An efficient noble metal-free Ce-Sm/SiO2 nano-oxide catalyst for oxidation of benzylamines under ecofriendly conditions

Article abstract of DOI:10.1039/c4ra04397a

A nanosized Ce-Sm/SiO₂ catalyst was found to show an outstanding performance in the oxidation of benzylamines into valuable dibenzylimine products with almost 100% selectivity with O₂ as the green oxidant under solvent-free condition

Full text of DOI:10.1039/c4ra04397a

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An efficient noble metal-free Ce–Sm/SiO2 nano-
oxide catalyst for oxidation of benzylamines under
ecofriendly conditions†
Cite this: RSC Adv., 2014, 4, 46378
Received 11th May 2014
Accepted 15th September 2014
*
Putla Sudarsanam, A. Rangaswamy and Benjaram M. Reddy
DOI: 10.1039/c4ra04397a
www.rsc.org/advances
A nanosized Ce–Sm/SiO2 catalyst was found to show an outstanding commercial exploitation in auto-exhaust purication, but also
performance in the oxidation of benzylamines into valuable dibenzy- due to their signicant use for various organic
limine products with almost 100% selectivity with O2 as the green transformations.18,19
oxidant under solvent-free conditions, which is attributed to the
One of the most notable features of ceria is that it easily
presence of abundant strong acidic sites, enhanced oxygen vacancy forms ample oxygen vacancy defects by preserving its uorite-
concentration, and superior BET surface area.
structure when an appropriate metal ion (e.g., Sm3+, Fe3+, etc.)
is introduced into the ceria lattice.20–22 Consequently, the
structural and redox properties of ceria are greatly improved,
resulting in remarkable catalytic performance. Moreover, owing
to their unique and fascinating physicochemical properties, the
ceria-based oxides can selectively activate organic molecules to
yield the desirable products.18 It was also shown that reducing
the ceria particle size to nanoscale range leads to a decrease in
oxygen vacancy formation energy and thereby, an improved
catalytic activity towards oxidation reactions.20 The dispersion
of ceria-based mixed oxides on an inert support (e.g., SiO2) is
one of the efficient approaches to improve the CeO2 properties
further as well as to reduce its particle size.23,24
Motivated by the above observations, we developed a novel
and promising ceria-based catalyst i.e., Sm-doped CeO2
dispersed on SiO2 (Ce–Sm/SiO2) for the aerobic oxidation of
various benzylamines under ecofriendly reaction conditions.
The selection of Sm in the present work is mainly due to its
resemblance in the ionic radius and electro negativity with
respect to Ce.21 The nano-structured Ce–Sm/SiO2 catalyst
showed an interesting performance in the aerobic oxidation of
various benzylamines with ꢀ100% selectivity to imine products.
We employed a highly practicable synthesis procedure based on
precipitation of metal nitrate precursors for the preparation of
the investigated Ce–Sm/SiO2 nano-oxides (ESI†). For compar-
ison purpose, pure CeO2 and Sm-doped CeO2 samples were also
prepared by adopting the same synthesis procedure. A linear
correlation was found between the concentration of strong
acidic sites, BET surface area, oxygen vacancy concentration,
and the oxidation performance of the synthesized catalysts. A
thorough multi-technique analysis detailing structural and
electronic properties of the synthesized samples was also
undertaken (ESI†).
Selective oxidation reactions are the most fundamental func-
tional group transformations in organic chemistry.1–4 In
particular, the selective oxidation of amines into imines is a
topic of signicant research investigation in recent years due to
extensive use of imines in the chemical industry.5,6 Imines are
vital electrophilic reagents for a number of organic reactions,
which include alkylation, condensation, reduction, cycloaddi-
tion, etc. Imines are also crucial intermediates for the synthesis
of medicines and biologically active compounds.7 Several
oxidation procedures that make use of hazardous stoichio-
metric oxidants, such as 2-iodoxybenzoic acid and N-tert-butyl-
phenylsulnimidoyl chloride, along with Ru-, Ir-, and Cu-based
homogeneous catalysts have been reported for amine oxida-
tion.7–10 Alternatively, numerous heterogeneous catalysts, espe-
cially based on precious metals (e.g., Au, Ru, and Pd) have been
developed for amine oxidation.7,10–15 Although these methods
are quite interesting, the combination of a promising noble
metal-free heterogeneous catalyst with molecular O2 as green
oxidant is essential for amine oxidation from the viewpoints of
both economical and environmental concerns.5,16,17
Development of ceria-based materials with favourable
properties is of immense research interest to the scientic
community during the past few years, not only due to their
Inorganic and Physical Chemistry Division, CSIR–Indian Institute of Chemical
Technology, Uppal Road, Hyderabad–500 007, India. E-mail: bmreddy@iict.res.in;
mreddyb@yahoo.com; Fax: +91 40 2716 0921; Tel: +91 40 2719 3510
† Electronic supplementary information (ESI) available: Details concerning the
synthesis and characterization results of ceria-based materials; TEM, XPS, FTIR,
time-on-stream and reusable analysis of the CeO2-based samples for
benzylamine oxidation. See DOI: 10.1039/c4ra04397a
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The XRD and Raman proles of the investigated CeO2-based the nanoscale range (ꢀ8–12 nm). The selected area electron
materials are shown in Fig. 1A and B, respectively. It was found diffraction patterns of Ce–Sm/SiO2 reveal cubic structured CeO2
that all samples exhibit the characteristic XRD patterns of with (111), (200), (220), and (311) planes, supporting the
uorite-structured CeO2.25,26 Raman spectra show a sharp peak observations made from the XRD studies (Fig. 1A). The Ce–Sm/
at ꢀ450–460 cmꢁ1 for all samples, attributed to Raman-active SiO2 sample is well crystalline and the shape of the most crys-
F2g mode of uorite-structured CeO2, in line with the XRD tallites is compatible with an octahedron dened by (111) facet
results.27 Interestingly, no XRD and Raman peaks correspond- at 3.1 A (Fig. 1D). TEM and HRTEM images of the Ce–Sm
ꢀ
ing to Sm2O3 were found, which conrm the doping of Sm sample provide evidence of the formation of nanosized particles
cations into the CeO2 lattice. It was obvious from Fig. 1A and B in the range of 10–15 nm (Fig. S1, ESI†).
that the addition of dopant (Sm) and support (SiO2) strongly
To our surprise, the Ce 3d spectrum of Ce–Sm/SiO2 sample is
modies the XRD patterns as well as Raman spectrum of pris- noticeably different from that of the Ce–Sm and CeO2 samples
tine CeO2. The estimated lattice parameter, average crystallite (Fig. 2A). Very much low binding energy of u/// peak (ꢀ912.6 eV)
size, and F2g peak position of the samples conrm the above was found for the Ce–Sm/SiO2 compared with Ce–Sm (916.9 eV)
observations (Table 1). Hence, the incorporation of Sm3+ ions and CeO2 (917.5 eV), which strongly indicates the facile reduc-
into the CeO2 lattice could favourably modify its structural ible nature of the Ce–Sm/SiO2 sample.22 It is generally accepted
properties for the aerobic oxidation of benzylamines as dis- that the reducible nature of CeO2 (i.e., Ce4+ / Ce3+) highly
cussed in the later paragraphs.
depends on generation of oxygen vacancies.22,28 Hence, there is
Two additional Raman bands are found for Ce–Sm and Ce– a huge possibility for the formation of more number of oxygen
Sm/SiO2 samples. The appearance of Raman band at ꢀ560 cmꢁ1 vacancies in the Ce–Sm/SiO2 compared with the Ce–Sm oxide,
indicates the presence of oxygen vacancies in the ceria-based in line with Raman results (Fig. 1B). Two peaks were found in
materials, whereas the band at ꢀ606 cmꢁ1 reveals the SmO8 the O 1s XP spectra of the ceria-based samples (Fig. S2, ESI†).28,29
defect complex.21 We estimated the concentration of oxygen The lower energy peak denotes the ceria lattice oxygen, which is
vacancies from the ratio of oxygen vacancy band (Ov) to F2g band considerably varied for the Ce–Sm/SiO2 sample in comparison
(Ov/F2g) in such a way that a higher ratio means a higher oxygen to Ce–Sm and CeO2 samples. Appearance of another peak at
vacancy concentration. It is interesting to note that the Ce–Sm/ higher binding energy side signies adsorbed oxygen species of
SiO2 sample shows higher oxygen vacancy concentration (Ov/F2g hydroxyl and carbonate groups on the catalyst surface, as
¼ 0.1053) than that of the Ce–Sm sample (Ov/F2g ¼ 0.0948). conrmed by FT-IR study (Fig. S3, ESI†).
Moreover, the Ce–Sm/SiO2 catalyst exhibits superior BET
surface area (188 m2 gꢁ1) compared with Ce–Sm (84 m2 gꢁ1) and show a peak at higher binding energy side, which conrm the
pristine CeO2 (41 m2 gꢁ1) samples (Table 1). presence of Sm in the 3+ oxidation state (Fig. 2B).30 Another
The Sm 3d5/2 XP spectra of Ce–Sm and Ce–Sm/SiO2 samples
As can be noted from the Fig. 1C and D, the Ce–Sm/SiO2 peak was found at lower binding energy side due to the strong
sample contains nearly spherical shaped particles and fall in charge transfer effect of the unpaired 4f electrons in Sm2O3.
Interestingly, the intensity of the charge transfer band is much
higher for the Ce–Sm/SiO2 compared with that of the Ce–Sm
sample.30–32 This unusual observation reveals that the number
of unpaired 4f electrons increases by more than one during the
creation of the 3dꢁ1 core hole in the Ce–Sm/SiO2 sample, thus
making it more positively charged. This result also explains the
Sm 3d5/2 peak shi of the Ce–Sm/SiO2 sample towards higher
binding energy side. It is therefore expected that these unusual
electronic modications of Ce, Sm and O could improve the
acidic properties of the Ce–Sm/SiO2 sample. To understand
this, the acidic properties of Ce–Sm and Ce–Sm/SiO2 samples
were investigated by means of NH3-TPD analysis (Fig. 2C). The
relative position of the peak maxima in Fig. 2C indicates the
magnitude of NH3 desorption activation energy, and therefore,
the relative strength of the acidic sites.33 As shown in Fig. 2C,
the NH3-TPD peaks can be assigned to before and aer 673 K,
corresponding to low-temperature (LT) and high-temperature
(HT) regions, respectively.34 The HT peak reveals NH3 desorp-
tion from the strong acidic sites, whereas the LT peaks are
attributed to release of NH3 from the weak acid sites. It was
apparent from Fig. 2C that the Ce–Sm/SiO2 sample contains
Fig. 1 (A) Powder XRD and (B) Raman profiles of CeO2-based mate-
rials (Ov-oxygen vacancy band); (C) TEM image of Ce–Sm/SiO2 and
the corresponding selected-area electron diffraction (SAED) pattern;
and (D) HRTEM image of Ce–Sm/SiO2 (inset: enlarged view of selected
both strong- and weak-acidic sites, whereas the Ce–Sm sample
has only weak acidic sites. This could be the reason for the
observed high catalytic efficiency of the Ce–Sm/SiO2 sample in
area).
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Table 1 BET surface area (BET SA), average crystallite size (D), lattice parameter (LP), and F2g peak position of CeO2-based catalysts. Standard
deviations for BET SA and D values are ꢂ5 m2 gꢁ1 and ꢂ0.5 nm, respectively
Sample
BET SA (m2 gꢁ1
)
D (nm)a
LP (nm)a
F2g peak position (cmꢁ1 b
)
CeO2
CeO2–Sm2O3
CeO2–Sm2O3/SiO2
41
84
188
8.9
7.6
5.1
0.541
0.545
0.543
461
456
454
a
From XRD studies. b From Raman spectra.
benzylamine. The presence of strong acidic sites, along with
abundant oxygen vacancies and larger BET surface area are
found to be the crucial factors for the observed enhanced
activity of the Ce–Sm/SiO2 catalyst.
We investigated the scope of the Ce–Sm/SiO2 catalyst for the
aerobic oxidation of various substituted benzylamines. As
summarized in Table 2, substituted benzylamines can also be
selectively oxidized into the corresponding imines using the Ce–
Sm/SiO2 catalyst. Amongst, the highest amine conversion was
achieved with p-MeOPhNH2 (Table 2, entries 3 & 4) attributed to
more basic nature of the amine and thereby, strong interaction
between the amine and Ce–Sm/SiO2 catalyst, resulting in high
amine conversion.11 Very low amine conversion was found for 2-
MePhNH2 (Table 2, entries 7 & 8) compared with 4-MePhNH2
(Table 2, entries 5 & 6), which might be due to steric hindrance
of the methyl group at 2-position. Reasonable amine conver-
sions were achieved in the case of halogen (Cl and F) substituted
amines (Table 2, entries 9–11), which are highly useful synthons
for the synthesis of chiral amines.35
We also studied the reusability of the Ce–Sm/SiO2 catalyst to
understand its stability for the aerobic amine oxidation (Fig. S5,
ESI†). The catalytic experiments were xed as follows: benzyl-
amine (0.2 mmol), catalyst amount (0.2 g), O2 bubbling rate (20
mL minꢁ1), reaction time (6 h), and temperature (393 K). Aer
each run, the catalyst was washed with acetone to remove
reaction gradients and then pre-activated at 423 K for 2 h. It was
found that the Ce–Sm/SiO2 catalyst could be used repeatedly up
to 4 cycles for amine oxidation without any signicant loss of
the activity, and ꢀ100 and 90% benzylamine conversions were
obtained for the 1st and 4th cycles, respectively. Aer 4th cycle,
the performance of the Ce–Sm/SiO2 catalyst was considerably
decreased. Approximately, 72 and 59% amine conversions were
obtained for the 5th and 6th cycles, respectively. Understanding
the reasons for the decrease in activity of the Ce–Sm/SiO2
catalyst aer several recycling experiments are under further
investigation. Despite the decrease in amine conversion with
repeated use of the catalyst, no variation in the selectivity of the
product was observed. On the whole, the Ce–Sm/SiO2 is a
promising and recyclable noble-metal-free catalyst for aerobic
oxidation of benzylamine under eco-friendly reaction
conditions.
Fig. 2 (A) Ce 3d and (B) Sm 3d5/2 XPS spectra of CeO2-based samples.
(C) NH3-TPD profiles of CeO2–Sm2O3 and CeO2–Sm2O3/SiO2
samples, and (D) catalytic performance of CeO2 (C), CeO2–Sm2O3
(CS), and CeO2–Sm2O3/SiO2 (CSS) samples for aerobic oxidation of
benzylamine. Reaction conditions: benzylamine (0.2 mmol), catalyst
amount (0.2 g), O2 bubbling rate (20 mL minꢁ1), reaction time (4 h), and
temperature (393 K).
the aerobic oxidation of benzylamine as discussed in the later
paragraphs.
Fig. 2D shows the catalytic performance of CeO2, Ce–Sm, and
Ce–Sm/SiO2 samples for the oxidation of benzylamine using O2
as a green oxidant under solvent-free conditions. Very low
benzylamine conversions were found for CeO2 (ꢀ16%) and Ce–
Sm (ꢀ31%) samples compared with the Ce–Sm/SiO2 sample
(ꢀ69%) for 4 h of reaction time. When the reaction time was
increased to 6 h, almost 100% amine conversion was found for
the Ce–Sm/SiO2 sample (Fig. S4, ESI†). In contrast, ꢀ38 and
72% of amine conversions were achieved with the bare CeO2
and Ce–Sm samples for 6 h. However, it must be mentioned
here that the activity of the Ce–Sm/SiO2 sample is very much low
at initial reaction times (i.e., ꢀ12% amine conversion for 1 h of
reaction time). A remarkable nding in the present investiga-
tion is that all samples exhibit superior selectivity of dibenzy-
limine product (ꢀ100%). Interestingly, when the reaction was
conducted in the absence of catalyst (blank test), only 9% of
amine conversion was observed. This result indicates the
signicance of ceria-based catalysts in the aerobic oxidation of
The aerobic oxidation of benzylamine into dibenzylimine
usually follows two steps (Scheme 1).10,11 First, an imine inter-
mediate (PhCH]NH) is formed through the oxidative dehy-
drogenation of benzylamine. Aerward, the unstable imine
intermediate reacts instantly with benzylamine to form an
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Table 2 Aerobic oxidation of various substituted benzylamines over the Ce–Sm/SiO2 catalyst
Entry
Substrate
Time (h)
Amine conversion (%)a
Imine selectivity (%)b
1
2
4
6
69
99.9
99.9
ꢀ100
3
4
5
6
4
5
4
6
90
99.8
99.6
99.9
99.7
ꢀ100
53
91
7
8
9
4
6
4
45
85
65
99.9
99.7
99.8
10
6
6
96
93
99.6
99.7
11
a
Reaction conditions: substituted benzylamine (0.2 mmol), catalyst amount (0.2 g), O2 bubbling rate (20 mL minꢁ1), and temperature (393 K).
Very small amount of nitrile product was formed.
b
aminal followed by release of NH3 to give the nal dibenzyli- formation of imine product through the aminal formation fol-
mine product. Alternatively, the dibenzylimine product can also lowed by NH3 release. It is a well-known fact in the literature
be formed by the transformation of imine intermediate into that acid catalysts show remarkable efficiency in the amine
benzaldehyde, which subsequently reacts with the available oxidation through activation of C]N bond of imine interme-
benzylamine to give dibenzylimine. It was obvious from Scheme diate.36–38 Hence, the strong acidic nature of the Ce–Sm/SiO2
1 that the intermediate imine has net positivity charge, thus catalyst is a key factor for its exceptional performance in the
there is a high possibility for the attack of benzylamine on aerobic oxidation of benzylamines.
imine intermediate due to the presence of lone pair on the
In conclusion, we have developed a novel nanostructured
nitrogen atom of the benzylamine rather than the formation of Ce–Sm/SiO2 catalyst for efficient aerobic oxidation of various
benzaldehyde from imine intermediate. As stated, no benzal- benzylamines under ecofriendly conditions. The Ce–Sm/SiO2
dehyde was found in the present study, conrming the catalyst showed almost 100% benzylamine conversion with
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