The selective high-yield conversion of methane using iodine-catalyzed methane bromination

Article abstract of DOI:10.1021/cs300775m

Methyl bromide is used as feed in a process that converts it to gasoline. It is prepared by the gas-phase reaction of CH₄ with Br₂, a reaction that produces, besides the desired CH₃Br, large amounts of CH₂Br₂. The latter cokes the catalyst used for gasoline production. The separation of CH₂Br₂ by distillation makes gasoline production too expensive. It is therefore important to increase the selectivity of the bromination reaction. We show that a small amount of I ₂ catalyzes the reaction CH₂Br₂ + CH ₄ → 2CH₃Br, which leads to higher CH₄ conversion and higher selectivity to CH₃Br. These findings are promising for developing a low-cost integrated bromine-iodine based dual-halogen pathway to convert stranded natural gas into fuels and chemicals.

Full text of DOI:10.1021/cs300775m

Research Article
pubs.acs.org/acscatalysis
The Selective High-Yield Conversion of Methane Using
Iodine-Catalyzed Methane Bromination
Kunlun Ding, Horia Metiu, and Galen D. Stucky*
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510, United States
S Supporting Information
*
ABSTRACT: Methyl bromide is used as feed in a process that converts it to gasoline. It is
prepared by the gas-phase reaction of CH with Br , a reaction that produces, besides the
4
2
desired CH Br, large amounts of CH Br . The latter cokes the catalyst used for gasoline
3
2
2
production. The separation of CH Br by distillation makes gasoline production too
2
2
expensive. It is therefore important to increase the selectivity of the bromination reaction. We
show that a small amount of I catalyzes the reaction CH Br + CH → 2CH Br, which leads
2
2
2
4
3
to higher CH conversion and higher selectivity to CH Br. These findings are promising for
4
3
developing a low-cost integrated bromine−iodine based dual-halogen pathway to convert stranded natural gas into fuels and
chemicals.
KEYWORDS: gas-to-liquid, methane bromination, dibromomethane, iodine catalysis
INTRODUCTION
The formation of polybromomethanes during bromination is a
serious problem, if the bromination products are used as feed for
a catalytic carbon-coupling reaction to produce higher hydro-
carbons, because the polybromomethanes deactivate the
catalyst. Separation of these polybromomethanes prior to
introduction into the coupling reactor makes the bromine-
based methane conversion process more expensive.
■
Most natural gas reserves are located far from industrial and
population centers, and transportation costs hinder its efficient
utilization. Conversion of natural gas into a liquid, at the site of
origin, would be an ideal solution for transport to distant places
1
9
1
−3
where it can be used. Most stranded gas fields are too small for
use in the current gas-to-liquid technology, which is economical
only if the plants are of very large size, mainly because of the high
energy cost of the production of the synthesis gas intermediate
via the reforming reactions (steam reforming and dry reforming).
A low-temperature, high-yield process using a small facility
that can be located close to the stranded natural gas source is
desirable. In the past few decades, several direct conversion
methods have been developed, including thermal and catalytic
The success of halogen-assisted methane activation hinges on
the ability to produce CH Br with highest selectivity (against
3
CH Br ). In this article we show that the addition of a small
2
2
amount of I catalyzes the reaction of Br with CH and
2
2
4
accelerates the reaction kinetics as equilibrium is approached.
Higher methane conversion and methyl bromide selectivity can
be achieved at a comparatively short reaction time. Gas-phase I2
in these experiments satisfies the traditional definition of a
catalyst: it participates in the reaction but it is not consumed by it.
4
,5
6,7
pyrolysis, oxidative coupling, selective oxidation to meth-
8
,9
10,11
anol, and others.
for commercial use.
The use of halogens in the first step in natural gas conversion has
However none is sufficiently economical
The idea of using I as a catalyst was suggested by our previous
2
20,21
work
in which I catalyzed the reaction of CH Br with
2
2
2
propane to produce CH Br and other compounds.
1
2−18
3
been studied for many years.
A typical halogen-based process
consists of three steps: methane halogenation, the conversion of
methyl halide to higher hydrocarbon or methanol/dimethyl ether,
EXPERIMENTAL SECTION
■
16−18
The reactions described here were performed in a glass tube at
atmospheric pressure. The configuration of the reaction system is
illustrated in Supporting Information, Figure S1. Argon was used
and halogen regeneration.
The oxygen- and halogen-based
processes for natural gas conversion as well as their energy profiles
are illustrated in Figure 1. Compared to the synthesis gas-based
GTL process, a halogen process has advantages. First, the whole
process can be carried out at rather low temperatures; second, no
highly exothermic and endothermic reactions are involved in the
process (Figure 1c); third, no oxygen plant is required.
as the carrier gas. CH , HBr, and Ar flow rates were controlled by
4
mass flow controllers. I was dissolved in liquid Br or in liquid
2
2
CH Br (depending on the kind of experiment performed) and
2
2
delivered by syringe pump. The liquid was vaporized in the head
space of the reactor. The effluent stream from the reactor was
passed through a fritted glass bubbler trap containing an organic
solution (10 wt % octadecane in hexadecane) and a 4 M NaOH
Bromine is the most suitable halogen for methane activation
because of higher equilibrium conversion and methyl bromide
14
selectivity and easier regeneration from HBr. Typically, at a
temperature of 525 °C and a CH /Br ratio of 1, the selective
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2
conversion of methane to methyl bromide is 70−80% with the
byproducts being primarily dibromomethane and small amounts
of tribromomethane.
Received: November 30, 2012
Revised: February 4, 2013
©
XXXX American Chemical Society
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dx.doi.org/10.1021/cs300775m | ACS Catal. 2013, 3, 474−477

ACS Catalysis
Research Article
Figure 1. Illustration of the oxygen-based (a) and halogen-based (b) methane upgrading processes for the production of higher hydrocarbons; (c)
Energy profiles of the oxygen-based and halogen-based methane upgrading processes at 327 °C. Here we use C H as a representative of higher
3
6
hydrocarbons.
solution. The remaining gaseous product was collected in a
gasbag after passing through a final base trap (4 M NaOH
solution) to prevent any residual HBr/HI from entering the bag.
For most of the experiments, the reactions were run for half an
hour with all the products collected. They were analyzed with
three GCs, which measured: (1) gaseous halocarbons; (2)
gaseous hydrocarbons C −C ; and (3) liquid halocarbons. All
intermediate nature of these compounds. These facts imply that I2
acts as a catalyst.
The dependence of the effluent composition on temperature is
shown in Figure 2b. The data were taken at a reaction time of 8 s,
which is a time when there is substantial amount of CH Br in the
2
2
system, at 500 °C (see Figure 2a). The feed composition is the
1
6
same as in Figure 2a. The presence of I is clearly beneficial (solid
2
the experiments reported here have carbon balances of 95−
05%.
lines): CH conversion is slightly higher, the selectivity to CH Br
4
3
1
is markedly improved, and the amount of CH Br is smaller when
2 2
compared to the performance in the absence of I (dotted lines).
2
RESULTS AND DISCUSSION
Note that in both cases (with I
2
or without it) the amount of
■
unwanted CH Br produced is maximum at about ∼480 °C and
2
2
Figure 2a shows the effluent composition as a function of the
reaction time. The temperature was held constant at 500 °C, the
CH /Br ratio was 1/1, and the I /Br ratio was 1/9. The solid
then it drops fairly rapidly as the temperature increases beyond
80 °C. It appears that optimizing with respect to both reaction
time and temperature will improve the performance.
4
4
2
2
2
lines give the effluent composition when I was present, and the
dotted lines give the effluent composition in the absence of I . At
the earliest time one makes very rapidly CH Br, CH Br , and
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22,23
Methane bromination is a chain reaction
formation of Br radicals through the dissociation of Br . The
initialized by the
2
2
3
2
2
details are described in the work of Kistiakowsky and van
CHBr . Very soon, however, the reproportionation reactions
3
22
Artsdalen, and we have collected relevant information on the
CH +CHBr → CH Br +CH Br and CH Br + CH → 2CH Br
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3
3
2
2
2
2
4
3
elementary steps in the chain reaction in the Supporting
Information, Table S1. The key mechanistic question for the
present work is: what are the main reactions through which
start taking place, which explains the increase in CH Br
concentration and the decrease in CH Br and CHBr concen-
trations. After about five seconds the increase in CH conversion
slows down, and the other trends are reversed: the amount of
3
2
2
3
4
catalysis by I takes place? Apparently the reaction kinetics of
2
methane bromination approaching the equilibrium is determined
by the reproportionation reaction between CH and CH Br ,
which is limited by the activation of the C−Br bond. The energy
barriers of Br-abstractions from the C−Br bond by iodine radical
are only slightly higher than those by bromine radicals
CH Br decreases and the amount of CH Br increases. In the earlier
2
2
3
stage (reaction time <5 s) the presence of I has no influence on the
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2
2
2
composition of the products. For reaction time longer than ∼5 s the
presence of I makes a difference: CH conversion is higher, more
2
4
CH Br is produced, and the amount of CH Br is reduced, as
3
2
2
(
Supporting Information, Figure S4), while the concentration
compared with the case when I is absent. The graph also shows the
2
of iodine radical is much higher than that of bromine radical
because of the comparatively lower bond dissociation energies of
I−I, Br−I, C−I, and H−I bonds. Therefore, the iodine radical
equilibrium conversion of CH and equilibrium selectivity to CH Br,
4
3
which are calculated from the thermodynamic data under
corresponding reaction conditions (wide temperature range
calculation results are given in Supporting Information, Figure
S2). The reaction does not reach equilibrium for the reaction times
used here. Only trace amount of iodine-containing halocarbons were
detected, mainly composed of CH I and CH BrI. The selectivities of
could readily abstract Br from CH
reactions, CH and CH Br are converted to CH
3
Br
. After a series of cascade
2
2
Br while the
4
2
2
iodine radical is regenerated. Generally speaking, iodine radicals
are engaged in the following reactions:
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2
CH I and CH BrI both reach peak values at short reaction time and
then drop (Supporting Information, Figure S3), indicating the
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2
•
I + CH Br → IBr + •CH Br
(1)
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2
2
4
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Figure 3. Product distribution in the reaction of CH with CH Br in the
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2
2
presence of I . The initial composition was 52.86% CH and 47.14%
2
4
CH Br . To this I was added so that the I /CH Br ratio was 5/95. The
2
2
2
2
2
2
reaction temperature was 500 °C, and the reaction time was 8 s. Note the
very small amount of I-containing compounds.
Note that the effluent contains 0.23% CH I and 0.07% CH BrI. The
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2
concentration of iodine-containing compounds is very small.
Finally, in a separate experiment we have shown that the
reaction eq 6 takes place when we run through the reactor
CH Br and Ar at 500 °C and a reaction time of 8 s.
2
2
Kinetic simulations using the “Chemical Kinetics Simulator”
CKS) software were performed to examine the validity of the
(
proposed mechanism. The reactions included in the calculations
and their rate constants are listed in the Supporting Information,
Table S1. Since most of the rate constants were measured at a
temperature lower than 300 °C, we used the Arrhenius equation to
calculate the rate constants at 500 °C. Figure 4 shows the simulation
results of methane bromination with and without iodine. Clearly the
simulation results in Figure 4 are qualitatively similar to the
Figure 2. Effluent composition as a function of (a) the reaction time and
(
b) temperature. (a) The temperature was held constant at 500 °C; (b) The
reaction time was 8 s. The CH :Br :Ar mole ratio was 7:7:14; I /Br ratio
4
2
2
2
was 1/9; CH input was 8.2 mmol. The dotted lines are the results obtained
4
in the absence of I ; the solid lines are the results when I was present.
2
2
Methane conversion is given as percentage of the inflow of methane
consumed in the reaction. For the brominated products the percentage is
with respect to the total number of moles of brominated compounds.
•
•
•
CH Br + HBr → CH Br + •Br
(2)
(3)
(4)
2
3
Br + CH → HBr + •CH3
4
CH + IBr → CH Br + •I
3
3
The sum of these reactions is
CH Br + CH → 2CH Br
(5)
2
2
4
3
Obviously the reactants and the products of the reactions 1−4
are engaged in other reactions, which are detailed in the
Supporting Information, Table S1. However, our hypothesis is
that these four reactions are the key to understanding the role
of I2.
Figure 4. Calculated effluent composition in the reaction of CH with
4
Br as a function of the reaction time when I is present (solid lines) or
2
2
absent (dotted lines). The conditions are the same as in the
measurements reported in Figure 2a.
An indication that these four reactions do convert CH and
4
CH Br into CH Br is provided by an experiment in which CH
2
2
3
4
is reacted with CH Br in the presence of I . The concentration
2
2
2
experimental results shown in Figure 2a. Furthermore, the CKS
simulated equilibrium iodine radical concentration is much higher
than the bromine radical concentration (Supporting Information,
Figure S5). These results agree well with our proposed mechanism.
To test the quality of the kinetic simulations we have also
performed kinetic measurements on the reaction of CH Br with
of the reactants was 47.14% CH Br and 52.86% CH . To this we
2
2
4
added I2 such that the I /CH Br ratio was 5/95. The
2
2
2
temperature was 500 °C and the reaction time 8 s. The products
of the reaction are shown as a pie chart in Figure 3. Two reactions
take place in the system: one is eq 5, the other is
2
2
2
CH Br → CH Br + CHBr
(6)
2
2
3
3
HBr and show the results in Figure 5a. The results of the simulation
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ACS Catalysis
Research Article
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the University of California
Discovery Grant Program GCP08-128649, GRT Inc., and the Air
Force Office of Scientific Research (FA9550-12-1-0333).
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Figure 5. Experimental (a) and calculated (b) effluent concentration in the
reaction “CH Br + HBr” at T = 500 °C. CH Br /HBr mole ratio = 1/2.
(
2
2
2
2
The solid lines show the results when I was present, and the I /CH Br
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CONCLUSIONS
We have found that a small amount of iodine (I /Br = 1/9)
improves the conversion of methane to CH Br and the selectivity
4
(
1
(
4
(
■
2
2
3
against formation of CH Br . The iodine affects the methane
2
2
bromination reaction only after the reaction has produced some
CH Br . The main effect of I is to catalyze the reaction of CH Br
2
2
2
2
2
Sardar, A.; Cross, S. E.; Sherman, J. H.; Stucky, G. D.; Ford, P. C. J. Phys.
Chem. A 2006, 110, 8695−8700.
with CH , to produce CH Br. We speculate that the use of iodine, as
4
3
a catalyst, may be beneficial when selective monobromination is
intended. The results, along with our previous work on the iodine
catalyzed “CH Br + C H ” reaction, provide a general model for
2
2
3
8
developing an integrated bromine−iodine dual-halogen pathway to
convert stranded natural gas into fuels and chemicals.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Reactor configuration, equilibrium and selectivity data from
experiments, thermodynamic and kinetic information concern-
ing the reaction mechanism. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E-mail: stucky@chem.ucsb.edu.
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*
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