Control of the Coordination Status of the Open Metal Sites in Metal

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Control of the Coordination Status of the Open Metal Sites in Metal−Organic Frameworks for High Performance Separation of Polar Compounds Yan-Yan Fu, Cheng-Xiong Yang, and Xiu-Ping Yan* State Key Laboratory of Medicinal Chemical Biology, and Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) with open metal sites have great potential for enhancing adsorption separation of the molecules with different polarities. However, the elution and separation of polar compounds on such MOFs packed columns using nonpolar solvents is difficult due to too strong interaction between polar compounds and the open metal sites. Here, we report the control of the coordination status of the open metal sites in MOFs by adjusting the content of methanol (MeOH) in the mobile phase for fast and high-resolution separation of polar compounds. To this end, highperformance liquid chromatographic separation of nitroaniline, aminophenol and naphthol isomers, sulfadimidine, and sulfanilamide on the column packed with MIL-101(Cr) possessing open metal sites was performed. The interaction between the open metal sites of MIL-101(Cr) and the polar analytes was adjusted by adding an appropriate amount of MeOH to the mobile phase to achieve the effective separation of the polar analytes due to the competition of MeOH with the analytes for the open metal sites. Fourier transform infrared spectra and X-ray photoelectron spectra confirmed the interaction between MeOH and the open metal sites of MIL-101(Cr). Thermodynamic parameters were measured to evaluate the effect of the content of MeOH in the mobile phase on the separation of polar analytes on MIL-101(Cr) packed column. This approach provides reproducible and high performance separation of polar compounds on the open metal sites-containing MOFs.

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INTRODUCTION Metal−organic frameworks (MOFs), constructed by metal ions and organic motifs, have attracted tremendous attention because of their fascinating structures, and intriguing potential applications in gas storage,1,2 catalysis,3 sensing,4 separation,5 and biomedical research.6,7 The large surface area, high adsorption affinity, diverse structures and pore topologies, accessible cages, and tunnels make MOFs promising as a stationary phase for chromatography.8 Recently, MOFs such as HKUST-1,9−11 MOF-5,12 MIL-53,13−17 MIL-47,18−21 MIL101,22,23 and ZIF-824 have been used as stationary phases for liquid chromatography, while MOF-508,25 MIL-101,26,27 and ZIF-828,29 are used as stationary phases for gas chromatography. However, most of these pioneering works regarding the utilization of MOFs as the stationary phases in chromatography have focused on nonpolar analytes, even though polar compounds, such as drug metabolites or intermediates, have © 2012 American Chemical Society

great importance in chemical and pharmaceutical industries. Thus, exploring the potential of MOFs for separating polar compounds is of great significance. MOFs with open metal sites have great potential for enhancing adsorption separation of the molecules with different polarities.30 However, the elution and separation of polar compounds, especially polar positional isomers, on the column packed with open metal sites-containing MOFs using nonpolar solvents as the mobile phase in liquid chromatography is difficult because of a too strong interaction between polar compounds and the open metal sites of MOFs. To achieve high-performance separation of polar compounds on the MOFs containing open metal sites, an appropriate Received: October 20, 2011 Revised: March 28, 2012 Published: April 5, 2012 6794

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Figure 1. Characterization of the prepared MIL-101(Cr), MIL-100(Fe), and MIL-53(Al): (a−c) XRD patterns (experimental, black; simulated, red); (d−f) TGA curves; (g−i) SEM images.

modification of MOFs is of tremendous necessity to tune the interaction between the polar analytes and the open metal sites in MOFs. Conventional postsynthetic modification of MOFs31−34 is likely a method of choice, but it is time-consuming, of high-cost, and therefore not suitable for continuously tuning the properties of MOFs-stationary phase. Here, we report the control of the coordination status of the open metal sites in MOFs by adjusting the content of methanol (MeOH) in the mobile phase for fast and high-resolution separation of polar compounds. For this purpose, MIL-101(Cr) was used as the stationary phase for high-performance liquid chromatography (HPLC). MIL-101(Cr) is built up from a hybrid supertetrahedral building unit, which is formed by terephthalate ligands and trimeric chromium octahedral clusters, possessing high surface area, large windows (12 Å and 16 Å × 14.5 Å), mesoporous pores (29 and 34 Å), excellent chemical and solvent stability, and open metal sites after evacuation (Figures 1 and S1).35−40 Nitroaniline, aminophenol and naphthol isomers, sulfadimidine, and sulfanilamide were chosen as the target polar compounds because they are important intermediates in chemical and pharmaceutical industries, and they are also toxic to the environment.41−48 In this work, the interaction between the open metal sites of MIL-101(Cr) and the polar analytes was adjusted by adding an appropriate amount of MeOH to the mobile phase for the efficient separation of polar compounds. Thus, baseline separation of nitroaniline, aminophenol and naphthol isomers, sulfadimidine, and sulfanilamide was achieved on the slurry-packed

MIL-101(Cr) column. Thermodynamic approach in combination with Fourier transform infrared spectrometry (FT-IR) and X-ray photoelectron spectroscopy (XPS) was used to elucidate the mechanism involved in the present study. MIL-100(Fe), another MOF containing open metal sites, was also used to demonstrate the universality of the present approach.

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EXPERIMENTAL SECTION

Materials and Chemicals. All chemicals and reagents used were at least of analytical grade. Ultrapure water (18.2 MΩ cm) was obtained from a WaterPro Water Purification System (Labconco Corp., Kansas City, MO). Cr(NO3)3·9H2O, Al(NO3)3·9H2O, Fe powder, terephthalic acid, trimesic acid, p-xylene, sulfadimidine, sulfanilamide, and hydrofluoric acid (40.0%) were purchased from Shanghai Aladdin Chemistry Co. Ltd. (Shanghai, China). Nitric acid (65−68%) was purchased from Tianjin Chemical Reagent No. 5 Plant (Tianjin, China). Nitroaniline, aminophenol and naphthol isomers, dichloromethane (DCM), isobuthanol (IBA), and N,N-dimethylformamide (DMF) were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). MeOH and acetonitrile (ACN) were purchased from Kangkede Fine Chemical Research Institute (Tianjin, China). Instrumentation. The X-ray diffraction (XRD) patterns were recorded with a D/max-2500 diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5418 Å). The TGA experiments were performed on a PTC-10A thermal gravimetric analyzer (Rigaku, Japan) from room temperature to 800 °C at a ramp rate of 10 °C min−1. The scanning electron microscopy (SEM) images were recorded on a Shimadzu SS-550 scanning electron microscope at 15.0 kV. Fourier transform infrared (FT-IR) spectra (4000−400 cm−1) were obtained 6795

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where t is the retention time, and t0 is the column void time, which was determined by injecting a small plug of hexane and recording the perturbation signal. Φ was calculated according to eq 4:

on a Magna-560 spectrometer (Nicolet, Madison, WI) in KBr plate. X-ray photoelectron spectroscopy (XPS) measurements were performed on an Axis Ultra DLD (Kratos Analytical Ltd. Britain). BET surface area was measured on an ASAP 2010 micropore physisorption analyzer (Micromeritics, Norcross, GA) using nitrogen adsorption at 77 K in the range 0.02 ≤ P/P0 ≤ 0.20, respectively. All HPLC separations were performed on a chromatographic system consisting of a Waters 510 HPLC pump and a 486 tunable absorbance detector. Data acquisition and processing was carried out on a N2000 chromatography data system. The Ameritech CO-5060 column heater was used to control the column temperature during HPLC separation. Synthesis of MIL-101(Cr), MIL-100(Fe), and MIL-53(Al). MIL101(Cr) was synthesized according to Férey et al.35 Typically, Cr(NO3)3·9H2O (800 mg, 2.0 mmol), terephthalic acid (332 mg, 2.0 mmol), and hydrofluoric acid (0.1 mL, 2.0 mmol) were mixed with 9.6 mL of ultrapure water in a Teflon-lined bomb. The bomb was then sealed and placed in an oven and heated at 220 °C for 8 h. The green solid was thus obtained. After being washed with DMF, the solid was washed with ethanol and collected by centrifugation at 10 000 rpm for 5 min. The procedure was repeated three times. Finally, the MIL101(Cr) solid was evacuated in a vacuum at 150 °C for 12 h to form dehydrated MIL-101(Cr). MIL-100(Fe) was hydrothermally synthesized according to Férey et al.49 Typically, 687.5 mg of trimesic acid, 277.5 mg of iron powder, 25 mL of ultrapure water, 200 μL of hydrofluoric acid, and 190 μL of concentrated nitric acid (1.0Fe:0.671,3,5-BTC:2.0HF:0.6HNO3:277H2O) were added in turn in a Teflon-lined bomb. The bomb was heated to 150 °C and kept for 12 h. The light orange solid product was recovered by filtration and washing with ultrapure water. The as-synthesized MIL-100(Fe) was further purified by two-step processes using hot water and ethanol. To decrease the amount of residual unreacted substances, the solid was immersed into water at 80 °C for 5 h and subsequently hot ethanol at 60 °C for 3 h until there was no detection of colored impurities in the mother liquor solution. The highly purified MIL100(Fe) was obtained. Finally, the MIL-100(Fe) solid was evacuated in a vacuum at 150 °C for 12 h to form activated MIL-100(Fe). MIL-53(Al) was synthesized under hydrothermal conditions.50 Typically, Al(NO3)3·9H2O (275 mg) and terephthalic acid (83 mg) were mixed with 14.4 mL of ultrapure water. The mixture was transferred to a 23 mL Teflon-lined bomb. The bomb was then sealed and placed in an oven and heated at 220 °C for 3 days. After being filtered and washed with ultrapure water, the resulting white product was ultrasonicated in ethanol to remove the unreacted acid. Finally, the MIL-53(Al) solid was evacuated in a vacuum at 330 °C for 3 days to form activated MIL-53(Al). Preparation of MIL-101(Cr), MIL-100(Fe), and MIL-53(Al) Packed Columns. 1.30 g of MIL-101(Cr), MIL-100(Fe), or MIL53(Al) was dispersed in 50 mL of DCM under ultrasonication for 5 min. The suspension was then packed into a stainless steel column (5 cm long × 4.6 mm i.d.) under 6000 psi for 10 min. The packed column was conditioned with DCM at a flow of 0.5 mL min−1 for 1 h on the HPLC apparatus before chromatographic experiments. Calculation of the Thermodynamic Parameters. Gibbs free energy change (ΔG, kJ mol−1), enthalpy change (ΔH, kJ mol−1), and entropy change (ΔS, J mol−1 K−1) for the transfer of the analytes from the mobile phase to the stationary phase MIL-101(Cr) were calculated from the van’t Hoff equation (eqs 1 and 2).

ln k′ = −

ΔH ΔS + + ln Φ RT R

ΔG = ΔH − T ΔS

Φ = VS/V0

where VS is the volume of the stationary phase in the column and was calculated on the basis of eq 5, while V0 is the void volume of the column and was evaluated according to eq 6.

VS = VCOL − V0

(5)

V0 = t0 × F

(6)

where VCOL is the geometrical volume of the column, and F is the flow rate of the mobile phase.

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RESULTS AND DISCUSSION The synthesized MIL-101(Cr), MIL-100(Fe), and MIL-53(Al) were characterized by XRD, TGA, and SEM. The XRD patterns of the synthesized MIL-101(Cr), MIL-100(Fe), and MIL53(Al) were in good agreement with the corresponding simulated one, indicating the successful preparation of MIL101(Cr), MIL-100(Fe), and MIL-53(Al) (Figure 1a−c). The TGA curves reveal that the MIL-101(Cr), MIL-100(Fe), and MIL-53(Al) frameworks were stable up to 300, 320, and 330 °C, respectively (Figure 1d−f). The SEM images show the cubic shaped crystals of MIL-101(Cr) and MIL-100(Fe), and lozenge shaped crystals of MIL-53(Al) (Figure 1g−i). The BET surface areas of MIL-101(Cr), MIL-100(Fe), and MIL-53(Al) were determined to be 2907, 1598, and 904 m2 g−1, respectively. An appropriate amount of MeOH was added to the mobile phase to control the interaction between the open metal sites of MIL-101(Cr) and the polar analytes for the efficient separation of polar compounds. To this end, nitroaniline isomers were first tested for HPLC separation on the MIL-101(Cr) packed column. Because of strong interaction between nitroaniline isomers and the open metal sites of MIL-101(Cr), all of the nitroaniline isomers were not eluted with pure DCM as the mobile phase even in 50 min (Figure 2A, a). Inclusion of 0.2% v/v MeOH in DCM led to the elution of all nitroaniline isomers within 22 min, but no resolution of m-nitroaniline and onitroaniline, and a very broad peak of p-nitroaniline (Figure 2A, b). An increase in the content of MeOH in the mobile phase to 1.3% v/v resulted in a significant decrease of the retention time and a baseline separation of all nitroaniline isomers (Figure 2A, c). However, further increase in the content of MeOH in DCM (≥6% v/v) led to almost coelution of nitroaniline isomers due to the overhold of the open metal sites (Figure 2A, d−f). The fact that all nitroaniline isomers were baseline separated using DCM/ MeOH (98.7:1.3) as the mobile phase indicates a suitable amount of MeOH in the mobile phase could control the interaction between nitroaniline isomers and MIL-101(Cr) for efficient separation of nitroaniline isomers. Baseline separation of aminophenol and naphthol isomers, and sulfadimidine and sulfanilamide, was also achieved by controlling the content of MeOH in the mobile phase (Figure 2B−D). The results show the feasibility of the use of MeOH to control the interaction between MIL-101(Cr) and the analytes for the efficient separation of polar compounds (Figure 3). This approach gave good reproducibility for the HPLC separation of the polar compounds on MIL-101(Cr) packed column (Figure 3). The relative standard deviations for 11 replicate separations of the polar compounds were 0.08−0.61%, 0.65−2.4%, and 0.33−2.4% for the retention time, peak height, and peak area, respectively (Table S1).

(1) (2)

where k′ is the retention factor, R is the gas constant, T is the absolute temperature, and Φ is the phase ratio. k′ was calculated according to eq 3: k′ = (t −t0)/t0

(4)

(3) 6796

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Figure 2. HPLC separation on the MIL-101(Cr) packed column (5 cm long × 4.6 mm i.d.) with different ratios of DCM/MeOH as the mobile phase at a flow rate of 0.5 mL min−1 at room temperature: (A) nitroaniline isomers; (B) aminophenol isomers; (C) naphthol isomers; (D) sulfadimidine (1) and sulfanilamide (2). o, ortho; m, meta; p, para.

Figure 3. Reproducible chromatograms for 11 replicate separations on the MIL-101(Cr) packed column (5 cm long × 4.6 mm i.d.): (a) nitroaniline isomers using DCM/MeOH (98.7:1.3) as the mobile phase at a flow rate of 0.5 mL min−1; (b) aminophenol isomers using DCM/MeOH (98.0:2.0) as the mobile phase at a flow rate of 0.5 mL min−1; (c) naphthol isomers using DCM/MeOH (97.0:3.0) as the mobile phase at a flow rate of 0.5 mL min−1; and (d) sulfadimidine and sulfanilamide using DCM/MeOH (97.0:3.0) as the mobile phase at a flow rate of 0.5 mL min−1.

In conventional HPLC, MeOH is usually used to adjust the polarity of the mobile phase and to tune the interaction between the mobile phase and the analyte. However, in the present work, the MeOH in the mobile phase served as the modifier to reduce the strong interaction between the open metal sites in MIL-101(Cr) and the polar analytes via competition for the open metal sites. As a consequence, MeOH not only affected the retention time, but also the peak area of the polar analytes (Figure 4a,b,d,e). The strong interaction between the polar analyte and the open metal sites in MIL-101(Cr) using nonpolar solvent or low content of polar solvent as mobile phase resulted in no or incomplete

elution of polar analytes from the open metal sites. Therefore, low contents of MeOH in the mobile phase caused irreversible retention of the polar analytes (m-nitroaniline or β-naphthol), in turn tailing and broadening chromatographic peaks coupled with small peak areas (Figure 4a,b,d,e). However, an increase of the content of MeOH in the mobile phase of DCM/MeOH enhanced the competition of MeOH for the adsorption on the open metal sites, and thus weakened the interaction between the open metal sites and the polar analytes. As a result, the increase of the content of MeOH facilitated the elution of the polar analytes, giving clear sharp peaks and increased peak areas (Figure 4a,b,d,e). In contrast, the peak area of a nonpolar 6797

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Figure 4. Effect of the content of MeOH in the mobile phase of DCM/MeOH on the chromatogram of (a) m-nitroaniline, (b) β-naphthol, and (c) p-xylene; the peak area of (d) m-nitroaniline, (e) β-naphthol, and (f) p-xylene on MIL-101(Cr) packed column (5 cm long × 4.6 mm i.d.).

Figure 5. (a) Effect of the content of MeOH in the mobile phase of DCM/MeOH at a flow rate of 0.5 mL min−1 on the peak area and retention time of o-nitroaniline on the MIL-101(Cr) packed column (5 cm long × 4.6 mm i.d.); and (b) effect of the content of ACN in the mobile phase of DCM/ACN at a flow rate of 0.5 mL min−1 on the peak area and retention time of o-nitroaniline on the MIL-101(Cr) packed column (5 cm long × 4.6 mm i.d.).

Figure 6. FT-IR (a−c) and XPS (d−f) spectra of the activated MIL-101(Cr) (activated-MIL-101(Cr)) and unactivated MIL-101(Cr) (synthesizedMIL-101(Cr)), and the activated-MIL-101(Cr) after immersing into MeOH or ACN and evaporating at room temperature (MeOH-MIL-101(Cr) or ACN-MIL-101(Cr)). 6798

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Figure 7. Effect of the content of IBA in the mobile phase of DCM/IBA at a flow rate of 0.5 mL min−1 on (a) the peak area of m-nitroaniline and β-naphthol; and the chromatogram of (b) m-nitroaniline and (c) β-naphthol on the MIL-101(Cr) packed column (5 cm long × 4.6 mm i.d.).

To further demonstrate the control of the open metal sites in MIL-101(Cr) with R−OH, we replaced MeOH by isobutanol (IBA) in the mobile phase to investigate the effect of the content of IBA on the elution of polar compounds. The polar compounds were not eluted with pure DCM, but could be eluted with a mixture of DCM and IBA (Figure 7), even though the polarity of IBA (3.0) is less than that of DCM (3.4)51 because IBA enables one to coordinate with the open metal sites in MIL-101(Cr), and to weaken the interaction between the polar analytes and MIL-101(Cr). The effective elution of the polar analytes with less polar mobile phase of DCM/IBA (as compared to 100% DCM) further confirms the control of the open metal sites in MIL-101(Cr) with R−OH. Thermodynamic parameters were measured to evaluate the effect of the content of MeOH in the mobile phase of DCM/ MeOH on the separation of polar compounds on MIL-101(Cr) packed column. The calculated Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) for the transfer of nitroaniline isomers from the mobile phase to the stationary phase of MIL-101(Cr) are summarized in Table 1.

compound (p-xylene) did not change with the content of MeOH in the mobile phase due to no significant interaction between the open metal sites and the nonpolar analyte (Figure 4c,f). The above results indicate that MeOH, as the organic modifier, affected the coordination between the open metal sites of MIL-101(Cr) and the polar analytes. Replacing MeOH in the mobile phase by ACN still did not elute all of the polar analytes except o-nitroaniline, even though the content of ACN in the mobile phase is high (50% v/v) and the polarity of ACN (6.2) is similar to that of MeOH (6.6) (Figure S2).51 In the case of o-nitroaniline, as the content of MeOH in the mobile phase of DCM/MeOH increased, the peak area also increased, but the retention time changed slightly (Figure 5a). In contrast, almost no change in the peak area, but remarkable reduction in the retention time of o-nitroaniline, was observed when the content of ACN in the mobile phase of DCM/ACN increased (Figure 5b). MeOH, as a proton donor, could chelate with the open metal sites of MIL-101(Cr), while as a proton acceptor, ACN could not interact with the open metal sites (see the following FT-IR and XPS results). Therefore, MeOH acted as the modifier in the mobile phase to control the interaction between the polar analytes and the open metal sites in MIL-101(Cr), and hence to change the peak areas of the polar analytes, while ACN simply served as the polar additive to adjust the solubility of the analyte in the mobile phase and thus to control the retention time of the analyte. To show if the open metal sites in MIL-101(Cr) existed and the interaction of MeOH or ACN with the open metal sites occurred, we performed FT-IR and XPS experiments. FT-IR spectra show much weaker intensity of the activated MIL101(Cr) than the unactivated MIL-101(Cr) at 3422, 1621, and 1548 cm−1 (the characteristic FT-IR bands of Cr−H2O in the regions of 3000−3800 and 1500−1700 cm−1) (Figure 6a).52,53 XPS results reveal the shift in the binding energy of the Cr 2p3 peak from 577.37 eV for the unactivated MIL-101(Cr) to 577.79 eV for the activated MIL-101(Cr), indicating the removal of H2O molecules from the Cr sites after activation at 423 K (Figure 6d). These results confirm the presence of the open Cr sites in the activated MIL-101(Cr). The enhanced band at 3420 cm−1 in the FT-IR spectra of MeOH-MIL101(Cr) resulted from the incorporation of stretching vibrations (2870, 2960 cm−1) of the −CH3 group54 into the broad band of 3800−2750 cm−1, revealing the coordination of MeOH to the open metal sites of MIL-101(Cr) (Figure 6b). However, the absence of characteristic bands of CN at 2253 and 2291 cm−1 in the activated MIL-101(Cr) after treating with ACN, and the similarity of the FT-IR spectra and the XPS of Cr 2p3 peak for the activated MIL-101(Cr) before and after treating with ACN (Figure 6c and f), show no interaction between ACN and the open metal sites of MIL-101(Cr).

Table 1. Effect of the Content of MeOH in the Mobile Phase of DCM/MeOH at a Flow Rate of 0.5 mL min−1 on the Values of ΔH, ΔS, and ΔG for the HPLC Separation of Nitroaniline Isomers on the MIL-101(Cr) Packed Column at 25 °C MeOH%

isomers

0.6

mopmopmopmop-

1.3

4.0

6.0

ΔH (kJ mol−1) 12.2 8.41 12.0 7.10 5.62 5.98 1.16 0.85 1.25 0.93 0.73 1.18

± ± ± ± ± ± ± ± ± ± ± ±

0.21 0.46 0.78 0.08 0.13 0.34 0.08 0.03 0.08 0.05 0.06 0.02

ΔS (J mol−1 K−1) 47.6 38.0 54.4 29.1 27.3 31.7 8.04 10.1 13.5 6.73 9.21 12.3

± ± ± ± ± ± ± ± ± ± ± ±

0.8 1.5 2.5 0.2 0.4 1.1 0.3 0.1 0.2 0.2 0.2 0.1

ΔG (kJ mol−1) −1.98 −2.91 −4.21 −1.57 −2.52 −3.47 −1.24 −2.16 −2.77 −1.08 −2.01 −2.49

± ± ± ± ± ± ± ± ± ± ± ±

0.20 0.46 0.78 0.08 0.13 0.34 0.08 0.03 0.08 0.05 0.06 0.02

The negative values of ΔG indicate the transfer of nitroaniline isomers from the mobile phase of DCM/MeOH to the stationary phase of MIL- 101(Cr) was a thermodynamically spontaneous process. Such process was controlled by positive ΔH and ΔS. As the content of MeOH increased, the values of ΔH and ΔS decreased, but those of ΔG became less negative. The above results show that the control of the open metal sites in MIL-101(Cr) with MeOH was unfavorable for the transfer of the polar compounds from the mobile phase to MIL101(Cr) because of the inhibition of the interaction between 6799

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analytes were not well separated on MIL-53(Al) packed even though the content of MeOH in the mobile phase of DCM/ MeOH increased (Figure 8). The results further show the important role of the open metal sites and their appropriate control with MeOH for effective separation of polar compounds. The above results indicate that the high performance separation of polar analytes on MIL-101(Cr) resulted from the competition of MeOH in the mobile phase with the polar analytes for the open metal sites in MIL-101(Cr). The available unsaturated Cr(III) sites in MIL-101(Cr) make such separation possible by controlling the interaction between the open metal sites and the polar analytes with electronic-rich functional groups.49 Meanwhile, MIL-101(Cr) can adsorb alcohols reversibly.55 Therefore, MeOH with a more electronegative oxygen and the ability to coordinate with the metal sites of MOFs53 can reversibly tune the open metal sites of MIL101(Cr) during chromatographic separation. The competition of MeOH for the open metal sites reduced the chance for the polar analytes to access to the open metal sites, and thus weakened the interaction between the polar analytes and the stationary phase of MIL-101(Cr), changed the resolution, and decreased the retention time. The difference in the stability or energy of analyte-metal center complexes affects the competitive ability, thereby resulting in different chromatographic retention behavior of the analytes. The universality of the present approach for the control of the open metal sites of MOFs with MeOH was demonstrated using another MOF containing open metal sites, MIL-100(Fe). Application of such approach to the MIL-100(Fe) packed column resulted in a baseline LC separation of toluidine isomers (Figure 9), demonstrating the feasibility of the present approach for the effective separation of polar compounds on other MOFs with open metal sites.

Figure 8. Effect of the content of MeOH in the mobile phase of DCM/MeOH on the peak area of (a) m-nitroaniline, (b) β-naphthol; and chromatograms of (c) nitroaniline isomers, (d) aminophenol isomers, (e) naphthol isomers, and (f) sulfadimidine and sulfanilamide on MIL-53(Al) packed column (5 cm long × 4.6 mm i.d.). Separation conditions are as in Figure 3.

the polar compounds and the open metal sites in MIL-101(Cr) resulting from the competition of MeOH for the open metal sites. To further demonstrate the role of the open metal sites and the necessity of the control of the open metal sites for the separation of polar compounds, another MOF, MIL-53(Al), was used as the stationary phase for HPLC separation of the four groups of polar analytes for comparison. MIL-53(Al) consists of Al(III) metal ions, terphthalate ligands, and OHgroups, but does not contain open metal sites. Because of no open metal sites and thus no interaction between MeOH or polar analytes and the metal sites in MIL-53(Al), the peak areas of m-nitroaniline and β-naphthol did not change, and the polar

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CONCLUSIONS

In summary, we have reported the use of MeOH to tune the coordination status of the open metal sites of MOFs for effective HPLC separation of polar compounds on the columns packed with open metal sites-containing MOFs. The control of the open metal site interaction with MeOH makes the MOFs with open metal sites promising for the HPLC separation of polar compounds. Further research should focus on the application of such MOFs for chiral separation.

Figure 9. (a) Effect of the content of MeOH in the mobile phase of DCM/MeOH on peak area response; and (b) chromatogram of toluidine isomers on the MIL-100(Fe) packed column (5 cm long × 4.6 mm i.d.) using DCM/MeOH (99.5:0.5, v/v) as the mobile phase at a flow rate of 0.5 mL min−1. 6800

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Grants 20935001, 21077057) and the Tianjin Natural Science Foundation (Grant 10JCZDJC16300).

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