Microporous materials synthetic

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

The use of clays as supports for hydroprocessing has been reported and summarized [9-11], Dibenzothiophene (DBT) diluted with hexadecane (0.75 wt% S) was the liquid feed for HDS tests. The pore diameter of the MSC catalysts is seen to have a strong effect on both the HDS activity and selectivity (Figure 4). A commercial catalyst (Crosfield 465, Co/Mo alumina) was also measured under these conditions where it gave 77% DBT conversion and 61% BP selectivity. In a previous study [12], other synthetic hectorites were compared using these conditions except that a 1 wt% S feed was utilized. One sample was a control made without template that consisted of only micropores. The DBT conversion and BP selectivity were very low for this microporous material. The Crosfield material has significant macroporosity (42% of the pore volume) in addition to a broad distribution of mesoporosity, and has clearly been optimized to perform well under these HDS conditions. 

The recent descriptions of the ALPO-n, SAPO-n and MeAPO-n families of microporous materials illustrate that hydrothermal syntheses can afford a wide and diverse range of four-coordinate framework structures based on nearregular tetrahedra [1,2]. As building blocks, octahedra and tetrahedra can also be combined, in various proportions, into a variety of structure types [3,4]. Reflecting the conditions used for conventional synthesis [3,4], most of these structures are condensed, with little accessible pore volume. There are, however, examples of both synthetic [5-7] and natural materials [8-11] that have microporous crystalline structures. Further, the formation chemistry of silicates and aluminosilicates [12,13] illustrates that the more open structures are generally produced under relatively mild conditions. Open octahedral-tetrahedral structures with large pore systems might therefore also be accessible under appropriate low temperature hydrothermal conditions. 

Microporous materials are typified by natural and synthetic zeolites that are crystalline 3D aluminosilicates with open channels or cages. Synthetic and structural concepts of zeolites have to a large extent shaped the development of microporous materials during the past 50 years. For example, the use of organic structure-directing agents in the synthesis of high-silica zeolites and their all-silica polymorphs contributed to... 

The synthesis of aluminophosphate molecular sieves in 1980s represents a breakthrough in the development of microporous materials. Since then, much of the worldwide synthetic efforts have been directed toward nonsilicate microporous materials. Many novel framework topologies could be found with phosphates, and many other elements could be incorporated into phosphates to produce additional new framework topologies or new compositions. 

The present book consists of nine chapters, with the synthetic and structural chemistry of microporous and mesoporous materials as the core. Five chapters (Chapters 3, 4, 5, 6, and 8) are allocated to cover the synthetic aspects of the topic. Chapter 3 introduces the synthesis and related fundamental principles, synthetic strategies, and techniques for the major microporous materials such as zeolites and microporous aluminophosphates. This Chapter serves as Part I of the synthetic aspects of the microporous compounds. 

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