Aqueous phase hydrogenation of furfural to tetrahydrofurfuryl alcohol over Pd/UiO-66

Article abstract of DOI:10.1016/j.catcom.2020.106178

A Pd/UiO-66 catalyst was synthesized with well-dispersed Pd nanoparticles. The obtained catalyst was tested in the hydrogenation of furfural to tetrahydrofurfuryl alcohol in various solvents, Water was found to be the most suitable solvent. Pd/UiO-66 exhibited much higher activity than Pd/SiO₂ and Pd/γ-Al₂O₃, completely converting furfural to tetrahydrofurfuryl alcohol with 100% selectivity under mild conditions. The hydrogenation of C[dbnd]O moiety in tetrahydrofurfural was rate-determining step. Static adsorption measurement indicated that the adsorption of furfural on UiO-66 was significantly stronger than that on SiO₂ or γ-Al₂O₃, suggesting that the adsorption play an important role in the gas-liquid-solid furfural hydrogenation reaction.

Full text of DOI:10.1016/j.catcom.2020.106178

CatalysisCommunications148(2021)106178
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Catalysis Communications
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Short communication
Aqueous phase hydrogenation of furfural to tetrahydrofurfuryl alcohol over
Pd/UiO-66
Chunhua Wanga,b, Anjie Wanga,b, Zhiquan Yua,b, Yao Wanga,b, Zhichao Suna,b, Victor M. Koganc,
⁎
Ying-Ya Liua,b,
a State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China
b Liaoning Key Laboratory of Petrochemical Technology and Equipment, Dalian University of Technology, Dalian 116012, PR China
c N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow 119991, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
A Pd/UiO-66 catalyst was synthesized with well-dispersed Pd nanoparticles. The obtained catalyst was tested in
the hydrogenation of furfural to tetrahydrofurfuryl alcohol in various solvents, Water was found to be the most
suitable solvent. Pd/UiO-66 exhibited much higher activity than Pd/SiO2 and Pd/γ-Al2O3, completely converting
furfural to tetrahydrofurfuryl alcohol with 100% selectivity under mild conditions. The hydrogenation of C]O
moiety in tetrahydrofurfural was rate-determining step. Static adsorption measurement indicated that the ad-
sorption of furfural on UiO-66 was significantly stronger than that on SiO2 or γ-Al2O3, suggesting that the
adsorption play an important role in the gas-liquid-solid furfural hydrogenation reaction.
Pd/UiO-66
Catalyst
Hydrogenation
Furfural
Tetrahydrofurfuryl alcohol
1. Introduction
hydrogenation of FAL [12,13]. When alcohols were used, acetals were
produced as the typical byproducts [7,14]. In contrast, water can avoid
Biomass and its derivatives have attracted increasing attention in
the production of renewable biofuels and value-added compounds due
to the environmental concerns [1]. Furfural (FAL) is mainly obtained by
acid-catalyzed dehydration of xylan, which mainly exists in the hemi-
cellulose components of lignocellulosic biomass [2]. Because of the rich
unsaturated bond in its structure, the catalytic hydrogenation of FAL
gives rise to a number of valuable chemicals [3], including furfuryl
alcohol (FOL), 2-methyl furan (2-MF), furan, tetrahydrofuran, cyclo-
pentanone (CPO) and tetrahydrofurfuryl alcohol (THFOL), the hydro-
genation products depend highly on the catalyst and the reaction con-
ditions [4].
the formation of acetalization product, but the variation of temperature
can influence the product distribution [15,16]. Therefore, developing
environmentally-friendly catalyst with high selectivity of THFOL under
mild condition is still a challenge [10].
Metal-organic frameworks (MOFs) have shown great promise in
heterogeneous catalysis, due to their high surface areas and well-de-
fined porosities [17]. Yin et al. reported a 3 wt% Pd@MIL-101(Cr)-NH2
could catalyze hydrogenation of FAL to THFOL at 40 °C and 2 MPa H2
in water. The initial selectivity was close to 100%, but it decreased to
90% in the second run [11]. UiO-66 is one of the considerably stable
MOFs, it is built up from [Zr6O4(OH)4(CO2)12] clusters linked with
terephthate ligand, to obtain a porous cubic architecture [18]. UiO-66
has relatively good chemical stability resistance to water and organic
solvents. Recently, Yang et al. prepared a Pd/UiO-66-v (v represents the
vinyl functional group on the MOF ligand), which showed high activity
in the hydrogenation of FOL, with 99% conversion and 90% selectivity
to THFOL at 0.5 MPa H2, 303 K for 12 h in water [19]. The high per-
formances of UiO-66 supported noble metal catalysts are believed to be
associated with the high dispersion of noble metals and favorable sur-
face properties for reactant adsorption [15,16,19].
THFOL is a total hydrogenation product, which is widely used as a
green solvent in pharmaceutical, industrial and agricultural sectors [3].
In industry, the production of THFOL from FAL involves two con-
secutive steps: hydrogenation of FAL to afford FOL and further hydro-
genation of FOL to THFOL [5]. It is highly desirable to synthesize
THFOL from FAL in one step [6,7]. Noble metal catalysts are commonly
used in hydrogenation of FAL to THFOL. Pd-based catalyst with specific
supports such as carbon [8], Al2O3 [9], hydroxyapatite [10], SiO2 [4],
MIL-101(Cr)-NH2 [11] have shown good catalytic activity, due to the
synergies of supports and active metal. Solvent significantly influences
the catalyst performance and the product selectivity in the
In this paper, Pd/UiO-66 was prepared by impregnation, and tested
in the hydrogenation of FAL, the catalytic performance with compared
⁎ Corresponding author at: School of Chemical Engineering, Dalian University of Technology, West Campus B225, 2 Linggong Road, Dalian 116024, PR China.
E-mail address: yingya.liu@dlut.edu.cn (Y.-Y. Liu).
https://doi.org/10.1016/j.catcom.2020.106178
Received 7 July 2020; Received in revised form 25 September 2020; Accepted 2 October 2020
Availableonline03October2020
1566-7367/©2020ElsevierB.V.Allrightsreserved.

C. Wang, et al.
CatalysisCommunications148(2021)106178
with Pd/SiO2 and Pd/γ-Al2O3 catalysts. The effects of solvent, catalyst
dosage, temperature, and hydrogen pressure in the reaction were in-
vestigated, and the reusability of Pd/UiO-66 was studied as well.
Table 1
The N2 adsorption and H2 chemisorption results of UiO-66 and the supported
Pd catalysts.
Sample
SBET (m2.g−1
)
H2 uptake (μmol.g−1
)
D (%)
UiO-66
1342
947
111
62
–
–
2. Experimental
Pd/UiO-66
γ-Al2O3
Pd/γ-Al2O3
SiO2
6.5
–
17.9
–
2.1. Characterization
4.4
–
12.3
–
232
126
Pd/SiO2
2.4
6.7
Powder X-ray diffraction (XRD) data was collected on a Rigaku D/
Max-2400 diffractometer with Ni-filtered Cu-Kα radiation at 40 kV and
100 mA. The BET specific surface area was calculated from the N2
adsorption/desorption isotherms which were measured at −196 °C on
a Micromeritics Tristar II 3020. The sample was out-gassed under va-
cuum at 120 °C for 3 h, prior to the adsorption measurement. FT-IR
spectrum of UiO-66 (KBr pellet) and Pyridine-adsorbed FT-IR spectrum
was recorded on an Equinox 55 FT-IR spectrometer (the detailed ex-
perimental procedures was given in Supplementary Information).
Elemental analysis was carried out by means of inductively coupled
plasma optical emission spectroscopy (ICP-OES) on an Optima 2000DV
device. The morphology observation of the sample was carried out on a
field-emission scanning electron microscope (FE-SEM) (FEI NOVA
NanoSEM 450), and the scanning was performed on the pre-dried
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of the synthesized UiO-66 and Pd/UiO-66, Pd/γ-
Al2O3 and Pd/SiO2 were shown in Fig. S1. All diffraction peaks of the
as-synthesized UiO-66 matched well with the simulated pattern ob-
tained from single crystal X-ray crystal diffraction data [18]. The
loading of Pd2+ into UiO-66 and subsequent reduction treatment did
not markedly change the crystallinity of the parent UiO-66. For γ-Al2O3
and SiO2 the diffraction peaks of each catalyst matched well with those
of the corresponding supports, the characteristic XRD peaks of all the
Pd crystalline phase were not detectable, probably because the Pd
particle size was below the detection limit.
sample powder and sputter-coated with
a thin layer of gold.
Transmission electron microscopy (TEM) images were acquired on
Tecnai G2 F30 transmission microscope operated at 300 kV. The sample
was suspended in ethanol and deposited on a copper grid prior to ob-
servation.
The BET surface area of UiO-66, γ-Al2O3 and SiO2 was 1343, 111
and 232 m2/g, respectively (Table 1). After reduction, the specific s
area of Pd/UiO-66, Pd/γ-Al2O3 and Pd/SiO2 was decreased moderately
to 947, 62 and 126 m2/g, respectively. This observation might be as-
cribed to the partial occupation of the cavities in supports.
The H2 chemisorption measurement was carried out using a dy-
namic pulse method on a Chembet-3000 instrument following the re-
ported procedure [20]. The H2 uptakes and metal dispersions of the
catalysts were calculated based on the equations given by Aramendía
et al. [21].
The SEM image of the synthesized UiO-66 is shown in Fig. S2a, the
obtained UiO-66 were octahedral crystallites of about 400 nm. The TEM
image of Pd/UiO-66 is shown in Fig. S2b. The lattice fringe with an
interplanar distance is 0.225 nm, corresponding to the Pd (111) plane.
The grain size of Pd particles was in the range of 2–3 nm, suggesting
that nanosized Pd particles were highly dispersed in UiO-66.
The metal dispersion was further characterized by H2 chemisorp-
tion, which showed that the H2 uptake of Pd/UiO-66 was 6.5 μmol/g,
giving the highest dispersion of 17.9 (Table 1). The average particle size
calculated based on the dispersion was 6.2 nm, which was larger than
that observed on TEM. It is likely that not all the surface metal sites of
Pd particles in the pores of UiO-66 were available for H2 adsorption due
to the pore blockage.
2.2. Catalyst preparation
UiO-66 was synthesized according to the procedure reported by
Lillerud and co-workers [18]. The Pd/UiO-66 catalyst was prepared by
impregnation. 0.2 g of UiO-66 was dispersed in 5 mL ethanol and so-
nicated for 30 min at room temperature. 200 μL aqueous solution of
1.5 wt% PdCl2 in diluted HCl was dissolved in ethanol, and the Pd-
containing solution was added dropwise to the UiO-66 suspension
under vigorous stirring. The resulting mixture was stirred at 50 °C for
24 h. Then the solid product was collected by centrifugation and the
supernatant was collected for complexometric titration of Pd. The ob-
tained Pd2+/UiO-66 was washed three times with ethanol, followed by
drying at 120 °C under vacuum for 12 h. Pd2+/UiO-66 sample was
reduced in a H2 flow (75 mL/min) at 300 °C for 2 h. After cooling down
to room temperature, the resultant catalyst was passivated in a gas flow
of 0.5 vol% of O2 in argon (20 mL/min) for 2 h prior to use.
Fig. 1 shows the FT-IR spectra of pyridine adsorbed on UiO-66 and
Pd/UiO-66 in the region 1400–1700 cm−1. In the spectrum of UiO-66,
the bands due to Lewis-bonded pyridine (1455 cm−1), Brønsted-bonded
pyridine (1536 cm−1) and the band assigned to pyridine associated
with both Brønsted and Lewis acid sites (1498 cm−1), were observed
[22]. The Lewis acidity of UiO-66 might originate from the accessible Zr
sites [23], as well as from the defects due to the missing organic linkers
between Zr6 clusters [18]. Furthermore, the Brønsted acidity could be
attributed to the protonation of the organic linkers in UiO-66. In the
spectrum for Pd/UiO-66, the three kinds of bands were all present,
without appreciable change of the peak intensity, suggesting that the
acidic sites were well preserved.
2.3. Catalytic performance
The catalytic hydrogenation of FAL was carried out in a 75 mL
stainless steel Parr autoclave. In a typical experiment, 50 mg of Pd/UiO-
66, 100 mg FAL and 9.9 g solvent were added into the autoclave, speed
of stirring 750 rpm, and was heated at a specific temperature under H2
pressure. After reaction, the reactor was cooled to room temperature.
The liquid product was centrifuged to separate the catalyst, and the
liquid was analyzed by a gas chromatography (Agilent 6890, FID) with
a HP-INNOWax capillary column (30 m×320 μm×0.5 μm). The carbon
balance of all examined catalysts was in the range of 95 to 98%.
3.2. Catalytic performance of Pd/UiO-66
In general, the production of THFOL by FAL hydrogenation pro-
ceeds mainly via two parallel pathways (Scheme 1): (1) C]O hydro-
genation followed by ring hydrogenation; (2) ring hydrogenation fol-
lowed by C]O hydrogenation. Therefore, FOL and THFAL are the main
intermediates.
2

C. Wang, et al.
CatalysisCommunications148(2021)106178
Table 2
Effect of solvent in FAL hydrogenation catalyzed by Pd/UiO-66.
Solvent
Conversion (%)
Selectivity (%)
THFOL
FOL
THFAL
FDA
Others
2-propanol
Water
100
100
100
100
62
88
1
11
-
-
-
100
75
-
-
-
Ethanol
n-butanol
Toluene
5
8
9
-
3
2
1
77
7
14
23
41
35
-
Reaction conditions: 50 mg of 1.2 wt% Pd/UiO-66, 100 mg FAL, 9.9 g solvent,
60 °C, 1.0 MPa, 4 h. THFOL: tetrahydrofurfuryl alcohol, FOL: furfuryl alcohol,
THFAL: tetrahydrofurfural, FDA: 2-furaldehyde diethyl acetal.
basic catalyst support [27]. Martin et al. [28]. reported that the se-
lectivity towards the undesired acetal decreased with increasing chain
length of the alcohol. In the present study, a small amount of 2-fur-
aldehyde diethyl acetal (FDA) was obtained in ethanol, whereas no
acetal was detectable in 2-propanol or n-butanol (Table 2). In addition,
a literature survey regarding the catalytic performance of different Pd-
based catalysts in the hydrogenation of FAL was presented in Table S2,
the hydrogenated intermediates (FOL and THFAL) were detected in all
the alcohol solvents (Entries 1–7), probably due to lowered hydro-
genation efficiency.
Fig. 1. FT-IR spectra of adsorbed pyridine on UiO-66 and Pd/UiO-66.
3.2.1. Effect of solvent
Solvents have a strong impact on the reaction rates and selectivities
in liquid phase [24]. Polar solvents have been reported to be favorable
for liquid phase hydrogenation of FAL [25].
Water is also often used as a solvent in the catalytic hydrogenation
of FAL. The advantage of water as the solvent is that no acetalization or
other side reaction occurs (Entries 8–10, Table S2). From Table 2, it is
seen that a perfect FAL full hydrogenation was obtained with 100% FAL
conversion and 100% THFOL selectivity. Zhao et al. [29]. proposed that
water participate directly in the kinetically relevant reaction step and
provides an additional channel for hydrogenation of the aldehyde
group. Wan et al. [30]. tested temporal H2 uptake profiles in water,
C1–C4 primary alcohols and aprotic solvents as a function of time by
monitoring the pressure drop in the external H2 reservoir. It was found
that water exhibited the highest H2 uptake rate. In the gas-liquid-solid
reaction of FAL hydrogenation over Pd/UiO-66, the dissolution of
gaseous H2 in the solvent is directly linked with the surface coverage of
H2 on the catalyst, which determines the overall reaction rate.
To investigate the solvent effects on conversion and product se-
lectivity, protic solvents (water and C2–C4 alcohols) and aprotic (to-
luene) were used in FAL hydrogenation catalyzed by Pd/UiO-66.
Table 2 shows that the solvents significantly influenced both FAL
conversion and the product distribution. When polar solvents were
used, full conversions of FAL were obtained, whereas only 62% FAL was
converted in toluene under same conditions. Hu et al. [26]. reported
that a shielding effect between the conjugated π system in both FAL and
toluene could be a major factor that negatively affected the hydro-
genation rates of both the furan ring and aldehyde group in FAL.
Alcohol solvents are frequently used for selective hydrogenation of
FAL, because the ability of hydrogen donation promotes the hydro-
genation rate. As a result, remarkably higher conversion was achieved
in an alcohol than in toluene. Nevertheless, when alcohol solvents are
used for aldehyde hydrogenation, acetals are usually obtained as the
byproducts [14]. The side-reaction of acetalization is correlated with
surface acidity of the catalyst support, and might be suppressed over
3.2.2. Effect of the support
Table 3 presents the conversion and product selectivity in aqueous
phase hydrogenation catalyzed by Pd supported on three different
porous materials: UiO-66, γ-Al2O3 and SiO2. The control experiment of
Scheme 1. Reaction network of FAL hydrogenation.
3

C. Wang, et al.
CatalysisCommunications148(2021)106178
Table 3
FAL hydrogenation over supported Pd catalysts.
Catalyst
Conversion (%)
Selectivity (%)
THFOL
FOL
THFAL
UiO-66
-
-
-
-
Pd/UiO-66
Pd/γ-Al2O3
Pd/SiO2
100
100
36
100
64
10
-
-
–
36
14
76
Reaction conditions: 50 mg of 1.2 wt% Pd-based catalyst, 100 mg FAL, 9.9 g
water, 60 °C, 1.0 MPa, 4 h.
UiO-66 catalyzed hydrogenation of FAL gives no activity, suggesting
that the originate UiO-66 did not contribute to the catalytic hydro-
genation. FAL conversion increased in the order: Pd/SiO2 (36%) < Pd/
γ-Al2O3 (100%) = Pd/UiO-66 (100%). In addition, the support had a
marked influence in the product selectivity. The main product was
THFOL for Pd/γ-Al2O3 (64%) and Pd/UiO-66 (100%), whereas FOL was
dominant in the product for Pd/SiO2. Recently, Mironenko et al. [31].
reported the effect of the Pd support on the reaction pathway in the
aqueous hydrogenation of FAL. They found that a basic MgAl oxide
support led to lower performance, and the major products were FOL
and THFOL (Entry 9, Table S2). In addition, the adsorption of FAL is
essential in determining the hydrogenation rate. Fig. S3 illustrates the
variation of FAL adsorbed on each support with time in static adsorp-
tion at different FAL concentrations. The adsorption of FAL was fast in
the first 30–60 min and then the adsorption gradually reached to sa-
turation. The adsorption capacity decreased in the same order: UiO-
66 > γ-Al2O3 > SiO2 at different FAL concentrations. In accordance
with the hydrogenation performance of the supported Pd catalysts. The
adsorption of FAL on MOFs were reported to be enhanced by the strong
interactions between FAL and the MOF support via the C]O bond and
their coordinatively unsaturated sites (CUSs) [32,33]. Wan et al. at-
tributed the enhanced adsorption of FAL to the acid sites from the
support [30]. Therefore, the presence of Brønsted and Lewis acid sites
in UiO-66 (Fig. 1) might be favorable for the adsorption of FAL, thus
improving the reaction rate.
Fig. 2. Variation of conversion and yield with reaction time in FAL hydro-
genation catalyzed by Pd/UiO-66. Reaction conditions: 50 mg Pd/UiO-66 cat-
alyst, 100 mg FAL, 9.9 g water, 60 °C, 1.0 MPa.
evident that the hydrogenation started in two parallel pathways, and
the ring hydrogenation and C]O hydrogenation took place simulta-
neously. However, the subsequent C]O hydrogenation of FOL was
considerably faster that the subsequent ring hydrogenation of THFAL.
The overall rate of the FAL-FOL-THFOL pathway was much faster than
that of FAL-THFAL-THFOL, which explained the absence of FOL in the
product for the Pd/UiO-66-catalyzed hydrogenation of FAL at full
conversion (Table 3). Fig. 2 indicates that the hydrogenation of C]O
moiety in THFAL was rate-limiting in the hydrogenation of FAL.
Krishna et al. [35]. proposed that the hydrogenation ability of the C]O
bond was weakened after the C]C bond hydrogenation. To investigate
the influence of diffusion limitation, hydrogenation of FAL over Pd/
UiO-66 were conducted by varying the stirring rates, from 500 rpm to
1200 rpm, while all other operating conditions were identical. As
shown in Table S1, the impact of changing stirring rate on the FAL
conversion and selectivity to the products are listed. Over different
stirring rates, the FAL conversion were constant, while product se-
lectivity changed slightly, suggesting that there is no obvious external
diffusion control.
Besides, the support affected the formation of hydrogenated inter-
mediates. It implies that the reaction rates of the two parallel pathways
might be determined by both active sites and the properties of the
supports. For Pd/γ-Al2O3, only THFAL was detected as the inter-
mediate. The absence of FOL might be due to the fast hydrogenation of
FOL. For Pd/SiO2, both THFAL and FOL were present in the hydro-
genation product, suggesting that Pd/SiO2 was much less active in the
hydrogenation.
Subsequently, the reaction conditions such as catalyst dosage, re-
action temperature and hydrogen pressure were optimized. FAL con-
version was kept at 100%, and the selectivity to THFOL reached 100%
when the catalyst dosage was larger than 0.05 g (Fig. S4). Increasing
reaction temperature can increase product selectivity, achieving a
100% THFOL selectivity when reaction temperature was higher than
50 °C (Fig. 3a). Meantime, 100% selectivity of THFOL was obtained
when the hydrogen pressure was increased to 1.0 MPa (Fig. 3b).
Additionally, the performance of the supported Pd catalyst might be
related with the dispersion of Pd nanoparticles. Pd/UiO-66 showed the
highest dispersion (Table 1) due to the Pd2+ that was anchored on the
external surface of UiO-66 via the Pd2+ with μ3-OH groups on the Zr6
nodes in the form of Zr-O-Pd bond [34]. So Pd/UiO-66 showed the
highest activity and selectivity to THFOL.
3.2.4. Catalyst recycling
To evaluate the stability of Pd/UiO-66 in aqueous phase FAL hy-
drogenation, the catalyst was reused for three times after separation of
the catalyst by centrifugation. In the fourth run (Fig. S5), the selectivity
to THFOL slightly decreased from 100% to 99%, indicating a slight
decrease of the hydrogenation activity. The concentration of Pd species
in the solution after the reaction was analyzed by ICP, the content of Pd
was below the detection limit, ruling out the possibility of Pd metal
leaching under the reaction conditions. In addition, the content of Pd in
the solution was further determined by complexometric titration, and
no Pd was titrated as well. In contrast, a tiny fraction of Zr (0.011 wt%)
was detected by ICP analysis. Fig. S6 shows the XRD pattern of the
spent Pd/UiO-66 catalyst after the four consecutive runs. Compared
with the fresh catalyst, the decreased intensity of XRD peaks of the
spent catalyst, suggesting there was slight loss in crystallinity, in
agreement with the fraction of Zr leaching from the UiO-66 framework.
3.2.3. Effect of reaction time
In order to identify the possible intermediates, the variation of FAL
conversion and product yield with reaction time was investigated
(Fig. 2). In the first 5 min, FAL reached 47% conversion with yield of
21% to FOL, 13% to THFOL, and 13% to THFAL, suggesting that FAL
hydrogenation to THFOL proceeded in both pathways (Scheme 1).
Afterwards, the yield to THFOL increased monotonically up to 100%
over time. The yield of FOL rapidly disappeared in 30 min, and no
maximum yield of FOL was observed in the time scale, indicating that
the subsequent ring hydrogenation of FOL was substantially fast over
Pd/UiO-66. On the other hand, the yield of THFAL passed through a
maximum at 20 min, and decreased slowly to zero until 250 min. It is
4

C. Wang, et al.
CatalysisCommunications148(2021)106178
Fig. 3. a) Effect of reaction temperature on conversion and product selectivity in FAL hydrogenation catalyzed by Pd/UiO-66. b) Effect of hydrogen pressure on
conversion and product selectivity in FAL hydrogenation catalyzed by Pd/UiO-66. Reaction conditions: 50 mg Pd/UiO-66 catalyst, 100 mg FAL, 9.9 g water, 4 h.
4. Conclusions
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The authors declare that they have no known competing financial
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Acknowledgements
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