Effect of pore size on the chemical removal of organic template molecules from synthetic molecular sieves

Abstract

Various aluminophosphate and silicoaluminophosphate molecular sieves were treated with methanolic HCl to see its effectiveness as an alternative route for removing organic template molecules from molecular sieves. Both calcined and methanolic HCl-treated samples were characterized by X-ray powder diffraction and water adsorption-desorption isotherm measurements. The effectiveness of methanolic HCl in the removal of template molecules was found to be dependent on the pore size of the molecular sieves; that is, the template molecules were extracted effectively in sieves having a pore size ≥6 Å. This method is not effective for molecular sieves with a pore size <6 Å. This differential behavior is apparently a result of steric hindrance, that is, difficulty in removing template molecules from the zeolite cages as a result of their large size compared with the cage opening. This behavior has been exploited to stabilize the structure of the molecular sieve SAPO-37 against water by selectively removing the template molecules from the large cages (α cages) but keeping them intact in the small cages (β cages).