The pyrolysis of diborane

The pyrolysis of diborane has been studied in single‐ and multi‐pass flow systems to determine the optimum conditions for its conversion to pentaborane. In a single‐pass flow system the products consisted of tetraborane, dihydropentaborane (B5H11), pentaborane (B5H9), decaborane and undesirable non‐volatile solid hydrides. The most economical conversion of diborane to pentaborane was obtained under the following conditions: temperature 250°, flow rate 400 c.c./min. and ratio of diborane/hydrogen 1 : 5 when 15·1% of the diborane was converted into pentaborane, 4·4% to solid hydrides and 80% recovered. In multi‐stage reactors designed on the basis of single‐pass experiments, the best yield of pentaborane was obtained in a 12‐unit reactor operating at 240°, flow rate 400 c.c./min. and ratio diborane/hydrogen 1 : 5. Under these conditions 57·0% of the diborane was converted into pentaborane, 14·1% to solid hydrides and 29·0% was recovered unchanged. Rate expressions are developed for the disappearance of diborane and activation energies calculated. The mechanism of the formation of pentaborane and decaborane is discussed.