Abstract
Silicalite-1 nanocrystals were synthesized with different sizes (40, 95 and 180 nm) from clear solutions and the organic structure directing agent removed by calcination. X-ray diffraction, scanning electron microscopy, nitrogen adsorption, Fourier transform infrared spectroscopy and thermogravimetric analysis were used for characterization. High-temperature solution calorimetry using lead borate (2PbO · B 2O 3) solvent at 974 K measured the enthalpies (Δ H tran) relative to quartz at 298 K. The Δ H tran values are 8.2 ± 0.4 kJ/mol (40 nm), 8.3 ± 0.3 kJ/mol (95 nm) and 8.0 ± 0.5 kJ/mol (180 nm). These values are essentially the same as Δ H tran for microsized MFI crystals, 8.0 ± 0.8 kJ/mol [J. Phys. Chem. B 104 (2000) 10001]. Thus the particle size has no effect on the framework energetics of silicalite. This unusual phenomenon, which is different from that of condensed nanocrystalline metal oxides, is discussed in terms of internal surface area and surface energy of zeolites.
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