A General One-Pot Methodology for the Preparation of Mono- and Bimetallic Nanoparticles Supported on Carbon Nanotubes: Application in the Semi-hydrogenation of Alkynes and Acetylene

Article abstract of DOI:10.1002/chem.201901041

A facile and straightforward methodology for the preparation of monometallic (copper and palladium) and bimetallic nanocatalysts (NiCu and PdCu) stabilized by a N-heterocyclic carbene ligand is reported. Both colloidal and supported nanoparticles (NPs) on carbon nanotubes (CNTs) were prepared in a one-pot synthesis with outstanding control on their size, morphology and composition. These catalysts were evaluated in the selective hydrogenation of alkynes and alkynols. PdCu/CNTs revealed an efficient catalytic system providing high selectivity in the hydrogenation of terminal and internal alkynes. Moreover, this catalyst was tested in the semi-hydrogenation of acetylene in industrially relevant acetylene/ethylene-rich model gas feeds and showed excellent stability even after 40 h of reaction.

Full text of DOI:10.1002/chem.201901041

A Journal of
Accepted Article
Title: A general one-pot methodology for the preparation of mono
and bimetallic nanoparticles supported on carbon nanotubes:
application in the semi-hydrogenation of alkynes and acetylene
Authors: Diego Alberto Lomelí-Rosales, Jorge A. Delgado, Miriam Díaz
de los Bernardos, Sara Pérez-Rodríguez, Aitor Gual, Carmen
Claver, and Cyril Godard
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A general one-pot methodology for the preparation of mono and bimetallic nanoparticles
supported on carbon nanotubes: application in the semi-hydrogenation of alkynes and acetylene
Diego A. Lomelí-Rosales,a Jorge A. Delgado,*b Miriam Díaz de los Bernardos,b Sara Pérez-Rodríguez,b
Aitor Gual,b Carmen Claver,b,c and Cyril Godard,*c
a Departamento de Química, Centro Universitario de Ciencias Exactas e Ingenierías, Universidad de
Guadalajara. Blvd. Marcelino García Barragán 1421, CP 44430, Guadalajara, Jalisco, México.
b Centre Tecnològic de la Química, C/Marceli Domingo, 43007 Tarragona, e-mail:
jorgealonso.delgado@urv.cat
c
Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, C/Marceli Domingo 1, 43007
Tarragona, e-mail: cyril.gordard@urv.cat
Abstract
A facile and straightforward methodology for the preparation of monometallic (copper and palladium)
and bimetallic nanocatalysts (NiCu and PdCu) stabilized by a N-heterocyclic carbene ligand is
presented. Both colloidal and supported nanoparticles (NPs) on carbon nanotubes (CNTs) were
prepared via a one-pot synthesis with outstanding control on their size, morphology and composition.
These catalysts were evaluated in the selective hydrogenation of alkynes and alkynols. PdCu/CNTs
revealed an efficient catalytic system providing highly selectivity in the hydrogenations of terminal and
internal alkynes. Moreover, this catalyst was tested in the semi-hydrogenation of acetylene in
industrially relevant acetylene/ethylene-rich model gas feeds and showed excellent stability even after
40 h of reaction.
Keywords: Ni, Cu, Pd, NiCu, PdCu NPs, bimetallic nanoparticles, selective hydrogenation, alkynes,
acetylene.
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Introduction
The development of novel metal nanoparticles (MNPs) has been intensively studied in the last decades
because of their unique physical and chemical properties,[1] and their potential utilization for various
applications such as catalysis,[2] where their efficiency in terms of productivity is mainly attributed to
their characteristic high surface-to-volume ratio compared to standard heterogeneous catalysts.[3]
Such a reactivity highly depends on the size and shape of the MNPs,[4] and can be additionally
modulated by the selection of the stabilizer or ligand,[2a] to provide the transformation of the substrate
into a specific product.[5] In this context, the preparation of metal nanoparticles via colloidal methods
can provide highly sophisticated functionalization of the active phase at structural and surface level.[3b]
Moreover, colloidal nanoparticles can be utilized as models for the characterization of the active phase
of catalysts, providing information which is hardly accessible when diluted onto solid carriers.
Among the transition metals reported in the selective hydrogenation of alkynes, palladium has been
identified as the most efficient metal in terms of activity and selectivity.[6] Nevertheless, if the reaction
is not interrupted once the alkyne substrate is consumed, pure Pd phases quickly over-hydrogenate
the alkene product. The dilution of the highly active palladium phase by a second metal with a low
affinity for hydrogen (e.g. Ag, Au, Cu, etc) is one of the strategies for enhancing the catalyst
performance in the selective hydrogenation of alkynes. The impact of such a dilution can be attributed
to geometric and electronic effects.[6-7] The geometric effect is attributed to the creation of suitable
ensembles of unique character or to the blockage of unselective active sites.[7b] The electronic effect
refers to the ability of a modifier to affect the electronic properties of the active metal in order to
enhance the thermodynamic selectivity and/or to suppress the formation of (sub)surface species.[6, 7c]
Among the most common bimetallic palladium based formulations tested in the selective
hydrogenation of alkynes, PdAg,[8] and PdAu,[9] revealed particularly efficient. Other combinations less
extensively studied are PdPt,[9a] PtZn,[9a] PdCu,[8a] PdRhP.[10] and PdB.[11] For instance, Kiwi-Minsker and
co-workers investigated the role of a second metal using colloidal Pd and bimetallic Pd-Ag and Pd-Cu
NPs as catalysts in the selective hydrogenation of dehydroisophytol (DIP) into isophytol (IP).[8a] The
bimetallic NPs were prepared using Pd/M ratios of 1.5-5.0 (M = Ag or Cu) by chemical reduction on
pre-formed Pd colloids stabilized by polyvinylpyrrolidone (PVP). The bimetallic Pd-Ag NPs adopted a
core (Pd)-shell (Ag) structure whereas a mixed alloy was obtained for Pd-Cu nanocrystals. In catalysis,
a significant increase in selectivity to the target alkenol from 91 % up to 97% (at 99% conversion) was
observed after incorporation of the second metal. This result was attributed to the dilution of the
surface Pd-sites by Ag or Cu and modification of the electronic properties of the nanoparticle surfaces.
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In addition to palladium based catalysts, very recently, Fedorov and co-workers reported the high
potential of a non-noble metal based catalyst consisting in silica supported copper nanoparticles
(capped with phosphorus or carbene ligands), in the semihydrogenation of alkynes.[2a, 12] The authors
evidenced high activity and alkene selectivity for a series of internal alkynes (at 50 bar H2 and 60 °C)
depending on the ligand used. In spite of these promising results, the preparation of well-defined
metallic copper nanoparticles usually involves the use of expensive copper precursors and complicated
synthetic methodologies.[2a, 12-13] In this regard, the development of synthetic methodologies for
copper nanoparticles (and bimetallic combinations of this metal) is of high interest.
Although there are available commercial options for the semihydrogenation of alkynes such as the
Lindlar, and the more recently commercialized Pd Nanoselect catalysts, they frequently suffer from
limitations in terms of robustness,[14] or accessibility.[15] In addition, the frequent requirement of the
presence of large amounts of nitrogen additives or the employment of highly toxic poisons such as Pb
have stimulated the research for the design of more selective and environmentally friendly catalytic
systems.[3b, 16]
In a previous contribution from our group, a new methodology was reported for the preparation of
small and well-defined nickel nanoparticles stabilized by N-heterocyclic carbene ligand (NHC), using a
“one-pot” synthesis based on the decarboxylation of 1,3-dimethylimidazolium-2-carboxylate
(Me2ImCO2).[16c] The high selectivity achieved to Z-alkenes in the hydrogenation of alkynes using these
nanocatalysts was correlated with the presence of N-heterocyclic carbene ligand (NHC) on the nickel
nanoparticle surface. In the present report, this synthetic methodology was extended to other metals
(Cu and Pd) and to bimetallic systems (NiCu and PdCu) and the resulting nanocatalysts applied to the
liquid phase semi-hydrogenation of alkynes and the gas phase semi-hydrogenation of acetylene in
ethylene-rich model gas feeds. As depicted in Scheme 1, this facile methodology, offers the possibility
to access well-defined PdCu bimetallic nanostructures using a one-pot approach, offering a simple
alternative to previous multistep routes.
Scheme 1. Synthetic approaches to the preparation of PdCu bimetallic catalysts
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Results and discussion
Preparation of nanocatalysts
Colloidal monometallic Cu, Pd and bimetallic NiCu and PdCu were prepared by decomposition of the
corresponding metallic precursors, Cu5Mes5, Pd(dba)2, and mixtures of Ni(COD)2/Cu5Mes5 or
Pd(dba)2/Cu5Mes5 respectively, under 3 bar of hydrogen in the presence of the NHC ligand precursor
(0.5 eq. vs. metal).
Specific conditions of metal concentration or reaction temperature (r.t./70 °C) were utilized depending
the metallic formulation (see Supporting information for details), but in all cases the reaction was
carried out in a one-pot approach. Transmission Electron Microscopy (TEM) images of the colloidal
nanoparticles are displayed in Figure 1. The series of colloidal nanoparticles exhibited in general
spherical shapes, with particle sizes between 2.7-6.4 nm. The particle sizes of monometallic CuNPs and
bimetallic NiCuNPs were 6.4 ± 1.4 and 5.2 ± 1.1 nm, respectively. The values obtained for the Pd and
PdCu NPs were 2.7 ± 0.5 nm and 2.8 ± 0.6 nm, respectively. It is noteworthy that NPs containing Pd
were smaller than those containing Cu and Ni. This observation could be explained by the faster
decomposition of the palladium precursor resulting in the formation of a large number of Pd-
nucleation clusters at early stages of the synthesis.
CNT-supported MNPs were also prepared following a one-pot approach via the reduction of the
corresponding metallic precursors in the presence of the stabilizer and multiwalled carbon nanotubes
(MWCNTs). Considering the difference of reactivity in the hydrogenation reaction of the metals
deposited, catalysts with different metal loadings were prepared to facilitate the catalytic evaluation.
Pd containing catalysts were prepared at 1 wt% Pd while Ni or Cu containing materials were
synthesized at 5 wt%. In all cases, the ligand to metal molar ratio employed for the synthesis was 0.5.
In a previous contribution, the effect of the NHC/Ni ratio was studied for catalysts prepared with 0.2,
0.5 and 1 equivalent of ligand per Ni.[16c] In view of the results obtained in terms of particle size,
distribution and catalytic performance, a NHC/M ratio of 0.5 was selected in the present study.
Figure 1 displays the TEM images obtained for the colloidal M-NPs and CNT-supported nanoparticles
counterparts (M-NP/CNTs). It is noteworthy that the particle sizes of colloidal and supported
nanoparticles of equivalent metal formulations are similar in mean diameter and distribution,
indicating that under the tested conditions, the support did not participate much in the stabilization
of these nanoparticles. Similar to our observations, Karousis et al. reported similar sizes for both,
colloidal and CNTs supported Pd NPs prepared by refluxing palladium acetate with SDS in water
(particle sizes ca. 2-4 nm).[17]
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Figure 1. TEM micrographs and size distributions of colloidal and CNTS supported MNPs.
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The fine structure of the CNTs supported catalysts was studied by STEM-HAADF. Representative
micrographs of each catalyst are displayed in Figure 2. In the case of Cu/CNTs and Pd/CNTs,
nanoparticles of ca. 6.3 and 2.2 nm were observed in agreement with bright field TEM analysis. For
NiCu/CNTs, analysis of the electron diffraction (FFT) of single NPs permitted the identification of (111)
and (220) crystallographic planes which d-spacings of 2.10 and 2.80 Å, respectively. Interestingly, these
spacings are slightly larger when compared to reference values of Cu-fcc and Ni-fcc phases, (see
comparison in the Supporting information), indicating an expansion of the crystalline lattice.
Considering that metal nanoparticles generally show bond length contraction with the decrease in
particle size, the opposite behavior could be indicative of the doping of the crystalline structure.[18] For
instance, Asakura and co-workers reported the expansion of the crystalline lattice in PVP stabilized Pd
NPs, a phenomena attributed to the formation of a palladium carbide phase at early stages of the
synthesis of the NPs.[19] Similarly, PdCu/CNTs displayed d-spacings slightly larger than the values of
pure Pd-fcc, suggesting the existence of the same phenomena observed in NiCu/CNTs. It is noteworthy
that ideal alloyed structures should display lattice parameters or d-spacing with average values of the
pure monometallic phases.
Figure 2. STEM-HAADF micrographs of monometallic and bimetallic CNTs supported catalysts.
The metallic composition and distribution at the nanoscale for bimetallic catalysts was studied by
HAADF-STEM coupled with EDX. Microanalysis of single particles demonstrated the existence of both
metals (Ni and Cu or Pd and Cu) in proportions similar to nominal values (see Supporting information).
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The metal distribution in single NiCu and PdCu NPs was studied by Energy Dispersive X-ray
Spectroscopy (EDX) of line. According to the observed profiles (Figure 3) the distribution of both metals
along the axes was consistent with bimetallic structures with uniform compositions.
Figure 3. STEM-HAADF micrographs of NiCu/CNTs and PdCu/CNTs catalysts with the corresponding elemental
analysis in line of single bimetallic nanoparticles.
The crystalline structure of the obtained nanomaterials (both, colloidal and supported catalysts), was
analyzed by XRD. Due to the high sensitivity to oxidation of colloidal nanoparticles, XRD analysis were
carried out under inert atmosphere. For the Cu series (Supporting Information), the colloidal
nanoparticles (CuNPs) displayed broad bands centered at 43° and 50°, in agreement with diffractions
planes (111) and (200) of metallic Cu-fcc structure (JCPDS: 00-004-0836). A diffraction peak at 38° was
attributed to the (111) plane of cuprite possibly generated during the isolation of the NPs (JCPDS: 00-
005-0667). The supported Cu/CNTs displayed a similar diffraction pattern than that obtained for the
CNTs alone, except for an additional broadening of the band centered at 43°, due to the presence of
the metallic copper phase. It is noteworthy that the absence of any feature close to 32° discards the
presence of cuprite in the supported catalyst. Similarly, the PdNPs also displayed broad bands (in
agreement with the small size of the NPs) at angles characteristic of Pd-fcc.
The diffractogram of the colloidal NiCuNPs displayed a single broad band centered at 45°(Figure 4a).
This angle is the average value of the diffraction planes (111) of pure monometallic Ni and Cu-fcc
phases, which is in agreement with an alloyed structure. The diffraction pattern observed for the
NiCu/CNTs corresponded to the sum of the XRD profiles of the colloidal NPs and the CNTs.
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For the case of the PdCu series (Figure 4b), the colloidal PdCuNPs exhibited a prominent broad
diffraction peak centered at 42°, value which is the average of the corresponding (111) planes of the
pure monometallic Cu and Pd-fcc structures. This observation confirmed the formation of fully alloyed
bimetallic PdCuNPs in agreement with STEM-EDX analysis. A shoulder at 40° also suggested the
presence of some Pd particles in the sample. For the PdCu/CNTs catalyst, a diffraction pattern very
similar to that of the CNT support was observed, in agreement with the low metal loading of this
catalyst (1 wt%). The absence of a sharp peak at 40° suggested the absence of monometallic PdNPs in
PdCu/CNTs catalyst. Simulation of experimental XRD patters with ideal monometallic or bimetallic fcc
structures (TOPAS® software) permitted to calculate the experimental lattice constant for several of
these nanomaterials (see Supporting information). Interestingly, the lattice constants determined for
NiCu/CNTs and PdCuNPs were very similar to the values expected for ideal Ni0.5Cu0.5 and Pd0.5Cu0.5
alloys (3.578 vs. 3.570, and 3.711 vs. 3.737, respectively), thus confirming the formation of fully alloyed
bimetallic nanostructures.
Figure 4. XRD analysis of colloidal and supported nanoparticles: a. NiCu; b. PdCu
The chemical composition and the oxidation state of the metals at the surface of the supported
catalysts was studied by XPS. Microanalysis of these catalysts performed by deconvolution of full XPS
spectra demonstrated the presence of nitrogen (from the carbene stabilizer) at the surface of these
nanomaterials. The N content was relatively higher in the case of the copper containing catalysts than
in the catalysts where copper was not present (1-2 % atom/atom vs. 0.3 % atom/atom, see Supporting
information).
The effect of copper alloying on the oxidation state of Pd was studied by deconvolution of Pd 3d XPS
spectra (see Supporting Information). Such an analysis revealed percentages of zero valent Pd of 73 %
for Pd/CNTs and 54% in the case of PdCu/CNTs. In the case of the bimetallic PdCu NPs, the presence
of copper could make the Pd phase more electron-deficient, and may affect crucial steps of the
selective alkyne hydrogenation, namely the coordination of the substrate/product and the activation
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of dihydrogen. Such an effect in the XPS binding energy has been previously reported for PdCu
systems.[8a]
Analysis of the Cu 2p spectra of copper containing catalysts evidenced some degree of electron
transfer when alloyed with Ni or Pd. For instance, comparing the Cu 2p XPS spectra of Cu/CNTs and
NiCu/CNTs, it was observed a shift of 1 eV towards higher binding energies for the alloyed catalyst,
possibly due to electron transfer from Cu to Ni.
The presence of the organic stabilizer in the prepared nanomaterials was studied by thermogravimetric
analysis (see data in Supporting information). In general, the colloidal nanoparticles displayed weight
losses of 20-45% in the range of 220-320 °C, that were attributed to the decomposition of the organic
stabilizer anchored on nanoparticle surfaces. For the case of supported catalysts, the presence of NHC
ligand was only detected in NiCu/CNTs (6-8 wt%), but not in PdCu/CNTs (less than 1 %) due to the low
metal loading in this latter case.
To determine experimental metal loadings, all the materials were analyzed by ICP (Table 1).
Table 1. Nominal and experimental metal loadings for
CNTs supported catalysts.
Nominal
ICP
Catalyst
M loading, wt%
M loading, wt%
Cu/CNTs
Pd/CNTs
NiCu/CNTs
10.0 Cu
1.0 Pd
5.0 Ni
5.2 Cu
1.0 Pd
0.7 Cu
6.6 Cu
0.9 Pd
3.6 Ni
3.8 Cu
0.8 Pd
0.5 Cu
PdCu/CNTs
In general, slightly lower metal loadings than nominal values were determined by ICP, in particular for
copper NPs. In contrast, palladium containing catalyst displayed loadings very close to nominal values.
For bimetallic catalysts, the relative percentages of the component metals were very close to the
nominal values (Ni0.50Cu0.50 and Pd0.47Cu0.53 for NiCu/CNTs and PdCu/CNTs respectively).
Summarizing, a general one-pot methodology for the preparation of a series of monometallic Cu, Pd
and bimetallic NiCu and PdCu stabilized by NHC ligands was successfully employed via the in-situ
decarboxylation of the corresponding imidazolium carboxylate. Metal nanoparticles in the range 2-7
nm were obtained by this approach. The particle size of the colloidal nanoparticles was similar to that
of the corresponding supported catalysts. Analysis of these materials by STEM-HAADF (coupled with
EDX) and XRD confirmed the formation of small size metallic Cu and Pd NPs (for the case of the
monometallic materials) and are in agreement with fully alloyed bimetallic NiCu and PdCu
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nanoparticles. The presence of NHC carbene was detected in the colloidal NPs by TGA. ICP analysis
confirmed the similar nominal and experimental metal loadings and relative metal percentages on
CNTs supported catalysts.
Catalysis
a. Screening of catalysts
The catalytic performance of the series of MNPs/CNT was evaluated in the semihydrogenation of 4-
octyne (Table 2). For comparison purposes other substrates such as 1-octyne and diphenylacetylene
were also tested in catalysis (see Supporting Information).
As previously reported, Ni/CNTs displayed excellent conversion and alkene selectivity for the
hydrogenation of internal alkynes under mild conditions (3 mol% Ni, 5 bar H2, 50 °C, 16h) (entry 1),[16c]
however under the same conditions this catalyst easily overhydrogenate terminal alkynes such as 1-
octyne (see Supporting Information). Cu/CNTs catalyst resulted inactive even using strong reaction
conditions (3 mol%Cu, 50 bar H2, 50 °C, 16h). The inactivity of pure copper catalysts in the semi-
hydrogenation of alkynes was also observed by other authors and mainly associated to the low activity
of copper in the activation of the hydrogen molecule.[20]
In contrast, Pd/CNTs easily isomerize 4-octyne under very mild reaction conditions (0.1 mol% Pd, 3 bar
H2, 30 °C, 3h; entry 3), evidencing the high activity of this metal and the associated selectivity issues.
Using the bimetallic catalysts, the first observation when testing NiCu/CNTs was the remarkably lower
activity when compared to the monometallic Ni/CNTs (entries 4 vs. 1). However, high conversion could
be reached by increasing the reaction pressure from 5 to 50 bar while the alkene selectivity was good
(93%), thus demonstrating the beneficial effect of alloying the nickel phase with copper, probably due
to the more electron deficient NiCu-surface.[3b] The moderation of the activity was also observed on
the bimetallic PdCu/CNTs catalyst (entry 5). The alkene selectivity was exceptional at full conversion
of the alkyne (95 %), in agreement with the promoting effect of copper on the selectivity. Interestingly,
under the same conditions, no over-hydrogenation was observed even after prolonged reaction times
(Supporting Information).
For comparison purposes, two commercial catalysts traditionally used in the hydrogenation of liquid
alkynes, namely the Lindlar and Pd Nanoselect catalysts, were also tested.[15] The palladium loadings
employed were 0.05 and 0.01 mol%, for the Lindlar and Pd Nanoselect respectively. Although full
conversions were obtained (entries 6 and 7), isomerization issues were observed for both commercial
references under the same reaction conditions of PdCu/CNTs (3 bar H2, 30 °C, 0.5 h, entries 13-15).
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Table 2. Hydrogenation of 4-octyne catalyzed by Ni, Cu, Pd, NiCu and PdCu/CNTs and commercial references.[a]
M
mol%[b]
3
3
0.01
P,
bar
5
50
3
50
3
3
Time,
h
16
16
3
Sel. Z-
A, %
99
-
66
93
95
66
84
Sel. E-A/
isomers, %
Sel. B,
E.
Catalyst
T, °C
Conv, %[c]
%
1
-
3
7
1
4
0
1[d]
2
3
4
5
Ni/CNTs
Cu/CNTs
Pd/CNTs
NiCu/CNTs
PdCu/CNTs
Lindlar
50
50
30
50
30
30
30
>99
-
>99
0
-
31
0
4
30
16
3
16
93
0.05
0.05
0.01
0.5
0.5
0.5
>99
>99
>99
6
7
Nanoselect
3
[a] Reaction conditions: 5 ml THF, 600 rpm. [b] Metal loading vs. Substrate; for bimetallic catalyst this percentage
is referred to either, Ni or Pd. [c] Conversion and selectivities determined by GC-MS. [d]Reported in [16c]
.
b. Effect of the PdCu composition
In view of the catalytic results obtained with the bimetallic PdCu/CNTs in terms of activity and
resistance to over-hydrogenation, the effect of the PdCu composition was studied. For this purpose,
apart from the above described monometallic Pd and Cu-nanocatalyst and the bimetallic PdCu/CNTs
(Pd0.50Cu0.50) nanocatalysts, two additional supported bimetallic catalysts, Pd0.25Cu0.75 and Pd0.75Cu0.25
in molar fractions, were prepared and subsequently tested in the hydrogenation of 1-octyne and 4-
octyne (Table 3). The employed synthetic methodology was analogous to that used for PdCu/CNTs,
and the Pd/Cu molar ratios were adjusted accordingly (Supporting Information). The particle sizes
measured for the series of PdCu NPs varied as a function of the copper content, with diameters of ca.
2.0 nm for Cu contents lower than 50% and of 4.4 and 6 nm for Pd0.25Cu0.75 and Pd0Cu100,
respectively: Pd1.00Cu0 (2.5 ± 0.5 nm) ≈ Pd0.75Cu0.25 (2.1 ± 0.4 nm) ≈ Pd0.50Cu0.50 (2.0 ± 0.6 nm) <
Pd0.25Cu0.75 (4.4 ± 1.1 nm.) < Pd0Cu100 (6.0 ± 1.3 nm). The observed trend in particle size is a consequence
of the competing processes of nucleation and growth which highly depend on the metal composition
of the reaction mixture.
When 1-octyne was the substrate (entries 1-4), full conversions were observed for the case of the pure
monometallic Pd and the bimetallic Pd0.75Cu0.25 and Pd0.50Cu0.50, while Pd0.25Cu0.75 only reached 22% of
conversion under the same conditions. In terms of selectivity, palladium rich catalysts (monometallic
Pd and Pd0.75Cu0.25) displayed strong over-hydrogenation and isomerization issues. The catalyst with
equimolar Pd/Cu ratio (Pd0.50Cu0.50) provided 92% alkene selectivity at full conversion (entry 4) while
the catalyst with lower Pd/Cu molar ratio (Pd0.25Cu0.75) reached excellent alkene selectivity (>99%) at
only 22 % conversion (entry 5).
Similar results were obtained in the semi-hydrogenation of 4-octyne. It was therefore concluded that
low Cu contents (below 50 mol%) did not moderate appropriately the selectivity, while a high Cu
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content (above 50%) decreased excessively the reactivity of the palladium phase. Considering that
both, terminal and internal alkynes were selectively hydrogenated by Pd0.5Cu0.5 at full conversions, this
catalyst formulation was selected for subsequent studies.
Therefore, these results indicated the crucial role of the metal composition on the surface properties
of bimetallic catalysts and how it affects their catalytic performance. For both substrates, the Pd0.5Cu0.5
provided excellent alkene selectivity at full conversion. Other authors have also studied the effect of
the PdCu composition in the semi-hydrogenation of alkynes. For instance, Liu et al. studied the
hydrogenation of phenylacetylene on PdCu/C with different Pd/Cu ratios (Pd0.67Cu0.33/C to
Pd0.25Cu0.75/C). In line with our observations, the authors observed a remarkable lowering of the
[20]
reaction rate for the catalysts with copper molar fractions higher to Pd0.50Cu0.50
.
Kiwi-Minsker and
co-workers studied the range between Pd0.6Cu0.4 to Pd0.16Cu0.84 for the semi-hydrogenation of
dehydroisophytol (DIP). In their study, the alkene selectivity was not affected by the amount of
modifier for the studied range.[8a]
Table 3. Hydrogenation of 1 and 4-octyne by PdCu/CNTs supported catalysts of variable molar
percentages.[a]
E. Composition
mol/mol%
R1
R2
Conv, %[b] Sel. Z-
Sel. E-A/
Sel. B,
A, %
isom, %[c]
%
51
10
4
0
0
2
1
0
1
2
3
4
5
6
7
8
Pd1Cu0
CH3(CH2)5
CH3(CH2)5
CH3(CH2)5
CH3(CH2)5
CH3(CH2)2
CH3(CH2)2
CH3(CH2)2
CH3(CH2)2
H
H
H
>99
>99
>99
22
>99
>99
99
1
57
92
>99
80
97
95
49
33
4
0
18
1
Pd0.75Cu0.25
Pd0.50Cu0.50
Pd0.25Cu0.75
Pd1Cu0
Pd0.75Cu0.25
Pd0.50Cu0.50
Pd0.25Cu0.75
H
CH3(CH2)2
CH3(CH2)2
CH3(CH2)2
CH3(CH2)2
4
0
18
>99
[a] Reaction conditions: cat. (0.05 mol% Pd), 5 ml of dry THF, 1 bar H2, 30 °C, 0.5 h. [b] Determined by
GC-MS. [c] Z/E 2-octene and E-A, as isomers.
c. Evolution of reaction components over time
Next, to gain insights into the selectivity control of PdCu/CNTs at extended reaction times, the
transformation of a series of model substrates, namely 1-octyne, 2-octyne and 4-octyne, was
monitored over time. For this purpose, samples of the reaction crude were extracted during the first
3 h and the evolution of the reaction components was monitored by GC-MS. Comparing the conversion
profile of the three substrates, a defined trend was identified in the reaction rate: 1-octyne < 2-octyne
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< 4-octyne. This observation could indicate that for PdCu/CNTs, the electronic properties of the
substrate (or of the alkene product) may be more relevant than the steric hindrance.
Regarding the evolution of the alkene selectivity, a clear trend was also appreciable in this series: after
full conversion of the alkyne (which occurs in all the cases during the first 30 min), the alkene selectivity
seems to stabilize at a particular value and then remain fairly stable during the following hours without
experimenting excessive over-hydrogenation. In the literature only a few examples of successful self-
terminating catalytic systems for the hydrogenation of terminal alkynes (without the assistance of
nitrogenated additives) have been reported to date. For instance, Mitsudome and co-workers reported
total inhibition of the over-hydrogenation of terminal alkynes such as 1-octyne or phenylacetylene
when using nanocatalysts based on core-shell Pd@Ag/HAC (HAC = hydroxyapatite),[8c] or Au@CeO2,[21]
respectively.
In our study, the alkene selectivity after 3h of reaction increases in the following order: 1-octyne < 2-
octyne < 4-octyne (80/90/99 % respectively), a trend which matches the more facile over-
hydrogenation of the terminal alkene product in comparison with internal alkenes.[3b]
Interestingly, the internal alkynes exhibited Z/E-isomerization at early reaction times even before of
the full conversion of the alkyne. For instance, Z-selectivities of 80 % and 90% were obtained for 2-
octyne and 4-octyne at 84 % and 95% of conversion, respectively. This behavior possibly indicates that
the formation of the Z-product could follow a mechanistic selectivity pathway in which the produced
Z-alkene is isomerized before its desorption from the metal surface.[22]
Figure 5. Evolution of conversion and the reaction components during the time for the semi-hydrogenation of
a. 1-octyne, b. 2-octyne and c. 4-octyne catalyzed by PdCu/CNTs. Reaction conditions: 30 °C, 3 bar H2, 0-3 h and
0.05 mol% of Pd.
d. Substrate scope
In order to gain more information on the reactivity of the PdCu/CNTs catalyst, a series of alkynes and
alkynols containing various substituents such as aliphatic, aromatic or carbonyl groups were tested in
the semi-hydrogenation reaction (Table 4).
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Table 4. Substrate scope of selective hydrogenation of alkynes and alkynols catalyzed by
PdCu/CNTs.[a]
Sel.
Z-
Sel. E-
Time,
h
Sel.
B, %
E.
R1
R2
Bar, H2
Conv, %[b]
A/isom./others[c], %
A, %
97
90
87
86
89
88
98
77
99
-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH3CH2
CH3CH2
3
3
6
6
3
3
3
6
3
3
3
3
3
3
3
2.5
3
3
5
3
5
3
3
3
>99
>99
43
0
7
5
5
0
3
3
8
44
9
H
H
H
H
H
H
H
H
H
>99
>99
28
99
42
36
0
35
11
12
0
15
0
91
-
Ph
0
Ph(OH)CH
Ph(OH)CH
CH3(CH2)4(OH)CH
CH3(CH2)4(OH)CH=[d]
CH3(CH2)4(OH)CH=[d,e]
OH(CH2)3
OH(CH2)3
OH(CH2)2
CH3CH2CO2
2[c]
8[c]
1[c]
9[c]
-
3
17
0.5
2
3
3
-
>99
39
58
0
0
>99
>99
47
12
32
1
49
10
3
CH3CH2
CH3CH2CO2
15
96
[a]
[b]
[c]
Reaction conditions: cat. (0.05 mol% Pd), 5 ml of dry THF, 30 °C.
Determined by GC-MS. The
corresponding alkyl-ketone was observed. [d] 1-octen-3-ol. [e] Catalyst substituted by only CNTs.
For diphenylacetylene (Table 4, entries 1 and 2), 97% selectivity to the Z product was obtained at full
conversion after 2.5 h of reaction; however, at 3h the selectivity decreased to 90% because of Z/E
isomerization. For 1-phenyl-1-butyne (entries 3 and 4) a hydrogen pressure of 6 bar was required to
get reasonable hydrogenation rates, and ca. 44% of conversion was obtained after 3 and 5h of reaction,
thus evidencing the slow hydrogenation of this substrate.
The hydrogenation of phenylacetylene (entries 5 and 6) was performed with ca. 90% of selectivity at
full conversion. This value remained unchanged after 3 and 5 h of reaction similarly to the observed
for 1-phenyl-1-butyne. Very recently, Liu et al. reported the hydrogenation of this substrate by an
activated carbon supported bimetallic catalyst (Pd0.5Cu0.5/C) at r.t. and atmospheric pressure.[20] The
authors claimed that under the tested conditions the activity of this catalyst was remarkably low that
reaction times above 30 h were required to approach the full conversion. The low activity was
attributed to the excess of the promoter, since formulations with less copper such as Pd0.67Cu0. 33/C,
converted 99% of the substrate in 4h with a 90% of alkene selectivity.
Interestingly, when 1-phenyl-2-propyn-1-ol was tested at 3 bar of H2 and 3h (entry 7), the formation
of propiophenone was observed (ca. 2 %) despite the low conversion of the alkynol (only 28%).
Increasing the pressure from 3 to 6 bar (entry 8) allowed the full conversion of the alkynol although
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the alkenol selectivity dropped to 77 % (with the concomitant formation of the ketone and the
overhydrogenated product in 8 and 15 %, respectively). The fast over-hydrogenation of vicinal alkynols
has been already observed. For instance, Kiwi-Minsker reported the semi-hydrogenation of
dehydroisophytol (DIP) catalyzed by colloidal Pd0.6Cu0.4 NPs at 4 bar H2 and 80 °C. Under the tested
conditions, the alkene selectivity quickly dropped when approaching the full conversion of the alkynol,
thus evidencing the difficult in preventing the over-hydrogenation for these particular substrates.[8a]
In our particular case, these observations suggested that during the hydrogenation process of vicinal
alkynols, the corresponding allylic alcohol products are susceptible to isomerize to the corresponding
ketone. To confirm the correlation between the alkynol structure with the ketone formation, 1-octyn-
3-ol was also tested at 3 bar and 3h, resulting also in the formation of small amounts of the
corresponding ketone product (1%, octan-3-one) at partial alkynol conversion (44 %, entry 9, Table 4).
To gain additional insights about this peculiar isomerization process, a control test employing 1-octen-
3-ol was performed (as the product of partial hydrogenation of 1-octyn-3-ol) in the presence of
PdCu/CNTs catalyst (entry 10). Under the tested conditions, 36 % of the alkenol was converted into
the ketone and the saturated alcohol in a 9/91 ratio respectively, thus indicating that the isomerization
or the hydrogenation of the allylic alcohol are competitive processes occurring at the NPs surface.
The palladium catalyzed isomerization of allyl alcohols to carbonyl compounds has been previously
documented in the literature.[23] For instance, Young-Seok and co-workers reported the catalytic
isomerization or hydrogenation of allyl alcohol catalyzed by PdNPs (in the presence of hydrogen) and
the dependence of the regioselectivity with the reaction solvent or the capping agent.[23b] First, the
presence of H2 gas appeared to be essential for the efficient catalytic isomerization reaction thus
suggesting that the mechanism involved a Pd-alkyl intermediate rather than an η3 π-allyl Pd hydride
intermediate. Regarding the effect of the solvent, in general, nonpolar or weakly polar solvents such
as benzene and chloroform promoted the isomerization of allyl alcohol to propanal presumably via the
formation of the branched Pd-alkyl intermediate, while, polar protic solvents such as methanol and
water foster the hydrogenation to 1-propanol involving the steric induced formation of a linear Pd-
alkyl intermediate.
Here, the transformation of 1-octen-3-ol into the isomerization and hydrogenation products in a 9/91
ratio (entry 10) could indicate a preferred linear coordination of the substrate on the NPs surface,
possibly because of steric hindrance exerted by the carbene ligand. To discard any possible role of the
CNTs in the isomerization, the same hydrogenation test was performed employing CNTs as catalyst.
No isomerization was observed even after extended times (17h, entry 11).
The reactivity of non-vicinal alkynols was also tested. For instance, the semihydrogenation of 4-pentyn-
1-ol (entry 12, Table 4) resulted in a 35 % of conversion with full selectivity towards the alkenol after
0.5h. Curiously, lengthening the reaction time to 2h resulted in the full conversion of the alkyne but
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with a considerably loss of the alkene selectivity (39%, entry 13, Table 4) due to overhydrogenation
and isomerization processes. Considering the structural similitude of this substrate with 1-octyne, the
significant over-hydrogenation/isomerization issues observed for the case of the alkynol (which are
absent for the case of 1-octyne) can be justified by a stronger adsorption of the alkenol product to the
metal surface because of the presence of the hydroxyl group. The influence of hydroxyl groups in the
reactivity of alkynols in the selective hydrogenation was previously reported by Chaudret and co-
workers.[24]
This behavior was also observed when 3-hexyn-1-ol was tested (entry 14). Despite the natural
resistance against over-hydrogenation or isomerization of the alkenol product, important amounts of
these byproducts are observed. Finally, the semi-hydrogenation of diethyl acetylenedicarboxylate was
also tested (entry 15). After 3h of reaction, 50% conversion was observed with high alkene selectivity
(96%).
e. Semi-hydrogenation of acetylene in the presence of ethylene
The bimetallic PdCu/CNTs catalyst was evaluated in the selective hydrogenation of acetylene in the
presence of ethylene. The employed reaction feed stream consisted in 1 % C2H2, 20 % C2H4, 5 % H2, 1 %
Ar and balance N2 to roughly approximate the tail-end feed of an ethylene cracker.[25] This reaction
was carried out in a micro fixed-bed reactor at 70 °C and a total pressure of 1 bar. The catalytic test
was performed at iso-conversion mode at a fixed value of 70%, and then reaction parameters such as
the reaction rate and selectivity were monitored during the first 40 h (for details, see the Supporting
Information).[26]
The bimetallic Pd0.5Cu0.5/CNTs catalyst exhibited a stable rate of ca. 300 molC2H2molPd-1h-1 during 40h,
demonstrating a remarkable stability under the tested reaction conditions (Figure 6). The ethylene
selectivity displayed a similar profile, starting at ca. 60%, and then decreasing slightly during the run.
Such a decrease was concomitant with a slight increase of the ethane selectivity (ca. 30%). The
production of C4´s was stable during the run and maintained at a value of 10%.
For comparison purposes, a commercial palladium catalyst was also tested under similar reaction
conditions. The reference consisted in a Pd/Al2O3 catalyst (0.5 wt% Pd) containing PdNPs of 3 nm. Due
to the very high initial activity of this monometallic Pd catalyst, the employed reaction temperature
was 50 °C (in contrast to the 70 °C required for the bimetallic PdCu/CNTs catalyst). Although the initial
-
activity of Pd/Al2O3 catalyst was higher compared to that of PdCu/CNTs (2500 vs. 1200 molC2H2molPd
1h-1 respectively), it quickly decreased during the first 4h until a value of 500 molC2H2molPd-1h-1,
evidencing a fast and extensive deactivation (see Supporting Information). Such a deactivation
observed on Pd based catalysts is frequently attributed to carbon deposits onto the metal surface of
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the PdNPs.[26] Ethylene selectivities in the range of 40-32% were registered during the run. The
selectivity towards ethane and C4s was ca. 60 and 5% respectively.
The higher performance in terms of stability and ethylene selectivity of the bimetallic PdCu/CNTs
catalyst in comparison to the commercial reference can be ascribed to the promoting effect of the
alloying copper, which exert positive geometric and electronic effects on the active Pd phase.[3b]
Previous studies have reported the promotion of palladium based catalysts with copper for the
selective hydrogenation of acetylene in ethylene rich mixtures.[27] For instance, Friedrich et al. studied
the effect in catalysis of disordered and ordered modifications of an unsupported Pd0.40Cu0.60 catalyst
(prepared by the use or not of thermal crystallization treatments).[27a] The ordered modification
corresponded to a fully alloyed structure and exhibited a high selectivity to ethylene (>90%, 20% higher
than the disordered structure) at ca. 90% conversion, and remained stable for at least 10 h on stream.
The authors attributed the superior performance to the degree of ordering in the bulk crystal structure
that determines the surface structure and the adsorption properties. More recently, the employment
of single-atom Pd catalysts in the semi-hydrogenation of acetylene was reported.[27c, 28] Pei et al.
reported the application of a silica supported Cu-alloyed Pd single-atom catalyst (SAC) for the
semihydrogenation of acetylene under simulated front-end conditions.[27c] The Cu-alloyed Pd SAC
showed ∼85% selectivity to ethylene at full acetylene conversion at 160 °C. Based on microcalorimetric
measurements, the authors proposed that the isolation of Pd by Cu and the electron transfer from Cu
to Pd not only promoted the dissociation of H2 but also resulted in the weak adsorption of C2H4, giving
high selectivity to ethylene at high acetylene conversion. However, it is noteworthy that relatively high
reaction temperature is frequently required for this type of catalyst (>160 °C),[27c, 29] in comparison to
nanoparticle based catalysts which usually operate under milder reaction conditions.
Figure 6. Evolution of the reaction rate and selectivities during the time obtained for PdCu/CNTs catalyst.
Conditions: 0.1 mg Pd, 70 °C, 1 bar (iso-conversion at 70%).
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Conclusions
The one-pot synthetic methodology recently reported by our group for the formation of NHC-stabilized
Ni-NPs was extended for the preparation of both colloidal and carbon supported monometallic Cu, Pd
and bimetallic NiCu and PdCu NPs. This facile methodology demonstrated to also be useful for the
variation of the PdCu composition, evidencing its robustness. These materials were fully characterized,
and the alloyed structure of the bimetallic catalysts was confirmed by XRD and STEM-EDX.
From the series of catalysts, the bimetallic PdCu/CNTs demonstrated superior properties in the
selective hydrogenation of model substrates such as 1-octyne and 4-octyne even when compared with
commercial references. This result was attributed to the electronic and geometric promotion of copper
in the moderation of the reactivity of the active Pd phase. The equimolar composition of Pd0.5Cu0.5
demonstrated to be the optimal for the activity and selectivity in this reaction. In terms of substrate
scope, this catalyst exhibited excellent alkene selectivities for both terminal and internal alkynes
bearing alkyl substituents. Alkynols underwent side reactions such as isomerization or over-
hydrogenation depending on the structural characteristics of the substrate.
When PdCu/CNTs was evaluated in the semi-hydrogenation of acetylene in the presence of ethylene,
excellent stability was observed even after 40 h of reaction with moderate to good ethylene selectivity.
This result evidences the applicability of bimetallic catalysts conceived purely by colloidal techniques,
not only in the semi-hydrogenation of substrates in liquid phase, but also other substrates of industrial
relevance in the gas phase.
Acknowledgements
The authors acknowledge the Ministerio de Economia y Competividad and the Fondo Europeo de
Desarrollo Regional FEDER (CTQ2016-75016-R, AEI/FEDER,UE) and the Generalitat de Catalunya
(SGR2014) for financial support. Authors also thank the Servei of Recursos Cientifics i Tècnics of
Universitat Rovira i Virgili for the analytical assistance (specially the Unitat de Caracterització de
Materials) and to the Advanced Microscopy Laboratory at Instituto de Nanociencia de Aragón for the
access to STEM facilities. D. A. L-R. is grateful for financial support (Grant 552256) by Consejo Nacional
de Ciencia y Tecnologia (CONACYT, Mexico).
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Graphical abstract
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