The construction of novel and efficient hafnium catalysts using naturally existing tannic acid for Meerwein-Ponndorf-Verley reduction

Article abstract of DOI:10.1039/c9ra10317a

The conversion of carbonyl compounds into alcohols or their derivatives via the catalytic transfer hydrogenation (CTH) process known as Meerwein-Ponndorf-Verley reduction is an important reaction in the reaction chain involved in biomass transformation. The rational design of efficient catalysts using natural and renewable materials is critical for decreasing the catalyst cost and for the sustainable supply of raw materials during catalyst preparation. In this study, a novel hafnium-based catalyst was constructed using naturally existing tannic acid as the ligand. The prepared hafnium-tannic acid (Hf-TA) catalyst was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetry (TG). Hf-TA was applied in the conversion of furfuraldehyde (FD) to furfuryl alcohol (FA) using isopropanol (2-PrOH) as both the reaction solvent and the hydrogen source. Both preparation conditions and the effects of the reaction parameters on the performance of the catalyst were studied. Under the relatively mild reaction conditions of 70 °C and 3 h, FD (1 mmol) could be converted into FA with a high yield of 99.0%. In addition, the Hf-TA catalyst could be reused at least ten times without a notable decrease in activity and selectivity, indicating its excellent stability. It was proved that Hf-TA could also catalyze the conversion of various carbonyl compounds with different structures. The high efficiency, natural occurrence of tannic acid, and facile preparation process make Hf-TA a potential catalyst for applications in the biomass conversion field.

Full text of DOI:10.1039/c9ra10317a

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PAPER
The construction of novel and efficient hafnium
catalysts using naturally existing tannic acid for
Meerwein–Ponndorf–Verley reduction†
Cite this: RSC Adv., 2020, 10, 6944
Xiaolu Wang,a Jianxiu Hao,a Lijuan Deng,b Hongye Zhao,a Quansheng Liu,a Na Li,a
a
Runxia He,a Keduan Zhia and Huacong Zhou
*
The conversion of carbonyl compounds into alcohols or their derivatives via the catalytic transfer
hydrogenation (CTH) process known as Meerwein–Ponndorf–Verley reduction is an important reaction
in the reaction chain involved in biomass transformation. The rational design of efficient catalysts using
natural and renewable materials is critical for decreasing the catalyst cost and for the sustainable supply
of raw materials during catalyst preparation. In this study,
a novel hafnium-based catalyst was
constructed using naturally existing tannic acid as the ligand. The prepared hafnium–tannic acid (Hf–TA)
catalyst was characterized by scanning electron microscopy (SEM), transmission electron microscopy
(TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetry (TG).
Hf–TA was applied in the conversion of furfuraldehyde (FD) to furfuryl alcohol (FA) using isopropanol (2-
PrOH) as both the reaction solvent and the hydrogen source. Both preparation conditions and the
effects of the reaction parameters on the performance of the catalyst were studied. Under the relatively
mild reaction conditions of 70 ꢀC and 3 h, FD (1 mmol) could be converted into FA with a high yield of
99.0%. In addition, the Hf–TA catalyst could be reused at least ten times without a notable decrease in
activity and selectivity, indicating its excellent stability. It was proved that Hf–TA could also catalyze the
conversion of various carbonyl compounds with different structures. The high efficiency, natural
Received 9th December 2019
Accepted 29th January 2020
DOI: 10.1039/c9ra10317a
occurrence of tannic acid, and facile preparation process make Hf–TA
applications in the biomass conversion field.
a potential catalyst for
rsc.li/rsc-advances
attracted great attention. These catalysts have high efficiency for
hydrogen transfer in biomass-derived carbonyl compounds under
relatively milder conditions. Recently, hafnium-based catalysts
were developed with even higher efficiencies than those of Zr-
based catalysts under mild conditions for the CTH reactions.13
For example, Corma et al. reported hafnium-based metal–organic
frameworks (MOFs) for the CTH of furfuraldehyde in isopropanol
(2-PrOH), with which a 97% yield of furfuryl alcohol was ob-
tained.24 Song et al. found that the CTH reaction of levulinic acid
and its ester to produce g-valerolactone could proceed over
a hafnium-based catalyst prepared using an amino tri(methylene
phosphonic acid)-containing phosphate group.25 Wu et al.
discovered that a phosphonate-hafnium solid catalyst played a key
role in the transfer hydrogenation of ethyl levulinate (EL) to g-
valerolactone (GVL).26 Zhang and coworkers designed a hafnium-
based DUT-67 (Zr6O6(OH)2(tdc)4(Ac)2) catalyst that enhanced the
efficiency of the CTH of EL to GVL.27 In spite of the effectiveness of
these catalysts, several obstacles still exist and need to be over-
come. These include low stability of the catalyst, high reaction
temperature, long reaction time and high preparation tempera-
ture. Besides, the organic ligands developed for the preparation of
hafnium-based catalysts are still very limited, and most of the re-
ported hafnium catalysts are prepared using carboxyl group-
Introduction
During the catalytic conversion of biomass, the hydrogenation of
carbonyl compounds into alcohols or their derivatives is an
important reaction step. The catalytic transfer hydrogenation
(CTH) process also known as Meerwein–Ponndorf–Verley (MPV)
reduction using isopropanol as the hydrogen source is commonly
used. Various catalysts, such as Ni,1 Co–Pd,2 Cu,3 Ru,4,5 Fe,6 Ir,7 beta
zeolite,8 and Pd,9 have been applied for the CTH reaction of
carbonyl compounds. The common transition metal-based cata-
lysts oen suffer from complex preparation processes and high
reaction temperatures.10,11 Generally, noble metal-based catalysts
perform well under milder conditions, but their costs are oen
high. In recent years, the use of zirconium-based catalysts, such as
ZrO2,12 Zr–lignosulfonate polyphenolic polymers,13 ZrO(OH)2,14 Zr-
SBA-15,15–17 Zr-PhyA,18 Zr-SRf,19 Zr-MOF,20,21 and Zr-beta,22,23 has
aCollege of Chemical Engineering, Inner Mongolia University of Technology, Inner
Mongolia Key Laboratory of High-Value Functional Utilization of Low Rank Carbon
Resources, Hohhot 010051, Inner Mongolia, China. E-mail: hczhou@imut.edu.cn
bHohhot No. 2 High School, Hohhot 010010, Inner Mongolia, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra10317a
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containing functional chemicals, which are oen obtained from (FeCl3, $99.9%), copper chloride (CuCl2, 98%), tin chloride
fossil resources. A few reports have focused on the construction of (SnCl4, 99%) and chromium chloride (CrCl3, 98%) were
hafnium catalysts for the hydrogenation reaction using phenolic purchased from Beijing Innochem Technology Co. Ltd. Ethyl
hydroxyl groups. Therefore, exploring novel organic ligands, levulinate (EL, 99%) and g-valerolactone (GVL, 98%) were
especially those derived from renewable biomass resources, is bought from Beijing J&K Scientic Ltd. Trimethylamine and
highly desirable for the construction of hafnium-based catalysts.
ethyl alcohol were obtained from the Beijing Institute of
Tannic acids (or tannins, TA) as naturally existing macromol- Chemical Reagents. Ethanol (AR) and triethylamine (AR) were
ecules, oen derived from a variety of plants, including mono- obtained from Sinopharm Reagent Co. Ltd.
cots, dicots and ferns.28 Depending on their structure and
properties, tannins can be classied into two major groups,
hydrolysable and condensed tannins. Commercially, hydro-
lysable tannins are extracted from Chinese gall, Sicilian sumac
leaves, Turkish gall, Tara pods, myrobalan nuts and chestnuts.29
Hydrolysable tannins contain abundant phenolic hydroxyl groups
in their structures.20 It has been proven that the hydroxyl group on
the benzene rings in TA can coordinate with metal ions.30 Bulut
et al. used the OH groups in TA to reduce Ag+ to Ag0, producing
silver nanoparticles with average sizes between 40–70 nm.31
Sahiner et al. used TA as a biopolymer to obtain poly(TA) (p(TA))
microgels by cross-linking TA with trimethylolpropane triglycidyl
ether (TMPGDE) in a water-in-oil microemulsion system.32 Most
tannins have multiple hydroxyl groups, and can combine metal
ions through coordination interaction. Therefore, TA is an
excellent candidate for the construction of organic–inorganic
hybrid catalysts.
Preparation of the catalysts
The catalyst developed in this work was prepared as follows.
HfCl4 (4 mmol) and TA (0.5 mmol) were added into DMF (100
mL) and stirred until completely dissolved. Then, triethylamine
(37.5 mmol) was dissolved into DMF (40 mL). Next, the solution
of trimethylamine was added dropwise in the mixed solution of
TA and HfCl4 and stirred for 3 h at room temperature, which
played the role of deprotonation of the TA linker.33 Then, the
mixtures were aged under static conditions at 60 ꢀC for 5 h.
Finally, the light brown precipitate was separated by ltration,
thoroughly washed with DMF and ethanol, dried at 80 ꢀC under
vacuum for 20 h, and then, ground into powder for use (denoted
as Hf–TA). The ratios of the hafnium precursor and trimethyl-
amine to TA were varied to optimize the preparing condition of
the catalyst. Similar preparation processes were adopted for the
other M–TA catalysts (M ¼ Zr, Al, Fe, Cr, Sn, and Cu).
In this work, TA was used as an organic ligand to construct
a hafnium-based catalyst. The prepared Hf–TA catalyst was
applied in the CTH reaction of biomass-derived carbonyls with
Catalyst characterization
isopropanol (2-PrOH) as the hydrogen source and solvent Scanning electron microscopy (SEM) was conducted using
(Scheme 1). Both the preparation conditions and the effects of a Hitachi SU8220 scanning electron microscope operated at 20
the reaction parameters on the performance of the catalyst were kV and equipped with an energy dispersive spectrometer (EDS)
studied. The prepared Hf–TA catalyst exhibited high activity and apparatus. The transmission electron microscopy (TEM) images
stability for the CTH reaction of furfuraldehyde (FD), as well as were captured using a JEOL JEM-2010 instrument. The Hf
other carbonyl compounds with different structures. With the elemental composition in Hf–TA was determined by inductively
advantages of high efficiency and the sustainable supply of TA, coupled plasma-optical emission spectroscopy (ICP-OES) on
the Hf–TA catalyst may nd potential applications in CTH a PerkinElmer optimum 5300 DV instrument. The X-ray
reactions implicated in the eld of biomass conversion.
diffraction (XRD) patterns of the powder were collected on
a Rigaku D/tex Ultra 250 X-ray diffractometer using Cu Ka (l ¼
0.154 nm) radiation in the 2q range from 5ꢀ to 130ꢀ. The Fourier
transform infrared (FT-IR) spectra were obtained using a Nexus
870 spectrometer, and the samples were prepared by the KBr
pellet method in the wavenumber range of 400–4000 cmꢁ1. The
thermal property of solid samples was investigated by ther-
mogravimetry (TG) analysis on a HENVEN instrument under N2
at a ow rate of 25 ꢀC minꢁ1 in a programmed temperature
range of 25–900 ꢀC and a heating rate of 10 ꢀC minꢁ1. X-ray
photoelectron spectroscopy (XPS) analysis was carried out on
a ThermoScientic ESCALAB 250Xi spectrometer, and the C 1s
orbital (284.6 eV) was used to correct the binding energies. The
acidity property of the catalyst was studied via the temperature-
programmed desorption of NH3 (NH3-TPD) using a Micro-
meritics AutoChem II 2920 chemisorption analyzer coupled
with a thermal conductivity detector. Briey, 100 mg of the
sample was degassed at 150 ꢀC for 2 h and cooled to 50 ꢀC under
10 mL minꢁ1 He. NH3 was adsorbed for 1 h, aer which He (50
mL minꢁ1) was introduced to purge the system and remove
physically adsorbed NH3 for 1 h. The resulting sample was
Material and methods
Materials
Furfuraldehyde (FD, 99%), furfuryl alcohol (FA, 98%), isopropyl
alcohol (2-PrOH, 99.9%), tannic acid (TA, 98%), N,N-dimethyl
formamide (DMF, 99.9%), hafnium chloride (HfCl4, $98%), n-
decane (98.5%), aluminum chloride (AlCl3, 99%), iron chloride
Scheme 1 (a) The structure of tannin acids (TA) and (b) the preparation
procedure of the Hf–TA catalyst.
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heated to 250 ꢀC at a rate of 10 ꢀC minꢁ1 under a He ow of 50
mL minꢁ1
.
In the present study, the prepared Hf–TA catalyst was used to
catalyze the conversion of furfuraldehyde (FD) to furfuryl
alcohol (FA). The reaction was conducted in a 10 mL Teon-
lined stainless steel autoclave equipped with a magnetic
stirrer. A mixture of 1.0 mmol FD, 5 mL 2-PrOH, and 200 mg
catalyst was added into the autoclave and placed into a pre-
ꢀ
heated oil bath at a set temperature of 70 C. Aer the reactor
was sealed, the reaction mixture was stirred at the desired
temperature for the desired time. When the reaction was
complete, the reactor was cooled down using ice water to
quench the reaction, and the reaction solution was transferred
and diluted with 2-PrOH. The products were analyzed quanti-
tatively using a gas chromatography (GC, Techcomp 7900)
system equipped with a ame ionization detector (FID) and
using decane as the internal standard (42.7 mg). Substrate
conversion, product yield and selectivity were calculated
according to the product analysis methods presented in the
ESI.† The products were identied by GC-MS (Agilent 7890B-
7000D), as well as by comparing the retention times with the
respective standards in GC traces. In order to investigate the
heterogeneity of the catalyst, the solid catalysts were removed
from the reaction mixture by centrifugation aer reacting for
a certain period of time, and the supernatant obtained aer
separation was allowed to react to determine if the product yield
of the reaction continued to increase in the absence of the solid
catalyst. To check the reusability of Hf–TA, the catalyst was
recovered by centrifugation and washed with 2-PrOH (5 mL ꢂ 3
Fig. 1 Effects of the preparation conditions on the performance of the
as-prepared Hf–TA catalyst, different molar ratios of tannic
acid : HfCl4 (a), molar ratios of tannic acid : triethylamine (b), prepa-
ration of catalyst solvent types (c), aging time (d). Reaction conditions:
FD 1 mmol, 2-PrOH 5 mL, catalyst 200 mg, reaction temperature
70 ꢀC, reaction time 5 h (a–c), reaction time 1 h (d).
The prepared Hf–TA sample was characterized by SEM, TEM,
XRD, and SEM-EDS, as shown in Fig. 2 and S1.† From the SEM
and TEM images, it could be seen that Hf–TA was composed of
spherical particles34 with an average particle size of around
68.4 nm. EDS mapping characterization showed that the
elements Hf, O, and C were distributed uniformly in the catalyst
(Fig. S1†). The Hf content in Hf–TA was detected by ICP-OES,
and the composition of Hf–TA was analyzed by elemental
analysis. The results showed that the elemental contents were
39.0 wt% Hf, 33.2 wt% C, 26.8 wt% O, and 1.0 wt% H, and thus
the chemical formula of Hf–TA could be speculated approxi-
mately to be Hf6C76O46H2. The powder XRD pattern (Fig. 2c)
indicated that Hf–TA had low crystallinity, probably due to the
irregular connectivities of Hf–TA and the non-uniform structure
of the TA molecules.35 The specic surface area and pore
structure were analyzed by the nitrogen adsorption–desorption
isotherms (Fig. 2c–e). The result showed that Hf–TA was porous
with mesopores and micropores centered at about 30 nm and
0.8 nm, respectively. The BET specic surface area, average pore
size, and average pore volume of Hf–TA were calculated to be
87.7 m2 gꢁ1, 12.8 nm, and 0.28 cm3 gꢁ1, respectively. The
micropores could have formed from the skeleton structure of
TA, and the mesopores of Hf–TA might have been caused by the
gap between the aggregated particles.36
As shown in Fig. 3, the chemical state of the Hf species in Hf–
TA was examined by XPS and compared with that in HfO2. The
binding energy values of Hf 4f and O 1s in Hf–TA were higher
than those in HfO2, inferring higher positive charge on the Hf
atoms in TA–Hf, which would result in stronger Lewis acidity.37
The FT-IR spectra of TA and Hf–TA displayed the interaction
mode between TA and Hf (Fig. 3c). The bending vibration of the
hydroxyl group at 1318 cmꢁ1 and the breathing vibrations of the
aromatic ring at 1612 and 1533 cmꢁ1 had blue-shied to 1364,
1658, and 1585 cmꢁ1 in Hf–TA,35 respectively. The absorption
peaks at 1201 and 1084 cmꢁ1 originated from the vibration of
ꢀ
times). Aer drying under vacuum at 80 C for 20 h, the recov-
ered catalyst was reused for the next cycle.
Results and discussion
Catalyst screening and characterization
Firstly, the preparation conditions of the catalyst were investi-
gated in this study. The catalysts were prepared using different
mole ratios of tannic acid and hafnium chloride, including
1 : 2, 1 : 4, 1 : 6, 1 : 8, and 1 : 10. The results are shown in
Fig. 1a. The performance of the Hf–TA catalyst prepared using
a 1 : 8 mole ratio of TA and HfCl4 was higher than those of the
other Hf–TA catalysts. Trimethylamine played a key role in the
preparation of the catalyst. It can be seen from Fig. 1b that the
catalytic activity of the catalysts increased with the content of
triethylamine. The molar ratio of 1 : 75 for tannic acid and
triethylamine was selected to prepare the catalyst. Fig. 1c shows
the inuence of the solvent used in the preparation process on
the activity of the catalyst. The conversion of FD and the yield
and selectivity of FA of the catalyst prepared using DMF as the
solvent were obviously higher than those of the catalyst
prepared using water. Fig. 1d shows the performance of the Hf–
TA catalyst under different aging times. It could be seen that the
conversion was the highest when the aging time was 5 h. Thus,
Hf–TA prepared under the conditions of 1 : 8 molar ratio of TA
to HfCl4 1 : 8, 1 : 75 molar ratio of TA to triethylamine, DMF as
the solvent, and aging under 60 ꢀC for 5 h was characterized and
used for further studies.
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Fig. 2 (a) SEM image, (b) TEM image, (c) powder XRD patterns, (d) nitrogen adsorption–desorption isotherm, (e) the pore size distribution of
mesopores, and (f) the pore size distribution of micropores of Hf–TA.
decomposition of TA could be divided into three stages. Below
ꢀ
203 C, the weight loss was 3% and most probably due to the
loss of bound water and organic solvents, and the weight loss
between 203 and 300 ꢀC was due to the decomposition of
oxygen-containing functional groups, such as the hydroxyl and
ester groups.46–50 Beyond 300 ꢀC, tannic acid continued to break
down and was carbonized.51 As seen in Fig. 3d, the TG result
indicated that the introduction of Hf metal salts increased the
thermal stability of the TA compound, and the Hf–TA catalyst
ꢀ
was stable enough at the reaction temperature (<100 C).
Several different metal ions that are commonly used to
construct MPV catalysts according to the literature were also
prepared.25 As observed from Table S1,† the prepared Hf–TA
catalyst showed the best catalytic activity (Table S1,† entry 1),
with an FD conversion of 97.2%, a yield of 96.2% and selectivity
of 98.9% at 70 ꢀC in a reaction time of 5 h. For comparison,
Fig. 3 XPS spectra of (a) Hf 4f and (b) O 1s in Hf–TA and HfO2, and (c)
other metal (Zr, Al, Cr, Sn, Fe, Cu)–TA ligands with similar
FT-IR and (d) TG of TA and the prepared Hf–TA catalyst.
structural characteristics were synthesized.18 Al–TA (Table S1,†
entry 3), Cr–TA (Table S1,† entry 4), Sn–TA (Table S1,† entry 5),
Fe–TA (Table S1,† entry 6) and Cu–TA (Table S1,† entry 7)
the substituted benzene rings.38,39 Compared to TA, the inten-
exhibited low activity for the CTH reaction of FD to FA. The Zr–
sity of the C–OH stretching or –OH bending peak at 1028 cmꢁ1
TA catalyst (Table S1,† entry 2) showed moderate activity with an
decreased, implying the combination of Hf4+ with the organic
FD conversion of 62.6% and an FA yield of 45.4%. Hf(OH)4
ligand.40,41 On the other hand, the peak at 1499 cmꢁ1 corre-
showed the lowest activity with an FA yield of 1.0% (Table S1,†
sponded to –N–CH3 stretching.42 The characteristic peak of the
entry 10), while HfO2 showed an FA yield of 4.0%, and HfCl4
C–N bonds was observed for Hf–TA, indicating that the solvent
showed an FA yield of 3.6%. The strong acidic property of HfCl4
DMF was not completely removed during the catalyst prepara-
led to poor selectivity for the reaction under identical reaction
tion process.42,43 The bands around 758 and 835 cmꢁ1 were
conditions due to the formation of humins.24 The results in
assigned to the Hf–O bonds in Hf–TA.44,45 Although the char-
Table S1† indicated that Hf–TA was highly efficient in FA
acteristic band of the Hf–O bond in Hf–TA at 758 cmꢁ1 partially
production via the MPV reduction (CTH reaction) of FD.
overlapped with that of the phenyl ring vibration (742 cmꢁ1),
The prepared Hf–TA catalyst was compared with analogues
the distinct blue-shi of the band from 669 cmꢁ1 in HfO2 to
reported in the literature (Table 1). The Hf–TA catalyst (Table 1,
693 cmꢁ1 in Hf–TA showed the existence of the Ar–O–Hf bonds,
entries 1–2) performed well under much milder conditions than
i.e. Hf4+ had connected with the phenolic hydroxyl groups in
the other Hf- or Zr-based catalysts (Table 1, entries 3–9).
TA.46 TG analysis was conducted to study the thermal stability of
Compared with other hydrogenation catalysts, the prepared Hf–
TA and Hf–TA in the range of 50–750 ꢀC. The thermal
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TA catalyst showed higher TOF (5.6 hꢁ1) at a lower temperature stretching vibration (1454 cmꢁ1) of DMF were obvious for fresh
(70 ꢀC) (Table 1, entry 1). These results in Table 1 indicated that Hf–TA,42,54,55 but they were found to weaken and gradually
Hf–TA could catalyze the CTH reaction of FD under mild disappear aer 3 reuses. TG indicated that the weight loss of the
conditions with high efficiency.
fresh catalyst was slightly higher than that of the reused cata-
Subsequently, the effects of the reaction conditions on the lysts under identical temperatures (Fig. 6b). In combination
performance of Hf–TA were investigated (Fig. 4). It could be with the FTIR result of the fresh catalyst (Fig. 6a), it was inferred
seen that both catalyst dosage and the temperature had that there was DMF residual in the pore structures of the cata-
a signicant inuence on the performance of Hf–TA. As shown lyst aer preparation, and the residual DMF was hard to be
in Fig. 4a, with increasing catalyst dosage, both conversion and completely removed during the drying process. During the
yield increased obviously, which was contributed to the initial uses, the residual DMF in the pore structures could be
increasing number of available catalytic sites during the reac- replaced by 2-PrOH gradually, and more active sites could
tion.25 As seen in Fig. 4b, the conversion, selectivity, and yield contact 2-PrOH, leading to an increase in activity. The conver-
increased with an increase in the reaction temperature. In order sion and yield decreased slightly with the increasing of aer the
to both shorten the reaction time and reduce energy 4th use, and it could be attributed to the catalyst loss during the
consumption, it seemed that the reaction temperature of 70 ꢀC centrifugation recycling processes. The performance for the
was high enough under the present conditions. Therefore, we 10th use was still better than that for the rst use. The hetero-
chose an Hf–TA dosage of 0.2 g and 70 ꢀC reaction temperature geneity of the catalyst was also studied. The reaction was
to study the effect of reaction time. From Fig. 4c, it is evident stopped at 0.5 h by removing the solid catalyst from the reaction
that the conversion and yield of the reaction increased almost mixture, and the solution was allowed to react under the same
linearly with the reaction time within the initial 2 h, indicating reaction conditions to see if the FA yield further increased
that the reaction proceeded fast in the initial stage. Aer 2 h, the without the solid catalyst. However, there was no further
increase in conversion and yield was not very obvious, and increase in the FA yield aer the solid catalyst was removed
nearly all of the substrate could be converted at 3 h.
(Fig. 5b). The Hf contents in the fresh and recycled catalysts
The reusability and heterogeneity of Hf–TA in the catalytic were analyzed by ICP-OES, and there was no obvious decrease
reaction process were investigated (Fig. 5). It could be seen that aer recycling (Table S2†). This result conrmed that the active
there was no obvious decrease in FD conversion, FA yield, and sites in Hf–TA did not leach into the reaction mixture, and it was
selectivity aer ten cycles compared to the rst (Fig. 5a). Inter- a heterogeneous catalytic process. Based on the above results,
estingly, the activity of Hf–TA increased gradually in the initial the prepared Hf–TA catalyst exhibits excellent stability in terms
three cycles. To disclose the possible reasons for the increase in of both structure and performance during the recycling process.
activity, fresh and recycled Hf–TA catalysts were characterized.
Besides FD, the highly efficient MPV reduction of other
Based on SEM (Fig. S2a†) and TEM (Fig. S2b†) characterization, carbonyl compounds is also of great signicance. In order to
the morphology of the recycled catalysts had no obvious broaden the substrate scope, the CTH reactions of different
changes. XRD indicated that the skeleton structure was main- substrates were investigated with the prepared Hf–TA catalyst.
tained well aer reuse, as shown in Fig. S3a.† The BET surface The structures of the products were analyzed by GC-MS. As
areas and pore structures of the fresh and recycled Hf–TA shown in Table 2, it could be seen that Hf–TA could catalyze the
catalysts are compared in Table S2.† The BET surface area of the CTH reactions of all the substrates studied, but the product
recycled catalyst showed a slight decrease compared with that of distribution was different for different substrates. For the
the fresh sample. The Hf 4f and O 1s XPS spectra (Fig. S4†) conversion of FD, no obvious byproducts were detected using
showed that the chemical states of the elements Hf and O had GC-MS. For veratraldehyde (Table 2, entry 2), phenyl-
no obvious changes aer the reaction. FTIR analysis (Fig. 6a) acetaldehyde (Table 2, entry 3), and 4-chlorobenzaldehyde
showed that the NH2 stretching vibration (1560 cmꢁ1) and C–N (Table 2, entry 4), high yields of 98.9%, 95.0%, and 94.0% were
Table 1 MPV reaction of FD to FA catalyzed by various catalysts
t
(h)
Conversion
(%)
Yield
(%)
Selectivity
(%)
Entry
Catalysts
T (ꢀC)
TOFc (hꢁ1
)
Ref.
1a
2b
3
4
5
6
7
8
9
Hf–TA
Hf–TA
50
70
120
100
80
10
3
2
2
3
9
2
2
5
81.2
96.2
99.2
100
97
98.0
—
99.9
97.7
80.3
94.8
97.6
—
96
80.0
97
98.9
98.5
98.4
>99
97
81.6
—
98.2
98.9
1.4
5.6
1.7
4.7
0.3
0.04
5.2
0.95
0.4
This work
This work
PhP–Hf
FDCA–Hf
Zr–LS
Zr–HAs
Hf-MOF-808
MZCCP
BZC
46
35
13
47
24
52
53
70
100
140
80
98.2
96.7
a
Reaction conditions: furfuraldehyde 1 mmol, 2-PrOH 5 mL, catalyst 100 mg. b Reaction conditions: furfuraldehyde 1 mmol, 2-PrOH 5 mL, catalyst
200 mg. c TOF is dened as mol (main product)/[mol (total metal added) ꢂ h (time)].
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Fig. 4 The effects of reaction parameters on the performances of Hf–TA: (a) the amount of catalyst, (b) reaction temperature, and (c) reaction
time. Typical reaction condition: FD 1 mmol, 2-PrOH 5 mL, other conditions as shown in the figures.
a change in the reduction rate,57 which could be inferred from
Table 2 (entries 4, 8 and 10). For EL, product 5a and by-product
5b were produced from the hydrogenation and trans-
esterication of EL with 2-PrOH, respectively, in the reaction
process.24,58 The reaction was also tested with 2-hexanone (Table
2, entry 6), and the yield of 2-hexanol (6a) was 82.9%. The
hydrogenation of 1-naphthaldehyde was conducted and
produced a yield of 82.4%, and the detected by-products
included an aldehyde–ketone condensation product (7b) and
naphthalene (7c) (Table 2, entry 7). The by-product 7c might
have formed from 7 by a decarbonylation reaction in the pres-
Fig. 5 (a) Reusability and (b) heterogeneity of the Hf–TA catalyst.
Reaction conditions: FD 1 mmol, 2-PrOH 5 mL, catalyst 200 mg,
reaction temperature 70 ꢀC, reaction time 0.5 h, and DMF solvent.
ence of the Lewis acid sites of Ar–O–Hf and the solvent 2-
PrOH.59 Besides the target product and the detectable byprod-
ucts mentioned above, humins were another important class of
byproducts (3b, 6b, 8b and 9b).60 Overall, these results showed
that Hf–TA was efficient for the conversion of various substrates
with different structures.
Mechanism
Through the characterization of NH3-TPD (Fig. S5†) techniques,
it was speculated that the formation of acid–base catalytic sites
in Hf–TA. The acid sites mainly originated from the Hf4+ and the
basic sites resulted mainly from O2ꢁ in the phenate groups.61
Based on the experimental results and some previous
reports,18,50,61,62 a possible mechanism for the CTH reaction of
FD to produce FA over Hf–TA has been proposed (Scheme 2). In
the rst step, 2-PrOH is adsorbed on Hf–TA, resulting in its
dissociation into the corresponding alkoxide and hydrogen at
the Ar–O–Hf sites.63 Meanwhile, the deprotonation of 2-PrOH is
enhanced by the basic phenate O2ꢁ ions, and the carbonyl
group of FD is activated by the catalytic Hf4+ sites.43 The acti-
vated aldehyde group interacts with the nearby activated 2-
PrOH to form a six-linked intermediate.45 The alcohol product is
formed, and 2-PrOH is converted into acetone. In the second
step Ar–O–Hf sites interacted with the C–O group of acetone,
increasing its polarization and facilitating the alcohol group of
by-product was attacked.64 The carbonyl species of substrate
was able to react with acetone resulted the aldol condensation
to form the main byproduct (2b, 4b, 7b, 10b, 11b and 12b).18,56
These results indicated that the designed Hf–TA catalyst in this
Fig. 6 (a) FT-IR spectra of the fresh Hf–TA and recycled Hf–TA
catalysts after 1, 3 and 10 cycles; (b) TG curves of the fresh Hf–TA and
recycled Hf–TA catalyst after 10 cycles.
obtained, respectively. For substrates 2 and 4, small amounts of
byproducts formed by the aldehyde–ketone condensation of the
reactant with the co-product acetone were detected, while for
the main products 2a and 4a, yields higher than 90% were
achieved. Similar byproducts were also detected for a few other
substrates (10, 11 and 12). For by-product 12c, it was probably to
imply that the cinnamaldehyde molecule containing conju-
gated C]C and C]O group had the strongest adsorption on
Ar–O–Hf sites, followed by the hydrocinnamaldehyde molecule
containing an isolated C]O group and further being hydroge-
nated into 12c.56 It has been reported that the introduction of
electron-withdrawing substituents in benzaldehyde causes
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Table 2 CTH reduction of different carbonyl compounds to alcoholsa
Main productb
By-productc
Number
t
(h)
Conv.
(%)
Number, yield
(%)
Entry
1
Reactant
T (ꢀC)
70
4
7
99.1
99.9
—
2
3
4
110
110
100
Humin
3b
12
99.9
98.9
6
5
6
160
150
46
24
92.2
92.8
Humin
6b
7
110
12
98.3
Humin
8b
8
9
110
100
10
6
99.5
97.7
Humin
9b
10
110
7
97.2
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Table 2 (Contd.)
Main productb
By-productc
Number
t
(h)
Conv.
(%)
Number, yield
(%)
Entry
11
Reactant
T (ꢀC)
100
10
10
96.7
98.0
12
100
a
b
Reaction conditions: substrate 1 mmol, 2-PrOH 5 mL, and Hf–TA catalyst 200 mg. Conversion (Conv.) and yield were quantied by GC using
decane as the internal standard. c By-products were qualitatively analyzed using GC-MS.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21968021, 21606134, 21676149,
21868021, 21566028, 21868020), CAS “Light of West China”
Program, and the Innovative and Entrepreneurial Talents
Grassland Talents Engineering of Inner Mongolia.
Notes and references
Scheme 2 Possible mechanism of the Hf–TA-catalyzed CTH reaction
of FD.
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