Influence of functionalization of terephthalate linker on the catalytic activity of UiO-66 for epoxide ring opening

Article abstract of DOI:10.1016/j.molcata.2016.10.022

A series of five isostructural zirconium terephthalate UiO-66 metal organic frameworks bearing different functional groups on the terephthalate linker (UiO-66-X; X = H, NH₂, NO₂, Br, Cl,) have been successfully prepared and characterized. UiO-66-X materials were evaluated as heterogeneous catalysts for the epoxide ring opening of styrene oxide by methanol, observing an increase in the initial reaction rate from UiO-66-H to UiO-66-Br, over one order of magnitude. The reactivity order, however, does not follow a linear relationship between the Hammett constant value of the substituent and the initial reaction rate. UiO-66-Br exhibits a wide scope, its activity depending on the structure of epoxide and nucleophile. The absence of Zr leaching to the solution together with the preservation of the UiO-66-X crystallinity confirms the stability of the framework under the reaction conditions. Nevertheless, UiO-66 undergoes a progressive deactivation upon reuse that was attributed to a strong adsorption of the reaction product.

Full text of DOI:10.1016/j.molcata.2016.10.022

Accepted Manuscript
Title: Influence of functionalization of terephthalate linker on
the catalytic activity of UiO-66 for epoxide ring opening.
Author: Juan F. Blandez Andrea Santiago-Portillo Sergio
´
Naval o´ n M o´ nica Gim e´ nez-Marqu e´ s Mercedes Alvaro Patricia
Horcajada Hermenegildo Garc ´ı a
PII:
S1381-1169(16)30444-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.022
MOLCAA 10084
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
31-7-2016
18-10-2016
19-10-2016
Please cite this article as: Juan F.Blandez, Andrea Santiago-Portillo, Sergio Naval o´ n,
´
M o´ nica Gim e´ nez-Marqu e´ s, Mercedes Alvaro, Patricia Horcajada, Hermenegildo
Garc ´ı a, Influence of functionalization of terephthalate linker on the catalytic activity
of UiO-66 for epoxide ring opening., Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.10.022
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HIGHLIGHTS

UiO-66s with substituents on the terephthalate (H, Br, Cl, NO and SO H) linker were
2
3
prepared.



The reaction tested with epoxide ring opening.
Substituted UiO-66 preserve crystallinity and do not leach Zr to the liquid phase
The activity of substituted UiO-66 increases with substituent electron withdrawing
strength

Bromo substituted UiO-66 is the most active catalyst of the series
1

Influence of functionalization of terephthalate linker on the catalytic
activity of UiO-66 for epoxide ring opening.
[
a]
[a]
[a]
Juan F. Blandez, Andrea Santiago-Portillo, Sergio Navalón, Mónica Giménez-
[
a]
[a]
[c,d]
[a,b]
Marqués, Mercedes Álvaro, Patricia Horcajada, and Hermenegildo García*
.
a
Department of Chemistry and Instituto de Tecnología Química (ITQ, CSIC-UPV) Universitat Politécnica de Valencia C/Camino
de Vera, s/n, 46022, Valencia (Spain)
E-mail: hgarcia@qim.upv.es
b
Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia.
c
Institut Lavoisier, UMR CNRS 8180
Université de Versailles Saint-Quentin-en-Yvelines 45 avenue des Etats-Unis, 78035 Versailles cedex, France.
d
IMDEA Energy Avda. Ramon de la Sagra, 3, 28035 Mostoles, Madrid, Spain.
Keywords: heterogeneous catalysis, metal organic frameworks, UiO-66, epoxide ring opening.
Abstract
A series of five isostructural zirconium terephthalate UiO-66 metal organic frameworks bearing different
functional groups on the terephthalate linker (UiO-66-X; X = H, NH , NO , Br, Cl,) have been successfully
2
2
prepared and characterized. UiO-66-X materials were evaluated as heterogeneous catalysts for the epoxide ring
opening of styrene oxide by methanol, observing an increase in the initial reaction rate from UiO-66-H to UiO-
6
6-Br, over one order of magnitude. The reactivity order, however, does not follow a linear relationship
between the Hammett constant value of the substituent and the initial reaction rate. UiO-66-Br exhibits a wide
scope, its activity depending on the structure of epoxide and nucleophile. The absence of Zr leaching to the
solution together with the preservation of the UiO-66-X crystallinity confirms the stability of the framework
under the reaction conditions. Nevertheless, UiO-66 undergoes a progressive deactivation upon reuse that was
attributed to a strong adsorption of the reaction product.
1
. Introduction
Metal organic frameworks (MOFs), where the lattice is constituted by metal nodes coordinated to
polycomplexant organic linkers [1],[2], are among the most porous materials, ranking at the top of the list of
2

solids with high specific surface area [2],[3],[4]. Besides their use as gas adsorbents [5] and in gas [6] and
liquid [7] phase separations, their large metal content together with the fact that, in some MOFs, there are
exchangeable coordination positions around the metal nodes, make possible the use of these materials as
heterogeneous catalysts for liquid phase reactions under conditions compatible with their chemical and
structural stability [3],[8],[9],[10]. Particularly interesting as catalysts are the highly chemically and thermally
stable metal carboxylates MIL-100, [11],[12] MIL-101,[11],[13] and UiO-66 (UiO stands for University of
Oslo) [14],[15] structures, among others [3],[4],[8],[9]. One of the major advantages of MOFs as catalysts is
their chemical versatility, enabling to obtain isostructural materials based on diverse cations or organic linkers
substituted by different functional groups [16]. The electron donating or withdrawing ability of the functional
groups on the aromatic linkers can modulate the electron density of the complexing groups (e.g. carboxylates)
coordinated to the cations and, consequently, influence the catalytic activity of the MOF through controlling
the electron density of the active metal site [15],[17],[18]. As previously shown, the ligand functionalization of
the microporous UiO-66, a cubic close packed structure based on Zr O (OH) oxoclusters linked together by
6
4
4
terephthalate linkers, strongly affects its Lewis acid catalytic activity [18]. The catalyst activity data have to be
compared cautiously because the influence of the substituent probably overlaps with other possible differences
in crystallinity and differences in the density of lattice defects [19]. Considering the large number of appealing
liquid phase reactions that can be promoted by MOF-based solid catalysts, it is of interest to expand these types
of studies to other class of industrially relevant reactions [4],[9],[10],[20]. Epoxide ring opening, particularly by
H O as nucleophile, is one industrially important reaction, since epoxides of simple alkenes are produced in
2
very large amounts. Also from the synthetic point of view, epoxide ring opening is a versatile reaction to obtain
vic-diols, α-aminoalcohols, halohydrins and even for C-C bond formation.
In the present manuscript, we have prepared and characterized a series of five isostructural UiO-66 solids
based on functionalized terephthalate ligands, exhibiting contrasting electron withdrawing or donating effects,
for the evaluation of their catalytic activity in the epoxide ring opening by alcohols and amines. Although there
are many homogeneous as well heterogeneous Lewis acid catalysts for this reaction, whose mechanism either
S 1 or S 2, has been well studied, the purpose of the present work is to show how the presence of substituents
N
N
on the terephthalic acid linkers can increase the catalytic activity [21].
2
. Experimental
3

2
.1. Materials
All the reagents and solvents used in this work were of analytical or HPLC grade and supplied by Sigma-
Aldrich.
2
.2. Catalyst preparation
A series of UiO-66-X solids were prepared following reported procedures [21]. Briefly, the corresponding
terephthalic acid derivative (1.0 mmol) and ZrCl (0.233 g, 1 mmol) were added to a Teflon-lined autoclave
4
containing 3 mL of dimethylformamide (DMF). This autoclave was heated at the temperature and time shown
in Table 1. After cooling to room temperature, the resulting precipitate was washed with DMF and the solid
was introduced in a Soxhlet system using methanol as solvent. Finally the solid was dried in an oven at 100 ºC
for 24 h.
Table 1. Temperature and time employed in the synthesis of the various UiO-66-X solids [19].
UiO-66-X
UiO-66-H
Temperature (ºC)
Time (h)
220
220
100
100
100
12
24
24
20
24
UiO-66-NO2
UiO-66-NH2
UiO-66-Br
UiO-66-Cl
2
.3. Catalyst characterization.
Fourier transformed infrared (FTIR) spectra were collected using a Nicolet 6700 instrument (Thermo
–
1
scientific, USA) within the range of 4000-400 cm range. Thermogravimetric analyses (TGA) were performed
–
1
on a Perkin Elmer Diamond TGA/DTA STA 6000 in the 25-600 ºC temperature range under a 5 ºC·min scan
–
1
rate and an O flow of 20 mL·min . Powder X-ray diffraction (PXRD) patterns were collected in a D8
2
Advance Bruker diffractometer using Cu Kα1 radiation (λ = 1.54056 Å). Diffraction patterns were generally
collected in the 3° < 2θ < 30° range with 0.02° resolution in continuous mode. The simulated PXRD pattern for
4

the ideal UiO-66 was obtained using the ―Mercury‖ software. N sorption isotherms were obtained at 77 K
2
using a BELsorp Mini (Bel, Japan). Previous to the analysis, ca. 30 mg of the sample were activated at 140 °C
under primary vacuum for 3 h.
2
.4. Catalytic experiments
Typically, the required amount of catalyst (0.037 mmol of Zr) was added to a round-bottom flask (25 mL).
To remove absorbed water in the catalysts, the MOFs were pre-treated by heating them at 150 ºC under
vacuum overnight. The required amount of alcohol was introduced (5 mL) into the flask and the system
sonicated for 15 min. After this time, the epoxide was added (1 mmol). Subsequently, the reaction mixture was
placed in a preheated bath at the required temperature (i.e. 50 ºC) and stirred magnetically. In the case of amine
after the activation of the catalyst, acetonitrile (2.5 mL) was added as solvent, the catalyst was sonicated and
finally, the amine (1.0 mmol) and the styrene oxide (1.0 mmol) were added. At the end of the reaction the
catalyst was exhaustively washed with methanol to recover most of the adsorbed product. Product isolation was
carried out by purification with flash column chromatography on silica gel using hexane: dichloromethane as
eluent. The course of the reaction was periodically follow by extracting aliquot of the reaction mixture with a
syringe, diluting in methanol and injecting the mixture immediately in GC (6890 Network GC system Agilent
technologies) and using a calibration plot to determine the product concentration.
Reuse experiments were performed under the above reaction conditions. At the end of the reaction, the
catalyst was filtered (0.2 nm Nylon filter), the solid was introduced in a Soxhlet system and extracted using
methanol as solvent for 24 h. Then, the catalyst was dried at 100 ºC for 24 h (See Scheme S1). The used
catalyst was also activated before subsequent use, as commented before.
2
.5. Product analysis.
Previously filtered reaction aliquots were diluted in methanol. Subsequently, the aliquots were analyzed by
gas chromatography using a flame ionization detector (GC-FID). Quantification was carried out by using
1
calibration curves of authentic samples. In some cases selectivity was calculated using H-NMR spectra
(Bruker 400 MHz NMR spectrometer).
2
.6. Leaching experiments.
5

At the end of the reaction, the catalyst was removed by filtration. The organic phase mixed with aqueous
solution of 3 M HNO , and the system heated at 80 ºC for 24 h. The presence of zirconium in the aqueous
3
phase was analyzed by chemical analysis using an ICP-AES instrument.
3
. Results and discussion
3
.1. Substituted UiO-66-X
In order to evaluate the potential influence of terephthalate functionalization of UiO-66 on their catalytic
activity, a series of five isostructural UiO-66-X solids bearing different functional groups on the aromatic ring
of the terephthalate ligand (X = H, NH , NO , Br, Cl) were solvothermally synthesized from zirconium(IV)
2
2
chloride and the corresponding functionalized terephthalic acid in DMF, following the procedure reported in
the literature [22]. The resulting UiO-66-X solids were characterized by PXRD (Figure 1), infrared
spectroscopy (IR) (Figure S2), TGA (see Figure S.1 in supporting information) and nitrogen adsorption
measurements. All these data are in good agreement with the reported characterizations [22]. As expected, the
2
-1
3
-1
Brunauer-Emmett-Teller (BET) surface area and pore volume of the UiO-66-H (1150 m ·g and 0.43 cm ·g ,
respectively) decrease due to the presence of substituents. Table 2 lists the UiO-66-X solids prepared in the
present study and their main compositional and textural properties. Crystal structure of the functionalized
derivatives was confirmed by PXRD (Figure 1). All the PXRD patterns show the characteristic diffraction
peaks of UiO-66 structure with, however, a different broadening of the Bragg reflections as a consequence of a
different particle size and crystallinity. Similarly, the purity of the samples was confirmed by TGA, elemental
analysis and IR (Table 2 and Figure S.2 in the supporting information).
6

(111)
(
311) (511)
(933)
(200)
(600)
(531)
(711)
UiO-66-H
UiO-66-NO2
UiO-66-NH2
UiO-66-Br
UiO-66-Cl
Simulated
2
0
40
2
º
Figure 1. PXRD pattern for the five UiO-66 materials used as catalysts, together with the simulated pattern of
UiO-66. The baselines have been shifted in the vertical scale for clarify.
Table 2. Analytical and textural data of UiO-66-X materials
-
1
FT-IR (cm )
Metal content (%)
Pore
BET surface
UiO-66-X
volume
Theoreti
TGA
ICP-AES
2
-1
area (m ·g )
3
-1
(
cm ·g )
cal
UiO-66-H
1150
0.43
3200 (OH),
32.8
34.8
34.9
3
050 (C-H aromatic), 1650
(carboxylate) and 1555
(C=C aromatic)
UiO-66-NO2
UiO-66-NH2
UiO-66-Br
750
910
745
0.30
0.39
0.46
1575 and 1380 (NO2)
28.2
31.2
25.6
34.8
33.0
28.7
34.8
32.9
32.5
3450 (NH ) and 1630 (NH )
2
2
554 (C-Br)
7

UiO-66-Cl
610
0.23
760 (C-Cl)
29.2
35.8
28.0
SEM image for the different catalyst were presented in the supporting information Figure S3 together with the
image of catalyst UiO-66-Br reused.
3
.2. Influence of substituents on the catalytic activity
Then, the effect of ligand modification of UiO-66-X was evaluated on their catalytic activity for the
epoxide ring opening by methanol. Finally the best catalyst UiO-66-Br was evaluated for the epoxide ring
opening with alcohols and amines. A preliminary screening was initially carried out using styrene oxide and
methanol as substrate and nucleophile, respectively. After reaction, the only product observed in all cases was
2
-methoxy-2-phenylethanol, that corresponds to the product formed by S 1 mechanism [23]. In this type of
N
mechanism, the epoxide oxygen should be activated by a Brønsted or Lewis acid site to generate a partial
positive charge at the benzylic carbon, where the nucleophile methanol could attack. According with this
simplistic mechanism, based on the observation of full selectivity towards a single product regioisomer, the key
step of the catalysis might be the activation of styrene oxide by Zr (OH) O oxoclusters, acting here as Lewis
6
4
4
acids. Consequently, one could rationally expect that electron withdrawing substituents increase the acidity of
the Zr nodes.
The time-conversion plots for styrene oxide over a series of UiO-66-X catalyst are presented in Figure 2.
Remarkably, catalytic performances exhibit important variations, depending on the nature of functional group.
Control experiment in absence of catalyst shows that styrene oxide conversion was below 5% under the present
reaction conditions. In contrast, styrene oxide conversions were substantially higher in presence of UiO-66-X
solids and it depends on the UiO-66 functionalization, with in general following the order of reactivity Br > Cl
>
NH > H > NO .
2 2
8

1
00
7
5
2
5
0
5
0
500
1000
1500
Time (min)
Figure 2. Time-conversion plots for the epoxide ring opening catalysed by a series of UiO-66-X. UiO-66-H
■), UiO-66-NO (●), UiO-66-NH (▲), UiO-66-Br (▼), UiO-66-Cl (◄), blank (♦). Reaction conditions:
(
2
2
styrene oxide (1 mmol), methanol (5 mL), nitrobenzene as internal standard (0.5 mmol), catalyst (0.037 mmol
of Zr) temperature 50 º C.
9

The initial reaction rates, determined from the slope of the time-conversion plots at zero time, are listed in
Table 3.
[
a]
Table 3. Initial reaction rate for the epoxide ring opening catalysed by the series of UiO-66-X
1
-6
UiO-66-X
UiO-66-H
Initial rate (mol·s )×10
9.6
UiO-66-NO2
UiO-66-NH2
UiO-66-Br
UiO-66-Cl
20.0
50.1
143.1
128.8
a) Reaction conditions: styrene oxide (1 mmol), methanol (5 mL), nitrobenzene as internal standard (0.5
mmol), catalyst (0.037 mmol of Zr), temperature 50 ºC.
For a deeper understanding of the catalytic activity of the UiO-66-X derivatives, we further investigate the
effect of the functionalized ligand on the electronic density of Zr (OH) O clusters and, so, on its catalytic
6
4
4
activity. If one could expect a direct correlation of the catalytic activity of UiO-66-X and the Hammett constant
σ or σ+ of the functional group of UiO-66-X, being roughly true for Br, Cl and H derivatives, this is not the
case for UiO-66-NO and UiO-66-NH , which unexpectedly showed a much lower and higher activity,
2
2
respectively, than anticipated. However, taking into account the general mechanism of nucleophilic attack on
epoxides, we have to consider not only the inductive effect of the functional group on the Lewis acid sites,
activating the styrene oxide (Scheme 1), but also: i) the presence of basic sites, potentially activating the
nucleophile methanol, ii) the effect of steric hindrance coming from the volume of the substituent (effective
steric effect,  : (H<NH <Cl<Br<NO ), as already probed for other MOFs, iii) the location/orientation of the
ef
2
2
1
0

functional group within the framework (i.e. towards the small or large cage), which could modify the
accessibility of the metal sites and then, their reactivity and iv) the possible differences in the density of
framework defects. [24]
Scheme 1. Dual activation of epoxide ring opening. L.A.: Lewis acid at metal node site, L.B: Lewis basic at the
linker substituent.
All these parameters, among others, might play a crucial role on the catalytic activity of UiO-66-X, making
impossible to find a simple correlation between electronic density at the Zr ions and catalytic activity.
Particularly in the cases of UiO-66 it has been found that depending on the preparation protocol, samples with
different crystallinity and defects density can be obtained. There is an increasing interest in determining how
the catalytic activity of MOFs depends on lattice defects, particularly in the case of UiO-66. With regard to the
dual activation proposed in Scheme 1 there are precedents in the literature suggesting this possibility for
instance in the case of the Knovenagel condensation of malonitrile and benzaldehyde in Cu (BTC) as
3
2
catalyst[25].
3
.3. Reuse of UiO-66-Br
Considering the higher activity of UiO-66-Br, this solid was selected for further characterization, including
stability and reuse tests. Heterogeneity of the process was confirmed by performing a hot filtration test, in
where after 30 min of the reaction under the general conditions in the presence of the catalyst, UiO-66-Br was
filtered off at the reaction temperature. The conversion at this point was 28 % (see Figure S.4 in supporting
information). Then, the resulting clear solution in the absence of the solid catalyst was allowed to continue the
1
1

reaction. After the removal of UiO-66-Br, the reaction stops, indicating that the active catalytic species were in
the solid. Furthermore, the leaching of Zr ions from the UiO-66-X solids during the reaction was ruled out from
the fact that a very low Zr content (<0.001%) is detected in the liquid solution by inductively coupled plasma-
atomic emission spectroscopy (ICP-AES). This confirms a high chemical stability of the UiO-66-Br under the
reaction conditions
1
00
7
5
2
5
0
5
0
500
1000
1500
Time (min)
Figure 3. Time-conversion plots for consecutive uses of the UiO-66-Br. First use (■), second use (●), third use
(▲). Reaction conditions: styrene oxide (13.14 mmol), methanol (65.7 mL), nitrobenzene as internal standard
(6.57 mmol) catalyst (0.486 mmol of Zr) temperature 50 º C.
The catalyst stability was also studied by performing a series of consecutive runs using the same UiO-66-Br
sample. It was observed that the reuse of the sample results in a significant deactivation, as determined by a
decrease in both the initial reaction rate and the conversion at final time. Although one could consider the
catalyst degradation as a possible cause of this deactivation, PXRD of the reused catalyst shows that the
crystalline structure of the sample is largely preserved after its use in two consecutive reactions, ruling out
1
2

structure collapse as the origin of deactivation (Figure S5). Thus, we speculated that the significant deactivation
of the activity could be due to the presence of strongly adsorbed 2-methoxy-2-phenylethanol, acting as poison
of the active Lewis sites. To support this hypothesis, after carrying out the styrene oxide ring opening recation
by methanol, the UiO-66-Br sample was submitted to exhaustive Soxhlet solid-liquid extraction with methanol
(see experimental section) before proceeding to a subsequent reuse. Analysis by GC-FID and gas
chromatography-mass spectrometry (GC-MS) of the compounds present in the extracted liquor shows the
exclusive presence of 2-methoxy-2-phenylethanol, indicating that the UiO-66-Br solid retains some product
after the reaction.
If phenylacetaldehyde or other regioisomer of the epoxide ring opening were formed I small amounts, they
should have been detected by our extraction procedure. It is noteworthy that the Soxhlet extraction process
renders the solid significantly more active for epoxide ring opening than the analogous sample not submitted to
this purification treatment. Despite this extraction, the process was not efficient enough to fully recover the
initial catalytic activity of the fresh material. After the use and solid-liquid extraction, the time-conversion plot
of a subsequent reaction (Figure 3) shows still a gradual decrease in the catalytic activity
1
00
7
5
2
5
0
5
0
500
1000
1500
Time (min)
1
3

Figure 4. Activity test of fresh UiO-66-Br in absence (■) or presence of 0.30 (●) or 0.60 (▲) mmol of 2-
methoxy-2-phenylethanol. Reaction conditions: styrene oxide (1 mmol), methanol (5 mL), nitrobenzene as
internal standard (0.5 mmol), catalyst (0.037 mmol of Zr) temperature 50 ºC.
To provide further confirmation to catalyst poisoning as the main cause of deactivation, a series of three
successive catalytic tests were performed in which the reaction of styrene oxide and methanol was carried out
in the absence or presence of two different concentrations of 2-methoxy-2-phenylethanol. The results are
represented in Figure 4. In the presence of 2-methoxy-2-phenylethanol, the reaction rate significantly
decreases. The reaction product acts, thus, as a poison of the catalyst, decreasing the initial reaction rate of the
fresh sample when this compound is present at initial reaction time
.
FT-IR spectrum (Figure S6) of used UiO-66 shows an increase in the OH band intensity accompanied by an
enhancement of CH -O peak intensity. These changes are compatible with the presence of the reaction product
3
adsorbed in the solid after the reaction. The C/H ratio, calculated by elemental analysis, for the fresh and
recovered catalyst after washing is 9.26 and 13.40, respectively. The surface area of the used catalyst is
2
-1
2
-1
3
21 m ·g that is smaller than the specific surface area value of the fresh catalyst (745 m ·g ). All these
characterization data, together with the experimental results in Figure 4, indicate that adsorption of the reaction
product is must probably the main reason of catalyst deactivation.
3
.4. Internal location of active sites
In order to determine the influence of the particle size of UiO-66-Br, a control experiment was performed
with a suspension of catalyst that was submitted to sonication for 30 min in methanol previous to its use as
catalyst in the reaction. This pretreatment causes a decrease in the average crystallite size (see Figure S7 where
the different size of the crystals before and after sonication can be observed). Comparing the time-conversion
plots of the regular size UiO-66-Br with the pretreated catalyst, no significant difference in activity was
observed (Figure S8). This sugggests that the particle size of UiO-66-Br does not play a significant impact on
the catalytic activity of the material under the present experimental conditions. This lack of influence of
crystallite size is in agreement with the reaction taking place within the internal pores of the solid and not only
on the external surface.
The activation energy for the reaction of styrene oxide and methanol was determined by performing four
reactions under the same conditions at different temperatures in the range from 30 to 60 ºC. From the temporal
1
4

-
1
evolution of the conversion, the activation energy from the Arrhenius plot was estimated to 27.6 kJ mol
Figure 5).
(
1
00
-1,4
-1,6
-1,8
-2,0
-2,2
-2,4
5
0
0
500
1000
1500
Time (min)
3
0
31
32
33
4
-
1
(
1/T)·10 (K )
Figure 5. Arrhenius plot of the logarithm of the reaction rate constant (k) vs the inverse of the absolute
temperature (1/T). The inset shows the time-conversion plots of epoxide ring opening promoted by UiO-66-Br
at different temperatures 30 (■), 40 (●), 50 (▲) and 60 ºC(▼). b) Reaction conditions: styrene oxide (1
mmol), methanol (5 mL), nitrobenzene as internal standard (0.5 mmol), catalyst (0.037 mmol of Zr).
3
.5. Scope
Using UiO-66-Br as solid catalyst, the nucleophilic addition to styrene oxide was also screened for a series
of primary, secondary and tertiary alcohols, as well aromatic and aliphatic amines. These results are presented
1
5

in Table 4. Except for the aniline, in all cases a single product corresponding to the regioisomer derived from
SN1 mechanism was observed which is in agreement with previously shown results. For the case of the aniline,
the presence of the regioisomer derived from the attack of the nucleophile to the less substituted carbon atom of
the epoxide ring was also detected, although with lower selectivity than the S 1 product. This is in agreement
N
with the effect of the steric hindrance on the catalytic activity, either of the nucleophile or the functional ligand,
as previously proposed. Also using methanol as nucleophile, the reaction of a series of five different
asymmetric and symmetric epoxides was investigated.
As can be seen in Table 4, the styrene oxide reacts with primary alcohols and the reactivity decreases with
the increasing of the chain length at 50 ºC (i.e. ethanol, 1-propanol and 1-butanol). Iso-propanol also reacts
with styrene oxide, but higher reaction temperatures are required to achieve high yield of the product, while
with tert-butanol the conversion is significantly lower even after longer reaction time (48 h).
All these reactivity patterns are consistent with the negative influence of steric hindrance of the alcohol on
the final reactivity. Additionally, aliphatic hexylamine reacts to give the corresponding α,β-aminoalcohol faster
than the aromatic amine, in agreement with the relative nucleophilicity of the amine.
Finally, styrene oxide having a benzylic position reacts much faster than aliphatic monosubstituted or
disubstituted aliphatic epoxides. Further, stilbene oxide exhibits a slowest reactivity due to a combination of
steric encumbrance and electronic effects.
1
6

a
Table 4. Scope of epoxide ring opening for different substrates using UiO-66-Br as catalyst.
a
Table 4. Scope of epoxide ring opening for different substrates using UiO-66-Br as catalyst.
Entry
Epoxide
Nucleophille
T (ºC)
Product
Conver. %
Yield %
1
50
96
86
b
2
50
Reflux
50
44
56
43
91
12
91
---
97
8
41
58
40
87
10
88
----
96
4
b
3
4
b
5
Reflux
50
6
7
8
Reflux
50
9
Reflux
50
b
1
1
1
0
1
2
b
c
Reflux
Reflux
49
54
40
51
c
1
3
Reflux
90
90
1
7

1
1
1
4
5
6
50
50
50
42
50
61
40
47
46
b
1
1
7
8
50
50
22
13
20
11
b
a) Reaction conditions: UiO-66-Br (13.4 mg), epoxide (1 mmol), alcohol (5 mL), nitrobenzene as internal
standard (0.5 mmol) and 24 h. Conversion were calculated by GC using nitrobenzene as internal standard and
1
yield were calculated by H-NMR spectroscopy. b) Time reaction 48 h c) UiO-66-Br (13.4 mg), epoxide (1
mmol), amine (1 mmol), CH CN (2.5 mL), nitrobenzene as internal standard (0.5 mmol), 24 h.
3
a) Reaction conditions: UiO-66-Br (13.4 mg), epoxide (1 mmol), alcohol (5 mL), nitrobenzene as internal
standard (0.5 mmol) and 24 h. Conversion were calculated by GC using nitrobenzene as internal standard and
1
yield were calculated by H-NMR spectroscopy. b) Time reaction 48 h c) UiO-66-Br (13.4 mg), epoxide (1
mmol), amine (1 mmol), CH CN (2.5 mL), nitrobenzene as internal standard (0.5 mmol), 24 h.
3
4
. Conclusions
In the present study, it has been shown that the catalytic activity of the microporous zirconium terephthalate
UiO-66 can be tuned by in the initial reaction rate varying by more than two orders of magnitude depending of
the substituent introducing functional groups on the aromatic ring of the terephthalate linker. A linear
correlation between the values of the Hammett constant of the substituent and the initial reaction rate does not
however apply in the present case, suggesting that the reaction mechanism might involve more than one type of
active center or that other factors besides electron density have to be considered. The most active material was
1
8

the UiO-66-Br derivative that behaves as heterogeneous catalyst and exhibits a wide scope with the expected
influence of the substrate and nucleophile reactivity. Although the chemical and structural integrity of the
catalyst is maintained under the reaction conditions, deactivation of the catalyst is also not observed, probably
due to poisoning by the reaction product. Overall, this study constitutes an additional proof that more efficient
MOF catalysts can be simply obtained by changing the chemical nature of isostructural MOFs thorough ligand
functionalization.
5
. Acknowledgments
Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa and CTQ2015-
9153) and CTQ2014-53292-R and Generalitat Valenciana (Prometeo 2013/14) and is grateful acknowledged.
6
We also thank EU under the Being Energy contract for partial funding. J.F.B. thanks the Universitat Politécnica
de Valencia for a postgraduate scholarship. S.N. thanks the Spanish Ministerio de Educación, Cultura y
Deporte for José Castillejo mobility programme (CAS14/00067).
6
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TOC
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