Startseite Green synthesis of hydratropic aldehyde from α-methylstyrene catalyzed by Al2O3-supported metal phthalocyanines
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Green synthesis of hydratropic aldehyde from α-methylstyrene catalyzed by Al2O3-supported metal phthalocyanines

  • Fanfan Niu , Ying Jiang , Ping Chen , Licheng Zhan und Xiaoling Sun ORCID logo EMAIL logo
Veröffentlicht/Copyright: 27. April 2018
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Abstract

This report presents a study of synthesis of hydratropic aldehyde from α-methylstyrene employing Al2O3-supported metal phthalocyanines as catalyst, molecular oxygen as oxidant and isobutyraldehyde as co-oxidant. The catalytic system was applied in the synthesis of hydratropic aldehyde for the first time. Under optimal conditions, the conversion of α-methylstyrene reached 99%, whereas the selectivity of hydratropic aldehyde reached 85.14%. Possible reaction mechanism and the effects of different factors on oxidation reaction were investigated. Aluminum oxide-supported metal (M = Co, Ni, and Fe) phthalocyanine catalysts (MPc/Al2O3) were prepared using the “ship-in-a-bottle” method by synthesizing metallophthalocyanines in support holes. Obtained catalysts were characterized by N2 adsorption, Brunauer-Emmett-Teller surface area (BET), inductively-coupled plasma atomic emission spectrometry (ICP-AES), IR, UV-Visible (UV-Vis), and X-ray diffraction (XRD).

1 Introduction

Hydratropic aldehyde (2-phenyl propionic aldehyde) is a kind of precious spice, which does not exist in nature. Therefore, it belongs to synthetic spices. Hydratropic aldehyde has strong aromas of hyacinths and cloves, mainly used to deploy the daily-used essence [1]. Hydratropic aldehyde also has trace use in edible flavor. At the same time, it is also an important chemical raw material, widely used in the spice, medicine, dye, and pesticide industries [2].

At present, there are mainly three kinds of methods for synthesizing hydratropic aldehyde. The most commonly used industrial process is the Darzens reaction (Scheme 1). The intermediate compound of this reaction is 3-methyl-3-phenylglycidic acid ethyl ester, which needs to undergo a series of reactions, such as saponification, neutralization and hydrolysis, before giving hydratropic aldehyde [3].

Scheme 1: Synthesizing hydratropic aldehyde by the Darzens reaction.
Scheme 1:

Synthesizing hydratropic aldehyde by the Darzens reaction.

Secondly, hydratropic aldehyde can be synthesized by the addition reaction of benzimidazolium salt and Grignard reagent (Scheme 2) [4]. The intermediate compound is benzimidazoline, which is converted to hydratropic aldehyde after acidic aqueous hydrolysis. The materials of this method need to be prefabricated. So, the long route and harsh reaction conditions make this method hard to apply in practical production. α-Methylstyrene, as a kind of important organic chemical raw material, has been widely used in industry. In recent years, there have been many researches of using α-methylstyrene in synthetic spices at home and abroad [5]. α-Methylstyrene can be epoxied under the action of peracetic acid. The product α-methyl phenyl epoxy ethane becomes hydratropic aldehyde by isomerization (Scheme 3). Compared with the traditional method, this synthetic route has advantages of brief reaction steps, high production rate and low cost. However, the peracetic acid in this reaction is inflammable and explosive, and has strong corrosive and irritating properties.

Scheme 2: Synthesizing hydratropic aldehyde from benzimidazolium salt and Grignard reagent.
Scheme 2:

Synthesizing hydratropic aldehyde from benzimidazolium salt and Grignard reagent.

Scheme 3: Synthesizing hydratropic aldehyde from α-methylstyrene using peracetic acid as oxidant.
Scheme 3:

Synthesizing hydratropic aldehyde from α-methylstyrene using peracetic acid as oxidant.

Owing to the similarities in structure and biological properties with cytochrome P450, metal phthalocyanine (Figure 1) has been recognized as an efficient biomimetic catalyst used in olefin epoxidation reaction with molecular oxygen under mild conditions [6], [7], [8]. Because of rather inexpensive and simple preparation on a large scale and chemical and thermal stability, these kind of ligands, having a unique catalytic activity, are particularly attractive as potential catalysts for organic reactions such as oxidation under mild conditions. However, these phthalocyanine macromolecules typically exhibit a strong tendency toward aggregation in solutions, which possibly exerts a remarkable influence on the catalytic performance [9]. Upon self-association in solution, the photocatalytic activity of the phthalocyanine complexes has been reported to be decreased by five to 10 times [10], [11], [12]. To immobilize transition metal complexes in an inorganic solid matrix, the following procedures can be applied: (1) grafting or impregnation onto the surface of silica or alumina; (2) tethering to the surface of a solid, e.g. silica, via a spacer ligand; and (3) encapsulation of metal complexes in a solid matrix [13]. Ship-in-a-bottle synthesis provides a convenient route for the heterogenization of homogeneous catalytic processes, and the catalytic activities of these compounds have been found to be either enhanced or more selective than the same complex in solution form [14], [15]. The “ship-in-a-bottle” complexes formed in situ by bringing the metal and ligand species within the voids of carriers, such as NaY, NaEMT, or VPI-5, have been widely used [16], [17], [18].

Figure 1: The structure of phthalocyanine and metal-phthalocyanine complexes.
Figure 1:

The structure of phthalocyanine and metal-phthalocyanine complexes.

Herein, we try to synthesize Al2O3-supported metallophthalocyanines by the “ship-in-a-bottle” method. Like our previous work [19], this kind of supported catalyst was used in the catalytic epoxidation of a series of olefins with dioxygen as oxidant. When α-methylstyrene was used as substrate, we were somewhat surprised to discover that the product not only had the expected epoxide, but also had a small amount of aldehyde. Upon examination, we found that this kind of aldehyde was hydratropic aldehyde, which was a kind of precious spice. This prompted us to initiate studies to assess application as catalysts for synthesizing hydratropic aldehyde from α-methylstyrene. The CoPc/Al2O3, NiPc/Al2O3, and FePc/Al2O3 catalysts were characterized via IR, inductively-coupled plasma atomic emission spectrometry (ICP-AES), UV-Visible (UV-Vis), and X-ray diffraction (XRD). Possible reaction mechanism and the effects of different factors were investigated.

2 Materials and methods

All reagents (≥99.0%) were obtained from Shanghai Titan Scientific Co., Ltd. (Shanghai, China). Powder XRD patterns were recorded on a Panalytical PW 3040/60 X’Pert PRO diffractometer (PANalytical, Netherlands) with Cu Kα radiation. The diffraction patterns at 40 kV and 40 mA were recorded within the 5°–80° Bragg angle (2θ) range at a rate of 5°/min. Fourier transform infrared (FT-IR) spectra of catalysts were obtained from a Magana-IR 500 FT-IR spectrometer (Nicolet, USA). The IR spectra of the three catalysts were obtained by the KBr tableting method; UV-Vis spectra of catalysts were obtained from a CARY-100 ultraviolet-visible spectrophotometer (Shimadzu, Japan). The thermogravimetric-differential scanning calorimetry (TG-DSC) curves of catalysts were measured using a Diamond TG/DTA/DSC simultaneous thermogravimetric analyzer (USA). The determination of total Co content of catalyst was carried out by ICP-AES on a Thermo Electron Corporation instrument (USA). Textural properties of the immobilized complexes were determined from N2 adsorption isotherms measured using a TriStar II 3020 version 1.03 instrument (Micromeritics instrument Ltd., Shanghai, China).

2.1 Catalyst preparation

Phthalic anhydride, urea, ammonium molybdate, and metal salts (CoCl2 · 6H2O, NiCl2 · 6H2O, FeSO4 · 7H2O) used for catalyst preparation were commercial products of analytically pure grade. Acidic Al2O3, which was used as the carrier, was screened at 200–300 mesh for chromatography. The MPc/Al2O3 catalysts were prepared according to the method in the literature [20], [21]. A cobalt chloride solution was prepared by dissolving CoCl2 · 6H2O (0.03 mol) in ethanol. Aluminum oxide (50 g) was added to anhydrous ethanol to form a suspension. The resulting suspension was then impregnated with the above cobalt chloride solution at room temperature for 24 h, followed by calcining at 300°C in a muffle furnace for 4 h after suction filtration and drying. The calcinated samples were cooled to room temperature, and then impregnated for a second time. The resulting powdered solid was added into an agate mortar charged with phthalic anhydride, urea, and ammonium molybdate (the molar ratio of CoCl2 · 6H2O, phthalic anhydride, and urea were 0.25:1:25, molar fraction of ammonium molybdate to the total sample above was 2%). After being fully ground in the agate mortar, the mixture was transferred into a stainless steel pot and heated to its molten state at 180°C with vigorous stirring until the mixture became a black-purple sticky substance. This substance was then calcined in a muffle furnace at 240°C for 4 h. The crude product was purified by sequentially washing with dilute hydrochloric acid and sodium hydroxide solution to remove the unreacted organic amine and anhydride. After these steps, the catalysts were washed with distilled water, ethanol, and acetone until the filtrate was colorless, then dried at 60°C under vacuum. The Co content in the CoPc/Al2O3 catalyst was 0.45%, as determined by an ICP plasma emission spectrum.

2.2 Synthesis of hydratropic aldehyde

Catalytic oxidation of α-methylstyrene with molecular oxygen was conducted in a 100 ml round-bottomed flask equipped with a condenser and a magnetic stirrer. α-Methylstyrene (2.0 g, 0.017 mol) was added into the flask, along with a certain amount of metal phthalocyanine catalysts and the co-oxidant, isobutyraldehyde. The reaction temperature was maintained by a temperature-controlled oil bath with O2 bubbling at atmospheric pressure. Reactions were analyzed with gas chromatography (GC) using a flame ionization detector and fitted with an SE-54 capillary column (30 m×0.53 mm). The GC conditions were as follows: initial temperature, 80°C (1 min); temperature rate, 5°C/min; final temperature, 200°C; injector temperature, 80°C; and detector temperature, 200°C. After running the reaction for a certain time and filtering to remove solid catalyst, the sample was withdrawn, and then injected into the GC for the analysis of the oxidation products of α-methylstyrene.

3 Results and discussion

3.1 Catalyst characterizations

In this paper, metallophthalocyanines were formed inside the acidic alumina when the alumina consisting of a stoichiometric amount of metal ion, phthalonitrile, urea and catalytic amount of ammonium molybdate were heated to the molten state at 180°C and then calcinated in a muffle furnace at 240°C for 4 h. The obtained catalysts, Al2O3-supported metal phthalocyanines, were characterized by N2 adsorption, Brunauer-Emmett-Teller surface area (BET), ICP-AES, IR, UV-Vis, and XRD.

3.1.1 IR spectroscopy

FT-IR spectra of the prepared CoPc/Al2O3, NiPc/Al2O3, and FePc/Al2O3 catalysts are shown in Figure 2, from which the three kinds of Al2O3-supported metal phthalocyanines are shown to have the same absorption band at 1610–1620 cm−1 (for aromatic C-H stretching) and 1520 cm−1 (for aromatic C-H stretching), which indicates the formation of phthalocyanine complex within the host material. Meanwhile, the absorption band in the low frequency area at 862 cm−1, 915 cm−1, and 906 cm−1, respectively, verify the presence of the cobalt phthalocyanine [22], nickel phthalocyanine [23], and iron phthalocyanine [24]. Compared with free metal phthalocyanines [25], the characteristic peaks of supported metal phthalocyanines have slight redshift, which may be attributed to the twisting of the phthalocyanine plane after being supported.

Figure 2: IR spectra of MPc/Al2O3.
Figure 2:

IR spectra of MPc/Al2O3.

3.1.2 UV-Vis spectroscopy

UV-Vis spectra of the phthalocyanine complexes (Figure 2) exhibit characteristic Q and B bands, where the Q bands caused by α1u(π)-eg(π*) transitions were observed at 600–800 nm in the visible region, while the B-bands arising from α2u(π)-eg(π*) transitions were observed at 300–400 nm in the near-ultraviolet region [26]. In fact, the high energy absorption band near 600 nm is associated with the dimeric species and the low energy band near 670 nm is due to the monomeric species [27]. The results showed that the characteristic absorptions in the B band region at 339 nm for the cobalt phthalocyanine, 329 nm for nickel phthalocyanine, 334 nm for iron phthalocyanine, and the Q band absorptions of the three metal phthalocyanines were observed at 657 nm, 660 nm, and 602 nm, respectively [22], [23], [25]. It can be seen from Figure 3 that the CoPc and NiPc supported on Al2O3 is in a monomeric form, while that of FePc is in a dimer form. According to literature, free metal phthalocyanines mostly exist in dimmers, but after entering into holes of carriers they mostly exist in monomers [28]. The above data shows that metal phthalocyanines were successfully supported into the channels of Al2O3.

Figure 3: UV-Vis spectra of MPc/Al2O3 from pyridine extraction.
Figure 3:

UV-Vis spectra of MPc/Al2O3 from pyridine extraction.

3.1.3 XRD

Further characterization of supported cobalt phthalocyanine, nickel phthalocyanine, iron phthalocyanine, and the Al2O3 support was performed with XRD (Figure 4). It is clear that immobilization of these metal phthalocyanine complexes makes very little difference to the crystalline structure of the parent Al2O3. No new peaks were seen for the supported complexes, which confirms that metal phthalocyanine dispersed through pores does not change the Al2O3 structure.

Figure 4: X-ray diffraction (XRD) patterns of MPc/Al2O3 and Al2O3 supporter.
Figure 4:

X-ray diffraction (XRD) patterns of MPc/Al2O3 and Al2O3 supporter.

3.1.4 N2 adsorption/desorption

The textural properties were determined by N2-sorption studies carried out at 195.8°C . Figure 5A shows the N2 adsorption-desorption isotherm and corresponding pore diameter distributions of Al2O3 used as support in experiments. It exhibits a type IV isotherm according to the IUPAC classification, characteristic of mesoporous solids. A strong uptake of N2 as a result of capillary condensation is observed in a relative pressure (PP0−1) of 0.4. The pore size distribution curve of the Al2O3 sample is quite narrow and monomodal, showing a peak pore diameter at 3.7 nm. The dimension of square planar CoPc as found in the unsupported or “free” state from X-ray crystallographic studies is 1.5 nm (end-to-end diagonal) [29]. Therefore, the pores of the Al2O3 sample are big enough for loading metallophthalocyanines into them. Figure 5B shows the N2 adsorption-desorption isotherm and corresponding pore diameter distributions of CoPc/Al2O3. The CoPc/Al2O3 sample exhibits a type IV isotherm (in the UPAC classification) with a sharp inflection step at a relative pressure range from 0.2 to 0.4, characteristic of capillary condensation in uniform mesopores, which suggests that the CoPc/Al2O3 sample was successfully synthesized. The pore size distribution curve of the CoPc/Al2O3 sample shows a peak pore diameter at 5.7 nm. The increase of pore diameter indicates that CoPcs are stacked in channels of Al2O3 [22].

Figure 5: Nitrogen adsorption-desorption isotherm and pore diameter distributions of the Al2O3 (A) and CoPc/Al2O3 (B).
Figure 5:

Nitrogen adsorption-desorption isotherm and pore diameter distributions of the Al2O3 (A) and CoPc/Al2O3 (B).

3.1.5 The analysis of BET surface area and pore volume

Table 1 shows the changes of specific surface area and pore volume. It can be seen that the BET surface area and pore volume of CoPc/Al2O3 are significantly smaller than those of Al2O3. This is mainly because CoPcs supported in the channels of Al2O3 occupy the most space of channels. The huge molecular volume of metal phthalocyanines leads to obvious specific surface area and pore volume decrease. Additionally, the specific surface area and pore volume of CoPc/Al2O3 after reaction are bigger than those of CoPc/Al2O3 before reaction. This may due to the CoPc leaching or deactivation, which is consistent with the decrease of Co content in the CoPc/Al2O3 catalyst measured after reaction by the ICP plasma emission spectrum (from 0.45% to 0.33%). Similar nitrogen adsorption-desorption isotherms and parameters for the mesoporous materials were obtained for the catalysts FePc and NiPc supported in channels of Al2O3.

Table 1:

Texture parameters for the mesoporous materials before and after anchoring with CoPc sample.

SampleTimes of reuseS (m2·g−1)Pore volume (ml·g−1)
Al2O3139.90850.189763
CoPc/Al2O3Before reaction5.09270.009213
CoPc/Al2O3After reaction10.59860.023458

3.2 Synthesis of hydratropic aldehyde

3.2.1 Identification of the product

The qualitative and quantitative analyses of this reaction, respectively, used 1H NMR [Varian Mercury-500 (500 HZ) NMR Spectrometer, Bruker, Germany] and the area normalization method by GC (GC9790II gas chromatography, Shanghai Jing Yu Equipment Co., Ltd., China). The experimental results showed that the main oxidation products were α-methyl phenyl epoxy ethane, hydratropic aldehyde and acetophenone (Scheme 4).

Scheme 4: Synthesis of hydratropic aldehyde by catalytic oxidation of α-methylstyrene.
Scheme 4:

Synthesis of hydratropic aldehyde by catalytic oxidation of α-methylstyrene.

Pure hydratropic aldehyde was obtained through silica column chromatography (eluting with a mixture of ethyl acetate: petroleum ether [1:30]) as colorless liquid. 1H NMR (CDCl3, 500.15 MHz) δ: 1.54 (d, 3H), 3.81 (t, 1H), 7.27 (d, 1H), 7.29 (t, 2H), 7.40 (t, 2H), 9.72 (d, 1H); MS: 134, 115, 105, 91, 77, 63, 51. The data are in accordance with experimental results reported in the literature [3].

α-Methyl phenyl epoxy ethane was characterized via 1H NMR (CDCl3, 500.15 MHz) δ: 1.08 (s, 3H), 2.59 (s, 2H), 7.49 (t, 2H), 7.59 (t, 2H), 7.98 (d, 2H); MS: 134, 105, 91, 79, 63, 51, 35.

Acetophenone characterized via 1H NMR (CDCl3, 500.15 MHz) δ: 2.50 (s, 3H), 7.56 (t, 2H), 7.64 (t, 1H), 7.94 (t, 2H); MS: 120, 105, 91, 77, 63, 51.

3.2.2 Optimization of reaction conditions

3.2.2.1 The influence of reaction temperature

In the process of carrying out exploring experiments, we found that the temperature of tests had a great influence on the selectivity of hydratropic aldehyde. With the reduction of temperature, the yield of hydratropic aldehyde gradually increased within a certain range.

On this foundation, the reaction conditions were studied and optimized with hydratropic aldehyde as the target product.

The reaction consists of a gas-liquid-solid three-phase reaction; thus, the temperature is the crucial factor that affects the reaction. The effect of reaction temperature on the yield is summarized in Table 2.

Table 2:

The influence of reaction temperature.

Temperature (°C)Conversion ratesa (%)Yield (%)
Hydratropic aldehydeAcetophenone2-Phenyl cyclopropane
1017.212.63.460.13
20>9982.414.23.41
30>9980.5115.154.32
40>9977.1721.313.37
50>9974.6022.912.54
  1. aReaction conditions: α-methylstyrene 2.0 g, catalyst 3%, O2 35 ml/min, ratio of aldehyde and α-methylstyrene 2:1, 8 h.

The results presented in Table 2 show that during increase of the reaction temperature gradually from 10°C to 50°C, the main product is still hydratropic aldehyde, but its yield had a corresponding change. When the reaction temperature was 10°C, α-methylstyrene was hard to convert and the yield of hydratropic aldehyde was only 12.6% after 6 h. This suggests that low temperature is unfavorable to the reaction. This may be because the activity of catalyst is too low at low temperatures. Approximately 99% conversion of α-methylstyrene with 82.40% hydratropic aldehyde yield was achieved after the reaction temperature increased to 20°C. With the temperature continuing to rise, the yield of the target product dropped, and more acetophenone byproducts wee formed. Therefore, 20°C was determined as the optimal reaction temperature.

3.2.2.2 The influence of reaction time

Table 3 shows the effect of reaction time on the catalytic oxidation of α-methylstyrene. The reaction time also performs a highly relevant function in the oxidation reaction. When the reaction time extended from 1 h to 6 h, the conversion of α-methylstyrene increased from 16.2% to 99%. With a further increase in the reaction time to 9 h, the conversion slightly increased, while the yield of hydratropic aldehyde significantly declined from 82.40% to 70.70%. This yield decrease may be due to the isomerization of hydratropic aldehyde. Under a certain temperature and with the increase in reaction time, hydratropic aldehyde isomerized to acetophenone. The increasing yield of acetophenone with time also illustrates this point. Therefore, 6 h was determined as the optimal reaction time.

Table 3:

The influence of the reaction time.

Time (h)Conversion ratesa (%)Yield (%)
Hydratropic aldehydeAcetophenone2-Phenyl cyclopropane
116.213.282.390.5
219.915.783.110.97
344.8385.530.27
468.458.647.052.67
585.566.6612.146.66
6>9982.414.23.42
8>9975.6520.314.03
9>9970.723.745.61
  1. aReaction conditions: α-methylstyrene 2.0 g, catalyst 3%, O2 35 ml/min, ratio of aldehyde and α-methylstyrene 2:1, 20°C.

3.2.2.3 The influence of aldehyde amount

The added isobutyraldehyde into the reaction system was the initiator of the epoxidation reaction. With the increasing mass ratio of isobutyraldehyde to α-methylstyrene, the yield of hydratropic aldehyde gradually increased. Table 4 shows that 2:1 is the best volume ratio of isobutyraldehyde to α-methylstyrene. Side reactions are likely to happen when an excess amount of isobutyraldehyde is present, resulting in reduced selectivity for hydratropic aldehyde. Therefore, 2:1 was determined as the optimal mass ratio of isobutyraldehyde to α-methylstyrene.

Table 4:

The influence of aldehyde amount.

The mass ratio of initiator: SubstrateConversion ratesa (%)Yield (%)
Hydratropic aldehydeAcetophenone2-Phenyl cyclopropane
167.348.715.642.73
1.59770.3923.692.96
2>9985.1411.793.04
2.5>9981.9317.031.06
  1. aReaction conditions: α-methylstyrene 2.0 g, catalyst 2.5%, O2 35 ml/min, 20°C, 6 h.

3.3 Plausible reaction mechanism

Direct oxidation of α-methylstyrene to hydratropic aldehyde catalyzed by Al2O3-supported metal phthalocyanines is first studied in this paper. The possible mechanism of this reaction is illustrated in Scheme 5. Owing to the similarities in structure and catalytic properties with metalloporphyrins, based on the references [30], [31], [32], we speculate that the cobalt-phthalocyanine-catalyzed aldehyde formation reaction is preceded by a tandem epoxidation-isomerization pathway. The formation of 2-phenyl cyclopropane is a known epoxidation process [19]. There are three possible routes for the formation of acetophenone from α-methylstyrene. One possible route requires cleavage of alkene double bonds [33]; α-methylstyrene was directly oxidated to acetophenone catalyzed by cobalt phthalocyanine using molecular oxygen as an oxidant. The other two possible routes to acetophenone are via the cleavage of C-C in the hydratropic aldehyde [34] or 2-phenyl cyclopropane [35].

Scheme 5: The possible mechanism of the reaction.
Scheme 5:

The possible mechanism of the reaction.

4 Conclusions

The results presented here reveal a simple, efficient, and green catalyst system based on supported-CoPc/Al2O3 for the synthesis of hydratropic aldehyde from α-methylstyrene, using environmental molecular oxygen, under very mild conditions. The reaction occurred with high conversion (;99%) and reasonable selectivity (85.14%). CoPc/Al2O3, NiPc/Al2O3, and FePc/Al2O3 catalysts have been prepared and characterized. The effects of different factors on the reaction were investigated, and possible reaction mechanism was proposed.

Acknowledgements

The work was supported by the Shanghai Alliance Program (no. LM 201666), the Shanghai Students’ Science and Technology Innovation Activities Key Projects (SH 2016001) and the Capacity-Building Projects in Shanghai Local Universities (no. 15120503700). The authors are grateful to the Shanghai Institute of Technology for providing the laboratory. We also acknowledge all undergraduate students (especially Rongting Rong and Kaiqi Xue) for their efforts on this project.

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Received: 2018-01-27
Accepted: 2018-03-22
Published Online: 2018-04-27
Published in Print: 2019-01-28

©2019 Walter de Gruyter GmbH, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Artikel in diesem Heft

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