Eclectic Hydroxylation of Benzene to Phenol Using ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Eclectic Hydroxylation of Benzene to Phenol Using Ferrites of Fe and Zn as Durable and Magnetically Retrievable Catalysts A. M. Al-Sabagh, F. Z. Yehia, Gh. Eshaq,*,† and A. E. ElMetwally*,† Egyptian Petroleum Research Institute, Nasr City, Cairo 11727, Egypt S Supporting Information *

ABSTRACT: Ferrites of iron and zinc were prepared and examined as catalysts for the eclectic hydroxylation of benzene in the presence of hydrogen peroxide as an oxidant under mild conditions. The obtained results revealed that iron ferrite is more efficient than zinc ferrite or other catalysts used in benzene hydroxylation. The efficacy of iron ferrite is attributed to the random occupation of the octahedral sites by divalent and trivalent iron cations, which eventually boosts the redox reaction within the octahedral site without altering the spinel structure. The catalyst durability testing revealed that the catalysts sustain the employed experimental conditions for five successive runs. The mechanism of benzene hydroxylation in the presence of iron ferrite and zinc ferrite was also suggested. KEYWORDS: Ferrites, Catalysis, Hydroxylation, Benzene, Phenol

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very low. 15 Despite the efficiency of these catalysts, unfortunately, they cannot be easily recovered from the reaction system. To overcome this obstacle, several heterogeneous catalysts have been studied in the direct hydroxylation of benzene including molecular sieves, aluminum−iron binary phosphates, Fe-Al-silicate, vanadium-doped graphitic carbon nitride, VPO/GO and Cu-Ag composite.16−21 Moreover, iron ions were immobilized on different supports such as MCM-41, SBA-16, Al2O3, ZSM-5 zeolites, TiO2 and MgO, and tested as catalysts in benzene hydroxylation.22−27 These catalysts have provided some merits as catalysts in benzene hydroxylation; however, benzene conversion is still far from its summit, which may be attributed to the indolence of the tested support. Consequently, the development of a catalyst capable of enhancing the extent of benzene conversion to phenol remains a primary economic and industrial goal. The target is not only to achieve a maximum benzene conversion but also to maintain the phenol selectivity by minimizing the generation of side products, such as benzenediols. It must be point out that the presence of such compounds makes the separation of the resulting products very intricate. Magnetic nanoparticles have fascinated different scholars due to their inimitable properties, such as their size, magnetic recoverability, sustainability and high surface area.28 The inimitable features of magnetic separable nanoparticles as catalysts arose from their susceptibility for facile removal from reaction medium and even the one with large volumes using a

INTRODUCTION Phenol (carbolic acid) is a white crystalline organic compound that acts as a key precursor for diverse agrochemical and petrochemical products.1−3 Phenol is globally manufactured via a cumene process through the partial oxidation of cumene. Despite the fact that this process requires comparatively mild operating conditions and economical starting materials, the process suffers from several limitations including the huge energy consumption, inefficiency and the generation of undesirable byproduct.4 Nowadays, direct hydroxylation of benzene as an alternative method for the production of phenol has attracted the attention of scholars due to its advantages of using mild oxidants such as oxygen, nitrous oxide and hydrogen peroxide.5−7 Although the direct hydroxylation of benzene is considered as one of the most stubborn oxidation reactions, the synthesis of phenol by one-step process remains more convenient than the traditional processes.8 The hydroxylation of benzene in the absence of a catalyst is very sluggish process where the oxidant alone is not capable to oxidize benzene into phenol. Numerous homogeneous catalysts have been tested in benzene hydroxylation in the presence of hydrogen peroxide as an oxidant such as polyoxometalates, Schiff bases, ionic liquids and Fenton reagent.9−12 The utilization of catalysts based on iron ions is inspired from Fenton’s reaction where Fe2+ is oxidized to Fe3+ in the presence of hydrogen peroxide, forming a highly active free radical species such as hydroxyl radicals and hydroperoxyl radicals.13,14 An iron-catalyzed biphasic reaction system has been reported as an efficient catalyst in benzene hydroxylation where ferrous sulfate provided the ferrous ions, but the conversion of benzene was © 2017 American Chemical Society

Received: January 19, 2017 Revised: April 4, 2017 Published: May 15, 2017 4811

DOI: 10.1021/acssuschemeng.7b00214 ACS Sustainable Chem. Eng. 2017, 5, 4811−4819

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ACS Sustainable Chemistry & Engineering

Figure 1. TEM image and magnetization curve of iron ferrite (a and c) and TEM image and magnetization curves of zinc ferrite (b and d).

simple magnet. This capability saves the time needed for intricate filtration processes or centrifugation technique. The use of such magnetic catalysts is not only a time saver but also a problem solver for issues including the catalyst wastage, solvent dissipation and catalyst corrosion. Thus, these magnetic catalysts are worthy to be considered as a double green dream.29 Among the magnetic nanoparticles, magnetite is a typical magnetic oxide that can be prepared easily to produce a material with a very active surface for metal immobilization and adsorption. The unique features of magnetite make it a successful catalyst that can be easily retrieved by magnetic separation and a sustainable catalyst that can endure different reaction conditions.30 The substitution of the divalent iron by another divalent metal generates a class of magnetic materials known as metal ferrite. This modification permits the amplification of the catalytic application of magnetic nanomaterial while keeping iron as the source of magnetic capability.31,32 During the past decade, metal ferrites have attracted particular attention in various fields because of their adaptable chemical and physical

properties that can meet various applications requirements in several fields, including drug delivery, magnets, microwave instruments, high-density storage media and ferrofluids.33,34 Spinel ferrites are ferrimagnetic nonconductive ceramic materials with formula AB2O4 where A represents the site of tetrahedral cation whereas B represents the site of octahedral cation, and O represents the site of oxygen anion. Metal spinel ferrites possess a general formula of MFe2O4 where M = Fe, Ni, Co, Cu, or Zn, with face-centered cubic close packing structure.35 We postulate that this is the first report that utilizes ferrites of iron and zinc in the eclectic hydroxylation of benzene to phenol. In this article, ferrites of iron and zinc were prepared and characterized using TEM, VSM, XRD, Raman spectroscopy, N2-sorption and TGA analysis. The catalyst was tested in the catalytic hydroxylation of benzene and the operational conditions were optimized. Finally, the efficacy of the prepared catalyst was explained and the mechanism of benzene hydroxylation was suggested and studied by density functional theory (DFT) calculations. 4812



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EXPERIMENTAL SECTION

Materials. FeSO4·7H2O, FeCl3·6H2O, oleic acid, NaOH, ZnCl2· 6H2O and ethanol were purchased from Sigma-Aldrich. NH4OH (25%), benzene and phenol were obtained from Merck. Acetic Acid (70−80% w/w) was purchased from Fisher Scientific. Hydrogen Peroxide 30% was obtained from PanReac AppliChem. Iron Ferrite Preparation. Iron ferrite nanoparticles were prepared as described elsewhere.36 In particular, FeCl3·6H2O and FeSO4·7H2O in the stoichiometric ratio of 2:1 were dissolved under an atmosphere of nitrogen in double distilled water. The metal ions solution was strenuously stirred for 30 min and the solution temperature was raised to 80 °C. Next, ammonium hydroxide solution (25%) was added dropwise under stirring until the pH value of the solution became 9, and then the reaction mixture was kept under strenuous stirring for 30 min. The obtained solids were separated by filtration, washed with double distilled water and ethanol to remove the undesirable byproducts, and finally dried. Zinc Ferrite Preparation. Zinc ferrite nanoparticles were synthesized according to the procedure described elsewhere.37 FeCl3·6H2O solution and ZnCl2·6H2O solution were mixed with a stoichiometric ratio of 2:1 under stirring then few drops of the surfactant (oleic acid) were added.38 Thereafter, NaOH solution (3 M) was added dropwise until a pH level of 12 and the solution temperature was raised to 80 °C and stirred for 40 min. The resultant suspension was left to cool down and the solids were collected by filtration. The collected solid was repeatedly washed with double distilled water and ethanol to remove the undesired impurities, and the obtained solids were dried overnight at 80 °C. The zinc ferrite nanoparticles were eventually acquired after annealing in air at 450 °C for 4 h. Characterization. TEM micrographs of the prepared samples were taken using a JEOL JEM-2100 operating at 200 kV. The magnetization curves of the prepared ferrites were obtained using a vibrating sample magnetometer (VSM) model 9600-1 LDJ, USA, at room temperature. XRD analysis of the prepared samples was operated on a Philips powder diffractometer in the range 2θ = 4− 80° using Cu Kα radiation. A scanning speed of 2° θ/min was used to record the spectra with a 2θ step of 0.02°. Raman spectroscopic analysis was conducted using a Bruker Optics Raman spectrometer at exciting lines of 532 nm at room temperature. Thermal analysis data were obtained using a STD Q600 at a rate of 10 °C/min in the range of 25−800 °C under N2 atmosphere. Surface area, total pore volume and average pore size were determined by N2 sorption at −195.85 °C using a Quantachrome AS1Win Instrument. Catalytic Testing Procedure. The catalyst testing procedure was carried out using a round-bottom three-necked flask (25 mL) immersed in a thermally regulated oil bath and equipped with a condenser to preserve over the vapors that may be generated during the reaction. In a typical catalytic run, 0.9 mL of benzene, 1 mL of acetic acid (70%) and a specific amount of catalyst were loaded into the flask. Next, a particular amount of H2O2 was added to the previously heated reaction medium under stirring to initiate the reaction. The catalyst testing procedure was examined in the temperature range of 30−80 °C for a run time of 30−420 min. After each run, the catalyst was magnetically separated and the remaining liquid was analyzed using Agilent 7890 GC equipped with FID and an HP-5 capillary column (5%-phenyl)-methylpolysiloxane (30 m, 0.25 μm and 0.32 mm). The yield of phenol was calculated as the molar ratio of the generated phenol to the initial benzene whereas the selectivity of phenol was calculated as the molar ratio of the generated phenol to the converted benzene. The selectivity of pbenzoquinone was calculated as the molar ratio of the generated pbenzoquinone to the converted benzene. DFT Calculations. All the DFT calculations were performed on a Gaussian 09 program. The structure optimization was operated using the B3LYP/6-31++G(d) level of theory for intermediates and compounds of the reaction.

Research Article

RESULTS AND DISCUSSION

Catalysts Characterization. The shape of iron ferrite and zinc ferrite nanoparticles are described using the TEM micrographs displayed in Figure 1. The micrographs of the prepared iron ferrite and zinc ferrite show that the nanoparticles have a well-ordered cubic morphology. The magnetization curves of the iron ferrite and zinc ferrite (Figure 1) reveal that the prepared ferrites exhibit a high magnetization performance with a saturation magnetization of 58.28 and 48.21 emu g−1, respectively. The lack of a hysteresis loop manifests that the prepared ferrites exhibit a superparamagnetic behavior, which boosts the magnetic recoverability of the utilized catalysts. The XRD patterns of iron ferrite and zinc ferrite are displayed in Figure S1. The XRD pattern of the prepared iron ferrite exhibits diffraction peaks at 2θ = 30.25°, 35.75°, 43.07°, 53.63°, 57.15° and 62.77° that can be ascribed to the (220), (311), (400), (422), (511) and (440) reflection planes of iron ferrite, respectively (JCPDS card no.82-1533). The obtained data manifest that the prepared nanoparticles are pure cubicstructured iron ferrite. Also, the XRD pattern of zinc ferrite exhibits diffraction peaks at 2θ = 29.85°, 35.31°, 42.77°, 53.13°, 56.73° and 62.11° that can be ascribed to the (220), (311), (400), (422), (511) and (440) reflection planes of zinc ferrite, respectively (JCPDS card no. 022-1012). The reflection planes of zinc ferrite indicate that a single-phase of zinc ferrite is formed with a face-centered cubic structure that agrees well with the previously reported studies.39,40 The Raman spectra of iron ferrite and zinc ferrite are displayed in Figure S2. It is well-known that the spinel structure possesses five Raman active modes, which are A1g, Eg and 3T2g. These active modes comprise the oxygen ions motion and the oxygen and tetrahedral [A] site ions.41 The A1g mode corresponds to the oxygen atoms symmetric stretching over the iron−oxygen tetrahedral bonds. The Eg mode corresponds to the symmetric oxygen-bending whereas the T2g (1) mode corresponds to asymmetric-oxygen bending with respect to iron. T2g (2) mode corresponds to the asymmetric stretching of iron and oxygen whereas T2g (3) corresponds to the tetrahedron translational motion.42,43 The Raman spectrum of iron ferrite shows that the modes appear at 282, 490 and 590 are attributed to the T2g symmetry and those at 670 and 396 are attributed to A1g and Eg symmetries, respectively. On the other hand, the Raman spectrum of zinc ferrite shows that the modes appear at 219, 337 and 470 cm−1 correspond to T2g symmetry whereas the modes appear at 622 and 248 cm−1 correspond to A1g and Eg symmetries, respectively. The obtained data validate the structure of the prepared metal ferrites and agree well with previously reported data.41,44 The adsorption−desorption isotherms of iron ferrite and zinc ferrite can be assorted as type II and IV, respectively, which concur with IUPAC classification.45 The isotherms of the prepared ferrites display a significant hysteresis area, which indicates that the pore sizes are arranged in a fairly uniform way as shown in Figure S3. Brunauer−Emmett−Teller analysis was used to determine the total specific surface area of iron ferrite and zinc ferrite, which was found to be 147.5 and 51.059 m2 g−1, respectively. An average pore radius of 70.3 and 35.6 Å for iron ferrite and zinc ferrite, respectively, was observed. The pore volume of iron ferrite and zinc ferrite was found to be 0.51 and 0.18 cm3 g−1. The Barrett−Joyner−Halenda method was 4813


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Figure 2. Benzene hydroxylation conditions optimization. (a) Effect of hydroxylation time. Reaction conditions: Benzene (0.9 mL), acetic acid (1 mL),; benzene: H2O2 molar ratio of 1:1, catalyst (0.1 g), 60 °C, 760 mm-Hg. (b) Effect of benzene: H2O2 molar ratio. Reaction conditions: Benzene (0.9 mL), acetic acid (1 mL), catalyst (0.1 g), 60 °C, 760 mm-Hg, 300 min. (c) Effect of catalyst amount. Benzene (0.9 mL), acetic acid (1 mL); benzene: H2O2 molar ratio of 1:1, 60 °C, 760 mm-Hg, 300 min. (d) Effect of hydroxylation temperature. Reaction conditions: Benzene (0.9 mL), acetic acid (1 mL); benzene: H2O2 molar ratio of 1:1, catalyst (0.1 g), 760 mm-Hg, 300 min.

30−420 min as shown in Figure 2. It is clear that the phenol yield increases incrementally with increasing the hydroxylation time and after a particular time the phenol yield remains steady. Thus, it can be concluded that 300 min of benzene hydroxylation is quite enough to obtain a maximum yield of phenol where the radical chain reactions remain active. Beyond this time, the phenol yield slightly decreases because the active radicals become engrossed in the further hydroxylation of phenol, which competes with benzene hydroxylation. This hypothesis seems to concur with the phenol selectivity behavior upon the elongation of the hydroxylation time. The decline of the phenol selectivity with increasing the hydroxylation time makes clear that there exists a side reaction, which takes place alongside the hydroxylation of benzene. p-Benzoquinone is the only detectable side product, which may be formed via hydroquinone oxidation as shown in Figure S5. Furthermore, it is clear that zinc ferrite is more selective for phenol than iron ferrite at prolonged hydroxylation time because iron ferrite is more efficient than zinc ferrite in phenol oxidation. Consequently, it is essential to set the hydroxylation time at this point to reduce time and energy wastage. Effect of Benzene: H2O2 Molar Ratio. The optimization of the amount of hydrogen peroxide is considered one of the most intricate challenges in benzene hydroxylation. If a huge amount of hydrogen peroxide is added to the reaction medium enormous amount of active radicals are produced and thus it

used to obtain the pore size distribution curves of iron ferrite and zinc ferrite as shown in Figure S3. A broad peak with a maximum centered at 6.2 nm was observed in the pore size distribution curve of iron ferrite. The presence of two peaks with maxima centered at 1.3 and 9.9 nm in the pore size distribution curve of zinc ferrite indicates that there exist two sizes of pores. To avoid any unexpected transformation in the catalyst structure during the catalyst testing procedure, the thermal stability of the prepared catalysts was investigated using TGA analysis. The reusability of the catalyst also strongly depends on the catalyst durability; if the catalyst undergoes any transformational changes during the reaction then it will not be able to be reused any further. The TGA curves of iron ferrite and zinc ferrite manifest that the structure of the two catalysts did not undergo any transformational change in the range of the tested analysis temperature as shown in Figure S4. It can be deduced that the prepared catalysts can tolerate the reaction temperature without distorting the structure of the catalysts. Catalytic Testing. Effect of Hydroxylation Time. Hydroxylation time optimization is deemed an attempt to determine the time at which the radical chain reactions are terminated. When these chain reactions are terminated, the benzene molecules become not able to be oxidized any longer and the production of phenol comes to an end. To optimize the hydroxylation time, the effect of time was tested in the range of 4814


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situation. To explore this conflict, the effect of hydroxylation temperature was tested in the range of 30−80 °C as shown in Figure 2. The results show an incremental increase in the phenol yield with elevating the hydroxylation temperature. The phenol yield reaches its summit when the hydroxylation temperature is raised to 60 °C, beyond which the incrementing trend is hindered. Furthermore, when the hydroxylation was raised to 80 °C, the phenol yield declined to a lower value. This Sudden drop is ascribed to the thermal decomposition of hydrogen peroxide into water and oxygen before the generation of the hydroxyl radical and thus the oxidant is decomposed before the participation in the hydroxylation reaction. The uncontrolled decomposition of hydrogen peroxide greatly affects the phenol selectivity, which tends to decrease at elevated temperatures. Thus, optimizing the hydroxylation temperature is highly recommended. Catalysts Sustainability. The catalyst reusing is now considered as one of the most desirable demands for chemical industries. The catalyst reusing curbs expenses of the chemical industries by filling the need for purchasing a fresh catalyst and saving the time needed to substitute the consumed catalyst with the fresh catalyst. The catalyst durability testing was carried out by using the same catalyst separated from the reaction medium in each run under the same experimental conditions. When each catalytic run was complete, the magnetically retrievable catalyst was separated using a magnet, rinsed with ethanol and eventually dried. The catalysts reusing curve reveal that the activity results remain steady along the durability testing procedure and there was only a slight decrease in the activity after the fourth run as shown in Figure 3. Structural

is expected that the entire amount of benzene will be converted to phenol. Unfortunately, this scenario simply does not occur as expected. The generated active radicals will scavenge each other instead of attacking the benzene molecules and consequently this huge amount of hydrogen peroxide will dramatically decompose to water and molecular oxygen. The effect of benzene:H2O2 molar ratio on benzene hydroxylation was tested in the range of 1:0.5−1:2 as shown in Figure 2. It is clear that the phenol yield increases rapidly with increasing the amount of hydrogen peroxide. When the benzene:H2O2 molar ratio was adjusted to 1:1, the phenol yield reached a maximum value of 49.3% and 42.3% for iron ferrite and zinc ferrite, respectively. Moreover, the benzene:H2O2 molar ratio of 1:1 offers the highest hydrogen peroxide efficiency in case of iron ferrite and zinc ferrite as displayed in Figure S6. Beyond this amount, the phenol yield declines with increasing the amount of hydrogen peroxide. The increase of the phenol yield with increasing the hydrogen peroxide amount is ascribed to the enhanced production of hydroxyl radicals. On the contrary, the addition of excessive amount of hydrogen peroxide leads to the generation of enormous amounts of hydroxyl radicals, which tend to recombine with hydrogen peroxide (k = 2.7 × 107 M−1 s−1) to produce hydroperoxyl radical, which finally attack another hydroxyl radicle (k = 6 × 109 M−1 s−1) instead of attacking the benzene molecule (k = 7.8 × 109 M−1 s−1).46−48 Furthermore, the decomposition of hydrogen peroxide to water and molecular oxygen has a great influence on reducing the generation of hydroxyl radicals, which in turn reduces the yield of phenol.49 The decline of the phenol selectivity upon increasing the hydrogen peroxide is ascribed to the involvement of the active radicals in the further hydroxylation of phenol. Consequently, the optimization of the hydrogen peroxide is seriously indispensable to prevent the consumption of the generated radicals. Effect of Catalyst Amount. To inspect the effect of catalyst amount on the yield of phenol, a catalyst weight range of 0.05− 0.25 g was tested in benzene hydroxylation as shown in Figure 2. It is to be noted that the benzene hydroxylation cannot be proceeded in absence of catalyst as shown in Table S1. The obtained data reveal a noteworthy increase in the phenol yield with increasing the catalysts amount. This formidable increase may be attributed to accretion of immense amount of active sites, which hastens the generation of hydroxyl radicals and in turn increase the yield of phenol. It obvious that the yield of phenol reaches a maximum value when the catalyst amount is increased to about 0.1 g, peaking at 49.3% and 42.3% for iron ferrite and zinc ferrite, respectively. Surprisingly, when the catalysts amount transcends this amount, the phenol yield exhibits an unusual behavior. The sudden decline of the phenol yield might be ascribed to the excessive production of hydroxyl radicals over a huge number of active sites in a short period and consequently the hydrogen peroxide is consumed rapidly. The excessive amounts of hydroxyl radicals tend to scavenge each other instead of attacking the benzene molecules.50 The further hydroxylation of phenol is also participated as a third party, which dissipates the active radicals. This possible explanation might be the reason that lies behind the unusual tendency of phenol yield upon the increment of catalyst amount. Effect of Hydroxylation Temperature. The adjustment of hydroxylation temperature is considered a significant perplexity when the reaction is carried out in the presence of hydrogen peroxide. The competition between benzene hydroxylation and hydrogen peroxide decomposition actually complicates the

Figure 3. Reusing of catalysts. Reaction conditions: Benzene (0.9 mL), acetic acid (1 mL); benzene: H2O2 molar ratio of 1:1, catalyst (0.1 g), 60 °C, 760 mm-Hg, 300 min.

examination of the reused catalyst was performed using XRD analysis to ensure the stability of the catalyst structure during the durability testing procedure as shown in Figure S7. The XRD patterns of the retrievable catalysts after the fifth catalytic run manifest that the catalysts sustain the employed experimental conditions for five successive runs. Mechanism of Benzene Hydroxylation. The hydroxylation of benzene in the presence of iron ferrite and zinc ferrite is known to take place via heterogeneous Fenton, where hydrogen peroxide is decomposed on the catalyst surface. 4815


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Scheme 1. Mechanism of Benzene Hydroxylation and the Energy of the Optimized Structures Calculated Using DFT Method at the B3LYP/6-31++G(d) level

Hydrogen peroxide tends to form a surface complex FeIII OH with the iron ferrite surface, which then undergoes an electron transfer reaction accompanied by a release of water molecule and an electronically excited complex. The complex then dissociate giving rise to FeII and hydroperoxyl radical. The generated FeII reacts with hydrogen peroxide to produce hydroxyl radical and FeIIIOH complex. The existence of two types of radicals in the system proposed two different pathways for benzene hydroxylation as shown in Scheme 1. The first is the reaction between hydroxyl radical and benzene (step 1) to produce benzene-OH-adduct, which then losses hydrogen (step 2) to produce phenol. The other pathway is the reaction between hydroperoxyl radical and benzene (step 1′) to produce benzene-OOH-adduct, which tends to react with hydrogen peroxide (step 2′) producing benzene-hydrogentransferred OOH-adduct. Next, the adduct losses water to finally produce phenol (step 3′). DFT calculations were used to calculate the enthalpy and Gibbs free energies of reaction of each step and the calculated values are listed in Table S1. The obtained Gibbs free energies values show that step 1 is an exergonic reaction while that of step 1′ is an endergonic reaction. Thus, it is obvious that the reaction between hydroxyl radical and benzene is more favorable than that between hydroperoxyl radical and benzene, which required high energy to proceed (rate-determining step). Step 2′ is an also an uphill reaction but faster than the first one. Eventually, the benzene-

hydrogen-transferred OOH-adduct rapidly and spontaneously losses water to finally produce phenol through step 3′. On the contrary, the elimination of hydrogen from benzene-OHadduct through step 2 to produce phenol is an exergonic reaction. Lastly, the hydroxyl radicals are terminated through the reaction of hydroxyl radical with hydrogen peroxide to produce hydroperoxyl radical, while hydroperoxyl radicals are terminated by the disproportionation of two hydroperoxyl radicals to generate water and oxygen. The same pathway can be also suggested for zinc ferrite. Effect of Divalent Cation. Metal spinel ferrite possess a general formula of MFe2O4 where M = Fe, Ni, Co, Cu, or Zn, with face-centered cubic close packing structure. The divalent metal cations and trivalent metal cations occupy different crystallographic sites, which have octahedral and tetrahedral oxygen coordination. The tetrahedral coordinated sites are termed as A-sites and octahedral coordinated ones are termed as B-sites.51 Thus, it can be concluded that the structure of the spinel ferrite possesses two sites, which can be occupied by the metal cations. The metal cations that occupy the A-sites can form eight tetrahedral coordinates with oxygen whereas those occupy the B-sites can form 16 octahedral coordinates with oxygen. The metal ferrite that has a divalent cations occupying the A-sites and ferric cations occupying the B-sites is called normal spinel while that has ferric cations occupying the A-sites and both ferric cations and divalent cations in the B-sites is 4816


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ACS Sustainable Chemistry & Engineering called inverse spinel.51 Iron ferrite has an inverse spinel structure where the ferric cations occupy the A-sites and the ferrous and ferric cations with a ratio of 1:1 occupy the Bsites.52 On the contrary, zinc ferrite has a normal spinel structure where zinc ions entirely occupy the A-sites and all the ferric cations occupy the B-sites.53 It can be concluded that the octahedral sites of zinc ferrite are entirely occupied by the trivalent iron cations whereas those of iron ferrite are occupied by both divalent and trivalent iron cations. Studies show that the presence of divalent cations in the octahedral sites greatly enhances the reactivity toward hydrogen peroxide and consequently the overall catalytic oxidation reaction.54 The presence of the divalent and trivalent iron cations in the octahedral sites of iron ferrites allows the redox reactions of iron species to take place without altering the spinel structure. These special merits explain the efficacy of the iron ferrite over zinc ferrite or other catalysts used in benzene hydroxylation as presented in Table 1.

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catalyst

yield

reference

20.7% 29.0% 12.6% 9.7% 18% 22.7% 34.9% 17% 24.6% 49.3% 42.3%

55 55 56 57 58 59 21 60 61

*E-mail address: [email protected] (Gh. Eshaq). *Tel: +2011-15025588, Fax: +202-22747433. E-mail address: [email protected] (A. E. ElMetwally). ORCID

A. E. ElMetwally: 0000-0001-6746-6372 Present Address †

Petrochemical Technology Laboratory, Petrochemicals Department, Egyptian Petroleum Research Institute, 1 Ahmed ElZomor Street - El-Zohour Region, Nasr City, Cairo 11727, Egypt. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Ehrich, H.; Berndt, H.; Pohl, M.-M.; Jähnisch, K.; Baerns, M. Oxidation of benzene to phenol on supported Pt-VO x and Pd-VO x catalysts. Appl. Catal., A 2002, 230 (1), 271−280. (2) Lemke, K.; Ehrich, H.; Lohse, U.; Berndt, H.; Jähnisch, K. Selective hydroxylation of benzene to phenol over supported vanadium oxide catalysts. Appl. Catal., A 2003, 243 (1), 41−51. (3) Stöckmann, M.; Konietzni, F.; Notheis, J. U.; Voss, J.; Keune, W.; Maier, W. F. Selective oxidation of benzene to phenol in the liquid phase with amorphous microporous mixed oxides. Appl. Catal., A 2001, 208 (1), 343−358. (4) Hock, H.; Lang, S. Autoxydation von Kohlenwasserstoffen, IX. Mitteil.: Ü ber Peroxyde von Benzol-Derivaten. Ber. Dtsch. Chem. Ges. B 1944, 77 (3−4), 257−264. (5) Xin, H.; Koekkoek, A.; Yang, Q.; van Santen, R.; Li, C.; Hensen, E. J. A hierarchical Fe/ZSM-5 zeolite with superior catalytic performance for benzene hydroxylation to phenol. Chem. Commun. 2009, 48, 7590−7592. (6) Kusakari, T.; Sasaki, T.; Iwasawa, Y. Selective oxidation of benzene to phenol with molecular oxygen on rhenium/zeolite catalysts. Chem. Commun. 2004, 8, 992−993. (7) Chen, X.; Zhang, J.; Fu, X.; Antonietti, M.; Wang, X. Fe-g-C3N4catalyzed oxidation of benzene to phenol using hydrogen peroxide and visible light. J. Am. Chem. Soc. 2009, 131 (33), 11658−11659. (8) Niwa, S.-i.; Eswaramoorthy, M.; Nair, J.; Raj, A.; Itoh, N.; Shoji, H.; Namba, T.; Mizukami, F. A one-step conversion of benzene to phenol with a palladium membrane. Science 2002, 295 (5552), 105− 107. (9) Khatri, P. K.; Singh, B.; Jain, S. L.; Sain, B.; Sinha, A. K. Cyclotriphosphazene grafted silica: a novel support for immobilizing the oxo-vanadium Schiff base moieties for hydroxylation of benzene. Chem. Commun. 2011, 47 (5), 1610−1612. (10) Peng, J.; Shi, F.; Gu, Y.; Deng, Y. Highly selective and green aqueous−ionic liquid biphasic hydroxylation of benzene to phenol with hydrogen peroxide. Green Chem. 2003, 5 (2), 224−226. (11) Nomiya, K.; Yagishita, K.; Nemoto, Y.; Kamataki, T.-a. Functional action of Keggin-type mono-vanadium (V)-substituted heteropolymolybdate as a single species on catalytic hydroxylation of benzene in the presence of hydrogen peroxide. J. Mol. Catal. A: Chem. 1997, 126 (1), 43−53. (12) Jiang, W.; Zhu, W.; Li, H.; Chao, Y.; Xun, S.; Chang, Y.; Liu, H.; Zhao, Z. Mechanism and optimization for oxidative desulfurization of fuels catalyzed by Fenton-like catalysts in hydrophobic ionic liquid. J. Mol. Catal. A: Chem. 2014, 382, 8−14. (13) Wang, Y.; Zhao, H.; Li, M.; Fan, J.; Zhao, G. Magnetic ordered mesoporous copper ferrite as a heterogeneous Fenton catalyst for the degradation of imidacloprid. Appl. Catal., B 2014, 147, 534−545.

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CONCLUSIONS The eclectic benzene hydroxylation in the presence of hydrogen peroxide was achieved using iron ferrite and zinc ferrite as catalysts. The catalysts exhibited very high selectivity toward phenol (100%), but yet the iron ferrite showed higher activity than zinc ferrite toward phenol yield. A maximum yield of phenol (49.3%) was achieved using iron ferrite weight of 0.1 g; benzene: H2O2 molar ratio of 1:1 for 300 min of hydroxylation at 60 °C. The use of a magnetically recoverable iron ferrite in benzene hydroxylation is thought to be a felicitous endeavor to fulfill a durable and efficacious application of this reaction. The presence of the divalent and trivalent iron cations in the octahedral sites of iron ferrite is deemed the main reason that lies behind the higher performance of iron ferrite over zinc ferrite or other catalysts. This merit allows the redox reactions of iron species to take place within the octahedral sites without altering the spinel structure.

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AUTHOR INFORMATION

Corresponding Authors

Table 1. Comparison of Catalytic Activities with Other Catalysts for Benzene Hydroxylation V-HMS Cu0.90-V-HMS V-N-C catalysts VxOy@C mesostructured V/mp-C3N4 V/GO Cu-Ag/C [CuII(tmpa)]2+@Al-MCM-41 μ-(SCH(CH2CH3)CH2S)-Fe2(CO)5PCy3 iron ferrite (This work) zinc ferrite (This work)

peroxide efficiency, XRD patterns of the used catalysts, Gibbs free energy and enthalpy calculated by DFT method (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00214. XRD, Raman spectroscopy, N2 adsorption−desorption isotherm, TGA analysis of iron ferrite and zinc ferrite, selectivity toward p-benzoquinone during the optimization of benzene hydroxylation conditions, hydrogen 4817


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