(253b) Exploiting Unique Properties of Porous Polymers for Air Pollution Control

Microporous activated carbon (AC) and zeolites are industrially applied for air pollution control, but pore blockage, irreversible adsorption, and co-adsorption of moisture reduce their contaminant removal efficiencies and/or prevent reuse. Porous polymers are gaining traction in environmental engineering because they can have tunable physical and chemical properties that allow tailoring for specific applications. Our previous studies show that hydrophobic styrene-divinylbenzene (St-DVB) polymers maintain their catalytic NO oxidation performance when moisture is present, while AC fails due to competitive water adsorption caused by increased hydrophilicity and surface oxidation during use. It was also observed that polymers provide faster adsorption/desorption kinetics and higher desorption efficiency than AC at low temperatures. However, commercial St-DVB polymers have low micropore (d_p < 2 nm) volume, reducing adsorption capacity for low concentration contaminants. To overcome, the Friedel-Crafts reaction is applied to increase micropore volume by hyper-cross-linking polymer segments and reducing the network mesh size. The Friedel-Crafts reaction can also be directly used to synthesize microporous polymers from aromatic compounds such as benzene, phenol, or methylbenzene using external cross-linkers. This method not only produces cost-effective microporous polymers, but also allows user-defined surface functional groups by changing the aromatic building blocks or by post-synthesis modifications.

In this work, commercial St-DVB polymers were first hyper-cross-linked with dichloroalkanes to increase micropore volume by up to 333%, without sacrificing the inherent hydrophobicity of the material. Consequently, hexane adsorption capacity increased by up to 218% at relative pressure (p/p₀) = 0.05 and, in a separate series of experiments, catalytic NO oxidation rate increased by 100% ([NO] = 500 ppm_v). Next, microporous polymers were directly prepared using the Friedel-Crafts reaction with benzene or phenol monomers, formaldehyde dimethyl acetal or 4,4′-Bis(chloromethyl)-1,1′-biphenyl (BCMBP) cross-linkers, and FeCl₃ as the reaction catalyst. Maximum surface area and micropore volume were 1600 m²/g and 0.43 cm³/g, respectively, when BCMBP self-condensed (reacted as monomer and cross-linker). The surface area and micropore volume decreased by 50% and 95% when 0.2 ml and 2 ml benzene was used as a co-monomer, respectively. Using co-solvents of dichloroethane and hexane reduced surface area, micropore volume, and total pore volume by 50%, 40%, and 70%, respectively. Overall, this work describes novel polymer modification and production processes that highlight opportunities to tailor the product’s physical and chemical properties. The overarching goal is to exploit this flexibility, in an effort to present microporous polymers as competitive materials for use in air pollution control applications.