Development of mesoporosity in activated carbons via coal modification using Ca- and Fe-exchange

Abstract

The calcium and iron loading of coal by ion exchange was proposed as an efficient pretreatment to produce mesoporous activated carbons by carbonization and subsequent steam activation. Lignite (lignite A) was used as a kind of reference precursor due to natural ion-exchangeable ability. The incorporation of Fe into the Ca-loaded lignite led to the activated carbon (AC) with better-developed mesoporosity than produced from Ca- or Fe-exchanged lignite. For gas coal (hvBb), prior to calcium loading, two different treatments, i.e. oxidation with nitric acid and sulfonation with sulfuric acid, were applied to generate the ion-exchangeable groups on the coal surface. The impact of acid pretreatments on the coal structure and the porous texture of resultant activated carbons was determined. The sulfuric acid treatment resulted in a higher cation-exchangeable capacity compared with the use of nitric acid. As a result, the Ca loading level of sulfonated gas coal was higher than that of the oxidized coal. At a solution pH around 2.5, the loading of both Ca-preexchanged coals with iron gave approximately 1 wt.% of a highly dispersed iron in coal. The ACs from Ca/Fe-loaded lignite and gas coal are characterized by mesopore volumes between 0.405 and 0.496 cm 3 g −1 and very high mesopore fraction, in the range of 77–83%. For the AC from loaded sulfonated gas coal the mesopores are mainly distributed at 2–10 nm, whereas the AC from loaded oxidized coal shows a predominant contribution of pores between 10 and 50 nm. Acid washing of resultant mesoporous activated carbons resulted in an increase of mesopore volume, by 20–25%.