Gold particle size effect in biomass-derived lignan hydroxymatairesinol oxidation over Au/Al2O3 catalysts

Article abstract of DOI:10.1016/j.apcata.2014.12.051

The heterogeneously catalyzed aerobic selective oxidation of naturally occurring lignan hydroxymatairesinol (HMR) to another lignan oxomatairesinol (oxoMAT) was carried out at 70°C and atmospheric pressure over gold nanoparticles with different sizes supp

Full text of DOI:10.1016/j.apcata.2014.12.051

Applied Catalysis A: General 504 (2015) 248–255
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Gold particle size effect in biomass-derived lignan
hydroxymatairesinol oxidation over Au/Al2O3 catalysts
Martín Lópeza, Olga A. Simakovab, Elena V. Murzinab, Stefan M. Willförc, Igor Prosvirind,
Andrey Simakove, Dmitry Yu. Murzinb,∗
a Posgrado de Ciencia e Ingeniería de Materiales de la UNAM, Centro de Nanociencias y Nanotecnología, C.P. 22860, Ensenada, Baja California, Mexico
b Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi University, FI-20500 Åbo/Turku, Finland
c Laboratory of Wood and Paper Chemistry, Process Chemistry Centre, Åbo Akademi University, FI-20500 Åbo/Turku, Finland
d Boreskov Institute of Catalysis, Lavrentieva 5, Novosibirsk 630090, Russia
e Universidad Nacional Autónoma de México, Centro de Nanociencias y Nanotecnología, km. 107 Carretera Tijuana-Ensenada, C.P. 22860, Ensenada,
Baja California, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The heterogeneously catalyzed aerobic selective oxidation of naturally occurring lignan hydroxy-
matairesinol (HMR) to another lignan oxomatairesinol (oxoMAT) was carried out at 70 ◦C and atmospheric
pressure over gold nanoparticles with different sizes supported on alumina prepared from organo-
metallic precursor by the sol–gel method. Precursor of gold nanoparticles (i.e. gold hydroxide) was
supported on alumina by deposition–precipitation with urea from HAuCl4 aqueous solution followed by
washing with ammonium hydroxide. The size of the supported gold nanoparticles was changed by either
treatment under hydrogen flow at different temperatures or exposure to ultraviolet (UV) irradiation at the
wavelength of 254 nm. The catalysts were characterized by inductively coupled plasma atomic emission
spectroscopy (ICP-AES), nitrogen adsorption, X-ray diffraction (XRD), transmission electron microscopy
(TEM), X-ray photoelectron spectra (XPS), and UV–visible spectroscopy (UV–vis). Structure sensitivity in
HMR oxidation was studied showing that the reaction rate passes through a maximum when the gold
Received 15 September 2014
Received in revised form
28 December 2014
Accepted 29 December 2014
Available online 6 January 2015
Keywords:
Sol–gel
UV–visible spectroscopy
Oxomatairesinol (oxoMAT)
Structure sensitivity
cluster size is ca. 4 nm.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
chemicals. Biomass derived extractives can be found mainly in
knots and bark, which are normally considered to be a waste in
Catalysis using gold nanoparticles recently became attractive
for many researches since the moment when Haruta and Hutch-
ings independently and almost simultaneously in around 1987
discovered that gold served as an extraordinary good heteroge-
neous catalyst in the low-temperature oxidation of CO and the
hydrochlorination of ethyne to vinyl chloride, respectively [1–3].
Nowadays gold catalysts are well known for being highly active
and selective in different oxidation reactions, in particular, trans-
formations of alcohols to corresponding aldehydes, ketones and
acids [4–9]. These reactions are among the fundamental ones in
biomass catalytic conversion.
pulp and paper industry. Lignans are a group of phenolic com-
pounds found in different parts of plants, while the richest source is
knot wood. For example, Norway spruce (Picea abies) knots contain
a large amount of lignans including hydroxymatairesinol (HMR),
which is the most abundant, constituting 65–85 wt.% of the lignans.
Lignans having anticarcinogenic and antioxidative effects can be
applied for the cosmetic (skin- and hair-care products) and phar-
maceutical use. Thus, the lignan oxomatairesinol (oxoMAT), which
is valuable for such applications, can be synthesized via dehydro-
genation/oxidation of secondary alcohol group present in HMR
(Scheme 1).
Wood biomass offers a wide range of possibilities from the
view point of biosustainable chemical processes. It can be trans-
formed not only into fuels and basic chemicals, but also into fine
The formation of oxoMAT from HMR by light-irradiation
was reported in [10], while in [11] 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) was used as an oxidizing agent. Hence a
need for industrially applicable catalytic methods for the selective
synthesis of lignan oxoMAT exists. Commercially more attractive
heterogeneously catalyzed synthesis of oxoMAT was performed
over supported palladium [12] and gold catalysts [13,14]. The
∗
Corresponding author. Tel.: +358 22154985.
E-mail address: dmurzin@abo.fi (D.Yu. Murzin).
http://dx.doi.org/10.1016/j.apcata.2014.12.051
0926-860X/© 2015 Elsevier B.V. All rights reserved.

M. López et al. / Applied Catalysis A: General 504 (2015) 248–255
249
O
O
H3CO
HO
H3CO
O
O
HO
OH-
HO
O2
Au
OCH3
catalyst
H2O
OCH3
O
OH
-Coni
H3CO
HO
HMR 1 OH
α
O
OH-
O
OH-
O
O
O
H3CO
HO
H3CO
OCH3
OH
O2
Au
OH
HO
OH
OxoMAT
HO
H2O
catalyst
OCH3
OCH3
OH
OH
HMR 2
α
-ConiA
Scheme 1. Transformations of the lignan hydroxymatairesinol (HMR) over gold catalysts.
former studies demonstrated superior performance of gold cat-
alysts over palladium ones in terms of activity and selectivity.
Moreover, utilization of gold catalysts allowed carrying out the
reaction under environmentally benign conditions, using oxygen
from air as an oxidant, water–alcohol mixture as a solvent and
low temperature. Therefore, further investigation of the reaction
became focused on application of the heterogeneous gold catalysts,
in particular Au/Al2O3.
It is generally accepted, that catalytic activity of gold catalysts
is strongly dependent on gold particle size [15]. In [16,17] it was
pointed out that the largest effect on the activity is attributed to
the presence of low-coordinated Au atoms, which are abundant
on the smallest gold nanoparticles. The most reactive atoms are
located at the edges and corners of particles. These Au atoms are
suggested to be able to facilitate adsorption of CO and dioxygen
to perform CO oxidation reaction. The number of low-coordinated
corner and edges sites is decreasing with increasing of Au particle
size. Therefore, even the particle size changing slightly, the fraction
of active sites decreases dramatically. The other effects, such as
interactions with the support [18,19], charge transfer [20], strain
[21] were suggested to contribute, albeit to a lesser extent. Gold
catalysts are known to exhibit structure sensitivity in particular in
hydrogenation reactions [22]; oxidation of CO [23], glycerol [24],
alcohols [25] and sugars [26,27].
The aim of the present work was to investigate the structure
sensitivity of the biomass-derived lignan HMR oxidative dehydro-
genation on the Au nanoparticles (NPs) supported on alumina.
Recently it was reported that oxidation of HMR over Au NPs sup-
ported on the commercially available ␥-alumina demonstrated
volcano-shape relationship between the reaction rate and the size
of Au particles with the maximum at around 2 nm [28]. In [14] it
was found that alumina prepared by the sol–gel method possesses
appropriate acidity for improving activity of gold catalysts in the
selective oxidation of HMR. Since both, support acidity and Au par-
ticle size, impact the reaction rate it is important to investigate the
effect of the gold particle size in the case of a more acidic catalyst
support. Therefore, in the current work sol–gel alumina prepared
with an organo-metallic precursor was used bearing enhanced
acid properties as was already demonstrated in our previous work
[14].
2. Experimental
2.1. Catalyst preparation
Alumina support was prepared by a sol–gel method from a
metallo-organic precursor following a procedure reported pre-
viously [29]. 75 mL of aluminum sec-butoxide (Alfa-Aesar) were
mixed with 50 mL of 2-methyl-2,4-pentanediol (Alfa-Aesar), stay-
ing in reflux for 3 h, with moderate agitation at 80 ◦C. Hydrolysis
was performed by drop wise adding of 50 mL of deionized water.
The obtained gel was aged for 10 h. The sample was dried under
vacuum (about 10−3 Torr) at 100 ◦C for 12 h and then heated in N2
atmosphere at 450 ◦C for 12 h with a ramp rate 5 ◦C/min, followed
by treatment in O2 at 650 ◦C for 4 h for decomposition of organic
residuals.
Gold (1 wt.%) was supported by deposition–precipitation using
urea as a precipitation agent. 4 g of prepared alumina support were
added to 400 ml of aqueous solution of HAuCl4 (4.2 × 10−3 M) and
urea (0.42 M). The initial pH of solution was ca. 2. After vigorous
stirring at 80 ◦C for 4 h the suspension was filtered and washed with
ammonium hydroxide (25 M) for 30 min. The pH of the solution
after stirring with ammonium hydroxide was ca. 10. Finally, the
sample was washed in water until pH was 7, filtered and dried at
room temperature for 24 h. The obtained catalyst was denoted as
Au/Al2O3-F.
In order to form gold nanoparticles (Au NPs) with different sizes
certain amounts of freshly prepared Au/Al2O3-F were treated in
hydrogen flow at different temperatures or exposed to ultraviolet
(UV) irradiation at 254 nm, using a fully microprocessor controlled
unit UVIlink CL 508 Crosslinker (Uvitec Cambridge) equipped with
an integrating sphere. Temperature programmed reduction (TPR)
of freshly prepared gold samples was carried out in a flow reactor
under linear sample heating with ramp 20 ◦C/min up to 250, 350
and 600 ◦C using gas mixture 3 vol.% H2 in He.
2.2. Catalyst characterization
Chemical analysis for obtained gold samples was done with an
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) using ICP Liberty 110 Emission Spectrometer Varian. For each

250
M. López et al. / Applied Catalysis A: General 504 (2015) 248–255
measurement 30 mg of sample was dissolved in the 20 ml of acids
mixture (H2SO4:HCl:HNO3 = 6:6:3) and heated up to 150 ◦C.
The specific surface area and pore size distribution were deter-
mined by nitrogen adsorption measurements in a Tri-Star III device
applying BET method. Before analysis the samples were treated in
a vacuum (10−3 Torr) at 300 ◦C for 4 h.
mass transfer limitations) at reaction time set to zero and the first
sample was withdrawn.
2.4. Product analysis
The samples withdrawn from the reactor at different time
intervals were analyzed by GC (Perkin Elmer Instrumentary, Auto
System XL) equipped with autosampler using a HP-1 column
(length 25 m, inner diameter 0.20 mm, film thickness 0.11 ␮m) and
a flame ionization detector (FID) operating at 300 ◦C. Prior to the
GC analysis an internal standard containing mainly betulinol and
C21:0 fatty acid was added, the solvent was evaporated and the
samples were silylated. More details of the analysis procedure are
given in [13]. The initial temperature of the column was 120 ◦C
(for 1 min), and the temperature was increased at a rate 6 ◦C/min
to 300 ◦C (for 10 min). The peaks were verified by analysis with a
gas chromatograph–mass spectrometer (GC–MS, Hewlett Packard)
applying the same GC conditions.
X-ray diffraction (XRD) analysis was carried out by Philips X’pert
diffractometer equipped with a curved graphite monochroma-
tor applying CuK␣ (ꢀ = 0.154 nm) radiation. Transmission electron
microscopy (TEM) was performed by using JEOL 2010 microscope.
The sample was dispersed in isopropanol and dropped on a copper
grid coated with a carbon film. To determine the mean diame-
ter of gold particles more than 200 particles were chosen. The
mean diameter (dm) of particles was calculated using well known
procedure from histograms of particle size distribution with the
ꢀ
ꢀ
following formula: dm
particles with diameter di.
=
i(xidi)/ ixi, where xi is the number of
X-ray photoelectron spectra were recorded using SPECS spec-
trometer with PHOIBOS-150 hemispherical energy analyzer and
AlK␣ irradiation (hꢁ = 1486.61 eV, 200 W). All measured binding
energies (BE) were referred to C1s line of adventitious carbon at
284.8 eV. The pass energy of the analyzer was 20 eV.
3. Results and discussion
In situ UV–visible (UV–vis) spectroscopic analysis of the gold
species transformation under temperature-programmed reduction
(TPR) was carried out in a lab made set-up [30] with simulta-
neous analysis of gas phase components and recording of the
UV–vis spectra of the sample. Spectra were collected using an
AVANTES AvaSpec-2048 UV–visible spectrometer equipped with
an AvaLight-DHS light source and a high temperature optic fiber
reflection probe located close to the external wall of the quartz
reactor. Time of spectrum recording was about 5 ms. UV–vis spec-
tra recorded each 15 s were obtained by subtraction of the initial
spectrum recorded at room temperature from those recorded at
elevated temperatures. The reactor packed with MgO was used as
a reference.
3.1. Formation of Au NPs
In order to prepare Au NPs of a small size a freshly prepared
Au/Al2O3-F sample was treated by UV irradiation at room temper-
ature, which permits to avoid agglomeration of formed metallic
gold species. For the first time this technique was applied for the
formation of Au NPs in HAuCl4 aqueous solutions in [32]. Later
on ultraviolet irradiation was used for the formation of metallic
gold species supported on titania [33]. In the current study it was
anticipated that gold hydroxide being supported on alumina [37]
may also be reduced into Au NPs via ionization of pre-adsorbed
water under ultraviolet irradiation [34]. The formation of Au NPs
under UV irradiation was monitored by UV–vis spectroscopy ex
situ. The presence of metallic gold species could be easily detected
by the appearance of well resolved plasmon peak at the wavelength
of ca. 530 nm in the UV-visible spectrum, which is characteristic
for metal gold NPs [35–37]. It was observed, that the freshly pre-
pared Au/Al2O3-F catalyst did not contain supported metallic gold
species. UV–visible spectra obtained after different time of treat-
ment of Au/Al2O3-F catalyst are presented in Fig. 1 (left). After
the first hour of treatment by UV irradiation the peak of plasmon
appeared, which was associated with the initiation of metal Au par-
ticles formation. The dependence of the plasmon peak intensity and
position on time of treatment is presented in Fig. 1 (right). Since the
intensity of the plasmon peak was not changing after 10 h of the cat-
alyst treatment by UV irradiation the formation of Au NPs can be
considered to be complete by this time. According to a theoreti-
cal estimation described in [38] the observed blue shift of the peak
position under the sample treatment by UV irradiation corresponds
to the Au NPs growth in media with a dielectric constant below 3,
such as air. It implies that Au NPs were predominantly formed via
the attack of the external surface of gold hydroxide particles with
solvated electrons [34] yielded by the ionization of pre-adsorbed
water. The catalyst treated by UV irradiation for 12 h was denoted
as Au/Al2O3–UV.
Ex situ UV–vis spectroscopic analysis of the gold species trans-
formation was carried out at room temperature using the same
spectrometer and standard diffuse reflectance cell.
2.3. Hydroxymatairesinol (HMR) oxidation
Lignan HMR was extracted from ground Norway spruce knots
by acetone–water mixture as described in [31]. The extract was
concentrated in a rotary evaporator and then purified by flash chro-
matography. HMR was obtained as a mixture of two diastereomers
HMR 1 and HMR 2, and the HMR 2-to-HMR 1 molar ratio was 2. The
purity of HMR was determined by gas chromatography (GC) to be
95%. The major contaminants were lignans: ␣-conidendrin (Coni)
and ␣-conidendric acid (ConiA), which structures are presented in
Scheme 1. Both isomers, HMR 1 and HMR 2, could be oxidized, and
the isomerization between them occurs as well.
The reaction was performed in a stirred 200 ml glass reactor
under atmospheric pressure, equipped with a heating jacket (using
silicon oil as the heat transfer medium), a re-flux condenser (cooling
medium set at −20 ◦C), oil lock, pitched-blade turbine and stirring
baffles. In a typical experiment, the catalyst was charged into the
reactor in the amount corresponding to 5 mg of the active metal.
The catalyst grain size was 45–63 ␮m to suppress the internal mass
transfer limitations. Catalysts were tested using 2 vol.% propan-2-
ol (Sigma–Aldrich, 99.8%) in water as a solvent, which was shown
to be the most effective in one in the previous work of the authors
[13]. The reactant solution (100 ml) with an HMR concentration of
1 mg/ml was poured into the reactor. Synthetic air (20% oxygen
and 80% nitrogen, supplied by AGA, 99.999%) gas flow was set to be
100 ml/min. The stirring was started (1000 rpm to avoid external
In order to form larger Au NPs of various sizes, the freshly
prepared Au/Al2O3-F was exposed to the thermal treatment in
hydrogen flow at different temperatures (250–600 ◦C). The cata-
lyst treatment at 600 ◦C was conducted for extended period of time
such as 16 and 168 h to further increase particle size. The UV–vis
spectroscopy in situ was applied to monitor the formation of Au
NPs. The spectra obtained are presented in Fig. 2 (left). The appear-
ance of the plasmon peak in UV–vis spectra at temperatures above
50 ◦C manifested the start of the Au NPs formation (Fig. 2, left,

M. López et al. / Applied Catalysis A: General 504 (2015) 248–255
251
700
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1 h
2 h
3 h
4 h
5 h
6 h
7 h
8 h
9 h
10 h
11 h
12 h
0.6
0.4
0.2
0.0
650
600
550
500
0
1
2
3
4
5
6
7
8
9
10 11 12 13
300
400
500
600
700
Time, h
Wavelength, nm
Fig. 1. Ex situ UV–visible spectra of Au/Al2O3 catalyst under UV irradiation at 254 nm vs time (left). Intensity and position of the plasmon peak vs treatment temperature
(right).
marked with a white arrow). The changes of normalized intensity
and position of plasmon peak with temperature of thermal treat-
ment are presented in Fig. 2 (right). The plasmon peak intensity
almost reached its maximum at about 250 ◦C. In contrast to the
formation of Au NPs under UV irradiation, thermal treatment led
to the red shift of plasmon peak within the temperature interval
50–250 ◦C. Such shift of the plasmon peak corresponds to the for-
mation of Au NPs initially inside of gold hydroxide particles due to
their sequential thermal decomposition: Au(OH)3 → Au2O3 → Au
[39]. The sample obtained at 250 ◦C was denoted as Au/Al2O3-250
and selected for the catalytic test. Further thermal treatment of the
formed Au NPs resulted in the blue shift of the plasmon peak for
the temperature interval 250–350 ◦C. Since the media surrounding
supported Au NPs is characterized with the dielectric constant for
air (ca. 1), the plasmon peak shift was associated with agglomera-
tion of supported Au NPs. Therefore, sample obtained at 350 ◦C was
selected for further catalytic tests and was denoted as Au/Al2O3-
350. At temperatures above 500 ◦C some changes in the shape of
plasmon peak were observed. Intensity of the peak shoulder at high
wavelengths decreased with an increase of temperature (Fig. 2,
left). The latter may be assigned to the disappearance of small
Au NPs interaction between inter se due to their agglomeration
as it was noted in [40,41]. Thus, other samples for catalytic tests
were obtained by thermal treatment at 600 ◦C for 16 h or for 168 h
(7 days) and were denoted as Au/Al2O3-600-16 h and Au/Al2O3-
600-168 h, respectively.
The Au NPs found by TEM in the studied samples (dark features
marked with arrows) were characterized with almost spherical
shape practically without any well detectable crystallographic
planes (Fig. 3). Metallic Au NPs were observed in TEM images for
the as-prepared sample, although those particles were not detected
by UV–vis. Most likely Au NPs were formed as a result of gold
hydroxide decomposition/reduction under the sample evacuation
in the microscope vacuum chamber and/or due to the sample
exposure to the electron beam. The size of Au NPs for Au/Al2O3-
F sample seemed to be determined by the size of initial Au(OH)3
particles deposited on the alumina during the preparation. Anal-
ysis of TEM images showed that the average size of Au NPs in
Au/Al2O3-F (Fig. 3A) and Au/Al2O3–UV (Fig. 3F) was found to be
similar, ca. 1.8–1.9 nm. It was confirmed that conditions of UV-
radiation did not cause agglomeration of the primary deposited
Au species, although thermal treatment of catalysts at tempera-
tures of 250–600 ◦C resulted in the variations in Au NPs size (see
Table 1). The mean diameter of Au NPs prepared by catalyst treat-
ment at 250 ◦C (Au/Al2O3-250) was ca. 3 nm (Fig. 3B). Increase of the
treatment temperature by 100 ◦C resulted in a slight increase of Au
NPs size up to 3.2 nm in Au/Al2O3-350 (Fig. 3C), while an increase
of treatment temperature up to 600 ◦C as well as increasing time
Fig. 2. In situ UV–visible spectra of Au/Al2O3 catalyst under thermal treatment in hydrogen with a temperature ramp rate 20 ◦C/min (left). Normalized intensity and position
of the plasmon peak vs. treatment temperature (right).

252
M. López et al. / Applied Catalysis A: General 504 (2015) 248–255
Fig. 3. TEM micrographs and histograms of gold nanoparticles size distribution for Au/Al2O3 sample: A – fresh sample after drying (Au/Al2O3-F); B – thermally treated at
250 ◦C (Au/Al2O3-250); C – thermally treated at 350 ◦C (Au/Al2O3-350); D – thermally treated at 600 ◦C for 16 h (Au/Al2O3-600-16 h); E – thermally treated at 600 ◦C for
168 hours (Au/Al2O3-600-168 h); F – treated by UV irradiation at 254 nm during 12 h (Au/Al2O3-UV).

M. López et al. / Applied Catalysis A: General 504 (2015) 248–255
253
Table 1
of spectra Au 4f indicates Au 4f7/2 components at BE value around
83.2–83.5 eV, which can be attributed to Au0 species in according
to [42,43]. Some variations of peak position seem to be caused by
the presence of organic residuals on the alumina prepared from an
organo-metallic precursor.
Average size of the supported Au NPs according to TEM analysis.
Catalyst
Average Au particle size (nm)
Au/Al2O3-F
Au/Al2O3-UV
Au/Al2O3-250
Au/Al2O3-350
Au/Al2O3-600-16 h
Au/Al2O3-600-168 h
1.8 0.6
1.9 1.1
3.0 1.3
3.2 0.9
4.8 1.2
10.0 3.2
3.2. Hydroxymatairesinol (HMR) oxidation
3.2.1. Catalyst activity
Gold catalysts with different Au particle size (1.9–10 nm) were
evaluated in HMR selective oxidation. The obtained dependence
of oxoMAT concentration changes on time over catalysts with dif-
ferent particle sizes is presented in Fig. 5. The highest activity was
achieved over gold clusters of 3.2 nm (Au/Al2O3-350), while the cat-
alyst with the smallest gold particles (1.9 nm) demonstrated zero
activity (Au/Al2O3–UV).
Experiments were carried out several times to confirm repro-
ducibility of the obtained values. Catalytic activity defined as the
initial rate within 10 min was found to be dependent on the gold
cluster size and have a volcano relationship with the particle size
(Fig. 6). The values of initial rate were estimated taking into account
standard deviations for particle size (shown in Table 1) and errors
for the detection of the initial reagents and formed products by
gas chromatographic analysis (3–5%). The highest activity was
achieved over gold clusters of ca. 4 nm.
4f 5/2
4f 7/2
83.2 eV
D
C
B
A
It is important to emphasize that the reaction rate in the cur-
rent work at maximum is four fold higher than in the case of gold
catalysts supported on commercial ␥-alumina [28]. As was already
mentioned, higher catalytic activity is attributed to the presence
of the stronger Lewis acid sites on the prepared alumina surface
[14]. Another difference between the current work with sol–gel
alumina and the previous HMR selective oxidation reaction inves-
tigated in [28] with Au/␥-Al2O3 is that in the latter the maximum
of the reaction rate was observed with Au particle size of ca. 2 nm
[28]. Therefore, there is a clear connection between the position of
the rate maxima and the values of kinetic parameters.
82
84
86
88
90
92
BE, eV
Fig. 4. A4 4f XP spectra of Au/Al2O3 catalysts prepared by treatment under UV
irradiation at 254 nm during 12 h – Au/Al2O3-UV (A) or in hydrogen at 250 ◦C –
Au/Al2O3-250 (B), at 350 ◦C – Au/Al2O3-350 (C); at 600 ◦C – Au/Al2O3-600 (D).
of the treatment to 16 h (Au/Al2O3-600-16 h) or 168 h (Au/Al2O3-
600-168 h) significantly affected the size of Au NPs (Fig. 3D and
E).
The chemical state of the gold species formed on the surface
of the obtained catalysts was characterized by XPS technique. The
XPS spectra for Au 4f region are presented in Fig. 4. All spectra
were characterized by one doublet of two spin–orbit components
separated by 3.67 eV, i.e., Au 4f7/2 and Au 4f5/2. The curve fitting
The analysis of structure sensitivity in the case of HMR oxida-
tion is rather complex due to involvement of the support. As was
proposed before [14], metal oxide surface groups play an impor-
tant role in the coordination of bulky HMR molecule (ca. 1.5 nm)
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
3.2 nm
4.8 nm
o 10.0 nm
3.0 nm
1.9 nm
0
50
100
150
200
250
Time, min
Fig. 5. Dependence of oxomatairesinol (oxoMAT) concentration on time over Au/Al2O3 catalysts.

254
M. López et al. / Applied Catalysis A: General 504 (2015) 248–255
11
10
9
8
7
6
5
4
3
2
1
0
0
2
4
6
8
10
12
14
Au nanoparticles diameter, nm
Fig. 7. Dependence of selectivity in the lignan hydroxymatairesinol (HMR) selective
oxidation to oxomatairesinol (oxoMAT) on Au particle size of Au/Al2O3 catalyst.
Fig. 6. Dependence of initial rates in the lignan hydroxymatairesinol selective oxida-
tion on Au particle size of Au/Al2O3 catalyst: experimental data (squares), estimated
curve (solid line).
for Eq. (1) requiring Taylor expansion of exponential functions. In
any case difference in the location of the rate maximum can be
attributed to the values of Polanyi parameter, kinetic parameters p1
and p4 as well as the parameter p3. The latter reflects the magnitude
of so-called heterogeneity of gold clusters, which is related to the
differences in the strength of HMR adsorption on edges and terraces
for gold clusters supported on sol–gel and commercial ␥-alumina.
For example if due to changes in the support and interactions with
it, smaller gold clusters adsorb HMR differently on sol–gel alumina
and commercial ␥-alumina it is natural to expect changes in the
position of the rate maxima. This is a conclusion of general validity
not only related to HMR oxidative hydrogenation.
suitable for further transformation of the lignan over gold catalysts.
This step is considered to be essential for the HMR transforma-
tion. Once the molecule is in the right position, oxidation or in fact
oxidative dehydrogenation [44] proceeds rapidly, enhanced by the
presence of activated oxygen.
Whatever is the chemical explanation behind the structure sen-
sitivity it is possible to apply a phenomenological thermodynamic
approach to model the experimental data. Such approach is based
on the differences in adsorption strength on small and large clus-
ters due to changes in the fraction of terraces, edges and corners
with the size increase [45].
It was also clearly demonstrated in [45] that position of the max-
imum in TOF, if there is such a maximum, depends on the values of
rate parameters, such as kinetic constants, as well as the value of
the Polanyi parameter and the magnitude of the difference between
Gibbs energy of reactant adsorption on edges and terraces.
The kinetic model for description of the rate dependence in HMR
oxidation was developed in [28] giving in the case of experimentally
observed zero in oxygen and fractional order in HMR the following
rate equation
3.2.2. Product distribution
There are two competitive reactions during the transformation
of HMR over gold catalysts, namely oxidation of HMR and isom-
erization of diastereomers HMR 2 to HMR 1. The former one is
typically performed over the alumina surface even in the absence of
Au particles. However, in the presence of Au particles on the sup-
port surface the oxidation reaction influence is dominating [13].
Therefore, transformation of HMR over catalysts demonstrating
low oxidation activity is typically resulting in the formation of both,
isomerization and oxidation products. Selectivity toward HMR 1
and oxoMAT dependence on gold particles size is given in Fig. 7.
Fig. 7 clearly shows that selectivity to OxoMAT is increasing with
an increase of the cluster size. It was also observed, that the most
active catalysts with Au particles size of 3.2 nm (Au/Al2O3-350) and
10 nm (Au/Al2O3-600-168 h), respectively, had 100% selectivity to
oxoMAT, while the selectivity to oxoMAT of less active catalysts
(Au/Al2O3-250 and Au/Al2O3-600-16 h) was lower.
p
/d
p1ep
(1 + p4ep
2
3
cluster
cluster
r =
(1)
/d
3
)dcluster
with
p1 = kKHMRCHMR, p2 = ˛, p3 = ꢂHMR, p4 = KHMRCHMR
(2)
where k is the modified rate constant of rds, KHMR and CHMR are
the adsorption constant of HMR on terraces and concentration of
HMR, respectively, ˛ is the Polanyi parameter of rds, ꢂ is the dif-
ference between Gibbs energy of HMR adsorption on edges and
terraces divided by RT.
Comparison of experimental data with the calculations showed
rather good correspondence of them (Fig. 6). A model describing
volcano-shape dependence of the turnover-frequency on the Au
particle size has the maximum of catalytic activity corresponding
to the Au particle with diameter of ca. 3.5–4 nm. The values of the
parameters could not have been identified with certainty due to a
limited set of data. Analysis of the position maximum can be used in
principle to improve statistical identification of the parameters, as
in certain cases position of the rate maximum can be theoretically
determined by analyzing the derivative of the reciprocal function of
the reaction rate [46]. However, such analysis cannot be easily done
4. Conclusions
The synthesis of supported Au which allows control of Au NPs
particle size was applied. The growth of the Au particles under
influence of UV irradiation and temperature was thoroughly inves-
tigated. The reaction structure sensitivity of the selective oxidation
of the lignan hydroxymatairesinol (HMR) over gold catalysts was
demonstrated. A model was developed describing volcano-shape
dependence of the turnover-frequency on the Au particle size,
where the maximum of catalytic activity was corresponding to the
Au particle diameter of ca. 4 nm.

M. López et al. / Applied Catalysis A: General 504 (2015) 248–255
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