Unsaturated Mn(II)-Centered [Mn(BDC)]n Metal–Organic Framework

Communication Cite This: Inorg. Chem. 2018, 57, 13075−13078

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Unsaturated Mn(II)-Centered [Mn(BDC)]n Metal−Organic Framework with Strong Water Binding Ability and Its Potential for Dehydration of an Ethanol/Water Mixture Kanyaporn Adpakpang,† Warat Pratanpornlerd,‡ Panyapat Ponchai,† Wararat Tranganphaibul,† Sutarat Thongratkaew,§ Kajornsak Faungnawakij,§ Satoshi Horike,∥,⊥,# Theeranun Siritanon,¶ Apinpus Rujiwatra,∇ Makoto Ogawa,† and Sareeya Bureekaew*,† Downloaded via UNIV STRASBOURG on November 15, 2018 at 13:18:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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School of Energy Science and Engineering and ‡School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, 555 Moo 1 Payupnai, Wangchan, Rayong 21210, Thailand § National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), 111 Thailand Science Park, Pahonyothin Road, Klong Laung, Pathumthani 12120, Thailand ∥ Institute for Integrated Cell-Material Sciences, Institute for Advanced Study, ⊥Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, and #AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ¶ School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand ∇ Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand S Supporting Information *

pore shape/size, hydrophilicity, etc., can be rationally controlled.10 In addition, some MOFs possess framework flexibility responsive to the specific adsorbate and coordinatively unsaturated metal sites (CUSs) providing a strong interaction with the guest molecule. Integrating all of the above-mentioned characteristics, MOFs can be interesting materials for efficient gas- and liquid-phase separations.11 Herein, liquid-phase water/ethanol separation using a manganese 1,4-benzenedicarboxylate (Mn-BDC) framework was studied. The framework possesses a tunnel pore structure similar to the well-known MIL-53 [Al(OH)(BDC)] and also other reported Mn-based frameworks.12 Therefore, the breathing behavior responsive to the adsorbed guest molecule is also expected, as observed in MIL-53. In addition, the Mn framework offers CUSs (Figure 1a), which further enhances the strong and specific guest−framework interaction. The thermal stability of [Mn(BDC)·nDMF]n was probed by thermogravimetric analysis (TGA). The TGA profile (Figure S1) shows three steps of weight loss. The weight loss at ∼150 and ∼250−300 °C corresponds to the removal of noncoordinated solvent (possibly DMF or water) and Mncoordinated DMF, respectively. This observation agrees very well with the crystallographic data, showing one coordinated DMF on the Mn center. The weight loss at ∼400−550 °C demonstrates that the framework is stable up to ∼400 °C. To study the stability of the framework toward water, the obtained [Mn(BDC)·nDMF]n crystals were immersed in deionized water. The crystallinity of the hydrated powder was then probed by powder X-ray diffraction (PXRD). Figure 1b shows the PXRD pattern of the hydrated product [Figure 1b(i)] compared to the as-synthesized and simulated patterns in parts b(ii) and b(iii) of Figure 1, respectively. The observed

ABSTRACT: An unsaturated Mn(II)-centered metal− organic framework was synthesized. The presence of an unsaturated Mn(II) center, together with a guestresponsive structural changing feature, plays a crucial role for strong binding with water, leading to its potential application for water/ethanol separation. In addition, the present framework is thermally stable up to 400 °C, which is beneficial for the regeneration process after adsorption.

A

lcohol is known as a cleaner energy source compared to fossil fuel. To reduce the use of fossil fuel, alcohol (especially ethanol) has been increasingly used in automobile engines.1,2 This leads to the large-scale production of ethanol. For the utilization of ethanol as an engine fuel, ethanol should be homogeneously mixed with gasoline, meaning that it should be anhydrous or contain a very low amount of trace water.1,3 Typically, large-scale production of ethanol can be done by fermentation of biomass. In this case, the obtained ethanol is not concentrated. It usually contains a high content of water, which limits its industrial use. To increase the concentration of ethanol, distillation is usually applied.4 However, highly dehydrated ethanol cannot be achieved via conventional distillation due to an azeotropic property at the ethanol concentration of 95.6 wt %.5 This leads to the employment of modified distillation with an additional process and/or chemicals requiring complex instrumentation and large energy consumption.6 As an alternative to distillation, adsorption using a porous adsorbent is considered to be an effective process to produce high-purity ethanol for such a purpose. Several porous materials, including zeolite,7 silica gel,6 graphene,8 etc., were reported as effective adsorbents, attributed to their selective pore shape/size and hydrophilicity.7−9 Accordingly, metal−organic frameworks (MOFs) gain much attention because their properties, including © 2018 American Chemical Society

Received: August 9, 2018 Published: October 16, 2018 13075

DOI: 10.1021/acs.inorgchem.8b02245 Inorg. Chem. 2018, 57, 13075−13078

Communication

Inorganic Chemistry

Figure 1. (a) Mn coordination of the synthesized [Mn(BDC)·nDMF]. Mn, C, and O atoms are shown in green, gray, and red, respectively. (b) PXRD patterns of the (i) hydrated and (ii) as-synthesized forms compared to (iii) simulation.

diffraction peaks of the hydrated product confirm its maintained crystalline nature. The cell parameters determined based on the Le Bail refinement method using TOPAS software (Figure S2) are a = 13.8481 Å, b = 9.9008 Å, c = 17.5110 Å, α = γ = 90°, β = 92.169°, and cell volume = 2399 Å3. Different from MIL-53, which has a distinct reduction of the cell volume upon hydration, hydrated Mn-BDC shows a slight change of the cell parameters and cell volume (Table S2), implying a larger amount of adsorbed water of the framework.13,14 TGA of the hydrated form was also conducted. As can be seen in Figure S1, the TGA profile of the hydrated Mn-BDC framework exhibits weight loss at 150 and 400 °C, corresponding to the removal of water (1.5 H2O per formula unit) and decomposition of the framework, respectively. No weight loss of Mn-coordinated DMF is observed in the hydrated sample. To probe the complete exchange of DMF with water, 1H NMR measurement on digested hydrated Mn-BDC was performed. As a result, no peak related to DMF was observed (Figure S3). Together with an alteration of the XRD pattern after the water exchange, the complete exchange between Mncoordinated DMF and water is ensured. Note that, different from DMF, coordinated (1 H2O) and uncoordinated (0.5 H2O) water are not distinguishable from those in the TGA profile. This result agrees with the PXRD evidence that a small change in the cell parameters of the hydrated Mn-BDC framework is attributed to the accommodated water of 1.5 mol equiv of the Mn(II) center that is different from MIL-53, whose structure changes to accommodate water of 1.0 mol equiv of the metal centers.12a This ensures that the present framework is highly hydrophilic, and its structure can change to adapt to new accommodated water molecules. Therefore, ethanol dehydration over the framework through rehydration was speculated. To support the XRD results for alteration of the framework structure, the extended X-ray absorption fine structure (EXAFS) measurement of the hydrated and activated Mn-BDC was conducted to clarify the local geometry of the Mn(II) (Figure 2). The EXAFS data were fitted using the model structure derived

Figure 2. Mn K-edge EXAFS data of (a) hydrated and (b) activated Mn-BDC frameworks. Mn, C, OBDC, and Owater are shown in green, gray, red, and blue, respectively. The experimental and fitted data are shown as circles and solid lines, respectively.

from the single-crystal data of the as-synthesized Mn-BDC. Detailed fitting information is presented in Table S3. The Mn Kedge EXAFS data of the hydrated framework (Figure 2a) exhibit the six coordination of the Mn(II) ion, confirming that the Mn(II) ion can maintain its octahedral geometry after an exchange of DMF with water. This observation supports the expectation that water coordinates with the Mn(II) ion at the position where DMF occupied. The bond lengths of Mn−Owater, Mn−O (C), and Mn−O (Mn) are 2.219, 2.135, and 2.354 Å, respectively. Alternatively, the activated form shows a decreased Fourier-transformed (FT) amplitude compared to that of the hydrated one. According to an EXAFS equation, the FT amplitude is a function of the amplitude reduction factor (S02) and coordination number. Considering the constant S02 under the similar conditions of the EXAFS measurement, the reduced FT amplitude of the activated form is due to the decrease in the coordination number of the Mn(II) center. The good fitted data of the activated form were obtained by using the five O atoms coordinated to the Mn atom from the model (Figure 2b). While the coordination number decreases to 5, the bond lengths of Mn−O (C) and Mn−O (Mn) remain as 2.179 and 2.423 Å, respectively. This indicates that, after the activation, the Mn(II) ion possesses uncapped square-pyramidal geometry, providing one accessible CUS per one Mn(II). The local geometry information on the Mn(II) ion evidences that the hydrated MnBDC retains its coordination manner after the solvent exchange of DMF and water. After the removal of water, the Mn-BDC with an accessible CUS is expected to rapidly bind with a water molecule. 13076

DOI: 10.1021/acs.inorgchem.8b02245 Inorg. Chem. 2018, 57, 13075−13078

Communication

Inorganic Chemistry To investigate the behavior of the framework upon the interaction of water and ethanol, the vapor adsorption isotherm was measured. Figure S4 shows the water and ethanol vapor adsorption isotherms of the Mn-BDC measured at 298 K. The water adsorption isotherm (red line) possesses a two-step adsorption. The adsorption capacity at the first step corresponds to a 1:1 molar ratio of water to the Mn(II) center. This can be strong evidence for the preferential adsorption of water at the Mn(II) CUS. After complete adsorption of water at the Mn(II) site, the adsorption of water proceeds at higher relative pressure because of a pore-filling behavior. This stepwise adsorption isotherm indicates structural flexibility due to a breathing behavior of the present framework upon the interaction with water. Note that the activated Mn-BDC is in the closed form, as confirmed by the N2 sorption isotherm (Figure S5). Expansion of the closed form of the activated Mn-BDC allows guest molecules that have strong interaction with the framework to diffuse into the framework, eventually resulting in the open form. The water adsorption result agrees well with the TGA result that the amount of adsorbed water is greater than the Mn(II) CUS, ascribed to the framework flexibility. In contrast, ethanol adsorption shows significantly lower uptake (blue line). MnBDC with a pore size of 1.0 × 1.8 nm (the as-synthesized phase) exhibits different adsorption behavior for water (with a size of 0.28 nm) and ethanol (with a size of 0.4 nm). Regardless of the selective pore size of the framework, its interaction with water is superior to that with ethanol despite the smaller size of ethanol compared to that of the pore channel of the open form. This may be attributed to its structural flexibility, which is adjustable upon the different response to water or ethanol. Also, the presence of the Mn(II) CUS leads to selective binding with water. On the basis of this evidence, the present Mn-BDC framework is expected to have a high potential for dehydration of ethanol via the selective adsorption of water from a water/ethanol mixture in the liquid phase. To verify the potential application of Mn-BDC for dehydration of alcohol, an experiment on the selective adsorption of water from an ethanol/water mixture at room temperature was conducted. The hydrated Mn-BDC powder was first activated under vacuum at 150 °C prior to injection of an ethanol mixture (ca. 95 and 97.5 vol %). Then, the solution was collected and quantitatively analyzed by gas chromatography. According to adsorption at different reaction times (Figure S6), the framework can adsorb water rapidly. The Mn-BDC framework can adsorb 0.13 and 0.12 g of water/g of MOF at water concentrations of ca. 5 and 2.5 vol %, respectively. The adsorption capacities were 82 and 75% of the capacity calculated from the TGA result (0.16 g of water/g of MOF). In addition, the present framework shows a good reusability. By activation of the used framework materials at 150 °C for 5 h, the adsorption capacities of 0.11 and 0.10 g of water/g of MOF were obtained for the second and third cycles, respectively (Table S4). The adsorption capacity of the Mn-BDC, compared to other adsorbents examined under similar condition, is presented in Table 1. At similar activation conditions, the adsorption capacity of the Mn-BDC is higher than those of activated carbon (AC), activated alumina (Al2O3), and silica gel. This may due to their large pore sizes, which can allow coadsorption of ethanol with water, leading to their poor water adsorption selectivity at the present conditions. The adsorption capacities at water contents of ca. 5 and 2.5 vol % are comparable to those of zeolite 3A (3A). To achieve higher water adsorption capacity, 3A requires an

Table 1. Adsorption Ability of Water by the Present Mn-BDC Framework Compared to Those of Other Adsorbents adsorbent

Co water (%vol.)

capacity (g of water/g of adsorbent)

Co water (%vol.)

capacity (g of water/g of adsorbent)

Mn-BDC AC Al2O3 silica gel 3A 3A (ht)

4.93 5.18 5.16 5.19 5.19 5.25

0.13 N/A 0.01 0.01 0.13 0.17

2.01 2.55 2.53 2.59 2.59 2.66

0.12 N/A N/A N/A 0.10 0.17

activation at much higher temperature (500 °C, 8 h) compared to the Mn-BDC needed in activation/regeneration. In summary, the structurally flexible Mn-BDC with abundant unsaturated Mn(II) centers was synthesized. The framework is thermally stable up to 400 °C and stable toward water. Moreover, its hydrophilicity is highly active for dehydration of an ethanol/water mixture. Beneficial from its hydrophilicity attributed to abundant CUSs that can strongly bind with water, the framework can efficiently selectively separate water out of ethanol. In addition, activation or regeneration for repeated use can be done at low temperatures (150 °C), showing a great potential to be used as a desiccant. This finding highlights the usefulness of Mn-based MOF possessing CUSs for water/ ethanol separation. This provides information for the further improvement of membrane and/or energy technologies.

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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.8b02245. Detailed information for the synthesis, characterization, and water adsorption capacity (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.B.). Tel.: (+66)33014256. ORCID

Kajornsak Faungnawakij: 0000-0002-4724-0613 Satoshi Horike: 0000-0001-8530-6364 Apinpus Rujiwatra: 0000-0002-2364-4592 Makoto Ogawa: 0000-0002-3781-2016 Sareeya Bureekaew: 0000-0001-9302-2038 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a student grant and a postdoctoral fellowship from the Vidyasirimedhi Institute of Science and Technology, by the Thailand Research Fund (Grant RSA6080068), and by the Research Chair Grant 2017 (Grant FDA-CO-2560-5655) from the NSTDA, Thailand. The authors thank Dr. Suttipong Wannapaiboon (Multiple X-ray Techniques Beamline, BL1.1W) and Dr. Pinit Kidkhunthod (SUTNANOTEC-SLRI XAS Beamline, BL5.2) from the Synchrotron Light Research Institute, Thailand, for assistance on the XAS measurement and analysis and “Smart Materials Research Center” of Kyoto University−VISTEC collaboration. 13077

DOI: 10.1021/acs.inorgchem.8b02245 Inorg. Chem. 2018, 57, 13075−13078

Communication

Inorganic Chemistry

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(13) Munn, A. S.; Ramirez-Cuesta, A. J.; Millange, F.; Walton, R. I. Interaction of methanol with the flexible metal-organic framework MIL-53(Fe) observed by inelastic neutron scattering. Chem. Phys. 2013, 427, 30−37. (14) Mowat, J. P. S.; Miller, S. R.; Slawin, A. M. Z.; Seymour, V. R.; Ashbrook, S. E.; Wright, P. A. Synthesis, characterisation and adsorption properties of microporous scandium carboxylates with rigid and flexible frameworks. Microporous Mesoporous Mater. 2011, 142, 322−333.

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DOI: 10.1021/acs.inorgchem.8b02245 Inorg. Chem. 2018, 57, 13075−13078