Catalytic transfer hydrogenation of furfural to furfuryl alcohol using easy-to-separate core-shell magnetic zirconium hydroxide

Article abstract of DOI:10.1039/d0nj05638c

A hollow core-shell magnetic zirconium hydroxide catalyst was synthesized and employed for the catalytic transfer hydrogenation (CTH) ofnumerous biomass-derived platform molecules (furfural and other carbonyl compounds). 93.9% conversion of furfural and 97.3% selectivity of furfuryl alcohol was achieved under mild reaction conditions (160 °C, 4 h, 0.1 g catalyst) with 2-propanol as the H-donor. After 7 times of reaction cycles, the catalyst retained excellent conversion (91.1%) and selectivity (97.8%) and no structural damage was found. Furthermore, a scale-up experiment was carried out, and the results proved that the catalyst has a prospect for industrial applications in the CTH reaction.

Full text of DOI:10.1039/d0nj05638c

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Catalytic transfer hydrogenation of furfural to
furfuryl alcohol using easy-to-separate core–shell
magnetic zirconium hydroxide
Cite this: New J. Chem., 2021,
45, 2715
Pan Hou,a Mingwei Ma,a Peng Zhang,a Jingjie Cao,a Hui Liu,a Xingliang Xu,b
a
a
Huijuan Yue,
Ge Tian *a and Shouhua Feng
A hollow core–shell magnetic zirconium hydroxide catalyst was synthesized and employed for the catalytic
transfer hydrogenation (CTH) ofnumerous biomass-derived platform molecules (furfural and other carbonyl
compounds). 93.9% conversion of furfural and 97.3% selectivity of furfuryl alcohol was achieved under mild
reaction conditions (160 1C, 4 h, 0.1 g catalyst) with 2-propanol as the H-donor. After 7 times of reaction
cycles, the catalyst retained excellent conversion (91.1%) and selectivity (97.8%) and no structural damage
was found. Furthermore, a scale-up experiment was carried out, and the results proved that the catalyst has
a prospect for industrial applications in the CTH reaction.
Received 19th November 2020,
Accepted 20th December 2020
DOI: 10.1039/d0nj05638c
rsc.li/njc
The use of high-pressure hydrogen and environmentally
harmful catalysts are the main disadvantages. Current studies
1. Introduction
Nowadays, due to the outbreak of worldwide population and are usually performed with hydrogen as the H-donor using
extremely increasing demand of non-renewable fossil fuels, zero-valent noble metals (Pd,20,21 Ru,22,23 Rh,24 Pt25,26) or some
humans are confronted with energy crisis.1,2 Researchers are newly developed non-noble metals (Ni,8,27,28 Co,29,30 Cu31,32) as
eager to exploit eco-friendly and renewable energy for sustainable the catalysts. However, catalytic transfer hydrogenation (CTH)
development. Biomass (cellulose and lignocellulose), an by the Meerwein–Ponndorf–Verley (MPV) reaction offers an
inexhaustible, renewable natural resource, is an appropriate attractive approach for the hydrogenation of aldehydes and
substitute.3 Also, it can be transformed into monosaccharide ketones33–35 using alcohols as the H-donor. Clearly, the CTH
(glucose and xylose),4 which can be further converted into reaction seems to be a safer and more economical method
platform compounds. Furfural (FAL), the product of hydrolysis avoiding the use of hydrogen gas. In the reaction, the catalysts
and dehydration of xylan, has been identified by US Department for common use vary from oxides,9,36 hydroxides of transition
of Energy as one of the most encouraging chemicals, which can metals to zeolites, metal–organic frameworks (MOFs) and other
be converted into value-added fuels and chemicals.5 Among all catalysts containing Lewis acid or basic sites. Compared with
the strategies of upgrading furfural, hydrogenation is one of other non-noble transition metals, zirconium offers better
the most universal reactions to manufacture biofuels,6 including performance in the MPV reduction, which is attributed to its
2-methylfuran (2-MF),7,8 furfural alcohol (FOL),9,10 g-valerolactone strong Lewis acidity.37 The recent research on Zr-zeolite,38,39
(GVL),11,12 and further-hydrogenation product tetrahydrofurfuryl Zr-mesoporous silicon,40 Zr-MOFs41 and other Zr-containing
alcohol (THFOL),13,14 and 1,5-pentanediol (1,5-PDO).15–17 Also, materials40,42 has made zirconium one of the most suitable
these furan-based compounds are outstanding fuels in compar- elements for its fabulous effect in the hydrogenation reaction.
ison to the alcohols due to their higher energy density and octane
values.18
In addition to the catalytic performance, the recyclability of
materials should also be taken into account. Compared with the
FOL is produced industrially by the liquid- or vapour-phase traditional separation methods of filtration or centrifugation,
hydrogenation of furfural at around 180 1C and 70–100 bar the simple and fast separation of magnetic nanoparticles has
hydrogen over copper chromite catalysts (CuCr2O4ÁCuO; Cu–Cr).16,19 made the research on magnetic carriers loaded with active
centres receiving extensive attention. Wang et al. reported a
hydroxyapatite-encapsulated magnetic g-Fe2O3 acting on the
a State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
hydrogenation of FAL into FOL and achieved a conversion of
College of Chemistry, Jilin University, Changchun 130012, P. R. China.
96.2% at 180 1C for 10 h.43 He et al. developed a kind of
E-mail: tiange@jlu.edu.cn
b College of Chemistry and Material Science, Shandong Agricultural University,
aluminium and zirconium mixed oxides coated Fe3O4 that
exhibited a high FOL yield of 90.5% in the CTH of FAL.44
Taian 271018, P. R. China
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Moreover, He et al. also compared the catalytic activity of Fe3O4, was prepared by ZrOCl2 and NaOH in the alcohol solution and
CoFe2O4 and NiFe2O4 in the CTH reduction, and found that named as Zr(OH)4-A. The resulting products were dried at 80 1C
NiFe2O4 has the best catalytic performance among the three overnight.
materials from the perspective of conversion and selectivity.45
2.3 Catalytic conversion of FAL and product analysis
In this study, in order to make the best of the catalytic
properties of zirconium and the convenience of magnetic
materials, a magnetic zirconium hydroxide core–shell catalyst
was employed, with high thermal stability cobalt ferrite
(CoFe2O4) as the magnetic core.46 95.4% of the conversion
from FAL to FOL and 97.7% of FOL selectivity were achieved
at 160 1C for 4 h by adjusting the ratio of cobalt to zirconium
and optimizing the reaction conditions. In addition, several
industrial manufacturing trials have also been conducted,
including the reusability of catalysts and scale-up experiments.
The reaction of FAL was carried out in a steel alloy autoclave
(GB1220-92) with a volume of 35 mL. A mixture of substrates
(0.67 mmol), solvents (20 mL), and catalysts (10–200 mg) was
added to the reactor before the reactor was sealed. The reaction
was conducted at a definite temperature (120–180 1C) without
stirring. After a period of time (1–4 h), the reactor was cooled
down to room temperature and the liquid samples were analysed
by GC-MS (Thermo Scientific Trace ISQ) with a TR-WAX-MS
capillary column (30.0 cm  320 mm  0.25 mm).
2.4 Catalyst characterization
2. Experiment
2.1 Materials
A Rigaku D/max-2550 diffractometer with Cu-Ka radiation was
utilized for X-ray diffraction. The morphology of the samples
was investigated via scanning electron microscopy (SEM, Jeol
JSM-6700F) and transmission electron microscopy (TEM, Tecnai
F20). Brunauer–Emmett–Teller (BET) surface area was assessed
by N2 adsorption–desorption on an Asap 2420 surface adsorption
apparatus. The magnetic property was measured using a mag-
netic property measurement system (SQUID-VSM) with an
applied force from À30 000 to 30000 Oe at room temperature.
Fourier transform infrared (FT-IR) spectra between 400 to
4000 cmÀ1 was evaluated on a Bruker VERTEXV 80 V spectro-
meter. The thermogravimetric (TG) analysis was performed
under air atmosphere at the heating rate of 10 1C minÀ1 using
a thermogravimetric analysis system (PerkinElmer Instruments).
The elemental composition of the catalysts was determined by
inductively coupled plasma (ICP) with Optima 3300 DV (Perkin-
Elmer Instruments) and CHNS elemental analysis on Elementer
Vario Micro.
Furfural, furfuryl alcohol, other carbonyl compounds and the
corresponding hydrogenated products were bought from TCI
Chemical Reagent Company (Shanghai, China). ZrCl4, ZrOCl2Á
8H2O were obtained from Aladdin Reagent Co. Ltd (Shanghai,
China). AlCl3, CoCl2Á6H2O, FeCl3, benzoic acid, pyridine and all
the alcohols were purchased from Sinopharm Chemical
Reagent Company (Shanghai, China).
2.2 Catalysts preparation
2.2.1 Preparation of CoFe2O4. Magnetic CoFe2O4 particles
were prepared through
a well-established hydrothermal
method.47 First, 5 mmol CoCl2Á6H2O and 10 mmol FeCl3 were
dissolved into 35 mL of ethylene glycol solution. Then, 0.09 mol
urea was added in the solution under continuous stirring for
2 h until the solid disappeared. The mixture was transferred
into an autoclave and heated up at 200 1C for 12 h. The
resulting precipitate was washed with deionized water and
ethanol for three times and was allowed to dry at 80 1C
overnight.
3. Result and discussion
3.1 Catalyst screening for CTH reaction from FAL to FOL
2.2.2 Preparation of Zr@Co. 0.234 g of CoFe2O4 was mixed
into ethanol (40 mL) in which a certain amount of ZrOCl2Á8H2O
was dissolved, and dispersed ultrasonically for 15 min. Then,
the solution was mechanically stirred for 30 min and then
The catalytic performance of different metal hydroxides for the
CTH reaction from FAL to FOL is listed in Table 1. The results
show that zirconium hydroxide gives the best catalytic perfor-
mance among the four catalysts at 120 1C for 3 h (27.7%
conversion and 86.3% FOL selectivity, entries 1–4), which is
consistent with the literature.37 Moreover, the synthesis
20 mL of the sodium hydroxide aqueous solution (0.4 mol LÀ1
)
was dropped into it. After stirring for 2 h, the precipitate was
separated with a permanent magnet and washed with deionized
water and ethanol. Finally, the brown powder (product) obtained
was dried overnight at 80 1C and named Zr(OH)4@CoFe2O4,
abbreviated as Zr@Co. Different molar ratios of the feeding Zr to
Co in the catalyst were denominated as Zr@Co-0.5, Zr@Co-1 and
Zr@Co-2, respectively.
2.2.3 Preparation of other hydroxides. Other hydroxides
including Fe(OH)3, Al(OH)3, Co(OH)2, and Zr(OH)4 were synthe-
sized by traditional precipitation. A certain amount of corres-
ponding chlorides (FeCl3, AlCl3, CoCl2, and ZrOCl2) was
dissolved into deionized water and the pH value was changed
Table 1 Screening of different metal hydroxide catalystsa
Entry
Catalysts
Conversion (%)
Yield (%)
Selectivity (%)
1
2
3
4
Fe(OH)3
Co(OH)2
Al(OH)3
Zr(OH)4
Zr(OH)4-A
Zr(OH)4
5.5
17.5
12.4
27.7
53.5
48.3
1.5
11.6
7.2
23.9
47.8
42.3
27.3
66.2
58.1
86.3
89.3
87.6
5
6b
a
Reaction condition: 0.67 mmol furfural, 20 ml 2-PrOH, 0.1 g catalyst,
b
120 1C, 3 h. The zirconium hydroxide here is refluxed in ethanol for
by adding the sodium hydroxide solution. Also, an extra catalyst 6 h before the reaction.
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Table 2 Catalytic performance of different zirconium loading capacities
a
of CoFe2O4
Entry
Catalysts
Conversion (%)
Yield (%)
Selectivity (%)
1
2
3
CoFe2O4
Zr@Co-0.5
Zr@Co-1
Zr@Co-2
Zr@Co-2
Zr@Co-2
8.4
43.2
60.0
87.1
44.3
21.1
6.1
41.2
56.2
84.6
40.7
18.9
72.6
95.4
93.6
97.1
91.8
89.6
4
5b
6c
a
Reaction condition: 0.67 mmol furfural, 20 ml 2-propanol, 160 1C, 3 h.
1 mmol pyridine was added into the reaction. 1 mmol benzoic acid
b
c
was added into the reaction.
method has a crucial influence on the catalytic effect. Zr(OH)4-A
shows better catalytic performance (Table 1, entry 5) than the
one prepared from the traditional precipitation with respect to
conversion and selectivity (53.5% and 89.3%, respectively). It is
inferred that ethanol helped changing the surface properties of
the catalyst and enhanced the acid–base density by exposing
coordinatively unsaturated sites.48 To further confirm this
Fig. 1 XRD patterns of catalysts with different Zr loading.
inference,
a supplementary experiment was carried out
(Table 1, entry 6). Compared with the original one (entry 4),
the Zr(OH)4 refluxed in ethanol (entry 6) increased the conver-
sion from 27.7% to 48.3%. Therefore, zirconium hydroxide
synthesized in thre ethanol solution is the preferred catalyst
for the reaction.
Revealing in Table 2, CoFe2O4 alone has little catalytic effect
on the hydrogenation of FAL with a low conversion of 8.4%,
which is consistent with the previous work.45 Also, as expected,
with the increase in Zr(OH)4 loading, the catalytic activity of the
catalysts enhanced gradually, and the conversion reached to
87.1% with 97.1% selectivity. Therefore, Zr@Co-2 is the optimal
catalyst in the following researches. To further investigate the
influence of acid and base sites, the poisoning experiment was
conducted. A certain amount of pyridine or benzoic acid
(1 mmol) was added to the reaction to block the contact of
substrate and the acid or base sites in the catalyst, respectively.
Also, conversion sharply declined from 87.1% to 44.3%/21.2%,
indicating that the reaction is a catalytic process with concerted
effects of the acid and base sites.
Fig. 2 SEM images and particle size distribution charts of CoFe2O4 (a),
Zr@Co-0.5 (b), Zr@Co-1 (c) and Zr@Co-2 (d).
3.2 Characterization of the catalysts
The X-ray diffraction (XRD) patterns of the catalysts with
different Zr(OH)4 loading are displayed in Fig. 1. CoFe2O4 with
good crystallinity is synthesized (PDF JCPDS#22-1086). As the TEM image (Fig. 3d) can further confirm it. Fig. 3a and d
loading of Zr(OH)4 increases, the intensity of the CoFe2O4 illustrate the TEM images of cobalt ferrate and cobalt ferrate
diffraction peaks gradually decreases dramatically due to the coated zirconium hydroxide, respectively. The diameter of
amorphous phase of the coated zirconium hydroxide, which CoFe2O4 varies from 240 nm to 340 nm and mainly 280 nm
was verified by the broad peak of Zr@Co-2 from 201 to 401.
(Fig. 3a), which was in agreement with the SEM image (Fig. 2a).
The microstructure of the catalysts was characterized by The TEM image of CoFe2O4 (Fig. 3b) shows a hollow architec-
SEM and TEM (Fig. 2 and 3). Overall, CoFe2O4 reveals spherical ture with a bright centre. The dominant crystalline phase with a
nanoparticles with an average diameter of 260–310 nm (Fig. 2a). lattice distance of 0.297 nm corresponding to the lattice plane
Also, from the charts of particle size distribution, the introduc- [220] of CoFe2O4 (Fig. 3c) was also identified by the XRD
tion of Zr(OH)4 slightly increases the particle size to 300– characterization. The formation of the hollow structure can
350 nm and rough surfaces (Fig. 2b–d). In particular, some of be attributed to the Ostwald ripening process.47 Also, Zr(OH)4
the particles in Zr@Co-2 are stuck together (Fig. 2d) and the and CoFe2O4 have a manifest border marked in Fig. 3e and f.
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The magnetic property of the catalysts was detected by
a vibrating sample magnetometer and the magnetization
curves of the catalysts are given in Fig. 4b. The saturation
magnetization (SM) values of Zr@Co-0.5, Zr@Co-1, Zr@Co-2
are 93.14 emu gÀ1, 41.93 emu gÀ1, 15.11 emu gÀ1, respectively.
As expected, increase in the thickness of Zr(OH)4 shell resulted
in a decrease in the ratio of magnetic cores, causing the SM
value to drop sharply as the loading of Zr(OH)4 increases.
Although the SM value of Zr@Co-2 is not very high, it is easily
attracted by magnets.
3.3 Catalytic Performance of Zr@Co-2
3.3.1 Effect of reaction parameters on the CTH reaction
from FAL to FOL. The influence of the reaction temperature,
reaction time and catalyst dosage on the CTH reaction from
FAL to FOL over Zr@Co-2 is illustrated in Fig. 5. From Fig. 5a,
as the reaction temperature rose from 120 1C to 160 1C for 4 h,
the yield increased unilaterally (from 55.7% at 120 1C to 93.9%
at 160 1C), while the selectivity of FOL remained constant
(495%). Also, the elevation of reaction temperature from
Fig. 3 TEM images of CoFe2O4 nanoparticle (a and b) and Zr@Co-2
(d and e). The HRTEM image of CoFe2O4 (c) and Zr@Co-2 (f). The dotted
line is the boundary between Zr(OH)4 and CoFe2O4.
The N2 adsorption–desorption isotherms of the catalysts
are exhibited in Fig. 4a. Type-IV isotherms with a typical
hysteresis loop were found for all the catalysts, which are
typical of mesoporous catalysts. The specific surface area (Sbet
)
value of CoFe2O4 is very low (15.9 m2 À1, Table 3), but the
g
stacking of Zr(OH)4 around CoFe2O4 brings about a substantial
increase in the specific surface area due to the mesoporous
structure of Zr(OH)4 and the Sbet value of Zr@Co-2 reaches
192.2 m2 gÀ1
.
Fig. 4 The N2 adsorption–desorption isotherm (a) and the magnetization
curves (b) of the catalysts.
Table 3 Physicochemical property of different catalysts
Sbet (m2 gÀ1
)
Vpore (cm3 gÀ1
)
Dmean (nm)
a
b
c
Catalysts
CoFe2O4
Zr@Co-0.5
Zr@Co-1
Zr@Co-2
15.9
107.7
138.6
192.2
0.06
0.17
0.28
0.22
9.16
5.12
5.32
3.58
a
b
BET surface area was assessed by N2 physisorption. Volume of pores
was estimated by the BJH adsorption cumulative volume of pores.
Fig. 5 Effect of (a) temperature, (b) time and (c) catalyst dosage on the
production of FOL from FAL. Reaction conditions: FAL (0.67 mmol),
2-propanol (20 ml), catalyst (0.1 g).
c
Average pore size was obtained from the adsorption average pore
width.
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160 1C to 180 1C had a weak effect on the reaction. The turnover primary alcohols (ethanol, n-propanol and n-butanol) as the
frequency (TOF) is defined as mmol (converted FAL)/[mmol alcohol source, the selectivity of FOL is similar, but the con-
(total metal) Â h (time)] and the TOF value of Zr@Co-2 is version of FAL raises up with the growth of the carbon chain
0.842 hÀ1 at 160 1C for 4 h. It is worth noting that with the catalyst (from 33.5% of ethanol, 48.9% of n-propanol to 55.7% of
Zr@Co-2, the conversion of FAL at 120 1C (55.7%) is even better n-butanol). Owing to the steric effect, the transition state
than that of FAL with pure Zr(OH)4 as the catalyst (Table 1, entry formed between the catalyst and the H-donor is more stable
5). It reveals that the combination of zirconium hydroxide and as the carbon chain grows.41 As for secondary alcohols,
CoFe2O4 enhances the specific surface area of Zr(OH)4 and 2-propanol and 2-butanol exhibited a desired conversion
exposes more active sites. The influence of the reaction time at (87.1% and 77.1%) and selectivity (96.7% and 93.2%), which
160 1C is also discussed in Fig. 5b. The extension of the reaction are more effective H-donors in the CTH reaction. Previous
time (from 1 h to 4 h) resulted in the increase in both the studies demonstrated that b-hydride was easier to eliminate
conversion of FAL and the selectivity of FOL from 31.7% and from the intermediate alkoxide species of secondary alcohol in
85.8% in 1 h to 87.1% and 96.8% in 4 h, respectively. With the CTH process.49 It is noticeable that no FOL was generated
continuous extension of the reaction time to 5 h, the conversion with tert-butanol as the alcohol source, which is attributed to
and the selectivity remained unchanged. In the examination of the absence of hydrogen on the a-C of the tert-butanol, and a
the catalyst dosage (Fig. 5c), the blank experiment had little effect six-membered ring intermediate cannot be formed between
on the reaction (18.2% conversion of FAL and 16.4% selectivity of FAL and the active sites on the surface of the catalyst.36 There-
FOL), suggesting that the CTH reaction could barely take place fore, 2-propanol is the preferred source of alcohol.
without catalyst. In contrast, the FAL conversion and FOL
As Zr@Co-2 revealed excellent activity towards the catalytic
selectivity increased rapidly with the amount of catalyst, from transfer hydrogenation of FAL, a variety of other carbonyl
30.2% and 97.1% at 0.01 g to 93.9% and 97.3% at 0.1 g. A further compounds (saturated or unsaturated aldehydes/ketones) were
increase from 0.1 g to 0.2 g did not achieve a better catalytic tested as substrates (Fig. 6b). Clearly, the catalyst showed great
effect. In conclusion, the optimal reaction conditions for the catalytic activity towards these carboxides resulting in at least
reaction are 4 h at 160 1C with 0.1 g catalyst.
70% selectivity of the main products. Among them, two kinds
3.3.2 Effect of different alcohols as H-donors on the reaction of a, b-unsaturated aldehydes as substrates were tested as well.
and CTH reaction of various substrates. The catalytic performances Also, allylic alcohols, the selectively reduced products, were
of Zr@Co-2 with different alcohols (from primary alcohols to gathered with a selectivity of 90%. These results indicated that
tertiary alcohols) as hydrogen donors are displayed in Fig. 6a. Using the Zr@Co-2 catalyst exhibits good catalytic performance for
the CTH reaction of different types of aldehydes/ketones.
3.4 Reusability of catalyst
The reusability of Zr@Co-2 was tested under optimal reaction
conditions (160 1C, 4 h and 0.1 g) (Fig. 7). After the reaction, the
catalyst was separated by a magnet, and the digital image is
shown in Fig. 7a. After washing and drying at 80 1C, the catalyst
Fig. 6 CTH reaction of FAL to FOL using different alcohols as the H-
donor (a) and CTH reaction of various substrates (b) over the Zr@Co-2
catalyst. Reaction condition: 20 ml alcohol, 0.67 mmol substrate, 0.1 g
Fig. 7 Reusability tests (a) of the Zr@Co-2 catalyst and element content
catalyst, 160 1C, 4 h.
(b), FT-IR (c), TG curves (d) of fresh and used Zr@Co-2.
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was directly applied in the next cycle without further treatment.
From Fig. 7a, after 7 cycles, no obvious drop in the conversion
of FAL and the selectivity of FOL was observed (91.1% and
97.8%, respectively), indicating that the catalyst is a recyclable
catalyst with excellent catalytic performance and easy
separation.
To further confirm the structural stability of Zr@Co-2, some
characterizations of the fresh and used catalysts were accom-
plished (Fig. 7b–d). Compared to the fresh Zr@Co-2, carbon
content of the used catalyst increased by 1.75% due to the
accumulation of few organic compounds in the catalyst. More-
over, the weight percentage of zirconium decreases slightly
(from 16.83% to 14.94%) after 7 cycles, revealing the good
stability of the catalyst under the reaction conditions. In Fig. 7c,
the infrared spectra of fresh and used catalyst are similar. The
peak located in 463 cmÀ1 and 535 cmÀ1 belonged to the
stretching vibrations of Zr–O52 and Fe–O,53 respectively. The
bands observed at 1352 and 1528 cmÀ1 are typical character-
istics of the O–H bending vibration modes of water and Zr–OH,
respectively. More than that, the wide peak at 3401 cmÀ1 is
ascribed to the stretching modes of O–H. However, the two
extra peaks at 1475 and 1381 cmÀ1 result from the bending
vibrations of C–H on the used catalyst and confirm the carbon
accumulation of the catalyst as our previous work reported.52
Also, the thermogravimetric (TG) curve further confirmed that.
The adsorption of insoluble organic oligomers or polymers
produced from the side reaction is responsible for the differ-
ence in the weight loss percentage (2.6%) at 700 1C (Fig. 7d).
Scheme 1 Plausible reaction mechanism for hydrogen transfer of furfural
with the Zr@Co-2 catalyst.
the rising temperature, the conversion of FAL increased gradu-
ally and reached 88.2% in 4 h, which was consistent with our
previous results. The almost complete conversion of FAL and
over 95% selectivity to FOL at 8 h proved that the catalyst has
great potential in industrial applications.
3.6 Catalytic mechanism for the CTH reaction of FAL
Based on the above results and previous studies,52,54 the
plausible catalytic mechanism for the hydrogenation of FAL
was proposed (Scheme 1). The reduction of FAL adopted the
Lewis acid-mediated MPV reaction and passed through a
six-membered ring transition state. In this case, Zr(OH)4 plays
a critical role of the reduction of FAL as a Lewis acid.55 First,
2-propanol was adsorbed on the surface of Zr(OH)4 and inter-
acted with acid sites (Zr4+) and base sites (O2À) to form metal
alkoxide.39 Then, the oxygen atom on the carbonyl group of FAL
is activated and stabilized by Lewis acid sites.52 With the
formation of a six-membered ring transition state, the hydride
transfer from alkoxide to the carbonyl group of FAL was
accomplished.52,53 Finally, the newly formed FOL is desorbed
from the surface of the catalyst along with acetone and the
Lewis acid sites were re-exposed.
3.5 Scale-Up CTH reaction of FAL
As a preliminary attempt of industrial manufacturing, a 10-fold
scale-up experiment using Zr@Co-2 as the catalyst was per-
formed in a 300 mL Teflon-lined autoclave with magnetic
stirring (500 rpm). In the experiment, 1 g of Zr@Co-2 and
6.7 mmol of FAL were mixed in 200 mL of 2-propanol. The
mixture was sealed in a reactor and reacted at 160 1C for 8 h.
During the reaction, the sample was taken several times through
liquid extraction port and analysed by GC-MS. From Fig. 8, with
We also compared the catalytic activity of Zr@Co-2 with
other magnetic materials for the CTH reaction of FAL to FOL
(Table 4), and Zr@CO-2 gives competitive or even the best
catalytic performance.
4. Conclusion
A series of easy-to-separate hollow magnetic nanoparticles
Zr@Co were synthesized, among which Zr@Co-2 manifested
remarkable catalytic activity in the CTH process for the produc-
tion of FOL from FAL. Under the preferred reaction conditions
(160 1C, 4 h, and 0.1 g catalyst), 93.9% of FAL was hydrogenated
with 97.3% selectivity of FOL using 2-propanol as the hydrogen
donor. Seven cycle experiments from FAL to FOL revealed that
the catalyst still maintains high catalytic activity (91.1% FAL
conversion and 97.8% FOL selectivity). Furthermore, a ten-fold
scale-up experiment was carried out and obtained similar
Fig. 8 Scale-up CTH experiment of FAL to FOL with the Zr@Co-2
catalyst.
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Table 4 The comparison of Zr@Co-2 with other magnetic catalysts for the CTH reaction of FAL to FOL
Entry
Catalyst
H-Donor
Temperature (1C)
Time (h)
Conversion (%)
Selectivity (%)
Ref.
1
2
3
4
5
6
NiFe2O4
Fe3O4-12
g-Fe2O3@HAP
Al7/Zr3@Fe3O4
Fe3O4@C
2-Propanol
2-Propanol
2-Propanol
2-Propanol
2-Propanol
2-Propanol
180
160
180
180
200
160
6
5
10
4
4
4
99
95
45
50
43
44
97.5
96.2
99.1
93.6
93.9
92.4
95.3
91.3
98.9
97.3
51
Zr@Co-2
This work
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results as those of the batch experiments, proving that the
catalyst has industrial application potential in the CTH reaction
of FAL.
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Conflicts of interest
There are no conflicts to declare.
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).
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