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In this study, ultra-small MCM-41 mesoporous solids (<30 nm) were synthesized using colloidal silica and cesium hydroxide as cheap silica source and mineralizer, respectively. Four samples with a SiO 2 /Al 2 O 3 ratio of 5, 20, 30 and ∞ (designated as CsM-5, CsM-20, CsM-30 and CsM-∞, respectively) have been prepared. The effects of SiO 2 /Al 2 O 3 ratio on the formation, physico-chemical and catalytic properties of CsMCM-41 nanoparticles were studied. The results show that incorporation of high content of Al atoms into the framework reduces the ordering of the mesostructure, and hence the porosity of the solids decreases. By using CsOH as mineralizer, the morphology and the size of CsMCM-41 were also tuned from 3-D nanospheres (CsM-5: 29 nm and CsM-20: 27 nm) to hollow nanospheres with very thin walls (CsM-30: 25 nm, CsM-∞: 20 nm) when Al content was decreased. The basicity of CsMCM-41 nanoparticles was also affected by the Al and Cs contents. Hence, their catalytic behavior in the base-catalyzed reactions such as aldol condensation of heptanal with benzaldehyde, and Perkin condensation of benzaldehyde with acetic anhydride was evaluated under microwave heating conditions. CsM-20 displays the best catalytic activities among the samples studied, achieving 87.2% conversion and 78.4% selectivity to jasminaldehyde under optimum reaction conditions, while it also shows comparable catalytic performance as NaOH homogeneous catalyst in the Perkin condensation reaction under non-microwave instant heating conditions. Image 1 • One-pot synthesis of ultra-small AlMCM-41 nanoparticles (<30 nm) is reported. • Colloidal silica and cesium hydroxide are used as cheap silica source and mineralizer. • The effects of Al incorporation on MCM-41 nanoparticles are studied. • The changes in morphological and porosity properties of nanosolids are observed. • The nanoparticles exhibit good catalytic performance in aldol condensation reaction. [ABSTRACT FROM AUTHOR]