Polyoxometalate (POM)-layered double hydroxides (LDH) composite materials: design and catalytic applications

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

Shrink wrapping redox-active crystals of polyoxometalate open frameworks with organic polymers via crystal induced polymerisation

We report examples of crystal surface modification of polyoxometalate open frameworks whereby the use of pyrrole or aniline as monomers leads to the formation of the corresponding polymers via an oxidative polymerization process initiated by the redox active POM scaffolds. Guest-exchange experiments demonstrate that the polymers can finely tune the guest exchange rate and their structural integrity is retained after the surface modifications. In addition, the formation of polyoxometalate-based self-fabricating tubes by the dissolution of Keggin-based network crystals were also modulated by the polymers, allowing a new type of hybrid inorganic polymer with an organic coating to be fabricated

Modular polyoxometalate-layered double hydroxide composites as efficient oxidative catalysts

The exploitation of intercalation techniques in the field of two-dimensional layered materials offers unique opportunities for controlling chemical reactions in confined spaces and developing nanocomposites with desired functionality. In this paper, we demonstrate the exploitation of the novel and facile ‘one-pot’ anion-exchange method for the functionalization of layered double hydroxides (LDHs). As a proof of concept, we demonstrate the intercalation of a series of polyoxometalate (POM) clusters, Na3[PW12O40]•15H2O (Na3PW12), K6[P2W18O62]•14H2O (K6P2W18), and Na9LaW10O36•32H2O (Na9LaW10) into tris(hydroxymethyl)amino-methane (Tris) modified layered double hydroxides (LDHs) under ambient conditions without the necessity of degassing CO2. Investigation of the resultant intercalated materials of Tris-LDHs-PW12 (1), Tris-LDH-P2W18 (2), and Tris-LDH-LaW10 (3) for the degradation of methylene blue (MB), rhodamine B (RB) and crystal violet (CV) has been carried out, where Tris-LDH-PW12 reveals the best performance in the presence of H2O2. Additionally, degradation of a mixture of RB, MB and CV by Tris-LDH-PW12 follows the order of CV > MB > RB, which is directly related to the designed accessible area of the interlayer space. Also, the composite can be readily recycled and reused at least ten cycles without measurable decrease of activity

Rational design of a polyoxometalate intercalated layered double hydroxide: highly efficient catalytic epoxidation of allylic alcohols under mild and solvent-free conditions

Intercalation catalysts, owing to their modular and accessible gallery and unique interlamellar chemical environment, have shown wide application in various catalytic reactions. However, the poor mass transfer between the active components of the intercalated catalysts and organic substrates is one of the challenges that limit their further application. Herein, we have developed a novel heterogeneous catalyst by intercalating the polyoxometalate (POM) of Na9LaW10O36⋅32 H2O (LaW10) into layered double hydroxides (LDHs), which have been covalently modified with ionic liquids (ILs). The intercalation catalyst 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 % epoxide selectivity at 25 °C in 2.5 h. To the best of our knowledge, 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

A multicomponent assembly approach for the design of deep desulfurization heterogeneous catalysts

Deep desulfurization is a challenging task and global efforts are focused on the development of new approaches for the reduction of sulfur-containing compounds in fuel oils. In this work, we have proposed a new design strategy for the development of deep desulfurization heterogeneous catalysts. Based on the adopted design strategy, a novel composite material of polyoxometalate (POM)-based ionic liquid-grafted layered double hydroxides (LDHs) was synthesized by an exfoliation/grafting/assembly process. The structural properties of the as-prepared catalyst were characterized using FT-IR, XRD, TG, NMR, XPS, BET, SEM and HRTEM. The heterogeneous catalyst exhibited high activity in deep desulfurization of DBT (dibenzothiophene), 4,6-DMDBT (4,6-dimethyldibenzothiophene) and BT (benzothiophene) at 70 °C in 25, 30 and 40 minutes, respectively. The catalyst can be easily recovered and reused at least ten times without obvious decrease of its catalytic activity. Such excellent sulfur removal ability as well as the cost efficiency of the novel heterogeneous catalyst can be attributed to the rational design, where the spatial proximity of the substrate and the active sites, the immobilization of ionic liquid onto the LDHs via covalent bonding and the recyclability of the catalyst are carefully considered

Classical Keggin intercalated into layered double hydroxides: facile preparation and catalytic efficiency in Knoevenagel condensation reaction

The family of polyoxometalate (POM) intercalated layered double hydroxides (LDHs) composite materials has shown great promise for the design of functional materials with numerous applications. It is known that intercalation of the classical Keggin polyoxometalate (POM) of [PW12O40]3- (PW12) into layered double hydroxides (LDHs) is very unlikely to take place by conventional ion exchange methods due to spatial and geometrical restrictions. In this paper, such intercalated compound of Mg0.73Al0.22(OH)2 [PW12O40]0.04•0.98H2O (Mg3Al-PW12) has been successfully obtained by adopting a spontaneous flocculation method. The Mg3Al-PW12 has been fully characterized using a wide range of methods (XRD, SEM, TEM, XPS, EDX, XPS, FT-IR, NMR, BET). XRD patterns of Mg3Al-PW12 exhibit no impurity phase usually observed next to the (003) diffraction peak. Subsequent application of the Mg3Al-PW12 as catalyst in Knoevenagel condensation reactions of various aldehydes and ketones with Z–CH2-Z‘ type substrates (ethyl-cyanoacetate and malononitrile) at 60 oC in mixed solvents (Vi-propanol:Vwater = 2 : 1) demonstrated highly efficient catalytic activity. The synergistic effect between the acidic and basic sites of the Mg3Al-PW12 composite proved to be crucial for the efficiency of the condensation reactions. Additionally, the Mg3Al-PW12 catalysed Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate demonstrated the highest turnover number (TON) of 47980 reported so far

Efficient concurrent removal of sulfur and nitrogen contents from complex oil mixtures by using polyoxometalate-based composite materials

The increasingly stringent regulations in relation to the environmental impact of employed industrial processes make compulsory the development of alternative routes towards the reduction of sulfur and nitrogen contents in large scale chemical mixtures. Herein, we demonstrate for the first time the highly efficient application of polyoxometalate (POMs)/layered double hydroxide (LDHs) composites in deep desulfurization (1000 ppm) and denitrogenation (100 ppm) of a complex model oil system under mild conditions (65 °C), with a corresponding decrease of the content to less than 10 and 1 ppm, respectively. The high efficiency of the heterogeneous catalyst along with the high stability and easy recovery of the catalytic system renders them promising candidates for greener catalytic applications

Spontaneous formation of a chiral (Mo2O2S2)2+-based cluster driven by dimeric {Te2O6}-based templates

Utilization of [Mo2S2O2(H2O)6]2+ and a tellurite anion led to the formation of three new clusters, 1–3, with unique structural features. The tellurite anion not only templated the formation of [(Mo2O2S2)4(TeO3)(OH)9]3− 1 and [(Mo2O2S2)12(TeO3)4(TeO4)2 (OH)18]10− 3, but also the in situ generation of two different types of dimeric {Te2O6} based moieties induced the spontaneous assembly of the chiral [(Mo2O2S2)10(TeO3)(Te2O6)2(OH)18]8− anionic cluster, 2

Enabling single qubit addressability in a molecular semiconductor comprising gold-supported organic radicals

A bis(dithiolene)gold complex is presented as a model for an organic molecular electron spin qubit attached to a metallic surface that acts as a conduit to electrically address the qubit. A two-membered electron transfer series is developed of the formula [AuIII(adt)2]1−/0, where adt is a redox-active dithiolene ligand that is sequentially oxidized as the series is traversed while the central metal ion remains AuIII and steadfastly square planar. One-electron oxidation of diamagnetic [AuIII(adt)2]1− (1) produces an S = 1/2 charge-neutral complex, [AuIII(adt23−˙)] (2) which is spectroscopically and theoretically characterized with a near negligible Au contribution to the ground state. A phase memory time (TM) of 21 μs is recorded in 4 : 1 CS2/CCl4 at 10 K, which is the longest ever reported for a coordination complex possessing a third-row transition metal ion. With increasing temperature, TM dramatically decreases becoming unmeasurable above 80 K as a consequence of the diminishing spin-lattice (T1) relaxation time fueled by spin–orbit coupling. These relaxation times are 1–2 orders of magnitude shorter for the solid dilution of 2 in isoelectronic [Ni(adt)2] because this material is a molecular semiconductor. Although the conducting properties of this material provide efficient pathways to dissipate the energy through the lattice, it can also be used to electrically address the paramagnetic dopant by tapping into the mild reduction potential to switch magnetism “on” and “off” in the gold complex without compromising the integrity of its structure. These results serve to highlight the need to consider all components of these spintronic assemblies

Layered double hydroxide anchored ionic liquids as amphiphilic heterogeneous catalysts for the Knoevenagel condensation reaction

In recent years, great attention has been dedicated to the development of heterogeneous base catalysts providing a green and sustainable process in benign aqueous media. Herein, the ionic liquid modified layered double hydroxide (LDH) based catalysts of LDH-IL-Cn (n = 4, 8, 12) were prepared by adopting an exfoliation/assembly strategy, in which the ionic liquids (ILs) were covalently anchored onto the surface of LDH layers. The resulting LDH-IL-Cn were characterized by FT-IR, solid-state 13C NMR, 29Si CP/MAS NMR, 27Al-MAS NMR, XRD, TG-DTA, BET, XPS, SEM, CO2-TPD, contact angle experiments, etc. The catalytic performance of LDH-IL-C12 for the Knoevenagel condensation of a variety of aldehydes with ethyl cyanoacetate/malononitrile in H2O at room temperature showed excellent yields and selectivities. Moreover, the base catalyst of LDH-IL-C12 can be easily recycled and reused for at least 5 times without the decrease of its catalytic efficiency. The scaled-up experiments revealed that the catalyst retained its efficiency and robustness