Liquid Chromatography - The Fluidized Bed Method for Bonded Phase Synthesis 2
A steam generator, similar in form to the silanizing reagent vaporizer, is also attached to the preheater and consequently, can supply steam to the fluidized bed if, and when required. This means that the fluidized bed can be used to hydrothermally treat the silica prior to silanization. The nitrogen stream is continuously monitored by a flow-meter. All vapor and nitrogen streams are adequately fitted with valves to provide the maximum operating flexibility. The fluidizer is normally loaded with about 25 g of silica (which provides a fluidized bed about 5 cm long) and heated to 200oC for about 3 hr. in a fluidizing stream of preheated nitrogen.
The silanizing reagent is then heated to its boiling point in the vaporizer or to a temperature where it has a significant vapor pressure and the reagent blown through the fluidized bed by a stream of nitrogen. Excess, unreacted reagent is condensed and passed directly back to the vaporizer. The reaction is allowed to proceed for about 6 hours and then the reagent vapor flow stopped and replaced by a stream of pure nitrogen. When reagent is no longer returning to the vaporizer, the bed is allowed to cool to room temperature with the nitrogen still flowing. When cool, the nitrogen flow is stopped, and the bonded phase can be removed from the fluidizer. Alternatively, the silanizing reagent can be replaced with a capping reagent and the material capped by a similar procedure and the product then removed.
The fluidized bed synthesis gives a more reproducible product and, at the same time, eliminates many of the tedious operations that are involved in the alternative method, such as solvent removal and recovery, washing procedures and other manipulations. It also offers the possibility of carrying out very complicated syntheses involving multiple steps by a relatively simple procedure. However, the technique is more complicated, requires more elaborate apparatus and needs to be operated by an experienced technician.