Phase transformation and control

From a view point of reaction time, the typical preparation of mesoporous material can be divided three main steps (1) interaction between surfactant and silica (or other inorganic) species in solution and the formation of ordered mesostmcture (2) the further reaction (polymerization or condensation for silica) at a certain temperature for a time period. A possible phase transformation may occur (3) recovery of solid product by filtration, washing, and drying. The phase transformation may also occur in this step (4) removal of template from the solid product by calcination or extraction with solvent. The phase transformation is also possible even in this step. 

In fact, since the mesophase synthesis or reaction is a kinetically control process and the solid formed is not a thermally stable phase, the phase transformation is very common during the synthesis of mesoporous materials. The phase transformations include the transition from one structure or symmetry into another structure or symmetry, or the transition from a disordered phase to an ordered phase, or from an ordered phase to a disordered phase. The intermediate phase can be isolated as a product or be observed by analysis techniques. The phase transformation can occur during the synthesis process or in a post-synthesis treatment. The early famous example is the transition of lamellar mesophase of silicate into hexagonal mesophase in basic or near neutral media.[5] 

Once those parameters which have more influence on mesophase stability are known, the new synthetic pathways utilizing structural rearrangements to create new materials will become a reality. An investigation of phase transformation11581 under hydrothermal conditions indicated the most mildly basic conditions utilized (pH 9), which favor silica condensation, best inhibit the phase transformation, and thus produce the most kinetically 

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