Cubic octamer

FIGURE 2. Structures of silsesquioxanes. Left ladder structure middle double four-ring (= cubic octamer) right partial cage structure... 

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 the Structure of Silicon Sources. Hoebbel et al. used silicic acid sols, silicic acid gels, or Aerosil as a silica source of tetramethylammonium silicate aqueous solutions (9). In the solutions at the conditions that a N/Si ratio is 1.0 and Si02 concentration is ca. 1.4 mol dm-, the distributions of silicate anions are almost the same, and the cubic octamer is a dominant species, although the degradation rates of the silica sources are different. This suggests that the cubic octamer is formed in the tetramethylammonium silicate aqueous solution, regardless of the type of silica source with tetra-functionality. Tetraalkoxysilanes (Si(0R), R denotes an alkyl group) can be used as a silica source as well (4,12,14). 

Sakka, S., unpublished data.)- In the solutions, a number of methylsilsesquioxane species, formed by the hydrolysis of methyltriethoxysilane, with different structures are present even under the conditions where the cubic octamer is dominant in the aqueous silicate solutions. This indicates that the use of a silica source with tetra-functionality is required for the selective structure formation with the aid of organic quaternary ammonium ions. 

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-. ... 

Kuroda, K. Kato, C. J. Chem. Soc., Chera. Commun. in press.). Moreover, the amount of the cubic octamer formed in the methanolic system at higher SiC concentrations is larger than that in the aqueous system (25), suggesting that methanol increases the degree of polymerization of silicate species and stabilizes the higher molecular weight species. It is reported that dimethyl sulfoxide ((CH SO) also has such an effect (14,34). 

As the exothermal reaction occurs after 200 min of stirring, the cubic octamer and the species with molecular weights higher than the cubic octamer are recovered from the bottom phase. As indicated with the mixture containing sodium ions, it appears that the polymerization of silicate species following the hydrolysis of tetraethoxysilane proceeds abruptly when the exothermal reaction occurs. Actually, the amount of tetraethoxysilane in the top phase decreases pronouncedly as the temperature rises, and the mixture becomes a single phase within a few minutes of the exothermal reaction. 

Although the silicate skeleton structure of the (2-hydroxyethyl)trimethylammonium silicate solid, deposited from the solution, consists of the SigCUQ8- silicate structure solely, the recovery of the cubic octamer from the solution at the maximum temperature is only 10.3%, indicating that the selective formation of silicate structure in the solid is promoted very rapidly with lowering temperature. 

As partially illustrated for the cubic octamer, an organo-substituted Si occupies each vertex, and oxygen atoms are located at the midpoint of each edge T is a trifunctional RSiOj/j moiety. These structures can be prepared via alkaline-solution equilibration of the polymeric hydrolyzates of organotrichlorosilanes, and many (RSiOj/jls-ie mixtures are made in this way. Certain of these silsesquioxanes are obtained in almost quantitative yield when they can selectively crystallize from equilibrating solutions e.g., Me-Tg, Ph-Tg,Ph-Ti,. 

One of the entries in Table II enabled an initial assignment error in ref. (J) (for the cubic octamer) to be corrected in Table I, leaving one unassigned unsplit line at A634 = -25.60 ppm. It is possible that this peak is due to a 5- or 6-membered ring, or to a double -5- or double -6- membered cage (j[2), but chemical shift considerations make all these possibilities seem somewhat unlikely. 

The nature of the twelve species positively identified indicates a predilection for rings and cages, especially those involving three- and four-membered rings (whether in cages or not). We have not found any of the cages which may be considered as basic building blocks of zeolites, except for the cubic octamer, but we... 

Figure 9. Si NMR at 79,5 MHz and 5°C of alkaline aqueous tetramethylammo-nium silicate solution (9) with atomic ratio N Si = LO and concentration 2 Min SL Sample was enriched in Si to 95.3%. Peak assignments are indicated. Peak due to cubic octamer is not taken to its full height. Measurements show this peak is ca. 12 times as intense as that of monomer. (Reproduced, with permission, from Ref. 9. Copyright 1981.)...

If 95% of the silica reacts in 6 minutes the corresponding value of k is about 0.5. According to Figure 3. this is close to the value for the cubic octamer, Si0g0g(0H)g. This suggests that silica that reacts in less than six minutes and is not extractable, consists of monomer and oligomers. 

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