MoO3 supported on ordered mesoporous zirconium oxophosphate: An efficient and reusability solid acid catalyst for alkylation and esterification

Article abstract of DOI:10.1016/j.mcat.2017.10.028

A series of molybdenum oxide supported on ordered mesoporous zirconium oxophosphate (MoO₃/M-ZrPO) materials with different MoO₃ loadings (0–20 wt%) and calcination temperatures (500–900 °C) have been designed, synthesized and employed as solid acid catalysts in alkylation and esterification. The XRD, TG-DSC, H₂-TPR, N₂-physisorption and TEM characterizations were taken to investigate the structural properties and states of introduced MoO₃ species. The influence of MoO₃ loadings and calcination temperatures in catalytic performance was detailedly investigated and optimal catalytic activity was reached at 10 wt% MoO₃ loadings and treated at 700 °C. Moreover, MoO₃/M-ZrPO catalysts exhibited outstanding catalytic performance in Friedel-Crafts alkylation of different aromatic compounds and esterification of levulinic acid with 1-butanol. Furthermore, it was noteworthy that the catalyst had superior reusability and no noticeable declines were observed in catalytic performance even after seven runs.

Full text of DOI:10.1016/j.mcat.2017.10.028

Molecular Catalysis 444 (2018) 10–21
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Molecular Catalysis
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Editor’s choice paper
MoO supported on ordered mesoporous zirconium oxophosphate:
3
An efficient and reusability solid acid catalyst for alkylation and
esterification
∗
∗
Zhichao Miao, Zhenbin Li, Jinping Zhao, Weijiang Si, Jin Zhou , Shuping Zhuo
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 July 2017
Received in revised form
A series of molybdenum oxide supported on ordered mesoporous zirconium oxophosphate (MoO /M-
ZrPO) materials with different MoO3 loadings (0–20 wt%) and calcination temperatures (500–900 C) have
3
◦
been designed, synthesized and employed as solid acid catalysts in alkylation and esterification. The XRD,
TG-DSC, H2-TPR, N2-physisorption and TEM characterizations were taken to investigate the structural
properties and states of introduced MoO3 species. The influence of MoO3 loadings and calcination tem-
peratures in catalytic performance was detailedly investigated and optimal catalytic activity was reached
1
3 September 2017
Accepted 23 October 2017
Keywords:
◦
at 10 wt% MoO3 loadings and treated at 700 C. Moreover, MoO3/M-ZrPO catalysts exhibited outstanding
Mesoporous solid acid
MoO3 loadings
Calcination temperature
FC alkylation
catalytic performance in Friedel-Crafts alkylation of different aromatic compounds and esterification of
levulinic acid with 1-butanol. Furthermore, it was noteworthy that the catalyst had superior reusability
and no noticeable declines were observed in catalytic performance even after seven runs.
© 2017 Elsevier B.V. All rights reserved.
Esterification
1
. Introduction
As the backbone of petroleum and chemical processing
Friedel-Crafts (FC) alkylation of aromatic compounds is one of
the most important strategies to synthesis a wide variety of diary-
lalkanes in organic chemistry, which have significant commercial
value and key intermediates in the fields of petrochemicals, phar-
maceuticals, cosmetics, dyes and many other important chemical
industries [20–23]. Furthermore, as for alkylating agents, benzyl
industries, different kinds of acid-catalyzed reactions, such as iso-
merization [1,2], alkylation [3], dehydration [4,5] and esterification
[
6], are widely investigated. However, use of cheap and available
homogeneous acid catalysts (e.g. AlCl , FeCl , HCl and H SO ) has
alcohol, which only products H O as byproduct is more preferable
3
3
2
4
2
led to severe environmental problems [7,8]. Therefore, continu-
ous efforts have been taken to replace these conventional catalysts
with solid acid catalysts, which are easy separation, little corro-
sion, environmentally friendly and little pollution and waste [9,10].
As a new generation of solid acid catalyst, Zr-based solid acid cat-
alysts (e.g. WO /ZrO [11,12], MoO /ZrO2 [13,14] and B O /ZrO
than others [21,24,25]. Traditionally, strong Lewis acid catalysts
(AlCl , FeCl3 and BF ) and Brønsted acid catalysts (H SO , HCl and
3
3
2
4
HNO ) are employed in FC alkylation [26]. These years, different
3
solid acid catalysts which are environmentally friendly, such as
magnetic carbon [27], TiO nanosheets [28], zeolites [29] and MOFs
2
[30], are employed and investigated in FC alkylation reactions.
Esterification reaction is a significant reaction for production
of fuels and chemicals [31,32]. Levulinic acid esters, which can be
synthesized by esterification of levulinic acid (LA) with 1-butanol,
have drawn plentiful attentions owing to the potential applications
in domains of fuel additives, green solvents, fragrances, lubricants
and polymer productions [33–35]. More important, LA can be got-
ten from hydrolysis of renewable cellulosic materials and it is an
alternative renewable energy resource. These years, various solid
acid catalysts were utilized for esterification of LA including WO3-
SBA-16 [36], WOx/ZrO2 [37] and heteropolyacids [38].
3
2
3
2
3
2
[
15]), which have strong acidic properties and high stability, have
attracted considerable attention since their initial discovery in the
late 1980s. In addition, as reported by other workers, there exist dif-
ferent influence factors, such as loadings of modifiers (W, Mo and
B), calcination temperatures and crystalline phase of ZrO2 on cat-
alytic performance [16–19]. Therefore, it is important to investigate
these factors systematically to get an excellent solid acid catalyst.
In comparison of traditional bulk materials, ordered meso-
porous materials have attracted a lot of attentions, because of their
outstanding textural properties such as large accessible surface
∗ Corresponding author.
E-mail addresses: zhoujin@sdut.edu.cn (J. Zhou), zhuosp academic@yahoo.com
S. Zhuo).
(
http://dx.doi.org/10.1016/j.mcat.2017.10.028
468-8231/© 2017 Elsevier B.V. All rights reserved.
2

Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
11
area, big pore size and volume [39–42]. As for supported catalysts,
mesoporous materials are promising supports for improving the
dispersion of active component to provide abundant active sites
for reactant molecules. This might be in favor of the enhancement
of catalytic performance [43–45]. Therefore, mesoporous materials
have potential to be investigated in the field of support.
tributions were calculated using adsorption branches of nitrogen
adsorption-desorption isotherms by Barrett-Joyner-Halenda (BJH)
method.
Transmission electron microscopy (TEM) images and elemen-
tal mapping measurements were carried out on TECNAI G2 F20
high-resolution transmission electron microscopy under a working
voltage of 200 kV.
In this work, we focus on synthesizing a sequence of MoO3
supported on the ordered mesoporous zirconium oxophosphate
Temperature programmed desorption of ammonia (NH -TPD)
3
(
M-ZrPO) with various MoO3 loadings and taken as solid acid
was performed on a Finesorb 3010 (FINETEC INSTRUMENTS). The
typical experiment for TPD measurement was carried out as fol-
lows: prior to the test, the sample (0.1 g) was pretreated at 500 C
catalyst for FC alkylation of different aromatic compounds and
esterification of levulinic acid with 1-butanol. The aim of this
work is to investigate the influence of MoO3 loadings and calci-
nation temperatures on catalytic performance and get an excellent
◦
for 1 h under the flowing He to remove the moisture and absorbed
◦
impurities. After cooling to 100 C, NH -He (0.5-99.5 mol%) was
3
solid acid catalyst. In addition, the MoO /M-ZrPO materials were
introduced for 30 min. The physically absorbed NH3 in the sample
was removed by blowing in He at 100 C for 60 min. Afterwards, the
3
◦
detailedly characterized by N -physisorption, TEM, XRD and H -
2
2
TPR characterizations for mesoporous properties and states of Mo
species in the materials. Moreover, the possible structure-function
TPD experiment was carried out by increasing the temperature to
◦
◦
−1
500 C at a rate of 10 C min and the desorbed NH was measured
3
relationship was discussed and the influences of MoO loadings and
by TCD detector.
3
calcination temperatures on catalytic performance were detailedly
investigated.
The infrared spectra of adsorbed pyridine (Pyridine-IR spectra)
were taken on PE Frontier FT-IR spectrometer. Prior to the test, the
◦
−3
sample was pretreated at 400 C for 1 h under 5 × 10 Pa, after-
wards, the sample was cooled to room temperature and pyridine
2
. Experimental section
◦
was introduced. Then, the sample was raised to 200 C and held for
−
3
3
0 min under 5 × 10 Pa, after which the spectrum was recorded.
2.1. Catalyst preparation
The blank experiments were operated under the same conditions
and employed as the background to insure the accuracy of the
infrared spectra of adsorbed pyridine.
Ordered mesoporous zirconium oxophosphate (M-ZrPO) was
prepared as reported in the previous report [46]. The sup-
ported MoO3 solid acid catalysts were synthesized through the
equivalent-volume impregnation method with ammonium molyb-
date ((NH ) Mo7O ·4H O) aqueous solution as precursor of MoO .
X-ray fluorescence (XRF) spectra were recored on a ZSX-100e
(Rigaku Corporation).
4
6
24
2
3
2.3. Catalytic reaction
After the process of impregnation, the materials were treated
◦
◦
at 120 C for 12 h and calcined at 700 C for 4 h. The synthe-
The liquid phase Friedel-Crafts (FC) alkylation with different
sized catalysts were named as X wt% MoO /M-ZrPO and X wt%
3
aromatic compounds and alkylating agents was taken in round bot-
tom flask accompanied with a reflux condenser in a temperature
controlled oil bath. In a typical run for alkylation of anisole with
benzyl alcohol, anisole (10 mL), benzyl alcohol (1 mL) and catalysts
(
X wt% = mMoO3/(mMoO3 + mM-ZrPO) × 100%) stood for the loadings
of MoO . The 10 wt% MoO /M-ZrPO catalysts treated at different
temperatures were denotes as 10 wt% MoO /M-ZrPO-Y and Y stood
for calcination temperatures (500, 600, 700, 800 and 900 C).
3
3
3
◦
(0.1 g) were successively added into a round bottom flask. Then, the
reaction was performed under reflux condition and magnetic stir-
ring for 1 h. The used catalysts were separated and treated at 500 C
2.2. Characterization
◦
for 1 h and employed in next cycle. The conversion of reactants
and selectivity of products were detected by a gas chromatography
X-ray diffraction (XRD) patterns were recorded on an X’Pert Pro
Multipurpose diffractometer (Bruker AXS D8 Advance) with Cu K␣
(GC) instrument (Agilent-7890B; equipped with a flame ioniza-
◦
◦
radiation (0.15406 nm) at room temperature from 10.0 to 80.0
tion detector (FID) and PE-6 column (50 m × 0.32 mm × 0.25 ␮m)).
The qualitative analysis of reaction products was confirmed by gas
chromatography-mass spectroscopy (GC-MS) (GCMS-QP2010 SE,
SHIMADZU). The o-benzylanisoles (o-BA), p-benzylanisoles (p-BA)
and dibenzyl ether (DBE) were detected and no other products
were observed in this reaction. The conversion of benzyl alcohol
◦
(
4
voltage 40 kV, current setting 20 mA, step size 0.02 , count time
s).
Thermogravimetric-differential scanning calorimetry (TG-DSC)
characterizations were taken on a NETZSCH STA 449C thermogravi-
◦
metric analyzer from room temperature to 1000 C with the rate of
0 C min under air atmosphere.
◦
−1
1
(Xbenzylalcohol) and selectivity of BA (SBA) were calculated according
H2 temperature-programmed reduction (H -TPR) measure-
2
to the following equations:
ments were performed on a Chembet PULSAR TPR/TPD (Quan-
tachrome Instruments U.S.). The sample (0.1 g) was loaded in a
U-shaped quartz reactor. Prior to the test, sample was pretreated
mole of benzyl alcohol reacted
mole of benzyl alcohol initially
Xbenzyl alcohol(mol%) =
× 100 mol%
◦
−1
at 500 C for 1 h in flowing He gas (40 mL min ) to remove any
moisture and other adsorbed impurities. After cooling the reactor
o-BA + p-BA
o-BA + p-BA + DBE
◦
−1
SBA(mol%) =
× 100 mol%
to 40 C, a 5% H -Ar gas (40 mL min ) mixture was introduced. The
catalyst was heated to 1000 C at a rate of 20 C min and the H2
gas consumption was measured using a TCD detector.
2
◦
◦
−1
Moreover, esterification of levulinic acid (LA) with 1-butanol
was performed and analyzed as aforementioned method. In a
typical run, 0.1 g of catalyst was added to a mixture (1-butanol
◦
The nitrogen adsorption and desorption isotherms at −196 C
were recorded on a Micromeritics ASAP 2020 static volumetric ana-
◦
◦
lyzer. Prior to the test, the sample was pretreated at 200 C for
(10 mL) + LA (1 mL)). The reaction was carried out at 120 C for 4 h.
2
h. The specific surface areas were calculated via the Brunauer-
Emmett-Teller (BET) method in the relative pressure range of
.05-0.3; the single-point pore volume was calculated from the
adsorption isotherm at a relative pressure of 0.990; pore size dis-
The conversion of LA and selectivity of products were detected
as above-mentioned GC instrument. The normal-butyl levulinate
(n-BL) and pseudo-butyl levulinate (p-BL) were detected and no
other products were observed in this reaction. The conversion of
0

1
2
Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
◦
Fig 1. X-ray diffraction patterns: (1) MoO3/M-ZrPO with differernt MoO3 loadings treated at 700 C: (a) 0 wt% MoO3/M-ZrPO, (b) 5 wt% MoO3/M-ZrPO, (c) 10 wt% MoO3/M-
◦
◦
◦
◦
ZrPO, (d) 15 wt% MoO3/M-ZrPO, (e) 20 wt% MoO3/M-ZrPO; (2) 10 wt% MoO3/M-ZrPO treated at different temperatures: (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C, (e)
◦
9
00 C.
LA (XLA) and selectivity of n-BL (Sn-BL) were calculated according to
the following equations:
mole of LA reacted
mole of LA initially
XLA(mol%) =
× 100 mol%
n-BL
Sn−BL(mol%) =
× 100 mol%
n-BL + p-BL
. Results and discussion
.1. XRD analysis
XRD characterization is taken to investigate the crystalline
3
3
structure of materials. The patterns of MoO /M-ZrPO solid acid
3
catalysts with different MoO3 loadings are depicted in Fig. 1(1).
The sample without introducing MoO species, only presented two
3
Fig. 2. TG-DSC curves of the as-synthesized 10 wt% MoO3/M-ZrPO.
broad peaks, showing the existence of amorphous structure with-
out crystalline structure in M-ZrPO support. Moreover, materials
with MoO3 loadings under 10 wt% exhibited similar profile to M-
ZrPO. The lack of exclusive crystalline MoO3 peaks showed that
introduced Mo species had a highly dispersed state on the M-ZrPO
◦
with this, amorphous structure could exist even treated at 800 C
for M-ZrPO support (Fig. S1(2)). This might be owing to that intro-
ducing Mo species began to form crystalline aggregations at high
MoO3 loadings and calcination temperature and induced transfor-
mation of amorphous structure to crystalline structure for M-ZrPO
support.
surface or the MoO clusters were too small to be confirmed by XRD
3
characterization in these samples. However, with MoO3 loadings
increasing to 15 and 20 wt%, observable diffraction peaks appeared,
implying that some of introduced Mo species assembled on the
M-ZrPO and turned into crystalline MoO3 (JCPDF No. 05-0508).
3.2. H -TPR analysis
2
In addition, with the formation of crystalline MoO , amorphous
3
structure of M-ZrPO was destroyed and transformed to crystalline
H -TPR patterns are taken to analyse existing states of intro-
2
Zr O(PO ) (JCPDF No. 37-0155) structure (Fig. S1(1)).
duced Mo species. As displayed in Fig. 3(1), the H -TPR profiles
2
4
3
2
XRD patterns of 10 wt% MoO /M-ZrPO calcined at different tem-
of MoO /M-ZrPO solid acid catalysts with different MoO3 loadings
3
3
peratures are provided in Fig. 1(2). The highly dispersed Mo species
exhibited different patterns. For the sample without introducing
Mo species, there exhibited no obvious peak in 300–1000 C, show-
◦
◦
existed in samples treating at 500, 600 and 700 C. Afterwards,
◦
further increasing calcination temperature to 800 and 900 C, Mo
ing no reducible species existed on M-ZrPO. With introducing of Mo
◦
species aggregated and crystalline structure appeared. The same
conclusion also could be confirmed by TG-DSC technology of as-
species, reduction peak could be observed at about 450 C in pat-
tern of 5 wt% MoO /M-ZrPO, which was attributed to the reduction
3
synthesized 10 wt% MoO /M-ZrPO. As displayed in Fig. 2, a weigh
loss and endothermic peak under 400 C was attributed to the
of dispersed polymolybdate species [47–49]. Further increasing the
3
◦
MoO loadings to 10 wt%, a new reduction peak, which was ascribed
3
decomposition of precursors. In addition, the absence of obvi-
ous weight loss at 500–700 C implied that precursors completely
to the reduction of crystalline MoO3 species, could be observed at
about 900 C [47]. However, no obvious diffraction peaks owing to
◦
◦
decomposed and formed MoO species. The small weight loss with
an exothermic peak at about 750 C was ascribed to the pore wall
crystalline MoO species were discovered in XRD pattern of 10 wt%
3
3
◦
MoO /M-ZrPO. This might be owing to that the size of crystalline
3
transformation from amorphous to crystalline structure. Compared
MoO3 species was too small to be confirmed by XRD technology.

Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
13
◦
Fig. 3. The H2-TPR profiles: (1) MoO3/M-ZrPO with differernt MoO3 loadings treated at 700 C: (a) 0 wt% MoO3/M-ZrPO, (b) 5 wt% MoO3/M-ZrPO, (c) 10 wt% MoO3/M-ZrPO,
◦
◦
◦
◦
◦
(
d) 15 wt% MoO3/M-ZrPO, (e) 20 wt% MoO3/M-ZrPO; (2) 10 wt% MoO3/M-ZrPO treated at different temperatures: (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C, (e) 900 C.
◦
Fig. 4. Isotherm (1) and pore size distributions (2) of MoO3/M-ZrPO with differernt MoO3 loadings treated at 700 C: (a) 0 wt% MoO3/M-ZrPO, (b) 5 wt% MoO3/M-ZrPO, (c)
1
0 wt% MoO3/M-ZrPO, (d) 15 wt% MoO3/M-ZrPO, (e) 20 wt% MoO3/M-ZrPO.
As for 15 wt% MoO /M-ZrPO and 20 wt% MoO /M-ZrPO samples,
isotherms. As shown in Fig. 4(1), samples with MoO3 loadings
under 10 wt%, presented typical IV type isotherm with H1 shaped
hysteresis loops observed in the P/P0 range from 0.4 to 0.7. With
3
3
◦
obvious peaks could be found at about 900 C, showing Mo species
mainly existed as crystalline MoO3 phase in samples, further con-
firming the XRD analysis, which displayed obvious diffraction peaks
for crystalline MoO3 species.
MoO loadings increasing to 15 and 20 wt%, the isotherms began to
3
deform and hysteresis loop changed to the type between H1 and H2,
indicating existence of cage-like mesopores or cylindrical meso-
pores with partial distortion. This might be owing to the formation
of crystalline MoO3 particles led to the transformation of amor-
phous skeleton to crystalline structure and the ordered mesoporous
structure of M-ZrPO was destoryed. In addition, as presented in
Fig. 4(2), the samples with MoO3 loadings under 10 wt% showed
obvious distribution around 5 nm, proving the successful preserva-
H -TPR profiles of 10 wt% MoO /M-ZrPO solid acid catalysts
2
3
with different calcination temperatures are displayed in Fig. 3(2).
Although these samples had same MoO3 loadings, they exhibited
entirely different profiles. Specifically, as for the materials treated
◦
◦
at 500 and 600 C, reduction peaks only appeared at 400–700 C,
showing the existence of dispersed polymolybdates in the sam-
ples. With the increasing of calcination temperatures, intensity
◦
of peaks at 400–700 C became weaker and vanished in 10 wt%
tion of ordered mesostructure. Moreover, for 15 wt% MoO /M-ZrPO
3
MoO /M-ZrPO-800 and 10 wt% MoO /M-ZrPO-900 samples. Mean-
while, the peaks at 800–950 C appeared, indicating the highly
and 20 wt% MoO /M-ZrPO samples, absence of obvious pore size
distributions indicated the disappearance of ordered mesoporous
structure in materials.
3
3
3
◦
dispersed polymolybdates transformed to crystalline MoO species
3
at high calcination temperature, further confirming the aforemen-
tioned XRD characterizations.
Nitrogen adsorption-desorption isotherms and pore size distri-
butions of 10 wt% MoO /M-ZrPO treated at different temperatures
3
are given in Fig. 5. For samples with calcination temperature below
◦
7
00 C, ordered mesostructure with uniform pore size distribution
3
.3. Nitrogen adsorption-desorption analysis
were observed. However, with the temperature increasing to 800
and 900 C, mesoporous structure vanished and no obvious pore
◦
The textural properties and Mo atom density of MoO /M-
3
ZrPO samples are investigated by nitrogen adsorption-desorption

1
4
Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
◦
◦
◦
◦
◦
Fig. 5. Isotherm (1) and pore size distributions (2) of 10 wt% MoO3/M-ZrPO treated at different temperatures: (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C, (e) 900 C.
Table 1
Textural properties and acidity of the MoO3/M-ZrPO samples.
Mo atom density (atom nm 2)a
−
Total acidity (mmol/g)b
Samples
Nitrogen physisorption
Specific Surface Area (m2
−1
Pore Volume (cm3
g )
−1
g
)
Pore size (nm)
0
5
1
1
2
1
1
1
1
wt% MoO3/M-ZrPO-700
wt% MoO3/M-ZrPO-700
0 wt% MoO3/M-ZrPO-700
5 wt% MoO3/M-ZrPO-700
0 wt% MoO3/M-ZrPO-700
0 wt% MoO3/M-ZrPO-500
0 wt% MoO3/M-ZrPO-600
0 wt% MoO3/M-ZrPO-800
0 wt% MoO3/M-ZrPO-900
121
94
82
42
16
120
111
28
5.61
5.18
6.62
8.45
13.4
5.01
5.81
15.3
–
0.19
0.18
0.15
0.13
0.09
0.17
0.17
0.09
0.05
0
0.103
0.127
0.148
0.071
0.035
0.155
0.144
0.014
0.008
2.21
5.11
14.8
51.5
3.48
3.77
15.0
51.6
8
a
2
The Mo surface densities expressed as the number of Mo atoms per nanometer square area (Mo atoms nm ) and were calculated using the equation: Surface density of
23
2
1
18
Mo = {[MoO3 loadings (wt%)/100] × 6.023 × 10 }/{[144.0 (formula weight of MoO3) × SBET (m g ) × 10 ]}.
b
The acidity gotten from NH3-TPD measurements.
size distributions were observed, implying the absence of ordered
mesostructure at high calcination temperature.
The parameters of textural properties, such as specific surface
area, pore size, pore volume and Mo atom density are given in
Table 1. It could be clearly observed that textural properties of the
obvious diffraction peaks in XRD patterns. With the increasing of
MoO3 loadings to 15 and 20 wt%, the mesostructure destroyed and
obvious particles were observed. In order to give further considera-
tion to the dispersion of introduced Mo species, elemental mapping
of 10 wt% MoO /M-ZrPO was taken and shown in Fig. 7. It could
3
samples sharply decreased with MoO loadings up to 15 wt% and
calcination temperature to 800 C. Combined with XRD analyse, it
be clearly observed that O, Zr, P and Mo elements were uniformly
dispersed throughout the skeleton of material, showing that Mo
species were introduced as designed and highly dispersed in the
framework of M-ZrPO.
3
◦
could be summarized that MoO3 species were highly dispersed
on the surface of M-ZrPO when the Mo atom density was under
−
2
5
.11 atom nm . Moreover, crystalline MoO3 began to form and
ordered mesostructure was destroyed when the Mo atom density
−
2
exceeded 14.8 atom nm . This conclusion also could be demon-
strated by the MoO /M-ZrPO with different MoO loadings and
3
.5. Acidic properties
3
3
◦
treated at 600 C (shown in Fig. S2, S3 and Table S1). When the MoO3
loadings of samples were below 15 wt%, Mo atom density was
under 6.45 atom nm and Mo species existed as highly dispersed
states. Further increasing the MoO3 loadings up to 20 and 25 wt%,
crystalline structure appeared and mesostructure was destroyed.
The acidic properties of materials are evaluated by NH -TPD
3
characterization. NH -TPD profiles of MoO /M-ZrPO solid acid
−2
3
3
catalysts with different MoO3 loadings are given in Fig. 8(1). In
these profiles, obvious peaks which caused by the desorption of
NH3 interacted with acid sites in the materials, could be clearly
observed, implying the existence of abundant acid sites in the
3.4. TEM analysis
MoO /M-ZrPO materials. In addition, the intensity of peak and
3
corresponding acidity (shown in Table 1) were enhanced with
the MoO3 loading increasing from 0 to 10 wt% and reached the
The following TEM characterizations are constructed for pur-
−
1
pose of illustrating the morphology of catalysts. As shown in
Fig. 6a,b, obvious ordered mesoporous structure could be clearly
observed with MoO3 loadings under 10 wt%, implying ordered
mesoporous structure existed and this further confirmed the char-
maximum (0.148 mmol g ) for 10 wt% MoO /M-ZrPO-700. This
3
indicated the acidic properties were successfully improved by the
introduced Mo species which existed as highly dispersed states.
However, further increasing the MoO3 loading to 15 and 20 wt%,
the intensity of peak decreased and the acidity decreased to 0.071
acterization of N -physisorption, which predicted the presence
2
−
1
of ordered mesostructure. Moreover, it was noteworthy that no
obvious particles were found in these two samples, which had no
and 0.035 mmol·g . This might be owing to the aggregation of Mo
species, which led to the decrease of acid sites in the materials.

Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
15
◦
Fig. 6. TEM images of MoO3/M-ZrPO with differernt MoO3 loadings treated at 700 C: (a) 5 wt% MoO3/M-ZrPO, (b) 10 wt% MoO3/M-ZrPO, (c) 15 wt% MoO3/M-ZrPO, (d) 20 wt%
MoO3/M-ZrPO.
◦
Fig. 7. TEM element mapping of 10 wt% MoO3/M-ZrPO treated at 700 C.
The NH -TPD patterns of 10 wt% MoO /M-ZrPO treated at dif-
The type of acid sites in the MoO /M-ZrPO solid acid catalysts
3
3
3
ferent calcination temperatures are displayed in Fig. 8(2). The
samples calcined under 700 C exhibited similar pattern, indicating
are investigated by the pyridine-IR spectra. As displayed in Fig. 9(1),
the bands at 1540 or 1450 cm , which were the exclusive bands for
◦
−1
the acidic properties had little change. Nevertheless, the inten-
sity of peak decreased obviously when the calcination temperature
Brønsted or Lewis acid sites [36,50,51], could be observed, imply-
ing both Brønsted and Lewis acid sites existed in the MoO /M-ZrPO
3
◦
reached 800 and 900 C and the acidity largely decreased to 0.014
solid acid catalysts. In addition, with increasing of MoO3 loading
from 0 to 10 wt%, the intensity of bands, especially for the bands at
−1
and 0.008 mmol·g . This might be ascribed to that the highly dis-
−
1
persed Mo species aggregated and formed large MoO3 particles.
1540 cm , was gradually improved, indicating the enhancement

1
6
Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
◦
Fig. 8. NH3-TPD profiles: (1) MoO3/M-ZrPO with differernt MoO3 loadings treated at 700 C: (a) 0 wt% MoO3/M-ZrPO, (b) 5 wt% MoO3/M-ZrPO, (c) 10 wt% MoO3/M-ZrPO, (d)
◦
◦
◦
◦
◦
1
5 wt% MoO3/M-ZrPO, (e) 20 wt% MoO3/M-ZrPO; (2) 10 wt% MoO3/M-ZrPO treated at different temperatures: (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C, (e) 900 C.
◦
Fig. 9. IR spectra for adsorbed pyridine: (1) MoO3/M-ZrPO with differernt MoO3 loadings treated at 700 C: (a) 0 wt% MoO3/M-ZrPO, (b) 5 wt% MoO3/M-ZrPO, (c) 10 wt%
◦
◦
◦
◦
MoO3/M-ZrPO, (d) 15 wt% MoO3/M-ZrPO, (e) 20 wt% MoO3/M-ZrPO; (2) 10 wt% MoO3/M-ZrPO treated at different temperatures: (a) 500 C, (b) 600 C, (c) 700 C, (d) 800 C,
◦
(
e) 900 C.
Scheme 1. Friedel-Crafts alkylation reaction between substrates and benzyl alcohol.
of acidic properties by the introduced Mo species. Further increas-
ing the MoO3 loading to 15 and 20 wt%, the intensity decreased
gradually. In addition, as given in Fig. 9(2), the patterns of 10 wt%
MoO /M-ZrPO catalysts. In this reaction system, benzylanisoles
(BA) was produced as principal product and dibenzyl ether (DBE)
is by-product of benzyl alcohol self-etherification [23,25]. The
3
MoO /M-ZrPO solid acid catalysts with calcination temperature
catalytic performance of MoO /M-ZrPO solid acid catalysts with
3
3
◦
under 700 C had little change, however, the intensity decreased
various MoO loadings is depicted in Fig. 10(1). Difference in MoO3
3
◦
drastically when the temperature reached 800 and 900 C. This kept
loadings led to a distinct difference in catalytic performance for
consistent with the conclusion gotten from the NH -TPD character-
alkylation reaction. The 0 wt% MoO /M-ZrPO, without introduction
3
3
ization.
of Mo species, only exhibited 5.02% conversion of benzyl alco-
hol and 60.2% selectivity of BA. However, with introduction of
MoO3 species, an obvious improvement in conversion, selectivity
and yield was observed and achieved optimal activity at 10 wt%
3.6. Catalytic performance
MoO /M-ZrPO with 100% conversion of benzyl alcohol and 91.3%
selectivity of BA. The gradual enhancement in catalytic perfor-
mance might be ascribed to the increasing of acid sites, especially
3
The MoO /M-ZrPO materials were taken as solid acid catalysts in
alkylation reactions (as shown in Scheme 1). In the first set of exper-
iment, alkylation of anisole with benzyl alcohol was performed over
3

Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
17
Fig. 10. Friedel-Crafts alkylation reaction between anisole and benzyl alcohol catalyzed by MoO3/M-ZrPO: (1) different MoO3 loadings and (2) different calcination temper-
◦
atures. (Reaction conditions: anisole 10 mL, benzyl alcohol 1 mL, catalysts 0.1 g, 150 C, 1 h).
for Brønsted acid sites, which produced by introduction of Mo
species in MoO /M-ZrPO. However, further increasing MoO3 load-
3
ings to 15 and 20 wt%, the catalytic performance began to decrease.
This might be owing to the appearance of crystalline structure and
decrease of mesoporous structure, therefore, the amounts of active
sites decreased.
The catalytic activity of MoO /M-ZrPO with different calcina-
3
◦
tion temperatures (500–900 C) in alkylation reaction was also
researched. As given in Fig. 10(2), slight improvement in conver-
sion and selectivity could be found with calcination temperature
◦
increased from 500 to 700 C. Nevertheless, further increasing cal-
◦
cination temperature to 800 and 900 C, a “free-fall” decrease in
catalytic performance was clearly observed and there only showed
conversion 13.2% and 1.21% for 10 wt% MoO /M-ZrPO calcined at
3
◦
8
00 and 900 C. As aforementioned characterizations about 10 wt%
MoO /M-ZrPO materials treated at different temperatures, ordered
3
mesostructure completely vanished and the Mo species in pore wall
of mesostructure began to get together and form crystal MoO . The
3
recession in specific surface area and aggregation of Mo species
Fig. 11. Friedel-Crafts alkylation reaction catalyzed by 10 wt% MoO /M-ZrPO at dif-
3
might lead to reduction in performance of 10 wt% MoO /M-ZrPO
after treated at 800 and 900 C. Based on the above discussion, fol-
ferent times. (Reaction conditions: anisole 10 mL, benzyl alcohol 1 mL, catalysts 0.1 g,
150 C).
3
◦
◦
lowing conclusion could be made that both high specific surface
area and high dispersion of Mo species were beneficial for improve-
ment of catalytic activity. This speculation could also be verified by
◦
X wt% MoO /M-ZrPO treated at 600 C. As shown in Fig. S4, catalytic
3
performance of catalysts was gradually improved from 0 to 15 wt%
Fig. 12. Isotherm (1) and pore size distributions (2) of used 10 wt% MoO3/M-ZrPO catalyst.

1
8
Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
Fig. 13. TEM element mapping of used 10 wt% MoO3/M-ZrPO catalyst.
Fig. 14. Friedel-Crafts alkylation reaction catalyzed by MoO3/M-ZrPO: (1) different amount of catalyst; (2) different reaction temperature. (Reaction conditions: anisole
0 mL, benzyl alcohol 1 mL, 1 h).
1
Fig. 15. Friedel-Crafts alkylation reaction catalyzed by MoO3/M-ZrPO: (1) reused for seven cycles; (2) poisoned by 2,6-DTBP. (Reaction conditions: anisole 10 mL, benzyl
◦
alcohol 1 mL, catalysts 0.1 g, 150 C, 1 h).
with Mo species existed as highly dispersed states. Compared with
ZrPO and 25 wt% MoO /M-ZrPO, which had crystalline structure
3
this, the activity of catalyst began to reduce for 20 wt% MoO /M-
and poor mesoporous properties.
3

Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
19
Table 2
Alkylation of different arenes and alkylating agents over 10 wt% MoO3/M-ZrPO-700 .
Entry
Arenes
Alkylating agents
Alkylation products
Con. (%)
100
Sel. (%)d
1a
41:50
42:58
43:57
49:51
36:64
2a
3a
4a
5a
99.1
82.0
97.3
100
6b
7c
75.2
99.6
33:28
69:28
8c
9c
100
86.7
90.2
18:82
57
1
1
1
0c
1c
2c
37:25
100
100
100
74:18
Reaction conditions.
a
◦
arene: 0.10 mol, alkylation agent: 0.01 mol, catalyst: 0.1 g, 150 C, 1 h.
arene: 0.10 mol, alkylation agent: 0.005 mol, catalyst: 0.1 g, 110 C, 2 h.
arene: 0.10 mol, alkylation agent: 0.005 mol, catalyst: 0.1 g, 150 C, 2 h.
b
c
◦
◦
d
The by-product was dibenzyl ether (DBE) in these reactions.
3
−1
The progress of alkylation and heterogeneity of 10 wt% MoO /M-
size (7.08 nm) and pore volume (0.13 cm g ) had little change,
3
ZrPO was determined by researching products at different intervals.
As shown in Fig. 11, conversion of benzyl alcohol increased with
time from 0 to 60 min and gotten 100% at 60 min for 10 wt%
implying that 10 wt% MoO /M-ZrPO catalyst suffered little change
3
in process of acid-catalyzed reaction. Moreover, as given in Fig. 13,
ordered mesoporous structure with highly dispersed Mo species
still maintained in used catalyst. Meanwhile, Mo content of used
catalyst was investigated by XRF characterization. Compared with
MoO /M-ZrPO. Furthermore, as an excellent solid acid catalyst,
3
heterogeneity was very important, and the leaching test should
be investigated. As displayed in Fig. 11, the 10 wt% MoO /M-ZrPO
fresh catalyst (10.1 wt%), MoO loadings (10.3 wt%) of used catalyst
3
3
catalysts were removed at 20 min from the reaction system. After
that, few variations were observed in conversion of benzyl alcohol
changed little, indicating the excellent stability of catalysts and no
obvious Mo species were leached into liquid phase in process of
reaction.
and yield of BA, indicating that 10 wt% MoO /M-ZrPO was a truly
3
heterogeneous catalyst and no obvious active components were
leached into liquid phase in the progress of alkylation.
Furthermore, the used catalyst was researched by N2-
physisorption and TEM techniques to study stability of 10 wt%
In this section, details on influence of catalyst amounts and
reaction temperatures on FC alkylation reaction were researched
and the results are given in Fig. 14. It could be clearly observed
that conversion of benzyl alcohol, selectivity and yield of BA were
improved gradually with enhancing of catalyst amounts from 0.025
to 0.1 g. The 100% conversion and 91.3% selectivity were achieved
when amount of catalyst reached 0.1 g. Therefore, the presence of
MoO /M-ZrPO. As shown in Fig. 12, H1 typed hysteresis loops and
3
intensive pore size distribution still maintained in the used 10 wt%
2
−1
MoO /M-ZrPO. Meanwhile, specific surface area (72 m g ), pore
3

2
0
Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
Fig. 16. Esterification of levulinic acid with 1-butanol catalyzed by MoO3/M-ZrPO: (1) different MoO3 loadings and (2) different calcination temperatures. (Reaction conditions:
◦
1
-butanol 10 mL, levulinic acid 1 mL, catalysts 0.1 g, 120 C, 4 h).
adequate acid sites was in favor of improving the catalytic per-
formance. Further increasing catalyst amount to 0.125 g, catalytic
activity changed little. Moreover, conversion of benzyl alcohol and
selectivity of BA gradually enhanced with increasing of reaction
Moreover, MoO /M-ZrPO solid acid was employed in esterifi-
cation of levulinic acid (LA) with 1-butanol (1-BuOH) (as shown
in Scheme 2). As shown in Fig. 16, the catalytic activity (conver-
3
sion of LA, selectivity and yield of n-BL) of MoO /M-ZrPO solid
3
◦
temperatures and reached maximum at 150 C.
acid was improved with enhancement of MoO3 loadings and cal-
For sake of checking the reutilization of catalyst, a seven
cination temperature and 10 wt% MoO /M-ZrPO solid acid catalyst
treated at 700 C exhibited optimal catalytic performance (90.2%
3
◦
runs test over 10 wt% MoO /M-ZrPO-700 was carried out under
3
aforementioned reaction conditions (shown in Fig. 15(1)). It was
noteworthy that no significant declines in catalytic performance
were observed and the yield of BA could reach 90% even after
seven cycles compared with fresh catalyst, indicating that 10 wt%
conversion of LA and 97.6% selectivity of n-BL). Further increasing
MoO3 loadings to 15 and 20 wt%, calcination temperature to 800
◦
and 900 C, catalytic performance began to decrease. This further
confirmed the above conclusion. The influence of reaction times,
catalyst amounts and reaction temperatures on esterification is dis-
played in Fig. S5 and S6 . The catalytic performance was gradually
improved and gotten 90.2% conversion of LA and 97.6% selectiv-
ity of n-BL when the reaction time, catalyst amount and reaction
MoO /M-ZrPO-700 had excellent reusability and could be reused
3
as solid acid catalyst. It might be owing to excellent stability of cat-
alysts and no active sites were leached into liquid phase in process
of reaction.
◦
To further investigate influence of acid sites in FC alkylation,
temperature reached 4 h, 0.1 g and 120 C.
catalytic activity of 10 wt% MoO /M-ZrPO poisoned by differ-
3
ent amounts of 2,6-Di-tert-butylpyridine (2,6-DTBP), which was
thought to mainly adsorb on the accessible Brønsted acid sites
in solid acid catalysts [52–55], have been studied. As shown in
Fig. 15(2), with addition of 2,6-DTBP increasing from 0 to 3 ␮L, con-
version of benzyl alcohol and yield of BA gradually decreased. This
indicated that FC alkylation of anisole with benzyl alcohol mainly
4. Conclusion
reacted on the Brønsted acid sites in 10 wt% MoO /M-ZrPO. More-
over, yield of DBE changed little with addition of 2,6-DTBP and this
might be owing to that auto-etherification of benzyl alcohol mainly
3
In this paper, a novel MoO /M-ZrPO solid acid catalysts with
3
different MoO3 loadings (0–20 wt%) and different calcination tem-
◦
peratures (500–900 C) have been synthesized and employed in
happened on the Lewis acid sites of MoO /M-ZrPO.
3
Friedel-Crafts alkylation and esterification. All the results indicated
that the highly dispersed polymolybdate species and excellent
mesostructure were in favor of improving the catalytic perfor-
The alkylation of different arenes (methylbenzene, ortho-
xylene, meta-xylene, para-xylene, ethylbenzene, mesitylene, 4-
methylanisole) and alkylating agents (benzyl chloride, benzyl
mance of catalysts. In addition, MoO /M-ZrPO catalysts showed
3
bromide and dibenzyl ether) over 10 wt% MoO /M-ZrPO is shown
3
excellent catalytic performance in Friedel-Crafts alkylation of dif-
ferent aromatic compounds and esterification of levulinic acid with
-butanol. Moreover, it was noteworthy that the catalyst had supe-
in Table 2. It was noteworthy to mention that 10 wt% MoO /M-
3
ZrPO exhibited excellent catalytic performance for alkylation with
different arenes and alkylating agents. This indicated that 10 wt%
1
rior reusability and there were no obvious declines in catalytic
MoO /M-ZrPO was an excellent solid acid catalyst for alkylation
3
performance even after seven runs. Therefore, MoO /M-ZrPO was
3
reaction.
a promise and environmentally benign solid acid catalyst.
Scheme 2. Esterification of levulinic acid with 1-butanol.

Z. Miao et al. / Molecular Catalysis 444 (2018) 10–21
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