Fine control of crystal morphologies of all-silica DDR in ethylenediamine-free gel with inorganic base as mineralizing agent

The size and morphology control of zeolite crystals is important which might have significant impact on their adsorption, separation and catalytic properties. Traditional hydrothermal synthesis of zeolite DDR usually yields big diamond-shaped crystals and there are few reports about its size and morphology control. In this study, microwave synthesis of all-silica DDR zeolite was systematically studied with 1-adamantane amine as template and inorganic base as mineralizing agent (KOH, NaOH and LiOH). The use of co-solvent (ethylenediamine) and fluorides were avoided which is environment-friendly. The use of inorganic bases also reduced synthesis cost. High quality micron sized DDR crystals with high yield were obtained in only a few hours which is the synergy of microwave heating and seeding, both are powerful methods on reducing synthesis time. A rich variety of crystal morphologies, like diamond, bipyramid, hexagonal prism and sphere, were obtained which can be controlled by the addition of metal cations, gel alkalinity, synthesis temperature and gel dilution. 29Si NMR, FT-IR and nitrogen adsorption confirmed the formation of zeolite framework and its porosity. The fine control of crystal size and morphology of DDR zeolite may facilitate its application in adsorption, membrane separation and catalysis.