Journal of Non-Crystalline Solids 400 (2014) 6–11

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Journal of Non-Crystalline Solids

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Using bagasse ash as a silica source when preparing silica aerogels viaambient pressure drying

N. Nazriati, Heru Setyawan ⁎, Samsudin Affandi, Minta Yuwana, Sugeng WinardiDepartment of Chemical Engineering, Faculty of Industrial Technology, Sepuluh Nopember Institute of Technology, Kampus ITS Sukolilo, Surabaya 60111, Indonesia

⁎ Corresponding author. Tel.: +62 31 5946240; fax: +E-mail address: sheru@chem-eng.its.ac.id (H. Setyawa

http://dx.doi.org/10.1016/j.jnoncrysol.2014.04.0270022-3093/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 January 2014Received in revised form 22 April 2014Available online xxxx

Keywords:Bagasse ash;Silica aerogel;Ambient pressure drying;Surface modification;Silylation

Silica aerogels were prepared using bagasse ash as a silica source through a route involving the surface modifica-tion of wet gels and ambient pressure drying (APD). The silica in the bagasse ash was extracted using a NaOHsolution to form the sodium silicate precursor for the silica aerogels. Trimethylchlorosilane (TMCS) andhexamethyldisalazane (HMDS) were used to replace the surface silanol groups with alkyl groups to preventthe condensation and shrinkage of the gel structure during APD. The prepared silica aerogels exhibited stronghydrophobicity. The silica aerogels aged in water possess a higher surface area than those aged in hexane. Themixing time for the silylating agents with the silica precursor influences the surface area of the silica aerogels.The highest surface area and pore volume of the silica aerogels were 1114 m2/g and 2.16 cm3/g, respectively.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Bagasse is the cellulose fiber remaining after the extraction of thesugar-bearing juice from sugarcane, making it a major by-product ofthe sugar industry and abundantly available. Similar to most other bio-mass materials, bagasse has a high volatile organic content. Therefore,bagasse is a cheap potential energy source; most, if not all, sugarfactories, including those in Indonesia, use bagasse as a primary fuel,generating almost enough energy to operate the plants. Burning ba-gasse to generate energy leads produces large amounts of ash waste,causing disposal problems. Out of overall concern for environmentand the need for safe disposal methods for bagasse ash, various usesfor bagasse ash have been suggested: adsorbents [1], filler for buildingmaterials [2], and high-purity mesoporous silica gels [3].

Our method for producing high-purity mesoporous silica gel frombagasse ash was based on the unique solubility properties of silica [3].The solubility of amorphous silica is very low at pH b 10, but increasessharply at pH 10 [4]. Therefore, the silica in bagasse ash could be dis-solved in a boiling alkali solution to form sodium silicate and subse-quently precipitated after adding acid to lower the pH. The silicahydrogel was washed and dried to produce a high-purity mesoporoussilica gel. However, the surface area of these silica gels was relativelylow (~160 m2/g), hindering their use as adsorbents and catalystsupports.

The silica gel surface is terminated by silanols (\SiOH). These surfacesilanol groups cause the gel to collapse via irreversible condensationduring ambient pressure drying. To avoid collapsing the gel structure

62 31 5999282.n).

during drying, the surface silanol groups can be silylated [5–7]. Themodified silica gels can be dried at ambient pressure without shrink-age, producing a silica aerogel. Silica aerogels are unique porous ma-terials consisting of a three-dimensional mesoporous network ofsilica nanoparticles. They exhibit unique properties, such as a largesurface area, a large pore volume, a low bulk density and a low ther-mal conductivity. Therefore, silica aerogels have many applications,including catalysis [8], adsorption [9], thermal insulation [10] anddrug delivery system [11].

Silica aerogels are usually synthesized using organic alkoxides,which are expensive and harmful precursors, and a supercritical dryingprocess, which is an energy-extensive process. To reduce the costs ofpreparation, two approaches are available: using cheap precursors ordeveloping novel drying techniques at ambient pressure. Waterglass(sodium silicate) may be a viable and inexpensive alternative to alkox-ide precursors [12,13]. Therefore, sodium silicate produced from bio-mass ash, such as bagasse ash, rice hull ash and fly ash, is potentialcandidate due to its very low cost and facile production. Rice hull ashhas been used as the silica source for silica aerogels that required su-percritical drying [14]. Using supercritical drying methods limits thecommercial applications of these materials. Industrial fly ash fromcoal power plants has been used to prepare silica aerogels via ambientpressure drying [15]. However, the route to produce sodium silicate isenergy-intensive because the fly ash at must be pre-treated at 600 °Cand hydrothermally processed.

In this work, a process for preparing silica aerogels from bagasse ashwas investigated. Sodium silicate was prepared as a precursor for silicaaerogels using a method developed in our laboratory [3]. The silica sur-face must be modified using silylation agents for successful ambient-pressure drying. Therefore, the effects of various process parameters,

Fig. 1. The schematic procedure for preparing silica aerogels from bagasse ash.

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including the type of solvent used during aging, the mixing time for thesilylating agents with the silica precursor and the use of a co-precursorfor silylation, on the specific surface area, pore volume and pore size dis-tribution, were investigated. Compared with the above-mentionedmethods, our proposed method is muchmore promising to prepare sil-ica aerogels from the view point of very low cost and facile productionmethod because our method utilizes low-energy alkali extraction com-bined with ambient pressure drying.

2. Materials and methods

2.1. Materials

The bagasse ash was obtained from PG. Kebon Agung, a canesugar mill located in Malang, Indonesia. All chemicals used to pre-pare the silica aerogel from the bagasse ash were reagent grade andused without further purification. The sodium hydroxyde (NaOH),trimethylchlorosilane (TMCS), hexamethyldisilazane (HMDS), ammo-nium hydroxide (NH4OH) and pyrydine were supplied by Merck. Thesilica content of the bagasse ash was 50.36 wt.%, and the main impuri-ties were K2O (19.34 wt.%), Fe2O3 (18.78 wt.%) and CaO (8.81 wt.%).Demineralized water was used during all synthesis and treatmentprocesses.

2.2. Silica extraction

The silica in bagasse ash was extracted using ourmethod. Briefly, 10g of bagasse ash was dispersed in 60 ml of 2 N NaOH in a 250 ml Erlen-meyer flask. Themixturewas boiled for 1 hwith constant stirring to dis-solve the silica and produce a sodium silicate solution. The suspensionwas allowed to cool to room temperature and filtered through theWhatman no. 41 ashless filter paper. The filtratewas the sodium silicatesolution used to prepare the silica aerogel.

2.3. Production of silica aerogels

The sodium silicate solution prepared abovewas used to prepare thesilica aerogel. The procedures used to produce the silica aerogel areshown in Fig. 1. Two methods were employed to induce gel formation:adding NH4OH solution until pH 4 (Method I) and dropping silicic acidinto hexane before adding pyridine (Method II). The procedure usedto prepare silicic acid for both methods was the same: the sodium sili-cate solution was mixed with an acidic ion exchange resin in a 1:2 vol-umetric ratio and stirred for 1 h to produce the silicic acid (SA) (pH ~ 2).TMCSwas added to the SAwith constant stirring; the stirring continuedfor several minutes after the TMCS was added. Afterward, HMDS wasadded, and the mixture was stirred for several minutes. The continuedstirring allowed the reactions to equilibrate with water.

In Method I, the mixture was titrated with 1 N NH4OH to induce gelformation. The hydrogel was aged at 40 °C for 18 h and 60 °C for 1 h tostrengthen the gel network. Afterward, the silylated gel was dried at80 °C under ambient pressure for 24 h.

In Method II, after successively adding TMCS and HMDS, the hydro-gel was dropped slowly into hexane. TMCS and HMDS do not react withhexane, allowing it to act as a neutral medium [16]. Because hexane isimmisciblewithwater, thewater containing TMCS andHMDS remainedinside the silica pores during aging, possibly influencing the silylationreaction. To induce gel formation, pyridine was added. The gel wasaged at 40 °C for 18 h and 60 °C for 1 h to strengthen the gel network.Finally, the silylated gel was separated from the hexane using filtrationand dried at 80 °C under ambient pressure for 24 h.

2.4. Characterization

The specific surface area, pore volume and pore size were deter-mined using a nitrogen gas adsorption-desorption method (Nova

1200, Quantachrome). Initially, the silica aerogels were degassed at573 K for 3 h, and the adsorption–desorption isotherms were obtainedat 77 K. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET)method at P/P0 b 0.3. The cumulative pore volumewas calculated from the adsorption–desorption profiles. The averagepore diameter and pore size distribution were estimated using theBarrett–Joyner–Halenda (BJH) method with the desorption branch.The error in the measurements for surface area, pore volume andpore size was calculated as a standard deviation based on the entire

population using the formulaffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi∑ x−xð Þ2=n

q, where x and x represent

the actual and mean values, respectively, and n is the sample size.The results reported in this paper are based on the averages of threesets of experiments. The morphology of the silica aerogel was ob-served using scanning electron microscopy (SEM) (S-5000, Hitachi).Fourier transform infrared spectroscopy (FTIR 8400 s, Shimadzu)was used to confirm the surface modification of the silica aerogels.The weight loss of the silica aerogel was studied using thermo gravi-metric and differential thermal analysis (TG–DTA, Shimadzu) in airfrom room temperature to 600 °C. The contact angle of the aerogelswas measured by photographing a water droplet on the surface ofthe pressed aerogel.

Table 1Physical properties of silica aerogels prepared from bagasse ash for various volume ratiosof SA:TMCS:HMDS and solvent type.

Sample no. Volume ratio of SA:TMCS:HMDS

SBET (m2/g) Pore volume(cm3/g)

Pore diameter(nm)

Aged in waterA134 1:0.03:0.04 441.02 ± 7.1 0.75 ± 0.02 3.85 ± 0.04A135 1:0.03:0.05 537.85 ± 8.9 1.75 ± 0.06 3.80 ± 0.01A136 1:0.03:0.06 1113.76 ± 10.9 2.13 ± 0.04 3.77 ± 0.07A146 1:0.04:0.06 619.19 ± 8.2 1. 80 ± 0.18 3.81 ± 0.01A156 1:0.05:0.06 455.15 ± 20.8 1.09 ± 0.15 3.39 ± 0.03

Aged in hexaneAH134 1:0.03:0.04 551.32 ± 12.8 0. 75 ± 0.01 5.34 ± 0.06AH135 1:0.03:0.05 606.82 ± 13.3 0.86 ± 0.07 5.51 ± 0.11AH136 1:0.03:0.06 544.86 ± 5.9 0.82 ± 0.02 6.04 ± 0.04AH146 1:0.04:0.06 466.35 ± 10.7 0.77 ± 0.03 7.38 ± 0.30AH156 1:0.05:0.06 635.12 ± 13.2 0.76 ± 0.05 5.10 ± 0.08

Fig. 2. FTIR spectra of the silica aerogels frombagasse ash preparedusing different solventsduring aging.

8 N. Nazriati et al. / Journal of Non-Crystalline Solids 400 (2014) 6–11

3. Results and discussion

3.1. Surface modification

The silica aerogels can be prepared using ambient pressure dryingonly through a surface modification step. FTIR studies of the aerogelsprepared using the abovemethodswere conducted to verify the forma-tion of the Si\C and C\H bonds in the aerogels, confirming the modi-fication of the silica surface. Fig. 2 shows the typical FTIR spectra ofsilica aerogels prepared using the two methods. Sample A146 refers toan aerogel prepared using Method I with a 1:0.04:0.06 ratio of SA:TMCS:HMDS and AH156 refers to one prepared using Method II witha 1:0.05:0.06 ratio of SA:TMCS:HMDS. The FTIR spectra are similar toone another. In addition to the bands corresponding to the Si\O\Sibonds at 1091 and 466 cm−1, other bands corresponding to Si\C andC\H bonds are apparent. The band at 848 cm−1 can be attributed toSi\C bonds, and the bands at 1257 cm−1 and 2962 cm−1 correspondto C\H bonds. These signals indicate that the silanol groups at the silicasurface have been replaced with \CH3 groups using Methods I and II.

3.2. Effect of solvent during aging

Fig. 3 shows the SEM images of silica aerogels aged in water (a) andin hexane (b). Both types of silica aerogels exhibit a porous networked

Fig. 3. SEM images of silica aerogel prepared from bagasse ash us

structure. The microstructures of the porous aerogels were examinedusing nitrogen adsorption/desorption.

The specific surface area, total pore volume and average porediameter of silica aerogels synthesized under various conditions arepresented in Table 1. The mixing time for both cases was 10 min aftercompleting the addition of TMCS and HMDS. For the silica aerogelsaged in water, the BET surface area ranged from approximately 450 to1114 m2 g−1. The surface area increased when increasing the amountof HMDS. The pore volume of the silica aerogels exhibited a similartrend, increasing when increasing the amount of HMDS and rangingfrom approximately 0.75 to 2.16 m3 g−1. However, the BET surfacearea and pore volume of the silica aerogels decreased when increasingthe amount of TMCS. The surface area decreased from approximately1114 to 488m2 g−1, and the pore volume decreased from approximate-ly 2.16 to 1.11 m3 g−1. The largest surface area (1114 m2 g−1) wasobtained at a 1:0.03:0.06 ratio of SA:TMCS:HMDS. The pore diameterwas not significantly influenced by the volumetric ratio of SA:TMCS:HMDS, averaging between 3.40 and 3.83 nm.

The surface area of the silica aerogels aged in hexanewas lower thanthat of the aerogels aged in water, ranging from approximately 480to 625 m2 g−1. In contrast to silica aerogels aged in water, the vol-ume ratio of SA:TMCS:HMDS did not significantly influence the sur-face area, the pore volume or the pore diameter. The pore volumeand pore diameter remained relatively constant regardless of theSA:TMCS:HMDS ratio. The average pore volume and diameter wereapproximately 0.8 m3 g−1 and 5.9 nm, respectively. These valuesare smaller than those obtained after aging the gels in water are.Fig. 4 shows the pore size distribution of the silica aerogels aged inwater (a) and in hexane (b). The pore distribution of the silica aerogels

ing (a) Methods I (sample A136) and (b) II (sample AH136).

Fig. 4. Pore size distribution for the silica aerogels produced from bagasse ash at differentvolumetric ratios of SA:TMCS:HMDS aged in: (a)water and (b) hexane. Lines are drawn asguides to the eyes.

9N. Nazriati et al. / Journal of Non-Crystalline Solids 400 (2014) 6–11

aged in water is narrower than that of the aerogels aged in hexane,indicating that the pore size of the aerogels aged in water is moreuniform.

The kinetics of the condensation reaction used to form silica sol–gelin the aqueous silicate solution is very slow at room temperature, oftenrequiring several days for reaction completion. An acidic or base catalystis usually added to increase the rate of reaction in sol–gel process. In thiswork, NH4OH and pyridine were used to increase the reaction rates inMethod I and Method II, respectively. NH4OH is soluble in water andcan generate a homogeneous aqueous solution with silicic acid. There-fore, the condensation reactions used to form the silica clusters andgel networks are faster. In Method II, however, the pyridine must firstdiffuse from the hexane into the aqueous phase containing the silicicacid before catalyzing the condensation reaction. Afterward, the con-densation reaction is slow, allowing the silylation reaction to occurfirst. Consequently, the silanol groups on the silica surface have onlybeen modified partially, hindering the aggregation needed to form thegel networks. These effects may explain why the surface area and porevolume of the silica aerogels aged in hexane were lower and the porediameters were larger.

The typical N2 adsorption-desorption isotherms at 77 K for the silicaaerogels prepared from bagasse ash with ambient pressure drying arepresented in Fig. 5. The isotherms are all typical type IV isotherms ac-cording to the IUPAC classification. These isotherms are characterizedby the disappearance of the saturation limit with a hysteresis. Thistype of isotherm indicates an indefinite multilayer forms after themonolayer and that the materials are mesoporous. The hysteresis ob-served in the silica aerogels aged in water (Fig. 5a and b) is the H4type; the two branches remain nearly horizontal and parallel over awide range of P/P0. The type H4 hysteresis occurs with narrow-slitpores. The hysteresis of the silica aerogels aged in hexane (Fig. 5cand d) is a H2 type hysteresis; this type exhibits steeper hysteresisloops for the desorption branch than those of the adsorption branch.This type of materials has poorly defined pore shape and sizedistribution.

The characteristics, including thermal stability and hydrophobicity,of silica aerogels prepared using Method I with material from bagasseash will be presented. In addition, the effect of the mixing time forTMCS andHMDSon theproperties of the silica aerogelwill be described.

3.3. Thermal analysis of silica aerogels

Fig. 6 shows the results of a typical thermogravimetric analysis of sil-ica aerogels prepared using material from bagasse ash and an ambientpressure drying (APD) method. A weight loss is detected from roomtemperature to approximately 150 °C, indicating the loss of physical-ly adsorbed water. From 150 °C to approximately 350 °C, the weightof the silica aerogels remains relatively constant. A further increasein the temperature causes a weight loss of the sample due to the oxida-tion of the surface methyl groups installed during surface modification[9].

3.4. Hydrophobicity of silica aerogels

Fig. 7 shows a water droplet on the surface of pressed silica aerogelsprepared from bagasse ash usingMethod I: (a) A136 and (b) A156. Thefigures obviously demonstrate that the silica aerogels are strongly hy-drophobic with a contact angle of 140° in both cases due to the surfacemethyl groups. All of the silica aerogel samples prepared at different SA:TMCS:HMDS ratios exhibit a strong hydrophobic character, as indicatedby the contact angles (N130°). The surface modification was confirmedby FTIR, as shown in Fig. 1.

3.5. Effect of mixing time

When studying the effects of themixing time, the volumetric ratio ofSA:TMCS:HMDS was fixed at 1:0.03:0.04. The mixing time used afteradding TMCS and HMDS was the same. Fig. 8 shows the influence ofthe mixing time on the surface area of the silica aerogels. The surfacearea tends to increase when increasing the mixing time, peaking atapproximately 30 min and remaining constant afterward.

When TMCS was added to the aqueous silicic acid solution, the pos-sible reactions included the following [17]:

2 CH3ð Þ3SiCl TMCSð Þ þH2O⇌ CH3ð Þ3Si‐O‐ CH3ð Þ3Si HMDSOð Þ þ 2HCl ð1Þ

CH3ð Þ3SiCl þ HO−Si ≡ → ≡ Si−O−Si− CH3ð Þ3 þ HCITMCS Surface modified silica:

ð2Þ

The TMCS reacted with water to yield hexamethyldisiloxane(HMDSO) and HCl after a spontaneous and isothermic reaction; theproducts reacted further to form TMCS andwater (Eq. (1)). The equi-librium of reaction (1) is shifted toward the HMDSO (right) side. Thesecond possible reaction was the reaction between TMCS and the hy-droxyl groups on the silica surface (Eq. (2)). Similarly, the reactions

Fig. 5.Nitrogen adsorption–desorption isotherms for the silica aerogels produced frombagasse ashpreparedusingMethod I ((a) A134 and (b) A136) andMethod II ((c) AH134 and (d)AH136). Lines are added to serve as a visual guide.

Fig. 6. Thermogravimetric analysis of silica aerogels prepared from bagasse ash and driedat ambient pressure.

10 N. Nazriati et al. / Journal of Non-Crystalline Solids 400 (2014) 6–11

that occurred when HMDS was added into the aqueous silicic acidsolution were

CH3ð Þ3Si−NH− CH3ð Þ3Si þ H2O → CH3ð Þ3Si−O− CH3ð Þ3Si þ 2NH3HMDS HMDSO

ð3Þ

CH3ð Þ3Si−NH−Si CH3ð Þ3 þ HO−Si ≡ → ≡ Si−O−Si− CH3ð Þ3 þ NH2Si CH3ð Þ3HMDS Surface modified silica:

ð4Þ

HMDS reacted with water to produce HMDSO and ammonia; how-ever, unlike the spontaneous reaction of TMCS with water, this reactionis slow. The presence of HMDSO generated by reaction (1) may hinderreaction (3). Consequently, HMDS acts more as a modifying agent dur-ing aging than TMCS. As shown in Table 1, the effect of HMDS on the po-rosity of the silica aerogel is more pronounced than that of TMCS. TMCSmay act as a catalyst for the HMDS.

When the pH was increased by adding NH4OH solution, silica beganto precipitate due to a condensation reaction, specifically the formation

Fig. 7.Water contact angle of the silica aerogels: (a) sample A136 and (b) sample A156.

Fig. 8. Effect of mixing time on the specific surface area of a silica aerogel prepared frombagasse ash. Error bars show the uncertainty in the measurement. Line is drawn as aguide for the eyes.

11N. Nazriati et al. / Journal of Non-Crystalline Solids 400 (2014) 6–11

of a siloxane linkage between the surface silanol groups. Afterward, theparticles increased in size and decreased in number in an Oswald ripen-ing process. In addition, the electrostatic interaction between chargedparticles favored their aggregation (coagulation) to form a porousthree-dimensional silica network. Both processes occurred simulta-neously, competing with the silylation process. Therefore, when themixing time is relatively short, the dissociation of HMDS has not yetcompleted, enabling competition between HMDS dissociation andsilylation. Consequently, the gels tend to collapse during ambient pres-sure drying, generating a smaller surface area. The surface area tends toincrease when increasing the mixing time, indicating that the HMDSdissociation proceeds further. When the HMDS has reached equilibrium,the surface area tends to remain constant.

4. Conclusions

Hydrophobic silica aerogels with high surface area can be preparedwhen using bagasse ash as the silica source through the surface modifi-cation of wet gels and an APD route. The type of solvent used duringaging influences the properties of the silica aerogels. Silica aerogelsaged in water possess a higher surface area than those aged in hexane.The mixing time for the silylating agents with the silica precursor influ-ences the surface area of the silica aerogels. The surface area of silicaaerogels was from 450 to 1114 m2/g, and the pore volume was from0.75 to 2.16 cm3/g.

Acknowledgments

This work was supported by the Directorate General of Higher Edu-cation (DGHE), the Ministry of Education and Culture, Indonesia(013508/IT2.7/PN.08.01/2013) through Hibah Kompetensi. One of the

authors (N.N.) would like to thank DGHE for a doctoral scholarshipthrough BPPS.

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