Fabrication of Poly(p-Phenylene)/Zeolite Composites and Their Responses Towards Ammonia

Poly(p-phenylene) (PPP) was chemically synthesized via oxidative polymerization using benzene and doped with FeCl3. The electrical conductivity response of the doped PPP (dPPP) towards CO, H2 and NH3 is investigated. dPPP shows no electrical conductivity response towards the first two gases (CO and H2), but it shows a definite negative response towards NH3. The electrical conductivity sensitivity of dPPP increases linearly with increasing NH3 concentration. To improve the sensitivity of the sensor towards NH3, ZSM-5 zeolite is added into the conductive polymer matrix. The electrical sensitivity of the sensor increases with increasing zeolite content up to 30%. The effect of the type of cation in the zeolite pores is investigated: namely, Na+, K+, NH4+ and H+. The electrical conductivity sensitivity of the composites with different cations in the zeolite can be arranged in this order: K+ < no zeolite < Na+ < NH4+ < H+. The variation in electrical sensitivity with cation type can be described in terms of the acid-base interaction, the zeolite pore size and surface area. The PPP/Zeolite composite with H+ possesses the highest electrical sensitivity of −0.36 since H+ has the highest acidity, the highest pore volume and surface area, which combine to induce a more favorable NH3 adsorption and interaction with the conductive polymer

Exploitation of Unique Properties of Zeolites in the Development of Gas Sensors

The unique properties of microporous zeolites, including ion-exchange properties, adsorption, molecular sieving, catalysis, conductivity have been exploited in improving the performance of gas sensors. Zeolites have been employed as physical and chemical filters to improve the sensitivity and selectivity of gas sensors. In addition, direct interaction of gas molecules with the extraframework cations in the nanoconfined space of zeolites has been explored as a basis for developing new impedance-type gas/vapor sensors. In this review, we summarize how these properties of zeolites have been used to develop new sensing paradigms. There is a considerable breadth of transduction processes that have been used for zeolite incorporated sensors, including frequency measurements, optical and the entire gamut of electrochemical measurements. It is clear from the published literature that zeolites provide a route to enhance sensor performance, and it is expected that commercial manifestation of some of the approaches discussed here will take place. The future of zeolite-based sensors will continue to exploit its unique properties and use of other microporous frameworks, including metal organic frameworks. Zeolite composites with electronic materials, including metals will lead to new paradigms in sensing. Use of nano-sized zeolite crystals and zeolite membranes will enhance sensor properties and make possible new routes of miniaturized sensors