Zeolite crystal growth on the International Space Station

Zeolites L, Beta, and ZK-5 were grown in the low gravity environment of the International Space Station from precursor solutions held unmixed until activation on orbit. Resultant particle sizes of Beta and ZK-5 were larger and crystals of L smaller on average for the flight-grown zeolites than for their earth-grown controls. Different purity of the flight and terrestrial/control zeolite L samples, which depended on the degree of mixing, is hypothesized to be due to the potentially different behavior of the poorly mixed synthesis mixtures under low gravity conditions (less convective mixing). Differences in mixing characteristics of the viscous zeolite ZK-5 synthesis mixture in the low and the normal gravity environment were inferred from the fact that several flight autoclaves were only partially activated. The flight and terrestrial/control zeolite Beta products were pure zeolite Beta, had identical morphology, surface and framework Si/A1 ratio, and unit cell dimensions, but the less intense terminal silanol IR band acquired for the flight particles confirms their larger average size, and may also indicate their lower roughness. The flight and terrestrial/control zeolite ZK-5 had the same intergrown/twinned cubes morphology, however the flight particles were larger on average and composed of individual crystallites that appeared larger than these forming terrestrial controls. The flight and terrestrial/control products had the same surface Si/A1 ratio and unit cell dimension, however, a less intense isolated internal silanol IR band after heat treatment for the flight ZK-5 indicates a lower concentration of structural defects and suggests a higher thermal stability of the flight framework.