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Abstract

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Mesoporous alumina (MA)was synthesized by sol–gel based evaporation-induced self-assembly process using aluminum isopropoxide as alumina source in the presence of three different types of triblock copolymers (TBCs), F68, F127, and L64. The role of different TBCs as surfactants on thermal, crystallization, textural, and microstructural properties of the alumina powders was studied. To understand the effects of different copolymers, the adsorption efficiency of the samples for Congo red (CR) was studied. For all the surfactants, the XRD results showed the stability of γ-Al2O3 phase up to 1000°C for 1 h dwell time. A maximum value (431.8 m2/g) of Brunauer–Emmet–Teller surface area was obtained for the 400°C-treated powder prepared from F68 surfactant. The transmission electron microscopy micrograph exhibited worm-like mesoporous structures of the 400°C-treated powders prepared from F68 and F127 surfactants. The adsorption performance for CR of the 400°C-treated powders for different surfactants was in the order of F68 > F127 > L64. A tentative mechanism was illustrated to understand the roles of different block copolymers on the properties of the prepared MA.

I. Introduction

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

The discovery of ordered mesoporous silica (the M41S family)[1] using templates represented by surfactant assemblies has been a breakthrough in the research field for its potential applications in catalysis and in other realms of chemistry. In extension, the efforts have been made for the synthesis of other group of mesoporous oxides in nonsilica systems. In comparison to mesoporous silica, mesoporous alumina (MA) is important for its wide applications as adsorbents, catalysts, catalyst supports, membranes, ceramics, etc. The first successful synthesis of ordered MA was reported by Vaudry et al.[2] using long-chain carboxylic acids as the structure directing agents. For preparing MA, many other synthesis routes have been developed such as aerosol generation of particles using block copolymers (BCPs),[3] modified sol–gel method using organic structural agents,[4, 5] cationic[6] and anionic surfactants,[2] BCPs,[7, 8] mesoporous carbon templates,[9] colloidal precursors with amine based structural agents,[10] and spray pyrolysis method.[11] Recently, we have prepared mesoporous γ-alumina with nanostructured morphology from boehmite sol.[12, 13] Mesoporous γ-alumina nanorod was synthesized by reverse microelulsion process.[14] MA was obtained by evaporation-induced self-assembly method using aluminum alkoxides and TBCs such as poly(ethylene oxide)–poly(propylene oxide) –poly(ethylene oxide) (PEO–PPO–PEO) (P123).[15, 16]

The potential applications of alumina depend upon its structural, textural, and morphological characteristics. There has been a great interest in obtaining MA with high surface area, large pore volumes, crystalline pore walls, and good thermal stability for favorable enhancement of catalytic and adsorptive performances. The porosity of the materials depends on the way the hydrolysis–polycondensation reaction of aluminum alkoxides with the aid of coassembly of surfacatant.[17] The nature of interactions between polymerizing inorganic species and surfactant molecules is important in tailoring the porosity and pore structures of MA. Because of the biodegradable property, nonionic BCPs have recently been exploited for the preparation of porous materials with well-defined nanopores. Kurahashi et al.[18] have recently shown the role of block copolymer surfactant on the pore formation in methylsilsesquioxane aerogel. The nonionic poly(ethylene oxide)–block-poly(propylene oxide)–block-poly(ethylene oxide) (PEO–b-PPO–b-PEO) of different types having their different hydrophilic (PEO) and hydrophobic (PPO) polymeric units, and hydrophilic–lipophilic balance (HLB) values could interact with the aluminum alkoxides in various ways for the formation of different pore structures of alumina. Therefore, study of the role of block copolymer surfactants on the physicochemical properties of MA is of utmost important.

Environmental problem relating to the water pollution is a challenging issue in recent time. The pollution of water resources by dyes from textiles and mining industries has become a serious concern. It is also mentioned worthy that the textural and framework properties related to surface area, pore volume, porosity, pore size distribution (PSD), ordering of the pores of alumina could affect on its adsorption behaviors toward the removal of toxic and carcinogenic dyes, Congo red (CR). In our previous papers, we have synthesized mesoporous γ-alumina using boehmite sol in the presence of P123 as TBC, and studied its adsorption efficiency for CR.[12, 13] The above study helps us design to investigate the role of different types of TBCs for the synthesis of MA. Keeping this view in mind, in this investigation we have first studied the roles of different types of TBCs as surfactants on the physicochemical properties of mesoporous Al2O3 and its adsorption capacity for CR removal. In this study, F68 (EO78PO30EO78), F127 (EO106PO70EO106), and L64 (EO13PO30EO13) have been chosen as TBCs with their decreasing order of HLB values and PEO/PPO ratios, respectively, which could influence in different properties of synthesized MA.

II. Experimental Procedure

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

For the synthesis of MA, 0.8 g of each TBC surfactant, F68, F127, and L64 (Sigma-Aldrich Chemie GmbH, Steinheim, Germany) was dissolved in 20 mL absolute ethyl alcohol (Merck, Darmstadt, Germany) under stirring for 60 min at 35°C. Then, 2.04 g aluminum isopropoxide (Sigma-Aldrich Chemie GmbH, Steinheim, Germany) dissolved in 20 mL absolute ethyl alcohol and 1.4 mL of 67 wt% nitric acid (Merck, Mumbai, India) was added into the former solution. The mix solution was allowed to stir for 5 h to obtain a homogeneous sol. It was then poured into a Petri dish and kept in an oven at 60°C for 48 h for solvent evaporation. The dried as-prepared powders were calcined at 400°C for 4 h with a heating rate of 1°C/min, and at 700°C, 900°C, 1000°C, 1100°C with a heating rate of 1°C/min up to 400°C/4 h followed by heating rate of 5°C/min up to those temperatures with 1 h dwell time each.

The thermal behaviors of the prepared powders were studied by differential thermal analysis (DTA) and thermogravimetry (TG) (Shimadzu 50, Kyoto, Japan) from 30°C to 1000°C in air atmosphere at the heating rate of 10°C/min. The wide-angle XRD measurements were performed for the as-prepared and calcined samples using powder diffraction technique by a Philips X'Pert Pro XRD (Model: PW 3050/60; Philips, Almelo, the Netherlands) with Ni-filtered CuKα radiation (λ = 0.15418 nm), operating at 40 kV and 30 mA. The low-angle XRD measurements of the calcined samples (400°C) were recorded by Rigaku Smartlab (Tokyo, Japan) (9 kW) using CuKα radiation, operating at 45 kV and 200 mA. Nitrogen adsorption–desorption measurements were conducted at 77 K with a Quantachrome (ASIQ MP, Quantachrome, Boynton Beach, FL) instrument. The powders were outgassed under vacuum at 250°C for 4 h prior to the measurement. The surface area was obtained using Brunauer–Emmet–Teller (BET) method within the relative pressure (P/Po) range 0.05–0.20, and the PSD was calculated by Barret–Joyner–Halenda (BJH) method. The nitrogen adsorption volume at the relative pressure (P/Po) of 0.99 was used to determine the pore volume. The samples were imaged by a transmission electron microscopy, TEM using a Tecnai G2 30ST (FEI, Eindhoven, the Netherlands) instrument operating at 300 kV. For TEM analysis, the powder samples were dispersed in ethanol by moderate sonication followed by dropping on the carbon-coated copper grid.

For adsorption experiments with CR, in a typical experiment, 20 mg of the as-prepared M-γA was mixed with 20 mL of aqueous solution of CR (100 mg/L) under vigorous stirring at room temperature. Analytical sample was taken from the suspension after different adsorption times and separated by centrifugation (5000 rpm). The supernatant solutions were analyzed with UV–vis spectroscopy (Cary 50; Varian, Mulgrave, Vic., Australia) to obtain the concentration of CR in solution. The characteristic absorption of CR at around 500 nm was selected to monitor the adsorption process. The concentrations of CR were determined using a linear calibration curve over 6.25–100 mg/L based on the absorbance value at 500 nm. The quantity (qt) of CR adsorbed (mg CR/g adsorbent) by the sample (adsorbent) was determined as: qt = (Co − Ct) V/m, where, Co and Ct are the initial and time (t) CR concentration (mg/L), respectively, V is the volume of solution (mL), and m is the sorbent weight (gm) in dry form.

III. Results and Discussion

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

Figures 1(a)–(c) shows TG and DTA curves of the uncalcined alumina powders prepared by using F68, F127, and L64 as surfactants, respectively. In the DTA curves, the endothermic peaks at ~95°C–110°C were attributed to the removal of adsorbed water, and the same at ~375°C for F127 and L64, and at ~350°C for F68 were due to the release of structural water molecules and hydroxide ions. The exothermic peaks appeared at ~200°C–205°C and at ~255°C–270°C indicated the decompositions of organic surfactants[18] and alkoxide precursor of alumina,[19] respectively. The TG curves for all the samples reveal that the maximum weight loss of ~76 wt% occurred up to 400°C which corroborated to the removal of water, hydroxides, organic surfactants, and organic moiety (alkoxides) of alumina precursors. In this study, no remarkable changes were observed by changing the surfactants.

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Figure 1. Thermogravimetry and differential thermal analysis curves of the alumina gel powders prepared from (a) F68, (b) F127, and (c) L64 surfactants.

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The XRD patterns [Figs. 2(a)–(c)] of the calcined powders obtained from F68, F127, and L64 as surfactants show that γ-Al2O3 phase (JCPDS file no. 10-425) became stable up to 1000°C for 1 h, while transformation to α-Al2O3 (JCPDS file no. 48-366) phases occurred at 1100°C. However, up to 700°C, the alumina was not well-crystalline. It was noticed that there were no significant changes in the crystallization behaviors using different surfactants. The low-angle XRD patterns [Fig. 2(d)] of the samples calcined at 400°C exhibited 2θ peaks at 0.60 (d100 = 14.82 nm), 0.61 (d100 = 14.54 nm), and 0.65 (d100 = 13.64 nm) for the surfactants, F68, F127, and L64, respectively, which indicated the presence of mesopores in the samples. For the sample using L64 surfactant, a slight shifting of the low-angle peak to higher value accompanying with a lower d-spacing reflected a lesser organization of pores.[20] It was supported by the TEM images discussed latter.

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Figure 2. X-ray diffraction (XRD) patterns of calcined alumina powders prepared from (a) F68, (b) F127, and (c) L64: •: γ- Al2O3, ▲: α-Al2O3; and (d) low-angle XRD patterns of the corresponding powders calcined at 400°C/4 h each.

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Figure 3 shows the nitrogen adsorption–desorption isotherms of the powders obtained from different surfactants followed by their calcination at (a) 400°C, (b) 700°C, and (c) 1000°C each. All the curves display type IV isotherm according to IUPAC classification, which indicated mesoporous characteristic of the sample. It is to be noted that for 400°C-treated samples, a steep increase in adsorption took place from a P/Po of about 0.8, indicating a very large volume of mesopores.[21] Table 1 shows the textural properties of the powders prepared from different surfactants after calcination at 400°C, 700°C, and 1000°C. It exhibits that BET surface area and pore volume are maximum, i.e., 431.8 m2/g and 1.771 cm3/g, respectively, for the sample prepared from F68 surfactant at 400°C followed by those obtained from F127 and L64 surfactants. For 700°C-treated [Fig. 3(b)] and 1000°C-treated [Fig. 3(c)] samples, the nature of hysteresis loops was H2 type indicating ink-bottle-like mesopores and/or pore constrictions, where pore mouth is smaller than the pore body. This type of pore is generated as a consequence of the interconnectivity of pores. It was observed that with increase in temperature, a more bulging of the hysteresis loops appeared indicating much narrower pore mouth (neck) and higher network connectivity of the pores. It is to be noted that the samples calcined at higher temperatures required more energy for desorption/evaporation of the capillary condensate from the pore body through the much narrower pore mouth and higher network channel of the pores. A greater extent of pore mouth constriction was observed from the samples prepared from F127 and L64 surfactants. Figure 4 shows the PSDs of the powders obtained from different surfactants followed by their calcination at (a) 400°C and (b) 700°C, and (c) 1000°C each. The curves reveal the PSD centered at ~3.9 nm derived from BJH desorption model. The standard deviation of the pore diameters of all the samples is 0.416 nm. For F127-treated powders calcined at 700°C, a wider pore size of 5.053 nm renders the high standard deviation of the pore size values. It is interesting to point out that the physical properties of the surfactants (Table 2) could have impact on the textural properties of the MA. The hydrophilicity (high HLB value and PEO/PPO ratio) of the surfactants increased in the order of L64 < F127 < F68, which enhanced more interactions with the alumina sol through hydrogen bonding rendering higher surface area as well as pore volumes of the powders prepared from different surfactants following the order of L64 < F127 < F68. Instead of longer PEO and PPO chains of F127 compared with F68, the BET surface area and pore volume of F127-derived powder were lower because of its lower hydrophilicity. The trend in changing the BET surface area and pore volume of the samples mostly holds good in all temperatures (400°C, 700°C, and 1000°C) with a little deviation in BET surface area values for the samples obtained from F68, F127, and L64 at 700°C. The measurement error in determining the surface properties could be the reason for the little deviation of the results.[22]

Table 1. Textural Properties of the Alumina Powders Obtained from Different Surfactants, Calcined at 400°C, 700°C, and 1000°C Each
Surfactant-temperature (°C)SBET (m2/g)aVp-Total (cm3/g)bDp (nm)c
  1. a

    BET surface area.

  2. b

    Total pore volume.

  3. c

    Pore diameter (BJH desorption).

  4. Standard deviation of pore diameters is 0.416 nm.

F68-400431.81.7713.921
F68-700289.40.41413.930
F68-1000148.20.30853.715
F127-400278.71.1323.701
F127-700227.80.37895.053
F127-1000141.70.2633.712
L64-400273.80.27793.923
L64-700296.80.22103.927
L64-10001340.21563.930
Table 2. Physical Properties of Triblock Copolymers (Surfactants) Used for the Preparation of Mesoporous γ-Alumina
Molecular structureMolecular weightPEO/PPOHLB valuea
  1. a

    Hydrophilic–lipophilic balance.

F68 (EO78PO30EO78)84005.2024–29
F127 (EO106PO70EO106)126003.0318–23
L64 (EO13PO30EO13)29000.8712–18
image

Figure 3. Nitrogen adsorption–desorption isotherms of the alumina powders prepared from F68, F127, and L64 surfactants, each calcined at (a) 400°C, (b) 700°C, and (c) 1000°C.

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Figure 4. Pore size distributions of the alumina powders prepared from F68, F127, and L64 surfactants, each calcined at (a) 400°C, (b) 700°C, and (c) 1000°C.

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Figures 5(a)–(c) show the TEM images of MA obtained from F68, F127, and L64 surfactants after calcination at 400°C, and Figs 5(d)–(f) exhibit the images of the corresponding samples after calcination at 1000°C. It is clear that for 400°C-treated sample, worm-like mesostructures alumina was obtained prepared from (a) F68 and (b) F127 surfactants, whereas sponge-like mesostructure was found in the presence of L64 surfactant [Fig. 5(c)]. Because of longer chain molecules of F68 and F127 surfactants, the higher degree of hydrogen bonding formation renders worm-like structure of MA. However, sponge-like mesostructure of alumina formed for L64 was due to its short-chain molecule. With increasing temperature up to 1000°C, all the mesoporous structures changed to sponge-like. The selected-area electron diffraction (SAED) patterns [insets in Figs. 5(a)–(c)] for 400°C-treated samples show that the diffraction rings were not well resolved, indicating low crystallinity of the powder. The corresponding SAED patterns [insets in Figs. 5(d)–(f)] of 1000°C-treated samples reveal mesoporous γ-alumina with the crystalline walls. These results corroborated with the XRD studies. However, it is to be pointed out that after several hours of calcination at 1000°C, alumina could lose high surface area and start transforming to the α-alumina phase. The substantial loss of BET surface area and pore volume of the samples calcined at 1000°C for 1 h could help us conceive that this temperature (1000°C) is vulnerable to the phase transformation to α-alumina.

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Figure 5. Transmission electron microscopy images of 400°C-treated alumina powders obtained from (a) F68, (b) F127, and (c) L64, and 1000°C-treated alumina powders obtained from (d) F68, (e) F127, and (f) L64; the corresponding selected-area electron-diffraction images are in the insets.

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UV–vis absorption spectroscopy measurement of the samples calcined at 400°C (Fig. 6) was performed to determine the CR concentration before and after adsorption. It demonstrates that with increasing time, the absorbance values decreased indicating decrease concentration of CR, i.e., high adsorptivity of the samples. This effect became more pronounced for the samples obtained from (a) F68 and (b) F127, whereas for L64 surfactant, the changes were less significant [Fig. 6(c)]. Figure 6(d) shows that adsorption capacity increased with time. However, the order of increase in adsorption capacity for different surfactants followed as F68 > F127 > L64. It was observed that after 2 h, the percentages of the CR removal at room temperature were 63%, 56%, and 11% for the surfactants, F68, F127, and L64, respectively. It is important to mention that MA with adsorbed CR could be regenerated by a simple thermal treatment in air at 400°C for 4 h, as observed by their constant adsorption performance. Therefore, the synthesized MA powders can be recycled for adsorption of CR in waste water. It is mentioned worthy that the efficiency for CR removal was due to the higher BET surface area and pore volumes of the samples which followed as F68 > F127 > L64. The removal of CR with alumina is associated with the electrostatic attraction and surface complexation through hydrogen bonding between the hydroxyl groups of the samples and amine group of CR molecules.[23] Therefore, more hydrophilicity (higher HLB values) of the surfactants enhanced hydrogen bonding formation rendering higher adsorption efficiency for CR.

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Figure 6. Absorption spectra of CR after different time intervals using mesoporous alumina obtained from (a) F68, (b) F127, and (c) L64 after calcination at 400°C/4 h each; and (d) the corresponding adsorption capacities.

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We proposed a mechanism for the formation of mesostructure of the alumina powder in the presence of block copolymers as surfactants (Fig. 7). The surfactants in ethanol self-assembled together to form micelles (Fig. 7-I) in which hydrophobic polypropylene oxide (PPO) remains in the core surrounded by hydrophilic polyethylene oxide (PEO) in the corona. The hydroxyl (-OH) groups in alumina sol particles obtained by the hydrolysis of aluminum isopropoxide in acidic medium interact with the hydrophilic PEO of the surfactant molecules through hydrogen bonding (Fig. 7-II–III). With increase in PEO/PPO ratio and HLB values, i.e., with high hydrophilicity of the surfactants, the probability of hydrogen bonding formation is increased. It was also reported that alkylene oxide segments can form crown-ether-type complexes with inorganic metal ions through weak coordination bonding.[7] Therefore, in the present case both the hydrogen bonding and coordination interactions could exist between the surfactants and alumina particles. Under slow evaporation of the sol, dried gel is formed in which the self-assembled surfactant molecules get entrapped in the alumina matrix (Fig. 7-III) as template. After heating, the entrapped surfactants are removed rendering mesopores in the structure (Fig. 7-IV).

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Figure 7. Schematic representation for the formation of mesoporous γ-alumina using different triblock copolymers as surfactants.

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In this work, we have synthesized mesoporous γ-alumina using aluminum isopropoxide in the presence of three different TBCs, F68, F127, and L64, which showed higher surface area and pore volume than those obtained from boehmite sol as alumina source in the presence of P123 as TBC as reported in our previous papers.[12, 13] The 400°C-treated powders obtained from different surfactants exhibited relatively ordered (worm- or sponge-like) pore structure with smaller pore size, while the powder prepared from boehmite precursor in the presence of P123 rendered larger and disordered pore structure. However, because of the fiber- and rod-like structure of γ-alumina, the adsorption efficiency for CR of γ-alumina derived from boehmite sol is better than that of the same synthesized from aluminum isopropoxide in this study. Therefore, it is inferred that adsorption efficiency for CR of mesoporous γ-alumina depends not only on its textural properties and pore structure but also on its morphology.

IV. Conclusions

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

The role of different TBC surfactants namely, F68, F127, and L64 for the synthesis mesoporous γ-Al2O3 was investigated. The polymeric chain lengths of PEO, PPO, and HLB of the surfactants played an important role on the textural and microstructural properties, and CR adsorption efficiency of the prepared MA powders. With increase in PEO/PPO ratio and HLB values, i.e., with high hydrophilicity of the surfactants, the BET surface area and the adsorption efficiency for CR of the alumina powders increased in the order of L64 < F127 < F68. The worm-like mesostructure of alumina was obtained for F68 and F127 surfactants, whereas L64 exhibited sponge-like MA because of long-chain molecules of the former and short-chain molecule of the latter surfactant. Different block copolymers as surfactants could have a significant role on the properties of other types of mesoporous oxides such as silica, zirconia, titania, etc.

Acknowledgments

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References

The authors thank the Director of this institute for his kind permission to publish this study. They also acknowledge the help rendered by Sensor & Actuator division, Nanostructured Materials Division, X-ray Characterization, and Electron Microscopy sections of the Institute for material characterization. One of the authors (S. Ghosh) is thankful to CSIR for his fellowship. The work was funded by the DST-SERB project (no. GAP 0616), Government of India.

References

  1. Top of page
  2. Abstract
  3. I. Introduction
  4. II. Experimental Procedure
  5. III. Results and Discussion
  6. IV. Conclusions
  7. Acknowledgments
  8. References