Doping Pd/SiO2 with Na+: Changing the reductive etherification of CO to furan ring hydrogenation of furfural in ethanol

Article abstract of DOI:10.1039/c9ra05281j

The production of biofuels and chemicals by hydrogenation of furfural has attracted much attention recently. Herein the effect of Na⁺ doping on the catalytic performance of Pd/SiO₂ in hydrogenation and reductive-etherification of furfural in ethanol was systematically studied. Two Pd/SiO₂ catalysts with and without the modification by Na⁺ were prepared by impregnation and calcination. Their catalytic properties were compared for the hydrogenation of furfural and furfural diethyl acetal under mild conditions. The silanol groups on Pd/SiO₂ catalysed the acetalization of furfural and alcohol and the resulted acetal underwent hydrogenolysis on Pd nanoparticles (NPs) with an average particle size of 8 nm, leading to a moderate yield (～58%) of furfuryl ethyl ether. Doping Na⁺ on Pd/SiO₂ led to the diminishing of silanol groups as well as strong interaction between Na⁺ and Pd NPs. No acetalization occurred on Na⁺ modified Pd/SiO₂ due to the exchange of H⁺ of Si-OH with Na⁺, thus the reductive etherification of CO group in furfural was completely inhibited. Meanwhile the hydrogenation of furan-ring over Na⁺ coordinated Pd NPs could proceed with very high selectivity (>90%) forming tetrahydrofurfural in high yield. Kinetics study on the hydrogenation of furfural diethyl acetal over Pd/SiO₂ and Na⁺ doped Pd/SiO₂ suggested that the Na⁺ greatly impeded the hydrogenolysis of C-O-C bond of acetal, while the hydrogenation of the furan ring took place selectively.

Full text of DOI:10.1039/c9ra05281j

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Doping Pd/SiO2 with Na+: changing the reductive
etherification of C]O to furan ring hydrogenation
of furfural in ethanol†
Cite this: RSC Adv., 2019, 9, 25345
Yinshuang Long, Yun Wang, Haihong Wu,
Teng Xue, Peng Wu
*
and Yejun Guan
The production of biofuels and chemicals by hydrogenation of furfural has attracted much attention
recently. Herein the effect of Na+ doping on the catalytic performance of Pd/SiO2 in hydrogenation and
reductive-etherification of furfural in ethanol was systematically studied. Two Pd/SiO2 catalysts with and
without the modification by Na+ were prepared by impregnation and calcination. Their catalytic
properties were compared for the hydrogenation of furfural and furfural diethyl acetal under mild
conditions. The silanol groups on Pd/SiO2 catalysed the acetalization of furfural and alcohol and the
resulted acetal underwent hydrogenolysis on Pd nanoparticles (NPs) with an average particle size of
8 nm, leading to a moderate yield (ꢀ58%) of furfuryl ethyl ether. Doping Na+ on Pd/SiO2 led to the
diminishing of silanol groups as well as strong interaction between Na+ and Pd NPs. No acetalization
occurred on Na+ modified Pd/SiO2 due to the exchange of H+ of Si–OH with Na+, thus the reductive
etherification of C]O group in furfural was completely inhibited. Meanwhile the hydrogenation of furan-
ring over Na+ coordinated Pd NPs could proceed with very high selectivity (>90%) forming
tetrahydrofurfural in high yield. Kinetics study on the hydrogenation of furfural diethyl acetal over Pd/
SiO2 and Na+ doped Pd/SiO2 suggested that the Na+ greatly impeded the hydrogenolysis of C–O–C
bond of acetal, while the hydrogenation of the furan ring took place selectively.
Received 10th July 2019
Accepted 2nd August 2019
DOI: 10.1039/c9ra05281j
rsc.li/rsc-advances
been put forward to increase the furfuryl ether yield.6,7 For
instance, lower the etherication temperature of FA to 40 ꢁC by
Introduction
adding trimethyl orthoformate (TMOF) or triethyl orthoformate
(TEOF) as a sacricial reagent has been found to be an efficient
method.6 Alternatively, ether formation by reductive ether-
ication starting with furfural (FAL) can also take place under
mild conditions.7 We have recently found that the choice of
support is very crucial for the reductive etherication of furfural
over supported Pd catalysts and 0.7 wt% Pd/C catalyst was
found to be the best one.7d By contrast, conicting results have
been reported in the case of Pd/SiO2 catalyst. In our previous
study, the Pd/SiO2 catalyst reduced by NaBH4 showed very low
selectivity toward the reductive-etherication.7d Other studies
on FAL hydrogenation over Pd/SiO2 also showed that Pd NPs on
SiO2 were very selective toward the hydrogenation of furan ring.8
On the other hand, some reports have found that Pd/SiO2 was
an excellent catalyst for the reductive etherication of C]O
bond in aldehydes like cyclohexanone and etc.9 We speculate
that the introduction of Na during the catalyst preparation may
have a profound role in this process. Moreover, in a recent study
by Wu and coworkers, the presence of Na also signicantly
altered the reductive etherication activity of I–Pd/Al2O3 cata-
lyst.7c Thus a deep understanding on how catalyst preparation,
reaction conditions and metal structures affect the catalytic
performance of Pd/SiO2 catalyst in FAL hydrogenation is highly
The production of high-value chemicals and/or transportation
fuels from lignocellulosic biomass has attracted wide interest in
the past decades.1 One of the most selective chemical conver-
sions of biomass is based on the transformation of platform
compounds via hydrogen-involving reactions aiming at
tailoring the oxygen content to full the requirement of prac-
tical application.2 For example, biomass-derived etheric
oxygenates have been proposed to be effective gasoline-
additives, which can decrease the vehicle exhaust pollution by
enhancing the combustion of fuels.3 Among the proposed bio-
ethers, furfuryl alcohol (FA) derived ethers have attracted
much attention. Conventionally, the furfuryl ethers are
prepared by acid catalyzed etherication between FA and the
other alcohols.4 However, this route has a major drawback in
that the decomposition of ethers readily takes place under the
reaction conditions.5 Therefore, mild reaction conditions have
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry
& Molecular Engineering, East Normal University of China, North
Zhongshan Road 3663, Shanghai, China. E-mail: yjguan@chem.ecnu.edu.cn
† Electronic supplementary information (ESI) available: Textural properties, XPS,
and acetalization and hydrogenation activity of Pd catalysts. See DOI:
10.1039/c9ra05281j
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desirable, whereas few studies have been carried out in this be about 1.7. These PdO phases were readily reduced upon
regard.10 In this study, we have found that the Na+ introduced in exposure to H2 at room temperature according to the TPR
the preparation of Pd/SiO2 indeed plays a vital role in control- proles (Fig. 1C), in which the formation of PdH was clearly
ling the reaction pathways of furfural hydrogenation. The noticed according to the negative peak in TPR proles of both
results suggest that Pd/SiO2 without Na+ is active and selective catalysts. The Pd 3d5/2 peak for reduced Pd/SiO2 and Pd–Na/SiO2
for reductive etherication, whereas Pd/SiO2 containing Na+ shied to 335.1 and 335.6 eV, respectively.
selectively catalyses the hydrogenation of furan ring. Moreover,
The particle size of reduced catalysts was further studied by
this effect also applies to K+,9c pointing to a general role of TEM (Fig. 2). The Pd/SiO2 catalyst contains Pd NPs with an
alkaline cations in this process. These results shed new insights average diameter about 8.7 nm. Small particles with average
on the structure–activity relationship of Pd/SiO2 catalyst in FAL size about 4.6 nm were obtained for Pd–Na/SiO2. The particle
hydrogenation process.
sizes of both catalysts estimated by TEM are smaller than that
determined by XRD. Nonetheless, both techniques showed that
the modication with Na+ may inhibit the sintering of PdO
particles in calcination. The dispersion of Pd NPs on Pd/SiO2
and Pd–Na/SiO2 was estimated to be 5.9% and 7.4% according
to CO chemisorption, respectively.
Results and discussion
Catalyst characterisation
The Pd loading of Pd/SiO2 and Pd–Na/SiO2 was determined to
be 2.2 wt% and 2.0 wt% by ICP-AES, respectively. The textural
properties of both catalysts are listed in Table S1.† The chemical
structure of catalysts was characterized by XRD, XPS, H2-TPR
and CO-IR. The XRD patterns of two catalysts (Fig. 1A) suggested
that the oxidic PdO phase was mainly present on fresh Pd/SiO2
and Pd–Na/SiO2 catalysts. The particle size of PdO was calcu-
lated from the full width at half maximum (FWHM) of 33.8ꢁ due
to PdO (101) diffraction by using Debye–Scherrer equation. The
Pd nanoparticle size of Pd/SiO2 and Pd–Na/SiO2 is about 15 and
7.2 nm, respectively. The oxidic nature of Pd can be also
conrmed by XPS spectra (Fig. 1B). The Pd 3d5/2 peak of Pd/SiO2
appeared at 336.7 eV, corresponding to Pd2+ of bulk PdO. The
Pd 3d5/2 of Pd–Na/SiO2 appeared at 338.4 eV. This phenomenon
has been observed previously on Pd/NaZSM-5 and the higher
binding energy was proposed to be related to PdO2 species.11
The presence of Na+ on Pd–Na/SiO2 catalyst was conrmed by
XPS (Fig. S1†) and the atomic ratio of Na/Pd was determined to
The electronic structure of Pd NPs before and aer Na+
modication was studied by CO adsorption IR. Fig. 3 shows the
FTIR spectra of adsorbed CO on Pd catalysts. For fresh Pd/SiO2
catalysts. Very weak bands at 2110, 2090, 1988, 1963 cmꢂ1 due
to the adsorbed CO were observed. These peaks are likely
assigned to CO adsorbed on atop or bridge PdOx.12 Aer pre-
treated in H2 at 120 ꢁC, the intensity of bands at 1988 and
1963 cmꢂ1 attributed to the CO adsorbed on bridge sites of
metallic Pd surface was increased remarkably, meaning that
most PdO species were reduced. For fresh Pd–Na/SiO2, bands
centered at 2078 and 1964 cmꢂ1 with very lower intensity were
observed, meaning that PdOx was also the dominating species
on calcined Pd–Na/SiO2 catalyst. Aer reduction by H2 at 120 ꢁC,
the peaks due to CO adsorbed on bridged sites of Pd metal
appeared at 1980 and 1890 cmꢂ1. It should be noted that the
value of I1890/I1980 was much higher when Na+ was introduced,
which has been previously assigned to the interaction of Pd–
CO–Na+.12a,13 These characterization results suggest that the
electronic property of Pd NPs or the structure of exposed Pd
atoms has been signicantly changed by addition of Na+.
Furfural hydrogenation over Pd/SiO2 and Pd–Na/SiO2
As an initial test, we investigated the catalytic performance of
furfural hydrogenation over Pd/SiO2 catalyst and the results
regarding to FAL conversion and products distribution at
different reaction time are shown in Fig. 4. One can see that
Fig. 1 The (A) XRD patterns, (B) XPS spectra, (C) H2-TPR profiles and
(D) N2 adsorption–desorption isotherms of 2.2 wt% Pd/SiO2 and
2.0 wt% Pd–Na/SiO2 catalysts.
Fig. 2 TEM images and particle size distribution of Pd/SiO2 and Pd–
Na/SiO2 catalysts.
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Scheme 1 The major reaction pathways of furfural hydrogenation
over Pd/SiO2 and Pd–Na/SiO2 at temperature between 80–120 ꢁC
under 0.3 MPa H2. FAL: furfural; THFAL: tetrahydrofurfural; FA: furfury
alcohol; THFA: tetrahydrofurfuryl alcohol; FDA: furfural diethyl acetal;
THFDA: tetrahydrofurfural diethyl acetal; EFE: ethyl furfural ether;
THEFE: tetrahydroethyl furfural ether.
Fig. 3 FTIR spectra of CO adsorbed on fresh and reduced (120 ꢁC in
H2) Pd/SiO2 and Pd–Na/SiO2.
(50–58%) than that at 80 ꢁC (32%). The low EFE yield at 80 ꢁC is
likely due to the low activity of hydrogenolysis of FDA, sug-
gesting that the hydrogenolysis step is temperature-dependent.
Fig. 5 shows the conversion and product selectivity for
hydrogenation of FAL over Pd–Na/SiO2 as a function of reaction
ꢁ
time at 120 C and 0.3 MPa H2. Aer reaction for 10 min, the
FAL conversion was about 32% and the products consisted of
THFAL (74%), FA (13%) and THFA (13%). These products were
all produced by hydrogenation of furan ring or C]O bond, with
hydrogenation of furan-ring being the predominant reaction
(87%). The furfural conversion increased steadily with the
increase of reaction time. Full conversion of furfural was ach-
ieved within 90 min. The product distribution did not change
with reaction time. We also explored the furfural hydrogenation
activity of Pd/SiO2 and Pd–Na/SiO2 in isopropanol and the
results are shown in Fig. S2.† For Pd/SiO2, the formation of
ether, acetal, FA and THFA were all observed in the rst 10 min.
Interestingly, the selectivity toward ether and acetal was 34%
and 30%, respectively, meaning that the acetalization and
reductive etherication proceed simultaneously from the
beginning. This behaviour differs to the above results observed
in ethanol, wherein a consecutive-process was found. We
consider that the structure of isopropanol plays an important
role in this process, which may be not favourable to the
Fig. 4 Furfural (FAL) conversion and products distribution as a func-
tion of reaction time for the hydrogenation of FAL in ethanol over Pd/
SiO2. Reaction conditions: 10 mL of 0.24 M FAL ethanol solution;
120 ꢁC; 0.3 MPa H2; 100 mg catalyst.
ꢁ
81% FAL was converted in the rst 10 min at 120 C, with fur-
furyl diethyl acetal (FDA) being the predominant product. The
other compound was ethyl furfuryl ether (EFE). Aer reaction
for 30 min, the FDA selectivity dropped to 16% whereas the
selectivity of EFE increased to 54%. The EFE selectivity reached
60% aer reaction for 60 min. Further prolonging the reaction
time to 120 min resulted in dramatic decrease of EFE selectivity
to 14% in 2 h. The byproducts were mainly those compounds
formed by the decomposition or over-hydrogenation of EFE
under the reaction conditions. The overall reaction network is
shown in Scheme 1. This result demonstrated that Pd/SiO2
prepared by impregnation is active and selective for EFE
synthesis and short reaction period is preferred to insure a high
yield of EFE. We investigated the effects of temperature and
hydrogen pressure on the EFE yield by keeping the reaction
time at 1 h and the results are shown in Table S2.† One can see
that the total yield of FA and tetrahydrofurfuryl alcohol (THFA)
was almost the same (about 16–20%) at 80–120 ꢁC under
hydrogen pressure below 0.5 MPa, meaning that the hydroge-
nation of furan ring was not affected by reaction temperature.
Fig. 5 Furfural hydrogenation activity as a function of reaction time
over Pd–Na/SiO2 in ethanol. Reaction conditions: 10 mL of 0.24 M FAL
ethanol solution; 120 ꢁC; 0.3 MPa H2; 100 mg catalyst.
ꢁ
By contrast, the EFE yield was much higher at 100 and 120 C
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Table 1 Catalytic performance of Pd/SiO2 for the hydrogenation of
FDA at 0.3 MPa H2 under different reaction conditions
acetalization because of steric effect. The direct formation of
ether in isopropanol has also been reported previously over
several Pd catalysts.7c,7e With the prolonging of the reaction
time, the furfural conversion and ether selectivity reached 92%
and 61%, respectively. This result further illustrates the appli-
cation of Pd/SiO2 in ether formation. In the case of Pd–Na/SiO2,
only hydrogenation pathway was noticed regardless of solvent
used. Moreover, the hydrogenation activity in isopropanol is
relative lower in comparison with that in ethanol. We surmise
that the hydrogen dissociation ability of Pd surface might be
lowered in isopropanol.
Products sel.e (%)
Entry Time (min) Conv.d (%) THFDA EFE THEFE THFA Others
1a
2b
3c
10
30
10
30
10
30
100
100
95
100
32
38
39
52
50
79
78
0
0
10
0
7
3
23
25
25
32
10
16
3
3
2
2
3
2
36
33
11
16
0
84
0
a
b
Reaction temperature 120 ꢁC, catalyst loading 100 mg. Reaction
c
temperature 120 ꢁC, catalyst loading 20 mg. Reaction temperature
Effect of Na+ doping on the acetalization reaction
d
80 ꢁC, catalyst loading 10 mg. Conversion of furfural diethyl acetal.
e
THFDA: tetrahydrofurfural diethyl acetal; EFE: ethyl furfural ether;
This study clearly shows that Pd–Na/SiO2 showed distinct
behavior to Pd/SiO2. For Pd/SiO2 catalyst, FDA was formed as
the predominant product in the rst 10 min, meaning that
acetalization of furfural with ethanol proceeded fast over Pd/
SiO2 surface, whereas the acetalization could not take place over
Pd–Na/SiO2 catalyst. Stimulated by this nding, we have
compared the acetalization activity of Pd/SiO2 and Pd–Na/SiO2
and the results shown in Table S2† suggested that the acetali-
zation could take place smoothly over Pd/SiO2. SiO2 gave 51%
furfural conversion (Table S3†) aer reaction for 30 min. Aer
loading Pd, the Pd/SiO2 showed furfural conversion of 44%
under the identical reaction conditions. No acetalization was
observed for Pd–Na/SiO2 catalyst. Previous studies have shown
that the acetalization could be catalyzed by Brønsted acid sites
on zeolites including USY, beta and H-mordenite.14 We believe
that the weak acidity of silanol groups on SiO2 was active for
acetalization and its activity was not affected by Pd deposition.
Once the SiO2 was treated with NaOH solution, the exchange of
Si–OH and Na+ would lead to the formation of Si–ONa species,
which explains the loss of acetalization activity of SiO2–Na and
Pd–Na/SiO2. This result is consistent with the nding that Na-
mordenite was nearly inert toward acetalization.14 Since the
production of EFE is strongly associated to the formation of
FDA in the reductive-etherication process, it is expected that
no EFE could be produced during the furfural hydrogenation
over Pd–Na/SiO2. Similarly, no EFE formation was observed for
Pd–K/SiO2 catalyst where K+ instead of Na+ is introduced
(Fig. S3†), pointing to the general role of alkaline cations.
THEFE: tetrahydroethyl furfural ether; THFA: tetrahydrofurfuryl
alcohol; others: products formed by EFE decomposition.
reaction for another 20 min. Then we decreased the catalyst
amount to 20 mg and the FAL conversion was 95% aer 10 min
reaction. The products included THFDA (52%), THEFE (25%)
and EFE (10%). Further decrease of the reaction temperature to
80 ꢁC and the catalyst amount to 10 mg gave FDA conversion of
32% aer 10 min reaction, with THFDA being the dominating
product (sel. 79%). The total selectivity of EFE and THEFE were
17% in 10 min. Further prolonging the reaction time to 30 min
led to 84% conversion of FDA without any change in product
distribution.
By comparing the product distribution of reactions starting
with furfural and FDA on Pd/SiO2, two ndings should be noted.
First, THFDA was formed in a moderate yield starting with FDA,
while it was observed in a minute yield starting with FAL. We
propose that the competitive adsorption of FAL and/or FA with
FDA over Pd surface occurs, thus inhibiting the hydrogenation
of furan ring in FDA while favoring the hydrogenolysis of C–O–C
bond. This assumption has been veried by testing the hydro-
genation activity of FDA in presence of FAL or FA over Pd/SiO2
catalyst. The results in Table S4† showed that the FDA conver-
sion decreased signicantly aer adding 20 mL of furfural or FA.
Moreover, the EFE selectivity was signicantly improved by
adding both reagents. These results well explained the stable
selectivity of EFE in the rst 90 min when furfural was used as
reactant. Second, EFE was only detected when less amount of
catalyst or lower reaction temperature were employed. The
possible explanation is that over-hydrogenation of EFE was very
fast so that the produced EFE could not be observed under
higher reaction temperatures.
Effect of Na+ doping on the hydrogenolysis of FDA
Another unsolved issue is that whether the presence of Na+
affects the hydrogenation or hydrogenolysis of FDA over Pd NPs,
which is the second step of reductive-etherication process. To
clarify this point, we have compared the hydrogenation
performance of Pd/SiO2 and Pd–Na/SiO2 by using FDA as
substrate. Table 1 shows the FDA conversion and product
distribution over Pd/SiO2 catalyst under different reaction
We also tested the activity of Pd–Na/SiO2 in FDA hydroge-
ꢁ
nation at 80 C and 0.3 MPa H2. The results in Fig. 6 suggests
that THFDAwas always the major product in 120 min (sel. >
95%), meaning that the hydrogenation of furan-ring dominated
the reaction process over Pd–Na/SiO2. These results suggest that
Pd nanoparticles coordinated with Na+ are highly selective in
hydrogenation of furan ring. One could also see that the reac-
tion rate of FDA hydrogenation on Pd–Na/SiO2 was half of that
on Pd/SiO2 (Table 1, Entry 3). We may conclude that the pres-
ence of Na+ partially deactivates the hydrogenation activity of
ꢁ
conditions. We rst tested the activity at 120 C and 0.3 MPa,
which is the optimum reaction condition for EFE synthesis in
this study. One can see that total conversion of FDA was ob-
tained within 10 min when 100 mg catalyst was used. The
products were mainly composed of THFDA (38%) and THEFE
(23%). The product distribution did not change upon further
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1 h. 0.57 mL of H2PdCl4 aqueous solution (21.512 gPd Lꢂ1) was
added to the mixture, and then the mixture was stirred for 1 h.
The pH value of the suspension was adjusted to 10 by adding
NaOH solution (1 M). The slurry was stirred for another 1 h,
ltered, washed with deionized water, and nally dried at 80 ꢁC
ꢁ
overnight and calcined in air at 550 C for 5 h. The Pd loading
was determined by inductively coupled plasma (ICP) with
Thermo IRIS Intrepid II XSP atomic emission spectrometer. The
catalysts were digested using certain amount of aqua regia and
the obtained solutions were diluted to the desired concentra-
tion with deionized water before test.
Fig. 6 Catalytic performance of Pd–Na/SiO2 for the hydrogenation of
FDA. Reaction conditions: 10 mg catalyst, 10 mL of 0.24 M FDA ethanol
solution; 80 ꢁC, 0.3 MPa H2.
Materials characterization
Nitrogen adsorption–desorption isotherms at ꢂ196 ꢁC were
obtained on a BELSORP-Max equipment. Prior to the test, the
samples were rst degassed at 150 ꢁC under vacuum for 6 h.
Specic surface areas (SSA) were calculated according to the
Brunauer–Emmett–Teller (BET) method using ve relative
pressure points in the interval of 0.05–0.30. The powder X-ray
diffraction (XRD) patterns were collected on Rigaku Ultima IV
furan ring while totally inhibits the hydrogenolysis reaction
pathway. This behaviour is also consistent with previous nding
over I–Pd/Al2O3 and NaI–Pd/Al2O3.7c Apparently the residue Na+
affects the electronic property of Pd nanoparticles according to
the CO-IR adsorption. However, the nature of Na+–Pd interac-
tion is still not clear at this stage. Further study on the coordi-
nation of C]C or C–O–C group on Pd surface would be helpful
for elucidating the competition between hydrogenation and
hydrogenolysis. Nevertheless, the observation in this study well
explains the low selectivity of Pd catalysts with residue Na+ in
reductive-etherication of furfural.
˚
X-ray diffractometer using Cu Ka radiation (l ¼ 1.5405 A)
operated at 35 kV and 25 mA. Transmission electron micros-
copy (TEM) images were taken on FEI Tecnai G2 F30 microscope
operating at 300 kV. The average Pd particle size was calculated
P
P
by dTEM¼ ( nidi3)/( nidi2) by measuring at least 100 particles.
The X-ray photoelectron spectroscopy (XPS) was recorded on an
ESCALAB 250xi spectrometer with a monochromated Al Ka X-
ray source (hn ¼ 1486.6 eV) at base pressure of 8 ꢃ 10ꢂ10 Pa.
Fourier transform infrared (FT-IR) CO adsorption on fresh and
reduced samples was performed on a Nicolet iS50 spectrometer
equipped with a home-made in situ cell (32 scans with resolu-
tion of 4 cmꢂ1). Catalyst samples were pressed to self-
supporting wafers about 9–12 mg with diameter of 11 mm.
For CO adsorption on fresh sample, the wafer was dehydrated in
Conclusions
In summary, we herein systematically studied the
hydrogenation/reductive-etherication of furfural in alcohol
over Pd/SiO2 catalysts with/without Na+ modication. The Pd/
SiO2 catalyst without Na+ was active and selective for reductive
etherication under mild conditions by forming furfural acetal
as key intermediate. The Pd/SiO2 modied with Na+ was found
to be solely active in hydrogenation of furan ring and C]O
group. The role of Na+ in changing the reaction pathways of
furfural hydrogenation can be explained in two folds: (i) the ion-
exchange of Na+ and Si–OH leads to for formation of Si–ONa,
which is not active for acetalization; (ii) the structure/electronic
property of Pd nanoparticles was strongly altered by doping Na+,
thus showing high selectivity in hydrogenation of furan ring
while losing its activity in hydrogenolysis of C–O–C bond.
N2 with ow rate of 30 mL minꢂ1 at 100 C for 30 min. Aer
ꢁ
cooling to room temperature, a spectrum was recorded and
used as background for subtraction. Then, CO was introduced
into the in situ cell by owing high purity CO gas with ow rate
about 10 mL minꢂ1 for 3 min. Aerwards, the in situ cell was
ushed by N2 at room temperature for certain period to remove
the gas phase CO. For CO adsorption on reduced sample, the
wafer was ushed in 30 mL minꢂ1 N2/H2 at 120 C for 30 min
and then cooled down to 30 C in N2.
ꢁ
ꢁ
Catalytic tests
Experimental
Catalyst preparation
The reductive etherication/hydrogenation of furfural was
conducted in a Teon-lined (60 mL) steel batch reactor. No
The SiO2 support was Evonik Degussa Aerosil® 200 (surface pretreatment on the catalysts was conducted prior to each
area: 208 m2 gꢂ1). Pd/SiO2 catalysts were prepared by wet reaction. The reactor was charged with 9.8 mL of ethanol,
impregnation method. In a typical preparation, 0.6 g of silica 0.2 mL of furfural and 10–100 mg of catalyst, and the mixture
support was impregnated with 2 mL ethanol containing was stirred under 0.3–1 MPa H2 pressure at 80–120 ꢁC. Aer
0.57 mL H2PdCl4 aqueous solution (21.512 gPd Lꢂ1). The sample reaction for desired period, H2 was released and the liquid was
was dried at 80 ꢁC for 10 h and calcined in air at 550 ꢁC for 5 h. diluted with ethanol before analysis. The products were
Pd–Na/SiO2 catalyst was prepared by a typical deposition– analyzed with a ame ionization detector (FID) and the capillary
precipitation method. In a typical preparation, 0.6 g silica column of DB-FFAP (30 m length and 0.25 mm internal diam-
support was dispersed in H2O (20 mL) with vigorous stirring for eter). The mass spectra of some products were recorded on
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a Shimadzu GCMS-QP2010SE instrument equipped with an Rxi-
5Sil MS (30 m ꢃ 0.25 mm ꢃ 0.25 mm).
5 G. M. Gonzalez Maldonado, R. S. Assary, J. Dumesic and
L. A. Curtiss, Energy Environ. Sci., 2012, 5, 8990.
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Conflicts of interest
7 (a) M. Balakrishnan, E. R. Sacia and A. T. Bell, Green Chem.,
2012, 14, 1626; (b) R. Pizzi, R. van Putten, H. Brust,
S. Perathoner, G. Centi and J. C. van der Waal, Catalysts,
2015, 5, 2244; (c) D. Wu, W. Y. Hernandez, S. W. Zhang,
E. I. Vovk, X. H. Zhou, Y. Yang, A. Y. Khodakov and
V. V. Ordomsky, ACS Catal., 2019, 9, 2940; (d) Y. Wang,
Q. Cui, Y. Guan and P. Wu, Green Chem., 2018, 20, 2110;
(e) C. Li, G. Xu, X. Liu, Y. Zhang and Y. Fu, Ind. Eng. Chem.
Res., 2017, 56, 8843.
8 (a) Y. Nakagawa, K. Takada, M. Tamura and K. Tomishige,
ACS Catal., 2014, 4, 2718; (b) R. R. Chada, S. S. Enumula,
K. S. Koppadi, V. R. B. Gurram, S. R. R. Kamaraju and
D. R. Burri, ChemistrySelect, 2018, 3, 9946.
There are no conicts to declare.
Acknowledgements
We acknowledge the nancial support from China Ministry of
Science and Technology under contract of 2016YFA0202804, the
National Natural Science Foundation of China (Grant No.
21773067, 21533002) and the Open Research fund of Shanghai
Key Laboratory of Green Chemistry and Chemical Processes.
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25350 | RSC Adv., 2019, 9, 25345–25350
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