Metatungstate and tungstoniobate-containing LDHs: Preparation, characterisation and activity in epoxidation of cyclooctene

https://doi.org/10.1016/j.jpcs.2007.05.012Get rights and content

Abstract

Polyoxometalates (POMs) H2W12O406− and W4Nb2O194− have been intercalated between the brucite-like layers of Mg, Al and Zn, Al hydrotalcites by anion exchange, starting from the corresponding nitrate precursors. The solids have been characterised by Powder X-ray Diffraction (PXRD), Fourier Transform infrared (FT-IR) spectroscopy, N2 adsorption–desorption at −196 °C and thermogravimetric (TG) and differential thermal analyses (DTA), and have been tested in the epoxidation of cyclooctene using H2O2 or t-BuOOH as oxidants. The results show that both anions are effectively located in the interlayer space maintaining their pristine structures without depolymerisation. Upon intercalation of such large anions microporosity is developed and subsequently an increase in the specific surface areas is also observed. In general, the prepared materials possess catalase and epoxidation activity, with ZnAl-intercalated H2W12O406− giving the best results in terms of epoxide yield (17% at 24 h). Product selectivity is different for the intercalated and free POMs, the latter yielding 1,2-cyclooctanediol as the only product, whereas the former produces only the epoxide. The epoxidation reaction seems to be catalysed in homogeneous phase by the POM.

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.

References (27)

  • V. Rives et al.

    Coord. Chem. Rev.

    (1999)
  • A. Vaccari

    Catal. Today

    (1998)
  • E. Gardner et al.

    Appl. Catal. A: Gen.

    (1998)
  • Y. Watanabe et al.

    J. Mol. Catal. A

    (1999)
  • D. Carriazo et al.

    Microporous Mesoporous Mater.

    (2006)
  • J. Palomeque et al.

    Appl. Catal. A: Gen.

    (2006)
  • M. del Arco et al.

    J. Solid State Chem.

    (2000)
  • M.R. Weir et al.

    Microporous Mesoporous Mater.

    (1998)
  • B.C. Lippens et al.

    J. Catal.

    (1965)
  • D.E. de Vos et al.

    Adv. Synth. Catal.

    (2003)
  • M.T. Pope

    Heteropoly and Isopoly Oxometalates

    (1983)
  • C.L. Hill

    Chem. Rev.

    (1998)
  • V. Rives

    Layered Double Hydroxides: Present and Future

    (2001)
  • Cited by (29)

    • Molybdate Stabilized Magnesium‐Iron Hydrotalcite Materials: Potential Catalysts for Isoeugenol to Vanillin and Olefin Epoxidation

      2021, Applied Catalysis A: General
      Citation Excerpt :

      The intercalation of different poly-oxometallates facilitated the development of catalysts with attractive acid bases and redox properties [10–14]. Poly-oxometallate-intercalated layered double hydroxides have been used for epoxidation, esterification, photo-degradation, peroxidation, and alkoxylation reactions [10–12]. To date, the limited studies are known on catalytic activity of molybdate-intercalated HT materials for organic processes, such as propane dehydrogenation [13], tertiary butanethiol oxidation [14], and styrene oxidation with air [15].

    • Synthesis and characterization of 12-tungstophosphoric acid intercalated layered double hydroxides and their application as esterification catalysts for deacidification of crude oil

      2017, Applied Clay Science
      Citation Excerpt :

      It is caused by the reaction between metal ions in the layer and HPW anions in the interlayer. Other researchers (Carriazo et al., 2007; Kwon and Pinnavaia, 1992) also found the similar thermal decomposition behaviors of Keggin ions intercalated MgAl and ZnAl LDH. For Ni2Al-PW, no exothermic peak in this temperature range is observed, which indicates that the products derived from the reaction of mixed metal oxides and interlayer PW12O403 − anions are stable, as reported by Ballesteros et al. (2008).

    • Comparative study of Keggin-type polyoxometalate pillared layered double hydroxides via two synthetic routes: Characterization and catalytic behavior in green epoxidation of cyclohexene

      2017, Applied Clay Science
      Citation Excerpt :

      It has been demonstrated that LDH are ideal inorganic supports for immobilizing bulky POM catalysts, as the interlayer spaces are very flexible to accommodate guest molecules of different sizes (Rives and Ulibarri, 1999). Over the past few decades, a variety of POM/LDH compounds have been synthesized by the conventional methods such as co-precipitation, ion-exchange and show excellent catalytic performance including selective oxidation of sulfide (Liu et al., 2015; Zhang et al., 2015), alkenes (Carriazo et al., 2007; Liu et al., 2008) and alcohols (Hasannia and Yadollahi, 2015). However, the relevant intercalation hybrids face two serious challenges: 1) it is almost impossible to obtain POM/LDH compounds with no impurity; (2) the oxidants generally are strong ones like H2O2, tert-butyl hydroperoxide.

    View all citing articles on Scopus
    View full text