Atmospherical oxidative coupling of amines by UiO-66-NH2 photocatalysis under milder reaction conditions

Article abstract of DOI:10.1016/j.catcom.2019.03.011

To assess the mechanism and milder conditions for amines to imines, UiO-66-NH₂ was chosen as an efficient photocatalyst for the transformation under light irradiation. A series of experiments for the catching of the active species were performe

Full text of DOI:10.1016/j.catcom.2019.03.011

Catalysis Communications 124 (2019) 108–112
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Atmospherical oxidative coupling of amines by UiO-66-NH photocatalysis
2
under milder reaction conditions
a
a
a
a
a
b
a
Ruxue Liu , Shuangyan Meng , Yali Ma , Litong Niu , Shuanghong He , Xueqing Xu , Bitao Su ,
a
a,⁎
a,⁎
Dedai Lu , Zhiwang Yang , Ziqiang Lei
a
Key Laboratory of Polymer Materials of Gansu Province， Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education, College of Chemistry
and Chemical Engineering, Northwest Normal University, Lanzhou 730070, PR China
b
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
To assess the mechanism and milder conditions for amines to imines, UiO-66-NH was chosen as an efficient
2
UiO-66-NH
2
photocatalyst for the transformation under light irradiation. A series of experiments for the catching of the active
Heterogeneous photocatalysis
Amines coupling reaction
Free radicals
+
−
species were performed, and it was proved that holes (h ) and superoxide radicals (%O
2
) were involved when
the reaction was performed under air. The reaction also occurred because of the presence of nitrogen-centered
+
radical cations and carbon-centered radicals, which were induced by photogenerated hole (h ) with consider-
able conversion and selectivity under anaerobic conditions. Our experiments revealed that the catalyst can be
recycled for four times.
1
. Introduction
reaction to obtain milder conditions and have a better idea of the re-
action mechanism.
Semiconductor-based photocatalysis have attracted great interest
Metal-organic frameworks (MOFs), a fascinating class of crystal
materials because of their special advantages has launched in many
fields [13,14] such as gas storage [15], drug delivery [16], and che-
mical sensors [17]. There also have been reports that MOFs can be
applied for the transformation of amines to imines. For example, Garcia
et al. reported that MIL-100(Fe) can achieve this transformation but the
reaction can occur in the presence of heat [18]. Sun et al. also reported
from researchers for their capacity of environmental remediation and
solution of resource shortage [1–3]. Among these studies, hetero-
geneous catalysis for selective organic transformations under light has
attracted great interest from scholars. To date, some selective redox
organic transformations in this field have been developed, such as the
oxidation of alcohols [4], the reduction of nitroaromatic [5], and the
oxidation of toluene [6].
that NH -MIL-125(Ti) was an efficient photocatalyst for amines to
2
Mostly, imines are formed from amines and aldehyde by con-
densation; it is always difficult to control the reaction because of the
high activity of aldehyde [7]. Heterogeneous photocatalysis seems to be
a desirable and green method for the direct conversion of benzylamines
into imines [8]. These photocatalysts exhibit excellent catalytic per-
formance under light irradiation but the requirements of concentration
of oxygen and temperature are strict. When the mesoporous graphite
imines [19]. However, oxygen was necessary for the transformation.
UiO-66-NH is derived from UiO-66 through ligand functionalization
for a wider response to the spectrum, owing to its high thermal stability,
2
we choose UiO-66-NH as an efficient photocatalyst for the transfor-
2
mation of amines to imines. It can be noted that imines could be de-
tected with considerable conversion whatever under the air or anae-
robic condition. Meanwhile, corresponding mechanisms were
proposed, as for this situation, the conditions for the conversion of
amines to imines is milder and the mechanism is clearer.
carbon nitride (mpg-C
3
N ) was used as photocatalyst, high oxygen
4
pressure (0.5 MPa) and relative temperature were required [9]. When
WS was used as a photocatalyst, the yield of the imines decreased
significantly because the reaction was performed in the air rather than
in O [10]. Although core-shell structured CdS@C [11] and PCN-
2
2. Experimental
2
N
3 4
2
22 [12] were used as photocatalysts for this transformation in air, the
UiO-66 and UiO-66-NH
2
were synthesized using the same procedure
O
2
is still needed as an oxidant. Therefore, it is necessary to assess the
reported previously [20,21]. The detailed experimental procedures are
⁎
Corresponding authors.
E-mail address: yangzw@nwnu.edu.cn (Z. Yang).
https://doi.org/10.1016/j.catcom.2019.03.011
Received 29 December 2018; Received in revised form 11 March 2019; Accepted 12 March 2019
Available online 14 March 2019
1566-7367/ © 2019 Published by Elsevier B.V.

R. Liu, et al.
Catalysis Communications 124 (2019) 108–112
described in the Supporting Information.
Table 1
Comparison between UiO-66-NH
2
and UiO-66 as catalysts.
UiO-66
3
. Results and discussion
a
Entry
UiO-66-NH
2
bCon. (%)
cSel. (%)
bCon. (%)
cSel. (%)
3.1. Characterization
1
83
66
34
99
99
99
42
15
7
99
99
99
Fig. S1 showed the XRD patterns of pure UiO-66 and UiO-66-NH
2
.
d
e
2
The diffraction peaks of UiO-66 matched well with the calculated ones,
3
providing a clear proof that the sample was synthesized successfully.
a: Catalyst (15 mg), benzylamine (0.1 mmol), reaction time (10h); light (Xenon
lamp irradiation with the full spectrum), air, 10 h;
Further, it was obvious that UiO-66-NH and UiO-66 had the same
2
diffraction peaks; this revealed that the functionalization of the organic
linker did not affect the crystal structure of UiO-66, which matched
previously reported [22]. The morphology of the samples is shown in
Fig. S2. The figure showed that there was no obvious difference be-
b and c: Calculated by GC analysis.
d: Catalyst (15 mg), benzylamine (0.1 mmol), reaction time (10 h); light (Xenon
lamp irradiation with a 365 nm cutoff filter), air, 10 h.
e: Catalyst (15 mg), benzylamine (0.1 mmol), reaction time (10 h); light (Xenon
lamp irradiation with a 420 nm cut-off filter), air, 10 h.
tween UiO-66-NH and UiO-66 in morphology.
2
The FT-IR spectra were also obtained to analyze the structure of
UiO-66 and UiO-66-NH . As shown in Fig. S3, for UiO-66, the peaks
that appeared at 1573 cm
asymmetric and symmetric vibrations of carboxyl groups, respectively.
The peaks due to the OeH and CeH vibration in the H BDC ligand
2
(
Table 1, Entries 2, 3). All the results indicated that UiO-66-NH
more efficient photocatalyst than UiO-66 for the coupling of amines.
The reason is that UiO-66-NH shows a wider response to the spectrum
by the modification of -NH
2
was a
−
1
−1
and 1435 cm
are produced by the
2
2
2
group on the organic ligand. The experi-
−
1
appeared at 819, 769, and 665 cm , respectively. Compared with UiO-
mental results are consistent with the UV–vis DRS, which was showed
in Fig. S6, Therefore, Xenon lamp with a full spectrum was chosen for
the following studies if there was no specific explanation.
6
6, for UiO-66-NH , the two absorption bands detected at 3450 and
2
−
1
3
375 cm were attributed to the stretching vibrations of NeH, and the
−1
peak at 768 cm
was easily considered as characteristic wagging vi-
−1
To obtain the influence of solvents on photocatalytic coupling of
benzylamine, the reaction was examined in various solvents. The results
are shown in Table 2. It was clear that the solvents affected the results.
The adsorption, oxidation, and polarity of solvent are often considered
for the factors affecting the process of photocatalytic reaction [27]. For
example, polarity affects the condensation reaction process for produ-
cing imines. Among the different polar solvents, ethyl acetate, di-
chloromethane, and DMF have relatively higher polarity, but the lower
conversion was obtained (48%, 51%, and 43%, respectively, Table 2,
Entries 3, 6, 9). The reason was that competitive adsorption between
the solvent and the substrate, which may lead to a slow reaction rate.
When reaction was proceeded in toluene and hexane, not only the
lower conversion were provided (65% and 36%, respectively, Table 2,
Entries 4, 8), but also the inferior selectivity achieved (82% and 87%,
respectively, Table 2, Entries 4, 8). Lower conversion but not very high
selectivity was appeared when benzotrifluoride was involved in the
conversion (Table 2, Entry 5). When acetonitrile, ethanol, and 1,2-di-
chloroethane were used as the reaction medium (Table 2, Entries 1, 2,
brations of NeH. The stronger band at 1258 cm
was produced by
CeN stretching absorption. The peaks that the asymmetric and sym-
−
1
metric stretching vibrations of C]O appeared at 1655 and 1497 cm
,
respectively.
Detailed information of chemical composition and elemental states
about UiO-66-NH was obtained by XPS. It is demonstrated that Zr, N,
2
C, and O elements were existed in the sample and the results were
showed in XPS survey spectrum (Fig. S4a). The high-resolution XPS
spectrum of Zr3d indicated that there were two peaks located at around
1
82.94 eV and 185.30 eV (Fig. S4b), respectively, which belong to
4+
Zr3d5/2 and Zr3d3/2, was indicating the existence of Zr [23]. Fig. S4c
is the spectrum of N1s, the peak at 399.58 eV was assigned to N species
in the NH group. Three peaks, located at 288.80 eV, 285.37 eV, and
2
2
84.60 eV in the C1s spectrum (Fig. S4d), were attribute to the carbonyl
carbon (C]O), carboxylate carbon (OeC]O), and C]C bond of the
carbon components in H ATA linkers [24]. The O1s spectrum can be
deconvoluted into three peaks appeared at 531.02 eV, 531.89 eV, and
32.69 eV (Fig. S4e), respectively. The peak at 532.69 eV was due to the
hydroxyl groups, while another two peaks at 531.02 eV and 531.89 eV
were the fitted peaks of the ZreO bonds in UiO-66-NH and the car-
boxylate groups in the H ATA linkers [25].
Fig. S5 is the nitrogen adsorption-desorption isotherms of UiO-66
and UiO-66-NH . The typical IV isotherms show that the two samples
bear the mesoporous structure. The BET specific surface area of the as-
2
5
7
), the highest conversion and selectivity were obtained when acet-
onitrile was selected (83% and 99%, respectively, Table 2, Entry 1).
Considering all the above results, acetonitrile was chosen as the best
medium for the following studies.
2
2
To further explore the UiO-66-NH
2
as photocatalyst for photo-
2
catalytic coupling of amine, a series of amines under light irradiation,
the results were listed in Table 3. It showed that benzylamines with
2
−1
prepared UiO-66-NH is 1030 m g , although it is less than that of the
2
electron-donating groups (such as CH
3
and OCH ) and electron-
3
2
−1
UiO-66 (1418 m g ) due to the involving of the eNH
2
group, it is
bigger than that of reported previously (832 m g ) [26]. All the re-
sults indicated that UiO-66-NH was obtained successfully.
2
−1
2
Table 2
Photo-oxidation coupling of benzylamine in different solvents.
3
.2. Photo-oxidation coupling of amines
a
b
cSel.%
Entry
Sol.
Con.%
The activity of the as-prepared samples under light illumination was
1
2
3
4
5
6
7
8
9
acetonitrile
ethanol
83
81
48
65
71
51
81
36
43
99
99
99
82
98
98
99
87
86
investigated through the photo-oxidation coupling of amines and ben-
zylamine was selected as the model substrate first. To explore the role
ethyl acetate
toluene
of -NH
2
group on the organic ligand, UiO-66-NH and UiO-66 were
2
benzotrifluoride
dichloromethane
1,2-dichloroethane
hexane
applied in the reaction under different wavelengths of light. The results
are listed in Table 1. When UiO-66-NH was used as catalyst, with the
2
illumination of a full spectrum Xenon lamp, the catalytic efficiency of
the reaction was much higher than when UiO-66 was used (Table 1,
Entry 1). When a Xenon lamp with a 365 nm cut-off filter or a 420 nm
cut-off filter was used as the light source, higher conversion was ob-
DMF
a: Conditions: UiO-66-NH
time (10 h);
2
(15 mg), benzylamine (0.1 mmol), light, reaction
tained when UiO-66-NH
2
was used as photocatalyst than that of UiO-66
b and c: Calculated by GC analysis.
109

R. Liu, et al.
Catalysis Communications 124 (2019) 108–112
Table 3
Photo-oxidation coupling of various substituted benzylamine over UiO-66-NH .
2
aEntry
Substrate
Product
b
Conv. (%)
c
Sele. (%)
99
NH
2
N
1
83
52
58
43
29
68
44
43
83
99
40
NH
2
N
2
3
4
5
6
7
8
9
99
89
90
86
99
99
99
97
93
76
NH
2
N
N
MeO
MeO
OMe
NH
2
OMe
OMe
OMe
NH
OMe
2
N
O
MeMe
O
NH
2
N
N
N
F
F
F
NH
2
Cl
Cl
Cl
NH
2
F
3
C
F3C
CF3
S
NH
2
N
N
S
S
O
NH
2
1
0
O
O
11
NH2
N
N
N
N
a: Conditions: UiO-66-NH
2
(15 mg), substrate (0.1 mmol), light, reaction time (10 h).
b and c: Calculated by GC analysis.
withdraw groups (such as F, Cl, and CF
3
) had no definite influence both
Table 4
Active species trapping experiments of the photocatalytic coupling of amines by
in the conversion and selectivity (Table 3, Entries 1–8), suggesting that
the catalysis of UiO-66-NH .
2
there was no clear electronic effect in this system. Benzylamines with
–
OCH
3
at different position of phenyl ring showed decreased conver-
a
bCon. (%)
cSel. (%)
Entry
Condition.
sion in the order of orth- > meta- > para- position (Table 3, Entries
1
2
3
O
2
Air
85
83
61
14
8
99
99
99
97
99
98
99
92
94
3
–5), indicating that steric hindrance affected this system significantly.
Benzylamines with heteroatoms (such as O, S, and N atom), especially
heterocyclic amines containing sulfur atoms, which usually considered
to be poison agent for most metal catalysts, could also proceed at the
same conditions. And it was clearly that the conversion of both thienyl-
and furyl- containing methylamine was higher than others.
N
2
BQ
4
d
5
Air
e
6
Air
11
22
3
7
KI
f
8
TEMPO
KI
f
9
18
3.3. Photo-oxidation coupling mechanism
a: Conditions: UiO-66-NH
(10 h).
2
(15 mg), benzylamine (0.1 mmol), reaction time
To study the possible mechanism of photocatalytic coupling of
amines, a series of control experiments were carried out, the results are
shown in Table 4. The conversion from starting substrate into final
product was very little, which can be negligible when the reaction was
b and c: Calculated by GC analysis.
d: The reaction was performed in dark.
e: The reaction was performed without UiO-66-NH
2
.
f: The reaction was performed in the N .
2
performed in dark (Table 4, Entry 5). In the absence of UiO-66-NH , a
2
similar result was observed (Table 4, Entry 6), suggesting that both
light and UiO-66-NH were vital for the reaction. Obviously, there was
no significant difference in the conversion whether under O or air
holes (h+) played in the coupling, the reaction was carried out in the
presence of KI (Table 4, entry 7), which is known to be the hole sca-
venger in photocatalytic reactions, the conversions of benzylamine was
2
2
(
85% and 83%, respectively, Table 4, Entries 1, 2). Therefore, the
+
condition under air was chosen for the following studies if there was no
specific explanation.
reduced to 22%, which indicated that h was also the vital active
species for photocatalytic coupling of amines. These results provided
–
+
To confirm the active species involved in the photocatalytic cou-
the evidence that%O
2
and h are important in the photocatalytic
pling of amines, p-benzoquinone (BQ), as a scavenger of superoxide
coupling of amines in the air conditions.
−
radical (%O
2
) [28], was added to the reaction system (Table 4, entry
.
Based on all the above results, we proposed the possible mechanism
4
1
). The yield of N-benzylidene benzylamine decreased significantly to
–
for the photocatalytic coupling of amines in the air. As shown in
−
+
4%, indicating that%O
2
is an essential active species in the photo-
Scheme 1, photoinduced electrons (e ) and h were generated when
UiO-66-NH was illuminated by light, H ATA acted as an antenna to
oxidation coupling of amines. To investigate the role the photoinduced
2
2
110

R. Liu, et al.
Catalysis Communications 124 (2019) 108–112
Scheme 1. The proposed mechanism for photocatalytic coupling of amine by
Scheme 2. The proposed mechanism for photocatalytic coupling of amine by
the UiO-66-NH
2
in the air.
the UiO-66-NH2 under anaerobic condition.
process under the anaerobic conditions. To testify this conclusion,
2
,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), a typical radical sca-
venger, was added to the reaction system to capture free radicals
(
Table 4, Entry 8). The yield of the products decreased significantly to
about 3%, indicating that free radicals were involved in the reaction.
When KI was also added, the conversion of amines decreased to 18%
+
(
Table 4, Entry 9). The above results proved that free radicals and h
were involved in the process. In other words, the presence of free ra-
+
dical and h could also promote the transformation.
When the reaction was performed in N , the proposed reaction
2
mechanism was shown in Scheme 2. Under the light illumination, the
+
h
was transferred to amines producing nitrogen-centered radical ca-
tions (Step 1 in Scheme 2). Then, carbon-centered radicals were gen-
erated from nitrogen-centered in a proton transfer (PT) process [32]
(
Step 2 in Scheme 2). Amino radical cations were then produced
through the coupling between nitrogen-centered radical cations and
carbon-centered radicals (Step 3 in Scheme 2). With a subsequent PT
process to form aminals (Step 4 in Scheme 2). In general, a hole-assisted
elimination of amine group occurred in the final process, and N-ben-
zylidene benzylamine was formed as the final product (Step 5 in
Scheme 2).
Fig. 1. The Mott-Schottky plots of UiO-66-NH .
2
absorb light and the energy shifted from H
conium-oxygen clusters [29]. As shown in Fig. 1, the flat-band potential
of UiO-66-NH was measured using Mott-Schottky plots. It showed that
was −0.82 V vs. Ag/AgCl
2
ATA to the inorganic zir-
3.4. Recyclability and stability of the catalyst
2
the flat-band potential of UiO-66-NH
2
The stability of the catalyst was verified by performing the recycle
experiments. The catalyst was washed with ethyl alcohol and cen-
trifuged three times after the first run; it was then dried at 100 °C for
12 h, and the recovered catalysts were used in the next run. As shown in
Fig. S7a, after four cycles of reuse in the photocatalytic oxidation of
benzyl amine, the conversion of benzylamine had no significant de-
creases and the selectivity of N-benzylidene benzylamine was main-
tained. XRD and FT-IR was used to estimate the structural stability of
(
−0.60 V vs NHE). The positive slope of the plot revealed an n-type
semiconductor feature of UiO-66-NH
2
. For n-type semiconductors, the
conduction band (CB) position was very close to their flat band po-
tentials [30]. Therefore, the CB of UiO-66-NH
2
was −0.60 V (vs NHE),
–
which was more negative than the oxidation potential of O
2
/%O
2
−
(
−0.33 V vs NHE); thus, the e was transformed to O
2
to producing one
−
of the main active species of %O
2
, (Step 1 in Scheme 1). Meanwhile,
+
the h were transferred to amines, nitrogen-centered radical cations
UiO-66-NH
intensity of the peaks of XRD were maintained well after the reaction.
This suggested that the crystal structure of UiO-66-NH was not de-
stroyed after the recycling. Fig. S7c showed the FT-IR spectra of UiO-
2
. As shown in Fig. S7b, the absorption position and the
were produced in this process (Step 2 in Scheme 1). Intermediate imine
–
was then generated by the reaction between the %O
2
and nitrogen-
2
centered radical cations (Step 3 in Scheme 1), which was reactive in the
system. These intermediate imines were next attacked by the unreacted
amines to generate the next corresponding aminals (Step 4 in Scheme
66-NH after four cycles. No obvious changes were observed. Based on
2
the above results, it is suggested that UiO-66-NH
reusability in the reaction system.
2
possess excellent
1
). Finally, N-benzylidene benzylamines were obtained by hole-assisted
elimination of amine group from aminals (Step 5 in Scheme 1).
To investigate the role of O , further experiments were carried out.
Interestingly, the photocatalytic coupling of amines could still be
achieved after the removal of O from the system by purging with N
2
4. Conclusions
2
2
With the catalysis of the UiO-66-NH under light irradiation, the
2
(
61%, Table 4, Entry 3). Considering the further research performed by
transformation of amines to imines was successfully achieved with
considerable conversion and selectivity under air and anaerobic con-
ditions. The possible mechanisms were proposed. The work provides
Wang et al. [31], to explain this phenomenon, we speculate that the
conversion of benzylamines to the corresponding product was a radical
111

R. Liu, et al.
Catalysis Communications 124 (2019) 108–112
clearer mechanisms for amines to imines and advance a new MOFs
based green photocatalysts system for the transformation, Furthermore,
it is met the requirements of green chemistry and the sustainable energy
development of society. On the other hand, the research also provided
the great potential for the application of MOFs-based materials in
photocatalytic organic reactions.
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