The UV–vis spectra of the TS-1-s and TS-1#
xACh samples are shown in Fig.4(b), which indicate the Ti coordination. A distinct peak at approximately 210 nm was attributed to the framework Ti
4+, which originated from p
π–p
π charge transfer transitions between O
2– and isolated tetrahedral Ti species in the TS-1 zeolite framework [
33]. For the TS-1#0.2ACh and TS-1#0.4ACh samples, the characteristic band at 260 nm indicated formation of Ti species with different coordination geometries, such as pentacoordinated and hexacoordinated [
44]. Notably, for the TS-1#0.05ACh sample, another obvious absorption peak at approximately 330 nm was detected, which was attributed to extra-framework bulk TiO
2 [
34,
45]. However, generation of the extra-framework TiO
2 was avoided as much as possible with the TS-1 catalyst and was due to undesired decomposition of the H
2O
2 solution. Therefore, isolated framework Ti species were dominant in the TS-1-s, TS-1#0.075ACh and TS-1#0.1ACh samples, especially in TS-1#0.1ACh, and can be considered the catalytic active centers in these epoxidation reactions. The surface element electronic states of the TS-1#0.1ACh sample were further characterized with XPS measurements. As shown in Fig.4(c), the Ti 2p XPS data contained two strong characteristic peaks at 460.0 and 464.8 eV, which were assigned to the binding energies of the 2p
3/2 and 2p
1/2 states for Ti
4+ species in the TS-1#0.1ACh sample, respectively [
46]. Additionally, peaks at approximately 103.5 and 532.8 eV were observed in the Si 2p (Fig. S3, cf. ESM) and O 1s (Fig.4(d)) spectra, respectively. The acid properties of the catalyst surface were also determined by NH
3-TPD, and the results are presented in Fig. S4 (cf. ESM). There were three desorption peaks located at approximately 160, 300 and 480 °C, and these corresponded to weak acid sites, medium acid sites and strong acid sites, respectively [
28]. Clearly, the NH
3-TPD results showed that the amount of acid in the TS-1#0.1ACh sample, which contained weak acid and medium acid sites, was slightly higher than that of the TS-1-s sample, but neither sample showed obvious strong acid sites. Therefore, the TS-1#0.1ACh catalyst may promote more acid-catalyzed side reactions than the TS-1-s catalyst.