High-Capacity Sulfur Dioxide Absorbents for Diesel Emissions Control

High-capacity sulfur dioxide absorbents based on manganese oxide octahedral molecular sieves (OMS) have been identified. These materials are based on MnO 6 octahedra sharing faces and edges to form various tunnel structures (2 × 2, 2 × 3, 2 × 4, 3 × 3) differentiated by the number of octahedra on a side. The SO 2 capacities of these materials, measured at 325 °C with a feed containing 250 ppmv SO 2 in air, are as high as 70 wt % (w/w), remarkably higher than conventional metal oxide based SO 2 absorbents. Among the OMS materials, the 2 × 2 member, cryptomelane, exhibits the highest capacity and absorption rate. Its SO 2 absorption behavior has been further characterized as a function of temperature, space velocity, and feed composition. The dominant pathway for SO 2 absorption is through the oxidation of SO 2 to SO 3 by Mn 4+ followed by SO 3 reaction with Mn 2+ to form MnSO 4 . Absorption can occur in the absence of gas-phase oxygen, with a moderate loss in overall capacity. The inclusion of gases NO and CO in the feed does not reduce SO 2 capacity. The absorption capacity decreases at high space velocity and low absorption temperature. A color change of cryptomelane from black to yellow-brown after SO 2 absorption can be used as an indicator of absorption progress. Cryptomelane can be synthesized using MnSO 4 as a reagent. Therefore, after full SO 2 absorption, the product MnSO 4 can be reused as raw material for a subsequent cryptomelane synthesis. Cryptomelane has a similarly high capacity toward SO 3; therefore, it can be used for removal of all SO x species generated from a variety of combustion sources. Cryptomelane may find application as a replaceable absorbent for the removal of SO x from diesel truck exhaust, protecting downstream emissions control devices such as particulate filters and NO x traps.