Research Article www.acsami.org
Superficial Chiral Etching on Achiral Metal−Organic Framework for Enantioselective Sorption Xudong Hou, Tingting Xu, Yang Wang, Shengjun Liu, Jing Tong, and Bo Liu* Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *
ABSTRACT: Chiral recognition and separation is of general research interests in natural product separation and the pharmacy industry. In this work, we develop a novel strategy to modify achiral porous metal−organic framework (MOF) surfaces via a superficial chiral etching process (SCEP), in which reacting a presynthesized achiral MOF with a chiral ligand produces an achiral@chiral MOF core−shell hybrid composition. SCEP creates chiral species on an achiral porous MOF surface but does not change the porosity and pore structure, enabling core−shell composition enantioselective sorption. Reacting (+)-camphoric acid, (+)-Cam, and 1,4-diazabicyclo[2.2.2]octane (Dabco) with [Cu3(Btc)2] microcrystals leads to a chiral MOF of [Cu2((+)-Cam)2Dabco] crystallites attached on the surface of [Cu3(Btc)2] (Btc = 1,3,5-benzenetricarboxylate). The resulting [Cu3(Btc)2] @[Cu2((+)-Cam)2Dabco] core−shell composition displays preferred sorption kinetics toward (S)limonene against (R)-limonene, with a similar discrimination effect with pure chiral [Cu2((+)Cam)2Dabco]. Superficial chiral etching of the porous achiral MOF represents an economic and efficient strategy for enantioselective separation. KEYWORDS: metal−organic framework, core−shell, enantioselective, superficial chiral etching process, isochoric technique
1. INTRODUCTION Homochiral metal−organic frameworks (h-MOFs) have been extensively studied because of their structural tunability and diversity and consequently promising applications in the drug industry, enantioselective separation, asymmetric catalysis, etc.1−3 Lin et al. have summarized the strategies for h-MOF syntheses:2 (i) self-resolution from achiral ligands during crystal growth,4−6 (ii) chiral template induction,7,8 and (iii) syntheses directly from chiral linker or auxiliary ligands.9−11 Strategy i is promising but challenging with few examples, whereas strategies using pure chiral organics are often costly because of complexity and difficulty for preparation of enantiopure ligands. h-MOFs are a kind of ideal candidate for chiral separation owing to their chiral pores and high porosity. There are two steps in a typical chiral separation process by h-MOFs. Chiral surface/pores discriminates a racemic mixture first, depending on their different steric configurations. Subsequently, an enantiopure species passes through the channels at a different time scale, namely, separation. The channels in the second step are not necessarily chiral as they could only function as mass transfer passages. Therefore, it is rational to infer that anchoring a chiral gate (h-MOF) on achiral MOFs could achieve the aim of enantioselective separation/sorption, but using very expensive, less enantiopure chiral molecules. Among >20 000 reported MOF structures, h-MOF is a very small portion. The preparation of pure homochiral MOFs often involves a large amount of costly homochiral ligands. Therefore, enabling separation by achiral MOFs of chiral molecules by superficial chiral modification will significantly extend MOFs’ applications in chiral separation and the pharmacy industry. © 2017 American Chemical Society
Hybrid MOFs, including multivariate metal−organic frameworks,12−14 MOF-on-MOFs,15 core−shell MOFs,16−18 etc., have been developed to introduce complementary properties by assembling organic ligands bearing the same coordination function but different substitution groups, or different metal ions having the same coordination geometry, into one single piece of crystal. Success of this kind of hybrid MOF relies on the lattice matching in different single MOF components. Rosi and co-workers have demonstrated that a mixture of bio-MOF11 and bio-MOF-14 as the core successfully induced bio-MOF14 shell growth for selective CO2 capture; however, bio-MOF14 cannot directly grow on bio-MOF-11 because of lattice mismatching.19 For comparison, the postsynthetic modification strategy dramatically decreases pore sizes and the porosity of parent MOFs.20−22 On the other hand, monolayer modification on the MOF outer surface has been achieved via a stepwise liquid epitaxial method23 and “substitution” reaction.14,24 However, it remains a great challenge to prepare chiral−achiral hybrid MOFs, and there is no such report so far, to the best of our knowledge. In this work, we proposed a superficial chiral etching process (SCEP) which not only etches the surface but also creates a new phase to prepare an achiral MOF@homochiral MOF core−shell hybrid structure, and we illustrated the strategy with [Cu3(Btc)2]@[Cu2((+)-Cam)2Dabco] using [Cu3(Btc)2]25 and [Cu2((+)-Cam)2Dabco]26 as the achiral core and chiral Received: July 12, 2017 Accepted: August 25, 2017 Published: August 25, 2017 32264
DOI: 10.1021/acsami.7b10147 ACS Appl. Mater. Interfaces 2017, 9, 32264−32269
Research Article
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Figure 1. Schematic illustration of the superficial chiral etching process. Superficial layers of a presynthesized MOF of [Cu3(Btc)2] provides a Cu(II) source to react with (+)-Cam and Dabco, producing [Cu2((+)-Cam)2Dabco] on the surface of the achiral parent MOF, leading to an achiral MOF@ homochiral MOF core−shell hybrid structure (Btc = 1,3,5-benzenetricarboxylate; (+)-Cam = (+)-camphoric acid; Dabco = 1,4-diazabicyclo[2.2.2]octane). Octahedra represent [Cu3(Btc)2] (left) and [Cu3(Btc)2]@[Cu2((+)-Cam)2Dabco] (right). for 12 h to remove possible guest molecules (H2O) and air, and then cooled down to 278 K. A glass tube with calibrated volume (V = 157.78 mL) was introduced with limonene vapor to 60 Pa at 278 K. The valve was opened to connect both tubes to allow adsorption to start, and the pressure drop was recorded against time. The pressure drop was transferred to an adsorbed limonene amount. The adsorption rates were evaluated by slope of the linear part of the adsorption curve.
shell components, respectively (Figure 1). The resulting [Cu3(Btc)2]@[Cu2((+)-Cam)2Dabco] composition represents the first example of an achiral@chiral hybrid MOF. It exhibited a similar saturated sorption capability to (R)- and (S)-limonene (the information on molecular structures is provided in Figure S2), but a preferred (S)-limonene kinetic sorption rate against (R)-limonene. Thus, we achieved enantioselective sorption using an achiral MOF via a superficial chiral etching process.
3. RESULTS AND DISCUSSION We obtained the [Cu3(Btc)2]@[Cu2((+)-Cam)2Dabco] core− shell hybrid via a one-pot solvothermal reaction of presynthesized [Cu3(Btc)2] microcrystallites with (+)-Cam and Dabco. In this process, [Cu3(Btc)2] (Figure S4) behaves as not only core components but also the Cu(II) precursor for growth of [Cu2((+)-Cam)2Dabco] (Figure S5). It is speculated that the reaction mechanism is as follows: superficial [Cu3(Btc)2] is decomposed at reaction conditions, and Cu(II) is released to react with (+)-Cam and Dabco producing [Cu2 ((+)Cam)2Dabco] on the surface of unreacted [Cu3(Btc)2] and resulting in a core−shell hybrid structure. We defined this process as a superficial chiral etching process (SCEP). Powder Xray diffraction (PXRD) patterns of [Cu3(Btc)2] after etching with (+)-Cam and Dabco for 12 h (denoted as SCEP-12) confirmed the existence of both [Cu3(Btc)2] and [Cu2((+)Cam)2Dabco] phases (Figure 2a). To verify the core−shell structure, we observed the morphology of [Cu3(Btc)2] before and after etching, which gives direct evidence of the core−shell structure as shown in Figure 2. Consistent with the reported literature,29 [Cu3(Btc)2] displays a regular octahedral morphology with a relatively smooth surface (Figure 2a−c). After etching for 12 h, the octahedra shape remains, but dense microcrystalline blocks appeared on its surface (Figure 2d−f); no obvious individual [Cu3(Btc)2] particles were observed. According to the SCEP definition, it is expected that changing reaction conditions could control the etching depth. We systematically investigated etching time’s influence over the final products and monitored this by PXRD and SEM (Supporting Information, Figures S6−10). After etching for 6 h, we started to see obvious particles scattered on [Cu3(Btc)2] octahedra. Etching for 12 h results in a dense [Cu2((+)Cam)2Dabco] layer on [Cu3(Btc)2] particles. However, when the etching process is overdone, separated [Cu2 ((+)Cam)2Dabco] microcrystallites were observed by SEM. When
2. EXPERIMENTAL SECTION 2.1. Materials. The following materials were used: copper nitrate trihydrate (99.95% metals basis powder, Macklin); copper acetate monohydrate (99.95% metals basis powder, Macklin); trimesic acid (AR, 98%, Macklin); (+)-camphoric acid (99%, Macklin); 1,4diazabicyclo[2.2.2]octane (98%, Macklin); (+)-limonene ((R)-limonene; >95%, TCI); and (−)-limonene ((S)-limonene; >95%, TCI). 2.2. Preparation of [Cu2((+)-Cam)2Dabco], [Cu3(Btc)2], and SCEP-x. 2.2.1. Syntheses of [Cu3(Btc)2] and [Cu2((+)-Cam)2Dabco]. [Cu3(Btc)2] was synthesized according to modified methods from the literature.25 Cu(NO3)2·6H2O (0.4356 g 1.8 mmol) was heated with TMA-H3 (0.21 g, 1.0 mmol) in 12 mL of H2O and EtOH (v/v = 1:1) at 120 °C for 12 h in the screw-cap Teflon autoclave (TMA = trimesic acid). After cooling to room temperature, the blue powder was collected by centrifugation, washed with methanol and water, and dried under vacuum. [Cu2((+)-Cam)2Dabco] was synthesized according to reported methods.26 A mixture of Cu(AcO)2·2H2O (100 mg, 0.5 mmol), (+)-H2camphoric acid [(+)-Cam, 100 mg, 0.5 mmol], and 1,4-diazabicyclo[2.2.2]octane (Dabco, 28 mg, 0.25 mmol) in 5 mL of N,N′-dimethylformamide (DMF) was heated in the screwcap Teflon autoclave at 110 °C for 2 days. The block-shaped crystals were collected, washed with DMF, and dried on air. 2.2.2. Superficial Chiral Etching Process: Preparation of [Cu3(Btc)2]@[Cu2((+)-Cam)2Dabco]. In a 20 mL Teflon autoclave, presynthesized [Cu3(Btc)2] (0.080 g, 0.06 mmol), Dabco (0.012 g, 0.01 mmol), and (+)-Cam (0.02g, 0.01 mmol) were mixed in DMF (10 mL). The mixture was stirred for 30 min at room temperature, and the reaction mixture was heated at 110 °C for 6, 12, 24, or 48 h. After cooling to room temperature, the precipitates were collected by centrifugation, washed with methanol, and dried under vacuum. According to the reaction time, the samples were denoted SCEP-x (x represents the reaction time). 2.3. Limonene Sorption Kinetic Rate Measurements. We adopted an isochoric technique which has been widely employed for VOC sorption study in the literature.27,28 (See the Supporting Information for full experimental details.) Activated [Cu3(Btc)2], [Cu2((+)-Cam)2Dabco], and SCEP-12 (10 mg for each sample) was loaded into a small glass tube (V = 1.59 mL), degassed again at 120 °C 32265
DOI: 10.1021/acsami.7b10147 ACS Appl. Mater. Interfaces 2017, 9, 32264−32269
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Figure 3. N2 sorption isotherms of [Cu3(Btc)2] (black), [Cu2((+)Cam)2Dabco] (red), and SCEP-12 (green) at 77 K (●, adsorption; ○, desorption).
sorption behavior with a specific BET surface area at 1311 cm3 g−1. This suggested that superficial etching did not dramatically decrease the BET surface area of the parent MOF [Cu3(Btc)2]. 3.2. (R)- and (S)-Limonene Adsorption and Enantioselectivity. For an investigation into the chiral gating effect on achiral MOFs, a pair of chiral probe molecules (S)- and (R)limonene were selected as probe molecules to study their adsorption behaviors in the SCEP-12 sample. Limonene does not bear active groups that may significantly affect physical adsorption behavior, and their molecular size matched the pore size of [Cu2((+)-Cam)2Dabco] and [Cu3(Btc)2]. Adsorption of (S)- and (R)-limonene over [Cu2((+)-Cam)2Dabco] has been demonstrated.33 The MOF thin film coupled with quartz crystal microbalance (QCM) technology has been extensively employed to investigate volatile organic compound (VOC) sorption behaviors.34−37 The attempts to disperse the SCEP-12 microcrystallite sample on a QCM sensor to measure the limonene sorption by QCM failed because of the loose interaction between powder sample and sensor, where mass change in the absorbent cannot be effectively transferred to the QCM sensor and then detected by QCM. We adopted an isochoric technique to study the sorption kinetics of limonene (vapor) in the [Cu3(Btc)2], [Cu2((+)-Cam)2Dabco], and SCEP-12 which has been widely employed for VOC sorption study in the literature.27,28 By exposing activated absorbent to limonene vapor and monitoring the pressure drop at constant temperature, we can calculate and make the plot of limonene sorption amount against time (see experimental details in the Supporting Information). The control experiment without absorbent gave rise to a negligible pressure change (Supporting Information, Figure S13). Limonene sorption over [Cu3(Btc)2] and [Cu2((+)-Cam)2Dabco] was carried out for comparison (Figure 4). As expected, sorption behaviors of (S)- and (R)limonene over achiral [Cu3(Btc)2] are almost identical (Figure 4a), because [Cu3(Btc)2] contains much larger pore sizes and has no discrimination toward limonenes with (S)- or (R)configuration. [Cu2((+)-Cam)2Dabco] bearing stereogenic centers exhibited a much faster adsorption for (S)-limonene than for (R)-limonene (Figure 4c). Hybrid SCEP-12 displayed a similar shape of adsorption curve toward both limonene enantiomers with [Cu2((+)-Cam)2Dabco] but a distinct saturated sorption amount due to the coexistence of a chiral center from [Cu2((+)-Cam)2Dabco] and a large pore volume from [Cu3(Btc)2] (Figure 4b). The control experiment of the
Figure 2. (top) PXRD patterns of [Cu3(Btc)2] (blue), [Cu2((+)Cam)2Dabco] (red), and SCEP-12 (black). (insert) Enlarged PXRD pattern in the range 9.0−9.8°. (bottom) (a−c) SEM images of [Cu3(Btc)2] at different magnifications and (d−f) SEM images of SCEP-12 at different magnifications.
reaction time was prolonged to 24 and 48 h, most [Cu3(Btc)2] octahedra were digested; separated [Cu2((+)-Cam)2Dabco] powder appeared, and the core−shell structure was destroyed. To determine the [Cu2((+)-Cam)2Dabco] amount on the surface, we digested the 12 h etching sample for 1HNMR measurements, which indicated the molar ratio of ligand (+)-Cam to Btc is 0.21 (Supporting Information, Figure S11). Hereafter, we selected SCEP-12 as an example to study its sorption isotherms and enantioselective sorption properties. As [Cu2((+)-Cam)2Dabco] and [Cu3(Btc)2] have different lattice parameters, we obtained the separated single components when we directly reacted presynthesized [Cu3(Btc)2] microcrystallites and Cu(II), (+)-Cam, and Dabco (Supporting Information, Figure S12). This implied that the etching process is slower than the reaction between Cu(II) and ligands of (+)-Cam and Dabco. In contrast, the reaction of Cu(II), Btc, (+)-Cam, and Dabco together gave rise to a mixed phase composed of [Cu2((+)-Cam)2Dabco] and [Cu3(Btc)2] (Figure S12). As a control experiment, we found that [Cu3(Btc)2] microcrystallites did not solely react with (+)-Cam or Dabco at the same conditions (Figure S12). 3.1. Gas Adsorption. According to N2 sorption isotherms as displayed Figure 3, Brunauer−Emmett−Teller (BET) specific surface areas were estimated to be 1591 and 414 cm3 g−1 for [Cu3(Btc)2] and [Cu2((+)-Cam)2Dabco], respectively. This is in accordance with the values reported in the literature.26,30 It is noted that the high sorption at relative P/ P0 close to 1 arose from the space among particles for [Cu2((+)-Cam)2Dabco].31,32 SCEP-12 exhibits a typical type-I 32266
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nonlinearity of adsorption curves (two-stage adsorption in Figure 4b,c).35 Herein, we chose a direct and facile way to analyze the adsorption kinetics according to the reported process by the literature,37,41 where adsorption rates were evaluated by the slope of each linear part of the adsorption curve. In general, adsorption amount and rate are proportional to the surface area and pore volume of absorbents in the sequence of [Cu3(Btc)2] > SCEP-12 > [Cu2((+)-Cam)2Dabco]. As presented in Figure 4a, [Cu3(Btc)2] with a large pore shows nearly the same adsorption amounts and rate for (S)-limonene (load, 56.8 mg g−1; slope, 3.3 mg g−1 s−1) and (R)-limonene (load, 56.4 mg g−1; slope, 3.2 mg g−1 s−1). There is no enantioselectivity for [Cu3(Btc)2] because of the lack of chiral centers. [Cu2((+)Cam)2Dabco] shows a much smaller saturated adsorption capacity in comparison with [Cu3(Btc)2] (Table 1). It is noteworthy that the two-stage (R)-limonene adsorption was observed for [Cu2((+)-Cam)2Dabco]. This may correspond to adsorption at the entrance (0.085 mg g−1 s−1) and diffusion inside of the channel (0.38 mg g−1 s−1) of [Cu2((+)Cam)2Dabco]. Gu and co-workers also reported this twostage adsorption behavior.33 [Cu3(Btc)2] did not show such behavior, whereas SCEP-12 containing [Cu 2 ((+)Cam)2Dabco] also exhibited two-stage adsorption. The initial adsorption rate of (R)-limonene in SCEP-12 (0.19 mg g−1 s−1) is faster than that in [Cu2((+)-Cam)2Dabco] (0.085 mg g−1 s−1). This suggested an incomplete coverage of [Cu2((+)Cam)2Dabco] on the surface of [Cu3(Btc)2]. In contrast, all samples showed a one-step linear adsorption toward (S)limonene. In terms of chiral separation, enantioselectivity is a critical parameter to evaluate the performance of absorbents. Here, enantioselectivity was roughly evaluated as the ratio of adsorption rate of (S)-limonene to (R)-limonene (second stage, in the case of [Cu2((+)-Cam)2Dabco] and SCEP-12). As summarized in Table 1, [Cu3(Btc)2] exhibited no enantioselectivity, whereas SCEP-12 led to a similar separation effect of homochiral [Cu2((+)-Cam)2Dabco] toward limonene enantiomers. It is clearly shown to us that installation of a chiral gate on an achiral channel achieves the same separation effect as homochiral MOFs.
Figure 4. (S)-Limonene (black) and (R)-limonene (red) sorption curves over (a) [Cu3(Btc)2], (b) SCEP-12, and (c) [Cu2((+)Cam)2Dabco] at 278 K. The adsorption rates (mg g−1 s−1) are evaluated by the slope of each linear part of the sorption curve.
enantioselective sorption is conducted by a mixture (molar ratio, [Cu2((+)-Cam)2Dabco]/[Cu3(Btc)2] = 0.21:1), and their performances are provided in the SI (Figures S17 and S18). Ideally, the same saturated adsorption amounts for both limonene enantiomers over three samples should be reached at equilibrium conditions, considering the same limonene volume in both the (R)- and (S)-configuration. This is consistent with our data in Figure 4. The adsorption data (adsorption amount and kinetics) are summarized in Table 1. However, because of the stereogenic effect ((+)-Cam ligand in this work), [Cu2((+)Cam)2Dabco] and SCEP-12 gave rise to distinct adsorption kinetics toward limonene enantiomers. This is the fundamental idea of chiral separation. Different physical models have been employed to fit and understand adsorption kinetics processes in microporous absorbents.38−40 We attempted to employ the linear driven force (LDF) model to fit the adsorption curves. Unfortunately, we failed to acquire a satisfied regression coefficient (Supporting Information, Figures S19−22 and Table S1). The discrepancy between the experimental data and the prediction of these models is largely due to the
4. CONCLUSION In summary, we developed a superficial chiral etching process to install a chiral porous gate on an achiral MOF surface, and we illustrated the strategy with a [Cu3(Btc)2]@[Cu2((+)Cam)2Dabco] core−shell hybrid. Superficial layers of presynthesized [Cu3(Btc)2] behave as not only a core material but also a copper precursor for [Cu2((+)-Cam)2Dabco] shell synthesis. This is the key to the success of preparing a core−
Table 1. Limonene Adsorption Data (Amount and Rate) over [Cu3(Btc)2], SCEP-12, [Cu2((+)-Cam)2Dabco], and a Mixture (Molar Ratio, [Cu2((+)-Cam)2Dabco]/[Cu3(Btc)2] = 0.21:1) amount [mg g−1] adsorption ratea [mg g−1 s−1] enantioselectivityb
(S)-limonene (R)-limonene (S)-limonene (R)-limonene
[Cu3(Btc)2]
[Cu2((+)-Cam)2Dabco
SCEP-12
mixture
56.8 56.4 3.3 3.2 1.03
24.4 23.9 0.57 0.085/0.38 1.5
36.7 36.6 1.42 0.19/0.81 1.75
38.9 38.2 1.79 1.57 1.14
Adsorption rates (mg g−1 s−1) are evaluated by the slope of each linear part of the adsorption curve. bEnantioselectivity was roughly evaluated as the ratio of adsorption rate of (S)-limonene to (R)-limonene (second stage in the case of [Cu2((+)-Cam)2Dabco] and SCEP-12). a
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shell MOF hybrid structure with unmatched lattice parameters. Chiral shell modification does not change the porosity of the core MOF but dramatically alters its surface properties and hence allows enantioselective sorption toward limonene enantiomers. Superficial etching could be a universal approach for preparing such hybrid MOFs. The SCEP strategy will pave a novel way to explore achiral MOF materials for enantioselective adsorption and separation in an economic way in comparison with costly homochiral MOFs.
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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/acsami.7b10147. Experimental details, additional figures and images, sorption curves, PXRD patterns, SEM images, and 1 HNMR data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bo Liu: 0000-0002-1150-3709 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from National Natural Science Foundation of China (NSFC, 21571167, 51502282), Chinese Academy of Sciences, the Fundamental Research Funds for the Central Universities (WK2060190053), and Anhui Province Natural Science Foundation (1608085MB28). Mr. Junxiang Zhang is thanked for his valuable discussion.
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REFERENCES
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