The comparison of two classes of bifunctional SBA-15 supported platinum-heteropolyacid catalysts for the isomerization of n-hexane

Article abstract of DOI:10.1039/c5nj00453e

Mono- and bifunctional catalysts composed of silicotungstic acid (H₄SiW₁₂O₄₀) and/or platinum nanoparticles supported on SBA-15 mesostructured silica were comparatively tested for the isomerization of n-hexane. Monofunctional catalysts were produced by impregnating the SBA-15 support with either H₄SiW₁₂O₄₀ or the platinum particle precursor, H₂PtCl₆. In the bifunctional catalysts, both metallic and acidic functions were combined either by mechanically mixing the monofunctional solids yielding a multiphase bifunctional material (HSiW/SBA-15 + Pt/SBA-15), or by dual impregnation of H₄SiW₁₂O₄₀ and Pt within a single material leading to the monophasic bifunctional catalyst, HSiW/Pt/SBA-15. These hybrid materials, with the exception of the monofunctional platinum catalyst, were all active for the gas-phase isomerization of n-hexane. The two bifunctional catalyst systems showed high activity and selectivity for branched isomers with no catalyst degradation over 3 days of reaction.

Full text of DOI:10.1039/c5nj00453e

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The comparison of two classes of bifunctional
SBA-15 supported platinum–heteropolyacid
catalysts for the isomerization of n-hexane†
Cite this:DOI: 10.1039/c5nj00453e
a
a
b
a
Teresa Pinto, Philippe Arquilliere, Gerald P. Niccolai, Frederic Lefebvre* and
´ ´
´
`
a
Veronique Dufaud*
Mono- and bifunctional catalysts composed of silicotungstic acid (H
supported on SBA-15 mesostructured silica were comparatively tested for the isomerization of n-hexane.
Monofunctional catalysts were produced by impregnating the SBA-15 support with either H SiW12 40 or
the platinum particle precursor, H PtCl . In the bifunctional catalysts, both metallic and acidic functions
were combined either by mechanically mixing the monofunctional solids yielding a multiphase bifunctional
material (HSiW/SBA-15 + Pt/SBA-15), or by dual impregnation of H SiW12 40 and Pt within a single
4
SiW12O40) and/or platinum nanoparticles
4
O
2
6
Received (in Montpellier, France)
2
3rd February 2015,
4
O
Accepted 29th April 2015
material leading to the monophasic bifunctional catalyst, HSiW/Pt/SBA-15. These hybrid materials, with
the exception of the monofunctional platinum catalyst, were all active for the gas-phase isomerization
of n-hexane. The two bifunctional catalyst systems showed high activity and selectivity for branched isomers
with no catalyst degradation over 3 days of reaction.
DOI: 10.1039/c5nj00453e
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to mono-branched products such as 2-methylpentane (2MP)
and 3-methylpentane (3MP) would raise the octane number
Introduction
5 6
Skeletal isomerization of linear paraffins (C –C ) is a key refining from 25 to 74. Further ramification to the di-branched dimethyl-
process producing clean gasoline. The reaction consists of the butane isomers (2.2DMB and 2.3DMB) leads to octane numbers
transformation of straight-chain paraffins into branched-chain of around 100.
isomers without cracking, thus improving the overall octane
The isomerization of linear alkanes is generally carried out
number (RON) of hydrocarbon fuels. A high octane number over bifunctional catalysts comprising a metallic phase with
allows for better fuel combustion which means better performance hydrogenating/dehydrogenating properties and an acid phase
1,2
of the internal combustion engine. Thus, improved technology on which isomerization and/or cracking may occur. The metal
for alkane isomerization can provide for higher performance phase, in the presence of hydrogen, allows for a better stabili-
gasoline with reduced reliance on highly regulated additives such zation of the catalytic activity as well as significant enhancement
1
,2
as aromatics, olefins or methyl tert-butyl ether (MTBE).
of the selectivity for isomerization by disfavoring the bimolecular
4
An on-going challenge in this field of research seeks the mechanism pathway responsible for coke formation.‡ Now-
development of highly active catalysts for n-alkane isomeriza- adays, the industrial catalysts consist of platinum supported on
tion capable of operating under moderate temperatures while chlorinated alumina or acidic zeolites such as H-mordenite.
5
,6
keeping high selectivity towards branched isomeric products. Although the former can operate at temperatures as low as
Indeed, under these mild reaction conditions, the thermodynamic 120–180 1C, it is highly sensitive to contaminants in the feed
3
equilibrium favors high concentrations of branched isomers.
(i.e. water, sulfur) and requires additional chlorine treatment
For instance, in the particular case of n-hexane, isomerization to maintain stable acidity and hence constant activity. On the
other hand, zeolites are relatively stable with respect to poison-
5
,6
a
ing by water and sulfur, but less acidic than chlorinated
alumina. As a consequence, zeolites require more stringent
Laboratoire de Chimie, Catalyse, Polym `e res, Proc ´e d ´e s, CNRS,
Universit ´e Claude Bernard Lyon 1, CPE Lyon, 43 Bd du 11 november 1918,
6
9616 Villeurbanne Cedex, France. E-mail: Veronique.Dufaud@univ-lyon1.fr,
operating conditions (220–400 1C, 15–30 bar H ) for the activation
2
Frederic.Lefebvre@univ-lyon1.fr; Fax: +33 472-431-795
b
ICAR, UMR 5191 CNRS, Universit ´e Lumi `e re Lyon 2, ENS Lyon,
1
5 parvis Ren ´e -Descartes, 69342 Lyon Cedex 07, France
Electronic supplementary information (ESI) available: XRD patterns before and
‡ In the bimolecular mechanism, the carbocation leads to an olefin which can
further react with another carbocation yielding not only isomerization but also
cracking through a rearrangement and/or breaking of C–C bonds. Thus, the role of
†
after catalysis, nitrogen sorptions, FT-IR spectra, HRTEM images and evolution of C6
products as a function of temperature and flow rate. See DOI: 10.1039/c5nj00453e
2
the Pt–H system would be to reduce the alkene concentration by hydrogenation.
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of C
5
–C
6
alkanes, which in turn is unfavorable for the produc-
In a previous report, we have shown that SBA-15 type silica
7
–10
tion of branched isomers.
More recently, other solid acid could also be a suitable carrier for use in isomerization reac-
2
8
materials, displaying stronger acid strength than zeolites, have tions of light alkanes. Compared to MCM-41 based materials,
been successfully applied to the skeletal isomerization of light its large pore volume and diameter (up to 80 Å) and thick walls
alkanes. These include sulfated zirconia,
11,12
13
tungstated zirconia
allow for better reactant and product diffusion as well as provide
thermal and mechanical resistance which are key parameters in
14–17
and heteropolyacids (HPA)
among others.
HPA are particularly well-suited as catalysts for the isomer- view of industrial applications. With respect to n-hexane iso-
ization of n-alkanes owing to their very strong Brønsted acidity merization, SBA-15 supported H 40 (HPW) and H SiW12
3
PW12
O
4
O
40
and high thermal stability. In the past few years, several (HSiW) were found to be efficient catalysts affording branched
catalytic studies were undertaken using HPA in combination isomers with high selectivity (490%), the supported silico-
with a noble metal, mainly platinum, to perform the skeletal tungstic acid being intrinsically more selective than its phos-
1
8–20
isomerization of n-alkanes in the C –C range.
Generally, photungstic acid counterpart. However, as already mentioned
4
10
these bifunctional catalytic systems, whether supported or not, for monofunctional acid catalysts, in the absence of any
were able to provide high conversion and selectivity towards noble metals, a deactivation of the catalysts was noticed with
2
1–25
branched isomeric products.
shown that promotion with platinum considerably improved by coke formation.
the selectivity and the stability with time on stream. For In this contribution, we explore the influence of noble metal
instance, Miyaji et al. used the Pt–Cs2.5 40 catalyst for addition on the activity and stability of our previous mono-
In most of the cases, it was time on stream which was attributed to poisoning of acid sites
H
0.5PW12O
the isomerization of n-butane. The results showed high selec- functional acid materials and examine whether the proximity
tivity for isomerization products (490%) suggesting that between HPA and platinum functions has an impact on the
this catalyst operated through a monomolecular mechanism overall performance of the system for the isomerization of n-hexane.
5
pathway. Ivanov et al. investigated the catalytic activity of For this purpose, two approaches were developed to produce
systems based on Pt–HPW Keggin and Dawson tungstic acid bifunctional Pt–HPA catalytic systems, either by a simple mixing
supported on zirconia. High activity for n-hexane isomerization of the two monofunctional solids (separate metallic and acidic
was achieved with yield and selectivity for iso-hexanes of, phases), or by including both HPA and platinum functions
2
2
respectively, 80% and 96–98% at 190 1C. However, in most of within a single material made in a two-step process. Silicotungstic
the cases, the initial structure of the HPW compounds was acid was used for its strong acidity and high selectivity for this
partially damaged due to the insufficient stability of initial reaction based on our previous results. Platinum was chosen
acids in contact with the ZrO2 support resulting in lower for its well-known hydrogenating/dehydrogenating properties.
concentration of active sites. Lately, Gherib et al. described a The present work also investigates the effect that hydrogen,
promotion effect of reduced Pt–Ce oxides in the isomeriza- reaction temperature and gas flow rate have on activity and
tion of n-hexane for silica supported Keggin-based HPA, which product distributions. A detailed physico-chemical character-
was attributed to the enhanced hydrogenating/dehydrogenat- ization of the obtained materials before and after catalytic runs
2
6
ing properties of the final composites. A similar promotion was carried out using different techniques. These include X-ray
effect on n-pentane isomerization was observed by doping powder diffraction at small and wide angles, elemental analysis,
zirconia supported Pt–H PW O catalysts with lanthanum or High Resolution Transmission Electron Microscopy (HRTEM)
3
12 40
cerium. The benefit of La or Ce addition on activity and and High Angle Annular Dark Field Scanning Transmission
selectivity, although strongly dependent on the Pt loadings, was Electron Microscopy (HAADF-STEM), infrared spectroscopy, H
2
2
7
ascribed to a better dispersion of HPA over the solid surface.
In a series of publications, Xu et al. investigated in detail the
skeletal isomerization of n-pentane using Pt–HPA immobi-
chemisorption and nitrogen adsorption.
lized onto various oxides. With ZrO
version of n-pentane reached 65% at 200 1C and atmospheric
2
as the support, the con- Experimental
General
pressure, which is nearly the equilibrium value, with 97%
4
Tetraethoxysilane (TEOS), poly(ethylene oxide)–poly(propylene
selectivity. When supported over high surface area SiO or
2
oxide)–poly(ethylene oxide) block copolymer (Pluronic 123, MW:
g-Al
observed for the Pt–HPA bifunctional catalyst. The greater
efficiency of the Pt–HPA/ZrO catalyst was attributed to its
ability to generate stronger acid sites and to promote the disper-
2 3
O , a marked decrease in the catalytic performance was
5000), 12-silicotungstic acid hydrate (H
4
SiW12
2
O40ꢀxH O), chloro-
platinic acid hexahydrate (H PtCl O) and n-hexane (Z99%
2
ꢀ6H
6 2
2
purity) were purchased from Aldrich Chemical and used without
further purification. Anhydrous ethanol (Z99.9% purity) was
obtained from Carlo Erba. SBA-15 silica was prepared according
2
7
sion of noble metal. The use of mesoporous silica materials
such as MCM-41 and SBA-1 was also investigated by the same
2
9–31
4
to the procedure reported by Zhao et al.
authors. While the SBA-1 based Pt–HPA catalyst exhibited the
highest activity, no noticeable changes in selectivity were
observed, the difference in activity probably arising from dis-
Preparation of monofunctional catalysts
tinct textural properties (e.g. pore connectivity, surface area) of The monofunctional acid catalyst, HSiW/SBA-15, was obtained,
4
28
the supports.
as previously described, by simply impregnating the SBA-15
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support with a methanol solution of H
4
SiW12
O
40ꢀxH
2
O (HSiW) Transmission Electron Microscopy) and HAADF-STEM (High
to achieve the desired tungsten loading of B25 wt%. The Angle Annular Dark Field Scanning Transmission Electron
preparation of the metallic catalyst, Pt/SBA-15, was carried Microscopy) with a JEOL JEM 2100F microscope operating at
out using H PtCl ꢀ6H O as the metal precursor following the 200 kV with a point to point resolution of, respectively, 0.23 nm
2
6
2
3
2
method recently described by Li et al. Typically, a desired and 0.19 nm. The size distribution histograms were obtained
amount of H
2
PtCl
6
ꢀ6H
2
O was dissolved in 20 mL of ethanol and from the manual measurement with a J Image of at least
then dispersed drop by drop onto the SBA-15 support. The 200 different nanoparticles assuming a near spherical shape
quantities were adjusted to reach a Pt loading of B1 wt%. The and random orientation. Log normal fits were used to approx-
resulting suspension was stirred at room temperature over- imate experimental size distribution histograms. The chemical
night. After removal of the solvent, the solid was dried in an composition of the samples was analyzed using the energy-
oven at 80 1C during 15 hours. The dried material was next pre- disperse X-ray (EDX) method. Before observation, two methods
treated at 300 1C for 2 hours in a flowing N atmosphere. As were used to prepare the samples: the powders were (i) sus-
2
reported by the authors, this step is crucial in obtaining well- pended in toluene and transferred onto a Cu grid (200 mesh
dispersed Pt nanoparticles into the SBA-15 channels. To further coated with Formvar and carbon films) or (ii) embedded within
control the generation of Pt particle size in the 2–6 nm range, an EPON resin. After polymerization, thin slices (90 nm) were
2
an additional reduction step (H flow, 300 1C, 2 hours) was cut by ultramicrotomy with a diamond knife. The slices were
introduced at the end of the synthesis.
then deposited over Cu grids (200 mesh) previously coated
with a holey-carbon film. The last method was performed on
selected samples to better visualize the Pt nanoparticles present
Preparation of bifunctional catalysts
Catalysts containing both acidic and metallic functional groups on the surface and for the HAADF-STEM images. The platinum
were prepared using two different approaches. The simplest dispersion was determined by chemisorption of hydrogen. The
method, the mechanical mixture, includes fine grinding in an experiments were made on a Micromeritics ASAP 2020 instru-
agate mortar of an appropriate amount of the two previous ment. The samples were first activated at 110 1C under vacuum
monofunctional powders to obtain the desired multiphase for 1 h and then under hydrogen at 350 1C. Finally, nitrogen
bifunctional catalyst, denoted (HSiW/SBA-15 + Pt/SBA-15). In was passed on the samples at 350 1C in order to remove any
the second approach, both types of functions were combined adsorbed hydrogen. The isotherm was made at 55 1C after
in the same material following a two-step process, involving cooling the sample.
first the formation of the metallic phase prior to the introduc-
tion of the acid one. In a typical synthesis, the necessary
quantity of HSiW to reach a W loading of B25 wt% was
Catalytic tests
dissolved in 5 mL of distilled water and added drop wise over The n-hexane isomerization took place in a fixed bed dynamic
the monofunctional Pt/SBA-15 material previously heat-treated flow reactor at atmospheric pressure. The introduction of gas
s
at 300 1C. The slurry was stirred at 500 rpm at room temperature (argon or hexane) was monitored by a Bronkhorst mass flow-
overnight. After elimination of the water, the solid was dried at meter. The reaction products were analyzed online by gas
8
0 1C during 15 hours. Finally, the material was reduced at 300 1C chromatography (GC) equipped with a flame ionization detec-
under H for 2 hours leading to the final monophasic bifunctional tor (FID) and a nonpolar capillary column KCl/Al (Varian
2 3
O
2
catalyst labeled as HSiW/Pt/SBA-15.
capillary column; 50 mm ꢂ 530 mm ꢂ 15 mm). It was possible
to sample the gas upstream and downstream of the reactor.
The experiments were conducted during 12 hours or 72 hours
Catalysts characterization
Low and wide-angle X-ray powder diffraction (XRD) data were at temperatures ranging between 140 and 200 1C. Typically, the
acquired on a Bruker D5005 diffractometer using the Cu Ka synthesized hybrid materials were carefully weighed and charged
1
2
monochromatic radiation (l = 1.54184 Å). N
2
adsorption– into a stainless steel ’’ cylindrical reactor (inside diameter
desorption measurements were performed on an ASAP 2020 1 cm and 20 cm of length) to produce a catalyst bed 5 cm thick.
ꢁ
1
Micromeritics system. The samples were evacuated at 350 1C The reactor was then heated under a flow of argon (5 mL min
)
for 12 hours prior to the experiment. The surface area was for 2 hours at 200 1C to remove any adsorbed water. If
obtained from the BET equation and the pore size distribution necessary, the reactor was re-equilibrated at the desired reac-
was calculated by the BJH method applied to the desorption tion temperature. The gas flow was then switched to hydrogen
branch of the nitrogen adsorption–desorption isotherm. bubbled through hexane at room temperature, which corres-
ꢁ
1
Fourier transform infrared (FT-IR) spectra were recorded from ponds to a partial pressure of n-hexane of 151 mmHg (5 mL min
,
ꢁ
1
KBr pellets, in the 400–4000 cm range, on a Nicolet 5700 hexane/hydrogen ratio = 0.25). All light hydrocarbons excepting
spectrometer in the absorbance modes. Elemental analyses methane and ethane (which are minor products and neglec-
were made by ICP-AES with an ICP spectroflamme-D from a ted in the following calculations) have been identified and
solution obtained by treatment of the solid catalyst with a standardized for the flow reactor under dynamic flow condi-
3 2 4
mixture of HF, HNO and H SO
in a Teflon reactor at 150 1C. tions. Mass concentration and GC peak area are directly corre-
The morphology of the supported catalysts and platinum lated. Conversion is defined as the sum of the mass
nanoparticles was examined by HRTEM (High Resolution concentrations of all products (cracking and isomerization)
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divided by the sum of all the products and the remaining
n-hexane reactant:
The monofunctional acid catalyst, HSiW/SBA-15, was pre-
pared as previously described by impregnating the SBA-15
2
8
P ½
Piꢃ
support with a methanol solution of H SiW O ꢀxH O to produce
4
12 40
2
28
Conversion ð%Þ ¼ P ½
a material with 25 wt% tungsten. In the previous report, this
material was characterized by highly dispersed and intact
Keggin units available for the catalytic reaction.
Piꢃ þ ½hexaneꢃ
remaining
Selectivity of each product is the mass concentration of that
The simplest bifunctional catalyst containing both acidic
product divided by the sum of all product mass concentrations: and metallic functional groups was obtained by finely grind-
ing an appropriate amount of the tungsten monofunctional
material with the platinum material in an agate mortar, result-
ing in the multiphase bifunctional catalyst, denoted (HSiW/
Pi
SelectivityPi ð%Þ ¼ P ½
Piꢃ
These values were used to calculate the global selectivity to SBA-15 + Pt/SBA-15).
isomerization products and to cracking products reported in
In the second approach, metallic and acidic functions were
Tables 2–4 and the corresponding figures.
combined according to a stepwise two-step process leading to
The relative molar fractions among C6 products are of the monophase bifunctional catalyst HSiW/Pt/SBA-15 having
particular interest. This corresponds to the mass concentration approximately the same charge in tungsten and in platinum as
of each C6 isomer (n-hexane and the four C6 isomerization the multiphase system mentioned above. The synthesis initially
products) divided by the sum of those concentrations.
proceeds as for the monofunctional Pt/SBA-15 material, with
the addition of an aqueous solution of H SiW O ꢀxH O before
PC isomer
4
12 40
2
6
i
Molar FractionC isomer ð%Þ ¼ P ꢀ
ꢁ
6
i
the final reduction step producing the monophase bifunctional
catalyst, HSiW/Pt/SBA-15.
PC isomer
6
i
Mono- and bifunctional catalysts were characterized by
various physico-chemical and spectroscopic techniques. The
results showed highly ordered and well dispersed materials, as
proved by X-ray powder diffraction (XRD) measurements. All
Results and discussion
Preparation and characterization of catalysts
The synthesis of highly dispersed Pt nanoparticles onto high catalysts exhibited typical diffractograms characteristic of hex-
3
3
surface area supports has recently attracted much interest. In agonally ordered mesophases with the presence of three well-
addition to metal dispersion, metal particle size is also known resolved peaks in the 2y-range of 0.61–31 indexed as (100), (110)
to greatly influence the activity, selectivity as well as the catalyst and (200) reflections (ESI,† Fig. S1). The presence of higher order
3
3
lifetime. Several methods have lately appeared in the litera- reflections as well as the absence of significant changes in the
ture to prepare stabilized and nano-sized Pt nanoparticles in d-spacing (Table 1) indicated that the long-range structuration
the range of 1–10 nm inside the cavities of SBA-15 mesoporous of the materials was not affected by functionalization and that
3
3
silica. These include the conventional incipient wetness the principle structural features of the SBA-15 silica matrix were
impregnation technique, the deposition–precipitation route, retained. The wide-angle XRD patterns of catalysts are shown in
the in situ incorporation of Pt colloids during the oxide synth- Fig. S2 (ESI†). Neither mono- nor bifunctional catalysts displayed
esis, the ion-exchange method from an organically modified diffraction peaks in the 10–701 2y-range evidencing that both
surface and the chemical vapor deposition of volatile metal HSiW Keggin ions and Pt nanoparticles were finely dispersed
3
3
compounds followed by their subsequent decomposition.
throughout the solids. In the case of the monophase bifunc-
Among all these approaches, we have identified the method tional catalyst, HSiW/Pt/SBA-15, it also demonstrates that the
reported by Li et al. as a reliable and convenient way to provide initial high Pt dispersion, achieved in the first step of the
3
2
Pt/SBA-15 catalysts with highly-dispersed Pt nanoparticles.
synthesis, was not notably affected by subsequent immobiliza-
Using an adapted Li synthesis, we obtained our platinum tion of the acidic HSiW phase.
hybrid material, Pt/SBA-15, with a platinum loading of 0.8 wt%.
The textural data derived from the BET analysis of nitrogen
Note that this loading is relatively low when compared to adsorption and desorption experiments are presented in Fig. S3
Li’s materials.
(ESI†) for pure SBA-15 and mono and bifunctional hybrids.
Table 1 Textural and physical properties of catalysts
a
b
0
c
d
p
3
ꢁ1
e
m
3
ꢁ1
f
p
2
ꢁ1
g
Catalyst
Pt (wt%) W (wt%)
d
100 (Å)
a
(Å) Wall thickness (Å)
V
(cm g
)
V
(cm g
)
D
(Å)
S
BET (m g
)
C
BET
SBA-15
Pt/SBA-15
HSiW/Pt/SBA-15 0.7
HSiW/SBA-15
—
0.8
—
—
22
25
99
101
99
114
116
115
112
48
52
50
47
1.1
1.1
0.5
0.6
0.06
0.06
0.04
0.04
75
76
75
77
859
844
423
503
151
168
302
266
—
97
a
b
c
d(100) spacing (the error is estimated as 3 Å).
a
0
= 2d(100)/O3 Hexagonal lattice parameter calculated from XRD. Calculated by a
0
– pore size.
d
e
f
Total pore volume at P/P
0
= 0.980. Microporous volume determined using the t-plot method. Pore size from the adsorption branch applying
g
the BJH pore analysis. Derived from the BET equation of the nitrogen adsorption isotherms.
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All materials exhibited type IV isotherms characteristic of The distribution of Pt particles in Pt/SBA-15 was rather uniform
mesoporous materials with a steep capillary condensation loop with no obvious bulk aggregates of the Pt metal which is in
appearing at a relative pressure range of 0.6 to 0.8 P/P which good agreement with the wide-angle XRD data. The HRTEM
0
34,35
suggests a narrow mesopore size distribution in the samples.
micrographs revealed quasi-spherical shaped Pt particles,
The absence of substantial shift in the inflection position when mainly located inside the channels of the SBA-15 host (ESI,†
compared to parent SBA-15 indicated that the mean pore Fig. S6). The particle mean size estimated both from the Gaussian
diameters remained relatively unchanged upon functionaliza- distribution of particles size and H chemisorption measurement
2
tion (75–77 Å for hybrid materials versus 75 Å for plain SBA-15, was found to be, respectively, 5 and 5.3 ꢄ 1.0 nm, which matches
Table 1). However, unlike Pt/SBA-15, the HSiW derived materials, well the SBA-15 pore width of 7.6 nm derived from the BJH
regardless of whether they did or did not contain Pt nano- method. EDX chemical analysis performed over a large number
particles, showed a marked decrease in both specific surface of sample areas of Pt/SBA-15 (ESI,† Table S1) showed that Pt
areas and pore volumes with respect to parent SBA-15 (respec- particles were uniformly present over most of the ordered
2
ꢁ1
2
ꢁ1
3
ꢁ1
tively, from 859 m g to 423–503 m g and from 1.1 cm g to surface with, on average, 0.28 atom Pt per 100 atoms Si. This is
0
3
ꢁ1
.5–0.6 cm g , Table 1) which is consistent with the presence of consistent with the elemental analysis of a milligram scale
a large extent of HSiW clusters in the pore channels. This was sample of Pt/SBA-15 which also showed a value of 0.25 atom Pt
further demonstrated by the increase in the CBET parameter (from per 100 atoms Si for a Pt loading of 0.8 wt%.
1
51 for SBA-15 to 302 and 266 for respectively HSiW/Pt/SBA-15 and
HSiW/SBA-15) indicating a change in the surface polarity due to magnification (Fig. 1, bottom, middle), showed an ‘‘island-
the presence of HSiW species. type’’ distribution of the acidic HSiW phase over the whole
The HAADF-STEM images of HSiW/Pt/SBA-15, taken at low
The FT-IR spectra of all materials are displayed in Fig. S4 surface with, in some cases, HSiW particles of 20–30 nm in
(ESI†), together with those of bulk HSiW and bare SBA-15. The diameter fulfilling some of the pores. At higher magnification,
FT-IR spectra of the HSiW/SBA-15 and HSiW/Pt/SBA-15 catalysts however, one can also observe some small bright areas at the
clearly showed the characteristic bands of the Keggin ion at 978 surface or within the core of the silica framework, attributed to
ꢁ
1
(
(
n
n
as(W–Od)), 928 (nas(Si–Oa)), 878 (nas(W–Ob–W)), and 790 cm
the presence of metallic clusters of 1–3 nm size (Fig. 1, bottom,
3
6
as(W–Oc–W)), indicating that the structural integrity of the right). EDX analysis confirmed the simultaneous presence of
HSiW Keggin units was preserved in all cases during the prepara- both Pt and W in these sample areas with a Pt/W atomic ratio
tion of the samples. A confirmation of this behavior was also varying between 0.04–0.08 which is a clear indication that both
obtained by measurement of the acidity of the HSiW/SBA-15 acidic and metallic functions in HSiW/Pt/SBA-15 can coexist in
system which was found to be highly acidic (Fig. S5 in ESI†).
very close proximity. HRTEM images of HSiW/Pt/SBA-15 showed
Further insight into nanoparticle size and morphology, an average platinum nanoparticle size of 2.9 ꢄ 1.1 nm (ESI,†
chemical composition and spatial distribution over the support Fig. S7) with a narrower particle size distribution when compared
was provided by transmission electron microscopy. Represen- to the Pt/SBA-15 catalyst. This may be due to a better dispersion of
tative HAADF-STEM images of the platinum-derived catalysts the Pt nanoparticles during the reduction step due to the presence
(Pt/SBA-15 and HSiW/Pt/SBA-15) after reduction are displayed of a large amount of HSiW (25 wt% W). One can also evoke the
in Fig. 1. In all cases, one can see that the highly ordered hexagonal synergistic participation of tungsten to the reduction process of Pt
structure of SBA-15 was retained after the preparation processes. allowing for an easiest and fastest generation of Pt nanoparticles,
37
and consequently much smaller Pt NPs. Both Pt/SBA-15 and
HSiW/Pt/SBA-15 catalysts revealed the presence of crystalline Pt
particles. Comparison with known metallic and oxide Pt based
structures and their synthetic protocols allow us to probe the
formation of metallic cubic Pt nanoparticles, as evidenced by the
atomic resolution HRTEM micrograph and Fourier transform
diffraction pattern exemplified for Pt/SBA-15 in Fig. S8 (ESI†).
Catalytic results
In this study, our goal was to evaluate the performance of these
different catalyst systems and probe more specifically the effect
of bifunctional catalysts with respect to their monofunctional
counterparts. Catalytic tests were performed by passing a
carrier gas containing n-hexane through a bed of the solid
catalyst in a dynamic reactor. Reactants and products were
sampled at regular intervals. Some key basic performance
results for each catalytic test are summarized in Table 2.
In a previous publication, we had shown that the HSiW/SBA-15
material was active for n-hexane isomerization but that the catalyst
activity degraded with time on stream, which we attributed to the
Fig. 1 Representative HAADF-STEM images of the platinum-derived catalysts:
Pt/SBA-15 (top) and HSiW/Pt/SBA-15 (bottom).
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Table 2 Conversions and selectivities of the mono- and bifunctional catalysts
Catalysts
Conversion
0 h
Selectivity (12 h)
Entry
HPA phase
Pt phase
Carrier gas
12 h
C
6
C –C
3 5
1
2
3
4
5
6
0.5 g HSiW/SBA-15
—
0.5 g HSiW/SBA-15
0.5 g HSiW/SBA-15
0.5 g HSiW/SBA-15
0.5 g HSiW/Pt/SBA-15
—
H
H
H
Ar
H
H
2
2
2
12
0
37
15
24
27
7
0
63
10
59
55
93
—
96
93
94
98
7
—
4
7
6
0.5 g Pt/SBA-15
0.5 g Pt/SBA-15
0.5 g Pt/SBA-15
0.05 g Pt/SBA-15
2
2
2
a
ꢁ1
Reaction conditions: 1 bar, 200 1C, carrier gas flow 5 mL min , n-hexane partial pressure approximately 151 mmHg.
2
8
poisoning of acid sites by coke formation. As aforementioned,
several authors have proposed the introduction of hydrogen
and hydrogenation catalysts to such systems to prevent coking.
Thus, based on the optimal results from that study, HSiW/SBA-
15 was used as a catalyst for the isomerization of n-hexane in a
hydrogen stream at 200 1C (Table 2, entry 1), with results
similar to the previous reports. Initially, the catalyst showed
moderate activity (12% conversion) but this activity dropped
steadily over the first 12 hours on-stream to 7% (Fig. 2, top,
square curve). This significant degradation is nevertheless less
than that seen when using argon as a carrier gas. Selectivity
increases slightly with time on-stream to achieve 93% isomer-
ization vs. 7% hydrocracking after 12 hours on-stream (Fig. 2,
bottom, square curve).
As expected, the monofunctional platinum catalyst Pt/SBA-
15 had no significant activity for n-hexane isomerization both
initially and after 12 hours on-stream (Table 2, entry 2 and
Fig. 2, top, triangle curve).
A marked improvement was immediately observed for a
mechanical mixture of the two monofunctional materials, the
multiphase bifunctional catalyst (HSiW/SBA-15 + Pt/SBA-15).
This material showed high and steadily increasing n-hexane
conversion (63% after 12 hours on stream) with high selectivity
for n-hexane isomers (96%) with no measurable olefin formation
(Table 2, entry 3). Under these reaction conditions, one can see
that the distribution of n-hexane isomers approached the
thermodynamic equilibrium mixture (vide infra, Table 3 and
Fig. S10, ESI†).
As previously stated, and demonstrated by other authors, the selectivity to C
use of hydrogen is also a key to reduce coking. As a control, (HSiW/
SBA-15 + Pt/SBA-15) was tested using argon as a carrier gas
Fig. 2 Conversion of n-hexane as a function of reaction time (top) and
isomers as a function of the conversion (bottom) for mono-
6
ꢁ1
and bifunctional catalysts. Reaction conditions: 1 bar, 200 1C, 5 mL min
hydrogen flow, hexane/hydrogen ratio = 0.25, catalyst (0.5 g). For the
mechanical mixture, 0.5 g of each monofunctional catalyst was introduced.
(Table 2, entry 4). In this case, n-hexane conversion after 12 hours
was only 10% with 93% selectivity for isomerization. This perfor-
mance is very similar to the monofunctional tungsten catalyst sufficient to reduce catalyst degradation, and this point could
HSiW/SBA-15 (Table 2, entry 1, 7% conversion and 93% selectivity), be optimized if one were to pursue the use of the multiphase
thus the presence of platinum does not have decisive effects on bifunctional catalysts.
catalyst performance in the absence of hydrogen.
The evolution of the stability of the multiphase bifunctional
When using less of the monofunctional platinum catalyst in catalyst (HSiW/SBA-15 + Pt/SBA-15) was studied over a longer
the mechanical mixture, thus reducing the overall platinum time on-stream, up to 80 hours (see ESI,† Fig. S9). Conversion
loading, conversion and selectivity were very slightly affected. continues to increase with time on stream, but the increases are
The mixture of 0.5 g of HSiW/SBA-15 with 0.05 g of Pt/SBA-15 very slight after 24 hours as one is approaching the thermo-
(
1
(
0.04 wt% Pt, Table 2, entry 5) produced 59% conversion after dynamic equilibrium mixture of hexane isomers at this tempera-
2 hours on stream (versus 63% in entry 3) and 94% selectivity ture (72% conversion after 80 hours versus 86% conversion at
versus 96% in entry 3). Thus, lesser quantities of platinum are equilibrium). At this longer time-on-stream, one also observes
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Table 3 Conversions and selectivities of multiphase and monophase bifunctional catalysts as a function of temperature
6
Mole fraction among C compounds (%)
Catalyst
T (1C) Conversion (%) Selectivity iC
6
(%) Selectivity C –C (%) n-Hexane 2,2-DMB 2,3-DMB 3-MP 2-MP
3 5
HSiW/SBA-15 + Pt/SBA15 140
7
13
37
63
98
98
98
96
2
2
2
4
93
87
63
37
0
0
1
4
1
2
4
7
4
7
20
33
2
4
11
19
1
1
2
60
80
00
HSiW/Pt/SBA-15
160
9
29
55
99
99
98
1
1
2
90
71
45
0
0
2
1
3
6
6
17
30
3
10
18
1
2
80
00
a
ꢁ1
Reaction conditions: 1 bar, 140–200 1C, carrier gas flow 5 mL min , hexane/hydrogen ratio = 0.25, 0.5 g catalyst (0.5 g of each monofunctional
solids in the case of (HSiW/SBA-15 + Pt/SBA-15)).
that selectivity for isomerization is slightly decreasing as 0.2% (HSiW/SBA-15 + Pt/SBA-15) and HSiW/Pt/SBA-15 to determine the
conversion to cyclohexane was detected.
activation energy of the reaction and consider the distribution of
To test the importance of platinum proximity to the tung- isomers obtained in each case. The data were obtained under
sten site, the monophase bifunctional catalyst HSiW/Pt/SBA-15 steady-state conditions, that is, at 200 1C after 12 h reaction.
was also evaluated under similar conditions. Let’s recall that Results are reported in Table 3.
this catalyst was prepared such that it contained approximately
For each catalyst, increasing the temperature leads not only
the same charge in tungsten and in platinum as the multiphase to higher conversion of n-hexane but also to more lighter
system (HSiW/SBA-15 + Pt/SBA-15) tested above. The mono- hydrocarbon cracking products. Similar results were obtained
2
2
phase catalyst, HSiW/Pt/SBA-15, shows lower initial activity for Pt/HPW/ZrO
than (HSiW/SBA-15 + Pt/SBA-15) (27% vs. 37%, Table 2, entries catalyst HSiW/Pt/SBA-15 was slightly less active than the multi-
and 6), but again conversion steadily increases over time, phase catalyst (HSiW/SBA-15 + Pt/SBA-15), but it was slightly
2
catalytic systems. Overall, the monophase
3
closing the gap with that of the multiphase catalyst (Fig. 2, top, more selective for isomerization of n-hexane over hydrocrack-
circle curve). Furthermore, selectivity of the monophase catalyst ing. The apparent activation energies calculated from these
ꢁ
1
ꢁ1
for n-hexane isomerization was superior to that of the multi- data (72 kJ mol for HSiW/Pt/SBA-15 and 66 kJ mol for
phase catalyst at all conversions observed (Fig. 2, bottom, circle (HSiW/SBA-15 + Pt/SBA-15), Table S2 in ESI†) compare well with
ꢁ
1 38
curve). Different explanations are available concerning these those reported for zeolites (around 60 kJ mol ).
differences. For example, the proximity of platinum to tungsten Given the very high selectivity for isomerization over hydro-
could minimize diffusional limitations favoring rapid olefin cracking (between 1–4% of reactor outflow depending on the
hydrogenation once formed and, thus, reducing coking. experimental conditions) observed under these conditions, one
The presence of tungsten could also affect platinum disper- can analyze the C product distributions in a meaningful manner.
6
sion during the catalyst reduction phase of the preparation of Fig. S10 (ESI†) represents the data contained in Table 3 for C6
HSiW/Pt/SBA-15, leading to a more efficient hydrogenation products only, for each catalyst over several temperatures and
catalyst. Indeed, HRTEM analysis (comparison Fig. S6 and S7, the theoretical thermodynamic proportion of each product as a
3
9
ESI†) seems to indicate a smaller particle size for this material function of temperature. This representation illustrates the
compared to the Pt/SBA-15 catalyst used in the multiphase reactivity difference between the catalysts noted above in the
bifunctional system.
difference between the n-hexane curves, but one also notes that
Comparative runs at varying temperatures were undertaken at 200 1C, some of the isomerization products are approaching
under isomerization conditions for the two bifunctional catalysts their expected equilibrium values. Most notably, 2-methyl and
Scheme 1 Monomolecular mechanism of n-hexane isomerization over acid catalysts.
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Table 4 Conversions and selectivities of (HSiW/SBA-15 + Pt/SBA-15) as a function of contact time
Mole fraction among C
6
compounds (%)
Catalyst
Flow rate (mL min) Conversion (%) Selectivity iC
6
3 5
(%) Selectivity C –C (%) n-Hexane 2,2-DMB 2,3-DMB 3-MP 2-MP
HSiW/SBA-15 + Pt/SBA15
5
0
0
61
50
35
96
97
98
4
3
2
39
51
65
4
2
1
6
6
4
32
26
19
19
16
11
1
2
a
ꢁ1
Reaction conditions: 1 bar, 200 1C, carrier gas flow 5–20 mL min , hexane/hydrogen ratio = 0.25, 1 g of the catalyst composed of 0.5 g of each
monofunctional solids.
3-methyl pentane are at their expected values, while 2,3-dimethyl maintained with no significant changes of the particle size distribu-
butane is not quite there, and especially the 2,2-dimethyl tion with average Pt particles size of 5.3 ꢄ 1.7 nm and 2.3 ꢄ 0.6 nm,
butane (or neohexane) is furthest from equilibrium. These respectively, for multiphase (HSiW/SBA-15 + Pt/SBA-15) and mono-
phenomena are coherent with the conventional acid cata- phase HSiW/Pt/SBA-15 catalysts (Fig. S15 and S16, ESI†). Thus, these
lyzed monomolecular isomerization mechanism (Scheme 1). bifunctional catalysts proved to have excellent thermal behavior and
In the mechanism, methyl pentane products are produced stability under our reactions conditions.
by the first cycle of protonation and b-methyl transfer. The
dimethyl butane products (or the corresponding carbocations,
which lead to them) can only be formed from these initial Conclusions
products by a second b-methyl transfer step. One also notes
Bifunctional Pt–HSiW catalysts supported onto SBA-15 silica were
that 2,2-dimethyl butane, which Fig. S10 (ESI†) shows as
prepared using two different approaches, either by mechanically
being furthest from the equilibrium, is formed via a secondary
mixing monofunctional HSiW and Pt containing solids leading to
carbocation, whereas the other products are formed through
separate metallic and acidic phases, or by including both HSiW
presumably more stable tertiary carbocations.
and platinum functions nearby within a single material made in a
The dependence of catalyst performance and product dis-
two-step process. Hybrid materials with intact HSiW Keggin
tributions on contact time was probed by varying the flow rate
structures and highly dispersed Pt nanoparticles throughout
of the substrate stream through the dynamic reactor for the
the solid surface were produced as evidenced by FT-IR, XRD
multiphase bifunctional catalyst (HSiW/SBA-15 + Pt/SBA-15).
and HRTEM/STEM analyses. Both catalysts proved to be active
Results are summarized in Table 4 and illustrated by Fig. S11
for the gas-phase isomerization of n-hexane at 200 1C with
(ESI†). For comparison, Fig. S11 (ESI†) also plots the theoretical
conversion and selectivity for branched isomeric products super-
ior to the monofunctional HSiW material, and furthermore that
performance was maintained for long times on stream, in
marked contrast to the monofunctional material. In the case
of the mechanical mixture, even small amounts of platinum led
to this increased performance, but only when the isomerization
was carried out in the presence of hydrogen. Comparison
between the two bifunctional catalyst systems showed that
the monophase catalyst, HSiW/Pt/SBA-15, initially exhibited a
lower activity than the multiphase bifunctional catalyst (HSiW/
SBA-15 + Pt/SBA-15), but its activity steadily increased over time-
on-stream toward that of the multiphase catalyst, but with clearly
superior selectivity for isomerization. The study of various reaction
parameters showed that 200 1C presents the best compromise
between activity and the potential for hydrocracking reactions,
equilibrium values of the molar fraction for each C product at
6
200 1C at zero flow rate (infinite contact time) on the horizontal axis.
As expected, in the predominance of n-hexane at the relatively
low conversions produced under these conditions, conversion
depends directly on the flow rate, that is, the longer the effective
contact time, the more n-hexane is converted. One also notes that at
a low flow rate, hydrocracking is slightly more significant. Concen-
trating on the isomerization product distributions (Fig. S11, ESI†),
one remarks that n-hexane, 2-methyl pentane, 3-methyl pentane,
and 2,3-dimethyl butane selectivities all tend toward their expected
thermodynamic equilibrium values whereas 2,2-dimethyl butane
does not. This again is in accord with the monomolecular acidic
isomerization mechanism in that the latter product is produced only
through the formation of a secondary carbocation, which is unfavor-
able at this reaction temperature.
and that given a sufficient contact time, mixtures of C products
6
approaching thermodynamic equilibrium can be obtained.
Characterization of the used catalysts shows no evidence for
catalyst modification after 3 days of reaction, further demon-
strating their potential for industrial application.
Characterization of catalysts after catalytic runs
Examination of the mono- and multiphase bifunctional catalysts
after 3 days on stream showed a high degree of hexagonal meso-
scopic organization of the solids with no evidence of active phase
clustering as demonstrated in the low- and wide-angle XRD
patterns displayed in Fig. S12 and S13 (ESI†). The integrity of the
HSiW Keggin ions was fully retained after the reaction as assessed
Notes and references
by FT-IR which exhibits the main bands associated with H
Fig. S14, ESI†). HRTEM measurements also indicated that the high
initial dispersion of the Pt nanoparticles in these catalysts was
4
SiW12
O
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