(659a) Synthesis of Nanostructured Metals in MF Membranes with Applications to Toxic Organic Dechlorination

Traditionally, MF membranes are used for separations based on size exclusion; membrane functionalization can extend their area of application to reactive processes. Current paper deals with bimetallic (Fe/Pd) nanoparticle synthesis inside the membrane pores and application for catalytic dechlorination of toxic organic compounds form aqueous streams.

Membranes have been used as platforms for nanoparticle synthesis, their open structure and high internal surface area ensure a high nanoparticle loading and the ease of the active site accessibility. Also, the nanoparticle synthesis inside of a polymer matrix reduces the agglomeration, encountered in solution phase synthesis and leads to a dramatic loss of reactivity. The synthesis consists of four steps: membrane modification with polymers containing ion-exchange groups, iron deposition via ion exchange, reduction to zero-valent state and addition of a dopant metal (Pd), to increase the nanoparticle reactivity and stability.

In our studies the membrane support, polyvinylidene fluoride (PVDF) was modified by in situ polymerization of acrylic acid and the polymerization reaction was carried out in aqueous and organic (toluene) phases. In order to ensure the proper wetting, hydrophilic or hydrophobic PVDF membranes with a thickness of 125 micrometer and nominal pore sizes of 650 nm and 450 nm, respectively, were used. In organic phase, the polymerization solution contained toluene, acrylic acid monomer, cross-linker ? trimethylolpropane triacrylate, and initiator - benzoyl peroxide. For the aqueous phase polymerization, the solution contained acrylic acid monomer, potassium persulfate as an initiator and polyethylene glycol added as a cross-linking agent. In both cases, the cross linker was added to ensure the chain stability (polyacrylic acid leaching from the membrane). After coating with polyacrylic acid, subsequent steps included ion exchange with Fe2+, reduction to zero valent Fe with sodium borohydride and Pd deposition.

Various techniques, such as SEM, TEM, EDX and BET surface area, were used for characterization and showed the formation of bimetallic (Fe/Pd) nanoparticles with an average size of 20-30 nm inside of the membrane pores and 50-70 nm on the external membrane surface. The agglomeration occurs more readily on the outer membrane area; inside, due to restricted chain mobility more discreet particles are formed. Polymer chain rigidity has a significant impact on the particle size and can be modulated by varying the degree cross-linking.

The Fe/Pd?modified membranes were used for dechlorination of toxic organics and showed a high reactivity toward model compounds, trichloroethylene (TCE) and dichlorobyphenyl. In a Fe/Pd bimetallic system, the role of Fe (reactant) in zero-valent form is to generate hydrogen by corrosion reaction and Pd acts as a hydrogenation catalyst. On different membranes the iron particle loading varied from 48 to 85 mg/cm3 and 62 to 108 mg/g of membrane, respectively. Experimental results showed a strong dependence of dichlorobyphenyl (typically 10mg/L feed concentration) degradation on Pd content, with the surface-normalized rate constant, kSA values increasing from ~0 for 0.3 wt% (of Fe) Pd to 0.07 L/sqm h for 2.6 wt%. Another important aspect is the ability to regenerate and reuse the Fe/Pd bimetallic systems by washing with a solution of sodium borohydride, because the iron becomes inactivated (corroded) as the dechlorination reaction proceeds.

This work is supported by NIEHS.