The morphology control of zeolite ZSM-5 by regulating the polymerization degree of silicon and aluminum sources

• Polymerization degree of silica sol and alumina were regulated by NaOH and H2SO4.
• The morphology of ZSM-5 was controlled by different Si and Al sources.
• The crystallization mechanism was revealed in gels by different Si and Al sources.

This study was focused on the regulation of the polymerization degree of silica sol and alumina with NaOH and H2SO4, and the role of the polymerization degree of Si and Al sources in determining the morphology of zeolite ZSM-5. Results show that, NaOH and H2SO4 were useful for decreasing the polymerization degree of silica sol and alumina, respectively. Micro-sized large ZSM-5 crystal can be obtained when the highly polymerized silica sol and alumina (slightly depolymerized by NaOH and H2SO4) were used. On the other hand, fine ZSM-5 crystal can be obtained when the highly depolymerized silica sol and alumina (deeply depolymerized by NaOH and H2SO4) were used. It is possible to fine tune the polymerization degree of silica sol and alumina by NaOH and H2SO4 dosage, and therefore to fine tune the crystal size of zeolite ZSM-5. Raman spectra disclosed that, there existed only very small amount of five-membered structure species in the gels formed by the highly polymerized Si and Al sources. However, monomeric silicate anions could be found in these gels. In contrary, in the gels formed by the deeply depolymerized Si and Al sources, abundant five-membered structure species were seen together with a small amount of monomeric aluminate anions. Such differences of the two kinds of gels accounted for the remarkable distinction of their crystallization processes and product morphologies.

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