physico-chemical properties of ambiently dried sodium silicate...

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 (<2M) both the protons and OH- groups are present in less

amount leading to very slow gelation of the sol. At higher 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 <90

2 1,470 91 85 2.9 0.150 <90

3 10 86 84 2.8 0.151 135

4 5 77 87 3.6 0.137 138

5 5 73 89 4.4 0.118 145

(b) Nitric acid (HNO3)

1 30,420 86 95 9.7 0.101 140

2 2,940 89 86 3.3 0.145 <90

3 90 86 88 3.8 0.146 137

4 5 81 86 3.1 0.147 134

5 5 77 88 3.9 0.125 147

(c) Sulfuric acid (H2SO4)

1 33,480 95 92 6.1 0.112 140

2 2,700 80 91 5.7 0.114 140

3 1,800 84 85 3.1 0.147 100

4 5 86 78 1.8 0.170 100

5 5 84 79 2.0 0.165 110

Table 5.1 Effect of strong acids on physico-chemical properties of the silica aerogels


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 6

0.096

0.144

0.192

0.240

0.288

0.090

0.135

0.180

0.225

0.270

0.186

0.248

0.310

0.372

0.434

Acid concentration (M)

Den

sit

y (

g/c

c)

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

4.43×10-3.

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

(<4M) and then remained constant. And for the acids C3H6O2, C4H6O6,

C6H8O7.H2O the gelation time first decreased with an increase in

concentration and then increased. The increase in gelation time at higher

concentration may be because of the presence of large amount of protons

and less amount of OH- groups. The lower gelation times for the weak acids

are due to the fact that the anions of weak acids are basic [5].

Fig. 5.4 FTIR spectra of aerogels prepared using strong acids (2M) a) HCl, b) HNO3, c) H2SO4

a

b

c

Wavenumber (cm-1)

4000 3000 2000 1500 1000 500

%

of

tra

nsm

issio

n (

arb

itra

ry u

nit

)

-OH

C-H

C-H -OH

Si-C

Si-O-Si


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

sit

y (

g/c

c)

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

0.102

0.136

0.170

0.204

0.238

0.105

0.140

0.175

0.210

0.245

0.084

0.126

0.168

0.210

0.252

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 2.23×10-3.


De

ns

ity (

g/c

c)

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<HCOOH< C3H6O2<

H3PO4<C4H6O6<C6H8O7.H2O. This indicates that the silica aerogels

catalyzed with citric acid (3M) are hydrophobic in character.

Fig. 5.7 TEM micrographs of the silica aerogels prepared using weak acid (a) Citric-2M (b) Citric-3M

(a)

(b)

50 nm

50 nm


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 t

ran

sm

issio

n (

arb

itra

ry u

nit

)

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); θ=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)

Weig

ht

(%)

Tem

pera

ture

Dif

fere

nce (

oC

/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)

Vo

lum

e a

dso

rbed

(cc/g

)

(a) (b)

Vo

lum

e a

dso

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.


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physico-chemical properties of ambiently dried sodium silicate...

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