J. Phys. Chem. B 2004, 108, 5263-5268
5263
Plugged Hexagonal Templated Silica (PHTS): An In-Depth Study of the Structural Characteristics Ellen Van Bavel,* Pegie Cool, Katrien Aerts,† and Etienne F. Vansant UniVersity of Antwerp, Department of Chemistry, Laboratory of Adsorption and Catalysis, UniVersiteitsplein 1, 2610 Wilrijk, Belgium, and UniVersity of Antwerp, Department of Chemistry, Micro and Trace Analysis Centre, UniVersiteitsplein 1, 2610 Wilrijk, Belgium ReceiVed: January 14, 2004; In Final Form: February 27, 2004
Hexagonal mesoporous silica materials (PHTS) are synthesized with a high TEOS/EO20PO70EO20 ratio containing microporous nanocapsules or plugs inside the mesoporous channels. This PHTS material exhibits a tunable amount of open and plugged pores, has a high micropore volume (up to 0.24 mL/g), and is more stable than conventional micellar templated structures known so far. This study focuses on a thourough investigation of the influence of the amount of TEOS, the stirring temperature, the aging temperature, and the stirring time on the material characteristics using N2 sorption, X-ray diffraction, and scanning electron microscopy. PHTS materials with different amounts of TEOS were synthesized at stirring temperatures between 60 and 100 °C. At 60 °C, materials synthesized with a molar ratio TEOS/P123 of 125 proved to have a more rigid silicate framework than materials synthesized with lower amounts of TEOS resulting in PHTSs with a smaller diameter. The amount of TEOS and the stirring temperatures proved to play an important role in the material formed. Unlike SBA-15, the micropore volume of PHTS tended to increase with increasing temperature where the maximum value was found for the highest amount of TEOS used (TEOS/P123 ) 125). The influence of the aging temperature on the structural characteristics of PHTS is the same as that of the stirring temperature but less pronounced. An aging temperature of 80 °C improved the structural characteristics greatly and also enlarged the particle size, whereas 4 h was shown as the minimum stirring time required.
Introduction Mesoporous molecular sieves have attracted increasing attention because of their high surface area, large pore volume, and tunable uniform pore size. These special features make them very suitable for applications in catalysis, separation, and adsorption.1,2 Since the discovery of the M41S materials, several new mesophase structures have been developed, e.g., FSM-16,3 HMS,4 PCH,5 MSU,6 KIT,7 SBA,8 and MCF9 materials, of which the 2-D hexagonally ordered SBA-15 received a lot of attention over the past few years. This is due to its high thermal and hydrothermal stability compared to M41S8,10 and other materials,10 tunable pore size,8,11-17 and the presence of microporosity.17,18 The coexistence of meso- and micropores is one of the interesting properties of SBA-15. Materials with combined micro- and mesoporosity can offer significant supplementary advantages such as an improved diffusion rate for transport in catalytic processes, multifunctionality to process a large variety of feedstocks, or the capability to encapsulate waste in the micropores.19 The existence and location of the micropores in SBA-15 were first reported by Ryoo et al.18 who found that SBA-15 synthesized with the triblockcopolymer EO20PO70EO20 consists not only of large, uniform, and ordered mesoporous channels but also of micropores and smaller mesopores which provide connectivity between the mesochannels. This was revealed through carbon20 and platinum18 replication: platinum and carbon were prepared in SBA-15 which resulted in the formation of 2-D hexagonally ordered platinum and carbon nanowires that retained this structure upon removal of the SBA* Author to whom correspondence should be adressed. Tel.: +32 3 8202379; Fax: +32 3 8202375; E-mail:
[email protected]. † Department of Chemistry, Micro and Trace Analysis Centre.
15 template. The micropores in the silica wall originate from the penetration of the EO blocks into the silica wall17 during synthesis so that microporosity is generated upon calcination. Recently, the existence of micropores was confirmed by several structural elucidation studies.16,21,22 X-ray diffraction16 showed the existence of a microporous corona around the mesopores; NMR investigations21 have also shown deep occlusion of the EO blocks into the silica wall, and EPR spectroscopy22 demonstrated that the majority of the PEO chains are located in the micropores. Several research groups have reported the systematic control of the structural characteristics of SBA-15.8,11-17,22-25 Increasing the synthesis temperature results in a decrease of the micropore volume and an increase in the mesopore diameter,14-17 owing to the partial dehydration of the EO blocks at elevated temperatures.17 As a consequence, the EO blocks become more hydrophobic, increasing the hydrophobic core of the micelle resulting in an increased mesopore diameter.15,17 On the other hand the partial dehydration of the EO blocks decreases the interactions between micelles through the PEO chains so that these become less occluded into the silica wall with the resulting decrease in microporosity.15,17 The influence of the EO- and PO-block lengths was also clarified.8,13,23 The length of the hydrophilic EO-block determines the silica mesostructure (lamellar, hexagonal, or cubic) and influences the wall thickness of SBA-15. The hydrophobic PO-block affects the pore diameter and the templating ability as longer PO-blocks result in more highly ordered domains and more well-defined particles. Finally, the total polymer length determines the unit cell parameter. Upon increasing the salt concentration the pore volume, pore size, specific surface area, and micropore volume were found to
10.1021/jp049815a CCC: $27.50 © 2004 American Chemical Society Published on Web 04/06/2004
5264 J. Phys. Chem. B, Vol. 108, No. 17, 2004 decrease.24 Sodium chloride is reported to play the role of structure maker during micellization of PEO-PPO-PEO block copolymers because of its self-hydration through hydrogen bonding. As a consequence, the PEO blocks are expected to contain less water, resulting in samples with lower structural characteristics compared to those synthesized in the absence of sodium chloride.24 Studies on the variation of the TEOS/ triblockcopolymer ratio were also performed.14,22,25,26 A higher ratio results in an increased micropore volume,14,22 an increased pore wall thickness,14,26 and a morphology change from the curved cylinders, typical of the hexagonal SBA-15, to spheres.25 Control of the network connectivity was also reported.26 However, recently it was demonstrated that when a high TEOS/ EO20PO70EO20 ratio was used, a part of the channels contained internal microporous nanocapsules or plugs.19,27-29 This could be inferred from the two-step desorption branch.19,27,28 These materials, referred to as Plugged Hexagonal Templated Silica or PHTS, exhibit a tunable amount of open and plugged pores, have a very high micropore volume, and are more stable than the conventional micellar templated structures known so far.19,27-29 This, in combination with the high stability of PHTS, makes this material very promising. Recently, Kruk et al.30 investigated the structural characteristics of plugged SBA-15 materials through argon adsorption indicating that all constrictions are likely to exhibit diameters above ∼4 nm. Up to now in the literature, no systematic study was performed of the influence of the synthesis parameters on the structural characteristics of PHTS. In this paper a detailed investigation of the effects of the amount of TEOS, the stirring temperature, the aging temperature, and the stirring time on the structural properties of PHTS materials will be performed for the first time. Experimental Section Synthesis. SBA-15 was synthesized as follows: 4 g of P123 (EO20PO70EO20, BASF) was dissolved in 30 g of water and 120 g of a 2 M HCl solution with stirring at 45 °C. Then 8.5 g TEOS (tetraethoxysilane, 98% Acros) was added. The resulting mixture was stirred for 8 h at 45 °C and aged overnight under static conditions at 80 °C unless specified otherwise. PHTS materials were prepared as follows: 4 g of P123 was dissolved in 30 g of water and 120 g of a 2 M HCl solution with stirring at a fixed temperature between 60 and 100 °C. Then an amount of TEOS of 12 g, 15 g, or 18 g (TEOS/P123 ) 84, 104 and 125, respectively) was added. The resulting mixture was stirred for 8 h between 60 and 100 °C and aged overnight under static conditions at 80 °C unless specified otherwise. Determination of the Cloud Point. The cloud point of P123 in the H2O/HCl solution was visually determined as follows. The P123 solution was sealed in an ampule and was put in a beaker filled with water. The temperature of the water was increased and the turbidity of the solution was visually checked every 5 °C against a black background for contrast because the solution turns white at the cloud point. Characterization. X-ray diffractograms were recorded on a Philips PW1840 powder diffractometer, using Ni-filtered Cu KR (0.154 nm) radiation. Porosity and surface area studies were performed on a Quantachrome Autosorb-1-MP automated gas adsorption system using nitrogen as the adsorbate at liquid nitrogen temperature (77 K). All samples were outgassed under vacuum for 16 h at 200 °C before adsorption measurements. The pore diameter was obtained from the N2 adsorption branch using the BJH method. The surface area was calculated using
Van Bavel et al.
Figure 1. Nitrogen adsorption-desorption isotherms of (×) PHTS8.5/60/80, (2) PHTS-12/60/80, (]) PHTS-15/60/80, and (b) PHTS18/60/80. The isotherms were offset vertically by 0, 250, 450, and 600 cm3 g-1, respectively.
the BET method and the micropore volume was calculated using the t-plot method. SEM images were recorded using a JEOLJSM-6300 scanning electron microscope operating at an accelerating voltage of 20-30 kV. The samples were sputtered with a thin film of gold. Results and Discussion Various PHTS samples have been prepared by changing the stirring temperature, aging temperature, the stirring time, and the amount of TEOS. PHTS materials will be denoted as PHTSx/y/z in which x is the quantity of TEOS in grams, y is the stirring temperature in degrees centigrade, and z is the aging temperature in degrees centigrade. All the isotherms of the samples discussed in this article that are not shown are available as Supporting Information. Influence of the Stirring Temperature and the Amount of TEOS. PHTS samples with different amounts of TEOS were prepared at stirring temperatures ranging from 60° to 100 °C. When materials are synthesized with copolymers it is important to know the cloud point of the copolymer since synthesis conducted near and above the cloud point lead to disordered materials.23 The cloud point is the onset of macrophase separation caused by the dehydration of the PEO corona31 resulting in the clustering of micelles into large aggregates.32 For a 1% P123 aqueous solution this cloud point is at 90 °C,33 but since it is influenced by the weight percentage of the surfactant and the acidity of the solution31 it was also determined here for this solution. It was not possible to determine the cloud point through DSC, because all the water was already evaporated before reaching the cloud point. Alternatively it was visually determined. At 70 °C the solution started to get foggy and ultimately between 85 and 90 °C it turned white, which indicated the macrophase separation of the surfactant from the solution and consequently the cloud point. PHTS materials were prepared both near and above the cloud point at stirring temperatures between 80 and 100 °C so that the effect on the porosity properties of PHTS materials could be determined. Consequently, the results of the PHTSs synthesized between a stirring temperature of 60 and 80 °C (below the cloud point), and 80 and 100 °C (above the cloud point) will be discussed separately. a. Below the Cloud Point (60-80 °C). Figure 1 shows the nitrogen sorption isotherms of PHTS materials synthesized with
Plugged Hexagonal Templated Silica (PHTS)
Figure 2. X-ray diffractogram of (a) PHTS-8.5/60/80, (b) PHTS-12/ 60/80, (c) PHTS-15/60/80, and (d) PHTS-18/60/80.
varying amounts of TEOS (8.5, 12, 15 and 18 g) which were stirred and aged at, respectively, 60 °C and 80 °C. The X-ray diffraction patterns from these samples are shown in Figure 2. The reflections can be assigned to the (100), (110), and (200) diffractions of the 2-d hexagonal space group p6mm.8 As can be seen in Figure 1, all the materials exhibit a two-step desorption isotherm. Combined with the p6mm XRD pattern for a hexagonally ordered structure, the adsorption-desorption behavior is consistent with a structure comprising both open (first desorption step) and blocked (second desorption step) cylindrical mesopores.19,27,28 For detailed information about this sorption behavior see ref 19. The nitrogen sorption data of the PHTS samples with different amounts of TEOS prepared at stirring temperatures ranging from
J. Phys. Chem. B, Vol. 108, No. 17, 2004 5265 60° to 100 °C are shown in Figure 3 a-d. Note that the data of samples synthesized with 8.5 g of TEOS (as in a typical SBA15 synthesis) are not shown. Synthesis of materials with this amount of TEOS at stirring temperatures higher than 60 °C give rise to materials with a very broad pore size distribution and the absence of a two-step desorption isotherm so that these data are excluded. Figure 3a represents the change in diameter as a function of the stirring temperature for different amounts of TEOS. For the samples synthesized at 60 °C, the diameter is smaller when a larger amount of TEOS is used. This can also be visualized in Figure 1 by the shifting of the capillary condensation step toward higher relative pressures with a decreasing amount of TEOS. However, this trend does not continue for the samples synthesized at higher stirring temperatures. If one looks more closely to the data in Figure 3, the diameter for 12 g TEOS is the same for stirring temperatures between 60 and 80 °C, stagnates for 15 g at 70 °C, and shows a maximum for 18 g at 80 °C. The stagnation of the diameter can be explained as follows. Figure 3b shows the open mesopore volume as a function of the stirring temperature from which can be deduced that the open mesopore volume for PHTS-18/ 60/80 and PHTS-18/70/80 is generally lower than that of their 12 g and 15 g analogues. Consequently it can be assumed that PHTS-18/60/80 and PHTS-18/70/80 have more plugs. Another aspect that needs to be considered is the synthesis conditions. The samples of 60 °C stirring temperature are stirred for 8 h and are aged at 80 °C for 16 h so that the impact of the aging is considerable larger than the stirring. In combination with the data from Figure 3b, it can be concluded that PHTS-18/60/80 and PHTS-18/70/80 have a more rigid silicate framework than their 12 g and 15 g analogues. For these samples the impact of aging on the structure is less, so that the diameter formed during stirring is still maintained during aging. Another structural parameter under investigation was the micropore volume. As can be seen from Figure 3c, the micropore volume of PHTS materials increases with increasing stirring temperature with a maximum of 0.24 mL/g for PHTS-18/70/80. This behavior is opposed to that of SBA-15 because in this case the micropore volume decreases with increasing synthesis temperature.15,17
Figure 3. Influence of the stirring temperature on (a) the diameter, (b) the open mesopore volume, (c) the micropore volume, and (d) the total pore volume of PHTS materials synthesized with (2) 12 g, (]) 15 g, and (b) 18 g of TEOS.
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Van Bavel et al. TABLE 1: Structural Characteristics of SBA-15 and PHTS Samples as a Function of the Ageing Temperaturea sample
D (Å)
SBET (m2/g)
Vmicro (cm3/g)
Vtotal (cm3/g)
PHTS-18/60/60 62 400 0.06 0.42 PHTS-18/60/80 69 656 0.16 0.62 PHTS-18/60/90 72 712 0.17 0.69 SBA-15-8.5/45/60 51 606 0.10 0.44 SBA-15-8.5/45/80 69 740 0.14 0.81 SBA-15-8.5/45/90 75 864 0.11 1.04 a Notation: D: pore diameter from the adsorption branch; SBET: BET specific surface area; Vmicro: micropore volume; Vtotal: total pore volume.
Figure 4. Adsorption isotherms of PHTS-12/80/80 (2), PHTS-15/80/ 80 (]), PHTS-18/80/80 (b), PHTS-18/90/80 (O), and PHTS-18/100/ 80 (×). The isotherms were offset vertically by 0, 170, 270, 530, and 700 cm3 g-1, respectively.
Since the microporosity in the pore wall tends to decrease with increasing temperature, the increase in micropore volume must be the consequence of the presence of more microporous plugs or an increase of the microporosity of the plugs themselves. Since the hydration of the PEO and PPO blocks is temperature sensitive31 the latter explanation seems more plausible. Kruk et al.30 proposed the plug formation as a partial or complete solubilization of unhydrolyzed or partially hydrolyzed excess TEOS into the micelles. Subsequently, as the temperature is increased the PEO blocks will withdraw themselves into the hydrophobic micellar core and into the plugs rendering these more microporous. Synthesis of PHTSs near the cloud point, e.g. at 70 and 80 °C, could account for the high micropore volumes found. Because of the high dehydration of the PEO corona there is a large withdrawal of the PEO blocks into the plugs. Comparing the data of the total pore volume (Figure 3d) and the open mesopore volume (Figure 3b) for the different amounts of TEOS shows an interesting observation: the data for 12 and 15 g of TEOS show an opposite trend compared to this of 18 g of TEOS. On going from 70 to 80 °C, both the total pore volume and the open mesopore volume decrease distinctively for the 12 and 15 g TEOS samples while the samples synthesized with 18 g show an increase in these parameters. Second, as can be seen in Figure 4 the isotherms of PHTS-12/80/80 and PHTS-15/80/80 show a broad hysteresis between P/P0 0.75 and 1, whereas samples synthesized with 18 g TEOS show this phenomenon only from a 90 °C stirring temperature on. These hystereses are indicative of pore blocking effects, and clearly the amount of TEOS plays an important role here. From all these data it can be concluded that the porosity of PHTS materials is strongly influenced by the amount of TEOS and the stirring temperature. b. AboVe the Cloud Point (80-100 °C). Several interesting observations can be made showing the remarkable effect of the amount of TEOS. Beside the cloud point, another element should be taken into consideration at these high synthesis temperatures. The gelling rate will increase because of the temperature increase34 so that, as a result, TEOS will be readily hydrolyzed and condensed upon addition to the synthesis mixture. Moreover, the synthesis is conducted above the cloud point so that the PEO chains are dehydrated.31 Consequently, these factors will influence the properties of the synthesized PHTS materials at these high temperatures.
Figure 5. SEM images of SBA-15 synthesized at an aging temperature of (a) 60 °C and (b) 90 °C.
A direct consequence of conducting syntheses above the cloud point is a decrease in the micropore volume which is observed for all samples from 80 °C on (Figure 3c). For the other investigated properties the samples synthesized with 18 g of TEOS showed an opposite trend compared to the ones synthesized with 12 and 15 g of TEOS. The most distinct difference is the high and low open mesopore volume of PHTSs synthesized with 18 g of TEOS and with 12 and 15 g of TEOS, respectively. The very low open mesopore volume of PHTSs synthesized with 12 g and 15 g of TEOS indicate large poreblocking effects which can also be deduced from the isotherms in Figure 4. As the stirring temperature increases, the second
Plugged Hexagonal Templated Silica (PHTS)
J. Phys. Chem. B, Vol. 108, No. 17, 2004 5267
Figure 6. Influence of the stirring time on (a) the diameter, (b) the open mesopore volume, (c) the micropore volume, and (d) the total pore volume of PHTS materials synthesized with 18 g of TEOS and stirred at (2) 60 °C, (]) 70 °C, and (b) 80 °C.
step in the desorption isotherm becomes less distinct for these materials and the isotherm becomes analogous to the one for PHTS-18/100/80. For PHTS-15/100/80 the second desorption step disappeared so that the open mesopore volume could not be determined for this sample. Taking the low open mesopore volume (Figure 3b) and the pore-blocking effects into consideration, it can be concluded that PHTS materials synthesized with 12 and 15 g of TEOS at stirring temperatures above 80 °C are of minor quality. PHTS-18/90/80 and PHTS-18/100/80 show also pore-blocking effects in their isotherms (see Figure 4), whereas the open mesopore volume remains very high. This indicates a different nature of the pore blocking of PHTSs synthesized with 18 g of TEOS and those synthesized with 12 and 15 g of TEOS. Influence of the Aging Temperature. For the study of the influence of the aging temperature on the characteristics of PHTS materials, only samples of 18 g of TEOS were synthesized. By taking the largest amount of TEOS, the influence of the presence of the plugs on the structural characteristics becomes more obvious. Additionally, the lowest investigated stirring temperature (60 °C) was taken as a constant so that the impact of the aging temperature on the structural characteristics is highlighted. To this end, aging temperatures of 60, 80, and 90 °C were chosen. The obtained data are compared to the plugfree analogue of PHTS, namely SBA-15. The latter was synthesized with 8.5 g of TEOS and during the same stirring and aging times as PHTS but at a stirring temperature of 45 °C. This is necessary in order to still obtain a SBA-15 material since synthesis at 60 °C yields a plugged material (see Figure 1). The results of nitrogen sorption on the PHTS and SBA-15 materials are displayed in Table 1. Generally, the influence of the aging temperature on the structural characteristics of PHTS is the same as that of the stirring temperature but to a lesser extent. The increase of the diameter of PHTS materials with increasing aging temperature is less than for SBA-15 materials. This is the result of their more rigid silicate framework as explained for the stirring temperature. Going from an aging temperature of 60 to 90 °C, the unit cell parameter of PHTS
(9.35 and 10.38 nm respectively) increases to the same extent as the diameter resulting in a constant wall thickness of 32 Å. For SBA-15, however, the wall thickness decreases (from 38 to 22 Å) as reported before.8,11,13,15,25 As already mentioned in the Introduction, SBA-15 materials show a decrease in micropore volume with increasing synthesis temperature.14-17 These data show that there is an optimum for SBA-15 at an aging temperature of 80 °C. An explanation for this behavior can be found in the dehydration of the PEO blocks at higher temperatures on one hand and the low synthesis temperature on the other hand. The latter influences the hydrolysis and condensation rate of the silica source34 which has repercussions on the structure formed. As can be seen from Table 1, also other characteristics improve tremendously on going from an aging temperature of 60 to 80 °C; the diameter of SBA-15 increases by 15 Å, and the total pore volume nearly doubles. The large increase in micropore volume of PHTS with these same aging temperatures demonstrates the similar effect of stirring and aging temperature on this characteristic. It should be noted, though, that the maximum micropore volume by varying the aging temperature (0.17 mL/g for PHTS-18/60/90) is smaller than the maximum micropore volume obtained by varying the stirring temperature (0.24 mL/g for PHTS-18/70/80). The increase in total pore volume with increasing aging temperature for both SBA-15 and PHTS can be explained as combined effects of the changes in pore diameter and micropore volume. For SBA15, the large increase in the total pore volume results mainly from the increase in pore diameter because the micropore volume does not change that much. For PHTS the opposite is true. Clearly, aging temperatures of 80 and 90 °C improve the structural characteristics of PHTS as well as of SBA-15. The SEM images of PHTS-18/60/60 and PHTS-18/60/90 demonstrate this as well. It can be seen from Figure 5a that the particles of PHTS synthesized at an aging temperature of 60 °C are much smaller than the large particles formed at 90 °C (Figure 5b). This is the result of Ostwald ripening,34 which involves a simultaneous process of dissolution and recondensation of the silica particles. Because small particles are more soluble than larger ones, the particles grow in average size and diminish in
5268 J. Phys. Chem. B, Vol. 108, No. 17, 2004 numbers as the smaller ones dissolve and are deposited upon the larger ones. Since the solubility of silica increases with increasing temperature,34 samples synthesized at higher temperatures will form larger particles. Influence of the Stirring Time. Investigating the stirring time is important because during this time the preliminary structure of the material is formed after which during aging it is consolidated. PHTS materials were synthesized with 18 g of TEOS at stirring temperatures of 60, 70, and 80 °C for 2, 4, and 8 h. Aging occurred at 80 °C overnight. Figure 6a displays the evolution of the diameter as a function of stirring time. For stirring temperatures of 60 and 70 °C, it shows that the pore diameter decreases with increasing stirring time which could be ascribed to the condensation of the silica framework. The overall decreasing open mesopore volume of these samples (Figure 6b) and the strongly increasing micropore volume on going from a stirring time of 2 h to 4 h (Figure 6c) indicate the formation of the plugs during this period of time. The PHTSs synthesized at a stirring temperature of 80 °C behave somewhat differently than the ones synthesized at 60 and 70 °C. The data for all studied parameters remain constant, except for the increase of the diameter and the micropore volume on going from a stirring time of 4 h to 8 h (Figure 6a and c). The simultaneous increase in diameter and micropore volume could explain why the total and open mesopore volume remain constant. If the strong increase of the micropore volume is in accordance with the formation of the plugs, then a simultaneous increase in the pore diameter will ensure that part of the pore channels remain open. In this way the open mesopore and the total pore volume remain constant. From Figure 6 it can be concluded that a minimum stirring time of 4 h is required in order to obtain materials with a high microporosity. This is in accordance with the findings of Ruthstein et al.22 who investigated the formation of the SBA15 structure for a TMOS/P123 ratio of 58 as a function of the stirring time through EPR spectroscopy. They concluded that the SBA-15 structure is already formed after 2 h and that higher TEOS/P123 ratios require longer stirring times. Conclusions Four important parameters in the PHTS synthesis were investigated: the amount of TEOS, the stirring temperature, the aging temperature, and the stirring time. PHTS materials synthesized with a high amount of TEOS (18 g) proved to have a more rigid silicate framework than materials synthesized with lower amounts. This resulted in PHTSs with smaller diameters when synthesized at lower stirring temperatures. The micropore volume tends to increase with increasing temperature with a maximum value of 0.24 mL/g for the highest amount of TEOS used (18 g). As the stirring temperature was increased till 100 °C, the porous characteristics of the PHTS materials synthesized with different amounts of TEOS differentiated. The data indicated a different nature of pore blocking of PHTSs synthesized with 18 g of TEOS and those synthesized with 12 g and 15 g of TEOS. The influence of the aging temperature on the structural properties of PHTS is the same as that of the stirring temperature but to a lesser extent. An aging temperature of 80 °C improved the structural characteristics greatly and also enlarged the particle size. The minimum stirring time required was 4 h in order to obtain materials with a high microporosity. Acknowledgment. P. Cool acknowledges the FWO-Vlaanderen (Fund for Scientific Research-Flanders -Belgium) for financial support.
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