development and evaluation of ordered mesoporous ...

AIAA 2011-5197

41st International Conference on Environmental Systems 17 - 21 July 2011, Portland, Oregon

DEVELOPMENT AND EVALUATION OF ORDERED MESOPOROUS CARBONS FOR RESORCINOL REMOVAL Ruixuan Guo1, Lin Lu1, Victoria C. Hover2, and Daniel D. Gang3 University of Louisiana at Lafayette, Lafayette, LA, 70503, USA

Ordered mesoporous carbons (OMCs) show great potential for environmental improvement for their ability to remove inorganic and organic contaminants from liquid and gas. OMCs have controlled mesopores. This results in much higher adsorption capacity and faster adsorption kinetics than regular activated carbonbased materials. OMCs represent innovative and promising adsorption materials for treatment of hygiene and urine waste streams generated during space missions by replacing the Granular Activated Carbon (GAC) currently used in the water recovery system (WRS) on the International Space Station. OMC preparation involves the synthesis of and ordered silica template SBA-15, introduction of carbon precursors into the silica template, carbonization, and finally silica template removal. Three different carbon precursors were investigated in this study. Low angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that SBA15 has an ordered structure with hexagonal symmetry.

The synthesized OMC

products exhibit highly ordered structure according to TEM photos. Resorcinol, a typical total organic carbon (TOC) model compound, was selected to evaluate the adsorption behavior of OMCs. The adsorption results indicated that the OMC made from glucose have a higher adsorption capacity than that from acrylic acid and sucrose, with an adsorption capacity of 37.4 mg/g.

I.

INTRODUCTION

One of the major challenges of a spacecraft life support system is to supply the crew with sufficient water for potable and hygiene uses. This fresh water typically makes up the majority of the weight associated with the resupply of life support consumables1. Therefore, recycling the initial water supply represents a significant cost saving by minimizing resupply payloads. Physical/chemical methods for recycling wastewater have been well explored due to their gravity independence, high reliability, relatively high-energy efficiency, design flexibility, technological maturity, and regenerative nature2. However, these systems have limitations and often require their own expendables in the form of chemical additives or filters. The current water recovery system (WRS) for

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Graduate Student, Civil Engineering, Madison Hall, Room 127 Assistant Professor, Department of Geology, Madison Hall, Room 224B 3 Associate Professor, Civil Engineering, Madison Hall, Room 254N, P. O. Box 42291 1 American Institute of Aeronautics and Astronautics 2

Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

providing potable water to the International Space Station (ISS) crew consists of units of vapor compression distillation, adsorption, and ion exchange3. The adsorption materials are expendable media, which require resupply from the Earth. Such resupply of life support consumables limits human exploration of even our own moon4. Decreasing the resupplies and reducing the weight, volume, and power needed for packing materials are critical in order to fulfill the U.S. long-duration space exploration missions5. Therefore, there is an urgent need for the development of innovative adsorption materials used in WRS, which have with high adsorption capacity, and less weight and volume for long and short-term space missions and the International Space Station. Ordered mesoporous carbon materials are of considerable interest because of their high specific surface area and controlled mesopore size. Historically, activated carbon with a large fraction of micropores (< 2 nm) has been used for contaminant adsorption. The material’s applications, however, can be limited by slow contaminant diffusion kinetics and the inaccessibility of the sorption sites. Ordered mesoporous carbons (OMCs), on the other hand, can dramatically increase the adsorption capacity and adsorption kinetics because of their high surface areas and controlled mesoporous size (5 to 50 nm). Recently, increasing attention has been focused on innovative ordered mesopors and ordered mesoporous carbon frameworks with high thermal stability. Ordered large pores and channels facilitate diffusion within the frameworks and adsorption behavior of mesoporous materials are of central importance to the practical applications6. Confined-space effects inside the mesostructured channels would certainly modify the unique physical and chemical properties of carbons, which have inspired chemists and material scientists to create uniform mesoporous carbons7. Ryoo and coworkers first fabricated ordered mesoporous carbon using ordered mesoporous silicate templates8. Carbon precursors were first introduced into the channels of the mesoporous silicate templates through either a solution-phase or a vapor-phase reaction. After the removal of the hard silica templates, the resultant mesoporous carbon nanowire, nanorod, or nanotube arrays form an inverse replica of the ordered mesostructures of silicates9. Of all the reported mesoporous carbons prepared by templating mesostructured silica materials, mesoporous carbons obtained from SBA-15 silica have attracted the most attention11. Because of the well-ordered structure of SBA-15 silica (i.e. hexagonally ordered cylindrical nanochannels or nanorods), the carbons obtained as inverse replicas of the template are also highly ordered. In our ongoing research to develop a simple procedure for synthesizing OMCs, we used Pluronic P123 as a structure-directing agent in the preparation of the SBA-15 silica template and three different carbon sources (acrylic acid, sucrose, and glucose). Sucrose and glucose are inexpensive and environmentally friendly carbon precursors as opposed to acrylic acid. Herein, a new and simple route for OMCs preparation was investigated by carbonization of sulfuric-acid treated silica/triblock copolymer/sucrose composites, followed by silica removal. The pore structures of SBA-15 and OMC were characterized in detail by low angle X-ray diffraction (XRD) and transmission electron microscopy (TEM)10. The study also provides a much-improved understanding of contaminants fate and transport in the surface of the mesoporous material. The overall objective of this research was to develop and test the efficiency of an innovative OMC material for removing organic matters commonly found in hygiene and urine waste streams produced in space. In this study, resorcinol, a typical mid-molecular weight total organic carbon (TOC) was selected as the primary model compounds to evaluate the adsorption behavior of the adsorbent. The specific objectives of this project are: 1.

Prepare ordered mesoporous carbon (OMC) and optimize the pore size of OMCs that are suitable for removing organic matters in hygiene and urine waste streams.

2.

Investigate the adsorption capacity of the ordered mesoporous carbon material for TOC removal. 2 American Institute of Aeronautics and Astronautics  

 

II.

MATERIALS AND METHODS

2.1 Preparation of silica template In this study, SBA-15, a mesoporous silica molecular sieve was selected as a template. Two different routes were used to synthesize the SBA-15 silica template: one is in aqueous solutions with a temperature of 99C and the other in non-aqueous ethanol solution at room temperature. In a typical aqueous solutions synthesis, 100 ml of concentrated hydrochloride acid (HCL, 37%) was added into 545 ml of distilled water with magnetically stirring. The silica source (tetraethyl orthosilicate, TEOS, 98%, Aldrich) was added to an aqueous solution along with a surfactant, which acts as the organic structure-directing agent. The surfactant used was Pluronic P123 (EO20PO70EO20, BASF), nonionic oligomeric alkyl-ethylene oxide) (starting quantities: 20 g Pluronic P123, 48.5 mL TEOS, 100 mL HCl and 545 mL distilled water). The mixture was magnetically stirred until the TEOS was dissolved. Then, it was placed in a constant temperature water bath (Precision Scientific) for 4 h at 40C, followed by aging at temperature of 90C for 24 h. The solid product was filtered, washed with distillate water, dried at 40C and calcined in air at 550C for 8 h. In the non-aqueous ethanol solution synthesis, 46.5 ml of the silica source (TEOS, 98%, Aldrich) and 20 g structure-directing agent Pluronic P123 were dissolved in 100 ml ethanol and mixed completely, and then transferred into a petri dish under room temperature (between 25C to 40C) under humidity of 40% to 60%. The petri dish was put into a fuming hood, overnight until the ethanol evaporated completely. The white and ceraceous mixture was then transferred to a crucible and was calcined in a muffle furnace at 550C for 8 hrs. After that, the white silica template was stored in desiccators for the preparation of OMC. 2.2 Preparation of mesoporous carbons It was believed that the use of carbon precursors was responsible for the formation of the pore walls of ordered mesoporous carbon9. The synthesis of OMCs was performed by means of the impregnation of silica nanopores by carbon precursors. Three different carbon precursors were used: acrylic acid, sucrose, and glucose. In one set of the experiments, the synthesis of OMC was accomplished by in situ polymerization of acrylic acid (>99%, Aldrich) in the porous structure of silica template SBA-15 in an aqueous solution. Ninety ml of acrylic acid was added into 180 ml of distilled water with stirring, and then 9 g of SBA-15 was added and suspended in acrylic acid solution. After the mixture was stirred for 30 min, 0.02 g of 2.2-azobisisobutyronitrile (AIBN) was added as a free radical initiator to produce polyacrylic acid. The mixture was then heated to 60C for the in situ polymerization. The resultant polyacrylic acid (PAA) and silica template was dried in an oven at 200C overnight. Afterwards, the composite was heated under N2 flow at a temperature ramp rate of 5C min-1 to 700C and held for 8 hrs for carbonization. The resulting carbon–silica composite was immersed in 50 ml of 48% HF (Aldrich) at room temperature for 15 hrs to remove the silica template. The recovered OMC obtained as an insoluble fraction was washed with distilled water to remove the residual HF and then dried in the oven at 90 ~100C overnight. In a second set of experiments using glucose and sucrose as carbon precursors we employed sulfuric acid (>98%, Aldrich) as a catalyst. The meso-structured silica was impregnated with glucose and sucrose. The mixtures were dried and then carbonized under N2 at a rate of 10C /min to 780C over 8 hrs. The samples were held for 1 hr at 780C. The resulting carbon–silica composite was immersed in 48% HF at room temperature for 24 hrs to remove the silica template. The OMC obtained as an insoluble fraction was washed with distilled water and then dried in air at 98C overnight. The furnace used is Thermolyne Economy Solid-Tube Furnace (Thermo Scientific). 3 American Institute of Aeronautics and Astronautics    

2.3 Characterization of structures Low angle X-ray diffraction (XRD) patterns were obtained on a Diano 2100E instrument operated at 45 Kv and 30 mA and using Cu Ka12 radiation (λ=1.5418 Ångstoms). Measurements were carried out to obtain resolved XRD patterns of diffraction intensity versus 2θ incident angle from 0.5° to 3.5° 2θ. The scanning rate was 0.03°/step, and 15 second/step. The aperture slit (soller slit) was 0.4°. Transmission electron microscopy (TEM) studies reveal information not only about structures and structural relationship, but also important information regarding structural units or building blocks. Our TEM images were obtained using a Hitachi 7600 Transmission Electron Microscope. The acceleration voltage was 100 kv. The TEM samples were prepared by dispersing a large number of particles in ethanol and dispensing some drops of this solution on a polymeric TEM grid. The TEM image was taken from the thin edges of carbon particles mounted on the grid. 2.4 Batch adsorption study Resorcinol, a typical TOC model compound, was selected to evaluate the adsorption behavior of the OMCs. The adsorptive capacities of all OMCs were measured by determining the amount of adsorption of resorcinol from aqueous solution. The absorption experiments were conducted on an Incubator Shaker (New Brunswick Scientific) at 25C. The concentration of resorcinol remaining in solution after each experiment was analyzed by UV-VIS Absorption Spectroscopy (Varian). The amount resorcinol adsorbed by OMCs can be determined by subtracting the final concentration from the initial concentration using the following formula:

 

  Where, q is the adsorption capacity (mg/g); Ci is the initial concentration of resorcinol in solution (mg/L); Cf is the final concentration of resorcinol in treated solution (mg/L); V is the volume of the solution taken (L); M is the weight of the adsorbent OMCs (g).

III.

RESULTS AND DISCUSSION

3.1 X-ray diffraction (XRD) image of SBA-15 The structure of the SBA-15 template made by aqueous method and structure of the corresponding OMCs were studied using low angle X-ray diffraction (XRD). Powder X-ray diffraction patterns were obtained from 0.5° to 3.5°° 2θ. Figure 1 shows the XRD pattern for the mesostructured silica SBA-15 used as a template. The low-angle XRD reflections show the periodic nature of pore openings in the silica template. The diffraction reflection at 2θ = 0.9° (d = 9.6 nm) is the characteristic of ordered mesostructured silica SBA-15. The three well resolved XRD reflections at 0.9° 2θ (9.6 nm), 1.6° 2θ (5.4 nm), and 1.8° 2θ (4.8 nm) are similar to previously reported data for hexagonally ordered SBA-15 templates and can be assigned to (100), (110), and (200) reflections, respectively. The intensity of the (100) reflection is the strongest of the three reflections.

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Figure 1. Powder XRD pattern of SBA-15 by the aqueous method. 3.2 The TEM images of SBA-15 The TEM images of the SBA-15 are shown in Figure 2. Figure 2a shows the elongate rod-like nanochannel structure of the silica substrate. Figure 2B also shows the hexagonal arrays of silicon nanochannel rods, which have an approximately 10 nm diameter (or repeat from center to center) and about 6.5 nm wall thickness. These nanochannel diameters, all thicknesses and overall structure are similar to those reported in the literature for SBA-15 template material. The repeat distance of (or diameter) of the nanochannels of approximately 10 nm is consistent with the 9.6 nm periodicity determined by XRD methods above. SBA-15 made in aqueous solution has larger mesopores than that from non-aqueous solution.

a 

b

Figure 2. TEM of template made from (a) aqueous method and (b) non-aqueous Method 3.3 The TEM images of OMC The TEM images of OMCs synthesized on the SBA-15 template above are shown in Figure 3. The images reveal that the OMCs possess a highly ordered nanorod array (Figures 3A and 3C) with some images showing the characteristic 2-D hexagonal mesostructures with pore size from about 3 to 5 nm in diameter (Figure 3B). The microscopic structure and morphology of mesoporous materials depended on the type of carbon precursors 5 American Institute of Aeronautics and Astronautics    

and preparation conditions. OMC made from glucose possessed more ordered hexagonal arrays of carbon rods (Figure 3B) than OMCs from sucrose (Figure 3C) and poly-acrylic acid (Figure 3A).

a

b

c 

Figure 3. TEM of OMCs from Aqueous Method: from (a) Acrylic Acid, (b) Glucose and (c) Sucrose 3.4 X-ray diffraction (XRD) of OMCs OMCs should have a hexagonal structure, which corresponds to the negative replication of the silica template. The XRD patterns of OMCs are similar to that of SBA-15 as shown in Figures 5, 6, and 7. The OMCs products also show the periodic nature but with weaker diffraction intensities compared to the SBA-15 template. This may due to defects in the carbon frameworks during the carbonization and etching processes. And the (100) reflections of the OMCs occur between about 0.9-1.0° 2θ. The corresponding d-values of the (100) reflection for OMCs made from acrylic acid is 9.0 nm, from sucrose is 9.6 nm, and from glucose is 10.0 nm. Weak (110) and (200) are also observed (Figures 5, 6, and 7).

Figure 4. Powder XRD pattern of OMC made from acrylic acid carbon precursor on the SBA-15 template made by the aqueous method 6 American Institute of Aeronautics and Astronautics    

Figure 5. Powder XRD pattern of OMC made from sucrose carbon precursor on SBA-15 template made by the aqueous method

Figure 6. Powder XRD pattern of OMC made from glucose carbon precursor on SBA-15 template made by the aqueous method The (100), (110) and (200) reflections from the hexagonal structures, are similar to those of SBA-15 template, which indicate the long-range ordering of the carbon material. The long-range ordering of OMCs is maintained after the removal of the silica template, which proves that the replication process did not influence 7 American Institute of Aeronautics and Astronautics    

the ordered pore arrangement of SBA-15 template 3.5 TOC Model Compound Removal The adsorption capacities were calculated according to formula. The Cf values using six different kinds of OMCs were gained from UV-Vis Absorption Spectrophotometry, and they were recorded in Table 1. Table 2 is the calculated adsorption capacity values of six different kinds of OMCs: Table 1. Cf values of six different kinds of OMCs. Cf (mg/L) carbon sources

Synthesis Methods average ethanol method 4.25 3.86 3.85 3.90 3.40 3.80 3.70 3.13 3.35

aqueous method 3.90 3.83 3.35 3.45 3.25 3.00

acrylic acid sucrose glucose

average 4.05 3.85 3.53

Table 2. Adsorption capacities values of six different kinds of OMCs. q (mg/g) carbon sources

acrylic acid sucrose glucose

Synthesis Methods aqueous method ethanol method 22.8 19 32 23 37.4 29.4

Figure 7 shows the adsorption results of six different kinds of OMCs for resorcinol. It can be seen that the OMCs made from aqueous method SBA-15 show higher adsorptive ability than OMCs made from SBA-15 from non-aqueous method under the same carbon resources. Comparing the three kinds of OMCs made from the same template with different carbon precursors, the OMC from glucose showed higher adsorptive ability than OMCs from acrylic acid and sucrose. Overall, the OMC made from glucose and SBA-15 made from aqueous method shows the best performance.

Figure 7. Adsorption capacity (mg/g) of OMCs for resorcinol. 8 American Institute of Aeronautics and Astronautics    

IV.

CONCLUSION

In summary, mesoporous carbons were synthesized using SBA-15 silica templates produced from two different methods two different methods. The following conclusions can be drawn from the research: 1.

SBA-15 was prepared using two different methods: the aqueous method and the non-aqueous (ethanol) method.

The product made by the aqueous method produced a structure with long-range ordered

arrangement of mesopores. The product made by the ethanol method produced a less ordered structure. OMCs made by template from liquid-crystal method possess more ordered hexagonal arrays of carbon structures than OMCs made by template from non-aqueous method. 2.

Three different carbon precursor compounds were used to synthesize OMCs: acrylic acid, sucrose and glucose. Acrylic acid has the smallest molecular weight. Sucrose has the largest molecular weight and glucose has an intermediate molecular weight. The XRD pattern of OMC made from acrylic acid indicated the most intense (100) reflections relative to the patterns obtained from glucose and sucrose. The results suggest that the pores of SBA-15 can be filled more completely by smaller molecular compounds such as acrylic acid, whereas the larger molecular weight carbon sources such as glucose and sucrose are not able to fill the template completely, yielding less intense (100) reflections.

3.

From the batch adsorption study, all kinds of OMCs could remove resorcinol from aqueous solutions. But, the OMCs made on the SBA-15 template made from aqueous solution have better absorption performance than those made from the non-aqueous template. OMCs made from glucose on the aqueous SBA-15 template showed best performance in adsorption of resorcinol, with an adsorption capacity of 37.4 mg/g carbon.

ACKNOWLEDGMENTS Financial support from LaSPACE is gratefully acknowledged.

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