Zn-doped W/aluminium oxide catalyst: Efficient strategy towards sustainable oxidation of alcohols

Article abstract of DOI:10.1016/j.mcat.2020.111114

Bifunctional catalysts have been considered to have vital importance in catalytic chemical process, but there is still some developing room for convenient materials with dual active sites. These catalysts have a notorious reputation for inhibiting mutual neutralization and controlling the distribution of active sites in order to perform their functions. We tailor a series of W-Zn-Al₂O₃ catalysts by modulating the doping density of metal species, which can boost the catalytic process of alcohols into corresponding carbonyl compounds in an additive-free-condition. Test results indicate that the proper content of zinc element can promote the overall activities, and subsequent adjustment of doping zinc can dramatically increase the electronic interaction and change the distribution of chemical active sites. Also, a plausible reaction mechanism was proposed to better understand the acid-base bifunctional catalytic process. Theoretical results confirm this system can provide certain references for similar reactants. Present reaction system is a green procedure and features a broad substrate scope, which reveals a sustainable method to process oxidative dehydrogenation reaction.

Full text of DOI:10.1016/j.mcat.2020.111114

Molecular Catalysis 494 (2020) 111114
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Zn-doped W/aluminium oxide catalyst: Efficient strategy towards
sustainable oxidation of alcohols
a
b
a,
a
a
a
a
Menglu Cai , Jun Li , Xiaozhong Wang *, Ming Zhang , Yangyang Fang , Yu An , Yingqi Chen ,
a,
Liyan Dai *
a
College of Chemical and Biological Engineering, Zhejiang University, Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology,
Hangzhou, 310027, PR China
b
Institute for Micro Process Engineering (IMVT). Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen, 76344, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
Bifunctional catalysts have been considered to have vital importance in catalytic chemical process, but there is
still some developing room for convenient materials with dual active sites. These catalysts have a notorious
reputation for inhibiting mutual neutralization and controlling the distribution of active sites in order to perform
Alcohol oxidation
Synergic catalysis
Zn-W oxides
2 3
their functions. We tailor a series of W-Zn-Al O catalysts by modulating the doping density of metal species,
Heterogeneous catalysis
which can boost the catalytic process of alcohols into corresponding carbonyl compounds in an additive-free-
condition. Test results indicate that the proper content of zinc element can promote the overall activities, and
subsequent adjustment of doping zinc can dramatically increase the electronic interaction and change the dis-
tribution of chemical active sites. Also, a plausible reaction mechanism was proposed to better understand the
acid-base bifunctional catalytic process. Theoretical results confirm this system can provide certain references
for similar reactants. Present reaction system is a green procedure and features a broad substrate scope, which
reveals a sustainable method to process oxidative dehydrogenation reaction.
1. Introduction
aldehydes [14]. Traditionally, this chemical transformation uses cor-
rosive or toxic oxidants to facilitate the reaction, which violates the
Recently, efficient and sustainable chemoselective dehydrogenation
purpose of green chemistry [15]. Through the lens of green process,
“soft” oxidants (e.g., O and H ) appear to be better choices to con-
for alcohols has attracted extensive attentions, considering the en-
vironment issues[1] Numerous heterogeneous noble metal-based cata-
lysts (e.g., Au [2], Ag[3], Ru [4] and Pd[5]) as well as some non-noble
transitional metal catalysts (e.g., Cu[6], Co [7], Ni [8], Fe [9] and Mn
2
2 2
O
duct this catalytic transformation, which can minimize toxic pollutants
and wastes [16]. So, using cleaner oxidants combined with cheaper and
environmental-friendly bifunctional catalysts to conduct the dehy-
drogenation reaction efficiently under mild conditions were highly
appreciated.
Considering the synergetic effect between supports and immobilized
metal species on dehydration performance, much more attention has
been brought to synthesize supported bifunctional catalysts [17]. For
example, Y. Iwasawa et al. selected organic amine functional group to
modify silica/alumina oxides surface and created an acid-base catalyst,
which was capable of accelerating the CeC coupling process []. Al-
though, the coexistence of acid-base sites has made diverse bifunctional
catalysts, catalysts decorated with dual active sites do face some chal-
lenges, such as the cross-intersection of acid/base sites and uneven
distribution of the dual centers, which hindered the practical applica-
tion [18].
[
10]) have been developed recently to generate the selective oxidation
reaction of aryl or alkyl alcohols. But high costs, scarcity of noble metal,
the limited efficiency of non-noble catalyst and harsh reaction condi-
tions to some extent confine their application in the chemical industry.
To date, bifunctional catalysts have sparked increasing attention and
have been widely applied in organic reactions [11] or used as an
electrocatalyst [12] due to their acid-base properties and higher cata-
lytic activities [13]. For instance, C. Hammond et al. compared the
catalytic conversion in a bifunctional system with a monofunctional
catalytic system. The bifunctional catalytic system exhibited more se-
lective and higher catalytic performance and obtained (butoxy) methyl
b
furan in an excellent yield.[13] With regard to safety and cost-effi-
ciency, bifunctional catalyst, broadly feasible in organic synthesis, has
been recognized as a promising candidate for the production of
Attempts have been made to tackle these obstacles. As regards an
⁎
Corresponding authors.
E-mail addresses: wangxiaozhong@zju.edu.cn (X. Wang), dailiyan@zju.edu.cn (L. Dai).
https://doi.org/10.1016/j.mcat.2020.111114
Received 12 May 2020; Received in revised form 22 June 2020; Accepted 6 July 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
efficient and a green preparation method, using polymeric aggregates
to encapsulate acidic and basic group or incorporating acidic and basic
groups over metal-organic framework is also a practical method to
prepare bifunctional catalysts.[19] For example, a core-shell structured
catalyst was synthesized by WG. Song et al. The inner core (Mg-Al
mixed oxides) served as basic sites, and Al-containing mesoporous silica
acted as an outer shell to provide acid sites. Due to the well-dispersion
of active sites in prepared catalyst, this catalytic system performed well
in Knoevenagel condensation reaction []. However, the preparation
processes were generally complex and lengthy. In addition, they could
result in uncertainty of catalytic activity, which caused high operation
cost and large amount of energy consumption [20]. Thus, a straight-
forward preparation method of acid-base bifunctional catalysts is
highly desirable. It is also of great necessity to select a suitable support
for the high dispersion and the incorporation of the active sites [,21]
the dispersion onto a carbon − coated copper (Cu) grid. Field emission
scanning microscopy (FESEM) images were collected on a Hitachi
SU − 70 microscope combined with energy dispersive X − ray spec-
trometer (EDS) conducted on X − MaxN 80 T, Oxford Instruments, UK.
FT − IR spectra of samples were obtained on an IRAffinity−1S
(Shimadzu, Japan) spectrometer using KBr pellets in a range of
400 − 4000 cm . X−ray photoelectron spectroscopy (XPS) was per-
formed on a Thermo Scientific Escalab 250Xi spectrometer using
monochromatic Al Kα radiation (1486.6 eV), and the obtained XPS data
was calibrated according to the reference C 1s peak at 284.8 eV. The
nitrogen adsorption−desorption test was performed at −196 °C with a
Micromeritics ASAP 2460 Version 2.01 to get the textual properties of
samples. Prior to the tests, all samples were outgassed at 200 °C for 3 h.
The surface area of each catalyst was obtained using the Brunauer,
Emmett and Teller (BET) method. The pore volumes and pore diameters
were calculated by the BJH method, respectively. Inductively coupled
plasma (ICP) emission was used to analyze the content of metal ions of
prepared samples and detect the leaching of metal species in reaction
(ICP–OES, Optima 2100DV). To quantify the concentration and
−
1
2 3
Herein, we successfully tailored W/Zn-Al O bifunctional catalysts
in a straightforward synthesis method by means of changing the in-
troducing amount of zinc species. Benefiting from the coexistence of
2 3
active sites in 15W-2.3Zn-Al O and the promotional synergistic effect,
the oxidative dehydrogenation reaction proceeded efficiently of aryl or
alkyl alcohols without the need for additives. The catalyst can be easily
regenerated without significant activity loss. Moreover, it is an inter-
esting discovery that introducing a certain amount of metal species can
adjust the acid − base property over catalyst surface as well as control
the distribution of chemical active sites, thereby accelerating the oxi-
dative dehydrogenation reaction under mild conditions.
3 2
strength of acid and base sites, the heat of adsorption of NH and CO
were carried out using an AutoChemⅡ 2920 (Micromeritics, USA).
2.4. Activity tests
The catalytic oxidation of benzyl alcohol was conducted in a sealed
regular glass reactor at 80 °C under magnetic stirring. Typically,
1
.0 mmol of benzyl alcohol, 16 mg of catalyst and 1 mL of solvent were
added into the reactor. While stirring, 1.7 mmol of hydrogen peroxide
30 wt% aq. solution) was added into mixture solvent, then increasing
2. Experimental section
(
2
.1. Materials
temperature up to 80 °C to start the reaction. After completion of the
oxidative reaction, collecting the solid catalyst by centrifugation when
the system cooled to room temperature. The products were identified
by comparing their retention time with standard substances by a gas
chromatograph (Agilent 6820, flame ionization detector, 30 m
OV − 1701 capillary column) and reconfirmed by GC − MS (Agilent
6890 − 5973). In order to assure the reproducibility, the content ana-
lysis of products was calculated with an external standard method at
least three times.
Solvents and substrates used in reaction were purchased from
Sinopharm Chemical Reagent Co., Ltd and Aladdin without further
purification. Zinc acetate dehydrate (Zn(CH COO) ·2H O), ammonium
tungstate (H40 12·xH O), Al (99.99 % metals basis, powder)
and H aqueous solution (30 wt%) were commercially available.
3
2
2
N
10
O
41
W
2
2 3
O
2 2
O
2
.2. Catalyst preparation
The W − Zn − Al
lowing process. Taking the preparation of 15W − 2.3Zn − Al
illustration, Zn(CH COO) ·2H O (170 mg) as the source for zinc was
dissolved in 100 mL deionized water containing Al (2.0 g). After
2
O
3
catalysts were prepared according to the fol-
3. Results and discussion
2 3
O
as an
3.1. Catalyst characterization
3
2
2
2 3
O
vigorous stirring for 1 h, the slurry was evaporated to remove the water
and then calcined in a muffle oven at 300 °C for 2 h to obtain
X − ray diffraction (XRD) patterns of Zn − Al
a series of W/Zn Al catalysts were investigated as shown in Fig. 1.
Al exhibited (012), (104), (110), (113) and (116) characteristic
2 3
peaks in all samples (JCPDS No. 10 − 0173). As regards Zn − Al O ,
2 3 2 3
O , 15W − Al O and
2 3
O
Zn − doped Al
aqueous solution containing Zn − doped Al
2
O
3
. Afterwards, 60 mL of H40
N
O
10
O
41
W
12·xH
2
O (276 mg)
2 3
O
2
3
(1.0 g) was also treated
under vigorous stirring for 4 h. The solution was aged for 20 h, followed
by evaporating excess water at about 70 °C under continuous stirring.
The resulting solid was ground into fine powder and then treated by air
calcination at 400 °C for 3 h to give the catalyst. For convenience, the
the peaks located at 2θ = 31.77°, 34.42°, 36.25°, 47.54° and 56.60° can
be ascribed to (100), (002), (101), (102) and (110) planes of hexagonal
2 3
ZnO phase (JCPDS NO. 36 − 1451), respectively [22]. In 15W − Al O
catalyst, the strong peaks appeared at 2θ = 23.08°, 23.71°, 24.10°,
28.77° and 34.02°, corresponding to (001), (020), (200), (111) and
calcined samples were labelled as xW − yZn − Al
mass percentage of W, Zn, metal base). For comparison, monometallic
oxide catalysts with Al as support were prepared with the same
method above using the referring metal precursors, and nominated as
W − Al and Zn − Al respectively.
2
O
3
, (x, y represent
(220) planes of the orthorhombic − phase WO
20 − 1324) [23]. Adding ZnO to Al leads to the intensity change of
WO from orthorhombic phase to low detection (Fig. 1a). As supported
in Fig. 1b, the phase of ZnWO peaks (JCPDS NO. 15 − 0774) was not
3
(JCPDS NO.
2
O
3
2 3
O
3
2
O
3
2
O
3
4
observed clearly for samples with low zinc concentrations (2.3 wt%),
indicating the mono − dispersion of W and Zn species might highly
relied on the unbalanced content. FT − IR spectra show that there are
no characteristic bands owning to dopant W metal, which are in good
consistent with XRD results (Figure S1). With the addition of W and Zn
2
.3. General
X − ray diffraction (XRD) spectra were recorded on a PANalytical X’
Pert diffractometer (Almelo, Netherlands) employing a Cu Kα radiation
−
1
(
λ=1.5406 Å). Transmission electron microscopy (TEM) and
species, the band at 628 cm
occurred, which can be related to the
high − resolution TEM (HRTEM) measurements were conducted to
vibration of tetrahedral M − O bond. With an increase contents of Zn
2
−1
observe the morphology of the catalyst on a FEI Tecnai G F20, oper-
element, the half − peak breadth of the bands at 514 cm get broader
ating at 200 kV. Prior to the test, the samples were ground to powder
and ultrasonic dissolved into anhydrous ethanol, followed by dropping
while intensity of the representing Al
compelling evidences that the dopants have certain interaction with
2 3
O reduced. These results become
2

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
Fig. 1. XRD patterns of samples calcined at 400 °C.
Al
2
O
3
or even incorporated into the structure of Al
SEM and TEM measurements were used to investigate the mor-
phology of the prepared W/Zn − Al catalysts. The resulting images
2
O
3
[24].
distribution of prepared catalyst were characterized by the elemental
mapping (Fig. 2e) and the EDS results in Figure S2a verified the rela-
tively homogeneous distributions of those elements. Fig. 2f-h shown the
2 3
O
indicated that the morphological structure could be controlled and
tuned by introducing different amount of metal dopants. Tungsten
shows as an oxide and successfully loads on the support surface rather
2 3
TEM, STEM and HRTEM images of the 15W − 2.3Zn − Al O catalyst.
The lattice spacing of obtained sample was calculated from high − re-
solution TEM (Fig. 2h, Fig. S2b), however, the interface between W, Zn
and the support matrix could not be identified clearly, indicating the
existence of the strong metal − support interactions.[14]b
HAADF − STEM was employed to investigate the internal structure,
than into the structure of Al
Fig. 2a). It can be seen in Fig. 2c that some square nanostructures (the
yellow rectangle represents WO ) are loaded on the support when the
2 3
O if the zinc element is insufficient
(
3
introduction amount of zinc element is about 4.3 wt%. When the con-
tent ratio of W and Zn is 1:1, ZnO phase became clearly as regards the
aggregation of Zn species (Fig. 2d). Interestingly, no W or Zn oxides are
found when the introduction amount of zinc element is about 2.3 wt%,
which is also a favorable evidence of increasing interaction between
dopant ions and support (Fig. 2b). The W, Zn, Al, and O elements
Fig. 2g shown the whole morphology feature of 15W − 2.3Zn − Al
and no WO or ZnO species were presented of the surface. It surprised
us that some metal oxides particles were clearly occurred at the thinner
edges of the Al support, indicating W and Zn species were surly
incorporate into the structure of Al [25].
.XPS was further used to analyze the elemental composition and
2 3
O
3
2 3
O
2 3
O
Fig. 2. FESEM images of (a) 15W − 0.97Zn − Al
2
O
3
, (b) 15W − 2.3Zn − Al
2
O
3
, (c) 15W − 4.3Zn − Al
2
O
3
, (d) 15W−15Zn − Al
2 3
O , (e) the corresponding ele-
mental distribution mapping of 15W − 2.3Zn − Al
2
O
3
. TEM (f), STEM (g) and HRTEM (h) images of 15W − 2.3Zn − Al O
2 3
sample. The inset in (g) displayed a
marked area.
3

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
Fig. 3. High resolution XPS spectrum of (a) Al 2p of pure Al
2
3
O , 15W − 2.3Zn − Al
2 3 2 3 2 3
O and 15W−15Zn − Al O , (b) O 1s of 15W−2.3Zn−Al O , (c) W 4f spectra of
WO
3
, 15W − Al
2
O
3
and 15W − 2.3Zn − Al
2
3
O , (d) Zn 2p spectra of Zn − Al
2
O
3
and 15W − 2.3Zn − Al O
2 3.
chemical state of the obtained samples. The wide − range XPS spec-
trum in Fig. S3 clearly demonstrates the presence of Al, O, W and Zn
revealed the possibility of electronic interaction modulation [].
The N adsorption − desorption isotherms and pore size distribu-
tions of pure Al 15W − Al 15W − 2.3Zn − Al and
15W−15Zn − Al were depicted in Fig. S4, to investigate zinc
2
elements in the 15W − 2.3Zn − Al
2
O
3
catalyst, which is well consistent
2
O
3
,
2
O
3
,
2 3
O
with the EDS results. Fig. 3a shows the binding value at 70.4, 74.6 and
2 3
O
7
1
4.3 eV assigned to Al 2p doublets for pure Al
2
O
3
,
doping influence of the textural parameters. The occurrence of the ty-
pical type V isotherm with a H3 hysteresis loop can be found in each
sample [31]. As listed in Table S1, all samples shown the low surface
area and the small pore value. Though, the low specific surface area of
the prepared samples might be ascribed to the limited number of pores,
it was reasonable to assume prepared materials were non-porous.
The temperature − programmed desorption of NH and CO have
3 2
been extensively used to test the acid − base properties over the cata-
lysts surface, in order to verify the improved catalytic activity with
5W − 2.3Zn − Al , and 15W−15Zn − Al , respectively [26].
2
O
3
2 3
O
The O 1s spectrum can be divided into two peaks, as shown in Fig. 3b,
one peak can be indexed as the O − W bond (530.8 eV) and the other at
5
31.8 eV is corresponding to the hydroxyl groups (−OH) over the ob-
tained sample surface, respectively [27]. Pristine WO was obtained via
direct pyrolysis of H40 12·xH O. The spectra of W 4f in Fig. 3c,
as for WO and 15W − Al , two distinct characteristic peaks at 35.5
and 37.5 eV are respectively corresponding to W 4f7/2 and W 4f5/2
3
N
10
O
41
W
2
3
2
O
3
6
+
doublets, indicating the presence of W
28]. Notably, the binding energy values of
5W − 2.3Zn − Al are obviously larger than 15W − Al
introducing Zn species, and two peaks assigned to the W − Al bond and
WO move to higher binding energy (36.0 and 38.1 eV). This is a great
consequence to verify the interaction between WO and Al , such as
connected by the W − Al bond [29]. The bands at 1021.7 and
044.8 eV are characterized to Zn 2p3/2 and Zn 2p1/2 respectively
2 3
Fig. 3d), and the binding energy of Zn 2p in 15W − 2.3Zn − Al O
in those prepared samples
4f doublets in
when
doping W/Zn species on Al
[32]. Broad desorption peaks can be observed for either CO
2
O
3
and modulation of the acid − base sites
or NH
3
[
1
W
2
2
O
3
2
O
3
profiles, which can be deconvoluted to three peaks based on the des-
orption temperature. As shown in Fig. 4, three desorption peaks in
3
CO
erate site (220 − 350 °C) and strong base site (above 350 °C), respec-
tively [22,33]. NH −TPD profiles are also deconvoluted to three peaks
2
−profile can be identified as the weak site (150 − 220 °C), mod-
3
2 3
O
3
1
(
with maximal temperature in the region of 110 − 135 °C, 220 − 260 °C,
330 − 400 °C, corresponding to weak, moderate and strong acid sites,
respectively [].
both shift to higher position [22]. These phenomena may be attributed
to the formation of the Al − OeM bonds in the crystal lattice, causing
the change of the electron binding force and then the shift of the
electron binding energy [30]. Furthermore, the surface proportions of
In general, the amount of desorbed NH
centration of the acid/base sites. It was observed that the dominant
sites in 15W − 2.3Zn − Al (400) were the weak acid/base sites and
3 2
/CO represented the con-
2 3
O
W and Zn in 15W − 2.3Zn − Al
the calculated contents of 5.8 % and 2.37 %, respectively. The decrease
content of W element demonstrates the formation of the strong W − Al
2 3
interactions or dopant tungsten is precisely penetrated into the Al O
lattice as well [,25]. Overall, the binding energy shift of various zinc
loading samples indicated a strong metal − support interaction and
2
O
3
are further detected by XPS with
the strong acid/base sites. Furthermore, the type of dominant
acid − base sites changed obviously due to the mutual neutralization
with the addition of Zn as well as the occurrence of new phases at
higher calcination temperature. Specifically, increasing zinc loading
can reduce the strong acid sites, and the high calcination temperature
can eliminate the strong base/acid sites but maintaining the moderate
4

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
3 2 2 3 2 3 2 3 2 3
Fig. 4. (a) NH −TPD and (b) CO −TPD profile of 15W − Al O , 15W − 2.3Zn − Al O , 15W−15Zn − Al O and 15W − 2.3Zn−500 (15W − 2.3Zn − Al O
calcined at 500 °C), respectively.
Fig. 5. High resolution XPS spectrum of (a) W 4f spectra of WO
Zn − Al , 15W − 2.3Zn − Al calcined at 400 °C and 500 °C, respectively. (c) XRD patterns of 15W − 2.3Zn − Al
d) FESEM image of 15W − 2.3Zn − Al calcined at 500 °C.
3
, 15W − Al
2
O
3
, 15W − 2.3Zn − Al
2
O
3
(400) and 15W − 2.3Zn − Al
2
O
3
(500). (b) Zn 2p spectra of
(500).
2
O
3
2
O
3
O
2 3
(400) and 15W − 2.3Zn − Al O
2 3
(
2 3
O
and weak sites. According to the literatures,[34] low − coordinated
2 3
acid − base properties. As listed in the Table 1, 15W − 2.3Zn − Al O
oxygen anions may account for the strong basic sites in catalysts, which
catalyst exhibits excellent conversion (97.5 %) and selectivity (95.2 %)
for this chemical transformation (Table 1, Entry 4). Compared with
can be supported by XPS analysis. Because ZnO
x
ubiquitously used as
amphoteric oxides and WO possessed the strong Lewis acidity,[35] the
3
2 3 2 3
15W − 2.3Zn − Al O , Zn-Al O catalysts with different amount of W
2
−
2+
O
surrounded on the Zn
basic sites.[36] The occurrence of WO
5W − 2.3Zn − Al (500), as detected by XRD (Fig. 5c), might pro-
might be a reason for the occurrence of
can also catalyze the reaction but with lower conversion or selectivity
(Table 1, Entry 8, 9). To better exhibit the effect of W content in cat-
alytic performance, we also have carried out the reaction using varied
3
and ZnWO phases in
4
1
2 3
O
moted the mutual neutralization of acid/base sites, weakening the in-
tensity of strong acid and base sites.
2 3
mass percentage of W − Al O catalysts (Table 1, Entry 11–15). With
the increasing tungsten amount, the conversion increased first and then
decreased steadily. In short, the catalytic performance of monometallic
oxide catalysts was inferior than bi-component catalyst and the doping
of zinc could enhance the whole catalytic activity. The catalytic activity
is unsatisfactory in the absence of the catalyst (Table 1, Entry 16) and
the yield of desired product was only 13.5 %. Upon the increase of
catalyst amount (16 mg), both the conversion of benzyl alcohol and the
3.2. Catalytic Activity of the prepared catalysts
Herein, the oxidation of benzyl alcohol in the absence of additives
was used as a probe reaction to give intuitive observation into the sy-
nergistic effect of prepared metal − doped catalysts and their
5

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
Table 1
Table 3
Catalytic activity of prepared catalyst .
a
a
Catalytic performance of prepared catalysts .
Entry
Catalyst
Conv. (%)
Sel. (%)
Yield (%)
Catalyst
Conv. (%)
Sel. (%)
TOF (h−1)
1
2
3
4
5
6
7
8
9
15W-Al
2.3Zn-Al
15W-0.97Zn-Al
15W-2.3Zn-Al
15W-4.3Zn-Al
15W-9.0Zn-Al
15W-15Zn-Al
12W-2.3Zn-Al
18W-2.3Zn-Al
15W-2.3Zn-Al
2
O
3
60.6
11.8
99.2
97.5
88.6
76.6
33.4
94.3
27.5
64.2
38.2
58.5
63.8
55.8
52.4
13.5
99.9
99.9
89.5
95.2
76.7
99.9
78.0
96.5
99.9
89.3
99.9
91.5
95.5
99.9
99.9
100
60.5
11.8
88.8
92.9
68.0
76.5
26.1
91.0
27.5
57.3
38.2
53.5
60.9
55.7
52.3
13.5
15W-Al
2.3Zn-Al
15W-2.3Zn-Al
15W-15Zn-Al
15W-2.3Zn-Al
2
O
3
11.3
3.9
26.3
5.5
99.9
99.9
99.9
99.9
99.9
26.0
20.7
42.2
3.3
2
O
3
2
O
3
2
O
3
2
O
3
2
2
2
O
3
O
3
O
3
2
O
3
b
O
2 3
5.6
9.0
a
2
O
3
2 2 3
Reaction conditions: benzyl alcohol (1.0 mmol), H O (1.7 mmol), CH CN
2
2
2
O
3
O
3
O
3
(
1 mL), 16 mg of catalyst, 80 °C, 1/3 h.
b
The catalyst was calcined at 500 °C.
b
1
1
1
1
1
1
1
0
1
2
3
4
5
6
5W-Al
2
O
3
almost 1.5 times larger than sample calcined at 400 °C. Additionally,
the profiles in NH −TPD and CO −TPD (Fig. 4) indicating that bi-
10W-Al
12W-Al
18W-Al
25W-Al
None
2
2
2
2
O
3
3
3
3
O
O
O
3
2
functional property has an important contribution of dehydrogenation
activity. The sample with same concentration of the W and Zn species
but calcined at 500 °C, displayed inferior catalytic activity due to the
aggregation of active sites, if not, the unbalanced distribution of acid
and base sites over the support surface, compared to the sample cal-
cined at 400 °C.
a
2 2 3
Reaction conditions: benzyl alcohol (1.0 mmol), H O (1.7 mmol), CH CN
(
1 mL), 16 mg of catalyst, 80 °C, 6 h.
b
The catalyst was calcined at 500 °C.
Moreover, experiments
some
were
conducted
using
selectivity of benzaldehyde have no significant increase (Fig. S5).
15W − 2.3Zn − Al O as the representative catalyst over different re-
2 3
Furthermore, we compared current catalytic system with other
catalytic system for the selective oxidation of benzyl alcohol, and the
data was presented in Table 2. The data clearly verified that present
catalyst had remarkable catalytic activities in oxidative dehydrogena-
tion reaction in an additive-free-condition.
action parameters, such as solvent nature, oxidant dosage, reaction
temperature and reaction time, to investigate the catalytic activity in
detail. The solvent effect is very important for the selective oxidation
performance. Fig. 6a exhibited that the conversion of benzyl alcohol
was poor when using ethyl acetate as a solvent, which might be account
for the poor miscibility of oxidant and the low reaction temperature. In
addition, tetrahydrofuran or methanol was not suitable for this che-
mical transformation. The selectivity of benzaldehyde was excellent
Moreover, we calculated the turnover frequency (TOF) of some ty-
pical catalysts in the present system from the first 20 min.. The TOF
−
1
2 3
value of 15W − 2.3Zn − Al O (42.2 h ) was distinctly larger than
that of other catalysts, which was consistent with the catalytic perfor-
mances (Table 3). The aggregation of active species on the surface of
3
when using CH CN as a solvent, which might be due to the suppression
of the formation of other by − products by its property, and good so-
lubility for the reactant and product as well [37]. Notably, hydrogen
peroxide alone is regarded as a poor oxidant in reaction and usually
needs to be activated by a catalyst so that efficiently perform the oxi-
dation. Nitriles have been employed as an accelerant for this activation
as reported [38]. Deming, PH et al. came up with ideas that hydrogen
1
5W−15Zn − Al
identical reaction system. Attention should be paid that when
5W − 2.3Zn − Al sample was calcined at 500 °C, the catalytic ac-
2 3
O may account for such a low TOF value under
1
2 3
O
tivity decreased dramatically and the yield of target product was 1.6
times smaller than using the catalyst calcined at 400 °C (Table 1, Entry
10). As regards, XRD, SEM, XPS, NH
3
−TPD and CO
2
−TPD measure-
3
peroxide in combination with acetonitrile (CH CN) as the solvent
ments were used to explore structural difference and neutralization in
these two catalysts. Interestingly, the binding energy values of W and
Zn doublets were obviously shift to lower position compared with the
low − temperature calcined sample (Fig. 5a, 5b). The values of W
generated a reactive intermediate (peroxyimidic acid), which could also
act as an effective oxidant for various reactions []. Some experiments
were performed to verify the terminal oxidant (H O or peroxyimidic
2 2
acid) in our catalytic system (Fig. 6b). We compared the catalytic
performance of our reaction system in the following three solvents in
detail: acetonitrile, ethyl cyanoacetate and N,N − Dimethylformamide,
doublets in 15W − 2.3Zn − Al
2
O
3
(500) were almost same as
1
5W − Al (400), indicating that the formation of strong W − Al
2 3
O
bond highly depended on the calcination temperature and the partici-
pation of dopant ions. Because W and Zn elements have the tendency to
sinter into larger species during high temperature calcination tem-
perature, some active metal have aggregated over the support surface in
finding
the
results
in
the
order
acetonitrile >
N,N − Dimethylformamide > ethyl cyanoacetate. If hydrogen per-
oxide in our system is activated by the nitrile and then produce a
peroxycarbonmidic acid, the yield of desired product in ethyl cyanoa-
cetate will same as acetonitrile to some degree and the reaction per-
formance will have no difference with and without the participation of
catalyst. If the true oxidant is peroxyimidic acid, using N,N − Di-
methylformamide as a solvent will have a negative effect in catalytic
1
5W − 2.3Zn − Al
the 15W − 2.3Zn − Al
Fig. 5d). Furthermore, the content ratio of W, Zn is increased to 3.60
2
O
3
(500), leading to the difference morphology of
2
O
3
(500) and 15W − 2.3Zn − Al (400)
2 3
O
(
2 3
over the 15W − 2.3Zn − Al O surface when it is calcined at 500 °C,
Table 2
2 3
The comparison of catalytic performance of 15W − 2.3Zn − Al O with other published data in benzyl alcohol oxidation.
Entry
Catalyst
Oxidant
Solvent
Reaction Conditions
Conv. (%)
Sel. (%)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
15W-2.3Zn-Al
2
O
3
H
None
2
O
2
CH
Toluene
tert-amyl alcohol
n-hexane
3
CH CN
None
Toluene
CH
3
CN
80 °C, 6 h
110 °C, 6 h, base
9 bar, 160 °C, continuous flow
69 °C, 24 h, K
70 °C, 6 h
90 °C, 6 h
100 °C, 1 h, KOH
40 °C, 2 h, isobutyraldehyde
97.5
98
50
> 99
> 99
70.0
> 99
100
95.2
98
92.9
96.0
50.0
> 99
> 99
53.9
> 99
98
Hereina
Ag/WO
Ru /γ-Al
Pd-STO
3
(PEG 4000)
3a
4b
5a
6c
7
0
2
O
3
O
O
2
99.9
> 99
> 99
77.0
> 99
98
2
2
CO
3
CuCl
Pd/CoMagSBA
Fe(NO ·9H O, TEMPO
MnP-AMP (S345
2
@MOF-NH
2
Air
O
2
3
)
3
2
Air
Air
9
10
)
3
CN
a
Reaction conditions: benzyl alcohol (1.0 mmol), H
2
O
2
(1.7 mmol), CH
3
CN (1 mL), 16 mg of catalyst, 80 °C, 6 h.
6

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
Fig. 6. (a) Effects of solvents nature, (b) deep insights of the reaction using hydrogen peroxide as an oxidant. Reaction conditions: benzyl alcohol (5.0 mmol), H
8.5 mmol), Solvent (5 mL), 80 °C, 6 h, 80 mg of 15W − 2.3Zn − Al catalyst or 0 mg.
2 2
O
(
2 3
O
performance. Because the amide could not react with H
peroxycarboximidic acid intermediate []. However, these are not the
case, using DMF as a solvent also have a good conversion (Fig. 6b).
2
O
2
to give the
Table 4
2 3
Oxidation of various alcohols catalysed by 15W − 2.3Zn − Al O
a.
Entry
Substrate
Product
Conv. (%)
Sel. (%)
Moreover, the conversion in the condition that using H
dant, CH CN or DMF as a solvent but in the absence of the catalyst is
poor, which may confirm that H in our system is activated by
5W − 2.3Zn − Al catalyst rather than the acetonitrile. Meanwhile,
these results verify that 15W − 2.3Zn − Al have a good catalytic
2 2
O as an oxi-
1
2
3
97.5
96.7
93.6
95.2
98.8
99.0
3
2 2
O
1
2 3
O
2 3
O
performance and can facilitate the reaction process well.
The influence of the oxidant dosage was also investigated, as pre-
sented in Fig. 7a. The conversion increased rapidly with increasing
4
76.6
100
5
6
72.6
71.3
100
100
molar ratio of H
But the excess amount of oxidant, especially H
2
O
2
to benzyl alcohol and then reached a steady stage.
to benzyl alcohol
2 2
O
7
52.1
100
molar ratio reached 2:1, would decrease the selectivity of benzaldehyde
significantly due to the further oxidation to form benzoic acid. The
reaction temperature effect in this catalytic reaction was also in-
vestigated (Fig. S6). The conversion of reactant was obviously increased
and the selectivity to benzaldehyde kept steady with the increasing
reaction temperature, indicating this reaction might be sensitive to the
change of the temperature and this reaction was endothermic. Reaction
time was also an important factor needed to be studied, and we found
that the conversion of reactant reached 93.1 % while proceeded for 6 h
a
Reaction conditions: substrate (1.0 mmol),
, 80 °C, 6 h.
2 2 3
H O (1.7 mmol), CH CN
(
1 mL), 16 mg of 15W − 2.3Zn − Al
2
O
3
1
2 3
5W − 2.3Zn − Al O catalyst to verify the general applicability of
present catalytic system (Table 4). Interestingly, substrates were oxi-
dized to the corresponding products with moderate to excellent yields
regardless of substituents nature, the electron − donating or the elec-
tron − withdrawing groups [39]. The only by − product of those sub-
strates was the corresponding acid, which was confirmed by GC − MS
(
Fig. 7b). Prolonging the reaction time, the conversion kept constant
but had a tendency to form benzoic acid, resulting in the decrease of the
selectivity.
(
Fig. S7). Analyzing the results more deeply, we found that elec-
tron − withdrawing groups have a slightly negative influence of the
catalytic activity compared with electron − donating groups (Table 4,
Entry 2 − 4). In the meantime, the dehydrogenation reaction of ali-
phatic alcohol (pentan−2−ol) was also proceed efficiently and gained
3
.3. General applicability
Encouraged by the favorable experiment results, we extended the
reaction using different types of alcohols as the substrate over the
Fig. 7. Effects of (a) the molar ratios of H
2 2 2 3
O to reactant and (b) reaction time on catalytic performances over 15W − 2.3Zn − Al O catalyst. Reaction conditions:
benzyl alcohol (5.0 mmol), H (8.5 mmol), CH
2
O
2
3 2 3
CN (5 mL), 80 mg of 15W − 2.3Zn − Al O catalyst, 80 °C, 6 h.
7

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
conductive to the proton transformation from the hydroxyl group and
the acidic sites are crucial for the dehydration step [,43]. Moreover, the
acid sites can prevent the further oxidation of the substrates []. Ac-
cording to the CO
2
−TPD and NH
3
−TPD, 15W − 2.3Zn − Al
2
O
3
(400)
possesses more acid and basic sites than 15W − 2.3Zn − Al
2
O
3
(500),
and the catalytic activity are extremely different. Additionally, electron
density of the substrates can support the above assumption of the me-
δ−
δ+
chanism. Because when the value of O −H group, or more speci-
fically, the electronegativity of the O species in molecular, is smaller,
the reactants tend to be inert to the catalyst surface, so the reaction is
less active. Therefore, the comparable catalytic performance in present
catalytic system may be related to the surface acid/base sites on the
catalysts.
3.5. Catalyst reusability
The study about the nature of prepared catalysts was performed by
using 15W − 2.3Zn − Al O as the representative catalyst under the
2 3
optimal reaction conditions. A hot filtration test was performed firstly,
in which the catalyst was removed from the system after 40 min. of
reaction, and the filtrate continued to react. As indicated in Fig. 7b, the
conversion increased slightly without the participation of
2 3
15W − 2.3Zn − Al O at the beginning and then no further grown up.
The conversion change might due to the leaching of the metal species or
the existence of the active oxygen species. When the reaction was
completed, the catalyst could be recovered by a simple phase separa-
tion, then was washed thoroughly and followed by drying at 70 °C
overnight for next use. It was worth mentioning that there was no
significant loss of the catalytic activity after six consecutive runs
Fig. 8. Charge distribution and the electron density of the substrates calculated
by Gaussian: (a) benzyl alcohol, (b) 4−methyl benzyl alcohol, (c) 4−meth-
oxybenzyl alcohol, (d) 4−chlorobenzyl alcohol, (e) pentan−2−ol, (f)
4
−pyridinemethanol and (g) α−methylbenzyl alcohol.
(
Fig. 9). The XRD spectra of the used catalyst after six runs was shown
in Fig. S8, confirming that the structure and morphology of the reused
catalyst were nearly unchanged. To ensure the validity of the research,
ICP-OES (Optima 2100DV) was further performed to calculate the ratio
of the zinc and tungsten ions after each run in the reusability experi-
ment (Table S2). The Zn/W ratio slightly increased to 0.1605 after 3
cycles, leading to a little decrease of the benzyl alcohol conversion in
the fourth cycle. This phenomenon indicated that the tungsten species
in prepared catalyst had a slightly leaching tendency after a few times
of reuse but could maintain a good catalytic performance. Generally,
a moderate yield (Table 4, Entry 5). However, less efficient catalytic
performance was found when using 4−pyridinemethanol as a substrate
(
Table 4, Entry 6). In order to get a relationship of the substrate
structure and their reaction activity, we investigated the charge dis-
tribution and the electron density of the substrates at the B3LYP/
6
− 31 G level of theory, using the Gaussian 09, Revision D.01 [40],
finding that the theoretical results were in accordance well with the
experimental conclusions. Substituent groups influenced the electron
density of both the oxygen and adjacent carbon on the alcohol hydroxyl
group, and the reaction activity might rely on the charge density dif-
ference between the oxygen and adjacent carbon (Fig. 8). In particular,
the excellent yield was occurred when the electron density and charge
value of substrates was similar to benzyl alcohol, and the low charge
density (the yellow area) of oxygen on the alcohol hydroxyl group
tended to decreased the reaction activity. In general, present system is
favorable for both aryl and alkyl alcohols oxidation reaction, and can
provide certain references for similar reaction.
2 3
15W − 2.3Zn − Al O had a good recyclability and exhibited excellent
catalytic performance, which highlighted the application potential for
oxidative dehydrogenation without additives in a future industrial
scale.
4. Conclusion
In summary, an efficient and sustainable catalytic oxidation system
of various alcohols without additions was proposed. Zn-doped bime-
tallic W − Zn − Al catalysts were prepared via an eco-friendly
3.4. Proposed reaction mechanism
2 3
O
method. Controlling the amount of zinc will prevent the active metal
Based on the characterization of the prepared catalysts, we assume
from aggregation so that the resulting catalyst can exhibit significantly
that the reaction is processed on the external surface of the
5W − 2.3Zn − Al , and the reaction mechanism has been proposed
to better understand how the acid-base sites affects the oxidative de-
hydrogenation reaction. As illustrated in Scheme 1. Firstly, the hy-
droxyl group (−OH) in benzyl alcohol structures is prone to adsorbed
2 3
enhanced catalytic activity. 15W − 2.3Zn − Al O catalyst, exhibited
1
2
O
3
the highest catalytic activity, remarkable stability and is a potential
candidate for selective catalytic oxidation of alcohols. Based on the
characterization, XPS verified an interesting coordinating phenomenon
between dopant ions and support. TPD measurement indicated that the
acid − base property over the catalyst surface was sensitive to the
catalytic performances, which could be modulated by changing the
doping density of metal species. Moreover, the oxidation of a wide
substrate scope without additions is a feature of present catalytic
system. Therefore, the present work presents a sustainable process for
obtaining carbonyl compounds by dehydrogenation reaction since it
demonstrates a simple and environmental-friendly method for elim-
inating mutual neutralization and controlling the distribution of che-
mical active sites over bifunctional catalyst.
on 15W − 2.3Zn − Al
the absorbed OeH bond cleaved to present as an alcoholate inter-
mediate [41]. As referred above, H in our system is activated by
5W − 2.3Zn − Al catalyst rather than the acetonitrile, and the
strong acid sites of 15W − 2.3Zn − Al can activate the H to give
2 3
O with the medium-strong acid-base sites, and
2 2
O
1
2 3
O
2
O
3
2 2
O
the active oxygen species [42]. Subsequently, the adsorbed reactant
molecules react with neighboring reactive oxygen species to proceed
the cleavage of α-C–H bond, and thus form the benzaldehyde.
Based on the literatures, the surface basicity of prepared catalysts is
8

M. Cai, et al.
Molecular Catalysis 494 (2020) 111114
2 3
Scheme 1. Possible reaction mechanism for the oxidative dehydrogenation reaction over the 15W − 2.3Zn − Al O catalyst.
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