Angewandte
Research Articles
Chemie
idine (bipy)) membrane which was synthesized via an in situ
MIL-53-NH2 in the form of nanocrystals with an average
growth method.[9] This membrane exhibited an ee value of
32.5% for racemic (Æ)-2-methyl-2,4-pentanediol (R-enantio-
mer in excess). Recently, our group developed a homochiral
zeolitic imidazolate framework (ZIF-8) membrane by incor-
porating l-histidine (l-His) into the ZIF-8 frameworks, which
exhibited a higher ee value up to 76% for chiral separation of
racemic 1-phenylethanol.[10] Although current homochiral
MOF membranes have demonstrated the feasibility to
achieve enantioselectivity, fabrication of high-quality homo-
chiral MOF membranes remains a challenge and requires
complex fabrication processes. Alternatively, the use of mixed
matrix membranes (MMMs) that combine the advantages of
a polymeric matrix and the molecular selectivity of micro-
porous materials can provide a simpler approach to achieve
enhanced permeability and selectivity. MOFs are ideal
candidates as filler in the polymer matrix to form MMMs
due to the presence of terminal organic ligands and versatile
chemical functionalities, which ensure good interfacial com-
patibility.[11] Compared with polycrystalline pure MOF mem-
branes, MOF-based MMMs have shown attractive properties
in terms of easy processing, satisfactory mechanical proper-
ties and potential of scalability, which are desirable for the
practical applications of membrane-based separation. High-
performance MOF-based MMMs have been widely reported
for potential applications such as gas separation and nano-
filtration.[12] Despite these successes, to our best knowledge,
no studies of homochiral MOF-based MMMs with high
enantioselective performance have been reported.
Herein, we report a new type of enantioselective MMM
derived from homochiral MIL-53 nanocrystals with polye-
thersulfone (PES) as a polymeric matrix. The homochiral
MIL-53 nanocrystals were successfully synthesized by graft-
ing amino acid (l-His for example) into the frameworks of
MIL-53-NH2. As-obtained MIL-53-NH-l-His nanocrystals
showed excellent enantioselective adsorption for R-(+)-1-
phenylethanol over S-(À)-1-phenylethanol. Notably, the
MIL-53-NH-l-His-based MMMs exhibited an ee value up to
100% for separation of racemic 1-phenylethanol, demon-
strating a facilitated transport mechanism for chiral resolu-
tion. Additionally, MIL-53-NH-l-Glu-PES MMM fabricated
by the same method also showed a high ee value up to 100%
for racemic 1-phenylethanol, which demonstrates the versa-
tility of fabricating homochiral MOF-based MMM. These
results pave a new way to explore the application of
homochiral MOF-based MMMs for efficient chiral resolu-
tion.
crystal size of 375 Æ 110 nm was synthesized through a typical
solvothermal approach.[14] Trapped ligands within MOF
cavities were removed through an activation process to
increase the accessibility of pores.[15] l-His molecules were
immobilized into MIL-53-NH2 by an amidation reaction
between carboxylic groups of amino acids and free amino
groups on MOF ligands (See Note S1 in the Supporting
Information). There is no apparent change in crystal mor-
phology of MIL-53 after modification with l-His, which is
shown in the scanning electron microscope (SEM) images of
MOF crystals (Figure S1). The powder X-ray diffraction
(PXRD) pattern (Figure 1c) of as-prepared MIL-53-NH2
(MIL-53-NH2_as) is consistent with the PXRD pattern of
the large pore (lp) configuration, suggesting that there are
trapped ligands and solvent inside the MOF cavities.[16] After
the activation process, the PXRD pattern of MIL-53-NH2
matches with the narrow pore (np) configuration due to the
adsorption of water from air, demonstrating that trapped
ligands were successfully removed from the MOF pores.[16,17]
Moreover, we observed that the PXRD pattern of the l-His-
functionalized MIL-53-NH2 (MIL-53-NH-l-His) is similar to
that of the MIL-53-NH2_as. This indicated the incorporation
of amino acids into the cavities of the MIL-53-NH2 induced
the crystalline structure transformation of the MIL-53. The
successful incorporation of l-His into the MIL-53-NH2
framework was further confirmed by Fourier-transform infra-
red (FTIR) spectroscopy. As shown in Figure S4, MIL-53-
NH-l-His showed an absorption peak at 1670 cmÀ1, which
corresponded to the amide group formed between the
carboxylic group of l-His and amino group on the MIL-53
framework, whereas FTIR spectroscopy of MIL-53-NH2 did
not have this absorption peak. The amount of l-His
(23.1 wt%) incorporated into the MIL-53-NH2 was estimated
by thermogravimetric analysis (TGA) (Figure S5 and Note S2
in the Supporting Information).
Circular dichroism (CD) spectroscopy was also performed
to characterize the homochirality of the MIL-53-NH-l-His
nanocrystals. Compared with unmodified MIL-53-NH2, there
is an adsorption peak at 215 nm in the CD spectrum of MIL-
53-NH-l-His, demonstrating the homochirality of the MIL-
53-NH-l-His (Figure S6). The Brunauer–Emmett–Teller
(BET) surface area of MIL-53-NH-l-His is 84.7 m2 gÀ1, which
is much lower than the BET surface area of pristine MIL-53-
NH2 (SBET = 786.8 m2 gÀ1), suggesting that most of the pore
volume is occupied by the l-His (Figure S7). The reduction of
microporosity was confirmed by positron annihilation life-
time spectroscopy (PALS), in which the average pore
diameter of MIL-53-NH-l-His (5.39 Æ 0.08 ꢀ) is lower than
that of pristine MIL-53-NH2 (6.94 Æ 0.44 ꢀ) (Table S1). Solid
state NMR was also performed to verify the successful
incorporation of l-His into the MIL-53 (Figure 1d). In
contrast with the unmodified MIL-53-NH2, additional peaks
attributed to the l-His can be observed from the 13C NMR
spectra of MIL-53-NH-l-His crystals along with several peak
shifts, which confirmed the incorporation of the l-His into the
MIL-53 framework.
Results and Discussion
The homochiral MIL-53 was synthesized by post-synthetic
modification of achiral MIL-53-NH2 (Al) with l-His via
a classic EDC/NHSS coupling reaction (Figure 1). l-His is
a natural amino acid that is cheap and accessible in the
market, thus it was selected to be the incorporated enantio-
pure molecule. Due to the flexible framework structure and
reversible breathing effect of MIL-53-NH2, this MOF is an
ideal candidate as a host to immobilize guest molecules.[13]
Adsorption properties of MIL-53-NH-l-His nanocrystals
for racemic 1-phenylethanol were investigated prior to
Angew. Chem. Int. Ed. 2019, 58, 2 – 10
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