Pd/UiO-66(Hf): A highly efficient heterogeneous catalyst for the hydrogenation of 2,3,5-trimethylbenzoquinone

Article abstract of DOI:10.1016/j.catcom.2018.05.010

The conversion of 2,3,5-trimethylbenzoquinone (TMBQ) to 2,3,5-trimethylhydroquinone is regarded as one of the most important reactions for the production of vitamin E. Here, we report on a Hf-based metal–organic framework (MOF), UiO-66(Hf), with supported palladium nanoparticles as a highly active, selective, and recyclable catalyst for the direct hydrogenation of TMBQ with H₂ under mild reaction conditions. The developed Pd/UiO-66(Hf) catalyst exhibited a marked increase in activity compared with its Pd/UiO-66(Zr) counterpart and conventional Pd/C catalysts. Its excellent catalytic performance is ascribed to the large number of acid sites in the UiO-66(Hf) framework, which promote hydrogenation.

Full text of DOI:10.1016/j.catcom.2018.05.010

Accepted Manuscript
Pd/UiO-66(Hf): A highly efficient heterogeneous catalyst for the
hydrogenation of 2,3,5-trimethylbenzoquinone
Xiaoxiong Cai, Jianping Pan, Gaomei Tu, Yanghe Fu, Fumin
Zhang, Weidong Zhu
PII:
S1566-7367(18)30192-4
doi:10.1016/j.catcom.2018.05.010
CATCOM 5404
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
18 January 2018
18 May 2018
19 May 2018
Please cite this article as: Xiaoxiong Cai, Jianping Pan, Gaomei Tu, Yanghe Fu, Fumin
Zhang, Weidong Zhu , Pd/UiO-66(Hf): A highly efficient heterogeneous catalyst for the
hydrogenation of 2,3,5-trimethylbenzoquinone. The address for the corresponding author
was captured as affiliation for all authors. Please check if appropriate. Catcom(2017),
doi:10.1016/j.catcom.2018.05.010
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ACCEPTED MANUSCRIPT
Pd/UiO-66(Hf): A highly efficient heterogeneous catalyst for the hydrogenation of
2
,3,5-trimethylbenzoquinone
Xiaoxiong Cai, Jianping Pan, Gaomei Tu, Yanghe Fu, Fumin Zhang* and Weidong Zhu*
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
Corresponding Authors: E-mail: zhangfumin@zjnu.edu.cn; weidongzhu@zjnu.cn

ACCEPTED MANUSCRIPT
ABSTRACT
The conversion of 2,3,5-trimethylbenzoquinone (TMBQ) to 2,3,5-trimethylhydroquinone is
regarded as one of the most important reactions for the production of vitamin E. Here, we
report on a Hf-based metal–organic framework (MOF), UiO-66(Hf), with supported
palladium nanoparticles as a highly active, selective, and recyclable catalyst for the direct
hydrogenation of TMBQ with H2 under mild reaction conditions. The developed
Pd/UiO-66(Hf) catalyst exhibited a marked increase in activity compared with its
Pd/UiO-66(Zr) counterpart and conventional Pd/C catalysts. Its excellent catalytic
performance is ascribed to the large number of acid sites in the UiO-66(Hf) framework,
which promote hydrogenation.
Keywords: Metal–organic framework; UiO-66(Hf); 2,3,5-Trimethylbenzoquinone; Pd
nanoparticle; Catalytic hydrogenation

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1
. Introduction
Vitamin E (VE) is an organic compound with biological activity that is indispensable
to humans and animals [1,2]. In recent years, the market demand for VE has increased
considerably because of its beneficia l biological and chemical functionality.
2
,3,5-Trimethylhydroquinone (TMHQ) is an important intermediate in the production of
VE, as condensation of TMHQ with isophytol yields VE [3]. The quality and price of VE
vary depending on those of the TMHQ used as feedstock. Conventionally, TMHQ is
obtained via the reduction of 2,3,5-trimethylbenzoquinone (TMBQ) using a chemical
reductant such as sulfur dioxide and sodium hydrosulfite. Unfortunately, this reduction
method is hindered by several deficiencies, including the use of expensive reductants,
relatively low TMHQ yield, and the presence of by-products, which can be difficult to
remove from the final product [3]. An alternative strategy involves hydrogenation of
TMBQ using molecular hydrogen as the H-donor in the presence of a Pd catalyst, as
shown in Scheme S1 in the Supplementary Information (SI) [4–7]. This strategy has
yielded good results [4,5,7]. From the viewpoint of basic research, developing highly
active and stable catalysts for the hydrogenation of TMBQ remains an active, albeit
challenging, pursuit.
Metal–organic frameworks (MOFs) constructed via the coordination of metal
cations/clusters with organic bridging linkers have emerged as an incredibly useful type
of crystalline porous material. MOFs have been intensively investigated in
heterogeneous catalysis due to their well-defined structures, tunable pores, and
convenient post modification [8–10]. Additionally, MOFs with three-dimensional

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structures are appropriate for the encapsulation of ultrasmall metal nanoparticles (NPs),
minimizing NP aggregation [11–13]. Furthermore, both the cavities as well as channels
of MOFs can control the size of NPs and simultaneously ensure accessibility for
reactants to the catalytically active sites [11–13]. In this context, using MOFs as supports
for loading noble metal (e.g., Pt, Pd, Au, and Ru) NPs has been widely studied as an
effective strategy for catalysis for a variety of reactions [11–20].
Among the various MOFs, the Zr-based UiO-66 (UiO stands for University of Oslo)
has received significant attention from both academia and industry due to its facile
synthesis and high thermal and chemical stability [21]. UiO-66 is a three-dimensional
MOF constructed from Zr6O4(OH)4 oxoclusters and the rigid 1,4-benzenedicarboxylic
acid ligand. The framework of UiO-66 is composed of tetrahedral and octahedral cavities
in a 2:1 ratio with diameters close to 0.75 and 1.2 nm, respectively, and the cavities are
interconnected by narrow triangular windows with a diameter close to 0.6 nm [21]. Hf-
and Zr-based MOFs should share similar physical properties because both Hf and Zr
belong to the same elemental group. In fact, UiO-66(Hf) possesses thermal and chemical
stability as high as that of UiO-66(Zr), as well as the same topological structure [22,23].
−1
Moreover, because the bond dissociation energy of Hf−O (802 kJ mol ) is higher than
−1
that of Zr−O (776 kJ mol ) [22], Hf would be more oxophilic, and the μ3-OH groups in
the Hf-containing secondary building units of UiO-66(Hf) can possibly function as
Brønsted acid sites, which are stronger than those in UiO-66(Zr) [24,25]. This
fascinating feature renders UiO-66(Hf) more intriguing properties as a heterogeneous
catalyst in many types of chemical reactions [24–31].

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To the best of our knowledge, there is no report on the use of UiO-66(Hf) as a host
to support metal NPs as a solid catalyst. In the present work, we prepared a
Pd/UiO-66(Hf) catalyst via a facile wetness impregnation method and subsequent
sodium borohydride reduction. The resultant Pd/UiO-66(Hf) catalyst exhibited high
catalytic activity and exclusive selectivity in the liquid-phase hydrogenation of TMBQ to
TMHQ under mild reaction conditions, with its activity surpassing that of its
Pd/UiO-66(Zr) counterpart as well as commercially available Pd/C catalyst.
2
. Experimental
2
.1. Catalyst Preparation
All catalysts were prepared via a wet impregnation method. Detailed information
regarding the chemicals, methods, and catalysts preparation can be found in the SI.
2
.2. Hydrogenation of TMBQ
The hydrogenation reaction was performed in a high-pressure Teflon-lined autoclave (50
mL) equipped with a pressure gauge and a magnetic stirrer. Typically, TMBQ (1.5 g),
catalyst (20 mg, 0.038 mol% of Pd), and i-propanol (20 mL) as solvent were charged into the
autoclave and repeatedly (6 times) purged with H2 at 25 °C. The autoclave was then heated
quickly to a given reaction temperature (45 °C), after which hydrogen was introduced and a
constant pressure (0.4 MPa) was maintained in the autoclave. Finally, agitation was initiated
with a stirring speed of 1080 rpm. A steady pressure (0.4 MPa) was maintained throughout
the reaction period. Aliquots were withdrawn at regular time intervals and analyzed by gas
chromatography. The recyclability of the 2.0% Pd/UiO-66(Hf) catalyst was tested after its
separation from the reaction system by centrifugation, rinsing with a large quantity of ethanol,

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and drying in a vacuum desiccator at 120 °C for 4 h. More information on the reactor, and
how the aliquots were withdrawn can be found in SI.
3
. Results and discussion
The physicochemical properties of the prepared Pd catalysts were measured and
compared. As displayed in the powder X-ray diffraction (XRD) patterns shown in Fig.
A, no significant loss of crystallinity was observed for the Pd/UiO-66(Hf) catalyst
1
compared with the pristine UiO-66(Hf) support, indicating that the integrity of the
UiO-66(Hf) framework was maintained after immobilizing the Pd NPs via the facile
wetness impregnation method applied in the current work [23]. In addition, no diffraction
peaks attributable to palladium species were observed, possibly because of the low Pd
content and/or the small-sized nature of the Pd NPs within the Pd/UiO-66(Hf)
framework. It should be pointed out that the peak at ca. 13–15 degree for the prepared
UiO-66(Hf) and Pd/UiO-66(Hf) is a characteristic peak of UiO-66, as demonstrated by
the local magnification pattern (see the embedded figure in Fig. 1A).
Furthermore, a substantial decrease in the adsorbed quantity of N2 was observed for
Pd/UiO-66(Hf) relative to the parent UiO-66(Hf) framework, implying that the cavities
of UiO-66(Hf) were partially blocked by the dispersed Pd NPs (Fig. 1B). Moreover, a
transmission electron microscopy (TEM) image of the 2.0% Pd/UiO-66(Hf) catalyst
confirmed that the Pd NPs were highly distributed within the UiO-66(Hf) framework
(Fig. 1C). On the basis of counting more than 100 NPs, the sizes of the Pd NPs were
mostly in the approximate diameter range of 1.0–3.0 nm (Fig. 1D), which is conducive to
stabilize (or become embedded in) the inner cavities of UiO-66(Hf). However, because

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most of the observed Pd NPs were larger than the cavity diameters of the MOF, it should
be noted that some of the Pd NPs were deposited on the outer surfaces of the UiO-66
crystals in the prepared Pd/UiO-66 catalyst. As a consequence, catalysis most likely
occurs on the surface of the prepared MOF-based catalyst. High-resolution TEM clearly
shows that the spacing between the lattice fringes is 0.224 nm, which corresponds to the
(111) planes of face-centered cubic Pd (Fig. S3).
Fig. 1 (A) XRD patterns and (B) N2 adsorption isotherms of the UiO-66(Hf) support and the
prepared Pd/UiO-66(Hf) catalyst; (C) TEM image of the prepared Pd/UiO-66(Hf) catalyst
and (D) the corresponding size distribution plot of Pd NPs.
A scanning electron microscopy (SEM) image of the Pd/UiO66(Hf) indicated that
the crystal sizes of the catalyst were roughly in the range of 100 nm to 250 nm (Fig. S4).
To explore this system further, elemental mapping was performed in a selected area on

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2
.0% Pd/UiO-66(Hf). As seen in Fig. S5, large numbers of ultrasmall Pd NPs, as well as
Hf and O in the same selected area, were evenly distributed within the UiO-66(Hf)
framework. Further analysis of Pd NPs within the MOF crystals requires other
sophisticated techniques. Based on the above EDS characterization, it should be noted
that this analysis is most likely mapping the outer layers of the crystal, which agrees well
with the location of the Pd NPs on the surface of the MOFs.
A Fourier transform infrared (FTIR) spectrum of the synthesized UiO-66(Hf)
support is shown in Fig. S6, with the main infrared bands matching previously reported
−1
values rather well. Representative vibrational peaks at 1580 and 1400 cm ,
−1
corresponding to the carboxylate groups, and peaks at 750 and 670 cm , corresponding
to Hf-(μ3)O, could be observed in the FTIR spectrum [20]. No peak shift was observed
after impregnation of the Pd NPs into the support, suggesting that the UiO-66(Hf)
structure was well preserved for the supported catalyst. Thermogravimetric analysis
(
TGA) demonstrated the integrity of the UiO-66(Hf) framework after loading the Pd
NPs; thermal stability up to 500 °C for Pd/UiO-66(Hf) was achieved (Fig. S7), which is
very close to that of the parent support [23].
0
The presence of Pd in the catalyst was confirmed by X-ray photoelectron
spectroscopy (XPS) (Fig. S8B), as clearly demonstrated by the presence of 3d bands at
ca. 335.8 and 340.9 eV, which is characteristic of zero-valent Pd species [7,18]. No peak
associated with PdO species was observed at a binding energy of approximately 342.4
2
+
eV [7], demonstrating that the Pd precursors were completely reduced by NaBH4 in the
Pd/UiO-66(Hf) catalyst. In addition, Pd/UiO-66(Hf) and the control catalysts contained a

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similar Pd loading (ca. 2.0% in weight), which is very close to the nominal amount
calculated from the added metal precursors to the MOF, as determined by inductively
coupled plasma-atomic emission spectrometry (ICP-AES) (Table S1 in the SI). This
2
+
result indicates that the Pd cations were fully incorporated into the MOF framework
during the impregnation procedure and the subsequent reduction process did not cause
loss of Pd species.
The catalytic activity of 2.0% Pd/UiO-66(Hf) with several control samples were
compared (Fig. 2). Under the evaluated conditions, the selectivity for TMHQ was 100% over
all the investigated catalysts, and no by-products were detected by gas chromatography.
Apparently, no reaction occurred in the presence of pure UiO-66(Hf), suggesting that metal
sites are indispensable to hydrogenate TMBQ. Among the various catalysts investigated,
2
.0% Pd/UiO-66(Zr) exhibited the lowest catalytic activity (curve b in Fig. 2A), while 2.0%
Pd/UiO-66(Hf) showed a relative higher conversion rate (curve f). The calculated TOF value
of 2.0% Pd/UiO-66(Hf) was as high as 135 (Fig. 2B), which is much higher than that of 2.0%
Pd/UiO-66(Zr), 2.0% Pd/MIL-101(Cr), and even better than the 5.0% Pd/C catalyst (102).
The textural properties of the various catalysts were compared (Table S1 and Figs. S9–S11 in
the SI), among which the specific surface area and pore volume of Pd/UiO-66(Hf) were the
lowest. Additionally, the dispersion of the Pd NPs on the control supports was also examined
by TEM (Figs. S12 and S13); no significant differences in the Pd NP distribution were
detected for the MOF-supported Pd catalysts. For 5.0% Pd/C (Figs. S14 and S15), the
relatively slightly larger size of the Pd NPs on the activated carbon surface is partially
ascribed to its relatively high Pd loading. For comparison, we also tested the activity of a

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physical mixture of 5.0% Pd/C + UiO-66(Hf); this mixture gave a comparably low TOF value
(125) relative to that over 2.0% Pd/UiO-66(Hf). It is plausibly that a combination of Pd NPs
size, distribution, and appropriate amount of acidity within MOF what makes Pd/UiO-66(Hf)
slightly better than the physical mixture and Pd/C.
150
(A)
(B)
1
35
5
100
125
4
(
f)
e)
d)
c)
120
80
60
40
20
0
(
102
4
4
(
9
0
0
(
64
6
36
3
(
b)
30
0
(
a)
0
0
20
40
60
80 100
(a) (b) (c) (d) (e) (f)
Catalyst
Reaction time (min)
Fig. 2 (A) Comparison of TMBQ vs reaction time over (a) UiO-66(Hf), (b) 2.0%
Pd/UiO-66(Zr), (c) 2.0% Pd/MIl-101(Cr), (d) 5.0% Pd/C, (e) physical mixture of 5.0% Pd/C
and UiO-66(Hf), and (f) 2.0% Pd/UiO-66(Hf). (B) Corresponding TOFs calculated in the
range of initial kinetic behavior. In the case (d), 8.0 mg of 5.0% Pd/C was used; in the case
(e), 8.0 mg of 5.0% Pd/C and 20 mg of UiO-66(Hf) were used.
It was reported that the presence of appropriate acidic sites in the catalysts could favor
the adsorption and activation of TMBQ due to the interaction between the aromatic substrate
and these sites [17–19], thereby boosting the catalytic activity. To clarify the different
performances of 2.0% Pd/UiO-66(Hf) and the control catalysts, the acid concentrations in
UiO-66(Hf) and UiO-66(Zr) were measured using acid-base titrations [25]. It was found that

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−1
UiO-66(Zr) exhibited a mild acidity of 0.01 mol mol (i.e., moles of acidic proton per mole
of MOF based on the molecular formula). In contrast, UiO-66(Hf) possessed 5 times more
−1
acidic protons (0.06 mol mol ), which strongly suggests that its higher acidity is derived
from its μ3-OH groups [31]. Regarding MIL-101(Cr), its acidity concentration is theoretically
–
6
−1
less than 3×10 mol mol [19], which is exceedingly small relative to that of UiO-66(Zr)
and UiO-66(Hf), and it may therefore be ignored. Thus, besides the highly dispersed Pd NPs
within the MOF framework, whose function is to adsorb and activate hydrogen molecules,
the pronounced high activity of 2.0% Pd/UiO-66(Hf) for TMBQ hydrogenation is likely
ascribed to the higher acid concentration in the UiO-66(Hf) support [7]. As to the slightly
lower activity of the physical mixture catalyst (i.e., 5.0% Pd/C + UiO-66(Hf)) relative to
2
.0% Pd/UiO-66(Hf), this is likely caused by the close proximity of metal/acidity sites. This
close proximity means that less diffusion is required before catalysis can proceed,
demonstrating the advantage of integrating acid sites within the Pd/UiO-66(Hf) catalyst.
The influences of temperature, hydrogen pressure, and substrate concentration on the
conversion of 2.0% Pd/UiO-66(Hf) were also screened (Figs. S16–S18). A plot of
–
ln(1–XTMBQ) versus reaction time (Fig. S18B) gave a distinct linear relationship between the
initial concentration of TMBQ and reaction time, suggesting that the reaction possesses
first-order dependence on TMBQ [4]. Additionally, a plot of ln (TOF) versus the inverse of
the reaction temperature gave the apparent activation energy (Figs. S19 and S20), yielding a
–
1
Ea of 31.1 kJ mol for 2.0% Pd/UiO-66(Hf). The activation energy measured in this work is
lower than the values reported using Raney nickel, Pd/MIL-101, and Pd@NH2-UiO-66(Zr) as
catalysts (Table S2).

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Recyclability of a solid catalyst is prerequisite for its practical application. To this end,
we also performed cycle tests of 2.0% Pd/UiO-66(Hf) for the hydrogenation of TMBQ. As
shown in Fig. 3, the catalytic activities were essentially unchanged during five successive
cycles for reaction times of either 30 or 45 min, demonstrating the excellent reusability of
2
.0% Pd/UiO-66(Hf). In addition, the XRD pattern, N2 adsorption isotherm, XPS, FTIR
spectrum, TGA curve, and elemental mapping of the spent 2.0% Pd/UiO-66(Hf) catalyst were
almost identical with those of the fresh one (Figs. S21–S26). Notably, no Pd species were
detected in the reaction liquor by ICP-AES analysis, thus further confirming that the
Pd/UiO-66(Hf) catalyst possesses excellent stability under the current catalytic conditions.
3
4
0 min reaction
5 min reaction
100
80
60
40
20
0
1
2
3
4
5
Run
Fig. 3 Recycling performance of 2.0% Pd/UiO-66(Hf) in the catalytic hydrogenation of
TMBQ.
4
. Conclusions
We demonstrate that Pd/UiO-66(Hf) is a highly efficient catalyst for the hydrogenation of
TMBQ to TMHQ under mild reaction conditions. The excellent performance of 2.0%

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Pd/UiO-66(Hf) is attributed to the cooperation between the highly dispersed Pd nanoparticles
within the MOF and an appropriate amount of available acid sites in UiO-66(Hf). The
function of the Pd nanoparticles is to adsorb and activate the hydrogen molecules, whereas
the acid sites likely favor the activation of TMBQ. This work is believed to be important for
the development of high-performance solid catalysts for hydrogenation reactions.
Acknowledgments
This work was supported by the National Natural Science Foundation of China
(21576243) and the Natural Science Foundation of Zhejiang Province (LY18B060006,
LY18B030006).

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Graphical Abstract

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Highlights

A catalyst that combined UiO-66(Hf) and Pd NPs is prepared by wet impregnation and
reduction


Pd/UiO-66(Hf) exhibits a high performance in hydrogenation of TMBQ to TMHQ
The acidity in UiO-66(Hf) improves the catalytic performance of Pd/UiO-66(Hf)