Metatungstate and tungstoniobate-containing LDHs: Preparation, characterisation and activity in epoxidation of cyclooctene
Introduction
Layered double hydroxides (LDHs) or hydrotalcite-type solids are lamellar materials that structurally consist of brucite-like layers where a partial divalent/trivalent substitution has taken place, thus providing a positive charge to the layers, which is balanced by anions located, together with water molecules, in the interlayers [1], [2]. These interlayer anions can be easily exchanged and following this route several hydrotalcite-like materials intercalated with different sorts of anions (organic or inorganic) have been prepared during the last decades [3], [4]. Upon intercalation of POMs between the brucite-like layers of LDH solids, not only an improvement in their thermal stability is achieved, but also acid and redox sites are also developed in the basic hydrotalcite structure, making them suitable catalysts in some oxidation reactions [4], [5].
Catalytic epoxidation of alkenes is deserving special interest not only from an academic point of view, but also from an industrial perspective, since valuable intermediates are produced through this reaction. Although homogeneous catalysts have already proved their efficiency in this process, there is still a huge interest in the research of solid materials able to catalyse epoxidation easily using available oxidants, such as hydrogen peroxide or organic peroxides, and environmentally friendly solvents. Solids such as Amberlite, MCM-41 or SiO2 loaded with different metal cations (e.g., Ti, Mn, W, V, Mo, etc.) have been tested and showed high selectivities for the epoxidation of several alkenes and/or unsaturated alcohols [6]. Due to their acidic and redox properties, many polyoxometalates (POMs), mainly those containing Mo, W or V [7], [8], have also been used in these processes, either on their own, supported over SiO2, MCM or titania, or intercalated in LDHs.
Some authors have reported the benefits of using different LDHs intercalated with POMs, mainly based on molybdenum or tungsten, as heterogeneous catalysts in some epoxidation reactions [9], [10], [11], [12], [13]. Tatsumi et al. [9] have found a high selectivity for the epoxidation of 2-hexene when they used LDH–POM (paramolybdate or paratungstate) as catalyst and they propose that the substrate (alkene) enters into the interlayer to interact with the reactive sites of intercalated POM units. However, Gardner et al. [10] confirmed that the temperature at which the samples used by Tatsumi are dried destroys the layered structure and consequently the active catalysts are the decomposition products. Moreover, even if the LDH–POM structure were retained, the increase in the selectivity cannot be produced through the insertion of the substrate in the interlayer because of the limited access to the solvated anions in the gallery under the reaction conditions; so these authors assigned the increased selectivity to other facts, such as a selective adsorption based on the substrate polarity. Watanabe et al. [11] tested Zn3Al–SiW11O398− and Zn3Al-SiW12O404− LDHs (where Zn3Al stands for an LDH containing Zn and Al in a 3:1 molar ratio in the brucite-like layers) in the epoxidation of cyclohexene using H2O2 or O2 as oxidants and they observed a higher selectivity to the epoxide when using the first LDH if hydrogen peroxide was used, and the results are the opposite when using O2 instead H2O2 as the oxidant. Palomeque et al. [14] carried out the insertion of hydrophylic paratungstate anions and two hydrophobic anion species (derived from the complexation of peroxotungstenic acid with phenyl and dodecylphosphonic acids) into hydrotalcite and tested them in the epoxidation of cyclohexene, concluding that an allylic oxidation was the main reaction when intercalating the phosphonatotungstate compounds.
Recently, some of us have studied the LDH systems ZnAl, MgAl and NiAl-heptamolybdate in the epoxidation of bicycloalkenes, and showed that these materials are suitable catalysts for this process. The selectivity was found to depend on the hydrotalcite composition as well as on the nature of the solvent [13].
In this work, we report the synthesis of LDHs containing Mg or Zn and Al within the layers, intercalated with H2W12O406− or W4Nb2O194− anions which have been prepared by the ion exchange method, starting from the corresponding nitrate-intercalated precursor. The materials have been characterised by means of element chemical analyses, powder X-ray diffraction (PXRD), Fourier Transform infrared (FT-IR) spectroscopy, thermal analyses (TG and DTA) and nitrogen adsorption–desorption at −196 °C for surface texture assessment, and tested in the epoxidation of cyclooctene.
Section snippets
Catalyst preparation
All reagents used for the catalyst preparation were purchased from Panreac, except ammonium metatungstate, which was supplied by Fluka, and were used as received without any further purification.
Hydrotalcites intercalated with different POMs were prepared by ion exchange starting from the corresponding nitrate precursors. These LDH–NO3 precursor samples were prepared following a procedure similar to that reported previously in the literature [15].
Catalyst characterisation
The results obtained from the element chemical analyses for the prepared catalysts as well as the calculated formulae are summarised in Table 1, and show that the layer composition is maintained after the exchange process in all cases. For all POM-intercalated samples the layer charge is well balanced by the POM charge, suggesting that the exchange was complete in all cases.
The PXRD patterns of the as-synthesised catalysts are included in Fig. 1. All of them show profiles, which are
Conclusions
In this work, hydrotalcite-type-layered systems with different metals within the layers (Al3+ and Mg2+ or Zn2+) and metatungstate (H2W12O406−) or tungstoniobate (W4Nb2O194−) in the interlayers were prepared by an ion exchange method. The formulae calculated for all systems together with the absence of other layered crystalline phases suggest that the exchange was complete in all cases. The intercalation of such large anions gives rise to gallery heights of 7.2 and 9.7 Å, much larger than that
Acknowledgements
Financial support from MEC-Spain (grant MAT2006-10800-C02-01) and ERDF are acknowledged. D.C. thanks a grant from Universidad de Salamanca. S.L. is grateful to the FCT-MCTES/Portugal for a post-doctoral grant.
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