Article pubs.acs.org/cm
Colloidal Nanoclusters of MgO Coated with Alkali Metal Nitrates/ Nitrites for Rapid, High Capacity CO2 Capture at Moderate Temperature Takuya Harada and T. Alan Hatton* Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ABSTRACT: Colloidal nanoclusters of metal oxides constitute a promising new class of building blocks for a range of advanced functional materials. Herein, we report on the development of colloidal nanoclusters of MgO for CO2 capture prepared by a novel nonhydrolytic sol−gel reaction followed by the deposition of alkali metal salts by methanol evaporation-induced surface precipitation. The CO2 uptake exceeded 11.7 mmol g−1 (514.8 mg CO2 per 1 g of adsorbent) in 30 min in the presence of 100% dry CO2 under ambient pressure (1 bar) at 340 °C and reached 15.7 mmol g−1 (690.8 mg CO2 per 1 g of adsorbent) in 4 h. Colloidal nanoclusters possessing multiple inner grain boundaries and rough surfaces allowed for a dramatic increase in active surface area of MgO coated with thin layers of alkali metal salts and enabled the rapid conversion of MgO to MgCO3 with high conversion ratio. It was also discovered that the CO2 uptake loading and the regenerability of the sorbents can be enhanced on introduction of nitrite salts to the coating layer through the formation of magnesium nitro or nitrate species, which increased the critical thickness of product layers and mitigated the degradation of nanoclusters over the repeated sorption/desorption cycles.
1. INTRODUCTION Recent progress in advanced methods for nanoparticle synthesis and controlled assembly have opened up tremendous possibilities for the design of new functional materials.1−3 Colloidal nanoparticle clusters,4 which are also known as “colloidal atoms”1 or “supraparticles,”5 in particular, are promising for their unique potential to deliver effective bioseparation media,6 highly active catalysts,7 strong surface plasmon resonance agents,8,9 bright fluorescent imaging elements,10,11 and high density magnetic storage media.12 Here, we report on the attractive features of colloidal nanoparticle clusters of magnesium oxide (MgO) coated with thin layers of alkali metal nitrates and nitrate/nitrite mixtures as a new class of CO2 adsorbents that enable rapid uptake of significant amounts of CO2 and regeneration at moderate temperatures (Abs. at ∼300 °C, Reg. at ∼400 °C); these adsorbents show significant potential for the treatment of hot gaseous emissions and the effective utilization of medium and low temperature heat sources. MgO is a valid candidate for future advanced solid CO2 adsorbents for both postcombustion CO 2 capture and precombustion CO2 separation13−16 because it has an operational adsorption range at intermediate temperatures (∼300 °C)17 and has a low energy cost for regeneration (MgCO3(s) → MgO(s) + CO2(g); ΔH0 = 97.2 kJ·mol−1 at 450 °C) relative to other representative metal oxide based adsorbents (e.g., CaCO3(s) → CaO(s) + CO2(g); ΔH0= ∼170 kJ mol−1, Li2CO3(s) + Li2SiO3(s) → Li4SiO4(s) + CO2(g); ΔH0 = ∼142 kJ mol−1).18,19 Recently, we have shown that CO2 uptake by © XXXX American Chemical Society
MgO can be accelerated dramatically when the particles are coated with alkali metal nitrates in their molten state.20 These significant enhancements in CO2 reactivity were attributed to the role played by the molten nitrates in preventing the formation of a rigid CO2-impermeable product layer of unidentate carbonates on the MgO surfaces while promoting the generation of carbonate ions (CO32−) in the molten salts. However, further improvements in CO2 uptake rates and loading capacities are desirable for the establishment of a versatile carbon capture, utilization, and storage (CCUS) system for the reduction of global CO2 emissions and their adverse effect on the global climate.21,22 To this end, we have focused on colloidal nanoclusters of MgO in our further development of advanced CO2 adsorbents; these nanoclusters have multiple inner grain boundaries and rough surfaces that are expected to contribute to a large active surface area for the MgO particles which we have coated with thin layers of alkali metal salts. We have also focused on exploring the effects of the anion species (nitrate and nitrite) in the salts used to coat the nanoparticles as they modify the overall reactivity of MgO with CO2. Colloidal nanoclusters of MgO were prepared by a newly developed one-step nonhydrolytic sol−gel process in a highboiling point nonaqueous solvent (benzyl ether), followed by calcination in air to remove the surfactants. The alkali metal salt Received: October 9, 2015 Revised: November 7, 2015
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DOI: 10.1021/acs.chemmater.5b03904 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 1. Procedure for the preparation of colloidal nanoclusters of MgO coated with alkali metal nitrates or nitrate/nitrites mixtures. remove the organic surfactants on the surfaces of the nanoclusters. The calcined MgO nanoclusters were dispersed in either a methanol solution in which a ternary mixture of alkali metal nitrates was dissolved to prepare (Li−Na−K)NO3-coated MgO with [LiNO3]: [NaNO3]:[KNO3] = 0.30:0.18:0.52 or in a solution with a mixture of lithium nitrate and sodium and potassium nitrites for the preparation of LiNO3−(Na−K)NO2-coated MgO ([LiNO3]:[NaNO2]:[KNO2] = 0.30:0.18:0.52). Typically, 1.5 mmol of calcined MgO clusters was dispersed in 20 mL of methanol solution with the dissolved alkali metal salts. The mixture was ultrasonicated to obtain a well-dispersed colloidal suspension of MgO nanoclusters, followed by evaporation of the methanol solvent at 50 °C under vigorous stirring in a rotary evaporator to deposit the alkali metal salts on the surfaces of the MgO nanoclusters. The dried particles were redispersed in 5 mL of ethanol, dried overnight again in the oven at 90 °C, and ground with an agate mortar to obtain the final product. The procedure for the sample preparation is summarized in Figure 1. 2.3. Sample Characterization and Analysis of CO2 Uptake. The size and morphology of the nanoclusters were characterized by transmission electron microscopy (TEM: JEOL JEM-2010). The phase composition and crystallographic structure of each sample was examined by powder X-ray diffractometry (XRD: PANalytical X’Pert Pro Multipurpose Diffractometer). The average size of the crystal domains in the nanoclusters (D) was evaluated from the halfbandwidth of the peak in the XRD spectra through the Scherrer equation (D = Kλ/(β cos θ), where K is the Scherrer constant for spherical particles (=0.9), λ is the X-ray wavelength (1.54060 Å), β = (B2 − b2)1/2, B is the full-width at half-maximum (FWHM) of the diffraction peak, b is the instrumental line broadening, and θ is the Bragg angle).23 Textural features of the particles were examined on a BET surface area and porosimetry system (BET: Micromeritics ASAP 2020). Molecular species on the samples were analyzed by Fourier transform infrared spectroscopy (FT-IR: Nicolet NEXUS 470). CO2 uptake by the nanocluster adsorbents was determined by isothermal weight variations in samples immersed in the flow of dry CO2 at ambient pressure (1 bar), as measured by thermogravimetric analysis (TGA: TA Instruments TGA-Q50). Concentration of CO2 in the flow gas was regulated by mixing 100% dry CO2 with 100% dry nitrogen, and the concentration was confirmed by an iSense CO2 monitor (CO2Meter Inc. NEMA4). All samples were precalcined at 450 °C for 1 h under the flow of nitrogen in the TGA sample chamber to remove
coatings on the MgO nanoclusters were introduced by methanol evaporation-induced surface precipitation of the salts from a well-dispersed colloidal suspension of MgO nanoclusters in a methanol solution of dissolved alkali metal salts. Sample characterization was performed by TEM, XRD, BET, and FT-IR. The CO2 uptake of the samples was measured by thermogravimetric analysis (TGA) by following the isothermal variation in weight of the adsorbents placed in a flowing stream of dry CO2 at ambient pressure (1 bar).
2. EXPERIMENTAL SECTION 2.1. Materials. Magnesium acetylacetonate dihydrate (Mg(acac)2· 2H2O, 98%), 1,2-tetradecandiol (CH3(CH2)11CH(OH)CH2OH, 90%), oleic acid (CH3(CH2)7CHCH(CH2)7COOH, 90%), oleylamine (CH3(CH2)7CHCH(CH2)7CH2NH2, 70%), benzyl ether ((C6H5CH2)2O, 98%), lithium nitrate (LiNO3, Reagent-Plus), sodium nitrate (NaNO3, 99%), sodium nitrite (NaNO2, 97%), potassium nitrate (KNO3, 99%), and potassium nitrite (KNO2, 96%) were purchased from Sigma-Aldrich. Methanol (CH3OH, anhydrous, MACRON AR ACS Reagent grade), ethanol (C2H5OH, 200 proof anhydrous, KOPTEC, 99.5%), and hexane (H 3C(CH2 )4 CH3 , MACRON AR ACS Reagent grade) were purchased from VWR. All chemicals were used as received without further purification. The amounts of chemicals used for the synthesis of colloidal nanoclusters of MgO compensated for the purity of the reagents. 2.2. Sample Preparation. Colloidal nanoclusters of MgO were prepared by a nonhydrolytic sol−gel reaction in a high-boiling point nonaqueous solvent in the presence of two types of surfactants (oleic acid and oleyl amine), with subsequent calcination for surfactant removal. In a typical procedure, 20 mmol of magnesium acetylacetonate dihydrate, 20 mmol of 1,2-tetradecandiol, 10 mmol of oleic acid, and 20 mmol of oleylamine were mixed with 20 mL of benzyl ether in a 100 mL 3-neck flask. The mixture was stirred vigorously under flowing nitrogen and heated at 200 °C for 2 h, followed by reflux at 300 °C for 1 h. The solution was then cooled to room temperature and centrifuged after methanol was added. The precipitants were rinsed with hexane, recentrifuged after the addition of ethanol, and dried overnight at room temperature. The assynthesized MgO nanoclusters were calcined at 500 °C in air for 6 h to B
DOI: 10.1021/acs.chemmater.5b03904 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 2. Colloidal nanoclusters of MgO as-synthesized by nonhydrolytic sol−gel reaction. (a) TEM image and size distribution (inset), (b) XRD spectra, where D is the average size of the crystal domains indicated by the Scherrer equation, and (c) FT-IR spectra of the clusters.
Figure 3. Variations in colloidal nanoclusters of MgO following surfactant removal by calcination in air at 500 °C for 6 h and surface coating with 15 mol % (Li−Na−K)NO3 and LiNO3−(Na−K)NO2 by methanol evaporation-induced salt precipitation on the calcined nanoclusters in (a) TEM images, (b) XRD spectra, and (c) FT-IR spectra. preadsorbed species (atmospheric CO2, water, etc.) on the samples prior to CO2 uptake measurements.
particles. In the FT-IR spectrum, peaks ascribed to C−H stretching vibrations for alkyl-chains (−CH2−) at 2928 and 2856 cm−1 and CO stretching vibrations for carboxylates (COO − ) at 1583 cm −1 (asymmetric) and 1431 cm −1 (symmetric) are identified. The appearance of these peaks suggests that the MgO nanoparticles in the clusters were protected by deprotonated carboxylate molecules (R-COO−).24 A shoulder at ∼3727 cm−1 and peaks at 3625 and 3486 cm−1 are also identified in the FT-IR spectrum; these absorbances are ascribed to O−H stretching vibrations for 3-, 4-, and 5coordinated hydroxyl groups.25 The result indicates that the oxygen sites at the surface plane, edge, and corner of MgO particles were partially protonated. The absence of a sharp peak in the range between 3720 and 3780 cm−1 suggests that 1coordinated hydroxyl species (−OH group coordinated directly to magnesium sites on the surface of MgO) were not generated on the MgO nanoparticles.25,26
3. RESULTS AND DISCUSSION Figure 2 (a) shows a TEM image of colloidal nanoclusters of MgO synthesized via the nonhydrolytic sol−gel reaction procedure. It is clear that spherical colloidal nanoclusters with sizes ranging from 40 to 160 nm (∼95 nm average) were generated. The size of the primary nanoparticles comprising the colloidal clusters was about 10 nm on average. There was no secondary aggregation of the clusters nor was there intercluster coalescence. Figure 2 (b) and (c) shows XRD and FT-IR spectra of the colloidal nanoclusters, respectively. The XRD spectrum reveals that the colloidal nanoclusters were comprised of well-crystallized single phase 10 nm MgO nanoparticles (consistent with the TEM observations) and that no intermediate products, such as magnesium hydroxides or residual starting materials, were incorporated in the nanoC
DOI: 10.1021/acs.chemmater.5b03904 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
Figure 4. CO2 uptake by (Li−Na−K)NO3- and LiNO3−(Na−K)NO2-coated colloidal nanoclusters of MgO in 100% CO2 at atmospheric pressure (1 bar), where (Li−Na−K)NO3 and LiNO3−(Na−K)NO2 represent the ternary mixture of the nitrates ([LiNO3]:[NaNO3]:[KNO3] = 0.30:0.18:0.52) and the mixture of LiNO3 and (Na−K)NO2 at the same cation ratio ([LiNO3]:[NaNO2]:[KNO2] = 0.30:0.18:0.52), respectively. (a) Uptake by (Li−Na−K)NO3-coated colloidal nanoclusters of MgO at 340 °C with different amounts of the coatings, and the loading capacities in 4 h (inset); (b) Uptake by 15 mol % (Li−Na−K)NO3-coated colloidal nanoclusters of MgO (0.15·(Li−Na−K)NO3−MgO) at different temperatures; (c) Comparison of the uptake at 340 °C by (Li−Na−K)NO3-coated colloidal nanoclusters of MgO and by LiNO3−(Na−K)NO2coated colloidal nanoclusters of MgO; (d) Uptake by 15 mol % LiNO3−(Na−K)NO2-coated colloidal nanoclusters of MgO (0.15·(LiNO3−(Na− K)NO2)−MgO) at different temperatures.
Figure 3 (a)-(c) shows the variations in morphology, crystallographic phase compositions, and surface species of the colloidal nanoclusters of MgO after the surfactants were removed by calcination and the particles were coated with alkali metal nitrates ((Li−Na−K)NO3) or nitrate/nitrites (LiNO3− (Na−K)NO2). The TEM images shown in Figure 3 (a) reveal that the primary nanoparticles in the clusters grew into larger grains on calcination for surfactant removal. The grain growth of the primary nanoparticles resulted in the formation of colloidal nanoclusters with rough surfaces. Nevertheless, there were no significant variations in the size of the clusters nor was there intercluster coalescence during calcination. The textural features of the colloidal nanoclusters after the calcination were also examined in terms of the BET surface area and porosimetry. The average size of the grains and the pores determined by BET were 162.4 and 15.1 nm, respectively. The size of the grains was slightly larger than that determined by TEM, a discrepancy that may be due to the influence of weak aggregation between the nanoclusters. The pore size in the nanoclusters determined by BET was consistent with the morphological features identified by TEM. The specific surface area of the nanoclusters determined by BET was 37.0 m2·g−1. It is considered that the roughness of the surface of the nanoclusters contributed to the increase of the specific surface area. During the deposition of the alkali metal nitrates or nitrate/nitrites mixture, the salts precipitated as a thin layer on the surfaces of the MgO nanoclusters. Since no independent crystals of the nitrate or nitrite salts are identified in the TEM
images, it is concluded that precipitation of the salts initiated preferentially on the MgO nanocluster surfaces under our experimental conditions. XRD results shown in Figure 3 (b) indicate that the single phase cubic structure of MgO was maintained even after the calcination step and that the clusters were coated with well-crystallized nitrates or nitrate/nitrite salts after the coating treatment. In the FT-IR spectra shown in Figure 3 (c), the postcalcination clusters yielded only traces of the peaks for alkyl-chains at around 2928 and 2856 cm−1, indicating that most of the organic surfactants on MgO were decomposed and removed by the calcination in air. Peaks for the carboxylate groups identified in the spectrum of assynthesized MgO nanoclusters were merged into a single deformed peak with a maximum at 1525 cm−1 and a shoulder at 1430 cm−1 following calcination. The maximum and the shoulder correspond to asymmetric and symmetric stretching vibrations of unidentate carbonates, respectively.27−29 The peak deformation suggests that the organic surfactants with carboxylate groups were decomposed during the calcination step and that new carbonate species were generated that coordinated chemically on the MgO surface. Following the deposition of (Li−Na−K)NO3, a peak ascribed to N−O stretching vibration for nitrate (NO3−) groups appeared at 1384 cm−1. For the case of LiNO3−(Na−K)NO2, two peaks ascribed to N−O stretching vibrations were observed at 1384 and 1269 cm−1 for the nitrate (NO3−) and nitrite (NO2−) groups, respectively. In the spectrum for the clusters coated with LiNO3−(Na−K)NO2, other peaks and shoulders appeared D
DOI: 10.1021/acs.chemmater.5b03904 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials at 1635 cm−1, 1544 cm−1, and 1100 cm−1, ascribed to asymmetric and symmetric stretching vibrations for nitro (Mg-NO2) or nitrato (Mg−O−NO2, Mg−O2NO, (Mg− O)2NO) species,30,31 indicating that the nitrite salts reacted with MgO chemically to form several surface species of MgO− NOx on MgO. Figure 4 (a)-(d) summarizes the CO2 uptake performance of MgO nanoclusters coated with alkali metal nitrates and nitrate/ nitrite mixtures in 100% CO2 at atmospheric pressure (1 bar). The uptake at 340 °C by the nitrates-coated MgO nanoclusters (x(Li−Na−K)NO3−MgO) with different amounts (x) of nitrate salts is shown in Figure 4 (a). Here, the proportions of the three cations in the nitrates were fixed at [Li]:[Na]:[K] = 0.30:0.18:0.52. The results demonstrate that the CO2 uptake by MgO nanoclusters was improved dramatically by the nitrate salt coatings. In contrast to the quite low uptake of 0.03 mmol g−1 after 4 h of reaction with CO2 by the MgO nanoclusters without salt coatings (x = 0), the uptake by MgO clusters with x = 0.10 exceeded 14.4 mmol g−1 under the same reaction conditions and exposure time. For the nitrates-coated MgO nanoclusters, the uptake quickly leveled off at a low value (
