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