Zeolite ZSM-5 synthesized in space: catalysts with reduced external surface activity

Abstract

Two samples of zeolite ZSM-5 were prepared in the free-fall environment (10 −4–10 −6 g) of low earth orbit aboard the space shuttle Columbia. Using identical solutions and protocols of in situ mixing and hydrothermal treatment, similar samples were prepared on earth using identical hardware. The solutions processed in space produced three principal crystal morphologies: cubic twins, cubic single and elongated prismatic crystals. The terrestrial solutions produced elongated prismatic crystals and cubic twins which settled under gravity into intergrown sheets; unlike the space-grown samples, no single cubic crystals were found. On average, crystals grown in space were larger than those grown on earth. Ignoring multiple intergrowths, the largest crystals from space were cubic, measuring ≈ 300×250×150 μm 3 , while the largest terrestrial crystals were prismatic and about 300×100×50 μm 3 . Little difference in structure between low-gravity and terrestrial crystals was found using single crystal X-ray diffraction. The unit cells of the space samples were up to 0.05% larger in linear dimension than those of similar terrestrial crystals. These results were consistent with data from electron probe micro-analysis, which indicated that the aluminum concentration per unit cell in the space crystals was, on average, 2–2.5 times higher than in the terrestrial crystals. Preliminary catalytic testing revealed that the crystals grown in low gravity have lower activity to non-shape selective reactions than those grown on earth. AFM showed that the external surfaces of the space crystals were substantially smoother than those of the terrestrial crystals, while the concentrations of surface hydroxyl groups were found to be significantly higher for the earth-grown crystals. The reduction in the external surface area of the space crystals and the concomitant reduction in hydroxyl group concentrations across the surface could be of significant commercial importance for shape-selective reactions.