Steam stability, high alumina silica-aluminas

The stability of MCM-41 is of great interest because, from the practical point of view, it is important to evaluate its potential application as a catalyst or adsorbent. It is known that purely-siliceous MCM-41 (designated here as PSM) has a high thermal stability in air and in oxygen containing low concentration (2.3 kPa) of water vapor at 700 °C for 2 h [1], However, the uniform mesoporous structure of PSM was found to be collapsed in hot water and aqueous solution due to silicate hydrolysis [2], limiting its applications associated with aqueous solutions. After MCM-41 samples were steamed in 100% water vapor at 750°C for 5 h. their surface areas were found to be lower than amorphous silica-alumina and no mesoporous structure could be identified by XRD measurement [3]. In addition, PSM showed poor stability in basic solution [4]. 

Amazingly, under hydrothermal conditions in the temperature range of 373 K up to 493 K the hydrophilic high-alumina NaCaA and NaY zeolites are significantly more stable than the hydrophobic high-silica modifications DAY-S and DAY-Tg generated by dealumination. In steam of saturated pressure a shift of the stability region by 100 K and more can be observed... 

Operationally, the basic difference between the Type X and Type Y structures was largely a matter of thermal stability, although acid strength-related differences in selectivity were also found. While the Type Y zeolite was much more stable, it was also much more expensive. Davison initially marketed their XZ-15 catalyst, containing steam-stabilized Y zeolite admixed with low-alumina silica-alumina, at a price of 800 per ton. This was quite a high price at the time, but despite that, about 15 refiners tried it. At the same time, Filtrol introduced their Grade 800 catalyst, and although they didn t officially claim that it contained molecular sieves, the product distributions clearly pointed to their presence (67). 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

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