Porosity of acid activated Porto Santo bentonites assessed by n-nonane pre-adsorption and nitrogen adsorption

In previous work, we found that the materials produced by acid treatment of a bentonite collected at Serra de Dentro (Porto Santo Island, Portugal) exhibited high catalytic activity in the oxidation of limonene to p-cymene. Although a considerable increase in surface area was observed for the more active samples, a detailed description of the porosity is necessary to understand its role in the catalytic performance. Unfortunately, due to the complexity of the porosity of clay materials, the information provided by the usual methods of isotherm analysis, such as the alfa-s and t methods, is not straightforward. In order to overcome this problem, we have explored the possibility of using the n-nonane pre-adsorption method which was first introduced by Gregg and Langford and proved to be very useful for the characterisation of the micropore structure of activated carbons. We demonstrate that the n-nonane pre-adsorption method can be applied with success also to clay materials, providing useful information to elucidate the complex porosity of acid activated clay materials, with special reference to primary and secondary microporosity