Evaporation-induced self-assembly synthesis of nanostructured alumina-based mixed metal oxides with tailored porosity

Abstract

The one-pot synthesis of nanostructured ternary mixed oxides is challenging due to the heterogeneous nature of the hydrolysis and condensation processes of all metal oxide precursors. In addition, the solvents and additives used can affect these processes too. Herein, we report the effect of different solvents (ethanol, 1- and 2-propanol, or butanol) and additives (citric acid or 1,3,5-triisopropylbenzene) used on the formation of binary and ternary alumina-based oxides, NiO-Al2O3, NiO-TiO2-Al2O3, and NiO-ZrO2-Al2O3 in the presence of triblock copolymer Pluronic P123 used as a soft template. For comparison, this study includes also mesoporous Al2O3 prepared at the same conditions. It is shown that the kinetics of hydrolysis and condensation processes of metal alkoxides, and consequently, the properties of the resulting alumina-based mixed metal oxides are controllable by varying the solvents used. The use of propanol instead of ethanol affords mixed metal oxides with improved degree of mesostructure uniformity as evidenced by narrower pore size distributions. This finding is attributed to the smaller exchange of propanol with propoxide groups in Al(OPri)3, Ti(OPri)4, and Zr(OPrn)4 which results in an enhanced stability of the formed mesophase. Furthermore, the addition of citric acid leads to smaller pore sizes without significant changes in the textural properties of metal oxides, while addition of 1,3,5-triisopropylbenzene affords oxides with enlarged pores. The mixed metal oxides studied feature large specific surface areas (310–460 m2·g−1), large pore volumes (0.5–0.75 cm3·g−1), and uniform mesopores with widths ranging from 5 to 18 nm. Solid-state kinetic studies performed by thermal analysis using both isoconversional and model fitting methods reveal the complexity of the mesophase formation. The thermal decomposition of condensed oxoalkoxide species into metal oxides is mainly diffusion-controlled and affected by the type of solvent used too. This study shows that there are tremendous opportunities in tailoring porous structures of mixed metal oxides prepared via evaporation induced self-assembly (EISA) by selecting proper solvents and additives, and thermal treatment.