12 Chapter 5

Chapter - 5

Physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids

Physico-chemical properties of Chapter - 5 81

Chapter - 5 Physico-chemical properties of ambiently dried sodium

silicate based aerogels catalyzed with various acids

5.1 Introduction Generally silica aergels are produced by hydrolysis and condensation

of silicon alkoxides like TMOS or TEOS in the presence of an acidic or basic catalyst followed by supercritical drying in an autoclave. Since, various research groups have prepared monolithic and transparent silica aerogels applying the same method [1-5]. However, this method of preparation of aerogels in an autoclave is very risky and expensive. Because the drying occurs at high temperature and pressure with evacuation of highly flammable gases and the chemicals used are hazardous for health and costly. Hence, for commercialization, it is necessary to produce silica aerogels using low cost inorganic precursor and drying the gels at ambient pressure. Therefore, for easy availability we prepared the silica aerogels using sodium silicate precursor and dried the gels at ambient pressure. So, in this chapter we studied the effect of various acid catalysts for the preparation and characterization of silica aerogels.

5.2 Experimental The schematic presentation of the silica aerogels catalyzed with various acids is shown in fig. 5.1. Wet gels were prepared from commercial sodium silicate precursor of specific gravity 1.05 diluted from specific gravity of 1.39 (Na2SiO3, s-d fine chemicals, India, Na2SiO3 content 36 wt%, Na2O:SiO2=1:3.33) using various strong and weak acids as catalysts keeping molar ratio of Na2SiO3:H2O constant at 1:146.67. The sols were prepared by adding acid dropwise in sodium silicate solution while stirring and were kept for gelation at 50oC in a temperature controlled oven to form a gel. The formed gels were aged for 3 h at 50oC to give strength to the gel network. To study the effect of various acids, the monolithic gels were first cut into very small pieces then exchanged with 50 ml water four times so that the sodium salt trapped in the pores of gel will come out and once with


methanol in 24 h respectively. The subsequent surface chemical modification was carried out using a mixture of methanol (MeOH), hexane (Merck, India) and trimethylchlorosilane (TMCS, Fluka, Pursis grade, Switzerland) in volume ratio of 1:1:1 over a period of 24 h. After completion of surface modification, the gels were exposed to ambient air for 24 h. Dry gels obtained by ambient drying were heated at 50oC and 200oC for 1 h each, and were taken out for characterization after cooling of oven to room temperature as hydrophobic silica aerogels.

To obtain hydrophobic and low density silica aerogels, we prepared the gels varying the acid catalysts like strong acids namely hydrochloric (HCl), nitric (HNO3), sulfuric (H2SO4) acids and weak acids namely hydrofluoric (HF), acetic (CH3COOH), formic (HCOOH), propionic (C3H6O2), orthophosphric (H3PO4), tartaric (C4H6O6), citric (C6H8O7.H2O) acids along with their concentrations.

Dried at room temperature for 24 h and 50 0C & 200 0C for 1 h each

Exchanged with methanol once in 24 h at 50 0C

Gelation at 50 oC

Aging for 3 h at 500C

Hydrogel

Aged Gel

Hydrogel

Silylated alcogel

Alcogel

Hydrophobic Silica Aerogel

Washed with water for 4 times in 24 h at 50 0C

20 ml of 1.05 specific gravity of Sodium silicate + 2 ml of acid

Silylated with 1:1:1 volume ratio of methanol, hexane and TMCS for 24 h at 50oC

Fig. 5.1 Preparation of silica aerogels


5.3 Method of characterizations The density of as prepared aerogels was calculated by ratio of mass

of aerogel to its volume where mass is measured by microbalance (Dhona 100 DS, 10-5 accuracy) and volume is measured by filling the aerogel beads in a cylinder of known volume. The volume shrinkage (%), porosity (%) and pore volume were calculated from the formulae, which have been reported elsewhere [6]. Thermal conductivity of aerogel was measured using C-T meter, USA. The hydrophobicity of aerogel was tested by measuring the contact angle using contact angle meter, rame-hart, USA. Further, it was confirmed by observing the Fourier Transform Infrared Spectra (FTIR) of the aerogels. The microstructural studies were carried out using Transmission Electron Microscopy (TEM), Philips TECNAI F20 FEI. The quantity of the sodium present in the pores of aerogels is estimated by the Atomic Absorption spectroscopy (AA), Perkin Elmer, USA. The thermal stability of aerogel was checked by Thermo Gravimetric-Differential Thermal analyses (TGA-DTA). Lastly the surface area, pore volume and average pore diameter were measured using a N2 adsorption-desorption BET surface analyzer, Micromeritics Tristar 3000.

5.4 Results and discussion Acid or base catalyst can influence both the hydrolysis and

condensation rates and the structure of condensed product. Hydrolysis goes to completion when sufficient water is added. Acids serve to protonate negatively charged groups, enhancing the reaction kinetics by producing good leaving groups, and eliminating the requirement for proton transfer within the transition state. Acid catalyzed condensation is directed preferentially towards the end rather than the middles of chains, resulting in more extended, less highly branched polymers. A Bronsted acid is a compound that produces H+ ions in the solution. The strength and concentration of acids play an important role in the chemical reactions. According to Bronsted-Lowry, the strengths of acids are given by the equilibrium constant (KA) as given below.


where [B] is the concentration of base, [H+] is the concentration of protons and [A] is the acid concentration. Further, it is evidenced that the degree of dissociation or hydrolysis increases with increasing the dilution means decreasing the concentration of acid as given in the formula [7].

where x is the degree of dissociation or hydrolysis and c is the concentration of acid. Aelion et. al observed that the rate and extent of hydrolysis reaction was most influenced by the strength and concentration of the acid or base catalyst [8, 9]. They found that all strong acids behaved similarly, whereas weaker acids required longer reaction times to achieve the same extent of reaction. Therefore, in the present chapter, we studied the physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids.

The reaction mechanism of acid clarifies at what extent the acid replaces the various groups from the precursor to form the silanols. From the following acid reaction mechanism, it is very clear that the branched structure of sodium silicate is transferred to the silicic acid. As an example, weak acid (HF) has been considered (Equation 5.3). Initially, the attack of H2O on sodium silicate involves the displacement of two NaO groups via a bimolecular nucleophilic (SN2-Si) mechanism forming the NaOH as byproduct. In the next step, F- attacks the Si atom from back side resulting in a pentavalent intermediate, which on further nucleophilic attack of H2O displaces the first O atom of O-Si-O chain producing HF as byproduct. In the third step, again F- attacks the Si atom from front side which follows the second step to replace the second O atom of the O-Si-O chain giving the final product as silicic acid. The byproducts NaOH formed in the beginning of reaction get neutralized with HF regenerated during the reaction to produce the sodium fluoride salt as final byproduct.

[B][H+] [A]

KA = --- (5.1)

KA = cx2

1-x --- (5.2)


Acid reaction mechanism

Si

NaO

O O

HO H+.OH-

H+

Si

NaO-

O O

HO OH

(-)

Si

OH HO

O O

+ NaOH

F- Si

OH HO

O O

Si

OH HO

O O

F- H2O Si

OH HO

O O

F

(-)

Si

OH HO

O OH

Si

OH HO

O OH2

F-

(-)

+ HF

Si

OH HO

O OH

F- Si

OH HO

O OH

F- Si

OH HO

O OH

F

(-)

H2O

H+.OH-

Sodium silicate

Si

O Na NaO

O O

Si

O- Na NaO

O O

HO

H+ (-)

Si

NaO

O O

HO + NaOH

Pentavalent inetrmediate

Si

OH HO

H2O OH

F

(-)

Si

OH HO

HO OH

+ HF ---(5.3)


Therefore, the general sol-gel reactions (hydrolysis and condensation) for various acids are as follows:

Since, the formed silica gels contain the hygroscopic OH groups on their surface which are modified with TMCS as in the following reaction.

Hence, the hydrophobic silica aerogels were obtained by the replacement of H from the surface OH groups with the inert CH3 groups.

5.4.1 Effect of strong acids The effect of strong acids HCl, HNO3, H2SO4 and their concentration

on the physico-chemical properties of the silica aerogels have been studied as shown in table 5.1. While preparing the sols using all the acids, the molar ratio of Na2SiO3:H2O was kept constant at 1:146.67. An interesting fact noted is that the gelation time of sols strongly depends on the concentration of acid. From table 5.1 it is observed that all strong acids show same behavior in gelation means as their concentration increased to 5M, the gelation time decreased to 5 minutes. This may be because of the rates of hydrolysis and condensation reactions taking place during gel formation. In

Silica surface Trimethylchlorosilane

+

Si (CH3)3 Cl

OH Si

OH Si

O

Si (CH3)3 Cl Modified silica surface

Si (CH3)3

Si (CH3)3 O Si

O Si

O + 2HCl ---(5.6)

Surface chemical modification

Acid Na2SiO3 + H2O Si (OH)4 + Sodium salt ---(5.4) Sodium silicate Silicic acid

Hydrolysis

Condensation

+ 2n H2O ---(5.5)

Silica gel OH Si

OH Si

O n + Si HO

OH

OH

OH

n Si HO

OH

OH

OH


general, for a condensation process (i.e. gelation) a maximum amount of OH- groups and minimum amount of protons are needed [5]. At low acid concentration (3M), the gelation was faster due to presence of equal amount of protons and OH- groups. The gelation of sol takes place at isoelectric point of silica. At this point the surface charge is zero and the rate of condensation is least. The isoelectric point of silica depends on the acid used to make gel [10].

Concen-tration of acid (M)

Gelation time

(min.)

Volume shrin-

kage (%)

Poro-sity (%)

Pore volume (cc/g)

Thermal conductivity

(W/m.K)

Contact angle (deg.)

(a) Hydrochloric acid (HCl) 1 19,920 95 95 10.0 0.100


As the strength and concentration of acids affect the gelation, in the same way it influences the density of the silica aerogels. All strong acids produced only dense aerogels. Fig. 5.2 shows the effect of concentration of acids on the density of silica aerogels. It was observed that as the concentration of acid increased (>1M), the density of aerogel increased, which further decreased with an increase in concentration. At lower concentration (~1M), low density aerogels were obtained. The reason for this may be the turbid nature of the gels due to formation of silica clusters instead of a three dimensional network. At concentration greater than 2M, dense aerogels were obtained because of very fast hydrolysis and condensation reactions. Among all these three acids, H2SO4 produced low density silica aerogels (0.160 g/cc).

0 1 2 3 4 5 60.0960.1440.1920.2400.288

0.0900.1350.1800.2250.270

0.1860.2480.3100.3720.434

Acid concentration (M)

Den

sity

(g/

cc)

HCl

HNO3

H2SO4

Fig. 5.2 Effect of strong acid concentration on the density of the aerogel


TEM micrographs of the aerogels prepared using H2SO4 are shown in fig. 5.3. For the aerogels prepared using 5M H2SO4, the structure is denser than that of aerogels prepared using 2M H2SO4. The weight ratio of Na/Si estimated using AA spectroscopy in 2M H2SO4 catalyzed silica aerogel is 4.4310-3.

FTIR spectra of the silica aerogels prepared using strong acids (2M) catalyst are shown in fig. 5.4. It is evident from fig. 5.4 that the intensity of the peaks around 3400 and 1630 cm-1 correspond to O-H absorption band

Fig. 5.3 TEM micrographs of the aerogels prepared using strong acid (a) H2SO4 - 2M (b) H2SO4 - 5M

(a)

(b)

50 nm

50 nm


[11] decreased in the manner HCl> HNO3> H2SO4. And the intensity of absorption peaks at 2923 and 1450 cm-1 correspond to terminal CH3 and peak at 845 cm-1 correspond to Si-C groups [12], increased in the manner HCl< HNO3< H2SO4. The presence of absorption peak at 1096 cm-1

correspond to Si-O-Si is expected for the silica materials [13].

5.4.2 Effect of weak acids The effect of weak acids and their concentration on the physico-chemical properties of silica aerogels have been studied as given in the table 5.2. From table 5.2 it is observed that for the acids HF, CH3COOH, HCOOH and H3PO4, the gelation time decreased with increase in concentration (


Concen-tration of acid (M)

Gelation time

(min.)

Volume shrinka-ge (%)

Poro-sity (%)

Pore volume (cc/g)

Thermal conductivity

(W/m.K)

Contact angle (deg.)

(a) Hydrofluoric acid (HF) 1 450 84 89 4.5 0.117 130 2 5 81 85 3.0 0.149 133 3 3 81 82 2.5 0.156 100 4 2 81 83 2.6 0.153 100 5 2 77 89 4.4 0.118 140

(b) Acetic acid (CH3COOH) 1 20,220 96 95 10.1 0.100 146 2 540 89 84 2.8 0.152 148 3 15 86 86 3.3 0.147 144 4 5 84 85 2.9 0.150 147 5 5 80 88 3.9 0.125 140

(c) Formic acid (HCOOH) 1 24,360 98 93 6.7 0.110 147 2 1,980 84 90 4.9 0.117 143 3 35 89 85 3.0 0.149 144 4 5 84 86 3.1 0.147 145 5 5 77 89 4.2 0.120 146

(d) Propionic acid (C3H6O2) 1 4,140 97 96 12 0.090 145 2 135 84 88 4.0 0.122 135 3 5 82 89 4.1 0.121 147 4 2 77 89 4.2 0.121 147 5 7 55 94 7.8 0.107 148

(e) Orthophosphoric acid (H3PO4) 1 17,460 96 95 10.1 0.100 145 2 2,220 86 92 6.1 0.115 137 3 630 82 90 4.6 0.119 145 4 247 82 91 5.1 0.116 147 5 5 84 95 3.6 0.127 145

(f) Tartaric acid (C4H6O6) 1 734 77 93 7.0 0.112 147 2 2 82 95 3.7 0.127 144 3 30 50 94 8.0 0.105 136 4 85 59 92 6.3 0.115 147

(g) Citric acid (C6H8O7.H2O) 1 315 82 89 4.2 0.121 145 2 2 77 87 3.6 0.127 143 3 20 34 95 11 0.092 148 4 45 39 95 9.9 0.100 148

Table 5.2 Effect of weak acids on physico-chemical properties of the silica aerogels


Figs. 5.5 and 5.6 show the effect of weak acid concentration on the density of the silica aerogels. From fig. 5.5 it clears that for the silica aerogels prepared using the acids HF, CH3COOH, HCOOH and C3H6O2, the variation in the density is same as that of strong acids. But for the aerogels prepared using the acids H3PO4, C4H6O6 and C6H8O7.H2O, the density increased with increase in concentration, then decreased, and further increased with concentration of acid as seen from the fig. 5.6.

0 1 2 3 4 5 6

0.2100.2450.2800.3150.350

0.1040.1560.2080.2600.312

0.1400.1750.2100.2450.280

0.0700.1050.1400.1750.210

Weak acids produce low density aerogels because of the systematic formation of the network during gelation. The growth of silica particles occur more slowly than with strong acids, forming a well connected network. Among all the weak acids, the citric acid (3M) catalyzed silica aerogels have


Den

sity

(g/

cc)

HF

CH3COOH

HCOOH

C3H6O2

Fig. 5.5 Effect of weak acid concentration on the density of the aerogel


low density (0.086 g/cc). These aerogels were obtained for the molar ratio of Na2SiO3:H2O:Citric acid:TMCS at 1:146.67:0.72:9.46.

0 1 2 3 4 5

0.1020.1360.1700.2040.238

0.1050.1400.1750.2100.245

0.0840.1260.1680.2100.252

TEM images clarify the differences between the silica aerogels prepared using 2M and 3M citric acid as shown in fig. 5.7. The aerogels prepared using citric acid 2M shows the dense and compact texture (fig. 5.7a) while using citric acid 3M shows the loosely connected particles to a well tailored three dimensional network of silica. Na/Si weight ratio quantified using AAS in the silica aerogels catalyzed with 3M citric acid is 2.2310-3.


De

ns

ity (g/

cc)

C6H8O7.H2O

H3PO4

C4H6O6

Fig. 5.6 Effect of weak acid concentration on the density of the aerogel


Fig. 5.8 shows the FTIR spectra of silica aerogels catalyzed with weak acid (3M). It is observed that there is not much noticeable difference in the intensity of the peaks around 3400 and 1630 cm-1 which correspond to O-H absorption band [11]. But the intensity of absorption peaks at 2923 and 1450 cm-1 correspond to terminal CH3 and peak at 845 cm-1 correspond to Si-C groups [13], increased in the manner CH3COOH


Further, the hydrophobicity is confirmed by measuring the contact angle of water on the surface of aerogel. Fig. 5.9 illustrates the drop of water on a hydrophobic aerogel surface catalyzed with citric acid (3M) showing 148o contact angle.

Fig. 5.8 FTIR spectra of the aerogels prepared using weak acids (3M) a) HF, b) CH3COOH, c)HCOOH, d)C3H6O2, e)H3PO4, f)C4H6O6, g)C6H8O7.H2O

e f

g

Wavenumber (cm-1) 4000 3000 2000 1500 1000 500

% o

f tr

ansm

issi

on

(ar

bitr

ary

un

it)

d c

b

a

-OH

-OH

C-H

C-H Si-C

Si-O-Si

Fig. 5.9 Water drop on the aerogel surface catalyzed with citric acid (3M); =148o


The as prepared silica aerogels have the thermal stability up to around 420 oC as measured by the TGA-DTA as seen from the fig. 5.10. Three major weight losses were observed in thermal analysis plot. The first weight loss was attributed to the removal of moisture and adsorbed water from the system. This resulted in endothermic peak in the DTA plot, centered at the temperature of 100oC. The second weight loss was observed as an exothermic peak at 420oC. This was attributed to the oxidation of organic groups. The third weight loss was observed in the temperature range between 420oC and 1000oC. This gradual and continuous weight loss was attributed to the condensation of silanol groups. It increased rapidly between 420oC and 700oC and to a small extent above 700oC.

5.4.3 N2 adsorption-desorption of silica aerogels Fig. 5.11 shows the N2 adsorption-desorption isotherms of the

aerogels prepared using H2SO4-2M (fig. a), 5M (fig. b) and citric acid-2M (fig. c), 3M (fig. d). From the shape of the isotherms it can be observed that the materials exhibited the type IV isotherms for all the four samples, which is

Fig. 5.10 TG-DT analyses of the aerogel prepared using citric acid (3M) Temperature (oC)

Wei

ght (%

)

Tem

pera

ture

D

iffer

ence

(o C

/mg)

420oC

0 200 400 600 800 1000 90

92

94

96

98

100

0

0.4

- 0.2

0.2

- 0.4

0.6 TGA DTA


associated with capillary condensation taking place in mesopores, and a limiting uptake over a range of high p/po. From figs. (a) and (c), it can be seen that the hysteresis loop is of type H1 consisting of agglomerates of approximately uniform spheres in fairly regular array. Also, figs. (b) and (d) indicate the H2 type hysteresis loop consisting of pores with narrow necks and wide bodies (ink-bottle pores) [14]. The maximum amount of the N2 gas adsorbed by a porous solid depends on the volume of the pores present in that material. From fig. (a), it seems that the aerogels prepared with H2SO4-2M

adsorbed maximum amount of gas saturated at 900 cc/g corresponding to a pressure of 750 mmHg. And the aerogels prepared with H2SO4-5M adsorbed 750 cc/g gas at the same pressure (fig. b). As shown in figs. (c) and (d), it can be seen that the citric acid 2M and 3M catalyzed aerogels adsorbed almost same volume of N2 gas (1450 cc/g).

Fig. 5.11 N2 adsorption-desorption isotherms of the aerogels prepared using H2SO4-2M (a), 5M (b) and Citric acid-2M (c), 3M (d)

(c) (d)

Pressure (mmHg)

Volu

me

adso

rbed

(cc

/g)

(a) (b)

Volu

me

adso

rbed

(cc

/g)

Pressure (mmHg)


The results of fig. 5.11 are illustrated in table 5.3. It is observed that for the silica aerogels prepared using H2SO4, the increase in the concentration shifts the BET surface area towards maximum (458 m2/g) due to decrease in the particle size. Further, for the aerogels prepared using citric acid, the BET surface area increased from 448 to 719 m2/g when the concentration of citric acid increased from 2 to 3M. The increase in the BET surface area with decrease in the particle size can be explained on the model proposed by Zarzycki et al. [15]. According to this model, the specific surface area of a dry gel is related to particle and pore sizes. Assuming that the particles are spheres having uniform size, the specific surface area has been related to the inverse of the particle radius. Among all the strong and weak acids, the weak acid, citric acid-3M produced low density (0.086 g/cc) silica aerogels with large specific surface area (719 m2/g).

5.5 Conclusions Silica aerogels were obtained by catalyzing the hydrolysis and

condensation of sodium silicate with different acid catalysts varying their concentration followed by simultaneous solvent exchange, surface modification and ambient pressure drying. Strong acids requires a longer gelation time than the weak acids. In particular, the citric acid produced low density (0.086 g/cc) silica aerogels. These aerogels are obtained for the molar ratio of Na2SiO3:H2O:Citric acid:TMCS at 1:146.67:0.72:9.46 repectively. They have low thermal conductivity (0.09 W/m.K) with good hydrophobicity (148o). TEM spectra expressed the well connected network of silica particles with high porosity. These aerogels exhibited large specific surface area (719 m2/g) with mesopores in their network. And these aerogels are thermally stable up to a temperature of around 420oC.

Physical Properties H2SO4

2M H2SO4

5M C6H8O7.H2O 2M

C6H8O7.H2O 3M

BET surface area (m2/g) 458 283 448 719

Table 5.3 BET surface area, pore volume and average pore diameter of the aerogels catalyzed using H2SO4 and citric acids.


References [1] S. Henning and L. Svensson, Phys. Scr., 23 (1981) 697 [2] G. Poelz and R. Riethmuller, Nucl. Instrum. Methods, 195 (1982) 491 [3] M. Prassas, J. Phalippou, and J. Zarzycki, J. Mater. Sci., 19 (1984) 1665 [4] G. M. Pajonk, A. Venkateswara Rao, P. B. Wagh, and D. Haranath, J. Mater. Synt. and Processing, 5 (1997) 6 [5] A. Venkateswara Rao, G. M. Pajonk, N. N. Parvathy, J. Mater. Sci., 29 (1994) 1807 [6] A. Parvathy Rao, G. M. Pajonk, A. Venkateswara Rao, J. Mater. Sci., 40 (2005) 3481 [7] R. P. Bell F. R. S., Acids and Base: Their Quantitative Behavior, Ch. 2, p. 17 [8] R. Aelion, A. Loebel, and F. Eirich, J. Am. Chem. Soc., 72 (1950) 5705 [9] R. Aelion, A. Loebel, F. Eirich, Recueil Travaux Chimiques, 69 (1950) 61 [10] Z. Z. Vysotskii and D. N. Strazhesko, Wiley, New York, (1973) [11] C. J. Lee, G. S. Kim, and S. H. Hyun, J. Mater. Sci., 37 [11] (2002) 2237 [12] S. S. Prakash, C. J. Brinker, and A. J. Hurd, J. Non-Cryst. Solids, 190[3] (1995) 264 [13] F. Shi. L. J. Wang, J. X. Liu, S. H. Li, New Building Materials, 6 (2004) 9 (in Chinese) [14] K. S. W. Sing, D. H. Everett, R. A. W. Haw, L. Moscow, R. A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem., 57(4) (1985) 603 [15] J. Zarzycki, M. Prassas and J. Phalippou, J. Mater. Sci. 17 (1982) 3371

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12 Chapter 5