Tungsten-substituted Silicalite-1 with an interconnected hollow structure for catalytic epoxidation of cyclohexene

A novel hierarchical tungsten-substituted Silicalite-1 zeolite with the highly interconnected hollow structure (HWS-1) was synthesized for the first time by post-treating the purely microporous tungsten-substituted Silicalite-1 (WS-1) through simultaneous desilication and tungstation, in which a combination of sodium hydroxide and tetrapropyl ammonium hydroxide (TPAOH) was used to achieve a systematic balance between dissolution and recrystallization and further promote the synchronous tungstation with using sodium tungstate dihydrate as a tungsten source. Under an optimum addition ratio of NaOH/TPAOH, the resulted HWS-1 exhibited not only well retained microporosity and crystallinity but also obviously enlarged external surface area and mesopore volume in contrast to the parent WS-1. Furthermore, the effective tungstation also facilitated the substitution and increased the amount of coordinated W species in zeolite frameworks. The catalytic performance of HWS-1 was evaluated by the epoxidation of cyclohexene with using H 2 O 2 as oxidant. Benefited by the highly interconnected hollow structure and more accessible active sites on the enlarged external surface, the HWS-1 catalyst revealed a higher catalytic activity with improved cyclohexene conversion than WS-1. Moreover, it was found that a significant positive effect on promoting epoxidation was achieved by tungsten-substituted Silicalite-1 (WS-1 and HWS-1) catalysts reflecting at a much higher selectivity of epoxide (>79%) in comparison with titanium-substituted Silicalite-1 (TS-1) (~50% selectivity of epoxide), due to their superior hydrophobicity and suitably weaker acidity, which further demonstrated the potential application of the developed hierarchical tungsten-substituted Silicalite-1 as a candidate catalyst for epoxidation and other selective oxidation reactions.