Materials Letters 63 (2009) 2721–2724

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Materials Letters

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Preparation and characterization of silica aerogels from oil shale ash

Gui-Mei Gao, Li-Na Miao, Gui-Juan Ji, Hai-Feng Zou, Shu-Cai Gan ⁎College of Chemistry, Jilin University, 6 Ximinzhu street Changchun 130026, PR China

⁎ Corresponding author. Tel.: +86 431 88502259.E-mail address: gansc@jlu.edu.cn (S.-C. Gan).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.09.053

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 July 2009Accepted 21 September 2009Available online 29 September 2009

Keyword:Oil shale ashSilica aerogelMicrostructureSol–gel preparation

The silica aerogels were successfully synthesized using oil shale ash which is a by-product of oil shaleprocessing via ambient pressure drying. The physical and textural properties of the silica aerogels have beeninvestigated and discussed. The results showed that the organic modification of hydrogels was a crucial stepduring the processing which preserved mesopores in ambient pressure drying. The unmodified hydrogelunderwent tremendous shrinkage during the drying and yielded microporous silica aerogel. Using this novelroute, it could produce silica aerogel with low tapping density of 0.074 g/cm3, high specific surface (909 m2/g) and cumulative pore volume of 2.54 cm3/g. From the industrial point of view, the present process is quitesuitable for a large scale production of powdered silica aerogel. Furthermore, it provides a new way to solvethe problem of oil shale ash pollution.

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1. Introduction

Silica aerogels are novel mesoporous materials with many desir-able properties, such as low bulk density (~0.1 g/cm3), continuousporosities, high specific surface area ( 500 m2/g) and extremely lowthermal conductivity(~0.02 W m−1 K−1) [1–4]. Because of theseproperties, silica aerogels are promising materials as super-thermalinsulators, catalytic supports, adsorbents and host materials for drugdelivery systems [5–7].

The conventional route to prepare silica aerogels is sol–gel processcombined with supercritical drying, usually using expensive tetra-ethylorthosilicate (TEOS) as silica source. Supercritical drying processcan avoid capillary stress and associate drying shrinkage, which areusually prerequisite of obtaining aerogels structure [8]. However, thesupercritical drying is relatively an expensive process due to its high-pressure operating condition. The high costs of organic silicon pre-cursors and supercritical drying cumber considerably the large-scaleproduction and application of silica aerogels. Using cheap siliconsource and avoiding supercritical drying are the main efforts in thesilica aerogel research and development.

Oil shale ash (OSA), a by-product of oil shale processing, is rich insilica [9,10]. It could be used as a cheap silicon source to produce ahigh-grade silica product, such as silica nanoparticles and silicaaerogels. In our previous studies, the silica nanoparticles weresuccessfully synthesized using oil shale ash [11,12].

In the present work, the silica aerogels were synthesized usingOSA as a new silicon source via ambient pressure drying. The physicaland textural properties of the aerogels were investigated.

2. Experimental

2.1. Raw material and chemicals

The OSA used in this experiment was taken from oil retortingfactory of Jilin province. The major chemical compositions of theOSA is as follows: silicon dioxide (SiO2) 64.72%, alumina (Al2O3)15.85%, ferric oxide (Fe2O3) 8.61%, magnesia (MgO) 1.80%, lithia (K2O)1.17%, calcium oxide (CaO) 0.99% etc. [12]. All chemical were ana-lytical grade reagents supplied by Beijing Chemical Reagent ResearchInstitute.

2.2. Experimental procedure

2.2.1. Preparation of sodium silicate solutionFirstly, the OSA was calcined at 550 °C for 2 h in an electric furnace

to remove incorporated hydrocarbons. The calcined OSA (10 g) wasmixed with 30 wt.% sulfuric acid solution (50 g). The mixture washeated up to at 100 °C for 2 h with constant stirring. Then, themixturewas filtered and the residue was washed with distilled water until thepH value equaled 7. Secondly, a 40 g portion of 30 wt.% sodiumhydroxide solution was added to the resultant residue and boiled for5 h with constant stirring to dissolve the silica and produce a sodiumsilicate solution. This solution was filtered, and the residue waswashed with boiling water. The amount of silica in the sodium silicate

Fig. 1. FE-SEM images of the silica aerogel: (a) unmodified silica; (b) modified silica.

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solution was determined and the result showed that the extractionyield of silica was 68%.

2.2.2. Preparation of silica aerogelsFirstly, the resulting sodium silicate solution was passed through a

column filled with an ion exchange resin (Amberite, IR-120, China) ata rate of 20 mL/min. The prepared silicic acid has a pH ranging from2.4 to 2.8. A base catalyst (NH4OH) was then used to bring the pHvalue of the silica sol to 5.0. After gelation, the hydrogels wereimmersed into hexane at 50 °C for 24 h so as to strengthen the gelnetwork. The surface modification was then carried out by immersingthe gels in 5% hexamethyldisilazane (HMDZ)/hexane solution at 50 °Cfor 24 h. Finally, the surface-modified wet gels were washedrepeatedly by hexane in order to remove the remaining surfacemodification agents and dried at 80, 120 and 180 °C for 2 h respec-tively in the oven.

2.3. Methods of characterization

The bulk density of the aerogel was measured from weight tovolume ratio. The Fourier transform infrared (FT-IR) spectroscopy(Iraffinity-1, Shimadzu, Japan) was used to confirm the surfacechemical structure of the aerogels. The pore size distributions andspecific surface area of aerogels were measured using the BarretteJoynere Halenda (BJH) cumulative pore volume method (ASAP 2010,Micromeritics, USA). The microstructure of the aerogels was observedby field emission scanning electron microscopy (FE-SEM, FEI XL30,America). The thermal stability of the aerogel was observed from theTG-DTA (Rigaku TG-8120).

3. Results and discussion

The physical properties of the aerogel powders synthesized wereinvestigated. The unmodified silica aerogel showed much highertapping density (0.43 g/cm3) than that of modified silica powder(0.074 g/cm3). This is due to the irreversible shrinkage of the unmod-ified hydrogel during the drying resulting in an increase in the densityof dried silica. As a consequence, the unmodified silica aerogel ex-hibited micropores in its structure with a very less cumulative porevolume of 0.37 cm3/g and it was found to increase significantly up to2.54 cm3/g for the modified silica powders. Further, the BET specificsurface area analysis revealed that the surface area of the modifiedsilica aerogel (909 m2/g) was comparatively higher than that of un-modified silica aerogel (644 m2/g).

Fig. 1 depicts the FE-SEM images obtained for the unmodified andmodified silica aerogels, respectively. The unmodified silica aerogelexhibited highly dense microstructure with no mesopores presentin it (Fig. 1a). This is due to the fact that the unmodified hydrogelshrunk to a great extent during the ambient pressure leading to theformation of xerogel and thereby losing the mesopores present in it.Whereas, the modified silica aerogel showed mesoporous structurewith pores in the range of 5–20 nm (Fig. 1b). This positive effectstems from the reversible shrinkage of the hydrogel during thedrying as a consequence of organic modification by trimethylsilylgroups.

The N2 adsorption–desorption isotherms and pore size distribu-tions (PSD) are shown in Fig. 2. A significant change has beenobserved in the texture of the unmodified and modified silicaaerogels.

For the unmodified silica aerogel (Fig. 2a), the adsorption anddesorption branches of the isotherm coincided with each other withno hysteresis during desorption cycle. This refers to a Type I isothermwhich is a characteristic feature of microporous materials [13].ThePSD obtained for this aerogel showed a narrow distribution of thepores and the peak pore diameter resided at 1.96 nm (Fig. 2c). Fig. 2bshows the N2 physisorption isotherm obtained for modified silica

aerogel. The desorption branch of this isotherms exhibited hysteresisand correspond to the type IV isotherms which is the characteristicfeature of the mesoporous materials [14]. The desorption cycles of theisotherms showed a hysteresis loop is attributed to the capillarycondensation occurring in the mesopores. This fact could also beobserved from the PSD profile obtained for modified silica aerogel(Fig. 2d). The PSD of modified silica aerogel was in the mesoporerange (pore size, 14.6 nm).

The presence of silanol groups (i.e. Si–OH) on the silica surface isthe main cause of the irreversible shrinkage of the gel during thedrying at ambient pressure. This is due to the fact that the surfacesilanol groups undergo condensation reaction forming new siloxanebonds between the adjacent silica clusters. An organic modification ofthe silica surface by non-polar groups such as alkyl or aryl avoids theirreversible shrinkage of the gel. The trimethylsilyl groups present inHMDZ get attached to the silica surface according to the followingchemical reaction:

2ðSi−OHÞ + ðCH3Þ3Si−NH−SiðCH3Þ3→2ðSi−O−SiðCH3Þ3Þ + NH3

The grafting of the ―Si(CH3)3 groups to the silica surface wasconfirmed by means of FT-IR spectroscopy. Fig. 3 shows the FT-IRspectra obtained for the unmodified (a) and trimethylsilyl modified(b) silica aerogels. It is seen from Fig. 3 that apart from the Si–O–Siabsorption peak at 1060 cm−1, the modified silica powder showedSi―CH3 peaks at 840 and 1260 cm−1 [15]. The presence of Si―CH3

peaks in the FT-IR spectra of the modified silica aerogel conforms theattachment of ―SiCH3 groups to silica surface and hence the organic

Fig. 2. N2 adsorption–desorption isotherms of unmodified (a) and modified (b) silica aerogels; pore size distribution of unmodified (c) and modified (d) silica aerogels.

Fig. 3. FT-IR spectra of the silica aerogels: (a) unmodified silica aerogel; (b)modified silicaaerogel.

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modification. The silica powders were hydrophobic since they werenot wetted by water.

Fig. 4a shows the TG-DTA curve of unmodified silica aerogel. Thesharp decline in the weight observed at 80–110 °C accompanied byan endothermal peak in theDTA curve at the same temperature is dueto the evaporation of watermolecule. Fig. 4b shows the TG-DTA curveof modified silica aerogel. The weight loss observed up to 200 °C isdue to the evaporation of the residual solvent and water moleculefrom the aerogel. The exothermic peak in the DTA curve at 270 °C canbe attributed to the oxidation of the residual organics in the aerogelpowder. Further, the weight loss observed around 400 °C isattributed to the thermal decomposition of the organic component,i.e. ―CH3 groups. An exothermic peak was also observed in the DTAcurve at this temperature which is due to the oxidation of ―(CH3)3groups and consequently the aerogel loses its hydrophobicity[14,16].

4. Conclusions

The silica aerogel with high specific surface area (909 m2/g) andlarge pore volume (2.54 cm3/g) were successfully synthesized usingoil shale ash via ambient pressure drying. The organic modification ofthe hydrogel was an indispensable step during the processing whichpreserved the mesoporosity in the ambient pressure dried silicapowders. The N2 physisorption isotherm of modified silica aerogelexhibited hysteresis loop during the desorption cycle which revealedthat the silica aerogel was mesoporous material. Moreover, themesoporous structure was also evidenced from the FE-SEM image of

modified silica aerogel. The TG-DTA analysis revealed that the aerogelretain its hydrophobic nature up to amaximum temperature of 400 °Cand heating them above this temperature resulted in oxidation of thesurface Si―CH3 groups reasonable for the aerogel hydrophobicity. Theresults obtained in the mentioned method prove that the oil shale ashcan be used for production of silica aerogels.

Fig. 4. TG-DTA curves of silica aerogels: (a) unmodified silica aerogel; (b) modified silicaaerogel.

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Acknowledgement

This work was supported by foundation from the scientificresearch program No: 20051015 and No: 20070405, DevelopmentProgram of China (863 Program, Grant 2007AA06Z202).

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