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faujasite composite adsorbent
Microporous and Mesoporous Materials 268 (2018) 243–250
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of a graphene oxide/faujasite composite adsorbent Dae Woo Kim
a,b,∗∗,1
, He Han
a,c,1
a
c
, Hanim Kim , Xinwen Guo , Michael Tsapatsis
T
a,∗
a
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN, 55455, USA Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, Liaoning Province, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeolite Faujasite Graphene Adsorbent Template-free growth
Template-free synthesis of faujasite type zeolite was investigated in the presence of graphene oxide (GO) in the synthesis sol. Nano-sized FAU/EMT, nano-sized FAU, and micro-sized FAU can be obtained depending on synthesis conditions such as precursor molar ratios, crystallization temperature, and the use of freeze-drying to alter water content. The obtained GO/Faujasite powder was shaped in the form of a disk by vacuum filtration. The shaped GO/Faujasite powder is electrically conductive (∼0.4 S/m & 2000–3000 Ω/sq), mechanically stable (Young's modulus: ∼7 MPa) and contains micropores, mesopores, and macropores. Furthermore, the GO/ Faujasite disk exhibits high methylene blue adsorption capacity and can be regenerated by thermal annealing at 300 °C without significant capacity loss.
1. Introduction For industrial applications such as gas storage [1], adsorption [2], and catalysis [3,4], zeolites are usually shaped as a pellet. To ensure industrial operation, the shaped zeolites should meet requirements for mechanical strength, thermal conductivity, and chemical stability [5]. Electrically conductive zeolite composites may extend zeolite utilization for photo-electronic applications such as a sensor, electrocatalyst, capacitive deionization, supercapacitor, battery electrodes, and photocatalyst [6,7]. Although multiple additives have been used for structuring zeolite powders including lubricant, porogen, binder, filler, modifier and so on [7,8], they are only effective to enhance physical, chemical, and mechanical properties, while electrical conductivity is still challenging to achieve. Recently, two-dimensional carbon materials have been utilized to provide electrical conductivity to microporous materials such as zeolites and metal-organic frameworks [9–13]. Particularly, graphene is an effective additive because of its excellent thermal, mechanical properties, chemical stability as well as electronic conductivity [14]. In this context, silicalite-1/graphene oxide (GO) powder with micrometer diameter was synthesized using TPAOH as a structure directing agent [10] and multi-quaternary ammonium (C22-6-6) was used to synthesize two-dimensional silicalite-1/GO composite [15]. NaA zeolite/GO powder was also synthesized using halloysite nanotubes by blending
∗
with GO via a hydrothermal reaction [16]. The organic template-based synthesis requires calcination at a high temperature at 500 °C or above to activate micropores, resulting in the decomposition of graphene. Moreover, due to GO sheets being embedded in the zeolite particles, the role of graphene to provide properties such as electrical/thermal conductivity can be hindered [10,15,16]. Therefore, new synthesis methods are required in order to synthesize nano-sized zeolite particles/graphene composites without the use of organic templates. Herein, we explore the use of GO to make GO/Faujasite composite disks that exhibit electrical conductivity, mechanical stability, and micro/meso/macroporosity. First, we studied the organic-free synthesis of faujasite type zeolite in the presence of GO to assess the influence of the graphitic materials on the crystallization of faujasite zeolite. We adopted earlier approaches to control size and crystal type (FAU/EMT) and compared their effectiveness in the presence of GO in the synthesis sol. It was determined that nano-sized FAU/EMT, nano-sized FAU, and micro-sized FAU can be obtained with graphene sheets depending on the reaction conditions such as precursor molar ratio, crystallization temperature, and the use of freeze-drying to alter water content. Because organic additives were not used for the faujasite synthesis, the as-prepared GO/Faujasite powder showed well-developed micro-porosity from faujasite particles and meso/macroporosity from the aggregated graphene and faujasite particles. In addition, the GO/Faujasite powders were shaped to fabricate a mechanically stable and electrically
Corresponding author. Corresponding author. Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455, USA. E-mail addresses: [email protected] (D.W. Kim), [email protected] (M. Tsapatsis). 1 D.W. Kim and H. Han contributed equally to this work. ∗∗
https://doi.org/10.1016/j.micromeso.2018.04.034 Received 2 January 2018; Received in revised form 10 April 2018; Accepted 23 April 2018 Available online 27 April 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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2.5. Dye adsorption
conductive disk, which can be used as regenerable adsorbent for methylene blue removal from aqueous solution.
Graphite (FP 99.95% pure, Graphit Kropfmühl AG) was oxidized using a modified Hummer's method [17]. Graphite (1 g) was mixed with concentrated sulfuric acid (98%, 150 mL), and 3.5 g of KMnO4 was added to the solution in an ice box. After 5 h reaction at 35 °C, DI water, and hydrogen peroxide were added sequentially to the solution in an ice bath. The solution was centrifuged at 10000 rpm for 20 min and the supernatant solution was removed. And the remaining reactant was redispersed in the DI water. This cleaning procedure was repeated more than 5 times to remove remaining ion and acids. After the cleaning process, the oxidized graphite was freeze dried.
All dye adsorption tests were carried out in polypropylene tubes containing various concentrations of methylene blue in water under mild stirring. Before the dye adsorption test, all adsorbents were thermally annealed at 200 °C for 1 h to remove the adsorbed water and stored in a bottle. To determine the adsorption rate by adsorbents, 500 mg of adsorbent was immersed in the 50 mL of MB solution with a concentration of 10 mg/L (10 ppm). To obtain MB adsorption isotherms, 100 mg of adsorbent were immersed in 5 mL of MB solution with various concentrations from 10 mg/L (10 ppm) to 2 g/L (1000 ppm). Dye solutions containing powder type adsorbents were centrifuged at 6000 rpm and the supernatant solutions were drawn off to be measured by UV-vis spectrophotometer (Thermo Scientific, evolution 220). Adsorbent disks were simply separated from solution with a tweezer. The dye removal percentage (%) and adsorption capacity were calculated by the following equations.
2.2. Synthesis of faujasite and GO/Fauasite
R=
2. Experimental section 2.1. Synthesis of GO
Chemicals used were a reagent grade sodium silicate solution (10.6% Na2O, 26.5% SiO2, 62.9% H2O, Sigma-Aldrich), sodium aluminate (Sigma-Aldrich) and NaOH (Sigma-Aldrich). For typical synthesis, 0.72 g Na2Al2O4 and 1.58 g NaOH were dissolved in 7 mL water in a polyethylene bottle, 12 g of the Na2SiO3 solution were prepared in a polyethylene bottle. After placing both solutions in an ice bath for 30 min, the solutions were mixed slowly under vigorous stirring. The obtained sol was then aged for 24 h at room temperature, and then it was crystallized at 50 °C for 2 days. The crystals were washed by repeated centrifugation and redispersion until the pH of the solution became ∼9. The details of the synthesis conditions are included in the supporting Tables 1 and 2 To prepare GO/Fauasite, the same procedure was conducted with the addition of GO in the sol after aging. Before crystallization, the sol containing graphene oxide was sonicated for 1 h to exfoliate GO.
qe =
(Co − Ce ) V M
C0 (mg/L) and Ce are the initial and equilibrium concentrations of MB solution, respectively. V (L) is the volume of MB solution and M (g) is the weight of adsorbents. We assumed no water adsorption and no volume change by adsorption. The Concentration of MB solution was measured by the intensity of UV-vis absorbance spectra at the wavelength of 660 nm. 2.6. Regeneration of GO/Fauasite disk Thermal annealing was conducted to degrade the adsorbed MB molecules. Before annealing, the GO/Fauasite disk was dried in a 70 °C oven to remove the remaining water. After drying, the GO/Fauasite disk was heated to 300 °C in air with the heating rate of 5 °C/min and maintained for 2 h before cooling to room temperature.
2.3. Fabrication of GO/Fauasite disk
2.7. Characterization
The GO/Fauasite powder (around 2 g) was redispersed in the 100 mL DI water and the solution was sonicated until the powder was well dispersed without large particles. A polycarbonate (PC) filter (Whatman, 0.2 μm pore) was placed on a vacuum filtration set-up to filter the GO/Fauasite solution. Finally, the prepared GO/Fauasite disk was dried in 70 °C oven. In order to increase the electrical conductivity, the prepared GO/Fauasite disk was placed at 200 °C more than 2 h in the N2 environment to remove the adsorbed water molecules. The temperature was then increased to the desired temperature (500 °C and 800 °C) with a heating rate 50 °C/min and maintained for 30 min.
X-ray diffraction (XRD) patterns were obtained by a Bruker-AXS (Siemens) D5005 with 2.2 kW sealed Cu Source (wavelength: 1.54 Å). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) mapping images were obtained with a FEG-SEM (Hitachi SU8230). X-ray photoelectron spectroscopy (XPS) measurements were conducted with a surface science SSX-100. Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai T12 with accelerating voltage 120 kV. Ar adsorption isotherms were obtained at 87.3 K using a commercially available automatic manometric sorption analyzer (Quantachrome Instruments AutosorbiQ MP). Prior to adsorption measurements, the samples were outgassed at 573 K for 16 h under turbomolecular pump vacuum. The electrical resistance of GO/ Faujasite disk was measured by 4-point probe method using PRF-914B (PROSTAT). 3-point bending test was conducted with RSA G2 solid analyzer (TA instruments). Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using Nicolet 6700 (Thermo Scientific).
2.4. Gas permeance measurement The gas permeance of prepared disk was measured by differential pressure method. First, the disk was placed on a stainless steel ring and fixed by epoxy to seal leaks between the disk and stainless ring. The permeation cavity was divided into the disk and stainless steel. The one side of the disk was evacuated while the target gas was flowing on the other side. A pressure change by the permeated gas was measured by a vacuum gauge. The permeance of gas thorough disk was calculated by the equation.
Permeance =
100(Co − Ce ) Co
3. Results and discussion 3.1. Characterization of graphene oxide/faujasite powder
dp Vc ∗ R∗T ∗Pa ∗A dt
Fig. 1A–C is a schematic illustration of the approach used here to synthesize nano-sized faujasite with graphene oxide (GO) and then to prepare faujasite disk containing GO. For most of the experiments, a clear sol suspension was prepared with a molar composition of 14 SiO2: 1 Al2O3: 12 Na2O: 214 H2O and aged for 24 h at room temperature. The
Vc represents the volume of vacuum side, T is room temperature, A is the effective transmission area, Pa is atmosphere pressure, dp/dt is the pressure variation on vacuum side per unit time, and R is the gas constant. 244
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Fig. 1. (A–C) Schematics of the one-pot method to prepare GO/Faujasite disk with micro/meso/macropores. (D) and (E) Photographic images for faujasite and GO/ Faujasite powder and dispersion, respectively. (F) SEM image of GO/Faujasite powder (14 wt% of GO) after synthesis. (G) TEM image for GO/Faujasite powder and inset is a magnified TEM image of faujasite on graphene oxide.
particles with 100 nm–200 nm diameter are composed of smaller nanoparticles with around 20 nm diameter in agreement with earlier reports in the absence of GO [19,20]. The TEM image indicates crystalline faujasite as reconfirmed by the typical XRD pattern of nano-sized faujasite zeolite (Fig. S3). Broad full width half maximum (FWHM, 1.2°) of the peak at 6.12° indicates that it is composed of peaks from EMT [100], FAU [111], and EMT [101] due to the coexistence of EMT and FAU [19,20]. The FAU content can be increased by freeze-drying the sol used for FAU/EMT synthesis to reduce the molar ratio of water [21], resulting in a molar composition of 14 SiO2: 1 Al2O3: 12 Na2O: 100 H2O (Fig. S4). A molar composition [3.4 SiO2: 1 Al2O3: 4.3 Na2O: 211 H2O] was also used in order to synthesize micrometer size FAU particles. Longer crystallization time (more than 6 days) was required to prepare highly crystalline FAU nanoparticles with freeze-drying when the concentration of GO was high, around 16 wt% (Fig. S5). At the shorter time (2 days), only amorphous nanoparticles were found on the surface of GO (Figs. S5 and S6). The crystallization temperature needs to be maintained at 50 °C for pure FAU crystal, otherwise sodalite (SOD) begins to form at 70 °C (Fig. S5) [21]. Unlike earlier studies that report adhesion of MFI on GO surface or enveloped GO within MFI crystals [15,16], we found that the GO/Faujasite powder was a dispersed and loosely connected mixture. Moreover, the faujasite morphology was not affected by the presence and amounts of GO (Fig. S4). It seems that interactions between the zeolite sol and GO are not enough to trigger the growth of faujasite particles on the GO surface. This can be
GO was mixed and exfoliated in the sol by sonication (Fig. 1A). Then, crystallization was conducted at 50 °C for 2 days. After drying, some faujasite nanoparticles lying on the surface of GO along with unattached faujasite nanoparticles were obtained (GO/Faujasite, Fig. 1B). The concentration of GO was limited up to 20 mg/mL (Fig. S1). Otherwise, GO exfoliation is not achieved [18]. The GO/Faujasite disk was fabricated by filtering the GO/Faujasite powder dispersed in water. Below we show that it is thermally and electrically conductive due to the dispersed graphene, and contains micropores from faujasite and inter-crystalline meso/macropores (Fig. 1C). The faujasite powder without GO was white and formed milky suspension in water (Fig. 1D), but GO/Faujasite powder was dark gray showing a dark dispersion in water as shown in Fig. 1E. Centrifuging the suspension resulted in precipitation of both faujasite and GO (Fig. S2). Fig. 1F is a scanning electron microscopy (SEM) image obtained from the GO/Faujasite powder (14 wt% GO). Aggregated faujasite nanoparticles with a diameter of 100 nm can be seen near the surface of GO. Transmission electron microscopy (TEM) imaging was conducted to observe the crystalline structure of faujasite particles and their distributions on the surface of GO (Fig. 1G). Diluted GO/Faujasite solutions were dropped on a carbon-coated Cu grid and dried at room temperature. The obtained TEM images reveal that the faujasite nanoparticles (dark regions) were well-dispersed in graphene layers. The arrow in Fig. 1G is pointing the edge of GO sheet under Faujasite particles. The TEM image of the inset shows that aggregated faujasite
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reduction of GO during faujasite crystallization enhances the van der Waals interaction between sp2 carbon region of adjacent reduced GO sheets (Figs. S7 and S8). The electrical conductivity of GO/Faujasite disk was investigated in Fig. 2F. 5 mm thickness GO/Faujasite disks were used to measure the electrical conductivity and 5 samples were measured to obtain average values. As expected, faujasite disk (without GO) is an insulator, but electrical conductivity of GO/Faujasite disks increased from 4*10−5 S/ m (4 wt% of GO) to 2.7*10−4 S/m (19 wt% of GO). Because reduction of GO by NaOH was not effective comparing to other reducing chemicals such as hydrazine and HI [22], the electrical conductivity of GO/ Faujasite disks can be further improved by thermal annealing at 500 °C in a nitrogen environment [26], showing around 0.4 S/m (2000–3000 Ω/sq) at 19 wt% of GO. Unfortunately, while the conductivity of GO/Faujasite disk can be further improved to ∼2.6 S/m (∼400 Ω/sq) by increasing the temperature up to 800 °C [12,27], the crystal structure of faujasite collapsed at 800 °C (Figs. S15 and S16) [25]. The relatively lower electrical conductivity of GO/Faujasite disk, compared to that of reduced graphene oxide (100–1000 S/m), indicates that faujasite nanoparticles prevented graphene sheets from being stacked [28]. Mechanical properties of GO/Faujasite disk were also investigated by 3-point bending test. Fig. 2G shows the stress vs strain curves obtained from faujasite disk and GO/Faujasite (14 wt%) disk, respectively. For the measurement, specimens were cut from the disks with dimensions of 1 mm thickness, 10 mm length, and 10 mm width. The specimen from a faujasite-only disk was very brittle and easily broken at the low strain around 0.25%. The GO/Faujasite disk was also brittle and broken at low strain around 0.75%. However, Young's modulus, defined as the slope of the stress vs strain curve in the elastic region, was enhanced from ∼1 MPa of faujasite disk to ∼7 MPa of GO/Faujasite disk. The Young's modulus of GO/Faujasite disk was much lower than the intrinsic value of single-layer graphene oxide (around 207 GPa) [29]. However, the GO/Faujasite disk exhibits sufficient mechanical strength to be used in practical applications [30].
attributed to the negatively charged surface of both sol and GO surfaces due to the deprotonation of COOH groups of GO at high pH [22,23]. Although GO does not alter significantly faujasite growth, GO itself is altered during synthesis. Raman spectra, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results show the reduction of oxygen functional groups of GO during crystallization (Figs. S7 and S8). The reduction of GO can be attributed to the highly concentrated alkaline environment. It is known that NaOH can deoxygenate the surface of GO [22]. Based on the above results, we conclude that a powder composed of faujasite particles and reduced GO can be prepared by faujasite crystallization in the presence of GO. Earlier developed methods to control the structure type (FAU/EMT) and size of faujasite can be used in the presence of graphene materials by increasing crystallization time as the concentration of GO increases. Despite the well-mixed state of the obtained powder, there is no evidence for adhesion of faujasite to GO. The former appears to be loosely packed within in GO layers. 3.2. Characterization of graphene oxide/faujasite disk The obtained GO/Faujasite powder was redispersed in water and vacuum-filtered to fabricate a GO/Faujasite disk (Fig. 2A). The thickness and diameter of the GO/Faujasite disk can be controlled by adjusting the filter diameter and amount of GO/Faujasite dispersion used. In our experiment, 5 cm diameter and 1–5 mm thickness disks were readily obtained. While the color of faujasite only disk was white, the color of GO/Faujasite disk became darker by increasing the amount of GO (Fig. S9). The GO/Faujasite disk was mechanically stable. Interestingly, the GO/Faujasite disk showed ultrafast absorption of a water droplet in 30 ms of contact time, possibly due to the meso/macroporous structure of stacked nanoparticles and GO, in addition to the hydrophilic property of faujasite (Fig. 2B). The structure of GO/Faujasite disk was further investigated by SEM and energy dispersive spectroscopy (EDS) mapping as shown in Fig. 2C and E. Fig. 2C is the top-view structure of GO/Faujasite disk, displaying well-mixed GO and faujasite. Pores between packed faujasite nanoparticles are clearly observed as well as macro voids around graphene sheets. Fig. 2D and E shows crosssection images of the GO/Faujasite disk at low and high magnification, respectively. The sheet-like texture is observed in the entire region of magnified SEM image of Fig. 2E and is typical of graphene-based composite materials such as graphene-polymer composites [24]. The inset of Fig. 2E shows that aggregated faujasite nanoparticles were surrounded by graphene sheets. EDS mappings obtained from the SEM image of Fig. 2E for C, O, Si and Al atoms also show that graphene (carbon: purple), and faujasite particles (O: green, Si: red, and Al: blue) are uniformly distributed throughout the GO/Faujasite disk (at the resolution limit of the technique). Interestingly, aggregation of GO sheets was not observed in the GO/Faujasite disk when the weight percent of GO increased to 19 wt%. The hindering of aggregation and re-stacking of GO sheets was also confirmed by XRD patterns, obtained from GO/ Faujasite powder and disk, showing no diffraction peak from GO (001) (Figs. S3 and S10). The absence of GO (001) peak in GO/Faujasite could be attributed to the absence of long range order of stacked GO layers. However, the GO (001) peak appears in the disk made by directly mixing GO with faujasite (14 wt% GO) and then filtering the mixture, indicating the aggregation of GO (Fig. S10). In addition, because directly mixed GO sheets encapsulated the aggregated faujasite particles and covered the entire surface of the GO/Faujasite disk (Figs. S11 and S12), fast water absorption was not observed (Fig. S13). The mixture of faujasite nanoparticles and GO obtained by one-pot synthesis prevents the re-stacking of GO [25], resulting in the uniform dispersion of GO. The disk made from directly mixed powders was not stable in water because GO can be redispersed in the water due to the oxygen functional groups. On the other hand, GO/faujasite disk prepared by onepot synthesis method can be stable in water even after sonication for 90 min (Baransonic 5510, 135 W, Fig. S14) possibly because the
3.3. The porosity of graphene oxide/faujasite composite The porosity of GO/Faujasite disk (14 wt% of GO) was investigated by using Ar adsorption and desorption isotherms measured at 87 K (Fig. 3A). The faujasite powder displays a typical isotherm of Ar in the micropores of faujasite. On the other hand, GO/Faujasite powder, GO/ Faujasite disk, and GO/Faujasite disk made by direct mixing of GO and faujasite, show increased meso/macroporosity. The surface area and pore volume of each sample are summarized in Table 1. Pore size distribution analysis was carried out with density functional theory (DFT) method in Fig. 3B. All samples show a sharp peak at the pore diameter from 7 Å to 10 Å, originating from the well-developed micropore of faujasite nanoparticles. Also, we observed the formation of new peaks in the pore diameter range from 20 nm to 50 nm, indicating the presence of mesopores in the disks. The intensity of the peaks and pore volumes were increased as GO was mixed with faujasite particles: 0.41 cm3/g (faujasite powder), 0.64 cm3/g (GO/Faujasite powder), 1.01 cm3/g (GO/Faujasite disk), and 0.81 cm3/g (GO/Faujasite disk made by direct mixing). Because large pores were dominantly formed by the voids between graphene sheet and faujasite particles rather than aggregated faujasite particles, the GO/Faujasite disk with well-dispersed graphene showed a large external pore volume (1.01 cm3/g) than other samples. To investigate the influence of meso/macropores generated by the graphene sheet on the mass transport of gas molecules through the disk, the gas permeance of GO/Faujasite disk was compared to faujasite disk, and GO/Faujasite disk made by direct mixing, in Fig. 3C. To measure the gas permeance by differential pressure method [31], each disk with around 5 mm thickness was mounted on a stainless steel disk with a 12.7 mm circular hole. The support side of the disk was sealed with an epoxy adhesive as described in the schematic of Fig. 3D. The GO/ 246
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Fig. 2. (A) Photographic images of GO/Faujasite (14 wt% GO) disk. (B) Water absorption behavior of GO/Faujasite disk. (C) Top view SEM image of GO/Faujasite disk. (D) Low magnification cross-section SEM image of GO/Faujasite disk. (E) Magnified cross-section SEM image of GO/Faujasite disk and corresponding EDS mappings. (F) The electrical conductivity of GO/Faujasite disk for different GO amounts and after thermal annealing at 500 °C in nitrogen. (G) Stress and strain curves of faujasite and GO/Faujasite disk (14 wt% GO) obtained by 3 point bending test. Inset is averaged Young's modulus from 5 samples, respectively.
Faujasite disk was strong enough to withstand the pressure drop of 100 KPa (1 atm) during the permeance measurement. The faujasite disk shows a permeance around 1–3*10−8 mol Pa−1m−2s−1 for the He, N2, CO2, and Air. The permeances increased to 1–3*10−7 mol Pa−1m−2s−1 in the case of GO/Faujasite disk. On the other hand, the permeances of GO/Faujasite disk made by direct mixing were significantly lower, 6*10−12–1.6*10−11 mol Pa−1m−2s−1. The enhanced permeance of
GO/Faujasite disk can be attributed to the formation of mesopores and macropores around graphene sheets as shown schematically in Fig. 3D, while faujasite nanoparticles were densely packed in the faujasite-only disk (Fig. S17). The hindered permeance of GO/Faujasite disk made by direct mixing can be attributed to the graphene sheets covering the aggregated faujasite particles as shown in Figs. S11 and S12, i.e., acting as a barrier. 247
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Fig. 3. (A) and (B) Ar isotherm and corresponding pore size distribution of faujasite powder, GO/Faujasite powder, GO/Faujasite disk, GO/Faujasite disk made by direct mixing and graphene oxide, respectively. (C) The gas permeance of faujasite, GO/Faujasite and GO/Faujasite disk made by direct mixing. (D) Schematic for the gas permeation test set-up and schematic of gas flow through GO/Faujasite disk. The GO weight percent was 14%. Disks with around 5 mm thickness were used for the gas permeation test.
disks (faujasite disk, GO/Faujasite disk, GO/Faujasite disk made by direct mixing) were determined as shown in Fig. 4B. 500 mg of adsorbent was immersed in the 50 mL of MB solution with a concentration of 10 mg/L (10 ppm) for the desired time under mild stirring at room temperature. Nano-sized faujasite, micro-sized faujasite and GO/Faujasite powder show fast dye adsorption in 5 min with 99%, 93%, and 99% of dye removal, respectively. The dye adsorption of powder samples reached saturation after 5 min. Nano-sized faujasite was more
3.4. Dye adsorption of graphene oxide/faujasite composite A GO/Faujasite disk (14 wt% GO) was utilized to adsorb dye molecules from their solutions in water (Fig. 4). Methylene blue (MB, positively charged, 1.6 nm * 0.7 nm * 0.33 nm) was used to test dye adsorption because the size of MB molecule is similar to the pore size (7.4 Å) of faujasite zeolite (Fig. 4A). The adsorption vs time behavior of powders (nano-sized faujasite, micro-sized faujasite, GO/Faujasite) and
Table 1 Textural properties of prepared materials including surface areas and pore volumes. Sample
SBETa (m2/g)
Smicrob (m2/g)
Sexternal (m2/g)
Vtotalc (cm3/g)
Vmicrod (cm3/g)
Vexternal (cm3/g)
Faujasite powder GO/Faujasite powder GO/Faujasite disk GO/Faujasite mixed disk GO
726 488 610 725 30
558 353 434 501 0.0
168 135 176 224 30
0.62 0.77 1.17 1.00 0.03
0.21 0.13 0.16 0.19 0.0
0.41 0.64 1.01 0.81 0.03
a
Brunauer–Emmett–Teller (BET) surface areas calculated over the pressure range (P/P0) of 0.01–0.12. Micropore surface areas calculated from the Ar adsorption isotherms using the t-plot method. c Total pore volume obtained at P/P0 = 0.99. d Micropore volume calculated using the t-plot method. The external pore volume and surface area are obtained by subtracting micropore volume and surface area from the total values. The weight percent of GO was around 14 wt%. b
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Fig. 4. (A) Schematics for the chemical structure of MB molecule and faujasite. (B) Dye removal percentage of each sample depending on the contact time. 10 ppm MB solutions were used. (C) Photographic image of dye solutions before and after adsorption with GO/Faujasite disk. Numbers indicate the initial concentration of dye solution (ppm). (D) MB adsorption isotherm of GO/Faujasite and faujasite disk. The red line is a Freundlich plot for GO/Faujasite disk. All GO/ Faujasite disk used in dye adsorption contained 14 wt% of GO. (E) Photographic images of GO/Faujasite disk (14 wt% GO) after adsorption test with 1000 ppm MB solution and after regeneration. (F) Variation of adsorption capacity and dye removal percentage depending on the recycle number. Regeneration was conducted at 300 °C for 2 h in air. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
conducted with 1000 ppm MB solution and the results are shown in Fig. 4E and F. 1 g of GO/Fauasite disk was immersed in 50 mL of MB solution with a concentration of 1 g/L (1000 ppm) for 4 days. Regeneration was conducted by thermal annealing at 300 °C in air for 2 h. The annealing temperature was carefully selected to avoid the significant degradation of graphene at temperatures above 400 °C (Fig. S18). The temperature needs to be higher than 300 °C to ensure the decomposition of MB. Photographic images in Fig. 4E show that the color of GO/Fauasite disk changed from dark gray to dark brown after MB adsorption and the dark gray color was recovered after thermal annealing. The adsorption capacity of GO/Fauasite disk was well preserved after four regenerations, showing a stable capacity of ∼50 mg/g (Fig. 4F).
effective than micro-sized FAU possibly due to the reduced intra-crystalline diffusion and larger surface area exposed to the MB solution [32,33]. The adsorption rate decreased when a powder was shaped into a disk. Dye removal by each disk gradually increased and saturated to 99% after 120 min. The initial (at 2 min) fast dye adsorption by GO/ Faujasite disk, made by direct mixing could be attributed to adsorption by GO on the outer surface of the disk (GO has high adsorption capacity (240 mg/mL) for MB molecules [34]). However, because the GO surrounding the disk hindered the diffusion of dye molecules into the disk, eventually the adsorption was delayed. We conclude that while the adsorption rate by GO/Faujasite disk is slower than that from powder, the GO/Faujasite disk can be still effective to adsorb dye molecules. Fig. 4C shows photographic images of MB solutions before and after adsorption with GO/Faujasite disk and Fig. 4D is a MB adsorption isotherm of GO/Faujasite and faujasite disk. The concentration of MB solutions was varied from 10 mg/L (10 ppm) to 2 g/L (2000 ppm). As shown in the photographic images in Fig. 4C, MB solutions became clear or less colored after immersing the GO/Faujasite disk, resulting in the dye removal percentage from 92% to 99% depending on the initial concentration of dye solution. All samples were immersed in the dye solutions for 7 days to ensure equilibrium. The adsorption capacity of GO/Faujasite disk increased by increasing the initial concentration of MB: ∼1 mg/g at 10 ppm, ∼50 mg/g at 1000 ppm, 65.9 mg/g at 1400 ppm, and 92.4 mg/g at 2000 ppm starting concentration. The isotherm of GO/Faujasite disk is well fitted (R2 = 0.998) by the Freundlich isotherm equation (qe = Kf Ce1/ n ) with n = 1.45 [35]. The adsorption capacity of GO/Fauasite disk was similar to that of the faujasite disk, while previous binder materials such as polymer and alumina significantly reduced the adsorption capacity [36]. The reduced micropore volume of GO/Fauasite disk due to the added GO may be compensated by the high adsorption ability of GO for cationic dye molecules [16,34]. The MB adsorption capacity (92.4 mg/g at 152 mg/ L) of GO/Fauasite disk is the highest value when compared to the other zeolite materials such as MCM-22 (57.57 mg/g at saturation) [37], desilicated zeolite (47.3 mg/g at saturation) [38], and zeolite NaA/RGO composite (53.3 mg/g at 300 mg/L) [16]. The ability to regenerate zeolite adsorbents is one of their main advantages [39]. Thus, a regeneration test of GO/faujastie disk was
4. Conclusions Template-free synthesis of faujasite type zeolite was conducted in the presence of graphene oxide. It was found that previous approaches to control the size and structure type (FAU/EMT) can be applied. Because GO can be uniformly mixed with faujasite nanoparticles and reduced during crystallization of faujasite, re-stacking and aggregation of reduced GO can be highly suppressed, resulting in a GO/Fauasite composite disks with well dispersed reduced GO. On the other hand, faujasite particles were enveloped by GO sheets and the GO sheets became aggregated in GO/Fauasite disks made by direct mixing of GO and faujasite, resulting in highly hindered mass transport. One-pot synthesis enables the preparation of a well-dispersed GO/Fauasite composite. The obtained GO/Fauasite disk was electrically conductive (∼0.4 S/m & 2000–3000 Ω/sq) and mechanically stable with Young's modulus of ∼7 MPa. It is composed of micropores from the crystalline structure of faujasite and meso/macropores from the aggregated faujasite particles and graphene sheets. We demonstrated that the prepared GO/Fauasite disk can be used to adsorb dye molecules and that it is regenerable and stable upon regeneration. Acknowledgements This work was supported by the Center for Gas Separations Relevant 249
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to Clean Energy Technologies, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under DE-SC0001015. Dae Woo Kim was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1A6A3A04057367). He Han was supported by the China Scholarship Council (CSC) (File No. 201606060063). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of a graphene oxide/faujasite composite adsorbent Dae Woo Kim
a,b,∗∗,1
, He Han
a,c,1
a
c
, Hanim Kim , Xinwen Guo , Michael Tsapatsis
T
a,∗
a
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN, 55455, USA Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, Liaoning Province, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeolite Faujasite Graphene Adsorbent Template-free growth
Template-free synthesis of faujasite type zeolite was investigated in the presence of graphene oxide (GO) in the synthesis sol. Nano-sized FAU/EMT, nano-sized FAU, and micro-sized FAU can be obtained depending on synthesis conditions such as precursor molar ratios, crystallization temperature, and the use of freeze-drying to alter water content. The obtained GO/Faujasite powder was shaped in the form of a disk by vacuum filtration. The shaped GO/Faujasite powder is electrically conductive (∼0.4 S/m & 2000–3000 Ω/sq), mechanically stable (Young's modulus: ∼7 MPa) and contains micropores, mesopores, and macropores. Furthermore, the GO/ Faujasite disk exhibits high methylene blue adsorption capacity and can be regenerated by thermal annealing at 300 °C without significant capacity loss.
1. Introduction For industrial applications such as gas storage [1], adsorption [2], and catalysis [3,4], zeolites are usually shaped as a pellet. To ensure industrial operation, the shaped zeolites should meet requirements for mechanical strength, thermal conductivity, and chemical stability [5]. Electrically conductive zeolite composites may extend zeolite utilization for photo-electronic applications such as a sensor, electrocatalyst, capacitive deionization, supercapacitor, battery electrodes, and photocatalyst [6,7]. Although multiple additives have been used for structuring zeolite powders including lubricant, porogen, binder, filler, modifier and so on [7,8], they are only effective to enhance physical, chemical, and mechanical properties, while electrical conductivity is still challenging to achieve. Recently, two-dimensional carbon materials have been utilized to provide electrical conductivity to microporous materials such as zeolites and metal-organic frameworks [9–13]. Particularly, graphene is an effective additive because of its excellent thermal, mechanical properties, chemical stability as well as electronic conductivity [14]. In this context, silicalite-1/graphene oxide (GO) powder with micrometer diameter was synthesized using TPAOH as a structure directing agent [10] and multi-quaternary ammonium (C22-6-6) was used to synthesize two-dimensional silicalite-1/GO composite [15]. NaA zeolite/GO powder was also synthesized using halloysite nanotubes by blending
∗
with GO via a hydrothermal reaction [16]. The organic template-based synthesis requires calcination at a high temperature at 500 °C or above to activate micropores, resulting in the decomposition of graphene. Moreover, due to GO sheets being embedded in the zeolite particles, the role of graphene to provide properties such as electrical/thermal conductivity can be hindered [10,15,16]. Therefore, new synthesis methods are required in order to synthesize nano-sized zeolite particles/graphene composites without the use of organic templates. Herein, we explore the use of GO to make GO/Faujasite composite disks that exhibit electrical conductivity, mechanical stability, and micro/meso/macroporosity. First, we studied the organic-free synthesis of faujasite type zeolite in the presence of GO to assess the influence of the graphitic materials on the crystallization of faujasite zeolite. We adopted earlier approaches to control size and crystal type (FAU/EMT) and compared their effectiveness in the presence of GO in the synthesis sol. It was determined that nano-sized FAU/EMT, nano-sized FAU, and micro-sized FAU can be obtained with graphene sheets depending on the reaction conditions such as precursor molar ratio, crystallization temperature, and the use of freeze-drying to alter water content. Because organic additives were not used for the faujasite synthesis, the as-prepared GO/Faujasite powder showed well-developed micro-porosity from faujasite particles and meso/macroporosity from the aggregated graphene and faujasite particles. In addition, the GO/Faujasite powders were shaped to fabricate a mechanically stable and electrically
Corresponding author. Corresponding author. Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455, USA. E-mail addresses: [email protected] (D.W. Kim), [email protected] (M. Tsapatsis). 1 D.W. Kim and H. Han contributed equally to this work. ∗∗
https://doi.org/10.1016/j.micromeso.2018.04.034 Received 2 January 2018; Received in revised form 10 April 2018; Accepted 23 April 2018 Available online 27 April 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved.
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2.5. Dye adsorption
conductive disk, which can be used as regenerable adsorbent for methylene blue removal from aqueous solution.
Graphite (FP 99.95% pure, Graphit Kropfmühl AG) was oxidized using a modified Hummer's method [17]. Graphite (1 g) was mixed with concentrated sulfuric acid (98%, 150 mL), and 3.5 g of KMnO4 was added to the solution in an ice box. After 5 h reaction at 35 °C, DI water, and hydrogen peroxide were added sequentially to the solution in an ice bath. The solution was centrifuged at 10000 rpm for 20 min and the supernatant solution was removed. And the remaining reactant was redispersed in the DI water. This cleaning procedure was repeated more than 5 times to remove remaining ion and acids. After the cleaning process, the oxidized graphite was freeze dried.
All dye adsorption tests were carried out in polypropylene tubes containing various concentrations of methylene blue in water under mild stirring. Before the dye adsorption test, all adsorbents were thermally annealed at 200 °C for 1 h to remove the adsorbed water and stored in a bottle. To determine the adsorption rate by adsorbents, 500 mg of adsorbent was immersed in the 50 mL of MB solution with a concentration of 10 mg/L (10 ppm). To obtain MB adsorption isotherms, 100 mg of adsorbent were immersed in 5 mL of MB solution with various concentrations from 10 mg/L (10 ppm) to 2 g/L (1000 ppm). Dye solutions containing powder type adsorbents were centrifuged at 6000 rpm and the supernatant solutions were drawn off to be measured by UV-vis spectrophotometer (Thermo Scientific, evolution 220). Adsorbent disks were simply separated from solution with a tweezer. The dye removal percentage (%) and adsorption capacity were calculated by the following equations.
2.2. Synthesis of faujasite and GO/Fauasite
R=
2. Experimental section 2.1. Synthesis of GO
Chemicals used were a reagent grade sodium silicate solution (10.6% Na2O, 26.5% SiO2, 62.9% H2O, Sigma-Aldrich), sodium aluminate (Sigma-Aldrich) and NaOH (Sigma-Aldrich). For typical synthesis, 0.72 g Na2Al2O4 and 1.58 g NaOH were dissolved in 7 mL water in a polyethylene bottle, 12 g of the Na2SiO3 solution were prepared in a polyethylene bottle. After placing both solutions in an ice bath for 30 min, the solutions were mixed slowly under vigorous stirring. The obtained sol was then aged for 24 h at room temperature, and then it was crystallized at 50 °C for 2 days. The crystals were washed by repeated centrifugation and redispersion until the pH of the solution became ∼9. The details of the synthesis conditions are included in the supporting Tables 1 and 2 To prepare GO/Fauasite, the same procedure was conducted with the addition of GO in the sol after aging. Before crystallization, the sol containing graphene oxide was sonicated for 1 h to exfoliate GO.
qe =
(Co − Ce ) V M
C0 (mg/L) and Ce are the initial and equilibrium concentrations of MB solution, respectively. V (L) is the volume of MB solution and M (g) is the weight of adsorbents. We assumed no water adsorption and no volume change by adsorption. The Concentration of MB solution was measured by the intensity of UV-vis absorbance spectra at the wavelength of 660 nm. 2.6. Regeneration of GO/Fauasite disk Thermal annealing was conducted to degrade the adsorbed MB molecules. Before annealing, the GO/Fauasite disk was dried in a 70 °C oven to remove the remaining water. After drying, the GO/Fauasite disk was heated to 300 °C in air with the heating rate of 5 °C/min and maintained for 2 h before cooling to room temperature.
2.3. Fabrication of GO/Fauasite disk
2.7. Characterization
The GO/Fauasite powder (around 2 g) was redispersed in the 100 mL DI water and the solution was sonicated until the powder was well dispersed without large particles. A polycarbonate (PC) filter (Whatman, 0.2 μm pore) was placed on a vacuum filtration set-up to filter the GO/Fauasite solution. Finally, the prepared GO/Fauasite disk was dried in 70 °C oven. In order to increase the electrical conductivity, the prepared GO/Fauasite disk was placed at 200 °C more than 2 h in the N2 environment to remove the adsorbed water molecules. The temperature was then increased to the desired temperature (500 °C and 800 °C) with a heating rate 50 °C/min and maintained for 30 min.
X-ray diffraction (XRD) patterns were obtained by a Bruker-AXS (Siemens) D5005 with 2.2 kW sealed Cu Source (wavelength: 1.54 Å). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) mapping images were obtained with a FEG-SEM (Hitachi SU8230). X-ray photoelectron spectroscopy (XPS) measurements were conducted with a surface science SSX-100. Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai T12 with accelerating voltage 120 kV. Ar adsorption isotherms were obtained at 87.3 K using a commercially available automatic manometric sorption analyzer (Quantachrome Instruments AutosorbiQ MP). Prior to adsorption measurements, the samples were outgassed at 573 K for 16 h under turbomolecular pump vacuum. The electrical resistance of GO/ Faujasite disk was measured by 4-point probe method using PRF-914B (PROSTAT). 3-point bending test was conducted with RSA G2 solid analyzer (TA instruments). Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using Nicolet 6700 (Thermo Scientific).
2.4. Gas permeance measurement The gas permeance of prepared disk was measured by differential pressure method. First, the disk was placed on a stainless steel ring and fixed by epoxy to seal leaks between the disk and stainless ring. The permeation cavity was divided into the disk and stainless steel. The one side of the disk was evacuated while the target gas was flowing on the other side. A pressure change by the permeated gas was measured by a vacuum gauge. The permeance of gas thorough disk was calculated by the equation.
Permeance =
100(Co − Ce ) Co
3. Results and discussion 3.1. Characterization of graphene oxide/faujasite powder
dp Vc ∗ R∗T ∗Pa ∗A dt
Fig. 1A–C is a schematic illustration of the approach used here to synthesize nano-sized faujasite with graphene oxide (GO) and then to prepare faujasite disk containing GO. For most of the experiments, a clear sol suspension was prepared with a molar composition of 14 SiO2: 1 Al2O3: 12 Na2O: 214 H2O and aged for 24 h at room temperature. The
Vc represents the volume of vacuum side, T is room temperature, A is the effective transmission area, Pa is atmosphere pressure, dp/dt is the pressure variation on vacuum side per unit time, and R is the gas constant. 244
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Fig. 1. (A–C) Schematics of the one-pot method to prepare GO/Faujasite disk with micro/meso/macropores. (D) and (E) Photographic images for faujasite and GO/ Faujasite powder and dispersion, respectively. (F) SEM image of GO/Faujasite powder (14 wt% of GO) after synthesis. (G) TEM image for GO/Faujasite powder and inset is a magnified TEM image of faujasite on graphene oxide.
particles with 100 nm–200 nm diameter are composed of smaller nanoparticles with around 20 nm diameter in agreement with earlier reports in the absence of GO [19,20]. The TEM image indicates crystalline faujasite as reconfirmed by the typical XRD pattern of nano-sized faujasite zeolite (Fig. S3). Broad full width half maximum (FWHM, 1.2°) of the peak at 6.12° indicates that it is composed of peaks from EMT [100], FAU [111], and EMT [101] due to the coexistence of EMT and FAU [19,20]. The FAU content can be increased by freeze-drying the sol used for FAU/EMT synthesis to reduce the molar ratio of water [21], resulting in a molar composition of 14 SiO2: 1 Al2O3: 12 Na2O: 100 H2O (Fig. S4). A molar composition [3.4 SiO2: 1 Al2O3: 4.3 Na2O: 211 H2O] was also used in order to synthesize micrometer size FAU particles. Longer crystallization time (more than 6 days) was required to prepare highly crystalline FAU nanoparticles with freeze-drying when the concentration of GO was high, around 16 wt% (Fig. S5). At the shorter time (2 days), only amorphous nanoparticles were found on the surface of GO (Figs. S5 and S6). The crystallization temperature needs to be maintained at 50 °C for pure FAU crystal, otherwise sodalite (SOD) begins to form at 70 °C (Fig. S5) [21]. Unlike earlier studies that report adhesion of MFI on GO surface or enveloped GO within MFI crystals [15,16], we found that the GO/Faujasite powder was a dispersed and loosely connected mixture. Moreover, the faujasite morphology was not affected by the presence and amounts of GO (Fig. S4). It seems that interactions between the zeolite sol and GO are not enough to trigger the growth of faujasite particles on the GO surface. This can be
GO was mixed and exfoliated in the sol by sonication (Fig. 1A). Then, crystallization was conducted at 50 °C for 2 days. After drying, some faujasite nanoparticles lying on the surface of GO along with unattached faujasite nanoparticles were obtained (GO/Faujasite, Fig. 1B). The concentration of GO was limited up to 20 mg/mL (Fig. S1). Otherwise, GO exfoliation is not achieved [18]. The GO/Faujasite disk was fabricated by filtering the GO/Faujasite powder dispersed in water. Below we show that it is thermally and electrically conductive due to the dispersed graphene, and contains micropores from faujasite and inter-crystalline meso/macropores (Fig. 1C). The faujasite powder without GO was white and formed milky suspension in water (Fig. 1D), but GO/Faujasite powder was dark gray showing a dark dispersion in water as shown in Fig. 1E. Centrifuging the suspension resulted in precipitation of both faujasite and GO (Fig. S2). Fig. 1F is a scanning electron microscopy (SEM) image obtained from the GO/Faujasite powder (14 wt% GO). Aggregated faujasite nanoparticles with a diameter of 100 nm can be seen near the surface of GO. Transmission electron microscopy (TEM) imaging was conducted to observe the crystalline structure of faujasite particles and their distributions on the surface of GO (Fig. 1G). Diluted GO/Faujasite solutions were dropped on a carbon-coated Cu grid and dried at room temperature. The obtained TEM images reveal that the faujasite nanoparticles (dark regions) were well-dispersed in graphene layers. The arrow in Fig. 1G is pointing the edge of GO sheet under Faujasite particles. The TEM image of the inset shows that aggregated faujasite
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reduction of GO during faujasite crystallization enhances the van der Waals interaction between sp2 carbon region of adjacent reduced GO sheets (Figs. S7 and S8). The electrical conductivity of GO/Faujasite disk was investigated in Fig. 2F. 5 mm thickness GO/Faujasite disks were used to measure the electrical conductivity and 5 samples were measured to obtain average values. As expected, faujasite disk (without GO) is an insulator, but electrical conductivity of GO/Faujasite disks increased from 4*10−5 S/ m (4 wt% of GO) to 2.7*10−4 S/m (19 wt% of GO). Because reduction of GO by NaOH was not effective comparing to other reducing chemicals such as hydrazine and HI [22], the electrical conductivity of GO/ Faujasite disks can be further improved by thermal annealing at 500 °C in a nitrogen environment [26], showing around 0.4 S/m (2000–3000 Ω/sq) at 19 wt% of GO. Unfortunately, while the conductivity of GO/Faujasite disk can be further improved to ∼2.6 S/m (∼400 Ω/sq) by increasing the temperature up to 800 °C [12,27], the crystal structure of faujasite collapsed at 800 °C (Figs. S15 and S16) [25]. The relatively lower electrical conductivity of GO/Faujasite disk, compared to that of reduced graphene oxide (100–1000 S/m), indicates that faujasite nanoparticles prevented graphene sheets from being stacked [28]. Mechanical properties of GO/Faujasite disk were also investigated by 3-point bending test. Fig. 2G shows the stress vs strain curves obtained from faujasite disk and GO/Faujasite (14 wt%) disk, respectively. For the measurement, specimens were cut from the disks with dimensions of 1 mm thickness, 10 mm length, and 10 mm width. The specimen from a faujasite-only disk was very brittle and easily broken at the low strain around 0.25%. The GO/Faujasite disk was also brittle and broken at low strain around 0.75%. However, Young's modulus, defined as the slope of the stress vs strain curve in the elastic region, was enhanced from ∼1 MPa of faujasite disk to ∼7 MPa of GO/Faujasite disk. The Young's modulus of GO/Faujasite disk was much lower than the intrinsic value of single-layer graphene oxide (around 207 GPa) [29]. However, the GO/Faujasite disk exhibits sufficient mechanical strength to be used in practical applications [30].
attributed to the negatively charged surface of both sol and GO surfaces due to the deprotonation of COOH groups of GO at high pH [22,23]. Although GO does not alter significantly faujasite growth, GO itself is altered during synthesis. Raman spectra, Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results show the reduction of oxygen functional groups of GO during crystallization (Figs. S7 and S8). The reduction of GO can be attributed to the highly concentrated alkaline environment. It is known that NaOH can deoxygenate the surface of GO [22]. Based on the above results, we conclude that a powder composed of faujasite particles and reduced GO can be prepared by faujasite crystallization in the presence of GO. Earlier developed methods to control the structure type (FAU/EMT) and size of faujasite can be used in the presence of graphene materials by increasing crystallization time as the concentration of GO increases. Despite the well-mixed state of the obtained powder, there is no evidence for adhesion of faujasite to GO. The former appears to be loosely packed within in GO layers. 3.2. Characterization of graphene oxide/faujasite disk The obtained GO/Faujasite powder was redispersed in water and vacuum-filtered to fabricate a GO/Faujasite disk (Fig. 2A). The thickness and diameter of the GO/Faujasite disk can be controlled by adjusting the filter diameter and amount of GO/Faujasite dispersion used. In our experiment, 5 cm diameter and 1–5 mm thickness disks were readily obtained. While the color of faujasite only disk was white, the color of GO/Faujasite disk became darker by increasing the amount of GO (Fig. S9). The GO/Faujasite disk was mechanically stable. Interestingly, the GO/Faujasite disk showed ultrafast absorption of a water droplet in 30 ms of contact time, possibly due to the meso/macroporous structure of stacked nanoparticles and GO, in addition to the hydrophilic property of faujasite (Fig. 2B). The structure of GO/Faujasite disk was further investigated by SEM and energy dispersive spectroscopy (EDS) mapping as shown in Fig. 2C and E. Fig. 2C is the top-view structure of GO/Faujasite disk, displaying well-mixed GO and faujasite. Pores between packed faujasite nanoparticles are clearly observed as well as macro voids around graphene sheets. Fig. 2D and E shows crosssection images of the GO/Faujasite disk at low and high magnification, respectively. The sheet-like texture is observed in the entire region of magnified SEM image of Fig. 2E and is typical of graphene-based composite materials such as graphene-polymer composites [24]. The inset of Fig. 2E shows that aggregated faujasite nanoparticles were surrounded by graphene sheets. EDS mappings obtained from the SEM image of Fig. 2E for C, O, Si and Al atoms also show that graphene (carbon: purple), and faujasite particles (O: green, Si: red, and Al: blue) are uniformly distributed throughout the GO/Faujasite disk (at the resolution limit of the technique). Interestingly, aggregation of GO sheets was not observed in the GO/Faujasite disk when the weight percent of GO increased to 19 wt%. The hindering of aggregation and re-stacking of GO sheets was also confirmed by XRD patterns, obtained from GO/ Faujasite powder and disk, showing no diffraction peak from GO (001) (Figs. S3 and S10). The absence of GO (001) peak in GO/Faujasite could be attributed to the absence of long range order of stacked GO layers. However, the GO (001) peak appears in the disk made by directly mixing GO with faujasite (14 wt% GO) and then filtering the mixture, indicating the aggregation of GO (Fig. S10). In addition, because directly mixed GO sheets encapsulated the aggregated faujasite particles and covered the entire surface of the GO/Faujasite disk (Figs. S11 and S12), fast water absorption was not observed (Fig. S13). The mixture of faujasite nanoparticles and GO obtained by one-pot synthesis prevents the re-stacking of GO [25], resulting in the uniform dispersion of GO. The disk made from directly mixed powders was not stable in water because GO can be redispersed in the water due to the oxygen functional groups. On the other hand, GO/faujasite disk prepared by onepot synthesis method can be stable in water even after sonication for 90 min (Baransonic 5510, 135 W, Fig. S14) possibly because the
3.3. The porosity of graphene oxide/faujasite composite The porosity of GO/Faujasite disk (14 wt% of GO) was investigated by using Ar adsorption and desorption isotherms measured at 87 K (Fig. 3A). The faujasite powder displays a typical isotherm of Ar in the micropores of faujasite. On the other hand, GO/Faujasite powder, GO/ Faujasite disk, and GO/Faujasite disk made by direct mixing of GO and faujasite, show increased meso/macroporosity. The surface area and pore volume of each sample are summarized in Table 1. Pore size distribution analysis was carried out with density functional theory (DFT) method in Fig. 3B. All samples show a sharp peak at the pore diameter from 7 Å to 10 Å, originating from the well-developed micropore of faujasite nanoparticles. Also, we observed the formation of new peaks in the pore diameter range from 20 nm to 50 nm, indicating the presence of mesopores in the disks. The intensity of the peaks and pore volumes were increased as GO was mixed with faujasite particles: 0.41 cm3/g (faujasite powder), 0.64 cm3/g (GO/Faujasite powder), 1.01 cm3/g (GO/Faujasite disk), and 0.81 cm3/g (GO/Faujasite disk made by direct mixing). Because large pores were dominantly formed by the voids between graphene sheet and faujasite particles rather than aggregated faujasite particles, the GO/Faujasite disk with well-dispersed graphene showed a large external pore volume (1.01 cm3/g) than other samples. To investigate the influence of meso/macropores generated by the graphene sheet on the mass transport of gas molecules through the disk, the gas permeance of GO/Faujasite disk was compared to faujasite disk, and GO/Faujasite disk made by direct mixing, in Fig. 3C. To measure the gas permeance by differential pressure method [31], each disk with around 5 mm thickness was mounted on a stainless steel disk with a 12.7 mm circular hole. The support side of the disk was sealed with an epoxy adhesive as described in the schematic of Fig. 3D. The GO/ 246
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Fig. 2. (A) Photographic images of GO/Faujasite (14 wt% GO) disk. (B) Water absorption behavior of GO/Faujasite disk. (C) Top view SEM image of GO/Faujasite disk. (D) Low magnification cross-section SEM image of GO/Faujasite disk. (E) Magnified cross-section SEM image of GO/Faujasite disk and corresponding EDS mappings. (F) The electrical conductivity of GO/Faujasite disk for different GO amounts and after thermal annealing at 500 °C in nitrogen. (G) Stress and strain curves of faujasite and GO/Faujasite disk (14 wt% GO) obtained by 3 point bending test. Inset is averaged Young's modulus from 5 samples, respectively.
Faujasite disk was strong enough to withstand the pressure drop of 100 KPa (1 atm) during the permeance measurement. The faujasite disk shows a permeance around 1–3*10−8 mol Pa−1m−2s−1 for the He, N2, CO2, and Air. The permeances increased to 1–3*10−7 mol Pa−1m−2s−1 in the case of GO/Faujasite disk. On the other hand, the permeances of GO/Faujasite disk made by direct mixing were significantly lower, 6*10−12–1.6*10−11 mol Pa−1m−2s−1. The enhanced permeance of
GO/Faujasite disk can be attributed to the formation of mesopores and macropores around graphene sheets as shown schematically in Fig. 3D, while faujasite nanoparticles were densely packed in the faujasite-only disk (Fig. S17). The hindered permeance of GO/Faujasite disk made by direct mixing can be attributed to the graphene sheets covering the aggregated faujasite particles as shown in Figs. S11 and S12, i.e., acting as a barrier. 247
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Fig. 3. (A) and (B) Ar isotherm and corresponding pore size distribution of faujasite powder, GO/Faujasite powder, GO/Faujasite disk, GO/Faujasite disk made by direct mixing and graphene oxide, respectively. (C) The gas permeance of faujasite, GO/Faujasite and GO/Faujasite disk made by direct mixing. (D) Schematic for the gas permeation test set-up and schematic of gas flow through GO/Faujasite disk. The GO weight percent was 14%. Disks with around 5 mm thickness were used for the gas permeation test.
disks (faujasite disk, GO/Faujasite disk, GO/Faujasite disk made by direct mixing) were determined as shown in Fig. 4B. 500 mg of adsorbent was immersed in the 50 mL of MB solution with a concentration of 10 mg/L (10 ppm) for the desired time under mild stirring at room temperature. Nano-sized faujasite, micro-sized faujasite and GO/Faujasite powder show fast dye adsorption in 5 min with 99%, 93%, and 99% of dye removal, respectively. The dye adsorption of powder samples reached saturation after 5 min. Nano-sized faujasite was more
3.4. Dye adsorption of graphene oxide/faujasite composite A GO/Faujasite disk (14 wt% GO) was utilized to adsorb dye molecules from their solutions in water (Fig. 4). Methylene blue (MB, positively charged, 1.6 nm * 0.7 nm * 0.33 nm) was used to test dye adsorption because the size of MB molecule is similar to the pore size (7.4 Å) of faujasite zeolite (Fig. 4A). The adsorption vs time behavior of powders (nano-sized faujasite, micro-sized faujasite, GO/Faujasite) and
Table 1 Textural properties of prepared materials including surface areas and pore volumes. Sample
SBETa (m2/g)
Smicrob (m2/g)
Sexternal (m2/g)
Vtotalc (cm3/g)
Vmicrod (cm3/g)
Vexternal (cm3/g)
Faujasite powder GO/Faujasite powder GO/Faujasite disk GO/Faujasite mixed disk GO
726 488 610 725 30
558 353 434 501 0.0
168 135 176 224 30
0.62 0.77 1.17 1.00 0.03
0.21 0.13 0.16 0.19 0.0
0.41 0.64 1.01 0.81 0.03
a
Brunauer–Emmett–Teller (BET) surface areas calculated over the pressure range (P/P0) of 0.01–0.12. Micropore surface areas calculated from the Ar adsorption isotherms using the t-plot method. c Total pore volume obtained at P/P0 = 0.99. d Micropore volume calculated using the t-plot method. The external pore volume and surface area are obtained by subtracting micropore volume and surface area from the total values. The weight percent of GO was around 14 wt%. b
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Fig. 4. (A) Schematics for the chemical structure of MB molecule and faujasite. (B) Dye removal percentage of each sample depending on the contact time. 10 ppm MB solutions were used. (C) Photographic image of dye solutions before and after adsorption with GO/Faujasite disk. Numbers indicate the initial concentration of dye solution (ppm). (D) MB adsorption isotherm of GO/Faujasite and faujasite disk. The red line is a Freundlich plot for GO/Faujasite disk. All GO/ Faujasite disk used in dye adsorption contained 14 wt% of GO. (E) Photographic images of GO/Faujasite disk (14 wt% GO) after adsorption test with 1000 ppm MB solution and after regeneration. (F) Variation of adsorption capacity and dye removal percentage depending on the recycle number. Regeneration was conducted at 300 °C for 2 h in air. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
conducted with 1000 ppm MB solution and the results are shown in Fig. 4E and F. 1 g of GO/Fauasite disk was immersed in 50 mL of MB solution with a concentration of 1 g/L (1000 ppm) for 4 days. Regeneration was conducted by thermal annealing at 300 °C in air for 2 h. The annealing temperature was carefully selected to avoid the significant degradation of graphene at temperatures above 400 °C (Fig. S18). The temperature needs to be higher than 300 °C to ensure the decomposition of MB. Photographic images in Fig. 4E show that the color of GO/Fauasite disk changed from dark gray to dark brown after MB adsorption and the dark gray color was recovered after thermal annealing. The adsorption capacity of GO/Fauasite disk was well preserved after four regenerations, showing a stable capacity of ∼50 mg/g (Fig. 4F).
effective than micro-sized FAU possibly due to the reduced intra-crystalline diffusion and larger surface area exposed to the MB solution [32,33]. The adsorption rate decreased when a powder was shaped into a disk. Dye removal by each disk gradually increased and saturated to 99% after 120 min. The initial (at 2 min) fast dye adsorption by GO/ Faujasite disk, made by direct mixing could be attributed to adsorption by GO on the outer surface of the disk (GO has high adsorption capacity (240 mg/mL) for MB molecules [34]). However, because the GO surrounding the disk hindered the diffusion of dye molecules into the disk, eventually the adsorption was delayed. We conclude that while the adsorption rate by GO/Faujasite disk is slower than that from powder, the GO/Faujasite disk can be still effective to adsorb dye molecules. Fig. 4C shows photographic images of MB solutions before and after adsorption with GO/Faujasite disk and Fig. 4D is a MB adsorption isotherm of GO/Faujasite and faujasite disk. The concentration of MB solutions was varied from 10 mg/L (10 ppm) to 2 g/L (2000 ppm). As shown in the photographic images in Fig. 4C, MB solutions became clear or less colored after immersing the GO/Faujasite disk, resulting in the dye removal percentage from 92% to 99% depending on the initial concentration of dye solution. All samples were immersed in the dye solutions for 7 days to ensure equilibrium. The adsorption capacity of GO/Faujasite disk increased by increasing the initial concentration of MB: ∼1 mg/g at 10 ppm, ∼50 mg/g at 1000 ppm, 65.9 mg/g at 1400 ppm, and 92.4 mg/g at 2000 ppm starting concentration. The isotherm of GO/Faujasite disk is well fitted (R2 = 0.998) by the Freundlich isotherm equation (qe = Kf Ce1/ n ) with n = 1.45 [35]. The adsorption capacity of GO/Fauasite disk was similar to that of the faujasite disk, while previous binder materials such as polymer and alumina significantly reduced the adsorption capacity [36]. The reduced micropore volume of GO/Fauasite disk due to the added GO may be compensated by the high adsorption ability of GO for cationic dye molecules [16,34]. The MB adsorption capacity (92.4 mg/g at 152 mg/ L) of GO/Fauasite disk is the highest value when compared to the other zeolite materials such as MCM-22 (57.57 mg/g at saturation) [37], desilicated zeolite (47.3 mg/g at saturation) [38], and zeolite NaA/RGO composite (53.3 mg/g at 300 mg/L) [16]. The ability to regenerate zeolite adsorbents is one of their main advantages [39]. Thus, a regeneration test of GO/faujastie disk was
4. Conclusions Template-free synthesis of faujasite type zeolite was conducted in the presence of graphene oxide. It was found that previous approaches to control the size and structure type (FAU/EMT) can be applied. Because GO can be uniformly mixed with faujasite nanoparticles and reduced during crystallization of faujasite, re-stacking and aggregation of reduced GO can be highly suppressed, resulting in a GO/Fauasite composite disks with well dispersed reduced GO. On the other hand, faujasite particles were enveloped by GO sheets and the GO sheets became aggregated in GO/Fauasite disks made by direct mixing of GO and faujasite, resulting in highly hindered mass transport. One-pot synthesis enables the preparation of a well-dispersed GO/Fauasite composite. The obtained GO/Fauasite disk was electrically conductive (∼0.4 S/m & 2000–3000 Ω/sq) and mechanically stable with Young's modulus of ∼7 MPa. It is composed of micropores from the crystalline structure of faujasite and meso/macropores from the aggregated faujasite particles and graphene sheets. We demonstrated that the prepared GO/Fauasite disk can be used to adsorb dye molecules and that it is regenerable and stable upon regeneration. Acknowledgements This work was supported by the Center for Gas Separations Relevant 249
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to Clean Energy Technologies, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under DE-SC0001015. Dae Woo Kim was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1A6A3A04057367). He Han was supported by the China Scholarship Council (CSC) (File No. 201606060063). Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.
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