Preparation and applications of catalytic magnetic nanoparticles

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2009.

It is critical to decontaminate organophosphate compounds in large scale economically, including OP pesticides in groundwater system and chemical nerve agents on the battle field. Homogeneous or micellar decomposition systems with various nucleophiles improve reaction rates significantly without affording the recovery and reuse of the nucleophiles. This research focuses on developing functional magnetic particles to carry strong [alpha]--nucleophilic groups, which are able to catalyze the decomposition reaction and can be recycled and reused.The amidoxime modified magnetic particles were prepared first. The original particles were synthesized with the two-step procedure to obtain average particle size of around 80nm for effective capture by high gradient magnetic separation (HGMS). The precursor molecule cyanoacetohydrazide reacted with the free carboxyl groups on the particle surfaces and subsequently the nitrile groups were transformed into amidoxime groups. The modified particles were of similar average hydrodynamic diameter as the original ones and colloidally stable over a wide range of solutin pH. The amidoxime-modified particles accelerated the hydrolysis reaction of p-nitrophenyl acetate (PNPA). They were easily recycled by HGMS without loss of reactivity. Higher reactivity of the particle system than homogenous amidoxime systems was attributed to the increased concentration of the substrate on the particle surface due to the presence of hydrophobic centers using pseudo-phase exchange model.Stronger nucleophilic groups, hydroxamic acid, were then attached on the particle surfaces. Original particles were prepared with unsaturated carboxylic acid as the second coating in the two-step procedure. The acrylamide monomers were copolymerized with the second coating and the attached amide groups were converted into the hydroxamic acid groups.

(cont.) The reaction was very efficient. Crosslinking increased the particle size to 200nm and therefore the particles were effectively captured by HGMS. The modified particles significantly accelerated the hydrolysis reaction of PNPA. They were five times more reactive than the amidoxime modified particles based on the same weight of materials. The acetylated particles were only partially regenerated due to the Lossen rearrangement of the acetylated hydroxamic acid groups.During the hydrolysis of OP substrates, including diisopropyl fluorophosphate, methylparaoxon and ethyl-paraoxon, the added a-nucleophiles, 2-PAM and acetohydroxamic acid, only attacked the phosphorus atom to substitute the p-nitrophenol groups in methyland ethyl- paraoxon and the fluoride ions in DFP through second order nucleophilic substitution. Reactions between all three substrates and both nucleophiles yielded the same hydrolysis products as the spontaneous hydrolysis with no detected intermediates, indicating the unstable nature of any intermediates that may be formed. The hydroxamic acid modified particles accelerated the hydrolysis of methyl- and ethyl- paraoxon with relatively modest reactivity. Similar to polyhydroxamic acid, the reactivity was much lower than that of monomeric hydroxamic acid due to the steric hindrance from the polymer chains. The particles lost their reactivity after the reaction due to Lossen rearrangement of the phosphoryl hydroxamic acid.