Aluminosilicates, crystalline metal

Molecular sieves are crystalline metal aluminosilicates (1). Openings in their crystal structure permit passage of many gas constituents while preferentially adsorbing large, polar, or unsaturated compounds. Acid gas compounds may be adsorbed by certain types of molecular sieves. When used for H2S removal, the sieve is regenerated by a thermal swing cycle CL), being heated to release the H2S for downstream sulfur recovery. 

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]. 

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. 

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. 

Zeolites are crystalline aluminosilicates with porous, framework structures made up of linked [Si04] and [A104] tetrahedra that form channels and cages of discrete size [24]. The framework structures of zeolites bear a net negative charge, which must be balanced by positively charged species, typically alkali or alkaline earth metal cations these cations maybe exchanged for one another under appropriate experimental conditions. Zeolites are capable of... 

The concept zeolites conventionally served as the synonym for aluminosilicates with microporous host lattice structures. Upon removal of the guest water, zeolites demonstrate adsorptive property at the molecular level as a result they are also referred to as molecular sieves. Crystalline zeosils, AlPO s, SAPO s, MAPO s (M=metal), expanded clay minerals and Werner compounds are also able to adsorb molecules vitally on reproval of any of the guest species they occlude and play an Important role in fields such as separation and catalysis (ref. 1). Inclusion compounds are another kind of crystalline materials with open framework structures. The guest molecules in an inclusion compound are believed to be indispensable to sustaining the framework structure their removal from the host lattice usually results in collapse of the host into a more compact crystal structure or even into an amorphous structure. 

Zeolites are crystalline aluminosilicates whose primary structure is formed by Si04 and A104 tetrahedra sharing the edges . Their tertiary structure forms uniform channels and cavities of molecular dimensions that are repeated along the zeolite lattice. Due to the lower valence of the aluminium relative to silicon, the excess negative charge (one per A1 atom) is balanced by alkali metal cations, mainly Na". An important class of the zeolite family are the faujasites, known as zeolites X and Y, which have the typical composition for the unit cell as follows ... 

From a theoretical point of view, the gel layer is a barrier that reduces further hydrolysis of the silicate network, and is supposed to be more stable than the glass matrix, thus reducing the overall rate of corrosion. However, gel exfoliation may momentarily re-activate corrosion, at least locally. No clear trend was observed for the presence of the crystalline secondary phases identified at the surface of the corroded HT samples. The most abundant minerals are aluminosilicates, calcium phosphates, Fe- and Mg-rich minerals, and zeolites their role in the scavenging or release of metals remains ambiguous, although many mineral phases identified bear traces of metals. 

C at pressures of about 250—400 kPa (36—58 psi). The two types of catalysts, the amorphous silica—alumina (52) and the crystalline aluminosilicates called molecular sieves or zeolites (53), exhibit strong carboniumion activity. Although there are natural zeolites, over 100 synthetic zeolites have been synthesized and characterized (54). Many of these synthetic zeolites have replaced alumina with other metal oxides to vary catalyst acidity to effect different type catalytic reactions, for example, isomerization. Zeolite catalysts strongly promote carboniumion cracking along with isomerization, disproportionation, cyclization, and proton transfer reactions. Because butylene yields depend on the catalyst and process conditions, Table 7 shows only approximations. 

The Effects of Ionizing Radiation on Solid Catalysts Ellison H. Taylor Organic Catalysis over Crystalline Aluminosilicates P. B. Venuto and P. S. Landis On the Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of tr Complex Multicenter Processes G. N. SCHRAUZER... 

Most metals occur in nature as minerals, the crystalline, inorganic constituents of the rocks that make up the earth s crust. Silicates and aluminosilicates (Section 19.7) are the most abundant minerals, but they are difficult to concentrate and reduce and are therefore generally unimportant as commercial sources of metals. More important are oxides and sulfides, such as hematite (Fe203), rutile (Ti02), and cinnabar (HgS) (Figure 21.1), which yield iron, titanium, and... 

Zeolite Zeolites are crystalline aluminosilicates, which exhibit high selectivity for ammonia, but can also remove some heavy metals (or add small amounts of calcium to a sample). Effluent samples are passed through a column containing zeolite and then tested for toxicity. 

We discuss crystalline inorganic ion exchangers, such as [97,98,104,134-146] zeolites, hydrotal-cites, alkali metal titanates, titanium silicates, and zirconium phosphates, and also organic ionic exchangers such as the ion-exchange resins. However, details are given only in the case of the archetypal crystalline ionic exchangers, that is, aluminosilicate zeolites [97,98,104,134]. 

Zeolites (crystalline aluminosilicates) supporting oxidizing metals (Fe or Cu) have been introduced by Garcia and co-workers [106]. In particular, they showed that FeMCM-41 affords binaphthol from 2-naphthol as virtually the only product. Unfortunately, this compound is trapped in the zeolite pores and the isolated yields are lower than expected. 

---