Self-Exfoliated Synthesis of Transition Metal Phosphate Nanolayers for Selective Aerobic Oxidation of Ethyl Lactate to Ethyl Pyruvate

Article abstract of DOI:10.1021/acscatal.9b04452

Two-dimensional (2D) transition metal nanosheets are promising catalysts because of the enhanced exposure of the active species compared to their 3D counterparts. Here, we report a simple, scalable, and reproducible strategy to prepare 2D phosphate nanosheets by forming a layered structure in situ from phytic acid (PTA) and transition metal precursors. Controlled combustion of the organic groups of PTA results in interlayer carbon, which keeps the layers apart during the formation of phosphate, and the removal of this carbon results in ultrathin nanosheets with controllable layers. Applying this concept to vanadyl phosphate synthesis, we show that the method yields 2D ultrathin nanosheets of the orthorhombic β-form, exposing abundant V⁴⁺/V⁵⁺ redox sites and oxygen vacancies. We demonstrate the high catalytic activity of this material in the vapor-phase aerobic oxidation of ethyl lactate to ethyl pyruvate. Importantly, these β-VOPO₄ compounds do not get hydrated, thereby reducing the competing hydrolysis reaction by water byproducts. The result has superior selectivity to ethyl pyruvate compared to analogous vanadyl phosphates. The catalysts are highly stable, maintaining a steady-state conversion of ～90% (with >80% selectivity) for at least 80 h on stream. This "self-exfoliated" synthesis protocol opens opportunities for preparing structurally diverse metal phosphates for catalysis and other applications.

Full text of DOI:10.1021/acscatal.9b04452

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Article
Self-exfoliated Synthesis of Transition Metal Phosphate Nanolayers
for Selective Aerobic Oxidation of Ethyl Lactate to Ethyl Pyruvate
Wei Zhang, Paula Oulego, Sandeep Kumar Sharma, Xiulin
Yang, Lain-Jong Li, Gadi Rothenberg, and N. Raveendran Shiju
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b04452 • Publication Date (Web): 19 Feb 2020
Downloaded from pubs.acs.org on February 27, 2020
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Self-exfoliated Synthesis of Transition Metal Phosphate Nanolayers for
Selective Aerobic Oxidation of Ethyl Lactate to Ethyl Pyruvate
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Wei Zhang, 1 Paula Oulego, 2 Sandeep K Sharma,3 Xiu-Lin Yang,4 Lain-Jong Li,4 Gadi
Rothenberg 1 and N. Raveendran Shiju 1*
1 Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam,
The Netherlands.
2 Department of Chemical and Environmental Engineering, University of Oviedo, C/ Julián Clavería, s/n. E-33071,
Oviedo, Spain.
3 Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
4 Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal
23955-6900, Kingdom of Saudi Arabia.
The main state-of-the-art methods for preparing few-
ABSTRACT: Two-dimensional (2D) transition metal
nanosheets are promising catalysts because of the
layer nanosheets are gas/liquid exfoliation, ion-intercalation
or mechanical cleavage.7 These top-down approaches are
enhanced exposure of the active species compared to their
suitable for stacked materials with interplanar van der Waals
3D counterparts. Here we report a simple, scalable and
interactions, such as graphene, boron nitride and carbon
reproducible strategy to prepare 2D phosphate nanosheets
nitride.8 Synthesizing 2D nanosheets from non-layered
by forming a layered structure in situ from phytic acid and
materials is much more difficult.9 It requires harsh conditions
transition metal precursors. Controlled combustion of the
and gives non-uniform thicknesses and low yields.10
organic groups of phytic acid results in interlayer carbon
Alternatively, 2D non-layered nanosheets can be produced
which keeps the layers apart during the formation of
through template-assisted synthesis,11-12 surfactant self-
phosphate and the removal of this carbon results in ultrathin
assembly,13-14 oriented attachment growth15 and inorganic-
nanosheets with controllable layers. Applying this concept to
organic lamellar hybrid intermediates.16 Yet making high-
vanadyl phosphate synthesis, we show that the method
quality ultrathin nanosheets of non-layered inorganic
materials remains a challenge.17-18
yields 2D ultrathin nanosheets of the orthorhombic -form,
exposing abundant V4+/V5+ redox sites and oxygen
For example, vanadium phosphates (VPOs) are
vacancies. We demonstrate the high catalytic activity of this
comprised of alternating vanadium octahedra (VO6) and
material in the vapor-phase aerobic oxidation of ethyl lactate
phosphate tetrahedra (PO4).19 Several crystal structures in
to ethyl pyruvate. Importantly, these -VOPO4 do not get
different oxidation states are known, such as the V5+ vanadyl
hydrated, thereby reducing the competing hydrolysis reaction
Ⅴ
phosphate (i.e.,  -,  -, -, -, δ-, - and -V OPO4) and
Ⅰ
Ⅱ
by water by-product. The result is superior selectivity to ethyl
pyruvate compared to analogous vanadyl phosphates. The
catalysts are highly stable, maintaining a steady-state
conversion of ~ 90% (with >80% selectivity) for at least 80 h
on stream. This ‘self-exfoliated’ synthesis protocol opens
opportunities for preparing structural-diverse metal
phosphates for catalysis and other applications.
Ⅳ
the V4+ vanadyl pyrophosphate [(V O)2P2O7].20-24 The 
phase is thermodynamically the most stable.25 But since this
compact structure has only fewer accessible active sites, its
catalytic activity is low.26 We hypothesized that this problem
could be avoided by structuring -VOPO4 as thin
nanosheets, thus exposing more surface V4+/V5+ redox
couples.27 VOPO4 nanosheets are currently prepared by an
intercalation-exfoliation of bulk -VOPO4·2H2O exploiting the
weak hydrogen bonds between layers.28 However, unlike
layered - VOPO4, the 3D network of non-layered  phase is
unsuitable for this method, giving no control over the number
of layers.
Key words: metal phosphate •  phase • ultrathin
nanosheets • heterogeneous catalysis • aerobic oxidation •
lactic acid
INTRODUCTION
Here we report a new template-free and scalable
method for preparing 2D -VOPO4 ultrathin nanosheets with
controlled layers. These sheets are made by self-assembly
of vanadyl sulfate (VOSO4) and phytic acid (PTA) precursors,
which are abundant and inexpensive; vanadyl sulphate is a
byproduct of crude oil refining and phytic acid is a renewable
plant-based acid. The phytic acid molecules are the key to
this synthesis: (i) they react with VOSO4 as strong chelating
agents, suppressing the agglomeration of vanadium species;
(ii) they form carbon layers between the vanadium-
phosphate complexes from the cyclohexane rings during the
Ultrathin two-dimensional (2D) materials are attracting
increased attention for several applications thanks to their
distinctive electronic, optical, semiconducting and catalytic
properties.1-2 Single- or few-layer 2D sheets expose more
interior atoms than their bulk counterparts, with abundant
surface-active sites and vacancy defects.3-4 The 2D
confinement effect also can shorten mass and heat diffusion
5
pathways.
This makes them promising candidates for
designing efficient heterogeneous catalysts.6
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Figure 1. (a) Schematic summary of the synthesis procedure for 2D -VOPO4 nanosheets. (b) X-ray diffraction pattern of 2D -VOPO4
nanosheets (the inset shows the model of layered -VOPO4); (c-d) SEM images of -VOPO4 flakes stacked by layered structure; (e) AFM image
of few-layer -VOPO4 nanosheets (inset: the corresponding 3D demonstration); (f) The thickness of nanosheets derived from AFM
measurement.; (g) Representative TEM images of -VOPO4 nanosheets; (h-j) Magnified HRTEM images of -VOPO4 nanosheets taken along
[101], [102] and [110] direction.
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hydrothermal process. Subsequent pyrolysis removes the
We hypothesized that PTA plays a key role in the
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PTA,
formation of 2D -VOPO4 nanosheets. To test this, we ran a
control experiment where, instead of PTA, we used
phosphoric acid (H3PO4) as P precursor (all other conditions
were identical). The resulting VOPO4 material is denoted as
PA-VOP O4. As shown in Figure 2a, the XRD pattern of PA-
VOPO4 is almost identical to that of the PTA-derived VOPO4
nanosheets (denoted as PTA-VOPO4), yielding a typical 
structure. Unlike layered -VOPO4 with a 2D anisotropic
growth (the adjacent layers are connected with weak van der
Waals force),28 diffraction peaks do not shift in the  phase.29
However, the ratios of both (011)/ (101) and (002)/(201)
planes in PTA-VOPO4 are higher than that of PA-VOPO4
(Figure 2a and Figure S4), probably due to the lamellar
layered structure of PTA-VOPO4.
creating more accessible surface, and increasing the number
of V4+/V5+ redox sites and oxygen vacancies. This self-
exfoliating concept is also general, giving access to various
thin transition metal phosphate sheets. We used this method
to make active and selective catalysts from inactive forms of
phosphates by structuring them as thin nanosheets, thereby
exposing more surface active species. The resulting 2D -
VOPO4 ultrathin sheets are excellent catalysts for the vapor-
phase air oxidation of ethyl lactate to ethyl pyruvate (Figure
S1, see the Supporting Information for full experimental
details).
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However, an additional peak appeared at 20.3° in PA-
VOPO4, which belongs to crystalline V2O5. This indicates that
some of the vanadyl species are aggregated into V2O5.
Indeed, a representative HRTEM image of PA-VOPO4
(Figure 2b) confirmed that the V2O5 nanocrystallites are
dispersed on the VOPO4 matrix. The inset clearly shows
lattice fringes of 0.218 nm, corresponding to the (200) lattice
space of crystalline V2O5. The chemical mapping by energy-
dispersive X-ray (EDX) spectroscopy showed the mean
atomic V/P ratio of surface PTA-VOPO4 is 0.9 (Table S1),
much lower than that of PA-VOPO4 (1.41, see Table S2),
indicating the aggregation of vanadia species on the surface
of PA-VOPO4.
RESULTS AND DISCUSSION
Synthesis
nanosheets.
and
Characterization
of
-VOPO4
Figure 1a illustrates the synthesis strategy for 2D -VOPO4
ultrathin sheets. In the first step of this simple two-step
process, PTA and VOSO4 are dissolved in aqueous
solutions, separately. After mixing the two solutions, the
vanadium-PTA coordination precursors are formed by the
self-assembly of vanadyl ions and PTA. Subsequently,
ammonia was added to the mixture adjusting the pH of ~6.
Adding ammonia accelerates the complexation, as well as
helps to form cross-linked networks by surrounding the
vanadium-PTA micelles. The amorphous mixture is then
subjected to a hydrothermal treatment (Figure S2). Then, the
cyclohexane part of PTA is carbonized, forming VOP@C
hybrids. Note that the growth of carbon was restricted
between the inter-layers of VOP-PTA hybrids, forming
extended 2D carbon layers. In the second step, the in-situ
formed carbon templates were removed by heat treatment at
550 °C, yielding 2D -VOPO4 nanosheets.
Both PTA and phosphoric acid are strong chelating
agents and coordinate to VOSO4. As shown in Figure 2c,
PTA comprises six phosphoric acid attached to
a
cyclohexane ring. Unlike plain phosphoric acid, the steric
hindrance of PTA prevents aggregation during the co-
assembly process. Moreover, during the hydrothermal
treatment the cyclohexane segments of PTAs are carbonized
into a carbon framework, suppressing the agglomeration of
vanadium species to V2O5. Indeed, after the hydrothermal
treatment, the nitrogen sorption isotherm of PTA-derived
VOP (PTA-VOPHT) showed increased N2 uptake in the low
relative pressure (P/P0 <0.1) as well as a hysteresis loop in
the region of P/P0 >0.4, suggesting the formation of a carbon
rich in micropores and mesopores (Figure S5a). In contrast,
the PA-VOPHT gave negligible N2 adsorption. Subsequent
calcination removes the carbon, giving PTA-VOPO4 showing
a steep rise in the range of P/P0 >0.9 (Figure S5b), which can
be assigned to the interlayer voids. We also prepared PA-
VOPO4 samples with different VOSO4/PA molar ratios (0.5
and 0.8) under otherwise identical conditions, trying to avoid
the formation of crystalline V2O5. As is evident from the XRD
patterns in Figure S6 in supporting information, crystalline
V2O5 was present in these samples, further verifying the vital
role of phytic acid (PTA) precursor. We conclude that owing
to the confined carbonization of PTA, the formed carbon acts
as an in-situ template, giving the desired few-layer
nanosheets. Control experiments wherein the vanadium-
PTA complex was directly calcined, without any hydrothermal
treatment led to the formation of amorphous VPO (Figure
S7), upholding our conclusion.
Powder X-ray diffraction (PXRD) analysis confirmed the
formation of pure orthorhombic -VOPO4 phase (Figure 1b,
cf. PDF#71-0859). Scanning electron microscopy (SEM)
images show the stacked 2D plates with smooth surface,
shaped edges and corners, indicating a typical lamellar
layered morphology (Figure 1c and 1d). Analysis of the
sample by atomic force microscopy (AFM) (Figure 1e and
Figure S3) and the corresponding AFM height profile (Figure
1f) indicate that the VOPO4 samples comprise three stacks
(see inset in Figure 1e), each with the same average
thickness of ~6 nm. This confirms that we successfully
synthesized the -VOPO4 nanosheets with controlled
thickness of about 7~8 atomic monolayers. The ultrathin and
nearly transparent feature of VOPO4 nanosheets are also
shown by transmission electron microscopy (TEM) (Figure
1g), upholding the AFM results. Moreover, the high-
resolution TEM (HRTEM) images show the clear lattice
fringes with an interplanar distance of 0.53, 0.32 and 0.40 nm
(Figure 1h-1j), which can be assigned to the (101), (102) and
(110) planes of the -VOPO4 structure, respectively.
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Figure 2. (a) Comparison of the X-ray diffraction patterns of two -VOPO4 catalysts: phytic acid-derived VOPO4 nanosheets (PTA-VOPO4)
and the corresponding phosphoric acid-derived VOPO4 (PA-VOPO4); (b) Magnified HRTEM images of PA-VOPO4, the inset shows the V2O5
nanoparticles with the (200) lattice space. (c) Schematic of self-assembly process of VOSO4 and P precursors: phytic acid (PTA) and phosphoric
acid (PA).
over-oxidised on catalyst surface at such high temperatures,
We then studied the catalytic performance of these -
VOPO4 materials in the vapor-phase oxidative
33
lowering the product selectivity.
A series of control
experiments confirmed that the reaction is in the kinetic
regime, with no mass-transfer limitations. Then, we
measured the selectivity to ethyl pyruvate against ethyl
lactate conversion over phytic acid-derived VOPO4
nanosheets (PTA-VOPO4) and phosphoric acid-derived 3D
VOPO4 nanoparticles (PA-VOPO4) (Figure 3a). PTA-VOPO4
outperformed PA-VOPO4 at identical reaction conditions. At
the same conversion of ethyl lactate, PTA-VOPO4 is more
dehydrogenation of ethyl lactate with air to give ethyl
pyruvate in a fixed-bed reactor (eq.1). Lactates are biomass-
derived “platform molecules” and direct aerobic oxidation
30
of lactate is a sustainable route to bio-based pyruvate, an
important
intermediate
in
the
food,
cosmetics,
pharmaceuticals and agrochemicals sectors.31-32 Previously,
we showed that aerobic oxidation of ethyl lactate requires
relatively high temperatures. However, the pyruvate is easily
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selective for ethyl pyruvate than PA-VOPO4. The ethyl
Figure S9). As shown in Figure 4b, the spectra of V 2p can
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pyruvate yield at different reaction temperatures were much
higher over PTA-VOPO4 nanosheets (Figure 3b), confirming
the enhanced catalytic activity. Previously, we demonstrated
that this reaction is kinetically favoured in the presence of
isolated amorphous vanadium oxide sites, while crystalline
V2O5 can catalyse this reaction but not selective.34-35 PA-
VOPO4 features well-dispersed V2O5 nanocrystals on the
surface, yet its catalytic activity and selectively are lower
compared to PTA-VOPO4. This is probably because the
surface V4+/V5+ active sites and oxygen vacancies of PA-
VOPO4 are less accessible than those in PTA-VOPO4. We
then reasoned that the reactivity enhancement on PTA-
VOPO4 is due to the exposed V4+/V5+ redox sites and oxygen
vacancies.
be deconvoluted into two peaks centered at 517.2 eV and
518.0 eV, which are associated with V4+ and V5+ species,
respectively. The PTA-VOPO4 nanosheets gave a much
higher V4+/(V4++V5+) ratio of 40%, in comparison to 30% in
bulk PA-VOPO4 material. Thus, ultrathin nanosheets expose
more accessible surface sites, thereby increasing the
number of surface V4+ species. The enhanced catalytic
activity can be assigned to the increased V4+/V5+ redox active
sites of -VOPO4 nanosheets.36 From redox perspectives,
introducing V4+ into -VOPO4 nanosheets increases the
number of defects and oxygen vacancies. Elsewhere, we
have reported that the oxidation of ethyl lactate follows a
Mars-van Krevelen mechanism: ethyl lactate adsorbed on
the catalytic surface is oxidized by the lattice oxygen, then
the resultant oxygen vacancies are replenished by gas-
phase oxygen during the oxidation reaction.33 Thus, both
surface lattice oxygens and oxygen vacancies play key roles
in aerobic oxidation of ethyl lactate to ethyl pyruvate. These
two species can be roughly estimated from the O1s XPS
spectrum (Figure 4c): the peak at ~532.5 eV can be attributed
to the lattice oxygen (OI), and the peak at ~531.0 eV to the
adsorbed oxygen species at the vacancy sites (OII).4 The OI
peak has a larger area for the PTA-VOPO4 nanosheets than
that of PA-VOPO4, indicating that the former exposes more
lattice oxygen atoms. Additionally, OII oxygens can enhance
the mobility of oxygen species; they are more easily reduced
and favorable for the oxidation reaction. PTA-VOPO4 has a
higher OII/(OI+OII) ratio, indicating abundant structural
defects and oxygen vacancies. Moreover, as the structure
defects and oxygen vacancies decrease the electron charge
density around phosphorus of PTA-VOPO4, the P 2p1/2 and
P 2p3/2 peaks shift slightly towards a higher binding energy,
in comparison with PA-VOPO4 (Figure 4d).37
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OH
O
gas-phase
O
O
(1)
+
H O
2
1/2 O2
+
O
O
Ethyl pyruvate
Ethyl lactate
Further information on the structural defects was
obtained from Positron Annihilation Lifetime Spectroscopy
(PALS). Figure 4e shows a typical PALS spectrum of PTA-
VOPO4. All the PALS spectra could be fitted to two positron
lifetime components with a reasonable variance of fit (1.0–
1.1). The first positron lifetime (τ1) in the range of ~208–240
ps is attributed to positron annihilation in the bulk of the
materials (see inset in Figure 4e). The longer lifetime (τ2) in
the range of ~ 422-478 ps indicates the presence of larger
size defects i.e., vacancy clusters present either in the bulk
or at the grain boundaries of the samples. The τ1 values of
PTA-VOPO4 and PA-VOPO4 are nearly the same, showing
the identical lattice structure. The intensity corresponding to
larger components (I2) is higher (40%) for nanosheets
compared to bulk PA-VOPO4 (36%), suggesting more
vacancy defects for PTA-VOPO4 nanosheets. Positrons
trapped at the defect sites predominantly annihilate with the
surrounding elements and hence provides information about
the chemical surrounding. They are efficiently trapped either
at negatively charged or neutral open volume defects such
as vacancy clusters. According to the crystal structure of the
samples, cation vacancy defects (e.g. V or P based vacancy
defects) are surrounded by oxygen atoms. Figure S10 shows
the ratio curves of these samples with respect to a reference
Si, in which the peak at PL ~ 10x10-3 moc indicates the
annihilation with the surrounding oxygen atoms at the defect
sites. The corresponding peak intensity of PTA-VOPO4 is
higher than that of PA-VOPO4, indicating that the defects
present in the nanosheets have more O atoms in the
surrounding lattice sites. Thus, our ultrathin VOPO4
nanosheets expose more lattice oxygen and oxygen
Figure 3. The vapor-phase oxidative dehydrogenation of ethyl
lactate with air to give ethyl pyruvate over various -VOPO4 catalysts:
(a) Selectivity to ethyl pyruvate plotted against conversion over PTA-
VOPO4 and PA-VOPO4. Reaction conditions: Ethyl lactate
WHSV=6.25 h-1, air carrier flow rate= 2.25 L/h; (b) the corresponding
temperature-resolved yield profile of ethyl pyruvate. All data were
taken after 2 h on stream.
Factors governing activity and selectivity.
We further characterized the -VOPO4 sheets to
understand this enhanced activity. The full X-ray
photoelectron spectroscopy (XPS) survey spectra showed V,
P and O in all samples (Figure S8), in accordance with
element mapping from high-angle annular dark-field
scanning TEM (HAADF-STEM) analysis (Figure 4a and
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vacancies (Figure 4f), which can explain their high catalytic
activity.
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Figure 4. (a) STEM image and corresponding elemental mappings of V, P, and O of PTA-VOPO4 nanosheets. XPS studies showing high-
resolution V 2p spectra (b), high-resolution O 1s spectra (c), and high-resolution P 2p spectra (d) of PA-VOPO4 and PTA- VOPO4 nanosheets.
(e) The PALS spectra of PTA-VOPO4 nanosheets (the inset shows the positron annihilation lifetimes and the corresponding intensities in PA-
VOPO4 and PTA- VOPO4 nanosheets, respectively). (f) Schematic diagram of defects and vacancies, originating from the removal of carbon
layers, in PTA-VOPO4 nanosheets.
Self-exfoliated synthesis of other transition metal
phosphate nanolayers for catalytic aerobic oxidation of
ethyl lactate to ethyl pyruvate.
analysis (Figure 5, a-c and Figure S11). The combination of
XRD, SEM and EDX analysis showed that these phosphates
have lamellar layered morphologies with high crystallinity,
purity and uniformity (Figure S12-S13 and Table S3-S5, in
the supporting information). Thus, our synthesis method is
Based on this ‘self-exfoliated’ synthesis protocol, we
successfully made V, Ni, Co and Fe-based phosphates (see
the experimental section in the Supporting Information for
details). These desired few-layer nanosheets with the
thickness of 2–6 nm were confirmed by HRTEM and AFM
general, facile and scalable via
a two-step process
(hydrothermal and calcination treatment).
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Figure 5. Morphological and microstructural characterization of various phytic acid-derived phosphate nanosheets (a-c); NiPO4 nanosheets:
(a1-a3) TEM images, (a4) AFM image; CoPO4 nanosheets: (b1-b3) TEM images, (b4) AFM image; FePO4 nanosheets: (c1-c3) TEM images,
(c4) AFM image. Note: the corresponding height profles of phosphate nanosheets are shown in Figure S11, which derived from AFM
measurement.
Figure 6. (a) Comparisons of ethyl lactate conversion and ethyl pyruvate selectivity over various phytic acid-derived metal phosphate
nanosheets: VOPO4, FePO4, CoPO4 and NiPO4. Reaction conditions: Ethyl lactate WHSV=8 h-1, air flow rate= 2.25 L/h, reaction temperature:
300°C, 325 °C and 350 °C. (b) Arrhenius plots for steady-state ethyl lactate consumption rate over various phosphate catalysts, the apparent
activation energy (Ea) was measured at a series of temperatures below 15 % ethyl lactate conversion.
All the phytic acid-derived phosphate nanosheets were
then tested in the vapor-phase aerobic oxidation of ethyl
lactate with air at different reaction temperatures (300°C, 325
°C and 350 °C). As shown in Figure 6a, control experiments
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confirmed that Ni and Co showed some conversion but
corresponding Ea value for VOPO4 (35 kJ/mol) is much
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selectivity to ethyl pyruvate was low, owing to the hydrolysis
of ethyl pyruvate on Co- and Ni- phosphate.36 Intriguingly,
VOPO4 nanosheets exhibited the best catalytic performance
among all the phosphate catalysts tested in this study, giving
smaller than that for other phosphates: FePO4 (83 kJ/mol),
CoPO4 (102 kJ/mol) and NiPO4 (114 kJ/mol). This result
confirmed that VOPO4 is intrinsically more active for ethyl
lactate oxidation.
a
remarkably high activity and selectivity. To better
Comparisons of catalytic efficiency of various
vanadium phosphorus oxides for aerobic oxidation to
ethyl lactate.
understand this, the apparent activation energies (Ea) were
calculated based on Arrhenius plots (Figure 6b), from the
data collected below 15% ethyl lactate conversion. The
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Figure 7. (a) Selectivity to ethyl pyruvate plotted against conversion for different VPO catalysts: 2D -VOPO4 nanosheets, (VO)2P2O7,
VOHPO4·0.5 H2O and VOPO4·2H2O. (b) Comparison of the selectivity of various products at ethyl lactate conversion of ~ 6% (-VOPO4
nanosheets: 6.0 %, (VO)2P2O7: 6.2 %, VOHPO4·0.5 H2O: 5.7 % and VOPO4·2H2O: 6.4 %). The carbon balances were > 98%. (c) Reaction
pathway for the aerobic oxidation of ethyl lactate on the VPO catalysts. (d) The mass-specific activity for pyruvate formation over VPO catalysts
in the temperature range 250-325°C. (e) Comparisons of the area-specific production rate of pyruvate over 2D -VOPO4 nanosheets and
(VO)2P2O7. (f) Stability test of the 2D -VOPO4 nanosheets at optimized conditions (WHSV=3 h-1, T=300 °C).
Olier et al. reported that all vanadium phosphorus oxides
(VPO) can be hydrated except for -VOPO4, owing to its
highly stable structure.38 This is consistent with our XPS
measurements (Figure 4c), where no surface chemisorbed
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ACS Catalysis
water was detected (~533 eV). As a result, the competing
salts are also cheap; therefore, these nanosheets are
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industrially viable catalysts cost wise as well.
hydrolysis is suppressed, which may explain why the -
VOPO4 nanosheets gave such high selectivity to pyruvate
compared with other metal phosphate catalysts.
CONCLUSIONS
To test this, we prepared a series of bulk VPO catalysts
for comparison with our nanosheets: vanadyl pyrophosphate
[(VO)2P2O7], vanadyl phosphate dihydrate (VOPO4·2H2O)
We report the synthesis of 2D ultrathin phosphate
nanosheets by a new template-free ‘self-exfoliated’ strategy
using renewable phytic acid (PTA). PTA acts as a strong
chelating agent, but can also be carbonized in situ into
carbon templates, which are responsible for the precisely
controlled few-layer nanosheets. Application of this method
to vanadium phosphate produces -VOPO4 ultrathin
nanosheets, which expose abundant V4+/V5+ redox sites and
oxygen vacancies. Importantly, -VOPO4 does not get
hydrated, thereby reducing the competing hydrolysis by
water byproduct. These features result in a superior catalytic
activity and selectivity in the aerobic oxidation of ethyl lactate
to ethyl pyruvate compared to the classical vanadium
phosphate. The inexpensive -VOPO4 nanosheets show
good long-term stability and facile recovery. These
nanosheets are not only among the best heterogeneous
catalysts for the vapor-phase oxidation of lactate to pyruvate,
they also show for the first time that the “inert” -VOPO4
phase can be an efficient oxidation catalyst under the right
conditions. Note that this is a general synthesis method,
giving access to various metal phosphate nanosheets, such
as Ni, Co and Fe. Therefore, this work opens new avenue for
the synthesis of new transition metal phosphate nanosheets
for catalysis and other applications.
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and
vanadyl
hydrogen
phosphate
hemihydrate
(VOHPO4·0.5H2O, see supporting information for full
experimental details). Their crystalline structures were
confirmed by XRD and Raman spectroscopy (Figure S14 and
Figure S15).39-40 Figure 7a shows the selectivity-conversion
plots. All the VPO catalysts were active in lactate-to-pyruvate
reaction. Interestingly, -VOPO4 nanosheets showed highest
ethyl pyruvate selectivity, reaching over 90% at ethyl lactate
conversion of ~25%. Even at a high ethyl lactate conversion
of ~80%, the selectivity is as high as 80% compared with
~60% for (VO)2P2O7. Control experiments were performed at
a steady-state conversion of ~ 6% for all the tested catalysts
(the carbon balances were > 98%) to better differentiate the
influence of the VPO phases on product selectivity. As shown
in Figure 7b, -VOPO4 nanosheets gave over 99% selectivity
to ethyl pyruvate, while a series of by-products were detected
on other three VPO catalysts, such as acetaldehyde, ethanol,
acetic acid, ethyl acetate and COx. This indicates that except
for  phase, the VPO catalysts undergoes the competing
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overoxidation,
hydrolysis,
decarbonylation
and
decarboxylation (see Figure 7c). The by-product distribution
is different among different catalysts. VOHPO4·0.5 H2O and
VOPO4·2H2O gave higher selectivity to ethanol than
(VO)2P2O7, owing to the hydrolysis of ester on their hydrated
surfaces.
AUTHOR INFORMATION
Corresponding Author
We also compared the mass-specific activity (calculated
as grams of pyruvate produced per gram of catalyst per hour)
over VPO catalysts at different temperatures (Figure 7d). Our
-VOPO4 catalyst outperformed the classical phosphates,
especially at high reaction temperatures over 300 °C.
Moreover, layered -VOPO4 and (VO)2P2O7 gave similar
specific BET areas of 33 m2/g and 25 m2/g, respectively,
much higher than VOHPO4·0.5 H2O (16 m2/g), VOPO4·2H2O
(9 m2/g). We then plotted the area-specific catalytic rates for
pyruvate production over -VOPO4 and (VO)2P2O7 catalysts.
As shown in Figure 7e, the area-specific activity for -VOPO4
nanosheets is almost twice higher than (VO)2P2O7 at 300
°C.The stability and regenerability are key factors for a
heterogenous catalyst in its practical application. We tested
the stability of our -VOPO4 nanosheets at optimized
conditions (WHSV=3 h-1, T=300 °C, see Table S6). As shown
in Figure 7f, this catalyst is highly stable, with a steady-state
conversion of ~ 90% (over 80% selectivity) for at least 80 h
without significant loss of activity. We also used the same
catalyst bed for a series of testing studies, and for this, the
catalyst was cleaned and regenerated by simply passing air
at 500 C for 2 h and switching off the ethyl lactate feed. The
XRD, TEM and XPS analysis further confirmed that the
structure was well preserved after multiple regenerations
(Figure S16-S18 in supporting information). Most of 2D
nanosheets are still far from commercialisation, because
their cost is a problem to scale-up. Our 2D -VOPO4
nanosheets are promising in this regard, because they can
be readily achieved from inexpensive starting materials such
as VOSO4 and phytic acid. VOSO4 is a byproduct of crude oil
refining (ca. 2000-5000 $/ton) and PTA is a renewable
inexpensive plant-based acid (ca. 6500 $/ton).41 The metal
*E-mail: n.r.shiju@uva.nl
ORCID
Wei Zhang: 0000-0003-4506-6583
Gadi Rothenberg: 0000-0003-1286-4474
N. Raveendran Shiju: 0000-0001-7943-5864
Notes
There are no conflicts to declare.
ASSOCIATED CONTENT
Supporting Information
Additional data were listed in the Supporting Information.
Experimental procedures, additional characterization data of
vanadyl phosphate catalysts including XRD patterns,
nitrogen adsorption−desorption isotherms, SEM/TEM/AFM
images, EDX spectra, XPS spectra, PALS spectra, Raman
spectra and results of reaction parameters on aerobic
oxidation of ethyl lactate with air. This information is available
free of charge on the ACS Publications website.
ACKNOWLEDGMENT
W. Zhang thanks the China Scholarship Council
(201506140058) for a PhD fellowship. P. Oulego thanks the
Spanish Ministry of Economy and Competitiveness
(MINECO) (Project CTM2015-63864-R) and European
Union (FEDER) for funding. This work is part of the Research
Priority Area Sustainable Chemistry of the University of
Amsterdam, http://suschem.uva.nl.
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