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 1520 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 ¹ 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 ³3 © 10 ¹6 and ³0.03, respectively attributed to the initial adsorption in micropores and the densification of adsorbates in micropores.

Twinning by Merohedry and Thermal Expansion of Zeolitic Clathrasil Deca-dodecasil 3R

“…The good agreement of the observed and calculated angles confirmed that the rhombohedral particle consists of six {101̅1} faces ({100} faces in the rhombohedral representation) and the crystal faces A, B, and C can be indexed as (101̅1), (11̅01̅), and (011̅1̅), respectively. These crystal faces are almost parallel to the 8-membered rings, as also shown in Figure , middle. Thus, the rhombohedral crystal particles of DDR have the highest surface density of openings for the microporous channels and are the most suitable for applications in separation engineering.…”

“…The fractions of the two twin domains were ∼0.5 and nearly equal. In an earlier report, this binary twin operation did not improve the fit well ( R = 0.093) because a hexagonal platelike particle, which additionally contains penetration twinning, was used. These observations indicate that the rhombohedral particle exhibits twinning by merohedry but is free from penetration twinning.…”

“…The data of thermal expansions are also limited to a powder sample, although their importance in the evaluation of expansion mismatch between DDR-based separation membranes and their supports. In addition, the crystal faces of the rhombohedral particles had not been indexed until the recent study on crystal morphology and single-crystal X-ray diffraction . This study indicated that the index of the crystal faces is {101̅1}, whereas this identification was conducted under the restriction of h , k , and l ≤ 5 and did not exclude the possibility that these faces have higher indices.…”

“…We recently investigated the effects of synthesis conditions on the size and morphology of DDR crystal particles. 17 Hexagonal platelike crystal particles are obtained when crystal growth is fast and growth along the [0001] axis is suppressed, for example, by high hydrothermal temperatures and the presence of fluoride ions. We confirmed that the hexagonal platelike particles suffer from penetration twinning generated by the twin law 2 [001] .…”

Comparative study on different strategies for synthesizing all-silica DD3R zeolite crystals with a uniform morphology and size

“…Bai et al 49 developed an environment-friendly synthesis method to prepare DD3R crystals, in which only the silica source, a small amount of template, and a trace amount of water were used in the synthesis, without adding a toxic solvent such as ethylenediamine or the mineralizing reagent uoride. Without seeding, Kajihara et al 50 used the essential reagents, that is, a silica source, water, and 1-ADA, hydrothermally to synthesize DD3R crystals and found that the synthesis time could be signicantly reduced from 15-20 d at a hydrothermal temperature of 433 K to 5 d at a hydrothermal temperature of 473 K. Most recently, Wang et al 51 took advantage of the synergy of microwave heating and seeding to signicantly reduce the synthesis time of DD3R crystals to a few hours, and in addition, an inorganic base, such as KOH, NaOH, or LiOH, was used as a mineralizing reagent instead of ethylenediamine and uoride, which is comparatively more environment-friendly and economical. Although signicant progress in the synthesis of DD3R crystals, in terms of the synthesis time, cost, phase purity and so forth, has been made in recent years, unfortunately, most of the synthesized crystals were not uniform in morphology and size.…”