Isomerization of n-hexane over Pt‐H3PW12O40/SBA-15 bifunctional catalysts: Effect of the preparation method on catalytic performance

Article abstract of DOI:10.1016/j.apcata.2016.09.013

Pt-H₃PW₁₂O₄₀ based bifunctional catalysts immobilized on SBA-15 were prepared either by mechanically mixing of acidic (HPW/SBA-15 or HPW@SBA-15) and metallic (Pt/SBA-15) monofunctional materials or by incorporation of the two active phases within the same composite using dual encapsulation (Pt@HPW@SBA-15) and encapsulation/impregnation (Pt/HPW@SBA-15) methodologies. The phase structure, chemical composition and surface physico-chemical properties were characterized in details by XRD, FT-IR, ³¹P MAS NMR, N₂ adsorption‐desorption and HRTEM, STEM and EDX microscopy techniques. All hybrid materials showed highly ordered mesostructuration of the SBA-15 matrix with homogeneous dispersion of both metallic and HPW phases. In the case of the monophase bifunctional catalysts, Pt/HPW@SBA-15 and Pt@HPW@SBA-15, both functions were found to coexist in close vicinity, with the HPW crystallites mainly located in the silica walls whereas Pt was found either in the channels or as part of the walls depending on the preparation route. Their catalytic performance was then evaluated in the isomerization of n-hexane. The catalysts prepared by mechanical mixing exhibited higher activity than those where the Pt and HPW were integrated within a single solid. These results were interpreted by assuming that the carbocations in the latter case were hydrogenated faster on the Pt particles thus decreasing their preponderance at the steady-state compared to the mechanical mixture.

Full text of DOI:10.1016/j.apcata.2016.09.013

Accepted Manuscript
Title: Isomerization of n-hexane over Pt-H PW O /SBA-15
3
12 40
bifunctional catalysts: Effect of the preparation method on
catalytic performance
Author: Teresa Pinto Philippe Arquilli e` re V e´ ronique Dufaud
Fr e´ d e´ ric Lefebvre
PII:
S0926-860X(16)30477-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.09.013
APCATA 16009
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
8-7-2016
14-9-2016
21-9-2016
Pleasecitethisarticleas:TeresaPinto, PhilippeArquilli e` re, V e´ roniqueDufaud, Fr e´ d e´ ric
Lefebvre, Isomerization of n-hexane over Pt-H3PW12O40/SBA-15 bifunctional
catalysts: Effect of the preparation method on catalytic performance, Applied Catalysis
A, General http://dx.doi.org/10.1016/j.apcata.2016.09.013
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Isomerization of n-hexane over Pt-H PW12O40/SBA-15 bifunctional catalysts:
3
Effect of the preparation method on catalytic performance
Teresa Pinto, Philippe Arquillière, Véronique Dufaud *, Frédéric Lefebvre *
Laboratory of Chemistry, Catalysis, Polymers and Processes (C2P2), University of Lyon 1, CNRS,
CPE, UMR 5265, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne, France.
*
Corresponding authors. Tel.: +33 472431792 (V. Dufaud); Tel.: +33 472431807 (F. Lefebvre);
Fax: +33 472431795.
E-mail addresses: Veronique.Dufaud@univ-lyon1.fr; Frederic.Lefebvre@univ-lyon1.fr
1

Graphical abstract
Highlights



Development of Pt-H PW12O40/SBA-15 bifunctional catalysts using dual encapsulation or a
3
combination of encapsulation/impregnation methods
Their performance in n-hexane isomerization was compared to that of bifunctional catalysts
obtained by mechanically mixing of acidic and metallic monofunctional materials
The bifunctional catalysts where the Pt and HPW were immobilized within the same solid
exhibit lower activity but higher selectivity than when the metal and HPW phases were
physically separated

All these results were interpreted by assuming a monomolecular mechanism for the
isomerization and in terms of relative rates of initiation, propagation and termination.
Abstract
Pt-H PW12O40 based bifunctional catalysts immobilized on SBA-15 were prepared either by
3
mechanically mixing of acidic (HPW/SBA-15 or HPW@SBA-15) and metallic (Pt/SBA-15)
monofunctional materials or by incorporation of the two active phases within the same composite
2

using dual encapsulation (Pt@HPW@SBA-15) and encapsulation/impregnation (Pt/HPW@SBA-
5) methodologies. The phase structure, chemical composition and surface physico-chemical
1
3
1
properties were characterized in details by XRD, FT-IR, P MAS NMR, N adsorption-desorption
2
and HRTEM, STEM and EDX microscopy techniques. All hybrid materials showed highly ordered
mesostructuration of the SBA-15 matrix with homogeneous dispersion of both metallic and HPW
phases. In the case of the monophase bifunctional catalysts, Pt/HPW@SBA-15 and
Pt@HPW@SBA-15, both functions were found to coexist in close vicinity, with the HPW
crystallites mainly located in the silica walls whereas Pt was found either in the channels or as part
of the walls depending on the preparation route. Their catalytic performance was then evaluated in
the isomerization of n-hexane. The catalysts prepared by mechanical mixing exhibited higher
activity than those where the Pt and HPW were integrated within a single solid. These results were
interpreted by assuming that the carbocations in the latter case were hydrogenated faster on the Pt
particles thus decreasing their preponderance at the steady-state compared to the mechanical
mixture.
Keywords: isomerization, n-hexane, heteropolyacids, SBA-15, encapsulation, bifunctional catalyst,
platinum
1
. Introduction
Current specifications for the production of cleaner fuels impose strong restrictions on the
content of sulfur, aromatic compounds and olefins as well total interdiction of lead in the gasoline
pool [1]. These restricted conditions led to the loss of the performance of internal combustion
engine as these additives served as octane number enhancers [1, 2]. In this regard, the skeletal
isomerization of linear alkanes represents a promising alternative since, with this process, low-
octane straight chain paraffins are directly converted into high-octane branched molecules.
Isomerization of linear alkanes can, thus, be considered as an efficient and economically acceptable
way of increasing the octane number of motor gasoline and, unlikely aromatics and/or other toxic
additives, the gasoline isomerizate is an ideal product that can satisfy the most stringent
environmental legislation requirements.
Skeletal isomerization generally proceeds over acidic catalysts and is equilibrium-limited with
branched isomers favored at low temperatures [3]. Bifunctional catalysts with both metallic and
acid phases are currently being considered as key catalytic systems for this reaction. The metal
3

promotion, in presence of hydrogen, allows for the stabilization of the catalytic activity as well as
enhancement of the selectivity to isomerization [4]. Indeed, this metal-hydrogen system proceeds
through the monomolecular mechanism pathway where the final products are alkane isomers,
without cracking [5]. Nowadays, the industrial catalysts consist of platinum supported on acidic
zeolites [6, 7]. The zeolites have a high resistance to water and sulfur, making them stable catalysts
with a long effective lifetime [6]. However, zeolites are less acidic than chlorinated alumina
(commonly used in industry until very recently). As a consequence, the isomerization reaction
requires high temperature (220-400 °C) and a high hydrogen pressure (15-30 bar) to proceed which
is less favorable for the production of branched isomers [8-11]. Other acidic solids such as sulfated
zirconia [12, 13], tungstated zirconia [14] and heteropolyacids (HPAs) [15-17] have also been
applied to the skeletal isomerization of light alkanes. In particular, heteropolyacids, when used in
conjunction with a noble metal, mainly platinum, were found to be highly efficient catalysts for the
isomerization of n-alkanes in the C -C10 range [18-22]. Their strong acidic strength allowed for the
4
isomerization of alkanes at low temperature via the monomolecular reaction pathway, thus
providing high conversion and selectivity to the isomeric products [22-25]. For instance, A. Miyaji
et al. used Pt-Cs2.5
H0.5PW12
O40 as catalyst for the isomerization of n-butane with high selectivity (>
9
0%) [5]. Y. Xu et al. reported that the system Pt-H
3
PW12
O40/ZrO
2
provides a high and stable
conversion (60 %) of n-pentane after 60 h of reaction at 200 °C [26]. A.V. Ivanov et al. also
investigated the catalytic activity of Pt-HPW Keggin and Dawson tungstic acids supported on
zirconia. The results showed a high activity for n-hexane isomerization with a yield of 80% and a
selectivity of 96-98% at 190 °C [24]. However, partial degradation of the HPW structures was
observed upon immobilization on the ZrO
In a previous work, we have reported on the acid-catalyzed isomerization of n-hexane by
SBA-15 supported H 40 (HPW) and H 40 (HSiW) Keggin heteropolyacids [27]. The
2
support resulting in lower concentration of active sites.
3
PW12
O
4
SiW12O
catalysts were found to be active at 200 °C exhibiting high selectivity (90%) towards isomerization,
but a rapid deactivation was observed due to coke formation [27]. In a follow-up report, we studied
the effect of the preparation method of these monofunctional acidic catalysts on the catalytic
performance in n-hexane isomerization [28]. For this purpose, two strategies were considered for
the synthesis of HPA modified hybrid materials: the classical impregnation technique where the
HPA entities are held at the silica pores and the direct synthesis yielding embedded HPA clusters
directly within the silica framework. The impregnated and encapsulated systems based on
H
3
PW12
O40 exhibited nearly the same activity and selectivity while those based on H
4
SiW12
O
40
showed markedly different behavior. The HSiW catalyst prepared by classical impregnation
displayed an activity even higher than both HPW systems while that prepared by direct synthesis
4

was almost inactive. These results were correlated to acidity measurements by pyridine adsorption
and it was proposed that in the encapsulated materials the heteropolyacid was located at the surface
of the walls for HPW while, for HSiW derivatives, the active sites were more deeply embedded in
the walls. Experiments in the presence of platinum and hydrogen as the carrier gas were also
carried out with the impregnated HSiW based catalyst resulting in high and stable activity
overtime with selectivity to isomerization as high as 95 % at 200 °C [29]. No catalyst deactivation
was observed even after 3 days on stream in contrast to monofunctional catalysts emphasizing the
crucial role of both Pt and hydrogen to achieve high reactivity and prevent any catalyst deactivation.
As a continuation of this work, we wish to further explore the effect of the preparation
method of the bifunctional catalysts on the catalytic reactivity in n-hexane isomerization. Indeed,
the metal and HPA phases can be physically separated (mechanical mixture of the two
monofunctional catalysts) or present in close proximity within the same solid phase. In the latter
case, the two components were introduced by direct encapsulation during the oxide synthesis or by
a combination of both encapsulation and impregnation. One expects the catalytic reactivity to be
substantially dependent on these synthetic parameters as the distance between the two functions
may vary considerably. As the silicotungstic acid encapsulated material was not active in the
isomerization of n-hexane, these studies were pursued by considering exclusively phosphotungstic
acid based catalysts.
2
. Experimental
2
.1. Synthesis of the catalysts
40 (HPW) and H
H
3
PW12
O
2
PtCl
6
.6H
2
,
O were purchased from Aldrich Chemical and used as
received. Monofunctional HPW/SBA-15 (impregnated) and HPW@SBA-15 (encapsulated)
catalysts were produced respectively by classical impregnation and sol-gel encapsulation methods
following procedures described previously [28]. The HPW amount in these hybrid materials was
determined from ICP-AES to be 33 wt.%. Monofunctional Pt/SBA-15 was obtained by
impregnation of H
2
PtCl
6
.6H O over SBA-15 through a solid-liquid route as reported elsewhere [29,
2
3
0] with a platinum content of 0.8 wt.%. Platinum was also incorporated in the monofunctional
HPW@SBA-15 catalyst by the same procedure, at quite the same loading (0.7 wt. % Pt), yielding
the bifunctional catalyst Pt/HPW@SBA 15. Another type of bifunctional material was also
prepared through the dual encapsulation of both platinum and HPW by adapting the procedure
previously developed for HPW@SBA-15 [28]. Typically, the silica precursor (TEOS) was
introduced to an acidic solution (HCl 1.9 M) of a mixture of non-ionic (Pluronic 123) and ionic
(CTAB) structure directing agents. After a short period of prehydrolysis (45 min), the
5

phosphotungstic acid was first added to the mother solution followed, after a 15 min period, by
PtCl .6H O. The resulting as-made solid was calcined under humid air at 400 °C for 20 h and
then pre-treated at 300 °C for 3 h under nitrogen. Finally, the material was reduced under H at
H
2
6
2
2
3
00 °C for 3 h leading to the dual encapsulated material referred to as Pt@HPW@SBA-15.
2
.2. Characterization of the catalysts
KBr pellets were prepared and absorbance FTIR spectra were run on a Nicolet 5700
3
1
spectrometer. P MAS NMR spectra were recorded on a Bruker DSX-500 spectrometer. A Bruker
D5005 diffractometer operating with the Cu Kα monochromatic radiation ( = 1.54184 Å) was
used for the obtention of X-ray diffraction patterns. were performed on a A Micromeritics ASAP
2
020 system was used for determination of the N adsorption-desorption isotherms. Elemental
2
analyses were performed by ICP-AES with an ICP spectroflamme-D. HRTEM (High Resolution
Transmission Electron Microscopy) studies were made with a JEOL JEM 2100F microscope. The
size distribution histograms were obtained from a manual measurement with J Image of at least 200
different nanoparticles assuming near spherical shapes and random orientations. Log normal fits
were used to approximate the experimental size distribution histograms. The microscope was also
equipped with a High Angle Annular Dark Field (HAADF) STEM detector with a resolution of
0
.19 nm allowing to determine in situ the chemical composition of the samples using energy-
dispersed X-ray (EDX). The platinum dispersion was determined by chemisorption of hydrogen on
a a Micromeritics ASAP2010 instrument. More informations on the methods used can be found in
our previous works [27-29].
2
.3. Catalytic experiments at atmospheric pressure
The catalytic experiments were made by using the same experiment than previously [29].
They were conducted during 12 hours at 200°C. The reactor was heated under a flow of argon (5
-1
mL.min ) for 2 hours at 200 °C to remove any adsorbed water. The gas flow was then switched to
-1
hydrogen bubbled through hexane at room temperature (5 mL.min , hexane/hydrogen ratio = 0.25).
The carbon balances were determined in all cases in order to prove that all compounds were
detected. In presence of platinum they were always very good. The reproducibility was also
checked by repeating some experiments.
3
. Results and discussion
3
.1. Characterization of the catalysts
6

Four catalysts were evaluated in the isomerization of n-hexane: a mechanical mixture
composed of impregnated Pt/SBA-15 with either impregnated HPW/SBA-15 or encapsulated
HPW@SBA-15, a solid where platinum was subsequently impregnated on the encapsulated
HPW@SBA-15 and denoted Pt/HPW@SBA-15 and a solid where both HPW and platinum were
incorporated during the oxide synthesis leading to Pt@HPW@SBA-15. Prior to their use in
catalysis, all solids were characterized by various physicochemical and spectroscopic methods. The
texture and ordering of the materials was determined by X-ray diffraction and nitrogen sorption
3
1
measurements. The molecular scale properties of the catalysts were analyzed by FT-IR and P
MAS NMR. Finally, the platinum particles size and dispersion were characterized by
chemisorption of hydrogen and electron microscopy (HRTEM and STEM with EDX analysis).
The low-angle powder XRD patterns of the monofunctional systems HPW/SBA-15,
HPW@SBA-15 and Pt/SBA-15 were reported previously. In the three cases, the catalysts showed
the three well-resolved peaks characteristics of the hexagonally ordered mesostructure of SBA-15
silica [27-29]. The low-angle XRD patterns of the bifunctional systems Pt/HPW@SBA-15 and
Pt@HPW@SBA-15 are quite similar to those of the monofunctional ones and exhibit also the
three reflection peaks at low scanning angles indexed as (100), (110) and (200) (Fig. 1). These
results indicate that all solids had retained the long-range SBA-15 structure.
<
Fig. 1 >
The wide-angle powder XRD patterns of monofunctional HPW/SBA-15, HPW@SBA-15
and Pt/SBA-15 did not show any peaks in the 10 - 70° 2rangeindicating that both HPW and
platinum were finely dispersed throughout the solids [27-29]. The wide-angle XRD patterns of the
bifunctional catalysts are shown in Fig. 2. As for the monofunctional materials, no diffraction peaks
were found for Pt/HPW@SBA-15 evidencing again highly dispersed metallic and acidic phases all
over the solid. On the other hand, Pt@HPW@SBA-15 displayed small broad diffraction peaks at
2
 values of 24°, 33°, 41°, 50° and 54°characteristic of monoclinic WO [31]. These peaks are
3
probably due to a partial decomposition of the polyoxometalate ions which may occur during the
thermal treatment and/or the reduction step [32]. However, no characteristic peaks associated with
either HPW or Pt crystallites were observed which is indicative of well-dispersed active sites in the
mesoporous SiO framework [33].
2
<
Fig. 2 >
7

Table 1 lists the textural data deduced from X-ray diffraction and N adsorption-desorption
2
isotherms together with chemical analyses. For all materials, the isotherms are characteristic of
mesoporous materials with uniform cylindrical channels and narrow pore size distribution (see for
example Supporting Information, Fig. S1-S2 for encapsulated based hybrids, while data for
impregnated materials have already been reported elsewhere [28]). As expected, all isotherms
present a hysteresis loop at quite the same relative pressure range than SBA-15 (P/P
between 0.6 and 0.8). The d100 and a parameters are quite the same for all catalysts if we take into
account the errors on the measurements. In contrast, the BET surface areas vary between 360 and
0
values
0
2
-1
3
-1
8
44 m .g and the porous volume between 0.5 and 1.1 cm .g . For the two supported catalysts
HPW/SBA-15 and Pt/SBA-15, the surface area variation can be directly correlated to the real
amount of silica in the samples respectively of 75 wt. % and ≈ 99 wt. %. As HPW clusters are
located in the channels, the porous volume is lower than for Pt/SBA-15. For the encapsulated
materials, all results are comparable if we take into account the fact that the Pt/HPW@SBA-15
solid was prepared from another batch of HPW@SBA-15.
<
Table 1 >
3
1
The P MAS NMR spectra of Pt/HPW@SBA-15 and Pt@HPW@SBA-15 (Fig. 3) showed
that the primary Keggin structure was mostly retained. Indeed, in the case of the Pt/HPW@SBA-15,
the 31P MAS NMR spectrum showed a sharp peak at -15 ppm which is characteristic of the
[
PW12
O
40]3- anion [34]. Another weak resonance was also observed at -14.1 ppm which can be
ascribed to HPW in slight interaction with the silica surface as suggested by Yun and Hamad [35,
6]. In addition to the resonances at -15 and -14.1 ppm, the Pt@HPW@SBA-15 catalyst showed
3
two other small peaks at 0.4 and -6.1 ppm. The resonance around 0 ppm is clearly identified with
phosphate species suggesting that partial decomposition of some HPW has occurred during the
material synthesis. This result is in good agreement with the wide-angle XRD data which showed
the formation of WO
3
confirming some degradation of the Keggin structure into its composition
elements. The weak resonance observed at -6.1 ppm could be due to some degraded HPW species,
for example one isomer of the lacunary PW
9
polyoxometalate.
<
Fig. 3 >
8

The FT-IR spectra of SBA-15, Pt@HPW@SBA-15, Pt/HPW@SBA-15 and pure HPW are
displayed in Fig. 4. Pure HPW shows the typical IR bands at 1080 (as(P-Oa)), 982 (as(W-Od)),
-1
8
90 (as(W-Ob-W)), and 800 cm (as(W-Oc-W)) associated with Keggin anions [37]. The FT-IR
spectrum for pure SBA-15 displays the classical absorption bands at 1080, 800 and 465 cm-1
assigned to Si-O-Si vibrations. These latter are clearly observed for the bifunctional catalysts.
Pt/HPW@SBA-15 also clearly exhibits the characteristic bands of the Keggin ions indicating that
the structural integrity of the HPA Keggin unit was preserved during the two-step preparation
process. In the case of the Pt@HPW@SBA-15 material however, these bands are broadened and
difficult to observe, in agreement with a partial decomposition of the Keggin unit.
<
Fig. 4 >
Classically, the dispersion of metallic catalysts is determined by hydrogen chemisorption.
We thus used this method for the platinum containing solids, Fig. S3. For the Pt/SBA-15 solid, the
Pt dispersion was found to be 25 %, which corresponds to a particle size of 4.5 nm if we assume a
spherical shape. These results are in agreement with data reported by Seok Oh, et al., who prepared
Pt/SBA-15 catalysts with 1 wt. % of Pt by impregnation (with H
2
PtCl as the precursor) [38]. They
6
measured a dispersion of 29.4 %, a value close to that obtained in this work [38, 39]. In the case of
the bifunctional systems Pt/HPW@SBA-15 and Pt@HPW@SBA-15 unreliable results were
obtained. Blank experiments were then undertaken with the supported heteropolyacid alone.
Clearly, the polyoxometalate reacts with hydrogen as the material instantaneously turned to a dark
blue color. This reaction is only partially reversible thus preventing any conclusive determination
of the Pt dispersion in presence of the polyoxometalate.
As a consequence, other methods needed to be used for the characterization of the
bifunctional catalysts. This can be accomplished by transmission electron microscopy.
The morphology and distribution of platinum nanoparticles in the HPW containing
materials were assessed by HRTEM and dark-field STEM images coupled with EDX analysis. The
bifunctional catalysts presented the typical hexagonal organized channels of SBA-15 in agreement
with the low-angle XRD data which showed highly ordering of the mesostructure. Typical HRTEM
images are shown in Fig. 5 and 6. They provide direct observation of the distribution of Pt
nanoparticles in the SBA-15 solids.
<
Fig. 5 and 6 >
9

The Pt/HPW@SBA-15 (0.7%wt Pt) solid showed the presence of well dispersed and narrow-
sized nanoparticles of 2.3 ± 0.6 nm, generally spherical and mostly inside the channels of the host
SBA-15 (Fig. 5). The Pt@HPW@SBA-15 catalyst (0.4%wt Pt) (Fig. 6) exhibited also well-
dispersed platinum nanoparticles with a mean size of 2.8 ± 1.2 nm which is close to
Pt/HPW@SBA-15 but with a broader distribution of particles size. In both cases, the particle size
was smaller than that achieved on the Pt/SBA-15 catalyst (4.5 nm). This could be ascribed to the
presence of tungsten, as already mentioned in other reports, which may play an important role in
the nucleation-growth process during the nanoparticles formation [40].
Representative HAADF-STEM images of Pt/HPW@SBA-15 and Pt@HPW@SBA-15
catalysts are displayed in Fig. 7 and 8 respectively. For both catalysts, one can see the highly
ordered pore structure of SBA-15 with the presence of very small white dots uniformly dispersed
throughout the organized domains which can be either Pt or W heavy elements given that the
intensity in HAADF images depends on the atomic number. HAADF images taken at higher
magnification provide direct evidence of the particles location. Fig. 7 (right) shows that for
Pt/HPW@SBA-15 the white particles were mainly confined in the channels of the SBA-15
whereas in the case of dual encapsulated Pt@HPW@SBA-15 catalyst they were chiefly located at
the surface or within the core of the silica framework (Fig. 8, right). To get deeper insight into the
chemical composition of these white particles, EDX analysis was performed on both samples. Fig.
S4 shows that the white spots in Pt/HPW@SBA-15 were mainly constituted of Pt with an atomic
ratio Pt/Si of ca. 45 % while the W/Si one was found at ca. 5 %. In the dark gray areas, only Si, O
and W could be detected with a W/Si atomic ratio of ca. 5 %. This confirmed that the HPW clusters,
introduced by encapsulation during the gel synthesis, are mainly located within the walls of the
materials with the Pt nanoparticles mostly in the channels or above the walls. In the case of
Pt@HPW@SBA-15 (Fig. S5), one clearly sees the presence of very small crystallites of 1 to 2.5
nm size exclusively located within the framework walls of the solid. EDX analysis of the different
zones indicated that the brighter particles were more likely attributed to Pt nanoparticles whereas
the lighter ones corresponded to W. The dual encapsulation method of both Pt and HPW has thus
produced a hybrid material where Pt and HPW active sites are found in very close vicinity as a
structural part of walls.
<
Fig. 7 and 8 >
1
0

Note that for both Pt/HPW@SBA-15 and Pt@HPW@SBA-15 catalysts, crystalline Pt
nanoparticles with a face-centered cubic structure were obtained (Fig. S6).
3
.2. Catalytic experiments
The skeletal isomerization of alkanes is an acid-catalyzed stepwise process leading to the
formation of branched isomers. In order to achieve good conversions without catalyst deactivation
the experiments are usually carried out under hydrogen in the presence of platinum particles [41-
4
3]. In previous work, we have shown that bifunctional Pt-HSiW based systems provided high
selectivity to isomerization (> 90%) with a stable n-hexane conversion (60%) even after 3 days on
stream [29].
The use of the Pt-H system in addition to the HPA acidic phase prevents the formation of
2
olefins, improving the selectivity towards isomerization. Furthermore, elimination of the olefins
will also suppress the formation of coke and so deactivation of the catalyst by this way [44-50].
Another consequence is that the presence of the Pt-H system leads to the monomolecular
2
mechanism [4] which favors the formation of only hexane isomers. The acid sites protonate the
alkane and form a secondary carbocation which rearranges into a tertiary one. Upon reaction with a
new alkane molecule, the initial carbocation is reformed. However, there is probably a great
influence of the distance between the two functions (metal and acid site) on the performance of the
catalysts. Indeed, if this distance is infinite no effect of the metal should be expected and the
mechanism could be bimolecular. Keeping this in mind, our bifunctional catalysts, where both the
distance between sites and their spatial location can strongly vary, were then evaluated in the
isomerization of n-hexane. In order to allow for a direct and meaningful comparison, all systems
contained the overall same amount of tungsten and platinum. All the experiments were carried out
with a dynamic reactor during 12 hours in order to study the deactivation of the catalyst. The global
conversions and selectivities to C isomers for each catalyst are given in Table 2.
6
<
Table 2 >
The bifunctional catalyst prepared by a mechanical mixture of Pt/SBA-15 and
HPW/SBA-15 (entry 1) showed an initial global conversion around 44 % which gradually
increased until 62 % and remained stable after 12 h of reaction. The selectivity to isomerization of
up to 95 % is in agreement with the proposed monomolecular mechanism. The selectivity of each
isomer is constant with the conversion, the mono-branched isomers being the major products with
more than 50 % for 2-methylpentane (2MP) and 30 % for 3-methylpentane (3MP). These two
1
1

products stem from a single carbon shift, whereas the minor 2,2- and 2,3-dimethylbutanes
2.2DMB and 2.3DMB) necessitate two successive rearrangements and thus are produced more
slowly.
(
The other bifunctional catalyst prepared by a mechanical mixture of Pt/SBA-15 and
HPW@SBA-15 (entry 2), showed similar performance. Initially, the conversion was rather low,
around 19 %, which can be due to slow initiation due to the HPW localization which is held more
deeply in the pore walls of the encapsulated HPW@SBA-15 material. n-Hexane conversion
steadily increased with time on stream up to 66 % after 12 h, where it remained stable. The
distribution of the various hexane isomers was quite similar to that achieved with the (Pt/SBA-15 +
HPW/SBA-15) catalyst, in agreement with previous results for this type of catalyst [27].
Both bifunctional catalysts where the metallic and acidic functions are immobilized in
closer proximity within a single composite showed lower activity compared to the mechanically
mixed catalysts but exhibited higher selectivity. The Pt/HPW@SBA-15 catalyst, entry 3, presented
an initial conversion around 19 % which increased and remained stable at up to 25 % after 12 h of
reaction. This catalyst showed also a high selectivity to C
6
isomers (97 %). The Pt@HPW@SBA-
1
5, entry 4, had an initial conversion of 20 % which increased to 34 % in the initial four hours and
then remained stable over 12 h on stream with a selectivity of 98.6 %. As in this material some
HPW had been destroyed, part of the lower conversion compared to the mechanical mixtures can
be ascribed to the lower amount of acid sites. However this explanation cannot explain the factor 2
between these conversions as only a small part (and not 50 %) of HPW has been destroyed, as
shown by all analyses.
When looking at all these results, two conclusions can be pointed out. Firstly, in all cases
the activity increases during the first several hours on stream. This fact could be explained by a
modification of the catalyst which becomes more active with time but it seems more reasonable to
search an explanation in the mechanism of isomerization itself. In a first step (initiation), a n-
hexane molecule is activated by a proton of the heteropolyacid leading to a carbocation which will
then rearrange. Upon reaction with another hexane molecule, the isomerized product is formed and
the carbocation is regenerated (propagation). The initial increase in activity should then be related
to the increase of the number of carbocations in the catalyst. When the termination rate is equal to
the initiation one the activity becomes constant. The formation of hexene by dehydrogenation of
hexane on the metallic phase can be excluded in our conditions as a blank experiment performed
with Pt/SBA-15 did not show any trace of hexenes. The variation of the conversion of the four
catalysts as a function of time is shown in Fig. 9. If one compares the two mechanical mixtures,
clearly the rate of carbocations formation is higher on HPW/SBA-15 than on HPW@SBA-15
1
2

based system. This phenomenon is probably related to the accessibility of the HPA. Indeed, in the
impregnated material all HPA are located at the pore surface and can readily react with n-hexane
while in the case of the encapsulated material they are more likely located inside the walls, near the
surface and so are less accessible to the reactants.
<
Fig. 9 >
Secondly, when looking at the steady-state activities, the catalysts can be separated into two
groups: the two mechanical mixtures which exhibit similar catalytic activity and the two materials
where platinum and HPW sit nearby within the same support for which differences in activity can
be noticed probably arising from the lower tungsten amount in the Pt/HPW@SBA-15 material (see
Table 1). By assuming the same explanation as aformentioned, the steady-state activity is related to
the propagation rate (reaction of the carbocation with n-hexane leading to the isomer and
reformation of a carbocation). It is reasonable to assume that the rate of reaction of a carbocation is
the same in all materials as all experiments were performed under the same reaction conditions and
all materials are based on the same SBA-15 scaffold. As a consequence, the steady-state activity is
directly related to the number of carbocations present at the steady-state. We can then conclude that,
at the steady-state, the materials where Pt and HPW are included within the same support have a
lower amount of carbocations than those where they are located in separate matrices. The
explanation could also be found in the mechanism. The number of carbocations is obtained when
the initiation rate is equal to the termination rate. The termination rate can be the elimination of a
proton of the carbocation leading to an alkene molecule or the hydrogenation of the carbocation
leading to the alkane. This second possibility is the most reasonable as the reaction is run in
presence of hydrogen. As platinum is a well-known hydrogenation catalyst, a marked increase of
the termination rate, and so a decrease of the amount of carbocations at equilibrium, is expected
when both Pt and HPW are immobilized in close proximity rather than apart from each other in
separate solids.
4
. Conclusion
Different strategies were developed towards the synthesis of Pt-HPW bifunctional catalysts
supported on SBA-15, either by mechanically mixing Pt and HPW containing monofunctional
solids or by combining both functions within the same material via encapsulation and
encapsulation/impregnation methodologies. After thorough characterization to attest of the
structural ordering and the state of the metallic and acidic inclusions, all materials were tested in n-
1
3

hexane isomerization. The present results confirmed the essential role of both Pt and hydrogen to
achieve efficient and stable catalytic systems. In all cases, the activity increased initially before
reaching a steady-state. The bifunctional catalysts prepared by mechanical mixture were highly
active and selective, whatever the nature, impregnated or encapsulated, of the HPW derived
material. The bifunctional catalysts prepared with the two active phases immobilized within the
same solid exhibited a lower catalytic activity. The main conclusion of this work is then that in
contrast to what could be expected, the separation of the two phases, metallic and acidic, is required
to achieve the best catalyst.
All results were interpreted by assuming a monomolecular mechanism for the isomerization
and in terms of relative rates of initiation, propagation and termination.
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1
5

FIGURE CAPTIONS
Fig. 1. Low-angle XRD patterns of (a) Pt@HPW@SBA-15 and (b) Pt/HPW@SBA-15.
Fig. 2. Wide-angle XRD patterns of (a) Pt@HPW@SBA-15 and (b) Pt/HPW@SBA-15.
3
1
Fig. 3. P MAS NMR of Pt/HPW@SBA-15 and Pt@HPW@SBA-15.
Fig. 4. FT-IR spectra of (a) SBA-15, (b) Pt@HPW@SBA-15, (c) Pt/HPW@SBA-15 and (d) pure
HPW.
Fig. 5. HRTEM image and particle size distribution of Pt/HPW@SBA-15.
Fig. 6. HRTEM image and particle size distribution of Pt@HPW@SBA-15.
Fig. 7. Representative HAADF-STEM images of Pt/HPW@SBA-15.
Fig. 8. Representative HAADF-STEM images of Pt@HPW@SBA-15.
Fig. 9. Variation of the conversion as a function of the reaction time for the four catalysts. Reaction
-1
conditions: 1 bar, 200 °C, 5 mL.min hydrogen flow, ratio hexane/hydrogen = 0.25, 0.5
g of catalyst (0.5 g of each monofunctional solids in the case of the mechanical mixture).
1
6

(100)
(110)
(200)
(
a)
b)
(
0
1
2
3
4
5
6
7
2
θ (°)
<
Fig. 1 >
1
7

(
a)
b)
(
10
20
30
40
50
60
70
2
θ (°)
<
Fig. 2 >
1
8

Pt/HPW@SBA-15
Pt@HPW@SBA-15
<
Fig. 3 >
1
9

SiO (1080)
P-Oa (1080)
SiO (800)
SiO (465)
W-Oc-W (800)
(
a)
W-Od
W-Ob-W
(982)
(890)
(
(
b)
c)
(
d)
1200
1000
800
600
400
-1
Wavenumber (cm )
<
Fig. 4 >
2
0

<
Fig. 5 >
2
1

<
Fig. 6 >
2
2

<
Fig. 7 >
2
3

<
Fig. 8 >
2
4

1
00
Pt/SBA-15 + HPW/SBA-15
Pt/SBA-15 + HPW@SBA-15
Pt/HPW@SBA-15
8
6
4
2
0
0
0
0
0
Pt@HPW@SBA-15
0
5
10
Reaction time (h)
<
Fig. 9 >
2
5

Table 1
Textural and physical properties of Pt-HPW catalysts.
Wall
thickness
a
b
d
e
wt. % wt. %
d
(Å)
100
a
0
V
p
D
p
S
BET
c
f
Catalysts
C
BET
3
W
Pt
(Å)
(cm /g)
(Å) (m²/g)
(Å)
SBA-15
-
-
99
97
101
96
95
99
114 48
111 46
117 52
111 35
109 33
114 38
1.13
0.9
1.1
0.6
0.5
0.7
75
65
64
76
76
77
859
639
844
451
360
474
151
204
168
203
261
163
HPW/SBA-15
Pt/SBA-15
25
-
-
0.8
-
HPW@SBA-15
26
Pt/HPW@SBA-15 g 20
0.7
0.4
Pt@HPW@SBA-15 26
a
b
c
d
e
f
d
(100) spacing (the error is estimated as 3 Å).
= 2d(100)/√3, hexagonal lattice parameter calculated from XRD.
Calculated by a – pore size.
Total pore volume at P/P = 0.980.
a
0
0
0
Pore size from desorption branch applying the BJH pore analysis.
Derived from the BET equation of the nitrogen adsorption isotherms.
The parent HPW@SBA-15 used to impregnate the Pt is different from HPW@SBA-15 used in the mechanical
g
mixture.
2
6

Table 2
Conversions and selectivities of the Pt-HPW bifunctional catalysts.
Select.
Distribution of C
6
isomers
Entry
Catalysts
Conversion (%)
iC
6
(%) (%)
2
.2
2.3
DMB
3
MP
2
MP
0
h
12 h
62.4
66.5
12 h
95.7
96.4
DMB
Pt/SBA-15
HPW/SBA-15
Pt/SBA-15
+
+
1
2
44.4
5.1
10.3
11.3
30.0 50.3
29.6 49.7
18.9
18.9
5.8
HPW@SBA-15
3
4
Pt/HPW@SBA-15
Pt@HPW@SBA-15
24.7
34.4
97.5
98.6
1.4
1.9
8.1
32.4 55.6
31.2 55.2
19.8
10.3
-
1
Reaction conditions: 1 bar, 200 °C, 5 mL.min hydrogen flow, ratio hexane/hydrogen = 0.25, 0.5 g of catalyst (0.5 g of
each monofunctional solids in the case of the mechanical mixture).
2
7