Development of a novel one-pot reaction system utilizing a bifunctional Zr-based metal-organic framework

Article abstract of DOI:10.1039/c3cy00917c

A novel one-pot reaction system is developed by utilizing a bifunctional metal-organic framework photocatalyst (Zr-MOF-NH₂). Zr-MOF-NH ₂ promotes sequential photocatalytic oxidation and Knoevenagel condensation reaction to produce benzylidenemalononitrile from benzyl alcohol and malononitrile under UV-light irradiation.

Full text of DOI:10.1039/c3cy00917c

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Development of a novel one-pot reaction system
utilizing a bifunctional Zr-based metal–organic
framework†
Cite this: Catal. Sci. Technol., 2014,
, 625
4
Takashi Toyao, Masakazu Saito, Yu Horiuchi* and Masaya Matsuoka*
Received 12th November 2013,
Accepted 10th December 2013
DOI: 10.1039/c3cy00917c
www.rsc.org/catalysis
A novel one-pot reaction system is developed by utilizing a
bifunctional metal–organic framework photocatalyst (Zr-MOF-NH ).
Zr-MOF-NH promotes sequential photocatalytic oxidation and
have been actively studied for many applications such as gas
storage, gas separation, sensing and catalysis. In addition,
6
2
2
since the topology and surface functionality of MOFs can
be readily tuned by modifying or varying the constituent
metal–oxo clusters and bridging organic linkers, MOFs
have emerged as an interesting platform to engineer molecu-
Knoevenagel condensation reaction to produce benzylidene-
malononitrile from benzyl alcohol and malononitrile under UV-light
irradiation.
7
lar solids for multifunctional catalysts. Although several
In recent years, much attention has been paid to one-pot
reactions as an attractive synthetic concept for improving
reaction systems have been proposed so far based on the
multifunctionality of organic linkers or coordinatively unsat-
urated metal sites in MOF catalysts, there is no report
describing multifunctionality combined with photocatalytic
and catalytic activities utilizing MOF materials.
1
overall process efficiency and reducing production wastes.
Designing new catalysts with spatially isolated multiple active
sites is required to progress multi-step reaction cascades.
Various catalytic systems have been proposed for the realiza-
tion of one-pot reactions, but many of these employ homo-
geneous catalysts, which generally suffer from product
Herein we report the development of a novel one-pot reac-
tion system utilizing photocatalytic and basic properties of
an amino-functionalised Zr-based MOF (UiO-66-NH ) here
2
2
contamination and limited recyclability. Therefore, develop-
2 2
denoted as Zr-MOF-NH . Zr-MOF-NH promotes sequential
ment of heterogeneous catalysts that promote one-pot reac-
photocatalytic oxidation and Knoevenagel condensation reac-
tion: the conversion of benzyl alcohol into benzaldehyde
through photocatalytic oxidation over Zr–oxo clusters and
Knoevenagel condensation of benzaldehyde with malononitrile
3
tions is currently the focus of intensive research.
Inorganic–organic hybrid materials have also attracted
considerable attention since they not only combine the
respective beneficial characteristics of inorganic and organic
components, but also often exhibit unique properties that
exceed the expectations for a simple mixture of the compo-
over –NH groups, as shown in Scheme 1.
2
8
Zr-MOF-NH
Briefly, a mixture of ZrCl
acid (H BDC-NH ), ion-exchanged water and DMF was
2
was prepared according to the literature.
4
, 2-aminobenzene-1,4-dicarboxylic
4
nents. The construction of hybrid materials offers an almost
2
2
infinite number of chemical and structural possibilities,
namely, the structural diversity of inorganic–organic hybrid
materials allows for accurate material design. Among
them, metal–organic frameworks (MOFs), also called porous
coordination polymers (PCPs), have been of great interest
because of their attractive properties including high specific
surface areas, well-ordered porous structures and structural
reacted under solvothermal conditions at 393 K for 24 h
under autogenous pressure.
XRD, N adsorption, diffuse reflectance UV-Vis and FT-IR
2
measurements were conducted to confirm the formation
of Zr-MOF-NH . Zr-MOF-NH exhibits a diffraction pattern
2
2
5
designability. Taking advantage of these features, MOFs
Department of Applied Chemistry, Graduate School of Engineering, Osaka
Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan.
E-mail: horiuchi@chem.osakafu-u.ac.jp, matsumac@chem.osakafu-u.ac.jp;
Fax: +81 072 254 9910; Tel: +81 072 254 9288
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 One-pot sequential photocatalytic oxidation and Knoevenagel
condensation reaction over Zr-MOF-NH
c3cy00917c
2
.
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consistent with the previously reported pattern of the UiO-66
as the product. The results obtained from the above effort
type structure (see Fig. S1 in ESI†). The specific surface area
show that Zr-MOF-NH possesses both photocatalytic activity
and basicity that play key roles for respective photocatalytic
oxidation and Knoevenagel condensation reactions.
2
2
−1
of Zr-MOF-NH
2
was determined to be 1071 m g using BET
(Brunauer–Emmett–Teller) method based calculations on its
N2 adsorption isotherm (Fig. S2†). This value is similar to
The potential use of Zr-MOF-NH as a bifunctional catalyst
2
that of Zr-MOF which has also the same UiO-66 type struc-
ture (1141 m g ). The diffuse reflectance UV-Vis spectrum
also indicates the successful formation of Zr-MOF-NH2
for the one-pot reaction was investigated next. For this pur-
2
−1
pose, Zr-MOF-NH was employed to promote one-pot synthe-
2
sis of benzylidenemalononitrile (3) from benzyl alcohol and
malononitrile that takes place through sequential photocata-
lytic oxidation and Knoevenagel condensation reaction under
UV-light irradiation at 363 K. Inspection of the time course of
the process displayed in Fig. 1 shows that benzylidene-
malononitrile is efficiently generated from benzyl alcohol (1)
via a pathway involving initial formation of benzaldehyde (2),
and that the yield of 3 reaches 91% after a 48 h reaction
time. It is also shown that the reaction does not take place in
the absence of a catalyst (entry 11 in Table 1). These observa-
(
Fig. S3†). In addition, the FT-IR spectrum of Zr-MOF-NH
2
−
1
shows bands at 3388 and 3512 cm corresponding to the
symmetric and asymmetric stretching of primary amines
2
(see Fig. S4 in the ESI†), suggesting that the –NH groups
are free without coordination. These results clearly indicate
the successful formation of Zr-MOF-NH2.
In an investigation exploring the potential catalytic activity
of Zr-MOF-NH , photocatalytic oxidation of benzyl alcohol
2
was performed under UV-light irradiation (Fig. S5†). For
comparison with the Zr-MOF-NH
2
catalyst, the reaction
2
tions clearly demonstrate that Zr-MOF-NH serves as an effec-
was also performed over Zr-MOF without –NH groups and
tive bifunctional catalyst for this one-pot reaction. However,
an excess amount of malononitrile (3 mmol) is used for the
one-pot reaction compared with benzyl alcohol (0.1 mmol).
Interestingly, UV-light irradiation decreases the reaction rate
of the second step of the reaction (Knoevenagel condensa-
tion), as shown in Fig. S7.† Therefore, the excess amount of
malononitrile is required for the efficient progress of the
reaction. A detailed study into the reason why UV-light irradi-
2
9
Al-MOF-NH
2
(MIL-53(Al)-NH
2
)
which does not have photo-
and
catalytic activity. As shown in Fig. S5,† Zr-MOF-NH
2
Zr-MOF catalyse the reaction to produce benzaldehyde as
the product, whereas benzaldehyde is not generated over
Al-MOF-NH . This result suggests that Zr–oxo clusters
2
within Zr-MOF-NH2 and Zr-MOF behave as photocatalysts
under UV-light irradiation. Subsequently, the Knoevenagel
condensation reaction of benzaldehyde with malononitrile
1
3
ation decreases the reaction rate is now underway.
was carried out at 363 K over Zr-MOF-NH , Zr-MOF and
The stability of Zr-MOF-NH2 was investigated by XRD
measurements before and after the reaction, as shown in
Fig. 2. The diffraction peaks corresponding to the UiO-66
structure is maintained even after the reaction, indicating
that Zr-MOF-NH possesses high durability toward the one-
2
pot reaction cascades.
Moreover, a reusability test was carried out after the first
run. Although the reaction rate is decreased, an 89% yield
of the final product is achieved after reaction for 72 h.
This result suggests that the catalyst can be reused as a
heterogeneous catalyst with slight decrease in the activity.
2
Al-MOF-NH
proceeds over Zr-MOF-NH
2
, as shown in Fig. S6.† The reaction efficiently
and Al-MOF-NH . It is well-
2
2
known that the Knoevenagel condensation reaction pro-
ceeds through the formation of an imine intermediate on
the catalysts containing organic amines, followed by the
1
0
addition of an active methylene group.
reaction is promoted more efficiently over the catalysts
with –NH groups than over the catalysts without –NH
groups. It should be noted that Al-MOF-NH exhibits higher
Therefore, the
2
2
2
catalytic activity for the reaction than Zr-MOF-NH
both MOFs have –NH groups within their structures. This
result can be attributed to the large pore size of Al-MOF-NH
2
, although
2
2
1
1
8
(
2
0.85 nm) compared to that of Zr-MOF-NH (0.6 nm). The
large pore size enables facile diffusion of the substrates and
products, resulting in the higher catalytic activity. Further-
2 2
more, the total basicities of Zr-MOF-NH and Al-MOF-NH
−
1
were also determined to be 1.25 and 1.58 mmol g , respec-
tively, using an acid–base titration method. This basicity
difference also accounts for the difference in the activity
between Zr-MOF-NH
ingly, the reaction is also catalysed by Zr-MOF that does not
contain –NH groups. In general, Knoevenagel condensation
reactions are promoted not only by base catalysts but also by
2 2
and Al-MOF-NH . In addition, interest-
2
1
2
Lewis acid catalysts.
It has been widely studied that
coordinatively unsaturated metal sites in MOFs can act as
Lewis acid catalysts. These facts suggest that coordinatively
unsaturated metal sites in MOFs catalyse the Knoevenagel
condensation reaction, generating benzylidenemalononitrile
Fig. 1 Time course of the one-pot sequential photocatalytic oxidation
and Knoevenagel condensation reaction over Zr-MOF-NH
2
under
UV-light irradiation at 363 K: benzyl alcohol (◆), benzaldehyde (●),
benzylidenemalononitrile (■).
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Table 1 One-pot sequential photocatalytic oxidation and Knoevenagel
than the mixture of Zr-MOF and amino-functionalised
a
condensation reaction using various catalysts
14,15
MCM-41 (MCM-41-NH2)
MCM-41-NH
or the mixture of ZrO2 and
2
, suggesting that the combination of a Zr–oxo
cluster and a BDC-NH2 unit existing close to each other
would be favourable for the one-pot reaction. On the other
hand, the reaction hardly occurs over Al-MOF-NH that does
2
not exhibit photocatalytic activity. These results clearly dem-
onstrate that the existence of both photocatalytic activity
and basicity, like those found in Zr-MOF-NH , is necessary
2
for the promotion of the one-pot benzylidenemalononitrile
forming process. Moreover, Zn-MOF-NH2 (IRMOF-3) and
Yield (%)
2
3
Entry
1
Catalyst
Conv. (%)
Zr-MOF-NH
Zr-MOF-NH
Zr-MOF-NH
Zr-MOF
2
2
2
100
100
0
96
100
71
77
0
2
89
0
91
1
b
2
3
c
2 2
Ti-MOF-NH (MIL-125(Ti)-NH ) were also prepared and employed
0
1
6
4
5
6
7
8
9
62
75
6
2
0
28
2
61
59
0
for the reaction (entry 9, 10). Since both Zn-MOF-NH and
2
ZrO
Zr-MOF + MCM-41-NH
2
Ti-MOF-NH2 possess –NH2 groups and are known to show
d
2
17
e
2
photocatalytic activities under light irradiation, it is expected
ZrO
Al-MOF-NH
Zn-MOF-NH
2
+ MCM-41-NH
that the one-pot reaction also proceeds efficiently as well
2
2
37
100
0
28
51
0
7
32
0
as over Zr-MOF-NH . However, their catalytic activities are
lower than that of Zr-MOF-NH . These facts are rationalized
2
by the stability of the MOF catalysts under the reaction
conditions. XRD measurements revealed that the structures
2
1
1
0
1
Ti-MOF-NH
No catalyst
2
a
Reaction conditions: benzyl alcohol (0.1 mmol), malononitrile
3 mmol), p-xylene (4 mL), catalyst (100 mg), 363 K, UV-light irradia-
tion, 48 h. The reaction was performed at 298 K. The reaction
was performed without UV-light irradiation. A mixture of Zr-MOF
(
2 2
of Zn-MOF-NH and Ti-MOF-NH are collapsed during the
b
c
d
reaction, whereas the structure of Zr-MOF-NH is maintained
2
e
2
under the same conditions. Therefore, Zr-MOF-NH exhibits
(50 mg) and MCM-41-NH
2
(50 mg) was employed as the catalyst.
A
mixture of ZrO
the catalyst.
2
(50 mg) and MCM-41-NH
2
(50 mg) was employed as
higher activity toward one-pot sequential photocatalytic oxi-
dation and Knoevenagel condensation reaction.
2
Finally, the Zr-MOF-NH catalyst was applied to another
one-pot reaction of benzyl alcohol and ethyl cyanoacetate to
produce ethyl α-cyanocinnamate in order to investigate the
substrate applicability. The reaction was carried out under
the same conditions as the one-pot benzylidenemalononitrile
forming process except that ethyl cyanoacetate was used in
place of malononitrile. Ethyl α-cyanocinnamate as the final
product is generated via benzaldehyde as the intermediate
product over Zr-MOF-NH , and the yield reaches 89% after a
2
reaction time of 96 h. This fact suggests that the developed
system can be applied to various one-pot processes.
2
Fig. 2 XRD patterns of Zr-MOF-NH before and after the one-pot reaction.
Conclusions
In summary, we have prepared and studied a bifunctional
metal–organic framework photocatalyst consisting of a Zr–oxo
cluster and 2-aminobenzene-1,4-dicarboxylic acid as an
For comparison purposes, some control reactions and
one-pot reactions catalysed by conventional solid and other
MOF catalysts were performed (Table 1). When the reaction
is performed under UV-light irradiation at 298 K (entry 2),
the second step (Knoevenagel condensation of benzaldehyde
with malononitrile) does not proceed efficiently whereas the
first step of the reaction (photocatalytic oxidation of benzyl
alcohol) proceeds to produce benzaldehyde. It was also found
that the reaction between benzyl alcohol and malononitrile
does not occur without UV-light irradiation (entry 3), because
the first step in the pathway does not take place. From these
results, the UV-light irradiation and heating up to 363 K were
confirmed to be required to progress the one-pot reaction
organic linker (Zr-MOF-NH ). First, photocatalytic oxidation
2
of benzyl alcohol and Knoevenagel condensation of benzal-
dehyde with malononitrile were explored as test reactions
for the evaluation of the photocatalytic and basic properties
of Zr-MOF-NH , respectively. Zr-MOF-NH was found to catal-
2
2
yse both reactions, indicating that Zr-MOF-NH
2
possesses
both photocatalytically active and basic sites. Subsequently,
Zr-MOF-NH was applied to the one-pot reaction to produce
2
benzylidenemalononitrile through sequential photocatalytic
oxidation and Knoevenagel condensation under UV-light
irradiation. Benzylidenemalononitrile as the final product
was efficiently generated via benzaldehyde as the intermedi-
sequences. In addition, when Zr-MOF or ZrO
2
is employed
(entry 4, 5), the second step does not take place efficiently.
ate product over Zr-MOF-NH . In this process, the Zr–oxo
2
It is noteworthy that Zr-MOF-NH
2
exhibits higher activity
cluster catalyses the first step of the reaction (photocatalytic
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oxidation of benzyl alcohol), and subsequently the –NH2
group catalyses the second step (Knoevenagel condensation
of benzaldehyde with malononitrile), resulting in the prog-
ress of the one-pot reaction. To the best of our knowledge,
this is the first example of one-pot reaction systems utilizing
the photocatalytic and basic properties of MOF materials.
The observations made in this investigation should offer
new insight into the design and manipulation of specifically
functioning MOF catalysts.
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Acknowledgements
The present work is supported by a grant-in-aid for Scientific
Research (KAKENHI) from the Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan (no. 21550192).
T. T. thanks the JSPS Research Fellowships for Young Scientists.
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628 | Catal. Sci. Technol., 2014, 4, 625–628
This journal is © The Royal Society of Chemistry 2014