Studies on MeAPSO-5: An investigation of physicochemical and acidic properties

This work reports physicochemical and acidic properties of MeAPSO-5 (Me = chromium or iron), regardless their performance in the production of isobutane. Chromium and iron were partially introduced in SAPO-5 by direct hydrothermal synthesis. The solids were characterized by XRD, chemical analysis, N2 adsorption isotherms, EPR, XPS, and calorimetry of ammonia adsorption. Their activity was also evaluated in the isomerization of n-butane. The purity of hydrothermally synthesized SAPO-5 was identified in all solids. EPR and XPS showed that chromium and iron exist as Cr2O3 and Fe2O3, respectively, mostly distributed on the solid surface. However, no reflections characteristic of these oxides were found in the XRD patterns. The chromium and iron cause different effects on the acidity: chromium increases the solid acidity whereas iron reduces drastically the acidic features. While distinct activity were observed between CrSAPO-5 and the FeSAPO-5 in n-butane isomerization, it is noteworthy that the pure SAPO-5 is not active in the reaction due to the too low strength of its acid sites. Results suggest that the n-C4 isomerization activity of CrSAPO-5 may be ascribed to its enhanced acidity while, contrary to what was observed in the case of mordenite or sulfated zirconia, Iron addition to SAPO-5 did not enhance the catalytic activity via a possible participation of redox steps.