Elsevier

Microporous and Mesoporous Materials

Volume 159, 1 September 2012, Pages 36-41
Microporous and Mesoporous Materials

A facile rout to synthesis lamellate structure mesoporous alumina using polyethylene glycol 6000 (PEG, molecular weight = 6000) as structure directing agent

https://doi.org/10.1016/j.micromeso.2012.04.002Get rights and content

Abstract

In this study, Al2(SO4)3 and NaAlO2 were used as aluminum precursors for preparing lamellate structure mesoporous alumina with crystalline framework walls in the presence of non-ionic surfactant PEG6000. The characterization by SEM, TEM, TG-DSC and FTIR revealed that PEG surfactant induced the formation of the lamellate structure boehmite with relatively lower water content. XRD and SAED results conveyed the formation of well-crystallined mesoporous γ-alumina after the samples were calcined at 600 °C for 3 h. N2 physisorption showed that the mesoporous alumina prepared in this way displayed very rich porosities with large mesopores, and both the pore volumes and the pore sizes increased with the addition of such surfactant in the precipitation process. The obtained γ-alumina with lamellate morphology exhibited a large surface area of 279 m2/g, and the powder has no obvious agglomeration. The improved textural parameters in the samples should be attributed to the formation of lamellate structure nanoparticles.

Highlights

► PEG6000 were used as the structure directing agents to preparing mesoporous γ-Al2O3. ► The mesoporous γ-alumina with crystalline framework walls were appeared lamellate structure. ► The obtained γ-alumina with lamellate morphology exhibited improved textural properties.

Introduction

The research and development of new methods to produce porous alumina are making it extensively applied in: catalytic process [1], [2], [3]; aqueous effluents treatment containing heavy metals [4], [5]; liquid chromatography technique; ceramic material; electronic substrates; cells transplantation devices [6]; ultrafiltration membranes; arsenic [7] and so on, many of which depend on morphological characteristic such as textural properties, shape and structure. In particular, γ-AlOOH and γ-Al2O3 hollow structured materials have received much attention owing to their low densities, high specific surface areas, and closely packed interpenetrating networks [8], [9].

The catalytic performances of alumina-supported catalysts are largely dependent on the textural properties of the alumina supports. Alumina supports with high thermal stability, large surface areas, large pore volumes, narrow pore size distributions within the mesoporous range, as well as suitable surface acidic-basic properties can often result in favorable enhancement in the catalytic performance, which could not only increase the dispersion of the active catalytic species, but also enhance the diffusion efficiency and mass transfer of reactant molecules [10], [11]. Therefore, synthesis of this material with improved textural properties, crystalline framework walls and high thermal stability has attracted much attention [12], [13], [14].

In recent years, various of approaches have been developed for the synthesis of ordered mesoporous materials [15], [16], [17]. In general, ordered mesoporous materials are synthesized using either soft-templates (usually surfactant, such as hexadecyl trimethyl ammonium bromide (CTAB) [18], [19] and tri-block copolymer (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) [20]) or hard templates (nanocasting) that have mesoporous structures. Mesoporous silica, carbon, polymer and metal oxides are frequently synthesized by using the hard templates [21], [22]. Differing from ordered mesoporous silica many mesoporous alumina materials prepared via surfactant templates are lacking in long-range order, and the inorganic walls are often amorphous [10], [12], [23]. Organic–inorganic assemblies involving complicated sol–gel processes by using surfactants as structure-directing agents are regarded as one of the most promising approaches [24], [25]. Various of neutral and ionic surfactants have been used as templates for the preparation of mesoporous alumina. Following the S0I0 mechanism, the hydrolysis of an alumina alkoxide precursor in an organic solvent in the presence of non-ionic surfactant has been claimed to render aluminas with a high surface area after calcinations at 773 K [26], [27].

Motivated by the results that introduce non-ionic surfactants (such as Triton X family and Tergitol family) into the synthesis process would lead to a better textural property and well ordered mesoporous aluminas [28], [29], we have developed a facile and economic rout for preparing mesoporous γ-Al2O3 by using unexpensive aluminous saline (Al2(SO4)3, NaAlO2) as precursor and non-ionic surfactants PEG6000 as the structure directing agents. Finally, well-crystallined mesoporous γ-alumina with lamellate structure morphology as well as improved texture properties were obtained.

Section snippets

Experimental

The synthesis was carried out in a 5 L crystallizer equipped with a precise temperature controller, an agitator, and a peristaltic pump.

A desired amount of surfactant PEG6000 was dissolved in distilled water at room temperature. A NaAlO2 solution and a Al2(SO4)3 solution were added to PEG6000 solution by the peristaltic pump at the same time under vigorous stiring, the adding rate of Al2(SO4)3 solution was 10 ml/min. The reactants were fully mixed by the agitator at 3000 rpm. The mixture system

Characterization

Scanning electron microscopy (SEM) images were get from a JSM-7500F electron microscope. Gold were vapor-deposited on the samples before analysis.

Transmission electron microscopy (TEM) images were obtained on a JEM-2100 electron microscope operating at an accelerating voltage of 200 kV. The samples were first dispersed ultrasonically in ethanol and then dropped onto the carbon-coated copper grids prior to the observation.

Nitrogen adsorption–desorption measurement were performed at liquid

Effect of PEG surfactant on morphology and mesostructure

It is known that the boehmite layers, formed by Al3+ surrounded octahedrally by O2− and OH, are linked through hydrogen bonds and packed to give the particularly morphology [11], [30]. Fig. 1 and Fig. 2 give us the SEM and TEM images of the two samples prepared with and without surfactant. The SEM image (Fig. 1A) indicate the presence of lamellate particles obviously corresponding to the TEM image (Fig. 2A). We can see the length and width of the nanosheets clearly in this TEM image. The S4

Discussion

The above characterization results revealed that there exist interaction between boehmite and PEG6000 molecules, and the interaction had non-negligible impact on the morphology of the boehmite particles. In this mechanism, the surfactant amount and other variable (such as temperature, pH, reagent concentration, synthesize time, aging duration etc.) might impose influences on the morphology as well as the textural parameters of the resulting γ-Al2O3.

When the surfactant is dispersed in a polar

Conclusions

Lamellate structure morphology boehmite were prepared with unexpensive aluminous saline (Al2(SO4)3, NaAlO2) and non-ionic surfactant PEG6000. After the samples were calcined at 600 °C for 3 h, well-crystallined mesoporous γ-alumina with lamellate structure morphology as well as improved textural properties (with high surface area about 279m2/g, larger pore volume about 0.88 mL/g and average pore width about 12.5 nm) were obtained. TG-DSC results revealed that the dried sample prepared with PEG

Acknowledgments

Supports of Fushun Research Institute of Petroleum and Petrochemicals (FRIPP), SINOPEC for scholars are gratefully acknowledged.

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