Aluminosilicates, crystalline

Zeolites are aluminosilicate crystallines consisting of pores of molecular dimensions, interconnected by small windows(5-8A diameter). Strict regularity of the pore structure enables higher slectivities to be achieved in both catalysis and sorption processes. The intrazeolite circumstances alike a "solid-solvent" accomodate the selected reactant molecules and promote some inorganic and organic synthetic reactions, similarly in solution. 

The current synthesis of ethyl alcohol eliminates sulfuric acid and uses phosphoric acid suspended on zeolite substrates. Zeohtes are porous aluminosilicate crystalline minerals. The use of phosphoric acid as a catalyst allows the direct hydrolysis of ethylene into ethyl alcohol C2H4 + H20 —> C2H5OH. Industrial alcohol is rendered inconsumable by adding a small amount of a poisonous substance such as methanol or acetone to it. Alcohol unfit for consumption because of a poisonous additive is termed denatured alcohol. 

In addition to aluminosilicates, crystalline microporous materials can be phosphate-based. The aluminophosphate (A1P04) framework is electroneutral (analogue of Si02), and the aluminum and/or phosphorus tetrahedral atoms can be substituted by a number of metal and non-metal atoms that result in producing charged frameworks [1-3], e.g. Si+4 substitution for P+5. In addition, numerous other metal oxide, and nitride based microporous materials have been reported recently [4, 5]. 

The unique three-dimensional aluminosilicate crystalline lattice of zeolites gives rise to three intriguing characteristics (104). These characteristics are high cation-exchange capacity, sensitive molecular recognition (size and shape selectivity), and good catalytic activity. These properties give rise to the use of zeolite modified electrodes in sensor development, and electrocatalysis (106). These and other applications are outlined in Table 8.6. More detailed descriptions can be found in recent reviews (7, 96, 102-106), with extensive lists compiled by Rolison (Table 11 in (102)) and Walcarius (Tables 1 and 3 in (105) and Table 1 in (106)). 

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

Additional to tire aluminosilicate-based zeolites, a number of otlier crystalline microporous tliree-dimensional oxides have been syntliesized [25]. Most prominent among tliese are tire aluminophosphates (ALPO series) [26,... 

Sodium alumiaate is widely used in the preparation of alumina-based catalysts. Aluminosilicate [1327-36-2] can be prepared by impregnating siHca gel with alumiaa obtained from sodium alumiaate and aluminum sulfate (41,42). Reaction of sodium alumiaate with siHca or siHcates has produced porous crystalline alumiaosiHcates which are useful as adsorbents and catalyst support materials, ie, molecular sieves (qv) (43,44). 

Type 3A sieves. A crystalline potassium aluminosilicate with a pore size of about 3 Angstroms. This type of molecular sieves is suitable for drying liquids such as acetone, acetonitrile, methanol, ethanol and 2-propanol, and drying gases such as acetylene, carbon dioxide, ammonia, propylene and butadiene. The material is supplied as beads or pellets. 

Type 13X sieves. A crystalline sodium aluminosilicate with a pore size of about 10 Angstroms which enables many branched-chain and cyclic compounds to be adsorbed, in addition to all the substances removed by type 5A sieves. 

Molecular sieves are an adsorbent that is produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves inter-crystalline cavities into which normal paraffin molecules are selectively retained and other molecules are excluded. This process is used to remove normal paraffins from gasoline fuels for improved combustion. Molecular sieves are used to manufacture high-purity solvents. 

Zeolites have the following characteristics (ref. 1) they are crystalline aluminosilicates (tetrahedral connection) with accessibility ranging from. 3-.8 ran. All atoms are exposed to the pore system which can consist of parallel channels (1-D) or of a threedimensional system (3-D). Some common zeolites with their accessibility and minimum Si/Al ratios are given in Table 1. 

Zeolites form a unique class of oxides, consisting of microporous, crystalline aluminosilicates that can either be found in nature or synthesized artificially [J.M. Thomas, R.G. Bell and C.R.A. Catlow in Handbook of Heterogeneous Catalysis (Ed. G. Ertl, H. Knbzinger and J. Weitkamp) (1997), Vol. 1, p. 206, VCH, Weinheim.]. The zeolite framework is very open and contains channels and cages where cations, water and adsorbed molecules may reside and react. The specific absorption properties of zeolites are used in detergents, toothpaste, and desiccants, whereas their acidity makes them attractive catalysts. 

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pA J 2-15,7-1 and 12-4. 

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