Piperazine-promoted gold-catalyzed hydrogenation: The influence of capping ligands

Article abstract of DOI:10.1039/c9cy02016k

Gold nanoparticles (NPs) combined with Lewis bases, such as piperazine, were found to perform selective hydrogenation reactions via the heterolytic cleavage of H₂. Since gold nanoparticles can be prepared by many different methodologies and using different capping ligands, in this study, we investigated the influence of capping ligands adsorbed on gold surfaces on the formation of the gold-ligand interface. Citrate (Citr), poly(vinyl alcohol) (PVA), polyvinylpyrrolidone (PVP), and oleylamine (Oley)-stabilized Au NPs were not activated by piperazine for the hydrogenation of alkynes, but the catalytic activity was greatly enhanced after removing the capping ligands from the gold surface by calcination at 400 °C and the subsequent adsorption of piperazine. Therefore, the capping ligand can limit the catalytic activity if not carefully removed, demonstrating the need of a cleaner surface for a ligand-metal cooperative effect in the activation of H₂ for selective semihydrogenation of various alkynes under mild reaction conditions.

Full text of DOI:10.1039/c9cy02016k

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Piperazine-promoted gold-catalyzed
hydrogenation: the influence of capping ligands†
Cite this: DOI: 10.1039/c9cy02016k
a
a
a
Jhonatan L. Fiorio,
Eduardo C. M. Barbosa,
Danielle K. Kikuchi,
a
b
Pedro H. C. Camargo,
A. Stephen K. Hashmi
Matthias Rudolph,
b
and Liane M. Rossi *a
Gold nanoparticles (NPs) combined with Lewis bases, such as piperazine, were found to perform selective
hydrogenation reactions via the heterolytic cleavage of H . Since gold nanoparticles can be prepared by
2
many different methodologies and using different capping ligands, in this study, we investigated the
influence of capping ligands adsorbed on gold surfaces on the formation of the gold–ligand interface.
Citrate (Citr), polyIJvinyl alcohol) (PVA), polyvinylpyrrolidone (PVP), and oleylamine (Oley)-stabilized Au NPs
were not activated by piperazine for the hydrogenation of alkynes, but the catalytic activity was greatly
enhanced after removing the capping ligands from the gold surface by calcination at 400 °C and the
subsequent adsorption of piperazine. Therefore, the capping ligand can limit the catalytic activity if not
carefully removed, demonstrating the need of a cleaner surface for a ligand–metal cooperative effect in
Received 7th October 2019,
Accepted 28th November 2019
DOI: 10.1039/c9cy02016k
rsc.li/catalysis
the activation of H for selective semihydrogenation of various alkynes under mild reaction conditions.
2
by capping ligands, until very recently, ligands were often
removed either by extraction or pyrolysis before catalytic
Introduction
16–18
Colloidal metal nanoparticles (NPs) are often prepared by
colloidal synthesis methods and stabilized in solution by the
interaction of specific capping ligands with the surface, which
ensures that the nanoparticles do not coalesce. This type of
methodology has received attention for the preparation of
supported catalysts, allowing the preparation of metal
catalysts with high control over particle size, shape, and
applications.
capping ligands could be a dynamic component of the active
However, it has been discussed that the
19–26
catalyst which improves selectivity and activity.
We recently
1
,2
demonstrated an enhancement in the catalytic activity of gold
nanocatalysts when using amine ligands as an additive in the
reaction medium. The combination of the added amine ligand
(Lewis base) and the gold surface (Lewis acid) triggered the
heterolytic activation of molecular hydrogen via a frustrated Lewis
pair-like mechanism, allowing an unexpectedly high activity in an
3–5
surface composition.
The major concern of using colloidal
synthetic methods is the influence of capping ligands
adsorbed on the metallic surface both on the immobilization
efficiency and on the catalyst performance, due to their
binding affinity to the metallic surface.
presence of capping ligands can also decrease the
27
alkyne semi-hydrogenation reaction. This behavior has been
demonstrated with gold catalysts prepared by an impregnation–
reduction method, therefore free of capping ligands; however,
since capping ligands are generally present when the synthesized
colloidal nanoparticles are used in catalysis, a systematic study
analyzing the effect of capping ligands on the formation of the
6
–8
Overall, the
accessibility of substrates to the metal surface or metal–
9
–11
support interface,
causing low catalytic activity, since
28
reactants cannot bind to surface atoms and undergo chemical
transformations into the desired products.
Based on the assumption that the catalytic activity is
diminished if the surface of metal nanoparticles is partly covered
piperazine–gold interface is missing.
1
2–15
Herein, we investigate how capping ligands adsorbed on
the surface of Au NPs can hinder the formation of the
piperazine–gold interface needed to activate H2 in
hydrogenation reactions. Our studies reveal that the presence
of capping ligands blocks the adsorption of the amine onto
the gold surface, thereby leading to low catalytic activity.
When the capping ligands were removed from the catalyst
surface and the piperazine ligand was added, enhanced
catalytic activity was observed.
a
Departamento de Química Fundamental, Instituto de Química, Universidade de
São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil
b
Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg University,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany. E-mail: lrossi@iq.usp.br
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cy02016k
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Experimental section
Preparation of AuPVA/C
sodium citrate (150 mL, 2.2 mM), tannic acid (0.1 mL, 2.5
mM) and K CO (1 mL, 150 mM), and the resulting solution
2
3
was heated in a heating mantle to 70 °C. Once the
temperature reached 70 °C, 1 mL of HAuCl4 (25 mM) was
injected. The solution was kept at 70 °C for an additional 5
min to ensure complete reaction of the gold precursor. After,
The gold catalyst stabilized with PVA (average molecular
−
1
weight MW = 9000–10 000 g mol , 80% hydrolyzed) was
1
6,29
prepared as previously described in the literature
follows: to an aqueous HAuCl
as
−
4
−1
4
solution (5.08 × 10 mol L ),
1
g of the carbon support was added, and the mixture was
the required amount of a PVA solution (1 wt%) was added
stirred overnight to immobilize the Au NPs. The catalyst was
collected by centrifugation, washed three times with acetone
and dried under vacuum. The metal content was 0.4 wt% as
determined by FAAS.
(
PVA/(Au) (w/w) = 1.2), and a freshly prepared solution of
−
1
NaBH
4 4
(0.1 mol L , NaBH /(Au)IJmol/mol) = 5) was then
added to form a dark-brown sol. After 30 min of sol
generation, 1 g of the carbon support (Vulcan XC 72R) was
added to the colloid, and the mixture was vigorously stirred
overnight to immobilize the Au NPs in the support. The
slurry was filtered, and the catalyst was washed three times
with acetone and dried under vacuum. The final metal
loading was 0.7 wt% as determined by FAAS.
Thermal treatment to remove the ligand
Typically, to remove the ligand from the catalyst surface a
certain amount of the as-prepared catalyst was placed in a
crucible and the solid was thermally treated in an oven at a
rate of 10 °C per minute and held at 400 °C for 2 h under a
2
N atmosphere. Instead of being calcined, a set of catalysts
Preparation of AuPVP/C
were refluxed with water as previously described in the
The gold nanoparticles stabilized with PVP were prepared
according to a procedure reported elsewhere.
synthesis of 1 g of AuPVP/C, an aqueous solution of HAuCl
16
literature to remove the excess ligand, and then dried under
3
0,31
In a typical
vacuum.
4
·
3H
2
O (0.8 mL, 25 mg HAuCl
4 2
·3H O per mL Sigma Aldrich,
Catalytic investigations
Catalytic hydrogenation reactions
≥
49.0%) was added to 400 mL of deionized water under
vigorous stirring conditions, followed by the addition of a 1
wt% aqueous solution of PVP (Sigma Aldrich, average
In a modified Fischer–Porter 100 mL glass reactor, the
substrate (1 mmol), piperazine (1 mmol) and gold catalyst (0.5
mol%) were mixed with 2 mL of ethanol. The reactor was
−
1
molecular weight MW = 10 000 g mol ). Next, a 0.1 M freshly
prepared solution of NaBH was added (mol NaBH per mol
4
4
Au = 5) to form a red sol. After 30 min of sol generation, the
colloid was immobilized by adding 1 g of carbon support
closed and purged five times with H , leaving the vessel at 6
2
bar of H . All the reactions were performed at 80 °C; the
2
(Vulcan XC 72R). The mixture was stirred overnight. After,
temperature was maintained with an oil bath on a stirring hot
the slurry was filtered, and the catalyst was washed three
times with acetone and dried under vacuum. The metal
loading was 0.5 wt% as determined by FAAS.
plate. After the completion of the reaction, the catalyst was
removed by centrifugation and the products were analyzed by
GC with an internal standard to determine the conversion of
the substrate and the selectivity for the product.
Preparation of AuOleylamine/C
32
Recycling experiment
In a typical synthesis as described elsewhere, 10 mL of
tetralin, 10 mL of oleylamine and 278.4 mg of HAuCl were
added to a three-neck round-bottom flask and the mixture was
magnetically stirred under a N flow for 10 min, at 35 °C. After,
a reducing solution containing 0.5 mmol of TBAB, tetralin (1
mL), and oleylamine (1 mL) was prepared by sonication and
injected all at once into the precursor solution. The reduction
was instantaneously initiated, and the solution changed to a
deep purple color. The mixture was allowed to react at 35 °C for
4
In a typical recycling experiment, the substrate (1 mmol),
piperazine (100 eq.) and gold catalyst (1 mol%) were added
to a Fischer–Porter 100 mL glass reactor equipped with a
magnetic stirring bar, the reactor was then closed, purged
five times with H , leaving the vessel at 6 bar of H , and the
resulting reaction mixture was heated at 80 °C for 24 h. After
the reaction was complete, the reactor was cooled to room
temperature and the hydrogen was released. An internal
standard (biphenyl) was added to the solution to determine
the conversion and selectivity by GC analysis of the mixture.
The catalyst separated by centrifugation was washed with
ethanol three times, then dried under vacuum, and reused
for the next reaction.
2
2
2
1
h before 1 g of carbon support was added and vigorously
stirred overnight. The slurry was filtered, washed three times
with acetone and dried under vacuum. The metal content was
0.5 wt% as determined by FAAS.
Preparation of AuCitrate/C
Product analysis of catalytic experiments
To prepare the gold catalyst stabilized with citrate, we
3
3
adopted a previously described method in the literature.
A
All the products were determined by GC analyses carried out
with a Shimadzu GC-2010 equipped with an RTx-5 column
(30 m × 0.25 mm × 0.25 mm) and a FID (all GC yields are
typical procedure is as follows: to a three-neck round-bottom
flask was added a freshly prepared aqueous solution of
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Fig. 1 TEM images of nanoparticles before and after thermal treatment at 400 °C (A and E) AuPVA/C, (B and F) AuPVP/C, (C and G) AuOley/C, and (D
and H) AuCitr/C.
average values from at least 2 runs). Method: Ti = 40 °C, Tf =
00 °C, 17 min. Internal standard: biphenyl. The conversion
and selectivity were determined from the total amount of the
detected products and reactant.
evaluate the effect of each capping ligand on the catalytic
activity while avoiding possible size-dependent effects. The
colloidal Au NPs with different capping ligands were
immobilized on carbon and the TEM analyses of the
supported Au/C catalysts confirmed the uniform dispersion
over the support and the control over the NP size (Fig. 1A–D).
The corresponding particle size distributions (Fig. S2†)
indicate that the mean particle size of each sample is ∼3 nm.
To remove the ligand from the catalyst surface and compare
the adsorption of the amine ligand on a clean surface with
that on a surface occupied by capping ligands, we conducted
a thermal treatment in a portion of the as-prepared catalyst.
An obvious increase in size was observed for all the catalysts
2
High pressure NMR studies
All the reactions were analyzed using a Bruker Avance DRX
3
00 MHz at 80 °C. The AuPVA nanoparticles were extensively
washed to remove the excess stabilizer before analysis. The
gold colloid, piperazine and deuterated solvent (D O) were
2
added to a high pressure NMR tube, which was pressurized
with 10 bar of H and heated to the desired temperature. The
2
1
reactions were monitored using H NMR spectroscopy.
(
Fig. 1E–H), and the mean diameter after thermal treatment
was ∼8 nm (Fig. S2†). Different UV-vis spectrum profiles for
the catalysts were noticed (Fig. S1†), suggesting that the
electronic structure of the Au NPs can be disturbed by the
surrounding organic ligands or it might be related to the
Results and discussion
Colloidal Au NPs with various capping ligands but with
similar sizes were prepared by different methodologies to
a
Table 1 Catalytic activity of Au catalysts in the hydrogenation of phenylacetylene 1a
Entry
Catalyst
Calcined at 400 °C
Amine
Conversion (%)
Selectivity to 2a (%)
1
2
3
4
5
6
7
8
9
AuPVA/C
AuPVA/C
AuPVA/C
AuPVA/C
AuCitr/C
AuCitr/C
AuCitr/C
AuCitr/C
AuOley/C
AuOley/C
AuOley/C
AuOley/C
AuPVP/C
AuPVP/C
AuPVP/C
AuPVP/C
No
No
Yes
Yes
No
No
Yes
Yes
No
No
Yes
Yes
No
No
Yes
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
6
3
8
>99
5
3
2
70
2
3
4
68
3
6
8
>99
>99
>99
>99
>99
>99
>99
94
>99
>99
>99
85
10
11
12
13
14
15
16
>99
>99
>99
>99
82
a
Reaction conditions: 1 mmol of alkyne, 0.5 mol% Au, 1 mmol of piperazine, 2 mL of EtOH, 80 °C, 6 bar of H
2
, 24 h. Conversion and
selectivity were determined by GC using the internal standard technique.
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Fig. 2 A) Time course studies comparing the reaction profile of the calcined catalysts at 400 °C with addition of piperazine in the hydrogenation
of phenylacetylene; B) AuPVA/C; C) AuPVP/C; D) AuCitr/C; E) AuOley/C; reaction conditions: 4 mmol of alkyne, 0.5 mol% Au, 4 mmol of piperazine, 8
2
mL of EtOH, 80 °C, 6 bar of H .
differences in size distribution; both factors may cause a shift
in the surface plasmon resonance dipole mode of the Au
(Table 1, entries 4, 8, 12 and 16). This trend suggests that the
capping ligands restrict the free access of the reactants to the
catalytically active sites on the Au NP surface and/or prevent
the adsorption of piperazine, which is crucial to lowering the
34–36
NPs.
Characteristic absorption bands related to the
corresponding protective agents were observed in the FTIR
spectra of the as-prepared samples (Fig. S3a–d†). In the
thermally treated catalyst, no typical band of the capping
ligand was observed, confirming the removal from the
catalyst surface (Fig. S4†).
2
7
energy barrier for H
2
dissociation. Once these capping
It is anticipated that the capping ligands on the surface of
the Au NPs might affect the adsorption of piperazine,
possibly blocking the formation of the piperazine–gold
3
7,38
interface.
phenylacetylene as a model transformation under conditions
optimized elsewhere (6 bar of H , piperazine as a ligand and
0 °C) using the as-prepared supported Au NP catalysts
Here, we examined the hydrogenation of
2
2
7
8
(mean diameter size ∼3 nm) with different capping ligands
(PVA, citrate, oleylamine and PVP) and their thermally treated
counterparts (mean diameter size ∼8 nm). All the catalysts
as-prepared and thermally treated) reached very low
conversions in the absence of piperazine (Table 1, entries 1,
, 5, 7, 9, 11, 13 and 15). Typically, the as-prepared Au
catalysts, which contain capping ligands and smaller NPs,
were less active than the thermally treated Au catalysts
(
3
Fig. 3 Reaction rate performance of the AuPVA/C catalyst thermally
treated at different temperatures in the hydrogenation of
phenylacetylene.
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ligands were removed from the Au NP surface, the addition
of piperazine promoted the favorable activation of H2,
leading to a high conversion of 1a and a high selectivity to
the alkene product 2a (Table 1, entries 4, 8, 12 and 16). The
behavior reproduces that of the Au–piperazine ligand system
reported before, suggesting that piperazine plays a dual role
in the activation of Au NPs as an activity and selectivity
enhancer.
Kinetic studies were carried out with the four most active
catalytic systems (Fig. 2). Notably, the thermally treated
AuPVA/C in the presence of piperazine is the most active
catalytic system, reaching full conversion and high selectivity
to 2a. The reaction rates for the thermally treated AuPVP/C,
AuCitr/C, and AuOley/C are significantly lower. The initial lower
activity (activation time) might be due to carbonaceous
residues formed during the thermal treatment. However,
considering that all the catalysts are supported on carbon, it
is difficult to quantitatively determine any change in the
carbon content before and after calcination (Table S2†). No
significant size change was noticed among the different
capping ligands (∼8 nm) to justify the difference in activity
2
7
(Fig. S2†). Instead of calcination, the as-prepared Au
and
PVA
Aucitr catalysts were subjected to a milder method to remove
1
6,17
the capping ligands, as reported in the literature.
Both
catalysts were washed with water under reflux at 90 °C to
remove the excess capping ligand and then tested in the
hydrogenation reaction with and without piperazine (Table
S3†). The washed catalysts became slightly more active, but
the addition of piperazine promoted an activity enhancement
as seen in the calcined samples, suggesting that this method
Table 2 Semihydrogenation of various alkynes to alkenesa
a
2
Reaction conditions: 1 mmol of alkyne, 0.5 mol% Au (AuPVA/C calcined at 400 °C), 1 mmol of piperazine, 2 mL of EtOH, 80 °C, 6 bar of H .
Conversion and selectivity were determined by GC using the internal standard technique. In all cases, the selectivity was >99%. Isolated yields
b
c
1
in parenthesis. 1 mol% Au. Determined by H NMR.
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Fig. 4 A) Hot filtration test and B) recycling experiments for hydrogenation of phenylacetylene.
is also effective in removing the capping ligands for improving
the accessibility of piperazine to the Au NPs surface.
maintained through all the cycles (Fig. 4B). After each
reaction, the catalyst was separated by filtration and the
filtrate was analyzed by inductively coupled plasma atomic
emission spectrometry (ICP AES) analysis, which revealed
that no leaching of Au species in the reaction mixture
occurred (detection limit: 0.1 ppm) (Table S1†). TEM analysis
of the recycled catalyst (Fig. S6†) revealed that the decrease
in the catalytic activity can be due to a significant increase in
the mean diameter size of the Au NPs to ∼20 nm. Therefore,
as previously described, the capping ligand might play an
important role in preventing the agglomeration of the
nanoparticles and increasing the lifetime of the catalyst.
However, once the capping ligand is completely removed, a
At this point, we studied the influence of the calcination
temperature on the reaction rate, using the AuPVA/C catalyst
as a model. As shown in Fig. 3, the calcination temperature
and catalytic activity exhibit a volcano-type relationship. The
reaction rate increases with the calcination temperature
(ranging from 200 to 400 °C), reaching its maximum, and
abruptly decreases at higher temperatures. The drop in the
reaction rate at higher temperatures is due to nanoparticle
sintering as verified by TEM analysis of the catalysts (Fig.
16
S5†).
The low catalytic activity at lower calcination
temperatures may happen due to the presence of capping
ligands blocking the adsorption of the amine ligand.
The catalytic potential of AuPVA/C thermally treated at 400
6
drop in the activity is observed over the recycling process.
Altogether, the above catalytic results imply that, as
previously described, piperazine once adsorbed on the Au
°C, which exhibited the best performance, was investigated
using various alkynes. Ethynylbenzenes bearing electron-
donating and electron-withdrawing groups were selectively
hydrogenated to the corresponding terminal alkenes with
excellent selectivity, and reducible functional groups, such as
halogen and hydroxyl moieties, were tolerated (Table 2,
entries 2a–2t). Various aliphatic terminal alkynes were also
efficiently converted into their corresponding terminal
alkenes without isomerization into internal alkenes (Table 2,
entries 2u–2x). Importantly, alkynes with an alkene moiety
were smoothly hydrogenated, whereas alkene moieties in the
parent and product molecules were completely intact
surface facilitates the heterolytic H
2
activation at the Au
2
4,38–42
NPs.
However, the presence of capping ligands on the
surface of Au NPs avoids the adsorption of the amine ligand
(Fig. 5), providing an unfavorable situation for hydrogen
activation. Effective removal of the capping ligand by thermal
treatment exposes the nanoparticles' surfaces and thus
enhances the catalytic activity, as a result of the successful
formation of the piperazine–gold interface, which promotes
the molecular hydrogen splitting, the rate-determining step
of the reaction.
To gain insight into the nature of the catalytic active
species, NMR investigations were carried out using the AuPVA
(Table 2, entries 2q–2t). Moreover, a series of internal alkynes
were smoothly hydrogenated to the Z-alkenes in excellent
diastereoselectivity (Table 2, entries 2y–2ae).
Considering that heterogeneous catalysts, especially in the
presence of ligands, may act as a reservoir for highly active
leached species, a hot filtration test was carried out in order
to detect whether soluble active Au species were responsible
for the observed catalytic activity. After the first reaction
under standard conditions, the catalysts was filtered off when
the second reaction reached average 50% conversion and no
further conversion was observed when the reaction was
continued using the hot filtrate solution only (Fig. 4A). This
experiment excludes the presence of active solubilized
species. The recovered solid catalyst was reusable, but a
significant decrease in the catalytic activity was noticed after
the third cycle, though the selectivity towards the alkene was
Fig. 5 Scheme showing the differences in the activity of Au NPs with
and without a capping ligand.
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represents
a
clear demonstration of the ligand–metal
cooperative effect in the activation of H , where the nature of
2
the capping ligand plays a crucial role and can limit the
catalytic activity if not carefully removed.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to INCT-Catálise and the Brazilian
government agencies FAPESP (2016/16738-7, 2015/21366-9,
and 2015/26308-7), CNPq, the Serrapilheira Institute (grant
number Serra-1709-16900), and CAPES for financial support.
E. C. M. B. thanks FAPESP for the fellowship (2015/11452-5)
and J. L. F. also thanks CAPES-DAAD-CNPQ for his
scholarship (Grant 88887.161404/2017-00).
1
Fig. 6 Overlay of the H NMR spectra of (A) piperazine, (B) piperazine/
2 2
H (10 bar), (C) piperazine/Au colloid, (D) piperazine/Au colloid/H (10
+
2
bar) and (E) piperazine/Au colloid/H in D O.
Notes and references
NPs and piperazine. The Au colloid was thoroughly washed
to remove the excess capping ligand before analysis. Fig. 6
shows the proton NMR spectra of piperazine solutions in
1 N. T. K. Thanh, N. Maclean and S. Mahiddine, Chem. Rev.,
2014, 114, 7610–7630.
2 Y. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem.,
Int. Ed., 2009, 48, 60–103.
D O at 80 °C with and without the Au colloid. Either the free
2
ligand (Fig. 6A) or the free ligand under H
exhibits similar signals around 2.7 (4H, N–H) and 4.8 ppm
8H, CH ). In the presence of the Au colloid, the signal
remains at 2.7 ppm, but decreases in intensity, most
probably due to H/D-exchange (Fig. 6C). As observed in
Fig. 6D, when the solution of piperazine with Au colloid was
2
pressure (Fig. 6B)
3 P. Sonström and M. Bäumer, Phys. Chem. Chem. Phys.,
2011, 13, 19270–19284.
4 C.-J. Jia and F. Schüth, Phys. Chem. Chem. Phys., 2011, 13,
2457–2487.
5 L. S. Ott and R. G. Finke, Coord. Chem. Rev., 2007, 251,
1075–1100.
(
2
pressurized with H
ppm. A signal with a similar chemical shift was also observed
when a piperazine solution was acidified (Fig. 6E), which is
in agreement with the literature value. Therefore, the signal
at 3.1 ppm may be assigned to protonated piperazine species,
which are formed in acidic media and through dissociation
2
(10 bar), a new signal appeared at 3.1
6 A. Villa, D. Wang, G. M. Veith, F. Vindigni and L. Prati,
Catal. Sci. Technol., 2013, 3, 3036.
7 R. Y. Zhong, K. Q. Sun, Y. C. Hong and B. Q. Xu, ACS Catal.,
2014, 4, 3982–3993.
8 P. Munnik, P. E. de Jongh and K. P. de Jong, Chem. Rev.,
2015, 115, 6687–6718.
4
3
of H
2
. Therefore, these results suggest that a ligand to metal
dissociation, via a heterolytic
9 C. Kartusch and J. A. van Bokhoven, Gold Bull., 2009, 42,
343–348.
cooperative effect causes the H
2
mechanism, leading to the protonation of the ligand, which
is in sharp agreement with the mechanism suggested in
previously reported studies.
10 T. Whittaker, K. B. S. Kumar, C. Peterson, M. N. Pollock,
L. C. Grabow and B. D. Chandler, J. Am. Chem. Soc.,
2018, 140, 16469–16487.
2
7,44–46
1
1 J. E. Bruno, K. B. Sravan Kumar, N. S. Dwarica, A. Hüther, Z.
Chen, C. S. Guzman, E. R. Hand, W. C. Moore, R. M. Rioux,
L. C. Grabow and B. D. Chandler, ChemCatChem, 2019, 11,
1650–1664.
Conclusions
In conclusion, we have studied the limitations of using Au
NPs with different capping ligands for the preparation of
piperazine–gold interfaces necessary to boost gold activity
towards hydrogenation reactions. The presence of capping
agents blocks the adsorption of the amine ligand leading to
low catalytic activity. The removal of the stabilizing agents by
thermal treatment, which allows the successful adsorption of
the amine ligand, leads to an increase in the catalytic activity.
Moreover, the catalytic system enables the highly selective
semi-hydrogenation of various alkynes under mild reaction
conditions, with excellent selectivity for alkenes. This work
12 A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed.,
2006, 45, 7896–7936.
13 L. M. Rossi, J. L. Fiorio, M. A. S. Garcia and C. P. Ferraz,
Dalton Trans., 2018, 47, 5889–5915.
14 S. Campisi, M. Schiavoni, C. Chan-Thaw and A. Villa,
Catalysts, 2016, 6, 185.
15 Q. Wang, M. Ming, S. Niu, Y. Zhang, G. Fan and J.-S. Hu,
Adv. Energy Mater., 2018, 1801698, 1801698.
16 J. A. Lopez-Sanchez, N. Dimitratos, C. Hammond, G. L.
Brett, L. Kesavan, S. White, P. Miedziak, R. Tiruvalam, R. L.
This journal is © The Royal Society of Chemistry 2019
Catal. Sci. Technol.

View Article Online
Paper
Jenkins, A. F. Carley, D. Knight, C. J. Kiely and G. J.
Catalysis Science & Technology
31 L. Abis, S. J. Freakley, G. Dodekatos, D. J. Morgan, M.
Sankar, N. Dimitratos, Q. He, C. J. Kiely and G. J. Hutchings,
ChemCatChem, 2017, 9, 2914–2918.
32 S. Peng, Y. Lee, C. Wang, H. Yin, S. Dai and S. Sun, Nano
Res., 2008, 1, 229–234.
Hutchings, Nat. Chem., 2011, 3, 551–556.
1
1
7 Z. Niu and Y. Li, Chem. Mater., 2014, 26, 72–83.
8 G. Collins, F. Davitt, C. O'Dwyer and J. D. Holmes, ACS Appl.
Nano Mater., 2018, 1, 7129–7138.
1
2
2
9 J. Zhang, L. D. Ellis, B. Wang, M. J. Dzara, C. Sievers, S.
Pylypenko, E. Nikolla and J. W. Medlin, Nat. Catal., 2018, 1,
33 J. Piella, N. G. Bastús and V. Puntes, Chem. Mater., 2016, 28,
1066–1075.
34 X.-K. Wan, J.-Q. Wang, Z.-A. Nan and Q.-M. Wang, Sci. Adv.,
2017, 3, e1701823.
35 K. L. Kelly, E. Coronado, L. L. Zhao and G. C. Schatz, J. Phys.
Chem. B, 2003, 107, 668–677.
36 R.-Y. Zhong, X.-H. Yan, Z.-K. Gao, R.-J. Zhang and B.-Q. Xu,
Catal. Sci. Technol., 2013, 3, 3013.
1
48–155.
0 M. Makosch, W.-I. Lin, V. Bumbálek, J. Sá, J. W. Medlin, K.
Hungerbühler and J. A. van Bokhoven, ACS Catal., 2012, 2,
2
079–2081.
1 G. Chen, C. Xu, X. Huang, J. Ye, L. Gu, G. Li, Z. Tang, B. Wu,
H. Yang, Z. Zhao, Z. Zhou, G. Fu and N. Zheng, Nat. Mater.,
2
016, 15, 564.
37 L. Wang, G. Kehr, C. G. Daniliuc, M. Brinkkötter, T.
Wiegand, A.-L. Wübker, H. Eckert, L. Liu, J. G. Brandenburg,
S. Grimme and G. Erker, Chem. Sci., 2018, 9, 4859–4865.
38 Y. Ma, S. Zhang, C.-R. Chang, Z.-Q. Huang, J. C. Ho and Y.
Qu, Chem. Soc. Rev., 2018, 47, 5541–5553.
39 D. Ren, L. He, L. Yu, R.-S. Ding, Y.-M. Liu, Y. Cao, H.-Y. He
and K.-N. Fan, J. Am. Chem. Soc., 2012, 134, 17592–17598.
40 K. K. Ghuman, L. B. Hoch, T. E. Wood, C. Mims, C. V. Singh
and G. A. Ozin, ACS Catal., 2016, 6, 5764–5770.
41 D. W. Stephan, Science, 2016, 354, aaf7229.
42 G. Lu, P. Zhang, D. Sun, L. Wang, K. Zhou, Z.-X. Wang and
G.-C. Guo, Chem. Sci., 2014, 5, 1082–1090.
2
2
2 P. Hao, S. Pylypenko, D. K. Schwartz and J. W. Medlin,
J. Catal., 2016, 344, 722–728.
3 S. G. Kwon, G. Krylova, A. Sumer, M. M. Schwartz, E. E.
Bunel, C. L. Marshall, S. Chattopadhyay, B. Lee, J. Jellinek
and E. V. Shevchenko, Nano Lett., 2012, 12, 5382–5388.
4 B. Wu, H. Huang, J. Yang, N. Zheng and G. Fu, Angew.
Chem., Int. Ed., 2012, 51, 3440–3443.
5 N. Kaeffer, H. J. Liu, H. K. Lo, A. Fedorov and C. Copéret,
Chem. Sci., 2018, 9, 5366–5371.
6 J. L. Fiorio, R. V. Gonçalves, E. Teixeira-Neto, M. A. Ortuño,
N. López and L. M. Rossi, ACS Catal., 2018, 8, 3516–3524.
7 J. L. Fiorio, N. López and L. M. Rossi, ACS Catal., 2017, 7,
2
2
2
2
2
2
3
43 P. I. Nagy, A. Maheshwari, Y.-W. Kim and W. S. Messer,
J. Phys. Chem. B, 2010, 114, 349–360.
44 N. Kaeffer, K. Larmier, A. Fedorov and C. Copéret, J. Catal.,
2018, 364, 437–445.
2
973–2980.
8 S. Arndt, M. Rudolph and A. S. K. Hashmi, Gold Bull.,
017, 50, 267–282.
2
9 F. Porta, L. Prati, M. Rossi, S. Coluccia and G. Martra, Catal.
Today, 2000, 61, 165–172.
0 N. Agarwal, S. J. Freakley, R. U. McVicker, S. M. Althahban,
N. Dimitratos, Q. He, D. J. Morgan, R. L. Jenkins, D. J.
Willock, S. H. Taylor, C. J. Kiely and G. J. Hutchings, Science,
45 W. Liu, Y. Chen, H. Qi, L. Zhang, W. Yan, X. Liu, X. Yang, S.
Miao, W. Wang, C. Liu, A. Wang, J. Li and T. Zhang, Angew.
Chem., Int. Ed., 2018, 57, 7071–7075.
46 R. Lin, D. Albani, E. Fako, S. K. Kaiser, O. V. Safonova, N.
López and J. Pérez-Ramírez, Angew. Chem., Int. Ed., 2019, 58,
504–509.
2
017, 358, 223–227.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2019