Alkali-modified silica

Mn(N03)2 4H20 (Merck, P.A.) was slowly added to the supports to give solids with a content of about 6 wt.% Mn. The solids were subsequently dried at 393 K for 16 h and calcined at 773 K for 4 h in a muffle. A series of acid and alkali-modified silica-supported manganese oxide catalysts was also prepared. The silica support was first treated with acid (H2SO4 or citric acid) or alkali (Na or Cs) in a way similar to the one for the bulk manganese oxides, and then impregnated by incipient wetness with an aqueous solution of manganese, dried at 393 K for 16 h and calcined at 773 K for 4 h. 

For a more reliable study of this series of supported catalysts, the blank supports were also treated with 50 cm of deionised water, then evaporated, dried and calcined in the same way as for the acid and alkali-modified silica-supported catalysts. These catalysts are designated hereafter as Mn/(Sup)X, where Sup refers to the commercial support used, and X refers to the additive used in the case of the silica-supported catalysts. 

Figure 7-5. Schematic structure of silica a) cristobalite b) amorphous silica c) alkali modified silica. Silicon atoms are small dark circles. Oxygen atoms are open circles. Large shaded circles in c) represent impurity ions such as sodium. (Adapted from Lamkin et al., 1992.)...

Fig. 3. Total alkalis versus silica (TAS) and (b) AFM plot of Irvine and Baragar (1971), (c) modified Zr/Ti02-Nb/Y plot (Pearce, 1996) of Winchester and Floyd (1977), (d) Rock/chondrite-normalized REE diagram for rocks of amli-llica pluton and (e) rock/MORB-normalized spidergrams, (f) Th/Yb vsTa/Yb diagram.

Nakajima, M., Kimura, K., and Shono, T. (1983) liquid Chromatography of Alkali and Alkaline Earth Metal Salts on Poly(benzo-15-crown-5)- and Bis(benzo-15-crown-5)-modified Silicas, Anal. Chem. 55, 463-467. 

Lauth, M. and Gramain, P. (1987) Chromatographic Separations of Alkali and Alkaline Earth Metal Cations and Some Anions on Benzo-18-crown-6-modified Silicas, J. Chromatogr. 395, 153-158. 

Fig. A.2. Total alkali vs. silica (TAS) classification diagram of volcanic rocks (modified from Le Maitre 1989). Rocks falling in the shaded area may be subdivided as shown in the inset table, on the basis of their sodic (Na2O-2.0 > K20) or potassic (Na2O-2.0 < K20) affinity.

Alkali metal hydroxides, dissolution rate effect, 521-523f Alkoxides, formation of uniform precipitates, 451-464 Aluminum, silicic acid effect on adsorption in food, 612/ 613 Aluminum in biological systems, 604, 605f, 606 Aluminum-modified silica sol, formation, 62, 63/ Aluminum-silicon interactions in biology,... 

In a series of patent applications [28-31] Davies et al. show that the problems caused by high amounts of alkali metal ions could be overcome by using silica sols as the water-based binder instead of solutions of alkah silicates. The preferred sols had a particle size between 5 and 10 nm and a Si02/Na20 mole ratio of about 50 1. They also found that they could increase the pot life of the shop primer by using aluminum-modified silica sols. 

A batch equilibrium method, as a function of pH and shaking time, was performed to characterize the metal uptake properties of silica gel-immobiUzed-aminophenol and aminobenzoic acid. Studies were made and evaluated on the basis of column application of immobilized chelating sorbents for the removal of various metal ions from sugar cane molasses. These revealed that they have higher preference toward Mn(II), Fe(III), Co(II), Cu(II), and Zn(n) rather than alkali and alkaline earth metals.The structures of the modified silica gel phases are shown in Scheme 10. 

Poly (crown ether) as well as the corresponding mono-cyclic crown ethers was immobilized on silica surface by covalent bonding. The chromatographic behavior of the modified silica gel phases for alkali metal salts was demonstrated. Poly (crown ether)-modified silica provided better separation of the metal salts compared to the corresponding monocyclic crown ether-modified one. Other examples of modified silica and their applications (Table 1). 

Poly(benzo-15-crown-5) and hixfbenzo-lS-crown-S) modified silica gel Extraction and chromatographic separation of alkali and alkaline earth metal ions [81]... 

Let us consider an allcali-modified silica glass. Because of mobile modifiers, the glass will undergo ion exchange, alkali being leached. Let us consider for instance sodium. Water diffusion (in feet HsC ) into the glass network is as follows ... 

Beside alkaline non-surface-modified colloidal silica there are also deionized, or salt-free, silica dispersions, which are zero charged at about pH 2 [6]. Such dispersions are not as long-term stable as alkali-stabilized coUoidal silica dispersions, but can stiU be stable for years if stored at low temperatures [7]. In general, the colloidal stability for dispersions of non-surface-modified silica particles is dependent on pH, salt concentration, counter ion valence... 

A similar type of catalyst including a supported noble metal for regeneration was described extensively in a series of patents assigned to UOP (209-214). The catalysts were prepared by the sublimation of metal halides, especially aluminum chloride and boron trifluoride, onto an alumina carrier modified with alkali or rare earth-alkali metal ions. The noble metal was preferably deposited in an eggshell concentration profile. An earlier patent assigned to Texaco (215) describes the use of chlorinated alumina in the isobutane alkylation with higher alkenes, especially hexenes. TMPs were supposed to form via self-alkylation. Fluorinated alumina and silica samples were also tested in isobutane alkylation,... 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

In the alkali silicate glasses, the decay time appears to increase with increasing ionic radius of the alkali metal modifier. The decay time also increases with increasing silica content, and apparently reaches a saturation value of 1.03 msec at a silica content of about 85 mole per cent. Figure 37 shows this result. 

Synthesis of MCM-41 with Additives. The hydrothermal crystallization procedure as described earlier [10] was modified by adding additional salts like tetraalkylammonium (TAA+) bromide or alkali bromides to the synthesis gel [11]. Sodium silicate solution ( 14% NaOH, 27% Si02) was used as the silicon source. Cetyltrimethylammonium (CTA) bromide was used as the surfactant (Cl6). Other surfactants like octadecylltrimethylammonium (ODA) bromide (C,8), myristyltrimethylammonium (MTA) bromide (C,4) were also used to get MCM-41 structures with different pore diameter. Different tetralkylammonium or alkali halide salts were dissolved in little water and added to the gel before addition of the silica source. The final gel mixture was stirred for 2 h at room temperature and then transferred into polypropylene bottles and statically heated at 100°C for 4 days under autogeneous pressure. The final solid material obtained was washed with plenty of water, dried and calcined (heating rate l°C/min) at 560°C for 6 h. 

Kimura, K., Harino, H., Hayata, E., and Shono, T. (1986) Liquid chromatography of alkali and alkaline-earth metal ions using octadecylsilanized silica columns modified in situ with lipophilic crown ethers,... 

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