Selective aerobic oxidation of glycerol over zirconium phosphate-supported vanadium catalyst

Article abstract of DOI:10.1016/j.mcat.2019.110404

The zirconium phosphate (ZrP)-supported vanadium catalysts have been prepared by the mechanochemical synthesis, and were characterized in detail using various techniques including XRD, SEM, HRTEM, EDX, NH₃-TPD, H₂-TPR, pyridine-absorbed FT-IR and solid ³¹P NMR spectra. XRD, SEM and HRTEM images revealed the vanadium species were highly dispersed on ZrP support. Solid ³¹P NMR technique indicated that there was a strong interaction between vanadium species and ZrP. The calcination temperature affected significantly the distribution for oxidation states of vanadium, and the presence of vanadium (V) species were more favorable to the glycerol oxidation reaction. The resulting catalysts exhibited high catalytic activity for glycerol oxidation and the selectivity to formic acid in glycerol oxidation by using molecular oxygen as the terminal oxidant under the aqueous phase and base-free conditions. Both acid sites and oxidizing sites played a synergistic role in producing the compounds in this work. The reaction can proceed smoothly with high glycerol conversion up to 80–90% and the selectivity towards formic acid to 62%, respectively. After reaction, the catalyst can be facilely recovered and reused with stable activity. On the basis of the above results, the reaction mechanism was proposed accordingly. This study provides a prominent catalytic process for the large scale production of formic acid from biorenewable glycerol.

Full text of DOI:10.1016/j.mcat.2019.110404

MolecularCatalysis474(2019)110404
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Selective aerobic oxidation of glycerol over zirconium phosphate-supported
vanadium catalyst
Difan Lia, Honghui Gonga, Lina Linb, Wenbao Maa, Qingqing Zhoua, Kang Konga, Rong Huangb,⁎
,
Zhenshan Houa,⁎
a Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and
Technology, Shanghai, 200237, China
b Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai, 200062, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Zirconium phosphate
Vanadium
Mechanochemical synthesis
Glycerol oxidation
Formic acid
The zirconium phosphate (ZrP)-supported vanadium catalysts have been prepared by the mechanochemical
synthesis, and were characterized in detail using various techniques including XRD, SEM, HRTEM, EDX, NH3-
TPD, H2-TPR, pyridine-absorbed FT-IR and solid 31P NMR spectra. XRD, SEM and HRTEM images revealed the
vanadium species were highly dispersed on ZrP support. Solid 31P NMR technique indicated that there was a
strong interaction between vanadium species and ZrP. The calcination temperature affected significantly the
distribution for oxidation states of vanadium, and the presence of vanadium (V) species were more favorable to
the glycerol oxidation reaction. The resulting catalysts exhibited high catalytic activity for glycerol oxidation and
the selectivity to formic acid in glycerol oxidation by using molecular oxygen as the terminal oxidant under the
aqueous phase and base-free conditions. Both acid sites and oxidizing sites played a synergistic role in producing
the compounds in this work. The reaction can proceed smoothly with high glycerol conversion up to 80–90%
and the selectivity towards formic acid to 62%, respectively. After reaction, the catalyst can be facilely recovered
and reused with stable activity. On the basis of the above results, the reaction mechanism was proposed ac-
cordingly. This study provides a prominent catalytic process for the large scale production of formic acid from
biorenewable glycerol.
1. Introduction
accompanied by increasing interest in the development of potential
valorization strategies of glycerol [5,6].
With the progressively depleting of fossil fuel, such as the tradi-
tional petroleum, coal and natural gas resources, a large number of
problems are brought forward [1]. The main disadvantage of fossil
resource is nonrenewable, implying significant negative side effects are
related to the sustainable development of the global economy and so-
ciety, as well as exacerbating environmental pollution. In this respect, it
is important to search alternative resources and develop new processes
for the production of fine chemicals and fuels [2]. In recent years, one
of the main renewable resources is biodiesel. Its main advantages lie in
significant potential as a renewable energy source, biodegradability and
less emission of air pollutants, which can be converted to more valuable
compounds [3]. During the production process of biodiesel, an abun-
dant and inexpensive glycerol is produced as a by-product by transes-
terification of fats and oils (triglycerides) with methanol, resulting in a
high surplus flooding the market over the last decades (10% of the plant
product) [4]. Therefore, the growing production of biodiesel is
Great efforts have been devoted on the catalytic conversion of gly-
cerol into platform molecules and subsequently high value added che-
micals by selective catalytic transformation owing to the fact that it is
regarded as a highly functionalized molecule with three hydroxyl
groups [7]. On one hand, because of its good water solubility and hy-
groscopicity, it can be widely used as a humectant in the food, medi-
cine, cosmetics and tobacco industries. On the other hand, in order to
substantially increase the demand and the price of crude glycerol and,
hence, make the biodiesel production more economically feasible, it is
necessary to investigate the transformation of glycerol into different
derivatives and intermediates by new technologies and catalytic routes,
such as hydrogenation, hydrolysis, oxidation, chlorination, etherifica-
tion, esterification, transesterification and reforming [8]. Among those
catalytic conversion processes, the selective oxidation of glycerol in
liquid phase has been widely studied during the last decades with
various catalytic systems involving precious metals (Au, Pt, Pd, Ag)
⁎ Corresponding authors.
E-mail addresses: rhuang@ee.ecnu.edu.cn (R. Huang), houzhenshan@ecust.edu.cn (Z. Hou).
https://doi.org/10.1016/j.mcat.2019.110404
Received 17 November 2018; Received in revised form 14 April 2019; Accepted 13 May 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

D. Li, et al.
MolecularCatalysis474(2019)110404
[9–13]. However, the use of noble metals presents economic challenges,
owing to high costs, scarce resources, and these metal particles are
easily agglomerated during the reaction and limit availability of large-
scale application and production. In addition, the transition metals (Cu,
Ni, Co, Fe) as catalysts are also explored [14–18], although the cost of
this type of catalyst is not high, the catalytic activity is normally poor.
These catalysts cannot be applied to practical production without fur-
ther complicated modification. Besides, many catalytic systems involve
the basic reaction medium. Due to its basicity, it produces a lot of or-
ganic acid salts and thus the mixtures after reaction need further neu-
tralization and acidification so as to obtain the target products.
Therefore, the exploration of new materials based on non-noble metal
and base-free highly active systems become the focus of the catalysis
field.
Formic acid (FA) is one important source of C1 raw materials in
high demand for the chemical, leather, textile, pharmaceutical, agri-
cultural industries. Particularly, FA is explored as a promising medium/
carrier for hydrogen storage and an energy-efficient alternative for the
sustainable hydrogen production based on reversible FA−CO2 inter-
conversion [19]. A diverse range of highly attractive products and
building blocks can be derived from FA as well [20]. FA salts are also
widely used for environmentally friendly runway de-icing and fuel in
fuel cells [21]. Therefore, FA as a versatile renewable reagent for green
and sustainable chemical synthesis has attracted substantial research
interest in recent years. Industrial production technologies of FA gains
mainly from fossil resources, which have an unfavorable impact on the
environment [22]. Up to now, FA production from biomass has been
demonstrated to be a promising process [23]. Developing alternative
routes to produce FA directly from glycerol is desirable from both
economic and ecological perspectives.
The various vanadium-based catalysts have been reported to cata-
lyze the selective oxidation of alkanes, alkenes, arenes, alcohols, alde-
hydes, ketones, and sulfur species, as well as oxidative C − C or C − O
bond cleavage, C − C bond formation, deoxydehydration, hydrogena-
tion, dehydrogenation and polymerization [24]. Especially, the vana-
dium-based catalysts were highly active for the oxidations of biomass-
derived carbohydrates [22,25–28] and saccharides [29,30]. Tre-
mendous efforts have also been devoted to the selective oxidation of
glycerol to FA with different oxidizing agents. In our previous report,
the silica-encapsulated heteropolyacid (H4PMo11VO40) can convert
glycerol to FA selectively, but the process needs the aid of extra ad-
ditives and also an excess of H2O2 as oxidant [31]. The vanadium-
substituted phosphomolybdic acids could exhibit exceptionally high
conversion efficiency in highly concentrated aqueous solutions with
molecular oxygen as oxidant, but the isolation of water-soluble catalyst
is difficult after reaction [32].
Zirconium phosphate (ZrP) as a family of transition metal phosphate
materials, increasingly attracts research interest due to its outstanding
physical and chemical properties, including an extremely high ion-ex-
change capability and excellent thermal stability. These superior
properties, along with simple preparation and easy functionalization,
making ZrP-based materials as promising candidates for a wide range of
applications [33,34]. In this work, novel vanadium-based ZrPs are
prepared without using any solvents by mechanochemical transforma-
tion. The ZrP can be found to act as an efficient support to immobilize
vanadium source, wherein the surface P(OH) groups provide strong
interaction toward vanadium species. Owing to such strong interaction,
the obtained ZrP-supported vanadium catalysts show high activity and
good reusability for the selective oxidation of glycerol to FA using
molecular oxygen as the oxidant. Systematic activity tests and catalyst
characterizations have been applied to identify the roles of ZrP and
vanadium sites in FA formation in detail. Finally, the relationship of
structure with catalytic activity is presented herein and possible reac-
tion mechanism is proposed as well.
2. Experimental
2.1. Materials
All chemicals and solvents were commercially available and used as
received without further purification. Zirconium oxychloride
(ZrOCl2·8H2O, 98 wt.%), hydroxyacetone (HA, 95 wt.%) and dihy-
droxyacetone (DHA, 99 wt.%) were provided by Macklin. Vanadium
(IV) oxide sulfate (VOSO4, AR) was acquired from Meryer. Vanadyl
acetylacetonate (VO(acac)2), ammonium vanadate (NH4VO3) and so-
dium metavanadate (NaVO3) were all analytical reagent grade and
purchased from Aladdin. Ammonium dihydrogen phosphate
(NH4H2PO4) and glycerol, were also analytical reagent grade and sup-
plied by Sinopharm. Formic acid (FA, 98 wt.%), ethylene glycol (EG,
98 wt.%), acetic acid (AA, AR) and concentrated sulphuric acid (H2SO4,
98 wt.%) were all obtained from Lingfeng. High purity O2 (99.9%). was
supplied by Shangnong Gas Factory.
2.2. Catalyst preparation
2.2.1. Preparation of amorphous ZrP
The amorphous ZrP was prepared by a precipitation method ac-
cording to the previous procedure [35]. Briefly, an aqueous solution of
NH4H2PO4 (1.0 mol L−1, 64 mL) was added dropwise to an aqueous
solution of ZrOCl2·8H2O (1.0 mol L−1, 32 mL) at a molar ratio of P/
Zr = 2. The mixture was stirred overnight at room temperature, then
filtered, and washed with copious deionized water until the pH of the
filtrate reached to neutral and free of Cl−, which was detected by
aqueous AgNO3 solution. The resulting material was dried at 100 °C for
12 h, followed by calcination at 400 °C for 4 h in a muffle furnace and
the solid powder was denoted as ZrP.
2.2.2. Preparation of catalysts
The ZrP-supported vanadium catalysts can be achieved by me-
chanochemical synthesis. As a typical example, 0.5 g ZrP and a certain
amount of VOSO4 were mixed in agate mortar, and then continuously
grinded for 30 min. The resulting material was calcinated at 550 °C for
3 h in a muffle furnace, unless indicated otherwise. The obtained
sample was named as xV/ZrP-m, x stands for the loading contents of
vanadium on ZrP support. According to a similar method, NH4VO3,
NaVO3 and VO(acac)2 as vanadium precursors afforded the corre-
sponding catalysts 2 V1/ZrP-m, 2 V2/ZrP-m, 2 V3/ZrP-m, respectively.
No solvents were employed in the course of mechanochemical synth-
esis. For the sake of comparison, the ZrP-supported vanadium catalysts
with the same contents were prepared by the incipient wetness method,
and the resulting material was designated as 2 V/ZrP-i arisen from
VOSO4 as vanadium precursors.
2.3. Characterization
Powder X-ray diffraction (XRD) patterns were collected on a
SmartLab diffractometer from Rigaku equipped with a 9 kW rotating
anode Cu source at 45 kV and 100 mA (5–80°, 0.2° s−1). Scanning
electron microscope (SEM) accompanied by energy dispersive X-ray
spectrometry (EDX; accelerated voltage: 20 kV) was used to study the
morphology and the elements distribution (JEOL JSM-6360LV, Japan).
High resolution transmission electron microscopy (HRTEM) was per-
formed in a JEOL JEM 2010 transmission electron microscope oper-
ating at 200 kV with a nominal resolution of 0.25 nm. The samples for
HRTEM were prepared by dropping the aqueous solutions containing
the NPs onto the carbon-coated Cu grids. Nitrogen (N2) adsorption
isotherms and pore size distribution curves were measured at −196 °C
on a BELSORP-MINI analyzer. The samples were degassed at 200 °C for
1 h to a vacuum of 10−3 Torr before analysis, The BET surface area and
pore size distribution of the samples were calculated using the BET
(Brunauer–Emmett–Teller)
equation
and
the
BJH
2

D. Li, et al.
MolecularCatalysis474(2019)110404
(Barrett–Joyner–Halenda) model, respectively. A Perkin Elmer Pyris
Diamond was used for the TGA measurements. The samples were he-
ated from RT to 800 °C (heating rate: 10 °C min−1) under the flow of
anhydrous air (flow rate: 20 mL min−1). NH3-Temperature pro-
grammed desorption (TPD) results were recorded using a BELCAT-B
temperature programming unit equipped with a thermal conductivity
detector (TCD). The forming CO2 during the reaction was detected by
gas chromatography (GC) with TCD. As a typical run, 0.1 g of catalyst
was placed in a U-shaped quartz cell and preconditioned at 200 °C in He
for 2 h, followed by cooling down to 40 °C in He flow. Then, the catalyst
was saturated with ammonia (5% NH3 balanced with He) at 40 °C for
45 min. Afterwards, the sample was exposed to He for removing of the
physically adsorbed ammonia on the surface of the sample.
Subsequently, the TPD profile was recorded upon heating the sample at
a rate of 10 °C min−1 up to 800 °C. H2-Temperature programmed re-
duction (TPR) was carried out using the glass flow system. TPR runs
were performed in flowing 10% H2/Ar (30 cm3 min−1), ramping the
temperature at 10 °C min−1 and using a Gow-Mac TCD. Solid 13C NMR
spectra were recorded on a Bruker AVANCE-III spectrometer in a
magnetic field strength of 9.4 T corresponding to the Larmor frequency
of 500 MHz for 13C nuclei with a CP/MAS unit at room temperature
(spinning rate: 5 kHz; contact time: 2 ms; pulse width: 2 ms; spectral
width: 42.6 kHz; acquisition time: 48.1 ms; 700 scans for each spec-
trum). The 13C chemical shifts were referred by using tetramethylsilane
as internal standard. Solid 31P MAS NMR spectra were obtained with a
VNMRS400WB spectrometer (162 MHz for 31P nuclei) equipped with a
standard 4 mm MAS NMR probe head. The 31P chemical shifts were
referred to NH4H2PO4. The surface chemical composition was de-
termined by X-ray photoelectron spectroscopy (XPS) on ESCALAB
250Xi equipped with Al Kα radiation (1486.6 eV). Metal contents were
detected by an Agilent ICP-725 inductively coupling plasma atomic
emission spectrometer (ICP-AES). The sample was put in a plastic
beaker mixed with a certain amount of aqua regia and HF at 120 °C for
4 h to dissolve the sample easily, followed by diluted with deionized
water. The FT-IR spectra of the pyridine-adsorbed on catalyst was ob-
tained in the transmission mode using a Nicolet Model 710 spectro-
meter. First, the catalyst was ground into the powder and pressed into a
very thin self-supporting wafer. The disc was mounted in a quartz IR
cell equipped with a CaF2 window. Then the catalyst was pressed into a
self-supporting disk and placed in an IR cell attached to a closed glass
circulation system. The catalyst disk was dehydrated by heating at
400 °C under vacuum in order to remove physisorbed moisture. The IR
spectrum background was recorded at room temperature when the cell
cooled down. Pyridine vapor was then introduced into the cell at room
temperature until equilibrium was reached. Subsequent evacuations
were performed at room temperature for 60 min followed by spectral
acquisitions at room temperature using the background recorded be-
fore. The quantification of acid sites was performed using the same
expressions (1) and (2) as those in the research article of Angela S.
Rocha:
2.4. Catalytic reaction
Glycerol oxidation was performed in a high-pressure batch auto-
clave of stainless steel with a 25 mL polytetrafluoroethylene inlet. The
autoclave was equipped with gas supply system and a magnetic stirrer.
Typically, a certain amount of catalyst was suspended in 5 mL aqueous
glycerol (10 wt.%) and the autoclave was purged with oxygen for three
times. Then it was sealed with the required oxygen pressure and heated
to the set temperature in 30 min. The zero time was taken when the
medium in the reactor was heated to the desired temperature. After the
reaction was finished, the reactor was quenched in an ice-water bath to
stop the reaction and the reaction mixture was diluted with deionized
water then filtered with a 0.22 mm membrane filter before analysis. The
liquid samples were analyzed by high performance liquid chromato-
graphy (HPLC) using equipped with a refractive index detector in series
with Bio-Rad Aminex HPX-87H column together with a guard cartridge
was employed for product separation. The column oven temperature
was 50 °C, the mobile phase was diluted with a concentration of 5 mM
H2SO4 aqueous solution and 0.5 mL min−1 flow rate, 20 μL of each
sample was injected and peaks were detected with refractive index
detector. The gas phase from the high-pressure batch autoclave was
collected and analyzed quantitatively with TCD. All values determined
for selectivity and carbon mass balance were provided by HPLC ana-
lysis. The conversion of glycerol and yield towards products were cal-
culated as follows:
amount of glycerol converted (mole)
Conversion (%) =
Selectivity (%) =
× 100%
total amount of glycerol (mole)
amount of a product (mole)
amount of glycerol converted (mole)
number of carbon atoms in the product
3
×
× 100%
carbon atoms found in the products (mole)
carbon atoms of glycerol converted (mole)
Carbon mass balance (%) =
× 100%
3. Results and discussion
3.1. Catalyst characterization
The phase structures of the pristine VOSO4, ZrP-supported vana-
dium catalysts are determined by the XRD patterns. As illustrated in
π
r2
ω
⎛
⎝
⎞
⎠
CB = kBA1540
CL = kLA1450
=
=
A1540
IMECB
(1)
π
r2
ω
⎛
⎝
⎞
⎠
A1450
IMECL
(2)
CL and CB are the concentrations of Lewis acid and Brönsted acid
sites in μmol g−1; A1450 and A1540 are the integrated areas of bands at
1450 and 1540 cm−1 in the original data of FTIR spectra, as shown in
the section of Results and discussion. KL and KB are molar extinction
constants for Lewis and Brönsted acid sites; IMECL and IMECB are in-
tegration molar extinction coefficients, 2.22 and 1.67 cm μmol−1 for
Lewis and Brönsted acids, respectively; r is the wafer radius in cm and ω
is the wafer weight in g of the self-supporting catalyst disk.
Fig. 1. XRD patterns of (a) VOSO4 (b) ZrP, (c) 2 V/ZrP-m, (d) 2 V/ZrP-i and (e)
the spent 2 V/ZrP-m.
3

D. Li, et al.
MolecularCatalysis474(2019)110404
Fig. 2. SEM images of (a) ZrP, (b) 2 V/ZrP-m and (c) the spent 2 V/ZrP-m catalyst.
Fig. 1a, neat VOSO4 displays a set of well resolved sharp diffraction
peaks for the typical crystal structure of VOSO4 (JCPDS No. 019–1411)
[36]. The catalysts including ZrP, 2 V/ZrP-m and 2 V/ZrP-i mainly show
two broad peaks in the ranges of 10–40° and 40–70°, indicating that
they are all amorphous nature and the introduction of vanadium by
either mechanochemical synthesis or incipient wetness method actually
has no effect on the crystal structure of ZrP (Fig. 1b–d). In addition,
either increasing vanadium contents (from 1% to 10%), or using the
different vanadium precursors have no effect on the amorphous phase
of ZrP (Figure S1 and S2). Notably, the diffraction peaks from vanadium
species are almost not observed in all samples, indicating that vana-
dium species are highly dispersed on the surface of ZrP even if vana-
dium contents on ZrP are loaded as high as 10%.
which gain insight into the acidic properties of the catalysts. As dis-
played in Fig. 5a, in the case of NH3-TPD profile of ZrP, three deso-
rption peaks are observed in a wide range of temperature from 50 to
700 °C. The peak at temperatures less than 200 °C is attributed to the
interaction of the NH3 with weak acid sites. The shoulder peak observes
at temperatures between 200 and 500 °C is attributed to medium strong
acid sites, while the high-temperature desorption peak observe at
temperatures greater than 500 °C is attributed to strong acid sites
[39,40]. The total acidity from the NH3-TPD patterns for ZrP, 2 V/ZrP-
m, 2 V/ZrP-i are found to be 1013.9, 995.3 and 1006.5 μmol g−1 re-
spectively (Table 1). The NH3-TPD patterns of both vanadium-modified
ZrP catalysts show two broad desorption peaks. One broad peak centers
at 150 °C, and the other around 350 °C represent the weak and medium
acid sites, respectively. The latter peak becomes broader and weak
dramatically, as compared with the pristine ZrP support, which is
suggestive of decreased acidity. Furthermore, the strong acid sites al-
most disappear after the introduction of vanadium species, suggesting
the strong interaction may occur between vanadium precursor and ZrP.
Overall, the introduction of vanadium species not only influence the
acidic distribution but also decrease the total surface acidity of ZrP.
The nature of acidity is further characterized using pyridine-ad-
sorbed FT-IR spectroscopy which has been recorded in the range of
1800–1400 cm−1. As given in Fig. 5b, pyridine-adsorbed FT-IR spec-
trum of the ZrP shows a band at 1545 cm−1, which is the characteristic
band of the typical pyridinium ion (PyH+), confirming the presence of
Brönsted acid sites, which can be due to the presence of a reasonable
amount of P(OH) groups in the ZrP [41]. Further, the band at about
1438 cm−1 corresponds to the adsorbed pyridine at the Lewis acid site
(PyL), while the band at 1488 cm−1 can be attributed to the adsorption
of pyridine in both Brönsted and Lewis acid sites [42]. The quantifi-
cation of acid sites is deduced by using Eqs. (1) and (2), and the results
are shown in Table 1. The density of Brönsted acid sites and Lewis acid
Fig. 2 shows the morphology of ZrP-derived catalysts revealed by
the SEM images. Pristine ZrP reveals its amorphous nanostructure are
arranged almost irregular morphology in large domains with rough
surface (Fig. 2a). When the vanadium is loaded on ZrP, the amorphous
nanostructure seem to become thicker (Fig. 2b), indicating the effect of
vanadium introduction. HRTEM is also employed to reveal the mor-
phology of ZrP-derived catalysts (Fig. 3). It is observed that the ZrP
support actually consists of some extremely thin nanosheets
(50–100 nm), which are stacked together with no special regular
structure (Fig. 3a). As presented in Fig. 3b, the P, Zr, V are distributed
uniformly on the surface of 2 V/ZrP-m nanosheets as reflected by ele-
mental mapping analysis (Fig. 3e). Moreover, the P, Zr, V on 2 V-ZrP-i
catalyst are also observed clearly in spite of slight agglomeration
(Fig. 3c and f), as compared with that of 2 V-ZrP-m. Notably, the highly
dispersed oligomeric vanadium species are hardly discernable from
TEM images (Fig. 3b and c). Moreover, the vanadium species cannot be
observed from XRD likely owing to the small size of vanadium species
(< 4 nm), which is beyond the detection limit of XRD techniques [37].
Next, the textural properties of catalysts are determined by N2 ad-
sorption-desorption analysis. The isotherms and pore size distribution
curves are displayed in Fig. 4 and corresponding Brunauer-Emmett-
Teller (BET) surface area, average pore size and total pore volume re-
sults are summarized in Table 1. All of the samples possess a typical
type IV isotherm with a clear H1-type hysteresis loop in the relative
pressure P/P0 ranging from 0.8 to 0.9, index of the formation of large
amounts of mesoporous structure. The pristine ZrP sample exhibits the
sites of ZrP are determined as 217.9 μmol g−1 and 717.9 μmol g−1
,
respectively. Particularly, it is found that after the modification by
vanadium precursor, the densities of both Lewis acid and the Brönsted
acid on the catalysts are reduced. Especially, the density of Brönsted
acid sites on the 2 V/ZrP-m catalyst decreases dramatically from
217.9 μmol g−1 to 169.2 μmol g−1, accompanying with a decrease of
the total acid density (Table 1). In addition, it indicates that as the
calcination temperature increasing, both the density of Brönsted (at
∼1545 cm−1) and Lewis (∼1438 cm−1) acid sites show an obvious
decrease on the vanadium-modified ZrP samples (Figure S3). Notably,
Brönsted acid sites decrease significantly after calcination at 650 °C,
which accounts for the strong interaction between vanadium species
and (PeOH) (Brönsted acid sites), most likely due to the formation of
VeOeP linkages [37].
The redox properties of the catalysts are crucial for the catalytic
oxidation performance. As seen in Fig. 5c, the amorphous ZrP is silent
because of no reducible sites. In contrast, 2 V/ZrP-m and 2 V/ZrP-i
present only one peak at ca. 586 °C and 555 °C respectively, which are
attributable to the one-step reduction of the monomeric and/or low
oligomeric vanadium species [43]. It is noticeable that the vanadium
species on 2 V/ZrP-m are more difficult to be reduced than that on 2 V/
highest BET surface area (314 m2 g−1) pore volume (0.57 cm3 −1) and
g
pore size (7.2 nm), which reduced obviously due to the addition of
vanadium species during the post-treatment (Table 1). Compared to
ZrP, the pore diameters of 2 V/ZrP-m and 2 V/ZrP-i decrease to 6.9 and
7.1 nm, respectively. The decrease of pore size can be ascribed to the
partial coverage of the larger pores by the highly dispersed vanadium
species [38]. On the basis of the analysis above, it suggests that the
vanadium modified ZrP by mechanochemical synthesis owns more
opening pore than that by the impregnation method, which is in
agreement with that of morphology features as seen from the image of
HRTEM.
The density and strength of the acid sites (Brönsted and Lewis acid)
have a crucial effect on catalytic activity. Therefore, the acidic prop-
erties of the catalysts are investigated firstly by NH3-TPD method,
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D. Li, et al.
MolecularCatalysis474(2019)110404
Fig. 3. HRTEM images of (a) ZrP, (b) 2 V/ZrP-m, (c) 2 V/ZrP-i and (d) the spent 2 V/ZrP-m. (e), (f) and (g) are corresponding to the P/Zr/V elemental mappings of
2 V/ZrP-m, 2 V/ZrP-i and the spent 2 V/ZrP-m catalysts, respectively.
Fig. 4. N2 adsorption isotherms (left) and pore size distribution curves (right) of (a) ZrP, (b) 2 V/ZrP-m and (c) 2 V/ZrP-i.
5

D. Li, et al.
MolecularCatalysis474(2019)110404
m and 2 V/ZrP-i catalysts enable supported vanadium species exist
mainly in its high valence state (V).
Table 1
Physicochemical properties of different catalysts.
Subsequently, the coordination state of the phosphorous center is
evaluated by solid 31P MAS NMR spectra. As seen from Fig. 7a, the
spectra of the amorphous ZrP shows the three resolved resonance peaks
at –11.8 ppm, –19.9 ppm and –26.7 ppm, which can be corresponded to
the presence of the tetrahedral phosphates connected with two
[(OH)2P–(OZr)2] zirconium atom, [(OH)P–(OZr)3] and phosphate with
four P–O–Zr bonds, respectively [47,48]. The three resolved resonance
peaks at –11.8 ppm, –19.9 ppm and –28.3 ppm are observed if ZrP was
calcined under a higher temperature (550 °C) (Fig. 7b). This indicated
the further dehydration process from a partial condensation between
–POH and –ZrOH groups could be possible as reflected from a re-
sonance signal shift from –26.9 to –28.3 ppm (Fig. 7a vs b).
On one hand, it should be noted that the peak of P–O–Zr linkage
shows upfield shifts from –28.3 ppm to –29.6 ppm after vanadium
modification (Fig. 7c and d). This can be explained that introduction of
vanadium increases the electron density around phosphorus core of ZrP
bulk, resulting in the more shielding effect. On the other hand, the
peaks at –11.8 ppm and –19.9 ppm become very weak (Fig. 7c and d),
reflecting that the introduction of vanadium changes the environment
of the phosphate group. Hence, it is highly believable that vanadium
modification has a significant effect on the linkage of chemical bond in
(OH)mP–(OZr)n (m + n = 4). With regard to the results above, a
covalent linkage such as ((VeOeP)m–(OZr)n) has formed (Inset in
Fig. 7), which is in accordance with pyridine adsorbed FT-IR spectra
that the density of Brönsted acid sites (PeOH) decreased due to vana-
dium modification, and the presence of the dominant vanadium (V)
species from XPS analysis.
Properties
Catalysts
ZrP
2 V/ZrP-m
2 V/ZrP-i
SBET (m2 g−1
)
)
314
0.57
7.2
1013.9
217.9
717.9
3.29
175
0.36
6.9
995.3
169.2
638.1
3.77
807.3
143
0.25
7.1
1006.5
181.7
671.9
3.69
a
b
VP (cm3 g−1
dAV (nm)c
Total acidity (μmol NH3 g−1
)
B acid (μmol g−1
)
)
d
L acid (μmol g−1
e
L/B
Total acidity (μmol pyridine g−1
)
935.8
853.6
a
BET surface area.
Total pore volume.
b
c
Average pore size distribution estimated from BJH method.
B acid = Brönsted acid.
L acid = Lewis acid.
d
e
ZrP-i, which reveals that less reducible vanadium species are formed
during the reduction process [44]. This may further reflect that the
chemical interaction between vanadium species and ZrP support exhibit
a great difference on two catalysts.
The surface state of vanadium catalysts is further characterized by
XPS. The full survey spectra display similar element species of P, Zr, O
and V (Fig. 6a). The XPS spectra of two catalysts present homologous
peaks and the observed P 2p and Zr 3d signals centered at 133.8, and
183.3 eV confirm the presence of ZrP structure. In addition, the V 2p3/2
peaks centered at 517.0–517.3 and 516–516.5 eV, are characteristic of
V5+ and V4+ species [45,46]. As given in Fig. 6b, 2p3/2 XPS peak for V
at 517.3 eV is observed clearly, which indicates that the vanadium is
mainly present in the vanadium (V) state with a satellite peak 2p1/2 at
about 254.2 eV. However, a weak peak at 516.4 eV (2p3/2) reveals the
presence of a tiny amount of the vanadium (IV) species. The existence
of this vanadium (IV) phase is possibly due to a residue from the va-
nadium source (VOSO4) and not completely oxidized to vanadium (V)
species. The above results additionally prove that the present 2 V/ZrP-
3.2. Glycerol oxidation in aqueous phase
The vanadium-modified ZrP catalysts are evaluated for the selective
oxidation of glycerol to FA in the aqueous phase under base-free con-
dition. The catalytic activity are obtained in terms of glycerol conver-
sion and product selectivity using molecular oxygen as an oxidant. The
Fig. 5. (a) NH3-TPD profiles, (b) Pyridine-adsorbed FT-IR spectra and (c) H2-TPR curves of the different catalysts.
6

D. Li, et al.
MolecularCatalysis474(2019)110404
Fig. 6. (a) Survey and (b) V 2p XPS spectra of the vanadium-modified ZrP catalysts.
catalytic reaction results are listed in Table 2. The reaction does not
proceed in the absence of a catalyst or under nitrogen atmosphere,
showing an aerobic processes is required for this catalytic system (en-
tries 1 and 6). Control reaction manifests that the vanadium precursor
itself (VOSO4) is active for glycerol oxidation and a homogeneous
nature is observed visually during the reaction (entry 2). However, in
addition to the intrinsic disadvantages of homogeneous catalyst in
catalyst recovery and product purification, the homogeneous vanadium
compound is unstable and easily hydrolyzes by the water under the
present condition [49]. Although the sole ZrP gives low glycerol con-
version due to the lack of oxidative sites (entry 3), FA, AA and a trace of
HA indeed are detected with 13.8% 11.3% and 1.6% of selectivity,
respectively. Notably, the vanadium-modified ZrP catalysts show
moderate to good catalytic activity for glycerol oxidation to FA (entries
4, 5, 7–9), which reveals that both acidic sites and oxidizing sites are
important for glycerol selective oxidation to FA. Among of them, 2 V/
ZrP-m catalyst shows the highest catalytic activity with 85.6% con-
version and 62.5% selectivity to FA (entry 4), suggesting that the
modification of vanadium species by mechanochemical synthesis pro-
vides more efficient approach for glycerol oxidation, as compared with
that using incipient wetness method (entries 4 vs 5). This could result
from the higher molar ratio of L to B on 2 V/ZrP-m catalyst (Table 1),
which implies that Lewis acid sites play a more crucial role in activating
primary hydroxyl group of glycerol over the present catalysts [50].
Besides, the different vanadium precursors introduced by mechan-
ochemical synthesis seem not to influence the catalytic activity and FA
selectivity significantly (entries 7–9), implying that chemical state of
vanadium on ZrP support can be similar after calcination in spite of the
different vanadium sources. It was found that a certain amount of CO2
derived from the overoxidation was detected (Table 2). Notably, the
vanadium-modified ZrP catalysts seemed to suppress effectively the
formation of CO2, as compared with that of a sole VOSO4 (entries 2, 4
and 5). Besides, as shown in Table 2, although carbon mass balance of
the glycerol oxidation was normally higher than 92%, the formation of
a small amount of polymer or coking under the present reaction con-
dition could not be excluded [51,52]. After reaction, the spent 2 V/ZrP-
m catalyst was subjected to TGA analysis (Figure S5). It can be seen that
the spent 2 V/ZrP-m showed a weight loss (10.8%), which actually was
close to that of the fresh one (9.2%). This revealed that the coking
might be not a very serious problem on the present catalyst.
Fig. 7. Solid 31P NMR spectra of (a) ZrP, (b) ZrP was calcined at 550 °C, (c) 2 V/
ZrP-m and (d) 2 V/ZrP-i. Inset showed the V–O–P linkage as reflected from 31P
NMR spectra and XPS analysis.
Table 2
Catalytic performance of various catalysts for selective oxidation of glycerol.a.
Entry Catalysts
Con. (%) Sel. (%)
FA Yield
(%)
CMB (%)
FA
AA
HA CO2
1
2
3
4
5
6b
7
8
9
None
VOSO4
ZrP
2 V/ZrP-m
2 V/ZrP-i
2 V/ZrP-m
0
89
5.1
85.6
72.4
0
0
55
0
0
0
1
0
0
100
29.5 49
0.7
18.8 53.5
1.5 14.6 48.7
92.5
98.3
95.3
97.6
100
92.5
94.9
95.8
13.8 11.3 1.6
62.5 6.2
67.2 4.6
0
1
0
0
0
0
0
2 V1/ZrP-mc 81.4
2 V2/ZrP-md 81.3
2 V3/ZrP-me 80.7
57.5 1.1
60
65.6 3.9
1.2 23.2 46.8
1.1 21.1 48.8
1.3 17.3 52.9
5
The reaction conditions are optimized by systematically exploring
the influence of various reaction parameters, including reaction tem-
perature, time, pressure, vanadium loadings on ZrP support and the
reaction results are shown in Fig. 8. The reaction is normally performed
over 2 V/ZrP-m catalysts due to its high catalyst performance as de-
monstrated Table 2. It can be seen that as the reaction temperature
varies from 140 to 180 °C, the conversion of glycerol and the FA se-
lectivity increases significantly, accompanying with the formation of a
a
Reaction conditions: 5 mL glycerol aqueous solution (10 wt.%), 25 mg cata-
lyst, 170 °C, 4 h, 3 MPa O2. FA: formic acid, AA: acetic acid, HA: hydro-
xyacetone. CMB: Carbon mass balance; b 1 MPa N2; c–e NH4VO3, NaVO3 and VO
(acac)2 have been adopted as vanadium precursors for catalyst preparation,
respectively.
7

D. Li, et al.
MolecularCatalysis474(2019)110404
Fig. 8. Influence of (a) reaction temperature, (b) reaction time, (c) pressure and (d) vanadium loading on catalytic activity for selective oxidation of glycerol over
2 V/ZrP-m catalyst. Reaction conditions: 5 mL glycerol aqueous solution (10 wt.%), 25 mg catalyst, 170 °C, 4 h, 3 MPa O2.
trace of AA and HA (Fig. 8a). However, the FA selectivity affords an
optimum around 150 °C, and then decreases slightly with higher tem-
perature, revealing that FA cannot be stable and decompose into CO2 at
high reaction temperature. The current oxidation reaction actually
happens quite quickly at 170 °C, giving 70% conversion and 70% se-
lectivity of HA within 2 h (Fig. 8b), but the FA selectivity declines
slightly with the prolonged time up to 4 h because of the decomposition
of FA. The oxygen pressure has a positive effect on conversion of gly-
cerol when oxygen pressure is below 3 MPa. Nevertheless, higher
oxygen pressure brings about lower yield of FA due to the occurrence of
over-oxidation reaction (Fig. 8c). GC analysis confirms that CO2 is in-
deed present in gas phase, but no CO is detected. Furthermore, it is
found that the vanadium loadings on ZrP support have a negative im-
pact on the conversion of glycerol but a complicated influence on the
selectivity of FA (Fig. 8d). Especially, when the vanadium loadings is
over 2%, the negative effect on FA selectivity is observed, which reveals
that the presence of excess of the vanadium species on ZrP surface
contributes to decomposition of FA. Overall, the reaction can be pro-
Fig. 9. Catalytic recyclability over 2 V/ZrP-m for selective oxidation of gly-
cerol. Reaction conditions: 5 mL glycerol aqueous solution (10 wt.%), 25 mg
catalyst, 170 °C, 4 h, 3 MPa O2.
ceeded smoothly over 2 V/ZrP-m under 170 °C, 4 h and 3 MPa O2
conditions.
3.3. Catalyst recycling
catalyst (Fig. 2b vs c), although a slight aggregation of vanadium spe-
cies occur from HRTEM image and the elemental mapping (Fig. 3e and
The reusability of the catalysts is determined because catalyst re-
g). ICP-AES analysis results indicate the leaching of the vanadium
cyclability exhibits an indispensable part in the catalytic performance
species is neglectable (< 50 ppm), which reveals that the current 2 V/
evaluation of compound transformations. The 2 V/ZrP-m catalyst has
ZrP-m catalyst is highly stable and leaching-resistant even under aqu-
been employed for evaluating catalytic reusability in glycerol oxida-
tion. In each cycle, the catalyst can be facilely recovered by cen-
trifugation from the reaction mixture, followed by washing with deio-
nized water and dry at 100 °C for 12 h, and then sintering at 550 °C for
eous phase condition. The excellent recyclability of the present vana-
dium-modification catalyst clearly arises from the robust immobiliza-
tion of vanadium by forming V–O–P linkages on ZrP surface, as
reflected by the determination of Brönsted acid sites (Table 1) and solid
3 h. The recovered catalyst can be reused directly in the subsequent run
31P NMR spectra (Fig. 7). Overall, the ZrP support not only retains the
without adding fresh catalyst. In a five-run recycling test, no apparent
oxidation activity of vanadium species but also enhances the stability
due to the strong covalent interaction.
decline of FA yield is observed, confirming the excellent reusability of
the catalyst (Fig. 9). The XRD pattern of the spent catalyst after the 5th
run (Fig. 1e) is almost the same as that of the fresh one. The image of
SEM displays no obvious difference between the fresh and spent
8

D. Li, et al.
MolecularCatalysis474(2019)110404
3.4. Insight into the reaction mechanism
The reactivity of different substrates over 2 V/ZrP-m catalyst has
been investigated in order to understand the possible pathway, the
representative substrate molecules including EG, HA, DHA and AA are
employed for oxidation under the same reaction conditions. The reac-
tion results are summarized in Table S1. It is found that EG affords a
low reactivity, but offers 46.6% selectivity toward to FA (entry 1). Both
HA and DHA are proved to be highly reactive molecules for oxidation,
leading to the formation of FA and AA (entries 2 and 3). However, AA
cannot be oxidized under the present condition (entry 4). This result
indicates that both of FA and AA are ultimate oxidation product and HA
may be the intermediate of glycerol oxidation. Notably, the present
catalyst can be applicable to the oxidation of high glycerol concentra-
tion in aqueous solution (20 wt.%), which provides 89% glycerol con-
version and 63.6% selectivity to FA (entry 5).
As indicated above, the vanadium-modified catalyst 2 V/ZrP-m ex-
hibits superior catalytic performance in the selective oxidation of gly-
cerol in the aqueous phase under base-free condition, affording the
conversion and FA selectivity as high as 85.6% and 62.5%, respectively.
The highly active, stable oxidative sites on the 2 V/ZrP-m catalyst is
firstly attributed to large surface area, which enables the good disper-
sion of active oxidative sites and good accessibility to the substrates,
accelerating mass transfer in the catalytic process. Besides, the covalent
interaction between vanadium species and the surface P(OH) groups on
ZrP contributes to the excellent catalytic stability.
It should be pointed out that the vanadium species on the ZrP sur-
face are the catalytically active sites for glycerol oxidation. The active
sites are highly related with the chemical state of vanadium on the ZrP
surface. Thus one problem arises whether the concentration of vana-
dium (V) on the surface is a decisive factor on catalytic activity. With
regard to this issue, the 2 V/ZrP-m catalyst has subjected to different
calcination temperature and the XPS spectra of the corresponding cat-
alysts are presented in Figure S4. Interestingly, the contents of vana-
dium (V) species increase with the calcination temperature, indicating
that the calcination at higher temperature is favorable to forming more
vanadium (V) species. However, the high temperature calcination can
also results in the aggregation of the active vanadium species, the cal-
cination temperature is set to less than 650 °C in this work. The con-
centration of vanadium (V) species on surface is derived from the XPS
spectra (Fig. 6 and Figure S4) and summarized in Table 3. It demon-
strates clearly that FA yield is highly relevant to the concentration
vanadium (V) species, the higher vanadium (V) species, the higher FA
yield.
Fig. 10. Solid 13C NMR spectra of the glycerol-absorbed on 2 V/ZrP-m catalyst.
(a) Glycerol (15 wt.%) was absorbed on catalyst by immersing catalyst in gly-
cerol solution of methanol, followed by drying for 1 h under the vacuum at
30 °C; (b) After (a) procedure, the sample was heated at 70 °C under the air.
secondary carbon atom of glyceraldehyde. Moreover, a signal at
134.8 ppm corresponds with CeC double bond in acrolein molecule
[54,55], which is well consistent with the previous results originated
from dehydration of glycerol to 3-hydroxypropanal, followed by the
further dehydration to acrolein [56,57].
Based on the activity evaluation and solid 13C NMR characteriza-
tion, a possible reaction pathway on the present catalysts is proposed,
as shown in Scheme 1. There are two possible reaction pathways to
generate FA. The first route involves in Lewis acid-catalyzed dehydra-
tion of glycerol into hydroxyacetone, which is further oxidized to
pyruvic acid and then degrades into FA and AA by CeC cleavage, re-
spectively [58]. In the second route, glycerol first was dehydrogenated
into glyceraldehyde or dihydroxyacetone on active vanadium (V) spe-
cies, and these compounds are instable intermediate which can then be
oxidized to glyceric acid, and then underwent carbon-carbon cleavage
and further oxidation to oxalic acid. Since the two carboxyl groups in
oxalic acid are in adjacent positions and not very stable, decarboxyla-
tion is possible and formed further to FA [59–61]. However, FA could
be decomposed into CO2 under the present reaction condition (Scheme
1). Thus, to examine the stability of FA in this catalytic system, FA
(10 wt.% in water) is used as the substrate under the same oxidation
conditions, and it is found that only 13.8% of FA is decomposed. This
result reveals that FA is rather stable in this catalytic system, leading to
high selectivity towards FA (Table 2) [32].
To identify the intermediate product of this reaction, solid 13C NMR
spectra is carried out on the glycerol-absorbed. As shown in Fig. 10a,
the resonance signal at 66.6 and 76.2 ppm can be attributed to the
primary carbon and secondary carbon atom of glycerol molecule [53],
respectively. New signals are appeared after the glycerol-absorbed is
further subjected to heating under the air. As depicted in Fig. 10b, a
clear signal at 23.5 ppm is observed, matching with methyl group of
hydroxyacetone or pyruvic acid. The resonance peaks ranging from 165
to 195 ppm result from carbonyl groups of aldehydes, carboxylic acids
and ketones, while the characteristic peak at 95.2 ppm is assigned to the
Overall, this is the first time that vanadium-modified ZrP by me-
chanochemical synthesis has demonstrated a considerable activity for
selective oxidation of glycerol. The superior performance can be at-
tributed to the high dispersion of vanadium species on the ZrP surface;
such active sites are easily accessible to the substrate, thus dramatically
promoting the oxidation of glycerol into FA.
Table 3
The effect of surface vanadium (V) concentration on vanadium-modified ZrP
catalysts for FA yield.
4. Conclusions
a
Catalysts
V5+/(V5++V4+
)
Con. (%)
FA Yield (%)
Vanadium modified-ZrPs are directly synthesized from an en-
vironmentally friendly mechanochemical synthesis strategy and serve
as highly efficient catalyst. The as-obtained 2 V/ZrP-m catalyst exhibits
high glycerol conversion (85.6%) and FA yield (53.5%), as well as good
reusability for the liquid phase oxidation of glycerol. The remarkable
performance results from the strong covalent interaction of ZrP with
vanadium species that enable active vanadium species to be highly
2 V/ZrP-m, calcined at 350 °C
2 V/ZrP-m, calcined at 450 °C
2 V/ZrP-i, calcined at 550 °C
2 V/ZrP-m, calcined at 550 °C
2 V/ZrP-m, calcined at 650 °C
0.56
0.61
0.70
0.90
0.99
58.3
66.9
72.4
85.6
87.7
35.3
42.2
48.7
53.5
57.6
a
The values were determined by XPS analysis.
9

D. Li, et al.
MolecularCatalysis474(2019)110404
Scheme 1. Reaction mechanism for selective oxidation of glycerol to FA.
dispersed on surface, promoting both the activity and the stability of
catalyst. It demonstrates that Lewis acid sites of ZrP support exert a
great effect on glycerol activation, and meanwhile vanadium species
are responsible for glycerol dehydrogenation and subsequent oxidation
of the instable intermediates. The catalysts prepared with different
methods allow us to shed light on the relationship between the prop-
erties and catalytic activity. Notably, solvent-free and cost-effective
route for catalyst preparation are of paramount importance from the
practical point of view. This work provides a new alternative synthetic
route to fabricate non-noble metal doped ZrP heterogeneous catalyst for
biomass conversion to important commodity chemicals.
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Acknowledgements
The authors are grateful for support from the National Natural
Science Foundation of China (21373082, 21773061) and Innovation
Program of Shanghai Municipal Education Commission (15ZZ031), and
the Fundamental Research Funds for the Central Universities.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110404.
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