R. Fang, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
on the internal surface, but would collapse during the reaction pro-
cesses, far from meeting the regulations of long-term applications. In
these regards, hierarchical porous materials (such as hierarchical MOFs,
denoted as H-MOFs), taking both advantages of microporous and me-
soporous materials, are believed to be promising candidates [25–30].
However, seldom attention has been paid on the synthesis of highly
active and durable MOFs-based catalysts for liquid-phase biomass-re-
lated conversions.
Herein, we demonstrate the synthesis of a series of transition-metal
catalysts supported on a hierarchical porous MOF (i.e., H-UiO-66) for
highly efficient transformation of FFA to FOL. Benefitting from the
uniformly distributed Pd nanoparticles (NPs) and unique hierarchical
porosity of the H-UiO-66 support, the as-prepared Pd/H-UiO-66 cata-
lysts achieved a complete conversion of FFA with > 99 % FOL se-
lectivity under 60 °C and 0.5 MPa H2. The TOF value was as high as
66.7 h−1, outperforming the previously reported supported metal cat-
alysts. More importantly, the integrality of the catalyst retained well
even after up to ten runs, and no obvious activity loss was observed.
Additionally, the effects of various reaction parameters (including
metal loadings, reaction temperatures, H2 pressures and solvents) on
the catalytic performances were also investigated.
of H2. The Pd contents in the samples were measured quantitatively by
atomic absorption spectroscopy (AAS) on a HITACHI Z-2300 instru-
ment. The size and morphology of materials were studied by scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM). SEM was carried out on a Zeiss Merlin instrument. TEM and
high-angle annular dark-field scanning transmission electron micro-
scopy (HAADF-STEM) were recorded on a JEOL JEM-2010 F instrument
equipped with EDX analysis (Bruker XFlash 5030 T) operated at 200 kV.
XPS measurements were performed in a ultra-high vacuum (UHV)
multipurpose surface analysis system (SpecsTM model Germany) op-
erating at pressures < 10−10 mbar using a conventional X-ray source
(XR-50, Specs, Mg Kalpha, 1253.6 eV) in a “stop-and-go” mode to re-
duce potential damage due to sample irradiation. Binding energies were
referenced to the C1s line at 284.6 eV from adventitious carbon.
Deconvolution curves for the XPS spectra were obtained using software
supplied by the spectrometer manufacturer. ATR-IR measurements
were carried out on a Thermo FisheriS10 equipped with a liquid ni-
trogen cooled MCT detector. The spectra were obtained by averaging
32 scans at the resolution of 1 cm−1. The thin film of the catalyst
powder deposited on the ZnSe element for the ATR-IR spectroscopic
study was prepared as follows. A suspension of ca. 132 mg of the cat-
alyst powder and 5 mmol of FFA in 5 mL H2O was stirred overnight to
eliminate any agglomeration, after which, the slurry was dropped onto
the ZnSe internal reflection element (IRE) for recording the spectra. The
IR spectrum of the pure catalyst in the solvent was acquired as the
background.
2. Experimental
2.1. Materials
All reagents were of analytical grade and were used without further
purification.
2.3. General procedures for FFA hydrogenation
2.1.1. Synthesize of UiO-66
In a typical run, FFA (5.0 mmol) and catalyst (metal 0.3 mol%) were
added to H2O (5.0 mL) in an autoclave equipped with magnetic stirrer.
After purging the reactor several times with H2, the reactor was heated
to 60 °C and subsequently purged with 0.5 MPa H2 with a stirring speed
of 600 rpm. During the reaction, samples were taken and analyzed by
GCMS (7890 GC/5975C MS) equipped with a HP-innowax capillary
column (30 m ×0.25 mm). The first sample was taken when the H2
pressure was purged to 0.5 MPa. After reaction, the reactor was cooled
to room temperature and the catalyst was isolated from the solution by
centrifugation, washed with methanol and reused directly. The con-
version and selectivity were evaluated on the basis of the amounts of
furfural. The conversion of furfural (mol%), products yield (mol%) and
furfuryl alcohol selectivity (mol%) were calculated using the following
equations:
UiO-66 was synthesized through a solvothermal strategy. In a ty-
pical synthesis, ZrCl4 (133.6 mg), terephthalic acid (100.0 mg), con-
centrated hydrochloric acid (360 μL) and N,N-dimethylformamide
(DMF) (40 mL) were sealed and heated at 120 °C for 24 h. The resultant
powder was obtained after washing with DMF and drying under va-
cuum at 50 °C overnight.
2.1.2. Synthesis of H-UiO-66
The H-UiO-66 was synthesized following the above procedures with
some modifications. Typically, ZrCl4 (133.6 mg), terephthalic acid
(50.0 mg), benzoic acid (36.8 mg), N,N-dimethylformamide (DMF)
(40 mL) and concentrated hydrochloric acid (360 μL) were sealed and
heated at 120 °C for 20 h. The resultant powder was obtained after
washing with DMF and drying under vacuum at 50 °C overnight.
Furfural conversion = (1- nfurfural/nfurfural loaded) × 100 %
2.1.3. Synthesis of supported M/UiO-66 and M/H-UiO-66 (M = Pd, Pt, Au
and Cu)
Moles of product
Moles of furfural converted
Product selectivity =
nproduct/nfurfural converted
The catalysts were synthesized through the same impregnation
method: a certain amount of MClx salts and MOFs (UiO-66 and H-UiO-
66) (300 mg) were dissolved in 3 mL H2O and stirred for 24 h at room
temperature. Then, the resulting powder was isolated and heated at
250 °C for 2 h under H2 flow (N2 was utilized in advance to remove the
residual air). The as-prepared materials were denoted as M/UiO-66 and
M/H-UiO-66 (M = Pd, Pt, Au and Cu).
× 100%
Moles of cyclopentanone
Furfuryl alcohol yield =
nfurfuryl alcohol/nfurfural loaded
Moles of furfural loaded
× 100%
3. Results and discussion
2.2. Catalyst characterization
3.1. Synthesis and characterization of M/H-UiO-66
Powder X-ray diffraction (PXRD) patterns of the samples were ob-
tained on a Rigaku diffractometer (D/MAX-IIIA, 3 kW) using Cu Kα
radiation (40 kV, 30 mA, λ =0.1543 nm). BET surface areas and pore
size measurements were performed with N2 adsorption/desorption
isotherms at 77 K on a Micromeritics ASAP 2020 M instrument. Before
measurements, samples were degassed at 100 °C for 12 h. The metal
dispersion was characterized by H2 chemisorption experiments. Two
isotherms were measured at 77 K with the intermediary evacuation
treatment, the difference of which gave the irreversible chemisorption
The highly stable UiO-66 and its hierarchically porous derivative H-
UiO-66 employed in this work were synthesized through a solvothermal
strategy [31]. The supported M/UiO-66 and M/H-UiO-66 (M = Pd, Pt,
Au, Cu) catalysts were prepared by using an impregnation method,
during which the M ions were firstly adsorbed within MOF crystals and
subsequently reduced to form NPs. The powder X-ray diffraction (XRD)
patterns of the as-prepared materials matched well with that of the
parent MOF, indicating the crystal structure was well preserved after
2