A Resol-Assisted Cationic Coordinative Co-assembly Approach to Mesoporous ABO3 Perovskite Oxides with Rich Oxygen Vacancy for Enhanced Hydrogenation of Furfural to Furfuryl Alcohol

Article abstract of DOI:10.1002/anie.202012416

It is a challenge to obtain ABO₃ perovskite oxides with favorable crystal phase and well-defined porous structure via existing approaches. Here, we design an effective and versatile strategy to construct mesoporous ABO₃ perovskite oxides with functionalized nanocrystal frameworks and abundant oxygen vacancy sites via a resol-assisted cationic coordinative co-assembly approach. The as-prepared oxygen vacancy-rich mesoporous LaMnO₃ as heterogeneous catalyst exhibits remarkable catalytic activity and stability for hydrogenation of furfural to furfuryl alcohol, including over 99 % conversion and 96 % selectivity. Combined with density functional theory calculation, the catalytic mechanism is elucidated, revealing that porous LaMnO₃ nanocrystal framework is conducive to expose oxygen deficiency sites, which can facilitate the interaction between catalyst surface and catalytic substrate, leading to lower barrier in hydrogenation process.

Full text of DOI:10.1002/anie.202012416

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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International Edition
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Accepted Article
Title: A Resol-assisted Cationic Coordinative Co-assembly Approach to
Mesoporous ABO3 Perovskite Oxides with Rich Oxygen Vacancy
for Enhanced Hydrogenation of Furfural to Furfuryl Alcohol
Authors: Yuenan Zheng, Rui Zhang, Ling Zhang, Qinfen Gu, and Zhen-
An Qiao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202012416
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10.1002/anie.202012416
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RESEARCH ARTICLE
A Resol-assisted Cationic Coordinative Co-assembly Approach to
Mesoporous ABO3 Perovskite Oxides with Rich Oxygen Vacancy
for Enhanced Hydrogenation of Furfural to Furfuryl Alcohol
Yuenan Zheng,[a] Rui Zhang,[a] Ling Zhang[b], Qinfen Gu[c] and Zhen-An Qiao*[a]
Abstract: The design of ABO3 perovskite oxides with porous
mesostructure and unique surface active sites for heterogeneous
catalysts holds great promise in various catalytic applications.
However, it is still a challenge to get ABO3 perovskite oxides with
favourable crystal phase and well-defined porous structure via
existing approaches. Here, we design an effective and versatile
strategy to construct mesoporous ABO3 perovskite oxides with
functionalized nanocrystal frameworks and abundant oxygen vacancy
sites via a resol-assisted cationic coordinative co-assembly approach.
The as-prepared oxygen vacancy-rich mesoporous LaMnO3 as
heterogeneous catalyst exhibits remarkable catalytic activity and
stability for hydrogenation of furfural to furfuryl alcohol, including over
99% conversion and 96% selectivity. Combined with density
functional theory calculation, the catalytic mechanism is elucidated,
revealing that porous LaMnO3 nanocrystal framework is conducive to
expose oxygen deficiency sites, which can facilitate the interaction
between catalyst surface and catalytic substrate, leading to lower
barrier in hydrogenation process.
to the significantly enhanced activity for low-temperature CO
oxidation. This provides inspirations for the adsorption and
activation of oxygen-containing functional group of substrates on
the catalyst surface with oxygen vacancies in the catalytic
reaction and may lead to extraordinary catalytic activity.
Until to now, widely used methods to conduct oxygen defects
into perovskite oxides mainly involve thermal annealing under
vacuum or inert atmosphere or hydrogen gas, wet chemical
reduction method and some means of porous engineering and
reducing size of samples.[16-20] Commonly, crystalline perovskite
oxides are obtained via high temperature calcination treatment,
causing products with dense structure composed of large
particles, inferior surface reactivity even compromised catalytic
activity. From the perspective of these issues, the construction of
porous architecture of perovskite oxide catalyst is considered to
be an effective way to overcome above problems and provide
opportunities for boosting catalytic activity. Moreover, porous
ABO3 perovskite oxide composed of functional nanocrystalline
framework will hold a significant advantage for providing oxygen
defect sites to promote catalytic activity. However, until now it is
still a huge challenge to construct mesoporous perovskite oxides
by molecular assembly methods,[21-23] due to the following issues.
First, the synthesis system is complicated in terms of dynamics,
because it includes multifarious metal precursors, additive and
block copolymers. And strictly controlling of reaction condition is
generally required to regulate co-assembly process between
inorganic precursors and block copolymer to avoid phase
separation, due to their complicated hydrolysis and condensation
nature.[24-27] In addition, thermodynamically, a high-temperature
(>500 °C) calcination is necessary to drive organic-inorganic
composite to crystallize into perovskite oxides. But the
mesostructures tend to collapse and agglomeration during high-
temperature crystallization process, it will result in poor porous
structure and low thermal stability of perovskite oxides.[28, 29]
Herein, we develop an effective molecular assembly strategy to
solve above challenges for synthesis of mesoporous perovskite
oxides via a resol-assisted cationic coordinative co-assembly
approach. In this approach, resol is used as building block to co-
assemble with block copolymer F127 by H-bonds. Meanwhile, the
citric acid as ligand with abundant hydroxyl attaches to resol
through H-bonds, wherein citric acid simultaneously coordinate
with La3+ and M2+ (M= Mn, Co, Ni, Fe) homogeneously at the
atomic scale. And the coordination between the metal ion and
citrate can control the hydrolysis and condensation rates of metal
precursors and avoid phase separation. The slow assembly
between template micelle and inorganic metal chelate affords the
formation of well-defined mesostructure by solvent evaporation.
Furthermore, resol is also employed as a stabilizer to maintain the
stability of mesostructure of ABO3 perovskite oxides during high
temperature calcine process (up to 650 oC), endowing the
Introduction
Perovskite oxides have emerged as promising catalysts in various
catalytic application due to their low cost, high abundance, and
considerable activity and durability.[1-4] The crystal structure
composition as well as surface chemical property of perovskite
oxides are flexible, thus enabling their nature to be tailored
preferably for targeting catalytic reactions.[5-8] As a powerful
method, oxygen vacancy engineering has been developed to
regulate electronic structure and surface chemical property of
perovskite oxides and consequently enhance their catalytic
performance.[9-12] For instance, the modified perovskite oxide
La0.8Sr0.2CoO3 with surface oxygen defects and surface Lewis
acid sites was served as a catalyst to effectively improve the
activity of propane oxidation.[13] Furthermore, some researches
demonstrate that O2 is more favorably adsorbed and activated on
surface oxygen vacancies of La0.8Sr0.2MnO3,[14, 15] which gives rise
[a]
[b]
[c]
Y. Zheng, R. Zhang, Prof. Z.-A. Qiao
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun, Jilin 130012 (China)
E-mail: qiaozhenan@jlu.edu.cn
L. Zhang
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun,
Jilin 130012 (China)
Q. Gu
Australian Synchrotron, ANSTO, 800 Blackburn Rd, Clayton,
Victoria 3168 (Australia)
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.202012416
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RESEARCH ARTICLE
perovskite oxides with high crystallinity and porosity
simultaneously.
oxide structure. These are significant because the Lewis acid and
base sites on catalyst are the key factors to catalyze the CTH
reaction in the Meerwein-Ponndorf-Verley (MPV) mechanism,
and oxygen vacancies can facilitate the interaction between
carbonyl of substrate molecules and Mn sites in LMO catalyst. For
these unique nature of as-prepared LMO and combined with the
results of previous researches, the CTH reaction is selected to
probe the property of as-prepared mesoporous LMO catalyst and
what contributes more to improved activity over catalyst will be
explored. Remarkably, up to 99% conversion and 96% selectivity
of furfural hydrogenation to FOL were obtained, which is attributed
to the rich oxygen vacancies and mesoporous nanocrystalline
framework structure of catalyst. Density functional theory (DFT)
calculations are applied and suggest that furfural is more
favorably adsorbed and activated on mesoporous LaMnO3
surface with rich oxygen vacancies and the lower barrier has to
be overcome in the hydrogenation process.
As a promising chemical platform compound, furfuryl alcohol
can be effectively converted to biochemical and high-value
chemicals, which has received extensive researches.[30-33]
Catalytic transfer hydrogenation reaction (CTH) offers an efficient
and safe route for hydrogenation of furfural to furfuryl alcohol
(FOL), wherein organics (e.g., alcohols) rather than H2 are served
as hydrogen source in hydrogenation processes avoiding the risk
in transportation and storage. A few researches have reported
that ABO3 perovskite oxides shown catalytic activity in some
liquid-phase organic reactions, such as, acid/base-catalyzed
reactions, hydrogenation of aromatic nitro compounds and so on,
which are attributed to their flexible crystal structure composition
of metal ions at the A/B sites and surface chemical property. [6,34-
36] Importantly, our as-prepared LaMnO3 (LMO) with mesoporous
nanocrystalline framework possesses rich oxygen vacancies and
acid-base sites with moderate strength on the basis of perovskite
Scheme 1. Illustration of the resol-assisted cationic coordinative co-assembly route for the synthesis of OV-rich meso-LMO and as the hydrogenation catalyst for
furfural to FOL.
separation. With the continuous evaporation of solvent, block
copolymer F127 gradually arranges into homogeneous micelles.
Meanwhile, resol interacts with the hydrophilic part of F127 by H-
bonds, and La3+/M2+-citrate chelate are linked to phenolic hydroxyl
Results and Discussion
groups of resol by hydrogen-bonding, and thus result in co-
assembly of precursors around the surfactant micelles forming a
continuous organic-inorganic composite mesostructure. The -OH
peak in Fourier transform infrared spectra (FT-IR) of organic-
inorganic composite compared with alone F127 and citric acid
shows a distinct red shift, confirming the formation of H-bonds
between F127 and resol as well as resol and citric acid in
La3+/Mn2+-citrate chelate (in Supporting Information Figure S1,
Fig. S1).[26, 28, 38, 39] After further solidification in oven at 100 °C
and removing the triblock copolymer by annealing under air
As shown in Scheme 1, two metallic nitrates (cations of A and B
site) as the metal sources and citric acid as coordination agent
were dissolved in an acidic EtOH/H2O/F127/resol mixture solution,
stirring for 3 hours, then the resultant homogeneous and
transparent sol was dried in oven for several hours to get product
membrane. In this system, HNO3 as charge intermediate was
applied to hinder condensation between inorganic metal source
and surfactant.[26, 37] During the progress, La3+ and M2+ (M= Fe,
Co, Ni, Mn) simultaneously coordinate with citric acid, which
stabilizes the growth of inorganic metal sources and avoids phase
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RESEARCH ARTICLE
Fig. 2c-2d. The interconnected LaMnO3 nanoparticles form the
wall of the worm-like mesopores and the nanocrystals are
estimated to be ~18 nm. The adjacent lattice plane of 0.276 nm in
HRTEM is indexed to the (110) plane of LaMnO3. The N2 sorption
isotherm of OV-rich meso-LMO present characteristic type-IV
curves in Fig. 2e, which manifest the mesoporous feature of
prepared OV-rich meso-LMO. And OV-rich meso-LMO possesses
similar BET surface area of 46 m2 g-1 with meso-LMO (45 m2 g-1),
demonstrating the pore structure is maintained after NaBH4
solution treatment. The energy-dispersive spectroscopy (EDS)
elemental mapping images (Fig. 2f) of the OV-rich meso-LMO
reveal homogeneous distribution of La, Mn and O within the whole
mesoporous framework.
The X-ray photoelectron spectroscopy (XPS) measurement
were performed to investigate the surface oxygen defects and
chemical state of the prepared LMO samples. Bulk LaMnO3 (bulk
LMO) was prepared in order to compare and expound clearly the
surface property and defects (The detailed data of bulk LMO in
Fig.S3). As shown in Fig. 3a, the O1s spectra of LMO samples
are composed of three components of oxygen species. The peaks
located at about 529.3 eV are assigned to lattice oxygen (O2-),
while that at ~530.9 eV are usually ascribed to the oxygen atoms
in the vicinity of oxygen vacancies deemed as vacancy oxygen
species and the peaks at ~531.9 eV corresponded to hydroxyls
species (OH-).[40-42] The amount of vacancy oxygen species
characterized by peak area integration of O1s are 43.7% of OV-
rich meso-LMO and 45.5% of OV-rich bulk LMO higher than
22.9% of meso-LMO and 19.3% of bulk LMO, respectively
(detailed data in Fig. S4 and Table S2), which verifies that the
prepared LMO samples can obtained increased concentration of
oxygen vacancies after NaBH4 treatment of meso-LMO and bulk
LMO.[10, 43] The divided Mn 2p3/2 peaks at 641.7 eV and 642.7 eV
were ascribed to Mn3+ and Mn4+, respectively.[8] As seen from Fig.
3b and Table S3, the relative content of Mn3+ grow from 16.4% of
bulk LMO to 29.5% of meso-LMO and 34.5% of Ov-rich meso-
LMO, showing an increasing trend along with the enhanced
concentration of oxygen vacancies, which can be attributed to
electrons transfer induced by the formation of oxygen vacancy.[8,
Figure 1. a-d) SEM images of as-synthesized mesoporous LaMnO3 (meso-
LMO) a), La0.8Sr0.2MnO3 b), LaFeO3 c), LaCoO3 d). e) XRD patterns and f) N2
sorption isotherms of as-synthesized mesoporous ABO3 perovskite oxide. The
isotherm curves of mesoporous LaMnO3, La0.8Sr0.2MnO3, LaFeO3 and LaCoO3
in (f) are shifted by up to 175, 120, 40 and 20 cm3 g-1, respectively, for clarity.
atmosphere, the ABO3 perovskite oxides with unique mesoporous
structure composed of nanocrystal framework can be obtained.
As-synthesized mesoporous LaMnO3, LaFeO3, LaCoO3, LaNiO3,
La1-XSrXMnO3 all showed a single perovskite oxide phase
withoutextra diffraction peak in powder X-ray diffraction (XRD)
patterns (Fig. 1e). Scanning electron microscopy (SEM) images
(Fig.1a-1d) of these samples all exhibited abundant worm-like
mesopores (transmission electron microscopy (TEM) images in
Fig. S2).The porous framework is composed of self-linked ABO3
nanocrystals with a diameter of 11-59 nm and the pore sizes are
estimated to be about 4.8-31.8 nm. The N2 sorption isotherms of
samples have characteristic type IV curves assigned to the
feature of mesoporous structure. The Brunauer-Emmett-Teller
(BET) surface area of a series of mesoporous ABO3 perovskite
oxide is in a range from 13 to 45 m2 g−1, and the pore volume
ranged from 0.06 to 0.34 cm3 g−1. The detailed specific surface
area SBET of these samples in Table S1.
To further increase the concentration of oxygen vacancy of
samples, a NaBH4 aqueous post-treatment was conducted on
prepared meso-LMO to achieve the oxygen vacancy-rich
mesoporous LaMnO3 (OV-rich meso-LMO). The OV-rich meso-
LMO has been successfully synthesized (as shown in Fig.2a).
The XRD patterns in Fig. 2b confirm that no phase change occurs
in OV-rich meso-LMO after NaBH4 treatment of meso-LMO. The
SEM and TEM images of OV-rich meso-LMO were presented in
44]
Fig. 3c shows the temperature-programmed desorption of O2
(O2-TPD) spectra of LMO samples, in which vacancy oxygen
species and their content are presented by the integration of
different peaks and the detailed data are summarized in Table S4.
The desorption peaks below 350 °C are commonly relevant to
−
surface adsorbed O2 and O− species, which are ascribed to
surface oxygen vacancies, and the peaks above 400 °C are
assigned to lattice oxygen O2-.[45] Obviously, OV-rich meso-LMO
exhibits a broad and strong desorption feature in the range of 50-
350 °C (peak at 98.1 °C and peak at 287.6 °C), demonstrating
high contents of oxygen vacancy species on OV-rich meso-LMO.
The concentration of oxygen vacancy on OV-rich meso-LMO
reaches 1.32 and 3.34 fold than that of meso-LMO and bulk LMO,
respectively, in good agreement with the O 1s XPS analysis.
Furthermore, the ratio of adsorbed oxygen species/lattice oxygen
species from O2-TPD spectra of OV-rich meso-LMO, meso-LMO
and bulk LMO are 1.20, 0.895 and 0.211, respectively, confirming
that Ov-rich meso-LMO possess the larger proportion oxygen
vacancies in all oxygen species.[14]
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10.1002/anie.202012416
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vacancy sites, our prepared porous LMO perovskite oxide is a
promising catalyst for CTH reaction. Herein, the activity and
selectivity measured for CTH of furfural to FOL on our LMO
catalysts with isopropanol as a hydrogen donor were investigated.
As shown in Fig. 4a, the hydrogenation activity is extremely weak
under no catalyst, with a furfural conversion of only 5.4% at
180 °C for 3 hours under 1 Mpa N2 condition. The addition of bulk
LMO only exhibited a furfural conversion of 17.3%. Then an
improved activity of 75.3% conversion and 92.7% FOL selectivity
is brought by meso-LMO catalyst for the reaction. This great
increase of catalytic activity compared with bulk LMO is attributed
to porous framework of LMO and the defect sites provided by
nanocrystal pore wall. Encouragingly, OV-rich meso-LMO applied
as catalyst, up to 99% conversion and FOL selectivity 96.1% can
be achieved under the same condition. Prominently, OV-rich
meso-LMO presents the highest furfural conversion among these
LMO catalysts. As a comparison sample, the OV-rich bulk LMO
catalyst under the same catalytic condition shows an growing
furfural conversion of 38.7% than bulk LMO, confirming that the
surface vacancy oxygen species on the catalyst are contributed
to the promotion of catalytic activity. The catalytic performance
test based on surface area of catalyst was conducted (Fig. S6),
which also convincingly demonstrate the oxygen vacancies make
a great contribution to the CTH of furfural. On the other hand,
mesoporous structure and high surface-area feature of catalysts
play an extremely crucial role in the CTH reaction of furfural, and
that also can be perceived from the furfural conversion of 75.3%
on meso-LMO and above comparison result, which conforms that
the presence of a well-defined porous structure is favorable for
Figure 2. a) Schematic image of OV-rich meso-LMO crystal structure. b) XRD
patterns of Ov-rich meso-LMO and meso-LMO. c-d) SEM and TEM images of
Ov-rich meso-LMO. The inset in d) is HRTEM image of Ov-rich meso-LMO. e)
N2 sorption isotherms of Ov-rich meso-LMO and meso-LMO. f) Scanning TEM
image and EDS mapping images of elemental La, Mn and O of Ov-rich meso-
LMO.
To get insights on the role of oxygen vacancies, DFT modeling
is useful to disclose the atomic-level distinction of LMO perovskite
oxide with different concentration of oxygen vacancy.[42,46,47]
Accordingly to above data analyzation, three models were
constructed for LMO: perfect surface, the surface with one oxygen
vacancy and the surface with two oxygen vacancies on behalf
ofbulk LMO, meso-LMO and OV-rich meso-LMO, respectively (Fig.
3d-3f). And the band structure and electronic density of states
(DOS) of them are shown in Fig. 3g-3i, it is obvious that the band
gaps decrease from bulk LMO to meso-LMO when oxygen
vacancy occurs. With increasing oxygen vacancies, the
conduction band appears around 0.5 eV in defect level for Ov-rich
meso-LMO. The increase of defects can boost the reactivity of
catalyst and facilitate the catalytic reaction.[48,49]
The CTH of furfural pertains to a type of Meerwein-Ponndorf-
Verley (MPV) reaction. The hydrogenation processes of furfural
involve mainly both C=O (furfural) and C-OH (isopropanol) groups
, wherein acid-base pair sites play a significant role in the CTH
process with Lewis acid sites involved in the coordination of the -
OH and C=O. At the same time, the base site subtracts the H of
the -OH and promotes the final hydrogen transfer to furfural.[50-54]
Benefiting from the unique mesoporous structure and rich oxygen
Figure 3. a-c) O 1s and Mn 2p of XPS spectra, O2-TPD profiles of Ov-rich meso-
LMO, meso-LMO and bulk LMO, respectively. Constructed DFT models of bulk
LMO d), meso-LMO e), Ov-rich meso-LMO f). The La, Mn and O atoms are
marked with blue, purple and red, respectively. Calculated DOS of bulk LMO g),
meso-LMO h) and Ov-rich meso-LMO i), respectively.
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Table 1. Catalytic activity of Ov-rich meso-LMO for CTH reaction of various
carbonyl substrates using isopropanol as hydrogen donor.[a]
Temp.
(°C)
Time
(h)
Conv.
(%)
Sel.
(%)
Yield
(%)
Main
Substrate
product
180
180
3
5
>99
96.1
>99
95.1
90.0
90.5
180
180
15
10
87.7
98.8
92.2
82.8
80.9
81.8
Figure 4. a) Activity and selectivity of various catalysts at 180 °C for 3 hours
under 1 Mpa N2, isopropanol as hydrogen donor for the CTH of furfural to FOL.
b) Histogram of conversion of furfural at different reaction temperatures with
three LMO catalysts. c) Reaction kinetics plot and Arrhenius plot for Ov-rich
meso-LMO catalyst. d) Cycling tests of CTH of furfural to FOL over Ov-rich
meso-LMO catalyst.
180
15
90.1
>99
89.2
the catalytic reaction as well.[55] Because mesoporous structure of
catalysts increases the density of catalytic active sites with high
accessibility and facilitates the diffusion of reactants and products.
Thus, the obtained excellent catalytic performance of OV-rich
meso-LMO not only depends on abundant oxygen vacancy sites,
but also rests with the unique porous mesostructure. With
increasing reaction temperature from 150 °C to 180 °C (Fig. 4b),
the conversion of hydrogenation reaction increase as well by
using three different catalysts. Notably, OV-rich meso-LMO shows
much larger catalytic conversion than meso-LMO and bulk LMO
at each corresponding temperature, which strongly demonstrates
that its superiority of high catalytic activity is temperature
independent. In Fig. 4c, the CTH reaction of furfural on OV-rich
meso-LMO shows a relatively high k value of 0.054. From the
Arrhenius equation ln k = −Ea/RT + ln A, the activation energy (Ea)
of the hydrogen transfer of furfural to FOL over OV-rich meso-LMO
catalyst was calculated to be 38.86 kJ mol−1, which is
advantageous than meso-LMO (42.44 kJ mol−1), bulk LMO (49.29
kJ mol−1) and most reported catalysts (Fig. S7).[56,57] The low Ea is
attributed to well-defined mesoporous structure and abundant
oxygen vacancy active sites of OV-rich meso-LMO catalyst. The
reaction runs are free from mass transfer limitation (Fig. S8). The
recyclability and stability of OV-rich meso-LMO catalyst on furfural
are further evaluated (Fig. 4d). After 5 cycles, the selectivity and
conversion are almost no attenuation compared to fresh catalyst,
indicating the catalyst has excellent stability for hydrogenation of
furfural to FOL (Fig. S9).
[a] Reaction conditions: 0.42 mmol catalytic substrate, 50 mg catalyst, 15 mL
isopropanol, 180 °C, 1 MPa N2.
selectivity than simple aldehydes, which is assigned to steric
hindrance and electron-donating nature of these substrate
molecules.[58] And a summary about the catalytic hydrogen
transfer of biomass-derived carboxides upon various catalysts is
summarized in Table S5. Obviously, our OV-rich meso-LMO
catalyst shows predominant catalytic activity. Moreover, the
hydrogenation catalysis performance of furfural to FOL by
prepared a series of porous ABO3 perovskite oxides were
presented in Fig. S10.
Having shown that the abundant oxygen vacancies in OV-rich
meso-LMO lead to a noticeable enhancement in hydrogenation
activity, in the following part, we investigate the catalytic
mechanism in determining the hydrogenation performance by a
combination of DFT calculations and temperature-programmed
gas desorption experiments. DFT calculations were performed to
gain more atomistic insights into the superiority of OV-rich meso-
LMO among meso-LMO and bulk LMO catalysts. Fig. 5 shows the
optimized stable adsorption configuration of furfural adsorbed on
LMO, and corresponding adsorption energies (Eads) and bond
lengths are summarized in Table S6. As shown in Fig 5 a-c, the
adsorption process on the OV-rich meso-LMO surface (-1.15 eV)
is more favorable than that on the meso-LMO (-0.72 eV) and bulk
LMO (-0.51 eV) suggesting the stronger binding of furfural to the
OV-rich meso-LMO surface. The charge density distributions at
the interfaces were presented in Fig. 5 d-f. It was observed that
the electrons appeared around the Mn atoms and O atoms of
furfural, which further verified that the affinity of carbonyl in furfural
and Mn is enhanced as oxygen vacancy sites grow. It can be seen
that furfural adsorbed on the OV-rich meso-LMO surface is more
favorable, and stronger electron transfer happens between O
atom of carbonyl and Mn atoms from LMO catalyst with abundant
To demonstrate the general adaptability of our OV-rich meso-
LMO catalyst, the CTH reaction on other carbonyl derivatives
were subject to the similar conditions and the result is shown in
Table 1, which exhibits decent catalytic performance for
hydrogenation of different carboxides. It can be seen ketones or
aldehydes with complex structures need a longer reaction time or
rigorous conditions to achieve enhanced conversion and
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oxygen vacancies. To deeply understand the behavior of bonding
and hybridization at the furfural/LMO catalyst interfaces, Fig. 5g-i
shows calculated projected-DOS for the carbonyl group of furfural
and metal atoms of LMO catalysts. With oxygen vacancies
increasing in LMO catalysts, the interaction between carbonyl
group of furfural and O of LMO catalysts become weak, while the
bonding interactions between O in the carbonyl group of furfural
and Mn of OV-rich meso-LMO have formed according to the
hybridization of their states around -4 eV as demonstrated in
Fig.5i. In particular, the π*-states of carbonyl centered near -1.6
eV (adsorbed on the bulk-LMO surface) below the Fermi level are
further down shifted and significantly broadened when adsorbed
on the OV-rich meso-LMO surface. This shift favors the electron-
back donation from the metal 3d-states to the anti-bonding π*-
states of the carbonyl,[59] which is in keeping with the increased
results of C=O bond length (Table S6). Therefore, we believe that
the catalytic activity of CTH reaction on the LMO catalyst could be
promoted by regulating the surface oxygen vacancy of catalyst.
Elucidating the underlying catalytic reaction process and the
enhancement of catalytic activity on elaborate OV-rich meso-LMO
surface for hydrogenation process from furfural to FOL, the
process via proposed pathways and the energy potential of each
elementary step is depicted in Fig.5j. In all calculated cases, H*
from isopropanol as hydrogen donor is directly used. The
optimized catalysts surface with dissociative C4H3(CHO)O is
selected to start this reaction. Firstly, the reaction of C4H3(CHO)O*
Figure 5. DFT-calculated configuration of adsorption states of a furfural molecule on a) bulk LMO, b) meso-LMO and c) Ov-rich meso-LMO. Electron localization
function maps of a furfural molecule adsorbed on d) bulk LMO, e) meso-LMO and f) Ov-rich meso-LMO. g-i) Calculated projected-DOS for furfural adsorbed on bulk
LMO, meso-LMO and Ov-rich meso-LMO surface, corresponding to the adsorption configuration in Fig. 5 a-c, respectively. j) Potential energy profiles for
hydrogenation of furfural to FOL on LMO based catalysts.
+H*→C4H3(CHOH)O* proceeds with the energy barriers Ea of 0.51
eV on OV-rich meso-LMO surface, lower than bulk LMO and
meso-LMO (0.83 eV and 0.64 eV). As shown in reaction energy
C4H3(CH2O)O.[60] Then the remaining H* would react with
C4H3(CHOH)O*
to complete hydrogenation reaction,
C4H3(CH2OH)O obtained. Calculated Ea of elementary steps are
profile, this step is rate-determining step in whole reaction process, presented with respect to adsorbed reactants (Table S7).
and formation of C4H3(CHOH)O is kinetically and
thermodynamically more favorable than the formation of
Compared to hydrogenation reaction process on different LMO
catalysts, the reaction of C4H3(CHO)O*+2H*→C4H3(CH2OH)O on
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10.1002/anie.202012416
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RESEARCH ARTICLE
OV-rich meso-LMO displays the lowest Ea among meso-LMO and
bulk LMO surface. It is obvious that the value of Ea from DFT is
almost consistent with experimentally measured Ea, following OV-
rich meso-LMO< meso-LMO< bulk LMO (Table S8). Based on
above analysis, the enhancement of catalytic activity mainly
benefits from the following features: (1) designed oxygen
vacancies strengthens furfural bonding to the catalysts Mn sites,
the adsorption energy from -0.51 eV (bulk LMO) to -0.72 eV
(meso-LMO), -1.15 eV (OV-rich meso-LMO), respectively; (2) the
barriers in hydrogenation process are greatly decreased on OV-
rich meso-LMO catalyst, especially for the rate-determining step.
In short, DFT analysis reveals that the catalyst with rich oxygen
vacancies can facilitate reactants interaction and better balance
thermodynamics and kinetics for the catalytic reaction process,
which renders OV-rich meso-LMO catalyst to show a higher
catalytic activity of furfural hydrogenation.
In this case, Lewis acid and base sites on solid catalysts
mediate reaction processes and facilitate the reaction between
isopropanol and carbonyl group of furfural. The surface chemistry
property of catalyst would change after oxygen vacancy
engineering via the treatment of NaBH4. In our cases, Mn as the
electron-deficient metal center plays the role of Lewis acid site,
which easily bond to the electron-rich oxygen in isopropanol and
carbonyl group of furfural, respectively, whereas the adjacent
base site attracts the proton in the hydroxyl and weakens the O-
H bond of isopropanol. Furthermore, the adsorption energies of
isopropanol and furfural simultaneously adsorbed on OV-rich
meso-LMO with -2.39 eV is larger than meso-LMO and bulk LMO
(-1.92 eV and -1.03 eV, Fig. S11), which indicate strong affinity
exists among isopropanol, furfural, Lewis acid and base sites of
OV-rich meso-LMO, benefiting for the subsequent hydrogen
transfer reaction. And the acid-base properties of LMO catalysts
were evaluated by performing NH3-TPD and CO2-TPD
experiments (see Fig.S12-S13 and Table S9-S10). As expected,
the as-prepared OV-rich meso-LMO catalyst possesses abundant
acid-base sites with moderate strength, which makes it easier to
adsorb catalytic substrates and proceed the hydrogen transfer
reaction of biomass-derived carboxides. The surface alkalinity is
believed to be related closely to surface oxygen defects and the
chemisorbed oxygen species.[61] It verifies that the analysis of
surface alkaline property is consistent with the result active
oxygen species from O2-TPD and XPS.
to expose oxygen deficiency and rich oxygen vacancies can
facilitate adsorption and activation of furfural molecule, leading to
lower barrier in the hydrogenation process. This work develops a
general synthesis method for mesoporous perovskite oxides and
suggests the activity of ABO3 catalysts can be effectively
optimized via the fine control of the surface oxygen nature of
catalyst.
Acknowledgements
This work was supported by the Young Thousand Talented
Program and the National Natural Science Foundation of China
(21671073 and 21621001), the “111” Project of the Ministry of
Education of China (B17020), Program for JLU Science and
Technology Innovative Research Team.
Keywords: molecule co-assembly process • mesoporous
materials • perovskite oxides • catalytic transfer hydrogenation
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10.1002/anie.202012416
Angewandte Chemie International Edition
RESEARCH ARTICLE
RESEARCH ARTICLE
Yuenan Zheng,[a] Rui Zhang,[a] Ling
Zhang[b], Qinfen Gu[c] and Zhen-An
Qiao*[a]
Page No. – Page No.
A
Resol-assisted
Cationic
Coordinative Co-assembly Approach
to Mesoporous ABO3 Perovskite
Oxides with Rich Oxygen Vacancy for
Enhanced Hydrogenation of Furfural
to Furfuryl Alcohol
An effective and versatile strategy to construct mesoporous ABO3 perovskite oxides
via a resol-assisted cationic coordinative co-assembly approach is developed. The
as-prepared mesoporous LaMnO3 with functionalized nanocrystal frameworks and
abundant oxygen vacancy sites as catalyst exhibits remarkable catalytic activity and
stability for catalytic transfer hydrogenation of furfural to furfuryl alcohol.
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