Fabrication of silicalite-1 crystals with tunable aspect ratios by microwave-assisted solvothermal synthesis

A new method was developed to finely tune the aspect ratios of silicalite-1 (Si-MFI) crystals. This method involved the utilization of co-solvent of diols and the microwave heating of a reaction system of TEOS–TPAOH–H2O–Diol with the molar ratio of SiO2:TPAOH:EtOH:Diol:H2O = 1.0:0.357:4.0:x:y, where x = 6–9, y = 21.55–45.55. The diols included the ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (tEG). Under microwave radiation the Si-MFI crystals with tunable sizes, shapes, and aspect ratios were crystallized. An excellent linear correlation between the aspect ratio of the Si-MFI crystals and the “concentration” of the C/OH, denoted as CC/OH, where C/OH is the ratio of the number of carbon and hydroxyl of the diols of the co-solvent, was observed. On the basis of this linear relationship, the aspect ratios of the well defined Si-MFI crystals could be effectively tuned. The possible mechanism on how the co-solvent of diols affects the growth kinetics of the Si-MFI crystals is discussed. The resulting products were further characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).