Oxidative lactonization of diethylene glycol to high-value-added product 1,4-dioxan-2-one promoted by a highly efficacious and selective catalyst ZnO-ZnCr2O4

Article abstract of DOI:10.1016/j.mcat.2019.110643

For the first time, the desired product 1,4-dioxan-2-one (PDO) was successfully synthesized via the oxidative lactonization of diethylene glycol (DEG) under mild conditions. After screening several catalysts (M-Cr-O), we found ZnO-ZnCr₂O₄ (Zn-Cr-O) catalyst exhibited excellent catalytic performance and this chemical transformation obtained moderate to excellent selectivity (96.22%) and conversion (81.95%) within a 4 h reaction time. Subsequently, the morphology of calcined M-Cr-O was investigated by FT-IR, XRD, FESEM, TEM, and N₂ adsorption-desorption tests for further study on catalytic performances. The strength and quantity of acid and base sites over Zn-Cr-O were also detected by NH₃-TPD and CO₂-TPD, and it was worth noting that the acid/base sites over ZnO-ZnCr₂O₄ (Zn-Cr-O) catalyst could promote this catalytic process well. Recycle studies demonstrated exceptional stability and recyclability of the prepared catalyst without significant efficiency and selectivity loss after 10 consecutive cycles.

Full text of DOI:10.1016/j.mcat.2019.110643

MolecularCatalysis480(2020)110643
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Oxidative lactonization of diethylene glycol to high-value-added product 1,
4-dioxan-2-one promoted by a highly efficacious and selective catalyst ZnO-
ZnCr2O4
Menglu Cai, Xiaozhong Wang, Yingqi Chen, Liyan Dai⁎
Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University,
Hangzhou, 310027, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oxidative lactonization
1,4-dioxan-2-one
ZnO-ZnCr2O4catalyst
Heterogeneous catalysis
For the first time, the desired product 1,4-dioxan-2-one (PDO) was successfully synthesized via the oxidative
lactonization of diethylene glycol (DEG) under mild conditions. After screening several catalysts (M-Cr-O), we
found ZnO-ZnCr2O4 (Zn-Cr-O) catalyst exhibited excellent catalytic performance and this chemical transfor-
mation obtained moderate to excellent selectivity (96.22%) and conversion (81.95%) within a 4 h reaction time.
Subsequently, the morphology of calcined M-Cr-O was investigated by FT-IR, XRD, FESEM, TEM, and N2 ad-
sorption-desorption tests for further study on catalytic performances. The strength and quantity of acid and base
sites over Zn-Cr-O were also detected by NH3-TPD and CO2-TPD, and it was worth noting that the acid/base sites
over ZnO-ZnCr2O4 (Zn-Cr-O) catalyst could promote this catalytic process well. Recycle studies demonstrated
exceptional stability and recyclability of the prepared catalyst without significant efficiency and selectivity loss
after 10 consecutive cycles.
1. Introduction
scientific community pays more attention to a robust and flexible cat-
alytic system capable of manufacturing high-quality lactones. Recently,
Lactones and their derivatives have attracted extensive attention to
both academic and industrial chemists. Due to the promising and in-
teresting biological activities, their applications are in a wide range of
yields, including polymer production, drug discovery, as building
blocks for synthesis etc [1–8]. In general, oxidative lactonization of
diols is considered as one of the most advanced methodologies for in-
dustrial process with enormous economic efficiency. However, practical
applications are notoriously facing the limitations of requiring fierce
reaction conditions as well as employing costly and toxic oxidants.
These shortcomings currently have been proved to be the biggest ob-
stacles in industrial manufacture [9]. With an environmental view to
the oxidative lactonization, proposing a mild catalytic protocol in-
corporative uses “soft” oxidants (e.g. O2, H2O2) to tackle the above
mentioned problems is of great importance in chemical industry
[10,11].
researchers have studied the synthesis of lactones under bifunctional
PCP pincer catalyst system [17] and other M/TEMPO system [18] with
O2 as oxidants. Many noble metals, such as gold [19–21], palladium
[22,23], ruthenium [24,25] and iridium [26] have been also reported,
exhibiting excellent catalytic performances in the process of aerobic
oxidative lactonization of diols. Compared with noble metals, non-
noble transitional metals performed less active in this chemical trans-
formation. But employing cheap and effective economic transition
metals like Cu [27], Co [28,29], are an urgent matter due to the scarcity
and price of noble metals in chemical industry. Zhu Q et al. synthesized
WO3/ZrO2 catalyst and applied it in the oxidative lactonization of 1,2-
benzenedimenthanol with H2O2. This catalyst exhibited high activities
in the first run. With the leaching of WO6 units and the appearance of
WOx–O–Zr bonds, however, the catalytic selectivity decreased when the
recovered catalyst participated in the reaction [30]. Indeed, an efficient
oxidative lactonization that uses H2O2 as an oxidant in combination
with economical, reusable, and more efficacious catalysts under mild
conditions is still an unmet challenge in catalytic transformation.
Recently, a mixed metal oxide, prepared by LDH precursor, has
attracted significant attention. It displays broad application prospects in
Among the lactones, 1,4-dioxan-2-one (PDO), an industrially valu-
able precursor to Poly (1,4-dioxan-2-one) (PPDO), is a fundamental
building block for ranges of well-known derivative materials such as
degradable surgical sutures, osteosynthesis products, and drug-delivery
supporters [12–16]. Considering the degradation of ether bond, the
⁎ Corresponding author.
E-mail address: dailiyan@zju.edu.cn (L. Dai).
https://doi.org/10.1016/j.mcat.2019.110643
Received 25 August 2019; Received in revised form 18 September 2019; Accepted 20 September 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

M. Cai, et al.
MolecularCatalysis480(2020)110643
the field of catalyst and exhibits potential abilities on the dehy-
drocyclization reactions [31–35]. In addition, acid sites accelerate the
dehydration reactions and basic sites promote the dehydrogenation
reactions. These properties make them competitive in catalysis field
compared to other catalysts [36–38]. For example, Venugopal A et al.
successfully produced 2-methylpyrazine using prepared ZnO-ZnCr2O4
(Zn-Cr-O) catalyst obtained by Zn-Cr-HT precursor. Meanwhile, ex-
tensive and comprehensive studies toward the influences of acid-base
sites on that catalyst were also included [39]. Min Wei et al. obtained
an acid-base-promoted Ni nanocatalyst on the basis of NiAl-layered
double hydroxide precursor, and successfully applied it in the dehy-
drogenation reaction of 2-octanol to obtain 2-octanone. Both excellent
catalytic performances (the yield of the target product was nearly
100%, 3.9 times larger than typical catalysts) and characterization re-
sults revealed a vital role of the acid-base sites [40]. As mentioned in
the literature, acid-base bifunctional catalysts can facilitate the forma-
tion of C–C bond in order to obtain a complex and stable molecule
[41–44]. Ramesh C. Deka et al. synthesized several mixed metal oxides
calcined from their precursors and found NiMgAl was the best catalyst
for nitro-aldol condensation in comparison to other prepared catalysts.
The condensation reaction catalyzed by NiMgAl was completed in two
hours at room temperature with excellent conversion (99%). Mean-
while, the catalyst had good activities in the reaction of nitromethane
with other different aldehydes as well [45]. These remarkable perfor-
mances make mixed oxides catalyst a better choice to conduct a highly
efficient catalytic process.
Here, we successfully tailored mixed metal oxides (M-Cr-O) and
firstly applied them in the oxidative latonization of DEG to obtain PDO
under mild conditions. With diligent investigations of the reaction
parameters, we found that ZnO-ZnCr2O4(Zn-Cr-O) obtained by Zn-Cr-
HT exhibited highly catalytic activity and excellent reusability.
Especially, accompanied with the acid and base sites, diethyl glycol
(DEG) can undergo either dehydration or dehydrogenation reactions to
produce PDO, and the highest conversion of DEG could reach 81.95%
and the selectivity of PDO was 96.22% within a 4 -h reaction time.
Accordingly, different characterization methods were employed to re-
veal the correlations between the catalyst properties and the catalytic
activities toward oxidative lactonization of DEG.
produce hydrotalcite structure. Subsequently, an alkaline mixture of
NaOH and Na2CO3 was slowly added to the metallic solution until the
pH = 9 under vigorous stirring. The resulting gel was then washed to
neutral with deionized water, and filtered followed by drying at 120 °C
in an oven for 12 h. After that, the samples were collected and calcined
at specific temperature (350, 450, 550 and 650 °C) for 5 h. For con-
venience, the Zn-Cr-O samples calcined at different temperatures were
nominated as ZC350, ZC450, ZC550, and ZC650, respectively. Under
the same condition, Mn-Cr-O, Fe-Cr-O and Cu-Cr-O samples were also
prepared to compare the catalytic performances.
2.3. Activity tests
All catalysts were evaluated for the oxidative lactonization of die-
thylene glycol (DEG) to obtain 1,4-dioxan-2-one (PDO). The reaction
was carried out in a three-neck flask (25 ml) equipped with a reflux
condenser at 80 °C under magnetic stirring. First of all, 0.5 mmol of
DEG, 1.2 ml of CH3CN and 0.2 mmol of NaOH (0.2 M) were added into
a three-neck flask with vigorous stirring at room temperature. 15 min
later, the catalyst (5 mol%) was added. After stirring for 30 min, 30 wt.
% aqueous H2O2 (1.5 mmol) was dropwise added into the above mix-
ture and maintained the temperature at 80 °C for 4 h. In addition, a
series of experiments have been conducted to optimize the reaction
conditions such as hydrogen peroxide dosage, catalyst amounts, and
solvent nature. The solid catalyst was separated by filtration when the
reaction was completed, then washed with deionized water and ethanol
thoroughly, followed by drying in an oven at 120 °C overnight for reuse.
With external standard method, the filtrate was analyzed by GC (FULI
9790Ⅱ, FID detector, 30 m AT-FFAP capillary column).
3. Results and discussion
3.1. Catalyst characterization
The crystalline phase and the structure of the Zn-Cr-O samples
calcined at different temperatures ranged from 350 to 650 °C were
characterized by XRD (Fig. 1). Better crystallinity was obtained when
the samples were calcined at higher temperature. The peaks appeared
at 31.78°, 34.42°, 36.35°, 47.54°, 56.60°, 62.86°, and 66.38° might re-
veal the (100), (002), (101), (102), (110), (103), and (200) planes of
ZnO phase [JCPDS NO. 36-1451] and the resulting ‘d’ values of
0.28 nm, 0.26 nm, 0,25 nm, 0.19 nm, 0.16 nm, 0.15 nm and 0.14 nm,
respectively. The peak appeared at 30.30°, 35.73°, 37.36°, 43.41°,
53.90°, 57.46°, 63.11°, 66.31°, and 71.60° were attributed to the (220),
(311), (222), (400), (422), (511), (440), (531), and (620) facets of
ZnCr2O4 [JCPDS NO. 22-1107] phase, and the ‘d’ values were
2. Experimental section
2.1. General
Transmission electron microscopy (TEM) and high-resolution TEM
were performed on a FEI Tecnai G2 F20 operated at 200 kV. Field
emission scanning microscopy (FESEM) imagines were collected on a
Hitachi SU-70 microscope and energy dispersive X-ray spectrometer
(EDS) connected with FESEM was employed for elemental distribution.
The amount of Zn and Cr species in catalysts was measured by ICP
method (Varian-730ES). XRD was carried out on PANalytical X’ Pert
diffractometer with Cu Kα radiation. X-ray photoelectron spectra (XPS)
were recorded on a Thermo Scientific Escalab 250Xi spectrometer using
Al Kα radiation at 1486.6 eV, and the binding energies were calibrated
at 284.8 eV from C 1s peak. FT-IR spectra were tested in an IRAffinity-
1S (Shimadzu, Japan) spectrometer using KBr pellets in a scope of
400–4000 cm−1. UV–vis spectra of the samples were recorded on a UV/
vis UNICO UV-3820 spectrophotometer. N2 adsorption-desorption test
was performed on a Micromeritics ASAP 2020 analyzer at −196 °C to
determine the surface areas, pore size and volume of the catalysts using
the Brunauer-Emmett-Teller (BET) method and BJH method.
2.2. Catalyst preparation
A series of Zn-Cr-O catalysts were synthesized via a facile co-pre-
cipitation method. Typically, dissolving Zn(CH3COO)2·2H2O and Cr
(NO3)3·9H2O in deionized water with a constant Zn/Cr ratio of 2 to
Fig. 1. XRD patterns of the Zn-Cr-O catalysts calcined at various temperatures:
(a) ZC350, (b) used ZC450, (c) ZC450, (d) ZC550 and (e) ZC650.
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M. Cai, et al.
MolecularCatalysis480(2020)110643
Table 1
The textural parameters of catalysts.
[a]
[a]
Catalysts
SBET(m2 g−1
)
Vp(cm3 g−1
)
rp(nm)[a]
Crystallite size, nm[b]
ZnO(100)
ZnCr2O4(311)
ZC350
ZC450
ZC550
ZC650
118.03
53.29
30.81
21.47
0.2863
0.2428
0.1467
0.0619
9.702
18.23
19.05
11.53
nd
nd
26.69
38.95
44.97
16.94
19.13
23.27
[a] Determined from N2 adsorption-desorption at -195.96 °C and calculated using the BET and BJH methods (SBET and rp), t-polt method (Vp), respectively. [b]
Crystallite size calculated from XRD patterns by Scherrer’s equation.
calculated of 0.30 nm, 0.25 nm, 0.24 nm, 0.21 nm, 0.17 nm, 0.16 nm,
0.15 nm, 0.14 nm, and 0.13 nm, respectively. These findings were also
consistent with the results of other authors [46–48]. However, there
were no detection of ZnO and ZnCr2O4 peaks in ZC350, probably
caused by the pre-existence of amorphous phase [49]. Table 1 illu-
strated the average crystallite sizes of ZnO (100) phase and ZnCr2O4
(311) phase of the calcined catalysts and the XRD patterns of the M-Cr-
O catalysts were shown in Fig. S1. Compared to the Zn-Cr-O, there was
no diffraction peaks for MCr2O4 phase in M-Cr-O, indicating that the
spinel phase of them might be too small to be detected by XRD.
Meanwhile, Cr2O3 phase unexpectedly occurred in Mn-Cr-O and Fe-Cr-
O catalysts.
Fig. 2 depicts the N2 adsorption-desorption isotherms and the pore
distributions of Zn-Cr-O samples. A type of IV isotherm with the oc-
currence of an evident H3 hysteresis loop in each sample, inferring that
the structure was mesoporous, and slit-shaped pore channels were ex-
isted in our catalysts [50]. At low relative pressure, the adsorption
amount gradually decreased along with increasing calcination tem-
perature. All the detailed textural parameters of calcined catalysts were
listed in Table 1. It could be seen that only mesopores exhibited in our
samples and the specific surface area and the pore volumes (Vp) of Zn-
Cr-O catalysts obviously decreased with increasing calcination tem-
perature.
Fig. 3. FT-IR spectra of the Zn-Cr-O catalysts calcined at various temperatures:
(a) ZC350, (b) used ZC450, (c) ZC450, (d) ZC550 and (e) ZC650.
FI-IR spectra of ZnO-ZnCr2O4 (Zn-Cr-O) samples calcined at dif-
ferent temperatures were shown in Fig. 3. The vibration of tetrahedral
M–O and the octahedral M–O bond was presented at the bands v1 and
v2 at 627 and 502 cm−1 [51], which are in good correlation with XRD
results for the formation of a spinel phase. With increasing calcination
temperature, the absorption bands of Zn-Cr-O apparently enhanced and
became sharped, but the broad absorption bands in the range of
3500–3400 cm−1 were weakened. Meanwhile, a small absorption band
at 1636 cm−1 was related to the hydroxyl deformation mode of water
[52] and the band at around 928 cm−1 was an evidence of Cr6+ species,
Fig. 4. UV–vis spectra of Zn-Cr-O samples calcined at different temperatures:
(a) ZC350, (B) used ZC450, (C) ZC450, (d) ZC550, and (e) ZC650.
which were partly decreased with increasing calcination temperature
[53,54].
UV-visible spectroscopy can identify the Cr3+ and Cr6+ species in
the calcined Zn-Cr-O samples, as presented in Fig. 4, the charge trans-
ferred from O 2p→Cr6+ of the samples leading two characteristic bands
located at about 280 and 360 nm [55]. With increasing calcination
temperature, there was a blue-shift of the band around 360 nm, sug-
gesting a decreasing concentration of the surface chromate species [56].
This result was consistent with the phenomenon in FT-IR analysis. Same
tendency was also found in the spectra of recovered Z450, and this
Fig. 2. (a) N2 adsorption-desorption isotherms and (b) Pore size distributions of
the calcined catalysts.
3

M. Cai, et al.
MolecularCatalysis480(2020)110643
Fig. 5. TEM and FESEM images (a, b) of ZC450, HRTEM imagines (c) of ZC450 sample, selected area diffraction (SAED) pattern (e) recorded at the marked area of
(d), and the corresponding elemental distribution mapping(g–i) of (f), Zn(orange), Cr(green), O(red). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article).
might be the reason why the XRD result of used ZC450 was similar to
the high-temperature calcined one (Fig. 1b). An absorption band at
around 220 nm was attributed to the ZnO phase, presenting in all
samples [57]. Importantly, there are no bands located at around
440 nm, implying no occurrence of Cr2O3 phase in our samples, and
XRD results did not observe Cr2O3 phases in all the catalysts as well.
The microstructure of the ZC450 was tested by TEM and FESEM
measurements (Fig. 5), the FESEM image of ZC450 in Fig. 5b shown
that there was clearly existing the ZnO entity and ZnCr2O4 particles on
ZC450 surface. The lattice fringes are periodical, meaning a good single
crystal in prepared catalyst. And the measured inter-planar spacing of
0.251 nm was identical with the dominant exposed plane (311) of
ZnCr2O4, and the inter-planar spacing of 0.244 nm, 0.275 nm and
0.293 nm matched well with the (101), (100), and (220) facets of ZnO,
respectively (Fig. 5c). These results were consistent fitly with the peaks
shown in XRD spectra. Additionally, the corresponding SADE pattern
(Fig. 5e) was indicating a polycrystalline structure of ZC450. Mean-
while, we collected the corresponding energy dispersive spectroscopy
(EDS) spectra (Fig. S2) and further analyzed the component elements.
As presented in Fig. 5g-i, only three elements of Zn, Cr and O were
identified in the spectrum. In addition, the amount of metal species was
characterized by ICP-OES, giving a percentage of Zn and Cr of 0.56.
The X-ray photoelectron spectra of Zn-Cr-O samples are presented in
Fig. 6, and the existence of Zn, Cr and O elements are distinctly iden-
tified by the wide-range XPS spectrum of ZC450 in Fig. S3. The Zn 2p
spectra of Zn-Cr-O calcined at various temperatures are shown in
Fig. 6a. The BE values of Zn in these samples ranging from 1021.5 to
1021.7 eV along with 1044.9 eV are characteristic to ZnO and ZnCr2O4,
respectively. Meanwhile, a shift in BE towards the low energy region
with increasing calcination temperature is evident for the formation of
ZnO-ZnCr2O4. Chromium shows a binding energy at 575.9 eV along
with 585.4 eV (Fig. 6b) attributed to Cr3+ species for all catalysts, and
this phenomenon means the formation of ZnCr2O4 as well. Cr6+ species
was responded to 578.9–579.6 eV [58–60], and the shift in Cr 2p peak
towards low BE with increasing calcination temperature means the
transition of Cr6+ to Cr3+. These observations are also found in the FT-
IR spectra (Fig. 3) of high temperature treated samples.
To gain a better understanding of our calcined catalysts, the tem-
perature-programed desorption (TPD) of NH3 and CO2 were used to
evaluate the correlations between the quantity of acid/base sites and
their chemical performances. According to the desorption temperature,
these profiles were deconvoluted to several peaks. As shown in Fig. 7a,
Zn-Cr-O samples displayed a broad CO2 desorption peak in the tem-
perature region of 100–300 °C with a shoulder at 250–270 °C. Three
desorption peaks were clearly noticed in ZC450 and ZC550, those were
weak base site (150–160 °C), moderate base sites (250–270 °C) and
strong base sites (350–370 °C). In addition, the peak appearing at and
above 250 °C corresponded to moderate base sites were slightly greater
for ZC450 when compared to ZC550. The low temperature peak (weak
base sites) might cause by the occurrence of OH− groups, because
ZC350 with the pre-existence of amorphous hydroxyl zinc chromate
phase showed the highest intensity [53,54]. Similarly, the NH3-TPD
profiles of all samples were shown the peaks with maximal tempera-
tures in the regions of 100–105 °C (weak acid sites), 245–255 °C
(moderate acid sites), and 395–405 °C (strong acid sites).
TPD studies demonstrated that all catalysts possessed both acid and
base sites in different calcination temperatures, and total number of
weak and moderate acid/base sites decreased while strong acid/base
sites formed at higher calcination temperature (Table 2). We thought
the increasing crystallite size in ZnO and ZnCr2O4 species of high-
temperature-treated catalyst may cause the decrease of acid-base
strength. In addition, Tmax of CO2 desorption curves (Fig. 7a) in high-
temperature-treated samples were slightly shifted to low temperatures
and Tmax of NH3 desorption curves were moved toward high
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M. Cai, et al.
MolecularCatalysis480(2020)110643
Fig. 6. XPS analysis (a) Zn2p, (b) Cr2p spectra of calcined Zn-Cr-O.
temperature in contrary (Fig. 7b). It was worth noting that the mod-
erate acid site in ZC550 disappeared and strong acid site occurred. We
also calculated the ratio of acid to base sites, as listed in Table 2, the
ratio value of ZC350 was inferior to others. ZC450 and ZC550 had si-
milar ratio values, which were approximately 4 times higher than their
basicity. Reportedly the surface acid (base) sites were conductive to
either dehydration or dehydrogenation reactions [36–38]. These find-
ings could help us make a deliberation that DEG was first oxidized to
the monoaldehyde, then the carbonyl group might be activated by the
acid sites of ZC450. After receiving a nucleophilic attack on the car-
bocation, the intermolecular cyclization occurred to form the inter-
mediate product 1,4-dioxan-2-ol (Fig. S4). The cooperativity between
the basicity and acidity might be an important factor for the formation
of PDO in Zn-Cr-O catalysts.
increasing oxidant dosage. However, the selectivity observed above 6.0:
1 oxidant: substrate ratio was no appreciable increased. It was specu-
lated that a competitive reaction of the chain degradation might occur
to hamper the formation of PDO at high ratio of oxidant to substrate.
This phenomenon might be explained by the acidity of Zn-Cr-O and the
generation of OH radicals from the excess amounts of H2O2 [61].
Also, we have attempt the oxidative lactonization of DEG over
ZC450 with the bubble of O2, but the reaction failed to give the desired
product. Considering both catalytic performance and H2O2 utility, the
best dosage of H2O2 was chosen to be 3.0 equivalents in the following
investigations.
For comparison, a series of four M-Cr-O catalysts were prepared by a
simple co-precipitation method, and the samples were collected and
calcined in static air at 450 °C for 5 h. The DEG conversion and PDO
selectivity were depicted in Fig. 9, indicating ZC450 became the best
choice as the yield of PDO was superior to other catalysts. The FI-IR
spectra of M-Cr-O samples calcined at different temperatures were also
shown in Fig. S5. The brand at 501 cm−1 can be assigned to the vi-
bration of the octahedral MeO bond [51]. When we used XRD to
characterize the crystalline phase and the structure of the M-Cr-O
samples (Fig. S1), only Cu-Cr-O have formed a combination of MxOy-
MCr2O4 species, and MxOy and Cr2O3 phases formed rather than MxOy-
MCr2O4 species in Fe-Cr-O and Mn-Cr-O. Also, the FESEM image of M-
Cr-O catalysts in Fig. S6 told us that the morphology of M-Cr-O
(M = Cu, Fe, Mn) were different from Zn-Cr-O. There was no composite
structure of MCr2O4 particles existed and agglomerate spherical parti-
cles could be seen clearly in prepared M-Cr-O catalysts. The ZnO par-
ticles were well embedded into the pores of ZnCr2O4 particles in ZC450,
3.2. Catalytic oxidative lactonization of diethyl glycol (DEG)
The main product 1,4-dioxan-2-one (PDO) is successfully prepared
via the catalytic oxidation of DEG according to the GC analysis after
compared to a standard sample. And the occurrence of main by-product
glycol may due to the R-O-R chain scission. The catalytic performances
we concerned about are based on the yield of PDO.
The catalytic performances of the oxidation reaction over various
molar ratios of H2O2 to DEG were verified and the results were shown
in Fig. 8. With increasing amount of H2O2 to DEG molar ratio, i.e., 1.0
to 2.0, the conversion of DEG kept steady (65%) might on account of
the theoretically needs 2 mol H2O2 for lactonization of 1 mol DEG. Then
the conversion ascended continuously from 65% to 95% with
Fig. 7. TPD patterns of calcined samples. (a) CO2-TPD and (b) NH3-TPD of calcined samples.
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M. Cai, et al.
MolecularCatalysis480(2020)110643
Table 2
Amounts of acid and base sites according to NH3-TPD and CO2-TPD patterns.
Catalysts
The amounts of Base/Acid sites (mmol/g)
Ratio of acid to base site
[c]
[d]
BCO2
Bw
BM
BS
ANH3
AW
AM
AS
/
0.0204
0.2326
ZC350
ZC450
ZC550
0.2031
0.1428
0.1162
0.1613
0.0801
0.0612
0.0418
0.0442
0.0237
/
0.6464
0.5508
0.4553
0.2778
0.1970
0.2227
0.3686
0.3334
/
3.18
3.86
3.92
0.0185
0.0313
[c] CO2 uptakes measured by TPD of CO2, BW, BM, and BS are the amounts of weak, moderate, and strong base site, respectively. [d] NH3 uptake calculated by TPD of
NH3, AW, AM, and AS are the amounts of weak, moderate, and strong acid site, respectively.
Fig. 10. The effect of amounts of base on the reaction. Reaction condition: DEG
(0.5 mmol), H2O2/DEG molar ratio (6.0), 8 mg of ZC450 (5 mol %), reaction
temperature (80 °C), reaction time (4 h).
Fig. 8. Effect of H2O2 dosage on catalytic performances of oxidation of DEG
over ZC450.
calcination temperature. For the basicity of the reaction played a very
assignable role both in PDO selectivity and catalyst efficiency.
As shown in Fig. 10, the reaction didn’t go well without base ad-
ditive, maybe basicity environment had a positive impact upon the acid
sites in prepared catalyst and promoted the catalytic function of the
acid and base sites. Meanwhile, the influence of the base type of oxi-
dative lactonization reaction was also studied (Fig. S8). It can be found
that both DEG conversion and PDO selectivity were very poor when
Et3N or pyridine as an additive. When inorganic bases participated in
the reaction, the target product was obtained with a satisfactory yield.
Considering all the information, we have a hypothesis that prepared
catalysts which have detected the ZnO and ZnCr2O4 peaks could pro-
vide more active sites when they have larger specific surface areas and
are more stable than non-calcined precursor. ZC450 possessed mod-
erate basic sites and slightly lower acidity, but ZC550 had the position
of strong acid sites. The inferences are that an appropriate amounts of
acid/base sites and a larger surface area of effective catalytic compo-
nents which can adsorb more reactant molecules can exhibited higher
catalytic oxidation performance, and the ZnO and ZnCr2O4 phases
make prepared catalysts more stable in the cycling reaction [62].
Some studies were investigated to attest to that the oxidative lac-
tonization of DEG was a catalytic process (Fig. 11). The experiment in
the absence of Zn-Cr-O catalyst was first carried out with a poor con-
version and selectivity, proving that the particular oxidation process
was catalytic. Better performance was observed in the case of the cat-
alyst. With a high dosage of ZC450, the DEG conversion rose apparently
whereas the PDO selectivity decreased from 95.36% to 80.29% by the
decomposition of DEG. It could be concluded that the optimal dosage of
ZC450 was 5 mol% under the typically identical conditions. It was also
pleasure to find that ZC450 exhibited the highest selectivity of 95.36%
with a conversion of 74.82% when used the optimized dosage we have
found.
Fig. 9. Catalytic performances of oxidation of DEG over various catalysts.
but it couldn’t be found in other catalysts.
The performance of catalysts exposed to various calcination tem-
peratures was examined, suggesting 450 °C was the optimum calcined
temperature of the catalyst. The yield of PDO increased firstly and then
decreased steadily over prepared catalysts as the calcination tempera-
ture increased (Fig. S7). As Jiangping Xiao et al. reported the long-term
stability of the ZnCr2O4 species in the catalytic oxidation of HCl to
produce Cl2 [62]. The prepared catalyst calcined at 350 °C do shows
comparable results with the rest of the samples but the absence of the
ZnO and ZnCr2O4 peaks make it less stable after the reaction. Mean-
while, the pores or channels of prepared catalysts were in favor of the
absorption of active components, but they collapsed as calcination
temperature increased (Fig. S6). That mean the decrease of the PDO
yield might be explained by the changing density of acid/base sites and
the increasing crystallite size in both ZnO and ZnCr2O4 species in high
Hot filtration tests are carried out in order to verify that our catalyst
is indeed heterogeneous, and to ensure that there is no existence of
homogeneous catalysis owning to the leaching of Cr into the solution.
6

M. Cai, et al.
MolecularCatalysis480(2020)110643
Fig. 12. Recycling experiments of ZC450 catalyst.
Fig. 11. The effect of catalyst dosage on catalytic performance of ZC450.
desired product PDO. Fat-soluble solvents (Table 4, Entry 2, 3, 6) were
also tested in the same reaction conditions but the results were dis-
astrous. All the results revealed a strong dependence on the nature of
used solvent and the presence of acetonitrile was essential to this re-
action.
Table 3
The results of hot filtration tests[e]
.
Entry
A
Reaction time (h)
Cat. dosage
Conv. (%)
Sel. (%)
1 h
8 mg
0 mg
0 mg
8 mg
0 mg
69.63%
69.41%
69.17%
72.33%
71.53%
95.80%
95.46%
95.45%
95.53%
95.31%
1 h + 1 h
1 h + 3 h
2 h
3.2.1. Catalyst reusability
B
The life cycle and stability of the heterogeneous catalysts are crucial
in practical applications. Hence, recycle studies were carried out to
assess the stability of ZC450 catalyst for the oxidative lactonization of
diethylene glycol (DEG) (Fig. 12). After the completion of reaction,
ZC450 was separated from the reaction mixture by simple filtration,
then washed thoroughly with ethanol and dried at 120 °C in an oven for
next use. It was worth mentioning that ZC450 had a robust reusability
for it could be reused more than ten runs without significant loss in
catalytic activity. XRD analysis confirmed that ZC450 maintains its
stability after the reaction and even more stable than prepared. FI-IR,
UV–vis, and SEM spectra of used ZC450 were in good agreement with
that of freshly prepared sample (Figs. 1, 3 and S6). All the results in-
dicated a better recyclability of present catalyst system, and verified its
application potential in chemical transformation.
2 h + 2 h
[e] Reaction conditions: DEG (0.5 mmol), H2O2/DEG molar ratio (3.0), CH3CN
(1.2 ml), 0.2 M NaOH/DEG molar ratio (0.4), 8 mg of ZC450 (5 mol %), reaction
temperature (80 °C).
Considering that the reaction is completed in four hours (Fig. S9), we
performed the tests in one (Table 3, Entry A) and two hours (Table 3,
Entry B), respectively. Firstly, 0.5 mmol of DEG, 1.2 ml of CH3CN and
0.2 mmol of NaOH (0.2M) were added into a three-neck flask with
vigorous stirring at room temperature. 15 min later, ZC450 (5 mol%)
was added, after stirring for 30 min, 30 wt. % aqueous H2O2 (1.5 mmol)
was dropwise added into the above mixture. The catalyst was separated
from the hot reaction mixture after maintaining the temperature at
80 °C for 1 h (2 h). Then filtrate continued to be heated at 80 °C. After
the completion of 4 h, the filtrate was analyzed by GC and no increase
in the conversion of DEG was achieved. These results clarified that the
reaction heterogeneous in nature.
4. Conclusion
A facile, recyclable and effective method for the oxidative lactoni-
zation of diethylene glycol (DEG) has been proposed for the first time in
order to prepare the high-valued product 1,4-dioxan-2-one (PDO). With
H2O2 as an oxidant, the Zn-Cr-O oxide catalyst exhibited good perfor-
mances on the target reaction and well received the product PDO in a
satisfactory yield. We also used NH3-TPD and CO2-TPD to discover the
properties of acid/base sites over ZnO-ZnCr2O4 (Zn-Cr-O) catalyst and
found the cooperativity between the basicity and acidity might be an
important factor for the formation of PDO in Zn-Cr-O catalysts.
Although, the ZC350, which does not have the ZnO and ZnCr2O4 XRD
reflections, shows comparable results with the rest of the samples, those
have the ZnO and ZnCr2O4 peaks are more stable after the reaction. As
far as we knew, this was the first report of oxidative lactonization of
DEG with such an economical and efficacious catalyst under mild
conditions with H2O2 as oxidant. Furthermore, existing catalytic reac-
tion system had provided a safe, sustainable and low-cost process for
this chemical transformation and the prepared catalyst can be easily
reused more than ten runs without significant deactivation.
Accompanied with long term economic viability, this catalytic protocol
is a promising approach and may herald a new era for low-cost lactones
construction.
It was vital to investigate the solvent effect due to its significant role
in reaction. As listed in Table 4, the highest conversion and selectivity
were achieved when using CH3CN as a solvent. Unfortunately, other
water-soluble solvents such as tetrahydrofuran and methanol (Table 4,
Entry 4, 7) obtained chaotic systems and had no tendency to give the
Table 4
The effect of the different solvents over the reaction[f]
.
Entry
Solvents
T. °C
Conv. (%)
Sel. (%)
1
2
3
4
5
6
7
Acetonitrile
Cyclohexane
Toluene
80
80
80
80
60
60
60
81.95
51.83
46.74
18.95
/
96.22
10.74
12.56
29.77
/
Water
Tetrahydrofuran
Ethyl acetate
Methanol
48.82
/
17.25
/
[f] Reaction conditions: DEG (0.5 mmol), H2O2/DEG molar ratio (3.0), Solvent
(1.2 ml), 0.2 M NaOH/DEG molar ratio (0.4), 8 mg of ZC450 (5 mol %), reaction
time (4 h).
7

M. Cai, et al.
MolecularCatalysis480(2020)110643
Acknowledgments
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