Tesis
Propriedades de zeólitas x formadas por partículas micro- e nanométricas contendo cátions alquilamônio lineares
Fecha
2016-02-15Registro en:
Autor
Motta, Ingrid Lopes
Institución
Resumen
Nanosized NaX zeolites (FAU) (crystallite size = 16 nm; Si/Al ratio = 1,5) were
synthesized and modified through ion exchange with linear alkylammonium cations containing
from one to six carbons, with the intent to activate basic sites. In this work, commercial
microsized zeolites (Aldrich) were also ion exchanged for comparison matters and all samples
were characterized. Scanning electron microscopy analysis showed that both micro- and
nanosized zeolites containing sodium portray polycrystalline particles: the former presents
octahedral habit and particle size of 2952 nm and the latter presents an indefinite habit and
particle size of 426 nm. Among the ion exchanged samples, none presented complete exchange
of the Na+ cations due to steric hindrance. Nanosized zeolites presented higher exchange
degrees than the microsized ones, which is due to higher accessibility to the exchange sites.
Such factors also explain the exchange degrees’ reduction with the cations’ chain length, which
is less abrupt among the nanosized samples. This happens due to the nanosized samples’ higher
interparticular porosity, which has also allowed their micropore volumes’ constant decrease,
differently from the microsized samples, which faced a minimum value with butylammonium.
When the cations’ length is increased, the unit cell expands in comparison to the zeolites
containing sodium since bulkier cations cause variations in the length and angle of the of the
O-Si-O and O-Al-O bonds. Through thermogravimetric analysis in oxidant atmosphere, it was
observed that the longer the carbon chain length, the lower is the mass loss, which evidences
the lower diffusional limitations. The same was observed in inert atmosphere, which indicates
that the presence of O2 does not influence the thermal events. Nanosized zeolites illustrate
smoother mass loss profiles, which marks smaller diffusional limitations. The zeolitic materials
were catalytically evaluated along the Knoevenagel condensation between butyraldehyde and
ethyl cyanoacetate (3% m/m catalyst) during 1 h. The nanometric zeolites presented higher
conversions and higher reaction rates when t = 0 than the microcrystalline ones, confirming the
advantages of nanosized materials.