Effect of basicity on the catalytic properties of Ni-containing hydrotalcites in the aerobic oxidation of alcohol

Article abstract of DOI:10.1016/j.molcata.2016.10.017

A series of Ni-containing hydrotalcites with different basicities have been prepared by introducing different Mg²⁺ contents, and characterized by XRD, SEM, TG-DTG, ICP, FTIR, Hammett analysis, DR UV–vis, XPS, etc. The effects of basicity on the catalytic performance in the selective aerobic oxidation of alcohols and the mechanism have been studied. The results showed that substituting Ni²⁺ in the structure by Mg²⁺ ions significantly increased the surface basicity of the catalysts. The surface basicity of Ni-containing hydrotalcites could accelerate the first acid-base reaction step in the oxidation and improve the catalytic activity. Varied alcohols were tested and discussed in the reaction system to verify the effect, and the results indicated that the activity of α-C[sbnd]H bond is the key factor for the benzyl alcohol derivatives, while the first base-acid reaction step may be more important for aliphatic alcohols. The comparison results between the hydrotalcites and the calcined samples showed that the type of basic site have significantly influence on the catalytic activity, and only the Br?nsted OH basic sites accelerate the oxidation. In addition, a probable mechanism for the reaction was postulated based on catalytic results, Hammett and a series of controlled experiments. The main factors affecting the catalytic oxidation of varied alcohols using molecular oxygen as the ultimate oxidant have been discussed, which may be helpful in designing more efficient catalyst.

Full text of DOI:10.1016/j.molcata.2016.10.017

Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
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journal homepage: www.elsevier.com/locate/molcata
Editor’s choice paper
Effect of basicity on the catalytic properties of Ni-containing
hydrotalcites in the aerobic oxidation of alcohol
∗
Weiyou Zhou, Qianyun Tao, Jiugao Pan, Jie Liu, Junfeng Qian, Mingyang He , Qun Chen
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Changzhou 213164, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2016
Received in revised form 12 October 2016
Accepted 16 October 2016
Available online 18 October 2016
A series of Ni-containing hydrotalcites with different basicities have been prepared by introducing differ-
ent Mg contents, and characterized by XRD, SEM, TG-DTG, ICP, FTIR, Hammett analysis, DR UV–vis, XPS,
2+
etc. The effects of basicity on the catalytic performance in the selective aerobic oxidation of alcohols and
2+
2+
the mechanism have been studied. The results showed that substituting Ni in the structure by Mg
ions significantly increased the surface basicity of the catalysts. The surface basicity of Ni-containing
hydrotalcites could accelerate the first acid-base reaction step in the oxidation and improve the catalytic
activity. Varied alcohols were tested and discussed in the reaction system to verify the effect, and the
results indicated that the activity of ␣-C H bond is the key factor for the benzyl alcohol derivatives, while
the first base-acid reaction step may be more important for aliphatic alcohols. The comparison results
between the hydrotalcites and the calcined samples showed that the type of basic site have significantly
influence on the catalytic activity, and only the Brønsted OH basic sites accelerate the oxidation. In addi-
tion, a probable mechanism for the reaction was postulated based on catalytic results, Hammett and
a series of controlled experiments. The main factors affecting the catalytic oxidation of varied alcohols
using molecular oxygen as the ultimate oxidant have been discussed, which may be helpful in designing
more efficient catalyst.
Keywords:
Basicity
Ni-containing hydrotalcites
Magnesium
Alcohol oxidation
©
2016 Published by Elsevier B.V.
1
. Introduction
processes using O2 needed high pressure of oxygen, high reac-
tion temperature or additives, including base [21–23], coreductant
[24,25], NHPI [26,27] and TEMPO [28,29], etc.
Development of efficient catalyst for the selective oxidation
of alcohol to carbonyl compounds is one of the important topics
in synthetic and industrial chemistry, and has attracted so much
attention from chemists [1–3]. It is very difficult to control the
reaction because of over oxidations or side reactions under conven-
tional severe reaction conditions [4,5]. Therefore, many protocols
have been proposed for the catalytic transformation, including
direct dehydrogenation [6–8] and oxidation in the presence of var-
It should be very interesting if molecular oxygen is activated
under low temperature and atmospheric pressure without any
other additives in the oxidation of alcohol. To achieve the goal, it is
very important to develop a highly efficient catalyst. Actually, some
heterogeneous catalysts originated from noble metals for the reac-
tion system using O2 as the ultimate oxidant have been developed,
including Ru [30,31], Au [32–34], Pt [35] and Pd [36,37], etc. How-
ever, from the viewpoint of economy and sustainable chemistry,
it is especially important to develop economically catalysts for the
selective oxidation of alcohol, because noble metal catalysts are
highly expensive and scarce.
To date, some catalytic systems based on the non-noble met-
als for the transformation of alcohol to carbonyl by molecular
oxygen have been reported. Synthetic cryptomelane octahedral
molecular sieve (OMS-2) with a tunnel structure has been reported
as a heterogeneous catalyst in the oxidation of benzyl alcohol
ious oxidant, such as tert-butyl hydroperoxide (TBHP) [9,10], H O2
2
[
11–13], NaIO4 [14], meta-chloroperoxybenzoic acid (mCPBA) [15]
and O2 [16–20], etc.
Obviously, catalytic oxidation with molecular oxygen is par-
ticularly attractive from an economical and environmental point
of view. Moreover, heterogeneous catalysts in the liquid-phase
offer several advantages over homogeneous ones, such as an ease
of recovery and recycling, atom utility, and enhanced stability
in the oxidation reactions. Unfortunately, most of these catalytic
[
38,39]. The reaction proceeded via activation of molecular oxy-
gen through the lattice oxygen in manganese oxide associated
with the Mn(IV)/Mn(II) reduction-oxidation couple. Choudary [40]
and Kawabata [41] groups reported that activation of molecu-
∗ Corresponding author.
E-mail addresses: hemy cczu@126.com, hmy@cczu.edu.cn (M. He).
http://dx.doi.org/10.1016/j.molcata.2016.10.017
1
381-1169/© 2016 Published by Elsevier B.V.

2
56
W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
lar oxygen on nickel-containing hydrotalcite-like anionic clay also
took place in the oxidation of alcohols. Li et al. [42] have found
that NaOH treated VSB-5 phosphates molecular sieve possessed
efficient activities in the transformation. They thought treatment
by NaOH resulted in highly dispersed Ni(OH) active species. And
according to their proposed mechanism, the substrate alcohol first
coupled plasma analysis (ICP) was used to analyze the compositions
of samples in a Varian Vista-AX device. Diffuse reflectance ultravi-
olet visible spectra (DR UV–vis) were recorded in a Perkin-Elmer
Lambda 35 spectrophotometer, using BaSO4 as a reference. The IR
spectra were obtained on a Nicolet PROTÉGÉ 460 FTIR spectrometer
−
1
in the region 4000–400 cm using KBr pellets. Porosity and sur-
face area studies were performed on a micromeritics ASAP2010C
apparatus, using nitrogen as the adsorbate at liquid nitrogen tem-
2
+
−
bound to the Ni center through interacting with basic OH ion,
followed by hydride transfer from the alcoholate to the neighboring
Ni atom to generate the carbonyl product. These results indicated
that the surface basicity of the catalyst should have important effect
on the catalytic performance. In our previous research, we have also
found that the basicity of hydrotalcites could significantly affect
their catalytic performances in the alcohol oxidation [43,44].
Hydrotalcites is a typical basic material, the properties can be
fine-tuned via the adjustability of the cations and anions in the
brucite layer and interlayer [45,46]. Although the Ni-containing
hydrotalcite-type anionic clay catalysts have been reported in the
oxidation of alcohol, the effect of the surface basicity on the cat-
alytic performance in the transformation has not been studied. In
addition, the reaction mechanism is also debatable. Choudary [40]
and Kawabata [41] thought that peroxide formed in the reaction
and abstracted the ␣-H, while in Li’s catalytic system, the step
was replaced by the hydride transfer from the alcoholate to Ni
◦
perature (−196 C). All the samples were outgassed under vacuum
for 16 h at 298 K before the adsorption measurements. The sur-
face area was calculated using the BET method and the pore size
distributions were deduced from the adsorption branches of the
isotherms using the BJH method. The basic properties were deter-
mined by titration with 0.01 M benzoic acid solution in toluene
using 0.1 g of vacuum dried solid sample suspended in 2 mL of
phenolphthalein indicator solution [47].
2.3. Reaction procedure for benzyl alcohol oxidation
Liquid-phase catalytic oxidation of benzyl alcohol was carried
out in a 25 mL two-neck-flask with reflux condenser and magnet-
ically stirred autoclave heated in an oil bath under atmospheric
pressure. Dioxygen was bubbled (10 mL/min) through a solution of
toluene (8 mL), benzyl alcohol (216 mg, 2 mmol), and catalyst (1.0 g)
atom. On the other hand, CO32 was thought to be very impor-
−
tant in the Choudary’s catalytic system, while there was no CO32
−
◦
at 80 C. The product samples were drawn at regular time intervals
existed in the NaOH treated VSB-5 phosphates molecular sieve. The
knowledge of the basicity’s function and the mechanism should
be helpful in designing the economic catalysts with high catalytic
performance in the aerobic oxidation of alcohol.
and analyzed with a gas chromatography (Shimadzu GC-2010AF)
having SE-30 capillary column and FID detector. The products were
further confirmed using GC–MS (Shimadzu GCMS-2010) with a DB-
5MS capillary column. After the reaction, the resulting mixture was
cooled with ice bath and the catalyst was separated by centrifuga-
tion and washed with solvent. After drying at room temperature in
vacuum, the recycled catalyst can be reused in the next run under
the same conditions. The conversion, yield of benzaldehyde and
selectivity presented here are based on the GC calculations using
chlorobenzene as the internal standard reference compound.
In the present study, therefore, a series of Ni-containing hydro-
2
−
talcites (CO3 -Ni MgxAl-LDHs, x = 0, 0.5, 1.0, 1.5, and 2.0) with
2
2+
different basicities have been prepared by changing the Mg con-
tents in the structure. And the effect of surface basicity on their
catalytic properties in the selective aerobic oxidation of alcohols
has been detailedly studied. In addition, the influences of the main
factors including alcohol’s structure on the reaction and the possi-
ble mechanism have also been discussed.
2.4. Competitive reactions of benzyl alcohol and para-substituted
benzyl alcohols for Hammett plot
2
. Experimental
The reaction was carried out in the presence of an excess of
substrate. To a mixture of benzyl alcohol (5 mmol) and para-(X)-
substituted benzyl alcohol (5 mmol, X = OCH3, CH3, Cl, and
2.1. Preparation of catalysts
NO ), chlorobenzene (10 mmol, as the internal standard refer-
CO32 -Ni MgxAl LDHs with different Mg contents (x = 0, 0.5,
−
2
2
2−
ence compound), CO3 -Ni MgAl-LDH (0.5 g), and solvent toluene
2
1
.0, 1.5 and 2.0) were synthesized by a coprecipitation method.
8
mL was bubbled dioxygen (10 mL/min). The mixture was stirred
In a typical procedure, 0.04 mol of Ni(NO ) ·6H O, 0.02 mol of
3
2
2
◦
at 80 C and samples were drawn at regular time intervals and ana-
lyzed by GC. The relative reactivities were determined using the
Mg(NO ) ·6H O and 0.02 mol of Al(NO ) ·9H O were dissolved in
3
2
2
3
3
2
1
20 mL deionized water to form solution A, and solution B was pre-
following equation: kx/kH = CxX /CHH , where Cx and X are the con-
i
i
i
pared by dissolving 0.067 mol of Na CO3 and 0.2 mol of NaOH in
20 mL deionized water. The two solutions were added dropwise
with stirring to 120 mL deionized water at 60 C while the pH was
2
version and the initial concentration of benzyl alcohol, CH and Hi
are the conversions and the initial concentrations of substituted
benzyl alcohols.
1
◦
maintained 10.0 ± 0.5. After that, the resulting slurry was stirred for
◦
another 30 min and then digested at 80 C for 24 h. The precipitate
3
. Results and discussion
was washed with deionized water until the pH of the filtrate was
around 7.0. And then dried in an oven at 100 C for 12 h. The solid
obtained was named CO3 -Ni MgAl-LDH. The sample calcined
under 300 C for 4 h using CO3 -Ni MgAl-LDH as the precursor
was named Ni MgAl-LDO-300. Other samples with different Mg
content and calcination temperature were synthesized via similar
method.
◦
−
3
.1. Characterization of CO32 -Ni MgxAl- LDHs samples
2−
2
2
2−
◦
2
For the synthesis of LDHs compounds, the M2+/M3+ ratio has
2+
2
2−
significant effect on the structure. Therefore, we prepared CO3
-
Ni MgxAl-LDHs with the (Ni + Mg)/Al ratio between 2.0 to 4.0 to
2
obtain the pure hydrotalcite compounds.
The powder XRD patterns for CO32 -Ni MgxAl-LDHs are
−
2
2
.2. Characterization of catalysts
depicted in Fig. 1. The patterns of all the samples show char-
acteristic LDH reflections (sharp and symmetrical for (003) and
(006), broad and asymmetrical for (009), (015) and (018), respec-
tively) [40,48,49]. Furthermore, the distinguishable reflections
corresponding to planes (110) and (113) (recorded in the 2ꢀ range
Powder X-ray diffraction (XRD) patterns of these prepared
samples were obtained from a Rigaku D/max 2500 PC X-ray diffrac-
tometers with Cu-K␣ (1.5402 Å) radiation at 10 min . Inductively
−
1

W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
257
Table 1
Sample notation and chemical compositions of CO32−-Ni2MgxAl-LDHs.
Sample
Weight content (%)
Ni:Mg:Al
SBET
2
Pore volume
(cm /g)
Pore size
distribution (nm)
pHa
Basicity at
H = 7.6–10.0
mmol/g)b
3
(
m /g)
(
Ni
Al
Mg
CO32 -Ni2Al-LDH
−
31.25
27.17
26.26
24.95
23.91
7.95
7.23
6.49
5.71
5.47
–
1.8:0:1
109
105
156
95
0.15
0.26
0.20
0.20
0.33
3.8
8.01
8.32
8.75
8.97
9.05
0.323
0.435
0.650
0.912
1.151
CO32 -Ni2Mg0.5Al-LDH
−
2.99
5.61
7.56
9.44
1.7:0.5:1
1.9:0.9:1
2.0:1.5:1
2.0:1.9:1
2.8, 6.1
2.5, 5.5
2.4, 5.5
2.5, 5.5
CO32 -Ni2MgAl-LDH
−
2
−
CO3 -Ni2Mg1.5Al-LDH
CO32 -Ni2Mg2Al-LDH
−
110
a
Suspension of 0.3 g hydrotalcite in 20 mL deionized water.
b
0
.1 g hydrotalcite, suspended in 2 mL phenolphthalein indicator solution, is titrated with 0.01 M benzoic acid.
Fig. 2. FTIR spectra of CO32 -Ni2MgxAl-LDHs (a) x = 0; (b) x = 0.5; (c) x = 1; (d) x = 1.5;
(e) x = 2.
−
Fig. 1. XRD patterns of CO32 -Ni2MgxAl-LDHs (a) x = 0; (b) x = 0.5; (c) x = 1; (d)
−
x = 1.5; (e) x = 2.
◦
6
0–63 ) suggest a well ordering of both the anions of the inter-
in a disordered cation distribution, which is also verified by the
−
1
layer and the cations in the layers [50,51]. These results indicate
that Mg was incorporated into the hydrotalcite structures and
the cations were well dispersed. The basal spacings (d basal) of
CO3 -Ni MgxAl-LDHs calculated from the (003) reflection are
.65–7.87 Å, respectively, indicating that the interlayers of all LDHs
are mainly CO3 besides H O and OH . And the slight change of
the value may be due to the incorporation of Mg . All the samples
showed the similar morphologies, and the SEM image of CO3
Ni MgAl-LDH (Fig. S1) shows that the sample formed plate-like
appearance of the shoulder peak at 1475 cm [55,56].
TG/DTG curves of CO32 -Ni MgxAl-LDHs (x = 0, 0.5, 1, 1.5
−
2
and 2) (Fig. S2) show two main stages of weight loss around
2
−
◦
◦
(177.7–244.5 C and 358.0–373.3 C). The former peak corresponds
to the loss of absorbed water and interlayer water, whereas the lat-
ter can be put down to the dehydration of hydroxyl groups on the
2
7
2−
−
2
2+
brucite layers and also the loss of CO from the interlayer. Based on
2
2−
-
the first stage, the temperature peaks show a decreasing trend as
the Mg2+ content increases, while the second stages do not change
2
2+
agglomerated crystals, representing the character of layered mate-
rials [52,53]. The data of ICP analysis in Table 1 shows that the ratios
of Ni/Mg/Al are almost identical to the theoretical value.
a lot, indicating that the introduction of Mg might disturb the
orderly arranged gallery water [57].
For the purpose of exploring the textural parameters of the
The IR spectra of CO32 -Ni MgxAl-LDHs (x = 0, 0.5, 1, 1.5 and 2)
−
Mg2+ modified NiAl hydrotalcites, nitrogen sorption measurement
2
◦
2−
are shown in Fig. 2, it can be found that all the samples show a broad
and intense band between 3700 cm
at about 3500 cm due to the OH stretching vibration of layer
hydroxyl groups and interlayer water molecules [51]. These bands
shift to higher frequencies with increased Mg content, indicating a
decrease of Me–OH bond strength [54], the shift may also be due
to the increase of M /M ratio [55]. The absorption at 1637 cm
is assigned to the bending vibration of water, and 1384 cm to the
carbonate anion in the interlayers. In the 400–1000 cm region
there are some bands related to vibrations of cation–oxygen. The
broaden appearance at 616 and 566 cm and the shift at 809 cm
the lattice vibration modes of Me O in the LDH sheets) clearly
was carried out. The N adsorption-desorption isotherm at −196 C
2
−
1
−1
and 3400 cm
centered
and the corresponding pore size distribution curves for CO
-
3
−1
2+
Ni MgxAl-LDHs with different Mg contents were shown in Fig. 3.
2
We can see that the isotherm for these samples are all type IV
according to the IUPAC classification, conforming the mesoporous
structure of the materials. On the other hand, differences in the
hysteresis loops indicate different types of pore shapes which can
be correlated with the different composition of the layered sheets.
2
+
3+
−1
−
1
−
1
2−
For the CO3 -Ni Al-LDH and the samples with low Mg contents,
2
2+
2
−
H2 type hysteresis loops are observed, while CO3 -Ni Mg1.5Al-
2
−1
−1
2−
LDH and CO3 -Ni Mg Al-LDH show broad H3 type hysteresis
2
2
(
loops, attributed to aggregates of plate-like particles containing
slit-shaped pores [58].
2+
indicate that introduction of Mg into NiAl hydrotalcites resulted

2
58
W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
Fig. 3. N2 adsorption/desorption isotherms and pore size distribution (inserted picture) of CO32−-Ni2MgxAl-LDHs.
The data of the surface areas, pore volumes and pore sizes
of these samples are summarized in Table 1. After introducing
Mg , the BET surface area of these samples are all changed irreg-
incorporated into the hydrotalcite structures and the cations were
uniformly dispersed.
2+
2
−
ularly, and CO
1
-Ni MgAl-LDH exhibited the highest value of
3
2
3
.2. Oxidation of benzyl alcohol catalyzed by
2
56 m /g, markedly better than that reported in literature [41]. The
pore volumes of these Mg modified samples are all bigger than
CO3 -Ni Al-LDH, while the pore size distributions are widened.
Upon introduction of Mg into NiAl hydrotalcites, changes in
the porosity characteristics may be attributed to the alteration
of the microscopic morphology due to the presence of differ-
ent cations within the structure, consistent with the previously
reported results [58,59].
2−
CO3 -Ni MgxAl-LDHs samples
2
2+
2
−
2
In this present investigation, the oxidation of benzyl alcohol was
2+
2−
examined as a standard substrate catalyzed by CO3 -Ni MgAl-
2
LDH samples using molecular O2 as the sole oxidant. The blank
experiment (Table 2, entry 1) exhibited that the prepared sam-
ples performed as the catalyst in the oxidation. Before discussing
the effect of basicity on the catalytic activities of these catalysts,
the solvent, reaction temperature and catalyst amount have been
first optimized (the main data are summarized in Table 2). All the
selected solvent exhibited almost 100% selectivities of benzalde-
hyde, while the toluene showed the highest conversion and yield
Temperature-programmed desorption of carbon dioxide (CO2-
TPD) is an efficient method for measuring the basicity of samples,
but the high temperature needed during the pretreatment must
destroy the structure of LDHs samples. Therefore, the amount of the
basic sites in the catalysts was analyzed qualitatively using Ham-
mett indicators. The results in Table 1 show that the basic strength
and the total basic sites on the surface of these samples increased
(Table 2, entries 2–8). No product was observed without benzyl
alcohol as the substrate, indicating that the catalyst could not cat-
alyze the oxidation of toluene. As to the reaction temperature, it
is observed that (Table 2, entries 2, 9 and 11) low reaction tem-
perature significantly reduced the catalytic performance, the same
2+
with the enhancement of Mg content, indicating that changing
the Mg2 content is an effective method for adjusting the basicity
of these samples.
+
pattern was found in the influence of the amount of catalysts
DR UV–vis spectroscopy is an effective method to detect coordi-
nation states of incorporated nickel species in various materials. Fig.
◦
(
Table 2, entries 2, 12 and 13). 1.0 g of catalyst and 80 C were found
to be enough for the transformation, although no by-product ben-
2
−
S3 shows the DR UV–vis spectra of the prepared CO
-Ni MgxAl-
3
2
zoic acid was observed when the temperature was increased to
2
+
LDHs with different Mg content. The spectra of all the samples
are highly similar, with two split broad bands with two pairs of
maxima at 741, 649 and 412, 380 nm. The results are accordance to
the Kawabata’s that with different Ni/Mg/Al compositions, indicat-
◦
9
0 C. The yield of benzaldehyde reached to >99% under the opti-
mized conditions in the catalytic system. The recycled experiments
Table 2, entries 18 and 19) showed that the prepared catalyst could
(
be reused at least five times without changes of catalytic activ-
2
−
ing that Ni(II) is octahedrally coordinated in CO3 -Ni MgxAl-LDHs
2−
2
ity, indicating that the synthesized CO3 -Ni MgAl-LDH is a stable
2
[
41,60].
The XPS spectra were also used to analyze of these prepared
catalyst. The XRD pattern of the recycled catalyst in Fig. S5 shows
that the structure of the catalyst did not change in the catalytic
oxidation.
In addition, some typical heterogeneous catalytic systems for
the oxidation of benzyl alcohol using O2 as the sole oxidant have
been collected in Table 3 for comparision. To make the compari-
sion convenient, the concept “specific activity” was used instead of
TOF (turnover frequency). It can be found that non-noble metals
samples, and the Ni 2p spectra of these LDHs with different Mg
content are shown in Fig. 4. All these samples show similar spec-
tra. The Ni 2p3/2 (≈855.8 eV) an ied with two satellite bands and
a spin-energy separation of about 17.6 eV further indicate the
presence of Ni(II) in LDHs [61,62]. The O 1s spectra of these cat-
alysts also showed the similar spectra of the binding energies
between 531.3–531.5 eV (Fig. S4), corresponding to the Me OH
generally show less catalytic activity than precious metal catalysts.
2+
oxygen atom. These results further indicate that Mg has been
2−
However, CO3 -Ni MgAl-LDH gave the excellent conversion and
2

W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
259
Fig. 4. XPS survey spectra of CO32 -Ni2MgxAl-LDHs (x = 0, 0.5, 1, 1.5 and 2).
−
Table 2
Optimization of reaction conditions.
◦
Entry
Catalyst
Solvent
T ( C)
Amount of catalyst (g)
Conversion (%)
Selectivity of aldehyde (%)
1
2
3
4
5
6
7
8
9
No catalyst
toluene
toluene
dimethyl sulfoxide
DMF
cyclohexane
acetonitrile
1,4-dioxane
ethyl acetate
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
80
80
80
80
80
80
80
80
60
70
90
80
80
80
80
80
80
80
80
–
0
–
CO32 -Ni2MgAl-LDH
−
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.6
0.8
1.0
1.0
1.0
1.0
1.0
1.0
>99
28.9
16.7
77.8
34.2
19.9
36.8
83
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
2
−
CO3 -Ni2MgAl-LDH
CO32 -Ni2MgAl-LDH
−
2
−
CO3 -Ni2MgAl-LDH
CO32 -Ni2MgAl-LDH
−
2
−
CO3 -Ni2MgAl-LDH
CO32 -Ni2MgAl-LDH
−
CO32 -Ni2MgAl-LDH
−
CO32 -Ni2MgAl-LDH
−
90
>99
87
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
CO32 -Ni2MgAl-LDH
−
2
−
CO3 -Ni2MgAl-LDH
CO32 -Ni2MgAl-LDH
−
94
2
−
CO3 -Ni2Al-LDH
76.3
92.7
96.5
92.2
>99
>99
CO32 -Ni2Mg0.5Al-LDH
−
2
−
CO3 -Ni2Mg1.5Al-LDH
CO32 -Ni2Mg2Al-LDH
−
CO32 -Ni2MgAl-LDH
−
3rd
toluene
toluene
CO32 -Ni2MgAl-LDH
−
5th
Reaction conditions: benzyl alcohol 2 mmol; solvent 8 mL; O2 10 mL/min, 8 h.
Table 3
a
Some typical results of heterogeneous catalytic systems for the oxidation of benzyl alcohol using O2 as the sole oxidant.
Entry
Catalyst
Conversion (%)
Selectivity (%)
Specific activityb (mmol/g/h)
Ref.
CO32 -Ni2MgAl-LDH
VSB-5 nickel phosphate
Mg2.5Ni0.5Al-HT
Ni2Al-HT
−
>99
93
>99
99.9
97.8
0.25
0.31
0.35
0.10
4.9
0.86
33
25
This work
[42]
[41]
[40]
[39]
[63]
[64]
[31]
[37]
1
2
3
4
5
6
7
8
9
51.8
c
31
H-K-OMS-2
Au/CuO
Au nanoparticles
Ru(OH)x/TiO2(B)
Au@20Pd/SiO2
Au/MgCr-HT
97
100
99
85.7
c
99
99
94
96
85
99
88
>99
92
376
19.2
0.35
1
1
0
1
[32]
[35]
Au/MgAl-HT
a
b
c
See the reference for the detailed structure of catalyst.
Specific activity: mmol of product obtained per gram of catalyst per hour.
Yield of benzaldehyde.

2
60
W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
selectivity in the reaction with a comparable specific activity with
other non-noble catalysts and some of the precious samples.
3
.3. Investigation of the basicity’s function in the aerobic
2−
oxidation of alcohol by CO3 -Ni MgxAl-LDHs
2
3
.3.1. Effect of Mg2+ on the catalytic activity
Under the selected conditions, the effect of Mg2+ on the catalytic
performance was then discussed. The results in Table 2 show that
2
−
these CO3 -Ni MgxAl-LDHs samples could smoothly catalyze the
2
aerobic oxidation of benzyl alcohol with benzaldehyde as the only
product. Introduction of Mg2+ into the NiAl hydrotalcites exhibits a
significant influence on the catalytic performance (Table 2, entries
, 14–17). Comparing CO32−-Ni MgAl-LDH with CO3 -Ni Al-LDH,
2−
2
2
2
the conversion of benzyl alcohol significantly increased from 76.3%
to >99%, indicating that Mg2+ in the structure could efficiently
improve the catalytic activity. This phenomenon should be related
to the changes of LDHs’ properties, including SBET, pore volume,
pore size, surface basicity, etc. Concerning the ratio of Ni/Mg, we
found that the activity reached the highest when the ratio was 2.
Comparing the properties of these samples, the high activity might
be first ascribed to the high SBET. And the decrease of the activity
Fig. 5. Conversions of benzyl alcohol versus reaction time over different catalysts.
2
−
2−
Reaction conditions: CO3 -Ni2Al-LDH 0.5 g, CO3 -Ni2Mg0.5Al-LDH 0.577 g,
2−
2−
◦
2−
CO3 -Ni2MgAl-LDH 0.598 g, CO3 -Ni2Mg1.5Al-LDH 0.629 g, CO3 -Ni2Mg2Al-LDH
of CO32 -Ni Mg1.5Al-LDH and CO3 -Ni Mg Al-LDH might be due
−
2−
2
2
2
0.657 g, benzyl alcohol 2 mmol, 80 C, toluene 8 mL, O2 10 mL/min.
2
+
to the reducing of Ni content caused by the increase of Mg
.
2−
2−
When comparing CO3 -Ni Al-LDH with CO3 -Ni Mg Al-LDH
2
2
2
with similar SBET, we can conclude that surface basicity of catalyst
is helpful for the catalytic activity, because the basic OH site on the
surface first react with hydroxyl of alcohol [42]. Generally, there
carbonate anion cannot accelerate the reaction, in contrast to the
results observed by Choudary [40]. Concerning the results in Fig. 5,
we suppose that only the basic sites in the structure benefit the
catalytic performance, which will be further discussed combining
the supposed mechanism.
2−
are some different types of OH in the structure of CO3 -Ni MgAl-
2
LDH catalyst, respectively links to Ni, Mg and Al atom. Therefore,
we speculate that the basicity of the OH linked to Mg is stronger
than that to Ni and Al for the different electronegativity, and the
OH linked to Mg should exhibit higher activity in the reaction with
alcohol. If not, introduction of Mg should decrease the catalytic
activity, because the Ni concentration decreased. All the prepared
samples have been further investigated with the same Ni amount
in the oxidation of benzyl alcohol to elucidate the effect clearly, and
3.3.2. Effect of alcohols’ structure on the transformation
To further elucidate the effect of surface basicity on the catalytic
performance and the scope, various alcohols have been introduced
into the reaction system. In general, when benzyl alcohol and its
analogs were used as the substrates, the excellent yields were
obtained (Table 5, entries 1–7) without any byproducts, which are
more excellent than the reported results [40,41]. From the reac-
tion time needed for these substrates with different substituents,
it is easy to conclude that electronic variation on the aromatic sub-
stituents has some effects on the activity. Generally, benzyl alcohols
the amounts of catalyst were based on 0.5 g of CO32 -Ni Al-LDH. It
−
2
can be obviously observed that the conversion of alcohol increases
2
+
markedly as the Mg content increase (Fig. 5).
To further validate the effect, controlled experiments using
CO32 -Ni Al-LDH with K CO and KOH (the amount were calcu-
−
2
2
3
2−
lated based on the amount of basic site of CO3 -Ni MgAl-LDH) as
substituted with electron-donating groups such as CH , and OCH3
2
3
the catalysts have been performed. The conversion of benzyl alco-
hol were 77.3% and 76.8%, respectively (Table 4, entries 1 and 2),
nearly the same as that without base (Table 2, entry 4), indicat-
ing that the additive base in the studied system could not affect
the catalytic activity, in contrast to the P123-stabilized Au-Ag alloy
nanoparticles system [33]. The results also show that the added
(Table 5, entries 1, 4–6) exhibited higher activity as compared to
those with electron-withdrawing groups (Table 5, entries 2 and
3). An interesting result was found that the catalyst did not show
any catalytic activity for the 4-hydroxybenzyl alcohol (entry 8),
which should be related to the existence of the phenol hydroxyl
(the reason will be discussed in the mechanism segment).
Table 4
a
The performance of different catalytic system in the oxidation of benzyl alcohol.
Entry
Catalyst
Conversion (%)
Selectivity of aldehyde (%)
−
1
2
3
4
5
6
7
CO32 -Ni2Al-LDH + K2CO3b
77.3
76.8
11.8
54.7
61.4
>99
<5
>99
>99
>99
>99
>99
>99
>99
CO32 -Ni2Al-LDH + KOH
−
c
2
−
d
CO3 -Ni2Al-LDH
CO32 -Ni2MgAl-LDH
−
d
e
CO32 -Ni2MgAl-LDH
−
CO32 -Ni2MgAl-LDHf
−
CO32 -Ni2MgAl-LDH
−
g
a
Reaction conditions: benzyl alcohol 2 mmol, toluene 8 mL, catalyst 1.0 g, O2 10 mL/min, 8 h.
b
c
d
e
f
2−
CO3 -Ni2Al-LDH 1.0 g; K2CO3 22.6 mg.
2−
CO3 -Ni2Al-LDH 1.0 g, KOH 18.3 mg.
The reaction were performed under N2 atmosphere.
Same conditions were used except that the free radical inhibitor BHT (2 mmol) was added.
TEMPO (2 mmol) was added.
g
3
-Chlorophenol (2 mmol) was added.

W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
261
Table 5
Catalytic oxidation of varied alcohols under CO32−-Ni2MgAl-LDH.a
Entry
Substrate
Product
Conversion (%)
>99
Time (h)
1
4
2
3
4
>99
93.1
>99
4
9
4
5
6
>99
>99
2
6
7
98.5
8
8
9
0
10
8
>99
1
1
0
1
>99
2
95.8
12
1
2
61.8
12
1
1
3
4
>99
>99
6
4
1
1
5
6
92.4
60.9
18
8

2
62
W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
Table 5 (Continued)
Entry
Substrate
Product
Conversion (%)
Time (h)
6
17
18
19
5.5 (0.5%)b
9.2 (trace)b
12.5 (1%)b
12
16
a
2−
◦
Reaction conditions: CO3 -Ni2MgAl-LDH 1.0 g, substrate 2 mmol, 80 C, toluene 8 mL, O2 10 mL/min.
b
2−
Under the catalysis of CO3 -Ni2Al-LDH.
Table 6
The basicity of Ni2MgAl-LDO calcined under different temperatures.
◦
pHa
Basicity at H = 7.6–15.0 (mmol/g)b
Calcination temperature ( C)
300
400
500
10.65
10.44
10.42
1.62
1.58
1.50
a
Suspension of 0.3 g hydrotalcite in 20 mL deionized water.
b
0
.1 g hydrotalcite, suspended in 2 mL phenolphthalein indicator solution, is
titrated with 0.01 M benzoic acid.
On the other hand, the oxidation of secondary alcohol could also
lead to the corresponding ketone in excellent yields (Table 5, entries
9
–12). 1-phenyl ethanol, 1-phenyl propanol and 1-phenyl butanol
was transferred to corresponding ketone smoothly in the system,
significantly higher than that under the NaOH treated VBS-5 [42],
which may be related to the small pore size of the material (1.0 nm).
2+
By contrast, after the introduction of Mg , the data for our cata-
lysts have a widened distribution (Table 2), which could provide
sufficient space for the substrates to approach the active center. On
the other hand, from the results for the secondary alcohol with dif-
ferent carbons, it can be concluded that the steric effect also exist
in the catalytic system. Good to excellent yield for the aromatic
alcohols with hetero atoms (Table 5, entries 14 and 15) and diol
Fig. 6. Conversions of benzyl alcohol versus reaction time over Ni2MgAl-LDO cal-
cined under different termperatures.
Reaction conditions: benzyl alcohol 2 mmol, 80 C, catalyst 1 g, toluene 8 mL, O2
◦
10 mL/min.
(
Table 5, entry 16) were also obtained using the catalyst.
Further, we checked the applicability for the aerobic oxidation
cination, while the differences among these samples calcined at
different temperatures are not marked. After calcination, the basic
site (OH groups) probably changed, including metal-oxygen pairs,
of aliphatic alcohols (Table 5, entries 17–19), which have not been
investigated in the aerobic oxidation by Ni-containing catalytic sys-
tems [40–42]. Although considerable catalytic activity for these
substrates were not observed, the catalyst exhibited significantly
−
such as Mg O, Ni O and Al O, and low-coordinated oxygen atoms
−
(
O2 ions) [67,68]. Interestingly, their catalytic activities in the aer-
obic oxidation of alcohols significantly decreased (Fig. 6), indicating
that the reaction is affected not only by the basicity of the catalyst,
but also the type of the basic site.
2−
more efficient performance than CO3 -Ni Al-LDH, further indi-
2
cating that introduction of Mg can improve the catalytic activity.
The observation may offer a mentality for designing more efficient
catalyst.
3.4. Discussion of the mechanism of aerobic oxidation of alcohol
In summary, the CO32 -Ni MgAl-LDH exhibited high catalytic
−
2
under the catalysis of NiMgAl hydrotalcites
activity for the oxidation of varied alcohols. And the structure of
alcohol has a significant influence on the reaction activity.
Based on the above discussion, the reaction path of the aerobic
oxidation of alcohol will be discussed in this segment. To verify if
there is radical formed in the aerobic oxidation of alcohol under
3
.3.3. Effect of calcination on the catalytic activity
For the hydrotalcites compounds, calcination is also an efficient
2−
CO3 -Ni MgAl-LDH, radical scavengers (BHT and TEMPO) were
2
method to adjust their surface basicity. To further study the effect
introduced into the reaction, respectively. The results (Table 4,
entries 5 and 6) show that TEMPO had no effect on the reaction,
while the conversion decreased to 61.4% in the presence of BHT.
Then 3-chlorophenol, having stronger acidity than BHT, was inves-
tigated as the additive. A <5% conversion was observed (Table 4,
entry 7), indicating that the reduction of conversion was due to the
acidity of additives. The acidic additive neutralized the some basic
sites of catalyst and decreased the activity. These results show that
no radical formed during the reaction. Generally, the important
reaction steps in alcohol dehydrogenation mainly involve cleav-
age of O H and ␣-C H bonds. Liu’s [69] research on the Au/Cr-HT
2−
of basicity on the catalytic performance, we calcined the CO3
-
Ni MgAl-LDHs under different temperatures to obtain the catalysts
2
with different basicities. The XRD patterns in Fig. S6 of these cal-
cined samples show that the metal oxide (Ni MgAl-LDO) formed
2
◦
when the temperature was higher than 300 C, in according to the
results reported in literatures [65,66].
To compare with the precursors easily, the amount of the basic
sites in the LDO samples were also analyzed qualitatively using
Hammett method. The results in Table 6 show that the basic
strength and the total basic sites markedly increased after the cal-

W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
263
Fig. 7. Hammett plot of p-substituted benzyl alcohols.
Reaction conditions: 5 mmol p-X-benzyl alcohol (X = CH3O, CH3, H, Cl, NO2), 5 mmol
Scheme 1. The proposed mechanism of the aerobic oxidation of alcohol under
2−
2−
benzyl alcohol, CO3 -Ni2MgAl-LDH 0.5 g, toluene 8 mL, 1 mmol chlorobenzene,
CO3 -Ni MgAl-LDH.
2
◦
8
0 C, O2 10 mL/min.
catalysts showed that both O H and ␣-C H bond cleavages are
kinetically-relevant steps in alcohol oxidation.
bonyl compound. Therefore, the reactions were conducted under
2
− 2−
CO3 -Ni Al-LDH and CO3 -Ni MgAl-LDH, respectively, without
2 2
For the cleavage of O H, the basicity of catalyst is appar-
2+
ently very important, that’s why introducing Mg can improve
the catalytic activity of NiAl hydrotalcites. In opposite, for the
given catalyst, alcohols having different acidities should exhibit
different activities in the oxidation, because the acidity of sub-
strate affects its accessibility to the basic sites. Therefore, the low
activities of aliphatic alcohols should be ascribed to their weak acid-
ity. In a controlled experiment, benzyl alcohol and cyclohexanol
oxygen (replaced by nitrogen). The results (Table 4, entries 3 and 4)
2
−
show that the yield of aldehyde reached 11.8% under CO
-Ni Al-
3
2
LDH, while an interesting yield of 54.7% with CO32 -Ni MgAl-LDH
−
2
was obtained after 6 h. These results strongly proved that external
oxygen was not necessary in the single cycle production of carbonyl
compound, which is very different from that proposed by Choud-
hary [40] and Kawabata [41] for the Ni-containing hydrotalcites.
On the other hand, comparing with the results under oxygen, the
differences of the change range of conversion might be ascribed to
the different catalytic activities of the two samples. The results fur-
ther verify that introduction of Mg into the NiAl hydrotalcite can
improve the catalytic activity in the aerobic oxidation of alcohol.
According to the reported [42] and our results, a possible cat-
alytic reaction route for the oxidation of alcohol has been proposed
(Scheme 1). Alcoholate species form through the substrate alco-
hol interacting with Brønsted OH group (basic site) bound to the
(
lower OH acidity than benzyl alcohol) were introduced into the
reaction under N2 atmosphere using CO32 -Ni MgAl-LDH as the
−
2
catalyst. After 0.5 h, the conversion of benzyl alcohol reached15.8%,
markedly higher than that of cyclohexanol (5.1%), indicating the
first acid-base reaction is significantly important for the transfor-
mation. In addition, compared with CO32 -Ni MgAl-LDH, only a 1%
−
2
2−
conversion of cyclohexanol was obtained under CO3 -Ni Al-LDH,
2
further verified the above analysis. The same results were observed
in other two kinds of alcohols (Table 5, entries 17 and 18) [70].
On the other hand, for the cleavage ␣-C H bonds, a Hammett
experiment has always been performed [42]. A similar test of com-
2+
2−
Mg center (step 1) present in CO3 -Ni MgAl hydrotalcite. Then
2
hydride transfers from the alcoholate to the neighboring Ni atom
(step 2), simultaneously generates the carbonyl product and gives
the nickel hydride species (step 3). Finally the nickel hydride
species reacts with adsorbed molecular oxygen to regenerate the
OH group and complete the catalytic cycle (step 4). Li et al. [42]
thought that the vacant coordination site on the neighboring Ni
atom site may facilitate the hydride transfer to form the carbonyl
product (step 3). And in the present catalyst, the Mg atom hav-
ing weaker electronegativity (ꢂMg = 1.3, ꢂNi = 1.9) may increase the
electronic density of O atom and present higher activity than Ni.
Based on the proposed catalytic mechanism, it is easy to eluci-
date the less reactivity of 4-hydroxybenzyl alcohol in the catalytic
system, the reason should be due to the different acidic strength
of the phenol hydroxyl and alcohol hydroxyl. Because the acidity
of phenol hydroxyl is stronger than alcohol, the phenol hydroxyl
would first react with the basic sites on the surface of hydrotalcite
and anchor the molecule, hindering the further reaction of alco-
hol hydroxyl with the catalysts. The above results can also provide
the elucidation for the high selectivity of aldehyde, because the
aldehyde molecule cannot be activated under the catalytic condi-
tions. On the other hand, although the Lewis basic sites formed after
calcination can also react with alcohol to complete the first step,
petitive oxidations of para-substituted benzyl alcohols (X = CH O,
3
CH , H, Cl, NO ), which have sufficient acidic strength to react
3
2
with surface basic sites, have also been conducted under the catal-
2−
ysis of CO3 -Ni MgAl-LDH. In the competitive reaction, a large
2
excess of substrate was used to ensure that the rate is zero order
in substrate. The relative rates can be determined by the con-
versions of benzyl alcohol when conversions are low (<5%). The
Hammett plot is shown in Fig. 7, and the slope of the Hammett plot
(
ꢁ ≈ −0.38) was lower than that of NaOH-treated VSB-5 (ꢁ ≈ −0.93).
The accelerating effect of electron-donating substituents and the
decelerating effect of electron-withdrawing substituents indicated
that the dehydrogenation step (ˇ-hydride elimination) is rate-
limiting and the reaction is kinetically controlled [71].
In discussion of the reaction mechanism, after the formation of
alcoholate species, there are two possible paths proposed in lit-
eratures for the production of carbonyl compound. One contains
peroxide formed with Ni (Ni OOH) [40,41], which abstracts ␣-H
to form the product; and in the other path, the cleavage of ␣-C
H
bond is facilitated by vacant coordination site of metal atom [42]. To
distinguish them, we thought that a key issue was that whether the
molecular oxygen participates in the single-cycle formation of car-

2
64
W. Zhou et al. / Journal of Molecular Catalysis A: Chemical 425 (2016) 255–265
we supposed that the vacant coordination site of Ni atom cannot
facilitate the split of ␣-C H bond for the distance reason.
Combining the oxidation results of different substrates and the
above discussion, it may be concluded that, in the aerobic oxidation
under the catalysis of Ni-containing hydrotalcites, the activity of
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