Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10379-10386
Mesoporous Chitosan−SiO2 Nanoparticles: Synthesis, Characterization, and CO2 Adsorption Capacity Sayyid Mahdi Rafigh* and Amir Heydarinasab
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Department of Chemical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran ABSTRACT: Mesoporous chitosan−SiO 2 nanoparticles (NPs) were successfully synthesized. The prepared nanoparticles were characterized using TEM, FTIR, XRD, TGA, EDX, and (CHN) elemental analysis. From TEM micrograph, chitosan−SiO2 NPs were sphere-like, pretty uniformly distributed with coarse surface. The average size of chitosan−SiO2 NPs was determined as 211 nm with DLS, which was confirmed by TEM. The mesoporous structure of chitosan−SiO2 NPs was characterized with N2 adsorption/ desorption measurements. BET surface area was 621 m2 g−1 and the total pore volume was 0.71 m3 g−1. CO2 adsorption was evaluated by a volumetric method. Chitosan−SiO2 NPs showed a maximum CO2 adsorption capacity of 4.39 mmol g−1 at 25 °C and a high selective separation capacity for CO2 over N2 (SCO2/N2 = 15.46). The influence of amines on carbon dioxide adsorption was discussed. Stable CO2 adsorption/desorption was confirmed after six cycles of experiments. Therefore, chitosan− SiO2 NPs exhibit great potential for CO2 capture. KEYWORDS: Chitosan−SiO2 nanoparticles (NPs), CO2 adsorption, Mesoporous
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monolithic silica supports.14,15 The chitosan market is expected to be USD 4.22 billion in 2020.16 Recently, silica nanoparticles were incorporated into the polymers to enhance their mechanical and interfacial properties for several fields of application such as absorbent, reductive degradation, and reinforced polymeric materials.17−21 Our work focuses on the following key contributions: (1) synthesis of chitosan−SiO2 nanoparticles (NPs), (2) evaluation of chemical and physical properties of chitosan−SiO2 NPs using TEM, FTIR, XRD, DLS, TGA, EDX, and (CHN) elemental analysis, (3) investigating the N2 and CO2 adsorption capacity and microstructure properties of chitosan−SiO2 NP, and (4) studying the cyclic adsorption−desorption to examine the reusability potential of chitosan−SiO2 NPs.
INTRODUCTION CO2 emissions as the main source of global warming and climate change are a current environmental concern.1 Therefore, more attention has been paid to development of the methods and materials for capturing and sequestration of CO2.1−3 Among them, adsorption could be a useful technology because it is simple, low cost, and effective with an easy operation process. Various solid adsorbents have been studied for CO2 capture such as activated carbons, zeolites, mesoporous silicas, and metal−organic frameworks.4−7 The amine functionalization of porous materials such as silica and zeolite as sorbents has recently received significant attention by many researchers for high CO2 adsorption.2,5 Chitosan is a pseudonatural cationic biopolymer produced through deacetylation of chitin is the most abundant natural polysaccharide after cellulose.8,9 Chitosan has one amino group and two hydroxyl groups in a repeating glucosidic residue, and it has a higher proportion of amino groups than chitin.9 Chitosan has excellent properties such as good biocompatibility, biodegradability, high adhesion to the surface, high hydrophilicity, expressed chelating properties, nontoxicity, chemical stability, and antibacterial activity.9,10 It has been proved that chitosan is an effective adsorbent toward some heavy metals,11 dyes,12 and organic pollutants.13 The presence of a large number of amine groups on the chitosan structure can facilitate the acidic CO2 molecule to get adsorbed on the surface of the adsorbent molecule. Pure chitosan exhibited a negligible CO2 adsorption capacity, and the maximum CO2 adsorption was reported as 0.98 mmol g−1 for chitosan based© 2017 American Chemical Society
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MATERIALS AND METHODS
Materials. Tetraethyl orthosilicate (TEOS 99.9%), Tween 80 (polysorbate 80), absolute ethanol (EtOH 99.5%), and chitosan powder (medium molecular weight Mw = 161 000 g mol−1) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All other chemicals used were of analytical grade. Synthesis of Chitosan−SiO2 Nanoparticles (NPs). A two-step procedure was used for the sample preparation. The first step consisted of the preparation of silica nanospheres according to a modified method based on the reported protocol.22 Briefly, Tween 80 as a surface active material (0.3 g) was fully dissolved in an acidic Received: July 18, 2017 Revised: August 13, 2017 Published: September 24, 2017 10379
DOI: 10.1021/acssuschemeng.7b02388 ACS Sustainable Chem. Eng. 2017, 5, 10379−10386
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. TEM micrographs and particle size distribution for (a) SiO2 nanospheres and (b) chitosan−SiO2 NPs. TGA Analysis. The thermal properties of samples were analyzed using a thermogravimetric analyzer (TGA), Mettler Toledo TGA/ SDTA851 (Mettler Toledo Corp., Greifensee, Switzerland). Samples (10.0 mg in a platinum pan) were heated from 25 to 600 °C under a nitrogen atmosphere at a heating rate of 20 °C/min with a nitrogen flow of 30 mL/min. Particle Size Analysis. The mean diameter of particles was determined by at least five replicate measurements with dynamic light scattering (Zeta Plus; Brookhaven Instruments Corp., New York, USA) after appropriate dilution with double-distilled Milli-Q water. The mean ± standard deviation (SD) was assessed. Elemental Analysis and Energy-Dispersive X-ray Spectroscopy. Energy-dispersive X-ray spectroscopy (EDX) (JEOLJSM-600F) was used to determine the composition of the samples and further analyzed for the percent of carbon (C), hydrogen (H), and nitrogen (N) present in the samples by a 2400 CHNS analyzer (PerkinElmer, Series II). N2 and CO2 Adsorption Experiments. The N2 adsorption− desorption isotherms at −196 °C and CO2 adsorption isotherms at 0 and 25 °C were obtained under static conditions using an ASAP 2460 (Micromeritics) volumetric adsorption analyzer. Before analysis, samples were outgassed under vacuum at 60 °C overnight. Data were measured by admitting or removing a known quantity of adsorbing gas in or out of a sample cell containing chitosan−SiO2 NPs as the solid adsorbent maintained over a wide relative pressure range from 0.01 to 0.99 at a constant temperature. The specific surface area was calculated with the Brunauer− Emmett−Teller (BET) model from adsorption data in a relative pressure range of 0.05 < P/P0 < 0.25. The pore volume versus diameter distribution was derived from analyzing both the adsorption and the desorption branches of the isotherm using the Barrett− Joyner−Halenda (BJH) method.23 To check the repeatability and reproducibility of CO2 adsorption isotherms, measurements were carried out for several runs under similar operational conditions. Statistics. Statistical analysis was performed using a Statistical Analysis System (SAS, version 9.1). A one-way analysis of variance (ANOVA) was conducted with Duncan’s multiple range test at a 95%
solution containing hydrochloric acid (2 mL) and deionized water (25 mL) in a 250 mL Erlenmeyer flask at room temperature. It was then followed by slow addition of a TEOS mixture containing TEOS (3 mL) as the silica source, EtOH (32 mL), and deionized water (8.5 mL). Ammonium hydroxide (3.5 mL) as a catalyst was added dropwise to this mixture. The reaction was allowed to proceed under stirring at 30 °C for 25 min to give rise to white precipitates. The formed solids were collected by centrifugation at 4000 rpm and dispersed into an acidified ethanol solution to remove the surfactants. The white-colored silica nanospheres were then washed by deionized water and finally dried under vacuum at room temperature. In the second step, aqueous 1.5 wt % chitosan solution was prepared by dissolving chitosan (0.18 g) in 1 wt % acetic acid aqueous solution to produce a homogeneous mixture. Then a chitosan solution was immersed into SiO2 suspension containing SiO2 nanospheres (1 g) and distilled water (100 mL) under an ultrasonic bath for 20 min. To coat the chitosan on the SiO2 nanospheres, the mixture was kept at 60 °C for 6 h. The resulting suspension was placed in an ice water bath for 1 h. The as-produced nanoparticles (NPs) were collected by centrifugation at 5000 rpm. The chitosan−SiO2 NPs (0.41 g) that formed during the process were washed to a neutral pH with deionized water several times and dried at 50 °C overnight.
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CHARACTERIZATION
TEM Analysis. Transmission electron microscopy (TEM) analysis was performed on a JEOL JSM-2100F electron microscope (Tokyo, Japan) operated at 200 kV. FTIR Analysis. Fourier transform infrared spectra of the samples were recorded on a PerkinElmer 1000 spectrometer (PerkinElmer, Waltham, MA, USA) using a spectral range of 4000−400 cm−1 with a resolution of 4 cm−1 in the transmittance mode employing the KBr disk technique. XRD Analysis. X-ray diffraction spectra were recorded using a XRD 6000 diffractrometer (Shimadzu, Kyoto, Japan) from 2θ = 0° to 70° with a step of 0.05° and integration time of 5 s per step by Cu Kα as a radiation source (λ = 1.5405 Å) operating under a constant current of 30 mA at 40 kV. 10380
DOI: 10.1021/acssuschemeng.7b02388 ACS Sustainable Chem. Eng. 2017, 5, 10379−10386
Research Article
ACS Sustainable Chemistry & Engineering confidence level (p < 0.05). The data were expressed as mean ± standard deviation for each experiments conducted in triplicate (n = 3).
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RESULTS AND DISCUSSION Structural Characteristics of the Prepared Samples. The transmission electron microscopy (TEM) micrograph of SiO2 nanospheres was shown in Figure 1a. It can be observed that the SiO2 nanospheres are monodispersed with good sphericity and fine porous structure, which clearly shows the presence of a hexagonally packed porous structure embedded in the silica spherical particles. The average size of individual silica nanospheres is ∼190 nm. The TEM image of SiO2 nanospheres shows clear areas that correspond to the empty pores and darker regions that correspond to the pore walls. Figure 1b shows the fine porous structure of chitosan−SiO2 NPs as examined by TEM. The nanoparticles are porous and sphere-like and pretty uniformly distributed with a coarse surface with an average size of approximately 200 nm. As shown in Figure 1b, thin layers could be seen at the edge of chitosan−SiO2 NPs as the arrow pointed, which should be due to the chitosan coating on SiO2 nanospheres. It is observed from TEM that chitosan is practically filling up the pores as well as covering the outer surface of the particles. The particle size distribution of the chitosan−SiO2 NPs was measured by DLS (Figure 1b). The hydrodynamic particle sizes of the SiO2 nanospheres and chitosan−SiO2 NPs are narrowly distributed at 189 ± 21 and 211 ± 39 nm, respectively, which are very consistent with that measured from the TEM images. The FTIR spectra of pure chitosan are shown in Figure 2a. The infrared spectrum shows characteristic peaks at 3435 cm −1 (−OH stretch and N−H stretch, overlapped), 2871 cm −1 (C− H stretch), 1656 cm −1 (Amide I), 1596 cm −1 (amide II band (N−H)), 1320 cm −1 (Amide III), 1085 cm −1 (C−O stretch, secondary hydroxyl group), 1030 cm −1 (C−O stretch, primary hydroxyl group), and an absorption band at 894 cm −1 due to the β-(1,4) glycosidic in chitosan. The same results for the FTIR spectrum of chitosan have been previously reported.24 As shown in Figure 2a, the SiO2 nanospheres exhibit IR peaks at 3305 cm −1 (water and silanol OH stretching), 1639 cm −1 (water OH bending), 1107 cm −1 (Si−O stretching), 951 cm −1 (Si−OH stretching), 803 cm −1 (Si−O−Si symmetric stretching), and 451 cm −1 (Si−O−Si bending). The broad adsorption peak in the range of 3351 cm−1 is assigned to the stretching vibration of the silanol group, and characteristic peaks at 1107 and 951 cm−1 belong to the O−Si−O bonds stretching vibration, which confirms successful synthesis of SiO2 nanospheres.22 The FTIR results of chitosan−SiO2 NPs are displayed in Figure 2a. The FTIR spectrum shows adsorption peaks at 3351 cm −1 (−OH stretch), 2898 cm −1 (C−H stretch), 1632 cm −1 (water OH bending), 1363 cm −1 (CH2 bending), 1050 cm −1 (C−O−C bond), 590 cm −1 (O−H out-of-phase bending), and 455 cm −1 (Si−O−Si bending). It was indicated that chitosan was combined with SiO2 nanospheres. In addition, as compared with chitosan, the absorption peak of chitosan−SiO2 NPs at 1630 cm−1 was weaker. This result may be mainly attributed to the reduction of the hydrophilicity of chitosan−SiO2 NPs. The XRD patterns of all samples are shown in Figure 2b. The XRD pattern of the prepared silica nanospheres exhibits a typical broad peak associated with the amorphous SiO2 NPs.22 The crystallinity index (CrI) of chitosan was calculated according to the reported protocol.25 The X-ray diffraction
Figure 2. Comparative analysis among samples: (a) FTIR spectra, (b) XRD patterns, and (c) TGA curves.
profile of chitosan (Figure 2b) shows a sharp peak at 2θ = 9.3° and a broad peak located approximately at 2θ = 22.5° that symbolize semicrystalline chitosan. The same results for the XRD pattern of chitosan have been previously reported.26 The XRD pattern of chitosan−SiO2 NPs exhibits a minor diffraction peak at 2θ = 10.4° and a broad diffraction peak at 2θ = 23.6°. The XRD pattern of the chitosan−SiO2 NPs is similar to those for pure chitosan and silica nanospheres. The peak around 2θ = 10.4° could result from the presence of chitosan, with some shift compared with pure chitosan, and the slightly broader and weaker peak around 2θ = 23.6° could be assigned to the amorphous structure (from the SiO2 nanospheres). The decrease in the crystallinity could be due to incorporation of bulky chitosan, which demonstrates that the chitosan molecules were well coated on the silica nanospheres. To consider the thermal properties and the effect of chitosan attachment to the SiO2 nanospheres, TGA measurements were performed on the SiO2 nanospheres, chitosan, and chitosan− SiO2 NPs, and the results are shown in Figure 2c. The weight reduction of chitosan−SiO2 NPs takes place in three steps. The first weight loss (4%) appeared at around 100 °C, which was associated with evaporation of the absorbed water moisture from the surface due to physical absorption.27 The subsequent 10381
DOI: 10.1021/acssuschemeng.7b02388 ACS Sustainable Chem. Eng. 2017, 5, 10379−10386
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ACS Sustainable Chemistry & Engineering
Figure 3. EDX spectra of samples.
Figure 4. (a) N2 adsorption adsorption−desorption isotherms (at 77 K), and (b) pore size distributions.
confirms attachment of chitosan to the surface of SiO2 nanospheres. Mesoporosity of the SiO2 Nanospheres and Chitosan−SiO2 NPs. The N2 adsorption−desorption isotherms for the SiO2 nanospheres and chitosan−SiO2 NPs are displayed in Figure 4a and 4b, respectively. The N2 adsorption/desorption isotherms of all samples correspond to isotherms of type IV (IUPAC classification International Union of Pure and Applied Chemistry) and exhibit obvious hysteresis loops in the high relative pressure region, suggesting the mesoporosity is maintained in the adsorbent system. As shown in Figure 4a, the mesoporous structure of the particles is also confirmed by the TEM image. The steepness of the capillary condensation step demonstrates uniformity of
two weight loss steps were attributed to dehydration and complete decomposition of polymer chains (chitosan backbone).28 It can be clearly seen in Figure 2c that the residual weight of chitosan−SiO2 NPs (60.5%) after thermal decomposition is higher than pure chitosan (31.5%) at 600 °C. A rapid weight loss from 220 to 400 °C and a slow degradation from 400 to 600 °C representative of thermal degradation of deposited chitosan on the surface of the SiO2 nanospheres were observed. The significant increase in the weight residues at 600 °C illustrates successful incorporation of higher amounts of silica into the chitosan−SiO2 NPs and ultimately increases in thermal stability. Figure 3 represents the EDX spectra and quantitative elemental composition of the samples. The results show the presence of C, N, O, and Si in the chitosan−SiO2 NPs, which 10382
DOI: 10.1021/acssuschemeng.7b02388 ACS Sustainable Chem. Eng. 2017, 5, 10379−10386
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ACS Sustainable Chemistry & Engineering
a different amount of chitosan (Table 1). It is observed that an increase in the chitosan/silica ratio significantly decreases the
their mesopores. The sharpness of the isotherm indicates the narrow pore size distribution. Compared with the SiO2 nanospheres, although the adsorbed amount of is nitrogen reduced, the shape of the hysteresis loop remained unchanged, which reveals that the pore shape was not changed much. The morphology and structure of mesoporous particles were characterized. According to N2 sorption measurement, the total pore volume is 1.21 cm3 g−1 and the BET specific surface area is 1032 m2 g−1, indicative of a high porosity of the SiO2 nanospheres. For chitosan−SiO2 NPs, the estimated BET surface area is 630 m2 g−1 and the total pore volume is 0.72 m3 g−1. Figure 4b illustrates the pore size distribution (PSD) plots of the samples calculated by the BJH method showing that the pore size distribution of the spherical particles is relatively narrow with a uniform size of around 2.54 ± 0.26 and 1.41 ± 0.15 nm for the SiO2 nanospheres and chitosan−SiO2 NPs, respectively. The pore size uniformity is also confirmed by TEM images. The reduction of BET specific surface, pore size, and pore volume of the chitosan−SiO2 NPs are attributed to filling up the pores by chitosan. CO2 Adsorption. The CO2 adsorption experiments were performed at 0 and 25 °C under 1 bar pressure. The CO2 adsorption capacities are about 1.22 and 4.35 mmol g−1 adsorbent for the SiO2 nanospheres and chitosan−SiO2 NPs, respectively. The temperature dependence of the CO2 isotherm was considered, as shown in Figure 5. The slope of the initial
Table 1. Compositions and Textural Properties of Different Adsorbents
adsorbent
mass of chitosan loaded to MSNs (1 g) (g)
pore size (nm)
pore volume (cm3/g)
BET surfaces (m2 g‑1)
CO2 adsorption (mmol g‑1)
0 0.06
2.54 1.93
1.21 0.88
1032 931
1.22 2.84
0.12
1.66
0.86
844
3.45
0.18
1.41
0.72
630
4.35
0.33
1.37
0.71
621
4.39
0.42
1.30
0.66
609
4.11
0.51
1.21
0.63
556
3.82
SiO2 nanospheres chitosan−SiO2 NPs-1 chitosan−SiO2 NPs-2 chitosan−SiO2 NPs-3 chitosan−SiO2 NPs-4 chitosan−SiO2 NPs-5 chitosan−SiO2 NPs-6
pore size, pore volume, and BET surfaces. When the concentration of chitosan was varied from 0.06 to 0.33 g, the CO2 adsorption increased rapidly, and these could occur due to the high number of amine groups that were present in chitosan−SiO2 NPs. However, a higher composition of chitosan caused a decrease in CO2 adsorption. This behavior seems to be contradictory at first glance, which can be explained by the filled or plugged mesopores with chitosan and a decrease in the surface sites of the chitosan−SiO2 NPs for CO2 molecules. For better illustration, the compositions of the SiO2 nanospheres, pure chitosan, and chitosan−SiO2 NPs-4 were determined by a qualitative detection of the of (CHN) elements. Elemental analysis data (Table 2) showed that the content of N increased to 1.22% in chitosan−SiO2 NPs-4 compared with the SiO2 nanospheres. Table 2. Elemental Analysis of Samples
Figure 5. CO2 adsorption isotherms at 0 and 25 °C.
sample
C (%)
H (%)
N (%)
N/C ratio
SiO2 nanospheres chitosan chitosan−SiO2 NPs-4
0.75 45.27 17.62
1.93 7.26 2.54
11.41 1.63
0.25 0.93
Therefore, chitosan−SiO2 NPs-4 as absorbents with the optimum chitosan/silica ratio were chosen for further investigations. Effect of Initial CO2 Concentration. The effect of the initial CO2 loading was investigated for different concentrations of CO2 (20% and 100%), and other operating conditions were the same as the basic operation conditions. The CO 2 adsorption capacity of chitosan−SiO2 NPs-4 reached the maximum value (4.39 mmol g −1 ) faster at a higher concentration (100%) of CO2. The chitosan−SiO2 NPs-4 reached its saturation capacity at higher concentrations quickly, because more molecules were in contact with the adsorbent surface and saturation of opened pores of the chitosan−SiO2 NPs-4 occurred rapidly. Evaluation of CO2 Adsorption Capacity. To examine the adsorption capacity of chitosan−SiO2 NPs-4, the CO2 capture capacities of various previously reported CO2 adsorbents are
increases as well as the maximum uptake became smaller with increasing temperature. The CO2 uptake isotherms illustrates that an increase in the temperature of adsorption resulted in a marked decrease in the CO2 uptake of the chitosan−SiO2 NPs as adsorbent, indicating that exothermic physical adsorption is the underlying process. It could be due to an increase in the thermal energy of the CO2 molecules at higher temperatures, leading to lower adsorption uptakes.29 Effect of Chitosan Content on CO2 Adsorption. The CO2 adsorption capacity of pure chitosan has been reported to be very low, because of its nonporous structure and low specific surface area (SBET= 0.31 m2g−1).14 In addition, it is believed that the amino groups could increase the affinity toward CO2 adsorption.30 In order to elucidate the effect of chitosan content on CO2 adsorption, the parameters of textural properties were quantitatively determined for chitosan−SiO2 NPs with adding 10383
DOI: 10.1021/acssuschemeng.7b02388 ACS Sustainable Chem. Eng. 2017, 5, 10379−10386
Research Article
ACS Sustainable Chemistry & Engineering Table 3. Comparison of the CO2 Adsorption Capacity of Different Adsorbents (at 25 °C and 1 bar)a
a
adsorbent
pore size (nm)
pore volume (cm3 g‑1)
BET surfaces (m2 g‑1)
SCO2/N2
capacity (mmol g‑1)
ref
acid-treated bentonitic clay chitosan−silica support zeolite-like MOF (sodalite) mesoporous MgO microwave-activated carbon SAPO-34 molecular sieve activated template carbon sawdust-based porous carbon mesoporous SiO2 nanospheres mesoporous chitosan−SiO2 NPs
NR 0.23 NR 2.20 1.98 0.38
