Role of mixed metal oxides in catalysis science—versatile applications in organic synthesis

Abstract

A variety of mixed metal oxides (MMOs), containing alkali, alkaline, rare earth and noble metals, and their applications are presented. In this mini review, we summarize versatile applications of mixed metal oxides in organic synthesis. A variety of reactions such as reduction, oxidation, multicomponent, Mannich, alkylation, condensation, deprotection, cycloaddition, hydroxylation, dehydration, dehydrogenation, transesterification, reactions involving biomimetic oxygen-evolving catalysts and other important C–C bond forming reactions are well presented on the surface of mixed metal oxides under a variety of reaction conditions. The scope of MMOs in important organic reactions, industrial applications, and green chemistry and recent applications of MMOs are well presented in this review.

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Introduction

Metal oxides represent one of the most important and widely employed categories of solid catalysts, either as active phases or as supports. Metal oxides are utilized both for their acid–base and redox properties and constitute the largest family of catalysts in heterogeneous catalysis.¹ Metals and metal oxides form the bulk of catalysts employed for many synthetic conversions. Transition and noble group metals are frequently used as catalysts and their activity has been attributed to the outer electron configuration.²

Single metal oxides can crystallize with different morphologies (isotropic, anisotropic or amorphous) and local co-ordination. All metal oxides can crystallize at room temperature, but many phases may remain amorphous at modest calcination temperatures. The majority of one component metal oxides crystallize with an isotropic morphology (without preferential orientation) and the surface may terminate with M–OH, M–O–M, M=O or M-( ) functionalities where M-( ) represents an oxygen vacancy.

Among the metal oxide catalysts, those of transition metals occupy a predominant place owing to their low cost of production, easy regeneration and selective action. They are used in widely different types of organic reactions, such as oxidation, dehydration, dehydrogenation and isomerization. Their catalytic activity may be traced to the presence of partially filled d-shells of the metal ion and to the influence of the oxide ligand field on this partially filled d-shell.

In the view of a catalytic chemist, mixed metal oxides are oxygen-containing combinations of two or more metallic ions in proportions that may either vary or be defined by a strict stoichiometry. Solid solutions and mixed metal oxides are classified by physicochemists according to their crystalline systems. Mixed metal oxides are generally obtained in the form of powder or single crystals. They have a wide spectrum of industrial applications in ceramics, electronics, nuclear research and especially in catalysis.³ Since last one decade, there has been focus on synthesis and applications of mixed metal oxides. They were used for selective reduction of C=O in α,β-unsaturated carbonyls through catalytic hydrogen transfer reaction,⁴ study of chemical structures and performance of perovskite oxides,⁵ epoxidation on MoO₃/TiO₂ oxide,^6a study of effect of phosphate ions on the textural and catalytic activity of titania–silica mixed oxide.^6b Recently Jackson and Hargreaves published a book on mixed metal oxides.^7a

Organic conversions using mixed metal oxides are one of the widely studied classes of reactions in the pharmaceutical industry. Like other heterogeneous catalysts, mixed metal oxides also play very important role in the total catalysts system used^3c (Fig. 1). In this review, we focus on application of mixed metal oxides in organic synthesis, synthesis of fine chemicals, industry and green chemistry, which will be addressed briefly (Fig. 2).

Fig. 1 Industrially important heterogeneous catalysts.

Fig. 2 Applications of mixed metal oxides.

Mixed metal oxides

Oxides containing two or more different kinds of metal cations are known as mixed metal oxides. Oxides can be binary, ternary and quaternary and so on with respect to the presence of the number of different metal cations. They can be further classified based on whether they are crystalline or amorphous. If the oxides are crystalline the crystal structure can determine the oxide composition. For instance, perovskites have the general formula ABO₃; scheelites, ABO₄; spinels, AB₂O₄; and palmeirites, A₃B₂O₈. The different metal cations (M_I and M_II) are present as M_Iⁿ⁺-Ox and M_IIⁿ⁺-Ox polyhedra, which are connected in various possible ways, such as corner or edge sharing, forming chains M_I–O–M_II–O, M_I–O–M_I–O or M_II–O–M_II–O.

The arrangement of cations of a given element differs by the co-ordination and the nature of the neighboring cations and this governs the type of bonding between the cations. Different environment of the cation that constitutes an active center would give rise to different reactivity towards an approaching molecule. It is often not clear, which of the constituent metal cations plays a role of active center in mixed metal oxides. Although the industrial catalysts are usually multiphase systems, the presence of some particular phase appears to be critical to render the activity of the system for a given reaction. One of the most important results of the study of monophasic systems is the demonstration of different catalytic properties of different crystallographic faces in an oxide that provided an experimental proof for structure sensitivity phenomenon in the oxide systems.

There is much interest in understanding the phenomena for the deposition of one oxide on another oxide. There are various methods for the preparation of MMOs such as sol–gel,^7b–e wet impregnation,^7f,g mechanochemical synthesis,^7h,i hydrothermal method,^7j,k co-precipitation^7l,m and microwave irradiation.¹⁹ The mixed metal oxides are characterised by several spectroscopic and analytical techniques such as XRD, FT-IR, TG-DTA, BET, SEM-EDS, TEM, acidity and basicity measurements, XPS and Raman spectroscopy⁷ⁿ which are also additional techniques for catalyst characterization.

Why mixed metal oxides?

Mixed metal oxides play very important role in academic as well as industrial research due to their acid–base and redox properties, and constitute the largest family of catalysts in heterogeneous catalysis. Some of the important applications are depicted here, e.g. CaCoAl-HTlcs are used for NO_x capture, decomposition and reduction,⁸ a mixed oxide from Mo, Nb, Sb and V for the selective oxidation of propane,⁹ Ni/Mg–Al mixed metal oxide catalyst for steam reforming of ethanol,¹⁰ Mg–V–Al mixed metal oxides for oxidative dehydrogenation of propane on ,¹¹ mixed metal oxides La_2−xSr_xCuO_4±λ for hydroxylation of phenol,¹² Al₂O₃ supported mixed metal oxides for destructive oxidation of (CH₃)₂S₂,¹³ silica/titania mixed oxide-supported zirconocene catalyst for synthesis of linear low-density polyethylene,¹⁴ mesoporous Co₃O₄ and Au/Co₃O₄ catalysts for low-temperature oxidation of trace ethylene,¹⁵ Fe_xCe_1−xO₂ mixed metal oxides for N₂O decomposition,¹⁶ LaFe_1−xCo_xO₃ mixed-oxide for combined methane reforming^17aetc. Due to their broad scope in the catalysis field,^17b they are necessary for organic synthesis, industrial applications and in the pharmaceutical industry.

Notably, some of the mixed metal oxides are better in terms of their catalytic activity than component oxides in various reactions. Why is it so? It could be due to increasing active acidic or basic sites, and increasing surface area, which reduces the reaction time, increases the yield of reaction or conversions of reactants.^18,31,49a For example, consider liquid phase catalytic transfer hydrogenation of aromatic nitro compounds on La_1−xSr_xFeO₃ perovskites prepared by microwave irradiation. Notably, reduction of aromatic nitro compounds worked well in mixed metal oxides La_1−xSr_xFeO₃ as compared to their component oxides.¹⁹ The values of activation energy for LaFeO₃, La_0.8Sr_0.2FeO₃ and SrFeO₃ are 88.9, 73.9 and 84.4 kJ mol⁻¹, respectively, which are found to follow the order LaFeO₃ > SrFeO₃ > La_0.8Sr_0.2FeO₃.

Applications of mixed metal oxides in organic synthesis

Application of mixed metal oxides has an important role in organic transformations, due to their simplicity in handling, decreased reactor and plant corrosion problems, cost effectiveness and because most of the MMOs are reusable and recyclable. Some of the important reactions in organic synthesis are depicted below.

Antimicrobial drug tinidazole has been synthesized by condensation of 2-methyl-5-nitroimidazole with 2-ethyl-thio-ethanol over a MoO₃/SiO₂ catalyst to obtain 1-(2-ethyl-thio-ethanol)-2-methyl-5-nitro-imidazole (Scheme 1) which was further oxidized using hydrogen peroxide with the same catalyst to form tinidazole.²⁰

Scheme 1 Synthesis of tinidazole over MoO₃/SiO₂.

Tinidazole, an important pharmaceutical compound, was also synthesized by condensation/oxidation reaction using the MoO₃/SiO₂ catalyst without any use of acetic acid, tungstic acid or ammonium molybdate used in the conventional process. The MoO₃/SiO₂ catalyst could be recycled at least five times without any appreciable loss in conversion and selectivity, indicating its potential for use as a bifunctional catalyst.

Song and coworkers have reported a Cu–Mn mixed oxide catalyst supported on carbon which was prepared using the incipient wetness method according to the literature.²¹ Carbon supported copper–manganese oxide and TEMPO were used for the selective aerobic oxidation of alcohols to corresponding aldehydes or ketones. The synthesized Cu–Mn mixed oxide gave good selectivity, is reusable and separates easily²² (Scheme 2).

Scheme 2 Selective aerobic oxidation of alcohols to corresponding aldehydes or ketones over mixed metal oxides.

Perovskites are a class of mixed oxides, which have interesting catalytic and physicochemical properties. A novel method developed for the synthesis of perovskites of the type LaMO₃ (M = Mn, Fe, Co, Cr, Al) involves microwave irradiation for just 15 min using oxalate precursors. The LaMO₃ perovskites were used as catalysts for reduction of aromatic nitro compounds with propan-2-ol as hydrogen donor and KOH as promoter.²³ Notably, LaFeO₃ perovskite is the best catalyst under the same reaction conditions. The corresponding aniline derivatives are obtained in excellent yield (78–98%) in 2–6 hours (Scheme 3). The experimental results obtained for this research work allow us to conclude that perovskites prepared by the microwave irradiation method have activity values similar to those of the ones prepared by conventional methods. The method is simple and does not involve intermittent grindings and calcinations at elevated temperatures. Finally, we have achieved a considerable reduction in the time required for preparation of perovskites.

Scheme 3 Reduction of aromatic nitro compounds.

Wadsworth–Emmons reaction was reported on Mg–Al mixed oxides. The Mg–Al mixed oxide was prepared by a simple co-precipitation method by using Mg and Al nitrates and calcining at 923 K. The synthesized Mg–La mixed oxide was found to be an efficient solid base for the selective synthesis of α,β-unsaturated esters and nitriles with greater than 80% yields, avoiding Knoevenagel condensation and aldolisation even with aliphatic aldehydes.²⁴ The catalyst could be recycled once with high activity. The effect of substituents on the phenyl ring proves that the reaction proceeds via a base catalyzed mechanism with a low charge on intermediate species (Scheme 4).

Scheme 4 Wadsworth–Emmons reactions of aldehydes with phosphonates.

Cycloaddition of CO₂ to various epoxides was carried out on heterogeneous Mg–Al mixed oxides.²⁵ Mg–Al mixed metal oxides obtained by calcination of the hydrotalcites were effective catalysts for the fixation of CO₂ to various epoxides to form the corresponding five-membered cyclic carbonate. Particularly, mixed oxide with the Mg–Al ratio of 5 calcined at 400 °C showed the highest catalytic activity. This unique catalytic activity is due to the cooperative action of acid–base sites derived from the formation of Mg–O–Al bonds, i.e. the substitution of Al for Mg in the MgO matrix, on the surface of mixed oxides. The Mg–Al mixed oxide catalyst was reusable and could be proved effective for many organic reactions as environment-friendly acid–base catalysts (Scheme 8).

The possible reaction mechanism for the cyclo-addition reaction is depicted in Fig. 3. The addition reaction is initiated by adsorption of CO₂ on the Lewis basic sites to form a carbonate species, and independently, an epoxide is coordinated on the neighboring acid site on the surface. The coordinated epoxide is ring-opened by a nucleophilic attack of the carbonate species, which leads to an oxy anion species yielding the corresponding cyclic carbonate as a product. The Mg–Al mixed oxide has good catalytic activity compared to its physical mixture. Therefore, it is likely that a prominent feature of the active Mg–Al mixed oxide catalysts can be found to originate from the cooperative action of both the basic and acidic sites located in a neighbor on the surface.

Fig. 3 Mechanism of cycloaddition of CO₂ catalyzed by Mg–Al mixed metal oxides (ref. 25).

Recently, we have reported Al₂O₃–OK (K₂O supported on alumina) mixed metal oxide for N-alkylation of amines. The Al₂O₃–OK was prepared by a simple impregnation method and the synthesized Al₂O₃–OK catalyst was characterized by XRD, SEM–EDS, elemental analysis, particle size analysis, BET surface area, pore size and average pore diameter.²⁶

In order to know about the homogeneity of Al₂O₃–OK, SEM analysis was carried out. It has been noted that the K₂O base was finely and uniformly distributed over the Al₂O₃ support, which makes the catalyst more active (Fig. 4).

Fig. 4 Scanning electron micrograph of Al₂O₃–OK at 400 μm (left) and scanning electron micrograph of Al₂O₃–OK at 20 μm (right).²⁶

N-Alkylation of amines by alkyl halides using Al₂O₃–OK as a catalyst in acetonitrile at room temperature (30 °C) was described. The corresponding tertiary amines were obtained in good yield (65–95%) (Scheme 5).

Scheme 5 N-Alkylation of primary and secondary amines by alkyl halides.

A series of LaMO₃ (M = Cr, Co, Fe, Mn, Ni) perovskites were synthesized by a citrate precursor decomposition method and characterized by a XRD technique. The catalytic activity of the synthesized perovskites was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using TBHP as an oxidant.²⁷ Of various perovskites screened, LaCrO₃ exhibited remarkable catalytic activity, providing the oxidation of alkylarenes selectively at the benzylic position. The LaCrO₃/TBHP catalytic system provides excellent yields (84–97%, selectivity 97%) of the desired ketones under solvent-free conditions. The catalyst can be reused successfully up to six consecutive cycles with no significant loss in activity (Scheme 6).

Scheme 6 Oxidation of diphenyl methane to benzophenone.

A facile deprotection of oximes over mixed metal oxides (CeO₂–ZrO₂) under solvent-free conditions was reported. CeO₂–ZrO₂ was prepared by a co-precipitation method via an inexpensive precursor. The as-synthesized material was characterized by nitrogen adsorption–desorption, surface area, particle size, EDX, XRD and FT-IR techniques. The catalytic activity was tested for deprotection of oximes of ketones and aldehydes efficiently over CeO₂–ZrO₂ using tert-butyl hydrogen peroxide (TBHP) as an oxidant under solvent-free conditions.²⁸

This method is high yielding, clean, safe, cost effective and therefore very suitable for practical organic synthesis. The corresponding products were obtained in 30–94% yield (Scheme 7).

Scheme 7 Deprotection of oximes under solvent-free conditions.

Potassium iron zirconium phosphate [PIZP-K₂FeZrP₃O₁₂] was prepared by a sol–gel method.²⁹ The as-synthesized catalyst was characterized by crystal size, elemental analysis, nitrogen adsorption–desorption, pore volume, average pore diameter, XRD, FT-IR and DSC techniques. The SEM images of PIZP are depicted in Fig. 5. The particle size was found to be in the range of 1–3 μm. Benzophenone and its derivatives were synthesized over the PIZP mixed metal oxides catalyst under solvent-free conditions.

Fig. 5 Scanning electron micrograph of PIZP at 10 μm and 5 μm.

The catalytic activity of PIZP was tested for the benzoylation of different arenes using benzoyl chloride as the benzoylating agent. The reaction was carried out at different temperatures and catalyst loadings. The advantage of PIZP is that it can be prepared easily and does not lose its activity even after several runs. The synthesized benzophenone and its derivatives were obtained in excellent yield (87–96%) (Scheme 9).

Scheme 8 Cycloaddition of carbon dioxide to epoxides in the presence of Mg–Al mixed oxides.

Scheme 9 Friedel–Crafts benzoylation of arenes with benzoyl chloride.

Synthesis of polyfunctionalized 4H-pyrans was carried out in one pot through condensation of an aldehyde, malononitrile, and an active methylenic diketo compound using a heterogeneous strong basic Mg–La mixed oxide catalyst.³⁰ The advantages of this protocol are simple catalyst preparation, mild reaction temperature, easy recovery, and reusability of the catalyst with consistent activity and short reaction times (Scheme 10).

Scheme 10 Mg–La catalyzed synthesis of 4H-pyran derivatives.

The mechanism for the synthesis of 4H-pyrans is depicted in Fig. 6.

Fig. 6 A plausible mechanism of 4H pyran synthesis over Mg–La mixed metal oxides.

The synthesis of 4H pyrans involves a Knoevenagel condensation, catalysed by the as-strong basic sites on the catalyst surface, which initiate the reaction by abstracting a proton from the active methylene group from malononitrile. The aldol type product thus formed suffers a Michael addition from the enolate of the active methylenic diketo compound to yield after cyclization the polyfunctionalized 4H-pyrans

Recently, we reported NaLaTiO₄ and HLaTiO₄ as heterogeneous and reusable catalysts for O-tert-but-oxycarbonylation of various hydroxyl compounds under mild conditions.³¹ The observed comparable activity of NaLaTiO₄ and HLaTiO₄ can be explained on the basis of their structural features as both consist of single layers of TiO₆ octahedra that are separated in alternate layers by monovalent cations, which are ion-exchangeable, and La³⁺, which is non-exchangeable. Thus, the activity may be due to the anionic moiety [LaTiO₄]⁻ which is the same for both the catalysts. This was further proved when the influence of the component metal oxides on the reaction was studied. It was observed that both the component metal oxides (Table 1, entries 4 and 5) provided lower yields compared to the perovskites, indicating that the actual catalytic activity resides in the perovskite structure (Scheme 11).

Table 1 Influence of catalyst

Entry	Catalyst	Yield (%)
Reaction conditions: 4-methoxybenzyl alcohol = 1 mmol; Boc2O = 1.1 mmol; catalyst = 0.1 mmol; solvent = CH2Cl2 (1 mL); reflux; time = 7.3 h; yield determined by GC and GC–MS analysis.
1	None	—
2	LaNaTiO4	92
3	HLaTiO4	90
4	TiO2	37
5	La2O3	20
6	La2O3	20

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Scheme 11 O-tert-butoxycarbonylation of hydroxy compounds.

Hell has reported nickel(II) supported heterogeneous, magnesium–lanthanum mixed oxide for Kumada coupling reaction.³² The catalyst was used successfully in the Kumada coupling of aryl halides, especially aryl bromides. The synthesized compounds were obtained in 11–86% yield (Scheme 12). The Mg–La mixed oxide catalyst is easily separable from reaction mixture, thus the nickel contamination of the product can be avoided.

Scheme 12 Kumada coupling of Ni²⁺/Mg–La mixed metal oxides.

Role of mixed metal oxides in industrially important reactions

Several industrially important synthetic conversions have been carried out with mixed metal oxides. MMOs have been used in various important reactions such as reduction of nitroarenes,³³ NO reduction,³⁴ with NH₃, catalytic wet air oxidation of 2-chlorophenol,³⁵ liquid phase catalytic oxidation of alcohols,³⁶ alcohol dehydration,³⁷ dehydrogenation of 2-octanol,³⁸ transesterification on Na-based mixed metal oxide,³⁹ destructive oxidation of (CH₃)₂S₂,^40a solvent-free oxidation of primary alcohols to aldehydes using Au–Pd/TiO₂ catalysts,^40b ammonia adspecies and catalytic transformation,^40c and in some other important applications.^40d

Also, some of the mixed metal oxides such as Cu–Mn spinel type oxides are used in methanol steam reforming,^40e alumina based oxides for hydrodesulfurization,^40f nanostructured oxides for some applications,^40g and zinc based oxides for direct conversion of bio-ethanol to isobutene^40hetc.

Mannich reaction is a classical method for the preparation of β-amino carbonyl compounds. It is a key step in the synthesis of numerous pharmaceuticals and natural products.^41a,b With the increasing interest in developing environmentally benign reactions, the atom-economic catalytic process that employs unmodified carbonyl donor, amine and acceptor aldehyde as an idealized Mannich-type reaction is attracting more and more attention.^41c,d Very recently we have reported a one-pot three component Mannich reaction using aldehydes, amines and ketones over a solid MgO/ZrO₂ catalyst at 80 °C in acetonitrile.^41e Notably, all varieties of aldehydes and amines worked well over the MgO/ZrO₂ to afford the corresponding compounds in good yield (Scheme 13).

Scheme 13 Synthesis of β-amino carbonyl compounds by Mannich reaction using MgO–ZrO₂.

Scheme 14 Michael-addition reaction over magnesium–lanthanum mixed oxide.

Michael addition reactions were studied by Figueras and co-workers at room temperature with a variety of acceptors and donors, using a magnesium–lanthanum mixed oxide. Notably, all acceptors and donors worked well and obtained a very good yield.^42a The reaction of methyl acrylate and diethyl malonate is depicted in Scheme 14.

Isobutyl alcohol (ⁱBuOH) has gained an increasing interest in the last decade, owing to its potential use as precursor for the preparation of either gasoline additives, such as methyl-tert-butyl-ether and isooctane, or plasticizers. Synthesis of isobutyl alcohol from methanol and n-propanol through the Guerbet condensation has been studied in batch experiments using bifunctional heterogeneous systems based on a dehydrogenating/hydrogenating metal (Pd, Rh, Ni or Cu) and a basic Mg–Al mixed oxide derived from hydrotalcite-type (HT) precursors^42b (Scheme 15).

Scheme 15 Synthesis of isobutyl alcohol over mixed metal oxides.

Mestl has investigated MoVW mixed metal oxides catalysts for acrylic acid production.^42c (MoVW)₅O₁₄-type oxides were identified as the active and selective components in industrial acrylic acid catalysts. Tungsten is suggested to play an important role as a structural promoter for the formation and stabilization of this oxide. Vanadium is responsible for high catalytic activities but is detrimental for the stability of this oxide at the necessary high concentrations for optimum catalytic performance. The activity of mixed MoVW oxide catalysts for methanol, propene, and acrolein partial oxidation could be considerably improved, when the amount of the (MoVW)₅O₁₄-type oxide was increased by thermal annealing. In the model developed by Mestl (Fig. 7a), it can be assumed that the surface oxygen defects generated during catalysis may be replenished either by surface oxygen diffusion, by oxygen diffusion through the near surface layer or by oxygen bulk diffusion. He has also developed a core–shell model of MoVW mixed metal oxide (Fig. 7b).

Fig. 7 (a) Schematic drawing of different possible oxygen diffusion pathways in mixed oxide partial oxidation catalysts. (b) Schematic drawing of the proposed core–shell model of the MoVW mixed metal oxide catalyst. For detailed information on these models readers are referred to ref. 42c.

Ceria based mixed oxides have attracted much attention due to their versatile applications in various fields such as sensors, fuel cells, biomaterials and especially in catalysis, in automotive catalytic converters for treating emissions and due to some other industrial catalytic applications.^43a–k

Kaspar and co-workers have reported ceria-based oxide catalysts in the automotive three-way catalysts (TWCs). In this article they focused on a variety of roles of CeO₂ as a TWC promoter and in particular the oxygen storage/release capacities are discussed in detail. Their main focus was on the oxygen storage/release capacities of materials containing ZrO₂, which are employed in the last generation of catalytic automotive converters.^44a

In 2004, Kaspar and Monte have discussed the role of ceria based catalyst in oxygen storage and release capacity (OSC) in promoting the activity of noble metals in the three-way catalysts (TWCs). In this article they have explained the reduction mechanism for CeO₂ using H₂ as reducing agent.^44b

Kaspar and Monte have published a Feature article on advanced catalytic materials based on nanostructured CeO₂–ZrO₂ mixed oxides as oxygen storage promoters for the automotive three-way catalysts (TWCs).^44c In this article they focused on various factors related to the structures, texture and all other properties of oxide materials. For more detailed information the reader can refer to ref. 44c and d.

The combination of reduction/oxidation treatments yields a variety of different phases and features. Even a modest increase of the re-oxidation temperature easily leads to a disordered type of cation structure.

Applications of MMOs in green chemistry

Scientists need to generate a diverse array of exceptionally complex targets within a short span of time. Because of high molecular complexity in drug discovery accompanied by time constraints, the development of efficient and environmentally benign synthetic protocols becomes the primary driver of chemistry. This can be achieved by the proper choice of starting materials, atom economic methodologies with a minimum number of chemical steps, the appropriate use of greener solvents and reagents, and efficient strategies for isolation and purification.

In the current era there is a serious push towards developing processes that are eco-friendly. This process development is generally termed “Green Technology”⁴⁵ or “Sustainable Technology”. This necessitates a shift from the traditional concepts of process efficiency that focus exclusively on chemical yield to one that assigns economic value to eliminating waste and avoiding the use of toxic and/or hazardous substances and more environmentally acceptable processes.⁴⁶

The magnitude of the waste problem in the manufacture of chemicals measured as ‘E-Factor’⁴⁷ in terms of the amount of waste produced per kg of product is depicted in Table 2.

Table 2 E-factors in the chemical industry

Industry segment	Annual production/t	E-factor	Waste produced/t
Oil refining	10^6–10^8	ca. 0.1	10^5–10^7
Bulk chemicals	10^4–10^6	<1–5	10^4–5 × 10^6
Fine chemicals	10^2–10^4	5–50	5 × 10^2–5 × 10^5
Pharmaceuticals	10–10^3	25–100	2.5 × 10^2–10^5

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The waste mainly consists of inorganic salts used or formed during or after the reaction during the workup process. In organic chemistry, the ideal synthesis will be a combination of a number of environmental, health and safety, and economic targets (Fig. 8).

Fig. 8 The ideal synthesis.

To keep the green chemistry concern in mind, many industries are trying to synthesize target compounds by green chemistry routes. We report here, a few examples of mixed metal oxides used in green chemical processes.

Kantam has reported synthesis of β-hydroxy-α-diazo carbonyl compounds and β-amino-α-diazo carbonyl compounds on magnesium/lanthanum mixed oxide at room temperature in water to afford the corresponding products in good yields.⁴⁸ The catalyst was recovered and reused for several cycles with consistent activity. The corresponding compounds were obtained by condensation of a wide variety of aldehydes and imines with ethyl diazoacetate (Scheme 16).

Scheme 16 Synthesis of β-hydroxy-α-diazo carbonyl compounds and β-amino-α-diazo carbonyl compounds over mixed metal oxides.

The Knoevenagel condensation of several aldehydes with ethyl cyanoacetate and malononitrile was carried out at 60 °C on a MgO/ZrO₂ catalyst under solvent-free conditions.^49a The mixed metal oxides (MgO/ZrO₂) were prepared by using inexpensive precursors. The synthesized mixed metal oxides were well characterized by several analytical and physicochemical techniques. MgO/ZrO₂ worked well for the condensation reaction to obtain the corresponding compounds in good yield (65–98%) (Scheme 17).

Scheme 17 Knoevenagel condensation of aldehydes with ethyl cyanoacetate and malononitrile on MgO/ZrO₂.

The atom economy and environmental factor (E) of Knoevenagel reaction are depicted below:

Atom economy = M.Wt. of the desired product/M.Wt. of all products

=242/260 × 100

=93.07%

E = Sum of M.Wt. of the by-products/M.Wt. of the desired product

=18/242

=0.07

The theoretical atom efficiency and E-factor are 93.07% and 0.074 respectively. The experimental atom efficiency for the 10 mmol scale reaction under the given reaction conditions is 88.07% and the E-factor is 0.078.

The atom efficiency for the condensation reaction of benzaldehyde and malononitrile on several reported catalysts was calculated for comparison (Table 3). It can be seen that in terms of atom efficiency and E-factor the present mixed metal oxide catalyst system is comparable.

Table 3 Atom efficiency for the Knoevenagel condensation with different reported catalytic systems

Entry	Catalyst	E-factor
1	Silica gel^49b	0.090
2	ZnCl2^49c	0.084
3	RE-NaY^49d	0.094
4	MgO/ZrO2	0.078

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Very recently, we have reported MgO–ZrO₂ mixed metal oxides for the synthesis of dihydropyrimidinones under solvent-free conditions.^49e

The selective oxidation of alcohols is one of the most challenging reactions in green chemistry.⁵⁰ Cao has reported that mesostructured Ga–Al mixed-oxide solid solutions, which are characterized by unique dehydrogenation properties, may be used as new attractive supports for the fabrication of exceptionally active gold catalysts for the liquid-phase aerobic oxidation of alcohols under mild conditions.⁵¹ Aerobic oxidation of benzyl alcohol is depicted in Scheme 18.

Scheme 18 Selective oxidation of benzyl alcohol to benzaldehyde over mixed metal oxides.

Also, other various alcohols viz. 2-octanol, 1-phenyl ethanol etc. yield the corresponding aldehydes with good selectivity >99%.

These catalysts can even efficiently catalyze the aerobic oxidation of several alcohols under ambient conditions in the absence of water or base.

Kobayashi and co-workers recently reported PI/Au nanocatalysts as the most active Au catalysts in the current literature, with a very high turnover frequency (TOF) of 20 000 h⁻¹ for the conversion of 1-phenyl ethanol into acetophenone (at 160 °C).^52,53 Under these conditions, an Au/Ga₃Al₃O₉ catalyst gave an exceptionally high TOF of 25 000 h⁻¹, thus showing a great potential for practical applications (Scheme 19).

Scheme 19 Solvent-free aerobic oxidation of (±)-1-phenylethanol using an Au/Ga₃Al₃O₉ catalyst. Turnover frequency given for the following conditions: t = 30 min, (±)-1-phenylethanol (165.7 mmol), Au (4.0 × 10⁻⁴ mol%).

Lande and coworkers have established a series of mixed metal oxides, Ce₁Mg_xZr_1−xO₂ catalytic material, by a simple co-precipitation method and explored their potential use in the synthesis of 5-arylidine barbituric acid derivatives as a heterogeneous catalyst.⁵⁴ Catalytic activity results stated that, the Ce₁Mg_xZr_1−xO₂ (1 : 0.6 : 0.4) catalyst exhibits excellent catalytic activity as compared to the Ce₁Mg_xZr_1−xO₂ (1 : 0.2 : 0.8), Ce₁Mg_xZr_1−xO₂ (1 : 0.4 : 0.6) and Ce₁Mg_xZr_1−xO₂ (1 : 0.8 : 0.2) for the condensation of various aromatic aldehydes and barbituric acid and gave the yield 90 to 94% in short reaction time under microwave irradiation (Scheme 20). The plausible mechanism for the reaction is depicted in Fig. 9.

Scheme 20 Synthesis of 5-arylidine barbituric acid catalysed by CMZO mixed metal oxides.

Fig. 9 The plausible mechanism for the condensation reaction over CMZO.⁵⁴

Recent novel organic reactions on MMOs

The synthesis of mixed metal oxides has attracted much interest from various research groups. The reason for this is that such MMOs have found a wide range of applications in many critical areas of modern technology such as catalysis, solar energy conversion processes, oil recovery and especially in organic synthesis. We herewith present some important organic reactions over mixed metal oxides.

Mg–Al mixed-metal oxides were used for the transesterification of soyabean oil with methanol,⁵⁵ ternary Cu–Ni–Fe catalysts were used in alkyne hydrogenation,⁵⁶ Ni-based mixed oxide catalyst for carbon dioxide reforming of methane,⁵⁷ calcined Co–Mn–Al layered double hydroxide (LDH) for catalytic decomposition of N₂O, modified with slight amounts of alkali, rare earth, or noble metals,⁵⁸ nickel-based mixed-metal oxides with catalytic properties,⁵⁹ Cu–ZnO composite catalysts for selective hydrogenolysis of glycerol to propylene glycol,⁶⁰ nanosized Ti–V mixed metal oxides with anatase structure for the photo-elimination of toluene,⁶¹ nickel-based Ce_0.72Zr_0.28O₂ mixed oxide for methanation of carbon dioxide,⁶² calcium manganese(III) oxides (CaMn₂O₄·xH₂O) as biomimetic oxygen-evolving catalysts,⁶³ and some other recent applications of mixed metal oxides are depicted below.

Gold supported Cu–Mg–Al-mixed metal oxides of the 10–15 nm range were used for oxidation of a variety of alcohols.⁶⁴ The mixed-oxide-supported Au catalysts with noble metal loading of 0.6 ± 0.17 wt% were investigated concerning their structural properties and tested in the aerobic liquid-phase oxidation of 1-phenylethanol to phenyl–methyl ketone affording TOFs up to 1300 h⁻¹ (Scheme 21). The catalytic tests showed that the activity of these catalysts depends strongly on the composition of the support, with Cu and Mg being crucial components. Strongly enhanced catalytic activity was observed for gold supported on a ternary mixed oxide containing Cu, Mg, and Al at the molar ratio of 5 : 1 : 2. Extension of the catalytic tests to various structurally different alcohols indicated that the ternary mixed-oxide-supported gold catalyst has excellent catalytic properties in the aerobic oxidation of a broad range of structurally different alcohols, affording selectivities >98%.

Scheme 21 Aerobic liquid-phase oxidation of 1-phenylethanol to phenyl–methyl ketone over mixed metal oxides.

One-step synthesis of methyl isobutyl ketone (MIBK) from acetone and H₂ in the gas and liquid phase was carried out on Pd supported on Zn^II–Cr^III mixed oxide.⁶⁵ Due to the bifunctional nature of Zn^II–Cr^III mixed metal oxides, the reaction involves acid-catalyzed condensation of acetone to mesityl oxide, followed by its hydrogenation to MIBK (Scheme 22).

Scheme 22 Synthesis of methyl isobutyl ketone (MIBK) from acetone on mixed metal oxides.

Very recently Kantam and co-workers have reported effective heterogeneous Mg/La mixed oxide (Mg/La MO) for the one pot Wittig reaction involving aldehydes, α-halo esters and triphenylphosphine to afford α,β-unsaturated esters in good yields with high E-stereoselectivity, at room temperature⁶⁶ (Scheme 23). The corresponding products were obtained in good yields (49–99%).

Scheme 23 Synthesis of α,β-unsaturated esters on Mg/La mixed oxides.

Hell and coworkers reported copper-free Sonogashira reaction of alkynes and aryl halides by using Pd/MgLa mixed oxide.⁶⁷ This heterogeneous system allows the full recovery and reuse of the catalyst. Notably, palladium remains on the surface and no leaching was observed. The reaction of alkynes and aryl halides is shown in Scheme 24.

Scheme 24 Sonogashira cross-coupling reaction on Pd/MgLa mixed oxide.

The synthesis of benzimidazole structural derivatives has attracted much interest in medicinal chemistry as well as in various pharmacological activities.⁶⁸ Very recently, we have reported MoO₃/SiO₂ as a heterogeneous bifunctional catalyst for sequential oxidation of alcohols to their corresponding aldehydes/ketones using H₂O₂ as a green oxidant and its sequential condensation with O-phenylenediamine (OPDA) to yield benzimidazoles/benzodiazepines with minimum side product formation under mild reaction conditions.⁶⁹

The prepared catalysts were characterized using FT-IR, Raman spectroscopy, XRD, SEM and NH₃-TPD to study their surface properties. A SEM image of SiO₂ shows the presence of a smooth surface without any pores, however micrographs for MoO₃/SiO₂ clearly show marked difference in the morphology from that of SiO₂. The differences in SEM images thus illustrate that incorporation of MoO₃ changes the morphological structure of the SiO₂ surface considerably (Fig. 10).

Fig. 10 SEM images of (a) SiO₂ and (b) MoO₃–SiO₂.

Acidity measurements were carried out using NH₃-TPD analysis (Table 4), and it was found that the acidity of SiO₂ (entry 1) was very less compared to the Mo loaded SiO₂ (entries 2–7). This showed that the acidity of the catalyst is mainly due to Mo species. It was observed that as the percentage of Mo loading was increased, acidity of the catalyst also increased (entries 2–6).

Table 4 Acidity measurements and of MoO₃/SiO₂

Entry	Catalyst	NH3 desorbed/mmol g^−1	Yield (%)
1	SiO2	0.02	—
2	MoO3	0.13	28
3	5 MoO3/SiO2	0.43	42
4	10 MoO3/SiO2	0.62	63
5	15 MoO3/SiO2	0.77	77
6	20 MoO3/SiO2	0.82	73
7	25 MoO3/SiO2	0.80	72

---

However, the best results were obtained with 15 mol% Mo loadings (entry 4) although 20% Mo loading had higher acidity. Similar results were obtained by Wang and his co-workers.^70a

The decreased activity of the MoO₃/SiO₂ catalyst at higher MoO₃ loading was attributed to the aggregation of MoO₃ particles on the SiO₂ surface. MoO₃/SiO₂ shows good catalytic activity and reusability for both the reaction systems giving high yields (52–85%) of the desired products (Scheme 25).

Scheme 25 Sequential oxidation and condensation for synthesis of benzimidazole and benzodiazepine over MoO₃–SiO₂.

Very recently, we have prepared mixed metal oxides (MgO–ZrO₂) nanoparticles by an ultradilution–coprecipitation method and gradient heating.^70b The as-synthesized catalyst was characterized by several techniques such as XRD, TG-DTA, FT-IR, SEM and TEM. The SEM and TEM images are depicted in Fig. 11. TEM clearly indicates that the size of MgO–ZrO₂ particles are in the 20–35 nm range. The MgO–ZrO₂ catalyst is then used for various reactions such as cross-aldol condensation, Cbz protection of amines, reduction of nitroarenes and in synthesis of 1,5-bezodiazepines.

Fig. 11 (a) TEM image of MgO–ZrO₂ at 100 nm. (b) SEM image MgO–ZrO₂ at 1 μm.^70b

All reactions worked well under mild conditions and gave excellent yields of the corresponding products. The MgO–ZrO₂ catalyst was reused and recycled for all reactions without any loss of yield of the products (Scheme 26). The mechanism of Cbz protection of aniline over nano MgO–ZrO₂ is depicted in Fig. 12. In the first step, the –NH₂ group of aniline and Cbz–Cl adsorbed on the catalyst as shown in I. It is well known that functional groups containing electronegative atoms such as N, O, and S are adsorbed on metal or Lewis acid site and hydrogen adsorbed on the oxygen of the catalyst. Then in the second step electrophilicity of carbonyl carbon of Cbz–Cl is increased and it generates an intermediate state II, which, finally yields final product III along with liberation of HCl. The MgO–ZrO₂ catalyst participates in all these steps, weakening the chemical bonds of reactants and subsequently lowering the activation energy.

Scheme 26 Nano MgO–ZrO₂ catalyzed reactions.

Fig. 12 Mechanism for Cbz protection aniline over nano MgO–ZrO₂ mixed metal oxides.

The mechanism of reduction of aromatic nitrobenzene to aniline is shown in Fig. 13. During adsorption of propan-2-ol the hydrogen from the –OH group adsorbed as a proton and the hydrogen of C–H migrated via hydride transfer to the substrate. Hence the rate of reaction can show dependence on the strength of adsorption of both propan-2-ol and substrate. First nitrobenzene and isopropyl alcohol adsorbed on nano-MgO–ZrO₂I, then reduction of nitrobenzene with IPA as hydrogen source generated an intermediate II with elimination of an acetone molecule, then intermediate III with elimination of a water molecule and finally aniline Vvia intermediate IV.

Fig. 13 Mechanism of reduction of nitrobenzene to aniline over nano MgO–ZrO₂ mixed metal oxide.

Very recently, Baiker has reported metal-support for the hydrogenation of cinchonidine (CD) over acid- and base-doped flame-made Pt/alumina.⁷¹ They have studied the effect of support acidity/basicity on the adsorption geometry of CD at the surface of supported Pt nanoparticles. The model catalysts were prepared by the reported procedure.⁷² The chemo- and diastereoselective hydrogenation of CDH₂ are represented in Scheme 27.

Scheme 27 Chemo- and diastereoselective hydrogenation of CDH₂.⁷¹

The support effect of Cs₂O and SiO₂ over alumina on diastereoselective hydrogenation of CDH₂ to CDH₂-A is shown in Fig. 14. The support effect on the orientation of the alkaloid on Pt during hydrogenation of its heteroaromatic ring is depicted in Fig. 15.

Fig. 14 Diastereomeric excess (d.e.) in the hydrogenation of CDH₂ to CDH₆-A as a function of the promoter content in the alumina support.⁷¹

Fig. 15 Schematic drawing of the probable adsorption geometry of CDH₂ on a Pt (111) model surface located on acidic and basic supports. H—white, C—black, N—blue (gray), O—red (gray).⁷¹

The past few years have seen a development in the synthesis and characterization of mixed metal oxides and their versatile applications, making them very challenging and demanding in field catalysis science and technology.^73–84

Future scope of mixed metal oxides

Mixed metal oxides are the heart of heterogeneous catalysis and are being used in chemical industries, pharmaceutical industries as well as in academic research. They are modified day by day and are applied in several fields. Various combinations of noble metals with basic oxides can be used for Sonogashira reaction, Suzuki coupling, other important C–C, C–O, C–S and C–N bond forming reactions and some industrially important multicomponent reactions. Ferrites containing mixed metal oxides as well as pervoskites⁸⁵ (ABO₃) can be used for various name reactions and other important organic conversions. These ferrites containing mixed metal oxides can be separated magnetically and reused for next cycles.

In the field of mixed oxides, there are many kinds of fundamental structures (of double oxides and salts of oxoacids) and numerous compositions for each structure are possible. Scientists are reporting novel mixed metal oxides in various international journals of repute and those mixed metal oxides exhibit various valuable functions derived from a variety of electronic, magnetic and chemical properties. The mixed metal oxides shall be used in various fields, especially in catalysis and organic synthesis.

Conclusions

The aim of this review is to illustrate the significance of a variety of mixed metal oxides as catalysts and catalyst supports as employed for a wide variety of catalytic applications both in the liquid and gaseous phases. As discussed, these mixed metal oxides represent an interesting class of heterogeneous catalysts and catalyst supports for alkali, rare earth and noble metals which have been widely employed in reductions, oxidation, one-pot synthesis reactions, one-pot Mannich reactions, alkylations, condensations, deprotection, cycloadditions, hydroxylations, dehydration, dehydrogenations, transesterification, reactions involving biomimetic oxygen-evolving catalysts and other important C–C bond forming reactions. These mixed metal oxides are going to be modified in future and find numerous applications in the coming years both as catalysts and catalyst carriers.

Acknowledgements

We are grateful to CSIR, New Delhi, India, for providing financial support for the research work. Manoj Gawande is thankful to Faculty of Science and Technology, Lisbon, Portugal, for the award of Research grants (SFRH/BPD/64934/2009).

Notes and references

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