The Ionization Equilibrium Constant pKa=3.4.

39



Graphical TOC Entry

40


Page 40 of 40

Dependence of Dye Molecules Adsorption Behaviors on Pore

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