In situ growth of layered double hydroxide films on anodic aluminum oxide/aluminum and its catalytic feature in aldol condensation of acetone
Introduction
Layered double hydroxides (LDHs, [) is a large class of materials consisting of positively charged brucite-like layers ([) and exchangeable interlayer anions () 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 -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 as precipitation agent in our laboratory (Chen et al., 2006). Urea is a very weak Brønsted base (), 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–-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: , thickness: 0.1 mm) with a size of ca. 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 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 and planes, together with the 110 and 113 reflection peaks,
Conclusions
Thin MgAl–-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)
- et al.
Monolithic reactors for environmental applications: a review on preparation technologies
Chemical Engineering Journal
(2005) - et al.
Hydrotalcite-type anionic clays: preparation, properties and applications
Catalysis Today
(1991) - et al.
Activated hydrotalcites as catalysts for the synthesis of chalcones of pharmaceutical interest
Journal of Catalysis
(2004) - et al.
Comments on the validity and utility of the different methods for kinetic analysis of thermogravimetric data
Journal of Analytical and Applied Pyrolysis
(2001) - et al.
The kinetic interpretation of the decomposition of calcium carbonate by use of relationships other than the Arrhenius equation
Thermochimica Acta
(1996) - et al.
Preparation of hybrid films of an anionic Ru(II) cyanide polypyridyl complex with layered double hydroxides by the Langmuir–Blodgett method and their use as electrode modifiers
Thin Solid Films
(2001) - et al.
Isothermal kinetic analysis of the thermal decomposition of magnesium hydroxide using thermogravimetric data
Thermochimica Acta
(1998) - et al.
Gas chromatographic investigation of the competition between mass transfer and kinetics on a solid catalyst
Journal of Chromatography A
(2004) - et al.
Microstructure-controlled synthesis of oriented layered double hydroxide thin films: effect of varying the preparation conditions and a kinetic and mechanistic study of film formation
Chemical Engineering Science
(2007) - et al.
Investigation of the surface structure and basic properties of calcined hydrotalcites
Journal of Catalysis
(1992)