Synthesis and characterization of aluminium-free ZSM-5 type chromosilicates in different alkaline systems and investigation of their pore structures

• Mesoporous chromosilicates with MFI structure were synthesized by hydrothermal method.
• Three different alkaline system were used for the synthesis of chromosilicates.
• The type of alkaline cation affects the size of the pore.
• Aggregation of platelike particles leads to formation of slit like pores.

Aluminium-free ZSM-5 type chromosilicates as M[Cr]ZSM-5 (M = Na, K and Cs), were synthesized successfully from silicic acid, sodium (or potassium or cesium) carbonate and chromium (III) nitrate using hydrothermal procedure in the presence of tetrapropylammonium bromide (TPABr) as a template. These samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetry (TG), Fourier transform infrared spectroscopy (FT-IR) and N2 physical adsorption analysis. The experimental results show that M[Cr]ZSM-5 mesoporous molecular sieves were successfully synthesized. In the synthesis of M[Cr]ZSM-5, a pure MFI phase was obtained with a relatively low concentration of chromium in the synthesis gel. For higher chromium concentration in the synthesis gel, the final product was found to be amorphous. According to N2 physical adsorption analysis, the linear portion of the last section of t-plot, is extrapolated to the adsorption axis of the comparison plot, and this intercept is used to obtain the volume of the micropores. By using different alkalin cations in the synthesis process such as Na or K or Cs, the crystallinity of the zeolite, decreases with the order of Na > K > Cs; studying the porosity characterization of the system with different alkaline cations, also shows the same order for the BET surface area of the samples, but reverse order for the size of pores in the structures of chromosilicates. The effect of various factors, such as crystallization temperature, crystallization time and pH of reaction system on the crystallization process was investigated in detail.

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