The catalytic activity of pure and doped zinc oxides from single source precursors for CH activation of light alkanes
EuropaCat IV, Catalysis for a Sustainable World, Salamanca, Spain, 30 August - 04 September 2009, pp.74, (Full Text)
- Publication Type: Conference Paper / Full Text
- City: Salamanca
- Country: Spain
- Page Numbers: pp.74
- Akdeniz University Affiliated: Yes
Abstract
Introduction
The C-H activation in saturated alkanes is the crucial step for their functionalisation and
combustion[1]. Among the light alkanes methane is the most stable compound and
therefore the most difficult molecule to activate. The activation of ethane and propane is
less difficult, hence working at lower reaction temperatures is possible. As methane is
the major component of natural gas, its conversion into value added products is of high
interest for chemical industry[2]. The oxidative coupling of methane (OCM) is one way
to achieve this aim. As methane is so difficult to activate, the OCM reaction is
conducted at high temperatures, where many catalysts suffer from severe stability
problems.
A new strategy for the development of more active catalysts could be designing highly
active catalysts for the C-H activation of higher alkanes, via single source precursors,
and subsequent application for the OCM. We have chosen this strategy for pure and
doped zinc oxide catalysts, as we are able to control their properties on a molecular
level[3, 4]. Zinc oxides catalysts for the oxidative coupling of methane have been
described earlier[5, 6, 7], but in no publication the bulk properties were controlled. In
fact, many of them were calcinated at higher temperatures than the OCM reaction, what
results in substantial sintering and therefore deactivation.
Experimental
The catalysts were prepared via the decomposition of a mixture of lithiumheterocubanes
and pure zinc cubanes. Methane, ethane and propane, respectively, were
fed with synthetic air, as oxygen source, to the a tubular quartz reactor, which is filled
with 150 mg catalyst. The temperature was varied from 450 – 500 °C for ODP, 500 –
650 °C for ODE and 550 – 700 °C for the OCM reaction. The flow was varried between
30 to 180 ml/min. The reaction products were identified by a gaschromatograph.
Results and discussion
The application of undoped zinc oxides in OCM, ODE and ODP leads to high oxygen
conversion, while doping it with small amounts of lithium the oxygen conversion is
drastically reduced, the alkane conversion is also decreased, but except for OCM the
selectivity is drastically increased. Sintering experiments with pure commercial zinc
oxide showed that at 400 °C a severe loss of BET surface starts. For lithium doped zinc
oxide this effect should be even more pronounced[8].
Conclusions
Since pure and Li-doped zinc oxides are deactivated at temperatures above 400 °C, they
are not suitable for the ODP, ODE and OCM, they need substrates that can be activated
more easily.
Acknowledgements
We would like to thank the DFG, the Max Planck Society and UNICAT for financial
support.
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