Figure 1. Schematics of in situ hydrothermal synthesis of binderless zeolite X pellets.
Binderless zeolite X pellets were prepared by modifying the method we reported earlier to synthesize zeolite A (see Figure 1).35 First, a custom-made oil/water column for synthesis of the silica gel precursors was prepared. Specifically, thewater phase : sodium aluminate alkaline solution (250 mL) was prepared by dissolving sodium metaaluminate (24.6 g) and sodium hydroxide (24 g) into deionized water (230 g). Saturated CTAB aqueous solution (15 mL) served as the surfactant phase whilen -heptane (30 mL) was employed as the oil phase . These three phases were successively added in a beaker. Subsequently, the silica gel mixture, which was obtained by stirring the mixture of silica sol (45 g) and ammonium chloride solution (10 g, 10 wt%), was dripped into the aforementioned custom-made column through a 0.35 mm (id) syringe needle in order to form spherical silica gel precursors of 2~3 mm in size. Subsequently, the alkaline solution at the bottom as well as the silica gel precursors obtained were separated from the oil/water column followed by transfer to a Teflon-lined stainless steel autoclave. The molar composition of the prepared reaction system was 3Na2O: Al2O3: 2SiO2: 98H2O. After aging at 313 K for 0 to 6 h, the zeolite X products were crystallized at 373 K systematically for 0 to 24 h. Washing samples was triplicated with deionized water. The resulting products were collected through filtration and dried at 373 K overnight.