Catalysis plays a major role in chemical, physical and biological sciences. It has become, during the years, one of the most important fields in chemistry, as some 85%–90% of industrial processes include at least one catalytic step and, more recently, in environmental chemistry. The main role of a catalyst is to decrease the activation energy of a chemical reaction and, in a multiproduct reaction, to favour the most important one, i.e. to favour selectivity which is now a major concern to avoid at most the formation of unwanted by-products, even at the expense of the reaction activity.

Catalysis is important in the refinement of petroleum, in the manufacturing of synthetic fibres and plastics, in the production of many specialty chemicals, in the processing of foods, in the production of drugs and pharmaceutical compounds, etc. It helps in the decrease of atmospheric pollution, in the design of environmentally friendly catalytic technologies, and of new processes to generate energy, and it is considered as a pillar of green chemistry in the preservation of our environment. From the viewpoint of an academic scientist, catalysis is viewed as a cross-disciplinary field, including organometallic chemistry, physical chemistry, chemical engineering, surface science, solid-state chemistry, and theoretical/computational chemistry.

Among all catalysts and catalytic reactions, metal oxide catalysts and catalytic reactions are essential in most refining and petrochemical processes, in synthesizing specialty chemicals and more recently in improving environmental issues (depollution, high selectivity in reactions to avoid un-useful by-products). Metal oxide catlsysts became prominent in the mid-1950s, when they were found to effectively catalyse a wide variety of oxidation and acid-base reactions. In this review paper, major types of metal oxide catalysts and processes that use these catalysts are reviewed. I have decided to focus its content on heterogeneous catalysis (mainly gas/solid and liquid/solid), in heterogeneous media, using metal oxide-type catalysts, whatever bulk or supported catalysts. In other words, catalysis by metals, by multi-metallic catalysts, supported or bulk catalysts, or by carbons (graphene, carbides), sulfides, etc., is excluded, although they constitute a large part of the catalysis field, in particular for petroleum and petrochemicals.

Advanced research in catalysis with metal oxides has permitted the development of catalysts with high selectivity and with new preparation procedures. The performance of a solid catalyst relies on its grain size, shape, composition and preparation. New strategies for the preparation of metal oxide catalysts have resulted in the development of catalysts with single sites, that lead to 100% selectivity. The advent of nanoscience has promoted bottom-up over top-down strategies. Nanocrystals, with sizes of only a few nanometres, present the best catalytic efficiencies. However, the use of a support (silica, alumina, silica-alumina, carbons, etc.) could be important for the synergistic activation of substrates, electron conductivity enhancement in redox reactions, thermal conductivity enhancement for exo/endothermal reactions, etc. The activity, selectivity, resistance to deactivation, and ability for regeneration are the properties that characterise the usefulness of catalysts, and have been improved in the last fifty years by advanced research and by improved industrial processes in catalysis with metal oxides.

Metal oxide catalysts are involved in majority of industrial catalytic processes. They include simple oxides, such as silica, alumina, clays, zeolites, titania, zirconia, ZnO, and porous and mesoporous metal oxides, or complex oxides, such as polyoxometalates (POMs) of Keggin or Dawson type, phosphates (e.g., (VO)₂P₂O₇, FePO₄, silica phosphoric acid (SPA)), multicomponent mixed oxides (molybdates, antimonates, tungstates, MoVTe(Sb)Nb-O, etc.), perovskites, and hexaaluminates. Some of them correspond to specific arrangements of MO₆ (M = Mo, V, Te, Nb, etc.) octahedra as schematised in Fig. 1.

These metal oxides present specific properties such as acidity (Lewis or/and Brønsted types), basicity, redox features (when transition metal ions are present), which lead to specific catalytic properties. They are also the basis for metallic (mono- or pluri-metallic) catalysts, for hydrodesulfuration (HDS) catalysts (CoMoO₄-, NiMoO₄-, NiWO₄-based), for deNO_x and deSO_x reactions, as perovskites or ceria-based doped oxides. The main catalytic domains cover oxidation (selective or total), acid and base catalysis, photocatalysis, depollution and biomass conversion. Metal oxides are sometimes used simply as supports of active phases, such as silica, alumina, silica-alumina, mesoporous oxides, MOFs, etc., which may influence catalytic properties due to synergistic, electron conductivity or/and thermal conductivity effects, which stem from metal oxide-support interactions.

Metal oxides [1] constitute a class of inorganic materials, which exhibit peculiar and various properties and applications as sensors, catalysts, fuel cells, etc. Metal oxide surfaces terminate by oxygen O^2– anions, whose size is larger than that of Mⁿ⁺ cations. It follows that the symmetry, coordination and accessibility to reactant molecules of Mⁿ⁺ cations are lost at the surface. Moreover, the surface of an oxide may contain different types of defects and environments (kinks, steps, terraces), which may play a role in the catalytic phenomenon, for instance resulting in structure sensitivity of mletal oxides to catalytic reactions [5, 6]. This surface unsaturation is usually compensated by a reaction with water vapour, leading to the formation of surface hydroxyls according to O^2– + H₂O → 2 OH^–. These OH groups are conjugated acids of lattice oxygen ions O^2–, which are strong bases and conjugated bases of water molecules.

Single or complex metal oxides based on the first transition series present a wide variety of non-stoichiometric phenomena, which originate from the unfilled 3d electron shell. For instance, Fe sites can be vacant in FeO of rocksalt framework (Fe_1–xO with 0.05 < x < 0.18). In ABO₃ perovskite oxides, non-stoichiometry arises from a cation deficiency in A or B sites (A_xWO₃, A being an alkali ion) or oxygen anions excess. Cation A has a great influence on electrical properties. For instance, WO₃ is an insulator, while A_xWO₃ is a semiconductor at low x values and metallic at high x values. Oxygen-deficient perovskites have attracted much attention because of their oxygen storage ability and their redox properties, quite useful for total oxidation reactions. In fully oxidised CaMnO_3-δ and CaFeO_3-δ perovskites, Mn and Fe are at +4 oxidation state and δ = 0, the material can accommodate up to 17% oxygen vacancies without loosing its structure. The Sr_1–xLa_xCo_1–yFe_yO_3–δ series, with brownmillerite-type oxygen defects, exhibit high electronic/oxygen ion mobilities. In some perovskites a small oxygen excess can be accommodated by the formation of cation vacancies at A- or B-sites, leaving the oxygen sub-lattice intact. Electronic defects may be created upon reduction and oxidation of metal cations at different oxygen partial pressures (p_O2). At low p_O2, the material loses oxygen, which generates electrons, enhancing the n-type conductivity, according to O₀ ↔ (1/2) O₂ + V_O^•• + 2e^–. At high p_O2, oxygen is incorporated into an oxygen vacancy and takes two electrons from the valence band, leading to holes contributing to the p-type conduction, according to (1/2) O₂ + V_O^•• ↔ O₀ + 2h^•. A consequence of the trapping of electronic defects is that the solid becomes insulating at low temperatures. Defect associations between oxygen vacancies and acceptor dopants are observed in Y-ZrO₂, where V_O^•• are trapped by YZr^• defects, leading to a drastic decrease in oxide ion conductivity.

Protonic defects, associated with acceptor dopants, limit protonic conduction in metal oxides. As all metal oxides contain impurities, the defect mechanism of "pure" oxides is similar to that of doped oxides. For instance, strontium titanate, SrTiO₃, has a similar defect chemistry to acceptor-doped SrTiO₃ because the impurities are ions at lower oxidation states such as Fe³⁺, Al³⁺, Mg²⁺, and Na⁺, and their concentrations, typically ca. 10²–10³ ppm, are sufficient to dominate the defect chemistry, especially at high p_O2. All these aspects should be taken into account when considering the reaction mechanisms and kinetics on metal oxide catalysts, for instance in selective or total oxidation reactions on mixed metal oxides, where electron and oxygen anion mobilities and lattice oxygen diffusion or storage capacity are important, for instance in a redox or in total catalytic oxidation reactions.

Polyoxometallates (POMs) constitute a large class of nanosized transition metal-oxygen clusters intermediate between molecules and oxides [7]. They are formed of anions of metal-oxygen octahedra as basic structural units, linked together to constitute a stable and compact skeleton of polymeric oxoanions, as shown in the equation:

with M = metal (designated as addenda atom) of valency m, X = heteroatom of valency x, and s = the oxygen balance. The most common M atoms are Mo and W and less frequently Ta, V, Nb as d⁰ element. There are more than 20 structures incorporating 4 to 40 metal atoms and 1 to 9 heteroatoms. One distinguishes the structures of "Linqvist" [M₆O₁₉]^n– (O_h symmetry of 6 MoO₆ edge-shared octahedra), "Keggin" α-[(XO₄)M₁₂O₃₆]^n– (T_d symmetry of 4 M₃O₁₃ groups of 3 MO₆ octahedra), "Dawson" α-[(XO₄)₂M₁₂O₅₄^n– (D_h symmetry of 2 Keggin fragments α-XM₉O₃₄), "Anderson" [H_x(XO₆)O₁₈]^n– (D_3h symmetry planar arrangement of 7 edge-shared MO₆ octahedra), and (XO₁₂)M₁₂O₃₀ structure (I_h symmetry with an XO₁₂ icosahedron surrounded by 6 equivalent M₂O₉ groups of face shared MO₆ octahedra, linked by corner sharing tetravalent cations). The most common POMs used in catalysis are Keggin-type either as acid-base catalysts such as heteropolyacids (HPA) or as redox (for partial oxidation reactions) catalysts. Their properties are controlled, among a large range, by varying the chemical composition, e.g. changing the M element as in H_3+xPV_x^VM_12–x^ⅥO₄₀, the support, the extent of hydration and thermal treatment. For acid samples the following order of acidity range has been established [8]: H₃PW₁₂O₄₀ > H₄SiW₁₂O₄₀ ~ H₄GeW₁₂O₄₀ > H₃PMo₁₂O₄₀ > H₄SiMo₁₂O₄₀ ~ H₄GeMo₁₂O₄₀. Several industrial processes have ben developed, in particular in Japan, such as hydration of propylene to isopropanol in 1972, oxidation of methacrolein to methacrylic acid in 1982, direct oxidation of ethylene to acetic acid in 1997, and production of ethyl acetate in 2001, among other new processes. Giant heteropoyoxometalate compounds have also been synthesised [9], such as the Bielefeld "wheel" [Mo₁₂₆^ⅥMo₂₈^VO₄₆₂H₁₄(H₂O)₇₀]^14– or a Keplerate capsule superfullerene type [Mo_I72^VMo₆₀^VO₃₇₂(CH₃COO)₃₀(H₂O)₇₂]^42– [10] but with no industrial application yet, to my knowledge. Representations of the main heteropolyoxometalate structures are shown in Fig. 2, and some catalytic reactions are reported in Table 1.

This section addresses the broad topic of metal oxide syntheses of bulk and supported metal oxide catalysts. It corresponds to three main areas: (1) synthesis of simple metal oxides, (2) synthesis of mixed oxides, and (3) preparation of supported metal oxides. A typical industrial operation for manufacturing catalysts comprises: (1) precipitation at a given pH or other synthesis processes (e.g., sol-gel, solid-solid, flame hydrolysis, and vapour deposition); (2) hydrothermal transformation; (3) decantation, filtration, centrifugation, and washing; (4) crushing and grinding; (5) forming and/or shaping operations; (6) calcination, activation, and reduction.

Catalyst syntheses are schematically represented by: Chemicals (control impurities) → Catalyst precursor (texture) → Form (texture, shape attrition resistance) → Final catalyst (texture, attrition resistance).

For simple oxides, it is important to control the nature of the solid catalyst, its morphology, textural properties (surface area, porosity), and thermal, chemical and mechanical stabilities. Beside thermal methods, which do not permit a fine control of these parameters, three main preparation routes based on the reactions between water or an organic solvent and inorganic precursors are usually employed, namely aqueous-phase precipitation, sol-gel (hydrolytic and non-hydrolytic) process, solvothermal syntheses in the presence of organic molecules and templating procedures resulting in materials with controlled morphology and textural properties, in particular organized microporous, mesoporous, macroporous, and hierarchically-organized oxides. The complexation, solvent-free activated reactive synthesis, allows one to overcome the limited surface area reached after crystallization at high temperature. Organization of the porosity by hard or colloidal templating is a recent approach to maximise the specific surface area leading to mesoporous and hierarchical porosity materials.

The preparation of mixed oxide catalysts involves co-precipitation of two or more mixed salts in solution at a given pH, followed by classical heat treatments. One can also proceed by solid-solid interaction between both salts at high temperature.

Industrially one usually employs either bulk catalysts composed of active components but with some inert binder added (e.g. silica, alumina, silica-alumina) to aid the forming and/or shaping operation and to avoid important attrition, mainly in moving/fluidized bed reactors. This is the case, for instance, for silica-alumina for cat-cracking, copper and chromium oxide for the water gas shift (WGS) reaction, iron molybdate for the oxidation of methanol to formaldehyde, vanadyl pyrophosphate for butane oxidation to maleic anhydride, etc. A description of some structural components of some catalysts are shown in Fig. 2. However, in some cases, bulk catalysts are used as prepared, without the need for addition of the binder. Typically, this involves catalysts prepared by high temperature fusion, e.g., the iron-based ammonia synthesis catalyst. A schematic representation of the influence of preparation parameters is given in Fig. 3.

The main domains of heterogeneous catalysis by metal oxides cover acid, base, acid-base (bifunctional), selective oxidation reactions, total oxidation reactions and combustion, photocatalysis and biomass conversion. The first three domains have been described in detail in many reviews or books (see Refs. 1–3) and will not be discussed here. The main catalysts have been described above and include silica, alumina, silica-alumina, acid or base zeolites, polyoxometalates (POMs), hydrotalcites, haxaaluminates, perovskites, etc. The selective oxidation domain is very important and has been described in many papers and books [13]. It deals with transition metal ion oxides exhibiting redox properties, such as V, Mo, Cr, Te, Sb, Bi, and Fe, and usually proceeding via the famous Mars and van Krevelen mechanism [14]. In this mechanism shown in Scheme 1 and illustrated in Fig. 4, the lattice oxygen anions O^2– are incorporated into the hydrocarbon and give aldehydes or carboxylic acids or H₂O, leading to a reduction of the catalyst. The corresponding vacancy created is then replenished by gaseous oxygen in the re-oxidation step.

(VO) ₂P₂O₇ for butane to maleic anhydride, multicomponent Bi molybdate + Co(Fe) MoO₄ for propene to acrolein, MoVTeNb(Sb)-O for propane direct ammoxidation to acrylonitrile or oxidation to acrylic acid, and MoV-O for acrolein oxidation to acrylic acid are among the most important catalysts used industrially [15], as described in Section 7 below and assembled in Tables 2–4.

In the case of catalytic partial oxidation reactions mixed metal oxides based catalysts are mainly on V and Mo cations attached to other redox cations such as Sb, Nb, Te, and Ta. V₂O₅ and MoO₆ octahedra are assembled as shown in Fig. 1 and are often composed of several phases acting synergetically between them and giving complex chemical compositions. For instance, this is the case of the mixture of M1 and M2 phases in the MoVTeNb-O catalyst used for propane amm/oxidation to acrylonitrile/acrylic acid.

In these partial oxidation reactions, the selectivity to the desired compounds (olefins, aldehydes, carboxylic acids, nitriles, etc.) depends on the nature of surface oxygen anions (O^2–) being nucleophilic (selective) or nucleophilic (non selective) associated to the cation(s) Meⁿ⁺, on Me–O bond energy and their distribution on the surface. This may come from the fact that oxidation reactions are demanding, i.e., are structure-sensitive [4], when the number of electrons involved in the reaction is high. For instance, many catalysts are able to transform propane to propylene (reaction with 2e^–), whereas only three (Cu-O, Bi-Mo-O, Sn-Sb-O) are able to transform propylene to acrolein (4e^– reaction) and only one ((VO)₂P₂O₇) to oxidise butane to maleic anhydride (a 14e^– reaction) or (VTi-O) for o-xylene oxidation to phthalic anhydride (a 12e^–) reaction.

From the view point of chemicals and energy resources, utilization of the resources other than fossil resources is important in overcoming the environmental issues and for building a sustainable society. One of the possibilities for chemicals is biomass, in particular cellulosic biomass. Solid acids and bases should play important roles in conversion. Actually, the conversions of biomass and biomass-derived products have been studied extensively in recent years. Among them, biodiesel synthesis and utilization of glycerol are important topics [19, 20], though only one process has been industrialized for biodiesel production on ZnO-Al₂O₃ catalysts. Note that the solid acids and bases applicable to biomass conversions should be water-tolerant catalysts [21, 22].

Currently, acrolein is produced primarily by the gas-phase oxidation of propylene on multicomponent catalysts (e.g. BiFeCoMoO₄). However, as the use of propylene derivatives, particularly polymers, has recently been sharply increased, and since this trend is likely to continue in the foreseeable future, the availability and cost of propylene is going to increase. One approach to substitute propylene relies on the use of new raw materials. For instance, catalysts and processes have been developed for the direct conversion of glycerol to acrolein [23]. However, in such processes/catalyst two major difficulties were observed, namely rapid deactivation and selectivity limited to less than 80%. There is thus a need to develop novel routes for the production of acrolein. One approach could be the conversion of allyl alcohol, which is produced in an initial step either by biomass fermentation or through the dehydration of 1, 3-propanediol, or the dehydration/oxidation of glycerol [24, 25]. The MTO (methanol to olefins, mainly C₂⁼ and C₃⁼) has been successfully developed recently in China, using SAPO-34 catalyst [26]. The world first commercial unit of MTO process was constructed and started to operate by Shenhua Group in 2010, which was an important progress for coal to chemicals.

A series of Mo₃VO_x/H₄SiW₁₂O₄₀/Al₂O₃ catalysts, as bifunctional catalysts, have been prepared and applied for the one-step oxydehydration of glycerol to acrylic acid [27]. It was found that acetic acid was produced in parallel with acrylic acid at similar yield when adjusting Mo₃VO_x content in the catalysts. Increasing calcination temperature from 350 to 650 ℃ led to structural evolution of supported active species and subsequent activity change. Crystallinity of Mo₃VO_x increased with calcination temperature, and a large amount of (V, Mo)O_x hetero-polyoxo mixed oxides were formed above 550 ℃. The Keggin structure of H₄SiW₁₂O₄₀ began to dissociate around 450 ℃, causing the formation of various WO_x species. Above 450 ℃, the yield of acrylic acid dropped while that of acrolein reached its maximum value with increase of the catalyst calcination temperature. In contrast, the yield of acetic acid exhibited a minor change. Based on the catalytic and characterization data, possible active centres and reaction pathways have been proposed, while for the improvement of the bi-functional catalyst for the one-step conversion of glycerol, it was proposed that both the acidic and redox properties of catalysts should be finely tuned. The (V_0.07Mo_0.93)₅O₁₄ entities were suggested to be the active species for acrolein oxidation, while isolation of dehydration and oxidation active centres was also suggested to improve acrylic acid yield.

Photocatalysis is a relatively young science, which is already of great interest for green and environmental chemistries, as it operates at room temperature and does not need thermal energy, the catalyst being activated by the photons. The technique is able to excite electrons e^– from the conduction band or to generate electron holes h⁺ in the valence band. The catalysts, simple oxide as TiO₂ or doped oxides, sulfides, etc., are polyphasic: solid catalyst, liquid or gaseous reactants and electromagnetic (photons from solar light or UV-rays). The first example was discovered accidently from a thermogravimetric study of oxygen chemisorption on evacuated TiO₂, which was observed to lead to different weight increases and colour changes under oxygen, depending on the solar or unsolar day in the laboratory at Claude Bernard University (Lyon-1) in France. The same results were obtained by irradiating the sample with UV light. The pioneers (S. Teichner et al.) took advantage of this labile oxygen O^* species to study at room temperature selective oxidation of alkanes C_nH_2n+2 (with n ≥ 2) to the corresponding aldehydes and extended further to the oxidation of cycloalkanes, alkenes, substituted aromatics, amines, etc. Electrical photoconductivity experiments have shown that oxygen is photoadsorbed as ionic O₂^– and O^– species [33], O^– being the precursor of the activated O^* species according to O_ads^– + h⁺ → O_ads^*. In presence of water (gas or liquid) adsorbed H₂O molecules dissociate to OH_ads^– and H_ads⁺ and OH_ads^– + h⁺ → OH°_ads radical, which is very nucleophilic and thus non-selective in oxidation reactions. This aspect has led to the use of photocatalysis for pollution abatement of organic compounds such as pesticides, in particular organophosphorous compounds as phenitrothion [(CH₃)₂-P(=S)-O-C₆H₃(NO₂)-CH₃] or dyes.

Among all the semiconductor materials under investigation, metal oxides represent the most important candidates due to their unique physiochemical properties. Producing hydrogen fuel via photocatalytic water splitting on semiconductor materials has attracted intense interest in the past decades due to its potential to address important energy and environmental problems. As the main sector for implementing photocatalytic reactions, semiconductor materials have been the focus of the research efforts.

Since the report of using TiO₂ for photoelectrochemical water splitting, metal oxide-based semiconductors have received great attention in the field of solar energy conversion. In particular, significant advances have been achieved on using metal oxide semiconductors for photocatalytic water splitting reactions. The objective is the development of novel metal oxide-based photocatalysts with appreciable solar light absorption and electronic properties. The tuning of the electronic structures of existing photocatalysts to obtain enhanced light absorption and more efficient charge separation and transportation has been intensively studied. However, there is still a huge gap between the actual solar-to-hydrogen (STH) efficiency achieved and the theoretical maximal value. Currently, the highest STH efficiency achieved on metal oxide photocatalysts is only ca. 1%, which is far away from what is required for any practical application. Therefore, there is still a long way to go before the large-scale application of photocatalytic water splitting for solar energy conversion. In the future, efforts should be devoted to the development of metal oxides with wide solar light absorption and appropriate band edge positions for water splitting. Recently, MIL-101 (Fe) particles have been coated with an amorphous shell of titania and were observed [34] to lead to hydrogen production from water using visible light, while neither component alone was able to do it.

Industrial applications of metal oxide catalysts cover mainly the synthesis of petrochemicals (chemical intermediates such as acrylonitrile, maleic anhydride, acrolein, acrylic acid, and substituted aromatic compounds) [36], in which the V element appears as a key partner and for fuel manufacture. Many processes have been developed as summarised in Table 4.

Currently, most of the acrylic acid produced world-wide is made using the same process, namely the two-stage oxidation of propylene. In this process, propylene is oxidised to acrolein in a first reactor on scheelite Bi molybdate-based catalyst (defect structure of scheelite (Bi_2/3□_1/3Mo₃O₁₂, where □ represents a cation vacancy in the ideal ABO₄ structure) containing Co and Fe molybdates [37], and acrolein is further oxidised to acrylic acid in a second reactor without intermediate separation. The second stage catalysts are MoVWCu-O mixed oxides as key formulation. These catalysts are mostly X-ray amorphous and contain V in a reduced state.

It was observed that an oxide catalyst consisting of molybdenum, vanadium and niobium was able of yield high conversion of ethane and high selectivity to both ethylene and acetic acid products [38]. The best catalyst was found to be Mo_0.61V_0.31Nb_0.08. Further, two defect oxide structures of molybdenum and vanadium were identified as the likely catalysts for the selective oxidation of ethane. These two phases are uniquely described by the stoichiometry Mo₄V₆O₂₅ and Mo₆V₉O₄₀ with the inclusion of niobium to stabilise the structure and promote microcrystallinity. Propane direct oxidation to acrylic acid has been studied [39] on mixed oxides, such as MoVTe(Sb)Nb-O of structure derived from the above one MoVNb-O for ethane to ethylene, as well as the dehydration of lactic acid and 3-hydroxypropionic but have not been commercialised yet. Recently, a crucial discovery [40] was the calcination of the mixed oxide, which was previously done in air at around 350 ℃, in a non-oxygen containing (i.e. inert) atmosphere (nitrogen gas) at around 620 ℃. This resulted in acrylonitrile yields of up to 59% (92% propane conversion; 64% acrylonitrile selectivity), which made the system competitive with the previous processes starting from propene. The active phase was identified as the so-called M1 phase [41]. Acrylonitrile hydrolysis, as well as the Reppe chemistry to get acrylic acid, has been used in the past but is no longer economically attractive. At variance, direct ammoxidation of propane to acrylonitrile on the same MoVTe(Sb)Nb-O catalysts has been successively industrialised. Among the disclosed compositions is Mo₁V_0.4Te_0.2Nb_0.1O_x with reported per pass yield of acrylonitrile of ca. 14% (23% propane conversion; 61.5% acrylonitrile selectivity). Yields were improved to up to 25% (58% propane conversion; 44% acrylonitrile selectivity) with the addition of various elements as promoters.

Recently glycerol dehydration to acrolein has received much attention to generate a route to renewable acrylic acid. A shift to new reactor technologies, such as microstructured reactors or thermoplate reactors, able to better handle the reaction heat triggered the need for more active catalysts. However, currently incremental improvements of the catalyst and process are to be expected, since all the installed capacities are using the same type of technology: multitubular fixed-bed reactors operating slightly above the upper flammability limit, over MoVWCu-mixed oxide catalysts.

The nitrile functionality serves as a useful reactive centre for transformation into a wide spectrum of commercially valuable chemical products, via polymerization, hydrogenation, hydration and condensation, the production of aromatic nitriles using mixed metal oxide catalysts. Commercial application of alkyl aromatic ammoxidation is based on three categories of feedstocks, i.e. toluene, xylenes, and picolines (alkyl pyridines). Catalysts for alkyl aromatic ammoxidation are vanadium oxide based. The ammoxidation reaction produces the corresponding aromatic nitrile. The nitrile products are intermediates for industrially important chemicals including amines, isocyanates for polyurethanes, phthalocyanine dyes and vitamin B compounds (niacin).

An interesting case is the upgrading of methane to ethane by oxidative coupling (OCM), which has led to huge research worldwide since the pioneered early 1970s work by Lunsford at Texas A & M on Li/MgO-based catalysts. Unfortunately, none industrial processes were developed using such types of catalysts and technology. Recently Siluria Technologies Inc. [42] has developed a process to convert methane to ethylene by OCM. The catalyst is prepared with rare earth oxides either unsupported or supported on MgO, CaO or AlPO₄ and containing at least two dopants (a metal element, a semi-metal element and a non-metal element). A demonstration plant has been set in La Porte, Texas, and a commercial plant is scheduled for 2019.

Metal oxide catalysis is a still growing field reflecting the wide range of catalytic reactions involved. It appears that, although great improvements and new developments have been dicovered in the second part of the 20th century, it remains plenty of room for young researchers from academia and industry to discover and develop new metal oxide catalysts and new or improved processes. It can be expected that the first part of the 21st century will see development of new catalysts and development of commercial processes related to environmental issues and legislation constraints.

The challenge of catalysis in the 21st century is to achieve 100% selectivity of the desired product molecules in uni- or multi-path reactions (designated as "green chemistry") and develop renewable energy-based processes. In catalyst synthesis nanoscience, synthetic methods have been developed to produce monodisperse metal oxide nanoparticles (NPs) in the 0.8–10 nm range with controlled shape, oxidation states, and composition; these NPs can be used as selective catalysts since chemical selectivity appears to be dependent on all of these experimental parameters. New spectroscopic and microscopic techniques have been developed that analyse and characterise catalysts under reaction conditions and reveal the dynamic change of molecular structure of catalysts and adsorbed molecules as the reactions proceed with changes in reaction intermediates, catalyst compositions at the surface, and oxidation states. Photo water splitting, and methane and carbon dioxide conversion are important research directions in renewable energy conversion.

Let us end this paper by analysis of the current trends in applied heterogeneous catalysis [44]. Since the 1990s major themes of research and developments have emerged, for instance, auto exhaust catalysts for CO, NO_x and hydrocarbons eliminations from cars and trucks, catalysts for electrochemistry (utilising solar energy with cheap and stable materials, like reduction of water to H₂, of CO₂ to methanol) and photochemistry (like photo water splitting or solar driven clean-up of environmentally harmful by-products using TiO₂ as photocatalyst), quest for enantioselective catalysts by asymmetric sites on heterogeneous catalysts, immobilised transition metal complexes to heterogeneise homogeneous catalysts, immobilised enzymes and cells, catalytic antibodies, catalytic oxidation of widely abundant methane to syngas CO + H₂ and further methanol and onward to hydrocarbon mixtures (MTO and MTG processes), alkane up-grading, in particular by selective partial oxidation, e.g. propane oxidation/ammoxidation to acrylic acid/ acrylonitrile, etc. Some perspectives are summarized in Figs. 5 and 6.

In short, sustainability, efficiency, oil refinery, energy, transport, hydrogen synthesis and stocking, CO₂ and CH₄ up-grading and catalytic transformations, (e.g. dry reforming), alkanes up-grading to olefins, aldehydes or carboxylic acids, gaseous noxious emissions, greenhouse gases emissions, new catalysts (e.g. MOFs [45]), new processes, and biocatalysis are, at present, the main drivers in catalysis related R&D.