Vermiculita modificada pela troca por cátions cromo (III): síntese e atividade catalítica na decomposição do peróxido de hidrogênio.

Abstract

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.