J. He et al. / Journal of Catalysis 309 (2014) 280–290
281
Apart from the traditional homogeneous or heterogeneous acid
and base catalyzed hydrolysis of diphenyl ether in high tempera-
ture water, the metal catalyzed selective hydrogenolysis of diphe-
catalysts. Transmission electron micrographs (TEM) were recorded
on a JEM-2010 Jeol transmission microscope operated at 120 kV. Be-
fore TEM measurement, the samples were prepared by depositing a
drop of an ultrasonicated methanol suspension of the solid material
onto a carbon-coated Cu grid for TEM measurement. Scanning elec-
tron microscopy (SEM) was recorded on a JEOL 500 SEM-microscope
with accelerating voltage 25 kV. The power samples were used
nyl ether is also feasible. Hartwig et al. reported that a Ni(COD)
2
t
complex combined with ligand SIPrꢂHCl and NaO Bu can selectively
cleave diphenyl ether in m-xylene at 393 K and 0.1 MPa H , attain-
2
ing 99% yields of benzene and phenol after 16 h [13,14]. While the
homogeneous catalyst has minimal steric constraints on interac-
tions with lignin model compounds and allows extremely mild
conditions, the TOF with the complex Ni catalyst is as low as
without any pretreatment. N
out at 77.3 K using a PMI automatic Brunauer–Emmett–Teller
(BET) sorptometer. For the H chemisorption measurement, the Ni
catalysts were reduced under 0.1 MPa H at 733 K for 4 h prior to
measurement. They were activated in vacuum at 588 K for 1 h before
the H chemisorption and then cooled to ambient temperature. The
adsorption isotherms (chemisorption and physisorption) were
2
adsorption–desorption was carried
2
ꢁ1
0
.34 h [13]. The catalysts are also sensitive to high concentra-
2
tions of water, which is ubiquitous in raw biomass. To establish a
more sustainable, stable, water-tolerant, and applicable process,
2
we have developed a supported Ni/SiO
2
catalyst to quantitatively
H
2
convert diphenyl ether to benzene and cyclohexanol at 393 K and
measured at 1–40 kPa. Following the first isotherms, the samples
were outgassed at ambient temperature for 1 h to remove the phys-
ꢁ1
0
2
.6 MPa H in the aqueous phase, achieving a TOF of 26 h [15].
By changing the catalyst to Ni/HZSM-5, we have achieved a one-
pot conversion of diverse lignin-derived phenolic monomers and
aryl ethers to cycloalkanes at 523 K in the aqueous phase via cas-
cade reactions of hydrolysis, hydrogenolysis, dehydration, and
hydrogenation [16]. However, the detailed mechanisms of the
C–O bond cleavage of di-phenolic ethers over the heterogeneous
Ni catalysts have not been established. Therefore, in this
contribution, we investigate the mechanisms of C–O bond cleavage
2
isorbed H , followed by measuring another adsorption isotherm
(physisorption). The Ni dispersions were calculated from the differ-
ence between extrapolated intercepts of the first and second iso-
therms with the assumption of H:Ni atomic ratio = 1.
2.4. Preparation of Ni/SiO
(DP) method
2
catalyst using deposition–precipitation
in p- (H–, CH
3
–, OH–) substituted diphenyl ethers over SiO
2
, ZrO
2
,
An aqueous solution (250 mL) containing Ni(NO
(0.14 M, 10.2 g) was first divided in two fractions. To one 50 mL
portion was added urea (0.42 M, 6.3 g). The other 200 mL portion
3
)
2
ꢂ6H
2
O
and Al supported Ni catalysts in the aqueous phase by investi-
2
O
3
gating individual steps. Modeling using density function theory
helps to elucidate the mechanism for 4,4 -dihydroxydiphenyl ether
0
2 3
together with SiO (1.1 g) and HNO (65%, 0.02 M, 0.32 mL) was
conversion.
put into a flask thermostated at 353 K. The first part with urea
was slowly added into the flask, and the suspension was rapidly
heated to 363 K. After reaching 363 K, the suspension was magnet-
ically stirred for 10 h. Then, the suspension was cooled to 298 K,
and the solids were filtered and washed three times with distilled
water (5/1 = water/slurry). Finally, the sample was dried at 363 K
2
. Experimental section
2.1. Reagents
ꢁ1
All chemicals were obtained from commercial suppliers: diphe-
nyl ether (Sigma–Aldrich, >99% GC assay), di-p-tolyl ether (TCI Eur-
ope, >98% GC assay), 4,4 -dihydroxydiphenyl ether (TCI Europe,
for 24 h, and calcinated in flowing air (100 mL min ) at 973 K
ꢁ1
2 2
and reduced in flowing H (100 mL min ) at 733 K. The Ni/SiO
0
catalyst had a Ni content of 57 wt.% as analyzed by AAS.
>
98% GC assay), cyclohexyl phenyl ether (Sigma–Aldrich, >95%
GC assay), p-cresol (TCI Europe, >99% GC assay), hydroquinone
TCI Europe, >99% GC assay), 1,4-cyclohexanediol (TCI Europe,
cis- and trans-mixture, >99% GC assay), ethyl acetate (Roth,
99.9% GC assay), phenol (Sigma–Aldrich, >99% GC assay), benzene
Fluka, >99.5% GC assay), Ni(II) nitrate hexahydrate (Sigma–
Aldrich, P98.5%), urea (Sigma–Aldrich, BioReagent), HNO
Sigma–Aldrich, >65%), 5 wt.% Pd/C (Sigma–Aldrich), SiO (Aerosil
00, Evonik-Degussa), (Westfalen AG, 99.999 vol%),
Westfalen AG, 99.999 vol%), synthetic air (Westfalen AG,
2.5. Catalytic tests
(
In a typical experiment, the catalytic reactions were carried out
in a slurry autoclave reactor with Ni/SiO catalyst using H O as sol-
vent at 393 K in the presence of 0.6 MPa H . The diphenyl ether
>
(
2
2
2
ꢁ3
3
(0.010 mol), 57 wt.% Ni/SiO
(80 mL) were added into a Parr reactor (Series 4848, 300 mL). After
the reactor was flushed with H three times, the autoclave was
charged with 0.6 MPa H and the reactions were conducted at
393 K with a stirring speed of 700 rpm. It heats up 9 min from
ambient temperature to 393 K. As H O remained liquid under
2
(0.30 g, 2.91 ꢃ 10 mol Ni), and H
2
O
(
2
(
2
H
2
N
2
2
2
9
1
9.999 vol%), ultra pure water system (EASYpure II, resistivity:
8.2 M cm).
X
2
these conditions, two phases were formed as the reaction pro-
ceeded, requiring to determine the composition of both phases
by stopping the reaction at different times and analyzing the mix-
ture. At the selected times, the reactor was quenched by ice to
ambient temperature, and the organic products were extracted
by ethyl acetate and analyzed by gas chromatography (GC) and
GC–mass spectroscopy (GC–MS) analysis on a Shimadzu 2010 gas
chromatograph with flame ionization detector and a Shimadzu
QP 2010S GC–MS, both of them equipped with a HP-5 capillary
2
.2. Synthesis of dicyclohexyl ether
Diphenyl ether (20.0 g) and 5 wt% Pd/C (1.0 g) were loaded in a
Parr reactor (series 4848, 300 mL). After the reactor was flushed
with H three times, the hydrogenation reaction was conducted
at 423 K in the presence of 5.0 MPa H (ambient temperature) for
8 h with a stirring speed of 700 rpm. The catalysts were separated
2
2
1
from liquid phase by centrifugation, and the product was purified
though distillation under vacuum. Purity: >99% (detected by GC),
M
column (30 m ꢃ 250
lm). The calculations of conversion and
ꢁ1
22O. The 1H, C, and COSY NMR
13
w
: 182 g mol , Formula: C12
H
selectivity were on carbon mole basis. Conversion = (the amount
of raw-material decrease during reaction/original amount) ꢃ 100%.
Selectivity = (C atoms in each product/total C atoms in the prod-
ucts) ꢃ 100%. Internal standards (i.e., 2-isopropylphenol for the or-
ganic phase and acetone for the aqueous phase) were used to
determine the product concentration and carbon balance. The
carbon balance was better than 95 ± 3%. TOFs for C–O cleavage
spectra are displayed in Figs. S1–S3.
2.3. Catalyst characterization
Atomic absorption spectroscopy (AAS) with a UNICAM 939
AA-Spectrometer determined the content of Ni in the supported