Evolution of iron species for promoting the catalytic performance of FeZSM-5 in phenol oxidation

Abstract

Facile modification of FeZSM-5 utilizing an organic directing agent (ODA) for iron species evolution toward better Fenton activity was developed. Pristine FeZSM-5, prepared hydrothermally using tetrapropylammonium bromide as the ODA, was thermally treated in nitrogen or air atmosphere for comparison. The as-prepared FeZSM-5 were evaluated by XRD, SEM, TGA, Ar physical adsorption, H₂-TPR, UV-Vis, XPS and Mössbauer spectra, which confirmed well-dispersed iron species and partial transformation of framework iron to non-framework iron and Fe(III) to Fe(II) during thermal treatment in N₂. The phenol oxidation reaction was applied as a probe reaction and phenol conversion can be improved from 44% to 90% within 60 minutes. By thermal treatment in N₂ atmosphere, the ODA plays an important role in modifying non-framework iron species from Fe(III) to Fe(II) ions which is desirable for improving catalytic activity in Fenton-like systems. The strategy utilizing the thermal reduction effect of the ODA on iron species is a reliable method for preparing well-dispersed and valence tunable iron oxide nanoparticles encapsulated in ZSM-5.

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1 Introduction

Advanced oxidation processes (AOPs), using hydroxyl radicals (·OH), are one of the most effective methods for the destruction of organic pollutants in wastewater, and so have drawn much attention in an extensive range of studies.^1–3 Compared with the conventional homogeneous Fenton oxidation system, the activity and stability of heterogeneous catalysts under near neutral conditions are problems that need to be solved.^4–7 Regeneration of Fe(II) species is the rate-limiting step in catalytic cycling^4,8 and strategies which introduce oxidation–reduction pairs to lower the energy barrier have been proposed.^9–11 Though various zeolites have been used due to their acidity and hydrothermal stability,^12,13 tuning the chemical state of iron species in zeolite-based catalysts without affecting its dispersion is still a problem for their application.^14,15 Therefore, finding a way to optimize the physicochemical state of iron for preventing agglomeration is of great importance.

Up to now, many kinds of heterogeneous catalysts have been investigated in the effluent treatment.^3,4,16 Metals and metal oxides are the most direct and simplest multiphase catalysts, but they suffer from poor metal dispersion¹⁴ and catalyst recovery problem.¹⁷ Host–guest materials, like yolk–shell materials,^18–20 iron-supported catalyst,^11,21–23 iron-exchanged catalyst,^13,24 etc., have their special advantages in developing the synergetic effect between host and guest. However, iron-supported catalyst, prepared from impregnation, suffers from metal oxide agglomeration on ZSM-5 external surface¹⁴ or from the lower synergetic effect of ordered mesoporous materials²⁵ because of its lower adjustable acidity and hydrothermal stability. In the case of iron-exchanged catalyst, the amount of iron loading greatly depends on the Si/Al ratio of zeolite and low iron content is often obtained. Fortunately, FeZSM-5 obtained from hydrothermal synthesis can not only ensure the great dispersion of iron species but also maintain appropriate iron content. However, both framework iron and non-framework iron species of conventional FeZSM-5 have no satisfactory activity because higher Fe(III) species and lower Fe(II) species exist, which is not desirable for Fenton-like system.^8,20,26 Therefore, the direction for modification of FeZSM-5 is improve the content of Fe(II) species.

Reduction of Fe(III) using the thermal reduction effect of organics is a significant method to improve Fe(II) content. By H₂ temperature programmed reduction, Fe₃O₄ can be prepared from Fe₂O₃, but those Fe₃O₄ nanoparticles are easy to be oxidized in air. In contrast, organics such as ethylene glycol, polyethylene glycol, and polyvinyl pyrrole, play a very important role in reducing Fe³⁺ which is similar to H₂ in preparing Fe₃O₄ nanoparticles from Fe(III) salt.²⁷ More importantly, those organics can also stabilize the Fe₃O₄ from oxidation.²⁷ According to this idea, our previous work¹⁵ focused on introducing polyethyleneimine (PEI) to optimize the physicochemical properties of iron oxides. While in order to simplify the preparation process, a more facile strategy was developed in the present work.

Herein, a facile strategy to preparing FeZSM-5 with improved Fenton activity has been developed which involves the use of organic direction agents (ODAs) to reduce non-framework iron species. Phenol oxidation reaction is applied to evaluate the catalytic activity and stability. The structure–activity relationship was also delineated based on the analytical characterization.

2 Experimental

2.1 Chemicals and materials

The chemicals used include silica sol (30 wt%), ethylamine (EA, 65 wt%), tetrapropylammonium bromide (TPABr), tetraethylorthosilicate (TEOS), tetrapropylammonium hydroxide (TPAOH), ferric chloride (FeCl₃·6H₂O), H₂O₂, and phenol. All above materials were purchased in reagent grade and used without further treatment.

Preparation of microporous ZSM-5. FeZSM-5 was synthesized hydrothermally with the assistance of active seeds from a gel with a molar ratio of 1 SiO₂ : 0.15 TPABr : 1 EA : 17 H₂O. The active seeding gel was prepared from a gel with a molar ratio of 1 TEOS : 0.36 TPAOH : 19 H₂O by a conventional hydrothermal synthesis route.¹¹ Typically, a mixture which consists of 50 g of TEOS, 70 g of TPAOH (25 wt%), and 30 g distilled water was stirred at 308 K and maintain for 5 h which followed by increasing temperature to 353 K and crystallize for another 72 h. After cooling down, the obtained seeding gel was directly used for the synthesis of FeZSM-5 zeolites without any treatment. The synthesis of FeZSM-5 was conducted by using the following procedure: first, 13.34 g of TPABr was dissolved with 66.67 g of silica solution and afterwards with a solution of 22.26 g of EA (65 wt%). The mixture was aged at room temperature for 30 min under mechanical stirring followed by addition of a solution of FeCl₃·6H₂O and 45 g of H₂O. 0.21 g as-prepared active seeding gel was then added into the above FeZSM-5 precursor and then introduced to the stainless steel autoclave (200 ml, filled volume 150 ml) with hydrothermal treatment at 443 K for 72 h. The resulting solid was filtered, washed, and dried overnight at 373 K. The as-synthesized zeolite was calcined in nitrogen or air atmosphere at 873 K for 4 h. The resulting sample was denoted as NX or AX, where N and A represent that the calcination atmosphere is nitrogen and air, respectively; X represents the Si/Fe molar ratio of FeZSM-5.

2.2 Catalytic testing

All catalyst evaluations were conducted in a 100 ml round-bottom flask equipped with a magnetic stirring bar. In a typical procedure, 20 mg catalyst, 50 ml phenol solution (1 g L⁻¹), and 10.76 ml H₂O₂ solution (0.69 M) were added together into the flask followed by maintaining at 50 °C for the appropriate time. The products were analyzed by liquid chromatography. The molar ratio of H₂O₂/phenol was 14. Phenol oxidation reaction was chosen to evaluate the catalytic activity of the as-prepared catalyst. In a typical evaluation, 50 mg of the as-prepared catalyst, 10.76 ml of H₂O₂ solution (0.77 M), and 50 ml of phenol solution (1 g L⁻¹) were added together into a 100 ml round-bottom flask and stirred at 65 °C water bath. The product was sampled every other 15 minutes and analyzed by liquid chromatography (Agent 1200 series) for calculating the phenol conversion.

2.3 Characterization

The crystalline structures of the as-prepared catalyst were determined by a Rigaku Smartlab diffractometer with a nickel-filtered Cu Kα X-ray source. The scattering angel was between 5° and 80° at a scanning rate of 0.02°. Morphological properties were obtained on a Hitachi 5-5500 instrument. Prior to the measurement, the samples were sputtered with a thin film of gold. The textual properties including the surface area, pore volume and pore size distribution were measured using a Quantachrome autosorb-iQ2 gas adsorption analyzer with argon at 87 K. Prior to the measurement, the calcined samples were degassed in vacuum at 573 K for 8 h. The total surface area (S_BET) was measured by the Brunauer–Emmett–Teller (BET) method. Microporosity and mesoporosity was determined by the t-plot method. H₂-TPR measurements were carried out using ChemBET Pulsar TPR/TPD equipment (Quantachrome, USA). The iron content of the as-prepared samples was measured by ICP-OES after digestion with the mixture of HCl, HNO₃, HF, HClO₄. Ultraviolet-visible diffuse reflectance (UV-Vis) spectra were acquired on a Jasco UV-550 spectrometer, and pure BaSO₄ was used as reference. Deconvolution of the UV-Vis spectra into individual iron species' bands was conducted according to the procedure detailed in literature.²⁸ Thermal gravimetric analysis (TGA) was performed on a SDT Q600 (TA Instruments, U.S.A.) with a heating rate of 10 °C min⁻¹ in air or N₂ atmosphere. X-ray photoelectron spectra (XPS) were obtained with a Thermo VG ESCALAB250 instrument using Al Kα radiation. The ⁵⁷Fe Mossbauer spectra were recorded using a Topologic 500 A spectrometer at room temperature with a proportional counter.

3 Results and discussion

3.1 Material synthesis and characterization

The XRD patterns of FeZSM-5 series with different Si/Fe ratio and different thermal treatment atmosphere are shown in Fig. 1. Five characteristic diffraction peaks at 7.8°, 8.8°, 23.0°, 23.9° and 24.4° clearly exhibit pure MFI topology of FeZSM-5. No diffraction peaks associated with iron phase were detected, indicating the high dispersion of iron species. Fig. 2 shows the SEM images of the six catalysts with the inset showing their particle size distribution. All six catalysts have a uniform particle size distribution around 700 nm.

Fig. 1 XRD patterns of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200.

Fig. 2 SEM images of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200.

Fig. 3 shows the Ar adsorption–desorption isotherms of FeZSM-5 at 87 K with the pore size distribution presented in Fig. 4. The adsorption branch of the six samples shows typical type-I adsorption line which indicates their microporosity. Typically, pore distribution at around 0.54 nm represents the oval-shaped and Z-shaped channel, and pore distribution at around 0.9 nm represents the channel intersection.²⁹ The three samples calcined in N₂ atmosphere all present a relatively lower pore distribution compared with samples calcined in air which definitely indicate the existence of something blocking their channel intersection. At the same time, total pore volume (V_pore) (shown in Table 1) present the same principle and a relatively smaller pore volume exists in samples calcined in N₂. The substance which exists in the intersection is either coke-like species³⁰ or iron species. According to TG curves of sample N40 and A40 (shown in Fig. S1†), weight loss above 600 °C is approximately 0.5 wt% indicating that the organic directing agents of those samples have almost been removed meaning and merely have residual of coke-like species, from which we infer the existence of iron species in channel intersection for three samples calcined in N₂.

Fig. 3 Ar adsorption and desorption isotherms at 87 K (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200. The isotherms of each sample are displaced vertically by 100 cm³ g⁻¹.

Fig. 4 Pore size distributions of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200. The pore size distributions were determined by non-local density functional theory (NLDFT). The pore size distributions of each sample are displaced vertically by 0.5 cm³ g⁻¹.

Table 1 Textural properties of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200

Sample code	Smicro^a [m^2 g^−1]	Smeso^a [m^2 g^−1]	SBET^b [m^2 g^−1]	Vmicro^c [cm^3 g^−1]	Vpore^d [cm^3 g^−1]
a t-Plot method.b BET method.c NLDFT method.d p/p0 = 0.99.
N40	313	61	374	0.182	0.28
A40	319	98	417	0.197	0.32
N80	307	104	411	0.193	0.29
A80	318	104	422	0.199	0.31
N200	311	112	423	0.199	0.28
A200	341	107	448	0.213	0.31

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Fig. 5 shows the TGA and DTG curves of the uncalcined samples in N₂ and air atmosphere. Weight loss below 300 °C can be attributed to adsorbed water and crystal water, and above 300 °C attributed to the decomposition of TPA⁺. It can be seen that samples with the same Si/Fe ratio do have a difference when hydrothermal treatment was conducted in different atmosphere. The temperature for decomposition of TPA⁺ for samples calcined in air is slightly lower than those in N₂ which is corresponding with the ref. 31. Owing to the existence of O₂ under air atmosphere, the oxidation of TPA⁺ would release energy. On the contrary, without the help of O₂, the decomposition of TPA⁺ in N₂ is merely a pyrolysis process.^30,31

Fig. 5 TG curves of uncalcined FeZSM-5 with different Si/Fe ratio. (a) Si/Fe = 40, (b) Si/Fe = 80, (c) Si/Fe = 200. Measure condition: air or N₂ atmosphere, 10 °C min⁻¹.

UV-Vis spectra between 190–800 nm were conducted to investigate the nature of Fe(III) species of FeZSM-5,²⁸ and importantly that Fe(II) species do not contribute to the UV-Vis spectra absorbance because the absorbance of Fe(II) species falls in the near infrared range around 1000 nm.³² Fig. 6 shows the UV-Vis spectra of the as-prepared catalysts. For Fe³⁺ sites, two charge-transfer (CT) bands associated to t₁ → t₁ and t₁ → e transitions are expected.³² Bands at 215 nm and 241 nm can be attributed to framework iron species,³³ and a band at 278 nm is attributed Fe³⁺ species in octahedral sites. Octahedral Fe³⁺ ions in small oligo nuclear clusters give rise to broad bands between 300 and 450 nm and bands above 450 nm are characteristic for large Fe₂O₃ particle.^28,33 All six samples contain both framework and non-framework iron species but with different contents. According to the ICP-OES analysis (Table 3), samples with lower Si/Fe ratio showed higher iron contents, so it is reasonable that samples with lower Si/Fe ratio show higher intensity in UV-Vis spectra. Importantly, in the case of samples with the same iron contents, for N40 and A40 instance, compared with A40 which was treated under air atmosphere, sample N40 shows weaker absorbance which can be attributed to that sample N40 contained some of Fe(II) ions. Therefore, it can be concluded that the decomposition of TPA⁺ under N₂ result in the generation of Fe(II) species in sample N40. On the other hand, from the deconvolution of N40 and A40 (shown in Table 2), there is very limited change in composition of Fe(III). So there exists such a transformation of iron species to maintain the Fe(III) composition unchanged: framework iron → non-framework iron → Fe(II). And the content of non-framework iron is much higher for samples calcined in N₂ than in air.

Fig. 6 UV-Vis spectra of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200.

Table 2 Percentage of the area of the bands (I₁ at λ < 280 nm, I₂ at 280 < λ < 400 nm, and I₃ at λ > 400 nm) derived from deconvolution of the UV-Vis spectra in Fig. 5. The values do not take into account the contribution of eventual Fe²⁺ species present in the as-prepared samples

Sample	I1^a (%)	I2^b (%)	I3^c (%)
a Isolated Fe ions in tetrahedral and octahedral coordination.b Oligo-nuclear FexOy clusters (x ≥ 2).c Bulk Fe nanoparticles.
N40	62	26	12
A40	59	27	14
N80	58	25	17
A80	56	27	17
N200	58	24	18
A200	85	15	0

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Table 3 The composition of the synthesis gel and the real composition

Sample	Amount of metal in synthetic gel (wt%)		Amount of metal in samples^a
	Fe	Si/Fe	Fe	Si/Fe
a Measured by ICP analysis.
N40	2.26	40	3.01	29.6
A40	2.26	40	3.09	28.9
N80	1.15	80	1.72	52.9
A80	1.15	80	1.85	49.1
N200	0.46	200	0.65	143.1
A200	0.46	200	0.66	139.8

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To investigate the reducibility of the materials, H₂-TPR experiments were conducted, as shown in Fig. 7. For all six samples, the three H₂ consumption peaks (α₁, α₂, α₃) are attributed to Fe³⁺ and Fe₂O₃ to Fe^(3−δ)+ (α₁), with intermediate valence as that in Fe₃O₄, and then further reduction to Fe²⁺ (α₂). At higher temperature, the peaks represent partial reduction of Fe²⁺ to Fe⁰ (α₃). All the above-mentioned iron species in FeZSM-5 that could be reduced by H₂ is the non-framework iron species, and higher temperature leads to more framework iron's transformation to non-framework species. According to the H₂-TPR profiles of the catalysts, H₂ consumption of samples treated in N₂ is apparently higher than samples treated in air indicating higher amount of non-framework iron species, in accordance with the UV-Vis analysis. On the other hand, the reduction temperature of sample N40 slightly shifts to lower temperature which shows that iron species in sample N40 are easier to be reduced. This indicates that non-framework iron species in samples treated in N₂ atmosphere have a better dispersion.

Fig. 7 H₂-TPR of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200.

Fig. 8 shows the Fe 2p XPS spectra of N40 and A40. From the deconvolution of the XPS spectra for Fe 2p_3/2, the peak centered at 709.8 eV is attributed to Fe²⁺.³⁴ For the peak centered at 714.0 eV, it is attributed to the framework iron with the Fe–O–Fe bonds that make the peaks shifting to higher binding energy. In the case of sample A40, as some framework iron species were converted to non-framework species after thermal treatment, the binding energy correspondingly shifts to the higher value centered at 712.4 eV.

Fig. 8 XPS spectra of (a) N40, (b) A40.

To further investigate the nature of iron species, ⁵⁷Fe Mössbauer spectroscopic characterization was conducted. Fig. 9 shows the Mössbauer spectra of N40 and A40 at room temperature, with the hyperfine interaction parameters summarized in Table 4. In general, the nanoparticle size of iron species can be discussed by the peak shape including doublet and sextet.^35,36 Nanoparticles with high dispersion would present super-paramagnetic phenomenon therefore the peaks would be doublet. From Fig. 9, doublets are observed both N40 and A40 which imply well-dispersion of iron species. Both of the Mössbauer spectra of samples N40 and A40 could be fitted well by two doublets. In the case of N40, the doublet with IS = 0.38 mm s⁻¹ and QS = 0.92 mm s⁻¹ could be attributed to Fe(III) species, and doublet with IS = 0.92 mm s⁻¹ and QS = 2.51 mm s⁻¹ could attributed to Fe(II) species.^37–39 In contrast, according to the ⁵⁷Fe Mössbauer parameters, both of the doublets of A40 only could be attributed to Fe(III) species which is consistent with UV-Vis and XPS analysis.

Fig. 9 ⁵⁷Fe Mössbauer spectra of (a) N40, (b) A40 samples at room temperature.

Table 4 ⁵⁷Fe Mössbauer parameters of N40 and A40 samples

Sample	Oxidation state	Sub-spectrum	IS (mm s^−1)	QS (mm s^−1)	RI (%)
N40	Fe(III)	Doublet	0.38	0.92	77.3
	Fe(II)	Doublet	0.92	2.51	22.7
A40	Fe(III)	Doublet	0.32	0.76	23.8
	Fe(III)	Doublet	0.34	1.18	76.2

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3.2 Catalytic performance

As Fenton-like system is very sensitive to the iron chemical state, phenol oxidation reaction is applied as a probe reaction to evaluate the as-prepared FeZSM-5. Fig. 10 and 11 show the phenol and H₂O₂ conversion, respectively, in the presence of series of FeZSM-5 samples. According to the initial conversion of phenol and H₂O₂, the samples with higher iron contents exhibit higher activity, and, importantly, samples calcined in N₂ exhibit higher catalytic activity than that in air. The obtained products for phenol oxidation is mainly CO₂ and H₂O, and the intermediate products including catechol, hydroquinone, o-benzoquinone, p-benzoquinone, oxalic acid, acetic acid.

Fig. 10 Phenol conversion in the presence of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200.

Fig. 11 H₂O₂ conversion in the presence of (a) N40, (b) A40, (c) N80, (d) A80, (e) N200, (f) A200.

3.3 Discussion

Hydroxyl radical (·OH), produced from H₂O₂ in chain reaction initiated by Fe²⁺ (Table 5), is widely accepted as a strong oxidant for its high redox potential in Fenton system. Higher amount of Fe²⁺ as well as higher dispersion is beneficial for improving catalytic activity of iron-based catalyst,^4,8 and many efforts have been made by using Fe₃O₄ as the active phase in the previous work.^26,41,42

Table 5 Chain reaction initiated by Fe²⁺.⁴⁰

	Reaction	Rate constant at pH = 3 (mol^−1 s^−1)
1	Fe^2+ + H2O2 → Fe^3+ + ˙OH + OH^−	63
2	H2O2 + Fe^3+ → Fe^2+ + HO2˙/O2˙^− + H^+	2.0 × 10^−3
3	˙OH + H2O2 → HO2˙/O2˙^− + H2O	3.3 × 10^7
4	Fe^3+ + HO2˙/O2˙^− → Fe^2+ + O2 + H^+	7.8 × 10^5
5	Fe^2+ + ˙OH → Fe^3+ + OH^−	3.2 × 10^8
6	HO2˙/O2˙^− + HO2˙/O2˙^− → H2O2 + H2O	2.3 × 10^6
7	˙OH + HO2˙/O2˙^− → H2O + O2	7.1 × 10^9
8	˙OH + ˙OH → H2O2	5.2 × 10^9

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FeZSM-5, obtained from hydrothermal synthesis, is with the ODAs in its channel system and with good iron dispersion. The decomposition process of TPA⁺ used as ODAs in this work is shown in Fig. 12. The decomposition of TPA⁺ involves two gradual template decomposition steps. At a low temperature, tripropylamine (Pr₃N) and propene obtained from a Hofmann elimination of TPA⁺. In a second step, Pr₃N was further degraded into lower amines and consequently into propene. Finally, the unsaturated products like propene undergo oligomerization, cyclization and aromatization.³⁰ The oligomerization of the template derivatives finally lead to the coke-like species which have relatively strong interaction with ZSM-5 framework. When FeZSM-5 was calcined in air, the coke-like species would likely convert easily to gaseous products with the help of O₂ and iron species are seldom influenced. In contrast, when calcined in N₂ atmosphere, the temperature for those carbon species to break up is much higher. Higher temperature leads to the conversion of iron species from framework to non-framework. At the same time, the coke-like species can play a role in reducing the non-framework iron species.

Fig. 12 Evolution process of TPA⁺ during detemplation of ZSM-5.

The strategy involving the organic structure directing agents as a reductant to tune the iron species is also reliable in other zeolites.

4 Conclusions

A superior FeZSM-5 catalyst with excellent dispersion and valence tunable iron species was obtained from hydrothermal synthesis utilizing tetrapropylammonium bromide as the ODA in its channel system. Through simple thermal treatment in nonoxidative atmosphere, some framework iron converts to non-framework species. At the same time, the coke-like species obtained from oligomerization of ODAs play an important role in tuning the iron species to metastable state which is good for phenol oxidation. The catalytic activity of samples calcined in N₂ is totally higher than that in air and the phenol conversion can improve from 44% to 90% within 60 min. The strategy involving using the organic directing agents as a reductant to tune the iron species and using the confinement effect of microporous structure to get higher dispersion is a reliable method to prepare well dispersed zeolite-encapsulated iron oxide for phenol oxidation.

Acknowledgements

This work was supported by the State Key Program of National Natural Science Foundation of China (grant no. 21236008).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03552c

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