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

The formation of silicate particles upon gradual addition of TEOS to concen trated TPAOH and TBAOH solutions, and upon dilution with water and aging, w as studied with in situ X-ray scattering (XRS) and gel permeation chromatog raphy (GPC). The samples for the GPC analyses were obtained by extraction o f the particles from solution via a sequence of acidification with HCl, sal ting out with NaCl, and phase transfer into tetrahydrofuran. XRS and GPC re veal the presence of populations of particles with discrete sizes and numbe r molecular weights, respectively, growing through aggregation. The entitie s forming in TPAOH are identified on the basis of their size and number mol ecular weight relationships with species previously identified in these sus pensions by Si-29 NMR and other independent techniques. A mathematical expr ession for the X-ray scattering function is derived for particles with slab shape. When dimensions of 1.3 x 4.0 x 4.0 nm, corresponding to the nanosla b with MFI structure, or integer multiples of them are assumed, the derived function resembles the measured intensity pattern in shape and position. T he observed scattering at 6.6 degrees 2 theta at very early stages is assig ned to a trimer of tetracyclic undecamer. All larger particles including th e nanoslab observed by XRS and GPC are multiples of this trimer. The evolut ion of the system in TBAOH also obeys a stacking sequence of particles of t he same size, number and molecular weights, although the kinetics are total ly different. Up to the formation of nanoblocks, TBAOH and TPAOH affect the reaction mixture in a very similar way, indicating that the first molecula r steps in the early stages of formation of MFI and MEL zeolite structures in these systems are very similar. The nanoslabs representing a specific fr agment of MFI or MEL structures have channel intersections which are at lea st to two sides open and contain TPA or TEA molecules, respectively. The su rface of these Silicalite-l and -2 fragments is decorated on the ne and be planes with alkyl groups sticking out of the surface. In nanoslabs with TEA , unfavorable template-template interaction suppresses aggregation along a and b. The final product is a "double" nanoslab, probably connected along t he later c direction of the MEL structure in which there is only a very sma ll repulsion of the butyl chains. TPA favors particle aggregation to give l arger particles measuring up to 15.6 x 8 x 8 nm, even at room temperature.