doi.org/10.26434/chemrxiv.11862243.v1

Advancement of Actinide Metal-Organic Framework Chemistry viaSynthesis of Pu-UiO-66Ashley Hastings, Debmalya Ray, WooSeok Jeong, Laura Gagliardi, Omar K. Farha, Amy Hixon

Submitted date: 17/02/2020 • Posted date: 20/02/2020Licence: CC BY-NC-ND 4.0Citation information: Hastings, Ashley; Ray, Debmalya; Jeong, WooSeok; Gagliardi, Laura; Farha, Omar K.;Hixon, Amy (2020): Advancement of Actinide Metal-Organic Framework Chemistry via Synthesis ofPu-UiO-66. ChemRxiv. Preprint. https://doi.org/10.26434/chemrxiv.11862243.v1

We report the synthesis and characterization of the first plutonium metal-organic framework (MOF).Pu-UiO-66 expands the established UiO-66 series, which includes transition-metal, lanthanide, and earlyactinide elements in the hexanuclear nodes. The thermal stability and porosity of Pu-UiO-66 wereexperimentally determined and multi-faceted computational methods were used to corroborate experimentalvalues, examine inherent defects in the framework and decipher spectroscopic signatures. The crystallizationof a plutonium chain side product provides direct evidence of the competition that occurs between modulatorand linker in MOF syntheses. Ultimately, the synthesis of Pu-UiO-66 demonstrates adept control of Pu(IV)coordination under hydrolysis-prone conditions, provides an opportunity to extend trends across isostructuralUiO-66 frameworks and serves as the foundation for future plutonium MOF chemistry.

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1

Advancement of Actinide Metal–Organic Framework Chemistry via Synthesis 1

of Pu-UiO-66 2

Ashley M. Hastings,1 Debmalya Ray,2 WooSeok Jeong,2 Laura Gagliardi,2 Omar K. Farha,3 and 3

Amy E. Hixon*1 4

1Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 5

301 Stinson-Remick, Notre Dame, Indiana 46556, USA 6

2Department of Chemistry, Chemical Theory Center and Minnesota Supercomputing Institute, 7

University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, USA 8

3Department of Chemistry, International Institute of Nanotechnology, Northwestern University, 9

2145 Sheridan Road, Evanston, Illinois 60208, USA 10

KEYWORDS: actinides, computational chemistry, metal-organic frameworks, microporous 11

materials, plutonium(IV) 12

ABSTRACT: We report the synthesis and characterization of the first plutonium metal-organic 13

framework (MOF). Pu-UiO-66 expands the established UiO-66 series, which includes transition-14

metal, lanthanide, and early actinide elements in the hexanuclear nodes. The thermal stability and 15

porosity of Pu-UiO-66 were experimentally determined and multi-faceted computational methods 16

were used to corroborate experimental values, examine inherent defects in the framework, and 17

decipher spectroscopic signatures. The crystallization of a plutonium chain side product provides 18

direct evidence of the competition that occurs between modulator and linker in MOF syntheses. 19

Ultimately, the synthesis of Pu-UiO-66 demonstrates adept control of Pu(IV) coordination under 20

hydrolysis-prone conditions, provides an opportunity to extend trends across isostructural UiO-66 21

frameworks, and serves as the foundation for future plutonium MOF chemistry. 22

2

INTRODUCTION 23

Over the past two decades, metal-organic frameworks (MOFs) have been developed into an 24

extensive class of porous materials. Because of their structural diversity and tunability, MOFs have 25

offered a platform to answer many important questions for applications in gas storage, separations, 26

sensing, catalysis, and more.1–4 However, these characteristics are also desirable in the context of 27

the nuclear fuel cycle, namely for fission product capture,5–7 actinide/lanthanide separations,8 and 28

the nanoscale control of mixed-metal actinide-analogous materials.9 Actinide-based MOFs thus 29

far exhibit autoluminescence,10 record-breaking low density,11 and unprecedented clusters as 30

secondary building units12,13 and serve as stable platforms for studying actinide electronic 31

properties.14,15 These exotic characteristics add to the repertoire already demonstrated by MOF 32

materials and are due to the radioactive properties of the elements as well as the intersection of the 33

f- and d-orbitals.16 Depending on the oxidation states and coordination geometry of actinide 34

elements, these actinide MOFs manifest as either isostructural to previously-discovered transition-35

metal/lanthanide frameworks or unique topologies. The latter are dominated by the presence of the 36

linear dioxo actinyl cation, which is particular to high valence (i.e., +V or +VI) actinide 37

elements.11,17 While MOF structures are known for thorium,18–20,10,21,13,22 uranium,23,17,24,11,7,25,26 38

neptunium,12,27 and americium,15 there are no previous reports on plutonium-based MOFs. 39

The rich chemistry of the actinides reaches its pinnacle with plutonium, which possesses a 40

multiplicity of oxidation states, four of which can exist in solution simultaneously.28 The 41

tetravalent state is most dominant and environmentally relevant but has a tendency to hydrolyze at 42

pH > 1, depending on the plutonium concentration, presence of complexing agents, and 43

temperature.28,29 This hydrolysis, if uncontrolled, can lead to the rapid formation of Pu(IV) 44

colloidal species, but if controlled can form discrete metal-oxide nanoclusters of 6, 16, 22, and 38 45

metal centers.30–32 The smallest cluster is capped with glycine32 or dodecane tetraacetic acid33 46

(DOTA) whereas the larger clusters are capped by chloride and water ligands.30,31 47

Disproportionation reactions make it difficult to maintain the desired oxidation states.28 In 48

addition, the radiation risks of working with an alpha-emitting nuclide require a reduction in the 49

scale of each synthesis and limit available characterization techniques. In short, the synthesis of 50

plutonium-based compounds is notoriously difficult. 51

3

Since its discovery, UiO-66 (UiO stands for University of Oslo) has become a popular choice for 52

variations in conventional synthesis routes, functionalization, and specialized applications.34–37 53

The structure is composed of 12-connected hexanuclear nodes and ditopic carboxylate linkers 54

resulting in the fcu topology.34 The original structure was zirconium based34 but has since been 55

expanded to incorporate other tetravalent metals, such as hafnium,38 uranium,39 cerium,40 56

thorium,18 and neptunium.27 UiO-66 established an isostructural sweep across various tetravalent 57

metal types—transition-metal, lanthanide, and actinide—and was the first transuranic-based 58

MOF.20,27 This synthetic foundation offers the opportunity of extending the UiO-66 framework to 59

plutonium chemistry. 60

The existence of a plutonium-based MOF is of interest to the many facets of the nuclear fuel cycle. 61

Such a structure could provide opportunities for the simultaneous sequestration of plutonium and 62

fission product or other actinides into a stable form. Incorporating an alpha emitter as the metal 63

node could satisfy more fundamental curiosities regarding the radiation stability of MOFs. While 64

MOF materials are known to possess certain defects (e.g., missing linkers and/or missing clusters), 65

these properties have been well explored and can be engineered for desired applications.41 66

Actualization of a plutonium MOF provides a platform for nanoscale control of a high-profile 67

nuclear material and has been executed for the first time in this work. We report the synthesis and 68

characterization of the plutonium metal-organic framework, Pu-UiO-66, a foundational structure 69

for exploring the chemistry of tetravalent plutonium. 70

RESULTS AND DISCUSSION 71

We successfully modified the synthesis of UiO-66 to incorporate plutonium into the metal node. 72

The synthesis yielded 2–10 µm octahedral crystals which were not suitable for standard single-73

crystal X-ray diffraction (XRD) (Figure 1a). While some octahedrons showed distortion, the 74

particles were mostly monodispersed on the micro-scale and energy dispersive X-ray analysis 75

confirmed the presence of plutonium in the crystals (Figure S2). As a result of the necessary 76

experimental configurations for reduction of radiological hazards, the resulting resolution of the 77

powder patterns were not of the quality necessary for structural determination, nor could they be 78

indexed by standard software. Hence, the patterns were indexed manually (Figure S3). Using the 79

hkl plane peak assignments from Wu et al.42 for Zr-UiO-66, 15 peaks were used to calculate the 80

lattice parameter, a (Table S4). We report the average of these values. 81

4

82

Figure 1. a) Scanning electron micrograph of Pu-UiO-66 showing the signature octahedral crystal 83

habit. b) Powder diffractograms for Pu-UiO-66 from a single-crystal X-ray diffractometer. The as-84

synthesized bulk material, sample activated for gas sorption, and heat-treated sample for thermal 85

stability exploration as compared to the simulated powder pattern from the DFT-optimized crystal 86

structure without defects. 87

Even with the LaB6 correction, the experimental data still showed a monotonic shift to higher 2θ 88

when compared with the simulated pattern (Figure 1b). This shift was uniform for samples where 89

data was collected on two different XRD instruments and is thus attributed to the sample. While 90

this shift was systematic, it did result in a slight deflation of the experimentally-calculated lattice 91

parameter, but within error our data still agree with the expected value. Thus, for our experimental 92

data we calculated a = 20.97(35) Å as compared to the simulated value, a = 21.55(1) Å, obtained 93

by the same indexing method on the simulated powder diffractogram. The simulated lattice 94

parameter is also in good agreement with the density functional theory (DFT)-computed lattice 95

parameter of 21.55 Å. Similar analysis of the Pu-UiO-66 from one vial, as opposed to the compiled 96

material yields a = 21.28(16) Å, so there is variation between individual syntheses. As seen in 97

Table 1, the experimental lattice parameters for the M-UiO-66 series are slightly overestimated by 98

the PBE-D3 functional used in this study, and they are in concord with the actinide contraction.16 99

It is important to note that the powder diffractograms for the as-synthesized and activated materials 100

show good agreement (Figure 1b), indicating our activation conditions did not disturb the integrity 101

of the framework. The material heated to 300 °C showed significant reduction in crystallinity while 102

5

bearing resemblance to the framework. We also analyzed a sample of the powder that remained in 103

its mother solution for about four months. No noticeable change in composition or crystallinity 104

(e.g., damage due to alpha radiolysis) was observed by XRD (Figure S5). 105

Table 1. Experimental and computational lattice parameter comparison for the cubic cells of the 106

M-UiO-66 series. 107

M-UiO-66 Zr Hf Ce Th U Pu

Lattice Parameter, a (Å) [Experimental] 20.76 20.70 21.47 21.96 21.52 20.97

Lattice Parameter, a (Å) [DFT] 20.89 20.80 21.65 21.92 21.55 21.55

The experimental values are rounded from the original reports of each M-UiO-66, [zirconium,34 hafnium,38

cerium,40 thorium,18 uranium,39 and plutonium (this work)]. The DFT values were computed in this work.

108

The nitric acid wash of the plutonium starting material resulted in a variable amount of water per 109

synthesis because the stock is heated to “near-dryness”. Efforts to be consistent in heating duration 110

were made but, nevertheless, there was inherent variation between vials of the synthesis. One 111

instance that had slightly higher water content resulted in a greenish product, believed to be the 112

plutonium polymer,43 along with the pink powder of the MOF material. This vial was not compiled 113

with the others for the bulk material analysis, but a powder pattern of it still undeniably showed 114

the presence of Pu-UiO-66, with no additional crystalline phase contributions (Figure S5). This 115

suggests that the MOF assembly can compete with hydrolysis of the tetravalent actinide during the 116

synthesis. 117

Also vying with the MOF formation is a chain configuration, which crystallized alongside the 118

MOF powder in one vial (Figure 2). The structure is composed of infinite chains of plutonium 119

atoms bridged by the carboxylate groups of benzoate ligands. There are three crystallographically-120

unique plutonium atoms, all exhibiting square antiprismatic 8-fold coordination, and two planes 121

of perpendicular chain assemblages stack to form the lattice. It is isostructural to one of two 122

thorium polytypes (β = 116°) reported by Falaise et al.44 The existence of this co-product in the 123

MOF synthesis illustrates the competition that occurs between the modulator and linker for the 124

metal centers during framework assembly. While the modulator typically serves to slow down 125

framework assembly by ligand competition and inhibit catenation, in our system it also promoted 126

plutonium solubility in the organic media and prevented polymerization by complexation. Benzoic 127

6

acid (pKa = 4.204) is commonly used as a modulator with 1,4-benzenedicarboxylic acid (BDC) 128

(pK1,2 = 3.54, 4.34) as a linker.45 We have also used acetic acid (pKa = 4.756) as a modulator to 129

produce Pu-UiO-66 (Figure S1/Table S3).45 While MOF syntheses are often frequently modulated 130

by trifluoroacetic acid (TFA, pKa = 0.52), we did not use it in any syntheses of Pu-UiO-66 because 131

previous plutonium MOF synthetic attempts with TFA precipitated what we believed to be a 132

plutonium fluoride compound. 133

134

135

Figure 2. Representation of the plutonium benzoate chain structure, Pu[O2C-C6H5]4. Hydrogen 136

atoms have been omitted for clarity. The purple polyhedra represent the plutonium centers, red 137

spheres denote oxygen atoms of arbitrary radius, and black spheres compose the carbon atoms of 138

the benzoate molecules. 139

The execution of transuranic MOF N2 adsorption/desorption measurements at 77 K has been 140

historically difficult, but given the yield and purity of our synthesis, we were able to complete the 141

analysis. As expected, the data exhibited a Type I isotherm (Figure 3a). The resulting BET 142

(Brunauer, Emmett, and Teller) surface area was 709(3) m2 g-1 (for 0.0005 < P/P0 < 0.0212) and 143

the micropore volume was 0.30 cm3 g-1. The measured N2 uptake was comparable to that of the 144

other UiO-66 structures, when accounting for the increased mass of the plutonium atoms, and 145

matched well with the saturation quantity of the simulated isotherms (using Grand Canonical 146

Monte Carlo simulations).46,47 We should note, however, that the experimental data achieves 147

saturation at much higher relative pressure than the simulated models, which can indicate material 148

7

defects. Pore size distribution modelling for the DFT-optimized MOF structures showed the 149

typical tetrahedral and octahedral cavities of the UiO-66 structure (Figure 3b).48 We took the 150

opportunity to explore potential defects through computational modeling of missing linker 151

frameworks. 152

153

154

Figure 3. a) Nitrogen isotherms of Pu-UiO-66 collected at 77 K. Closed and open symbols indicate 155

adsorption and desorption measurements, respectively. The simulated data (DDEC and no charge 156

models) were from Grand Canonical Monte Carlo calculations. The crystal structure of Pu-UiO-157

66 is inset with plutonium nodes represented by the purple polyhedra, carbon rings in stick form, 158

oxygens of arbitrary radius as red spheres, and hydrogen atoms omitted for clarity. b) Comparison 159

of DFT modeled pore size distributions from the Pu-UiO-66 experimental N2 isotherm data to 160

missing-linker computational models of Pu-UiO-66. The ideal structure has 12-connected nodes, 161

but linker deficiencies resulting in 11- or 10-connected notes are not uncommon. There are three 162

different 10-linker models to account for different vacated positions for the two missing linkers 163

(see Figure S12). 164

The ideal M-UiO-66 structure (where M = Zr, Hf, Ce) has 12 linkers coordinated to each 165

hexanuclear node, but it is common for these structures to have a linker deficiency distributed 166

randomly throughout the structure, which results in an average of 11 linkers per node.42 In some 167

instances, because of the linker deficiency there can be 10 linkers per node, which can take three 168

different geometries (Figure S12). The pore size distributions calculated for each deficiency type 169

8

did not perfectly align with our experimental data; our limited sample size for the porosity 170

measurement could explain the inflated second pore width. No discussion of missing linkers is 171

included in the reports of U-UiO-6639 and Np-UiO-66.27 The Th-UiO-66 structure, however, has 172

only 9-fold coordination of the nodes because terminal waters are present.18 It was synthesized in 173

a mixed DMF/H2O system and, in contrast, we intentionally limited the amount of water in our 174

synthesis to prevent preemptive precipitation of the plutonium. Hence, we do not believe the 175

reduced connectivity of Th-UiO-66 applies to the plutonium system. 176

Another common UiO-66 defect is that of the missing cluster.48 This phenomenon is thought to be 177

the result of partially-deprotonated linkers caused by excess protons in the solution, which 178

passivate the carboxylic acid functional groups of the linker.48 The strong acid treatment of our 179

plutonium stock solution, which prevents polymerization and promotes dissolution of the 180

plutonium, may have resulted in excess protons. This type of defect can be ordered or randomly 181

distributed throughout the lattice.41 There is no discernable peak ingrowth at low 2θ below the 182

prominent [111] and [002] peaks and the pore size distribution does not show an enlarged third 183

pore, so while the missing-cluster defect is possibly descriptive of our material, it is below 184

detection with our limited sample size. If present, it would explain the increase in N2 uptake that 185

begins at P/P0 > 0.95 (Figure 3a) and reduce the thermal stability of the compound. 186

The thermal stability of Pu-UiO-66 was explored with thermogravimetric analysis (TGA) up to 187

900 °C (Figures S8-S9). The mass loss below 200 °C was attributed to both H2O and N,N’-188

dimethylformamide (DMF) molecules and comprised approximately 16% of the as-synthesized 189

sample and 13% of the activated sample. It is not uncommon to use higher activation temperatures 190

for M-UiO-66 structures, even up to 250 °C, because of their eminent high thermal stability, but 191

this can result in structural dehydration.49 In the interest of being conservative with the minimal 192

amount of produced material, an activation temperature of 100 °C was used. A separate portion of 193

sample was prepared in the same manner but only heated to 300 °C in the TGA instrument. This 194

material was analyzed by XRD and showed a significant reduction in crystallinity as compared to 195

the as-synthesized material (see Figure 1). Thus, Pu-UiO-66 is only thermally stable to about 200 196

°C, which is much lower than that of the archetypical Zr-UiO-66 (i.e., 375 °C in air).49,50 The steep 197

curve from 500–550 °C represents the decomposition of the linkers and conversion to poorly-198

crystalline PuO2, as evidenced by XRD on the post-analysis material (Figure S4). Under N2(g) 199

9

atmosphere, the material does not achieve complete combustion, and the presence or extent of the 200

missing-cluster defect is unknown, so the weight loss percent cannot be used to accurately quantify 201

any potential concurrent linker deficiency in the material. 202

Fourier transform infrared spectroscopy by attenuated total reflectance (FTIR-ATR) was also used 203

to evaluate the activation of Pu-UiO-66. It is important to note that while spectra bear resemblance 204

to that of the other M-UiO-66 structures, the Infineum™ oil that allows safe analysis of the 205

dispersible, radioactive material somewhat masked the data, especially around 1000 cm-1 because 206

of hydrocarbon contributions. We collected FTIR-ATR on the as-synthesized, activated, and 207

heated material and used the formate-truncated cluster computational model to assist in the 208

interpretation of the spectra (Figure S13). Formate was not intentionally added into the reaction 209

solution but is a decomposition product of the solvent, DMF, at elevated temperatures. Therefore, 210

it is possible to have formate-capped defect sites in UiO-66.51 We presumed the dominant bands 211

at 1390 and 1560 cm-1 to be the symmetric and asymmetric carboxylate stretches. A small peak at 212

1699 cm-1 indicated the minor presence of free BDC ligand remaining in the pores. Peaks typically 213

indicative of residual DMF are visible at 1650 and 2850 cm-1, the latter resembling the strong 214

formate C-H stretch at 2790 cm-1 in the computational model. The slightly broader peaks around 215

2925 cm-1 are evidence of hydrogen bonding. The reduction of the broad intercrystalline water 216

peak at 3300 cm-1 showed water removal from the system. The peak that corresponds to the µ3-217

hydroxo ligands of the hexanuclear nodes (~3650 cm-1) was not clearly evident in the data (Figure 218

S14).20 In the as-synthesized sample, it was likely masked by the dominant hydration peak. The 219

activated sample had the emergence of a peak shoulder at 3605 cm-1, but it lacked the sharp 220

character of the µ3-OH peak from the more sensitive diffuse reflectance infrared Fourier transform 221

(DRIFTS) analysis. The original report of Th-UiO-66 also has this broad peak at 3604 cm-1 from 222

FTIR-ATR, which is attributed to µ3-OH.18 Later DRIFTS analysis of Th-UiO-66 shows the 223

experimental µ3-OH peak at 3653 cm-1 as compared to the calculated 3644 cm-1.20 We calculated 224

(using M06-L functional)52 the predicted position of the µ3-OH peak for the Pu-UiO-66 structure 225

to be at 3633 cm-1. The sample treated to 300 °C had likely undergone dehydroxylation, in which 226

the node was converted from Pu6O4(OH)4 to Pu6O6.49,50 This conversion would not result in any 227

change detectable by XRD.50 228

10

The Raman spectrum of Pu-UiO-66 showed the anticipated vibrational modes of the aromatic and 229

carboxylate stretches (Figure S16).53 These peaks have been assigned in Table S10. While our 230

computational, truncated cluster Raman data cannot be used to comment on aromatic linker peaks, 231

the most intense peaks are observed around 500 cm-1 in the both the experimental and theoretical 232

spectra. Our computed Raman spectrum using the M06-L functional52 reveals that this peak is due 233

to a combined motion of Pu-O (µ3-oxo, µ3-hydroxo, and carboxylate oxygen) stretches present in 234

the MOF. Similarly computed models for Zr- and Th-UiO-66 do not show this dominant peak 235

(Figure S17). We attribute the emergence of the peak at 471 cm-1 in our experimental data as the 236

conjunction of Pu-O stretches and ingrowth of plutonium laser-induced breakdown products. Our 237

efforts to penetrate the glass cover slip to interact with the sample likely resulted in the formation 238

of PuO2, which has a prominent T2g phonon peak at the location of interest.54 239

We have evidence which indicates our Pu-UiO-66 deviates from pristine, 12-connected UiO-66, 240

resulting in a lower thermal stability than expected. These discrepancies could take the form of (i) 241

missing linkers, (ii) missing clusters, or (iii) some combination thereof, and the defective sites 242

could be passivated by -OH/-OH2 pairs, formate, excess modulator, or partially-deprotonated 243

linkers.20,48 Given available techniques and limited material, it is difficult to quantify or adequately 244

describe this defect. However, if these defect sites were capped with formate, we would expect 245

better agreement between our experimental FTIR-ATR data and the formate-truncated cluster 246

model. We are unable to discern the source(s) of defects in Pu-UiO-66, but it is reasonable to 247

conclude it is likely a minor combination of both missing linkers and clusters. 248

CONCLUSION 249

We report the first plutonium MOF, Pu-UiO-66, as well as the first transuranic MOF N2 sorption 250

measurement at 77 K, which yielded a BET area of 709(3) m2 g-1. The co-crystallized plutonium 251

benzoate chain is direct evidence of competition that occurs between modulator and linker in MOF 252

synthesis. It is also interesting to observe how the synthesis conditions that result in Pu-UiO-66 253

can produce the plutonium polymer, which exposes the grapple between the controlled hydrolysis 254

to hexanuclear node formation and the uncontrolled polymerization that thwarts so many synthetic 255

efforts for plutonium in the tetravalent state. Understanding the nuances of this tetravalent tug-of-256

war is essential for mastering rational design of plutonium coordination. This work serves to 257

11

underpin future endeavors in plutonium synthesis and lays the foundation for plutonium-based 258

MOFs with novel topologies. 259

EXPERIMENTAL METHODS 260

Synthesis. CAUTION: Plutonium is radioactive! All handling of weapons-grade plutonium was 261

executed by trained workers in approved facilities. Minimization of material was necessary in the 262

syntheses to reduce the associated risks, and thus affects the characterization capabilities of 263

products. 264

Pu-UiO-66 (1) was synthesized over a range of synthetic conditions (see Table S3). The following 265

synthesis resulted in the best yield and purity of the MOF powder. A 42.9 ± 9.4 mM Pu(IV) stock 266

solution in 2 M HCl was dried down in a Teflon vial, washed with 5 M HNO3, then subsequently 267

heated again to near dryness to produce a nitrate salt. A pre-dissolved solution of 1,4-268

benzenedicarboxylic acid (BDC) and benzoic acid (BA) in dry N,N’-dimethylformamide (DMF) 269

(1 Pu: 2 BDC: 40 BA: 500 DMF) was added to the vial and swirled to promote plutonium 270

dissolution. The vial was transferred to a Parr reaction vessel and heated statically at 130 °C for 271

24 hours. The pink powder of Pu-UiO-66 was evident upon removal of the vial from the Parr 272

reaction vessel. This material was washed with an aliquot of DMF to remove unreacted synthesis 273

components and three aliquots of acetone for solvent exchange. It was dried under ambient 274

laboratory conditions and stockpiled for bulk characterization. On one occasion, pink plate-like 275

crystals of Pu[O2C-C6H5]4 (2) co-crystallized. These were selected for analysis by single-crystal 276

X-ray diffraction (SC-XRD) but decomposed before further characterization could be conducted. 277

Attempts to reproduce this product have been unsuccessful. 278

X-ray Diffraction (XRD). In a glovebox, the powder of 1 was mixed in epoxy and mounted as a 279

bulb on the tip of a glass fiber. Powder patterns were collected on a Bruker APEX-II Quazar 280

diffractometer using a Mo Kα micro-source sealed X-ray tube. Debye rings were integrated from 281

2–60° 2θ with a region height of 500 and a 0.01° step. Data were corrected to align with a LaB6 282

standard. Resulting patterns were converted to Cu Kα 2θ values for ease of comparison. 283

Crystals of 2 (CCDC 1983230) were hand-picked and mounted on glass fibers with epoxy for SC-284

XRD. A hemisphere of diffraction data was collected at 298 K on a Bruker APEX-II Quazar 285

diffractometer with a Mo Kα sealed source X-ray tube. Data processing was executed with the 286

12

APEX-3 software package.55 Absorption corrections were applied with the SADABS function. 287

The structure was solved by intrinsic phasing and refined anisotropically in SHELXLE64. 288

Hydrogen atoms were placed in calculated positions. Some of the phenyl rings had inflated thermal 289

parameters due to torsion and were modeled accordingly for the slight disorder. 290

N2 Isotherm Analysis. Approximately 40 mg of 1 was activated on the degas port at 100 °C for 291

24 hours prior to N2 isotherm analysis at 77 K on a Micromeritics ASAP2020 physisorption 292

analyzer. 293

Thermogravimetric Analysis (TGA). About 10 mg of 1 was loaded into an alumina crucible for 294

thermogravimetric analysis on a TA Instruments Q50. The sample was heated to 900 °C at a ramp 295

rate of 10 °C min-1 for the as-synthesized material and 2 °C min-1 for the activated material. These 296

analyses were executed under flow of high-purity N2 gas. 297

Infrared (IR) and Raman Spectroscopies. Infrared spectra were collected using a Bruker 298

LUMOS microscope in attenuated total reflectance (ATR) mode. The instrument has an 8x-299

Schwarzschild objective and liquid nitrogen-cooled PermaVac mercury cadmium telluride (MCT) 300

detector. Powder was secured to a glass slide with Infineum™ oil. Data were collected from 600 301

to 400 cm-1 with 128 scans and a resolution of 4 cm-1. 302

Raman data were collected on a Renishaw inVia Raman Microscope using a thermoelectrically-303

cooled CCD detector outfitted with a 785 nm laser source. Ten, 20-second exposures were 304

accumulated over an extended scan of 100–2000 cm-1 with 10% of the 300 mW laser. The sample 305

was placed in a welled glass slide and contained with a glass cover slip. Resulting contributions to 306

the Raman spectrum from the cover slip were background subtracted from the sample. 307

Scanning Electron Microscopy (SEM). Images and energy dispersive X-ray spectroscopy (EDS) 308

were conducted with a JEOL-6000 Plus Neoscope Benchtop Scanning Electron Microscope using 309

an accelerating voltage of 10 kV. Images were collected in secondary electron mode. 310

Periodic Calculations. We started with the primitive cell of the Zr-UiO-66 MOF,56 replaced Zr 311

with Hf, Ce, Th, U, and Pu, and optimized the structures using the VASP package.57–60 Projected 312

augmented wavefunction (PAW) pseudopotentials61,62 were used for all our calculations. PBE 313

functionals63,64 with Becke Johnson dispersion corrections65 were used for structural 314

13

optimizations. For structural optimization of the primitive cell, we used a gamma-centered 2×2×2 315

k-point grid. A planewave energy cut-off of 520 eV, energy convergence criteria of 10-5 eV, and 316

force convergence criteria of 0.02 eV Å-1 were used for our calculations. Further primitive 317

structures were converted to the cubic unit cells for direct comparison with experimental lattice 318

parameters. In our calculation, each Pu(IV) center has 4 unpaired f-electrons and ferromagnetic 319

configuration is found to be the most stable electronic configuration. 320

The reduction in the thermal stability of 1 compared to other M-UiO-66 materials necessitated an 321

exploration of potential framework defects. Thus, we also considered various missing linker 322

defects on the Pu-UiO-66 structure by comparison of defective models, as conducted on Zr-UiO-323

66 by Momeni et al.56 (see Figure S13). These defective structures were optimized using a 1×1×1 324

k-point grid and a similar level of theory mentioned earlier. 325

For the primitive Pu-UiO-66 structure and various missing linker defective structures, pore 326

volumes, pore sizes, and pore size distributions (PSD) were computed via the Zeo++ program.66–327

68 A kinetic radius of 1.82 Å for N2 was used for the spherical probe. For the generation of PSD 328

histograms, the bin size of 0.33333 Å was used for comparison with experimental data. 329

We further simulated the N2 adsorption isotherms for Pu-UiO-66 at 77 K by performing Grand 330

Canonical Monte Carlo (GCMC) simulations using the RASPA code.46,47 Details of these 331

simulations can be found in the Supporting Information. 332

Cluster Calculations. We isolated clusters of M-UiO-66 (M = Zr, Th, and Pu) from their periodic 333

structures and truncated the organic BDC linkers to formate in order to decrease our computational 334

cost. We further replaced one of the linkers of UiO-66 with OH and H2O group as, shown in Figure 335

S15, in order to model the defect in site 1 of the UiO-66 MOF. The cluster was then optimized 336

using the Gaussian 0969 software package and M06-L functional.52 Def2SVP basis sets were used 337

for C, H and O atoms and SDD pseudopotentials with SDD basis sets were used for the Zr, Th, 338

and Pu metal centers.70 In our cluster calculation, each Pu(IV) center is quintet and the overall spin 339

multiplicity of the cluster was 25-et. During structural optimization, atomic positions were relaxed 340

and the nature of the structures were verified by analytical computation of vibrational frequencies. 341

Vibrational frequencies were scaled with a scaling factor of 0.940. 342

ASSOCIATED CONTENT 343

14

Supporting Information. This material is available free of charge via the Internet at 344

http://pubs.acs.org. 345

More details on synthetic materials and methods, computational methods, characterization by 346

SEM-EDS, XRD, TGA, FT-IR, Raman, crystallographic data, and manual indexing (PDF) 347

Optimized computational structure package for Pu-UiO-66 (CIFs) 348

Experimental data for Pu(IV) benzoate chain (CIF) 349

AUTHOR INFORMATION 350

Corresponding Author. *E-mail: ahixon@nd.edu 351

Author Contributions. All authors have given approval to the final version of the manuscript. 352

Funding Sources. This material is based upon work supported by the Department of Energy, 353

National Nuclear Security Administration under Award Number DE-NA0003763 and the Arthur 354

J. Schmitt Leadership Fellowship (A. Hastings). 355

Notes. The authors declare no competing financial interest. 356

ACKNOWLEDGEMENTS 357

We thank the assistance of Dr. Allen Oliver for his crystallographic expertise, Dr. Zhijie Chen, Dr. 358

Timur Islamoglu, and Dr. Xuan Zhang for their MOF characterization advice, and Dr. Ginger 359

Sigmon and Jennifer Szymanowski for their time and experience. 360

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Supporting Information for:

Advancement of Actinide Metal-Organic Framework Chemistry via Synthesis of

Pu-UiO-66

Ashley M. Hastings,1 Debmalya Ray,2 WooSeok Jeong,2 Laura Gagliardi,2 Omar K. Farha,3 and Amy E. Hixon1

1Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 301 Stinson-Remick, Notre Dame, Indiana 46556 USA 2Department of Chemistry, Chemical Theory Center and Minnesota Supercomputing Institute, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, USA 3Department of Chemistry, International Institute of Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA

S2

Table of Contents Synthesis Procedures ...................................................................................................................... 3

Materials ..................................................................................................................................... 3

Plutonium Stock ...................................................................................................................... 3

Synthetic Variations .................................................................................................................... 4

Manual Indexing Method ................................................................................................................ 6

Additional Powder Patterns ............................................................................................................ 8

Crystallographic Data ..................................................................................................................... 9

Thermal Ellipsoid Plot .............................................................................................................. 10

Crystal Picture ........................................................................................................................... 10

Selected Bond Distances ........................................................................................................... 11

Bond Valence Calculations ....................................................................................................... 11

Thermogravimetric Analysis ........................................................................................................ 12

N2 Adsorption Simulations ........................................................................................................... 13

Missing Linker Models ................................................................................................................. 15

Fourier-Transform Infrared Spectra .............................................................................................. 16

Raman Spectra .............................................................................................................................. 18

References ..................................................................................................................................... 20

S3

Synthesis Procedures

Materials The following chemicals were used without further purification: nitric acid (ACS grade, HNO3,

BDH), benzoic acid (99.5%, C6H5COOH, Alfa Aesar), acetic acid (certified ACS, CH3COOH,

Fisher), and terephthalic acid (98%, H2BDC, Millipore Sigma). N,N-dimethylformamide (ACS

grade, DMF, BDH) was treated with molecular sieves to reduce the water content.

Plutonium Stock

Our plutonium stock solution was characterized and quantified by liquid scintillation counting.

Oxidation state analysis was performed in triplicate on an aliquot of the stock diluted in 2 M HCl.

The stock is approximately 43 mM Pu and 90% Pu(IV) in ~ 2 M HCl.

Table S1. Results from oxidation state analysis of the plutonium stock solution.

Aliquot A B C

Solvent Extraction

LaF3 Precipitation

Solvent Extraction

LaF3 Precipitation

Solvent Extraction

LaF3 Precipitation

% Pu(IV) 0.81 0.91 0.89 0.89 0.84 0.93

% Pu(V) 0.12 0.09

0.13 0.11

0.07 0.07

% Pu(VI) 0.07 -0.02 0.09

Concentration 49.1 mM 47.5 mM 32.1 mM

Average ± Std. Dev. 42.9 ± 9.4 mM Pu

Table S2. Isotopic distribution of weapons-grade plutonium as determined by gamma spectroscopy used to determine concentration from the liquid scintillation counting.

Isotope Percentage (%) 238Pu 0.00778 239Pu 93.62934 240Pu 6.30353 241Pu 0.0406 242Pu 0.01875

S4

Synthetic Variations The synthesis conditions described in Table S3 resulted in the pink powder of Pu-UiO-66 (1).

Synthesis B also produced Pu[O2C-C6H5]4 (2). Efforts were focused on promoting complete

dissolution of dried down starting Pu material in order to produce a homogenous product. Success

in this regard relies on the use of a modulator, in our case, benzoic acid (BA) or acetic acid (AA).

More important is the proper dry down timing and treatment (HNO3) of the Pu stock solution.

Table S3. Matrix of synthesis conditions.

Ratios Heating Conditions Comments A 1 Pu: 2 BDC: 40 BA: 500 DMF 130 °C, 24 hrs HNO3 wash on Pu;

complete Pu dissolution; Reported synthesis

B 1 Pu: 2 BDC: 72 BA: 500 DMF 130 °C, 48 hrs 1 and 2 co-crystallized C 1 Pu: 2 BDC: 67 AA: 500 DMF 130 °C, 48 hrs Pu did not dissolve

completely D 1 Pu: 2 BDC: 50 BA: 500 DMF 130 °C, 48 hrs Pu did not dissolve

completely E 1 Pu: 2 BDC: 40 BA: 500 DMF: 25 H2O 130 °C, 48 hrs Water spike; Pu did not

dissolve completely

Figure S1. Powder patterns for the above listed synthesis variations for Pu-UiO-66 (1). Note that as these powders were being produced, the powder on single-crystal X-ray diffraction method in our facility was still being developed, with the 2θ < 10° collection ability being a more recent development. Epoxy contributions to the patterns have been manually subtracted. Some smoothing was applied in these as the sample mounting procedure underwent improvement.

S5

Scanning Electron Microscopy & Energy Dispersive X-ray Spectroscopy

Figure S2. EDS spectrum collected on Pu-UiO-66 (1). The Pu M peak series is evident as well as the C and O contributions from the organic linker. C, Na, Al, and Si are all common trace elements from the sample preparation and holder.

S6

Manual Indexing Method

Figure S3. The HKL plane assignments, from Wu et al.,1 for the powder diffractogram peaks of interest. These were used to manually index the powder patterns of the simulated and experimental data for 1.

S7

Table S4. The peak positions (Cu Kα 2θ) used with Bragg’s law to calculate the lattice parameter, a, for both the simulated and as-synthesized material. The lower angle 2θ d-spacings may deviate from the rest on our experimental data, because the single-crystal X-ray diffractometer is not intended for such data collection, and thus may be more reliable and consistent at higher 2θ values.

Simulated Pattern As-Synthesized Pattern hkl Peak Position (2θ) a (Å) Peak Position (2θ) a (Å)

[111] 7.10 21.55 7.62 20.07

[002] 8.20 21.55 8.70 20.31

[022] 11.60 21.56 12.02 20.82

[113] 13.62 21.54 14.07 20.85

[004] 16.44 21.55 16.92 20.94

[133] 17.92 21.56 18.53 20.85

[115] 21.40 21.56 21.89 21.09

[044] 23.32 21.56 23.77 21.15

[006] 24.76 21.56 25.22 21.17

[335] 27.10 21.56 27.64 21.14

[444] 28.68 21.55 29.12 21.23

[117] 29.58 21.55 30.07 21.21

[246] 31.02 21.56 31.52 21.22

[355] 31.86 21.56 32.37 21.23

[008] 33.22 21.56 33.74 21.24

Average(σ) 21.55(1) Average(σ) 20.97(35)

S8

Additional Powder Patterns

Figure S4. Powder diffractogram of the as-synthesized 1 heated to 900 °C in an alumina crucible for thermogravimetric analysis. The identity of the material was confirmed to be poorly crystalline PuO2.

Figure S5. Powder diffractogram of the as-synthesized 1 that was compiled for bulk characterization compared to a four month-aged sample and powder taken from a vial that seemed to produce both the MOF and green plutonium polymer products. No noticeable change in composition or crystallinity was observed for either of the patterns.

S9

Crystallographic Data Table S5. Crystal data for the plutonium (IV) benzoate chain, 2.2–5

Chemical formula C28H20O8Pu

Mr 726.44

Crystal system, Space group Monoclinic, P21/c Temperature (K) 298

a, b, c (Å) 22.596 (3), 15.3727 (18), 25.369 (3) β (°) 116.200 (1)

V (Å3) 7907.1 (16)

Z 12

Radiation type Mo Kα

µ (mm-1) 2.55

Crystal size (mm) 0.08 × 0.05 × 0.02 Data collection

Diffractometer Bruker APEX-II CCD

Absorption correction Multi-scan SADABS (Krause et al., 2015)

No. of measured, independent and observed [I > 2σ(I)] reflections

68018, 11898, 8526

Rint 0.091

θmax (°) 23.7

(sinθ/λ)max (Å−1) 0.564 Refinement

R[F2 > 2σ(F2)], wR(F2), S 0.045, 0.107, 1.05 No. of reflections 11898

No. of parameters 910

H-atom treatment H-atom parameters constrained w = 1/[σ2(Fo

2) + (0.0424P)2 + 19.5573P] where P = (Fo

2 + 2Fc2)/3

Δρmax, Δρmin (e Å−3) 2.30, −1.52

S10

Thermal Ellipsoid Plot

Figure S6. Thermal ellipsoid plot of the plutonium benzoate chain, 2. Hydrogen atoms and positional phenyl ring disorder modelling have been omitted for clarity.

Crystal Picture

Figure S7. Pink plate-like crystal of 2 in Infineum oil prior to single-crystal X-ray diffraction analysis.

~100 µm

S11

Selected Bond Distances Table S6. Selected bond distances for Pu—O bonds in Å used for bond valence calculations of compound 2.

Pu1 Pu2 Pu3

O1 2.289(7) O6 2.311(6) O14 2.244(8)

O2 2.330(6) O8 2.374(7) O16 2.390(7)

O3 2.328(7) O10 2.267(7) O18 2.251(6)

O4 2.292(7) O12 2.335(6) O20 2.396(7)

O5 2.323(7) O13 2.388(7) O21 2.306(8)

O7 2.286(7) O15 2.245(7) O22 2.303(8)

O9 2.406(6) O17 2.390(7) O23 2.289(8)

O11 2.328(6) O19 2.256(7) O24 2.346(7)

Bond Valence Calculations Table S7. Bond valence sums for the three crystallographically unique Pu atoms of 2. Empirical values, R0 = 2.09 and b = 0.35, from Zachariasen6 were used.

Pu1 Pu2 Pu3

4.14 4.19 4.25

S12

Thermogravimetric Analysis

Figure S8. Thermogravimetric curve of the as-synthesized Pu-UiO-66, 1, under flowing N2(g) with a ramp rate of 10 °Cmin-1.

Figure S9. Thermogravimetric curve of the activated (100 °C for 24 hrs) Pu-UiO-66, 1, under flowing N2(g) with a ramp rate of 2 °Cmin-1.

S13

N2 Adsorption Simulations Grand Canonical Monte Carlo (GCMC) simulations7,8 were performed to obtain N2 adsorption

isotherms at 77 K using RASPA code.9 The 12-6 Lennard-Jones (LJ) with a cutoff radius of 12.8

Å (with no tail-corrections) and Coulomb potentials with the Ewald summation method were used

to calculate interatomic interaction energies:

𝑈𝑈�𝑟𝑟𝑖𝑖𝑖𝑖� = 4𝜀𝜀𝑖𝑖𝑖𝑖 ��𝜎𝜎𝑖𝑖𝑖𝑖𝑟𝑟𝑖𝑖𝑖𝑖�12

− �𝜎𝜎𝑖𝑖𝑖𝑖𝑟𝑟𝑖𝑖𝑖𝑖�6

� +𝑞𝑞𝑖𝑖𝑞𝑞𝑖𝑖

4𝜀𝜀𝑖𝑖𝑖𝑖𝑟𝑟𝑖𝑖𝑖𝑖

where U is the potential energy, ε is the well-depth, σ is the distance at which the intermolecular

interaction between atoms i and j is zero, rij is the distance between interacting atoms i and j, q is

the partial charge, and ε0 is the dielectric constant. The Universal Force Field (UFF)10 and the

TraPPE force fields11 were adopted for the MOF framework atoms and N2, respectively.

Interaction energies between disparate atoms were calculated by the Lorentz-Berthelot mixing

rule. For charge values of the MOF framework atoms, EQeq12 and DDEC13,14 charges were used,

if available (e.g., EQeq charge is not available for plutonium). Our simulated N2 isotherm results

for Zr-UiO-66 with EQeq and DDEC charge models showed no significant difference (see Figure

S10). All simulations used a 3×3×3 unit cell to avoid interaction between atoms and their periodic

images, and MOF framework atoms were fixed at their crystallographic positions during the

simulations. A total of 10,000 GCMC moves with equal initialization and production cycles were

conducted, and random insertion/deletion, rotation, translation, and reinsertion moves with equal

probability were allowed. For each Monte Carlo cycle N moves with a minimum limit of 20 are

performed, where N is the number of N2 molecules in a simulation box to fully equilibrate the

system containing many molecules. The experimental adsorption loading (i.e., nex, excessive

loading) was converted to total adsorption loading (i.e., ntot) for comparison with simulated

adsorption loading:15

𝑛𝑛𝑡𝑡𝑡𝑡𝑡𝑡 = 𝑛𝑛𝑒𝑒𝑒𝑒 + 𝑉𝑉𝑝𝑝 ∙ 𝜌𝜌𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏(𝑃𝑃,𝑇𝑇)

where Vp is the total pore volume calculated from experimental N2 isotherm at 77 K and P/P0=0.9,

and ρbulk is the bulk gas density at specific temperature and pressure, as obtained from the NIST

Refprop database.16 Fugacities used for the simulations were converted to pressure using the Peng-

Robinson equation of state.

S14

To investigate whether Coulombic interactions between MOF framework atoms and nitrogen

molecules have a negligible effect on N2 isotherm at 77 K, as reported in a previous study,17 we

have compared N2 adsorption isotherms with and without charges for the MOF framework atoms.

For Zr-UiO-66, there is almost no difference in N2 uptake regardless of presence and types of

charges for framework atoms. The computed N2 isotherms for Zr-UiO-66 are well matched with

the previous literature.18 In the case of Pu-UiO-66 (see Figure S11), the saturated N2 uptakes are

slightly larger (i.e., 7.5 cc/g) when considering the Coulombic interactions.

Figure S10. Simulated N2 isotherms of Zr-UiO-66 using DDED, EQeq and no charge models.

Figure S11. Comparison of experimental and simulated N2 isotherms of Pu-UiO-66 using DDEC and no charge models.

S15

Missing Linker Models

Figure S12. Representation of pristine (12-linker), mono (11-linker), and bi (10-linker) missing linker defective models of Pu-UiO-66 used for comparison with our experimental Pu-UiO-66. The missing linkers are highlighted in green.

S16

Fourier-Transform Infrared Spectra

Figure S13. Fourier transform infrared spectra of the as-synthesized, activated, and heat-treated Pu-UiO-66 (1) compared to the truncated cluster model. Contributions from Infineum™ oil swamp the spectra around 1000 cm-1.

Figure S14. IR spectra as in Figure S14 scaled for emphasis of the µ3-hydroxo region.

S17

Figure S15. Schematic representation of the defect in site 1 of Pu-UiO-66 used for truncated cluster calculations.

Table S8. Assignment of IR frequencies for defected M-UiO-66 series (in cm-1) using M06-L functional.

MOF OOH-HOH

μ3 O-H near site1

Other μ3 O-H’s

Oaqua-Haqua

Oaqua-HHB

Formate C-H

Formate COO-

Symmetric

Formate COO-

Asymmetric Pu-UiO-66 3668 3629 3633 3628 2913 2781-2794 1329-1343 1522-1595

Zr-UiO-66 3658 3621 3642 3640 2582 2809-2822 1337-1354 1526-1624

Th-UiO-66 3696 3630 3642 3647 2704 2789-2801 1339-1355 1540-1620

Table S9. Different types of O-H and C-H bond distances in Defected M-UiO-66 series (in Å) using M06-L functional.

MOF OOH-HOH μ3 O-H near site1

Other μ3 O-H Oaqua-Haqua

Oaqua-HHB

Formate C-H

Pu-UiO-66 0.963 0.965 0.965 0.966 1.004 1.114

Zr-UiO-66 0.964 0.966 0.964 0.965 1.025 1.111

Th-UiO-66 0.961 0.966 0.965 0.965 1.018 1.113

S18

Raman Spectra

Figure S16. Experimental Raman spectrum of Pu-UiO-66 (1) compared to the truncated cluster computational model. The inset graph was scaled to emphasize the MOF peaks. Background contribution from the glass cover slip were manually subtracted from the data.

Table S10. Major Raman peak assignments for Pu-UiO-66 and assignments based on Shearer et al.19

Experimental Vibrational Frequency (cm-1) Assignment

1610 in phase C-C aromatic stretch 1438 Carboxylate OCO symmetric stretch/

C-C aromatic to carboxylate stretch 1423 1145 C-C symmetric ring breathing 862 OH bending & CC symmetric breathing

634 C-C-C- carboxylate to aromatic in plane bending

471 PuO2 T2g phonon (Sarsfield et al.20) & Pu-O stretches from computational model

S19

Figure S17. Calculated Raman data from truncated cluster models of Zr, Th, and Pu-UiO-66. The dominance of the Pu-O stretches are highlighted to emphasize this spectral characteristic is unique to plutonium.

S20

References (1) Wu, H.; Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.; Yildirim, T.; Zhou, W.

Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal-Organic Framework UiO-66 and Their Important Effects on Gas Adsorption. J. Am. Chem. Soc. 2013, 135 (28), 10525–10532. https://doi.org/10.1021/ja404514r.

(2) Bruker. APEX3, SAINT, SADABS, XP; Bruker, AXS Inc.: Madison, Wisconsin, USA, 2015. (3) Sheldrick, G. M. CIFTAB; University of Gottingen, 2008. (4) Sheldrick, G. M. SHELXT; University of Gottingen, 2014. (5) Sheldrick, G. M. SHELXL; University of Gottingen, 2018. (6) Zachariasen, W. H. Bond Lengths in Oxygen and Halogen Compounds of d and f Elements.

J. Less-Common Met. 1978, 62, 1–7. https://doi.org/10.1016/0022-5088(78)90010-3. (7) Smit, B.; Maesen, T. L. M. Molecular Simulations of Zeolites: Adsorption, Diffusion, and

Shape Selectivity. Chem. Rev. 2008, 108 (10), 4125–4184. https://doi.org/10.1021/cr8002642.

(8) Frenkel, Daan; Smit, Berend. Understanding Molecular Simulation: From Algorithms to Applications, Second.; Academic Press, 2001.

(9) Dubbeldam, D.; Calero, S.; Ellis, D. E.; Snurr, R. Q. RASPA: Molecular Simulation Software for Adsorption and Diffusion in Flexible Nanoporous Materials. Mol. Simul. 2016, 42 (2), 81–101. https://doi.org/10.1080/08927022.2015.1010082.

(10) Rappé, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard III, W. A.; Skiff, W. M. UFF, a Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc 1992, 114 (25), 10024–10035.

(11) Potoff, J. J.; Siepmann, J. I. Vapor–Liquid Equilibria of Mixtures Containing Alkanes, Carbon Dioxide, and Nitrogen. AIChE J. 2001, 47 (7), 1676–1682. https://doi.org/10.1002/aic.690470719.

(12) Christopher E. Wilmer; Ki Chul Kim; Randall Q. Snurr. An Extended Charge Equilibration Method. J. Phys. Chem. Lett. 2012, 3 (17), 2506–2511. https://doi.org/10.1021/jz3008485.

(13) Thomas A. Manz; Nidia Gabaldon Limas. Introducing DDEC6 Atomic Population Analysis: Part 1. Charge Partitioning Theory and Methodology. RSC Adv. 2016, 6, 47771–47801. https://doi.org/10.1039/c6ra04656h.

(14) Nidia Gabaldon Limas; Thomas A. Manz. Introducing DDEC6 Atomic Population Analysis: Part 2. Computed Results for a Wide Range of Periodic and Nonperiodic Materials. RSC Adv. 2016, 6, 45727–45747. https://doi.org/10.1039/c6ra05507a.

(15) Jarad A. Mason; Mike Veenstra; Jeffrey R. Long. Evaluating Metal–Organic Frameworks for Natural Gas Storage. Chem. Sci. 2014, 5, 32–51. https://doi.org/10.1039/c3sc52633j.

(16) Lemmon, E. W.; Huberand, M. L.; McLinden, M. O. NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP; National Institute of Standards and Technology: Gaithersburg, Maryland, 2013.

(17) Youn-Sang Bae; A. Ozgur Yazaydın; Randall Q. Snurr. Evaluation of the BET Method for Determining Surface Areas of MOFs and Zeolites That Contain Ultra-Micropores. Langmuir 2010, 26 (8), 5475–5483. https://doi.org/10.1021/la100449z.

(18) Katz, M. J.; Brown, Z. J.; Colón, Y. J.; Siu, P. W.; Scheidt, K. A.; Snurr, R. Q.; Hupp, J. T.; Farha, O. K. A Facile Synthesis of UiO-66, UiO-67 and Their Derivatives. Chem. Commun. 2013, 49 (82), 9449–9451. https://doi.org/10.1039/C3CC46105J.

S21

(19) Shearer, G. C.; Chavan, S.; Ethiraj, J.; Vitillo, J. G.; Svelle, S.; Olsbye, U.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. Tuned to Perfection: Ironing Out the Defects in Metal–Organic Framework UiO-66. Chem. Mater. 2014, 26 (14), 4068–4071. https://doi.org/10.1021/cm501859p.

(20) Sarsfield, M. J.; Taylor, R. J.; Puxley, C.; Steele, H. M. Raman Spectroscopy of Plutonium Dioxide and Related Materials. J. Nucl. Mater. 2012, 427 (1), 333–342. https://doi.org/10.1016/j.jnucmat.2012.04.034.

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