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Green synthesis of nano‐silicalite‐1: Biomass fly ash as a silica source and mother liquid recycling

This study investigated the effect of silica from biomass fly ash, template (tetrapropyl ammonium hydroxide; TPAOH) reduction, and mother liquid recycling on the synthesis of nano‐silicalite‐1 by the clear gel solution method. The results showed that nano‐silicalite‐1 was synthesized by using silica from the biomass fly ash at the appropriate Si:TPAOH mole ratios, e.g., 25Si:9TPAOH and 30Si:9TPAOH, and synthesis conditions (80°C and 96 h). An increased ratio from 25Si:9TPAOH:0.16NaOH to 30Si:9TPAOH:0.16NaOH was suspected to increased gel viscosity. An increased NaOH content in the 30Si:9TPAOH:0.64NaOH increased the silicalite‐1 yield, possibly by reducing silica polymerization. Decreasing the TPAOH content increased silicalite‐1 yields, but provided unfavorable conditions for nanoparticles. Increasing the reaction time increased the silicalite‐1 yield, but the crystals tended to agglomerate. For the 25Si:7TPAOH, lower reaction temperature (70°C) gave a decreased silicalite‐1 yield and a smaller crystal size than the higher temperature (80°C). Adding silica residues and zeolite nuclei in the mother liquid recycling from the first synthesis batch increased the silicalite‐1 yield. For 25Si:9TPAOH, mother liquid recycling had less effect on the crystal size, which was attributed to there being a sufficient amount of TPAOH for synthesizing nanocrystal silicalite‐1. However, a considerable crystal size reduction was observed when using 25Si:5TPAOH with mother liquid recycling, due to more nuclei being added from the mother liquid. Therefore, a greener and more efficient synthesis for nano‐silicalite‐1 could be performed by using an alternative silica source from biomass fly ash and by recycling of the mother liquid. © 2014 American Institute of Chemical Engineers Environ Prog, 34: 188–193, 2015