Journal of Nanomaterials
Volume 2020 (2020), Article ID 7532767, 20 pages
https://doi.org/10.1155/2020/7532767

Review Article

Recent Development of Catalytic Materials for Ethylbenzene Oxidation

Md. Motiar Rahman,¹ Mst. Gulshan Ara,² Md. Sohanur Rahman,² Md. Sahab Uddin,3,4 May N. Bin-Jumah,⁵ and Mohammed M. Abdel-Daim6,7

¹Institute of Synthetic Biology, Shenzhen Institute of Advanced Technology (SIAT), Chinese Academy of Sciences (CAS), Shenzhen 518055, China
²Graduate School of Innovative Life Science, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
³Department of Pharmacy, Southeast University, Dhaka, Bangladesh
⁴Pharmakon Neuroscience Research Network, Dhaka, Bangladesh
⁵Department of Biology, College of Science, Princess Nourah Bint Abdulrahman University, Riyadh 11474, Saudi Arabia
⁶Department of Zoology, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
⁷Pharmacology Department, Faculty of Veterinary Medicine, Suez Canal University, Ismailia 41522, Egypt

Correspondence should be addressed to Md. Motiar Rahman; motiar_bio@yahoo.com

Received 13 November 2019; Accepted 26 December 2019; Published 25 February 2020

Guest Editor: Domenico Lombardo

Copyright © 2020 Md. Motiar Rahman et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Catalysts are well-known to convert alkylbenzenes at high thermal condition to a number of useful products. However, the current schemes of transformation are not suitable for the hazard-free industrial applications because their reactive intermediates are transformed to a variety of side products that often retard the optimum yield and cause environmental pollutions. It is also observed that the formation of products depends on a wide range of parameters which are extremely difficult to control and often incur extra cost. Recently, heterogeneous catalysts have received huge commercial interests for the oxidation of alkylbenzene into carbonyl compounds which are platform chemicals in various synthetics and fine chemicals. This review is an up-to-date documentary on various catalysts used for the oxidation of alkyl-substituted benzenes along with their reaction condition and selectivity profiles. This work updates our knowledge for the selection and/or design of novel catalysts for the chemists and engineers in the industrial and academic settings.

1. Introduction

The oxidations of hydrocarbons to its corresponding carbonyl group (aldehydes or ketones) have a substantial value in organic synthesis, both in laboratory and in industry [1, 2]. It is remarkable to note that the global production of carbonyl composites per year has exceeded 10⁷ tones and many of these are produced from the direct oxidation of hydrocarbons [3]. In this regard, the catalyst-assisted oxidation processes have made significant contributions in the production of various chemicals, cosmetics, drugs, and other useful compounds. Among the various substrates used in oxidation reactions, aryl alkanes, such as ethylbenzene, have huge interests because their essential oxidation products are a rich source of a number of drugs and synthetics. For example, the oxidation products of ethylbenzene such as acetophenone and 1-phenylethanol are the precursors of optically active alcohols [4], benzalacetophenones (chalcones) [5, 6], and hydrazones [7].

The oxidation process was traditionally carried out by a stoichiometric amount of oxidants such as permanganates [8, 9], chromium reagents [10–13], ruthenium (VIII) oxide [14, 15], activated dimethyl sulfoxide (DMSO) [16] or Dess–Martin periodinane [17], and TPAP/NMO (tetra-N-propylammonium perruthenate/N-methyl-morpholine-N-oxide) [18], and all these involve high temperature and/or pressure and corrosive and toxic chemicals and produce an equivalent amount of waste metals, incurring the environmental burden such as halogenated organic solvents (hydrocarbons) [19, 20]. On the other hand, noncatalytic transformation under supercritical conditions [21] is immature and unsuitable commercial application due to the lack of stability and selectivity (Figure 1).

Figure 1: Major approaches to alkylbenzene oxidation.

Therefore, the catalytic approaches which offer low temperature and more selective conversion, minimizing the uses and/or formation of undesirable byproducts, have evolved as the method of choice for hydrocarbon oxidation using air or clean molecular oxygen (O₂) as oxygen source (Scheme 1) [21]. Hydrogen peroxide (H₂O₂) and tert-butyl hydroperoxide (TBHP) could also be used as oxygen source due to its obvious advantages in oxidation reaction [22, 23]. The aim of this review is to provide a brief but comprehensive outline of the major catalytic approaches for alkylbenzene oxidation along with some other oxidation reactions.

Scheme 1: Catalytic schemes for the aerobic oxidation of ethylbenzene.

2. Nanotechnology for Ethylbenzene Oxidation

Nanotechnology has set immeasurable status in the oxidation of ethylbenzene. Various catalytic materials with high surface area, high surface to volume ratio, reactivity, tunable pore size, and hydrophilic and hydrophobic interfaces are promising in catalysis. Several nanometals, such as gold [23], copper [24], titanium [25], silver [26], nickel [27], manganese [28], cobalt [29] and tin [25], deposited on solid supports are suitable in oxidation reaction. Functionalized silica with homogeneous metal dispersion which prevents metal agglomeration is efficient in ethylbenzene oxidation [23]. Gold-mediated ethylbenzene oxidation perhaps initiates Au nanoparticle-based decomposition of reactive oxidant to oxidant radical species followed by two different oxidation reactions, forming ketones and secondary alcohols [23]. Moreover, nanomaterials have increased surface porosity that uses low free energy for the reaction to happen [30]. Nanoparticles oxidize not only ethylbenzene but also various alkanes with improved catalysis [31]. In addition, nanoparticles can activate reaction as bimetallic forms with several other metals and metal oxide with controllable shape and composition. Even though nanomaterials are appreciated in oxidation reaction, they have some limitations. Thermal instability, high-pressure requirement, metal agglomeration, pore blocking, slow reaction, poor conversion and selectivity, and the formation of side products are frequent matters in this perspective. Poor recyclability and hidden hazards to ecosystems are also leading concerns [32]. Therefore, a consistent, nontoxic, delicate, and economical oxidation process has become the most laborious task in ethylbenzene oxidation.

3. Catalytic Method

The major goal of using a catalyst is to speed up or accelerate the rate of a chemical reaction. It remains unchanged at the end of the reaction but minimizes the activation/free energy that is needed to attain the transition state while keeping the total free energy of the reactants and products unchanged in the course of the reaction [33, 34]. The major catalytic processes for alkylbenzene oxidation are briefly presented step-by-step.

3.1. Homogeneous Catalysis

Homogeneous catalysts are those which exit in the same phase with the reactants and products; the catalysts fully dissolve in the reaction medium exposing all the catalytic sites to interact with the substrates. Homogeneous catalysts are usually complexes, often consisting of a metal which is bound to several organic ligands. The ligands are responsible for providing the stability as well as the solubility of the catalyst complex metal, and they could be adjusted to enhance selectivity of a catalyst towards the synthesis of a specific desirable product [35]. The great achievement of a homogeneous catalyst is that it can make a product with >90% selectivity at a high conversion rate through the careful selection of the metal center, ligands, reaction parameters, and a suitable substrate [31]. Although there are widespread advantages of selectivity in homogeneous catalysis, scientists are paying enormous attention to heterogeneous catalysts; this is due to the difficulty in the separation of homogeneous catalysts. Homogeneous catalysts are also known to cause corrosion to the reaction vessels, and some of them are deposited onto the reactor wall. Thus, the workup procedure for homogeneous catalysts is not straightforward (Figure 1) [36]. Here, we represent some homogeneous catalysts used in alkylbenzene oxidation with conversion and selectivity to acetophenone (Table 1).

Table 1: Homogeneous catalysts in ethylbenzene oxidation.

Catalysts
Reaction conditions
Major products
Conversion (%)
Selectivity (%)
Ref.

Oxidant
Solvent
Temperature (°C)
Reaction time (hour)

1-Glycyl-3-methyl imidazolium chloride-copper (II) complex
2 mmol NaClO
Solvent free
25
10
Acetophenone
85
85
[37]

CrO₃/CeSO₄
H₅IO₆
Acetonitrile
30
1
Acetophenone
100
49
[38]

CrO₃
Ce(SO₄)₂
Acetic acid
50
5
Acetophenone
100
61
[39]

1,4-dichloro-1,4-diazoniabicyclo [2] octane bis-chloride
—
Water
40
10
Acetophenone
—
95
[40]

Mixed valent dirhodium(II,III) tetrakis(caprolactamate)
TBHP and dichloroethane
—
40
16
Acetophenone
42
20
[41]

48% HBr & 30% H₂O₂
H₂O₂
Dichloromethane
30
12
Acetophenone
95
75
[42]

Cobalt(II) phthalocyanine
Oxygen
Ionic liquid 1-butyl-3-methyl-midazoliumbromide
100
7
Acetophenone
—
77
[43]

3.2. Heterogeneous Catalysis

Heterogeneous catalysts are those which exist in different phase from the reactants and products. They offer several advantages over their homogeneous counterparts in terms of separation and recyclability [31, 44]. These catalysts are usually solid, but the reactants could be either solid or liquid; so they could be easily detached from the reaction mixture by simple centrifugation and washing, keeping the manufacturing cost at the minimal level. Currently, heterogeneous catalysis is dominating in industries for chemical transformation and energy generation. Approximately 90% of all industrial practices indulge in heterogeneous catalysis. The most recent applications of heterogeneous catalysts are summarized in Table 2. Consequently, scientists have drawn huge attention as the oxidation catalyst for alkylbenzene conversion with better conversion rate and higher selectivity towards ketone products which are essential intermediates for the synthesis of many specialty chemicals with high economic value such as agrochemicals, pharmaceuticals, and perfumes [45]. Herein, we describe some representative heterogeneous catalysts on various supports for ethylbenzene oxidation.

Table 2: Recently used heterogeneous catalysts in oxidation reactions.

Year
Catalysts
Method of catalysts synthesis
Application
Ref.

2019
Mn catalysts on various supports
Coprecipitation
Carbon monoxide oxidation
[50]

Copper promoted ceria
Hydrothermal
Carbon monoxide oxidation
[51]

Copper on titania aerogel
Wet impregnation
Carbon monoxide oxidation
[52]

Pd supported on CeO₂(100) and CeO₂(111) facets
Hydrothermal
Carbon monoxide oxidation
[53]

Copper on titania hollow sphere
Wet chemical
Methanol oxidation
[54]

Pt-based catalysts
Chemical reduction
Glycerol
[55]

RuO₂/TiO₂ catalyst
—
Hg° oxidation
[56]

MnO₂ hollow sphere
Precipitation
Formaldehyde oxidation
[57]

2018
Ruthenium catalyst
—
Water oxidation
[58]

Solid catalyst on various supports
Impregnation
1-Octonol oxidation
[59]

Bioinspired manganese catalysts
—
Enantioselective oxidation of spirocyclic compounds
[60]

Pt catalyst on carbon
—
Xylose
[61]

α-ZrP. Mn(II)
Ion exchange
Cyclohexane oxidation
[62]

2017
Pt-Sn on carbon support
Formic acid reduction
Ethanol and carbon monoxide oxidation
[63]

Pt-Ru/C
Colloidal method
Glycerol oxidation
[64]

Porphyrinic metal-organic framework
Postsynthetic modification
Cyclohexane
[65]

MnOx-CeO₂ supported on Co-N-C
Coprecipitation
Ethylbenzene oxidation
[66]

2016
Pd-Pt nanocubes
Wet impregnation
Carbon monoxide oxidation
[67]

Pt₃Ni alloy nanoparticles
Impregnation
Carbon monoxide oxidation
[68]

Boron-doped crystalline diamond
—
Aliphatic polyamine oxidation
[69]

Pd/graphene
Sonoelectrochemical and chemical-reduction
Glucose oxidation
[70]

PdO/graphene
Cyclic voltammetry (CV) and chronoamperometry (CA)
Ethanol oxidation
[71]

Graphene-supported palladium
—
Formaldehyde oxidation
[72]

2015
PtAg bimetallic alloy
Coreduction
Methanol oxidation
[73]

Pt/carbon xerogel and Vulcan carbon
Impregnation, microemulsion
Methanol oxidation
[74]

Nickel supported on nitrogen-doped carbon nanotubes
Hydrothermal
Hydrogen oxidation
[27]

Lanthanum-based perovskite supports for AuPt nanoparticles
SAS precipitation
Glycerol oxidation
[75]

2014
Au/MnO
Photochemical, electrochemical
Water oxidation
[76]

Pd-Cu nanoalloy
Soft chemical method
Methanol oxidation
[77]

Cu (II) functionalized Fe₃O₄
—
Sulfides and thiols
[78]

Pd-Cu bimetal
Coreduction
Ethanol oxidation
[79]

Pd nanohollow/Pt nanorod core/shell composite
Multistep crystalline growth
Methanol oxidation
[80]

2013
Au/Mg(OH)₂
Deposition
Carbon monoxide
[81]

Au/Al₂O₃, Au/C
Dispersion
Glucose oxidation
[82]

Au/Pt bimetallic nanoparticles
Dispersion, reduction
Glucose oxidation
[83]

Au/CuO
Coprecipitation
Carbon monoxide
[84]

2012
Au/C
Incipient wet impregnation
Glucose oxidation
[85]

Au/SiO₂
Stöber method
Cyclohexene and d-glucose oxidation
[86]

Au-Cu/SiO₂
Two-step method
Alcohol
[87]

Au-Pd/MgO
Sol immobilization, adsorption-reduction
Benzyl alcohol oxidation
[88]

2011
Pd-Ni electrocatalysts
Nanocapsule
Ethanol oxidation
[89]

2010
PtBi/C electrocatalysts
Borohydride reduction
Ethanol electrooxidation
[90]

PdIr/C
Simultaneous reduction
Ethanol oxidation
[91]

PtSn/C-Rh, PtSn/C-CeO
Alcohol reduction, polymeric precursor
Ethanol oxidation
[92]

2009
Gold nanoparticles
Reduction
Glucose and 1-phenyl ethanol
[93]

Supported gold nanoparticles
—
Silanol oxidation
[94]

2008
Metalloporphyrin and cobalt acetate
Condensation
p-Xylene oxidation
[95]

Au/TiO₂
Deposition precipitation
Alcohol oxidation
[96]

2007
Gold catalysts
—
Alcohol oxidation
[97]

CuO/mesoporous silica
Impregnation
Benzene oxidation
[98]

Supported gold catalysts
Deposition precipitation
Alcohol oxidation
[99]

2005
Gold with anionic ligand
Precipitation
Alcohol oxidation
[100]

3.2.1. Nanohybrid SiO₂/Al₂O₃ Support

Recently, the nanohybrid SiO₂/Al₂O₃ support is used for the synthesis of various metal complexes such as Mn [28, 45, 46], Fe [47], and Co [29, 48, 49] catalysts for catalytic oxidation of ethylbenzene (Table 3). Arshadi and Ghiaci [44] synthesized nanosized SiO₂-Al₂O₃ mixed oxide supports and functionalized it with 3-aminopropyl-triethoxysilane (3-APTES) and 2-aminoethyl-3-aminopropyltrimethoxysilane (2-AE-3-APTMS) linkers (Figure 2). Thus, functionalized oxide was further functionalized with Schiff base by conjugating it to Mn(OAc)₂ to fabricate immobilized Mn catalyst complex. This heterogeneous Mn catalysts exhibited 67% ethylbenzene conversion along with 93% selectivity towards acetophenone at 80°C using TBHP (tert-butyl hydroperoxide) as an oxidant in the absence of any solvent. Arshadi et al. [29] further prepared Cobalt(II) Schiff base complexes immobilized onto SiO₂-Al₂O₃ mixed oxide supports combining two diverse linkers, 3-APTES and 2-AE-3-APTMS (Figure 2). This heterogeneous Cobalt(II) complexes resulted in 86% conversion of ethylbenzene with 99% selectivity toward acetophenone.

Table 3: Oxidation of alkylbenzene by SiO₂/Al₂O₃ supports.

Name of catalysts
Substrate
Oxidant
Reaction time (h)
Reaction temperature (°C)/solvent
Conversion (%)
Selectivity (%)
Ref.

SiO₂/Al₂O₃-APTMS-BPK-Mn, NHPI
Ethylbenzene
O₂
8
100/acetic acid
53
74
[45]

SiO₂/Al₂O₃-APTMS-BPK-Co(II), NHPI
O₂
8
100/acetic acid
81
98
[45]

Nanohybrid SiO₂/Al₂O₃ supported cobalt, NHPI
O₂
8
100/acetic acid
64
82
[48]

Nanohybrid SiO₂/Al₂O₃ modified Fe nanocatalysts
TBHP
24
50/solvent free
40
89
[47]

Nanohybrid SiO₂/Al₂O₃ supported cobalt
TBHP
24
100/solvent free
47
79
[49]

Mn nanocatalysts modified on nanohybrid SiO₂/Al₂O₃
TBHP
24
100/solvent free
67
84
[28]

SiO₂-Al₂O₃ mixed oxide immobilized Mn catalysts
TBHP
24
80/solvent free
67
93
[44]

SiO₂-Al₂O₃ mixed oxide immobilized cobalt catalysts
TBHP
24
80/solvent free
86
99
[29]

Mn supported on SiO₂-Al₂O₃, scCO₂
TBHP
24
120/CO₂
86
88
[46]

NHPI = N-hydroxyphthalimide; BPK = bipyridylketone; APTMS = trimethoxysilylpropylamine; SC = supercritical.

Figure 2: Immobilization of metal (Mn/Co) ion onto functionalized SiO₂/Al₂O₃ using linker molecule. Adapted with permission from Elsevier [29, 44].

In 2012, Arshadi et al. [46] synthesized Mn catalysts on modified SiO₂-Al₂O₃ mixed oxide supports using 2-AE-3-APTMS (Figure 3); this performed oxidation under mild conditions with a lower oxidation potential and charge-transfer resistance but leads to a greater conversion (91%) and better selectivity (98%) in the presence of supercritical carbon dioxide under solvent-free atmosphere. The catalysts were reused for eight times with a minimum loss of activity. In another instance, Habibi and Faraji [28] synthesized heterogeneous Mn nanocatalysts anchored on SiO₂-Al₂O₃ hybrid supports using a bidentate ligand of nitrogen atoms (Figure 3). The catalysts showed remarkable activity in the oxidation of ethylbenzene (conversion rate 67% and selectivity 84%) in the absence of any chemical solvent. On the other hand, Co(II) nanocatalyst was prepared by attaching of cobalt ions on inert bipyridylketone over the nanohybrid SiO₂/Al₂O₃ mixed oxides (Figure 3) [49]. The catalytic oxidations of the prepared nanocatalyst towards ethylbenzene were assessed with TBHP as an oxidant in the absence of any solvent. Under optimal conditions, the nanocatalyst showed 79% selectivity towards the acetophenone with 47.2% conversion. Habibi and coworkers prepared another catalyst by immobilizing cobalt ion on SiO₂-Al₂O₃ support (Figure 3) [48]. This performed ethylbenzene oxidation in N-hydroxyphthalimide (2-hydroxy-1H-isoindole-1,3-dione (NHPI)) with 82% selectivity in an oxygen atmosphere and acetic acid solvent at 100°C. Very recently, SiO₂-Al₂O₃-APTMS-BPK-Mn(III) and SiO₂-Al₂O₃-APTMS-BPK-Co(II) catalysts were synthesized (Figure 3); these carried out the ethylbenzene oxidation in NHPI without using any reducing agent under an oxygen atmosphere. Conversion rates were 53% and 81% with selectivities 74% and 98% towards acetophenone, respectively [45].

Figure 3: Synthetic process of Si/Al-APTMS-BPK-Mn/Co catalyst. Adapted with permission from Elsevier [28, 48].

A novel and very simple Fe nanocatalyst on a modified nanoscale SiO₂₋Al₂O₃ (Figure 4) was studied for alkylbenzene oxidation [47]. Under optimal environments (substrate to the TBHP ratio (1 : 1), in the absence of solvent, at 50-120°C and 24 h reaction time), an Fe nanocatalyst exhibited 40% conversion and 89% selectivity.

Figure 4: Immobilization of Fe on nanohybrid SiO₂/Al₂O₃. Adapted with permission from Elsevier [47].

3.2.2. Silica (SiO₂) Support

Silica has achieved a great interest for many catalysts; this is due to their three-dimensional open-pore network structures, high surface to volume ratio, high reusability, and distinct optoelectronic and physiochemical properties; these provide well dispersion of metal nanoparticles and facilitate the transport of molecules, ions, or electrons through the nanopores/nanochannels, enhancing product yields with minimum cost and time. Mal and Ramaswamy [25] reported the synthesis and catalytic activity of three different metals (Ti, V, or Sn) on silica supports (a new hydrophobic crystalline silica molecular sieve) using hydrogen peroxide as an oxidant at 60-80°C [101]. Of the three metallosilicates, Sn-silicalite-I was very reactive with H₂O₂, accounting for 60% catalytic efficiency. On the other hand, TS-1 (Si/Ti) and VS-1 (Si/V) demonstrated only 36.2% and 20.10% conversion, respectively. These catalysts oxidize ethylbenzene in two different ways: first, by hydroxylating of arene at para-position and some extending to ortho-position and, second, by adding oxygen at the side chain of primary and secondary (α- and β-) carbon atom; the corresponding carbinols (primary/secondary), which result from the side chain oxidation, further undergo to yield aldehyde or ketone. Normally, the oxidation at β-carbon dominates over the α-carbon. In case of TS-1, the oxidation does not occur at α-carbon. On the other hand, both positions are oxidized by VS-1 and Sn-silicalite-1. These hydroxylation reactions proceed an ionic mechanism onto TS-1 and TS-2 surfaces [102, 103]. Nonetheless, the product distribution reveals that the side chain product is almost 4 to 5 times higher than that of aromatic ring oxidation. Ghiaci et al. [104] immobilized Mn(III) porphyrin complexes [Mn(TMCPP)][TMCPP:5,10,15,20-tetrakis-(4-methoxycarbonylphenyl)-porphyrin] onto organo-functionalized silica gel (Figure 5). This catalyst results in 40.8% conversion but 96.6% selectivity in the liquid phase oxidation of ethylbenzene using TBPH as oxidant and without any solvent at 150°C. They further tested the effect of reaction time and found the catalysts exhibit maximum activity in 24 hours.

Figure 5: Metalloporphyrin metal ion covalently bound to silica. Adapted with permission from Elsevier [104].

On the other hand, Rajabi and his colleagues [105] successfully prepared and employed silica supported Cobalt(II) salen complex (Figure 6); cobalt acetate was used as a source of Cobalt(II) ion, for the aerobic oxidation of ethylbenzene in presence NHPI at atmospheric pressure. The catalysts were recycled for at least four times, and in the first cycle, 78% product yield and 91% selectivity were realized.

Figure 6: Preparation of silica supported Cobalt(II) catalyst. Adapted with permission from Elsevier [105].

Biradar and Asefa [23] have stated the preparation method of gold nanoparticles as efficient catalysts for alkylbenzene oxidation by reducing Au(III) ions onto mesoporous silica functionalized by hemiaminal reducing agents (Figure 7). The supported nanoporous gold demonstrated efficient catalytic action for the oxidation of diverse range alkyl benzenes as well as linear alkanes in the presence of a TBHP oxidant. It provided unprecedented conversion (∼99%) and selectivity (∼100%) toward carbonyl ketones under mild conditions. They also found that the mesoporous silica supported gold nanocatalysts exhibit the highest activity for ethylbenzene oxidation in acetonitrile followed by THF, ethyl acetate, and toluene, showing that the polar solvents have positive impact on polarity and/or dielectric constant of the reaction intermediates. Moreover, certain solvents outperform others by undergoing a cooxidation process which results in a more powerful oxidizing agent in the course of the reaction.

Figure 7: General procedure for the synthesis of silica-supported metal catalysts.

Anand et al. [26] synthesized four different types of crystalline Ag nanoparticles by impregnating silica with aqueous silver nitrate (Figure 7) and subsequent evaporation at 100°C. The crystalline Ag nanoparticles of size 37 nm showed maximum conversion (92%) and selectivity (99%) towards acetophenone in the absence of any solvent at 90°C. Cobalt(II) Schiff base complexes with modified silica were prepared by refluxing silica gel with 3-aminopropyl-trimethoxysilane in dry dichloromethane wherein the silica was liganded with Co(CH₃COO)₂·4H₂O [106]. The catalysts exhibited 98% conversion and 99% selectivity towards ketone products in the presence of NHPI under an O₂ atmosphere. Neeli et al. [24] prepared Cu/SBA-15 catalysts by loading Cu via impregnation wherein Cu(NO₃)₂·3H₂O is the metal source (Figure 7). At 10% Cu loading, the maximum conversion (94%) and selectivity (99%) to acetophenone under solvent-free condition were achieved at 90°C.

In another instance, Dan-Hua et al. [36] immobilized manganese porphyrin onto silica nanoparticles on Fe₃O₄ solid matrixes. The catalysts become active upon the removal of the hard template of the silica supports. Metalloporphyrin was fixed onto the inner surface of hollow microspheres allowing a substrate to diffuse onto grafted manganese porphyrin through the pore of the silica shell. The catalysts were recycled for six times with the retention of high activity and stability. Bhoware et al. [107] prepared cobalt nanocatalysts onto hexagonal mesoporous materials (Co-HMS and Co/HMS) by grafting various cobalt contents via hydrothermal and postsynthesis methods. The catalysts exhibited good activity for liquid-phase ethylbenzene oxidation in the presence of the TBHP oxidant wherein H₂O₂ was inactive under solvent-free condition accounting for 49.5% and 39.0% by Co-HMS and Co/HMS, respectively, in a 24-hour reaction at 80°C; Co/HMS catalysts exhibited greater selectivity (59%) towards acetophenone. Sujandi et al. [108] immobilized Co(III) ion onto cyclam (macrocyclic ring, Scheme 2) complexed to functionalized SBA-15 with a chloropropyl group through surface substitution reaction. This cyclam group deposits Cobalt(II) into its cavity that facilitates ethylbenzene oxidation with better conversion efficiency (60%). The presence of a pyridine group to the axial site of Co(III) cyclam composite was further investigated, conferring that this group enhances the ethylbenzene conversion by 10% without losing the selectivity towards acetophenone. Major silica-based catalysts for ethylbenzene oxidation are summarized in Table 4.

Scheme 2

Table 4: Oxidation of alkylbenzene by SiO₂ supports.

Name of catalysts
Substrate
Oxidant
Reaction time (h)
Reaction temperature (°C)/solvent
Conversion (%)
Selectivity (%)
Ref.

Ti, V & Sn containing silicalite molecular sieves
Ethylbenzene
H₂O₂
24
80/tert-butanol, acetone, water
—
—
[25]

Silica gel supported cobalt, NHPI
O₂
24
100/acetic acid
98
99
[106]

Cu/SBA-15
TBHP
5
90/solvent free
94
99
[24]

Metalloporphyrin@SiO₂
O₂
—
100/solvent free
16
74
[36]

Ag/SBA-15
TBHP
5
90/solvent free
92
99
[26]

Au/SBA-15
TBHP
36
70/acetonitrile
79
93
[23]

Silica supported cobalt, NHPI
O₂
<12
100/acetic acid
78
91
[105]

SF-ATPS-Mn(III)TMCPP
TBHP
24
150/solvent free
40
96
[104]

CO/HMS

TBHP
24
80/solvent free
49
60
[107]

Co (III) cyclam functionalized mesoporous silica

Air flow
8
-/acetonitrile
20
60
[108]

Mn-metformin complex on modified magnetic SiO₂@Fe₃O₄ core/shell

O₂
8
100/acetic acid
85
98
[109]

TMCPP: 5,10,15,20-tetrakis(4-methoxy carbonyl phenyl) porphyrin; AMTS = aminopropyl-trimethoxysilane.

3.2.3. Miscellaneous

Apart from these, a good number of catalysts are being used for ethylbenzene oxidation (Table 5). Rebelo et al. [110] synthesized and studied the activity of five different types of Mn(III) porphyrin complexes in ammonium acetate as cocatalysts. Among these, Mn(β-NO₂TDCPP)Cl provided the highest conversion and selectivity due to the presence of a nitro group. The cocatalysts for hydrogen peroxide activation included the buffering substances, i.e., ammonium acetate [111], imidazole [112], and pyridine plus benzoic acid [113]. However, the evidence of pyridine oxidation was also observed [114]. Xavier et al. [115] reported Y-zeolite supported Co(II), Ni(II), and Cu(II) centers of dimethylglyoxime and N,N-ethylenebis (7-methylsalicylideneamine) which were prepared in situ by reaction of ion-exchanged metal ions with disulfide flexible ligands. However, Cu(II)-zeolite complexes demonstrated maximum efficiency wherein the reactivity of the complex is believed to be provided by the geometry of encapsulated molecules as well as the steric condition of active sites. The supported zeolite composites are highly stationary to be recycled and are apt to be used as catalysts for partial oxidation. Choudhary et al. [116] investigated the catalytic effect of MnO₄⁻¹-exchanged Mg-Al hydrotalcite which is a stable and green catalyst for the oxidation of a methylene group, covalently attached to an aromatic ring under an oxygen atmosphere. They found that the activity of methylene-to-carbonyl conversion by MnO₄⁻¹-exchanged hydrotalcite, the decomposition of H₂O₂, and the basicity of Mg-Al hydrotalcite rises with the raising Mg/Al ratio in the catalyst and the Mg/Al ratio at 10; the highest catalytic activity as well as selectivity (above 95%) was obtained for ethylbenzene oxidation to acetophenone and diphenylmethane to benzophenone. These reactions were fully heterogeneous, but no leaching of the active component(s) from the catalyst was observed. The recycled catalyst exhibited good performance after its first use in the oxidation reaction.

Table 5: Oxidation of ethylbenzene with various catalytic supports materials.

Name of catalysts
Substrate
Oxidant
Reaction time (h)
Reaction temperature (°C)/solvent
Conversion (%)
Selectivity (%)
Ref.

Manganese (III) porphyrin, ammonium acetate
Ethylbenzene

5.5
Room temperature/acetonitrile
66
66
[110]

Zeolite-encapsulated Cu (II)
H₂O₂
8
70/benzene
46
66
[115]

MnO 4-1 exchanged Mg-Al hydrotalcite
O₂
5
-/solvent free
22
98
[116]

Cu(tacn)(ClO₄)₂
TBHP
10
60/acetonitrile
49
91
[117]

Ni/Al hydrotalcites, CO₃²⁻
O₂
5
135/solvent free
47
99
[118]

Vanadia/ceria
H₂O₂
6
60/acetonitrile
20
72
[119]

Cobalt(II)(5,10,15,20-tetrakis (pentafluorophenyl))porphyrin
O₂
24
100/solvent free
38
94
[120]

Hemin/NHPI
O₂
9
100/acetonitrile
92
94
[121]

Supported nickel
O₂
5
150/solvent free
20
80
[122]

Metal-doped HS-ALF₃
TBHP
6
60/acetonitrile
70
72
[125]

Macrocyclic copper (II) complex
TBHP
10
60/acetonitrile
62
88
[126]

DAEP-bentonite-Pd (II)
TBHP
24
80/solvent free
92
93
[127]

Ni substituted copper chromite spinel
TBHP
8
70/acetonitrile
56
68
[128]

Vanadium complex/NHPI system
O₂
12
90/Benzonitrile
69
97
[142]

Mesoporous Cu-ZrPO
THBP
24
80/Benzonitrile
91
87
[145]

Immobilized bidentate Schiff base oxovanadium(IV) complex
O₂
14
110/solvent free
~40
98
[143]

Fe@CNT
O₂
3
155/acetonitrile
36
60
[144]

Supported Co₄HP₂Mo₁₅V₃O₆₂
H₂O₂
—
70/glacial acetic acid
72
95
[146]

μ-Oxo dimeric metalloporphyrins
O₂
2
65/solvent free
91
99
[147]

Cobalt-supported catalysts on modified MNPs
O₂
10
100/ethanol
88
98
[148]

Carbon nanotube
O₂
4
155/acetonitrile
40
62
[149]

Mn/N-C/Al₂O₃
O₂
6
120/solvent free
27
99
[150]

Mesosubstituted pyrazolyl porphyrin complexes
TBHP

80/water
99
99
[151]

Tacn = triazacyclononane; HMS = hexagonal mesoporous materials; MNPs = magnetic nanoparticles.

Bennur et al. [117] synthesized copper tri- and tetra-aza macrocyclic complexes by encapsulating Y-type zeolite. The “neat” and encapsulated complexes showed noble performance in ethylbenzene oxidation at 60°C using TBHP as an oxidant. While the encapsulated complexes showed enhanced selectivity towards acetophenone, a small quantity of o- and p-hydroxyacetophenones was also yielded, reflecting that C-H bond activation takes place both at benzylic and at aromatic ring carbon atoms. It is inferred that ring hydroxylation takes place more over the “neat” complexes than over the encapsulated complexes. This difference is due to the formation of various types of “active” copper-oxygen intermediates, such as bis-μ-oxo complexes and Cu-hydroperoxo species, at different proportions over the “neat” and encapsulated complexes. In 2006, Jana et al. [118] prepared different NiAl hydrotalcites by a conventional precipitation technique using Ni/Al at molar ratios of 2-5 in guest inorganic anions such as CO₃²⁻ and Cl⁻; these carried out the liquid-phase oxidation of the methylene group of ethyl-substituted benzene to acetophenone under an atmospheric oxygen as the sole oxidant in a solvent-free system at 135°C. In the presence of CO₃²⁻ anion and Ni/Al ratio 5 mol mol⁻¹, it showed higher activity for ethylbenzene oxidation with 99% selectivity towards acetophenone than those prepared using Cl⁻, NO₃⁻, or SO₄²⁻ anions. Other hydrotalcite congaing transition-metal solid catalysts such as CuAl-, ZnAl-, CoAl-, MgFe-, and MgCr- demonstrated higher activity than that of NiAl hydrotalcite. However, the active NiAl hydrotalcite presented better performance in the oxidation of a variety of alkylaromatics to their corresponding benzylic ketones under similar reaction conditions. Additionally, the preparation of NiAl hydrotalcite is very cheap and stable using commercially available reagents.

Radhika and Sugunan [119] impregnated ammonium metavanadate in oxalic acid solution to prepare vanadia/ceria catalysts (2-10% of V₂O₅) and carried out liquid-phase oxidation of ethylbenzene with H₂O₂. It was found that the activity was increased with loading of V₂O₅ up to 8%; however, it decreased after V₂O₅ content (10%). In some instances, catalytic activity increases even after vanadia loading beyond 10%, but selectivity towards acetophenone decreases. Product analysis indicated that when vanadia loading higher than 6%, it oxidizes the acetophenone to 2-hydroxyacetophenone. XRD and FT-IR analysis revealed the existence of extremely dispersed vanadia at lower loading, but formation of CeVO₄ when vanadia loading exceeded to 10% V₂O₅. Vanadia exhibits tetrahedral properties at lower loading, but it forms Ce-O-V species onto the support surface; this exhibited the existence of highly dispersed tetrahedral species at lower loading but agglomeration at the higher extremes. Benzaldehyde production predominates with Ce-O-Ce but acetophenone is the major product with V-O-V structure.

Li et al. [120] described the oxidation process of ethylbenzene with fluorinated metalloporphyrins under a normal environment without any additives. They synthesized three different types of [5,10,15,20-tetrakis(pentafluorophenyl)] porphyrin with Fe, Mn, and Co at the centers, and Co(II)(5,10,15,20-tetrakis(pentafluorophenyl))porphyrin was found to be the ideal metalloporphyrins; these exhibited 38.6% conversion for ethylbenzene 94% selectivity towards acetophenone along with 2719 turnover numbers under optimal conditions (at 100°C and 24 h). Ma et al. [121] investigated the selective oxidation of ethyl substituted benzene with iron-containing hemin (a biomimetic system of catalysts) with molecular oxygen in acetonitrile. They reported 90.32% conversion of ethylbenzene and 94.30% selectivity toward acetophenone at 100°C under 0.3 MPa O2 for 9 h. In situ formed 1-phenylethyl hydroperoxide (PEHP) can easily be decomposed via a hemolytic cleavage by Hemin complex to acetophenone.

Raju et al. [122] studied the aerobic oxidation of ethylbenzene without any solvent or any other additives using nickel on various supports such as SiO2, hydroxyapatite, SBA-15, and USY zeolites prepared by impregnation. Hydroxyapatite- and USY- (13% Na2O) supported nickel catalysts showed better conversion and selectivity toward acetophenone due to the presence of a proper amount of comparatively weak acid sites which accelerate the formation of a major product [123], whereas the strong acid sites fasten the formation of side products [124]. Accordingly, the sample having a greater number of intermediate strength acidic points presented high conversion with reasonable selectivity to acetophenone in comparison to samples having a greater number of strong acidic points. It is also identified that additional acidic catalysts usually favor the breaking of hydrocarbons, thereby expediting the formation of more undesirable by-products and coke that lead to the deactivation of catalysts.

In 2008, Murwani et al. [125] synthesized pure and doped high surface aluminum fluorides (HS-AlF3) with various reactive metals like iron (Fe), vanadium (V), manganese (Mn), and niobium (Nb) by means of sol-gel fluorination. Of the four different types of reactive metals, V and Fe-doped aluminum fluoride produced the best results in ethylbenzene conversion and selectivity towards acetophenone at 60°C and solvent to TBHP ratio 1 : 3 since these two metals are rich with surface Lewis acid sites having abundant chemisorption capability to oxygen. It was also found that acetonitrile is the standard solvent. Mn-doped sample contained little quantity of medium-weak Lewis acid sites; however, Nb-doped aluminum fluoride exhibited very high concentration of Bronsted acid sites, on the educts that could not be necessarily activated. The functions of acid sites include (a) the activation of the tert-butyl hydroperoxide on doped metal fluorides acting as Lewis acid and redox center and (b) the yielding of acetophenone from ethylbenzene as a major product. Unfortunately, vanadium ion containing HS-AlF3 which has a leaching effect did not show ethylbenzene conversion under reaction condition.

Salavati-Niasari [126] encapsulated copper(II) complexed with twelve-membered cyclic ligands containing three contributing atoms (N₂O₂, N₂S₂, and N₄) in macrocyclic ring in zeolite-Y nanocavity with a flexible ligand method in a two-step liquid phase reaction. This, first, adsorbs the ligand source, 1,2-di(o-aminophenyl-, amino, oxo, thio)ethane, N₂X₂ in the supercages of the Cu(II)-NaY and, finally, condenses the Cu(II) precursor complex [Cu(N₂X₂)]²⁺ with glyoxal or biacetyl. Good catalytic action (58.2%) with high selectivity was found in ethylbenzene oxidation by zeolite encapsulated ligand complexes at 60°C using TBHP oxidant; this is because the encapsulated complexes ensure uniform dispersion of metal complexes inside the nanoporous support which gives the structural integrity. The zeolite structure can retain the visitor multiplexes dispersed and inhibit their dimerization.

In 2010, Ghiaci et al. [127] synthesized the palladium nanotubes as well as nanoparticles onto bentonite (an absorbent) modified with 3,3-(dodecylazanediyl)-bis-(N-(2-(2-aminoethylamino)-ethyl)propanamide) (DAEP) having an aliphatic tail (C-12) and a hydrophilic head. This modified bentonite, called DAEP-bentonite, was operated as a nanoreactor for the synthesis of Pd²⁺ and Pd⁰ nanoparticles. They carried out oxidation of ethylbenzene and found that Pd²⁺ on functionalized bentonite along with cetylpyridinium bromide and DAEP showed higher activity compared to Pd(0) onto identical support materials under similar reaction condition.

George and Sugunan [128] synthesized five different types of spinels, namely, CCr, CNCr-1, CNCr-2, CNCr-3, and NCr depending on Cu, Ni, and Cr by a coprecipitation method with the use of three consecutive nitrates such as copper nitrate, nickel nitrate, and chromium nitrate. In the liquid-phase oxidation of ethylbenzene, CNCr-2 resulted in the maximum conversion (56.1%) and selectivity (68.7%) under the same reaction condition. They also tested the efficiency of various solvents on catalytic activity and found better product in the absence of a solvent. A mechanistic scheme revealed chromite would be a convenient and ecofriendly alternative for hazardous oxidants. NHPI can efficiently improve the aerobic oxidation of hydrocarbon by combining with various mediators such as metal compounds [129–132], hemin [121], oximes [133], anthraquinones [134], o-phenanthroline [135], azobisisobutyronitrile (AIBN) [136], Ce(IV) [137], alkaline-earth chlorides [138], I₂/HNO₃ [139], NO2 [140], and quaternary ammonium salts [141].

Qin et al. [142] investigated the conciliation effect of vanadium complexes on ethylbenzene oxidation using N-hydroxyphthalimide (NHPI) at 90°C in benzonitrile. Of the vanadium mediators used, a sequence of oxobis (8-quinolinolato) vanadium(IV) complexes synthesized by coordination of 8-hydroxyquinoline or its derivatives with oxobis (2,4-pentanedionate) vanadium(IV)(VIVO(acac)₂) exhibited a better mediation effect compared to VIV O(acac)₂·NH₄VO₃ and V₂O₅ giving 60-69% conversion of ethylbenzene and 97% selectivity towards ketone product under optimum reaction condition because of the dual effect of vanadium mediators on NHPI transformation to phthalimide-N-oxyl(PINO) radical as well as the breakdown of 1-phenylethyl hydroperoxide to acetophenone [142].

A new immobilized bidentate Schiff base oxovanadium(IV) complex was prepared [143] using chloromethylated crosslinked polystyrene microspheres (CMCPS microspheres), a starting carrier. First, the chloromethyl group of CMCPS microspheres was transferred to the aminomethyl group through Delépine reaction with a hexamethylenetetramine (HMTA) reagent forming aminomethylated (AM) microspheres (AMCPS). Secondly, the Schiff base reaction between the primary amino group of AMCPS and salicylaldehyde (SA) resulted in Schiff base-type resin microspheres (SAAM-CPS) on which bidentate Schiff base ligand SAAM were chemically attached. The subsequent coordination between the ligand SAAM of SAAM-CPS micropores and vanadyl sulfate (VOSO₄) formed heterogeneous oxovanadium(IV) complex catalyst, chemically immobilized bidentate Schiff base-type oxovanadium(IV) complex, and CPS-VO(SAAM)₂ microspheres. This complex efficiently carried out oxidation of ethylbenzene under mild conditions with excellent reusability.

Luo et al. [144] prepared an iron nanowire-filled carbon nanotube (Fe@CNT), a magnetic separable heterogeneous catalyst by chemical vapor deposition. Selective oxidation of ethylbenzene showed that iron nanowire competently improved CNTs activity by accelerating electron transfer in dioxygen. Besides, Fe@CNTs could be recycled after consecutive six cycles with no loss of its catalytic activity simply by applying external magnetic force. Miao et al. [145] synthesized a sequence of mesoporous Cu-ZrPO (M-Cu-ZrPO) catalysts for liquid-phase ethylbenzene oxidation with a surface area of ~200 m²/g, uniform pore size of ~7.8 nm, and various copper content (0-30%) by facile one-pot evaporation. They stated that M-Cu-ZrPO can retain its thermal stability, reusability (more than five cycles), and ordered mesostructure even after heating at 700°C. Due to its stability, the activity of M-Cu-ZrPO steadily increased with raising copper contents up to 30% with conversion 91.2% and selectivity 87% towards acetophenone.

Tang et al. [152] synthesized and looked into the effect of several N-doped graphene in ethylbenzene oxidation. In all these reactions yielded acetophenone as the major products and little amount of benzaldehyde and benzoic acid as by-products. In the N-doped graphene catalytic system, it is not only the nitrogen but also the graphitic nitrogen catalyzes the conversion of ethylbenzene to acetophenone because the graphitic nitrogen is liable for TBHP activation [153]. However, too much N-doped graphene demonstrated an adverse effect on the activity. Usually, at high temperature, N-doped graphene exhibited good catalytic activity in comparison to those reacted at low temperature. Tang et al. also correlated the total nitrogen content and N-doped graphite in yielding of acetophenone (Table 6).

Table 6: Activity of graphitic nitrogen materials in selective oxidation of ethylbenzene [152].

Entry
Sample
Acetophenone (%)
Nitrogen element (%)
Oxygen element (%)
Graphitic nitrogen (%)

1
Ac-250
55.9
0.59
11
0.21

2
Ac-450
84.1
4.8
7.9
1.2

3
Ac-650
87.9
3.8
4.0
1.4

4
Ac-850
81.6
6.1
3.0
3.4

5
Ac-850
49.9
6.1
3.0
3.4

6
Am-250
55.8
2.0
9.5
0.18

7
Am-450
86.5
2.6
7.9
0.35

8
Am-650
93.2
4.6
3.9
0.77

9
Am-850
57.9
5.2
4.6
1.1

10
Am-850
81.9
5.2
4.6
1.1

Reaction conditions: 1 mmol ethylbenzene, 3 mmol TBHP, 10 mg catalyst, and 3 mL H₂O were put into a 50 mL sealed pressure glass vessel with magnetic stirring (80°C (65°C)) 24 h.

Very recently, Yao et al. [151] reported the synthesis of three novel catalysts (CuPp, MnPp, and ZnPp) by solvothermal methods and measured their catalytic activity in terms of alkylbenzene conversion. The catalysts exhibited better activities for ethylbenzene oxidation and selectivity towards acetophenone. These catalysts can be recycled by simple filtration with no loss of catalytic ability and selectivity. Among these three catalysts, MnPp showed the highest catalytic ability and the selectivity (99%). Although the conversion rates of CoPp and CuPp catalysts were slightly lower, the selectivity exceeded more than 98%. On the other hand, ZnPp exhibited low catalytic ability in ethylbenzene conversion (40%).

4. Current Approach for Oxidation Reaction

Recently, scientists are paying more attention to use the catalysts especially heterogeneous for oxidation reactions some of them have been summarized in Table 2.

5. Future Prospects

Several heterogeneous metal complexes are available for the oxidation of alkylbenzenes at high temperatures (>300°C), but the majority of the systems are not suitable for industrial conversions since the reactive intermediates are converted to various by-products that incur additional purification cost. Besides, the product distribution further depends on several factors. The profit-making interest in industrial catalysis of ethylbenzene to carbonyl compounds is a priori and must receive due interest in recent catalytic chemistry. The key barrier is the harsh reaction condition, and hence, designing a catalytic system with low cost and readily available selectivity has been challenging. The innovation of a novel method and modification of the existing techniques having clear advantages will continue to receive attention in catalyst research.

6. Conclusions

The continuous importance of aerobic oxidation of the alkyl substituted benzene to its corresponding ketone has inspired researchers to develop an efficient, green, and novel catalyst. Homogeneous catalysts have been investigated for high selectivity and conversion rate for the ethylbenzene oxidation. However, the reusable features of the supported metal catalysts have got wider acceptability even though the catalytic ability of the heterogeneous catalysts is still lower than the homogeneous ones. Lots of improvements have been made in the development of SiO₂-Al₂O₃- and SiO₂-based catalytic system along with various metals and organic/semiorganic linkers, and the selectivity of these systems has been demonstrated with various activators as building units. The Au/SiO₂ systems have made numerous progresses in the aerobic oxidation of alkylbenzene. Apart from this, SiO₂-Al₂O₃-based catalysts have presented much reactivity and selectivity in a variety of oxidation processes. Because of the substantial improvements in product yield and catalyst reusability, heterogeneous catalysts have gained growing consideration in the recent years. The wider availability along with various physical features and porosities of several supports (e.g., mesoporous carbon hydroxyapatite, mesoporous silica, and microporous zeolite) attracts think-tank to design and generate catalytic systems as well as to explore their oxidation scheme. Porous supports along with channels and well-defined cages offer a nanoreactor environment, which can present shape selectivity for substrates, products, and transition states.

Conflicts of Interest

The authors confirm that this article content has no conflicts of interest regarding the publication of the journal.

Authors’ Contributions

This work was carried out in collaboration with all authors. All the authors read and approved the final manuscript.

Acknowledgments

This research was funded by the Deanship of Scientific Research at Princess Nourah bint Abdulrahman University through the Fast-track Research Funding Program.

References

B. Trost, “Conprehensive organic synthesis: selectivity, strategy & efficiency in modern organic chemistry,” Pergamon Press, vol. 3, 1991.

R. C. Larock, “Comprehensive organic transformations : a guide to functional group preparations,” vol. 68, Article ID A86, 1999.

J. K, “Industrial organic chemistry,” Nature, vol. 141, no. 3561, p. 211, 1938.

T. Mehler, W. Behnen, J. Wilken, and J. Martens, “Enantioselective catalytic reduction of acetophenone with borane in the presence of cyclic α-amino acids and their corresponding β-amino alcohols,” Tetrahedron: Asymmetry, vol. 5, no. 2, pp. 185–188, 1994.

R. T. Blickenstaff, W. R. Hanson, S. Reddy, and R. Witt, “Potential radioprotective agents--VI. Chalcones, benzophenones, acid hydrazides, nitro amines and chloro compounds. Radioprotection of murine intestinal stem cells,” Bioorganic & Medicinal Chemistry, vol. 3, no. 7, pp. 917–922, 1995.

M. E. Ali, M. M. Rahman, and S. B. A. Hamid, “Nanoclustered gold: a promising green catalysts for the oxidation of alkyl substituted benzenes,” Advanced Materials Research, vol. 925, pp. 38–42, 2014.

G. R. Newkome and D. L. Fishel, “Preparation of hydrazones: acetophenone hydrazone,” in Organic Syntheses, p. 102, John Wiley & Sons, Inc, Hoboken, NJ, USA, 2003.

S. L. Regen and C. Koteel, “Activation through impregnation. Permanganate-coated solid supports,” Journal of the American Chemical Society, vol. 99, no. 11, pp. 3837-3838, 1977.

F. M. Menger and C. Lee, “Synthetically useful oxidations at solid sodium permanganate surfaces,” Tetrahedron Letters, vol. 22, no. 18, pp. 1655-1656, 1981.

J. R. Holum, “Study of the chromium(VI) oxide—pyridine complex,” The Journal of Organic Chemistry, vol. 26, no. 12, pp. 4814–4816, 1961.

G. Cainelli and G. Cardillo, Chromium oxidations in organic chemistry; reactivity and structure concepts in organic chemistry, vol. 19, Springer Berlin Heidelberg, Berlin, Heidelberg, 1984.

D. G. Lee and U. A. Spitzer, “aqueous dichromate oxidation of primary alcohols,” The Journal of Organic Chemistry, vol. 35, no. 10, pp. 3589-3590, 1970.

J. Muzart, “Chromium-catalyzed oxidations in organic synthesis,” Chemical Reviews, vol. 92, no. 1, pp. 113–140, 1992.

A. Dijksman, A. Marino-González, A. M. i. Payeras, I. W. C. E. Arends, and R. A. Sheldon, “Efficient and selective aerobic oxidation of alcohols into aldehydes and ketones using ruthenium/TEMPO as the catalytic system,” Journal of the American Chemical Society, vol. 123, no. 28, pp. 6826–6833, 2001.

W. P. Griffith, “Ruthenium oxo complexes as organic oxidants,” Chemical Society Reviews, vol. 21, no. 3, p. 179, 1992.

A. J. Mancuso and D. Swern, “Activated dimethyl sulfoxide: useful reagents for synthesis,” Synthesis, vol. 1981, no. 3, pp. 165–185, 1981.

D. B. Dess and J. C. Martin, “Readily accessible 12-I-5 oxidant for the conversion of primary and secondary alcohols to aldehydes and ketones,” The Journal of Organic Chemistry, vol. 48, no. 22, pp. 4155-4156, 1983.

W. P. Griffith, S. V. Ley, G. P. Whitcombe, and A. D. White, “Preparation and use of tetra-n-butylammonium per-ruthenate (TBAP reagent) and tetra-n-propylammonium per-ruthenate (TPAP reagent) as new catalytic oxidants for alcohols,” Journal of the Chemical Society, Chemical Communications, no. 21, p. 1625, 1987.

M. Hudlicky, “Oxidations in organic chemistry,” American Chemical Society, vol. 186, p. 433, 1990.

M. Hudlicky, “Oxidations in organic chemistry,” Tech. Rep., Choice Reviews Online, USA, 1991.

A. Demirbas, “Biodiesel production via non-catalytic SCF method and biodiesel fuel characteristics,” Energy Conversion and Management, vol. 47, no. 15-16, pp. 2271–2282, 2006.

B.-Z. Zhan and A. Thompson, “Recent developments in the aerobic oxidation of alcohols,” Tetrahydron, vol. 60, no. 13, pp. 2917–2935, 2004.

A. V. Biradar and T. Asefa, “Nanosized gold-catalyzed selective oxidation of alkyl-substituted benzenes and n -alkanes,” Applied Catalysis A: General, vol. 435-436, pp. 19–26, 2012.

C. K. P. Neeli, A. Narani, R. K. Marella, K. S. Rama Rao, and D. R. Burri, “Selective benzylic oxidation of alkylaromatics over Cu/SBA-15 catalysts under solvent-free conditions,” Catalysis Communications, vol. 39, pp. 5–9, 2013.

N. K. Mal and A. V. Ramaswamy, “Oxidation of ethylbenzene over Ti-, V- and Sn-containing silicalites with MFI structure,” Applied Catalysis A: General, vol. 143, no. 1, pp. 75–85, 1996.

N. Anand, K. H. P. Reddy, G. V. S. Prasad, K. S. Rama Rao, and D. R. Burri, “Selective benzylic oxidation of alkyl substituted aromatics to ketones over Ag/SBA-15 catalysts,” Catalysis Communications, vol. 23, pp. 5–9, 2012.

Z. Zhuang, S. A. Giles, J. Zheng et al., “Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte,” Nature Communications, vol. 7, no. 1, p. 10141, 2016.

D. Habibi and A. R. Faraji, “Synthesis, characterization and application of a nano-manganese-catalyst as an efficient solid catalyst for solvent free selective oxidation of ethylbenzene, cyclohexene, and benzylalcohol,” Applied Surface Science, vol. 276, pp. 487–496, 2013.

M. Arshadi, M. Ghiaci, A. A. Ensafi, H. Karimi-Maleh, and S. L. Suib, “Oxidation of ethylbenzene using some recyclable cobalt nanocatalysts: the role of linker and electrochemical study,” Journal of Molecular Catalysis A: Chemical, vol. 338, pp. 71–83, 2011.

M. E. Ali, M. M. Rahman, S. M. Sarkar, and S. B. A. Hamid, “Heterogeneous metal catalysts for oxidation reactions,” Journal of Nanomaterials, vol. 2014, Article ID 192038, 23 pages, 2014.

J. R. H. Ross, “Heterogeneous catalysis–chemistry in two dimensions,” in Heterogeneous Catalysis, pp. 1–15, 2012.

X. Jin, J. Feng, S. Li et al., “A novel homogeneous catalysis-liquid/solid separation system for highly effective recycling of homogeneous catalyst based on a phosphine- functionalized polyether guanidinium ionic liquid,” Molecular Catalysis, vol. 475, p. 110503, 2019.

I. Fechete, Y. Wang, and J. C. Védrine, “The past, present and future of heterogeneous catalysis,” Catalysis Today, vol. 189, no. 1, pp. 2–27, 2012.

D. J. Cole-Hamilton, “Homogeneous catalysis--new approaches to catalyst separation, recovery, and recycling,” Science, vol. 299, no. 5613, pp. 1702–1706, 2003.

M. A. M. Hossain, M. E. Ali, S. B. Abd Hamid, and M. M. Rahman, “Catalytic oxidation of alkyl benzene,” Advances in Materials Research, vol. 1109, pp. 248–252, 2015.

S. Dan-Hua, J. Lin-Tao, L. Zhi-Gang, S. Wen-Bin, and G. Can-Cheng, “Ethylbenzene oxidation over hybrid metalloporphyrin@silica nanocomposite microspheres,” Journal of Molecular Catalysis A: Chemical, vol. 379, pp. 15–20, 2013.

P. Karthikeyan, P. R. Bhagat, and S. S. Kumar, “A novel, green 1-glycyl-3-methyl imidazolium chloride-copper(II) complex catalyzed C–H oxidation of alkyl benzene and cyclohexane,” Chinese Chemical Letters, vol. 23, no. 6, pp. 681–684, 2012.

G. L. Song, Y. Y. Liu, L. Chen, and H. J. Zhu, “Selective oxidation of alkyl benzenes using CrO₃ combined with Ce(SO₄)₂,” Asian Journal of Chemistry, vol. 22, pp. 7267–7274, 2010.

S. Yamazaki, “Chromium(VI) oxide-catalyzed benzylic oxidation with periodic acid,” Organic Letters, vol. 1, no. 13, pp. 2129–2132, 1999.

S. Habibzadeh and M. Tajbakhsh, “Oxidation of alkyl benzenes to corresponding carbonyl compounds with 1,4-dichloro-1,4-diazoniabicyclo [2,2,2] octane bis-chloride in water,” Journal of Chemical Research, vol. 2009, no. 11, pp. 696–698, 2009.

A. J. Catino, J. M. Nichols, H. Choi, S. Gottipamula, and M. P. Doyle, “Benzylic oxidation catalyzed by dirhodium(II,III) caprolactamate,” Organic Letters, vol. 7, no. 23, pp. 5167–5170, 2005.

A. T. Khan, T. Parvin, L. H. Choudhury, and S. Ghosh, “A simple synthetic protocol for oxidation of alkyl-arenes into ketones using a combination of HBr-H₂O₂,” Tetrahedron Letters, vol. 48, no. 13, pp. 2271–2274, 2007.

A. Shaabani, E. Farhangi, and A. Rahmati, “Aerobic oxidation of alkyl arenes and alcohols using cobalt(II) phthalocyanine as a catalyst in 1-butyl-3-methyl-imidazolium bromide,” Applied Catalysis A: General, vol. 338, no. 1-2, pp. 14–19, 2008.

M. Arshadi and M. Ghiaci, “Highly efficient solvent free oxidation of ethylbenzene using some recyclable catalysts: the role of linker in competency of manganese nanocatalysts,” Applied Catalysis A: General, vol. 399, no. 1-2, pp. 75–86, 2011.

D. Habibi, A. R. Faraji, M. Arshadi, H. Veisi, and A. Gil, “Manganese nanocatalyst and N-hydroxyphthalimide as an efficient catalytic system for selective oxidation of ethylbenzene, cyclohexene and oximes under aerobic condition,” Journal of Molecular Catalysis A: Chemical, vol. 382, pp. 41–54, 2014.

M. Arshadi, M. Ghiaci, A. Rahmanian, H. Ghaziaskar, and A. Gil, “Oxidation of ethylbenzene to acetophenone by a Mn catalyst supported on a modified nanosized SiO₂/Al₂O₃ mixed-oxide in supercritical carbon dioxide,” Applied Catalysis B: Environmental, vol. 119-120, pp. 81–90, 2012.

D. Habibi, A. R. Faraji, M. Arshadi, and J. L. G. Fierro, “Characterization and catalytic activity of a novel Fe nano-catalyst as efficient heterogeneous catalyst for selective oxidation of ethylbenzene, cyclohexene, and benzylalcohol,” Journal of Molecular Catalysis A: Chemical, vol. 372, pp. 90–99, 2013.

D. Habibi, A. R. Faraji, M. Arshadi, S. Heydari, and A. Gil, “Efficient catalytic systems based on cobalt for oxidation of ethylbenzene, cyclohexene and oximes in the presence of N-hydroxyphthalimide,” Applied Catalysis A: General, vol. 466, pp. 282–292, 2013.

D. Habibi and A. R. Faraji, “Preparation, characterization and catalytic activity of a nano-Co(II)-catalyst as a high efficient heterogeneous catalyst for the selective oxidation of ethylbenzene, cyclohexene, and benzyl alcohol,” Comptes Rendus Chimie, vol. 16, no. 10, pp. 888–896, 2013.

S. Mobini, F. Meshkani, and M. Rezaei, “Supported Mn catalysts and the role of different supports in the catalytic oxidation of carbon monoxide,” Chemical Engineering Science, vol. 197, pp. 37–51, 2019.

C. Papadopoulos, K. Kappis, J. Papavasiliou et al., “Copper-promoted ceria catalysts for CO oxidation reaction,” Catalysis Today, 2019.

P. A. DeSario, C. L. Pitman, D. J. Delia et al., “Low-temperature CO oxidation at persistent low-valent Cu nanoparticles on TiO₂ aerogels,” Applied Catalysis B: Environmental, vol. 252, pp. 205–213, 2019.

G. Spezzati, A. D. Benavidez, A. T. DeLaRiva et al., “CO oxidation by Pd supported on CeO₂(100) and CeO₂(111) facets,” Applied Catalysis B: Environmental, vol. 243, pp. 36–46, 2019.

C. Li, X. Yang, G. Gao et al., “Copper on the inner surface of mesoporous TiO2hollow spheres: a highly selective photocatalyst for partial oxidation of methanol to methyl formate,” Catalysis Science & Technology, vol. 9, no. 22, pp. 6240–6252, 2019.

A. El Roz, P. Fongarland, F. Dumeignil, and M. Capron, “Glycerol to glyceraldehyde oxidation reaction over Pt-based catalysts under base-free conditions,” Frontiers in Chemistry, vol. 7, pp. 1683–1690, 2019.

Y. Yang, J. Liu, Z. Wang et al., “A complete catalytic reaction scheme for Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst,” Journal of Hazardous Materials, vol. 373, pp. 660–670, 2019.

Y. Boyjoo, G. Rochard, J. M. Giraudon, J. Liu, and J. F. Lamonier, “Mesoporous MnO₂ hollow spheres for enhanced catalytic oxidation of formaldehyde,” Sustainable Materials and Technologies, vol. 20, p. e00091, 2019.

F. Yu, D. Poole III, S. Mathew et al., “Control over electrochemical water oxidation catalysis by preorganization of molecular ruthenium catalysts in self-assembled nanospheres,” Angewandte Chemie International Edition, vol. 57, no. 35, pp. 11247–11251, 2018.

A. H. Hussein, A. Khalil, M. Jahjah et al., “Catalytic properties of various oxides and mesoporous materials containing niobium and sulfate ions, in the oxidation reaction of 1-octanol,” International Journal of Organic Chemistry, vol. 8, no. 1, pp. 41–53, 2018.

B. Qiu, D. Xu, Q. Sun et al., “Highly enantioselective oxidation of spirocyclic hydrocarbons by bioinspired manganese catalysts and hydrogen peroxide,” ACS Catalysis, vol. 8, no. 3, pp. 2479–2487, 2018.

S. Sadula and B. Saha, “Aerobic oxidation of xylose to xylaric acid in water over pt catalysts,” ChemSusChem, vol. 11, no. 13, pp. 2124–2129, 2018.

S. Khare, P. Shrivastava, R. Chokhare, J. S. Kirar, and S. Parashar, “Catalytic liquid phase oxidation of cyclohexane with tert-butylhydroperoxide over transition metal exchanged α-zirconium phosphate,” Indian Journal of Chemical Technology-Section A, vol. 57A, pp. 427–434, 2018.

R. Rizo, D. Sebastián, M. J. Lázaro, and E. Pastor, “On the design of Pt-Sn efficient catalyst for carbon monoxide and ethanol oxidation in acid and alkaline media,” Applied Catalysis B: Environmental, vol. 200, pp. 246–254, 2017.

Y. Kim, H. W. Kim, S. Lee et al., “The role of ruthenium on carbon-supported PtRu catalysts for electrocatalytic glycerol oxidation under acidic conditions,” ChemCatChem, vol. 9, no. 9, pp. 1683–1690, 2017.

L. Li, Q. Yang, S. Chen et al., “Boosting selective oxidation of cyclohexane over a metal-organic framework by hydrophobicity engineering of pore walls,” Chemical Communications, vol. 53, no. 72, pp. 10026–10029, 2017.

X. Lin, S. Zhao, L. Fu, Y. Luo, R. Zhu, and Z. Liu, “Synthesis of Co-N-C grafted on well-dispersed MnOx-CeO₂ hollow mesoporous sphere with efficient catalytic performance,” Molecular Catalysis, vol. 437, pp. 18–25, 2017.

A. De Clercq, O. Margeat, G. Sitja, C. R. Henry, and S. Giorgio, “Core-shell Pd-Pt nanocubes for the CO oxidation,” Journal of Catalysis, vol. 336, pp. 33–40, 2016.

S. Y. Hwang, E. Yurchekfrodl, C. Zhang, and Z. Peng, “Low-temperature preferential oxidation of carbon monoxide on Pt3Ni alloy nanoparticle catalyst with engineered surface,” ChemCatChem, vol. 8, no. 1, p. 3, 2016.

M. Witek, M. D. Koppang, and G. M. Swain, “Aliphatic polyamine oxidation reaction mechanism at boron-doped microcrystalline and ultrananocrystalline diamond electrodes,” Electroanalysis, vol. 28, no. 1, pp. 151–160, 2016.

B.-D. Hong, K.-L. Hunag, H.-R. Chen, and C.-L. Lee, “Effect of defective graphene flake for catalysts of supported Pd nanocubes toward glucose oxidation reaction in alkaline medium,” Journal of the Electrochemical Society, vol. 163, no. 9, pp. H731–H737, 2016.

Y. Chen, M. Cui, S. Li et al., “The influence of potential scanning rate on the electrocatalytic activity of pyrolysis-treated PdO/graphene for ethanol oxidation reaction (EOR),” International Journal of Electrochemical Science, pp. 3285–3295, 2016.

A. Ejaz, M. S. Ahmed, and S. Jeon, “Synergistic effect of 1,4-benzenedimethaneamine assembled graphene supported palladium for formaldehyde oxidation reaction in alkaline media,” Journal of the Electrochemical Society, vol. 163, no. 5, pp. B163–B168, 2016.

A. B. Yousaf, M. Imran, A. Zeb et al., “Single phase PtAg bimetallic alloy nanoparticles highly dispersed on reduced graphene oxide for electrocatalytic application of methanol oxidation reaction,” Electrochimica Acta, vol. 197, pp. 117–125, 2016.

C. Alegre, M. Gálvez, R. Moliner, and M. Lázaro, “Influence of the synthesis method for Pt catalysts supported on highly mesoporous carbon xerogel and Vulcan carbon black on the electro-oxidation of methanol,” Catalysts, vol. 5, no. 1, pp. 392–405, 2015.

C. D. Evans, S. A. Kondrat, P. J. Smith et al., “The preparation of large surface area lanthanum based perovskite supports for AuPt nanoparticles: tuning the glycerol oxidation reaction pathway by switching the perovskite B site,” Faraday Discussions, vol. 188, pp. 427–450, 2016.

C.-H. Kuo, W. Li, L. Pahalagedara et al., “Understanding the role of gold nanoparticles in enhancing the catalytic activity of manganese oxides in water oxidation reactions,” Angewandte Chemie, vol. 127, no. 8, pp. 2375–2380, 2015.

K. Mandal, D. Bhattacharjee, P. S. Roy, S. K. Bhattacharya, and S. Dasgupta, “Room temperature synthesis of Pd-Cu nanoalloy catalyst with enhanced electrocatalytic activity for the methanol oxidation reaction,” Applied Catalysis A: General, vol. 492, pp. 100–106, 2015.

A. Ghorbani-Choghamarani, Z. Darvishnejad, and M. Norouzi, “Cu(II)-Schiff base complex-functionalized magnetic Fe3O4nanoparticles: a heterogeneous catalyst for various oxidation reactions,” Applied Organometallic Chemistry, vol. 29, no. 3, pp. 170–175, 2015.

P. Mukherjee, P. S. Roy, K. Mandal, D. Bhattacharjee, S. Dasgupta, and S. K. Bhattacharya, “Improved catalysis of room temperature synthesized Pd-Cu alloy nanoparticles for anodic oxidation of ethanol in alkaline media,” Electrochimica Acta, vol. 154, pp. 447–455, 2015.

S. Lai, C. Fu, Y. Chen et al., “Pt-content-controlled synthesis of Pd nanohollows/Pt nanorods core/shell composites with enhanced electrocatalytic activities for the methanol oxidation reaction,” Journal of Power Sources, vol. 274, pp. 604–610, 2015.

L. Wang, D. Yang, J. Wang, Z. Zhu, and K. Zhou, “Ambient temperature CO oxidation over gold nanoparticles (14 nm) supported on Mg(OH)₂ nanosheets,” Catalysis Communications, vol. 36, pp. 38–42, 2013.

I. V. Delidovich, B. L. Moroz, O. P. Taran et al., “Aerobic selective oxidation of glucose to gluconate catalyzed by Au/Al₂O₃ and Au/C: impact of the mass-transfer processes on the overall kinetics,” Chemical Engineering Journal, vol. 223, pp. 921–931, 2013.

H. Zhang and N. Toshima, “Synthesis of Au/Pt bimetallic nanoparticles with a Pt-rich shell and their high catalytic activities for aerobic glucose oxidation,” Journal of Colloid and Interface Science, vol. 394, pp. 166–176, 2013.

H. Wang, W. Fan, Y. He, J. Wang, J. N. Kondo, and T. Tatsumi, “Selective oxidation of alcohols to aldehydes/ketones over copper oxide- supported gold catalysts,” Journal of Catalysis, vol. 299, pp. 10–19, 2013.

M. Zhang, X. Zhu, X. Liang, and Z. Wang, “Preparation of highly efficient Au/C catalysts for glucose oxidation via novel plasma reduction,” Catalysis Communications, vol. 25, pp. 92–95, 2012.

P. Bujak, P. Bartczak, and J. Polanski, “Highly efficient room-temperature oxidation of cyclohexene and d-glucose over nanogold Au/SiO₂ in water,” Journal of Catalysis, vol. 295, pp. 15–21, 2012.

W. Li, A. Wang, X. Liu, and T. Zhang, “Silica-supported Au-Cu alloy nanoparticles as an efficient catalyst for selective oxidation of alcohols,” Applied Catalysis A: General, vol. 433-434, pp. 146–151, 2012.

G. Zhan, Y. Hong, V. T. Mbah et al., “Bimetallic Au-Pd/MgO as efficient catalysts for aerobic oxidation of benzyl alcohol: A green bio-reducing preparation method,” Applied Catalysis A: General, vol. 439-440, pp. 179–186, 2012.

Z. Zhang, L. Xin, K. Sun, and W. Li, “Pd-Ni electrocatalysts for efficient ethanol oxidation reaction in alkaline electrolyte,” International Journal of Hydrogen Energy, vol. 36, no. 20, pp. 12686–12697, 2011.

M. M. Tusi, N. S. O. Polanco, S. G. da Silva, E. V. Spinacé, and A. O. Neto, “The high activity of PtBi/C electrocatalysts for ethanol electro-oxidation in alkaline medium,” Electrochemistry Communications, vol. 13, no. 2, pp. 143–146, 2011.

S. Y. Shen, T. S. Zhao, and J. B. Xu, “Carbon-supported bimetallic PdIr catalysts for ethanol oxidation in alkaline media,” Electrochimica Acta, vol. 55, no. 28, pp. 9179–9184, 2010.

R. F. B. Souza De, M. M. Tusi, M. Brandalise et al., “Preparation of PtSn/C-Rh and PtSn/C-CeO₂ for ethanol electro-oxidation,” International Journal of Electrochemical Science, vol. 5, pp. 895–902, 2010.

T. Ishida, S. Okamoto, R. Makiyama, and M. Haruta, “Aerobic oxidation of glucose and 1-phenylethanol over gold nanoparticles directly deposited on ion-exchange resins,” Applied Catalysis A: General, vol. 353, no. 2, pp. 243–248, 2009.

T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa, and K. Kaneda, “Supported gold nanoparticle catalyst for the selective oxidation of silanes to silanols in water,” Chemical Communications, no. 35, pp. 5302–5304, 2009.

Q. Jiang, Y. Xiao, Z. Tan, Q.-H. Li, and C.-C. Guo, “Aerobic oxidation of p-xylene over metalloporphyrin and cobalt acetate: Their synergy and mechanism,” Journal of Molecular Catalysis A: Chemical, vol. 285, no. 1-2, pp. 162–168, 2008.

X. Yang, X. Wang, C. Liang et al., “Aerobic oxidation of alcohols over Au/TiO₂: an insight on the promotion effect of water on the catalytic activity of Au/TiO₂,” Catalysis Communications, vol. 9, no. 13, pp. 2278–2281, 2008.

H. Li, B. Guan, W. Wang et al., “Aerobic oxidation of alcohol in aqueous solution catalyzed by gold,” Tetrahedron, vol. 63, no. 35, pp. 8430–8434, 2007.

K. M. Parida and D. Rath, “Structural properties and catalytic oxidation of benzene to phenol over CuO- impregnated mesoporous silica,” Applied Catalysis A: General, vol. 321, no. 2, pp. 101–108, 2007.

T. Hayashi, T. Inagaki, N. Itayama, and H. Baba, “Selective oxidation of alcohol over supported gold catalysts: methyl glycolate formation from ethylene glycol and methanol,” Catalysis Today, vol. 117, no. 1-3, pp. 210–213, 2006.

B. Guan, D. Xing, G. Cai et al., “Highly selective aerobic oxidation of alcohol catalyzed by a gold(I) complex with an anionic ligand,” Journal of the American Chemical Society, vol. 127, no. 51, pp. 18004-18005, 2005.

E. M. Flanigen, J. M. Bennett, R. W. Grose et al., “Silicalite, a new hydrophobic crystalline silica molecular sieve,” Nature, vol. 271, no. 5645, pp. 512–516, 1978.

R. Ciriminna, V. Pandarus, F. Béland, Y.-J. Xu, and M. Pagliaro, “Heterogeneously catalyzed alcohol oxidation for the fine chemical industry,” Organic Process Research & Development, vol. 19, no. 11, pp. 1554–1558, 2014.

J. S. Reddy, S. Sivasanker, and P. Ratnasamy, “Hydroxylation of phenol over TS-2, a titanium silicate molecular sieve,” Journal of Molecular Catalysis, vol. 71, no. 3, pp. 373–381, 1992.

M. Ghiaci, F. Molaie, M. E. Sedaghat, and N. Dorostkar, “Metalloporphyrin covalently bound to silica. Preparation, characterization and catalytic activity in oxidation of ethyl benzene,” Catalysis Communications, vol. 11, no. 8, pp. 694–699, 2010.

F. Rajabi, R. Luque, J. H. Clark, B. Karimi, and D. J. Macquarrie, “A silica supported cobalt (II) Salen complex as efficient and reusable catalyst for the selective aerobic oxidation of ethyl benzene derivatives,” Catalysis Communications, vol. 12, no. 6, pp. 510–513, 2011.

L. Chen, B.-D. Li, Q.-X. Xu, and D.-B. Liu, “A silica gel supported cobalt(II) Schiff base complex as efficient and recyclable heterogeneous catalyst for the selective aerobic oxidation of alkyl aromatics,” Chinese Chemical Letters, vol. 24, no. 9, pp. 849–852, 2013.

S. S. Bhoware, S. Shylesh, K. R. Kamble, and A. P. Singh, “Cobalt-containing hexagonal mesoporous molecular sieves (Co-HMS): synthesis, characterization and catalytic activity in the oxidation reaction of ethylbenzene,” Journal of Molecular Catalysis A: Chemical, vol. 255, no. 1-2, pp. 123–130, 2006.

Sujandi, E. A. Prasetyanto, D.-S. Han, S.-C. Lee, and S.-E. Park, “Immobilization of Co(III) using tethered cyclam ligand on SBA-15 mesoporous silica for aerial oxidation of ethylbenzene,” Catalysis Today, vol. 141, no. 3-4, pp. 374–377, 2009.

A. R. Faraji, F. Ashouri, Z. Hekmatian, S. Heydari, and S. Mosazadeh, “Organosuperbase dendron manganese complex grafted on magnetic nanoparticles; heterogeneous catalyst for green and selective oxidation of ethylbenzene, cyclohexene and oximes by molecular oxygen,” Polyhedron, vol. 157, pp. 90–106, 2019.

S. L. H. Rebelo, M. M. Q. Simões, M. G. P. M. S. Neves, and J. A. S. Cavaleiro, “Oxidation of alkylaromatics with hydrogen peroxide catalysed by manganese(III) porphyrins in the presence of ammonium acetate,” Journal of Molecular Catalysis A: Chemical, vol. 201, no. 1-2, pp. 9–22, 2003.

A. Thellend, P. Battioni, and D. Mansuy, “Ammonium acetate as a very simple and efficient cocatalyst for manganese porphyrin-catalysed oxygenation of hydrocarbons by hydrogen peroxide,” Journal of the Chemical Society, Chemical Communications, no. 9, p. 1035, 1994.

P. Battioni, J. P. Renaud, J. F. Bartoli, M. Reina-Artiles, M. Fort, and D. Mansuy, “Monooxygenase-like oxidation of hydrocarbons by hydrogen peroxide catalyzed by manganese porphyrins and imidazole: selection of the best catalytic system and nature of the active oxygen species,” Journal of the American Chemical Society, vol. 110, no. 25, pp. 8462–8470, 1988.

A. M. d.’ A. Rocha Gonsalves and A. C. Serra, “On the mechanism of carboxylic acid co-catalyst assisted metalloporphyrin oxidations,” Journal of Molecular Catalysis A: Chemical, vol. 168, no. 1-2, pp. 25–32, 2001.

T.-C. Zheng and D. E. Richardson, “Manganese porphyrin catalyzed homogeneous aqueous oxidation of organic molecules by magnesium monoperoxyphthalate (MMPP),” Tetrahedron Letters, vol. 36, no. 6, pp. 837–840, 1995.

K. Xavier, J. Chacko, and K. Mohammed Yusuff, “Zeolite-encapsulated Co(II), Ni(II) and Cu(II) complexes as catalysts for partial oxidation of benzyl alcohol and ethylbenzene,” Applied Catalysis A: General, vol. 258, no. 2, pp. 251–259, 2004.

V. R. Choudhary, J. R. Indurkar, V. S. Narkhede, and R. Jha, “MnO4−1 exchanged Mg-Al hydrotalcite: a stable and reusable/environmental- friendly catalyst for selective oxidation by oxygen of ethylbenzene to acetophenone and diphenylmethane to benzophenone,” Journal of Catalysis, vol. 227, no. 1, pp. 257–261, 2004.

T. H. Bennur, D. Srinivas, and S. Sivasanker, “Oxidation of ethylbenzene over "neat" and zeolite-Y-encapsulated copper tri- and tetraaza macrocyclic complexes,” Journal of Molecular Catalysis A: Chemical, vol. 207, no. 2, pp. 163–171, 2004.

S. K. Jana, P. Wu, and T. Tatsumi, “NiAl hydrotalcite as an efficient and environmentally friendly solid catalyst for solvent-free liquid-phase selective oxidation of ethylbenzene to acetophenone with 1 atm of molecular oxygen,” Journal of Catalysis, vol. 240, no. 2, pp. 268–274, 2006.

T. Radhika and S. Sugunan, “Vanadia supported on ceria: characterization and activity in liquid-phase oxidation of ethylbenzene,” Catalysis Communications, vol. 8, no. 2, pp. 150–156, 2007.

X. G. Li, J. Wang, and R. He, “Selective oxidation of ethylbenzene catalyzed by fluorinated metalloporphyrins with molecular oxygen,” Chinese Chemical Letters, vol. 18, no. 9, pp. 1053–1056, 2007.

H. Ma, J. Xu, Q. Zhang, H. Miao, and W. Wu, “Selective oxidation of ethylbenzene by a biomimetic combination: Hemin and N-hydroxyphthalimide (NHPI),” Catalysis Communications, vol. 8, no. 1, pp. 27–30, 2007.

G. Raju, P. S. Reddy, J. Ashok, B. M. Reddy, and A. Venugopal, “Solvent-free aerobic oxidation of ethylbenzene over supported Ni catalysts using molecular oxygen at atmospheric pressure,” Journal of Natural Gas Chemistry, vol. 17, no. 3, pp. 293–297, 2008.

C. Ngamcharussrivichai, P. Wu, and T. Tatsumi, “Liquid-phase Beckmann rearrangement of cyclohexanone oxime over mesoporous molecular sieve catalysts,” Journal of Catalysis, vol. 227, no. 2, pp. 448–458, 2004.

T. Ushikubo and K. Wada, “Vapor-phase Beckmann rearrangement over silica-supported tantalum oxide catalysts,” Journal of Catalysis, vol. 148, no. 1, pp. 138–148, 1994.

I. Murwani, K. Scheurell, and E. Kemnitz, “Liquid phase oxidation of ethylbenzene on pure and metal doped HS- AlF₃,” Catalysis Communications, vol. 10, no. 2, pp. 227–231, 2008.

M. Salavati-Niasari, “Host (nanocavity of zeolite-Y)/guest ([Cu([R])₂-N₂X₂)]₂+ (R = H, CH₃; X = NH, O, S) nanocomposite materials: synthesis, characterization and catalytic oxidation of ethylbenzene,” Journal of Molecular Catalysis A: Chemical, vol. 284, no. 1-2, pp. 97–107, 2008.

M. Ghiaci, Z. Sadeghi, M. E. Sedaghat, H. Karimi-Maleh, J. Safaei-Ghomi, and A. Gil, “Preparation of Pd (0) and Pd (II) nanotubes and nanoparticles on modified bentonite and their catalytic activity in oxidation of ethyl benzene to acetophenone,” Applied Catalysis A: General, vol. 381, no. 1-2, pp. 121–131, 2010.

K. George and S. Sugunan, “Nickel substituted copper chromite spinels: preparation, characterization and catalytic activity in the oxidation reaction of ethylbenzene,” Catalysis Communications, vol. 9, no. 13, pp. 2149–2153, 2008.

T. Iwahama, G. Hatta, S. Sakaguchi, and Y. Ishii, “Epoxidation of alkenes using alkyl hydroperoxides generated in situ by catalytic autoxidation of hydrocarbons with dioxygen,” Chemical Communications, no. 2, pp. 163-164, 2000.

Y. Ishii, S. Sakaguchi, and T. Iwahama, “Innovation of hydrocarbon oxidation with molecular oxygen and related reactions,” Advanced Synthesis and Catalysis, vol. 343, no. 5, pp. 393–427, 2001.

Y. Yoshino, Y. Hayashi, T. Iwahama, S. Sakaguchi, and Y. Ishii, “Catalytic oxidation of alkylbenzenes with molecular oxygen under normal pressure and temperature byN-Hydroxyphthalimide combined with Co(OAc)₂,” The Journal of Organic Chemistry, vol. 62, no. 20, pp. 6810–6813, 1997.

A. Shibamoto, S. Sakaguchi, and Y. Ishii, “Aerobic oxidation of methylpyridines to pyridinecarboxylic acids catalyzed byN-Hydroxyphthalimide,” Organic Process Research & Development, vol. 4, no. 6, pp. 505–508, 2000.

G. Zheng, C. Liu, Q. Wang, M. Wang, and G. Yang, “Metal-free: an efficient and selective catalytic aerobic oxidation of hydrocarbons with oxime andN-Hydroxyphthalimide,” Advanced Synthesis and Catalysis, vol. 351, no. 16, pp. 2638–2642, 2009.

G. Yang, Y. Ma, and J. Xu, “Biomimetic catalytic system driven by electron transfer for selective oxygenation of hydrocarbon,” Journal of the American Chemical Society, vol. 126, no. 34, pp. 10542-10543, 2004.

X. Tong, J. Xu, and H. Miao, “Highly efficient and metal-free aerobic hydrocarbons oxidation process by ano-phenanthroline-mediated organocatalytic system,” Advanced Synthesis and Catalysis, vol. 347, no. 15, pp. 1953–1957, 2005.

O. Fukuda, S. Sakaguchi, and Y. Ishii, “A new strategy for catalytic Baeyer-Villiger oxidation of KA-oil with molecular oxygen using N-hydroxyphthalimide,” Tetrahedron Letters, vol. 42, no. 20, pp. 3479–3481, 2001.

S. Sakaguchi, T. Hirabayashi, and Y. Ishii, “First Ritter-type reaction of alkylbenzenes using N-hydroxyphthalimide as a key catalyst,” Chemical Communications, no. 5, pp. 516-517, 2002.

X. Yang, L. Zhou, Y. Chen et al., “A promotion effect of alkaline-earth chloride on N-hydroxyphthalimide- catalyzed aerobic oxidation of hydrocarbons,” Catalysis Communications, vol. 11, no. 3, pp. 171–174, 2009.

F. Minisci, F. Recupero, C. Gambarotti, C. Punta, and R. Paganelli, “Selective functionalisation of hydrocarbons by nitric acid and aerobic oxidation catalysed by N-hydroxyphthalimide and iodine under mild conditions,” Tetrahedron Letters, vol. 44, no. 36, pp. 6919–6922, 2003.

S. Sakaguchi, Y. Nishiwaki, T. Kitamura, and Y. Ishii, “Efficient catalytic alkane nitration with NO₂ under air assisted by N-hydroxyphthalimide,” Angewandte Chemie International Edition, vol. 40, no. 1, pp. 222–224, 2001.

K. Matsunaka, T. Iwahama, S. Sakaguchi, and Y. Ishii, “A remarkable effect of quaternary ammonium bromide for the N-hydroxyphthalimide-catalyzed aerobic oxidation of hydrocarbons,” Tetrahedron Letters, vol. 40, no. 11, pp. 2165–2168, 1999.

J. Qin, F. Zaihui, Y. Liu et al., “Aerobic Oxidation of Ethylbenzene Co-catalyzed by N -Hydroxyphthalimide and Oxobis(8-Quinolinolato) Vanadium (IV) Complexes,” Chinese Journal of Catalysis, vol. 32, no. 6-8, pp. 1342–1348, 2011.

B. Gao, Y. Li, and N. Shi, “Oxovanadium (IV) Schiff base complex immobilized on CPS microspheres as heterogeneous catalyst for aerobic selective oxidation of ethyl benzene to acetophenone,” Reactive and Functional Polymers, vol. 73, no. 11, pp. 1573–1579, 2013.

J. Luo, H. Yu, H. Wang, and F. Peng, “Enhancing the catalytic activity of carbon nanotubes by filled iron nanowires for selective oxidation of ethylbenzene,” Catalysis Communications, vol. 51, pp. 77–81, 2014.

Z. Miao, H. Zhao, J. Yang, J. Zhao, H. Song, and L. Chou, “Facile synthesis of ordered mesoporous Cu-ZrPO with high copper contents as catalyst for liquid phase oxidation of ethylbenzene,” Microporous and Mesoporous Materials, vol. 198, pp. 271–280, 2014.

G. Li, Y. Li, R. Mu, Y. Xu, and P. Dong, “Direct side-chain oxidation of ethylbenzene over supported Co4HP2Mo15V3O62 catalysts as a clean and highly efficient approach to producing acetophenone,” Reaction Kinetics, Mechanisms and Catalysis, vol. 109, no. 1, pp. 199–212, 2013.

C. Guo, Q. Peng, Q. Liu, and G. Jiang, “Selective oxidation of ethylbenzene with air catalyzed by simple μ-oxo dimeric metalloporphyrins under mild conditions in the absence of additives,” Journal of Molecular Catalysis A: Chemical, vol. 192, no. 1-2, pp. 295–302, 2003.

A. R. Faraji, S. Mosazadeh, and F. Ashouri, “Synthesis and characterization of cobalt-supported catalysts on modified magnetic nanoparticle: green and highly efficient heterogeneous nanocatalyst for selective oxidation of ethylbenzene, cyclohexene and oximes with molecular oxygen,” Journal of Colloid and Interface Science, vol. 506, pp. 10–26, 2017.

J. Luo, F. Peng, H. Yu, H. Wang, and W. Zheng, “Aerobic liquid-phase oxidation of ethylbenzene to acetophenone catalyzed by carbon nanotubes,” ChemCatChem, vol. 5, no. 6, pp. 1578–1586, 2013.

W. F. Xu, W. J. Chen, D. C. Li, B. H. Cheng, and H. Jiang, “Highly dispersed manganese based Mn/N–C/Al2O3Catalyst for selective oxidation of the C-H bond of ethylbenzene,” Industrial and Engineering Chemistry Research, vol. 58, no. 10, pp. 3969–3977, 2019.

Y. Yao, Y. Du, J. Li, C. Wang, Z. Zhang, and X. Zhao, “Crystal structure ofmeso‐substituted pyrazolyl porphyrin complexes and their highly active catalyst for oxidation of alkylbenzenes,” Applied Organometallic Chemistry, vol. 32, no. 3, article e4184, 2018.

P. Tang, Y. Gao, J. Yang, W. Li, H. Zhao, and D. Ma, “Growth mechanism of N-doped graphene materials and their catalytic behavior in the selective oxidation of ethylbenzene,” Chinese Journal of Catalysis, vol. 35, no. 6, pp. 922–928, 2014.

Y. Gao, G. Hu, J. Zhong et al., “Nitrogen-doped SP²-hybridized carbon as a superior catalyst for selective oxidation,” Angewandte Chemie International Edition, vol. 52, no. 7, pp. 2109–2113, 2013.