Studies on catalytic activity of MIL-53(Al) and structure analogue DUT-5(Al) using bdc- and bpdc-ligands functionalized with L-proline in a solid-solution mixed-linker approach

Article abstract of DOI:10.1016/j.mcat.2019.01.029

Proline-functionalized MOFs MIL-53(Al) (based on the benzene-1,4-dicarboxylate, bdc linker) and DUT-5(Al) (based on the biphenyl-4,4′-dicarboxylate, bpdc linker) were synthesized using a mixed-linker approach. The proline-functionalized bdc and bpdc ligand was mixed with both the non-functionalized ligand as well as the amino functionalized one. The influence of the length of the linker on the catalytic activity showed a counterintuitive confinement effect and advantageous increase of ee with longer linker and higher porosity, that is a change of ee values from no selectivity in the MIL-53(Al) MOFs up to 16% ee for 27-L-Pro/H₂N-DUT-5(Al) and 19% ee for 26-L-Pro/H-DUT-5(Al). The conversion of 4-nitrobenzaldehyde remains over 85% after three catalytic runs for L-proline functionalized MIL-53(Al) MOFs. For the DUT-5(Al) MOFs with L-proline groups this value even increases up to 97%.

Full text of DOI:10.1016/j.mcat.2019.01.029

MolecularCatalysis467(2019)70–77
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Studies on catalytic activity of MIL-53(Al) and structure analogue DUT-5(Al)
using bdc- and bpdc-ligands functionalized with L-proline in a solid-solution
mixed-linker approach
Sandra Nießing, Christoph Janiak⁎
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, 40204 Düsseldorf, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
Heterogeneous organo catalysis
Proline
Aldol reaction
MIL-53
Chiral aluminum MOFs
Proline-functionalized MOFs MIL-53(Al) (based on the benzene-1,4-dicarboxylate, bdc linker) and DUT-5(Al)
(based on the biphenyl-4,4′-dicarboxylate, bpdc linker) were synthesized using a mixed-linker approach. The
proline-functionalized bdc and bpdc ligand was mixed with both the non-functionalized ligand as well as the
amino functionalized one. The influence of the length of the linker on the catalytic activity showed a coun-
terintuitive confinement effect and advantageous increase of ee with longer linker and higher porosity, that is a
change of ee values from no selectivity in the MIL-53(Al) MOFs up to 16% ee for 27-L-Pro/H2N-DUT-5(Al) and
19% ee for 26-L-Pro/H-DUT-5(Al). The conversion of 4-nitrobenzaldehyde remains over 85% after three cata-
lytic runs for L-proline functionalized MIL-53(Al) MOFs. For the DUT-5(Al) MOFs with L-proline groups this value
even increases up to 97%.
1. Introduction
systematic tuning of the pore walls, they were able to override the
expected stereochemical preference of the native L-proline catalyst.
Their versatility regarding their metal and linker composition has
brought tremendous attention to metal-organic frameworks, MOFs
[1–5]. The possibility to incorporate various functional groups by
grafting to the metal or linker, either pre- or postsynthetically has
further widened their variability [6–8]. Among these, there are also
mixed metal or mixed linker approaches [9,10] as well as the in-
corporation of metal complexes by using additional linker functional-
ities [11,12]. Applications envisioned for MOFs include drug delivery
[13–15], gas adsorption [16], separation processes [17–23], sensing
[24–27], hosts for catalytic metal nanoparticles [28–31] and water
adsorption for heat transformation [32–37]. An important potential
application is heterogeneous catalysis [38–40]. Chiral MOFs can be
obtained by chiral induction [41] or by using chiral ligands [42]. They
offer the opportunity for enantioselective heterogeneous catalysis that
led them to be called artificial enzymes [43–47]. The incorporation of
easily available amino acids is one possibility to obtain chiral MOFs
[48–51].
Kutzscher et al. used a Boc-protected proline group on the ligand in a
modified variant of DUT-32 [57] and in UiO-67/68 [58]. For the latter,
they observed an in situ deprotection of the proline and a reversion in
diastereoselectivity in the aldol reaction of cyclohexanone with 4-ni-
trobenzaldehyde. Such changes in selectivity, as reported in the work of
Telfer et al. [56] and Kutzscher et al. [58] were also found by Zheng
et al. [59] and our group [60].
In another approach, L-proline was used as a modulator during
zirconium MOF syntheses that was subsequently incorporated as a
terminal ligand on the metal cluster SBU [61]. In the work of Feng et al.
L-proline modulated UiO-66 and -67 were used as catalysts for the aldol
addition reaction of cyclohexanone or acetone with 4-ni-
trobenzaldehyde [62].
Based on our previous work [60], we selected the isoreticular
(isostructural) MOF pair of MIL-53, first reported by Férey et al. [63]
and DUT-5, first synthesized by Kaskel et al. [64], to elucidate the effect
of the length of the linker on the catalytic activity for an otherwise
identical secondary building unit and MOF topology. MIL-53 with the
benzene-1,4-dicarboxylate (bdc) ligand and isoreticular DUT-5 with the
biphenyl-4,4′-dicarboxylate (bpdc) linker, are Al-based MOFs which are
widely used [65–68]. As Wang et al. remarked, these isoreticular MOFs
offer the opportunity for studying the relation between channel sizes
At the same time, the amino acid L-proline is a thoroughly used
organo catalyst for aldol, Mannich and Michael reactions [52–54].
Telfer et al. incorporated L-proline in IRMOFs [55] and MUF-77 [56] by
using functionalized linkers. They observed differences in selectivity of
the catalytic performance based on the spatial environment. By
⁎ Corresponding author.
E-mail address: janiak@uni-duesseldorf.de (C. Janiak).
https://doi.org/10.1016/j.mcat.2019.01.029
Received 21 December 2018; Received in revised form 22 January 2019; Accepted 23 January 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

S. Nießing, C. Janiak
MolecularCatalysis467(2019)70–77
So far, there are numerous reports of using these MIL-53 and DUT-
5 MOFs for catalytic applications with [74–77] and without further
modification [78–80], but to the best of our knowledge none of the
specific above MOFs have been functionalized with proline groups so
far.
Here we have prepared the standard MIL-53(Al) and the amino-
functionalized H2N-MIL-53(Al) with partially L-proline-functionalized
bdc in a mixed-linker approach (Fig. 1). The same was done with DUT-5
and its amino-variant. Usually mixed-linker describes the stoichio-
metric combination of ligands of different lengths, shapes, and co-
ordination features, as for example in pillar-layer MOFs [81,82]. Here,
we apply the partial substitution of a linker in a MOF by a similar one
under retention of the MOF structure in an isoreticular synthesis ap-
proach (Fig. 1). Burrows called it “solid solution” approach in which the
proportions of the ligands or metals can be continuously adjusted, al-
beit maybe not in every proportion [83–85].
For prevention of L-proline racemization, we avoided high tem-
peratures above 90 °C for MIL-53(Al) and above 110 °C for DUT-5 in the
MOF syntheses [57]. The obtained MOFs were tested and compared
regarding their catalytic activity in the aldol reaction of acetone with 4-
nitrobenzaldehyde (Scheme 1).
2. Experimental section
2.1. Materials and methods
All reagents and starting materials were commercially obtained and
used without further purification. 2-Aminoterephthalic acid (99%), L-
proline (99%) and aluminum nitrate nonahydrate (98–102%) were
bought from Alfa Aesar, aluminum chloride hexahydrate from Janssen
Chimica, 4-nitrobenzaldehyde, dimethyl-2-aminoterephthalate (99%)
and terephthalic acid (99%) from Acros Organics and dimethyl-(1,1‘-
biphenyl)-4,4‘-bicarboxylate (> 98%) from TCI.
Powder X-ray diffraction, PXRD patterns were recorded on a Bruker
AXS D2 Phaser using Cu-Kα1/α2 radiation with λ = 1.5418 Å at 30 kV
covering 2theta angles in the range 5-50°. Nitrogen sorption experi-
ments for BET surface area determinations (BET, Brunauer-Emmett-
Teller [86]) were performed with a Quantachrome Autosorb-6 instru-
ment at 77 K. BET surface areas were calculated using the NovaWin
11.03 software. Prior to the adsorption measurements, the samples
were degassed at 140 °C under vacuum for 3 h. Solution 1H NMR
spectra were recorded on a Bruker Avance III-300. MOF samples were
digested using NaOD/D2O. Scanning electron microscopy, SEM pictures
were acquired on a JEOL JSM-6510 Advanced electron microscope
(Jeol, Akishima, Japan) with a LaB6 cathode at 20 keV. The microscope
was equipped with an Xflash 410 (Bruker, Billerica, US) silicon drift
detector for elemental composition analysis by energy-dispersive X-ray
spectrometry.
Fig. 1. Linker combinations used in the synthesis of L-proline-modified mixed-
linker MOFs MIL-53(Al) and DUT-5. The L-proline linkers (L-Pro-bdcH2, L-Pro-
bpdcH2) were combined with the amino functionalized ones (H2N-bdcH2, H2N-
bpdcH2) and the non-functionalized ones (bdcH2, bpdcH2).
and catalytic results [69]. The inorganic secondary building unit of
these MOFs are chains of trans-corner sharing and μ-OH-bridged {AlO6}
octahedra. These infinite Al-OH-Al chains are then bridged by the di-
carboxylate linkers to give microporous, rhombohedral channels (see
Supp. Info. Fig. S1). A remarkable feature of MIL-53 is its breathing
effect giving a large and a narrow pore form [70]. DUT-5, in contrast,
has a rigid structure [71]. Both MOFs are permanently porous and their
amino functionalized form can also be synthesized [71,72]. Compared
to MIL-101 MOFs that can be synthesized from the same educts, the
MIL-53 structure is the thermodynamically favored one [73].
The conversion and ee values of the catalytic reaction were de-
termined by chiral high-performance liquid chromatography, HPLC
using a Shimadzu system LC 20 A T prominence with an SPD-M20 A
detector. The chiral column was
a Lu x 5 μm i-Amylose-1 from
Phenonemex. Method details for the chromatographic separation of the
catalysis products were: eluent n-hexane/dioxane 82.5/17.5 (v/v), flow
rate 1 mL/min, UV-Vis detector wavelength λ = 273 nm, retention time
t[(S)-4-hydroxy-4-(4-nitrophenyl)butan-2-one] = 17.6 min and t[(R)-4-
hydroxy-4-(4-nitrophenyl)butan-2-one] = 20.3 min. Method details for
the linker separations were: eluent n-hexane/ethanol 88/12 (v/
Scheme
1. Aldol
reaction
of
4-ni-
trobenzaldehyde with acetone in the presence
of the L-proline functionalized MOFs as cata-
lysts, yielding 4-hydroxy-4-(4-nitrophenyl)
butan-2-one as the main addition product and
4-(4-nitrophenyl)but-3-en-2-one as the con-
densation side product.
71

S. Nießing, C. Janiak
MolecularCatalysis467(2019)70–77
v) + 0.1 vol% diethanolamine (DEA), flow rate 1 mL/min, UV-Vis de-
tector wavelength λ = 273 nm.
For the MIL-53 MOFs the amount of incorporated L-proline is lower (20
and 23 mol%) than for DUT-5 (24 to 27 mol%) due to the larger spatial
difference between the L-Pro-containing and the (H2N-)bdc linker in
(H2N-)MIL-53. We note that there was no sense in preparing a pure L-
Pro-MIL-53 or even a pure L-Pro-DUT-5 as the L-proline substituent on
bdc or bpdc would fill or block the available pores in MIL-53 or DUT-5,
leaving no void space for substrate diffusion and catalytic action within
the framework. As a trade-off between a high enough amount of cata-
lytically active groups and sufficient porosity, the value of 30 mol% of
functionalized ligand was chosen for all MOF syntheses.
Despite the mixed-linker functionalization, the crystallinity and
underlying MIL-53 or DUT-5 topology is still preserved as shown by
powder X-ray diffractometry (PXRD). It is evident that both 20-L-Pro/H-
MIL-53(Al) and 23-L-Pro/H2N-MIL-53(Al) feature the (H2N-)MIL-53(Al)
structure (Fig. 2, top). Also the L-Pro/H-DUT-5 and L-Pro/H2N-DUT-
5 MOFs have very similar powder patterns, matching the DUT-5 simu-
lation (Fig. 2, bottom).
2.1.1. Brief syntheses of L-Pro-bdcH2 and L-Pro-bpdcH2 (see Supp. Info. for
more details)
1 eq. of the amino compound was dissolved in dry dichloromethane
and added slowly to a cooled solution of 1 eq. 4-(dimethylamino)pyr-
idine (DMAP), 1.5 eq. 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride (EDC∙HCl) and
2 eq. Boc-L-proline in dry di-
chloromethane. After stirring overnight, the solution was washed two
times with 0.5 M hydrochloric acid and once with saturated sodium
bicarbonate solution and dried over magnesium sulfate. After solvent
removal, the product was cleaned by column chromatography if ne-
cessary.
2.1.2. Typical syntheses of MIL-53(Al) MOFs (see Supp. Info. for more
details)
3 mmol Al(NO3)3∙9H2O were dissolved in water. The ligand(s) was
suspended in of DMF and added to the solution while it was heated to
90 °C. Stirring and heating was continued for 2 d. The separated pro-
duct was washed two times with DMF and several times with acetone,
and dried under vacuum at 60 °C overnight.
The L-Pro-modified MIL-53(Al) MOFs have broader reflections than
the non-proline single-linker MOFs, while the L-Pro- and single-linker
DUT-5 MOFs have a similar width of the reflections. Broader reflections
are usually associated with lower crystallinity or smaller particle size. L-
proline functionalization in (H2N-)MIL-53(Al) MOFs decreases the
crystallinity since the bulky proline group on the short terephthalate
linker will yield to more defects during the crystal growth process. For
the comparison of simulated and experimental diffractograms con-
cerning phase and phase purity, it is crucial to be aware that the si-
mulated diffractograms are often based on crystal data where the sol-
vent-derived electron density in the voids had been removed by the
SQUEEZE option in PLATON. Even if the crystal solvent in the voids is
disordered its electron density still contributes to diffraction. Hence, the
experimental diffractograms on the as-synthesized samples differ in
their intensities and can have additional peaks compared to the simu-
lated patterns (see Supp. Info. Fig. S1-S5).
2.1.3. Typical syntheses of DUT-5(Al) MOFs (see Supp. Info. for more
details)
1.5 mmol of the ligand(s) were dissolved in DMF, heated to 110 °C
and 3 mmol of AlCl3∙6H2O were added in small portions over several
hours. After continuous heating overnight, the product was washed
with DMF and ethanol and finally suspended in ethanol and refluxed for
a day. The MOF was dried at 60 °C in a vacuum oven.
2.1.4. Catalytic studies
To a solution of 50 mg (0.33 mmol) 4-nitrobenzaldehyde in 2 mL
(27 mmol) of acetone 40 μL (2 vol%, 2 mmol) water were added. The
addition of water enhances the reaction rate [87], but a further increase
would have a detrimental effect on the ee value [88]. For the reaction
15 mol% of the catalyst were used. The mixture was stirred at 30 °C in a
water bath. The catalysis results were monitored using chiral HPLC.
The SEM pictures (see Supp. Info.) show very small and agglomer-
ated particles. This formation of agglomerated particles is in accordance
to reports of other groups [91,92]. Overall, the linker composition does
not influence the particle size or morphology in either MIL-53 or DUT-5
series.
For the determination of the porosity, nitrogen sorption isotherms
were measured (Fig. 3). The observed steep rise at very low partial
pressures (pp0–1 < 0.05) is typical for Type I isotherms that correspond
to microporous materials [86]. But all of them show, more or less,
another steep rise at a high partial pressure (pp0–1 > 0.9) that changes
the course of the isotherms to Type II or III. This increase is caused by
condensation in interparticular macropores. The small hysteresis loops
between adsorption and desorption branches are of Type H4, which are
seen if the adsorption branch is a composite of Types I and II, and given
by aggregated particles [86].
The larger hysteresis loop for H2N-MIL-53(Al) (blue curve in Fig. 3)
was also observed in the literature [72,95]. The interparticle con-
densation causes the values of the pore volume at pp–1 = 0.95 to be
unrealistically high, hence we only report the micropore volume in
Table 1. The surface areas and crystallinities of the MIL-53 MOFs are
lower than those reported in literature due to lower reaction tem-
perature (90 °C) and shorter reaction time (2 d) to prevent racemiza-
tion. For H2N-MIL-53(Al), catalytic tests revealed remaining linker and
catalytic Lewis-acidic aluminum metal educts inside the pores (vide
infra), probably due to these synthesis conditions. Typical syntheses for
MIL-53(Al) use temperatures of 120 °C and above and reaction time of
more than three days [93,94]. BET surface areas in the literature are
then given as 1140 m2/g for MIL-53(Al) [70] and 940 m2/g for H2N-
MIL-53(Al) synthesized under these higher temperature and longer time
conditions [95]. The surface area of 1020 m2/g found here for MIL-
53(Al) is only slightly lower than the literature value of 1140 m2/g
when the measurement uncertainty of 50 m2/g is taken into account.
From the catalytic tests with MIL-53(Al) no significant educt residues
3. Results and discussion
3.1. MOF synthesis and characterization
In this work we used a mixture of terephthalic acid or aminoter-
ephthalic acid and 30 mol% of 2-(pyrrolidine-2-carboxamido)ter-
ephthalic acid (L-Pro-bdcH2) to obtain the L-proline-functionalized
mixed-linker MOFs L-Pro-/H-MIL-53(Al) and L-Pro/H2N-MIL-53(Al).
Similarly, from a mixture of biphenyl dicarboxylic acid or amino bi-
phenyl dicarboxylic acid and 30 mol% of 2-(pyrrolidine-2-carbox-
amido)biphenyl dicarboxylic acid (L-Pro-bpdcH2) the L-proline-func-
tionalized mixed-linker MOFs L-Pro-/H-DUT-5(Al) and L-Pro/H2N-
DUT-5(Al) (Fig. 1) were synthesized. The L-proline-functionalized lin-
kers were synthesized from the aminoterephthalic acid and the amino
biphenyl dicarboxylic acid, respectively, using the conditions of Ste-
glich esterification [89]. All MOFs were analyzed after washing and
drying procedures (see details in Supp. Info.). For comparison the
single-linker MOFs (H2N-)MIL-53(Al) and (H2N-)DUT-5 were also syn-
thesized.
The actual incorporated amount of L-proline-linker was determined
from solution NMR spectra of the mixed-linker MOF product which had
been dissolved ("digested") in NaOD/D2O (see Supp. Info. Fig. S17-S20).
In accordance to our previous work [60] and to the literature [90], the
L-proline-functionalized ligand is incorporated in a lower amount than
the 30 mol% that was provided, probably due to the higher steric im-
pact of the L-proline function. Only between 20 and 27 mol% of L-pro-
line functionalized ligand were found in the digested MOFs (Table 1).
72

S. Nießing, C. Janiak
MolecularCatalysis467(2019)70–77
Table 1
Results of NMR analysis and BET measurements of MIL-53(Al) and DUT-5(Al) products.
a
b
c
d
Ligands used in synthesis
MOF Product
BET-surface area [m2/g]
Micropore volume [cm3/g]
bdcH2
L-Pro-bdcH2, bdcH2
H2N-bdcH2
L-Pro-bdcH2, H2N-bdcH2
bpdcH2
L-Pro-bpdcH2, bpdcH2
L-Pro-bpdcH2, bpdcH2
H2N-bpdcH2
MIL-53(Al)
20-L-Pro/H-MIL-53(Al)
H2N-MIL-53(Al)
23-L-Pro/H2N-MIL-53(Al)
DUT-5(Al)
26-L-Pro/H-DUT-5(Al)
24-L-Pro/H-DUT-5(Al)
H2N-DUT-5(Al)
1020
920
480
0.31
0.23
0.12
0.17
0.56
0.56
0.47
0.57
0.60
0.50
640
1660
1650
1470
1670
1760
1490
L-Pro-bpdcH2, H2N-bpdcH2
L-Pro-bpdcH2, H2N-bpdcH2
27-L-Pro/H2N-DUT-5(Al)
25-L-Pro/H2N-DUT-5(Al)
a
Molar ratio used in case of mixed-ligand synthesis 30 : 70.
The number in front of the ligand gives the experimentally determined mol percentage of the proline ligand determined from solution NMR spectra after
b
digestive dissolution in NaOD/D2O (see Supp. Info. Fig. S17-S20).
Surface areas (BET) were calculated from five adsorption points in the pressure range pp0–1 = 0.05-0.20.
Calculated with deBoer V-t-plot method.
c
d
Fig. 2. PXRDs of the synthesized MOFs. CSD-refcodes of the simulated diffractograms: MIL-53as: SABVOH (CCDC 220475) [70], H2N-MIL-53(Al) hydrate (large pore
form): TEPJAP (CCDC 901254) [91] and DUT-5: CCDC 691979 [64].
could be seen inside the pores, hence, we trace the lower surface area to
the milder synthesis conditions. For the amino MOF H2N-MIL-53(Al),
there is also a report stating 675 m2/g [94].
3.2. Catalysis
In our previous work, we synthesized a H2N-MIL-101(Al) that was
For the DUT-MOFs, the values of the surface areas and pore volumes
(Table 1) are in good accordance to previous reports that gave a value
of 1613 m2/g and 0.81 cm3/g [64] for DUT-5(Al) and 1966 m2/g and
0.79 cm3/g [71]. We note that the introduction of L-Pro-functionalized
linkers in the amino-MOFs H2N-MIL-53(Al) and H2N-DUT-5(Al) can
lead to a counterintuitively higher surface area and pore volume
compared to the parent compounds. We rationalize this unexpected
trend to a larger number of defects in the amino-MOF structures, in
particular the rise in pore volume can only be explained this way. It is
foreseeable that the incorporation of the relatively bulky proline-con-
taining linkers may increase the number of defects in their vicinity,
such as missing linkers or missing metal ions.
functionalized with L-proline groups similar to this work. The obtained
22-L-Pro/H2N-MIL-101(Al) was applied to the same aldol reaction of 4-
nitrobendzaldehyde and acetone as a catalyst (Scheme 1). We observed
an interesting change of selectivity. While the organo catalyst L-proline
achieved 98% conversion with 66% ee of the R-configured enantiomer,
the L-proline functionalized 22-L-Pro/H2N-MIL-101(Al) gave 73% con-
version with 29% ee of the S-configured enantiomer after 24 h. In our
proposed mechanism the trinuclear aluminum metal cluster as the
secondary building unit coordinates two water molecules (aqua li-
gands) which offer the opportunity for an attack on the Si-site of the
aldehyde (Scheme 2). Using the MIL-53 structure, these aqua ligands do
not exist on the ideal inorganic building unit (see Supp. Info. Fig. S1)
73

S. Nießing, C. Janiak
MolecularCatalysis467(2019)70–77
Fig. 3. Nitrogen sorption curves at 77 K of the synthesized MOFs. Filled symbols for adsorption, empty symbols for desorption curves. The strong increase in nitrogen
sorption at high pp0–1, which was cut-off for clarity, is due to interparticle condensation in the aggregates. For full curves and also the isotherms for the 25- and 24-L-
Pro-DUT-5 materials, see Supp. Info. Fig. S6.
of nearly 60% of the 4-nitrobenzaldehyde was reached after just one
day and quantitative conversion (99%) after 7 d. The conversion of the
DUT-5(Al) MOFs after 24 h is comparable to the 66% reported by
Banerjee et al. with MIL-101(Cr) [99]. Feng et al. observed a faster
reaction with their UiO-66 MOF that was modulated with L-proline
during its synthesis. They raised the reaction temperature from room
temperature to 45 °C to increase their conversion from 62% to complete
conversion after one day [62]. Our results are similar to those of Liu
et al. using the MOF MUF-77 with proline-functionalized linkers [56]:
They determined as the best result a conversion of 44% after one day
when the proline was linked to bpdc and up to 55% conversion when
substituting the shorter bdc linker. For their IRMOF-3, Lili et al. re-
Scheme 2. Proposed mechanism for the aldol reaction of acetone and ni-
ported a conversion of 41% after the same time [96]. Even though the
trobenzaldeyde catalysed by 22-L-Pro/H2N-MIL-101(Al). Graphic taken from
linker used for the DUT-5(Al) MOFs are longer than those for the MIL-
ref. [60] with permission from the author. Copyright Elsevier, 2017.
53(Al) MOFs, the catalyzed reaction is still slower compared to 22-L-
Pro/H2N-MIL-101(Al) from our previous work with the shorter L-Pro-
and therefore it is expected that no reversion occurs. In a typical ex-
bdc linker [60]. The reason for lower conversion with MIL-53(Al)- and
periment, the MOF catalyst was suspended in a solution of 4-ni-
trobenzaldehyde in acetone with 2 vol% water and heated at 30 °C.
DUT-5(Al)-MOFs lie in smaller channels which restrict diffusion. The
channels in the large-pore form of MIL-53(Al) have a diameter of about
Fig. 4 depicts time-conversion plots of the functionalized MOFs.
When using the L-Pro-bdc linker in 20-L-Pro/H-MIL-53(Al) and 23-L-
Pro/H2N-MIL-53(Al) a time of three days is needed to reach a conver-
sion of 70%. When using the L-proline functionalized DUT-MOFs 26-L-
Pro/H-DUT-5(Al) and 27-L-Pro/H2N-DUT-5(Al) as catalyst a conversion
8.5 Å, for DUT-5(Al) 11.1 Å, compared to the smallest window diameter
of about 12 Å in the MIL-101 structure and the even larger pore dia-
meters of 29 Å and 34 Å (with all diameters taking the van der Waals
radii into account) [73].
However, the reaction from the aldol addition product to the aldol
condensation product (cf. Scheme 1) is increased when using a L-Pro-
DUT-5(Al) catalyst (Table 2) and therefore has a reduced selectivity.
The achieved selectivity values are in the range of those reported so far
in the literature. Feng et al. report of values from 70% to 96% for the
addition product with their proline-modulated UiO MOFs [62]. Lili
et al. got 28% of their conversion as addition product and 72% as the
condensation product [96]. We note that sometimes the selectivity
between the addition and condensation product is not given [54,55].
The resulting ee values of 16 and 19% for the R enantiomer can be
described only as low for the DUT-5 MOFs (Table 2). In their work of
postsynthetically L-proline-modified H2N-MIL-101(Al), Bonnefoy et al.
observed an ee-value of 18% though they were able to improve it to
25% ee by using glycine as spacer between the linker and the proline
group [54]. Using free L-proline as an organo catalyst also favors the R
enantiomer albeit only with an ee value of 66% [98]. Surprisingly, for
MIL-53 MOFs there is no enantioselectivity at all. Considering that even
if inside the pores there may be no selectivity due to the small space,
there should still be some selectivity caused by the L-proline groups on
the surface of the small particles. All of our values are lower than the
69% ee reported by Banerjee et al. who used a L-proline-functionalized
ligand that was coordinated (grafted) to a free metal site in MIL-101(Cr)
[99] and the 29% ee of Telfer et al. with their IRMOF using Boc-L-Pro-
bpdc as a linker that was postsynthetically deprotected [55]. During the
Fig. 4. Time-conversion plots for the aldol reaction with the proline-functio-
nalized mixed-linker MIL-53(Al) and DUT-5(Al) MOFs as catalysts. Values for
total conversion of 4-nitrobenzaldehyde are average values from two in-
dependent runs. Between the two independent runs there was at maximum a
difference of 4%. Conditions: 30 °C, 15 mol% catalyst. Total reaction times were
10 d for MIL-53(Al) MOFs and 7 d for DUT-5(Al) MOFs.
74

S. Nießing, C. Janiak
MolecularCatalysis467(2019)70–77
Table 2
Results of the catalyzed aldol reaction between 4-nitrobenzaldehyde and acetone. Reaction times were 10 d for MIL-53(Al) MOFs and 7 d for DUT-5(Al) MOFs.a
Catalyst
Total conversion [%] (mol% catalyst)
Time
[d]
Selectivity (addition product) [%]
ee [%] (R enantiomer)
Lit.
b
20-L-Pro/H-MIL-53(Al)
23-L-Pro/H2N-MIL-53(Al)
26-L-Pro/H-DUT-5(Al)
27-L-Pro/H2N-DUT-5(Al)
22-L-Pro/H2N-MIL-101(Al)
Al-MIL-101-NH-Pro
96
96
98
99
95 (13)
18 (15)
62 (20)
41 (n.d.)
10
10
7
7
4
7
1
3
92
93
83
86
1
2
this work
this work
this work
this work
60
54
62
96
b
19
16
29 (S)
18
63
n.d.c
96
UiO-66-LP-120
IRMOF-3-Pr(OP)
5
d
28
89
a
Conditions: 30 °C, 15 mol% catalyst. Each catalytic reaction was carried out in two independent runs and with three recycling runs where the ee values agree
within 1% to each other (cf. Fig. 6).
b
This value cannot be viewed as selectivity but shows a racemic mixture.
n.d. = not determined.
Lili et al. found 89% ee after using their IRMOF-3-Pr(OP) [96] but Canivet et al. questioned those due to varying retention times in the HPLC chromatograms
c
d
[97].
deprotection of their proline-modified DUT-32, Kutzscher et al. found
no selectivity due to racemization [57]. At the same time, there are
reports of even smaller ee values as in the work of Feng et al. whose ee
values were around 5–7% ee for the MOFs with the highest conversion
[62]. By putting proline on different linkers in MUF-77, Liu et al. ob-
served values of 4–27% ee [56].
A difference between the ee values of the L-Pro-MIL-53 and L-Pro-
DUT-5 MOFs was expected due to the linker length and the concomitant
pore sizes. Initially we anticipated a stronger stereogenic effect in the
narrower channels of MIL-53 due to the assumed increased confine-
ment. Instead and perhaps counterintuitively, there is essentially no ee
with MIL-53 but in the larger channels of DUT-5, the stereogenic center
of L-proline exerts a larger effect (Figs. 5 and 6). In our previous work
with MIL-101(Al) the even larger pores (see above) had resulted in an
ee of 29% [60]. In their work with proline functionalized UiO-67/68,
Kutzscher et al. used these MOFs as catalyst for the aldol reaction of 4-
nitrobenzaldehyde with cyclohexanone. They observed a change in
diastereoselectivity, which they attributed to the confined space af-
fecting the cyclic transition state in the aldol reaction; there was no
enantiomeric excess [58]. Thus, it is obvious that a too narrow confined
space prevents the substrates from assuming the preferred orientation
at the catalytic site. The orientation which leads to the preferred en-
antiomer requires sufficient space. Our explanation is supported by the
comparison of the ee values which were achieved with 26-L-Pro/H-
DUT-5(Al) (19% ee) and 27-L-Pro/H2N-DUT-5(Al) (16% ee). Both MOFs
have the same L-proline functionalization and differ only in the bpdc
versus H2N-bpdc linker. The higher congestion exerted by the amino-
bpdc linker has a decreasing ee effect as it increases the hindering for
assuming the preferred substrate orientation inside the pores.
To exclude a linker racemization during MOF synthesis or catalysis,
we digested the MOFs used for catalysis and esterified the ligand to
verify the stable and unchanged enantiopurity of the L-proline modified
linker following a method outlined by Lun et al. [55] (see Supp. Info.
for details).
Fig. 5. Illustration for the counterintuitive confinement effect and advanta-
geous increase of ee with higher porosity in the L-Pro-DUT-5(Al) over the L-Pro-
MIL-53(Al) MOFs.
To ensure that the incorporated L-proline moieties are the catalytic
active part of the MOFs, the non-functionalized single-ligand MOFs
MIL-53(Al), H2N-MIL-53(Al), DUT-5(Al) and H2N-DUT-5(Al) were used
as catalysts under the same reactions conditions. The conversion was
only 2% for DUT-5(Al) and 5% for H2N-DUT-5(Al) after 10 d. MIL-
53(Al) resulted in 6% conversion of 4-nitrobenzaldehyde with acetone
to the aldol addition product. However, H2N-MIL-53(Al) (BET area
480 m2/g) gave a conversion of 47% after 10 d. To look into this un-
expected results, aminoterephthalic acid (H2N-bdcH2) and aluminum
chloride were separately tested as catalysts with the chosen standard
conditions, albeit gave only less than 4% conversion. This low con-
version was probably due to the low solubility of both compounds in
acetone. We observed noteworthy conversion only for the soluble
physical mixture of aminoterephthalic acid (H2N-bdcH2) and aluminum
chloride mixture (40% conversion after 10 d). Thus, it is evident that
the conversion of H2N-MIL-53(Al) resulted from remaining linker and
catalytic Lewis-acidic aluminum metal [100] inside the pores which
also helps to explain the observed lower surface area (cf. Table 1).
Additionally, a second charge of H2N-MIL-53(Al) with a BET area of
580 m2/g (indicating less remaining linker and metal in the pores) was
tested as catalyst and found to only give a conversion of 15%.
To test their heterogeneous nature and to prove that no leaching
occurs, the catalysts were separated by centrifugation and filtration
(0.20 μm syringe filter). In case of the DUT-5(Al) MOFs, the reactions
stopped after 1 d with about 60% conversion by using a 6000 rpm
75

S. Nießing, C. Janiak
MolecularCatalysis467(2019)70–77
Fig. 6. Conversion in the aldol reaction and ee value for three recycling runs.
centrifugation for 3–4 min. For the separation of MIL-53(Al) MOFs
under these conditions after 2 d we still detected a significant con-
tinuation of the aldol reaction with 17 and 23% conversion for 20-L-
Pro/H-MIL-53(Al) and 23-L-Pro/H2N-MIL-53(Al), respectively, up to 10
d (Fig. S22 in Supp. Info.). When we prolonged the centrifugation time
to 10 min the continued conversion was reduced to 3% for 20-L-Pro/H-
MIL-53(Al) and to 10% for 23-L-Pro/H2N-MIL-53(Al) over the re-
maining 8 d. From the SEM images of the MIL-53(Al) versus the DUT-
5(Al) MOFs it is evident that the former contain particles less than
0.2 μm (see Supp. Info. Fig. S7-S10). Thus, the ongoing reaction can be
traced to nanosized MOF particles, which were not separated from the
reaction mixture. Catalytic diacetyl formation from benzaldehyde and
methanol with MIL-101(Cr) catalysts also showed that small nano-MOF
particles with a diameter below 0.2 μm are difficult to separate and
continued the catalysis in the reaction mixture after filtration and
6000 rpm centrifugation [101].
only small amounts of the subsequent condensation product.
Comparison of the ee values of the L-proline-functionalized MIL-53,
DUT-5 MOFs and MIL-101(Al) from previous work [60] unexpectedly
revealed a significant decrease in the stereogenic effect of the L-proline
group in smaller pores, that is, in more congested environments. Thus,
the largest porosity MIL-101 gave the highest ee with 29% while
smallest MIL-53 showed no more enantiomeric excess. Initially, we had
anticipated a stronger stereogenic effect in the narrower channels of
MIL-53 due to the increased confinement. The counterintuitive loss of
stereogenic effect in MIL-53 indicates that higher porosity in MOFs is
advantageous for chiral catalysis, as the substrates have to have the
ability to assume the preferred orientation, which may not be possible
in too confined pore environments.
The heterogeneous L-proline-functionalized MOFs can be recycled
and re-used over at least three runs with little to no loss in activity for
MIL-53 and DUT-5, respectively.
The synthesized L-proline functionalized MOFs were also tested for
catalyst recycling over three runs. The results are illustrated in Fig. 6.
For the MIL-53(Al) MOFs a slight decrease in conversion is visible from
96% to 88% and 84% from the first to the third run, respectively.
The DUT-5(Al) MOFs show constant conversion and ee values. Even
after the third run 27-L-Pro/H2N-DUT-5(Al) converts 97% of the 4-ni-
trobenzaldehyde. The selectivity with respect to the addition product
does not change within experimental error for both MIL-53 and DUT-
5 MOFs. We note that the conversion with 22-L-Pro/H2N-MIL-101(Al)
as a catalyst decreased from 95% to 67% over three runs [60]. A de-
crease in conversion during catalyst recycling was also observed in
comparable literature and is attributed to blocking of the catalytic sites
inside of the pores [58,62].
Subsequent to the three catalytic runs, the MOFs were tested for
their remaining crystallinity and porosity. The MOFs were apparently of
little changed crystallinity as shown by PXRD (Fig. S23, Supp. Info.). In
SEM pictures no morphological changes were visible. There was,
however, a loss in BET surface area and porosity, the more so for the
MIL-53 than the DUT-5 MOFs, concomitant with the more pronounced
decrease in conversion of the former (see Supp. Info. Table 3). In the 1H
NMR spectra of the digested MOFs after the third catalytic run there
were, however, no signals visible that correspond to aldol reaction
products which had remained in the pores, thereby ruling out pore-
blocking effects and possibly pointing to slight MIL-53 degradation.
We believe that our finding of the counterintuitive confinement
effect helps in the design of chiral MOF catalysts.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.01.029.
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