Tunneling effects in confined gold nanoparticle hydrogenation catalysts

Article abstract of DOI:10.1039/c9cp03012c

Clean surface gold nanoparticles (AuNPs) of ～6.6 nm that were confined in ionic liquid (IL) cages of hybrid γ-alumina (γ-Al₂O₃) displayed hydrogenation pathways in the reduction of trans-cinnamaldehyde distinct from those imprinted directly onto γ-Al₂O₃. Hydrogen activation proceeded via homolytic activation in IL-encapsulated AuNPs and via heterolytic cleavage for IL-free supported AuNPs. Higher negative apparent entropy (ΔS_app) values were obtained for the IL-confined AuNPs compared to the non-hybrid catalyst (Au/γ-Al₂O₃), suggesting a decrease in the number of microstates induced by the nano-confined environment. High kinetic isotope effect (KIE) values (k_H/k_D = 2.5-2.9 at 273 K) and Arrhenius convex curves were observed. Furthermore, differences of 5.6 and 6.2 kJ mol^-1 between the apparent activation energies of the deuteration and hydrogenation reactions (EDa-app - EHa-app) associated with pre-exponential factor ratios (A^D/A^H) of 4.6 and 5.1 provided strong evidence of the possible involvement of a tunneling pathway in the case of the confined AuNPs.

Full text of DOI:10.1039/c9cp03012c

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Determination of the thermal, oxidative and photochemical
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DOI: 10.1039/C9CP03012C
ARTICLE
Tunneling Effects in Confined Gold Nanoparticle Hydrogenation
Catalysts
Leandro Luza,*a Aitor Gual,b Jesum Fernandes,c Dario Eberhardt,d and Jairton Dupont*a
Received 00th January 20xx,
Accepted 00th January 20xx
Clean surface gold ~6.6 nm nanoparticles (AuNPs) that were confined in ionic liquid (IL) cages of hybrid γ-alumina (γ-Al2O3)
displayed hydrogenation pathways in the reduction of trans-cinnamaldehyde distinct from those imprinted directly onto γ-
Al2O3. Hydrogen activation proceeded via homolytic activation in IL-encapsulated AuNPs and by heterolytic cleavage for IL-
free supported AuNPs. Higher negative apparent entropy (∆Sapp) values were obtained for the IL-confined AuNPs compared
to the non-hybrid catalyst (Au/γ-Al2O3), suggesting a decrease in the number of microstates induced by the nano-confined
environment. High kinetic isotope effect (KIE) values (kH/kD = 2.5–2.9 at 273 K) and Arrhenius convex curves were observed.
Furthermore, differences of 5.6 and 6.2 kJ mol−1 between the apparent activation energies of the deuteration and
hydrogenation reactions (Ea-appD − Ea-appH) associated with the pre-exponential factors ratios (AD/AH) of 4.6 and 5.1 provided
strong evidence of the possible involvement of a tunneling pathway in the case of the confined AuNPs.
DOI: 10.1039/x0xx00000x
through them. This reactivity paradigm should be considered in
addition to thermodynamic and kinetic control as a factor that
can determine which of two or more possible products is likely
Introduction
The introduction of a confined space around an active
catalytic site could be a practical way to produce unusual
activities and selectivities in transition metal catalysts.1 These
encapsulated catalysts can change certain steps in the catalytic
cycle, giving rise to new kinetic profiles and altered selectivities,
thereby inducing shape-substrate selectivity.2 In particular,
metal nanoparticles (MNPs) supported in bare ionic liquids (ILs)
or IL hybrid materials displayed catalytic properties of nano-
confined devices.3, 4 In these restricted nano-environments, the
number of microstates may change under asymmetric dynamic
conditions (metal catalyst/ligand/support/reagent/product).
Furthermore, the reaction could proceed far from equilibrium5,
to be obtained.10 Hydrogen tunneling plays an important role in
chemical reactivity at room temperature in biochemistry,
organic chemistry, and catalysis. The tunneling effect is mainly
recognized based on kinetic isotope effect measurements, and
it is known that the chemical environment, particularly in
enzymatic catalysis, affects tunneling by the vibration of
enzymes, which compresses reaction barriers at the active
site.14
Therefore, NPs@ILs may constitute an adequate model to
investigate the possible contribution of the tunneling effect in
hydrogenation reactions over these metal surfaces. For this
reason, clean-surfaced, small, and well-distributed AuNPs
supported directly on γ-alumina (γ-Al2O3) and on IL-hybrid γ-
Al2O3 were prepared (Scheme 1). In addition, detailed kinetic
6
and exhibit various phenomena of temporal and spatial self-
organization7 and complex transitory structures.8,
For
9
example, for catalytic hydrogenation over NPs@ILs, curved
Arrhenius plots and relative high kinetic isotope effects (KIEs)
are usually obtained, which might also suggest the involvement
of a tunneling pathway.10-13
The term “tunneling control” denotes a reaction that passes
through a high but narrow potential energy barrier, leading to
the formation of a product that would be disfavored if the
reaction proceeded by passage over kinetic barriers rather than
a. Laboratory of Molecular Catalysis, Universidade Federal do Rio Grande do Sul, Av.
Bento Gonçalves, 9500, Porto Alegre, Brazil.
b. Unitat de Tecnología Química (UTQ)-EURECAT, Centre Tecnològic de la Química de
Catalunya (CTQC), c/ Marcel.lí Domingo, s/n, Building N5, Tarragona 43007, Spain.
c. School of Chemistry, University of Nottingham, NG7 2RD, Nottingham, UK.
d. PUCRS, Centro Interdisciplinar de Nanociências e Micro-Nanotecnologia, Av.
Ipiranga, 6681, Porto Alegre, Brazil.
Electronic Supplementary Information (ESI) available: characterization of the
catalytic materials (13C and 29Si CP-MAS NMR, FT-IR, N2 physisorption and XPS) as
well as the kinetic and isotopic experiments (1H and 2H NMR). See
DOI: 10.1039/x0xx00000x
Scheme 1. Au nanocatalysts prepared by sputtering deposition: Au/γ-Al2O3, Au/M1 (X:
Cl), and Au/M2 (X: NTf2). Adapted from reference 18.
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and isotopic experiments of the selective hydrogenation of
trans-cinnamaldehyde (1) have offered a significant indication
that, in these confined spaces (IL cages), a tunneling mechanism
could be operative. This is because the Arrhenius plots have
exhibited convex curves, and the primary isotope effect was
larger than 2.5 when calculated from equations based on semi-
classical models and transition-state theory. Additionally, the
differences between the apparent activation energies of the
deuteration and hydrogenation reactions (Ea-appD − Ea-appH) were
significantly above 5 kJ mol−1, and, finally, pre-exponential
factors ratios (AD/AH) were larger than 2.12, 13, 15
View Article Online
DOI: 10.1039/C9CP03012C
Figure 2. XPS spectra at (a) the Cl region of the M1 support and Au/M1 catalyst and (b)
the F region of the M2 support and Au/M2 catalyst.
Results and Discussion
of metallic gold (Au0) for all catalysts (Figure 1a), as determined
by X-ray photoelectron spectroscopy (XPS), with a slight positive
shift for Au binding energies (4f and valence band) in Au/γ-Al2O3
with respect to Au/M1 and Au/M2 (Figure 1b and Table S5 in
the ESI). This fact suggests that the interaction of the AuNPs
with the γ-Al2O3 support decreased due to the IL pair layers,
which formed an IL cage surrounding the AuNPs.18-21 Moreover,
a slight negative shift of 0.6 eV in the Cl 2p region of Au/M1 was
noted (197.9 and 197.3 eV for M1 and Au/M1, respectively)
(Figure 2). In the case of NTf2-bearing supports, the F 1s regions
of M2 and Au/M2 displayed a peak related to uncoordinated
NTf2 anion (688.9 and 688.1 eV, respectively), and a new
component appeared at 684.9 eV only for Au/M2.22 These
behaviors, assigned to the interaction of the contact ion pairs
with the AuNPs.23, 24
The IL-hybrid γ-Al2O3 supports M1 and M2 were prepared
by the reaction of hydroxyl groups of the γ-Al2O3 surface with 1-
methyl-3-(trimethoxysilylpropyl)-imidazolium chloride and by
simple M1 anion exchange with LiNTf2, respectively (Scheme
1).16, 17 All supports were decorated with AuNPs using a 3D
mixing-sputtering device (Scheme 1) and characterized by 13C
and 29Si solid-state cross-polarization magic angle spinning
nuclear magnetic resonance (CP-MAS NMR), Fourier transform
infrared (FT-IR), N2-physisorption, X-ray fluorescence (XRF),
Rutherford backscattering spectrometry (RBS), X-ray diffraction
(XRD), and transmission electron microscopy (TEM) (see ESI:
Section S1, Figures S1–S9, and Tables S1–S4). It is worth
mentioning that the AuNP surfaces were completely composed
The kinetics of the hydrogenation and deuteration of 1 were
explored using a modified Langmuir-Hinshelwood model (see
ESI, Section S2, Tables S6–S8) with an overall rate law expressed
as:
[ ] [
*
]
0
kX K1 - X
1
rx
=
(1) (with x = H or D)
[ ]
1 + K1 - X
1
Different reaction rate constants (kx) (Table 1 and Figure 3)
provided distinct KIE values for the Au nanocatalysts confined
in
the IL when applied at temperatures between 273 and 423 K
(Table 2).
Table 1. Reaction rate constants of the hydrogenation and deuteration of 1 catalyzed by
Au/γ-Al2O3, Au/M1, and Au/M2 at different temperatures.
kH and kD / mmol m−2 h−1[b]
Entry[a]
T/ K
Au/γ-Al2O3
23 and 18
83 and 72
Au/M1
5 and 2
Au/M2
5 and 1.7
1
2
3
4
5
273
323
348
373
423
59 and 30
221 and 158
352 and 239
756 and 542
46 and 23
144 and 89
233 and 156
509 and 377
176 and 139
332 and 219
777 and 599
Figure 1. XPS measurements at (a) Au 4f and (b) at Au valence band regions on the Au/γ-
Al2O3, Au/M1, and Au/M2 catalysts.
[a]Reaction conditions: Au (0.5 µmol), 1/Au = 250-4000, anisole (10 mL), 2.5 MPa of
H2 or D2, and 250 rpm; [b]Analyzed by gas chromatography (GC) and calculated
using a modified Langmuir-Hinshelwood model at conversions of approximately
5% from the slope of reaction rate vs. time.25, 26
2 | J. Name., 2012, 00, 1-3
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Figure 3. Hydrogenation and deuteration rate dependence on 1 concentrations catalyzed by (a) Au/γ-Al2O3, (b) Au/M1, and (c) Au/M2 at temperatures of 273 K (-■-),
323 K (-□-), 348 K (-●-), 373 K (-○-), and 423 K (-▲-).
A KIE value of 1.3 was observed over the entire temperature the ESI). As the hydrogenation of 3 provided only 2 by using the
range for the Au/γ-Al2O3 catalyst (Table 2, entry 1), which is Au/γ-Al2O3 catalyst (Scheme 2a), the generation of the
comparable to values observed in a previous work using Au/γ- corresponding enol followed by isomerization to aldehyde via a
Al2O3 in the hydrogenation of dienes to monoenes (kH/kD
=
keto-enol equilibrium could be corroborated (Scheme 2b).
1.1)18 and similar to those observed in the hydrogenation Interestingly, the C=O double bond was hydrogenated only
reactions using Au/TiO2,27 Pd/Al2O3,17 Pd/SiO2,19, 28 and Pd/C29 when conjugated to the C=C group, probably by means of the
(kH/kD = 1.3–1.6) catalysts. Consequently, it is suggested that η4 adsorption mode.32,
33
In this scenario, an ionic
H2/D2 activation is not the rate-determining step (RDS) of the hydrogenation mechanism could occur in which γ-Al2O3
hydrogenation reactions catalyzed by Au/γ-Al2O3. In contrast, cooperates with the Au surface sites, generating the heterolytic
higher KIEs were obtained as the reaction temperature activation of H2 and the transference of one proton to the
decreased, from 1.4 and 1.3 at 423 K to 2.5 and 2.9 at 273 K, by carbonyl group of 1 with formation of the Au-hydride (Scheme
the IL-hybrid Au/M1 and Au/M2 catalysts, respectively (Table 2, 3a).34, 35
3,4
entries 2 and 3), which points to a different reaction pathway in
Moreover, higher amounts of 2-d2
using the Au
these cases. This scenario is akin to that of micelle nano nanocatalysts confined in the IL Au/M1 and Au/M2 (87–94%)
(macro)reactors, compartmentalizing and (Table 3, entries 4 and 6 and Figures S12–S15 in the ESI) indicate
concentrating/separating reactants and hence altering the the following two possible hydrogenation pathways: (i) the
apparent rate and equilibrium constants.30
occurrence of 1,4-addition of H2 followed by isomerization of
At 273 K, the selectivity to hydrocinnamaldehyde (2) was
essentially the same (~97%) for all catalysts, whereas the
deuterated 2-d2 product underwent changes depending on the
Table 3. Selectivity in the hydrogenation of 1 catalyzed by confined Au nanocatalysts.
2,3
catalyst used (Table 3). Higher selectivities for 2-d2 (72%)
indicate preference for 1,2-addition at the carbonyl group
(C=O), followed by isomerization of the deuterated cinnamyl
Selectivity/ %[b]
Entry[a]
Catalyst
Au/γ-Al2O3
Au/M1
3,4
2,3
alcohol (3-d2 )
1,2 31 (Table 3, entry 2, and Figures S10 and S11 in
2-d0
98
―
96
―
2-d2
―
2-d2
1
2[c]
3
4[c]
5
6[c]
―
27
―
87
―
94
72
―
12
―
5
Table 2. Kinetic isotope effect in the hydrogenation of 1 catalyzed by Au nanocatalysts.
[a]
kH/kD
Entry
Catalyst
97
―
273 K
1.3
2.5
323 K
1.2
1.9
348 K
1.3
1.4
373 K
1.5
1.5
423 K
1.3
1.4
Au/M2
1
2
3
Au/γ-Al2O3
Au/M1
Au/M2
[a]Reaction conditions: Au (0.5 µmol), 1/Au = 4000, anisole (10 mL), 2.5 MPa of H2,
273 K, and 250 rpm; [b]Selectivity determined by gas chromatography (GC) and
nuclear magnetic resonance (NMR) analyzes at conversions of approximately 5%;
[c]By using 2.5 MPa of D2.
2.9
2.1
1.6
1.5
1.3
[a]Calculated from Table 2.
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Based on these deuterium labelling aVniedw ArtkicilenOentliicnes
DOI: 10.1039/C9CP03012C
experiments, and including the desorption of saturated
carbonyl compound 2 as an additional step to the classic
Horiuti-Polanyi mechanism,40 two different pathways are
possible in the hydrogenation of 1. For the hydrogenation of
C=O by the Au/γ-Al2O3 catalyst, the scenario probably involves
the following: (i) adsorption of the entire C=C–C=O system on
the Au surface, (ii) the first addition of H to the O atom
(providing the hydroxyallyl intermediate), and (iii) the second
addition of H to the carbonyl carbon (generating the allyl
alcohol),41 followed by its isomerization to the saturated
aldehyde (Scheme 4a). The second mechanism for the
hydrogenation of C=C by Au/M1 and Au/M2 catalysts possibly
includes the following: (i) adsorption of the C=C–C=O group on
the Au surface, (ii) the first addition of H to the C attached to
the phenyl group (producing the 1-formylphenethyl
intermediate), and (iii) the second addition of H to the second
ethylenic carbon (providing the saturated aldehyde)41 (Scheme
4b). These observations are in agreement with the fact that,
different from group 8–10 metals, AuNP surfaces are saturated
with hydrogen, since Au displays an increasing ability to
dissociate H2 homolytically and uptakes the H2 with an increase
Scheme 2. Isomerization of (a) 3 and (b) 3-d2 catalyzed by Au/γ-Al2O3. Reaction
conditions: Au (0.5 µmol), substrate/Au = 1000, anisole (10 mL), 2.5 MPa of H2 or D2, 273
K, and 250 rpm. Adapted from reference 18.
the enol formed, or (ii) a 3,4-addition directly to the ethylenic
double bond. In addition, conversions below 1% were obtained
C=C double bond undergoes hydrogenation solely when
conjugated with the C=O group, which implies a preferential η4
in the hydrogenation of 2 and 3 by both Au/M1 and Au/M2
catalysts. As in the case of Au/γ-Al2O3, these facts indicate that
adsorption pathway.32, 33 Here, there is the possible presence of
dissociative chemisorption of the H2 mechanism with the
formation of H atoms in bridge positions, which share the low
coordinated Au surface atoms without substantially affecting
the Au–Au distances (Scheme 3b).36, 37 These higher KIE values
3,4
and higher amounts of 2-d2 could suggest the possible
involvement of a tunneling mechanism in these restricted and
confined spaces.38, 39 Although this effect is rarely observed, the
IL environment probably allows the hydrogen to move from one
side of the energy barrier to the other. Further evidence of the
probable involvement of tunneling are the shape of the
Arrhenius plots, the apparent activation energies (Ea-app), and
the pre-exponential factor (A) values for the hydrogenation and
deuteration reactions (see below).12, 13
Scheme 4. Proposed mechanism for the hydrogenation of 1 catalyzed by (a) Au/γ-Al2O3,
and (b) Au/M1 and Au/M2. Adapted from reference 18.
Scheme 3. Proposed mechanisms for the (a) heterolytic and (b) homolytic activation of
hydrogen by confined Au nanocatalysts. Adapted from reference 18.
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Table 4. Kinetic and adsorption data for the hydrogenation of 1 catalyzed by confined Au nanocatalysts.
Ea-app/ kJ mol−1 [A (× 103)/ s−1][a]
∆Happ
/
∆Sapp/
J mol−1 K−1[b]
−21.8
Entry
Catalyst
Au/γ-Al2O3
Au/M1
kJ mol−1[b]
−10.7
−9.1
273–348 K
20.9 [224]
348–423 K
24.0 [737]
24.1 [552]
20.1 [227]
20.2 [170]
20.6 [178]
23.5 [303]
273–423 K
22.8 [491]
24.3 [597]
33.1 [13,565]
37.2 [33,666]
30.4 [3,775]
35.6 [13,046]
1
2[c]
3
21.3 [211]
−20.7
−31.3
−25.9
−32.6
39.2 [149,529]
44.8 [685,048]
34.8 [22,298]
41.0 [114,033]
−13.6
−11.9
−13.9
−12.4
4[c]
5
6[c]
Au/M2
−27.8
[a]A plot of ln kx vs. 1/T yields a straight line (Figure 4) with a slope of –Ea-app/R and a y-intercept of ln A (x = H or D); [b]a plot of ln K1-x vs. 1/T yields a straight line (Figure
5) with a slope of –ΔHapp/R and a y-intercept of ΔSapp/R (x = H or D);42 [c]by using D2.
in reaction temperature.43
temperatures (273–348 K), the intermediate 1-formylphenethyl
A plot of the natural log of kH and kD against the inverse of predominates both in hydrogenation (A of 149,529 × 103 and
reaction temperature enabled the estimation of the Ea-app for 22,298 × 103 s−1) and deuteration (A of 685,048 × 103 and
the hydrogenation and deuteration of 1 catalyzed by Au/γ- 114,033 × 103 s−1) with higher Ea-app values when compared to
Al2O3, Au/M1, and Au/M2 (Table 4 and Figure 4). The Arrhenius the Au/γ-Al2O3 catalyst (39.2 and 34.8 kJ mol−1 for the
plots obtained for the Au/γ-Al2O3 catalyst exhibited close Ea-app hydrogenation and 44.8 and 41.0 kJ mol−1 for the deuteration)
values at low and high temperature ranges in both (Table 4, entries 3–6, and Figures 4b and 4c, red dash). On the
hydrogenation and deuteration reactions (20.9–24.0 kJ mol−1 other hand, as the temperature increases (348–423 K), the
and 21.3–24.1 kJ mol−1, respectively) (Table 4, entries 1 and 2, metastable intermediate dissociates and becomes non-reactive
and Figure 4a). Furthermore, the fraction of molecules that (smaller A values by two-to-three orders of magnitude: 170–
possessed enough kinetic energy to react, expressed by A, 303 × 103 s−1), which disturbs the reaction rates and leads to
displayed relatively similar behavior, since the values achieved lower Ea-app values between 20.1 and 23.5 kJ mol−1 (Table 4,
were in the same order of magnitude (224–737 × 103 s−1 for H2 entries 3–6, Figures 4b and 4c, blue dash). The Ea-app values at
and 211–552 × 103 s−1 for D2, Table 4, entries 1 and 2). low temperatures were comparable to those reported for Pd
Interestingly, for Au/M1 and Au/M2 catalysts, the Arrhenius and AuNPs on similar supports (32 and 29 kJ mol−1,
plots exhibited a convex curve within the studied temperature respectively), but the Ea-app values at high temperatures (~20 kJ
range (Figures 4b and 4c). Remarkably, this fact may suggest the mol−1) were much higher when compared to Au (9 kJ mol−1) and
involvement of the tunneling effect12, 13 with a change in the were slightly lower than Pd (24 kJ mol−1).18, 19 Negative apparent
reaction pathway and, consequently, the formation of a enthalpies
different product.10 By using these catalysts at lower
Figure 4. Arrhenius plots of the hydrogenation and deuteration of 1 catalyzed by (a) Au/γ-Al2O3, (b) Au/M1, and (c) Au/M2.
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Figure 5. Van’t Hoff plots of the hydrogenation and deuteration of 1 catalyzed by (a) Au/γ-Al2O3, (b) Au/M1, and (c) Au/M2.
(∆Happ) were observed in the Van’t Hoff plots for the adsorption Au/M2 catalysts, respectively), which are significantly away
of 1 for all catalysts, with higher values when using Au/γ-Al2O3 from the tunneling/nontunneling limit (Table 5, entries 2 and 3).
(−10.7 and −9.1 kJ mol−1) and lower values for Au/M1 and Thus, these facts offer robust indications that the tunneling
Au/M2 (−13.9 to −11.9 kJ mol−1) (Table 4, Figure 5, and Table S9 correction could be considerable in these IL-hybrid γ-Al2O3
in the ESI). The same trend was detected for the apparent environments.
entropy (∆Sapp, related to the adsorption of 1, Table 4), which
suggests a reduction in the number of microstates in the nano-
confined environment in the Au/IL-hybrid γ-Al2O3 (Au/M1 and
Conclusions
Au/M2) as compared to the non-hybrid catalyst (Au/γ-Al2O3).
Besides the convex shapes displayed by the Arrhenius plots,
there were two other factors that could be associated with a
contribution from the tunneling effect: differences between the
apparent activation energies of the deuteration and
In summary, AuNPs in direct contact with an oxide support
the induction of the heterolytic cleavage of H2, whereas AuNPs
located preferentially away from the oxide support, i.e., in the
IL cage, homolytically activate the H2. These IL-nanocontainers
minimize the interaction between Au and γ-Al2O3, thus
exchanging the hydrogen activation mechanism. The
hydrogenation pathways are determined by the movement of
the IL layer, which rearranges and allows the hydrogen to tunnel
almost instantaneously. Most importantly, in these AuNPs/IL
cages, high KIEs values, convex Arrhenius plots, and Ea-appD − Ea-
D
hydrogenation reactions (Ea-app − Ea-appH) above 5 kJ mol−1
associated with ratios higher than 2 for their respective pre-
exponential factors (AD/AH).13, 15
Because of this, and based on the deuterium labelling and
kinetics experiments by using any of the catalysts at high
temperatures (348–423 K), these parameters were too far
below those required for the tunneling effect to have
significance (Table 5, entries 1–3). Interestingly, at low
temperatures (273–348 K), the values of 5.6 and 6.2 kJ mol−1
H considerably above 5 kJ mol−1 associated with AD/AH ratios
app
greater than 2 strongly suggest the possible involvement of a
tunneling effect. For these reasons, this study suggests that in
dynamic confined spaces the tunneling pathway can operate
more often than what is generally expected.
D
H
related to the Ea-app − Ea-app and AD/AH ratios of 4.6 and 5.1
were obtained only by the IL-confined AuNPs (Au/M1 and
Table 5. Contribution from tunneling in the hydrogenation of 1 catalyzed by confined Au nanocatalysts.
Ea-appD − Ea-appH/ kJ mol−1
AD/AH
Entry[a]
Catalyst
273–348 K
0.4
348–423 K
0.1
273–348 K
0.9
348–423 K
0.7
1
2
3
Au/γ-Al2O3
Au/M1
5.6
0.1
4.6
5.1
0.7
1.7
Au/M2
6.2
2.9
[a]Calculated from Table 3.
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Preparation of the AuNPs
Experimental
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DOI: 10.1039/C9CP03012C
As a general procedure for AuNP preparation by sputtering
deposition, 1.0 g of each support (γ-Al2O3, M1, and M2) was
placed into a conical aluminum flask inside a vacuum chamber
containing an electromagnetic oscillator with variable
controlled frequency, which allowed for constant movement of
the conical flask. Then, the chamber was closed, its pressure
was lowered to a base pressure of 0.4 Pa, and the supports were
evacuated at this pressure for 4 h. Then, the vacuum chamber
was placed under a sputtering working pressure of 0.4 kPa by
adding argon flow. The supports were continuously
homogenized by revolving the aluminum flask at a vibration
frequency of 24 Hz. The Au was sputtered onto the revolving
support at 35 mA of discharge current for 4.5 min to give the
Au/γ-Al2O3, Au/M1, and Au/M2 catalysts. After deposition, the
chamber was vented with nitrogen and the red powders were
recovered and stored under argon atmosphere for further
characterization and application.
General
All syntheses were performed using standard Schlenk
techniques under an argon atmosphere. Chemicals were
purchased from Sigma-Aldrich and used without further
purification. The ILs were prepared employing the original
procedures described elsewhere.44 H2 (>99.999%) and D2 (D>
99.8%) were purchased from White-Martins and Cambridge
Isotope Laboratories, Inc, respectively. Petrobras provided the
γ-Al2O3 used in this study. Synthesis and characterization of the
IL-hybrid γ-Al2O3 supports (M1 and M2) were as reported
elsewhere.16, 17 The 13C and 29Si solid-state cross-polarization
magic angle spinning nuclear magnetic resonance (CP-MAS
NMR) spectra were performed using a Bruker 400 MHz
spectrometer at the CNANO/UFRGS. Fourier transform infrared
(FT-IR) spectra were obtained using an ABB FTLA 2000
instrument with a resolution of 4 cm−1 with 128 cumulative
scans. The N2-physisorption of the catalysts, previously
degassed at 373 K under vacuum for 3 h, were obtained using
Tristar 3020 Micromeritics equipment. Specific surface areas
were determined by the Brunauer-Emmett-Teller (BET)
multipoint method, and the average pore size was obtained by
the Barrett-Joyner-Halenda (BJH) method. Au content was
determined by X-ray fluorescence (XRF) carried out using a
Shimadzu XRF-1800 sequential spectrometer. Samples were
prepared in KBr, and calibration was performed using bromine
as an internal standard. Rutherford backscattering
spectrometry (RBS) measurements were carried out in a 3 MV
Tandetron accelerator using a He+ ion beam of 1.5 MeV at
IF/UFRGS. The Si surface barrier detector was positioned at a
scattering angle of 165°. The X-ray diffraction (XRD) analyses
were carried out using a Philips X’Pert MPD diffractometer with
Bragg-Brentano geometry using a graphite curved crystal with
Cu Kα X-ray radiation (1.5406 Å). Transmission electron
microscopy (TEM) was performed on a JEOL-JEM 1200ExII
electron microscope operating at 120 kV. The samples were
prepared by the slow evaporation of a drop of each colloidal
solution deposited under an argon atmosphere onto a holey
carbon-coated copper grid. XPS measurements were performed
using a Kratos AXIS Ultra DLD instrument. The analysis chamber
pressure during the measurements was greater than 0.5 μPa.
Wide energy range survey scans were collected at a pass energy
of 80 eV in hybrid slot lens mode with a step size of 0.5 eV. High-
resolution data on the Au 4f, Au VB, Al 2p, Cl 2p, and F 1s
photoelectron peaks were collected at a pass energy of 20 eV
over energy ranges suitable for each peak, with collection times
of 5 min and step sizes of 0.1 eV. The X-ray source was a
monochromated Al Kα emission run at 10 mA and 12 kV (120
W). The high-resolution spectra were analyzed using a
Lorentzian asymmetric line shape convoluted with a Gaussian
function for each chemical component. The high-resolution
data was charge corrected to the reference peak of the Al 2p
signal of γ-Al2O3 at 74.5 eV.
Hydrogenation Reactions
As a general procedure for the hydrogenation reactions, the
catalyst (0.5 μmol Au), substrate (1/Au = 1000), and solvent (10
mL of anisole) were placed in a 25 mL stainless steel reactor.
The reaction vessel was pressurized with 2.5 MPa of H2 and
warmed to the desired temperature. Aliquots of 25 μL were
regularly taken during the reaction. After the reaction time, the
reactor was cooled to room temperature and depressurized.
The conversion and selectivity were determined by GC and NMR
analyzes of the reaction samples. GC analyses of the reaction
samples were run with an Agilent Technologies GC System 6820
with an injector and detector (FID) temperature of 533 K. N2 was
the carrier (1 mL min−1), the column head pressure was 70 kPa,
the temperature program was from 313 K (10 min) to 523 K at
a heating rate of 10 K min−1, and a DB-17 column (30 m × 0.25
mm × 0.25 µm) was used. 1H and 2H NMR analyses of the
samples obtained by D2 reduction of 1 catalyzed by AuNPs were
performed using
a Varian 400 MHz spectrometer at
CNANO/UFRGS. The incorporation of D in the reaction products
1
2
was quantified by comparing the H and H NMR spectra with
these obtained from standard samples. The KIEs values were
calculated from the slope of hydrogenation/deuteration
reaction rates vs. time at conversions of approximately 5% by
using a modified Langmuir-Hinshelwood model (Equation 1).25,
26 Plots of ln kx and ln K1-x vs. 1/T yield straight lines with a slope
of –Ea-app/R and a y-intercept of ln A and with a slope of –ΔHapp/R
and a y-intercept of ΔSapp/R, respectively (x = H or D).42
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are thankful to CAPES, FAPERGS, and CNPq-INCT for
financial support.
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Physical Chemistry Chemical Physics
Page 8 of 9
ARTICLE
Journal Name
29. P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D. M. Chevrier, P.
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Page 9 of 9
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C9CP03012C
The H2 activation proceeds via homolytic activation over AuNPs encapsulated in ionic liquid
hybrid support, and display a high kinetic isotope effect and a convex curve by the Arrhenius
plots, which suggests the possible involvement of a tunnelling pathway.