Crystal Morphology Control of LTL-Type Zeolite Crystals

Abstract

The effects of the chemical composition of the starting gel, aging time, and crystallization temperature on the crystal size and morphology of zeolite L were studied. The chemical variables were the water content, concentration of potassium hydroxide, substitution of potassium by sodium, introduction of small amounts of barium, and the Si/Al ratio of the initial gel. The changes in the crystal morphology were evaluated by the aspect ratio of the prismatic ((100) and (1−10)) and pinacoidal (001) faces. The water content in the system was found to have the most pronounced effect on this ratio, providing crystals with a prismatic/pinacoidal face ratio between 0.5 and 3. Among the other chemical parameters, the alkalinity of the system had a pronounced effect on the nucleation and thus on the ultimate crystal size. The effects of potassium/sodium substitution and the introduction of barium in the starting gel on the zeolite L morphology were negligible. The interdependence of the chemical variables, crystallization temperature, and aging time and their impact on the crystal size and morphology is discussed. The crystal size and morphology of zeolite L were also influenced by the variation of the aging time and crystallization temperature. Thus, long aging of the gel (1−4 weeks) under ambient conditions provided nanosized crystals with narrow particle size distribution, while the increase of crystallization temperatures resulted in the formation of large (4−5 μm) crystals with high prismatic/pinacoidal faces ratios. The study allowed a fine-tuning of the size (from 140 nm to 7 μm) and of the morphology of zeolite L crystals, which varied from long prismatic crystals terminated by a small pinacoid to coin-like crystals with very short prismatic faces and a well-developed pinacoid. The changes in the aspect ratio between prismatic/pinacoidal faces lead to changes in the number of pore openings per total external surface. This number, together with the variation of the length of the prismatic faces, is expected to have an impact on the performance of the material.