Seed-free hydrothermal synthesis of all-silica deca-dodecasil 3R with essential reagents

Abstract

Deca-dodecasil 3R (DDR) was synthesized hydrothermally from essential reagents, i.e., silica source (fumed silica), water, and 1-adamantanamine, without employing seed particles or any special treatments, such as microwave or ultrasonic irradiation. The variations in synthesis time, size, and morphology of DDR particles with reagent composition, fluorine addition, and hydrothermal temperature were examined. At 160°C DDR was synthesized in 15–20 d. Rhombohedral, star-like, and hexagonal plate-like particles were obtained and their crystallographic orientations were determined. The morphology changes in this order are caused by twinning and suppression of growth along the [0001] direction, and simultaneous increase in the average particle size is attributed to the suppression of nucleation. Similar morphology changes were also caused by adding F⁻ ions to the precursor mixture or increasing hydrothermal temperature. An increase in hydrothermal temperature to 200°C enabled a reduction of synthesis time in 5 d whereas the average particle size increased and hexagonal plate-like particles were formed dominantly. The rhombohedral particles synthesized at 160°C exhibited the largest micropore volume. The smaller micropore volumes for the hexagonal plate-like particles synthesized at 180 and 200°C are probably due to twinning and resultant narrowing and disruption of the microporous channels. In all samples high-resolution nitrogen adsorption isotherms exhibited two adsorption steps at relative pressures of ∼3 × 10⁻⁶ and ∼0.03, respectively attributed to the initial adsorption in micropores and the densification of adsorbates in micropores.