Zeolite synthesis alkali cations

Whereas the synthesis of zeolite occurs in nature and in the laboratory under strongly basic conditions (pH 9-11), they are widely used as catalysts in hydrocarbon chemistry under their acidic form. In order to obtain acidic zeolites, the alkali cations (K, Li, Na, Ca, etc.) are first exchanged by NH4+C1 followed by heating which, after release of ammonia, leaves the proton loosely attached to the framework on the Si-O-Al bridging group (Figure 2.19). 

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). 

Alkali ions (salts) influence the formation of the precursor gel for most of the synthetic zeolites (3,34,39,40). Na+ ions were shown to enhance in various ways the nucleation process (structure-directing role) (40-42), the subsequent precipitation and crystallization of the zeolite (salting-out effect) (JO and the final size and morphology of the crystallites (34,43). Informations on the various roles played by the inorganic (alkali) cations in synthesis of ZSM-5, such as reported in some recent publications (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines contradictory and essentially qualitative. 

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 templating theory is based on a stereospecificity which cannot be separated from the chemistry of the cation. Zeolites are crystallized in alkaline solutions, most readily at a pH greater than 11, limiting the cations used in zeolite synthesis to alkali, some alkaline earths, and organic cations... 

Zeolite Beta is usually obtained with Si AI ratios between 5 and infinity using tetraethylammonium (TEA+) as the template.1,2 Important parameters in the TEA-Beta synthesis are the alkali cation concentration and the type of cation used,810 the hydroxide concentration,8 10 11 the nature and the amount of the organic template,1 2,8 10 1217 the temperature10,18 and the type of silica source used.19 20... 

The synthesis mixtures were prepared using tetraethylammonium hydroxide (40 % aqueous solution, Alfa or 35 % aqueous solution, Aldrich) with a very low alkali cations content (Na<2ppm, K<0.5ppm), deionized water, tetraethylorthotitanate (TEOT) or tetrabutylorthotitanate (TBOT) as a source of Ti and amorphous silica (Aerosil 200, Degussa) or tetraethylorthosilicate (TEOS, Aldrich) as the source of silica. Depending on the synthesis method a source of aluminium can be directly added to the synthesis mixture (metal Al, aluminium halide, etc.) or incorporated in the seeds of aluminosilicate zeolite Beta. Four synthetic procedures were developed and Table 1 summarizes the typical ranges of chemical composition of the initial mixture and the typical results of the syntheses. The methods are denoted according to the nature of the silica source and, in one case, the use of seeds. The preparation of the reaction mixtures was as follows ... 

The work presented here was aimed at establishing the influence of different alkali metal cations on the distribution of silicate anions in aqueous solutions such as those used in zeolite synthesis. Changes in the extent of silicate oligomerization were identified... 

In these systems zeolite ZSM-48 does not form if HM " ions are absent (samples 5 and 6) or if their relative concentration is low (sample 4). Under such conditions the reaction leads to the formation of ZSM-5, probably because the formation of 5-1 SBU is favoured in a high-silica hydrogel containing both Al and Na" " [13,24,25]. The presence of a-quartz admixed with ZSM-5 is caused problably by the high synthesis temperature. Indeed if the synthesis temperature decreases no dense phases are detected (sample 6). In absence or with a low NaOH content even in the presence of NH4 ions, the starting hydrogel does not crystallize, showing that the presence of alkali cations is Indispensable for... 

Table 10.2 summarizes some examples of zeolites based on their classification by chemical composition. Low-silica zeolites (Si/Al < 5) are synthesized in basic conditions (pH >13) using a silicon source, an aluminum source, and alkali hydroxides at moderate temperatures, typically less than 120°C. The identity of the alkaU species used is a determining factor in which phase is obtained from synthesis, as the relative rates of (alumino)silicate hydrolysis and condensation reactions are dependent on the identity of the alkali cation. It is also believed that hydrated alkali cations effectively direct the assembly of (alumino)silicate precursors into fuUy connected three-dimensional structures. Sodium and potassium hydroxide have been used most frequently in low-silica zeolite syntheses due to their low cost and high solubility in... 

Figure 5.7 A van der Waals size comparison between the alkaline series of cations (top, from right to left Li, Na" ", K" ", Rb" " and Cs" ") and some organoammonium cations used as OSD As in the synthesis of zeolites (bottom, from right to left TMA, TEA+, TPA+ and 1,3,4-trimethylimidazolium). The size shown for TEA and TPA corresponds to one among many possible conformations. Note that in addition to large differences in their size and rigidity, alkali cations are considered featureless spheres...

A piece of experimental evidence to support the above idea can be drawn from zeolite syntheses in the presence of N,N,N,N, N, N -hexam-ethylpentanediammonium (Me -diquat-d) ion, a slightly shorter OSD A than Et6-diquat-5, the only known OSD A leading to the crystallisation of ZSM-57. This flexible diquaternary cation was reported to give at least four different zeolites i.e. EU-1 (EUO), ZSM-48 ( MRE), ZSM-12 (MTW), MCM-22(P)] depending on the A1 content and type and concentration of alkali cations in the synthesis mixtures. Since neither of... 

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