Low temperature selective methane activation to alkenes by a new hydrogen-accumulating system

Article abstract of DOI:10.1039/a901439j

A heterogeneous hydrogen-accumulating system containing porous titanium with 0.4 wt% Ni combined with high-purity titanium chips was tested for methane activation; methane conversion to C₁-C₄ hydrocarbons reached a value of ca, 20% over this material, working at 450 °C and 10 atm, after methane circulation across the system for 22 h; the split hydrogen was accumulated as TiH₂, being in solid solution with porous metallic titanium.

Full text of DOI:10.1039/a901439j

Low temperature selective methane activation to alkenes by a new
hydrogen-accumulating system
M. V. Tsodikov,a Ye. V. Slivinskii,a V. P. Mordovin,a O. V. Bukhtenko,a G. Colón,b M. C. Hidalgob
and J. A. Navío*b
a A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 117912-Moscow, Leninskii prosp.
29, Russia
b Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla and Dpto. de Química
Inorgánica, Universidad de Sevilla, Avda. Américo Vespucio, s/n. 41092-Sevilla, Spain. E-mail: navio@cica.es
Received (in Cambridge, UK) 22nd February 1999, Accepted 16th April 1999
A heterogeneous hydrogen-accumulating system containing
porous titanium with 0.4 wt% Ni combined with high-purity
titanium chips was tested for methane activation; methane
conversion to C1–C4 hydrocarbons reached a value of ca.
20% over this material, working at 450 °C and 10 atm, after
methane circulation across the system for 22 h; the split
hydrogen was accumulated as TiH2, being in solid solution
with porous metallic titanium.
around 20% after methane flow-circulation across the catalyst
bed at a temperature of 450 °C for 22 h. This degree of
conversion was constant even after 28 h of continuous flow-
circulation, as experimental controls showed (Fig. 1). It should
be mentioned that methane conversion into C2 hydrocarbons
was initiated at 330 °C. Table 1 summarises the composition of
the methane conversion products. As can be seen, after methane
flow-circulation for 1 h a conversion of about 4.5% was
reached, yielding predominantly ethylene and ethane (92.9%).
After prolonged reaction time an increase of C2–C4 olefins of
around 70–75% was observed, 50–55% of which was ethylene.
At this point, the level of hydrogen in the reaction volume was
not more than 0.01% according to GC (LKhM-8MD and
Biokhrom chromatographs) and MS (VIMS MS-7201) analy-
ses.
Methane is the principal constituent of natural gas, landfill gas
or coalbed methane. It is also a by-product of oil refining and
chemical processing. Many techniques have been developed
for improving the industrial processes which convert methane
into higher hydrocarbons, gasoline and olefins by both indirect
and direct conversion processes.1,2 However these industrial
processes involve expensive separation steps and/or require the
use of temperatures higher than 80 °C, with the subsequent
consumption of energy.1,2
Direct methane conversion eliminates the need (and subse-
quently reduces the cost) for the syngas preparation step.
However, since methane is a very stable molecule2,3 its
reactions generally have high activation energy and, once
activated, it is difficult to stop the reaction from going further
than desired.
Analysis by X-ray (DROM-3M, Cu-Ka) diffraction of the
composition of the catalyst showed that, in addition to the
reflections corresponding to the metal titanium phase [d = 2.24;
2.34; 2.55 A], titanium chips removed from the reactor under
anaerobic conditions showed well-resolved intense reflections
On the other hand, the non-favourable thermodynamic
parameters4 for methane self-interaction reactions led us to
predict that, without strong oxidants, it would be difficult to
transform methane into higher homologues at moderate tem-
peratures using conventional catalytic approaches. We pre-
sumed, however, that it should be possible to shift the
equilibrium via the introduction of separate stages, including
the formation of [CH3*] and [CH2*] intermediates using
heterogeneous systems with dual activities: a high activity for
C–H bond fission and simultaneous hydrogen-accumulating
properties. The latter might lead to an increase in the probability
of direct self-interactions between generated intermediates,
giving higher hydrocarbon molecules by a route which has not
been explored until now. The adsorption of H2 on transition
metals has been much studied.5 At the same time the adsorption/
reaction of hydrocarbons on metallic catalysts is a very well
known topic,5 with Ni metal having a capability for hydro-
genolysis of C–C and steam reforming of CH4.
Fig. 1 Conversion of methane into products summarised in Table 1.
Table 1 Methane conversion and the composition of products from its
transformation (T = 450 ºC, P = 10 atm)
With this goal in mind we used a heterogeneous system
containing porous high purity titanium (99.6%) with 0.4 wt% Ni
in the form of cylindrical pellets (10 mm long and 5 mm in
diameter) (I) combined with titanium chips (II) also obtained
from high purity titanium. The I:II ratio was 5.3. The
composite was thermally activated at 850 °C in vacuo (1025
torr) before being placed in a loop reactor with a total volume of
0.42 L in circulation mode with a flow rate of 5 L of gas per hour
at 10 atm. It was shown by laser (ML-2) and scanning high
temperature mass spectroscopy (VIMS MS-720) that vacuum
treatment is sufficient to clean up the surface. Using 19 g of this
catalytic material it was found that methane conversion reached
Composition (mass%)
Composition
(% mass)
1 h
5 h
22 h
28 h
CH4
95.3
2.5
1.9
0.1
0.2
85.3
8.2
3.0
2.5
0.4
0.4
0.2
79.8
10.0
4.8
3.1
0.4
80.1
10.8
4.4
2.1
0.3
C2H4
C2H6
C3H6
C3H8
C4H8
C4H10
< 0.1
< 0.1
1.3
0.6
1.7
0.6
Chem. Commun., 1999, 943–944
943

*
*
3 CH4 ? [CH2 ] + 2 [CH3 ] – 4 [H] ? C3H6
(4)
The C4 fraction is probably a result of ethylene dimerization.
We also propose that one of the limitations of this process is the
coordination of CH2* intermediates to the surface metal centres.
The interaction between closely coordinated [CH2–M] species
at the catalyst surface could be followed by their elimination
leading to ethylene formation. In this way we can explain the
high selectivity for C2 alkene. In fact this tentative mechanism
for the formation of olefins is in good accordance with the
concepts of metal complex homogeneous catalysis.6
In any case our results indicate that C2–C3 species can be
obtained by a single step, compared with direct or indirect
processes which involve multi-steps such as the syngas and
steam reactions. Of course, the problem of the hydrogen
spillover to the titanium chips requires further study, which is
now in progress.
Here we have reported a new finding which opens the way to
selective low temperature methane conversion to important
products by a new means using bifunctional hydrogen accumu-
lating systems. At the same time the process offers the
possibility of simultaneously obtaining high purity dihydrogen.
The authors acknowledge the Russian Science Foundation
(Project 970332028a), NATO (Project ref. ENVIR.LG 971292)
and DGICYT-Spain (Project PB96-1346) for supporting part of
this work.
Fig. 2 Dependence of methane conversion on the quantity of hydrogen
evolved and sorbed by the heterogeneous system.
due to a hydride phase (TiH2) [d = 2.22; 2.31; 2.53 A]. At the
same time, the reflections associated with the metal titanium
phase shifted by only 0.45–0.47° (2q) with respect to the
starting system for the porous titanium. This evidence for the
formation of a Ti–H interstitial solid solution during the
decomposition of methane.
Fig. 2 shows that the increase in the methane conversion, in
the range of operation, is proportional to the amount of
hydrogen sorbed by the system, this being calculated on the
basis of the stoichiometry of the reaction. This enables us to
conclude that the removal of hydrogen from the reaction zone is,
as expected, one of the main stages of the transformation of
methane into higher products.
On the basis of the balance of the identified products, we may
propose the pathways in eqns. (1)–(4) for the self-interaction
reactions between the intermediates formed from methane
activation.
Notes and references
1 J. M. Fox, Catal. Rev.-Sci. Eng., 1993, 35, 169.
2 A. Ye. Shilov and G. B. Shulpin, in Activation and Catalytic Reactions of
Hydrocarbons, ed. A. P. Purmal, Moscow, 1995, p. 399.
3 Methane, ed. F. A. Alekseev, Nedra, Moscow, 1978.
4 D. R. Stull, E. F. Westrum and G. C. Siuke, in The Chemical
Thermodynamics of Organic Compounds, Wiley, New York, 1969.
5 J. H. Ross, in Metal Catalysed Methanation and Steam Reforming, in
Catalysis, ed. G. C. Bond and G. Webb, The Royal Society of Chemistry,
London, 1985, vol. 7, p. 1.
*
6 M. L. H. Green, Organometallic Compounds, in The Transition
Elements, Methuen, London, vol. 2, 1968.
2 CH4 ? 2 [CH2 ] 2 4 [H] ? C2H4
(1)
(2)
(3)
*
2 CH4 ? 2 [CH3 ] 2 2 [H] ? C2H6
*
3 CH4 ? 3 [CH2 ] 2 6 [H] ? C3H6
Communication 9/01439J
944
Chem. Commun., 1999, 943–944
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