Importance of Zeolite Wettability for Selective Hydrogenation of Furfural over Pd@Zeolite Catalysts

Article abstract of DOI:10.1021/acscatal.7b03443

The metal-catalyzed selective hydrogenation of biomass-derived molecules is in great demand but is challenging due to the complex reaction pathways. Herein, we report a persuasive example for achieving selective hydrogenation of furfural over Pd catalysts by controllable sorption of molecules in zeolite micropores. The key to this success is fixation of Pd nanoparticles inside of silicalite-1 zeolite with controllable wettability (Pd@S-1-OH) by functionalizing silanol groups into the zeolite framework. In the hydrogenation of furfural as a model reaction, the Pd@S-1-OH catalyst with appropriate hydrophilicity exhibits extraordinary selectivity for the formation of furan, giving furan selectivity as high as >99.9% with a complete conversion of furfural, outperforming the conventional Pd nanoparticles supported on zeolite crystals (Pd/S-1) and S-1 zeolite fixed Pd catalysts without an artificially functionalized silanol group (Pd@S-1). The extraordinary performance of Pd@S-1-OH is reasonably attributed to the controllable diffusion of molecules within the hydrophilic zeolite micropores, which favors the adsorption of furfural and a series of byproducts but promotes the desorption of furan. Very importantly, Pd@S-1-OH is stable and gives the furan productivity of ～583.3 g g_Pd^-1 day^-1 in a continuous test.

Full text of DOI:10.1021/acscatal.7b03443

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Article
Importance of Zeolite Wettability for Selective
Hydrogenation of Furfural over Pd@Zeolite Catalysts
Chengtao Wang, zhiqiang liu, Liang Wang, Xue Dong, Jian Zhang, Guoxiong
Wang, Shichao Han, Xiangju Meng, Anmin Zheng, and Feng-Shou Xiao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03443 • Publication Date (Web): 30 Nov 2017
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Importance of Zeolite Wettability for Selective Hydrogenation of
Furfural over Pd@Zeolite Catalysts
Chengtao Wang,a Zhiqiang Liu,b,d Liang Wang,a* Xue Dong,c Jian Zhang,a Guoxiong Wang,a
† †
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Shichao Han,a Xiangju Meng,a Anmin Zheng,b* and Feng-Shou Xiaoa*
a Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou
310028, China.
b National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and
Molecular Physics and Mathematics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences,
Wuhan 430071, China.
c Department of Chemistry and Biochemistry, Texas Tech University, Texas 79409, United States.
d University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
ABSTRACT: The metal-catalyzed selective hydrogenation of biomass-derived molecules is great demand but challengea-
ble due to the complex reaction pathways. Herein, we report a persuasive example for achieving selective hydrogenation
of furfural over Pd catalysts by controllable sorption of molecules in zeolite micropores. The key to this success is fixation
of Pd nanoparticles inside of Silicalite-1 zeolite with controllable wettability (Pd@S-1-OH) by functionalizing silanol
groups into the zeolite framework. In the hydrogenation of furfural as model reaction, the Pd@S-1-OH catalyst with ap-
propriate hydrophilicity exhibits extraordinary selectivity for the formation of furan, giving furan selectivity as high
as >99.9% with a complete conversion of furfural, outperforming the conventional Pd nanoparticles supported on zeolite
crystals (Pd/S-1) and S-1 zeolite fixed Pd catalysts without artificially functionalized silanol group (Pd@S-1). The extraor-
dinary performance of Pd@S-1-OH is reasonably attributed to the controllable diffusion of molecules within the hydro-
philic zeolite micropores, which favors the adsorption furfural and a series of by-products but promotes the desorption of
furan. Very importantly, the Pd@S-1-OH is stable and gives the furan productivity at ~583.3 g·gPd-1·d-1 in a continuous test.
KEYWORDS: Biomass conversion; Zeolite; Wettability; Selective hydrogenation; Pd catalyst
of substrate and product molecules.39-50 One of the examples
is the enhanced catalytic activity of titanosilicate (e.g. TS-1,
1. INTRODUCTION
Ti-Beta) zeolite for the catalytic oxidation of organic mole-
cules by adjusting the wettability.45-55 Recent results show
Metal nanoparticle catalyzed hydrogenation has been re-
garded as a crucial story in the field of heterogeneous cataly-
that the acid or base centers in hydrophobic/hydrophilic
sis.1-10 Generally, the reactions occur on the surface of metal
environment significantly enhance the activity and selectivity
in heterogeneous catalysis.42,47,56,57 However, the potentiality
nanoparticles, where it is difficult to control the selectivity
when more than one reducible groups is present in the sub-
of wettability control for the supported metal nanoparticle
strates.9-16 Many strategies have been developed for improv-
catalysts is rarely studied, which might be due to the limita-
ing the hydrogenated selectivity over the metal catalysts,
tion of synthesis methodology for rationally localizing metal
such as adjusting the metal nanoparticles sizes,17-19 alloying
nanoparticles in environments with controllable wettabil-
with additional metals,20,21 enhancing the interaction with
oxide support,22,23 and selectively poisoning the active cen-
ters.24-26 On the basis of these strategies, an ocean of catalysts
have been synthesized and displayed improved selectivities
in the hydrogenation of chemicals.19,20,23,26,27 For example, the
Pd nanoparticle catalysts modified with long-tail organic
groups or metal oxides exhibit significantly enhanced selec-
tivities in the catalytic decarbonylation.26,28,29 However, it is
still challengeable to control the selectivity for the reactions
with complex pathways. For example, in the hydrogenation
of biomass-derived molecules, which is a key process for up-
grading biomass feedstock to valuable fuels and/or chemicals,
the yield of target product is usually low due to the for-
mation of various by-products.30-38
ity.40,43,58
Herein, we report a strategy for achieving wettability-
driven Pd-catalyzed hydrogenation of biomass-derived furfu-
ral as a model reaction (Scheme 1). The key to this success is
fixation of Pd nanoparticles within zeolite crystals to form
Pd@zeolite core-shell catalysts, where the wettability of zeo-
lite sheath is controllable by functionalizing silanol groups
into the framework. In the hydrogenation of furfural, the Pd
nanoparticles fixed inside of Silicalite-1 zeolite with appropri-
ate hydrophilicity (Pd@S-1-OH) exhibit simultaneously high
activities and superior selectivities for the production of bio-
furan, giving furan selectivity as high as >99.9% with com-
plete conversion of furfural. In contrast, the conventional
zeolite supported Pd catalyst is non-selective. The extraordi-
nary performances of Pd@S-1-OH are attributed to the fixed
structure with hydrophilic zeolite sheath, which facilitates
The catalyst wettability has emerged to be sensitive to the
performances of heterogeneous catalysts by constructing
hydrophobic or hydrophilic surface to influence the diffusion
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the adsorption and enrichment of furfural in the hydrophilic
Na2PdCl4 solution (Pd concentration at 8.6 mg/mL), followed
by ultrasonic treatment for 1 h and static treatment for 20 h.
Then, the solution was transferred into a 250 mL bowl to
remove the water under continuously stirring at 50-60 °C.
After drying at 80 °C for 4 h, calcining at 400 °C for 4 h in air,
and reducing at 300 °C in 10% H2/Ar for 2 h, the Pd/S-1-OH-
10 seeds were obtained. The Pd/S-1 seeds were synthesized
via the same procedures except for using S-1 zeolite instead
of S-1-OH-10 zeolite as the support.
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zeolite micropores, as well as promotes the desorption of
furan rather than other by-products. Very importantly, the
Pd@S-1-OH is stable and gives the furan productivity at
~583.3 g·gPd-1·d-1 in a continuous test.
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2.4 Synthesis of Pd@S-1-OH and Pd@S-1. Direct addition
of Pd salt (e.g. Na2PdCl4) into the zeolite crystallization sys-
tem caused the formation of bulky Pd particles and failed to
be fixed in the zeolite crystals because of the Pd aggregation
under the zeolite crystallization condition with high temper-
ature and strong basicity. In order to overcome this issue, the
seeds of Pd/S-1-OH (or Pd/S-1) were used to introduce Pd
species into the zeolite crystallization system. As a typical
run for the synthesis of Pd@S-1-OH-10, 0.3 g of Pd/S-1-OH-10
seeds, 0.3 g of SiO2-Me and 0.5 g of TPAOH were mixed and
grinded at room temperature for 15 min, then the mixture
was transferred into an autoclave and thermally treated at
180 °C for 3 days.59 After removal of the organic template and
transforming methyl groups to silanol by calcination at
550 °C for 4 h, the Pd@S-1-OH-10 was finally obtained. These
procedures are shown in Scheme 2. The Pd@S-1 was synthe-
sized following the same procedures except using Pd/S-1 and
pure fumed silica instead of Pd/S-1-OH-10 and SiO2-Me-10 as
seeds and silica sources, respectively.
2.5 Synthesis of Pd/Al2O3. 1 g solid support Al2O3 was
added into 50 mL of aqueous solution of urea (45 mg) and
Na2PdCl4 (molar ratio of urea/Pd at 20). After stirring at 80
°C for 4 h, the liquor was cooled to room temperature, fol-
lowed by the quick addition of 10 mL of newly made NaBH4
aqueous solution (0.1 M). After stirring for another 1 h, the
Pd/Al2O3 sample was obtained by filtering, washing with
distilled water, and drying at 100 °C for 12 h.
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Scheme 1. The catalytic strategy for producing bio-furan via
selective hydrogenation of furfural.
2. EXPERIMENTAL SECTION
2.1 Materials. Tetraethylorthosilicate (TEOS, >99%), di-
ethoxydimethylsilane (DEMS, 97%), sodium borohydride
(97%), ammonium hydroxide (AR, 25%~28%), Al2O3 (AR),
furfural (AR, 99%), furan (GC, 99.5%), furfuryl alcohol (97%),
tetrahydrofuran (98%), 2,3-Dihydrofuran (99%), tetrahydro-
furfuryl alcohol (AR, >98%(GC)), methylfuran (98%), n-butyl
alcohol (99.5%), and octane (GC, 99.5%) were obtained from
Aladdin Chemical Reagent Company. Ethanol (AR, 99.7%)
was obtained from Shanghai Lingfeng Chemical Reagent Co.
Ltd, tetrapropylammonium hydroxide (TPAOH, 40 wt%) was
supplied by Shanghai Cairui Chemical Technology Co. Ltd.
NaCl (AR, 99.5%), urea (99.5%) was obtained from J&K Chi-
na Chemical Ltd, and PdCl2 (59~60%) was obtained from
Zhejiang Metallurgical Research Institute.
2.2 Synthesis of zeolite S-1-OH. The hydroxyl groups
modified S-1 zeolite was synthesized using DEMS and TEOS
as precursors, and the final products was denoted as S-1-OH-
x, where x is the molar percentage of DEMS in the total
amount of silica in the starting solution.
2.6 Synthesis of Pd@S-1-OH-10-HF. 1 g of Pd@S-1-OH-10
sample was added to 10 mL of hydrofluoric acid solution (0.5
wt%), followed by stirring at room temperature for 20 min.
Then, the mixture was centrifuged, washed with a large
amount of water, and dried at 100 °C for 12 h to obtain the
Pd@S-1-OH-10-HF sample.
As a typical run for the synthesis of S-1-OH-10, 80 mL of
ethanol was added into 100 mL of water solution containing
6 mL of aqueous ammonia under stirring. After addition of
3.75 g of TEOS and 0.30 g of DEMS, the solution was stirred
at room temperature for another 8 h, followed by distilling
under vacuum to remove the water and ethanol and drying
at 100 °C overnight. Then, the solid powder of amorphous
silica modified with methyl group (SiO2-Me) was obtained.
The S-1-OH-10 zeolite was synthesized by solvent-free crys-
tallization with SiO2-Me in the presence of TPAOH. After
grinding the mixture of 0.5 g of TPAOH and 0.6 g of SiO2-Me
at room temperature for 10 min, the mixture was transferred
into an autoclave for crystallization at 180 °C for 3 days to
obtain the methyl group modified S-1 zeolite. After calcina-
tion at 550 °C for 4 h, the organic template was removed, and
the methyl groups are transformed into hydroxyl groups,48
which was denoted as S-1-OH-10. S-1-OH-20 was synthesized
in the same procedures except using 3.33 g of TEOS and 0.60
g of DEMS as the silica precursors.
Scheme 2. Synthesis of Pd@S-1-OH.
2.7 Characterization. X-ray diffraction (XRD) patterns
were collected on a Rigaku D/MAX 2550 diffract meter with
CuKα radiation (λ=1.5418Å). Pd loadings were determined by
inductively coupled plasma (ICP) analysis (Perkin-Elmer
3300DV). Nitrogen sorption isotherms were measured using
2.3 Synthesis of Pd/S-1-OH and Pd/S-1 seeds. As a typi-
cal run for the synthesis of Pd/S-1-OH, 1 g of S-1-OH-10 zeo-
lites were added into 1.1 mL of water and 0.93 mL of aqueous
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a Micromeritics ASAP 2020 system. Transmission electron
When a water droplet is brought into contact with the
surface of the S-1 zeolite, the contact angle is measured at 38°
(Figure 2A). In contrast, the contact angles of water droplet
on the Pd@S-1-OH samples are much smaller than that on S-
1. For example, Pd@S-1-OH-10 and Pd@S-1-OH-20 exhibit the
contact angles at 23° and 15°, respectively (Figure 2B and 2C),
respectively. The lower contact angles indicate more hydro-
philic surfaces of the Pd@S-1-OH-10 and Pd@S-1-OH-20.60,61
To compare their difference in the structure, we acquired the
29Si MAS NMR spectra of Pd@S-1-OH-10 and Pd@S-1, as
shown in Figure 2E. Obviously, the Pd@S-1-OH-10 display an
additional signal at -104 ppm, assigned to the Si species relat-
ed to hydroxyl groups. These results support the successful
hydrophilization of Pd@S-1-OH samples by functionalization
of hydroxyl group on the zeolite frameworks.62
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microscopy (TEM) images were performed on a JEM-2100F
electron microscopy (JEOL, Japan) with an acceleration volt-
age of 200 kV. Thermogravimetric curves (TG) were per-
formed on a SDT Q600 Simultaneous DSC-TGA in flowing
air with heating rate of 10 °C/min. The IR spectra of mole-
cules adsorbed on solid samples were performed on a Bruker
Vertex 70 (diffuse reflection mode) under in-situ condition.
As a typical run, 50 mg of Pd@S-1-OH sample was put into
the crucible and reduced under 10% H2/Ar at 300 °C for 2 h
prior to the experiment. A three-way valve was used to
switch gases through the crucible. Firstly, a certain amount
of furan (or other molecules such as tetrahydrofuran,
methylfuran, furfural, furfuryl alcohol, and tetrahydrofurfuryl
alcohol) was introduced to the crucible by Ar flow at room
temperature until adsorption saturation. Then the gas flow
was switched to pure Ar to remove the furan from the cruci-
ble. The spectra were recorded under pure Ar flow at room
temperature each 4 min.
2.8 Catalytic tests. The furfural hydrogenation was car-
ried out in a continuous fixed-bed glass vertical reactor
(length at 400 mm and diameter at 6 mm) with two inlets.
For each reaction run, quartz sands were placed into both
ends of the catalyst to maintain the bed height. 0.1 g of cata-
lyst (20-40 mesh) was diluted with 0.2 g of quartz sands (20-
40 mesh) in the catalyst bed. Prior to reaction test, the cata-
lyst was reduced in 10% H2/Ar at 300 °C for 2 h. 5 wt% furfu-
ral (mass ratio of furfural to n-butyl alcohol at 1:19) was in-
troduced to one inlet of the reactor by a pump (rate at 0.5
mL/h). Hydrogen (10% H2/Ar) was introduced to another
inlet of the reactor with a rate of 10 mL/min. The reaction
temperature was programmed by a temperature-controlled
instrument. During the reaction process, the reaction tem-
perature was increased from 150 °C with heating rate of
5 °C/min. At 175, 200, 225, 250, and 275 °C, the temperature
was constant for 20 min to collect the products with a cold
trap. Then the products were analyzed by a Shimadzu GC-
2014 chromatograph equipped with flame ionization detector
(FID). The gas phase products (the main products were for-
maldehyde with slight amount of CO, which are from the
cleavage of C-C bond in furfural) were analyzed by a Fu Li-
9790 chromatograph equipped with Thermal Conductivity
Detector (TCD) and a PPM-400 ST formaldehyde detector.
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(A)
(B)
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0
(a)
(b)
(a)
(b)
(c)
(c)
0.0
0.5
1.0
10
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Relative pressure (P/P0)
2 Theta (degree)
Figure 1. (A) XRD patterns and (B) N2 sorption isotherms of
(a) S-1, (b) Pd@S-1-OH-10, and (c) Pd@S-1-OH-20.
(A)
(B)
CA≈23º
CA≈38º
Pd@Sꢀ1ꢀOHꢀ10
(D)
Sꢀ1
(C)
CA≈22º
CA≈15º
Used
Pd@Sꢀ1ꢀOHꢀ10
Pd@Sꢀ1ꢀOHꢀ20
-114
-104
(E)
3. RESULTS and DISCUSSION
(a)
3.1 Characterization. Figure 1A shows XRD patterns of S-1,
Pd@S-1-OH-10, and Pd@S-1-OH-20 samples, giving typical
peaks associated with MFI structure. Notably, the peaks re-
lated to Pd nanoparticles are undetectable in the XRD pat-
terns, suggesting the high dispersion. Figure 1B shows the N2
sorption isotherms of S-1, Pd@S-1-OH-10, and Pd@S-1-OH-20
samples, giving typical steps in the very low relative pressure
(<0.01 P/P0). These samples have high surface area and large
pore volume (Supporting Information, Table S1). In addition,
the Pd@S-1 and Pd@S-1-OH-10 samples give very similar mi-
cropore sizes (Supporting Information, Figure S1), confirm-
ing that pore size is almost not changed by inner surface
modification. By ICP analysis, the Pd loading amount on
Pd@S-1-OH-x was 0.34-0.35 wt% (Supporting Information,
Table S1), which is almost the same as that (0.32-0.33 wt%) of
Pd/S-1 and Pd/Al2O3.
(b)
29Si Chemical Shifts (ppm)
Figure 2. Contact angles of water-droplet on the surface of
(A) S-1, (B) Pd@S-1-OH-10, (C) Pd@S-1-OH-20, and (D) used
Pd@S-1-OH-10 in a long-term catalytic test; (E) 29Si NMR
spectra of (a) Pd@S-1-OH-10 and (b) Pd@S-1.
Figure 3 shows tomogram-section transmission electron
microscopy (TEM) images of the Pd@S-1-OH-10, Pd@S-1 and
Pd/S-1, which provided sectioned view of the samples and
avoid the influence of Pd nanoparticles on the external sur-
face of zeolite crystals. In the TEM images of Pd@S-1-OH-10
(Figures 3A and 3B) and Pd@S-1 (Figures 3C and 3D), the Pd
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nanoparticles are directly observed, confirming that these
and furan selectivity at 97.1%. Additionally, the Pd/S-1 dis-
plays poor furan selectivities at 6.3-12.6% during the tests. All
these results demonstrate excellent catalytic performances of
Pd@S-1-OH-10 in the selective hydrogenation of furfural to
furan, which is even much better than that of industrial cata-
lyst, Pd/Al2O3 (Supporting Information, Table S2) reported
previously.26
In the catalytic tests over Pd@S-1-OH-10 catalyst, rich for-
maldehyde and slight amount of CO are also detected in the
gaseous products, which are resulted from the cleavage of C-
C bond in furfural. The formation of formaldehyde also indi-
cate that the reaction proceeds through a hydrogenation-
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particles are indeed fixed within the S-1 crystals. Additional
TEM images show that the size distribution of the Pd nano-
particles in the Pd@S-1-OH-10 and Pd@S-1 mainly falls in the
range 3-11 (Supporting Information, Figure S2A) and 5-11 nm
(Supporting Information, Figure S2B) with a mean size at 6
and 7 nm, respectively. On the contrary, the sectioned view
of the Pd/S-1 only gives the Pd nanoparticles on the side of S-
1 crystals, confirming that the Pd nanoparticles are localized
on the external surfaced of S-1 zeolite on Pd/S-1. Additionally,
the Pd/S-1 gives the Pd nanoparticle size distribution at 4-10
nm (Supporting Information, Figure S2C) with a mean size at
6.4 nm, which is similar to those on Pd@S-1 and Pd@S-1-OH-
10.
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→
decarbonylation route (C5H4O2 + H2
C4H4O + CH2O),
different from the general decarbonylation route without
hydrogen giving CO as major product.63,64,65 In addition, the
bulky molecules from the condensation of the furfural on the
Pd sites are formed as major products over the Pd/S-1 and
Pd/Al2O3 catalysts, but undetectable over Pd@S-1-OH-10 and
Pd@S-1 catalysts. This phenomenon might be assigned to
that the Pd sites of Pd@S-1-OH-10 and Pd@S-1 are fixed in
zeolite crystals, where the bulky molecules are difficult to
form in the zeolite micropores with diameter at only 0.55 nm.
Considering the Pd@S-1 and Pd@S-1-OH-10 catalysts have
similar Pd nanoparticle size distribution and loadings as well
as the same S-1 zeolite support, better catalytic properties of
Pd@S-1-OH-10 than those of Pd@S-1 are reasonably attribut-
ed to more hydrophilic zeolite sheath of Pd@S-1-OH-10 ra-
ther than other factors. Furthermore, Pd@S-1-OH-20 with
more hydrophilic zeolite sheath has been employed, but it is
shown lower furan selectivities (Supporting Information,
Table S3) than that of Pd@S-1-OH-10, which might be related
to low zeolite crystallinity of Pd@S-1-OH-20 (Supporting
Information, Figure S4). Lower zeolite crystallinity leads to
reduced surface area and micropore volume (Supporting
Information, Table S1), which might fail to control the mass
transfer of the reactants and products in the reaction. To
understand the importance of hydrophilic zeolite sheath, we
performed the treatment of Pd@S-1-OH-10 with hydrofluoric
acid to partly destroy the zeolite sheath (Supporting Infor-
mation, Figure S5), the HF-treated sample (Pd@S-1-OH-10-
HF) exhibits a significant reduction of both furfural conver-
sion (80.1%) and furan selectivity (57.7 %, entry 19), confirm-
ing the crucial role of S-1-OH zeolite sheath for contribution
of the conversion and selectivity in furfural hydrogenation
over Pd catalysts.
3.3 Kinetic study. Figure 4 shows kinetic studies in furfu-
ral hydrogenation over the Pd@S-1-OH-10 and Pd@S-1 cata-
lysts. The reactions were performed at low temperature at
60-80 °C for achieving low conversion of furfural, which ben-
efit the calculation of the turnover frequencies (TOFs) and
reaction rates. It is worth noting that the Pd@S-1-OH-10 al-
ways gives faster reaction rate than Pd@S-1. On the basis of
the initial reaction rate at different temperatures, the appar-
ent activation energies (Ea) of Pd@S-1-OH-10 and Pd@S-1 are
calculated to be 21 and 36 KJ/mol (Figure 4B), respectively.
These results confirm that the conversion of furfural over the
Pd@S-1-OH-10 is much easier than that over the Pd@S-1.66
Considering that the two catalysts have very similar Pd na-
noparticle size and loading, the distinguishable Ea values
should be reasonably assigned to the different mass transfer
efficiency in the catalysts.
Figure 3. Tomogram-section TEM images of (A and B)
Pd@S-1-OH-10, (C and D) Pd@S-1, (E and F) Pd/S-1.
3.2 Hydrogenation of furfural. Table 1 presents catalytic
data in the hydrogenation of furfural in continuous flow
fixed-bed reactor over various catalysts. The reaction in-
volves different pathways (Supporting Information, Scheme
S1), giving furfuryl alcohol, furan, dihydrofuran, tetrahydro-
furan, tetrahydrofurfuryl alcohol, and bulky ether molecules
as detectable products. The Pd@S-1-OH-10 exhibits the furfu-
ral conversion and furan selectivityd at 56.7 and 82.5% at
150 °C. By increasing the reaction temperature from 150 to
275 °C, both the furfural conversions and furan selectivities
are significantly enhanced (Supporting Information, Figures
S3). Very interestingly, the furan selectivities higher than
99.9% with full conversion of furfural are achieved at 250 and
275 °C. In contrast, the Pd@S-1 catalyst always gives lower
furfural conversions than Pd@S-1-OH-10 under the same
reaction conditions. The best performance over the Pd@S-1 is
obtained at 250 °C, giving the furfural conversion at 92.9%
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Table 1. Catalytic data in furfural hydrogenation over various catalysts.a
1
2
3
Selectivity (%)
Temp.
(°C)
Conv.
(%)
Entry
Catalyst
Balancec
Othersb
4
5
6
7
8
9
1
2
3
4
5
6
7
8
Pd@S-1-OH-10
Pd@S-1-OH-10
Pd@S-1-OH-10
Pd@S-1-OH-10
Pd@S-1-OH-10
Pd@S-1-OH-10
Pd@S-1
Pd@S-1
Pd@S-1
Pd@S-1
Pd@S-1
Pd@S-1
Pd/S-1
Pd/S-1
Pd/S-1
Pd/S-1
Pd/S-1
Pd/S-1
Pd@S-1-OH-10-
HFd
150
175
56.7
89.5
>99.9
>99.9
>99.9
>99.9
48.9
62.8
86.6
91.3
92.9
86.6
41.3
42.2
62.8
87.5
82.5
89.7
95.9
98.4
>99.9
>99.9
85.0
91.4
92.8
93.2
97.1
82.6
7.3
9.0
4.7
1.3
0.7
0.0
0.0
7.9
3.0
3.0
3.2
8.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
9.0
11.2
11.6
12.1
11.7
7.9
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.09
0.95
0.92
0.97
0.94
1.09
1.03
1.08
0.99
0.90
0.88
0.92
1.04
0.89
0.93
0.90
0.91
1.03
5.6
2.8
0.9
0.0
0.0
7.1
5.6
4.2
3.6
200
225
250
275
150
175
200
225
250
275
150
175
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
10
11
12
13
14
15
16
17
18
0.0
0.0
1.0
1.9
0.4
26.5
10.5
4.2
2.7
1.8
0.5
21.5
29.4
19.2
12.3
12.1
12.5
16.4
35.7
36.3
54.6
63.6
68.1
72.2
12 .6
10.4
9.3
6.3
6.8
200
225
250
275
89.1
93.6
0.6
19
200
80.1
57.7
1.0
11.4
18.0
1.9
10.0
1.06
a Reaction condition: 5 wt% furfural (mass ratio of furfural/n-butyl alcohol at 1:19), feed rate at 0.5 mL/h, 10% H2/Ar at 10 mL/min,
0.1 g of catalyst, atmospheric pressure; b Bulky molecules from the condensation of furfural and some others; c Molecular balance
during the reaction, calculated from the amount of furan and tetrahydrofuran rings in feeds and products. The coke formed in
the reactions was not included in calculating the balance values;
d
The HF-treated catalyst was used.
spectra for the desorption of other molecules are given in
Supporting Information, Figure S6. Notably, the furan de-
sorbs more significantly than other molecules on both sam-
ples under the same conditions, as evidenced by fast de-
crease of the C-O bond (1190-1203 cm-1). These results also
suggest that furan diffuses much faster than other molecules
in the zeolite micropores. For example, the removal percent-
age of furan is 67% over Pd@S-1 at desorption time of 12 min,
while the value reaches 90% over Pd@S-1-OH-10, demon-
strating that the relative hydrophilic zeolite channels benefit
the diffusion of furan. In contrast, the diffusion of furfural
molecule in the hydrophilic zeolite channels is strongly hin-
dered, as given by the decreased removal percentage from
8.9% over Pd@S-1 to 5.0% over Pd@S-1-OH-10 (Supporting
Information, Table S4 and Figure S7). The dependences of
removal percentage of various molecules on time over Pd@S-
1 and Pd@S-1-OH-10 catalysts are summarized in Figures 5E
and 5F.
On the basis of these data, it is concluded that the supe-
rior selectivity of Pd@S-1-OH-10 catalysts is from the com-
bined contribution of zeolite-fixed structure and hydrophilic
zeolite channels. In this case, the zeolite-fixed structure can
significantly improve the furan selectivity in Pd-catalyzed
furfural hydrogenation by controlling the diffusion of various
molecules, and the hydrophilic zeolite micropores could
maximize the effect in adsorbing the reactant of furfural, but
quickly desorb the product of furan.
(B)ꢀ1.6
(A)45
Pd@Sꢀ1
Pd@Sꢀ1ꢀOHꢀ10
Pd@Sꢀ1
Pd@Sꢀ1ꢀOHꢀ10
ꢀ1.8
ꢀ2.0
ꢀ2.2
ꢀ2.4
ꢀ2.6
ꢀ2.8
ꢀ3.0
40
35
30
25
20
15
60
65
70
75
80
2.75 2.80 2.85 2.90 2.95 3.00
T (oC)
1/T (10ꢀ3Kꢀ1)
Figure 4. Kinetic studies in furfural hydrogenation over
Pd@S-1-OH-10 and Pd@S-1 catalysts. (A) Reaction rates of
Pd@S-1-OH-10 and Pd@S-1 in hydrogenation of furfural at
different temperatures. (B) Experimental points correlating
the inverse of the absolute temperature (1/T) with the initial
reaction rates (ln r0) over the Pd@S-1-OH-10 and Pd@S-1
catalysts.
3.4 IR spectra of molecular sorption. To understand the
effect of zeolite wettability to the molecular sorption, we
performed the IR spectra for desorption tests of various mol-
ecules on the Pd@S-1 and Pd@S-1-OH-10 catalysts. At first,
the molecules of furfural, furan, tetrahydrofuran, methylfu-
ran, furfuryl alcohol, and tetrahydrofurfuryl alcohol were
adsorbed on Pd@S-1 and Pd@S-1-OH-10 catalysts. After de-
sorption under the same conditions, the intensity of the IR
band assigned to the C-O bond in the furan or tetrahydrofu-
ran rings of these molecules, is decreased on these catalysts.
The percentages of molecular desorption were calculated on
the basis of the decreased intensity of IR bands. Figure 5
shows in-situ FTIR spectra for desorption of furfural and
furan on the Pd@S-1 and Pd@S-1-OH-10 catalysts. The IR
3.5 Theoretical simulation. To further understand the
diffusion behavior of various reactant molecules in the S-1
zeolite with or without hydrophilic OH groups, adsorption
energies for each reactant have been theoretically calculated.
Figure 6A shows the average adsorption energy (including
adsorbate-zeolite and adsorbate-adsorbate potential experi-
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enced per molecule) inside S-1-OH (see the models in Sup-
porting Information, Figure S8) under saturation adsorption
at 303 K and 101 kPa.
strated that the hydrophilic property strongly promotes the
diffusion of furan in the zeolite micropores.
1
2
3
4
5
6
(A)
(B)
1312
Pd@Sꢀ1
Pd@Sꢀ1ꢀOHꢀ10
1200
1325
1203
20 min
20 min
16 min
12 min
16 min
12 min
7
8
9
8 min
4 min
8 min
4 min
0 min
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0 min
1300 1200 1100 1000 900 800
Wavenumber (cmꢀ1)
1300120011001000 900 800
Wavenumber (cmꢀ1)
(C)
(D)
Pd@Sꢀ1ꢀOHꢀ10
Pd@Sꢀ1
1304
991
1305
1190
1199
1168
993
1170
20 min
16 min
12 min
8 min
20 min
16 min
12 min
8 min
4 min
4 min
0 min
0 min
1300 1200 1100 1000 900 800
Wavenumber (cmꢀ1)
1300 1200 1100 1000 900 800
Wavenumber (cmꢀ1)
(E)
(F)
100
80
60
40
20
0
100
80
60
40
20
0
Furan
Tetrahydrofuran
Methylfuran
Figure 6. (A) Probability distribution of adsorption energy
for various molecules in zeolite S-1-OH-9; and (B) Probability
distribution of adsorption energy of furan in zeolites with the
varied hydroxyl concentration S-1-OH-x (x = 0, 9, 17, 27).
Furan
Tetrahydrofuran
Methylfuran
Furfural
Furfural alcohol
Tetrahydrofurfural alcohol
Furfural
Furfural alcohol
Tetrahydrofurfural alcohol
3.6 Catalyst stability. The Pd leaching in the catalytic
process is a general challenge for the supported Pd catalysts,
and their stabilities and regeneration properties are crucial
for the practical applications. Figure 7 shows the dependenc-
es of furfural conversion and furan selectivity on reaction
time over Pd@S-1-OH-10 catalyst at 200 °C. For the selectivi-
ty, it is very stable at higher than 95%. For the reduced con-
version, it come back again to higher than 99% after calcina-
tion at 550 °C for 4h in oxygen. The catalyst deactivation is
owing to the coke formation, which is supported by the
change in catalyst color and TG analysis (Supporting Infor-
mation, Figure S9). TEM images of the used Pd@S-1-OH-10
catalyst after the 90 h reaction still exhibit the Pd nanoparti-
cle diameter distribution at 3-11 nm with a mean size at 6.5
nm (Supporting Information, Figure S10), which is very simi-
lar to that of the fresh catalyst. ICP-OES analysis demon-
strates that the Pd leaching is negligible during this long-
term test. These phenomena should be assiged to that the
zeolite sheath is stable and prevent the sintering/leaching of
Pd nanoparticles.67,68
4
8
12 16 20
4
8
12 16 20
Time (min)
Time (min)
Figure 5. In-situ FTIR spectra of (A and B) furfural, (C and D)
furan on the Pd@S-1 and Pd@S-1-OH-10 catalyst during the
desorption test. Remove percentage of various molecules
over (E) Pd@S-1 and (F) Pd@S-1-OH-10 during the molecule
desorption IR test.
Clearly, the adsorption of furan molecule in zeolite (-14.7
kcal/mol) is much weaker than all of the other organic mole-
cules involved in the catalytic reaction (tetrahydrofuran,-16.8
kcal/mol; methylfuran,-17.5 kcal/mol; furfural, -20.6 kcal/mol;
furfuryl alcohol, -20.4 kcal/mol; tetrahydrofurfuryl alcohol, -
20.2 kcal/mol). According to experimental results (Figure 5F),
about 90% furan molecule has been removed from S-1-H-10
catalyst at desorption time of 12 min, while the desorption of
the other molecules is less than 20%, which are well con-
sistent with those of theoretical simulation. Apparently, it’s
indicated that these molecules can’t easily be removed and
diffused out of the S-1-H-10 zeolite micropores.
Furthermore, to gain insight the influence of hydroxyl
group concentration on the diffusion, we also investigated
the adsorption of furan in zeolite S-1-OH-x (x = 0, 9, 17 and
27) with the varied hydroxyl concentration. As shown in Fig-
ure 6B, the corresponding distribution curve of average ad-
sorption energies of furan are gradually decreased to -14.8, -
14.7, -14.5 and -14.1 kcal/mol upon the increasing of hydroxyl
groups in zeolite S-1-OH-x (x = 0, 9, 17 and 27). It is demon-
More importantly, the hydrophilicity of Pd@S-1-OH-10 is
also well maintained during the catalytic test, which is con-
firmed by the water droplet on the surface of used Pd@S-1-
OH-10 displaying the contact angle at 22° (Figure 2D). It is
worth mentioning that, during the long-term test, the
productivity of furan can reach as high as ~583.3 g·gPd-1·d-1,
which is potentially important for practical applications in
the future.
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Super Computation of the NSFC-Guangdong Joint Fund (the
1
second phase, U1501501).
2
3
4
5
6
7
8
9
100
80
60
40
20
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Calcination
Calcination
Furfural conversion
Furan selectivity
0
10 20 30 40 50 60 70 80 90
Time (h)
10
11
12
13
14
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17
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Figure 7. Dependence of conversion of furfural and selectivi-
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ral to furan. Reaction conditions are: 5 wt% furfural (mass
ratio of furfural/n-butyl alcohol at 1:19), feed rate at 0.5 mL/h,
10% H2/Ar at 10 mL/min, 0.1 g of Pd@S-1-OH-10, 200°C, and
atmospheric pressure.
4. CONCLUSIONS
In summary, we report a novel strategy to achieve selective
hydrogenation over a core-shell Pd@S-1 zeolite catalyst with
controllable wettability of the zeolite sheath. Because the
zeolite micropores with appropriate hydrophilicity change
the diffusion of various molecules, the Pd@S-1-OH catalysts
obtained from functionalizing the silanol groups on the zeo-
lite framework exhibit extraordinary activity, selectivity and
stability in the hydrogenation of furfural to furan. This work
delineates a persuasive example on wettability-driven selec-
tive hydrogenation over Pd nanoparticles, which might open
an avenue for developing a class of high selective metal cata-
lysts by rationally controlling the catalyst wettability.
ASSOCIATED CONTENT
Supporting Information. Computational methods; TEM,
FTIR, addition experiment results in catalytic tests and com-
putational simulations. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
(18) Quek, X.-Y.; Pestman, R.; van Santen, R. A.; Hensen, E. J.
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Email: liangwang@zju.edu.cn (L.W.)
Email: zhenganm@wipm.ac.cn (A.Z.)
Email: fsxiao@zju.edu.cn (F.-S.X.)
Author Contributions
†C.W. and Z.L. contributed equally.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
This work is supported by National Key Research and Devel-
opment Program of China (2017YFC0211101), National Natural
Science Foundation of China (91645105 and 91634201). A.
Zheng acknowledges the support from Key Research Pro-
gram of Frontier Sciences (QYZDB-SSW-SLH026), Key Re-
search Program of Frontier Sciences, CAS (Grant QYZDB-
SSW-SLH026), and Special Program for Applied Research on
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