Studies on MeAPSO-5: An investigation of physicochemical and acidic properties Alcineia C. Oliveira a, * , Nadine Essayem b , Alain Tuel b , Jean-Marc Clacens b , Younes Ben Taarit b a University Federal of Ceara ´, Campus do Pici-Bloco 940, Department of Chemical Analysis and Physico-Chemistry, Lab. Langmur de Adsorc ¸a ˜o e Cata ´lise, Fortaleza, Ceara ´, Brazil b Institut de recherches sur la catalyse et l’environnement de Lyon, IRCELYON, CNRS, 2 Av. Albert Einstein, 69626, Villeurbanne, France Available online 12 February 2008 Abstract 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, N 2 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 Cr 2 O 3 and Fe 2 O 3 , 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-C 4 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. # 2007 Elsevier B.V. All rights reserved. Keywords: SAPO-5; Chromium; Acid properties; Iron; n-Butane 1. Introduction Silicoaluminophosphates (SAPOs) were first reported by Union Carbide in 1984 [1,2] and they have been widely studied since then. These microporous solids exhibit not only characteristics of zeolites, but also novel physicochemical traits that are related to their unique compositions [3–5]. SAPO-5 (IUPAC structure type AFI) with 12-ring straight channels is the interest of many researchers because substitu- tion of silicon in a neutral aluminophosphate generates acid sites of different strengths, which may provide a much stronger interaction with the hydrocarbons [2,5]. The acid properties of SAPO strictly depend on the Si content, sitting and ordering in the lattice [5–8]. The literature reports the theoretical possible isomorphous substitution mechanisms (SM) in SAPO-5 materials [6]: Si substitutes Al (SM1), Si replaces P (SM2) and also, two Si atoms can substitute one Al atom and one P simultaneously (SM3). SM1 provides a framework of possible changes that have to be neutralized by anions. However, the SMl substitution cannot occur practically because Si–O–P linkages are considered unlikely [8,9]. In contrast, by SM2, a PO 2 + tetrahedron is replaced by SiO 2 then, the cation exchange capacity is generated because the framework becomes negatively charged. The potential Brønsted site per Si atom should be generated by SM2 mechanism because this substitution introduces one negative charge, which can be compensated by a proton to form an acid site [6,7]. The substitution of two Si atoms for a pair of Al and P (SM3) atoms cannot occur independently because of the formation of undesirable Si–O–P linkages like SMl. Indeed, the simultaneous substitution of one AlO 2 À and one adjacent PO 2 + tetrahedron by (SM3) does not involve changes in the lattice charge. However, SM3 can take place in combination with SM2 to avoid the formation of Si–O–P [9,10]. Although www.elsevier.com/locate/cattod Available online at www.sciencedirect.com Catalysis Today 133–135 (2008) 56–62 * Corresponding author. Tel.: +55 85 3366 90 51; fax: +55 85 3366 99 82. E-mail address: alcineia@ufc.br (A.C. Oliveira). 0920-5861/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.12.105