Zeolite synthesis precursors

Microporous nanoparticles with ordered zeolitic structure such as Ti-Beta are used for incorporation into walls or deposition into pores of mesoporous materials to form the micro/mesoporous composite materials [1-3], Microporous particles need to be small enough to be successfully incorporated in the composite structure. This means that the zeolite synthesis has to be stopped as soon as the particles exhibit ordered zeolitic structure. To study the growth of Ti-Beta particles we used 29Si solid-state and liquid-state NMR spectroscopy combined with x-ray powder diffraction (XRPD) and high-resolution transmission electron microscopy (HRTEM). With these techniques we monitored zeolite formation from the initial precursor gel to the final Ti-Beta product. 

Si-Al] synthesis and zeolite synthesis have similarities advances in the synthesis methods of high surface area amorphous [Si-Al] have been attained by the use of TPAOH in an alkali-free reaction mixture, such as the precursor gel for the synthesis of a ZSM-5 zeolite [15]. 

First, although the use of bulky organic bases clearly shifts the silicate equilibrium to the DnR species, there may be a large amount (up to more than 90%) of polymeric species present in silicate solutions. This is true especially at low OH/Si ratios (<0.5) or high Si concentrations (>2), i.e., normal values for a zeolite synthesis composition. This range of polymeric silicates cannot at present be characterized satisfactorily, and the presence of zeolite precursor species other than DnR silicates in this range cannot be excluded. 

Second, compared to the time scale of a zeolite synthesis, the rate of exchange between all small silicates is very fast. The concentration of possible zeolite precursor species such as the DnR in this silicate range is accordingly expected to be constant throughout a synthesis (vide infra). On this basis, it is very well possible that, for instance, the DnR silicates do play a precursor role however, a conclusive proof for this will be difficult to obtain (labelling, for example, is not possible). 

On the basis of the composition and dynamics of silicate solutions and zeolite synthesis mixtures only, a precursor role for the D5R silicates during both stages cannot be excluded since these species are present in relatively high concentrations during the zeolite synthesis process. 

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