Tetramethylammonium zeolites

Pyrolysis of Sodium-Tetramethylammonium Zeolite Omega. Preliminary calcinations of the Q zeolites showed that intracrystalline diffusion restrictions interfered greatly with transport both of oxygen and of calcination products. Under mild conditions, coking was observed, and even under favorable conditions (550° C, thin beds, good venting) the reaction was slow. Some samples of zeolite were pyrolyzed under vacuum, and the products were identified by low resolution mass spectrometry. 

We have adopted the use of N to refer to a synthetic zeolite which is prepared from systems that contain alkyl ammonium bases. Thus, N-A refers to a synthetic tetramethylammonium zeolite that has the type A framework (9). 

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

In zeolite synthesis, large cations such as tetramethylammonium (NMe4" ) and tetrapropylammonium (N(C3H7)4" ) can be used as a template around which the aluminosilicate framework crystallizes with large cavities to accommodate the ion. On subsequent heating the cation is pyrolysed, but the structure retains the cavities. Such structures formed around a single molecule template, with pore sizes between 200 and 2000 pm, are known as microporous. 

The formation of novel silicon-rich synthetic zeolites has been facilitated by the use of templates, such as large quaternary ammonium cations instead of Na+. For instance, the tetramethylammonium cation, [(CH3)4N], is used in the synthesis of ZK-4. The aluminosilicate framework condenses around this large cation, which can subsequently be removed by chemical or thermal decomposition. ZSM-5 is produced in a similar way using the tetra-.n-propyl ammonium ion. Only a limited number of large cations can fit into the zeolite framework, and this severely reduces the number of [AIO4] tetrahedra that can be present, producing a silicon-rich structure. 

Wu et al. (5) recently interpreted the thermal decomposition mechanism of tetramethylammonium-exchanged Y zeolite. The order of occurrence of the gaseous decay products is (mole %) (CH N (50), CH4 (11), (CH3)20 (10), CO (9), CH3OH (6), H2 (4), C4H8 (4), C2H4 (trace), for the decomposition carried out at 275°C under vacuum. At this temperature, a displacement reaction of water nucleophile on the tetramethyl cation, forming methanol and trimethylamine, is proposed ... 

Ion Exchange and Pyrolysis. A typical specimen of zeolite 0 contains approximately eight cations per unit cell, of which about six are sodium ions, and the remaining two are tetramethylammonium ions. The positions of these ions within the zeolitic framework can be determined by x-ray structural studies or by ion-exchange experiments if the structure of the framework is known. 

Effect of the Structure of Organic Quaternary Ammonium Ions. The tetramethylammonium ion (N C J ), first introduced in zeolite synthesis by Barrer and Denny (30), and Kerr and Kokotailo (21) is effective in forming the cubic octameric silicate anion (Sig02Q°, cubic octamer) (2-16). In the tetramethylammonium silicate aqueous solutions at higher S3.O2 concentrations or cation-to-silica molar ratios (abbreviated to the N/Si ratios), the cubic octamer is singularly formed. 

Effect of Temperature. The temperature of a silicate solution also affects the polymerization of silicate anions in the solution. The distribution of silicate anions in an organic quaternary ammonium silicate solution at a fixed N/Si ratio and SiC concentration varies with the temperature of the solution (7,8,13,14,16). Ray and Plaisted (8) reported the temperature dependence of the distribution of silicate anions in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 2/3 and a SiC>2 concentration of 1.0 mol dm. The amount of the cubic octamer in the solution decreases with increasing temperature, and the cubic octamer practically disappears above 50 °C, indicating that the cubic octamer is unstable at higher temperatures. However, Groenen et al. (14) found that the cubic octamer remained in a significant concentration even at 85 °C, which was close to the temperature of actual zeolite formation, in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 1.0 and a Si02 concentration of 1.3 mol dm-. ... 

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). 

Table II. Effect of Tetramethylammonium on the Nature of the Zeolite Product...

Magadiite apart from pentasil phases can be found in zeolites synthesized with mono-n-methy1-, -ethyl-, -propyl- or -butylamine, with di-n-methyl-, -ethyl- or -propylamine, with tri-n-methyl- or -ethylamine and with tetramethylammonium iodide. Table IV shows that the zeolite content increases with the number of carbon atoms per group and with the number of alkyl groups. 

Inelastic and quasielastic neutron scattering have special advantages for studying certain of the motional properties of protonated or organic species within zeolites and related microporous materials. These advantages and various experimental methods are outlined, and illustrated by measurements of torsional vibrations and rotational diffusion of tetramethylammonium (TMA) cations occluded within zeolites TMA-sodalite, omega, ZK-4 and S APO-20. 

The vibrational spectrum of the tetramethylammonium cation in the region 150 -550 cm l contains botii torsional and vibrational modes. The vg and V19 vibrational modes of E and T2 symmetry involve C-N-C bond angle bending. These modes are Raman active and have been studied for TMA+ in several zeolite environments, although little change in frequency is observed (51). The V4 and V12 torsional modes involve partial rotation about C - N bonds and form respectively a singlet (A2) and a triplet (Ti) which are both Raman inactive. These torsional modes are directly observed in the HNS spectra and prove to be sensitive to the character of the TMA+ cation (see Table 1) environment(52). 

In the search for zeolites with new pore dimensions and channel structure, researchers employed the use of templating molecules to direct the growth of the aluminosilicates. Tetramethylammonium was the first of these templates used to prepare sodalite and others soon followed [61]. Recently, spectroscopic studies have... 

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