Zeolites different structures

Ammonium salts of the zeolites differ from most of the compounds containing this cation discussed above, in that the anion is a stable network of A104 and Si04 tetrahedra with acid groups situated within the regular channels and pore structure. The removal of ammonia (and water) from such structures has been of interest owing to the catalytic activity of the decomposition product. It is believed [1006] that the first step in deammination is proton transfer (as in the decomposition of many other ammonium salts) from NH4 to the (Al, Si)04 network with —OH production. This reaction is 90% complete by 673 K [1007] and water is lost by condensation of the —OH groups (773—1173 K). The rate of ammonia evolution and the nature of the residual product depend to some extent on reactant disposition [1006,1008]. 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

The objective of this contribution is to investigate catalytic properties of zeolites differing in their channel systems in transformation of aromatics, i.e. toluene alkylation with isopropyl alcohol and toluene disproportionation. In the former case zeolite structure and acidity is related to the toluene conversion, selectivity to p-cymene, sum of cymenes, and isopropyl/n-propyl toluene ratio. In the latter one zeolite properties are... 

The catalyst used for the conversion of methanol to gasoline is based on a new class of shape-selective zeolites (105-108), known as ZSM-5 zeolites, with structures distinctly different from other well-known zeolites. Apparently, the pore dimensions of the ZSM-5 zeolites are intermediate between those of wide-pore faujasites (ca. 10 A) and very narrow-pore zeolites such as Zeolite A and erionite (ca. 5 A) (109). The available structural data indicate a lattice of interconnecting pores all having approximately the same diameter (101). Hydrocarbon formation... 

Zeolites form many different families in which common stmcture building units for the frameworks are found. They have a tendency to form intergrowths between different structures within the same families. 

N- and C-Methylanilines Formation on Zeolites with Different Structural and Acidic Properties... 

The authors suggest that the new method could be of some value for titanium containing zeolites with structures different from silicalite, for instance large pore zeolites which could be useful in the oxidation of large molecules which cannot be oxidized with TS-1. 

Furthermore, aluminosilicates are available in a variety of different structural types including lamellar clays and three-dimensional microcrystalline zeolites. Such solids can be useful... 

Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-7]. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. 

Synthesis of silica-based materials with controlled skeleton structures, such as zeolites, requires controlling the structure of oligomeric silicate species at the first reaction step. Organic quaternary ammonium ions, which are known as organic templates in zeolite synthesis (1 ), have a role in making up the specific structures of silicate anions, whereas silicate anions randomly polymerize in aqueous solutions containing alkali metal ions, resulting in the presence of silicate anions with different structures. 

Effect of Addition of Sodium Ions to Tetramethylammonium Silicate Aqueous Solution. In zeolite synthesis, alkali metal cations are combined with organic quaternary ammonium ions to produce zeolites with different structures from the one produced with only the organic quaternary ammonium ion (2) It is then expected that other types of silicate species are formed in the silicate solutions when organic quaternary ammonium ions and alkali metal cations coexist. In such silicate aqueous solutions, however, alkali metal cations only act to suppress the ability of the organic quaternary ammonium ions to form selectively silicate species with cage-like structures (13,14,28,29). 

The tetrahedral unit that forms the basis of the network of the zeolite makes many different structures possible. a-Quartz, the low temperature form of SiC>2, is one of the dense polymorphs. Many low-density structures can be formed, which are microporous and contain interconnected channels, bound by oxygen atoms that connect the cation-containing lattice tetrahedra. Approximately 60 different structures exist two examples of are given in Fig. 4.61a and 4.61b. The oxygen atoms are located in the middle of the lines connecting tetrahedrally... 

When Cd(CN)2 is crystallized in the presence of other molecules that can stuff cavities or tunnels, many different structures are formed depending on the size and shape of the guests that stuff the cavities. Similar behavior is, of course, found elsewhere, e.g., in gas hydrates and hydrothermal synthesis of zeolites. These cadmium cyanide structures may be considered as a new class of clathrates. 

The first breakthrough was provided by Flanigen et al. [7] who, playing on the similarity 2 Si4+ -o-A13+ -f P5+, synthesized microporous aluminophosphates (hereafter noted AlPOs) with structures related to those of zeolites. The structural studies [8] showed however a striking difference between the two families. As already mentioned, the framework of zeolites is built up exclusively from connected tetrahedra which can accept small amounts (<10% of substitution) of other metals, whereas in aluminophosphates and homologous gallophosphates, Al and Ga polyhedra can adopt five and sixfold coordinations, which change [9] the connectivity of the framework, and therefore the shape of the windows. 

The zeolites have been first used as catalyst in the 1960s for alkane cracking reactions in petroleum industry.They replaced favorably previously employed alumina based catalysts because of their better therm and mechanical stability. Moreover, they showed higher selectivity. The selectivity finds its source in the zeolite micropore structure with different... 

Zeolites of different structures and pore opening widths exhibit different reduction profiles for the same metal precursors and the same pretreatment... 

---