MAIA, D. F.; http://lattes.cnpq.br/3667151280922633; MAIA, Divanira Ferreira.
Resumo:
In the present work the potential use of physically and chemically modified
vermiculite as a catalyst for chemical processes is verified.
The natural vermiculite was submitted to granulomere separation, thermal
exfoliation and exchanged natural cations with those of chromium, cobalt, sodium and
hydrogen. The chromium and the cobalt cation exchanged samples were calcined at
different temperatures. The effect of theses modifications of the vermiculite was studied
making use of a model reaction of decomposition of hydrogen peroxide. The order of
reaction and apparent energy of activation were determined by studding the influence of
concentration and temperature respectively.
The results showed that particles of smaller size and thermal exfoliation
increased the catalytic activity because of increase in the surface area of the vermiculite.
In the first case this increase is due to the decrease in the size of the particles while in
the second it is due to increase in the porosity. In relation to the exchanged cations one
can perceive that the transition elements (chromium and cobalt ) are more active while
sodium cation makes the homogeneous decomposition by increasing the alkalinity of the
medium and the proton provoked a decrease of the activity due to vermiculite lixiviation.
The effect of exchanged chromium cation loading was noticeable, and was observed that
up to 140meq of chromium the activity increased with the amount of the chromium and
above this it decreased due to oligomer formation. In relation to calcination (between
200-800°C) a maximum of the activity appeared for 400°C which could be due to
existence of the different active oxidation state of the chromium as indicated in literature.
The kinetic parameter was determined to be of the first order of reaction and
the apparent energy of activation was in accordance with the catalytic behaviour of the
samples.