Abstract
Fluoro tetrasilicic mica (TSM), Na [Mg2.5 Si4O10F2] is an unique cation exchanger among-a variety of silicate minerals, because it has no acid sites on the silicate sheet. The studies on the methanol conversion with a metal ion-exchanged TSM (Me-TSM) have revealed that the silicate sheet of TSM is catalytically inactive and the catalytic activities of Me-TSM's are individually different by the exchanged metal ion. The facts suggest that TSM is a suitable support for studying the catalytic activity of metal ion in a heterogeneous catalysis.
Cu (II)-TSM catalyzed the dehydrogenation of methanol to form methyl formate, 2CH3OH→HCOOCH3+2H2 (1), selectively and the catalytic activity did not change at all by the long use for the catalysis. ESR studies showed that Cu (II) ions in Cu (II)-TSM retained the divalent state throughout the reaction, although they were exposed to methanol vapor at 240°C. The facts indicate that Cu (II) ion is the active species for the methanol dehydrogenation, (1). The high reduction resistivity is understandable, considering that the interlayer Cu (II) ion is a constituent of the layer structure to counterbalance the negative charge of silicate sheet.
The catalytic activity of Cu (II)-TSM is superior to that of the patent catalysts claimed for the synthesis of methyl formate. When the reactions were conducted with Cu (II)-TSM at 300°C and 400°C alternately, the higher activity was observed at the lower temperature. The facts suggest that the dehydrogenation of methanol is not facilitated only by the exposed Cu (II) ions but also by the Cu (II) ions located in the interlayer region of TSM especially at the temperatures below 300°C.
The dehydrogenation (1) was markedly retarded when ammonia occupied two or more of six coordination sites of Cu (II) ion. Since the two silicate sheets occupy the other two coordination sites, it is inferred that the selective dehydrogenation to methyl formate is catalyzed by the Cu (II) ion which has at least three vacant sites available for coordination of methanol. The inference is consistent with the reaction mechanism revealed by tracer studies with deutrated methanol. The reaction mechanism illustrated in the text (Scheme 2) shows that methanol requires three vacant sites to transform into the key intermediate of the reaction.
Besides this particular synthetic mica, TSM, we have a variety of clay minerals. Extensive investigations on the catalysis of clay derivatives are expected to afford a lot of useful informations to design a new catalyst with clay intercalates.
