Nanosized sulfated tin oxide (STO) particles dispersed in the micropores of Al-pillared clay were prepared by a precipitation-deposition method followed by heat treatment and sulfation. The Al-pillared clays (Al-P) was prepared by the intercalation of [Al13O4(OH)(24)(H2O)(12)](7+) clusters into the clay interlayers. The physicochemical characteristics of the STO/Al-P material were evaluated by using X-ray diffraction (XRD), infrared (IR), transmission electron microscopy (TEM) and sorptometric studies. XRD results indicate an expansion in the interlayer spacing upon pillaring. The layer structure is found to be retained after the dispersion of the STO particles. The TEM images indicate the presence of uniform STO particles with sizes in the 5-15 nm range. The sorptometric study show a type I isotherm for both the Al-P and STO/Al-P material, indicating the presence of micropores. The pore volume of the parent-pillared clay was found to decrease upon supporting the STO particles. The STO/Al-P material was used as an environmentally benign, recyclable and efficient catalyst for the solvent-free synthesis of 3,4-dihydropyrimidin-2(1H)-ones, thiochromans, with selective protection of aldehyde groups such as 1,1-diacetates and coumarins. The STO/Al-P catalyst was highly active for the synthesis of the above-mentioned molecules. The protocol developed using this catalyst has many advantages over those of the conventional mineral acids used as catalysts. (C) 2007 Elsevier B.V. All rights reserved.