Hydrothermal synthesis of MWW-type analogues using linear-type quaternary alkylammonium hydroxides as structure-directing agents

摘要

The borosilicates and titanosilicates with a crystalline structure analogous to the MWW topology were hydrothermally synthesized by using linear organic quaternary ammonium hydroxides, CH3(CH2)_nN~+(CH3)3OH with n = 5, 6 and 7, as structure-directing agents (SDAs) with the co-existence of boric acid and alkali hydroxide mineralizers. The MFI zeolite appeared to be the main competitive phase that affected the purity of the MWW structure. The Ti-MWW lamellar precursors were synthesized successfully at a greatly reduced amount of boric acid in comparison to the conventional system using piperidine. The SDA cations were confirmed to be incorporated into both the interlayer spacings and the intralayer sinusoidal 10-MR channels of the MWW sheets, keeping intact in their molecular structures. The Ti-MWW precursors contained both tetrahedral and octahedral Ti species. The octahedral Ti species were selectively removed by acid treatment, leading to active titanosilicate catalysts for the epoxidation of 1-hexene with H2O2.