Synthesis and Structural Properties of Zeolytic Nanocrystals I. MFI Type Zeolites

Abstract

MFI type zeolite nanocrystals with a SiO2/Al2O3 ratio of 120 were synthesized using soft-chemistry conditions, at 353 K, P = 582 mmHg, from clear solutions of tetrapropylammonium hydroxide (TPAOH), fumed silica (CAB-O-SIL), and aluminum sulfate. The formation of crystallites with a low dimensionality (10−30 nm) was examined in function of the reaction time by X-ray diffraction (XRD), Fourier transform infrared (FTIR) (with KBr), 27Al and 29Si nuclear magnetic resonance (NMR) magic-angle spinning (MAS), and high-resolution transmission electron microscopy (HRTEM). Complementary information on the critical parameters of the low-dimension crystals was obtained by HRTEM and Rietveld's method. The theoretical kinematical approach was used for calculation of the high-resolution electron microscopy images (Cerius2). The physicochemical properties of the zeolitic nanocrystallites were compared with macroscopic MFI type crystals; the unit cell parameters were a = 20.02 Å, b =19.89 Å, and c =13.38 Å, which means a slight contraction of about 0.25, 0.16, and 0.3% with respect to large MFI type crystals, respectively. HRTEM indicated the ZSM5 nanocrystals in its aluminosilicate matrix present a disklike shape, a Pnma symmetry, and pore dimensions in the (010) plane of about 0.56 nm. The mean crystallite sizes along [020] and [200] were 20 and 46 nm, respectively. The high surface charge associated with the nanocrystals influences the cell dimensions and crystal morphology, mainly.