Perfluorosulfonic Acid Membrane Catalysts for Optical Sensing of Anhydrides in the Gas Phase

Abstract

Continuous, on-site monitoring of personal exposure levels to occupational chemical hazards in ambient air is a long-standing analytical challenge. Such monitoring is required to institute appropriate health measures but is often limited by the time delays associated with batch air sampling and the need for off-site instrumental analyses. In this work, we report on the first attempt to use the catalytic properties of perfluorosulfonic acid (PSA) membranes to obtain a rapid, selective, and highly sensitive optical response to trimellitic anhydride (TMA) in the gas phase for portable sensor device application. TMA is used as starting material for various organic products and is recognized to be an extremely toxic agent by the National Institute for Occupational Safety and Health (NIOSH). Resorcinol dye is shown to become immobilized in PSA membranes and diffusionally constrain an orange brown product that results from acid-catalyzed reaction with more rapidly diffusing TMA molecules. FTIR, UV/vis, reaction selectivity to TMA versus trimellitic acid (TMLA), and homogeneous synthesis are used to infer 5,7- dihydroxyanthraquinone-2-carboxylic acid as the acylation product of the reaction. The color response has a sensitivity to at least 3 parts per billion (ppb) TMA exposure and, in addition to TMLA, excludes maleic anhydride (MA) and phthalic anhydride (PA). Solvent extraction at long times is used to determine that the resorcinol extinction coefficient in 1100 EW PSA membrane has a value of 1210 m(2)/g at 271.01 nm versus a value of 2010 m(2)/g at 275.22 nm in 50 vol% ethanol/water solution. The hypsochromic wavelength shift and reduced extinction coefficient suggest that the polar perfluorosulfonic acid groups in the membrane provide the thermodynamic driving force for diffusion and immobilization. At a resorcinol concentration of 0.376 g/L in the membrane, a partition coefficient of nearly unity is obtained between the membrane and solution concentrations and a maximum conversion rate of one ambient TMA molecule for every two membrane-immobilized resorcinol molecules is observed in 15 min.

Water-Resistant Polymeric Acid Membrane Catalyst for Acetone Detection in the Exhaled Breath of Diabetics

“…To achieve such a system, we first investigated the extent of TA immobilization in Nafion in a manner similar to that previously used to investigate resorcinol immobilization . Nafion membranes were exposed to solutions of known concentration of TA in mineral oil at room temperature and the TA then extracted in ethanol.…”

“…Nafion membrane (The Fuel Cell Store, Nafion 117, protonated, 0.007 in. thickness, 1100 EW) was used as received and immersed in a 12g L –1 solution of resorcinol (Acros Organics, 98%) in ethanol (Acros Organics, ACS spectroscopic grade, >95% purity) for 31 min as described in further detail in our previous work. , The membranes were rinsed with deionized water and allowed to air-dry. The membranes were then placed into a flask containing a concentration of vanillic acid (Fluka, HPLC grade, >97%), benzoic acid (Sigma-Aldrich, >99.5%), ferulic acid (Sigma-Aldrich, >99%), or tiglic acid (Sigma-Aldrich, >98%) in food grade mineral oil (Bayes, 100%).…”

“…The membranes were initially equilibrated to ambient conditions prior to exposure but not otherwise preconditioned. Studies of the equilibrated membrane response to acetone exposure were also performed using a more convenient ex-situ batch sampling method previously described . The membranes were suspended in a sealed nitrogen purged flask with a known concentration of acetone (Acros Organics, ACS spectroscopic grade, 99% purity).…”

“…We have previously demonstrated , that the optical response arising from chemical reaction between reagent molecules immobilized within Nafion membrane and volatized toxins in the ambient can be used for highly selective, sensitive (subppb), and rapid detection. In the case of acetone, the specific sensing mechanism involves the condensation reaction of acetone in the ambient with membrane-immobilized resorcinol to produce a flavan with highly selective optical characteristics in the visible region of the electromagnetic spectrum.…”

“…Water is ubiquitous in the natural environment and can imbibe into the polymer structure of Nafion causing swelling with attendant solvation of the PSA groups and proton transfer. An ambient relative humidity less than 15% has been found sufficient to completely deactivate an 1100 Equivalent Weight sample of Nafion for acetone gas condensation with resorcinol solid . This interference has limited the use of Nafion catalysts to toxic and flammable organic solvents or high temperatures and have thus prevented commercial use in real time environments as optical sensing elements for portable, point of contact gas analysis.…”