Title:
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A Dielectric study of the synthetic - A Zeolite
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The dielectric properties of several ion forms of the
synthetic crystalline zeolite Linde type-A have been studied
in the frequency range 5 Hz to 100 MHz in the presence of varying
amounts of adsorbed water. In addition the mixed Ca/Na form of
type-A (i.e. the commercial 5-A) has been studied with ammonia,
sulphur dioxide, carbon dioxide and n-pentane as the adsorbed
phase in the frequency range 5 Hz to 150 kHz.
A cell specially suitable for the above studies has been
designed, and constructed, and satisfactorily operated. The cell
enables the zeolite to be dehydrated at temperatures up to 5000C
under a vacuum of 10-5 torr and tben equilibriated with any
required pressure of any gaseous adsorbate before measurement.
In all the ion forms studied several regions of dielectric
relaxation have been observed. The results for water have been
interpreted by considering two relaxation regions as being due
to adsorbed water. A small proportion (about 20 per cent)
apparently behaves as liquid water, the remainder having an
ice-like character.
In all the ion forms a region of dielectric loss may be
attributed to movement, within the zeolite cavity, of the
interstitial cations. The frequency of occurrence of this
dielectric loss peak is dependent on the cation, the adsorbate
and the adsorbate content. Temperature variation of this peak shows a linear relationship between the activation energy of
the process and the radius of the cation concerned. The
behaviour of this activation energy with temperature appears
to imply a type of phase transition (freezing) of the adsorbed
water.
Dielectric isotherms for 5-A/I:I20, 5-A/NH3 and 5-A/S0
2
have been obtained and the results discussed in terms of the
available smtes for adsorption. The isotherms indicate that
a large proportion of 802 appears to have a relaxation time
similar to that in the bulk liquid. The different behaviour of
H20 and NH3 is attributed to the hydrogen bonding capabilities
of the latter.
Some variation of the dielectric properties of 4-A
between different batches has been observed; this is attributed
to varying amounts of sodium aluminate in the material.
Values of apparent d.c. conductivity obtained during
the course of the dielectric study, while in agreement with
previous dielectric studies, are notably different from some
published conductivity values.
While the investigation shows that studies of dielectric
relaxation can give considerable information concerning the
adsorbate and adsorbent, results of molecular behaviour by other
techniques are required to enable a convincing interpretation
of the results to be made; these are not available.
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