Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts

Article abstract of DOI:10.1039/c6sc05276b

Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially dehydroxylated silica (Cu/SiO_2-700) was obtained using parahydrogen. Hydrogenation of 1-butyne over Cu/SiO_2-700 yielded 1-butene with ≥97% selectivity. The surface modification of this catalyst with tricyclohexylphosphine (PCy₃) increased the selectivity to 1-butene up to nearly 100%, although at the expense of reduced catalytic activity. Similar trends were observed in the hydrogenation of 2-butyne, where Cu/SiO_2-700 provided a selectivity to 2-butene in the range of 72-100% depending on the reaction conditions, while the catalyst modified with PCy₃ again demonstrated nearly 100% selectivity. Parahydrogen-induced polarization effects observed in hydrogenation reactions catalyzed by copper-based catalysts demonstrate the viability of pairwise hydrogen addition over these catalysts. Contribution of pairwise hydrogen addition to 1-butyne was estimated to be at least 0.2-0.6% for unmodified Cu/SiO_2-700 and ≥2.7% for Cu/SiO_2-700 modified with PCy₃, highlighting the effect of surface modification with the tricyclohexylphosphine ligand.

Full text of DOI:10.1039/c6sc05276b

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ARTICLE
Pairwise Hydrogen Addition in the Selective Semihydrogenation
of Alkynes on Silica-Supported Cu Catalysts
Oleg G. Salnikov,a,b Hsueh-Ju Liu,c Alexey Fedorov,c Dudari B. Burueva,a,b Kirill V. Kovtunov,a,b,†
Christophe Copéretc and Igor V. Koptyuga,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially
dehydroxylated silica (Cu/SiO2-700) was obtained using parahydrogen. Hydrogenation of 1-butyne over Cu/SiO2-700 yielded 1-
butene with ≥ 97 % selectivity. The surface modification of this catalyst with tricyclohexylphosphine (PCy3) increased the
selectivity to 1-butene up to nearly 100 %, although at the expense of a reduced catalytic activity. Similar trends were
observed in hydrogenation of 2-butyne, where Cu/SiO2-700 provided selectivity to 2-butene in the 72–100 % range
depending on the reaction conditions, while the catalyst modified with PCy3 again demonstrated nearly 100 % selectivity.
For the first time, parahydrogen-induced polarization effects were observed in hydrogenation reactions catalyzed by
copper-based catalysts, demonstrating the viability of pairwise hydrogen addition over these catalysts. Contribution of
pairwise hydrogen addition to 1-butyne was estimated as at least 0.2–0.6 % for unmodified Cu/SiO2-700 and ≥ 2.7 % for
Cu/SiO2-700 modified with PCy3, highlighting the effect of the Cu surface modification with the tricyclohexylphosphine
ligand.
www.rsc.org/
That said, addressing the molecular mechanisms in
heterogeneous catalysis is inherently difficult because of the
low abundance of catalytically relevant sites and low
Introduction
Semihydrogenation of alkynes is a process used industrially to
remove acetylene from ethylene streams and thus avoid
poisoning downstream ethylene polymerization catalysts.1 It is
also an important reaction in organic chemistry applied for the
stereoselective synthesis of Z-olefins.2 Expensive Pd
semihydrogenation catalysts are currently used for alkyne
semihydrogenation, both in industry and in academia (e.g., the
Lindlar catalyst).3 Research efforts to reduce the required
amounts of the Pd semihydrogenation catalysts or even totally
replace them by cheaper and more sustainable metals
sensitivity of many conventional spectroscopic techniques,
especially when applied to surfaces.10 The latter is a long-
standing problem in nuclear magnetic resonance (NMR) and
magnetic resonance imaging (MRI).11 One strategy to increase
the sensitivity utilizes hyperpolarization of nuclear spins, which
creates nuclear spin polarization beyond the thermal
equilibrium.12 Dynamic nuclear polarization (DNP) is
a
technique that exploits transfer of polarization from electrons
to nuclei.13,14 DNP was successfully applied to study surfaces of
solid materials,10,15,16 including catalysts.17,18 Another
identified Cu-based catalysts as
a
viable alternative.4–8
hyperpolarization
polarization
technique
which
is
parahydrogen-induced
non-equilibrium
However, further improvement of activity and especially
chemo- and stereoselectivity is necessary before Cu can
replace established Pd systems. It has been recently
demonstrated by some of us that combining small silica-
supported Cu nanoparticles and the phosphorus-based ligand
allows one to achieve this goal.9 However, a more detailed
understanding of the reaction mechanism and the role of the
phosphine ligand is critically needed to aid rational
development of improved catalysts.
(PHIP),
creates
magnetization using high spin order of parahydrogen (p-H2).19–
21
Parahydrogen itself cannot be detected by NMR because of
its zero nuclear spin. But if p-H2-derived hydrogen atoms are
added to an unsaturated asymmetric substrate as a pair, the
corresponding NMR signals of the reaction product are
enhanced because the parahydrogen-derived protons retain
their spin correlation. This effect was demonstrated with
homogeneous19,22 and heterogeneous23,24 hydrogenation
catalysts, the latter were utilized both in the liquid25–27 and in
the gas23,24 phases. Applied to heterogeneous catalysis, PHIP
provides very valuable mechanistic28–32 and kinetic33 insights
and can be used for MRI visualization of operating catalytic
reactors.34,35
a.International Tomography Center, SB RAS, 3A Institutskaya St., 630090
Novosibirsk, Russia.
b.Novosibirsk State University, 2 Pirogova St., 630090 Novosibirsk, Russia.
c. Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-
Weg 1-5, CH-8093 Zürich, Switzerland.
† Corresponding authors: kovtunov@tomo.nsc.ru.
Here, we use PHIP to obtain mechanistic insight into selective
semihydrogenation of alkynes on the promising Cu catalysts, in
Electronic Supplementary Information (ESI) available: Additional NMR spectra and
data on conversion, NMR signal enhancements and percentages of pairwise
hydrogen addition. See DOI: 10.1039/x0xx00000x
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particular to probe whether or not the pairwise hydrogen
addition route operates on these systems and how the
phosphine ligand might be involved in modifying the reaction
pathway and thereby the selectivity of silica-supported Cu
nanoparticles.
Results and discussion
Cu/SiO2-700 catalyst was prepared by the surface
organometallic chemistry approach36 by grafting [Cu5Mes5]
onto SiO2-700, followed by reduction in H2 flow at 300 °C.9 This
synthetic
protocol
provided
silica-supported
Cu(0)
nanoparticles with an average size in the range 1.9±0.3 nm, as
determined by high-angle annular dark field scanning
transmission microscopy after the sample was slowly exposed
to air.9 Treatment of Cu/SiO2-700 with PCy3 afforded a surface-
modified catalyst (Cy3P-Cu/SiO2-700) with enhanced selectivity
in alkyne batch semihydrogenation.9 Both catalysts were then
used in PHIP experiments with 1-butyne, 1-butene or 2-
butyne.
Experimental
Cu/SiO2-700 and Cy3P-Cu/SiO2-700 catalysts were prepared by the
surface organometallic chemistry approach36 grafting
[Cu5Mes5] (Mes = mesityl) onto silica partially dehydroxylated
at 700 °C (SiO2-700), followed by the reduction in H2 flow at 300
°C to give Cu/SiO2-700. Subsequent impregnation of this
material with tricyclohexylphosphine yields Cy3P-Cu/SiO2-700
catalyst, as reported in detail elsewhere.9
Cu/SiO2-700 catalyst showed modest to fair activity in 1-butyne
hydrogenation at 350–550 °C, with overall conversions in the
Commercially available 1-butene, 1-butyne (Sigma-Aldrich, ≥
99%), 2-butyne (Sigma-Aldrich, 99%) and hydrogen were used
as received. For PHIP experiments, hydrogen gas was enriched
with parahydrogen to 50% by passing it through a layer of
FeO(OH) maintained at liquid nitrogen temperature (77 K). The
1:4 mixtures of 1-butene/p-H2 and 1-butyne/p-H2 or a 1:11:5
mixture of 2-butyne/p-H2/He were used in the hydrogenation
experiments.
37–81
% range, depending on the gas flow rate and
temperature (see Table 1), with no significant activity below
300 °C. High selectivity toward 1-butene (≥ 97 %) was always
observed (Table 1). Even though hydrogen adsorption on Cu
surface is dissociative,37,38 PHIP effects were detected in 1H
NMR spectra not only for vinylic protons of 1-butene but also
for protons of its ethyl moiety as a result of spontaneous
transfer of polarization in Earth’s magnetic field (Figure 1b,c).39
The observed signal enhancements (SE) at 7.1 T magnetic field
were 25–90 for CH and 13–60 for CH2 groups of 1-butene,
respectively, corresponding to 0.2–0.6 % and 0.1–0.4 % lower
boundary estimates for the percentages of pairwise hydrogen
addition (φp). These values are typical for supported metal
catalysts,24 and it should be emphasized that the actual
contribution of pairwise hydrogen addition is likely higher due
to the partial loss of polarization associated with relaxation
effects and incomplete adiabaticity of gas transfer into 7.1 T
field.40 The difference in the estimated percentages of pairwise
addition between CH and CH2 groups is explained by different
nuclear spin relaxation rates of these protons.
Catalysts (20-105 mg) were loaded in the 1/4’’ outer diameter
(OD) stainless steel reactor between two pieces of fiberglass
tissue in an argon-filled glove box. The reactor was then sealed
with two Swagelok valves inside the glove box. All lines
between the tank with the gas mixture and the reactor were
preliminarily evacuated. The reactor was heated in a tubular
furnace; the temperature was varied from 150 to 550 °C. The
reaction gas mixture was supplied to the reactor and then to
the NMR tube placed inside an NMR spectrometer through a
1/16’’ OD PTFE capillary. All hydrogenations were carried out
at atmospheric pressure. The gas flow rate was controlled
using an Aalborg rotameter. 1H NMR spectra of reaction
mixtures in the gas phase were acquired on a 300 MHz Bruker
AV 300 NMR spectrometer using a single π/2 radiofrequency
pulse.
To assess the stability of the catalyst’s performance, it was
cooled down to room temperature in H2 atmosphere and
tested in 1-butyne hydrogenation at 350 °C ca. 4.5 hours later.
The activity of the catalyst decreased dramatically (1-butyne
Table 1. Hydrogenation of 1-butyne with parahydrogen over Cu/SiO2-700 catalyst: 1-butyne conversion (X), selectivities to
different reaction products (S), signal enhancements (SE) calculated for vinyl CH and CH2 protons of 1-butene, and lower
estimates of percentages of pairwise hydrogen addition calculated using SE values.
Temperature, °C Flow rate, mL/s X, % S1-butene, % S2-butene, % Sbutane, % SE (CH) SE (CH2) φp (CH)a φp (CH2)a
350
350
350
450
450
450
3.8
5.1
6.5
3.8
5.1
6.5
81
70
37
71
62
55
98.5
98
1.1
1.5
0
0.4
0.6
0
25
35
88
42
67
79
13
29
59
42
41
49
0.18
0.25
0.63
0.31
0.48
0.57
0.09
0.21
0.43
0.31
0.30
0.35
100
99
0.8
0.6
2.0
0.1
0.3
0.9
99
97
aThe losses of polarization caused by relaxation on the way from the reactor to the NMR instrument and the incomplete
adiabaticity of this transfer were not taken into account. The difference in the percentages of pairwise addition between CH and
CH2 groups is explained by different relaxation rates of these protons.
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1
Figure 1. (a) Scheme of 1-butyne hydrogenation. (b-c) H NMR spectra acquired in 1-butyne hydrogenation with parahydrogen
over Cu/SiO2-700 catalyst (b) while the gas was flowing and (c) after rapid interruption of the gas flow and subsequent relaxation
of nuclear spins to thermal equilibrium. (d-e) 1H NMR spectra acquired in 1-butyne hydrogenation with parahydrogen over
Cy3P–Cu/SiO2-700 catalyst (d) while the gas was flowing and (e) after rapid interruption of the gas flow and subsequent relaxation
of nuclear spins to thermal equilibrium. All spectra were acquired with 8 signal accumulations and are presented on the same
vertical scale. Reaction temperature was 450 °C, and gas flow rate was 5.1 mL/s.
increase in the SE and φp values was observed (Table S1), isomerization than in hydrogenation. This activity in
indicating that the catalyst became less active but more hydrogenation of alkenes to alkanes agrees well with the
selective in terms of pairwise hydrogen addition.
behaviour usually observed in the batch hydrogenation of
Next, the more selective catalyst,9 Cy3P-Cu/SiO2-700, was tested alkynes, where olefins (primary products) get consumed as
in 1-butyne hydrogenation with parahydrogen (Figure 1d,e). soon as there is no more alkyne in the reaction mixture.9,41
Similar to Cu/SiO2-700, it showed hydrogenation activity only Noteworthy, no PHIP effects were observed in the
above 250 °C (Table S2). Cy3P-Cu/SiO2-700 was significantly less hydrogenation of 1-butene on Cu/SiO2-700 (Figure S5). This
active than Cu/SiO2-700 with conversion below 10 %. However, observation is in a sharp contrast to semihydrogenation of 1-
it provided higher PHIP enhancements of 1-butene NMR butyne under similar reaction conditions and signifies that
signals (ca. 3–6 fold compared to Cu/SiO2-700), and therefore is mechanistic pathways for hydrogenation of alkynes and
more prone for pairwise hydrogen addition, assuming that the alkenes on supported Cu nanoparticles are different.
presence of the ligand does not change the spin relaxation Similarly, Cy3P-Cu/SiO2-700 was tested in hydrogenation of 1-
rates significantly. The maximum signal enhancement values butene. The catalyst did not show any considerable activity in
were no less than 370 and the maximum contributions of isomerization or hydrogenation of 1-butene in the 150–550 °C
pairwise hydrogen addition were no less than 2.7 %, as temperature range. This result demonstrates that modification
observed at 350 °C and 6.5 mL/s gas flow rate (Figure S3, Table of Cu/SiO2-700 with PCy3 indeed makes it a very selective
S2). Note that as the signals of thermally polarized 1-butene catalyst toward hydrogenation of C≡C triple bonds, consistent
were below the noise level, SE and φp values were estimated with high selectivity to olefins featured by Cy3P-Cu/SiO2-700 in
using the signal-to-noise ratio as a reference. As already liquid phase batch alkyne semihydrogenation.9 Note, however,
mentioned above, losses of polarization due to relaxation and that Cy3P-Cu/SiO2-700 shows slightly higher activity than
incomplete adiabaticity were not taken into account and Cu/SiO2-700 in semihydrogenation of 1-phenyl-1-propyne at 40
therefore the actual SE and φp values can be significantly
°
activity of Cu/SiO2-700 in the flow conditions utilized in this
C in toluene solution,9 in contrast to the substantially higher
higher.
In order to get further insight into the mechanism of study. This difference is possibly due to the very different
hydrogenation of 1-butyne on these Cu-based catalysts, reaction conditions (gas vs. liquid phase and temperatures).
hydrogenation of 1-butene with parahydrogen was also Since hydrogenation of terminal and internal alkynes can
carried out. The unmodified Cu/SiO2-700 catalyst initially proceed via different mechanisms, Cu/SiO2-700 was tested in
showed substantial activity in both the isomerization of 1- the hydrogenation of 2-butyne with p-H2. Significant activity in
butene to 2-butene and hydrogenation to butane even at 150 formation of 2-butene and butane was observed starting at
°C (Table S4). However, in a short time (~2-4 min) its activity 250 °C (see Table S6 for conversions). The catalyst was
decreased dramatically (Table S4). In contrast, the catalyst selective toward 2-butene, but also produced some amounts
performance was stable at higher temperatures (250–450 °C). of butane at 250 and 350 °C. No PHIP effects were observed in
As can be seen from Table S4, the catalyst was more active in 2-butyne hydrogenation over Cu/SiO2-700 catalyst. Similarly,
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Conclusions
We reported the first study on selective semihydrogenation of
alkynes with silica-supported Cu nanoparticles and
parahydrogen. PHIP effects were observed for the
hydrogenation of 1-butyne on Cu/SiO2-700 and were very minor
for the hydrogenation of 1-butene and not observed for 2-
butyne hydrogenation, indicating that different mechanistic
manifolds operate with these substrates. Modification of
Cu/SiO2-700 catalyst with PCy3 leads to an increased selectivity
in 1-butyne hydrogenation to nearly 100 % (by-products below
the detection limit of NMR), although making this catalyst
significantly less active. The remarkable selectivity of
Cy3P-Cu/SiO2-700 also manifested in no activity in
hydrogenation of 1-butene, and no formation of butane from
Cu
Concerted
Stepwise
H
1ꢀC4H6
Ha Hb
H
C2H5
pairwise
addition
Ha
Hb
C2H5
Ha Hb
Ha Hb
Ha Hb
H3C
H
Cu Cu Cu Cu Cu
Cu Cu Cu Cu Cu
Hb
Ha
Spin scrambling
by Hꢀdiffusion
H
H
C2H5
Ha
Ha
C2H5
Hb Hb
Ha Ha
Hb Hb
2-butyne. Cy3P-Cu/SiO2-700 provided no less than 2.7
%
H3C
Ha
H
Cu Cu Cu Cu Cu
Cu Cu Cu Cu Cu
pairwise hydrogen addition to 1-butyne, the actual value is
likely higher due to polarization losses via relaxation. Our
results demonstrate that copper-based catalysts are promising
inexpensive alternatives to heterogeneous noble metal
catalysts for semihydrogenation of alkynes in the flow and the
production of hyperpolarized gases.
Ha
nonpairwise
addition
Compare:
Ha Hb
P
Ha Hb
Ha Hb
Ha Hb
vs.
Cu Cu Cu Cu Cu
Cu Cu Cu Cu Cu
pairwise addition
≥ 0.6% ≥ 2.7%
Cu/SiO2–700
Cy3P–Cu/SiO2–700
Acknowledgements
Scheme 1. Non-pairwise and pairwise routes of hydrogen
addition to 1-butyne over Cu/SiO2-700 and Cy3P-Cu/SiO2-700
catalysts.
ITC team thanks the Russian Science Foundation (grant 14-13-
00445) for the support of PHIP experiments, and Dr. Svyatoslav
E. Tolstikov (ITC) for providing access to the glove box
equipment. A.F. thanks the Holcim Stiftung for the Habilitation
fellowship. O.G.S. acknowledges the PhD scholarship from
Haldor Topsøe. The supported Cu nanoparticles were
developed in a project funded by CCEM and Swiss Electric
Research.
Cy3P-Cu/SiO2-700 catalyst showed significant activity above 250
°C. The catalyst was highly selective toward 2-butene (> 95%)
and in this case, no butane was formed (Table S7). Then the
temperature was increased to 300 °C. At the very first moment
at 300 °C and 6.5 mL/s gas flow rate, some catalytic activity
was observed in the hydrogenation of 2-butyne to 2-butene
(Figure S8, red trace). Unfortunately, the catalyst rapidly
deactivated at the reaction temperature, evidenced by a rapid
decrease of the signals of 2-butene (see Figure S8, blue trace)
that occurred within minutes. Any PHIP effects, if present,
were very small. Interestingly, hydrogenation of 2-butyne over
more common heterogeneous hydrogenation catalysts such as
Pd/TiO2 and Rh/TiO230 featured pronounced PHIP effects for 2-
butene and butane (Figure S9).
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