A Cu-BTC metal-organic framework (MOF) as an efficient heterogeneous catalyst for the aerobic oxidative synthesis of imines from primary amines under solvent free conditions

Article abstract of DOI:10.1039/c9nj05997k

A Cu-BTC (MOF-199) [copper(ii)-benzene-1,3,5-tricarboxylate] catalyst has been synthesized and evaluated for imine synthesis from amine compounds under neat conditions. The performance of the Cu-BTC MOF was significantly higher than that of the CuO supported on Al₂O₃, TiO₂ and SiO₂ catalysts. The role of surface Lewis acid sites on the catalyst in the formation of imine products was illustrated by the pyridine-IR studies. The recovered Cu-BTC catalyst demonstrated consistent activity for five cycles under similar experimental conditions. The physicochemical properties of the catalysts were analyzed by XRD, BET-SA, FT-IR, UV-DRS, SEM, TEM, XPS and pyridine adsorbed DRIFT spectroscopy.

Full text of DOI:10.1039/c9nj05997k

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
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DOI: 10.1039/C9NJ05997K
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Cu-BTC metal-organic framework (MOF) as an efficient
heterogeneous catalyst for aerobic oxidative synthesis of imines
from primary amines under solvent free conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Boosa Venua, Varimalla Shirishaa, Bilakanti Vishalia, Gutta Naresha, Ramineni Kishorea, Inkollu
Sreedharb and Akula Venugopala,*
DOI: 10.1039/x0xx00000x
The Cu-BTC (MOF-199) [copper(II))-benzene-1,3,5-tricarboxylate] catalyst has been synthesized and evaluated for imine
synthesis from amine compounds under neat condition. The performance of Cu-BTC MOF was significantly higher compared
to the CuO supported on Al2O3, TiO2 and SiO2 catalysts. Role of surface Lewis acid sites on the catalyst towards the formation
of imine products was illustrated by the pyridine - IR studies. The recovered Cu-BTC catalyst was demonstrated the
consistent activity for five cycles under similar experimental conditions. The physicochemical properties of the catalysts
were analyzed by XRD, BET-SA, FT-IR, UV-DRS, SEM, TEM, XPS and pyridine adsorbed DRIFT spectroscopy.
conditions. However, the major challenges involved with the
utilization of aforesaid catalysts are the catalyst recovery and
reusability. In addition to this, presence of more quantities of
1. Introduction
Imines are identified as one of the significant nitrogen
sources and their reactive intermediates can be employed for
the synthesis of variety of biologically active compounds which
finds widespread applications as anti-cancer and anti-
inflammatory agents.1 Conventionally imine synthesis is carried
out by reacting the amine with carbonyl functional groups
under acidic conditions, however, this route has a limitation of
using the dehydrating agents.2-4 The other alternative process
such as aerobic oxidation (using molecular oxygen) of amines
has however been attracted much attention recently owing to
its both environmental and economic view point of modern
chemistry. From industrial perspectives, solvents being play the
prominent role in terms of the overall process economics and
hence considered as the active area of research.5
Most of the previous reports revealed that the utilization of
costlier Ru- and Ir-based homogeneous catalysts for amine
oxidation by employing with hazardous oxidizing agents such as
2-iodoxybenzoic acid, N-tert-butylphenylsulfinimidoyl chloride
and high valence reagents such as chromate ions, manganese
dioxide, lead tetra acetate and permanganate ions as oxygen
sources.6,7 Further, homogeneous transition metal systems
such as Ru-complex,8 Fe/CCl4,9 Co-porphyrin complex,10
heteropoly acid NPV6Mo611 and VO(Hhpic)212 catalysts have also
been tested in single step synthesis of imines under mild
ligands, bases, alcohols and amine compounds lead to the
complexity in product separation. To address the
aforementioned challenges, an alternative strategy for the
synthesis of imines from amines in homo and/or cross-coupling
conditions has been developed.13 Previous reports reveal that
the supported noble metal heterogeneous catalysts,
particularly, Ru, Pd, Au, and Pt based catalysts can catalyse the
amine coupling to imine under oxidative conditions.14 On the
contrary it is reported that the Ru/hydroxyapatite and Ru/Al2O3
catalysts are more selective towards nitriles from primary
amines and they could be more selective to imines when
secondary amines are used as reactant.15,16 Further, supported
gold nano particles have also been reported recently for imine
synthesis.17-19 However, the drawbacks of aforesaid catalysts
are mainly involved with the poor catalytic activity or selectivity
and/or a limited substrate scope with molecular O2 as green
oxidant.20
Metal organic frameworks (MOFs) are the novel three
dimensional materials in which active inorganic metal ions are
connected with the organic ligands. MOFs, owing to their
special characteristic properties such as highly crystalline, well
defined structure, ultrahigh specific surface area, thermal
stability and the tunable functionalities, which have been
attracted much interest in various fields of drug delivery, gas
storage and separation, chemical sensor and catalysis.21,22
There have been reports using Cu based homogeneous catalysts
particularly, CuI and CuBr2 for the selective oxidation of
benzylamine, however the catalyst recovery is the main
drawback of the aforesaid catalysts. In addition, CuI catalyst has
been reported the normalized rate of 66.2 mmol (gCu)-1 h-1 with
N-benzylidenebenzylamine in the conversion of amines23 and
a.
Catalysis and Fine Chemicals Division, CSIR – Indian Institute of Chemical
Technology, Tarnaka, Hyderabad – 500 007, Telangana State, India.
Corresponding author’s E-mail: akula@iict.res.in (Akula Venugopal). Tel: +91-40-
27193165; Fax: +91-40-27160921.
b. Department of Chemical Engineering, BITS Pilani Hyderabad Campus, Hyderabad
500 078, Telangana State, India
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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CuBr2 catalyst exhibited rate of 46 mmol (gCu)-1 h-1 with N-
benzylidenebenzylamine for oxidation of benzylic amines.24 In
the present study, we achieved about 150.1 mmol (gCu)-1 h-1 of
N-benzylidenebenzylamine using Cu-BTC MOF catalyst. Hence,
the objective of this study is to synthesize, characterize and
evaluate the Cu-BTC MOF [MOF-199 copper(II))-benzene- 1,3,5
tricarboxylate] (Cu-BTC) for the selective aerobic oxidation of
benzylamine in the presence of molecular O2 without utilization
of solvent at moderate reaction temperatures.
View Article Online
DOI: 10.1039/C9NJ05997K
Scheme 1 Aerobic oxidation of Amines for imine synthesis over Cu-BTC MOF catalyst.
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3 Results and discussion
3.1 Catalytic performance measurements
2 Experimental
In the preliminary experiments a model for imine formation
from benzylamine (optimized loading of 7 mmol, catalyst
loading and reaction run time; see ESI) was performed over Cu-
BTC MOF catalyst and the results are reported in Table 1.
Among the various solvents tested, solvents such as CH3OH,
C2H5OH, DCM and CHCl3 afforded very low yields of imine
product (Table 1 entries 1, 2, 3 and 4). The other solvents
namely acetonitrile, H2O and 1,4- dioxane also demonstrated
lower yields towards the imine formation (Table 1 entries 5,
6and 7). On the contrary, solvents such as Toluene and DMF
exhibited a slightly increase in imine yields (Table 1, entries 8
and 9), which showed 35 and 30% of desired product
respectively. In order to know the influence of catalyst, a blank
experiment without catalyst was also performed. In this
experiment no product was formed even after 24 h of run (Table
1, entry 10) under similar conditions. More interestingly,
whenever the experiment was performed without a solvent, the
imine formation was drastically improved. In general, the
catalyst systems would exhibit better performance in the
absence of a solvent.
2.1 Synthesis and characterization of the Cu-BTC
MOF catalyst
In a typical preparation, solution A was prepared by
dissolving the benzene-1,3,5- tricarboxylic acid (4.91 g, 0.0234
mol) in ethanol and the solution B was prepared by mixing of
DMF (1:1) (25 mL), Cu(NO3)2 .3H2O (10.86 g, 0.0466 mol) and
deionized water (25 mL)25 in required amounts. Then the above
prepared solutions (A and B) were mixed thoroughly on a
magnetic stir. The combined solution was transferred into an
autoclave and kept for hydrothermal treatment at 85 °C for 24
h. The obtained mother liquor was decanted and washed with
DMF and ethanol. The isolated solid product was heat treated
under vacuum at 170 °C for 6 h. Finally, deep purple fine crystal
of Cu-BTC MOF was obtained. For brevity and also to avoid the
self-plagiarism the experimental conditions pertaining to
powder XRD, SEM, XPS, TG, FTIR, BET – surface area, UV-DRS,
ICP-OES and pyridine adsorbed IR studies of all the fresh and
some of the used (recovered after reaction) catalysts were
given in ESI.
Among the various catalysts examined, Cu-BTC MOF
showed a high activity (98%) compared to other catalysts (Table
1, entry 11). The copper precursor of Cu(NO3)2 .3H2O
CuCl2.2H2O and Cu(OAC)2.H2O exhibited some activity with
2.2 Conventional protocol for the synthesis of
imines:
Aerobic oxidation of amine experiments were conducted in yields of about 39, 19 and 23% (Table 1, entries 12-14)
a round bottomed (RB) flask which is connected to a condenser respectively. Interestingly, the Lewis acidic CuO/TiO2 catalyst
followed by oxygen balloon. Benzylamine (7 mmol) and Cu- BTC exhibited significant activity with 75% yield of the imine after 12
MOF (~ 0.050 g) were placed in a round necked - bottomed flask h (Table1 entry 15). Whereas the Al2O3 supported copper oxide
fitted with a reflux condenser and the experiments were a catalyst showed moderate yield of about 52% and the SiO2
performed at a temperature of 80 °C for a period of 12 h. After supported copper oxide offered moderate yield of 38% (Table
completion of the reaction, the product mixture was mixed with 1, entries 16, 17). The experimental results revealed that these
ethyl acetate followed by the centrifugation in order to catalysts are found to be less effective than Cu-BTC MOF
separate the catalyst. The obtained mother liquor was catalyst. In contrast when the experiment carried out under Ar
concentrated using rotary evaporator. Subsequently, product and N2 atmosphere the Cu-BTC MOF exhibited about 20 and
separation was performed using column chromatography using 18% yields respectively (Table 1, entries 18 and 19). The
neutral alumina. The separated catalyst was treated in excess reaction was also performed using various oxidizing agents such
amount of distilled water followed by the heat treatment in a as TBHP and H2O2 which showed lower activity (38% and 42%)
vacuum oven at 100 °C. Finally, the obtained catalyst was than in O2 (Table 1, entries 20 and 21).
utilized for the next cycle under aforesaid identical
experimental conditions. The same protocol was adopted for
the next five recycles.
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Table 1: Optimization of reaction parameters over various copper based catalysts a
desired product with 98% yield (Table 2, entry 1).VIinewpAartrictliecOunlalinre,
electron-rich amines such as 4-methoxy
DOI: 10.1a0n39d/C94N-Jm05e9t9h7yKl
Entry
Substrate
Catalyst
Solvent
CH3OH
Yieldh
(%)
15
benzylamines are produced the desire product of secondary
imines with high yield (Table 2, entries 2 and 5). The 3-methoxy
substituent on benzyl group influenced the reaction yield and
afforded 87% yield (Table 2, entry 3). Additionally, 2-
methylbenzylamine also gave an impressive yield of 82% (Table
2, entry 6). The ortho and meta substituted amines showed
slightly lower yield than the para-substituted one. The 4-fluoro,
4-chloro and 4-bromo benzylamines were also afforded high
yields (Table 2, entries 7, 8 and 9). The halo substituted imines
are generally used for the synthesis of synthons which are
important compounds for the chiral amines synthesis.26 The 4-
CF3 and 3-Cl benzylamines exhibited moderate yield towards
imine product (Table 2, entries 10 and 11). Interestingly, bulkly
groups such as 1-napthyl and ortho-methoxy benzylamine had
shown slightly reduced yields probably due to the more steric
hindrance (Table 2, entries 13 and 4). The 3,4-dichloro
benzylamine substituted substrate gave the moderate yield
(Table 2, entry 12). Surprisingly, hetero cyclothio amine
undergone oxidative dimerization and showed 80% of yield
(Table 2, entry 14). Whereas the 4-phenyl benzylamine also
showed the better yields of about 68% (Table 2, entry 15).
1
2
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
C2H5OH
DCM
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Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
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CHCl3
5
Acetonitrile 20
H2O 20
1,4-dioxane 22
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Toluene
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0.0
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Benzylamine
-
-
-
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-
-
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-
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-
Benzylamine Cu-BTC MOF
Benzylamine Cu(NO3)2. 3H2O
Benzylamine CuCl2. 2H2O
Benzylamine Cu(OAC)2. H2O
Benzylamine 25wt%CuO/TiO2
Benzylamine 25wt%CuO/Al2O3
Benzylamine 25wt%CuO/SiO2
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Cu-BTC MOF
Benzylamine Basolite C300
Table 2: Cu-BTC MOF catalyzed primary benzyl amines to iminea
a All the reaction conditions: Catalyst (0.050 g), benzylamine (7 mmol), oxidant (O2
b
c
balloon), without solvent, 12 h at 80 °C, Reaction run time 24 h; Experiment
conducted in Ar atmosphere, d Experiment conducted in N2 atmosphere, e Reaction
f
g
with TBHP (5-6 M in decane), Reaction with H2O2 (30% w/v). Activity over
commercial Basolite C300 catalyst. h Selectivity ~ 100%.
3.2 Substrate scope
The substrate scope of the reaction was studied by
performing the aerobic oxidative coupling of different benzyl,
alkyl, cycloalkyl amines and anilines over Cu-BTC MOF catalyst
in presence of O2. The aerobic oxidative coupling reaction
afforded excellent isolated yields for benzylamine with various
substituents on the phenyl ring (Table 2). The model substrate oxidant (O2 balloon), without solvent, 12 h at 80 °C. b Overall yield of adducts.
benzylamine readily undergo aerobic oxidation to afford the
a
Reaction conditions: Cu-BTC MOF catalyst (0.050 g), benzylamines (7 mmol),
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Under the similar experimental conditions, the reaction was
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3.3 Recyclability and hot filter tests
View Article Online
Reusability of the catalyst is examinDeOdI:o1v0e.1r03C9u/C-B9NTJC05M99O7KF
with benzylamine (7 mmol) under molecular O2 atmosphere at
a temperature of 80 °C in solvent free condition which exhibited
the consistent activity for the five consecutive cycles (Fig. S2). In
this procedure, firstly, the catalyst was separated from the
reaction mixture using centrifugation followed by washing with
ethyl acetate and double distilled water to remove residual salts
and organic solvents. Before using the catalyst for the next cycle
the catalyst was heat treated under vacuum at 100 °C for 12 h.
The recovered sample was also characterized by using XPS,
SEM, XRD, ICP-OES and TEM analysis which emphasized that no
structural changes were observed on the recovered catalysts.
The recyclability potential of Cu-BTC MOF catalyst showed that
there is no significant loss in activity even after five consecutive
cycles. ICP-OES analysis of fresh Cu-BTC MOF catalyst and after
2nd cycle (recovered catalyst) revealed 21.57 and 21.27% of
copper respectively. The hot filtration test was used for the
analysis of heterogeneous nature of Cu-BTC MOF catalyst. In
this experiment, a mixture of Cu-BTC MOF and benzylamine (7
mmol) was kept for stirring at 80 °C. After 6 h, Cu-BTC MOF
catalyst was removed and separated from the reaction mixture
through filtration that showed only 50% of product yield.
Subsequently the reaction was continued without catalyst by
maintaining the reaction temperature 80 °C for another 6 h.
After completion of the reaction there was no increase in the
product yield (50%) suggesting that the reaction did not
proceed further after filtration. These results proved that the
reaction actually catalysed on surface active sites. The
characterization data of the fresh and used catalysts
emphasized that there is no change in either bulk or the surface
properties of the Cu-BTC MOF catalyst showing its robust and
sustainable nature or homo coupling of amines in presence of
oxygen.
performed with different benzylamines to synthesize the cross
imine products. The screening conditions followed were as
follows: 3.5:3.5 mole ratios of aniline and different
benzylamines, O2 as oxidant and Cu-BTC MOF (50 mg) catalyst.
The oxidative cross coupling of aniline and benzylamine
afforded 70% yield of the desired product (Table 3, entry 1). The
2-methoxy benzylamine and aniline exhibited the moderate
yield of about 58% of desired product (Table 3, entry 2). The 4-
F and 4-Cl benzylamines with aniline showed 60% and 55% of
imine product respectively (Table3, entries 3 and 4) and the 4-
methyl benzylamine with aniline produced moderate yield of
cross imine product with 60% (Table 3, entry 5).
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Table 3: Cu-BTC MOF catalyzed oxidative coupling of benzylamines with aniline a.
a
Reaction conditions: Cu-BTC MOF catalyst (0.050 g), benzylamines (3.5 mmol),
anilines (3.5 mmol), oxidant (O2 balloon), without solvent, 12 h at 80 °C. b Overall
yield of adducts.
We also found that the Cu-BTC MOF catalyst showed better
catalytic activity for oxidative homo-coupling of anilines to form
diphenyl diazene products. The aerobic oxidative
dehydrogenative coupling of aniline, p-methoxy aniline and 2-
methyl aniline were successfully converted to give the self-
coupling azo compounds with better yields of about 80, 85 and
78% respectively (Table 4).
3.4 Characterization of catalysts
XRD patterns of the calcined 25wt%CuO/SiO2 catalyst
showed a broad peak at 21.7° that could be attributed to an
amorphous silica phase. In all the samples, diffraction signals
corresponding to crystalline CuO phase (JCPDS-05-0661) is
observed at 2휃 = 35.5°, 38.7° and 48.4°.27,28 The 25wt%CuO
supported on TiO2 sample exhibited the peaks at 2휃 values of
ca. 25.4°, 38.0°, 48.2°, and 54.4° attributed to TiO2 anatase form
[JCPDS-21-1272]. The diffraction reflections of the Cu–BTC MOF
sample showed sharp lines at 2휃 = 11.6°, 13.4°, 17.5°, 19.0°
assigned to the crystalline nature of Cu-BTC nanoparticles. The
XRD pattern of the used Cu-BTC MOF (recovered after the
second run) showed that the crystal structure remained similar
while compared to its fresh form and no significant change was
occurred explaining its stability and robust nature of the
catalyst. The two benzenetricarboxylic acid molecules and three
Cu2+ ions forms paddle-wheel units like structure of highly
crystalline Cu3(BTC)2.29 The thermal stability of the Cu-BTC MOF
analysed by TG analysis and the results are reported in Fig. S1.
Table 4: Cu-BTC MOF catalyzed aerobic dehydrogenative coupling of anilines a
a
Reaction conditions: Cu-BTC MOF catalyst (0.050 g), anilines (7 mmol), oxidant
(O2 balloon), without solvent, 12 h at 80 °C. b Overall yield of adducts.
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vibration band wherein the copper is coordinated with oxygen
View Article Online
DOI: 10.1039/C9NJ05997K
atom.32
The UV-DR spectra of BTC, Cu-BTC MOF of fresh and used
samples are presented in Fig. S4. The BTC showed an absorption
profile at around 265 nm and the other absorption band at 295
nm, which can be attributed to the transitions of π* → n and π*
→ π respectively. The Cu-BTC fresh and used samples exhibited
broad band similar to the absorption of BTC. In addition, in the
visible region Cu-BTC MOF fresh and used samples showed
signal around 495 nm which is ascribed to the d–d transitions of
metal ions. The visible region clearly indicated the carboxylate
oxygen atoms co-ordinate to Cu2+ ions in the Cu-BTC MOF
complex.33
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The XPS peaks observed in the survey scan as C 1s, Cu 2p3/2
,
Cu 2p1/2 and O 1s centered at binding energies of 284.5, 934.14,
953.8 and 531.1 eV respectively (Fig. 3). The Cu-BTC MOF
displayed two signals at 934.14 and 953.8 eV corresponding to
the binding energies of Cu 2p3/2 and Cu 2p1/2 respectively,
emphasizing the divalent nature of Cu-BTC MOF. The obtained
results are in good correlation with the XRD, UV-DRS and FT-IR
analyses. From Fig. 3C, XPS signal of O 1s is observed at the
binding energy of 531.1 eV that is due to the presence of oxygen
anion in the crystalline network.34
Fig. 1 Powder x-ray diffraction lines of the various Cu catalysts (a) 25wt%CuO/SiO2, (b)
25wt%CuO/Al2O3, (c) 25wt%CuO/TiO2, (d) Cu-BTC MOF (used) and (e) Cu-BTC MOF
(fresh) samples.
The surface morphology of the Cu-BTC MOF sample showed
cubo-octagonal shaped crystals (Fig. 2A) which are identical
with the previous reports.30,31 The SEM image of Cu-BTC MOF
clearly indicated that large number of CuO crystals which are in
pyramidal shape. It is very interesting to see that the
morphology and crystal structure were similar in both fresh and
the used sample (Fig. 2B). The XRD analyses of fresh and used
form of Cu-BTC MOF catalysts also confirmed the identical
phases before and after the reaction (Fig. 1d, 1e), emphasizing
the stability Cu-BTC MOF catalyst.
Fig. 3 X-ray photoelectron patterns of (a) Cu 2p, (b) C 1s and (c) O 1s signals Cu-BTC MOF
fresh and used samples.
Pyridine adsorbed FT-IR spectroscopy was employed to
determine the type of acidic sites (Lewis and/or Brønsted)
present on the copper based catalysts and the results are
represented in Fig. 4. It is a known fact that, pyridine
coordinated with Brønsted acid sites (BAS) shows the
vibrational band at ~1540 cm-1 and the peak appears at around
~1450 cm-1 is attributed to the Lewis acid sites (LAS).35 The
DRIFT spectrum shows that all the catalysts had a higher
amount of Lewis acid sites compared to the Brønsted acid sites
on the catalyst surface.36
Fig. 2 SEM images of (A) fresh and (B) used Cu-BTC MOF samples.
The FT-IR spectra of the used Cu-BTC MOF (recovered after
second run) catalyst is found to be similar the fresh catalyst (Fig.
S3). The IR bands showed that the C=O stretching vibrational
band at around 1725 cm-1 is appeared in BTC, is shifted to 1667
cm-1 after the addition of Cu to BTC. This illustrates that the
strong stretching vibration of carboxylate anions which
associated with the Cu2+ ions present in the Cu-BTC. Presence
of strong absorption bands due to the -COOH groups in the
range of 1760–1690 cm-1 indicate the stability of carboxylic acid
groups coordinated Cu2+ species in the Cu-BTC MOF. The
characteristic signal at 720 cm-1 is assigned to Cu–O stretching
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formed benzyl radicals immediately reacts with the TEMPO
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radicals consequently the reaction doesDnOoI:t10p.1r0o3c9e/eCd9NfJu0r5t9h9e7rK.
The free radical mechanism mainly involved with the
incorporation of O2 molecules followed by the formation of
peroxides during the reaction, which are considered as the
barriers for the up scaling the process owing to their highly
flammable and explosive nature. On the contrary, aerobic
oxidation of amines is preceded by ionic mechanism, where the
role of molecular O2 is to enhance the dehydration step.
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Scheme 2 Mechanical study of benzylamines with TEMPO.
To examine the influence of active copper species on the Cu-
BTC MOF catalyst; a controlled reaction was performed using
pyridine poisoning of the Cu-BTC MOF catalyst with
copper/pyridine mole ratio of 1:1. The reaction was carried out
under the identical experimental conditions (7 mmol of
benzylamine, O2 oxidant). The obtained experimental results
demonstrated a significant decrease in the product yield
(approximately 10%) was observed after 12 h. This result
indicates that the copper sites in the Cu-BTC MOF were blocked
after the pyridine treatment consequently the activity was
dramatically decreased. Moreover, after regeneration of the
catalyst (under vacuum at 170 °C for 3 h); the catalytic activity
is almost retained to 92% yield. The pyridine poisoning test thus
suggests that the adsorption of pyridine on the Lewis acid sites
of the copper species in the Cu-BTC MOF causes the decrease in
activity.
Fig. 4 Pyridine adsorbed DRIFT spectra of various Cu based catalysts (a) Cu-BTC MOF, (b)
CuO/Al2O3, (c) CuO/TiO2 and (d) CuO/SiO2 catalysts.
Transmission electron microscopy of the fresh and
recovered Cu-BTC MOF samples reported in Fig. S5. From Fig.
S5A the cubo-octagonal crystal of Cu-BTC can be seen. The
obtained TEM images confirm highly crystalline and disordered
porous structure. The obtained TEM micrographs clearly
demonstrated the fresh and used form of Cu-BTC MOF crystal
structure approximately similar. Thus emphasizing no
significant changes occurred during the course of reaction.37
3.5 Plausible reaction mechanism:
In the conversion of benzylamine to N-benzylbenzaldimine,
the mechanism proceeds through the following steps.6b,17
Initially,
primary
benzylamine
undergo
oxidative
dehydrogenation to afford primary imine (RCH=NH) (Scheme 3)
by eliminating H2O. This step is highly favourable on the Cu-BTC
MOF catalyst since it possess higher amount of Lewis acidic sites
(Fig. 4) on the catalyst surface.38 In the following step,
nucleophilic addition of benzyl amine with the highly unstable
imine intermediate (it possesses net positive charge) results to
the formation of aminal intermediate. Subsequently, aminal
intermediate losses NH3 molecule and homo-coupled imine
product was formed on the catalyst surface. It is noteworthy to
mention here that the formation of Benzaldehyde was not
observed during the reaction, as the reaction mechanism
proceeds via aminal formation. Overall, the Lewis acidic nature
of the Cu-BTC MOF catalyst played an important role for the
homo-coupling of amine to form the desired imine product.
To understand the reaction mechanism, the benzylamine
compound was subjected to the optimized experimental
reaction conditions in the presence of a radical scavenger
TEMPO [(2,2,6,6-tetramethyl piperidin-1-yl)oxyl] and achieved
about 94% yield towards the desired product (Scheme 2). These
results demonstrated that the reaction proceeds via ionic
mechanism rather than radical mechanism. It is a known fact
that, if the free radicals are generated during the reaction, the
Scheme 3 Reaction mechanism of imine formation.
3.6 General large-scale procedure for the aerobic
oxidation of benzylamine
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Fu and C. H. Yan, Chem. Commun., 2011, 47, 10148–10150.
(f) F. Su, S. C. Mathew, L. Mohlmann, M. AntonVieietwtiA,rXtic.leWOanlninge
and S. Blechert, Angew. Chem. Int. Ed.,D2O0I1: 110,.15003,96/C597N–J606509.97K
D. J. C Constable, A. D Curzons and V. L. Cunningham, Green
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For large scale production we have adopted the following
experimental procedure; briefly, a mixture of benzylamine (30
mmol; 3.214 g) and Cu-BTC MOF (~ 0.214 g) was placed in a 50
mL round two necked - bottomed flask. The Inlet was connected
to an oxygen cylinder through the mass flow controller (MFC)
and the outlet was connected to a condenser followed by a glass
trap. Then the O2 gas was continuously supplied at a flow rate
of 50 mL/min, which is represented in a Fig. S6 and cross
checked the outlet flow using a bubble meter. The reaction
mixture contained flask was kept on the magnetic stirrer at a
temperature of 80 °C for 12 h. After completion of the reaction,
the catalyst was separated by centrifugation and concentrated
the mixture under vacuum rotary evaporator. Then the desired
product N-benzylidenebenzylamine was isolated by column
chromatography by using silica gel with eluting solvents. The
obtained yield is found to be around 94% (3.010 g). The same
procedure was followed for further up scaling of the reaction by
taking the benzylamine in 100 mmol (10.715 g) and Cu-BTC MOF
of about 0.714 g that showed around 92% yield (9.850 g). These
results thus emphasized the robust nature of the catalyst for
aerobic oxidation reaction at moderately large scale operations.
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13 H. Naeimi, F. Salimi and K. Rabiei, J. Mol. Catal. A: Chem.,
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15 K. Mori, K. Yamaguchi, T. Mizugaki, K. Ebitan and K. Kaneda,
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4 Conclusions
In summary, the Cu-BTC MOF has been identified as novel
heterogeneous catalyst for imine formation under mild reaction
conditions. This method involves O2 as an environmentally
friendly oxidant at 80 °C without solvent and the Cu-BTC MOF
can be reused up to five cyclic experiments that showed
consistent catalytic activity. In addition, the Cu-BTC MOF
catalyst could demonstrate excellent yield of desired product at
moderately large-scale experiments. The synthesized imine
intermediates are important for the synthesis of various
biological active compounds. The Lewis acid sites present on the
Cu-BTC MOF catalyst played a key role for the enhanced imines
yields in the conversion of primary amines under aerobic
oxidative conditions.
Acknowledgements
All the authors thank DST New Delhi for funding under India-
Poland bilateral program.
CSIR – IICT Communication number: IICT/Pubs./2019/189.
23 B. Huang, H. Tian, S. Lin, M. Xie, X. Yu and Q. Xu, Tetrahedron
Lett., 2013, 54, 2861–2864.
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Cu-BTC MOF catalyst has been identified as an efficient and reusable heterogeneous catalyst for
imine formation under neat condition.