Highly Selective Zeolite Membranes As Pre-concentrators For Explosive Detection

After the tragic events of9/11in US, the need for increased security and damageprevention is now more sought out than ever. Terrorist threats and millions ofunexploded land mines buried around the world necessitate the development ofequipment for explosives detection that is rapid, reliable and compact enough to beincorporated into a hand-held device or a remote surveillance device. If can beconsidered the commonly employed devices, most are also not shielded hermetically,it is possible to detect the explosives molecular by a sensor. The critical problem fordetection by a sensor device is the low sensitivity caused by the low to ultra-lowconcentrations of nitro compounds under ambient conditions. The most commonlyused nitro-based explosives (TNT, NG, DNT, RDX, Tetryl, etc.) possess a smallvapor pressure. Furthermore, natural air movements dilute ppb level to ppt or evenlower. This problem is exacerbated in large and open areas, such as airports and trainstations, where even the best-engineered sensors will have a hard time detecting suchlow concentrations. A simple and effective solution is to design a preconcentrator forsensor device to improve sensitivity.Zeolite membranes have been developed for use in gas separation in recentdecades according to their pore size and selective adsorption. MFI type zeolite isconsidered as a promising membrane material for explosives separation because itspore size (0.51-0.56nm) is smaller than explosives molecules but much larger than aircomponents. Consequently, an ultrathin molecular sieving zeolite membrane couldfilter an air sample and serve as an explosives vapor concentrator. The membraneswill allow a large flux of air, thus leading to a concentration of the explosives vaporsby a large factor in a short time and will allow the detection of ultra-lowconcentrations (ppt or lower).The aim of this work is to synthesize continuous, pinhole-free, ultra thinmolecular sieving zeolite membranes with suitable pore sizes by a seeded growthmethod on porous stainless steel, porous nickel or α-Alumina substrates. Themembranes, after thermal calcination or UV-treatment, will be characterized in termsof their permeation rates and concentration efficiency for explosive molecules. Toavoid membrane cracking during high temperature calcination, an UV/ozone treatment method is employed as a low temperature alternative to remove the organictemplate. To examine the selectivity of the membrane,1,3,5-Trimethylbenzene (TMB)has been used as a surrogate of the explosive molecule. Toward that end, a mixture ofTMB/nitrogen feed will be used as the testing case for this desired application. To ourknowledge, this is the first application of zeolite membranes for TMB/nitrogen binarymixture separation.Higher selectivity and permeance are observed for samples with zeolite film onboth sides of the support, as compared to those with zeolite film on only one side ofthe support. In addition, by varying the type of support with different pore structures,it is shown that the support with large pore size can reduce the support resistance, andthereby increasing permeance. UV-treated membranes on both sides of α-Aluminasupport has the highest flux (13.5mmol/m2·s) and selectivity (10600) among allsamples tested.Furthermore, organic functionalization is a very common and efficient method tomodify the zeolite. Silane was reacted with hydroxyl group on the surface of zeolite,which not only can increase the hydrophobic properties but also the oleophobicproperties of zeolite. Meanwhile, the silane with fluorocarbon chain can furtherincrease the hydrophobic properties of zeolite. The contact angle to water of thehydrophobic coating is range from130to150°. At same condition, organicfunctionalized ZSM-5membranes can get higher contact angle than silicalite-1membranes, due to its stronger hydroxyl groups. Otherwise, the organic functionalgroups can surfer high temperature as high as300°C on the zeolite. Organicfunctionalization cannot dramatically increase the separation permeance or selectivityof zeolite, but can increase the durability of zeolite when separate organic mixture gasor moist air. In addition, the properties of substrate have a significant influence on thedurability of separation film. The durability of nitrogen permeance on silanizedα-Alumina supported zeolite is much better than that on silanized porous nickelsupported zeolite.