Study on the Course of Crystallization of Gallosilicate with MFI Structure

Abstract

The kinetics of nucleation and crystallization of zeolite Ga-ZSM-5 crystallized in the system 15 Na,O * xGa,O, * 100 SiO, * 13 TPABr * 4400 H,O is investigated. The course of crystallization is followed by different methods: XRD, MIR, TG-DTA, n-hexane adsorption, NH,-TPD and pHmeasurement.The low apparent activation energies of nucleation En = 42.5 KJ/mole and crystallization E, = 20.4 KJ/mole show that the incorporation of gallium into the framework of the zeolite is not as difficult as expected from the disruptive nature of this process. Other synthesis parameters, e.g. temperature, alkalinity and gel composition have a major influence on the crystallization process. Here the nucleation is pronounced influenced whereas the crystal growth is only marginally influenced.The physico-chemical characterization proves the formation of ,.reactive" and catalytic active gallosilicate species already in the gel, which are assumed to take part directly in the crystal growth process.

Incorporation of Gallium into Zeolites: Syntheses, Properties and Catalytic Application

“…Temperature mainly affects the rate of nucleation due to the increased dissolution of silicate/gallosilicate species and to the consequently enhanced supersaturation of the synthesis gel. This is in agreement with general expectations of the temperature dependence of the nucleation and the crystallization process (Figure ) 12 Arrhenius plots for (1) nucleation and (2) crystallization of [Ga]-ZSM-5 (Si/Ga = 50).…”

“…Immediately after the addition of small amounts of gallium to the silicate solution, new peaks appear in the NMR spectra that can be attributed to gallosilicate anions. This finding supports and explains IR results which conclusively prove the formation of Ga−O−Si gallosiloxane groups in the amorphous hydrogel during the nucleation period already. With gallium, only anions with Si(1Ga) groups are observed.…”

“…Studies on the temporal course of the crystallization process leading to gallium-substituted zeolites are relatively rare. The majority of available studies ,,− deal with MFI-type zeolites (Figure ). A detailed knowledge on the crystallization steps is essential for the optimization of synthesis parameters (like time, temperature, and composition), for the replacement of expensive gel ingredients, and for the control of the synthesis procedure.…”

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11 Influence of the Si/Ga ratio in the synthesis gel on the temporal course of crystallization of [Ga]-ZSM-5: (1) Si/Ga = 100, (2) Si/Ga = 25, (3) Si/Ga = 50. (Reprinted with permission from ref . Copyright 1992 Akademie Verlag GmbH.

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“…Apparent activation energies for nucleation and crystallization amount to 42.5 and 20.4 kJ/mol, respectively. (Reprinted with permission from ref . Copyright 1992 Akademie Verlag GmbH.…”