Oxidation of benzyl alcohol through eco-friendly processes using Fe-doped cryptomelane catalysts

Solid State Sciences 94 (2019) 145–154

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Oxidation of benzyl alcohol through eco-friendly processes using Fe-doped cryptomelane catalysts

T

S. Said∗, M. Riad Petroleum Refining Division, Egyptian Petroleum Research Institute, Nasr City, 11727, Cairo, Egypt

A R T I C LE I N FO

A B S T R A C T

Keywords: K-OMS-2 xFe-OMS-2 Oxidation reaction Benzyl alcohol TON

Due to the convenience of catalyst separation and solvent-free conditions, gas-phase oxidation of benzyl alcoholin particular to-benzaldehyde reaction is more attractive method for industrial applications. In this respect it is environmentally and scientifically important to develop a low cost, energy-saving, and green approach for benzyl alcohol oxidation reaction, as an alternative to conventional liquid phase aerobic oxidation. Herein, a facile method for enhancing the catalytic activity of K-OMS-2 for benzyl alcohol-to-benzaldehyde oxidation by adding a small amount of Fe2(SO4)3 to the mixture of KMnO4 & MnSO4 with Fe/Mn = 0, 0.03, 0.06, 0.08 and 0.10 was report. The physico-chemical characteristics of the prepared samples were studied by using powder XRD, UV-VIS, N2- physisorption, HR-TEM, FT-IR and H2-TPR techniques. The influence of the different iron amounts on the catalytic performance of the prepared xFe-OMS-2 samples at different reaction temperature (250oC–450 °C) using air as an oxidant carrier gas was investigated. H2-TPR results confirmed that iron doping facilities the Mn4+↔Mn3+ redox cycle for providing oxygen species that are necessary for benzyl alcohol oxidation reaction. Also the lower reducibility, Small particle sizes with higher iron species dispersion within the OMS-2 framework matrix also had a great influence on the catalytic activity effectiveness of the tested samples.

1. Introduction Oxidation reaction of alcohols to selectively convert into more valuable aldehydes, ketones, and carboxylic acids products is of great importance in industrial manufacturing [1]. Benzaldehyde is considered an important raw material in synthesis of pharmaceuticals, plastic additives, perfumes, favoring compounds and also in the preparation of certain aniline dyes that's why the direct oxidation of benzyl alcohol to benzaldehyde is an important reaction [2]. The common methods used for alcohol oxidation are toxic, corrosive, expensive oxidants and use a severe condition, like high pressure or temperature and strong mineral acids. With the increase of environmental concerns, selective oxidation of alcohols to the corresponding aldehydes with environmentally benign oxidants has attracted great attention [3] Many studies have been reported on the selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen where additions of supports and promoters, visible light irradiation, and process in combination with different oxidants have been used to improve the catalytic systems [4]. A significant number of studies on catalytic oxidation of benzyl alcohol using precious metal or metal based compounds as catalysts, such as Au [5–7], Ru [8], and Pt [9] have been reported. These catalysts suffer from high cost and limited availability and also structural ∗

stability, heterogeneity, deactivation rates and the recyclability of the noble metal catalysts are still critical impending to the high catalytic performance in liquid phase reactions [10]. Therefore, it is highly desirable to investigate and develop inexpensive catalysts consisting of‘3d’ transition metals, because these metals offer environmentally benign and cost-effective alternative to noble metal catalysts. Manganese oxide is one of the largest groups that are constructed of corner-and edge-shared [MnO6] octahedral units. Due to the diversity of their crystallographic structures, their multiple d orbital electrons and space confirmed transport phenomena [11], the 2 × 2 tunnel structured α-manganese oxide octahedral molecular sieve (OMS) known as Cryptomelane is the most useful and has been most widely studied. Cryptomelane (KMn8O16. nH2O) often named as OMS-2 with a pore size of 0.46 nm × 0.46 nm has been reported as an efficient catalyst in many oxidation reactions [12]. It can accommodate some other metal cations to tune its photosensitivity, catalytic activities and to stabilize its framework [13]. Besides, potassium ions and water molecules often reside in the 2 × 2 tunnels to provide structure stabilization and charge balance [14]. In the review of Suib [11], cryptomelane has mixed valence state of manganese [Mn] and has mobility of lattice oxygen in the cryptomelane surface structure. Inclusion of foreign metal cations such as Cu2+, Ni2+, Co2+, Fe3+ or V5+ into in the framework

Corresponding author. E-mail address: [email protected] (S. Said).

https://doi.org/10.1016/j.solidstatesciences.2019.05.020 Received 7 May 2019; Received in revised form 28 May 2019; Accepted 29 May 2019 Available online 06 June 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.

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S. Said and M. Riad

gas and the heating rate was set at 10oCmin−1 with scans from ambient temperature to 1000 °C. The transmission electron microscopy (TEM) images were obtained with a JEOL 2100F instrument. FT-IR spectra of the prepared samples were taken as KBr pellets on spectrometer PerkinElmer-Spectrum-1 in the range of 4000-400 cm−1. A JASCO V-750 Spectrophotometer was used to collect the diffuse reflectance UV–Vis spectra data. The spectra were collected in the range of 200–800 nm at room temperature with BaSO4 reference.

or tunnel structure of the cryptomelane can alter its structure and physicochemical properties by increasing the defects inside the catalyst surface thereby creating more number of active sites [15], leading to an increase specific surface area and pore volume [16], modifying Mn average oxidation states [17], enhance their thermal stability [18,19], and enhance the reactivity of cryptomelane towards the catalytic oxidation of various organic pollutants [20,21]. S. A.C. Carabineiro et al., synthesized Cs-K-OMS-2, Li-KOMS-2 and Ti-K-OMS-2 catalysts and found that addition of those metals to cryptomelane was detrimental in terms of CO oxidation catalytic activity [22]. Also, V.P. Santos et al., showed that the interplay of stabilized gold nanosized species and ceria nano-particles doped cryptomelane resulted in a highly active gold catalyst for the low-temperature oxidation of CO [23]. The cryptomelane can be used as photocatalyst. Liu et al. studied the Photocatalytic degradation of polyethylene film with cryptomelane as photocatalyst in the air under ultraviolet and visible light irradiation [24]. Iyer et al. evaluated the activities of various KOMS-2 and metal doped K-OMS-2 catalysts prepared by different synthesis procedures, and found that K-OMS-2 prepared by solvent free method had the highest activity for selective oxidation of 2-propanol to acetone under visible light irradiation [25]. Previously, cryptomelanes were synthesized hydrothermally, which is a problem in terms of safety and energy consumption. So, it is more favorable to synthesize cryptomelane by refluxing at low temperature and at ambient pressure [26,27].

2.4. The catalytic activity test The gas phase catalytic oxidation reaction of benzyl alcohol (Aldrich, P > 99%) was conducted under atmospheric pressure using a conventional fixed-bed Pyrex glass flow reactor containing 0.5 g of the catalyst. A syringe pump was used to inject the benzyl alcohol into the reactor at a fixed rate (1 ml h−1). The air gas flow was regulated with a mass flow controller with flow rate 30 ml min−1, and the reactor temperature was controlled with a tubular furnace equipped with a programmable temperature controller and a K-type thermocouple. Reaction temperature was varied in the range of 250–450 °C in order to change the reactant conversion. The gas-phase oxidation of benzyl alcohol was conducted for 1 h at a specified reaction temperature and the products were collected using a liquid trap. The liquid products were analyzed using an online gas chromatograph (Thermo-Scientific) equipped with FID and a HP-5 (capillary 30 m × 0.32 mm). The turnover numbers (TON) is defined as PhCH2OH converted moles per active site and is calculated according to the Equation: moles of PhCH 2OH converted TON = moles of metal in the catalyst × 100.

2. Experimental procedure 2.1. Materials

3. Results and discussions

All the starting materials; Fe2 (SO4)3, MnSO4·H2O, KMnO4 and acetic acid, were reagent grade, of 99.9% purity.

3.1. X-ray diffraction and infrared spectra 2.2. Preparation of pure K-OMS-2 & xFe-OMS-2 samples The pure K-OMS-2 exhibit XRD patterns (Fig. 1) at 2Ɵ = 12.9°, 18.2°, 28.9°, 37.7°, 49.9° and 60.5° that can be assigned to (110), (200), (310), (211), (411), (521) reflection planes, respectively, which agree well to the natural tetragonal structure of cryptomelane-type manganese oxide (KMn8O16, JCPDS 29-1020). After iron doping (Fig. 1), all the prepared xFe-OMS-2 samples presented the typical cryptomelane structure XRD peaks without any obvious impurities or additional phases, nor any shift of peaks to higher 2θ, which would suggest that the doped iron didn't form a separate compound and the iron ions are highly dispersed and homogeneously distributed into the unit cell of the K-OMS-2 structure. According to the published data, the incorporation of iron in the OMS-2 framework by substitution of manganese ions does not affect the diffraction patterns if the Fe/Mn molar ratio is less than 0.1 [29]. However, K-OMS-2 structural perturbations due to iron doping can be noticed by the increase in peak intensities of xFe–K-OMS2, suggesting that the xFe-OMS-2 samples have relatively high crystallinity degree that may be due to the crystallite size effect. As shown in Table 1, Fe doping likely induced the increase in CDS values calculated by Scherrer equation and also the average particle size determined by DLS technique of the prepared Fe-OMS-2. In other words, during the pre-incorporation process the Fe2+ species and the Mn4+ species are represented in the solution at the same time, the smaller Fe2+ radius (0.075 nm) replaces the K+ (0.152 nm) in the channels of the cryptomelane. And the iron ions did not occupy the center of the cube formed by the octahedron, but located on the common surface of the cube with four oxygen anions, whereas the atomic coordination in the channel reconstructed and the crystal structure was reformed [30,31]. FT-IR spectra (Fig. 2A and B) is used to study the effect of iron doping on the lattice spectral features of the pure K-OMS-2 and hence can provide complementary structural information since it is sensitive towards amorphous compounds [32]. K-OMS-2 and xFe-OMS-2 prepared samples displayed IR bands at around 705, 512 and 460 cm−1

Fe-doped cryptomelanes were synthesized by adding KMnO4 solution to mixtures of Fe2 (SO4)3, MnSO4 and acetic acid, by refluxing at ambient pressure [28]. In a typical procedure, 80 mL of 0.4375 moL/L KMnO4 solution was heated at 60 °C and poured into 100 mL mixtures of [0.5 moL/L Fe2 (SO4)3 + MnSO4 solutions with different atomic ratio (Fe/Mn = 0, 0.03, 0.06, 0.08 and 0.1)). A suitable amount of 2 moL/L acetic acid (preheated at 60 °C) was added to the mixture until the CH3COOH was smelled during the reaction. The mixtures were then heated and kept boiling under reflux under continuous stirring for 24 h. All samples were washed with distilled water, dried at 60 °C in oven and then grounded in an agate mortar to particle sizes of 0.125 mm. 2.3. Characterization methods The Surface area and pore size distribution of the pure K-OMS-2 and xFe-OMS-2 samples were determined from the N2 adsorption-desorption isotherms at liquid nitrogen temperature (−196 °C) using a NOVA 3200 S instrument. Before starting the process of nitrogen adsorption, all samples were degassed at 250 °C under vacuum for 8 h in nitrogen atmosphere. Evaluation of surface area was made according to data on nitrogen adsorption determined by the Brunauere-EmmetteTeller (BET) method and pore size distribution was estimated using the method of Barrette-Joynere-Halenda (BJH). The prepared samples structure and the phase purity were determined by X-ray diffraction (XRD) using a Shimadzu XD-1 diffractometer instrument with Cu Ka radiation and Ni filter was used to collect X-ray data. Also, the coherent scattering domain (CSD) size of the pure K-OMS-2 and xFe-OMS-2 crystals was determined from the (211) XRD peak at 2Ɵ = 37.6° using the Scherrer equation. Thermal stability of samples was studied with thermogravimetric (TGA) using a SDTQ-600 (TA-USA) thermo balance instrument. Air gas with flow rate of 100 mLmin-1 was used as carrier 146

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300

250

(211) 0.1 Fe-OMS-2 (110) (200) (310) (301) (411) (521) (600) (222)

200

0.08 Fe-OMS-2 0.06 Fe-OMS-2

100

0.03 Fe-OMS-2

I/Io

150

data. For the 0.06 & 0.08Fe-OMS-2 samples, a shoulder observed at 2975 cm−1 can be assigned to hydrogen bonds between water and Fe+3- O amorphous oxide phase. In addition, the band at ∼3368 cm−1 become more intense for the (0.03& 0.08)Fe-OMS-2 samples if compared to the other ones, indicating a considerable increase in the hydroxyl group amount. 3.2. HR-TEM and N2 adsorption–desorption The TEM images of pure K-OMS-2 and 0.1Fe-OMS-2 prepared sample are displayed in Fig.(3-a&b). The TEM image of K-OMS-2 is composed of uniform fibrous morphology with regular shape in which their ends are similar in diameter as their middle part. The TEM image of the iron rich content sample (0.1Fe-OMS-2) revealed the morphological change from fibrous morphology of the pure K-OMS-2 to rod like morphology (Fig. 3-b) with large particle size, in line with XRD and DLS data. Insertion of iron is thought to decrease the fibrous length and increase their width which is responsible for the reduction of the growth rate of fibers causing disorder in K-OMS-2. No iron oxide particles were detected in the rich iron-doped sample, confirming the homogeneous structure of the prepared sample. The corresponding SAED pattern (inset) proves that the prepared samples are single-crystalline and have a nano-rods structure lying on the (221) planes. Indeed, the distortion of the ideal tetragonal crystal structure of cryptomelane due to iron doping lowers the symmetry to the monoclinic geometry when iron cations species substitute the mixed-valent Mn in the octahedral framework [35]. The nitrogen adsorption–desorption isotherms of the prepared samples are represented in Fig. 4, and their textural parameters are summarized in Table 1. All samples exhibit a characteristic type II N2 adsorption-desorption isotherms (Fig. 4-A), with a hysteresis loop of type H3 (IUPAC classification), which is usually associated with the adsorption on aggregates of particles with a rod-like fibers morphology, forming slit-like pores with non-uniform sizes [36] as confirmed by TEM images (Fig. (3-a)) [37]. As shown in Table 1, the BET surface area, total pore volume and average pore diameter values of the xFeOMS-2 samples are lower than those of the pure K-OMS-2 sample. Moreover, the surface area, average pore diameter and total pore volume of xFe-OMS-2 prepared samples decreased with the rise in Fe/Mn ratio. The increase in Fe (III) incorporation could provoke a blockage of the OMS-2 pores by deposition of large amorphous Fe oxide crystal sizes, as confirmed by their pore size distribution curves (Fig. 4-b). (0, 0.03 & 0.06)Fe-OMS-2 samples pore size distribution curves (Fig. 4-B) show one broad peak, indicating a uniform mesopores size. According to TEM image (Fig. 2-a), the agglomeration of the nano-needles bunches shows mesopores between the needles, which responsible for the high average pore diameter of pure cryptomelane sample. On the other hand, the pore size distribution (PSD) curves of (0.08 and 0.1)Fe-OMS-2 samples behaves a bi-model pore size distribution, where a group of narrower mesopores start to appear with a pore diameter of 5 nm (which may be resulted from the assemblage of the extra frame work amorphous Fe oxide particles which are loosely coherent) beside the mesopores of pure K-OMS-2 with D = 12 nm, which demonstrates the presence of heterogonous mesopores with an average pore diameter sizes of 6 and 5 nm, respectively.

50

OMS-2 0 10

20

30

40

50

60

70

2 Fig. 1. The XRD patterns of the pure K-OMS-2 and xFe-OMS-2 prepared samples. Table 1 Structural parameters of pure K-OMS-2 & xFe-OMS-2 prepared samples. Sample

K-OMS-2 0.03Fe-K- OMS-2 0.06Fe-K- OMS-2 0.08Fe-K- OMS-2 0.1Fe- K-OMS-2

DLSe

XRD

N2 physisorption

CSDa (nm)

SbBET (m2g−1)

TVcp (ccg−1)

Ddp (nm)

(nm)

15 21 27 42 43

121 115 107 97 87

0.3 0.3 0.2 0.2 0.2

12 6 5 5 3

192 254 266 277 278

a

By sherrer equation. Total surface area calculated by BET method. c The total pore volumes were estimated at P/P0 = 0.98. d The mean pore size diameter of the samples calculated from the BJH method. e By DLS (Table 1). b

(Fig. 2-A) are involved in the lattice vibration modes of Mn-O in [MnO6] octahedra [33] and are characteristic of cryptomelane type structure. In addition to a broad band at ∼3376 cm−1 due to O–H stretching vibration (ν O–H) and the bending mode can be observed at 1606 cm−1 of H–O–H from H2O, confirming the presence of hydroxyl groups as well as water in the 2 × 2 tunnel structure that is characteristic for the pure K-OMS-2 sample (Fig. 2-B). Compared to the pure K-OMS-2 sample, the FT-IR spectrum of xFe-OMS-2 prepared samples show a slight decrease in the intensity of the peaks at 705 cm−1, 512 cm−1, 460 cm−1 and shifted to higher wavenumber, which probably ascribed to the change in Mn-O lattice vibration arising from insertion of the iron ions into the octahedral channels environments of OMS-2. With the increase of iron content up to 0.06, an absorption band is detected at ∼1080 cm−1 may be attributed to the oxygen bond formed by the interaction of Fe+3-O-Mn [34], indicating the existence of amorphous Fe+3-O as an impurity phase, in agreement with the XRD

3.3. Thermal analysis & H2-TPR Differences in thermal stability can be correlated with variations in morphology and or the surface area of K-OMS-2 as a result of iron doping. The thermal behavior of pure K-OMS-2 and xFe-OMS-2 prepared samples show similar TGA profiles between 50 and 1000 °C to that reported for K-OMS-2 (Fig. 5) [38,39]. Although the TGA profiles of xFe-OMS-2 prepared samples have similar shapes with pure K-OMS2, the regimes wherein major weight losses occur took place at lower temperatures if compared to those of pure K-OMS-2 (Fig. 5), suggesting 147

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B

A

0.1Fe-OMS-2

Transmittance,%

0.06Fe-OMS-2 0.03Fe-OMS-2 OMS-2

3500

849

1606

3376

4000

1080

2975

Transmittance,%

0.08Fe-OMS-2

705 3000

2500

Wavenumber

2000

1500

460 512

1000 900 800 700 600 500 400

1000

Wavenumber (Cm-1)

(Cm-1)

Fig. 2. FT-IR spectrum of pure K-OMS-2 and xFe-OMS-2 prepared samples.

a

b

Fig. 3. TEM images, the insets in the corresponding SAED patterns and EDX patterns of (a) K-OMS-2 & (b) 0.1Fe-OMS-2 samples.

350

0.7

300

0.6 0.5

OMS-2

200

d(v)/d(r)

V (cc g-1)

250

OMS-2 0.03 Fe 0.06 Fe 0.08 Fe 0.1 Fe

0.1 Fe-OMS-2

150

0.08 Fe-OMS-2

100

0.3 0.2

0.06 Fe-OMS-2

50

0.4

0.1

0.03 Fe-OMS-2

0

0 0

0.2

0.4

P/Po

0.6

0.8

1

0

10

20

30

Dnm

Fig. 4. N2 adsorption-desorption isotherms & PSD curves of the pure K-OMS-2 & xFe-OMS-2 samples. 148

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100

I

Weight loss,%

90

Table 2 Reduction temperatures and H2 consumption of the pure K-OMS-2 and xFeOMS-2 samples.

K-OMS-2

Sample

II

III

80

OMS-2 0.03Fe-OMS-2 0.06Fe-OMS-2 0.08Fe-OMS-2 0.1Fe-OMS-2

70

50 200

350

500

Temp.,oC

650

800

950

Fig. 5. TGA curves of pure K-OMS-2 and xFe-doped OMS-2 samples.

a decrease in thermal stability after Fe doping. Three major weight losses can be seen clearly between 50 and 950 °C. High weight loss (with 8.6% & 9.7%) at lower temperature (∼236 °C) is observed for 0.06 & 0.08 Fe-OMS-2 samples, respectively, indicating the desorption of large amounts of physisorbed and chemisorbed H2O while 0.03& 0.1Fe-OMS-2 samples show low weight loss with ∼6% at the same temperature. Second important high weight due to the loss of lattice oxygen at ∼560 °C which could lead to tunnel structural collapse [38] exhibited by 0.06& 0.08Fe-OMS-2 samples with ∼4.96 & 4.60%, respectively. Whereas, 0.03& 0.1Fe-OMS-2 samples exhibit 3.1& 2.4% weight losses, respectively at the same temperature indicating that the presence of Fe3+ with high amounts in the cryptomelane structure promotes the mobility and availability of the lattice oxygen at lower temperature in comparison to the pure K- OMS-2. Pure K-OMS-2 and xFe-OMS-2 prepared samples redox characteristics were studied by H2-TPR technique and their profiles are displayed in Fig. 6. The peak positions and H2 uptake are summarized in Table 2. The reduction of pure cryptomelane-type manganese oxide is often described by the following sequential process: MnO2 (Mn4+) → Mn2O3 (Mn3+) → Mn3O4 (Mn3+ & Mn2+) → MnO (Mn2+) [40]. H2-TPR profile of the pure K-OMS-2 sample showed a split peak in the range of 135–265 °C, suggesting the reduction of labile oxygen species without the collapse of the structure (i.e., generation of oxygen vacancies) [41]. In addition to, a strong peak appeared at ∼400 °C due to the reduction of MnO2 to Mn2O3. Besides, a broad weak split shoulder at temperature 0.1Fe

500

0.3Fe

400

OMS-2

773

300 0.06Fe 200 0.08Fe 100 0 0

200

400

600

800

LT

HT

LT

HT

Total

265 182 179 171 182

444 475 468 468 489

10.64 2.90 3.70 1.31 4.60

2.01 8.92 0.18 2.28 0.32

12.65 11.82 3.88 3.59 4.92

range 479–619 °C is related to the successive reduction of Mn2O3 to Mn3O4 (3MnO2 + 2H2→ Mn3O4 + 2H2O) and Mn3O4 to MnO (Mn3O4 + H2 → 3MnO + H2O). Finally, a broad peak at 800 °C was detected corresponds to the reduction of oxygen species that have different reactivity (i.e. tunnel species linked to protons or potassium cations). After iron doping, the TPR profile shape, peak intensity, area and position vary among the four prepared samples (Fig. 6 and Table 2), indicating different influence of the doping iron amount on the OMS-2. The shape of the reduction curve for 0.03Fe-OMS-2 and 0.1Fe-OMS-2 was nearly identical (Fig. 6) with an overlapped shoulder to a strong peak with maxima at ∼ 431 °C. While, (0.06-&0.08)Fe-OMS-2 samples showed a reduction profile with a split peak in the temperature range of 400–477 °C, besides an overlapped shoulder at 200–395 °C. These low temperature peaks can be ascribed to the hydrogen consumption during the formation of Mn2O3. The reduction peak at higher temperature assigned to the reduction of Mn2O3 to Mn3O4, and Mn3O4 to MnO [42]. It can clearly noticed that the second major peak (reduction of Mn2O3 to Mn3O4) was shifted to a higher temperature (∼400 °C) upon iron doping if compared with the pure prepared sample, suggesting lower reducibility of manganese oxide, lower reactivity of lattice oxygen presumably due to the influence of iron in agreement with TGA results. High peak intensities were observed for 0.03Fe and 0.1Fe-OMS2 established higher relative amounts of Mn3+ species in those samples than in other samples. A reduction peak at approximately 773 °C was detected whose intensity increased with the increase of iron doped amount may be related to the reduction process of Fe2O3→ FeO, suggesting that the Fe3+ species are reduced only to Fe2+ directly without the formation of Fe3O4 as intermediate [43]. The high temperature at which Fe3+ species are reduced gives account to the difficult reducibility of Fe3+ framework species thanks to the shielding of the OMS-2 framework structure which results in high stability for these species. Among the four doped iron OMS-2 samples, the split reduction peak for 0.08Fe-OMS-2 TPR profiles shifted to lower reduction temperature, indicating that the presence of Fe amount that promoted the reducibility of manganese oxide through oxygen Spill over [44]. In terms of reduction temperature, the low-temperature reducibility decreased in the order of: K-OMS-2 (265 °C) > 0.03Fe-OMS-2 (182 °C) ∼ 0.1Fe-OMS-2 (182 °C) > 0.06Fe-OMS-2 (179 °C) ˃ 0.08FeOMS-2 (171 °C). The higher reducibility of 0.08Fe-OMS-2 prepared sample indicates the higher mobility of the oxygen species in the sample. From Fig. 6 and Table 2, one can readily observe that 0.06& 0.08Fe-OMS-2 samples show the highest reducibility with the lowest H2 consumption (∼3.8 mmoL/g) and the lowest starting temperature of ∼170 °C. As nearly all Mn4+ over OMS-2 catalyst was converted to Mn2+ at a lower temperature. The promotion effect of Fe on the best low-temperature reducibility of (0.06&0.08)Fe-OMS-2 samples could be attributed to the formation of Fe–O–Mn bridge and the oxygen spillover from Fe atoms to manganese oxides [45] and hence is expected to show the enhanced catalytic performance for the oxidation of benzyl alchol.

60

50

H2 consumption (mmol gcat−1)

Temp.,oC

1000

Temp.,oC Fig. 6. H2- TPR profiles of K-OMS-2 and xFe-OMS-2 prepared samples. 149

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35

242 nm

30

Absorbance (A.u)

283 nm 25 0.1Fe-K-OMS-2 0.08Fe-K-OMS-2 0.06Fe-K-OMS-2 0.03Fe-K-OMS-2 K-OMS-2

20 15 10 5 0 200

400

600

800

Wavelength (CM-1) Fig. 7. UV-VIS spectra of the pure K-OMS-2 and xFe-OMS-2 prepared samples.

performed in the liquid or gas phase. The easy catalyst separation and solvent-free conditions make gas-phase process more attractive for industrial applications. Recent reports deal with the liquid phase aerobic oxidation with or without solvents [48]. However, the main disadvantage of PhCH2OH gas-phase oxidation reaction is the high reaction temperature that are generally above 300 °C, i. e, higher than the boiling point of benzyl alcohol(203 °C) with 100 °C, and reported over various catalysts (e.g., Au/SiO2,4a K/Ag/SiO2,4b Cu/Na/ZSM-5,4c and Au-Cu/SiO24d) [49] No benzyl alcohol gas-phase oxidation reaction is reported yet in literature below 250 °C. Such high temperature reaction suffers from low product stability problems, low product selectivity and the deactivation of active sites. So attempts to develop a successful gasphase industrial application have been made to overcome such problems. In the current study, the catalytic performance of the prepared xFeOMS-2 catalysts were investigated towards the gas-phase oxidation of PhCH2OH with air as carrier gas using fixed-bed flow system operated under atmospheric pressure at a reaction temperature range of 250–450 °C. The major converted products obtained were formed of benzaldehyde (PhCHO), benzoic acid (PhCOOH), benzylbenzoate, trace amount of benzene (PhH) and toluene (PhCH3) for all the tested catalysts. The effects of the reaction temperature and the different Fe: Mn atomic ratio amounts over the catalytic oxidation of PhCH2OH were studied. The catalytic activity is expressed in terms of PhCH2OH conversion (%), the selectivity (%) for PhCHO and PhCOOH over various

3.4. Uv–vis analysis The iron metal species present in the cryptomelane was analyzed by UV–Vis diffuse reflectance spectroscopy. The UV–Vis spectra of pure KOMS-2 and xFe-OMS-2 prepared samples are very similar as shown in Fig. 7. The UV–Vis spectra contain only one intense absorption peak at ∼242 nm that is attributed to the O2-/Mn + 3 charge transition in Mn3O4 in which Mn is octahedrally coordinated with oxygen [46], Suggesting the coexistence of Mn + 3 in the prepared samples. In addition to the absorption peak at ∼242 nm, the xFe-OMS-2 samples spectrum showed another shallow & diffuse absorption peak at ∼283 nm. According to published data [47], this peak may be due to the dЛ-pЛ charge transfer between Fe and O of the dispersed Fe3+ cations and linked to O in their incorporation in the cryptomelane matrix indicating the existence of amorphous Fe+3-O as confirmed by FT-IR spectrum. Also, the absence of absorption bands at wavelengths higher than 450 nm associated with the formation of large iron oxide (FeO)n clusters formed as extraframework species and/or small nanoparticles of Fe2O3 [34] is an another evidence of XRD results of the incorporation of iron into the cryptomelane framework structure. 3.5. The catalytic activity Depending on the thermal stability and volatility of the products produced, benzyl alcohol (PhCH2OH) oxidation generally can be

B

100

A

100

450oC

80

400oC

60

40 0.06Fe-OMS-2 0.08Fe-OMS-2 0.1Fe-OMS-2 0.03Fe-OMS-2

20

0 250

300

350

Temp.,oC

400

Tot. Conversion ,%

Tot. Conversion ,%

80 60

350oC 300oC

40

250oC

20

0

450

Fig. 8. The effect of (A) reaction temperature and (B) Fe/Mn atomic ratio on the total conversion Of PhCH2OH over xFe-OMS-2 catalysts. 150

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100 80 Yield,%

conver. PhCHO PhCOOH benzyl benzate

conver. PhCHO PhCOOH benzyl benzate

60

Conver. PhCHO PhCOOH benzyl benzate

Conver. PhCHO PhCOOH benzyl benzate

40 20

0.08Fe-OMS-2

0.06Fe-OMS-2

0.03Fe-OMS-2

0.1Fe-OMS-2

0 250

300

350

Temp.,oC

400

450 250

300

350

400

450

250

300

350

400

Temp.,oC

Temp.,oC

450 250

300

350

400

450

Temp.,oC

Fig. 9. The effect of the reaction temperature on the total conversion and the converted product. Distribution of the PhCH2OH oxidation over xFe-OMS-2 catalysts.

via the interaction of organic molecules with oxygen species at oxygen vacancies [50]. For transition metal oxide catalysts (α-MnO2), both the reduction of metal ions and the generation of surface oxygen vacancies take place concurrently during the oxidation process [51]. Thus it is speculated that the catalytic oxidation of PhCH2OH over xFe-OMS-2 catalysts consists of two reversible steps in presence of gaseous oxygen provided by air carrier gas: 1- the reduction of manganese cations giving up catalyst lattice oxygen to 2- oxidize PhCH2OH. Thus it is generally supposed that the cryptomelane OMS-2 catalyst as a reservoir of oxygen species due to the coexistence of mixed-valent manganese framework, i. e; Mn4+Ox phases on OMS-2 surface that reduced to Mn3+Ox species producing oxygen vacancies that are replenished by lattice oxygen moving from the catalyst bulk which is subsequently replenished by the oxygen from air carrier gas according to a Mars-VanKrevelen redox mechanism [52]. Dopping iron into the cryptomelane lattice structure provokes the presences of more redox cycles (Mn4+↔ Mn3+ and Fe3+ ↔ Fe2+), i. e, exhibited higher intrinsic activities and hence facilities the transfer of lattice oxygen from the catalyst bulk and the more formed oxygen vacancies is replenished more easily by increasing the oxygen mobility during the removal and replenishment of surface lattice oxygen as evidenced by H2-TPR and uv–vis analysis (Figs. 6 and 7). Continually exposing xFe-OMS-2 catalysts for oxidization and reduction environments generally avoid the deactivation of the metals active sites and a concurrent major advantage of their catalytic properties by minimizing the formation of coke deposition over the surface of catalyst admitting the positive effect of iron doping on getting more effective catalysts.

xFe-OMS- 2 catalysts and the results are graphically illustrated in Figs. 8 and 9, 10&11. Also, the pure K-OMS-2 sample showed a poor catalytic activity by itself under the same reaction conditions and for simplicity the curves are not given. 3.5.1. Effects of reaction temperature and the different Fe: Mn atomic ratios on PhCH2OH conversion The catalytic activity in terms of PhCH2OH conversion at reaction temperature range of 250–450 °C over different xFe-OMS-2 catalysts is depicted in Fig. 8-A&B. As shown in Fig. 8-A, the catalytic activity increases gradually for all the prepared catalyst with the increase of the reaction temperature demonstrating the temperature-dependent behavior of the oxidation reaction. Also, the catalytic activity increases with the successive increase in iron content amount up to Fe/Mn = 0.08 (Fig. 8-B) forming maximum and then decreases for Fe/Mn = 0.1. Therefore, it can be ascribed that the prepared 0.08Fe-OMS-2 catalyst is the most active one. Accordingly, a further increase in Fe amount to get Fe/Mn = 0.1 causes a significant decrease in activity at all reaction temperatures. This may be attributed to the whole change of the texture parameters of 0.1Fe-OMS-2 catalyst as evidenced from the decrease of BET surface area from 121 to 87 m2g-1 (Table 1) indicating a certain interaction between the iron oxide and manganese oxide species occurred resulting in an increase in the average particle size (from 15 to 42 nm, DLS results, Table 1), causing pore narrowing (the decrease of the average pore diameter from 12 to 3 nm), associated which the change in the structural morphology from fibrous morphology to rod like morphology (TEM image Fig. 3) inducing the poor dispersion of the amorphous Fe oxide species detected by FT-IR spectrum (Fig. 2). This explanation indicates that the improved catalytic activity is correlated well with the proper balance between the increase in amorphous Fe oxide dispersion and the easy Mn4+↔Mn3+ redox cycle for providing oxygen species that are necessary for the oxidation reaction. And consequently, the activity towards PhCH2OH oxidation increased effectively. From Fig. 8-A&B, a very similar catalytic activity conversion patterns were noticed for the studied xFe-OMS-2 catalysts, however the highest conversion with 90% & 76% at 450 °C was achieved for 0.08FeOMS-2 & 0.06Fe-OMS-2 catalysts, respectively. Meanwhile, on using the highest iron content catalyst (0.1Fe-OMS-2), the catalytic activity decreased to 63% reaching to 49% for 0.03Fe-OMS-2 catalyst. The change in the catalytic activity of different xFe-OMS-2 catalysts may be attributed to the richer defect-oxide species which function as nucleation sites causing enhancement in the oxygen mobility. This further suggests the good correlation between the redox properties of xFe-OMS2 with their catalytic activities in the order of: 0.08Fe-OMS-2 > 0.06Fe -OMS-2 > 0.1Fe -OMS-2 > 0.03Fe-OMS-2 as confirmed from H2-TPR results. It is well known that the oxidation of organic compounds proceeds

3.5.2. Effects of reaction temperature and the different Fe: Mn atomic ratios on converted products yield formation The distribution and the yield of the converted products resulted from PhCH2OH oxidation are presented in Fig. 9. The PhCHO, PhCOOH and benzyl benzoate yield formation increases gradually by increasing the reaction temperature regardless the iron content amount. The yield formation of PhCHO can be as high as ∼49% & 42% at 450 °C over 0.08Fe-OMS-2 and 0.06Fe-OMS-2, respectively. Meanwhile on using 0.03Fe-OMS-2 & 0.1Fe-OMS-2 catalysts, the yield formation of PhCHO dropped to ∼25% and 28% at the same reaction temperature, respectively. As shown in Fig. 9, the yield formation of PhCHO decreased in the following order: 0.08Fe-OMS-2 > 0.06Fe-OMS-2 > 0.03Fe-OMS2 > 0.1Fe-OMS-2. Complete oxidation of PhCH2OH to form PhCOOH increases with the reaction temperature up to 450 °C and its yield formation behavior is parallel with that of the PhCHO production. This possibly be due to the increase of reactive lattice oxygen amount and hence the reduction–reoxidization cycles of manganese proceed more efficiently which is known to favor overoxidation of benzyl alcohol to form benzoic acid [53]. PhCOOH is obtained in higher yield with yield formation ∼48% and 42% over 0.08Fe-OMS-2 and 0.06Fe-OMS-2 151

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Scheme 1. Reaction pathway for the catalytic oxidation of benzyl alcohol over xFe-OMS-2 catalysts.

catalysts, respectively if compared with the other two catalysts. Concurrently at high reaction temperature benzyl benzoates are formed in trace amounts resulting from the esterification reaction between the produced PhCOOH with the unreacted PhCH2OH which is available in much higher concentration, according to Scheme 1 [54]. Generally it is well known that the esterification reaction is an acid catalyzed reaction. As confirmed by FT-IR spectra (Fig. 2) the iron-doping into OMS-2 lattice decreases the –OH groups of the catalyst which is mainly Brønsted acid sites and hence the esterification reaction and the formation of benzyl benzoate product is not favored and is formed in limited quantities of ∼3%,4%,9%&2% for 0.03Fe-OMS-2, 0.06Fe-OMS2, 0.08Fe-OMS-2&0.1Fe-OMS-2,respectively.

3.5.3. Effects of reaction temperature and the different Fe: Mn atomic ratios on PhCHO & PhCOOH production selectivities Histogram in Fig. 10 shows the selectivities toward PhCHO & PhCOOH formation over xFe-OMS-2 catalysts at low reaction temperature (250 °C) and high reaction temperature (400 °C). The highest selectivity for PhCHO production at low reaction temperature is obtained on using 0.06Fe-OMS-2 catalyst (∼60%) and the lowest selectivity for PhCHO production is obtained over 0.03Fe-OMS-2 catalyst (∼30%). Whereas at high reaction (400 °C), the production of PhCHO is selective in the order of: 0.1Fe-OMS-2 (∼73%) > 0.06 Fe-OMS-2 (∼50%) > 0.03Fe-OMS-2 (∼43%) catalysts (Fig. 11). These differences are in good agreement with the aforementioned textural parameters results (Table 1). As the iron content amount increases, the pore become narrower and the bulkier PhCOOH diffusion become limited. So, one can assert that 0.06Fe-OMS-2 is highly active catalyst in the selective oxidation of PhCH2OH in particular to PhCHO (∼60%) at low reaction temperature (250 °C) with a conversion of ∼30% (Fig. 10). Such a slight improvement in PhCH2OH conversion and PhCHO selectivity at this low temperature in gas-phase benzyl alcohol oxidation might be due to the enhancement of redox properties of the active

100

Fig. 11. The selectivity toward PhCHO production over the different xFe-OMS2 catalysts at different reaction temperatures.

species of catalyst with the addition of this amount of iron as achieved from H2-TPR analysis (see Fig. 6). On the other hand, the dependence of the PhCHO production on the reaction temperature and is relevant to the iron content clearly demonstrated histogramically in Fig. 11. However, the only benefit from increasing the iron doping amount content to Fe/Mn = 0.1 is the increase in PhCHO production selectivity at high reaction temperature (400 °C) to about ∼73%. According to many studies, different metal content amount catalysts catalytic activity expressed by their turnover number (TON) can be compared best [55] TON values were calculated on the premise that iron metal present in the synthesized OMS-2 samples is considered as an active site at 250 °C & 400 °C. Histogram in Fig. 12 displays different

100

S.PhCHO (250) S.PhCOOH (250)

80

60

60

Selectivity,%

Selectivity,%

80

40 20 0

S.PhCHO (400) S.PhCOOH (400)

40 20 0

.

Fig. 10. The selectivity of PhCHO & PhCOOH production over the different xFe-OMS-2 catalysts and at low (250 °C) & high (400 °C) reaction temperatures. 152

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Fig. 12. TON values at different reaction temperature for different iron content catalysts.

TON values, anyway, at low reaction temperature (250 °C) 0.06FeOMS-2 & 0.08Fe-OMS-2 catalysts demonstrate high TON values, so their catalytic performance is much better than that of 0.03Fe-OMS-2 and 0.1Fe-OMS-2 catalysts having lower TON values. This in accordance with the previous results discussed in detail above. Meanwhile, at high reaction temperature (400 °C), the catalytic conversion of PhCH2OH is more preferred over lower iron amount content catalysts than on the higher amount iron catalyst (0.1Fe-OMS-2) in the order: 0.06Fe-OMS-2–0.03Fe-OMS-2 > 0.08Fe-OMS-2. This could be explained by considering the high proportion of iron metal forming extraframework Fe2O3 oxides species that was less dispersed in the cryptomelane structure which diminished its catalytic performance as it would form a separate active sites from the cryptomelane structure. 4. Conclusion In summary, a series of xFe-OMS-2 samples with different Fe/Mn atomic ratios were prepared by refluxing at ambient pressure and their catalytic performances for gas-phase benzyl alcohol oxidation were evaluated comparatively. Although, the physicochemical and morphological properties affected upon iron doping by increasing the average particle size, CDS increasing of the samples crystals and the decrease of the prepared samples textural parameters, the XRD patterns have shown that the iron dopant species were highly dispersed, as no peaks related to the dopant iron were detected suggesting that part of Fe(III) incorporated into the framework by replacing the crystallographic Mn (III) sites, and the rest adsorbed in the tunnel cavity. The synergism between reducibility and textural parameters is the key factor on getting more effective catalysts for benzyl alcohol gas-phase oxidation in particular to benzaldehyde. The catalytic activity results indicate the good correlation with the redox properties as confirmed from H2-TPR results and the more active catalyst in terms of benzyl alcohol catalytic conversion is obtained on using 0.08Fe-OMS-2 with ∼90% at 450 °C. At relatively low reaction temperature (250 °C), the catalyst 0.06Fe-OMS-2 selectively produces benzaldehyde with (∼60%) at benzyl alcohol conversion ∼ 76%. Conflicts of interest There are no conflicts to declare. 153

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