Structural, electronic and catalytic properties of palladium nanoparticles supported on poly(ionic liquid)

Article abstract of DOI:10.1016/j.apcata.2018.06.001

The structural, electronic and support effect on palladium nanoparticles (Pd NPs) prepared by sputtering deposition and chemical reduction of a Pd(II) precursor in/on a poly(ionic liquid) (PIL) was investigated in the selective hydrogenation of α,β-unsaturated carbonyl compounds and dienes. Sputtering deposition generates naked NPs with a narrow size distribution (3.2–3.8 nm) that are predominantly composed of Pd(0) (85–100%). Conversely, chemical reduction produces PIL-covered NPs with almost twice the average size (6.6 nm) and only 15% Pd(0). Regard the catalytic performance, support composition (by ionic liquid (IL) addition or not) and NP location are decisive. The best activity and selectivity was obtained with imprinted Pd NPs on a PIL/IL mixture (D-MPIL.NTf₂/IL-Pd catalyst). A kinetic investigation was conducted using 2-cyclohexen-1-one (CHN) and D-MPIL.NTf₂/IL-Pd catalyst revealing that this reaction follows the Langmuir-Hinshelwood mechanism. Enthalpies obtaining from a Van't Hoff plot show that the adsorption of the CHN substrate on the surface of the PIL-Pd catalyst is an exothermic process (-9 kJ mol^?1), whereas H₂ adsorption occurs by an endothermic process (12 kJ mol^?1). This distinct behavior is consistent with the rate determining step proposed, in which the independent adsorption of reagents is followed by the hydrogenation of a π-allyl intermediate on the catalyst surface.

Full text of DOI:10.1016/j.apcata.2018.06.001

Accepted Manuscript
Title: Structural, electronic and catalytic properties of
palladium nanoparticles supported on poly(ionic liquid)
´
Authors: Nathalia M. Simon, Gabriel Abarca, Jackson D.
Scholten, Josiel B. Domingos, David Mecerreyes, Jairton
Dupont
PII:
S0926-860X(18)30271-0
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.06.001
APCATA 16689
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
13-4-2018
29-5-2018
1-6-2018
Please cite this article as: Simon NM, Abarca G, Scholten JD, Domingos JB,
Mecerreyes D, Dupont J, Structural, electronic and catalytic properties of palladium
nanoparticles supported on poly(ionic liquid), Applied Catalysis A, General (2018),
https://doi.org/10.1016/j.apcata.2018.06.001
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Structural, electronic and catalytic properties of palladium nanoparticles
supported on poly(ionic liquid)
Nathália M. Simon,a Gabriel Abarca,a,b Jackson D. Scholten,a Josiel B. Domingos,c David Mecerreyesd and
Jairton Duponta*
aInstitute of Chemistry, Universidade Federal do Rio Grande do Sul. Av. Bento Gonçalves, 9500 Porto Alegre
91501-970 RS (Brazil)
bPresent address: Centro de Nanotecnología Aplicada, Facultad de Ciencias, Universidad Mayor. Camino
La Pirámide 5750, Huechuraba, Santiago (Chile)
cChemistry Department, Universidade Federal de Santa Catarina. Campus Trindade, Florianópolis 88040-
900 SC (Brazil)
dPOLYMAT, University of the Basque Country UPV/EHU, Joxe Mari Korta Center, Avda. Tolosa 72, 20018
Donostia-San Sebastian (Spain)
Graphical abstract
1

Highlights





Poly(ionic liquid) (PIL) supports were decorated with Pd nanoparticles (NPs)
Naked NPs composed mainly of Pd(0) were produced by sputtering deposition
Best catalytic performance was achieved with naked NPs over PIL/IL mixture support
Cyclohexenone hydrogenation by PIL/IL-Pd follows Langmuir-Hinshelwood mechanism
Rate determining step is the independent adsorption of reagents
ABSTRACT: The structural, electronic and support effect on palladium nanoparticles (Pd NPs)
prepared by sputtering deposition and chemical reduction of a Pd(II) precursor in/on a poly(ionic
liquid) (PIL) was investigated in the selective hydrogenation of α,β-unsaturated carbonyl
compounds and dienes. Sputtering deposition generates naked NPs with a narrow size
distribution (3.2–3.8 nm) that are predominantly composed of Pd(0) (85–100%). Conversely,
chemical reduction produces PIL-covered NPs with almost twice the average size (6.6 nm) and
only 15% Pd(0). Regard the catalytic performance, support composition (by ionic liquid (IL)
addition or not) and NP location are decisive. The best activity and selectivity was obtained with
imprinted Pd NPs on a PIL/IL mixture (D-MPIL.NTf2/IL-Pd catalyst). A kinetic investigation was
conducted using 2-cyclohexen-1-one (CHN) and D-MPIL.NTf2/IL-Pd catalyst revealing that this
reaction follows the Langmuir-Hinshelwood mechanism. Enthalpies obtaining from a Van't Hoff
plot show that the adsorption of the CHN substrate on the surface of the PIL-Pd catalyst is an
exothermic process (-9 kJ mol-1), whereas H2 adsorption occurs by an endothermic process (12
kJ mol-1). This distinct behavior is consistent with the rate determining step proposed, in which
the independent adsorption of reagents is followed by the hydrogenation of a π-allyl
intermediate on the catalyst surface.
KEYWORDS: palladium nanoparticles, poly(ionic liquid), PIL, hydrogenation, sputtering
1. Introduction
2

Supported ionic liquid phase (SILP) catalysis is emerging as an important alternative for the
generation of more active and selective catalysts for several transformations.1–3 Transition metal
compounds, nanoparticles (NPs) and enzymes supported in inorganic,4,5 polymeric,6 bio-based7,8
and hybrids materials9,10 display, in most cases, distinct and more efficient catalytic
performances compared to those of neat ionic liquids (ILs) (Table S1). A simple thin film of IL is
necessary to induce the desired properties for the catalytic device (stability, selectivity and so
on)11 and significantly reduce at mass transfer problems.12 These processes can involve the
modification of conventionally enzymatic, homogeneous, heterogeneous and nano-based
catalysts by non-covalent thin IL films. However, covalently grafting ILs onto the surface of solid
materials, as well as the use of poly(ionic liquid)s (PILs), offers significant opportunities to
investigate the support effect on homogeneous metal complexes or metal NPs. Indeed, the SILP
effect on the catalyst can be either geometrical and/or electronic, and also can act as a physical
barrier controlling the access or removal of reagents, intermediates and products from the
catalytically active site.13 It has been recently proposed that this process may be regarded as
flowing liquid(s) (reagents and products) through a chemically active membrane-like composed
of SILP NPs in a confined space.14–16 Hence, SILP catalysts under the multiphase regime
(“dynamic asymmetric mixture”)17 operate akin to catalytically active membranes18, i.e., away
from the thermodynamic equilibrium.19 On this basis, catalysts based on PILs offer a distinct
opportunity to investigate the SILP effect on metal NPs.
An ideal hydrophobic PIL20 polyDADMA.NTf2 derived from
a
commercial
poly(diallyldimethylamonium) chloride source was prepared. The obtained material is a white
powder, chemical, thermal and mechanical stable, which be easily process by a solvent film
casting method.21 This interesting characteristic has already made its application as a CO2
separation membrane22,23 and polymer electrolyte for batteries and supercapacitors24,25
possible. Moreover, in general, PILs have been used in chemical methods of metal NP
preparation, acting as stabilizing supports.6,26–29
The goal of this paper is to investigate the support effect on the catalytic proprieties of poly(ionic
liquid)/ionic liquid/Pd nanoparticle combinations. For this study, we prepared Pd NPs by
sputtering deposition over polyDADMA.NTf2 in both membrane and powder forms. This physical
method was chosen because it provides well-dispersed NPs with a clean surface and uniform
sizes.10,30 For comparison, we also synthesized NPs over this PIL by the chemical reduction of
Pd(II). Finally, the catalytic properties of the prepared composites were probed in hydrogenation
reactions of α,β-unsaturated carbonyl compounds and dienes. Also, a kinetic investigation
showed details regarding the mechanism of hydrogenation conducted by a PIL-Pd catalyst,
3

which differs from that observed in the hydrogenation by naked Pd NPs imprinted in inorganic
and hybrid supports.
2. Experimental Section
2.1 General methods
The reagents and solvents were purchased from commercial sources (Acros, Sigma
Aldrich, Tedia) and used without further purification. The infrared Fourier transform
spectroscopy (FT-IR) was performed on a Bruker alpha P. Solid-state cross-polarization
magic angle spinning carbon-13 nuclear magnetic resonance analysis (CP/MAS-NMR)
1
13
were obtained on a Varian 500 MHz spectrometer. H NMR and C NMR experiments
were performed on a Varian 400 MHz spectrometer. Textural characterization was
performed using N2 adsorption-desorption isotherms at liquid N2 boiling point
temperature, using a Tristar Kr 3020 Micromeritics equipment. The samples were
previously degassed at 90 °C under vacuum, for 30 h. The specific surface area was
determined by BET (Brunauer, Emmett and Teller) multipoint technique, and the pore
size distribution was obtained by using the BJH (Barret, Joyner and Halenda).31
Thermogravimetric analysis (TGA) were performed with a Q50 TA under N2 atmosphere.
Palladium deposition of nanoparticles was performed at Med 020 (Bal-Tech) in sputtering
mode, using a high purity target (99.99%). Transmission electronic microscopy (TEM)
analysis were carried out on a JEOL JEM-1200 EXII. For scanning electron microscopy
(SEM) studies, the sample (5 mg) was fixed in a carbon ribbon and analyzed in a JEOL JSM
5800 microscope operating at 15 kV, coupled with energy-dispersive X-ray spectroscopy
(EDS) detector. XPS measurements were obtained with an energy of 1840 eV, into the
SXS station of the National Laboratory of Synchrotron Light (LNLS), considering C 1s peak
value of 284.6 eV as reference. For metal quantification, the samples were decomposed
and analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) on
a Analyst 200 Perkin Elmer. GC analyses were run with an Agilent Technologies GC System
6820 with DB-17 and HP-5 columns. GC-MS analyses were run with a Shimadzu QP50
with a Rtx-5MS column employing ionising voltage of 17 eV. X-ray diffraction (XRD)
experiments were performed by using a Philips X'Pert MPD diffractometer with Bragg-
Brentano geometry using a graphite curved-crystal with the Cu Kα X-ray radiation (1.5406
Å).
2.2 Preparation of supports
4

The supports are based on neat polyDADMA.NTf2 or a mix of polyDADMA.NTf2 and
BMI.NTf2 (1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) (1:1 wt.).
Both PIL and IL were prepared according to procedures described in the literature.24,32 To
obtain the PIL.NTf2 support, the white powder polyDADMA.NTf2 were macerated. To
obtain MPIL.NTf2, 0.3 g of polyDADMA.NTf2 was dissolved in 5 mL of acetone. The final
solution was poured into a petri plate (diameter 3.5 cm). After evaporating the solvent
at room temperature, the formation of a white solid membrane was observed. To obtain
MPIL.NTf2/IL, 0.15 g of polyDADMA.NTf2 and 0.15 g of BMI.NTf2 were dissolved in 5 mL
of acetone. The solution obtained was stirred for 30 min and then poured into a petri
plate (diameter 3.5 cm). After evaporating the solvent at room temperature, the
formation of a seemingly solid membrane was observed.
2.3 Preparation of catalysts
Sputtering deposition was used to prepare the catalysts A-PIL.NTf2-Pd, C-MPIL.NTf2-Pd,
D-MPIL.NTf2/IL-Pd and E-MPIL.NTf2/IL-Pd. As a general procedure, a certain amount of
each support was placed in a petri plate and inserted into a MED 020 chamber, operating
in sputtering mode. After 30 min. under reduced pressure of 0.2 mbar, argon was
admitted into the chamber until the pressure reached 3 mbar. Then deposition was
initiated on the support, at 40 mA (465V) and during different times. When the
deposition was finished, the chamber pressure slowly returned to ambient conditions
and the catalyst was stored for later used.33 Further details of conditions can be viewed
in Table S2. To prepare the catalyst B-PIL.NTf2-Pd, a Fisher-Porter reactor was charged
with 0.5 g of PIL.NTf2 support, 0.01 g of Pd(OAc)2 and 10 mL of methanol. The reactor was
pressurized with 4 bar of H2, and the reaction was carried out for 1 h at 75 °C under
stirring. Finally, the system was cooled and depressurized. The grey solid formed was
removed from the solution by centrifugation, washed with methanol, dried under
reduced pressure and stored under argon.
2.4 Hydrogenation reactions
In a typical procedure, 10 mL of ethanol solution with 0.5 mmol of α,β-unsaturated
carbonyl compound was added in a Fisher-Porter reactor containing an appropriated
amount of catalyst (0.26 μmol Pd). The reactor was pressurized with 4 bar H2 and
5

maintained at 40 °C in an oil bath. Samples were collected during the reactions, and their
conversion and selectivity were determined by GC-analysis. Further characterization of
the products obtained were performed by GC-MS.
2.5 Recycling reactions
The stability of the catalyst D-MPIL.NTf2/IL-Pd was evaluated through recycling reactions.
After a typical reaction (40 °C, 4 bar H2, 8 h), the liquid phase was removed from the
Fisher-Porter reactor under reduced pressure. The catalyst was washed with three
portions of ethanol and dried under reduced pressure. After, a fresh solution of the
substrate was introduced into the reactor, which was pressurized and immersed in an oil
bath to restart the reaction.
3. Results and Discussion
3.1 Design of supports and catalysts
The composites were designed to evaluate the influence of support shape and
composition, the method of obtaining the NPs and the location/accessibility of NPs in the
catalyst on their structures and catalytic performances. The different pathways to
prepare each support and catalyst are shown in Scheme 1 (for more details see
Experimental Section). The PIL polyDADMA.NTf2 was prepared by anion exchange of
poly(diallyldimethylamonium) chloride,24 from which three types of supports were
prepared (Scheme 1). The neat polyDADMA.NTf2 was used as support in its original
powder shape (PIL.NTf2). PolyDADMA.NTf2 forms membranes when dissolved in the
appropriate organic solvent (acetone, for instance) and molded,21,23 and two other
supports were produced based on this property. One of the membranes was composed
of neat polyDADMA.NTf2 (MPIL.NTf2), and the other was a blend with BMI.NTf2 1:1 wt.
(MPIL.NTf2/IL). The naked Pd NPs were imprinted through the sputtering deposition
method33–36 on each support to form the A-PIL.NTf2-Pd, C-MPIL.NTf2-Pd and D-
MPIL.NTf2/IL-Pd catalytic materials. D-MPIL.NTf2/IL-Pd was re-dissolved and molded to
form E-MPIL.NTf2/IL-Pd. Also, over the PIL.NTf2 support was synthesized Pd NPs by the
classical reduction method using Pd(OAc)2 under 4 bar of hydrogen at 75 °C, thereby
producing the catalytic material B-PIL.NTf2-Pd.
6

3.2 Structural characterization of supports
FT-IR analyses (Fig. S1) of the PIL.NTf2 and MPIL.NTf2 supports show characteristics peaks
of the polyDADMA.NTf2 cation at 2900 and 1470 cm-1.24 The similarity of these spectra
suggests that the contact of polyDADMA.NTf2 with acetone to produce the membrane
MPIL.NTf2 did not promote any change in its structure. The MPIL.NTf2/IL support also
shows typical signals for an imidazolium ring between 3160–2880 and 1620–1580 cm-1.
-
Peaks associated with the NTf2 anion were found at 1350, 1170, 1130 and 1050 cm-1,
confirming anion exchange.37
Solid-state CP/MAS 13C-NMR (Fig. S2) corroborates the FT-IR results. Signals
corresponding to the pyrrolidine cation appear at around 12, 26, 36, 52 and 70 ppm in all
samples.38 Furthermore, the MPIL.NTf2/IL support exhibits characteristics peaks of
BMI.NTf2. Although most are superimposed by the PIL signals, the peak corresponding to
the carbon between nitrogens in the imidazolium ring stands out at 133 ppm. The signal
-
at 120 ppm can be assigned to the NTf2 anion.
The isotherms obtained by N2 adsorption-desorption experiments are characteristic of
non-porous solids31 (Fig. S3). It is noted that both the surface area and pore volume are
slightly improved by the presence of BMI.NTf2, when the supports with membrane shape
MPIL.NTf2 and MPIL.NTf2/IL are compared. IL addition also promoted a slight increase in
the porosity (Fig. S4).
The stability of the PIL polyDADMA.NTf2 and the support MPIL.NTf2/IL was tested by
means of TGA (Fig. S5). Both PIL and PIL/IL are stable up to 350 °C, which makes these
materials suitable for a wide range of applications in catalysis.
The SEM images (Fig. S6) show a clear difference in the morphologies of the PIL.NTf2
support (powder) and the MPIL.NTf2 and MPIL.NTf2/IL supports (membranes). In the first
case, the sample is composed of aggregate spherical particles. Meanwhile, in the second
and third cases, a denser superficial structure was observed, which is in agreement with
characteristics described for other polymeric membranes.39 In all cases, it is possible to
identify some macroporosity at the micrometric level.
3.3 Structural characterization of imprinted Pd NPs
7

The formation of small Pd NPs was confirmed by transmission electron microscopy (TEM)
and the images obtained are shown in Fig. S7. It was observed that sputtering generated
NPs with a very similar average size and regular distribution (3.2 ± 1.3 nm for A-PIL.NTf2-
Pd, 3.2 ± 1.7 nm for C-MPIL.NTf2-Pd, 3.2 ± 1.0 nm for D-MPIL.NTf2/IL-Pd and 3.8 ± 1.8 nm
for E-MPIL.NTf2/IL-Pd, Table 1). This trend is valid since the same sputtering deposition
conditions were used.30 Metal NPs in B-PIL.NTf2-Pd (prepared by the classical reduction
method) have twice the average size of others (6.6 ± 3.2 nm), which indicates that the
size of the NPs is related to the synthetic route employed. In fact, it is expected that each
method has its own specific and different ways of controlling the size of the NPs.35
Moreover, a high local concentration of metal precursor (Pd(OAc)2) may generate larger
NPs and/or more aggregate particles, which is consistent with the present data, once B-
PIL.NTf2-Pd has almost ten times more metal than other catalysts (Table 1).
The concentration of Pd in the catalysts was determined by ICP-OES and the percentages
found were 0.33%, 0.17%, 0.18% and 0.30% for A-PIL.NTf2-Pd, C-MPIL.NTf2-Pd, D-
MPIL.NTf2/IL-Pd and E-MPIL.NTf2/IL-Pd, respectively (Table 1). The values measured were
expected in each case, considering the relationship between the mass support and the
deposition time, since the same current was used for samples (Table S2).40
The X-ray powder diffraction (XRD) pattern of the D-MPIL.NTf2/IL-Pd catalyst is shown in
Fig. S8. The peak indexed to the (111) plane of face-centered cubic Pd diffractions (JCPDS,
Card N°. 05-0681) could be found at 2θ values of ~40°. The presence of the
crystallographic planes (200), (220) and (311) at 47°, 68° and 82°, respectively, were
almost indiscernible, which may be due to the low concentration of metal in the sample.41
The wide-scan X-ray photoelectron spectroscopy (XPS) spectra (Fig. S9) show the
presence of F, O, N, C and S in all samples, as expected. Their binding energies are in
agreement with the values reported in the literature.41,42Furthermore; the Pd 3d region
is not well evidenced for B-PIL.NTf2-Pd and is not detected under the analysis conditions
employed for E-MPIL.NTf2/IL-Pd. This result indicates that the Pd NPs are covered by the
support and located at the innermost PIL layers, since the photoelectrons generated from
Pd 3d at 1840 eV have an inelastic mean free path of λ ≈ 4 nm.10 This is also indicative
that “naked” Pd NPs are present on the surface of the A-PIL.NTf2-Pd, C-MPIL.NTf2-Pd and
D-MPIL.NTf2/IL-Pd catalysts. The main peak of Pd 3d5/2 for B-PIL.NTf2-Pd (Fig. 1 and Table
S3) has a positive shifted at 0.7 eV compared to the specimens of clusters of Pd(0) (335.0
eV).43 This behaviour is probably related to the Pd NPs size of this catalyst, which is largest
than others.44
8

An important aspect to be considered is the predominant metallic Pd component (87–
100% of Pd(0)) for the samples prepared by sputtering against the material synthesized
by a classical chemical reduction process (15% of Pd(0)) (Fig. 1 and Table 1). These results
evidenced the role of the synthetic technique employed and the nature of the supported
Pd. Indeed, imprinted Pd NPs on hybrid and inorganic supports usually display much
higher proportions of the Pd(II) surface component.10 Hence, this indicates that the PIL
provides better protection for the Pd(0) surface atoms than the other supports.
The F 1s region shows positive shifts above 0.3 eV for D-MPIL.NTf2/IL-Pd and E-
MPIL.NTf2/IL-Pd (Fig. S10; 0.88 eV and 1.04 eV, respectively) compared to the neat
supports. This result is attributed to the interaction between IL contact pairs45 and the
metal surface. Moreover, an additional component over the F 1s region (670.65 eV)
appears in D-MPIL.NTf2/IL-Pd (Fig. S11), confirming the interaction of the coordinated
contact pairs of the BMI.NTf2 with the Pd atoms.46,47 In this case, an XPS analysis was also
used to estimate the surface composition of the PIL over the Pd NPs. A ratio of 1:5 surface
IL/Pd was estimated from the ratio between the coordinated signals that appears in the
-
F 1s region (10% NTf2 ) and the Pd-X component.
3.4 Hydrogenation reactions using 2-cyclohexen-1-one and PIL-Pd catalysts
Initially, PILs containing Pd NPs were tested in the hydrogenation of 2-cyclohexen-1-one
(CHN), and the results are shown in Table 2. In all reactions, the only product observed
was cyclohexanone. A blank experiment, where no catalyst was used, did not show any
conversion. Also, no conversion was observed for the reactions employing only the
support, indicating that the hydrogenation activity comes from Pd NPs and not from the
PIL components. Regarding the activities observed in Table 2, the catalysts could be
divided into (i) those that have exposed NPs without IL on the support; (ii) those that
have NPs covered by PIL or PIL/IL and (iii) one that has exposed Pd atoms with IL in the
support.
In the first group, there are the A-PIL.NTf2-Pd and C-MPIL.NTf2-Pd catalysts (entries 1 and
3, Table 2), which obtained the lowest TOF values. In both cases, the composition of
supports are neat PILs and the method of obtaining NPs was by sputtering. This
combination may generate catalysts with more exposed surface Pd atom NPs, i.e.,
9

located at outermost PIL layer. The metal location at the outermost layer and hence less
“protected" may facilitate the forming aggregates during the reaction, decreasing the
catalytic activity. By TEM measurements it was possible to confirm the metal aggregation
after 8 h, since the NP size doubles (from 3.2 to 7.5 nm after the hydrogenation reaction,
see Fig. S12 (a)). In the second group, there are the B-PIL.NTf2-Pd and E-MPIL.NTf2/IL-Pd
catalysts (entries 2 and 5, Table 2), which have intermediate TOF values. Although they
have different supports and methods of metallization, they have in common their
preparation with the aid of a solvent. Thus, both had NPs dispersed in a solution of PIL,
and it is suggested that after removal of the solvent, the NPs were almost completely
covered by the support. These observations are reinforced by the XPS survey spectrum,
as mentioned earlier (Fig. S9). Regarding B-PIL.NTf2-Pd, the concentration of metal
detected by ICP-OES was higher than expected, considering the initial proportion
between the metal precursor and the support (see Experimental Section). This indicates
that part of the PIL.NTf2 support was dissolved under the conditions used for the
reduction of the metal precursor and removed in the washings with methanol, thereby
increasing the amount of NPs in the remaining solid. With this configuration, fewer NPs
should be agglomerated, which explains the superior activity compared to the catalysts
of entries 1 and 3 (Table 2). Simultaneously, NPs were located at the innermost PIL layers
and were less accessible to the substrate, leading to values lower than those obtained
with D-MPIL.NTf2/IL-Pd, as discussed below. The catalyst that showed a better response
to hydrogenation of CHN was D-MPIL.NTf2/IL-Pd (entry 4, Table 2). Compared to C-
MPIL.NTf2-Pd (entry 3, Table 2), the studied reaction was strongly influenced by the
presence of BMI.NTf2. In this case, the MPIL.NTf2/IL support displays more liquid-like
properties than the PIL due to the presence of the IL. Indeed, the use of IL has proved
effective for increasing the activity and stability of Pd NPs for the hydrogenation of CHN.48
To further investigate the effect of these two components, a reaction with C-MPIL.NTf2-
Pd and adding BMI.NTf2 in the reaction medium was performed. The TOF obtained was
3 min-1, which was the same obtained using C-MPIL.NTf2-Pd without the additive (entry
3, Table 2). This suggests that the catalytic activity achieved by D-MPIL.NTf2/IL-Pd should
be the result of a synergistic effect between PIL and IL. In addition, metal aggregation
after 8 h of reaction with D-MPIL.NTf2/IL-Pd was observed in smaller proportions than
that obtained with A-PIL.NTf2-Pd, since NPs size increase from 3.2 nm to 4.0 nm (Fig.
S12(b)). Recycling experiments for CHN hydrogenation using D-MPIL.NTf2/IL-Pd for five
cycles showed no loss in selectivity, although the conversion decreased significantly (97%
conversion for the first cycle down to 34% conversion in the fifth cycle).
10

3.5 Catalytic tests using other substrates
The scope of the catalytic reaction was investigated for other unsaturated compounds using the
best performance catalyst, D-MPIL.NTf2/IL-Pd. The results are presented in Table 3. It is notable
that α,β-unsaturated carbonyls with less sterically hindered C=C bonds provided excellent
conversions and selectivities through quick reactions (entries 1 and 2, Table 3). Simultaneously,
the presence of substituent groups (entries 3 and 4, Table 3) required vigorous reaction
conditions to achieve similar conversion than others. Otherwise, hydrogenation is not favored.
Among conjugated dienes (entries 5, 6 and 7, Table 3), moderate to excellent conversions and
activities were reached. It is noteworthy that the case of 1,3-cyclohexadiene (entry 5, Table 3)
formed a significant amount of benzene, which is a disproportionation product.30 This may be
indicative of agglomerated NPs in the reaction medium.49 NPs with larger diameters can favor
the coordination of two 1,3-cyclohexadiene molecules and the hydrogen transfer between
them, thus generating the disproportionation reaction.50 It is interesting to mention that when
this hydrogenation was tested in DMSO (not reported here), dissolving the MPIL.NTf2/IL support
completely, the selectivity for benzene doubles. This result reinforces the idea described above
since a total absence of support must generate an environment of even more agglomerated
NPs. Disproportionation of α-terpinene did not occur (entry 7, Table 3), probably because methyl
and isopropyl groups hinder the hydrogen transfer. It is worth highlighting, regarding conjugated
dienes, the important selectivities achieved for monoenes.
This result from the action of the SILP/Pd NP system, which can control the access or removal of
reagents, intermediates and products from the catalytically active site.13 In addition, it is known
that dienes are more soluble in IL than monoenes.11,51 These indicate that the IL layer in D-
MPIL.NTf2/IL-Pd favors the hydrogenation of diene to monoene. The nonconjugated diene γ-
terpinene (entry 8, Table 3), as well as the monoenes, are not reactive in the presence of D-
MPIL.NTf2/IL-Pd.
3.6 Kinetics of CHN hydrogenation using PIL-Pd catalysts
A kinetic study was conducted to gain information concerning the catalytic hydrogenation
mechanism of CHN. The most active catalyst, D-MPIL.NTf2/IL-Pd, was chosen to lead these
11

experiments. The influences of hydrogen pressure (Fig. S13) and substrate concentration at
different temperatures (Fig. S14) were evaluated and the initial rates of these reactions are
dependent on both factors. Given these initial catalytic evidences, the kinetics of the CHN
hydrogenation was investigated using the Langmuir−Hinshelwood kinetic model.52–56 This model
proposes some elementary steps for hydrogenation reactions that include: (I) substrate
adsorption, (II) H2 dissociation, (III) surface reaction and (IV) desorption of product. From this
information, the following simplified mechanism is suggested for the hydrogenation of CHN
(Scheme 2):
The kinetic model of Langmuir–Hinshelwood also provides a means to investigate the rate
determining step (RDS) of the studied reaction, by associating a particular equation with each
elementary step of the hydrogenation. In order for the model to be applied in the kinetic
investigation of CHN hydrogenation, some minor adaptations were made. Among them are the
subdivision of step III, so that it can cover two situations: (a) competitive adsorption10,54,57,58 and
(b) non-competitive adsorption53,55,56,59,60 between the substrate and H2 for the active sites of
the catalyst surface. In addition, the possibility of desorption of the cyclohexanone product
being the slow reaction step was discarded due to the small percentage of it in the reaction
medium during the determination of reaction rates (conversions below 20%). By the application
of linear forms of these model equations (Table S4) to our data, followed by the evaluation of
the best correlation coefficient (R2), the RDS suggested for CHN hydrogenation was the surface
reaction between the independently adsorbed substrate and hydrogen. Similar behavior was
observed when a Pt-Au/SiO2 catalyst was used in cinnamaldehyde hydrogenation.53 Indeed, H2
dissociation could not be the RDS, since only a secondary isotopic effect (TOF ratio H2/D2 = 1.1)
was found when the reaction was carried out under D2 pressure (Fig. S15).10 Moreover, if the
dissociation of H2 was the RDS, the reaction rate should decrease with CHN concentration
augmentation (entry 2, Table S4). Instead, the increase in the CHN concentration leads to an
increase in the rate with saturation behavior (Fig. S14), which also excludes CHN adsorption as
the RDS (entry 1, Table S4). Thus, the equation suggested by the Langmuir-Hinshelwood kinetic
model when the RDS is a surface reaction with independent adsorption (entry 4, Table S4) was
used for a nonlinear fitting of the concentration profiles at different temperatures (Fig. S14) and
determine the rate constant for the surface reaction (k3), the equilibrium constant for substrate
adsorption (K1) and the equilibrium constant for H2 adsorption (K2) (Table 4).
12

From the k3 values at different temperatures, it was possible to build an Arrhenius plot and
estimate the apparent activation energy (Ea) for the CHN hydrogenation. Fig. 2 shows that there
are two trends for Ea under the studied conditions: one related to the reactions at higher
temperatures (37 kJ mol-1), in agreement with the literature reports (36–57 kJ mol-1) for
hydrogenation of α,β-unsaturated compounds using Pd catalysts;61,62 and another associated
with lower temperatures (9 kJ mol-1), resulted in an Ea smaller than expected, related to the
enthalpy of the reaction.
The determined K1 and K2 values at different temperatures allowed for estimation of the
enthalpies of adsorption for CHN and H2 (ΔH1 and ΔH2, respectively) through the use of Van't
Hoff plots (Fig. 3 and 4). The calculated value for ΔH1 was -9 kJ mol-1, showing that the adsorption
of the substrate on the surface of the D-MPIL.NTf2/IL-Pd catalyst is an exothermic process. The
low enthalpic value is typical of physisorption and was also observed by other authors in similar
systems, such as chalcone adsorption in Ni NPs (ΔH = -6 kJ mol-1).58 However, the calculated
value for ΔH2 was 12 kJ mol-1, indicating the occurrence of an endothermic process. This distinct
behavior between CHN and H2 adsorption is consistent with the independent mechanism
proposed for the reaction. In addition, it can justify the two trends observed in the Arrhenius
plot (Fig. 2): at lower temperatures, the exothermic CHN adsorption step should be favored,
generating a lower value for Ea; however, at higher temperatures, the endothermic H2
adsorption step is promoted and the effect is a higher Ea. Although the H2 adsorption is
endothermic, the process is spontaneous due to a high entropy energy (ΔS = 55 kJ mol-1 K-1),
which suggests that the interaction between the support MPIL.NTf2/IL and H2 is accompanied
with a possible catalyst restructuration. The same trend was observed for hydrogenation of the
acetylene with Pd-Al2O3 catalyst and in the presence of a carbonaceous species57 and also
observed for Pd nanocrystals.63 Note that in the case of imprinted Pd NPs on hybrid IL containing
inorganic supports9 the catalyst restructuration is not significant as that observed for the Pd NPs
on PILs.
4. Conclusions
13

In this work, we prepared five catalysts based on polyDADMA.NTf2, which differ from each other
considering the following aspects: (i) presence (or not) of IL in the support, (ii) the shape (powder
or membrane) of the support and (iii) the method used to produce Pd NPs (chemical or physical).
Naked NPs with a narrow size composed mainly of Pd(0) were produced by the physical method.
In contrast, covered NPs with broad distribution and high percentage of Pd(II) were obtained by
the chemical method.
This imprinted Pd NPs on PILs are active catalysts for the selective hydrogenation of several
substrates, like dienes and α,β-unsaturated carbonyl compounds, especially towards the C=C
bond and under mild conditions (40 °C, 4 bar of H2). The relative order of activity observed for
the catalysts during the hydrogenation of CHN was found as catalysts with exposed NPs and
supported in pure PIL< covered NPs < exposed NPs and supported in a mixture of PIL and IL. The
presence of IL on the support promotes an increase in the activity and stability of Pd NPs for the
hydrogenation of CHN. The PIL imposes a physical barrier (membrane-like effect) for the
substrates to access the Pd catalytic active sites.
Indeed, a kinetic investigation based on the Langmuir-Hinshelwood mechanism shows that the
independent adsorption of substrates, followed by the hydrogenation of the π-allyl intermediate
on the PIL-Pd catalyst surface, is the RDS of this reaction. Moreover, and corroborating with the
independent adsorption of the substrates proposed, the Van't Hoff plot suggests that the
adsorption of the CHN substrate on the surface of the PIL-Pd catalyst is an exothermic process
(-9 kJ mol-1), whereas H2 adsorption occurs by an endothermic process (12 kJ mol-1). It also can
justify the two trends observed in the Arrhenius plot: at lower temperatures, the exothermic
CHN adsorption step should be favored, generating a lower value for Ea (9 kJ mol-1); however, at
higher temperatures, the endothermic H2 adsorption step is promoted and the effect is a higher
Ea (37 kJ mol-1).
Finally, the catalytic behavior of Pd NPs on PILs, hybrid and inorganic supports in ILs, can be
regarded as catalytic membrane-like devices. The membrane-like approach paves the way for
the design and synthesis of more efficient metal catalysts in/on IL-based materials.
Acknowledgements
The authors are grateful to CNPq, FAPERGS (16/2551-0000), CAPES, INCT-Catal. and Petrobras
for the financial support. This work was also supported by CMM/UFRGS and the National
Laboratory of Synchrotron Light (LNLS).
Notes
14

Supplementary Material available: additional characterization data (FT-IR, CP/MAS 13C-NMR,
BET, TGA, SEM, XPS, TEM) and other kinetic details.
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Scheme 1. Pathways to prepare supports and Pd NPs catalyst (i = maceration; ii = acetone, BMI.NTf2,
moulding in petri plate; iii = acetone, moulding in petri plate; iv = sputtering deposition; v = Pd(OAc)2,
methanol, 75 °C, 4 bar H2).
18

Scheme 2 Mechanism proposed for hydrogenation of CHN using PIL-Pd catalysts.
Fig. 1 XPS analysis performed on the Pd 3d region for (a) A-PIL.NTf2-Pd, (b) B-PIL.NTf2-Pd, (c) C-MPIL.NTf2-Pd
and (d) D-MPIL.NTf2/IL-Pd. Black line = Shirley background; blue line = fitting; red line = Pd(0); green line =
Pd(II); 0 = experimental data.
Fig. 2 Arrhenius plot of CHN hydrogenation over D-MPIL.NTf2/IL-Pd.
19

Fig. 3 Van’t Hoff plot from the equilibrium constant for substrate adsorption (K1) of CHN hydrogenation by D-
MPIL.NTf2/IL-Pd.
Fig. 4 Van’t Hoff plot from the equilibrium constant for H2 adsorption (K2) of CHN hydrogenation by D-MPIL.NTf2/IL-
Pd.
TABLES
20

Table 1 Summary of the physical and chemical characteristics of PIL-Pd catalysts.
Entry
Catalyst
ØPdNPs (nm)a
Pd (wt%)b
Pd(II)/Pd(0)c
1
2
3
4
5
A-PIL.NTf2-Pd
B-PIL.NTf2-Pd
3.2
6.6
3.2
3.2
3.8
0.33
1.50
0.17
0.18
0.30
0/100
85/15
09/91
13/87
-
C-MPIL.NTf2-Pd
D-MPIL.NTf2/IL-Pd
E-MPIL.NTf2/IL-Pd
aDetermined by TEM analysis. bDetermined by ICP-OES analysis. cDetermined by XPS analysis.
Table 2 Hydrogenation of CHN using PIL-Pd catalytic materials.a
Entry
Catalyst
Conversion (%)
TOF (min-1)
1
2
3
4
5
A-PIL.NTf2-Pd
B-PIL.NTf2-Pd
54
63
32
97
44
4.5
9.5
C-MPIL.NTf2-Pd
D-MPIL.NTf2/IL-Pd
E-MPIL.NTf2/IL-Pd
3.5
21.1
8.6
b
aConditions: 0.5 mmol of substrate, 0.26 µmol Pd, 10 mL of ethanol, 4 bar H2, 8 h of reaction. TOF = mol
CHN converted/(mol of surface Pd x time); calculated from the slope of plots of TON vs. time at 20% of
conversion.
Table 3 Hydrogenation of various substrates promoted by D-MPIL.NTf2/IL-Pd.a
21

Conversion
(%)
Entry
1
Substrate
TOFb
99
Products (% Selectivity)
-
>99c
>99d
90e
-
-
-
(>99)
(>99)
2
3
80
10
-
(96)
-
(4)
-
4
<1
-
-
5
6
>99
68
96
9
(84)
(98)
(15)
(2)
(1)
-
7
71
10
(43)
-
(34)
-
(23)
8
<1
-
-
b
aConditions: 0.5 mmol of substrate, 0.26 µmol Pd, 10 mL of ethanol, 40 °C, 4 bar H2, 8 h of reaction. TOF = mol
substrate converted/(mol Pd at surface x minute); calculated from the slope of plots of TON vs. time at low conversion.
c 2 h. d 5 h. e Substrate/metal molar ratio = 1000, 80 °C.
22

Table 4 Kinetic parameters for hydrogenation of CHN by D-MPIL.NTf2/IL-Pd catalyst.a,b
Entry
Temperature
(°C)
k3 x10-3
(mol L-1 min-1)
K1
(L mol-1)
K2
(L mol-1)
c
1
2
3
4
5
20
40
60
70
80
0.889
1.318
1.684
2.407
4.503
-
4.5
6.0
12.6
11.2
9.9
8.3
9.7
8.3
10.7
aReaction conditions: 0.26 µmol Pd, 10 mL of ethanol, 4 bar H2. bKinetic parameters determined by nonlinear fitting
c
using the equation described in entry 4, Table S3. The error from the adjustment did not allow an accurate
determination of this parameter.
23