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By now, it is widely accepted that the reactivity of TS-1 arises from tetrahedral [TiIV(OSi)4] sites that are isomorphously substituted into the zeolitic framework, and which may—or may not—be partially hydrolysed [7]. The active sites are proposed to exist in pseudo-tetrahedral geometry, comparable to conventional framework atoms, but are believed to be able to expand their co-ordination sphere to six following interaction with solvent molecules and/or the oxidant, due to the flexibility of the MFI framework [7]. Despite this, debate remains regarding the speciation of the Ti active sites, and the nature of the reactive intermediates formed during catalysis, which remains given that in situ spectroscopic study is complicated by the presence of water (as by-product), the condensed phase nature of the reaction (solvent spectators), and the lability of Ti-(hydro)peroxo complexes. Recent work has demonstrated that post-synthetic modification of TS-1 with NH4HF2 leads to exciting improvements for aromatic hydroxylation activity and selectivity [8–10]. Despite this, the impact of this treatment on: (i) the active site speciation; and (ii) the activity of TS-1 for other oxidation reactions, particularly epoxidation, remains almost unexplored. In this manuscript we demonstrate the positive impact of NH4HF2 treatment on the catalytic activity of TS-1 for olefin epoxidation. Kinetic measurements reveal that following post-synthetic modification, changes in epoxidation activity are observed, and H2O2 decomposition activity is almost entirely eliminated, leading to promising breakthroughs in H2O2-based selectivity. Through in situ spectroscopic study with resonance enhanced Raman spectroscopy, we also observe modifications to the Ti site speciation, which may be responsible for the changes in kinetic behaviour activity. 2. ResultsandDiscussion 2.1. Synthesis of TS-1 and Post-Synthetic Modification with NH4HF2 TS-1 was prepared according to a hydrothermal synthesis protocol, and XRD, porosimetry, UV-Vis and FTIR analysis confirm its successful synthesis (ESI Figures S1–S3, Table S1). The post-synthetic modification of calcined TS-1 with NH4HF2 and H2O2 was performed in an aqueous solution under the conditions optimised by Bianchi and co-workers for 4 h (catalyst henceforth denoted TS-1B) [8–10]. The treatment does not lead to any substantial differences in crystallographic structure, porosity or Ti loading, with each of these parameters remaining constant following NH4HF2 treatment. The material is clearly still an MFI type zeolite, and a micropore volume of 0.138 cm3·g−1 indicates that any potential modification undergone by TS-1 does not change its overall topology or porosity. The Ti loading remains constant, within experimental error, and the Si/Ti molar ratio remains comparable at 43.5 (Table S1). Clearly, any changes occurring to the material 101PDF Image | Zeolite Catalysis
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