Polyoxometalate (POM)-Layered Double Hydroxides (LDH) Composite Materials: Design and Catalytic Applications

catalysts

Review Polyoxometalate (POM)-Layered Double Hydroxides (LDH) Composite Materials: Design and Catalytic Applications

Tengfei Li 1, Haralampos N. Miras 2,* ID and Yu-Fei Song 1,3,* ID

1 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China; [email protected] 2 WestCHEM, School of Chemistry, the University of Glasgow, Glasgow G12 8QQ, UK 3 Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China * Correspondence: [email protected] (H.N.M.); [email protected] or [email protected] (Y.-F.S.); Tel.: +44-(0)-141-330-4375 (H.N.M.); +86-10-6443-1832 (Y.-F.S.); Fax: +86-10-6443-1832 (Y.-F.S.)

Academic Editor: Ivan V. Kozhevnikov Received: 9 August 2017; Accepted: 24 August 2017; Published: 1 September 2017

Abstract: Layered double hydroxides (LDHs) are an important large class of two-dimensional (2D) anionic lamellar materials that possess flexible modular structure, facile exchangeability of inter-lamellar guest anions and uniform distribution of metal cations in the layer. Owing to the modular accessible gallery and unique inter-lamellar chemical environment, polyoxometalates (POMs) intercalated with LDHs has shown a vast array of physical properties with applications in environment, energy, catalysis, etc. Here we describe how polyoxometalate clusters can be used as building components for the construction of systems with important catalytic properties. This review article mainly focuses on the discussion of new synthetic approaches developed recently that allow the incorporation of the element of design in the construction of a fundamentally new class of materials with pre-defined functionalities in catalytic applications. Introducing the element of design and taking control over the finally observed functionality we demonstrate the unique opportunity for engineering materials with modular properties for specific catalytic applications.

Keywords: layered double hydroxides; polyoxometalates; intercalated materials

1. Layered Double Hydroxides (LDHs)-Intro Background Layered double hydroxides (LDHs) or hydrotalcite-like compounds are a large family of two-dimensional (2D) anionic clay materials made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules, which can be II III x+ n− represented by the general formula [M1−x Mx (OH)2] [Ax/n] ·mH2O[1–4]. As shown in Figure1, the LDH layers incorporate divalent MII (e.g., MgII, FeII, CoII, CuII, NiII, or ZnII) and trivalent MIII metal cations (e.g., AlIII, CrIII, GaIII, InIII, MnIII or FeIII), forming positively charged layers. An− are mainly 2− − 2− − II III inorganic or organic anions (e.g., CO3 , Cl , SO4 , RCO2 ), where x is the molar ratio of M /M and generally in the range of 0.20–0.33, which occupy the space between the layers compensating for the positive charge and inducing stability in the overall structure [5,6]. The MII/MIII-based edge shared octahedra are used to construct 2D inﬁnite sheets, whereas hydroxyl groups are organized on the vertices of the octahedra with the hydrogen atoms pointing toward the interlayer region, as a consequence forming a complex network of hydrogen bonds with the interlayer anions and water molecules [1,7–10]. LDH-based materials are very attractive systems due to their robust structure,

Catalysts 2017, 7, 260; doi:10.3390/catal7090260 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 260 2 of 17

Catalysts 2017, 7, 260 2 of 16 wide range of compositions and ﬂexible properties which can be ﬁnely modulated by suitable selection selectionof metal of cations, metal functionalcations, functional interlayer interlayer compensating compensating anions, ratioanions, of MratioII/(M of IIM+II/(M MIIIII ),+ asMIII well), as as well the asrelative the relative weak weak interlayer interlayer bonding. bonding. All the All above the above physical physical and chemical and chemical properties properties render render the LDH the LDHas exceptional as exceptional candidates candidates for the for development the development of functional of functional materials materials with with versatile versatile physical physical and andchemical chemical properties. properties.

FigureFigure 1. SchematicSchematic representation representation of theof classicalthe classical LDHs LDHs structure. structure. Adapted Adapted from Ref. from [11], Ref. Copyright [11], Copyright1999, Elsevier. 1999, Elsevier.

LDHsLDHs have have been been known known for for more more than than 150 150 years years since since the the discovery discovery of of the the mineral mineral hydrotalcite, hydrotalcite, II II IIIIII [Mg[Mg6Al6Al2(OH)2(OH)16]CO16]CO3∙4H3·4H2O2 O[[11].11 ].Partial Partial substitution substitution of ofthe the Mg Mg ionsions by by Al Al ionsions in in hydrotalcite hydrotalcite compositionscompositions leads leads to to the the formation formation of of positively positively charged charged layers layers [12]. [12]. The The layers layers can can be be stacked stacked in in twotwo ways,ways, eithereither with with a rhombohedral a rhombohedral (3R symmetry) (3R symmetry) or hexagonal or hexagonal cell (2H symmetry). cell (2H Hydrotalcitesymmetry). Hydrotalcitecorresponds corresponds to the 3R symmetry, to the 3R symmetry, while the while 2H analogue the 2H analogue is referred is referred to as manasseite to as manasseite [12,13]. [12,13].In general, In general, LDH LDH can be can synthesized be synthesized in an in an aqueous aqueous medium medium employing employing a a varietyvariety of methods methods includingincluding co co-precipitation‐precipitation [14] [14] hydrothermal hydrothermal conditions, conditions, [15] [15] urea urea decomposition decomposition-homogeneous‐homogeneous precipitationprecipitation [16] [16] or or mechanochemical mechanochemical methods methods [17]. [17].

2.2. LDH LDH-Based‐Based Composite Composite Materials Materials InIn recent recent decades, decades, the the properties properties and and functionality functionality of of LDHs LDHs have have been been adjusted adjusted by by employing employing designdesign principles principles in in order order to to fabricate fabricate advanced advanced materials materials which which fulfill fulfill specific specific requirements requirements for for desirabledesirable applications applications in various fieldsfields [ [1].1]. For For example, example, LDHs LDHs were were used used as as additives additives in in polymers polymers [9], [9],as adsorbents as adsorbents for water for water remediation remediation [18], as[18], precursors as precursors for functional for functional materials, materials, applied inapplied the synthesis in the synthesisof pharmaceuticals of pharmaceuticals [19], in photochemistry [19], in photochemistry [20] and electrochemistry [20] and electrochemistry [21]. Moreover, [21]. LDHs Moreover, prepared LDHsdirectly prepared and their directly calcined and mixed their metalcalcined oxide mixed (MMOs) metal products oxide (MMOs) have been products widely have used been as an widely actual usedsolid as base an actual catalyst, solid and base promising catalyst, precursors and promising or supports precursors of catalysts or supports in a varietyof catalysts of fields in a variety including of fieldsorganic including synthesis, organic adsorption synthesis, of organic adsorption wastes or of heavy organic metals wastes ions, andor heavy the decomposition metals ions, ofand volatile the decompositionorganic compounds of volatile [22]. Recently, organic Kurodacompounds and co-workers[22]. Recently, prepared Kuroda novel and LDH co‐workers nanoparticles prepared with novelexceptionally LDH nanoparticles small particle with sizes exceptionally (ca. 10 nm) small by grafting particle asizes tripodal (ca. 10 ligand nm) ofby tris(hydroxymethyl) grafting a tripodal ligandaminomethane of tris(hydroxymethyl) (Tris) on the outer aminomethane surface of LDH (Tris) nanoparticles on the outer [23 surface–25]. The of TrisLDH molecules nanoparticles were [23– used 25].as a The surface-stabilizing Tris molecules agent were byused interacting as a surface with‐stabilizing the LDH layer agent through by interacting multiple coordination,with the LDH leading layer through multiple coordination, leading to effective control of the LDH nanoparticles’ growth. The resulting small‐sized LDH nanoparticles possess extremely high anion exchange abilities. Therefore,

Catalysts 2017, 7, 260 3 of 17 to effective control of the LDH nanoparticles’ growth. The resulting small-sized LDH nanoparticles possess extremely high anion exchange abilities. Therefore, their work provides a potentially new design for the development of LDH-based composite materials.

3. POM-LDH Composite Materials LDH materials possess relatively weak interlayer interactions and as a consequence they offer a remarkable opportunity for the development of methods that will allow the modulation of their functionality without compromising their structural features. In recent decades, increasing interest has been devoted to design and fabrication of novel intercalation composite materials with a variety of desirable physical and chemical properties. Recently, the adopted approach usually involves exfoliation of LDH layers and re-assembly in the presence of functional guest anions driven by electrostatic interactions. In particular, incorporation of catalytically active species, such as simple inorganic anions, complex ligands or biomolecules into the interlayer region of LDHs, is an effective immobilization method, and has been demonstrated to be an effective approach to enhance the catalytic stability and recyclability when compared to their corresponding homogeneous system. Moreover, the confined space of the LDHs interlayer region can impose restrictions to the geometry and accessibility of the catalytic active site in the interlayer region, resulting in the increased control of stereochemistry and dispersion of active components. Consequently, that led to increased reaction rates as well as modified product distributions [1]. For example, some intercalation composite materials have been applied in promotion of shape selective epoxidation reactions [26]. The intercalation of functional metal 6− 10− oxide-based anions with different molecular dimensions (for instance, Mo7O24 and W12O41 ) can control the gallery height of interacted materials. Tatsumi et al. reported different degrees of 6− gallery accessibility for the substrates based on the gallery dimensions [27]. The Mo7O24 -intercalated LDH exhibited slower epoxidation of cyclohexene owing to its smaller gallery height, preventing easy access to the inter-lamellar catalytic sites by the substrate. However, the decrease of the catalytic 10− activity of cyclohexene was less remarkable for W12O41 -intercalated LDH with a higher interlayer gallery. Additionally, the inherent basicity of brucite-like host LDH layers favored the production of epoxides over further hydrolysis, which leads to the formation of diols as principal products. Moreover, higher cis to trans ratio can be obtained for the oxidation of 2-hexene over intercalation composites. The observed enhancement of cis to trans ratio is probably due to the existence of steric constraints imposed by the robust interstitial environment. The gallery region is believed to provide a unique chemical environment for the modulation of selective catalytic reactions. Polyoxometalates (POMs) are a diverse family of compounds mainly constructed via Mo, W, V or Nb etc. at their highest oxidation states. Their formation is mainly based on the appropriate charge to ionic radius ratio and charge density which: (i) prohibits infinite polymerization; (ii) promotes formation of π-bonds with the O2− ligands; (iii) exhibits flexible and various coordination geometries. In the past decades, the properties of POMs have been extensively investigated due to a wide range of composition, sizes, rich redox chemistry, charge distribution and photochemistry to fulfill specific requirements in a variety of applications, including functional materials in catalysis, molecular electronics, magnetism, materials science and medicine [28–31]. Based on the above observations, various groups reported their efforts to design POM-based heterogeneous catalytic systems for the promotion of specific reactions. For example, Mizuno et al. reported the preparation, efficiency and stability of a peroxotungstate immobilized on ionic liquid-modified SiO2 heterogeneous catalyst for the epoxidation of a wide range of olefins [32]. In another case, Inumaru et al. demonstrated the efficiency of a POM-SiO2 heterogeneous catalyst where H3PW12O40 species immobilized the channels of mesoporous silica decorated with hydrophobic alkyl groups. The produced catalyst not only revealed exceptional stability but also showed exceptionally high catalytic activity for ester hydrolysis in aqueous medium [33]. Various elegant approaches have been adopted for the design of appropriate components for the development of new POM-based heterogeneous catalytic systems. The use of LDH also attracted the attention of research groups due to their relatively facile preparation, under mild Catalysts 2017, 7, 260 4 of 17

Catalysts 2017, 7, 260 4 of 16 conditions, cost-effective preparation and most importantly their chemical and structural modularity. physicochemicalRecently, the manifestation properties of superiorintercalation physicochemical composites due properties to cooperative of intercalation effects between composites LDHs due andto cooperative POMs attracted effects the between interest LDHsof research and POMs groups attracted since they the have interest exhibited of research great groupsadvantages since over they bothhave traditional exhibited greatPOMs advantages and LDH over compounds, both traditional especially POMs in andcatalytic LDH applications. compounds, especiallyA series of in intermolecularcatalytic applications. interactions, A series including of intermolecular electrostatic interactions, and hydrogen including bond electrostatic networks andetc. hydrogen, can be developedbond networks between etc., canthe bebrucite developed‐like layers between of theLDHs brucite-like and the layers intercalated of LDHs POM and theanions. intercalated More importantly,POM anions. the More confinement importantly, effect the of conﬁnement the LDH interlayer effect of gallery the LDH prevents interlayer the galleryPOM species prevents from the leachingPOM species into the from reaction leaching mixture into the and reaction increasing mixture the selectivity and increasing of the the catalytic selectivity reaction. of the A catalyticgeneral structurereaction. of A the general POM structure‐LDH composite of the POM-LDH is illustrated composite in Figure is illustrated2. in Figure2.

6− FigureFigure 2. 2. SchematicSchematic representation representation of of POM POM-LDHs‐LDHs structure (e.g., VV1010O286− intercalatedintercalated in in ZnAl ZnAl-LDH).‐LDH). AdaptedAdapted from from Ref. Ref. [12], [12], Copyright Copyright 2014, 2014, Elsevier. Elsevier.

3.1.3.1. Synthesis Synthesis of of POM/LDH POM/LDH Composites Composites

InIn 1988, 1988, Pinnavaia Pinnavaia et et al. al. reported reported the the preparation of POM-LDHPOM‐LDH composite ZnZn22AlAl-V‐V10OO2828 byby ion ion −− exchangeexchange of of the the LDH LDH-Cl‐Cl withwith [NH [NH4]46][V6[V1010OO28]28∙6H]·6H2O2 atO atpH pH 4.5. 4.5. As As shown shown in in Figure Figure 3,3 ,the the resulting resulting compositecomposite possesses possesses a a basal basal spacing spacing of of 11.9 11.9 Å, Å, corresponding corresponding to to a a gallery gallery height height of of 7.1 7.1 Å, Å, indicating indicating thatthat the the decavanadate’s decavanadate’s CC2 axis2 axis is isaligned aligned parallel parallel to LDH to LDH host host layers. layers. The Thegroup group verified veriﬁed further further the 6− 6− 51 51 successfulthe successful intercalation intercalation of the of the V10 VO1028O 28clustercluster using using V MASV MAS-NMR‐NMR spectrum spectrum of of intercalated intercalated ZnZn2Al2Al-V‐V1010OO28 28byby comparing comparing the the relevant relevant spectrum spectrum of of its its ammonium ammonium salt, salt, which which does does not not observe observe obviousobvious change change of of the the chemical chemical shifts; shifts; additionally, additionally, all all chemical chemical parameters parameters are are qualitatively qualitatively similar, similar, whichwhich indicates indicates that that the the decavanadate decavanadate retains retains its its structural structural integrity integrity in in the the intercalated intercalated state state [34]. [34]. ThisThis observation observation demonstrates demonstrates that that the the LDHs LDHs with with high high-charge‐charge density density can can be be intercalated intercalated by by guest guest POMPOM anions anions forfor thethe ﬁrst first time, time, providing providing the the intercalation intercalation of POMs of POMs with relativelywith relatively high charge high charge density. 6− density.Moreover, Moreover, the authors the reportedauthors thatreported Keggin that species Keggin and species their derivatives and their suchderivatives as H2W such12O4 Oas 6− 7− 7− Hand2W12 BVWO4O11 Oand40 BVWalso11O can40 be also successfully can be successfully intercalated intercalated into LDH, into affording LDH, affording the resulting the interactedresulting interactedderivatives derivatives with gallery with heights gallery of 9.6 heights Å. Since of then,9.6 Å. the Since development then, the of newdevelopment methodologies of new for methodologiesthe preparation for of the POM-LDH preparation composite of POM materials‐LDH composite and the investigationmaterials and of the their investigation functionalities of their has functionalitiesincreased exponentially. has increased exponentially. ToTo date, date, several several synthetic synthetic approaches approaches can can be be used used for for the the preparation preparation of of POM POM-LDH‐LDH composites. composites. AmongAmong the the most developeddeveloped onesones are are the the ion ion exchange exchange synthetic synthetic pathway pathway [34 –[34–37];37]; co-precipitation co‐precipitation [38], [38],reconstitution reconstitution [39] [39] et al. et al. and and have have been been discussed discussed into into detail detail elsewhere. elsewhere. Here,Here, we will will focus focus our our discussiondiscussion on on the the following following two two methods methods developed developed recently recently by by Song Song et et al. al. [40] [40]..

Catalysts 2017, 7, 260 5 of 17 Catalysts 2017, 7, 260 5 of 16

Figure 3. Orientation of decavanadate anion, V10O2866−−, in the interlayer region of layered double Figure 3. Orientation of decavanadate anion, V10O28 , in the interlayer region of layered double hydroxides, with its ‘main’ C2 axis aligned parallel to the brucite‐like layers. Adapted from Ref. [12], hydroxides, with its ‘main’ C2 axis aligned parallel to the brucite-like layers. Adapted from Ref. [12], CopyrightCopyright 2014, 2014, Elsevier. Elsevier.

3.1.1. Host Layer Modification Method 3.1.1. Host Layer Modification Method Although POM‐intercalated LDH composite materials exhibit superior performance and Although POM-intercalated LDH composite materials exhibit superior performance and modular modular functionality compared to individual components, the relevant intercalation process faces functionality compared to individual components, the relevant intercalation process faces serious serious challenges: (1) the preparation of POM‐LDH composites without impurities using traditional challenges: (1) the preparation of POM-LDH composites without impurities using traditional synthetic synthetic methods such as reconstitution and co‐precipitation pathways is an extremely challenging methods such as reconstitution and co-precipitation pathways is an extremely challenging task; task; (2) POM intercalation is directly related to the geometry, charge density and overall size of (2) POM intercalation is directly related to the geometry, charge density and overall size of POMs. POMs. Thus, POM anions that exhibit negative charge below four, such as the classical Keggin3− of Thus, POM anions that exhibit negative charge below four, such as the classical Keggin of [PW12O40] , [PW12O40]3−, are very difficult to intercalate into LDH galleries using conventional synthetic methods; are very difficult to intercalate into LDH galleries using conventional synthetic methods; (3) LDHs 2− (3) LDHs exhibit a high affinity for smaller anions 2such− as CO3 ; thus, ion exchange reactions for exhibit a high affinity for smaller anions such as CO3 ; thus, ion exchange reactions for preparation preparation of POMs intercalated into LDHs is not favored under air. These issues significantly limit of POMs intercalated into LDHs is not favored under air. These issues significantly limit the chemical the chemical tunability and intercalation ability of LDH‐based materials. Inspired by Kuroda et al tunability and intercalation ability of LDH-based materials. Inspired by Kuroda et al. work of LDH-NPs work of LDH‐NPs with exceptionally small size and enhanced anion exchangeability by modifying with exceptionally small size and enhanced anion exchangeability by modifying the surface using a the surface using a tripodal ligand Tris [23–25]. In a similar manner, our group developed recently a tripodal ligand Tris [23–25]. In a similar manner, our group developed recently a novel approach where novel approach where the host layer modified with tris(hydroxymethyl)aminomethane (Tris) prior the host layer modified with tris(hydroxymethyl)aminomethane (Tris) prior to the intercalation of to the intercalation of the classical Keggin cluster of Na3PW12O40∙15H2O (Na‐PW12) under ambient the classical Keggin cluster of Na3PW12O40·15H2O (Na-PW12) under ambient conditions without conditions without the necessity of degassing CO2 (Figure 4) [41]. Tris‐LDH‐CO3 (2 mg/mL) was the necessity of degassing CO2 (Figure4)[ 41]. Tris-LDH-CO3 (2 mg/mL) was re-dispersed in re‐dispersed in Na3[PW12O40] aqueous (0.1 M) and then reacted for 2 h at room temperature. The Na3[PW12O40] aqueous (0.1 M) and then reacted for 2 h at room temperature. The reaction mixture was reaction mixture was then filtered, washed by water and◦ acetone, and dried at 60 °C to obtain the then filtered, washed by water and acetone, and dried at 60 C to obtain the Tris-LDH-PW12. The host Tris‐LDH‐PW12. The host layer modification method can modulate the host‐guest interactions layer modification method can modulate the host-guest interactions including electrostatic interactions including electrostatic interactions and hydrogen bonding between host layers and guests in and hydrogen bonding between host layers and guests in interlayer gallery and consequently promote interlayer gallery and consequently promote the ion exchange process. the ion exchange process. 3.1.2. Exfoliation Assembly Method In the past few years, the delaminated LDHs have been employed successfully in the design and fabrication of advanced functional materials. For example, Kim et al. [42] reported a design strategy for the assembly of quantum dots (QDs) that was encapsulated by delaminated LDH nanosheets in formamide via electrostatic interactions. Hwang et al. [43] reported the preparation of mesoporous nanohybrids with excellent photocatalytic activity, by the assembly of positively charged 2D ZnCr‐LDH nanosheets and negatively charged layered titanium oxide. Inspired by these

Catalysts 2017, 7, 260 6 of 16

exfoliation and assembly strategies of LDH nanosheets, we envisaged that POM clusters, as a class of discrete anionic metal oxides with high negative charge density, should be able to assemble with LDH nanosheets using a similar approach. As shown in Figure 5, the first step involves the exfoliation of the LDH‐NO3 precursor in formamide solvent at 1:1 ratio (1 mg LDH‐NO3 per mL of formamide) in a flask; the mixture was sealed after purging with N2 in order to avoid carbonate contamination. The reaction mixture becomes clear upon quantitative exfoliation of the LDH precursor; subsequently, addition of POM clusters triggers the assembly of the components towards the formation of the intercalated material. The restacked POM‐LDH composite was collected by filtration and washed thoroughly with ethanol and water, and dried under vacuum [44]. It is remarkable that the exfoliation assembly pathway provides a novel and rational method to fabricate composite functional materials with a desirable and precisely controlled nanostructure. Moreover, Figure 4. Schematic representation of the host layer modification method to prepare POM‐LDH Figure 4. Schematic representation of the host layer modification method to prepare POM-LDH this method cancomposite prevent (Tris ‐LDHmetal‐PW ions12). Adapted leaching from Ref. out [41], of Copyright the host 2014, layer Royal during Society of the Chemistry. intercalation process and compositethereby successfully (Tris-LDH-PW control12). Adapted the final from metal Ref. [ 41ions], Copyright ratio (for 2014, example, Royal Society MgII/Al ofIII Chemistry.) in the resulting POM‐LDHs composite without co‐formation of impurity phases. 3.1.2. Exfoliation Assembly Method In the past few years, the delaminated LDHs have been employed successfully in the design and fabrication of advanced functional materials. For example, Kim et al. [42] reported a design strategy for the assembly of quantum dots (QDs) that was encapsulated by delaminated LDH nanosheets in Figure 5. The preparation process of POM‐LDHs composites (Mg3Al‐LnW10) using the Exfoliation formamide viaassembly electrostatic method. Reproduced interactions. from Ref. Hwang [44], Copyright et al. [ 2014,43] reportedElsevier. the preparation of mesoporous nanohybrids with excellent photocatalytic activity, by the assembly of positively charged 2D ZnCr-LDH nanosheets3.2. and Structural negatively Features charged layered titanium oxide. Inspired by these exfoliation and assembly strategies of LDHPOMs’ nanosheets, structural diversity we envisaged in terms of that size POMand features clusters, can asbe aused class constructively of discrete for anionic the metal oxides withengineering high negative of the chargestructure density, of the composite should be materials able to that assemble exhibit withspecific LDH gallery nanosheets height; using a additionally POMs with relatively high‐charge density should afford large lateral anion spacing, similar approach. As shown in Figure5, the first step involves the exfoliation of the LDH-NO precursor consequently providing the chance to access the LDH gallery region [34]. Therefore, POMs can be3 in formamideviewed solvent as ideal at functional 1:1 ratio guest (1 mg anions LDH-NO for intercalation,3 per mL ofwhich formamide) offers the inopportunity a flask; thefor mixture was sealed after purging with N2 in order to avoid carbonate contamination. The reaction mixture becomes clear upon quantitative exfoliation of the LDH precursor; subsequently, addition of POM clusters triggers the assembly of the components towards the formation of the intercalated material. The restacked POM-LDH composite was collected by filtration and washed thoroughly with ethanol and water, and dried under vacuum [44]. It is remarkable that the exfoliation assembly pathway provides a novel and rational method to fabricate composite functional materials with a desirable and precisely controlled nanostructure. Moreover, this method can prevent metal ions leaching out of the host layer during the intercalation process and thereby successfully control the final metal II III ions ratioFigure (for 4. example,Schematic Mgrepresentation/Al ) in of the the resulting host layer POM-LDHs modification composite method to without prepare co-formation POM‐LDH of impuritycomposite phases. (Tris‐LDH‐PW12). Adapted from Ref. [41], Copyright 2014, Royal Society of Chemistry.

Figure 5. The preparation process of POM‐LDHs composites (Mg3Al‐LnW10) using the Exfoliation Figure 5. The preparation process of POM-LDHs composites (Mg3Al-LnW10) using the Exfoliation assemblyassembly method.method. ReproducedReproduced fromfrom Ref.Ref. [[44],44], CopyrightCopyright 2014,2014, Elsevier. Elsevier.

3.2. Structural Features POMs’ structural diversity in terms of size and features can be used constructively for the engineering of the structure of the composite materials that exhibit specific gallery height; additionally POMs with relatively high‐charge density should afford large lateral anion spacing, consequently providing the chance to access the LDH gallery region [34]. Therefore, POMs can be viewed as ideal functional guest anions for intercalation, which offers the opportunity for

Catalysts 2017, 7, 260 7 of 17 Catalysts 2017, 7, 260 7 of 16 modulation3.2. Structural and Features fine‐tuning of the composite material’s properties. Additionally, synergistic effects betweenPOMs’ the structural constituents diversity of the in termsPOM ofintercalated size and features LDH cancomposites be used lead constructively to performance for the enhancementengineering of induce the structure emergence of the of composite new functionality materials as that well exhibit as offer specific the opportunity gallery height; to additionallyexpand the areasPOMs of with application relatively [12]. high-charge For instance, density the intercalation should afford of POM large clusters lateral anion into LDH spacing, layers consequently in aqueous mediaproviding results the in chance an insoluble to access self the‐assembled LDH gallery product region where [34 ].the Therefore, POM cluster POMs retains can their be viewed catalytic as propertiesideal functional and guestcan perform anions for efficiently intercalation, as component which offers of the the opportunity heterogeneous for modulation system. andThe combinatorialfine-tuning of effect the composite induced by material’s the structural properties. and chemical Additionally, characteristics synergistic of the effects POM‐ betweenintercalated the LDHconstituents materials of leads the POM to the intercalated performance LDH enhancement composites of leadthe final to performance product. enhancement induce emergence of new functionality as well as offer the opportunity to expand the areas of application [12]. 3.3. Oxidation Catalyst For instance, the intercalation of POM clusters into LDH layers in aqueous media results in an insolubleAs shown self-assembled in Figure 6, product we prepared where a the series POM of clusterPOM‐LDH retains composites their catalytic Mg3Al properties‐P2W17X (X and = Mn canIII FeperformIII, Zn efficientlyII, CoII, Cu asII component, NiII) by ofintercalating the heterogeneous a Dawson system. polyoxometalate The combinatorial (POM) effect inducedspecies [byα2‐ theP2W structural17O61(X∙OH and2)]n− chemical ([P2W17X] characteristicsn−, n = 7 or 8) into of the a Mg POM-intercalated3Al‐suberic precursor LDH [36]. materials Catalytic leads tests to thefor theperformance sulfoxidation enhancement of various of sulfides the final demonstrated product. that Mg3Al‐P2W17Zn exhibits superior catalytic efficiency and selectivity than the individual POM or LDH precursors under mild conditions. The uniform3.3. Oxidation and well Catalyst‐ordered dispersion of POM clusters in the confined gallery of LDHs as well as the multipleAs showninteractions in Figure between6, we POMs prepared and LDHs a series contribute of POM-LDH to the excellent composites catalytic Mg performance.3Al-P2W17X (Furthermore,X = MnIII FeIII Mg, Zn3AlII‐,P Co2WII17,Zn Cu isII, stable NiII) byand intercalating can be easily a Dawsonseparated polyoxometalate from the reaction (POM) system. species The n− n− recycled[α2-P2W 17MgO613Al(X‐P·OH2W172)]Zn material([P2W17 X]retains, n its= 7structural or 8) into features a Mg3Al-suberic and composition precursor of [the36]. individual Catalytic 31 components,tests for the sulfoxidation which has been of variousconfirmed sulfides by Fourier demonstrated transform that infrared Mg3Al-P (FT2‐IR),W17 XZn‐ray exhibits diffraction, superior P NMR,catalytic and efficiency X‐ray photoelectron and selectivity spectroscopy than the individual characterization. POM or LDH The precursors scaled‐up underexperiment mild conditions. provided furtherThe uniform supportand for well-ordered its potential dispersion use for industrial of POM applications. clusters in the confined gallery of LDHs as well as the multipleWater pollution interactions from between dyes is POMs a serious and environmental LDHs contribute problem to the excellentdue to the catalytic inherent performance. difficulties forFurthermore, their efficient Mg 3degradationAl-P2W17Zn in is conjunction stable and with can betheir easily high separated chemical fromstability the [45]. reaction Continuous system. Theresearch recycled interest Mg3 hasAl-P been2W17 focusedZn material on the retains development its structural of methods features andto treat composition dye effluents. of the Recently, individual a seriescomponents, of POM which‐intercalated has been LDHs confirmed (Tris‐LDHs by Fourier‐PW12, transformTris‐LDH‐LaW infrared10 and (FT-IR), Tris‐LDH X-ray‐P2 diffraction,W18) were prepared31P NMR, by and employing X-ray photoelectron the host layer spectroscopy modification characterization. method under The ambient scaled-up conditions experiment without provided the necessityfurther support of degassing for its potentialCO2. use for industrial applications.

n− n− Figure 6. SchematicSchematic of of the the synthesis synthesis of of [α [α2‐2P-P2W2W17O1761O(X61∙(XOH·OH2)] 2 )]intercalatedintercalated into LDHs. into LDHs. Adapted Adapted from Ref.from [36], Ref. Copyright [36], Copyright 2015, 2015,American American Chemical Chemical Society. Society.

TheWater heterogeneous pollution from catalytic dyes is systems a serious were environmental used for the problem efficient due degradation to the inherent of methylene difficulties blue for (MB),their efficient rhodamine degradation B (RB) and in conjunction crystal violet with (CV) their solution, high chemical where stabilityTris‐LDHs [45‐].PW Continuous12 exhibits researchthe best performanceinterest has been in the focused presence on theof H development2O2 [46]. As shown of methods in Figure to treat 7, almost dye effluents. complete Recently, dye degradation a series of can be achieved within 320 min, 140 min, and 5 min for RB, MB, and CV, respectively. Their POM-intercalated LDHs (Tris-LDHs-PW12, Tris-LDH-LaW10 and Tris-LDH-P2W18) were prepared by 2− degradationemploying the products host layer have modification been detected method and identified under ambient as non conditions‐ or less‐toxic without anions the such necessity as SO4 of, NO3−, C2O42−, CHO2− and C2H3O22−. Additionally, degradation of the mixture of RB, CV and MB by degassing CO2. Tris‐LDHsThe heterogeneous‐PW12 follows catalytic the order systems of CV were > MB used > RB, for thewhich efficient is directly degradation related of to methylene the designed blue accessible area of the interlayer space. (MB), rhodamine B (RB) and crystal violet (CV) solution, where Tris-LDHs-PW12 exhibits the best On the basis of increasing environmental impact of sulfur‐containing chemicals in fuel oils, the performance in the presence of H2O2 [46]. As shown in Figure7, almost complete dye degradation developmentcan be achieved of efficient within 320and min, environmentally 140 min, and friendly 5 min fordeep RB, desulfurization MB, and CV, processes respectively. has Theirbeen flagged as an urgent global challenge [47,48]. In recent years, POM‐LDH composites have been widely used in green chemistry industry [40,46], and currently, catalytic oxidation desulfurization has become an important task for environmental pollution control. However, the traditional

Catalysts 2017, 7, 260 8 of 16 hydro‐desulfurization (HDS) processes employed for the removal of DBT, 4,6‐DMDBT and BT etc. are quite challenging. Furthermore, the HDS processes need harsh conditions, (high temperature and pressure regimes), thus restricting its wide‐ and large‐scale application. Therefore, the removal ofCatalysts sulfur2017‐containing, 7, 260 compounds from fuel oils under mild conditions is a serious issue. We 8used of 17 Tris‐LDH‐PW12 for extraction and catalytic oxidation/desulfurization of fuel oils. We demonstrated that deep desulfurization of DBT can be achieved in 25 min at 75 °C using H2O2/DBT/Cat = 100:20:1.2− degradation products have been detected and identiﬁed as non- or less-toxic anions such as SO4 , Also −recently,2− we developed2− an intercalated2− hybrid material Mg3Al‐IL‐EuW10 that incorporates NO3 ,C2O4 , CHO and C2H3O2 . Additionally, degradation of the mixture of RB, CV and MB covalently anchored ionic liquids, and exhibits efficient deep desulfurization and high selectivity for by Tris-LDHs-PW12 follows the order of CV > MB > RB, which is directly related to the designed theaccessible removal area of 4,6 of‐ theDMDBT interlayer and space.DBT etc.

FigureFigure 7. ((aa)) Degradation time time of different dyes. Experimental condition: Tris Tris-LDH-PW‐LDH‐PW1212 5050 mg; mg; ◦ HH22OO22 == 500 500 μµL;L ;T T = = 30 30 °C;C; Dye Dye = = 10 10 mg/L mg/L (50 (50 mL); mL); (b ()b The) The UV UV−Vis−Vis spectra spectra of of degradation degradation mixed mixed dyes. dyes. ExperimentExperiment condition: Tris-LDH-PWTris‐LDH‐PW1212 50 mg; H22O2 == 500 500 μµL;L; total dye = 10 mg/L (50 (50 mL, mL, each each dye dye = 3.333.33 mg/L); mg/L); T T = = 30 30 °C.◦C. Reproduced Reproduced from from Ref. Ref. [46], [46], Copyright Copyright 2015, 2015, WILEY WILEY-VCH.‐VCH.

InOn addition the basis ofto increasingsulfur compounds, environmental diesel impact oil, ofgasoline sulfur-containing and jet chemicalsfuel contain in fuel many oils, nitrogenthe development‐containingof efficientcompounds and [49]; environmentally the combustion friendly of these deep substances desulfurization will release processes large amounts has been offlagged nitrogen as an oxides, urgent leading global challengeto the formation [47,48]. Inof recentacid rain years, with POM-LDH serious environmental composites have consequences been widely [50].used Although in green chemistry the concentration industry of [40 nitrogen,46], and‐containing currently, catalytic compounds oxidation in fuel desulfurization oils is relatively has low, become the competitionan important with task sulfur for environmental‐containing components pollution control. for the However, active sites the traditionalof the catalysts hydro-desulfurization [51,52], can lead to(HDS) the processesinhibition employed of the deep for the desulfurization removal of DBT, process. 4,6-DMDBT Therefore, and BT deep etc. aredesulfurization quite challenging. and denitrogenationFurthermore, the is HDS of processesgreat significance need harsh for conditions, the development (high temperature of clean and oil pressure products, regimes), and is thus a challengingrestricting its task. wide- We and also large-scale demonstrated application. that the Therefore, Tris‐LDH the‐LaW removal10 composite of sulfur-containing could be used compounds as green and highly efficient catalyst for simultaneous deep hydro‐desulfurization and from fuel oils under mild conditions is a serious issue. We used Tris-LDH-PW12 for extraction and hydrocatalytic‐denitrogenation oxidation/desulfurization of complex model of fuel oil oils. mixtures We demonstrated under mild conditions. that deep desulfurization As shown in Figure of DBT 8, the catalytic system of H2O◦ 2/[bmim]BF4/Tris‐LDH‐LaW10 can complete concurrent deep can be achieved in 25 min at 75 C using H2O2/DBT/Cat = 100:20:1. Also recently, we developed an desulfurization and denitrogenation in 60 min with S and N contents less than 10 and 1 ppm, intercalated hybrid material Mg3Al-IL-EuW10 that incorporates covalently anchored ionic liquids, and respectively,exhibits efficient at 65 deep °C [53]. desulfurization and high selectivity for the removal of 4,6-DMDBT and DBT etc. AnotherIn addition family to sulfur of organic compounds, compounds diesel oil, that gasoline received and much jet fuel attention contain manyin recent nitrogen-containing decades is the heterocycliccompounds N [49‐oxides]; the because combustion of their of wide these use substances as versatile will synthetic release intermediates, large amounts their of biological nitrogen significance,oxides, leading their to use the as formation protecting ofgroups, acid rainas oxidants, with serious as auxiliary environmental agents, as consequences ligands in metal [50]. complexes,Although theas surrogates concentration for heterocyclic of nitrogen-containing boronic acids, and compounds catalysts [54,55]. in fuel Our oils group is relatively successfully low, 11− preparedthe competition a Lewis with acid sulfur-containing POM‐LDH catalyst components by intercalating for the active[La(PW sites11O39 of)2] the anions catalysts into [ 51LDH,52], modifiedcan lead with to the tris(hydroxymethyl)aminomethane inhibition of the deep desulfurization (Tris) process.[56]. As shown Therefore, in Figure deep 9, desulfurization the resulting Trisand‐LDH denitrogenation‐La(PW11)2 composite is of great shows significance excellent for selectivity, the development high yield, of cleanand high oil products, efficiency and for isthe a N‐oxidation of pyridines and its derivatives at room temperature in the presence of H2O2. Moreover, challenging task. We also demonstrated that the Tris-LDH-LaW10 composite could be used as green Trisand‐LDH highly‐La(PW efficient11)2 catalyst‐based catalysts for simultaneous have been deep applied hydro-desulfurization in the denitrogenation and hydro-denitrogenationof a model oil mixture inof the complex presence model of H2O oil2 and mixtures 1‐butyl under‐3‐methylimidazolium mild conditions. tetrafluoroborate. As shown in Denitrogenation Figure8, the catalytic can be achieved in 40 min at 75 °C. system of H2O2/[bmim]BF4/Tris-LDH-LaW10 can complete concurrent deep desulfurization and denitrogenation in 60 min with S and N contents less than 10 and 1 ppm, respectively, at 65 ◦C[53].

Catalysts 2017, 7, 260 9 of 17 Catalysts 2017, 7, 260 9 of 16

Catalysts 2017, 7, 260 9 of 16

FigureFigure 8. Proposed 8. Proposed schematic schematic of of extraction extraction andand catalytic desulfurization desulfurization and and denitrogenation denitrogenation route route

Another family of organic compounds that received much attention in recent decades is the heterocyclic N-oxides because of their wide use as versatile synthetic intermediates, their biological significance, their use as protecting groups, as oxidants, as auxiliary agents, as ligands in metal complexes, as surrogates for heterocyclic boronic acids, and catalysts [54,55]. Our group successfully 11− prepared a Lewis acid POM-LDH catalyst by intercalating [La(PW11O39)2] anions into LDH modified with tris(hydroxymethyl)aminomethane (Tris) [56]. As shown in Figure9, the resulting Tris-LDH-La(PW11)2 composite shows excellent selectivity, high yield, and high efficiency for the N-oxidation of pyridines and its derivatives at room temperature in the presence of H2O2. Moreover, Figure 8. Proposed schematic of extraction and catalytic desulfurization and denitrogenation route Tris-LDH-La(PW11)2-based catalysts have been applied in the denitrogenation of a model oil mixture with H2O2/[bmim]BF4/TrisLDH‐LaW10. Adapted from Ref. [53], Copyright 2016, Royal Society of in the presenceChemistry. of H2O2 and 1-butyl-3-methylimidazolium tetrafluoroborate. Denitrogenation can be achieved in 40 min at 75 ◦C.

Figure 9. Proposed reaction mechanism for oxidation of pyridines by Tris‐LDH‐La(PW11)2. Adapted

One‐pot cascade reactions are highly desirable for organic synthetic chemists [57,58], because they can drastically simplify multi‐step synthetic pathways and thereby reduce the waste production and energy consumption [59,60]. A feasible pathway for efficient progression of a tandem reaction is based on the design and fabrication of bifunctional catalyst with spatially isolated multiple active sites, promoting simultaneously different reactions within the same system. The Knoevenagel reaction between an aldehyde/ketone and active methylene‐containing compounds is one of the most important reactions leading to carbon‐carbon bond formation [61,62], which has been utilized for the syntheses of important products or intermediates for pharmaceuticals, perfumes and calcium antagonists [63,64]. Taking into consideration the fact that POMs possess

excellentFigure 9. redoxProposed properties reaction while mechanism the LDHs for oxidationhave a number of pyridines of active by Tris-LDH-La(PW basic sites that can11)2 .efficiently Adapted Figure 9. Proposed reaction mechanism for oxidation of pyridines by Tris‐LDH‐La(PW11)2. Adapted from Ref. [56]. Copyright 2015, Willey-VCH. from Ref. [56]. Copyright 2015, Willey‐VCH.

Catalysts 2017, 7, 260 10 of 17

One-pot cascade reactions are highly desirable for organic synthetic chemists [57,58], because they can drastically simplify multi-step synthetic pathways and thereby reduce the waste production and energy consumption [59,60]. A feasible pathway for efficient progression of a tandem reaction is based on the design and fabrication of bifunctional catalyst with spatially isolated multiple active sites, promoting simultaneously different reactions within the same system. The Knoevenagel reaction between an aldehyde/ketone and active methylene-containing compounds is one of the most important reactions leading to carbon-carbon bond formation [61,62], which has been utilized for the syntheses of important products or intermediates for pharmaceuticals, perfumes and calcium antagonists [63,64]. TakingCatalysts 2017 into, 7 consideration, 260 the fact that POMs possess excellent redox properties while the LDHs10 of 16 have a number of active basic sites that can efficiently promote Knoevenagel condensation reactions, wepromote envisaged Knoevenagel that a POM-LDH condensation composite reactions, could we be envisaged used as a novelthat a bifunctionalPOM‐LDH composite catalyst, which could can be catalyzeused as botha novel Knoevenagel bifunctional reactions catalyst, and which the oxidation can catalyze of substrates. both Knoevenagel Recently, we reactions prepared and a series the ofoxidation bifunctional of substrates. catalysts ofRecently, the general we formula prepared Tris-LDH-X a series of4(PW bifunctional9)2 (X = Mn, catalysts Fe, Co, Ni,of the Cu andgeneral Zn) 10− 10− viaformula intercalating Tris‐LDH [X‐X4(H4(PW2O)9)22(PW (X =9 OMn,34) 2Fe,] Co,anions Ni, Cu into and the Zn) Tris-modified via intercalating LDH [X using4(H2O) the2(PW host9O34 layer)2] modificationanions into the method Tris‐modified reported LDH above using [65]. the Among host layer all the modification members ofmethod the designed reported bifunctional above [65]. catalysts,Among all the the Tris-LDH-Zn members of4 the(PW designed9)2 composite bifunctional revealed catalysts, the highest the Tris activity‐LDH and‐Zn4(PW selectivity9)2 composite in the tandemrevealed reaction the highest for the activity preparation and of selectivity benzylidene in ethylthe cyanoacetatetandem reaction via an for oxidation-Knoevenagel the preparation of condensationbenzylidene ethyl between cyanoacetate benzyl alcoholvia an oxidation and ethyl‐Knoevenagel cyanoacetate condensation under mild between and solvent-base-free benzyl alcohol conditions.and ethyl cyanoacetate As shown under in Figure mild 10 and, initially, solvent the‐base formation‐free conditions. of benzaldehyde As shown in can Figure be promoted 10, initially, by the POMformation species of inbenzaldehyde the presence can of H be2O promoted2 via oxidation by the of POM benzyl species alcohol. in Simultaneously,the presence of theH2O basic2 via sitesoxidation from of Tris benzyl and LDH alcohol. layers Simultaneously, promote the activation the basic sites of ethyl from cyanoacetate Tris and LDH to formlayers a promote nucleophile, the whichactivation rapidly of ethyl reacts cyanoacetate with the benzaldehyde to form a nucleophile, leading to the which formation rapidly of the reacts corresponding with the benzaldehyde benzylidene ethylleading cyanoacetate to the formation product. of the corresponding benzylidene ethyl cyanoacetate product.

Figure 10. Schematic of the reaction pathway of the one‐pot tandem reaction catalyzed by Figure 10. Schematic of the reaction pathway of the one-pot tandem reaction catalyzed by Tris-LDH-Zn4 Tris‐LDH‐Zn4 (PW9)2. Reproduced from Ref. [65], Copyright 2016, WILEY‐VCH. (PW9)2. Reproduced from Ref. [65], Copyright 2016, WILEY-VCH.

The poor mass transfer between the active components of the intercalated catalysts and organic The poor mass transfer between the active components of the intercalated catalysts and organic substrates is one of the main challenges that limit their further application. On the basis of substrates is one of the main challenges that limit their further application. On the basis of summarizing summarizing the traditional intercalation catalysts of POMs and in order to improve the mass the traditional intercalation catalysts of POMs and in order to improve the mass transfer, we reported transfer, we reported the design and synthesis of a novel intercalation POM‐LDH catalyst the design and synthesis of a novel intercalation POM-LDH catalyst (Mg3Al-ILs-C8-LaW10) by (Mg3Al‐ILs‐C8‐LaW10) by intercalating Na9LaW10O36∙32H2O (LaW10) POM clusters into LDHs, which intercalating Na LaW O ·32H O (LaW ) POM clusters into LDHs, which have been covalently have been covalently9 modified10 36 with2 ionic10 liquids (ILs) [66]. As shown in Figure 11, the grafting of modiﬁed with ionic liquids (ILs) [66]. As shown in Figure 11, the grafting of ILs on the LDH not only ILs on the LDH not only induces flexibility to the catalyst but also allows easy accessibility of the induces ﬂexibility to the catalyst but also allows easy accessibility of the substrates to the active center substrates to the active center during the liquid–solid reaction system, which improves dramatically the mass transfer. The catalyst Mg3Al‐ILs‐C8‐LaW10 demonstrates high activity and selectivity for the epoxidation of various allylic alcohols in the presence of H2O2. For example, trans‐2‐hexen‐1‐ol undergoes up to 96% conversion and 99% selectivity for the formation of epoxide at 25 °C in 2.5 h. The Mg3Al‐ILs‐C8‐LaW10 composite material constitutes one of the most efficient heterogeneous catalysts for the epoxidation of allylic alcohols (including the hydrophobic allylic alcohols with long alkyl chains) reported so far.

Catalysts 2017, 7, 260 11 of 17 during the liquid–solid reaction system, which improves dramatically the mass transfer. The catalyst Mg3Al-ILs-C8-LaW10 demonstrates high activity and selectivity for the epoxidation of various allylic alcohols in the presence of H2O2. For example, trans-2-hexen-1-ol undergoes up to 96% conversion ◦ and 99% selectivity for the formation of epoxide at 25 C in 2.5 h. The Mg3Al-ILs-C8-LaW10 composite material constitutes one of the most efﬁcient heterogeneous catalysts for the epoxidation of allylic Catalystsalcohols 2017 (including, 7, 260 the hydrophobic allylic alcohols with long alkyl chains) reported so far. 11 of 16

Figure 11. Top Top:: schematic schematic diagram diagram of of the the catalytic catalytic process; Bottom:: the the water water contact contact angle

measurements of: (a)) MgMg33Al-LaWAl‐LaW1010;;( (b) MgMg33Al-ILs-CAl‐ILs‐C4‐-LaWLaW1010; ;((cc)) Mg3AlAl-ILs-C‐ILs‐C88‐-LaWLaW1010; ;and and ( (dd)) Mg3AlAl-ILs-C‐ILs‐C1212‐LaW-LaW1010 ReproducedReproduced from from Ref. Ref. [66], [66], Copyright Copyright 2017, 2017, WILEY WILEY-VCH.‐VCH.

3.4. Acid Acid-Base‐Base Synergistic Synergistic Catalyst Aldoximes and ketoximes are valuable chemical and biopharmaceutical intermediates that are widely appliedapplied in in chemical chemical synthesis synthesis [67 ],[67], and theyand arethey usually are usually synthesised synthesised by oximation by oximation of aldehydes of aldehydesor ketones usingor ketones mineral using salt ofmineral hydroxylamine, salt of hydroxylamine, such as NH2OH such·HCl as or NH NH2OH2OH∙HCl·H2 SOor 4NH[682].OH However,∙H2SO4 [68].the abovementioned However, the abovementioned pathway suffers frompathway two drawbacks:suffers from the two high drawbacks: cost of hydroxylamine the high cost along of hydroxylaminewith the increasing along environmental with the increasing concern environmental accompanied concern with the accompanied disposal of with co-produced the disposal large of coamounts‐produced of inorganic large amounts salts [69 of]. inorganic To overcome salts such [69]. problems, To overcome many such efforts problems, have been many dedicated efforts tohave the beendeveloppment dedicated of to salt-free the developpment oximation methods of salt‐free using oximation aqueous methods H2O2 as anusing oxidant aqueous in the H oximation2O2 as an oxidantreactions in of the aldehydes oximation to reactions aldoximes of and aldehydes Ketoximes to aldoximes [70,71]. An and additional Ketoximes key [70,71]. issue associated An additional with keythe formationissue associated of aldoximes with the is thatformation they can of undergoaldoximes an is acid-catalyzed that they can dehydration undergo an toacid yield‐catalyzed nitriles dehydrationor go through to anyield acid-catalyzed nitriles or go Beckmann through rearrangementan acid‐catalyzed to formBeckmann amides, rearrangement leading to decreased to form amides,product leading selectivity. to decreased product selectivity. ItIt has beenbeen observedobserved that that the the water-soluble water‐soluble POM POM cluster cluster Na Na12[WZn12[WZn3(H3(H2O)2O)2[ZnW2[ZnW9O9O3434))22]]·∙46H2O can act as an efficient efﬁcient catalyst to promote the oximation of aldehydes in the presence of H2OO22 [72].[72]. However, thethe inherent inherent acidity acidity of POMs of canPOMs promote can sidepromote reactions side and reactions induce further and induce transformations. further transformations.Therefore, achieving Therefore, high selectivity achieving during high theselectivity oximation during reaction the ofoximation aromatic reaction aldehydes of remainsaromatic a aldehydeschallenging remains task. Li anda challenging co-authors [task.37,73 Li] reported and co‐ thatauthors the self-assembled [37,73] reported POM-intercalated that the self‐assembled LDHs are POMactive‐intercalated catalysts for LDHs the epoxidation are active reactions, catalysts andfor the the basicepoxidation hydroxyl reactions, groups ofand the the LDHs basic can hydroxyl prevent groupssubsequently of the the LDHs acid-catalyzed can prevent ring subsequently opening hydrolysis the acid of‐catalyzed the epoxides. ring Inspired opening by hydrolysis this, we prepared of the epoxides.three POM-intercalated Inspired by LDHsthis, we of Znprepared3Al-Zn5 WO,three Zn POM3Al-Zn‐intercalated2Mn3WO andLDHs Zn 3Al-Znof Zn2Fe3Al3‐WOZn5WO, and Zn3Al‐Zn2Mn3WO and Zn3Al‐Zn2Fe3WO and studied their efficiency in the oximation of aromatic aldehydes in the presence of H2O2 in the absence of organic solvents [7]. Interestingly, utilization of the POM‐intercalated catalyst for the oximation reaction of aromatic aldehydes revealed an improvement of the selectivity compared to the homogeneous catalytic system promoted by the aqueous POMs solution. The enhanced selectivity of the desirable products is attibuted to the cooperative effect between the components of the composite materials which modulate the inherent acidity of the POM clusters, which would otherwise promote the production of amides and nitriles via acid‐catalyzed side reactions. The reaction pathway is illustrated in Figure 12.

Catalysts 2017, 7, 260 12 of 17

studied their efﬁciency in the oximation of aromatic aldehydes in the presence of H2O2 in the absence of organic solvents [7]. Interestingly, utilization of the POM-intercalated catalyst for the oximation reaction of aromatic aldehydes revealed an improvement of the selectivity compared to the homogeneous catalytic system promoted by the aqueous POMs solution. The enhanced selectivity of the desirable products is attibuted to the cooperative effect between the components of the composite materials which modulate the inherent acidity of the POM clusters, which would otherwise promote the production of amides and nitriles via acid-catalyzed side reactions. The reaction pathway is

Catalystsillustrated 2017, in 7, 260 Figure 12. 12 of 16

FigureFigure 12. SchematicSchematic of of the the synergistic synergistic effect effect of of LDH LDH-POM‐POM catalysts catalysts for for promotion promotion oximation oximation of of aromaticaromatic aldehydes aldehydes [7]. [7]. Adapted Adapted from from Ref. Ref. [12], [12], Copyright 2014, Elsevier.

TheThe cyanosilylationcyanosilylation of of aldehydes aldehydes or ketonesor ketones with with trialkylsilyl trialkylsilyl cyanides cyanides is an example is an ofexample important of importantreactions in reactions industry andin syntheticindustry chemistry.and synthetic The cyanosilylation chemistry. The promotes cyanosilylation the formation promotes of new C-Cthe formationbonds while of protecting new C‐C thebonds hydroxo while functional protecting groups the hydroxo [74]. Moreover, functional the correspondinggroups [74]. Moreover, cyanohydrins the correspondingproducts can be cyanohydrins transformed products into a variety can be of transformed building blocks, into such a variety as β-amino of building alcohols, blocks,α-hydroxy such as β‐ketonesamino andalcohols,α-hydroxy α‐hydroxy acids [ketones75], which and are α‐ widelyhydroxy applied acids in[75], synthetic which chemistry. are widely Hence, applied many in syntheticefforts have chemistry. been dedicated Hence, many to investigating efforts have the been cyanosilylation dedicated to reaction, investigating where the both cyanosilylation acid [76] and reaction,base catalysts where [ 77both,78 ]acid were [76] utilized and base to promotecatalysts the[77,78] cyanosilylation were utilized reaction. to promote Therefore, the cyanosilylation fabrication reaction.of the acid–base Therefore, synergistic fabrication catalyst of the is a acid–base promising synergistic method to developcatalyst systemsis a promising that could method catalyze to developefﬁciently systems the cyanosilylation that could catalyze reaction. efficiently Recently, the cyanosilylation we prepared a seriesreaction. of MgRecently,3Al-LnW we10 prepared(Ln = Dy, a seriesEu, Tb) of catalystsMg3Al‐LnW employing10 (Ln = Dy, the Eu, exfoliation Tb) catalysts assembly employing method the [44 exfoliation]. The temperature-programmed assembly method [44]. Thedesorption temperature of carbon‐programmed dioxide (CO desorption2-TPD) and of ammoniacarbon dioxide (NH3 -TPD)(CO2‐TPD) demonstrated and ammonia that the (NH resulting3‐TPD) demonstratedPOM-intercalated that catalysts the resulting possess POM both‐intercalated acid and basic catalysts properties. possess As both such, acid the Mg and3Al-EuW basic properties.10 catalyst Ashas such, been the successfully Mg3Al‐EuW employed10 catalyst in the has catalysis been successfully of cyanosilylation employed of variousin the catalysis aldehydes of cyanosilylation or ketones with ofexcellent various catalytic aldehydes activity or ketones under with solvent-free excellent conditions. catalytic activity The Mg under3Al-EuW solvent10 can‐free be easilyconditions. separated The Mgand3Al reused‐EuW10 for can 10 be times easily without separated obvious and decrease reused for of the10 times catalytic without performance; obvious thedecrease composition of the catalyticand structure performance; of Mg3Al–EuW the composition10 catalyst and remained structure stable of Mg during3Al–EuW the catalytic10 catalyst cycle. remained In addition, stable duringthe Mg 3theAl–EuW catalytic10 catalyzed cycle. In cyanosilylationaddition, the Mg of3 hexanalAl–EuW and10 catalyzed benzaldehyde cyanosilylation revealed the of highesthexanal TON and benzaldehydeof 119906 and 119950,revealed respectively. the highest TON of 119906 and 119950, respectively.

4.4. Conclusions Conclusions-Perspectives‐Perspectives InIn summary, summary, over recent recent decades, several different methods have been developed developed for for the the design design ofof POM-LDH-intercalatedPOM‐LDH‐intercalated composites composites by by various various research research groups groups in an in effort an toeffort overcome to overcome the inherent the inherentchallenges challenges associated associated with the process: with the (i) process: Some POMs (i) Some are hydrolytically POMs are hydrolytically unstable under unstable weakly under acidic weaklyto basic acidic pH environment to basic pH leading environment to formation leading of to impurities formation during of impurities the intercalation during the reactions; intercalation (ii) the reactions;intercalation (ii) processthe intercalation is closely process related tois closely the charge related density, to the size, charge and density, geometry size, of the and POM geometry clusters; of the(iii) POM leaching clusters; of the (iii) metal leaching ions (M of IItheor metal MIII) from ions the(MII host or M layerIII) from can the occur host under layer neutral can occur to slightly under neutral to slightly acidic environments. In contrast to the traditional methods, we recently developed the host layer modification and exfoliation assembly methods demonstrating the advantages of these approaches and effectively tackling the aforementioned limitations to some extent. These two new methods not only modulate the host‐guest interactions and overcome its spatial and geometrical restrictions of POMs (such as the intercalation of Keggin species, [PW12O40]3−, of low negative charge) but also prevent the leaching of MgII/AlIII cations during the intercalation process promoting the formation of the POM‐LDH composite without co‐formation of the impurity phase. The POM‐LDH composites have been widely utilized successfully in different catalytic processes, such as oxidation catalysis, acid catalysis and acid‐base synergistic catalysis etc. The as‐prepared POM‐LDH composites show superior catalytic performance comparing to the relevant performance of its parent components. This is due to synergistic effects arising from intermolecular

Catalysts 2017, 7, 260 13 of 17 acidic environments. In contrast to the traditional methods, we recently developed the host layer modification and exfoliation assembly methods demonstrating the advantages of these approaches and effectively tackling the aforementioned limitations to some extent. These two new methods not only modulate the host-guest interactions and overcome its spatial and geometrical restrictions of 3− POMs (such as the intercalation of Keggin species, [PW12O40] , of low negative charge) but also prevent the leaching of MgII/AlIII cations during the intercalation process promoting the formation of the POM-LDH composite without co-formation of the impurity phase. The POM-LDH composites have been widely utilized successfully in different catalytic processes, such as oxidation catalysis, acid catalysis and acid-base synergistic catalysis etc. The as-prepared POM-LDH composites show superior catalytic performance comparing to the relevant performance of its parent components. This is due to synergistic effects arising from intermolecular interactions such as electrostatic and network hydrogen bonds, which prevent agglomeration and enhance the overall performance. The active components are uniformly dispersed in the 2D confined space defined by the LDH layers. Additionally, the design of the robust confined space in combination with the intermolecular interactions induces further stability to the system, prevents leaching of the POM species into the reaction mixture and increases the catalyst’s selectivity. Further investigation and expansion of the family of POM-LDH composite materials will offer great opportunities for the development of economical and versatile methods for designing specific structural features that could drive reactions towards the desirable direction and modulate the electron transfer properties of the POM-LDH intercalated composite in a controlled manner. Finally, the exploration of alternative design approaches will pave the way for the construction of POM-LDHs composites that exhibit new chemical and physical properties, emergence of functionalities as well as broaden the catalytic applications of this family of novel materials.

Acknowledgments: This research was supported by the National Basic Research Program of China (973 program, 2014CB932104), National Nature Science Foundation of China (U1407127, U1507102, 21521005, 21625101), International Joint Graduate-Training Program of Beijing University of Chemical Technology, and Fundamental Research Funds for the Central Universities (ZY1709). H.N.M. acknowledges the financial support from the University of Glasgow. Author Contributions: Y.-F.S., and H.N.M. conceived and designed the structure of the review article. T.F.L. conducted the literature search and collected the relevant material. T.F.L., Y.-F.S. and H.N.M co-write and revised the article. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Fan, G.L.; Li, F.; Evans, D.G.; Duan, X. Catalytic applications of layered double hydroxides: Recent advances and perspectives. Chem. Soc. Rev. 2014, 43, 7040–7066. [CrossRef][PubMed] 2. Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124–4155. [CrossRef][PubMed] 3. Sideris, P.J.; Nielsen, U.G.; Gan, Z.; Grey, C.P. Mg/Al Ordering in Layered Double Hydroxides Revealed by Multinuclear NMR Spectroscopy. Science 2008, 321, 113–117. [CrossRef][PubMed] 4. Yin, H.J.; Tang, Z.Y. Ultrathin two-dimensional layered metal hydroxides: An emerging platform for advanced catalysis, energy conversion and storage. Chem. Soc. Rev. 2016, 45, 4873–4891. [CrossRef] [PubMed] 5. Williams, G.R.; Khan, A.I.; O’Hare, D. Mechanistic and Kinetic Studies of Guest Ion Intercalation into Layered Double Hydroxides Using Time-resolved, in situ X-ray Powder Diffraction. Struct. Bond. 2006, 119, 161–192. 6. Evans, D.G.; Slade, R.C.T. Structural Aspects of Layered Double Hydroxides. Struct. Bond. 2006, 119, 1–87. 7. Zhao, S.; Xu, J.H.; Wei, M.; Song, Y.-F. Synergistic catalysis by polyoxometalate-intercalated layered double hydroxides: Oximation of aromatic aldehydes with large enhancement of selectivity. Green Chem. 2011, 13, 384–389. [CrossRef] 8. Guo, X.X.; Zhang, F.Z.; Xu, S.L.; Evans, D.G.; Duan, X. Preparation of layered double hydroxide ﬁlms with different orientations on the opposite sides of a glass substrate by in situ hydrothermal crystallization. Chem. Commun. 2009, 28, 6836–6838. [CrossRef][PubMed] Catalysts 2017, 7, 260 14 of 17

9. Evans, D.G.; Duan, X. Preparation of layered double hydroxides and their applications as additives in polymers, as precursors to magnetic materials and in biology and medicine. Chem. Commun. 2006, 485–496. [CrossRef][PubMed] 10. Xu, Z.P.; Stevenson, G.S.; Lu, C.Q.; Lu, G.Q.; Bartlett, P.F.; Gray, P.P. Stable Suspension of Layered Double Hydroxide Nanoparticles in Aqueous Solution. J. Am. Chem. Soc. 2006, 128, 36–37. [CrossRef][PubMed] 11. Rives, V.; Ulibarri, M.A. Layered double hydroxides (LDH) intercalated with metal coordination compounds and oxometalates. Coord. Chem. Rev. 1999, 181, 61–120. [CrossRef] 12. Omwoma, S.; Chen, W.; Tsunashima, R.; Song, Y.-F. Recent advances on polyoxometalates intercalated layered double hydroxides: From synthetic approaches to functional material applications. Coord. Chem. Rev. 2014, 258–259, 58–71. [CrossRef] 13. Duan, X.; Zhang, F.Z. Intercalation Assembly and Functional Materials; Chemical Industry Press: Beijing, China, 2007. 14. Theiss, F.L.; Ayoko, G.A.; Frost, R.L. Synthesis of layered double hydroxides containing MgII, ZnII, CaII and AlIII layer cations by co-precipitation methods—A review. Appl. Surf. Sci. 2016, 383, 200–213. [CrossRef] 15. Li, K.W.; Kumada, N.; Yonesaki, Y.; Takei, T.; Kinomura, N.; Wang, H.; Wang, C. The pH effects on the formation of Ni/Al nitrate form layered double hydroxides (LDHs) by chemical precipitation and hydrothermal method. Mater. Chem. Phys. 2010, 121, 223–229. [CrossRef] 16. Inayat, A.; Klumpp, M.; Schwieger, W. The urea method for the direct synthesis of ZnAl layered double hydroxides with nitrate as the interlayer anion. Appl. Clay Sci. 2011, 51, 452–459. [CrossRef] 17. Qu, J.; Zhang, Q.W.; Li, X.W.; He, X.M.; Song, S.X. Mechanochemical approaches to synthesize layered double hydroxides: A review. Appl. Clay Sci. 2016, 119, 185–192. [CrossRef] 18. Zubaira, M.; Daud, M.; McKay, G.; Shehzad, F.; Al-Harthi, M.A. Recent progress in layered double hydroxides (LDH)-containing hybrids as adsorbents for water remediation. Appl. Clay Sci. 2017, 143, 279–292. [CrossRef] 19. Bi, X.; Zhang, H.; Dou, L.G. Layered Double Hydroxide-Based Nanocarriers for Drug Delivery. Pharmaceutics 2014, 6, 298–332. [CrossRef][PubMed] 20. Feng, Y.J.; Li, D.Q.; Wang, Y.; Evans, D.G.; Duan, X. Synthesis and characterization of a UV absorbent- intercalated Zn-Al layered double hydroxide. Polym. Degrad. Stab. 2006, 91, 789–794. [CrossRef] 21. Wang, X.; Sumboja, A.; Lin, M.F.; Yan, J.; Lee, P.S. Enhancing electrochemical reaction sites in nickel–cobalt layered double hydroxides on zinc tin oxide nanowires: A hybrid material for an asymmetric supercapacitor device. Nanoscale 2012, 4, 7266–7272. [CrossRef][PubMed] 22. Chen, Y.; Song, Y.-F. Highly Selective and Efﬁcient Removal of Cr(VI) and Cu(II) by the Chromotropic Acid-Intercalated Zn–Al Layered Double Hydroxides. Ind. Eng. Chem. Res. 2013, 52, 4436–4442. [CrossRef] 23. Kuroda, Y.; Miyamoto, Y.; Hibino, M.; Yamaguchi, K.; Mizuno, N. Tripodal Ligand-Stabilized Layered 2− Double Hydroxide Nanoparticles with Highly Exchangeable CO3 . Chem. Mater. 2013, 25, 2291–2296. [CrossRef] 24. Kuroda, Y.; Koichi, T.; Muramatsu, K.; Yamaguchi, K.; Mizuno, N.; Shimojima, A.; Wada, H.; Kuroda, K. Direct Synthesis of Highly Designable Hybrid Metal Hydroxide Nanosheets by Using Tripodal Ligands as One-Size-Fits-All Modiﬁers. Chem. Eur. J. 2017, 23, 5023–5032. [CrossRef][PubMed] 25. Oka, Y.; Kuroda, Y.; Matsuno, T.; Kamata, K.; Wada, H.; Shimojima, A.; Kuroda, K. Preparation of Mesoporous Basic Oxides through Assembly of Monodispersed Mg–Al Layered Double Hydroxide Nanoparticles. Chem. Eur. J. 2017, 23, 9362–9368. [CrossRef][PubMed] 26. Pinnavaia, T.J.; Chibwe, M.; Constantino, V.R.L.; Yun, S.K. Organic chemical conversions catalyzed by intercalated layered double hydroxides (LDHs). Appl. Clay Sci. 1995, 10, 117–129. [CrossRef] 27. Tatsumi, T.; Yamamoto, K.; Tajima, H.; Tominaga, H.-O. Shape selective epoxidation of alkenes catalysed by polyoxometalate-intercalated hydrotalcite. Chem. Lett. 1992, 21, 815–818. [CrossRef] 28. Song, Y.-F.; Tsunashima, R. Recent advances on polyoxometalate-based molecular and composite materials. Chem. Soc. Rev. 2012, 41, 7384–7402. [CrossRef][PubMed] 29. Miras, H.N.; Vilà-Nadal, L.; Cronin, L. Polyoxometalate based open-frameworks (POMOFs). Chem. Soc. Rev. 2014, 43, 5679–5699. [CrossRef][PubMed] 30. Wang, S.S.; Yang, G.Y. Recent advances in polyoxometalate-catalyzed reactions. Chem. Rev. 2015, 115, 4893–4962. [CrossRef][PubMed] 31. Zhou, Y.; Guo, Z.J.; Hou, W.; Wang, Q.; Wang, J. Polyoxometalate-based phase transfer catalysis for liquid–solid organic reactions: A review. Catal. Sci. Technol. 2015, 5, 4324–4335. [CrossRef] Catalysts 2017, 7, 260 15 of 17

32. Yamaguchi, K.; Yoshida, C.; Uchida, S.; Mizuno, N. Peroxotungstate immobilized on ionic liquid-modiﬁed silica as a heterogeneous epoxidation catalyst with hydrogen peroxide. J. Am. Chem. Soc. 2005, 127, 530–531. [CrossRef][PubMed] 33. Inumaru, K.; Ishihara, T.; Kamiya, Y.; Okuhara, T.; Yamanaka, S. Water-tolerant, highly active solid

acid catalysts composed of the Keggin-type polyoxometalate H3PW12O40 ιmmobilized in hydrophobic nanospaces of organomodiﬁed mesoporous silica. Angew. Chem. Int. Ed. 2007, 46, 7625–7628. [CrossRef] [PubMed] 34. Kwon, T.; Tsigdinos, G.A.; Pinnavaia, T.J. Pillaring of Layered Double Hydroxides (LDH’s) by Polyoxometalate Anions. J. Am. Chem. Soc. 1988, 110, 3653–3654. [CrossRef] 35. Yun, S.K.; Pinnavaia, T.J. Layered Double Hydroxides Intercalated by Polyoxometalate Anions with Keggin 6− 6− 10− (α-H2W12O40 ), Dawson (α-P2W18O62 ), and Finke (Co4(H2O)2(PW9O34)2 ) Structures. Inorg. Chem. 1996, 35, 6853–6860. [CrossRef][PubMed] 36. Liu, K.; Yao, Z.X.; Song, Y.-F. Polyoxometalates Hosted in Layered Double Hydroxides: Highly Enhanced Catalytic Activity and Selectivity in Sulfoxidation of Sulﬁdes. Ind. Eng. Chem. Res. 2015, 54, 9133–9141. [CrossRef] 37. Liu, P.; Wang, C.H.; Li, C. Epoxidation of allylic alcohols on self-assembled polyoxometalates hosted in

layered double hydroxides with aqueous H2O2 as oxidant. J. Catal. 2009, 262, 159–168. [CrossRef] 38. Narita, E.; Kaviratna, P.D.; Pinnavaia, T.J. Direct synthesis of a polyoxometallate-pillared layered double hydroxide by coprecipitation. J. Chem. Soc. Chem. Commun. 1993, 60–62. [CrossRef] 39. Jana, S.K.; Kubota, Y.; Takashi, T. Cobalt-substituted polyoxometalate pillared hydrotalcite: Synthesis and catalysis in liquid-phase oxidation of cyclohexanol with molecular oxygen. J. Catal. 2008, 255, 40–47. [CrossRef] 40. Li, T.F.; Wang, Z.L.; Xu, Y.Q.; Wei, C.; Song, Y.-F. Recent progress in polyoxometalate-intercalated layered double hydroxides composite materials. Sci. China Chem. 2017, 47, 451–464. [CrossRef] 41. Chen, Y.; Yan, D.P.; Song, Y.-F. Tris(hydroxymethyl)aminomethane modified layered double hydroxides greatly facilitate polyoxometalate intercalation. Dalton Trans. 2014, 43, 14570–14576. [CrossRef][PubMed] 42. Cho, S.; Kwag, J.; Jeong, S.; Baek, Y.; Kim, S. Highly Fluorescent and Stable Quantum Dot-Polymer-Layered Double Hydroxide Composites. Chem. Mater. 2013, 25, 1071–1077. [CrossRef] 43. Gunjakar, J.L.; Kim, T.W.; Kim, H.N.; Kim, I.Y.; Hwang, S.-J. Mesoporous Layer-by-Layer Ordered Nanohybrids of Layered Double Hydroxide and Layered Metal Oxide: Highly Active Visible Light Photocatalysts with Improved Chemical Stability. J. Am. Soc. Chem. 2011, 133, 14998–15007. [CrossRef] [PubMed] 44. Jia, Y.Q.; Zhao, S.; Song, Y.-F. The application of spontaneous flocculation for the preparation of lanthanide-containing polyoxometalates intercalated layered double hydroxides: Highly efficient heterogeneous catalysts for cyanosilylation. Appl. Catal. A Gen. 2014, 487, 172–180. [CrossRef] 45. Kuo, W.G. Decolorizing dye wastewater with Fenton’s reagent. Water Res. 1992, 26, 881–886. [CrossRef] 46. Chen, Y.; Yao, Z.X.; Miras, H.N.; Song, Y.-F. Modular Polyoxometalate-Layered Double Hydroxide Composites as Efficient Oxidative Catalysts. Chem. Eur. J. 2015, 21, 10812–10820. [CrossRef][PubMed] 47. Jiang, B.; Yang, H.W.; Zhang, L.H.; Zhang, R.Y.; Sun, Y.L.; Huang, Y. Efficient oxidative desulfurization of diesel fuel using amide-based ionic liquids. Chem. Eng. J. 2016, 283, 89–96. [CrossRef] 48. Xu, Y.Q.; Xuan, W.M.; Zhang, M.M.; Miras, H.N.; Song, Y.-F. A multicomponent assembly approach for the design of deep desulfurization heterogeneous catalysts. Dalton Trans. 2016, 45, 19511–19518. [CrossRef] [PubMed] 49. Xu, X.D.; Moulijn, J.A.; Ito, E.; Wagemans, R.; Makkee, M. Deep Desulfurization of Fossil Fuels by Air in the Absence of a Catalyst. ChemSusChem 2008, 1, 817–819. [CrossRef][PubMed] 50. Shannon, M.S.; Irvin, A.C.; Liu, H.Q.; Moon, J.D.; Hindman, M.S.; Turner, C.H.; Bara, J.E. Chemical and

Physical Absorption of SO2 by N-Functionalized Imidazoles: Experimental Results and Molecular-level Insight. Ind. Eng. Chem. Res. 2015, 54, 462–471. [CrossRef] 51. Kaernbach, W.; Kisielow, W.; Warzecha, L.; Miga, K.; Klecan, R. Inﬂuence of petroleum nitrogen compounds on hydrodesulphurization. Fuel 1990, 69, 221–224. [CrossRef] 52. Macaud, M.; Sévignon, M.; Favre-Réguillon, A.; Lemaire, M. Novel Methodology toward Deep Desulfurization of Diesel Feed Based on the Selective Elimination of Nitrogen Compounds. Ind. Eng. Chem. Res. 2004, 43, 7843–7849. [CrossRef] Catalysts 2017, 7, 260 16 of 17

53. Yao, Z.Y.; Miras, H.N.; Song, Y.-F. Efficient concurrent removal of sulfur and nitrogen contents from complex oil mixtures by using polyoxometalate-based composite materials. Inorg. Chem. Front. 2016, 3, 1007–1013. [CrossRef] 54. Bamoharram, F.F.; Heravi, M.M.; Roshani, M.; Tavakoli, N. N-oxidation of pyridine carboxylic acids using 14− hydrogen peroxide catalyzed by a green heteropolyacid catalyst: Preyssler’s anion, [NaP5W30O110] . J. Mol. Catal. A 2006, 252, 219–225. [CrossRef] 55. Malkov, A.V.; Bell, M.; Castelluzzo, F.; Koˇcovský, P. METHOX: A New Pyridine N-Oxide Organocatalyst for the Asymmetric Allylation of Aldehydes with Allyltrichlorosilanes. Org. Lett. 2005, 7, 3219–3222. [CrossRef] [PubMed] 56. Liu, K.; Yao, Z.X.; Miras, H.N.; Song, Y.-F. Facile Immobilization of a Lewis Acid Polyoxometalate onto Layered Double Hydroxides for Highly Efficient N-Oxidation of Pyridine-Based Derivatives and Denitrogenation. ChemCatChem 2015, 7, 3903–3910. [CrossRef] 57. Corma, A.; Ródenas, T.; Sabater, M.J. Monoalkylations with alcohols by a cascade reaction on bifunctional solid catalysts: Reaction kinetics and mechanism. J. Catal. 2011, 279, 319–327. [CrossRef] 58. Liu, P.; Li, C.; Hensen, E.J. Efficient Tandem Synthesis of Methyl Esters and Imines by Using Versatile Hydrotalcite-Supported Gold Nanoparticles. Chem. Eur. J. 2012, 18, 12122–12129. [CrossRef][PubMed] 59. Zhu, F.X.; Wang, W.; Li, H.X. Water-Medium and Solvent-Free Organic Reactions over a Bifunctional Catalyst

with Au Nanoparticles Covalently Bonded to HS/SO3H Functionalized Periodic Mesoporous Organosilica. J. Am. Chem. Soc. 2011, 133, 11632–11640. [CrossRef][PubMed] 60. Felpin, F.X.; Fouquet, E. Heterogeneous Multifunctional Catalysts for Tandem Processes: An Approach toward Sustainability. ChemSusChem 2008, 1, 718–724. [CrossRef][PubMed] 61. Elhamifar, D.; Kazempoor, S.; Karimi, B. Amine-functionalized ionic liquid-based mesoporous organosilica as a highly efﬁcient nanocatalyst for the Knoevenagel condensation. Catal. Sci. Technol. 2016, 6, 4318–4326. [CrossRef] 62. Zhang, L.N.; Wang, H.; Shen, W.Z.; Qin, Z.F.; Wang, J.G.; Fan, W.B. Controlled synthesis of graphitic carbon nitride and its catalytic properties in Knoevenagel condensations. J. Catal. 2016, 344, 293–302. [CrossRef]

63. Zhao, S.; Chen, Y.; Song, Y.-F. Tri-lacunary polyoxometalates of Na8H[PW9O34] as heterogeneous Lewis base catalysts for Knoevenagel condensation, cyanosilylation and the synthesis of benzoxazole derivatives. Appl. Catal. A Gen. 2014, 475, 140–146. [CrossRef] 64. Ezugwu, C.I.; Mousavi, B.; Asraf, M.A.; Luo, Z.X.; Verpoort, F. Post-synthetic modified MOF for Sonogashira cross-coupling and Knoevenagel condensation reactions. J. Catal. 2016, 344, 445–454. [CrossRef] 65. Liu, K.; Xu, Y.Q.; Yao, Z.Y.; Miras, H.N.; Song, Y.-F. Polyoxometalate-Intercalated Layered Double Hydroxides as Efficient and Recyclable Bifunctional Catalysts for Cascade Reactions. ChemCatChem 2016, 8, 929–937. [CrossRef] 66. Li, T.F.; Wang, Z.L.; Chen, W.; Miras, H.N.; Song, Y.-F. Rational Design of a Polyoxometalate Intercalated Layered Double Hydroxide: Highly Efficient Catalytic Epoxidation of Allylic Alcohols under Mild and Solvent-Free Conditions. Chem. Eur. J. 2017, 23, 1069–1077. [CrossRef][PubMed] 67. Xu, X.D.; Henninger, T.; Abbanat, D.; Bush, K.; Foleno, B.; Hilliard, J.; Macielag, M. Synthesis and antibacterial activity of C2-fluoro, C6-carbamate ketolides, and their C9-oximes. Bioorg. Med. Chem. Lett. 2005, 15, 883–887. [CrossRef][PubMed] 68. Jain, N.; Kumar, A.; Chauhan, S.M.S. Metalloporphyrin and heteropoly acid catalyzed oxidation of C=NOH bonds in an ionic liquid: Biomimetic models of nitric oxide synthase. Tetrahedron Lett. 2005, 46, 2599–2602. [CrossRef] 69. Bars, J.L.; Dakka, J.; Sheldon, R.A. Ammoximation of cyclohexanone and hydroxyaromatic ketones over titanium molecular sieves. Appl. Catal. A Gen. 1996, 136, 69–80. [CrossRef] 70. Song, F.; Liu, Y.M.; Wu, H.H.; He, M.Y.; Wu, P.; Tatsumi, T. A novel titanosilicate with MWW structure: Highly effective liquid-phase ammoximation of cyclohexanone. J. Catal. 2006, 237, 359–367. [CrossRef] 71. Li, Z.H.; Chen, R.Z.; Xing, W.H.; Jin, W.Q.; Xu, N.P. Continuous Acetone Ammoximation over TS-1 in a Tubular Membrane Reactor. Ind. Eng. Chem. Res. 2010, 49, 6309–6316. [CrossRef] 72. Sloboda-Rozner, D.; Neumann, R. Aqueous biphasic catalysis with polyoxometalates: Oximation of ketones and aldehydes with aqueous ammonia and hydrogen peroxide. Green Chem. 2006, 8, 679–681. [CrossRef] Catalysts 2017, 7, 260 17 of 17

73. Liu, P.; Wang, H.; Feng, Z.C.; Ying, P.L.; Li, C. Direct immobilization of self-assembled polyoxometalate catalyst in layered double hydroxide for heterogeneous epoxidation of oleﬁns. J. Catal. 2008, 256, 345–348. [CrossRef] 74. Brunel, J.M.; Holmes, I.P. Chemically Catalyzed Asymmetric Cyanohydrin Syntheses. Angew. Chem. Int. Ed. 2004, 43, 2752–2778. [CrossRef][PubMed] 75. Sreekanth, P.; Kim, S.-W.; Hyeon, T.; Kim, B.M. A Novel Mesoporous Silica-Supported Lewis Acid Catalyst for C-C Bond Formation Reactions in Water. Adv. Synth. Catal. 2003, 345, 936–938. [CrossRef] 76. Procopio, A.; Das, G.; Nardi, M.; Oliverio, M.; Pasqua, L. A Mesoporous ErIII-MCM-41 Catalyst for the Cyanosilylation of Aldehydes and Ketones under Solvent-free Conditions. ChemSusChem 2008, 1, 916–919. [CrossRef][PubMed] 77. Song, J.J.; Gallou, F.; Reeves, J.T.; Tan, Z.L.; Yee, N.K.; Senanayake, C.H. Activation of TMSCN by N-Heterocyclic Carbenes for Facile Cyanosilylation of Carbonyl Compounds. J. Org. Chem. 2006, 71, 1273–1276. [CrossRef][PubMed] 78. Denmark, S.E.; Chung, W.J. Lewis Base Catalyzed Addition of Trimethylsilyl Cyanide to Aldehydes. J. Org. Chem. 2006, 71, 4002–4005. [CrossRef][PubMed]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).