If a conductive mayenite compound having a large specific surface area is obtained, the usefulness thereof in respective applications is remarkably increased. A conductive mayenite compound powder having a conduction electron density of 10.sup.15 cm.sup.3 or more and a specific surface area of 5 m.sup.2g.sup.1 or more is produced by: the following steps: (1) forming a precursor powder by subjecting a mixture of a starting material powder and water to a hydrothermal treatment; (2) forming a mayenite compound powder by heating and dehydrating the precursor powder; (3) forming an activated mayenite compound powder by heating the compound powder in an inert gas atmosphere or in a vacuum; and (4) injecting electrons into the mayenite compound through a reduction treatment by mixing the activated mayenite compound powder with a reducing agent.

The invention related to a material that can stably hold an imide anion (NH.sup.2) therein even in the atmosphere or in a solvent, and a method for synthesizing the material and a use of the material. A mayenite-type compound into which imide anions are incorporated at a concentration of 110.sup.18 cm.sup.3 or more are provided. The mayenite-type compound can be produced by heating a mayenite-type compound including electrons or free oxygen ions in a cage thereof, in liquefied ammonia at 450 to 700 C. and at a pressure of 30 to 100 MPa. The compound has properties such that active imide anions can be easily incorporated into the compound and the active imide anions can be easily released in the form of ammonia from the compound, and the compound has chemical stability.

If a conductive mayenite compound having a large specific surface area is obtained, the usefulness thereof in respective applications is remarkably increased. A conductive mayenite compound powder having a conduction electron density of 10.sup.15 cm.sup.3 or more and a specific surface area of 5 m.sup.2g.sup.1 or more is produced by: (1) a step for forming a precursor powder by subjecting a mixture of a starting material powder and water to a hydrothermal treatment; (2) a step for forming a mayenite compound powder by heating and dehydrating the precursor powder; (3) a step for forming an activated mayenite compound powder by heating the compound powder in an inert gas atmosphere or in a vacuum; and (4) a step for injecting electrons into the mayenite compound through a reduction treatment by mixing the activated mayenite compound powder with a reducing agent.

A particulate, heterogeneous solid CO.sub.2 absorbent composition, comprising decomposition products of Ca.sub.3Al.sub.2O.sub.6 after having been heated to a temperature between 500 C. and 925 C. in the presence of H.sub.2O and CO.sub.2 for a period of time sufficient to allow the Ca.sub.3Al.sub.2O.sub.6 to react and form the particulate, heterogeneous absorbent composition which exhibits a higher concentration of aluminum than calcium in the particle core but a higher concentration of calcium than aluminum at the particle surface. The invention also comprises a method for preparing the particulate, heterogeneous product as well as a method for utilizing the composition for separating CO.sub.2 from a process gas.

A molded sintered body containing a mayenite type compound, an inorganic binder sintered material, and a transition metal, wherein a content of the inorganic binder sintered material is 3 to 30 parts by mass with respect to 100 parts by mass of the molded sintered body, and the molded sintered body has at least one pore peak in each of a pore diameter range of 2.5 to 20 nm and a pore diameter range of 20 to 350 nm. A method for producing the molded sintered body, including mixing a precursor of a mayenite type compound and a raw material of an inorganic binder sintered material to prepare a mixture; molding the mixture to prepare a molded body of the mixture; firing the molded body to prepare a fired product; and supporting a transition metal on the fired product to produce a molded sintered body.