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Research Article
Cite This: ACS Catal. 2018, 8, 3516−3524

pubs.acs.org/acscatalysis

Accessing Frustrated Lewis Pair Chemistry through Robust Gold@NDoped Carbon for Selective Hydrogenation of Alkynes
Jhonatan Luiz Fiorio,† Renato Vitalino Gonçalves,‡ Erico Teixeira-Neto,§ Manuel A. Ortuño,∥
Núria López,∥ and Liane Marcia Rossi*,†

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†

Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000
São Paulo, São Paulo, Brazil
‡
Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, São Paulo Brazil
§
Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in Energy and Materials (CNPEM),
13083-970 Campinas, São Paulo, Brazil
∥
Institute of Chemical Research of Catalonia, ICIQ, The Barcelona Institute of Science and Technology (BIST), Av. Països Catalans
16, 43007, Tarragona, Spain
S
* Supporting Information

ABSTRACT: Pyrolysis of Au(OAc)3 in the presence of 1,10phenanthroline over TiO2 furnishes a highly active and selective Au
nanoparticle (NP) catalyst embedded in a nitrogen-doped carbon
support, Au@N-doped carbon/TiO2 catalyst. Parameters such as
pyrolysis temperature, type of support, and nitrogen ligands as well
as Au/ligand molar ratios were systematically investigated. Highly
selective hydrogenation of numerous structurally diverse alkynes
proceeded in moderate to excellent yield under mild conditions.
The high selectivity toward the industrially important alkene
substrates, functional group tolerance, and the high recyclability
makes the catalytic system unique. Both high activity and selectivity
are correlated with a frustrated Lewis pairs interface formed by the
combination of gold and nitrogen atoms of N-doped carbon that,
according to density functional theory calculations, can serve as a basic site to promote the heterolytic activation of H2 under very
mild conditions. This “fully heterogeneous” and recyclable gold catalyst makes the selective hydrogenation process
environmentally and economically attractive.
KEYWORDS: gold, N-doped carbon, hydrogenation, alkyne, heterolytic cleavage, frustrated Lewis pairs

■

INTRODUCTION

catalyst still suﬀer from the lack of activity, mainly due to
limited capability to dissociate molecular hydrogen (H2), which
usually is the rate-limiting step.3 Cooperation between the
coordinatively unsaturated Au atoms on the Au NPs and the
acid−base pair site on Al2O3,12,13 basic sites on hydroxyapatite14,15 and on ceria16,17 has been suggested as a strategy for
the H2 dissociation to yield a H+/H− pair at the metal−support
interface, which is then transferred to the organic moiety. Such
heterolytic H2 splitting has been proposed to occur on pure
oxides,18−20 on gold complexes,21−23 or due to a cooperation
between gold and adsorbed basic ligands.24−30 We have
recently systematically studied the activation of gold via the
heterolytic dissociation of H2 based on the adsorption of
nitrogen-containing bases on gold surfaces.24 The activation of
H2 was proposed to occur at the metal−ligand interface
forming a tight ion pair induced by a frustrated Lewis pair
(FLP) like structure,31−35 which was able to promote the

Alkenes have an extensive range of applications from
intermediates in the chemical industry to their primary use as
comonomers in the production of polymers. High-purity alkene
feedstocks (alkyne free) for the polymer industry are obtained
by a very ﬁne-tuned catalytic hydrogenation process; the
challenging selective hydrogenation of impurities (alkyne to
alkene), while inhibiting the hydrogenation of the alkene to the
alkane step and double-bond isomerization.1 The semihydrogenation of alkynes to alkenes is also known as a valuable
synthetic strategy for drug design.2 Typically, platinum group
metal catalysts are very active for hydrogenations, but they
usually suﬀer from the lack of selectivity to alkenes.3,4
Approaches to improve the chemoselectivity relied on addition
of ligands,5,6 usually nitrogen, phosphorus, sulfur, or addition of
a second less-active metal.7,8 In the context of hydrogenations,
gold was less explored,9,10 but is expected to be intrinsically
selective for hydrogenation of triple bonds in alkyne−alkene
mixtures, because of the preferential adsorption of only one of
these components, the alkyne.11 Nevertheless, gold-based
© 2018 American Chemical Society

Received: February 27, 2018
Published: March 15, 2018
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conversion of 1a (Table 1, entry 1). The best activity was
obtained by performing the pyrolysis at 400 °C (Table 1, entry
2). On increasing the pyrolysis temperature to 600 and 800 °C,
the activity of the resulting catalyst decreased signiﬁcantly
(Table 1, entries 3 and 4). We next studied the inﬂuence of the
N-containing ligands on the stabilization and catalytic activity
of the various materials obtained after pyrolysis at 400 °C
(Table 2).

heterolytic activation of H2, allowing a high activity and
selectivity for the hydrogenation of alkynes into alkenes. In the
proposed mechanism, identiﬁed by density functional theory
(DFT), the H2 molecule splits forming a quaternary N center
(amine protonation) and a hydride on the gold surface; both
H− and H+ were then transferred to the organic moiety in a synfashion controlled by electrostatic interactions.
Nitrogen-doped carbon materials aﬀect the catalytic activity
and selectivity of transition metals signiﬁcantly, such as
cobalt,36,37 nickel,38 palladium,39,40 ruthenium,41 and iron/
iron oxide-based catalysts.42,43 The incorporation of nitrogen
atoms in the carbon architecture combined with metal
nanoparticles have enabled application in a wide range of
catalytic reactions including reductions, oxidations, and H2
generation.44,45 N-Doped carbon structures have also emerged
as an important component of catalysts for hydrogenation
reactions. 36,42,46,47 More interestingly, a heterolytic H 2
activation has been invoked to explain the hydrogenation
reaction mechanism.48−50 Although it is not explicit in the
previous studies, it is likely that the metal-N-doped carbon
interface can be understood as a FLP, with an active
participation of the support basic nitrogen atoms in the H2
activation. Based on our interest in this concept, we decided to
explore the synthesis of gold nanoparticle catalysts embedded
on N-doped carbon for studies in selective hydrogenations. The
main advantage with respect to previous studies24 is to avoid
the addition of external ligands, in large excess, for the
activation of gold surfaces via FLP, making the whole process
environmentally and economically attractive.

Table 2. Catalytic Activity in the Hydrogenation of
Phenylacetylene of Au@N-Doped Carbon Catalysts
Obtained with Diﬀerent N-Containing Ligands, after
Pyrolysis at 400 °Ca

entry

■

RESULTS AND DISCUSSION
We commenced our study by preparing a series of gold
catalysts inspired by the method developed by Beller’s group
for the preparation of Ni, Co, and Fe-catalysts.36,42,47,51,52 The
gold catalysts were prepared by pyrolysis of the gold precursor
at diﬀerent temperatures in the range from 200 to 800 °C in the
presence of 1,10-phenantroline (L1) and TiO2 support under
N2 atmosphere (Figure S1). The thermal behavior TG-DTG of
the Au-L1/TiO2 material under N2 indicated that the pyrolysis
proceeds in a multistage fashion (Figure S2). We studied the
inﬂuence of the pyrolysis temperature on the catalytic activity
of the various materials obtained for the benchmark hydrogenation of phenylacetylene 1a using molecular hydrogen (H2)
(Table 1). The material pyrolyzed at 200 °C lead to 51% of

pyrolysis temperature (°C)

conversion of 1a (%)

200
400
600
800

51
100
27
5

yield of 2a (%)

100
47
8
18
65
17
0
4
0

>99 (95)
88
79
94
93
89
>99
-

Reaction conditions: 0.14 mmol of 1a, 2 mol % of Au catalyst, 2 mL
of ethanol, 100 °C, 6 bar of H2, 20 h. Conversion and yield determined
by GC using internal standard technique; numbers in parentheses refer
to isolated yield. bCatalyst not pyrolyzed.

Among the diﬀerent N-containing ligands tested, 1,10phenanthroline (L1) was found to give the most-active system
(Table 2, entry 1), resulting in a catalyst that exhibited superior
reactivity in comparison with those obtained from other ligands
such as 2,2′-bipyridine (L2), ethylenediamine (L5), diethylenetriamine (L6) (Table 2, entries 2, 5, and 6). The catalyst
obtained using piperazine (L4) showed the lowest activity
(Table 2, entry 3). Moreover, the model reaction does not
occur to any extent in the presence of the material not
submitted to pyrolysis (Table 2, entry 7). Notably, the catalyst
prepared without addition of any ligand (Au/TiO2) showed
signiﬁcant low catalytic activity (Table 2, entry 8), and no
conversion was observed with a material obtained via pyrolysis
of the support with 1,10-phenantroline (L1/TiO2) but without
the gold precursor (Table 2, entry 9). We further investigated
the potential eﬀect of supports (C, CeO2, MgO, Fe3O4, SiO2,
and TiO2) in the catalytic activity (Table S1). The catalysts
based on carbon, cerium oxide, iron oxide, and magnesium
oxide showed low conversion (≤10%). The catalyst supported
on silica reaches 71% of conversion, but the fully hydrogenated
product 3a was formed. Titanium dioxide proved to be the best
support under the studied conditions. The gold precursor

yield of 2a (%)

1
2
3
4

conversion of 1a (%)

Au-L1/TiO2
Au-L2/TiO2
Au-L3/TiO2
Au-L4/TiO2
Au-L5/TiO2
Au-L6/TiO2
Au-L1/TiO2
Au/TiO2
L1/TiO2

a

Table 1. Catalytic Activity in the Hydrogenation of
Phenylacetylene of Au@N-Doped Carbon Catalysts
Obtained from Au-L1/TiO2 after Pyrolysis at Diﬀerent
Temperaturesa

entry

catalyst precursor

1
2
3
4
5
6
7b
8
9

>99
>99
>99
65

a

Reaction conditions: 0.14 mmol 1a, 2 mol % of Au@N-doped
catalyst, 2 mL of ethanol, 100 °C, 6 bar H2, 20 h. Conversion and yield
determined by GC using internal standard technique.
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the overlap of the majoritarian Ti signal at 0.452 keV with the
N signal at 0.392 keV (Figure S6).
The type of N-doping was investigated in more detail by Xray photoelectron spectroscopy (XPS). The XPS survey
spectrum of catalytic material obtained by pyrolysis at 400 °C
revealed that the samples surface contains Au, C, Ti, N, and O.
The Au 4f spectrum exhibit two peaks centered at binding
energy 87.0 and 83.3 eV, which correspond to the Au0
oxidation state53,54 (Figure 1c). Valuable information was
obtained by analysis of the binding energy peak of the N 1s
region (Figure 1d and Figure S7−S9). The N 1s spectrum of
the most active Au@N-doped carbon catalyst (pyrolysis at 400
°C) was ﬁtted with three components located at binding
energies of 399.5, 400, and 401.3 eV (Figure 1d). The peak at
401.3 eV can be attributed to graphitic nitrogen, while the peak
at 399.5 eV can be attributed to the pyridinic nitrogen
species.44,55,56 However, the peak at 400 eV is more diﬃcult to
attribute, as it can be both a result of Au−N interaction (see
results for Au-L1/TiO2 (before pyrolysis), Figure S9) or due to
pyrrolic nitrogen species, very often present in N-doped carbon
systems,46 and also present in the XPS N 1s spectrum of the
sample L1/TiO2 (without gold) (Figure S7). This component
at ca. 400 eV was found in all catalysts prepared by pyrolysis
with other ligands (L2−L6) at 400 °C (Figure S8) and for Au/
L1-TiO2 at other pyrolysis temperatures (Figure S9). The asprepared catalyst precursor Au-L1/TiO2 (before pyrolysis)
displays two nitrogen species at 399.3 and 399.8 eV (Figure
S9), which are tentatively attributed to pyridinic nitrogen atoms
of free ligand and of the ligand coordinated and/or adsorbed on
gold (Au−N), respectively. The 1,10-phenanthroline ligand
adsorbed on TiO2 (L1/TiO2 without gold) displays only one
nitrogen species at 399.4 eV (Figure S7), typically of a pyridinic
nitrogen atom (free ligand).
The kinetic study shown in Figure 2b indicates full
conversion of 1a to 2a in about 8 h and suppression of further
hydrogenation of 2a to 3a, even at long reaction times. This
high selectivity for the alkene 2a is an intrinsic property of our
catalyst and it was not observed for the well-known Pd Lindlar
catalyst (Pd/CaCO3 + Pb(OCOCH3)2 + quinoline), which
produces 3a at high conversions (Figure S10). It is worthwhile
to note that a sixty-fold reaction rate enhancement was
observed when comparing the catalytic activity of similar Au
catalysts prepared with L1 (Figure 2b) (reaction rate = 0.273
mmol gcat−1 h−1) or without any ligand (Figure 2c) (reaction
rate = 0.00472 mmol gcat−1 h−1). In order to further examine
the stability as well as recyclability of the “fully heterogeneous”
catalytic material, the hydrogenation of 1a was repeated up to
10 times using the same catalyst portion without any further
treatment or ligand addition. As depicted in Figure 2d, the
desired product styrene 2a was obtained in a 99% yield even
after ten runs and no signiﬁcant loss of activity was observed
(see also Figure S11). A hot ﬁltration test (Figure 2e) revealed
that the obtained activity is not related to any leaching of the
catalytically active metal (no remaining activity in the
supernatant). ICP AES analysis of the recycling experiments
indicated no undesired leaching processes of gold occurred
during the course of the catalytic transformation. Furthermore,
no aggregation of Au NPs was observed by STEM of the
recycled catalyst after the 10th recycle (Figure S12).
Because Au@N-doped carbon/TiO2 catalyst (Au-L1/TiO2
pyrolyzed at 400 °C) displayed the best activity for the
production of styrene, it was explored in the scope study under
the optimized conditions. A variety of terminal and internal

inﬂuence of the catalytic activity was also evaluated (Table S2).
The catalyst prepared using gold acetate showed the best result,
full conversion of 1a, and a slight decrease in catalytic activity
was noticed when using hydrogen tetrachloroaurate(III) (68%
of conversion) or potassium dicyanoaurate(I) (90% of
conversion). An increase in the Au/L1 molar ratio from 1:2
to 1:5 or 1:10 led to complete loss of catalytic activity (Table
S3). After a further survey of common reaction parameters
(solvent, reaction temperature, molecular hydrogen pressure;
for more details, see Tables S4, S5 and S6 at the Supporting
Information), the best reaction conditions were ethanol as
solvent, 6 bar of H2 at 100 °C. The most active catalytic
material obtained by pyrolysis at 400 °C was characterized in
detail. The active catalyst contains 1.1 wt % Au, 0.7 wt % C and
0.2 wt % N, as determined by elemental analysis and FAAS.
Images by scanning transmission electron microscopy (STEM)
of the material pyrolyzed with the L1 ligand revealed formation
of Au NPs with a mean diameter of 4.5 nm, in addition, a few
larger particles of 10 nm were observed occasionally (Figure
S3a−c). In complete contrast, the catalyst prepared without the
presence of the ligand, a signiﬁcant increase in the gold NPs
size was observed, with a mean diameter about 17 nm (Figure
S3d−f). UV−vis diﬀuse reﬂectance spectra were recorded for
both solids (with and without L1), showing a typical gold SPR
band (Figure S4). The X-ray diﬀraction (XRD) pattern
corresponding to Bragg diﬀractions of Au (200) at 44°, Au
(220) at 65°, or Au (311) at 78°, conﬁrmed the metal
reduction and anatase as the predominant phase, without
signiﬁcant phase conversion after thermal treatment (Figure
S5). EDX spectrum imaging (elemental mapping) of the AuL1/TiO2 material after pyrolysis at 400 °C provided a clear
evidence for the presence of carbon deposited on the surface of
the Au NPs (Figure 1). No conclusive information on the
location of nitrogen in the sample could be obtained because of

Figure 1. (a) Bright-ﬁeld STEM image of a representative area of the
Au@N-doped carbon/TiO2 catalyst (Au-L1/TiO2 material after
pyrolysis at 400 °C) and (b) corresponding EDX elemental maps.
Carbons appear as the red spots deposited mostly on the yellow gold
particles. (c) Au 4f XPS spectrum and (d) N 1s XPS spectrum for the
Au@N-doped carbon/TiO2 catalyst.
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alkynes was readily hydrogenated to the desired alkene and cisalkene with moderate to excellent yield (Scheme 1) and,
notably, diﬀerent from what occur with other transition metals,
without any over-reduction to alkane. Moreover, both electrondeﬁcient substituents, such as esters (2ab) and carboxylic acids
(2ad) moieties, and electron-rich groups, for example amino
(2s and 2t) and methoxy (2c and 2r), were tolerated well. The
developed catalyst was even able to reduce only the alkyne unit
in molecules with alkene moieties (2k, 2v, 2w, and 2x), without
any detectable concurrent reduction of the alkene moieties
both in the parent and product molecules. The results depicted
in Scheme 1 conﬁrmed that a broad range of sensitive and
reducible functional groups, including halide (2d, 2e, 2p, and
2q), ketones (2u), and aldehydes (2ac), were tolerated in the
alkyne hydrogenation process. With an increase in H2 pressure
(10 bar H2), (Z)-alkenes were mostly formed from internal
alkynes (2y−2af). The catalyst system was also applicable for
scaled-up conditions, where 5 mmol of 1a (0.51 g) was
successfully converted into the alkene 2a (0.49 g, 94%).
Considering full conversion, the turnover number (TON) of
1700 was reached, with a TOF of 70 mol mol−1 h−1. This TOF
value is an order of magnitude higher than the values reported
for heterogeneous gold-based catalysts using molecular hydrogen as hydride source and without the use of ligands.
Furthermore, this TOF is also high when taking into account
values for systems using other sources of hydride and ligands to
boost the catalytic activity (Table S7).

Figure 2. (a) Time course of hydrogenation of phenylacetylene 1a
catalyzed by (b) Au@N-doped carbon/TiO2 catalyst and (c) Au/TiO2
catalyst; (d) recycling experiments and (e) hot ﬁltration test to
determine homogeneous catalysis contribution in hydrogenation of 1a.
Reaction conditions: 0.14 mmol of 1a, 2 mol % of Au, 2 mL of
ethanol, 100 °C, 6 bar of H2.

Scheme 1. Au@N-Doped Carbon/TiO2 Catalyst for the Semihydrogenation of Alkynes to Alkenesa, b

a

Reaction conditions: 0.14 mmol of alkyne, 2 mol % of Au catalyst, 2 mL of ethanol at 100 °C, 6 bar of H2. bGC yields are shown; values in
parentheses refer to isolated yields. Selectivity was >99%. c8 bar of H2. d80 °C. e10 bar of H2.
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To further check the chemoselectivity of the developed
catalyst, we performed experiments using 1a as substrate in the
presence of molecules functionalized with other sensitive
groups (Scheme 2). Total chemoselectivity was observed for
Scheme 2. Selective Hydrogenation of 1a in the Presence of
Molecules with Other Reducible Functional Groups by Au@
N-Doped Carbon/TiO2 Catalysta

Figure 3. Hammett plot for the hydrogenation of various parasubstituted alkynes catalyzed by Au@N-doped carbon/TiO2 and
Lindlar catalyst.

0, which indicates a lack of substituent eﬀect in the
hydrogenation reaction and that no charge is built or lost
during the catalytic cycle. Those results give support to a
heterolytic H2 dissociation mechanism for the Au@N-doped
carbon catalyst, in contrast to the homolytic H2 activation
mechanism for Lindlar catalyst. Moreover, theoretical calculation and experimental results have demonstrated that
nitrogen doping structures could also enhance the stability of
Au catalyst against sintering, preserving the catalytic activity,54,58,59 which explains the high recyclability and lack of
particle aggregation.
On the basis of the results described above, we propose that
the nitrogen atoms doped on carbon have a major role in the
heterolytic activation of H2 via frustrated Lewis pairs. To
support such hypothesis, we present a computational study of
the hydrogenation mechanism of alkyne 1a. Following previous
experiments (Table 2, entry 1), we model a Au(111) surface
functionalized with phenanthroline L1. Taking into account the
high coverage situation of the experiments, the π-system of L1
may adsorb in two main conﬁgurations: (i) parallel (ﬂat) to the
surface, model A, or (ii) perpendicular to it, model A2. Model
A contains one L1 in the supercell, while model A2 can ﬁt up to
three (Figure S13). Both situations present the same adsorption
energy (Eads) per ligand, (−0.74 and −0.75 eV, respectively,
Table S9), with the shortest Au···N distances of ca. 3.3 and 3.5
Å, respectively. At high coverage situations, the packing
predicted in model A2 would be expected, and in operando
conditions, ligands would be replaced by reactant molecules via
0.75 eV. Such a continuous release of ligand compromises the
robustness of the catalyst. After pyrolysis treatment, however,
aromatic rings agglomerate to form extended graphene-like
sheets. From literature we know that the Eads of benzene on
Au(111) is 0.63 eV (experiment)60 and 0.48 eV (theory);61 in
other words, ca. 0.08 eV per sp2 atom. The Eads of L1 (14 sp2
atoms) on Au(111) in model A is 0.74 or 0.05 eV/sp2,
somehow smaller than that of benzene. The hypothetical
merging of two L1 ligands (28 sp2 atoms) would render a new
species with a signiﬁcantly larger Eads of ∼1.4 eV. In contrast to
free ligand conditions, where ligands can be easily displaced,
extended graphene-like sheets anchor to the surface, thus
creating a stable and robust system while maintaining the
nature of the catalytic active site.

a

Reaction conditions: 0.14 mmol 1a, 0.14 mmol of sensitive molecule,
2 mol % of Au catalyst, 2 mL of ethanol, 24 h, 100 °C, 6 bar H2.
Conversion was determined by GC using internal standard technique.

the reduction of the model alkyne in the presence of potentially
reducible groups, such as nitro, aldehydes, epoxides, ketones,
nitriles, esters, heteroaromatic N-oxides, and amide, because
not even traces of conversion of these functional groups were
detected. In the absence of the alkyne, some of those functional
groups such as epoxides, nitro, aldehydes, heteroaromatic Noxides, and sulfoxides can be eﬃciently reduced as well (Table
S8).
On the basis of the experimental results and previous work,
we believe that pyridinic, graphitic, and pyrrolic N atoms play a
pivotal role in boosting the catalytic activity of our gold-based
catalyst. The reactivity pattern displayed by the Au@N-doped
carbon catalyst, prepared via pyrolysis of Au-L1/TiO2, allows us
to postulate that H2 activation may proceed by a heterolytic
pathway, due to the interaction of Au NPs and the nitrogen
atoms of the N-doped carbon around the metallic nanoparticles, following the concept of frustrated Lewis pair
interface suggested before for Au NPs and externally added
N-containing ligands.24 This type of H2 activation at N-doped
carbon supports was previously described for Pd,48 boron−
nitrogen codoped graphene,57 and Co catalyst. 49 The
introduction of N atoms might aﬀect the properties of the
supported catalyst, such as tunable acid−base character at the
support surface, which could cause a better interaction of
metal−support−reactant, as well as charge transfer phenomena
at the metal−support junction.44
In order to gain more insight into the mechanism of the
reaction, we conducted a Hammett correlation study using the
developed catalyst and Lindlar catalyst to verify the inﬂuence of
various para substituents on aromatic alkynes. The Hammett
plot for the reaction catalyzed by Au@N-doped carbon catalyst
reveals that the reaction is sensitive to the substituent (Figure
3). The reaction rate was enhanced in the presence of electronwithdrawing groups, and the positive value of ρ (ρ = +1.73; R2
= 0.93) highlights the buildup of negative charge during the
catalytic cycle. The value of ρ with Lindlar catalysts was close to
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catalyst, which will be economically and environmentally
attractive. The Au@N-doped carbon catalyst tolerates other
reducible functional groups, including C−C double bonds,
nitro, aldehydes, epoxides, ketones, nitriles, esters, heteroaromatic N-oxides, and amides, and they can be recycled without
loss of activity. The surface-FLP concept is in its infancy when
compared to main group-FLP32,63 and transition metal-based
FLP,64 but has shown many interesting features. The expansion
of this concept for a broad range of applications in the ﬁeld of
heterogeneous catalysis is encouraged. New reactivity patterns,
not limited to activation of H2, will be soon explored.

Herein we use model A (Figure 4 inset) to properly mimic
the N-doped (pyridinic) graphene-like sheets experimentally

■

EXPERIMENTAL SECTION

Procedure for Preparation of Au-L/Support. Typically,
Au(OAc)3 (19 mg, 0.05 mmol) and amine ligand (18 mg, 0.1
mmol) (molar ratio Au:L = 1:2) were stirred in 20 mL of
ethanol for approximately 5 min at 60 °C. The support was
then added and the mixture was stirred for more 20 min.
Afterward, the solvent was dried by rotary evaporation. The asobtained solid was ground to a ﬁne powder and then pyrolyzed
under nitrogen atmosphere in the desired temperature in an
oven at a rate of 10 °C per minute and held at temperature for
2 h.
General Procedure for Hydrogenation of Alkynes.
Unless otherwise stated, a typical procedure for the semihydrogenation of alkynes is as follows: alkyne (0.14 mmol), Au
catalyst (2 mol % Au), and 2 mL of solvent were placed in a
modiﬁed Fischer−Porter 100 mL glass reactor. The reactor was
purged ﬁve times with H2, leaving the vessel at 6 bar. The
resulting mixture was vigorously stirred, and the temperature
was maintained with an oil bath. After the desired time, the
catalyst was removed by centrifugation, and the products were
analyzed by GC with an internal standard to determine the
conversion of alkyne and the selectivity for alkene. To
determine the isolated yield of the obtained products, after
the reaction was complete, the solvent was evaporated, and the
crude reaction mixture was puriﬁed by column chromatography
on silica gel to aﬀord corresponding alkenes. 1H conﬁrmed the
purity of the isolated products.
Computational Details. All calculations were performed at
Density Functional Theory (DFT) level using the Vienna Abinitio Simulation Package (VASP).65,66 We use the PBE
functional67 supplemented with Grimme’s D2 dispersion
scheme68 and reﬁned parameters for transition metals.61 Core
electrons were described by projector augmented wave (PAW)
pseudopotentials69 and valence electrons by a kinetic energy
cutoﬀ of 450 eV. The Au(111) surface was modeled by a (4 ×
4) supercell with four-layer thick slabs, where the two upper
layers were relaxed and the two bottom ones were ﬁxed to
mimic the bulk. The Brillouin zone was sampled by a 3 × 3 × 1
k-points mesh, generated through the Monkhorst−Pack
method.70 To avoid spurious interactions between the periodic
images, a vacuum region between the slabs of 15 Å and dipole
correction along z were included.71 Transition states were
located with the climbing image nudged elastic band (CI-NEB)
method.72 The assessment of the minima and transition states
was performed by diagonalizing the numerical Hessian matrix
obtained by ±0.015 Å displacements. Metal and ligand atoms
were ﬁxed during frequency calculations. All inputs and ﬁnal
structures can be found in the ioChem-BD repository.73,74

Figure 4. Computed reaction energy proﬁle for the hydrogenation of
1a on L1-functionalized Au(111) surface (Au = yellow, N = blue, C =
gray, H = white).

observed after pyrolysis. The proposed reaction mechanism is
shown in Figure 4. First, the alkyne 1a interacts with the surface
A with an energy of −0.69 eV (B). Although subsequent H2
adsorption (C) is slightly endothermic by 0.23 eV, the Nassisted heterolytic H2 activation (TS-C) is essentially
barrierless, with a relative energy barrier of 0.07 eV above C
(0.30 eV above B). Moreover, the formation of D is
signiﬁcantly exothermic by 0.71 eV. These results are in
sharp contrast with the H2 dissociation on pristine Au(111),
where the energy barrier is 1.45 eV and the reaction energy is
endothermic by 0.69 eV.24 The absence of a direct interaction
between the lone pair of the nitrogen and the gold surface
(Au···N distances of ca. 3.3 Å) creates a unique interface that
promotes the heterolytic cleavage. In that sense, we can relate
this behavior to gold-based FLP chemistry.62 Then, the hydride
from the surface D eventually transfers to the alkyne (TS-D)
with a barrier of 0.36 eV and forms the alkenyl intermediate E.
Final proton transfer from nitrogen to carbon has a barrier of
1.09 eV (TS-E) and produces intermediate F, from which the
product 2a quickly desorbs (0.62 eV) and regenerates the initial
species A. Overall, the present mechanism is in line with
experiments and ilustrates the major role of nitrogen atoms
during the H2 activation process.

■

CONCLUSIONS
We have prepared, for the ﬁrst time, a N-doped carbon
supported gold-based catalyst that exhibited excellent activity
and selectivity for the hydrogenation of alkynes to alkenes, in a
broad scope of substrates, at mild reaction condition. The
intriguing structure of the novel gold heterogeneous catalyst,
which could be understood as frustrated Lewis pairs interfaces
according to computational studies, enables the heterolytic
activation of molecular hydrogen, boosting gold catalytic
activity while avoiding addition of external ligands to improve
activity and selectivity. The graphenization of the ligand ensures
that it is present on the surface and not removed by the solvent
or the reactants, which explains the robustness of the material
under reaction conditions. We demonstrated that the
heterolytic dissociation of H2 can occur in a fully heterogeneous
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ACS Catalysis

■

Research Article

ASSOCIATED CONTENT

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S
* Supporting Information

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.8b00806.
Experimental details, additional experimental results,
characterization of the catalyst (XRD, TEM, UV−vis),
and L1/Au adsorption energies (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for L.M.R.: lrossi@iq.usp.br.
ORCID

Jhonatan Luiz Fiorio: 0000-0001-7435-7430
Renato Vitalino Gonçalves: 0000-0002-3372-6647
Manuel A. Ortuño: 0000-0002-6175-3941
Núria López: 0000-0001-9150-5941
Liane Marcia Rossi: 0000-0001-7679-0852
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The authors are grateful to the Brazilian government agencies
FAPESP (Grant 2016/16738-7), CNPq, INCT-Catalise, and
CAPES for ﬁnancial support. We thank LNNano-CNPEM
(Campinas, Brazil) for the use of the FEI Titan Themis 60-300
microscope and Brazilian Synchrotron Light Laboratory
(LNLS) for use of its SXS experimental facilities (proposal
SXS- 20170205). We thank MINECO (CTQ2015-68770) for
ﬁnancial support and BSC-RES for computational resources.
M.A.O. also thanks the MINECO Juan de la Cierva−
Incorporación postdoctoral program (IJCI-2016-29762).

■

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