Effect of the addition of Sn to zirconia on the acidic properties of the sulfated mixed oxide

Abstract

Zirconium hydroxide, tin hydroxide and the mixed hydroxide of tin and zirconium (1:9) have been prepared, sulfated with a 0.1 Maqueous H ₂ SO ₄ solution and calcinated at 873 K. The chemical composition of these solids has been characterized by chemical analysis, differential thermal analysis (DTA), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX–) scanning transmission electron microscopy (STEM), and their structural and textural properties have been studied by BET surface area and XRD techniques. The nature of the acid sites (Brønsted or Lewis) was characterized by in situ IR study of pyridine adsorption and desorption and their amount was found to depend on the sulfate content. Catalytic properties have been studied for the isomerization reaction of n-butane to isobutane, in the 423 to 523 K range and for propan-2-ol conversion, in the 373 to 473 K range. Addition of Sn to ZrO ₂ (SnO ₂ :ZrO ₂ =1:9) led to a solid solution and enhanced slightly (ca. 30%) the weak acid features of ZrO ₂ and removed the basic properties of pure SnO ₂ which was shown to convert propan-2-ol to acetone. Upon sulfation of ZrO ₂ , SnO ₂ –ZrO ₂ [1:9] and SnO ₂ , the acidity of the oxides was sharply enhanced. SO ₄ ^2- /SnO ₂ , which was inactive for the isomerization reaction, was only 60% less active than SO ₄ ^2- /ZrO ₂ for propan-2-ol dehydration. This shows that sulfation of SnO ₂ generates acid sites of moderate strength. The presence of only 10% of SnO ₂ in ZrO ₂ decreased the reaction rates per SO ₄ ^2- by a factor of six for the initial n-butane isomerization reaction and by a factor of seven for the propan-2-ol dehydration to propene and diisopropyl ether. It is suggested that the presence of Sn decreases the electron acceptor properties of Zr and thus its acidity strength. Since the rate per sulfated species of both reactions decreased in the same proportion, one may consider that on SO ₄ ^2- /ZrO ₂ and SO ₄ ^2- /SnO ₂ –ZrO ₂ samples there is no site of moderate strength able to dehydrate propan-2-ol without isomerising n-butane.