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Preparation of hydrophobic mesoporous silica powder with a high specific surface area by surface modification of a wet-gel slurry and spray-drying
Powder Technology 197 (2010) 288–294
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Preparation of hydrophobic mesoporous silica powder with a high specific surface area by surface modification of a wet-gel slurry and spray-drying Pradip B. Sarawade a, Jong-Kil Kim b, Askwar Hilonga a, Hee Taik Kim a,⁎ a b
Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea E&B Nanotech. Co., Ltd, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 June 2009 Received in revised form 15 September 2009 Accepted 10 October 2009 Available online 17 October 2009 Keywords: Mesoporous silica powder Sodium silicate Surface modification Spray-drying Surface area
a b s t r a c t A hydrophobic mesoporous silica powder was prepared by surface modification of a sodium silicate-based wet-gel slurry. The effects of the volume percentage (%V) of trimethylchlorosilane (TMCS), used as surfacemodifying agent, on the physicochemical properties of the silica powder were investigated. We observed that as the %V of TMCS in the simultaneous solvent exchange and surface modification process increased, so did the specific surface area and cumulative pore volume of the resulting silica powder. Hydrophobic silica powder with low tapping density (0.27 g/cm3), high specific surface area (870 m2/g), and a large cumulative pore volume (2.2 cm3/g) was obtained at 10%V TMCS. Surface silanol groups of the wet-gel slurry were replaced by non-hydrolysable methyl groups (–CH3), resulting in a hydrophobic silica powder as confirmed by FT-IR spectroscopy and contact angle measurements. We also employed FE-SEM, EDS, TG–DTA, and nitrogen physisorption studies to characterize the silica powders produced and to compare the properties of modified and unmodified silica powders. Moreover, we used a spray-dying technique in the present study, which significantly reduced the overall processing time, making our method suitable for economic and largescale industrial production of silica powder. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Mesoporous materials with high surface area are used as dielectric materials, elastomeric materials (such as tires), in flat panel displays, sensors, filters for exhaust gases, filters for automobile exhaust systems, industrial pollutants, adsorbents, separations, biomedicine, censors, drug delivery systems, oil-spill clean-up [1–5], and heterogeneous catalysts in various chemical reactions [6]. Aerogels are the most highly porous material available, and are normally prepared by supercritical extraction of pore liquid from wet gels [7–10]. However, economic large-scale industrial production of silica aerogels is hampered by three main factors: (i) the use of expensive autoclaves for the supercritical drying process that involves heating and evacuation of highly inflammable solvents; this is potentially risky because the process is performed at a high temperature (260 °C) and pressure (∼ 100 bars); (ii) the use of high-cost and hazardous alkoxides [tetraethoxysilane (TEOS) or trimethoxysilane (TMOS)] in the sol–gel synthesis, and (iii) the long synthesis period required (up to 7 days in some cases). Moreover, aerogels are very sensitive to moisture (hydrophilic); thus, it is essential to modify the surface of the aerogels using non-hydrolysable organic groups. The silanol (Si– OH) groups present on the gel structure are the main source of
⁎ Corresponding author. Tel.: +82 31 400 5493; fax: +82 31 500 3579. E-mail address: [email protected] (H.T. Kim). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.10.006
hydrophilicity and they promote a condensation reaction that produces H2O; the structure therefore deteriorates over time. The replacement of the hydrogen (H) in the Si–OH group with an hydrolytically stable organofunctional group such as Si–R (R = CH3, C2H5, etc.), alkoxysilanes, or alkylsilazanes and alkylchlorosilanes of the type RnSiX4 − n, can be used to produce hydrophobic silica material that is not affected by moisture [11,12]. It is reasonable to predict that aerogels in powder form might have a high surface area and show a general improvement in undesirable properties caused by the presence of hydrophobic groups. However, this has not been thoroughly investigated, and powder-form aerogels have not been compared with conventional hydrophobic aerogels produced by a supercritical drying process or by ambient pressure drying [13–17]. To the best of our knowledge, few reports have investigated the versatile and cost–effect synthesis of hydrophobic silica powders. Kim et al. synthesized hydrophobic silica powders by surface modification of wet gels [18]; however, the authors used expensive silica alkoxides such as TEOS. Recently, Bhagat et al. synthesized silica powders using sodium silicate silica as a precursor and an expensive silylating agent (hexamethyldisilazane) using a derivatization method [19]. The silica powders they prepared had a low surface area (700 m2/g) and low pore volume (0.9 cm3/g). Furthermore, their method was fairly complicated and may not be suitable for economical large-scale industrial production of silica powder. To develop a versatile and cost-effective method to produce hydrophobic mesoporous silica powders with a high surface area and
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large pore volume, we used an inexpensive silica precursor (sodium silicate). Moreover, we employed a spray-drying technique to speedup drying of the final product. These modifications make our proposed method suitable for large-scale industrial production of silica powder. Basically, the heating mechanism used differentiates the spray-drying technique from the conventional ambient pressure-drying (APD) method. In the conventional APD method, the heat energy is transferred to the interior of the gel by conduction. The rate of heat conduction depends on the thermal conductivity of the wet-gel slurry, and decreases significantly as the moisture content in the gel decreases, thereby prolonging the drying process [12,20–22]. In contrast, in the spray-drying process, size-controlled drops of liquid or slurry are dispersed through the nozzle into a hot air chamber. Liquid (ethanol) present throughout the wet-gel slurry absorbs the hot air, facilitating uniform heating of the entire mass dispersed through the nozzle. Ultimately, this accelerates the drying process and thereby reduces the total drying time as well as the production cost. This drying process is a one-step rapid process that eliminates the need for additional processing, and is therefore suitable for use in large-scale industrial production of silica powders. 2. Experimental 2.1. Materials Sodium silicate (24% SiO2, 7.4% Na2O) was purchased from Shinwoo Materials Co. Ltd., South Korea. Sulfuric acid (98%), trimethylchlorosilane (TMCS), and n-hexane were purchased from Duksan Chemicals. All reagents were used without further purification. 2.2. Preparation of a wet-gel slurry by drop-wise addition of sulfuric acid Scheme 1 shows the flow chart for the preparation of silica wet-gel slurry from aqueous sodium silicate solution by drop-wise addition of sulfuric acid. The reaction took place in aqueous alkaline conditions at
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room temperature (∼ 25 °C). Initially, 24% sodium silicate solution was mixed with 4% sodium chloride (NaCl) solution in a reaction vessel under continuous stirring. Sodium chloride solution was used to control the polymerization rate during the reaction between sodium silicate and sulfuric acid. The reaction was carried out by adding sulfuric acid (8%) solution to the aqueous sodium silicate solution. The sulfuric acid was added at a constant drop-rate (200 ml/ min) by a chemical feed pump under constant stirring speed (500 rpm). The sulfuric acid solution was added until the reaction pH became ∼7. The reaction was conducted in a 3-l heating mantle. The admixing of reactants (typically completed within 10 min) was done at room temperature before increasing the reaction temperature up to 60 °C; this temperature was then maintained for 2 h. In all cases, bulk bottom NaCl solution (4%) (equivalent to half of the total weight of the other components) was first introduced at the bottom of the heating mantle. 2.3. Washing (removal of by-products), surface modification, and spraydrying The prepared wet-gel slurry was thoroughly washed and filtered using a filter press (laboratory scale) to obtain the wet cake. The removal of Na+ from the washed cake was confirmed using a sodium ion detector (NeoMet, ISTEK, pH/ISE meter). Furthermore, the washed cake was chemically modified with trimethylchlorosilane in the presence of n-hexane/ethanol solution. For this purpose, the obtained cake (100 g) was mixed with n-hexane (100 ml) solution with different %V of TMCS and ethanol for 12 h at room temperature (∼25 °C). In all cases, the molar ratio of ethanol/TMCS (M1) was fixed at 1. Ethanol was used to slow down the reaction between pore water and TMCS. After surface modification of the wet cake, the slurry was removed and washed with fresh n-hexane to remove unreacted TMCS. The wet-gel slurry was then dried using a spray-drying technique. For this purpose, 10% (200 ml) of modified wet-gel slurry was mixed with fresh ethanol (solvent). Then, it was continuously stirred and fed into the spray-drier with a peristaltic pump that operates at a speed of 10 rpm. The liquid was atomized at 10 kPa in a nozzle using the airflow of 75 m2/min. The spray-drier's outlet and inlet temperatures were maintained at 98 and 180 °C, respectively. These processing conditions were maintained throughout the drying process. The total drying period for 200 ml slurry was 20 min. This process (spray-drying) is a one-step rapid procedure that eliminates the additional processing and makes the present route a suitable alternative for large-scale industrial production. The mesoporous silica powder obtained through the present route had a maximum oil absorption of more than 3.4 ml/g, as measured by dioctyl phthalate (DOP). The oil absorption amount was measured as follows: 10 g of a sample was placed on a polyethylene plate (which does not absorb oil), DOP was added drop-by-drop to the center of a sample from a burette, and the sample and DOP were thoroughly kneaded with a spatula after each drop. The operation of dropping and kneading was repeated by setting the end point to the time when the entire amount became a solid lump of putty. The amount of DOP used was determined and the oil absorption amount was finally expressed in terms of ml/g (that is, volume of oil absorbed/weight of the sample). 2.4. Characterization methods
Scheme 1. A flow chart showing experimental procedures for the synthesis of mesoporous silica powder.
The hydrophobicity of the samples was tested by measuring the percentage of water absorbed by placing the mesoporous silica powder directly on top of a water surface. The changes in weight of the samples over time were then recorded using an electronic microbalance (Model OHAUS EPG214C USA) with an accuracy of 10− 5 g. The tapping density of mesoporous silica powder was determined by filling a cylindrical column of known volume with silica powder; densities were then calculated from the mass of the silica powder to
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volume ratio. Observations were taken three times per sample and the average value of these three measurements was recorded as the tapping density of the silica powder. The morphology and the atomic percentage of residual components in the silica powders were investigated using field-emission scanning electron microscopy (FESEM) (Hitachi, S-4800) and X-ray energy dispersive spectroscopy (EDS), respectively. The latter was also employed to examine the purity of the final product. The specific surface areas of mesoporous silica powders were analyzed using the Brunauer–Emmet–Teller (BET) method while the pore size distributions (PSDs) were determined using the BJH method (ASAP 2020, Micromeritics, USA). BET analysis based on the amount of N2 gas adsorbed at various partial pressures (ten points 0.05 < p/po < 0.3, nitrogen molecular cross sectional area = 0.162 nm2) was used to determine the surface area, and a single condensation point (p/po = 0.99) was used to determine the pore size and pore volume. Before N2 adsorption, samples were degassed at 200 °C. Pore size distributions were calculated from the desorption isotherms [12]. To investigate the thermal stability of the silica powders (in terms of retention of hydrophobicity), the powders were examined by thermogravimetric and differential thermal analysis (TG–DTA). Ten milligrams of the hydrophobic nanoporous silica powder was heat-treated in air — from room temperature (25 °C) up to 1000 °C at a controlled heating rate of 1.5 °C min− 1 using a microprocessor-based Parr temperature controller (Model 4846) connected to a muffle furnace (A.H. JEON Industrial Co. Ltd. Korea). Here, thermal stability refers to the temperature up to which the silica powder retained its hydrophobicity [23]. Surface modifications were confirmed using Fourier transform infrared (FT-IR) spectroscopy (Perkin-Elmer spectrophotometer, Model No. 783). For this purpose, silica powders were mixed with KBr, and pressed to form a sample pellet for FT-IR measurements. The contact angle (θ) measurements were performed using a contact angle meter (CAM 200 automated contact angle analyzer, Finland) to quantify the degree of hydrophobicity. For this purpose, a water droplet of 12 μl was placed at three different places on the surface of the hydrophobic silica powder pellet and the average value was then taken as the contact angle (θ).
3. Results and discussion
2ðCH3 Þ3 Si–Cl½TMCS þ H2 O→ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 ½HMDSO þ 2HCl ð2Þ 2ðCH3 Þ3 –Si–O–CH2 CH3 þ H2 O→ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 þ 2C2 H5 OH ð3Þ ðCH3 Þ3 –Si–Cl þ ≡Si–OH→≡Si–O–SiðCH3 Þ3 þ HCl:
ð4Þ
Thus, TMCS reacts with ethanol (Eq. (1)), pore water (Eq. (2)), and the OH group of the silica surface (Eq. (3)), to form ethyltrimethoxysilane [ETMS] and hexamethyldisiloxane (HMDSO); consequently, the hydrophilic surface of the silica network becomes hydrophobic. As the reaction proceeds, transferable liquids (HCl/residual ethanol) spontaneously come out of the wet gel [25]. During the simultaneous solvent exchange/surface modification process, the reaction between ethanol and TMCS can help to slow down the reaction rate of TMCS with the OH groups on the silica surface. This process spontaneously replaces pore water with n-hexane. The chemical surface modification of hydrogels with non-polar alkyl/aryl groups is an indispensable step before the drying process, as it prohibits the formation of new siloxane bonds between the adjacent silica cluster and therefore irreversible shrinkage of the gel [26,27]. 3.2. FT-IR studies and thermal stability of the mesoporous silica powder Chemical bonding in the sodium silicate-based mesoporous silica powders was studied using FT-IR spectroscopy. FT-IR spectra of the samples synthesized at various %V of TMCS are shown in Fig. 1. Generally, the intensity of most of the peaks increased with an increase in the %V of TMCS. The broad absorption peak centered at 3450 cm− 1 is attributed to the O–H stretching band of hydrogenbonded water molecules (H–O–H…H2O). The strong absorption peak at 2970 cm− 1 corresponds to the terminal –CH3 group, and those at 1260 and 850 cm− 1 are due to Si–C bonding. These bands, which are not present in the spectra of unmodified silica powder, are attributed to and provide substantial support for surface modification of the wetgel slurry by TMCS [25,28]. In contrast, in unmodified silica powder, a strong –OH peak at 3540 cm− 1 indicates that a significant fraction of the Si atoms exist on the surface of a gel slurry network as
Sodium silicate (Na2SiO3) has been and probably always will be the cheapest source of relatively pure silicic acid from which silica gel can be made. Sodium silicate reacts with water and acid (sulfuric acid) to give silicic acid. The silicic acid condenses to form small silica particles, chains, and consequently a silica network (silica gel/slurry). The preparation of hydrogels involves a sol to gel transition. This transition is referred to as gelation; the sol becomes highly viscous and seizes to move, even if the beaker is tilted or inverted [24]. The gelation time is highly dependent on the amount of catalyst added to the sol form.
3.1. Mechanism of simultaneous solvent exchange and surface modification In the present work, the surface of the sodium silicate-based wetgel slurry was organically modified with trimethyl groups present in trimethylchlorosilane by a simultaneous solvent exchange/surface modification process. The substitution of the H of Si–OH is essential to produce hydrophobic mesoporous silica powder, as described in the Introduction of this article. The expected chemical reactions for the one-step solvent exchange and surface modification of the hydrogels are as follows: C2 H5 OH þ ðCH3 Þ3 –Si–Cl→ðCH3 Þ3 –Si–O–CH2 CH3 ½ETMS þ HCl
ð1Þ
Fig. 1. FT-IR spectra of (a) unmodified silica powder together with those modified with (c) 4 (e) 8 and (f) 10%V of TMCS.
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hydroxylated species. In the modified silica gel slurry, 4, 6, and 10%V TMCS exhibited significantly reduced –OH peak intensities [18]. The weak absorption peak at 1640 cm− 1 corresponds to deformation vibrations of the adsorbed water molecules [29]. The strong peaks at 1100 and 471 cm− 1 correspond to Si–O–Si bonding and they are the most informative about the structure of the silica network [9,30]. As a consequence of surface modification, the mesoporous silica powder exhibited hydrophobic behavior. In unmodified silica powder, Si–OH groups react to produce Si–O–Si bonds by a condensation process (as discussed in the introduction). The formation of Si–O–Si bonds is known to cause the collapse of the pore network of the wet gel because of the high capillary pressure that these bonds induce [31]. Fig. 2 demonstrates the purity and hydrophobicity of the final product. Fig. 2(a) shows the EDS spectra — no peaks for other elements are evident, except for the expected peaks for Si, O, and C, emphasizing the purity of the final product. Hydrophobicity was assessed by placing the silica powder directly on the surface of water (Fig. 2(b)). The powder floated on the surface for a number of days without absorbing significant amounts of water (see Table 1 for details). The wettability of materials has been shown to depend on their surface chemistry [32]. Our water absorption studies showed that the percentage of water absorption of silica powder decreased with an increase in the %V of TMCS. Table 1 shows the contact angles of various solvents on pellets of the powder products. The contact angle of water on silica pellets was nearly 0° for unmodified silica — it quickly absorbs water droplets due to the presence of hydrolysable –
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OH groups on the surface; however, the contact angle increased up to 155° for the modified silica powder (Fig. 2(c)).
3.3. TG–DTA curves To investigate the thermal stability and the hydrophobic nature of the mesoporous silica powder, 10 mg of silica powder was heated in a furnace at various temperatures. It has been observed that silica powders retain their hydrophobic behavior up to a maximum temperature of 345 °C; above this temperature, they become hydrophilic. This is due to the fact that at temperatures above 345 °C, the surface methyl groups (–CH3) are oxidized, resulting into the formation of hydrophilic silica powder. The thermal stability of the silica powder and oxidation temperature for the –CH3 groups were estimated by thermogravimetric (TG) and differential thermal (DT) analyses, respectively. Fig. 3 shows the TG–DT analysis of the mesoporous silica powder surface modified with 10%V TMCS in nhexane in the presence of ethanol solvent via a simultaneous solvent exchange and surface modification process in an oxygen atmosphere up to 1000 °C. The curves clearly show that the modified mesoporous silica powder exhibited negligible weight loss up to a temperature of 345 °C; above this temperature, the silica powder showed significant weight loss. This decrease in weight is due to oxidation of the surface methyl groups. This is clearly demonstrated by the two sharp exothermic peaks in the DTA curve that appear when the temperature is raised above 345 °C, confirming the oxidation of surface methyl
Fig. 2. (a) EDS spectra of the sodium silicate-based mesoporous silica powder. (b) Photograph showing superhydrophobicity of mesoporous silica powder on water surface. (c) Water droplet placed on the hydrophobic silica surface (pellet form). It shows a large contact angle (up to 155°).
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Table 1 Effects of %V of TMCS on the physicochemical properties of the sodium silicate-based mesoporous silica powder prepared by simultaneous solvent exchange/surface modification of the wet-gel slurry and spray-drying. No.
Volume % of TMCS
Taping density (g/cm3)
BET surface area (m2/g)
Pore diameter (nm)
Pore volume (cm3/g)
Contact angle (θo)
Oil absorption (ml/g)
Water absorption (after six months) (ml/g)
(a) (b) (c) (d) (e) (f) (g)
0 2 4 6 8 10 12
0.96 0.71 0.59 0.46 0.38 0.27 0.26
610 746 765 789 813 870 879
8.3 10.1 12.3 12.9 13.7 16.4 17.1
0.7 1.2 1.4 1.7 1.9 2.2 2.3
∼0 135 138 141 149 155 156
2.10 2.89 3.06 3.17 3.23 3.40 3.41
2.30 0.49 0.33 0.27 0.20 0.14 0.11
groups [33]. In contrast, unmodified silica powder showed continuous weight loss with increasing temperature.
during the final stages of drying also serves to expand the surface, allowing partial recovery of the wet-gel slurry structure [21]. 3.5. FE-SEM studies
3.4. Tapping density and the physical properties of mesoporous silica powder The tapping densities of mesoporous silica samples (unmodified and modified with different %V of TMCS) are summarized in Table 1. The tapping density of mesoporous silica powder decreased from 0.96 to 0.27 g/cm− 3 (modified silica samples) with an increase in the %V of TMCS. The tapping density of the silica powder modified with 10%V TMCS was 0.27 g/cm3. This is much lower than that of unmodified silica powder (0.96 g/cm3), and nearly equal to that of ambient pressure-dried silica aerogel (0.10 g/cm3) [34]. As illustrated in Table 1, the specific surface area of mesoporous silica powder gradually increased as the %V of TMCS increased. The silica gel modified with 10%V TMCS had a high surface area (870 m2/g), large pore size (16.4 nm), and high pore volume (2.2 cm3/g). In contrast, the unmodified silica powder had a low specific surface area (610 m2/g), low pore volume (0.96 cm3/g), and small pore size (8.3 nm). This indicates that the hydrophobic surface of the pore network reduces capillary pressure by lowering the surface tension and consequently causes less shrinkage during the aging and drying stages. This phenomenon has also been reported in previous studies [20]; in other words, the increase in %V of TMCS increased the extent of surface modification of silica powder and consequently increased the surface area of the powder. Furthermore, it has been reported that the repulsion between –CH3 groups on the surface of the powder
Fig. 3. TG–DTA curves of (a) unmodified and (b) 10%V of TMCS-modified mesoporous silica powder.
The mesoporous structure of the silica powders was confirmed by field-emission scanning electron microscopy (FE-SEM). Fig. 4 shows the pore characteristics and structural morphology of mesoporous silica based on FE-SEM micrographs of (a) unmodified and (b) 10%V TMCS-modified silica powders. Generally, the unmodified silica powder was characterized by dense aggregates of spheres. The modified silica powder had a highly porous structure (Fig. 4(b)). This is because the unmodified wet-gel silica slurry shrunk more during spray-drying at ambient pressure than the modified silica slurry, leading to the formation of dense, dried silica powder microstructures and the loss of mesopores present in it [22]. All the samples modified with TMCS had mesopores with an average pore diameter in the range of 10–17 nm. This is because during drying of the modified silica powder, reversible shrinkage of the wet-gel slurry occurred as a consequence of organic modification of trimethyl groups on the silica surface. The unmodified silica powder showed more dense aggregates of silica particles while the product modified with 10%V TMCS had a highly porous silica network with uniform pore size. 3.6. Nitrogen physisorption studies The porosity properties of mesoporous (unmodified and modified) silica powder obtained at various V% of TMCS were further compared by examining nitrogen adsorption–desorption isotherms and pore size distributions. Fig. 5 shows the N2 adsorption/desorption isotherms of unmodified and modified (4, 8, and 10%V TMCS) silica powder. The unmodified samples showed the typical adsorption/ desorption curve of xerogels with hysteresis behavior, which indicates the presence of ink bottle-shaped pores. In contrast, the curves for the modified silica were very similar to those of the aerogels prepared by the supercritical method, although with a relatively low surface area and pore size. The physisorption isotherms obtained for all the mesoporous silica powder exhibited hysteresis loops due to the characteristic features of mesoporous materials (Type IV isotherms) [35,36]. Fig. 6 shows pore size distribution (PSDs) curves of unmodified and modified silica powders. The pore size distribution of the unmodified silica powder had a narrow distribution of pores with a peak pore diameter of 8.3 nm. In contrast, the pore size distribution of the modified silica powders had a broad distribution of pores with the peak pore diameters ranging between 10 and 17 nm. The peak pore diameter increased as the volume of TMCS increased from 2 to 10%. This enlargement of pores is considered to be due to the increase in non-polar alkyl groups (–CH3) on the silica surface that reduce drying shrinkage and thereby increase pore size. As per the IUPAC classification of pores [37], all the silica powders showed a pronounced peak in the mesopore region (2–50 nm), which indicates that the
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Fig. 4. FE-SEM micrograph (low and high magnification) of (a) unmodified and (b) TMCS-modified (10%V of TMCS) mesoporous silica powders.
modified silica powders maintained a mesoporous structure even after spray-drying. 4. Conclusions Hydrophobic mesoporous silica powder was obtained by surface modification of wet-gel slurry with different %V of trimethylchlor-
Fig. 5. N2 adsorption/desorption isotherms of (a) unmodified silica powder together with those modified with (c) 4, (e) 8 and (f) 10%V of TMCS.
osilane (TMCS) followed by a spray-drying process. We established that the properties of the final product were strongly affected by the % V of the surface-modifying agent (TMCS). The properties examined were the surface area, pore diameter, hydrophobicity, and morphology. The surface area of the obtained mesoporous silica powders ranged from 600 to 900 m2/g, and increased as the %V of TMCS
Fig. 6. Pore size distributions of (a) unmodified silica powder together with those modified with (c) 4, (e) 8 and (f) 10%V of TMCS.
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increased. A hydrophobic mesoporous silica powder with superior properties in terms of surface area (870 cm2/g), pore volume (2.2 cm3/g), hydrophobicity (contact angle 155°), pore diameter (16.4 nm), and density (0.27 g/cm3) was obtained when 10%V TMCS was used for surface modification. Taken together, our results demonstrate that our synthesis route, which exploits a low-cost silica source (sodium silicate) and spray-drying technique, is suitable for economic and large-scale industrial production of silica powders.
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Preparation of hydrophobic mesoporous silica powder with a high specific surface area by surface modification of a wet-gel slurry and spray-drying Pradip B. Sarawade a, Jong-Kil Kim b, Askwar Hilonga a, Hee Taik Kim a,⁎ a b
Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Korea E&B Nanotech. Co., Ltd, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 June 2009 Received in revised form 15 September 2009 Accepted 10 October 2009 Available online 17 October 2009 Keywords: Mesoporous silica powder Sodium silicate Surface modification Spray-drying Surface area
a b s t r a c t A hydrophobic mesoporous silica powder was prepared by surface modification of a sodium silicate-based wet-gel slurry. The effects of the volume percentage (%V) of trimethylchlorosilane (TMCS), used as surfacemodifying agent, on the physicochemical properties of the silica powder were investigated. We observed that as the %V of TMCS in the simultaneous solvent exchange and surface modification process increased, so did the specific surface area and cumulative pore volume of the resulting silica powder. Hydrophobic silica powder with low tapping density (0.27 g/cm3), high specific surface area (870 m2/g), and a large cumulative pore volume (2.2 cm3/g) was obtained at 10%V TMCS. Surface silanol groups of the wet-gel slurry were replaced by non-hydrolysable methyl groups (–CH3), resulting in a hydrophobic silica powder as confirmed by FT-IR spectroscopy and contact angle measurements. We also employed FE-SEM, EDS, TG–DTA, and nitrogen physisorption studies to characterize the silica powders produced and to compare the properties of modified and unmodified silica powders. Moreover, we used a spray-dying technique in the present study, which significantly reduced the overall processing time, making our method suitable for economic and largescale industrial production of silica powder. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Mesoporous materials with high surface area are used as dielectric materials, elastomeric materials (such as tires), in flat panel displays, sensors, filters for exhaust gases, filters for automobile exhaust systems, industrial pollutants, adsorbents, separations, biomedicine, censors, drug delivery systems, oil-spill clean-up [1–5], and heterogeneous catalysts in various chemical reactions [6]. Aerogels are the most highly porous material available, and are normally prepared by supercritical extraction of pore liquid from wet gels [7–10]. However, economic large-scale industrial production of silica aerogels is hampered by three main factors: (i) the use of expensive autoclaves for the supercritical drying process that involves heating and evacuation of highly inflammable solvents; this is potentially risky because the process is performed at a high temperature (260 °C) and pressure (∼ 100 bars); (ii) the use of high-cost and hazardous alkoxides [tetraethoxysilane (TEOS) or trimethoxysilane (TMOS)] in the sol–gel synthesis, and (iii) the long synthesis period required (up to 7 days in some cases). Moreover, aerogels are very sensitive to moisture (hydrophilic); thus, it is essential to modify the surface of the aerogels using non-hydrolysable organic groups. The silanol (Si– OH) groups present on the gel structure are the main source of
⁎ Corresponding author. Tel.: +82 31 400 5493; fax: +82 31 500 3579. E-mail address: [email protected] (H.T. Kim). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.10.006
hydrophilicity and they promote a condensation reaction that produces H2O; the structure therefore deteriorates over time. The replacement of the hydrogen (H) in the Si–OH group with an hydrolytically stable organofunctional group such as Si–R (R = CH3, C2H5, etc.), alkoxysilanes, or alkylsilazanes and alkylchlorosilanes of the type RnSiX4 − n, can be used to produce hydrophobic silica material that is not affected by moisture [11,12]. It is reasonable to predict that aerogels in powder form might have a high surface area and show a general improvement in undesirable properties caused by the presence of hydrophobic groups. However, this has not been thoroughly investigated, and powder-form aerogels have not been compared with conventional hydrophobic aerogels produced by a supercritical drying process or by ambient pressure drying [13–17]. To the best of our knowledge, few reports have investigated the versatile and cost–effect synthesis of hydrophobic silica powders. Kim et al. synthesized hydrophobic silica powders by surface modification of wet gels [18]; however, the authors used expensive silica alkoxides such as TEOS. Recently, Bhagat et al. synthesized silica powders using sodium silicate silica as a precursor and an expensive silylating agent (hexamethyldisilazane) using a derivatization method [19]. The silica powders they prepared had a low surface area (700 m2/g) and low pore volume (0.9 cm3/g). Furthermore, their method was fairly complicated and may not be suitable for economical large-scale industrial production of silica powder. To develop a versatile and cost-effective method to produce hydrophobic mesoporous silica powders with a high surface area and
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large pore volume, we used an inexpensive silica precursor (sodium silicate). Moreover, we employed a spray-drying technique to speedup drying of the final product. These modifications make our proposed method suitable for large-scale industrial production of silica powder. Basically, the heating mechanism used differentiates the spray-drying technique from the conventional ambient pressure-drying (APD) method. In the conventional APD method, the heat energy is transferred to the interior of the gel by conduction. The rate of heat conduction depends on the thermal conductivity of the wet-gel slurry, and decreases significantly as the moisture content in the gel decreases, thereby prolonging the drying process [12,20–22]. In contrast, in the spray-drying process, size-controlled drops of liquid or slurry are dispersed through the nozzle into a hot air chamber. Liquid (ethanol) present throughout the wet-gel slurry absorbs the hot air, facilitating uniform heating of the entire mass dispersed through the nozzle. Ultimately, this accelerates the drying process and thereby reduces the total drying time as well as the production cost. This drying process is a one-step rapid process that eliminates the need for additional processing, and is therefore suitable for use in large-scale industrial production of silica powders. 2. Experimental 2.1. Materials Sodium silicate (24% SiO2, 7.4% Na2O) was purchased from Shinwoo Materials Co. Ltd., South Korea. Sulfuric acid (98%), trimethylchlorosilane (TMCS), and n-hexane were purchased from Duksan Chemicals. All reagents were used without further purification. 2.2. Preparation of a wet-gel slurry by drop-wise addition of sulfuric acid Scheme 1 shows the flow chart for the preparation of silica wet-gel slurry from aqueous sodium silicate solution by drop-wise addition of sulfuric acid. The reaction took place in aqueous alkaline conditions at
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room temperature (∼ 25 °C). Initially, 24% sodium silicate solution was mixed with 4% sodium chloride (NaCl) solution in a reaction vessel under continuous stirring. Sodium chloride solution was used to control the polymerization rate during the reaction between sodium silicate and sulfuric acid. The reaction was carried out by adding sulfuric acid (8%) solution to the aqueous sodium silicate solution. The sulfuric acid was added at a constant drop-rate (200 ml/ min) by a chemical feed pump under constant stirring speed (500 rpm). The sulfuric acid solution was added until the reaction pH became ∼7. The reaction was conducted in a 3-l heating mantle. The admixing of reactants (typically completed within 10 min) was done at room temperature before increasing the reaction temperature up to 60 °C; this temperature was then maintained for 2 h. In all cases, bulk bottom NaCl solution (4%) (equivalent to half of the total weight of the other components) was first introduced at the bottom of the heating mantle. 2.3. Washing (removal of by-products), surface modification, and spraydrying The prepared wet-gel slurry was thoroughly washed and filtered using a filter press (laboratory scale) to obtain the wet cake. The removal of Na+ from the washed cake was confirmed using a sodium ion detector (NeoMet, ISTEK, pH/ISE meter). Furthermore, the washed cake was chemically modified with trimethylchlorosilane in the presence of n-hexane/ethanol solution. For this purpose, the obtained cake (100 g) was mixed with n-hexane (100 ml) solution with different %V of TMCS and ethanol for 12 h at room temperature (∼25 °C). In all cases, the molar ratio of ethanol/TMCS (M1) was fixed at 1. Ethanol was used to slow down the reaction between pore water and TMCS. After surface modification of the wet cake, the slurry was removed and washed with fresh n-hexane to remove unreacted TMCS. The wet-gel slurry was then dried using a spray-drying technique. For this purpose, 10% (200 ml) of modified wet-gel slurry was mixed with fresh ethanol (solvent). Then, it was continuously stirred and fed into the spray-drier with a peristaltic pump that operates at a speed of 10 rpm. The liquid was atomized at 10 kPa in a nozzle using the airflow of 75 m2/min. The spray-drier's outlet and inlet temperatures were maintained at 98 and 180 °C, respectively. These processing conditions were maintained throughout the drying process. The total drying period for 200 ml slurry was 20 min. This process (spray-drying) is a one-step rapid procedure that eliminates the additional processing and makes the present route a suitable alternative for large-scale industrial production. The mesoporous silica powder obtained through the present route had a maximum oil absorption of more than 3.4 ml/g, as measured by dioctyl phthalate (DOP). The oil absorption amount was measured as follows: 10 g of a sample was placed on a polyethylene plate (which does not absorb oil), DOP was added drop-by-drop to the center of a sample from a burette, and the sample and DOP were thoroughly kneaded with a spatula after each drop. The operation of dropping and kneading was repeated by setting the end point to the time when the entire amount became a solid lump of putty. The amount of DOP used was determined and the oil absorption amount was finally expressed in terms of ml/g (that is, volume of oil absorbed/weight of the sample). 2.4. Characterization methods
Scheme 1. A flow chart showing experimental procedures for the synthesis of mesoporous silica powder.
The hydrophobicity of the samples was tested by measuring the percentage of water absorbed by placing the mesoporous silica powder directly on top of a water surface. The changes in weight of the samples over time were then recorded using an electronic microbalance (Model OHAUS EPG214C USA) with an accuracy of 10− 5 g. The tapping density of mesoporous silica powder was determined by filling a cylindrical column of known volume with silica powder; densities were then calculated from the mass of the silica powder to
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volume ratio. Observations were taken three times per sample and the average value of these three measurements was recorded as the tapping density of the silica powder. The morphology and the atomic percentage of residual components in the silica powders were investigated using field-emission scanning electron microscopy (FESEM) (Hitachi, S-4800) and X-ray energy dispersive spectroscopy (EDS), respectively. The latter was also employed to examine the purity of the final product. The specific surface areas of mesoporous silica powders were analyzed using the Brunauer–Emmet–Teller (BET) method while the pore size distributions (PSDs) were determined using the BJH method (ASAP 2020, Micromeritics, USA). BET analysis based on the amount of N2 gas adsorbed at various partial pressures (ten points 0.05 < p/po < 0.3, nitrogen molecular cross sectional area = 0.162 nm2) was used to determine the surface area, and a single condensation point (p/po = 0.99) was used to determine the pore size and pore volume. Before N2 adsorption, samples were degassed at 200 °C. Pore size distributions were calculated from the desorption isotherms [12]. To investigate the thermal stability of the silica powders (in terms of retention of hydrophobicity), the powders were examined by thermogravimetric and differential thermal analysis (TG–DTA). Ten milligrams of the hydrophobic nanoporous silica powder was heat-treated in air — from room temperature (25 °C) up to 1000 °C at a controlled heating rate of 1.5 °C min− 1 using a microprocessor-based Parr temperature controller (Model 4846) connected to a muffle furnace (A.H. JEON Industrial Co. Ltd. Korea). Here, thermal stability refers to the temperature up to which the silica powder retained its hydrophobicity [23]. Surface modifications were confirmed using Fourier transform infrared (FT-IR) spectroscopy (Perkin-Elmer spectrophotometer, Model No. 783). For this purpose, silica powders were mixed with KBr, and pressed to form a sample pellet for FT-IR measurements. The contact angle (θ) measurements were performed using a contact angle meter (CAM 200 automated contact angle analyzer, Finland) to quantify the degree of hydrophobicity. For this purpose, a water droplet of 12 μl was placed at three different places on the surface of the hydrophobic silica powder pellet and the average value was then taken as the contact angle (θ).
3. Results and discussion
2ðCH3 Þ3 Si–Cl½TMCS þ H2 O→ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 ½HMDSO þ 2HCl ð2Þ 2ðCH3 Þ3 –Si–O–CH2 CH3 þ H2 O→ðCH3 Þ3 –Si–O–Si–ðCH3 Þ3 þ 2C2 H5 OH ð3Þ ðCH3 Þ3 –Si–Cl þ ≡Si–OH→≡Si–O–SiðCH3 Þ3 þ HCl:
ð4Þ
Thus, TMCS reacts with ethanol (Eq. (1)), pore water (Eq. (2)), and the OH group of the silica surface (Eq. (3)), to form ethyltrimethoxysilane [ETMS] and hexamethyldisiloxane (HMDSO); consequently, the hydrophilic surface of the silica network becomes hydrophobic. As the reaction proceeds, transferable liquids (HCl/residual ethanol) spontaneously come out of the wet gel [25]. During the simultaneous solvent exchange/surface modification process, the reaction between ethanol and TMCS can help to slow down the reaction rate of TMCS with the OH groups on the silica surface. This process spontaneously replaces pore water with n-hexane. The chemical surface modification of hydrogels with non-polar alkyl/aryl groups is an indispensable step before the drying process, as it prohibits the formation of new siloxane bonds between the adjacent silica cluster and therefore irreversible shrinkage of the gel [26,27]. 3.2. FT-IR studies and thermal stability of the mesoporous silica powder Chemical bonding in the sodium silicate-based mesoporous silica powders was studied using FT-IR spectroscopy. FT-IR spectra of the samples synthesized at various %V of TMCS are shown in Fig. 1. Generally, the intensity of most of the peaks increased with an increase in the %V of TMCS. The broad absorption peak centered at 3450 cm− 1 is attributed to the O–H stretching band of hydrogenbonded water molecules (H–O–H…H2O). The strong absorption peak at 2970 cm− 1 corresponds to the terminal –CH3 group, and those at 1260 and 850 cm− 1 are due to Si–C bonding. These bands, which are not present in the spectra of unmodified silica powder, are attributed to and provide substantial support for surface modification of the wetgel slurry by TMCS [25,28]. In contrast, in unmodified silica powder, a strong –OH peak at 3540 cm− 1 indicates that a significant fraction of the Si atoms exist on the surface of a gel slurry network as
Sodium silicate (Na2SiO3) has been and probably always will be the cheapest source of relatively pure silicic acid from which silica gel can be made. Sodium silicate reacts with water and acid (sulfuric acid) to give silicic acid. The silicic acid condenses to form small silica particles, chains, and consequently a silica network (silica gel/slurry). The preparation of hydrogels involves a sol to gel transition. This transition is referred to as gelation; the sol becomes highly viscous and seizes to move, even if the beaker is tilted or inverted [24]. The gelation time is highly dependent on the amount of catalyst added to the sol form.
3.1. Mechanism of simultaneous solvent exchange and surface modification In the present work, the surface of the sodium silicate-based wetgel slurry was organically modified with trimethyl groups present in trimethylchlorosilane by a simultaneous solvent exchange/surface modification process. The substitution of the H of Si–OH is essential to produce hydrophobic mesoporous silica powder, as described in the Introduction of this article. The expected chemical reactions for the one-step solvent exchange and surface modification of the hydrogels are as follows: C2 H5 OH þ ðCH3 Þ3 –Si–Cl→ðCH3 Þ3 –Si–O–CH2 CH3 ½ETMS þ HCl
ð1Þ
Fig. 1. FT-IR spectra of (a) unmodified silica powder together with those modified with (c) 4 (e) 8 and (f) 10%V of TMCS.
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hydroxylated species. In the modified silica gel slurry, 4, 6, and 10%V TMCS exhibited significantly reduced –OH peak intensities [18]. The weak absorption peak at 1640 cm− 1 corresponds to deformation vibrations of the adsorbed water molecules [29]. The strong peaks at 1100 and 471 cm− 1 correspond to Si–O–Si bonding and they are the most informative about the structure of the silica network [9,30]. As a consequence of surface modification, the mesoporous silica powder exhibited hydrophobic behavior. In unmodified silica powder, Si–OH groups react to produce Si–O–Si bonds by a condensation process (as discussed in the introduction). The formation of Si–O–Si bonds is known to cause the collapse of the pore network of the wet gel because of the high capillary pressure that these bonds induce [31]. Fig. 2 demonstrates the purity and hydrophobicity of the final product. Fig. 2(a) shows the EDS spectra — no peaks for other elements are evident, except for the expected peaks for Si, O, and C, emphasizing the purity of the final product. Hydrophobicity was assessed by placing the silica powder directly on the surface of water (Fig. 2(b)). The powder floated on the surface for a number of days without absorbing significant amounts of water (see Table 1 for details). The wettability of materials has been shown to depend on their surface chemistry [32]. Our water absorption studies showed that the percentage of water absorption of silica powder decreased with an increase in the %V of TMCS. Table 1 shows the contact angles of various solvents on pellets of the powder products. The contact angle of water on silica pellets was nearly 0° for unmodified silica — it quickly absorbs water droplets due to the presence of hydrolysable –
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OH groups on the surface; however, the contact angle increased up to 155° for the modified silica powder (Fig. 2(c)).
3.3. TG–DTA curves To investigate the thermal stability and the hydrophobic nature of the mesoporous silica powder, 10 mg of silica powder was heated in a furnace at various temperatures. It has been observed that silica powders retain their hydrophobic behavior up to a maximum temperature of 345 °C; above this temperature, they become hydrophilic. This is due to the fact that at temperatures above 345 °C, the surface methyl groups (–CH3) are oxidized, resulting into the formation of hydrophilic silica powder. The thermal stability of the silica powder and oxidation temperature for the –CH3 groups were estimated by thermogravimetric (TG) and differential thermal (DT) analyses, respectively. Fig. 3 shows the TG–DT analysis of the mesoporous silica powder surface modified with 10%V TMCS in nhexane in the presence of ethanol solvent via a simultaneous solvent exchange and surface modification process in an oxygen atmosphere up to 1000 °C. The curves clearly show that the modified mesoporous silica powder exhibited negligible weight loss up to a temperature of 345 °C; above this temperature, the silica powder showed significant weight loss. This decrease in weight is due to oxidation of the surface methyl groups. This is clearly demonstrated by the two sharp exothermic peaks in the DTA curve that appear when the temperature is raised above 345 °C, confirming the oxidation of surface methyl
Fig. 2. (a) EDS spectra of the sodium silicate-based mesoporous silica powder. (b) Photograph showing superhydrophobicity of mesoporous silica powder on water surface. (c) Water droplet placed on the hydrophobic silica surface (pellet form). It shows a large contact angle (up to 155°).
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Table 1 Effects of %V of TMCS on the physicochemical properties of the sodium silicate-based mesoporous silica powder prepared by simultaneous solvent exchange/surface modification of the wet-gel slurry and spray-drying. No.
Volume % of TMCS
Taping density (g/cm3)
BET surface area (m2/g)
Pore diameter (nm)
Pore volume (cm3/g)
Contact angle (θo)
Oil absorption (ml/g)
Water absorption (after six months) (ml/g)
(a) (b) (c) (d) (e) (f) (g)
0 2 4 6 8 10 12
0.96 0.71 0.59 0.46 0.38 0.27 0.26
610 746 765 789 813 870 879
8.3 10.1 12.3 12.9 13.7 16.4 17.1
0.7 1.2 1.4 1.7 1.9 2.2 2.3
∼0 135 138 141 149 155 156
2.10 2.89 3.06 3.17 3.23 3.40 3.41
2.30 0.49 0.33 0.27 0.20 0.14 0.11
groups [33]. In contrast, unmodified silica powder showed continuous weight loss with increasing temperature.
during the final stages of drying also serves to expand the surface, allowing partial recovery of the wet-gel slurry structure [21]. 3.5. FE-SEM studies
3.4. Tapping density and the physical properties of mesoporous silica powder The tapping densities of mesoporous silica samples (unmodified and modified with different %V of TMCS) are summarized in Table 1. The tapping density of mesoporous silica powder decreased from 0.96 to 0.27 g/cm− 3 (modified silica samples) with an increase in the %V of TMCS. The tapping density of the silica powder modified with 10%V TMCS was 0.27 g/cm3. This is much lower than that of unmodified silica powder (0.96 g/cm3), and nearly equal to that of ambient pressure-dried silica aerogel (0.10 g/cm3) [34]. As illustrated in Table 1, the specific surface area of mesoporous silica powder gradually increased as the %V of TMCS increased. The silica gel modified with 10%V TMCS had a high surface area (870 m2/g), large pore size (16.4 nm), and high pore volume (2.2 cm3/g). In contrast, the unmodified silica powder had a low specific surface area (610 m2/g), low pore volume (0.96 cm3/g), and small pore size (8.3 nm). This indicates that the hydrophobic surface of the pore network reduces capillary pressure by lowering the surface tension and consequently causes less shrinkage during the aging and drying stages. This phenomenon has also been reported in previous studies [20]; in other words, the increase in %V of TMCS increased the extent of surface modification of silica powder and consequently increased the surface area of the powder. Furthermore, it has been reported that the repulsion between –CH3 groups on the surface of the powder
Fig. 3. TG–DTA curves of (a) unmodified and (b) 10%V of TMCS-modified mesoporous silica powder.
The mesoporous structure of the silica powders was confirmed by field-emission scanning electron microscopy (FE-SEM). Fig. 4 shows the pore characteristics and structural morphology of mesoporous silica based on FE-SEM micrographs of (a) unmodified and (b) 10%V TMCS-modified silica powders. Generally, the unmodified silica powder was characterized by dense aggregates of spheres. The modified silica powder had a highly porous structure (Fig. 4(b)). This is because the unmodified wet-gel silica slurry shrunk more during spray-drying at ambient pressure than the modified silica slurry, leading to the formation of dense, dried silica powder microstructures and the loss of mesopores present in it [22]. All the samples modified with TMCS had mesopores with an average pore diameter in the range of 10–17 nm. This is because during drying of the modified silica powder, reversible shrinkage of the wet-gel slurry occurred as a consequence of organic modification of trimethyl groups on the silica surface. The unmodified silica powder showed more dense aggregates of silica particles while the product modified with 10%V TMCS had a highly porous silica network with uniform pore size. 3.6. Nitrogen physisorption studies The porosity properties of mesoporous (unmodified and modified) silica powder obtained at various V% of TMCS were further compared by examining nitrogen adsorption–desorption isotherms and pore size distributions. Fig. 5 shows the N2 adsorption/desorption isotherms of unmodified and modified (4, 8, and 10%V TMCS) silica powder. The unmodified samples showed the typical adsorption/ desorption curve of xerogels with hysteresis behavior, which indicates the presence of ink bottle-shaped pores. In contrast, the curves for the modified silica were very similar to those of the aerogels prepared by the supercritical method, although with a relatively low surface area and pore size. The physisorption isotherms obtained for all the mesoporous silica powder exhibited hysteresis loops due to the characteristic features of mesoporous materials (Type IV isotherms) [35,36]. Fig. 6 shows pore size distribution (PSDs) curves of unmodified and modified silica powders. The pore size distribution of the unmodified silica powder had a narrow distribution of pores with a peak pore diameter of 8.3 nm. In contrast, the pore size distribution of the modified silica powders had a broad distribution of pores with the peak pore diameters ranging between 10 and 17 nm. The peak pore diameter increased as the volume of TMCS increased from 2 to 10%. This enlargement of pores is considered to be due to the increase in non-polar alkyl groups (–CH3) on the silica surface that reduce drying shrinkage and thereby increase pore size. As per the IUPAC classification of pores [37], all the silica powders showed a pronounced peak in the mesopore region (2–50 nm), which indicates that the
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Fig. 4. FE-SEM micrograph (low and high magnification) of (a) unmodified and (b) TMCS-modified (10%V of TMCS) mesoporous silica powders.
modified silica powders maintained a mesoporous structure even after spray-drying. 4. Conclusions Hydrophobic mesoporous silica powder was obtained by surface modification of wet-gel slurry with different %V of trimethylchlor-
Fig. 5. N2 adsorption/desorption isotherms of (a) unmodified silica powder together with those modified with (c) 4, (e) 8 and (f) 10%V of TMCS.
osilane (TMCS) followed by a spray-drying process. We established that the properties of the final product were strongly affected by the % V of the surface-modifying agent (TMCS). The properties examined were the surface area, pore diameter, hydrophobicity, and morphology. The surface area of the obtained mesoporous silica powders ranged from 600 to 900 m2/g, and increased as the %V of TMCS
Fig. 6. Pore size distributions of (a) unmodified silica powder together with those modified with (c) 4, (e) 8 and (f) 10%V of TMCS.
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increased. A hydrophobic mesoporous silica powder with superior properties in terms of surface area (870 cm2/g), pore volume (2.2 cm3/g), hydrophobicity (contact angle 155°), pore diameter (16.4 nm), and density (0.27 g/cm3) was obtained when 10%V TMCS was used for surface modification. Taken together, our results demonstrate that our synthesis route, which exploits a low-cost silica source (sodium silicate) and spray-drying technique, is suitable for economic and large-scale industrial production of silica powders.
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