- Email: [email protected]
Preparation of hierarchically structured porous aluminas by a dual soft template method
Microporous and Mesoporous Materials 132 (2010) 268–275
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of hierarchically structured porous aluminas by a dual soft template method Leandro Martins, Marinalva A. Alves Rosa, Sandra H. Pulcinelli, Celso V. Santilli * Instituto de Química, UNESP – Univ Estadual Paulista, P.O. Box 355, 14800-900 Araraquara, SP, Brazil
a r t i c l e
i n f o
Article history: Received 11 January 2010 Received in revised form 4 March 2010 Accepted 5 March 2010 Available online 12 March 2010 Keywords: Macro–mesoporous aluminas Micelle Emulsion Sol–gel and template
a b s t r a c t A general and potentially easy method of synthesizing aluminas possessing hierarchical pore structure is presented in this paper. This method is based on the integration of the sol–gel process with micelles of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer and oil droplets of decahydronaftalen (DHN) as dual pore templates. With the addition of n-pentanol as co-surfactant, DHN droplets diameter could be controlled within the range of ca. 2–12 lm. Mercury intrusion porosimetry shows that the produced alumina ceramics possess hierarchical structure composed of two families of pores, namely, the macro- (from 0.1 to 6 lm) and mesopores (from 8 to 10 nm). The relative population of each family and the average size of macropores could be easily controlled by adjusting the DHN quantity. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The conjugation of macropores with interconnected mesopores framework and controlled micro texture are required in an increasing number of functional advanced materials [1]. For instance, in catalysis the larger surface density of active sites is often achieved by the high specific surface of the micropores (<2 nm), while the mesopores (2-50 nm) and macropores (>50 nm) substantially facilitate the mass flow and reduce the transport limitations [2]. Indeed, the transport of small molecules inside the macroporous materials has diffusion rates comparable to those achieved in open medium. Additionally, in order to attain a higher performance, such material should possess macropores interconnected with mesoporous walls, i.e. hierarchically organized and not just segregated pores. This becomes particularly important for chemical processes involving large molecules or viscous liquids, in which the diffusion rates are low. Consequently, large molecules would not only interact at their external surfaces, but throughout the bulk of the porous material. Most synthetic methods used in the production of porous solids are based on multi-step processes through repeated templating, in which a porous solid material, e.g. mesoporous silica MCM-48 or SBA-15 is used as a sacrifice hard template [3]. However, these templates need to be previously synthesized by soft templating procedures that usually use long chain surfactant micelles, hence only a limited range of pores size can be mimicked. * Corresponding author. Tel.: +55 16 33016645; fax: +55 16 33016692. E-mail address: [email protected] (C.V. Santilli). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.03.006
An emerging technique for the production of inorganic solids showing macroporous architectures involves the combination of the sol–gel process with emulsions as soft template [4]. This templating method consists of the emulsification (dispersion of oil droplets in a continuous aqueous phase, see Scheme 1) of the inorganic precursors solution, followed by the gelation of the sol around the oil droplets and the subsequent conversion of the gel into xerogel by drying [5]. This method takes advantage of the fact that the oil droplets are both highly deformable and easily removable. In addition, emulsification conditions can be adjusted to produce different sized droplets. Irrespective of the self-assembling surfactants used to direct the structure of the mesopores [6], a large extent can be reached, thereby allowing the independent control of macro- and mesopore dimensions. Emulsified systems are non-equilibrium assemblies that can undergo phase separation into two liquid layers. However, they can be kinetically stabilized by surfactants that form films at the surface (Scheme 1b) [7]. In this case, the surfactants play two main functions: (1) to alter the energetic situation at oil/water interface (O/W) increasing the emulsion stability and (2) to form micelles, thus working as soft template for the mesopores formation (Scheme 1a). Furthermore, the addition of a co-surfactant, usually an alcohol of medium chain length, can also lead the emulsion to a long-term kinetic stability or to a thermodynamically stable microemulsion [7]. Nowadays various approaches regarding the synthesis of mesoporous alumina have been described including the self-assembly of anionic, cationic or neutral surfactants. They all provide mesoporous aluminas that offer structural features attractive in several
269
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
a a
sol
c C
oil droplet surfactant micelle
macropores mesopores
1. pH correction 2. aging 3. calcination
1
b
d Protection against coalescence
2
Facilitated diffusion (from macropores to mesopores)
Scheme 1. Dual soft templating method used to obtain solids presenting hierarchical macro–mesopores: (a) Oil droplets and surfactant micelles dispersed in aqueous sol, (b) surfactant at the droplets interface, (c) porous solid obtained after gelation, aging and calcination and (d) macro–mesoporous solid presenting facilitated diffusion from macropores to mesopores (from 1 to 2).
applications [8]. Among various synthesis routes, those employing poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers [(EO)x(PO)y(EO)x] as soft templates attract attention because they are inexpensive, commercially available, and afford materials with relatively uniform pores. In the case of Pluronic P123 (EO20PO70EO20), the hydrophilic PEO blocks are found to be much shorter as compared to the hydrophobic PPO portion, hence leading to a decrease in the repulsion among the PEO–PPO–PEO chains, and thereby favoring the formation of large micelles [9]. Herein, we present a straightforward method of synthesizing porous aluminas through the conjugation of the sol–gel route as well as a dual soft template technique that consists of dispersed oil droplets and block copolymers micelles. By using this strategy, a series of aluminas with hierarchical macro–mesopores and presenting controlled amount of macroporosity are readily obtained.
2. Experimental
Gelation was induced by adding 1.4 cm3 of NH4OH solution (29 wt.%) drop by drop into the above medium under mechanical stirring. This caused an increase in pH from 3 to 4, inducing the sol–gel transition. The gelated emulsions were aged at 25 °C for 7 days in closed flasks, before drying at 50 °C for 2 days. Calcination was then carried out in a conventional muffle oven by increasing the temperature 5 °C/min from room temperature to 190 °C for 2 h (for the removal of DHN) and heating (1 °C/min) to 600 °C for 2 h. The molar quantity of the co-surfactant n-pentanol was made to vary as follows: 0.015, 0.1 and 0.3, resulting in n-pentanol/pluronic molar ratio of 0, 1, 7 and 20, respectively. The quantity of DHN was also made to vary within the following range: 15, 30, 50, 60, 70 and 80% of the sol mass (n-pentanol/pluronic molar ratio fixed to 7). Based on these quantities, the samples were named as Al-x, where x denotes DHN weight percentage. Occasionally the samples were denoted as Al-x (Ry), where y stands for n-pentanol/pluronic molar ratio.
2.1. Macro–mesoporous aluminas synthesis
2.2. Emulsion characterization
Pluronic P123 (Maverage = 5800 g/mol, EO20PO70EO20), aluminum iso-propoxide (AlðOPir Þ3 ) and decahydronaphthalene (C10H18, DHN) were purchased from Aldrich and Sigma–Aldrich, nitric acid (65% m/m) and n-pentanol from Vetec and ammonium hydroxide from Mallinckrodt. All of them were used as received. In a typical synthesis, 1.36 g of the Pluronic P123 surfactant was dissolved in 8.97 g of Milli-Q water at room temperature. After complete dissolution, 0.17 cm3 of n-pentanol was added as co-surfactant. Then 3.18 g of aluminum iso-propoxide was further added into the above solution under magnetic stirring followed by the addition of 2.25 g of HNO3 (caution: highly exothermic reaction!). The mixture was then covered with polyethylene film and stirred at room temperature for 5 h. HNO3 was used to catalyze the hydrolysis of the alkoxide to produce aluminum hydroxide: Al(OR)3 + 3H3O+ ? Al(OH)3 + 3ROH + 3H+ [10,11]. A homogeneous and clear sol was obtained with the following molar composition:
The oil droplets present in the emulsions were observed in an optical microscope (Olympus BX41) before gelation. For observation purposes, a portion was put on a sample holder and then observed directly without further treatment at room temperature. Droplet-size distributions were determined by evaluating at least fifty droplets in microscopic images. The electrical conductivity of emulsified alumina sol with the desired composition was measured at 22 °C using a Marconi conductivity meter (Model CA-150).
0.015Plunonic P123:1AlðOPir Þ3 :0.1n-pentanol:1.5HNO3:35H2O After the sol preparation, emulsification was performed by adding decahydronaphthalene (DHN) under stirring and dispersed for 5 min (Scheme 1a). The quantity of DHN used was made to vary between the range of 15–80%, according to the formula %DHN ¼ massDHN 100: massDHN þmass sol
2.3. Macro–mesoporous alumina characterization The crystalline phases present in calcined samples were analyzed by X-ray diffraction powder (XRD) using a Siemens D5000 diffractometer, Cu Ka radiation selected by a curved graphite monochromator. The phase identification was done using the program X’Pert High Score and the crystallographic pattern files [70238] and [04-0875] for c-Al(OH)3 and c-Al2O3, respectively. The thermal behavior of the dried solids was investigated by thermo-gravimetric analysis (TGA), performed from room temperature up to 800 °C under oxygen flow (50 cm3/min), using a SDT600 TA Instruments at a heating rate of 10 °C/min. Nitrogen adsorption–desorption isotherms were recorded at liquid nitrogen temperature and relative pressure interval between 0.001 and 0.998 on the equipment supplied by Micromeritics
270
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
(ASAP 2010). Samples were evacuated prior to measurements at 200 °C for 12 h under vacuum of 10 lPa. Surface areas were calculated following the BET equation [12]. The pore size distribution was then determined from mercury intrusion porosimetry using the AUTOPORE III equipment (Micromeritics). All the samples were degassed before analysis at a vacuum pressure below 50 lPa. The pore diameter was calculated from the Washburn equation [13], using surface tension and contact angle of 0.489 N/m and 135o, respectively. The alumina macropores were observed by scanning electron micrographs, using a Philips XL 30 equipment. The samples were further deposited on aluminum sample holder and sputtered with gold. 3. Results and discussion 3.1. Effect of co-surfactant/surfactant ratio A series of emulsions containing aluminum iso-propoxide and 30% of DHN were synthesized and the kinetic stability of the O/ W emulsions before gelation was monitored as a function of the Pluronic P123 block copolymer addition. Prompt macroscopic
phase separation was observed in the system emulsified in the absence of this surfactant. In contrast, a huge increase in kinetic stability of the DHN droplets was achieved due to the addition of Pluronic P123. Here, a macroscopic flotation of the polar phase was found to occur only after 12 h of the emulsification step. A further increase in the emulsion stability can be achieved with the addition of the co-surfactant n-pentanol. The effect of the co-surfactant/surfactant molar ratio (n-pentanol/Pluronic P123) on the structural feature of emulsions containing aluminum iso-propoxide and 30% of DHN is depicted in the optical micrographs shown in Fig. 1a. Clearly, an increase in the n-pentanol content dramatically reduces the droplets average diameter (Fig. 1b), thereby modifying their size distribution. This feature evidences the amphiphilic nature of n-pentanol, i.e. a short hydrophobic chain with terminal hydroxyl group. This makes it to interact with surfactant monolayers at the interface of oil droplets, affecting the packing of the interface and in turn influencing the curvature of the interface as well as the interfacial energy. The net result is a more densely packed interfacial layer with stronger lateral interactions between the surfactant and co-surfactant molecules, which provides a barrier to the collision coalescence and,
Fig. 1. (a) Optical micrographs of emulsions before gelation prepared with 30 wt.% of DHN and with different n-pentanol/Pluronic P123 M ratio of 0, 1, 7 and 20, and (b) mean droplet diameter of emulsions as a function of n-pentanol/Pluronic P123 ratio.
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
thus, droplets with narrow distribution sizes are obtained as the proportion of n-pentanol is increased. In this studied system, the surfactant and the co-surfactant agents do not offer long-term stability against coalescence, however, they initially stabilize the O/W emulsions for the period of time necessary to proceed with gelation by increasing the pH from 3 to 4. Once the abrupt addition of ammonium hydroxide solution results in fast reaction, consequently leading to uncontrolled phase separation of the organic and inorganic phases as well as the considerable precipitation of aluminum hydroxide. This step was carried out slowly drop by drop until gelation. No macroscopic phase separation was observed in the emulsion within a time period of more than one month after emulsification and gelation. This long-term stability can be attributed to the accumulation of condensed aluminum hydroxide shell at the O/W interface [14,15]. As condensation of the aluminum hydroxide proceeds during gelation with ammonium hydroxide, the viscosity of the emulsion increases, thereby improving the kinetic stability of the emulsified system. Fig. 2 shows the thermogravimetric curve and the weight derivative for the partially dried (50 °C) Al-30 (R7) sample. Weight loss below 70 °C corresponds to propanol and water evaporation. This sample exhibited about 50% weight loss at around 190 °C, which corresponds to the DHN boiling point. From 190 to 600 °C
271
(Fig. 2b), the weight loss was close to 15%. This was attributed primarily to the decomposition of the surfactant (250 °C) as well as the release of water or iso-propanol formed from the secondary condensation reaction of aluminum hydroxide and/or aluminum iso-propoxide present in the alumina framework (condensation to form Al–O–Al bridges at 360 °C). These two decomposition steps were confirmed by testing a reference sample (Al-ref), which was synthesized without surfactant and DHN. X-ray diffraction patterns of samples before and after thermal treatment at 600 °C (not shown) also confirmed the transition of c-Al(OH)3 to c-Al2O3. Considering the TGA result and the relatively high difference between the vapor pressure of DHN (1.5 mm Hg at 22 °C) and water (23.8 mm Hg at 25 °C), we conclude that the free water and alcohol present in the emulsified gel can be evaporated by drying at 110 °C, while keeping DHN in the pores. As the pores are filled with surfactant and DHN, careful calcination is required in order to avoid a catastrophic damage of the porous structure. Moreover, further condensation of Al–O–Al bridges that occurs during firing is probably decisive to achieve stable porous alumina. This is the reason why we have decided to perform slow sample calcinations, i.e. at 1 °C/min. The pore volume and size distribution of the calcined aluminas evaluated using mercury porosimetry revealed three intrusion steps (Fig. 3), being two in the domain of macropores (i) >7 lm
Fig. 2. (a) Thermogravimetric curve and (b) weight derivative for the dried Al-30 (R7) sample.
Fig. 3. Pore size distribution and cumulative pore volume determined by Hg-porosimetry of Al-30 series with different n-pentanol/Pluronic P123 M ratio: (a) 0, (b) 1, (c) 7, and (d) 20.
272
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
and (ii) from 0.1 to 1.1 lm and a third one in the domain of mesopores (iii) from 3 to 15 nm. Two reference samples prepared without surfactant and DHN (Fig. 4a) and only with surfactant (Fig. 4b) were also included in this analysis. The former one presents only a low mercury intrusion volume in the region of 7–50 lm, which is also present in all of the samples. It indicates that this large pore family does not arise from the micelle/emulsion template process, being probably associated with the presence of cracks or air bubbles formed during the xerogel processing. Furthermore, the pore size distribution of the sample prepared with the addition of surfactant (Fig. 4b) occurs within the mesopore range, nonetheless, the modal pore size obtained is equivalent to that of the value observed for all the samples. Regarding these features, we attributed the origin of the second macropores family as well as the mesopores to the oil droplets and to the micelle templating process, respectively. This provides evidence for the additive contribution of O/W emulsion and surfactant micelle on the total porosity, thus demonstrating the viability of this method in the production of ceramic materials with hierarchical porous structure. Table 1 shows some characteristics of the porous aluminas such as macro/mesopore volume and size, including the BET surface area. The aluminas macropore sizes that were generated as a result of the presence of DHN have been adjusted in the range of 0.1– 1.1 lm, but these observed pore diameters are very below than those exhibited by the emulsions before gelation. Two hypotheses can be outlined to explain this result: (1) after calcinations, the shrinkage of large pores occurs thereby reducing the overall porosity and (2) the emulsion droplets size produced at the end of gelation probably diminishes as a consequence of the drying shrinkage as well as the condensation of aluminum hydroxide shell around the DHN droplets. Unfortunately, the last hypothesis could not be confirmed yet because the samples turned to opaque after gelation and the droplets could not be observed by optical microscopy. It was also found out that the co-surfactant addition in the emulsion did not considerably affect the final size of the macropore family templated by oil droplet. However, the molar ratio of 7 was the one that optimized the formation of macropores (Vmacro/Vme2 so = 0.7) and gave the highest BET surface area (339 m /g). The low-
er macroporosity was obtained for the sample prepared with cosurfactant/surfactant molar ratio of 20. 3.2. Effect of DHN quantity The influence of the volume of DHN on the droplet diameter and size distribution was also investigated. The co-surfactant/surfactant molar ratio was kept constant at 7. The average droplet diameter was found to increase from ca. 4 to 82 lm as the quantity of DHN increases from 15% to 80% (Fig. 5). Also, for the higher DHN quantity, the droplet diameter shows a broader distribution (Fig. 6) while the relative amount of droplets was found to diminish. Since the macroscopic separation into two liquid layers was not observed even for the highest DHN addition, it leads us to believe that the O/W structure changes to a W/O structure, where the aqueous droplets are dispersed in the DHN continuous phase. In order to confirm the compositional domains of W/O and O/W emulsions formation, the electrical conductivity was measured after each DHN addition. These measurements show the occurrence of three different behaviors of the specific conductivity as a function of the amount of DHN, as clearly shown in Fig. 7. For DHN < 70%, the specific conductivity decreases continuously with the oil addition. This behavior is typical of the decreasing amount of continuous water phase in O/W structure. The conductivity presents a small linear decrease with the DHN addition in the 70% < DHN < 77% region (see the amplified region shown in the inset of Fig. 7). This kind of behavior is typical of emulsions presenting a bi-continuous structure for which aqueous and oil domains are randomly interconnected to form a sponge like microstructures, where both oil and water act as continuous phases while remaining distinct from each other. Bi-continuous phases may exist in the intermediate phase region between O/W and W/O emulsions. The further increase in the amount of added DHN produces a sharp decrease in the specific conductivity, attesting to the fact that the aqueous phase becomes discontinuous rightly as expected of a W/O emulsion structure. The effect of DHN quantities on the cumulative and differential pore size distribution of fired aluminas is shown in Fig. 8, while the
Fig. 4. Pore size distribution and cumulative pore volume determined by Hg-porosimetry of the reference samples (a) without surfactant and DHN and (b) only with surfactant.
Table 1 Pore volume of Al-30 samples synthesized with different n-pentanol quantity.
a
Sample
Vmeso (cm3/g)
Vmacro (cm3/g)
V macro V meso
Dmacroporea (lm)
Dmesopore (nm)
SBET (m2/g)
R0 R1 R7 R20
0.36 ± 0.02 0.59 ± 0.03 0.51 ± 0.03 0.42 ± 0.02
0.20 ± 0.01 0.28 ± 0.01 0.36 ± 0.02 0.16 ± 0.01
0.6 0.5 0.7 0.4
0.40 ± 0.02 0.30 ± 0.02 0.70 ± 0.04 0.30 ± 0.02
8.0 ± 0.4 8.0 ± 0.4 8.0 ± 0.4 10.0 ± 0.5
225 ± 7 254 ± 8 339 ± 10 247 ± 7
Pore diameter determined at maximum DV/Dlog(D).
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
273
Fig. 5. Optical micrographs of emulsions before gelation prepared with different weight percentage of DHN and fixed n-pentanol/Pluronic P123 M ratio of 7.
Fig. 6. Mean droplet diameter of emulsions as a function of the DHN weight percentage.
Fig. 7. Normalized electrical conductivity of emulsion as a function of the DHN quantity. The insert shows the amplified region of DHN > 70%.
values of the porous structure characteristic parameters are displayed in Table 2. The contribution of the DHN in the formation of macropores becomes detectable by mercury intrusion porosime-
try for the sample with initial DHN contents above 15%. The average macropores diameter increases monotonically from 0.7 to 1.7 lm as the initial amount of DHN in the emulsion increases from 30% to 60 %, respectively. As already mentioned, the macropore diameters are very below than those exhibited by the microstructures of emulsions before gelation. Otherwise, in this DHN range, the average mesopores size formed by the micelles template process stays essentially independent on the amount of oil in the initial emulsion. This demonstrates how viable the fine-tuning of the structural feature of the macropores family is, once it does not produce significant changes on the mesoporous sub-structure. Fig. 9 compares the effect of the initial amount of DHN in the emulsified sol on the contribution of each pores family to the total pores volume. For DHN <60% the mesopore volume of the samples was effectively independent on the amount of DHN. However, the macropore volume increases slightly for the DHN range between 0% and 50% and strongly between 50% and 70%. The total pore volume and the specific surface area (SBET in Table 2) reached maximum values for the DHN amount of 60%. The transformation of O/W into W/O emulsion structure verified for the DHN addition superior to 70% may explain the apparent reduction in the macropore volume, which is in fact the consequence of the dominant contribution of the pore size superior to 10 lm (Fig. 8) of the total porosity. Representative scanning electron microscopy (SEM) images of these aluminas (Fig. 10) provide evidence for the presence of macropores for the samples prepared with DHN contents superior to 15%. Alumina morphology is composed of circular cross section pores with diameter varying from 0.1 to 2 lm. The surface density of macropores increases continuously as the DHN amount increases from 30% to 70%. In SEM images, it is interesting to note that after calcination in air at 600 °C, the macropores of these materials were retained and no significant amounts of cracking were observed. Furthermore, the structure of the walls separating the macropores reveals the presence of a porous texture, consistent with the existence of the mesoporous family of approximately 9 nm (Table 2). This hierarchical pore structure with interconnected macropores and texture walls can enhance the permeability of fluids through the porous ceramic, making this material very suitable for several applications. It is noteworthy that this hierarchical pore structure presents a good thermal-stability as revealed by the analysis of sample Al-60
274
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
Fig. 8. Differential pore size distribution and cumulative pore volume determined by mercury porosimetry of samples prepared with different DHN quantity: (a) Al-15, (b) Al30, (c) Al-50, (d) Al-60, (e) Al-70 and (f) Al-80.
Table 2 Pore volume of samples synthesized with different DHN quantity (co-surfactant/ surfactant molar ratio of 7).
a b
Sample
VDHN/VSola
V macro V meso
Dmacroporeb (lm)
Dmesopore (nm)
SBET (m2/g)
Al-ref Al-0 Al-15 Al-30 Al-50 Al-60 Al-70 Al-80
0 0 0.2 0.5 1.1 1.7 2.6 4.4
0.1 0.3 0.3 0.7 1.2 3.1 30.1 6.8
nd nd nd 0.7 ± 0.1 1.2 ± 0.1 1.7 ± 0.1 1.3 ± 0.1 >10
nd 8.0 ± 0.4 9.0 ± 0.5 8.0 ± 0.4 8.0 ± 0.4 9.0 ± 0.5 nd nd
362 ± 11 352 ± 11 276 ± 8 339 ± 10 267 ± 8 473 ± 14 417 ± 13 391 ± 12
Volume DHN per volume of alumina sol. Macropore diameter determined at maximum DV/Dlog(D).
treated at different temperatures until 1100 °C. Despite the c-Al2O3 to a-Al2O3 transformation occurring near to 1000 °C, the macroand mesopores families were stable up to this temperature. At 1100 °C the mesopores collapse, however the shrinkage of the macroporous framework does not take place according to mercury intrusion porosimetry measurements. As expected from the W/O structure of emulsion prepared with 80% of DHN, the Al-80 sample shows a different morphology, apparently composed of small agglomerates. Besides, the optical microscopy images of emulsions Al-70 have shown the presence of sparse multiple emulsions domains (Fig. 11). Multiple emulsifi-
Fig. 9. Pores volume as a function of DHN weight percentage, for samples prepared with n-pentanol/Pluronic P123 M ratio of 7.
cation (designed as W/O/W) is composed of water droplets dispersed in larger DHN droplets, which are then dispersed in the continuous phase (water) [8]. Due to the variety of interfaces, multiple emulsions are inherently less stable compared to the simple emulsions [8], nonetheless, this emulsion microstructure takes place when the emulsion nears the borderline that defines the do-
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
275
Fig. 10. Scanning electron micrograph of alumina samples prepared with different DHN mass percentage.
version by firing at 600 °C. The well-defined mesopores family has shown an average size of 9 nm irrespective of the surfactant/ co-surfactant ratio as well as the oil content used in the initial template step. The average pore size and the pore volume corresponding to the macropore family could be adjusted within the range of 0.7–1.7 lm and 0.15–1.63 cm3/g, respectively, varying the initial oil content from 15% to 60%. The phase inversion from W/O to O/ W structure occurs at 78% of oil leading to drastic structural modifications of the porous alumina ceramic. The high specific surface area (470 m2/g) associated to the hierarchical pore structure of such alumina material may be useful in applications such as catalysis and separation processes, especially in macromolecules and viscous systems where large pores are needed to improve the mass transport throughout the pore structure. Results reported here are only based on alumina, however, it should be possible to extend this dual template method to other ceramic materials. Fig. 11. Multiple emulsification observed in the preparation of sample Al-70.
mains of W/O and O/W emulsions formation. Therefore, the image displayed in Fig. 5 for the sample Al-80 is related to a W/O system, i.e. droplets of sol dispersed in DHN, conforming to the VDHN/VSol higher than 4 as presented in Table 2. This inverted emulsion leads to the formation of small alumina agglomerates with large interagglomerate pores, which was confirmed with the mercury intrusion results shown in Fig. 8f. 4. Conclusions Macro–mesoporous aluminas presenting hierarchical structure of pores were synthesized using one-pot pathway based on the dual templating process of pores by micelles and oil droplets. Pluronic P123 micelles and decahydronaphthalene oil droplets were used as soft template agents for the meso- and macropores structuring, respectively. A series of emulsions containing aluminum iso-propoxide and decahydronaphthalene were synthesized and the emulsions micrographies have presented different oil droplets size that was dependent on n-pentanol/ Pluronic P123 molar ratio and on decahydronaphthalene quantity. The templated bimodal pores size distributions were retained after xerogel–ceramic con-
Acknowledgments We are deeply grateful to CAPES/COFECUB, CNPq, and FAPESP for the financial support offered. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
R. Backov, Soft Matter 2 (2006) 452. A. Vantomme, A. Leonard, Z.Y. Yuan, B.L. Su, Colloids Surf. A 300 (2007) 70. M. Tiemann, Chem. Mater. 20 (2008) 961. M.A.A. Rosa, E.P. Santos, C.V. Santilli, S.H. Pulcinelli, J. Non-Cryst. Solids 354 (2008) 4786. I. Akartuna, A.R. Studart, E. Tervoort, L.J. Gauckler, Adv. Mater. 20 (2008) 4714. Z.Y. Yuan, B.L. Su, J. Mater. Chem. 16 (2006) 663. D. Myers, Surfactant Science and Technology, third ed., John Wiley & Sons, 2006. C. Marquez-Alvarez, N. Zilkova, J. Perez-Pariente, J. Cejka, Catal. Rev. – Sci. Eng. 50 (2008) 222. S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science 269 (1995) 1242. N.Y. Turova, E.P. Turevskaya, V.G. Kessler, M.I. Yanovskaya, The Chemistry of Metal Alkoxides, Kluwer, Academic Publishers, 2002. Q. Yuan, A.-X. Yin, C. Luo, L.-D. Sun, Y.-W. Zhang, W.-T. Duan, H.-C. Liu, C.-H. Yan, J. Am. Chem. Soc. 130 (2008) 3465. S. Brunauer, P.H. Emmet, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. E. Washburn, Phys. Rev. 17 (1921) 273. R. Aveyard, B.P. Binks, J.H. Clint, Adv. Colloid Interface Sci. 100 (2003) 503. B.P. Binks, Modern Aspects of Emulsion Science, The Royal Society of Chemistry, 1998.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of hierarchically structured porous aluminas by a dual soft template method Leandro Martins, Marinalva A. Alves Rosa, Sandra H. Pulcinelli, Celso V. Santilli * Instituto de Química, UNESP – Univ Estadual Paulista, P.O. Box 355, 14800-900 Araraquara, SP, Brazil
a r t i c l e
i n f o
Article history: Received 11 January 2010 Received in revised form 4 March 2010 Accepted 5 March 2010 Available online 12 March 2010 Keywords: Macro–mesoporous aluminas Micelle Emulsion Sol–gel and template
a b s t r a c t A general and potentially easy method of synthesizing aluminas possessing hierarchical pore structure is presented in this paper. This method is based on the integration of the sol–gel process with micelles of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer and oil droplets of decahydronaftalen (DHN) as dual pore templates. With the addition of n-pentanol as co-surfactant, DHN droplets diameter could be controlled within the range of ca. 2–12 lm. Mercury intrusion porosimetry shows that the produced alumina ceramics possess hierarchical structure composed of two families of pores, namely, the macro- (from 0.1 to 6 lm) and mesopores (from 8 to 10 nm). The relative population of each family and the average size of macropores could be easily controlled by adjusting the DHN quantity. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The conjugation of macropores with interconnected mesopores framework and controlled micro texture are required in an increasing number of functional advanced materials [1]. For instance, in catalysis the larger surface density of active sites is often achieved by the high specific surface of the micropores (<2 nm), while the mesopores (2-50 nm) and macropores (>50 nm) substantially facilitate the mass flow and reduce the transport limitations [2]. Indeed, the transport of small molecules inside the macroporous materials has diffusion rates comparable to those achieved in open medium. Additionally, in order to attain a higher performance, such material should possess macropores interconnected with mesoporous walls, i.e. hierarchically organized and not just segregated pores. This becomes particularly important for chemical processes involving large molecules or viscous liquids, in which the diffusion rates are low. Consequently, large molecules would not only interact at their external surfaces, but throughout the bulk of the porous material. Most synthetic methods used in the production of porous solids are based on multi-step processes through repeated templating, in which a porous solid material, e.g. mesoporous silica MCM-48 or SBA-15 is used as a sacrifice hard template [3]. However, these templates need to be previously synthesized by soft templating procedures that usually use long chain surfactant micelles, hence only a limited range of pores size can be mimicked. * Corresponding author. Tel.: +55 16 33016645; fax: +55 16 33016692. E-mail address: [email protected] (C.V. Santilli). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.03.006
An emerging technique for the production of inorganic solids showing macroporous architectures involves the combination of the sol–gel process with emulsions as soft template [4]. This templating method consists of the emulsification (dispersion of oil droplets in a continuous aqueous phase, see Scheme 1) of the inorganic precursors solution, followed by the gelation of the sol around the oil droplets and the subsequent conversion of the gel into xerogel by drying [5]. This method takes advantage of the fact that the oil droplets are both highly deformable and easily removable. In addition, emulsification conditions can be adjusted to produce different sized droplets. Irrespective of the self-assembling surfactants used to direct the structure of the mesopores [6], a large extent can be reached, thereby allowing the independent control of macro- and mesopore dimensions. Emulsified systems are non-equilibrium assemblies that can undergo phase separation into two liquid layers. However, they can be kinetically stabilized by surfactants that form films at the surface (Scheme 1b) [7]. In this case, the surfactants play two main functions: (1) to alter the energetic situation at oil/water interface (O/W) increasing the emulsion stability and (2) to form micelles, thus working as soft template for the mesopores formation (Scheme 1a). Furthermore, the addition of a co-surfactant, usually an alcohol of medium chain length, can also lead the emulsion to a long-term kinetic stability or to a thermodynamically stable microemulsion [7]. Nowadays various approaches regarding the synthesis of mesoporous alumina have been described including the self-assembly of anionic, cationic or neutral surfactants. They all provide mesoporous aluminas that offer structural features attractive in several
269
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
a a
sol
c C
oil droplet surfactant micelle
macropores mesopores
1. pH correction 2. aging 3. calcination
1
b
d Protection against coalescence
2
Facilitated diffusion (from macropores to mesopores)
Scheme 1. Dual soft templating method used to obtain solids presenting hierarchical macro–mesopores: (a) Oil droplets and surfactant micelles dispersed in aqueous sol, (b) surfactant at the droplets interface, (c) porous solid obtained after gelation, aging and calcination and (d) macro–mesoporous solid presenting facilitated diffusion from macropores to mesopores (from 1 to 2).
applications [8]. Among various synthesis routes, those employing poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymers [(EO)x(PO)y(EO)x] as soft templates attract attention because they are inexpensive, commercially available, and afford materials with relatively uniform pores. In the case of Pluronic P123 (EO20PO70EO20), the hydrophilic PEO blocks are found to be much shorter as compared to the hydrophobic PPO portion, hence leading to a decrease in the repulsion among the PEO–PPO–PEO chains, and thereby favoring the formation of large micelles [9]. Herein, we present a straightforward method of synthesizing porous aluminas through the conjugation of the sol–gel route as well as a dual soft template technique that consists of dispersed oil droplets and block copolymers micelles. By using this strategy, a series of aluminas with hierarchical macro–mesopores and presenting controlled amount of macroporosity are readily obtained.
2. Experimental
Gelation was induced by adding 1.4 cm3 of NH4OH solution (29 wt.%) drop by drop into the above medium under mechanical stirring. This caused an increase in pH from 3 to 4, inducing the sol–gel transition. The gelated emulsions were aged at 25 °C for 7 days in closed flasks, before drying at 50 °C for 2 days. Calcination was then carried out in a conventional muffle oven by increasing the temperature 5 °C/min from room temperature to 190 °C for 2 h (for the removal of DHN) and heating (1 °C/min) to 600 °C for 2 h. The molar quantity of the co-surfactant n-pentanol was made to vary as follows: 0.015, 0.1 and 0.3, resulting in n-pentanol/pluronic molar ratio of 0, 1, 7 and 20, respectively. The quantity of DHN was also made to vary within the following range: 15, 30, 50, 60, 70 and 80% of the sol mass (n-pentanol/pluronic molar ratio fixed to 7). Based on these quantities, the samples were named as Al-x, where x denotes DHN weight percentage. Occasionally the samples were denoted as Al-x (Ry), where y stands for n-pentanol/pluronic molar ratio.
2.1. Macro–mesoporous aluminas synthesis
2.2. Emulsion characterization
Pluronic P123 (Maverage = 5800 g/mol, EO20PO70EO20), aluminum iso-propoxide (AlðOPir Þ3 ) and decahydronaphthalene (C10H18, DHN) were purchased from Aldrich and Sigma–Aldrich, nitric acid (65% m/m) and n-pentanol from Vetec and ammonium hydroxide from Mallinckrodt. All of them were used as received. In a typical synthesis, 1.36 g of the Pluronic P123 surfactant was dissolved in 8.97 g of Milli-Q water at room temperature. After complete dissolution, 0.17 cm3 of n-pentanol was added as co-surfactant. Then 3.18 g of aluminum iso-propoxide was further added into the above solution under magnetic stirring followed by the addition of 2.25 g of HNO3 (caution: highly exothermic reaction!). The mixture was then covered with polyethylene film and stirred at room temperature for 5 h. HNO3 was used to catalyze the hydrolysis of the alkoxide to produce aluminum hydroxide: Al(OR)3 + 3H3O+ ? Al(OH)3 + 3ROH + 3H+ [10,11]. A homogeneous and clear sol was obtained with the following molar composition:
The oil droplets present in the emulsions were observed in an optical microscope (Olympus BX41) before gelation. For observation purposes, a portion was put on a sample holder and then observed directly without further treatment at room temperature. Droplet-size distributions were determined by evaluating at least fifty droplets in microscopic images. The electrical conductivity of emulsified alumina sol with the desired composition was measured at 22 °C using a Marconi conductivity meter (Model CA-150).
0.015Plunonic P123:1AlðOPir Þ3 :0.1n-pentanol:1.5HNO3:35H2O After the sol preparation, emulsification was performed by adding decahydronaphthalene (DHN) under stirring and dispersed for 5 min (Scheme 1a). The quantity of DHN used was made to vary between the range of 15–80%, according to the formula %DHN ¼ massDHN 100: massDHN þmass sol
2.3. Macro–mesoporous alumina characterization The crystalline phases present in calcined samples were analyzed by X-ray diffraction powder (XRD) using a Siemens D5000 diffractometer, Cu Ka radiation selected by a curved graphite monochromator. The phase identification was done using the program X’Pert High Score and the crystallographic pattern files [70238] and [04-0875] for c-Al(OH)3 and c-Al2O3, respectively. The thermal behavior of the dried solids was investigated by thermo-gravimetric analysis (TGA), performed from room temperature up to 800 °C under oxygen flow (50 cm3/min), using a SDT600 TA Instruments at a heating rate of 10 °C/min. Nitrogen adsorption–desorption isotherms were recorded at liquid nitrogen temperature and relative pressure interval between 0.001 and 0.998 on the equipment supplied by Micromeritics
270
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
(ASAP 2010). Samples were evacuated prior to measurements at 200 °C for 12 h under vacuum of 10 lPa. Surface areas were calculated following the BET equation [12]. The pore size distribution was then determined from mercury intrusion porosimetry using the AUTOPORE III equipment (Micromeritics). All the samples were degassed before analysis at a vacuum pressure below 50 lPa. The pore diameter was calculated from the Washburn equation [13], using surface tension and contact angle of 0.489 N/m and 135o, respectively. The alumina macropores were observed by scanning electron micrographs, using a Philips XL 30 equipment. The samples were further deposited on aluminum sample holder and sputtered with gold. 3. Results and discussion 3.1. Effect of co-surfactant/surfactant ratio A series of emulsions containing aluminum iso-propoxide and 30% of DHN were synthesized and the kinetic stability of the O/ W emulsions before gelation was monitored as a function of the Pluronic P123 block copolymer addition. Prompt macroscopic
phase separation was observed in the system emulsified in the absence of this surfactant. In contrast, a huge increase in kinetic stability of the DHN droplets was achieved due to the addition of Pluronic P123. Here, a macroscopic flotation of the polar phase was found to occur only after 12 h of the emulsification step. A further increase in the emulsion stability can be achieved with the addition of the co-surfactant n-pentanol. The effect of the co-surfactant/surfactant molar ratio (n-pentanol/Pluronic P123) on the structural feature of emulsions containing aluminum iso-propoxide and 30% of DHN is depicted in the optical micrographs shown in Fig. 1a. Clearly, an increase in the n-pentanol content dramatically reduces the droplets average diameter (Fig. 1b), thereby modifying their size distribution. This feature evidences the amphiphilic nature of n-pentanol, i.e. a short hydrophobic chain with terminal hydroxyl group. This makes it to interact with surfactant monolayers at the interface of oil droplets, affecting the packing of the interface and in turn influencing the curvature of the interface as well as the interfacial energy. The net result is a more densely packed interfacial layer with stronger lateral interactions between the surfactant and co-surfactant molecules, which provides a barrier to the collision coalescence and,
Fig. 1. (a) Optical micrographs of emulsions before gelation prepared with 30 wt.% of DHN and with different n-pentanol/Pluronic P123 M ratio of 0, 1, 7 and 20, and (b) mean droplet diameter of emulsions as a function of n-pentanol/Pluronic P123 ratio.
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
thus, droplets with narrow distribution sizes are obtained as the proportion of n-pentanol is increased. In this studied system, the surfactant and the co-surfactant agents do not offer long-term stability against coalescence, however, they initially stabilize the O/W emulsions for the period of time necessary to proceed with gelation by increasing the pH from 3 to 4. Once the abrupt addition of ammonium hydroxide solution results in fast reaction, consequently leading to uncontrolled phase separation of the organic and inorganic phases as well as the considerable precipitation of aluminum hydroxide. This step was carried out slowly drop by drop until gelation. No macroscopic phase separation was observed in the emulsion within a time period of more than one month after emulsification and gelation. This long-term stability can be attributed to the accumulation of condensed aluminum hydroxide shell at the O/W interface [14,15]. As condensation of the aluminum hydroxide proceeds during gelation with ammonium hydroxide, the viscosity of the emulsion increases, thereby improving the kinetic stability of the emulsified system. Fig. 2 shows the thermogravimetric curve and the weight derivative for the partially dried (50 °C) Al-30 (R7) sample. Weight loss below 70 °C corresponds to propanol and water evaporation. This sample exhibited about 50% weight loss at around 190 °C, which corresponds to the DHN boiling point. From 190 to 600 °C
271
(Fig. 2b), the weight loss was close to 15%. This was attributed primarily to the decomposition of the surfactant (250 °C) as well as the release of water or iso-propanol formed from the secondary condensation reaction of aluminum hydroxide and/or aluminum iso-propoxide present in the alumina framework (condensation to form Al–O–Al bridges at 360 °C). These two decomposition steps were confirmed by testing a reference sample (Al-ref), which was synthesized without surfactant and DHN. X-ray diffraction patterns of samples before and after thermal treatment at 600 °C (not shown) also confirmed the transition of c-Al(OH)3 to c-Al2O3. Considering the TGA result and the relatively high difference between the vapor pressure of DHN (1.5 mm Hg at 22 °C) and water (23.8 mm Hg at 25 °C), we conclude that the free water and alcohol present in the emulsified gel can be evaporated by drying at 110 °C, while keeping DHN in the pores. As the pores are filled with surfactant and DHN, careful calcination is required in order to avoid a catastrophic damage of the porous structure. Moreover, further condensation of Al–O–Al bridges that occurs during firing is probably decisive to achieve stable porous alumina. This is the reason why we have decided to perform slow sample calcinations, i.e. at 1 °C/min. The pore volume and size distribution of the calcined aluminas evaluated using mercury porosimetry revealed three intrusion steps (Fig. 3), being two in the domain of macropores (i) >7 lm
Fig. 2. (a) Thermogravimetric curve and (b) weight derivative for the dried Al-30 (R7) sample.
Fig. 3. Pore size distribution and cumulative pore volume determined by Hg-porosimetry of Al-30 series with different n-pentanol/Pluronic P123 M ratio: (a) 0, (b) 1, (c) 7, and (d) 20.
272
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
and (ii) from 0.1 to 1.1 lm and a third one in the domain of mesopores (iii) from 3 to 15 nm. Two reference samples prepared without surfactant and DHN (Fig. 4a) and only with surfactant (Fig. 4b) were also included in this analysis. The former one presents only a low mercury intrusion volume in the region of 7–50 lm, which is also present in all of the samples. It indicates that this large pore family does not arise from the micelle/emulsion template process, being probably associated with the presence of cracks or air bubbles formed during the xerogel processing. Furthermore, the pore size distribution of the sample prepared with the addition of surfactant (Fig. 4b) occurs within the mesopore range, nonetheless, the modal pore size obtained is equivalent to that of the value observed for all the samples. Regarding these features, we attributed the origin of the second macropores family as well as the mesopores to the oil droplets and to the micelle templating process, respectively. This provides evidence for the additive contribution of O/W emulsion and surfactant micelle on the total porosity, thus demonstrating the viability of this method in the production of ceramic materials with hierarchical porous structure. Table 1 shows some characteristics of the porous aluminas such as macro/mesopore volume and size, including the BET surface area. The aluminas macropore sizes that were generated as a result of the presence of DHN have been adjusted in the range of 0.1– 1.1 lm, but these observed pore diameters are very below than those exhibited by the emulsions before gelation. Two hypotheses can be outlined to explain this result: (1) after calcinations, the shrinkage of large pores occurs thereby reducing the overall porosity and (2) the emulsion droplets size produced at the end of gelation probably diminishes as a consequence of the drying shrinkage as well as the condensation of aluminum hydroxide shell around the DHN droplets. Unfortunately, the last hypothesis could not be confirmed yet because the samples turned to opaque after gelation and the droplets could not be observed by optical microscopy. It was also found out that the co-surfactant addition in the emulsion did not considerably affect the final size of the macropore family templated by oil droplet. However, the molar ratio of 7 was the one that optimized the formation of macropores (Vmacro/Vme2 so = 0.7) and gave the highest BET surface area (339 m /g). The low-
er macroporosity was obtained for the sample prepared with cosurfactant/surfactant molar ratio of 20. 3.2. Effect of DHN quantity The influence of the volume of DHN on the droplet diameter and size distribution was also investigated. The co-surfactant/surfactant molar ratio was kept constant at 7. The average droplet diameter was found to increase from ca. 4 to 82 lm as the quantity of DHN increases from 15% to 80% (Fig. 5). Also, for the higher DHN quantity, the droplet diameter shows a broader distribution (Fig. 6) while the relative amount of droplets was found to diminish. Since the macroscopic separation into two liquid layers was not observed even for the highest DHN addition, it leads us to believe that the O/W structure changes to a W/O structure, where the aqueous droplets are dispersed in the DHN continuous phase. In order to confirm the compositional domains of W/O and O/W emulsions formation, the electrical conductivity was measured after each DHN addition. These measurements show the occurrence of three different behaviors of the specific conductivity as a function of the amount of DHN, as clearly shown in Fig. 7. For DHN < 70%, the specific conductivity decreases continuously with the oil addition. This behavior is typical of the decreasing amount of continuous water phase in O/W structure. The conductivity presents a small linear decrease with the DHN addition in the 70% < DHN < 77% region (see the amplified region shown in the inset of Fig. 7). This kind of behavior is typical of emulsions presenting a bi-continuous structure for which aqueous and oil domains are randomly interconnected to form a sponge like microstructures, where both oil and water act as continuous phases while remaining distinct from each other. Bi-continuous phases may exist in the intermediate phase region between O/W and W/O emulsions. The further increase in the amount of added DHN produces a sharp decrease in the specific conductivity, attesting to the fact that the aqueous phase becomes discontinuous rightly as expected of a W/O emulsion structure. The effect of DHN quantities on the cumulative and differential pore size distribution of fired aluminas is shown in Fig. 8, while the
Fig. 4. Pore size distribution and cumulative pore volume determined by Hg-porosimetry of the reference samples (a) without surfactant and DHN and (b) only with surfactant.
Table 1 Pore volume of Al-30 samples synthesized with different n-pentanol quantity.
a
Sample
Vmeso (cm3/g)
Vmacro (cm3/g)
V macro V meso
Dmacroporea (lm)
Dmesopore (nm)
SBET (m2/g)
R0 R1 R7 R20
0.36 ± 0.02 0.59 ± 0.03 0.51 ± 0.03 0.42 ± 0.02
0.20 ± 0.01 0.28 ± 0.01 0.36 ± 0.02 0.16 ± 0.01
0.6 0.5 0.7 0.4
0.40 ± 0.02 0.30 ± 0.02 0.70 ± 0.04 0.30 ± 0.02
8.0 ± 0.4 8.0 ± 0.4 8.0 ± 0.4 10.0 ± 0.5
225 ± 7 254 ± 8 339 ± 10 247 ± 7
Pore diameter determined at maximum DV/Dlog(D).
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
273
Fig. 5. Optical micrographs of emulsions before gelation prepared with different weight percentage of DHN and fixed n-pentanol/Pluronic P123 M ratio of 7.
Fig. 6. Mean droplet diameter of emulsions as a function of the DHN weight percentage.
Fig. 7. Normalized electrical conductivity of emulsion as a function of the DHN quantity. The insert shows the amplified region of DHN > 70%.
values of the porous structure characteristic parameters are displayed in Table 2. The contribution of the DHN in the formation of macropores becomes detectable by mercury intrusion porosime-
try for the sample with initial DHN contents above 15%. The average macropores diameter increases monotonically from 0.7 to 1.7 lm as the initial amount of DHN in the emulsion increases from 30% to 60 %, respectively. As already mentioned, the macropore diameters are very below than those exhibited by the microstructures of emulsions before gelation. Otherwise, in this DHN range, the average mesopores size formed by the micelles template process stays essentially independent on the amount of oil in the initial emulsion. This demonstrates how viable the fine-tuning of the structural feature of the macropores family is, once it does not produce significant changes on the mesoporous sub-structure. Fig. 9 compares the effect of the initial amount of DHN in the emulsified sol on the contribution of each pores family to the total pores volume. For DHN <60% the mesopore volume of the samples was effectively independent on the amount of DHN. However, the macropore volume increases slightly for the DHN range between 0% and 50% and strongly between 50% and 70%. The total pore volume and the specific surface area (SBET in Table 2) reached maximum values for the DHN amount of 60%. The transformation of O/W into W/O emulsion structure verified for the DHN addition superior to 70% may explain the apparent reduction in the macropore volume, which is in fact the consequence of the dominant contribution of the pore size superior to 10 lm (Fig. 8) of the total porosity. Representative scanning electron microscopy (SEM) images of these aluminas (Fig. 10) provide evidence for the presence of macropores for the samples prepared with DHN contents superior to 15%. Alumina morphology is composed of circular cross section pores with diameter varying from 0.1 to 2 lm. The surface density of macropores increases continuously as the DHN amount increases from 30% to 70%. In SEM images, it is interesting to note that after calcination in air at 600 °C, the macropores of these materials were retained and no significant amounts of cracking were observed. Furthermore, the structure of the walls separating the macropores reveals the presence of a porous texture, consistent with the existence of the mesoporous family of approximately 9 nm (Table 2). This hierarchical pore structure with interconnected macropores and texture walls can enhance the permeability of fluids through the porous ceramic, making this material very suitable for several applications. It is noteworthy that this hierarchical pore structure presents a good thermal-stability as revealed by the analysis of sample Al-60
274
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
Fig. 8. Differential pore size distribution and cumulative pore volume determined by mercury porosimetry of samples prepared with different DHN quantity: (a) Al-15, (b) Al30, (c) Al-50, (d) Al-60, (e) Al-70 and (f) Al-80.
Table 2 Pore volume of samples synthesized with different DHN quantity (co-surfactant/ surfactant molar ratio of 7).
a b
Sample
VDHN/VSola
V macro V meso
Dmacroporeb (lm)
Dmesopore (nm)
SBET (m2/g)
Al-ref Al-0 Al-15 Al-30 Al-50 Al-60 Al-70 Al-80
0 0 0.2 0.5 1.1 1.7 2.6 4.4
0.1 0.3 0.3 0.7 1.2 3.1 30.1 6.8
nd nd nd 0.7 ± 0.1 1.2 ± 0.1 1.7 ± 0.1 1.3 ± 0.1 >10
nd 8.0 ± 0.4 9.0 ± 0.5 8.0 ± 0.4 8.0 ± 0.4 9.0 ± 0.5 nd nd
362 ± 11 352 ± 11 276 ± 8 339 ± 10 267 ± 8 473 ± 14 417 ± 13 391 ± 12
Volume DHN per volume of alumina sol. Macropore diameter determined at maximum DV/Dlog(D).
treated at different temperatures until 1100 °C. Despite the c-Al2O3 to a-Al2O3 transformation occurring near to 1000 °C, the macroand mesopores families were stable up to this temperature. At 1100 °C the mesopores collapse, however the shrinkage of the macroporous framework does not take place according to mercury intrusion porosimetry measurements. As expected from the W/O structure of emulsion prepared with 80% of DHN, the Al-80 sample shows a different morphology, apparently composed of small agglomerates. Besides, the optical microscopy images of emulsions Al-70 have shown the presence of sparse multiple emulsions domains (Fig. 11). Multiple emulsifi-
Fig. 9. Pores volume as a function of DHN weight percentage, for samples prepared with n-pentanol/Pluronic P123 M ratio of 7.
cation (designed as W/O/W) is composed of water droplets dispersed in larger DHN droplets, which are then dispersed in the continuous phase (water) [8]. Due to the variety of interfaces, multiple emulsions are inherently less stable compared to the simple emulsions [8], nonetheless, this emulsion microstructure takes place when the emulsion nears the borderline that defines the do-
L. Martins et al. / Microporous and Mesoporous Materials 132 (2010) 268–275
275
Fig. 10. Scanning electron micrograph of alumina samples prepared with different DHN mass percentage.
version by firing at 600 °C. The well-defined mesopores family has shown an average size of 9 nm irrespective of the surfactant/ co-surfactant ratio as well as the oil content used in the initial template step. The average pore size and the pore volume corresponding to the macropore family could be adjusted within the range of 0.7–1.7 lm and 0.15–1.63 cm3/g, respectively, varying the initial oil content from 15% to 60%. The phase inversion from W/O to O/ W structure occurs at 78% of oil leading to drastic structural modifications of the porous alumina ceramic. The high specific surface area (470 m2/g) associated to the hierarchical pore structure of such alumina material may be useful in applications such as catalysis and separation processes, especially in macromolecules and viscous systems where large pores are needed to improve the mass transport throughout the pore structure. Results reported here are only based on alumina, however, it should be possible to extend this dual template method to other ceramic materials. Fig. 11. Multiple emulsification observed in the preparation of sample Al-70.
mains of W/O and O/W emulsions formation. Therefore, the image displayed in Fig. 5 for the sample Al-80 is related to a W/O system, i.e. droplets of sol dispersed in DHN, conforming to the VDHN/VSol higher than 4 as presented in Table 2. This inverted emulsion leads to the formation of small alumina agglomerates with large interagglomerate pores, which was confirmed with the mercury intrusion results shown in Fig. 8f. 4. Conclusions Macro–mesoporous aluminas presenting hierarchical structure of pores were synthesized using one-pot pathway based on the dual templating process of pores by micelles and oil droplets. Pluronic P123 micelles and decahydronaphthalene oil droplets were used as soft template agents for the meso- and macropores structuring, respectively. A series of emulsions containing aluminum iso-propoxide and decahydronaphthalene were synthesized and the emulsions micrographies have presented different oil droplets size that was dependent on n-pentanol/ Pluronic P123 molar ratio and on decahydronaphthalene quantity. The templated bimodal pores size distributions were retained after xerogel–ceramic con-
Acknowledgments We are deeply grateful to CAPES/COFECUB, CNPq, and FAPESP for the financial support offered. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
R. Backov, Soft Matter 2 (2006) 452. A. Vantomme, A. Leonard, Z.Y. Yuan, B.L. Su, Colloids Surf. A 300 (2007) 70. M. Tiemann, Chem. Mater. 20 (2008) 961. M.A.A. Rosa, E.P. Santos, C.V. Santilli, S.H. Pulcinelli, J. Non-Cryst. Solids 354 (2008) 4786. I. Akartuna, A.R. Studart, E. Tervoort, L.J. Gauckler, Adv. Mater. 20 (2008) 4714. Z.Y. Yuan, B.L. Su, J. Mater. Chem. 16 (2006) 663. D. Myers, Surfactant Science and Technology, third ed., John Wiley & Sons, 2006. C. Marquez-Alvarez, N. Zilkova, J. Perez-Pariente, J. Cejka, Catal. Rev. – Sci. Eng. 50 (2008) 222. S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science 269 (1995) 1242. N.Y. Turova, E.P. Turevskaya, V.G. Kessler, M.I. Yanovskaya, The Chemistry of Metal Alkoxides, Kluwer, Academic Publishers, 2002. Q. Yuan, A.-X. Yin, C. Luo, L.-D. Sun, Y.-W. Zhang, W.-T. Duan, H.-C. Liu, C.-H. Yan, J. Am. Chem. Soc. 130 (2008) 3465. S. Brunauer, P.H. Emmet, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. E. Washburn, Phys. Rev. 17 (1921) 273. R. Aveyard, B.P. Binks, J.H. Clint, Adv. Colloid Interface Sci. 100 (2003) 503. B.P. Binks, Modern Aspects of Emulsion Science, The Royal Society of Chemistry, 1998.