Efficient catalytic transfer hydrogenation of biomass-based furfural to furfuryl alcohol with recycable Hf-phenylphosphonate nanohybrids

Article abstract of DOI:10.1016/j.cattod.2018.04.056

An acid-base bifunctional nanohybrid phenylphosphonic acid (PhP) - hafnium (1:1.5) was synthesized through assembly of PhP with HfCl₄ for catalytic transfer hydrogenation of furfural (FUR) to furfuryl alcohol (FFA) using 2-propanol as both reaction solvent and hydrogen donor source. An FFA yield of 97.6 % with formation rate of 9760 μmol g^?1 h^?1 at 99.2 % FUR conversion was obtained with the reaction system at 120 °C for 2 h reaction time. Activation energy (Ea) was estimated to be 60.8 kJ/mol with respect to FUR concentration, which is comparable with or even lower than Ea values attained over metal catalysts. The pronounced catalytic activity of PhP-Hf (1:1.5) is attributed to its moderate acidity and relatively strong basicity. The PhP-Hf (1:1.5) catalyst was demonstrated to maintain its activity for five consecutive reuse cycles.

Full text of DOI:10.1016/j.cattod.2018.04.056

Accepted Manuscript
Title: Efficient catalytic transfer hydrogenation of
biomass-based furfural to furfuryl alcohol with recycable
Hf-phenylphosphonate nanohybrids
Authors: Hu Li, Yan Li, Zhen Fang, Richard L. Smith Jr.
PII:
S0920-5861(18)30517-0
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.04.056
CATTOD 11412
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Catalysis Today
Received date:
Revised date:
Accepted date:
10-12-2017
10-3-2018
24-4-2018
Please cite this article as: Li H, Li Y, Fang Z, Smith RL, Efficient
catalytic transfer hydrogenation of biomass-based furfural to furfuryl alcohol
with recycable Hf-phenylphosphonate nanohybrids, Catalysis Today (2010),
https://doi.org/10.1016/j.cattod.2018.04.056
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Efficient catalytic transfer hydrogenation of biomass-based furfural to
furfuryl alcohol with recycable Hf-phenylphosphonate nanohybrids
Hu Li1,2,3, Yan Li2, Zhen Fang1*, Richard L. Smith Jr.3
1
Biomass Group, College of Engineering, Nanjing Agricultural University, 40
Dianjiangtai Road, Nanjing, Jiangsu 210031, China
2
State-Local Joint Engineering Laboratory for Comprehensive Utilization of Biomass,
Center for R&D of Fine Chemicals, Guizhou University, Guiyang 550025, China
3 Tohoku University, Research Center of Supercritical Fluid Technology, Graduate School
of Environmental Studies, Aramaki Aza Aoba 6-6-11, Aoba-ku, Sendai 980 8579, Japan
*Author for correspondence
Zhen Fang (zhenfang@njau.edu.cn)
Web: http://biomass-group.njau.edu.cn/
Other emails: Hu Li (huli_0414@126.com)
Yan Li (1246613573@qq.com)
Richard L. Smith Jr. (smith@scf.che.tohoku.ac.jp)
Revised submission for a special issue of Catalysis Today
March 2018
Graphic Abstract:
1

Mesoporous PhP-Hf(1:1.5) nanohybrid material bearing acidic and basic sites
promotes catalytic transfer hydrogenation of furfural to furfuryl alcohol with 97.6%
yields with 2-propanol as solvent and hydrogen donor source at 120 ºC in 2 h reaction
time.
Highlights:
 Solvothermal method used to prepare mesoporous PhP-Hf(1:1.5) nanohybrid
 Yields of 97.6% furfuryl alcohol obtained from furfural with hybrid material
 Acid-base sites of hybrid material exhibit synergism
 Density and strength of acid-base sites in hybrid material affect activity
 PhP-Hf (1:1.5) recyclable with little change in activity or morphology
ABSTRACT
An acid-base bifunctional nanohybrid phenylphosphonic acid (PhP) – hafnium (1:1.5)
was synthesized through assembly of PhP with HfCl4 for catalytic transfer
hydrogenation of furfural (FUR) to furfuryl alcohol (FFA) using 2-propanol as both
reaction solvent and hydrogen donor source. An FFA yield of 97.6% with formation
rate of 9760 μmol g-1 h-1 at 99.2% FUR conversion was obtained with the reaction
2

system at 120 ºC for 2 h reaction time. Activation energy (Ea) was estimated to be
60.8 kJ/mol with respect to FUR concentration, which is comparable with or even
lower than Ea values attained over metal catalysts. The pronounced catalytic activity
of PhP-Hf (1:1.5) is attributed to its moderate acidity and relatively strong basicity.
The PhP-Hf (1:1.5) catalyst was demonstrated to maintain its activity for five
consecutive reuse cycles.
Keywords: heterogeneous catalysis; biomass conversion; furfural; acid-base catalysis;
transfer hydrogenation
1. Introduction
Lignocellulosic biomass is the most abundant organic carbon source in the nature.
Many efforts are being made to transform lignocellulosic biomass into biofuels and
chemicals with catalytic processes for replacement of petroleum processes [1-3].
Among the catalytic processes, emphasis is being made for upgrading furanic
compounds such as furfural (FUR) and 5-hydromethylfurfural (HMF) via
hydrogenation, oxidation, alkoxylation and rehydration [4-8]. In particular, increasing
the hydrogen content in oxygenates using H-donor sources is one of crucial steps for
producing energy-intensive molecules efficiently and safely [9-12]. Noble metals (e.g.,
Au, Pd and Pt) are typically used as catalysts to obtain sufficient yields of target
products [13-17]. Nevertheless, the development of low-cost solid functional
materials active for catalytic hydrogenation of biomass-based chemicals is required to
relieve the demand for noble metals.
One of the most important components in bio-oil, FUR, can be obtained via
dehydration of carbohydrates, especially xylose, in the presence of an acid catalyst
[18]. This approach, however, is susceptible to humin formation or to the formation of
soluble polymers via condensation of the reactive aldehyde group with alcohols,
carbonyl and phenolic compounds at high temperatures (> 300 ºC) even without a
3

catalyst [19]. Catalytic hydrogenation of C=O to C–OH can be used to avoid the
occurrence of unwanted side reactions [19] and may also enrich transformation routes
for FUR upgrading [20]. A number of metallic catalysts (e.g., Pd, Ir, Pt and Ru) have
been reported to be efficient for partial hydrogenation of the aldehyde group in FUR,
selectively producing furfuryl alcohol (FFA) in yields as high as 98% [21-25].
However, it would be desirable to have non-noble metal (e.g., Cu, Co and Ni) [26-30],
with solid materials such as zirconia [31,32], hydrotalcite [33-35], hydroxyapatite
[36], and zeolites [37-40] as heterogeneous catalysts for biomass transformations,
since these materials are widely available and can contribute to sustainable
development .
Trivalent and tetravalent metal organophosphonates are highly insoluble and stable in
neutral and acidic reaction media [41,42]. In the present study, a series of hafnium
phenylphosphonates (PhP-Hf) were prepared by solvothermal assembly of
phenylphosphonic acid (PhP) with HfCl4 in different P/Hf molar ratios (1:2-2:1) that
were characterized to be nanosized and mesoporous with irregular interlayer spacings,
as well as having moderate acidity and basicity. Especially, the nanohybrid
PhP-Hf(1:1.5) was found to exhibit pronounced catalytic performance in the transfer
hydrogenation of FUR to FFA under relatively mild conditions. Reaction conditions
were optimized to understand structure-activity relationships, reaction mechanism and
reusability of the catalyst.
2. Experimental
2.1. Materials
Furfural (FUR, >99.5%), furfuryl alcohol (FFA, >98%), phenylphosphinic acid
(PhP, >98%), HfCl4 (>99.9%), HfO2 (99.9%), SiO2 (>99.8%), NiO (99.9%), MgO
(99.9%), CaO (99.9%), Al2O3 (99.9%), ZrO2 (99.9%), dimethyl formamide (DMF,
99.8%), naphthalene (>99.7%), methanol (>99.9%), ethanol (>99.5%) and 2-propanol
(99.5%) were purchased from Aladdin Industrial Inc. (Shanghai). Other
4

general-purpose chemicals were purchased from chemical companies and employed
without further treatment, unless otherwise noted.
2.2. Preparation of Hf-phenylphosphonate hybrids
Hf-phenylphosphonate (PhP-Hf) hybrids with various P/Zr ratios were prepared by a
solvothermal method using different moles of phenylphosphonic acid (PhP) and HfCl4
as starting materials. In the general procedure, 316.2 mg PhP (2 mmol) was initially
added into a Teflon test tube (100 mL) containing 80 mL DMF, which was stirred
(500 rpm) and allowed to be completely dissolved before addition of 906.9 mg HfCl4
(3 mmol). Upon stirring for another 20 min, the Teflon tube was transferred into a
stainless steel autoclave and sealed, which was then placed into a muffle furnace and
statically heated at 120 ºC for 24 h. The resulting precipitate was separated by
filtration, followed by successively washing with DMF (30 mL × 3), methanol (25 mL
× 2) and ethanol (20 mL × 3) to remove the unreacted precursors and residual DMF
solvent inside pores, and then dried under vacuum at 80 ºC for 8 h. The obtained
Hf-phenylphosphonate hybrid was denoted as PhP-Hf (1:1.5), where the values in
brackets represent the molar ratio of P to Hf. Other Hf-phenylphosphonate hybrids
with different P/Hf molar ratios of 1:2, 1:1, 1.5:1 and 2:1 were prepared and are
designated as PhP-Hf (1:2), PhP-Hf (1:1), PhP-Hf (1.5:1) and PhP-Hf (2:1),
respectively.
2.3. Catalyst characterization
Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer 1710
spectrometer (KBr disks). Content of Hf and P species were determined by
inductively coupled plasma-optical emission spectrometer (ICP-OES) on a
PerkinElmer Optima 5300 DV. X-ray diffraction (XRD) patterns were recorded on a
D/max-TTR III X-ray powder diffractometer (radiation source: Cu Kα). Sample
images were obtained with a transmission electron microscope [(HR)-TEM;
JEM-1200EX]. Particle size distribution based on about 100 particles was estimated
with software (Nano Measurer 1.2, Visual Basic 6.0). Scanning transmission electron
5

microscope and high-angle annular dark-field (STEM-HAADF) mappings were
acquired with an aberration corrected FEI Tecnai G2 F30 S-Twin (S)TEM operating
at 300 kV, fitted with energy dispersive X-ray (EDX) spectrometer.
Brunauer-Emmett-Teller (BET) surface area, pore size and volume were determined
from N2 physisorption measurements at liquid N2 temperature on a Micromeritics
ASAP 2020 instrument. Thermogravimetry (TG) analyses were carried out on a
NETZSCHSTA 429
instrument
under
an
N2
atmosphere
for
a
programmed-temperature range of 50-600 °C (heating ramp: 10 °C/min). X-ray
photoelectron spectroscopy (XPS) measurements were recorded on a Physical
Electronics Quantum 2000 Scanning ESCA Microprobe (Al Kα anode). Amount of
basic and acidic sites in the materials were determined by CO2-temperature and
NH3-temperature programmed desorption (TPD), respectively, using a Micromeritics
AutoChem 2920 chemisorption analyzer. In detail, solid samples placed in a quartz
reactor were initially degassed at 150 °C for 60 min under a flow of He (50 mL min−1).
Subsequently, samples were allowed cool via natural convection to 50 °C, and then
the treated samples were flushed with 10% CO2 or NH3 in He (60 mL min−1) for 1 h,
followed by purging with pure He (60 mL min−1) for the same period. Then,
desorption of CO2 or NH3 was recorded by a thermal conductivity detector (TCD) at
one-second intervals for a programmed temperature range of 50-300 °C (10 °C min−1)
under He (50 mL min−1), and with a hold temperature of 300 °C for 60 min.
2.4. Typical reaction procedure for conversion of FUR to FFA
All reactions were carried out in Ace borosilicate glass pressure test tubes (15 mL) in
an oil bath. Typically, 1.0 mmol FUR, 5 mL 2-propanol, and 0.05 g catalyst were
added into the test tube, and placed into an oil bath that was pre-heated to a set
temperature of 80-140 ºC. The reaction mixture was magnetically stirred at 600 rpm
for a specific reaction time of 0.25-10 h. After a given reaction time, the test tube was
cooled down to room temperature with water in a beaker. The resulting liquid samples
were collected with an injector and passed through a filter membrane (pore size: 0.22
μm) prior to quantitative analysis by gas chromatography (GC).
6

Liquid samples were identified by GC-MS (Agilent 6890N GC / 5973 MS). FUR,
FFA and byproducts (e.g., furfural diisopropyl acetal, 2-isopropoxymethylfuran, and
isopropyl levulinate) were analyzed with GC (Agilent 7890B) using a HP-5 column
(30 m × 0.320 mm × 0.25 μm) equipped with a flame ionization detector (FID) by
addition of naphthalene (ca. 20 mg) as internal standard. The conversion of FUR and
FFA yields were calculated on the basis of standardized curves with five points made
from commercial samples.
After each reaction, the solid residue in the solution was separated by centrifugation
(12000 rpm) for 3 min, followed by successively washing with ethanol (25 mL × 3)
and acetone (20 mL × 2) under ultrasonic treatment, and drying at 80 ºC for 6 h. The
obtained catalyst was directly used for the next run. To examine leaching of the Hf
and P species, ICP analyses of the supernatant liquids that were volumetrically diluted
with water were performed.
3. Results and discussion
3.1. Catalyst characterization
Structural functionalities of the prepared PhP-Hf (1:1.5) hybrid and commercial HfO2
were initially examined with FT-IR (Fig. 1). Both PhP-Hf (1:1.5) and HfO2 had a
wide stretching region of 3200-3600 cm-1 and bending vibration at 1640 cm-1, which
can be ascribed to the presence of –OH species and water molecules, respectively [43].
The partially overlapping band near 1600 cm-1 belongs to phenyl group stretching,
while the bands at 1485 and 1435, and 3000 cm-1 are assigned to the skeletal vibration
and C-H stretching vibration of the aromatic ring, respectively [44]. It should be noted
that the bands around 2770 and 2400cm-1 are possibly due to the stretching vibration
of O-H species in monohydrogen phosphonate groups [45]. Bands at around 560 and
760, 695 and 750, 900-1120, and 1165 cm-1 are characteristic of Hf-O bonds,
monosubstituted phenyl ring vibrations, asymmetric and symmetric P-O stretching
7

vibrations of tetrahedral C-PO3 groups, and P-C stretching vibration, respectively [46].
Although the characteristic band at 760 cm-1 of Hf-O bond in PhP-Hf (1:1.5) was
partially overlapped with that of the phenyl ring vibration (750 cm-1), the distinct
blueshift of the band from 520 cm-1 for HfO2 to 560 cm-1 for PhP-Hf (1:1.5) showed
that a relationship existed between Hf-O with P species.
<Fig. 1>
XRD patterns of HfO2 showed that the material was crystalline and mixed with
tetragonal (t) and monoclinic (m) structures, while PhP-Hf (1:1.5) had an amorphous
structure (Fig. 1B). Notably, an additional reflection with d-spacing of 14.9 Å at 2θ of
5.9º was observed in the case of PhP-Hf (1:1.5), which was most likely due to the
interlayer distance of Hf layers being held apart by PhP [47]. Correspondingly,
nitrogen adsorption-desorption isotherms (Fig. 1C) gave H1-type and H4-type loops
for HfO2 and PhP-Hf(1:1.5), respectively, illustrating an increase in BET surface area
(214 m2/g) and pore volume (0.21 cm3/g) but a decrease in average pore size (3.4 nm)
of PhP-Hf (1:1.5), as compared with those of HfO2 (Table 1). These results
substantiated the formation of extra interlayers via assembly that significantly
contributed to improving surface texture to have more substrate-accessible active sites.
The molar ratio of P/Hf in the PhP-Hf (1:1.5) hybrid was found to be 1.07 (epiphase)
by XPS quantitative analysis, which was much higher than that (0.68, bulk phase)
obtained by ICP analysis. The difference of P/Hf molar ratio in epiphase and epiphase
of PhP-Hf (1:1.5) indicated that the organic ligand (PhP) preferred to be located on
the surface of the formed hybrid, while Hf was mainly encapsulated inside the
material. The thermal stability of PhP-Hf (1:1.5) and HfO2 is shown in Fig. 1D. Both
PhP-Hf (1:1.5) and HfO2 were stable up to 300 ºC, so that weight loss in the
temperature range of 300-550 ºC can be attributed to removal of weakly bonded –OH
species (Fig. 1A). When samples were brought to temperatures above 550 ºC, large
weight loss was observed for PhP-Hf (1:1.5) (Fig. 1D), that can be attributed to the
decomposition of organic species.
<Table 1>
8

In view of the PhP-Hf (1:1.5) hybrid being highly stable below 300 ºC, its acidic and
basic densities were evaluated by NH3- and CO2-TPD over the
temperature-programmed range of 50-300 ºC and at a maximum holding temperature
of 300 ºC (Table 1). Basic (0.32 mmol/g) and acidic (0.27 mmol/g) content of PhP-Hf
(1:1.5) was observed to be relatively higher than that of HfO2 (Table 1); in particular,
a significant increase in the content of acid sites relative to that of basic sites lead to
the formation of additional acidic –OH species possibly derived from Zr-OH and
P-OH groups [48]. To further elucidate the basic and acidic strength of PhP-Hf (1:1.5)
and HfO2, XPS analyses were performed (Fig. 2). The binding energies of Hf 4f in
PhP-Hf (1:1.5) at 17.4 and 18.9 eV were slightly larger than those (16.9 and 18.5 eV)
of Hf 4f in HfO2, respectively (Fig. 2A), showing the formation of Hf species in the
hybrid with strong Lewis acidity [49]. On the other hand, a relatively higher binding
energy of O 1s (531.3 eV) assigned to P-O-Hf interaction in PhP-Hf (1:1.5) than that
of Hf(H)-O-Hf interaction in HfO2 (Fig. 2B) due to the much smaller negative charge
on the oxygen species [50,51], which was directly correlated with lower base strength
of PhP-Hf (1:1.5) compared with that of HfO2. These results showed that the
introduction of organic ligand PhP could stabilize the Hf species with respect to both
acid content and strength of PhP-Hf (1:1.5) [52], and that the formed P-O-Hf
framework in PhP-Hf (1:1.5) with interlayers appeared to increase surface area and
pore volume, thus increasing its basic content achieved by its superior accessibility to
HfO2. The Hf-O-Hf framework or hydroxide species in HfO2 seemed to give
enhanced basic strength in comparison with PhP-Hf (1:1.5).
< Fig. 2>
HR-TEM images indicated the presence of amorphous and crystalline [tetragonal (t)
and monoclinic (m)] structures in PhP-Hf (1:1.5) and HfO2, respectively (Fig. 3),
which was consistent with XRD analyses (Fig. 1B). Worm-like stripes present in Fig.
3A were most likely the result of disordered interlayers of the PhP-Hf (1:1.5)
nanohybrid materials. Moreover, STEM-HAADF images with corresponding
9

elemental mappings in Fig. 4 demonstrated good spatial arrangement of Hf, C, O and
P species in PhP-Hf (1:1.5), showing even dispersion and connection of Hf and
C–PO3 moieties throughout the nanohybrid material. All of these unique
physicochemical characteristics make PhP-Hf (1:1.5) a promising material for
heterogenous transfer hydrogenations.
<Fig. 3>
<Fig. 4>
3.2. Transfer hydrogenation of FUR to FFA with different catalysts
Preliminary studies were conducted with different oxides (SiO2, NiO, MgO, CaO,
Al2O3, ZrO2 and HfO2), the prepared PhP-Hf (1:1.5) nanohybrid material and the
corresponding synthetic precursors (PhP and HfCl4) as catalysts for transfer
hydrogenation of FUR to FFA using 2-propanol as H-donor source at 120 ºC for 2 h
(Table 2). Neither weakly acidic oxides (e.g. SiO2) nor weakly basic metal oxides (e.g.
NiO, MgO and CaO) could efficiently catalyze transfer hydrogenation of FUR
(Entries 1-4, Table 2), which is in accordance with literature results for transfer
hydrogenation of ethyl levulinate to γ-valerolactone using metal hydroxides [53]. In
these catalytic systems, furfural diisopropyl acetal derived from acetalization of FUR
with 2-propanol was detected to be the dominant byproduct, particularly in the
presence of an acidic catalyst (e.g., SiO2) [54]. In sharp contrast, metal oxides bearing
both basic and acidic sites were active for FFA synthesis (Entries 5-7, Table 2),
wherein basic sites were postulated to assist in the formation of isopropoxide from
2-propanol on acidic sites by proton abstraction, thus giving a six-membered
intermediate among acid sites, alcohol and aldehyde groups to transfer β-H and finally
yield FFA [55]. The prepared PhP-Hf (1:1.5) nanohybrid material, which had higher
strength acidic and lower strength basic sites in comparison with HfO2 (Table 1), was
able to produce FFA in almost quantitative yield (97.6%) and at high formation rate
(9760 μmol g-1 h-1) from FUR at 99.2% conversion (Table 2, entry 8). When a strong
base (NaOH) was used as catalyst, FUR was also converted but via Cannizzaro
reaction [56], affording mixed products of FFA and 2-furoic acid in nearly same
10

yields of ca. 21.3% (entry 9). The strong acidic precursors of PhP-Hf (1:1.5) including
PhP and HfCl4 could not selectively catalyze FUR (Entries 10-11, Table 2), while
furfural diisopropyl acetal, 2-isopropoxymethylfuran and isopropyl levulinate were
found to be the major products under identical reaction conditions. Therefore, the
presence of both acid-base sites having appropriate content and strength seemed to be
key factors for the selective production of FFA from FUR with the prepared catalytic
materials. Compared with PhP-Hf (1:1.5), the lower catalytic activity of PhP-Ni
(1:1.5), PhP-Al (1:1.5), and PhP-Zr (1:1.5) (Table 2, Entries 12-14) clearly indicated
the unique role that the Hf species have in the formation of acid-base sites that
promote transfer hydrogenation. Notably, the obtained FFA selectivity (98.4%) over
the prepared PhP-Hf (1:1.5) hybrid (Table 2, Entry 8) is superior to 5 wt% Pd/C
(87.1%) and comparable to 5% Pd–5% Cu/MgO (99.0%) at 130 ºC under 0.8 MPa H2
after 100 min and 55 min reaction time, respectively [21].
<Table 2>
3.3. Effect of acidity/basicity on catalytic transfer hydrogenation of FUR to FFA
To further investigate the influence of both content and strength of acid-base sites on
the catalytic transfer hydrogenation of FUR to FFA, a series of PhP-Hf hybrids with
different P/Hf molar ratios were prepared (Table 3). XPS analyses showed a slightly
higher P/Hf ratio than the actual feed ratio that was supported by ICP analysis,
demonstrating that the organic ligand PhP rather than Hf species was prone to be
assembled on the hybrid surface in accordance with results in vide supra. With an
increase in the relative Hf content (Table 3), the reactivity of the corresponding
hybrids became greatly enhanced, with a maximum yield (97.6%) and selectivity
(98.4%) of FFA with a turnover number (TON) value (33.6) being obtained when the
P/Hf ratio was 1:1.5. With a decrease in the P/Hf ratio of 2:1 to 1:2, both acid-base
site contents were characterized separately by NH3- and CO2-TPD and found to
steadily increase from 0.44 to 0.65 mmol/g, while acid/base site ratios slightly
increased from 0.82 to 0.87 (Fig. 5, Table 3). This indicates that the acid-base content
of 0.59 mmol/g with acid/base site ratio of 0.84 in the case of PhP-Hf (1:1.5) is
11

favorable for the selective production of FFA from FUR. The strength of acidic and
basic sites of these PhP-Hf hybrids as given by XPS (Fig. 6) were found to be
negatively correlated with the P/Hf ratio (Fig. 6A), while the base strength was
intially enhanced with the reduction of the P/Hf ratio from 2:1 to 1:1.5, and then
became weak as the P/Hf ratio further decreased to 1:2 (Fig. 6B). Considering the
above results, it could be concluded that the catalytic performance of the PhP-Hf
(1:1.5) hybrid in the transfer hydrogenation of FUR to FFA (Table 3) could be
attributed to its moderate content of acid-base sites and medium acidity and relatively
stronger basicity, despite the basicity of the sites being weaker than those of HfO2
(Fig. 2B).
<Table 3>
<Fig. 5>
<Fig. 6>
A reaction mechanism can be proposed analogous to the catalytic pathway of
Meerwein-Ponndorf-Verley reduction [57-59] that considers the synergistic role of
acid (Hf4+) and base (O2-) sites of PhP-Hf (1:1.5) in the transfer hydrogenation of
FUR to FFA. A key transition state, which is shown in Fig. 7, is proposed to be
formed in situ among FUR, 2-propanol, and acid-base coupled species (Hf4+–O2-). In
general, a pair of acid-base sites (Hf4+–O2-) assists in the adsorption of 2-propanol
onto PhP-Hf (1:1.5) and is capable of leading to the generation of isopropoxide and
hydride by dissociation. The aldehyde group in FUR is probably activated by acidic
Hf4+ species, thus resulting in the formation of a transition state with six links to fulfill
the hydrogen transfer route to yield FFA and acetone (Fig. 7). In the transformation,
some side products such as furfural diisopropyl acetal, 2-isopropoxymethylfuran,
isopropyl levulinate, and 2-furoic acid may be formed in the presence of relatively
stronger acidic and basic sites, respectively (Fig. 7).
<Fig. 7>
3.4. Effect of reaction temperature and time on catalytic conversion of FUR to FFA
12

Typically, reaction rate and product distribution are closely correlated with reaction
temperature and time. Table 4 summarizes catalytic results of FUR-to-FFA conversion
with PhP-Hf (1:1.5) at 80-140 ºC after 0.25-10 h. Both FUR conversion and reaction
rates were sensitive to reaction temperature in which the highest TOF value (68.5 h-1)
that could be obtained occurred at 140 ºC for 0.25 h reaction time, and more than 90%
conversion was obtained in less than 1.5 or 1 h as the reaction took place at relatively
high temperatures of 120 or 140 ºC. Furfural diisopropyl acetal was observed to be the
dominant co-product either at relatively low temperaures (80 and 100 ºC) or at short
reaction times (0.25-1 h), thus leading to lower yields and selectivities of FFA from
FUR. Interestingly, the formed acetal could be reversibly transformed into FUR by
prolonging the reaction duration, which was verified by the gradual increase in FFA
selectivity (Table 4). Under relatively harsh reaction conditions (e.g., 140 ºC for 4 or 6
h), some undesirable products (e.g., furan-based esters, ethers and polymers) in small
amounts derived from side reactions such as Cannizzaro and polycondensation were
observed [60] that resulted in the decrease of FFA yield and selectivity. The carbon
balance for all the reactions was found to be no less than 85%. From the point of view
of saving energy, reactions proceeding at moderate conditions (e.g. 120 ºC, 2 h
reaction time) are most likely to be close to optimal for the catalytic transfer
hydrogenation of FUR. Importantly, it was observed that the TOF value of 16.8 h-1
obtained at 120 ºC was superior to those at 80 ºC (8.2 h-1) and 100 ºC (12.7 h-1), and
comparable to that (16.9 h-1) obtained at a higher temperature of 140 ºC for the same
reaction time of 2 h.
<Table 4>
The catalytic hydrogenation of FUR has been reported to be pseudo-first order with
respect to FUR concentration [61,62]. At first, the initial reaction rate constants (k) of
PhP-Hf (1:1.5) at different reaction temperatures of 353, 373 and 393 K were
calculated by plotting the values of -ln(1 - X) (X = FUR conversion) against variable
reaction times (t) in the range of 15 to 120 min (Fig. 8A). From an Arrhenius plot of
the data (Fig. 8B), the apparent activation energy (Ea) was estimated to be 60.8
13

kJ/mol, which is comparable to and even lower than previously reported Ea values
using metal catalysts for the same reaction including e.g., supported Ni (QD3): 40-60
kJ/mol [63], ZrPN: 70.5 kJ/mol [49], Cu-Mg-Al: 66 kJ/mol [64], and CuMgAl: 127
kJ/mol [65]. The pronounced catalytic results of PhP-Hf (1:1.5) clearly demonstrates
that the prepared materials are highly active for transfer hydrogenation of FUR to
FFA.
<Fig. 8>
3.5. Effect of catalyst dosage on transfer hydrogenation of FUR to FFA
The effect of PhP-Hf (1:1.5) dosage on catalytic transfer hydrogenation of FUR to
FFA was studied (Table 5). It could be observed that almost no FUR (<1.0%) was
converted into FFA in the absence of PhP-Hf (1:1.5), implying that the reaction could
not occur spontaneously (Entry 1, Table 5). Once the addition of 12 mg PhP-Hf (1:1.5)
(FUR/catalyst mass ratio: 8.0), FFA yield reached 40.7% (TON: 60.2) under identical
conditions (Entry 2, Table 5). The catalytic activity regarding the yields of FFA
linearly increased by further increasing the catalyst dosage (Entries 2-4, Table 5), and
high FFA yields of 97.6% and FUR conversions of 99.2% with moderate TON of 33.6
were obtained using PhP-Hf(1:1.5) at a FUR/catalyst mass ratio of 1.9 (Entry 4, Table
5). Then, both FFA yields and FUR conversions leveled off, giving low TON values
(Entries 5-6, Table 5). It should be noted that FFA selectivity showed little change,
regardless of the amount of catalyst. In this regard, PhP-Hf (1:1.5) with a dosage of 50
mg (FUR/catalyst mass ratio: 1.9) seems to be a robust candidate for the synthesis of
FFA from FUR.
<Table 5>
3.6. Catalyst recycle study
At the above optimized reaction conditions for 50 mg catalyst at 120 ºC for 2 h, the
recycle of PhP-Hf (1:1.5) in the transfer hydrogenation of FUR to FFA was studied
(Fig. 9). As shown in Fig. 10, FFA yields were only slightly reduced from 97.6% to
92.3% at FUR conversions of 93.6-99.2% over PhP-Hf (1:1.5) in five consecutive
14

reaction cycles. Stable FFA selectivities in the range of 98.3-98.8% were observed
during the recycle runs, further indicating the robust acid-base sites in the PhP-Hf
(1:1.5) hybrid. ICP analyses showed that less than 1.2 and 1.8 ppm of Hf and P
species were leached into the alcoholic solution after the first round of reactions.
There was no significant change in textural structure (surface area, pore size and
volume) and acid-base properties of PhP-Hf (1:1.5) before and after five cycles of
reactions (Table 1). The slight decrease in FFA yield and FUR conversion is probably
caused by the partial adsorption of organic species such as isopropoxide derived from
2-propanol, which is supported by the appearance of additional bands at around 2900
cm-1 (Fig. 10) possibly assigned to C-H stretching vibrations of methyl and ethyl
groups in isopropoxide, and the reduced intensity that is probably due to the coverage
of exogenous organic species of the bands at near 1600 cm-1 belonging to the
stretching vibration of phenyl groups in PhP moieties.
<Fig. 9>
<Fig. 10>
Conclusions
Hybrid PhP-Hf (1:1.5) catalysts that are highly-active for transfer hydrogenation can
be prepared by solvothermal treatment of phenylphosphonic acid (PhP) with HfCl4 in
a molar ratio of 1:1.5 at 120 ºC for 24 h treatment time. The PhP-Hf (1:1.5) materials
were characterized and found to be nanosized (13.7 nm), mesoporous (average pore
size: 3.4 nm), and bifunctionalized with acid (0.27 mmol/g) and base (0.32 mmol/g)
sites. The PhP-Hf (1:1.5) nanohybrid was demonstrated to have good catalytic
performance in transfer hydrogenation of FUR with 2-propanol, giving FFA in yields
of up to 97.6% after reaction for 2 h at 120 ºC. The superior catalytic activity with
moderate activation energy (Ea = 60.8 kJ/mol) could be attributed to the moderate
acidity and relatively strong basicity of the catalytic sites in the PhP-Hf (1:1.5)
nanohybrid materials. In the reaction mechanism, a key transition state forms among
FUR, 2-propanol, and acid-base coupled species (Hf4+–O2-). The PhP-Hf (1:1.5)
15

nanohybrid catalyst was determined to be recyclable with no distinct loss in activity
for at least five reaction cycles.
Acknowledgments
This work is financially supported by Nanjing Agricultural University (68Q-0603),
International Postdoctoral Exchange Fellowship Program of China (20170026),
Postdoctoral Science Foundation of China (2016M600422), and Jiangsu Postdoctoral
Research Funding Plan (1601029A).
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Table and Figure Captions:
Table 1. Textural and acid-base properties of HfO2 and PhP-Hf (1:1.5)
Table 2. Transfer hydrogenation of FUR to FFA with different catalysts
Table 3. Effect of different P/Hf ratios on transfer hydrogenation of FUR to FFA
Table 4. Effect of reaction time and temperature on conversion of FUR to FFA
19

Table 5. Effect of PhP-Hf (1:1.5) dosage on transfer hydrogenation of FUR to FFA
Fig. 1. FT-IR spectra (A), XRD patterns (B), N2 adsorption-desorption isotherms (C), and TG
curves (D) of PhP-Hf (1:1.5) and HfO2
Fig. 2. XPS spectra of (A) Hf 4f and (B) O 1s in PhP-Hf (1:1.5) and HfO2
Fig. 3. HR-TEM images of (A) PhP-Hf (1:1.5) and (B) HfO2
Fig. 4. STEM-HAADF images (a & b), and (c) Hf, (d) C, (e) O & (f) P elemental mappings of
PhP-Hf (1:1.5)
Fig. 5. NH3- and CO2-TPD patterns of different PhP-Hf catalysts
Fig. 6. XPS spectra of (A) Hf 4f and (B) O 1s in different PhP-Hf catalysts
Fig. 7. Possible mechanism for transfer hydrogenation of FUR to FFA catalyzed by PhP-Hf (1:1.5)
Fig. 8. (A) Kinetic profiles and (B) Arrhenius plot of PhP-Hf (1:1.5)-catalyzed conversion of FUR
to FFA; reaction conditions: 96 mg FUR (1 mmol), 0.05 g PhP-Hf (1:1.5), and 5 mL 2-propanol
Fig. 9. Recycling study of PhP-Hf (1:1.5) in transfer hydrogenation of FUR to FFA; reaction
conditions: 96 mg FUR (1 mmol), 0.05 g PhP-Hf(1:1.5), 5 mL 2-propanol, 120 ºC for 2 h
Fig. 10. FT-IR spectra of fresh and recovered (after 5 cycles) PhP-Hf (1:1.5) hybrids
Table 1. Textural and acid-base properties of HfO2 and PhP-Hf (1:1.5)
SBET
(m2/g)
23
Vpore
(cm3/g)
0.17
Basicity
(mmol/g)[a]
0.24
Acidity
(mmol/g)[a]
0.16
Acid/base
ratio
Catalyst
Dmean (nm)
HfO2
28.9
3.4
0.67
PhP-Hf (1:1.5)
PhP-Hf (1:1.5)[b]
214
0.21
0.32
0.27
0.84
203
0.19
3.6
0.35
0.25
0.71
[a] Basicity and acidity were determined by CO2- and NH3-TPD, respectively
[b] Recovered PhP-Hf (1:1.5) after five consecutive cycles
SBET: BET surface area, Vpore: volume of pores, Dmean: average pore size
20

Table 2. Transfer hydrogenation of FUR to FFA with different catalysts
Formation rate
Entry
Catalyst
FFA yield (%)
FUR conv. (%) FFA selec. (%)
(μmol g-1 h-1)[a]
1
2
SiO2
NiO
0.6
3.1
19.0
8.6
2.9
36.2
15.1
75.6
74.5
83.4
85.5
98.4
-
60
310
3
MgO
0.5
3.6
50
4
CaO
1.4
1.8
140
5
Al2O3
22.3
23.6
27.8
97.6
21.3
<0.2
1.1
29.9
28.3
32.5
99.2
52.6
37.4
93.3
17.4
45.2
86.8
2230
2360
2780
9760
2130
<20
6
ZrO2
7
HfO2
8
PhP-Hf (1:1.5)
NaOH
9
10
11
12
13
14
PhP
<0.5
1.2
HfCl4
110
PhP-Ni (1:1.5)
PhP-Al (1:1.5)
PhP-Zr (1:1.5)
7.5
43.1
81.6
89.4
750
36.9
77.6
3690
7760
Reaction conditions: 96 mg FUR (1 mmol), 0.05 g catalyst, 5 mL 2-propanol, 120 ºC for 2 h
[a] FFA formation rate is defined as (mol of formed FFA) / (catalyst amount × time)
21

Table 3. Effect of different P/Hf ratios on transfer hydrogenation of FUR to FFA
P/Hf
ratio[a]
2.53
FFA yield
(%)
FUR
conv. (%)
50.8
FFA selec.
(%)
Acid-base content
Acid/base
site ratio
0.82
Catalyst
TON[c]
(mmol/g)[b]
PhP-Hf (2:1)
PhP-Hf (1.5:1)
PhP-Hf (1:1)
PhP-Hf (1:1.5)
PhP-Hf (1:2)
46.8
92.2
0.44
23.1
26.7
32.0
33.6
30.7
1.99
62.5
66.1
94.5
0.49
0.82
1.45
85.6
88.1
97.2
0.55
0.83
1.07
97.6
99.2
98.4
0.59
0.84
0.80
97.3
99.8
97.5
0.65
0.87
Reaction conditions: 96 mg FUR (1 mmol), 0.05 g catalyst, 5 mL 2-propanol, 120 for 2 h
[a] Contents of P and Hf were obtained by XPS analyses
[b] Total contents of acid and base sites were determined by NH3- and CO2-TPD
[c] TON (turnover number) = (mole of converted FUR) / (mole of acid-base sites)
22

Table 4. Effect of reaction time and temperature on conversion of FUR to FFA
FFA yield (%)
FUR conv. (%) FFA selec. (%)
TOF(h-1)[a]
Temp. (ºC) Time (h)
0.25
0.5
1
2.8
11.7
15.7
25.3
34.3
48.1
65.6
79.3
86.7
90.3
11.5
30.7
51.9
70.2
75.2
83.3
23.9
40.1
62.1
69.1
77.5
84.5
86.3
87.8
90.3
56.5
61.6
74.6
85.8
92.2
94.4
15.9
10.6
8.6
6.3
15.7
23.7
37.3
55.5
68.4
76.1
81.5
6.5
1.5
7.8
80
2
4
8.2
5.6
6
4.5
8
3.7
10
0.25
0.5
1
3.1
15.6
20.8
17.6
15.9
12.7
7.1
18.9
38.7
60.2
69.3
78.6
100
1.5
2
4
6
0.25
0.5
1
1.5
2
87.9
15.4
52.5
74.8
86.2
97.6
98.3
99.9
43.9
63.6
84.3
90.9
98.3
96.5
95.2
92.4
23.7
65.1
86.5
94.5
99.2
99.7
100
50.5
71.2
92.3
98.6
99.8
100
95.1
65.0
80.6
86.5
91.2
98.4
98.6
99.9
86.9
89.3
91.3
92.2
98.5
96.5
95.2
5.2
32.1
44.1
29.3
21.4
16.8
8.4
120
140
4
6
5.6
0.25
0.5
1
1.5
2
68.5
48.3
31.3
22.3
16.9
8.5
4
6
100
5.6
Reaction conditions: 96 mg FUR (1 mmol), 0.05 g PhP-Hf (1:1.5), 5 mL 2-propanol
[a] TOF (turnover frequency) = (mole of converted FUR) / [(mole of acid-base sites) × time]
23

Table 5. Effect of PhP-Hf (1:1.5) dosage on transfer hydrogenation of FUR to FFA
FUR/catalyst
mass raio
FFA yield
(%)
FUR conv.
(%)
FFA selec.
(%)
TON[a]
Entry
1
2
3
4
5
6
0
-
<1.0
42.6
-
-
8.0
4.0
1.9
1.3
1.0
40.7
95.5
60.2
64.3
97.6
66.2
99.2
97.1
98.4
46.8
33.6
98.5
99.2
99.7
100
98.7
99.2
23.5
17.7
Reaction conditions: 96 mg FUR (1 mmol), 5 mL 2-propanol, 120 ºC for 2 h
[a] TON (turnover number) = (mole of converted FUR) / (mole of acid-base sites)
24

Fig. 1
25

Fig. 2
26

Fig. 3
27

Fig. 4
28

Fig. 5
29