Magnetic Fe3O4 nanoparticles as easily separable catalysts for efficient catalytic transfer hydrogenation of biomass-derived furfural to furfuryl alcohol

Article abstract of DOI:10.1016/j.apcata.2020.117709

Magnetic Fe₃O₄-12 nanoparticles, an easily separable heterogeneous catalyst, exhibits high catalytic performance in the catalytic transfer hydrogenation (CTH) reaction of biomass-derived furfural (FAL) to furfuryl alcohol (FOL) with 2-propanol (2-PrOH) as the hydrogen source (90.1 percent FOL yield at 160 °C for 5 h). Comparing Fe₃O₄ nanoparticles with different particle sizes found that the smaller the nanoparticles, the larger the BET surface area, pore volume and pore diameter, resulting in more acid-base active sites through BET, SEM, TEM, NH₃-/CO₂-TPD and other characterizations. Moreover, a scaled-up CTH experiment of FAL and a stepwise reaction from xylose to FOL verified that the catalytic system has great industrial application prospects. A feasible flowchart of the chemical process from xylose to FOL was designed, which can realize the regeneration of biomass resources (xylose derived from corncob), the reuse and recycling of catalysts and solvents, and minimize the amount of solvents and pollutant emissions.

Full text of DOI:10.1016/j.apcata.2020.117709

Journal Pre-proof
Magnetic Fe3O4 nanoparticles as easily separable catalysts for efficient
catalytic transfer hydrogenation of biomass-derived furfural to furfuryl
alcohol
Mingwei Ma (Conceptualization) (Methodology) (Validation)
(Investigation) (Resources) (Writing - original draft) (Visualization),
Pan Hou (Validation) (Writing - review and editing) (Visualization),
Peng Zhang (Validation) (Writing - review and editing)
(Visualization), Jingjie Cao (Validation) (Writing - review and editing)
(Visualization), Hui Liu (Validation) (Writing - review and editing)
(Visualization), Huijuan Yue (Resources) (Writing - review and
editing), Ge Tian (Funding acquisition) (Conceptualization)
(Methodology) (Investigation)
(Resources)<ce:contributor-role>Writing - Review and editing)
(Supervision) (Project administration), Shouhua Feng (Resources)
(Writing - review and editing)
PII:
S0926-860X(20)30302-1
DOI:
https://doi.org/10.1016/j.apcata.2020.117709
APCATA 117709
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
5 April 2020
25 May 2020
17 June 2020
Please cite this article as: Ma M, Hou P, Zhang P, Cao J, Liu H, Yue H, Tian G, Feng S,
Magnetic Fe3O4 nanoparticles as easily separable catalysts for efficient catalytic transfer
hydrogenation of biomass-derived furfural to furfuryl alcohol, Applied Catalysis A, General
(2020), doi: https://doi.org/10.1016/j.apcata.2020.117709

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Magnetic Fe3O4 nanoparticles as easily separable
catalysts for efficient catalytic transfer hydrogenation
of biomass-derived furfural to furfuryl alcohol
Mingwei Ma, Pan Hou, Peng Zhang, Jingjie Cao, Hui Liu, Huijuan Yue, Ge Tian,* Shouhua Feng
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,
Jilin University, Changchun 130012, P. R. China.
Corresponding author: * tiange@jlu.edu.cn.
Graphical abstract
A stable and easily separable Fe3O4-12 nanoparticles exhibited the desired catalytic performance
in the catalytic transfer hydrogenation reaction of furfural to furfuryl alcohol with 2-propanol as
the hydrogen source. Moreover, a feasible flowchart from xylose to FOL was designed. Utilizing
the process can realize the regeneration of biomass resources, the reuse and recycling of catalysts
and solvents, and minimize the amount of solvents and pollutant emissions.
1

Highlights:
 Magnetic Fe3O4-12 nanoparticles exhibited excellent catalytic performance for the CTH
reaction of FAL to FOL.
 A scaled-up CTH experiment and a stepwise reaction from xylose to FOL verified that
the catalytic system has great industrial application prospects.
 A feasible flowchart from xylose to FOL was designed, which can realize the valorization
of biomass resources.
 Fe3O4-12 nanoparticles have good recyclability and universality for different carbonyl
compounds.
Abstract
Magnetic Fe3O4-12 nanoparticles, an easily separable heterogeneous catalyst, exhibits high
catalytic performance in the catalytic transfer hydrogenation (CTH) reaction of biomass-derived
furfural (FAL) to furfuryl alcohol (FOL) with 2-propanol (2-PrOH) as the hydrogen source (90.1 %
FOL yield at 160 °C for 5 h). Comparing Fe3O4 nanoparticles with different particle sizes found
that the smaller the nanoparticles, the larger the BET surface area, pore volume and pore diameter,
resulting in more acid-base active sites through BET, SEM, TEM, NH3-/CO2-TPD and other
characterizations. Moreover, a scaled-up CTH experiment of FAL and a stepwise reaction from
xylose to FOL verified that the catalytic system has great industrial application prospects. A
feasible flowchart of the chemical process from xylose to FOL was designed, which can realize
the regeneration of biomass resources (xylose derived from corncob), the reuse and recycling of
catalysts and solvents, and minimize the amount of solvents and pollutant emissions.
Keywords: catalytic transfer hydrogenation, magnetic nanoparticles, heterogeneous catalyst, acid-
base catalysis, biomass conversion
2

1. Introduction
At present, with the continuous consumption of fossil energy and increasingly serious ecological
damage on a global scale, the development of renewable energy has been an urgent and important
issue to solve these problems.[1, 2] Biomass, as an eco-friendly renewable resource with enormous
storage capacity, can be a good alternative to traditional fossil fuels. Therefore, exploring an
efficient catalytic conversion of biomass and its platform compounds has attracted widespread
attention in modern industry.[3, 4] And a wide range of important biomass-derived chemicals,
including furfural (FAL), 5-hydroxymethylfurfural, levulinic acid, glycerol, etc., are obtained in
large quantities from agricultural waste resources (cellulose and hemicellulose).[5-8] Among them,
FAL derived from C5 or C6 sugars through well-established methods is considered to be one of
the top 30 platform chemicals derived from biomass.[9-12] Degradation of FAL, the hydrogenation
of FAL to furfuryl alcohol (FOL), can be utilized to produce various chemicals including synthetic
fibers, resins, foundry binders, binders and other important platform molecules (Scheme 1).[13-16]
Scheme 1. Routes for the production of FOL from biomass and upgrading of FOL to other
biomass-derived platform molecules.
Traditionally, a harsh reaction condition (130-200 °C with 30 bar H2) is applied to the synthesis
of FOL from the gas phase and liquid phase hydrogenation of FAL with a toxic copper-chromium
catalyst.[17-19] Further, the catalyst is replaced by various transition metals (including Pt,[20, 21]
Pd,[22] Ir,[23-25] Cu,[26, 27] Ni,[28, 29] etc.). However, the use of explosive H2 at high temperature or
high pressure required the use of specialized equipment, and has potential safety issues.[30]
Recently, as a substitute for the hydrogenation reaction, a catalytic transfer hydrogenation (CTH)
reaction utilizing a H-donor (formic acid and alcohols) as a hydrogen source has been proposed.[13,
3

31-33]
Especially, the alcohols can provide both a hydrogen source and a solvent for the reaction.
Several catalytic systems with different alcohols as the hydrogen source have been developed for
the CTH reaction of FAL in the presence of various catalysts such as NiO,[34] Zr-HAs,[35] ZrPN,[36]
Co-Ru/C,[37] MgO,[38] etc.
In these catalytic systems, the conventional filtration method for separating heterogenous
catalyst from solution is inconvenient to handle in industrial production, which allows the easily
separated magnetic catalytic materials to enter our field of view.[15, 39-41] And some wonderful
works on the CTH reaction from FAL to FOL with magnetic materials as catalysts are listed as
follows. Yang et al. found that magnetic ferrite nickel nanoparticles and a series of acid/base
bifunctional magnetic Al–Zr@Fe mixed oxides can efficiently catalyze the CTH reaction of FAL
[42, 43]
to FOL with 90% yield of FOL at 180 °C for 4 h.
Zhang et al. studied hydroxyapatite-
encapsulated magnetic γ-Fe2O3 (γ-Fe2O3@HAP) as a magnetic base to promote the CTH reaction
of FAL and obtained 91.7% yield of FOL at 180 °C for 10 h.[30] Xu et al. synthesized a magnetic
zirconium-cyanuric acid coordination polymer (MZCCP) catalyst capable of efficiently converting
FAL to FOL at 140 °C for 2 hours (98.2 % yield of FOL).[40]
In this study, magnetic Fe3O4 nanoparticles were synthesized and exhibited excellent catalytic
performance for the CTH reaction of FAL to FOL with 2-propanol (2-PrOH) as hydrogen source
and solvent under relatively mild reaction conditions (120-160 °C). And the smaller the Fe3O4
particles, the larger the BET surface area and the pore diameter, and the better the catalytic
performance. Further, a scaled-up CTH experiment of FAL verified that the catalytic reaction
system has the potential to large-scale production. Combined with a two-phase reaction system,
FOL can be produced directly from xylose. In addition, reaction kinetics and a possible mechanism
were also studied.
2. Experimental
2.1 Materials
Furfural, furfuryl alcohol, methyl levulinate, γ-valerolactone, cyclohexanone, cyclohexanol,
acetophenone, 1-phenylethanol, cinnamaldehyde and cinnamyl alcohol were purchased from TCI
Chemical Reagent Company (Shanghai, China). Fe(NO3)3·9H2O, NH4OH, ethylene glycol,
4

benzoic acid, pyridine, NaCl, tetrahydrofuran (THF), AlCl3·6H2O and all solvents were purchased
from Sinopharm Chemical Reagent Company (Shanghai, China). Anhydrous sodium acetate,
FeCl3·6H2O and FeSO4·7H2O were purchased from Aladdin Industrial Corporation. Fe3O4
(200nm) was purchased from Tianjin Huadong Reagent Factory (Tianjin, China). All chemicals
were used without further purification.
2.2 Catalyst preparation
12 nm Fe3O4 nanoparticles were prepared by the co-precipitation method. To do this, 2.22 g
FeSO4·7H2O and 5.72 g Fe(NO3)3·9H2O were put into a three-necked bottle and dissolved with
160 mL deionized water by ultrasonication for 10 min. Then, the bottle was heated in a water bath
and was stirred at 80 ° C for 30 min under N2 atmosphere. Next, 10 mL of NH4OH (25 %) was
quickly added to the solution and stirring was continued for 30 min. After reaction, the bottle was
cooled to room temperature. Black precipitate was collected by a magnetic separation method and
washed three times with water and alcohol, respectively. Finally, the sample was dried in vacuum
at 60 °C and designated Fe3O4-12.
200 nm Fe3O4 nanoparticles (Fe3O4-200) were synthesized by well-established hydrothermal
method, referred to the relevant literature.[44] Briefly, 8 g anhydrous sodium acetate and 2.4 g
FeCl3·6H2O were dissolved in 60 mL ethylene glycol with stirring for 30 min. Then the liquid was
transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 200°C for 12 h.
After cooling to room temperature, the sample was collected by a magnetic separation method,
washed (ethanol/water), and dried in vacuum at 60 °C.
In addition, 300 nm Fe3O4 nanoparticles (Fe3O4-300) were commercially available.
2.3 CTH reaction and sample analysis
CTH reaction of FAL to FOL was carried out without stirring in a 35 mL steel alloy autoclave
(Fe-Cr-Ni alloy, GB1220-92). Typically, FAL (0.67 mmol), solvents (20 mL) and catalyst (0.1 g)
are moved into the reactor, then sealed and heated to the target temperature (120-160 °C) for a
certain reaction time (1-8 h). After the reaction, the autoclave was taken out and cooled to ambient
temperature. In addition, a scaled-up CTH experiment of FAL was performed in 300 mL stainless
steel autoclave (JULABO) with heating jacket (Figure 7a). Typically, in a scale-up experiment,
5

FAL (60 mmol), 2-PrOH (150 mL), and catalyst (1 g) were charged into the autoclave, then sealed
and heated to 180 °C for 1-8 h.
Identification of liquid products in the reaction mixture was achieved by the TRACE ISQ GC-
MS (Thermo Scientific Co, TR–WAX–MS column 30.0 m × 320 μm × 0.25 μm). The temperature
program started at 60 °C for 1 min, then increased from 60 °C to 230 °C at a rate of 15 °C /min
and held for 2 min.
2.4 A production process from xylose to FOL through the two-phase reaction and the CTH
reaction
The xylose-derived FOL was prepared by a two-step process. One step was the formation of
FAL from xylose by a two-phase reaction. In detail, the reaction was carried on in a stainless steel
autoclave (40 mL) heated in a temperature-controlled oil bath. The mixture of xylose (0.1 g),
solvents (biphase: 15 mL 2-butyl alcohol (2-BuOH) as extract phase and 15 mL saturated NaCl
solution as reaction phase) and AlCl3 (0.1 g) were put into the autoclave with a magnetic stirring
bar, then sealed and heated to the target temperature (140 °C) for a certain reaction time (1-3 h).
After reaction, the reactor was cooled to room temperature. The organic phase (extract phase) was
separated and 2-BuOH in organic phase was removed by a rotary evaporator for reuse. The
remaining liquid is high-purity furfural for subsequent reactions. In the inorganic phase (reaction
phase), a certain amount of xylose and 15 mL of 2-BuOH were added to the phase to continue the
reaction. The other step was to further convert the purified FAL to FOL through the CTH reaction
with 2-PrOH as solvent, which is the same as the CTH reaction of pure FAL.
2.5 Catalyst characterization
X-ray diffraction (XRD) was carried out via a Rigaku D/max-2550 diffractometer using Cu-Kα
radiation. The wavelength of X-ray source was set at 0.154 nm, and the step width was 0.02 ° with
a scanning speed of 4 ° per minute between two thetas of 5 ° and 80 °. The thermogravimetric
(TG) analysis was performed in a thermogravimetric analysis system (TG: PerkinElmer
Instruments) under air atmosphere at a heating rate of 10 °C·min-1. The magnetic property was
assessed using a magnetic property measurement system (SQUID-VSM) with an applied force of
−30ꢀ000 to 30ꢀ000 Oe at room temperature. Brunauer-Emmett-Teller (BET) surface area was
measured on an Asap 2420 surface adsorption apparatus. NH3-TPD and CO2-TPD (temperature-
6

programmed desorption) were conducted on a Micromeritics AutoChem II2920 automated
chemisorption analyzer to assess the surface acidity and basicity of catalysts, respectively. X-ray
photoelectron spectroscopy (XPS) measurements were performed using ESCALAB250
photoelectron spectrometer equipped with a charge neutralizer and an Mg Kα X-ray source. The
morphology of the samples was investigated by transmission electron microscopy (TEM; Tecnai
F20) and scanning electron microscopy (SEM; Jeol JSM-6700F). The elemental compositions of
catalysts were determined by inductively coupled plasma (ICP) on an Optima 3300 DV
(PerkinElmer Instruments).
3. Results and discussion
3.1 Characterization of catalysts
Figure 1. The characterization of different particle Fe3O4 nanoparticles. XRD patterns (a). XPS
spectra (b). Hysteresis loops (c). N2 adsorption-desorption isotherm (d).
7

XRD patterns of Fe3O4 nanoparticles with different particle sizes are shown in Figure 1a. For
Fe3O4-12, Fe3O4-200 and Fe3O4-300, the XRD diffraction peaks at 2θ=18.2, 30.0, 35.2, 37.0, 43.0,
53.3, 56.9, 62.5, 65.7, 70.9, 73.9, 74.9 and 78.9°, are assigned to the (111), (220), (311), (222),
(400), (422), (511), (440), (531), (620), (533), (622) and (444) planes of Fe3O4, respectively, which
is consistent with JCPDS card 19-0629. Especially the commercial Fe3O4-300, its crystallinity is
significantly higher than synthesized Fe3O4. The Fe3O4 nanoparticles were further analysed by
XPS spectrum (Figure 1b), and the photoelectron peaks at 711 and 725 eV correspond to Fe 2p3/2
(without satellite peaks) and Fe 2p1/2, respectively, indicates the formation of Fe3O4 other than γ-
Fe2O3.[41] And then the two peaks at 711 and 725 eV split into four peaks, two of them at 710.3
and 723.4 eV ascribing to the Fe 2p3/2 and Fe 2p1/2 orbits of Fe2+, and the other two peaks at 711.6
and 724.9 eV corresponding to the Fe2p3/2 and Fe2p1/2 orbits of Fe3+ (Figure S1a). Moreover, the
peak area ratios of Fe3+ to Fe2+ in Fe3O4-12, Fe3O4-200 and Fe3O4-300 are approximately 2.131,
2.138 and 2.117, respectively, which is very close to the stoichiometric ratio of Fe3+ to Fe2+ in
Fe3O4. From Figure S1b (XPS spectra of O 1s), the binding energies at around 530.4 and 532.5 eV
are attributed to Fe-O bonds and C-O bonds, respectively, which manifests that the O species in
Fe3O4-12 are substantially coordinated with iron compared to Fe3O4-200 and Fe3O4-300.
Additionally, XPS spectra of C 1s are also displayed in Figure S1c.
8

Figure 2. SEM image (a) and TEM (b) of Fe3O4-12 nanoparticles. SEM image (c) and TEM (d)
of Fe3O4-200 nanoparticles. SEM image (e) and TEM (f) of Fe3O4-300 nanoparticles.
The magnetic properties of Fe3O4-12, Fe3O4-200 and Fe3O4-300 catalysts were assessed using
a vibrating sample magnetometer (VSM) at 300 K (Figure 1c). From the magnetic hysteresis loops,
it can be concluded that the saturation magnetizations (Ms) of Fe3O4-12, Fe3O4-200 and Fe3O4-300
are 78, 90 and 85 emu g−1, respectively, indicating that these materials have high magnetic
properties and facilitate separation of the catalyst from the catalysis system.
9

N2 adsorption-desorption isotherm of the Fe3O4 samples measured by the N2 physisorption
(Figure 1d) displays type IV isotherms with H3-type hysteresis loop, which is attributed to the
irregular mesoporous structure between the nanoparticles. Further analysis of the sample (Table
1), BET surface area, pore volume and pore size of Fe3O4-12 (76.8 m2 g-1, 0.312 cm3 g-1 and 16.25
nm) are significantly greater than those of Fe3O4-200 (18.4 m2 g-1, 0.071 cm3 g-1 and 15.34 nm)
and Fe3O4-300 (2.8 m2 g-1, 0.009 cm3 g-1 and 12.77 nm), indicated that the smaller the particle size,
the larger the BET surface area, the pore volume and the pore size.
Table 1. Physicochemical properties of catalyst.
BET surface
area[a]
Pore
Acid
Base
Pore size[a]
(nm)
Entry
Catalyst
volume[a]
(cm3/g)
concentrations[b]
(mmol/g)
concentrations[b]
(mmol/g)
(m2/g)
1
2
Fe3O4-12
76.8
18.4
0.312
0.071
16.25
15.34
0.828
0.652
0.642
0.349
Fe3O4-
200
Fe3O4-
300
0.337
3
2.8
0.009
12.77
0.101
[a] Derived from N2 physisorption. [b] Derived from NH3- or CO2-TPD.
To further confirm the morphologies and particle size distribution of the nanoparticles, the Fe3O4
samples were observed by SEM and TEM (Figure 2). From Figures 2, the size distribution range
of the three nanoparticles (Fe3O4-12, Fe3O4-200 and Fe3O4-300) are 6-24 nm (12 nm at most), 80-
280 nm (200 nm at most) and 190 to 430 nm (300 nm at most) respectively.
10

Figure 3. NH3-TPD (a) and CO2-TPD (b) profiles of different particle Fe3O4 nanoparticles.
Table 2． Hydrogen transfer reaction of FALto FOL with 2-propanol as solvent over various Fe3O4
nanoparticles
Formation rate [a]
Entry
Catalyst
Conversion (%) Yield (%)
Selectivity (%)
TOF [b] (h-1)
(mmol g-1 h-1)
1[c]
2
0
<1
0
0
0
0
Fe3O4-12
Fe3O4-200
Fe3O4-300
Fe3O4-12
Fe3O4-12
Fe3O4-12
Fe3O4-12
97.5
47.6
8.9
90.1
44.3
7.5
92.4
93.1
84.3
84.1
82.4
96.3
87.9
1.21
0.101
0.049
0.009
0.088
0.014
0.153
0.069
3
0.59
4
0.10
5[d]
6[e]
7[f]
8[g]
85.5
13.1
73.9
>99
71.9
10.8
71.2
87.9
0.96
0.14
1.91
0.79
Conditions: catalyst (0.1 g), FAL (0.67 mmol), 2-PrOH (20 mL), 160 °C, 5 h.
[a] The FOL formation rate is defined as mmol (formed FOL)/ [g (catalyst amount) × h (time)] [b] TOF is
defined as mmol (converted FAL)/ [mmol (total metal) × h (time)]. [c] Reaction without any catalyst. [d] 5
mmol of pyridine was added additionally. [e] 5 mmol of benzoic acid was added additionally. [f] The amount
of catalyst is 0.05g. [g] The amount of catalyst is 0.15g.
Since acid-base sites of the catalysts play an important role in the CTH reaction of carbonyl
compounds,[36, 45] the acid-base properties of the Fe3O4 nanoparticles were measured by NH3- and
CO2-TPD (Figures 3a and 3b), and the detailed data are compiled in Table 1. The acid and base
concentrations of Fe3O4-12 are 0.828 and 0.642 mmol g-1, respectively, which are evidently higher
than those of Fe3O4-200 and Fe3O4-300 (Table 1). From the NH3- / CO2-TPD profiles (Figure 3),
two broad desorption peaks appear at 50-150 ℃and 300-500 ℃, representing the weak acid / weak
base sites and strong acid / strong base sites of the catalysts, respectively. Therefore, compared
with Fe3O4-200 and Fe3O4-300, Fe3O4-12 has the strongest acid / base sites. Combined with BET
analysis, it was concluded that the smaller the Fe3O4 nanoparticles, the larger the BET surface
11

area, pore volume and pore size, resulting in more acid-base active sites of the catalysts. Therefore,
Fe3O4-12 should have the best catalytic activity in the CTH reaction of FAL to FOL.
3.2 CTH reaction of FAL to FOL over various Fe3O4 nanoparticles
In the blank experiment, no reaction occurred without catalyst at 160 °C for 5 h (Table 2, entry
1). With Fe3O4-12 as catalyst, 97.5 % conversion of FAL and 90.1 % yield of FOL were obtained
(Table 2, entry 2). The increase of the particle size of the catalyst to 200nm (Fe3O4-200) reduced
the catalytic activity (47.6 % conversion of FAL and 44.3 % yield of FOL, Table 2, entry 3). When
the particle size was further increased to 300 nm (commercial Fe3O4-300), the yield and the
conversion were dropped sharply (8.9 % conversion of FAL and 7.5 % yield of FOL, Table 2,
entry 4). Similarly, the TOF value of FAL and the formation rate of FOL of Fe3O4-12 (1.21 mmol
g-1 h-1 and 0.101 h-1) are also higher than those of Fe3O4-200 (0.59 mmol g-1 h-1 and 0.049 h-1) and
Fe3O4-300 (0.10 mmol g-1 h-1 and 0.009 h-1). However, the selectivity of the reaction maintains at
a high level with the three nanoparticles as catalysts. And a small amount of by-products, 2-
(isopropoxymethyl)furan and 4-(2-Furyl)-3-buten-2-one, were detected by GC-MS (Figure S2).
Combined with the characterization of BET, SEM, TEM and NH3- / CO2-TPD, it can be inferred
that the smaller Fe3O4 nanoparticles, the more active sites (eg. acid-base sites) are exposed,
resulting in better catalytic performance of the catalysts. Subsequently, in order to investigate the
effect of the acid-base sites on the catalytic performance, pyridine and benzoic acid were separately
added to the reaction system for poisoning of the acid-base sites of the catalysts without changing
the reaction conditions (Table 2, entry 5 and 6). The results revealed that the catalytic activity was
greatly reduced, which further confirmed that the CTH reaction was a catalytic reaction with a
synergistic effect of acid-base sites. Experiments on the effect of catalyst dosage on the reaction
(Table 2, entry 2,7 and 8) confirmed that 0.1 g was the preferred amount of catalyst with the best
yield of FOL (90.1%). However, as the amount of catalyst was reduced to 0.05 g, the formation
rate of FOL (1.91 mmol g-1 h-1) and the TOF value of FAL (0.153 h-1) increased significantly,
which may be attributed to an increase in the utilization of active sites per unit mass of catalyst.
3.3 Effect of reaction temperature and time on the catalytic performance of Fe3O4-12
nanoparticles and reaction kinetics study
12

The reaction temperature and time have a remarkable effect on the activity of the reaction. Thus,
the CTH reaction of FAL to FOL was carried out at different reaction temperatures, ranging from
120 to 160 °C, within a certain period of time (from 1 h to 8 h) (Figure 4a). It is well known that
increasing the temperature accelerates the yield of FOL and the conversion of FAL. In detail, at
120 °C, the conversion of FAL and the yield of FOL increased with increasing reaction time,
reaching 70.9% and 65.5% at 8 h, respectively. Increasing the temperature to 140 °C, it takes only
4 h to achieve similar conversion of FAL and the yield of FOL at 120 ° C for 8 h, and the conversion
of FAL and the yield of FOL are 94.1% and 88.3% at 8 h, respectively. When the temperature goes
up to 160 ° C, the conversion of FAL quickly reached 100% and remains constant, while the yield
of FOL decreased to 85.4% at 8 h after reaching a maximum of 90.1% at 5 h, since the generated
FOL was further etherified to 2-(isopropoxymethyl)furan with 2-PrOH. Therefore, 160 °C and 5
h are the optimal reaction conditions for the CTH reaction of FAL to FOL over Fe3O4-12.
Figure 4. Effect of reaction temperature on the performance of Fe3O4-12 nanoparticles (a) and
first-order kinetic fit for the CTH reaction of FAL to FOL at different temperatures (b). Reaction
conditions: FAL (0.67 mmol), 2-PrOH (20 mL), catalyst (0.1 g).
In order to provide more detailed information about the reaction, the kinetics of the reaction
were calculated at three temperatures of 120, 140 and 160 °C. In the catalytic system, since the
amount of the catalyst is constant with an excess of 2-PrOH, we assume that the CTH reaction of
FAL to FOL is a first-order reaction as a function of FAL concentration. From Figure 4b, the
experimental results reveal a linear relationship between ln(C0/Ct) and reaction time, and the
relevant reaction rate constants (k) are calculated at each temperature (120, 140 and 160 °C) from
the corresponding slope of the plots, which corresponds to 0.1324, 0.2669, and 0.5320 h-1,
13

respectively. Based on the Arrhenius plot, the activation energy (Ea) is calculated to be 49.16 kJ
mol-1 over the Fe3O4-12 for the CTH reaction of FAL to FOL with 2-PrOH as the hydrogen source,
which is close to or below the reported Ea value for the reaction with other catalysts.[43, 45, 46] The
lower activation energy is attributed to smaller particle sizes, larger pore volumes, higher BET
surface area and more acid-base sites.
3.4 Influence of different hydrogen donor
Figure 5. Influence of hydrogen donor for the CTH reaction of FAL to FOL over Fe3O4-12
nanoparticles. Reaction conditions: FAL (0.67 mmol), alcohol (20 mL), catalyst (0.1 g), 160 °C,
5 h.
Catalytic performance of Fe3O4-12 with different alcohol sources was summarized in Figure 5.
With primary alcohols as the hydrogen source, the acetalization reaction dominates the
competition with the CTH reaction of FAL, and especially when methanol was used as a hydrogen
source. As the carbon chain grows, the hydrogen supply capacity increases, from methanol to
ethanol to n-propanol, and finally to n-butanol, the yield of acetal products gradually decreases,
and the yields of the corresponding four products (furfural dimethyl acetal, furfural diethyl acetal,
furfural dipropyl acetal and furfural dibutyl acetal) are 80.8%, 41.2%, 40.6% and 32.9%,
respectively. The reason is explained according to the reaction mechanism. The elimination of β-
hydride is difficult to occur after the alcohol formed an alkoxy species on the surface of Fe3O4,
resulting in the primary alcohols being poor H-donors in the CTH reaction. Similarly, tertiary
14

alcohols are also weak H-donors and fail to produce the corresponding carbonyl compounds
because no hydrogen is close to the hydroxyl group.[33, 47, 48] Therefore, primary and tertiary
alcohols are not suitable as hydrogen sources for the reaction. Instead, the high conversion of FAL
and the high yield of FOL occur in a reaction with secondary alcohols as a hydrogen source, which
indicates that the dehydrogenation of the secondary alcohols is easier than those of the primary
and tertiary alcohols. Therefore, 2-PrOH, represented as secondary alcohols, is used as a suitable
hydrogen source and a reaction solvent for the reaction.
3.5 Reusability and heterogeneity of the Fe3O4-12 nanoparticles
Figure 6. Reusability (a) and leaching test (b) of Fe3O4-12 nanoparticles for the CTH reaction of
FAL to FOL. Reaction conditions: FAL (0.67 mmol), 2-PrOH (20 mL), catalyst (0.1 g), 160 °C, 5
h.
Reusability and heterogeneity of the Fe3O4-12 nanoparticles were carried out to evaluate its
practical application. After completion of each cycle (Figure 6a), the magnetic Fe3O4-12
nanoparticles, separated from the reaction system by magnet, were washed twice with water and
ethanol, and vacuum-dried for reuse. The catalyst exhibited almost constant performance after five
consecutive cycles, with the yield of FOL slightly decreasing from 90.1% to 87.8%. And no iron
ions were detected in the solution after the reaction by ICP analysis, confirming that the catalyst
has good stability under the reaction conditions. Moreover, the heterogeneity of the Fe3O4-12
nanoparticles in the catalytic progress was tested by removing the catalyst from the mixed system
after 2 h at 160 °C and maintaining the autoclave in the oven for a further period of time (1-3 h)
without the catalyst (Figure 6b). The yield of FOL did not continue to increase and no other
reactions occurred, stating that no active sites entered the liquid and the catalyst was
15

heterogeneous. Subsequently, the removed Fe3O4-12 nanoparticles were re-added to the catalyst-
free system and maintaining the autoclave in the oven for a further period of time (1-3 h).
Eventually, the yield of FOL continued to increase and reached 89.4 % after 3 h. The result further
supports the above conclusion (ie: heterogeneity of the catalyst).
Table 3． CTH reaction of carbonyl compounds over Fe3O4-12 nanoparticles
Entry
Reactant
Main product
Temperature (℃)
Time (h) Conv (%) Yield (%)
1
160
5
97.5
40.3
90.1
38.6
2
3
180
160
10
10
56.4
50.3
4
5
180
180
7
87.5
86.7
85.6
10
87.16
Conditions: catalyst (0.1 g), substrate (0.67 mmol), 2-PrOH (20 mL).
The stability of Fe3O4-12 nanoparticles is also exposed by XRD, BET, VSM, TG, SEM and
TEM. The XRD diffraction peaks and the saturation magnetization of the catalyst after 5 cycles
are almost consistent with those of the fresh catalyst, indicating that the structure and magnetic
properties of the catalyst do not change before and after the reaction (Figure S3a and S3b). From
the BET analysis (Figure S3c), the same N2 adsorption-desorption isotherm of the catalyst before
and after the reaction revealed no change in the pore structure of the catalyst, and the BET surface
area decreased slightly from 76.8 m2 g-1 to 58.9 m2 g-1, which may be caused by carbon
accumulation. From the TG analysis (Figure S3d), the fact that less carbon accumulation on the
catalysts was evidenced by the similar weight loss of the spent catalyst with the fresh catalyst.
16

Images of SEM and TEM (Figure S3e and S3f) reveal that the particle size of the catalyst did not
agglomerate after five cycles. In summary, the catalyst is stable.
3.6 CTH reaction of other carbonyl compounds and a scaled-up CTH reaction of FAL over
the Fe3O4-12 nanoparticles
In order to examine the universality of the catalyst, the CTH reaction of other carbonyl
compounds with 2-PrOH was attempted at 160-180 °C. From the Table 3, the catalyst had good
catalytic activity for the examined compounds. Particularly for FAL, cyclohexanone and methyl
levulinate, the yields of their target products were all above 80%. Therefore, Fe3O4-12
nanoparticles have a good feasibility for the CTH reaction of different carbonyl compounds.
Combined with its simple, economical and easy to separate features, the catalyst has excellent
application prospects.
Figure 7. Scaled-up CTH reaction of FAL to FOL over Fe3O4-12 nanoparticles. Reaction device
of the scaled-up experiment (a). Effect of reaction time for the scaled-up experiment (b). Cyclicity
of the scaled-up experiment (c). Reaction conditions of (b): FAL (60 mmol), 2-PrOH (150 mL),
catalyst (1 g), 180 °C. Reaction conditions of (c): FAL (60 mmol), 2-PrOH (150 mL), catalyst (1
g), 180 °C, 7 h.
In order to assess the industrial application potential of the catalyst for CTH reaction of FAL to
FOL, a scaled-up experiment of FAL (close to 100 times magnification) was performed with 60
mmol of FAL as substrate (Figure 7). From Figure 7 b and 7 c, the yield of FOL still remained
above 90 % at 180 °C for 7 h and the catalytic activity of catalyst did not decrease significantly
after five cycles. After the reaction, the reaction solution was subjected to rotary distillation to
17

obtain pure FOL (Figure S4). Accordingly, the Fe3O4-12 catalyst has potential for large-scale
production of FOL from FAL.
3.7 A production process from xylose to FOL through the two-phase reaction and the CTH
reaction
The upstream feedstock of FAL is xylose derived from corncob (see the supplementary
materials for details). In order to verify the industrial production feasibility of biomass conversion,
xylose is served as a raw material to produce FOL through two steps of two-phase reaction system
(dehydration) and CTH reaction (hydrogenation). First, a two-phase reaction system was
established to synthesize FAL from xylose (Figure 8). After the reaction, an optimum yield of
67.8% FAL is acquired from organic phase at 140 ° C for 2.5 hours (Figure 8a). Further, the solvent
in the organic phase was removed by rotary evaporation to obtain FAL (containing an small
amount of water, 2-BuOH, NaCl, by-products, etc.), and the distilled solvent (2-BuOH) and the
saturated NaCl solution with catalyst (AlCl3) are returned to sample mixer to continue to produce
FAL as the extract phase and the reaction phase, respectively (Figure 8c). After two-phase system
cycle test (Figure 8b), the accumulation of FAL in the recycled organic phase caused by the
formation of an azeotrope of FAL with the solvent (2-BuOH) during the steaming process resulted
in an increase in the yield of FAL from 67.8% to 99.5% after five cycles. Although humin and
FAL changed the organic and inorganic phases to pale yellow, they did not affect the system.
Finally, the above-mentioned unpurified FAL derived from xylose was used as a raw material for
the subsequent CTH reaction, and a yield of 86.7% FOL was obtained at 160 ° C for 5 h.
18

Figure 8. Effect of reaction time on the yield of FAL for the two-phase reaction (a). Cyclicity of
the two-phase reaction (b). A simple process flow chart from xylose to FOL (c). Reaction
conditions of (a): xylose (0.1 g), saturated NaCl solution (15 mL), 2-BuOH (15 mL), AlCl3 (0.1
g), 140 °C. Reaction conditions of (b): xylose (0.1 g), saturated NaCl solution (15 mL), 2-BuOH
(15 mL), AlCl3 (0.1 g), 140 °C, 2.5 h.
The above experimental results elucidate that the intention to produce FOL from xylose using a
two-phase system and a CTH reaction is feasible. And a simple process flow chart from xylose to
FOL is designed (Figure 8c). First, the reaction raw material is mixed into the sample mixer (4)
and injected into a two-phase reactor (6). After the reaction, the organic phase enters one
fractionation tower (7), and the inorganic phase is returned to the reactor 4 for reuse. Next, after
passing through the fractionation tower, the solvent (2-BuOH) is reinjected into the mixer 4 for
reuse, and the purified FAL is introduced into the CTH reactor (9) and undergoes the CTH reaction
19

with Fe3O4 and 2-PrOH (8) to form FOL. Finally, the catalyst is fixed by a magnet (10) after the
reaction, and the liquid is transferred into the other fractionation tower (11) to separate the product
FOL from the solvent (2-PrOH) which is returned to apparatus 8 for reuse.
Scheme 2. Plausible reaction mechanism for the CTH reaction of FAL to FOL over Fe3O4-12
nanoparticles.
3.8 Mechanism analysis
Based on the above results and relevant literature,[31, 49] a possible mechanism was proposed,
assuming that the hydrogenation of the FAL undergoes a six-membered ring coordination process.
Detailed picture in Scheme 2, first, 2-PrOH was adsorbed on the surface of Fe3O4, and formed an
alkoxide by interacting with the acid (Fe2+ or Fe3+) and base sites (O2-) of Fe3O4. Meanwhile, C=O
of FAL was also adsorbed and activated by the acid-base sites. Further, hydrogen transfer took
place between the dissociated alkoxide and the activated FAL via a concerted process involving a
six-link intermediate to form FOL and acetone. Finally, the FOL and acetone were obtained. In
addition, the by-product 2-(isopropoxymethyl)furan was the etherification product of FOL and 2-
PrOH under the action of acid sites.[50] Another by-product, 4-(2-Furyl)-3-buten-2-one, was the
aldol condensation product of FAL with acetone under the action of base sites.[51, 52]
20

Table 4． Comparison of the catalytic performance of Fe3O4-12 nanoparticles with other magnetic catalyst in the production
of FOL from FAL with alcohols as H-donor.
Entry
Catalyst
NiFe2O4
H-Donor
2-propanol
2-propanol
2-propanol
2-propanol
2-butanol
Temperature (℃)
Time (h)
Conv (%)
99
Yield (%)
95
Ref.
[43]
1
2
3
4
5
6
180
180
180
180
140
160
6
7.5
4
Pd/Fe2O3
68
41
[53]
Al7Zr3@Fe3O4
γ-Fe2O3@HAP
MZCCP
99.1
96.2
99.9
97.5
90.5
91.7
98.2
90.1
[42]
10
2
[30]
[40]
Fe3O4-12
2-propanol
5
This work
It is well known that magnetic catalysts have the advantage of being easily separated from the
reaction system, including simple magnetic materials having active sites themselves and composite
magnetic materials covered with active sites on magnetic materials. From the Table 4, [26-30] the
magnetic catalysts have high catalytic activity for CTH reaction of FAL, especially for composite
magnetic materials. However, the preparation process of composite magnetic materials is
complicated and costly compared to a simple magnetic material. Therefore, the facile, low-cost
and green preparation method together with high catalytic performance render the Fe3O4-12
nanoparticles a promising substitute of magnetic catalysts for the CTH reaction.
4. Conclusions
Magnetic Fe3O4 nanoparticles as easily separable catalysts exhibited excellent catalytic
performance for the CTH reaction of FAL to FOL with 2-propanol (2-PrOH) as hydrogen source
under mild reaction conditions. Comparing Fe3O4 nanoparticles with different particle sizes, the
minimum Fe3O4-12 catalyst had the best catalytic performance, and the yield of FOL reached
90.1% with 97.5% conversion of FAL at 160 °C for 5 h. Combined with the characterization of
BET, SEM, TEM, NH3- and CO2-TPD, it can be inferred that the catalytic performance of Fe3O4-
12 nanoparticles is better than that of Fe3O4-200 and Fe3O4-300 nanoparticles, which is also related
to the relatively low activation energy (49.16 kJ mol-1). Moreover, a scaled-up CTH experiment
of FAL and a production process from xylose to FOL manifested that the catalytic system has the
potential for large-scale production. The simple, low-cost and high-efficiency catalyst in this work
is a potentially attractive catalyst for conversion reaction from xylose to FOL.
21

Credit Author Statement
Mingwei Ma: Term, Conceptualization, Methodology, Validation, Investigation, Resources,
Writing - Original Draft, Visualization. Pan Hou, Peng Zhang, Jingjie Cao and Hui Liu:
Validation, Writing - Review & Editing, Visualization. Huijuan Yue and Shouhua Feng:
Resources, Writing - Review & Editing. Ge Tian,*: Funding acquisition, Conceptualization,
Methodology, Investigation, Resources, Writing - Review & Editing, Supervision, Project
administration.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by grants from the National Natural Science Foundation of China (No.
21771086) and Natural Science Foundation of Jilin Province, China (20180101293JC).
Appendix A. supplementary data
Supplementary data associated with this article can be found, in the online version.
22

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