Structural Evolution of Polyimide-Derived Carbon during Phosphoric Acid Activation

Abstract

Carbon adsorbents were obtained by carbonization of polyimide polymer with and without the presence of phosphoric acid at temperatures in the range of 400–1000 °C. Carbons produced in the presence of phosphoric acid have been demonstrated to contain up to 13.2% phosphorus. The structure of phosphorus-containing compounds was investigated by XPS and 31P MAS NMR methods. Deconvolution of the P 2p peak with variable binding energy showed the presence of only phosphates/polyphosphates. However, a low value of the O/P ratio is an indirect indication of the possible presence of phosphonates. A 31P MAS NMR study revealed the existence of several kinds of phosphates as well as a minor quantity (1–9%) of phosphonates. All discovered phosphorus-containing compounds are acidic and therefore give carbon the ability to absorb metal cations. The study of copper ion adsorption demonstrated that phosphorus-containing carbon shows a significant adsorption capability even in extremely acidic conditions. At pH 3–6, phosphorus-containing carbon may completely remove copper from the aqueous solution. Phosphorus-containing carbon has a higher adsorption capacity for copper ions than ion exchange resins with carboxyl or sulfo groups.

Keywords

• phosphorus-containing carbon
• XPS
• ³¹P MAS NMR
• surface groups
• acid-base properties
• metal ion binding