Iridium catalysts for C-C and C-O hydrogenolysis: Catalytic consequences of iridium sites

Article abstract of DOI:10.5935/0103-5053.20150354

The support effect on the properties of iridium catalysts for C-C and C-O hydrogenolysis was investigated. Cyclohexane conversion and glycerol hydrogenolysis were used to compare the behavior of iridium catalysts in terms of C-C and C-O cleavage. The nature of the support influenced the catalyst performance in both cyclohexane conversion and glycerol hydrogenolysis. This effect was more evident on the product selectivity. Ir/SiO₂ catalyst presented the highest cyclohexane hydrogenolysis activity and the highest selectivity to minor hydrocarbons formed from hexane re-adsorption. For glycerol hydrogenolysis, all catalysts displayed a higher selectivity to products formed by C-O cleavage, mainly 1,2-propanediol (1,2-PDO). Ir/ZrO₂ catalyst presented the highest activity in all reaction conditions and the lowest selectivity to minor alcohols produced by C-C cleavage like methanol, ethanol and ethylene glycol. The results were explained in terms of the requirements and the structure of the catalytic sites. Printed in Brazil -

Full text of DOI:10.5935/0103-5053.20150354

http://dx.doi.org/10.5935/0103-5053.20150354
J. Braz. Chem. Soc., Vol. 27, No. 5, 958-966, 2016.
Printed in Brazil - ©2016 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
Iridium Catalysts for C–C and C–O Hydrogenolysis: Catalytic Consequences of
Iridium Sites
Aracelis J. Pamphile-Adrián, Pedro P. Florez-Rodriguez and Fabio B. Passos*
Departamento de Engenharia Química e de Petróleo, Universidade Federal Fluminense,
Rua Passos da Pátria, 156, 24210-040 Niterói-RJ, Brazil
The support effect on the properties of iridium catalysts for C–C and C–O hydrogenolysis
was investigated. Cyclohexane conversion and glycerol hydrogenolysis were used to compare
the behavior of iridium catalysts in terms of C–C and C–O cleavage. The nature of the support
influenced the catalyst performance in both cyclohexane conversion and glycerol hydrogenolysis.
This effect was more evident on the product selectivity. Ir/SiO2 catalyst presented the highest
cyclohexane hydrogenolysis activity and the highest selectivity to minor hydrocarbons formed from
hexane re-adsorption. For glycerol hydrogenolysis, all catalysts displayed a higher selectivity to
products formed by C–O cleavage, mainly 1,2-propanediol (1,2-PDO). Ir/ZrO2 catalyst presented
the highest activity in all reaction conditions and the lowest selectivity to minor alcohols produced
by C–C cleavage like methanol, ethanol and ethylene glycol. The results were explained in terms
of the requirements and the structure of the catalytic sites.
Keywords: iridium catalysts, hydrogenolysis, C–C cleavage, C–O cleavage, support effect
Introduction
of cyclohexane hydrogenolysis activity and reaction
mechanism on factors like dispersion, particle size and
H2/cyclohexane ratio.13,14,21 Locatelli et al.21 investigated
cyclohexane hydrogenolysis over Ir/SiO2 and proposed a
reaction mechanism in which the key step is a concerted
electronic transfer in a dimetallacyclopentane intermediate.
Thus, after hexane formation subsequent hydrogenolysis
steps define the minor hydrocarbons that are formed.
On the other hand, less attention have been paid to
fundamental investigation of C–O hydrogenolysis over Ir
catalysts. Ir-ReOx/SiO2 are active in C–O hydrogenolysis
of substrates like glycerol, tetrahydrofurfuryl alcohol,
tetrahydrofuran and tetrahydropyran.22-26 Among these
substrates, glycerol is a versatile feedstock that is
mainly obtained as a by-product of biodiesel production.
Consequently, due to the fast growth of biodiesel production
worldwide, catalytic transformation of glycerol into high-
value products has gained importance in the last decade.
Glycerol hydrogenolysis may yield 1,2- and
1,3-propanediol (1,2-PDO and 1,3-PDO), high-value raw
materials for the chemical industry.27 The formation of these
and other alcohols requires one or more C–O cleavages,
and particularly, 1,3-PDO is difficult to obtain due to the
difficult cleavage of the secondary C–O bond.
Iridium catalysts have been tested for several reactions
like isomerization,1,2 hydrogenation,3-6 hydrazine
decomposition and several authors reported their high
activity for hydrocarbon hydrogenolysis reactions.7-12
For example, McVicker et al.12 studied the selective
rupture of naphthenic rings (methylcyclopentane
(MCP), methylcyclohexane (MCH), 1,1-, 1,2- and
1,3-dimethylcyclopentane (1,1-DMCP, 1,2-DMCP and 1,3-
DMCP), pentylcyclopentane (PCP) and ethylcyclopentane
(ECP)) on Pt/SiO2, Pt/Al2O3, Ni/Al2O3, Ru/Al2O3, Rh/Al2O3
and Ir/Al2O3. Among the mentioned catalysts, Ir/Al2O3
presented the best activity and selectivity for cleavage of
non-substituted C–C in five-membered naphthenic rings
as MCH and PCP.12
Cyclohexane, a model six-membered naphthenic
molecule, has been used to study the direct endocyclic C–C
bond cleavage.13,14 Additionally, conversion of cyclohexane
is typically used to probe the catalyst surface properties
such as apparent dispersion (dehydrogenation to benzene)
and surface structure (hydrogenolysis).15-20
Several studies of cyclohexane conversion on Ir
catalysts were performed to understand the dependence
Nakagawa et al.22 studied the 1,3-PDO production
over Ir-ReOx/SiO2 and found the addition of several acids
*e-mail: fbpassos@vm.uff.br

Vol. 27, No. 5, 2016
Pamphile-Adrián et al.
959
as H2SO4, silica-alumina, zeolites and ion-exchange resin
promoted the activity of the catalyst.24 According to these
authors, the mechanism of glycerol hydrogenolysis to
1,3-propanediol occurs through attack of hydride species
at the interface between Ir metal surface and ReOx to the
adsorbed glycerol on the ReOx cluster.23
In this work, we report cyclohexane conversion and
glycerol hydrogenolysis over iridium catalysts supported
on γ-Al2O3, SiO2 and ZrO2, comparing these catalysts in
terms of C–C and C–O cleavage. We also address the effect
of support on the activity and product selectivity in both
reactions. This study contributes to a better understanding
of the catalytic consequences for both reactions of
modification of Ir catalytic sites.
150 °C for 30 min under He flow (30 mL min-1) and cooled
to room temperature. Then, the samples were submitted to
a 5% H2/Ar gas flow (30 mL min-1) and the temperature
was raised to 1000 °C at a heating rate of 10 °C min-1. For
H2-TPD experiments, the samples were dried as mentioned
before and were reduced at 250 °C. After reduction, a He
stream (30 mL min-1) was passed through the catalyst during
30 min at the reduction temperature and then cooled to
27 °C. H2 pulses from a 100 μL loop were injected in a He
stream (30 mL min-1) until the area of the pulses remained
constant, indicating saturation of the catalyst surface. Then,
the temperature was increased to 1000 °C at a heating rate
of 10 °C min-1, under He flow, while H2 desorption was
monitored.
X-Ray photoelectron spectroscopy (XPS) analysis was
carried out using an ESCALAB 250Xi spectrometer (Thermo
Fisher Scientific) with monochromatic Al Kα X-rays (spot
size of 900 μm). Surface charging of the electrically
insulating materials was compensated using the integrated
flood gun with approximate spot size of 900 μm. Survey
spectra were obtained with a pass energy of 100 eV and
region spectra (Ir4f, Al2p, Si2p, Zr3d) using a pass energy
of 58 eV. The base pressure of the analysis chamber was
7 × 10-10 mbar. Spectra were acquired, analyzed and peak
fitted using theAvantage software with Lorentzian/Gaussian
product function and a Shirley type background. Ir4f spectra
were fitted with an energy constraint for Ir4f5/2 within a
3 eV shift relative to the Ir4f7/2 peak, an area ratio of 0.75:1,
respectively, and a full width at half maximum (FWHM) in
the range 0.5-2.5 eV. The residual standard deviation of the
fitted Ir4f spectra was around 0.9 for all catalysts.
Fresh and used catalysts were characterized by Raman
spectroscopy with a WITec alpha 300 system. Experiments
were performed at 25 °C using 50× objective lens
(numerical aperture = 0.75) and a Nd:YAG green laser with
532 nm wavelength with an aperture of 800 cts to avoid
the decomposition of possible organic compounds in the
sample. The integration time was 0.777 s and the number
of scans was 600.
Experimental
Catalyst preparation
Commercial γ-Al2O3 (Puralox HP-14) and SiO2
(DAVICAT), used as catalyst supports, were calcined at 600
and 900 °C, respectively, during 4 h (5 °C min-1). ZrO2 was
synthesized by precipitation of Zr(OH)4. NH4OH (25% v/v)
was added to an aqueous solution of ZrO(NO3)2∙xH2O until
a pH equal to 14 was reached. The precipitated solid was
filtered, washed with deionized water and dried overnight
at 100 °C. The prepared Zr(OH)4 was calcined at 500 °C
during 4 h (5 °C min-1). The calcination temperatures were
chosen to obtain similar surface areas of the supports. The
catalysts were prepared by incipient wetness impregnation
technique using a solution of H2IrCl6∙xH2O (Sigma-
Aldrich) to yield 2 wt.% Ir. The impregnated solids were
dried at 70 °C for 16 h due to the low melting point of
H2IrCl6∙xH2O (65 °C). Finally, the catalysts were calcined
at 400 °C during 4 h (5 °C min-1).
Catalyst characterization
Nitrogen adsorption isotherms of the samples were
measured using a Micromeritics ASAP 2020 equipment.
Surface areas were calculated using the Brunauer-
Emmett-Teller (BET) equation. X-Ray diffraction (XRD)
experiments were performed using a Miniflex RIGAKU
spectrometer (Cu Kα radiation). The diffractograms were
obtained between 2θ = 10 and 80° using a 0.04° step size
(1 s per step).
Temperature programmed reduction (TPR), H2 pulse
chemisorption and temperature programmed desorption
(H2-TPD) experiments were performed with a multipurpose
unit coupled to a Prisma quadrupole mass spectrometer
(Pfeiffer). For TPR analysis, the samples were dried at
Cyclohexane conversion
This reaction was performed under atmospheric
pressure in a continuous flow microreactor. The pre-
treatment of the samples (10 mg) consisted of drying at
150 °C under 30 mL min-1 of He flow for 30 min, followed
by reduction at 250 °C under H2 flow (30 mL min-1). The
reactant mixture was obtained by bubbling hydrogen
through a saturator containing cyclohexane at 11 °C
(H2/C6H12 = 14.2). The total flow rate was 100 mL min-1.
At these conditions, the conversion was kept below 10%.

960
Iridium Catalysts for C–C and C–O Hydrogenolysis
J. Braz. Chem. Soc.
The reaction system was gradually heated and samples
were collected at 250, 260, 270, 280, 290, 300 and 310 °C.
A descending temperature sequence was also performed
to detect any deactivation. The reaction products were
analysed in a gas chromatograph (GC) HP GC-5890
Series II, equipped with an HP-Innowax capillary column
and a flame ionization detector. The products were also
analysed in a Micro-GC 490 (Agilent), equipped with
three columns: M5A for permanent gases (H2, O2, N2,
CH4 and CO), 5CB for hydrocarbons (C3-C6) and PPU
for CO2 and ethane.
iridium sites. Product selectivity was calculated on carbon
basis using the following equation:
ji × yi
Selectivity (%) =
×100
(2)
ji × yi
∑
i
in which ji is the number of carbons of product i and yi is
the molar fraction of product i.
For all reactions, gas phase products (CO, CO2 and CH4)
were not considered for calculations, since their selectivities
were negligible (< 0.5%).
The conversion was kept below 10%, which enabled
comparison of the selectivities at similar conversions.16 The
selectivity for the hydrogenolysis products was calculated
as the ratio between the amount of cyclohexane converted
to a hydrogenolysis product and the total amount of reacted
cyclohexane, according to the equation below:
Results and Discussion
Characterization of catalysts
X-Ray diffractograms of the catalysts (not shown)
displayed the respective main peaks of the supports: SiO2,
γ-Al2O3 and ZrO2 (tetragonal and monoclinic phases).
However, characteristic peaks of iridium species were
not observed due to the low iridium content or to a high
dispersion of iridium species on the support.28,29
ji × F
i
Selectivity (%) =
× 100
(1)
6F × XA
A0
Hydrogenolysis
in which ji is the number of carbons in product i, Fi is the
exit flow rate of product i, F is the initial flow rate of
XPS spectra for Ir4f of the calcined catalysts are
presented in Figure 1. In the case of the Ir/γ-Al2O3
(Figure 1b), the Ir4f doublet was not resolved, showing
a first evidence of the effect of the support on the
characteristics of the surface of the catalysts. The presence
of two different iridium species on the surface of the catalyst
was evidenced by the deconvolution of the Ir4f peaks. The
two doublets (including Ir4f5/2 and Ir4f7/2) were identified
in Figure 1 as A and B. The doublets identified as A, of
lower energy, can be attributed to surface IrO2.30-32 The
doublets identified as B in all spectra can be attributed to
the presence of surface iridium chloride species (IrCl3 or
IrCl4), as reported by Folkesson33 and Kim and Hatfield.34
The presence of these chloride species was observed in the
survey spectrum of all catalysts, with spectral lines of Cl2p
(200 eV) and Cl2s (270 eV), indicating that after calcination
the chloride species remain on the surface of the catalyst. In
the case of Ir/ZrO2, the spectrum showed a higher amount of
IrCl4 than IrO2, 87 and 13%, respectively (Table 1), which
confirms that the calcination of the catalyst was not efficient
to completely eliminate the metal precursor.
A0
cyclohexane and XA
is the cyclohexane conversion
Hydrogenolysis
to hydrogenolysis products.
Glycerol hydrogenolysis
Glycerol hydrogenolysis reactions were performed in
a 300 mL autoclave reactor (Parr Instruments Co.). The
catalyst was reduced in the autoclave prior the reaction.
The reactor was loaded with the necessary amount of water
and catalyst, and then was filled with H2, with 200 psi of
pressure, the stirring speed was adjusted to 500 rpm and
the temperature was set to 250 °C. After 2 h, the reactor
was cooled to 25 °C, opened, and glycerol was quickly
loaded into the reactor. Then, the reactor was filled with
the desired pressure of H2, both the stirring speed and
pressure were adjusted, and the temperature was set to the
desired value. After the reaction time (12 h), the reactor
was cooled to room temperature, gas phase was collected
and analyzed in the Micro-GC 490. The liquid phase
products were quantified in a gas chromatograph-mass
spectrometer (GC-MS) QP2010Plus (Shimadzu) equipped
with a capillary column (Rtx-Wax). The products were
identified using GC-MS Solutions software comparing
with the National Institute of Standards and Technology
NIST05 and NIST05s libraries. Initial turnover frequency
(TOF) values were estimated for each catalyst from the
initial reaction rate, assuming a first order reaction, and
using H2 chemisorption to estimate the number of surface
Surface and bulk Ir/Si, Ir/Al and Ir/Zr atomic ratios are
shown in Table 1. The surface to bulk ratio increased in the
order Ir/Zr < Ir/Al < Ir/Si, the same order obtained by H2
pulse chemisorption.
TPR profiles (Figure 2a) of the catalysts showed
an increment in the reduction temperature peaks in the
sequence Ir/ZrO2 < Ir/SiO2 < Ir/γ-Al2O3 (100, 195 and
254 °C, respectively), which can indicate an increase in

Vol. 27, No. 5, 2016
Pamphile-Adrián et al.
961
Table 1. XPS results for prepared catalysts
Ir/Ma
Ir4f7/2 (IrO2)
BE / eV
Ir4f7/2 (IrClx)
BE / eV
Catalyst
Bulk
At.%b
At.%b
Surface
Surface
× 100
/ %
Bulk
Ir/SiO2
0.008
0.004
0.013
0.0056
0.0026
0.0077
69.3
61.6
61.3
60.3
72
60
13
63.7
63.1
28
40
87
Ir/γ-Al2O3
Ir/ZrO2
65.3
58.8
62.1
b
aCalculated from atomic percentages obtained in survey spectra using Ir4f, Si2p, Al2p and Zr3p peaks; M = Si, Al, Zr; calculated atomic percentages
using respective Ir4f7/2 peaks. BE: binding energy.
Figure 1. XPS spectra of the Ir4f spectral line of iridium catalysts:
(a) Ir/SiO2; (b) Ir/γ-Al2O3; (c) Ir/ZrO2. The doublets can be attributed to
(A) surface IrO2 or (B) surface IrCl3 or IrCl4.
the interaction of the metal precursor and the support.35
In all cases, the reduction peaks can be attributed to the
reduction of iridium oxide and iridium chloride species with
different interactions with the support,4 whose presence
was also confirmed by the XPS experiments. In the case
of Ir/γ-Al2O3 (Figure 2a), the reduction peak was broader
than the reduction peaks of the other two catalysts. The
surface areas obtained for all catalysts were within the
desired range for this investigation: Ir/ZrO2 (81 m2 g-1),
Ir/SiO2 (167 m2 g-1) and Ir/γ-Al2O3 (119 m2 g-1).
Hydrogen chemisorption results are summarized in
Table 2. In general, metal dispersion followed the same
trend as surface area of the supports, since higher surface
areas often lead to higher metal dispersion.36,37 Ir/SiO2
presented the highest surface area and dispersion among
the studied catalysts, but the values were similar to those
obtained for Ir/γ-Al2O3. H2 chemisorption uptakes were
used to estimate iridium average particle diameters
(Table 2), resulting in similar sizes for Ir/γ-Al2O3 and
Ir/SiO2 and somewhat larger size for Ir/ZrO2. H2-TPD
profiles for the catalysts are presented in Figure 2b. The
Figure 2. (a) H2-TPR and (b) H2-TPD profiles of iridium catalysts.
TPD profile of Ir/SiO2 showed a broad peak between 50
and 360 °C due to desorption of H2 from different types
of active sites present on the catalyst surface, indicating
the surface is non-uniform. In the case of Ir/ZrO2, the TPD
profile showed two main desorption peaks. The first one,
placed at around 100 °C, can be attributed to desorption
of H2 from the metal surface, and the second peak can be
due to desorption of spillover hydrogen from the support.
Finally, the TPD profile of Ir/γ-Al2O3, Figure 2b, showed a
main peak around 304 °C, with a shoulder at 220 °C. The
peak can be attributed to strongly adsorbed hydrogen on
metallic iridium, and the shoulder can be due to weakly
adsorbed hydrogen. This profile also presented a peak
around 630 °C that is attributed to desorption of spillover
hydrogen from the support.38 Comparing the TPD profiles
of all catalysts, H2 desorption temperature increased in
the sequence Ir/ZrO2 < Ir/SiO2 < Ir/γ-Al2O3, which can

962
Iridium Catalysts for C–C and C–O Hydrogenolysis
J. Braz. Chem. Soc.
Table 2. Surface characterization of fresh Ir supported catalysts by H2 pulse chemisorption and cyclohexane conversion
Cyclohexane rate of
TOFc / s-1
Ea / (kcal mol-1)
H2 uptake / (H/Ir) × 100
-1
reactionb / (mmol h-1 gcat
)
Catalyst
dpa / nm
-1
(μmol gcatalyst
)
/ %
Dehyd.
29.3
Hydr.
2.9
Dehyd.
0.12
Hydr.
0.012
0.018
0.011
Dehyd.
Hydr.
17
Ir/γ-Al2O3
Ir/SiO2
35
37
30
67
71
57
1.5
1.4
1.8
20
17
21
22.6
4.4
0.09
11
Ir/ZrO2
44.0
2.8
0.18
15
aParticle size calculated as dp = 1 / (H / Ir); bcalculated at 260 °C; cturnover frequency calculated from H2 chemisorption. Ea: activation energy; Dehyd.:
cyclohexane dehydrogenation; Hydr.: cyclohexane hydrogenolysis.
indicate different binding strength between hydrogen and
the metal sites.
Cyclohexane conversion
Cyclohexane conversion results are summarized in
Table 2. Although the dehydrogenation reaction rate is
often used as an indicator of metal dispersion, the highest
dehydrogenation rate was obtained with Ir/ZrO2. This
catalyst presented the lowest dispersion as measured by
H2 chemisorption. This was probably caused by the Ir–Zr+
interactions, which could provide the formation of new
interfacial sites, which are active for dehydrogenation but
inactive for hydrogen adsorption.39
The results for cyclohexane hydrogenolysis rate showed
a different trend. The higher hydrogenolysis rate and the
lower activation energy obtained for Ir/SiO2 are consistent
with the increase of TOF with the dispersion observed
for small Ir particles.14,40 The structure sensitivity for
hydrogenolysis varies with metal and type of reactant. For
Figure 3. Product selectivity for cyclohexane hydrogenolysis at 260 °C.
example, Sinfelt41 studied the hydrogenolysis of ethane
reaction over a series of SiO2 supported Rh catalysts
with different metal concentrations (varying from bulk
to 0.1 wt.% Rh). For that series of catalysts, the catalytic
activity increased with the decrease of particle size for bulk
Rh (256.0 nm), 5 wt.% Rh/SiO2 (sintered, 12.7 nm) and
1-10 wt.% Rh/SiO2 (1.2-4.1 nm) catalysts. Within the latter
series, the specific activity for the ethane hydrogenolysis
varied from 8 to 16 mmol of ethane converted per hour
per square meter of rhodium, increasing with the decrease
of particle size.
Figure 3 displays the dependence of product selectivity
of cyclohexane hydrogenolysis with temperature for all
catalysts.A first difference is observed between Ir/SiO2 (that
presented the best activity for cyclohexane hydrogenolysis)
and the other catalysts regarding the formation of minor
hydrocarbons from hexane fragmentation. The selectivity
to methane increased with temperature, showing significant
cyclohexane fragmentation for this catalyst, in agreement
with the results reported by Locatelli et al.21 for cyclohexane
(a) Ir/SiO2; (b) Ir/γ-Al2O3; (c) Ir/ZrO2.
conversion over Ir/SiO2. Methane and pentane selectivities
did not follow complementary trends for temperatures
higher than 280 °C. This indicates methane was formed
via successive terminal hydrogenolysis of intermediate
hexane.21
On the other hand, Ir/ZrO2 and Ir/γ-Al2O3 catalysts
showed a different behavior (Figures 3b and 3c). For both
catalysts, hexane was the main hydrogenolysis product
throughout the temperatures range, showing a lower ability
of these catalysts for C–C cleavage from hexane, which
is in agreement with the values of TOF and reaction rate
obtained for this reaction. For these two catalysts, the
fragmentation of cyclohexane occurred by C–C cleavage
from hexane re-adsorption, giving similar selectivities
for the methane-pentane and ethane-butane pairs. This
mechanism could be explained by the formation of two
different types of hexane re-adsorption on the catalyst
surface, one that leads to a CI–CII cleavage (to form

Vol. 27, No. 5, 2016
Pamphile-Adrián et al.
963
pentane and methane), and another one that leads to a
CII–CIII cleavage (to form ethane and butane) or a CIII–CIV
cleavage (to form propane).35 This way, after the primary
hydrogenolysis of cyclohexane to hexane, while on
Ir/SiO2 catalysts methane is further formed through terminal
C–C hydrogenolysis, in the case of Ir/γ-Al2O3 and Ir/ZrO2
internal C–C cleavage was favored. This can be explained
by the formation of metal-support adlineation sites on
Ir/γ-Al2O3 and Ir/ZrO2 catalysts which hinder the required
intermediate for terminal C–C cleavage.
1,2-PDO and/or 1,3-PDO.43,44 Among the three catalysts,
Ir/γ-Al2O3 presented the greater ability for C–C cleavage,
forming methanol and ethylene glycol with selectivities of
9.0 and 3.2%, respectively. This result can indicate a better
carbon adsorption on the catalyst surface to allow the C–C
cleavage and the consequent formation of the mentioned
products, which can be related to the properties of the
supports. Delgado et al.45 found similar results for glycerol
hydrogenolysis reaction on a series of Pt catalysts supported
on TiO2, Al2O3 and SiO2-Al2O3. Pt/TiO2, with less acidic
sites than the other catalysts, presented higher selectivity
to C3 products, whereas the catalysts with more acidic sites
yielded the formation of minor alcohols like methanol. The
same argumentation could explain the behavior of our series
of catalysts, in terms of the difference on the formation
of the minor alcohols from subsequent hydrogenolysis of
1,2-PDO.
Ir/SiO2 catalyst was used to investigate the effect of
increasing the H2 initial pressure from 360 to 550 psi
(Table 4). Increasing H2 pressure caused a decrease in
glycerol conversion and in selectivity to 1,2-PDO, which
decreased from 86.7 to 82%. An interesting difference
between these two reactions is observed in the selectivity
to acetol, a glycerol dehydration product, which was higher
for the reaction performed at higher pressure. This result
could indicate that higher pressures have an unfavorable
effect on the subsequent hydrogenation of acetol to form
1,2-PDO. For this series of spent catalysts, no characteristic
bands of carbon compounds were observed in Raman
spectroscopy analysis.
Glycerol hydrogenolysis
Table 3 lists the conversion and selectivity of glycerol
hydrogenolysis over the tested catalysts. Total mass balance
closed between 97 and 101% for all reactions. The activity
of the catalysts was measured by the TOF based on H2
chemisorption uptakes. All catalysts presented a very
low activity for the hydrogenolysis of glycerol in these
reaction conditions compared with Ru catalysts tested
in the same conditions.42 However, the results can be
used to better understand the requirements of the iridium
catalytic sites. TOF and conversion values increased in
the sequence Ir/SiO2 < Ir/γ-Al2O3 < Ir/ZrO2. The trend in
glycerol hydrogenolysis activity was the same as the one
observed for the dehydrogenation of cyclohexane, which
could indicate that there is an influence of the number of
available Ir active sites on the catalysts surfaces on glycerol
adsorption or products desorption. Besides, the presence
of adlineation metal-support sites on Ir/γ-Al2O3 and
Ir/ZrO2 favored glycerol conversion. Raman spectroscopy
analysis of spent catalysts did not evidence any presence
of carbon compounds. For all catalysts, 1,2-propanediol
(1,2-PDO) was the main product. Despite the low activity,
Ir/SiO2 showed the highest selectivity to 1,2-PDO (86.7%).
Ir/γ-Al2O3 and Ir/ZrO2 presented 1,2-PDO selectivities of
79.4 and 71.6%, respectively. These results indicate that
all catalysts favored the selective primary C–O cleavage
over C–C cleavage.
The support effect on glycerol hydrogenolysis was
also studied at higher temperature and pressure conditions
(Table 5). The total mass balance closed between
98 and 102% for all reactions. In these conditions,
glycerol conversion increased in the following sequence:
Ir/SiO2 < Ir/γ-Al2O3 < Ir/ZrO2. The higher activity of
Ir/ZrO2 is in agreement with several studies which showed
Ir/ZrO2 is an active catalyst for hydrogenation reactions.46,47
In this series of reactions, the increase on conversion and
the formation of esters, carboxylic acids and long chain
alcohols (C4+) could be due to the elevated temperature of
For all catalysts, the formation of propanol and methanol
can be explained by the subsequent hydrogenolysis of
Table 3. Effect of the support on conversion and selectivity of glycerol hydrogenolysis. Reaction conditions: T = 200 °C; PH2 = 360 psi; mcatalyst = 1 g;
150 mL of 20% glycerol solution; treaction = 12 h
Product selectivity / %
Catalyst
Conversion / %
TOF / s-1
1,2-PDO
86.7
1,3-PDO
0.0
PrOH
6.4
EG
1.1
3.2
2.8
MeOH
0.0
Acetol
5.9
Others
0.0
Ir/SiO2
3.9
7.7
8.3
0.004
0.009
0.012
Ir/γ-Al2O3
Ir/ZrO2
79.4
1.1
5.1
9.0
2.1
0.0
71.6
2.2
16.2
0.0
3.7
3.5
TOF: turnover frequency; PDO: propanediol; PrOH: propanol; EG: ethylene glycol; MeOH: methanol.

964
Iridium Catalysts for C–C and C–O Hydrogenolysis
J. Braz. Chem. Soc.
Table 4. Effect of H2 initial pressure on conversion and selectivity of the glycerol hydrogenolysis over Ir/SiO2 catalyst. Reaction conditions:
T = 200 °C; mcatalyst = 1 g; 150 mL of 20% glycerol solution; treaction = 12 h
Product selectivity / %
PH2 / psi
Conversion / %
TOF / s-1
1,2-PDO
86.7
EtOH
0.0
MeOH
0.0
PrOH
6.4
EG
1.1
0.0
Acetol
5.9
360
550
3.9
1.5
0.004
0.003
82.0
0.9
0.6
7.0
9.4
TOF: turnover frequency; PDO: propanediol; EtOH: ethanol; MeOH: methanol; PrOH: propanol; EG: ethylene glycol.
Table 5. Effect of support on conversion and selectivity of glycerol hydrogenolysis. Reaction conditions: T = 250 °C; PH2 = 550 psi; mcatalyst = 1 g; 150 mL
of 50% glycerol solution; treaction = 12 h
Product selectivity / %
Alcohols
Catalyst
Conversion / %
TOF / s-1
Alcohols
C4+
C1-C3
Ketones
Esters
Ethers
Aldehydes
C–C cleavagea
C–O cleavageb
71.8
Ir/SiO2
21.2
57.2
65.3
0.05
0.16
0.22
2.9
3.8
1.6
1.5
8.1
7.2
7.4
7.9
0.4
1.0
0.9
Ir/γ-Al2O3
Ir/ZrO2
42.3
10.0
12.4
18.9
19.7
11.7
10.5
34.5
15.5
aC–C cleavage alcohols: methanol, ethanol, ethylene glycol; bC–O cleavage alcohols:propanol, 1,2-propanediol, 1,3-propanediol. TOF: turnover frequency.
reaction, since the pressure increase had an insignificant
effect on conversion and selectivity over Ir/SiO2.
results can indicate that both C–O and C–C hydrogenolysis
are influenced by acidic properties of the support, the
electronic state of the active phase and the different metal-
support interactions.48
The distribution of oxygenated products formed in
the reactions with Ir/γ-Al2O3 and Ir/ZrO2 was interesting.
Ir/ZrO2 presented a selectivity to esters of 15.5%, whereas
Ir/γ-Al2O3 displayed a selectivity of 7.4%. This fact can
indicate that Ir/ZrO2 favored the oxidation of hydrogenolysis
products, like alcohols and ketones, by the water present
in the reaction media to form more oxygenated products
including esters and carboxylic acids.49
In summary, the main products for the three catalysts
were short chain alcohols (C1-C3), the typical glycerol
hydrogenolysis products, including methanol, ethanol,
propanol, ethylene glycol, 1,2-PDO and 1,3-PDO. Ir/SiO2
showed the highest selectivity to C1-C3 alcohols (74.7%),
whereas the Ir/γ-Al2O3 and Ir/ZrO2 catalysts had selectivity
for short chain alcohols of 46.1 and 36.1%, respectively.
For all catalysts, 1-propanol was the main product, with
selectivities of 46.7% for Ir/SiO2, 30.9% for Ir/γ-Al2O3 and
29.6% when using Ir/ZrO2. Besides, 1,2-PDO was the product
with the second highest selectivity among the alcohols:
24.4% for Ir/SiO2, 8.5% for Ir/γ-Al2O3 and 2.8% for Ir/ZrO2.
Furthermore, 1,3-PDO was also formed with a selectivity of
0.6% for Ir/SiO2, 2.7% for Ir/γ-Al2O3 and 1.8% for Ir/ZrO2.
These results can be attributed to the elevated temperature
of the reaction, which could have led 1,2-PDO and 1,3-
PDO to further react and form 1-propanol by dehydration,
according to reaction mechanisms studies starting from the
main products of glycerol hydrogenolysis.43,44
In these reaction conditions, C–O cleavage was
favored, and the trend for C–C cleavage in the selectivity
of short chain alcohols was Ir/γ-Al2O3 > Ir/SiO2 > Ir/ZrO2
(Table 5). Ir/γ-Al2O3 and Ir/SiO2 catalysts presented similar
selectivities for alcohols produced by C–C cleavage
(methanol and ethylene glycol), higher than those obtained
for Ir/ZrO2, as observed for cyclohexane hydrogenolysis,
where Ir/ZrO2 presented the lowest selectivity for products
formed by subsequent hexane hydrogenolysis. Thus, these
Conclusions
Cyclohexane conversion and glycerol hydrogenolysis
were used to compare the behavior of iridium catalysts
in terms of C–C and C–O cleavage. For cyclohexane
hydrogenolysis, Ir/SiO2 showed the best activity and high
selectivities for minor hydrocarbons formed by subsequent
hexane hydrogenolysis. On the other hand, Ir/ZrO2 catalyst
showed the lowest activity for cyclohexane hydrogenolysis
and the lowest selectivities for minor hydrocarbons. These
differences could be attributed to surface characteristics
like particle size, adlineaton sites and electronic state of
Ir on the supports, which affect directly the re-adsorption
of hexane to form minor hydrocarbons. For glycerol
hydrogenolysis, the activity increased in the sequence
Ir/SiO2 < Ir/γ-Al2O3 < Ir/ZrO2 in all reaction conditions, the
same trend observed for the number of surface active sites

Vol. 27, No. 5, 2016
Pamphile-Adrián et al.
965
estimated by cyclohexane dehydrogenation model reaction.
The effect of the nature of support was more evident on
the product selectivity. Although all catalysts favored C–O
cleavage, forming products like 1,2-PDO and propanol,
Ir/γ-Al2O3 presented the higher selectivity to minor alcohols
formed by C–C cleavage such as methanol and ethylene
glycol. Ir/ZrO2 catalyst showed the highest activity in all
reaction conditions and the lowest selectivity to minor
alcohols produced from C–C cleavage. The differences in
the state of surface iridium and support properties were
responsible for the catalytic behaviors.
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Supplementary Information
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F. B.; Fuel Process. Technol. 2011, 92, 2075.
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Supplementary information (liquid products formed in
high temperature reactions) is available free of charge at
http://jbcs.sbq.org.br as PDF file.
Acknowledgments
23. Amada, Y.; Shinmi, Y.; Koso, S.; Kubota, T.; Nakagawa, Y.;
Tomishige, K.; Appl. Catal., B 2011, 105, 117.
24. Nakagawa, Y.; Ning, X.; Amada, Y.; Tomishige, K.; Appl.
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The authors gratefully acknowledge funding from
CAPES and CAPES/CNPq-IEL Nacional-Brasil
(scholarships for P. P. F. R. and A. J. P. A.), FAPERJ and
CNPq.
25. Chen, K.; Mori, K.;Watanabe, H.; Nakagawa,Y.; Tomishige, K.;
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