Efficient aerobic oxidation of alcohols catalyzed by NiGa hydrotalcites in the absence of any additives

Article abstract of DOI:10.1039/c7nj03895j

The aerobic oxidation of alcohol catalyzed by NiGa hydrotalcites in the absence of any additives has been studied in detail. The research results revealed that the surface basicity significantly affected the catalytic performance. Moreover, the Br?nsted OH basic site on Ni-containing hydrotalcites was suggested to be the key active site and accelerated the oxidation. The catalytic system had good tolerance for various alcohols, and an excellent selectivity of aldehyde could be obtained for the oxidation of primary alcohol. A probable non-radical reaction path for the transformation has been proposed according to the catalytic results, isotope labelling experiments and Hammett experiments.

Full text of DOI:10.1039/c7nj03895j

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C7NJ03895J
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Efficient aerobic oxidation of alcohols catalyzed by NiGa
hydrotalcites in the absence of any additives
Received 00th January 20xx,
Accepted 00th January 20xx
Fu‘an Sun, Jiacheng Zhou, Weiyou Zhou,*Jiugao Pan, Junfeng Qian, Mingyang He* and Qun Chen
The aerobic oxidation of alcohol catalyzed by NiGa hydrotalcites in the absence of any additives has been detailedly
studied. The research results revealed that the surface basicity significantly affected the catalytic performance. Moreover,
the Brønsted OH basic site on Ni-containing hydrotalcites was suggested to be the key active site and accelerated the
oxidation. The catalytic system had good tolerance for varied alcohols, and excellent selectivity of aldehyde could be
obtained for the oxidation of primary alcohol. A probable non-radical reaction path for the transformation has been
proposed according to the catalytic results, isotope labelling experiments and Hammett experiments.
DOI: 10.1039/x0xx00000x
www.rsc.org/
catalytic property in detail in case of NiMgAl hydrotalcites [19]. The
results indicated that the reaction was not through a radical path
and the surface basicity had significantly effect on the catalytic
activity.
On the basis of the obtained results, we speculated that
enhancing the basicity of catalyst might be a breakthrough point to
design more efficient catalyst. The hydrotalcites’ basicity is mainly
affected by the composition, including cations in the brucite layers
and anions in the interlayers. Recently we have communicated a
result of alcohol oxidation/ Knoevenagel condensation tandem
reaction over NiGa hydrotalcites [20]. Excellent catalytic activity for
the NiGa LDHs in the tandem reaction has been observed. In the
present research, the catalytic behavior of NiGa LDHs in the alcohol
oxidation and the probable mechanism were investigated in detail.
1. Introduction
Developing high-efficiency catalytic system for the transformation
of alcohol to corresponding carbonyl compound using molecular
oxygen without any additives is particularly attractive in synthetic
and industrial chemistry. Evident progress has been made with the
noble metals, for example, Ru [1,2], Au *3‒5+, Pt *6+, and Pd *7‒9+,
etc. However, only a few catalytic systems originated from non-
noble metals could be found in literatures, including OMS-2
(cryptomelane octahedral molecular sieve) [10,11], Mn-Ca oxide
[12], NiAl hydrotalcite-like anionic clay [13,14], and VSB-5
phosphates molecular sieve treated with NaOH [15]. Developing
economical and easy-manufactured catalysts for the oxidative
transformation is still highly desirable from the perspective of
application.
2. Experimental Section
Layered double hydroxides (LDHs) with the hydrotalcite
structure is a typical basic material, consisted of M2+ and M3+
cations in the brucite layer, and anions in the interlayer [16,17].
These components can be adjusted through certain methods to
tune the physicochemical properties of the material. These features
make the hydrotalcite an ideal material of catalyst or the support of
catalyst. Actually, the hydrotalcite-like compounds have been
investigated in varied reactions, and efficient catalytic
performances have been observed [18]. To the best of our
knowledge, only two types of Ni-containing materials have been
found effective in the oxidation of alcohol via molecular oxygen
[13,14], and the catalytic efficiency were not satisfactory. To find
more efficient catalyst for the transformation, we have studied the
relation between Ni-containing hydrotalcites structure and the
2.1. Materials and methods
The preparation method for CO32‒-NixGa-LDHs (x = 2, 3, 4 and 5)
samples can be found in the literature [20]. Ni3Ga-LDO-200 and
Ni3Ga-LDO-400 were obtained by calcining CO32‒-Ni3Ga-LDH under
200 ºC or 400 ºC for 5 h, respectively.
2.2. Catalysts characterization
The Powder X-ray diffraction (XRD) was recorded in a Rigaku D/max
2500 PC X-ray diffractometer. The element chemical analysis was
performed by ICP-OES in
adsorption/desorption isotherms of the catalysts were collected on
a Micromeritics ASAP 2010C instrument at liquid N2 temperature.
Titration was used to analyze the surface basicity of the samples
a Varian Vista-AX device. N2-
using 0.025
M benzoic acid solution as the titrant and
phenolphthalein as the indicator [21].
2.3. Catalytic oxidation of alcohol
In a typical procedure, the oxidation of alcohol was performed in a
carousel reaction tube under molecular oxygen. A mixture of
toluene (5 mL), benzyl alcohol (108 mg, 1 mmol), chlorobenzene
(0.5 mmol) and catalyst (0.5 g) at 80 ºC was magnetically stirring
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under oxygen. The products were analyzed through a GC (Shimadzu
GC-2010AF) and GC-MS (Shimadzu GCMS-2010). When the reaction
finished, the catalyst was filtered and washed with solvent, and
then dried at 120 ºC before being recycled. The TOF (turnover
frequency) values were obtained on the basis of the total Ni in the
catalysts.
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DOI: 10.1039/C7NJ03895J
2.4. Procedure for Hammett study
Benzyl alcohol (10 mmol) and para-(X)-substituted benzyl alcohol
(10 mmol, X = –OCH3, –CH3, –Cl, and –NO2), chlorobenzene (10
mmol, internal standard), CO32‒-Ni3Ga-LDH (0.25 g), and toluene (5
mL) was mixed and stirred at 80 ºC under oxygen. The reaction
mixture was regularly sampled and analyzed. The equation kx/kH
=
CxXi/CHHi (Cx and CH are the conversions of the two substrates, and
Xi and Hi are the initial concentrations) was applied to determine
the relative reactivities of the substituted substrates.
2.5. 18O-labeled experiments
Fig. 1 N2 adsorption/desorption isotherms and pore size distribution
18O-labeled benzylalcohol (93% 18O enriched, prepared according to
reported method [22], 110 mg, 1 mmol), CO32‒-Ni3Ga-LDH (0.5 g),
and toluene (8 mL) was mixed and stirred under oxygen at 80 ºC.
The reaction mixture was sampled and analyzed through GC-MS.
The 16O and 18O compositions in benzaldehyde were calculated
according to the relative abundance of mass peaks at m/z = 106 for
16O and m/z = 108 for 18O.
(inserted picture) of CO32‒-NixGa-LDHs.
the brucite layer [23,24].
According to the IUPAC classification, the N2 adsorption-
desorption isotherm depicted in Fig. 1 indicate that all the NiGa
LDHs exhibit type IV isotherm, suggesting the mesoporosity of the
materials. Concerning the hysteresis loops, the NiGa LDHs samples
with Ni/Ga ratios of 2 and 3 show H2-type hysteresis. In contrast,
3. Results and discussion
3.1. Catalyst characterization studies
2‒
broad H3 type hysteresis loops can be observed for the CO3
-
Ni4Ga-LDH and CO32‒-Ni5Ga-LDH, suggesting plate-like particles
aggregated during the synthesis [25,26]. These phenomena imply
that the different composition of the brucite layer results in the
diverse types of pore shapes.
The characterization of CO32‒-NixGa-LDHs samples, including XRD,
FTIR, DR UV-vis, TG-DTG and SEM, can be found in our previous
reported literature [20]. And the results indicated that the
hydrotalcite structure formed for all the samples and the cations
were well dispersed. The data of ICP analysis in Table 1 shows that
the Ni/Ga ratios are very close to the theoretical value. Nitrogen
sorption measurement was also performed to explore the textural
parameters of the NiGa hydrotalcites (Table 1). The analysis results
show that the BET surface area of these NiGa LDHs change
irregularly, while the pore volumes and the average pore diameter
of these NiGa hydrotalcites with different Ni/Ga ratio are close to
each other. Upon replacing Al by Ga in hydrotalcites, the average
pore diameter increases, which may be ascribed to the change of
the microscopic morphology for the introduction of Ga3+ cations in
To elucidate the probable significant influence of basicity on
the catalytic activity, the surface basicity of these prepared samples
should be analyzed. The method of Hammett indicator instead of
CO2-TPD analysis was applied to determine the amount of the basic
sites. Because the structure of LDHs samples must be destroyed
during the pretreatment process (high temperature is needed) in
the CO2-TPD analysis. The results presented in Table 1 indicate that
both of the basic strength and the total amount of basic sites are
significantly affected by the Ga content, and CO32‒-Ni3Ga-LDH
exhibits the strongest basicity and the largest number of basic sites.
The enhancement of the basicity in comparision with NiAl
hydrotalcite suggests that substituting the Al by Ga is an efficient
Table 1 Sample notation and chemical compositions of CO32‒-NixGa-LDHs
Weight content (%)
SBET
(m2/g)
Pore volume
(cm3/g)
Average pore
diameter (nm)
Basicity at H_ =
7.6~10.0 (mmol/g)b
Sample
Ni︰Ga
pHa
Ni
Ga
CO32‒-Ni2Ga-LDH
CO32‒-Ni3Ga -LDH
CO32‒-Ni4Ga -LDH
CO32‒-Ni5Ga -LDH
CO32‒-Ni2Al-LDH
Ni3Ga-LDO-200
Ni3Ga-LDO-400
32.2
35.5
40.1
41.9
33.6
34.6
42.4
17.6
13.8
11.2
9.3
2.2︰1
3.1︰1
4.3︰1
5.4︰1
2.0︰1
3.3︰1
3.1︰1
122
109
133
151
109
51
0.32
0.28
0.28
0.32
0.15
0.10
8.7
8.8
6.7
7.5
3.8
7.8
9.2
7.78
8.36
7.82
7.77
7.94
9.14
9.72
0.16
0.20
0.16
0.13
0.02
0.23
-
7.92
12.4
16.1
100
0.23
a
b
Suspension of 0.3 g hydrotalcite in 20 mL deionized water. 0.1 g hydrotalcite, suspended in 2 mL phenolphthalein
indicator solution, is titrated with 0.025 M benzoic acid.
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way to enhance the basicity of these samples.
oxidation, comparable to the best results reported in tVhieew lAitrteicrleatOunrlienes.
The TOF of the reaction was also calculated in the study to make
3.2. Catalytic activity of CO32‒-NixGa-LDHs
DOI: 10.1039/C7NJ03895J
In the course of benzyl alcohol oxidation under the CO32‒-NixGa- the comparision more precise. Three Au-containing catalysts exhibit
LDHs with molecular oxygen as the sole oxidant, no byproduct was evidently high catalytic activity in the reaction (entries 8 and 9), but
observed under the optimized conditions. The time-plots of the the present catalytic system provides higher selectivity. The TOF of
conversion for these hydrotalcites catalysts with different Ni/Ga CO32‒-Ni3Ga-LDH in the oxidation is higher than the previous
ratio are illustrated in Fig. 2. The CO32‒-Ni3Ga-LDH exhibits the reported catalysts based on non-noble metals.
highest catalytic activity, which should be due to its highest content
With this optimization in hand, the applicability of CO32‒-Ni3Ga-
of basic site [20]. And the higher activity of CO32‒-Ni5Ga-LDHs than LDH was tested in the oxidation of a range of representative
CO32‒-Ni2Ga-LDHs and CO32‒-Ni4Ga-LDHs may be ascribed to the substrates (Table 3). In general, oxidation of the benzyl alcohol and
higher Ni content of the sample.
its analogs efficiently proceeded to give the corresponding
Further, some representative heterogeneous catalytic systems aldehydes (entries 1‒8) without the formation of any byproducts,
have been collected in Table 2 for comparision. It can be seen that significantly better than some reported results [13,14]. The reaction
CO32‒-Ni3Ga-LDH provided the excellent catalytic results for the times required for the complete conversion of the benzyl alcohol
analogues were varied, obviously indicating that electronic effect
existed during the oxidation. Interestingly, both electron-donating
substituents (entries 1,
5 and 6) and electron-withdrawing
substituents (entries 2–4) enhanced the reactivity. However,
although excellent yields were also obtained in cases of 4-
tertbutylbenzyl alcohol and 1-naphthalenemethanol (entries 7 and
8), the reactivity decreased concerning the reaction time, which
was likely to be due to the steric effect. An exceptional case was
found that CO32‒-Ni3Ga-LDH could not catalyze the transformation
of the 4-hydroxybenzyl alcohol (entry 9).
Delightedly, excellent yields (entries 10‒14) of the
corresponding ketone have also been obtained for the secondary
alcohol. 1-phenyl ethanol and the analogues could be smoothly
transformed to the corresponding products under the catalytic
system, significantly better than the NaOH treated VBS-5 [15]. The
lower activity might be due to the smaller pore size of the VBS-5
(1.0 nm), while the data for NiGa hydrotalcites has a widened
distribution (Table 1), and these bulky substrates can easily
approach the active site. Further, steric effect was also observed in
the catalytic transformation, because different reaction times were
required for the substrates with various size.
Fig. 2 Catalytic oxidation of benzyl alcohol over different NixGa
hydrotalcites. Reaction conditions: benzyl alcohol 1 mmol; toluene
5 mL; catalyst 0.5 g; oxidant oxygen; reaction temperature 80 ℃.
Table 2 Some typical results of heterogeneous catalytic systems for
the oxidation of benzyl alcohol using O2 as the sole oxidant.a
TOFd
Entry
Catalyst
Cb (%)
Sc (%)
Ref.
(×10-3 s-1)
Table 3 Catalytic oxidation of varied alcohols under CO32‒-Ni3Ga-
LDH.a
This
work
1
2
CO32‒-Ni3Ga-LDH
Ca0.16MnO2·2H2O
97
>99
82e
20.6
time
(h)
Entry
1
Substrate
Product
Conv. (%)
>99
100
0.6
12
VSB-5 nickel
phosphate
3
4
5
6
93
99.9
-
-
-
-
15
14
13
11
0.75
Mg2.5Ni0.5Al-HT
51.8
97.8e
Ni2Al-HT
H-K-OMS-2
Pd-(VO)4P2W18
/mAPO
31f
>99
>99
1.5
3
97
100
99
2
7
68.6
-
9
8
9
10
Au/Cr4-HT
Au/CuO
Au nanoparticles
NAP-Mg-Pd(0)
76
844.4
1.8
-
27
28
29
3
4
85.7
99
99f
>99
>99
0.5
0.5
11
100
91
99
87g
2.9
1
13
14
Au@8Pd/SiO2
Au/MgCr-HT
7666.7
522.2
8
2
5
6
76
a See the reference for the detailed structure of catalyst. b Conversion of
c
d
benzyl alcohol. Selectivity of benzaldehyde. Based on the observed
reaction rate. e The by-product is benzoic acid. f Yield of benzaldehyde. g
The other products are toluene (4.6%), benzoic acid (4.3%) and benzyl
benzoate (3.7%).
>99
0.5
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important fragrances and organic synthetic intermediates, and
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always produced by oxidation of the correDspOoI:n1d0i.n10g3a9/lcCo7hNoJl0s3.8T9h5eJ
present catalytic system can also provide excellent yields (entries 16
and 17), significantly better than commercial results and some
reported results [30–32].
7
>99
6
8
>99
8
Moreover, the present catalytic system was also effective in
the aerobic oxidation of some heterocyclic alcohols. Excellent yields
were obtained for 3-thienylmethanol and 3-pyridinemethanol
(entries 18 and 19), although 24 h was needed for the complete
conversion of 3-pyridinemethanol. Good yield for the diol (entry 21)
was also obtained under the catalytic system.
Moreover, the aliphatic alcohols (entries 22‒23) have also
been tested under the catalytic system, although these substrates
are very challenging and have not been studied under the Ni-
containing catalytic systems [13–15]. Although the NiGa catalyst
only gave a moderate yield of 2-pentanone, the observation was
very interesting.
9
trace
10
10
>99
>99
2
2
11
12
>99
>99
4
4
3.3. Discussion of the mechanism
The radical scavenger experiments have excluded the formation of
radical intermediate [20]. In addition, benzyl alcohol together with
α-phenethyl alcohol with same amount was introduced into the
reaction and the results could be found in Fig. 3. The reaction rate
of primary alcohol was faster than that of secondary substrate,
suggesting that an alcoholate species between the catalyst and the
alcohol formed in the oxidation [2,33,34].
13
14
>99
8
In the oxidative transformation of benzyl alcohol to
benzaldehyde, the cleavage of O–H and α-C–H bonds [4,5,8,9,27]
are generally involved. For the cleavage of O–H, only the basic sites
located in the catalysts were suggested to be effective by the
controlled experiments [20]. To further unravel the effect of the
type of basic site, CO32‒-Ni3Ga-LDH was calcined at 200 and 400 ºC
in air for 5 h, respectively, and the corresponding samples (Ni3Ga-
LDO-200 and Ni3Ga-LDO-400) were obtained (XRD patterns are
displayed in Fig. 4A) [35,36]. Hammett method was also used to
analyze the surface basicity of the LDO samples. The results in Table
1 show that the basic strength markedly increased after the
calcination, whereas it was difficult to analyze the total basic sites
15
16
>99
>99
2
3
17
18
>99
>99
3
2
19
20
>99
24
36
79.8
89.8
21
22
22
(87.1%) b
43.8
24
24
14.5
23
(trace) c
Reaction conditions: CO32‒-Ni3Ga-LDH 0.5 g, substrate 1 mmol, 80 ºC,
a
toluene 8 mL, O2. The selectivities of the corresponding carbonyl products
b
c
for these substrates were all 99%. Selectivity. Under the catalysis of
CO32‒-Ni2Al-LDH.
Fig. 3. Comparision of the conversions of benzyl alcohol and α-
phenethyl alcohol. Reaction conditions: catalyst 0.5 g, benzyl
alcohol 0.5 mmol, α-phenethyl alcohol 0.5 mmol, toluene 8 mL, 80
ºC, O2.
Allylic alcohol such as cinnamyl alcohol produces
cinnamaldehye selectively without formation of any epoxide
product (entry 15). Additionally, geranial and perilla aldehyde are
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DOI: 10.1039/C7NJ03895J
Fig. 4 XRD patterns of the calcined samples and their catalytic
performance. Reaction conditions: benzyl alcohol 1 mmol; toluene
5 mL; catalyst 0.5 g; oxidant oxygen; reaction temperature 80 ºC.
for the black of color Ni3Ga-LDO-400. In addition, the basic site
probably changed from Brønsted OH groups to metal-oxygen pairs,
‒
namely Ni‒O and Ga‒O, and low-coordinated oxygen atoms (O2
Fig. 5 Hammett plot of p-substituted benzyl alcohols. Reaction
conditions: 5 mmol p-X-benzyl alcohol (X = CH3O, CH3, H, Cl, NO2), 5
mmol benzyl alcohol, CO32‒-Ni3Ga-LDH 0.5 g, toluene 8 mL, 1 mmol
chlorobenzene, 60 ºC, O2.
ions) [37,38]. Surprisingly, an even reduced catalytic activity was
observed for these calcined samples (Fig. 4B), especially for the
Ni3Ga-LDO-400, suggesting that OH groups as Brønsted basic sites in
the prepared hydrotalcite were necessity for accelerating the
oxidation. These observations implied that the reaction was
affected by both of the basic strength and the type of the basic site
of the catalyst. Moreover, the compared experiment of calcined
samples suggested that Brønsted basic OH groups presented in HT-
like lattice were particularly important for the oxidation of alcohol
by molecular oxygen.
behavior was quite different from that we previous reported
NiMgAl samples [19]. We speculated that the phenomenon might
be due to the different basicity of the two catalysts. Under the
catalysis of NiGa samples, the relative reaction rates of the O–H and
α-C–H bond cleavages were depended on the substrate. For the
benzyl alcohols with electron-donating substituents, the α-C–H
bond cleavage was the rate-controlled step, while the O–H bond
cleavage was slower for the electron-withdrawing groups
substituted benzyl alcohols. These results suggest that both O–H
and α-C–H bond cleavages might be kinetically-relevant steps in
transformation catalyzed by NiGa hydrotalcites.
In prior to the Hammett experiment, the existence of pore
diffusion limitations inside the catalysts was examined. And a
Weisz-Prater criterion (expressed in eqn (1)) has been introduced in
the catalytic system [39–41].
r Rp2
a
WP

Further, isotope labelling experiments were conducted by using 18O
labelled benzyl alcohol as the substrate. The results in Table 4
Cs Deff
(1)
The calculation was performed according to ref. 40 for its similar indicate that only about 5.7% of the produced benzaldehyde was
liquid-phase reaction (see SI for the detailed calculation procedure). labelled by 18O after 5 min and 60 min, which was quite strange. If
WP
WP|O
=
1.0×10-3, the oxygen in benzaldehyde was not from benzyl alcohol, there
2
The value of
for each reactant
(

WP|benzylalcohol = 2.6×10-4) is less than 0.3, which excludes the should be no 18O labelled product was observed; on the other hand,
presence of significant pore diffusional limitations during the if the oxygen originated from benzyl alcohol, the observed content
reaction under the selected conditions [40].
of the 18O labelled benzaldehyde was obviously too low. We
Then, Hammett experiment was performed to elucidate the speculated that the low content of the 18O labelled benzaldehyde
cleavage α-C–H bonds [15]. In the present study, the competitive might be due to the fast exchange of the oxygen atom between
oxidations of para-substituted benzyl alcohols were conducted by benzaldehyde and H2O formed in the reaction. To check the
CO32‒-Ni3Ga-LDH to explain the effect of substituents on the speculation, excess H216O was added into the reaction after 5 min,
t r a n s f o r m a t i o n . T o e n s u r e t h a t t h e r e a c t i o n and only 1.1% of 18O labelled benzaldehyde was remained after 5
proceeded in a zero grade mode during the Hammett reaction, min, indicating that the exchange reaction existed and the
excess amount of substrates were introduced. The relative rates equilibrium was established rapidly. Further, we investigated the
were calculated based on the conversions of substrate when the exchange reaction of oxygen atom between benzaldehyde
values are <5%. Interestingly, the Hammett plots depicted in Fig. 5 consisted of 16O and H218O under the similar reaction conditions.
indicate that both electron-donating and electron-withdrawing The results in Table 4 indicate that the CO32‒-Ni3Ga-LDH certainly
substituents can accelerate the oxidation, different from the can accelerate the exchange reaction (a possible reaction path for
reported catalytic systems [42]. Concerning the electron-donating the oxygen exchange between benzaldehyde and water is shown in
substituents such as the –OCH3 and –CH3, the results suggest the Scheme 1). These results show that the 18O labelled benzaldehyde
formation of the carbocation-type transition state via the hydride was formed by the oxidation of 18O labelled benzyl alcohol in the
abstraction. The enhancement of the –NO2 and –Cl substituted present catalytic system. However, because the fast exchange
benzyl alcohol might be due to the increase of the substrates’ reaction of oxygen atom between benzaldehyde and H2O produced
acidity in the situation, because the strong acidity of the alcohol in the oxidation under the catalytic system, only a part of the 18O
benefited for the first acid-base reaction step. The catalytic labelled benzaldehyde was detected.
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Table 4 The results of isotope-label experiments.a
4. Conclusions
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DOI: 10.1039/C7NJ03895J
Content of 18O-
Developing efficient heterogeneous catalysts originated from non-
noble metals for the aerobic oxidation of alcohols in the absence of
any additives has been a highly research attractive topic. In the
present work, the catalytic oxidation of alcohol by molecular
oxygen under NiGa hydrotalcites has been studied in detail. The
results suggested that the enhanced catalytic activity in the
transformation is associated with the surface basicity of the NiGa
hydrotalcites, including both of the type of basic sites and the
strength. Further, the controlled experiments implied that the
Brønsted OH basic site plays the key role in the reaction. An
interesting Hammett plot was observed, and the results suggested
that both O–H and α-C–H bond cleavages are kinetically-relevant
steps in the aerobic oxidation of alcohol. According to the obtained
results, a probable mechanism has been proposed.
Entry
Substrate
Additive
benzaldehyde /%
5.7 (5.8)c
1.1
1
2
3b
4
18O - benzyl alcohol
18O - benzyl alcohol
16O - benzaldehyde
16O - benzaldehyde
-
H216O
H218O
H218O
1.1(2.7)c
17.5 (18.8)c
Reaction conditions: substrate 0.5 mmol, H216O or H216O 10 mmol,
CO32‒-Ni3Ga-LDH 0.5 g, toluene 8 mL, 1 mmol chlorobenzene, 60 ºC, O2,
a
b
c
reaction 5 min. Without catalyst. The data in bracket was analyzed
after 60 min.
Scheme 1 A possible reaction path for the oxygen exchange
between benzaldehyde and water.
Acknowledgements
This work was supported by Advanced Catalysis and Green
Manufacturing Collaborative Innovation Center of Changzhou
University and National Science Foundation of China
(21403018).
On the basis of the obtained and reported results
[13,14,15,19,20], a possible reaction mechanism for the catalytic
alcohol oxidation was proposed in Scheme 3. The reactant alcohol
firstly reacts with Brønsted basic OH group linked to the Ga3+ (step
1) in NiGa hydrotalcite to form alcoholate species. Then the α- H of
the alcoholate transfers to the neighboring Ni atom (step 2), and
the carbonyl product simultaneously forms as well as the nickel
hydride species (step 3). The species then react with adsorbed O2 to
generate OOH group (step 4), and the catalytic cycle finished after
releasing of 1/2O2 (step 5). According to the reaction path, the low
reactivity of 4-hydroxybenzyl alcohol should be attributed to the
stronger acidity of phenolic hydroxyl, which reacted with Brønsted
basic OH group faster than alcohol hydroxyl. The high selectivity of
aldehyde can also be explained by the proposed mechanism,
because the produced carbonyl compounds cannot be activated by
the catalyst for the further oxidation under the reaction conditions.
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C7NJ03895J
The surface Brønsted OH basic site on NiGa hydrotalcites was suggested to be the key active site
for the oxidation.