Solid–Gas Carbonation Coupled with Solid Ionic Liquids for the

Jun 20, 2013 - The effect of temperature, pressure, time, solid ionic liquid (SIL) mass ... International Journal of Molecular Sciences 2014 15 (7), 1...
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Solid−Gas Carbonation Coupled with Solid Ionic Liquids for the Synthesis of CaCO3: Performance, Polymorphic Control, and SelfCatalytic Kinetics Abdul-Rauf Ibrahim, Yanan Gong, Xiaohui Hu, Yanzhen Hong, Yuzhong Su, Hongtao Wang, and Jun Li* Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen 361005, People’s Republic of China S Supporting Information *

ABSTRACT: The effect of temperature, pressure, time, solid ionic liquid (SIL) mass (here, the SIL used is tetra-n-heptylammonium bromide, abbreviated as THepAmBr), reuse of THepAmBr, type of SIL, and additives on the solid−gas (Ca(OH)2− CO2) carbonation system was investigated. Results showed that conversion increased as the temperature increased from 25.0 °C up to 50.0 °C and then decreased thereafter; but increased consistently with increasing pressure. Similarly, conversion increased as the mass of THepAmBr increased, leading to complete conversion at a THepAmBr/Ca(OH)2 mass ratio of 0.1:1 with the production of rhombohedral calcites. Stability tests revealed that THepAmBr was active and stable. The effect of BmimBr and BmimCl indicated that they gave conversions of >96%. Furthermore, studies on polymorphic control to aragonite gave interesting results: 30.2% aragonite was synthesized with MgCl2 and 54.7% with PEG6000. Moreover, time-dependent conversion showed that the reaction mechanism for the system was self-catalytic. Consequently, the reaction rate equation that was derived described the experimental conversion satisfactorily.

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INTRODUCTION Crystalline calcium carbonate is a mineral that has three discrete forms but with the same chemical formula: vaterite, aragonite, and calcite.1−3 The former two compounds are metastable forms that can transform to the latter, which is the most desirable form; in fact, its demand has been increasing, because of many applications in tremendous human endeavors. Yet, the applications of calcium carbonate are solely guided by several parameters, such as surface area, particle size, purity, and morphology,3 which can be enhanced by cautiously controlling reaction conditions: temperature, pressure, time, and composition of reactants.4−6 Solid−gas carbonation (SGC) reaction using Ca(OH)2 involves passing CO2 gas into dry Ca(OH)2 powder under agitation with an overall reaction expressed as Ca(OH)2 (s) + CO2 (g ) → CaCO3(s)↓ + H 2O(l /v)

( 0.02634 s−1 (e.g., 0.05 s−1) provides the calculated conversion as in curve (d) in Figure 11, indicating a fast reaction. The experimental points (see the open circle symbol (○) on curve (d) in Figure 11) support the calculations (also see Figure 12). In contrast, Mk < 0.02634 s−1 (e.g., 0.015

(7)

Substituting eq 7 into eq 4 yields −

d[Ca(OH)2 ] = k′[Ca(OH)2 ]([M]0 − [Ca(OH)2 ]) dt (8)

Solving eq 8 results in ⎧ ⎫ [Ca(OH)2 ][H 2O]0 ⎬ = −[M]0 k′t ln⎨ ⎩ [Ca(OH)2 ]0 ([M]0 − [Ca(OH)2 ]) ⎭ (9)

Meanwhile, the conversion of Ca(OH)2 to CaCO3 can be expressed as XCa(OH)2 =

1 − exp( −[M]0 k′t ) 1+

(

[Ca(OH)2 ]0 [H 2O]0

) exp(−[M] k′t) 0

(10)

Equation 10 contains the pseudo rate constant, which can be fitted into the experimental data through an error minimization algorithm. Yet, the initial concentrations ([Ca(OH)2]0, [H2O]0) are not easy to determine due to the immeasurable volume of the hydration layer; therefore, two parameters in eq 10, namely, Mk = [M]0k′ and R0 = [Ca(OH)2]0/[H2O]0 are fitted into the experimental data (see Table S3 in the Supporting Information) at 50.0 °C and 15.0 MPa. Curve a in Figure 11 shows a comparison of the experimental conversions and the modeling data with the optimized parameters (R0 = 104.7 and Mk = 0.02634 s−1) obtained from a program coded by C++ language (Figure 11 also shows the effect of R0 and Mk on the conversion). As shown, the modeling matches almost perfectly with the experimental results, indicating the validity of the first-order assumption in eq 3. What this means is that, with the appropriate amount of initial water (involved in R0) and CO2 pressure (involved in Mk), the conversion equation is able to describe the experimental results satisfactorily. Discussion on Kinetics. The appropriate initial amount of water (from R0 = 104.7) was necessary for the simulation to describe the experimental results perfectly (curve a in Figure 11). Based on this information, we simulated situations for the presence of large and small amounts of initial water in the system. As shown in curve c in Figure 11, with relatively small amount of initial water, R0 will be large (R0 > 104.7), leading to sigmoid conversion curve with slow increase of the conversion until completion over relatively long time (see the experimental point with “dewatering” (solid triangle in Figure 11), illustrating this situation). On the other hand, if the amount of initial water in the system is relatively large, R0 will be small (R0 < 104.7); in such a case, the conversion equation (eq 10) fails to provide sigmoid change of the conversion (see curve b in Figure 11) but shows quick conversion, which seems to conflict with the results from BmimBr and the experiments with the addition of 10 mL of water.19 The presence of a relatively large amount of initial water makes the system behave like a slurry system, which leads to mass-transfer restrictions that result in decreased conversion.19 Therefore, we implemented another “hydration” reaction but with exactly 1 mL of water for 5 min, and a conversion of 98.7% was realized; however, when the reaction was allowed for only 2 min, a conversion of 49.9% was achieved (see the open triangle symbol (△) at curve (b) in Figure 11). Yet, a conversion of 17.6% was obtained within 2

Figure 12. XRD patterns of CaCO3 produced with extreme reaction conditions at (a) “dewatering”,19 (b) 0.1 MPa in 5 min, (c) 1 mL hydration in 2 min, (d) 20.0 MPa in 2 min, (e) 1 mL hydration in 5 min, and (f) 20.0 MPa in 5 min.

s−1) provides the calculated conversion as in curve e in Figure 11, indicating a slow reaction. The experimental point (see the solid circle symbol (●) near curve e in Figure 11) is in agreement with the calculation. Reaction Rate Constant. Previously, when the reaction was implemented without THepAmBr, a negligible conversion of 0.5% was realized in 5 min; despite the presence of absorbed water on the solid Ca(OH)2 surface.19 Presently, with the introduction of THepAmBr, a conversion of 7.2% was obtained within 1 min (see Table S3 in the Supporting Information). It is therefore safer to assume that initially absorbed water on the solid Ca(OH)2 surface does not contribute substantially to the conversion and that the absorbed water on the THepAmBr has the maximum effect on the conversion; such that the initial water concentration [H2O]0 can be calculated with the weight of absorbed water on the THepAmBr (450 μg/g SIL):19 [H 2O]0 =

450 × 10−6ρ 18

(11)

where ρ is the density of THepAmBr at 50.0 °C (700.04 g/L). Equation 11 assumes that the volume of the mixture of water and THepAmBr is approximately equal to that of pure THepAmBr, because of the small quantity of water, giving [H2O]0 = 0.01750 mol/L, with which [Ca(OH)2]0 is found to 9522

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be 1.832 mol/L from the optimized R0 = 104.7. Therefore, the total concentration of the reactants at the initial stage [M]0 is then calculated to be 1.850 mol/L. Meanwhile, from the optimized Mk = 0.02634 s−1, k′ is found to be 0.01424 L/(mol s). The reaction rate constant is then calculated with k=

k′ [CO2 ]

(12)

From the measured solubility of CO2 in THepAmBr at 15.0 MPa and 50.0 °C (0.516 g CO2/g THepAmBr), the value of k is calculated to be 0.00173(L/mol)2/s at 15.0 MPa and 50.0 °C.

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CONCLUSIONS Carbonation reactions via dry Ca(OH)2 with supercritical carbon dioxide (sc-CO2) result in relatively low conversions, because of a lack of water and the formation of a dense and protective CaCO3 layer around the starting materials, limiting its access to CO2. In this study, we have reported the performance, polymorphic control, and self-catalytic kinetics of a SGC coupled with a solid ionic liquid (SIL) (such as tetra-nheptyl-ammonium bromide, denoted as THepAmBr) and concluded the following: (1) The presence of the THepAmBr facilitated a condition where the dissolution rate of Ca(OH)2 was less than or equal to that of CO2, such that the rate of production of Ca2+ species was less than or equal to the formation of CO32− species (Ca2+/CO32− ≤ 1) leading to fast production of stoichiometric {104} rhombohedral calcite under all of the investigated conditions. (2) The THepAmBr was recyclable and stable over three reaction cycles. (3) Polymorphic control was possible for the SGC system with solid structure-directing substances (SDS). (4) The imidazolium-based SILs (1-butyl-3-methylimidazolium bromide (BmimBr) and 1-butyl-3-methylimidazolium chloride (BmimCl)) performed equally well within the system. (5) The system was controlled by the availability of water, indicating self-catalytic kinetics. (6) The modeling described the experimental results satisfactorily. These results could revolutionize conventional synthesis with CO2 especially, its capture, mineralization, and storage, as well as the applications of CaCO3.

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LIST OF SYMBOLS BmimBr = 1-butyl-3-methylimidazolium bromide BmimCl = 1-butyl-3-methylimidazolium chloride [Ca(OH)2] = concentration of Ca(OH)2, mol/L [CO2] = concentration of CO2, mol/L [H2O] = concentration of water, mol/L IL = ionic liquid [M] = total concentration of the reactants, mol/L Mk = parameter in eq 10, namely, Mk = [M]0k′, s−1 k = reaction rate constant, (L/mol)2/s k′ = the pseudo rate constant; k′ = k[CO2]), L/(mol s) PEG6000 = polyethylene glycol with a molecular weight of 6000 r = reaction rate, (mol/L)/s R0 = parameter in eq 10; namely, R0 = [Ca(OH)2]0/[H2O]0 SDS = structure-directing substance SGC = solid−gas carbonation SLGC = solid−liquid−gas carbonation SIL = solid ionic liquid SLS = sodium lauryl sulfate t = reaction time, s THepAmBr = tetra-n-heptyl-ammonium bromide XCa(OH)2 = conversion of Ca(OH)2 to CaCO3

Greek Symbol

ρ = density of THepAmBr, g/L Subscript

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0 = initial

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ASSOCIATED CONTENT

S Supporting Information *

Supplemental figures, tables, and specifications for phase and crystallite size determinations from XRD patterns referenced in the text. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: (+86) 592 2183055. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (No. 21276212) and SRFDP (No. 20100121110009). 9523

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