Hydrogen separation preparation techniques

As an alternative approach towards the above requirement, Somorjai introduced the method of electron lithography [119] which represents an advanced HIGHTECH sample preparation technique. The method ensures uniform particle size and spacing e.g. Pt particles of 25 nm size could be placed with 50 nm separation. This array showed a uniform activity similar to those measured on single crystal in ethylene hydrogenation. The only difficulty with the method is that the particle size is so far not small enough. Comprehensive reviews have been lined up for the effect of dispersion and its role in heterogeneous catalysis [23,124,125]. 

E. Gardeniers, H. V. Jansen, F. C. Gie-lens, M. C. Elwenspoek, Preparation of palladium-silver alloy films by a dual-sputtering technique and its application in hydrogen separation membrane,... 

Finally, SINTEF has developed a technique for the manufacture of Pd-based hydrogen separation membranes based on a two-step process, allowing a reduction in membrane thickness. First, a defect-free Pd-alloy thin film is prepared by magnetron sputtering onto a silicon wafer. In the second step the film is removed from the wafer. These films may subsequently be either used self-supported or integrated with various supports of different pore size, geometry and size. This allows, for example, the preparation of... 

A membrane separation device was prepared by Wilhite et al. by micro-electromechanical techniques [526]. The palladium/silver membrane deposited onto a silicon oxide support was only 20-nm thick, which was possibly the lowest membrane thickness ever reported for hydrogen separative purposes. A lanfhanum/nickel/cobalt oxide catalyst (LaNi0.95Co0.05O3) catalyst for partial oxidation of methanol was deposited onto the membrane. At a O/C ratio of 0.86 and 475 °C reaction temperature, up to 64% methanol conversion and more than 90% hydrogen selectivity could be achieved. These workers claimed a deviation from Sievert s law (see Section 5.2.4) for their membrane. N amely, the hydrogen flux did not depend by a power of 0.5 of the retenate and permeate pressure but rather by a power of 0.97, which they attributed to the absence of internal solid-state diffusion limitations in their ultra-thin membrane. 

S. Uemiya, M. Koseki, T. Kojima, Preparation of highly permeable membranes for hydrogen separation using a CVD technique, in Proceedings of the 3rd International Cortference on Inorganic Membranes, Y.H. Ma, ed., Worcester, MA, July 10-14,1994, p. 545. 

Although the electroplating technique has been used to deposit Pd-Au films, it suffered from the formation of micro-cracks and has not been extensively used for Pd-Au alloy membrane preparation for hydrogen separation. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

Analyses for the Saxitoxins. Early methods for analysis of the saxitoxins evolved from those used for toxin isolation and purification. The principal landmarks in the development of preparative separation techniques for the saxitoxins were 1) the employment of carboxylate cation exchange resins by Schantz et al. (82) 2) the use of the polyacrylamide gel Bio-Gel P2 by Buckley and by Shimizu (5,78) and 3) the development by Buckley of an effective TLC system, including a new solvent mixture and a new visualization technique (83). The solvent mixture, designated by Buckley as "E", remains the best for general resolution of the saxitoxins. The visualization method, oxidation of the saxitoxins on silica gel TLC plates to fluorescent degradation products with hydrogen peroxide and heat, is an adaptation of the Bates and Rapoport fluorescence assay for saxitoxin in solution. Curiously, while peroxide oxidation in solution provides little or no response for the N-l-hydroxy saxitoxins, peroxide spray on TLC plates is a sensitive test for all saxitoxin derivatives with the C-12 gemdiol intact. 

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

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