Effect of the synergetic interplay between the electrostatic interactions

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Effect of the synergetic interplay between the electrostatic interactions, size of the dye molecules, and adsorption sites of MIL-101(Cr) on the adsorption of organic dyes from aqueous solutions Wei Zhang, Run-Zhi Zhang, Yong-Qing Huang, and Ji-Min Yang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01340 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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Crystal Growth & Design

Effect of the synergetic interplay between the electrostatic interactions, size of the dye molecules, and adsorption sites of MIL-101(Cr) on the adsorption of organic dyes from aqueous solutions Wei Zhang†, Run-Zhi Zhang† Yong-Qing Huang‡ and Ji-Min Yang*,† †School ‡State

of Chemistry & Chemical Engineering, Linyi University, Linyi 276005, China

Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong

Province and the Ministry of Science and Technology, College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China

Abstract: Herein, we report that the surface charge status and size of the metal-organic framework (MOF) MIL-101(Cr) can be

tailored

by

varying

the

concentration

of

tetramethylammonium hydroxide. The adsorption capacity of MIL-101(Cr)-2 towards the anionic dye Congo red (CR) was found to be significantly higher than that of other MIL-101(Cr) adsorbents, although MIL-101(Cr)-3 microcrystals exhibited the highest adsorption capacity towards the anionic dye methyl orange (MO) due to the different adsorption mechanisms involved. This is the first demonstration of the synergetic interplay between the electrostatic and π-π interactions, adsorption spaces and sites, and size of the dye molecules, in terms of the uptake capacity of an adsorbent. In addition, the obtained results shed new light on the adsorption mechanism towards organic dyes over MOFs. Moreover, the

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MIL-101(Cr) microparticles exhibited excellent adsorption capabilities towards CR, achieving a maximum capacity of 1367.1 mg g−1, which is higher than the majority of reported values for other materials. Therefore, these results indicate that the MIL-101(Cr) nanostructure exhibits potential for application in the treatment of anionic dye-containing wastewater.

1.

INTRODUCTION Metal-organic frameworks (MOFs) are inorganic-organic hybrid materials comprising unique

combinations of metal ions and organic ligands.1-6 They commonly exhibit excellent solvent stabilities, tunable pore sizes, high specific surface areas, and accessible coordinatively unsaturated sites. As such, MOFs have received significant attention for application as adsorbents for the removal of hazardous materials.7-15 Hitherto, the mechanisms for the adsorption of organic dyes over MOFs have been investigated in the context of electrostatic and π-π interactions.9,14-19 For example, the uptake capacity of a UiO-66-P composite toward methylene blue (MB) was improved by increasing the electrostatic attractions through the incorporation of phosphate ions.16 In addition, Wang et al. reported a negatively charged POM@MIL-101 composite that can improve the adsorption performance towards cationic dyes,12 while Jin et al. reported positively charged MIL-68(In) for the adsorption of the anionic dye Congo red (CR).17 Although electrostatic attractions are key in the adsorption of organic dyes by these MOFs, it has also been found that some MOFs exhibit excellent adsorption capacities towards anionic and cationic dyes simultaneously. For example, Yao et al. reported that UiO-66 can effectively adsorb anionic and cationic dyes.18 MOF-235,19 MIL-100(Cr), and MIL-100(Fe)20 have been employed as


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adsorbents to remove MB and methyl orange (MO). Remarkably, these MOFs can be employed to simultaneously adsorb the cationic and anionic organic dyes, despite the significant difference in the charge characteristics of these dyes. This indicates that electrostatic interactions are not the only primary driving force for dye adsorption. Indeed, Ma et al. reported that the sizes and charges of organic dyes can also affect the adsorption performance,12 thereby suggesting that the adsorption performances of porous MOFs are not only closely associated with electrostatic interactions, but also with π-π interactions, the type/number of adsorption spaces and sites (i.e., pore size and volume), and the size of the organic dye molecules. An understanding into the synergistic relationship between these factors is, therefore, crucial to elucidate the dye adsorption processes and improve the adsorption performances of MOFs towards organic dyes. Therefore, we herein report the controlled preparation of MIL-101(Cr) truncated octahedra with different sizes and surface charges through the addition of different quantities of tetramethylammonium hydroxide (TMAOH) to the reaction system. Owing to the high specific surface areas and pore volumes, good water stabilities, and excellent adsorption properties, porous MOF nano/microparticles are widely used in wastewater treatment to enrich, adsorb, and separate large quantities of pollutants, such as organic dyes and heavy metal ions.21-27 The adsorption performances of porous MOF nano/microparticles are closely related to the adsorption spaces and sites available on the MOF nano/microcrystals, the surface charge status of the adsorbents, the size of the dye molecules, and the π-π interactions between the MOF benzene rings and aromatic backbones of the organic dyes. In order to investigate the synergetic interplay between the abovementioned factors in the



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context of organic dye removal from an aqueous solution, we selected CR, MO, and MB as model compounds for the adsorption experiments. As a representative porous MOF having a large surface area, extra-large pore sizes, exceptional pore volumes, and a high thermal stability, we selected [Cr3(OH)(H2O)2(μ3-O)(BDC)3]·nH2O (BDC = benzene-1,4-dicarboxylate), which is generally denoted as MIL-101(Cr).4 This MOF contains two types of mesoporous cages, namely dodecaedric and hexacaidecaedric cages, which have diameters of ~29 and 34 Å, and are accessible through five-membered (12 Å) and six-membered (16 Å) windows, respectively. MIL-101(Cr) is, therefore, expected to be a potential adsorbent for the treatment of dye-polluted wastewater due to its high surface area, large pore channels, and accessible coordinative unsaturated sites. In this work, we investigated the adsorbent properties of the resulting MIL-101(Cr) microcrystals toward anionic organic dyes (e.g., CR and MO), and examined the obtained trends using zeta potential and Brunauer–Emmett–Teller (BET) surface area measurements. Finally, the mechanisms of anionic organic dye adsorption by MIL-101(Cr) in the context of electrostatic interactions, π-π interactions, the sizes of the organic dyes, and the nature and content of adsorption sites, were confirmed.

2.

EXPERIMENTAL SECTION

2.1 Sample preparation Materials

and

chemicals:

Chromium

chloride

(Cr(NO3)3·9H2O,

99%)

and

tetramethylammonium hydroxide (TMAOH, 10% aqueous solution) were purchased from Aladdin Chemical Reagent Co., Ltd. Terephthalic acid (H2BDC, 98%) was obtained from Sinapharm Chemical Reagent Co., Ltd. All chemicals were analytically pure and were used as received without further purification.


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MIL-101(Cr) was synthesized according to a modified previously reported procedure.28 In a typical procedure, H2BDC (0.22 g) was added to an aqueous solution of TMAOH (20 mL, 0.01 mol/L) and allowed to stir for 15 min at room temperature. Cr(NO3)3·9H2O (0.5 g) was then added, and the mixture was stirred for a further 30 min. After this time, the mixture was transferred to a 25 mL Teflon-lined autoclave, which was sealed and kept at 180 °C for 24 h. The reaction vessel was then removed from the oven and allowed to cool to room temperature. The particles were isolated by centrifugation and washed three times with deionized water. The obtained particles were referred to as MIL-101(Cr)-1. The coordination polymer particles (designated as MIL-101(Cr)-0, MIL-101(Cr)-2, and MIL-101(Cr)-3) were synthesized according to the same procedure, but with the use of 0, 0.02, and 0.03 mol/L TMAOH, respectively. Prior to carrying out any measurements, the as-obtained MIL-101(Cr) particles were immersed in methanol for 3 d. The methanol was replenished daily, and after 3 d, the particles were dried under vacuum at 100 °C for 5 h. 2.2 Characterization Powder X-ray diffraction (PXRD) data were obtained on a Bruker D8 Advance X-ray diffractometer with CuKα radiation (λ = 1.5418 Å) at room temperature. Transmission electron microscopy (TEM) images were obtained on a JEM-2100 employing an accelerating voltage of 200 kV. UV-Vis spectra were collected on a UV-3600 UV-vis spectrophotometer (Shimadzu). Fourier-transform infrared (FT-IR) spectra were recorded in the 400–4000 cm-1 range on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. Thermogravimetric analyses (TGA) were performed on a Mettler-Toledo (TGA/DSC1) thermal analyzer under a nitrogen atmosphere with a heating rate of 10 °C min-1. The zeta potential was measured with a Zetasizer Nano Z (Malvern Instruments) at 25 °C. Nitrogen sorption experiments were performed at 77 K on a Micromeritics ASAP2460 volumetric gas sorption instrument. Surface areas were



determined by the Brunauer–Emmett–Teller (BET) equation. Ultra-high-purity N2 was used in the adsorption study. 2.3 Adsorption studies The adsorption kinetics and adsorption isotherms were investigated at 25 °C by batch experiments. For the batch tests, MIL-101(Cr)-2 (5 mg) was dispersed in a dye solution (50 mL) at a known initial concentration. After adsorption for a pre-determined time, the different dye concentrations were determined at a wavelength of 497 nm using a UV-vis spectrophotometer, and the amounts of the adsorbed dye (qe, mg g–1) were calculated according to the following equation: 𝑞e =

(𝐶0 - 𝐶e)𝑉 𝑚

where C0 and Ce (mg/L) are the initial and equilibrium CR concentrations, respectively, V (L) is the volume of the CR solution, and m (g) is the mass of MIL-101(Cr)-2). In order to investigate the effect of initial solution pH on the CR adsorption, the pH of the dye solution was adjusted with dilute aqueous HCl or NaOH, the contact time was 24 h, and the initial concentration of CR was 50 mg L–1. 2.4 Desorption and reusability experiments After the CR adsorption, MIL-101(Cr)-2 was thoroughly cleaned by repeated ultrasonication in ethanol and centrifugation, and subsequently activated by heating in vacuum at 100 °C for 10 h. The activated material was reused for the next adsorption.

3.

DISCUSSION

3.1 Preparation and characterization


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Figure 1a shows the PXRD patterns recorded for the MIL-101(Cr) microcrystals obtained using different quantities of TMAOH. The data imply the presence of a unique crystalline phase, and are in good agreement with the simulated pattern obtained from the single-crystal data.4 In addition, the FT-IR spectra of the different MIL-101(Cr) species were comparable, further confirming that they are isostructures of one another (Figure 1b). More specifically, the characteristic bands at 1621 and 1380 cm–1 correspond to the νas(-COO–) and νs(-COO–) stretching vibrations of the carboxylate moiety of BDC2–. In addition, analysis of the MIL-101(Cr)-0 and MIL-101(Cr)-2 microcrystals by thermogravimetric analysis (TGA) yielded two main weight loss steps between 30 and 550 °C (Figure 2a). The first weight loss, which occurred in the temperature range of 30–300 °C, was attributed to the loss of guest molecules, while the second was attributed to the decomposition of the framework. The two main weight losses for the MIL-101(Cr)-0 and MIL-101(Cr)-2 microcrystals occurred at the same temperature, thereby confirming that the heat stability of the MIL-101(Cr) microcrystals was not influenced by the addition of TMAOH.

Figure 1 (a) Experimental and simulated PXRD patterns and (b) FT-IR spectra of the MIL-101(Cr) microcrystals. The morphologies of the different MIL-101(Cr) species were then investigated by TEM.



Interestingly, we found that the sizes of the prepared samples were easily tailored upon the addition of the base modulator TMAOH to the reaction mixture. In the absence of TMAOH, the particles were truncated octahedral with average sizes of ~155 nm (Figures 3a and 3b). Upon increasing the concentration of TMAOH, MIL-101(Cr) microcrystals with average lengths of approximately 135, 125, and 110 nm were obtained (Figures. 3c–3h), and the truncated octahedral morphologies remained. It has previously been considered that the crystal size is significantly influenced by the deprotonation rate of H2BDC.29,30 More specifically, it has been reported that the crystal size can be easily tuned by varying the concentration of TMAOH, which, in turn, alters the deprotonation rate of H2BDC. Upon increasing the concentration of TMAOH, the rate of deprotonation accelerated, ligands immediately coordinated to the metal ions, the nucleation rate increased, and smaller particles were obtained.

Figure 2 (a) TGA curves, and (b) N2 adsorption-desorption isotherms of the MIL-101(Cr) microcrystals. In order to evaluate the influence of the particle size on the adsorption properties of MIL-101(Cr), the N2 sorption isotherms at 77K were investigated at different concentrations of TMAOH, as shown in Figures 2b and S1. All MIL-101(Cr) species showed characteristic Type I


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sorption isotherms, which is a typical behavior for microporous materials with high amounts of N2 adsorption. The MIL-101(Cr) sample prepared without TMAOH exhibited the smallest BET specific surface area of 3147.5 m2 g−1, while those prepared with TMAOH concentrations of 0.01, 0.02, and 0.03 mol L-1 exhibited BET specific surface areas of 3179.0, 3278.6, and 3181.4 m2 g–1, respectively. Furthermore, the average channel size of the four MIL-101(Cr) sample was found to be the same, suggesting that their chemical structures were uniform (Figures S2). However, MIL-101(Cr)-2 exhibited the largest N2 uptake capacity, which was attributed to its uniform size and coherent crystalline structure.

Figure 3 TEM images of: (a, b) MIL-101-0, (c, d) MIL-101-1, (e, f) MIL-101-2, and (g, h) MIL-101-3.

Figure 4 Adsorption uptakes of different organic dyes by MIL-101(Cr) after 24 h, as determined by dispersing 5 mg of MIL-101(Cr) in solutions of the organic dyes (50 mL, 20–500 mg L–1) at room temperature.



3.2 Adsorption and separation of organic dyes Figure 4 shows that MIL-101(Cr)-2 exhibits good adsorption capabilities toward CR and MO, with respective maximum capacities of 1367.1 and 455.2 mg g–1 (1.96 and 1.39 mmol g–1), respectively, that are significantly higher than those of other porous materials (Tables S1 and S2),10,17,31-39 demonstrating that MIL-101(Cr)-2 can be employed in water treatment and for the adsorptive removal of organic dyes. For an initial CR concentration of 500 mg L−1, the adsorption capacities of MIL-101(Cr)-0, MIL-101(Cr)-1, and MIL-101(Cr)-3 were determined to be 1109.1, 1204.3, and 1223.6 mg g−1 (i.e., 1.59, 1.73, and 1.76 mmol g–1), respectively (Figure 4a). Additionally, the adsorption capacities of MIL-101(Cr)-0, MIL-101(Cr)-1, and MIL-101(Cr)-3 toward MO were 389.9, 406.1, and 475.3 mg g−1 (i.e., 1.21, 1.24, and 1.45 mmol g–1), respectively (Figure 4b). The obtained results suggest that the removal behavior is organic dye-specific, being closely associated with the MOF properties and the dye molecular structures. Indeed, the investigated dyes have different chemical structures (Figure S3); the MB molecule is a cationic dye, whereas CR and MO are anionic. Moreover, CR contains a greater number of aromatic rings than MO, and the CR anion bears a greater negative charge than that of MO. These differences are significant as they can affect the electrostatic and π-π interactions between the MOFs and organic dyes. In addition, the addition of TMAOH resulted in changes to the electrical properties. In this case, the adsorption capacities of CR were significantly greater than those of MO, as the electrostatic and π-π interactions between MIL-101(Cr) and CR are stronger. Interestingly, the cationic dye (i.e., MB) could not be adsorbed by the MIL-101(Cr) microcrystals (Figure S4), and the adsorption capacities of MIL-101(Cr)-2 toward the anionic dye (i.e., CR) were significantly higher than those of MIL-101(Cr)-0, MIL-101(Cr)-1, and MIL-101(Cr)-3, with MIL-101(Cr)-3 exhibiting the largest MO adsorption capacity. The adsorption process is depicted in Figure 5.


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Figure 5 Schematic illustration of the proposed formation mechanisms of the MIL-101(Cr) microcrystals. In order to investigate and understand a plausible adsorption mechanism to explain this behavior, the zeta potential was measured (Figure S5). The zeta potential of MIL-101(Cr)-0 was determined to be 8.19 mV, and upon increasing the quantity of TMAOH added, the zeta potentials decreased to 6.37 and 5.69 mV for MIL-101(Cr)-1 and MIL-101(Cr)-2, respectively. In addition, upon increasing the TMAOH concentration to 0.03 mol/L, the zeta potential of MIL-101(Cr)-0 increased to 8.14 mV. These results indicate that the surfaces of the four MIL-101(Cr) microparticles were positively charged, and hence, MB, which tends to exist as a cation, is poorly adsorbed by MIL-101 (Cr) due to electrostatic repulsion. Although the surface positive charge of MIL-101(Cr)-2 was the smallest of the four MIL-101(Cr) microcrystals examined, it had the highest BET surface area, which resulted in greater numbers of adsorption spaces and sites. In addition, the adsorption of CR by this species was favored over MO due to the greater number of aromatic rings on CR and its larger size, which promoted strong π-π interactions between the benzene rings of the MIL-101(Cr) nanostructures and the aromatic CR backbone. The abovementioned results indicate that the number of adsorption spaces and sites, and the strength of π-π interactions are key to determining the CR adsorption behavior, with the



electrostatic interactions between the adsorbent and CR molecules being less significant. However, MIL-101(Cr)-3 adsorbed greater quantities of MO than MIL-101(Cr)-2, because the smaller molecular size of MO resulted in less space being occupied in the adsorbent than in the case of CR, the π-π interactions between MIL-101(Cr) and MO were weaker than those between MIL-101(Cr) and CR, and the MIL-101(Cr)-3 surface was more positively charged than that of MIL-101(Cr)-2. Therefore, the adsorption by MIL-101(Cr)-3 exceeded that of MIL-101(Cr)-2, thereby implying that the effect of the adsorption spaces and sites on the MO adsorption was less than that for CR. In addition, we also note the importance of the electrostatic interactions between the adsorbent and MO molecules. Although the highest surface positive charge was recorded for MIL-101(Cr)-0, this species contained the smallest amount of adsorption spaces and sites due to its lower BET surface area. Thus, the synergistic interplay of electrostatic and π-π interactions, the size of the dye molecules, and adsorption spaces and sites, cause the adsorption capacity of MIL-101(Cr)-0 toward CR and MO to be the smallest among the various adsorbents, thereby suggesting that the electrostatic interactions between the adsorbent and dye molecules were not the only primary driving force for adsorption. More specifically, the synergistic interplay between the electrostatic and π-π interactions, the size of the dye molecules, and adsorption spaces and sites affected the uptake capacity of the adsorbent. In addition, the number of adsorption spaces and sites, and the strength of π-π interactions were key to determining the adsorption behavior for the large molecular size of CR. Moreover, the electrostatic and π-π interactions between the adsorbent and MO were important to the uptake capacity of the MO organic dye, with small molecular size. Furthermore, the N2 sorption isotherms at 77 K were investigated after the CR adsorption (Figure S6). In this respect, MIL-101(Cr)-2@CR showed a BET specific surface area of 871.7 m2 g–1, which was much smaller than that of the MIL-101(Cr)-2 nanostructures (3278.6 m2 g–1). Also, the average channel size of


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MIL-101(Cr)-2@CR became much smaller than that of MIL-101(Cr)-2 (Figure S2), due to the adsorption sites and spaces occupied by CR in the pore channel of the MOFs.

Figure 6 (a, d) Pseudo-first order and (b, c, e, and f) pseudo-second order fits of the experimental adsorption kinetics data. Next, the effect of the initial solution pH on the adsorption of CR by MIL-101(Cr)-2 was investigated (Figure S7). Kinetics and isotherm measurement experiments were performed at pH 7, as this pH value gave the optimal adsorption capacities. The adsorption kinetics curves obtained for the investigated dyes are shown in Figure 6, where it is apparent that the correlation coefficients (i.e., the R2 values) for the pseudo-second order kinetics model were higher than those of the pseudo-first order model, and the calculated equilibrium adsorption capacity, qe,cal, was in good agreement with the experimental data (Tables S3 and S4). It is, therefore, apparent that the adsorption of CR and MO by MIL-101(Cr) was a pseudo-second order process, in which the organic dyes adsorbed more rapidly on MIL-101(Cr)-2 and MIL-101(Cr)-3 than on MIL-101(Cr)-0 and MIL-101(Cr)-1 (Figure 6), due to the effects of the adsorption spaces and sites, and the electrostatic attractions between the dye and sorbent. All adsorption kinetics constants are listed in Table S4. Taking into account the intraparticle diffusion model,41 the plot



of the amount of adsorbed dye on the absorbent (qt) versus the square root of time (t1/2) should be linear. If these lines are well fitted to the complete time range, the intraparticle diffusion could be determined to be the only rate-controlling step. However, in this case, the plots were nonlinear in the time range of 2–80 min, possibly separated into multilinear curves (Figure 7), suggesting that multiple stages were involved in the organic dye adsorption process. This indicated that the adsorption of the CR and MO organic dyes onto the MIL-101(Cr) sample included more than one process, and that the intraparticle transport was not the rate-limiting step.41 Furthermore, the equilibrium adsorption isotherms were of particular importance for understanding the adsorption process, and were therefore commonly employed to evaluate the adsorption capacities. In this respect, Figure 8 shows the equilibrium adsorption isotherms for CR. The experimental adsorption data for the adsorption of CR by MIL-101(Cr) was in good agreement with the Langmuir isotherm model, which provided higher correlation regression coefficients than the Temkin and Freundlich models (Table S3). Furthermore, Figure 8 shows the equilibrium isotherms for the MO adsorption, and demonstrates that they fit poorly to the Temkin and Freundlich models at concentrations of 20–500 mgL–1, while they were in good accordance with the Langmuir model (Table S4). In addition, according to the Langmuir model, the calculated maximum adsorption capabilities of MIL-101(Cr) towards CR and MO were 1377.4 and 467.3 mg g−1, respectively. All adsorption isotherm constants are listed in Tables S3 and S4, along with the Gibbs free energy changes (ΔG) for dye adsorption. In all cases, the Gibbs free energy change was negative, thereby suggesting spontaneous adsorption under the experimental conditions employed herein.


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Figure 7 Weber-Morris intraparticle diffusion plots for the adsorption of CR and MO on the MIL-101(Cr) nanostructures.

Figure 8 Plots of the experimental data for CR adsorption fitted with the (a, b) Themkin, (c, d) Freundlich, and (e, f) Langmuir isotherm models. Finally, the reusability of the as-obtained adsorbents was determined. The XRD patterns and nitrogen adsorption isotherms indicated that the MIL-101(Cr) structures were maintained after the consecutive adsorption–desorption cycles of adsorbing the organic dye molecules (Figures S8 and S9). Following four consecutive adsorption-desorption cycles, the uptake capacity of MIL-101(Cr)-2 toward CR decreased by

Effect of the synergetic interplay between the electrostatic interactions

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