The effect of oxidation state of tungsten on hydrocracking of n-heptane over tungsten oxide

Abstract

Hydrocracking of n-heptane was investigated with tungsten oxide of various oxidation states under high pressure of hydrogen. It was found that the reaction products definitely change four times with changing oxidation state during the course of reduction of tungsten trioxide. In the presence of slightly reduced tungsten oxide WO 3 − a (WO 2.84, tungsten oxide reduced at 445 °C), isomerization and subsequent central cracking of n-heptane occurred predominantly, showing that the reaction proceeded by carbonium ion mechanism. As the oxidation state approaches tungsten oxide WO 3 − b (WO 2 or WO 2 + α tungsten oxide reduced at 500 °C to 525 °C), demethylation reaction to form methane and normal paraffins by the selective scission of terminal CC bond took place predominantly. Further reduced tungsten oxide WO 3 − c (WO 2 − β, tungsten oxide reduced at 580–610 °C) gave isomerized heptane and central cracked products as in the case of WO 3 − a . Tungsten oxide WO 3 − d reduced almost to tungsten metal above 680 °C again favors demethylation rather than isomerization and central cracking. These results suggest that the oxidation state of tungsten determined the reaction mechanism of hydrocracking of n-heptane.