Surface groups chromene

Reactions of the hydronium ion with porous active carbon have been investigated in aqueous systems. Hydronium-ion activity, specific-anion concentration, and carbon dosage have been among the major variables studied. Rates of reaction have been found to be limited by pore diffusion, as partially verified by activation energies of —(2 to 3) kcal. per mole-deg. The results can be interpreted partly in terms of a reaction of the hydronium ion and dissolved oxygen with a surface benzpyran (chromene) group to produce hydrogen peroxide and a surface benzopyrylium (carbonium) ion with a sorbed anion, and partly in terms of physical sorption of the acid on the carbon surface. 

Several studies have been directed toward examination of the interaction of acids and bases with active carbons (I, 8, 10, 17, 18, 19). Boehm (3), Garten and Weiss (9), and Snoeyink and Weber (21) have presented reviews on the subject. Garten and Weiss (8, 9, 10) have shown that acid and alkali sorption can be related to surface functional groups which form during the preparation of the carbon. Alkali sorption occurs principally on carbons activated at temperatures near 400°C., and is attributed to the presence of phenolic and lactone functional groups on the carbon surface. Carbons which sorb acid usually are activated at temperatures near 1000°C. the acid reaction in this case is assumed to take place with chromene (benzpyran) structures on the surface. 

Garten and Weiss (8) have postulated that strong acids react with chromene functional groups on the active carbon surface in the following manner ... 

Figure 2.1 Acidic and basic oxygen-containing functionalities of carbon surface (a) carboxyl groups, (b) lactone, (c) hydroxyl, (d) carbonyl, (e) quinone, (f) ether, (g) pyrone, (h) carboxylic anhydride, (i) chromene, (j) lactol, and (k) % electron density on carbon basal planes.

The acidic groups that have been postulated as carboxyls, lactones, and phenols render the carbon surface polar and hydrophilic, and the basic groups have been postulated as pyrones and chromenes structnres. 

---