Green synthesis and characterization of zeolite silicalite-1 from recycled mother liquor

Abstract

Researches on zeolite silicalite-1 have focused mainly on improving its performance, with little attention paid on green and economic synthesis. Herein, a novel route for green synthesis of pure silicalite-1 from the recycled mother liquor has been developed. This route starts from the hydrothermal reaction of natural quartz with NaOH to form a sodium silicate solution, which is then transformed into a silicic acid (H2SiO3) gel by hydrochloric acid titration. Then the solid silicic acid (H2SiO3) gel and fumed silica (SiO2) are mixed with water under stirring to form a highly active slurry, which can be hydrothermally “dissolved” in a dilute NaOH aqueous solution. Using this slurry with a high SiO2/Na2O molar ratio of 5 : 1 as the silica source as well as tetrapropylammonium bromide (TPABr) as the organic template, MFI-type pure silicalite-1 has been successfully synthesized. After each synthesis, the mother liquor of centrifuged supernatant was recovered and recycled. The silica source and organic template were added to the recycled mother liquors for the synthesis of pure silicalite-1. The minimum TPABr/SiO2 molar ratio needed to synthesize pure silicalite-1 in the initial run is 0.04 : 1. In comparison, only half the amount of the organic template is needed in subsequent runs with the recovered mother liquors. Through recycling, all reactants in the mother liquors were ultimately utilized without any waste, leading to not only effectively reducing the cost of silicalite-1 synthesis but also minimizing its environmental impact.