& Porous Materials | Hot Paper |

Noria: A Highly Xe-Selective Nanoporous Organic Solid

Rahul S. Patil,[a, b] Debasis Banerjee,[b] Cory M. Simon,[c] Jerry L. Atwood,[a] andPraveen K. Thallapally*[b]

Chem. Eur. J. 2016, 22, 12618 – 12623 Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim12618

CommunicationDOI: 10.1002/chem.201602131

Abstract: Separation of xenon and krypton is of industrial

and environmental concern; the existing technologies usecryogenic distillation. Thus, a cost-effective, alternative

technology for the separation of Xe and Kr and their cap-ture from air is of significant importance. Herein, we

report the selective Xe uptake in a crystalline porous or-

ganic oligomeric molecule, noria, and its structural ana-logue, PgC-noria, under ambient conditions. The selectivi-

ty of noria towards Xe arises from its tailored pore sizeand small cavities, which allows a directed non-bonding

interaction of Xe atoms with a large number of carbonatoms of the noria molecular wheel in a confined space.

Nuclear energy is a highly dense, clean, and affordable alterna-

tive for our rapidly depleting non-renewable fossil fuels.[1]

However, radioactive, used nuclear fuel (UNF) from energy

generation in nuclear power plants requires an economicallyviable process to recover precious isotopes and mitigate theamount of radioactive waste to be sequestered.[2] The produc-tion of volatile radioactive nuclides poses difficulties in the re-

processing of the UNF. These volatile radionuclides predomi-nantly include 3 H, 14C, 129I, and isotopes of Xe and Kr. Xe and

Kr are present in ppm concentrations in the off-gas of UNF re-

processing facilities.[2a] Cryogenic distillation is the incumbenttechnology to capture and separate these noble-gas isotopes

from air. However, cryogenic distillation is energy and capitalintensive, and the radiolytic formation of ozone during this

process poses an explosion hazard.[3] Thus, there is a need fora safer, more cost-effective approach to capture and separate

Xe and Kr from UNF off-gas.

The captured radioactive 85Kr has a long half-life of10.8 years and thus must be sequestered as radioactive waste

for an extended period of time.[2a] On the other hand, radioac-tive 127Xe has a short half-life of 36.3 days and, consequently,

after short-term storage, most of the Xe is non-radioactive.

Separating the Xe and Kr to obtain pure Xe thus would signifi-cantly reduce the volume of radioactive waste to be seques-tered. Because Xe has many industrial applications, such aslighting, medical imaging, anesthesia and neuro-protection, it

could be sold in the chemical market to offset operatingcosts.[1b] The conventional source of pure Xe and Kr for their

myriad of industrial applications is from the separation of airvia cryogenic distillation. As a by-product of the separation of

air, a stream of 20/80 mol % Xe/Kr exits the fractional distilla-tion column; at Air Liquide, this stream is stored in cylinders at

200 bar and subsequently shipped to another plant for anoth-er round of cryogenic distillation to further separate the Xe/Krmixture to obtain pure Xe and pure Kr. Thus, there is also anindustrial need for a more cost-effective process to separatea Xe/Kr mixture.[4]

Physisorption-based separation at room temperature utiliz-ing a porous material is a potentially cost-effective alternative

to the conventional cryogenic distillation process.[1b] BecauseXe will outcompete Kr for adsorption sites (as Xe has a largerelectron cloud and consequently a deeper potential well),[5] weseek a material that is selective for Xe over Kr to first remove

the Xe from the UNF reprocessing off-gas; Kr can then be re-moved from the Xe-free effluent in a second step.[1b, 6] In the

category of traditional porous materials, zeolites and activated

carbon have been tested for noble-gas capture; however, theysuffer from relatively low selectivity, lack of modularity, and fire

hazards.[7] More advanced and highly tunable classes of nano-porous materials, metal–organic frameworks (MOFs), porous

coordination polymers, and porous molecular crystals, havenow emerged with promising potential for Xe/Kr separa-

tions.[1b, 4, 6, 8] In particular, porous organic cage-like material CC3

exhibits a cage commensurate with the size of Xe and demon-strated a selectivity of 20 for Xe over Kr from a simulated UNF

reprocessing facility off-gas stream.[8a]

For further expansion of this class of porous organic materi-

als, herein, we have evaluated an organic oligomeric com-pound, “molecular waterwheel” noria, and its structural ana-

logue, PgC-noria, for their potential application in Xe and Kr

capture.[9] Computational studies have shown that the mostXe-selective materials have an optimal pore diameter large

enough to accommodate at most one Xe atom.[4, 8j, k, n] We pre-viously reported that the amorphous form of noria and its tert-

butyloxycarbonyl (BOC) derivative exhibit selectivity for CO2

over H2 and N2 at high pressure.[9c] Noria has a central molecu-

lar cavity of diameters in the range of 5–7 æ, which is close to

the atomic diameter of Xe (4.1 æ). Thus, noria (and its deriva-tives) can be considered as potential candidates for separatinga Xe/Kr mixture at ambient conditions.

We synthesized noria (chemical formula: C102H96O24) andPgC-noria (chemical formula: C102H96O36) from the previouslyreported acid-catalyzed condensation reaction between resor-

cinol/pyrogallol and glutyraldehyde in the presence of concen-trated HCl (see the Supporting Information, section 1).[9a, b] Al-though the crystal structure of noria was previously reported,

the crystal structure of PgC-noria has not been reported todate. The plate-like crystals of PgC-noria were grown in DMF

(Table S1 in the Supporting Information). The bulk phase purityof noria and PgC-noria were established by means of powder

XRD and 1H NMR spectroscopy (Figures S1–S4 in the Support-

ing Information).The crystal structure of noria is a double cyclic ladder-type

oligomer, in which each ring has three alternate resorcinol andmethylene units ; these two rings are connected through six re-

sorcinol units. This large macrocycle has six small cavities atthe periphery and a large hydrophobic cavity at the center

[a] R. S. Patil, Prof. J. L. AtwoodDepartment of Chemistry, University of MissouriColumbia, Missouri, 65211 (United States)

[b] R. S. Patil, Dr. D. Banerjee, Dr. P. K. ThallapallyFundamental and Computational Science DirectoratePacific Northwest National LaboratoryRichland, Washington, 99352 (United States)E-mail : Praveen.Thallapally@pnnl.gov

[c] C. M. SimonDepartment of Chemical & Biomolecular EngineeringUniversity of California, Berkeley, Berkeley, California, 94720 (United States)

Supporting information for this article can be found under http ://dx.doi.org/10.1002/chem.201602131.

Chem. Eur. J. 2016, 22, 12618 – 12623 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim12619

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(Figure 1, A). The previously reported crystal structure of noriaconfirms the diameter of the central cavity is 5–7 æ, and the in-

ternal volume was calculated to be about 160 æ3.[9c] Thedouble cyclic ladder-like macrocycle of PgC-noria is formed

with twelve pyrogallol and six methylene units (Figure 1, B). It

also has six small cavities at the periphery and one large hy-

drophobic cavity at the center similar to molecular cavities innoria. The nature of guest solvent/host framework interaction

in PgC-noria was found to be similar to that of noria: a singlePgC-noria molecule hosts seven DMF molecules in molecular

cavities and intermolecular void spaces in its crystal structure,while seven DMSO molecules occupy the space in the molecu-

lar cavities of noria (Figures S5 and S6 in the Supporting Infor-

mation).Thermogravimetric (TG) analysis of these bulk materials

showed an initial weight loss of 10 % until 50 to 70 8C, which isassociated with the removal of solvent molecules to the

porous form (or activated form; Figure S7 in the Supporting In-formation). The variable temperature in situ XRD studies on

PgC-noria and noria showed that both materials remain crys-

talline until at least 150 8C, with sharp peaks between angles5–158 and broad peaks between angles 15–258 (Figures S3 and

S4 in the Supporting Information). The broad peaks may be as-sociated with the lack of long-range ordering because of inef-

fective packing of the macrocycles. The change in the relativepeak heights in PXRD at different temperatures is associated

with loss of solvent molecules without losing the initial ar-rangement of cage molecules in the extended framework.

In addition to molecular pores commensurate with the size

of Xe, the high thermal stability of both noria and PgC-noriamake them potential candidates for use in a physisorption-

based Xe/Kr separation process. So, we measured the pure-component Xe and Kr adsorption/desorption curves in both

materials at 298, 288, and 278 K, up to a pressure of 1 bar(Figure 2, and the Supporting Information, section 4). Noria in

particular shows a strong preference for Xe adsorption over Kr

at all these temperatures. At 1 bar and 298 K, noria adsorbs1.55 mmol g¢1, compared to 0.64 mmol g¢1 Kr (Figure 2 A). In

contrast, PgC-noria does not have high (or selective) Xeuptake. The distinguished higher capacity of Xe adsorption by

noria is attributed to the molecular sized pores within thenoria molecules, as well as the less dense or porous nature of

the bulk crystalline noria. In contrast, the thirty six hydroxyl

groups at the outer rim of PgC-noria molecule enhance the in-termolecular hydrogen-bonding interactions, which induce

a more densely packed arrangement in the bulk PgC-noria. Asa result, PgC-Noria is less impermeable for gases than noria

(Figure 2 B). The isosteric heats of adsorption (Qst) of Xe and Kradsorption for noria were calculated from the temperature de-

pendence of the pure-component adsorption isotherms (278,

288, 298 K) using the Clausius–Clapeyron equation. The calcu-lated Qst values of Xe adsorption are 24.5–26.9 kJ mol¢1, higher

than the Qst of Kr adsorption (20.1–21.4 kJ mol¢1) within sameloading range. The Qst value for Xe adsorption in noria is slight-

ly lower than CC3 (27.80�1.20 kJ mol¢1)[8a] and cobalt formate(28 kJ mol¢1),[10] but higher or comparable to most of the

previously reported benchmark MOFs, such as IRMOF-1

Figure 1. A) Space-filling view of a pore in noria. B) Seven guest DMF mole-cules hosted in the cavities in and around PgC-noria in its crystal structure(hydrogen atoms are excluded for clarity in both A and B).

Figure 2. A) Xe and Kr adsorption isotherms in noria at 298 K. B) Xe adsorption in noria and PgC-noria at 298 K.

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(15 kJ mol¢1),[8l] NiMOF-74 (22 kJ mol¢1),[8l] HKUST-1(26.9 kJ mol¢1),[8d] MFU-4 L (20 kJ mol¢1),[11] and SBMOF-2

(26.4 kJ mol¢1).[8b] Nevertheless, the difference in Qst for Xe andKr lead us to multicomponent adsorption studies on noria.

Ideal adsorbed solution theory (IAST) is a widely adoptedmethod to predict multicomponent isotherms from pure-com-

ponent isotherms.[12] IAST can accurately predict binary Xe/Kradsorption isotherms in MOFs from pure-component Xe andKr adsorption isotherms. Herein, we employ IAST to estimate

the Xe/Kr adsorption selectivity for noria (see the SupportingInformation, section 5) by using pyIAST.[13] For a 20/80 mol %Xe/Kr mixture at 1 bar and 298 K, relevant to replacing theconventional cryogenic distillation process to obtain pure Xe

and pure Kr, the IAST Xe/Kr selectivity of Noria is 9.4. At diluteconditions (e.g. , 40 ppm Xe, 400 ppm Kr, relevant to UNF re-

processing), we took the ratio of the Henry coefficients of Xe

and Kr in noria to predict its Xe/Kr selectivity, which is consis-tent with IAST. Noria exhibits a Xe/Kr selectivity of 9.4 at dilute

conditions (see the Supporting Information, section 5), higherthan benchmark MOFs Ni-MOF-74,[8l] HKUST-1,[8d] SBMOF-2,[8b]

and IRMOF-1[8l] (Figure 3) and slightly lower than CC3 andSBMOF-1, the two leading sorbents for Xe/Kr separation from

nuclear reprocessing applications. Noria also has a reasonablyhigh Xe Henry coefficient compare to CC3 and SBMOF-1.[14]

Next, we utilize molecular models to identify the adsorptionsites for Xe and Kr in a noria unit. Using Lennard–Jones poten-

tials for Xe and Kr and the classical Dreiding force field for thenoria unit, we visualize the potential-energy contours of a Xe

and Kr adsorbate around a noria unit in Figure 4 (see the Sup-porting Information, Section 6).[5, 16] There are six adsorptionsites around the periphery. The strongest adsorption site is in

the center of the waterwheel unit. Note that Kr does not havea ¢35 kJ mol¢1 contour at the center or a ¢25 kJ mol¢1 contourat the periphery, whereas Xe does, indicating the stronger af-finity of the noria unit for Xe in both the center and periphery.

Although modeling and simulating adsorption in MOFs andzeolites is a mature field, modeling adsorption in porous mo-

lecular materials, such as noria, is a significant challenge, be-

cause the packing of the porous organic units is difficult topredict.[17] Bulk noria is particularly challenging to model be-

cause of the absence of long-range ordering of the waterwheelunits.[17b] To circumvent the challenge of modeling the struc-

ture of an amorphous material, we now assess if we can pre-dict the Xe/Kr selectivity of bulk noria by considering only

a single noria molecule. Although porous organic molecular

materials can exhibit both intrinsic porosity from a pore ina single unit and extrinsic porosity from the inefficient packing

of units, simulating adsorption in only a single noria unit domi-nantly considers its intrinsic porosity.[18] Instead of explicitly de-

fining a Gibbs dividing surface for a single noria unit,[19] wetook inspiration from experimental volumetric adsorption

measurements, in which helium is used as a non-adsorbing

gas to probe the accessible volume.[20] We place a single noriaunit inside a simulation box and perform Monte Carlo simula-

tions of Xe, Kr, and He adsorption in the grand canonical en-semble (see the Supporting Information, section 5). We then

subtract the He adsorption from the Xe and Kr adsorption toobtain the Gibbs excess gas uptake in noria.

Figure 5 shows the simulated excess Xe and Kr adsorption

isotherms in comparison to the experiment. At low pressures,the agreement between the simulated and experimental Xe

Figure 3. Survey of Xe/Kr adsorption isotherm data (see https://github.com/CorySimon/XeKrMOFAdsorptionSurvey/tree/Noria). We extracted Henry coef-ficients and resulting selectivities from experimental pure-component Xeand Kr adsorption isotherms at 298 K in the literature.[8a–e, l, 10, 14–15] A goodmaterial for Xe/Kr separations should exhibit both a high selectivity anda high Xe Henry coefficient († 297 K, †† 292 K)

Figure 4. Potential-energy contours of Xe (A) and Kr (B) adsorbates around a single noria unit. Shown are contours at ¢15, ¢25, and ¢35 kJ mol¢1 in progress-ing darkness.

Chem. Eur. J. 2016, 22, 12618 – 12623 www.chemeurj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim12621

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and Kr uptake is reasonable; IAST calculations using the simu-

lated adsorption isotherms predicted a Xe/Kr selectivity of ten,which is in good agreement with the ratio of Xe and Kr Henry

coefficients in the experimental isotherms (9.4). We attributethe match of our simulations to experiment at low pressures

to the dominance of the intrinsic porosity of noria in determin-

ing adsorption at low coverage. Energy contours in Figure 4confirm that the strongest adsorption site is in the center of

the noria unit. At higher pressures, the extrinsic porosity dueto packing becomes more important, and simulations over-

predict the adsorption. These results suggest that when intrin-sic porosity dominates, simulations of adsorption in a single

porous organic unit can reasonably predict low pressure

uptake. This may justify considering only a single unit ofporous organic molecules in high-throughput screenings for

low-pressure applications, such as capturing Xe and Kr fromUNF reprocessing facilities. Using the same simulation method,

we obtained excellent agreement with the Kr isotherm in CC3,but simulations underestimate the Xe adsorption, leading to

a predicted selectivity of 5.4 at dilute conditions (see the Sup-

porting Information, section 5). The simulated isosteric heat ofadsorption of Xe and Kr in noria at low pressures is 35 and

18 kJ mol¢1, respectively.Porous crystalline noria exhibits superior Xe adsorption com-

pared to PgC-noria. The hydrophobic cavity at the center andsmall cavities at the periphery of the noria molecule construct

adsorption sites for Xe and Kr capture at molecular level, aswas revealed by molecular models. Experimental adsorptiondata, isosteric heats of adsorption, and IAST calculations

showed that noria exhibits an outstanding Xe/Kr selectivity of9.4 at dilute conditions. By considering only the intrinsic poros-

ity in noria and simulating adsorption in a single unit, we pre-dicted a Xe/Kr selectivity that matched the experiment. The

high Xe/Kr selectivity and thermal stability of noria make it

a potential candidate for capturing Xe from UNF reprocessingoff-gas. To further improve the adsorption and selectivity of

Noria, future work will be focused on computationally inspiredmaterial discovery analogous to MOFs,[14] in which noria cage

will be functionalized with various groups, and predict the op-timum Xe and Kr adsorption, selectivity.

Acknowledgements

We thank the US Department of Energy (DOE), Office of Nucle-

ar Energy, and in particular, Jim Bresee and Kimberly Gray, fortheir support (DOE – HQ). Terry Todd (Idaho National Laborato-

ry), John Vienna (PNNL), Denis Strachan (PNNL) and Bob Jubin

(Oak Ridge National Laboratory) provided programmatic sup-port and guidance. C.M.S. is supported through the Center for

Gas Separations Relevant to Clean Energy Technologies, anEnergy Frontier Research Center funded by the U.S. Depart-

ment of Energy, Office of Science, Office of Basic Energy Scien-ces under Award DE-SC0001015. We thank Prof. Berend Smith

and Dr. Maciej Haranczyk for helpful discussions. Pacific North-

west National Laboratory is a multiprogram national laboratoryoperated for the US Department of Energy by Battelle Memori-

al Institute under Contract DEAC05-76L01830.

Keywords: adsorption · organic macrocyclic compounds ·selective gas separation · xenon

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Received: May 5, 2016

Published online on July 5, 2016

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