Thermochimica Acta 434 (2005) 81–87 Influence of cations on active sites of the alkaline earth salts of 12-tungstophosphoric acid: microcalorimetric study Lj. Damjanovi ´ c a,∗ , V. Raki ´ c b , U.B. Mio ˇ c a , A. Auroux c a Faculty of Physical Chemistry, Studentski trg 12-16, P.O. Box 137, 11000 Belgrade, Serbia and Montenegro b Faculty of Agriculture, Nemanjina 6, 11080 Zemun, Serbia and Montenegro c Institut de Recherches sur la Catalyse, 2 Av. A. Einstein, 69626 Villeurbanne, France Received 7 November 2004; received in revised form 31 December 2004; accepted 5 January 2005 Available online 1 February 2005 Abstract The acidic properties of the alkaline earth salts of 12-tungstophosphoric acid were investigated using ammonia absorption microcalorimetry. The number and the strength of acid sites of these catalysts as well as differential heats of ammonia absorption were determined. As a result of substitution of two protons with one alkaline earth cation, the decrease of both the total number of acid sites and the number of the strongest acid sites characterized by differential heats higher then 150 kJ/mol, compared to the values found in the case of initial 12-tungstophosphoric acid, were noticed. While all protons seem accessible for ammonia in Mg, Ca and Ba salts, in the case of Sr salt less then stoichiometric ammonia sorption was detected which can be explained by steric effect of these divalent cations. © 2005 Elsevier B.V. All rights reserved. Keywords: Microcalorimetry; Ammonia absorption; Keggin anion; 12-Tungstophosphoric acid; Alkaline earth salts 1. Introduction Keggin-type heteropoly compounds (HPC) have attrac- tive and important characteristics in terms of catalysis: the versatility of these materials and their compatibility with en- vironmentally and economically attractive conditions led to their utilization in a variety of reactions, e.g. they are known to be effective catalysts for various kinds of acid-catalyzed reactions [1–3]. Importantly, solid HPC have been used for synthetic processes in both homogeneous and heterogeneous phases [3–5]. These compounds exhibit high tunability to ex- perimental conditions: HPC have been used in both gas phase [2–5] and liquid phase reactions [6–8]. They consist of het- eropoly anions and counter cations such as H + , Cs + , NH 4 + , etc. When the counter cations are protons, they are called het- eropoly acids (HPA), also known as “super-acids”. An im- portant characteristic of HPA, like 12-tungstophosphoric acid H 3 PW 12 O 40 (WPA), is the presence of acid sites correspond- ∗ Corresponding author. Tel.: +381 113282394; fax: +381 11187133. E-mail address: ljiljana@ffh.bg.ac.yu (Lj. Damjanovi ´ c). ing to those classified as strong acids [9]. Heteropoly acids exhibit Brønsted acidity, significantly higher compared with the acidity of traditional mineral acid catalysts [10], which together with special structural properties has rendered this class of compounds as promising catalysts. HPA may po- tentially replace corrosive liquids in acid-catalyzed reactions thus leading to more environmentally friendly processes [11]. Furthermore, HPA and its derivatives have been shown to be valuable and significantly superior to HPC containing molyb- denum in the removal and conversion of nitrogen oxides [12]. Ammonium salt of WPA has been shown to be effective for the reduction process of NO x to N 2 [12]. Recently, it has been shown that heteropoly acids can act as photocatalysts in the processes of a decomposition of environmentally persistent pollutants [13]. But, characteristics of HPA strongly depend on temperature and relative humidity and it is necessary for industrial processes to have catalysts less sensitive to the sur- roundings [14–16]. The charge balancing protons of HPA can be substituted by cation exchange via salt formation. Substituted cations de- termine the crystallographic, secondary structure of the HPC 0040-6031/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2005.01.004