Dual activity of durable chiral hydroxyl-rich MOF for asymmetric catalytic reactions

Article abstract of DOI:10.1016/j.jcat.2019.08.015

The quest to prepare of asymmetric heterogeneous catalysts with both effective Br?nsted acid sites (BASs) and Lewis acid sites is very significant challenge. Herein, we report the construction of a chiral metal-organic framework with two kinds of catalytic active sites (Lewis acid/Br?nsted acid). It contains coordinative unsaturation metal centers and chiral functional groups that have cooperation in the catalytic activity. In the synthesized CMOF, the chiral decoration of metal node was performed through the practical method: anions exchange hypothesis (post-synthetic exchange). For this aim, the elimination of framework fluorides happened by using the enantiopure auxiliary anions (L-(+)-tartrate anion (tart^?)) that led to a chiral cationic MOF with eventual chemical formula [Cr₃tart(H₂O)₂O(bdc)₃]. XRD, BET, ¹H NMR, SEM and EDX were employed to characterize of the present CMIL. Despite the chiral tartrate anions generate a chiral environment, they have main role in the activating of epoxide ring due to hydrogen-bonding interaction. Experiments show that the enantiopure tartrate-functionalized MIL-101(Cr) as a green asymmetric catalyst has the considerable performance in the enantioselective reactions due to chiral modified surface without remarkable loss in activity.

Full text of DOI:10.1016/j.jcat.2019.08.015

Journal of Catalysis 378 (2019) 28–35
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Dual activity of durable chiral hydroxyl-rich MOF for asymmetric
catalytic reactions
⇑
Kayhaneh Berijani, Ali Morsali
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The quest to prepare of asymmetric heterogeneous catalysts with both effective Brønsted acid sites
(BASs) and Lewis acid sites is very significant challenge. Herein, we report the construction of a chiral
metal-organic framework with two kinds of catalytic active sites (Lewis acid/Brønsted acid). It contains
coordinative unsaturation metal centers and chiral functional groups that have cooperation in the cat-
alytic activity. In the synthesized CMOF, the chiral decoration of metal node was performed through
the practical method: anions exchange hypothesis (post-synthetic exchange). For this aim, the elimina-
tion of framework fluorides happened by using the enantiopure auxiliary anions (L-(+)-tartrate anion
(tartÀ)) that led to a chiral cationic MOF with eventual chemical formula [Cr3tart(H2O)2O(bdc)3]. XRD,
BET, 1H NMR, SEM and EDX were employed to characterize of the present CMIL. Despite the chiral tartrate
anions generate a chiral environment, they have main role in the activating of epoxide ring due to
hydrogen-bonding interaction. Experiments show that the enantiopure tartrate-functionalized MIL-101
(Cr) as a green asymmetric catalyst has the considerable performance in the enantioselective reactions
due to chiral modified surface without remarkable loss in activity.
Received 5 July 2019
Revised 10 August 2019
Accepted 12 August 2019
Keywords:
Chiral metal-organic framework
Brønsted acid
Lewis acid
Prochiral substrate
Asymmetric catalysis
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
cal technology [12], proton conductivity [13], gas sensing and stor-
age [14,15]. The CMOFs are synthesized via well-established
Chirality as an important geometric property is one of the fun-
damental features in the nature especially in the biological struc-
tures and drug industry, so the designing of the new chiral
systems has been considered [1]. The chiral compounds have use-
ful potential applications in different areas therefore; the asym-
metric catalytic reactions are one of the mandatory processes to
make of them [2–4]. These reactions are better performed in the
presence of the various chiral catalysts, homogeneous and hetero-
geneous, such as chiral metal salen complexes [5,6], chiral ketone-
derived organocatalysts [7], chiral metallo-porphyrin [8] and chiral
magnetite manganese-porphyrin [9]. But, the chiral porous sys-
tems as the fascinating platforms can reveal striking properties,
performance and unique characteristics. Among them, the chiral
metal-organic frameworks (CMOFs) with the different morpholo-
gies can be ideal candidates in asymmetric catalysis with adjusta-
ble components like linkers, nodes, functional groups [10]. The
designing of MOFs is very valuable specially the creation of CMOFs,
with the various stereotype components, is a new open window
toward more different applications as catalysis [11], pharmaceuti-
methods with three classifications: Direct synthesis, Chiral-
template synthesis and Post-synthesis [16,17]. Sometimes the chi-
ral agents are not necessarily to synthesize of CMOFs. For example,
in 1999, Aoyama and co-workers introduced a synthesized MOF
with achiral linker that its chirality was due to helical array [18].
But, the using of the chiral molecules is more effective way than
the inherent chiralization of the framework due to the complex
topology. In the first state, the starting organic linker can be chiral
that this procedure is difficult due to uncontrollable conditions and
diverse complexities. Also, the achiral linker can be chiralized with
functional groups as NH2 via post-synthetic modification (PSM)
method. Recently, five kinds of chiral MIL-101(Al)–NH2 were gen-
erated by using five various chiral compounds through PSM
approach [19]. However, CMOFs were constructed by using the
organic linkers that have chiral multifunctions [17,20]. Metal
nodes of MOFs can be also modified by the chiral species [16]. It
is concluded that the accessible positions on the surface or inside
pores of the achiral frameworks in the porous systems can be chi-
ralized by pre- and post-synthesis strategy [21]. Postmodification
is a simple and straight pathway to product the chiral functional
materials. This general method can be applied to construct of
CMOFs that have numerous catalytic chiral sites and they are
⇑
Corresponding author.
E-mail address: morsali_a@modares.ac.ir (A. Morsali).
https://doi.org/10.1016/j.jcat.2019.08.015
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

K. Berijani, A. Morsali / Journal of Catalysis 378 (2019) 28–35
29
employed in the asymmetric transformations. Even, the chiral
molecular catalysts can be also heterogenized by MOFs for the
facile recovery and reuse in the catalytic reactions [22]. Totally,
in the catalytic asymmetric transformations, designing, porosity,
functional groups, the accessible catalytic active sites, stability in
catalyst have a significant role in the generation of enantioselective
product [23]. These factors affect on the interaction between solid
catalyst and reactant.
Brønsted acidity sites is more complicated [26,27]. In comparison
to the previous reported structures, this kind of chiral MOF has
not been presented until now.
2. Results and discussion
2.1. Synthesis of MIL-101(Cr)-tart
Herein, we report the formation of a chiral MIL-101(Cr) as a
asymmetric heterogeneous catalyst (MIL-101(Cr)-tart: Cr3O
(BDC)3tart) via PSM for the enantioselective catalysis. There are
two separate catalytic active centers (Scheme 1), Brønsted and
Lewis acid which show catalytic activity for proceeding of the
enantioselective epoxidation and methanolysis reactions. Open
metal sites (Cr) as Lewis acid centers are generated from the
removing of the adsorbed solvent molecules by activation process
that MIL framework is not damaged during activation conditions.
The Brønsted acid centers (tartrate anion) are found in the different
ligands with the various functional groups like –OH and –SO3H
that they can attach to the open metal sites. To functionalize of
the open metal nodes with chiral ligand, we used tartrate anion
because it has been derived from tartaric acid which is as the foun-
dation of the various asymmetric organocatalysts. The synergistic
effect of coordinated chiral functional groups and open Cr sites of
MIL framework in the catalytic experiments was observed. These
centers can activate substrates in the asymmetric epoxidation/
methanolysis reactions. The Cr metals show a high activity as
Lewis acid centers in the oxidation of olefins. Unlike the unsatu-
rated metal sites, the functional groups of tartrate as Brønsted acid
display a high acidic activity in the asymmetric methanolysis since
they are more acidic [24]. Our aim was to chiralize a nonfunction-
alized MIL-101(Cr) as an acidic solid catalyst with mixing of the
catalytic active sites, Brønsted and Lewis acidity. These kinds of
MOFs have rarely been reported [25]. The findings showed that
not only the framework provides the accessible catalytic sites but
also the manipulation of them (surface or pore) with different
functional groups, can prepare new opportunities of catalytical
units. Several MOFs with Lewis acid centers were synthesized
and used in the various reactions but the preparing of MOFs with
Chemicals and characterization methods have been reported in
S1. The MIL-101(Cr) was constructed according to reported
hydrothermal method [28]. Without additional explanations, the
mixture of H2BDC (164 mg), Cr(NO3)3Á9H2O (400 mg), HF (1 mmol)
and D.IÁH2O (4.8 mL) was transferred into a 25 mL autoclave and
heated in an oven under optimized condition (220 °C/8h). The fil-
tered crystalline powder was washed by hot ethanol and NH4F
solution to eliminate of the unreacted components, metal and
organic linker. At the end, MIL-101(Cr)-tart was synthesized by
placing of the MIL-101(Cr)-F in the concentrated silver tartrate
solution over 20 h at 90 °C, then the obtained solid was repeatedly
washed with a certain amount of water, ethanol and, it was placed
in an oven under vacuum to be dried.
2.2. General procedure of asymmetric epoxidation, methanolysis and
reusing catalyst
The catalytic epoxidation experiments were carried out with
2 mmol styrene as a model substrate, 5 mmol isobutyraldehyde,
0.05 g catalyst, oxygen and 5 mL acetonitrile. Then this reaction
mixture was constantly stirred at 80 °C for 8 h. During catalytic
reaction, conversion, selectivity of products and enantiomeric
excess were investigated via injection to the chiral GC and finally
1H NMR was employed to corroborate of the generated products.
The methanolysis reaction was performed with 1 mmol styrene
oxide in 10 mL MeOH with 0.05 g catalyst for 9 h at 40 °C and
500 rpm in air atmosphere. The final reaction mixture was cooled
to room temperature and after removing the catalyst, it was ana-
lyzed by chiral GC and 1H NMR, too. To investigate of the reusabil-
ity of MIL-101(Cr)-tart in both of catalytic reactions, after the
finishing the reaction, the catalyst was separated from the reaction
Scheme 1. Synthetic route of chiral heterogeneous catalyst [MIL-101(Cr)-tart].

30
K. Berijani, A. Morsali / Journal of Catalysis 378 (2019) 28–35
mixture, washed with the specific solvents (acetonitrile and
methanol in the epoxidation and methanolysis, respectively) and
dried in oven at 80 °C. Then, the used MIL-101(Cr)-tart was repeat-
edly employed in new similar catalytic reaction with fresh sub-
strate under the optimized conditions. At the end, solutions of
catalytic reactions were injected to chiral GC.
IR of chiral MOF has been shown. (S2 in the Supporting Information
(SI)). The intense band at about 1700 cmÀ1 is related to the L-(+)-
tartrate anion (tartÀ) [29]. The other absorption bands can be
assigned to: CrAO vibration (566 cmÀ1), CAH deformation (734,
887, 950, 1124 cmÀ1), symmetric vibrations of OACAO
(1425 cmÀ1), C@C (1512 cmÀ1), ms(COO) and mas(COO) (1300–
1700 cmÀ1) [30].
Both PXRD pattern and SEM image of MIL-101(Cr)-tart have
been showed in Fig. 4a, that the perceptible changes were not spec-
ified. Therefore, MIL-101(Cr)-tart clearly retained their crystalline
structure and cubic morphology than parent MOF after postmodi-
fication by chiral component. Hence, the chiral post-modification
step not only did not change the structural features of MIL-101
(Cr) but also modified achiral MIL surface. To demonstrate of the
chemical composition of MIL-101(Cr)-tart and the degree of the
incorporated tartrate into achiral framework, 1H NMR spec-
troscopy of the digested CMOF (by D2SO4) was investigated (S3).
This technique is a useful characterization to detect of the chiral
ligand protons in CMOF. In addition to the hydrogens signal of
the linker, a new signal revealed at about 4.8 ppm that could be
assigned to chiral species. The ratio of tartrate to Cr was estimated
by 1H NMR: 1.16:3. To confirm the replacement of F by tartrate,
EDXs before and after modification have been studied (Fig. 1).
The decrease and increase of F and O peaks intensity at about
0.50 and 0.25 keV, respectively, show that the anion exchange hap-
pened in the concentrated silver tartrate solution. So these changes
indicated that the incorporation of tartrate anions has successfully
occurred with the formation of silver(I) fluoride.
2.3. Structural characterization of MIL-101(Cr)-tart
Before the catalytic investigations, the structure of MIL-101(Cr)-
tart as a new chiral heterogeneous catalyst was studied. To remove
of the first anions of framework (bonded F to Cr), the anions dis-
placement process was performed. To accomplish this aim, the
synthesized MIL-101(Cr) was soaked in the concentrated silver tar-
trate solution. The inclusion of the tartrate anion to the framework
structure was observed (bonded tartrate to Cr) (post-synthetic
modification via ion-exchange: F with tartrate). According to our
suggestion in the preparation process of MIL-101(Cr)-tart, the
special interactions can be considered. There is an electrostatic
attraction in the silver tartrate salt that has been synthesized from
L-(+)-tartaric acid and AgNO3. In the first step, to convert the
neutral framework of MIL-101(Cr) into cationic framework, the
stripping of FÀ anion was performed by using the concentrated
silver tartrate. The advantage of this method is the using of a sim-
ple chiral salt to eliminate of the F anion on Cr as a monovalent
anion. The non-covalent acid-base interaction between Ag and F
goes toward the formation of a new chiral MOF (Scheme 2). It
has been reported which the functionalized porous materials such
MOFs can be used as ion exchangers. Due to their chemical struc-
ture, they can be cation or anion exchangers.
The porosity of chiral MIL was determined by the N2 adsorption
at 77 K (S4). BET surface area and total pore volume of MIL-101
(Cr)-tart were 1760 m2 gÀ1 and 1.52 cm3 gÀ1, respectively.
Whereas, these parameters in MIL-101(Cr) are: BET surface area:
3133 m2 gÀ1, pore volume: 1.68 cm3 gÀ1. It is clear that the post-
synthetic modification has easily happened without changes in
the porosity of the synthesized CMIL. Additionally, Fig. 4a is a
The prepared sample was characterized by several techniques
that the results have been shown as follows. Normally, a routine
method to confirm PSM or the incorporation of a new ligand is IR
spectra. For comparing the MIL-101(Cr) versus MIL-101(Cr)-tart
and displaying of L-(+)-tartrate anion in the prepared CMOF, FT-
Scheme 2. Anatomy of the interactions in the synthesis of [MIL-101(Cr)-tart].

K. Berijani, A. Morsali / Journal of Catalysis 378 (2019) 28–35
31
methanol and ethane to ethanol [35], sulfoxidation of aryl sul-
phides [36]. So, MIL-101(Cr)-tart was used in the enantioselective
epoxidation of a series of olefins. Styrene was employed as model
substrate to determine the optimized conditions with three differ-
ent oxidants (Table 1). In the control experiments, the oxidation of
styrene was performed in H2O2 which the reaction was completed
with enantiomeric excess (ee) 68% ((S)-enantiomer) and epoxide
selectivity about 20% (Table 1, entry 2). The poor epoxide selectiv-
ity can be assigned to the generation of aldehyde products due to
the cleavage of epoxide bond (the most: benzaldehyde, the least:
phenylacetaldehyde).
In spite of these results, the oxidation styrene was also
investigated in presence of TBHP as oxidant, that the conversion
diminished (46%) and ee 65% obtained (the major isomer is
(S)-enantiomer) (Table 1, entry 3). Since, tert-Butyl hydroperoxide
is expensive and unfriendly environmentally, MIL-101(Cr)-tart
activity was evaluated in the presence of molecular dioxygen as
an available oxidant, cheap and safe [37,38].
(ii) Epoxidation by using IBA/O2
Fig. 1. EDX spectra of MIL-101(Cr) structure. (a) before and (b) after chiral
modification.
In comparing to the other oxidants, under identical conditions
(solvent, temperature and time) MIL-101(Cr)-tart showed better
catalytic activity in the styrene epoxidation by using O2 and isobu-
tyraldehyde (IBA) with conversion 87% and ee 89% to (S)-
enantiomer (Table 1, entry 1, TOF 167 hÀ1).
powerful reason to show the maintenance of MIL morphology
after chiral postmodification. Thereby, ion-exchange happened:
{[MIL-101(Cr)]+-[F]À converted to [MIL-101(Cr)]+-[tart]À} by
L-(+)-tartrate as
a simple chiral organic compound without
In point of catalysis, two parameters of epoxide selectivity/ee in
the catalytic reaction with H2O2 and conversion/ee in the catalytic
reaction with TBHP were lower than IBA/O2. So, we investigated
the preliminary reactions under aerobic condition with O2 as ter-
minal oxidant and isobutyraldehyde (Table 1). The asymmetric
epoxidation of styrene was performed in the absence of catalyst
which the negligible conversion was observed (conversion < 6%).
This reaction was performed to show the effect of catalyst nature.
The effect of co-catalyst and solvent was investigated in the pro-
gress of the reaction, too (entries 4–7). In the absence of IBA,
MIL-101(Cr)-tart did not catalyze the reaction because the activa-
tion of oxygen as essential factor did not happen (entry 4). In fact,
the used aldehyde is as a co-oxidant that has the activating role of
O2 with the producing of the acylperoxy radical. We used the com-
mon solvents of chemical reactions to indicate the polarity effect of
solvents in catalytic reaction: acetonitrile, ethylacetate, n-hexane
and methanol (entries 1, 5–7). According to the obtained evidences,
complexity. It seems that in addition to the Cr centers as acidic
catalysts, the functional groups in tartrate anions have catalytic
activity in the methanolysis via hydrogen bonding.
So the catalytic behavior of this chiral catalyst was investigated
in enantioselective epoxidation of olefins and methanolysis of styr-
ene epoxide that the evaluated results reported as follows.
2.4. Evaluation of enantioselective catalytic reactions under various
conditions
(i) Epoxidation by using H2O2 and TBHP
MIL-101 can be utilized as acidic catalyst in the different reac-
tions like oxidation reactions that are important processes in the
chemical transformations [31–33]. Some examples are the benzylic
oxidation of hydrocarbons [34], the conversion of methane to
Table 1
Control experiments for enantioselective epoxidation of styrene.a
Entry
Catalyst
Catalytic conditions
Conv. (%)b
Epoxide selectivity (%)
Ee (%) (Conf.)c
1
2
3
4
[MIL-101(Cr)-tart]
[MIL-101(Cr)-tart]
[MIL-101(Cr)-tart]
[MIL-101(Cr)-tart]
IBA/O2/CH3CN/80 °C/8 h
H2O2/CH3CN/80 °C/6 h
TBHP/CH3CN/80 °C/8 h
No co-catalyst/O2/
87
100
46
70
16
81
89(S)
68 (S)
65 (S)
N.R.
CH3CN/80 °C/8 h
5
6
7
8
9
[MIL-101(Cr)-tart]
[MIL-101(Cr)-tart]
[MIL-101(Cr)-tart]
[MIL-101(Cr)] without L-tartrate ion
[MIL-101(Cr)-tart]
IBA/O2/n-hexane/80 °C/8 h
IBA/O2/EtOAc/80 °C/8 h
IBA/O2/CH3OH/80 °C/8 h
IBA/O2/CH3CN/80 °C/8 h
IBA/O2/CH3CN/40 °C/8 h
28
49
6
79
42
23
37
17
68
24
44 (S)
61 (S)
34 (S)
61(S)
a
b
c
i
Reaction conditions: catalyst 50.0 mg, ratio of PrCHO/styrene: 3 mmol, solvents (CH3CN, DMF, Toluene and CH3OH) 5 mL, molecular oxygen1 atm, 80 °C.
Conversions are based on the starting substrate and determined by GC.
Enantiomeric excess (Ee%) was determined by GC on a chiral SGE-CYDEX-B capillary columnÁH2O2 6 mmol. TBHP 6 mmol.

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K. Berijani, A. Morsali / Journal of Catalysis 378 (2019) 28–35
acetonitrile with higher polarity and lower coordinating ability
provide the appropriate medium for the catalyst performance com-
pared to other solvents [9]. MIL-101(Cr) actvity was also studied
without any chiral agent and as expected, the enantioselectivity
was not observed (entry 8). However, the reaction was not com-
pleted at 80°, but we did the oxidation of styrene at 40 °C that
the reaction was not completed (cnov. 42%, epoxide selectivity
24% and ee 61% to S-enantiomer). As well as, the MIL-101(Cr) abil-
ity was surveyed before the water exiting from structure (non-
activated catalyst) which it did not show the catalytic activity
(conversion<10%) (not displayed in Table 1).
In order to understand the catalyst efficiency, the enantioselec-
tive epoxidation of a few unfunctionalized olefins were examined
(Table 2). In the styrene oxidation, benzaldehyde (BA) and 2-
phenylacetaldehyde (PAA) as side-products were identified
(Table 1, entry 1).
Also, the oxidation of the styrene derivative,
a-methyl styrene
was checked after 8 h with the conversion 83% and ee 85% (S)
(Table 2, entry 1, TOF 159 hÀ1). Probably the decreasing of epoxide
and enantioselectivity in this reaction than styrene is related to the
steric effect of alkyl group around double bond. MIL-101(Cr)-tart
was served to catalyze 1-phenyl-1-cyclohexene oxidation, which
Table 2
Enantioselective epoxidation of several olefins.a.
c
Entry
Substrate
-methyl styrene
1-Phenyl-1-cyclohexene
1-Octene
1-Decene
trans-stilbene
Converdion (%)b/Time (h)
Epoxide selectivity (%)
Ee (%) (Conf.)
1
2
3
4
5
a
83/8
79/8
77/8
69/8
68
85 (S)
100
100
100
100
78 (R,R)
100 (R or S)
91 (R)
100/30 min
75 (R,R)
a
i
Reaction conditions: catalyst 50.0 mg (0.0013 mmol Cr/g catalyst), PrCHO/substrate 3 mmol, CH3CN 5 mL, molecular oxygen1 atm. b and c was mentioned in Table 1.
Fig. 2. Enantioselective epoxidation of the unfunctionalized olefins in the controlled stirring speed (500 rpm) with the acquired enantiomeric excess values by chiral-GC.

K. Berijani, A. Morsali / Journal of Catalysis 378 (2019) 28–35
33
both epoxide selectivity and ee were 100% and 78%, separately
(entry 2, TOF 151 hÀ1).
In the epoxidaion of two aliphatic hydrocarbons, 1-octene and
1-decene, a decrease was observed in the conversions (77% and
69%, entries 3 and 4, TOF 148 and 132 hÀ1 respectively). It is inter-
esting that in both of them, the high epoxide selectivity and enan-
tiomeric excess were seen after 8 h. Since, 1-decene is longer than
1-octene in term of the length, so the steric effect affects on the
results, certainly. The oxidation of trans-stilbene as an organic
compound with (E) stereochemistry was performed that the trans-
stilbene epoxide obtained with selectivity 100%. In this oxidative
reaction, (R,R)-trans stilbene epoxide enantiomer appeared with
ee 75% after 30 min without the additional products (diol or ben-
zaldehyde) (entry 5, TOF 769 hÀ1). Fig. 2 demonstrates the gained
ee values and the effect of L-tartrate anion as chiral agent on the
sterogenic centre configuration of the produced epoxids.
The possible mechanism was suggested in S6a. Table 1 in S5
presents data of the carbon balance calculation in some of the alke-
nes epoxidation. The percentage of carbon from the initial time of
the reaction to final time has been shown. The recyclability of MIL-
101(Cr)-tart in the successive reactions of the styrene epoxidation
was also studied. Conversion and ee were almost constant to the
fourth step, but they decreased in the next step (76% and 82%,
respectively, Fig. 3).
The lack of Cr in recycle solutions to the fourth step was proved
by ICP. PXRD measurement revealed that the crystalline structure
of chiral MIL-101(Cr)-tart is retained after 4 cycles with closely
matching to PXRD pattern of fresh MIL-101(Cr)-tart (Fig. 4b). Also,
SEM image of MIL-101(Cr)-tart after four recycles showed that the
particles size of the used catalyst changed somewhat than fresh
catalyst (Fig. 4).
Fig. 4. The investigation of MIL-101(Cr)-tart structure. (a) and (b) PXRD patterns
and SEM images (a) before and (b) after asymmetric epoxidatin catalysis.
(iii) Methanolysis of styrene oxide
study with the other works, we expected that this chiral MOF to
show catalytic behavior in the asymmetric methanolysis due to
the density of the chiral functional groups (as Brønsted acid).
According to the constructed CMOF features, we focused on asym-
metric methanolysis of styrene epoxide. The experiments con-
firmed our assumption whereas the affinity between CMOF and
styrene epoxide was correct; the results showed that the epoxida-
tion of styrene over MIL-101(Cr)-tart in MeOH was negligible
because the Cr3+ nodes lost their performance. Solvent molecules
as nucleophile coordinated to unsaturated metal centers and
almost they turned off. But the chiral groups showed stronger
effect in compared to Cr sites in methanolysis of styrene epoxide.
The asymmetric methanolysis of styrene epoxide was done over
achiral MIL-101(Cr) at 40 °C with low conversion (Conv.%<20%)
Several MOFs as acidic catalysts were presented in the previous
reports with Brønsted acidity (BA), Lewis acidity (LA) or both of
them as active sites. They have been investigated in the different
chemical reactions such as ring-opening of the epoxide [39], con-
densation of aldehydes with alcohols [40] and alcoholysis of styr-
ene oxide [41]. Even, this capability (both of BA/LA) can be used
in the tandem reactions like the use of Hf-NU-1000 in the tandem
epoxidation and then the epoxide opening [42]. From comparative
for 48 h. But the present CMOF (combination of tartrate and Cr3+
)
demonstrated the high conversion and enantioselectivity values
(100% and 90% to S-enantiomer, TOF 221 hÀ1) for 9 h (Table 3,
entries 1 and 2). The methanolysis of styrene oxide at room tem-
perature was also investigated that the conversion was not com-
pleted (cnov. 41% and ee 53% to S-enantiomer for 9 h).
Separately, the catalytic activity of the tartrate was also studied
and it showed the high conversion close to 100%, ee 89% and enan-
tioselectivity 62% (main enantiomer: 2-methoxy-2-phenylethanol)
than MIL-101(Cr)-tart (it has not been shown in Table 3).
The ideal asymmetric induction has originated from the tartrate
anions with two chiral hydroxyl centers. These results show that
the product enantioselectivity is very sensitive to the pore/surface
surrounding of the catalysts [29]. To us, the host-guest interactions
can happen due to the proper porosity and available content of the
chiral group in the present chiral porous solid as host and styrene
epoxide as guest. In the crystal structure of MIL-101(Cr)-tart, tar-
trate anions have two different kinds of functional groups. They
can generate intermolecular hydrogen bonds toward epoxide ring
Fig. 3. MIL-101(Cr)-tart recyclability investigation for model substrate in the
asymmetric epoxidatin.

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K. Berijani, A. Morsali / Journal of Catalysis 378 (2019) 28–35
Table 3
Results of enantioselective methanolysis of styrene oxide.a
c
Entry
Catalytic center
Conversion (%)b/Time (h)
2-Methoxy-2-phenylethanol Selectivity (%)
Ee (%) (Conf.)
1
2
Cr3+ (Lewis acid)
[MIL-101(Cr)-tart]
10/48
100/9
100
100
90 (S)
(Contains Brønsted and Lewis acid)
a
Reaction conditions: catalyst 50.0 mg, styrene oxide 1 mmol, MeOH 10 mL, 40 °C. b and c was mentioned in Table 1.
of the reactant (tartrate O-H. . .O epoxide). In fact, the present
acidic centers can facilitate the ring opening of the SO due to: 1)
hydrogen bonding and 2) the proton transfer to the oxygen of
the epoxide ring. In the rich agreement with the mentioned
hypothesis, MIL-101(Cr)-tart is like the rest of the hydrogen-
bonding-based catalysts: probably, the formation of the 2-
methoxy-2-phenylethanol can be assigned to the ‘‘functional
groups-assisted” methanolysis due to hydrogen-bonds association.
Herein, it can be concluded that the non-covalent interactions can
affect on the improvement of the catalyst capability. These kinds of
interactions are very important particularly in drug delivery like
the electrostatic interactions between the functional groups of
the mesoporous structures and target molecules [43]. Thereby, it
should be noted that the combination of two abilities in the chiral
functional groups, as active centers and creation of the asymmetric
environment is an advantage for better asymmetric catalysis. Car-
bon balance was calculated in the enantioselective methanolysis of
styrene epoxide, too. In initial time of reaction, the carbon percent
was 79.97% and after 9 h, the reaction completed. In final time, the
carbon percentages were 75.97% and 4.00% related to (S)- and (R)-
enantiomer products, respectively.
To show of the maintenance of the recycled catalyst structure
and morphology, PXRD and SEM were used. As shown in Fig. 6,
the appeared changes were not sensible in the reused catalyst than
pristine MIL-101(Cr). These characterizations disclose that MIL-
101(Cr)-tart has remarkable stability. The methanolysis mecha-
nism has been shown in S6b.
Also, hot filtration test (HFT) as a general technique was per-
formed to indicate the catalyst effect in the progress of two studied
catalytic reactions (S7). In the epoxidation reaction of styrene after
2 h and methanolysis of styrene oxide after 3 h, the chiral catalyst
was removed from the catalytic reactions. Then, the obtained solu-
tions were stirred to optimized time at the determined tempera-
ture. The results showed that the reactions without the presence
of catalyst did not proceed, considerably. Therefore, this experi-
ment is a good procedure to find the heterogeneity. Of course,
the catalytic behavior of the present catalyst was compared to sev-
eral chiral heterogeneous catalysts in both of asymmetric catalytic
reactions: olefin epoxidation and ring-opening (S8 and S9).
It is worth that MIL-101(Cr)-tart can be recycled from reaction
mixture and reused in the same new catalytic reaction up to five
times without significant changes in the results and no detectable
Cr is seen by ICP. At the sixth step, the reduction of the conversion
and ee was observed to 89% and 86%, separately (Fig. 5).
Fig. 5. MIL-101(Cr)-tart recyclability investigation for model substrate in the
Fig. 6. The investigation of MIL-101(Cr)-tart structure. PXRD pattern and SEM
asymmetric methanolysis.
image after asymmetric methanolysis at the fifth cycle.

K. Berijani, A. Morsali / Journal of Catalysis 378 (2019) 28–35
35
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In summary, we reported an asymmetric cationic functional
MOF that has been fabricated by chiral post-synthetic modifica-
tion: incorporation of tartrate anions into the Cr3+ node of the neu-
tral framework with FÀ stripping. Chiral MIL with Lewis acids
(Cr3+) and the existent hydrophilic functional groups as Brønsted
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olefins and better catalytic behavior in the enantioselective
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abundant chiral centers. It can be used in the recycle experiments
without any damaging of the structure. In the future, we will
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