Zeolite membranes sensors

Three different ways in which a zeolite membrane can contribute to a better sensor performance can be distinguished (i) the add-on selective adsorption or molecular sieving layer to the sensor improves selectivity and sensitivity, (ii) the zeolite layer acts as active sensing material and adds the selective adsorption and molecular sieving properties to this, and (iii) the zeohte membrane adds a catalytically active layer to the sensor, improving the selectivity by specific reactions. 

Another form of zeolite membranes is a zeolite crystal layer that consists of isolated crystals deposited on a solid substrate (Fig. 1C). The substrate can be a variety of materials such as metal, ceramic, or silicon wafer. Crystal layers have to be supported. There has been exciting fundamental research carried out in this area, however, demonstrated applications have been limited to sensors. The organic linker approach appears very promising for the preparation of these types of membranes. ... 

Zeolite membranes have been demonstrated for many applications. Applications such as separation membranes, membrane reactors, adsorption, and catalysis have been covered in several reviews. In this entry, we focus on new applications including sensors, low-dielectric constant (low-k) films, corrosion resistant coatings, hydrophilic coatings, heat pumps, and thermoelectrics. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

In contrast to equilibrium-based sensing such as described above, it is also possible to use the zeolite film as a membrane controlling molecular access to an appropriate transduction mechanism. In this case, Pd-doped semiconductor gas sensors were used as a fairly non-selective sensor platform. After coating these sensors with a thin film of MFI-type or LTA-type zeolites, they were examined with respect to gas phase sensing of different analytes such as methane, propane and ethanol, at different humidity levels (Fig. 14).[121] The response of a zeolite-coated sensor towards the paraffins was strongly reduced compared to the non-coatcd sensor device, thus resulting in an increase of the sensor selectivity towards ethanol. 

In the middle of the last century, the original form of zeolite membranes were synthesized by dispersing the zeolite crystals in polymer membrane matrixes, which were used for gas separation and pervaporative alcohol/water separations. In the last few decades, the researches of polycrystalline zeolite membranes that supported on ceramic, glass, or metal substrates have grown into an attractive and abundant field. Their applications for gas separation, pervaporation, membrane reactors, sensors, low-k films, corrosion protection coatings, zeolite modified electrodes, fuel cells, heat pumps et al. have been wildly explored. In the following text, the applications of supported polycrystalline zeolite membranes for energy and fuels will be presented. 

Fong, Y. Y., Abdnllah, A. Z., Ahmad, A. L. and Bhatia, S. (2007) Zeolite membrane based selective gas sensors for monitoring and control of gas emissions. Sensor Letters 5,485-99. 

Zeolite molecular sieves are crystalline, porous inorganic solids, typically aluminosilicates, with channel diameters ranging from 0.3 to 1.2 nm and beyond, and crystal sizes typically between 0.5 and 5 /im. If species are selectively adsorbed into the well-defined channel systems of zeolites, the sensor response can be made selective for those species, while larger molecules are only adsorbed on the outer surface of the sensor membrane. Furthermore, many zeolites show ion exchange capability which introduces numerous possibilities for intrapore modifications. 

After this chapter. Part 11 is dedicated to zeolite, ceramic and carbon membranes and catalysts used in membrane reactors. In Chapter 6 (Algieri, Comite and Capannelli) the remarkable properties of zeolite membranes are illustrated. Moreover, the key role of zeolite membrane reactors to improve the yield and the selectivity of reactions is particularly emphasised. Furthermore, the possibility of using zeolite membranes as micro-reactors and sensors is also discussed. Chapter 7 (Tan and Li) deals with dense ceramic membrane reactors, which are made from composite oxides usually having perovskite or fluorite structures with appreciable mixed ionic (oxygen ion and/or proton) and electronic conductivity. This chapter mainly describes the principles of various configurations (disc/flat-sheet, tubular and hollow fibre membranes) of dense ceramic membrane reactors and the... 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Single-layer zinc-phosphate zeolite crystals were grown with more than 90% of their (111) faces oriented to a gold-coated silicon surface. Sudi oriented zeolite films might find application as membrane catalysts or as specific chemical sensors [66]. 

In addition to the more traditional applications, new applications of these versatile materials arc now being explored by an increasing number of research groups. These applications often utilize the unique spatial structuring of the zeolite channel system for novel concepts such as the stabilization of nanoscale forms of matter, size-selective chemical sensing, or separation of reaction spaces in electron transfer processes, to name just a few.[6,7] Furthermore, a number of novel applications depends not only on the control of pore structure and intrazeolite chemistry, but also on the ability to control the external morphology, for example in thin membranes for separations or in the design of sensor... 

The aim of using the ex situ techniques is to obtain a better control of the microstructure and a preferential orientation of the crystals in the membrane with a shortened crystallization time rendering highly selective and permeable membranes. Preferential orientation is needed, not only for separation purposes, when high fluxes are required, but also for size-selective chemical sensors (see Section 11.6.6.2). Due to the anisotropy in the pore geometry of the zeolite crystals, an orientation that shows the widest channels in the direction of the flux is... 

The synthesis of supported zeolite crystals has been ireported previously. Different supports such as stainless steel, alumiriium foil, mulKte and mica were combined with different types of zeolites [1-3]. Zeolite coatngs can be aippiiad as membranes, catalysts and sensors [4]. 

Ceramic thin films, sensors, nanoscale materials, multi-functional ceramic composites, optical fibers, ceramic membranes and many other products can be manufactured by the sol-gel process [1-3]. The major applications of sol-gel processing are in ceramic industry for fabrication of oxide ceramics and glasses. Several studies have been reported on the preparation of supported catalysts and zeolite granular particles using the sol-gel technique [4-10]. Sol-gel derived inorganic thin films and membranes have recently attracted attentions from both academia and industry [11-13]. Only limited studies have been carried out on the sol-gel fabrication of adsorbents for industrial separation or purification purposes. 

Membrane, filter Electrochemical, gas sensors A1,0, SiOj, zeolites... 

Some discrimination between different alcohols is evident (Innocenzi, 2001b). Spin coated silica films with a cubic structure have also been incorporated into a surface photovoltage gas sensor (Yamada, 2002). Dip coated mesoporous thin films on a macroporous or mesoporous support have been prepared for separation membrane applications (Kim, 2002, 2003). Spin coating has also been used to deposit CieTABr templated films where the silicate walls were composed of nanosized zeolite MFI crystals co-deposited with the TEOS solution (Petkov, 2003).The continuous film contained a wormlike mesostructure and zeolite nanocrystals aligned with the a-axis perpendicular to the substrate, however the surface roughness of the film was increased. 

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