Chapter - 5. Physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids

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

Physico-chemical properties of 81 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 TMS or TES in the presence of an acidic or basic catalyst followed by supercritical drying in an autoclave. nce, 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 (Na 2 3, s-d fine chemicals, India, Na 2 3 content 36 wt%, Na 2 : 2 =1:3.33) using various strong and weak acids as catalysts keeping molar ratio of Na 2 3 :H 2 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 50 o C in a temperature controlled oven to form a gel. The formed gels were aged for 3 h at 50 o C 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

Physico-chemical properties of 82 methanol in 24 h respectively. The subsequent surface chemical modification was carried out using a mixture of methanol (MeH), 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 50 o C and 200 o C 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 (HN 3 ), sulfuric (H 2 S 4 ) acids and weak acids namely hydrofluoric (HF), acetic (CH 3 CH), formic (HCH), propionic (C 3 H 6 2 ), orthophosphric (H 3 P 4 ), tartaric (C 4 H 6 6 ), citric (C 6 H 8 7.H 2 ) acids along with their concentrations. 20 ml of 1.05 specific gravity of Sodium silicate + 2 ml of acid Hydrogel Aged Gel Hydrogel Alcogel Gelation at 50 o C lylated alcogel Aging for 3 h at 50 0 C Washed with water for 4 times in 24 h at 50 0 C Exchanged with methanol once in 24 h at 50 0 C lylated with 1:1:1 volume ratio of methanol, hexane and TMCS for 24 h at 50 o C Dried at room temperature for 24 h and 50 0 C & 200 0 C for 1 h each Hydrophobic lica Aerogel Fig. 5.1 Preparation of silica aerogels

Physico-chemical properties of 83 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 N 2 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 (K A ) as given below.

Physico-chemical properties of 84 [B] [H + ] K A = --- (5.1) [A] 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]. K A = cx2 1-x --- (5.2) 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 H 2 on sodium silicate involves the displacement of two Na groups via a bimolecular nucleophilic (S N 2-) mechanism forming the NaH as byproduct. In the next step, F - attacks the atom from back side resulting in a pentavalent intermediate, which on further nucleophilic attack of H 2 displaces the first atom of -- chain producing HF as byproduct. In the third step, again F - attacks the atom from front side which follows the second step to replace the second atom of the -- chain giving the final product as silicic acid. The byproducts NaH formed in the beginning of reaction get neutralized with HF regenerated during the reaction to produce the sodium fluoride salt as final byproduct.

Physico-chemical properties of 85 Acid reaction mechanism H + (-) Na Na H +.H - Na H - Na Na H + NaH Sodium silicate Pentavalent inetrmediate Na H H +.H - H + Na - H H (-) H H + NaH H H F - (-) H H H H F - F H 2 F - H H (-) H H + HF H 2 H H H F - H H F - H H F (-) H 2 H H H H H (-) H H F + HF ---(5.3) H 2 H H H

Physico-chemical properties of 86 Therefore, the general sol-gel reactions (hydrolysis and condensation) for various acids are as follows: Hydrolysis Acid Na 2 3 + H 2 (H) 4 + Sodium salt ---(5.4) Sodium silicate Condensation licic acid H H H n H H + H H n + 2n H 2 ---(5.5) H H H lica gel nce, the formed silica gels contain the hygroscopic H groups on their surface which are modified with TMCS as in the following reaction. Surface chemical modification H Cl (CH 3 ) 3 (CH 3 ) 3 + + 2HCl ---(5.6) H Cl (CH 3 ) 3 (CH 3 ) 3 lica surface Trimethylchlorosilane Modified silica surface Hence, the hydrophobic silica aerogels were obtained by the replacement of H from the surface H groups with the inert CH 3 groups. 5.4.1 Effect of strong acids The effect of strong acids HCl, HN 3, H 2 S 4 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 Na 2 3 :H 2 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

Physico-chemical properties of 87 general, for a condensation process (i.e. gelation) a maximum amount of H - groups and minimum amount of protons are needed [5]. At low acid concentration (<2M) both the protons and H - 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 H - 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]. Table 5.1 Effect of strong acids on physico-chemical properties of the silica aerogels Concentration of acid (M) Gelation time (min.) Volume shrinkage (%) Poro -sity (%) Pore volume (cc/g) (a) Hydrochloric acid (HCl) Thermal conductivity (W/m.K) Contact angle (deg.) 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 (HN 3 ) 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 (H 2 S 4 ) 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

Physico-chemical properties of 88 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, H 2 S 4 produced low density silica aerogels (0.160 g/cc). Density (g/cc) 0.434 0.372 0.310 0.248 0.186 0.270 0.225 0.180 0.135 0.090 0.288 0.240 0.192 0.144 0.096 H 2 S 4 HN 3 HCl 0 1 2 3 4 5 6 Acid concentration (M) Fig. 5.2 Effect of strong acid concentration on the density of the aerogel

Physico-chemical properties of 89 TEM micrographs of the aerogels prepared using H 2 S 4 are shown in fig. 5.3. For the aerogels prepared using 5M H 2 S 4, the structure is denser than that of aerogels prepared using 2M H 2 S 4. The weight ratio of Na/ estimated using AA spectroscopy in 2M H 2 S 4 catalyzed silica aerogel is 4.43 10-3. (a) 50 nm (b) 50 nm Fig. 5.3 TEM micrographs of the aerogels prepared using strong acid (a) H 2 S 4-2M (b) H 2 S 4-5M 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 -H absorption band

Physico-chemical properties of 90 [11] decreased in the manner HCl> HN 3 > H 2 S 4. And the intensity of absorption peaks at 2923 and 1450 cm -1 correspond to terminal CH 3 and peak at 845 cm -1 correspond to -C groups [12], increased in the manner HCl< HN 3 < H 2 S 4. The presence of absorption peak at 1096 cm -1 correspond to -- is expected for the silica materials [13]. % of transmission (arbitrary unit) -H C-H c b a -H C-H -C -- 4000 3000 2000 1500 1000 500 Fig. 5.4 FTIR spectra of aerogels prepared using strong acids (2M) a) HCl, b) HN 3, c) H 2 S 4 5.4.2 Effect of weak acids Wavenumber (cm -1 ) The effect of weak acids and their concentration on the physicochemical 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, CH 3 CH, HCH and H 3 P 4, the gelation time decreased with increase in concentration (<4M) and then remained constant. And for the acids C 3 H 6 2, C 4 H 6 6, C 6 H 8 7.H 2 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 H - groups. The lower gelation times for the weak acids are due to the fact that the anions of weak acids are basic [5].

Physico-chemical properties of 91 Table 5.2 Effect of weak acids on physico-chemical properties of the silica aerogels Concen- Gelation Volume Poro Pore Thermal Contact tration of time shrinka- -sity volume conductivity angle acid (M) (min.) ge (%) (%) (cc/g) (W/m.K) (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 (CH 3 CH) 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 (HCH) 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 (C 3 H 6 2 ) 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) rthophosphoric acid (H 3 P 4 ) 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 (C 4 H 6 6 ) 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 (C 6 H 8 7.H 2 ) 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

Physico-chemical properties of 92 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, CH 3 CH, HCH and C 3 H 6 2, the variation in the density is same as that of strong acids. But for the aerogels prepared using the acids H 3 P 4, C 4 H 6 6 and C 6 H 8 7.H 2, the density increased with increase in concentration, then decreased, and further increased with concentration of acid as seen from the fig. 5.6. Density (g/cc) 0.210 0.175 0.140 0.105 0.070 0.280 0.245 0.210 0.175 0.140 0.312 0.260 0.208 0.156 0.104 0.350 0.315 0.280 0.245 0.210 0 1 2 3 4 5 6 Acid concentration (M) C 3 H 6 2 HCH CH 3 CH HF Fig. 5.5 Effect of weak acid concentration on the density of the aerogel 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

Physico-chemical properties of 93 low density (0.086 g/cc). These aerogels were obtained for the molar ratio of Na 2 3 :H 2 :Citric acid:tmcs at 1:146.67:0.72:9.46. Density (g/cc) 0.252 0.210 0.168 0.126 0.084 0.245 0.210 0.175 0.140 0.105 0.238 0.204 0.170 0.136 0.102 0 1 2 3 4 5 Acid concentration (M) C 6 H 8 7.H 2 C 4 H 6 6 H 3 P 4 Fig. 5.6 Effect of weak acid concentration on the density of the aerogel 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/ weight ratio quantified using AAS in the silica aerogels catalyzed with 3M citric acid is 2.23 10-3.

Physico-chemical properties of 94 (a) 50 nm (b) 50 nm Fig. 5.7 TEM micrographs of the silica aerogels prepared using weak acid (a) Citric-2M (b) Citric-3M 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 -H absorption band [11]. But the intensity of absorption peaks at 2923 and 1450 cm -1 correspond to terminal CH 3 and peak at 845 cm -1 correspond to -C groups [13], increased in the manner CH 3 CH<HCH< C 3 H 6 2 < H 3 P 4 <C 4 H 6 6 <C 6 H 8 7.H 2. This indicates that the silica aerogels catalyzed with citric acid (3M) are hydrophobic in character.

Physico-chemical properties of 95 % of transmission (arbitrary unit) g f e d c b a -H C-H -H C-H -C -- 4000 3000 2000 1500 1000 500 Wavenumber (cm -1 ) Fig. 5.8 FTIR spectra of the aerogels prepared using weak acids (3M) a) HF, b) CH 3 CH, c)hch, d)c 3 H 6 2, e)h 3 P 4, f)c 4 H 6 6, g)c 6 H 8 7.H 2 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 148 o contact angle. Fig. 5.9 Water drop on the aerogel surface catalyzed with citric acid (3M); θ=148 o

Physico-chemical properties of 96 The as prepared silica aerogels have the thermal stability up to around 420 o C 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 100 o C. The second weight loss was observed as an exothermic peak at 420 o C. This was attributed to the oxidation of organic groups. The third weight loss was observed in the temperature range between 420 o C and 1000 o C. This gradual and continuous weight loss was attributed to the condensation of silanol groups. It increased rapidly between 420 o C and 700 o C and to a small extent above 700 o C. 100 420 o C TGA DTA 0.6 Weight (%) 98 96 94 92 0.4 0.2 0-0.2 Temperature Difference ( o C/mg) 90-0.4 0 200 400 600 800 1000 Temperature ( o C) Fig. 5.10 TG-DT analyses of the aerogel prepared using citric acid (3M) 5.4.3 N 2 adsorption-desorption of silica aerogels Fig. 5.11 shows the N 2 adsorption-desorption isotherms of the aerogels prepared using H 2 S 4-2M (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

Physico-chemical properties of 97 associated with capillary condensation taking place in mesopores, and a limiting uptake over a range of high p/p o. From figs. (a) and (c), it can be seen that the hysteresis loop is of type H 1 consisting of agglomerates of approximately uniform spheres in fairly regular array. Also, figs. (b) and (d) indicate the H 2 type hysteresis loop consisting of pores with narrow necks and wide bodies (ink-bottle pores) [14]. The maximum amount of the N 2 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 H 2 S 4-2M adsorbed maximum amount of gas saturated at 900 cc/g corresponding to a pressure of 750 mmhg. And the aerogels prepared with H 2 S 4-5M 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 N 2 gas (1450 cc/g). (a) (b) Volume adsorbed (cc/g) (c) Volume adsorbed (cc/g) (d) Pressure (mmhg) Pressure (mmhg) Fig. 5.11 N 2 adsorption-desorption isotherms of the aerogels prepared using H 2 S 4-2M (a), 5M (b) and Citric acid-2m (c), 3M (d)

Physico-chemical properties of 98 The results of fig. 5.11 are illustrated in table 5.3. It is observed that for the silica aerogels prepared using H 2 S 4, the increase in the concentration shifts the BET surface area towards maximum (458 m 2 /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 m 2 /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 m 2 /g). Table 5.3 BET surface area, pore volume and average pore diameter of the aerogels catalyzed using H 2 S 4 and citric acids. Physical Properties H 2 S 4 2M H 2 S 4 5M C 6 H 8 7. H 2 2M C 6 H 8 7.H 2 BET surface area (m 2 /g) 458 283 448 719 3M 5.5 Conclusions lica 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 Na 2 3 :H 2 :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 (148 o ). TEM spectra expressed the well connected network of silica particles with high porosity. These aerogels exhibited large specific surface area (719 m 2 /g) with mesopores in their network. And these aerogels are thermally stable up to a temperature of around 420 o C.

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Physico-chemical properties of 100