Walter Reppe Revival – Identification and Genesis of Copper Acetylides Cu2C2 as Active Species in Ethynylation Reactions

Article abstract of DOI:10.1002/chem.202101932

More than six decades after proposing copper acetylide, Cu₂C₂, as catalytically active species in ethynylation reactions by Walter Reppe, the explosive species have been experimentally identified and investigated during catalysis in detail now. Taking into account specific safety precautions, unequivocal qualitative characterization was achieved by Raman spectroscopy and X-ray powder diffraction of supported copper catalysts Cu/Bi/SiO₂ during and after activation and catalysis in comparison to bulk Cu₂C₂ materials. Quantification of Cu₂C₂ succeeded by thermal analysis and Raman spectroscopy. Its formation in aqueous suspension is studied starting from copper(II) oxide catalysts including dissolution, reduction and precipitation steps. Copper acetylide formation can be correlated with catalytic performance in the ethynylation of formaldehyde to 1,4-butynediol.

Full text of DOI:10.1002/chem.202101932

Communication
doi.org/10.1002/chem.202101932
Chemistry—A European Journal
Walter Reppe Revival – Identification and Genesis of
Copper Acetylides Cu2C2 as Active Species in Ethynylation
Reactions
Tobias Bruhm,[b] Andrea Abram,[b] Johannes Häusler,[b, c] Oliver Thomys,[b] and Klaus Köhler*[a]
reaction scheme for the industrially important ethynylation
reaction of formaldehyde to 1,4-butynediol is depicted in
Abstract: More than six decades after proposing copper
acetylide, Cu2C2, as catalytically active species in ethynyla-
Scheme 1.[1]
tion reactions by Walter Reppe, the explosive species have
In industrial processes, 1,4-butynediol is hydrogenated to
been experimentally identified and investigated during
either 1,4-butenediol or 1,4-butanediol, which are used for
catalysis in detail now. Taking into account specific safety
production of solvents (e.g. tetrahydrofuran), Vitamin B6 and
precautions, unequivocal qualitative characterization was
other chemicals.[2]
achieved by Raman spectroscopy and X-ray powder
W. Reppe et al. developed Cu/Bi/SiO2 catalysts that are still
diffraction of supported copper catalysts Cu/Bi/SiO2 during
successfully used in current industrial processes. The most
and after activation and catalysis in comparison to bulk
commonly applied pre-catalysts are based on CuO.[1] Most of
Cu2C2 materials. Quantification of Cu2C2 succeeded by
them are supported catalysts, where Cu2C2 is probably formed
thermal analysis and Raman spectroscopy. Its formation in
during activation and stable under reaction conditions. Still
aqueous suspension is studied starting from copper(II)
missing identification and characterization of these active
oxide catalysts including dissolution, reduction and precip-
species may be due to (i) the explosive properties of Cu2C2,
itation steps. Copper acetylide formation can be correlated
which is only stable under wet conditions and in diluted state,
with catalytic performance in the ethynylation of
(ii) limited availability of suitable analytical tools, (iii) its ill-
formaldehyde to 1,4-butynediol.
defined polymeric structure, also known from more defined
analogous inorganic “Cu2C2”-compounds[3] and (iv) challenges
with its in situ formation in a complex 3-phase aqueous slurry
during catalysis.
The structurally complex polynuclear explosive copper acety-
lide, Cu2C2, was proposed and generally accepted as active
species in copper catalyzed reactions with acetylene in the
literature more than six decades ago, but it has never been
experimentally proven and investigated in detail up to now.
Walter Reppe et al. assumed that the active species in
ethynylation reactions could consist of three acetylene mole-
cules coordinated to a Cu2C2 molecule (Cu2C2 ·3C2H2). The
In contrast to this limited understanding of the catalyst,
there is a clearly increased interest in application of acetylene
based processes like the ethynylation of formaldehyde to 1,4-
butynediol as alternative to petrol chemistry for countries with
large coal reserves. This is reflected by an increased number of
recent scientific investigations on supported oxide catalysts and
structure-activity relationships. The influence of bismuth is
studied by Yang et al. who propose that bismuth weakens the
interaction between CuO and the support and promotes the
dispersion of CuO.[4] The influence of chlorine was investigated
by Gao et al.[5] Wang et al. investigated the influence of Bi and
they conclude that a strong CuOÀ Bi2O3 interaction leads to a
stabilization of Cu(I), being important for the formation of active
Cu2C2 species.[6] W. Reppe proposed that Bi inhibit the formation
[a] Prof. Dr. K. Köhler
Department of Chemistry
Technical University of Munich
Lichtenbergstr. 485748 Garching (Germany)
E-mail: klaus.koehler@tum.de
of cuprene which plugs the pipes in the industrial process.[7]
A
[b] T. Bruhm, A. Abram, J. Häusler, Dr. O. Thomys
Catalysis Research Center
relationship between catalytic activity and unspecified Cu(I)
Technical University of Munich
Ernst-Otto-Fischer-Str. 285748 Garching (Germany)
[c] J. Häusler
Forschungszentrum Jülich GmbH
Wilhelm-Johnen-Str., 52425 Jülich (Germany)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101932
Part of a Special Issue on Contemporary Challenges in Catalysis.
© 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial NoDerivs License, which permits use and distribution in any medium,
provided the original work is properly cited, the use is non-commercial and
no modifications or adaptations are made.
Scheme 1. Ethynylation reaction of acetylene and formaldehyde to 1,4-
butynediol and propargyl alcohol including the parameter range applied in
industrial processes.[1]
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surface centers (amorphous active cuprous species) was derived
from infrared spectroscopic experiments (CO adsorption), one
of the very few studies of spent catalysts in the literature
proposing a reaction mechanism.[8]
In Figure 1, the Raman spectrum of the freshly prepared
pure Cu2C2 is shown in comparison to a typical carbon reference
(with typical D-peak at 1350 cmÀ 1 and G-peak at 1600 cmÀ 1).[12]
Signals arise at 430 cmÀ 1 including a shoulder at higher
wavenumbers and at 1710 cmÀ 1. The signal at 430 cmÀ 1
corresponds to the CuÀ C bond. Garbuzova et al. and Aleksan-
yan et al. indicate the same wavenumber for CuÀ C σ-bonds in
other copper(I) acetylide compounds as for example (phenyl-
ethynyl)copper(I) (PEC, (CuC�CPh)x).[3a,b] The signal at 1710 cmÀ 1
corresponds to the C�C bond and is surprisingly shifted; C�C-
bonds are rather expected at 2100 cmÀ 1. Aleksanyan et al. found
C�C bond wavenumbers being lower on average by 180–
200 cmÀ 1 compared to the corresponding alkyne precursor
molecules in other copper(I) acetylide compounds.[3b] Sladkov
et al. propose that the C�C bond in copper(I) acetylide
compounds can be shifted towards lower frequencies by up to
300 cmÀ 1 because of the decreasing order of the C�C bond.[3d]
The even larger difference for Cu2C2 found in this investigation
can be explained by additional bonding partners. In contrast to
the molecular model compounds discussed above, triply bound
carbons do not only coordinate to one copper but to two, i.e.
on both sides of the C�C bond in addition to π-bond
coordination. The further decrease of the bond order
(1710 cmÀ 1) is thus reasonable.
Crystalline Cu2C2 can also be identified by X-ray powder
diffraction.[13] The powder diffractogram of pure Cu2C2 is given
in Figure S3. The pattern obtained for the freshly prepared
Cu2C2 match well with the pattern published by Judai et al.,[13]
who also calculated the structure of lowest energy of Cu2C2 via
DFT based on alkali metal acetylide compounds. Bond lengths
of 1.29 Å between triple and double bonds were predicted. The
C2-unit is surrounded by four end-on and four side-on
coordinated Cu atoms.[13] Several other polymer-like structures
of copper acetylides were published.[3a,d,14] A general formula is
given in Scheme 2 for illustration.
In all former experimental studies, the focus was on the
characteristics of the pre-catalysts only (supported copper(II)
oxides). The present study shall bridge the gap between these
pre-catalysts and catalysis by focusing on the active species
“Cu2C2” formed under activation conditions. We learned to work
safely with the explosive Cu2C2 species in the activated, working
and spent catalyst and identified Raman spectroscopy and X-
ray powder diffraction (XRD) as most suitable methods for its
characterization. Cu2C2 was doubtlessly identified and even
quantified (thermogravimetry, TG, and Raman spectroscopy) in
the activated and working catalysts, allowing correlation
between the progress of the reaction and the catalytic perform-
ance. Moreover, Cu2C2 formation could be followed during
typical activation procedures in the complex solid-liquid-gas
systems. All experiments were performed with silica supported
industrial-like Cu/Bi catalysts and wherever necessary or
possible with pure (bulk) or supported Cu2C2 for comparison.
Preparation and characterization of the pre-catalysts: The
SiO2 supported Cu/Bi catalysts were prepared by co-precipita-
tion in close analogy to industrial catalysts. The standard
loading of industrial catalysts is 30–60 wt% Cu and 2–4 wt%
Bi.[1c,9] X-ray powder diffractograms of a standard catalyst
°
containing 35 wt% Cu and 4 wt% Bi after drying (80 C) and
°
after calcination (4 h; 450 C (=standard calcination temper-
°
ature), and 700 C, denoted as Cu35Bi4-450 etc.) are given in
Figure S1.
°
The sample dried at 80 C (Cu35Bi4-80) consists of basic
copper nitrate (Cu2NO3(OH)3), the main product of co-precip-
itation. The calcined samples contain CuO. Table S1 summarizes
crystallite sizes calculated by Scherrer equation.[10]
Surface areas, pore sizes and pore volumes of the catalysts
as well as the real Cu and Bi loadings (ICP-OES) are summarized
in Supporting Information Table S1 too. As expected, the higher
calcination temperature reduces the BET surface area. The
Raman spectra of the catalyst after different thermal treatment
are shown in Figure S2. The spectra of the calcined samples fit
well with the CuO reference spectrum. The signals of Cu35Bi4-
80 match a Cu2NO3(OH)3 reference.
Model compounds: pure and supported copper acetylide,
Cu2C2: Besides the industrial-like Cu/Bi catalyst, pure Cu2C2 was
synthesized and characterized for direct comparison, taking
account of safety measures. Characterization of Cu2C2 is
challenging due to its explosive decomposition in dry state.
Characterization techniques are preferred, which allow to work
in (aqueous) suspension or at least in wet state. Raman
spectroscopy was found to best meet all the demands. Besides
experimental advantages compared to infrared, Raman allows
to identify the characteristic C�CÀ and CÀ H-bond vibrations
separately from water. Supported Cu2C2 catalysts were found to
be comparatively stable at dry (ambient) conditions at least up
to moderate loadings.
Figure 1. Raman spectrum of freshly prepared Cu2C2 (aqueous suspension)
from 115 cmÀ 1 to 1880 cmÀ 1. For comparison, a carbon reference (activated
charcoal from FLUKA) spectrum is shown. Spectra of CaC2 or Li2C2 are similar
to Cu2C2 and show characteristic signals below 500 cmÀ 1 and at around
1870 cmÀ 1 [11]
2850 cmÀ 1 is depicted in Figure S14.
.
The whole spectrum of Cu2C2 in the range between 150 and
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°
carried out at 100 C under slight pressure of acetylene
(1.2 bar).[1] The reactions occurring under these conditions are
summarized in scheme 4.
Before the reaction can start, the CuO precursor must be
activated, i.e. the pre-catalyst is transformed into the active
species Cu2C2. Industrially this is carried out either separately
before the reaction or in situ during the reaction by adding new
substrate and catalyst precursor to the reaction slurry. A typical
activation time is 5 h.[1c] During the activation both reactants
are present and reaction conditions are applied. Due to the
importance of this activation process for the formation and
properties of Cu2C2, we investigated this particular step in
detail.
Formation of Cu2C2 under typical reaction conditions (catalyst
activation). Pre-reduction of Cu(II) to Cu(I) is the first step
necessary to form the Cu2C2 phase. In the literature it is
generally accepted[5] that formaldehyde acts as reducing agent
being oxidized to formic acid. There are however no studies
investigating this reduction. We investigated the reduction of
Cu(II) to Cu(I) by EPR spectroscopy, which allows the simulta-
neous observation of paramagnetic Cu(II) species dissolved in
solution, as a solid as well as at interfaces. A first series of
experiments (Figure S6) shows clearly that reduction of Cu(II) to
Cu(I) does not take place, neither at temperatures of 25 nor at
Scheme 2. General structure of Cu acetylides, R=Cu, H, organic group.[3d]
The proposed structures match well with results obtained
by Raman spectroscopy. The signal at 1710 cmÀ 1 corresponds
to the C2-building block representing the triple bond with
decreased order caused by the π-bonded Cu atoms. The broad
signal at 430 cmÀ 1 corresponds to the CuÀ CÀ σ-bond. The
broadness of the signal including the shoulder is caused by the
variation of bond strengths between Cu and C and Cu and
triple bonds.
Besides the structural identification, the amount of Cu2C2
formed is of particular interest for catalytic properties. Thermog-
ravimetric analysis coupled with mass spectrometry (TG-MS)
was found to be the most useful approach to quantify Cu2C2. A
method was developed based on the decomposition of Cu2C2
in an oxygen containing atmosphere leading to different
defined decomposition reactions (Scheme 3) and corresponding
mass changes of the sample allowing determination of the
initial acetylide amount.[3c,15]
The TG results of supported Cu2C2 samples with different
CuO loadings (5 wt%, 15 wt%, 25 wt%) are exemplarily shown
in Figure S4 together with a detailed description of the
procedure. The mass increase by oxidation of Cu(I) to Cu(II)O
was used to calculate the amount of Cu2C2 in the catalysts. The
validation of the TG-MS results is demonstrated in Figure S5. All
experiments were carried out with supported Cu2C2 only,
because of the explosive decomposition of pure dry Cu2C2.
Ethynylation of formaldehyde catalyzed by (bulk) Cu2C2:
The activation and reaction conditions are selected to be as
equal/comparable as possible to the industrial process. The
reaction of formaldehyde and acetylene to 1,4-butyndiol was
°
100 C, i.e. formaldehyde does not act as reducing reagent for
Cu(II) under typical reaction conditions. When acetylene is
added (Figure S7), Cu(II) chloride is slowly reduced to Cu(I) at
°
25 C in 45 min and very fast and completely within 2 min at
°
100 C. Analogous reduction experiments with the standard
°
catalyst Cu35Bi4-450 performed at 100 C (Figure S8) show that
the intensities of the Cu(II)-EPR signals (double integrals of the
“powder” EPR spectra) decrease slowly over time (by about
20% per hour). Additional experiments showed that the CuO
catalyst can be reduced by acetylene alone (without
°
formaldehyde) at 100 C too. This agrees with reports of
Brameld et al., who synthesized copper acetylides from Cu(II)
solutions with acetylene.[16]
Formation of Cu2C2 as function of activation time can be
traced by Raman spectroscopy (Figure 2). The Raman spectra of
the activated Cu35Bi4-80 catalyst (not calcined), i.e. starting
from supported basic copper nitrate (Cu2NO3(OH)3) are shown
after certain reaction times.
During the purging period under N2 (two top spectra), only
signals of Cu2NO3(OH)3 appear. Addition of acetylene gives rise
to two signals at 430 cmÀ 1 (CuÀ C bond) and 1710 cmÀ 1 (C�C-
bond) of Cu2C2. Under acetylene gas, the signals get more
pronounced over time and a third signal becomes visible at
580 cmÀ 1. The signal at 430 cmÀ 1 of the activated catalyst is
narrower and more intense than the signal of the reference
(supported Cu2C2 is seemingly structurally better defined or
crystalline). The signal at 580 cmÀ 1 can only be observed with
the activated catalyst and corresponds to a �CÀ C-bond[3a]
indicating that coupling products like Cu polyynides were
formed. Controlled deposition of Cu2C2 in contrast to fast
precipitation of pure Cu2C2 (reference) from Cu(I) solutions
without reactants (formaldehyde) is the probable reason for
these differences.
Scheme 3. Proposed reactions occurring during thermogravimetry of Cu2C2
supported on SiO2.[3c]
Scheme 4. Reactions during catalytic ethynylation of formaldehyde.
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Figure 3. Probable formation of Cu2C2 during the activation of a Cu/Bi/SiO2
catalyst by copper(II) leaching, reduction and re-deposition onto the support
under typical activation conditions.
Figure 2. Raman spectra of the Cu35Bi4-80 catalyst after certain activation
times. Freshly prepared Cu2C2 and carbon (last two spectra) serve as
reference.
The progressive formation of crystalline Cu2C2 is additionally
confirmed by XRD of suspensions taken at different times
during activation procedure (Figure S9–S11). Quantitative deter-
mination of Cu2C2 amounts by thermal analysis (Figure S12)
shows that conversion of the Cu(II) precursor occurs continu-
ously and is completed depending on calcination conditions,
latest after 5–6 hours, which corresponds to activation times in
patents.[1c]
In order to study the Cu2C2 formation mechanism during
activation, the copper content in solution has been determined
by ICP-OES as function of time (Figure S13 for the dried (non-
calcined) catalyst); the results were confirmed by conductivity
measurements of the solutions with a Cu(II) specific electrode
(Figure S13b). During N2 purging, copper leaches from the Cu/Bi
catalysts into the reaction medium (e.g. up to 12 ppm=about
0.1% of the total copper content with Cu35Bi4-80, but up to 10
times less for catalysts calcined). This agrees with structural
changes of the catalysts observed by Raman spectroscopy
(Figure 2). With addition of C2H2 gas, the Cu(II) concentration
drops almost to zero indicating that sparingly soluble Cu2C2 is
precipitated. Raman spectroscopy and XRD revealed (Figure 2,
Figure S9–11) that formation of Cu2C2 gradually occurs during
activation (ca. 5 h). Copper acetylides are supposedly formed
from temporarily dissolved copper, the permanent copper
concentration during activation is determined to be 0.3 ppm.
This also agrees with the gradual formation of Cu2C2 and offers
an interesting starting point to optimize the final activated
catalyst. The proposed model is illustrated in Figure 3.
Figure 4. (a) Reaction of formaldehyde with pure (bulk) Cu2C2 as catalyst and
(b) reaction of formaldehyde and propargyl alcohol with Cu2C2 as catalyst.
that the addition of propargyl alcohol to the reaction mixture
leads to formation of 1,4-butynediol in the presence of Cu2C2,
i.e. Cu2C2 acts as catalyst for conversion of the intermediate
propargyl alcohol too. 1,4-butynediol yield increases faster
when acetylene is present. The results fit well with kinetic
studies of Kiyama et al. who obtained a zero order reaction
based on formaldehyde.[17]
Copper(I) acetylide activity in ethynylation reactions: Fig-
ure 4 shows the yields of propargyl alcohol and 1,4-butynediol
in the ethynylation of formaldehyde in the presence of (bulk)
Cu2C2.
Formaldehyde and Cu2C2 are not converted (stoichiometri-
cally) to either propargyl alcohol or 1,4-butynediol in the
absence of acetylene (a, left). After the addition of acetylene,
1,4-butynediol and propargyl alcohol are formed. Cu2C2 is not a
reaction intermediate but acts as catalyst. Part b) also shows
The experiments were also conducted with the supported
Cu2C2 catalyst Cu35Bi4-450 (Figure 5). During the N2 purging
period no products are formed. After the addition of acetylene,
the catalyst is activated forming Cu2C2, which catalyzes the
production of propargyl alcohol and 1,4-butynediol after an
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for the non-calcined catalyst (3.8 g1,4-butynediol ×gCuÀ 1 ×hÀ 1) than
for the calcined catalyst (3.1 g1,4-butynediol ×gCuÀ 1 ×hÀ 1) up to the
end of the activation period (300 min.) indicating a higher
number of active species, when the catalyst is not calcined.
In summary, copper acetylides Cu2C2 have been doubtlessly
proven experimentally as active species (Raman, XRD) in wet
supported (Cu/Bi/SiO2) and bulk copper oxide catalysts after
activation and during ethynylation of formaldehyde to 1,4-
butynediol for the first time. The structurally complex com-
pound is formed during catalyst activation in aqueous suspen-
sion by leaching of copper(II) (ICP-OES) followed by reduction
and reaction with C2H2 and deposition onto the support. The
reducing agent is acetylene, not formaldehyde (EPR, conductiv-
ity). Correlation of properties of Cu2C2 (Raman, XRD, thermal
analysis) with catalytic activity support the identification of
copper(I) acetylide as active species. Experimental proof and
quantification of copper acetylide bridges the gap between
well-characterized copper oxide pre-catalysts and its catalytic
performance and allows for advanced investigations of the
mechanism and more efficient parameter optimization of
copper catalyzed reactions with acetylene in the future.
Figure 5. Reaction of formaldehyde with Cu35Bi4-450 as catalyst.
induction period (about 20 min.). The yield of the intermediate
propargyl alcohol is very low in accordance with former kinetic
investigations. When acetylene is replaced by N2 after 180 min,
yields do not increase. Addition of propargyl alcohol after
300 min leads to the formation of 1,4-butyndiol again and
underlines that Cu2C2 represents the active phase in the Experimental Section
ethynylation reaction.
For more experimental details, see the Supporting Information.
The reaction kinetics during activation is shown in Figure 6
CuO/Bi2O3/SiO2 catalysts with 35 wt% Cu loading and 4 wt% Bi
loading were prepared by a common co-precipitation procedure[9]
based on copper nitrate, dissolved bismuth oxide and sodium
silicate. Pure or SiO2 supported Cu2C2 was synthesized from CuCl
and acetylene in ammoniacal solution.[16] XRD measurements were
carried out using an Empyrean diffractometer from PANalytical in
°
for the CuO/Bi/SiO2 catalysts after drying at 80 C (Cu35Bi4-80)
and after calcination (Cu35Bi4-450). A number of interesting
information can be concluded: the catalyst without calcination
does not show an activation period, i.e. formation of the
product(s) starts immediately after introduction of C2H2. The
calcined catalyst has an induction period of about 20 min. This
is in agreement with the observation that the formation of
Cu2C2 is faster for the non-calcined catalyst starting from
(supported) basic copper nitrate (Cu2NO3(OH)3) than from
copper oxide. This is in agreement with the differences in the
dissolved copper amount and supports the leaching mecha-
nism for the Cu2C2 formation. The reaction rate (slope) is higher
°
°
reflection mode and in an angle range between 5 and 70 in steps
°
of 0.008 . Raman spectra were recorded on an InVia Raman
Microscope from Renishaw with a laser intensity of 0.015 mW using
a 532 nm laser. EPR spectroscopy was measured using a JEOL JES-
°
RE 2X spectrometer on X-band frequency at À 120 C with a
microwave frequency of 9.20 GHz, 5 mW, a modulation amplitude
of 0.4 mT and a sweep-time of 4 min. The metal contents as well as
the leached Cu amounts were determined via elemental analysis
using an ICP-OES 700 from Agilent. For the conductivity measure-
ment a copper(II)-selective electrode by Metrohm was inserted and
an Ag/AgCl(aq) (3 M KCl) reference electrode was used. TG-MS
experiments were done with a STARe System TGA DSC3+, TGA 2
from Mettler Toledo GmbH connected to a ThermoStarTM mass
spectrometer from Pfeiffer Vacuum. The samples Àw1 ere heated from
°
°
25 C to 1000 C with a heating rate of 10 Kmin in synthetic air.
°
The standard catalyst activation was carried out at 100 C and
1.2 bar of acetylene for 5 h. The pH was kept constant using a
NaOH/NaH2PO4 buffer. For a standard activation procedure 5 g of
the catalyst was applied in 100 mL of a 37% aqueous formaldehyde
solution. Product analysis was done by GC using Agilent G1530A
equipped with a TCD. The catalysts were separated with a syringe
from the reaction suspension during the activation procedure. The
separated catalysts were washed with water, dried under vacuum
and stored in a glovebox for further characterization. The exact
description of the synthesis of the catalysts, the activation
procedure and the analytical methods are given in the Supporting
Information.
Figure 6. Reaction kinetics of supported copper oxide catalysts calcined at
°
°
different temperatures (80 C, 450 C).
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T.B. and A.A. thank the TUM Graduate School for financial
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: acetylides · copper · ethynylation · heterogeneous
catalysis · Reppe chemistry
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Manuscript received: June 2, 2021
Accepted manuscript online: July 16, 2021
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–7
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COMMUNICATION
T. Bruhm, A. Abram, J. Häusler, Dr. O.
Thomys, Prof. Dr. K. Köhler*
1 – 7
Walter Reppe Revival – Identification
and Genesis of Copper Acetylides
Cu2C2 as Active Species in Ethynyla-
tion Reactions
Copper acetylides, Cu2C2, have been
proposed as catalytically active
species in reactions with acetylene by
Walter Reppe decades ago. The
present study succeeded for the first
time with the doubtless experimental
identification of the explosive material
as active species in the ethynylation
of formaldehyde to 1,4-butynediol by
correlation with the performance of
supported Cu/Bi/SiO2 and bulk Cu2C2
catalysts.