Toward Exploiting the Behavior of Niobium-Containing Mesoporous Silicates vs. Polyoxometalates in Catalysis

Classification of polyoxometalates (POMs) is based on their chemical composition, basically represented by two general formulae: a) [MmOy]p− b) [XxMmOy]q−, where M is the main transition metal, O is the oxygen atom and X can be a non-metal atom such as Si. Additionally, in the most cases, the structure of the polyoxometalates is derived from a combination of octahedral units MO6 with a central metal atom M and the oxygen atoms placed at their corners. In such octahedra, oxygen atoms allow the condensation between two octahedral units, while one oxygen atom (or max. two atoms) makes double bond with the central metal atom and is not shared with other metal atoms within the complex (terminal oxygens). On the other hand, niobium-containing mesoporous silicates contain mainly MO4 tetrahedra and reveal superior activity in heterogeneous catalysis. Thus, the proper coordination of niobium is crucial for the catalytic activity and will be deeply discussed. The similarity in the catalytic behavior of niobium-polyoxometalates and heterogeneous niobium single-site catalysts in selective oxidations will be demonstrated.


INTRODUCTION
The family of polyoxometalates (POMs) consist of different anionic polynuclear metal-oxygen clusters which comprise the edge-sharing and corner-sharing pseudo-octahedral MO 6 units that form an ionic core and cover mainly early transition metals (Gumerova and Rompel, 2018). Up till now many compounds falling into this category have been synthesized in a great number of shapes and sizes, with the Lindqvist (1953) and Keggin (1933) geometries being the foremost studied. POMs can be also sub-categorized as isopolyanions, containing no heterometals, or additional metals (with Lindqvist ion as an example) and heteropolyanions enclosing heterometals (like in the Keggin ion) .
The Lindqvist ion (hexametalate, of the formula [M 6 O 19 ] p− ) can be characterized as a superoctahedron created from 6 edge-sharing octahedra with every octahedral metal connected to the oxygen atoms (Figure 1). On the other hand, the Keggin ion with a formula [XM 12 O 40 ] q− contains a central tetrahedral oxoanion (e.g., PO 4 , SiO 4 , AlO 4 ) in which each oxygen is the terminal atom for three edge-sharing MO 6 octahedra (Nyman, 2011). In the case of α-Keggin polymorph the four MO 6 trimers share corners, whereas in other isomers (labeled with the prefixes β-,γ-, δ-and ε-) these units are linked together in different ways (Baker and Figgis, 1970). and terminal (e.g., Nb-O t ) oxygen atoms, respectively (adopted from Bontchev and Nyman, 2006).
In literature a great exertion has been devoted to the synthesis and characterization of various POM structures (Pope and Kortz, 2012;Hutin et al., 2013), nevertheless these materials are also attractive regarding their applications, namely in catalysis (Lv et al., 2012;, bio-and nanotechnology (Yamase and Pope, 2002), medicine (Sarafianos et al., 1996;Rhule et al., 1998;Zhang et al., 2014), material sciences (Proust et al., 2008;Tong and Ye, 2010), macromolecular crystallography (Bijelic and Rompel, 2015) or electrochemistry (Sadakane and Steckhan, 1998). Many of these applications are based on the redox potential of POMs, since they possess high capacity to release and bear electrons (Botar et al., 2009;Su et al., 2017). Polyoxometalates may be considered as weak Lewis bases due to the presence of surface oxoligands that may attach Lewis acids. Nevertheless, one has to bear in mind that the basicity of these surface oxygens is very low. Still, the addenda metals present in polyanion skeletons may possess unoccupied orbitals and thus can generate Lewis acidic sites at the outside of POMs .
On the other hand, from among HPONbs, Keggin-type niobooxalates are the most widely studied and used as efficient precursors for creating functional materials. The first Keggintype HPONb [SiNb 12 O 40 ] 16− was synthesized in the presence of titanium cations, where a Ti 2 O 2 dimer was applied as a linker to connect the Keggin type units into a one dimensional chainlike framework (Nyman et al., 2002). Up till now a series of HPONbs have been synthesized in the presence of P, V, Ge, Si, and As, resulting in compounds like [V 3 Nb 12 O 42 ] 9− (Son et al., 2013a) [PV 2 Nb 12 O 42 ] 9− (Son et al., 2013b) (Abramov et al., 2017). Lately also series of niobium-tungsten-lanthanide heterometallic polyoxometalates have been prepared with Y, La, Sm, Eu, and Yb cations (Jin et al., 2016). However, it is still challenging to find papers broadening PONbs applications, beyond the Lindqvist ion, due to the limitations of chemistry in aqueous synthesis of PONbs (requirement of alkaline solution, lack of appropriate precursors) (Nyman, 2011).
Additionally, a new possibility will be exploited soon, i.e., the encapsulation of polyoxoniobates in inorganic substrates and the application in catalysis of the resulting materials can be expected. There are already examples of other than niobiumbased polyoxometalates (Lefebvre, 2013). Only recently a functionalized polyoxoniobate/g-C 3 N 4 nanoporous material has been synthesized with carbon nitride (g-C 3 N 4 ) and hexaniobate (K 8 Nb 6 O 19 ·10H 2 O) as starting materials (Gan et al., 2018).
Contrary, niobium-containing mesoporous silicates receive great attention due to the well-studied and convenient synthesis procedures in aqueous solutions. Moreover, they possess very attractive structural, textural, and morphological surface properties, like highly ordered structures, high surface areas, narrow pore size distributions as well as tunable pore sizes and structures, affecting their activity in catalysis, sorption, and separations (Nowak, 2012).
The best known mesoporous material is MCM-41. The synthesis of this material was described by scientists at the Mobil Oil Company in 1992. This procedure was realized by the application of surfactant micelles as structure determining agents in a sol-gel method. The surfactants (amphiphilic) organize themselves into cylindrical micelles that are encapsulated by silicate compounds. Finally, calcination process is applied in order to eliminate the organic surfactant without changes in a hexagonal organization of mesopores Kresge et al., 1992). Sol-gel technology concerns the creation of a solid phase through the gelation of a colloidal suspensionsol (Lev et al., 1997). Two possible routes for this process are known: inorganic and organic ones. Inorganic route is the hydrothermal technique that includes the creation of a sol from the inorganic, silicon-containing starting material. In the organic route the starting material possess an organic component, e.g., tetraethyl orthosilicate (TEOS) which is hydrolyzed to form a gel. The liquid crystal templating process employed by the Mobil Oil Company in 1992, engaged the dissolution of a various surfactant species in the pre-hydrolyzed inorganic precursor. This mechanism is highly influenced by electrostatic and steric interactions between the solvent, inorganic precursor and the self-assembled organic surfactants. In the synthesis of mesoporous materials, a wide variety of surfactants with various properties could be used, e.g., anionic, lipid, zwitterionic even two-tailed species. Surfactants are used to create the mesopores. The metal precursor could be added during the synthesis of materials or by wet impregnation (Huo et al., 1994). As a source of niobium, ammonium niobium oxalate NH 4 [NbO(C 2 O 4 ) 2 (H 2 O) 2 ]·3H 2 O, potassium niobate K 8 Nb 6 O 19 or niobium chloride NbCl 5 were commonly used (Yan et al., 2018).
There are several aspects that influence the synthesis of Nb-containing mesoporous molecular sieves, namely silicon and niobium source, their molar ratio, surfactant type, synthesis environment (pH, the presence of counterions), etc. General methodologies for the synthesis of niobium catalysts on mesoporous materials are based on the use of the hydrothermal method, e.g., Ziolek's and Nowak's groups described incorporation of niobium during the synthesis of mesoporous materials using hydrothermal method (Ziolek and Nowak, 1997;Kilos et al., 2004;Nowak and Jaroniec, 2005;Nowak, 2012). An overall procedure for the synthesis of various mesoporous materials is shown in Figure 2.
For the purposes of this review, we will focus only on the papers on the catalytic activity of polyoxometalates and niobiumcontaining mesoporous silicates published in the last decade.

CATALYSIS ON POLYOXONIOBATES
To date the great number of studies on POMs have been dedicated to the synthesis and characterization procedures. So far, in latest scientific literature a progress in investigating the applications of POMs in catalysis may be noticed. It is the most probably stimulated by the attractive features of these materials, comprising adjustable acidity and redox properties, fundamental resistance to the oxidative decomposition, significant thermal stability, and remarkable susceptibility to light and electricity.
These outstanding features have a strong connection to the structures and compositions of POMs (Wang and Yang, 2015).
Several POM systems, in both oxidized and reduced forms, can be used as robust catalysts due to their significant thermal-and photostability. Moreover, they can also be simply transformed to reactive forms by applying light and electricity (Gumerova and Rompel, 2018). The applications of polyoxometalate clusters as catalysts are many and varied, nonetheless they include mostly green H 2 O 2 -based epoxidation systems (Zhou et al., 2015), catalytic approaches to sustainable splitting of water , and also photocatalysis in various photoredox systems e.g., the oxidation of alkanes, alkenes or alcohols as well as in the light-induced mineralization of several organic and inorganic contaminants (Shen et al., 2009;Streb, 2012;Wang et al., 2017;Gan et al., 2018;Li et al., 2018).
When catalytic reactions are taken under consideration, PONbs are foremost less studied than their counterparts containing Mo, W, or V. Among others, it can be observed for epoxidation reactions, particularly those based on H 2 O 2 catalyzed by POMs, which have been thoroughly disscussed by groups of Neumann or Hill in many papers from the 1980's (e.g., Duncan et al., 1995;Vasylyev and Neumann, 2004 Greater catalytic activity of polyoxoniobates may be achieved, for example, by its functionalization with transition metals, but one has to bear in mind that PONbs are quite challenging materials because of insufficient recognition of their solution behavior, since they can be present only in strong alkaline aqueous solutions, whereas most of the transition metals (TM) cations precipitate as a result of the creation of respective oxides or hydroxides. Though, cations as Cu, V, Ti, W, Zn, Cr, Ni, Co, and Pt, have been successfully introduced into the PONb systems (Ohlin et al., 2008a;Chen et al., 2010;Niu et al., 2010Niu et al., , 2011Guo et al., 2011Guo et al., , 2012Huang et al., 2012b;Son et al., 2013b;Abramov et al., 2015a,b;Liu et al., 2017). Some examples of applications of TM-modified polyoxoniobates as catalysts will be also covered in this review.

Catalytic Splitting of Water
Last decades clearly showed that a transfer of energy sources to renewable and sustainable systems is essential, since the global warming induced by the so called greenhouse gases has become a vital issue. Photochemical systems, permitting the conversion of solar energy to useful chemical reactivity, appear to be quite promising in this respect. Particular effort has been made to the designing photocatalytic processes which can easily proceed in the presence of polyoxometalates with improved photoactivity. The photoactivity of POMs has been already acknowledged in literature, but it is focused mainly on Mo-based salts containing organoammonium counter ions that can easily undergo photoredox-reactions in the solid state (Streb, 2012).
Generally speaking, polyoxometalates are a class of highly redox-active compounds with semiconductor-like photochemical properties that may be involved in the lightinduced photoredox reactions with direct environmental impact. Their reactivity can result in the substrate reduction or oxidation, depending on the type of the used cluster (Figure 3).
Polyoxometalates have been involved in the conversion of a wide range of molecules that may proceed mostly with two types of photoreductive reactions, i.e., the homogeneous photoreductive activation of CO 2 or production of hydrogen, the latter one being the reductive aspect of the water splitting process, as well as with visible-light induced substrate oxidation (Streb, 2012).

Water Oxidation on PONbs
Water oxidation is a crucial step in the water splitting reaction. It can be used to produce H 2 toward application as a sustainable energy carrier. In order to reach the shortest way in conversion of solar light into its storable energy counterparts it is fundamental to create catalytic systems straightforwardly powered by sunlight and based on stable and economically feasible catalysts.
A few years ago, Feng et al. reported the synthesis procedure for obtaining a photoactive catalyst based on polyoxoniobate cluster [{Nb 2 (O) 2 (H 2 O) 2 }{SiNb 12 O 40 }] 10− . In this structure, which consists of infinite 1D chains created from Nb-based Keggin clusters interconnected by dinuclear {Nb 2 (O) 2 (H 2 O) 2 } moieties, it is possible to coordinate water molecule to each of the two bridging Nb sites. The Nb 5+ site in the linking {Nb 2 (O) 2 (H 2 O) 2 } unit is seven-coordinated with four O 2− anions from the Keggin unit, two bridging O 2− sites, which are connected to another Nb 5+ , and one H 2 O molecule. This specific seventh coordination of niobium atoms strongly affects the local coordination geometry of Nb 5+ as well as the chain arrangement and spatial packing. All these features establish K 10 [Nb 2 O 2 (H 2 O) 2 ][SiNb 12 O 40 ]·12H 2 O as a potential catalyst for efficient water oxidation (Zhang et al., 2011).
PONbs may also serve as compounds supporting the synthesis of water oxidation catalysts. Zhang and co-workers have developed an efficient method for electrodeposition of cobalt and nickel nanostructures in the presence of the Lindqvist ion [Nb 6 O 19 ] 8− . ICP-MS analysis revealed that the elemental ratio of Co:Nb and Ni:Nb was 1:1 and 1:3, respectively.
Raman spectra confirmed the presence of both [Nb 6 O 19 ] 8− and Co(OH) 2 /Ni(OH) 2 species. Further study indicated that Lindqvist ion provided electrostatic stabilization to Co(OH) 2 or Ni(OH) 2 and hence the films show exceptional stability and efficiency for electrocatalytic oxidation of water in the alkaline solution . in order to evaluate their activity in the water oxidation. They found out that the studied compounds were active electrochemical catalysts under basic condition with a high catalytic current. Computational methods were applied as well to estimate photoluminescence of these compounds. It was found that the calculated values are in a good agreement with the experimental data. The obtained results proved also the impact of clusters size and addenda atoms on the catalytic and fluorescent properties of PONbs (Ye et al., 2013).
Casey's group have examined how isopolyoxoniobates, namely [H x Nb 6 O 19 ] (8−x)− and [H x Nb 10 O 28 ] (6−x)− ions, react with hydrogen peroxide. By using ESI-MS and 17 O NMR spectroscopy the scientists could observe the differences in the behavior of hexaniobate and decaniobate anions in the presence of H 2 O 2 molecules. The reaction with the hexaniobate Lindqvist ion proceeds quite slowly, while with the decaniobate anion it completes almost immediately. In addition, contrary to the hexaniobate, the decaniobate anions dissociate in the presence of peroxide, yielding in peroxohexaniobate species. These results are of particular importance when it comes to the use of PONbs as water-splitting catalysts, as the oxygen atoms bound to the terminal Nb atom can be easily substituted with peroxy groups, which are a product of water oxidation (Ohlin et al., 2008b). It is also worth mentioning that similar studies were also performed for molybdenium and tungstate-based polyoxometaltes (e.g., Duncan et al., 1995;Vasylyev and Neumann, 2004).
Casey et al. have synthesized a new pentaphosphate niobate polyoxometalate cluster in the form of highly soluble and stable over a wide pH range tetramethylammonium (TMA) salt: (TMA) 9 H 3 Nb 9 P 5 O 41 ·28H 2 O. The researchers proved that this compound could be reversibly converted into the peroxo form when H 2 O 2 is added. This novel cluster may be regarded as a FIGURE 2 | General procedure for the synthesis of various Nb-containing mesoporous materials.
Frontiers in Chemistry | www.frontiersin.org molecular form of the niobium-phosphate solid, which is suitable for many catalytic applications, among others for the oxidation reactions (Son and Casey, 2015).

H 2 Evolution on PONbs
POMs may be considered as efficient functional modules for assembling molecular photocatalysts, since they can undergo stepwise, fast, and reversible reactions based on multielectrontransfer with no changes in their structures. As yet, several polyoxometalate catalysts containing mainly W and Mo have been exploited for light-driven H 2 evolution. Still, one always has to bear in mind that most of these POM-based catalysts cannot act as photocatalyst alone. Usually co-catalyst, such as Pt, NiO, and Co complexes, are necessary to lower the overpotential for H 2 evolution .
In the last years, the progress in PONbs chemistry was focused primarily on the synthesis and characterization procedures of novel types of polyoxoniobate compounds. However, Nb-based compounds such as niobates and niobium oxides have been already used as efficient photocatalysts and extensively studied in the water-splitting process for H 2 generation. Particular attention to the polyoxoniobates as photocatalysts has been paid in the last decade (Wu et al., 2015).
Feng O, and possessing molecular triangle, square, and cuboctahedral cage geometries, respectively, were successfully applied in the UV-light photocatalytic H 2 evolution. Catalytic systems involved also Co III (dmgH) 2 pyCl or Pt as co-catalysts and triethylamine as a sacrificial donor of electrons (Huang et al., 2012a).
An interesting survey of the photocatalytic activity of polyoxoniobates was presented by Son and co-workers. The same authors suggested that hexaniobate cluster and metallic tellurium, formed as photodecomposition products, act as co-catalyst. Furthermore, they proposed a mechanism of the studied photocatalytic reaction system (Figure 4).
It should be specified, that the reported TMA salt of the TeNb 5 molecule, when compared to an analogous Nb 6 unit, exhibits a similar solid-state structure and pH stability. Though, it reveals much higher H 2 -evolution activity due to a photodecomposition route to Nb 6 -based anion and Te(0) nanowires (Son et al., 2014).
It is worth mentioning that polyoxoniobates containing transition metals (TM-PONbs) are much less studied, as their synthesis procedures are quite problematic and may be FIGURE 3 | Polyoxometalate-catalyzed reaction systems with catalytic oxidation and photocatalytic reduction (adopted from Hutin et al., 2013).
Frontiers in Chemistry | www.frontiersin.org performed only in the narrow pH range suitable for the PONbs precursors. Nonetheless, many groups explore this problem, trying to design and synthesize new TM-PONbs, since these compounds are potentially attractive materials e.g., in water photolysis, base-catalyzed decomposition of biocontaminants and nuclear waste treatment (Wu et al., 2015). Su  O proved to be active in the studied catalytic system, especially in the presence of Pt species as a co-catalyst .
By means of a combination of hydrothermal and diffusional procedures, the same group obtained also a one-dimensional polyoxoniobate K 5 [H 2 AgNb 6 O 19 ]·11H 2 O, in which Lindqvist polyanions [Nb 6 O 19 ] 8− are connected to each other by silver cations forming an infinite chain. As reported by the above authors, this compound represents the first example of PONb in which an Ag atom is grafted on a hexaniobate anion. Moreover, photocatalytic tests revealed that it also enables acquiring high amounts of H 2 . (enMe = 1,2-diaminopropane). Due to the presence of vanadium atoms supporting the activation of surface oxygen atoms of heteropolyniobate-based frameworks, these compounds exhibit photocatalytic activity in the hydrogen evolution .
Polyoxoniobates modified with organic ligands also show potential in the visible-light-driven catalysis. In order to extend the light absorbed by oxoniobium clusters to the visible region, Cu(en) 2 (en = ethylenediamine) complex may be introduced into the structure of PONbs. The resulting dimer [Cu(en) 2 ] 11 K 4 Na 2 [KNb 24 O 72 H 9 ] 2 ·120H 2 O benefits with strong absorption from the UV to visible light region and therefore can be used in H 2 evolution systems comprising Co III (dmgH) 2 pyCl or Pt as co-catalyst. According to Wang an co-workers, a satisfying catalytic performance is associated with the synergistic effect of the oxoniobium clusters and Cu(en) 2 complex, which occurs during the photoexcitation step . Moreover, the results with [Cu(en) 2 ] 11 K 4 Na 2 [KNb 24 O 72 H 9 ] 2 ·120H 2 O catalyst are noticeably better than those obtained with the Cu-connected polyoxomolybdates (Fu et al., 2012).
Quite an interesting example of polyoxoniobate-based catalytic systems for H 2 evolution was presented by a group of Li and Xu. They synthesized a low-cost and stable K 7 HNb 6 O 19 -NiS/Cd 0.65 Zn 0.35 S photocatalytic system and proved that the presence of K 7 HNb 6 O 19 polyoxoniobate and NiS as co-catalysts affects strongly catalytic activity of Cd 0.65 Zn 0.35 S, increasing it to 3.4 times of that obtained with non-modified Cd 0.65 Zn 0.35 S. This improvement was ascribed to the positive synergetic effect between Nb 6 species and NiS in the co-catalysts, serving as electron collectors and adsorption active sites (Ma et al., 2014).

Epoxidation Reactions
Epoxidation of allylic alcohols with hydrogen peroxide as the oxidizing agent is a convenient method for obtaining epoxide alcohols, which are important final products or intermediates in the fine chemicals industry. PONbs, mostly functionalized with transition metals, have been also investigated as catalyst in the epoxidation process due to their exceptional features of great basicity and surface charge. Wang  } cluster are functionalized by ethanediamine molecules. The synthesized samples were investigated as catalyst in the epoxidation of allylic alcohols in the assistance of H 2 O 2 at room temperature in aqueous solution. Epoxidation of 3-methyl-2-buten-1-ol, which was selected as a model substrate, resulted in high conversion (98%) and selectivity (94%) at mild reaction conditions (25 • C, 5 min). Comparative studies with K 7 HNb 6 O 19 ·13H 2 O as a catalyst showed that the conversion of allylic alcohols was lower than that of Na 8 {Ni[Ni(en)] 2 Nb 10 O 32 }·28H 2 O. According to the same authors, it suggests that Nb atoms play crucial role in the considered epoxidation reactions. Furthermore, catalytic tests involving recycling of the catalyst have also been performed. The catalytic activity of PONb-based organicinorganic hybrid compound stayed almost unchanged even after the sixth catalytic run. On the other hand, the catalytic activity of K 7 HNb 6 O 19 ·13H 2 O was decreased already in the second run. Comparison of the FT-IR spectra of the fresh and reused catalysts showed noticeable differences at the peaks of 1,388 and 668 cm −1 , that could be attributed to bridging Nb-O b vibrations from Lindqvist hexaniobate structure. These results suggests that the framework of the catalyst was subjected to the partial decomposition and peroxidation by H 2 O 2 .
Epoxidation of styrene is an essential reaction for the production of styrene oxide, a key organic intermediate for producing valuable products such as surfactants, paints and epoxy resins. However, simultaneous achievement of significant conversion of the substrate and selectivity to the epoxide is quite challenging.
Researchers from Cui group have studied the epoxidation of styrene to styrene oxide in a batch reactor heated to 80 • C, with aqueous tert-butyl hydroperoxide (TBHP) and CH 3 CN as an oxidizing agent and solvent, respectively. and potassium clusters, indicated that the compound structure and the presence of other elements influence the catalytic effect of PONbs. Reusing of catalysts in styrene epoxidation proved good stability in catalytic experiments. As evidenced from FT-IR measurements, the characteristic bands attributed to ν(Nb-O t ) and ν(Nb-O b -Nb) were located at 852 and 696 cm −1 , respectively, and could be observed both for Nb 6 O 19 -based isopolyniobate and organic-inorganic hybrid PONb clusters, confirming the Lindqvist hexaniobate-related composition of the second group of compounds .

Base Catalysis
Base-catalyzed reactions such as the C-C bond creating ones (e.g., aldol condensation, Knoevenagel condensation, Michael addition, Henry reaction), chemical fixation of CO 2 , cyanosilylation of carbonyl compounds, oxidation, hydrogenation, isomerization of alkenes/alkynes, (trans)esterification or the Tishchenko reaction are very important to the bulk and fine chemicals industry (Kamata and Sugahara, 2017). In recent years much effort has been devoted to the activity of polyoxometalates in the acid-, photo-, and oxidation-catalysis, but base catalysis on these compounds has hardly been investigated . Nevertheless, new aspects of mono-and polyoxometalate base catalysts have also found coverage in literature.
The base catalysis by polyoxometalates may be attributed to the negativity of the surface oxygen atoms, mostly due to the great charge densities of the clusters. Thus, POMs possessing oxygen atoms with a higher negative charge density will reveal stronger base properties. The value of -n/y in [M x O y ] n− units characterizes a negative charge averaged over all oxygen atoms, but it was obtained not taking into account the electronic charge transfer from metal to oxygen. Consequently, this value delivers only a lower limit of the oxygen atom negativity. Polyoxoniobates, like hexaniobate ([Nb 6 O 19 ] 8− ) and decaniobate ([Nb 10 O 28 ] 6− ) cluster, have more negative -n/y values than polyoxotungstates (POWs) (Nyman, 2011;Wu et al., 2015). Assuming that the -n/y ratio may be considered as a quantification of the basicity of POMs, one can say that compounds with group V metals (V, Nb, Ta) are more favorable in base catalysts than their counterparts with group VI elements (Mo, W). Therefore, Tsukuda et al. have applied (TMA) 6 [Nb 10 O 28 ]·6H 2 O (TMA + = tetramethylammonium cation) as a homogeneous catalyst for the Knoevenagel condensation between benzaldehyde and various nitrile compounds having different pK a values. Knoevenagel condensation is a vital process among coupling reaction leading to the formation of new C=C bonds. It proceeds between carbonyl compounds (acceptors) and active methylene compounds (donors) such as nitriles. The key step in this reaction is abstraction of proton from donors performed by base catalysts, therefore a proper selection of catalyst is crucial for high yields of products. The results obtained by the group of Tsukuda showed that decaniobate cluster [Nb 10 O 28 ] 6− may act as a robust homogeneous catalyst not only for Knoevenagel condensation between benzaldehyde and p-methoxyphenylacetonitrile but also for Claisen-Schmidt condensation of benzaldehyde and acetophenone. This substantial catalytic activity was related to the high negative charge at the oxygen sites (−0.79 to −0.87) that originates primarily from the high over-all negative charge on the cluster. Density functional theory calculations, particularly natural bond orbital (NBO) analysis, were applied in order to obtain localized charge on the oxygen atoms. It was proved that NBO charge of O b (−0.873) in [Nb 10 O 28 ] 6− was the lowest through the composing oxygen atoms and thus the tetraalkylammonium salts of decaniobate cluster could successfully serve as catalysts in the Knoevenagel condensation and CO 2 fixation to epoxides (Hayashi et al., 2016(Hayashi et al., , 2017 (Ge et al., 2016).
On the other hand, the paper published lately by Niu and Wang describes the exceptional basicity of negatively charged Lindqvist type PONb (K 7 HNb 6 O 19 ·13H 2 O). Theoretical NBO calculations confirmed that the most negative NBO charge of oxygen in this compound equals −1.001. This value indicates outstanding basicity, since it is significantly higher than those reported in other polyoxoniobates. Thus, K 7 HNb 6 O 19 ·13H 2 O is likely to be used as a strong base catalyst. Experimental study, performed under mild conditions, confirmed that it can effectively catalyze Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate, disregarding the steric and electronic effect of aromatic aldehydes (Xu et al., 2018).
Although, it is worth mentioning that most of the above described catalytic systems for Knoevenagel condensation exhibit quite low relevance due to a narrow range of active methylene compounds with low pK a values. Hence, only a few papers on catalytic systems which may be applied to unreactive substrates have been published so far. It reveals that strongly negative NBO charges of PONbs play a crucial role in the Knoevenagel condensation (Kamata and Sugahara, 2017).
Synthesis of 4H-pyrans from an aldehyde, malononitrile and a β-dicarbonyl compound is an example of reaction in which a basic catalysts is required. In most cases this multicomponent process lead to extended reaction times and tiresome catalyst retrieval. Several categories of basic solids like mesoporous Ca-MCM, oxides or hydrotalcites have been employed in this reaction. Quite often microwave radiation is used in order to decrease the reaction times by causing a direct energy transfer to the reagents and initiating instant superheating. The study published by Martínez et al. focuses on the new application of the compound based on decaniobate [Nb 10 O 28 ] 6− , namely [N(CH 3 ) 4 ] 6 [Nb 10 O 28 ]·6H 2 O. It was successfully used as basic solid catalyst in multi-reagent, microwave-assisted and solventfree reaction to obtain several 4H-pyran derivatives. Different aldehydes with various electron-withdrawing or electron donor functionalities were tested, showing that the former ones favor shorter reaction times. The reuse of the catalyst, that could be  Frontiers in Chemistry | www.frontiersin.org FIGURE 7 | Cyclohexene oxidation pathway (adopted from Kilos et al., 2004). easily isolated from the reaction mixture, was evaluated as well. It was proved that the catalyst retained its initial activity and almost unchanged structure after four cycles of the reaction. FT-IR analysis exhibited that the representative bands of the decaniobate unit were maintained but with reduced intensity, what was perhaps caused by the adsorption of the substrate on the surface of the catalyst. It was also demonstrated that a low basicity is sufficient to promote the synthesis of 4H-pyrans, thus the reaction takes place disregarding the quantity of decaniobate phase in the catalyst's structure. Moreover, under microwave irradiation and solvent-free conditions the desired products were obtained in shorter reaction times (Gutierrez et al., 2018).

CATALYSIS ON NIOBIUM-CONTAINING MESOPOROUS SILICATES
Among abundant applications of metal-modified mesoporous materials, those containing niobium are preferred in catalytic processes. Mesoporous silicas are an ideal platform for providing an appropriate distribution of Nb species and may work as active and environment-friendly heterogeneous catalysts in conversion of quite large organic molecules. Introduction of Nb atoms to siliceous matrix is favored, as both lattice are matching to each other in nucleation and particle evolution. Consequently, the content of niobium in the framework can be enlarged without creating extra-framework Nb species. However, it is important to specify that the replacement of Nb(V) with Si(IV) in mesoporous silica creates in the lattice an excessive positive charge, which is presumed to be stabilized by the charge of -OH groups (e.g., Nowak, 2004Nowak, , 2012. Only dehydroxylation of such material generates active species, as it is shown in Figure 5. The presence of oxidative species (Nb-O δ− ) and Lewis acid sites (Nb δ+ ) was confirmed on the basis of ESR and NO/FTIR measurements, as well as catalytic tests. It was assumed that the detected oxidative properties of Nb-containing mesoporous sieve were caused by the occurrence of active lattice oxygen created during the dehydroxylation of the Nb-modified molecular silica. One should bear in mind, that the generation of this active lattice oxygen always goes along with the formation of Lewis acid sites (Nowak, 2012).

Oxidation of Various Unsaturated Compounds
Epoxidation of unsaturated compounds constitutes one of the principal transformations in the industry of fine chemicals (Sheldon and van Vliet, 2001;Cavani and Teles, 2009;Ivanchikova et al., 2017). Conventionally, epoxidation of organic compounds, such like alkenes or various alcohols is realized by oxidation in the presence of organic peracids or by the chlorohydrin method. Nowadays, the elimination of the use of hazardous substrates and reduction of wastes is a great challenge in the modern organic synthesis. For this reason, many scientists have been applying aqueous H 2 O 2 as an oxidant, as it generates only water as the byproduct (Grigoropoulou et al., 2003;Lane and Burgess, 2003;Sheldon et al., 2007;Clerici and Kholdeeva, 2013).
The activity of niobium-containing silicates in epoxidation reactions is related mainly to the occurrence of isolated Nb(V) sites in tetrahedral coordination, then the type of the acidity of the surface niobium species imposes the stability of formed reaction products . For example, the Lewis acid sites created by NbO 4 tetrahedra promote epoxidation, whereas the Brønsted acid sites, originating from distorted NbO 6 octahedra or nano-domains of Nb 2 O 5 (Turco et al., 2015), induce decomposition of hydrogen peroxide and promote the ring opening reaction of epoxide (Di Serio et al., 2012;Yan et al., 2014). Additionally, isolated and undercoordinated Nb(V) species are strongly engaged in the significant catalytic activity of materials containing niobium in selective oxidation with H 2 O 2 (Aronne et al., 2008;Thornburg et al., 2016).
Very interesting property of the niobium catalysts is regioselectivity toward epoxidation of the less electron-rich exocyclic C=C bond in terpenes (Gallo et al., 2013;Tiozzo et al., 2014). Ivanchikova et al. have confirmed that a very important factor in the regioselectivity of epoxidation of limonene over mesoporous niobium silicates is the nature of the solvent (Ivanchikova et al., 2015). The highest ratio of exo/endo epoxides was obtained in acetonitrile as solvent.
Nowak et al. described the selective oxidation of terpenes and terpenoids, such as geraniol, limonene, α-terpineol using H 2 O 2 as an oxidant over NbMSU-X-catalysts , which are characterized by a 3D interconnecting network of "worm-like" pores (Feliczak and Nowak, 2007). The oxidation of terpenes/terpenoids was performed in glass batch reactor upon dynamic stirring at 313 K for 23 h. Products of reaction were evaluated by GC and GC-MS. The oxidations of terpenes exhibited good site-and chemoselectivity, resulting in monoepoxides as the main products, e.g., geraniol can be epoxidized to epoxy-or diepoxygeraniol, limonene to 1,2-and 8,9-epoxylimonene.
The catalytic activity of niobium-containing materials (NbMCM-41) was studied also in oxidation of m-toluidine to m-aminobenzoic acid and m-aminobenzaldehyde with CO 2 -free air at 472-673 K (Nowak, 2012).

Knoevenagel Condensation
Knoevenagel condensation is a catalytic process, which requires the presence of basic active centers. Their activity and the selectivity of the reaction depend on the type of reagents used. This reaction states one of the most frequently used C-C forming processes (Corma and Martín-Aranda, 1991). The final products of this reaction could be used in perfumes, cosmetics, herbicides, insecticides, pharmaceuticals, polymers, etc. .
In general, the Knoevenagel condensation is carried out in the presence of different catalysts, including carbonyl compounds, aldehydes or ketones, and active methylene compounds. Lewis acid catalysts such as LaCl 3 (Narsaiah and Nagaiah, 2003), piperidine and alkali metal supported catalysts (Leelavathi and Kumar, 2004;Martín-Aranda et al., 2005;Perozo-Rondon et al., 2006), ionic liquids (Moriel et al., 2010), or alkali modified metal oxides , zeolites (Wada and Suzuki, 2003) and mesoporous materials (Zienkiewicz et al., 2009) or polymer-supported catalysts (Tamami and Fadavi, 2005) were used as catalysts. Bifunctional heterogeneous catalysts established on the basis of SBA-15 samples, modified with various metals, such as Zr, Nb, and Mo and functionalized with aminopropylsiliceous species were interesting materials in this reaction (Calvino-Casilda et al., 2016). According to literature, the role of amine group as basic centers was significant. Ziolek et al. described the application of imidazole as a source of basicity, loaded on mesoporous silicas and niobosilicates in Knoevenagel reaction (Kryszak et al., 2017a) as Lewis acid site involved in the above reaction.
Experimental study, concerning the development of basicity on different mesoporous metallosilicates (Nb-and Ce-) and silicas, has been reported by Ziolek et al. Their results proved a significant role of the catalyst structure, amount and nature of metal in the supports, and category of organic, nitrogencontaining modifying agent in adjusting catalysts' activity and selectivity in Knoevenagel condensation. It was also evidenced that niobium and cerium assumed different locations within the siliceous matrix: Nb was almost completely incorporated into the silica skeleton, while cerium was placed in pores in the form of crystalline CeO 2 . This led to entirely divergent performance of catalysts in the test reaction, since Nb-containing supports showed Lewis acidity, whereas cerium modified supports exhibited redox properties. These acidic and redox properties are vital in promoting Knoevenagel condensation, because Lewis acidity strongly supports this reaction via acidbase cooperation (the ion-pair mechanism) and redox properties play an important role in the enhancement of the support basicity. Moreover, the presence of niobium, contrary to ceria, improved the thermal stability of the nitrogen containing compounds attached to the silicas' surface by the chemical cooperation between niobium species (Lewis acid sites) and nitrogen compounds (source of basic sites) (Kryszak et al., 2017b).
Similar results were obtained in the catalytic tests with silica matrices of various chemical compositions and consequently with different acidity. It was proved that selectivity to the Knoevenagel products depends considerably on the nature of the support for each anchored amine, mostly due to the participation of acid sites in the withdrawing of hydroxyl groups from the reaction's intermediate (Blasco-Jiménez et al., 2010).
Further studies, involving catalytic tests of mesoporous silica with different Nb content and loaded aminosilane, proved that the chemical composition of the mesoporous supports strongly impact the basicity of the catalyst. The aminografted samples based on supports with high niobium-content revealed much higher activity in catalytic reactions (Knoevenagel condensation and Michael addition) than their counterparts with no incorporated niobium species (Blasco-Jiménez et al., 2009).

SUMMARY
The distinctive chemical properties of niobium designate it as a valuable promoter, active phase or support in numerous catalytic systems, e.g., niobium species are key factors in the liquid phase oxidation in the presence of H 2 O 2 as the oxidizing agent.
Materials containing niobium have been used in catalytic processes over the last few decades. At present, such materials may be considered as a substitute to the routinely used Ti-or Zrbased catalysts in several applications. The addition of niobium often increases surface redox properties in catalysts.
Nonetheless, it is crucial to point out that the behavior of Nb species strongly depends on the location and type of material into which it is incorporated. Polyoxometalates and niobium-containing mesoporous silicates are good examples illustrating the effects of niobium atoms localization and the character of matrix to which they are incorporated on the properties of particular materials. In POMs niobium occurs in octahedral units MO 6 , while in the silica matrix it prefers MO 4 tetrahedra. These different coordination types play a vital role in the catalytic activity. Therefore, PONbs are tested mainly in the photocatalytic systems, whereas Nb-containing mesoporous silicates are remarkably active catalysts in the oxidation/epoxidation reactions.
At present, the research concerning polyoxometalates, especially polyoxoniobates, seems to be entering a new exciting phase. On the basis of this review one can see that POM-based chemistry is aiming further than the synthesis and structural characterization of new clusters. The evolution of functionalized compounds, employing, among others, self-assembly aspects and intrinsic chemical and electronic properties, encourages to bring about gradually more complex molecular structures that could operate as tools with several adjustable utilities.
On the other hand, Nb-functionalized mesoporous silicates proved to be active in selective transformations of variety of reagents. They may be preferably employed in oxidation reactions, but after additional adjustment, they can serve as basic, acidic, bifunctional, and reducible or non-reducible catalysts. What is more, substantial corelations of niobium with other lattice components make Nb-containing mesoporous molecular sieves more stable and active even in the gas phase reactions.
The growing attention payed to Nb-based chemistry permits us to believe that further development, especially in the field of polyoxoniobates, may be expected and the performed study will bring exciting outcomes for the future in the area of catalysis and other applications.

AUTHOR CONTRIBUTIONS
AW, IN, AF-G contributed to the design of the paper layout, implementation of the bibliographical research and to the writing of the manuscript.

ACKNOWLEDGMENTS
The authors wish to thank the National Center for Science for financial support of the studies reported within the research project HARMONIA-5 (no. DEC-2013/10/M/ST5/00652).