Accessing Frustrated Lewis Pair Chemistry through Robust Gold@N-Doped Carbon for Selective Hydrogenation of Alkynes

Article abstract of DOI:10.1021/acscatal.8b00806

Pyrolysis of Au(OAc)₃ in the presence of 1,10-phenanthroline 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.

Full text of DOI:10.1021/acscatal.8b00806

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Accessing frustrated Lewis pair chemistry through robust
Gold@N-doped carbon for selective hydrogenation of alkynes
Jhonatan Luiz Fiorio, Renato V. Gonçalves, Erico Teixeira-
Neto, Manuel A. Ortuño, Nuria Lopez, and Liane Marcia Rossi
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00806 • Publication Date (Web): 15 Mar 2018
Downloaded from http://pubs.acs.org on March 15, 2018
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Accessing frustrated Lewis pair chemistry through
robust Gold@N-doped carbon for selective
hydrogenation of alkynes
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Jhonatan Luiz Fiorio,a Renato Vitalino Gonçalves,b Erico Teixeira-Neto,c Manuel A. Ortuño,d
Núria López,d Liane Marcia Rossi.a*
a Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av.
Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil.
b Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São Carlos,
SP, Brazil.
c Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in
Energy and Materials (CNPEM), Zip Code 13083-970, Campinas, Sao Paulo, Brazil.
d Institute of Chemical Research of Catalonia, ICIQ, The Barcelona Institute of Science and
Technology, Av. Països Catalans 16, 43007, Tarragona, Spain.
*Correspondence to: lrossi@iq.usp.br.
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ABSTRACT. Pyrolysis of Au(OAc)3 in the presence of 1,10-phenanthroline 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 and 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 DFT 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.
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KEYWORDS. Gold; N-doped carbon; hydrogenation; alkyne; heterolytic cleavage, frustrated
Lewis pairs.
INTRODUCTION
Alkenes have an extensive range of applications from intermediates in the chemicals industry
to their primarily use as co-monomers 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
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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 the preferential adsorption of only one of these components, the
alkyne.11 Nevertheless, gold-based 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 heterolytic activation of H2, allowing a high activity and selectivity for the
hydrogenation of alkynes into alkenes. In the proposed mechanism, identified by 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 syn-fashion controlled
by electrostatic interactions.
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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 in a
plethora of hydrogenation reactions.36,42,46,47 More interestingly, a heterolytic H2 activation has
been invoked to explain the hydrogenation reaction mechanism.48,49,50 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 nanoparticles
supported on N-doped carbon supports 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.
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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 multi-stage fashion (Figure S2). We studied the
influence of the pyrolysis temperature on the catalytic activity of the various materials obtained
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for the benchmark hydrogenation of phenylacetylene 1a using molecular hydrogen (H2) (Table
1). The material pyrolyzed at 200 °C lead to 51% of 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
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pyrolysis temperature to 600 °C and 800 C, the activity of the resulting catalyst decreased
significantly (Table 1, entry 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
oC (Table 2).
Table 1. Catalytic activity in the hydrogenation of phenylacetylene of Au@N-doped carbon
catalysts obtained from Au-L1/TiO2 after pyrolysis at different temperatures.a
Entr
Pyrolysis
Conversion of 1a
(%)
Yield of 2a
(%)
y
temperature (°C)
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4
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400
600
800
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100
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>99
>99
>99
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aReaction conditions: 0.14 mmol 1a, 2 mol% of Au@N-doped catalyst, 2 mL of ethanol, 100 °C,
6 bar H2, 20 h. bDetermined by GC using internal standard technique.
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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 oC.a
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Entry
Catalyst precursor
Au-L1/TiO2
Au-L2/TiO2
Au-L3/TiO2
Au-L4/TiO2
Au-L5/TiO2
Au-L6/TiO2
Au-L1/TiO2
Au/TiO2
Conversion of 1a (%)
Yield of 2a (%)
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>99 (95)
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-
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>99
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L1/TiO2
0
aReaction conditions: 0.14 mmol of 1a, 2 mol% of Au catalyst, 2 mL of ethanol, 100 °C, 6 bar of
b
H2, 20 h. Determined by GC using internal standard technique; numbers in parenthesis refer to
isolated yield. cCatalyst not pyrolyzed.
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Among the different N-containing ligands tested, 1,10-phenanthroline (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 (Table S1, entries 1–4)
showed low conversion (≤10%) of 1a. Although, the catalyst supported on silica reaches 71% of
conversion (Table S1, entry 5), the fully hydrogenated product 3a was formed. Titanium dioxide
proved to be the best support (Table S1, entry 6) under the studied conditions. Gold precursor
influence in the catalytic activity was also evaluated (Table S2), the catalyst prepared using gold
acetate (Table S2, entry 1) show the best result, full conversion of 1a, a slightly decrease in
catalytic activity was noticed when using hydrogen tetrachloroaurate(III) (68% of conversion of
1a, Table S2, entry 2) or potassium dicyanoaurate(I) (90% of conversion of 1a, Table S2, entry
3). An increase in the Au:L1 molar ratio from 1:2 to 1:5 or 1:10 lead to complete loss of catalytic
activity (Table S3, entries 2 and 3). After a further survey of common reaction parameters
(solvent, reaction temperature, molecular hydrogen pressure; for more details, see Tables S4, S5
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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 reductions and anatase as the predominant phase without significant
phase conversion after thermal treatment (Figure S5). EDX spectrum imaging (elemental
mapping) of the Au-L1/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, due to the overlap of the majoritarian
Ti signal at 0.452 keV with the N signal at 0.392 keV (Figure S6).
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Figure 1. (a) Bright field STEM image of a representative area of the Au@N-doped carbon
catalyst (Au-L1/TiO2 material after pyrolysis at 400 °C) and (b) corresponding EDX elemental
maps. Carbon appear as the red spots deposited mostly on the yellow gold particles. (c) Au 4f
XPS spectra and (d) N 1s XPS spectra for Au-L1/TiO2 material after pyrolysis at 400 °C.
The type of N-doping was investigated in more detail by X-ray 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 eV 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
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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 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
N1s 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 ºC (Figure S8)
and for Au/L1-TiO2 at other pyrolysis temperature (Figure S9). The as-prepared 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).
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The kinetic study shown in Figure 2b indicates full conversion of 1a to 2a in about 8 hours 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 worth to note that a sixty-fold reaction rate enhancement was
observed when comparing the catalytic activity of similar Au catalysts prepared with L1 (Figure
-1
2b) (reaction rate = 0.273 mmol gcat h-1) or without any ligand (Figure 2c) (reaction rate =
-1
0.00472 mmol gcat 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 ten times
using the same catalysts portion without any further treatment or ligand addition. As depicted in
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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 tenth recycle (Figure S12).
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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 using the
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Au@N-doped carbon/TiO2 catalyst; (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 oC, 6 bar of H2.
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Since 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 alkynes was readily hydrogenated to the desired
alkene and cis-alkene with moderate to excellent yield (Scheme 1) and, notably, different from
what occur with other transition metals, without any over-reduction to alkane. cis-Control was
also reported to be a consequence of FLP. Moreover, both electron-deficient substituents, such as
esters (2ab) and carboxylic acids (2ad) moieties, and electron-rich groups, for example amino
(2s and 2t) and methoxy (2c and 2o), 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 approximately 70 mol mol-1 h-1. Such a 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
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TOF is also high when taking into account values for system using other sources of hydride and
ligands to boost the catalytic activity (Table S7).
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Scheme 1. Au@N-doped carbon/TiO2 catalyst for the semihydrogenation of alkynes to
alkenes.a,b
aReaction conditions: 0.14 mmol of alkyne, 2 mol% of Au-L1/TiO2 catalyst, 2 mL of ethanol at
b
100 °C, 6 bar of H2. GC yields are shown; values in parentheses refer to isolated yields.
d
Selectivity was >99%. c8 bar of H2. 80 °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 diverse sensitive functionalized molecules (Scheme 2).
Total chemoselectivity was observed for 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, since 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 N-oxides and sulfoxides can be efficiently reduced too
(Table S8).
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Scheme 2. Selective hydrogenation of 1a in the presence of molecules with other reducible
functional groups by Au@N-doped carbon/TiO2 catalyst.a
aReaction conditions: 0.14 mmol 1a, 0.14 mmol of sensitive molecule, 2 mol% of Au catalyst, 2
mL of ethanol, 24 h, 100 oC, 6 bar H2. Conversion was determined by GC using internal standard
technique.
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Based on the experimental results and previous ascertain, 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 (FLP) 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 co-
doped 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
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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 electron-withdrawing groups and the positive
value of ρ (ρ = +1.73; R2=0.93) highlights the build-up of negative charge during the catalytic
cycle. The value of ρ with Lindlar catalysts was close to 0, which indicates a lack of substituent
effect in the hydrogenation reaction, and no charge 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
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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.
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Figure 3. Hammett plot for the hydrogenation of various para-substituted alkynes catalyzed by
Au@N-doped carbon/TiO2 and Lindlar catalyst.
Based on 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 eV and −0.75 eV, respectively, Table S9), with shortest Au···N distances of ca.
3.3 Å and 3.5 Å, respectively. At high coverage situations, the packing predicted in model A2
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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 eV 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.
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Herein we use model A (Figure 4 inset) to properly mimic the N-doped (pyridinic) graphene-
like sheets experimentally observed after pyrolysis. The proposed reaction mechanism is shown
in Figure 4. Firstly, 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 N-assisted
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
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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.
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Figure 4. Computed reaction energy profile for the hydrogenation of 1a on L1-functionalized
Au(111) surface (Au = yellow, N = blue, C = grey, H = white).
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 (FLP) interfaces
according to computational studies, enables the heterolytic activation of molecular hydrogen,
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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 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 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.
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EXPERIMENTAL SECTION
Procedure to 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 minutes at 60 °C. The support
was then added and the mixture was stirred for more 20 min. After, the solvent was dried by
rotary evaporation. The as obtained 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 hours.
General procedure for alkynes hydrogenation
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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
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by column chromatography on silica gel to afford corresponding alkenes. H confirmed the
purity of the isolated products.
Computational details
All calculations were performed at Density Functional Theory (DFT) level using the Vienna
Ab-initio 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
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fixed during frequency calculations. All inputs and final structures can be found in the ioChem-
BD repository.73,74
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ASSOCIATED CONTENT
Supporting Information. These materials are available free of charge via the Internet at The
Supporting Information is available free of charge on the ACS Publications website.
Experimental details, additional experimental results, characterization of the catalyst (XRD,
TEM, UV-vis), and L1/Au adsorption energies.
AUTHOR INFORMATION
Corresponding Author
*E-mail for L. M. R.: lrossi@iq.usp.br.
ACKNOWLEDGMENT
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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