Using Zeolitic Core@Shell Adsorbents for the Selective Removal of Free Glycerol from Crude Biodiesel

Abstract

Selective adsorption of free glycerol from crude biodiesel mixture has been investigated using mesoporous silica spheres coated with a thin shell of microporous silicalite-1. Various types of mesoporous silica spheres with different sizes (commercial silica gel spheres: 20-45 μm and 3 μm, HMS spheres: ~1.5 μm) were used as core templates. A polycrystalline silicalite-1 shell was formed upon first covering the external surface of the core templates with discrete silicalite-1 nanocrystals via electrostatic attractions, followed by short hydrothermal treatments in silica/TPAOH-containing gel to ensure shell coverage and uniformity. The synthesized materials were characterized SEM, TEM, XRD and nitrogen physisorption. Series of batch glycerol adsorption experiments were conducted to evaluate the ability of the final product in the selective removal of free glycerol from crude biodiesels with different compositions at various temperatures. Glycerol contents of the produced biodiesel were compared to those purified by using conventional adsorbents including bare mesoporous silica gel spheres, conventional zeolites, e.g. silicalite-1, pure siliceous beta (Si-BEA) and ZSM-5 (H) crystals as well as physical mixture of the constitutive materials, i.e., equally mixed silicalite-1 and silica gel spheres. Although mesoporous silica gel spheres showed slightly higher glycerol adsorption capacity, the mesoporous adsorbents tend to trap a significant amount of bulkier molecules (e.g., FAME) in their large pore network (d_pore= 7nm). However, the silicalite-1 shell provided a microporous membrane which hindered FAME diffusion into the mesopores of the composite adsorbent, while the large pore volume of the mesoporous core enabled a multi-layer glycerol adsorption. This property of the core@shell material significantly enhanced the dry washing performance in terms of purification yield and adsorption capacity, in comparison to other conventional sorbents (Glycerol:FAME ratio in sorbent improved from ~0.65 for silica gel up to 5.2 for core@shell particles).