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

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 treatm ent, similar samples were prepared on earth using identical hardware. The s olutions 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 sett led under gravity into intergrown sheets; unlike the space-grown samples, n o 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 approximate to 300 x 250 x 150 mum(3), while the largest terrest rial crystals were prismatic and about 300 x 100 x 50 mum(3). Little differ ence in structure between low-gravity and terrestrial crystals was found us ing single crystal X-ray diffraction. The unit cells of the space samples w ere up to 0.05% larger in linear dimension than those of similar terrestria l crystals. These results were consistent with data from electron probe mic roanalysis, which indicated that the aluminum concentration per unit cell i n the space crystals was, on average, 2-2.5 times higher than in the terres trial crystals. Preliminary catalytic testing revealed that the crystals grown in low gravi ty have lower activity to non-shape selective reactions than those grown on earth. AFM showed that the external surfaces of the space crystals were su bstantially smoother than those of the terrestrial crystals, while the conc entrations of surface hydroxyl groups were found to be significantly higher for the earth-grown crystals. The reduction in the external surface area o f the space crystals and the concomitant reduction in hydroxyl group concen trations across the surface could be of significant commercial importance f or shape-selective reactions. (C) 2001 Elsevier Science B.V. All rights res erved.