Vycor composites

Yeung K.L, Sebastian J.M, Varma A. Novel preparation of Pd/Vycor composite membranes. Catalysis Today 1995 25 231-236. 

The rates of hydrolysis of siliceous materials will be affected by several factors. For instance, the rate will be directly related to surface area, explaining the low rates observed for silica deposition from the Vycor apparatus. Also, the composition of the siliceous material will Influence the rate of hydrolysis, explaining the differing amounts of silica transferred from pure silica, silica alumina, zeolite, and the high alumina cracking catalyst. 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

Each sample was prepared in the same manner. A degassed quartz tube fitted with a Vycor standard taper joint was weighed on an analytical balance before and after loading with stoichiometric U02 (1 to 5 grams) and evacuated at 700° to 800° C. to 10-6 mm. Purified oxygen (99.5 mole %) was added to a pressure sufficient to attain the desired composition (50 to 600 mm.) pressures were meas-... 

For the x-ray work, wire samples were prepared by shearing 0.010-inch sheet of the desired uranium-zirconium alloy composition, keeping the width as close to the thickness as possible. A small batch of these wires was then hydrided at 600° C. to the desired hydrogen content along with a small block of the same alloy to bring the sample weight to a feasible value. The hydrided material of each composition was then sealed in an evacuated Vycor capsule and homogenized at 1000° C. 

The production of Vycor requires precise adherence to the technological procedures. Correct choice of the initial glass is of prime importance. Both the rate and resulting texture of separation and thus also the behaviour during leaching depend on the glass composition. 

Attempts have been made to synthesize an oxide that is intermediate in composition between Tc and TCO2. Mixtures of Tc and TCO2 were prepared in different ratios, wrapped in platinum foil, vacuum-sealed into Vycor tubes, and heated at about 970 °C for 4.5 days. The distorted rutile structure of TCO2 was noted in all X-ray patterns. In addition, one new phase was found in several samples, which could be indexed on the basis of a primitive pseudocubic unit cell with a=9.45 A. The same new pseudocubic phase was obtained when a mixture of Tc and TCO2 corresponding to the composition TcOo io was heated in a sealed silica tube up to 1250 °C over a... 

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