Modified carbons for enhanced nucleophilic substitution reactions of adsorbed methyl bromide

Abstract

Activated carbons are highly-effective adsorbents for the passive removal of organic pollutants from aqueous and gaseous phases. However, the pollutant remains chemically unchanged and poorly available to dissolved reagents. In this study, we developed quaternary ammonium (QA)-modified carbon adsorbent-catalysts that can catalyze direct reactions between adsorbed molecules and anionic reagents, thereby permitting a trap-and-degrade strategy. The concept was applied to base hydrolysis of methyl bromide (CH3Br, MeBr), a quarantine and pre-shipment fumigant of concern as an ozone-depleting gas. Modifications of the carbons included irreversible adsorption of the cationic polyelectrolyte, poly (diallyldimethylammonium chloride) (PDADMAC), and covalent grafting of QA groups via precursors (Quat188 and Quab360). In mixtures of the carbon in 1 M NaOH at 55 °C under conditions where hydrolysis in the aqueous phase was negligible, first-order rate constants were increased by up to a factor of 7.4 compared to the unmodified carbon. Combined PDADMAC/Quat188 modification was superior to the corresponding individual modification. Rate acceleration was due to attraction of hydroxide ions to the newly-created anion exchange sites. This is supported by effects of QA modification on anion exchange capacity, point of zero charge, and isoelectric point, as well as by the results of competition experiments with otherwise-inert monovalent anions. The modified catalysts are robust for at least five hydrolysis cycles. QA modification also greatly enhances hydrolysis at unadjusted pH (8.6–10.2) and nucleophilic reaction by thiosulfate. QA modification of carbons is a promising approach for in situ trapping and degrading MeBr or other hydrophobic compounds that are reactive toward anionic reagents.