Catalytic hydrogen-chlorine exchange between chlorinated hydrocarbons under oxygen-free conditions

Article abstract of DOI:10.1002/anie.200800270

(Figure Presented) Pass the parcel: Activity experiments show that LaCl₃ supported on carbon nanofibers is a highly active, selective, and stable catalyst for the H-Cl exchange reaction between CCl₄ and CH₂Cl₂ to form CHCl₃ (see scheme) in the absence of either lattice or gas-phase oxygen. Density functional calculations suggest that the reaction proceeds through the formation of weakly adsorbed Cl and H species which can be exchanged between the reactants.

Full text of DOI:10.1002/anie.200800270

Communications
DOI: 10.1002/anie.200800270
Oxygen-Free H–Cl Exchange
Catalytic Hydrogen-Chlorine Exchange between Chlorinated
Hydrocarbons under Oxygen-Free Conditions**
Alwies W. A. M. van der Heijden, Simon G. Podkolzin, Mark E. Jones, Johannes H. Bitter, and
Bert M. Weckhuysen*
Chlorinated hydrocarbons (CHCs) remain important indus-
trial chemical intermediates and solvents, especially for the
exploration of the potential of La-based materials for the
carbons, namely the reaction of CCl with methane to produce
4
[
8]
CH Cl over supported Pt catalysts. However, as the
3
reported catalysts also degrade due to conversion of their
oxide supports into chlorinated compounds, the authors
advocate addition of gas-phase oxygen to the feed to
remedy the coke formation even though this inevitably
leads to the formation of oxidation products and lower
selectivity.
[
1]
conversion of chlorinated waste compounds. The produc-
tion of industrially important CHCs frequently occurs with
concurrent formation of less desirable side-products. For
example, mixtures of chlorinated C and C hydrocarbons are
1
2
still formed as by-products in industrial processes such as the
[
2,3]
production of vinyl chloride monomer (VCM).
Another
Catalytic systems for activating CꢁH and CꢁCl bonds
example is carbon tetrachloride (CCl ) formation in the
generally use oxygen-containing compounds in the feed
because they provide a thermodynamic driving force, as in
4
production of chloroform (CHCl ) and other chlorinated
3
[
9,10]
methanes. The United States Clean Air Act and the Montreal
oxidation of methane to methanol and acetic acid
concurrent water formation in the oxidative coupling of
methane.
or
[
4,5]
Protocol limit the production and sale of CCl4,
therefore
[11]
methods to effectively recycle chlorinated side-products, in
Kinetically limiting oxidation and preventing
particular CCl , would be advantageous. The hydrogen–
thermodynamically favorable total combustion is difficult,
and even selective bond activation remains a challenge,
4
[
12,13]
chlorine exchange of CCl4 with other CHCs, such as
CH Cl , for the recycling of less desirable compounds into
therefore the development of more efficient catalysts for
complete oxidative destruction of hydrocarbons and chlori-
nated hydrocarbons remains an area of active research.
Recent examples in this field include combustion over
2
2
valuable products would be of particular interest.
The reaction thermodynamics favor the use of CCl as a Cl
4
source with methane or a chloromethane. The best known
way to run these reactions is thermal gas-phase radical
[
14]
uranium-oxide catalysts, reaction with H O over an iron
2
2
[
6,7]
[15]
chemistry,
chemistry is its low selectivity due to the formation of various
although the main disadvantage of radical
catalyst,
and our own publications on the destructive
adsorption over lanthanum-based catalysts.
[
16–21]
[
6,7]
chloromethanes and C (n ꢀ 2) coupling products.
Coke
Lanthanum-based catalysts have also recently been
reported to selectively activate hydrocarbons such as methane
n
formation at the temperatures required for radical generation
also lowers product yields and can foul the equipment. As a
result, the commercial application of this reaction does not
currently appear to be economically attractive, and inciner-
[
22,23]
and ethane.
Previous studies have suggested that the
presence of oxygen is critical for activation of both CꢁH and
CꢁCl bonds over these catalytic materials, therefore the
destructive adsorption reaction is proposed to proceed via an
ation is commonly used as a route for disposing of CCl . To
4
[
16–21]
our knowledge, there is only a single report on the catalytic
exchange of H and Cl atoms between chlorinated hydro-
exchange of two chlorine atoms for one oxygen atom.
If
lattice oxygen is depleted, the reaction stops. In the case of Cꢁ
H bond activation in the oxidative hydrochlorination of
methane, the reaction is proposed to proceed via exchange of
[
*] A. W. A. M. van der Heijden, Dr. J. H. Bitter,
Prof. Dr. ir. B. M. Weckhuysen
Inorganic Chemistry and Catalysis Group
Department of Chemistry
Utrecht University
[22]
a hydrogen atom for a chlorine atom.
This H-for-Cl
exchange, however, only takes place when the surface Cl
species are activated in the presence of O , and the reaction
2
stops without O . Herein we report that LaCl is an active and
2
3
Sorbonnelaan 16, 3584 CA Utrecht (The Netherlands)
Fax: (31)30-251-1027
E-mail: b.m.weckhuysen@uu.nl
stable catalyst for the hydrogen–chlorine exchange reaction
between CH Cl and CCl , selectively yielding CHCl under
2
2
4
3
oxygen-free conditions.
The gas-phase reaction between CCl and CH Cl under
Dr. S. G. Podkolzin, Dr. M. E. Jones
The Dow Chemical Company
Core Research and Development
Midland, Michigan 48674 (USA)
4
2
2
our experimental conditions yields CHCl3 in only trace
amounts. To correct for these gas-phase reactions, the results
from the catalytic experiments (Figure 1) were adjusted by
subtracting the results obtained under the same conditions
obtained from blank experiments with either quartz pellets or
carbon nanofibers (CNFs). Before testing, all catalysts were
chlorinated under the appropriate conditions to ensure
complete substitution of all lattice oxygen for chlorine to
[
**] The authors thank NWO-CW VICI for financial support. The
metathesis reaction between CCl and CH Cl was discovered by
4
2
2
Mark E. Jones at the Dow Chemical Company. The carbon nano-
fibers were synthesized by Arjan J. Plomp.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
5
002
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 5002 –5004

Angewandte
Chemie
magnitude more active than the unsupported materials. No
formation of C hydrocarbon products was detected during
2
the experiments, thereby indicating that the catalyst is highly
selective and that catalytic chemistry dominates over any gas-
phase radical reactions.
To evaluate the stability of the novel LaCl /CNF catalyst,
3
the reaction was run for 36 h at 4008C. The results shown in
Figure 2 indicate the presence of an induction period in the
first hour of reaction when a significant amount of coke is
Figure 1. Conversion of a mixture of CH Cl and CCl intoCHCl as a
2
2
4
3
function of temperature over bulk LaCl synthesized by chlorination of
3
LaCl ·7H O with CCl (*), bulk LaCl synthesized by chlorination of
3
2
4
3
LaOCl with HCl (~), and 20 wt% LaCl supported on CNF (
)
&
3
ꢁ1
(
GHSV=400 h ; inlet concentration: CCl =CH Cl =4.7 vol% in He).
4 2 2
Figure 2. Reaction mixture composition (CHCl : ^; C: ~) for the
3
metathesis of CCl and CH Cl at 4008C over 20 wt% LaCl supported
4
2
2
3
prevent destructive adsorption of the reactants. The concen-
tration of the chlorinating agent was determined by GC
analysis. Once the uptake of chlorinating agent by the catalyst
ceased, the material was deemed to be fully chlorinated. For
the first set of experiments, a bulk LaCl3 catalyst was
synthesized by chlorinating LaCl ·7H O with CCl . Figure 1
on CNF as a function of time. The concentration of carbon was
calculated based on the concentration of the gas-phase products
ꢁ1
before and after reaction (GHSV=400 h ; inlet concentration:
CCl =CH Cl =4.7 vol% in He).
4
2
2
formed along with HCl and Cl . Nevertheless, only CHCl is
3
2
4
2
3
shows that this catalyst has a low, but appreciable, activity for
H–Cl exchange between CCl and CH Cl (Reaction 1) with
produced as a gas-phase product. A possible explanation for
the induction period is that residual Ni from the CNF growth
catalyzes coke formation. CNFs are grown on a Ni/SiO2
4
2
2
CH Cl ðgÞ þ CCl ðgÞ ! 2 CHCl ðgÞ
ð1Þ
catalyst and then washed to remove Ni and SiO . A trace
2
2
2
4
3
amount of Ni becomes encapsulated in the CNF, however,
and therefore cannot be removed by washing. This Ni may
become accessible during the H–Cl exchange experiments as
a result of thermal expansion and the severe chlorination
conditions needed for the synthesis of LaCl3.
1
00% selectivity for CHCl at moderate temperatures (not
3
shown). At higher temperatures, however, the selectivity
decreases and coke formation is observed, thus suggesting the
poor long-term stability of this catalytic material.
Although surface area measurements are difficult due to
The likely reaction steps in the H–Cl exchange between
CCl and CH Cl (Reaction 1) were evaluated by density-
the hygroscopic nature of LaCl , it is reasonable to expect that
3
4
2
2
the surface areas of the precursor and the final chlorinated
material are correlated. The low conversion observed with the
catalyst prepared from LaCl ·7H O can therefore be attrib-
functional theory (DFT) calculations. These calculations
suggest that neither CCl4 nor CH Cl2 is likely to adsorb
2
molecularly on a fully chlorinated ideal surface of LaCl3.
However, it is energetically favorable for chloromethanes to
split off a Cl atom and donate it to the surface. Although
splitting off and donation of an H atom to the surface is
calculated to be endothermic, the process can be partially
compensated by exothermic chlorination of the resulting
hydrocarbon fragment. It is, therefore, reasonable to assume
that the reaction proceeds in two separate H–Cl exchange
steps: CH Cl exchanges an H for a surface Cl (Reaction 2)
3
2
2
ꢁ1
uted to its initially low surface area of around 1 m g . LaOCl,
in contrast, has a relatively high initial surface area (20–
2
ꢁ1
3
0 m g ), although the catalyst prepared using LaOCl as a
precursor chlorinated with HCl shows only a slight improve-
ment in activity (Figure 1). This suggests that the higher initial
surface area of the LaOCl precursor is mostly lost during the
chlorination pre-treatment. To efficiently increase the catalyst
surface area, LaCl was therefore impregnated onto CNFs
3
2
2
2
ꢁ1
(
140 m g ). This support was selected because of its stability
and then CCl exchanges a Cl for a surface H (Reaction 3).
4
under the chlorination conditions due to the absence of lattice
oxygen, which may lead to destructive adsorption of the
reactants. In addition, the deposition of non-metals on carbon
CH
Cl
ðgÞ þ ClꢁLaCl
ðsÞ ! CHCl
ðgÞ þ HꢁLaCl
ðsÞ
ð2Þ
ð3Þ
2
2
2
3
2
[
24]
CCl ðgÞ þ HꢁLaCl ðsÞ ! CHCl ðgÞ þ ClꢁLaCl ðsÞ
supports has been proven to be successful.
Figure 1
4
2
3
2
demonstrates that this catalyst exhibits the highest conversion
on a weight basis. Furthermore, on a volume basis (not
shown), the CNF-supported catalyst is at least two orders of
The calculated energies for Reaction 2, where H is
exchanged for surface lattice Cl, are high (310–
a
Angew. Chem. Int. Ed. 2008, 47, 5002 –5004
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 5003

Communications
ꢁ
1
3
24 kJmol ) regardless of the presence or position of any
reaction can proceed through the formation and exchange of
weakly adsorbed H and Cl species on the catalytic surface.
See the Supporting Information for full experimental and
computational details.
neighboring defects (Cl vacancy or F-center Cl vacancy). This
result suggests that gas-phase CHCs, such as CH Cl , are
unlikely to exchange their H for a Cl atom from the LaCl3
lattice. However, the calculations also suggest that the surface
2
2
of LaCl can have both terminal lattice Cl and weakly held
adsorbed Cl species (Figure 3a). These species are calculated
Received: January 18, 2008
Published online: May 27, 2008
3
Keywords: density functional calculations ·
.
heterogeneous catalysis · lanthanum · metathesis ·
supported catalysts
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004 www.angewandte.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 5002 –5004