The role of acid and metal sites in hydrodeoxygenation of guaiacol over Ni/Beta catalysts

Article abstract of DOI:10.1039/c9cy01970g

Hydrodeoxygenation (HDO) of guaiacol over Ni metal supported on zeolites (H-Beta and H-ZSM-5) with different Si/Al ratios (12.5, 25, 175) and different metal loadings (2.3-23.4 wt%) was investigated in order to elucidate the role of catalyst acidity and the structure of Ni in the HDO reaction. Results show that the deoxygenation activity and product selectivity depend on the choice of support (acidity, pore size), level of metal loading, and reaction conditions. Guaiacol was deoxygenated in the presence of hydrogen with a maximum cyclohexane yield of 76% at a guaiacol conversion level of 100% over 15.7 wt% Ni/Beta-12.5 catalyst. Compared to Ni/ZSM-5 catalysts, Ni/Beta catalysts with mesopores facilitated the formation of coupling products (1,1′-bicyclohexyl). Under differential reaction conditions, we observed a linear relationship between deoxygenation activity and concentration of acid sites. Over catalysts containing small Ni particles, cyclohexane was formed as a result of a consecutive reduction of guaiacol to catechol and cyclohexane. At higher Ni-loading and consequently larger Ni particles, the selectivity towards cyclohexane increases with increased Ni loading. A higher concentration of nickel hydrides compared to smaller Ni sites was observed by H₂-TPD and H₂-FTIR over larger Ni species, and the nickel hydrides are believed to be crucial intermediates in the hydrogenation reaction. The 15.7 wt% Ni/Beta (Si/Al = 12.5) exhibits a promising HDO activity due to its good synergistic effect of hydrogenation and deoxygenation functions (high concentration nickel hydrides of and acid sites). In addition, based on the product distribution over catalysts containing mainly small Ni species and the Ni nanoparticles, two different reaction pathways were proposed, and the role of the acid sites and metal sites for each reaction route was discussed.

Full text of DOI:10.1039/c9cy01970g

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Highly dispersed Pt nanoparticles supported on carbon
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The role of acid and metal sites in hydrodeoxygenation of guaiacol overDNOIi:/1B0.e10t3a9/C9CY01970G
catalysts
Penghui Yan,† Molly Meng-Jung Li,† Eric Kennedy,† Adesoji Adesina,‡ Guangyu Zhao,† Adi
Setiawan,§ and Michael Stockenhuber*,†
†Chemical Engineering, School of Engineering, The University of Newcastle, Callaghan,
NSW, 2308, Australia
‡Atodatech LLC, Camarillo, California 93010, United States
§Mechanical Engineering, Faculty of Engineering, Universitas Malikussaleh, Bukit Indah,
Lhokseumawe, 24352, Indonesia
*Corresponding author: michael.stockenhuber@newcastle.edu.au
Abstract
Hydrodeoxygenation (HDO) of guaiacol over Ni metal supported on zeolites (H-Beta and H-
ZSM-5) with different Si/Al ratios (12.5, 25, 175) and different metal loadings (2.3~23.4 wt%)
was investigated in order to elucidate the role of catalyst acidity and the structure of Ni in the
HDO reaction. Results show the deoxygenation activity and product selectivity depend on the
choice of support (acidity, pore size), level of metal loading, and reaction conditions. Guaiacol
was deoxygenated in the presence of hydrogen with a maximum cyclohexane yield of 76% at
a guaiacol conversion level of 100% over 15.7 wt% Ni/Beta-12.5 catalyst. Compared to
Ni/ZSM-5 catalysts, Ni/Beta catalysts with mesopores facilitated the formation of coupling
products (1,1’-bicyclohexyl).
Under differential reaction conditions, we observed a linear relationship between
deoxygenation activity and concentration of acid sites. Over catalysts containing small Ni
particles, cyclohexane was formed as a result of a consecutive reduction of guaiacol to
catechol and cyclohexane. At higher Ni-loading and consequently larger Ni particles, the
selectivity towards cyclohexane increases with increased Ni loading. A higher concentration
of nickel hydrides compared to smaller Ni sites was observed by H2-TPD and H2-FTIR over
larger Ni species, and the nickel hydrides are believed to be crucial intermediates in the
hydrogenation reaction. The 15.7 wt% Ni/Beta (Si/Al=12.5) exhibits a promising HDO activity
due to its good synergistic effect of hydrogenation and deoxygenation functions (high
concentration nickel hydrides of and acid sites).
In addition, based on the product distribution over catalysts containing mainly small Ni species
and the Ni nanoparticles, two different reaction pathways were proposed, and the role of the
acid sites and metal sites for each reaction route was discussed.
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DOI: 10.1039/C9CY01970G
1. Introduction
With the extensive use of crude oil to meet increasing energy demands and with more
research aimed at decreasing our dependence on fossil fuels, the development of processes
which synthesize products from renewable sources (biocrude) is an area of increased
research activity. Biocrude oil has attracted widespread interest as the most practical
alternative source for fossil fuels and chemical intermediates. Biocrude is an oxygenated
organic liquid often derived from the pyrolysis or liquefaction of biomass at elevated
temperatures [1-3]. However, biomass contains high level of oxygen, which engenders
undesirable properties such as high acidity, low heating value and high corrosivity in its
biocrude product [4, 5].
The majority of scientific research reported in the open literature has focused on the
development of techniques for upgrading biocrude oils primarily by carefully decreasing its
oxygen content while minimizing deleterious changes to the properties of bio-oil with respect
to hydrocarbon chain length (especially minimizing the rate of small hydrocarbons formation)
[6, 7]. Hydrodeoxygenation (HDO) is one of the more widely studied methods to upgrade
pyrolysis biocrude oil into green and value-added products. It generally employs
heterogeneous catalysts and hydrogen to selectively remove oxygen atoms from the biocrude
[8].
The hydrodeoxygenation catalyst mainly consists of two functional parts, i.e. a redox
component (metal) and acid sites [9, 10]. A synergy of these two components facilitates the
hydrogenation and hydrogenolysis processes (dehydration, isomerization, demethoxylation,
and demethylation). Conventional sulfided nickel and sulfided cobalt in conjunction with
sulfided molybdenum or sulfided tungsten are widely employed as hydrodeoxygenation
catalysts [11-15]. However, the catalytic activity of these conventional HDO catalysts is
relatively low. In addition, sulfided catalysts are quickly deactivated when used for low sulfur
content feedstock, moreover, the contamination of the product with sulfur from sulfided
catalysts is a serious problem [11, 16-18]. To overcome this problem, supported catalysts with
noble metals (Ru, Pt, Pd, Rh, etc.) as active components were studied and proven to have
good HDO activity [18-21], however, the high cost of noble-metal catalysts prevents their wide
application in industry. Therefore, metallic Ni, which is low cost, non-sulfided, and has the
advantage of high hydrogenation activity [22, 23], was selected as the metal component in
this study. TiO2, Al2O3, SiO2 and zeolites have been employed as catalyst supports for HDO
of model compounds of biocrude oil such as guaiacol, phenol, etc. [16, 24, 25]. Zeolite
catalysts attract increasing attention in this research field due to their tunable acidity, porosity,
and shape selectivity. ZSM-5 has been extensively investigated for the hydrodeoxygenation
of biocrude model compounds, however, its small pore diameter was found to restrict the
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entrance of large molecules in the biocrude oil, limiting the access of the moleDcOuIl:e10s.10to39/tCh9eCY01970G
active sites [26]. In this work, H-Beta zeolite, which contains a bimodal pore size distribution
that can allow a better accessibility of the bulky bio-oils toward the active sites [27], was
studied, and the result of the ZSM-5-based catalyst was also included as a comparison.
It has been reported that a proper balance between metallic sites and acid sites is essential
to decrease catalyst deactivation during HDO reaction [9, 10, 25, 27], as metal sites can
facilitate the hydrogenation of both olefinic and aromatic compounds, the intermediates during
the formation of carbonaceous deposits, therefore delaying catalyst deactivation [28]. In this
regard, we have focused on investigating the effect of acid sites and metal sites on HDO of
guaiacol through changing Si/Al ratio of zeolite and metal loading, respectively. The
relationships of the acidity, metal loading/particle size of the catalysts and the catalytic
performance will be discussed in detail. In addition, reaction pathways of the catalysts with
different active sites (acid sites and metal sites) are derived based on the products distribution
and characterization results.
2. Experimental method
2.1. Catalyst preparation
Beta and ZSM-5 zeolites (Zeolyst International) in the ammonium form were employed as the
support materials. Zeolite-supported metal catalysts were prepared using incipient wetness
impregnation method. In applying this technique, a predetermined mass of nickel nitrate
hexahydrate (Ni(NO3)2·6H2O) supplied by Sigma-Aldrich was dissolved in distilled water and
then impregnated onto pre-calcined Beta (Si/Al=12.5), Beta (Si/Al=25), Beta (Si/Al=175),
ZSM-5 (Si/Al=15) and ZSM-5 (Si/Al=140) zeolites. The wet solids were dried at 120 °C for 3
h, followed by calcination at 550 °C for 6 h. The calcined samples were coded as 2.3 wt%
Ni/Beta-12.5, 3.9 wt% Ni/Beta-12.5, 7.8 wt% Ni/Beta-12.5, 15.7 wt% Ni/Beta-12.5, 23.4 wt%
Ni/Beta-12.5, 15.7 wt% Ni/Beta-25, 15.7 wt% Ni/Beta-175, 15.7 wt% Ni/ZSM-5 (Si/Al=15) and
15.7 wt% Ni/ZSM-5 (Si/Al=140). In addition, ion-exchanged Ni/Beta-12.5 (2.4 wt% Ni) was
also prepared to obtain the Ni/Beta catalyst with smaller Ni species. In the ion exchange
procedure, 4 g of NH4Beta (Si/Al=12.5) was exchanged with 100 mL of Ni(NO3)2 (0.6 M)
aqueous solution. The ion exchange process was performed at 80 °C for 24 h in a glass flask.
After the ion exchange procedure, the suspension was filtered and washed using deionized
water, dried at 110 °C for 8 h and the exchange repeated two times. Following preparation,
the sample was calcined at 550 °C for 6 h in air and marked as ion-exchanged Ni/Beta-12.5.
2.2. Catalyst characterization
Nitrogen adsorption measurements were carried out in a Micromeritics Gemini surface area
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analyser at -196 °C. Prior to nitrogen adsorption, the samples were degassed using a
DOI: 10.1039/C9CY01970G
Micromeritics Vac Prep 061 sample preparation device. The t-plot and Barrett-Joyner-Halenda
(BJH) models were used to estimate micro and mesopore volume, as well as pore size
distribution. The Si/Al atomic ratio and Ni content of the catalysts as prepared were determined
using a Varian Radial 715-ES Inductively Coupled Plasma-Optical Emission Spectrometer
with a Sturman-Masters spray chamber, quartz torch and an SPS 3 autosampler. Sample
digestion took place in a Milestone Start D microwave digestion system using a mixed solution
of 4.5 mL HNO3 (65%), 4.5 mL HCl (37%) and 3 mL HBF4 (50%). The Ni dispersion of the
catalysts was measured by volumetric chemisorption. All samples were pretreated by H2 at
450 °C for 1 h. The chemisorption of carbon monoxide was performed at 35 °C over a pressure
range between 30 and 90 mbar, and the total number of gas adsorbed on the catalyst was
calculated based on the ideal gas law. The 15.7 wt% Ni/Beta-12.5 sample after the
chemisorption was tested by H2-TPR, and no reduction peak was detected, indicating NiO can
be totally reduced under the pre-treated conditions.
The crystal structure of the catalysts was determined using a Phillips X’pert Pro diffractometer
with Cu Kα radiation. The XRD patterns were acquired in the range of 5-90 ° 2θ. The metal
nanoparticle size was calculated by Scherrer equation. The Scherrer equation can be written
퐾휆
휏 = 훽푐표푠휃
as
Where:
휏 is the mean size of the crystalline domains. K is shape factor, The most common values for
K are: 0.94 for FWHM of spherical crystals with cubic symmetry; 0.89 for integral breadth of
spherical crystals with cubic symmetry. 휆 is x-ray wavelength, Cu Kα =1.54178 Å, β is the line
broadening at half the maximum intensity (FWHM) (2θ). θ is the Bragg angle.
Transmission electron microscopy (TEM) imaging was performed with a JEOL JEM-1200EXII
TEM (200 kV) microscope, equipped with an EDXS spectrometer. Samples were prepared
using an ethanol dispersion method and placed on a copper grid.
Catalysts acidity was studied by measuring the temperature programmed desorption (TPD) of
ammonia, using an apparatus described in detail elsewhere [29]. The amount of carbon
deposited on the catalysts was determined using a Mettler Toledo TGA/DSC, operating under
a flowing air stream. H2-TPD experiments were carried out in the same system that was used
for NH3-TPD measurements. 0.2 g catalyst was used and reduced in situ in a pure H2 at 550 °C
for 5 h, then the sample was cooled to 50 °C and saturated with H2. H2 desorption pattern was
obtained from 40 °C to 500 °C at a heating rate of 10 °C/min. The desorbed H2 was detected
by a mass spectrometer.
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Catalysis Science & Technology
To determine reducibility of the catalysts, temperature-programmed reductionD(OTI:P10R.10)3V9wie/wCa9AsCrtiYc0le1O97nl0inGe
carried out in a custom-built instrument. The reduction reactor was connected through two
independent mass flow controllers and an AGC-10-454 thermal conductivity detector (TCD).
The TCD is powdered via a TCD power supply and bridge control unit (AGC instrument, model
40-202) and the analogue TCD output signal is amplified by an indigenous DC linear amplifier.
200 mg sample was loaded in a quartz tube. The catalyst was first activated in air (20 mL/min)
at 500 °C for 30 min to remove water and other impurities, followed by cooling to 25 °C.
Reduction was carried out using a 2.04 vol% H2/Ar flow (50 mL/min) from room temperature
up to 1000 °C with a heating rate of 10 °C/min. The hydrogen consumption was monitored by
TCD. A trap containing dry ice was placed between the reactor and TCD to remove water.
FTIR spectra were recorded with a Bruker Tensor27 FTIR spectrometer. The self-supporting
wafers were prepared by application of a pressure of 5 t. The pressed wafers were placed in
a sample holder in the center of a furnace located in the FTIR cell. Prior to the adsorption
experiment, the sample was activated by calcination at 500 °C for 1 h under vacuum (10-6
mbar). For the reduced sample, the catalyst was first off-line reduced in pure H2 (50 mL/min)
at 550 °C for 5 h. Then the pre-reduced sample was ground and pressed to prepare a wafer,
the pre-reduced sample was activated at 500 °C by three repeated injection of
hydrogen/evacuation cycles. The spectra were recorded at 50 °C under vacuum (lower than
10-7 mbar). The IR spectra of adsorbed NH3 were recorded at 150 °C with a NH3 pressure
range between 1.0×10-4 mbar and 10 mbar. The IR spectra of adsorbed H2 were recorded at
50 °C with a H2 pressure range between 1.0×10-3 mbar and 10 mbar. For quantitative analysis
of the number of catalytically active sites, the intensities of bands of coordinatively bound
ammonia (1575~1649 cm-1) and to Brønsted acid sites (1340~1553 cm-1) were quantified
+
using the respective extinction coefficients: 0.11 cm2·μmol-1 for the NH4 band and 0.026
cm2·μmol-1 for the NH3-L band [30].
2.3. Catalytic reaction and product analysis
The hydrodeoxygenation of guaiacol was initially conducted in a 35 mL stainless steel batch
reactor operating without agitation. Prior to the reaction, calcined catalyst was reduced in H2
(50 mL/min) at 550 °C for 6 h. The reduced catalyst (0.2 g) and the feed (0.4 g guaiacol) were
then charged into the batch reactor, minimizing its exposure to air during transfer. The
experimental set up and photo of the mixed state of catalyst and feed were shown in Figure
S1. As displayed in Figure S1, the catalyst and guaiacol were totally mixed and stayed in the
reactor bottom. After purging the reactor three times with pure H2 gas, the hydrogen pressure
in the reactor was increased to 4.0 MPa, then the reactor temperature was increased to the
experiment temperature (250 to 300 °C). After reaction, the reactor was cooled to room
temperature, then gaseous products were collected and analyzed via an offline Varian 490-
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GC micro gas chromatograph. The moles of gas products were calculated baDsOeI:d10o.10n3V9iee/wCn9AdCrtiYc0le1O97nl0inGe
pressure of reactor and van der Waals equation. The liquid products were extracted and
collected from the reactor and residual catalyst with 20 mL of dichloromethane, then the
residual catalyst was dried at 110 °C for 3 h. The amount of carbon deposited on the catalyst
was estimated by TGA/Mass spec and the mass difference between reduced and used
catalysts.
Some catalysts were also tested in a continuous-flow reactor. 0.05g catalyst was placed in the
center of the tubular reactor and held in place with quartz wool. The thermocouple was placed
in the middle of the catalyst bed to monitor the reaction temperature. The Ni catalysts were
reduced at 550 °C for 6 h at a flow rate of 50 mL/min pure H2 before the reactivity test. After
catalyst pre-treatment, the temperature was adjusted to reaction temperature and the total
hydrogen pressure was increased to 4 MPa. The reactant (guaiacol) was introduced into the
reactor with a HPLC pump.
The hydrodeoxygenation of guaiacol on the reduced Ni/BEA catalyst surface involves transfer
of hydrogen into liquid phase and diffusion through the liquid phase to the catalyst through a
boundary layer surrounding the catalyst surface. To investigate the intrinsic activity of the
catalyst, the internal mass transfer were examined by using Ni/Beta catalysts with two different
particle sizes, the external mass transfer limitations were checked by using catalyst with
varying mass but kept the same GHSV. In addition, the mass transfer limitations were also
examined by means of the Weisz-Prater (CWP) and Mears Criterion (CM) [31]. Both the
experimental and calculation results showed the hydrodeoxygenation under the employed
reaction conditions is not limited by the mass transfer (the detailed experimental results and
calculations are present in the supporting information, Table S1, Table S2). Apart from mass
transfer limitations, heat transfer limitations should also be considered to ensure that the
kinetic data obtained only reflect kinetic effects. For this regard, the criteria established by
Mears were used to check whether heat transfer effects were remarkably affecting the
experimental results. Axial heat transport effect can be neglected if the ratio of axial length of
the reactor is 20 times greater than catalyst particle size. In this study, the reactor length at
reaction conditions is much higher than the catalyst particles size (>100 times), therefore, the
axial heat conduction can be neglected. The radial heat transfer effect was estimated by the
Mear criterion (CMH) [32], the calculation results indicated the CMH value was 2 orders of
0.4푅푇/퐸
magnitude lower than the
푎, therefore, radial heat-transfer effect in the HDO reaction
was not a factor (detailed calculation information was shown in supporting information).
The liquid products from HDO of guaiacol were analyzed by an Agilent GC-6980 MS via an
Rtx-200 MS (30 m length, 0.25 mm ID, and 0.5 µm film thickness) column. Helium was used
as carrier gas with a sample injection volume of 1 µL and a split ratio of 100:1 applied for all
samples. Injector and detector were maintained at 220 °C and 285 °C respectively. The initial
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oven temperature (40 °C) was held for 5 min, increasing to 250 °C at a rate of 10 °C /min. The
DOI: 10.1039/C9CY01970G
filament detector was turned off during the elution of the injection solvent (CH2Cl2). For
quantification and the analysis of methanol, GC-FID analyses were conducted in a 2014
Shimadzu GC, fitted with a Restek Stabilwax column (30 m, 0.32 mm ID with 1 µm film
thickness, using air, hydrogen and helium with a split ratio of 10:1. Injector and detector were
kept at 300 °C and 310 °C, respectively, while an initial temperature of 35 °C was held for 5
min, ramping to 230 °C at a rate of 8 °C/min, holding at this temperature for 9 min. The liquid
products (methanol, methyl-cyclopentane, cyclohexane, cyclohexene, cyclohexanol, 1,1’-
bicycohexyl, 2-methoxycyclohexanol, guaiacol, anisole, phenol) were calibrated by standard
chemicals from Sigma-Aldrich, other complex mono-aromatic products (2-methoxy-6-
methylphenol, creosol, etc.), which have similar structures with guaiacol, were calibrated using
the same response factor as guaiacol. Gaseous products from the continuous-reactor were
analyzed via an online Varian 490-GC micro gas chromatograph equipped with two columns
(10 m PPQ; 15 m 5CB), and a mixture of gaseous components (helium balanced, 2000 ppm
methane, 2000 ppm ethane, 2000 ppm ethene, 2000 ppm propane, 2000 ppm propene, 2000
ppm n-butane, 2000 ppm isobutene) was used as calibration gas for Micro-GC.
푔푢푎푖푎푐표푙 푖푛 푓푒푒푑 ― 푔푢푎푖푎푐표푙 푖푛 푝푟표푑푢푐푡푠
퐺푢푎푖푎푐표푙 퐶표푛푣푒푟푠푖표푛 =
푝푟표푑푢푐푡 푦푖푒푙푑 =
× 100% (1)
(2)
푔푢푎푖푎푐표푙 푖푛 푓푒푒푑
푐푎푟푏표푛 푚푎푠푠 표푓 푝푟표푑푢푐푡
× 100%
푐푎푟푏표푛 푚푎푠푠 표푓 푓푒푒푑
The guaiacol conversion was calculated using equation (1). Product yield was calculated by
equation (2). Product yield was calculated on the basis of the carbon mass.
3. Results and discussion
3.1. Effect of acidity on the hydrodeoxygenation of guaiacol
3.1.1. Characterization of the Ni-based catalysts with different zeolite supports
Figure 1 (a, b) displays the N2 adsorption-desorption isotherms at -196 °C for Ni/Beta and
Ni/ZSM-5 catalysts. The isotherms of Ni-based catalysts shown in Figure 1(a) disclose a type
I-type IV combination of isotherms with a clear H4 type hysteresis loop in the Beta zeolite
catalysts based on the IUPAC classification [33], confirming that Beta zeolite samples have
both micro- and mesoporous channels. In contrast, Ni/ZSM-5 catalyst presents a type I
isotherm [33], with a high uptake of nitrogen at low relative pressures, which is characteristic
of a microporous material. It is noticeable from the adsorption isotherm that Beta zeolite
contains both micropores and mesopores whereas ZSM-5 possesses only micropores. In line
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with the literature, Beta zeolite has a significantly higher concentration of mesopores than VMiewFAIr-ticle Online
DOI: 10.1039/C9CY01970G
based materials such as ZSM-5 zeolite [34]. In addition, it is also noted that the micropore
surface area of Ni-loading samples display a remarkable decrease compared to parent
supports, indicating part of Ni sites entered into the pores (as shown in Table S3).
Figure 1. Nitrogen adsorption isotherms for the reduced Ni/Beta (a) and Ni/ZSM-5 (b)
catalysts at -196 °C
The XRD patterns of calcined and reduced catalysts are shown in Figures 2 (a, b). As shown
in Figure 2a, both calcined and reduced Ni/Beta catalysts have broad reflections around 2-
theta= 22.5 °, 25.4 °, 27.0 °, 28.8 °, and 29.7 °, which are assigned to Beta zeolite based on
the published data [35, 36]. The catalysts supported on ZSM-5 zeolite (Figure 2b) show
reflections at 2-theta= 13.9 °, 15.9 °, 23.1 °, 24.0 °, 25.9 ° and 27 °, which are ascribed to the
MFI structure type [37]. In general, all the calcined catalysts (both Beta and ZSM-5 supports)
show five high intensity reflections at 37.3 °, 43.3 °, 62.9 °, 75.4 ° and 79.4 °, respectively,
corresponding to lattice planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) of NiO (JCPD no.
04-0835). On the other hand, the reduced catalysts have three reflections at 44.7°, 51.9° and
76.4°, which can be attributed to the lattice planes (1 1 1), (2 0 0) and (2 2 0) of metallic nickel
(JCPD no. 04-0850), and this suggests that metallic nickel was formed by the H2 pre-reduction
process and it could be the active phase during hydrodeoxygenation reaction. This is in good
agreement with the crystalline phase as identified by TEM (Figure S2), which show monoclinic
and cubic phases in calcined and reduced 15.7 wt% Ni/Beta-12.5 samples, respectively.
Besides, it is noted the particle size of some Ni species is bigger than 20 nm (as shown in
TEM), which is dramatically larger than the average pore size of catalysts, therefore, these
large particles cannot enter the pores and stay in the external surface.
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DOI: 10.1039/C9CY01970G
Figure 2. X-ray diffraction patterns of the calcined and reduced Ni/Beta (a) and Ni/ZSM-5 (b)
samples
To measure the acidity of the catalysts, NH3-TPD was performed. With NH3-TPD the number
and relative strength of acid sites in the catalysts can be determined [38]. The m/z=16 amu
was used to extract NH3 desorption curves (Figure 3). The heat of desorption is also calculated
from TPD based on Redhead [39]:
Δ 퐸푑푒푠
= ln휐1푇푚푎푥
-3.64
푅푇푚푎푥
훽
Where ΔEdes is the activation energy of desorption, Tmax is the temperature at which the
desorption rate is a maximum, R is the ideal gas constant, β is the heating rate. The frequency
factor is estimated to be constant, ν1=1013 s-1. The results are shown in Table S4.
For the calcined Ni/Beta catalysts with Si/Al ratio of 12.5 and 25, NH3-TPD profiles show three
desorption peaks where a low-temperature peak (LTP) in the range of 200~220 °C, a medium-
temperature peak (MTP) in the range of 250~295 °C and a high-temperature peak (HTP) in
the range of 428~439 °C. As for the Ni/Beta catalyst with the Si/Al ratio of 175, the LTP is
centered at 255 °C, whereas the MTP is approximately 373 °C. HBeta-12.5 zeolite without Ni
loading displays a large NH3 desorption peak combining the LTP and MTP in the range of
150~350 °C (Figure S3). The LTP is ascribed to weak Brønsted acid sites in zeolite BEA
catalysts [40-42]. As for the MTP peak, which is evidently shown in all Ni/Beta catalyst and H-
Beta, is normally assigned to strong Brønsted acid sites due to bridging Si(OH)Al groups [43].
The HTP peak is distinct over calcined Ni/Beta catalysts but absent in the reduced Ni/Beta
catalysts (Figure 3b), which can be attributed to Lewis acid sites relating to Ni2+ attached to
zeolite framework [44, 45]. It is noted that the amount of desorbed ammonia and the peak
position can be correlated to the aluminium content of the zeolite support and desorption
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energy. As expected, Figure 3 shows an increase in acid site concentration with decreasing
DOI: 10.1039/C9CY01970G
Si/Al ratio. In addition, it is clear that the peak positions of LTP and MTP slightly shifted to
higher temperatures with increasing Si/Al ratio, indicating the acid strength and desorption
activation energy increased with increasing Si/Al ratio (Table S4). It is known that the
dehydration and demethoxylation are typical acid-catalyzed reactions [46, 47], therefore, one
can expect that the catalyst with a higher amount of acid sites would facilitate the
deoxygenation reaction.
Figure 3. Ammonia temperature programmed desorption profiles for the calcined (a) and
reduced (b) Ni/Beta catalysts with different Si/Al ratios, m/z=16, the black line - original trace,
the red line - the composite sum of fitted peaks, and the green line - fitted peaks
3.1.2. Catalytic performance of Ni-based catalysts with different zeolite support
The supported metal catalysts were evaluated in HDO of guaiacol at different temperatures
using a batch reactor with an initial pressure of 4.0 MPa H2 gas. Ni/Beta zeolite catalysts with
the same Ni loading but at different Si/Al ratios were studied, and the performance results are
shown in Table 1. At a reaction temperature of 250 °C, the highest yield of cyclohexane
(68.9%) was observed in 15.7 wt% Ni/Beta-12.5 catalyst, followed by 15.7 wt% Ni/Beta-25
(45.4%) and 15.7 wt% Ni/Beta-175 (25.4%). The Ni/Beta catalysts with higher Si/Al ratios
display a high yield of 2-methoxycyclohexanol, which is nearly absent over 15.7 wt% Ni/Beta-
12.5 catalyst, indicating the initially produced 2-methoxycyclohexanol could quickly undergo
demethoxylation and dehydroxylation to form cyclohexane over the catalyst with lower Si/Al
ratio. When the reaction temperature was increased to 300 °C, the 2-methoxcyclohexanol was
not detected over all Ni/Beta catalysts, suggesting high reaction temperature facilitates the
deoxygenation activity. It is worth mentioning that the 15.7 wt% Ni/Beta-12.5 catalyst gives a
cyclohexane yield 76.0% with a methane yield of 11.2% under a guaiacol conversion of 100%
at 300 °C. The changes of cyclohexane yields in the catalysts with different Si/Al ratios can be
explained by the acidity of the catalysts. Based on NH3-TPD and NH3-FTIR analysis (Figure
10

Page 11 of 27
Catalysis Science & Technology
3b and Table 2), the total number of acid sites in the reduced catalysts (mainly BrønstedVaiewciAdrticle Online
DOI: 10.1039/C9CY01970G
sites) increases with a decrease in Si/Al ratio which is in good agreement with the literature
[48, 49]. And these acid sites were reported to play a key role in the dehydration and
demethoxylation of 2-methoxycyclohexanol [46, 47, 49]. Moreover, guaiacol can undergo
transalkylation reaction to form catechol via acid sites, which is more readily to be
hydrodeoxygenated [9]. In addition, a dramatically higher concentration of Lewis acid sites
was observed in reduced Ni/Beta catalysts compared to parent HBeta (Table S5) as indicated
by the symmetric bending vibrations of NH3 coordinated on Ni2+ [50, 51]. The presence of
Lewis bound ammonia suggests part of metallic Ni sites were re-oxidized during the wafer
preparation procedure and form cationic Ni which acts as Lewis site.
As the HDO activity is independent of the concentration of the accessible surface Ni atoms at
high metal loading [49], the rate of cyclohexane formation should mainly depend on catalyst
acidity. In addition, it is noted the batch reactor has no stirring system, therefore, the observed
catalyst performance could be mass transfer limited. To investigate the relationship between
catalyst acidity and the formation rate of cyclohexane, the Ni/Beta catalysts with different Si/Al
ratios were evaluated in a continuous-flow reactor in order to obtain an accurate formation
rate of cyclohexane at differential conditions (conversion <15%), with results summarized in
Table 2. It is shown in Table 2 that the ratio of cyclohexane formation rate to the total acid
concentration and the ratio of cyclohexane formation rate to the Brønsted acid sites,
represented as normalized rate and TOF-B acid, are nearly constant, which are around 4.7x10-2
(molcyclohexane·mol-1acid·s-1) and 5.5 x10-2 (molcyclohexane·mol-1acid·s-1), respectively. This again
confirms that the catalyst with a higher concentration of acid sites can provide an enhanced
deoxygenation activity and favour the formation of cyclohexane. On the other hand, the 15.7
wt% Ni/ZSM-5 catalysts with different Si/Al ratios were also evaluated to investigate the
influence of the Si/Al ratio and pore size on the HDO reaction, see Table 1. Similar to the result
for Ni/Beta catalysts, the Ni/ZSM-5 catalyst with a lower Si/Al ratio (Si/Al=15) also displays a
higher deoxygenation activity as well as a higher cyclohexane yield than that of the catalyst
with a higher Si/Al ratio (Si/Al=140). It is also worth mentioning that over all catalysts using
Beta zeolite as support a low yield of 1,1’-bicyclohexyl was formed via a coupling reaction
(Table 1). In contrast for catalysts using ZSM-5 as support, 1,1’-bicyclohexyl was not formed.
Physisorption analysis (Figure 1) shows both micropores and mesopores are detected in
Ni/Beta catalyst, while Ni/ZSM-5 catalyst only contains micropores. This suggests that the
formation of 1,1’-bicyclohexyl cannot occur in the micropores of Ni/ZSM-5 catalyst.
Considering the fact that the chemicals in biocrude oil are very complex and heavy, catalyst
with a larger pore size and a higher acidity is believed to be more suitable for effectively
processing larger compounds and producing alkanes with higher carbon number. Therefore,
11

Catalysis Science & Technology
Page 12 of 27
mesoporous Beta zeolite with Si/Al=12.5 was chosen as the support for furthDeOrI:s10tu.10d3Vy9ie/wCo9AnCrtiYc0le1O97nl0inGe
optimizing Ni loadings for HDO of guaiacol.
Compared to 15.7 wt% Ni/Beta-175, 15.7 wt% Ni/ZSM-5(Si/Al=15) and 15.7 wt% Ni/ZSM-
5(Si/Al=140) catalysts, the 15.7 wt% Ni/Beta-12.5 catalyst exhibited a significantly higher
methane yield (Table 1), which could be attributed to the higher concentration of surface Ni,
facilitating the hydrogenolysis of methanol to methane at high reaction temperature [52]. The
Ni-based catalysts are known to be very effective in methanation reaction (hydrogenation of
carbon oxides or alcohols to methane), which property could be related to the methane
formation in this reaction system [52, 53].
In addition, it was observed the pressure increased to 7~8 MPa from 4 MPa when the reactor
was heated to 250 °C, then the pressure dropped to 5.5~6.5 MPa during the reaction and the
end-pressure at room temperature was around 2.9~3.4 MPa varying depending on the
reaction conditions. The molar ratio of H2 to guaiacol was around 20:1 and one mole of
guaiacol would consume 6 mole H2 when the guaiacol was totally converted to cyclohexane,
methane and H2O, then the calculated end-pressure should be around 2.8 MPa. This was in
accordance with the observed value of end-pressure (Table 1).
Table 1. The catalytic performance of HDO of guaiacol over Ni/Beta and Ni/ZSM-5 catalysts
using the batch reactor[a]
Catalyst
T
End P
X-
Y-
Y-
meth
anol
Y-
Y-
Y-2-
methox
Y-1,1’-
bicyclo
hexyl
Others[
b]
(°C) (MPa) guaiaco Methan
cyclohe cyclo
xane
l
e
hexan ycycloh
ol
exanol
15.7 wt%
Ni/Beta-12.5
15.7 wt%
Ni/Beta-25
15.7 wt%
Ni/Beta-175
15.7 wt%
Ni/Beta-12.5
15.7 wt%
Ni/Beta-25
15.7 wt%
250
250
250
300
300
300
300
3.0
3.3
3.5
2.9
2.9
3.0
2.9
99.5%
94.9%
66.2%
100%
100%
97.2%
100%
1.2%
1.3%
1.2%
13.0%
12.2%
3.4%
3.5%
8.3%
8.6%
5.3%
0
68.9%
45.4%
25.4%
76.0%
76.9%
74.0%
82.4%
7.3%
1.4%
1.7%
0
2.3%
6.0%
6.1%
6.3%
3.1%
8.2 %
5.2%
8.8%
22.9%
5.1%
14.8%
0
0
0
0
0
0
0
0
0
3.5%
2.3%
4.1%
0
0
4.4%
3.7%
Ni/Beta-175
15.7 wt%
Ni/ZSM-
5(Si/Al=15)
15.7 wt%
300
3.1
96.3%
1.7%
4.5%
72.5%
1.8%
3.1%
0
8.9%
Ni/ZSM-
5(Si/Al=140)
[a] Reaction conditions: initial hydrogen pressure (4.0 MPa), reaction time (200 min), 0.2 g catalyst, 0.4
g guaiacol.
[b] Others include methylcyclohexane, 1,2-cyclohexanediol, (cyclopentylmethyl)cyclohexane and
tercyclohexyl, deposited carbon.
12

Page 13 of 27
Catalysis Science & Technology
Table 2. The catalytic performance of hydrodeoxygenation of guaiacol over Ni/Beta cataVlyiewstAsrticle Online
DOI: 10.1039/C9CY01970G
using the continuous-flow reactor[a]
Catalyst
X-
Y-
Oth Rate-
LTP
acid
MTP
acid
Total
acid
B[e]
(mm
ol/gc
Normali
zed
TOF-B
acid
[g]
ers[b
guai
cyclohex
cyclohex
] (%)
con.
con.[d]
con.[d]
Rate [f]
acol
ane
ane [c]
[d]
(%
(%)
(mmo (mmol/gc
)
at
)
(mmo l/gcat)
l/gcat)
)
at
15.7 wt%
Ni-Beta-
12.5
11. 5.38
1
5.6
3.32
0.39
0.32
0.71
0.60
4.68
5.53
15.7 wt%
Ni-Beta-25
11. 4.42
0
4.6
4.1
2.73
1.47
0.27
0.13
0.30
0.18
0.57
0.31
0.50
0.26
4.79
4.74
5.46
5.65
15.7 wt%
Ni-Beta-
175
9.1 2.37
[a] Reaction conditions: PH2=4.0 MPa, 0.05 g pre-reduced catalyst, WHSV (3.12 min-1), H2 flow rate
(120 mL/min), T=230 °C.
[b] Others include methane, 2-methoxycyclohexanol, phenol, p-cresol, 1,2-dimethoxybenzene, etc.
[c] Rates for cyclohexane formation base on the weight of catalyst, and the rate is an average over the
20~40 minutes, unit: (10-5·molcyclohexane·gcat-1·s-1).
[d] The acid concentrations of LTP and MTP in reduced catalysts were determined by quantifying the
desorbed NH3 with NH3-TPD.
[e] The concentration of Brønsted acid was calculated by NH3-FTIR.
[f] Normalized rate=Rate for cyclohexane formation base on the total acid concentration of reduced
catalyst, unit: 10-2·molcyclohexane·mol-1acid site· s-1.
[g] TOFB acid = Normalised rate for cyclohexane formation based on the Brønsted acid concentration of
reduced catalysts, unit: 10-2·molcyclohexane·mol-1 B acid site· s-1.
3.2. Effect of Ni loading/particle size on the HDO of guaiacol
The above results have revealed the influence of the acidity and porosity of the Ni/Beta and
Ni/ZSM-5 catalysts on the catalytic performance of HDO of guaiacol. Among all the catalysts
that were evaluated, mesoporous Beta zeolite with Si/Al=12.5 is believed to be the most
suitable support for effectively catalyzing deoxygenation reaction due to its suitable acidity and
porosity. On the other hand, the redox active component, plays a vital role in the
hydrodeoxygenation reaction because metal sites are active for both hydrogenation and
dehydrogenation reactions [54]. However, the influence of Ni loading/particle size on the
hydrodeoxygenation performance is still unclear. In the literature, the isolated nickel atoms
prepared by ion-exchange were found to be active sites in many reactions, such as catalytic
cracking and oligomerization [50, 55, 56]. It is also noted that the nickel catalyst prepared by
impregnation with low metal loading (<0.4 wt%) is also regarded to contain isolated Ni species
occupying exchange positions in the zeolite framework, which provides similar activity as the
Ni catalysts prepared by ion-exchange [50, 56]. On the other hand, some researchers suggest
that the increased Ni loading can enhance the catalytic activity of dehydrogenation,
hydrodeoxygenation and hydroreforming reaction, which could be attributed to the contribution
of bulk Ni particles in lowering the activation of H2 dissociation thus enhancing the
hydrogenation activity [27, 54, 57-59]. Therefore, it is necessary to investigate the roles of the
13

Catalysis Science & Technology
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small Ni atoms and the larger Ni nanoparticles in the HDO reaction for further opDtiOmI:i1z0a.1t0i3o9n/C9oCfY01970G
the catalyst.
In this section, ion-exchange technology was employed to prepare a catalyst with mainly small
sites (2.4 wt% Ni), while a series of Ni/Beta-12.5 catalysts with Ni loading from 2.3% to 23.4%
were also prepared via the conventional impregnation method.
3.2.1. Characterization of the Ni/Beta-12.5 catalysts prepared by different synthesis methods.
Figure 4a shows TEM images of the ion-exchanged Ni/Beta catalyst, which exhibits small and
dispersed Ni-containing particles after calcination and no crystalline NiO was detected,
suggesting the Ni2+ species were finely distributed on the Beta support. This is in line with XRD
results in Figure S4 that the diffraction patterns of HBeta support and the ion-exchanged
Ni/Beta show the diffraction peaks of Beta zeolite, while the diffraction peaks from Ni-
containing phase were not observed. The EDS mapping images of 15.7 wt% Ni/Beta and ion-
exchanged Ni/Beta samples were presented in Figure 4b and 4c. It is evident from Ni K
mapping result that Ni species formed aggregates of large Ni particles (over 10 nm) as
observed in the sample prepared by impregnation technique. As for the ion-exchanged Ni/Beta
catalyst, the Ni species were highly dispersed throughout the support, suggesting the
formation of small or isolated Ni species in ion-exchanged materials.
(a1)
(a2)
(b2)
(c1)
(b1)
(c2)
Figure 4. TEM of ion-exchanged Ni/Beta-12.5 sample (a); and EDS mapping images of 15.7
wt% Ni/Beta-12.5 (b) and ion-exchanged Ni/Beta-12.5 (c) samples
14

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Catalysis Science & Technology
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In-situ FTIR spectra of the HBeta-12.5 zeolite, ion-exchanged Ni/Beta-12.5 and rDeOdI:u1c0.e10d39i/oCn9C-Y01970G
exchanged Ni/Beta-12.5 catalysts (Figure 5) show four different types of hydroxyl bands. A
small band was observed at 3779 cm-1 over HBeta-12.5, suggesting the formation of
framework defects [60, 61]. The high-intensity absorption band around 3744 cm-1 is assigned
to internal silanol groups [60-63]. It is clear that the intensity of this peak decreased after ion-
exchange and recalcination under vacuum in infrared cell, which was also noticed by others
and explained as the facile annealing of the hydrogen-bonded silanol groups generated on
aluminium extraction [63, 64]. On the other hand, the band at 3661 cm-1 represents OH groups
bonded with extra-framework aluminium species (Al-OH) [60, 62]. The absorption band
around 3605 cm-1 is ascribed to the O-H stretching vibration of Brønsted acid OH groups from
the bridging Si(OH)Al in the zeolite framework [65-67]. The intensity of this absorption band
+
decreases over calcined and reduced ion-exchanged Ni/Beta, indicating that the NH4 ions in
zeolite (NH4Beta) were successfully replaced by Ni2+ ions during the ion-exchange procedure
[60, 65, 68, 69].
3
Black line (HBeta Si/Al=12.5)
Red line (ion-exchanged Ni/Beta-12.5)
2.5
Blue line (reduced ion-exchanged Ni/Beta-12.5)
2
1.5
1
.5
0
3500
3000
2500
2000
1500
Wavenumber (cm-1)
Figure 5. IR spectra of HBeta-12.5, ion-exchanged Ni/Beta-12.5 and reduced ion-
exchanged Ni/Beta-12.5 samples
IR spectra obtained from adsorption of NH3 on HBeta-12.5 and ion-exchanged Ni/Beta-12.5
are presented in Figure 6. The adsorption of NH3 on both samples displays two main peaks in
the range of 1300-1700 cm-1. The peak located in 1448 cm-1 accounts for the majority
+
+
vibrations of NH4 , which is assigned to NH4 molecules formed by reaction with Brønsted acid
sites and coordinated to the zeolite framework [70-72]. The intensity of this peak displays a
remarkable decrease in the ion-exchanged Ni/Beta catalyst, indicating the bridging Si(OH)Al
+
groups decreased due to the ion exchange of NH4 with Ni2+. The addition of Ni2+ ions
+
eliminates a fraction of protons on Al3+ sites (as indicated by the lower intensity of the NH4
vibration at 1450 cm-1) via the ion exchange process, and in turn generates new Ni2+ Lewis
15

Catalysis Science & Technology
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acid sites. Therefore, the band centered at approximately 1622 cm-1 can be assigned to
DOI: 10.1039/C9CY01970G
symmetric bending vibrations of NH3 coordinated on Lewis acid sites (Ni2+, Al3+) [50, 51]. Since
the band increases significantly after Ni ion exchange, the band is most likely due to NH3
bound to the Ni sites (also see TPD results). The NH3-TPD profiles of ion-exchanged Ni/Beta-
12.5 and non-metal loading HBeta zeolite are presented in Figure 7. In addition, the acidity of
other common catalyst supports such as Al2O3 and SiO2 were also measured for a comparison.
It is evident that the acidic properties of the zeolite samples changed after the ion exchange
+
of NH4 (in NH4Beta) with Ni2+. The majority of ammonia desorption peaks over the ion-
exchanged Ni/Beta-12.5 were centered at 250 °C and 448 °C, whereas the ammonia
desorption of HBeta samples was mainly at 250 °C (similar to that of the Al2O3 sample). The
higher ammonia desorption temperature (448 °C) found in the ion-exchanged Ni/Beta-12.5
suggests the occurrence of Lewis acid sites resulting from the formation of accessible Ni2+
+
cations as a result from the exchange of NH4 ions by Ni2+ ions [73], which is in line with the
formation of coordinatively bound NH3 as observed by FTIR (Figure 6b). Besides, it is noted
that the ammonia desorption peak was not detected in SiO2, indicating the SiO2 sample
contains no acid sites.
.4
.3
.2
.1
150 °C
(b)
(a)
150 °C
.5
.4
.3
.2
.1
250 °C
300 °C
250 °C
300 °C
350 °C
400 °C
450 °C
350 °C
400 °C
450 °C
0
0
1650
1600
1550
1500
1450
1400
1650
1600
1550
1500
1450
1400
Wavenumber (cm-1)
Wavenumber (cm-1)
Figure 6. IR spectra for NH3 temperature program desorption over (a) HBeta and (b) ion-
exchanged Ni/Beta-12.5
16

Page 17 of 27
Catalysis Science & Technology
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DOI: 10.1039/C9CY01970G
Figure 7. Ammonia temperature programmed desorption profiles for ion-exchanged Ni/Beta-
12.5 and non-metal loading supports, m/z=16
H2-TPR profiles of Ni/Beta-12.5 samples prepared by ion-exchange and impregnation with
various Ni loadings are presented in Figure 8a. Ion-exchanged Ni/Beta-12.5 exhibits one
reduction peak at around 700 °C, which is attributable to the reduction of isolated Ni cations
(Ni2+) charge-compensating the framework [56, 74], this kind of Ni2+ ions prefer to occupy
hexagonal prisms (S1) in octahedral symmetry, which are reduced at a higher temperature
than those required for reducing nickel oxide particles [57, 75]. Interestingly, 2.3 wt% Ni/Beta-
12.5 prepared by impregnation shows a similar reduction behaviour but a lower reduction
temperature (around 650 °C) compared to what was observed in ion-exchanged Ni/Beta-12.5.
With the increasing metal loading in the catalysts prepared by impregnation, another more
intense reduction peak was observed at around 500-550 °C, which can be ascribed to the
reduction of the small nickel particles and some nickel oligomeric species located in the zeolite
channels [76]. When the metal loading was increased to 15.7 wt%, a main reduction peak at
472 °C with a shoulder peak centered at 343 °C was detected, attributing to the reduction of
NiO nanoparticles presented on the zeolite external surface [56, 57]. In addition, the pre-
reduced 15.7 wt% Ni/Beta-12.5 catalyst was also investigated by H2-TPR to confirm the
chemical state of Ni under reaction conditions, the calcined 15.7 wt% Ni/Beta-12.5 sample
was pre-reduced at 550 °C for 6 h, which is the same pre-treatment procedure that was
employed in the micro-reactor studies. As shown in Figure 8a, no reduction peak could be
detected in reduced 15.7 wt% Ni/Beta-12.5, indicating all Ni2+ species were reduced by the
pre-treatment procedure.
Figure 8b shows H2-TPD profiles of the reduced ion-exchanged Ni/Beta-12.5, reduced 2.3 wt%
Ni/Beta-12.5 and reduced 15.7 wt% Ni/Beta-12.5 catalysts. The traces were normalized to the
moles of surface Ni (loading × dispersion). The reduced ion-exchanged Ni/Beta and reduced
2.3 wt% Ni/Beta display a low amount of H2 desorption per surface Ni atom compared to the
reduced 15.7 wt% Ni/Beta at high temperature. On the other hand, a large desorption peak
centered around 180 °C was observed over the reduced 15.7 wt% Ni loading sample, which
could be attributed to the desorption of H in the bulk Ni nanoparticles. To confirm this, FTIR
spectra of hydrogen absorbed on reduced 15.7 wt% Ni/Beta-12.5 was investigated (shown in
Figure 9). A distinct peak around 1615 cm-1 was detected, which could be attributed to nickel
hydride (Ni-H) [77]. To further confirm this, the FTIR of H2 and CO adsorbed on the reduced
15.7 wt% Ni/Beta-12.5 with different adsorption sequence was investigated, with results
shown in Figure 10. The band around 2209 cm-1 could be attributed to the Ni2+(CO) species
[78], indicating part of surface Ni particles were re-oxidized during the wafer preparation
procedure, which is well in line with the NH3-FTIR analysis. The XPS profile of reduced 15.7
17

Catalysis Science & Technology
Page 18 of 27
wt% Ni/Beta-12.5 also showed that the surface Ni was almost totally re-oxidized as meVtiaewllAicrticle Online
DOI: 10.1039/C9CY01970G
Ni 2p3/2 (852.6 eV) was absent (Figure S5).
When H2 was adsorbed firstly, the intensity of Ni-H bond was not significantly influenced by
the outgassing and CO-adsorption processes. In comparison, when CO was adsorbed firstly,
there was no peak around 1618 cm-1 detected after 10 mbar H2 adsorption. These results
suggest the CO was strongly adsorbed on the metal surface and nickel hydrides could not
form when the metal surface was covered by CO. In addition, there are no nickel hydrides
observed in calcined 15.7 wt% Ni/Beta-12.5 catalyst (without reduction), suggesting Ni2+ ions
do not facilitate the formation of Ni-H. As the surface of reduced Ni/Beta catalyst was re-
oxidized during the wafer preparation procedure, it can be deduced that the H2 penetrates the
surface Ni2+ and forms the Ni-H with inside metallic Ni. And H2 cannot penetrate deeper when
the metal surface was covered by CO. Effect of H2 and CO adsorption pressure on the amount
of desorbed H2 and desorbed CO was also investigated to confirm the existence of Ni-H in the
nickel crystal, with results shown in Figure 8c and Figure 8d, respectively. As displayed, the
amount of desorbed H2 increased with the increasing H2 adsorption pressure, while the
amount of desorbed CO did not exhibit significant change with the increasing CO pressure,
indicating H penetrated the nickel crystal and high pressures facilitated the H penetrated
deeper. CO chemisorption occurred in the Ni surface and did not affected by adsorption
pressure.
In addition, the differential curve of H2-TPD spectra of the adsorbate extracted from 1575 to
1656 cm-1 presented in Figure 9(2) is comparable to the gas phase H2-TPD profile of reduced
15.7 wt% Ni/Beta-12.5 (Figure 8b), suggesting the desorption peak centered around 180 °C
can be ascribed to the breaking of Ni-H bonds. Compared to the smaller Ni species formed in
the reduced ion-exchanged Ni/Beta and low Ni loading catalysts, the Ni nanoparticles
produced at high Ni loadings, i.e., 15.7 wt% Ni/Beta-12.5, facilitate the formation of nickel
hydride. The nickel hydrides are crucial intermediates in hydrogenation reaction [77, 79, 80].
Therefore, it is expected that 15.7 wt% Ni/Beta-12.5 catalyst can provide a better
hydrogenation activity than the catalyst with a low Ni loading.
18

Page 19 of 27
Catalysis Science & Technology
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DOI: 10.1039/C9CY01970G
Figure 8. (a) H2-TPR profiles of ion-exchanged Ni/Beta-12.5 and Ni/Beta-12.5 prepared by
impregnation with various Ni loadings; (b) H2-TPD profiles of the reduced ion-exchanged
Ni/Beta, reduced 2.3 wt% Ni/Beta and reduced 15.7 wt% Ni/Beta, the traces were
normalized to the moles of effective surface Ni (loading*dispersion);(c) effect of H2
adsorption pressure on the amount of desorbed H2 and (d) effect of CO adsorption pressure
on the amount of desorbed CO
(e)
0
-.02
-.04
-.06
-.08
-.1
(2)
(1)
1615
.2
(d)
(c)
.15
.1
(b)
(a)
.05
0
50
100
150
200
250
300
1680 1660 1640 1620 1600 1580 1560
Wavenumber (cm-1)
Temperature (°C)
Figure 9. (1) FTIR spectra of hydrogen absorbed on reduced 15.7 wt% Ni/Beta-12.5 with
increased dosage of H2, T= 50 °C: (a) PH2=1.5×10-6 mbar, (b) PH2=1.5×10-1 mbar, (c) PH2=1.0
mbar, (d) PH2=10 mbar, (e) PH2=3.5×10-6 mbar (outgassing after 10 mbar H2 absorbed); and
(2) differential curve of H2-FTIR spectra extracted from 1575 to1656 cm-1 as a function of
temperature over reduced 15.7 wt% Ni/Beta-12.5
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DOI: 10.1039/C9CY01970G
.2
(a)
.2
(b)
.15
.15
.1
.05
0
.1
.05
0
2200 2100 2000 1900 1800 1700 1600 1500
Wavenumber (cm-1)
2200 2100 2000 1900 1800 1700 1600 1500
Wavenumber (cm-1)
Figure 10. FTIR spectra of (a) 10 mbar H2 firstly adsorbed at 50 °C on reduced 15.7 wt%
Ni/Beta-12.5 then 10 mbar CO adsorbed after the degassing of H2, black line- outgassing
after 10 mbar H2 adsorbed (6.5×10-6 mbar), red line-10 mbar CO adsorbed after the
outgassing of 10 mbar H2 adsorbed, blue line- outgassing after 10 mbar CO and 10 mbar H2
adsorbed; (b) 10 mbar CO firstly adsorbed at 50 °C on reduced 15.7 wt% Ni/Beta-12.5 then
10 mbar H2 adsorbed after the degassing of CO, black line-10 mbar CO adsorbed, red line-
outgassing after 10 mbar CO adsorbed (6.1×10-6 mbar), blue line-10 mbar H2 adsorbed after
degassing of 10 mbar H2 adsorbed;
3.2.2. Catalytic performance of Ni/Beta-12.5 – smaller Ni catalysts
The HDO of guaiacol over Ni/Beta-12.5 catalysts and non-metal loading supports was
performed in a flow reactor with different reaction temperatures being evaluated. The results
are summarized in Table 3. At the reaction temperature of 230 °C, the guaiacol conversion is
extremely low and cyclohexane was not detected over the ion-exchanged Ni/Beta catalyst,
indicating the small Ni particles/cationic species have little HDO activity at this reaction
temperature. However, with a higher reaction temperature (300 °C) employed, the guaiacol
conversion over the ion-exchanged Ni/Beta catalyst increased along with the formation of
catechol (Y-Catechol =3.6%) and 1,2-dimethoxybenzene (Y-1,2-Dimethoxybenzene=4.6%). Besides, 0.8 %
yield of cyclohexane was also detected. Interestingly, among all the catalysts that were
evaluated, the formation of cyclohexane was only observed in the ion-exchanged Ni/Beta
catalyst. In addition, the yield of catechol over ion-exchanged Ni/Beta catalyst is lower than
that of in other zeolite catalysts, suggesting that catechol molecules can be converted to
cyclohexane via HDO reaction on small Ni sites. Based on the H2-TPR (Figure 8a) and H2-
TPD (Figure 8b) analysis results, a higher temperature (> 600°C) was required to reduce the
small NiO, indicating a stronger metal-support interaction in this catalyst. Moreover, the
reduced small Ni particles display a low concentration of nickel hydrides, which implies a weak
hydrogenation activity on the surface of the small Ni species. Although the small Ni particles
cannot effectively catalyse hydrogenation of guaiacol, the catechol can be formed from
conversion of guaiacol over the ion-exchanged Ni/Beta-12.5 catalyst. In addition, it is noted
that the formation of 1,2-dimethoxybenzene and catechol is observed not only on the ion-
20

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Catalysis Science & Technology
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exchanged Ni/Beta-12.5 and 2.3 wt% Ni/Beta-12.5 catalysts but also on the H-DBOeI:t1a0.1z0e39o/Cli9teCY01970G
supports without adding metal, suggesting that this reaction is catalysed by acid sites instead
of metal sites. To confirm this, various non-metal loading supports with different acidity have
been evaluated under the same reaction conditions as for HDO and the results are
summarized in Table 3. It can be seen that both HBeta and Al2O3 catalysts have the ability to
catalyse guaiacol to 1,2-dimethoxybenzene and catechol, whereas SiO2 shows no conversion
of guaiacol under the same reaction conditions due to lack of acidity (Figure 7).
Table 3. The performance results of HDO of guaiacol over Ni/Beta-12.5 and supports using
a continuous flow reactor[a]
Sample
Ni
T
X-guaiacol
Y-
cyclohexane
Y-
cyclohexene
Y-1,2-
Y-catechol
Others
[c]
loading[b] (°C)
(wt%)
dimethoxy-
benzene
0.2%
Ion-exchanged
Ni/Beta-12.5
Ion-exchanged
Ni/Beta-12.5
2.3 wt%
2.4
2.4
2.3
230
300
300
0.9%
19.5%
23.4%
0
0.8%
0
0
0.6%
0
0.1%
3.6%
6.9%
0.6%
7.8%
4.6%
5.0%
11.4%
Ni/Beta-12.5
HBeta-12.5
HBeta-25
HBeta-175
Al2O3
-
-
-
-
-
-
300
300
300
300
300
300
14.9%
13.9%
11.7%
17.8%
0
0
0
0
0
0
0
0
0
0
0
0
0
3.4%
3.2%
3.9%
1.2%
0
4.7%
4.3%
4.1%
5.4%
0
6.3%
3.4%
3.7%
8.4%
0
SiO2
No catalyst
0
0
0
0
[a] Reaction conditions: WHSV (0.26 min-1), catalyst (0.1 g), guaiacol (0.026 g/min), 4.0 MPa
H2, H2 flow rate (120 ml/min).
[b] ICP- AES measurements.
[c] Others include 2-methoxy-6-methylphenol, methyl-1,2-benzenediol, etc., detailed
information can be seen in Table S6.
Based on the above results, the pathway for catalysing guaiacol to form cyclohexane over the
smaller Ni particles is deduced and shown in Figure 11. Firstly, the methoxy- group in one
guaiacol molecule was substituted with a hydroxyl- group in another guaiacol molecule (via
an acid catalysed process) to form 1,2-dimethoxybenzene and catechol on the acid sites,
followed by the hydrogenation of catechol and dehydroxylation of 1,2-cyclohexanediol to form
cyclohexane on those small Ni and acid sites presented in the ion-exchanged Ni/Beta catalyst.
Note that 1,2-dimethoxybenzene and catechol are observed in similar quantities, suggesting
the acid catalysed disproportionation. It is noted that this reaction pathway mainly occurs at
higher reaction temperatures (as 300 °C), whereas under low reaction temperature the acid
sites and small Ni sites show almost no activity in HDO of guaiacol.
21

Catalysis Science & Technology
Page 22 of 27
View Article Online
DOI: 10.1039/C9CY01970G
Figure 11. Reaction pathway for HDO of guaiacol over ion-exchanged Ni/Beta-12.5 catalyst
and acid-containing supports
3.2.3. Catalytic performance of Ni/Beta-12.5 – low reaction temperature
The small Ni sites display a very low hydrodeoxygenation activity at low reaction temperature
due to their reduced ability to activate hydrogen. To effectively catalyze HDO of guaiacol under
mild reaction conditions, Ni/Beta catalyst with a higher Ni loading/larger particle size is
required to provide the catalytic sites with higher concentration of active hydrogen. However,
the relationship between the metal loading and HDO activity is still unclear. Therefore, Ni/Beta
catalysts with different metal loadings were prepared and tested.
The performance results of different metal loading/Ni particle size catalysts on HDO of
guaiacol in a continuous-flow reactor were summarized in Table 4. As shown in Table 4, there
was no cyclohexane detected in 2.3 wt% Ni/Beta-12.5 catalyst. When Ni loading was
increased to 3.9 wt%, a low yield of cyclohexane (1.6 wt%) was observed. Both guaiacol
conversion and cyclohexane yield increase with increasing Ni loading, and the highest
guaiacol conversion (28.7 %) and cyclohexane yield (12.3%) can be obtained over 23.4 wt%
Ni/Beta-12.5 catalyst. It is evident that the guaiacol conversion and normalized rate to
cyclohexane increase with the increased metal loading. Based on the analysis of H2-TPR
(Figure 8a), the Ni clusters were observed when the Ni loading was above 3.9 wt%, suggesting
the presence of bigger Ni clusters in the high-metal-loading catalysts facilitate the formation
of cyclohexane. This is in agreement with the H2-TPD result (Figure 8b) showing that a higher
Ni loading is required for the formation of nickel hydride. Moreover, it was reported the (1 0 0),
(1 1 0) and (1 1 1) surfaces of Ni, which were clearly shown in the XRD patterns of the high
Ni loading samples (Figure S6), could reduce the activation energy of H2 dissociation [59, 81,
82], thus facilitating the hydrogenation activity. However, when using 23.4 wt% Ni/Beta, similar
values of guaiacol conversion and normalized rate for cyclohexane formation were observed
as those for the 15.7 wt% Ni/Beta catalyst. The activity of HDO of guaiacol did not increase
further when increased the Ni-loading higher than 15.7 wt%. The average Ni diameter is
around 9.2 nm in 15.7 wt% Ni/Beta and 13.4 nm in 23.4 wt% Ni/Beta. As the Ni loading
increased, more of the pores are blocked (as evidenced by a decrease micropore volume)
and decrease the accessibility of guaiacol molecules to the active sites. This also can be
confirmed by the textural properties of 23.4 wt% Ni/Beta and 15.7 wt% Ni/Beta (Table S3), the
22

Page 23 of 27
Catalysis Science & Technology
reduced 23.4 wt% Ni/Beta displayed an increase in average pore diameter compDaOreI: d10.t1o03V19ie/5wC9.A7CrtiYc0le1O97nl0inGe
wt% Ni/Beta, suggesting an increased number of small pores were blocked. Therefore, an
optimum Ni loading for the HDO of guaiacol under the low-temperature reaction condition can
be found (15.7 wt%). This result also indicates that the optimization of the catalyst with suitable
metal loading and metal particle size is very important to effectively catalyze HDO of guaiacol
under mild reaction conditions.
Table 4. Effect of metal loading on the HDO of guaiacol[a]
[
Cat DNi
alyst
dNi
(n
dNi-
XRD
Surf
ace
X-
guai
Y-
meth
Y-
cyclohe
Y-
cyclo
Y-2- Y-1,2- Y- Oth Rate Norm
b]
ers[f]
(%)
-
alized
methox
dimeth
cate
(Ni/ (%) m)[ (nm) Ni[e]
rate
acol
ane
xane
hexe
ycycloh
oxyben
chol
cycloh
Beta
)
(10-4 (% (%
mol/
g)
(%)
(%
)
c]
[d]
[
[h]
ne
exanol
zene
exane
g]
)
)
(%
)
(%)
(%)
2.3
wt%
3.9
wt%
7.8
39. 1.6
3
25. 2.5
1
11. 5.4 12.9 1.50 15. 1.4
4.4
7.3
1.54 4.7
1.66 6.3
0
0
0
0
0
2.0
0.8
0.8
1.4
1.8
1.
1
0
0.7
2.1
5.6
0
0
1.6
5.5
0.6
1.2
0
2.5
8.5
1.5
5.6
9.8
10.4
0.1
0.2
0.4
0
0
0
wt%
4
7
15.7 6.7 9.2 14.1 1.78 26. 2.7 11.3 2.0
wt%
23.4 4.6 13. 17.6 1.82 28. 2.9 12.3 1.8
wt%
4.9 17.4
4.8 18.9
7
4
7
[a] Reaction conditions: guaiacol (0.065 g/min), catalyst (0.05 g), reaction temperature (230 °C), H2 flow
rate (120 mL/min), PH2=4.0 MPa, WHSV=1.3 min-1.
[b] Calculated by chemisorption, dispersion of Ni was calculated by assuming a stoichiometry of
CO/Ni=1/1.
[c] The average Ni crystallite size was estimated from CO chemisorption.
[d] The average Ni crystallite size was estimated from XRD spectrum by Scherrer's equation.
[e] Effective surface Ni sites were calculated by (total moles of Ni loading * Dispersion)/catalyst mass,
-1
unit: molsurface Ni · gcat
.
[f] Others include phenol, 2-methoxy-6-methylphenol, etc.
[g] Rate for cyclohexane formation based on the weight of catalyst, and the rate is an average over the
30~60 minutes, unit: (molcyclohexane·gcat-1·min-1·10-4).
[h] Normalized rate is defined as rate for cyclohexane formation base on the effective moles of surface
Ni, unit: min-1
Based on the product distribution of HDO of guaiacol (Table 4) along with the references of
the mechanistic studies reported in the literature, the proposed reaction pathways for the HDO
of guaiacol are shown in Figure 12. In the dominant route (A), guaiacol initially undergoes
hydrogenation to 2-methoxycyclohexanol on metallic Ni sites [22, 49, 83], followed by the rapid
demethoxylation and dehydroxylation of 2-methoxycyclohexanol to form cyclohexene,
methanol, and H2O on acid sites, then hydrogenation of cyclohexene to cyclohexane on metal
sites, which is in line with literature [11, 25, 84-86]. Subsequently, methanol can be further
converted to methane at higher temperatures on metallic Ni sites [87]. In addition, it is noted
the reaction pathway is slightly different with the studies by Song [49], in which the guaiacol
undergoes hydrogenolysis to phenol was more favorable over Ni/ZSM-5 catalyst caused by
23

Catalysis Science & Technology
Page 24 of 27
steric constraints. However, the hydrogenation of the aromatic ring of guaiacol wDaOsI:r1e0.g10a3Vr9ide/wCe9AdCrtiYc0le1O97nl0inGe
as the dominant reaction pathway in many studies [88-90], the reasons leading to the
difference in reaction pathways could be attributed to the reaction pressure and nickel particles
sites. Small Ni particles are prone to be formed by deposition-precipitation method, which
could possess a low hydrogenation activity per surface nickel.
On the other hand, combined with the performance results (see Table 3&4), we propose an
alternative pathway for the conversion of guaiacol to cyclohexane (route B). When the catalyst
has little hydrogenation activity (as is the case in the absence of Ni hydrides on nanoparticles),
the guaiacol is first converted to 1,2-dimethoxybenzene and catechol on acid sites, then the
catechol rapidly undergoes hydrogenation-dehydroxylation to form cyclohexane, while the 1,2-
dimethoxybenzene slowly undergoes the hydrogenation-demethoxylation to form
cyclohexane.
Figure 12. Reaction scheme for the hydrodeoxygenation of guaiacol on high Ni loading
catalysts
4. Conclusion
Guaiacol upgrade to cyclohexane derivatives over Ni-based catalysts has been thoroughly
investigated in this study. 15.7 wt% Ni/Beta-12.5 was shown to be the most effective catalyst
for the conversion of guaiacol, attributed to the optimized balance between active
24

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Catalysis Science & Technology
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hydrogenation (metal) and acid sites. The formation rate of cyclohexane was found to have a
DOI: 10.1039/C9CY01970G
positive relationship with the total number of acid sites, suggesting demethoxylation and
dehydroxylation is catalyzed by Brønsted acid sites. On the other hand, the small Ni particles
prepared by ion exchange have a stronger interaction with the support, and do not readily form
nickel hydride. As a consequence, the catalysts containing small Ni particles exhibit a lower
hydrogenation activity at low reaction temperatures. As for the samples with higher Ni loading
(prepared by impregnation), the normalized rate for cyclohexane formation based on surface
Ni increases with the increasing Ni loading, and the catalyst that shows the best performance
contains 15.7 wt% Ni. This is due to the high concentration of nickel hydrides on larger Ni
nanoparticles, which can facilitate the hydrogenation activity under mild reaction conditions.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Funding from the Australian Research Council is gratefully acknowledged. PY would like to
thank the University of Newcastle for a post graduate research scholarship. Authors thank the
Electron Microscope and X-ray Unit of the University of Newcastle for the use of their facilities.
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