Direct Hydroxylation of Benzene with Hydrogen Peroxide Using Fe Complexes Encapsulated into Mesoporous Y-Type Zeolite

Mesoporous Y-type zeolite (MYZ) was prepared by an acid and base treatment of commercial Y-type zeolite (YZ). The mesopore volume of MYZ was six times higher than that of YZ. [Fe(terpy)2]2+ complexes encapsulated into MYZ and YZ with different Fe contents (Fe(X)L-MYZ and Fe(X)L-YZ; X is the amount of Fe) were prepared and characterized. The oxidation of benzene with H2O2 using Fe(X)L-MYZ and Fe(X)L-YZ catalysts was carried out; phenol was selectively produced with all Fe-containing zeolite catalysts. As a result, the oxidation activity of benzene increased with increasing iron complex content in the Fe(X)L-MYZ and Fe(X)L-YZ catalysts. The oxidation activity of benzene using Fe(X)L-MYZ catalyst was higher than that using Fe(X)L-YZ. Furthermore, adding mesopores increased the catalytic activity of the iron complex as the iron complex content increased.

Substantial progress has been made in the synthesis, characterization, and catalytic exploitation of hierarchically structured variants of Faujasite (X, Y, and USY) zeolites [37][38][39]. Verboekend et al. [37,38] reported that mesoporous zeolite could be prepared from Y-type zeolite by acid and base treatments. The mesoporous zeolite prepared by acid-base treatment of Y-type zeolite exhibited higher catalytic activity for the alkylation of toluene with benzyl alcohol than untreated Y-type zeolite.
The mesoporous zeolite support for [Fe(terpy) 2 ] 2+ @Na-Y was prepared by an acidbase treatment of Y-type zeolite to improve the catalytic activity for benzene oxidation. In this study, the catalytic activity for the oxidation of benzene with H 2 O 2 was investigated using [Fe(terpy) 2 ] 2+ complexes encapsulated into mesoporous Y-type zeolite.

Preparation and Characterization of Mesoporous Y-Type Zeolite
The mesoporous Y-type zeolites were prepared from commercial Y-type zeolite (YZ) by sequential acid (H 4 EDTA), base (NaOH), and acid (Na 2 H 2 EDTA) treatments. The acid-and base-treated zeolites to produce mesoporous Y-type zeolite were called MYZ-t (t = 0-24 h), where t is the base treatment time (hours (h)). Figure 1 shows X-ray diffraction (XRD) patterns of MYZ-t (t = 0-24 h) and original YZ. The XRD patterns of MYZ-t were similar to that of the original YZ, suggesting that the structure of Y-type zeolite had been maintained after the acid and base treatments. The N 2 absorption-desorption isotherms of MYZ-t and YZ ( Figure S1) provided IV and H4 type isotherms [40], suggesting that MYZ-t and YZ have cylinder-like pores. Table 1 lists the N 2 absorption-desorption parameters of MYZ-t and YZ. The mesopore volumes (V meso ) of MYZ-t were much larger than those of the original YZ. The mesopore volume of MYZ-t increased to t = 1.0 (base treatment = 1.0 h) and decreased with further increases in t. Both total pore volume (V total ) and mean pore diameter (d p ), and the mesopore-specific surface area (S meso ) of MYZ-1.0 were also the largest among the samples tested. Figure 2 shows the pore distributions of MYZ-t and YZ. In the case of MYZ-0, approximately 4 nm pores formed. On the other hand, in the case of MYZ-t (t > 0), the number of pores less than 10 nm in size decreased with a concomitant increase in the number of 20-30 nm mesopores. The number of 20-30 nm mesopores of MYZ-t increased up to t = 1.0 and decreased with further increases in t. Thus, among MYZ-t, MYZ-1.0 was adopted as a typical mesoporous Y-type zeolite.

Preparation and Characterization of the Fe Complexes Encapsulated into the Zeolite
The mesoporous Y-type zeolite catalysts (Fe(X)L-MYZ-t) and untreated zeoli lysts (Fe(X)L-YZ) with different contents of iron complexes were prepared using a m reported elsewhere [30][31][32][33]. The X calculated by inductively coupled plasma-atomi sion spectroscopy (ICP-AES) represents the weight percentage concentration (wt. % in Fe(X)L-MYZ-t and Fe(X)L-YZ catalysts. The ICP-AES and CHN elemental a

Preparation and Characterization of the Fe Complexes Encapsulated into the Zeolite
The mesoporous Y-type zeolite catalysts (Fe(X)L-MYZ-t) and untreated zeolite catalysts (Fe(X)L-YZ) with different contents of iron complexes were prepared using a method reported elsewhere [30][31][32][33]. The X calculated by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) represents the weight percentage concentration (wt. %) of Fe in Fe(X)L-MYZ-t and Fe(X)L-YZ catalysts. The ICP-AES and CHN elemental analysis

Preparation and Characterization of the Fe Complexes Encapsulated in
The mesoporous Y-type zeolite catalysts (Fe(X)L-MYZ-t) and u lysts (Fe(X)L-YZ) with different contents of iron complexes were prep reported elsewhere [30][31][32][33]. The X calculated by inductively coupled sion spectroscopy (ICP-AES) represents the weight percentage conce in Fe(X)L-MYZ-t and Fe(X)L-YZ catalysts. The ICP-AES and CHN

Preparation and Characterization of the Fe Complexes Encapsulated into
The mesoporous Y-type zeolite catalysts (Fe(X)L-MYZ-t) and untr lysts (Fe(X)L-YZ) with different contents of iron complexes were prepar reported elsewhere [30][31][32][33]. The X calculated by inductively coupled pla sion spectroscopy (ICP-AES) represents the weight percentage concentr in Fe(X)L-MYZ-t and Fe(X)L-YZ catalysts. The ICP-AES and CHN e )).

Preparation and Characterization of the Fe Complexes Encapsulated into the Zeolite
The mesoporous Y-type zeolite catalysts (Fe(X)L-MYZ-t) and untreated zeolite catalysts (Fe(X)L-YZ) with different contents of iron complexes were prepared using a method reported elsewhere [30][31][32][33]. The X calculated by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) represents the weight percentage concentration (wt. %) of Fe in Fe(X)L-MYZ-t and Fe(X)L-YZ catalysts. The ICP-AES and CHN elemental analysis indicated that the Fe ion in MYZ-t and YZ was coordinated with two terpy (terpy/ Fe = 2) ligands, suggesting the formation of [Fe(terpy) 2 ] 2+ ions in the Fe(X)L-MYZ-t and Fe(X)L-YZ catalysts. Figure 3 shows XRD patterns of the Fe(X)-MYZ-1.0 and Fe(X)L-MYZ-1.0 catalysts. The zeolite structure was maintained after the introduction of each metal complex. The empirically derived relationship between the relative peak intensities of the (220) and (311) reflection in the XRD pattern confirmed the formation of a large metal ion in a supercage of faujasite-type zeolite; the intensity of (220) for the zeolite containing large complexes is lower than that for the original zeolite Y, while the intensity of (311) for former is greater than that for the latter [12,31,33]. As shown in Figure 3, the intensities of the (220) reflection (2θ = 10 • ) for the Fe(X)L-MYZ-1.0 catalysts were lower than those for the corresponding Fe(X)-MYZ-1.0 samples, while the intensities of the (311) plane (2θ = 12 • ) in the former were greater than those of the latter. Similar changes in XRD peak intensity before and after ligand introduction were obtained for Fe(X)L-YZ ( Figures S2 and S3) and FeL-MYZ-t ( Figures S4 and S5) catalysts. The relative intensity of the (220) reflection to the (331) reflection (2θ = 16 • ) decreased with increasing Fe content in the Fe(X)L-MYZ-1.0 and Fe(X)L-YZ catalysts, while the (311) to (331) intensity ratio increased (Figure 4). These results provide clear evidence of the formation of metal complex ions within the supercage.
percage of faujasite-type zeolite; the intensity of (220) for the zeolite containing large complexes is lower than that for the original zeolite Y, while the intensity of (311) for former is greater than that for the latter [12,31,33]. As shown in Figure 3, the intensities of the (220) reflection (2θ = 10) for the Fe(X)L-MYZ-1.0 catalysts were lower than those for the corresponding Fe(X)-MYZ-1.0 samples, while the intensities of the (311) plane (2θ = 12) in the former were greater than those of the latter. Similar changes in XRD peak intensity before and after ligand introduction were obtained for Fe(X)L-YZ ( Figures S2 and S3) and FeL-MYZ-t ( Figures S4 and S5) catalysts. The relative intensity of the (220) reflection to the (331) reflection (2θ = 16) decreased with increasing Fe content in the Fe(X)L-MYZ-1.0 and Fe(X)L-YZ catalysts, while the (311) to (331) intensity ratio increased (Figure 4). These results provide clear evidence of the formation of metal complex ions within the supercage.   The catalysts showed no absorption in the ultraviolet to the visible region without the metal complex, Fe-YZ and YZ [33]. The absorption spectra of Fe(X)L-MYZ-1.0, Fe(X)L-YZ, and FeL-MYZ-t catalysts ( Figure 5) produced two bands in the regions of 400-650 nm and 250-400 nm, which can be assigned to metal-to-ligand (d-π*) charge-transfer (MLCT) and a π-π* transition of terpy ligand of the [Fe(terpy) 2 ] 2+ complex, respectively, similar to that of [Fe(terpy) 2 ](ClO 4 ) 2 [33]. The UV-vis spectral behaviors of Fe(X)L-YZ ( Figure S6

Oxidation of Benzene with Hydrogen Peroxide
The partial oxidation of benzene with H2O2 using Fe(X)L-MYZ-1.0 and Fe(X)L-YZ catalysts proceeded, and phenol was produced selectively on all catalysts. Figure 6 shows the time course for the oxidation of benzene with H2O2 using Fe(X)L-MYZ-1.0 and Fe(X)L-YZ catalysts. The phenol yield for all Fe complex-containing catalysts increased with increasing time. The initial catalytic activities of the Fe(X)L-MYZ-1.0 catalysts for the oxidation of benzene were higher than those of the Fe(X)L-YZ ones. Figure 7 shows the catalytic

Oxidation of Benzene with Hydrogen Peroxide
The partial oxidation of benzene with H 2 O 2 using Fe(X)L-MYZ-1.0 and Fe(X)L-YZ catalysts proceeded, and phenol was produced selectively on all catalysts. Figure 6 shows    The partial oxidation of benzene with H 2 O 2 using the FeL-MYZ-t and FeL-YZ catalysts with the same Fe content (0.9-1.2 wt. %) in Table S1 proceeded. Phenol was produced as the main product, while o-catechol and traces of hydroquinone were produced as by-products (Figure 9). The catalytic activities of the FeL-MYZ-t catalysts were higher than those of the FeL-YZ catalyst. The catalytic activity of FeL-MYZ-t increased to t = 5.0 and decreased with further increases in t. The phenol selectivity of FeL-MYZ-t increased with increasing t. These results suggest that the catalytic activity of FeL-MYZ-t and phenol selectivity increased with the increasing mesopore size of the zeolite support (average pore diameter (d p ), mesopore volume (V meso ), and surface area (S meso ) in Table 1 and pore distribution in Figure 2), supporting the hypothesis shown in Figure 8. The FeL-MYZ-5.0 catalyst exhibited the best catalytic activity for the oxidation of benzene, with H 2 O 2 to phenol among the catalysts tested. . Reaction condition: Fe in catalysts (7.9 mol), benzene (7.9 mmol), 30% aqueous H2O2 (7.9 mmol), CH3CN (10 mL), 50 °C and Ar atmosphere.  The partial oxidation of benzene with H2O2 using the FeL-MYZ-t and FeL-YZ catalysts with the same Fe content (0.9-1.2 wt. %) in Table S1 proceeded. Phenol was produced as the main product, while o-catechol and traces of hydroquinone were produced as byproducts (Figure 9). The catalytic activities of the FeL-MYZ-t catalysts were higher than those of the FeL-YZ catalyst. The catalytic activity of FeL-MYZ-t increased to t = 5.0 and decreased with further increases in t. The phenol selectivity of FeL-MYZ-t increased with increasing t. These results suggest that the catalytic activity of FeL-MYZ-t and phenol se-
ICP-AES and CHN elemental analyses of all catalysts were carried out after the sample was dissolved into a HF solution. The powder XRD patterns of the catalysts were collected on a Rigaku MiniFlex II diffractometer using CuK radiation. The Brunauer-Emmet-Teller (BET) surface area measurements were conducted to determine the specific surface areas and pore diameters of the samples by performing N2 adsorption experiments at 77 K using a BEL Japan Bellsorp-max instrument. The UV-vis spectra were recorded on a Hitachi U-4000 spectrometer for solid samples. Gas chromatography (GC, Shimadzu GC-2014) was performed using a flame ionization detector equipped with a DB-1MS capillary column (internal diameter = 0.25 mm and length = 30 m) at the nature of the nonpolar liquid phase.
ICP-AES and CHN elemental analyses of all catalysts were carried out after the sample was dissolved into a HF solution. The powder XRD patterns of the catalysts were collected on a Rigaku MiniFlex II diffractometer using CuKα radiation. The Brunauer-Emmet-Teller (BET) surface area measurements were conducted to determine the specific surface areas and pore diameters of the samples by performing N 2 adsorption experiments at 77 K using a BEL Japan Bellsorp-max instrument. The UV-vis spectra were recorded on a Hitachi U-4000 spectrometer for solid samples. Gas chromatography (GC, Shimadzu GC-2014) was performed using a flame ionization detector equipped with a DB-1MS capillary column (internal diameter = 0.25 mm and length = 30 m) at the nature of the non-polar liquid phase.

Preparation of Mesoporous Y-Type Zeolite
The mesoporous Y-type zeolites were prepared by the sequential processes of acid (H 4 EDTA), base (NaOH), and acid (Na 2 H 2 EDTA) treatments of commercial Y-type zeolite (Na-Y, YZ) as follows.
A suspension of YZ (6.7 g), ethylenediaminetetraacetate acid (3.2 g), and water (100 mL) was stirred at 100 • C for 6 h followed by drying at room temperature under vacuum after filtration. A suspension of the obtained solid, sodium hydroxide (0.8 g), and water (100 mL) was stirred at 65 • C for a set time (t = 0-24 h), followed by filtration and drying at room temperature under vacuum. A suspension of the obtained solid, ethylenediaminetetraacetate acid disodium salt (4.1 g) and water (100 mL) was stirred at 100 • C for 6 h followed by filtration and drying at room temperature under vacuum to obtain a white powder. The white powder was called the acid and base-treated zeolite (MYZ-t).

Catalytic Oxidation of Benzene
The catalytic oxidation of benzene was carried out in a glass tube reactor. A typical procedure was as follows: catalyst (Fe: 7.9 or 16 µmol), MeCN solvent (5 or 10 mL), and benzene (7.9 mmol) were charged, and 30% aqueous hydrogen peroxide (7.9 mmol) was added to a glass tube reactor under an Ar atmosphere. The reaction was carried out at 50 • C. After the reaction, triphenylphosphine as a quencher and o-dichlorobenzene as an internal standard was added to a glass tube reactor. The products were identified by comparing the peak intensity and retention time for GC-FID with authentic samples. The turnover number (TON) is defined as the total yield [mol] per amount of Fe [mol] contained in the catalyst.

Conclusions
Mesoporous Y-type zeolite (MYZ) was prepared by the acid-base treatment of commercial Y-type zeolite (YZ) at various base treatment times. The mesopore volume was a maximum when the base treatment time was 1.0 h. MYZ had six times higher mesopore volume than YZ. The mesoporous zeolite catalyst (Fe(X)L-MYZ) with the iron complex encapsulated in mesoporous zeolite was characterized. Fe(X)L-MYZ and Fe(X)L-YZ catalysts with different iron complex contents were used for benzene oxidation, with H 2 O 2 as the oxidant. Phenol was selectively obtained with all catalysts. For the same amount of iron complex, Fe(X)L-MYZ catalyst had higher catalytic activity than Fe(X)L-YZ catalyst. For both catalysts, the catalytic activity increased with increasing iron complex content. The effect of the iron complex content was greater for the Fe(X)L-MYZ catalyst than for the Fe(X)L-YZ catalyst.