Low-silica X zeolites

In 1989, Chao [2] reported that LiLSX (lithium ion-exchanged low silica X zeolite, having a Si/Al ratio close to I.O) showed an unexpected high capacitiy and selectivity for nitrogen over oxygen. He found a Li exchange threshold value in LiNaLSX at about 2/3. Below... 

Basaldella, E.I. Kikot, A., and Tara, J.C.. Effect of pellet pore size and synthesis conditions in the in-situ synthesis of low-silica X-zeolite, Ind. Eng. Chem. Res., 34(9). 2990-2992 (1995). 

Cation sites are well defined in type A zeolite but are not well defined in nearly all other zeolites, including LSX (low silica X, Si/Al = 1). Most of the cation sites are hidden and are not accessible for interactions with the adsorbate molecules. 

Prior to ca. 1980, PSA systems were used with both adsorption and desorption pressures well above atmospheric. These systems were low in capital (due to simplicity) but high in power consumption (since both N2 and O2 in the feed are compressed compared with vacuum swing adsorption (VSA) where only the waste gas is evacuated). The availability of improved sorbents and lower cost vacuum equipment led to the development of VS A, which is typically operated with adsorption pressure slightly above atmospheric and desorption pressure of typically 0.2 atm. A further breakthrough occurred in 1989, with the invention of LiLSX zeolite (low silica X, with Si/Al = 1) (Chao, 1989). The LiLSX is currently the best commercial sorbent for air separation and will be discussed... 

The low silica zeolites represented by zeolites A and X are aluminum-saturated, have the highest cation concentration and give optimum adsorption properties in terms of capacity, pore size and three-dimensional channel systems. They represent highly heterogeneous surfaces with a strongly hydrophilic surface selectivity. The intermediate Si/Al zeolites (Si/Al of 2-5) consist of the natural zeohtes eri-onite, chabazite, clinoptilolite and mordenite, and the synthetic zeolites Y, mordenite, omega and L. These materials are still hydrophilic in this Si/Al range. 

For zeolites composed only of clusters of the third type, [Al2Si2AlSi3]n, the ratio Si/Al is 1.7. This is just the border between Y- and X-type faujasites. Therefore, for low-silica-containing Y zeolites and X zeolites, one might expect the presence of both OH-III and OH-II groups. The former should predominate in X-type zeolites. 

Low-temperature (e.g., ambient temperature) aging can increase the rate of nucleation from the parent mixture, which is equivalent to a low-temperature reaction. However, the growth rate of a crystal at ambient temperature is very slow and can be neglected. Previous studies indicate that the aging process is necessary for the synthesis of both high-silica zeolites (e.g., TS-1) and low-silica zeolites (e.g., zeolite A and X). 

Linde type X and Y zeolites are two typical microporous materials with the FAU framework. So far, both low-silica type X and high-silica type Y molecular sieves are... 

In the case of the same zeolites containing alkaline earth cations, by way of contrast, the red shift increased in magnitude with the polarising power (charge/radius ratio) of the cations for mordenite, but this trend was found to be reversed in zeolite X (Khodakov et al., 1992). The difference was ascribed to the importance in low silica zeolites of hydrolysis during... 

One of the most thoroughly studied zeolite crystallisations is that of the synthesis of the pure silica form of ZSM-5 from a clear solution using TP A as the structure-directing agent. A series of low-angle X-ray scattering experiments by de Moor et indicate the presence of populations of scattering... 

The first natural zeolites were discovered more than 200 years ago, and to date over 40 types of natural zeolites have been found [94]. Because natural zeohtes cannot meet the huge industrial demand, the synthesis of zeolite was started at the end of the 1940s. Low silica zeolites, such as zeolites A (Hnde type A, LTA) and X (faujasite, FAU) were first synthesized, and by 1954 began to be produced... 

The search for enhanced builder properties has led to the development of zeolite AX. This is a cocrystallisate of 20% A-type zeolite and 80% zeolite LSX. It is a zeolite of the X-type having low silica content. Its chemical composition is reported as 10% NajO, 1% KjO, 43% SiOj, 32% AljOj, and 23% Hp [105]. 

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