aqueous ion separations by selective...
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Aqueous Ion Separationsby Selective Crystallization
Oak Ridge National Laboratory
A Research Agenda for a New Era in Separations ScienceIrvine CA, May 8, 2018
Radu Custelcean
Ion separation by selective crystallizationWhy crystallization?- High level of molecular recognition ➪ high selectivity- Crystal lattice exerts structural constraints leading to rejection of mismatched ions- Crystal structure is directly relevant to the observed separation selectivity- Low energy input and environmental impact
Materials for Separation TechnologiesDOE Report, 2005
Custelcean, R. Ion Separation by Selective Crystallization of Organic FrameworksCurr. Opin. Solid State Mater. Sci. 2009, 13, 68.
Sulfate separation by crystallizationBarite mineral (BaSO4)
Pnmaa = 8.870b = 5.453c = 7.151
Insoluble inorganic sulfatesKsp
CaSO4 4.93 x 10–5
SrSO4 3.44 x 10-7
PbSO4 2.53 x 10–8
BaSO4 1.08 x 10–10
RaSO4 3.66 x10–11
Solubility and anion selectivity are predetermined by theion sizes and hydration energies – not open to design
Selective crystallization via encapsulation
Angew. Chem. Int. Ed. 2008, 47, 1866J. Am. Chem. Soc. 2010, 132, 7177Chem. Comm. 2013, 49, 2173
MgSO4(L)2(H2O)6 Mg2+ + SO42– + 2L + 6H2O
Ksp = γ±2[Mg2+][SO42–][L]2
Ksp = 2.0(3) x 10–17
[SO42–] = 4.7 x 10–5 M
ΔHcryst = –99.1 kJ/mol; TΔHcryst = –3.8 kJ/mol
Thermodynamics
2
MgSO4(L)2(H2O)6
SO4(urea)62–
J. Am. Chem. Soc. 2010, 132, 7177
SO42– binding SO3
2– binding
Anion crystallization selectivity
Na2(H2O)42+
Na2SO4
H2O2
Na2SO4(L)2(H2O)4
Sodium sulfate crystallization
– d[SO42–]/dt = kapp([SO4
2–] – [SO42–]eq)
kapp = 0.050(16) h-1
t1/2 ≃ 14 h
Ea = 14.1 ± 0.5 kcal/molCaSO42H2O: Ea = 15.0 kcal/mol
Cryst. Growth Des. 2011, 11, 2702; 2015, 15, 517.
90%
L2H2O (s)
Na2SO4(L)2(H2O)4
Sulfate separation from nuclear wasteTake advantage of the high Na+ concentration (≃6 M) in the waste
Cryst. Growth Des. 2011, 11, 2702; 2015, 15, 517.
Drive SO42– crystallization/stripping by ionic strength swings
50 mM
Sulfate separation by guanidine crystallization
guanidinium sulfate[SO4
2–] = 9.6 MPol. J. Chem. 2000, 74, 1637.
Bis-iminoguanidines (BIGs)
Sulfate crystallization:Ksp = 3.2(5) x 10-7
SrSO4: Ksp = 3.4 x 10-7
Aminoguanidinium sulfate: 7.3 M
in situ ligand synthesisone pot synthesis + crystallization
GBIG-SO4(H2O)5GBIG cation stacking
3.15 Å
[SO4(H2O)52–]n
8 water H-bonds/SO42–Angew. Chem. Int. Ed. 2015, 54, 10525
Chem. Eur. J. 2016, 22, 1997.
BBIG-SO4(H2O)2
[(SO4)2(H2O)4]4–
3.39 Å
Cluster binding:20 NH---O H-bonds
SO42– coordination:
7 NH---O H-bonds4 OH---O H-bonds
Crystallization of sulfate-water clusters
BBIG-SO4: Ksp = 2.4(6) x 10-10
BaSO4: Ksp = 1.1 x 10-10
ΔHcryst = 3.7 kJ/molCrystallization is entropy driven
Sulfate separation cycle
Chem. Eur. J. 2016, 22, 1997.
Sulfate separation from seawater (≈ 30 mM)Relevant to scale prevention in oil field operations
BBIG equiv SO42– removed (%)
1 881.1 951.5 992 99
CO2 separation by carbonate crystallization
H2O
CO2 (air)
CO32– coordination:
5 NH---O H-bonds4 OH---O H-bonds1.7071.687
1.827
1.6871.759
1.895 1.732
1.800
1.784
[CO3(H2O)42–]n
1.7321.800
1.7841.895
1.876
1.773
1.8141.814
1.7841.895
1.773
1.814
1.895
1.732
1.773
Single-crystal neutron diffraction
Angew. Chem. Int. Ed. 2017, 56, 1042.
Ksp = 1.0 × 10–9 (CaCO3: 3.4 × 10–9)ΔHcryst = – 47.1 kJ/mol
PyBIG
CO2 (air)80 –120 oC– CO2– 5 H2O
Angew. Chem. Int. Ed. 2017, 56, 1042.Nature Energy 2018, DOI: 10.1038/s41560-018-0150-z
K = 1/pCO2 = 105 atm–1
PCO2(air) = 4 x 10–4 atm–1Ksp = 1.0 ± 0.3 x 10–2 Ksp = 1.0 ± 0.3 x 10–9
ΔHdes = 223 kJ/mol
ΔHabs = –71 kJ/molCO2 separation cycle
(148 kJ/mol for H2O desorption)
absorptiondesorption
Thermodynamics of CO2 absorption
CO2 separation by carbonate crystallization
Outstanding challenges (research opportunities)
Understand and predict aqueous solubilities – crystal lattice energies,
ionic and molecular solvation, interactions with matrix components
Quantify intermolecular interactions in crystals
Enthalpy vs entropy of crystallization
Kinetics and mechanism of crystallization
Role of water of crystallization
Develop new mechanisms of stripping
Acknowledgement
US DOE, Office of Science, Basic Energy SciencesChemical Sciences, Geosciences, and Biosciences Division