Mechanisms of catalytic cleavage of benzyl phenyl ether in aqueous and apolar phases

Article abstract of DOI:10.1016/j.jcat.2013.10.024

Catalytic pathways for the cleavage of ether bonds in benzyl phenyl ether (BPE) in liquid phase using Ni- and zeolite-based catalysts are explored. In the absence of catalysts, the C-O bond is selectively cleaved in water by hydrolysis, forming phenol and benzyl alcohol as intermediates, followed by alkylation. The hydronium ions catalyzing the reactions are provided by the dissociation of water at 523 K. Upon addition of HZSM-5, rates of hydrolysis and alkylation are markedly increased in relation to proton concentrations. In the presence of Ni/SiO₂, the selective hydrogenolysis dominates for cleaving the C_aliphatic-O bond. Catalyzed by the dual-functional Ni/HZSM-5, hydrogenolysis occurs as the major route rather than hydrolysis (minor route). In apolar undecane, the non-catalytic thermal pyrolysis route dominates. Hydrogenolysis of BPE appears to be the major reaction pathway in undecane in the presence of Ni/SiO₂ or Ni/HZSM-5, almost completely suppressing radical reactions. Density functional theory (DFT) calculations strongly support the proposed C-O bond cleavage mechanisms on BPE in aqueous and apolar phases. These calculations show that BPE is initially protonated and subsequently hydrolyzed in the aqueous phase. DFT calculations suggest that the radical reactions in non-polar solvents lead to primary benzyl and phenoxy radicals in undecane, which leads to heavier condensation products as long as metals are absent for providing dissociated hydrogen.

Full text of DOI:10.1016/j.jcat.2013.10.024

Journal of Catalysis 311 (2014) 41–51
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Mechanisms of catalytic cleavage of benzyl phenyl ether in aqueous
and apolar phases
a
a
a,
⇑
b
a,b,
⇑
Jiayue He , Lu Lu , Chen Zhao , Donghai Mei , Johannes A. Lercher
a
Department of Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstraße 4, 85747 Garching, Germany
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic pathways for the cleavage of ether bonds in benzyl phenyl ether (BPE) in liquid phase using
Ni- and zeolite-based catalysts are explored. In the absence of catalysts, the CꢀO bond is selectively
cleaved in water by hydrolysis, forming phenol and benzyl alcohol as intermediates, followed by
alkylation. The hydronium ions catalyzing the reactions are provided by the dissociation of water at
Received 9 August 2013
Revised 28 October 2013
Accepted 30 October 2013
523 K. Upon addition of HZSM-5, rates of hydrolysis and alkylation are markedly increased in relation
to proton concentrations. In the presence of Ni/SiO , the selective hydrogenolysis dominates for cleaving
2
Keywords:
Lignin
the CaliphaticꢀO bond. Catalyzed by the dual-functional Ni/HZSM-5, hydrogenolysis occurs as the major
route rather than hydrolysis (minor route). In apolar undecane, the non-catalytic thermal pyrolysis route
dominates. Hydrogenolysis of BPE appears to be the major reaction pathway in undecane in the presence
Hydrolysis
Hydrogenolysis
Ether cleavage
Alkylation
Pyrolysis
2
of Ni/SiO or Ni/HZSM-5, almost completely suppressing radical reactions. Density functional theory
(DFT) calculations strongly support the proposed CꢀO bond cleavage mechanisms on BPE in aqueous
and apolar phases. These calculations show that BPE is initially protonated and subsequently hydrolyzed
in the aqueous phase. DFT calculations suggest that the radical reactions in non-polar solvents lead to
primary benzyl and phenoxy radicals in undecane, which leads to heavier condensation products as long
as metals are absent for providing dissociated hydrogen.
Ó 2013 Elsevier Inc. All rights reserved.
1
. Introduction
The most abundant CꢀO bonds in lignin are
, b-1, and 5–5 linkages [11], in which the -O-4 bond is the most
active and thermally unstable one due to the lowest bond dissoci-
a-O-4, b-O-4, 4-O-
5
a
Lignin is a three-dimensional amorphous polymer with meth-
ꢀ1
oxylated phenyl-propane units, which are randomly connected
by CꢀO or CꢀC linkages [1]. It would be ideal to catalytically and
efficiently convert lignin to liquid transportation fuels or fine
chemicals without destroying its inherent benzene structure. How-
ever, currently lignin is primarily used as a low-grade fuel to pro-
vide heat [2,3].Three major approaches, i.e., gasification of lignin to
synthesis gas [4,5], liquefaction (pyrolysis) of lignin to bio-oil [6,7],
and hydrolysis of lignin to monomeric or oligomeric units [8–10],
are used to depolymerize lignin. As the hydrolysis route leads to
relatively selective products under much milder reaction condi-
tions, it is rational to select lignin model compounds such as aryl
ether dimers to investigate the principal fundamental chemistry
of the hydrothermal depolymerization of lignin.
ation energy of the aliphatic CꢀO bond (218 kJ mol ) among these
bonds [12–15]. To explore the mechanism of cleaving the -O-4
a
bond, BPE has been selected as the model compound in this work.
The non-catalyzed pyrolysis of BPE has been extensively inves-
tigated in the past [16]. It is initiated by homolytic scission of the
weak CaliphaticꢀO bond above 548 K. Generally, pyrolysis was, how-
ever, carried out under more severe conditions above 593 K in the
vapor phase or in supercritical solvents such as methanol [17], tol-
uene [18], and tetralin [18]. The pyrolysis process produces high
concentrations of benzyl and phenoxy radicals, which led to tolu-
ene and phenol formation as primary products, together with
dimer and high molecular weight products derived from the
recombination of the initial radicals. The recombination yield
was usually higher than 15% with a carbon balance lower than
8
0%. Water, in contrast, leads to higher hydrothermal conversion
rates of BPE than neat pyrolysis. Klein et al. [19] reported that
the pyrolysis rate of BPE in the presence of water was approxi-
mately four times higher than that in neat pyrolysis at 605 K. We
have explored BPE conversion in supercritical water (between
⇑
Corresponding authors. Address: Department of Chemistry and Catalysis
Research Center, Technische Universität München, Lichtenbergstraße 4, 85747
Garching, Germany (C. Zhao and J.A. Lercher). Tel.: +49 89 289 13540; fax: +49 89
2
89 13544.
5
43 K and 623 K) as well via analyzing the influence of salts (alkali
E-mail addresses: chenzhao@mytum.de (C. Zhao), johannes.lercher@ch.tum.de
(
J.A. Lercher).
carbonates) toward BPE conversion in aqueous phase [20].
0
021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.10.024

4
2
J. He et al. / Journal of Catalysis 311 (2014) 41–51
Recently, we have reported that diverse lignin-derived aryl
ethers can be cleaved with Ni/HZSM-5 or Ni/SiO at 393–523 K
by ultrasonic treatment. After that, the dispersion was dropped on
a copper grid-supported carbon film.
2
via combined hydrogenolysis, hydrolysis, and hydrodeoxygenation
integrated steps in the aqueous phase [21–24]; however, the
mechanisms for cleaving of CꢀO bonds in these ethers have not
been unequivocally established in the presence of mono- and
dual-functional metal and acid catalysts. In this work, we will,
therefore, investigate the roles of gas atmosphere, acid and metal
sites, as well as their cooperative action in the conversion of BPE
in the aqueous phase at 523 K via exploring the detailed kinetics
together with the analysis of reaction pathways and mechanisms
for BPE conversion at specifically varied conditions. In addition,
density functional theory (DFT) modeling is comparatively
employed to explore the ether cleavage mechanisms in the
aqueous and apolar phases as well.
IR spectra of adsorbed pyridine: IR spectroscopy with pyridine as
probe molecule was used to determine the acid site concentrations
and distributions. The IR spectra were measured with
a
PerkinElmer 2000 spectrometer operated at resolution of
a
ꢀ
1
4 cm . The sample was activated at 723 K for 2 h in vacuum,
and a background spectrum was recorded after the temperature
decreased to 423 K. The activated sample was exposed to pyridine
ꢀ5
vapor (1.0 ꢂ 10 MPa) at 423 K for 0.5 h. After removing
physisorbed pyridine by outgassing at 423 K for 0.5 h, the spectra
were recorded. For quantification, molar integral extinction
ꢀ1
ꢀ1
coefficients of 0.73 cm
lmol and 0.96 cm lmol were used for
Brønsted (BAS) and Lewis acid sites (LAS), respectively.
2.4. Catalytic test
2
. Experimental section
The detailed reaction conditions are described in the corre-
2
.1. Chemicals
sponding figures as footnotes. In a typical experiment, the catalytic
reactions were carried out in a slurry autoclave reactor loaded with
Ni/HZSM-5 in the water solvent at 523 K in the presence of 4 MPa
The chemicals were purchased from commercial suppliers
and used as received: benzyl phenyl ether (TCI, >98% GC assay),
nickel(II) nitrate hexahydrate (Sigma–Aldrich, P98.5% GC assay),
H (STP). A mixture of benzyl phenyl ether (0.010 mol), 10 wt.% Ni/
2
ꢀ5
HZSM-5 (0.050 g, 8.62 ꢂ 10 mol Ni), and H O (80 mL) was firstly
2
SiO
5), Ni/SiO
undecane (Sigma–Aldrich, >99% GC assay), H
99.999 vol.%), N (Westfalen AG, >99.999 vol.%), synthetic air
Westfalen AG, >99.999 vol.%), and water (EASYpure II,
resistivity:18.2 M cm).
2
(Aerosil 200, Evonik-Degussa), HZSM-5 (Clariant AG, Si/Al =
(Acros, Ni loading 70%), H SO (ROTH, P95.5%),
(Westfalen AG,
added into a Parr reactor (Series 4848, 300 mL). After the reactor
4
2
2
4
was flushed with H2 by three times, the autoclave was charged
with 4 MPa H2 (ambient temperature) and the reaction was con-
ducted at 523 K with a stirring speed of 700 rpm at different reac-
tion times. The temperature of the autoclave was increased in
approximately 20 min. from ambient to reaction temperature
2
>
2
(
X
(
523 K). Because it is a two-phase reaction, the kinetics data are
collected at different duration times. After reaction, the reactor
was cooled by ice to ambient temperature, and the organic prod-
ucts were extracted by ethyl acetate and analyzed by GC and MS.
The products were analyzed by a gas chromatography (GC) and
GC–mass spectroscopy (GC–MS) on Shimadzu 2010 gas chromato-
graph with flame ionization detector and a Shimadzu QP 2010S
GC–MS, both of them equipped with a HP-5 capillary column
2
.2. Preparation of Ni/HZSM-5
Ni/HZSM-5 catalyst was synthesized by the wetness impregna-
tion method as follows, Ni(NO O (5.6 g) was first dissolved in
ꢁ6H
O (5.0 g) as a transparent green solution, and subsequently, such
aqueous solution was slowly dropped into zeolite HZSM-5 powder
10 g) with stirring. After metal incorporation for 2 h, the catalyst
was sequentially dried at 373 K for 12 h, air-calcined (flow rate:
3
)
2
2
H
2
(
(
30 m ꢂ 250
lm). Internal standard, i.e., 2-isopropylphenol, was
ꢀ1
used to determine the liquid product concentration and carbon
balance. The carbon balance for all reported experiments in liquid
phase was better than 95 ± 3% in this work. The calculations of con-
version and selectivity were based on carbon mole basis. Conver-
sion = (the amount of raw materials change during reaction/total
amount of raw materials) ꢂ 100%. Selectivity = (C atoms in each
product/total C atoms in the products) ꢂ 100%. Rate = (moles of
reactants cleaved)/(reaction time in hour). TOF = (moles of reac-
tants cleaved)/(moles of surface active sites ꢂ reaction time in
hour).
1
1
00 mL min ) at 673 K for 4 h, and hydrogen-reduced (flow rate:
ꢀ1
00 mL min ) at 733 K for 4 h.
2.3. Catalyst characterization
Atomic absorption spectroscopy (AAS): A UNICAM 939 AA-
spectrometer was used to measure the Ni concentrations of
Ni/SiO and Ni/HZSM-5.
BET surface area: The surface areas and pore diameters were
determined by the nitrogen sorption measurement. PMI
2
A
automated BET sorptometer was used to measure the nitrogen
adsorption at 77 K and before measurement, the samples were first
outgassed at 523 K for 20 h.
2.5. Density functional theory (DFT) calculation
H
2
chemisorption: The catalysts were first activated at 733 K for
h in H and 1 h in vacuum and then cooled to 313 K. An isotherm
of H adsorption (chemisorption and physisorption) was measured
within a pressure range from 1 kPa to 40 kPa. Then, the physi-
sorbed H was removed by outgassing the sample at the same
temperature for 1 h, and another adsorption isotherm (physisorp-
tion) was taken. The concentration of chemisorbed H on the metal
was determined by extrapolating the differential isotherm to zero
H2, and this value was used to calculate the dispersion of Ni with
All the quantum chemical calculations of BPE in hot water and
undecane were performed using Gaussian 09 program [25]. The
DFT/B3LYP functional with 6-311++G(d,p) basis set was applied.
The accuracy of the B3LYP functional was checked by comparing
with the second-order Möller–Plesset perturbation theory (MP2)
[26]. The transition state of each reaction pathway is located using
the linear (LST) and quadratic synchronous transit (QST) methods
[27,28]. A series of single point energy calculations of interpolated
structures between the initial and the final states along the reac-
tion pathway are performed. The maximum energy structure along
this reaction path is used as an estimate transition state in the QST
calculation. Each identified transition state was confirmed by the
sole imaginary vibrational frequency. The solvation effects on the
hydrolysis in water and the pyrolysis in undecane were described
3
2
2
2
2
P
the assumption of H:Ni atomic ratio = 1.
Transmission electron microscopy (TEM):
A JEM-2010 JEOL
transmission electron microscope operating at 120 kV was used
to record the TEM images. Before measurement, the catalyst was
ground and then suspended in ethanol and subsequently dispersed

J. He et al. / Journal of Catalysis 311 (2014) 41–51
43
by the polarizable continuum model (PCM) model with dielectric
constants of 78.3553 for water and 1.9910 for undecane [29].
2
(0.1 MPa N ). Without catalyst, hydronium ions from dissociated
water at 523 K (pH = 5.5) catalyze the hydrolysis [31,32]. At the se-
lected conditions, the conversion increased linearly to 31% as a
function of time up to 110 min, and the yields of phenol and benzyl
alcohol synchronously increased to around 13%. Besides Ph and BA,
some products with 13 C atoms such as 4-benzyl phenol and 2-
benzyl phenol were also observed. It was reported that these hea-
vier products were dimers or trimers formed from BPE pyrolysis or
hydrolysis via recombination of radical intermediates [20,33].
However, Siskin et al. proposed a different mechanism on BPE con-
version at different temperature ranges, i.e., above 623 K, the rad-
ical pathway dominated, but at the lower temperature 523 K in the
aqueous phase, the contribution of the ionic chemistry was con-
cluded to play a significant role [34]. Siskin et al., however, also ob-
served the formation of phenol (41.0%), benzyl alcohol (26.4%), and
alkylated products (29.6%, including 19.6% dimers and 10% trimers)
at 523 K. The higher yield of alkylated products, especially of tri-
mers in their case, is attributed to the much longer reaction time
3
. Results and discussion
3.1. Catalysts characterization
The physicochemical properties of the supported metal catalyst
(
Ni/SiO
alyst (Ni/HZSM-5) are compiled in Table 1. The Ni/SiO
a Ni content of 70 wt.%, a BET-specific surface area of 82 m g
2
), the acid catalyst (HZSM-5), and the dual-functional cat-
2
catalyst had
2
ꢀ1
,
3
ꢀ1
pore volume of 0.12 cm g , and a pore diameter of 9.2 nm. The
average size of supported Ni particles in Ni/SiO was around
.0 nm determined from the TEM image (Supporting information
2
8
Fig. S1b). The parent HZSM-5 had
a
BET surface area of
2
ꢀ1
ꢀ1
3
95 m g and a total acid site concentration of 0.360 mmol g
determined by TPD of ammonia (TPDꢀNH
3
). Probed by IR of ad-
sorbed pyridine (PyꢀIR), the Brønsted acid site (BAS) and Lewis
(
5.5 days) than the 110 min in our case. In our work, the yield of
acid site (LAS) concentration of HZSM-5 was 0.266 and
ꢀ1
the CꢀC coupled products continuously increased to 5% after
0
.048 mmol g , respectively. Impregnated with Ni, the BET sur-
2
ꢀ1
achieving the specific reaction equilibrium. When the gas atmo-
face of Ni/HZSM-5 (Ni content: 10 wt.%) decreased to 374 m g
.
sphere was altered to H
in 110 min, which was close to the conversion obtained in the
presence of 0.1 MPa N . The major products of Ph and BA were
2
(Fig. 1c), 34% conversion was attained
The concentration of the Brønsted acid sites decreased by approx-
imately 20% after Ni incorporation due to the ion exchange of
Brønsted acid sites for Ni2+, while the Lewis acid site concentration
2
formed via hydrolysis by hydronium in the high-temperature
water, and the CꢀC alkylation products kept a stable yield of 5%.
This indicates that gas atmosphere (P = 0.1 MPa) exerts a negligible
influence to the hydrolysis of BPE in the aqueous phase.
was almost unchanged compared to parent zeolite. The average Ni
particle size was 20 nm on Ni/HZSM-5 prepared by impregnation
as determined by the TEM image (Supporting information Fig. S1a).
With high-pressure N
rates were significantly accelerated with a high conversion of 41%
in 110 min. At a high H pressure (4 MPa) under hydrothermal con-
ditions, the cleavage rates on BPE were further enhanced leading to
1% conversion in 110 min. (Fig. 1d); equal concentrations (23%) of
phenol and benzyl alcohol were formed. In addition, the CꢀC cou-
pling products also attained a maximum yield of 5% in water, and
this constancy implied that the CꢀC coupling is primarily domi-
nated by the acid-catalyzed alkylation, but not by the thermal
free-radical reaction [35]. Summarizing these results leads to the
conclusion that the aqueous environment favors the ionic pathway
in the CꢀO bond cleavage of BPE at 523 K. BPE is then cleaved to
phenol and benzyl alcohol by hydrolysis, which is catalyzed by
the protons of high-temperature water in the absence or the pres-
2
(4 MPa, Fig. 1b), however, the hydrolysis
3
.2. Benzyl phenyl ether conversion in the aqueous phase
2
High-temperature water has a density and polarity at 573 K
similar to acetone at ambient temperature [30] and acts conse-
quently as a suitable reaction medium for dissolving organic com-
pounds. The dielectric constant of water drops rapidly from 78 at
2
5
high-temperature water could act as a moderately strong acid as
well as a base. We structure the investigations of the influence of
gas atmosphere, of a solid acid introduced in the form of HZSM-
5
ꢀ11
93 K to 27 at 523 K, and the ionic product of water is 10
at
ꢀ14
23 K compared to 10
at 293 K [31,32]. This indicates that the
5
2
, of metal sites introduced by Ni/SiO , and the presence of both
sites by introducing Ni/HZSM-5, on BPE conversion at 523 K in four
sections, i.e., (i) BPE conversion in aqueous phase in the absence
2
ence of H . The subsequent alkylation between phenol and alcohol
reaches a maximum yield of 5%.
and the presence of H
in aqueous phase in the presence of H
on BPE conversion in the presence of H
bifunctionality with acid and metal sites on BPE conversion in
the presence of H
2
, (ii) the influence of acid on BPE conversion
, (iii) the influence of metal
, (iv) the influence of
2
The relationship between the hydrolysis rate and the gas
pressure in the aqueous phase has been systematically investi-
gated by Whalley et al. It has been shown that in the ether
hydrolysis based on A2 mechanism (with the rate-determining
step being the bimolecular hydrolysis), the higher gas pressure
enhances the ether hydrolysis rate in the aqueous phase due
to a better stabilization of the transition state via the increased
gas pressure [36,37]. The A2 mechanism of hydrolysis of BPE is
described in equations 1–4. In agreement with Whalley et al.
2
2
.
3
.2.1. BPE conversion in aqueous phase in the absence and the
presence of H
Fig. 1a shows that BPE was equally cleaved into phenol (Ph) and
2
benzyl alcohol (BA) in water at 523 K in the absence of H
2
Table 1
Physicochemical properties of catalysts.
Catalyst
Ni/SiO
2
HZSM-5 (Si/Al = 45)
Ni/HZSM-5 (Si/Al = 45)
Metal loading (wt.%)
BET surface area (m
70
82
0.12
9.2
8.0
–
–
–
6.9
–
10
2
ꢀ1
g
)
)
395
0.26
3.6
374
0.24
3.6
3
ꢀ1
Pore volume (cm
Pore diameter (nm)
Metal particle diameter (nm)
g
–
20
ꢀ
1
Acidity (mmol g ) (TPD-NH
3
)
0.360
0.266
0.048
–
0.296
0.228
0.048
0.84
ꢀ
1
Conc. BAS (mmol g
)
Conc. LAS (mmol gꢀ1)
Dispersion of metal (H
2
chem.%)

4
4
J. He et al. / Journal of Catalysis 311 (2014) 41–51
(
a) In N atmosphere (0.1 MPa)
(b) In N atmosphere (4.0 MPa)
2
2
2
2
1
1
5
0
5
0
5
0
40
25
20
15
10
5
45
4
3
3
2
2
1
1
5
0
0
5
0
5
0
5
0
3
3
2
2
1
1
5
0
5
0
5
0
5
0
Conversion
Conversion
Benzyl alcohol
Benzyl alcohol
Phenol
Phenol
Alkylation
Alkylation
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)
(
c) In H
2
atmosphere (0.1 MPa)
(d) In H atmosphere (4.0 MPa)
2
2
2
1
1
5
0
5
0
5
0
40
30
60
50
40
30
20
10
0
3
3
2
2
1
1
5
0
5
0
5
0
5
0
2
2
5
0
Conversion
Conversion
15
Benzyl alcohol
Benzyl alcohol
Phenol
1
0
5
0
Phenol
Alkylation
Alkylation
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)
Fig. 1. Product yields for conversion of benzyl phenyl ether in aqueous phase in the absence and the presence of H
ether (1.84 g), H O (80 mL), 523 K, stirring at 700 rpm, (a) 0.1 MPa N , (b) 4 MPa N , (c) 0.1 MPa H , (d) 4 MPa H
2
as a function of time. Reaction conditions: benzyl phenyl
2
2
2
2
2
.
[
37], our results also show that the TOF of BPE hydrolysis
cage for the reactant, facilitating reaction from the solvent cage,
enhancing in turn the reaction rate with H in comparison with
. At present, however, we cannot fully exclude a direct positive
3
ꢀ1
ꢀ1
increased from 7.6 ꢂ 10 mol mol
þꢃH
h
at 0.1 MPa
H
2
to
2
½
H
2
O
4
ꢀ1
ꢀ1
1
.1 ꢂ 10 mol mol½
þ
h
2
ꢃH O
at 4 MPa H
2
.
N
2
H
effect of the H
2
pressure on the acid-catalyzed reactions.
ð1Þ
3.2.2. The influence of acid on BPE conversion in the aqueous phase in
the presence of H
2
The impact of the acid after loading low (0.03 g) and high
0.15 g) concentrations of HZSM-5 was subsequently investigated
at 523 K in the conversion of BPE at 4 MPa H (Fig. 2a and b). With
2
(
ð2Þ
a small concentration of HZSM-5 (0.03 g), the conversion was in-
creased from 51% to 75% compared to in the neat aqueous phase
with 4.0 MPa H in 110 min (Fig. 2a). Meanwhile, the cleavage
2
yields to phenol and benzyl alcohol (30%) were comparable to
those (23%) in the absence of acid. But it is interesting to find that
the small addition of the acid remarkably enhanced the alkylation
yield from 5% to 15% at 523 K. It supports again that C13 products
are formed by acid-catalyzed alkylation and not by free-radical
reactions.
ð3Þ
ð4Þ
The higher CꢀO cleavage rate of BPE in H
2
compared to N
2
with
In a comparison, with a high concentration of solid acid (HZSM-
5, 0.15 g), the conversion increased to 87% in the first 10 min, and
further increased to 98% at 110 min, which was much higher than
the conversion (only 51%) without introducing the HZSM-5 at
identical conditions (Fig. 2b). This confirms that hydrolysis is in-
deed the major reaction pathway for BPE conversion in aqueous
phase. It should be emphasized that the alkylation yield reached
50% under these conditions, which is much higher than that in neat
a pressure of 4 MPa may be originated from the different solubili-
ties of the gases in the aqueous phase. When a reaction takes place
in solvents, the reactant is surrounded by solvent molecules and
needs to overcome the cage of solvents to collide with reactants
or catalysts. This is called ‘‘cage effect.’’ We speculate that the high-
2 2
er solubility of H compared to N leads to a lower equilibrium
pressure and, hence, less compression and a more loose solvent

J. He et al. / Journal of Catalysis 311 (2014) 41–51
45
(
a) With low concentrations
(b) With high concentrations
of acids
of acids
35
30
25
20
15
10
5
0
80
60
100
98
96
94
92
90
88
86
Conversion
Conversion
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
50
Phenol
Benzyl alcohol
40
30
Alkylation
Phenol
Benzyl alcohol
2
0
0
0
Alkylation
1
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)
Fig. 2. Product yields for conversion of benzyl phenyl ether with HZSM-5 in aqueous phase in the presence of H
ether (1.84 g), H O (80 mL), 523 K, 4 MPa H , stirring at 700 rpm, (a) HZSM-5 (0.03 g), (b) HZSM-5 (0.15 g).
2
as a function of time. Reaction conditions: benzyl phenyl
2
2
water with or without hydrogen (5%), or with small concentrations
of HZSM-5 (15%). This indicates that with a sufficient concentra-
tion of acid sites, the initial hydrolysis and following alkylation
rates are both highly accelerated.
With the addition of HZSM-5, the major pathway was still
hydrolysis for BPE in water at 523 K. The rate of hydrolysis and fol-
lowing alkylation between the intermediate fragments were both
enhanced with a small concentration of HZSM-5. Adding a higher
concentration of acid sites (higher amount of HZSM-5 added) pro-
moted both the hydrolysis and alkylation rates to a larger extent.
cleaving of the other side of CaromaticꢀO bond (bond dissociation
ꢀ1
energy: 314 kJ mol ) [14] were not observed. Five percentage of
alkylation products (catalyzed by hydronium ions formed by disso-
ciated water at 523 K) were formed as well, indicating that the
hydrolysis/alkylation route contributes to a very minor part. The
alkylation yield was quite constant at 5% without the acid, as the
pH value in such aqueous system is stable at 5.5 [31,32].
The observed yield of phenol was much lower than the yield of
ꢀ1
toluene, as the hydrogenation rate of phenol (2400 h ) was much
ꢀ1
2
faster than the hydrogenation rate of toluene on Ni/SiO (135 h )
at 523 K. (Supporting information Table S1). After phenol was
hydrogenated to cyclohexanone and cyclohexanol on Ni, cyclohex-
anol was subsequently dehydrated to cyclohexene catalyzed by
hydronium ions and then hydrogenated or dehydrogenated to
cyclohexane or benzene, respectively. However, all these hydro-
deoxygenated intermediates from phenol were detected with
yields lower than 7% (not shown in Fig. 3).
3
.2.3. The influence of metal on aqueous-phase BPE conversion in the
presence of H
The metal site (Ni/SiO
way of BPE conversion in water at 523 K (see Fig. 3). After
10 min, BPE was quantitatively cleaved to 50% phenol and 50%
2
2
) expectedly catalyzed a different path-
1
toluene. This suggests that the weaker CaliphaticꢀO bond (bond
In summary, the Ni-catalyzed direct hydrogenolysis on BPE is
the dominating pathway for producing equal concentrations of
phenol and toluene. Subsequently, phenol was hydrogenated at a
relatively fast rate, forming cyclohexanone and cyclohexanol as
intermediates. In turn these, oxygenates were deoxygenated to
cyclohexene, cyclohexane, and benzene catalyzed by the joint
ꢀ1
dissociation energy: 218 kJ mol ) [14] is selectively cleaved by
Ni-catalyzed hydrogenolysis, but not by acid-catalyzed hydrolysis.
Note that the alternative products (benzene and benzyl alcohol) by
5
4
3
2
1
0
0
0
0
0
0
100
80
action of metal (Ni/SiO
temperature water).
2
) and acid (hydronium ions in the high-
Conversion
Toluene
3.2.4. The influence of bifunctionality with acid and metal sites on
aqueous-phase BPE conversion in the presence of H
2
Phenol
The role of acid and metal sites in proximity on Ni/HZSM-5 was
investigated in the next step. Fig. 4 shows the variations in the con-
centration in the reactant product mixture of BPE over Ni/HZSM-5
as a function of time. The products included 50% phenol, 40% tolu-
ene, 2% benzyl alcohol, as well as 8% C13 alkylated products, with
quantitative conversion after 240 min. With metal and acid sites
in Ni/HZSM-5, phenol and toluene (major products) were produced
by metal-catalyzed hydrogenolysis, while the parallel acid-
catalyzed hydrolysis (minor) pathway led to the formation of
phenol and benzyl alcohol as well as some alkylation products.
The hydrolysis was catalyzed by the acid sites of Ni/HZSM-5 and
protons in high-temperature water, attaining a maximum alkyl-
ation yield of 10%. It is noted that the turnover frequencies of
protons/hydronium ions in the zeolite channels (Table 2, Entry
60
40
20
0
Alkylation
0
20
40
60
80
100
120
Time (min)
Fig. 3. Product yields for conversion of benzyl phenyl ether with Ni/SiO
phase in the presence of H as a function of time. Reaction conditions: benzyl
2
(70 wt.%, 0.010 g), H O (80 mL), 523 K, 4 MPa H ,
2
in aqueous
2
4
–6) were approximately one magnitude lower than those of water
phenyl ether (9.20 g), Ni/SiO
stirring at 700 rpm.
2
2
dissociation. The hydrogenolysis rate was at least four times as fast

4
6
J. He et al. / Journal of Catalysis 311 (2014) 41–51
100
6
5
4
3
2
1
0
0
0
0
0
0
0
3.3. Different mechanisms for cleaving of the CꢀO bond of BPE at
Conversion
varying conditions in aqueous phase
8
6
4
2
0
0
0
0
0
The variations in product distributions and reaction rates on the
varying conditions in the aqueous phase are compiled in Table 2. In
Phenol
N
2
(P = 0.1 MPa), BPE was cleaved into phenol and benzyl alcohol
by hydrolysis under the hydrothermal conditions with an initial
Toluene
ꢀ
1
cleavage
rate
of
0.0017 mol h
;
and
a
TOF
of
3
ꢀ1
ꢀ1
6
.8 ꢂ 10 mol mol
þꢃH
h
5% of alkylation products were
by H
½
H
2
O
formed at such conditions (Table 2, Entry 1). Replacing N
2
2
ꢀ
1
(
P = 0.1 MPa), the CꢀO bond cleavage rate (0.0019 mol h ) and
Alkylation
3
ꢀ1 ꢀ1
TOF (7.6 ꢂ 10 mol mol
þ
h
) (Table 2, Entry 3) were compa-
½H ꢃH O
2
ꢀ1
rable to these reactions in 0.1 MPa N
2
(0.0017 mol h
and
3
ꢀ1
ꢀ1
Benzyl alcohol
6.8 ꢂ 10 mol mol
½
H
þꢃH
O
h ). This implies that gas atmosphere
2
does very minor impact to the ether hydrolysis rate at a pressure
0
100 200 300 400 500 600 700
Time (min)
of 0.1 MPa. Employing a high-pressure in the presence of N
(4.0 MPa), the reaction rate was accelerated leading to a conversion
2
ꢀ
1
of 41%, with an increased initial cleavage rate of 0.0022 mol h
Fig. 4. Product yields for conversion of benzyl phenyl ether with 10% Ni/HZSM-5 in
aqueous phase in the presence of H as a function of time. Reaction conditions:
benzyl phenyl ether (9.20 g), 10 wt.% Ni/HZSM-5 (0.050 g), H O (80 mL), 523 K,
MPa H , stirring at 700 rpm.
3
ꢀ1
ꢀ1
and TOF of 8.8 ꢂ 10 mol mol
þꢃH
h
(Table 2, Entry 2). With
½
H
2
O
2
4
2
.0 MPa H , 51% conversion was reached under identical condi-
2
ꢀ
1
4
2
tions with an increased initial cleavage rate of 0.0028 mol h
4
ꢀ1
ꢀ1
and TOF of 1.1 ꢂ 10 mol mol
þꢃH
h
(Table 2, Entry 4). The yield
½
H
2
O
of alkylation was maintained at the stable level of around 5% in the
as that of hydrolysis, as the yield of the hydrogenolyzed products
80%) was much higher than that of hydrolyzed (20%). Trace
aqueous phase in the presence of H
ation equilibrium exists [35].
2 2
or N , suggesting the alkyl-
(
amounts (yields < 2%) of hydrodeoxygenation intermediates from
phenol and toluene such as cyclohexanone, cyclohexanol, and
methylcyclohexane were observed as well (not shown in Fig. 4).
With both catalytic sites present, the hydrogenolysis route cat-
alyzed by metallic Ni was the major reaction pathway for cleaving
the CꢀO bond of BPE, with producing phenol and toluene as major
products. Hydrolysis was only the minor route, but hydrodeoxy-
genation of phenol and toluene occurred to a small extent with
Ni/HZSM-5 as well. Alkylation of the cleaved products (phenol
and benzyl alcohol) took place with HZSM-5 catalyst in the high-
temperature water.
The addition of even a small amount of acid (HZSM-5, 0.030 g)
increased the cleavage rate of BPE threefold in the presence
ꢀ1
of H
2
, i.e., it attains an initial cleavage rate of 0.0090 mol h com-
ꢀ1
pared to 0.0028 mol h without the solid acid (Table 2, Entry 5).
Yields of hydrolysis and alkylation were increased by 5–10% in
1
0
10 min. Employing a higher concentration of acid (HZSM-5,
3
ꢀ
1
H
ꢀ1
.15 g), the initial TOF (1.3 ꢂ 10 mol mol
þꢃHZSM-5 h ) (Table 2,
½
Entry 6) was comparable to that with a small amount of acid
3
ꢀ1
ꢀ1
(
1.1 ꢂ 10 mol mol
þꢃHZSM-5 h ). However, the reaction rate was
higher and the reaction reached full conversion with equal yields
of hydrolysis (50%) and alkylation products (50%) in 110 min
½H
Table 2
The effect of gas atmosphere, HZSM-5 (acid), and Ni/SiO
2
(metal) on the reaction of BPE in the aqueous phase at 523 K.a
Rate (mol h 1
ꢀ
)
TOF (mol mol
ꢀ1 ꢀ1
h )
Entry
Gas
Active site
1b
c
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
N
N
H
H
H
2
2
2
2
2
H
2
O
0.0017
3
6
8
7
1
1
.8 ꢂ 10 mol mol
þ
h
h
h
h
½
H
ꢃH
ꢃH
ꢃH
ꢃH
2
2
2
2
O
O
O
O
n (½Hþꢃ ) = 2.5 ꢂ 10ꢀ7 mol
2
H O
2b
3b
4b
5b
ꢀ1
þ
H
H
2
O
0.0022
0.0019
0.0028
0.0090
3
.8 ꢂ 10 mol mol
½
n (½Hþꢃ ) = 2.5 ꢂ 10ꢀ7 mol
2
H O
c
ꢀ1
þ
H
2
H O
3
.6 ꢂ 10 mol mol
½
n (½Hþꢃ ) = 2.5 ꢂ 10ꢀ7 mol
H
2
2
O
2
H O
4
ꢀ1
þ
H
.1 ꢂ 10 mol mol
½
þ
ꢀ7
n (½H
HZSM-5, H O
2
ꢃ
) = 2.5 ꢂ 10 mol
H
O
3
ꢀ1
ꢀ1
ꢀ1
.1 ꢂ 10 mol mol
þ
h
h
½
H
ꢃHZSM-5
+
ꢀ6
n ([H ]HZSM-5) = 8.0 ꢂ 10 mol
þ
ꢀ7
n (½H
ꢃ
) = 2.5 ꢂ 10 mol
2
H O
6b
ꢀ1
þ
H
H
2
2
HZSM-5, H
2
O
0.0522
0.0542
3
1
1
.3 ꢂ 10 mol mol
½
ꢃHZSM-5
ꢀ1
+
ꢀ5
n ([H ]HZSM-5) = 4.0 ꢂ 10 mol
n (½Hþꢃ ) = 2.5 ꢂ 10 mol
ꢀ7
2
H O
7d
ꢀ1
þ
H
H
H
2
SO
4
þ
, H
2
O
3
.4 ꢂ 10 mol mol
h
½
ꢃ
ꢀ5
n (½H ꢃH2 SO4 ) = 4.0 ꢂ 10 mol
þ
ꢀ7
n (½H
70 wt.% Ni/SiO
ꢃ
) = 2.5 ꢂ 10 mol
2
H O
8b
9b
ꢀ1
ꢀ1
H
2
2
2
, H
2
O
0.1410
0.0777
4
1
1
.7 ꢂ 10 mol mol
h
Surf Ni
n ([Surf. Ni]) = 8.5 ꢂ 10 mol, n (½HþꢃH2 O) = 2.5 ꢂ 10ꢀ7 mol
ꢀ6
ꢀ1
ꢀ1
H
10 wt.% Ni/HZSM-5, H
2
O
5
.1 ꢂ 10 mol mol
h
Surf Ni
ꢀ
7
n ([Surf. Ni]) = 7.4 ꢂ 10 mol
+
ꢀ5
n ([H ]HZSM-5) = 1.0 ꢂ 10 mol
þ
ꢀ7
n (½H ꢃH2 O) = 2.5 ꢂ 10 mol
a
b
c
Gas pressure: 4.0 MPa.
The initial rate and TOF are obtained via the kinetics in the aqueous phase in Figs. 1–4.
Gas pressure: 0.1 MPa.
d
Rate and TOF are calculated based on the data listed in Supporting information Table S2.

J. He et al. / Journal of Catalysis 311 (2014) 41–51
47
1
0
9
8
7
6
5
4
3
2
1
0
HZSM-5,
-1
E = 47 kJ•mol
a
H SO
2
4
-1
E = 163 kJ•mol
a
1
.8
1.9
2
2.1
3
-1
1
/T (×10 ) (K )
Fig. 5. Arrhenius plots for the ln TOF (hꢀ1) with H
temperature range of 483–543 K in the presence of 4 MPa H
SO
2 4
and HZSM-5 catalysts in the
. Reaction conditions:
2
4
ꢀ
benzyl phenyl ether (18.4 g), H
2
SO
4
solution (c = 2.5 ꢂ 10 mol/L), H
2
O (80 mL),
2
0 min.; benzyl phenyl ether (1.84 g), H
2
O (80 mL), HZSM-5 (0.03 g), 30 min.
Scheme 1. The major reaction pathways for conversion of BPE with different
catalyst components in the aqueous phase in the presence of hydrogen at 523 K.
(
Fig. 2b). We speculate that the higher concentration of HZSM-5
in the aqueous phase at 523 K. In high-temperature water with or
without HZSM-5 addition, hydrolysis is the major pathway for
cleaving the CꢀO bond, and the following C–C coupling of
alkylation takes place between the cleaved phenol and alcohol
fragments. When Ni is employed with or without HZSM-5,
Ni-catalyzed hydrogenolysis dominates throughout the overall
C–O bond cleavage reaction (Scheme 1).
enhances the in situ formed concentrations of phenol and benzyl
alcohol and increases so alkylation yields. Note that the TOF of
4
ꢀ1
ꢀ1
ether hydrolysis in water (1.1 ꢂ 10 mol mol
þꢃH
h ) was one
½
H
2
O
magnitude higher than that with HZSM-5 in the aqueous phase
3
ꢀ1
ꢀ1
(
1.1–1.3 ꢂ 10 mol mol
þꢃHZSM-5
h
(Table 2, Entries 4–6), which
½
H
is speculated to the fact that liquid protons in the high-
temperature water are more accessible than acid sites in the solid
zeolite. To verify this hypothesis, BPE conversion with equivalent
amount of liquid acid (H
2
SO
4
) was carried out at the same
3.4. Cleavage of CꢀO bond of BPE in undecane at 523 K
conditions, attaining an initial comparable cleavage TOF of
3
ꢀ1
ꢀ1
1
.4 ꢂ 10 mol mol
þ h
as HZSM-5 (Table 2, Entry 7 and Support-
When changing the aqueous to apolar phase, the fundamental
chemistry for cleaving of CꢀO bond of BPE is changed. Without cat-
alyst either in N or in H atmospheres (4.0 MPa), the CꢀO bond of
½
H ꢃ
ing information Table S2). Tested at the temperature range from
4
83 to 543 K, the apparent activation energy of BPE hydrolysis
2
2
ꢀ
1
ꢀ1
with H
2
SO
4
was 163 kJ mol , which was 116 kJ mol higher than
BPE was cleaved by pyrolysis in undecane, but not by hydrolysis
because of the absence of water. The cleavage rates in undecane
were much slower than those in the aqueous phase, i.e., 0.0014
ꢀ1
a
that with HZSM-5 (E = 47 kJ mol ) (Fig. 5). The much lower
apparent activation energy with HZSM-5 can be ascribed to the
added heat of adsorption of BPE on the HZSM-5. The pre-
ꢀ1
ꢀ1
vs. 0.0022 mol h in 4.0 MPa N , and 0.0019 vs. 0.0028 mol h
2
19
ꢀ1
ꢀ1
exponential factor for H
2
SO
4
was 6.4 ꢂ 10 molecules mol
þ
h
,
in 4.0 MPa H , respectively (Table 3, Entries 1 and 2). This indicates
½
H
ꢃ
2
whereas the corresponding value for HZSM-5 was merely
that the rate of ether hydrolysis in water is much faster than pyro-
7
ꢀ1
ꢀ1
4
.5 ꢂ 10 molecules mol
þ h
, strongly indicating that the acces-
SO is much more abundant than the
lysis of ether in undecane, agreeing with the observation of Klein
et al. [19] that in the presence of water, the cleavage rate of BPE
was fourfold higher than that during pyrolysis of the neat substrate
at 605 K. The conversion reached 9.7% for the thermal pyrolysis of
½
H ꢃ
sible acid sites with liquid H
2
4
confined solid acid sites in HZSM-5. Having excluded the thermal
pyrolysis route on BPE in the aqueous phase, the higher hydrolysis
rate in neat water can be also attributed to either OH ions from
-
BPE at 523 K for 30 min. in the presence of H , and the main prod-
2
ionization of water which can act as base to efficiently catalyze
ether hydrolysis [38] or the in situ produced phenol enhances
the acidity of high-temperature water during reaction at 523 K.
When 70 wt.% Ni/SiO was introduced to the reaction mixture,
2
Ni catalyzed the specific hydrogenolysis route on BPE to phenol
ucts were heavier products (selectivity: 92.6%) as well as small
amounts of toluene (selectivity: 3.1%) and phenol (selectivity:
4.3%) (Table 3, Entry 2), indicating that the non-catalytic pyrolysis
breaks down the weakest bond of CaliphaticꢀO of BPE. The heavier
products were formed from the free-radical reaction, dominating
the thermal pyrolysis process (Scheme 2). When an acid compo-
and toluene formation. In addition, the CꢀO bond cleavage rate
ꢀ
1
(
0.1410 mol h ) was nearly 50 times as fast as the rate in the pres-
nent of HZSM-5 was added in the presence of H , the cleavage rate
2
ꢀ1
ꢀ1
ꢀ1
ence of 4.0 MPa H
cleavage TOF of Ni/SiO
2
(0.0028 mol h ) (Table 2, Entry 8). The bond
was increased to 0.0050 mol h
compared to 0.0019 mol h
4
ꢀ1
ꢀ1
2
(1.7 ꢂ 10 mol mol
Surf Ni
h
) was one mag-
without catalysts (Table 3, Entry 3). Similarly, such rate with
3
ꢀ1
nitude higher than that with 0.03 or 0.15 g HZSM-5 (1.1 ꢂ 10 and
HZSM-5 in undecane (0.0050 mol h ) was slower than that in
3
ꢀ1
ꢀ1
ꢀ1
1
.3 ꢂ 10 mol mol
þ h
H ꢃHZSM-5
, respectively). These results imply
water (0.0090 mol h ), which infers again that the hydrolysis
½
that Ni atoms are the major active sites for BPE cleavage when
Ni and HZSM-5 components are present together. Adding 10 wt.%
Ni/HZSM-5 (Table 2, Entry 9) verifies the hypothesis that the major
reaction pathway for C–O bond cleavage of BPE was Ni-catalyzed
hydrogenolysis and that the hydrolysis route only contributed to
a minor part (Fig. 4).
route in the aqueous phase is much faster than the pyrolysis path-
way in undecane.
However, the metallic Ni/SiO selectively catalyzes the hydrog-
2
enolysis of CꢀO bond of BPE with
a
cleavage rate of
ꢀ1
0.2172 mol h , which was 114 times as fast as the rate of pyroly-
ꢀ1
sis without catalyst in the presence of 4.0 MPa H (0.0019 mol h
2
)
Combing these results allows us drawing the general reaction
pathways on conversion of BPE with different catalyst components
(Table 3, Entry 4). Over Ni sites, the primary products were toluene
and phenol (selectivity: >80%), while the free-radical products

4
8
J. He et al. / Journal of Catalysis 311 (2014) 41–51
Table 3
The effect of gas atmosphere, HZSM-5 (acid), and Ni/SiO
2
(metal) on the reaction of benzyl phenyl ether (BPE) in undecane at 523 K.a
TOF (mol mol 1
ꢀ
h
ꢀ1
)
Entry Active center in gas atmosphere Conv.
Selectivity (C%)
Initial rate
(mol h )
ꢀ
1
(
%)
Heavier
products
1
2
3
N
H
H
2
2
2
6.8
9.7
1.8
3.1
3.6
3.8
4.3
11.0
94.4
92.6
85.4
0.0014
0.0019
0.0050
–
–
2
ꢀ
þ
H
1
ꢀ1
, HZSM-5
25.2
14.5
25.9
6
2
3
.3 ꢂ 10 mol mol
h
ꢃHZSM-5
½
+
ꢀ6
n ([H ]HZSM-5) = 8.0 ꢂ 10 mol
, 70 wt.% Ni/SiO
N ([Surf. Ni]) = 8.5 ꢂ 10 mol
, 10 wt.% Ni/HZSM-5
4 b,c
5b
ꢀ1
ꢀ1
H
2
2
45.5 0.6 19.1 5.7
32.4 0.6 19.4 2.9
9.9
2.7
2.3
2.1
3.2
3.3
13.7
36.6
0.2172
0.0259
4
.6 ꢂ 10 mol mol
h
Surf Ni
ꢀ
6
H
2
4
ꢀ1
ꢀ1
.5 ꢂ 10 mol mol
h
Surf Ni
ꢀ
7
n ([Surf. Ni]) = 7.4 ꢂ 10 mol
+
ꢀ5
n ([H ]HZSM-5) = 1.0 ꢂ 10 mol
a
Reaction conditions: Benzyl phenyl ether (1.84 g), undecane (80 mL), HZSM-5 (Si/Al = 45, 0.030 g), Ni/SiO
2
(70 wt.%, 0.010 g), Ni/HZSM-5 (10 wt.%, 0.050 g), gas pressure
(
4 MPa), 30 min., stirring at 700 rpm.
b
Benzyl phenyl ether (9.20g).
c
2
min.
Ni/HZSM-5 the CꢀO bond cleavage TOF in undecane (3.5 ꢂ
4
ꢀ1
ꢀ1
1
(
0 mol mol
h
) (Table 3, Entry 5) was much lower in water
Surf Ni
5
ꢀ1
ꢀ1
1.1 ꢂ 10 mol mol
h
), which may be attributed to the fact
Surf Ni
that BPE is protonated in the high-temperature water which makes
the further CꢀO bond cleavage much easier, which is supported by
the DFT calculations discussed in the following part.
3
.5. Theoretical calculations for cleavage of CꢀO bond of BPE in the
aqueous and aploar phases
Accompanying with the experimental investigation, DFT calcu-
lations explore potential reaction routes for cleaving the CꢀO
bonds of BPE in aqueous and apolar phases. In the aqueous phase,
the direct cleavage of the CꢀO bond of the BPE into benzyl cation
þ
ꢀ
(
C
6
H
5
CH ) and phenoxy anion (C
H
6 5
O ) was found to be difficult,
2
ꢀ1
requiring a high endothermic energy of 238 kJ mol . This agrees
perfectly with the reported bond strength of CaliphaticꢀO bond in
ꢀ1
BPE (234 kJ mol ) [41]. In an alternative way, BPE was facilitated
to be protonated by the adjacent protons, and subsequently, the
protonated BPE could be readily cleaved forming phenol and ben-
zyl cation with a low activation barrier of 9.9 kJ mol (Fig. 6).
Compared with the direct C–O cleavage with a high endothermic
energy, the C–O cleavage of the protonated BPE is slightly exother-
mic. The formed benzyl cation was very active, combining with
phenol or water in the aqueous phase in the next step. In essence,
in the first route, the benzyl cation combines with phenol to
Scheme 2. The reaction pathways for conversion of BPE with different catalyst
components in the non-polar solvent undecane in the presence of hydrogen at
ꢀ1
5
23 K.
were highly suppressed (selectivity: 13.7%) at a conversion of
4.5%. In addition, the partial hydrogenation of aromatic ring of
BPE also occurred on Ni/SiO in undecane, producing cyclohexylm-
ethyl phenyl ether (selectivity: 2.3%) and benzyl cyclohexyl ether
1
2
produce 2-benzylphenol or 4-benzylphenol, while in the latter
þ
(
(
selectivity: 3.2%) (Table 3, Entry 4). The bond cleavage TOF of Ni
pathway, the benzyl alcohol (C
6
H
5
CH
2
OH ) formed via combina-
2
4
ꢀ1
ꢀ1
2.6 ꢂ 10 mol mol
h
) was 40 times as fast as that with
tion of protonated benzyl cation and water is deprotonated to
benzyl alcohol. Then, the produced proton is transferred to the
neighboring water. Our calculations support the second route. It
shows that the coupling between the benzyl cation and water
Surf Ni
2
ꢀ1
ꢀ1
HZSM-5 (6.3 ꢂ 10 mol mol
þꢃHZSM-5
h ). It should be addressed
½H
that in the non-polar solvent TOF for CꢀO bond cleavage on
4
ꢀ1
ꢀ1
Ni/SiO
2
(2.6 ꢂ 10 mol mol
h
) was slightly higher than that
Surf Ni
4
ꢄ
act
ꢀ1
ꢀ1
ꢀ1
in the aqueous phase (1.7 ꢂ 10 mol mol
h
). It has been con-
(
D
G
= 9.0 kJ mol ) was almost comparable to that of coupling
Surf Ni
ꢄ
sistently demonstrated that Ni surface atoms are the active sites
for hydrogenolysis of CꢀO bond of BPE in the aqueous and apolar
solvent (Schemes 1 and 2). The lower activity of Ni in aqueous
phase may be attributed to the weaker adsorption of the reacting
substrates in the presence of water [39]. It has been reported that
in the aqueous phase, the Ni catalyst would deactivate due to
water induced formation of Ni-bridged hydroxo species [40]. With
Ni/HZSM-5, the major reaction pathway was hydrogenolysis of the
CꢀO bond of BPE in undecane to produce toluene (initial
selectivity: 33%) and phenol (initial selectivity: 25%) (Scheme 2).
Free-radical reactions via pyrolysis were enhanced after HZSM-5
addition (together with Ni) with selectivity up to 36.6%. With
between the benzyl cation and phenol
14.2 kJ mol , respectively), whereas the deprotonation of C H
6 5
(
D
G
= 12.5 and
act
ꢀ
1
ꢄ
+
ꢀ1
CH
2
OH
2
(
DG
= 53.1 kJ mol ) was much more favored than the
act
ꢄ
ꢀ1
other two CꢀC coupling steps (
DG
= 71.6 and 82.4 kJ mol ,
act
respectively) although the deprotonation step is lightly endother-
mic. The calculated energy profile of BPE conversion in the aqueous
phase (Fig. 6) clearly demonstrates that phenol and benzyl alcohol
were the major products while 2-benzylphenol and 4-benzylphe-
nol were formed in minor parts. This is consistent with our
experimental observation that hydrolysis dominates for CꢀO bond
cleavage in BPE, while the following CꢀC coupling reactions played
a minor role. Our calculation also shows that the required energy

J. He et al. / Journal of Catalysis 311 (2014) 41–51
49
+
TS (-4.0) / 53.1 / 32.9
TS (+3.2) / 71.6 / 90.2
TS (+0.7) / 82.4 / 122.9
+
TS (-21.8) / 14.2 / 59.9
TS (-23.5) / 12.5 / 44.9
TS (-27.0) / 9.0 / 30.1
+
+
TS (-24.7) / 9.9 / 11.3
-34.6
-36.0
-36.9
+
+
+
-
57.1
protonation
-68.4
C-O scission
+
-
81.7
-87.0
+
+
-122.2
+
deprotonation
+
Fig. 6. Energy profile of benzyl phenyl ether conversion in the aqueous phase calculated by DFT.
ꢀ
1
was 655.5 kJ mol for forming benzyl and phenoxy radicals from
BPE in the aqueous phase, so that the pyrolysis route was not fa-
vored in this sense.
demonstrated that the dominant intermediates were benzyl and
phenoxy radicals in undecane obtained by the CꢀO bond cleavage
of the BPE, due to the much lower bond dissociation energy
ꢀ1
In contrast to the major hydrolysis pathway of BPE in water,
based on the experimental results, the conversion of BPE followed
the pyrolysis route in the apolar solvent such as undecane. Our the-
oretical calculations support this conclusion as well. It
(184.3 kJ mol ) compared to that of phenyl and benzoxy radicals
ꢀ1
(332.9 kJ mol ) and that of benzyl cation and phenoxy anion
ꢀ1
(627.7 kJ mol ) (Fig. 7a). The formed two radicals (benzyl and
phenoxy radicals) spontaneously recombined together with an en-
ꢀ
1
ergy drop of ꢅ150 kJ mol (Fig. 8), resulted in producing 2-C
OCH and 4-C OCH radicals. In turn, the H atoms in
-C OCH and 4-C OCH radicals stepwise shifted
6 5
H
2
C
H
6 5
H
6 5
2 6 5
C H
H
2
6
H
5
C
2 6
H
5
6
5
2 6 5
C H
to the Oradical atom via H atom transfer forming the final products
of 2-benzylphenol and 4-benzylphenol (Fig. 8). The free activation
(
a) C−O bond cleavage mode of BPE in undecane
O
ꢀ1
barriers for the H atom transfer were 216.1 and 210.4 kJ mol in
-C OCH and 4-C OCH , respectively. Comparing
2
6
H
5
2
C
6
H
5
6
H
5
2 6 5
C H
+
ꢀ1
to the relative low activation barriers (9.9 kJ mol ) in the hydroly-
sis path of BPE in water, the activation barriers in the pyrolysis of
the BPE in undecane were much higher (184.3 kJ mol ), which is
in line with the experimental results that the rates of BPE conver-
sion were slower in undecane than in water. In addition, the
formation of phenol and toluene was thermodynamically favorable
ꢀ
1
O
+
O
2
in undecane (Fig. 7b). However, the high H dissociation energy
results in the quite low concentrations of hydrogen radical without
the aid of metals, which eventually leads to a low selectivity to
phenol and toluene. In summary, CꢀO bonds of BPE in undecane
are majorly cleaved by pyrolysis for producing phenyl and benzoxy
radicals as intermediates, and the subsequent coupling of CꢀC
O
-
+
+
bonds forms via radical recombination producing 2-C
6
5 2 6 5
H OCH C H
(
b) Mechanism of phenol and toluene formation in undecane
and 4-C
results.
6 5 2 6 5
H OCH C H , fitting well with the previous experimental
ΔE = +415.0
H2
H
+
H
O
4. Conclusion
OH
ΔE = -310.4
ΔE = -390.3
In the aqueous phase, the CꢀO cleavage of BPE proceeds with an
H
H
+
+
ionic pathway at 523 K, but not via a thermal/free-radical route. In
the neat water in the absence or the presence of H
2
, the acid
(
hydronium ions from dissociated water)-catalyzed hydrolysis
route dominates, and subsequently, the alkylation between the
phenol and alcohol fragments follows to a small extent. In the
presence of HZSM-5, both hydrolysis and alkylation rates are sub-
stantially enhanced. Ni/SiO
and selective hydrogenolysis of CaliphaticꢀO bond to form phenol
2
, in contrast, leads to the quantitative
Fig. 7. (a) Three CꢀO bond cleavage modes of BPE in undecane, and (b) proposed
mechanism of phenol and toluene formation in undecane.

5
0
J. He et al. / Journal of Catalysis 311 (2014) 41–51
TS (+251.8) / 216.1 / 287.3
O
TS (+233.4) / 210.4 / 273.8
+
TS (+198.8) / 163.1 / 24.8
TS (+183.7) / 9.9 / 160.7
173.8
+
184.3
+
0.0
+35.5
+
35.7
+
23.0
-35.5
-40.3
Fig. 8. DFT calculated energy profile of benzyl phenyl ether conversion in undecane.
and toluene in the presence of H
2
. In the presence of metal and
Appendix A. Supplementary material
acid sites, metal (Ni)-catalyzed hydrogenolysis plays a more
important role for cleaving the ether bond in BPE. In apolar
undecane at 523 K, non-catalytic pyrolysis cleaves the
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2013.10.024.
C
aliphaticꢀO bond of BPE, and thus, the free-radical products
dominate. When metals such as Ni/SiO and Ni/HZSM-5 are
involved, hydrogenolysis dominates the conversion, markedly
suppressing the radical reaction. Ni/SiO leads to higher hydrog-
enolysis rates in undecane than in water, probably because of
the much weaker adsorption of reactants onto active Ni sites
in the aqueous phase.
2
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