Defining Reactivity of Solid Sorbents: What Is the Most Appropriate Metric?

A series of solid sorbent materials, including alumina (Al2O3), magnesia (MgO), titania (TiO2), silica (SiO2), and carbon, of widely varying physical properties, have been studied as sorbents for two toxic substances: sulfur dioxide gas (SO2) and chemical warfare surrogate 2-chloroethyl ethyl sulfide (2-CEES, ClCH2CH2SCH2CH3). Sorbent surface areas, average pore sizes, pore volumes, surface hydroxyl groups, and nitrogen adsorption-desorption isotherms were measured. Surface areas varied from 18 m(2)/g to over 1000 m2/g, pore volumes from 0.04 to 1.2 cm(3)/g, and pore diameters from 1.7 to 4.9 nm. Breakthrough studies of SO2 and 2-CEES sorption yielded information about the effectiveness of each sorbent. Carbon samples worked well for 2-CEES but not SO2, while silica samples were poor for both. The best MgO and TiO2 samples were good for both SO2 and 2-CEES, and overall, the highest surface area (459 m(2)/g) TiO2 sample was the superior sorbent. The important features for an effective sorbent under the conditions employed are high surface area and high pore volume, possessing isolated surface -OH groups, mesoporous nature, and a polar surface (Lewis base and Lewis acid sites).