Surfactant-free Pd nanoparticles immobilized to a metal-organic framework with size- and location-dependent catalytic selectivity

Article abstract of DOI:10.1039/c4cc09139f

Surfactant-free Pd nanoparticles, immobilized to a metal-organic framework (MIL-101), have been used for the first time as highly active and durable catalysts in water for biomass refining (hydrodeoxygenation of vanillin, a typical compound of lignin) with metal nanoparticle size- and location-dependent catalytic activity and selectivity. This journal is

Full text of DOI:10.1039/c4cc09139f

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Surfactant-free Pd nanoparticles immobilized to metal-
organic framework with size- and location-dependent
catalytic selectivity
Cite this: DOI: 10.1039/x0xx00000x
Arshad Aijaz,a Qi-Long Zhu,a Nobuko Tsumori,b Tomoki Akitaa and Qiang Xua*
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/
of biofuels; however, a suitable catalyst with high activity is always
demanded.12 In this regard, ultrafine MNPs immobilized within
pores could provide highly active sites for decreasing the oxygen
contents from bio-oils by hydrodeoxygenation reaction.
Herein, we chose vanillin (4-hydroxy-3-methoxybenzaldehyde), a
large component of pyrolysis oil derived from the lignin fraction, as
a reactant to study the catalytic hydrodeoxygenation reaction with
MOF-immobilized Pd NPs. Although reactant’s size and shape
selectivity with porous material-based catalysts have been
successfully reported thus far, to the best of our knowledge, for any
catalytic organic reactions, this is the first example that location and
size of MNPs immobilized into nanopores can control the reaction
Surfactant-free Pd nanoparticles, immobilized to metal-
organic framework (MIL-101), have been used for the first
time as a highly active and durable catalyst in water for
biomass refining (hydrodeoxygenation of vanillin, a typical
compound of lignin) with metal nanoparticle size- and
location-dependent catalytic activity and selectivity.
In recent years, catalysis using metal nanoparticles (MNPs) has
attracted great interest in search of selective and enhanced catalytic
performances.1,2 Immobilizing MNPs to porous materials could
afford heterogeneous catalysts having surfactant-free active sites
with the advantages of controlling particle nucleation and growth to
a nanosize region in the confined pores and reducing particle
aggregation.3 MOFs have particularly demonstrated their potentials
for immobilizing fine MNPs within their pores for heterogeneous
catalysis.4,5 MOFs are crystalline porous materials, composed of
metal ions or metal clusters with multitopic organic ligands, usually
thermally robust and have nanoporous spaces that could be accessed
by guest molecules.6-8 Although many studies have demonstrated
that the catalytic activity can be enhanced by decreasing the MNP
sizes owing to their high surface area and surface roughness, very
few studies have shown changes in product selectivity.1a Reactant
size selectivity during catalytic transformation is now common
phenomenon in porous material-based catalysts, modulated by the
pore sizes of used porous support.9 Recently MNP size- and
location-controlled activity has been reported with MOF as
support,5a whereas controlling of reaction pathway and product
selectivity by the surfactant-free MNP size and location have not yet
been attempted with any porous host materials.
pathway and product selectivity. Scheme
1
shows the
immobilization of Pd nanoparticles into/onto MIL-101 and their use
for one- and two-step catalytic hydrodeoxygenation reactions of
vanillin.
In the past decades, production of fuels from renewable biomass
has drawn considerable attention.10 The continued depletion of fossil
fuels has marked serious concerns for searching renewable and
sustainable fuel sources. Lignin, which constitutes ~30 wt % of
woody biomass, is the second most abundant bio-source on earth
after cellulose.11 Compared to cellulose-derived pyrolysis oil, it is
more challenging to deoxygenate lignin-derived pyrolysihs oil
because of its higly complicated structure, which consists of oxygen-
rich subunits derived from phenol, p-coumaryl, coniferyl, and
sinapyl alcohols typically connected with ether linkages.
Hydrodeoxygenation is one of the best ways to improve the quality
Scheme 1 Schematic representation of the immobilization of Pd
nanoparticles into/onto MIL-101, and their use for catalytic
hydrodeoxygenation reactions of vanillin.
MIL-101, a chromium-based MOF with molecular formula
Cr3F(H2O)2O[(O2C)C6H4(CO2)]3.nH2O (n = ~25), was chosen for
immobilizing MNPs because of its high stability in water, large
surface area, and two giant zeotypic cavities with free diameters of
2.9 and 3.4 nm accessible through two pore windows of 1.2 and 1.6
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nm.13 Double solvents method (DSM) was used to immobilize (HRSTEM) analysis showed that the Pd NPs are crystalline with an
ultrafine Pd NPs within MIL-101 cavities,5b while simple deposition average spacing of 2.4 Å corresponding to Pd (1 1 1) (Fig. S5). On
of Pd precursor on non-activated MIL-101 led to larger and outer- the other hand, STEM images of Pd/MIL-101 showed broad
surface deposited Pd NPs.14 Briefly, for loading Pd NPs inside the distribution of larger Pd NPs throughout the outer surfaces of MIL-
cavities of MIL-101, 100 mg of green MIL-101 powder activated by 101 with an average size of 5.0 ± 0.5 nm, larger than the cavity sizes
heating at 150 °C for overnight under vacuum was suspended in of MIL-101 (Fig. 1d), while PXRD and BET measurements of
hexane (20 mL), to which an aqueous H2PdCl4 solution (0.15 mL) Pd/MIL-101 suggested the retention of MIL-101 framework (Figs.
with required concentrations was added dropwise under continuous S1, S2).
vigorous stirring. After careful filtration, the green powder was dried
in air at room temperature followed by further drying at 150 °C for
overnight and subsequent reduction in a stream of H2/He (50:50 mL
min–1) at 200 °C for 5 h to yield the sample designated as Pd@MIL-
101. For immobilizing Pd NPs to the outer surfaces of MIL-101, 100
mg of MIL-101 without pre-activation was mixed with aqueous
H2PdCl4 solution (0.5 mL) by stirring. After drying at room
temperature, the green powder was further dried at 150 °C for
overnight and subsequently reduced in a stream of H2/He (50:50 mL
min–1) at 200 °C for 5 h to yield the sample designated as Pd/MIL-
101.14
Scheme
2
Possible
reaction
pathways
for
vanillin
hydrodeoxygenation.
Fig. 1 (a) TEM and (b, c) HAADF-STEM images of Pd@MIL-101;
(d) HAADF-STEM image of Pd/MIL-101.
Fig. 2 Evolution of reactant and products contents with reaction time
in the reaction of vanillin over 2 wt% Pd immobilized inside the
cavity of MIL-101 (Pd@MIL-101) at 75 °C and 2 bar H2.
There was no loss of crystallinity in the powder X-ray diffraction
(PXRD) patterns after reduction, suggesting that the integrity of the
MIL-101 framework was maintained after MNP deposition in
Pd@MIL-101 (Fig. S1). The decreases in the amount of N2
adsorption and the pore volume indicated that the cavities of MIL-
101 were occupied by dispersed Pd NPs and/or blocked by the Pd
NPs located at the MIL-101 surface (Fig. S2). The X-ray
photoelectron spectrum (XPS) of Pd@MIL-101 at Pd 3d levels
exhibited metallic Pd peaks (Fig. S3). The high-angle annular dark-
field scanning transmission electron microscopy (HAADF-STEM)
and transmission electron microscopy (TEM) images (Figs. 1a-1c),
energy-dispersive X-ray spectroscopy (EDX) analyses (Fig. S4) and
electron tomographic reconstruction (a movie provided as ESI) of
Pd@MIL-101 exhibited the three-dimensional uniformity of the Pd
NPs in the cavities of MIL-101 crystalline framework. The TEM
images of Pd@MIL-101 showed that the sizes of the Pd NPs were in
the range of 1.0-2.2 nm with an average size of 1.8 ± 0.2 nm, which
are small enough to be accommodated in the two mesoporous
cavities of MIL-101; no large Pd particle aggregations have been
observed in the TEM and STEM images. High resolution STEM
To examine the catalytic activity of Pd@MIL-101, biofuel
upgrade was investigated in aqueous medium. Water is a desirable
green solvent for chemical transformations for reasons of cost, safety,
and environmental impacts. Vanillin, a large component of pyrolysis
oil derived from the lignin fraction, was chosen as a reactant to study
the catalytic hydrodeoxygenation reaction.14 Transformation of
carbonyl group into a methyl group can proceed in three pathways:
(1) hydrogenation/dehydration, (2) hydrogenation/hydrogenolysis,
and (3) direct hydrogenolysis.15 Since after hydrogenation vanillin
alcohol has no hydrogen atom at the adjacent position to the
hydroxyl group, dehydration is not possible in this reaction.
Therefore, the transformation of vanillin to 2-methoxy-4-
methylphenol must proceed via pathway (2) and/or pathway (3)
(Scheme 2). Fig. 2 shows the evolution of the reactant and products
contents with the reaction time. The catalytic reaction was carried
out in a stainless steel reactor inbuilt with a pressure gauge setup.
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The contents of products and reactant were determined by GC-FID 4-methylphenol via hydrogenolysis (Fig. 4, S9). Even at 3 h of
based on authentic samples. Briefly, vanillin (100 mg), Pd@MIL- reaction, 25% of vanillin itself was observed, suggesting that
101 catalyst (100 mg, 2 wt% Pd) and water (5 mL) were added into hydrogenolysis of vanillin alcohol together with vanillin direct
the reactor. The reactor was evacuated with vacuum pump for 15 hydrogenolysis occurred, ie, the process is not selective for any of
min at room temperature to remove any O2 or air and then available two paths.
pressurized with H2 at a desired pressure. The temperature of the
reactor was maintained with heating bath. After reaction, the reactor
was removed from the heating bath and cooled down to room
temperature. Ethyl acetate was used to extract the organic
compounds from aqueous solution and then condensed and solved in
ethanol before supplied for GC-FID analysis.14 The reaction was
accompanied by almost proportional increase and decrease in 2-
methoxy-4-methylphenol and vanillin, respectively, with only a very
small amount of vanillin alcohol (~ 2%) observed in initial 3 h,
suggesting that mainly the direct hydrogenolysis of vanillin occurred
(Fig. 2). The transformation into 2-methoxy-4-methylphenol was
Fig.
4
Reactant and products distributions for vanillin
hydrodeoxygenation under identical conditions (2 bar H2, 75 oC, 3 h)
with (1) Pd@MIL-101 and (2) Pd/MIL-101.
completed in 7 h (Fig. S6). These observations were consistent with
both ethyl acetate and aqueous phases GC analysis (Figs. S7, S8).
This finding, associated with ultrafine Pd NPs embedded within the
MOF pores, is different from the very recently reported observations
associated with larger MNPs dispersed on the surfaces of carbon
supports.16 In these recent studies, the conversion of vanillin to 2-
methoxy-4-methylphenol proceeded with hydrogenation to vanillin
alcohol followed by hydrogenolysis. For example, in a biphasic
system (water and decalin) the Pd@SWNT-SiO2 catalyst afforded a
82% conversion of vanillin in 1 h at 100 oC with 17 and 83 %
selectivities for 2-methoxy-4-methylphenol and vanillin alcohol,
respectively.16b With similar kinetics, Wang and co-workers
demonstrated 90% vanillin conversion at 90 oC and 1 bar H2 in water
with 22 and 78 % selectivities for 2-methoxy-4-methylphenol and
vanillin alcohol, respectively, over nitrogen-doped carbon-
immobilized Pd catalyst (Pd@CN0.132).16a In comparison to these
reports, Pd@MIL-101 has higher catalytic activity with 49%
conversion in initial 1 h with ~100 % selectivity for 2-methoxy-4-
methylphenol under very mild conditions (75 oC and 2 bar H2).
The higher activity of Pd@MIL-101 might be owing to the
smaller size of Pd NPs confined within the cavities of the MIL-101.
The striking selectivity of Pd@MIL-101 for 2-methoxy-4-
methylphenol via direct hydrogenolysis could be associated with the
steric hindrance and strong interactions of the reactant and
intermediate (vanillin alcohol) caused by the encapsulation of
ultrafine Pd within the MIL-101 cavities, which resulted in the
blocking of the reaction pathway to vanillin alcohol by steric
constraints. Without any nano-confinement effect Pd is better
accessible in case of Pd/MIL-101, which leads to weak interactions
with the reactant and thus results in the observed higher quantity of
vanillin alcohol. It is proposed that the adsorbate-surface interactions
become stronger over smaller nanoparticles and, other than catalytic
activity, dramatic changes in product selectivity occur with MNPs in
the size region of 1-10 nm.1 Therefore, other than nanoconfinement
and strong reactant-catalyst interaction effects, the Pd particle size
could also be a determining factor for such higher selectivity, as Pd
in Pd@MIL-101 has much smaller sizes (~1.8 nm) than those in
Pd/MIL-101 and the previously reported Pd/carbon based catalysts
(~5 nm). Furthermore, Pd@MIL-101 can be easily separated from
the reaction solution by simple filtration. The catalyst showed high
stability and could be reused for several cycles with complete
conversion of vanillin. After catalytic reactions, the crystallinities of
MIL-101 frameworks as well as metallic Pd state remain unchanged,
as confirmed by the PXRD and XPS patterns, respectively (Figs.
S10, S11). TEM measurements of Pd@MIL-101 after catalysis
showed no significant changes in the size and morphologies of Pd
NPs with retention of the MIL-101 framework (Fig. S12). The
ultrafine sub-2 nm sizes of the Pd stably immobilized within MIL-
101 zeotypic cavities with exposed (111) planes largely contributed
to the high catalytic activities.
In conclusion, ultrafine Pd NPs have been immobilized within the
cavities of a mesoporous MOF, MIL-101, which showed high
activity and selectivity for the hydrodeoxygenation of vanillin, a
common component in lignin-derived bio-oil, under mild reaction
conditions with water as a green solvent. The high catalytic
performance has been attributed to the nano-confinement effect and
strong interactions between reactants and confined Pd NPs. This is a
good example showing the significance of size/location of MNPs
immobilized to nanoporous materials. The present results bring light
to new opportunities in the development of high performance
heterogeneous catalysts for more selective organic transformations.
These catalysts hold promising potential for biofuel upgrade
processes, and further work will be directed toward the applications
to natural lignin and other model compounds.
Fig. 3 Reactant and products distributions with Pd@MIL-101 for
o
vanillin hydrogenolysis for 2 h reaction time at (a) 100 C (2 bar H2
pressure) and (b) different H2 pressures (75 oC).
We subsequently conducted the reaction at higher temperature and
pressure, and observed enhanced reaction rates in both cases. As
shown in Fig. 3, raising the temperature or pressure led to higher rate
of vanillin conversion to 2-methoxy-4-methylphenol without
observing any significant formation of vanillin alcohol. However, at
higher temperature some decarbonylation of the aldehyde group was
observed, leading to
a deeper hydrogenolysis product o-
methoxyphenol (guaiacol).
On the other hand, Pd/MIL-101 which has larger Pd size and is
located on the outer surfaces of MIL-101, showed the formation of
significant amounts of vanillin alcohol under similar conditions,
following the reaction path of previously reported Pd/Carbon based
catalysts. The reaction was accompanied by formation of vanillin
alcohol in initial hours (26% conversion in 3 h at 75 oC and 2 bar H2)
via hydrogenation, which was subsequently converted to 2-methoxy-
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