A new approach to the study of the reactivity of solid-acid catalysts: The application of constant rate thermal analysis to the desorption and surface reaction of isopropylamine from NaY and HY zeolites

A new and potentially valuable approach to the study of the acidity and reactivity of the surfaces of solid acid materials is described. The technique uses a combination of constant rate thermal analysis (CRTA) with the rate jump technique (CRTA-RJ) for temperature programmed desorption (TPD) and reaction (TPR_x). CRTA, where the sample temperature is changed to force the rate of reaction to remain constant through a process, offers a number of advantages over conventional linear heating thermal methods by minimizing temperature and concentration gradients in the sample and reducing the effects of diffusion. CRTA and CRTA-RJ were applied to the TPD and TPR_x of isopropylamine from NaY zeolite and its acidic form, HY. Activation energies of both the physical desorption of the amine and the surface-catalyzed Hoffman elimination reaction were calculated as a function of surface coverage and the advantages and limitations of the techniques are discussed. Comparisons are made with analogous TPD experiments using conventional linear heating methods. It is concluded that the TPR_x approach reflects the reactivity of strong acidic catalytic sites directly and so usefully complements base adsorption microcalorimetry in characterizing acid solid catalysts.