Supercritical fluid deposition of Ru nanoparticles onto SiO2 SBA-15 as a sustainable method to prepare selective hydrogenation catalysts

Article abstract of DOI:10.1039/c5ra04969e

Ru nanoparticles were successfully deposited onto mesoporous SiO₂ SBA-15 using supercritical CO₂ (scCO₂). The use of scCO₂ favoured the metal dispersion and Ru nanoparticles uniformly distributed throughout the support were obtained. Different precursors and methodologies were employed: impregnation with Ru(tmhd)₂(COD) in scCO₂ at 80°C and 13.5 and 19.3 MPa and further reduction in H₂/N₂ at 400°C at low pressure, reactive deposition of Ru(tmhd)₂(COD) with H₂ in scCO₂ at 150°C and reactive deposition of RuCl₃·xH₂O with ethanol in scCO₂ at 150 and 200°C. The size of the particles was limited in one dimension by the pore size of the support. The metal loading varied with the methodology and experimental conditions from 0.9 to 7.4% Ru mol. These materials exhibited remarkable catalytic activity. The Ru/SiO₂ SBA-15 materials prepared by reactive deposition with H₂ in scCO₂ were selective catalysts for the hydrogenation reactions of benzene and limonene, allowing the production of partly hydrogenated hydrocarbons that may serve as building blocks for more complex chemicals. scCO₂ is shown to be a green solvent that allows the preparation of efficient heterogeneous catalysts to design sustainable processes. Furthermore, in the hydrogenation of limonene, scCO₂ was also used as the solvent.

Full text of DOI:10.1039/c5ra04969e

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PAPER
Supercritical fluid deposition of Ru nanoparticles
onto SiO2 SBA-15 as a sustainable method to
prepare selective hydrogenation catalysts†
Cite this: RSC Adv., 2015, 5, 38880
a
b
a
a
a
*
J. Morere, M. J. Torralvo, C. Pando, J. A. R. Renuncio and A. Cabanas
`
˜
Ru nanoparticles were successfully deposited onto mesoporous SiO2 SBA-15 using supercritical CO2
(scCO2). The use of scCO2 favoured the metal dispersion and Ru nanoparticles uniformly distributed
throughout the support were obtained. Different precursors and methodologies were employed:
impregnation with Ru(tmhd)2(COD) in scCO2 at 80 ꢀC and 13.5 and 19.3 MPa and further reduction in
H2/N2 at 400 ꢀC at low pressure, reactive deposition of Ru(tmhd)2(COD) with H2 in scCO2 at 150 ꢀC and
reactive deposition of RuCl3$xH2O with ethanol in scCO2 at 150 and 200 ꢀC. The size of the particles
was limited in one dimension by the pore size of the support. The metal loading varied with the
methodology and experimental conditions from 0.9 to 7.4% Ru mol. These materials exhibited
remarkable catalytic activity. The Ru/SiO2 SBA-15 materials prepared by reactive deposition with H2 in
scCO2 were selective catalysts for the hydrogenation reactions of benzene and limonene, allowing the
production of partly hydrogenated hydrocarbons that may serve as building blocks for more complex
chemicals. scCO2 is shown to be a green solvent that allows the preparation of efficient heterogeneous
catalysts to design sustainable processes. Furthermore, in the hydrogenation of limonene, scCO2 was
also used as the solvent.
Received 20th March 2015
Accepted 21st April 2015
DOI: 10.1039/c5ra04969e
www.rsc.org/advances
Partly hydrogenated terpenes are interesting building blocks for
ne chemicals.5
1. Introduction
Ruthenium catalysts have been supported on amorphous
alumina, silica, titania or active charcoal,6 mesoporous silica
materials MCM-41, SBA-15 and HMS,7 KL zeolite,8 porous
metal–organic frameworks,9 carbon nanotubes, ZnO,4 ZrO2,8
and montmorillonite,10 among others. The activity and selec-
tivity of the catalyst depend on the metal concentration, metal
particle size and its distribution, as well as on the chemical
nature of the support, its morphology and the metal–support
interactions. In principle, the better the dispersion, the higher
the activity of the catalyst.
The synthesis of supported metal nanoparticles on solid
porous supports and the different preparations routes have
been recently reviewed.11 Among the different preparation
routes, the use of supercritical uids deserved especial
attention.
Zhang and Erkey have reviewed the preparation of supported
metallic nanoparticles using supercritical uids.12,13 Although
in principle any supercritical uid can be used, most experi-
ments have been performed using CO2 (Tc ¼ 31 ꢀC and Pc ¼ 7.4
MPa). The use of scCO2 presents a number of advantages in
materials processing and synthesis.14 Supercritical CO2 (scCO2)
has densities intermediate between those of liquids and gases,
but transport properties (diffusivity and viscosity) similar to
gases. This combination of properties makes possible to intro-
duce precursors dissolved in the supercritical uid inside highly
Ruthenium catalysts have been widely used in heterogeneous
catalysis particularly in hydrogenation reactions. In comparison
to other traditional metal catalysts such as Pd, Pt, or Rh, Ru has
been shown to perform better in selective hydrogenation reac-
tions.1 Ru catalysts have been used for the partial hydrogenation
of aromatics2,3 and the selective hydrogenation of carbonyl
groups in the vicinity of either a double bond or an aromatic
ring.1 These are very important processes for both environ-
mental and economic reasons. For example, the partial hydro-
genation of benzene to cyclohexene provides a more efficient
and low cost route for the production of chemical intermediates
for nylon.4 Another interesting example of the use of Ru cata-
lysts is the selective hydrogenation of terpenes such as a- and
b-pinene, 1,8-cineol, citral and limonene. These compounds
can be extracted from renewable sources and are very cheap
precursors of fragrances, avours, drugs and agrochemicals.
aDepartment of Physical Chemistry I, Universidad Complutense de Madrid, 28040
Madrid, Spain. E-mail: a.cabanas@quim.ucm.es; Fax: +34 91 3944135; Tel: +34 91
3945225
bDepartment of Inorganic Chemistry I, Universidad Complutense de Madrid, 28040
Madrid, Spain
† Electronic supplementary information (ESI) available: TGA of the precursor
Ru(tmhd)2(COD) and the impregnated samples. N2-adsorption–desorption
isotherms and pore size distributions of some samples. See DOI:
10.1039/c5ra04969e
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porous inorganic substrates.15,16 On the other hand, the high
Ru thin lms were successfully deposited onto silicon wafers
solubility of scCO2 in amorphous polymers leads to swelling of by the H2 reduction of bis(2,2,6,6-tetramethyl-3,5-heptanedio-
the polymer and a decrease in its glass transition temperature nato)(1,5-octadiene)ruthenium(II) [Ru(tmhd)2(COD)]22,25 and
and enhances the chain mobility of the polymers, making also bis-cyclopentadienylruthenium [Ru(Cp)2]21,26 at temperatures
possible the incorporation of materials within polymeric between 250–350 ꢀC in scCO2. Similarly, Ru nanoparticles were
substrates.17 Furthermore, CO2 properties can be tuned with deposited onto carbon nanotubes (CNT) by the H2 reduction of
small changes of pressure and temperature, and the charac- ruthenium acetylacetonate [Ru(acac)2] in pure scCO2 at 250 ꢀC,27
teristics of the composite material can be controlled in the same and from RuCl3$3H2O in supercritical CO2–methanol solutions
way.18 From an environmental point of view, CO2 is considered a at 200 ꢀC.28 Ru nanoparticles were also immobilized into metal–
green solvent because it has moderate critical parameters, it is organic framework nanorods from RuCl3$3H2O in supercritical
cheap, non-toxic, non-ammable and can be recycled. CO2 is a CO2–methanol solutions at 200 ꢀC.29 The alcohol acted as
gas at ambient pressure and can be eliminated easily by simple cosolvent as well as reductant. The same precursor was used to
depressurization without leaving any residue.
deposited Ru nanoparticles onto CNTs in supercritical meth-
The Supercritical Fluid Deposition technique (SFD) was anol at 300 ꢀC (ref. 30) and in supercritical water at 400–450 ꢀC
originally proposed by Watkins et al.19,20 The method involves (ref. 31) and to produce Ru/graphene composites in supercrit-
the dissolution of a metal precursor in the supercritical uid ical water at 400 ꢀC.32 Similarly, Yen et al. used a hybrid
and its adsorption onto a given support (planar or porous). approach and impregnated a mesoporosus SiO2 SBA-15 with a
Then the metal precursor is decomposed, either in the super- solution of Ru(acac)2 in THF followed by drying under vacuum
critical uid (by addition of a reducing agent such as H2 or an and H2 reduction in scCO2 at 200 ꢀC.33 In all these examples, the
alcohol, or simply by heat treatment), or aer depressurization precursor reduction was carried out at supercritical conditions.
of the system under controlled atmosphere, yielding the metal
Others have followed a different approach and used scCO2 as
or metal oxide/support composite materials. By controlling the the solvent to impregnate the metal precursor into the support.
reaction conditions, lms or nanoparticles can be deposited The precursor is then decomposed aer depressurization of the
into planar and porous substrates.
system by thermal treatment under a reducing atmosphere. In
In this paper we study the deposition of metal nanoparticles this way, Ru(acac)3 and Ru(tmhd)2(COD) dissolved in scCO2
ꢀ
into a highly porous support, mesoporous silica SBA-15. were impregnated into carbon aerogels (CA) at 80 C and then
Traditional preparative methods in liquid solution oen reduced in N2 at low pressure.23 The thermodynamic and
yield inhomogeneous materials due to the high surface adsorption kinetics of Ru(tmhd)2(COD) on CA were also repor-
tension of most liquids, the slow diffusion of the metal ted.34 Using the same technique, polydimethylsilosane (PDMS)
ꢀ
precursor within the support pores and the potential damage lms were also impregnated at 40 C with the same precursor
of the support during the drying process. On the other hand, and further decomposed in N2 atmosphere.35
A similar
gas based processes such as Chemical Vapour Deposition approach was used to produce silica aerogel–Ru composites
(CVD) tend to yield non uniform materials mainly because of using Ru(acac)3.36 This precursor was also used to deposit Ru
volatility constrains, which lead to mass transport-limited onto nanoporous silica FSM-16.37 The support was impregnated
conditions and poor step coverage. The use of scCO2 in with Ru(acac)3 dissolved in scCO2 at 150 ꢀC using acetone as the
metallization processes presents several advantages over the cosolvent, followed by thermal reduction at low pressure in H2/
conventional techniques. Beside the environmental benets, N2. RuCl3$3H2O and Ru(acac)3 were also used in combination
ꢀ
the transport properties of scCO2 favour the penetration of with ethanol to impregnate activated carbon at 45 C and 10.0
scCO2 and its solutions into nanostructures and nanopores. In MPa. The impregnated material was reduced in H2/N2 at 350 ꢀC
this way, metal nanoparticles can be introduced within the at low pressure.38 Similarly, CNT were impregnated with
micro and mesopores of different substrates in a much more RuCl3$3H2O at 140 ꢀC and 8.0 MPa in a supercritical CO2–
efficient way than the conventional processes in both liquid ethanol solution and further reduced in H2 at 400 ꢀC at low
and gas phases.
pressure.39
Although a relatively large number of publications on the
The deposition of Ru onto different porous and planar
supports using scCO2 has been pursued because of their deposition of Ru nanoparticles in supercritical uids has been
numerous applications in microelectronics, catalysis and elec- published, there is not a comprehensive study comparing the
trochemistry.21–24 In these studies, different precursors and different reaction routes and precursors. In this work, we carry
methodologies have been used. All these methods require the out this comparative study and perform impregnation, H2-
solubilisation of the metal precursor in the supercritical uid reduction and alcohol reduction experiments using
mixture. The particular choice of precursor and uid deter- Ru(tmhd)2(COD) and RuCl3$3H2O on mesoporous SiO2 SBA-15
mines the solubilisation temperature and pressure used. Then, as support. The aim of this work is to elucidate the role that
if the decomposition of the precursor is carried out under the different variables have on the nal material and hopefully
supercritical conditions, addition of a reducing agent and/or serve as a selection guide to deposit Ru from supercritical
increase of the temperature and pressure are required. The solutions.
decomposition of the precursor can be also carried out aer
Furthermore, we demonstrate that these materials serve as
depressurization of the reactor by thermal treatment of the selective catalysts in the hydrogenation reactions of benzene
impregnated support in a controlled atmosphere.
and limonene. In the hydrogenation of limonene, the reaction
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was performed in scCO2. Due to its tuneable solvent properties
and its green nature, scCO2 is a very attractive medium for
chemical reactions.40–42 scCO2 and H2 are fully miscible43 and
limonene can be dissolved in such a mixture at moderate
temperatures and pressures. Bogel-Lukasik et al. studied the
phase behaviour of the ternary system CO2/H2/limonene and
shown that selectivity in the hydrogenation of limonene in
scCO2 can be tuned by changing the pressure.44 These authors
have also studied the effect of the catalyst in this reaction and
performed experiments using Pt/C, Pd/C and Ru/Al2O3
combined with an ionic liquid.44–46 In this work, the Ru catalysts
produced using scCO2 have been tested in the hydrogenation
reaction of limonene in CO2 at supercritical conditions.
2. Experimental
2.1. Materials
Tetraethylorthosilicate (TEOS, +99% pure), poly(ethylene
glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)
(Mw ¼ 5800) (PEO–PPO–PEO), dichloromethane (99, 99%),
hexane (+99%), (R)-(+)-limonene (97%), ZnSO4$7H2O and
RuCl3$xH2O (+99.98%) were obtained from Sigma-Aldrich.
Benzene (+99.5%) was obtained from Panreac and ethanol
(+99.8%) was supplied by Scharlau. Ru(tmhd)2(COD) was
provided by Strem chemicals (99%). CO2 (purity > 99.99%) and
H2 (purity > 99.999%) were supplied by Air Liquide. 5% H2/N2
forming gas was supplied by Contse.
Scheme 1 Summary of the Ru deposition experiments performed by
impregnation and reactive deposition using H2 and EtOH.
Mesoporous silica SBA-15 was prepared following the
procedure described by Zhao et al.47,48 In a typical experiment,
4.0 g of PEO–PPO–PEO were dissolved in 30 g of water and 120 g
temperature up to 13.5 or 19.3 MPa. The temperature was
measured using a K-type thermocouple. The pressure was
measured using a pressure gauge. Impregnation experiments
were carried out in scCO2 under stirring for 24 hours. The
reactor was then depressurized through a needle valve in
1 hour. The Ru impregnated SiO2 SBA-15 samples were then
decomposed in a tube furnace under N2/H2 atmosphere for
ꢀ
of 2 M HCl solution with stirring at 35 C. Then 8.5 g of TEOS
was added into the solution with stirring at 40 ꢀC for 20 hours.
ꢀ
The mixture was aged at 100 C without stirring for a further
12 hours. The solid residue was ltered, washed with ethanol
several times and calcined in air at 550 ꢀC ꢁfo1r 6 hours. Heating
ꢀ
rate from room temperature was 1 C min
.
5 hours at 400 ꢀC and atꢁm1ospheric pressure. Heating rate in
ꢀ
2.2. Materials preparation
both cases was 10 C min
.
Reactive deposition experiments using H2 were carried out on
the SiO2 support using Ru(tmhd)2(COD). The experiments were
conducted in the 100 mL stirred stainless-steel high-pressure
reactor previously described in the batch mode. SiO2 SBA-15
(ca. 150 mg) and Ru(tmhd)2(COD) (ca. 70 mg) were loaded
into the reactor (Ru : SiO2 molar ratio close to 1 : 20). Excess H2
(50 fold excess) was added to the reactor using a ca. 30 mL
auxiliary cell constructed from Swagelok 3/4 inch pipe and lled
with 15 bar H2, by ushing CO2 from the thermostated Isco
high-pressure syringe pump through the auxiliary cell up to a
nal pressure of 140 bar. System was kept at these conditions
for 2 hours for complete dissolution. At these conditions,
reduction of the precursor did not take place. To promote the
Ru deposition into mesoporous silica SBA-15 was carried out in
supercritical CO2 following three different procedures:‡
(a) impregnation, (b) reactive deposition using H2 and (c)
reactive deposition using EtOH. Ru(tmhd)2(COD) and RuCl3-
$xH2O were used as the metal precursors. The experimental
procedure is summarized in Scheme 1.
Most experiments were conducted in a ca. 100 mL stirred
high-pressure reactor (Autoclave Eng. Inc.) in the batch mode.
In the impregnation experiments, approximately 150 mg of the
support and 70 mg of Ru(tmhd)2(COD) were loaded into the
reactor (Ru : SiO2 molar ratio close to 1 : 20). The reactor was
then heated by a heating jacket connected to a PDI controller to
80 ꢀC and was then lled with CO2 using a high-pressure syringe
pump (Isco, Inc. Model 260D) thermostated at the same
ꢀ
precursor reduction, the temperature was increased at 150 C.
Reduction was complete in 3 hours and depressurization was
carried out through a needle valve in 1 hour. Samples were
further extracted to remove unreacted precursor and/or the
hydrogenated ligand.
‡ Caution: these experiments involve high-pressure and/or temperature and they
should be only performed with caution using appropriate high-pressure
equipment and safety precautions.
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Reactive deposition experiments using EtOH were carried out on were obtained on a Perkin-Elmer Pyris 1 at a heating rate of
the mesoporous SiO2 using Ru(tmhd)2(COD) and RuCl3$xH2O as 10 ꢀC minꢁ1 in N2 ow (100 cm3 minꢁ1). Ru content on selected
precursors. Approximately 150 mg of the support and 10–20 mg samples was determined by XRF. A PANalytical Wavelength
of RuCl3$xH2O or 70 mg of Ru(tmhd)2(COD) (Ru : SiO2 molar dispersive X-ray uorescence spectrometer (4 kW) was used
ratio ranging from 1 : 48 to 1 : 20) and a small amount of EtOH placing the samples in plastic holders in powder form. Quan-
(10% mol in CO2) were loaded into the 100 mL stirred high- tication was performed using internal standards of the
pressure reactor. In the Ru(tmhd)2(COD) experiments the instrument.
ꢀ
reactor was then heated by the heating jacket to 80 C and was
lled with CO2 using the high-pressure syringe pump thermo-
stated at the same temperature to a nal pressure of 13.5 MPa. In
the RuCl3$xH2O experiments, the reactor was however loaded at
35 ꢀC and 8.5 MPa. In this case, the temperature was kept low to
avoid decomposition. Experiments performed in a view cell
previously described49 showed that RuCl3$xH2O at these condi-
tions is soluble in the 10% EtOH–CO2 mixture but it starts to
decompose at temperatures as low as 60 ꢀC. In one experiment
with RuCl3$xH2O, the mass of support was reduced to 50 mg to
increase the precursor to support molar ratio to 1 : 6 but
assuring complete precursor solubility. The system was kept
under stirring at 35 ꢀC and 8.5 MPa for 2 hours to promote
dissolution of the precursor in the supercritical mixture and its
impregnation on the support. Then the reactor was heated at
150–200 ꢀC for another 2–4 hours for its decomposition. During
these experiments the pressure was kept below 30.0 MPa (which
is the maximum pressure rating of the equipment) by venting a
small amount of the CO2 solution from 100 ꢀC. Then, the heater
was turned off and the reactor was depressurized through a
needle valve in 1 hour.
2.4. Catalytic tests
Catalytic tests of selected materials were performed.‡ The
catalytic hydrogenations of benzene (Scheme 2) and limonene
(Scheme 3) were chosen as model reactions. In both reactions
the intermediate hydrogenated compounds are difficult to
obtain using other conventional hydrogenation catalysts such
as Pt and Pd.
The hydrogenation of benzene was carried out using a ca.
10 mL high-pressure batch reactor constructed from Swagelok 3/4
inch stainless steel pipe. The reactor was connected to a pressure
transducer and provided with a Swagelok safety valve. Two
different procedures were used: hydrogenation without solvent in
pure H2 and hydrogenation in aqueous ZnSO4 solution.
For the hydrogenation of benzene in pure H2, a given
amount of Ru-catalyst (30–50 mg, depending on the catalyst)
and 2 mL of benzene were charged into the reactor along with a
stirring bar. The reactor was sealed and purged with H2 at low
pressure several times. The reactor was heated at 40 ꢀC using a
Teon heating tape (Omegalux SRT051-040) connected to a PID
controller (Microomega, model CN77322) using a type J cali-
brated thermocouple attached to the reactor wall. To start the
reaction, H2 was added to the reactor up to a pressure of
2.0 MPa and kept at these conditions under stirring for a given
time (15–40 minutes). During the catalytic test, the reactor was
connected to a H2 reservoir to keep pressure constant. The
reaction was terminated by removing the heating tape and
immersing the reactor into an ice bath. Then the system was
quickly depressurised. In other experiments a 60 mL custom
made high-pressure stainless steel reactor heated with a custom
made furnace was employed following the same procedure.
In order to improve selectivity to cyclohexene, the hydroge-
nation of benzene in aqueous ZnSO4 solutions was also tried. It
has been reported that this salt is chemisorbed on the surface of
the Ru catalyst increasing the hydrophilicity of the catalyst and
that, in the presence of a water layer, induces desorption of the
partial hydrogenated product before complete reduction,
improving selectivity towards cyclohexene.52 The procedure
used in this case is described next. A given amount of Ru-
catalyst (30–50 mg, depending on the catalyst), along with
1.0 mL of benzene and 2.0 mL of a 0.400 M ZnSO4 solution were
charged into the 60 mL high-pressure stainless steel reactor
along with a stirring bar. The reactor was sealed and purged
2.3. Materials characterization
Materials were characterized using transmission electron
microscopy (TEM), N2-adsorption, X-ray diffraction (XRD),
thermogravimetric analysis (TGA). Selected samples were
studied by X-ray uorescence spectrometry (XRF). TEM were
carried out on a JEOL JEM 2100 electron microscope working at
200 kV and a JEOL-JEM 3000F electron microscope operating at
300 kV. Both TEM microscopes were equipped with a double tilt
specimen holder (ꢂ25ꢀ) and Energy-dispersive detection X-ray
analysis (EDX) (Oxford INCA). Samples were dispersed in 1-
butanol over copper grids and dried in air. N2 adsorption–
desorption isotherms at 77 K were obtained using a Micro-
meritics ASAP-2020. Prior to adsorption measurements,
samples were out-gassed at 110 ꢀC and ꢃ10ꢁ1 Pa for 6 h.
Isotherms were analysed using standard procedures. The BET
equation was used for specic surface calculations.50 The total
pore volume was estimated from the amount adsorbed at a
relative pressure of 0.995. The pore size distributions were
calculated using the Barrett, Joyner and Halenda (BJH) method
for a cylindrical pore model51 corrected by the statistical thick-
ness using the adsorption and desorption branches of the
isotherms. The actual pore size was estimated from the
adsorption branch.
Wide angle XRD patterns of the composite materials were
collected using a X'PERT MPD diffractometer with Cu Ka radi-
ation on the conventional Bragg–Brentano geometry at 2q
values between 10 and 80ꢀ. TGA of the impregnated supports Scheme 2 Reaction pathway for the hydrogenation of benzene.
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and limonene hydrogenation reactions, respectively. The solid
catalyst was separated from the reaction mixture by ltration
and when necessary, organics were recovered by liquid extrac-
tion using a separating funnel. Reaction products were analysed
by a GC-2010 Plus Shimadzu gas chromatograph equipped with
a ame ionization detector (FID). A Zebron ZB-1HT capillary
column (20 m ꢄ 0.18 mm i.d. ꢄ 0.18 mm lm thickness) was
used for the separation. N2 was used as carrier gas. For the
hydrogenation of benzene, oven temperature was programmed
at 35 ꢀC for 10 minutes. Injector and detector temperature was
ꢀ
280 C with a split ratio of 150. Identication of the products
Scheme
3 Main reaction pathway for the hydrogenation of
was performed by comparison with high-purity standards. In
the hydrogenation of limonene, the oven temperature was
programmed from 87–91 ꢀC ramp at 0.5 ꢀC minꢁ1, and 91–240
ꢀC ramp at 20 ꢀC minꢁ1. Injector and detector temperature was
limonene.58
with low pressure H2 several times. Then the reactor was heated
to 150 ꢀC with a custom made furnace connected to a PID
controller (Microomega, model CN77322) using a type J cali-
brated thermocouple attached to the reactor wall. At this
temperature, H2 was added to the reactor until a pressure equal
to 4.0 MPa. Reaction was kept at these conditions under stirring
for 40 minutes. The reaction was terminated by quickly
removing the reactor from the furnace and immersing it into an
ice bath.
ꢀ
250 C with a split ratio of 300. A GC/MS CP-3800 coupled to a
MS Varian model Saturno 2200 Ion Trap equipped with a
Zebron ZB-5MS capillary column (30 m ꢄ 0.25 mm i.d. ꢄ
0.25 mm lm thickness) was used for the product identication.
He gas was used as carrier at 1 mL minꢁ1. Oven temperature
ꢀ
ꢀ
was programmedꢁa1t 55 C for 2 minutes, then from 55–80 C
ꢁ1
ramp at 3 C min and 80–290 ꢀC ramp at 20 C min
.
ꢀ
ꢀ
The hydrogenation of limonene was performed in scCO2 in a
batch reactor. A given amount of Ru-catalyst (20–50 mg,
depending on the catalyst), 1 mL of limonene and a stirring bar
were placed in the 60 mL high-pressure reactor. The reactor was
sealed and purged with H2 at low pressure several times. Then,
the reactor was heated at 50 ꢀC and 16.0 MPa of CO2 were added
from a thermostated ISCO syringe pump (Isco, Inc. Model
260D). The reaction was started by adding 4.0 MPa of H2 to the
reactor and was kept at these conditions under stirring for 15–
60 minutes. The reaction was terminated by quickly removing
the reactor from the furnace and immersing it into an ice bath.
Reaction mixture was recovered by washing the reactor with
small amounts of dichloromethane or hexane for the benzene
3. Results and discussion
Ru was deposited on mesoporous SiO2 SBA-15 using scCO2
following the different procedures previously outlined. The
materials were then tested in hydrogenation reactions.
A summary of the experiments conducted is given in Table 1.
3.1. Ru deposition experiments by impregnation
Deposition experiments by the impregnation method were
performed using Ru(tmhd)2(COD) on SiO2 at 80 ꢀC and 13.5 and
19.3 MPa. The amount of precursor adsorbed on the support
was determined by TGA analysis of the impregnated samples in
N2 ow and values close to 30 and 13 mass% were obtained for
Table 1 Summary of Ru deposition experiments on SiO2 SBA-15 using scCO2
SiO2
Ru/SiO2
Methodology
Sample scCO2
% Ru mol SBET
(EDX)
Pore size Vp
SBET
Pore size Vp
(m2 gꢁ1
)
(nm)
6.8
6.8
6.7
6.8
6.8
7.0
(cm3 gꢁ1
)
(m2 gꢁ1
)
(nm)
6.6
6.4
6.5
6.1
6.2
6.4
(cm3 gꢁ1
)
a
Precursor
Ru : SiO2
1
2
3
4
5
6
Impregnation 80 ꢀC Ru(tmhd)2(COD) 1 : 19
1.5
571
0.80
0.80
0.81
0.80
0.80
0.78
571
0.78
0.73
0.66
0.47
0.67
0.69
13.5 MPa
Impregnation 80 ꢀC Ru(tmhd)2(COD) 1 : 19
19.3 MPa
0.9
571
544
H2 reactive
Ru(tmhd)2(COD) 1 : 20
6.0
581
438
deposition 150 ꢀC
EtOH reactive
RuCl3$xH2O
RuCl3$xH2O
RuCl3$xH2O
1 : 48
1 : 28
1 : 6
0.8
571
447
deposition 150 ꢀ
EtOH reactive
C
C
C
3.2
571
470
deposition 200 ꢀ
EtOH reactive
7.4
564
435
deposition 150 ꢀ
a
Initial Ru : SiO2 molar ratio.
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samples 1 and 2 obtained at 13.5 and 19.3 MPa, respectively at
the same Ru : SiO2 molar ratio (see ESI†). Considering that the
total weight loss is due to the precursor ligands, the amount of
Ru(tmhd)2(COD) remaining in CO2 aer adsorption at these
conditions is below its solubility limit.49 The amount adsorbed
decreased as the pressure and density of the supercritical phase
increased, in agreement with previous reports.15 At high pres-
sure, both the solubility of the precursor in the uid phase and
the concentration of CO2 increased and consequently, the
partition coefficient of the precursor changed, lowering its
adsorption on the surface.
Aer impregnation, samples were reduced in N2/H2 at
ꢀ
400 C. XRD analysis of samples 1 and 2 obtained by impreg-
nation at 80 ꢀC and 13.5 and 19.3 MPa and further reduction are
shown in Fig. 1. Wide angle XRD reveals the presence of a very
broad and intense reection at 2q ca. 22 which is due to the
amorphous SiO2 support, as well as minor peaks at ca. 42 and
44ꢀ assigned to the (002) and (101) reexions of hexagonal Ru
(PDF 06-0663). Ru peaks are very broad suggesting that particles
are very small. The intensity of the Ru peaks is much lower for
sample 2 impregnated at the higher pressure.
TEM images of the Ru/SiO2 samples 1 and 2 obtained by
impregnation in scCO2 at 80 ꢀC and further reduction in H2/N2
are shown in Fig. 2. Mesoporous silica SBA-15 is a highly porous
support formed by an hexagonal array of one-dimensional
cylindrical mesopores interconnected through smaller micro
and mesopores.53,54 The mesoporous channels of the SiO2
SBA-15 structure along with small darker Ru nanoparticles may
be observed in Fig. 2. Particles are very small, more or less
spherical and homogeneously dispersed. Similar results have
been previously obtained for the deposition of Pd nanoparticles
on mesoporous SiO2.15,55 The particle diameter is well below the
pore size of the support, particularly for sample 2 impregnated
at the higher pressure. TEM images clearly show that the
number and size of the Ru nanoparticles are much larger when
the impregnation is performed at the lower pressure in agree-
ment with previous results. At the 1 : 19 Ru : SiO2 molar ratio,
Fig. 2 TEM images of Ru/SiO2 SBA-15 samples obtained by impreg-
nation of Ru(tmhd)2(COD) in scCO2 at 80 ꢀC and: (a and b) 13.5 MPa
(sample 1) and (c and d) 19.3 MPa (sample 2), after reduction in H2/N2.
the Ru content determined by EDX analysis varied from 0.9 to
1.5% mol for the samples prepared at 80 ꢀC and 13.5 and
19.3 MPa, respectively (average of several images).
For selected samples the Ru content was also determined by
XRF as previously described. For a sample containing a 0.9% Ru
mol determined by EDX, 1.2% Ru mol was measured by XRF.
Considering the good agreement among the different tech-
niques and the errors associated to each of them, for compar-
ison purposes, the % Ru determined by EDX was used to
quantify the Ru content in the samples as shown in Table 1.
The expected Ru mol percentage considering the precursor
uptake measured by TGA (see ESI†) was higher than the Ru mol
percentage measured in the samples reduced by the different
techniques, which indicated the partial loss of the precursor
during the thermal reduction. TGA analysis of the precursor
Ru(tmhd)2(COD) showed that this compound sublimes between
200–275 ꢀC in N2. However, TGA analysis of a SiO2 support
impregnated with Ru(tmhd)2(COD) revealed different weight
loss events, the rst one related to the sublimation of the
precursor adsorbed on SiO2 at temperatures below 250 ꢀC.
At higher temperatures, the weight loss is associated to the
decomposition of the precursor to its metal form. In contrast,
when Ru(tmhd)2(COD) was adsorbed on CA in scCO2, the
sublimation of the precursor did not take place in N2 atmo-
sphere.23 These results indicate that the hydrophilic SiO2
support interacts weakly with the precursor.
3.2. Ru deposition experiments by reactive deposition with
H2
Fig. 1 XRD of the Ru/SiO2 SBA-15 samples obtained by impregnation
of Ru(tmhd)2(COD) in scCO2 at 80 ꢀC and: (a) 13.5 MPa (sample 1) and
(b) 19.3 MPa (sample 2), after reduction in H2/N2.
H2-reduction of Ru(tmhd)2(COD) on SiO2 SBA-15 was also per-
formed. The precursor dissolution was carried out at 80 ꢀC and
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3.3. Ru deposition experiments by reactive deposition with
EtOH
The deposition of Ru on SiO2 SBA-15 was also attempted in
CO2–EtOH mixtures with EtOH as the reducing agent. Experi-
ments were performed using Ru(tmhd)2(COD) and RuCl3$xH2O
as precursors. Dissolution was carried out at 80 ꢀC and 13.5 MPa
for Ru(tmhd)2(COD) and at 35 ꢀC and 8.5 MPa for RuCl3$xH2O
in the EtOH–CO2 mixture. In both cases the reduction was
initiated by heating the reaction mixture. Experiments per-
formed using Ru(tmhd)2(COD) showed the incomplete decom-
position of the precursor even at 200 ꢀC, which could be
assessed by the weak colour change of the SiO2 support from
white to slightly grey and the dark brown colour of the solution
obtained aer venting the supercritical phase through acetone
(not shown in Table 1). On the contrary, Ru was successfully
deposited on the support from RuCl3$xH2O at 150–200 ꢀC in the
supercritical mixture (10% mol EtOH in CO2) yielding dark grey/
black products (samples 4–6).
Fig. 3 XRD pattern of a Ru/SiO2 SBA-15 sample obtained by the H2-
reduction of Ru(tmhd)2(COD) in scCO2 at 150 ꢀC (sample 3).
XRD patterns of the different Ru/SiO2 samples obtained
following this procedure are shown in Fig. 5. XRD of the samples
shown in Fig. 5a and b were obtained using similar or lower
RuCl3$xH2O to support molar ratios than in previous experi-
ments (1 : 28 and 1 : 48) and at the decomposition temperature
of 150 and 200 ꢀC, samples 4 and 5 respectively. However, XRD of
sample 6 shown in Fig. 5c corresponds to a material obtained at
the lower temperature using a much higher precursor to support
ratio (1 : 6). Apart from the wide reection of the support, XRD of
the sample obtained with the higher precursor to support molar
ratio (sample 6) showed the presence of very broad bands at 2q
values ca. 38.4, 42.2 and 44.0 due to Ru (PDF 06-0663). In
contrast, the samples produced at the lower ratios at both
temperatures (samples 4 and 5) did not show clearly the pres-
ence of Ru in the XRD pattern, which may be related to the low
metal concentration and the small metal particle size.
13.5 MPa and the reduction was performed at 150 ꢀC in the
H2–CO2 mixture. XRD analysis of a Ru/SiO2 SBA-15 sample
obtained following this procedure (sample 3) is shown in Fig. 3,
showing strong reections due to Ru (PDF 06-0663). The
intensity of the peaks is much higher than that observed in the
samples obtained by impregnation. Nevertheless, the peaks are
very broad, which suggests that particles are very small.
TEM images of sample 3 obtained by H2-reduction of
Ru(tmhd)2(COD) are shown in Fig. 4. As in previous examples,
small dark Ru nanoparticles are deposited into the mesopores
of the support. The size of the particles is constrained by the
pore size of the support. The average Ru content by EDX on this
sample was ca. 6.0% mol Ru. Comparison with images shown in
Fig. 2a and b for materials obtained by impregnation in scCO2
at 80 ꢀC and 135 bar and reduction in H2/N2 revealed that
particles in Fig. 4 have grown slightly and turned into small
rods. The deposition of Ru is a self-catalytic process and once it
is started, the small Ru nanoparticles act as catalytic centres for
the precursor reduction. Nevertheless, this material remained
very homogeneous. Considering the amounts of Ru(tmhd)2-
(COD) and substrate loaded into the reactor, the precursor
decomposition in H2–CO2 is complete.
Fig. 5 XRD pattern of the Ru/SiO2 SBA-15 samples obtained by the
alcohol reduction of RuCl3$xH2O in scCO2 at different temperatures
and/or precursor to support molar ratios: (a) 150 ꢀC and 1 : 48
Fig. 4 TEM images of a Ru/SiO2 SBA-15 sample obtained by the H2- Ru : SiO2 molar ratio (sample 4), (b) 200 ꢀC and 1 : 28 Ru : SiO2 molar
reduction of Ru(tmhd)2(COD) in scCO2 at 150 ꢀC (sample 3).
ratio (sample 5), (c) 150 ꢀC and 1 : 6 Ru : SiO2 molar ratio (sample 6).
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Fig. 6 shows TEM images of the samples obtained by this
RSC Advances
Ru nanoparticles for the samples deposited at 150 ꢀC and the
method at 150 and 200 ꢀC. In every case, mesopores in the different concentrations were homogeneously distributed
support were lled with Ru metal nanoparticles whose size was throughout the support. In contrast, in sample 5 obtained at
limited by the pore size of the support. The amount of Ru 200 ꢀC, Ru nanoparticles arranged together forming long
determined by EDX in the different samples varied as a function nanowires. Ru nanoparticles deposited acted as catalytic sites
of deposition temperature and concentration. The amount of for the precursor reduction and less homogeneous materials
ꢀ
Ru deposited at 150 C increased as the Ru : SiO2 molar ratio were obtained at 200 ꢀC. At 150 ꢀC this effect was not so
increased, and values of 0.8% and 7.4% mol Ru were measured important and uniformly distributed particles were obtained at
by EDX for sample 4 (Fig. 6a and b) and sample 6 (Fig. 6e and f), the different compositions studied.
respectively. These values were lower than those expected taking
EDX analysis revealed the presence of chlorine impurities in
into account the amounts of RuCl3$xH2O and support loaded the different samples deposited using EtOH, which may be due
into the reactor; 2.0 and 10.0% Ru mol for samples 4 and 5, to the unreacted precursor or to reaction by-products impurities.
ꢀ
respectively. The lower Ru loads obtained at 150 C indicated
the incompꢀlete precursor decomposition. For sample 5 depos-
3.4. N2 adsorption–desorption isotherms
ited at 200 C, the percentage Ru mol determined by EDX was
equal to 3.2% (Fig. 6c and d). This value is very similar to the The porosity of the Ru/SiO2 SBA-15 composite materials was
maximum expected (3.4% mol). Increasing the deposition further studied by N2-adsorption. Table 1 shows SBET, pore
temperature at 200 ꢀC favours the incorporation of the metal volume (Vp) and pore size obtained from the adsorption
into the support.
isotherms for the SiO2 and Ru/SiO2 samples. Fig. 7 compares the
adsorption isotherms and pore size distributions of the Ru/SiO2
samples prepared by impregnationꢀ at 80 ꢀC and 13.5 MPa
(sample 1), and H2 reduction at 150 C (sample 3). Data for the
rest of the samples presented in Table 1 are given as ESI.†
Isotherms exhibit a type IV, subtype H1, hysteresis loop which is
found in mesoporous materials with well-dened cylindrical-like
pore channels. BET surface area of the support was 571 m2 gꢁ1
and pore volume was 0.80 cm3 gꢁ1. Adsorption isotherms for the
samples produced by impregnation at 13.5 and 19.3 MPa
(samples 1 and 2) were very similar to those of the support (for
clarity not shown here), particularly in the adsorption branch of
the isotherms. The desorption branch of the isotherm however
changed, as it will be explained later, due to the presence of Ru
nanoparticles into the mesopores. Due to the small amount of
Ru deposited and the small particle size, deposition of Ru by
impregnation into the support (samples 1 to 2) led to SBET and Vp
values very similar to those of the SiO2 sample. However, SBET
measured for sample 3 obtained by H2-reduction was reduced to
438 m2 gꢁ1 and the pore volume to 0.66 cm3 gꢁ1 most likely due
to the much larger Ru content. Analysis of the pore size distri-
bution of the SiO2 SBA-15 support obtained from the adsorption
branch of the isotherm gave a narrow pore size distribution with
a maximum at 6.8 nm. In comparison, there was only a slight
reduction of the pore size in all the Ru/SiO2 samples.
Similar results were found for samples 4 to 6 reduced using
EtOH (see Table 1 and ESI†). For sample 6, with the highest Ru
content, SBET was reduced to 435 m2 gꢁ1 and the pore size
decreased from ca. 7.0 to 6.4 nm. For samples 5 and 6, SBET was
reduced in a similar way. Comparison of samples 2 and 4,
containing very similar Ru mol percentages but prepared by
different techniques, showed much lower SBET and Vp values in
sample 4 deposited using EtOH. This sample showed the
highest reduction in the pore volume. This could be related to
the presence of unreacted RuCl3$xH2O or reaction by-products
in the samples deposited using EtOH that were already detec-
ted by EDX and suggests the need to incorporate a washing step
when the reaction is performed by reactive deposition with
EtOH.
Fig. 6 TEM images of the Ru/SiO2 SBA-15 samples obtained by the
alcohol reduction of RuCl3$xH2O in scCO2 at different temperatures
and/or precursor to support molar ratios: (a and b) 150 ꢀC and 1 : 48
Ru : SiO2 molar ratio (sample 4), (c and d) 200 ꢀC and 1 : 28 Ru : SiO2
molar ratio (sample 5), (e and f) 150 ꢀC and 1 : 6 Ru : SiO2 molar ratio
(sample 6).
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Fig. 7 N2 Adsorption–desorption isotherms (a) and pore size distributions obtained from the adsorption (b) and desorption (c) branches of the
isotherm for: (B) Ru/SiO2 SBA-15 obtained by impregnation of Ru(tmhd)2(COD) in scCO2 at 80 ꢀC and 13.5 MPa and further reduction in H2/N2
(sample 1) and (,) Ru/SiO2 SBA-15 obtained by H2-reduction of Ru(tmhd)2(COD) in scCO2 at 150 ꢀC (sample 3).
The pore size distributions estimated from the desorption
Total conversion (% C), selectivity (% Si) and yield (% Yi) of
branch of the isotherm of all the Ru/SiO2 composite materials product i were dened as:
X
showed a new maximum at ca. 3.6 nm. This phenomenon is
oen referred as tensile strength effect and it is due to ink-bottle
like sections created by the nanoparticles in the mesopores.
CP
Ci
X
X
Cð%Þ ¼
ꢄ 100% Sið%Þ ¼
ꢄ 100%
ꢄ 100%
CP þ CRun
CP
Similar adsorption–desorption isotherms have been previously
reported for partially plugged hexagonal templated silica
(PHTS).54 The maximum at 3.6 nm depends on the adsorptive
used (in our case N2). These data indicate that the presence of Ru
nanoparticles narrows at least part of the mesopores in SBA-15 in
all the samples. However, the fact that the pore volume remains
high in the Ru/SiO2 samples indicates that the pores are still
accessible to the gas molecules aer deposition. The inter-
connected mesopores in SiO2-SBA-15 may facilitate this process.
Ci
X
Yið%Þ ¼
CP þ CRun
where Ci is the concentration of the intermediate product
P
(cyclohexene or p-menthene), CP is the total concentration of
the hydrogenation products and CuRn is the concentration of the
unreacted reactant. Quantication was performed by integrated
peak area normalization.
The Turnover Frequency (TOF) was estimated considering
the Ru content measured by EDX as follows
mole reactant ꢄ Cð%Þ
TOF ¼
3.5. Catalytic tests
mol Ru ꢄ timeðhÞ
The catalytic performance of some of the Ru/SiO2 SBA-15 samples
synthesized using supercritical CO2 was assessed for the hydro-
genation reactions of benzene and limonene (Schemes 2 and 3,
respectively). Tables 2 and 3 summarise the results obtained. The
catalytic activity was compared to that of a 5% mass Ru on carbon
commercial catalyst purchased from Strem Chemical.
Table 2 summarizes the benzene hydrogenation catalytic
tests. Cyclohexene and cyclohexane were identied by GC-FID
as the only reaction products, by comparison of the retention
Table 2 Summary of the benzene hydrogenation experiments using different Ru catalysts
Time
(min)
C
(%)
Sb
(%)
Yb
(%)
TOF ꢄ 10ꢁ3
Method
Sample
3
Methodology scCO2
% Ru EDXa
Benzene : Ruc
(hꢁ1
)
(1) No solvent
H2 reactive deposition
9.7
5.0
9.7
5.0
15
40
15
40
40
<1
64
7
100
17
0
2
0
0
0
1
0
0
860
800
1040
1120
890
—
150 ꢀ
Ru/C
C
76
27
170
23
Strem
3
(2) ZnSO4 (aq.)
H2 reactive deposition
100
17
150 ꢀ
Ru/C
C
Strem
40
30
0
0
500
22
a
Percentage by mass. b S and Y for cyclohexene. c Molar ratio.
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Table 3 Summary of the limonene hydrogenation experiments using different Ru catalysts
Time
(min)
C
(%)
Sb
(%)
Yb
(%)
TOF ꢄ 10ꢁ3
Sample
3
Methodology scCO2
% Ru EDXa
9.7
Limonene : Ruc
(hꢁ1
)
H2 reactive deposition 150 ꢀ
C
60
30
15
15
60
30
15
100
100
90
100
100
100
93
4
4
480
560
660
550
420
540
540
48
63
77
0
6
41
62
63
69
0
6
41
58
110
240
220
42
110
200
1
Impregnation 80 ꢀC and 13.5 MPa
Ru/C
2.5
5.0
Strem
a
Percentage by mass. b S and Y for p-menthene. c Molar ratio.
times with external standards. Experiments were performed catalyst gave a yield to p-menthene equal to 63%. On the other
without solvent at 40 ꢀC and in a ZnSO4 aqueous solution at hand, the catalyst obtained by impregnation in scCO2 (sample
150 ꢀC for 15 and 40 minutes. The reaction without solvent 1) rendered a conversion equal to 100% aer 15 minutes but no
proceeded at 40 ꢀC to completion to the fully hydrogenated selectivity to p-menthene.
product cyclohexane in 40 minutes when the commercial Ru/C
For the commercial Ru/C and the Ru/SiO2 catalyst obtained
catalyst was used. Comparatively, the Ru/SiO2 catalyst prepared by H2-reduction (sample 3), the conversion decreased to 90% at
by reactive deposition in H2–CO2 (sample 3) gave a lower 15 min, but the selectivity increased. The highest selectivity was
conversion without much selectivity to cyclohexene. Diffusion obtained for the Ru/SiO2 catalyst obtained by H2-reduction with
of reactants to the inner surface of the mesoporous catalyst may a value close to 80%, which represents a yield to p-menthene of
be hindered in the liquid phase at such a low temperature. On 69%. This reaction yield is larger than those obtained by Nunes
the other hand, when the reaction was carried out in the ZnSO4 da Ponte and co-workers at very similar conditions using other
aqueous solution at 150 ꢀC using the same Ru/SiO2 catalyst, Pd/C and Pt/C catalysts44,45,58,59 and slightly lower than those
although the conversion was low, total selectivity to cyclohexene obtained by the same group for Ru/Al2O3 coated with screened
was obtained. The conversion was higher when the Ru/C cata- imidazolium ionic liquids catalysts using larger Ru to limonene
lyst was used, but the process was not selective to cyclohexene. ratios.46
Furthermore, the benzene to Ru molar ratio was lower and as a
The results suggest a correlation between conversion, selec-
result the TOF was lower. However, cyclohexene yield using the tivity and Ru particle size. The particle size in the catalyst
Ru/SiO2 SBA-15 catalyst prepared in this study was lower than obtained by impregnation in scCO2 (sample 1) is very small and
those obtained using other Ru/SiO2 catalysts at similar condi- the metal dispersion is very good, and as a result the conversion
tions.56 The partial degradation of the SiO2 SBA-15 support at to the fully hydrogenated product is very high aer 15 minutes.
the reaction conditions in the aqueous medium may cause the On the other hand, the catalyst obtained by H2-reduction
lower conversion.57 Due to the limitations of the support for this (sample 3) is not so active but more selective to the intermediate
reaction, no further optimization of the process was performed. product. Ru nanoparticles in this case are larger and slightly
The Ru/SiO2 materials obtained by the alcohol reduction in elongated. Other authors have suggested a similar relationship
scCO2 were also tested in the benzene hydrogenation. Conver- between selectivity and particle size for other selective hydro-
sion in this case was very low even for the catalysts with a very genation reactions.60 Further experiments should be conducted
high Ru content, which could be related to the presence of in order to conrm these ndings.
chlorine impurities in these samples (determined by EDX).
Further washing of the samples in this case would be required.
4. Conclusions
To avoid the use of water and to improve the mass transport,
the hydrogenation of limonene at 50 ꢀC in scCO2 was chosen as The deposition of Ru nanoparticles on a mesoporous SiO2 SBA-
a model reaction. Table 3 summarises the results. Reactions 15 support has been successfully carried out using
were performed for 15, 30 and 60 min employing the Ru/SiO2 Ru(tmhd)2(COD) and RuCl3$xH2O in scCO2. Three different
catalysts produced by H2-reaction (sample 3) and impregnation reaction routes have been tried: impregnation in scCO2 and
(sample 1) and were compared to the same reaction using the reduction in H2/N2 at low pressure, reactive deposition with H2
commercial Ru/C catalyst. Reaction products are the interme- in scCO2, and reactive deposition with ethanol in scCO2. In
diate product p-menthene (p-menth-1-ene and p-menth-3-ene) every case, Ru nanoparticles were deposited within the meso-
and the fully hydrogenated compounds cis-p-mentane and pores of the SiO2 SBA-15. When scCO2 was used only as
trans-p-menthane in a 4 : 5 ratio in agreement with previous impregnation medium, Ru load was controlled by the adsorp-
reports.46 Complete conversion was achieved with every catalyst tion equilibrium of the precursor on the support and, at 80 C
ꢀ
at 60 and 30 min. At these reaction times, selectivity to p-men- samples with 1.5 and 0.9% mol Ru were obtained at 13.5 and
thene was very low for the Ru/C catalyst but it was higher for the 19.3 MPa, respectively. Particles were very small and appeared
Ru/SiO2 catalyst obtained by H2-reduction. Aer 30 min, this very homogeneously distributed throughout the mesoporous
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support. Furthermore, due to the small particle size, there was
almost no reduction of the BET surface area. Similar Ru/SiO2
SBA-15 materials were obtained by the reactive deposition with
3 P. Kluson, J. Had, Z. Belohlav and L. Cerveny, Appl. Catal., A,
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at very mild conditions (150–200 C). This is a very promising
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and easier to handle than organometallic compounds. In this
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RuCl3$xH2O salt in scCO2 and, at the same time, the reducing
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´
catalyst prepared by H2-reduction in scCO2 was more selective 16 Y. Sanchez-Vicente, S. L. C. Pando, M. J. Torralvo,
˜
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´
´
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´
Espectrometrıa de Masas” at UCM for technical assistance.
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