Surfactant-assisted synthesis of mesoporous hafnium- imidazoledicarboxylic acid hybrids for highly efficient hydrogen transfer of biomass-derived carboxides

Article abstract of DOI:10.1016/j.mcat.2019.110611

Catalytic transfer hydrogenations of biomass-derived carbonyl compounds to produce corresponding alcohols are important pathway for biomass transformation. Herein, a facile route was developed to synthesize the surfactant-assisted heterogeneous acid-base bifunctional 4,5-imidazoledicarboxylic acid-hafnium hybrid catalyst (Hf-H₃IDC-T) by hydrothermal self-assembly method. The as-prepared Hf-H₃IDC-T was characterized by SEM and TEM, FT-IR spectra, N₂ adsorption-desorption, X-ray diffraction patterns (XRD), X-ray photoelectron spectroscopy (XPS), Thermogravimetry analysis (TG), NH₃/CO₂-TPD, NMR, GC[sbnd]MS, ICP-OES and elemental analysis. Hf-H₃IDC-T hybrid had mesoporous structure and acid-base couple sites. A quantitative yield (99.2%) of furfuryl alcohol (FFA) was obtained from furfural (FUR) over Hf-H₃IDC-T using 2-propanol as the hydrogen source under mild conditions. It's found that the amino groups on the imidazole ring is beneficial to enhance the base sites of catalyst. Meanwhile, the addition of hexadecyl trimethyl ammonium bromide (CTAB) as template agents can improve the specific surface area of the catalyst. Dynamic analysis showed that the apparent activation energy of FUR reduction was as low as 50.89 kJ / mol. The as-prepared catalyst has good stability and can be recycled. Finally, the catalyst also has a good catalytic effect on the hydrogenation reaction of aldehydes and ketones of biomass-derived compounds.

Full text of DOI:10.1016/j.mcat.2019.110611

MolecularCatalysis479(2019)110611
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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Surfactant-assisted synthesis of mesoporous hafnium- imidazoledicarboxylic
acid hybrids for highly efficient hydrogen transfer of biomass-derived
carboxides
⁎⁎
Fanglin Daia,b, Shenghui Zhoub, Xingzhen Qina,⁎, Detao Liub, Haisong Qib,c,
a School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
b State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China
c Guangdong Engineering Research Center for Green Fine Chemicals, Guangzhou, 510640, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Catalytic transfer hydrogenations of biomass-derived carbonyl compounds to produce corresponding alcohols
are important pathway for biomass transformation. Herein, a facile route was developed to synthesize the
surfactant-assisted heterogeneous acid-base bifunctional 4,5-imidazoledicarboxylic acid-hafnium hybrid catalyst
(Hf-H3IDC-T) by hydrothermal self-assembly method. The as-prepared Hf-H3IDC-T was characterized by SEM
and TEM, FT-IR spectra, N2 adsorption-desorption, X-ray diffraction patterns (XRD), X-ray photoelectron spec-
troscopy (XPS), Thermogravimetry analysis (TG), NH3/CO2-TPD, NMR, GCeMS, ICP-OES and elemental ana-
lysis. Hf-H3IDC-T hybrid had mesoporous structure and acid-base couple sites. A quantitative yield (99.2%) of
furfuryl alcohol (FFA) was obtained from furfural (FUR) over Hf-H3IDC-T using 2-propanol as the hydrogen
source under mild conditions. It's found that the amino groups on the imidazole ring is beneficial to enhance the
base sites of catalyst. Meanwhile, the addition of hexadecyl trimethyl ammonium bromide (CTAB) as template
agents can improve the specific surface area of the catalyst. Dynamic analysis showed that the apparent acti-
vation energy of FUR reduction was as low as 50.89 kJ / mol. The as-prepared catalyst has good stability and can
be recycled. Finally, the catalyst also has a good catalytic effect on the hydrogenation reaction of aldehydes and
ketones of biomass-derived compounds.
Surfactant-assisted
Acid-base bifunctional
Catalytic transfer hydrogenation
Furfural
Furfuryl alcohol
1. Introduction
Due to its high reactivity, FUR can be converted to other high value-
added products by the several chemical routes [7,10]. In the catalytic
Over the past years, the development of renewable energy as a
substitute for diminishing fossil fuels has gradually attracted the at-
tention of the chemical field [1,2]. As a sustainable and readily avail-
able resource, biomass feedstock is an unique alternative to traditional
fossil resources from which energy, fuels and chemicals can be obtained
[3]. In the existence of acidic catalysts, simultaneously combined with
basic/metallic sites, sugars and lignin components can be selectively
converted into a variety of products, such as organic acids (e.g., levu-
linic acid and lactic acid) [4], furanic compounds (e.g., furfural (FUR),
5-hydroxymethylfurfural (HMF), and 5-ethoxymethylfurfural (EMF))
[5], polyols (e.g., sorbitol, mannitol, xylitol, and ethylene glycol) [6],
and phenolics (e.g., vanillin and veratraldehyde) [7].
process, the focus is on the further use of FUR by hydrogenation, oxi-
dation, alkylation and rehydration. Especially, the use of H-donor
sources to increase the hydrogen content of oxygenates is one of the
significant steps in the efficient and safe production of energy-intensive
molecules [11].
Nevertheless, an effective alternative to conventional catalytic hy-
drogenation using hydrogen as a reducing agent is catalytic transfer
hydrogenation, which has been proposed for the conversion of ketones
or aldehydes in alcohols because it avoids the use of high hydrogen
pressure [12–14]. As reported, Meerwein-Ponndorf-Verley (MPV) can
reduce aldehydes and ketones by secondary alcohols in the presence of
Lewis acidic or basic catalysts [15]. The Lewis acid sites is capable of
activating the C]O bond of the aldehyde or ketone, while the alcohol is
deprotonated. Moreover, it is speculated that the formation of the six-
membered intermediate is due to the participation of hydrogen at the
FUR is one of the main products derived from the conversion of
lignocellulosic biomass. The presence of aldehyde groups and aromatic
rings makes FUR become a multi-purpose chemical intermediate [8,9].
⁎ Corresponding author.
⁎⁎ Corresponding author at: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China.
E-mail addresses: xzhqin2009@163.com (X. Qin), qihs@scut.edu.cn (H. Qi).
https://doi.org/10.1016/j.mcat.2019.110611
Received 24 July 2019; Received in revised form 6 September 2019; Accepted 6 September 2019
Availableonline25September2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

F. Dai, et al.
MolecularCatalysis479(2019)110611
Cα position, and the aldehyde/ketone and the deprotonated alcohol
being directly activated on the Lewis acid sites [16–18]. The heteroa-
toms-modified zeolites and mesoporous silica can be used to prepare
Lewis acid catalyst, with which FUR conversion was carried out by MPV
reaction [19]. Hydrogenation of the FUR produces FFA, which can be
reacted with secondary alcohol to form furfuryl ethers. Noble metals
such as Au, Pt and Pd, and non-noble metals such as Cu, Co and Ni are
commonly used as catalysts to obtain a target product in a sufficient
yield [20,21]. In addition, Such as zirconia, hydrotalcite, hydro-
xyapatite, zeolite, etc., solid materials are also used as heterogeneous
catalysts for biomass conversion [22,23]. Some reports indicated that
the assembly of organic molecules (such as polyphenols, porphyrins,
and phytic acids) with inorganic metal ions (such as Cu2+, Co2+, and
Zr4+) could mimic natural structures and generate a series of active
frameworks with good stability, which was a more convenient and
controllable alternative for the MPV reaction [24–27]. For this purpose,
the preparation of FFA produced by MPV reaction with high efficiency
and easy recovery of the organic-inorganic framework structure catalyst
still is a subject worthy of study.
When considering the use of MOFs as catalysts for the reaction
process, the metal oxide nodal component of the MOFs framework is
usually used as the active site [28,29]. In general, the introduction of
more basic sites to enhance the basicity of the catalyst is an attractive
method to improve catalytic efficiency for the MPV reaction of FUR to
produce FFA [30]. In our work, we synthesized a new surfactant-as-
sisted, heterogeneous and acid-base functional organic-inorganic Hf-
based/nitrogen-containing hybrid catalyst (named Hf-H3IDC-T) based
on HfCl4 and H3IDC in N, N-dimethylformamide (DMF) by hydro-
thermal method (Scheme S1). It provides the possibility to design ef-
fective catalysts with high basicity by using H3IDC as a structural unit.
Additionally, surfactant (hexadecyl trimethyl ammonium bromide
(CTAB)) was used as template agents for synthesis of Hf-H3IDC-T. The
surfactant is commonly used to augment the porous structure of metal-
organic framework topology [30]. The catalytic process uses 2-propanol
as the hydrogen source to catalyze the MPV reaction to provide a sa-
tisfactory FFA yield. The structure and performance of the prepared
catalysts were comprehensively investigated.
order to further expand the structure of Hf-H3IDC, CTAB was added as a
co-reagent under otherwise identical conditions, and the obtained cat-
alyst was defined as Hf-H3IDC-T. In addition, the CTAB was removed as
a co-reagent by filtration and washing. For comparison, Hf-FDCA was
also synthesized by hydrothermal method of FDCA with metal chloride
(HfCl4) in DMF, according to previously reported procedures [23,28].
HfO2 was calcined at 150 ℃ for 12 h. All solid samples were dried at
90 °C for 5 h prior to catalytic performance testing and catalyst char-
acterization.
2.3. Catalyst characterization
The structure and morphology of the catalyst were examined using a
scanning electron microscope (SEM) and a high-resolution transmission
electron microscope (JEM-2100F) equipped with an EDX analyzer op-
erated at 200 kV. FT-IR spectra were recorded on a Perkin Elmer 1710
spectrometer (KBr disk) with a wavenumber range of 4000–400 cm−1
.
X-ray photoelectron spectroscopy (XPS) analysis was performed on a
physical electron Quantum 2000 scanning ESCA microprobe (Al-Kα
Mono, hv = 1486.6 eV), and the binding energy was corrected using
C1 s orbital (284.8 eV). X-ray diffraction (XRD) patterns of the powder
samples were obtained using a Rigaku International D/max-TTR III X-
ray powder diffractmeter with Cu-Kα (λ = 1.542 Å) radiation and 2θ
scanned from 5° to 90°. Thermogravimetry (TG) was used to determine
the thermal properties of catalysts on a NETZSCHSTA 429 instrument
under N2 in a flow rate of 30 ml/min at a programmed temperature
range of 50–900 °C with a heating ramp of 10 °C/min. BET surface area,
Barrett-Joyner-Halenda (BJH) pore size and volume were measured by
N2 physisorption (TriStar II 3flex) at liquid N2 temperature. The de-
termination of acidity and basicity of different samples by NH3 and CO2
(NH3
/
CO2-TPD) temperature programmed desorption by
a
Micromeritics AutoChem II 2920 equipment. The catalyst was firstly
degassed under a flowing He (30 ml/min) at 150 °C for 2 h, and then the
system was cooled to room temperature in the presence of He. After the
solid samples were adsorbed with NH3 or CO2, the system was purged
at 50℃ heat flow and the TPD results were gained from 50 °C to 300 °C
under flowing He. The ratios of Brønsted and Lewis acid sites of the
solid catalysts were determined by pyridine adsorption FT-IR fitted
with a Bruker VERTEX V70v system, based on integral area and the
characteristic peak position in the wavenumber range of
1400–1700 cm−1. The Hf elemental composition was measured by
Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES)
equipment (Spectro Arcos FHX22).
2. Experimental section
2.1. Materials
4,5-imidazoledicarboxylic acid (H3IDC, 97%), Hafnium chloride
(HfCl4, 99.5%), Hafnium oxide (HfO2,99.9%), 2-propanol-d8 (2-PrOH-
d8,99.5%)were purchased from Macklin Corporation, Furfural
(FUR, > 99.5%), Furfuryl alcohol (FFA, 98%), 5-hydroxymethylfurfural
2.4. Catalytic reaction
The MPV reaction of the biomass-derived compounds with 2-pro-
panol was carried out in an oil-heated condition in a 15 ml Ace pressure
tube (Synthware, Beijing). Typically, aldehydes (1.0 mmol), catalysts
(0.1 g), and 2-propanol (10 mL) were added into the reactor, and then
placed into the oil bath at stated temperature of 80–140 °C, followed by
the magnetic stirring for specific time at 600 rpm. After completion, the
reaction tube was cooled to room temperature with cold water in a
beaker. The reaction mixture was centrifuged and collected for analysis.
Quantitative analysis of reactants and products on a standard sample
using toluene as an internal standard on a GC (Shimadzu Nexis GC-
2030) equipped with an HP-5 capillary column (30.0 m × 250 mm ×
0.25 mm) and a flame ionization detector. Identification of products
were observed using GC–MS (GCMS-QP2010 Ultra) equipped with HP-
5MS capillary column (30.0m × 250 mm × 0.25 mm).
(HMF, > 99.0%),
5-methylfurfural
(> 99%),
cinnamaldehyde
(> 99.5%), citral (> 97%), veratraldehyde (> 99%), and 2-propanol
(> 99.0%), hexadecyl trimethyl ammonium bromide (CTAB, 99%),
2,5-Furandicarboxylic acid (FDCA, 98%) were purchased from Aladdin
Corporation. All other reagents were used as received unless otherwise
stated and were not treated at all.
2.2. Catalyst preparation
The catalyst (Hf-H3IDC-T) was prepared by self-assembly of H3IDC
with metal chloride (HfCl4) in DMF under hydrothermal conditions.
The preparation process is as follows, H3IDC (6 mmol, 0.94 g) and HfCl4
(6 mmol, 1.92 g) were dissolved in DMF (31 ml, 0.4 mol), respectively.
The two solutions, which are completely dissolved, are then mixed and
transferred to a 100 ml stainless steel reactor. The sealed autoclave was
placed in an oven at 120 °C for 24 h under static conditions. After
completion of the reaction, the obtained white precipitate was removed
by filtration, and then washed with DMF and ethanol for 6–8 times until
a colorless transparent filtrate was observed, and then vacuum dried at
100 °C overnight to obtain a target sample designated as Hf-H3IDC. In
2.5. Reusability of the prepared Hf-H3IDC-T
In order to research the reusability of Hf-H3IDC-T, the catalyst was
recovered by the centrifugation, and washed five times with DMF and
ethanol. After drying at 100 °C for 5 h in an oven, the recovered catalyst
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F. Dai, et al.
MolecularCatalysis479(2019)110611
was reused for the next cycle of the reaction. The solid residue was
removed by filtration, and then the Hf element content immersed in the
reaction solution was analyzed by ICP, and the filtrate was diluted with
water to an equal volume.
respectively. This indicated that the introduction of organic species
(i.e., H3IDC) greatly increased the adsorbed quantity of N2 and gener-
ated more distribution of micropores and mesopores, possibly due to
the formation of assembled organic-inorganic networks (Fig. 2a),
especially in the case of Hf-H3IDC-T. Hf-FDCA had inferior surface area
(9.73 m2 g−1) and pore volume (0.0326 m3 g−1) (Table 1), although its
average pore size is mesoporous (14.02 nm). In contrast, Hf-H3IDC and
Hf-H3IDC-T showed much higher surface areas (ca.89.59 and
153.50 m2 g−1) and average pore sizes of 14.49 and 12.94 nm, re-
spectively (Table 1, Fig. 2b). Therefore, the CTAB used in the pre-
paration of Hf-H3IDC-T can enlarge the surface areas of the catalysts,
and improving the accessibility of active sites for the substrates [30].
The composition of the catalyst elements was determined by ICP-OES
and elemental analysis in Table S1. The results showed that the molar
ratio (HfCl4:H3IDC = 1:1) of the synthetic catalyst is reasonable co-
ordination pattern (Scheme S1).
FT-IR spectra of HfO2, Hf-H3IDC, and Hf-H3IDC-T in Fig. 2c showed
the characteristic signals of HfeO bonds at 800 cm−1. For Hf-H3IDC
and Hf-H3IDC-T, the enhanced absorptions at 1470 and
1500–1700 cm−1 shows symmetric and asymmetric stretching vibra-
tions of the eCO2− group, respectively [32,33]. The bridging co-
ordination mode of H3IDC and Hf (Scheme S2) can be demonstrated by
splitting about 93 cm−1 between symmetric and asymmetric peaks,
which is consistent with the Hf-FDCA structure (Fig. S3) [34]. In the
case of pure H3IDC, only a single stretching vibrations having a split of
205 cm−1 was observed, which was very consistent with the mono-
dentate CeOH (2500–3200 cm−1) of the carboxyl group in H3IDC [30].
In addition, the broadband at 3300 cm−1 for Hf-H3IDC and Hf-H3IDC-T
not only were from the protonated carboxyl group (−COOH) of the
H3IDC moieties that are incompletely coordinated with Hf, but also are
ascribed to adsorbed water and other hydroxyls [32]. The use of the
CTAB for preparation of Hf-H3IDC-T had enhanced the binding of the
carboxylate compound to Hf, and its weaker band was well supported at
about 3300 cm−1 compared to Hf-H3IDC [30,33]. Meanwhile, the blue
shift in the stretching vibrations of the hydroxyl group of Hf-H3IDC-T
means that there is an unstable eOH group, possibly corresponding to a
weak Brønsted acid site, compared to pure H3IDC. Fig. 2d illustrated the
XRD patterns of HfO2, Hf-H3IDC, and Hf-H3IDC-T. Compared to the
HfO2 having high crystallinity (Fig. 2d) and the clear crystal lattice,
these Hf-containing materials hybrids was all amorphous. The intense
2.6. Isotopic labelling experiments
For Isotopic kinetic study of the MPV reaction of FUR to FFA, 1H
NMR spectra of the reaction mixture after different time durations were
performed in cosolvent tert-butanol and 2-propanol-d8 with a molar
ratio of 3:1 on a JEOL-ECX 500 NMR spectrometer operating at
500 MHz at room temperature. To reveal the occurrence of direct hy-
drogen transfer other than metal hydride route, mass fragmentation
analysis was further conduced with GCeMS using a cosolvent tert-bu-
tanol and 2-propanol-d8 with a molar ratio of 3:1. In the resulting
medium, tert-butanol is not able to act as hydrogen source due to the
absence of β-H, while it can exchange most D species of eOD group in
2-propanol-d8 to eOH group and replace active D on metal surface with
H [31].
3. Results and discussion
3.1. Characterization of catalysts
The synthetic procedure for Hf-H3IDC and Hf-H3IDC-T were de-
scribed in detail in the experimental section. The synthesized Hf-H3IDC
and Hf-H3IDC-T was primarily characterized by SEM, TEM, STEM-
HAADF, as shown in Figs. 1 and S1. The Hf-H3IDC-T hybrid was small
particles with porous surface (Fig. 1a and b). By comparison, the HfO2
was in small ball structure (Fig. S2). Compared to the SEM and TEM
images of Hf-H3IDC hybrid (Fig. S1), the introduction of the CTAB in
preparation process not only caused the formation of the network layer
structure (Fig. 1c), but also change the surface and porous structure of
Hf-H3IDC-T. As observed from the corresponding elemental mappings
and the STEM-HAADF image in Fig. 1d and the Hf, O, N and C com-
positions (i.e., Hf4+ and eCO2− moieties) in Hf-H3IDC-T are evenly
distributed in Fig. 1e–h.
The structural and textural of the as-prepared catalysts were further
investigated. Fig. 2a and b showed the N2 adsorption-desorption iso-
therms and pore volume of HfO2, Hf-H3IDC and Hf-H3IDC-T,
Fig. 1. SEM images (a), TEM images (b) and HR-TEM (c) images of Hf-H3IDC-T; STEM-HAADF image (d) and corresponding elemental mappings of Hf(e), C(f), N(g)
and O(h) of Hf-H3IDC-T.
3

F. Dai, et al.
MolecularCatalysis479(2019)110611
Fig. 2. N2 adsorption-desorption isotherm (a), pore size distribution by N2 adsorption-desorption isotherm (b), FTIR spectra (c), XRD patterns (d) of the various
catalysts.
information about the acid/base sites for these solid hybrids (Fig. 3a
and b, Table 1). Hf-H3IDC and Hf-H3IDC-T have relatively higher con-
tents of both basic and acidic species, as compared with those of HfO2.
The formation of acid-base sites in these catalysts could be attributed to
the formation of robust Hf-O-C frameworks with the better heat re-
sistance both acidic (Hf4+) and basic (O2−) species during thermal
treatment of H3IDC and HfCl4 [35]. Furthermore, Hf-H3IDC and Hf-
H3IDC-T possesses relatively higher contents of basic than Hf-FDCA
(Scheme S3). This may be due to the presence of amino groups on the
imidazole ring in Hf-H3IDC and Hf-H3IDC-T. (Scheme S1).
Table 1
Texture and acid-base properties of Hf-containing catalysts.
c
Catalyst
SBET
Vpore
Dpore nm
Basicity
Acidity
Base/
acid
ratio
m2 g−1
m3 g−1
mmolg−1
mmolg−1
a
b
d
d
HfO2
8.59
89.59
153.50
0.0226
0.3135
0.4791
0.2274
0.0326
11.18
14.49
12.94
7.31
0.37
0.66
0.78
0.72
0.48
0.81
1.51
1.41
1.38
0.68
0.46
0.44
0.55
0.52
0.71
Hf-H3IDC
Hf-H3IDC-T
Hf-H3IDC-Te 133.77
Hf-FDCA
9.73
14.02
In addition, it was demonstrated that the presence of Brønsted
acidic sites was also beneficial for the Lewis acid-mediated hydrogen-
transfer process [36–38]. As shown in Fig. 4a, pyridine-adsorbed FT-IR
spectra of both HfO2, Hf-H3IDC and Hf-H3IDC-T were recorded. The
bands at wavenumber of 1450 and 1545 cm−1 were attributed to the
pyridine interacting with Lewis acid sites (LA) and Brønsted acid sites
(BA), respectively (Fig. 4a). Compared to HfO2, the acid density of Hf-
H3IDC and Hf-H3IDC-T was increased obviously. Hf-H3IDC and Hf-
H3IDC-T exhibited relatively larger bands than those of HfO2 at around
1580 and 1620 cm−1, which were assigned to hydrogen-bonded pyr-
idine and pyridine-adsorbed on Lewis acidic centers, respectively. It
further demonstrated that Hf-H3IDC-T had higher Lewis and Brønsted
acid strength. The results of pyridine-adsorbed FT-IR proved the si-
multaneous presence of Hf-O-C frameworks and protonated carboxyl
groups (eCO2H) of H3IDC moieties, which contributed to the formation
of Lewis acid/base sites and Brønsted acidic species, respectively
[37,39]. Apart from the nature and density/content of acid and base
sites, the acid-base strength of Hf-H3IDC, Hf-H3IDC-T, Hf-FDCA, and
HfO2 was proved by analysis the local environment of the Hf and O
element in different materials by XPS technique. The survey spectra
further confirmed the presence of the Hf, O, C and N elements in Hf-
H3IDC-T (Fig. 4b). A higher Hf 4f binding energy corresponds to the
larger positive charge of Hf species, demonstrating that the Lewis
a
BET surface area determined by N2 adsorption isotherm.
Pore volume estimated by BJH adsorption cumulative volume of pores.
Average pore size was estimated from the adsorption average pore dia-
b
c
meter.
d
Basicity and acidity of the catalysts were determined by CO2 and NH3-TPD,
respectively.
e
Recovered Hf-H3IDC-T in the sixth cycle.
peaks of Hf-H3IDC and Hf-H3IDC-T were typical reflection, which is
similar to that of Hf-FDCA (Fig. S4) [34].
The thermal stability of selected HfO2, Hf-H3IDC and Hf-H3IDC-T
samples was investigated by thermogravimetric (TG) analysis at
50–900 °C (Fig. S5). As indicated by these results, Hf-H3IDC and Hf-
H3IDC-T were relatively stable at 50–300 °C. Particularly, in the tem-
perature range of 50–300 °C, the weight of Hf-H3IDC and Hf-H3IDC-T
are reduced by about 10%, which is slightly higher than HfO2. This is
probably own to the removal of extra Brønsted acidic species (eOH)
and organic solvent residues (e.g., DMF) inside pores of Hf-H3IDC and
Hf-H3IDC-T besides physically adsorbed water, and it is consistent with
results gained by N2 adsorption-desorption and FT-IR (Fig. 2a–c). The
results indicated that the as-prepared catalysts Hf-H3IDC and Hf-H3IDC-
T were stable under the reaction conditions which will be discussed in
the later parts [35]. CO2 and NH3-TPD were carried out to obtain more
4

F. Dai, et al.
MolecularCatalysis479(2019)110611
Fig. 3. CO2-TPD (a) and NH3-TPD (b) of the various catalysts.
acidity of these Hf centers was increased (Fig. 4c) [40,41]. Additionally,
the binding energy of O 1s belonging to Hf-O-C coordination interac-
tions in hafnium-based hybrid was higher than for Hf-O-Hf interactions
in HfO2 (Fig. 4d) [40,42]. The medium binding energy of O 1s assigned
to negative charge on the oxygen atoms indicated the medium basic
strength of Hf-H3IDC-T. Evidently, these nitrogen-containing groups in
Hf-H3IDC-T could facilitate the formation of the basic sites. These re-
sults showed the functional groups in H3IDC could increase the basic
contents and stabilize Lewis acidic Hf4+ species achieved by the for-
mation of porous Hf-O-C framework structure [43]. As consistent to FT-
IR (Fig. 2c), the results of pyridine-adsorbed FT-IR (Fig. 4a) certified the
simultaneous presence of Hf-O-C frameworks and protonated carboxyl
groups (eCO2H) of H3IDC moieties, which contributed to the formation
of Lewis acid/base sites and Brønsted acidic species, respectively.
3.2. Catalytic performance evaluation of the various catalysts
FUR derived from lignocellulose-derived carbohydrates is one of key
components in bio-oil, while its reactive aldehyde group is susceptible
to formation of humins by condensation reactions [44]. Herein, the
conversion of FUR to FFA was selected as a model reaction to study the
catalytic performance of Hf-H3IDC-T in MPV reduction. (Table 2). No
significant conversion of FUR was detected without catalysts (Table 2,
Entry 1). Then, different hafnium-containing compounds (HfCl4 and
HfO2) were tested (Table 2, Entry 2–3). Neither Lewis acidic (HfCl4) nor
HfO2 with enhanced basic strength could efficiently catalyze this re-
action. Furfural diisopropyl acetal (FDIA) obtained by acetalization of
FUR with 2-propanol was detected as major by-product. And the H3IDC
precursor showed little conversion to FUR, indicating that the building
block for catalyst synthesis were not active in this reaction under the
Fig. 4. FT-IR spectra after pyridine adsorption and evacuation at 110 and 250 °C (a) of various catalysts; Survey spectrum of different samples (b) and XPS spectra of
Hf 4f peaks (c) and O 1s peaks (d) of various catalysts.
5

F. Dai, et al.
MolecularCatalysis479(2019)110611
Table 2
a
MPV reduction of FUR to FFA with various catalysts.
.
b
b
c
d
e
Entry
Catalyst
Conv (%)
FFAYield (%)
Select (%)
IPMA yield (%)
FDIA yield (%)
FFA formation rate (μmolg−1 h−1
)
1
none
0
0
0
0
0
–
2
3
4
HfCl4
HfO2
H3IDC
79.0
26.6
1.1
0.43
0.47
0
0.55
1.77
0
14.3
0
0
36.0
26.2
1
43
47
–
5
6
Zr-H3IDC
Hf-H3IDC
Hf-FDCA
Hf-H3IDC-T (0.5:1)
Hf-H3IDC-T
Hf-H3IDC-T (2:1)
Hf-H3IDC-T
Hf-H3IDC-T
Hf-H3IDC-T
65.7
84.5
63.6
53.5
100
97.8
63.9
75.6
96.4
50.2
70.8
46.8
27.2
99.2
97.2
44.3
64.8
91.6
76.3
83.8
73.5
50.8
99.2
99.3
69.3
85.7
95.1
6.51
7.86
2.6
0
0
0.64
0
9.06
3.09
14.2
26.3
0.8
0
10.3
2.7
5015
7084
4679
2717
9920
9719
4438
6487
9160
7
8f
9
10g
11h
12i
13j
0
2.97
1.81
a
Reaction conditions unless specified otherwise: 1 mmol FUR in 10 ml 2-propanol,100 mg catalyst, 120 °C, 1 h.
Conv is FUR conversion, and Select is FFA selectivity.
IPMA is 2-(isopropoxy)methyl furan.
FDIA is furfural diisopropyl acetal (the acetalization product of FUR with 2-propanol).
FFA formation rate is defined as (mol of formed FFA)/ (catalyst amount × time).
the molar ratio of the synthetic catalyst (HfCl4:H3IDC = 0.5:1).
b
c
d
e
f
g
h
i
the molar ratio of the synthetic catalyst (HfCl4:H3IDC = 2:1).
Base sites of Hf-H3IDC-T were poisoned by adding 100 mg benzoic acid.
Acid sites of Hf-H3IDC-T were poisoned by adding 100 mg pyridine.
Scaled-up reaction: 5 mmol FUR in 50 ml 2-propanol, 500 mg catalyst, 120 °C, 1 h.
j
studied conditions (Table 2, Entry 4). Zr-H3IDC exhibited a slightly
partial poisoning of acidic sites [46,49]. We found the similar results in
our work, that is, the effect of poison on basic sites was higher than that
on acidic sites for the catalytic performance of Hf-H3IDC-T and evi-
dently displaying the concerted eff ;ect of acid-base couple sites for this
reaction. Even the reaction (Table 2, Entry 9) was carried out at a
magnification of five times, interestingly, there still exhibited a high
conversion of 96.4% and selectivity of 95.1% (Table 2, Entry 13, Fig.
S6). This indicated that the reaction has great potential to be scaled-up.
lower FUR conversion (65.7%) and FFA selectivity (76.3%), as com-
pared with Hf-H3IDC (Table 2, Entry 5–6). This indicated that hafnium-
based hybrid has better catalytic performance compared to zirconium-
based hybrid. In addition, Hf-H3IDC also showed a relatively higher
catalytic performance than Hf-FDCA (Table 2, Entry 7). The interaction
between the intrinsic amino groups and the hydroxyl groups could also
activate the dissociation of 2-propanol to corresponding alkoxide and
proton, which was beneficial for the MPV reaction [45]. It should be
noted that, the prepared Hf-H3IDC-T (Table 2, Entry 9), which had both
the higher strength and the higher content of acidic and basic sites,
manifested the highest conversion (100%), FFA formation rate
(9920 μmolg−1 h−1) with high selectivity (99.2%) among these ex-
amined catalysts. Due to the surfactant (CTAB) as template agents for
preparation of Hf-H3IDC-T can enlarge surface areas, and facilitate the
diffusion and mass transfer of the substrate. Moreover, the conversion
of FUR was obviously increased from 53.5% to 100% as the initial
HfCl4/H3IDC mole ratios being increased from 0.5:1 to 1:1 (Table 2,
Entry 8–10). However, further increasing HfCl4-initial loading amounts
had slightly reduced the FFA selectivity. This is because the amount of
carboxylic acid groups in H3IDC is constant. H3IDC could only co-
ordinate with the certain content of Hf4+ (Table S1). Based on the re-
sults mentioned above, the existence of both the acidic and basic sites
with suitable content in catalysts was the main reason for the high
selective MPV of FUR into FFA. In order to understand the effect of the
acid-base active sites on the MPV reaction in Hf-H3IDC-T, pyridine and
benzoic acid were separately introduced into the reaction system as
poisoning additives. The catalysts were poisoned with benzoic acid and
pyridine respectively before the experiment. In the case of the addition
of benzoic acid, a sharp drop in FUR conversion and FFA selectivity was
observed, indicating that Hf-H3IDC-T catalyzed MPV reduction was
closely related to the basic sites on the catalyst surface (Table 2, Entry
11). It was possible that benzoic acid possessed a higher acidity and
poisoned a large number of basic sites, destroying the synergistic effect
of the acid-base coupling sites and reducing the activity of the catalyst.
However, the presence of pyridine also reduced the conversion of FUR
but had few negative effects on the selectivity of FFA (Table 2, Entry
12). It was reported that the poisoning eff ;ect of pyridine (for acidic
sites) is weak when compared with benzoic acid (for basic sites), the
addition of pyridine only slightly reduced the conversion of FUR due to
3.3. Effects of hydrogen donor and the dosage of surfactant (CTAB)
Different alcohols were investigated as hydrogen sources in the MPV
of FUR to FFA with Hf-H3IDC-T at 120 °C for 1 h, as are showed in
Fig. 5a. An extremely low conversion of FUR was found when tert-bu-
tanol was regarded as H-donor. This is because there is no hydrogen
atom next to the hydroxyl group in the tert-butanol, which provides a
cyclic six-membered transition state with FUR at the acid-base site of
the catalyst as a prerequisite for the hydrogen transfer process [47,48].
In contrast, both primary and secondary alcohols could treat as H-do-
nors for the MPV of FUR. However, the secondary alcohols shown the
higher FFA selectivity than primary alcohols (Fig. 5a) [48]. Moreover,
the conversion of FUR decreased slightly with the increasing of carbon
number in secondary alcohol. The lower FUR conversions in 2-butanol
and 3-pentanol compared to 2-propanol were attributed to steric eff ;ect
from the relative long carbon chain of 2-butanol and 3-pentanol, which
prevents the substrate molecules from diffusing into the channel of Hf-
H3IDC-T. We also found similar results from other references [10,44].
Hence, 2-propanol was the best H-donor and solvent for MPV reduction
of aldehydes with Hf-H3IDC-T as the catalyst.
As it was well-known, the performance of the catalyst is highly
depended on the structure. This work, the structure of catalysts (Hf-
H3IDC) are modified by adding different molar amount of CTAB. As
shown in Fig. 5b, the conversion of FUR and the selectivity of FFA were
both increased when the catalyst was treated with CTAB. The use of
surfactant (CTAB) as structure-directing agents enhance surface areas,
and facilitate the diffusion and mass transfer of the substrate, therefore
improve the catalytic performance of Hf-H3IDC-T. It should be noted
that the performance of catalysts was increased with the molar amount
of CTAB used. However, when the molar amount of CTAB used being
above 1.8 mmol, the catalytic performance did not change obviously.
6

F. Dai, et al.
MolecularCatalysis479(2019)110611
Fig. 5. Influence of hydrogen donor on MPV reduction of FUR catalyzed by Hf-H3IDC-T. Reaction conditions:1 mmol FUR in 10 ml solvent, 100 mg Hf-H3IDC-T
(21 mol% Hf), 120 °C, 1 h (a); Effect of the CTAB dosage used for Hf-H3IDC-T preparation on the conversion of FUR to FFA; Reaction conditions:1 mmol FUR in 10 ml
2-propanol, 100 mg catalyst, 120 °C, 1 h (b).
3.4. Effects of the reaction time and temperature and reaction kinetics study
393 K (Fig. S9a). This indicated that the equilibrium transition from the
acetalization reaction to the MPV reduction could be improved by in-
creasing the reaction temperature [49]. However, the FFA selectivity
gradually declined due to the etherification of FFA with 2-propanol to
2-(isopropoxy) methyl furan when the reaction temperature was further
increased to 413 K (Fig. S9b). Additionally, the reaction time also had a
great influence on the MPV reduction. It could reach nearly full con-
version of FUR (100%) and 99.2% of FFA selectivity when the reaction
time was prolonged to 1 h at 393 K.
To further confirm the apparent activation energy (Ea) of the FUR
transfer hydrogenation, the kinetics were studied at temperatures ran-
ging from 353 to 413 K. Since the amount of the catalyst was constant,
and 2-propanol had an extremely high concentration compared to FUR,
The effect of reaction time and temperature on FUR conversion to
FFA with Hf-H3IDC-T was also investigated. As shown in Fig. 6a and b,
the reaction temperature indicated obvious effect on the catalytic per-
formance of Hf-H3IDC-T for the MPV reaction of FFA from 353 K to
393 K. When the reaction was maintained at a relatively low reaction
temperature (353 K) for 30 min, both the FUR conversion (15.4%) and
FFA selectivity (87.2%) were relatively low. As the reaction tempera-
ture increased to 413 K, the higher 85.1% conversion of FUR with
92.5% of FFA selectivity could be achieved. Meanwhile, the acetaliza-
tion product of FUR and 2-propanol (furfural diisopropyl acetal) gra-
dually decreased when the reaction temperature rose from 353 K to
Fig. 6. Effects of the reaction time and temperature for the conversion of FUR (a) and selectivity of FFA (b) over the Hf-H3IDC-T. First-order kinetic fit (c) for the MPV
of FUR to FFA and the corresponding Arrhenius plots (d) for Hf-H3IDC-T. Reaction conditions: 1 mmol FUR in 10 ml 2-propanol, 100 mg Hf-H3IDC-T (21 mol% Hf).
7

F. Dai, et al.
MolecularCatalysis479(2019)110611
the formula for the rate of FUR transfer hydrogenation could be ex-
pressed by the Eq. (1) Assuming that the reaction was the first-order
reaction (γ = 1), c0 was the initial concentration of FUR conversion,
and ct was the FUR concentration after the reaction time t, Eq. (1) could
be expressed as Eqs. (2) or (3). Based on the results in Fig. 6c, ln(c0/ct)
had a linear relationship with t. The reaction rate constant (k) were
determined from the corresponding slope of the linear relationship at
each temperature. The value of Ea was calculated to be 50.89 kJ mol−1
according to the Arrhenius equation and its variable equation (Eq. (4)),
and the corresponding curve presented in Fig. 6d. This value was lower
than that of Zr-LS for the same reaction in our previous work (52.25 kJ/
mol) [49]. It is also comparable to the latest reported catalysts for FFA
production from FUR which were summarized in Table S2.
There were mainly three reasons accounting for the excellent cat-
alytic performance of Hf-H3IDC-T with high selectivity: (1) The Hf-
H3IDC-T catalyst exhibited high activity, with conversion (100%) and
selectivity (99.2%) in the MPV reduction for the conversion of FUR to
FFA using 2-propanol as the hydrogen source under mild conditions
compared to most solid catalyst. (2) Poisoning experiments authenti-
cated that the catalyst with high content of basic sites was crucial to
improve the catalytic activity for MPV reduction. Therefore, we chose
4,5-imidazoledicarboxylic acid with a basic group as an organic ligand.
There are the amino groups on the imidazole ring structure in the
catalyst Hf-H3IDC-T, and the interaction between the amino groups (as
the base site) and the hydroxyl group can also activate the dissociation
of 2-propanol into the corresponding alkoxide and proton, which is
conducive to the MPV reaction. (3) BET experiments and catalytic ex-
periments indicated that we introduced surfactants (CTAB) as template
agents to modify metal organic structures. It can enhance the binding of
carboxylate to Hf and enlarge surface areas, and facilitate the diffusion
and mass transfer of the substrate, therefore improve the catalytic
performance. From the viewpoint of the reaction conditions and the
design for catalyst preparation, the Hf-H3IDC-T catalyst had obvious
advantages and showed excellent catalytic activity.
H3IDC-T dosages (25–100 mg). It was found that the hemi-acetalization
of FUR with 2-propanol is the main reaction pathway for the con-
sumption of substrates [50], thus resulting in low to moderate FFA
selectivity at relatively high FUR conversions for relatively low catalyst
dosages (25–75 mg). Although a large dosage of 150 mg of Hf-H3IDC-T
could promote complete conversion of FUR (Fig. 7a) without the pro-
duction of hemiacetal by-products, the downstream product (IPMA)
was formed by the etherification of FFA and 2-propanol by excessive
acid and base sites. Experimental results proved that the 100 mg
(21 mol% Hf) would be an appropriate dosage under this reaction
condition.
To investigate the catalytic behavior of Hf-H3IDC-T in MPV reaction
from FUR to FFA, two parallel experiments were performed using a
solid catalyst at an optimal dosage of 100 mg, with or without separa-
tion of the catalyst from the thermal reaction mixture after 30 min in 2-
propanol at 120 °C (Fig. 7b). After stirring for an additional 1.5 h under
the same conditions, Hf-H3IDC-T continued to smoothly catalyze the
conversion of FUR to FFA, while almost no reaction was observed for
the reaction system with the filtered catalyst. Inductively coupled
plasma optical emission spectroscopy (ICP-OES) indicated that both
filtrates contained very low Hf species, clearly proved the heterogeneity
of Hf-H3IDC-T catalysis in the MPV reaction.
The recyclability of solid catalysts is one of the most important
characteristics for evaluating its potential applications in industrial
production [49,51]. In six consecutive cycles of experiments, Hf-H3IDC-
T catalyst could be simply recovered by washing with DMF and ethanol,
and remained capable of producing FFA in yields of 91–99% at almost
complete FUR conversion under the same reaction conditions (Fig. 8).
In addition, the concentration of Hf immersed in the reaction mixture
was extremely low as detected by ICP-OES. These results confirmed the
high durability of Hf-H3IDC-T for the MPV of FUR to FFA. The struc-
tural changes in Hf-H3IDC-T before and after MPV for sixth cycle was
characterized by SEM (Fig. S7), FT-IR, XRD, TG, N2 adsorption-deso-
rption, and NH3/CO2-TPD (Fig. S8). There was only a slight change of
the structural and textural for the used catalysts, (BET surface area:
153.5 vs 133.77 m2/g), which could be attributed to the adsorption of
organic substances into the cavities of Hf-H3IDC-T during the reaction
[30]. The acidic/basic properties (acidity: 1.41 vs 1.38 mmol/g; basi-
city: 0.78 vs 0.72 mmol/g; base/acid ratio: 0.55 vs 0.52) of the used Hf-
H3IDC-T catalysts had no significant change after six consecutive cycles.
It was worth noting that the slight weight loss of the recovered Hf-
H3IDC-T catalyst compared to the fresh catalyst indicated a small ad-
sorption of organic matter, which might partially hinder the entry of
the active site, resulting in a marginal decrease in catalytic activity
[49].
β
r= -dct /dt = k·[FUR]γ, k = k0·[Cat]α· [2-propanol]
(1)
(2)
(3)
(4)
r= - dct/dt = k· [FUR]
ln(ct/c0) = -k·t
Ea = (lnA - lnk) ·RT
3.5. Effects of the catalyst dosage, heterogeneity and recyclability for the
MPV of FUR to FFA
The effect of the dosages of Hf-H3IDC-T catalyst on the reaction
activity was researched at 120 °C for 1 h. As shown in Fig. 7a, the FUR
conversion and FFA selectivity increased rapidly with an increase in Hf-
Fig. 7. Effect of Hf-H3IDC-T dosage on conversion of FUR to FFA (a); and time-yield plots for MPV reaction of FUR with Hf-H3IDC-T (21 mol% Hf) or removing Hf-
H3IDC-T after 0.5 h (b). Reaction conditions: 1 mmol FUR, 10 ml 2-propanol, 120 °C, and 1 h.
8

F. Dai, et al.
MolecularCatalysis479(2019)110611
absence of the β-H, but it can exchange most D of eOD group in the
isotopic 2-propanol with eOH and replace the D on the metal surface
with H (Scheme S4). By GC–MS analysis, an additional 1 amu (m/
z = 99) of the molecular ion peak of FFA was detected, whereas D could
not be used for the metal hydride route, thus confirming that the re-
action undergoes direct hydrogen transfer with the Hf-H3IDC-T
(Scheme S4, Fig. S10). As noted above, the Lewis acid-base coupling
sites and the Brønsted acid sites were critical for the MPV reaction from
the carbonyl compounds to the corresponding alcohols. Fig. 10 shown
the rational reaction mechanism for Hf-H3IDC-T catalytic reduction of
carbonyl compounds. Firstly, 2-propanol was adsorbed on Hf-H3IDC-T,
resulting in its dissociation to alkoxide and hydrogen by the acid-basic
coupled sites (Hf4+−O2−). Meanwhile, the deprotonation of 2-pro-
panol was increased by the basic O2− atoms and amino groups. The
carbonyl group in the substrate molecules was activated by Lewis acidic
Hf4+ species. Next, the six-membered ring transition state was formed
with carbonyl groups and 2-propanol, leading to direct hydride transfer
via β-H elimination and yielding corresponding alcohols and acetones
[31,49].
In the process of FUR conversion to produce FFA, Hf-H3IDC-T cat-
alyst showed excellent catalytic performance, and the target product
with higher yield was obtained. Inspired by this reaction, we researched
the probability of the MPV reactions of other carbonyl compounds
using the Hf-H3IDC-T catalyst (Table 3). All chosen aldehydes could be
efficiently catalyzed by Hf-H3IDC-T to produce their corresponding al-
cohols in high yields. For example, the developed Hf-H3IDC-T exhibited
superior catalytic performance for 5-methylfurfural and 5-hydro-
xymethylfurfural, resulting in high conversions (more than 90%) and
high selectivities (96% and 98%, Table 3, entry 3–4). In addition, Hf-
H3IDC-T was also widely applicable to MPV of other bio-derived alde-
hydes, the conversions of bio-derived aldehydes exceeding 95%, and
the selectivity to the corresponding alcohol also exceeding 95%
(Table 3, entry 5–8). These corresponding alcohols are important che-
mical intermediates widely used in the preparation of pharmaceuticals,
perfumes and cosmetics in the industries. In addition to testing catalytic
biomass-derived chemicals, other commercial aldehydes and ketones
were tested to assess their catalytic potential. As expected, the aliphatic
and aromatic aldehydes could be effectively hydrogenated to corre-
sponding alcohols under the moderate conditions (100 °C) in 2 h
(Table 3, entry 9–10). The wide substrate scope indicates that the Hf-
H3IDC-T catalyst was a highly versatile catalyst for MPV reduction of
both biomass-based platform compounds and other commercial alde-
hydes to alcohols.
Fig. 8. Recyclability of Hf-H3IDC-T from FUR to FFA (Reaction conditions:
1 mmol FUR, 100 mg Hf-H3IDC-T (21 mol% Hf), 10 ml 2-propanol, 120 °C, and
1 h).
4. Conclusions
In summary, the porous heterogeneous nitrogen-containing acid-
base bifunctional catalyst Hf-H3IDC-T hybrids were synthesized by a
facile surfactant-assisted route. These catalysts possessed high surface
area and the strong Lewis acid-base couple sites (Hf4+−O2−)/ mod-
erate basic sites (the inherent amino groups), which exhibited excellent
catalyst activity, with conversion of 100%) and selectivity of 99% in the
MPV reduction for the conversion of FUR to FFA. There are two special
points about the catalyst: (1) The interaction between the amino groups
on the imidazole ring (as basic sites) and the hydroxyl group can acti-
vate the dissociation of hydrogen source into the corresponding alk-
oxide and proton, which is conducive to the MPV reaction. (2) The
surfactant (CTAB) as template agents for preparation of Hf-H3IDC-T can
enhance the binding of carboxylate to Hf and enlarge surface areas, and
facilitate the diffusion and mass transfer of the substrate, therefore
improve the catalytic performance of Hf-H3IDC-T. The Ea of the MPV
reaction of FUR was as low as 50.89 kJ/mol, indicating that the MPV
reaction is a high reaction rate. The direct hydrogen transfer from α-C
of 2-propanol to the α-C of FUR was verified to be the dominant re-
action pathway and the rate-determining step. In addition, the as-pre-
pared Hf-H3IDC-T hybrid has high thermal stability and chemical
Fig. 9. Reaction kinetics study from FUR to FFA hydrogenation in cosolvent
tert-butanol and isotopic 2-propanol-d8 with a molar ratio of 3:1 using 1H NMR;
other unmarked major bonds in 1H NMR spectra belonged to the solvent, and in
situ-formed acetone with constant chemical shifts.
3.6. Reaction mechanism study and MPV reduction of other carbonyl
compounds
To clarify the reaction mechanism, the MPV reaction from FUR to
FFA in the cosolvent tert-butanol and 2-propanol-d8 with a molar ratio
of 3:1 was monitored by ex situ NMR spectroscopy at different reaction
times. As shown in 1H NMR spectrum (Fig. 9), the protons in the FUR
(1a) aldehyde group (eCHO) gradually disappeared as the amount of
deuterium (D) gradually increased from 2-propanol-d8 to its C]O
bond, which resulting in the formation of a eCH2e moiety in FFA (1b).
However, due to the non-selective addition of D, it was not clear
whether the reaction route was the direct hydrogen transfer or the
metal hydride route. In order to clearly distinguish the two hydro-
genation routes, tert-butanol cannot be used as an H-donor due to the
9

F. Dai, et al.
MolecularCatalysis479(2019)110611
Fig. 10. Plausible reaction pathway involved in the MPV reduction of FUR to FFA over Hf-H3IDC-T.
Table 3
a
MPV of different biomass-derived carbonyl compounds to alcohols
.
b
b
b
Entry
Substrate
Product
Temperature (°C)
Time (h)
Conversion (%)
Yield (%)
Selectivity (%)
1
2
120
100
1
3
100
97
99
92
99
95
3
4
100
100
4
4
97
94
93
92
96
98
5
120
4
95
91
95
6
7
120
120
3
4
98
95
94
90
97
95
8
120
4
95
91
96
9
100
100
2
2
96
98
93
96
97
98
10
a
Reaction conditions: 1 mmol substrate; 0.1 g Hf-H3IDC-T and 10 ml of 2-propanol.
Conversion, yield and Selectivity were quantified using GC, determined by GC analysis using an internal standard.
b
stability, and can maintain excellent catalytic performance after six
consecutive cycles of reaction. Moreover, Hf-H3IDC-T as efficient cat-
alyst for MPV reaction can be extrapolated to other similar lig-
nocellulose-derived substrates for upgrading biomass.
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Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
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