Catalytic hydrochlorination of acetylene on PdCl2/C supported catalysts: Kinetic isotopic effect of HCl/DCl, stereoselectivity, and mechanism

Article abstract of DOI:10.1134/S0023158417050135

Two routes of catalytic hydrochlorination of acetylene were found by the isotopic label method for systems with supported palladium K₂PdCl₄/C and Н₂PdCl₄/C catalysts: with formation of the products of syn- and a

Full text of DOI:10.1134/S0023158417050135

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 5, pp. 533–540. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © T.V. Krasnyakova, D.V. Nikitenko, E.V. Khomutova, S.A. Mitchenko, 2017, published in Kinetika i Kataliz, 2017, Vol. 58, No. 5, pp. 551–558.
Catalytic Hydrochlorination of Acetylene on PdCl2/С
Supported Catalysts: Kinetic Isotopic Effect of HCl/DCl,
Stereoselectivity, and Mechanism
T. V. Krasnyakovaa, b, D. V. Nikitenkoa, E. V. Khomutovaa, and S. A. Mitchenkoa, c,
*
aLitvinenko Institute of Physical Organic Chemistry and Coal Chemistry, Donetsk, 830114 Ukraine
bTaras Shevchenko National University of Lugansk, Lugansk, 91011 Ukraine
cPlatov South-Russian State Polytechnic University (NPI), Novocherkassk, 346400 Russia
*e-mail: samit_RPt@mail.ru
Received February 5, 2017
Abstract—Two routes of catalytic hydrochlorination of acetylene were found by the isotopic label method for
systems with supported palladium K2PdCl4/C and Н2PdCl4/C catalysts: with formation of the products of
syn- and anti-addition of the H(D)Cl molecule to the triple bond of acetylene. Two isotopic effects that differ
in magnitude were determined for these systems from the reaction kinetics and the ratio of the yields of the
nondeuterated and monodeuterated isotopomers of the product, which are due to the participation of the
H(D)Cl molecule in the two reaction stages: limiting chloropalladation and rapid protodemetallation. The
effective activation energies and kinetic isotope effects coincided within the experimental errors, which suggests
that the reaction mechanisms are similar and the active centers of the catalysts in systems with K2PdCl4/C and
Н2PdCl4/C are of the same nature.
Keywords: acetylene, hydrochlorination, PdCl2/C catalyst, kinetic isotope effect, stereoselectivity
DOI: 10.1134/S0023158417050135
Polyvinyl chloride occupies the third place after defined [5]. The reaction starts with π coordination of
polyethylene and polypropylene in the world output. the triple С≡С bond to the metal center. Further
There are two main methods of the industrial produc- attack of the external nucleophile at π-coordinated
tion of the vinyl chloride monomer: combined method acetylene leads to the formation of an intermediate
based on the chlorination and oxychlorination of eth- product of anti-addition of the nucleophile and metal
ylene and direct catalytic hydrochlorination of acety- complex to the triple bond. An intermediate product
lene [1]. The homogeneous catalytic systems for of syn-addition forms by an attack of the coordinated
hydrochlorination of acetylene (for example, platinum nucleophile. The protolysis of these intermediate
and palladium chloride complexes in basic solvents [2, complexes gives the final product with preserved ste-
3] and rhodium chloride in N-methylpyrrolidone [4]) reochemistry of the intermediate.
demonstrate high activity, selectivity, and stability, but
heterogeneous catalysts are preferable in industry.
HgCl2 on activated coal is traditionally used as a cata-
lyst [1, 5]. This catalyst is quickly deactivated by subli-
mation of highly toxic mercury, which leads to a nega-
tive impact on the environment [1, 6–9]. That is why
in recent years, active search for alternative environ-
mentally friendly catalytic systems is under way. The
overwhelming majority of studies [10–14] in this
direction dealt with the detection and modification of
supported catalysts of acetylene hydrochlorination,
but paid little attention to the reaction mechanisms,
whereas their rationalization would allow directed
design of new catalytic systems.
For heterogeneous catalytic systems, there is no
conventional mechanism for alkyne hydrochlorina-
tion. One of the tools for investigating the mechanisms
of chemical reactions is the kinetic isotopic effect
(KIE) method, which makes it possible to determine
the limiting stage of the reaction and the structure of
the transition state [15]. Previously, as a model reac-
tion, we considered the gas-phase catalytic hydrochlo-
rination of acetylene on the mechanically activated
(MA) K2PdCl4 salt (system 1) [16–20]. The use of the
H(D)Cl isotope labels revealed that the reaction was
stereoselective [16]: only the product of trans-addition
of chlorine and deuterium atoms to the triple bond of
acetylene formed in this system. In addition, it was
The mechanism of catalytic hydrochlorination of shown by the KIE method that the catalytic reaction
acetylene under homogeneous conditions is well involves the H–Cl bond cleavage at two stages:
533

534
KRASNYAKOVA et al.
chlorometallation of π-coordinated acetylene (limit- The relative concentration of acetylene (ϕ
ing stage) and protodemetallation of the σ-chlorovinyl
) was
C2H2
determined as the ratio of the areas of the chromato-
graphic peaks of C2H2 and internal standard (meth-
ane). The observed reaction rate constants were deter-
mined by the equation
palladium derivative (fast stage) [17].
A similar approach can be used to determine the
mechanism of gas-phase hydrochlorination of acety-
lene involving supported palladium catalysts, the use
of which makes it possible to substantially reduce the
cost of the expensive metal. The goal of the present
study was to determine the tendencies in the catalytic
hydrochlorination of acetylene in systems with acti-
vated coal-supported palladium catalysts K2PdCl4
(system 2) and H2PdCl4 (system 3).
k = –d (lnϕС Н ) dt.
2
2
The kinetic isotope effect of gas-phase hydrochlori-
nation of acetylene was calculated [15] as the ratio
kHCl
kDCl
KIE =
of the observed rate constants of the reactions occur-
ring in the HCl (kHCl) and DCl (kDCl) atmosphere. As
pure deuterium chloride is difficult to obtain because
of the fast isotopic exchange between the DCl mole-
cules and the atmospheric moisture traces, the reac-
tion was carried out in an isotopic DCl/HCl mixture.
The observed rate constant of this reaction k depends
on the mole fraction χHCl in the DCl/HCl mixture and
is related to the reaction rate constants in the atmo-
sphere of HCl and pure DCl by the equation [15]
EXPERIMENTAL
Preparation of Support
To prepare the catalyst, we used activated coal of
AG-3 grade. The coal was ground in an agate mortar
and sieved through 0.25 and 0.1 mm sieves to separate
the 0.25–0.1 mm fraction. The resulting fraction was
boiled in concentrated HCl for 5 h, then washed with
distilled water until neutral reaction of rinsing water
was achieved and dried at 120°C for 5 h.
k − kHCl
1 − χ
χ
kDCl
=
.
Preparation of Catalysts
The mole fraction (χ) of the HCl isotopomer in the
HCl/DCl gas mixture was determined by IR spectro-
photometry from the integrated intensity ratio of the
HCl and DCl bands, taking into account that under
the same conditions, the band intensities of HCl are
approximately two times higher than those of DCl
[22]. For this purpose, an FT/IR-4200 Fourier spec-
trophotometer (JASCO, Japan) was used, whose soft-
ware allows the integration of the IR signals.
The isotope effect of protolysis was determined from
the ratio of the yields of deuterium-free (H) and
monodeuterated trans-DHC=CHCl (E) and cis-
DHC=CHCl (Z) products formed in the DC1/HCl
isotope mixture, including the mole fraction χ of the
HCl isotopomer in it:
A K2PdCl4 sample (32.6 mg, 0.1 mmol) was dis-
solved in 3 M hydrochloric acid (3 mL). An activated
coal sample (1 g) was impregnated dropwise with the
prepared solution while constantly stirring it, and the
sample was kept at room temperature for 2 h. The
resulting catalyst was dried at 120°C for 15 h.
A PdCl2 sample (35.25 mg, 0.2 mmol) was dis-
solved in 3 M hydrochloric acid (5 mL), and an acti-
vated coal sample (2 g) was impregnated in a similar
way and dried. The indicated amounts of K2PdCl4 and
PdCl2 were calculated based on the condition that the
same palladium charge was obtained.
Determination of the Observed Rate Constant
of Acetylene Hydrochlorination
1 − χ
E + Z χ
H
γ =
.
A catalyst (or activated coal) sample (0.1 g) was
placed in a closed glass reactor, flushed with hydrogen
chloride gas (HCl or HCl/DCl mixture), and then
sealed; and acetylene (1 mL) and methane as an inter-
nal standard were injected through a rubber sealing
gasket. Hydrogen chloride (HCl or DCl) was obtained
from calcinated KCl and H2SO4 (D2SO4); acetylene
was obtained by the procedure of [21]. The reactor was
thermostatted in the temperature range 4–80°C. The
kinetic mode of the reaction was provided by shaking
the reactor. The consumption of acetylene was moni-
tored by GLC on an LKhM-8MD chromatograph
(Chromatograph, Russia) with a flame ionization
detector and a packed column filled with the Silochrome
S-120 adsorbent. The data were collected and processed
NMR Studies
The 1H NMR spectra of the reaction products were
recorded on an AVANCE-II-400 spectrometer
(Bruker BioSpin, Germany) at an operating frequency
of 400 MHz. After the reaction was completed, the gas
phase contained in the reactor was transferred with a
dry argon current (50 mL) into deuterochloroform
(1 mL). The spectra were processed using TOPSPIN
software.
1H NMR spectrum of trans-DHС=CHCl, δ, ppm:
6.32 (d, JН–Н = 14.8 Hz), Ha1, 5.54 (d, JН–Н = 14.8
using the MultiChrome system (Ampersend, Russia). Hz), Hb1.
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CATALYTIC HYDROCHLORINATION OF ACETYLENE
535
1H NMR spectrum of cis-DHС=CHCl, δ, ppm: 6.32
ln ϕC H
2
2
(d, JH–H = 7.0 Hz), Ha2, 5.45 (d, JH–H = 7.0 Hz), Hc2.
1H NMR spectrum of H2C=CHCl, δ, ppm: 6.32
(dd, Jcis(H–H) = 7.0 Hz, Jtrans(H–H) = 14.8 Hz,), Ha3; 5.55
(dd, Jtrans(H–H) = 14.8 Hz, Jgem(H–H) = 1.5 Hz), Hb3; 5.46
(dd, Jcis(H–H) = 7.0 Hz, Jgem(H–H) = 1.5 Hz), Hc3.
1
0
RESULTS AND DISCUSSION
Kinetics of Catalytic Hydrochlorination of Acetylene
on Supported PdCl2/C Catalysts
–1
K2PdCl4 and H2PdCl4 supported on activated coal
(see, e.g., [10]) catalyze the hydrochlorination of acet-
ylene with gaseous hydrogen chloride. The kinetics of
acetylene consumption from the gas phase of a closed
reactor with excess HCl (DCl) in systems 2 and 3 cor-
responds to the first-order kinetic equation (Fig. 1).
After more than 10 catalytic cycles based on loaded
palladium, there was no any significant reduction of
the catalytic activity. Note that in the presence of acti-
vated coal alone (support), acetylene was not con-
sumed at a noticeable rate.
0
500
1000
1500
2000
2500
t, s
Fig. 1. Typical kinetics of acetylene consumption in the
hydrochlorination of acetylene in systems with supported
catalysts.
= 0.1 g, 25°C.
m
K2PdCl4
C
lnk [s–1]
1
2
–5.5
The effective activation energies and pre-exponen-
tial factors of the Arrhenius equation in systems 2 and
3 were determined from the temperature dependence
of the experimental rate constant of catalytic hydro-
chlorination of acetylene: Е2 = 30 1 kJ/mol and Е3 =
32 3 kJ/mol, lnA2 = 4.9 0.4 and lnA3 = 6.2 1.1,
respectively (Fig. 2). The effective activation energies
in systems 2 and 3 coincide within the limits of exper-
imental errors, so the high reaction rate for system 3
compared with that for 2 (Table 1) is obviously
explained by the relationship between the pre-expo-
nential factors A3 > A2.
–6.0
–6.5
–7.0
–7.5
0.0028 0.0030 0.0032 0.0034 0.0036
1/T, K–1
Stereoselectivity of Reaction
Fig. 2. Temperature dependence of the observed rate con-
stant of acetylene hydrochlorination in systems with sup-
ported catalysts (1) K PdCl /C and (2) H PdCl /C.
The reaction in the HCl/DCl isotope mixture in
systems 2 and 3 is accompanied by isolation of the
products of syn- and anti-addition of chlorine and
deuterium atoms to the C≡C triple bond of acetylene
along with nondeuterated vinyl chloride (Fig. 3).
Based on the stereoselectivity of hydrochlorina-
tion, it was assumed [5] that the source of the Cl atoms
in the anti-addition product is the hydrogen chloride
molecule because there are no other “external” chlo-
ride ions in the system in an amount sufficient for the
catalytic process. The syn-addition product is formed
[5] by the intraspheric attack of the chloride ligand on
the π-acetylene complex of palladium(II) (vinyl chlo-
ride includes the Cl atom from the coordination
sphere of the metal complex).
2
4
2
4
1
result of anti-addition can be evaluated from the H
NMR spectrum of the reaction products:
ε = Е/(Е + Z).
The group of low-field signals in the spectrum
(Fig. 4) corresponds to the proton Ha located at the
carbon atom bound to the chlorine atom and rep-
resents a set of signals of all the three products:
H2C=CHCl, trans-DHC=CHCl, and cis-DHC=CHCl.
The central group is a superposition of the signals of
the proton Hc in the cis position relative to the chlorine
atom in the isotopomers H2C=CHCl and trans-
DHC=CHCl. The high-field group of signals corre-
sponds to the trans-protons Hb in the H2C=CHCl and
If we assume that the isotope effects of the protol-
ysis stage of the vinyl palladium derivatives obtained
by anti- (E) and syn- (Z) chloropalladation are the
same, the fraction (ε) of vinyl chloride obtained as a cis-DHC=CHCl isotopomers (Fig. 4). If we assume
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536
KRASNYAKOVA et al.
Table 1. Experimental rate constants kHCl (kDCl) of catalytic hydrochlorination of acetylene in an HCl(DCl) atmosphere
and kinetic isotope effects of the limiting stage at 25°C
kHCl × 103, s–1
kDCl × 103, s–1
System
KIE
1. K2PdCl4 (МА)
2. K2PdCl4/C
3. Н2PdCl4/C
0.064 0.003
1.07 0.02
2.40 0.10
0.036 0.004
0.86 0.02
1.86 0.22
1.78 0.25
1.25 0.06
1.29 0.20
that the integrated intensity of the low-field group of because in this case, the limiting state involves cleav-
signals is 1, the relative integrated intensities of the two age of the H–Cl bond in the gaseous hydrogen chlo-
subsequent groups of signals will be 0.87 and 0.63, ride molecule. Using the KIE value of hydrochlorina-
respectively (Fig. 4). Then the ratio of the products of tion in system 1, which leads exclusively to the product
anti- and syn-addition is E : Z = 7 : 3, and the fraction ε of anti-addition [19], and the evaluated fraction ε for 2
of vinyl chloride obtained as a result of anti-addition for and 3, we obtain the expected KIE equal to 1.3 in these
systems 2 and 3 is 0.7.
systems, which coincides with the experimental value
(Table 1).
The kinetic isotope effect determined from the
ratio of the yields of the nondeuterated and monodeu-
terated isotopomers of the reaction product that accu-
mulated under the competitive conditions in the
DCl/HCl atmosphere is γ = 2.8 0.3, which consider-
ably exceeds the KIE values given above for systems 2
and 3 (Table 1).
Kinetic Isotope Effect
The experimental rate constants of acetylene con-
sumption from the gas phase of a closed reactor mark-
edly decrease for the reaction occurring in the atmo-
sphere of the HCl/DCl isotope mixture compared
with the rate constants for the reaction in the HCl
atmosphere in all the three systems. The KIE values of
the gas-phase hydrochlorination of acetylene in the
systems are listed in Table 1.
Reaction Mechanism
The KIE values obtained for systems 2 and 3 are
smaller than for 1. The reason for this decrease in KIE
is the presence of two routes of acetylene hydrochlori-
nation: with formation of the products of syn- and
anti-addition. The contribution to KIE can be only
from the formation of the product of anti-addition
The presence of two isotope effects of HCl/DCl in
systems 2 and 3, like in the case of mechanically acti-
vated K2PdCl4, K2PtCl4, and K2PtCl6 [20], points to
bond cleavage in the hydrogen chloride molecule at
two stages of acetylene hydrochlorination: chloropalla-
dation of π-coordinated acetylene with intermediate
formation of the chlorovinyl derivative of palladium(II)
and protodemetallation of the latter species. At the
chloropalladation stage, the kinetic isotope effect is
observed during the H–Cl bond cleavage, and the
product includes the chlorine atom; during protode-
palladation, the isotope effect γ is observed, and the
product includes the hydrogen atom. The isotope
effect γ differs considerably from KIE determined
from the reaction kinetics. Consequently, the limiting
stage of the catalytic hydrochlorination of acetylene by
gaseous HCl is the chlorometallation stage. This
agrees with the fact that protodemetallations of
σ-organic derivatives of platinum group metals are
generally fast (see, e.g., [19, 20, 23, 24]). Note that the
situation is slightly different under the homogeneous
conditions [25]: the palladium(II) chloride complexes
in aqueous solutions also catalyze hydrochlorination
of acetylene, and it is believed [25] that chloropallada-
tion of acetylene is quasiequilibrium, and the reaction
is limited by protodemetallation of the intermediate
chlorovinyl derivative of palladium(II). The reason for
this difference under the homogeneous and heteroge-
neous conditions is the absence of free chloride ions in
heterogeneous systems in amounts close to the amount
Cl
Hc1 Cl
Ha2
D
Cl
Hc3
Hb3
Ha1
D
Hb2 Ha3
Hc3
Hc1
Hb3
Ha3
Ha1
Hb2
Ha1
5.36
5.26
5.52
Chemical shift, ppm
1
Fig. 3. Typical H NMR spectrum of the reaction products
of gas phase hydrochlorination of acetylene in systems with
supported K PdCl /C and Н PdCl /C catalysts in the
2
4
2
4
HCl/DCl isotope mixture.
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CATALYTIC HYDROCHLORINATION OF ACETYLENE
537
Hc1 Cl
Ha2
D
Cl
Hc3
Hb3
3.8 Å
Cl
Ha1
D
Hb2 Ha3
Ha3
Ha1
Hc3
Hb3
Hb2
Hc1
Ha2
6.0
5.5
1.00
0.87
0.63
Chemical shift, ppm
1
Fig. 4. Line diagram of the H NMR spectrum of the reac-
tion products of gas phase hydrochlorination of acetylene.
b
Pd
Cl
of the product. As a result, the stage of chloropallada-
tion of acetylene becomes slow.
The KIE values and effective activation energies for
systems 2 and 3 coincide within the experimental
errors, which suggests that the reaction mechanisms in
these systems are similar, and the active form of the
catalyst is the same. The state of the activated coal-
supported Pd(II) chloride complexes was studied in
detail in [26–31]. It was shown that the adsorption of
Н2PdCl4 by the activated coal surface leads to the for-
mation of palladium dichloride complexes with active
centers (А) of the coal matrix and liberation of hydro-
gen chloride [31]:
c
a
Fig. 5. Structure of the unit cell of α-PdCl .
2
~4.15 Å (Fig. 5). The indicated distances 3.8 and
4.15 Å in the structure of α-PdCl2 are close to the dis-
tance between the chlorine atoms in the neighboring
palladium complexes in system 1 (which is 4.11 Å [33])
involved in the formation of the product of anti-addi-
tion. Because of this coincidence of interatomic dis-
tances, we can assume that the mechanism in systems 2
and 3 is similar to the mechanism of the reaction in
system 1 [19], which occurs on two neighboring palla-
dium complexes (united by a brace in Scheme 1), one
of which is a complex with a vacancy in the coordina-
tion sphere. We do not consider the possibility of
replacing the chloride ligand by acetylene from the
coordination sphere of palladium under heteroge-
neous conditions because this situation is thermody-
namically unfavorable: the evolved chloride ion is
not solvated, and the palladium chloride complexes
do not form stable π complexes [18, 34].) Acetylene
is π-coordinated to the Pd(II) complexes with an
uncrowded coordination sphere; then at the limiting
stage, chloropalladation of π coordinated acetylene
occurs under the action of the HCl molecule with the
aid of the adjacent palladium complex, which leads to
the chlorovinyl derivative of Pd(II) and regenerates the
complex with the coordination vacancy. Note that π
coordination of acetylene does not change the charged
state of the complex, which has a local positive charge.
The polar НСl molecule is bound by its chlorine atom
to π coordinated acetylene by means of electrostatic
H2PdCl4 + A ꢀ PdCl2 ⋅ A + 2HCl.
As a result, (PdCl2)n fragments of two types form
on the activated coal surface [28]. About 60% Н2PdCl4
is adsorbed by the surface of activated coal in the form
of the α-PdCl2 (Х) clusters of an orthorhombic struc-
ture with sizes of 16–18 Å and with the number of
units in the chain n ≈ 70 [27]. The remaining number
(Y) of (PdCl2)n complexes are present on the support
surface in the form of monomers, dimers, or trimers
[28]. It can be assumed that the deposition of K2PdCl4
on the activated coal surface leads to the formation of
the PdCl2. А states, which also exist in two forms: as α-
PdCl2 clusters and small molecular groups of palla-
dium dichloride (n = 1–3).
In the structure of bulky α-PdCl2 (Fig. 5), the
oligomer chains containing four-coordinated palla-
dium atoms pass along the edges of the unit cell of the
crystal parallel to the [001] direction via the center of
the unit cell [32]. The distance between the chlorine
atoms belonging to the neighboring chains in the (010)
plane is 3.8 Å [32]. The unit cell parameters are а =
3.81 Å and b = 11.04 Å; the Pd–Cl bond length (2.3 Å)
in the chain and the angle (~23°) between the chain
plane and the [010] direction [32] allow us to evaluate
the distance between the chlorine atoms in the neigh-
boring oligomer chains (one of which passes along the
edge of the unit cell and the other across its center): forces near the palladium complex. In the electrostatic
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538
KRASNYAKOVA et al.
field, the Н–Сl bond is polarized to a still greater tion of the chlorovinyl derivative of Pd(II) gives the
extent, which facilitates its cleavage. Protodemetalla- final product of anti-addition.
H
Cl
H
C2H2
Cl
C
C
*H
Pd
Pd
Cl
Cl
Cl
Cl
*HCl
H
H
C
Cl
Cl
Cl
C
C
Cl
Cl
Pd
C
H
Pd
Pd
Cl
Cl
Cl
Cl
H
Cl
Cl
Cl
Pd
Cl
Cl
+
Cl
Cl
H
+
Cl
C
HCl
Pd
C
HCl
Cl
Cl
Cl
Cl
Cl
H
H
Pd
Cl
Cl
Scheme 1. Mechanism of formation of the product of anti-addition in systems with supported K2PdCl4/C and
Н2PdCl4/C catalysts.
The product of syn-addition is obtained by chloro- metal (Scheme 2). Further fast protodemetallation
metallation of π-coordinated acetylene by the chlorine during the attack of the carbon atom of the Pd(II)
atom from the coordination sphere of the same metal vinyl derivative by the HCl molecule leads to the for-
complex, forming the chlorovinyl derivative of palla- mation of the final product and regeneration of the
dium and a vacancy in the coordination sphere of the starting metal complex.
H
H
C2H2
C
C
Cl
Cl
*H
Cl
Pd
Cl
*HCl
H
H
C
C
Cl
Cl
C
Cl
Cl
H
Pd
C
H
Pd
Cl
Cl
+
+
H
Cl
C
Pd
C
H
Cl
Cl
Scheme 2. Mechanism of formation of the product of syn-addition in systems with supported K2PdCl4/C and
Н2PdCl4/C catalysts.
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CATALYTIC HYDROCHLORINATION OF ACETYLENE
In this case, the active centers of the catalyst may REFERENCES
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ACKNOWLEDGMENTS
This study was financially supported by the Russian
Scientific Foundation (project no. 14-23-00078).
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2017