Preparation of fibrous porous materials by chemical activation: 1. ZnCl2 activation of polymer-coated fibers

Fibrous porous materials (FPMs) have been prepared by coating a glass fiber with a solution of polymer and ZnCl₂, followed by stabilization in air and heat treatment in N₂. The ZnCl₂ was then removed by washing with D.I. water and HCl. Four kinds of polymers, a phenolic resin, polyacrylonitrile, poly(vinyl alcohol) and cellulose, were used to prepare solutions with ZnCl₂. The results showed that ZnCl₂ acts as a dehydration agent to promote the thermal cross-linking of polymer at a much lower temperature, leading to FPMs having much higher char yields and very high surface areas. The porosity was created in part by dissolution of the ZnCl₂ left in the charred coating. The activation temperature and ZnCl₂ concentration play an important role in porosity development. In the early stage of heating, the specific surface area, micropore and mesopore volumes increased with increasing temperature. As the activation temperature increases above 450°C, ZnCl₂ begins to volatilize out of the coating, and further charring and aromatization of the coating results in a dimensional contraction leading to a decrease in the micropore and mesopore volumes. It was observed that the specific surface area, as well as micropore and mesopore volumes, increased with increasing ZnCl₂ concentration. Pore size analysis showed that the FPMs activated with ZnCl₂ were mainly microporous. For FPMs activated with concentrated ZnCl₂ (66 wt.%), there is a remarkable and large mesopore size distribution in addition to the typical micropore size distribution. In addition, such FPMs have very high specific surface area, more than 1600 for PAN-based and 2500 m²/g of coating for cellulose-based FPMs.