Methyl chloride production from methane over lanthanum-based catalysts

Article abstract of DOI:10.1021/ja066913w

The mechanism of selective production of methyl chloride by a reaction of methane, hydrogen chloride, and oxygen over lanthanum-based catalysts was studied. The results suggest that methane activation proceeds through oxidation-reduction reactions on the surface of catalysts with an irreducible metal-lanthanum, which is significantly different from known mechanisms for oxidative chlorination. Activity and spectroscopic measurements show that lanthanum oxychloride (LaOCl), lanthanum trichloride (LaCl₃), and lanthanum phases with an intermediate extent of chlorination are all active for this reaction. The catalyst is stable with no noticeable deactivation after three weeks of testing. Kinetic measurements suggest that methane activation proceeds on the surface of the catalyst. Flow and pulse experiments indicate that the presence of hydrogen chloride is not required for activity, and its role appears to be limited to maintaining the extent of catalyst chlorination. In contrast, the presence of gas-phase oxygen is essential for catalytic activity. Density-functional theory calculations suggest that oxygen can activate surface chlorine species by adsorbing dissociatively and forming OCI surface species, which can serve as an active site for methane activation. The proposed mechanism, thus, involves changing of the formal oxidation state of surface chlorine from -1 to +1 without any changes in the oxidation state of the underlying metal.

Full text of DOI:10.1021/ja066913w

Published on Web 02/13/2007
Methyl Chloride Production from Methane over
Lanthanum-Based Catalysts
Simon G. Podkolzin,† Eric E. Stangland,† Mark E. Jones,† Elvira Peringer,‡ and
Johannes A. Lercher*,‡
Contribution from The Dow Chemical Company, Core Research and DeVelopment, Midland,
Michigan 48674, and Department of Chemistry, Technische UniVersita¨t Mu¨nchen,
Lichtenbergstrasse 4, D-85747 Garching, Germany
Received September 26, 2006; E-mail: Johannes.Lercher@ch.tum.de
Abstract: The mechanism of selective production of methyl chloride by a reaction of methane, hydrogen
chloride, and oxygen over lanthanum-based catalysts was studied. The results suggest that methane
activation proceeds through oxidation-reduction reactions on the surface of catalysts with an irreducible
metal-lanthanum, which is significantly different from known mechanisms for oxidative chlorination. Activity
and spectroscopic measurements show that lanthanum oxychloride (LaOCl), lanthanum trichloride (LaCl3),
and lanthanum phases with an intermediate extent of chlorination are all active for this reaction. The catalyst
is stable with no noticeable deactivation after three weeks of testing. Kinetic measurements suggest that
methane activation proceeds on the surface of the catalyst. Flow and pulse experiments indicate that the
presence of hydrogen chloride is not required for activity, and its role appears to be limited to maintaining
the extent of catalyst chlorination. In contrast, the presence of gas-phase oxygen is essential for catalytic
activity. Density-functional theory calculations suggest that oxygen can activate surface chlorine species
by adsorbing dissociatively and forming OCl surface species, which can serve as an active site for methane
activation. The proposed mechanism, thus, involves changing of the formal oxidation state of surface chlorine
from -1 to +1 without any changes in the oxidation state of the underlying metal.
1. Introduction
based catalysts can similarly be used to selectively activate other
alkanes: for example, ethane can be directly converted to vinyl
chloride.7
Methane is not widely used as a feedstock by the chemical
industry because selective activation of the C-H bond presents
a formidable challenge.1-3 The need to utilize methane ef-
ficiently as an alternative chemical feedstock is becoming more
urgent due to diminishing proven reserves and increasing
consumption of crude oil. Here, we report on a new catalytic
chemistry for selective production of methyl chloride by a
reaction of methane, hydrogen chloride, and oxygen. Our results
suggest that methane activation proceeds through oxidation-
reduction reactions on the surface of a catalyst that has,
surprisingly, only an irreducible metal lanthanum. Methyl
chloride can be converted to olefins with known catalysts,4,5
and the hydrogen chloride formed during olefin production can
be recycled for reaction with methane. In this way, methane
can be converted to olefins via a methyl chloride intermediate
with no net consumption of hydrogen chloride.6 Lanthanum-
Natural gas is an abundant resource that can be used as fuel
or chemical feedstock, yet 30-60% of its reserves are classified
as stranded, meaning that such gas cannot be used locally and
cannot be efficiently transported to market due to a lack of
infrastructure.8,9 When stranded gas is produced in the course
of crude oil production, it is usually burned at the well or vented
as a means of disposal. In recent years, approximately 150
billion m3 of natural gas have been flared or vented worldwide
annually,10 which is equivalent to about 5% of the global annual
natural gas consumption11 or about 25% of the consumption in
the United States.10 The environmental aspect of this issue is
being addressed by the introduction of a voluntary standard on
flaring and venting reduction adopted in 2004 under the auspices
of the World Bank.12 Methane is a major component of natural
gas, and development of practical technologies for methane
utilization at remote locations is a critical component for the
success of these efforts.
† The Dow Chemical Company.
‡ Technische Universita¨t Mu¨nchen.
(1) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507.
(2) Periana, R. A.; Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. J. Science
2003, 301, 814.
(3) Lunsford, J. H. Catal. Today 2000, 63, 165.
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L. M. J. Catal. 1993, 143, 32.
(5) Taylor, C. E. Stud. Surf. Sci. Catal. 2000, 130D, 3633.
(6) Podkolzin, S. G.; Stangland, E. E.; Schweizer, A. E.; Jones, M. E.
International Patent Application WO 2006118935 A2, 2006.
(7) Jones, M. E.; Olken, M. M.; Hickman, D. A. U.S. Patent 6909024 B1,
2005.
(8) Thackeray, F.; Leckie, G. Pet. Econ. 2002, 69, 10.
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(10) Reducing the Gas Burning. World Bank Weekly Update, June 26, 2006.
(11) BP Statistical ReView of World Energy; British Petroleum Co.: London,
2006.
(12) A Voluntary Standard for Global Gas Flaring and Venting Reduction; The
World Bank: Washington, DC, 2004.
9
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J. AM. CHEM. SOC. 2007, 129, 2569-2576
2569

A R T I C L E S
Podkolzin et al.
Production of chemicals from methane is dominated by
technologies that require first the formation of a mixture of
carbon monoxide and hydrogen, which is usually referred to as
synthesis gas:
on synthesis gas production:
CH4 + HCl + 0.5O2 f CH3Cl + H2O
2CH3Cl f C2H4 + 2HCl
(5)
(6)
CH4 + 0.5O2 f CO + 2H2
CH4 + H2O f CO + 3H2
(1)
(2)
The overall reaction here is identical to the ethylene formation
via oxidative coupling (eq 3). Furthermore, propylene and higher
hydrocarbons can be produced from the CH3Cl intermediate with
reactions analogous to eq 6:4,5
Synthesis gas production is capital intensive, and the breaking
of C-H bonds in eqs 1 and 2 has to be substantially reversed
in subsequent steps to produce hydrocarbons. A promising
alternative technology is oxidative methane coupling, where C2
and higher products are produced directly from methane in the
presence of oxygen:
3CH3Cl f C3H6 + 3HCl
(7)
Oxidative chlorination chemistry is practiced commercially
in the production of vinyl chloride. Catalysts for this reaction
are based on a reducible metal, usually copper.17,18 The catalytic
cycle is believed to proceed through the reduction of CuCl2 to
CuCl on the surface of the catalyst by ethylene followed by
oxidation back to CuCl2 by HCl and O2.18 Multiple attempts19-23
to adapt this catalytic chemistry for methane conversion (eq 5)
had limited success because a reducible metal catalyzes the
formation of chlorine gas according to the Deacon reaction:
2CH4 + O2 f C2H4 + 2H2O
(3)
Methane coupling technology has not, however, been com-
mercialized because of low product yields. Reaction 3 occurs
partially on the catalytic surface and partially in the gas phase,
and the product selectivity rapidly declines as methane conver-
sion increases due to side reactions.3 New ways of activating
methane over a heterogeneous catalyst can be advantageous
compared to these and other known chemistries, because solid
catalysts are usually more economical than liquid-phase ones
and since reactions on the surface can be more selective than
radical transformations in the gas phase. However, even total
methane combustion over solid catalysts presents a major
scientific challenge.13,14
One of the methods for functionalizing methane is chlorina-
tion, which can proceed in the gas phase without a catalyst.
The selectivity to methyl chloride (CH3Cl) is usually low,
because it is a radical reaction where incorporation of each
successive chlorine makes the product more reactive, that is,
the rate of chlorination increases in the following order:
2HCl + 0.5O2 f H2O + Cl2
(8)
and methane chlorination proceeds, at least partially, in the gas
phase, yielding low selectivity. Furthermore, copper salts are
volatile and unstable at the temperatures required for methane
activation. Our group at The Dow Chemical Company has
discovered that this chemistry can be efficiently catalyzed by
irreducible and thermally stable lanthanum compounds. Spe-
cifically, lanthanum oxychloride (LaOCl) and derivative com-
pounds with a higher extent of chlorination were found to be
selective in catalyzing the conversion of methane to methyl
chloride6 and ethane to vinyl chloride,7 and also to be useful
for functionalizing higher alkanes.24 The fully chlorinated
form of this catalyst, lanthanum trichloride (LaCl3), is known
to be a promoter for various copper-based formulations,18,19,25,26
but it has never been considered as a catalyst by itself, because
it is generally accepted that the active catalyst site has to be
based on a reducible metal. We here present evidence that
catalysis can occur with a metal that does not change its
oxidation state (La+3) and propose a reaction mechanism that
involves a catalytic oxidation-reduction cycle with surface
chlorine species.
CH4 < CH3Cl < CH2Cl2 < CHCl3
(4)
It has been shown15,16 that this trend can be reversed, and
selectivities to methyl chloride close to 100% can be achieved
by using a catalyst that can generate a chlorine atom with a
partial positive charge (Clδ+). Since incorporation of Cl atoms
into methane makes the carbon progressively electropositive,
the electrophilic reaction with Clδ+ becomes progressively less
favorable. Efficient solid catalysts have been identified for this
chemistry,16 but only with chlorine gas as a reactant. It is
preferable to use HCl as a chlorine source, because it is usually
generated during the second step: the conversion of CH3Cl to
other chemicals. For example, a process where CH3Cl is
produced by oxidative chlorination (eq 5) and then utilized for
ethylene production (eq 6), and HCl formed in the second stage
(eq 6) is recycled to the first process stage (eq 5) has the
potential of being less capital intensive than a process based
2. Results and Discussion
2.1. Kinetic Measurements. Kinetic measurements were first
used to confirm that the oxidative chlorination of methane (eq
5) is actually catalyzed by LaOCl by comparing reaction rates
for two identical parallel reactors: one reactor was filled with
LaOCl and another one with quartz chips, which are considered
chemically inert but increase the contact surface area to make
a valid blank experiment. For all reported experimental condi-
(19) Pieters, W. J. M.; Conner, W. C.; Carlson, E. J. Appl. Catal. 1984, 11, 35.
(20) Ohtsuka, Y.; Tamai, Y. J. Catal. 1978, 51, 169.
(21) Noceti, R. P.; Taylor, C. E. Prepr.-Am. Chem. Soc., DiV. Pet. Chem. 1992,
37, 281.
(13) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65.
(14) Park, S.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265.
(15) Olah, G. A.; Gupta, B.; Farina, M.; Felberg, J. D.; Ip, W. M.; Husain, A.;
Karpeles, R.; Lammertsma, K.; Melhotra, A. K.; Trivedi, N. J. J. Am. Chem.
Soc. 1985, 107, 7097.
(22) Rozanov, V. N.; Gvozd, E. V.; Babich, N. F.; Trushechkina, M. A.; Treger,
Yu. A. Khim. Prom-st. (Moscow) 1989, 10, 726.
(16) Olah, G. A. Acc. Chem. Res. 1987, 20, 422.
(23) Vlasenko, V. M.; Chernobrivets, V. L. Zh. Prikl. Khim. 1998, 71, 1321.
(24) Schweizer, A. E.; Jones, M. E.; Hickman, D. A. U.S. Patent 6984763 B2,
2006.
(17) Olah, G. A.; Molna´r, AÄ . Hydrocarbon Chemistry, 2nd ed.; Wiley Inter-
science: Hoboken, NJ, 2003.
(18) Hodnett, B. K. Heterogeneous Catalytic Oxidation: Fundamental and
Technological Aspects of the SelectiVe and Total Oxidation of Organic
Compounds; Wiley & Sons: Chichester, U.K., 2000.
(25) Garcia, C. L.; Resasco, D. E. J. Catal. 1990, 122, 151.
(26) Taylor, C. E.; Noceti, R. P.; Schehl, R. R. Stud. Surf. Sci. Catal. 1988, 36,
483.
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2570 J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007

Methyl Chloride Production from Methane
A R T I C L E S
Figure 2. Effect of feed components on the reaction rate over LaCl3 at
4.8 kPa CH4, 2.4 kPa O2, and 24 kPa HCl with balance He with a total
flow rate of 14 sccm at 675 K. The reaction stops without gas-phase O2 (2
and 5) but can be sustained without HCl in the feed by Cl diffusion from
the bulk onto the catalyst surface (4).
Figure 1. Comparison of reaction rates over LaCl3 for Cl incorporation
into methane and for Cl evolution as Cl2 without methane in the feed.
tions, the reaction rate without the catalyst did not exceed 5%,
and it was usually below 1%, of the reaction rate with the
catalyst. The irreducibility of lanthanum (the fact that it does
not change its nominal +3 oxidation state) under evaluated
reaction conditions was tested with X-ray photoelectron spec-
troscopic (XPS) measurements. For these measurements, catalyst
samples were transferred into a quartz cell under a vacuum of
10-6 Pa, and the positions of the La3d peaks were determined.
The spectra were similar to those reported for pure and
chlorinated La2O3.27,28 The positions of the La3d 5/2 doublet
at 836.2 and 840.4 eV and the 3/2 doublet at 853.1 and 857.2
eV were practically constant in all tests.
We then considered if the reaction may proceed through
generation of gas-phase chlorine (Deacon reaction, eq 8). To
test this, the rates of chlorine incorporation into methane and
chloromethanes (mostly eq 5) at various conditions were
compared to those of chlorine evolution as Cl2 when there was
no methane in the feed (eq 8). No Cl2 formation was detected
in experiments with methane. A comparison of the reaction rates
with and without methane in the feed is provided in Figure 1
as a function of O2 and CH4 partial pressures at 675 K. Although
the rate of Cl incorporation into C1 products at the highest tested
oxygen partial pressure of 39 kPa is comparable to the rate of
Cl evolution as Cl2 in the absence of methane (Figure 1a), the
data in Figure 1 show that there is no correlation between the
rates of the two reactions. For example, without methane, the
rate of Cl2 formation was below the detection limit at O2
pressures below 4.8 kPa, but considerable quantities of CH3Cl
were produced at the same conditions when methane was present
(Figure 1b). The same kinetic experiments were repeated at 750
K,29 and, similarly, there was no correlation between the rates
Figure 3. In situ Raman spectra of LaOCl as a function of exposure to
HCl flow at 725 K: (a) theoretical vibrational frequencies calculated with
the LaOCl model in Figure 5, (b) LaOCl spectrum prior to exposure to
HCl flow, (c) after exposure to HCl flow for 2 min, (d) 4 min, (e) 46 min,
and (f) 4 h. The spectrum (f) is essentially that of LaCl3. DFT calculations
suggest that the bands at 179 and 206 cm-1 are associated with motion of
Cl atoms, and the bands at 330 and 432 cm-1 with motion of O atoms.
of the two reactions, suggesting that methane reacts with
chlorine on the surface of the catalyst.
The reaction mechanism was further examined by considering
the importance of the feed components. When O2 was eliminated
from the feed mixture, the reaction rate measured over LaCl3
abruptly declined practically to zero, as shown in Figure 2
(segment 2). When O2 flow was restored, the reaction rate
immediately returned to its original value (segment 3), showing
that the presence of gas-phase O2 is essential for the chemistry.
When HCl was eliminated from the feed (segment 4), the
reaction rate gradually declined and then stabilized at a lower
value. This result shows that chlorine can be supplied by the
catalyst surface. When the surface is not regenerated by HCl, a
transformation of LaCl3 into LaOCl begins, as evidenced by in
situ Raman measurements (not shown). This transformation
proceeds by diffusion of chlorine from the bulk onto the surface,
and it is the reverse process of LaOCl chlorination, for which
Raman and pulse reaction measurements are presented in
Figures 3 and 4. Thus, in the absence of HCl, the reaction rate
is limited by the chlorine bulk diffusion, and it can be sustained
(27) Gruenert, W.; Sauerlandt, U.; Schloegl, R.; Karge, H. G. J. Phys. Chem.
1993, 97, 1413.
(28) Weckhuysen, B. M.; Rosynek, M. P.; Lunsford, J. H. Phys. Chem. Chem.
Phys. 1999, 1, 3157.
(29) Peringer, E.; Podkolzin, S. G.; Jones, M. E.; Olindo, R.; Lercher, J. A.
Topics Catal. 2006, 38, 211.
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A R T I C L E S
Podkolzin et al.
Figure 5. Proposed reaction mechanism for oxidative chlorination of
methane over LaOCl catalyst based on DFT calculations: (a) gas-phase
oxygen above LaOCl catalyst, (b) dissociative oxygen adsorption with the
formation of OCl surface species, (c) reaction of gas-phase methane with
surface OCl species, (d) surface OH and gas-phase CH3Cl. Numbers next
to element symbols show partial atom charges calculated with the Hirshfeld
method.
Figure 4. Mass spectrometer traces for mass 50 (CH3Cl) and mass 18
(H2O) collected with 30-s reaction pulses: (1) CH4 + O2 pulse over LaOCl
injected after collection of the spectrum in Figure 3b, (2) CH4 + O2 + HCl
pulse over LaOCl injected after completion of the first pulse, (3) CH4 +
O2 + HCl pulse injected after collection of the spectrum in Figure 3c
collected over LaOCl exposed to HCl flow for 2 min. Results of consecutive
pulses collected after additional exposure to HCl were similar to (3). The
relative scale of the mass 50 traces is increased 10 times for clarity.
For example, a nuclear magnetic resonance (NMR) study on
the exchange and diffusion of F and OH anions in LaF3,
LaF2(OH), LaF(OH)2, and La(OH)3 was published in 1980.30
Just recently, a reaction of CCl4 decomposition to CO2 over
La2O3 and LaOCl was reported.31,32 This reaction can be used
to convert chlorohydrocarbon impurities from waste streams into
carbon oxides, and the chemistry can be run catalytically, in a
pulse or continuous mode, by dechlorinating the catalyst surface
with steam and forming HCl as another product. This chemistry
can be viewed as the reverse of methyl chloride formation
reported here in the sense that the surface and, if not regenerated,
the whole bulk of the catalyst can exchange O for Cl anions,
resulting in the transformation of La2O3 to LaOCl and eventually
to LaCl3. This year, the same concept of anion exchange and
diffusion was reported for a pulse reaction of H2S over La2O3.33
In this case, the catalyst exchanges O for S anions, and it can
later be regenerated with steam.
Our preliminary results show that methane chlorination can
be effectively run not only over LaOCl and LaCl3 but also over
similar compounds of other lanthanide metals.6 An interesting
case is Ce, which, unlike La, can change its formal oxidation
state from +3 to +4. For example, XPS spectra reported for
chlorination with CCl4 show a transformation of CeO2 into
CeCl3.28 Our preliminary comparison between La- and Ce-based
catalysts, conducted similarly to the experiments in Figure 1,
indicates that the rates of Cl evolution as Cl2 without CH4 are
significantly higher as a fraction of the rates for Cl incorporation
into C1 products with the full feed over chlorinated CeO2. This
suggests that, in contrast to La, the Deacon reaction (eq 8) may
play a significant role for Ce-based catalysts.
for a considerable period of time. Without gas-phase O2
(segment 5 in Figure 2), the rate again became negligibly small.
Finally, when O2 and HCl flows were restored (segment 6),
the reaction rate rapidly reached its original value. The reaction
was run for three weeks without noticeable catalyst deactivation.
The kinetic experiments of Figure 2 were repeated with another
LaOCl sample that had a lower surface area due to a preparation
with an organic solvent, and the same qualitative results for
the effect of O2 and HCl were reproduced.29
The ability of the catalyst to sustain methane chlorination in
the absence of HCl for an extended period of time (segment 4
in Figure 2) demonstrates that the lanthanum structure can serve
as a chlorine reservoir capable of an efficient exchange of lattice
chlorine for another anion, in this case oxygen. Notably, as
methane conversion decreased in the absence of HCl under the
tested conditions, only methyl chloride was detected as a product
and, furthermore, the selectivity to methyl chloride remained
at 100% as the catalyst continued to dechlorinate. This catalytic
property of sustaining the reaction at high selectivity without
HCl can be utilized by operating the system in a pulse or swing
mode, that is by exposing the catalyst to alternating separate
feed streams of first CH4 plus O2 and then HCl. Such a mode
has a potential advantage of reducing product separation costs.6
Whether the role of HCl is limited only to rechlorination of the
catalytic surface and whether the product formation is affected
by the separation of the feed components can be further studied
by comparing selectivities to methyl chloride with and without
HCl at similar methane conversion levels. For this comparison,
additional data will have to be collected similarly to the results
in Figure 6 but only without HCl in the feed.
(30) Buznik, V. M.; Komissarova, L. N.; Moskvich, Yu. N.; Pushkina, G. Ya.
Zh. Neorg. Khim. 1980, 25, 1488.
(31) Van der Avert, P.; Weckhuysen, B. M. Angew. Chem., Int. Ed. 2002, 41,
4730.
(32) Van der Avert, P.; Podkolzin, S. G.; Manoilova, O.; De Winne, H.;
Weckhuysen, B. M. Chem.-Eur. J. 2004, 10, 1637.
(33) Flytzani-Stephanopoulos, M.; Sakbodin, M.; Wang, Z. Science 2006, 312,
1508.
The ability of La structures to serve as anion reservoirs with
efficient bulk diffusion has been known for more than 20 years.
9
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Methyl Chloride Production from Methane
A R T I C L E S
2.2. Raman Spectroscopy and Reaction Pulse Measure-
ments. The nature of the catalytic phase and details of catalyst
transformation from LaOCl to LaCl3 were examined with in
situ Raman measurements and reaction pulses. Results of a
typical run at 725 K are illustrated in Figures 3 and 4. In this
run, a Raman spectrum of LaOCl sample was first collected
under He flow. This experimental spectrum (Figure 3b) is
compared to the theoretical vibrational frequencies (Figure 3a),
which were obtained with density-functional theory (DFT)
calculations using the LaOCl model in Figure 5. The calculated
frequencies are shown in Figure 3a as lines of arbitrary height
because the DFT calculations allowed us to estimate the
frequencies but not the intensities of the vibrational modes. The
experimental bands in Figure 3b are in agreement with the
theoretical frequencies in Figure 3a and previously reported
experimental results for LaOCl.28,32 The calculated frequency
at 126 cm-1 in Figure 3a, previously reported based on
experimental results,28,32 was outside of our current experimental
data collection range. The DFT calculations suggest that the
experimental bands at 179 and 206 cm-1 in Figure 3b are
associated with motion of Cl atoms in LaOCl, and the bands at
330 and 432 cm-1 with motion of O atoms.
ratio became essentially 1:3, indicating the completion of the
transformation to LaCl3.
After the second pulse in Figure 4.2, the catalyst was exposed
to a flow of HCl for 2 min, which was equivalent to 0.2 mol of
HCl per mol of catalyst, and then the Raman spectrum in Figure
3c was collected. This spectrum shows that the intensity of the
bands associated with Cl motion increased and those associated
with O motion decreased, confirming a higher extent of catalyst
chlorination after the reaction with HCl. Following the spectrum
acquisition, the catalyst was again probed with another pulse
of CH4, O2, and HCl (Figure 4.3). The sequence of exposing
the catalyst to HCl flow for 2 min, then acquiring a spectrum
and interrogating with a reaction pulse was repeated multiple
times. Selected spectra from this sequence, acquired after 4 min,
46 min, and 4 h of cumulative exposure to HCl flow, are
presented in Figures 3d-f. These spectra show a trend of gradual
catalyst chlorination. The last shown spectrum collected after
4 h of HCl exposure (Figure 3f) with two dominant bands at
176 and 205 cm-1 is consistent with those reported for
LaCl3.28,32 Thus, the experimental spectral series in Figure 3
represent a gradual transformation of LaOCl to LaCl3. The
results of subsequent pulse measurements in the series were all
similar to the traces shown in Figure 4.3. CH3Cl formation was
detected during the reaction pulse in quantities comparable to
those for the initial LaOCl, and H2O formation was dominated
by the catalyst chlorination. Significant amounts of H2O were
evolving from the catalyst even after the HCl flow was stopped
and the catalyst was kept under He flow during spectra
acquisition and prior to the reaction pulse introduction, as can
be seen in the segment from 0 to 50 s in Figure 4.3. During the
pulse, the concentration of H2O in the outlet actually decreased,
which can be attributed to suppression of the continuous and
relatively slow catalyst chlorination and subsequent dehydration
by methane activation. After the completion of the reaction
pulse, the segment after 80 s in Figure 4.3, the H2O evolution
from the catalyst resumed. The delay and relatively slow rate
of H2O evolution from the catalyst after exposure to HCl flow
is similar to the trailing of the H2O trace after the completion
of the very first pulse with HCl in Figure 4.2. Importantly,
however, the Raman spectra in Figure 3 and the results of pulse
measurements in Figure 4 suggest that LaOCl, LaCl3, and La
phases with an intermediate extent of chlorination are all
catalytically active and that the only essential element for
methane activation is the presence of chlorine surface species
that can be activated by gas-phase oxygen.
After collection of the initial spectrum, the catalyst was
probed with a short pulse of CH4 and O2 (15.0 and 7.5 mol %,
respectively, in He) to minimize changes caused by any reactions
to the surface composition. The total amount of CH4 in the pulse
was equivalent to 0.02 mol per mol of catalyst. The mass
spectrometer traces for H2O and CH3Cl collected during the
pulse interrogation are presented in Figure 4.1. The concentration
of these products was negligible prior to the injection of reactants
(0-50 s). Then, during the reaction pulse, between 50 and 80
s, CH3Cl (red trace in Figure 4.1) and H2O (blue trace) were
formed. This result suggests that Cl species on the surface of
LaOCl can react with CH4 in the presence of gas-phase O2.
This reaction, proceeding through surface Cl species, is similar
to the one observed with CH4 and O2 flow over LaCl3 (segment
4 in Figure 2).
After the first pulse, the catalyst was probed with a pulse of
CH4, O2, and HCl (15.0, 7.5, and 15.0 mol %, respectively, in
He) in the stoichiometric ratios based on eq 5, which again
produced CH3Cl and H2O (Figure 4.2). The total amount of
CH4 introduced in the second pulse was the same as in the first
one. Similarly to the first pulse, the conversion of CH4 and O2
was low (below 5 mol %), and the amounts of CH3Cl produced,
based on the trace curve areas, were comparable in the two
pulses. The difference in the second pulse was the complete
consumption of HCl, which resulted in a different H2O evo-
lution trace: water was slowly evolving long after the comple-
tion of the reaction pulse. This behavior can be attributed to
catalyst chlorination (eq 9) based on the following additional
experiments.
2.3. Density-Functional Theory Calculations. A possible
reaction mechanism was evaluated with electronic-structure
calculations using DFT, and the results are summarized in Figure
5. Figure 5a shows a gas-phase O2 molecule above a model
LaOCl surface terminated with Cl atoms. Calculations suggest
that molecular oxygen can react with the surface by splitting
and adsorbing dissociatively with the formation of OCl- surface
species (Figure 5b, eq 10). The formal oxidation state of the
activated surface Cl atom in this step changes from -1 to +1,
and this transformation is reflected in the change of the
calculated partial atomic charge on the Cl atom from 0.17- to
0.12+, as shown in, correspondingly, Figures 5a and 5b. Since
our attempts to characterize these OCl- species with static
adsorption, temperature-programmed desorption (TPD), and
infrared and Raman spectroscopic measurements were not
LaOCl + 2HCl f LaCl3 + H2O
(9)
In a separate run, a sample of LaOCl was exposed to a flow
of HCl and analyzed ex situ for the La-to-Cl ratio by neutron
activation. A complete HCl conversion was observed up until
the La-to-Cl mol ratio reached a value of about 1:2, and only
during the subsequent chlorination was HCl detected in the outlet
stream. The HCl consumption stopped when the La-to-Cl mol
9
J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007 2573

A R T I C L E S
Podkolzin et al.
90% or higher, mostly 98% or higher, with the rest of the
product being CH2Cl2. Additional selectivity measurements were
performed at methane conversion levels up to 34% with the
HCl-to-O2 feed ratio of 2:1 (based on the stoichiometry of eq
5). At those conditions, CO2 formation was observed for
methane conversions above 1 mol %, and CO formation was
observed for methane conversions above 10 mol %. The main
product was always CH3Cl followed by CH2Cl2; small amounts
of CHCl3 were observed, and CCl4 was never detected. The
absence of CO or CO2 at low methane conversion suggests that
these products are formed not directly from methane but through
decomposition of chloromethanes. This supposition was tested
with temperature-programmed reaction (TPR) measurements
over a prechlorinated catalyst.29 In the first TPR experiment
with CH4 and O2 as reactants, the formation of CH3Cl was
observed at about 600 K, while the formation of carbon oxides
was not detected until the temperature reached about 675 K. In
the second TPR experiment with CH2Cl2 and O2 as reactants,
carbon oxides were observed already at 625 K, and no other
products were detected except for trace amounts of CHCl3.
These TPR results are consistent with methane first forming
CH3Cl, and then CH3Cl either chlorinating further or decompos-
ing to carbon oxides:
Figure 6. Dependence of selectivity to CH3Cl on CH4 conversion. The
data were collected by varying the space velocity and feed composition at
800 K and also by varying the temperature from 680 to 825 K at a constant
space velocity with LaOCl catalysts prepared with different bases (Table
1): (0) NH4OH in water under Ar, (×) NH4OH in ethanol, (]) TEAOH,
(+) TPAOH, (O) TBAOH.
successful, we conclude that these species are transient (i.e.,
oxygen readily desorbs into the gas phase or displaces chlorine
and becomes a part of the lattice). Additional calculations
suggest that methane can react with OCl- by exchanging a
proton for Cl+ (Figure 5c) and by forming a hydroxyl group
on the surface (Figure 5d, eq 11). Although the intermediate
structure in Figure 5c was obtained by linear geometric
interpolation of the configurations in Figures 5b and 5d and
was not confirmed as a transition state, it shows a possible
mechanism of methane activation by surface OCl- species. The
catalytic site in Figure 5d can be regenerated by the reaction
between the hydroxyl group and gas-phase HCl with the
formation of gas-phase water and restoration of the Cl-
terminated surface of Figure 5a (eq 12).
kCH
kCH
3
CH4 48 CH3Cl
Cl8 CH2Cl2 + CO + CO2 (13)
Published studies on the relative activity of chloromethanes
in decomposition over La-based catalysts indicate that the
reactivity increases with the extent of hydrocarbon chlorina-
tion.34 Therefore, if a significant fraction of CH2Cl2 decomposes
to carbon oxides under our oxidative chlorination conditions,
any formed CHCl3 can be expected to produce exclusively
carbon oxides, so that CCl4 would never form or would exist
only as a transient intermediate. The active site for the formation
of carbon oxides in oxidative chlorination is likely to be a
terminal lattice oxygen atom, the active site proposed for
decomposition of chloromethanes over dechlorinated surfaces
of La-based catalysts.32,35 Our reported DFT model for the
LaOCl terminal lattice oxygen site was generated by substituting
two surface Cl atoms in the model in Figure 5a for one O
atom.35,36 Our similarly constructed partially dechlorinated
surface of LaCl3 was also computationally shown to be active
in decomposition of CCl4.35 The absence of carbon oxides in
our kinetic experiments with a large HCl excess (Figure 1) is
consistent with this proposed mechanism where decomposition
of chloromethanes requires at least partial surface dechlorination
to form active oxygen sites.
0.5O2 (gas) + Cl- (surface) f OCl- (surface) (10)
CH4 (gas) + OCl- (surface) f CH3Cl (gas) +
OH- (surface) (11)
HCl (gas) + OH- (surface) f H2O (gas) + Cl- (surface)
(12)
The partial atomic charge on surface La atoms for all the
structures in Figure 5 changes only slightly: within 0.53+ and
0.59+. The proposed catalytic cycle, thus, involves oxidation
of surface chlorine without any changes in the oxidation state
of the underlying metal, which is significantly different from
known mechanisms for oxidative chlorination.
2.4. Methyl Chloride Selectivity Measurements and Mod-
eling. Since a reaction between methane and electrophilic
chlorine surface species (Clδ+ in Figure 5c) should provide an
opportunity for achieving high selectivities to methyl chloride,
we attempted to compare reaction selectivities over La-based
catalysts and those reported for radical gas-phase chlorination.
In kinetics experiments in Figure 1, methane conversion was
kept below 10 mol % by adjusting the flow rate, so that a
differential reactor model could be used in the data analysis. In
addition, the catalyst was kept in the chlorinated LaCl3 form
by supplying a large HCl excess, with the state of the catalyst
being confirmed by in situ Raman spectroscopy measurements
and by control reaction rate measurements at even larger HCl
concentrations. At these conditions, the selectivity to CH3Cl was
The analysis of eq 13 can be simplified by assuming that the
main product of CH3Cl is CH2Cl2 and, correspondingly, that
only minor quantities of carbon oxides are produced directly
from CH3Cl. If we further assume that the reaction rates for
CH4 and CH3Cl are first-order in the concentrations of the
respective hydrocarbon and the source of chlorine, the depen-
dence of selectivity to CH3Cl (S) on CH4 conversion (C) for
this consecutive reaction can be derived as a closed-form
(34) Van der Avert, P.; Weckhuysen, B. M. Phys. Chem. Chem. Phys. 2004, 6,
5256.
(35) Podkolzin, S. G.; Manoilova, O. V.; Weckhuysen, B. M. J. Phys. Chem. B
2005, 109, 11634.
(36) Manoilova, O. V.; Podkolzin, S. G.; Tope, B.; Lercher, J.; Stangland, E.
E.; Goupil, J.-M.; Weckhuysen, B. M. J. Phys. Chem. B 2004, 108, 15770.
9
2574 J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007

Methyl Chloride Production from Methane
A R T I C L E S
Table 1. Properties of Catalysts Prepared with Different Bases
solution that depends only on the ratio of rate constants kCH Cl
/
3
surface area of
initial LaOCl,
activitya of chlorinated
phase at 800 K,
k
CH . Modeling our tubular reactor as an ideal plug flow reactor
4
base used in LaOCl
preparation
gives the following dependence:
1
1
m2 g-
mol CH4 (g cat)-1 s-
kCH
Cl
4
1.5 × 10-7
6.8 × 10-7
9.4 × 10-7
1.1 × 10-6
6.5 × 10-7
3
NH4OH in water under Ar
NH4OH in ethanol
TEAOH
TPAOH
TBAOH
39
21
125
118
78
1 - C/100 - (1 - C/100) k
CH
S )
100
(14)
k
CH3Cl
C/100
- 1
(
)
k
CH4
a Activity measured with feed CH4, HCl, O2 at, respectively, 20, 20, 10
mol % in He.
where S ) [(CH3Cl)out]/[(CH4)in - (CH4)out]100, mol % and C
) [(CH4)in - (CH4)out]/[(CH4)in]100, mol %.
at similar conversion levels would be impractical due to process
stability and the requirement of a separate stage for HCl
conversion to Cl2. Moreover, La-based catalysts exhibit stability
and complete conversion of reacting chlorine species into
chloromethanes, in contrast with known reducible-metal cata-
lysts, which are volatile and, therefore, unstable and which
usually produce Cl2 in addition to chloromethanes. The new
catalytic chemistry, thus, presents a promising opportunity for
functionalizing alkanes and, specifically, for selective production
of methyl chloride from methane.
This expression was used to fit the selectivity versus
conversion data in Figure 6. The measurements in Figure 6 were
collected with multiple catalysts by varying the space velocity
and feed composition at 800 K and also by varying the
temperature from 680 to 825 K at a constant space velocity.
Catalyst samples for this study were prepared by using different
bases in the precipitation of LaOCl, resulting in materials with
BET surface areas between 39 and 125 m2 g-1 based on N2
adsorption, as summarized in Table 1. All catalysts were
converted to LaCl3 by exposure to HCl prior to activity
measurements. The surface area after chlorination could not be
easily measured due to hygroscopicity of LaCl3, but it was
estimated to be on the order of 5-20 m2 g-1. Despite the
dramatic reduction in the surface area on the transformation from
LaOCl to LaCl3, the differences in activity for the catalysts in
Table 1 suggest that the morphology of the initial LaOCl
material can still influence the reactivity of the chlorinated phase.
The similarity of the trend for selectivity versus conversion for
catalysts with various activities in Figure 6 and the accuracy of
the fit based on eq 14 suggest that the reaction mechanism is
the same for all the materials and that this mechanism is
consistent with eq 13. The fit in Figure 6 was obtained with
the ratio of rate constants kCH Cl/kCH of 3.9. Our analysis of
3. Methods
LaOCl was prepared by reacting LaCl3‚7H2O (Aldrich, 99.999%)
with an aqueous solution of ammonium hydroxide (Aldrich, ACS
reagent) under argon (BOC, 99.999%) atmosphere. The resulting
precipitate was washed with water, dried, and then calcined under
synthetic air at 823 K for 8 h. For selectivity measurements in Figure
6, additional LaOCl catalysts were prepared with ammonium hydroxide
in ethanol under ambient conditions and with three organic bases (Table
1): tetraethyl ammonium hydroxide (TEAOH, Merck, 20% aqueous
solution), tetrapropyl ammonium hydroxide (TPAOH, Merck, 40%
aqueous solution), and tetrabutyl ammonium hydroxide (TBAOH,
Aldrich, 20% aqueous solution). For these additional catalysts, the
precipitate was centrifuged, washed with ethanol, freeze-dried, and
finally calcined. The LaOCl composition was confirmed with X-ray
diffraction and with elemental analysis by neutron activation.
3
4
literature data37-39 on the kinetics of radical gas-phase chlorina-
tion of CH4 and CH3Cl suggests that the ratio of rate constants
Catalytic activity was tested in a tubular fixed-bed nickel-alloy
reactor with a length-to-diameter ratio of 28.6. The catalyst was sized
to ensure greater than 10 particles per reactor diameter. The reactor
outlet pressure was atmospheric. The effluent was analyzed with gas
chromatography, which allowed closure of carbon, oxygen, and chlorine
mass balances. The detection limit for feed and product components
was 0.01 mol %. Prior to kinetic measurements with CH4 (BOC,
99.999%), O2 (BOC, 99.6%), HCl (BOC, 99.99%), and N2 (BOC,
99.999%), the catalyst was chlorinated in situ with 5 mol % HCl in
He (BOC, 99.999%) at the total weight hourly space velocity (WHSV)
of 0.53 h-1 at 675 K for 24 h. The complete transformation to LaCl3
was confirmed with Raman spectroscopy, X-ray diffraction, and
elemental analysis. For kinetic measurements in Figure 1, the order of
experiments was randomized, and measurements at each of the reported
conditions were collected at least twice. Selectivity measurements in
Figure 6 were performed at 800 K for the catalyst prepared with
NH4OH in water under Ar using the following CH4, HCl, O2, and N2
inlet concentrations in mol %: 76.0, 16.0, 8.0, 0; 88.0, 8.0, 4.0, 0;
94.0, 4.0, 2.0, 0; 94.0, 4.0, 2.0, 0; 80.0, 16.0, 4.0, 0; and 36.0, 36.0,
18.0, 10.0 at WHSV of 0.12-7.38 h-1. Selectivity measurements for
other catalysts were collected by first pretreating in He at 823 K for 1
h, chlorinating in situ with 20% mol HCl in He at total WHSV of 1.89
h-1 at 675 K for 10-12 h, and then running the reaction by increasing
the temperature from 680 to 825 K using a constant feed composition
of CH4, HCl, O2, N2, and He in mol %: 20.0, 20.0, 10.0, 10.0, 40.0 at
WHSV of 0.92 h-1. The catalyst activity throughout the text is quoted
based on the weight of LaOCl initially loaded for testing.
k
CH Cl/kCH for the consecutive reaction with Cl2 at 250-450 K
3
4
is 3.2-4.2. Since the value of 3.9 for the oxidative chlorination
falls within this range, it appears that catalysts based on pure
lanthanum compounds do not provide a selectivity advantage
compared to gas-phase chlorination. We note that this conclusion
does not contradict our proposed mechanism for methane
activation through surface Clδ+ species because methyl chloride
can potentially react through an additional pathway. Specifically,
while methane has to be activated to be present on the surface
and to react with chlorine, methyl chloride, in contrast, should
be coordinated, even if very weakly, with the surface on
formation and, therefore, can potentially react further through
an additional pathway. We further note that, unlike gas-phase
radical chemistry, properties of active sites on the surface of
La-based catalysts can be modified, for example with promoters,
to tune the chemistry by changing the site geometry, its charge
distribution, and propensity for dechlorination to achieve better
selectivities. Furthermore, the chemistry over La-based catalysts
can be operated at low methane conversions with the selectivity
approaching 100%, whereas operation of gas-phase chlorination
(37) McBee, E. T.; Hass, H. B.; Neher, C. M.; Strickland, H. J. Ind. Eng. Chem.
(Washington, D. C.) 1942, 34, 296.
(38) Clyne, M. A. A.; Walker, R. F. J. Chem. Soc., Faraday Trans. 1 1973, 69,
1547.
(39) Keyser, L. F. J. Chem. Phys. 1978, 69, 214.
9
J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007 2575

A R T I C L E S
Podkolzin et al.
Pulse and spectroscopic Raman measurements were performed with
a reactor system that had two feed lines, each equipped with a separate
set of mass flow controllers. The two feed lines were connected with
a multiport switch valve, which allowed the feed composition to be
changed in a pulse mode. The temperature of the catalyst was initially
increased at a ramp rate of 10 K min-1 to 950 K under He flow, held
at 950 K for 2 h, lowered to 725 K, and then maintained at this value
for all measurements. The He flow between reaction pulses and
exposure to HCl flow was maintained at WHSV of 0.31 h-1. The
exposure to HCl in flow mode was performed in 2-min increments at
the concentration of 20 mol % in He at total WHSV of 1.35 h-1. In
situ Raman spectra were collected with a Kaiser Optical Systems, Inc.,
Holoprobe 532/633 spectrometer equipped with a frequency-doubled
Nd/YAG 532-nm laser, grating, and EEV CCD detector chip with 278
× 1024 pixels. The exposure time and number of accumulations were
optimized for an 80% CCD detector pixel fill. The laser power output
was about 35 mW.
At methane conversions below 1-2 mol %, the catalyst was
100% selective toward CH3Cl. At higher conversions, CH3Cl
was always the main product but CH2Cl2, CO, and CO2 were
also observed. Only trace amounts of CHCl3 were detected, and
CCl4 was never detected. The dependence of CO and CO2
formation rates on methane conversion and the results of TPR
experiments, which compared the stability of CH4 and CH2Cl2
at the reaction conditions, suggest that carbon oxides are formed
through decomposition of chloromethanes. The dependence of
selectivity to CH3Cl as a function of CH4 conversion for La-
based catalysts appears to be in line with that expected for gas-
phase chlorination based on kinetic modeling of relative reaction
rates for CH4 and CH3Cl chlorination (Figure 6).
Kinetic measurements were performed for the reactions of
(1) Cl incorporation into C1 hydrocarbons when CH4, O2, and
HCl were fed and (2) Cl evolution as Cl2 when O2 and HCl
were fed (Figure 1). Formation of Cl2 was not detected in the
presence of CH4. When CH4 was not present in the feed, the
rates of Cl2 formation were usually significantly lower than those
for CH4 chlorination under similar conditions. In addition,
apparent reaction orders in reactants were different for the two
reactions, suggesting that they are not correlated and that CH4
activation proceeds through a surface reaction.
Oxygen adsorption measurements were performed with a Micromer-
itics 2010 apparatus. Infrared spectroscopic and TPD measurements
for oxygen adsorption were conducted similarly to characterization of
CO2 on the surface of LaOCl.36 X-ray photoelectron spectra were
collected with a Kratos AXIS instrument using monochromatic Al KR
radiation with an anode current of 15 mA and 14 keV and an aperture
slot of 3 × 10 mm. The pass energy was set at 80 eV for the survey
and 40 eV for the analyzer scans. The analysis area for 16-84% signal
level was 0.4 × 0.2 mm. Binding energies were referenced to the C1s
peak of adventitious carbon, which was set to 285.0 eV.
Flow and pulse experiments using CH4 and O2 as feed show
that CH3Cl can be formed using chlorine from the catalyst
surface (Figures 2 and 4). The presence of HCl, thus, is not
required for activity, and its role appears to be limited to
maintaining the extent of catalyst chlorination. In contrast, the
presence of gas-phase O2 is essential for catalytic activity (Figure
2). Based on this evidence, methane chlorination is proposed
to proceed through the surface chlorine species that can be
activated by oxygen.
DFT calculations suggest that O2 can activate surface Cl-
species by dissociatively adsorbing and forming OCl- species,
with the formal oxidation state of Cl changing from -1 to +1
(Figure 5). The formation of OCl- species was not detected
using static adsorption, TPD, and infrared and Raman spectro-
scopic measurements, and therefore, these species are assumed
to be transient. DFT calculations suggest that these OCl- species
can serve as an active site for methane activation. The proposed
catalytic cycle, thus, involves oxidation of a surface chlorine
without any changes in the oxidation state of the underlying
metal (eqs 10-12), which is significantly different from known
mechanisms of oxidative chlorination.
The DMol3 code in Materials Studio 2.1 software by Accelrys was
used to perform gradient-corrected spin-polarized periodic DFT calcula-
tions with the same model and settings as those utilized in the study of
LaOCl surface acidity and basicity and relative activity of CCl4
decomposition over La phase.35,36 An infinite LaOCl(001) stoichiometric
model was constructed using a 2 × 2 unit cell with 12 layers (48 atoms
total) with a vacuum spacing of 1.5 nm. Only half of the surface layers
are shown in Figure 5 for clarity. The bottom eight layers were
constrained during the calculations to simulate bulk properties, and the
remaining top layers were optimized with an adsorbate. The calculations
used the double numerical with polarization (DNP) basis set and the
generalized gradient-corrected Perdew-Wang (GGA PW91) functional.
Tightly bound core electrons for La were represented with semi-
core pseudopotentials. Reciprocal-space integration over the Bril-
louin zone was approximated through k-point sampling with a separation
of 0.5 nm-1 using the 2 × 2 × 1 Monkhorst-Pack grid. An orbital
cutoff distance of 0.5 nm was used for all atoms. All adsorption and
surface reaction energies were calculated at 0 K without zero-energy
corrections.
4. Conclusions
Activity and spectroscopic measurements demonstrate that
La-based catalysts (LaOCl, LaCl3, and La phases with an
intermediate extent of chlorination) are active for methane
conversion to methyl chloride in the presence of hydrogen
chloride and oxygen (Figures 2-4). These La-based catalysts
appear to be very stable: no deactivation was detected after
three weeks of testing. Although LaCl3 was known as a catalyst
promoter for hydrocarbon chlorination, it was never considered
as a catalyst by itself because it was believed that a reducible
metal was required for a catalytic site. The irreducibility of La
(the fact that it does not change its nominal +3 oxidation state)
under evaluated reaction conditions was tested with XPS
measurements.
Acknowledgment. This work would not have been possible
without the support of The Dow Chemical Company and many
Dow colleagues. In particular, we thank D. M. Millar for
anaerobic LaOCl preparation, M. A. Leugers and J. J. Maj for
spectroscopic measurements, M. M. Olken for adsorption
measurements, and S. E. Beyer for both spectroscopic measure-
ments and GC experimental expertise. The work at the Tech-
nische Universita¨t Mu¨nchen was partially funded by The Dow
Chemical Company.
JA066913W
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2576 J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007