Narrowing the mesopore size distribution in hierarchical TS-1 zeolite by surfactant-assisted reorganization

Abstract

Hierarchical TS-1 zeolites exhibiting a uniform mesoporosity were obtained by means of the sequential combination of both bottom-up and top-down synthesis approaches. First, the parent hierarchical TS-1 zeolites were synthesized following the silanization of protozeolitic units procedure, whereby an additional porosity is formed due to the inhibitor crystal growth effect caused by a silanization agent. After that, the materials were subjected to a reorganization hydrothermal alkali treatment in presence of cationic surfactants. Here, the zeolite lattice is locally dissolved, being the nanometric dispersed species reorganized around surfactant micelles. The influence of the reorganization time (15–25h), surfactant molecule length (C10–C18) and textural properties of the starting hierarchical TS-1 zeolite were explored. The reorganized hierarchical TS-1 zeolites so obtained show an uniform mesoporosity around 40Å, in contrast to the more heterogeneous secondary porosity exhibited by the parent hierarchical TS-1 materials, comprised between 10 and 100Å. Nevertheless, a reduction in BET surface area and surface attributed to the secondary porosity was found. Likewise, the materials obtained after the reorganization treatment preserve a high crystallinity degree and Ti leaching is not observed. However, though in the reorganized samples the titanium atoms are mostly in tetrahedral coordination, titanium species with higher coordination number were also detected. The catalytic properties of these materials were tested in cyclohexene (C6H10), dibenzothiophene (DBT) and 2-methylbenzothiophene (2-MBT) oxidation reactions using tert-butylhydroperoxide (TBHP) as oxidizing agent. A beneficial effect of the narrower mesoporosity is observed in the catalytic activity of the reorganized materials compared to that exhibited by the parent zeolites.