Activated carbon-mediated base hydrolysis of alkyl bromides

Abstract

Activated carbon (AC) is widely used as an adsorbent in water and air purification, but recent studies show that AC can also mediate chemical reactions at ambient temperature, including electron-transfer, redox, free radical, and dehydrohalogenation. Here, we expand the repertoire of such reactions to SN2 base hydrolysis of alkyl bromides. Detailed studies were carried out on the quarantine and pre-shipment fumigant, methyl bromide (MeBr), whose removal from fumigation vent streams is sought due to its ozone-depleting potential. Mixed with a 1M solution of NaOH, ACs are effective adsorbents, but also participate in hydrolysis of MeBr to bromide, methanol, and dimethyl ether. MeBr decay is first order in MeBr and obeys a two-term rate law corresponding to aqueous- and adsorbed-state reactions. The adsorbed-state reaction is, i) unassisted by surface groups, as shown by cycling experiments; ii) 0.83 order in aqueous hydroxide concentration suggesting participation of adsorbed hydroxide; iii) accelerated by pre-adsorption of quaternary ammonium surfactants, which also shift the zeta potential into the positive region; and iv) inhibited by competing inert anions in the order of their chromatographic retention time on ion-exchange resins: Br−<NO3−<ClO4−. The results support an anion exchange mechanism in which hydroxide is attracted to positive sites on the carbon. Normalized to aqueous-phase rate constants, adsorbed-state rate constants for RBr followed the order, R=methyl>ethyl>propyl.