Mechanism of Transformation of Precursors into Nanoslabs in the Early Stages of MFI and MEL Zeolite Formation from TPAOH−TEOS−H2O and TBAOH−TEOS−H2O Mixtures

Abstract

The formation of silicate particles upon gradual addition of TEOS to concentrated TPAOH and TBAOH solutions, and upon dilution with water and aging, was studied with in situ X-ray scattering (XRS) and gel permeation chromatography (GPC). The samples for the GPC analyses were obtained by extraction of the particles from solution via a sequence of acidification with HCl, salting out with NaCl, and phase transfer into tetrahydrofuran. XRS and GPC reveal the presence of populations of particles with discrete sizes and number molecular weights, respectively, growing through aggregation. The entities forming in TPAOH are identified on the basis of their size and number molecular weight relationships with species previously identified in these suspensions by 29Si NMR and other independent techniques. A mathematical expression for the X-ray scattering function is derived for particles with slab shape. When dimensions of 1.3 × 4.0 × 4.0 nm, corresponding to the nanoslab with MFI structure, or integer multiples of them are assumed, the derived function resembles the measured intensity pattern in shape and position. The observed scattering at 6.6° 2θ at very early stages is assigned to a trimer of tetracyclic undecamer. All larger particles including the nanoslab observed by XRS and GPC are multiples of this trimer. The evolution of the system in TBAOH also obeys a stacking sequence of particles of the same size, number and molecular weights, although the kinetics are totally different. Up to the formation of nanoblocks, TBAOH and TPAOH affect the reaction mixture in a very similar way, indicating that the first molecular steps in the early stages of formation of MFI and MEL zeolite structures in these systems are very similar. The nanoslabs representing a specific fragment of MFI or MEL structures have channel intersections which are at least to two sides open and contain TPA or TBA molecules, respectively. The surface of these Silicalite-1 and -2 fragments is decorated on the ac and bc planes with alkyl groups sticking out of the surface. In nanoslabs with TBA, unfavorable template−template interaction suppresses aggregation along a and b. The final product is a “double” nanoslab, probably connected along the later c direction of the MEL structure in which there is only a very small repulsion of the butyl chains. TPA favors particle aggregation to give larger particles measuring up to 15.6 × 8 × 8 nm, even at room temperature.