Steam-assisted crystallized Fe-ZSM-5 materials and their unprecedented activity in benzene hydroxylation to phenol using hydrogen peroxide

Journal of Catalysis 368 (2018) 354–364

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Steam-assisted crystallized Fe-ZSM-5 materials and their unprecedented activity in benzene hydroxylation to phenol using hydrogen peroxide Meysam Shahami b, Kerry M. Dooley a, Daniel F. Shantz b,⇑ a b

Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803, United States Department of Chemical and Biomolecular Engineering, Tulane University, 6823 St. Charles Avenue, New Orleans, LA 70118, United States

a r t i c l e

i n f o

Article history: Received 19 June 2018 Revised 7 September 2018 Accepted 6 October 2018

Keywords: Steam-assisted crystallization Fe-ZSM-5 Benzene oxidation Phenol

a b s t r a c t Iron-containing, hierarchical steam-assisted crystallized MFI materials were found to possess unprecedented reactivity for benzene hydroxylation to phenol. Numerous characterization methods were used to confirm the crystallinity, composition, textural properties and iron coordination in these samples. In-situ Fourier-transform infrared spectroscopy (in-situ FT-IR) of methanol and nitric oxide adsorption were also used to probe the materials. These catalysts were then studied in the catalytic oxidation of benzene to phenol using hydrogen peroxide as oxidant at mild conditions. Microporous materials showed lower activity (
4% phenol yield after 8 h), in agreement with previous reports, whereas hierarchical Fe-ZSM-5 zeolites, as synthesized here, exhibited superior catalytic performance. The best catalyst in this report gave a benzene conversion of 25.5% with a phenol selectivity of 90% corresponding to a turnover number of 82. This finding is twice the best zeolite-based catalyst in the open literature. Suitable control experiments ruled out homogeneous catalysis. Hydrogen peroxide consumption was also higher for the hierarchical samples, up to 95% at eight hours, compared to conventional catalysts. The obtained results revealed that the active sites for this reaction were located predominantly in the micropores of the mesoporous Fe-ZSM-5 zeolites. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction Phenol, also known as phenolic acid, is an important commodity chemical used as a precursor for the production of plastics and related materials such as nylon, phenolic resins, pharmaceutical drugs, epoxies, and detergents [1]. The cumene process is the predominant route of phenol production, currently at the level of approximately 10 million metric tons a year. This is a prototypical ‘two for one’ process in that one equivalent of acetone is produced per equivalent of phenol [2,3]. Market demand for the two chemicals does not align with this ratio. Further, the cumene process involves multiple reactions and a complex separations train to separate the acetone and phenol produced. Given these challenges, the ability to directly oxidize benzene to phenol without other byproducts would be viewed as technologically and economically significant [3–6]. This reaction, however, is non-trivial and has been described as one of the ten most demanding transformations in the petrochemical industry. There have been many prior investigations of the direct oxidation of benzene to phenol. However, in broad strokes much of ⇑ Corresponding author. E-mail address: [email protected] (D.F. Shantz). https://doi.org/10.1016/j.jcat.2018.10.011 0021-9517/Ó 2018 Elsevier Inc. All rights reserved.

the prior work in the heterogeneous catalysis literatures can be organized into a few bins. There has been much work around the gas phase oxidation of benzene to phenol [7–9]. Panov and coworkers found that benzene readily reacts with N2O over FeZSM-5 zeolites [7], which is now referred to as the ‘‘Alphox process”. The key to this chemistry is that the Fe site in the zeolite pore decomposes the N2O forming N2 and active oxygen centers that have been referred to as a-oxygen [7,10]. Mesoporous Fe-ZSM-5 zeolites have also been reported to improve the catalytic properties compared to conventional microporous zeolitic materials in this reaction due to less diffusion resistance [1,11]. Rapid deactivation of Fe-ZSM-5 catalysts as well as the high price of N2O are serious challenges in realizing a commercial process for phenol production via Alphox process. Titanium-based zeolites have been investigated in some depth in the liquid phase as well, given their industrial use in the hydroxylation of phenol to make catechols and quinones [2,6,12,13]. However, they do not effectively catalyze the benzene hydroxylation reaction, due to the lower activity of titanium sites for hydrogen peroxide decomposition. Because of the aforementioned challenges in utilizing N2O as oxidant, an alternative route for one-step oxidation of benzene to phenol in the liquid phase is desired. MFI zeolites have attracted significant attention in benzene oxidation reactions due to the fact that the

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diameter of the benzene molecule is comparable to the MFI pore size (5.5 Å), potentially imparting enhanced phenol selectivity. Other works have investigated loading metals into zeolites; a series of reports on rhenium-containing zeolites is particularly noteworthy [14,15]. Other labs also have studied the one-step oxidation of benzene via reaction with molecular oxygen O2 [15–19]. For instance, Chen et al showed that molecular oxygen can be activated over vanadium oxide nano-spheres and react with benzene to produce 4% phenol [19]. Because of the inability to selectively activate molecular oxygen and subsequent low phenol yield, this approach has met with limited success. In this report, steam-assisted crystallized (SAC) Fe-MFI is synthesized and tested for benzene hydroxylation using H2O2 as an oxidant. The best sample synthesized here gives a benzene conversion of 25.5% with phenol selectivity of 90%. This result is twice as good (on a yield basis) of any zeolite-based catalyst we are aware of that has been previously reported for benzene to phenol using hydrogen peroxide as the oxidant. The detailed characterization results provide insight on the different textural properties such as mesopores volumes, crystal sizes, and acidity for SAC samples as compared to conventional Fe-ZSM-5 and TS-1 samples, which show much lower activity. These encouraging results indicate SAC samples have potential to be used as solid catalysts in benzene oxidation. 2. Experimental 2.1. Materials Tetraethoxysilane (TEOS, 99.99%) from Alfa Aesar and aluminium hydroxide from Sigma-Aldrich (reagent grade) were used as silica and aluminium sources, respectively. Iron (III) chloride hexahydrate (Sigma-Aldrich, 97 wt%) and iron (III) nitrate nonahydrate (Alfa Aesar, >98 wt%) were used as iron sources. Tetra-npropylammonium hydroxide (40 wt% in water) from Alfa Aesar was used as the structure-directing agent. Pluronic P123 (BASF, EO20PO70EO20, MW = 5800) was used as a soft template in the synthesis of steam-assisted crystallized materials. Benzene (SigmaAldrich, 99.8%), hydrogen peroxide (BDH, 30 vol%), and acetonitrile (EMD Millipore, HPLC grade) were used for the catalytic testing. Methanol (BDH, ACS grade) was employed to quench the reaction for gas chromatography (GC) analysis. Phenol (Sigma-Aldrich, ACS reagent), 1,4-benzoquinone (TCI AMERICA, >97%), 1,2dihydroxybenzene (Sigma-Aldrich, >99%), and hydroquinone (Sigma-Aldrich, >99%) were used as GC calibrants. Vanadium oxide (V2O5, 99.6% min) and sulfuric acid (95–98%, ACS grade) from Alfa Aesar and BDH were used for H2O2 analysis. Deionized (DI) water used for syntheses (18 MX cm) and washing (15 MX cm) was obtained from a MilliporeSigma purification system. All chemicals were used as received. 2.2. Synthesis Mesoporous Fe-ZSM-5 zeolites with Si/Fe molar ratios of 150, 50, and 25 were prepared by the stean assisted crystallization SAC method according to a procedure reported in the literature with some modifications [20]. The gel molar compositions were 100 SiO2: (0.67, 2, 4) FeCl36H2O: 2 Al(OH)3: 37 TPAOH: 0.5 P123: 4600 H2O. As an example, Fe-ZSM-5 with a Si/Fe molar ratio of 50 was prepared as follows: 1.473 g of Pluronic P123 were dissolved in 36 g of DI water at 313 K. After obtaining a clear solution, 0.0764 g Al(OH)3 and 10.4 g TEOS were added to the solution which was stirred vigorously for 20 min. 0.278 g of FeCl36H2O were then added to the stirred solution, resulting in a decrease of the pH to approximately 2. After 1 h stirring at 313 K, 9.5 g TPAOH

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(40 wt%) was added drop-wise, resulting in an increase of the pH to above 12. This solution was stirred for 24 h at the same temperature. The stirring was then stopped and the solution aged at 333 K overnight. Finally, the solution was uncapped and the temperature was increased to 353 K for 12 h to dry the solution by evaporation. The dry solution was washed with 100 mL DI water and then added to a 10 mL borosilicate glass beaker in order to perform the SAC method using a 5500 HP compact Parr reactor. The mass ratio of dry gel to DI water around the beaker was 1.5. The dry gel was steamed for 16 h at 448 K. The steaming step was essential to crystallize the catalyst (Fig. S2). The dry powder was calcined in air by heating to 373 K with a heating rate of 2 K/min, held at this temperature for 1 h, then heated to 823 K at a heating rate of 2 K/min and held at that temperature for 4 h. Obtained powders were labeled as Fe-ZSM-5 (a)-S, where a is the molar ratio of Si to Fe. For comparison, Fe-ZSM-5 materials were hydrothermally synthesized using standard methods [21–23]. The molar gel compositions were 100 SiO2: (0.67, 2) Fe(NO3)39H2O: 2 Al(OH)3: 27 TPAOH: 3000 H2O. As an example, 0.3245 g of Fe(NO3)39H2O were dissolved in 20 mL of DI water. To this, 8.9 mL of TEOS were added drop-wise and stirred until the solution became uniform. 0.0626 g of Al(OH)3 were added to this mixture. Afterwards, 4.07 mL of TPAOH (40 wt%) were added slowly. The solution was stirred at 500 rpm for approximately 2 h. The resulting solution was then transferred to a Teflon lined autoclave and heated at 443 K with agitation. After 5 days, the material was collected by centrifugation (5000 rpm, 3 min) and washed with DI water until the pH of the wash solution was between 7 and 8. The obtained materials were dried in the oven at 353 K overnight. Calcination was identical to the calcination procedure for steam-assisted crystalized Fe-ZSM5 samples. Hereafter, the standard MFI materials will be denoted as Fe-ZSM-5 (b)-C, where b represents the Si/Fe molar ratio. 2.3. Characterization Powder X-ray diffraction was carried out on a Rigaku Benchtop MiniFlex X-ray diffractometer with Cu-Ka (k = 1.5418 Å) radiation operating at 40 kV and 15 mA. Materials were scanned in the range of 5°  2h  50° with a scanning speed of 0.19°/min. The metal content of the catalysts was determined by inductively coupled plasma optical emission spectrometry performed by Galbraith Laboratories. Energy-dispersive X-ray spectroscopy (EDS) also was conducted using a Hitachi S3400 system (30 V, 100 lA) for elemental analysis of the conventional samples. Nitrogen adsorption measurements were performed using a Micromeritics ASAP 2020 system at 77 K. Approximately 0.03 g of calcined sample was degassed at 573 K for 12 h under high vacuum prior to each analysis. Micropore volumes were calculated by the t-plot method (0.15  p/p0  0.3). Total pore volumes were calculated at a relative pressure of 0.98. Field-emission scanning electron microscopy (FE-SEM) analyses were carried out using a Hitachi 4800 highresolution scanning electron microscope operating at 3 kV. Ultraviolet visible spectroscopy (UV–Vis) was performed in the range of 200–1000 nm using a Thermo Scientific Evolution 300 equipped with a praying mantis stage from Harrick Scientific. A barium sulfate disk was used as the reference standard. The IR spectra were recorded on a Nicolet iS50R Fourier transform infrared spectroscopy (FT-IR) spectrometer equipped with a mercurycadmium-telluride (MCT) detector from Thermo Scientific. All diffuse-reflectance IR (DRIFTS) measurements were performed using a reaction cell (ZnSe windows) housed in a praying mantis stage, both from Harrick Scientific. Spectra were obtained by averaging 128 scans with a resolution of 4 cm1. Samples were mixed with KBr to 5 wt% and then were pressed into a pellet. Prior to the introduction of the probe molecule, catalysts were treated by heat-

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ing to 773 K for 2 h followed by evacuation (103 Torr) and then flowing N2 to remove water and other contaminants. Afterwards, the sample was cooled to room temperature and then methanol or NO (130 lmoles) were introduced on the pellet surface. To confirm that KBr does not have any influence on probe molecule physisorption features, a pure KBr pellet was placed in the IR cell and showed no methanol or NO adsorption. The IR spectrum of the sample was used as the background in the case of NO adsorption. X-ray absorption (XANES and XAFS) spectra were measured in fluorescence mode on undiluted samples at the HEXAS beamline of the LSU Center for Advanced Microstructures and Devices, for the ring operating at 1.3 GeV and a maximum current of 90 mA. A Ge (2 2 0) double crystal monochromater was used. The scan range (relative to the Fe edge at 7112 eV) was 200 to 30 eV (5 eV step), 30 to 30 eV (0.45 eV step), 30–100 eV (1 eV step), all with 4 s integration time.

2.4. Catalytic testing The oxidation of benzene to phenol using hydrogen peroxide was performed in a borosilicate glass vial with a screw cap. Typically, 40 mg of the catalyst were dried at 423 K for 1 h. Then, catalyst (40 mg), benzene (0.4 mL) and acetonitrile (4 mL) were loaded into the vial. This mixture was kept at 333 K in an oil bath under vigorous stirring at 500 rpm (Scheme 1). After about 10 min, the desired amount of H2O2 (30 vol%) was added drop-wise into the above suspension over the course of 30 min. All reactions were performed with a molar ratio of hydrogen peroxide to benzene of 1.85. To avoid losing any components during the reaction, the vial was cooled down to room temperature using an ice bath before each analysis. Then, 60 lL aliquots of sample were withdrawn from the reaction mixture every 1 h for analysis and filtered using an Acrodisc PTFE syringe membrane. To quench the reaction, methanol was added to the filtered mixture. 1 lL of the resulting mixture was injected into the GC. Prior to GC analysis and addition of methanol, 8 lL of reacted mixture were added into 3 mL of aqueous sulfuric acid-V2O5 slurry to determine the extent of H2O2 remaining in the reaction mixture by UV–Vis spectroscopy. This led to formation of a peroxovanadate complex (red-brown color) which had a maximum absorptivity at 449 nm (Fig. S1). Reaction products were analyzed with a 7890B Agilent GC system equipped with a flame ionization detector and equipped with a HP-5 capillary column (30 m, 0.32 mm, 0.25 lm). Ultra-high purity helium (average velocity = 35 cm/s) was used as the carrier gas. The GC temperature program was between 318 K and 358 K applying a heating rate of 4 K/min, then increased to 423 K at a heating rate of 50 K/min and held at this temperature for 3 min. The split ratio was 35 to 1. The injector and detector temperatures were both 523 K. Quantitative analyses were based on an internal standard (I.S., here methanol) method. In all reactions, phenol was detected as the major product. In order to check for iron leaching, hot filtration along with XPS were done.

Scheme 1. Hydroxylation of benzene catalyzed by Fe-ZSM-5 zeolites. x is the molar ratio of Si to Fe.

3. Results and discussion 3.1. Catalyst characterization The powder X-ray diffraction (PXRD) patterns of all calcined FeZSM-5 samples are presented in Fig. 1. All samples are phase pure MFI except for Fe-ZSM-5 (25)-S which appears to be amorphous (Fig. S2). Comparing these patterns with a-Fe2O3 in the range of 32°  2h  50°, there are no peaks corresponding to iron oxide particles (2h = 33.3°, 35.9°, 41.1°, 49.7°) (Fig. S3). Samples with higher iron contents exhibit less sharp peaks. One possible reason for this is that higher iron content samples possess smaller MFI domains. Another possibility is that the iron containing zeolites have a higher X-ray absorption coefficient [23]. Table 1 summarizes the composition of the calcined samples. These results indicate that 71–76% of the aluminum added to the initial gel is incorporated into the final materials. Conventional samples show lower iron uptake in comparison with SAC samples. Fe-ZSM-5 (150)-C shows the lowest Fe uptake and this explains its higher XRD peak intensities. The nitrogen adsorption isotherms of the samples are shown in Fig. 2. The conventional MFI preparations yield materials that exhibit a Type I isotherm, typical for microporous materials, with minimal N2 uptake at elevated pressures. In contrast, the SAC samples exhibit a hysteresis loop in the range of 0.7  p/p0  0.99 (type IV). Fe-ZSM-5 (50)-S possess the largest mesopore volume (0.36 cm3/g, Table 2). The standard samples and SAC samples have essentially the same (±0.02 cm3/g) micropore volumes. Fig. 3 shows the FE-SEM micrographs of the samples. The conventional Fe-ZSM-5 zeolites are spherical objects approximately a micron in size which appear to be agglomerates of smaller crystals. The SAC samples are considerably smaller in size than the conventional samples. Mean crystals size of Fe-ZSM-5 (150)-S and FeZSM-5 (50)-S are approximately 176 and 256 nm, respectively (Fig. S4). Moreover, increasing the iron content correlates with an increase in particle size. UV–Vis spectroscopy was used to investigate the coordination and aggregation state of the iron in the samples. Fig. 4 shows the UV–Vis spectra of the calcined Fe-ZSM-5 samples as well as aFe2O3 particles. In all Fe-ZSM-5 materials, the spectra are domi-

Fig. 1. PXRD patterns of calcined Fe-ZSM-5 zeolites.

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M. Shahami et al. / Journal of Catalysis 368 (2018) 354–364 Table 1 The chemical compositions of Fe-ZSM-5 samples. a) EDS results (error: ±0.05%) and b) ICP results. Sample Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5

Al wt%

Fe wt%

a

(150)-C (50)-C (150)-S (50)-S

a

0.66 0.64a 0.62b 0.65b

0.43 1.50a 0.55b 1.71b

Fig. 2. Nitrogen adsorption isotherms for calcined Fe-ZSM-5 samples, (blue circle) Fe-ZSM-5 (150)-C, (red square) Fe-ZSM-5 (50)-C, (green triangle) Fe-ZSM-5 (150)-S, and (black diamond) Fe-ZSM-5 (50)-S.

Table 2 Textural properties of calcined Fe-ZSM-5 samples. Mesopore volume was calculated by subtraction of micropore volume from total pore volume. Sample Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5

V (150)-C (50)-C (150)-S (50)-S

Micropore

0.14 0.13 0.14 0.12

(cm3/g)

V

Mesopore

(cm3/g)

0.07 0.06 0.11 0.36

nated by intense bands in the ultra-violet region (210–250 nm) which can be assigned to isolated tetrahedral Fe3+ species [23,24]. There is also a clear correlation between signal intensity and iron content. A weak shoulder between 300 and 400 nm can also be observed likely due to different coordinated Fe such as octahedral Fe3+ in small oligomeric FeOx clusters or dinuclear [Fe (III)-lO2-Fe(III)]2+ [23–26]. The intensity of this band correlates with the iron content. Absorption peaks above 400 nm can be seen for the Fe-ZSM-5 (50)-S sample. Comparing this feature with the spectra of a-Fe2O3, this could be attributed to Fe2O3 present in the sample with domains that are too small to be detected by XRD [27]. However, this is inconsistent with the nitrogen adsorption results which indicate very similar micropore volumes for all samples. Probe molecule adsorption was used to investigate iron and acid centers in the materials. IR spectroscopy using methanol as a probe molecule was carried out to investigate differences in hydroxyl groups or Brönsted acid sites between conventional and SAC materials. Fig. 5 displays the spectra of calcined Fe-ZSM-5 samples before (red line) and after (blue lines) methanol dosage.

Al uptake (%)

Fe uptake (%)

76 73 71 74

71 83 92 95

All these spectra are in good agreement with those of MFI zeolites, displaying framework vibration overtones at 1880 cm1 (Fig. S9) [22]. In all the cases, there is also a noticeable peak at 3745 cm1, corresponding to terminal silanol groups [28]. This peak is somewhat broad in Fe-ZSM-5 (150)-C sample due to presence of different silanol groups (geminal, vicinal, etc.) [22]. The low silanol peak intensity for the conventional samples is evident. This is consistent with the large crystal sizes of these materials as observed by FE-SEM. The intensity of the silanol peak decreases after methanol loading suggesting condensation of silanol groups after prolonged exposure to vacuum [29]. Methanol dosage on the surface leads to growth of new peaks in the range of 3550– 3200 cm1, corresponding to hydrogen bonding between methanol and silanols. A peak at 3623 cm1 also becomes visible, associated with chain formation caused by hydrogen bonding between several methanol molecules [29]. There are two weak bands at 3598 cm1 and 3679 cm1 in some of the samples. The former is assigned to Brönsted acid sites (T-OH-Al) and the latter is a characteristic feature of extra-framework aluminum species (Al-OH) (Fig. S11) [22,29,30]. Mitigation of the Brönsted acidity peak intensity for samples with higher iron contents can be ascribed to the either substitution of more iron into the framework, or possibly cationic iron species (Fig. S11). We believe the former is more likely, as for the SAC samples there is no increase in absorption in the region where one would expect Fe-OH. The only other possible explanation for the elimination of the Brönsted O-H peaks is exchange by Fe2+, i.e. the latter option. However, that would require a framework Al in close proximity to another framework Al or to a framework Fe, which given the aluminum content in these samples appears unlikely. The methoxy species formed after methanol loading cause the disappearance of these peaks. The asymmetric (masy) and symmetric (msy) stretching vibrations of CH3 groups at 2951 cm1 and 2843 cm1, respectively, also appear [30]. A weak band at 2911 cm1 can be seen, ascribed to the first overtone of the bending modes of the methoxy group [22,30]. The center rotational band appears at 2990 cm1. Desorption of more methanol from the surface shifts the masy and msy to higher frequencies [29]. This is due to delocalization of the protons. All the spectra in the 2100–1300 cm1 range are presented in Fig. S9. There is a band at 1460 cm1 after methanol dosage, attributed to the asymmetric bending mode of C-H out of plane of methoxy groups [30]. As expected, its intensity decreases after methanol desorption. Methoxy groups are anchored since the methanol proton affinity is not enough at room temperature to completely deprotonate the zeolite. Furthermore, the absence of a Fermi resonance effect (A, B, and C triplet) indicates that these materials possess mild Brönsted acid sites [30,31]. In-situ IR spectroscopy using NO as a probe molecule was carried out to gain insight into the coordination chemistry of Fe3+/ Fe2+ cationic sites in these samples. Fig. 6 shows the spectra of adsorbed NO at room temperature. Certain peaks appearing in the range of 1910–1750 cm1 are related to formation of various iron nitrosyls. The IR bands observed during NO adsorption can be categorized into several subgroupings. The bands at 1850 cm1 and 1842 cm1 are attributed to m(NO) of dinitrosyl iron species, Fe3+/Fe2+(NO)2 [20,24,31–33]. Calis et al showed that

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Fig. 3. FE-SEM images of calcined Fe-ZSM-5 samples; (a) Fe-ZSM-5 (150)-C, (b) Fe-ZSM-5 (50)-C, (c) Fe-ZSM-5 (150)-S, and (d) Fe-ZSM-5 (50)-S. The scale bar in all images is 1 lm.

Fig. 4. UV–Vis spectra of calcined Fe-ZSM-5 zeolites and a-Fe2O3.

these bands are due to adsorption of NO in b (six-membered rings) and c (five-membered rings) sites, respectively [34]. But adsorption of NO on a-Fe2O3 also shows a weak band in this region (Fig. S12). Upon evacuation, this peak disappears. Due to the small amounts of iron in the Fe-ZSM-5 samples, the contribution of aFe2O3 to the mononitrosyl band will be negligible, and we believe given the low iron content in the Si/Fe = 150 samples in particular

supports this feature being due to interactions of NO with framework iron species. Furthermore, incorporation of Al3+ and Fe3+ into the zeolite framework leads to the appearance of an intense band at this region [28]. Based on its appearance, some Al and Fe must be substituted into the framework in these samples. A weak band at 1905 cm1 and a band at approximately 1815 cm1 are due to symmetric and asymmetric vibrations of NO of iron dinitrosyl species (Fe2+(NO)2) at straight channels. Another explanation is that the feature at 1815 cm1 is due to trinitrosyl clusters on Fe(II) centers [35,36]. The shoulder at 1746 cm1 corresponds to Fe2+(NO) adsorbed on the same structural unit responsible for the dinitrosyl species [28,33,34,36]. Samples with a molar ratio of Si to Fe of 150 show a significant peak at 1815 cm1. On evacuation, the dinitrosyl species are converted to the mononitrosyl species with an absorption band at 1766 cm1 [28,33,34,36,37]. SAC samples show a more intense peak at 1766 cm1, attributed to more accessible iron sites in these samples. Makkee et al and later Mul et al. found that the IR shoulder at around 1880 cm1 arises due to a variety of Fe species, hydrated and dehydrated, but in close proximity to aluminum [35]. This band is noticeable in all samples, but particularly in Fe-ZSM-5 (50)-S. Calis et al put forth that these species are responsible for enhanced activity in NOx reduction [34]. A band at 1821 cm1 and shoulders at 1830 cm1, 1839 cm1, and 1859 cm1 can also be found in the IR spectra of SAC samples. These bands could be due to Fe2+(NO)2 and they disappear after long evacuation [35]. These might be associated with interactions of NO with iron incorporated in the zeolites but in different environments, or due to NO interacting with extra-framework iron [28,35,38]. The band appearing at 2132 cm1 is assigned to NOd+ or NO+ adsorbed at Brönsted acid sites (Fig. S13) [28,33–38]. The intensity of the peak at 3598 cm1 decreases with NO dosage

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0.1 a.u.

Fig. 5. In-Situ IR spectra of samples activated at 773 K for 3 h (red curves, spectra obtained at 294 K under vacuum), after introduction of methanol at 294 K (highest blue line), desorption of methanol after 5 min (second blue line) up to 30 min (rest of blue lines) under high vacuum (103 Torr). From top to bottom, clockwise: Fe-ZSM-5 (150)-C, Fe-ZSM-5 (50)-C, Fe-ZSM-5 (150)-S, and Fe-ZSM-5 (50)-S, respectively. The indicator shows the scale of the Kubelka-Munk function (a.u.).

showing the substitution of some hydroxyl bridges with cationic NO (Fig. S14) [28,34]. Iron K-edge XANES of Fe-ZSM-5 (50)-S and the most relevant oxide standards is shown in Fig. 7. In XANES, the normalized Fe K-edge spectra display two sets of features: the pre-edge representing 1s-3d transitions and the main absorption edge representing the 1s-4p transition. A noncentrosymmetric (e.g., Td) environment has a more intense pre-edge feature than a centrosymmetric (e.g. Oh) one, due to 4p mixing into 3d orbitals, which imparts some dipole-allowed 1s-4p character to the transition [32,39–41]. The position of the pre-edge also shifts to higher energy with increasing oxidation state [39,42]. Therefore, the position and shape of the pre-edge feature provides useful information on oxidation state, site distortion, and coordination number. In Fig. 7, the pre-edge of Fe-ZSM-5 (50)-S is relatively intense, with a well-defined peak indicative of Td coordination. There are clear differences from Fe(II) octahedral FeO and also from Fe2O3, with all octahedral Fe(III). The pre-edge centroid at 7114 eV is in the correct location for ferrosilicate framework Fe(III) [43–45], but also for octahedral Fe (III). However, the normalized intensity at the pre-edge (
0.08) is too small to indicate purely framework Fe(III) [43]. There is apparently also some octahedral, extra-framework Fe(III). The breadth of the pre-edge suggests some small amount of Fe(II) as well, but the edge position (7128 eV) confirms that the Fe is mostly Fe(III). The coordination environment of the Fe was further explored by XAFS; the R-plot is given in Fig. S15. The shell whose maximum is at 1.3 Å represents both framework terminal and extra-framework Fe-O for Fe(II) [46–48]. As seen from both first shell and multishell (all scattering paths to Reff = 3.2 Å included) fits to pure ferrisilicate MFI, there appear to be more such sites than are typical, consistent with the small crystal size of Fe-ZSM-5 (50)-S. The Fe-O shell with maximum
1.7–1.8 Å arises from internal framework sites and Fe (III) extra-framework sites [44,47,48], while the first shell (Fe-O) and second shell (mostly Fe-Fe) maxima at 2.1 and 2.5 Å [44,46,47,49], respectively, are of size, position and breadth consistent with the presence of some octahedral extraframework Fe. The shell maximum at 3.1 Å arises from several scattering paths (including Fe-Si second-shell interactions and multiple scattering)

[44,47,48], but clearly cannot be fully explained solely by framework scattering around a central Fe atom. Extra-framework Fe-O and Fe-Fe bonds typically show relatively high Debye-Waller factors in XAFS [43], which accounts for the increased breadth of the shells when extra-framework Fe is present. In conclusion, while much of the XAFS function can be explained by first- and second-shell scattering paths of framework ferrisilicate, the fits are inadequate and the results further reinforce the conclusion that some extra-framework material is present, in agreement with the XANES, UV–Vis and some of the IR data. 3.2. Benzene hydroxylation reaction Fig. 8 shows the benzene conversion and the phenol selectivity versus time for the different catalysts. All the reactions were performed at 333 K and atmospheric pressure under vigorous stirring. A blank reaction (no catalyst) and an experiment performed with ZSM-5 (no Fe) were carried out and no benzene conversion was observed. This shows that iron is necessary and is the active center. For all of the Fe-ZSM-5 materials studied, 1,4-benzoquinone was detected as the sole by-product except for Fe-ZSM-5 (150)-S, which had 100% phenol selectivity. In the control experiment where iron oxide particles were used as the catalyst, 1,4benzoquinone and hydroquinone were minor and major sideproducts, respectively. Results for the conventional samples are consistent with previous results [3]. As compared to the Fe-ZSM-5 samples prepared via conventional syntheses, the SAC samples show higher activity. The benzene conversion was 9.5% after 8 h for the Fe-ZSM-5 (150)-S sample. The Fe-ZSM-5 (50)-S catalyst showed the highest benzene conversion at 25.5% with a phenol selectivity of 90% after 8 h. For comparison, Ti-MFI samples with molar ratios of Si to Ti of 150 and 50 were synthesized (described in supporting information) and used in the same reaction conditions. After 4 h, TS-1 (150) and TS-1 (50) gave conversions of 0.85% and 1.1% and phenol selectivities of 78% and 63%, respectively (Fig. S18). Furthermore, the direct catalytic oxidation of benzene to phenol of SAC samples was compared with certain iron/titanium-based materials as shown in Table 3. The phenol yield over the SAC zeolites are much higher than the previous published results, and consistent with our

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Fig. 6. In-Situ IR spectra of samples activated at 773 K for 1 h (gray), after introduction of NO at 294 K (red line), desorption of methanol up to 10-2 (blue lines). From top to bottom, clockwise: Fe-ZSM-5 (150)-C, Fe-ZSM-5 (50)-C, Fe-ZSM-5 (150)-S, and Fe-ZSM-5 (50)-S, respectively. The indicator shows the scale of the Kubelka-Munk function (a.u.).

Fig. 7. Fe K-edge XANES of Fe-ZSM-5 (50)-S with oxide standards. Left: full region. Right: pre-edge only.

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Fig. 8. Benzene conversion/phenol selectivity versus time over calcined Fe-ZSM-5 samples and a-Fe2O3 particles at 333 K.

Table 3 Comparison of catalytic performance of different iron/titanium based materials for benzene hydroxylation to phenol using H2O2 as the oxidant at mild conditions. X is the desired products formed benzene conversion except for TS-PQTM, which is H2O2 conversion. Turn over number was calculated as follows: TON ¼ Moles ofMoles . of Fe or Ti Catalyst

Solvent

T (K)

t (h)

X (%)

Y

TS-1 TS-PQTM TS-1B TS-1A TS-1 (50) TS-1 (150) [Fe, Al]-MFI Fe/SBA-16 Fe-SBA-15 AC-Fe30 Fe3O4/CMK-3 Fe/GO Fe/MWCNTs a-Fe2O3 Fe(NO3)39H2O Fe-ZSM-5 (150)-C Fe-ZSM-5 (50)-C Fe-ZSM-5 (150)-S Fe-ZSM-5 (50)-S

Acetonitrile Water Sulfolane Methanol/Sulfolane Acetonitrile Acetonitrile Acetonitrile Acetonitrile Acetonitrile Acetonitrile Acetonitrile Acetic Acid Acetonitrile Acetonitrile Acetonitrile Acetonitrile Acetonitrile Acetonitrile Acetonitrile

333 343 373 353 333 333 333 338 333 303 333 338 333 333 333 333 333 333 333

3 3 1 2 4 4 3 8 8 7 4 3 2.5 8 8 8 8 8 8

1.80 19.40 8.60 14.00 1.10 0.85 2.80 12.10 10.67 19.60 18.00 15.90 10.80 3.70 8.46 3.48 4.18 9.50 25.50

<1.44 – 8.08 13.00 0.69 0.66 >2.74 11.66 9.39 17.50 16.56 14.96 10.31 3.13 7.34 3.41 4.05 9.50 22.95

claims that the reactivity we observed is unprecedented for a zeolite-based catalyst when using hydrogen peroxide as an oxidant. The turnover number (TON) values are also shown in Table 3 and as can be seen the SAC samples possess higher TON values than those reported for other catalysts in the literature. To rule out the possible contribution of homogeneous catalysis, the issue of iron leaching was investigated. The first experiment carried out was the hot filtrate test, the results of which are shown in Fig. 9. After specific times (5 h for Fe-ZSM-5 (150)-S, 2.5 h for FeZSM-5 (50)-S), the catalysts were filtered and separated from the mixture. The filtered reaction solution, without catalyst, was heated for additional time and the conversion/selectivity monitored. No further reactivity was observed. This observation is consistent with the zeolite being solely responsible for the benzene conversion observed. To further examine possible leaching, the reaction solution after testing was analyzed by ICP. ICP analyses showed no iron present. Finally, Fe(NO3)39H2O was used as a catalyst to investigate the activity of dissolved Fe salts. Fig. S19 shows the benzene conversion/phenol selectivity for this reaction. Over the course of the reaction, the selectivity was constant at approximately 85%, with a benzene conversion of 8.5% after 8 h. In this case, hydroquinone and 1,4-benzoquinone were produced as major

Phenol

(%)

TON

Reference

13.53 30.00 14.66 17.88 3.77 8.64 34.95 7.61 39.31 14.63 34.80 22.60 17.04 0.33 3.85 50.10 17.34 105.63 81.74

[3] [50] [6] [51] Present Present [3] [52] Present [53] [54] [55] [56] Present Present Present Present Present Present

work work

work

work work work work work work

and minor by-products. These experiments demonstrate that the Fe-ZSM-5 zeolite is responsible for the reactivity observed and there is no appreciable contribution to the results observed in Fig. 8 from homogeneous catalysis involving leached iron species. Having ruled out artifacts due to homogeneous catalysis, multiple explanations could be invoked for the superior performance of the SAC samples. One possibility is that the SAC samples have much less diffusion resistance than the conventional samples, leading to an increase in the observed rates. The second possibility is that iron centers generated in the SAC samples are different than what is formed in the conventional samples and this leads to the observed reactivity. If the latter is true, then understanding which part of the reaction pathway is facilitated by these superior iron centers needs to be deduced. Specifically, is the enhanced conversion in the SAC samples due to enhanced hydrogen peroxide activation, enhanced complexation with benzene, or both? The role of the mesoporosity was first explored. In order to better understand the role of the mesopores, a sample was prepared where instead of calcining after synthesis, the sample was Soxhlet extracted with methanol (supporting information). This was done in order to remove the Pluronic soft template from the mesopores but leave TPA+ in the micropores. TGA analysis of this sample along

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Fig. 9. Benzene conversion/phenol selectivity for SAC samples from the hot filtrate test where the catalyst was removed after 150 minutes on stream.

Fig. 11. Hydrogen peroxide decomposition as a function of time over calcined FeZSM-5 samples and a-Fe2O3 particles at 333 K.

with nitrogen adsorption indicate a sample with open mesopores but no microporosity, consistent with our aims (Fig. S16). The catalytic results for this sample are shown in Fig. 10. This catalyst shows a benzene conversion of 4% with 99% selectivity to phenol. The conclusion from this experiment is that the majority of the active sites for the benzene to phenol reaction are located in the micropores of the SAC samples, prior to calcination of the catalyst. Given this control did not include a high temperature calcination step we cannot conclusively rule out that the calcination step is needed to form the iron centers that are active for this chemistry. We have also attempted to understand if the active sites of the conventional and SAC samples were different, and if this could be the reason for the observed enhancement in reactivity. One ques-

tion was whether the hydrogen peroxide reactivity was substantively different between samples; these results are shown in Fig. 11. Samples with higher iron contents show higher H2O2 consumption rates. The H2O2 decomposition rates over the conventional samples are consistent with the results of Hirsekorn et al. [3]. SAC samples show higher hydrogen peroxide consumption by
10–20% above that of the conventional samples, at times greater than two hours. Whether this is a result of lower diffusion resistance, presence of more active sites in the SAC samples, or that the iron species in the SAC samples are more stable is unclear. However, we speculate that diffusion resistances are not a key issue, given that the difference between the SAC and conventional samples widens at longer times (>4 h), where the mass transfer effects should be less pronounced. We speculate that the iron centers formed in the SAC samples are stable and less prone to deactivation (vide infra). Fe-ZSM-5 (50)-S shows 95% H2O2 conversion after 8 h. Iron oxide particles show the lowest hydrogen peroxide consumption. Reusability of the catalyst is a crucial factor for a heterogeneous catalysis. In this regard, the reusability of the SAC zeolites was evaluated over three benzene oxidation runs, with results shown in Fig. 12. After reaction, the catalysts were separated by centrifugation, washed several times with methanol and dried at 373 K overnight. The recovered catalyst was then used under the same conditions as previous reactions. As can be observed, the catalyst activity was maintained over three runs. Moreover, the UV–Vis spectra of these samples after three runs showed no significant variations (Fig. S17). This finding, we believe, supports our speculation above that the iron centers formed in the SAC samples are less prone to deactivation. 4. Conclusions

Fig. 10. Benzene oxidation using Soxhlet extracted Fe-ZSM-5 (50)-S as well as calcined Fe-ZSM-5 (50)-S.

Mesoporous and microporous Fe-ZSM-5 were synthesized successfully by steam-assisted crystallization and conventional methods, respectively. XRD confirms all samples are phase pure MFI and porosimetry was used to quantify the textural differences of the samples. FE-SEM images showed that mesoporous Fe-ZSM-5 consists of much smaller aggregates than do conventionally synthesized samples. For the calcined samples, UV–Vis and XANES/XAFS

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Fig. 12. Benzene conversion/phenol selectivity versus number of runs at the same conditions as before. Left) Fe-ZSM-5 (150)-S and right) Fe-ZSM-5 (50)-S.

suggest that most of the iron centers are isolated framework Fe3+ sites, except for the SAC sample with a molar Si/Fe of 50, where small quantities of what are likely small extraframework FeOx clusters can be detected. In-situ IR spectroscopy of adsorbed methanol on the surface of these materials displayed the presence of Brönsted acidity. Catalytic testing of SAC Fe-ZSM-5 in benzene oxidation at mild temperature indicate that these materials are potential catalysts for phenol production. Based on use of a Soxhlet extracted catalyst in this reaction, and hot filtration results confirming no Fe leaching into solution, it can be concluded that iron species distributed inside the micropores are the active centers. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding sources Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2018.10.011. References [1] L.Q. Meng, X.C. Zhu, E.J.M. Hensen, Stable Fe/ZSM-5 nanosheet zeolite catalysts for the oxidation of benzene to phenol, ACS Catal. 7 (2017) 2709–2719. [2] L. Balducci, D. Bianchi, R. Bortolo, R. D’Aloisio, M. Ricci, R. Tassinari, R. Ungarelli, Direct oxidation of benzene to phenol with hydrogen peroxide over a modified titanium silicalite, Angewandte Chemie-International Edition 42 (2003) 4937–4940. [3] M.L. Neidig, K.F. Hirsekorn, Insight into contributions to phenol selectivity in the solution oxidation of benzene to phenol with H2O2, Catal. Commun. 12 (2011) 480–484. [4] J.S. Choi, T.H. Kim, K.Y. Choo, J.S. Sung, M.B. Saidutta, S.O. Ryu, S.D. Song, B. Ramachandra, Y.W. Rhee, Direct synthesis of phenol from benzene on ironimpregnated activated carbon catalysts, Appl. Catal. A-General 290 (2005) 1–8. [5] B.M. Abu-Zied, W. Schwieger, A. Unger, Nitrous oxide decomposition over transition metal exchanged ZSM-5 zeolites prepared by the solid-state ionexchange method, Appl. Catal. B-Environ. 84 (2008) 277–288.

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