High-Capacity Sulfur Dioxide Absorbents for Diesel Emissions Control

High-capacity sulfur dioxide absorbents based on manganese oxide octahedral molecular sieves (QMS) have been identified. These materials are based on Mn06 octahedra sharing faces and edges to form various tunnel structures (2x2,2x3,2x4,3x3) differentiated by the number of octahedra on a side. The S02 capacities of these materials, measured at 325 °C with a feed containing 250 ppmv S02 in air, are as high as 70 wt % (w/w), remarkably higher than conventional metal oxide based 802 absorbents. Among the OMS materials, the 2x2 member, cryptomelane, exhibits the highest capacity and absorption rate. Its 802 absorption behavior has been further characterized as a function of temperature, space velocity, and feed composition. The dominant pathway for 802 absorption is through the oxidation of 802 to 803 by Mn^4+ followed by 803 reaction with Mn^2+ to form Mn804. 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 802 capacity. The absorption capacity decreases at high space velocity and low absorption temperature. A color change of cryptomelane from black to yellow-brown after 802 absorption can be used as an indicator of absorption progress. Cryptomelane can be synthesized using Mn804 as a reagent. Therefore, after MI 802 absorption, the product Mn804 can be reused as raw material for a subsequent cryptomelane synthesis, Cryptomelane has a similarly high capacity toward 803; therefore, it can be used for removal of all S0x species generated from a variety of combustion sources. Cryptomelane may find application as a replaceable absorbent for the removal of S0x from diesel truck exhaust, protecting downstream emissions control devices such as particulate filters and NOx traps.