Applied Catalysis A: General 406 (2011) 59–62 Contents lists available at ScienceDirect Applied Catalysis A: General j ourna l ho me page: www.elsevier.com/locate/apcata Influence of framework composition over SAPO-34 and MeAPSO-34 acidity Fagner Calegario Sena, Bianca Figueirôa de Souza, Núbia Caroline de Almeida, Jonathan Simonace Cardoso, Lindoval Domiciano Fernandes ∗ Departamento de Engenharia Qumica, Universidade Federal Rural do Rio de Janeiro, BR465, km7, Seropédica - RJ, 23890-000, Brazil a r t i c l e i n f o Article history: Received 28 April 2011 Received in revised form 4 August 2011 Accepted 9 August 2011 Available online 19 August 2011 Keywords: Silicoaluminophosphates Low pore size molecular sieves a b s t r a c t SAPO-34 and MeAPSO-34 samples were prepared based on a standard literature procedure [1]. In fact, several kinds of transition metal were incorporated in the SAPO-34 structure, during the synthesis step, beginning with the same gel. As a result, samples of SAPO-34 and MeAPSO-34, presenting good crys- tallinity and also having similar surface area and microporosity, were obtained. It was observed that acid sites concentration depended strongly on their metal content. Metal incorporation in the SAPO-34 structure generally increased acid sites concentration but did not affect the acid strength distribution. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Crystalline microporous aluminophosphates (named AlPO 4 -n), formed by AlO 4 and PO 4 tetrahedra connected by their vertices, form a molecular sieves family with adsorption capacity similar to that presented by the zeolites. However, its application in het- erogeneous catalysis is limited by the fact that their structures are electrically neutral and, consequently, do not present active sites. However, the isomorphous substitution of phosphorous atoms by silicon atoms results in a negatively charged framework, requir- ing the presence of extra-lattice cations to compensate for these charges [2]. When the compensation cation is the hydrogen, the resulting material presents Brönsted acidity, being active for acid catalysis purposes. The isomorphous substitution of aluminum atoms by transition metal atoms (like Fe, Ni, Co and Mn) results in materials presenting different acidity properties called MeAPSO-n [1,3–13]. Three mechanisms were initially proposed for the incorporation of silicon atoms in aluminophosphates crystalline framework [14]. In the first one, called SM1, an aluminum atom is replaced by a sil- icon atom, forming a positively charged crystalline framework. In the second one, called SM2, a phosphorous atom is replaced by a silicon atom resulting in a negatively charged framework. In the last one, called SM3, two silicon atoms replace one aluminum and one phosphorous atoms, resulting in an electrically neutral structure. As so far only negativally charged frameworks could be synthesized, the SM1 mechanism seems not to occur [12]. The replacement a ∗ Corresponding author. Fax: +55 21 3787 3742. E-mail address: lindoval@ufrrj.br (L.D. Fernandes). pair of aluminum and phophosrous atoms does not take place due to the instability of the Si–O–P bond. Martens et al. [15] observed by 29 Si MAS NMR that samples of SAPO-5, SAPO-11 and SAPO-37 contains aluminosilicate (SA) domains, where silicon atoms is con- centrated, next to silicoaluminophosphate (SAPO) domains, where all phosphorous atoms is located. In principle, Brønsted acid sites can be present in the SA parts, in the SAPO parts, and at the bound- aries between these different domains [15]. The SAPO part can be considered to be derived from an AlPO 4 framework, with a strict alternation of aluminum and phosphorous atoms, in with part of the phosphorous atoms are replaced with silicon. Thus each sili- con atom in the SAPO domains appears in Si(4Al) environment and generates a SAPO-type Brønsted acid site. The SA part of the crystal does not contain phosphorous. Each aluminum atom introduced a net negative lattice charge and one SA-type Brønsted acid site. On the SA side of the interphases between SA and SAPO domains, sil- icon environments are present and such interphase-type Brønsted acid site may be important for acid catalysis [15]. A transition metal atom incorporation in the silicoaluminophos- phate framework influence the silicon coordination states and consequently the acid properties of the resulting MeAPSO samples [7,16,17]. Recently, the microporous crystalline silicoaluminophosphates and their derivatives, presenting chabazite structure, i.e. CHA type (FeAPSO-34, CoAPSO-34, NiAPSO-34 and MnAPSO-34), have been investigated as selective catalysts to methanol to olefins conversion [7–9,11,18–23]. These selectivity is due to pore size of about 3.8 ˚ A of the CHA type structure, which constrains the ocorrence of sec- ondary reactions that can leave to heavier hydrocarbons molecules. In an earlier work, a SAPO-34 and MeAPSO-34 synthesis and characterization study was done [24]. In that work, it was observed 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.08.010