The effect of Na2SO4 salt on the synthesis of ZSM-5 by template free crystallization method

ZSM-5 was synthesized by template-free method using sodium silicate as a silica source and the content of Na2O in mother liquid was controlled by H2SO4. To see the effect of Na2SO4 generated by added H2SO4, hydrothermal crystallization was carried out with two types of mother liquids having low and high Na2SO4 contents at 170 °C. The physical and chemical properties of ZSM-5s prepared in two different conditions were compared by XRD, EDS, FTIR, micropore analyzer and particle size analyzer. High crystalline, isometrical shaped and uniform 1–2 μm sized ZSM-5s with relative crystallinity around 100% could be obtained successfully from the mother liquids having the molar range of (11.0–21.0) Na2O:(21.4–11.4) Na2SO4:100.0 SiO2:2.0 Al2O3:4000.0 H2O. Through the crystallization in the absence and presence of extra Na2SO4, it was found that the condensation rate of aluminosilicate was increased by the Na2SO4 and resulted in the formation of amorphous phase instead of crystalline ZSM-5. The formation of crystalline HZSM-5 was prohibited critically if the total amount of Na2SO4 in mother liquid was higher than 24.4 mole based on 100.0 mole of SiO2. The enhanced condensation reaction and the preferential formation of agglomerated amorphous aluminosilicates in the presence of Na2SO4 could be explained by the oxyanion effect of SO42- generated by Na2SO4.