154 / 390
Figure 10:
Adiponitrile
Summary and Conclusions
It could be shown that aluminum chloride dissolved in the silane product mixture of the Müller-
Rochow direct synthesis is the main cause for the formation of solid precipitates clogging filters,
pipes and tubing of equipment used in its distillation. The strong Lewis acid reacts with other
components of the product mixture to yield tetrachloroaluminate salts which are scarcely
soluble in silanes and thus precipitate in several stages of the distillation. This can be prevented
by either removing aluminum chloride from the silane mixture or by blocking its Lewis-acidic
site with a strong aprotic Lewis base. Removal of aluminum chloride is best achieved by
addition of urea or imidazolium chlorides resulting in the formation of ionic liquids which are
barely soluble in and can be easily separated from the silane mixture. To block the reactivity of
aluminum chloride, strong Lewis bases like adiponitrile can be used. Already a very low
concentration of this readily available solvent in the product mixture prevents the formation of
solid precipitates without the risk of contaminating the desired product. If filters, pipes and
tubing have already started to plug, aprotic high polarity solvents like tertiary amines or nitriles
can be used to dissolve the precipitates and clean the equipment.
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