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The effect of a novel D-camphoric acid-based MOF on chiral separation
Solid State Sciences 98 (2019) 106032
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The effect of a novel D-camphoric acid-based MOF on chiral separation Ji Wang a, b, Jiaojiao Chen a, b, c, *, Tianyu Xu a, c a
School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China c Tianjin Key Laboratory of Green Chemical Technology and Process Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal-organic framework Chiral resolution Phenylalanine Adsorption separation
A new metal organic framework [Cu6(D-cam)8(TMDPY)4Cd2Na(OH)4] (D-cam ¼ D-Camphoric acid, TMDPY ¼ 4,40 -Trimethylenedipyridine, MOF 1) was synthesized for the first time under hydrothermal condi tions. It has three different metal clusters that are connected by a D-cam to form a two-dimensional structure. The layers are connected by TMDPY to form a three-dimensional structure. We used this material to separate racemic phenylalanine by adsorption, indicating that it has a much higher adsorption effect on D-phe (D-phenylalanine) than L-phe (L-phenylalanine). The adsorption kinetics of the adsorption process was studied, which indicated that the adsorption of phenylalanine by MOF 1 accords with the Langmuir isotherm adsorption model. Finally, the ratio of L-phe to D-phe in the solution adsorbed by MOF 1 is about 9:1. It is further illustrated that MOF 1 has a good separation effect on phenylalanine.
1. Introduction Since the 1990s, porous materials of metal-organic framework ma terials self-assembled by coordination of metal nodes and organic ligands have been synthesized and studied in large quantities [1–11]. In 1999, Y. Aoyama et al. [12] successfully synthesized optically pure chiral MOFs for the first time, and chiral MOFs were gradually recognized. Based on the diverse spatial topologies [13–15], chiral pore environments [16–18] and easy functionalization of chiral MOFs [19–23], they exhibit incred ible potential in enantiomeric separation analysis [24,25], asymmetric catalysis [26–28], nonlinear optics [29–32], drug delivery [33,34], and chiral sensing [35–39]. In recent years, the use of MOFs containing chiral molecules for the resolution of enantiomers has been extensively studied. In 2007, A.L. Nuzhdin et al. [40] first used [Zn2(bdc)(L-lac)(dmf)]⋅DMF (bdc ¼ Benzene-p-dicarboxylic acid, dmf ¼ N,N0 -Dimethylformamide, L-lac ¼ L-lactic acid) as a liquid chromatography column packing to prepare a chiral column for the separation of chiral sulfoxides. S.M. Xie et al. [41]used [{Cu(sala)}n] (H2sala ¼ N-(2-hydroxybenzyl)-L-alanine) with a three-dimensional left-handed spiral channel as a GC chiral sta tionary phase to rapidly resolve 11 chiral compounds such as amino acids, alcohols, aldehydes and organic acids. Similarly, Li Wang et al. [42] used (Me2NH2)2[Mn4O(D-cam)4]⋅(H2O)5 as a high performance liquid chromatography stationary phase for resolution (�)-ibuprofen
and (�)-1-phenyl 1,2-ethanediol. The Jian Zhang project is composed [NO3 ]}⋅[DMF] (5-eatz¼ into {[CuI2CuII(5-eatz)2(CN )(H2O)] (1S)-1-(5-tetrazolyl)-ethylamine). This chiral MOF has a rhombic tunnel and is successfully separated (R,S)-1-phenylethanol and (R,S)-1-phenyl propanol [43]. However, there are some disadvantages in using MOF as a stationary phase of a liquid chromatography column [44]. For example, the particle size and shape of the MOF are difficult to control, and the column efficiency may be poor when the liquid chromatography column is filled. The Song Weiguo project [45] combines ZIF-8 with D-histidine, which has an amazing resolution effect on alanine, glutamic acid and �nchez et al. [25] immersed [Cu(GHG)] lysine. Similarly, Jos� e Navarro-Sa (GHG ¼ tripeptide Glycine-L-Histidine-Glycine) in a racemic meta mphetamine solution and then determined the resolution of the chiral molecule by measuring the e.e. (enantiomeric excess) value of the su pernatant. Not only that, but they also used solid phase extraction to verify that [Cu(GHG)] also has excellent separation effect for ephedrine. It is well known that the application fields of the two configurations of phenylalanine are quite different, so the resolution of phenylalanine is of great significance. The use of MOF materials to resolve phenylalanine is very rare. Nengsheng Ye et al. [46] synthesized [Zn2(D-Cam)2(4, 40 -bpy)]n (bpy ¼ bipyridyl) as the stationary phase for capillary elec trochromatography. This is the first time that phenylalanine has been resolved with MOF materials. Unfortunately it works but the experiment
* Corresponding author. School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China. E-mail address: [email protected] (J. Chen). https://doi.org/10.1016/j.solidstatesciences.2019.106032 Received 26 June 2019; Received in revised form 4 October 2019; Accepted 5 October 2019 Available online 17 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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Solid State Sciences 98 (2019) 106032
Fig. 1. The first coordination environment of Cu ions (a), the structure observed along the axial direction of two Cu ions in (a) (b), the second coordination environment of Cu ions (c), the structure observed along the axial direction of two Cu ions in (c) (d), the coordination environment of Cd ions and Na ions. All hydrogen atoms are omitted for clarity.
is very complicated. First, it is necessary to modify the MOF with APTES ((3-Aminopropyl)triethoxysilane) so that it can be bonded to the capil lary. It is also necessary to try different buffer solutions to achieve the best separation. In our work, we synthesized a three-dimensional MOF 1 based on camphoric acid and 4,40 -Trimethylenedipyridine ligand by hydrothermal synthesis. The MOF 1 directly immersed in a racemic phenylalanine solution. And then we used the adsorption kinetics method to explore the chiral separation performance of MOF 1, and then determined its resolution by polarimeter, which showed excellent res olution for phenylalanine. This method not only avoids the tedious process of modification, compared to [Zn2(D-Cam)2(4,40 -bpy)]n has a better separation effect.
UV data were collected on a model TU-1901 dual-beam UV–visible spectrophotometer. 2.2. Synthesis of MOF 1 Cu(NO3)2⋅3H2O(0.0604 g, 0.025 mmol),Cd(NO3)2⋅4H2O (0.1542 g, 0.05 mmol), D-cam(0.05 g, 0.025 mmol), NaOH(0.7 mL, 1 mol/L) and TMDPY (0.0991 g, 0.05 mmol) were suspended in a 10 mL mixed solvent of H2O and EtOH (1:1, v/v) and stirred for 30 min. The final suspending solution was put into a Teflon-lined stainless steel container (25 mL) and heated at 140 � C for 5 days. The container was then allowed to cool to room temperature and a green square-like crystal was obtained. Elemental analysis found (calcd) % for C132H173Cd2Cu6N8NaO35: C, 51.64(51.76); H, 5.255 (5.65); N 3.68(3.66).
2. Experimental 2.1. Materials and general measurements
2.3. Chiral separation experiment
All the reagents were purchased and not further purified. Single-crystal X-ray diffraction data were collected at 193 (2) K on a Bruker SMART APEX II-CCD diffractometer equipped with a Ga Kα radiation (λ ¼ 1.34139 Å). The structure was solved by the direct method and refined on F2 by full-matrix least-squares methods using the SHELX program package [47]. The structure was checked with PLATON for missing symmetry el ements [48]. The powder XRD pattern was collected on a Bruker D8 ADVANCE Xray diffractometer using Cu Kα radiation in the 2θ angular range of 3–90� . The PXRD patterns is shown in Fig. S1 of the support information. The simulated powder diffraction pattern of the crystal is given by the software Mercury. The results of the experiments and simulations shown in Fig. S1 are basically consistent. The results of elemental analysis were obtained by the vario El cube test produced in Germany. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere using NETZSCH STA449F3 equipment with a heating rate of 10 � C min 1. The electron microscope used in the experiment was produced by the German company ZEISS and the model was Gemini SEM500.EDS is mounted on SEM500 and the model is OCTANE SUPER. The FT-IR data was collected at 400 to 4000 cm 1 using the ATR accessory of the Nicolet iS50 manufactured by ThermoFisher Scientific. The optical rotation of the solution was measured by a SWGzz-2 thermostat automatic polarimeter manufactured by Shanghai Shen guang Instrument Co., Ltd.
2.3.1. UV absorption working curve Different concentrations of D-/L-phe were scanned by UV–visible spectrophotometer in the wavelength range of 200 nm–400 nm, and then the appropriate wavelength (257 nm) was selected. The scatter plot is then plotted with the concentration of the phenylalanine solution as the abscissa and the absorbance A at 257 nm as the ordinate. Linear fitting of the scatter plot to obtain the UV absorption curve. 2.3.2. Adsorption kinetics experiment Add 100 mg of MOF 1 to both 50 mmol/L D- and L-phenylalanine solutions(methanol: water ¼ 1:1, V%). The solution was placed in a constant temperature shaker, and 1 mL of the supernatant was accu rately transferred at 25 � C at different times. The supernatant was then made to a volume of 10 mL, and the absorbance of the solution obtained at 257 nm at different times was measured. The concentration of the solution at different adsorption times was determined according to the UV working curve, and the adsorption amount of MOF 1 for D-/L-phe at different times was calculated according to formula (1). Finally, the adsorption kinetic curve was drawn [49]. Q ¼ ðC0
CT Þ � V=m
(1)
where Q is the adsorption amount at different times (μmol/g); C0 is the initial concentration of the solution (mmol/L); CT is the concentration of the solution at time T (mmol/L); V is the volume of the test solution (mL); m is the mass of MOF 1 in the solution. 2
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Fig. 2. Two-dimensional structure of MOF 1 (a), Structure observed along the c-axis(b), Structure observed along the b-axis(c), A simplified two-dimensional structure of MOF 1(d) Simplified three-dimensional structure observed along the c-axis(e), Simplified structure observed along the b-axis(f). And the red polyhedron represents CuO, the blue polyhedron represents CuN, the purple polyhedron represents CdNa, the yellow line represents D-camphoric acid, and the blue line represents TMDPY. shown in Fig. 2(d). The layers are then joined by TMDPY to form a three-dimensional structure of MOF. The structure observed along the c-axis of MOF 1 is shown in Fig. 2 (b), with a mesmerizing shape of regular arrangement of flowers. The structure observed along the b axis of MOF 1 is shown in Fig. 2 (c). TMDPY is simplified into a blue line, and its structure is shown in Fig. 2 (f). The connection mode between layers can be intuitively observed. Calculated by PLATON pores of void volume (5437 Å3) occupy about 30.7% of the unit cell volume (17722 Å3). (For interpretation of the refer ences to colour in this figure legend, the reader is referred to the Web version of this article.)
2.3.3. Chiral separation performance test 100 mg of MOF 1 was separately added to 50 mL of a 50 mmol/L solution of racemic phenylalanine (methanol: water ¼ 1:1, V%). The solution was placed in a constant temperature shaker, and 1 mL of the supernatant was accurately transferred at 25 � C at different times. The supernatant was then made up to 10 mL and the specific rotation of the solution obtained at different times was measured. The percentage of L-/ D-phe in the solution is then calculated by specific rotation. See the support information for specific formulas.
CuN, CdNa, respectively, and they are connected by camphoric acid to form a two-dimensional structure, as shown in Fig. 2(a). Then all of its secondary unit structures and molecules are represented as polyhedrons and lines respectively, and the simplified results are. 3.2. Thermogravimetric analysis The thermogravimetric curve indicates that the porous frameworks of MOF 1 is relatively stable before 242 � C with almost no mass loss. The frameworks of the MOF began to collapse at 242 � C. Then the quality of the MOF tends to be stable until about 600 � C. The thermogravimetric curve indicates that MOF 1 has good thermal stability and is not prone to collapse at room temperature. Thermogravimetric curve is shown in Fig. 3.
3. Results and discussion 3.1. Crystal structure of MOF 1 A single-crystal X-ray diffraction study reveals that compound 1 crystallizes in the tetragonal space group P4/ncc. Crystal data and structure refinement information for 1 are given in Table S1 in the Supporting Information. CIF data files are deposited at the Cambridge Crystallographic Data Center (deposition number: CCDC 1888692). Six Cu ions, eight camphoric acid ligands, four TMDPY ligands, two Cd ions, one sodium ion and four OH groups constitute an asymmetric unit. Fig. 1 shows the coordination environment for each metal ion. Copper ions have two different coordination environments, as shown in Fig. 1 (a) and (c). The copper metal clusters formed are not significantly different from those previously reported [50]. The difference between (a) and (c) in Fig. 1 is that the coordination atoms in the axial direction of the metal cluster are respectively the free oxygen atoms in the solu tion and the nitrogen atoms in the TMDPY ligand. It is worth noting that the angle of distortion between the upper tetrahedron and the lower tetrahedron in Fig. 1(b) is 15.102� along the axial direction of the metal cluster. The upper and lower tetrahedrons in Fig. 1(c) are symmetrical. The coordination environment of cadmium ions and sodium ions is shown in Fig. 1(e). Two camphoric acids provide four oxygen atoms in coordination with cadmium ions, and the TMDPY ligands are vertically coordinated with cadmium ions in the axial direction. The sodium ions act as a connection, and the angle of rotation of the upper and lower planes where the cadmium ions are present is 90� . Three different metal clusters (a), (c) and (e) in Fig. 1 are named CuO,
Fig. 3. Thermogravimetric (TG) curve of MOF 1. 3
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3.3. Fitting of the UV working curve
L-phe solution. It can be seen from Fig. 5(b) that as the concentration of the phenylalanine solution increases, the amount of MOF 1 adsorbed to D/L-phe also increases. It is worth noting that when the concentration of the solution reaches 100 mmol/L, Qe (equilibrium adsorption amount) gradually becomes gentle. This is because MOF 1 is about to reach the maximum amount of adsorption. The isothermal adsorption model de scribes the relationship between the equilibrium adsorption amount and the equilibrium concentration at a constant temperature. The Langmuir and Freundlich isotherm adsorption models are the two most widely used models (see the supporting information for the model). The ob tained data was taken into two models for linear simulation. As shown in Fig. 5(c) and (d), by comparing the fitting coefficients, it can be found that the adsorption of phenylalanine by MOF 1 is in accordance with the langmuir isotherm adsorption model. This indicates that the adsorption of phenylalanine by MOF belongs to the specific adsorption of monolayer.
The ultraviolet spectra of different concentrations of phenylalanine solution in the wavelength range of 200–400 nm were measured. The resulting spectrum is shown in Fig. 4 (a). It can be seen from Fig. 4(a) that the maximum absorption wave length is 257 nm. The fixed wavelength is 257 nm. The absorbance of the two configurations of phenylalanine solution is determined. The results are shown in Tables S2 and S3. Then the absorbance and solution con centration were fitted to obtain the UV working curve. The results are shown in Fig. 4(b) and (c). 3.4. Adsorption kinetics study Adsorption kinetics is an important means to study the adsorption process. In our work, MOF was added to different concentrations of D-/ L-phe solution, and then the adsorption amount of D-phe and L-phe at different times of MOF was calculated by formula (1). The resulting adsorption curve is shown in Fig. 5(a). It can be seen from Fig. 5(a) that the adsorption of phenylalanine by MOF 1 tends to be gentle after 300 min, and the adsorption capacity for D-phe is far better than that of L-phe. This also indicates that MOF 1 may have a separation ability for phenylalanine. To further investigate the adsorption of phenylalanine by MOF, we added equal amounts of MOF to different concentrations of D-/
3.5. Chiral separation performance test In this experiment, the specific rotation of the solution was used to investigate the resolution of the enantiomer of phenylalanine by MOF at different times, and to evaluate the resolution of MOF 1 to racemic phenylalanine. It is known that the specific rotation of optically pure Lphe is negative and the specific rotation of D-phe is positive. It can be
Fig. 4. UV spectra of different concentrations of phenylalanine solution (a). UV working curve of D-phe solution (b). UV working curve of L-phe solution (c). 4
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Fig. 5. The adsorption curve of MOF 1 to D-/L-phe at different times (a). The equilibrium adsorption curve of MOF 1 in D-/L-phe solution at different concentrations (b). (The red dot represents the adsorption of L-phe by MOF 1, and the black represents the adsorption of D-phe by MOF 1) Langmuir isotherm adsorption linear simulation (c). Freundlich isotherm adsorption linear simulation (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
seen from Fig. 6 (a) that as the adsorption time of MOF 1 for the mixed phenylalanine solution increases, the specific rotation of the solution is also gradually reduced. After 300 min of adsorption, the specific optical rotation of the solution does not change substantially. The optical purity and percentage of L-phe in the solution can be obtained by the formula
(S2), (S3) and (S4) in support information. In general, o.p. (optical pu rity) and e.e. (enantiomeric excess) are numerically identical and can approximate e.e., but not equal to e.e. The optical purity of the enan tiomer is more scientific with e.e., and o.p. is calculated by using the specific rotation. It is more convenient to express the purity of the
Fig. 6. Change in specific optical rotation of DL-phe solution at different times (a). Infrared spectrum of MOF 1. (b) The red curve represents the infrared spectrum of MOF 1, and the blue curve represents the infrared spectrum after adsorption of phenylalanine. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 5
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References
Table 1 Optical purity and D/L-phe percentage of DL-phe solution at different times. Time(min)
L-phe(%)
D-phe(%)
o.p.(%)
0 10 20 30 40 50 60 70 80 90 100 110 120 180 240 300
50 56.425 60.165 68.66 72.845 76.605 81.19 83.85 85.16 86.645 88.205 89.22 89.44 89.63 89.765 89.805
50 43.575 39.835 31.34 27.155 23.395 18.81 16.15 14.84 13.355 11.795 10.78 10.56 10.37 10.235 10.195
0 12.85 20.33 37.32 45.69 53.21 62.38 67.7 70.32 73.29 76.41 78.44 78.88 79.26 79.53 79.61
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enantiomer by o.p. The enantiomeric content in the solution at different times is shown in Table 1, indicating that MOF 1 has a certain ability to split phenylalanine, providing a new way of thinking and pathway for chiral separation. Similarly, we collected infrared data for MOF 1 and MOF 1 adsorbed with phenylalanine, as shown in Fig. 6(b). It can be clearly seen that there is one more peak at 3340 cm 1 in the infrared spectrum. This peak is the stretching vibration of the N–H bond in the –NH2 group. The peak which is more than 3030 cm 1 is the stretching vibration of the unsat urated C–H bond on the benzene ring. Moreover, there are two more peaks at 747 cm 1 and 695.24 cm-1, which are the out-of-plane bending of the C–H bond on the benzene ring and the position of the peak indicating a single substitution on the benzene ring. The comparison of the map further illustrates that MOF 1 has an adsorption effect on phenylalanine. 4. Conclusion In summary, our group successfully synthesized a novel threedimensional structure of MOF 1. It has excellent separation of phenyl alanine. The performance of the adsorption of D-phe and L-phe solutions showed that MOF 1 has a much stronger adsorption capacity for D-phe than L-phe. The adsorption process conforms to the Langmuir isotherm adsorption model, indicating that the adsorption of phenylalanine by MOF 1 is a monolayer adsorption. Finally, we measured the specific optical rotation of MOF 1 containing racemic phenylalanine solution at different times to obtain a ratio of L-phe to D-phe in the solution after adsorption for 5 h was about 9:1. This method is not only relatively simple but also superior in separation efficiency with respect to chro matographic separation. This has great significance for the application of phenylalanine separation. Declaration of competing interest The authors declare that they have no conflict of interest. Acknowledgement This work was supported by the National Natural Science Foundation of China (21374078, 51308390 and 51303132). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solidstatesciences.2019.106032.
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Solid State Sciences journal homepage: http://www.elsevier.com/locate/ssscie
The effect of a novel D-camphoric acid-based MOF on chiral separation Ji Wang a, b, Jiaojiao Chen a, b, c, *, Tianyu Xu a, c a
School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China State Key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China c Tianjin Key Laboratory of Green Chemical Technology and Process Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal-organic framework Chiral resolution Phenylalanine Adsorption separation
A new metal organic framework [Cu6(D-cam)8(TMDPY)4Cd2Na(OH)4] (D-cam ¼ D-Camphoric acid, TMDPY ¼ 4,40 -Trimethylenedipyridine, MOF 1) was synthesized for the first time under hydrothermal condi tions. It has three different metal clusters that are connected by a D-cam to form a two-dimensional structure. The layers are connected by TMDPY to form a three-dimensional structure. We used this material to separate racemic phenylalanine by adsorption, indicating that it has a much higher adsorption effect on D-phe (D-phenylalanine) than L-phe (L-phenylalanine). The adsorption kinetics of the adsorption process was studied, which indicated that the adsorption of phenylalanine by MOF 1 accords with the Langmuir isotherm adsorption model. Finally, the ratio of L-phe to D-phe in the solution adsorbed by MOF 1 is about 9:1. It is further illustrated that MOF 1 has a good separation effect on phenylalanine.
1. Introduction Since the 1990s, porous materials of metal-organic framework ma terials self-assembled by coordination of metal nodes and organic ligands have been synthesized and studied in large quantities [1–11]. In 1999, Y. Aoyama et al. [12] successfully synthesized optically pure chiral MOFs for the first time, and chiral MOFs were gradually recognized. Based on the diverse spatial topologies [13–15], chiral pore environments [16–18] and easy functionalization of chiral MOFs [19–23], they exhibit incred ible potential in enantiomeric separation analysis [24,25], asymmetric catalysis [26–28], nonlinear optics [29–32], drug delivery [33,34], and chiral sensing [35–39]. In recent years, the use of MOFs containing chiral molecules for the resolution of enantiomers has been extensively studied. In 2007, A.L. Nuzhdin et al. [40] first used [Zn2(bdc)(L-lac)(dmf)]⋅DMF (bdc ¼ Benzene-p-dicarboxylic acid, dmf ¼ N,N0 -Dimethylformamide, L-lac ¼ L-lactic acid) as a liquid chromatography column packing to prepare a chiral column for the separation of chiral sulfoxides. S.M. Xie et al. [41]used [{Cu(sala)}n] (H2sala ¼ N-(2-hydroxybenzyl)-L-alanine) with a three-dimensional left-handed spiral channel as a GC chiral sta tionary phase to rapidly resolve 11 chiral compounds such as amino acids, alcohols, aldehydes and organic acids. Similarly, Li Wang et al. [42] used (Me2NH2)2[Mn4O(D-cam)4]⋅(H2O)5 as a high performance liquid chromatography stationary phase for resolution (�)-ibuprofen
and (�)-1-phenyl 1,2-ethanediol. The Jian Zhang project is composed [NO3 ]}⋅[DMF] (5-eatz¼ into {[CuI2CuII(5-eatz)2(CN )(H2O)] (1S)-1-(5-tetrazolyl)-ethylamine). This chiral MOF has a rhombic tunnel and is successfully separated (R,S)-1-phenylethanol and (R,S)-1-phenyl propanol [43]. However, there are some disadvantages in using MOF as a stationary phase of a liquid chromatography column [44]. For example, the particle size and shape of the MOF are difficult to control, and the column efficiency may be poor when the liquid chromatography column is filled. The Song Weiguo project [45] combines ZIF-8 with D-histidine, which has an amazing resolution effect on alanine, glutamic acid and �nchez et al. [25] immersed [Cu(GHG)] lysine. Similarly, Jos� e Navarro-Sa (GHG ¼ tripeptide Glycine-L-Histidine-Glycine) in a racemic meta mphetamine solution and then determined the resolution of the chiral molecule by measuring the e.e. (enantiomeric excess) value of the su pernatant. Not only that, but they also used solid phase extraction to verify that [Cu(GHG)] also has excellent separation effect for ephedrine. It is well known that the application fields of the two configurations of phenylalanine are quite different, so the resolution of phenylalanine is of great significance. The use of MOF materials to resolve phenylalanine is very rare. Nengsheng Ye et al. [46] synthesized [Zn2(D-Cam)2(4, 40 -bpy)]n (bpy ¼ bipyridyl) as the stationary phase for capillary elec trochromatography. This is the first time that phenylalanine has been resolved with MOF materials. Unfortunately it works but the experiment
* Corresponding author. School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, PR China. E-mail address: [email protected] (J. Chen). https://doi.org/10.1016/j.solidstatesciences.2019.106032 Received 26 June 2019; Received in revised form 4 October 2019; Accepted 5 October 2019 Available online 17 October 2019 1293-2558/© 2019 Elsevier Masson SAS. All rights reserved.
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Fig. 1. The first coordination environment of Cu ions (a), the structure observed along the axial direction of two Cu ions in (a) (b), the second coordination environment of Cu ions (c), the structure observed along the axial direction of two Cu ions in (c) (d), the coordination environment of Cd ions and Na ions. All hydrogen atoms are omitted for clarity.
is very complicated. First, it is necessary to modify the MOF with APTES ((3-Aminopropyl)triethoxysilane) so that it can be bonded to the capil lary. It is also necessary to try different buffer solutions to achieve the best separation. In our work, we synthesized a three-dimensional MOF 1 based on camphoric acid and 4,40 -Trimethylenedipyridine ligand by hydrothermal synthesis. The MOF 1 directly immersed in a racemic phenylalanine solution. And then we used the adsorption kinetics method to explore the chiral separation performance of MOF 1, and then determined its resolution by polarimeter, which showed excellent res olution for phenylalanine. This method not only avoids the tedious process of modification, compared to [Zn2(D-Cam)2(4,40 -bpy)]n has a better separation effect.
UV data were collected on a model TU-1901 dual-beam UV–visible spectrophotometer. 2.2. Synthesis of MOF 1 Cu(NO3)2⋅3H2O(0.0604 g, 0.025 mmol),Cd(NO3)2⋅4H2O (0.1542 g, 0.05 mmol), D-cam(0.05 g, 0.025 mmol), NaOH(0.7 mL, 1 mol/L) and TMDPY (0.0991 g, 0.05 mmol) were suspended in a 10 mL mixed solvent of H2O and EtOH (1:1, v/v) and stirred for 30 min. The final suspending solution was put into a Teflon-lined stainless steel container (25 mL) and heated at 140 � C for 5 days. The container was then allowed to cool to room temperature and a green square-like crystal was obtained. Elemental analysis found (calcd) % for C132H173Cd2Cu6N8NaO35: C, 51.64(51.76); H, 5.255 (5.65); N 3.68(3.66).
2. Experimental 2.1. Materials and general measurements
2.3. Chiral separation experiment
All the reagents were purchased and not further purified. Single-crystal X-ray diffraction data were collected at 193 (2) K on a Bruker SMART APEX II-CCD diffractometer equipped with a Ga Kα radiation (λ ¼ 1.34139 Å). The structure was solved by the direct method and refined on F2 by full-matrix least-squares methods using the SHELX program package [47]. The structure was checked with PLATON for missing symmetry el ements [48]. The powder XRD pattern was collected on a Bruker D8 ADVANCE Xray diffractometer using Cu Kα radiation in the 2θ angular range of 3–90� . The PXRD patterns is shown in Fig. S1 of the support information. The simulated powder diffraction pattern of the crystal is given by the software Mercury. The results of the experiments and simulations shown in Fig. S1 are basically consistent. The results of elemental analysis were obtained by the vario El cube test produced in Germany. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere using NETZSCH STA449F3 equipment with a heating rate of 10 � C min 1. The electron microscope used in the experiment was produced by the German company ZEISS and the model was Gemini SEM500.EDS is mounted on SEM500 and the model is OCTANE SUPER. The FT-IR data was collected at 400 to 4000 cm 1 using the ATR accessory of the Nicolet iS50 manufactured by ThermoFisher Scientific. The optical rotation of the solution was measured by a SWGzz-2 thermostat automatic polarimeter manufactured by Shanghai Shen guang Instrument Co., Ltd.
2.3.1. UV absorption working curve Different concentrations of D-/L-phe were scanned by UV–visible spectrophotometer in the wavelength range of 200 nm–400 nm, and then the appropriate wavelength (257 nm) was selected. The scatter plot is then plotted with the concentration of the phenylalanine solution as the abscissa and the absorbance A at 257 nm as the ordinate. Linear fitting of the scatter plot to obtain the UV absorption curve. 2.3.2. Adsorption kinetics experiment Add 100 mg of MOF 1 to both 50 mmol/L D- and L-phenylalanine solutions(methanol: water ¼ 1:1, V%). The solution was placed in a constant temperature shaker, and 1 mL of the supernatant was accu rately transferred at 25 � C at different times. The supernatant was then made to a volume of 10 mL, and the absorbance of the solution obtained at 257 nm at different times was measured. The concentration of the solution at different adsorption times was determined according to the UV working curve, and the adsorption amount of MOF 1 for D-/L-phe at different times was calculated according to formula (1). Finally, the adsorption kinetic curve was drawn [49]. Q ¼ ðC0
CT Þ � V=m
(1)
where Q is the adsorption amount at different times (μmol/g); C0 is the initial concentration of the solution (mmol/L); CT is the concentration of the solution at time T (mmol/L); V is the volume of the test solution (mL); m is the mass of MOF 1 in the solution. 2
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Fig. 2. Two-dimensional structure of MOF 1 (a), Structure observed along the c-axis(b), Structure observed along the b-axis(c), A simplified two-dimensional structure of MOF 1(d) Simplified three-dimensional structure observed along the c-axis(e), Simplified structure observed along the b-axis(f). And the red polyhedron represents CuO, the blue polyhedron represents CuN, the purple polyhedron represents CdNa, the yellow line represents D-camphoric acid, and the blue line represents TMDPY. shown in Fig. 2(d). The layers are then joined by TMDPY to form a three-dimensional structure of MOF. The structure observed along the c-axis of MOF 1 is shown in Fig. 2 (b), with a mesmerizing shape of regular arrangement of flowers. The structure observed along the b axis of MOF 1 is shown in Fig. 2 (c). TMDPY is simplified into a blue line, and its structure is shown in Fig. 2 (f). The connection mode between layers can be intuitively observed. Calculated by PLATON pores of void volume (5437 Å3) occupy about 30.7% of the unit cell volume (17722 Å3). (For interpretation of the refer ences to colour in this figure legend, the reader is referred to the Web version of this article.)
2.3.3. Chiral separation performance test 100 mg of MOF 1 was separately added to 50 mL of a 50 mmol/L solution of racemic phenylalanine (methanol: water ¼ 1:1, V%). The solution was placed in a constant temperature shaker, and 1 mL of the supernatant was accurately transferred at 25 � C at different times. The supernatant was then made up to 10 mL and the specific rotation of the solution obtained at different times was measured. The percentage of L-/ D-phe in the solution is then calculated by specific rotation. See the support information for specific formulas.
CuN, CdNa, respectively, and they are connected by camphoric acid to form a two-dimensional structure, as shown in Fig. 2(a). Then all of its secondary unit structures and molecules are represented as polyhedrons and lines respectively, and the simplified results are. 3.2. Thermogravimetric analysis The thermogravimetric curve indicates that the porous frameworks of MOF 1 is relatively stable before 242 � C with almost no mass loss. The frameworks of the MOF began to collapse at 242 � C. Then the quality of the MOF tends to be stable until about 600 � C. The thermogravimetric curve indicates that MOF 1 has good thermal stability and is not prone to collapse at room temperature. Thermogravimetric curve is shown in Fig. 3.
3. Results and discussion 3.1. Crystal structure of MOF 1 A single-crystal X-ray diffraction study reveals that compound 1 crystallizes in the tetragonal space group P4/ncc. Crystal data and structure refinement information for 1 are given in Table S1 in the Supporting Information. CIF data files are deposited at the Cambridge Crystallographic Data Center (deposition number: CCDC 1888692). Six Cu ions, eight camphoric acid ligands, four TMDPY ligands, two Cd ions, one sodium ion and four OH groups constitute an asymmetric unit. Fig. 1 shows the coordination environment for each metal ion. Copper ions have two different coordination environments, as shown in Fig. 1 (a) and (c). The copper metal clusters formed are not significantly different from those previously reported [50]. The difference between (a) and (c) in Fig. 1 is that the coordination atoms in the axial direction of the metal cluster are respectively the free oxygen atoms in the solu tion and the nitrogen atoms in the TMDPY ligand. It is worth noting that the angle of distortion between the upper tetrahedron and the lower tetrahedron in Fig. 1(b) is 15.102� along the axial direction of the metal cluster. The upper and lower tetrahedrons in Fig. 1(c) are symmetrical. The coordination environment of cadmium ions and sodium ions is shown in Fig. 1(e). Two camphoric acids provide four oxygen atoms in coordination with cadmium ions, and the TMDPY ligands are vertically coordinated with cadmium ions in the axial direction. The sodium ions act as a connection, and the angle of rotation of the upper and lower planes where the cadmium ions are present is 90� . Three different metal clusters (a), (c) and (e) in Fig. 1 are named CuO,
Fig. 3. Thermogravimetric (TG) curve of MOF 1. 3
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3.3. Fitting of the UV working curve
L-phe solution. It can be seen from Fig. 5(b) that as the concentration of the phenylalanine solution increases, the amount of MOF 1 adsorbed to D/L-phe also increases. It is worth noting that when the concentration of the solution reaches 100 mmol/L, Qe (equilibrium adsorption amount) gradually becomes gentle. This is because MOF 1 is about to reach the maximum amount of adsorption. The isothermal adsorption model de scribes the relationship between the equilibrium adsorption amount and the equilibrium concentration at a constant temperature. The Langmuir and Freundlich isotherm adsorption models are the two most widely used models (see the supporting information for the model). The ob tained data was taken into two models for linear simulation. As shown in Fig. 5(c) and (d), by comparing the fitting coefficients, it can be found that the adsorption of phenylalanine by MOF 1 is in accordance with the langmuir isotherm adsorption model. This indicates that the adsorption of phenylalanine by MOF belongs to the specific adsorption of monolayer.
The ultraviolet spectra of different concentrations of phenylalanine solution in the wavelength range of 200–400 nm were measured. The resulting spectrum is shown in Fig. 4 (a). It can be seen from Fig. 4(a) that the maximum absorption wave length is 257 nm. The fixed wavelength is 257 nm. The absorbance of the two configurations of phenylalanine solution is determined. The results are shown in Tables S2 and S3. Then the absorbance and solution con centration were fitted to obtain the UV working curve. The results are shown in Fig. 4(b) and (c). 3.4. Adsorption kinetics study Adsorption kinetics is an important means to study the adsorption process. In our work, MOF was added to different concentrations of D-/ L-phe solution, and then the adsorption amount of D-phe and L-phe at different times of MOF was calculated by formula (1). The resulting adsorption curve is shown in Fig. 5(a). It can be seen from Fig. 5(a) that the adsorption of phenylalanine by MOF 1 tends to be gentle after 300 min, and the adsorption capacity for D-phe is far better than that of L-phe. This also indicates that MOF 1 may have a separation ability for phenylalanine. To further investigate the adsorption of phenylalanine by MOF, we added equal amounts of MOF to different concentrations of D-/
3.5. Chiral separation performance test In this experiment, the specific rotation of the solution was used to investigate the resolution of the enantiomer of phenylalanine by MOF at different times, and to evaluate the resolution of MOF 1 to racemic phenylalanine. It is known that the specific rotation of optically pure Lphe is negative and the specific rotation of D-phe is positive. It can be
Fig. 4. UV spectra of different concentrations of phenylalanine solution (a). UV working curve of D-phe solution (b). UV working curve of L-phe solution (c). 4
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Fig. 5. The adsorption curve of MOF 1 to D-/L-phe at different times (a). The equilibrium adsorption curve of MOF 1 in D-/L-phe solution at different concentrations (b). (The red dot represents the adsorption of L-phe by MOF 1, and the black represents the adsorption of D-phe by MOF 1) Langmuir isotherm adsorption linear simulation (c). Freundlich isotherm adsorption linear simulation (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
seen from Fig. 6 (a) that as the adsorption time of MOF 1 for the mixed phenylalanine solution increases, the specific rotation of the solution is also gradually reduced. After 300 min of adsorption, the specific optical rotation of the solution does not change substantially. The optical purity and percentage of L-phe in the solution can be obtained by the formula
(S2), (S3) and (S4) in support information. In general, o.p. (optical pu rity) and e.e. (enantiomeric excess) are numerically identical and can approximate e.e., but not equal to e.e. The optical purity of the enan tiomer is more scientific with e.e., and o.p. is calculated by using the specific rotation. It is more convenient to express the purity of the
Fig. 6. Change in specific optical rotation of DL-phe solution at different times (a). Infrared spectrum of MOF 1. (b) The red curve represents the infrared spectrum of MOF 1, and the blue curve represents the infrared spectrum after adsorption of phenylalanine. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 5
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References
Table 1 Optical purity and D/L-phe percentage of DL-phe solution at different times. Time(min)
L-phe(%)
D-phe(%)
o.p.(%)
0 10 20 30 40 50 60 70 80 90 100 110 120 180 240 300
50 56.425 60.165 68.66 72.845 76.605 81.19 83.85 85.16 86.645 88.205 89.22 89.44 89.63 89.765 89.805
50 43.575 39.835 31.34 27.155 23.395 18.81 16.15 14.84 13.355 11.795 10.78 10.56 10.37 10.235 10.195
0 12.85 20.33 37.32 45.69 53.21 62.38 67.7 70.32 73.29 76.41 78.44 78.88 79.26 79.53 79.61
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enantiomer by o.p. The enantiomeric content in the solution at different times is shown in Table 1, indicating that MOF 1 has a certain ability to split phenylalanine, providing a new way of thinking and pathway for chiral separation. Similarly, we collected infrared data for MOF 1 and MOF 1 adsorbed with phenylalanine, as shown in Fig. 6(b). It can be clearly seen that there is one more peak at 3340 cm 1 in the infrared spectrum. This peak is the stretching vibration of the N–H bond in the –NH2 group. The peak which is more than 3030 cm 1 is the stretching vibration of the unsat urated C–H bond on the benzene ring. Moreover, there are two more peaks at 747 cm 1 and 695.24 cm-1, which are the out-of-plane bending of the C–H bond on the benzene ring and the position of the peak indicating a single substitution on the benzene ring. The comparison of the map further illustrates that MOF 1 has an adsorption effect on phenylalanine. 4. Conclusion In summary, our group successfully synthesized a novel threedimensional structure of MOF 1. It has excellent separation of phenyl alanine. The performance of the adsorption of D-phe and L-phe solutions showed that MOF 1 has a much stronger adsorption capacity for D-phe than L-phe. The adsorption process conforms to the Langmuir isotherm adsorption model, indicating that the adsorption of phenylalanine by MOF 1 is a monolayer adsorption. Finally, we measured the specific optical rotation of MOF 1 containing racemic phenylalanine solution at different times to obtain a ratio of L-phe to D-phe in the solution after adsorption for 5 h was about 9:1. This method is not only relatively simple but also superior in separation efficiency with respect to chro matographic separation. This has great significance for the application of phenylalanine separation. Declaration of competing interest The authors declare that they have no conflict of interest. Acknowledgement This work was supported by the National Natural Science Foundation of China (21374078, 51308390 and 51303132). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solidstatesciences.2019.106032.
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