Article pubs.acs.org/cm
Alkali Metal Nitrate-Promoted High-Capacity MgO Adsorbents for Regenerable CO2 Capture at Moderate Temperatures Takuya Harada,† Fritz Simeon,‡ Esam Z. Hamad,‡ and T. Alan Hatton*,† †
Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡ Research & Development Centre, Saudi Aramco, Dhahran 31311, Kingdom of Saudi Arabia S Supporting Information *
ABSTRACT: Regenerable high capacity CO2 sorbents are desirable for the establishment of widespread carbon capture and storage (CCS) systems to reduce global CO2 emissions. We report on the marked effects of molten alkali metal nitrates on CO2 uptake by MgO particles and their impact on the development of highly regenerable CO2 adsorbents with high capacity (>10.2 mmol g−1) at moderate temperatures (∼300 °C) under ambient pressure. The molten alkali metal nitrates are shown to prevent the formation of a rigid, CO2-impermeable, unidentate carbonate layer on the surfaces of MgO particles and promote the rapid generation of carbonate ions to allow the high rate of CO2 uptake.
1. INTRODUCTION The establishment of advanced technologies to halt the increase in atmospheric CO2 levels is a crucial challenge today if serious ongoing global climate changes are to be mitigated.1 The International Energy Agency (IEA) reports that CO2 emissions by fossil fuel combustion continue to increase year-by-year, and currently over 30 Gt/year of CO2 are released to the atmosphere globally.2 While there has been significant progress in the development of new carbon-free energy conversion systems and in the utilization of various alternative energy sources, fossil fuels will still be an indispensable source of energy in the foreseeable future to sustain the growing demands of human activity. The introduction of versatile carbon capture and storage (CCS) systems to contain the introduction of CO2 to the environment at various emission sources is currently the most attractive and practical solution to the greenhouse gas problem.3,4 Herein, we report a new regenerable high capacity CO2 adsorbent operative at moderate temperatures that could contribute to the establishment of advanced CCS systems. Aqueous solutions of CO2 absorbents such as monoethanol amine (MEA) and sterically hindered amines (KS-1) have played an important role in the capture of CO2 from natural and flue gases. These compounds were the first commercially available absorbents, pioneered in 1930,5 and have been utilized in hundreds of coal- and gas-fired power plants to date.6 Aqueous ammonia7 and alkali hydroxide solutions8 have also been considered as potential candidates for CO2 capture.9 Systems based on these aqueous absorbents are limited, however, by their high energy and capital costs.10 As alternative materials, various types of solid adsorbents have been proposed for use under ambient or close to ambient conditions,9,11−13 © XXXX American Chemical Society
including surface functionalized micro- or mesoporous materials, such as zeolites,14,15 porous silica,16,17 activated carbons,18,19 and metal organic frameworks.20,21 These porous materials have the advantage of rapid CO2 uptake and low energy requirement for regeneration but suffer from low CO2 capacity and easy deterioration by water or other contaminants. For systems operated at high temperatures (>500 °C), basic metal oxides such as CaO22,23 and Li2ZrO324,25 have been proposed. These metal oxide-based adsorbents have the advantage of high theoretical capacities (e.g., 17.8 mmol·g−1 for CaO and 6.53 mmol·g−1 for Li2ZrO3) but are hampered by their slow reaction kinetics and low regenerabilities. Recently, hydrotalcite-like anionic clays have been investigated as adsorbents for use at intermediate temperatures in the vicinity of ∼400 °C.26−28 Such hydrotalcites can store CO2 intercalated between the layers of the clay frameworks and show relatively rapid uptake and good regenerability. These intermediatetemperature operative adsorbents appear to be particularly advantageous for the establishment of versatile systems for CO2 capture from natural gas reserves9 and from the exhausts of mobile transportation systems.29 The capacities of the hydrotalcites are still very low, however, which limit their appeal for such applications. We have explored a new class of intermediate-temperature CO2 adsorbents with a focus on MgO-based materials as potential candidates for this carbon capture. MgO adsorbs CO2 by the formation of the carbonates at around 300 °C, and, while Received: September 7, 2014 Revised: November 19, 2014
A
DOI: 10.1021/cm503295g Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
Figure 1. CO2 uptake by alkali metal nitrate-coated MgO particles in 100% dry CO2 at atmospheric pressure (1 bar). (a) Uptake at 300 °C for noncoated MgO particles and 10 mol % alkali metal nitrate-coated MgO particles (0.1RNO3·MgO; R = Na, K, (Na−K), and (Li−Na−K)), where (Na−K)NO3 and (Li−Na−K)NO3 represent binary ([Na]:[K] = 0.50:0.50, m.p. = 222 °C) and ternary ([Li]:[Na]:[K] = 0.30:0.18:0.52, m.p. = 133 °C) mixtures of the nitrates, respectively; (b) temperature dependence of the CO2 uptake after 4 h reaction of CO2 with noncoated MgO particles and 10 mol % alkali metal nitrate-coated MgO with different nitrate compositions; (c) uptake at 300 °C in (Li−Na−K)NO3-coated MgO particles with different amounts of the nitrates; (d) uptake in 15 mol % (Li−Na−K)NO3-coated MgO particles over repeated cycles of CO2 adsorption (at 300 °C in 100% CO2 for 1 h) and particle regeneration (at 350 °C in 100% N2 for 30 min).
its capacity in theory is quite high (24.8 mmol·g−1), it has been reported that the actual uptake of CO2 by pure MgO is very low ( 10 min) is described well by the Ginstling−Brounschtein product layer diffusion model45,52 given by 1−
2 α − (1 − α)2/3 = k 3t 3
(4)
where k3 is a diffusion constant. The good fit of this model to the CO2 uptake data is shown in Figure 3a-3, from which we infer that the reaction over this period was controlled by the E
DOI: 10.1021/cm503295g Chem. Mater. XXXX, XXX, XXX−XXX
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The amount of CO2 taken up did not deteriorate, but rather increased slightly, upon repeated adsorption by and regeneration of the particles. In general, CO2 adsorption decreases with the repeated cycles of CO2 absorption and regeneration in metal oxide based absorbents due to the sintering of the particles during the regeneration at high temperature.23 In our case of alkali metal nitrate-coated MgO particles, the temperature required for the regeneration was around 350 °C, which is much lower than the temperature needed to regenerate CaObased absorbents (∼850 °C),23 which we attribute to the lower enthalpy of formation for MgCO3 than for CaCO3.63 In addition, it has also been reported that the dissociation of carbonate ions is accelerated in molten alkali metal nitrates by the favorable solvation of the oxide ion.64 It is reasonable, therefore, to conclude that the molten nitrates may also facilitate the rapid decomposition of MgCO3 at low temperatures, allowing regeneration of the particles without sintering, and with concomitant excellent cyclic regenerability of these samples. The slight improvement in the uptake following repeated adsorption−desorption cycles may be attributed to a homogenization of the nitrate distributions in the grains through the repeated changes in particle sizes during the reactions.
4. CONCLUSION We have performed a detailed study of the effects of alkali metal nitrates on the adsorption of CO2 by MgO particles. The results demonstrate that the uptake can be improved dramatically when done in the presence of an alkali metal nitrate coating in the molten state. A high capacity of over 450 mg·g−1 (10.2 mmol·g−1) at 300 °C was attained, and the adsorbents showed excellent regenerability over repeated cycles of CO2 adsorption and regeneration. The peculiar effects of alkali metal nitrates were attributed to the presence of a high concentration of oxide ions in the molten alkali metal nitrates that restricted the formation of rigid surface layers of unidentate carbonates and facilitated the generation of carbonate ions (CO32−), resulting in the rapid formation of MgCO3 and ease of regeneration of the particles at moderate temperatures.
Figure 4. Schematic illustration of the possible reaction occurred in molten alkali metal nitrate coated MgO.
magnesium salts includes many lattice defects or vacancies.61 The existence of these defects may also contribute to the rapid growth of MgCO3 in this stage. The different effects of the nitrates depending on alkali metal compositions can be attributed to the differences in melting temperatures of the nitrates and the CO2 solubility in these salts. Our results suggest that the enhancement of CO2 uptake by MgO particles due to the presence of the nitrates was facilitated at temperatures above the melting points of the nitrates through the dramatic increase of CO2 solubility on phase transition from the solid to liquid state of the nitrates. In the case of KNO3-coated MgO particles, the nitrate salt coating is in the solid state at the CO2 adsorption temperature owing to its high melting point (=329 °C). Low CO2 uptake rates by KNO3-coated MgO particles may be attributed to the low solubility of CO2 in the solid KNO3. Furthermore, it is reported that the CO2 solubility in molten alkali metal nitrates increases as the ionic radius of the alkali metal cations decreases.57,62 Thus, it is reasonable to suggest that the dissolution of CO2 proceeded most quickly in (Li−Na−K)NO3 among the nitrates examined in this study, and the rapid CO2 dissolution in (Li− Na−K)NO3 may have induced the highest supersaturation of carbonate ions and hence the highest dimensional nucleation and nuclei growth of MgCO3. The excellent regenerability is also an important advantage of coating the MgO particles with molten alkali metal nitrates.
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ASSOCIATED CONTENT
S Supporting Information *
Temperature dependencies of power index n obtained by the fitting of eq 3 to the CO2 uptakes in the second jump stage for 10 mol % alkali metal nitrate coated MgO; FT-IR spectra of noncoated MgO before (“As synthesized”) and after CO2 absorption and assignments of the peaks in FT-IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Saudi Aramco under the MIT Energy Initiative program. F
DOI: 10.1021/cm503295g Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
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DOI: 10.1021/cm503295g Chem. Mater. XXXX, XXX, XXX−XXX
