Synergetic catalysis of palladium nanoparticles encaged within amine-functionalized UiO-66 in the hydrodeoxygenation of vanillin in water

Article abstract of DOI:10.1039/c5gc02615f

Ultrasmall palladium nanoparticles (1.5-2.5 nm) encapsulated in metal-organic frameworks (MOFs) have been prepared by introducing a palladium precursor into a highly porous and hydrothermally stable amine-functionalized UiO-66 (NH₂-UiO-66) via a direct anionic exchange and subsequent H₂ reduction. The prepared Pd@NH₂-UiO-66 catalyst was then applied in the hydrodeoxygenation of vanillin (a typical model compound of lignin) at a low H₂ pressure in aqueous media. Excellent catalytic results (100% conversion of vanillin with exclusive selectivity for 2-methoxy-4-methylphenol) could be achieved over the developed 2.0 wt% Pd@NH₂-UiO-66 catalyst under mild conditions. Furthermore, the catalytic activity and selectivity were not affected after six reaction cycles indicating excellent stability and reproducibility of this catalyst system. It was found that the presence of free amine groups in the framework of NH₂-UiO-66 plays a key role in the formation of uniform, well-dispersed and leaching resistant palladium nanoparticles within the MOF host. Moreover, the developed Pd@NH₂-UiO-66 exhibits a novel synergetic catalysis in the hydrodeoxygenation due to the cooperation between the well-dispersed metallic Pd sites and the amine-functionalized MOF support, in which Pd offers hydrogenation activity and the MOF support facilitates hydrogenolysis of the intermediate vanillin alcohol to the 2-methoxy-4-methylphenol product.

Full text of DOI:10.1039/c5gc02615f

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Page 1 of 23
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DOI: 10.1039/C5GC02615F
Synergetic Catalysis of Palladium Nanoparticles Encaged within
Amine-functionalized UiO-66 in Hydrodeoxygenation of Vanillin in Water
ab
a
a
a
a
b
Fumin Zhang,* Shuang Zheng, Qiang Xiao, Yijun Zhong, Weidong Zhu, Andrew Lin, and
b
M. Samy El-Shall*
a
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical
Chemistry, Zhejiang Normal University, 321004 Jinhua, People’s Republic of China
b
Department of Chemistry, Virginia Commonwealth University, 23284 Richmond, VA, United States
Corresponding Authors: Tel.: +86 579 82288919; Fax: +86 579 82282234; E-mail:
zhangfumin@zjnu.edu.cn, fzhang5@vcu.edu (F. Zhang), mselshal@vcu.edu (M. Samy
El-Shall).
ABSTRACT
Ultrasmall palladium nanoparticles (1.5 - 2.5 nm) encapsulated in metal−organic frameworks (MOFs)
have been prepared by introducing a palladium precursor into a highly porous and hydrothermally
stable amine-functionalized UiO-66 (NH -UiO-66) via direct anionic exchange and subsequent H
2
2
reduction. The prepared Pd@NH -UiO-66 catalyst was then applied in the hydrodeoxygenation of
2
vanillin (a typical model compound of lignin) at low H pressure in aqueous media. Excellent
2
catalytic results (100% conversion of vanillin with exclusive selectivity for
2
-methoxy-4-methylphenol) could be achieved over the developed 2.0 wt.% Pd@NH -UiO-66
2
catalyst under mild conditions. Furthermore, the catalytic activity and selectivity were not affected
after six reaction cycles indicating excellent stability and reproducibility of this catalyst system. It
was found that the presence of free amine groups in the frameworks of NH -UiO-66 plays a key role
2
in the formation of uniform, well-dispersed and leaching resistant palladium nanoparticles within the
MOF host. Moreover, the developed Pd@NH -UiO-66 exhibits a novel synergetic catalysis in the
2
hydrodeoxygenation due to the cooperation between the well-dispersed metallic Pd sites and the
amine-functionalized MOF support, in which Pd offers hydrogenation activity and the MOF support
facilitates hydrogenolysis of the intermediate vanillin alcohol to the 2-methoxy-4-methylphenol
product.
KEYWORDS: Metal–organic frameworks; NH -UiO-66; Synergetic catalysis; Hydrodeoxygenation;
2
Vanillin
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1
. Introduction
The search for renewable and sustainable fuel sources has drawn great attention because of the
continued depletion of fossil fuels [1]. The conversion of biomass into fuels and chemical feedstocks
is an alternative to fossil fuels because of the availability and low cost of biomass [2]. Lignin, which
constitutes ~30 wt.% of woody biomass, is one of the most abundant bio-sources on earth [3].
Compared to cellulose-derived pyrolysis oil, it is more challenging to deoxygenate lignin-derived
pyrolysis oil due to its highly complex structure, which consists of oxygen-rich subunits derived
from phenol, p-coumaryl, coniferyl, and sinapyl alcohols typically connected with ether linkages [4].
Catalytic hydrodeoxygenation is considered to be the most important and feasible strategy to upgrade
the quality of biofuels [5-8]. Various heterogeneous catalysts, mainly those consisting of supported
noble metal catalysts such as Pt-Ni/γ-Al O [9], Pt-Co/γ-Al O [9], Pd/γ-Al O [10], Pd/TiO [10],
2
3
2
3
2
3
2
Pd/MgO [10], Pd/CeO [10], Pd/C [10], Pd/N-doped carbon [10], Ru/CNT [11], Au/CNTs [12],
2
Pd/carbonaceous microspheres [13], Pd/SWNT-SiO [14], and Pd/MSMF (MSMF: superhydrophilic
2
mesoporous sulfonated melamine–formaldehyde resin) [15], have been tested in the
hydrodeoxygenation of biomass compounds. In addiction, hydrodeoxygenation of anisole, a
methoxy-rich lignin model compound, has been investigated over a series of Ni-containing catalysts
supported on activated carbon, SBA-15, SiO , and γ-Al O [16], and the cleavage of lignin-derived
2
2
3
4
-O-5 aryl ethers has also been conducted over Ni nanoparticles supported on niobic acid-activated
carbon composite [17]. However, many of these catalysts exhibited low catalytic activity and poor
reusability. Therefore, the development of highly active and stable catalysts for the
hydrodeoxygenation is desirably required.
Metal–organic frameworks (MOFs) represent a new class of porous materials assembled from
inorganic nodes and organic linkers [18,19]. Due to their well-ordered crystalline structure, high
porosity, tunable pore size, and modifiable surface properties, the applications of MOFs in
heterogeneous catalysis have recently attracted tremendous attention [20,21], particularly, those
using MOFs as supports for noble metal (e.g., Pd, Au, Ru, and Pt) nanoparticles [22-36]. Despite the
many investigations of the MOFs’ role in catalysis, there are only a limited number of studies of
bifunctional metal@MOFs systems for tandem catalysis [37-43]. For example, Pan et al. prepared
Pd@MIL-101 for the one-pot synthesis of methyl isobutyl ketone from acetone [37]; Zhao et al.
2

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designed a core−shell Pd@IRMOF-3 and used it as a multifunctional catalyst for a cascade
condensation-hydrogenation reaction [40]; recently, Chen et al. developed a multifunctional
PdAg@MIL-101 for one-pot multistep cascade conversion of nitroarene to secondary arylamine
through the combination of host-guest cooperation and guest bimetallic synergy [41]. In light of
these reports, the development of MOFs-based multifunctional materials for tandem catalysis will
exert a significant impact on conventional chemical industrial processes [44,45].
The metal-organic framework UiO-66 is built up from [Zr O (OH) (CO ) ] clusters linked with
6
4
4
2 12
1
,4-benzenedicarboxylic acid [46,47]. The framework is comprised of tetrahedral and octahedral
cages, in a 2:1 ratio, with diameters close to 0.75 and 1.2 nm, respectively. Access to these cages is
provided by narrow triangular windows with a diameter close to 0.6 nm [46,47]. UiO-66 and its
amin-functionalized isostructure NH -UiO-66 show great potential for many applications due to their
2
good thermal stability, resistance toward a variety of organic solvents as well as acidic and basic
aqueous media [48-51]. Besides the above mentioned features, NH -UiO-66, as a host matrix to
2
support nanoparticle catalysts, has several unique characteristics including [27,28,43,46]: i) the large
pore volume of NH -UiO-66 is appropriate for the loading of metal precursors, ii) the amine groups
2
on terephthalic acid linkers in NH -UiO-66 could provide coordination sites for metal ions, and iii)
2
NH -UiO-66 possesses numerous potentially unsaturated metal Zr sites having Lewis acidity upon
2
activation in vacuum. The Lewis acidity of the NH -UiO-66 support has been demonstrated to play
2
significant roles in promoting the reactivity of aromatic substrates [48-51].
In this paper, we report the development of a new catalyst system consisting of Pd nanoparticles
incorporated within the NH -UiO-66 framework by using a direct anionic exchange and subsequent
2
gentle reduction approach. The developed 2.0 wt.% Pd@NH -UiO-66 catalyst shows excellent
2
catalytic properties in the hydrodeoxygenation of vanillin (a typical model compound of lignin), as a
model reaction to explore the principal hydrogenation and hydrogenolysis routes (Scheme 1), due to
the synergistic catalysis between the MOF host and the metal nanoparticles guest. The MOF
promotes transformation of the intermediate vanillin alcohol formed in situ, while Pd nanoparticles
offer hydrogenation activity, much superior to the analogue catalysts reported in the literature.
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CHO
OH
CH2OH
CH3
Hydrogenation
H2
Hydrogenolysis
H2
OCH3
OCH3
OCH3
OH
OH
A
B
C
Scheme 1. Possible reaction pathways for hydrodeoxygenation of vanillin (A) to
2
-methoxy-4-methylphenol (C).
2
2
. Experimental section
.1. Chemicals
All reagents with AR purity (analytical reagent grade) were purchased and used as received
without further purification. Deionized water with a resistance larger than 18.2 MΩ was obtained
from a Millipore Milli-Q ultrapure water purification system.
2
.2. Catalyst preparation
The synthesis of NH -UiO-66 was based on the procedure reported by Lillerud and coworkers
2
with some modification [46]. Typcially, ZrCl (0.24 g, 1.029 mmol) was dissolved in 60 mL of
4
N,N′-dimethylformamide (DMF) by sonication for 5 min. Then, the ligand 2-aminoterephthalic acid
(0.186 g, 1.029 mmol) and 0.15 mL deionized water were added into the clear solution under
vigorous stirring for 10 min at room temperature. Aftwards, the solution was transferred into a
teflon-lined stainless steel autoclave. The tightly capped autoclave was kept in an oven at 120 °C
under static conditions. After 24 h, the solutions were cooled to room temperature and the
precipitates were isolated by centrifugation. The solids were washed with 40 mL of DMF three times
and followed by washing with 40 mL of methanol three times. During each wash, the suspension was
kept at room temperature for 24 h before being centrifuged. The solvent was decanted. Finally, the
solids were dried at 150 °C under vacuum. Pure UiO-66, was synthesized by the same method as
aforementioned for the preparation of NH -UiO-66 by using terephthalic acid as the organic ligand.
2
To prepare 2.0 wt.% Pd@NH -UiO-66, 0.5 g of vacuum-dried NH -UiO-66 was first dispersed
2
2
in 30 mL of deionized water at room temperature, and treated with appropriate amount of
hydrochloric acid to tune the pH of the slurry around 4. Then 1.68 mL aqueous H PdCl (0.0552
2
4
4

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mol/L) was added dropwise to the above solution under vigorous stirring. The slurry was stirred at
room temperature for 20 h. Then the impregnated NH -UiO-66 was washed with 30 mL of water
2
three times by centrifugation and then dried at 80 °C in a vacuum desiccator for 10 h. The resulting
2
+
+
2−
dried sample, the Pd impregnated NH -UiO-66 ([NH -UiO-66] [PdCl ] ), was reduced in a 20%
2
3
2
4
H /N flow (H /N = 10/40 mL/min) at 200 °C for 4 h to obtain Pd nanoparticles confined inside the
2
2
2
2
cavities of NH -UiO-66. For controlled experiments and comparisons of catalyst’s locations, Pd
2
nanoparticles inside UiO-66, and Pd nanoparticles supported on the outer surface of UiO-66-NH2
(
the support UiO-66-NH without purification by DMF and methanol) were also prepared by the
2
same method.
2
.3. Catalyst characterization
The X-ray powder diffraction (XRD) patterns were obtained on a Philips PW3040/60
diffractometer, using CuKα radiation (λ = 0.1541 nm) in a scanning range of 5~50° at a scanning rate
of 1°/min.
N adsorption isotherms were obtained at -196 °C using a Micromeritics ASAP 2020
2
instrument. The samples were outgassed under vacuum at 150 °C for 10 h, prior to the adsorption
measurements.
The amount of the Pd in NH -UiO-66 was measured by an IRIS Intrepid II XSP inductively
2
coupled plasma-atomic emission spectrometer (ICP-AES). Because the emission of zirconium in
NH -UiO-66 is similar to that of the palladium, it is difficult to detect the palladium amount directly
2
by ICP-AES. Instead, we measured the amount of Pd that was washed away during the washing steps
and calculated the actual palladium remaining in Pd@NH -UiO-66. When loading the 2.0 wt.% Pd in
2
NH -UiO-66, the exact initial weight of the palladium precursor was recorded first. Every time after
2
washing, the washing solution that contained the palladium residue was collected. Finally, ICP-AES
was performed for the washing solution, and the palladium content was calculated. The difference
between added and removed Pd (during washing steps) is the actual palladium amount that was
introduced in NH -UiO-66. The same method was also used to determine the actual Pd amount in
2
Pd@UiO-66.
The infrared (IR) spectra were collected on a Nicolet NEXUS670 Fourier transform IR
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spectrophotometer in KBr disks at room temperature.
The surface electronic states were investigated by X-ray photoelectron spectroscopy (XPS,
Thermo VG ESCALAB250 using AlKα radiation). The XPS data were internally calibrated, fixing
the binding energy of C 1s at 284.6 eV.
Scanning electron microscopy (SEM) was performed on a Hitachi S-4800 apparatus which was
performed on a sample powder previously dried and sputter-coated with a thin layer of gold. In
addition, energy-dispersive X-ray spectroscopy (EDX) analyses were also performed to confirm the
presence and the distribution of Pd nanoparticles.
The transmission electron microscopy (TEM) observations were carried out on a JEOL
JEM-1200 working at 200 kV. The sample was diluted in ethanol to give a 1 : 5 volume ratio and
sonicated for 10 min. The ethanol slurry was then dropped onto a Cu grid covered with a thin film of
carbon.
Temperature-programmed desorption (TPD) profiles of NH from the MOF catalysts were
3
measured using a Micromeritics AutoChem II 2920 instrument. The sample cell loaded with 0.1 g of
sample was initially treated in a helium flow of 40 mL/min at 200 °C for 2 h, then cooled to 100 °C,
and subsequently exposed to pulses of ammonia until saturation and then purged with helium flow at
1
00 °C for 1 h. Afterwards, the sample cell was heated to 400 °C with a ramp of 10 °C/min. The
concentration of the desorbed ammonia was monitored continuously with a TCD detector. The total
amount of the desorbed NH for the 2.0 wt.% Pd@NH -UiO-66 catalyst was determined by a
3
2
reaction with an excess of dilute HCl solution and back titration with NaOH solution, according to
the procedure described in our previous work [54].
2
.4. Hydrodeoxygenation of vanillin
The catalytic hydrodeoxygenation of vanillin was carried out in a 50 ml Teflon-lined autoclave
equipped with stirrer, heater, and sample port. In a typical experiment, 2 mmol of vanillin, 0.05 g of
catalyst, and 20 ml water as solvent were charged into the autoclave and were repeatedly purged with
hydrogen gas at room temperature. Afterwards, the autoclave was heated at 90 ºC and then
pressurized with hydrogen up to 0.5 MPa. Finally, the agitation was initiated with a stirring speed of
1
000 rpm. A steady pressure was maintained throughout the reaction period. The mixture was
6

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extracted with ethyl acetate and the catalyst was taken out from the system by centrifugation before
being analyzed by GC (Agilent 6820) equipped with an FID detector and a capillary column (DB-5,
3
0 m × 0.45 mm × 0.42 μm). Additionally, the reaction conditions with respect to temperature, time,
hydrogen pressure, and molar ratio of the catalyst amount to vanillin were studied.
In order to test the catalytic reusability of 2.0 wt.% Pd@NH -UiO-66, after the reaction, the
2
catalyst was separated by filtration, washed with ethanol, and dried in a vacuum desiccator at 150 °C
for 5 h, and then the recovered catalyst was used in the next run reaction.
2
.5. Hydrogenation of n-hexylene and cyclooctene
The encapsulation of Pd nanoparticles in NH -UiO-66 was indirectly verified by the
2
hydrogenation of n-hexylene and cyclooctene. Typically, 0.03 g catalyst, 2.0 mmol substrate, and 20
mL ethanol were added into a stainless steel autoclave. After replacing the air in the autoclave with
o
argon, 2 MPa H was injected, and the catalytic tests were then performed at 80 C for 120 min. The
2
products were analyzed by GC (Agilent 6820) with a OV-101 capillary column.
2
.6. Adsorption Experiments
NH -UiO-66 and UiO-66 were dried at 150 °C under vacuum for 24 h before adsorption
2
experiments. Adsorption of vanillin was investigated by addition of activated MOFs (0.05 g) to the
solutions of vanillin (1.0 mmol) in water (100 ml). The mixture was stirred at 25 °C for 48 h. The
amount of adsorbed vanillin over MOFs was determined by measuring the UV-vis spectra of the
solution. The adsorption of vanillin alcohol over NH -UiO-66 and UiO-66 was performed with the
2
same procedure to that of vanillin except that vanillin was replaced by vanillin alcohol.
3
3
. Results and discussion
.1. Catalyst characterization
Pd@NH -UiO-66 was prepared by incorporating a Pd precursor into the amine-functionalized
2
metal-organic framework NH -UiO-66 via direct anionic exchange and subsequent H reduction. The
2
2
prepared catalysts were characterized by means of various techniques. As shown in the XRD patterns
in Fig. 1, the encapsulation of Pd into NH -UiO-66 does not influence the crystallinity of the parent
2
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NH -UiO-66 and characteristic XRD peaks of Pd nanoparticles are not observed, mainly due to the
2
low loading and high dispersion of the ultrasmall Pd nanoparticles inside the MOF matrix. Fig. 2
shows the N adsorption isotherms of various catalysts at -196 °C, from which the textural properties
2
were determined, and the results are summarized in Table S1. The specific surface area of
2
3
Pd@NH -UiO-66 is 769.2 m /g with a micorpore volume of 0.35 cm /g, which are lower than those
2
2
3
of unloaded NH -UiO-66 support (931.5 m /g with a micorpore volume of 0.38 cm /g). The decrease
2
in the surface area and pore volume of Pd@NH -UiO-66, in comparison with those of NH -UiO-66,
2
2
may be ascribed to the partial occupation of the cavities in NH -UiO-66 by the deposited Pd
2
nanoparticles.
d
c
b
a
5
10
15
20
25
30
35
40
45
2
Theta / degree
Fig. 1 XRD patterns of UiO-66 simulated from crystal structure data (a), the synthesized
NH -UiO-66 (b), 2.0 wt.% Pd@NH -UiO-66 (c), and the spent 2.0 wt.% Pd@NH -UiO-66 catalyst
2
2
2
after the sixth-run reaction (d).
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600
500
400
300
200
100
0
UiO-66
NH -UIO-66
2
fresh Pd@NH -UiO-66
2
used Pd@NH -UiO-66
2
0.0
0.2
0.4
P/P0
0.6
0.8
1.0
Fig. 2 N adsorption isotherms of unloaded UiO-66 and NH -UiO-66 as well as the as prepared
2
2
(
fresh) and the spent (used) Pd@NH -UiO-66 catalysts.
2
The FTIR spectrum shows that the characteristic −OH stretching frequency in the carboxylic
−
1
acid group of the NH -H BDC linker centered at 2970 cm disappears after formation of the
2
2
4
+
Pd@NH -UiO-66 catalyst, suggesting occurrence of the coordination interaction between Zr ions
2
and carboxylic acid group of NH -H BDC to form NH -UiO-66 (Fig. 3). In addition, two peaks at
2
2
2
−
1
around 3470 and 3360 cm , attributing to the asymmetric and symmetric stretching absorptions of
primary amine group, can be clearly discerned over the prepared NH -UiO-66 and Pd@NH -UiO-66
2
2
-1
-1
(
curves b and c in Fig.3). These two peaks are down shifted from 3500 cm and 3400 cm
respectively in comparison with the pure NH -H BDC (curve a in Fig.3), indicating that the amino
2
2
groups in the NH -UiO-66 are retained [40]. The presence of NH group within the framework of
2
2
NH -UiO-66 was further analyzed using N1s spectra from the XPS data (Fig. S1 in Supporting
2
Information). The N1s peak for amine group appeared at 400 eV.
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d
c
3
360 3470
b
a
2970
3500
3400
500
1000 1500 2000 2500 3000 3500
-
1
Wavenumber / cm
Fig. 3 FTIR spectra of (a) organic linker NH -H BDC; (b) NH -UiO-66; (c) the synthesized
2
2
2
Pd@NH -UiO-66 and (d) the spent 2.0 wt.% Pd@NH -UiO-66 catalyst after the sixth-run reaction.
2
2
The loading amount of Pd in (or on) Pd@NH -UiO-66 and Pd/NH -UiO-66 are 1.97 wt.% and
2
2
1
.92 wt.%, respectively, as determined by the ICP-AES analysis, very close to the nominal amount
of 2.0 wt.%. However, in Pd@UiO-66, only 0.4 wt.% Pd was detected, when we anticipated to
introduce 2.0 wt.% Pd on the MOF. The majority of Pd precursor for Pd@UiO-66 was lost during
the washing steps (probably due to the weak interaction between UiO-66 and the Pd precursor).
These results imply that the amine functional groups in NH -UiO-66 can strongly favor the loading
2
of metal nanoparticles as reported in other MOFs [42].
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a Pd@NH -UiO-66
b Zr
2
c N
d Pd
Fig. 4 EDX mapping on the same selected area of the 2.0 wt.% Pd@NH -UiO-66.
2
From the EDX mapping of a selected area on the 2.0 wt.% Pd@NH -UiO-66 catalyst,
2
homogeneous distribution of Pd, Zr and N on the same selected area was observed, suggesting that
ultrasmall Pd nanoparticles were effectively encapsulated within the NH -UiO-66 MOF (Fig. 4). The
2
dispersion was uniform, and no sign of aggregation was observed. The TEM images of the 2.0 wt.%
Pd@NH -UiO-66 catalyst, shown in Fig. 5, also confirms the uniformity of the Pd nanoparticles in
2
the matrix of NH -UiO-66. The sizes of the Pd nanoparticles were mainly within a range of 1.5 – 2.5
2
nm in diameter, which are well-dispersed (or embedded) inside the cavities of the NH -UiO-66 [32].
2
The slightly larger Pd nanoparticles size than the cage size of NH -UiO-66 is probably due to the
2
close contact between Pd nanoparticles with the Zr cluster [24]. It should be noted that the increase
6
of the cage size of NH -UiO-66 due to defects in the MOF crystals will facilitate the encapsulation of
2
the Pd nanoparticles within the cages [46,47]. Fig. 5d shows that the Pd nanoparticles are crystalline
with a spacing of 0.224 nm, corresponding to the Pd (111) interplanar spacing [31]. On the other
hand, the encapsulation of Pd nanoparticles in NH -UiO-66 can also be indirectly verified by the
2
hydrogenation of n-hexylene and cyclooctene [55], in which 2.0 wt.% Pd@NH -UiO-66 had
2
extremely high activity in the hydrogenation of n-hexylene for its linear structure making it possible
to pass through the narrow window of NH -UiO-66. At the same time, the ring reactant (cyclooctene)
2
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can barely be converted under the same conditions (Fig. S2). The results indirectly proved that most
of the Pd particles are encapsulated by NH -UiO-66.
2
Fig. 5 (a) and (b) TEM images of 2.0 wt.% Pd@NH -UiO-66. (c) High resolution image of the 2.0
2
wt.% Pd@NH -UiO-66 catalyst showing that the average size of the Pd nanoparticles is in the range
2
of 1.5 – 2.5 nm. (d) Lattice spacing of the Pd nanoparticle (0.224 nm) corresponding to the Pd (111)
interplanar spacing.
The acidities of NH -UiO-66 and 2.0 wt.% Pd@NH -UiO-66 measured by the NH -TPD are
2
2
3
shown in Fig. S3. The NH -TPD results indicate that the acidity of the NH -UiO-66 support can be
3
2
classified into three groups: weak (110–200 °C), medium (210–300 °C) and strong (330–430 °C),
very similar to the results reported by Kim et al [50]. Similarly, the 2.0 wt.% Pd@NH -UiO-66
2
catalyst also exhibits the three kinds of acidic sites, although the desorption temperatures of these
acidic sites are slightly offset to low temperatures. On the other hand, it should be noted that the
medium acidity sites of the 2.0 wt.% Pd@NH -UiO-66 catalyst seem to be much higher than those of
2
the unloaded NH -UiO-66 support. Obviously, the major acidity of the 2.0 wt.% Pd@NH -UiO-66
2
2
catalyst is originated from the accessible metal Zr sites having Lewis acidity [48-51], as well as the
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defects due to the missing of organic linkers between Zr clusters [46,47]. Furthermore, a portion of
6
the Brønsted acidity can be attributed to the protonation of the amino-functionalized links in the
MOF during the synthesis of the 2.0 wt.% Pd@NH -UiO-66, with the charge being balanced by
2
chloride anions [42,52]. The IR analysis was conducted for the sample after desorption (Fig. S4). It
has been reported that the UiO-66 series of MOFs show exceptionally high thermal stability (up to
4
50 °C in air) and strong resistance towards water, organic solvents and other chemicals [46,47].
However, compared with parent UiO-66, it should be admitted that thermal stability of NH -UiO-66
2
would decrease to some extent. Nevertheless, the total acid sites number of the 2.0 wt.%
Pd@NH -UiO-66 catalyst was measured to be 3.0 mmol/g based on the desorbed NH .
2
3
3
.2. Hydrodeoxygenation of vanillin
Considering that water is a desirable green solvent for chemical transformations for reasons of
environmental impact, safety and cost, we first conducted the catalytic hydrodeoxygenation in the
presence of water. Fig. 6 shows the evolution of the reactant and product contents as a function of
reaction time over the 2.0 wt.% Pd@NH -UiO-66 catalyst. The reaction was accompanied by a rapid
2
decrease in vanillin and increase in vanillin alcohol in the first 15 min. Later, hydrogenolysis of
vanillin alcohol proceeded rapidly.
In general, transformation of a carbonyl group into a methyl group can proceed via three
pathways: (1) hydrogenation/dehydration, (2) hydrogenation/hydrogenolysis, and (3) direct
hydrogenolysis. Since after hydrogenation vanillin alcohol has no hydrogen atom at the position
adjacent 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). Fig. 6 illustrates that vanillin is mainly hydrogenated to vanillin alcohol in the first step
then converted into 2-methoxy-4-methylphenol.
In order to demonstrate the validity of pathway (2) for the hydrodeoxygenation of vanillin over
the 2.0 wt.% Pd@NH -UiO-66 catalyst, we further investigated the effect of the reaction
2
temperatures and molar ratios of substrate and catalyst on the conversion and selectivity of vanillin
as a function of reaction time. The results are shown in Figs S5 to S8 in Supporting Information.
These results consistently prove that the hydrodeoxygenation of vanillin over the 2.0 wt.%
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Pd@NH -UiO-66 catalyst follows pathway (2). Furthermore, it should be noted that 100%
2
conversion of vanillin with exclusive selectivity for 2-methoxy-4-methylphenol could be achieved
over the 2.0 wt.% Pd@NH -UiO-66 catalyst within 75 min when the reaction was carried out at
2
9
0 °C and hydrogen pressure of 0.5 PMa, and no by-product formed even when the reaction time
was extended to 300 min (Fig. 6). In contrast, the Pd@UiO-66 catalyst exhibits a low catalytic
activity, only 49.3% vanillin was converted and the selectivity for 2-methoxy-4-methylphenol was
4
3.1% within 70 min (Fig. 7). This can be explained by the limited amount of Pd nanoparticles
encaged in the cavities of UiO-66 as determined by ICP-AES.
100
80
60
40
20
0
vanillin
vanillin alchohol
2-methoxy-4-methylphenol
0
50
100
150
200
250
300
Reaction time / min
Fig. 6 Changes in the reactant and product distributions as a function of reaction time over 2.0 wt.%
Pd@NH -UiO-66 catalyst. (Reaction conditions: vanillin, 2 mmol; water, 20 ml; amount of catalyst,
2
5
0 mg; S/C = 200; hydrogen pressure, 0.5 MPa; reaction temperature, 90 °C).
On other hand, Pd nanoparticles loaded on the outer surface of NH -UiO-66 (2.0 wt.%
2
Pd/NH -UiO-66) was also prepared. The TEM images of prepared Pd/NH -UiO-66 showed broad
2
2
distribution of Pd nanoparticles throughout the outer surfaces of NH -UiO-66 with an average size of
2
about 5.5 nm (Fig. S9), larger than the cavity size of NH -UiO-66. N adsorption measurement of
2
2
Pd/NH -UiO-66 suggested the BET surface and pore volume of the NH -UiO-66 framework were
2
2
almost remained (Table S1). However, the Pd/NH -UiO-66 catalyst showed rather low catalytic
2
14

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activity and selectivity, only 74.4% vanillin was converted and the selectivity for
2
-methoxy-4-methylphenol was 15.2% when the reaction was carried out at 90 °C for 60 min and
hydrogen pressure of 0.5 PMa (Fig. S10), much lower than the counterpart catalyst encapsulated
within the cavities of NH -UiO-66.
2
100
80
60
40
20
0
vanillin
vanillin alchohol
2-methoxy-4-methylphenol
0
60
120
180
240
300
360
420
Reaction time / min
Fig. 7 Changes of reactant and product distributions as a function of reaction time over 0.4 wt.%
Pd@UiO-66 (Reaction conditions: vanillin, 2 mmol; water, 20 ml; amount of catalyst, 50 mg; S/C =
1
000; hydrogen pressure, 0.5 MPa; reaction temperature, 90 °C).
Additionally, the pH value of the reaction solution using 2.0 wt.% Pd@NH -UiO-66 as a
2
catalyst was measured to be in the range of 3.8 to 4.1. As discussed above, The acid sites in
UiO-66-NH could be derived from the open Zr(IV) sites, the presence of defects in NH -UiO-66
2
2
crystals, and a portion of the Brønsted acidity introduced during the catalyst preparation. Whatever
the origin of the acid sites is, the presence of acid sites could explain the superior performance of the
Pd@NH -UiO-66 catalyst for the hydrodeoxygenation of vanillin. Thus, the high catalytic properties
2
of the 2.0 wt.% Pd@NH -UiO-66 catalyst in this cascade reaction under mild conditions, is probably
2
correlated to the combination of host-guest cooperation [42,43], where Pd nanoparticles as the guest
offer hydrogenation activity and the support host affords acidity as measured by NH -TPD, thus
3
promoting the hydrogenolysis. The same promotion effect of Lewis acidity was also observed by Li
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et al when they conducted the selective hydrogenation of phenol to cyclohexanone in water by using
Pd nanoparticles supported onto MIL-101 as catalysts [38].
To provide further evidence for the favorable effect of the acid sites in the NH -UiO-66 support
2
for the hydrodeoxygenation, a control experiment was performed over the 2.0 wt.%
Pd@NH -UiO-66 catalyst in the presence of a stoichiometric amount of NaOH with respect to the
2
number of acid sites available on the catalyst (the amount of acid sites determined by NH -TPD) for
3
the hydrogenolysis of vanillin alcohol (the intermediate in the hydrodeoxygenation of vanillin). As
can be seen in Fig. S11, the activity of 2.0 wt.% Pd@NH -UiO-66 almost fully vanished when the
2
acid sites were poisoned by the presence of NaOH, and this remarkable decrease in catalytic activity
is mainly due to the poisoning of the NH -UiO-66 active sites by the strong interaction of NaOH
2
with the acid sites of NH -UiO-66 [53,54]. This result demonstrates the significant importance of
2
acid sites on the catalytic hydrodeoxygenation.
1
1
20
00
2-methoxy-4-methylphenol
Vanillin alcohol
Vanillin
8
6
4
2
0
0
0
0
0
0
1
2
3
4
5
6
Run
Fig. 8 Reusability of 2.0 wt.% Pd@NH -UiO-66 in the catalytic hydrodeoxygenation of vanillin
2
(Reaction conditions: vanillin, 2 mmol; water, 20 ml; amount of catalyst, 50 mg; hydrogen pressure,
S/C = 200; 0.5 MPa; reaction temperature, 80 °C; reaction time, 60 min).
More importantly, the 2.0 wt.% Pd@NH -UiO-66 catalyst exhibits excellent recyclability (Fig.
2
8
). After being recycled for six times, the catalytic activity and selectivity could be well preserved.
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2
DOI: 10.1039/C5GC02615F
The BET surface area of the spent Pd@NH -UiO-66 catalyst decreased from 769.2 to 734.7 m /g
2
(Fig.2 and Table S1), which may be due to the presence of residual reactants or products trapped
inside the NH -UiO-66 support. In addition, we measured the XRD pattern of the spent catalysts and
2
found the crystalline structure of the NH -UiO-66 was still maintained after the sixth cycle (pattern d
2
in Fig. 1). The corresponding FTIR spectra for the spent catalyst is almost the same as those for the
fresh one (curve d in Fig.4). TEM image of used 2.0 wt.% Pd@NH -UiO-66 catalyst showed only
2
slight aggregation of Pd nanoparticles (Fig. S12), which does not affect the activity of the catalysts
within six repeated cycles. Moreover, the concentration of Pd in the reaction solution was negligible
based on the ICP-AES analysis, implying that no Pd leaching occurred during the catalyst repeated
reuse.
Moreover, the NH -UiO-66 support showed higher adsorption uptake of the substrate and the
2
intermediate than those of UiO-66 (Fig. S13), although the former MOF possesses lower BET
specific surface area and pore volume than the latter. It is suggested that the adsorptions of vanillin
or vanillin alcohol over amine-functionalized MOFs are enhanced by the hydrophilic nature of
substrate molecule as well as the interactions between the substrate molecule and
amine-functionalized MOF support [42].
Based on the above discussion, we propose a possible reaction mechanism to illustrate the
unique function of the Pd@NH -UiO-66 catalyst and demonstrate the reaction sequence for the
2
hydrodeoxygenation of vanillin. In the initial stage, H is presumably activated by the palladium
2
nanoparticles stabilized within the framework of NH -UiO-66 [42]. The aldehyde group in vanillin is
2
therefore hydrogenated to the hydroxyl group, and accordingly vanillin is converted into vanillin
alcohol. Afterwards, the in situ obtained vanillin alcohol is further converted to
2
-methoxy-4-methylphenol promoted by acidic sites highly dispersed within the support framework
as demonstrated by our controlled experiment.
3
.3. Catalytic activity comparison
Furthermore, we compared the catalytic activity and selectivity over the developed 2.0 wt.%
Pd@NH -UiO-66 catalyst with those reported in the literatures (Table 2). As can been see, a 100%
2
conversion of vanillin with 100% selectivity for 2-methoxy-4-methylphenol could be obtained over
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2
2
.0 wt.% Pd@NH -UiO-66 within 60 min (entry 1 in Table 2). The selectivity for
2
-methoxy-4-methylphenol over the prepared 2.0 wt.% Pd@NH -UiO-66 is much higher than the
2
result reported by Xiao et al over 4.5 wt.% Pd/MSMF under the same reaction conditions with a
similar conversion of vanillin [15] (entry 2), as well as higher than 2.0 wt.% Pd/SO H-MIL-101 [56]
3
(
entry 3). Additionally, 2.0 wt.% Pd@NH -UiO-66 is also superior to the N-doped carbon-supported
2
Pd catalyst [14] (entries 4 and 5) and 2.0 wt.% Pd@MIL-101 [57] (entries 6 and 7). On the other
hand, Resasco and co-workers found that an 85% conversion of vanillin within 0.5 h with 47%
selectivity (entry 8) for 2-methoxy-4-methylphenol could be achieved in the hydrodeoxygenation of
vanillin by depositing palladium onto SWNT-inorganic oxide hybrid (Pd/SWNT-SiO ) in a water/oil
2
system [13]. In contrast, our 2.0 wt.% Pd@NH -UiO-66 showed remarkably high catalytic activity
2
and selectivity for 2-methoxy-4-methylphenol under the same conditions (entry 9). The superior
performance for Pd@NH -UiO-66 is mainly due to the cooperation between the well-dispersed
2
metallic Pd sites and the amine-functionalized MOFs promoted by the acidity highly distributed
throughout the framework of NH -UiO-66.
2
Table 2. Hydrodexygenation of vanillin over different catalysts
a
Conversion
%)
Selectivity (%)
Entry
Catalyst
(
B
C
b
1
2.0 wt.% Pd@NH -UiO-66
100
> 99.5
96.1
99.1
65
0
45.8
9.1
21.4
31
100
54.2
90.9
78.6
69
2
b
2
4.5 wt.% Pd/MSMF [15]
b
3
2.0 wt.% Pd/SO H-MIL-101 [56]
3
c
4
2.0 wt.% Pd@NH -UiO-66
2
c
5
Pd/CN0.132 [14]
d
6
2.0 wt.% Pd@NH -UiO-66
100
50
0
100
98
2
d
7
2.0 wt.% Pd@MIL-101 [57]
2
e
8
5.0 wt.% Pd/SWNT-SiO [13]
85
53
47
2
e
9
2.0 wt.% Pd@NH -UiO-66
100
24.8
75.2
2
a
B is the hydrogenation product, vanillin alcohol, and C is the hydrogenolysis product,
-methoxy-4-methylphenol (see Scheme 1).
Reaction conditions: water, 20 ml; S/C (molar ratio of substrate to catalyst) = 200; 0.5 MPa;
reaction temperature, 100 °C; reaction time, 60 min.
2
b
c
Reaction conditions: water, 80 ml; S/C = 350; hydrogen pressure, 1.0 MPa; reaction temperature,
9
0 °C; reaction time, 60 min.
d
Reaction conditions: water, 5 ml; S/C = 35; hydrogen pressure, 0.2 MPa; reaction temperature,
18

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7
5 °C; reaction time, 60 min.
e
Reaction conditions: 1:1 (v/v) water/decalin, 20 ml; S/C = 100; hydrogen pressure, 0.35 MPa;
reaction temperature, 100 °C; reaction time, 30 min.
4
. Conclusions
In summary, we have developed an efficient heterogeneous catalyst consist of ultrasmall
palladium nanoparticles immobilized on amine-functionalized metal−organic frameworks (2.0 wt.%
Pd@NH -UiO-66) by using a direct anionic exchange and subsequent gentle reduction approach.
2
The presence of free amine moieties in the frameworks of Pd@NH -UiO-66 is assumed to play a key
2
role on the formation of uniform, well dispersed and leaching resistant palladium nanoparticles on
the support. The prepared Pd@NH -UiO-66 catalyst exhibits excellent catalytic properties in the
2
hydrodeoxygenation of vanillin in aqueous media. The superior performance for Pd@NH -UiO-66 is
2
due to the cooperation between the metallic Pd sites and the amine-functionalized MOFs. The
research thus highlights new perspectives for amine-functionalized MOF material for biomass
transformation.
Acknowledgements
This work was supported by National Natural Science Foundation of China (21576243, 21471131),
and Zhejiang Provincial Natural Science Foundation of China (LY13B030002).
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Graphical Abstract
Synergetic Catalysis of Palladium Nanoparticles Encaged within
Amine-functionalized UiO-66 in Hydrodeoxygenation of Vanillin in Water
Vanillin
Vanillin alcohol
CHO
CH OH
2
H2
OCH3
OH
OCH3
OH
H2
CH3
Pd
OCH3
OH
-Methoxy-4-methylphenol
2