Conversion of magadiite to pure-silica-Nu-1 and an insight into the changes in its medium-range structure during crystallization

Abstract

Pure-silica-Nu-1 has been synthesized by a magadiite conversion method with the reactive gel composition SiO₂ : 0.2 TMAOH : 5 H₂O at 170 °C for 7 h. Its crystallization behaviors and the changes in its medium-range structure were discussed. The results demonstrate that part of the structure of magadiite decomposed. However, the 5-membered rings and 6-membered rings in magadiite were retained as SBUs in the system and participated in the formation of zeolite Nu-1, which accelerated the conversion. 4-Membered rings were present in the product after heating for 1 h, proving the formation of pure-silica-Nu-1. The conditions of the silicate species both in the solid product and in solution were investigated. Based on the results obtained from ¹³C NMR spectroscopy and TG/DTA, the state and amount of TMA⁺ in the solid product were examined. In addition, the influences of temperature, alkalinity and amount of TMA⁺ in the starting gels were studied.