Superficial Chiral Etching on Achiral MetalâOrganic Framework for

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Superficial Chiral Etching on Achiral MetalOrganic Framework for Enantioselective Sorption Xudong Hou, Tingting Xu, Yang Wang, Shengjun Liu, Jing Tong, and Bo Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10147 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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ACS Applied Materials & Interfaces

Superficial Chiral Etching on Achiral MetalOrganic 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

ABSTRACT: Chiral recognition and separation is of general research interests in natural product separation and pharmacy industry. In this work, we develop a novel strategy to modify achiral porous metal-organic framework (MOF) surface via a superficial chiral etching process (SCEP), in which reacting pre-synthesized achiral MOF with chiral ligand produces an achiral@chiral MOF core-shell hybrid composition. SCEP creates chiral species on achiral porous MOF surface but does not change the porosity and pore structure, enabling the core-shell composition enantioselective sorption. Reacting (+)-camphoric acid (+)-Cam and 1,4diazabicyclo[2.2.2]octane (Dabco) with [Cu3(Btc)2] microcrystals leads to a chiral MOF of [Cu2(+Cam)2Dabco]

crystallites

attached

on

surface

of

[Cu3(Btc)2]

(Btc

=

1,3,5-

benzenetricarboxylate). The resulting [Cu3(Btc)2]@[Cu2(+Cam)2Dabco] core-shell composition displays preferred sorption kinetic towards (S)-limonene against (R)-limonene, with a similar discrimination effect with pure chiral [Cu2(+Cam)2Dabco]. Superficial chiral etching of 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

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1. INTRODUCTION Homochiral metal-organic frameworks (h-MOFs) have been extensively studied owing to 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 syntheses2: (i) self-resolution from achiral ligands during crystal growth,4-6 (ii) chiral template induction7,8 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 due to complexity and difficulty for preparation of enantiopure ligands. h-MOFs are a kind of ideal candidates for chiral separation owing to their chiral pores, high porosity. There are two steps in a typical chiral separation process by h-MOFs. Chiral surface/pores discriminates racemic mixture firstly, depending on their different steric configurations. Subsequently, enantiopure species passes through the channels at different time scale, namely separation. The channels in the second step are not necessarily chiral as it could only function as a 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 much expensive less enantiopure chiral molecules. Among > 20,000 reported MOF structures, h-MOF is very small portion. The preparation of pure homochiral MOFs often involves large amount of costly homochiral ligands. Therefore, enabling achiral MOFs separating chiral molecules by superficial chiral modification will significantly extend MOFs’ applications in chiral separation and pharmacy industry. Hybrid MOFs, including multivariate metal organic frameworks,12-14 MOF-on-MOFs,15 coreshell MOFs16-18 etc., have been developed to introduce complementary properties by assembling organic ligands bearing the same coordination function but different substitution groups or


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different metal ions having the same coordination geometry into one single piece of crystal. Success of this kind of hybrid MOFs relies on the lattice matching in different single MOF components. Rosi and coworkers has demonstrated mixture of bio-MOF-11 and 14 as core successfully induced bio-MOF-14 shell growth for selective CO2 capture, however, bio-MOF-14 cannot directly grow on bio-MOF-11 due to lattice mismatching.19

Figure 1. Schematic illustration of superficial chiral etching process. Superficial layers of presynthesized MOF of [Cu3(Btc)2] provides Cu(II) source to react with (+)-Cam and Dabco, producing [Cu2(+Cam)2Dabco] on surface of achiral parent MOF, leading to a achiral MOF@homochiral MOF core-shell hybrid structure. Octahedra represent [Cu3(Btc)2] (left) and [Cu3(Btc)2]@[Cu2(+Cam)2Dabco] (right), respectively. In comparison, post-synthetic modification strategy dramatically decreases pore sizes and porosity of parent MOFs.20-22 On the other hand, monolayer modification on MOF outer surface has been achieved via stepwise liquid epitaxial method23 and “substitution” reaction.14,

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However, it remains a great challenge to prepare chiral-achiral hybrid MOFs and there is no such reports so far, to best of our knowledge. In this work, we proposed a superficial chiral etching process (SCEP) which not only etches surface but also creates new phase to prepare achiral MOF@homochiral MOF core-shell hybrid structure and illustrated the strategy by [Cu3(Btc)2]@[Cu2(+Cam)2Dabco] using [Cu3(Btc)2]25 and [Cu2(+Cam)2Dabco]26 as achiral core and chiral shell components, respectively (Figure 1). The resulting [Cu3(Btc)2]@[Cu2(+Cam)2Dabco] composition represents the first example of achiral@chiral hybrid MOF. It exhibited a similar saturated sorption capability to (R)- and (S)limonene (the information of molecular structures is provided in Figure S2), but a preferred (S)limonene kinetic sorption rate against (R)-limonene. Thus, we achieved enantioselective sorption using achiral MOF via superficial chiral etching process. 2. EXPERIMENTAL PART 2.1. Materials. 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); 1,4-diazabicyclo[2.2.2]octane (98%, Macklin); (+)-Limonene (R-Limonene) (>95%, TCI), (-)-Limonene (S-Limonene) (>95%, TCI). 2.2. Preparation of [Cu2(+Cam)2Dabco], [Cu3(Btc)2] and SCEP-xx: 2.2.1 Syntheses of [Cu3(Btc)2] and [Cu2(+Cam)2Dabco]. [Cu3(Btc)2] was synthesized according to a modified methods from literature25. 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 hours in a in the screwed-cap Teflon autoclave. 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 methods26. Mixture of


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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 screwed-cap Teflon autoclave at 110 oC 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, pre-synthesized [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, the reaction mixture was heated at 110 °C for 6, 12, 24 or 48 hours. 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-xx (xx represents for reaction time). 2.3 Limonene sorption kinetic rate measurements. We adopted an isochoric technique which has been widely employed for VOC sorption study in literature.27-28 (See Supporting Information for full experimental details) Activated [Cu3(Btc)2], [Cu2(+Cam)2Dabco] and SCEP12 (10 mg for each sample) was loaded into a small glass tube (V=1.59 mL) and degassed again at 120 oC for 12 hours 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. Open the value connect both tubes to allow adsorption start and record the pressure drop against time. The pressure drop was transferred to adsorbed limonene amount. The adsorption rates were evaluated by slope of linear part of the adsorption curve. 3. RESULTS AND DISCUSSION We obtained [Cu3(Btc)2]@[Cu2(+Cam)2Dabco] core-shell hybrid via one-pot solvothermal reaction of pre-synthesized [Cu3(Btc)2] microcrystallites with (+)-Cam and Dabco. In this


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process, [Cu3(Btc)2] (Figure S4) behaves as not only core components but also Cu(II) precursor for growth of [Cu2(+Cam)2Dabco] (Figure S5). The reaction mechanism is speculated that 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 surface of unreacted [Cu3(Btc)2] and resulting in core-shell hybrid structure. We defined this process as a superficial chiral etching process (SCEP). Powder X-ray diffraction (PXRD) patterns of [Cu3(Btc)2] after etching with (+)-Cam and Dabco for 12 hours (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 give direct evidence of the core-shell structure as shown in Figure 2. In consistent with literature report,29 [Cu3(Btc)2] displays regular octahedral morphology with a relatively smooth surface (Figure 2a-c). After etching for 12 hours, the octahedra shape remains but dense micro-crystalline blocks appeared on its surface (Figure 2d-f), and no obvious individual [Cu3(Btc)2] particles were observed.


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Figure 2. Top: PXRD patterns of [Cu3(Btc)2] (blue), [Cu2(+Cam)2Dabco] (red) and SECP-12 (black). Insert: enlarged PXRD pattern in range from 9.0 to 9.8°. Bottom: (a)-(c) SEM images of [Cu3(Btc)2] at different magnification and (d)-(f) SEM images of SECP-12 at different magnification. According to SCEP definition, it is expected that changing reaction conditions could control the etching depth. We systematically investigated etching time influence over the final products and monitored by PXRD and SEM (Supporting information, Figure S6-10). After etching for 6 hours, we started to see obvious particles scattered on [Cu3(Btc)2] octahedra. Etching for 12


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hours 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 prolonging reaction time to 24 and 48 hours, most [Cu3(Btc)2] octahedra were digested; separated [Cu2(+Cam)2Dabco] powder appeared and the core-shell structure has been destroyed. In order to determine the [Cu2(+Cam)2Dabco] amount on surface, we digested the 12 hours etching sample for 1HNMR measurements, which indicated the molar ratio of ligand of (+)-Cam to Btc is 0.21 (Supporting information, Figure S11). Hereafter, we selected SCEP-12 as 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 pre-synthesized [Cu3(Btc)2] microcrystallites and Cu(II), (+)-Cam, Dabco (Supporting information, Figure S12). This implied that etching process is slower than the reaction between Cu(II) and ligands of (+)-Cam and Dabco. In contrast, reaction of Cu(II), Btc, (+)-Cam, Dabco together gave rise to a mixed phases composed of [Cu2(+Cam)2Dabco] and [Cu3(Btc)2] (Figure S12). As a control experiment, we found that [Cu3(Btc)2] microcrystallites didn't solely react with (+)-Cam or Dabco at the same conditions (Figure S12). 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 for [Cu3(Btc)2] and [Cu2(+Cam)2Dabco], respectively. This is accordance with the values reported in literature.26, 28 It is noted the high sorption at relative P/P0 close to 1 arisen from the space among particles for [Cu2(+Cam)2Dabco].31,32 SCEP-12 exhibits a typical type I sorption behavior with a specific BET surface area at 1311 cm3/g, This suggested superficial etching did not dramatically decreased the BET surface area of parent MOF ([Cu3(Btc)2]).


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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 (solid symbols: adsorption; empty symbols: desorption). (R)- and (S)-Limonene adsorption and enantioselectivity. In order to investigate 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 SCEP-12 sample. Limonene does not bear active groups that may significantly affect physical adsorption behavior, and their molecular size matched with 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


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Figure 4. (S)-limonene (black) and (R)-limonene (red) sorption curves over (a) [Cu3(Btc)2], (b) SECP-12 and (c) [Cu2(+Cam)2Dabco] at 278 K. The adsorption rates (mg·g-1·s-1) is evaluated by slope of each linear part of sorption curve. MOF thin film coupled with quartz crystal microbalance (QCM) technology has been extensively employed to investigate volatile organic compounds (VOCs) sorption behaviours.3437

The trials to disperse SCEP-12 microcrystallite sample on a QCM sensor to measure the

limonene sorption by QCM failed owing to the loose interaction between powder sample and sensor, where mass change in absorbent can not be effectively transferred to 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 literature.27-28 By exposing activated absorbent to limonene


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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 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] were 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 a much larger pore sizes and has no discrimination towards limonenes with (S)- or (R)- configuration. [Cu2(+Cam)2Dabco] bearing stereogenic centers exhibited a much faster adsorption for S-limonene than R-limonene (Figure 4c). Hybrid SCEP-12 displayed a similar shape of adsorption curve towards both limonene enantiomers with [Cu2(+Cam)2Dabco] but distinct saturated sorption amount due to coexistence of chiral center from [Cu2(+Cam)2Dabco] and large pore volume from [Cu3(Btc)2] (Figure 4b) and the control experiment of the enantioselective sorption are conducted by Mixture (Molar ratio: [Cu2(+Cam)2Dabco] : [Cu3(Btc)2] = 0.21 : 1), and their performance are provided in SI ( Figure S17-18). 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 (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, due to the stereogenic effect (+cam ligand in this work), [Cu2(+Cam)2Dabco] and SCEP-12 gave rise to distinct adsorption kinetics towards limonene enantiomers. This is the fundamental 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 Linear Driven Force (LDF)


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model to fit the adsorption curves. Unfortunately, we failed to acquire satisfied regression coefficient (Supporting information, Figure S19-22, Table S1). The discrepancy between the experimental data and the prediction of these models is largely due to the nonlinearity of adsorption curves (two-stage adsorption in Figure b and c).35 Table 1: Limonene adsorption data (amount and rate) over [Cu3(Btc)2], SECP-12, [Cu2(+Cam)2Dabco] and Mixture (Molar ratio: [Cu2(+Cam)2Dabco] : [Cu3(Btc)2] = 0.21 : 1). The adsorption rates (mg·g-1·s-1) are evaluated by slope of each linear part of adsorption curve and the enantioselectivity was roughly evaluated as the ratio of adsorption rate of (S)-limonene to (R)-limonene (second stage in case of [Cu2(+Cam)2Dabco] and SCEP-12). [Cu3(Btc)2]

[Cu2(+Cam)2Dabco

SCEP-12

Mixture

Amount

(S)-limonene

56.8

24.4

36.7

38.9

[mg·g-1]

(R)-limonene

56.4

23.9

36.6

38.2

Adsorption rate

(S)-limonene

3.3

0.57

1.42

1.79

[mg·g-1·s-1]

(R)-limonene

3.2

0.085/0.38

0.19/0.81

1.57

1.03

1.5

1.75

1.14

Enantioselectivity

Herein, we chose a direct and facile way to analyze the adsorption kinetics according to the reported process by literautres,37, 41 where adsorption rates were evaluated by slope of each linear part of adsorption curve. In general, adsorption amount and rate are proportional to the surface area and pore volume of absorbents in 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] owing to lack of chiral centers. [Cu2(+Cam)2Dabco] shows a much smaller saturated


12

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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 channel (0.38 mg·g-1·s-1) of [Cu2(+Cam)2Dabco], respectively. Gu and coworkers also reported this two-stage adsorption behavior.33 [Cu3(Btc)2] did not show such behavior, whereas SCEP-12 containing [Cu2(+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·g1

·s-1). This suggested an incomplete cover of [Cu2(+Cam)2Dabco] on surface of [Cu3(Btc)2]. In

contrast, all samples showed a one-step linear adsorption towards (S)-limonene. In term of chiral separation, enantioselectivity is 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 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 to homochiral [Cu2(+Cam)2Dabco] towards limonene enantiomers. It is clearly showed us that installation of chiral gate on achiral channel achieves the same separation effect as homochiral MOFs. 4．CONCLUSION In summary, we developed a superficial chiral etching process to install chiral porous gate on achiral MOF surface, and illustrated the strategy by a [Cu3(Btc)2]@[Cu2(+Cam)2Dabco] coreshell hybrid. Superficial layers of pre-synthesized [Cu3(Btc)2] behave as not only core material but also copper precursor for [Cu2(+Cam)2Dabco] shell synthesis. This is the key to success of preparing core-shell MOF hybrid structure with unmatched lattice parameters. Chiral shell modification doesn't change porosity of core MOF but dramatically alter its surface properties


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and hence allows enantioselective sorption towards 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. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx xxx. Experimental details, additional figures and images, PXRD patterns, SEM images, 1HNMR etc. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT We gratefully acknowledge financial support from National Natural Science Foundation of China (NSFC, 21571167, 51502282), Chinese Academy of Sciences, and the Fundamental Research Funds for the Central Universities (WK2060190053), Anhui Province Natural Science Foundation (1608085MB28). Mr. Junxiang Zhang is thanked for his valuable discussion.

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TOC

We develop a novel strategy to modify achiral porous metal-organic framework (MOF) surface via a superficial chiral etching process (SCEP), in which reacting pre-synthesized achiral MOF with chiral ligand produces an achiral@chiral MOF core-shell hybrid composition. Chiral shell modification doesn't change porosity of core MOF but dramatically alter its surface properties and hence allows enantioselective sorption towards limonene enantiomers. We proved that installation of chiral gate on channel in achiral MOF achieve the same separation effect as homochiral MOF itself. Superficial chiral etching of porous achiral MOF represents an economic and efficient strategy for enantioselective separation.


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