Catalytic reaction of methane with CBrF3

Article abstract of DOI:10.1039/a900972h

The catalytic reaction of CH₄ with CBrF₃ over Co, Cu and Mn ZSM-5 zeolites is described; major products (at low temperatures) are those expected for simple hydrodebromination: CH₃Br and CHF₃.

Full text of DOI:10.1039/a900972h

Catalytic reaction of methane with CBrF3
Kai Li,a Eric Kennedy,*a Bogdan Dlugogorskia and Russell Howeb
a Department of Chemical Engineering, University of Newcastle, Callaghan, NSW 2308, Australia.
E-mail: cgek@cc.newcastle.edu.au
b School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
Received (in Cambridge, UK) 4th February 1999, Accepted 10th March 1999
The catalytic reaction of CH4 with CBrF3 over Co, Cu and
Mn ZSM-5 zeolites is described; major products (at low
temperatures) are those expected for simple hydrodebromi-
nation: CH3Br and CHF3.
The hydrodehalogenation reaction of chlorofluorocarbons
(CFCs) and bromofluorocarbons or bromochlorofluorocarbons
(halons) are potentially important reactions for both disposal of
ozone depleting chemicals and production of hydrofluor-
ocarbon replacements. Previous work in this area has focussed
on reactions of CFCs with hydrogen over supported palladium
catalysts.1–12 Little attention has been paid to the corresponding
hydrodebromination of halons. Here we describe for the first
time the catalytic reaction of halon 1301 (CBrF3) with methane
over transition metal exchanged ZSM-5 zeolites. This novel
Fig. 2 Steady state conversion of CBrF3 and CH4 versus temperature over
MnZSM-5.
catalytic reaction offers potential new routes to hydrofluor-
ocarbons using methane as the hydrogen source.
Cobalt, copper and manganese exchanged ZSM-5 zeolites
were prepared from a commercial ZSM-5 (PQ, Si/Al = 25), and
charged into an alumina plug flow microreactor. Reactant gas
stream (CH4–CBrF3–N2 = 1:1:10) was flowed over the
catalyst at a GHSV of 3500 h21, and products were analyzed by
on-line gas chromatography after passing through a caustic
scrubber.
Fig. 1 shows typical CBrF3 conversions versus time on
stream over four different zeolite catalysts at 873 K. Also shown
is the corresponding conversion in the homogeneous reaction,
as reported elsewhere.13 The gas phase reaction between CH4
and CBrF3 begins at ca. 800 K, and increases dramatically
above 900 K. A striking feature of the reaction in the presence
of zeolite catalysts is the initial high conversion of CBrF3 which
then falls to a steady state value. Our preliminary investigations
of this breakthrough period suggest reaction of CBrF3 with the
zeolite which generates the active phase for the hydro-
dehalogenation reaction. 27Al MAS NMR measurements show
that framework tetrahedral aluminium in the zeolite is com-
pletely transformed into an as yet unidentified form, although
XRD analysis shows no change in the zeolite structure. In the
case of HZSM-5, the steady state conversion approaches that
found in the absence of catalyst. For the transition metal
exchanged zeolites, however, there is a very evident influence
of catalyst on the steady state conversion. It is important to note
that zeolite crystallinity is fully retained during this reaction,
even after 36 h on stream.
Fig. 2 plots the steady state CBrF3 and CH4 conversions
versus reaction temperature over MnZSM-5; similar profiles
were measured for the other transition metal zeolites. The
CBrF3 conversion exceeds that of CH4 at all temperatures,
suggesting that the reaction pathway is more complex than that
predicted for the simple 1:1 stoichiometry of reaction (1):
CBrF3 + CH4 ? CH3Br + CHF3
(1)
In separate experiments, we have noted that CBrF3 undergoes
pyrolysis reactions over the same catalysts, producing C2F6 and
Br2,14 and C2F6 was detected as a minor product also in the
reactions with methane. The major reaction products detected in
the reactions with methane at lower temperatures are those
expected from reaction (1), as illustrated in Fig. 3. Above 873 K,
however, the yield of CH3Br declines, and other reaction
products appear, (notably hydrocarbons C2H4 and C2H2), due to
further reaction of CH3Br. Other products identified include
C2H2F2, C2H3Br, CH2BrF, and trace (less than 5 ppm)
quantities of C2HBr2F3, CH2Br2, C2F6, C2H3F3, C6H5Br and
H2. Mass balances for all reactions were generally better than ±
Fig. 1 Conversion of CBrF3 versus time on stream at 873 K over various
catalysts.
Fig. 3 Steady state conversion of CBrF3 to major products CHF3 and CH3Br
versus temperature over MnZSM-5.
Chem. Commun., 1999, 709–710
709

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3% and usually ± 1%, and suggest any as-yet unidentified
species represent only a small fraction of reaction products.
The reaction mechanisms involved in this intriguing chem-
istry are not yet clear. The high initial conversions of CBrF3,
and the results of preliminary characterization experiments on
catalysts exposed to CBrF3 plus CH4 or CBrF3 alone for short
periods14 suggest that initial reaction of CBrF3 with fresh
catalysts generates the catalytically active species. The high
reaction selectivity (at low temperatures) may result from the
selective cleavage of C–Br bonds in preference to C–F bonds in
CBrF3, and reflects the lower relative strength of the C–Br bond
(295 kJ mol21 15 compared with C–F bonds (ca. 460 kJ mol21
) ,
estimated from analogous F–CF2Cl bond strength) in CF3Br.
Low temperature cleavage of C–Br would occur before
evidence of C–F bond cleavage, leaving the CF3 moiety
intact.
Further work is needed to optimize catalysts, identify active
sites and determine reaction pathways. At present we are also
investigating the reactions of CFCs and other halocarbons with
methane. The results presented here offer a new route for
methane utilization in the disposal and upgrading of ozone
depleting materials.
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14 K. Li, F. Oghanna, E. M. Kennedy, B. Z. Dlugogorski, A. Fazeli, S.
Thomson and R. F. Howe, to be published.
This work has been supported by the Australian Research
Council.
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Notes and references
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Communication 9/00972H
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Chem. Commun., 1999, 709–710