Pd/Lewis Acid Synergy in Macroporous Pd@Na-ZSM-5 for Enhancing Selective Conversion of Biomass

Article abstract of DOI:10.1002/cctc.202000868

Pd nanometal particles encapsulated in macroporous Na-ZSM-5 with only Lewis acid sites have been successfully synthesized by a steam-thermal approach. The synergistic effect of Pd and Lewis acid sites have been investigated for significant enhancement of the catalytic selectivity towards furfural alcohol in furfural hydroconversion.

Full text of DOI:10.1002/cctc.202000868

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Pd/Lewis acid synergy in macroporous Pd@Na-ZSM-5 for
enhancing selective conversion of biomass
Authors: Jia-Wen Liu, Si-Ming Wu, Li-Ying Wang, Ge Tian, Yuan Qin,
Jing-Xian Wu, Xiao-Fang Zhao, Yan-Xiang Zhang, Gang-
Gang Chang, Lu Wu, Yue-Xing Zhang, Zhao-Fei Li, Cheng-
Yu Guo, Christoph Janiak, Silvia Lenaerts, and Xiao-Yu Yang
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.202000868
Link to VoR: https://doi.org/10.1002/cctc.202000868
01/2020

10.1002/cctc.202000868
ChemCatChem
COMMUNICATION
Pd/Lewis acid synergy in macroporous Pd@Na-ZSM-5 for
enhancing selective conversion of biomass
Jia-Wen Liu,†[a] Si-Ming Wu,†[a, b] Li-Ying Wang,†[c] Ge Tian,[a] Yuan Qin,[a] Jing-Xian Wu,[a] Xiao-Fang
Zhao, [a] Yan-Xiang Zhang,[a] Gang-Gang Chang,[a] Lu Wu,[e] Yue-Xing Zhang,[e] Zhao-Fei Li,[f] Cheng-
Yu Guo,[f] Christoph Janiak,[g] Silvia Lenaerts[h] and Xiao-Yu Yang*[a, d ,i]
[a]
J. W. Liu, S. M. Wu, G. Tian, Y. Qin, J. X. Wu, X. F. Zhao, Y. X. Zhang, G. G. Chang, X. Y. Yang
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing & School of Materials Science and Engineering & School of
Chemical Engineering and Life Science
Wuhan University of Technology
122 Luoshi Road, Wuhan, 430070, China
E-mail: xyyang@whut.edu.cn
[b]
[c]
[d]
S. M. Wu
Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) & School of Chemical Engineering and Technology
Sun Yat-sen University (SYSU)
Zhuhai, 519000,China
L. Y. Wang
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics
Wuhan Institute of Physics and Mathematics
Chinese Academy of Sciences, Wuhan, 430071
X. Y. Yang
School of Engineering and Applied Sciences
Harvard University
Cambridge, Massachusetts 02138, USA
E-mail: xyyang@seas.harvard.edu
L. Wu, Y. X. Zhang
[e]
College of Chemistry and Chemical Engineering
Hubei University
Wuhan, 430062, China
[f]
Z. F. Li, C. Y. Guo
Petrochemical Research Institute of PetroChina
Changping District, Beijing, 102206, China
C. Janiak
[g]
Institut für Anorganische Chemie und Strukturchemie
Heinrich-Heine-Universität Düsseldorf
40204 Düsseldorf, Germany
[h]
[i]
S. Lenaerts
Research Group of Sustainable Energy and Air Purification (DuEL), Department of Bioscience Engineering
University of Antwerp
Antwerp, Belgium
X. Y. Yang
Qingdao National Laboratory for Marine Science and Technology
Qingdao, 266237, China
†These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Pd nanometal particles encapsulated in macroporous Na-
ZSM-5 with only Lewis acid sites have been successfully synthesized
by a steam-thermal approach. The synergistic effect of Pd and Lewis
acid sites have been investigated for significant enhancement of the
catalytic selectivity towards furfural alcohol in furfural hydroconversion. such as adsorption promotion of reactants[1,9] and electronically
influence of Pd.[15] In the meantime, the confined Pd particles in
selectivity, and outstanding stability.[7-10] Zeolite-confined Pd show
unique cascade effects on a molecular scale, and thus enables
spatially confined catalysis similar to enzyme catalysis.[11-14] The
acid sites play a key role in sustainable production of biofuels,
the acidic zeolite will also strongly influence the state of the acid
Nanostructured palladium (Pd) metal, as one of the most
sites,[16] contributing to the synergy effects of Pd and acid sites.
efficient catalysts for hydrogenation reactions, has been widely
Brønsted acid sites, arising from the framework Al species in
used for biomass conversion.[1-3] However, many issues need to
highly crystalline zeolites, have been studied for selective
be addressed for the practical application of Pd nanoparticles,
conversion.[17-18] However, the strong Brønsted acid sites often
such as fast kinetics, high stability, and especially selectivity
lead to the formation of coke and the low stability of Al sites in the
enhancement.[4-6] Confinement of Pd nanometal in a zeolite matrix
zeolite framework, which would decrease the catalyst lifetime.[19]
is a promising way to a catalyst design that shows significant
Lewis acidity might circumvent these two problems due to its
synergy by combination of well-defined acid sites, shape
weaker acidity property and higher stability.[18] It is therefore of
1
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great interests to understand the role of Lewis acid sites in
Pd/zeolites catalysts, and thus achieve the high activity,
selectivity and stability of Pd/zeolites during selective
transformation of renewable biomass-derived feedstocks.[20-22]
However, most studies focus on the Brønsted acid sites in
zeolite catalytic systems,[23-25] and there are rare reports on the
design and study of Pd/Lewis acid sites. Generally, Brønsted acid
sites are attributed to the framework Al in highly crystallized
zeolites, which is commonly obtained by hydrothermal
process.[26,27] Therefore, to design Pd/Lewis sites system, we
present a direct steam-thermal crystallization approach to obtain
low crystallized Pd/macroporous ZSM-5 (denoted as Pd@ZSM-
5) with a large amount of extra framework Al. For decreasing the
number of Brønsted acid sites, H3O+ ions in ZSM-5 are
exchanged by Na+ ions. Macroporosity is introduced in ZSM-5, for
increasing the contact of the extra framework Al. The catalysts are
then applied in the hydroconversion of furfural (FFL), which is an
important and highly unsaturated intermediate in the production
of biofuels.[28-31]
SEM and TEM images of Pd@ZSM-5 show 2-3 μm
vesicle-like particles, consisting of intergrown primary
particles of about 500-700 nm and windows of 500-700 nm
(Figure 1, S1 and S2, ESI†). Some of the large Pd particles
are obviously in the macropores (Figure 1b, S1 b-d) and
some smaller Pd particles can be observed inside the zeolite
can be observed inside the zeolite particle with average size
of 5-10 nm (Figure 1c). Furthermore, from TEM-EDX
element mapping, it can be observed that Pd nanoparticles
are dispersed throughout the ZSM-5 particle. XRD patterns
match with the MFI structure of ZSM-5 (Figure 2a). The
absorption band at 550 cm-1 in FT-IR spectra is also associated
with the asymmetric stretching mode of the double five-rings in
the MFI structure (Figure S3). Notably, no reflections associated
with metallic Pd (2theta 40.1°and 46.7°) are observed in the XRD
patterns, which is possibly due to the very small particles and the
low Pd loading amount (Figure S4). The microporosity of
Pd@ZSM-5 was confirmed from N2 adsorption isotherms and the
pore size distribution indicates that the formation of Pd
nanoparticles does not clearly affect the microporosity (Figure 2b,
Table S1).
Figure 1a illustrates the procedure for the synthesis of
Pd@ZSM-5. First K2PdCl4 was impregnated in mesoporous silica
spheres (MS), then NaAlO2 and tetrapropyl ammonium hydroxide
solution (TPAOH) were added. The mixture gelated, and was then
transferred into an autoclave for steam-assisted crystallization.
After calcination in air and reduction under hydrogen atmosphere,
Pd@ZSM-5 was obtained (see ESI† for details).
Figure 2. (a) XRD patterns of Pd@ZSM-5, Pd@ZSM-5-W and Pd@ZSM-5-S.
(b) N2 adsorption/desorption isotherms of Pd@ZSM-5, Pd@ZSM-5-W and
Pd@ZSM-5-S; the latter two isotherms are displaced as indicated for clarity. (c)
and (d) SEM image of Pd@ZSM-5-W and Pd@ZSM-5-S.
Moreover, the hydrothermal stability and steam stability of
Pd@ZSM-5 is assessed by exposing it to water at 110 °C and
steam at 110 °C for 3 days (designed as Pd@ZSM-5-W and
Pd@ZSM-5-S, respectively). After the thermal treatment, the MFI
structure is well-maintained and the microporisity has no obvious
change (Figure 2a, b, Table S1). The SEM images (Figure 2c, d)
display the well-maintained vesicle-like structure, indicating
excellent hydrothermal and steam stability. The EDX analysis in
Figure S4 indicates the existence of Al element in Pd@ZSM-5.
The signal at 20 ppm in 27Al MAS NMR spectrum of Pd@ZSM-5
(Figure S5) shows the presence of large amount of the extra-
frame Al sites, indicating the low crystallized structure for the
formation of Lewis acid sites. In order to investigate the effect of
acidic sites in ZSM-5 zeolite, Na+ ion exchange was applied for
decreasing the amount of Brønsted acid sites (designed as
Pd@Na-ZSM-5). As comparison, the samples include Pd@H-
ZSM-5 (treatment of NH4NO3 exchange), Pd@S-1 (Silicalite-1
with MFI structure and no any acidic sites using our method),
Figure 1. (a) Schematic illustration of the synthesis of Pd@ZSM-5. (b), (c) TEM
images of Pd@ZSM-5, Inset: The size distribution of Pd nanoparticles in the
zeolite. (d) HAADF-STEM image of Pd@ZSM-5 and (e) the corresponding EDX
mapping images for Si, Al, Pd, O elements.
Pd/H-ZSM-5 (macroporous ZSM-5 supported Pd via
+
impregnation-reduction-NH4
ion
change),
Pd/Na-ZSM-5
2
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(macroporous ZSM-5 supported Pd via impregnation-reduction-
Na+ ion exchange), Pd/S-1 (macroporous Silicalite-1 supported
Pd via impregnation-reduction) (Figure S6) and commercial Pd/C
are also obtained.
ZSM-5 show larger amount of weak acid sites (1.8 mmol NH3/g).
In the meantime, an additional peak ranging of 300-500 °C of
Pd@H-ZSM-5 indicates the existence of strong acid sites in it (0.7
mmol NH3/g). Solid-state NMR is also a very powerful tool for
probing the acid sites.[34,35] To further study the Brønsted and
Lewis acid sites and to gain insight into the spatial
proximity/interaction between the acid sites in ZSM-5, 1H magic-
angle-spinning (MAS)-NMR and 2D 1H double-quantum (DQ)
MAS-NMR, both highly sophisticated pulse techniques, have
been performed. And to avoid the influence of absorbed water, a
pre-dehydration is required before measurement. Figure 4a, b
The unique catalytic properties of zeolite-encapsulated Pd
nanoparticles are investigated with the model reaction of
hydroconversion of FFL,[6] where various products will form based
on the diverse catalytic steps derived by different active sites
(Figure 3a).[32] Pd@Na-ZSM-5 shows the highest yield (57%) and
selectivity (74%) of furfuryl alcohol (FAL) in comparison with
Pd@H-ZSM-5 (14%, 18%) and Pd@S-1 (22%, 30%), Pd/Na-
ZSM-5 (7%, 10%), Pd/H-ZSM-5 (16%, 19%), Pd/S-1 (11%, 13%),
and commercial Pd/C (13%, 14%) (Figure 3b, Table S2).
Compared with the reported catalysts (Table S3), higher
selectivity of furfural alcohol is obtained in our work. Note that the
by-productions of the catalysts (Pd/Na-ZSM-5, Pd/H-ZSM-5,
Pd/S-1, and commercial Pd/C) using impregnation-reduction are
considerably high (up to 86%), while the catalysts (Pd@Na-ZSM-
5, Pd@H-ZSM-5 and Pd@S-1) using direct transformation show
very low amounts of by-products (up to 40% over Pd@S-1). This
suggests that the catalysts designed by our synthesis method
greatly decreases the by-production and enhances the yield of
value-added productions 2-methylfuran (2-MF) and 1,5-
pentanediol (PDO) compared with samples obtained by common
impregnation method. Very interestingly, only Pd@Na-ZSM-5
show significant selectivity of FAL, which is possibly due to the
synergistic effect of Pd and acid sites in Pd@Na-ZSM-5.
Figure 4. (a) 1H solid-state MAS NMR spectra of Pd@Na-ZSM-5, Pd/Na-ZSM-
5 and Na-ZSM-5, (b) 1H solid-state MAS NMR spectra of Pd@H-ZSM-5, Pd/H-
ZSM-5, H-ZSM-5. (c)-(h) 1H DQ-SQ MAS NMR spectra of (c) Pd@Na-ZSM-5,
(d) Pd@H-ZSM-5, (e) Pd/Na-ZSM-5, (f) Pd/H-ZSM-5, (g) Na-ZSM-5 and (h) H-
ZSM-5.
Figure 3. (a) Proposed reaction pathways for hydrogenation of furfural. (b)
Catalytic performance of furfural hydroconversion over samples. Reaction
conditions: 0.03 g of catalyst, 0.8 mmol of furfural, 5 mL of 2-propanol as solvent,
1 MPa H2, T = 150 °C.
shows the 1H MAS NMR spectra of Pd@Na-ZSM-5, Pd/Na-ZSM-
5 and Na-ZSM-5 (Figure 4a), and Pd@H-ZSM-5, Pd/H-ZSM-5
and H-ZSM-5 (Figure 4b). Typically, protons residing at Brønsted
acid sites, for example, bridging hydroxyls in zeolites, possess 1H
in range of 3.6-5.6 ppm, whereas weakly acidic terminal hydroxyls,
possibly caused by Lewis sites, tend to appear at 1.5-2.0 ppm.[36]
It is obvious that all Na+ ion exchanged ZSM-5 (Pd@Na-ZSM-5,
Pd/Na-ZSM-5 and Na-ZSM-5) present the sharp and symmetrical
The acidic microenvironment of this zeolite might significantly
modulate the reaction pathways and product distribution in FFL
hydroconversion.[7] Firstly, the acid-base sites provided by the
zeolite environment are investigated by NH3 temperature-
programed desorption (TPD).[33] The ammonia desorption peak in
the temperature range of 50-200°C (1.0 mmol NH3/g) in the NH3-
TPD profile of Pd@Na-ZSM-5 (Figure S7) is observed, which is
generally assigned to the weak acid sites. As comparison, Pd@H-
3
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signal peak near 2.4 ppm (Figure 4a), indicating the successful
control of only Lewis acid sites. Meanwhile, the all H-ZSM-5
based catalysts (Pd@H-ZSM-5, Pd/H-ZSM-5 and H-ZSM-5)
show relatively strong signal of Brønsted acid sites at 5.1 ppm and
weak signal of Lewis acid sites (Figure 4b).
To further obtain the information about interaction between acid
sites of zeolite and Pd, the highly sophisticated pulse technique,
1
2D H DQ-SQ MAS NMR, is performed. As shown in Figure 4c,
the signal at (2.4, 2.4+2.4) of Pd@Na-ZSM-5 indicates the spatial
proximity of hydroxyl groups associated with external framework
Al in the supercage, while the strong signal at (5.4, 5.4+5.4) of
Pd@H-ZSM-5 (Figure 4d) indicates the main existence of
neighboring Brønsted acid sites. Compared with Pd/Na-ZSM-5
1
and Na-ZSM-5, the H DQ-SQ MAS NMR spectrum of Pd@Na-
ZSM-5 shows only the signal of Lewis acids, suggesting that the
direct transformation could create the interaction between the Pd
and Lewis acid sites, and the corresponding synergy of Pd and
Lewis acid sites leads to significant enhancement of catalytic
selectivity.
For further understanding the interaction between the Pd and
Brønsted/Lewis acid sites, the X-ray photoelectron spectroscopy
(XPS) (Figure 5a) and EPR (Figure 5b) are tested. The two peaks
at around 334.5 and 339.9 eV of Pd@Na-ZSM-5 are
corresponding to the Pd 3d5/2 and Pd 3d3/2 of Pd (0), respectively
(Figure S8). Chemical shift of Pd 3d spectra reflects the different
states of Pd in each sample. Compared with Pd@S-1, the Pd 3d3/2
binding energy values of Pd@Na-ZSM-5 and Pd@H-ZSM-5
zeolites shift toward higher values by 0.3 eV and 0.4 eV,
respectively (Figure 5a), which is due to the higher electron
transfer from palladium to the zeolite framework in the acid
environment. Similarly, the stronger signal of the EPR spectra in
Pd@Na-ZSM-5 and Pd@H-ZSM-5 (Figure 5b) is also attributed
to more active free-electrons than Pd@S-1.[4] Considering the
very high hydrogen activation of Pd, the saturation hydrogenation
occurs over the Pd@H-ZSM-5, which is in good agreement with
the catalytic result (the 2-MF % is higher than the Pd@Na-ZSM-
5). The possible mechanism of selective hydrogenation by
Pd/Lewis acid and Pd/Brønsted acid models are proposed in
Figure 5c. In a Pd/Lewis acid system (Figure 5c, I), the synergistic
effect of Pd and Lewis acid sites will promote the adsorption of H2
and the hydrogenation reaction, resulting in the formation of FAL.
For Pd/ Brønsted acid system (Figure 5c, II), the protons from
bridging hydroxyls in H-ZSM-5 (Si-OH-Al) are more likely to attack
the oxygen atoms in furan rings, resulting in the ring opening
reaction. Pd that provides excellent hydrogen activation through
homolytic H2 cleavage, leads to a saturation hydrogenation and
even total hydrogenation to products like PDO. Therefore,
different product distributions are obtained in Pd/Lewis acid and
Pd/ Brønsted acid system.
Figure 5. (a) Chemical shift of Pd 3d3/2 XPS spectra of Pd@S-1, Pd@Na-ZSM-
5 and Pd@H-ZSM-5. (b) EPR spectra of Pd@S-1, Pd@Na-ZSM-5, Pd@H-
ZSM-5 and Pd/C. (c) Schematic illustration of the selective hydrogenation of
FFL over the model of (I) Pd/Lewis acid and (II) Pd/Brønsted acid.
In summary, we have successfully designed Pd-encapsulated
macroporous ZSM-5 zeolite with high stability. Pd@Na-ZSM-5
shows high selectivity of FAL in the biomass conversion. The
interactions between Pd and the acid sites in zeolite has been
investigated with multiple techniques and the mechanism of
synergistic effect of Lewis acid and Pd is proposed. It is believed
that this work provides a new insight for the investigation of highly
selective zeolite-encapsulated metal catalysts.
Acknowledgements
We acknowledge a joint DFG-NSFC project (DFG JA466/39-1,
NSFC grant 51861135313). This work was also supported by
National Key R&D Program of China (2017YFC1103800), NSFC
(U1662134, 21711530705), Jilin Province Science and
Technology Development Plan (20180101208JC), HPNSF
(2016CFA033), FRFCU (19lgzd16) and ISTCP (2015DFE52870).
Keywords: Pd cluster • Lewis acids • confinement effect •
synergistic effect • biomass conversion
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Entry for the Table of Contents
The synergy effect of Pd and Lewis acid sites confined in Pd@macroporous Na-ZSM-5 significantly enhance the catalytic selectivity
in biomass conversion.
6
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