Silver initiated hydrogen spillover on anatase TiO2 creates active sites for selective hydrodeoxygenation of guaiacol

Article abstract of DOI:10.1016/j.jcat.2018.11.033

Selective hydrocleavage of aromatic C–O bond while preserving the aromatic ring is a desired catalytic process in hydrodeoxygenation (HDO) of biomass derived aromatic oxygenates to high value biochemicals. Guaiacol with typical methoxy and phenolic hydroxyl groups is a representative aromatic di-oxygenates of lignin fragments. Sub-nanometer sized silver, a poor hydrogenation metal catalyst, was found to initiate H₂ dissociation and H spillover onto anatase TiO₂ (TiO₂-A) surface. The spillover hydrogen enhanced the partial reduction of TiO₂-A surface at a substantially lowered temperature as compared to the reduction of TiO₂-A by molecular H₂. The reduction of TiO₂-A surface by spillover hydrogen was enhanced at Ag loading as low as 0.01 wt% on TiO₂-A. The ratio of spillover H to Ag was as high as 675 at Ag loading of 0.01 wt% and the ratio decreased with increasing Ag loading. We found that the yield of phenolic products was linearly correlated with the amount of spillover hydrogen that created oxygen vacancy (Ov) through partial reduction of TiO₂-A surface. Methanol was formed as the main side-product, indicating that the direct hydrocleavage of aromatic C-OCH₃ in guaiacol is favored over that of aromatic C-OH on the created site.

Full text of DOI:10.1016/j.jcat.2018.11.033

Journal of Catalysis 369 (2019) 396–404
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Silver initiated hydrogen spillover on anatase TiO creates active sites for
2
selective hydrodeoxygenation of guaiacol
Kairui Liu a,b, Peifang Yan , Hong Jiang , Zhi Xia , Zhanwei Xu , Shi Bai c,d, Z. Conrad Zhang a,
a
a
a
a
⇑
a
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics， 457 Zhongshan Road, Dalian 116023, Liaoning, China
University of Chinese Academy of Sciences, Beijing 100049, China
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, United States
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective hydrocleavage of aromatic CAO bond while preserving the aromatic ring is a desired catalytic
process in hydrodeoxygenation (HDO) of biomass derived aromatic oxygenates to high value biochemi-
cals. Guaiacol with typical methoxy and phenolic hydroxyl groups is a representative aromatic
di-oxygenates of lignin fragments. Sub-nanometer sized silver, a poor hydrogenation metal catalyst,
Received 21 August 2018
Revised 24 November 2018
Accepted 26 November 2018
was found to initiate H
hydrogen enhanced the partial reduction of TiO
compared to the reduction of TiO -A by molecular H
gen was enhanced at Ag loading as low as 0.01 wt% on TiO
2
dissociation and H spillover onto anatase TiO
-A surface at a substantially lowered temperature as
. The reduction of TiO -A surface by spillover hydro-
-A. The ratio of spillover H to Ag was as high as
2 2
(TiO -A) surface. The spillover
Dedicated to the 70th anniversary of Dalian
Institute of Chemical Physics, CAS
2
2
2
2
2
Keywords:
Hydrodeoxygenation
Silver
Titania
Hydrogen spillover
Guaiacol
675 at Ag loading of 0.01 wt% and the ratio decreased with increasing Ag loading. We found that the yield
of phenolic products was linearly correlated with the amount of spillover hydrogen that created oxygen
vacancy (Ov) through partial reduction of TiO -A surface. Methanol was formed as the main side-product,
2
indicating that the direct hydrocleavage of aromatic C-OCH
C-OH on the created site.
3
in guaiacol is favored over that of aromatic
Ó 2018 Published by Elsevier Inc.
1
. Introduction
Various catalysts have been studied for the HDO of guaiacol,
such as metals, metal sulfides, metal phosphides, metal carbides,
and metal nitrides [1]. Noble metals are known to catalyze the full
hydrogenative saturation of aromatics at typical HDO conditions
[3–5], to products such as cyclohexane and cyclohexanol. Olcese
Lignocellulosic biomass is an abundant renewable feedstock for
the production of biofuels, biochemicals and biomaterials. Lignin
as the most abundant aromatic biopolymer in nature, accounting
to 20–30 wt% of lignocellulose [1], is a sustainable resource for
the production of aromatic chemicals. Different processes,
including pyrolysis, have been reported for the depolymerization
of lignin to oxygenated aromatics. Guaiacol, which contains a
methoxy group and a phenolic hydroxyl group, is a representative
structural component of lignin [2].
Through catalytic hydrodeoxygenation (HDO) of guaiacol, hydro-
carbon products such as benzene, toluene, and xylene (BTX) or com-
plete hydrogenation product alkanes may be obtained [1].
Completely hydrogenative saturation of aromatics consumes excess
amount of hydrogen by producing alkanes [1]. It may be envisioned
that selective hydro-demethoxylation of guaiacol not only produces
phenol in a direct process by consuming only one mole of hydrogen,
but also exhibits high atom efficiency as compared to the traditional
cumene oxidation process in producing phenol.
2
et al. reported HDO of guaiacol on Fe/SiO catalyst with a good
selectivity to benzene, toluene, and xylene (BTX), but the phenolic
product yield is low [2]. Metal sulfide catalysts have better
selectivity to aromatic products, but low phenolics selectivity,
and suffer from the deactivation due to loss of sulfur. Continuous
addition of sulfur causes serious problems for the downstream
purification [6–8]. Metal phosphides catalysts, while also produc-
ing aromatics, are more active than metal sulfides catalysts in
the conversion of guaiacol, but coke formation was a serious issue
due to a high concentration of Lewis acid sites on the catalysts
2
[9–11]. Chen et al. investigated Mo C as the catalyst, high BTX pro-
duct yields with a small amount of saturated hydrocarbons (<10%)
were obtained, with low selectivity to phenolic products [12].
Support properties have been recognized for having great influ-
2 3
ence in the HDO process [1]. Acidic supports such as Al O promote
the transalkylation reactions, so favors the formation of catechol
and methylated products. However, the Lewis acid sites also cause
coke formation that resulted in catalyst deactivation [7,13].
⇑
Corresponding author.
E-mail address: zczhang@dicp.ac.cn (Z.C. Zhang).
https://doi.org/10.1016/j.jcat.2018.11.033
021-9517/Ó 2018 Published by Elsevier Inc.
0

K. Liu et al. / Journal of Catalysis 369 (2019) 396–404
397
Benzene was obtained on a Pd/ZrO
ates formed from phenol. However, Pd supported on Al
SiO catalyzed benzene ring saturated product, producing cyclo-
hexanone as the main product. Evidently, the ZrO support altered
the reaction pathway [14]. Furthermore, Griffin et al. [15] studied
m-cresol HDO reaction over Pt catalyst on different supports;
2
catalyst with keto intermedi-
ꢀ196 °C of liquid nitrogen. Each sample was degassed for 2 h at
300 °C before measurement. X-ray diffraction (XRD) measure-
ments were conducted using PIXcel1D detector and PreFIX inter-
face with an empyrean tube Cu LFF operated at 40 kV. The
scanning range is in 20ꢀ85° with a step of 0.013°.
2 3
O
or
2
2
Temperature programmed reduction (TPR) experiments were
performed on Micromeritics AutoChem II 2920 system with a ther-
mal conductivity detector (TCD). Samples that were calcinated at
400 °C in air for 3 h separately, and were pre-dried at 150 °C for
1 h in air prior to the TPR experiments. The reducing gas was 5%
toluene was produced over a Pt/TiO
2
(anatase phase) catalyst at
high temperature and low pressure.
Silver supported on TiO and SiO have been reported to cat-
2 2
alyze highly selective hydrogenation of crotonaldehyde to desired
unsaturated alcohol products [16]. A range of chloronitrobenzenes
were hydrogenated to their corresponding chloroanilines without
hydrogenation of benzene ring [17].
2
H /Ar; TPR experiments were carried out by increasing the temper-
ature to 800 °C with a linear heating ramp of 10 °C/min. Water
vapor produced during the reduction was collected by a cold trap.
For the TPR-MS experiment, the water vapor wasn’t cooled down.
Recently, we reported the remarkably high selectivityto phenolic
compounds in guaiacol conversion over TiO
2
-A in the presence of
2
For the measurement of H consumption at specified reduction
pre-made large Au nanoparticle [18]. A low Au surface area associ-
ated with low Au loadings (0.1 to 0.7 wt%) and large Au particles
temperature (325 °C or 400 °C), the temperature was increased to
the setting temperature (10 °C/min), and kept at the temperature
(
2.7–41 nm) were sufficient to activate the TiO
2
-A in H
2
for the cat-
for 2 h to simulate the pre-reduction process. The H
for the partial reduction of TiO -A was obtained through the inte-
gration of the peak area between 100 °C and the set end tempera-
ture in the corresponding profile. The H consumption in the pre-
reduction of Ag/TiO -A surface was used to account the reducible
sites. The calculation was based on the H calibration curve
through external standard method by correlating the TCD signal
intensities with the corresponding concentrations of H gas in Ar
2
consumption
alytic HDO of guaiacol. Ag is a less studied metal in hydrogenation
for its low catalytic hydrogenation activity. In addition, silver is a
much less expensive metal than gold. It was also particularly easy
to synthesize highly dispersed Ag atoms on a support. Therefore,
Ag was chosen in this work to investigate its capability in initiating
2
2
2
2
H
2
dissociation and H spillover in order to gain better understanding
on the role of spillover H on an oxide support like TiO in HDO of gua-
iacol. It was hypothesized that the quantitatively measured H con-
sumptions for the partial reduction of TiO -A surface at
progressively increasing Ag loading on a TiO -A may help under-
stand the performance of the Ag/TiO -A catalysts, and therefore
2
2
2
gas using the gas concentration calibration program of Micromerit-
ics AutoChem II 2920 system.
2
2
The surface composition and the binding energy (B.E.) of the
catalysts were determined by X-ray photoelectron spectra (XPS)
on an ESCALAB250 X-ray photoelectron spectrometer with con-
taminated C as the internal standard (C1s = 284.6 eV). The curve
fitting was done using XPS41 program. All of the samples were
compressed into thin tablets before reduction. After reduction at
2
may allow us to establish the role of spillover hydrogen on the
HDO performance.
2
. Experimental
3
2
25 °C for 2 h in 10% H /Ar, a sealed transfer cell was used to trans-
port the ex-situ reduced samples from a glove box to the analysis
chamber without exposure to air.
2.1. Materials and chemicals
High resolution transmission electron microscope (HRTEM)
images of the catalysts were taken to observe the trend of particle
size change and morphology of the supported Ag using a JEOL JEM
Guaiacol (99.0%), TiO
AC (active carbon) and SiO
Sigma-Aldrich Corporation, Phenol (99%), catechol (98%). N-
heptane (99.5%), n-decane (99%) and acetic ether (99.5%) were pur-
chased from SinopharmChemical Reagent Co., Ltd. Cresols, Xylenols,
and trimethylphenols were obtained from Aladdin Industrial Inc.
2
-A (anatase, 40 nm), TiO
2
-R (rutile, 40 nm),
2
(20–50 nm) were purchased from
2
100 microscopy at 200 kV. Prior to observation, the samples were
ultrasonically dispersed in ethanol, and a few droplets of the sus-
pension were put on copper grids and dried at room temperature.
Above 200 nanoparticles were measured to get the size distribu-
tions of Ag in the TEM image using a NanoMeasure of 1.2.
CH
4
as calibration gas was obtained from DL-Gas Co., Ltd.
A leveled attenuated total reflectance (ATR) accessory with a 3
mm diameter diamond plate purchased from Pike Technologies
was used for the infrared spectroscopy measurement on a Thermo
Scientific Nicolet iS50 Adv FTIR Spectrometer equipped with a liq-
uid nitrogen cooled MCT detector. The spectra were recorded in the
range from 4000 to 650 cm at 4 cm resolution and 64 scans per
sample. A steady flow of nitrogen was purged above the sample to
maintain a dry atmosphere during the measurement. The catalyst
was collected through filtration after reaction, and dried at 40 °C
for 6 h before measurement.
Thermo gravimetric analysis (TGA) was performed on a thermal
analyzer (STA 449 F3, NETZSC, Germany). The used catalyst was
heated from room temperature to 900 °C at a rate of 10 °C/min in
air. The catalyst was collected through filtration after reaction,
and dried at 40 °C for 6 h before measurement.
2
.2. Catalyst preparation
Ag/TiO
nation method with an aqueous solution of AgNO
TiO -A catalysts (0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2 and 3 wt%) were
synthesized by varying the concentration of AgNO solution. After
impregnation, the powder was vacuum dried at 80 °C for 12 h and
oven dried at 120 °C for another 12 h. The sample was then calcined
at 400 °C for 3 h. Before catalytic evaluation, the catalyst was pre-
reduced at 325 °C in 10% H
catalyst, two reduction temperatures were chosen: 325 and
4
0
3
2
-A catalyst was synthesized by incipient wetness impreg-
3
. A series of Ag/
ꢀ1
ꢀ1
2
3
2 2
/Ar for 2 h. For the 0.05 wt% Ag/TiO -A
2 2
00 °C. Ag/TiO -R, Ag/AC and Ag/SiO with a silver loading of
.5 wt% were synthesized by the same method and reduced at
25 °C prior to catalytic evaluation. All the catalysts after reduction
were sealed in black bottles and stored at room temperature (RT) in
glove box.
2.4. Measurement of catalytic performance
2.3. Catalyst characterization
The reactions were carried out in a 50 mL batch reactor. In a
typical reaction, 1.5 g guaiacol, 0.5 g reduced catalyst (for 0.5 wt%
ꢀ
3
The BET surface area was measured by N
Micromeritics ASAP 2020HD88 instrument by N
2
adsorption on a
adsorption at
Ag/TiO
2
-A, Ag/guaiacol = 1.9 ꢁ 10 mol/mol), and 18.5 g n-
2
heptane (solvent) were introduced into the reactor. The reactor

3
98
K. Liu et al. / Journal of Catalysis 369 (2019) 396–404
was repeatedly flushed with 3 MPa H
2
for 6 times before being
X-ray diffractograms of the Ag/TiO
ings along with that of the commercial TiO
in Fig. S1. No rutile was found in the XRD pattern of TiO
confirming the pure anatase phase of the TiO -A support
(2h = 25.28, 48.05, 55.06. JCPDS NO. 21-1272). The XRD pattern of
Ag/TiO -A is nearly identical to that of TiO -A even when the Ag
loading was increased to 3 wt%. The absence of metallic silver
peaks suggests highly dispersed Ag over TiO -A support [20]. The
X-ray diffractograms of Ag/TiO -R, Ag/AC and Ag/SiO are shown
2
-A with different Ag load-
-A powder are shown
-A sample,
sealed and heated to 300 °C. The agitation speed was 700 r/min
after verification that it is sufficient to eliminate diffusion of reac-
tant to the catalyst particles (Fig. S6). After reaction for 1 h, the
reactor was cooled to ambient temperature by external water.
2
2
2
3
0 mL of ethyl acetate was added into the solution to enhance
2
2
the solubility of products and guaiacol, and 0.6 g n-decane was
added as the internal standard reference for quantitative analysis.
The products and guaiacol in the solvent were quantitatively ana-
lyzed by an Agilent Technologies 7890A GC equipped with a flame
ionization detector (FID) and a DB-FFAP column. The products
were confirmed by an Agilent Technologies 7890A/5975C GCMS
2
2
2
in Fig. S2. No metallic silver peaks appeared except for the Ag/AC.
High resolution transmission electron microscopy images of Ag/
TiO -A catalysts with different Ag loadings are shown in Fig. 1 and
2
calibrated by standard compounds. The gas phase product CH
was analyzed by an Agilent 7890A gas chromatography equipped
with two TCD detectors and one FID detector.
4
Fig. S3. No silver nanoparticles (NPs) were observed until the Ag
loading was increased to 2 wt%, although energy dispersive X-ray
spectroscopy (EDX) analysis confirmed the presence of Ag on
The product yield, selectivity of phenolic compounds, catechol
and others were calculated as follows:
TiO
loading was 2 wt%, the average diameter of Ag is 1.35 nm
Fig. 1e), and increased to 1.73 nm at 3 wt% Ag loading (Fig. 1f).
The metal particle size in this range is below the detection sensitiv-
ity of X-ray diffraction [21], which is consistent with the XRD
results (Fig. S1).
2
-A in the samples with low Ag loadings (Table S1). When Ag
(
mol of product produced
mol of guaiacol feed
Yield ð%Þ ¼
ꢁ 100%
mol of product produced
mol of guaiacol consumed
H
2
-TPR experiments were performed for TiO
catalysts to study the effect of Ag loading on the enhanced reduc-
tion of the TiO -A support (Fig. 2). In the absence of supported Ag,
2 2
-A and Ag/TiO -A
Selectivity ð%Þ ¼
ꢁ 100%
2
The TOF for the phenolics yield was calculated as follows:
one major peak at high temperature (584 °C) appeared, which is
consistent with the reported value in literature [22]. The peak
appeared at around 80 °C corresponds to the reduction of silver
oxides [23]. The anatase reduction peak was moved to a markedly
lowered temperature (524 °C) even when trace amount of Ag
ꢀ ꢀ
1
ꢁ
mol of phenolics yield
mol of hydrogen consumption
1
TOF
h
¼
ꢁ t ðt : reaction timeÞ
The hydrogen consumption was for TiO
reduction process.
2
-A reduction in the pre-
xylenols and
(0.01 wt%) was loaded on TiO -A. With increasing silver loading,
2
Phenolic
products:
phenol,
cresols,
the H2 consumption profile shifted to further lowered tempera-
tures (331 °C at 3 wt% Ag loading). When the silver loading was
>0.1 wt%, more than one peaks were observed for the partial reduc-
trimethylphenols.
Others: anisole, 1,2-dimethoxybenzene and 2-methoxy-4-
methylphenol.
Notes: the phenolics yield was given directly in mmol where
appropriate.
tion of TiO -A surface. The peaks between 200 and 300 °C may be
2
attributed to the reduction of Ag-O-Ti interface or the reducible
sites on TiO -A close to Ag [24,25], and the peaks above 300 °C
2
belongs to the reduction of the reducible sites on TiO
Ag [26,27]. Here, these peaks above 100 °C were all classified as the
partial reduction of TiO -A surface. Evidently, the presence of silver
greatly lowered the reduction temperature of TiO -A surface.
However, even when the silver loading was as low as 0.01 wt%,
the reduction of the TiO -A surface began at around 300 °C. Only
one main peak appeared at >100 °C lower than that of TiO -A
reduction without Ag. We attribute the enhanced reduction of
TiO -A in the TPR profile of 0.01 wt% Ag/TiO -A as compared to that
of pure TiO -A to the spillover H originated from Ag or the Ag-TiO
A interface [28]. The hydrogen consumption for the partial reduc-
tion of TiO -A surface was substantially higher than the silver load-
ing amount as demonstrated by the rather high n(H)/n(Ag) value of
75, which is a clear evidence for hydrogen spillover onto the TiO
A surface originated from Ag initiated H dissociation. Concomitant
with the partial reduction of TiO -A surface, H O was detected
Fig. S4). The high H O intensity indicated that the hydrogen con-
sumed for the partial reduction of TiO -A surface mainly con-
tributed to the generation of Ov on TiO -A surface [25].
The gradual shift of the main H consumption peak to lower
2
-A apart from
2
.5. The reuse of the catalyst
2
0
.5 wt% Ag/TiO -A was used in the catalyst reuse. After each
2
2
run, the catalyst was collected through filtration (washed by ethyl
acetate), and dried at 100 °C for 6 h in oven. The catalyst was
reduced again in 10% H
reaction.
2
2
/Ar at 325 °C for 2 h before subsequent
2
2
2
2
2
-
3
. Results and discussion
2
3.1. Catalyst characterization
6
2
-
Table 1 shows the BET surface area of the catalysts and support.
2
TiO
0 nm. The surface areas of Ag/TiO
increasing Ag loading, and this may come from the addition of
2
-A support used in this work have an average diameter of
2
2
4
2
-A catalyst decreased with
(
2
2
2
Ag on the surface of TiO -A [19].
2
2
Table 1
BET surface area of Ag/TiO
temperature in the TPR profiles with increasing Ag loading may
be attributed to two factors: (1) at low Ag loading, spillover H
2
-A catalysts.
2
migrates over a longer distance on the TiO
temperature was set to linearly increase with time. The rate of spil-
lover H traveling to remote reducible surface TiO sites during
2
-A surface while the
Catalyst
BET surface area (m /g)
TiO
2
-A
77.8
73.5
71.0
70.9
69.1
62.6
56.3
0
0
0
1
2
3
.1 wt% Ag/TiO
.2 wt% Ag/TiO
.5 wt% Ag/TiO
wt% Ag/TiO
wt% Ag/TiO
wt% Ag/TiO
2
2
2
-A
-A
-A
-A
-A
-A
2
increasing temperature makes the TPR peak appear at higher tem-
perature; (2) at increasing Ag loading, the coverage of Ag particles
on the TiO surface was increased, and a larger pool of dissociated
2
H proportional to the Ag loading may be expected. The increased
concentration of available H atoms and the shortened distance of
2
2
2

K. Liu et al. / Journal of Catalysis 369 (2019) 396–404
399
Fig. 1. HRTEM images of (a) 0.1 wt% Ag/TiO
Ag/TiO -A.
2
-A; (b) 1 wt% Ag/TiO
2
-A; (c) 2 wt% Ag/TiO
2
-A; (d) 3 wt% Ag/TiO
2
-A and the size distribution of (e) 2 wt% Ag/TiO
2
-A and (f) 3 wt%
2
the H atoms to reach a reducible TiO
profile toward lower temperatures.
The chemical state of Ag on Ag/TiO
XPS measurements. The XPS spectrum of 0.1 wt% Ag/TiO
showed the binding energies of Ag3d5/2 and Ag3d3/2 peaks corre-
sponding to 368.2 and 374.2 eV respectively (Fig. 3), which is con-
sistent with the metallic Ag. At the same time, the gap between the
Ag3d5/2 and Ag3d3/2 peaks is 6.0 eV, which further indicates the
metallic state of Ag atoms [29].
2
-A surface sites shift the TPR
sponding supports are shown in Fig. S7, Ag on these two supports
showed very low yield and selectivity to phenolics. Catechol yield
was slightly increased with 0.5 wt% Ag on AC compared to that
2
-A catalyst was analyzed by
-A
2
with AC only, concomitant with the formation of CH
sistent with the Ag catalyzed hydrogenolysis of the CArO-CH
4
, which is con-
bond.
3
The low yield and selectivity of phenolics on Ag/AC and Ag/SiO
indicates the weak catalytic activity of Ag on HDO of guaiacol.
2
Guaiacol conversion was very low on TiO
2
-A. However, When
silver was supported on TiO -A, both of the guaiacol conversion
2
and the phenolic product yield were increased (Fig. 4a). The differ-
ence (about 5%) between conversion and yield may be partly due to
the adsorbed guaiacol and products on the catalysts (about 1.8 wt
%) and the heavy products (3.2 wt%) which are undetectable by GC
or GC/MS (Fig. S8 and Fig. S9). Neither complete HDO products,
BTX, nor benzene ring saturation products were detected on
TiO -A supported Ag catalysts. With increasing Ag loading from
2
0.05 wt% to 0.2 wt%, the conversion of guaiacol and phenolic pro-
duct yield increased sharply (Fig. 4a, from 18 to 34%). Increasing
3
.2. Catalytic performance
2
The HDO of guaiacol on Ag/TiO -A catalyst was optimized first.
2
The H pressure chosen was 3 MPa, above which the reaction was
not affected (Fig. S5). The agitation speed of 700 r/min was suffi-
cient for the full mixing of the content in the reactor (Fig. S6).
The typical reaction temperature for the HDO of guaiacol was
3
2
00 °C. Guaiacol conversions on Ag/AC, Ag/SiO and the corre-

4
00
K. Liu et al. / Journal of Catalysis 369 (2019) 396–404
loading. The hydrogen consumption increased sharply from
.05 wt% to 0.2 wt%, and then slowed between 0.2 and 1 wt%. The
value became stabilized around 0.47 mmol/g, corresponding to the
total reducible sites on TiO -A surface at 325 °C. With the rising of
0
2
hydrogen consumption, the phenolic product yield was synchroni-
cally increased (Fig. 4c). Remarkably, a linear relationship between
phenolic product yield and H
of the TiO -A surface was found (Fig. 4b). To further confirm the lin-
ear correlation between phenolic product yield and H consumption
in the pre-reduction process, the pre-reduction temperature of
.05 wt% Ag/TiO -A was increased from 325 °C to 400 °C (Fig. 4a,
b). H consumption was increased 77.17% (from 0.14 mmol/g to
.25 mmol/g), and correspondingly the phenolic product yield was
2
consumption in the pre-reduction
2
2
0
2
2
0
increased by 71.45% (from 8.30% to 14.23%).
The results indicate that the dissociated hydrogen spilt over
onto TiO
water evolution (Fig. S4). The linear correlation of the phenolics
yield with the amount of spillover H measured in the H pretreat-
ment indicates that the generated Ov represents the catalytic HDO
sites in subsequent guaiacol conversion. During the reaction phase,
2
-A surface, and generated Ov [30,31], as evidenced by
Fig. 2. H
and the corresponding n(H)/n(Ag) ratio (mol/mol). (n(H) is the number of
dissociated hydrogen atom in mol consumed in the partial reduction of TiO -A,
and the H consumed for the reduction of silver oxides was excluded in the n(H)
value even when the hydrogen consumption peak for the reduction of silver oxide is
very small).
2 2 2
-TPR profiles of TiO -A and Ag/TiO -A catalysts with different Ag loadings
2
2
2
2
the supported Ag served as the center of H dissociation and con-
tinuously supplied spillover H for the HDO reaction. The results
in Fig. 4c further demonstrate the critical role of very low Ag load-
ing in facilitating H
sible for the generation of active site for guaiacol HDO by partial
reduction of TiO -A surface. Therefore, the amount of spillover H
consumed in the generation of Ov in the catalyst pretreatment in
may also be viewed as a titration of the available catalytic sites
on TiO -A under the pretreatment conditions. The average TOF
2
dissociation and spillover, which was respon-
2
H
2
2
ꢀ1
value of 7.51 h for phenolics yield further revealed the Ov site
should be the catalytic sites for the phenolics production (Table 2).
The hydrogen consumption for the reduction of TiO
the pre-reduction process was 0.44 mmol (0.5 wt% Ag/TiO
2
-A surface in
-A),
2
and the corresponding phenolics yield (300 °C, 1 h) was 3.25 mmol,
which was 7.34 times of the possible catalytic sites as counted by
the hydrogen consumption in the pre-reduction step. The depen-
dence of phenolics yield on the measured H
sponding to the pretreatment of the 0.05 wt% Ag on TiO
25 and 400 °C as shown in Fig. 4b further supports this general
2
consumption corre-
2
-A at
3
finding. The excess catalytic sites were generated by the higher
pre-reduction temperature (400 °C), as indicated by the TPR profile
2
for the 0.05 wt% Ag on TiO -A catalyst. Although the oxygen vacan-
cies to which oxygenates such as guaiacol and catechol molecules
may adsorb and be converted by spillover hydrogen, removal of the
2
Fig. 3. Ag3d XPS spectra of 0.1 wt% Ag/TiO -A catalyst.
3 2
oxygenate species as CH OH and H O concomitant with the forma-
Ag loading from 0.2 to 1 wt% only increased the guaiacol conver-
sion moderately (from 34 to 39%). However, further increase Ag
loading up to 3 wt% did not result in appreciable increase in guaia-
col conversion (40%). Although no discernable Ag particles were
observed in Ag loading of 1 wt% in the TEM micrograph (Fig. 1b),
Ag particle size of around 1.35 nm was observed for the 2 wt% Ag
loading (Fig. 1c). Therefore, highly dispersed silver in very low
tion phenolic products after hydrogenation by spillover H presum-
ably requires lower energy than that is required to generate an
oxygen vacancy site from TiO -A [25]. Therefore, although higher
2
pre-reduction temperature may be required to generate Ov sites
for the very low Ag loading catalysts as indicated by the TPR pro-
file, the active sites are maintained by repeatedly recovering the
sites because lower energy is needed to remove CH OH and H O
3
2
loadings was most effective in enhancing the activity of Ag/TiO
A (from 0 to 0.2 wt%). Evidently, the activity of Ag/TiO -A catalyst
did no correlate with Ag loading. Ag or Ag-TiO -A interface [28]
appeared playing a dominant role in H dissociation which was
responsible for the activation of the TiO -A surface.
We quantified the H consumption for the partial reduction of
TiO -A surface through simulation of the pretreatment of Ag/TiO
A catalysts in H at 325 °C for 2 h. The H consumption in the pre-
reduction of Ag/TiO -A surface was used to account the reducible
sites, particularly when the amount of H consumed was in large
2
-
from these sites at the reaction temperature of 300 °C.
Resasco et al. reported the enhanced activity of Ru/TiO -A in the
HDO of guaiacol and proposed the creation of active sites on the
2
2
2
2
TiO surface or at the Ru-Ti interface [26]. Crossley et al. attributed
2
2
the high activity of Ru/TiO2 systems in guaiacol conversion to
defect sites on TiO2 [32]. A better performance was observed on
2
2
2
-
TiO2 (P25, containing both anatase and rutile TiO ) supported Ru
2
2
2
as compared to Ru/TiO -A and was attributed to surface defects
2
2
on TiO -R. With supported weak hydrogenation Ag metal catalyst
2
2
in this work, however, the nanosized TiO -R did not appear to
2
access to the loaded Ag. The amount of hydrogen consumed in the
HDO of guaiacol was verified to be much larger than the amount
show such effect, and TPR results showed that oxygen vacancies
could not be generated on Ag/TiO -R (Fig. S10) [27]. Similarly, Siev-
2
of H
2
consumed in the pre-reduction. Fig. 4c shows the change of
consumption with increasing Ag
ers et al. reported that Ov on CeO –ZrO promoted the HDO of gua-
iacol on CeO –ZrO [33].
2 2
2
2
the phenolic product yield and H
2

K. Liu et al. / Journal of Catalysis 369 (2019) 396–404
401
Fig. 4. Guaiacol hydrodeoxygenation on Ag/TiO
Ag/TiO -A was pre-reduced in 10% H /Ar at 400 °C for 2 h before reaction; all other catalysts were pre-reduced at 325 °C for 2 h). Reaction conditions: 300 °C, 3 MPa H
00 r/min. (b) A linear relationship of phenolic product yield with H consumption determined in the partial reduction of TiO -A surface (exclude the H consumption for the
reduction of silver oxide) during the pre-reduction step, and sliver loading (j) was given in the graph. (c) Phenolic product yield (j), H consumption ( ) in the partial
consumption for the reduction of silver oxide) during the pre-reduction step, and the corresponding n(H)/n(Ag) ratio ( ) at different
2
-A catalyst with different silver loadings: (a) Guaiacol conversion (j) and product yield (bar graph) (0.05–400: the 0.05 wt%
2
2
2
, 1 h,
7
2
2
2
2
reduction of TiO
silver loadings.
2 2
-A (excluding the H
Table 2
2
TOF values in phenolics production on Ag/TiO -A catalysts with different silver loading.
Ag loading/%
0.05
7.18
0.05–400
6.95
0.1
0.2
0.5
1
3
TOF (h 1
ꢀ
)
7.42
7.86
7.34
7.72
8.09
2 2 2
Note: 0.05–400: 0.05 wt% Ag/TiO -A was pre-reduced in 10% H /Ar at 400 °C for 2 h before reaction. Reaction condition: 300 °C, 3 MPa H , 1 h, 700 r/min.
The HDO of guaiacol on the pre-reduced (550 °C for 2 h) pure
TiO -A was negligible. In the absence of Ag that continuous sup-
plies spillover H that regenerates Ov on TiO -A surface, the initial
catalytic sites may be passivated at the initial step of the reaction.
We recycled Ag/TiO -A for repeated tests. The results in Fig. 5a
the reaction conditions. After the 3rd reuse of the catalyst, the
hydrogen consumed in the HDO reaction is about 22 times of the
hydrogen consumed in the pre-reduction step for the creation of
2
2
the active sites on TiO
2
-A.
2
show that the guaiacol conversion and the products yield did not
suffer from repeated use. Moreover, prolonged reaction time
3.3. Mechanism of guaiacol hydrogenation on Ag/TiO -A catalyst
2
(
Fig. 5b) showed nearly linear increase in guaiacol conversion
There are three possible pathways in guaiacol conversion lead-
ing to phenolics (Fig. 6). During the HDO process, negligible anisole
and product yield, indicating that the catalyst was stable under

4
02
K. Liu et al. / Journal of Catalysis 369 (2019) 396–404
2 2
Fig. 5. Catalyst recycled use and the effect of reaction time. Reaction conditions: 0.5 wt% Ag/TiO -A, 300 °C, 3 MPa H , 700 r/min. For catalyst recycle, the reaction time is 1 h.
The numbers in Fig. 5a are the TOF values.
Fig. 6. Possible pathways leading to phenol from guaiacol.
Fig. 7. Methane yield (j) and the ratio of methane to converted guaiacol (
change with Ag loading. Reaction conditions: 300 °C, 3 MPa H , 1 h, 700 r/min.
)
2
(
(
0.5%) was detected. It was found that only trace amount of phenol
3%) could be obtained from anisole under the same reaction con-
to increase even when the guaiacol conversion stopped increasing
(Fig. 4a, at Ag loading of 1–3 wt%), indicating that the contribution
of path 3 in the overall HDO increases with Ag loading. Therefore,
ditions (Fig. S11). Therefore, the production of phenol via path 1 is
negligible, in agreement with the higher CArAOH bonding energy
than that of CArAOCH
3
[34].
direct correlation of CH
CH was produced on Ag surface and/or at Ag-TiO
However, even when the silver loading was increased to 3 wt%,
the CH yield only accounted for 15% of the converted guaiacol.
The higher contribution of path 2 through direct hydrodemethoxy-
lation by forming CH OH as the side product at low Ag loading sug-
4
production with Ag loading indicates that
Methanol was detected in 20–32% selectivity (path 2), indicat-
ing that phenol can be obtained directly by hydrodemethoxylation
of guaiacol through path 2. This pathway is consistent with our
4
2
-A interface.
4
previous report about the HDO of guaiacol on Au/TiO
18].
Catechol and CH
col. Catechol may form through the hydrodemethylation of the
Me-OCAr bond (path 3) or the acid-catalyzed bimolecular
transalkylation, similar to that observed in Au/TiO -A catalyst
18]. Because catechol can be further hydrogenated to phenol,
and the rate of catechol HDO is much faster than that of guaiacol
18], the yield of CH as a side product is an appropriate measure
of catechol formation for the assessment of the contribution of
path 3 in the overall HDO reaction of guaiacol. No CH was
detected on pure TiO -A. However, with the increase of Ag loading,
both the CH yield and the ratio of CH /converted guaiacol were
found to increase with the silver loading (from 0.05 to 0.5 wt%,
Fig. 7). In addition, the ratio of CH /converted guaiacol continued
2
-A catalyst
[
3
4
were all detected in the conversion of guaia-
gests that path 2 is a dominant mechanism in Ov, which is
indicative of the reverse Mars-van Krevelen mechanism. In our
C
2
previous work on Au/TiO -A [20], low Au loading (0.1–0.7 wt%)
2
was used and large Au particles ranged from 2.7 to 41 nm. The
[
HDO reaction proceeded mainly via pathway 2, as little CH
formed.
4
was
[
4
Similar to Au/TiO
xylenols and trimethylphenols) were also produced. On Au/TiO
2
-A catalyst [18], methylated phenols (cresols,
-A,
2
4
the bimolecular transmethylation between phenols and guaiacol
was found to account for the methyl transfer for the formation of
methylated phenolics.
In view of the results of this work, the catalytic pathways of
2
HDO of guaiacol on Ag/TiO -A catalyst are proposed as shown in
2
4
4
4

K. Liu et al. / Journal of Catalysis 369 (2019) 396–404
403
2
Fig. 8. Mechanism of guaiacol conversion over Ag/TiO -A catalyst.
Fig. 8. Ag facilitated the dissociation of H
2
to active H. The spillover
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