Journal of Non-Crystalline Solids 356 (2010) 1466–1469

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Journal of Non-Crystalline Solids

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Influence of reaction conditions on the properties of sodium alumino silicatesynthesized by simultaneous addition of precursors

Askwar Hilonga a, Jong-Kil Kim b, Pradip B. Sarawade a, Hee Taik Kim a,⁎a Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Koreab E&B Nanotech. Co., Ltd, Republic of Korea

⁎ Corresponding author. Tel.: +82 31 400 5493; fax:E-mail address: khtaik@yahoo.com (H.T. Kim).

0022-3093/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2010.04.039

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 November 2009Received in revised form 18 April 2010Available online 28 May 2010

Keywords:Sodium alumino silicate;pH;BET;EDS

In this study we have examined the properties of sodium alumino silicate (SAS) synthesized by simultaneousaddition of sodium silicate and aluminate under controlled reaction conditions. The varied conditions includethe concentration of the reactants and the pH (low, neutral, and high); while the stirring speed, precipitatingtemperatures, and the drying conditions were kept constant. XRD, SEM, and EDS results indicate that thematerials developed were amorphous and have irregularly-textured particles. They were rich in Si, Al, O, andNa; and had Na/Al molar ratios of about 0.4 to 1. The SiO2:Al2O3 molar ratio in the final product ranges from2.2 to 5.8 as determined by EDS. FTIR studies confirmed the formation of Si–O–Al bond. Nitrogenphysisorption studies were done to examine the surface area and porosity of the final product. The SASsamples examined have bulk density greater than 2.5 g/cm3, pore volume up to 0.85 cm3/g, and the porediameters between 13 to 20 nm. The BET surface area ranges from 80 to 318 m2/g, and oil absorption from 75to 200 ml/100 g. The products obtained have desirable properties and are expected to be a potential low-costalternative as functional fillers for paper and paint industries.

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1. Introduction

In recent years a considerable number of synthesis techniques foramorphous sodium alumino silicate (SAS) pigments have beenreported [1–3]. The basic chemical reaction involves the condensationreactions between aluminate and silicate species [4]. In this kind ofreaction, concentration and alkalinity of the reactants have asubstantial influence on the final product. The condensation betweensilicate species themselves, and between aluminate and silicatespecies is worthy of consideration. Silicate speciation and theircondensation with [Al(OH)4]− have been described in a previousreport [5]. In summary, it was concluded that monomeric silicateanions tend to condense to form oligomeric species in concentratedalkaline silicate solutions, which further conform to a cyclicconfiguration due to the influence of partial charge distribution, thenumber of hydroxyl groups, and electrostatic and steric effects.Simultaneous addition of [Al(OH)4]− anions and alkaline silicatesolutions generally tend to promote the condensation process. Thecondensation products between aluminate and silicate species aremostly found to have cyclic structures, corresponding to theformation of cyclic silicate species [6]. Thus, the sodium aluminosilicate structure is expected to be initiated by condensation between[Al(OH)4]− anions and mainly oligomeric silicate species. This

condensation process results in numerous alumino silicate oligomers,and consequently the network structure gradually forms by furthercondensation between oligomers.

The SAS compositions can be controlled to develop pigmentsthat are useful in a wide range of applications such as fillers andreinforcing pigments for rubber compounds, plastics, paper and papercoating compositions, paints and adhesives [7,8]. While suchamorphous SAS pigments have been found to be useful in suchapplications, their optimum utilization is hampered by the lack ofsufficient research-based information on the appropriate reactionconditions for the synthesis of the product with desired properties.The desired properties for SAS are: high total pore volume (exceeding0.2 cm3/g), a relatively narrow pore size distribution (efficientfor scattering visible light), and oil absorption value that is withinthe range of 100 to 200 ml/100 g. Most of the existing literature hasno sufficient information on the optimum reaction conditions forthe synthesis of SAS materials with those properties. Limitedinformation is available on the material produced at wider range ofpH, simultaneous addition of reactants, stirring speed, and reactiontemperature. The relationship between the reaction conditionsand the structure developed will be a useful tool to propose thefinal application of the synthesized material.

In an attempt to develop materials with the desired properties forpaint and paper industries, a systematic study was conducted on thesynthesis of amorphous sodium alumino silicate under controlledreaction conditions. Results on the relationship between the reactionconditions and the structure of the final material are discussed. The

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products obtained are expected to be a potential low-cost alternativeas functional fillers, as TiO2 extenders, as silica extenders or asreinforcing agents for paper, paint, rubber, plastics and specialtyproducts.

2. Experimental

2.1. Materials

Sodium silicate (24% SiO2, 7.4% Na2O) was from ShinwooMaterialsCo. Ltd., South Korea. Aluminate (Al2O3 6.55% H2SO4 18.41%), dioctylphthalate (DOP) and NaOH were purchased from Duksan Chemical.

2.2. Synthesis

Various SAS samples were prepared at low, neutral, and high pH assummarized in Table 1. In all cases, sodium silicate has SiO2/Na2Omole ratio composition of 3.24. A process for the preparationof amorphous sodium alumino silicate comprises three steps:(1) separately mixing sodium silicate and sodium hydroxide pelletsin aqueous solution to develop “container A” componentswith weightratios of: Na2SiO3:NaOH:H2O=563:53:160 (2) separately mixingaluminate and water in “container B”: Al2O3:H2O=460:140. (3) Thenthe components of containers A and B were simultaneously mixedslowly at a constant stirring speed (700 rpm) while maintaining pH atdesired value (from 3 to 9.5). The final gel composition depends onthe selected pH. Thus the actual final amount of the reactants (SiO2

and Al2O3) in the final product was determined by EDS as reported inTable 1. Note that various properties of the final product depend onthe starting pH of the reaction media. Sample SAS-5 was prepared atlower concentration of NaOH (half of the one used for other samples).

The SAS reactions were conducted in a 3-L heating mantle. Theadmixing of reactants (typically done within 5 to 10 min) was done atroom temperature before increasing the reaction temperature to50 °C and maintained for 2 h. In all cases, bulk bottom water(equivalent to the half of the total weight of the other components)was first introduced at the bottom of the heating mantle. Theprepared sample was thoroughly washed and dried at 110 °C for 2 h.The SAS obtained by the present investigation have an oil absorptionamount of more than 1.0 ml/g as measured by using dioctyl phthalate(DOP). The oil absorption amount was measured as follows: 1 g of asample was placed on a polyethylene plate (which does not absorboil), DOP was dropped little by little on the center of sample from aburette and it was thoroughly kneaded with a spatula every eachdropping. The operation of dropping and kneading was repeated bysetting the end point to the time when the entire amount becomes asolid putty lump. The amount of DOP usedwas determined and the oil

Table 1Properties of sodium alumino silicate prepared at low, neutral, and high pH.

SAS-1 SAS-2 SAS-3 SAS-4 *SAS-5 SAS-6

pH 3.0 5.5 7.0 7.0 7.0 9.5**SiO2/Al2O3 (by EDS) 2.2 3.3 4.2 3.2 5.8 3.5DOP oil absorption 110 150 200 150 75 200Bulk density—g/cm3 2.8 2.7 3.0 2.5 1.0 3.0Na % (by EDS) − 6.0 5.8 7.5 3.2 6.8Al % (by EDS) 11.8 8.5 8.2 9.4 7.8 6.8Si % (by EDS) 26.0 27.4 34.6 30.4 45.6 23.7O % (by EDS) 62.2 58.1 51.4 52.7 43.4 62.7Na/Al (ratio) − 0.7 0.7 0.8 0.4 1.0BET surface area (m2/g) 80 191 227 216 318 174Pore diameter (nm) 20.4 13.1 13.0 13.7 3.8 20.5Pore volume (cm3/g) 0.38 0.56 0.67 0.67 0.25 0.85

*SAS-5: Prepared at lower concentration of NaOH (half of the amount used for the othersamples).**SiO2/Al2O3 (by EDS): It means the actual molar ratio of Si oxide to aluminum oxide inthe final product — measured using the EDS technique.

absorption amount was finally expressed in terms of ml/g (that is,volume of oil absorbed/weight of the sample).

2.3. Characterization

Themorphology of the sampleswas characterized byfield emissionscanning electron microscopy (FE-SEM) (Hitachi, S-4800) coupledwith X-ray energy dispersive spectroscopy (EDS). The acceleratingvoltage was 15 kV. X-ray diffraction patterns (XRD-6000, Shimadzu)were used to determine the crytallinity of the composite over the scanrange of 20–70°. The accelerating voltage and applied current were40 kV and 100 mA, respectively. The Brunauer–Emmett–Teller (BET)surface area and the porosity of the sampleswere studied by a nitrogenadsorption instrument (Micrometrics ASAP 2020). Pore size distribu-tion and specific desorption pore volume were determined by BJH(Barrett–Joyner–Halenda)method. The formation of the Si–O–Al bondin the prepared material was confirmed by FTIR spectra obtained by aBomem MB-100 spectrometer.

3. Results

3.1. FE-SEM, TEM, and EDS

The FE-SEM, TEM, FTIR and the qualitative EDS analyses confirmedthe formation of alumino silicates in the present study. The resultsfrom the qualitative EDS analyses showed that almost all newlyformed solid phases had similar chemical compositions; they wererich in Si, Al, O, and Na. After 2 h the microstructures of SAS werecharacterized by dense and homogeneous phases (Figs. 1 and 2)which also accounted for their large BET surface area. The micro-graphs are similar for all samples as portrayed in Fig. 2 which showsTEM image of SAS-6. Neither long-range ordered alumino silicatephase in the form of the particles in crystalline phase [9] nor smallnano-sized zeolite crystals [10] were observed in the final product.Fig. 3 shows X-ray diffraction patterns of SAS-6. Generally the XRDpattern is similar for all samples. It indicates a broad “maximum”

characteristic of true amorphous alumino silicates [11]. FTIR spectrumof SAS-6 sample (Fig. 3) reveals strong bands at 1100 cm−1 and472 cm−1 which belong to asymmetric stretching and bendingmodesof Si–O–Si, respectively [12]. The vibration at 810 cm−1 confirms theformation of Si–O–Al bonds [13]. Since IR vibrations of zeolite(crystalline) skeleton are intense for agglomerates of even a fewunit cells, the absence of the intense band at 556 cm−1 (assigned toexternal vibrations related to D-4 rings in zeolite A framework)

Fig. 1. FE-SEM micrograph of SAS-6 (inset is the EDS micrograph).

Fig. 4. The nitrogen adsorption–desorption isotherm of all SAS samples.Fig. 2. TEM micrograph of SAS-6.

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reveals that the vibrations observed are for the true amorphousnature of the SAS samples [14].

3.2. Nitrogen physisorption studies

The coalescence of primary particles results into the formation ofSAS agglomerates which have internal void volume or porosity. As aresult of this porosity, the measurement of surface area by the BETmethod includes contributions from external, as well as internal,surface area. The internal porosity or void volume is typicallymeasured by oil absorption methods (e.g., dioctyl phthalate (DOP)).The measurement of oil absorption (ml/100 g) gives a directindication of the degree of structure: very high (N200), high (175–200), medium (125–175), low (75–125), and very low (b75). The oilabsorption results confirm that the absorptivity of the SAS has a directcorrelation with the ratio of Na/Al (Table 1). The optimum value wasobtained at ratio 1.0. On the other hand, SiO2/Al2O3 molar ratio wasfound to have a direct correlation with the BET surface area obtainedin this study.

The N2 adsorption–desorption isotherms of SAS samples (Fig. 4)show a wide hysteresis cycle starting from a relative pressure of

Fig. 3. FTIR result of SAS-6. The inset shows XRD pattern.

0.4, which suggests the presence of a mesoporous material [15].Generally, they exhibit characteristic type IV curves according to theIUPAC classification. Pore size distribution of the amorphous SAS wasobtained by applying the BJHmodel on the absorption branch of its N2

adsorption–desorption isotherms (Fig. 5). The average pore sizedistribution of the SAS-5 is generally smaller than that of the rest ofSAS samples prepared in this study. The reason for this is pointed outin discussion section. The amorphous SAS is a mesoporousmaterial, asshown by the pore size distribution, possessing inter-particlemesopores of 3.8 to 20.5 nm diameter with 0.25 to 0.85 cm3/g porevolume;while the BET surface area is as high up to 318 m2/g (Table 1).A small amount of relatively larger pores at around 44.1 nm diameterwith average pore volume of about 0.4 cm3/g is also observed. Theselarge pores are formed by aggregation of SAS particles. The formationof agglomerates could happen through binding of surface hydroxyl

Fig. 5. The BJH pore size distributions of SAS-1, 2, 3, and 5.

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groups (Si–OH) of neighboring particles which can be facilitated athigh temperatures [16].

4. Discussion

The present study reasonably demonstrates how various reactionconditions affect final properties of sodium alumino silicate. Irrespec-tive of initial molar ratios of ingredients in the range investigated inthis study, the final product was always amorphous. The results of thisstudy demonstrated some clear correlations between composition,microstructure and BET surface area. From the BET surface areaobtained in this study, it is evident that themolar ratio of SiO2 to Al2O3

has a significant impact to the properties of the final product. Highconcentration of SiO2 in the initial components tends to promoteformation of material with high BET surface area (Table 1). HighestBET surface area (318 m2/g) was observed in sample SAS-5 (SiO2/Al2O3=5.8). The latter also have a narrow pore size distribution withaverage pore diameter of 3.8 nm.

The gelation process proceeds most slowly at the lowest pH (3),probably because the Si(OH)4 monomers are all fully protonated;polymerization is catalyzed by H+ [17]. The precipitating gel is formedby the aggregation of very small (2 nm) primary silica particles [2].Moreover, while aluminum is very soluble at acidic and alkaline pH[18], it only coagulates at alkaline pH. At acid medium (pH 3), theprevalent species is Al3+ and AlOH2+; Na+ was not observed (SAS-1).Rather than Si–O–Al bonds, these species favor the formation ofalkaline salts such as Na2Al(OH)2. When the pH is raised from 3 to 5.5,the silicic acid becomesmore dominant and it favors the condensationof slightly condensed species-monomers, dimmers and trimers. Inany event, since at these pH values silicate solubilization rates arelow, dimer formation and hence condensation reactions proceedslowly. Particles precipitate once they reach an estimated diameterof 2–4 nm. At higher pH the concentration of Al3+ cations, in turn,declines, and aluminate ion content rises (SAS-6). Considering,moreover, that aluminum is also highly soluble at alkaline pH, suchmedia likewise favor the reactions whereby aluminum is included inthe gel structure. But at the same time the bonding energy of the silicastructure is altered, for Si–O covalet bond energy decreases as Alcontent increases, i.e., as Al–O ionic units are incorporated.

Another relevant development in connection with gels synthe-sized at alkaline pH is the Na/Al ratio, which remains constant andequal to 1.0 (SAS-6). The reason is that certain other ions must beincluded in the structure to offset the electrical imbalance generatedwhen Al3+ ions replace Si4+ ions in the polymer. If all the aluminumadded to the system were to be included, an equal number of sodiumions would also have to be incorporated in the structure. The previousreports provide empirical evidence that particle growth in the acidicrange is the result of a combination of aggregation and polymeriza-tion. The higher the pH, the higher the solubility of silicate ions;consequently, the reaction rate can be expected to rise. Under theseconditions, particles grow to colloid size as a result of Ostwaldripening [19], in which large particles grow at the expense of smallerones to form a sol. The colloids themselves form cross-links,generating 3-D structures. In other words, at alkaline pH, all thecondensed species are ionized. This causesmutual repulsion, wherebyparticles grow more as a result of the addition of monomers to themore condensed species than through particle aggregation, sincethe very small particles–nanometrical species–would be subjected torepulsive forces, a situation that is, of course, altered by the presenceof aluminum and sodium. These phenomena were clearly observedand reported by Fernández-Jiménez et al. [20].

The reaction temperature, pH, and the aging time appear to haveplayed a very important role in controlling the chemistry (composi-tion, microstructure, and crystallinity) of the final products. As thereactions were conducted at constant stirring speed (700 rpm) and

uniform heat treatment (reactants added at room temperature thenaged for 2 h at 50 °C), it is possible that the crystallinity should havebeen developed at some stage at a particular initial compositions andreaction conditions. However, all XRD results indicate the formationof amorphous product. Perhaps the time period investigated in thisstudy was insufficient to develop crystalline phase; this may needfurther investigations if such materials are desired for applicationsother than ones intended in this study. This is beyond the scope of thepresent study.

5. Conclusion

A novel, versatile, and reproducible route has been introduced toprepare amorphous sodium alumino silicate with suitable propertiesfor paper and paint industries. The characterization techniquesemployed reveal the formation of amorphous SAS with suitableproperties. The bulk density greater than 2.5 g/cm3, pore volume upto 0.85 cm3/g and the pore diameters between 13 to 20 nm wereobtained. The BET surface area ranges from 80 to 318 m2/g, and oilabsorption from 75 to 200 ml/100 g. Our products had similar Si–O–Albond intensity and XRD pattern at all molar ratios for the studiedreaction conditions. We presume that, there might be some queriesconcerning the correlation between Si–O–Al bond intensity and themolar ratios in relation to other microstructures such as the BETsurface area and the pore size. While these might have somerelationship, it is not clearly evident, in the present research, whetherthe molar ratios of Si/Al affect the bond intensity of the final product.Further investigations may be required to provide research-basedexplanation of this phenomenon; this is beyond the objectives of ourpresentwork. Overall, the products obtained have desirable propertiesand are expected to be a potential low-cost alternative as functionalfillers, as TiO2 extenders, as silica extenders or as reinforcing agents forpaper, paint, rubber, plastics and specialty products.

Acknowledgements

This work (Grant No. 2008-00028464) was supported by theBusiness for International Cooperative R&D between Industry,Academy, and Research Institute for funding Korea Small andMediumBusiness Administration in 2008.

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