Selective hydrogenolysis of glycerol over Ir-Ni bimetallic catalysts

Article abstract of DOI:10.1016/j.cattod.2016.07.026

The effect of the addition of Ni to Ir/γ-Al₂O₃ was studied for glycerol hydrogenolysis reaction. The catalysts were characterized by textural analysis, XRD, H₂-TPR, XPS and TEM. In all cases, the addition of Ni to Ir/γ-Al₂O₃ produced a significant increase in glycerol conversion, and a high selectivity to 1,2-PDO was maintained. TOF values increased from 0.4?×?10^?2?s^?1 for Ir monometallic catalyst, to 4.3?×?10^?2?s^?1 for bimetallic IrNi₂_C catalyst. The latter was the most active catalyst among the calcined bimetallic catalysts. Selectivity to 1,2-PDO had an insignificant decrease from 89.9% for Ir monometallic catalyst to 83.1% for IrNi₂_C catalyst. However, non-calcined IrNi₂ catalyst presented a TOF value of 1.3?×?10^?2?s^?1. This result indicates that the calcination step was fundamental to obtain a strong Ir-Ni interaction that lead to better performances of the bimetallic catalysts. Ir-Ni interaction was evidenced by XPS analyses of the calcined series of catalysts, where the presence of an Ir^δ+ species was observed (0?

Full text of DOI:10.1016/j.cattod.2016.07.026

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Selective hydrogenolysis of glycerol over Ir-Ni bimetallic catalysts
Aracelis J. Pamphile-Adriána, Pedro P. Florez-Rodrigueza, Mattheus Henrique M. Piresa,
Geronimo Perezb, Fabio B. Passosa,∗
a Departamento de Engenharia Química e de Petróleo, Universidade Federal Fluminense, Rua Passos da Pátria, 156, 24210-040, Niterói, RJ, Brazil
b Instituto Nacional de Metrologia (INMETRO), Divisão de Metrologia de Materiais (DIMAT), Av. Nossa Senhora das Grac¸ as, 50, 25250-020, Xerém, Duque
de Caxias, RJ, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of the addition of Ni to Ir/␥-Al2O3 was studied for glycerol hydrogenolysis reaction. The catalysts
were characterized by textural analysis, XRD, H2-TPR, XPS and TEM. In all cases, the addition of Ni to
Ir/␥-Al2O3 produced a significant increase in glycerol conversion, and a high selectivity to 1,2-PDO was
maintained. TOF values increased from 0.4 × 10−2 s−1 for Ir monometallic catalyst, to 4.3 × 10−2 s−1 for
bimetallic IrNi2 C catalyst. The latter was the most active catalyst among the calcined bimetallic catalysts.
Selectivity to 1,2-PDO had an insignificant decrease from 89.9% for Ir monometallic catalyst to 83.1% for
IrNi2 C catalyst. However, non-calcined IrNi2 catalyst presented a TOF value of 1.3 × 10−2 s−1. This result
indicates that the calcination step was fundamental to obtain a strong Ir-Ni interaction that lead to better
performances of the bimetallic catalysts. Ir-Ni interaction was evidenced by XPS analyses of the calcined
series of catalysts, where the presence of an Ir␦+ species was observed (0 < ␦ < 4), suggesting an electronic
density transferring from Ni to Ir.
Received 15 May 2016
Received in revised form 9 July 2016
Accepted 27 July 2016
Available online xxx
Keywords:
Glycerol hydrogenolysis
Iridium catalyst
Bimetallic catalysts
Nickel catalyst
Biodiesel
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
sented about 90% of glycerol conversion after 48 h of reaction and a
maximum yield of 1,3-PDO of 38% at 36 h of reaction. Based in the
The investigation of renewable energy alternatives has been
an important concern recently and has led to the exploration of
biodiesel as an alternative to fossil fuels. Biodiesel is obtained from
fat oils by the transesterification with an alcohol [1], and crude
glycerol is obtained as a by-product [2]. In this context, the search
for synthesis routes aiming the transformation of glycerol into high
value products has grown in the last years. Catalytic hydrogenoly-
sis is an interesting route for obtaining high value materials such as
1,2-propanediol (1,2-PDO), 1,3-propanediol (1,3-PDO), and other
alcohols.
Different supported noble and non-noble metal catalysts have
been employed for the selective hydrogenolysis of glycerol. Low
activity for hydrogenolysis of glycerol with supported iridium cat-
alysts have been reported [3–6]. Nakagawa et al. [3] reported a low
conversion of glycerol for a 4% Ir/SiO2 catalyst, however, a great
increase in activity and selectivity for 1,3-PDO was observed when
the catalyst was promoted with ReOx. Ir-ReOx/SiO2 catalyst pre-
results of this work, the authors proposed a reaction mechanism
that involves the glycerol adsorption on the ReOx cluster and the
attack of an activated hydrogen on metallic Ir to explain the forma-
tion of 1,3-propanediol. Subsequently, other investigations have
been developed by this research group to elucidate the effect of
reaction parameters like Re/Ir rate, type of acid co-catalyst, as well
as determining the structure of Ir-ReOx/SiO2 catalyst and the effect
of the addition of Ru as a promoter [7–10]. Furtheremore, the activ-
ity of this bifunctional catalyst have been studied for C O bonds
hydrogenolysis of different biomass derived substrates using sol-
vents as water and alkanes [11,12]. The activity of the catalyst was
strongly depedent of the solvent used. Alkane solvent was advanta-
geous on water in the hydrogenolysis of secondary mono-alcohols
due the stronger adsorption of the substrate on the catalyst surface,
leading to a two-setep indirect mechanism (acid-catalyzed dehy-
dration and subsequent hydrogenation) [11]. In the presence of
water, hydrogenolysis was promoted by the addition of acid cocat-
alysts, which was explained by the promoter effect of Re alkoxide
formation under an acidic atmosphere, leading to the C O bond
hydrogenolysis by direct mechanism [12].
∗
Corresponding author.
Recently, other properties of Ir-Re catalysts have been studied
as suport [13] and structural [14,15] effects. It was found that Ir-
Re supported on alumina-silica material (ASA) was nearly inactive,
E-mail addresses: aracelispamphile@id.uff.br (A.J. Pamphile-Adrián),
pedropablofr@gmail.com (P.P. Florez-Rodriguez), mhenrique mp@hotmail.com
(M.H.M. Pires), perezgeronimo@hotmail.com (G. Perez), fbpassos@vm.uff.br,
fabiopassos@id.uff.br (F.B. Passos).
http://dx.doi.org/10.1016/j.cattod.2016.07.026
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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which was attributed to the strong Re-support interaction that pro-
motes the dispersion of Re species, difficulting the sinegy between
Ir and Re. However, Ir-Re supported on de-aluminated ASA (ASA
treated with 0.05 or 0.5 M HNO3) showed an enhanced activity and
selectivity to 1,3-PDO, as a result of the formation of active Ir-Re
alloy nanoparticles [13]. This Ir-Re alloy was shown to be more
active than Ir-ReOx structure (both supported on mesoporous silica
KIT-6). Besides the enhanced activity, Ir-Re alloy presented double
1,3-PDO formation rate, and enhanced resistance against particle
sintering [14].
Previously, we have studied the activity and selectivity of Al2O3,
SiO2 and ZrO2 supported iridium catalysts in C C and C O cleavage
in cyclohexane conversion and glycerol hydrogenolysis [16]. The
effect of support, H2 initial pressure and temperature reaction on
glycerol conversion and products selectivity were explored. When
reaction was performed at 200 ◦C, glycerol conversion was 8.3% for
Ir/ZrO2 catalyst, 7.7% for Ir/␥-Al2O3 catalyst and 3.9% for Ir/SiO2 cat-
alyst, and a maximum selectivity for 1,2-PDO of 86.7% was obtained
with Ir/SiO2 catalyst. For a higher reaction temperature (250 ◦C),
higher values of conversion were obtained, however, a decrease in
selectivity to 1,2-PDO was also observed. At this temperature, sev-
eral undesired products as short and long chain alcohols, ketones,
esters, ethers and aldehydes were formed. The results showed that
both C O and C C hydrogenolysis were influenced by acidic prop-
erties of the support, the electronic state of the active phase and
the different metal-support interactions [16]. Consequently, the
present manuscript addresses the addition of Ni as a second metal
to Ir/␥-Al2O3 aiming the increase of the catalyst activity with a high
selectivity to 1,2-PDO. Mono- and bimetallic Ni based catalysts have
shown activity for glycerol conversion reactions [17–19].
as “Ir” catalyst. Finally, “Ir C” catalyst was prepared by calcination
of “Ir” catalyst at 400 ◦C during 4 h (10 ◦C/min).
For bimetallic catalysts Ir content was maintained at 2 wt.%, and
Ni content was adjusted for obtaining Ni/Ir = 0.5; 1.0 e 2.0 molar
ratios.
For the preparation of these solids, first the support was impreg-
nated with a Ni(NO3)2·6H2O (Sigma-Aldrich) aqueous solution.
Then, the precursors were dried at 100 ◦C during 16 h and calcined
at 500 ◦C (10 ◦C/min) during 4 h. Finally, the solids were impreg-
nated with an H2IrCl6·xH2O (Sigma-Aldrich) aqueous solution, and
the precursors were dried at 70 ◦C during 16 h. Finally, calcination
of bimetallic catalyst was performed at 500 ◦C (10 ◦C/min) during
4 h.
Non-calcined and calcined solids (IrNix and IrNix C, respec-
tively) were reduced and passivated in a fixed bed reactor prior
to catalytic runs. The solids were dried at 150 ◦C for 30 min under
He flow (30 mL/min) and cooled to room temperature. The reduc-
tion was performed at 500 ◦C in H2 flow (30 mL/min) during 2 h,
then catalysts were passivated with a 5% O2/He flow at liquid N2
temperature.
2.2. 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 (CuK␣ radiation). The diffractograms were
obtained between 2␪ = 10◦ and 80◦ using a 0.02◦ step size (1 step/s).
Temperature programmed reduction (TPR) experiments were
The performances of bimetallic Ni-Ir catalysts have been inves-
tigated in several reactions involving C H cleavage like methane
dissociation [20], and H2 production by hydrated hydrazine decom-
position [21] and partial oxidation of methane [22]. Additionally,
the addition of Ir to TiO2 supported Ni catalysts produced an activ-
ity increase in the cinnamaldehyde hydrogenation due the strong
interaction between Ni and Ir that modified the electronic structure
of the surface Ni [23]. A similar effect was obtained in the ammonia
decomposition, where the addition of Ir to a Ni/␥-Al2O3 catalyst
caused an increase of 40% in conversion, suggesting the presence
of a synergic effect that reduces the interaction of the active phase
with the support and favors the formation of more active sites [24].
On the other hand, the addition of Ni to Ir/␥-Al2O3 catalysts was
shown to be efficient in suppressing substituted C C cleavage in
1,3-dimethylcyclohexane ring opening, allowing the formation of
products with better cetane number and moderate vapor pressure
[25].
performed with
a multipurpose unit coupled to a Prisma
quadrupole mass spectrometer (Pfeiffer). The samples were dried
at 150 ◦C for 30 min under He flow (30 mL/min) and cooled to room
temperature, then the samples were submitted to a 5% H2/Ar gas
flow (30 mL/min) and the temperature was raised to 1000 ◦C at a
heating rate of 10 ◦C/min.
CO chemisorption analyses were performed in a Micromeritics
ASAP 2010 device by volumetric method. Samples (about 600 mg)
were dried in He flow (30 mL/min) at 100 ◦C during 30 min, and
reduced in H2 (30 mL/min) at 500 ◦C during 2 h. A first outgas was
performed at 500 ◦C during 60 min, followed by an outgas at 35 ◦C
during 30 min. Total and reversible CO adsorption isotherms were
measured at 35 ◦C and CO/Metal was calculated from irreversible
CO uptake.
XPS analysis was carried out using an ESCALAB 250Xi spectrom-
eter (Thermo Fisher Scientific) with monochromatic Al K␣ X-rays
(spot size of 900 ␮m). Surface charging of the electrically insulat-
ing materials was compensated using an integrated flood gun with
approximate spot size of 900 ␮m. Survey spectra were obtained
with a pass energy of 100 eV and region spectra (Ir4f, Al2p, Ni2p)
using a pass energy of 25 eV. The base pressure of the analysis cham-
ber was 7 × 10−10 mbar. Spectra were acquired, analyzed and peak
fitted using the Avantage software with Lorentzian/Gaussian prod-
uct 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 FWHM
in the range 0.5–2.5 eV. Ni2p spectra were fitted with an energy
constraint for Ni2p3/2 within a 17.5 eV shift relative to the Ni2p1/2
peak, an area ratio of 1:0.52, respectively, and a FWHM in the range
1.0–2.7 eV. The residual STD of the fitted spectra was around 0.9
for all catalysts. The binding energy (BE) calibration was done by
adventitious carbon signal (C1s = 284.8 eV).
In this work, the effect of the addition of different amounts of
Ni to an Ir/␥-Al2O3 catalyst for glycerol hydrogenolysis reaction
was investigated. The effect of the Ir-Ni interactions on glycerol
conversion and selectivity to 1,2-PDO was studied.
2. Experimental
2.1. Catalyst preparation
Commercial ␥-Al2O3 (Puralox HP-14) calcined at 600 ◦C
(5 ◦C/min during 4 h) was used. The monometallic iridium and
nickel catalysts were prepared by incipient wetness impregna-
tion technique, using solutions of H2IrCl6·xH2O (Sigma-Aldrich)
and Ni(NO3)2·9H2O, to yield 2 wt.% Ir and 3 wt.% Ni, respectively.
“Ni C” catalyst was obtained after drying of the impregnated solid
at 100 ◦C for 16 h, and calcination at 500 ◦C during 4 h (10 ◦C/min).
Iridium impregnated solid was dried at 70 ◦C for 16 h due the low
melting point of the H2IrCl6·xH2O (65 ◦C), this solid was denoted
Morphological and nanostructural characterization of passi-
vated catalysts were performed using a FEI Tecnai G2 Spirit Twin
TEM with LaB6 filament operating at 120 kV.
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Fig. 1. XRD patterns for mono- and bimetallic calcined catalysts.
Fig. 2. Temperature programmed profiles for mono- and bimetallic calcined cata-
lysts.
2.3. Glycerol hydrogenolysis
Glycerol hydrogenolysis was performed in a 300 mL autoclave
reactor (Parr Instruments Co.). The reactor was initially loaded with
10.6 g of glycerol (Vetec Brasil), 40.6 g of deionized water and 1.6 g
of catalyst (glycerol/Ir molar ratio of 714). The reactor was purged
with a H2 flow of 100 mL/min in order to remove air present in
the system. After 10 min, H2 pressure was raised to 360 psi, stir-
ring speed was adjusted to 500 rpm and temperature was set to
200 ◦C. After the reaction time (12 h), the reactor was cooled to
room temperature and the gas phase was collected and analyzed
in a Micro-GC 490 (Agilent) equipped with three columns: M5A
for permanent gases (H2, O2, N2, CH4 and CO), 5CB for hydrocar-
bons (C3-C6) and PPU for CO2 and ethane. The liquid phase products
were quantified in a GC–MS QP2010Plus (Shimadzu) equipped with
a capillary column (Rtx-Wax). The products were identified using
“GC–MS Solutions” software comparing with NIST05 and NIST05 s
libraries. The identified products were 1,2 and 1,3 propanediol,
propanol, ethylene glycol, isopropanol, ethanol, methanol and ace-
tone. Initial TOF values were estimated for each catalyst from the
initial reaction rate, assuming a first order reaction, and using CO
chemisorption to estimate the number of surface metal sites. Prod-
uct selectivity was calculated on carbon basis using the following
equation:
Table 1
Surface characterization of prepared catalysts by BET surface area, CO chemisorption
and XPS analysis.
Catalyst
SBET (m2/g)
Irrev. CO uptake (␮mol/gcat
)
CO/M
␥-Al2O3
Ir C
Ni C
IrNi0.5
IrNi1.0
IrNi2
Ir
IrNi0.5
IrNi1.0
IrNi2
123
119
113
108
108
108
–
–
–
–
–
–
50.0
17.5
10.0
10.0
11.0
19.3
41.0
58.4
41.5
0.48
0.03
0.06
0.05
0.04
0.19
0.26
0.28
0.13
C
C
C
teristic peaks of IrO2 at 27.6◦, 34.5◦ and 53.7◦ (PDF N◦431019),
and characteristic peaks of nickel species were not observed. This
result was not expected, since iridium content of bimetallic cata-
lysts was the same as that of monometallic iridium catalyst (2 wt.%),
but it could be explained by preferential sintering of iridium at
calcination temperature of 500 ◦C [29]. On the other hand, X-ray
diffractograms of non-calcined bimetallic catalysts (not shown)
showed only ␥-Al2O3 characteristic peaks, confirming that the
agglomeration of iridium particles occurred due to the calcination
step.
Temperature programed reduction profiles of prepared cata-
lysts are shown in Fig. 2. The reduction profile of Ni C catalyst
presented three broad reduction peaks at 170, 580 and 850 ◦C. The
first peak can be attributed to the reduction of well-dispersed NiO
particles with weak interactions with support [30]. The second peak
is due to reduction of NiO particles interacting with support and
the last peak can be attributed to the reduction of nickel alumi-
nate species (NiAl2O4) [31,32]. The TPR profile of monometallic
iridium catalyst presented a main broad peak at 255 ◦C ascribed
to the reduction of iridium oxide and iridium chloride species [16].
The TPR profiles of the bimetallic catalysts showed well-defined
reduction peaks at 263 ◦C (IrNi0.5 C), 254 ◦C (IrNi1.0 C) and 230 ◦C
(IrNi2 C). In all cases, the peaks can be attributed to the simultane-
ous reduction of iridium and nickel oxide species [24]. This effect
is produced by the strong interaction between both metals, which
has been observed previously [24,25].
ji × yi
ꢀ
Selectivity
% =
( )
× 100
iji × yi
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%).
3. Results and discussion
3.1. Catalyst characterization
Fig. 1 presents X-ray diffractograms of prepared catalysts. In
the case of calcined monometallic Ir and Ni catalysts, there were
characteristic peaks of ␥-Al2O3 (2␪ = 32.7◦, 36.9◦, 39.5◦, 45.6◦ and
67.0◦. PDF N◦066558). However, there were not any characteris-
tic peaks of iridium or nickel species, probably due to low metal
content, a high dispersion of metallic species on the support [26]
or formation of an amorphous nickel aluminate, which could dif-
ficult the reduction of NiO species [27,28]. X-ray diffractograms
of calcined IrNix C bimetallic catalysts (Fig. 1) displayed charac-
Table 1 depicts the results obtained for CO chemisorption on
the studied catalysts. In all cases a linear adsorption was assumed
(CO/M = 1, M = Ir or Ni) [33]. Iridium monometallic calcined cata-
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Table 2
XPS results for monometallic Ir and Ni catalysts and bimetallic IrNix C and IrNix C P series.
Catalyst
Ir4f7/2
Ni2p3/2
Ni/Ira
Ir0
Ir␦+
IrO2
IrClx
Ni0
NiO
B.E. (eV)/At%
Ir C
Ni C
–
–
–
–
61.3/60
–
63.7/40
–
–
–
–
856.0/100
856.2/100
855.5/100
856.1/100
856.0/88
–
IrNi0.5
IrNi1.0
C
C
–
–
–
–
60.5/45
60.4/27
60.5/28
–
60.6/39
60.8/31
60.8/30
61.9/47
61.2/49
61.7/65
–
62.7/7
62.0/21
62.6/2
63.4/8
62.6/24
63.3/7
–
–
–
–
–
–
–
1.1
0.7
3.6
–
–
1.1
5.1
IrNi2
C
Ni C P
851.8/12
–
853.1/36
852.9/19
IrNi0.5 C P
IrNi1.0 C P
IrNi2 C P
59.7/54
59.6/48
59.7/68
855.8/64
855.4/81
a
Ni/Ir calculated from At.% of Ir4f and Ni2p peaks in survey spectra.
Fig. 3. XP spectra of the Ir4f spectral line of bimetallic IrNix C and IrNix C P series.
lyst (Ir C) presented the highest CO/M ratio (CO/Ir = 0.48), while
monometallic Ni calcined catalyst presented the lowest CO/M ratio
(CO/Ni = 0.05), about 10 times lower than for Ir C catalyst. In gen-
eral, for bimetallic calcined catalysts, the amount of adsorbed CO
decreased with the addition of Ni. This behaviour was expected due
to the low CO adsorption capacity presented by the monometallic
Ni catalyst [34] and due to the sintering of Ir during calcination at
500 ◦C as observed by XRD. However, an increasing trend of CO/M
ratio with the amount of Ni in the catalyst was observed. This trend
could be attributed to Ir-Ni interactions that affect the adsorption of
CO on the active sites [35]. Non-calcined series catalysts presented
higher CO adsorption uptakes with the exception of monometallic
Ir catalyst. For the latter, the low adsorption of CO can be related to
the weak metal-support interactions due the absence of the calcina-
tion of the solid, which have a direct influence on the CO adsorption.
In the case of the bimetallic non-calcined catalysts, there was an
increase in the CO/M ratio, compared to calcined catalysts. This
result is consistent with IrO2 lines obtained in X-ray diffractograms
and suggests the presence of large crystallites that could lead to a
lower dispersion of the active phase on the support. Thus, the lower
CO/M ratios obtained for the bimetallic Ir-Ni calcined catalysts is
another evidence of sintering of the iridium surface particles caused
by the elevated calcination temperature.
Table 2 depicts the results obtained for XPS analysis of
monometallic iridium catalyst and bimetallic IrNix C and IrNix C P
series. For monometallic Ir C catalyst, the deconvolution of Ir4f evi-
denced the presence of two iridium species. The doublets having
lower binding energies (with Ir4f7/2 at 61.3 eV) are attributed to
surface IrO2 [36–38], and the doublets with higher binding ener-
gies (with Ir4f7/2 at 63.7 eV) can be attributed to the presence of
surface iridium chloride species (IrCl3 or IrCl4) [39,40]. On the other
hand, for bimetallic IrNix C and IrNix C P series of catalysts, Fig. 3,
the deconvolution of Ir4f peaks evidenced three iridium species on
the surface of the catalysts. For IrNix C series, besides the IrO2 and
IrClx species, also observed in the monometallic catalyst, a third 4f
doublet was found, with Ir4f7/2 at 60.4 eV, for IrNi1.0 C, and 60.5 eV,
for IrNi0.5 C and IrNi2 C samples. This low binding energy doublet
could be explained by the presence of an Ir␦+ species (0 < ␦ < +4),
product of the strong interaction between Ir and Ni, and can sug-
gest that Ni is acting like an electronic density donor, leading Ir to
an electron rich state, closer to metallic state. In the case of pas-
sivated catalysts, IrNix C P series, the presence of metallic iridium
was confirmed, with Ir4f7/2 at 59.6 eV, for IrNi C P, and 59.7 eV for
IrNi0.5 C P and IrNi2 C P catalysts. It is important to note that this
binding energy is 0.8 eV lower than that obtained for the Ir␦+ species
founded for IrNix C series. This difference confirms that Ir␦+ is an
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Fig. 4. XP spectra of the Ni2p spectral line of bimetallic IrNix C and IrNix C P series.
100 mL of 20% glycerol solution, at 200 ◦C and 290 psi of H2 dur-
ing 12 h. Furthermore, Ueda et al. [43] have reported an increasing
on activity and stability of a Ni/␥-Al2O3 catalyst for hydrogenolysis
of glycerol by modification with a small amount of Pt, which was
attributed to the formation of a Pt-Ni alloy.
In relation to product selectivity, Table 3, monometallic Ir cata-
lyst exhibited a higher selectivity to 1,2-PDO than Ni monometallic
catalyst, which presented the higher selectivity to propanol and
acetol, which are produced by 1,2-PDO and glycerol dehydration,
respectively [44]. In the case of the bimetallic catalyst, the selectiv-
ity to 1,2-PDO slightly decreased with the increase in the amount
Ir species with an electronic state between Ir4+ and Ir0, produced
by the strong interaction between Ir and Ni, as stated before. Addi-
tionally, for IrNix C P, surface IrO2 was obtained, which could be
formed in the passivation process of the samples.
Ni2p spectra for calcined monometallic Ni and bimetallic Ir-Ni
catalysts are displayed in Fig. 4. It can be observed that for both
calcined and passivated series of catalysts, lower content of Ni lead
to less resolved peaks. This can be an evidence of the enrichment of
the catalysts surface with Ir, turning peak fitting difficult. Spectra
of passivated catalysts showed the presence of Ni0 in low quan-
tities, confirming the difficulty of the reduction of this series of
catalysts. Additionally, the interaction between Ni and Ir was also
confirmed by these spectra, were Ni0 peaks were observed at 853.1
and 852.9 eV for IrNi C P and IrNi2 C P catalysts, respectively. These
values are higher than binding energy values typically obtained for
Ni0 [17,41], which can be attributed to transference of electronic
density from Ni to Ir, in agreement with the observations for Ir4f
spectra.
of Ni. However, it remained high (a minimum of 83.1% for IrNi2
C
catalyst). The opposite trend was observed for the selectivity to
propanol, which slightly increased with the amount of added Ni.
This result is consistent with that found for monometallic Ni cat-
alyst, since the amount of propanol formed increased with the
decrease of 1,2-PDO, which suggests that the addition of Ni pro-
motes the dehydration of 1,2-PDO to form propanol.
Recycle tests of Ir-Ni calcined catalysts are displayed in Table 3,
where a decrease between 3.7 and 5.7% in glycerol conversion was
observed for all catalysts. It has been demonstrated that hot liquid
water has a negative influence on the stability of ␥-Al2O3, leading
to a phase transition to bohemite [45,46]. However, Pt/Al2O3 cata-
lysts prepared by impregnation of H2PtCl6 showed a resistance to
this transition. The expected total bohemite fraction is about 0.6
for 1 wt% of Pt [45,46]. This behaviour could be expected to occur
for impregnated Ir/Al2O3 catalysts. However, the results obtained
for recycle catalysts showed that the phase transition of Al2O3 to
bohemite is not relevant, since the decrease in activity would be
more pronounced.
For the reactions performed with non-calcined catalysts differ-
ent trends for catalyst activity were observed (Table 4). For IrNi1.0
and IrNi2, the values of glycerol conversion remained almost con-
stant. However, a marked difference is observed for TOF values. In
all cases, the TOF was about five times lower than that obtained
for calcined catalysts. The higher TOF value was obtained for IrNi2
catalyst, higher than that obtained for monometallic Ir catalyst, but
lower than that obtained with IrNi2 C catalyst.
3.2. Glycerol hydrogenolysis
Application of Weisz-Prater criterion indicated the catalytic
results were free from internal mass transfer limitations. Table 3
displays the results of conversion and TOF for the glycerol
hydrogenolysis over IrNix C catalysts. For comparison, monometal-
lic Ir and Ni calcined catalysts were also tested, and both catalysts
presented low glycerol conversion (5.9% for Ir C and 2.4% for Ni C)
and consequently low TOF values.
Bimetallic catalysts presented a significant increase in glyc-
erol conversion compared to Ir and Ni monometallic catalysts.
This increase in catalytic activity with the addition of Ni may be
attributed to a synergic effect between both metals promoted by
the modification of the iridium electronic state in the presence
of nickel as observed in the XPS analysis, with a possible alloy
formation after reduction. This result is in agreement with the
results found by Jiang et al. [42] with a Pd-Ni catalyst in glycerol
hydrogenolysis reaction. The authors prepared a bimetallic Pd-Ni
catalyst (5 wt.% Pd), which catalytic activity and stability was much
higher than that obtained for pure Ni using 1.0 g of catalyst in
Please cite this article in press as: A.J. Pamphile-Adrián, et al., Selective hydrogenolysis of glycerol over Ir-Ni bimetallic catalysts, Catal.
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A.J. Pamphile-Adrián et al. / Catalysis Today xxx (2016) xxx–xxx
Table 3
Glycerol conversion and product selectivity in glycerol hydrogenolysis reaction for calcined Ir, Ni and Ir- Ni catalysts.
Catalyst
Cycle
Conv. (%)
(-rA
)
0 × 10−7 (mol/s gcat
)
TOFa × 10−2 (s−1
)
Product Selectivity (%)
1,2-PDO
1,3-PDO
PrOH
EG
Acetol
Others
Ir C
Ni C
IrNi2
1
1
1
2
1
2
1
2
5.9
2.4
2.2
2.0
4.7
3.4
4.2
3.4
3.9
3.2
0.4
1.1
4.3
3.1
4.2
3.4
3.9
3.2
89.9
70.9
83.1
85.0
87.2
90.5
88.9
90.2
1.3
0.4
2.2
1.1
1.8
0.5
2.1
0.5
5.4
10.5
7.0
6.3
4.3
2.9
3.2
3.4
2.6
4.7
5.8
5.5
4.7
4.5
4.5
4.3
0.4
8.5
0.1
0.4
0.2
0.5
0.2
0.3
0.5
3.8
1.8
1.6
1.8
1.2
1.2
1.2
C
24.3
20.6
21.2
16.7
19.5
13.8
IrNi1.0
IrNi0.5
C
C
PDO = Propanediol; PrOH = Propanol; EG = Ethylene glycol; Others: Isopropanol, Ethanol, Methanol, Acetone. Reaction conditions: T = 200 ◦C; PH2 = 360 psi; mcatalyst = 1.6 g;
50 mL of 20% glycerol solution; treaction = 12 h.
a
Based on total liquid phase products normalized to the CO chemisorption of the fresh catalysts.
Table 4
Glycerol conversion and product selectivity in glycerol hydrogenolysis reaction for non-calcined Ir and Ir-Ni catalysts.
Catalyst
Conv. (%)
(-rA
)
0 × 10−7 (mol/s gcat
)
TOFa × 10−2 (s−1
)
Product Selectivity (%)
1,2-PDO
1,3-PDO
PrOH
EG
Acetol
Others
Ir
6.6
1.2
5.4
4.8
3.1
0.6
1.3
0.8
0.8
39.9
80.8
72.0
77.8
6.5
1.7
3.0
2.8
48.5
10.9
20.1
16.4
1.8
5.0
3.5
2.3
2.0
0.0
0.0
0.1
1.3
1.6
1.5
0.8
IrNi2
IrNi1.0
IrNi0.5
22.3
23.0
15.7
Reaction conditions: T = 200 ◦C; PH2 = 360 psi; mcatalyst = 1.6 g; 50 mL of 20% glycerol solution; treaction = 12 h. PDO = Propanediol; PrOH = Propanol; EG = Ethylene glycol; Others:
Isopropanol, Ethanol, Methanol, Acetone.
a
Based on total liquid phase products normalized to the CO chemisorption of the fresh catalysts.
Selectivity to 1,2-PDO also decreased for non-calcined catalysts.
For monometallic Ir catalyst, selectivity to 1,2-PDO decreased about
50%, and a high amount of propanol was formed as a product of
1,2-PDO dehydration. Within non-calcined bimetallic catalysts, the
higher selectivity to 1,2-PDO was obtained for IrNi2 catalyst, how-
ever, it was lower than that obtained for IrNi2 C catalyst. For all
non-calcined bimetallic catalysts, a higher quantity of propanol
was formed, compared to calcined catalysts, especially for IrNi1.0
catalyst, which had the highest selectivity to propanol among the
bimetallic catalysts.
All these observations suggest that the process of calcination is
fundamental to determine the course of the glycerol hydrogenol-
ysis reaction, in terms of activity and product selectivity, even for
Ir monometallic catalyst. Spheroidal Ir-Ni particles (dark contrast)
supported on ␥-Al2O3 particles of irregular shapes are observed in
multibeam TEM micrographs of passivated IrNi1.0 C and IrNi1.0 cat-
alysts, presented in Fig. 5, where the differences in particles sizes
are notorious.
metal size effect for glycerol hydrogenolysis reactions in the lit-
erature. Low metal dispersion (i.e. large metal particle size) has
been reported as being favorable for this reaction [48,49]. On the
other hand, in other catalytic systems smaller particles have yielded
better activity [50,51]. Thus, a morphological effect caused by the
agglomeration of Ir-Ni particles may also be acting as a favorable
factor for the activity of bimetallic catalysts.
Electronic interactions between the components of bimetallic
catalysts have a direct influence on the adsorption of the differ-
ent reactants and products on the metallic surface. There are three
main proposed mechanisms for glycerol hydrogenolysis that con-
sist in dehydrogenation-dehydration-hydrogenation [52], direct
hydrogenolysis [5] and dehydration-hydrogenation [44]. This last
route involves the formation of acetol by glycerol dehydration as
an intermediate that is subsequently hydrogenated to form 1,2-
PDO. This mechanism has been reported to occur in the presence
of acidic catalysts [53–56], which is the case of noble metal ␥-Al2O3
supported catalysts [57].
IrNi1.0 catalyst displayed small and well dispersed particles,
between 1.9 and 3.5 nm. On the other hand, IrNi1.0 C catalyst pre-
sented slightly larger particles, with sizes between 3.0 and 5.9 nm,
suggesting that the non-calcined catalyst has more available active
sites than the calcined catalyst, in agreement CO chemisorption
analysis, where IrNi1.0 catalyst showed the highest CO/M ratio.
These observations are also in agreement with the results obtained
for XRD analysis, where characteristic peaks of Ir species were
obtained for IrNi1.0 C catalysts, suggesting the presence of larger
iridium particles. This results could be an evidence of the impor-
tance of the calcination process of the catalysts to obtain a strong
Ir-Ni interaction that favors the glycerol conversion.
The fact that acetol was obtained in low quantities in most of
performed reactions, including those performed with Ir and Ni
monometallic catalysts, could indicate that what determines the
course of reaction is the glycerol adsorption on the catalyst surface,
after which, acetol and subsequently 1,2-PDO are formed. Thus, the
higher activity of bimetallic catalysts compared to the monometal-
lic Ir and Ni catalysts is probably due to the effect produced by
the electronic interactions between both metals. These interactions
can be given in the form of chemical bonding, charge transferring
or polarization effect [35]. In this case, the electronic transference
from Ni to Ir that confers an electronic enrichment on Ir active sites
was observed in XPS analysis.
The effect produced by bimetallic catalysts in the course of the
reaction can be due to a change of the number of atoms available for
the reaction [47] or due to effects occasioned by electronic inter-
actions between the components of bimetallic catalyst [35]. The
notorious difference in the particle size of non-calcined and cal-
cined catalysts may suggest that, besides electronic interactions, a
structural effect could be responsible for the higher activity of cal-
cined catalysts. However, apparently there is a controversy about
In this context, it can be suggested that calcination process
is fundamental to create a strong electronic interaction between
Ir and Ni that provides active sites, which are more suitable for
glycerol adsorption, and subsequently formation of products. Addi-
tionally, a structural effect could also be playing an important role,
considering that the higher activity was obtained with the catalyst
containing larger metal particles.
Please cite this article in press as: A.J. Pamphile-Adrián, et al., Selective hydrogenolysis of glycerol over Ir-Ni bimetallic catalysts, Catal.
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Fig. 5. Multi beam TEM micrographs of passivated IrNi1.0 C and IrNi1.0 catalysts.
4. Conclusions
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The authors gratefully acknowledge the funding from CAPES
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