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Figure 3:
Survey (left) and close-up SEM images of solid precipitates from filters,
survey scan showing rust particle (white)
Scattered in the samples a few rust particles were found accounting for a fraction of the iron
contents of the solids as found in ICP-AES. The rust particles show very different morphologies
and compositions compared to the rest of the sample and are therefore easily identified (Figure
4).
Since aluminum is a major constituent of the precipitates, solid state
27
Al-NMR was employed
to identify the actual aluminum species present. For this, only rust-free samples were used. The
peaks in the spectra were assigned by reference measurements. Samples from filters early in
the distillation process contain two different aluminum species which could be identified as
tetrachloroaluminate and aluminum chloride. Later in the process, only tetrachloroaluminate
was found in samples from filters and evaporators (Figure 5).
There are a few metals that can act as possible cations for the tetrachloroaluminate anion, with
copper and iron being the most abundant in the precipitates. Generally, all tetrachloroaluminate
salts are scarcely soluble in the product mixture of the Müller-Rochow direct synthesis, so
precipitation of the identified compounds is not surprising. Rather unexpectedly, the
precipitates comprise a high carbon content. The nature of the carbon species in the precipitates
could not be elucidated; thermogravimetric analysis in combination with mass spectrometry
(TGA-MS) shows the elimination of chloroalkanes and alkanes at elevated temperatures (>
175°C). Since the precipitates are solids, the carbon species must be of high molar masses or
polymeric or both. Since no evidence for elemental carbon was found and the hydrogen content
of the precipitates suggests a saturated linear organic compound of high molar mass, polymers
are most likely. They have to originate from reactions of the alkanes, alkenes and chloroalkanes
in the product mixture. Such reactions could be those of chloroalkanes with the Lewis-acidic
aluminum chloride to form tetrachloroaluminate and carbocations which then further react with
alkenes to form polymers via cationic polymerization.
The silicon species present were found to be mainly highly crosslinked siloxanes probably
resulting from the reaction of silane residues with moisture which occurred while taking the
samples from filters or evaporators in air.
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