Heterogeneous Metal Catalysts for Oxidation Reactions

Abstract

Oxidation reactions may be considered as the heart of chemical synthesis. However, the indiscriminate uses of harsh and corrosive chemicals in this endeavor are threating to the ecosystems, public health, and terrestrial, aquatic, and aerial flora and fauna. Heterogeneous catalysts with various supports are brought to the spotlight because of their excellent capabilities to accelerate the rate of chemical reactions with low cost. They also minimize the use of chemicals in industries and thus are friendly and green to the environment. However, heterogeneous oxidation catalysis are not comprehensively presented in literature. In this short review, we clearly depicted the current state of catalytic oxidation reactions in chemical industries with specific emphasis on heterogeneous catalysts. We outlined here both the synthesis and applications of important oxidation catalysts. We believe it would serve as a reference guide for the selection of oxidation catalysts for both industries and academics.

Md. Eaqub Ali 1 and Md. Motiar Rahman 1 and Shaheen M. Sarkar 2 and Sharifah Bee Abd Hamid 1

Academic Editor:Amir Kajbafvala

1, Nanotechnology and Catalysis Research Centre (NanoCat), Universiti of Malaya, 50603 Kuala Lumpur, Malaysia
2, Faculty of Industrial Sciences and Technology, University Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia

Received 11 August 2014; Accepted 9 October 2014; 22 December 2014

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.

1. Introduction

In the past, efforts were made for the oxidation of alkyl substituted benzene to useful products such as benzylic and allylic ketones by adding stoichiometric amounts of strong oxidants such as chromium (IV) reagents, permanganates, tert -butyl hydroperoxide (TBHP), selenium oxide (SeO₂ ), ruthenium (VIII) oxide, hydrogen peroxide, nitric acid, and oxygen [7-9]. However, most of these chemicals are either toxic or corrosive to reactor wall, unstable in atmospheric conditions, nonspecific in actions, which produce many undesirable side products, and that increases the purification cost and environment pollutant [7-9]. These traditional transformation schemes are also time consuming and cannot be recycled [10].

The green chemistry approaches must meet health and environmental safeties and use very little chemicals reducing both cost and time [11]. Catalytic approaches might be considered as green since specific chemical transformation could be achieved within very short time with the addition of very little catalysts, significantly reducing production cost as well as health and environmental risks [12, 13]. According to the North American Catalysis Society, approximately 35% of global GDP rest on catalysts and the use of catalysts in industry are increasing 5% per year [14]. Currently, more than 60% of chemical synthesis and 90% of chemical transformations in chemical industries are using catalysts [15, 16]. In 2013, the sales of catalysts were between 15.5 billion USD and the turnover in industries using catalyst was 14 trillion USD.

Homogeneous catalyst has been extensively used in the oxidative process for the manufacturing of bulk as well as fine chemicals. This is because of its efficiency in bringing huge influences in chemical conversion via the same phase catalysis reaction [17]. In the recent time, some transition metal ion complexes have shown high selectivity, efficiency, and reproducibility to catalyze the reaction under mild conditions. The single catalytic entity in homogeneous catalysts can act as a single active site which can speed up reaction and reduce the reaction time [18]. However, homogeneous catalytic processes produce huge waste materials, significantly disrupting the environmental and ecological stability [19-21]. One of the main disadvantages to the use of these types of catalysts is the ease of separating of the comparatively affluent catalysts from the reaction mixtures at the end of reaction [9, 19, 22]. Homogeneous catalysts also cause corrosion to the industrial materials and some of them are deposited on the reactor wall. To get rid of these problems and minimize environmental hazards, the homogenous catalysts could be prepared by the dispersion of metal on an insoluble solid supports via covalent anchoring to keep the metal on the surface where catalysis reaction takes place [18, 22].

Oxidation reactions play a pivotal role in chemical industry for the production of many crucial compounds [1]. For example, selective oxidation of alkyl substituted benzene produces alcohol and ketones which have significant biological and mechanistic interest in modern organic synthesis [2]. Ethylbenzene is a representative compound of various linear and phenyl-substituted alkanes and is a model substrate to study alkane oxidation reactions. The oxidation products of ethylbenzene include acetophenone and 1-phenylethanol which have been used as precursors for the synthesis of a wide variety of drugs, such as hydrogel [3], optically active alcohols [2], hydrazones [4], benzalacetophenones (chalcones) [5], tear gas, and resins [6].

Heterogeneous catalyst is considered to be a better choice for the synthesis of commodity materials [23-25]. Nowadays, silica, carbon, clay, zeolite, metal oxide polymers, and other mesoporous materials are being used as inorganic solid supports [26, 27]. Supported materials can be obtained as complexes with transition metals and Schiff base ligands by heterogenization process [28]. The application of supported polymers in catalytic oxidation has gained much attention because of their inertness and nontoxic, nonvolatile, and recyclable criteria [29]. Among inorganic supports, the mesoporous materials have been proven to be ideal catalyst supports due to their three-dimensional open pore network structures, high surface area and porosity, high reusability and heat stability, and uniform and interconnected pores which offer a reliable and well-separated atmosphere for the deposition of dynamic components and interactive surfaces between the catalysts and reactants [30-38]. Various support materials along with their major features are presented in Table 1.

Table 1: Major features, advantages, and disadvantages of the commonly used support materials.

Supports materials	Features	Advantages	Disadvantages	References
Alumina	(1) Hardness (2) High melting point and high compression strength (3) Resistant to abrasion and chemical attack (4) High thermal conductivity	(1) Thermally stable (2) Randomly ordered (3) High surface area and pore volume (4) Well-ordered pore (5) Narrow pore size	(1) Difficult to control the hydrolysis rate of aluminum precursors	[73]
Silica	(1) Tendency to form large networks (2) Found in nature and living organisms (3) Hardness	(1) High efficiency (2) High selectivity (3) Highly stable (4) Mechanical strength	(1) Low compatibility (2) Formation of aggregates/agglomerates	[74]
Zeolite	(1) Microporous (2) Inertness (3) Excellent electron conductivity	(1) Highly effective (2) Less or no corrosion (3) No waste or disposal problems (4) High thermo stability (5) Easy set-up of continuous processes (6) Great adaptability to practically all types of catalysis	(1) Irreversible adsorption or steric blockage of heavy secondary products. (2) Impossibility of using microporosity (3) Difficult to exploit the shape selectivity	[75, 76]
Carbon	(1) Nonmetallic (2) Tetravalent (3) Porous structure	(1) High mechanical strength (2) Large surface area (3) Excellent electron conductivity (4) Good elasticity (5) Thermal stability (6) Inertness	(1) High temperature physical activation (2) Expensive (3) Emission of greenhouse gasses during pyrolysis	[77, 78]

Heterogeneous catalysts promote oxidation reactions via attracting oxygen from oxidants, such as TBHP (tert -BuO₂ H) and HP (H₂ O₂ ) [39, 40]. In the last decade, TBHP has been used as oxidant for various oxidation reactions such as alkyl benzene and benzyl alcohol oxidation. In this review, we described heterogeneous catalysts, their synthesis schemes on various supports, and applications in selected oxidation reactions. The comparative features of homogeneous and heterogeneous catalysts are presented in Figure 1.

Figure 1: Special features, advantages, and disadvantages of homo- and heterogeneous catalysts.

[figure omitted; refer to PDF]

2. Heterogeneous Catalysts

In heterogeneous catalysis reaction, the catalysts and reactants exist in different phases. In reality, the vast majority of heterogeneous catalysts are solids and the vast majority of reactants are either gases or liquids [14]. A phase separation catalysis reaction greatly helps in reactant, product, and catalyst separation at the end of the reaction. Heterogeneous catalysts are also easier to prepare and handle. These catalysts consist of fine nanosized powders supported on technically inert oxide substrates exhibiting all possible crystallographic faces. The catalyst is often a metal to which chemical and structural promoters or poisons are added to enhance the efficiency and/or the selectivity. 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.

Table 2: Recent scenario in heterogeneous catalysis.

Year	Catalyst	Method of preparation	Major applications	References
2013	Fe nanocatalyst	Immobilization	Ethylbenzene, cyclohexene, and benzyl alcohol oxidation	[18]
2013	Au/Al2 O3 , Au/C	Deposition-precipitation, cationic adsorption	Glucose oxidation	[79]
2013	Au/Pt bimetallic nanoparticles	-	Glucose oxidation	[80]
2013	Gold nanoparticles supported on Mg(OH)2 nano sheets	Colloidal deposition	CO oxidation	[81]
2013	Au/TiO2 supported on ferritic stainless steel monoliths	Direct anionic exchange	CO oxidation	[82]
2013	Nanoporous gold	Electrolytic dissolution	CO oxidation	[83]
2013	P123-stabilized Au-Ag alloy	Co reduction	Benzyl alcohol oxidation	[84]
2013	Alumina-supported gold-ruthenium bimetallic catalysts	Incipient wetness Impregnation Deposition Precipitation	CO oxidation	[54]
2013	Au/CuO catalysts	Coprecipitation	Alcohol oxidation	[85]
2013	Cerium modified silver	Impregnation	Alkyl aromatic compounds	[86]
2013	Pd-Au catalyst	Dealloying	Methanol electrooxidation	[87]
2013	Au/ZnO and Au/TiO2 catalysts	Colloidal deposition	Methanol oxidation	[88]
2013	Microstructured Au/Ni-fiber catalyst	Incipient impregnating	Alcohol oxidation	[89]
2013	Nanocrystalline Ag and Au-Ag alloys supported on titania	Deposition Precipitation	CO oxidation	[90]
2013	Nanosized Au supported on 3-D ordered mesoporous MnO2	Deposition Precipitation	Oxidation of carbon monoxide, benzene, and toluene	[91]
2013	Au/ [figure omitted; refer to PDF]	Coprecipitation	CO oxidation	[92]
2013	Nanosized ruthenium particles decorated carbon nanofibers	Sol gel	p -Cymene oxidation	[93]
2012	Au/C	Incipient wetness impregnation	Glucose oxidation	[94]
2012	CeAlPO-5 molecular sieves	-	Diphenylmethane oxidation	[10]
2012	Nanosized gold on SiO2	Stöber	Cyclohexene and D-glucose oxidation	[95]
2012	Au/SiO2	Dispersion	Silanes oxidation	[47]
2012	Nano gold-mesoporous silica	-	CO oxidation, benzyl alcohol oxidation	[96]
2012	Nanosized gold	Dispersion	Alkyl benzene oxidation	[40]
2012	Ag/SBA-15	Impregnation	Alkyl substituted aromatics	[35]
2012	Bimetallic Au-Pd/MgO	Sol-immobilization (SI) and adsorption-reduction (AR)	Benzyl alcohol oxidation	[97]
2012	Inverse Fe2 O3 /Au(111) model catalysts	-	CO oxidation	[98]
2012	Silica-supported Au-Cu alloy	-	Alcohol oxidation	[99]
2012	Gold nanoparticles supported on MgO	Deposition-precipitation	Alcohol oxidation	[100]
2012	Silica-supported Au- [figure omitted; refer to PDF]	Oxidative dealloying	Ethanol oxidation	[101]
2011	Au/Al2 O3	Incipient wetness impregnation	Glucose oxidation	[102]
2011	Au-Pd/C	Impregnation	Glyoxal and glucose oxidation	[103]
2011	Pd-Te supported catalysts	Repeated impregnation	Glucose oxidation	[104]
2011	Gold nanoparticles supported on functionalized mesoporous silica	One-pot Synthesis	Cyclohexane oxidation	[105]
2011	Silica supported cobalt (II) salen complex	Immobilization	Alkyl benzene oxidation	[70]
2011	Gold nanowires	-	Oxidation of benzylic compounds	[106]
2011	Cu3/2 [figure omitted; refer to PDF] PMo12 O40 [figure omitted; refer to PDF] /SiO2	Incipient wetness impregnation	Benzylic alcohol	[107]
2010	Gold nanoparticles deposited on cellulose	Deposition-reduction, grinding method	Glucose oxidation	[41]
2010	Metalloporphyrin bound to silica	Immobilization	Ethylbenzene oxidation	[68]
2010	Hydrophobized palladium	Vapor deposition	Glucose oxidation	[108]
2010	Supported gold catalysts	Colloidal gold deposition	CO oxidation	[109]
2010	Au/HMS catalysts	Impregnation and direct synthesis	Benzyl alcohol oxidation	[63]
2010	Mobilized gold nanoparticles	Gold sol	Secondary alcohols oxidation	[67]
2010	Mesoporous Co3 O4 and Au/Co3 O4 catalysts	Nanocasting	Ethylene oxidation	[110]
2010	Metal-organic framework supported gold nanoparticles	Colloidal deposition	Alcohol oxidation	[111]
2010	Pt/Al2 O3	Impregnation	Heavy hydrocarbons oxidation	[112]
2009	Au/TiO2	Deposition-precipitation	Alcohol oxidation	[113]
2009	Co(AcO)2 /Mn(AcO)2	Direct condensation	p -xylene oxidation	[114]
2009	Nickel substituted copper chromite spinels	Coprecipitation	Alkyl substituted benzene oxidation	[9]
2007	Gold catalysts	Deposition-precipitation	Alcohol oxidation	[115]
2007	MCM-48 molecular sieve modified with SnCl2	Post-synthesis modification	Alcohol oxidation	[65]
2007	CuO-impregnated mesoporous silica	Impregnation	Benzene oxidation	[116]
2006	Supported gold catalysts	Deposition-precipitation	Alcohol oxidation,	[117]
2006	Au-CuO/Al2 O3 , Pt/Al2 O3 catalysts	Deposition-precipitation, impregnation	Propene and propane oxidation	[118]
2006	Manganese containing mesoporous MCM-41 and Al-MCM-41 molecular sieves	Impregnation	p -isopropyltoluene oxidation	[119]
2005	Gold catalysts	-	Alcohol oxidation	[120]
2005	Au/C	Immobilization	Glucose oxidation, Alcohol oxidation	[64]
2005	Gold immobilized mesoporous silica	Immobilization	Cyclohexane oxidation	[121]
2005	Nitrous oxide over MFI zeolites	Hydrothermal	Benzene oxidation	[122]
2005	CoAPO-5 molecular sieves	Hydrothermal	Cyclohexane oxidation	[123]
2004	Carbon-supported gold	Gold sol	Glucose oxidation	[124]
2004	Mn-containing MCM-41	Impregnation	Ethylbenzene oxidation	[72]
2003	[figure omitted; refer to PDF] /CeO2	Coprecipitaion	Carbon monoxide oxidation	[125]
2002	Gold catalysts	Immobilization	Glucose oxidation	[126]
2002	Mn (Salen)/MCM-41	-	Olefins epoxidation	[127]
2002	Nanostructured [figure omitted; refer to PDF] /CeO2	Gas-condensation	Carbon monoxide oxidation	[128]
2002	Nano-Au Catalysts	-	Carbon monoxide oxidation	[55]
2001	Au/TiO2 , Au/TiO2 /SiO2	Deposition-precipitation	Propene epoxidation	[129]
2000	Gold-titania catalysts	Deposition-precipitation	Propylene oxidation	[130]
1999	Gold dispersed on TS 1 and other titanium-containing supports	Dispersion	Propene epoxidation	[131]
1998	Gold-titania catalysts	Deposition-precipitation	Propylene epoxidation	[61]
1996	Heteropoly catalysts containing Ru(III) and Rh(III) particles	-	Alkane oxidation	[132]
1996	Gold supported on ZnO and TiO2	Coprecipitation & Deposition-precipitation	Carbon monoxide oxidation	[133]
1996	Au-TiO2	Incipient wetness impregnation	Carbon monoxide oxidation	[134]
1995	Bismuth promoted palladium catalysts	Ion exchange	Glucose oxidation	[42]

3. Heterogeneous Metal Catalysts in Oxidation Reactions

Over the last few decades, scientists have paid tremendous attention to heterogeneous catalysts to overcome the limitations of their homogeneous counterparts to increase products yields and minimize side reactions. Herein, we reported a summary of selected oxidation reactions catalyzed by supported metal catalysts.

3.1. Conversion of Glucose to Gluconic Acid

Recently, the aerobic oxidation of glucose to gluconic acid (Figure 2) has gained much consideration because of its water-soluble cleansing properties and application in food additives and beverage bottle detergents [41]. In the past, the oxidation of glucose was carried out via biochemical pathways which are cumbersome, multistep process, not recyclable, and expensive [42]. The development of catalytic route is probably an alternative pathway for the large scale production of gluconic acid from glucose. In 1970s, researchers used to dope Pt or Pd onto some heavy metals such as bismuth. However, several limitations, such as instability, poor selectivity, and low conversion rate, were encountered with this procedure without any supporting materials [42]. On the other hand, bismuth on palladium or Pt/Pd on carbon supports demonstrated high selectivity and stability and excellent conversion rate, overcoming the limitations of the heavy metal supports. Some features such as catalyst type and the role of bismuth support are still a disputed issue [42].

Figure 2: Conversion of glucose to gluconic acid.

[figure omitted; refer to PDF]

Prati and Rossi (1997) [43] studied the oxidation of 1,2-diols and found excellent selectivity with gold catalyst over platinum and palladium catalysts. The gold catalyst showed unusual selectivity in the oxidation of alcohol to its corresponding carboxylates whereas Pd or Pt showed lower selectivity to oxidize ethane-1,2-diol. From this observation, they also concluded that Au is less sensitive to overoxidation and/or self-poisoning than Pd or Pt. Gold clusters and nanoparticles (NPs) deposited on the metal oxide surface such as Al₂ O₃ and ZrO₂ demonstrated unexpected catalytic activity in the oxidation of glucose with better turnover frequency (TOF, reaction rate per Au atom surface). In addition to carbon and metal oxide supports, some inorganic polymers such as silica could be used as catalytic supports for small Au nanoparticles (>10 nm in diameter) [43]. The catalytic effect of Au nanoparticles (2.5 nm) held by polymer gel was demonstrated by Ishida et al., [44]. Polymer supported AuNPs exhibited higher catalytic performance than Au/C in the oxidation of primary alcohols such as benzyl alcohol to benzaldehyde in absence of base [45]. The catalytic activity of various catalysts for glucose oxidation is summarized in Table 3.

Table 3: Oxidation of glucose by various catalysts.

Name of catalysts	Preparation method	Reaction condition	Main product	Selectivity (%)	References
Substrate	Oxidant	Reaction time (h)	Reaction temperature (°C)	pH	Solvent
Gold nanoparticles on cellulose	Deposition-reduction		O2	-	60	9.5	Water	Gluconic acid	-	[41]
Au/Al2 O3	Deposition-precipitation		O2	7	60	9.0	Water	Gluconic acid	97	[79]
Au/C	Cationic adsorption		O2	7	60	9.0	Water	Gluconic acid	97	[79]
Au-Pd/C	Impregnation		O2	20	50	9.25	-	Gluconic acid	-	[103]
Au/Al2 O3	Incipient wetness impregnation	Glucose	H2 O2		40	9.0	-	Sodium D-gluconate	99	[102]
Au/C	Gold sol		-	30	50	9.5	-	Gluconic acid	45	[124]
Nanosized Au/SiO2	Stöber		H2 O2	24	30	9.2	Water	Gluconic acid	80	[95]
Pb-Te/SiO2	Repeated impregnation		O2	1.5	60	9.0	-	Gluconic acid	88.4	[104]
Au/Pt bimetallic nanoparticle	Vacuum drying		O2	2	60	9.5	-	Gluconic acid	-	[80]

3.2. Selective Oxidation of Silanes to Silanols

Silane is an inorganic compound having the silicon atom with chemical formula SiH₄ . It is a colorless flammable gas with a sharp and repulsive smell, somewhat similar to that of acetic acid. Silane has interest as a precursor of silicon metal. Silane may also be referred to many compounds containing silicon, such as trichlorosilane (SiHCl₃ ), trimethyl(phenyl)silane (PhSi(CH₃ )₃ ), and tetramethylsilane (Si(CH₃ )₄ ) (Scheme 1).

Scheme 1: [figure omitted; refer to PDF]

The oxidation of silane to corresponding silanols (as for example dimethylphenylsilane to dimethylphenylsilanol, Scheme 2) is a key reaction to manufacture building blocks for the synthesis of silica based polymers [46] and nucleophilic couplers in organic synthesis. In the past, silanols synthesis was often carried out by stoichiometric oxidation of organosilanes, hydrolysis of halosilanes, or alkali treatment of siloxanes which incurred environmental hazards. In contrast, the catalytic oxidation of silanes with water is an ecofriendly process since it produces silanols with high selectivity, producing only hydrogen as a by-product. Supported gold nanoparticles have shown higher catalytic activity and selectivity on silane oxidation over other transition metal catalysts [47]. Mitsudome et al. [48] oxidized aliphatic silanes to silanols using hydroxyapatite supported AuNPs in water at 80°C. Nanoporous gold also showed high reactivity and selectivity towards silanes in acetone at room temperature [49].

Scheme 2: [figure omitted; refer to PDF]

Recently, John et al. [50] have synthesized carbon nanotube-supported gold nanoparticles which showed turnover frequency (TOF) of 18,000 h^-1 for silane oxidation in tetrahydrofuran (THF) at room temperature. However, the preparation of Au CNT (carbon nanotube) hybrids involved a multistep layer-by-layer assembly which needed expensive reagents which have limited its practicability. Li et al. [47] prepared silica supported gold catalysts for the selective oxidation of silanes. However, they observed that silica supported gold catalysts are more active than reducible oxides (TiO₂ , Fe₂ O₃ , CeO₂ , etc.) supported AuNPs. Highly dispersed silica supported gold catalysts override the reducible oxides supported AuNPs due to superior adsorption of silane substrate on silica support. Surprisingly, for the oxidation of dimethylphenylsilane in THF at room temperature, the Au/SiO₂ catalyst afforded a TOF of 59,400 h^-1 , which is the highest TOF reported to date.

The other oxide supported gold catalysts, such as Au/TiO₂ , Au/ZnO, and Au/Fe₂ O_3, were less active than Au/SiO₂ , and they afforded a maximum conversion of 90%. However, the activity of Au/CeO₂ catalyst was very similar to the Au/SiO₂ catalyst (Table 4).

Table 4: Comparison of supported gold catalysts for the oxidation of triethylsilane [47].

Catalysts	Reaction condition	Conversion rate (%)	Yield (%)
Substrate	Solvent	Reaction temperature	Time (min)	Au/substrate (mol%)
Au/SiO2	Triethylsilane	Water	25°C	3	0.4	99	99
Au/TiO2	Water	25°C	3	0.4	81	81
Au/Fe2 O3	Water	25°C	3	0.4	36	36
Au/ZnO	Water	25°C	3	0.4	89	89
Au/CeO2	Water	25°C	3	0.4	98	98

3.3. Oxidation of Hydrogen to Hydrogen Peroxide (H₂ O₂ )

H₂ O₂ is an essential chemical which has long been used mainly as strong oxidant in various oxidative reactions and bleaching agent as well as a disinfectant. It is a green oxidant since its sole by-product is water. In the current decades, a lot of attention has been paid to the green catalysts and green chemicals to ensure safety issues in health and environment. Industries have been using supported Pd catalysts for more than 90 years for the direct synthesis of H₂ O₂ from H₂ and O₂ . However, the synthesized H₂ O₂ is unstable and undergoes low-temperature decomposition or hydrogenation to water (Scheme 3) [51]. Recently, Edwards et al. [52] used Au-catalysts synthesized via coprecipitation or deposition-precipitation method and found very low H₂ O₂ conversion rate. They also observed that the addition of Au to Pd catalysts by impregnation enhances H₂ O₂ formation. They compared five different catalyst supports, namely, Al₂ O₃ , Fe₂ O₃ , TiO₂ , SiO_2, and carbon, and found the high conversion with carbon-supported Au-Pd (Au-Pd/C).

Scheme 3: Hydrogen peroxide formation, hydrogenation, and decomposition.

[figure omitted; refer to PDF]

In 2010, Song et al. [53] observed that KMnO₄ treated activated carbon in an acidic solution enhances H₂ O₂ production (78%) from hydroxylamine due to the creation of surface active quinoid species during oxidation. Structure and surface analyses revealed that KMnO₄ treatment produced more phenolic but less carboxylic groups on the activated carbon under acidic condition, confirming the crucial role of the quinoid groups. It was also proposed that the quinoid groups served as electron acceptors and redox mediators in the formation of H₂ O₂ [53].

3.4. Carbon Monoxide (CO) Oxidation

In the last decade, CO oxidation has become an important research area because of its involvement in a number of processes, such as methanol synthesis, water gas shift reaction, carbon dioxide lasers, and automotive exhaust controls [54]. Carbon monoxide is a lethal gas for animal life and toxic to the environment [55]. The oxidation of CO is a difficult process and hence a highly active oxidation catalyst is required for its efficient removal from the environment [55]. In the past, the gold was considered to be inert for CO oxidation [56].

However, Haruta et al. [57] demonstrated that highly dispersed gold prepared on various metal oxide supports by coprecipitation and deposition-precipitation methods is highly active in CO oxidation even below 0°C temperature. They found that catalytic performance significantly depends on the catalysts preparation methods and the highest activity was demonstrated by TiO₂ supported gold or platinum catalysts prepared by deposition-precipitation (DP). The gold catalysts prepared by photodeposition (PD) and impregnation (IMP) methods were less active than those prepared by deposition-precipitation. This is because the catalysts prepared by DP method contain higher loading of Au (>2 wt%) on smaller particles and are with better dispersion. Collectively, these features enable the catalyst to show higher activity, oxidizing ~100% of CO at temperatures below -20°C. In 1997, Yuan et al. [58] synthesized highly active gold catalysts for CO oxidation simply by grafting Au-phosphine complexes (AuL₃ NO₃ or Au₉ L₈ (NO₃ )₃ ; L = PPh₃ ) onto precipitated Ti(OH)₄ surfaces. This Au-phosphine-Ti(OH)₄ complex was active even below the 0°C. Apart from this, Na⁺ ions positively and Cl^- ions negatively affect the Au-catalyzed CO oxidation. Figure 3 represents the initial stages of CO oxidation at the edge of an active gold particle.

Figure 3: Plausible mechanism for CO oxidation on oxide supported gold catalyst. On the left, a CO molecule is chemisorbed onto a low coordination number gold atom (yellow sphere), and a hydroxyl ion is moved from the oxide support (pink sphere) to an Au (III) ion, creating an anion vacancy. On the right they have reacted to form a carboxylate group, and an oxygen molecule occupies the anion vacancy as O^2- (white sphere). This then oxidizes the carboxylate group by abstracting a hydrogen atom, forming carbon dioxide, and the resulting hydroperoxide ion [figure omitted; refer to PDF] then further oxidizes carboxylate species to form another carbon dioxide, restoring two hydroxyl ions to the support surface, completing the catalytic cycle. (Adapted with permission from Springer) [145].

[figure omitted; refer to PDF]

3.5. Epoxidation of Propene

The oxidation of propene to epoxide is an important reaction for the synthesis of various industrial chemicals such as polyether polyols (precursor of polyurethane or foams), propene glycol, and propene glycol ethers (Scheme 4) [59]. In the past, chlorohydrin and hydroperoxide mediated processes were used for the synthesis of propene epoxide. Chlorohydrin process produces environmentally hazardous chlorinated by-products and the hydroperoxide process is much expensive and produces styrene and tert -butyl alcohol as by-products. Silver catalysts were used in this reaction but poor selectivity and turnover were observed [60]. However, titania supported gold efficiently catalyzed the epoxidation reaction at 30-120°C with more than 90% selectivity in the presence of hydrogen [61].

Scheme 4: Synthetic products from propene epoxidation reaction.

[figure omitted; refer to PDF]

3.6. Oxidation of Alcohol

The oxidation of alcohols to its corresponding aldehydes or ketones is a crucial reaction in organic synthesis. Ketones, specially, acetone, are widely used in the production of various organic as well as fine chemicals [62]. Traditional chemical routes use stoichiometric chemicals such as chromium (VI) reagents, dimethyl sulfoxide, permanganates, periodates, or N -chlorosuccinimide which are expensive and hazardous. Several homogeneous catalysts such as Pd, Cu, and Ru are found to selectively catalyze alcohol oxidation. However, homogeneous catalysis requires high pressure oxygen and/or organic solvent, incurring cost and environmental burdens [63]. The present ecological deterioration has forced researchers to look for novel and environmentally friendly catalytic schemes for the oxidation of alcohol. Prati and Porta [64] demonstrated that Au/C catalyst shows higher selectivity toward aldehyde in the oxidation of primary alcohols. Subsequently, Endud and Wong [65] synthesized porous Si/Sn bimetallic catalyst through postsynthesis modification of rice husk ash as Si precursor and SnCl₂ as tin source. Using TBHP oxidant, the tin modified MCM-48 showed much selectivity toward aldehyde or ketone in the oxidation of benzyl alcohols [65].

Chaki et al. [66] looked into the catalytic activity of gold by adding silver (5-30% Ag content) into gold particles for aerobic oxidation of alcohols. It showed that <10% Ag accelerates the catalytic activity of Au. Recently, Kidwai and Bhardwaj [67] described that gold nanoparticles (AuNP) are highly active in alcohol oxidation with hydrogen peroxide as oxidant. They observed that AuNPs, with extended surface area, exhibit higher catalytic activity over others. Additionally, gold catalyzed reactions are free from chemical hazards and toxic solvents and produce water as the only side product. This methodology was a great contribution towards the development of sustainable green chemistry.

4. Heterogeneous Catalysts in the Oxidation of Alkyl Substituted Benzene

In this Section, we described various catalysts, their synthetic schemes, and performance for the oxidation of alkyl substituted benzenes which are an important compound in organic synthesis.

4.1. Fe Nanocatalysts

Habibi et al. [18] synthesized Fe nanocatalyst which oxidized alkyl substituted benzene. They prepared the heterogeneous nano-Fe catalyst on the SiO₂ /Al₂ O₃ supports through the covalent immobilization of ferrocenecarboxaldehyde which acts as iron source (Figure 4). In the presence of tert -butyl hydroperoxide (TBHP) oxidant, this catalyst produces acetophenone, benzaldehyde, and benzoic acid from ethylbenzene with 89% selectivity to acetophenone (Scheme 5).

Scheme 5: Products from the catalytic oxidation of ethyl aromatic with novel Fe nanocatalysts.

[figure omitted; refer to PDF]

Figure 4: Synthesis of heterogeneous Fe nanocatalysts by the immobilization of Fe on functionalized SiO₂ -Al₂ O₃ mixed oxide 3-aminopropyltrimethoxysilane (3-APTMS). Adapted with permission from Elsevier [18].

[figure omitted; refer to PDF]

This catalytic scheme provided certain benefits including the low cost raw materials, commercially available simple chemicals, and catalysts reusability for the further oxidation of ethylbenzene. The side chain carbonyl group is produced by TBHP oxidant without any solvent at a substrate/TBHP ratio of 1 : 1, at 50-120°C in a day.

This novel Fe nanocatalyst exhibited higher conversion rate (>84%) of ethylbenzene with 90% selectivity toward acetophenone which is the precursor of many products such as resins, chalcones, drugs, fine chemicals, and optically active alcohols. The comparative performances of various catalysts for alkyl benzene oxidation are given in Table 5.

Table 5: Catalysts for alkyl benzene oxidation.

Name of catalysts	Substrate	Oxidant	Reaction time (h)	Reaction temperature (°C)/solvent	Preparation method	Main product	Selectivity (%)	References
Fe nanocatalysts on the surface SiO2 /Al2 O3		TBHP	24	50/-	Immobilization	Acetophenone	89	[18]
Ag/SBA-15		TBHP	5	90/-	Impregnation	Acetophenone	99	[35]
Nickel substituted Cu chromite spinel		TBHP	8	70/CH3 CN	Coprecipitation	Acetophenone	69	[9]
Silica supported cobalt, NHPI		O2	24	100/CH3 COOH	Immobilization	Acetophenone	91	[70]
Au/SBA-15	Ethylbenzene	TBHP	36	70/CH3 CN	In situ impregnation	Acetophenone	93	[40]
Mn-containing MCM-41U	O2	-	350/	Impregnation	Acetophenone	93.6	[72]
[figure omitted; refer to PDF] Fe(tpa) (MeCN)2 [figure omitted; refer to PDF] (ClO4 )2		O2	24	75°C/2-butanone	-	Acetophenone	54	[135]
a TPFPPFeCl		O2	24	100/-	-	Acetophenone	82.8	[18]
Fe/MgO, b NHPI		O2	20	25/-	-	Acetophenone	52	[18]
Fe (salen)-c POM		H2 O2	5	80/CH3 CN	-	Acetophenone	100	[18]

[figure omitted; refer to PDF] Fe (5, 10, 15, 20-tetrakis (pentafluorophenyl)) porphyrin; ^b N -hydroxyphthalimide; ^c Kegging type polyoxometalate (K8SiW11O39) [17]. U = unwashed.

4.2. Manganese (III) Porphyrin Complexes in the Oxidation of Alkyl Substituted Benzene

Silica bound manganese (III) porphyrin complexes, [Mn(TMCPP)](TMCPP: 5, 10, 15, 20-tetrakis -(4-methoxycarbonylphenyl)-21,23H -porphyrin], selectively catalyzes the oxidation of alkyl substituted benzene to its corresponding ketone. Ghiaci et al. [68] synthesized manganese porphyrin complexes by immobilization onto silica support. This catalyst complex showed high selectivity and efficiency toward hydrocarbon oxidation due to its shape selectivity toward substrate and matrix support that provided special atmosphere for C-H oxidation [69]. For catalysts synthesis, the silica gel was made active at high temperature (500°C) followed by modification with 3-aminopropyltriethoxysilane that acts as silica source under inert gas (N₂ ) atmosphere. The details of the preparation of this catalyst are described elsewhere (Figure 5). The effects of various parameters such as oxidants, solvents, and temperature on the oxidation of substituted benzene were studied and the maximum catalysis was obtained with TBHP oxidant at 150°C under solvent free conditions.

Figure 5: The synthetic scheme of manganese porphyrin complex by immobilization on silica support (Adapted with permission from Elsevier [68]).

[figure omitted; refer to PDF]

4.3. Ag/SBA-15 Catalysts in the Oxidation of Alkyl Substituted Benzene

The C-H bond of alkyl substituted benzene can be selectively oxidized to its corresponding ketones by Ag/SBA-15 catalysts with TBHP as oxidant. Recently, Anand et al. [35] synthesized the silica supported Ag catalysts by impregnation method and found that Ag/SBA-15 is an environmentally friendly catalyst for the breaking of alkyl benzene C-H bond. They used tetraethyl orthosilicate as silica source and silver nitrate as silver source. The schematic of the synthetic scheme is given in Figure 6, and the details could be obtained from bibliography [35]. The prepared catalyst showed the best conversion rate in presence of tert -butyl hydroperoxide oxidant with 92% and 99% selectivity towards ketone under solvent free condition (Table 6).

Table 6: Effect of various solvents on the Ag/SBA-15 catalyzed oxidation of alkyl substituted benzene at 90°C in presence of 70% TBHP oxidant [35].

Solvent	Conversion (%)	Selectivity (%)
Acetophenone	1-phenylethanol
Toluene	92	92	8
DMF	15	80	20
Acetonitrile	85	86	12
Water	65	89	10
No solvent	92	99	1

Figure 6: Synthesis of Ag/SBA-15 catalysts by impregnation method.

[figure omitted; refer to PDF]

4.4. Nickel Substituted Copper Chromite Spinels

Another form of catalysts, called nickel substituted copper chromite (Cu₂ Cr₂ O₅ ) spinels, can efficiently catalyze the oxidation of alkyl substituted benzene. George and Sugunan (2008) [9] synthesized nickel substituted copper chromite spinels using copper nitrate, nickel nitrate, and chromium nitrate via coprecipitation method. In the first step, a solution of copper, nickel, and chromium nitrate was prepared in water. The pH of the solution adjusted to 6.5-8.0 with the stepwise addition of 15% ammonium solution under constant stirring. The precipitate was maintained at 70-80°C for 2 h and aged for 24 h. Finally, the precipitate was filtered, washed, and dried at 353 K for 24 h and calcined at 923 K for 8 h to get the spinels. Figure 7 depicts the complete procedure for the synthesis of nickel substituted copper chromite spinel. The recipe of George and Sugunan (2008) [9] for the preparation of nickel substituted copper chromite spinels catalyst is given in Table 7.

Table 7: Recipe for the preparation of various nickels substituted copper chromite spinels [9].

Catalysts composition ( [figure omitted; refer to PDF] Cr2 O4 )	Designation
CuCr2 O4 ( [figure omitted; refer to PDF] = 0)	CCr
Cu0.75Ni0.25Cr2 O4 ( [figure omitted; refer to PDF] = 0.25)	CNCr-1
Cu0.5Ni0.5Cr2 O4 ( [figure omitted; refer to PDF] = 0.5)	CNCr-2
Cu0.25Ni0.75Cr2 O4 ( [figure omitted; refer to PDF] = 0.75)	CNCr-3
NiCr2 O4 ( [figure omitted; refer to PDF] = 1)	NCr

Figure 7: Synthesis of nickel substituted copper chromite spinels.

[figure omitted; refer to PDF]

Catalytic activity of each spinel for the oxidation of ethylbenzene was studied in detail [9] and it was found that CNCr-2 type chromite spinel provides the maximum conversion rate (56.1%) with 68.7% selectivity towards acetophenone (Table 8) under solvent free conditions [9]. Nickel substituted chromites were compared with those simple chromites, and the nickel chromites demonstrated superior activity.

Table 8: Oxidation of ethylbenzene by nickel substituted copper chromite spinels [9].

Catalysts	Conversion (%)	Selectivity (%)
Acetophenone	1-phenylethanol	Others
CCr	32.9	13.9	83.4	2.7
CNCr-1	44.7	51.9	46.4	1.7
CNCr-2	56.1	68.7	28.1	3.2
CNCr-3	55.5	55.6	39.6	4.8
NCr	20.2	59.1	19.4	21.5

Reaction conditions: temperature 70°C, time 8 h, EB: TBHP ratio 1 : 2, catalyst weight 0.1 g, solvent 10 mL acetonitrile [9].

4.5. Silica Supported Cobalt (II) Salen Complex

The aerobic oxidation of alkyl substituted benzene was successfully carried out over silica supported cobalt (II) salen complex in presence of O₂ in N -hydroxyphthalimide (NHPI) solvent [70]. Rajabi et al. [70] prepared the silica supported cobalt salen complexes by chemical modification of di-imine cobalt complex using cobalt acetate as a source of cobalt ion (Figure 8). At first Salicylaldehyde was added to the excess amount of absolute MeOH at room temperature and the 3-aminopropyltrimethoxysilane was added to the mixture. The solution turned into yellow color due to the formation of imine which contains a carbon-nitrogen double bond, a hydrogen atom (H), or an organic group is attached to the nitrogen. The addition of cobalt (II) acetate to the imine compound allows the new ligands to complex the cobalt. Prior to surface modification, nanoporous silica was activated by inserting into concentrated HCl and subsequent washing with deionized water (Figure 8).

Figure 8: Preparation of silica supported cobalt (II) catalysts by surface chemical modification. Adapted with permission from Elsevier [70].

[figure omitted; refer to PDF]

Rajabi et al. [70] also investigated the catalytic activity of immobilized cobalt catalysts for ethylbenzene oxidation with O₂ in N -hydroxyphthalimide and other solvents and acetic acid was found to be the best solvent. The selectivity and the conversion rate were increased with temperature. The heterogeneous catalysts were reused four times and a little change in activity was observed (Table 9).

Table 9: Oxidation reaction of ethylbenzene by reused silica supported Co(II) catalysts.

Entry	Run	Temperature (°C)	Selectivity (%)	Yield (%)
Alcohol	Acetophenone
1	First	100	9	91	78
2	Second	100	10	90	78
3	Third	100	10	90	77
4	Fourth	100	10	90	70

4.6. Nanosized Gold-Catalysts

Materials in nanometer size show properties distinct from their bulk counterparts, because nanosized clusters have electronic structures that have high dense states [71]. Biradar and Asefa (2012) [40] described the oxidation of alkyl substituted benzene over silica supported gold nanoparticles. Supported AuNPs were prepared by in situ impregnation method [40] to keep the catalyst well dispersed on the support surfaces. Briefly, a solution of Pluronic P-123 was added to water and hydrochloric acid. Desired amount of TEOS (tetraethoxysilane) was added to the aqeous acidic Pluronic P-123 solution under stirring. The resulting precipitates was subsequently filtered and washed several time under ambient state to get mesostructured SBA-15. For the synthesis of SBA-15 supported gold catalysts, HAuCl₄ solution was made in ethanol/water (1 : 4 ratios) and was well dispersed on the silica support (Figure 9). The lower sized AuNPs demonstrated higher TON (turnover number) and lower TOF (turnover frequency) (Table 10). Solvent effects on oxidation reaction were studied and acetonitrile appeared to be the best solvent. It produced 79% conversion with 93% selectivity towards the ketone products.

Table 10: Oxidation of ethylbenzene by three different types of Au/SBA-15 catalysts [40].

Entry	Catalysts/sample (Au average size)	Wt.% (mmol Au/g)	Conversion (%)	Selectivity (%)	TON	TOF (h-1 )
Ketone	Alcohol
1	SBA-15	-	~0	~0	~0	~0	~0
2	Au/SBA-15 catalyst (5.4 ± 1.2 nm)	1.08% (54.8 μ mol/g)	68	94	6	764	23
3	Au/SBA-15 catalyst(6.9 ± 1.7 nm)	3.86% (196.0 μ mol/g)	79	93	7	274	8
4	Au/SBA-15 catalyst(8.4 ± 2.3 nm)	4.56% (231.5 μ mol/g)	89	94	6	256	7

Reaction condition: substrate, ethylbenzene, 1 mmol; oxidant: 80% TBHP (aq.), 2 mmol; solvent: acetonitrile, 10 mL; catalyst: Au/SBA-15 sample with 15 mg overall mass; reaction temperature: 70°C; internal standard: chlorobenzene (0.5 mL); reaction time: 36 h; and reaction atmosphere: air [40].

Figure 9: Schematic diagram for the synthesis of SBA-15 supported gold catalysts.

[figure omitted; refer to PDF]

4.7. Mn-Containing MCM-41 Catalyst for the Vapor Phase Oxidation of Alkyl Substituted Benzene

Vapour-phase oxidation of alkyl substituted benzene was performed with carbon dioxide-free air as an oxidant over MnO₂ impregnated MCM-41 catalysts [72]. Vetrivel and Pandurangan [72] synthesized MCM-41 on C₁₆ H₃₃ (CH₃ )₃ N⁺ Br^- template. The Mn containing MCM-41 mesoporous molecular sieves were prepared by impregnating MCM-41 into manganese acetate solutions under stirring overnight. Finally, the solution was filtered, washed, evaporated, and calcined at a specific temperature to obtain Mn containing MCM-41 (Figure 10). They also optimized the reaction conditions by varying reaction temperature, weight hourly space velocity, and time on stream. They carried out a number of reactions with the six types of washed and unwashed Mn containing catalysts. In every case, acetophenone was the major products which increase with the increase of metal content in the catalysts. The high conversion rate to acetophenone was obtained with Mn-MCM-41 catalysts with high Mn content. The unwashed catalysts showed higher reactivity than that of washed one due to the high density of active site in the unwashed catalysts.

Figure 10: Schematic diagram for the synthesis of Mn containing MCM-41 catalysts.

[figure omitted; refer to PDF]

5. Preparation Method of Supported Metal Catalysts

A high number of methods have been proposed for the synthesis supported heterogeneous metal catalysts [71]. Table 11 is a summary of the major methods frequently used in catalysts synthesis.

Table 11: Major methods of catalysts synthesis.

Method	Brief description	Limitations	References
Deposition-precipitation	(a) Deposition-precipitation method is easier for the synthesis of various supported metal catalyst complexes in presence of excess alkali. (b) In alkaline media the [Au(en )2 ]3 + cations are deposited on anionic oxide (TiO2 , Fe2 O3 , Al2 O3 , ZrO2 , and CeO2 ) surfaces having high isoelectric point (PI > 7.00). (c) Functionalization of oxides may take part in the reaction as co-catalysts for the enhancement of the catalytic activity. (d) It is a very good method for the oxidation of alkanes to epoxides.	(a) It is a multistep processes for the deposition of metal onto the oxide surface.(b) It cannot integrate AuNPs on metal oxides of low isoelectric point (IEP ~2) such as SiO2 . (c) It is limited to maximum 1 wt% Au-loading. (d) It requires multiple washing steps to eliminate excess chloride	[40, 136, 137]
Cocondensation	(a) It simultaneously forms mesostructure to anchor gold. (b) It easily forms hexagonal array of mesopores and metal crystallites of 3-18 nm in diameter. (c) It is a simple method to insert gold nanoparticles onto the surface of oxides. (d) It permits the formation of particles in metallic state surrounded by chloride ions. These Cl- ions are the basic species for catalysts activation during acetonylacetone (AcAc) transformation (cyclization/dehydration) in gaseous state and also act as promoters for electron transfer to O2 during NO reduction with propene in presence of oxygen.	(a) The surface area of catalysts, prepared by this method, is low.	[136, 138]
Anion adsorption	(a) Aqueous anions (sulfate, arsenates, and anionic functional groups of biomolecules) are adsorbed on the electrically charged metal oxide surfaces (b) Optimum gold loading takes place at 80°C. (c) It is a simple method, with no need for expensive instrumentations and expert personnel.	(a) Gold loading cannot exceed 1.5 wt%. (b) It requires multiple washing steps.	[137, 139, 140]
Cation adsorption	(a) Catalyst can be prepared at room temperature to avoid decomposition of the metal complex and reduction of gold. (b) Higher loading of gold (3 wt%) can be achieved and cation adsorption with metal leads to smaller particles (~2 nm) when the solution/support contact time is moderate (1 h)	(a) In general, the Au loading did not exceed 2 wt%.	[139, 141]
Incipient wetness impregnation	(a) Interaction of gold precursors and the support surface takes place between the oxygen atoms of Me2 Au (acetonylacetone) and the OH groups of the SiO2 surface at high temperature (~300°C). (b) Strong interaction between the metal catalyst and support oxides. Thus catalyst is not easily lost.	(a) The chlorides on support promote the aggregation of AuNPs and frequently poison the active sites of the catalyst. (b) Low pH ( [figure omitted; refer to PDF] 1) and high temperature are prerequisite ( [figure omitted; refer to PDF] 300°C). Contains higher amount of chloride impurities. (c) It cannot produce homogeneous and stable particles.	[136, 137, 139]
Dispersion	(a) it is an attractive method to control the aggregation of AuNPs. (b) Particle size is preserved during the immobilization step. (c) Particles size can easily be controlled. (d) It is highly selective and efficient.	(a) It requires extensive washing steps to remove excess chloride impurities.	[40, 136]
Chemical vapor deposition	(a) Supports are evacuated in vacuum at 200°C for 4 h to remove the adsorbed water (b) In general, OMCVD method involved in a system where the proportion between the substrate area and gas phase volume gets larger, so that the surface reactions hold a key parameter.	(a) It is expensive, requires special equipment, and the amount of metal incorporated by this method is somehow limited by pore volume of inert solid support.	[142, 143]
Etching	(a) It is synthetic methods for yolk-shell nanoparticles (b) It is efficient, cheaper and simple method	(a) Catalysts work only at low temperature.	[40, 144]

6. Concluding Remark

This review provides an extensive overview of the literature regarding the applications and synthesis of some heterogeneous catalysts for oxidation catalysis. Advantages and disadvantages of certain candidature support materials are presented. Special emphasis is given to heterogeneous catalysis, specially the metal-support synergy. The role of appropriate solvent that codissolves the catalysts and substrate to ease the pretreatment and oxidation process is tabulated for better understanding. In line with the goal of industrial process, reaction conditioning and utilization of appropriate and cheap catalysts are briefly outlined. Future research should focus on the synthesis and application of more efficient heterogeneous catalysts as well as synergizing the catalyst cost for large scale synthesis.

Acknowledgment

The authors acknowledge the University of Malaya Fund no. RP005A-13 AET.

Conflict of Interests

The authors declare that they have no conflict of interests regarding the publication of this paper.

References

[1] K. Hemalatha, G. Madhumitha, A. Kajbafvala, N. Anupama, R. Sompalle, S. Mohana Roopan, "Function of nanocatalyst in chemistry of organic compounds revolution: an overview," Journal of Nanomaterials , vol. 2013, 2013.

[2] T. Mehler, W. Behnen, J. Wilken, 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.

[3] V. N. Hasirci, "PVNO-DVB hydrogels: synthesis and characterization," Journal of Applied Polymer Science , vol. 27, no. 1, pp. 33-41, 1982.

[4] G. Newkome, D. Fishel, "Preparation of hydrazones: acetophenone hydrazone," Organic Syntheses , vol. 50, pp. 102-102, 1988.

[5] R. T. Blickenstaff, W. R. Hanson, S. Reddy, 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.

[6] M. Ali, M. Rahman, 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.

[7] I. Kani, M. Kurtça, "Synthesis, structural characterization, and benzyl alcohol oxidation activity of mononuclear manganese(II) complex with 2,2[variant prime]-bipyridine: [Mn(bipy)₂ (ClO₄ )₂ ]," Turkish Journal of Chemistry , vol. 36, no. 6, pp. 827-840, 2012.

[8] P. Gallezot, "Selective oxidation with air on metal catalysts," Catalysis Today , vol. 37, no. 4, pp. 405-418, 1997.

[9] K. George, 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.

[10] S. Devika, M. Palanichamy, V. Murugesan, "Selective oxidation of diphenylmethane to benzophenone over CeAlPO-5 molecular sieves," Chinese Journal of Catalysis , vol. 33, no. 7-8, pp. 1086-1094, 2012.

[11] G. Centi, S. Perathoner, "Catalysis and sustainable (green) chemistry," Catalysis Today , vol. 77, no. 4, pp. 287-297, 2003.

[12] J. H. Clark, D. J. Macquarrie, "Heterogeneous catalysis in liquid phase transformations of importance in the industrial preparation of fine chemicals," Organic Process Research & Development , vol. 1, no. 2, pp. 149-162, 1997.

[13] Y. Wang, X. Wang, M. Antonietti, "Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to multipurpose catalysis to sustainable chemistry," Angewandte Chemie International Edition , vol. 51, no. 1, pp. 68-89, 2012.

[14] D. Cole-Hamilton, R. Tooze, "Homogeneous catalysis-advantages and problems," Catalyst Separation, Recovery and Recycling , pp. 1-8, Springer, 2006.

[15] N. R. Shiju, V. V. Guliants, "Recent developments in catalysis using nanostructured materials," Applied Catalysis A: General , vol. 356, no. 1, pp. 1-17, 2009.

[16] I. Fechete, Y. Wang, J. C. Vedrine, "The past, present and future of heterogeneous catalysis," Catalysis Today , vol. 189, no. 1, pp. 2-27, 2012.

[17] A. Zapf, M. Beller, "Fine chemical synthesis with homogeneous palladium catalysts: examples, status and trends," Topics in Catalysis , vol. 19, no. 1, pp. 101-109, 2002.

[18] D. Habibi, A. R. Faraji, M. Arshadi, 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.

[19] M. R. Maurya, A. Kumar, J. Costa Pessoa, "Vanadium complexes immobilized on solid supports and their use as catalysts for oxidation and functionalization of alkanes and alkenes," Coordination Chemistry Reviews , vol. 255, no. 19, pp. 2315-2344, 2011.

[20] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, "Metal-organic frameworks as heterogeneous catalysts for oxidation reactions," Catalysis Science and Technology , vol. 1, no. 6, pp. 856-867, 2011.

[21] Q. Yin, J. M. Tan, C. Besson, Y. V. Geletii, D. G. Musaev, A. E. Kuznetsov, Z. Luo, K. I. Hardcastle, C. L. Hill, "A fast soluble carbon-free molecular water oxidation catalyst based on abundant metals," Science , vol. 328, no. 5976, pp. 342-345, 2010.

[22] A. Sivaramakrishna, P. Suman, E. V. Goud, S. Janardan, C. Sravani, C. S. Yadav, H. S. Clayton, "Recent progress in oxidation of n-alkanes by heterogeneous catalysis," Research and Reviews in Materials Science and Chemistry , vol. 1, no. 1, pp. 75-103, 2012.

[23] P. Sudarsanam, L. Katta, G. Thrimurthulu, B. M. Reddy, "Vapor phase synthesis of cyclopentanone over nanostructured ceria-zirconia solid solution catalysts," Journal of Industrial and Engineering Chemistry , vol. 19, no. 5, pp. 1517-1524, 2013.

[24] A. Kajbafvala, H. Ghorbani, A. Paravar, J. P. Samberg, E. Kajbafvala, S. K. Sadrnezhaad, "Effects of morphology on photocatalytic performance of Zinc oxide nanostructures synthesized by rapid microwave irradiation methods," Superlattices and Microstructures , vol. 51, no. 4, pp. 512-522, 2012.

[25] K.-H. Kim, S.-K. Ihm, "Heterogeneous catalytic wet air oxidation of refractory organic pollutants in industrial wastewaters: a review," Journal of Hazardous Materials , vol. 186, no. 1, pp. 16-34, 2011.

[26] A. Corma, H. Garcia, F. X. Llabres I Xamena, "Engineering metal organic frameworks for heterogeneous catalysis," Chemical Reviews , vol. 110, no. 8, pp. 4606-4655, 2010.

[27] A. Kajbafvala, S. Zanganeh, E. Kajbafvala, H. R. Zargar, M. R. Bayati, S. K. Sadrnezhaad, "Microwave-assisted synthesis of narcis-like zinc oxide nanostructures," Journal of Alloys and Compounds , vol. 497, no. 1-2, pp. 325-329, 2010.

[28] M. Yoon, R. Srirambalaji, K. Kim, "Homochiral metal-organic frameworks for asymmetric heterogeneous catalysis," Chemical Reviews , vol. 112, no. 2, pp. 1196-1231, 2012.

[29] K. C. Gupta, A. K. Sutar, C.-C. Lin, "Polymer-supported Schiff base complexes in oxidation reactions," Coordination Chemistry Reviews , vol. 253, no. 13-14, pp. 1926-1946, 2009.

[30] A. Kumar, V. P. Kumar, B. P. Kumar, V. Vishwanathan, K. V. R. Chary, "Vapor phase oxidation of benzyl alcohol over gold nanoparticles supported on mesoporous TiO₂ ," Catalysis Letters , vol. 144, no. 8, pp. 1450-1459, 2014.

[31] D. R. Burri, I. R. Shaikh, K.-M. Choi, S.-E. Park, "Facile heterogenization of homogeneous ferrocene catalyst on SBA-15 and its hydroxylation activity," Catalysis Communications , vol. 8, no. 4, pp. 731-735, 2007.

[32] S. Sreevardhan Reddy, B. David Raju, V. Siva Kumar, A. H. Padmasri, S. Narayanan, K. S. Rama Rao, "Sulfonic acid functionalized mesoporous SBA-15 for selective synthesis of 4-phenyl-1,3-dioxane," Catalysis Communications , vol. 8, no. 3, pp. 261-266, 2007.

[33] D. J. Kim, B. C. Dunn, P. Cole, "Enhancement in the reducibility of cobalt oxides on a mesoporous silica supported cobalt catalyst," Chemical Communications , no. 11, pp. 1462-1464, 2005.

[34] R. Burri, K.-W. Jun, Y.-H. Kim, J. M. Kim, S.-E. Park, J. S. Yoo, "Cobalt catalyst heterogenized on SBA-15 for p -xylene oxidation," Chemistry Letters , vol. 31, no. 2, pp. 212-213, 2002.

[35] N. Anand, K. H. P. Reddy, G. V. S. Prasad, K. S. Rama Rao, 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.

[36] J. H. Nam, Y. Y. Jang, Y. U. Kwon, J. D. Nam, "Direct methanol fuel cell Pt-carbon catalysts by using SBA-15 nanoporous templates," Electrochemistry Communications , vol. 6, no. 7, pp. 737-741, 2004.

[37] M. Arsalanfar, A. A. Mirzaei, H. R. Bozorgzadeh, A. Samimi, R. Ghobadi, "Effect of support and promoter on the catalytic performance and structural properties of the Fe-Co-Mn catalysts for Fischer-Tropsch synthesis," Journal of Industrial and Engineering Chemistry , vol. 20, no. 4, pp. 1313-1323, 2014.

[38] A. Kajbafvala, M. R. Shayegh, M. Mazloumi, S. Zanganeh, A. Lak, M. S. Mohajerani, S. K. Sadrnezhaad, "Nanostructure sword-like ZnO wires: rapid synthesis and characterization through a microwave-assisted route," Journal of Alloys and Compounds , vol. 469, no. 1-2, pp. 293-297, 2009.

[39] P. J. Kropp, G. W. Breton, J. D. Fields, J. C. Tung, B. R. Loomis, "Surface-mediated reactions. 8. Oxidation of sulfides and sulfoxides with tert-butyl hydroperoxide and OXONE," Journal of the American Chemical Society , vol. 122, no. 18, pp. 4280-4285, 2000.

[40] A. V. Biradar, 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.

[41] T. Ishida, H. Watanabe, T. Bebeko, T. Akita, M. Haruta, "Aerobic oxidation of glucose over gold nanoparticles deposited on cellulose," Applied Catalysis A: General , vol. 377, no. 1, pp. 42-46, 2010.

[42] M. Besson, F. Lahmer, P. Gallezot, P. Fuertes, G. Fleche, "Catalytic oxidation of glucose on bismuth-promoted palladium catalysts," Journal of Catalysis , vol. 152, no. 1, pp. 116-121, 1995.

[43] L. Prati, M. Rossi, "Chemoselective catalytic oxidation of polyols with dioxygen on gold supported catalysts," Studies in Surface Science and Catalysis , vol. 110, pp. 509-515, 1997.

[44] T. Ishida, H. Watanabe, T. Bebeko, M. Haruta, "Aerobic oxidation of glucose over gold nanoparticles deposited on cellulose," Applied Catalysis A: General , vol. 377, no. 1-2, pp. 42-46, 2010.

[45] T. Ishida, S. Okamoto, R. Makiyama, 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.

[46] R. Murugavel, M. G. Walawalkar, M. Dan, H. W. Roesky, C. N. R. Rao, "Transformations of molecules and secondary building units to materials: a bottom-up approach," Accounts of Chemical Research , vol. 37, no. 10, pp. 763-774, 2004.

[47] W. Li, A. Wang, X. Yang, Y. Huang, T. Zhang, "Au/SiO₂ as a highly active catalyst for the selective oxidation of silanes to silanols," Chemical Communications , vol. 48, no. 73, pp. 9183-9185, 2012.

[48] T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa, K. Kaneda, "Supported gold nanoparticle catalyst for the selective oxidation of silanes to silanols in water," Chemical Communications , no. 35, pp. 5302-5304, 2009.

[49] N. Asao, Y. Ishikawa, N. Hatakeyama, "Nanostructured materials as catalysts: nanoporous-gold-catalyzed oxidation of organosilanes with water," Angewandte Chemie , vol. 49, no. 52, pp. 10093-10095, 2010.

[50] J. John, E. Gravel, A. Hagège, H. Li, T. Gacoin, E. Doris, "Catalytic oxidation of silanes by carbon nanotube-gold nanohybrids," Angewandte Chemie-International Edition , vol. 50, no. 33, pp. 7533-7536, 2011.

[51] P. Landon, P. J. Collier, A. J. Papworth, C. J. Kiely, G. J. Hutchings, "Direct formation of hydrogen peroxide from H₂ /O₂ using a gold catalyst," Chemical Communications , no. 18, pp. 2058-2059, 2002.

[52] J. K. Edwards, A. Thomas, B. E. Solsona, P. Landon, A. F. Carley, G. J. Hutchings, "Comparison of supports for the direct synthesis of hydrogen peroxide from H₂ and O₂ using Au-Pd catalysts," Catalysis Today , vol. 122, no. 3-4, pp. 397-402, 2007.

[53] W. Song, Y. Li, X. Guo, J. Li, X. Huang, W. Shen, "Selective surface modification of activated carbon for enhancing the catalytic performance in hydrogen peroxide production by hydroxylamine oxidation," Journal of Molecular Catalysis A: Chemical , vol. 328, no. 1-2, pp. 53-59, 2010.

[54] O. A. Kirichenko, E. A. Redina, N. A. Davshan, I. V. Mishin, G. I. Kapustin, T. R. Brueva, L. M. Kustov, W. Li, C. H. Kim, "Preparation of alumina-supported gold-ruthenium bimetallic catalysts by redox reactions and their activity in preferential CO oxidation," Applied Catalysis B: Environmental , vol. 134-135, pp. 123-129, 2013.

[55] T. V. Choudhary, C. Sivadinarayana, C. C. Chusuei, A. K. Datye, J. P. Fackler Jr., D. W. Goodman, "CO oxidation on supported nano-Au catalysts synthesized from a [Au6(PPh₃ )₆ ](BF₄ )₂ complex," Journal of Catalysis , vol. 207, no. 2, pp. 247-255, 2002.

[56] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, "Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon monoxide," Journal of Catalysis , vol. 115, no. 2, pp. 301-309, 1989.

[57] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet, B. Delmon, "Low-temperature oxidation of CO over gold supported on TiO₂ , α -Fe₂ O₃ , and CO₃ O₄ ," Journal of Catalysis , vol. 144, no. 1, pp. 175-192, 1993.

[58] Y. Yuan, A. P. Kozlova, K. Asakura, H. Wan, K. Tsai, Y. Iwasawa, "Supported Au catalysts prepared from Au phosphine complexes and as-precipitated metal hydroxides: characterization and low-temperature CO oxidation," Journal of Catalysis , vol. 170, no. 1, pp. 191-199, 1997.

[59] B. K. Min, C. M. Friend, "Heterogeneous gold-based catalysis for green chemistry: low-temperature CO oxidation and propene oxidation," Chemical Reviews , vol. 107, no. 6, pp. 2709-2724, 2007.

[60] T. A. Nijhuis, M. Makkee, J. A. Moulijn, B. M. Weckhuysen, "The production of propene oxide: catalytic processes and recent developments," Industrial and Engineering Chemistry Research , vol. 45, no. 10, pp. 3447-3459, 2006.

[61] T. Hayashi, K. Tanaka, M. Haruta, "Selective vapor-phase epoxidation of propylene over Au/TiO₂ catalysts in the presence of oxygen and hydrogen," Journal of Catalysis , vol. 178, no. 2, pp. 566-575, 1998.

[62] Y.-H. Kim, S.-K. Hwang, J. W. Kim, Y.-S. Lee, "Zirconia supported ruthenium catalyst for efficient aerobic oxidation of alcohols to aldehyde," Industrial & Engineering Chemistry Research , vol. 53, no. 31, pp. 12548-12552, 2014.

[63] C. Y. Ma, J. Cheng, H. L. Wang, Q. Hu, H. Tian, C. He, Z. P. Hao, "Characteristics of Au/HMS catalysts for selective oxidation of benzyl alcohol to benzaldehyde," Catalysis Today , vol. 158, no. 3-4, pp. 246-251, 2010.

[64] L. Prati, F. Porta, "Oxidation of alcohols and sugars using Au/C catalysts: part 1. Alcohols," Applied Catalysis A: General , vol. 291, no. 1-2, pp. 199-203, 2005.

[65] S. Endud, K.-L. Wong, "Mesoporous silica MCM-48 molecular sieve modified with SnCl₂ in alkaline medium for selective oxidation of alcohol," Microporous and Mesoporous Materials , vol. 101, no. 1-2, pp. 256-263, 2007.

[66] N. K. Chaki, H. Tsunoyama, Y. Negishi, H. Sakurai, T. Tsukuda, "Effect of Ag-doping on the catalytic activity of polymer-stabilized Au clusters in aerobic oxidation of alcohol," The Journal of Physical Chemistry C , vol. 111, no. 13, pp. 4885-4888, 2007.

[67] M. Kidwai, S. Bhardwaj, "Application of mobilized gold nanoparticles as sole catalyst for the oxidation of secondary alcohols into ketones," Applied Catalysis A: General , vol. 387, no. 1-2, pp. 1-4, 2010.

[68] M. Ghiaci, F. Molaie, M. E. Sedaghat, 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.

[69] I. N. Lykakis, M. Orfanopoulos, "Photooxidation of aryl alkanes by a decatungstate/triethylsilane system in the presence of molecular oxygen," Tetrahedron Letters , vol. 45, no. 41, pp. 7645-7649, 2004.

[70] F. Rajabi, R. Luque, J. H. Clark, B. Karimi, 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.

[71] A. D. Banadaki, A. Kajbafvala, "Recent advances in facile synthesis of bimetallic nanostructures: an overview," Journal of Nanomaterials , vol. 2014, 2014.

[72] S. Vetrivel, A. Pandurangan, "Vapour-phase oxidation of ethylbenzene with air over Mn-containing MCM-41 mesoporous molecular sieves," Applied Catalysis A: General , vol. 264, no. 2, pp. 243-252, 2004.

[73] P. Kim, Y. Kim, H. Kim, I. K. Song, J. Yi, "Synthesis and characterization of mesoporous alumina for use as a catalyst support in the hydrodechlorination of 1,2-dichloropropane: effect of preparation condition of mesoporous alumina," Journal of Molecular Catalysis A: Chemical , vol. 219, no. 1, pp. 87-95, 2004.

[74] I. Mora-Barrantes, A. Rodriguez, L. Ibarra, L. Gonzalez, J. L. Valentin, "Overcoming the disadvantages of fumed silica as filler in elastomer composites," Journal of Materials Chemistry , vol. 21, no. 20, pp. 7381-7392, 2011.

[75] G. Perot, M. Guisnet, "Advantages and disadvantages of zeolites as catalysts in organic chemistry," Journal of Molecular Catalysis , vol. 61, no. 2, pp. 173-196, 1990.

[76] A. Nezamzadeh-Ejhieh, S. Khorsandi, "Photocatalytic degradation of 4-nitrophenol with ZnO supported nano-clinoptilolite zeolite," Journal of Industrial and Engineering Chemistry , vol. 20, no. 3, pp. 937-946, 2014.

[77] A.-N. A. El-Hendawy, "Surface and adsorptive properties of carbons prepared from biomass," Applied Surface Science , vol. 252, no. 2, pp. 287-295, 2005.

[78] Z. Z. Chowdhury, S. B. A. Hamid, R. Das, M. R. Hasan, S. M. Zain, K. Khalid, M. N. Uddin, "Preparation of carbonaceous adsorbents from lignocellulosic biomass and their use in removal of contaminants from aqueous solution," BioResources , vol. 8, no. 4, pp. 6523-6555, 2013.

[79] I. V. Delidovich, B. L. Moroz, O. P. Taran, N. V. Gromov, P. A. Pyrjaev, I. P. Prosvirin, V. I. Bukhtiyarov, V. N. Parmon, "Aerobic selective oxidation of glucose to gluconate catalyzed by Au/Al₂ O₃ and Au/C: impact of the mass-transfer processes on the overall kinetics," Chemical Engineering Journal , vol. 223, pp. 921-931, 2013.

[80] H. Zhang, 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, no. 1, pp. 166-176, 2013.

[81] L. Wang, D. Yang, J. Wang, Z. Zhu, 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.

[82] V. G. Milt, S. Ivanova, O. Sanz, M. I. Dominguez, A. Corrales, J. A. Odriozola, M. A. Centeno, "Au/TiO₂ supported on ferritic stainless steel monoliths as CO oxidation catalysts," Applied Surface Science , vol. 270, pp. 169-177, 2013.

[83] S. Röhe, K. Frank, A. Schaefer, A. Wittstock, V. Zielasek, A. Rosenauer, M. Baumer, "CO oxidation on nanoporous gold: a combined TPD and XPS study of active catalysts," Surface Science , vol. 609, pp. 106-112, 2013.

[84] X. Huang, X. Wang, X. Wang, X. Wang, M. Tan, W. Ding, X. Lu, "P123-stabilized Au-Ag alloy nanoparticles for kinetics of aerobic oxidation of benzyl alcohol in aqueous solution," Journal of Catalysis , vol. 301, pp. 217-226, 2013.

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

[86] M. J. Beier, B. Schimmoeller, T. W. Hansen, J. E. T. Andersen, S. E. Pratsinis, J.-D. Grunwaldt, "Selective side-chain oxidation of alkyl aromatic compounds catalyzed by cerium modified silver catalysts," Journal of Molecular Catalysis A: Chemical , vol. 331, no. 1-2, pp. 40-49, 2010.

[87] X. Wang, B. Tang, X. Huang, Y. Ma, Z. Zhang, "High activity of novel nanoporous Pd-Au catalyst for methanol electro-oxidation in alkaline media," Journal of Alloys and Compounds , vol. 565, pp. 120-126, 2013.

[88] K. Kahler, M. C. Holz, M. Rohe, A. C. van Veen, M. Muhler, "Methanol oxidation as probe reaction for active sites in Au/ZnO and Au/TiO₂ catalysts," Journal of Catalysis , vol. 299, pp. 162-170, 2013.

[89] G. Zhao, M. Deng, Y. Jiang, H. Hu, J. Huang, Y. Lu, "Microstructured Au/Ni-fiber catalyst: Galvanic reaction preparation and catalytic performance for low-temperature gas-phase alcohol oxidation," Journal of Catalysis , vol. 301, pp. 46-53, 2013.

[90] X. Bokhimi, R. Zanella, V. Maturano, A. Morales, "Nanocrystalline Ag, and Au-Ag alloys supported on titania for CO oxidation reaction," Materials Chemistry and Physics , vol. 138, no. 2-3, pp. 490-499, 2013.

[91] Q. Ye, J. Zhao, F. Huo, "Nanosized Au supported on three-dimensionally ordered mesoporous β -MnO₂ : highly active catalysts for the low-temperature oxidation of carbon monoxide, benzene, and toluene," Microporous and Mesoporous Materials , vol. 172, pp. 20-29, 2013.

[92] L. Li, A. Wang, B. Qiao, J. Lin, Y. Huang, X. Wang, T. Zhang, "Origin of the high activity of Au/FeO_x for low-temperature CO oxidation: direct evidence for a redox mechanism," Journal of Catalysis , vol. 299, pp. 90-100, 2013.

[93] P. R. Makgwane, S. S. Ray, "Nanosized ruthenium particles decorated carbon nanofibers as active catalysts for the oxidation of p -cymene by molecular oxygen," Journal of Molecular Catalysis A: Chemical , vol. 373, pp. 1-11, 2013.

[94] M. Zhang, X. Zhu, X. Liang, Z. Wang, "Preparation of highly efficient Au/C catalysts for glucose oxidation via novel plasma reduction," Catalysis Communications , vol. 25, pp. 92-95, 2012.

[95] P. Bujak, P. Bartczak, 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.

[96] A. C. Sunil Sekhar, K. Sivaranjani, C. S. Gopinath, C. P. Vinod, "A simple one pot synthesis of nano gold-mesoporous silica and its oxidation catalysis," Catalysis Today , vol. 198, no. 1, pp. 92-97, 2012.

[97] G. Zhan, Y. Hong, V. T. Mbah, J. Huang, A. R. Ibrahim, M. Du, Q. Li, "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.

[98] T. Yan, D. W. Redman, W.-Y. Yu, D. W. Flaherty, J. A. Rodriguez, C. B. Mullins, "CO oxidation on inverse Fe₂ O₃ /Au(1 1 1) model catalysts," Journal of Catalysis , vol. 294, pp. 216-222, 2012.

[99] W. Li, A. Wang, X. Liu, 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.

[100] V. V. Costa, M. Estrada, Y. Demidova, I. Prosvirin, V. Kriventsov, R. F. Cotta, S. Fuentes, A. Simakov, E. V. Gusevskaya, "Gold nanoparticles supported on magnesium oxide as catalysts for the aerobic oxidation of alcohols under alkali-free conditions," Journal of Catalysis , vol. 292, pp. 148-156, 2012.

[101] J. C. Bauer, G. M. Veith, L. F. Allard, Y. Oyola, S. H. Overbury, S. Dai, "Silica-supported Au-CuO_x hybrid nanocrystals as active and selective catalysts for the formation of acetaldehyde from the oxidation of ethanol," ACS Catalysis , vol. 2, no. 12, pp. 2537-2546, 2012.

[102] R. Saliger, N. Decker, U. Prüße, "D-Glucose oxidation with H₂ O₂ on an Au/Al₂ O₃ catalyst," Applied Catalysis B: Environmental , vol. 102, no. 3-4, pp. 584-589, 2011.

[103] S. Hermans, A. Deffernez, M. Devillers, "Au-Pd/C catalysts for glyoxal and glucose selective oxidations," Applied Catalysis A: General , vol. 395, no. 1-2, pp. 19-27, 2011.

[104] I. Witonska, M. Frajtak, S. Karski, "Selective oxidation of glucose to gluconic acid over Pd-Te supported catalysts," Applied Catalysis A: General , vol. 401, no. 1-2, pp. 73-82, 2011.

[105] P. Wu, P. Bai, Z. Lei, K. P. Loh, X. S. Zhao, "Gold nanoparticles supported on functionalized mesoporous silica for selective oxidation of cyclohexane," Microporous and Mesoporous Materials , vol. 141, no. 1-3, pp. 222-230, 2011.

[106] L. Hu, X. Cao, J. Yang, M. Li, H. Hong, Q. Xu, J. Ge, L. Wang, J. Lu, L. Chen, H. Gu, "Oxidation of benzylic compounds by gold nanowires at 1 atm O₂ ," Chemical Communications , vol. 47, no. 4, pp. 1303-1305, 2011.

[107] H. Aliyan, R. Fazaeli, A. R. Massah, H. J. Naghash, S. Moradi, "Oxidation of benzylic alcohols with molecular oxygen catalyzed by Cu_3/2 [PMO₁₂ O₄₀ ]/SiO₂ ," Iranian Journal of Catalysis , vol. 1, no. 1, pp. 19-23, 2011.

[108] M. Rosu, A. Schumpe, "Oxidation of glucose in suspensions of moderately hydrophobized palladium catalysts," Chemical Engineering Science , vol. 65, no. 1, pp. 220-225, 2010.

[109] T. Benko, A. Beck, O. Geszti, R. Katona, A. Tungler, K. Frey, L. Guczi, Z. Schay, "Selective oxidation of glucose versus CO oxidation over supported gold catalysts," Applied Catalysis A: General , vol. 388, no. 1-2, pp. 31-36, 2010.

[110] M. Chun Yan, Z. Mu, J. J. Li, Y. G. Jin, J. Cheng, G. Q. Lu, Z. P. Hao, S. Z. Qiao, "Mesoporous co₃ o₄ and AU/CO₃ o₄ catalysts for low-temperature oxidation of trace ethylene," Journal of the American Chemical Society , vol. 132, no. 8, pp. 2608-2613, 2010.

[111] H. Liu, Y. Liu, Y. Li, Z. Tang, H. Jiang, "Metal-organic framework supported gold nanoparticles as a highly active heterogeneous catalyst for aerobic oxidation of alcohols," Journal of Physical Chemistry C , vol. 114, no. 31, pp. 13362-13369, 2010.

[112] F. Diehl, J. Barbier Jr., D. Duprez, I. Guibard, G. Mabilon, "Catalytic oxidation of heavy hydrocarbons over Pt/Al₂ O₃ . Influence of the structure of the molecule on its reactivity," Applied Catalysis B: Environmental , vol. 95, no. 3-4, pp. 217-227, 2010.

[113] X. Yang, X. Wang, C. Liang, W. Su, C. Wang, Z. Feng, C. Li, J. Qiu, "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.

[114] Q. Jiang, Y. Xiao, Z. Tan, Q.-H. Li, 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.

[115] H. Li, B. Guan, W. Wang, "Aerobic oxidation of alcohol in aqueous solution catalyzed by gold," Tetrahedron , vol. 63, no. 35, pp. 8430-8434, 2007.

[116] K. M. Parida, 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.

[117] T. Hayashi, T. Inagaki, N. Itayama, 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.

[118] A. C. Gluhoi, N. Bogdanchikova, B. E. Nieuwenhuys, "Total oxidation of propene and propane over gold-copper oxide on alumina catalysts: comparison with Pt/Al₂ O₃ ," Catalysis Today , vol. 113, no. 3-4, pp. 178-181, 2006.

[119] S. Vetrivel, A. Pandurangan, "Aerial oxidation of p -isopropyltoluene over manganese containing mesoporous MCM-41 and Al-MCM-41 molecular sieves," Journal of Molecular Catalysis A: Chemical , vol. 246, no. 1-2, pp. 223-230, 2006.

[120] B. Guan, D. Xing, G. Cai, X. Wan, N. Yu, Z. Fang, L. Yang, Z. Shi, "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.

[121] K. Zhu, J. Hu, R. Richards, "Aerobic oxidation of cyclohexane by gold nanoparticles immobilized upon mesoporous silica," Catalysis Letters , vol. 100, no. 3-4, pp. 195-199, 2005.

[122] E. J. M. Hensen, Q. Zhu, R. A. J. Janssen, P. C. M. M. Magusin, P. J. Kooyman, R. A. Van Santen, "Selective oxidation of benzene to phenol with nitrous oxide over MFI zeolites: 1. on the role of iron and aluminum," Journal of Catalysis , vol. 233, no. 1, pp. 123-135, 2005.

[123] R. Zhang, Z. Qin, M. Dong, G. Wang, J. Wang, "Selective oxidation of cyclohexane in supercritical carbon dioxide over CoAPO-5 molecular sieves," Catalysis Today , vol. 110, no. 3-4, pp. 351-356, 2005.

[124] Y. Önal, S. Schimpf, P. Claus, "Structure sensitivity and kinetics of D-glucose oxidation to D-gluconic acid over carbon-supported gold catalysts," Journal of Catalysis , vol. 223, no. 1, pp. 122-133, 2004.

[125] M. Kang, M. W. Song, C. H. Lee, "Catalytic carbon monoxide oxidation over CoO_x / CeO₂ composite catalysts," Applied Catalysis A: General , vol. 251, no. 1, pp. 143-156, 2003.

[126] S. Biella, L. Prati, M. Rossi, "Selective oxidation of D-glucose on gold catalyst," Journal of Catalysis , vol. 206, no. 2, pp. 242-247, 2002.

[127] S. Xiang, Y. Zhang, Q. Xin, C. Li, "Enantioselective epoxidation of olefins catalyzed by Mn (salen)/MCM-41 synthesized with a new anchoring method," Chemical Communications , no. 22, pp. 2696-2697, 2002.

[128] B. Skårman, D. Grandjean, R. E. Benfield, A. Hinz, A. Andersson, L. Reine Wallenberg, "Carbon monoxide oxidation on nanostructured CuO_x /CeO₂ composite particles characterized by HREM, XPS, XAS, and high-energy diffraction," Journal of Catalysis , vol. 211, no. 1, pp. 119-133, 2002.

[129] G. Mul, A. Zwijnenburg, B. van der Linden, M. Makkee, J. A. Moulijn, "Stability and selectivity of Au/TiO₂ and Au/TiO₂ /SiO₂ catalysts in propene epoxidation: an in situ FT-IR study," Journal of Catalysis , vol. 201, no. 1, pp. 128-137, 2001.

[130] E. E. Stangland, K. B. Stavens, R. P. Andres, W. N. Delgass, "Characterization of gold-titania catalysts via oxidation of propylene to propylene oxide," Journal of Catalysis , vol. 191, no. 2, pp. 332-347, 2000.

[131] T. A. Nijhuis, B. J. Huizinga, M. Makkee, J. A. Moulijn, "Direct epoxidation of propene using gold dispersed on TS-1 and other titanium-containing supports," Industrial and Engineering Chemistry Research , vol. 38, no. 3, pp. 884-891, 1999.

[132] Y. Matsumoto, M. Asami, M. Hashimoto, M. Misono, "Alkane oxidation with mixed addenda heteropoly catalysts containing Ru(III) and Rh(III)," Journal of Molecular Catalysis A: Chemical , vol. 114, no. 1-3, pp. 161-168, 1996.

[133] F. Boccuzzi, A. Chiorino, S. Tsubota, M. Haruta, "FTIR study of carbon monoxide oxidation and scrambling at room temperature over gold supported on ZnO and TiO₂ · 2," Journal of Physical Chemistry , vol. 100, no. 9, pp. 3625-3631, 1996.

[134] M. A. Bollinger, M. A. Vannice, "A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au-TiO₂ catalysts," Applied Catalysis B: Environmental , vol. 8, no. 4, pp. 417-443, 1996.

[135] S. Furukawa, Y. Hitomi, T. Shishido, T. Tanaka, "Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme Fe(II) complexes without any reductant," Inorganica Chimica Acta , vol. 378, no. 1, pp. 19-23, 2011.

[136] L.-F. Gutierrez, S. Hamoudi, K. Belkacemi, "Synthesis of gold catalysts supported on mesoporous silica materials: recent developments," Catalysts , vol. 1, no. 1, pp. 97-154, 2011.

[137] A. Hugon, N. E. Kolli, C. Louis, "Advances in the preparation of supported gold catalysts: mechanism of deposition, simplification of the procedures and relevance of the elimination of chlorine," Journal of Catalysis , vol. 274, no. 2, pp. 239-250, 2010.

[138] W. R. Glomm, G. Øye, J. Walmsley, J. Sjöblom, "Synthesis and characterization of gold nanoparticle-functionalized ordered mesoporous materials," Journal of Dispersion Science and Technology , vol. 26, no. 6, pp. 729-744, 2005.

[139] R. Zanella, S. Giorgio, C. R. Henry, C. Louis, "Alternative methods for the preparation of gold nanoparticles supported on TiO₂ ," Journal of Physical Chemistry B , vol. 106, no. 31, pp. 7634-7642, 2002.

[140] D. A. Sverjensky, K. Fukushi, "Anion adsorption on oxide surfaces: inclusion of the water dipole in modeling the electrostatics of ligand exchange," Environmental Science & Technology , vol. 40, no. 1, pp. 263-271, 2006.

[141] R. Zanella, L. Delannoy, C. Louis, "Mechanism of deposition of gold precursors onto TiO₂ during the preparation by cation adsorption and deposition-precipitation with NaOH and urea," Applied Catalysis A: General , vol. 291, no. 1-2, pp. 62-72, 2005.

[142] M. Okumura, S. Nakamura, S. Tsubota, T. Nakamura, M. Azuma, M. Haruta, "Chemical vapor deposition of gold on Al₂ O₃ , SiO₂ , and TiO₂ for the oxidation of CO and of H₂ ," Catalysis Letters , vol. 51, no. 3-4, pp. 53-58, 1998.

[143] Y.-S. Chi, H.-P. Lin, C.-Y. Mou, "CO oxidation over gold nanocatalyst confined in mesoporous silica," Applied Catalysis A: General , vol. 284, no. 1-2, pp. 199-206, 2005.

[144] J. Lee, J. C. Park, H. Song, "A Nanoreactor framework of a Au@SiO₂ yolk/shell structure for catalytic reduction of p-nitrophenol," Advanced Materials , vol. 20, no. 8, pp. 1523-1528, 2008.

[145] D. T. Thompson, "An overview of gold-catalysed oxidation processes," Topics in Catalysis , vol. 38, no. 4, pp. 231-240, 2006.