Styrene is one of the most important monomers inpolymer chemistry. Global styrene monomer (SM) consumption is driven by theproduction of its polymer or copolymer products, such as polystyrene (PS),polystyrene-butadiene rubber (SBR), polyacrylonitrile-butadiene-styrene (ABS).Styrene-based polymer materials offer unique characteristics, such as durability,high performance, versatility of design, simplicity of production and economy.They can also provide excellent hygiene, sanitation, and safety benefits. Owingto its usefulness, the demand of SM has been increasing every year. The SMcapacity in 2012 was estimated to be over 32.7 million tonnes (Merchant Researchand Consulting Group). To provide feedstock for products, the SM capacity usagerates are predicted to grow rapidly in the near future and are expected to reach~94% (Merchant Research & Consulting, 2015).

Catalytic dehydrogenation, one of the 10 mostimportant petrochemical processes, produces more than 90% of the SM requiredfor application ^[1]. The dehydrogenation of ethylbenzene (EB) is performed overa K-promoted iron oxide catalyst at around 600-650 °C, just below thedecomposition temperature of the reactant ^[2]. Owing to the high endothermic(125 kJ/mol) nature and volume increasing character of the reaction, in anadiabatic reactor, a large amount of superheated and pressurized steam(steam/EB = 7-15 mol/mol) is necessary to supply the heat and lower the partialpressure of reaction to shift the equilibrium towards the SM product ^[3]. Thelarge amount of steam acts as a diluent to lower the partial pressure of theproduct mixture, to shift the equilibrium to SM and to avoid (or remove) theformation of carbon deposits ^[4]. However, at the gas-liquid separator, a largeamount of latent heat steam is wasted ^[5]. Consequently, the estimated hike inproduction cost is about 10% and, in addition, the development of hotspotsabates the overall activity and catalyst lifetime. Several technologicaladvancements have emerged for the improvement of the existing commercialprocess by varying the other diluents apart from steam, such as co-feeding of O₂and by introducing coupled reactions (SM Monomer Advanced Reheat Technology,SMART, by Lummus/UOP) ^[1]. Unlike conventional steam-reheat processes, theSMART process involves reactants that leave the dehydrogenation catalyst bedbeing reheated by selectively oxidizing part of the co-produced H₂^[6]. The reaction is fed by air or by oxygen-rich air and takes place over aproprietary UOP high selectivity catalyst with a selectivity of 92%-96%, andconversion in the range of 77%-93%. The exothermic oxidation reaction of H₂with O₂ provides the heat required for the subsequent EB dehydrogenation.The removal of H₂ from the process shifts the reaction equilibriumin the dehydrogenation unit to substantially increase the single-pass EBconversions while maintaining high SM selectivity. Advantages of the Styro-Plusprocess include high conversion, high SM selectivity and reduced steamrequirements, thus making it an energy efficient process. Disadvantages of theprocess include the need for two catalysts in the reactor, and that the reactordesign and catalyst loading are more complicated for this system. Additionally,there is a danger of explosion owing to the side reaction of the completeoxidation reaction and the violent reaction of oxygen and the reactants.

To overcome such barriers of the oxidativedehydrogenation (ODH) of EB, a variety of oxidizing agents (such as air, CO₂,N₂O, SO₂ and so forth) have also been widely investigatedover novel catalysts ^{[7, 8, 9, 10]}.Recently, there have been reports on O₂-promoted dehydrogenationwith metal-free carbon nanotubes. Su et al. ^[11] reported a very highly stableand selective catalyst for the synthesis of C4 olefins from n-butane,and the same group reported the hierarchical carbon materials which can beoperated at lower temperatures, below 350 °C, under O₂. A few otherpromising catalytic systems have been developed with other soft oxidantconcepts. First, sulfur was reported as a soft oxidant for methane activationreported by Neurock and Marks with co-workers. The formation of metal sulfideslowered the activation energy which improved the selectivity of the products inmethane coupling. This sulfur system exhibited a decrease in surface cokeformation and high carbon balance ^[12]. Later, N₂O and N₂were also reported as soft oxidants in the ODH of EB to SM ^{[13, 14]}. Theseinnovative findings open new possibilities for building new systems anddeveloping catalysts suitable for these reactions, which may include a combinationof these components. After all the research on the ODH, a lot of questions havearisen regarding whether the present oxidative system, with CO₂ assoft oxidant or in combination with other oxidants, is commercially credible.

As an emerging alternative technology, ODH of EBwith CO₂ instead of steam has been under intensive investigation ^{[11, 12, 13, 14, 15]}. Although the dehydrogenation of EBprocess with CO₂ has currently only been performed on a laboratoryscale, it is of interest, as this process can have a direct impact on both theeconomics and the environmental aspect during commercialization.

Dehydrogenation of light alkanes or alkane groupsof aromatics is a very important process in petroleum, polymer and industrialchemistry ^[11]. CO₂ is used as a feedstock replacing the previouslyused partial oxygen-steam or oxygen-nitrogen streams. CO₂ possessesa much weaker oxidizability as compared with the oxygen molecule but can stillbe used as a mild oxidant to remove the H₂ by-product in thedehydrogenation unit and lower the reaction temperature without decreasing theconversion and selectivity.

There are two possible pathways for EBdehydrogenation with CO₂. One is ODH of EB through the oxygenspecies dissociated from CO₂ in a single step route and the other isdehydrogenation of EB coupled with a reverse water-gas shift reaction (RWGS) inthe two-step process ^{[16, 17, 18, 19, 20]}.The result of the reaction is that the CO₂ is not hydrogenated butconverted to CO and H₂O molecules.

The estimated energies required in this CO₂applied process is 190 kcal/kg of SM (EB/CO₂ = 1/9). This is muchlower than the commercial steam process which is 1500 kcal/kg of SM (EB/H₂O= 1/9). Consequently, the dehydrogenation in the presence of CO₂should be an energy-saving process. As an estimation, a simple replacement ofwater steam with CO₂ in an SM installation should decrease theenergy consumption from 6.4 to 0.8 GJ/t of SM ^[3]. Obviously, using CO₂as a mild oxidant, ODH of EB to SM possesses both environmental and economicbenefits ^{[11, 21]}.

CO₂ use offers several advantages incatalysis, such as acceleration of the reaction rate, enhancement in productselectivity, alleviation of thermodynamic equilibrium limitations, suppressionof total oxidation, prolonging of catalyst lifetime and prevention of hot spotson the catalyst surface ^{[3, 17, 22]}. Based on these unique characteristics, CO₂has been coined as a soft oxidant ^{[16, 23]}. Additionally, industrial use of CO₂is increasingly important in many aspects especially the greenness of the processwhere CO₂ is consumed during the process ^[14].

In view of these points, CO₂ applied tothe EB dehydrogenation process could be considered as an industrially viable process.

Commercial Fe-based catalysts are not effectivefor the dehydrogenation of EB in the presence of CO₂ owing to the decompositionof the active phase of the catalyst ^{[2, 24]}. Intensive studies have beenperformed to improve the commercial catalytic system of alkali metal-promotediron oxides, by supporting them onto Al₂O₃ and activatedcarbon materials ^{[18, 25, 26, 27, 28, 29]}.These catalysts showed compatible catalytic activities effectively activatingCO₂. Recently, various metal or metal oxide systems wereinvestigated further for exploitation of ODH of EB by means of CO₂as soft oxidant ^{[5, 10, 15, 30, 31, 32, 33, 34]}.In this direction, a variety of active metal components (Fe-, Cr-, V-,La-oxides and so forth.) have been investigated, supported on differentmaterials such as carbonaceous materials, MgO, SiO₂ (mesoporoussilica), Al₂O₃, Ga₂O₃, ZrO₂,TiO₂ hydrotalcite-type compounds, zeolites and so forth ^{[35, 36, 37, 38, 39, 40, 41, 42]}. The most recent studies have relatedto active carbon sites from carbonaceous deposits ^{[43, 44]} and synergeticeffects from redox active sites ^{[45, 46]}.

Park and co-workers have studied several highperformance catalytic systems for CO₂ applied effective ODH of EB,such as Fe-, V- and Cr-based catalysts ^[34]. These systems were based ondesigning a redox surface to activate CO₂, dissociate it on thesurface of catalysts and use the oxygen for the RWGS reaction. Among thesecatalysts, Al₂O₃-supported vanadium and vanadium-antimonyoxide catalysts have been intensively studied and the results showed reasonableactivities ^{[47, 48, 49, 50, 51, 52, 53, 54]}. The significanceof these catalyst systems for the title reaction has been consolidated.However, the deactivation owing to coke deposition and the deep reduction ofthe surface vanadium species still restrains the practical use of VO_x/Al₂O₃-basedcatalysts.

To design a better catalyst system in terms ofactivity, selectivity and stability for time-on-stream experiments, Park et al.^{[5, 55, 56, 57, 58, 59, 60]} developed several ZrO₂-basedcomposite oxides; in particular, MnO₂-ZrO₂, TiO₂-ZrO₂,CeO₂-ZrO₂,SnO₂-ZrO₂, effect of Cl on Fe₂O₃/TiO₂-ZrO₂,effect of redox promoters (V₂O₅, CeO₂, Sb₂O₃)and effect of alkali doping in TiO₂-ZrO₂. These catalystswith acid-base properties exhibited interesting and improved catalyticproperties for this reaction. Based on these catalytic achievements, thecontinuous work was supported by SABIC toward commercialization of thiseffective system. In this review, we will briefly summarize our recent basicstudies on ZrO₂-based catalysts for CO₂ applied ODH of EBand the further application to light paraffin for the production of olefins(Scheme 1).

Based on the systemic research ofalumina-supported vanadia (VO_x/Al₂O₃)catalysts with redox surface properties, we developed ZrO₂-basedmetal oxide catalysts for ODH of EB with CO₂, and proposed that theacid-base surface of zirconia metal oxides could activate EB and CO₂simultaneously, to observe the positive effects of CO₂ in directingthe EB conversion, product selectivity and prolonged catalytic activity.

In a primary study, Park et al. ^[61] found amphotericZrO₂ was active for the dehydrogenation of EB, especially in thepresence of CO₂. This positive effect of CO₂ was highlydependent on the tetragonal phase of crystalline ZrO₂. A higher EBconversion and SM selectivity were obtained with increased surface area and CO₂affinity, which was related to surface basicity.

Additionally, mixing the pure oxides is beneficialto either modulate the properties of the component oxides or to create newactive sites. Several theories have been proposed for the enhanced acid-baseproperties and their activities ^{[62, 63, 64, 65]}.

In the exploration of ZrO₂-based mixedmetal oxide catalysts, our studies have focused on the various ZrO₂-basedbinary metal oxide catalysts at the same time. These binary oxides are namelyTiO₂-ZrO₂, MnO₂-ZrO₂, CeO₂-ZrO₂and SnO₂-ZrO₂.

In our recent report, we described the influenceof CO₂ over ZrO₂, wherein an enhancement in theconversion of EB and the selectivity towards SM in the presence of CO₂oxidant were observed ^[61]. Furthermore, the catalytic activity of ZrO₂was significantly improved when it was mixed with other transition metal oxides^{[66, 67, 68]}. Our study on MnO₂-ZrO₂binary metal oxides was focused on enhancing the acid-base properties and alsothe introduction of the redox nature.

MnO₂-ZrO₂mixed oxides with various compositions (5-50 wt%)have been prepared by a co-precipitation method ^[55]. The precipitate waswashed thoroughly with water and dried at 120 °C for 12 h, then calcined at 550°C for 6 h. MnO₂-ZrO₂ mixed oxides are X-ray amorphous orpoorly crystalline, whereas both MnO₂ and ZrO₂ are purelycrystalline. The increase in surface area of the mixed oxide suggested the formationof solid solution. The surface areas of all the mixed oxides were more than 4times higher than those of the individual oxides. Additionally, the temperatureprogrammed desorption (TPD; NH₃ and CO₂) measurementsreveal that the 10% MnO₂-ZrO₂ mixed oxide possessed ahigher number of acidic as well as basic sites compared with the individualoxides.

As a result, MnO₂-ZrO₂binary metal oxides exhibited greatly elevated catalytic activity for theconversion of EB than those of the individual oxides in the presence of CO₂(Fig. 1). The MnO₂-ZrO₂ binary metal oxide catalystexhibited conversion of 73% with a selectivity of 98% at 650 °C. However, in the absence of CO₂,poor catalytic activity and stabilities were observed. A gradual enhancement ofactivity was demonstrated for the higher CO₂ to EB ratios. Theincrease or optimization of the CO₂/EB ratios showed an increase inconversion along with a preservation of the high selectivity. Hence, CO₂had an active role as a soft oxidant by improving both activity and stabilityin the dehydrogenation of EB over MnO₂-ZrO₂ mixed oxidecatalysts.

The superior activity of MnO₂-ZrO₂mixed oxide over its respective individual oxides might arise from thegeneration of active sites during the stages of mixed oxide catalyst preparationas described above. The combination of redox and acid-base bi-functionalmechanism might be responsible for the higher activity of MnO₂-ZrO₂mixed oxides. However, further studies are necessary for the elucidation of themechanism.

The combination of TiO₂-ZrO₂has attracted considerable attention as an active catalyst as well as support fora wide variety of reactions, as was summarized in a recent review by Reddy etal. ^[69]. The mixing of TiO₂ and ZrO₂ exhibits highsurface area, improved surface acid-base properties and enhanced hydrothermaland mechanical robustness. TiO₂-ZrO₂ mixed oxides areproven to be highly active and selective for dehydrogenation reactions.Particularly, TiO₂-ZrO₂ catalysts were used for theoxidative and non-oxidative dehydrogenation of EB and cyclohexane. We havestudied TiO₂-ZrO₂ binary metal oxides with variouscompositions, modified its catalytic properties with K and B, and finallycombined surface modification with textural property control in the sol-gelprocess by using the digestion method ^{[56, 57, 58, 70]}.

We have prepared TiO₂-ZrO₂binary metal oxide catalysts with different compositions by theco-precipitation method then calcined at 550 °C ^[56]. XRD patterns showed themixed metal oxides phase changed from monoclinic ZrO₂ to tetragonal,amorphous and TiO₂ anatase phase with incremental increase of TiO₂content from 10% to 30%. Between the ratios 40%-60%, the catalysts showedamorphous nature and further increase of the Ti content only showed anatase TiO₂phase. The amorphous phase was crystallized to clear TiZrO₄ phasewhen the calcination temperature was raised to higher than 650 °C. Through theaddition of TiO₂ to ZrO₂, the surface area increasedsharply and reached to about 5-9 times that of its individual oxides. The NH₃ and CO₂-TPDprofiles demonstrated that the strength and number of acid-base sites ofTiO₂-ZrO₂ binary metal oxides were much higher than thatof its individual oxides (TiO₂ and ZrO₂).

In the dehydrogenation of EB with CO₂,the 60% TiO₂-ZrO₂ amorphous catalyst exhibited superioractivity than that of other loadings (Fig. 2). The amorphous TiO₂-ZrO₂gave the compatible catalytic sites that are comparable to TiZrO₄phase catalysts obtained at high temperature calcination ^[56]. The catalyactivity was almost double those of the individual oxides and stable even atelevated temperatures in the presence of CO₂. The high selectivityarose from the suppressed cracking and hydrogenolysis reactions. Formation ofbenzene and toluene by-products was minimized.

The formation of amorphous TiZriO₄phase, enhancement in the specific surface area, and increase in the number andstrength of acid-base sites are some of the reasons for the observed high catalytic activity andselectivity ^[68]. These results indicate that the dehydrogenation of ethylbenzene to SM followed the acid-base bi-functional mechanism, wherein Zr ionacts as Lewis acid site and Ti ion as a base site ^[64]. The coke deposition onthe surface of the catalyst was lower in the presence of CO₂ thanthat of its absence and resulted in better catalytic activity and selectivity.

The dehydrogenation of EB to SM in the presence ofCO₂ has been shown to proceed effectively on acid-base bi-functionalcatalysts with a balanced strength and number of acidic and basic sites. Various amounts of B₂O₃ (2.5-15 wt%)were supported on to amorphous TiO₂-ZrO₂ bifunctionalcatalysts with the wet-impregnation method ^[57]. The impregnated sampleswere dried, then calcined at 550 °Cto activate the surface. The specific surface area of the TiO₂-ZrO₂decreased with the impregnation of B₂O₃. The XRD patternsrevealed that the addition of B₂O₃ influenced thecrystallization and stabilization of the anatase TiO₂. The additionof B₂O₃ increases the acidic strength and number ofacidic sites on TiO₂-ZrO₂ catalyst. The 5 wt% B₂O₃/TiO₂-ZrO₂catalyst did not exhibit good conversion or selectivity in comparison to TiO₂-ZrO₂catalysts. Benzene and toluene were the two major side products, which mainlyarise from the influence of the strong acid-base centers on the surface of thecatalysts ^[66]. Strong basic sites can abstract the β-H of the adsorbedEB which facilitates the cleavage of the C-C bond of the side chain resultingin the formation of toluene (Scheme 2), whereas the strong acid centersabstract the α-H of EB and facilitate the cleavage of the phenyl-C bondresulting in the production of benzene (Scheme 2).

B₂O₃ could not promote theTiO₂-ZrO₂ catalysts for the CO₂ applieddehydrogenation of EB. However, these results indicate an alternative directionfor promoting TiO₂-ZrO₂ with base moieties.

Based on the studies of TiO₂-ZrO₂binary metal oxide catalysts, we realized that these TiO₂-ZrO₂binary metal oxide catalysts could be further improved for the dehydrogenationof EB with CO₂ by neutralizing the strong acidic sites. AmorphousTiO₂-ZrO₂ binary metal oxide with a specific composition(60:40 molar ratio) was impregnated with 1, 2, 3, and 4 wt% K₂O^[58]. After the impregnation, all these samples were dried at 120 °C for 12 h,and calcined at 600 °C for 6 h in air.

An increased amount of K₂O loading willdiminish the BET surface areas of TiO₂-ZrO₂ binary metaloxide which might arise from the blockage of micropores. Through the additionof K₂O, excess TiO₂ in the amorphous TiO₂-ZrO₂binary metal oxide was crystallized into the anatase phase ^[71]. The motivationwas to use a basic alkali promoter for the activation of CO₂. Forthe over 3% K₂O/TiO₂-ZrO₂ catalyst, unifiedeffects of K₂O and CO₂ were observed. At a minimumloading (1 wt%) of K₂O, the strong acidic sites were poisoned, whileincreasing K₂O loading resulted in the neutralization of some weakto moderate acidic sites ^{[3, 4]}. In the presence of N₂, theconversion of EB over the TiO₂-ZrO₂ catalyst was 52.19%.In the presence of CO₂, the conversion of EB over a 3% K₂O/TiO₂-ZrO₂catalyst was increased to 71.95%. The increase in the conversion is almost 20%and selectivity had almost reached 100% (99.63%). Deactivation of the K₂O/TiO₂-ZrO₂catalyst was prolonged and the benzene formation was almost negligible. Suchhigh selectivity was usually observed at lower conversion levels of EB ^[72].The high SM selectivity of the K₂O-promoted catalyst arose from thesuppression of the dealkylation reaction by neutralizing the strong acidicsites and converting hydrogen into water by the coupling of the RWGS reaction(Fig. 3) ^[58].

As mentioned earlier, an acid-base bi-functionalmechanism was proposed by several authors for the dehydrogenation of EB ^{[5, 10]}.It is clear that there should be a balance of number and strength betweenacidic and basic sites either for simple dehydrogenation or ODH of EB with CO₂.

AmorphousTiO₂-ZrO₂ binary metal oxides will provide more acid-baseactive sites by increasing their surface area. This will bring compatiblebenefits to the dehydrogenation of EB proposed by an acid-base bi-functionalmechanism. There is a well-known method to improvesurface area of metal oxides by amorphization with alkali metal doping (Scheme3) ^[70].

Using KOH solution as a hydrolyzing agent as wellas a digestion medium, a high surface area TiO₂-ZrO₂(50%) was prepared. Highly crystalline phases of ZrO₂ are observedfor the undigested TiO₂-ZrO₂ catalyst (calcined at 600°C) ^[70]. After digestion with KOH solution, TiO₂-ZrO₂was transformed to amorphous TiZrO₄ phase, which was confirmed fromthe Raman spectra ^[70]. After digestion, the particle size decreased from 25 to5 nm. Surprisingly, the specific surface area was increased to 166 m²/gwhich was higher than previously reported for the undoped catalyst. The phasetransformation from the crystalline to amorphous phase is the crystallization-amorphization-reorganizationprocess which is the Ostwald ripening process induced by K (or Na) in thedigestion media ^[73]. The alkali metal was doped into TiO₂-ZrO₂metal oxides during digestion which showed a broadness in the O1sspecies of the catalysts as evidenced in the references which consequentlyshowed the increase in activity and selectivity along with improved CO₂conversion ^[70]. The digestion process helped in adjusting the acid-baseproperties of the catalysts.

The catalysts were applied to the dehydrogenationof methylethlbenzene. The highest conversion, selectivity and CO₂consumption were observed simultaneously over properly digested TiO₂-ZrO₂catalyst.

TiO₂-ZrO₂catalysts were also prepared by NaOH digestion^[74]. Compared with KOH digestion, a higher surface area of 188 m²/gwas achieved. The particle size of the TiO₂-ZrO₂ catalystwas even decreased to 3-5 nm, which is slightly smaller than that observed withKOH digestion. Acid-base sites were increased in both number and strength. Inthe dehydrogenation of EB with CO₂, during 15 h time-on-stream, NaOHdigested catalyst showed high conversion (>60%), good selectivity (>95%)and apparent CO₂ consumption (>10%) (Fig. 4).

Digested TiO₂-ZrO₂ is highlydesirable toward dehydrogenation reactions, especially for CO₂applied ODH of EB.

CeO₂ is one of the most populartransition metal oxides in the application of heterogeneous catalysis as theactive component, additives or the support ^{[75, 76]}. CeO₂ and ZrO₂mixed oxides could generate definite benefits to the catalytic properties owingto the redox properties of CeO₂, acid-base properties of ZrO₂and the formation of new crystal phase ^{[65, 77]}. The CeO₂ phase isable to store more active oxygen species in the catalyst system and is expectedto suppress the coke formation more effectively.

A catalyst comprised of 25 mol% CeO₂and 75 mol% ZrO₂ (25CZ),as a binary metal oxide, was prepared by the co-precipitation method ^[59].According to the XRD patterns of 25CZ, the solid solution formed withtetragonal symmetry, and no tetragonal or monoclinic forms of ZrO₂were observed ^[64]. This arose from a facile solid phase reaction between CeO₂and ZrO₂. The surface area of 25CZ binary metal oxide (37 m²/g) isapproximately the same as the sum of the constituent individual oxides (25 and19 m²/g for ZrO₂ and CeO₂, respectively) ^[59].

25CZ catalyst exhibited better activity than CeO₂and ZrO₂. The enhancement of catalytic activity could be explainedwhere a kind of stress between the bonded atoms (Ce-O-Zr) was developed byvariation of the ionic sizes (effective ionic radii of Ce⁴⁺ = 0.97Å, Ce³⁺ = 1.14 Å, Zr⁴⁺ = 0.84 Å). The mixed metal oxidetends to form fine particles or amorphous mixtures, and will improve theacid-base property. The catalytic result was not comparable to MnO₂-ZrO₂and TiO₂-ZrO₂ systems. Further investigations on CeO₂-ZrO₂binary metal oxides are necessary.

A series of SnO₂-ZrO₂nanocomposite catalysts were prepared over the full range from 0 to 100% of SnO₂by careful hydrolysis of both metal salts with NH₃ solution ^[60].The gel was precipitated and calcined at 600 °C. The sizes of the binary metaloxide particles were in the range of 10-15 nm with uniform sphericalmorphology. Pure monoclinic ZrO₂ phase and cassiterite SnO₂phase co-existed in the nanocomposite catalysts containing 10%-40% of SnO₂.However, at 50%-90% SnO₂ content, the nanocomposite catalystsexhibited only cassiterite SnO₂ phase. No significant XRDreflections were related to ZrO₂. There was a strong interactionbetween the two metal oxides. Owing to the strong interaction, the binary metaloxides showed greater surface acidity and moderate basicity compared with thoseof the single component oxides (Fig. 5). The highest catalytic activity wasobtained over 15% SnO₂-ZrO₂ (15% SnO₂, 85% ZrO₂).The beneficial role of CO₂ as a diluent and as a soft oxidant wasapparent over the SnO₂-ZrO₂binary metal oxide catalyst.

The properties of metal oxide nanoparticles weregreatly influenced by the number of atoms at the surface. However, at hightemperatures, the metal oxide nanoparticles were easily sintered and dramaticallylost their activities. One solution is to support the metal oxides on high surfacearea and thermally stable mesoporous materials, such as SBA-15 or mesoporoussilicalite-1 ^{[59, 78, 79]}.

Supported metal or metal oxides have been used andstudied in various reactions. Mesoporous silica, used as support, was chosen todisperse the mixed metal oxide system for overcoming the diffusion limitationto improve the activity. In the preparation of the CeO₂-ZrO₂/SBA-15catalyst, Ce(NO₃)₃ and ZrO(NO₃)₂were hydrolyzed with NH₃ solution in the presence of calcinedSBA-15. Mixed metal hydroxide gels will form inside the mesopore or on theouter surface of SBA-15. The 25/25CZS catalyst (25 wt% of 25CZ on the SBA-15)was calcined at 650 °C for 12 h ^[59]. Undoubtedly, the supported 25/25CZScatalyst is X-ray amorphous revealing that CeO₂-ZrO₂binary metal oxides were highly dispersed on the SBA-15. In the dehydrogenationof EB with CO₂, the 25/25 CZS catalyst showed superior activity thanthose of the individual oxides and binary metal oxides in terms of turn overfrequencies (TOF). Owing to the high dispersion of the binary metal oxides onthe SBA-15, the active sites were easily accessible to the reactants. The TOFof 25/25CZS catalyst was elevated 10-20 times when compared with its bulkoxides. The catalytic activity of this supported catalyst was stable during 10h of time-on-stream, while the activity of the unsupported metal oxides wasdecreased within 8 h of time-on-stream ^[59]. The improved lifetime of thesupported 25/25CZS catalyst was mainly owing to the suppressed sintering ofmetal oxide nanophases at high temperatures and facile transportation ofreactant or product in the mesoporous SBA-15. Further optimization of synthesisprocedures and more detailed characterization are required to improve thisnovel catalyst system.

Among our studies on ZrO₂-based binarymetal oxide bulk catalysts, the results over TiO₂-ZrO₂are promising. They were supported onto the thermallystable mesoporous silicalite-1 ^[79]. The XRD pattern of TZ/CS-1 did not showany crystalline phase of TiO₂ or ZrO₂, only a typical reflectancefor the MFI-type zeolite from mesoporous silicalite-1 support. TiO₂-ZrO₂binary metal oxides were highly dispersed within the silicalite-1 crystals andshowed a high proportion of strong acid-base sites which might arise from theformation of solid solutions (ZrTiO₄). In the dehydrogenation of EBto SM with CO₂, this catalyst showed poor catalytic activity andselectivity to SM. However, the amount of coke deposition was less owing to thewell-developed porosity of the mesoporous silicalite-1 support.

The TiO₂-ZrO₂ compositeoxides are of great interest as active catalysts as well as supports in thedesign of heterogeneous catalysts for a wide range of reactions ^[69]. Thesignificant features of TiO₂-ZrO₂-basedmixed oxides include a high specific surface area, improved acid-base propertieswhen compared with those of the individual oxides, and improved redoxproperties, together with a high thermal stability and strong mechanicalstrength. Our fundamental studies revealed that TiO₂-ZrO₂binary metal oxides were dependable catalysts with excellent catalytic activityand selectivity for dehydrogenation of EB with CO₂ as a soft oxidant^[5]. However, there is still some concern regarding the fast deactivation ofTiO₂-ZrO₂ binary metal oxide catalysts by carbonaceousdeposits ^[80].

O

ne of the generally accepted hypotheses for ODHis that the reaction proceeds over redox active sites following a Mars-VanKrevelen mechanism and the rate-determining step is the breaking of the C-Hbond (Schemes 4 and 5) ^{[81, 82]}. Further, it is generally agreed that hydrocarbonoxidation over metal oxide catalysts involves the participation of lattice oxygen species or oxygen vacancies.

Therefore, in the subsequent investigations, redoxmetal oxide-promoted MO_x/TiO₂-ZrO₂catalysts, where MO_x is Fe₂O₃, V₂O₅, CeO₂,V₂O₅-CeO₂ and V₂O₅-Sb₂O₅-CeO₂,were synthesized and evaluated for the ODH of EB using CO₂ as thesoft oxidant ^{[80, 83, 84, 85]}.

The Fe₂O₃/TiO₂-ZrO₂catalyst was explored and the influence of chloride ions in TiO₂-ZrO₂binary metal oxide was discussed (Fig. 6). Calcined TiO₂-ZrO₂,which was synthesized by a co-precipitation method, was wet impregnated with 15wt% Fe₂O₃. After calcination, the chlorine-containing TiO₂-ZrO₂showed less surface area and revealed a slightly crystalline phase of ZrTiO₄.The iron oxides were highly dispersed over the chlorine-containing TiO₂-ZrO₂support as evidenced by the XRD pattern. Fe₂O₃impregnation did not change the pore structure, crystal phase or particlemorphology of chlorine-containing TiO₂-ZrO₂. In thedehydrogenation of EB with CO₂, the order of activity on variouscatalysts was found to be Fe₂O₃/TiO₂-ZrO₂(Cl) > TiO₂-ZrO₂ (Cl) > TiO₂-ZrO₂.Cl ions exhibit a strong influence on the physicochemical and catalytic propertiesof the TiO₂-ZrO₂ and revealed a good catalytic efficiencyof the Fe₂O₃/TiO₂-ZrO₂ combination.From the results of high and stable activity without rapid deactivation, theiron chloride species are expected to play a role as redox active sites andthis can be coupled with the acid-base Fe₂O₃/TiO₂-ZrO₂^[83].

The ceria component in the mixed metal oxides isable to promote the oxidizability of catalysts and can suppress catalystdeactivation by preventing coke formation in the ODH reactions ^[80]. In thehigh temperature gas phase dehydrogenation reaction, bulk cerium and vanadiumoxides undergo sintering easily and deactivate rapidly. The combination ofvanadia (known for its redox properties) and ceria (known for its oxygenstorage and release functions) is expected to provide better catalytic systems.

On the acid-base TiO₂-ZrO₂binary metal oxides, vanadium oxide and cerium oxide were supportedindividually or together for the ODH of EB to SM using CO₂ as a softoxidant. Here, by control of the sol-gel process, a high specific surface area(207 m²/g) TiO₂-ZrO₂ was synthesized by aco-precipitation method. The TiO₂-ZrO₂ was calcined at550 °C. Based on the surfacearea of TiO₂-ZrO₂, a monolayer equivalent of V₂O₅(15 wt%), CeO₂ (15 wt%) and CeO₂-V₂O₅(7.5 wt% + 7.5 wt%) were deposited by adopting a standard wet impregnation method,then calcined again at 550 °C. These mixed metal oxides showed an X-rayamorphous phase, but contained an amorphous TiZrO₄ phase and smalleramounts of ZrV₂O₇ and CeVO₄ phase. In the CeO₂-V₂O₅/TiO₂-ZrO₂catalyst, the co-addition of ceria with vanadium oxide onto the TiO₂-ZrO₂support gave rise to a strong solid-state reaction, which influenced theparticle size of the TiO₂-ZrO₂, and promoted the vanadiumoxide dispersion and the reducibility.

The CeO₂-V₂O₅/TiO₂-ZrO₂catalyst showed superior activity with higher selectivity than those of theindividual metal oxide-supported catalysts (Fig. 7). In this combinationcatalyst system, 56% conversion and 98% product selectivity were achievedduring 10 h time-on-stream. Addition of CeO₂ to V₂O₅/TiO₂-ZrO₂prevented catalyst deactivation and helped to maintain a high and stablecatalytic activity.

The redox behavior of V₂O₅as a catalyst is known in ODH. Sb₂O₅ is able to stabilizehighly dispersed V₂O₅ species on the catalyst support. Antimonyoxides and highly dispersed vanadium oxides could react in the solid phase andprovide a new mixed oxide phase, which is likely to induce a facile redoxtransformation between vanadium redox pairs, and result in improved catalystactivity and stability ^[16]. Similar results were also observed in the catalystof V stabilized with CeO₂-ZrO₂ solid solution (Fig. 8)^[43].

Based on the positive role of Sb and Ce to thesupported vanadium oxides, we designed a V₂O₅-Sb₂O₅-CeO₂/TiO₂-ZrO₂catalyst, under the concept of combining the acid-base bi-functional naturewith the redox properties ^[85]. In the mixed metal oxides, the highly dispersedvanadium species induced the formation of a well-crystalline ZrTiO₄compound and some ZrV₂O₇ phase ^[84]. The existence ofhighly active VSbO₄ species was also observed.

The addition of antimony oxide to the former V₂O₅-CeO₂/TiO₂-ZrO₂ catalyst will give a selectivity of SM higherthan 96% at the conversion levels of EB at about 50%-65%. This result is betterthan the data previously reported by our group.

Acid-base and redox properties, includingsustained stability of active species (V⁵⁺), are responsible for thehigh activity of the V₂O₅-Sb₂O₅-CeO₂/TiO₂-ZrO₂catalyst. The redox cycle is related to the dispersed V⁵⁺ andlattice reduced vanadium site in the VSbO₄ phase ^[85]. The antimonyoxide inhibits the easy redox cycle between different vanadia species.

The concept of acid-base bi-functionality andredox nature has been extended to other alkyl aromatics and lower alkanes,especially, TiO₂-ZrO₂-based catalysts have been appliedto other reactions, such as CeO₂ promoted TiO₂-ZrO₂nano-oxide catalysts for the ODH of p-diethylbenzene and a novel VO_x/SnO₂-ZrO₂ catalyst for C4 olefins from n-butane.Both catalytic systems developed provide further proof that acid-basebi-functional/redox catalysts can undoubtedly be used for C-H activation ^{[15, 86, 87]}.

CeO₂ promoted TiO₂-ZrO₂(CTZ-MW) mixed oxides were synthesized by the microwave hydrothermal method andcompared with the conventional hydrothermal method (CTZ-HT) in thedehydrogenation of p-diethylbenzene with CO₂ ^[86].Interestingly, microwave prepared CTZ-MW showed an improved specific surfacearea compared to CTZ-HT, which is not common in the case of the TiO₂-ZrO₂binary metal oxide system. CeO₂ is highly dispersed onto TiO₂-ZrO₂.Over the CTZ-MW catalyst, the conversion and selectivity of p-diethylbenzenewere 74% and 98%, respectively. The time-on-stream was stable up to 8 h. Theunique activity of CTZ-MW was achieved through CeO₂ promotion and improvedtextural properties with MW synthesis (Fig. 9).

Vanadium oxide was supported onto the ZrO₂,SnO₂ and SnO₂-ZrO₂ mixed oxide for the ODH of n-butaneemploying CO₂ as the soft oxidant ^[87]. The composition of catalystwas investigated and the 7.5 wt% V₂O₅/SnO₂-ZrO₂(SnO₂ is 15% in the SnO₂-ZrO₂) catalystexhibited the best catalytic activity. This optimized catalyst has a highspecific surface area and a higher number of medium strength acid-base sitesthan the others. Additionally, supported-vanadia species were reduced at arelatively lower temperature. The CO₂ as a diluent or oxidant alsoexhibited a significant promotion effect on the selective conversion of n-butaneto C4 olefins over 7.5 wt% V₂O₅/SnO₂-ZrO₂catalyst with 22.34% conversion of n-butane and 36.63% selectivity to C4olefins.

ODH of EB to SM is of great potential andeconomical interest as a means of producing high yields of SM. EB dehydrogenationwith CO₂ as a soft oxidant was studied intensively over ZrO₂-basedbinary metal oxide catalysts. The catalytic activity of the mixed oxides isgreater in the presence of CO₂ compared with its absence. Thesemixed metal oxides are essentially nanoparticles with improved texturalproperties and the newly developed acid-base bi-functionality.

The development of a catalyst system that showspromising catalytic activity for EB with selectivity towards SM and CO₂conversion has been studied. CO₂ played a vital role in the activityand improving stability of the catalyst. The role of CO₂ can bedefined as a soft oxidant, which is oxidizing the surface of the catalyst andmaintaining the surface oxygen content, removing the coke on the surface of thecatalyst by reverse Boudouard reaction, removing the hydrogen product by RWGSreaction and providing high heat capacity to the reaction system, which helpsin overcoming the equilibrium limitation and achieve high conversions ^{[45, 54, 73, 88, 89, 90, 91, 92]}.

All the catalysts studied above are illustrationsof ZrO₂-based catalytic systems with improved and modified acid-baseproperties along with redox properties. These modifications lead to animprovement of the development of catalysts with high thermal stability,improved activities and CO₂ activation. The best result was observedfrom the CeO₂−V₂O₅/ TiO₂−ZrO₂catalyst, which combines the stabilized redox property with acid-basebi-function. This multi metal oxide system is improved further by the additionof Sb as a redox stabilizer. Doping of alkali and alkaline earth metals hasenhanced the conversions and selectivities along with CO₂ conversionbased by optimizing the acid-base properties and increased surface areas. Insummary, based on the present status of the catalyst and the process, there isa lot of scope for further studies to meet requirements such as high per passconversion above 75% and selectivity greater than 98% with CO₂conversion around 30%. From the above discussion, the strategies that should befocused on are:

(1) Stabilization and improvement of the presentcatalyst system for a greater than 24 h single run;

(2) Reducing coke formation;

(3) Regeneration of catalyst;

(4) Improving CO₂ activation andconversion.

These developed catalysts provide a foundation forscaling up the reaction systems to the commercial scale. Using CO₂as feedstock has been a farfetched dream which can be run by using the out gasfrom petrochemical oxidation or the reforming industry. The high selectivity ofSM in this selective dehydrogenation process suggests a $2.7 million saving for0.6 Mt of SM monomer per year could be made, which arises from the input andthe output energies in the case of the industrial steam process, and is muchhigher than that of the CO₂- mediated process.

The catalyst systems for the ODH have been testedonly to the lab scale and are mainly based on acid-base catalysts and redoxcatalysts. We conclude that the oxidative dehydrogenation of EB with CO₂is an economic and environmentally-benign process which may serve theincreasing SM demand in the near future. Although using the CO₂process has the advantage of being an energy saving process, there are stillchallenges that need to be overcome, such as coke formation, longevity of thecatalyst and high per pass conversion. These challenges provide a lot of scopefor research in this area for the development of catalysts. Another importantaspect of the ODH with CO₂ is the production of CO byproducts, whichis a very valuable chemical able to be used in many processes. A clear understandingof the mechanisms of the reaction, coke formation and CO₂ activationwill enable us to obtain a potential catalyst that can be industrially viable.We can envisage that using CO₂ in this process may reduce theoverall CO₂ production in the production of SM.

Acknowledgments

This work is funded by Saudi Arabia BasicIndustries Corporation (Kingdom of Saudi Arabia), the BK21 Plus Project in 2014and Nano-Technology Fusion Center (Inha University, Chemistry & ChemicalEngineering, Lab of Nano-Green Chemistry).