Partial Reduction and Selective Transfer of Hydrogen Chloride on Catalytic Gold Nanoparticles

Article abstract of DOI:10.1002/anie.201700282

HCl in solution accepts electron density from Au NPs and partially reduces at room temperature, as occurs with other simple diatomic molecules, such as O₂ and H₂. The activation can be run catalytically in the presence of alkynes to give exclusively E-vinyl chlorides, after the regio- and stereoselective transfer of HCl. Based also on this method, vinyl chloride monomer (VCM) can be produced in a milder and greener way than current industrial processes.

Full text of DOI:10.1002/anie.201700282

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International Edition: DOI: 10.1002/anie.201700282
German Edition: DOI: 10.1002/ange.201700282
Heterogeneous Catalysis
Partial Reduction and Selective Transfer of Hydrogen Chloride on
Catalytic Gold Nanoparticles
Judit Oliver-Meseguer, Antonio Domꢀnech-Carbꢁ, Mercedes Boronat, Antonio Leyva-Pꢀrez,*
and Avelino Corma*
Abstract: HCl in solution accepts electron density from
Au NPs and partially reduces at room temperature, as occurs
with other simple diatomic molecules, such as O2 and H2. The
activation can be run catalytically in the presence of alkynes to
give exclusively E-vinyl chlorides, after the regio- and stereo-
selective transfer of HCl. Based also on this method, vinyl
chloride monomer (VCM) can be produced in a milder and
greener way than current industrial processes.
Au NPs.[9] UV/Vis measurements showed the expected
absorption band for the excited reduced state of HCl at
about 210–220 nm (ca. 5 eV higher than HCl; Supporting
Information, Figure S1),[10] which is well differentiated from
the 1,4-dioxane solvent (ca. 190 nm), HCl itself (ca. 180 nm),
and Au NPs (> 500 nm; Supporting Information, Figure S2).
To check this result, measurements were repeated with the
trityl cation (Ph3C+) as a probe, which is able to accept in situ
generated electrons and sequentially forms the well-defined
radical cation (Ph3C+C), anion (Ph3CÀ), and finally the radical
anion (Ph3CCÀ), with its associated oxygenate species
(Ph3CO2CÀ).[11] The results in Figure 1A clearly show the
H
ydrogen chloride is a multi-ton produced chemical that is
mostly employed in the manufacture of hydrochloric acid but
also in the production of vinyl chloride monomer (VCM),[1]
alkyl chlorides, and chlorobenzenes.[2] In these processes, the
À
H Cl bond is generally activated on catalytic metals by
oxidation rather than by reduction, despite the s*H-Cl lower
unoccupied molecular orbital (LUMO) being, in principle,
energetically available to accept electron density (Supporting
Information, Figure S1).[3] To trigger this process, the metal
must enable a symmetry-allowed interaction with the LUMO
of HCl while having the lowest affinity towards dissociated
HCl, to avoid formation of stable metal–chlorine bonds.
These requirements are fulfilled by late-heavy transition
À
metal Au NPs, with one of the lowest M Cl bond energy
Figure 1. A) UV/Vis spectrum obtained after addition of HCl to a dis-
persion of Au-TiO2 and trityl(tetrapentafluorophenyl)borate
tabulated.[4] In fact, this is exactly what occurs when Au NPs
activate, dissociate, and transfer on a surface a related
diatomic molecule such as O2,[5] which is particularly produc-
tive in the presence of soft electrophiles, such as carbon
monoxide,[6] alkynes,[7] and thiols.[8]
(Ph3C·C24BF20) in anhydrous 1,4-dioxane under ambient conditions.
B) Cyclic voltammogram of Au-TiO2 contact-probe Pt microelectrode
with a HCl solution in 1,4-dioxane (ca. 0.8m). Potential scan rate
50 mVsÀ1
.
Following this initial hypothesis, the possible interaction
between a HCl solution in 1,4-dioxane solvent and homo-
appearance of the radical anion Ph3CCÀ and its oxygenated
form at expenses of the Ph3C+ cation, only when HCl is added
to Au NPs. A linear correlation between the HCl added and
the formation of the reduced trityl radical species was found
dispersed (2.8–3.2 nm) Au0 NPs on TiO2 (Au TiO2), both
À
commercially available, was studied by two different in situ
techniques: UV/Vis absorption spectroscopy (UV/Vis) and
À
À
cyclic voltammetry (CV). The choice of Au TiO2 is based on
the well-known metallic character of these supported
in an excess of Au TiO2 (Supporting Information, Figure S3),
À
and blank experiments without Au TiO2, without HCl or
with TiO2 + HCl do not show any reduced species.
À
CV measurements of Au TiO2 in HCl solution show the
[*] Dr. J. Oliver-Meseguer, Dr. M. Boronat, Dr. A. Leyva-Pꢀrez,
Prof. A. Corma
enhancement of signals at microelectrodes for the one-
[12]
electron oxidation of ClC to Cl2 (+ 0.85 V, AHCl) and the
Instituto de Tecnologꢁa Quꢁmica, Universidad Politꢂcnica de
Valꢂncia-Consejo Superior de Investigaciones Cientꢁficas
Avda. de los Naranjos s/n, 46022 Valꢂncia (Spain)
E-mail: anleyva@itq.upv.es
reduction of trace protons[13] (rising cathodic current between
+ 0.5 and À0.5 V, CH/AH in the Supporting Information,
Figure S4, black lines), which is consistent with the catalytic
effect exerted by metal NPs in solution[14] and attached to
electrodes.[15] Besides, a s-shaped continuous curve with
a series of disruptive blip features appears in the linear scan
voltammogram, which corresponds to the collision of HCl
acorma@itq.upv.es
Prof. A. Domꢀnech-Carbꢃ
Departamento de Quꢁmica Analꢁtica, Universitat de Valꢂncia
Dr. Moliner, 50, 46100 Burjassot (Valꢂncia) (Spain)
Supporting information (including experimental section, product
characterization, and additional figures) and the ORCID identifica-
tion number(s) for the author(s) of this article can be found under
https://doi.org/10.1002/anie.201700282.
molecules with Au TiO2.[16] To permit a very sensitive record
À
of the process occurring at the immediate vicinity of the
electrolyte/solid material/electrode three-phase boundary,[17]
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contact-probe experiments were performed with the Pt
À
microelectrode pressed on a fine deposit of Au TiO2 at the
bottom of the glass cell, and Figure 1B shows that a new
oxidation peak (Arc) with the expected increase of voltage
(+ 0.35 V) for a reduced HCldÀ species appears, well differ-
entiated from the signals between 0.0 and À0.5 V (CH/AH)
associated to hydrogen species generated during the initial
oxidation of HCl, and with the characteristic crossover for the
À
Au TiO2 catalyzed process, marked by a dotted arrow. Thus,
the UV/Vis and CV results support the transfer of electrons
from Au NPs to HCl.
Alkynes are soft electrophiles that adsorb well on Au NPs
and react with activated O2 and H2 on surface.[7,9] Thus, it is
possible that the HCl activated on the Au NP could be
transferred to an alkyne, thus liberating the Au surface for
a next catalytic cycle. The resulting reaction, the hydro-
chlorination of alkynes, has not found use in the preparation
of vinyl chlorides beyond VCM[1] since the non-catalyzed
reaction proceeds with low yield and selectivity under harsh
reaction conditions[18] and, only very recently, some homoge-
nous metal complexes have shown catalytic activity for the
reaction.[19] Vinyl chlorides are naturally occurring products
with broad biological activities,[20] and their synthesis still lays
on waste-generating stoichiometric halogenating reagents
such as thionyl or phosphorus chloride.[21a] Scheme 1 shows
that not only different alkynes react well in the hydro-
À
chlorination reaction with Au TiO2 catalyst but also that the
Scheme 1. Top: Au-TiO2 catalyzed reaction of internal and terminal
alkynes with HCl. Reaction conditions: 1 mmol of alkyne, 1.2 equiv of
HCl in 1,4-dioxane (4m), 1,4-dioxane (1 mL), 3 ꢄ M.S. and the
reaction proceeds regioselectively, to the Markovnikov prod-
uct for terminal alkynes (products 2a–j), and stereoselec-
tively, to the E-alkenyl chloride for internal alkynes (products
2k–n; see also the Supporting Information, Figure S5). The
structure of the hydrochlorination product of deuterated
terminal alkyne [D]1a (Supporting Information, Figure S6)
confirms that the syn addition also operates for terminal
alkynes.
À
corresponding amount of Au TiO2 (5 mol%). Yield of isolated pro-
duct.[21b] Regioselectivity is >95% in all cases. [a] Tenth use of the
À
catalyst. Bottom: One-pot hydrochlorination C C bond-forming reac-
tions, isolated yield for 3a and GC yields for 3b–d.
basis of thermogravimetry results (Supporting Information,
Figure S9A), where the presence of residues that volatilize at
about 4508C together with about 3 nm Au NPs that melt
between 650 and 9508C was observed.[24] This volatile residue
could be assigned to aromatic polyalkynes and polyalkenes
A Hammett plot for different phenylacetylenes show the
generation of a partial positive charge on the internal carbon
during reaction (1 = À1.50; Supporting Information, Fig-
ure S7), and whereas electron donor substituents increase
the chlorination rate, very strong electron-withdrawing
groups decrease the rate and also switch the reaction to the
anti-Markovnikov products (products 2o,p). It is difficult to
explain the low reactivity of meta-substituted phenylacety-
lenes, although the so-called ortho effect[22a,b] and the possible
activation of the Au surface by particularly electron-rich
ortho- and para-substituted phenyacetylenes[22c] may be
invoked to explain this reactivity. Other Au catalysts tested
including AuI and AuIII salts (halides, oxides, triflates), AuI
complexes (phosphines and carbenes), and bare Au clusters
(3–7 atoms)[22d] did not show any significant activity for the
reaction. Notice that E-alkenyl products will not be expected
on the basis of typical anti-hydroadditions to alkynes[9]
catalyzed by Au+, but resemble more to the syn addition
products found for H2 on catalytic metal surfaces.[23]
À
strongly adsorbed on the recovered Au TiO2 catalyst,
according to IR spectroscopy (Supporting Information, Fig-
À
ure S9B). Measurements of the reused Au TiO2 by high-
angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM), before and after calcination,
shows that the size of the Au NPs is not affected throughout
the reuse (Supporting Information, Figure S9C), and hot
filtration tests and inductively coupled plasma atomic emis-
sion spectroscopy analysis of the reused catalyst confirmed
that no catalytically active Au species were present in solution
during reaction, below the detection limit of the technique
(< 10 ppm; Supporting Information, Figure S9D).
The use of vinyl chlorides as starting materials for the
synthesis of more elaborated compounds has been severely
limited for the unsuitability of synthetic methods available so
far and the sensitiveness of the products to polymerization,
despite the fact that they present great potential in paramount
reactions in organic synthesis, such as rearrangements and
cross-couplings.[25] Scheme 1 also shows that the vinyl chloride
Au-TiO2 could be reused 10 times for the hydrochlorina-
tion of 1a without depletion of the catalytic activity, filtering
the solid after complete conversion (19 h), and then calcining
the solid in air at 5008C (Scheme 1; Supporting Information,
Figure S8). This calcination temperature was chosen on the
À
products 2a,h,i engage well in C C bond-forming reactions
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such as the Claisen rearrangement to give products 3a,b, the
Pd-catalyzed Suzuki coupling to give product 3c, and
a cyclization coupling to give product 3d, in one-pot after
the hydrochlorination step without need of isolation, to give
products otherwise tedious to obtain by multistep synthetic
routes.[26] These results illustrate the practicality of activating
HCl on Au surfaces.
Experimental mechanistic studies were performed in
order to assess that the selective hydrochlorination of alkynes
on Au NPs comes and is controlled by HCl activation. First,
cyclic and horizontal touch voltammetry measurements[27]
confirm that the transfer of electron density from Au NPs to
HCl also occurs in the presence of the alkyne (Supporting
Information, Figures S4, S10). Second, the hydrochlorination
of 1a follows a first-order reaction rate for HCl and also Au
catalyst (Supporting Information, Figure S11). Third, the
hydrochlorination rate increases linearly with the number of
Au0 atoms on the NP, since Au0 is more prone to give
electrons (Supporting Information, Figure S12). For this
À
study, three samples of Au CeO2 with variable amounts of
Au0 according to X-ray photoelectron spectroscopy (XPS)
and in situ CO probe IR spectroscopy (Supporting Informa-
tion, Figure S13) were prepared, since the metallic-to-cationic
À
À
Au ratio can be much easily varied on Au CeO2 than in Au
TiO2.[28] A possible participation of TiO2 in the hydrochlori-
nation of alkynes was also discarded, since the kinetic profile
of the reaction under UV/Vis light (Hg lamp) is very similar
to the thermal reaction (Supporting Information, Figure S14).
And fourth, an inverse kinetic isotopic effect KIEH = 0.71(1)
was found for the hydrochlorination of phenylacetylene (PA)
Figure 2. Optimized geometries of the structures involved in the
hydrochlorination of phenylacetylene (PA) over an Au38 nanoparticle
and calculated energy profiles for PA (black line), methylacetylene
(MA; blue line), and dimethylacetylene (DMA; purple line) hydro-
chlorination yielding the Markovnikov product. Attempts to obtain the
anti-Markovnikov product for PA are depicted in red. Distances are
given in ꢄ. Optimized geometries of all structures plotted in the
energy profile are given in the Supporting Information, Figures S16–
S18.
À
with DCl catalyzed by Au TiO2, which strongly supports the
idea that the H atom is bound to a heavy atom, Au0 in this
case, during the reaction, in accordance to the location of the
s*H-Cl LUMO of HCl on the H atom.[3,10]
Density functional theory (DFT) calculations, shown in
À
Figure 2, support the Au···H Cl electron transfer, since all the
optimized geometries for HCl adsorbed on Au38 NPs show the
H atom pointing to the Au surface, with the Cl atom far from
cally competitive (solid red line in Figure 2). However, no
transition states could be found for the subsequent attack of
Cl to either M5 or M10, suggesting that these highly stable
intermediates should accumulate on the gold NPs, in agree-
ment with the spectroscopic observation of aromatic poly-
alkenes strongly adsorbed on the catalyst surface after
reaction. On the other hand, initial addition of Cl to the
terminal C atom yielding M7 is energetically disfavored
(dotted red line in Figure 2), in agreement with the high
regioselectivity experimentally observed. Similar structures
and energy profiles were obtained for methylacetylene (MA)
and dimethylacetylene (DMA) hydrochlorination (Support-
ing Information, Figures S17, S18). Experimentally, neither
alkenes (hydrogenation product) nor polychlorinated com-
pounds (from Cl2) are found during reaction, and the lack of
reactivity of styrene as a substrate discards the formation and
polymerization of simple alkenes from alkynes. These results
support that the regio- and stereoselective hydrochlorination
event occurs very rapidly on the Au NP surface, without any
spillover of the dissociated atoms of the HCl molecule.
Taking into account the experimental and theoretical
evidence shown above, a plausible mechanism for the hydro-
À
the metal, and the H Cl bond weakens and increases its
length from 1.285 ꢀ in the isolated molecule up to 1.344 ꢀ
when adsorbed on Au (Supporting Information, Figure S15).
À
À
Once the Au···H Cl interaction occurs, dissociation of H Cl
is an exothermic process by 8–10 kcalmolÀ1, with activation
energy barriers of 5–7 kcalmolÀ1 depending on the presence
or not of co-adsorbed alkyne (Figure 2; Supporting Informa-
tion, Figure S15). The experimental values found for DH##
and Ea, obtained by an Eyring plot, are 8.6(8) and 10.0-
(5) kcalmolÀ1, respectively, which approach reasonably well
the values given by calculations.
After HCl dissociation yielding structure M2, the Cl atom
attacks the internal position of the alkyne via transition state
TS23, forming the intermediate system M3 with an activation
energy of 12.5 kcalmolÀ1. Then, addition of the hydride
occurs stereoselectively via transition state TS34 to give the
adsorbed Markovnikov product M4 with an activation barrier
of 20.2 kcalmolÀ1. Initial H addition to form intermediate
species M5 involves a higher activation barrier (dotted black
line in Figure 2), while initial H addition to the terminal C
atom yielding a stable M10 intermediate species is energeti-
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chlorination of alkynes in solution catalyzed by Au NPs is
shown in Scheme 2. The reaction starts with the transfer of
electron density to adsorbed HCl, which then dissociates on
À
À
À
À
the partially charged Au NP to generate a H Au (Au)n Au
Scheme 2. Proposed mechanism for the syn-addition of HCl to alkynes
in solution catalyzed by Au NPs.
Figure 3. Hydrochlorination of acetylene in flow by feeding a gas
mixture of HCl and acetylene in a continuous flow stirred-tank reactor
(CSTR) of Au-TiO2 catalyst dispersed in 1,4-dioxane at 808C, for two
different catalyst masses. Data could be reproduced in a second run. A
mixture of pure acetylene, pure HCl, and part-per-million amounts of
toluene as additive passed through a fixed-bed tubular reactor (FBTR)
with supported Au-TiO2 gave very similar results.
Cl species. The presence of co-adsorbed alkyne does not
modify significantly this process. Then, the regioselective
addition of the Cl atom occurs and the adsorbed transient
species stereoselectively captures the H atom, to form the syn
product and regenerate the Au0 atoms.
The mechanism proposed here resembles more the metal-
catalyzed pairwise addition of H2[23] than the classical electro-
philic activation of alkynes.[29a] In the described mechanism,
the electron density flows from the metal to the nucleophile
(HCl) and then to the electrophile (alkyne), thus the
nucleophile is activated before the electrophile. This mech-
anism bypasses the typical protodemetalation step in Au-
catalyzed electrophilic hydroadditions that leads to anti
addition products. Indeed, this reaction mechanism occurs
a more efficient and greener production of the precursor of
PVC.
In conclusion, catalytic Au0 atoms in supported Au NPs
adsorb and dissociate HCl in anhydrous organic solvents at
À
À
À
À
room temperature, to give H Au (Au)n Au Cl transient
species that reacts regio- and stereoselectively with alkynes
adsorbed on surface, leading to E a-vinyl chlorides (syn-
hydrochlorination products). The solid catalyst can be reused
and employed in flow for the conversion of acetylene to VCM
under much milder reaction conditions than currently
employed in industry. Notice that although the hydrochlori-
nation of acetylene on carbon-supported Au catalysts was
already proposed by Hutchings in 1985,[1] and subsequently
brought to industrial scale, the approach presented herein is
clearly different, since the reaction is in a liquid rather than in
À
for the cis addition of some heteroelements (Het Het and
[29b,c]
À
À
Het H bonds) to alkynes with Au TiO2
and Au nano-
pore[29d–f] catalysts.
The Minamata Convention in 2013 states that mercury
catalysts will not be allowed in new VCM plants and that all
plants will have to remove them definitively in 2022. Thus, the
search of eco-friendly alternative catalysts for VCM produc-
tion is mandatory.[30] Despite exothermic in nature, the
hydrochlorination of acetylene is currently run at > 1508C,
to partially desorb the acetylene molecules of the alkynophilic
Hg2+ or Au3+ catalytic sites and allow HCl to interact with the
metal. In clear contrast, here it is shown that the adsorption
and activation of HCl on Au NPs occurs prior to the alkyne at
lower temperatures. Thus, VCM production was attempted
with Au-TiO2 catalyst either in a continuous flow stirred-tank
reactor (CSTR) or in a fixed-bed tubular reactor (FBTR).
Figure 3 shows that the process works well, and acetylene
transforms at 808C to vinyl chloride monomer (VCM) and
other chlorinated products, including C2 polychlorides and
vinyl chloride oligomers, in high conversions after long
reaction times on stream. Considering that polyvinyl chloride
(PVC) is the third worldwide plastic in terms of production
volume per year (> 30 million tons),[31] the results here shown
might constitute a step forward towards the development of
À
the gas phase, the H Cl bond is activated by reduction rather
than by oxidation, and Au0 active sites rather than Au+ are
involved.
Acknowledgements
J.O.-M. thanks ITQ for a contract. Financial support by
“Severo Ochoa” program is acknowledged.
Conflict of interest
The authors declare no conflict of interest.
Keywords: alkynes · gold · heterogeneous catalysis ·
hydrochlorination · vinyl chlorides
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Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Heterogeneous Catalysis
A helping hand from gold: Hydrogen
chloride is partially reduced by supported
gold nanoparticles at room temperature.
It is catalytically added to alkynes to give,
regio- and stereoselectively, E-a-vinyl
chlorides, including the synthesis of vinyl
chloride monomer from acetylene in
continuous mode.
J. Oliver-Meseguer, A. Domꢁnech-Carbꢂ,
M. Boronat, A. Leyva-Pꢁrez,*
A. Corma*
&&&& — &&&&
Partial Reduction and Selective Transfer
of Hydrogen Chloride on Catalytic Gold
Nanoparticles
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!