Materials Letters 107 (2013) 344–347

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Materials Letters

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journal homepage: www.elsevier.com/locate/matlet

Gel-derived porous alumina systems

Katalin Sinkó n

Institute of Chemistry, L. Eötvös University, Pazmany Street 1/a, Budapest H-1117, Hungary

a r t i c l e i n f o

Article history:Received 2 April 2013Accepted 16 June 2013Available online 21 June 2013

Keywords:Porous materialFreeze dryingAlumina

7X/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.matlet.2013.06.048

+36 1 3722500; fax: +36 1 3722592.ail address: sinko@chem.elte.hu

a b s t r a c t

Highly porous alumina systems were aimed to produce by a low energy-consuming sol–gel technique.Using the newly developed method based on a freeze drying technique, the expensive supercriticalextraction can be avoided. Only two materials are needed for the synthesis: an inorganic Al salt and anorganic solvent. Basic or chelating agents are not necessary to apply for gelation. The cryogels producedby freeze drying were compared with the aerogels obtained by the supercritical technique. The porousalumina systems have different structures; the aerogels can be characterized with nanopores (10–20 nm)and the cryogels with hierarchical (macro- and meso-)pores (8–16 μm and 70–80 nm). The cryogelsystems keep their high porosity even above 1300 1C.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The aim of this study was to synthesize highly porous aluminasystems by a new low cost technique avoiding the expensivesupercritical extraction. Porous alumina materials are capable ofproviding thermal insulation over a large temperature range andpossess various applications in the field of heterogeneous catalysis.Two types of porous systems have been synthesized in this study:cryogels and aerogels. We applied a regular sol–gel technique forthe preparation of alumina alcogels, which were dried undersupercritical conditions to obtain aerogel systems. The possibilityto avoid a liquid vapor interface requires that the liquid first befrozen and then sublimed in vacuum. The freeze-drying methodis a useful, versatile, simple and efficient method to obtainporous materials or granules [1–4]. The supramolecular three-dimensional architecture of various porous alumina materials wasalso aimed to investigate. The small and wide angle X-ray scatter-ing (SAXS, WAXS), scanning electron microscopy (SEM), andporosity measurements provide the comparison of the novelcryogels with aerogels.

2. Experimental

Preparation of alumina cryogels: Present sol–gel technique startsfrom only two chemical compounds; from an aluminum salt(Al(NO3)3 �9H2O) and a solvent (1-propanol), and it does not adoptany basic or chelating agent. Thus, the time-consuming peptiza-tion can be avoided by this way. The typical synthesis pathway

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using Al nitrate is the precipitation of aluminum salt withammonia yielding aluminum oxide hydroxide powders [5]. Theuse of Al nitrate generally does not result in transparent gel bulks.This new sol–gel preparation method leads to optically clearmonolith hydrogels. The hydrolysis and the precondensationreactions of Al nitrate are performed by refluxing at about 80 1Cfor 11 h in 1-propanol. To make the condensation complete, theorganic solvent must be carefully evaporated. The solution turnsinto an transparent gel after 2–4 h. The second step is the freezedrying of monolith wet gels (at 233 K for 24 h).

Preparation of alumina aerogels: Alumina alcogels were pre-pared by one-step sol–gel method from alkoxide precursors. Inorder to control the hydrolysis rate of Al alkoxides and to hinderthe precipitation, the use of a complexing agent (such as glycols,β-diketones or ethyl acetoacetate) is required [6–9]. In the presentstudy Al isopropoxide provides as precursor and methanol orisopropanol as solvent. The dissolution and the hydrolysis ofprecursor were carried out at 70 1C in the presence of some water(3–5 mol H2O/Al) and ethyl acetoacetate (3–10 mol Etac/Al). In thesupercritical drying, the alcohol content of the wet gels wasexchanged with liquid CO2, the temperature increased up to45 1C and the pressure to ∼100 bar. The heat treatment of aluminaaerogels was performed at 500–800 1C for 2 h to remove theorganic compounds.

Characterization: The morphology has been studied by a FEIQuanta 3D FEG scanning electron microscope. The SEM images weretaken with an Everhart-Thornley secondary electron detector(ETD), its ultimate resolution is 1–2 nm.

The surface area and porosity were characterized by CCl4sorption analysis at 25 1C on an autosorb computer controlledsurface analyzer (ASAP 2010 Micrometrics).

Small- and wide angle X-ray scattering (SAXS, WAXS) experi-ments were conducted on a laboratory equipment operated with a

K. Sinkó / Materials Letters 107 (2013) 344–347 345

5.4 kW rotating anode X-ray generator (Nanostar, Bruker AXS), apinhole camera, and a 2D position sensitive detector (Vantec 2000,Bruker AXS). A 1D MYTHEN detector was used as a WAXS detector.The WAXS data were collected over the 2θ-range of 7–301 with astep size 0.02121. SAXS data were evaluated by Freltoft expression[10].

3. Results and discussion

Comparison of cryogels and aerogels: Aluminum-containingmonolith wet gels were prepared by two routes: from Al isopor-opoxide in alcoholic medium and from Al nitrate in aqueoussolution. Two types of sol–gel techniques are needed to producetwo types of porous systems. The supercritical drying methodfavors alcogels rather than hydrogels, while the freeze dryingprefers more the hydrogels than the alcogels. We made someeffort to produce cryogels by freeze drying from alcogels but theporosity of the products obtained by this way was very low. Thefrozen alcohol particles less can provide a template than ice grains,the organic alcohol sublimates poorly. The monolith hydrogelswere tried to dry under supercritical conditions. The water contentcannot be completely removed by supercritical CO2 washing stepowing to the good dissolution of CO2 in water. If the water contentwas exchanged by methanol before the washing with liquid CO2,the gel structure was collapsed. Direct drying of the hydrogelswith supercritical extraction requires extreme high pressure andtemperature.

Table 1 summarizes the data of structural investigations pro-viding the comparison. Drying the alumina wet gels under super-critical conditions ensures nanoporous structures. The SEM imagesconfirm 40–50% porosity and 10–20 nm pores in the case of

Table 1Results of structural investigations of cryogels and aerogels.

Sample name Pore size (nm) Aggregate s

SEM N2 sorption SEM

Aerogel in methanol 10–20 1575 100750Aerogel in isopropanol 10–20 2075 50725Cryogel, 6� waterc 16,000710,000 80710 10,00072000 –d

Cryogel, 4� waterc 14,000710,000, 70710 10,00071500 –d

Cryogel, 2� waterc 8000710000, 80710 500072000 –d

a Total pore volume.b BET specific surface area was measured by N2 sorption.c Mass ratio of water/dried Al-containing gel.d No aggregate.

Fig. 1. SEM images of c

aerogels. The SEM images of cryogels depict the high porosity(60–85%); the open macropores ordered in channel-like architec-ture (10–16 μm ∅) and the mesopores (∼80 nm ∅) closed in thewall. Macropores were generated by using ice crystals as atemplate, while the walls which surround the macropores wereformed as porous cryogels by freeze drying. The cryogels possesshigher porosity and total pore volume than the aerogels, contrarilythe aerogels have larger specific surface area due to the nanopores.The water amount of the swelling influences on the porosity ofcryogels. The larger the water content of hydrogels the higher theporosity and the total pore volume of cryogels are (Table 1). TheWAXS data indicate complete amorphous characters for cryo- aswell as aerogel samples. The structure of aerogels is built up fromrandomly connected aggregates. The aggregates are 3-D particleswith rough surfaces. The scattering data and the SEM images alsoverify the non-aggregate structure of cryogels (Fig. 1).

Structure of cryogels in the function of temperature: The SEMimages of cryogels heated at various temperatures (80–1300 1C)represent that the porous structure of cryogels is maintainedduring heating up to 1300 1C (Fig. 2). According to the literaturedata, the porous alumina structure collapses by the crystallizationof α-Al2O3 [6,11,12]. In our samples, only the macroporosityreduces from ~85% to ~62%.

The SAXS curves (Fig. 3) for cryogel systems do not indicate anystructural ordering in the size range of 1–50 nm confirming theiramorphous feature. Only a characteristic size can be observed from300 1C, which can be derived by the change of bond systems.At around 400 1C, the shared OH, H2O and H-bonds transform intooxygen bridges (Al–O–Al). At 1300 1C this characteristic lengthdisappears due to the crystallization of α-Al2O3 phase. Between 80and 200 1C, only amorphous structure can be identified by WAXS(Fig. 4). Some ordering appears at 300 1C due to the temporary

ize (nm) Porosity SEM (%) TPVa (cm3 g−1) Spec. surf. areab (m2 g−1)

SAXS

480 50710 1.170.2 30072045720 40710 0.870.2 500740–d 85720 3.870.5 350730–d 80720 3.170.3 280720–d 65720 1.970.3 150720

ryogel and aerogel.

Fig. 2. SEM images of cryogels heated at various temperatures.

Fig. 3. SAXS curves for cryogels heated at various temperatures.

Fig. 4. WAXS curves for cryogels heated at various temperatures.

K. Sinkó / Materials Letters 107 (2013) 344–347346

aluminum oxide hydroxide and aluminum hydroxide phases. Thescattering detects a regular crystalline phase (α-Al2O3) above1000 1C.

The most important conclusion of the structure investigationsis that the changes of the bonds system or the conversion ofamorphous structure into crystalline phase do not lead to collapseof the highly porous structure. General observation is that theamorphous porous structure disappears after the crystallizationabove 800–1000 1C [6,11,12].

4. Conclusion

In this study, a new low cost procedure of porous aluminasystem has been developed. Only two materials are applied for thesynthesis; an inorganic Al salt and an organic solvent. Basic orchelating agents are not needed in order to form a gel system

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directly from the initial solution. The wet gels are dried by freezedrying instead of expensive supercritical extraction. The synthesisof aerogels is provided in comparison with cryogels. The alcogelsobtained from Al alkoxides fit rather for supercritical drying whilethe hydrogels prefer the freeze drying more. The cryogels keeptheir highly porous structure even after the changes of bondsystems and the crystallization of the amorphous structure(41000 1C). The porous alumina network generally collapses bycrystallization.

The porous alumina systems have different structures; theaerogels can be characterized with an aggregate structure andnanopores (10–20 nm). The cryogels are composed from acontinuous glass-like skeleton and macropores (8–16 μm) orderedinto channel-like architecture and the mesopores (∼80 nm ∅)closed in the walls. The cryogels possess higher porosity and totalpore volume, while the aerogels have larger specific surface areadue to their nanoporous system.

Acknowledgments

This study has been supported by EU FP-7 CHATT ACP1-GA-2011-285117 and OTKA NK 101704 funds.

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