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

A series of solid sorbent materials, including alumina (Al 2 O 3 ), magnesia (MgO), titania (TiO 2 ), silica (SiO 2 ), and carbon, of widely varying physical properties, have been studied as sorbents for two toxic substances: sulfur dioxide gas (SO 2 ) and chemical warfare surrogate 2-chloroethyl ethyl sulfide (2-CEES, ClCH 2 CH 2 SCH 2 CH 3 ). 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 m 2 /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 SO 2 and 2-CEES sorption yielded information about the effectiveness of each sorbent. Carbon samples worked well for 2-CEES but not SO 2, while silica samples were poor for both. The best MgO and TiO 2 samples were good for both SO 2 and 2-CEES, and overall, the highest surface area (459 m 2 /g) TiO 2 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).