The design of new catalysts for the partial oxidation of methane to methanol.
The direct partial oxidation of CH4 to CH30H would offer considerable economic
advantages over the current two stage process. It would also facilitate the utilisation
of natural gas reserves in remote locations.
To date, the catalytic partial oxidation of CH4 to CH30H has been extensively
studied, however, it has proved to be an extremely demanding reaction which has
met with little success.
This study has adopted a design approach for the identification of new catalysts by
considering the efficacy of single oxides for CH4 activation, CH30H oxidation and
02 isotope exchange activity.
On the basis of CH30H stability Sb203 was the best oxide, showing only 3 %
CH30H conversion at 500°C. The majority of oxides totally combusted CH30H
below 400°C. Mo03, Nb20S, Ta20S and W03 showed high selectivity to HCHO
and (CH3hO with low levels of COx throughout the range of conversion. These
oxides were not considered unsuitable from the perspective of CH30H stability as
the products HCHO and (CH3hO are not considered undesirable by products from
a CH4 partial oxidation process. A weak but significant correlation was observed
between the combustion activity of the oxides and the oxygen exchange rate.
Using CH4/D2 exchange as an indication of CH4 activation it has been shown that
Ga203 was a particularly good catalyst, followed by ZnO and Cr203. A relationship
between exchange activity and oxide basicity was established for the rare earth
sesquioxides, MgO and CaO. This relationship indicak:d that CH4 activation took
place by H+ abstraction to form a surface CH3- species.
From these results and literature studies of oxygen isotope exchange, dual
component oxides have been formulated as catalysts for CH4 partial oxidation. The
best catalysts was Ga203/Mo03, prepared by a physical mixing process. This
catalyst showed an increased yield of CH30H over the homogeneous gas phase
oxidation of CH4 in a quartz chips packed reactor. This increased yield has been
attributed to the development of a cooperative effect between the two component
oxides. Comparison of the catalytic data with the homogeneous reaction in the
empty reactor tube showed that the presence of a catalyst had a detrimental effect on
the CH30H yield.