Article pubs.acs.org/IECR
Preparation and Characterization of Rectorite Gels Pengwu Zheng,† Peter R. Chang,‡ and Xiaofei Ma*,§ †
School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, China Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, Saskatchewan S7N 0X2, Canada § Chemistry Department, School of Science, Tianjin University, Tianjin 300072, China ‡
ABSTRACT: The mineral clay rectorite (REC) was used successfully to prepare ultralight gels by the simple process of freezing the aqueous gel precursor, exchanging the solvent (ice−ethanol), and performing ethanol drying rather than employing supercritical drying technology. The light bulk density of the rectorite (REC) gel was only 18.95 mg/cm3. Three-dimensional networks in REC gels were composed of REC sheets. Cationic guar gum could intercalate into the REC layers or attach on the REC surface and consolidate the REC gel networks. This novel porous REC gel material contained many macropores and a few mesopores. The REC gel could adsorb ethanol and soybean oil as much as 5.62 and 15.8 times the weight of the gel, respectively. In addition, the adsorptions of methylene blue dye and stearic acid (phase-change material) could reach 95.24 mg/g and 39.7 g/ g, respectively. REC gels thus have potential for applications in wastewater treatment and thermal energy storage.
1. INTRODUCTION Aerogels are generally prepared from a wet-gel precursor and subsequently dried to remove the solvent without collapsing the three-dimensional network.1 Aerogels exhibit unique characteristics, including ultralight weight, large pore volume, high surface area, and tunable porosity, and thus have potential for application in the fields of detectors, catalysis, electronic devices, adsorption, and so on.2 Many aerogels are fabricated from silica gel3 or pyrolysis of organic aerogels (such as resorcinol/formaldehyde and phenol/furfural) under an inert atmosphere.4,5 Bryning et al.1 reported the synthesis of carbon nanotube aerogels from aqueous gel precursors using criticalpoint drying and freeze-drying. Zou et al.6 used a reactive polymer, poly[3-(trimethoxysilyl)propyl methacrylate] (PTMSPMA), to functionalize and facilitate the dispersion of multiwalled carbon nanotubes (MWCNTs). Subsequent hydrolysis and condensation of PTMSPMA introduced strong and permanent chemical bonding between the MWCNTs. This interaction facilitated the formation of an MWCNT percolation network, which led to the gelation of MWCNT dispersions at ultralow MWCNT concentrations. After the liquid had been removed from the wet gel, an MWCNT aerogel monolith with a density of 4 mg/cm3 was obtained.6 Recently, work on the preparation of three-dimensional graphene gels and analysis of their functionalities has been reported.7−10 Among these efforts, chemical or physical cross-linkers, such as organic binders obtained by the sol−gel polymerization method, ion linkages, and ion coordination, were used to prepare monolithic graphene structures.11 A clay, sodium montmorillonite, has also been combined with poly(vinyl alcohol)12 and polyimide13 to prepare clay gels by freeze-drying. In this study, we investigate an easy method of creating rectorite (REC) gels from aqueous gel precursors by freezing, solvent exchange (ice−ethanol), and ethanol drying. The obtained REC gels are different from aerogels obtained by supercritical drying, cryogels obtained by freeze-drying, or xerogels obtained by vacuum/atmospheric drying without any © 2013 American Chemical Society
pretreatment (i.e., without solvent exchange or freezing prior to drying).14,15 Both supercritical drying and freeze-drying are rather expensive and difficult to perform, and the formation of capillary pressure during the preparation of xerogels causes shrinkage of the gel network and thereby reduces the pore size. The rectorite (REC) gels prepared ub this work are expected to overcome the weaknesses of these general gels. REC is a regularly interstratified clay mineral with alternating pairs of a dioctahedral mica-like layer (nonexpansible) and a dioctahedral smectite-like layer (expansible) existing in a 1:1 ratio. The REC structure can easily cleave between smectitelike interlayers. The thickness of a single REC layer is about 2 nm, whereas the width and length of a single REC layer can vary from 1 μm to several micrometers. REC has been widely utilized in superabsorbent materials to improve performance and reduce cost.16 The interlayer cations of REC can be exchanged easily by either organic or inorganic cations.17 Glucono-δ-lactone (GDL) is used to delay gelation to obtain bulk-form aerogels instead of fast partial gelation.18 The hydrolysis of GDL can slowly release hydrogen ions, reduce the pH, and accelerate the dissolution of cationic guar gum (CGG), which can interact with REC through a cationexchange reaction and gradually form a gel. This interaction can improve the stability of the three-dimensional network, which is composed of randomly oriented REC sheets. Neither REC gels nor this method for preparing clay gels has been reported before.
2. EXPERIMENTAL DETAILS 2.1. Materials. Sodium rectorite (REC) was purchased from Hubei Zhongxiang Rectorite Mine (Wuhan, China). Received: Revised: Accepted: Published: 5066
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in REC gels such as melting and solidification points, and latent heats were measured by the differential scanning calorimetry (DSC) technique (DSC204, HP, NETZSCH Corporation, Selb, Germany). The analyses were performed between the temperatures of 0 and 130 °C at a heating rate of 10 °C/min under a constant stream of nitrogen. The melting and solidification points were taken as the onset temperatures obtained by drawing a line at the point of maximum slope of the leading edge of the DSC peak and extrapolating the baseline on the same side as the leading edge of the peak. The latent heat was determined by numerical integration of the area of the peak of the thermal transition.
Cationic guar gum (CGG) JK140 (with quaternary ammonium cationic functions) was provided by Jingkun Chemistry Company, Jiangsu, China. The dye methylene blue (MB) was provided by Tianjin Benchmark Chemical Reagent Co., Ltd. All other reagents were of analytical grade and were purchased from Tianjin Chemical Reagent Factory (Tianjin, China). 2.2. Preparation of REC Gels. To prepare REC gels, 2 g of REC was dispersed in distilled water (100, 150, or 200 mL) using ultrasonication for 15 min, and GDL (1, 1.5, or 2 g, respectively) was dissolved in the suspension. Then, 0.5 g of CGG was added, and the suspension was stirred. The mixture was held at room temperature for 6 h for gelatinization of REC to occur. The resulting gels were frozen for 12 h at −10 °C. The frozen gels were immersed in ethanol at room temperature. Specifically, the gels were immersed three times, for about 2 h each time, and then dried at 50 °C for 6 h to remove the ethanol. The obtained REC gels are denoted as gel-100, gel150, and gel-200, corresponding to the amounts of distilled water used in their preparation. 2.3. Characterization of REC Gels. Fourier transform infrared (FTIR) spectra of REC, CGG, and REC gels were obtained on an FTS 3000 IR spectrometer (Bio-Rad Laboratories, Hercules, CA). REC and REC gel powders were placed in a sample holder for X-ray diffraction (XRD). XRD patterns were recorded in reflection mode in the angular range of 2−10° (2θ), at ambient temperature, on a Bruker D8S4 Pioneer instrument (Bruker AXS GmbH, Karlsruhe, Germany) operated at a Cu Kα wavelength of 1.542 Å. The morphologies of the REC gels were viewed with a Hitachi (Tokyo, Japan) S-4800 scanning electron microscope. The morphology of REC was examined by transmission electron microscopy (TEM; JEM-1200EX, JEOL, Tokyo, Japan). Nitrogen adsorption−desorption measurements were performed with an Autosorb-1 specific surface area analyzer (Quantachrome Instruments, Boynton Beach, FL). The REC gels were cut to obtain cubes, and the weights and volumes of the cubes were measured and used to obtain the apparent densities of the gels. The apparent density of the REC gels, also called the bulk density, is defined as the mass of material divided by the total volume it occupies. 2.4. Ethanol, Soybean Oil, and Stearic Acid Adsorption Capacities. The dried REC gels (weight = w0) were immersed in ethanol, soybean oil, or stearic acid for 0.5 h. The adsorptions of ethanol and soybean oil were performed at room temperature, whereas that of stearic acid was performed at 90 °C. The mixtures were filtered in a funnel under a vacuum for soybean oil and stearic acid and under gravity for ethanol. When ethanol, soybean oil, or stearic acid had stopped dripping from the filter paper, the gels were weighed (weight = w). The adsorption capacity was calculated as follows w − w0 adsorption capacity = w0
3. RESULTS AND DISCUSSION 3.1. FTIR Spectroscopy. Figure 1 shows the FTIR spectra of REC, REC gels, and CGG. The peak at 2920 cm−1 represent
Figure 1. FTIR spectra of REC, REC gels, and CGG.
the stretching vibration of CH2 in CGG. The peaks at 3640 and 3440 cm−1 in the FTIR spectrum of REC are ascribed to the bending vibration of the hydrogen band of the hydroxyl stretching of SiOH and interlaminar water.19 The peak at 1040 cm−1 is attributed to the in-plane Si−O−Si stretching vibration, and the peaks at 705 and 821 cm−1 are associated with Al−O out-of-plane motion and Si−O−Al bending, respectively. These peaks shifted to lower frequencies in the spectra of the REC gels. The O−H vibration band at 3436 cm−1 in CGG also shifted to a lower frequency in the REC gels. These shifts are indicative of a possible hydrogen-bonding interaction between REC and CGG.20 In addition, the band at 1480 cm−1 in CGG, ascribed to the methyl groups of the quaternary ammonium salt, was weakened in the gels, so the negatively charged OH− groups on the surface of REC could electrostatically interact with −N+(CH3)3 in CGG.21 3.2. XRD. XRD patterns of raw REC and REC gels were recorded from 2° to 10° (2θ) as shown in Figure 2. The distances d001 and d002 between the layers of REC were estimated according to the Bragg diffraction equation, 2d sin θ = λ. Raw REC showed a strong (001) diffraction peak at 2θ = 4.00° with a basal spacing (d001) of 2.21 nm and a (002) diffraction peak at 2θ = 8.01° (d = 1.02 nm). After the gels were formed, the REC (001) and (002) diffraction peaks shifted to lower 2θ = 3.71° (d001 = 2.38 nm) and 2θ = 7.45° (d002 = 1.18
2.5. Dye MB Adsorption. REC or REC gels (0.5 g L−1) and the dye (0.1 mmol L−1 MB) were added to glass bottles for adsorption experiments at 25 °C. Glass bottles containing 20 mL of solution were placed on a slow-moving platform shaker at the speed of 100 rpm. After 12 h, the dye concentration in each solution was analyzed at 662 nm by UV−vis spectrometry, and the relative dye adsorption for that reaction time was determined. 2.6. Differential Scanning Calorimetry (DSC) Analysis. The thermal properties of stearic acid and stearic acid adsorbed 5067
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Figure 2. XRD patterns of raw REC and two REC gels.
nm) for gel-100. Similarly, in an earlier work, it was found that a modified chitosan could intercalate REC and expand the REC interlayer distance by 0.09 or 0.21 nm with the different substitution of chitosan.22 This indicates that CGG was intercalated into the gallery of REC and that the layer gap increased in gel-100. The interlayer Na+ cations of montmorillonite- (MMT-) like layers in REC can be exchanged easily by −N+(CH3)3 cations in CGG, and CGG can intercalate the layers; therefore, REC exhibited swelling similar to that of MMT.22 In gel-150, the layer gap expanded further to d001 = 2.41 nm (2θ = 3.67°) and d002 = 1.21 nm (2θ = 7.32°). More water resulted in a lower viscosity of the precursor solutions and was propitious for the intercalation of CGG into REC. 3.3. Scanning Electron Microscopy (SEM). As shown by the SEM image of REC (Figure 3a), pores with a size of tens of micrometers lead to a three-dimensional network composed of randomly oriented REC sheets. Therefore, it is reasonable to conclude that a loose dynamic REC network existed in the original dispersion of REC sheets owing to the binding interaction.10,11 Closer observations (Figure 3b) showed that the surfaces and edges of REC sheets were attached by CGG. This indicates that CGG interacted with REC, through ion exchange between the Na+ ions of REC and the −N+(CH3)3 ions of CGG, as revealed by FTIR spectroscopy. The randomly oriented sheet-like structure of REC gels was consolidated by the presence of CGG, which intercalated into the REC layers, exfoliated the REC layers, and attached to the REC surface. The formation of REC gels is illustrated in Scheme 1. 3.4. Brunauer−Emmett−Teller (BET) Surface Area and Pore Size Distribution. The REC gels were found to contain many macropores and a few mesopores. The macropores were caused by the gradual growth of ice crystals during freezedrying.2 Macropores could be observed by SEM, whereas the presence of mesopores was determined by nitrogen adsorption/desorption, as shown in Figure 4. The Brunauer− Emmett−Teller (BET) surface areas, mesopore volumes, and mesopore average diameters of the gels are listed in Table 1. The pore size distribution, determined by the Barrett−Joyner− Halenda (BJH) method, showed that the pore volume of raw REC (0.0336 cm3 g−1) was in the 3−100-nm range, with a peak pore diameter of 4 nm (Figure 4). The BET surface area for
Figure 3. SEM images of gel-150 at (a) 200× and (b) 50000× magnifications.
raw REC was found to be 9.94 m2/g. The N2 adsorption and desorption isotherms of gel-150 exhibited an adsorption hysteresis, indicating that some mesopores existed in the REC gels. The pore size distribution, as determined by the BJH method, showed that much of the pore volume (0.1488 cm3 g−1) was in the 5−100-nm range, with a peak pore diameter of 30 nm (Figure 4). The BET surface area for gel-150 was 22.98 m2/g, much higher than that of raw REC. A new surface was produced in the gels, which could have resulted from the partial exfoliation of RECs in the gels. The introduction of CGG partially exfoliated the galleries of REC, and the thickness of REC in the gels decreased greatly in comparison with that of raw REC, as shown in Figure 5. There were still galleries that were not exfoliated in the gels, but the d spacing of REC was slightly increased. With increased water content in the precursor solutions, the gels exhibited larger BET surface areas and mesopore volumes, but lower mesopore diameters. 3.5. Adsorption of REC Gels. Table 1 also lists the bulk densities of raw REC and the REC gels, along with the adsorption results for ethanol, soybean oil, MB dye, and stearic acid. The bulk density of raw REC was about 912.5 mg/cm3, whereas the gels were found to be ultralight. Increasing the water content in the precursor solutions caused the bulk density of the gels to decrease. The bulk density of gel-200 was only 18.95 mg/cm3. More water in the precursor solutions 5068
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Scheme 1. Schematic Illustration of the Formation of REC Gels
Figure 5. TEM image of raw REC.
macropores could potentially withhold ethanol as much as 4.92−5.62 times the weight of the gel. Sample gel-150 exhibited a soybean oil adsorption of 15.8 g/g. These results indicate that the REC gels can effectively adsorb liquids. In contrast, for graphene gel in vegetable oil, the adsorption capacity was about 17 times its weight.11 In addition, silica aerogels absorbed organic liquids and oils by nearly 15 times their weight.23 REC was also a good adsorbent for the cationic dye, methylene blue (MB).17 Compared to raw REC (71.36 mg/g), the equilibrium adsorption (qt) of methylene blue by gel-200 apparently improved over that of raw REC and reached 95.24 mg/g. REC gels could be used to remove residual dyes from wastewaters. Latent-heat thermal energy storage is one of the most favorable methods of thermal energy storage. Phase-change materials (PCMs) are latent-heat storage materials. Heat is stored mostly by means of the latent heat of phase change of the medium. The temperature of the medium remains more or
Figure 4. (a) N2 adsorption (○) and desorption (●) isotherms of gel150 and REC. (b) BJH mesopore size distributions of gel-150 and REC.
meant that the REC sheets were randomly oriented in a larger hydrogel. When water was removed, a light three-dimensional gel network formed. The obtained gels were composed of many interconnected macropores, as revealed by SEM. These Table 1. Some Characteristics of REC and REC Gels
REC gel100 gel150 gel200
BET surface area (m2/g)
mesopore volume (cm3/g)
mesopore average diameter (nm)
bulk density (mg/cm3)
ethanol adsorption (g/g)
oil adsorption (g/g)
MB adsorption qe (mg/g)
stearic acid adsorption (g/g)
9.94 18.87
0.0336 0.1309
11.55 34.56
912.5 30.75
− 4.92
− 14.0
71.36 72.21
− 15.6
22.98
0.1488
31.79
23.95
5.69
15.8
84.19
22.6
26.56
0.1557
30.66
18.95
5.63
9.8
95.24
39.7
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less constant during the phase transition.24 Many materials with high latent heat are chosen as PCMs, such as paraffin and stearic acid. They are usually encapsulated to obtain form-stable composite PCMs that prevent the leakage of melted PCM when the composite is subjected to the solid−liquid phasechange process. The capsule materials are usually polymers,25 which greatly decrease the thermal storage per unit weight. It is desirable to prepare PCM storage containers of light weight and high capacity. The REC gels studied withheld stearic acid (Table 2). Each gram of gel-100, gel-150, and gel-200 adsorbed
GJJ12590) and the National Nature Science Foundation of China (No. 51162011).
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Table 2. DSC Data for Stearic Acid, Gel-100, Gel-150, and Gel-200 melting temperature (°C)
stearic acid gel-100 gel-150 gel-200
enthalpy of melting (J/g)
onset
218.4 190.8 189.8 197
crystallization temperature (°C)
peak
enthalpy of crystallization (J/g)
onset
peak
54.9
61.7
−217.1
41.7
45.7
54.3 54.6 54.4
59.9 61.7 60.3
−190.9 −191.5 −197.8
42.8 42.2 42.9
46.3 46.4 46.4
15.6, 22.6, and 39.7 g of stearic acid, respectively. The latent heats of melting and freezing of gel/stearic acid composites were very close to that of stearic acid. The gel-200 containing stearic acid had latent heats of 197 and −197.8 J/g for melting and crystallization, respectively. The high latent heats were maintained because of the high adsorption of stearic acid by the REC gels. The discrepancy between the melting and freezing peak temperatures was reduced when stearic acid was adsorbed by the REC gels. This might be related to the better thermal conductivity of REC than stearic acid. Clay minerals can improve the thermal conductivity of fatty acids;26 therefore, REC gels have good potential for use in thermal energy storage.
4. CONCLUSIONS A mineral clay, REC, was successfully used to prepare ultralight gels by a simple process. Three-dimensional REC sheets networks were formed in the REC gels and consolidated by CGG, which intercalated into the REC layers or attached to the REC surface. As novel porous materials, REC gels were composed of many macropores and a few mesopores and exhibited high adsorption of ethanol, oil, MB dye, and stearic acid. These REC gels have potential applications in wastewater treatment and thermal energy storage, for example. More research on the application of REC gels is worth doing. In addition, this method will be extended to the preparation of the gels of other layered clays (such as MMT).
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +86 22 27406144. Fax: +86 22 27403475. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Science and Technology Project of Jiangxi Provincial Office of Education (No. 5070
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