Effect of Mg²⁺ Ions on the Formation of Todorokite Type Manganese Oxide Octahedral Molecular Sieves

Abstract

The readily available permanganate, KMnO 4 , was successfully utilized in the synthesis of manganese oxide octahedral molecular sieve (OMS-1) with the todorokite type structure. The magnesium concentration in the starting materials was systematically varied to study the synthetic chemistry of Mg-OMS-1. The techniques such as powder XRD, ICP, TGA, TPD, and redox titrations were all applied to characterize the as-synthesized Mg-OMS-1 samples. The correlation between the amount of magnesium in the starting materials and the properties of Mg-OMS-1 results in differences in (1) colors, (2) crystalline purity, and (3) thermal stabilities. Magnesium(II) ions are believed to be doped into the framework of the OMS-1 structure, but the major amount of Mg 2+ mainly behaves as a template and is located in the tunnels of OMS-1. Two interesting Mg 2+ concentration windows have been observed, one for the starting materials and another for the Mg 2+ ions in the framework of Mg-OMS-1 which cover the magnesium concentration range where good quality Mg-OMS-1 can be formed.

Liquid-phase epoxidation of olefins by manganese oxide octahedral molecular sieves

“…TBHP, being a very bulky oxidant, does not get into the channels of OMS-2 as readily as H 2 O 2 because the decomposition is slow and there could be a chance for the formation of the oxo-metal complex, which provides the necessary catalytic active sites for oxidation of cyclooctene. The OMS-2 material has Lewis acid sites, which are mainly caused by vacancies of oxygen atoms, located at vertices of octahedral structural units of OMS-2 [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Lewis base sites are due to the lone electron pairs on the oxygen atoms of TBHP.…”

“…These are mixed valent materials, which are good semiconductors and oxidation catalysts [14][15][16][17][18]. These synthetic mixed valent manganese oxide materials have been modeled after naturally occurring manganese nodules.…”

“…The average manganese oxidation state OMS-2 is 3.8 due to the presence of a mixture of Mn 4+ , Mn 3+ , and Mn 2+ ions [20][21][22]. The mixed valency of OMS-2 along with the one-dimensional tunnel structure is capable of exhibiting interesting oxidation properties [14][15][16][17][18]. Pore-size distribution studies show that a wide range of micropores are present with the diameters of the pores ranging from ∼ 4.5 to 6.5 Å [23].…”

Synthesis of manganese oxide octahedral molecular sieves containing cobalt, nickel, or magnesium, and the catalytic properties for hydration of acrylonitrile

“…In the last two decades, there have been many reports of todorokite-type octahedral molecular sieves prepared by using magnesium salts and/or Mg (MnO 4 ) 2 [2,[10][11][12]. Some of divalent magnesium cations exist in the framework and play a role in maintaining the stable structure of these manganese oxide compounds [31]. However, the todorokite catalysts with magnesium ions can result in undesired by-products for some reactions because of the base catalysis of magnesium species.…”

Studies on the formation of todorokite-type manganese oxide with different crystalline birnessites by Mg2+-templating reaction

“…Suib and coworkers subsequently reported the preparation of a thermally stable TodMO by hydrothermal treatment of layer-structured manganese oxide, prepared by the reaction of MnO − 4 and Mn 2+ [9][10][11][12][13][14]. The catalytic, adsorptive, magnetic, and electrochemical properties of TodMO were studied in detail by the same authors [15,16]. Feng et al described another route to TodMO using the oxidation of Mn(OH) 2 with H 2 O 2 to prepare the Na-birnessite (Na-BirMO) precursor; the metal ion extraction/insertion reaction of TodMO in aqueous solutions was also studied [17,18].…”