In Situ Tailoring the Crystalline Defects of Titanium Silicalite-1 (TS-1) to Improve the 1-Butene Epoxidation Performance

Abstract

The crystalline defects are vitally important for the catalytic activity and stability of zeotype materials; however, the influences of which on the catalytic performance of TS-1 should further be studied. Herein, a modified hydrothermal synthesis process was developed to in situ tailor the framework defects of TS-1, and the physicochemical properties and catalytic performance were studied in detail. Generally, the n_H2O/n_SiO2 (5–20) of the precursor mixture was in situ tuned via distillation through the crystallization under 90–110 °C, and TS-1 with decreased framework defects can be directly synthesized during the following crystallization under 170 °C. When the n_H2O/n_SiO2 declined, the crystal size decreased; however, more intracrystalline mesopores (3.0–5.3 nm) can be introduced, the mesopore volume increased from 0.051 to 0.083 cm³/g. More importantly, the ratio of silicon species in Q4 (Si(OSi)₄) to Q3 [Si(OSi)₃(OH)] units increased from 29.9 to 55.0, and the hydroxyl groups in the nest or neighboring environments decreased more significantly than the surface silanol. Nevertheless, the titanium coordination states remained unchanged. Owing to the decreased framework defects, the weak acid sites declined from 191.5 to 160.5 μmol NH₃/g, the epoxide selectivity in butene epoxidation reaction increased from 97.4 to 98.1%, while the H₂O₂ efficiency was enhanced from 96.7 to 98.6%. More importantly, the catalytic stability of TS-1 was also improved.