In situ growth of layered double hydroxide films on anodic aluminum oxide/aluminum and its catalytic feature in aldol condensation of acetone

https://doi.org/10.1016/j.ces.2008.05.007Get rights and content

Abstract

Mg/Al layered double hydroxide (LDH) films were fabricated in situ with anodic aluminum oxide (AAO)/aluminum as both the substrate and the sole aluminum source by means of urea hydrolysis. The structure and morphology of the LDH films were investigated by X-ray diffraction (XRD), attenuated total reflectance-Fourier transform infrared (ATR-FT-IR) and scanning electron microscopy (SEM), which show that hexagonal plate-shaped LDH crystallites grow perpendicularly on the substrate via a strong chemical interaction. After being activated by a calcination/rehydration procedure, the rehydrated LDH (RLDH) platelets remain firmly immobilized on the AAO/aluminum substrate and retain the hexagonal platelet morphology of the LDH precursor. The catalytic activity of the resulting material was tested by using aldol condensation of acetone as a probe reaction, showing that the RLDH/AAO is superior in catalytic performance to the powdered RLDH analogue. The kinetic feature of this catalytic reaction was also analyzed, which suggests that it obeys the D1 kinetic model, a one-dimensional diffusion-controlled mechanism.

Introduction

Layered double hydroxides (LDHs, [M1-xIIMxIII(OH)2]x+(An-)x/n·yH2O) is a large class of materials consisting of positively charged brucite-like layers ([M1-xIIMxIII(OH)2]x+) and exchangeable interlayer anions (An-) with water (Braterman et al., 2004; Evans and Duan, 2006; Evans and Slade, 2006; Williams and O’Hare, 2006), which have potential applications in many fields (Cavani et al., 1991, Li and Duan, 2006). One of the most interesting features of these materials is their role as catalysts or precursors for catalysts. On calcination of the LDHs with compensating carbonate anions at about 773 K and subsequent rehydration of the calcined LDHs (CLDHs) at room temperature, the activated rehydrated LDHs (RLDHs), which are similar to the mineral meixnerite possessing the original brucite-like layers but with compensating anions being hydroxide rather than carbonate (Braterman et al., 2004), have base catalytic activity and have been studied as solid catalysts for Claisen–Schmidt condensation (Climent et al., 2004), Knoevenagel condensation (Kantam et al., 1998), Wittig (Sychev et al., 2001), Henry (Choudary et al., 1999), Michael addition (Ebitani et al., 2006) and aldol condensation (Tichit et al., 1998; Prinetto et al., 2000; Roelofs et al., 2000, Roelofs et al., 2001; Zhang et al., 2004; Abelló et al., 2005a, Abelló et al., 2005b; Winter et al., 2005) reactions. However, use of powder catalysts on an industrial scale gives rise to a number of problems, such as high pressure drop and difficult catalyst separation. They are therefore desired to be fabricated into robust macroscopic form to mitigate these problems. Immobilization of LDH films on a monolithic substrate is an ideal one of such possibilities. Such monolithic catalysts are the subject of increasing interest focused on their use in the areas of environmental protection and sustainable chemistry. For example, they are currently used in all car catalytic converters (Avila et al., 2005; Tomašić and Jović, 2006).

Recently, immobilization of layered double hydroxides (LDHs) has attracted considerable attention. There have been several reports of the preparation of LDH films on inorganic substrates, including deposition of LDH layers on mica from Langmuir–Blodgett films (He et al., 2001, He et al., 2002), deposition of extremely well oriented transparent films on glass from colloidal suspension of LDHs obtained through hydrolysis of LDHs containing methoxide anions (Gardner et al., 2001) and formation of a monolayer film of LDHs on Si (1 0 0) wafers with the LDH platelets having preferred orientation with their c-axis perpendicular to the substrate (Lee et al., 2003, Lee et al., 2004). Our group recently reported a simple method for the fabrication of highly ordered transparent self-standing LDH films (Wang et al., 2007). In general, the immobilization methods described above involve two steps; formation of the LDH aggregates in colloidal suspension as the first step, followed by deposition of the aggregates onto the inorganic substrate. As a result, the LDH films adhere relatively poorly to the substrate surface and are not sufficiently robust for application as monolithic catalysts. We have previously shown that oriented dense thin LDH films can be strongly attached to fully sulfonated polystyrene substrate (Lei et al., 2005), though the thermal stability of polystyrene is not sufficient to survive the calcination during the catalyst activation process. However, it shows that if LDHs are formed and immobilized in situ, much stronger forces can result between the LDHs and substrate. According to this principle, superhydrophobic NiAl-LDH films have been successfully prepared by using NH4OH/NH4NO3 as precipitation agent in our laboratory (Chen et al., 2006). Urea is a very weak Brønsted base (pKb=13.8), highly soluble in water (Costantino et al., 1998, Oh et al., 2002). The hydrolysis of urea gives ammonia and carbonate resulting in a pH of about 9 very suitable for the homogeneous precipitation of LDHs, and furthermore the rate of hydrolysis can be easily controlled by temperature. Herein, MgAl–CO3-LDH films are prepared by means of urea hydrolysis with anodic aluminum oxide (AAO)/aluminum, also known as porous alumina membrane, as both the substrate and sole aluminum source. After being activated, the catalytic properties of the resulting material are investigated by using the self-condensation of acetone as a probe reaction. The kinetic feature of this catalytic reaction is also discussed.

Section snippets

Preparation of the AAO/aluminum substrate

The AAO membrane was prepared by anodizing high purity aluminum foils (Patermarakis et al., 1999). Aluminum foils (Shanghai Jing Xi Chemical Technology Co., Ltd., purity: >99.5%, thickness: 0.1 mm) with a size of ca. 100×100mm were cleaned ultrasonically for 3 min, treated in a solution of NaOH for another 2 min, and then washed with deionized water. The freshly cleaned aluminum sheets were anodized in an electrolyte solution of 1 M H2SO4 with a lead cathode at a temperature of 298 K and a current

Structure and morphology of the LDH films

Fig. 1 shows the top- and edge-view SEM images of U-LDH/AAO. Slightly curved hexagonal platelets attached on, and approximately perpendicular to, the surface of the substrate were clearly observed. In order to determine the crystal structure of the hexagonal platelets, the U-LDH/AAO sample was studied by XRD. The XRD patterns of U-LDH/AAO and freshly anodized substrate are given in Fig. 2. The symmetric reflection peaks of 00l and 0kl planes, together with the 110 and 113 reflection peaks,

Conclusions

Thin MgAl–CO3-LDH films have been fabricated in situ with AAO/aluminum as both the substrate and sole aluminum resource by means of urea hydrolysis. SEM images show that the hexagonal plate-shaped LDH crystallites grow perpendicularly on the surface of the AAO/aluminum substrate. Activation of the as-prepared U-LDH/AAO sample by a calcination/rehydration procedure leads to, as expected, the interlayer carbonate anions being replaced by hydroxyl anions. The resulting RLDH platelets remain firmly

Acknowledgments

This work was financially supported by the Program for Chang Jiang Scholars and Innovative Research Team in University (Project no. IRT0406), the 111 Project (Project no. B07004), the National Science Foundation of China, the Program for New Century Excellent Talents in University, and Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University.

References (44)

  • J.M. Oh et al.

    The effect of synthetic conditions on tailoring the size of hydrotalcite particles

    Solid State Ionics

    (2002)
  • G. Patermarakis et al.

    Preparation of ultra-active alumina of designed porous structure by successive hydrothermal and thermal treatments of porous anodic Al2O3 films

    Applied Catalysis A

    (1999)
  • G.G. Podrebarac et al.

    A kinetic study of the aldol condensation of acetone using an anion exchange resin catalyst

    Chemical Engineering Science

    (1997)
  • F. Prinetto et al.

    Mg- and Ni-containing layered double hydroxides as soda substitutes in the aldol condensation of acetone

    Catalysis Today

    (2000)
  • J.C.A.A. Roelofs et al.

    Base-catalyzed condensation of citral and acetone at low temperature using modified hydrotalcite catalysts

    Catalysis Today

    (2000)
  • J.C.A.A. Roelofs et al.

    On the structure of activated hydrotalcites as solid base catalysts for liquid-phase aldol condensation

    Journal of Catalysis

    (2001)
  • M. Sychev et al.

    Hydrotalcites: relation between structural features, basicity, and activity in the Wittig reaction

    Applied Clay Science

    (2001)
  • D. Tichit et al.

    Aldol condensation of acetone over layered double hydroxides of the meixnerite type

    Applied Clay Science

    (1998)
  • V. Tomašić et al.

    State-of-the-art in the monolithic catalysts/reactors

    Applied Catalysis A

    (2006)
  • W.S. Yang et al.

    A study by in situ techniques of the thermal evolution of the structure of a MgAlCO3 layered double hydroxide

    Chemical Engineering Science

    (2002)
  • H. Zhang et al.

    Synthesis and characterization of a novel nano-scale magnetic solid base catalyst involving a layered double hydroxide supported on a ferrite core

    Journal of Solid State Chemistry

    (2004)
  • S. Abelló et al.

    Aldol condensations over reconstructed Mg–Al hydrotalcites: structure–activity relationships related to the rehydration method

    Chemistry—A European Journal

    (2005)
  • Cited by (0)

    View full text