From ill-resolved atomic to ZSM-5 type of ordering in mesoporous lamellar aluminosilica nanoparticles

Abstract

Mesoporous lamellar aluminosilicate–surfactant nanoparticles as small as 50 nm can be hydrothermally synthesized at 140 °C using a mixture of cationic and anionic surfactants as a templating agent and tetrapropylammonium cations (TPA⁺), known to be the structure directing agent of the MFI type of zeolite. The nanophases exhibit some ill-resolved atomic ordering after 1 to 3 d of crystallization. A definitive ZSM-5 ordering occurs for longer crystallization times, concomitant with a progressive contraction of the lamellar array. In most cases, calcination at 550 °C leads to a collapse of the lamellar structure, while the zeolitic ordering is preserved. For short crystallization times, the lamellar structure with its atomic ordering is maintained when the aluminium source is the nitrate salt, instead of Al(OH)₃ or Boehmite. Then, after 3 d of crystallization, highly porous systems are produced with a surface area of ∼650 m² g⁻¹ and pore volume of ∼1.1 cm³ g⁻¹. For longer crystallization time, the collapse of the lamellar phase under calcination does not affect the well-defined ZSM-5-like atomic ordering and leads to non-faceted nanoparticles of ca. 200 nm size with rugged shapes. These conclusion are supported by cross investigations using X-ray diffraction patterns, infrared spectra, scanning electron microscopy and high-resolution transmission electron microscopy with selected area electron diffraction. The ordering of the inorganic sheet of the aluminosilicate–surfactant mesophase is likely favoured by the high diffusion rate of TPA⁺ in the nanosized particles.