Pore Structure−Chemical Composition Interactions of New High Surface Area Manganese Based Mesoporous Materials. Materials Preparation, Characterization, and Catalytic Activity

The present article deals with pore structure−chemical composition interactions of new high surface area manganese based porous mixed oxides (MANPOs). These mixed oxidic materials were prepared by the dropwise addition of a solution containing metal nitrate into an acetonic solution containing the trinuclear complex of manganese [Mn 3 O(CH 3 COO) 6 (pyr) 3 ]ClO 4, which is water sensitive. The CPSM model (corrugated pore structure model) was employed in pore structure investigations of the MANPO materials containing Al, Fe, La, La + Ce, and La + Sr. Successful CPSM simulations of nitrogen sorption hysteresis loops for the MANPO solids enabled the evaluation of intrinsic pore size distributions (PSDs) and tortuosity factors. The latter vary over the approximate range τ CPSM = 3.0−5.7 (i.e. τ CPSM ≈ 3 for materials containing Al or Fe and τ CPSM = 4.7−5.7 for those containing La, La + Ce, or La + Sr). These values are typical of porous catalysts and are higher than those reported for MCM-41 materials (ca. τ CPSM = 1.00−2.35) and anodic aluminum oxide films (e.g. τ CPSM = 2.60) . The BET surface areas of the MANPO materials vary over the approximate range 180−900 m 2 /g, being higher than those reported for manganese oxide mesopore solids (MOMS), for example 59−170 m 2 /g. Intrinsic PSD predictions for the MANPO solids are of bimodal type with mean pore sizes systematically higher than the respective mean hydraulic diameters. The addition of Fe and La + Ce caused a PSD shift toward the micropore region. Such MANPO materials are amorphous after heating to 500 °C (except for the ones containing Fe) and extremely efficient catalysts for redox reactions such as lean de-NO x applications and NO−CO conversion, probably because of the extremely high dispersion of separate nanophases into the solid.