Res. Chem. Intermed.
, Vol. 32, No. 3–4, pp. 205–215 (2006)
Also available online - www.vsppub.com
Free radicals as intermediates in catalytic oxidation of light
alkanes: new opportunities
M. YU. SINEV
Semenov Institute of Chemical Physics, 4 Kosygin str., Moscow 119991, Russia
Received 31 December 2005; accepted 15 February 2005
Abstract—The analysis of catalytic partial oxidation of light alkanes indicates that processes
involving this group proceed via the formation and consecutive transformations of free radicals.
Depending on the properties of the catalytic system and reaction conditions the same primary
radical can give different ﬁnal products, oleﬁns, oxygenated organic substances and carbon oxides.
An approach to design a complex catalytic system for efﬁcient alkane partial oxidation based on
separation of zones where free radicals are formed and where they are transformed into desired
products is suggested and examples of its implementation are presented.
Keywords: Partial oxidation; free radical; light alkane.
About 13 years ago in the laboratory of Prof. Kiyoshi Otsuka an important
fact was observed [1, 2]. During methane oxidation over a series of metal
(Zn, Zr) phosphates selectivity to partial oxidation products (formaldehyde and
ethane) varied along with two reaction parameters, namely temperature and oxygen
concentration in a feed gas. Selectivity variations with oxygen concentration were
the most intriguing: whereas total selectivity to the sum of the above products
) remained practically constant over a given catalyst, formaldehyde fraction
increased linearly with oxygen concentration. It was concluded that all ﬁnal
products, formaldehyde, ethane and carbon oxides, are formed via formation and
consecutive reactions of one and the same intermediate. By that time the free-
radical nature of catalytic methane oxidation to formaldehyde and C
had been proven by several authors [3–6]. Due to this fact it was beyond discussion
that free alkyl (methyl) radical is a key intermediate that transforms into different
ﬁnal products. Relative amounts of formaldehyde and ethane depend on the rates of