Molecular sieving using zeolite

Five common desiccant materials are used to adsorb water vapor montmorillonite clay ([(Na,Cao.5)o.33(Al,Mg)2Si40io(OH)2 H20], silica gel, molecular sieves (synthetic zeolite), calcium sulfate (CaS04), and calcium oxide (CaO). These desiccants remove water by a variety of physical and chemical methods adsorption, a process whereby a layer or layers of water molecules adhere to the surface of the desiccant capillary condensation, a procedure whereby the small pores of the desiccant become filled with water and chemical action, a procedure whereby the desiccant undergoes a chemical reaction with water. 

Chemical prqperties are also used in the largest field of application for the rare earth elements as catalysts. Most important are the cracking catalysts for the petroleum industry. The rare earth elements are combined into molecular sieves (Y-Zeolite) and serve in fluid bed or fixed bed reactors to increase the yield of gasoline. In addition thereto, there are the combustion catalysts for automobiles and for air pollution control. 

In order to exclude any hydrolyzed molecules in the reaction mixture, silanes are commonly distilled under reduced pressure before use. The toluene solvent may also be distilled prior to use or be stored on molecular sieve (5A zeolite) to assure complete drying. For extreme precaution, the silanation reactions may be performed under dry nitrogen atmosphere. A CaCl2 guard tube may also be employed. 

Unfortunately, the use of TS1 (as well as TS2 discovered in 1990 by the group of Ratnasamy (27)) in catalytic oxidations is restricted to the relatively small substrates able to enter the pores of these zeolites (apertures 0.55 nm). Therefore, many research groups attempted to incorporate titanium in large pore molecular sieves BEA zeolites, mesoporous molecular sieves MCM41 and MCM48. Other transition metal zeolites were also synthesized and tested in oxidation one of the main problems of these systems is the release of redox cations in liquid phase (24). Progress remains to be made to develop molecular sieves catalyzing the oxidation... 

The specific properties of zeolites, coupled with the separation properties of membranes, open the field to many areas of research for the future. This explains why the preparation and application of zeolite membranes is the subject of intensive research. By combining their adsorption and molecular sieving properties, zeolite membranes have been used for the separation of mixtures containing nonadsorbing molecules, different organic compounds, permanent gas-vapor mixtures, or water-organic mixtures. 

Molecular sieving using dehydrated zeolites Air separation (N2 from O2 with Li-LSX)... 

It is this possibility for variation of the pore size that leads to an important property of zeolites, that of molecular sieving. A zeolite is able to accommodate or reject molecules based on their size. Using suitable drying methods, the water within a zeolite framework can be removed the resulting dehydrated zeolite has space to accommodate other small molecules. So, if we have a mixture containing some molecules of suitable size and shape to enter the pores of the zeolite, and other molecules that are not able to do so, an effective separation can be carried out. A mixture of straight-chain and branched-chain hydrocarbons is one such example. 

Another special application of adsorption in space is presented by Grover et al. (1998). Tire University of Washington has designed an in situ resomce utilization system to provide water to tire life-support system in the laboratory module of the NASA Mars Reference Mission, a piloted mission to Mars. In this system, the Water Vapor Adsorption Reactor (WAVAR) extracts water vapor from the Martian atmosphere by adsorption in a bed of type 3A zeolite molecular sieve. Using ambient winds and fan power to move atmosphere, the WAVAR adsorbs tire water vapor until the zeolite 3A bed is nearly saturated, and then heats the bed witliin a sealed chamber by microwave radiation to drive off water for collection. Tire water vapor flows to a condenser where it freezes and is later liquefied for use in tire life-support system. 

The molecular sieves used are artificial zeolites, which absorb both carbon dioxide and water vapour on the surface of structural cavities of molecular size. The zeolite can be regenerated and the method has the advantage of supplying stripped and dried gas in one operation. 

Multifunctional catalysis, in which reactions consisting of several reaction steps are carried out by a shorter synthesis route, is becoming increasingly important in organic synthesis. Molecular sieve catalysts, too, help to combine several catalytic steps and tailor them optimally to one another [15, 18, 24], In this respect, molecular sieves like zeolites can be used as carriers for catalytically active components such as transition metals, noble metals. In addition the catalytic behaviour of these components the intrinsic acidic or basic or redox properties of the zeolites combined with shape selective feature are still present. 

The commercial separation of air into N2 and O2, an industrially very important process, is achieved by either cryogenic distillation or pressure swing adsorption (PSA). The use of pillared clays forms an interesting alternative for the carbon molecular sieves and zeolites currently applied as adsorbents in PSA techniques. Both the capacity and the selectivity towards air components are very important features in gas adsorption applications. 

To explore the difficulties in practical implementation of the above concepts, mixed matrix membranes, with 20% molecular sieves (by volume), were prepared by solution deposition on top of a porous ceramic support. The ceramic supports used were Anodise membrane filters which had 200 A pores that open into 2000 A pores and offer negligible resistance to gas flow. Initially the molecular sieve media, zeolites (4A crystals) or carbon molecular sieves, was dispersed in the solvent, dichloromethane, to remove entrapped air. After two hours, Matrimid was added to the mixture, and the solution was stirred for four hours. The solutions used varied in polymer content from 1-5 wt %. The solution was then deposited on top of the ceramic support, and the solvent was evaporated in a controlled manner. The membranes were then dried overnight at 90°C under vacuum. This was followed by a reactive intercalation post treatment technique 15) to eliminate defects. This technique involves imbibing a reactive monomer (e.g. diamine) from an inert solvent (e.g. heptane) into any micro defects. Next, a second reactive monomer (e.g. acid chloride) was introduced to reactively close defects by forming a low permeability polymer. The membranes were dried again to remove the inert solvent. Individual membrane thickness was determined by weight gain and varied from 5 to 25 Jim. 

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