Silicalite-l Synthesis from Silicate Aqueous Solutions Including Amines as a Base

Aqueous silicate solutions could be prepared from a silicon alkoxide using amines as a base. The synthesis of silicalite-1 conducted by hydrothermally treating the solutions after the addition of NPr n 4Br (TPABr), was examined focusing on triethylamine (NEt3) as the amine. The aqueous solutions with chemical compositions of Si(OEt)4 (TEOS) : NEt3 : TPABr : H2O = 1 : ≥0.5 : ≥0.8 : 200 led to silicalite-1 crystallization upon hydrothermal treatment at 130C for 4 days. The resulting crystals had a small thin coffin shape. With increasing the added amounts of NEt3 and TPABr, the crystal size decreased. Among the investigated conditions, the smallest silicalite-1 crystal measuring about 1.0 × 0.7 × 0.4 um was obtained at starting compositions of NEt3/TEOS ≥2.0 and TPABr/TEOS ≥3.2. The increase in the TPA content of the starting solutions accelerated the crystal nucleation of silicalite-1, so that the obtained crystals became smaller. During the hydrothermal synthesis, an amorphous silica gel was formed before the crystallization commenced. The increase in the NEt3 content may suppress the formation of the silica gel, and/or facilitate its dissolution, thereby the supersaturation degree of silicate species in the solution was increased, giving small silicalite-1 crystals. When an aqueous suspension of the smallest silicalite-1 crystals was spread onto a glass substrate by a dip-coating method, an ordered mono-grain layer of the silicalite-1 crystals oriented with the straight channels normal to the substrate surface was obtained.