Synthesis, characterization and performance of bifunctional catalysts for the synthesis of menthol from citronellal

Article abstract of DOI:10.1039/c6ra25931f

The synthesis of a series of bifunctional catalysts (1 wt% Pt/W-TUD-1 (Technische Universiteit Delft-1) and 1 wt% Pt/WO₃/TUD-1) with different tungsten loadings (5-30 wt% WO₃) is described. They were characterized using ICP-OES, INAA, N₂ physisorption, XRD and TEM. Their catalytic performance (activity and selectivity) was evaluated in the two-step catalytic synthesis of menthol from citronellal using kinetic analysis. Introducing tungsten during the TUD-1 synthesis results in a high WO₃ dispersion, essential for the acidity of the catalyst. High tungsten dispersion is also critical for the Pt hydrogenation activity. Therefore, high dispersion combined with optimal tungsten loading resulted in the highest catalytic activity. The best performing catalyst was 1 wt% Pt/W-TUD-1 (silicon to tungsten ratio of 30), with the highest yields of menthol (96%).

Full text of DOI:10.1039/c6ra25931f

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Synthesis, characterization and performance of
bifunctional catalysts for the synthesis of menthol
from citronellal†
Cite this: RSC Adv., 2017, 7, 12041
a
b
a
c
a
*
J. ten Dam, A. Ramanathan, K. Djanashvili, F. Kapteijn and U. Hanefeld
The synthesis of a series of bifunctional catalysts (1 wt% Pt/W-TUD-1 (Technische Universiteit Delft-1) and 1
wt% Pt/WO3/TUD-1) with different tungsten loadings (5–30 wt% WO3) is described. They were
characterized using ICP-OES, INAA, N2 physisorption, XRD and TEM. Their catalytic performance (activity
and selectivity) was evaluated in the two-step catalytic synthesis of menthol from citronellal using kinetic
analysis. Introducing tungsten during the TUD-1 synthesis results in a high WO3 dispersion, essential for
the acidity of the catalyst. High tungsten dispersion is also critical for the Pt hydrogenation activity.
Therefore, high dispersion combined with optimal tungsten loading resulted in the highest catalytic
activity. The best performing catalyst was 1 wt% Pt/W-TUD-1 (silicon to tungsten ratio of 30), with the
highest yields of menthol (96%).
Received 27th October 2016
Accepted 9th February 2017
DOI: 10.1039/c6ra25931f
rsc.li/rsc-advances
a hydrodeoxygenation and decarboxylation. They showed that
acid impregnation (Mo) followed by hydrogenation metal (Co)
produced a more active catalyst.17
Introduction
Citronellal can be obtained from the fractional distillation of
natural citronella oil. This monoterpenoid is a versatile
building block for a series of organic syntheses.1 One extensively
researched process, is the one pot, 2-step synthesis of
menthol.2–7 This sequence relies on the acid catalysed Prins
cyclisation that converts citronellal into isopulegol.8–12 This
olen is subsequently hydrogenated to form menthol.2–7
The process requires both an acid (Brønsted or Lewis acid)
and a hydrogenation catalyst functionality. Bifunctional acidic
hydrogenation catalysts can be obtained in various ways. They
can be prepared by impregnating acidic supports/catalysts like
zeolites, acidic carbons, Al-TUD-1, B-TUD-1, SAPOs and acidic
resins with a hydrogenation metal precursor (Pt, Pd, Rh, Ru, Ir,
Cu, Ni and Co).2–5,13–16 The ratio of Lewis and Brønsted acid were
varied to investigate their inuence on the Prins cyclisation.
Only in the case of Zr and Al in one material some synergy was
displayed.13
This paper describes the synthesis of two series of bifunc-
tional acidic hydrogenation catalysts. The acidity is derived
from tungsten, while platinum is the hydrogenation centre.18–21
The rst series is based on the direct synthesis of W-TUD-1,
which is subsequently impregnated with a platinum precursor
solution. The second series is based on the consecutive
impregnation of TUD-1 with WO3 and with platinum precursor
solutions. Their catalytic activity is evaluated in the two-step
synthesis of menthol from citronellal (Scheme 1).2–7 In this
example the selectivity revolves around the acidic sites: they
should be acidic enough to catalyse the Prins cyclisation of
citronellal, but they should not be able to remove the formed
hydroxyl group through elimination.8–12
TUD-1 was chosen as a support because of its amorphous
structure and relatively large pore size. These characteristics aid
in overcoming diffusion limitations, which is particularly
important in liquid phase ne chemical synthesis because of
the molecular sizes involved.13,22–24 Previous work showed that
direct synthesis of W-TUD-1 resulted in smaller WO3 particles
(below XRD detection limit, i.e. 2–3 nm) leading to a more acidic
Another option is to consecutively impregnate an inert
carrier with an acid and a hydrogenation metal precursor
solution (or vice versa). Ferrari et al. studied the inuence of the
order of impregnation on the activity of a CoMo catalyst in
aBiocatalysis and Organic Chemistry, Department of Biotechnology, Del University of
Technology, Van der Maasweg 9, 2629 HZ Del, The Netherlands. E-mail: U.
Hanefeld@tudel.nl; Tel: +31 (0)15 27 82683
bCenter for Environmentally Benecial Catalysis (CEBC), The University of Kansas,
Lawrence, KS 66047, USA
cCatalysis Engineering, Department of Chemical Engineering, Del University of
Technology, Van der Maasweg 9, 2629 HZ Del, The Netherlands
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra25931f
Scheme 1 Menthol synthesis from citronellal.
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catalyst than the impregnation method (WO3/TUD-1).25 Pt was dissolved in a mixture of TEA, dry ethanol (8.0 g) and dry i-
chosen for its high hydrogenation activity. propanol (8.0 g) in a 250 mL polyethylene bottle. Under
The menthol synthesis from citronellal was chosen as model vigorous stirring 20.0 g TEOS (Aldrich, 98%) was slowly added
reaction to establish the catalytic activity and selectivity of the with a dropping funnel. Aer stirring for 2–3 h a solution of
synthesized catalysts. The advantage of this model reaction is TEAOH (20.1 g, Aldrich, 35 wt% aqueous solution) with addi-
that both the acid and hydrogenation catalysis can be tested tional demineralized H2O was added dropwise and the vigorous
individually: the conversion of citronellal to isopulegol is acid stirring was continued for another 1–2 h. The amounts of
catalysed and hydrogenation of isopulegol yields menthol.2–12
W(OEt)6, TEA and demineralized H2O were chosen so that the
nal molar gel composition was Si/W/TEA/H2O/TEAOH ¼
1 : n : 1 + 2n : 11 : 0.5. The resulting liquid was poured into
a porcelain dish and aged at room temperature for at least 24 h.
The resulting thickened gel was dried in an oven at 98 ꢁC for at
least 12 h. The dried sample was ground and hydrothermally
Experimental
Materials
Tetraethoxysilane (TEOS, Aldrich, 98%), triethanolamine (TEA,
Acros, 97%), tetraethylammonium hydroxide (TEAOH, Aldrich,
35 wt% aqueous solution), tungstic acid (Aldrich, >99%),
ammonium hydroxide (J. T. Baker, 25 wt% aqueous solution),
tungsten(VI) ethoxide (Alfa Aesar), dry ethanol (Merck), dry i-
propanol (Merck), chloroplatinic acid (H2PtCl6$6H2O, Aldrich,
$37.50% Pt basis), isopulegol (Acros, technical), (rac)-citro-
nellal (Acros, $95%), dry toluene (sure seal, Aldrich), trime-
thylbenzene (Acros, 99%).
ꢁ
treated at 180 C for 5 h in a stainless steel Teon-lined auto-
clave. Finally, calcination was performed at 600 ꢁC for 10 h with
a heating rate of 1 ꢁC minꢀ1 in a ow of dry air. These materials
are denoted by W-TUD-1x, where x (28, 16, 11, 9 or 7) represents
the WO3-loading in wt%, which is equivalent to the following Si/
W ratios, respectively: 10, 20, 30, 40 or 50.
Pt/W-TUD-1. W-TUD-1 samples (Si/W ¼ 50, 40, 30, 20 and 10)
were synthesized as described above. The incorporation of 1
wt% platinum was performed by incipient wetness impregna-
tion (pore volume determined by nitrogen physisorption) using
solutions containing appropriate amounts of aqueous chlor-
Catalyst preparation
WO3/TUD-1. TUD-1 was synthesized according to Heikkila oplatinic hexahydrate (H2PtCl6$6H2O, Aldrich, $37.50 wt% ꢁPt
et al.,26 using 20.0 g tetraethoxysilane (TEOS, Aldrich, 98%), basis). The impregnated material was dried overnight at 95 C
14.8 g triethanolamine (TEA, Acros, 97%), 5.1 g demineralized and calcined at 600 ꢁC for 2 h with a heating rate of 1 ꢁC minꢀ1
H2O and 20.1 g tetraethylammonium hydroxide (TEAOH, in a ow of dry air. These materials were then stored in a drying
Aldrich, 35 wt% aqueous solution). The nal molar gel oven at 80 ꢁC to prevent water adsorption from air, thereby
composition was SiO2/TEA/H2O/TEAOH ¼ 1 : 1 : 11 : 0.5. Three avoiding potential sintering upon recalcination. Catalysts are
samples of WO3/TUD-1 (5, 10 and 20 wt%) were prepared by not specically pre-reduced before reaction, but PtOx is readily
incipient wetness impregnation of TUD-1 (pore volume ¼ 0.897 reduced at reaction conditions (20 bar hydrogen pressure and
cm3 gꢀ1) using solutions of appropriate amounts of tungstic 80 ꢁC). These materials are denoted by Pt/W-TUD-1x, where x
acid (Aldrich) in aqueous ammonium hydroxide (J. T. Baker, 25 (28, 16, 11, 9 or 7) represents the WO3-loading in wt%, which is
wt% aqueous solution). The material was dried overnight at equivalent to the following Si/W ratios, respectively: 10, 20, 30,
95 ꢁC and calcined at 600 ꢁC for 10 h with a temperature ramp of 40 or 50.
1 ꢁC minꢀ1 in a ow of dry air. These materials are denoted by
WO3/TUD-1x, where x (5, 10 or 20) represents the wt% of WO3 on
the TUD-1 support.
Catalyst characterization
Pt/WO3/TUD-1. WO3/TUD-1x samples (5, 10 and 20 wt%)
ICP-OES. Elemental analysis for Pt was performed using
were synthesized as described above. The incorporation of 1 Inductively Coupled Plasma-Optical Emission Spectrometry
wt% platinum was performed by incipient wetness impregna- ICP-OES (Optima 4300DV, Perkin Elmer USA). Samples were
tion (pore volume determined by nitrogen physisorption) using prepared by adding 5 to 10 mg catalyst to 1 mL concentrated
solutions containing appropriate amounts of aqueous chlor- hydrochloric acid. This dispersion was le overnight and was
oplatinic hexahydrate (H2PtCl6$6H2O, Aldrich, $37.50 wt% Pt then diluted using 50 mL 1.0% hydrouoric and 1.5% sulfuric
basis). The material was dried overnight at 95 ꢁC and calcined at acid. Agitation for 24 h led to a homogenous solution.
600 ꢁC for 2 h with a heating rate of 1 ꢁC minꢀ1 in a ow of dry
INAA. Elemental analysis for Si and W was established by
air. These materials were then stored in a drying oven at 80 ꢁC to Instrumental Neutron Activation Analysis (INAA) and was per-
prevent water adsorption from air, thereby avoiding potential formed at the Reactor Institute Del (RID). The sample was
sintering upon recalcination. Catalysts are not specically pre- irradiated with neutrons (neutron ux of 1.6 ꢂ 1017 neutrons
reduced before reaction, but PtOx is readily reduced at reac- sꢀ1 cmꢀ2) in the Hoger Onderwijs Reactor, Del. In this process,
tion conditions (20 bar hydrogen pressure and 80 ꢁC). These stable isotopes were converted into radioactive isotopes. These
materials are denoted by Pt/WO3/TUD-1x where x (5, 10 or 20) isotopes emit gamma radiation, which was measured with
represents the wt% of WO3 on the TUD-1 support.
semi-conductor gamma-ray spectrometers equipped with
W-TUD-1. A series of W-TUD-1 (Si/W ¼ 50, 40, 30, 20 and 10) a germanium semiconductor. The wavelength is specic for
was synthesized using tungsten(VI) ethoxide (W(OEt)6, Alfa each element. The amount of this element was determined
Aesar) as tungsten precursor. Initially, tungsten(VI) ethoxide was from the signal area of the sample and a calibration standard.
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N2 physisorption. Specic surface areas and pore charac-
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Table 1 Chemical composition and pore structure
teristics of the materials were determined using the BET and
BJH models from nitrogen sorption measurements on a Quan-
tachrome Autosorb-6B at ꢀ196 ꢁC.27 Prior to the measuremenꢁts,
WO3,
wt%
Pt (ICP),
wt%
SBET
,
Vpore
,
Dpore
A
,
m2 gꢀ1
cm3 gꢀ1
˚
Catalyst
the samples were degassed overnight under vacuum at 350 C TUD-1
—
—
—
—
—
—
—
—
—
655
210
225
245
420
635
710
635
720
220
200
255
415
605
655
590
670
0.58
0.33
0.36
0.42
0.48
0.83
0.81
0.90
0.91
0.33
0.33
0.44
0.49
0.72
0.72
0.84
0.74
35
60
65
70
45
50
45
55
50
60
65
70
50
45
45
55
45
WO3/TUD-120
18.2a
8.8a
using a Quantachrome Autosorb degasser.
WO3/TUD-110
WO3/TUD-15
W-TUD-128
W-TUD-116
W-TUD-111
W-TUD-19
W-TUD-17
Pt/WO3/TUD-120
Pt/WO3/TUD-110
Pt/WO3/TUD-15
X-ray diffraction. X-ray diffraction patterns of the W-TUD-1
samples were recorded on a Bruker-AXS D8 Advance diffractom-
eter with Cu-Ka radiation, which was operated at 25 mA andꢀ415
5.3c
31.0a
17.0a
12.3a
9.6a
kV. The measuring step size was 0.0387ꢁ with a step time of 1 s
.
The diffraction spectrum was taken over a range from 5ꢁ to 90ꢁ 2q.
X-ray diffraction patterns of the WO3/TUD-1, Pt/W-TUD-1
and Pt/WO3/TUD-1 samples were measured using a Bruker D8
Advance diffractometer with a Lynxeye detector and Cu Ka
8.1a
—
14.3b
10.6b
5.3b
1.01
1.01
0.84
1.06
0.89
0.72
0.74
0.61
radiation. Measuring range from 5ꢁ to 95ꢁ 2q with a step size of Pt/W-TUD-128
17.0b
13.8b
10.6b
9.4b
0.02ꢁ and a scan speed of 0.15 sꢀ1
.
Pt/W-TUD-116
Pt/W-TUD-111
Pt/W-TUD-19
Pt/W-TUD-17
Electron microscopy. High-resolution transmission electron
microscopy (HR-TEM) was performed on a Philips CM30UT
electron microscope with a LaB6 lament as the source of
electrons operated at 300 kV. Samples were prepared by placing
a few droplets of a suspension of ground sample in ethanol on
the grid, followed by drying at ambient conditions.
7.5b
INAA. b ICP. c ICP on Pt/WO3/TUD-15.
a
NH3-temperature programmed desorption.
A calcined
sample was pretreated in a continuous ꢀs1tream of He (10 mL
minꢀ1) at 250 C (ramping at 10 C min from room temper-
ature). Aer returning to 100 ꢁC, the sample was ushed for
30 min with ammonia (9.982 mol% ammonia in He, 10 mL
minꢀ1). Subsequently the physically adsorbed ammonia was
removed by ushing with He (10 mL minꢀ1) for 30 min at the
same temperature. Subsequently the temperature was ramped
ꢁ
ꢁ
ꢀ1
ꢁ
ꢁ
ꢁ
(1 C min ) from 100 C to 600 C and ammonia desorption
was recorded on a Micromeritics Autochem 2910 equipped with
a Thermal Conductivity Detector.
Catalytic performance testing
Isopulegol hydrogenation. Isopulegol hydrogenation was
performed in a PolyBLOCK 8 (HEL group), a parallel autoclave
reactor system consisting of eight 16 mL vessels. Technical
isopulegol (308.5 mg, 2.0 mmol), dry toluene (4.0 mL) and
catalyst (powder, 50 mg) were added to the reactor. The auto-
clave was purged three times with nitrogen (20 bar) and three
times with hydrogen (20 bar) and then pressurized with
hydrogen (20 bar). Theꢁreactor was magnetically stirred (800
rpm) and heated to 80 C within 10 minutes and kept at this
temperature for 16 h. Stirring was stopped and the reactors were
allowed to cool down to room temperature. 1,3,5-Trime-
thylbenzene (100 mL, 0.716 mmol) was added to the reaction
mixture as internal standard. A GC sample was prepared by
diluting 40 mL reaction mixture with 960 mL dry toluene.
The apparent turnover frequency (TOFPt) for hydrogenation
is dened as mmol isopulegol converted per mol Pt per hour,
based on a rate constant k that is derived from a rst order rate
approximation (Appendix A).
Fig. 1 (Top) isotherms of Pt/W-TUD-1 samples (bold) and W-TUD-1
Isopulegol hydrogenation – kinetic prole. A kinetic prole samples (dashed); (bottom) isotherms of Pt/WO3/TUD-1 samples
(bold) and WO3/TUD-1 samples (dashed).
was obtained by using the isopulegol hydrogenation procedure
that is described above by operating 8 parallel reactions in the
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PolyBLOCK 8. Pt/W-TUD-111 (50 mg) was used as a catalyst and
the reactions were individually stopped at the indicated reac-
tion times.
Recycling experiment. Aer the menthol synthesis the reac-
tion mixture was ltered off and the catalyst was rinsed using
dry toluene. Then (rac)-citronellal (308.5, 2.0 mmol) and dry
toluene (4.0 mL) were added and the reaction procedure for
menthol synthesis was repeated.
Menthol synthesis. Menthol synthesis was performed in two
stages in a PolyBLOCK 8. In the rst stage (rac)-citronellal
(308.5 mg, 2.0 mmol), dry toluene (4.0 mL) and catalyst (powder,
50 mg) were added to the reactor. The autoclave was purged
three times with nitrogen (20 bar) and then pressurized with
nitrogen (20 bar). The ꢁreactor was magnetically stirred (800
rpm) and heated to 80 C within 10 minutes and kept at this
temperature for 5 h. Stirring was stopped and the reactors were
allowed to cool down to room temperature. A GC sample was
prepared by diluting 40 mL reaction mixture with 960 mL dry
toluene and 1,3,5-trimethylbenzene (1.0 mL, 7.16 mmol) was
added to the GC sample as an internal standard.
The apparent turnover frequency (TOFw) for the Prins cycli-
sation is dened as mmol citronellal converted per mol W per
hour, based on a rate constant k that is derived from a rst order
rate approximation (Appendix A).
For stage 2 the reactor was closed again and the same proce-
dure as described for isopulegol hydrogenation was followed.
GC analysis. The GC samples were analyzed on a Shimadzu
ꢁ
GC-17A gas chromatograph (16 miꢀn1at 140 C isothermal, fol-
lowed by a ramp of 50 ꢁC min
isothermal) equipped with an injector at 250 C, a Cyclodex-B
to 250 ꢁC, and 1 min
ꢁ
column (60 m ꢂ 0.25 mm ꢂ 0.25 mm) and using a FID
ꢁ
detector at 270 C. The retention times observed are: 8.0 min
1,3,5-trimethylbenzene, 13.4 and 13.5 min (rac)-citronellal, 14.8,
15.0, 15.1, 15.2, 16.4 and 16.6 min (rac)-isopulegol, 16.3, 16.3,
16.8 and 16.8 min (rac)-menthol, 12.0 and 16.6 min 3,7-
dimethyloctan-1-ol, 17.7 min 3,7-dimethyl-6-octen-1-ol.9,11,22
The Prins cyclisation of citronellal in this study is not stereo-
selective due to the relatively large pores of TUD-1 and forms
a thermodynamic distribution of isomers.13,22 Moreover, this
study utilizes (rac)-citronellal and was therefore not focused on
stereoisomeric distribution of reaction products.
Fig. 2 Pore size distribution of Pt/W-TUD-1 samples (top). (Dashed)
adsorption (solid) desorption. Red lines are samples without Pt. Pore
size distribution of Pt/WO3/TUD-1 samples (bottom). (Dash) adsorp-
tion (solid) desorption. Red lines are samples without Pt. (distribution of Fig. 3 XRD patterns of Pt/W-TUD-1 samples (top); XRD patterns of Pt/
different samples shifted by +0.02 units).
WO3/TUD-1 samples (bottom).
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around 45 and 55ꢁ, indicating that the Pt particles on the
material are well dispersed and close to the detection limit of
XRD (Fig. 3).
Results and discussion
Catalysts characterization
The TEM images before and aer Pt addition show that the
supports are not affected by the impregnation procedure
(Fig. 4). Platinum has been well dispersed on the W-TUD-17
support and is present in <25 nm particles. The Pt/WO3/TUD-110
material shows a typical Pt particle size of 10 nm diameter.
Fig. 5 shows more detailed TEM images and EDX analyses of Pt
loaded WO3/TUD-1 samples, and shows the differences between
Pt and WO3 particles. Pt particles are identied by an EDX
signal at 2.0 keV and can be recognized by their dark colour and
sharp edges.30 The WO3 particles are identied by EDX through
the W signal at 1.8 keV and are generally more vague than the
sharp outlined Pt particles.
The particles on Pt/WO3/TUD-15 that were assigned with an
‘a’ in the top row of Fig. 5 were identied as WO3 particles by
EDX. The Pt particles, which tend to be smaller than the WO3
particles, were tagged with a ‘b’.
The Pt particle in the middle image in the middle row of
Fig. 5 can be clearly distinguished from the WO3 particle in the
image on the right hand side.
The chemical composition of the samples is determined by
a combination of INAA and ICP-OES. It shows a good correlation
between the amounts of tungsten and platinum (0.9 ꢃ 0.2 wt%)
used for the synthesis of the catalysts and the amounts that
were present in the materials (Table 1).
The pore structures of the materials are summarized in
Table 1 and the nitrogen isotherms and pore size distributions
are presented in Fig. 1. All samples show a type IV isotherm,
typical for mesoporous materials where capillary condensation
in mesopores occurs in a relative pressure range of 0.4–0.8. All
materials show a H2 type hysteresis loop (Fig. 1).28
The W-TUD-1 samples have a larger pore volume and larger
BET area than the WO3/TUD-1 samples. Impregnation of the W-
TUD-1 samples with platinum precursor resulted in slightly
lower pore volumes and BET areas, while impregnation of the
WO3/TUD-1 samples did not affect the physical properties of
this material (Table 1; Fig. 2). It is noted that in some cases the
adsorption–desorption hysteresis closes at 0.42 relative pres-
sure, which indicates the inuence of the tensile strength effect,
indicating that the distinct pore size visible for e.g. TUD-1 at
ꢄ3–4 nm is an artefact of this phenomenon.29
The TEM images of Pt/WO3/TUD-120 show relatively more
agglomerations of WO3, as a result of the higher tungsten
loading. The irregularly shaped forms (‘a’) were identied as
WO3, while the sharp dark dot proved to be a Pt particle (‘b’),
which shows that the tungsten loading has no effect on the
Pt particle size or shape. All of the Pt/WO3/TUD-1 materials,
irrespective of the tungsten loading, show relatively large
The XRD results show that the tungsten is better dispersed
on the Pt/W-TUD-1 samples. WO3 reections start only appear-
ing at Pt/W-TUD-116, whereas WO3 reections are already visible
in Pt/WO3/TUD-15. Addition of Pt to both the W-TUD-1 and the
WO3/TUD-1 materials results in barely visible Pt reections
Fig. 4 TEM images of W-TUD-17 (A and B) and Pt/W-TUD-17 (C) and WO3/TUD-110 (D and E) and Pt/WO3/TUD-110 (F).
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Fig. 5 TEM images of Pt impregnated WO3/TUD-1 and corresponding EDX analysis. Pt/WO3/TUD-15 (A–C); Pt/WO3/TUD-110 (D–F); bottom: Pt/
WO3/TUD-120 (G–I).
agglomerates of WO3. The higher the tungsten loading, the
larger these agglomerates become.
Catalyst performance
Isopulegol hydrogenation. A hydrogenation rate constant k
was calculated for the conversion of isopulegol to menthol
(Table 2), assuming a rst order dependence on isopulegol
concentration (vide infra). These rate constants are used to
calculate initial rates r0, which serve to compare the hydroge-
nation activity of the individual catalysts (Fig. 7). Both the Car-
berry number (extraparticle mass transfer) and the Weisz
modulus (intraparticle mass transfer) proved to be sufficiently
low to assure that the reactions proceeded without any mass
transfer limitations (Appendix B). The different Pt loadings are
taken into account by calculating a turnover frequency (TOFPt),
which is calculated by normalizing the initial rate for the Pt
content (Table 2). The only observed trend is that the Pt/W-TUD-
1 catalysts are more active hydrogenation catalysts than the Pt/
WO3/TUD-1 materials, with Pt/W-TUD-128 and Pt/W-TUD-19
As a direct result of the high tungsten loading, the Pt/W-
TUD-128 material shows the largest WO3 agglomerations (Fig. 6,
‘a’) of all the Pt/W-TUD-1 samples. This is also recognized from
the XRD results, which show a clear WO3 reection for this
material. Interestingly, there are not as much Pt particles (Fig. 6,
‘b’) visible on the Pt/W-TUD-128 (Fig. 6, 100 nm scale bar) as in
the image of Pt/W-TUD-116 with the same scaling. On the other
hand, the observed WO3 agglomerations in this image are much
smaller than in the Pt/W-TUD-128 material and become scarce
when the tungsten loading is lowered even further (Pt/W-TUD-
111, 9 and 7). The Pt particles on these materials do not seem to
change much in size and shape over the range of tungsten
loadings. Some EDX spectra of the TUD-1 background are
added as comparison (Fig. 6, ‘c’).
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Fig. 6 TEM images of Pt impregnated W-TUD-1 and corresponding EDX analysis. Pt/W-TUD-128 (A–C); Pt/W-TUD-116 (D), Pt/W-TUD-111 (E–G);
Pt/W-TUD-19 (H and I); Pt/W-TUD-17 (J–L).
being the most active in the series. However, there is no
It is important to note that WO3 by itself is not an active
observable trend for the hydrogenation activity and the WO3 hydrogenation catalyst and the activity of the individual cata-
content of the catalysts themselves. This could indicate that the lysts is likely to be dependent on the Pt metal surface and its
particle size of the WO3 that is present on the catalyst contrib- environment. This relates to the Pt dispersion on the catalyst,
utes to the hydrogenation activity.
but is difficult to quantify based on the XRD and TEM data. The
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Table 2 Conversion of isopulegol after 16 h hydrogenation over Pt/W-TUD-1 and Pt/WO3/TUD-1 catalysts and derived kinetic parameters.
Reaction conditions: 2.0 mmol isopulegol, 4.0 mL toluene, 50 mg catalyst, 20 bar H2, 80 ꢁC, 800 rpm
Conversion
(%)
Pt
(wt%)
k
r0
TOFPt
(mL gcat sꢀ1
)
(mmol gcat sꢀ1
)
(mmol molPt sꢀ1
)
ꢀ1
ꢀ1
ꢀ1
Catalyst
Pt/W-TUD-128
Pt/W-TUD-116
Pt/W-TUD-111
Pt/W-TUD-19
95.1
6.8
1.06
0.89
0.72
0.74
0.61
0.84
1.01
1.01
4.1
0.1
0.5
2.3
0.9
0.2
0.2
0.4
2.1
0.1
0.3
1.2
0.4
0.1
0.1
0.2
38
1.1
7.7
31
31.6
83.9
47.6
16.7
13.7
24.8
Pt/W-TUD-17
14
Pt/WO3/TUD-15
Pt/WO3/TUD-110
Pt/WO3/TUD-120
2.8
2.0
3.4
Fig. 8 Kinetic profiles of isopulegol hydrogenation using Pt/W-TUD-
111 as catalyst. Reaction conditions: 2.0 mmol isopulegol, 4.0 mL
toluene, 50 mg catalyst, 20 bar H2, 80 ꢁC. The line indicates the
menthol concentration development predicted based on the initial
rate constant k.
Fig. 7 Initial hydrogenation rate r0 of isopulegol over Pt/W-TUD-1 and
Pt/WO3/TUD-1 catalysts. Reaction conditions: 2.0 mmol isopulegol,
4.0 mL toluene, 50 mg catalyst, 20 bar H2, 80 ꢁC, 800 rpm.
isopulegol hydrogenation.3 Agglomeration of the Pt particles is
observed by TEM aer reaction, as can be seen in Fig. 9.
Obviously, the observed catalyst deactivation has an effect on
the accuracy of our assumption of rst order rate dependence.
However, this is still the best assumption that we can make.
Menthol synthesis. Hydrogenation of isopulegol only probed
the hydrogenation activity of the bifunctional Pt/W-TUD-1 and
Pt/WO3/TUD-1 catalysts. However, the acidic supports also
allow for the Prins cyclisation of citronellal to isopulegol. This
implies that menthol can be produced directly from citronellal,
thereby fully utilizing the capabilities of these bifunctional
catalysts (Table 3).
incipient wetness impregnation of the different catalysts must
have led to a varying Pt dispersion, resulting in a wide range of
hydrogenation activities.
Interestingly, no deoxygenated products are observed during
the reaction, showing that under these reaction conditions the
material is not acidic enough to remove the hydroxyl group that
is present in both isopulegol and menthol.
Kinetic prole. The kinetic prole of Pt/W-TUD-111 shows
that the conversion of isopulegol stabilizes around 40% (Fig. 8).
This is attributed to catalyst deactivation, as other catalysts have
shown higher conversions before (Fig. 7),31 so product inhibi-
tion is excluded as a cause. The deactivation starts in an early
stage of the reaction and becomes progressively worse. This is
visualized by the curve that predicts the menthol concentration
in case the initial rate constant k0 would be maintained
throughout the reaction. Since the mass balance is around 95%,
some polymerization and coking could be possible deactivation
mechanisms. Other possible deactivation mechanisms are
sintering and poisoning.31
The highest menthol yield was achieved over Pt/W-TUD-111
.
Overall, 96% citronellal could be converted into menthol by
sequential operation. The remaining 4% was identied as 3,7-
dimethyloctan-1-ol, resulting from the direct hydrogenation of
citronellal. This clearly shows that the acidity of these materials
does not lead to unwanted oxygen elimination.
The Prins cyclisation is performed under a nitrogen atmo-
sphere in the rst stage, as the hydrogenation of the carbonyl
and alkene by the platinum catalyst would reduce the menthol
selectivity signicantly (converting citronellal into dimethy-
loctenol and dimethyloctanol (Tables 3 and 5), thereby pre-
venting the formation of menthol). Overall, the Pt/W-TUD-1
Poisoning is not expected by using these clean feeds, and
product inhibition was not observed in previous studies of
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Fig. 9 TEM images and EDX analysis of spent (A, B and D) and fresh (D) Pt/W-TUD-116. WO3, Pt and background are labelled as a, b and c,
respectively.
catalysts show a higher conversion of citronellal compared to nm2) at which three-dimensional clusters are formed is
the Pt/WO3/TUD-1 catalysts. In this series, the catalysts with different for alumina and TUD-1 (a silica material). It is ex-
a low tungsten loading (Pt/W-TUD-17, 9 and 11) have a far higher pected that these clusters start to form at lower rWO because of
3
TOFw than catalysts with a higher tungsten loading (Pt/W-TUD- the weaker interaction of a silica surface compared to an
116 and 28). In fact, Pt/W-TUD-128 has a TOFw that is comparable alumina surface. A strong indication for this is the decrease in
to the TOFw of the Pt/WO3/TUD-1 catalysts. The Pt/WO3/TUD-1 surface area of the WO3/TUD-1 and Pt/WO3/TUD-1 materials
catalysts show an increasing TOFw with decreasing tungsten and W/TUD-128 and Pt/W-TUD-128 (Table 1).
loading. This is in agreement with earlier work with the W-TUD-
Aer 5 h the second stage, the hydrogenation of isopulegol to
1 and WO3/TUD-1 materials without Pt and is the result of the menthol, was started by replacing the nitrogen atmosphere by
better dispersion of tungsten (Table 4).25 The highest TOFw was the reducing hydrogen atmosphere. From this point on, the
observed for the Pt/W-TUD-111 catalyst, because this material Prins cyclisation and hydrogenation proceed simultaneously.
contains an optimal amount of small WO3 particles (Table 1), Aer 21 h, the citronellal is completely converted over the Pt/W-
without forming large, relatively non-acidic, WO3 particles. It TUD-1 catalysts (except for Pt/W-TUD-116) into menthol 3 and
exhibits the highest number of acid sites per tungsten (Table 4). dimethyloctanol 5. When Pt/W-TUD-116 was used, some iso-
The surface coverage of tungsten on an alumina support was pulegol and 4 remained. This illustrates that hydrogenation
recently reported by Garcıa-Fernandez et al.32 They evaluated the over this catalyst is not so efficient, as was also shown for the
tungsten surface density (rW, expressed in W atoms per nm2) neat hydrogenation (Table 2). Isopulegol and 4 are also
according to the Kerkhof–Moulijn model.33 It was found that observed when the Pt/WO3/TUD-1 catalyst samples were used,
this model was valid for a tungsten loading up to 9 wt%. At which is in agreement with the low TOFPt for these catalysts that
higher tungsten loadings the tungsten starts forming three- were determined for the hydrogenation (Table 2).
´
´
dimensional clusters. This is somewhat in agreement with our
The TOFPt's for the isopulegol conversion experiment (Table
observation that at higher WO3-loadings our catalysts are 2) are lower than the TOFPt's were observed during the menthol
becoming less acidic per tungsten atom (Table 4). Obviously, synthesis experiment (Table 3). It was assumed that the PtOx on
the WO3 surface density (rWO , expressed in WO3 molecules per the catalyst would readily reduce to the active metallic Pt under
3
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Table 3 Menthol synthesis in two stages. Stage 1 Prins cyclisation for 5 h, stage 2 consecutive hydrogenation for 16 ha
Stage 1
Stage 1 + stage 2
5 h Prins cyclisation
16 h hydrogenation
Yield (%) t ¼ 5 h
Yield (%) t ¼ 21 h
kb
TOFw
kd
TOFPt
c
e
Catalyst
2
1
2
3
5
4
(mL gcat sꢀ1
)
(mmol molWꢀ1 sꢀ1
)
(mL mgcat sꢀ1
)
(mmol molPt sꢀ1
)
ꢀ1
ꢀ1
ꢀ1
Pt/W-TUD-128
Pt/W-TUD-116
Pt/W-TUD-111
Pt/W-TUD-19
Pt/W-TUD-17
Pt/WO3/TUD-15 30.6
Pt/WO3/TUD-110 24.5
Pt/WO3/TUD-120 24.0
85.2
90.6
99.5
93.6
93.4
0
0
0
0
0
0
83.3 16.7
0
8.4
3.0
7.1
22.6
14.3
17.0
3.5
5.3
2.2
4.6
4.9
5.0
0.6f
0.8f
1.0f
38.6
22.9
65.2
59.3
77.2
4.8g
5.6g
7.6g
23.2 73.8 2.0 1.0 10.1
0
0
0
96.4 3.6
90.9 9.1
91.3 8.7
0
0
0
22.8
12.0
11.5
30.8 19.2 22.0 20.9 7.2 1.6
24.3 20.2 12.8 21.0 21.7 1.3
23.7 16.2 14.9 36.4 8.7 1.2
1.7
0.8
a
Reaction conditions Prins cyclisation: 2.0 mmol citronellal, 4.0 mL toluene, 50 mg catalyst, 80 ꢁC, 20 bar N2, 5 h; reaction conditions hydrogenation:
reaction mixture (including catalyst) of Prins cyclisation is used, 80 ꢁC, 20 bar H2, 16 h. b Prins cyclisation rate constant k is calculated from citronellal
conversion aer 5 h by assuming a rst order in citronellal concentration. c Prins cyclisation TOFw is calculated from the initial rate r0, and accounts
for the amount of W in the catalyst, dened as mol converted citronellal per mol W per s. d Isopulegol hydrogenation rate constant k is calculated
assuming a rst order in isopulegol concentration, and assuming no additional isopulegol is formed during hydrogenation (t > 5 h) (therefore rate
constants for Pt/WO3/TUD-1 catalysts cannot be calculated). e Isopulegol hydrogenation TOFPt is calculated from the initial rate r0, and accounts for
f
the amount of Pt in the catalyst, dened as mol converted isopulegol per mol Pt per s. Citronellal double bond hydrogenation rate constant k is
calculated for the Pt/WO3/TUD-1 catalysts, assuming a rst order in citronellal double bond concentration, and assuming no additional Prins
g
cyclisation occurs during hydrogenation. Citronellal double bond hydrogenation TOFPt is calculated from the initial rate r0, and accounts for
the amount of Pt in the catalyst, dened as mol converted double bond per mol Pt per s.
Table 4 NH3-TPD acidity23
The ve Pt/W-TUD-1 catalysts can be compared, as the initial
Prins cyclisation over these catalysts was almost complete, which
Total aciditya
Acidity/Wb
(molH+ molW
rWO
3
resulted in comparable starting concentrations of isopulegol.
Using a rst order rate approximation an isopulegol hydrogena-
tion rate constant can then be derived for these catalysts. This is
used to calculate initial rates r0 and TOFPt. This shows that the Pt/
W-TUD-1 catalysts have TOFPt values that are in the same order of
magnitude (2.3–7.7 ꢂ 10ꢀ2 mol per molPt per s) and also
comparable to those in Table 2, which is to be expected when
conversions are close to 100% and Pt concentrations in the
different experiments are comparable.
Catalyst
(mmol gcat
)
)
(WO3 units nmꢀ2
)
ꢀ1
ꢀ1
TUD-1
WO3 (ref. 34)
0.02
0.0
n.a.
n.a.
0.90
0.57
1.09
1.26
1.47
1.35
0.35
n.a.
n.a.
0.38
0.88
0.29
0.39
0.45
0.70
1.92
WO3/TUD-110 0.34
WO3/TUD-120 0.45
W-TUD-17
W-TUD-19
W-TUD-111
W-TUD-116
W-TUD-128
0.38
0.52
0.78
0.99
0.47
However, this approach cannot be applied to analysis of the
Pt/WO3/TUD-1 samples, as aer the initial Prins cyclisation
stage only 25–30% isopulegol was produced. In order to be able
to compare the hydrogenation activity under these conditions,
the concentration of double bonds in citronellal that is le aer
Prins cyclisation was dened. A double bond hydrogenation
(both carbon–carbon and carbon–oxygen double bonds) rate
constant k was then calculated, assuming a rst order rate
approximation and similar reactivity. From these k values,
initial rates r0 were calculated. These were normalized for Pt
content to get TOFPt values.
a
Total catalyst acidity determined over a temperature range of 100–
b
600 ꢁC. Acidity derived from WO3 can be considered as Brønsted
acid.35
20 bar of pure hydrogen at reaction temperature but it seems
that this was not the case. The 5 hours under nitrogen atmo-
sphere might have pre-reduced the PtOx on the catalyst,
resulting in a higher activity.
In this experiment, a direct comparison of the hydrogenation
efficiency of the eight acidic hydrogenation catalysts is difficult,
as the different catalysts are subjected to different substrate
concentrations, which is a direct result from the preceding
Prins cyclisation.
Recycling experiments. The catalysts were separated from the
reaction mixture and subjected to another round of menthol
synthesis to investigate their recyclability (Table 5). The initial
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Prins cyclisation clearly shows that the solid acid's activity is greatly
On the other hand, the TOFw's of Pt/W-TUD-128, 16 and 11 have
reduced aer one cycle. Obviously, a deactivation mechanism is at decreased by a factor of 5. This is in agreement with Fig. 9A,
play for all catalysts. TEM analysis shows that agglomeration of where an increase in Pt particle size can be seen in comparison
WO3 particles occurred, resulting in larger, less active particles with the fresh catalyst.
(Fig. 9). This was previously shown for the materials without Pt.25
Initially, the higher WO3 dispersion on the fresh Pt/W-TUD-1
The recycled Pt/WO3/TUD-1 catalysts all show a one order of catalysts compared to the fresh Pt/WO3/TUD-1 catalysts resulted
magnitude lower Prins cyclisation activity, independent of the in increased isopulegol formation and TOFw. However, catalysts
tungsten loading. This is likely due to relatively large WO3 that contain small WO3 particles, which are responsible for
particles of similar size that are present in these catalysts that initial high activity (Pt/W-TUD-1), are more affected by particle
agglomerate at comparable rates. The recycled Pt/WO3/TUD-1 agglomeration than the bulk WO3 phase that is present in the
catalysts are again outperformed by the Pt/W-TUD-1 catalysts in Pt/WO3/TUD-1 catalysts. As a result, the TOFw for catalysts that
this acid catalysed citronellal conversion. However, the TOFw's of contained these small WO3 particles decreased more than the
the two types of recycled catalyst are now of the same order of catalysts that contained more bulk WO3.
magnitude. Interestingly, the recycled Pt/W-TUD-1 catalysts with
The hydrogenation TOFPt is higher for the catalysts con-
higher tungsten loadings (Pt/W-TUD-128, 16 and 11) now exhibit taining nely dispersed WO3 particles, i.e. Pt/W-TUD-1, than for
higher citronellal conversion than the catalyst with the lowest the bulk WO3 containing Pt/WO3/TUD-1 catalysts. However,
tungsten loading, which in this recycle experiment only recycling of the catalysts showed that the hydrogenation activity
shows minimal Prins cyclisation activity. The TOFw of the only decreased for the spent Pt/W-TUD-1 catalysts. This indi-
Pt/W-TUD-111 and 7 catalysts had decreased by a factor 33 and 63, cates that the Pt on the Pt/W-TUD-1 catalysts differs from the Pt
respectively, while the TOFw of the Pt/W-TUD-128
only on the Pt/WO3/TUD-1 samples and that the Pt on the Pt/WO3/
and 16
decreased by a factor 10. This indicates that the small WO3 TUD-1 is relatively stable, but less active.
particles that were present on the original Pt/W-TUD-111 As a hypothesis, this could be due to the Pt being located on
and
7
materials are prone to agglomeration, resulting in decreased the nely dispersed WO3, resulting in a more active Pt phase,
catalytic activity. So, the decrease in Prins cyclisation activity is but one that is deactivated more readily upon agglomeration of
attributed to the agglomeration of the nely dispersed small WO3 the small WO3 particles during reaction.32,36
particles. The agglomeration of small WO3 particles has a relatively
larger effect on the acidity of the material, and hence the acidic
activity of the catalyst, than agglomeration of larger WO3 particles.
Conclusions
The reduced acidic activity of the bifunctional catalysts A series of bifunctional heterogeneous catalysts was prepared
impacts the overall menthol synthesis. As less isopulegol is through platinum impregnation of two different types of acidic
formed, less menthol is produced. The citronellal that is still tungsten oxide containing supports with different tungsten
present aer 5 h of Prins cyclisation can continue to be con- oxide loadings. A kinetic analysis was performed to compare the
verted into isopulegol during the hydrogenation stage. This is catalytic activities of the synthesized catalysts. These catalysts
observed for Pt/W-TUD-128,
and Pt/WO3/TUD-15. have proven to be effective in the acid catalyzed Prins cyclisation
16 and 11
However, larger amounts of 4 and 5 are now observed as of citronellal and the subsequent hydrogenation of isopulegol
citronellal can also be hydrogenated before it is converted into into menthol. The unwanted acid catalyzed dehydroxylation
isopulegol. This shows that all eight catalysts still exhibit was not observed for any of these catalysts.
hydrogenation activity.
Pt/W-TUD-1 materials are more active in the Prins cyclisation
As the hydrogenation TOFPt with regards to menthol cannot of citronellal than their Pt/WO3/TUD-1 counterparts. A good
be calculated for these recycled catalysts, it is difficult to compare dispersion of WO3 is critical for high acidity and introducing
the hydrogenation activities of the Pt/W-TUD-1 catalysts. the tungsten during the TUD-1 synthesis results in a higher
However, due to the lack of acid activity for these recycled cata- WO3 dispersion than aer impregnation of TUD-1. Unfortu-
lysts, the hydrogenation activity can still be evaluated using the nately, these small WO3 particles are more sensitive to
same approach that was used to determine the TOFPt in the fresh agglomeration during the reaction and the spent Pt/W-TUD-1
Pt/WO3/TUD-1 samples in Table 3. The concentration of double catalysts have TOFw's that are more resembling the activity of
bonds in citronellal that is still present aer Prins cyclisation was the less active bulk WO3. The highest activity was observed for
dened and a rst order rate approximation was assumed with the fresh Pt/W-TUD-111 catalyst, attributed to an optimum
respect to the hydrogenated products 4 and 5 to calculate between dispersion and WO3 loading, i.e. higher WO3 loading
a hydrogenation rate constant k for the different catalysts. Initial resulted in a lower dispersion, whereas a lower loading implies
rates r0 were calculated from these rate constants and normal- less available active tungsten.
izing them on Pt content provides TOFPt values.
The Pt/W-TUD-1 materials are also more active hydrogenation
These TOFPt values can only be compared to the TOFPt values catalysts than the Pt/WO3/TUD-1 materials. No correlation
of the fresh Pt/WO3/TUD-1 catalysts, as they are calculated in between tungsten loading and hydrogenation activity exists, but
the same way. This shows that the hydrogenation TOFPt for the there is a correlation between hydrogenation activity and the
Pt/WO3/TUD-1 is maintained aer the recycle for Pt/WO3/TUD- presence of small WO3 particles. The hydrogenation activity of
120 and Pt/WO3/TUD-110, while the TOFw for Pt/WO3/TUD-15 has the catalysts remains aer recycling for the Pt/WO3/TUD-1 cata-
slightly decreased, but still has the same order of magnitude.
lysts, while it decreased for the Pt/W-TUD-1 catalysts. This
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suggests that initially the Pt is present on the small WO3 particles 18 D. Liu, X.-Y. Quek, S. Hu, L. Li, H. M. Lim and Y. Yang, Catal.
in the Pt/W-TUD-1 catalysts, but this activity is lost during reac-
tion due to agglomeration of the small WO3 particles.
Today, 2009, 147, S51.
19 L. Tang, G. Luo, M. Zhy, L. Kang and B. Dai, J. Ind. Eng.
Chem., 2013, 19, 620.
20 S. K. Wilkinson, I. McManus, H. Daly, J. M. Thompson,
C. Hardacre, N. Sedaie Bonab, J. ten Dam,
M. J. H. Simmons, C. D'Agostino, J. McGregor,
L. F. Gladden and E. H. Stitt, J. Catal., 2015, 330, 362.
21 I. McManus, H. Daly, J. M. Thompson, E. Connor,
C. Hardacre, S. K. Wilkinson, N. Sedaie Bonab, J. ten Dam,
M. J. H. Simmons, E. H. Stitt, C. D'Agostino, J. McGregor,
L. F. Gladden and J. J. Delgado, J. Catal., 2015, 330, 344.
22 A. Ramanathan, M. C. C. Villalobos, C. Kwakernaak,
Acknowledgements
J. t. D. gratefully acknowledges nancial support from NWO
ASPECT (053.62.020).
Notes and references
1 E. J. Lenardao, G. V. Botteselle, F. de Azambuja, G. Perin and
R. G. Jacob, Tetrahedron, 2007, 63, 6671.
´
´
2 C. Barrales Cortes, V. Tamayo Galvan, S. Santiago Pedro and
´
S. Telalovic and U. Hanefeld, Chem.–Eur. J., 2008, 14, 961.
´
T. Viveros Garcıa, Catal. Today, 2011, 172, 21.
23 T. Yang, H. Ling, J.-F. Lamonier, M. Jaroniec, J. Huang,
M. J. Monteiro and J. Liu, NPG Asia Mater., 2016, 8, e240.
24 H. Ataee-Esfahani, J. Liu, M. Hu, N. Miyamoto, S. Tominaka,
K. C. W. Wu and Y. Yamauchi, Small, 2013, 9, 1047.
25 J. ten Dam, D. Badloe, A. Ramanatan, K. Djanashvili,
F. Kapteijn and U. Hanefeld, Appl. Catal., A, 2013, 468, 150.
26 T. Heikkilä, J. Salonen, J. Tuura, M. S. Hamdy, G. Mul,
N. Kumar, T. Salmi, D. Y. Murzin, L. Laitinen,
A. M. Kaukonen, J. Hirvonen and V. P. Lehto, Int. J.
Pharm., 2007, 331, 133.
3 Y. Nie, W. Niah, S. Jaenicke and G.-K. Chuah, J. Catal., 2007,
248, 1.
4 K. A. da Silvia Rocha, P. A. Robles-Dutenhefner,
E. M. B. Sousa, E. F. Kozhevnikova, I. V. Kozhevnikov and
I. V. Gusevskaya, Appl. Catal., A, 2007, 317, 171.
¨
5 J. Ploßer, M. Lucas and P. Claus, J. Catal., 2014, 320, 189.
6 A. Negoi, S. Wuttke, E. Kemnitz, D. Macovei, V. I. Parvulescu,
C. M. Teodorescu and S. M. Coman, Angew. Chem., Int. Ed.,
2010, 49, 8134.
´
7 F. G. Cirujano, F. X. Llabres I Xamena and A. Corma, Dalton
27 M. F. de Lange, T. J. H. Vlugt, J. Gascon and F. Kapteijn,
Microporous Mesoporous Mater., 2014, 200, 199.
Trans., 2012, 41, 4249.
8 G.-K. Chuah, S. H. Liu, S. Jaenicke and L. J. Harrison, J.
Catal., 2001, 200, 352.
28 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou,
´
R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure
9 Z. Yongzhong, N. Yuntong, S. Jaenicke and G.-K. Chuah, J.
Catal., 2005, 229, 404.
Appl. Chem., 1985, 57, 603.
29 J. C. Groen and J. Perez-Ramirez, Appl. Catal., A, 2005, 268,
10 M. Vandichel, F. Vermoortele, S. Cottenie, D. E. De Vos,
M. Waroquier and V. Van Speybroeck, J. Catal., 2013, 305,
118.
121.
30 NIST Standard Reference Database 20, http://srdata.nist.gov/
xps.
¨
¨
11 P. Maki-Arvela, N. Kumar, V. Nieminen, R. Sjoholm, T. Salmi
and D. Y. Murzin, J. Catal., 2004, 225, 155.
31 J. A. Moulijn, A. E. van Diepen and F. Kapteijn, Appl. Catal.,
A, 2001, 212, 3.
12 M. Fuentes, J. Magraner, C. de las Pozas and R. Roque-
Malherbe, Appl. Catal., A, 1989, 47, 367.
´
´
¨
32 S. Garcıa-Fernandez, I. Gandarias, J. Requies, M. B. Guemez,
S. Bennici, A. Auroux and P. L. Arias, J. Catal., 2015, 323, 65.
33 F. P. J. M. Kerkhof and J. A. Moulijn, J. Phys. Chem., 1979, 83,
1612.
´
13 S. Telalovic, A. Ramanathan, J. F. Ng, R. Maheswari,
C. Kwakernaak, F. Soulimani, H. C. Brouwer, G. K. Chuah,
B. M. Weckhuysen and U. Hanefeld, Chem.–Eur. J., 2011,
17, 2077.
34 D. Hua, S.-L. Chen, G. Yuan, Y. Wang, Q. Zhao, X. Wang and
B. Fu, Microporous Mesoporous Mater., 2011, 143, 320.
35 D. G. Barton, S. L. Soled and E. Iglesia, Top. Catal., 1998, 6,
87.
14 A. Ranoux, K. Djanashvili, I. W. C. E. Arends and
U. Hanefeld, RSC Adv., 2013, 3, 21524.
15 R. Nieguth, J. ten Dam, A. Petrenz, A. Ramanathan,
U. Hanefeld and M. B. Ansorge-Schumacher, RSC Adv.,
2014, 4, 45495.
16 Y. Chen, Z. Guo, T. Chen and Y. Yang, J. Catal., 2010, 275, 11.
17 M. Ferrari, B. Delmon and P. Grange, Carbon, 2002, 40, 471.
36 E. V. Ramos-Fernandez, C. J. M. Pieters, B. J. van der Linden,
J. Juan Alcaniz, P. Serra Crespo, M. W. G. M. Verhoeven,
J. W. Niemantsverdriet, J. Gascon and F. Kapteijn, J. Catal.,
2012, 289, 42.
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