C1 Coupling via bromine activation and tandem catalytic condensation and neutralization over CaO/zeolite composites

Article abstract of DOI:10.1039/b314118g

We demonstrate here an alternative scheme for C₁ coupling by way of methane bromination, followed by concurrent bromomethane condensation and quantitative HBr neutralization; regeneration of the metal oxide with O ₂ with recovery of

Full text of DOI:10.1039/b314118g

C1 Coupling via bromine activation and tandem catalytic condensation
and neutralization over CaO/zeolite composites†
Ivan Lorkovic,*a Maria Noy,a Mike Weiss,b Jeff Sherman,b Eric McFarland,bc Galen D. Stuckya and
Peter C. Forda
a Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106.
E-mail: lorkovic@chem.ucsb.edu; Fax: +1 805 893 4120; Tel: +1 805 893 5239
b Gas Reaction Technogies, Inc., 301B S. Wimbrow Dr. Sebastian, FL 32958
c
Department of Chemical Engineering, University of California, Santa Barbara, CA 93106
Received (in West Lafayette, IN, USA) 5th November 2003, Accepted 23rd December 2003
First published as an Advance Article on the web 6th February 2004
We demonstrate here an alternative scheme for C1 coupling by
way of methane bromination, followed by concurrent bromo-
methane condensation and quantitative HBr neutralization;
regeneration of the metal oxide with O2 with recovery of Br2
completes the cycle.
Fig. 1 shows the time dependent product output from two serial
continuous flow reactors for methane bromination (1) at 525 °C,
followed by reaction over a bed of CaO-ZSM-5 at 400 °C (2).
Retention of bromine within the second packed bed under these
conditions is better than 99.9%, while the product output is very
similar to that observed for MeOH coupling over Ca/ZSM-5.21
The reaction in Fig. 1 represents HBr sequestration to 50–75% of
neutralization capacity (5 hours, 5 cm3 min CH4, 0.5 cm3 min
Br2(g)) of the solid. After more than 10 runs and regenerations (525
C, 5 hours, 5 cm3 min O2, quantitative Br2 recovery), the coupling/
neutralization reactivity and product distribution of the regenerated
solid (50% selectivity to C2–C5) remained unchanged within
experimental error. Furthermore, these materials are also catalytic
[eqn. (3)] in that even after HBr breakthrough due to metal oxide
depletion, the conversion of bromomethanes under the conditions
shown in Fig. 1 continues (Figure S1†). This catalytic reactivity
allows use of more specialized auxiliary metal oxides for HBr
sequestration and Br2 recovery.
Although much of the world’s methane is “stranded,” utilization of
available methane as a chemical feedstock begins in practice with
its conversion to methanol by partial oxidation to syn gas followed
by recondensation. Subsequent conversion of CH3OH to olefins
was made possible by discoveries in the early seventies by Mobil
scientists,1–4 and later generalized by other workers to CH3X (X =
halide, SH, NH2, OCH3) condensation.5–14 The proposed mecha-
nism15,16 is an initial dehydrative coupling to form the actual
catalyst, a relatively ill-defined adsorbed cyclic hydrocarbocation
(carbon pool). The technology has been widely developed for
multiple product outputs, and may be considered an alternative to
the reliable but extreme steam cracking method of olefin production
from low and middle petroleum distillates. For CH3Cl, formed from
CH4 oxychlorination, the product in most cases is an aromatic-rich
liquid together with HCl and water,6,7 with some variation
depending on the promoters used.8–14 While this chloromethane
chemistry has been well characterized and the kinetics are
favorable, the process has not been commercialized.
(3)
Methane partial oxidation by free radical bromination leads to
significant buildup of CH2Br2 as well as some CHBr3 at
appreciable methane conversion. Over CaO/ZSM-5, CH2Br2
condenses predominantly to adsorbed carbon in the absence of
CH3Br while cross coupling between CH2Br2 and CH3Br is
manifest in the higher output of aromatics (best represented as
mesitylene C9H12), when both species are present (Fig. 2). Notably,
with pure CH3Br feed, the yield of C2–C5 is significantly higher,
and that of adsorbed carbon and aromatics significantly lower than
observed for a mixed bromomethane feed, Figure S2,† Fig. 2.
The advantage of Br2 over other halogens in this partial oxidation
scheme may be understood in terms of the reduction potential of
We previously reported a two step technique for partial oxidation
of alkanes by oxygen in which a two electron oxidation of a C–H
bond is effected by bromine, giving HBr and bromoalkane [eqn.
(1)].17–20
(1)
RH + Br2 ? RBr + HBr
These intermediates were further converted either to unsaturated
hydrocarbons (for C2+) or to oxygenates by reaction with a metal
oxide solid reactant. The metal oxide served to remove HBr
actively, and to direct the output to specific partial oxidation
products dependent upon the metal oxide composition and reaction
conditions. Complete recovery of bromine and regeneration of the
metal oxide was accomplished by reaction of the spent solid with
O2.
During the course of development of active and regenerable
metathesis materials, oxides of calcium were investigated because
of their lack of facile redox activity (leading to deep oxidation),
stoichiometric HBr neutralization capacity, and the nearly thermo-
neutral regeneration of CaBr2 with O2 to give CaO and Br2 (DG° =
14.9 kcal mol21). We observed that while activated HZSM-5 (Si :
Al = 80) rapidly lost catalytic CH3Br coupling activity over the
course of 10 minutes, calcium oxide zeolite composites‡ quantita-
tively neutralized HBr and effected the presumably superacidic
condensation of methyl bromide to higher olefins [eqn. (2)].
HBr + CH3Br + CaO/ZSM-5 ? 1/n (CnH2n) +
Fig. 1 Output of dual stage reactor with a feed of CH4 : Br2 (5 : 0.5 cm3
min21). First stage: plug flow reactor (1 3 100 mm glass tube, 500 °C,
space time = 0.3 s. Br2 conversion = 100%, CH4 conversion = 8.2%).
Second stage: fixed bed: (10 3 100 mm plug of 5 g Ca-ZSM-5, 400 C, space
time = 20 s, WHSV = 0.04 h21).
(2)
H2O + CaBr2/ZSM-5
† Electronic supplementary information (ESI) available: additional figures.
See http://www.rsc.org/suppdata/cc/b3/b314118g/
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T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Notes and references
‡ CaO/ZSM-5 composites were prepared by wet impregnation of 1 part
Ca(NO3)2 to 4 parts H-ZSM-5, (Si : Al = 80:1, obtained from Zeolyst
Corp.), drying at 125 °C. overnight, followed by calcination at 500 °C.
overnight. Catalytic coupling reactivity was not evident until after the first
CH3Br/HBr metathesis/regeneration cycle.
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Fig. 2 Comparison of product selectivity for methane/bromine reaction
product feed vs. pure CH3Br feed (conditions as in Fig. 1).
Br2 to Br2 (1.07 V vs. NHE), which in comparison with Cl2 (1.36
V) and I2 (0.54 V), makes alkane bromination significantly less
exothermic, yet spontaneous enough to go to completion. Bromine
also allows for utilization of a wider range of metal oxides as
bromide metathesis reagents because the reoxidation of metal
bromides by O2 (1.23 V) can be accomplished under relatively mild
conditions. Despite the slightly lower selectivity for monobromina-
tion versus monochlorination for comparable methane conversion,
a higher degree of reversibility is expected for the weaker C–Br
bonds22 than exists for C–Cl bonds for corresponding C1 species. In
addition, CH3Br and CH2Br2 are expected to be significantly easier
to separate from each other than are CH3Cl and CH2Cl2. Hence
polybrominated methanes are not necessarily lost from a methane
conversion process and may be induced to comproportionate with
CH4 feed, raising overall CH3Br, and ultimately olefin, yield.23
We direct further work towards establishing the generality of the
condensation reactivity of CH3Br over microporous solids to give
olefins or other products and draw on analogies to CH3OH
coupling.21 Ultimately our goal is to utilize the three step low
temperature route—bromination, coupling, regeneration—to
streamline the production of higher hydrocarbons from methane,
technology which is presently dominated by processes involving
MeOH or synthesis gas as intermediates. Selective bromination and
reactor configurations favoring comproportionation of methane and
CH2Br2 or CHBr3 to CH3Br are potential routes to improved carbon
utilization.
This research was funded by Gas Reaction Technologies, Inc.,
through a sponsored research agreement with the University of
California.
23 I. M. Lorkovic, P. Grosso, J. H. Sherman, E. M. Mcfarland, P. C. Ford
and G. D. Stucky, manuscript in preparation.
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