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do not involve changes to the crystalline structure, or a major loss of Ti atoms through leaching. In line with previous research, treatment of TS-1 in NH4HF2 does lead to a decrease in the main absorbance band at 210 nm—generally attributed to isomorphously substituted TiIV atoms [4]—and an increase in absorbance at a wavelength of ±270 nm (ESI Figure S2). Changes are also observed in the FTIR data (ESI Figure S3). The 960 cm−1 band—generally attributed to the framework TiIV atoms [11]—is partially eroded by NH4HF2 treatment (spectral intensity normalised to the Si–O–Si overtones of the zeolite lattice), with intensity decreasing by approximately 30% following treatment. According to this provisional characterisation, it may tentatively be attributed that NH4HF2 treatment leads to a change to the framework TiIV atoms. We note that the spectral data presented in Part 1 is in good agreement to the scattered reports of TS-1B in the open literature [8–10]. 2.2. Catalytic Studies of Allyl Alcohol Epoxidation Whilst previous studies of TS-1B have focused upon aromatic hydroxylation [8–10], we chose to investigate the impact of NH4HF2 treatment on the epoxidation activity of TS-1. Allyl alcohol was chosen as a model HPPO substrate [6]. Both TS-1 (Figure 1) and TS-1B (Figure 2) were found to be very active catalysts for this reaction. Both catalysts are able to reach full conversion in 120 min or less at the relatively mild reaction temperature of 50 ◦C. In both cases, glycidol is the main reaction product obtained with >95% selectivity, although 3-methoxy-1,2-propanediol (3M12PD) was also detected, almost fully accounting for the remaining carbon balance. Carbon balances of 95%–98% were always obtained, thus indicated that any other potential by-products below the detectability limit of our analytical protocols were not formed to a major extent. The observation of 3M12PD indicates that ring opening methanolysis of glycidol occurs across the least hindered end of the epoxide. Under a conventional acid catalysed mechanism, 2M13PD would be expected according to Markovnikov’s rules. This difference may be due to the active sites in TS-1 catalysing ring opening via a basic mechanism, or may alternatively be a consequence of shape selectivity favouring the linear methoxy-substituted diol over the branched. Although comparable in activity at 50 ◦C, further kinetic analysis at multiple temperatures reveals distinct differences in the activities of TS-1 and TS-1B. Initial rate analysis (ESI Figures S4 and S5), calculated from the linear region of the rate plot, and the resulting Arrhenius plot (Figure 3) clearly shows a change to both the temperature dependence and the pre-exponential factor following NH4HF2 treatment. The activation energy between 30 and 50 ◦C is found to increase from 39.1 kJ· mol−1 for TS-1, to 53.7 kJ·mol−1 for TS-1B. The Arrhenius barrier obtained for TS-1 is found to be in good agreement to previous theoretical studies [12,13]. Although this data indicates that TS-1B is a less active epoxidation 102PDF Image | Zeolite Catalysis
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