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