Hydrothermal: silica dissolved under pressure

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. 

The standard zeolite synthesis solution chosen [82,97,98] was characterised by the molar ratios 100 Si02 15 (TPA)20 5.3 Na2O 1420 H2O. The mixture was obtained by mixing sodium hydroxide (Merck, > 99%) and TPA hydroxide in water to which silica (Baker, 99.75%) was added. The silica was dissolved at 100°C in about 300 s after which the solution was cooled down to room temperature in 1.5 h and aged (at 25°C) for an additional 1.5 h. The pH of the solution was > 12.5. To control crystal size and layer thickness, hydrothermal treatments were applied in a stainless steel autoclave under autogenous pressure at temperatures of 40-180°C for 10-200 h with the support on the bottom of the autoclave. 

The reaction is carried out in water, but the reagents are solids. A solid is precipitated and by reaction with other components of the solid mixture or with dissolved species it is transformed into a new solid. The reaction takes place in a closed vessel (a hydrothermal bomb) usually built in stainless steel lined with Teflon. The reaction takes place at 150-500 °C (depending on the liner used) and high autogenous pressures. Water acts as a pressure transmitter and as a solvent. Seed crystals and a temperature gradient are sometimes used for crystal growth. Under these conditions, solubilization of very insoluble species (e.g., silica) talces place, and the reaction proceeds for instance ... 

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