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Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Summary
  7. Acknowledgments
  8. References

We present a simple synthetic route to hierarchically porous geopolymers using triglyceride oil for a reactive emulsion template. In the new synthetic method, highly alkaline geopolymer resin was first mixed with canola oil to form a homogeneous viscous emulsion which was then cured at 60°C to give a hard monolithic material. During the process, the oil in the alkaline emulsion undergoes a saponification reaction to be decomposed to water-soluble soap and glycerol molecules which were then extracted with hot water to finally yield porous geopolymers. Nitrogen adsorption studies indicated the presence of mesopores, whereas the SEM studies revealed that the mesoporous geopolymer matrix are dotted with spherical macropores (10–50 μm) which are due to oil droplet template in the emulsion. Various synthetic parameters including the precursor compositions were examined to control the porosity. BET surface area and BJH pore volume of the materials were up to 124 m2/g and 0.7 cm3/g, respectively, and the total pore volumes up to 2.1 cm3/g from pycnometry.


1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Summary
  7. Acknowledgments
  8. References

Over the past decades, geopolymers have received increasing attention as an attractive ceramic material due to the low-energy requirements in their production and their promising mechanical properties (compression strength, heat and chemical resistance, etc.).[1-4] More recently, new research efforts have been geared into utilizing the material for nontraditional applications such as evaporative cooling,[5] catalysis,[6] and drug delivery.[7] Success of such emerging applications of geopolymer materials further requires exploring new methods for controlling pore structures of the materials in nanoscale. In this communication, we demonstrate that a simple reactive emulsion templating with biorenewable oil can produce hierarchically porous geopolymer materials with coexisting controllable mesopores and spherical macropores, without a need of significantly modifying the conventional geopolymer synthetic process.

Geopolymers are typically produced by dissolving solid aluminosilicate precursors in a highly alkaline solution (typically with KOH or NaOH) to form a viscous solution (“geopolymer resin”) and subsequently curing the resin at ambient temperatures. Recent studies have shown that geopolymers are inherently a nanomaterial exhibiting a dense gel-like structure with 5–40 nm-sized amorphous aluminosilicate particles.[4, 8, 9] Their chemical structure consists of an amorphous, three-dimensional network of corner-sharing aluminate and silicate tetrahedra, with the negative charge due to Al3+ ions in the tetrahedral sites balanced by the alkali metal ions.[1, 3, 4] Figure 1 shows schematic diagrams for the reactive emulsion templating process employed in this work and for the final geopolymer product.[10] Emulsions are droplets of one fluid (e.g., oil) dispersed in a second immiscible fluid (e.g., water), which are often stabilized by a surfactant.[11] Mechanically induced droplet breakup generates (meta)stable emulsions with a distribution of droplet sizes. One novelty of the synthetic design in this work is that by employing a vegetable oil (mainly triglycerides[12]), mixing of a geopolymer resin with the oil generates carboxylate surfactants (soap molecules) in situ through the saponification reaction of the triglycerides with the highly alkaline geopolymer resin (hence reactive). The excess oil forms oil droplets which are then embedded in the geopolymer resin. Notably, it has been found in our work that the oil in the droplets continues to undergo saponification reaction during the curing of the mixture in our reaction condition, which turns the originally hydrophobic triglycerides all into soap and glyceride (CH2(OH)–CH(OH)–CH2(OH)). Those molecules are soluble in water and thus can be extracted by water from the cured solid material, resulting in a porous geopolymer material (Fig. 1(b), see 'Results and Discussion' for details).

image

Figure 1. (a) Scheme for the reactive emulsion templating of geopolymer with canola oil and (b) schematic diagram of the resulting hierarchically porous geopolymer with a random mesoporous matrix dotted with spherical macropores. The objects in the figures are not scaled.

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2 Experimental Procedure

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Summary
  7. Acknowledgments
  8. References

2.1 Synthesis

In the first step of the synthesis, a potassium silicate solution was prepared by dissolving an appropriate amount of KOH pellets (Sigma Aldrich, St. Louis, MO) in deionized water in a polypropylene cup in a water bath. A suitable amount of fumed silica (Cabot, CA-BO-SIL® EH-5, Bellerica, MA) was then added into the KOH solution and the mixture was stirred with an IKA (Wilmington, DE) mechanical mixer for 30 min at 800 rpm to give a clear solution. The geopolymer resins were then prepared by mechanically mixing metakaolinite into the potassium silicate solution to form a homogenous fluidic liquid. The metakaolinite was produced in advance by calcining kaolinite (Al2Si2O7·H2O, Alfa-Aesar, Ward Hill, MA) at 750°C for 10 h. Various samples were prepared with different water amounts and K/Al ratios but at a fixed Si/Al ratio of 2 (Table 1). The pH of the resins was about 14 for all the compositions. Canola oil (The J.M. Smucker Company, Crisco®, Orrvile, OH), waste vegetable oil (REV biodiesel, Gilbert, AZ) or paraffin oil (Alfa Aesar) were then added to the resin at a 1:1 oil-to-water volume ratio and mixed for an additional 15 min to give a homogeneous but viscous emulsion. The emulsion was transferred to a polypropylene cup and cured in a laboratory oven at 60°C for 24 h.

Table 1. Pore Properties of the Porous Products from Various Synthetic Conditions
SampleMole fraction of H2O (x)K/Al ratioOil usedSurface area (m2/g)Pore volumea (cm3/g)Average pore widthb (nm)Total pore volumec (cm3/g)Mesoporosityd (%)/total porositye (%)
  1. a

    From the pores with width no larger than 150 nm in the BJH desorption pore distribution.

  2. b

    4(BJH desorption pore volume)/(BET surface area).

  3. c

    Determined by pycnometry.

  4. d

    From pore volume.

  5. e

    From total pore volume§.

S10.632Canola690.44221.516/53
R0.682No oil620.1670.695.1/22
S20.682Canola970.53171.719/58
S30.682Canola + paraffin (1/1; v/v)420.40411.715/47
S40.682Paraffin50.03341.10.88/31
S50.682Waste vegetable oil1230.37141.615/65
S60.731Canola550.30171.113/49
S70.732Canola1240.61181.720/53
S80.733Canola840.70342.123/67

The cured product was then broken into small pieces (~1 cm × 1 cm × 1 cm) and subjected to extraction with hot deionized water, except for S4 for which hexanes were used. Three series of samples were prepared as shown in Table 1 to investigate the effect of three synthetic parameters on the resulting geopolymer; (1) type of oil (S2, S3, S4, and S5), (2) mole fraction of water (S1, S2, and S7), and (3) amount of potassium hydroxide (S6, S7, and S8). Paraffin oil was selected (S4) to examine the role of saponification, as paraffin oil is pure hydrocarbons and does not undergo a chemical reaction with geopolymer resin. To produce a “control” sample (R in Table 1), the same synthetic procedure was followed without adding any oil.

2.2 Characterization

Powder X-ray diffraction (PXRD) patterns of the finely ground samples were collected using a Siemens D5000 diffractometer with CuKα radiation (Berlin, Germany). Carbon–hydrogen–nitrogen (CHN) elemental analyses were performed by employing Perkin-Elmer 2400 Series II CHNS/O Analyzer (Waltham, MA) with a thermal conductivity detector. Samples for scanning electron microscopy (SEM) were prepared by placing small pieces of the products (approximate cubes of few millimeters in length) on a SEM stub using a copper conducting tape. Samples were then gold coated for 150 s and were studied using SEM-XL30 Environmental FEG (FEI, Hillsboro, OR) microscope operating at 10 kV. For transmission electron microscopy (TEM), colloidal suspensions of ground samples in ethanol were dried on to copper grids and were studied using JEOL TEM/STEM 2010F operating at 200 kV (Tokyo, Japan).

N2 sorption isotherms were obtained with a Micromeritics ASAP 2020 volumetric adsorption analyzer (Norcross, GA) at 77 K. Samples were degassed at room temperature for 10 h under vacuum until a residual pressure of ≤10 μmHg was reached. Specific surface areas were estimated using Brunauer–Emmett–Teller (BET) equation, in the relative pressure range from 0.06 to 0.2.[13] Pore volumes were calculated from the amount of nitrogen adsorbed at a relative pressure (p/po) of 0.99. Pore size distributions were obtained using the Barrett–Joyner–Halenda (BJH) method assuming a cylindrical pore model.[14] Total pore volume of the products was determined by pycnometry with deionized water at 23°C ± 2°C and ambient pressure, whose principle relies on the permeation of water through the open pore network of monolithic solid samples.

3 Results and Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Summary
  7. Acknowledgments
  8. References

Carbon–hydrogen–nitrogen analysis showed only small amounts of carbon (0.5 ± 0.3 wt%), hydrogen (1.3 ± 0.3wt%), and nitrogen (0.005 ± 0.002 wt%) in average for all samples, S1S8, with the maximum carbon content of 1.2 wt% found for S6. The values compare well with 0.83 wt%C, 1.4 wt%H, and 0.005 wt%N for the sample R, which indicates that the hot-water extraction removed the organics properly. Figure 2 shows SEM and TEM images of the sample S2 as a representative example. The material exhibits discrete spherical pores whose diameters range from about 5 to 40 μm in Fig. 2(a). A closer look in Fig. 2(b) reveals that the pore wall separating the spherical pores has a finer structure throughout the matrix. The corresponding TEM micrographs in Figs. 2(c) and (d) show the gel-like nanostructure of the material consisting of nanoparticles of about 20 nm that are strongly fused by necks, which is consistent with previous results.[4, 8] The materials were amorphous based on the largely featureless “hump” centered at approximately 27°–30° in 2θ, the unique feature of geopolymer, in their PXRD patterns (data not shown).[1] The combination of the SEM, TEM, and XRD results surmises that the geopolymer products exhibit a mesoporous geopolymer matrix made of rather leisurely connected amorphous aluminosilicate nanoparticles and that large spherical macropores are scattered over throughout the mesoporous matrix [Fig. 1(b)]. It is reminded that the extraction process removed the organic components completely, which strongly suggests that the mesopores in the geopolymer matrix are connected and open to allow the solvent and other molecules to flow in and out.

image

Figure 2. Scanning electron microscopy images in (a) and (b) (scale bar = 50 and 2 μm, respectively) and transmission electron microscopy images in (c) and (d) of sample S2.

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The samples were characterized further by applying BET and BJH analyses to N2 sorption isotherms for the samples, and the results are summarized in Table 1. Figure 3 shows the isotherms and BJH desorption pore distribution of the samples with the same precursor composition at K/Al = 2 and = 0.68 but with different types of oil (S2–S5) along with the control sample R prepared without oil. All the samples except S4 show a noticeable hysteresis in their isotherms [Fig. 3(a)], indicating the presence of mesopores.[15] The sizes of mesopores show a relatively narrow distribution in Fig. 3(b) centered in the mesopore region (10–50 nm). Excluding S4, the sample R shows the lowest BJH cumulative pore volume (0.16 cm3/g) and the smallest average pore diameter (7 nm), whereas the ones prepared with oil containing triglycerides (S2, S3, and S5) show a significantly higher porosity with the BJH pore volume up to 0.53 cm3/g and the average pore width up to 41 nm. The sample S4 prepared with paraffin oil shows a BJH pore volume even lower than the control sample R.

image

Figure 3. (a) Brunauer–Emmett–Teller isotherms and (b) BJH desorption pore size distribution curves of samples R1, S2, S3, S4, and S5. All samples have same composition but they differ in the type of oil (Table 1) used in their preparation.

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The presence of mesopores (10–50 nm) indicated from the gas sorption studies is consistent with the textural pores among the fused nanoparticles seen in Figs. 2(c) and (d). It is noted that the samples maintained their monolithic feature during the solvent extraction. When water was used for extraction, however, the original pieces broke into smaller monolithic particulates of about 3 mm in diameter, which might be due to the large capillary forces exerted by water in the microcracks that developed during curing. The solids were robust and did not lose their structural integrity during sample handling and soaking for water pycnometry, although the quantification of the mechanical strength of the materials is warranted in the future. Such a structural integrity is unusual for geopolymer materials with high total porosities up to 67% observed in our case (Table 1).The porosity from mesopore structure (given as nanoporosity in Table 1) is actually no greater than 23%, and hence the matrix itself possibly maintains its rigidity while the additional spherical macropores increase the total porosity of the materials.

The negligible nanoporosity (0.88%) for the sample S4 is intriguing because paraffin oil turned out in our experiments to mix well with the geopolymer resin and could produce porous geopolymer (total pore volume = 1.1 cm3/g). Detailed SEM studies on S4 (data not shown) indicate that the material indeed exhibits the spherical macropores (20–50 μm) like others, but interestingly the pore walls show additional macropores of about 2 μm instead of mesopores. It is suspected that the small macropores are open and connected together, as all the paraffin oil could be extracted out according to the CHN analyses. In any event, the presence of the small macropores in S4 instead of the mesopores found in other samples indicates that the saponification reaction does play a significant role in pore formation probably by providing the in situ formed surfactant and also water-soluble glycerol byproduct.

In addition to the oil type, the amount of water and K/Al ratio are shown to control porosity of the products as well. With a fixed ratio of K/Al = 2 and canola oil, samples S1, S2, and S7 show an increase in the BJH cumulative pore volume 0.44–0.61 cm3/g upon increasing the mole fraction of water from 0.63 to 0.73, whereas their pore widths are more or less the same (Table 1). Meanwhile, the increase in the K/Al ratio also significantly increases the pore volume and pore width. With canola oil and x fixed at 0.73, the samples S6S8, prepared with K/Al = 1, 2, and 3, show BJH cumulative pore volumes of 0.30, 0.61, and 0.70 cm3/g and the average pore widths of 17, 18, and 34 nm, respectively. The higher amount of KOH in the precursor solution may lead to a more extensive saponification reaction, which in turn provides a higher porosity in the final product. Despite the excess amounts of KOH in the precursor, it is worthy mentioning that all the products showed a neutral pH after the water extraction, indicating again that the pore structure is open for permeation of water in the matrix, hence enabling the removal of the excess alkaline component during the extraction. It is noted that the products were found to keep their structural integrity and the original porosity even after prolonged soaking in acidic solutions with a pH value as low as 3.

4 Summary

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Summary
  7. Acknowledgments
  8. References

We have demonstrated that a simple synthesis of hierarchically porous geopolymers is possible by employing emulsion templating with triglyceride oil. The coexisting distinctive mesopores and macropores were characterized using the N2 sorption, SEM, TEM, and pycnometric studies. We have also shown that the pore size and/or volume can be controlled by changing synthetic parameters such as oil type, and water and alkali contents in precursor solution. Further studies are due for elucidation of the precise role of those synthetic parameters and potentially others in controlling the porosity and also for quantitative examination of stability of this new class of porous ceramics under various physical and chemical stresses.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Summary
  7. Acknowledgments
  8. References

This work was supported by Mattium Corporation through NSF SBIR Phase II (award no. 1152665). D. M.'s research assistantship was partially supported by the Center for Bio-Inspired Solar Fuel Production, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DE-SC0001016. We gratefully acknowledge the use of facilities within the LeRoyEyring Center for Solid State Science at Arizona State University.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results and Discussion
  6. 4 Summary
  7. Acknowledgments
  8. References
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