Two-Step Pressure-Induced Superhydration in Small Pore Natrolite

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Two-Step Pressure-Induced Superhydration in Small Pore Natrolite with Divalent Extra-Framework Cations Donghoon Seoung, Yongmoon Lee, Chi-Chang Kao, Thomas Vogt, and Yongjae Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00506 • Publication Date (Web): 08 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015

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Chemistry of Materials

Donghoon Seoung,†‡ Yongmoon Lee,† Chi-Chang Kao,‡ Thomas Vogt,§ Yongjae Lee†* †Department of Earth System Sciences, Yonsei University, Seoul 120749, Korea ‡Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA §NanoCenter & Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA KEYWORDS: High-Pressure Chemistry, X-ray Diffraction, Pressure-Induced Hydration, Zeolites, Ion-Exchange

ABSTRACT: In-situ high pressure X-ray powder diffraction studies of natrolite (NAT) containing the divalent extraframework cations (EFC) Sr2+, Ca2+, Pb2+ and Cd2+ reveal that they can be super-hydrated in the presence of water. In the case of Ca-NAT, Sr-NAT and Pb-NAT pressure-induced hydration (PIH) inserts 40 H2O/unit cell into the zeolite compared to 32 in super-hydrated natrolites containing monovalent EFC. Cd-NAT is super-hydrated in one step to a zeolite containing 32 H2O/unit cell. PIH of Ca-NAT and Sr-NAT occurs in two steps. During PIH of Pb-NAT three distinct steps have been observed. The excess H2O in natrolites with divalent EFC are accommodated on sites no longer required for charge compensation. Two distinct families with ordered and disordered EFC-water topologies have been found. Our work established the importance of both size and charge of the EFC in PIH.

■INTRODUCTION Pressure-induced hydration (PIH) is a special case of pressure-induced insertion and been observed in small pore natrolites (NAT) containing monovalent Na+, Li+, K+, Rb+ and Ag+ as extra-framework cations (EFC)1-5 as well as in large-pore zeolites (i.e. zeolite Y),6 pyrochlores,7 clays,8 and graphite oxide.9 Reversible insertion of CO2,10 Ar,11 and Kr12 under pressure and temperature has been found in natrolites. Irreversible PIH has been established in a K-gallosilicate natrolite and for Xe in Ag-natrolite.12,13 PIH is achieved using diamond anvil cells with water present as part of alcohol-water mixtures or the sole pressure-transmitting fluid. Materials after PIH are referred to as “super-hydrated”. An atomistic model for PIH based on extensive crystallographic studies reveals that under pressure the pores through which H2O molecules can enter widen due to the anti-rotation of T5O10 units composed of TO4 (T = Si, Al, Ga) tetrahedral units connected by corner-sharing oxygen atoms which function as hinges. This concerted “rotating squares” mechanism results in pore and volume expansion.3 PIH can occur in materials with negative Poisson’s ratios (auxetic materials) allowing them to contract or expand in the direction perpendicular to the one they are compressed or stretched, respectively.14, 15 Grima et al.16 showed experimentally that

sodium natrolite, Na16Al16Si24O80 · 16H2O, is an auxetic material. DFT calculations provided structural models of natrolites containing monovalent EFC at ambient conditions and under pressure and agree well with those derived from diffraction experiments.17-19 In-situ studies of Na16Al16Si24O80·16H2O under pressure in water or ethanolmethanol-water mixtures reveals a two-step PIH process: near ~ 1 GPa an intermediate super-hydrated phase Na16Al16Si24O80·24H2O with a ~7% larger unit cell volume is found.1 Subsequently the unit cell volume decreases as the pressure increases and a fully super-hydrated Na16Al16Si24O80·32H2O phase is found above 1.5 GPa. This process is reversible. Originally Na16Al16Si24O80 · 16H2O was thought to only exist with Na+ as EFC. The known mineral scolecite (Ca8Al16Si24O80 ·24H2O)20, 21 containing Ca2+ also revealed experimental evidence for PIH. We established that when exchanging Na+ by K+ new natrolite phases with monovalent (Li+, Rb+, Cs+ and Ag+) and divalent EFC (Sr2+, Ba2+, Cd2+, Pb2+) can be synthesized.22-24 Natrolites containing divalent EFCs have been characterized using NMR,25 vibrational spectroscopy and computational techniques.26 In the following we present the first systematic investigation of PIH in natrolites with divalent EFC using in-situ high-pressure synchrotron x-ray powder diffraction experiments.

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■EXPERIMENTAL SECTION The starting material, K-natrolite (K-NAT), was prepared using a 4 M KNO3 (ACS reagent grade from Sigma-Aldrich) solution and the mineral, Na16Al16Si24O80 · 16H2O (San Juan, Argentina, from OBG International) in a 100:1 weight ratio. After 24 hours, the solid was separated from the solution by centrifugation. After K-exchange, further cation-exchange was done by stirring the mixture of the ground K-NAT and the respective nitrate solution (Ca, Sr, Pb, and Cd) in a 100:1 weight ratio in a closed system at 80 °C for 24 hours. The final product was air dried and elemental analysis using ICP and EDS revealed only small amounts (< 0.1%) of potassium ions. High-resolution synchrotron X-ray powder diffraction data on divalent-NAT materials were collected at beamline 3D at the Pohang Light Source II (PLS-II) at Pohang Accelerator Laboratory (PAL) using HERMES Si-strip detector and monochromatized incident beam with a wavelength of 1.23980(2) Å . In situ high-pressure synchrotron X-ray powder diffraction measurements on divalent-NAT materials were done at beamline 10-2 at the Stanford Synchrotron Radiation Lightsource (SSRL) at the SLAC National Accelerator Laboratory. At beamline 10-2, the radiation from the wiggler source impinges on a Si (111) crystal followed by 2 pin-holes in order to create an approximately 200 μm diameter beam of monochromatic X-rays with a wavelength of 0.774901(5) Å . A Pilatus 300Kw Si-diode CMOS detector manufactured by DECTRIS was used to collect powder diffraction data. The wavelength of the incident beam was determined using a LaB6 standard reference material (SRM 660a). A modified Merrill-Bassett type diamond anvil cell (DAC) with two opposing diamonds supported by tungstencarbide plates was used for high-pressure XRD measurements.27 The powdered sample was loaded into a 400 μm wide and 150 μm thick sample chamber obtained by electro-spark erosion in a pre-indented stainless steel gasket. A few ruby spheres of ~ 20 μm diameter were added as a pressure gauge. Subsequently, powder samples of either Ca-, Sr-, Pb- and Cd-NAT were inserted into the sample chamber and pure water was added as a pressuretransmitting medium (PTM). The pressure at the sample in the DAC was measured by detecting the shift of R1 emission line of the included ruby spheres (error: ±0.05 GPa).28 The sample was equilibrated for about 10 minutes in the DAC at each measured pressure. Changes in the unit cell lengths and volume were derived from a series of whole profile fitting procedure using the GSAS suite of programs.29 Pressure-dependent changes in the unit-cell lengths and volumes were derived from a series of whole profile fitting procedures using the EXPGUI suite of programs.30 The background was fitted with a Chebyshev polynomial using 20 coefficients, and the pseudo-Voigt profile function proposed by Thompson et al. was used to model the observed Bragg peaks.31 The structural models at selected pressures were then established by Rietveld methods.32 In order to reduce the number of parameters, isotropic

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displacement parameters in the ambient pressure models were refined by grouping the framework tetrahedral atoms, framework oxygen atoms, and non-framework cations and water oxygen atoms, respectively. In our high pressure structural models, all the isotropic displacement factors were grouped in the same way. Geometrical restraints on the T-O (T = Si, Al) and O-O bond distances of the tetrahedra were applied: the distances between Si-O and Al-O were restrained to target values of 1.620 ± 0.001 Å and 1.750 ± 0.001 Å , respectively, and the O-O distances to 2.646 ± 0.005 Å for the Si-tetrahedra and 2.858 ± 0.005 Å for the Al-tetrahedra. Based on successive difference Fourier syntheses, the distributions of the non-framework species in the channel were found. In the final stages of the refinements, the weights of the restraints were reduced to the range of 5 – 10 based on geometrical distance least squares (DLS). The CIF files of the final refined structures are deposited at CCDC website (see Supporting Information).

Figure 1. Drawing of (a) the structure of the natrolite framework, extra-framework cations (EFCs), and water molecules viewed along [001]. T5O10 tetrahedral building units are linked via bridging oxygen atoms to form the natrolite pores. M1, M2, W1, W2, W3, and W4 represent the ideal site locations of EFCs and H2O molecules inside pores, respectively. The rotation angle  is defined as  (b) Structures of monovalent-EFC containing natrolites at ambient and under hydrostatic pressure are shown.1, 2 Yellow and red circles represent EFC sites and water molecules, respectively.


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Figure 2. Polyhedral representations of (a) Ca- and (b) Sr-NAT showing structural and chemical changes during PIH. Cyan, green, and red circles indicate Ca2+, Sr2+, and oxygen atoms of H2O. Tetrahedra illustrate an ordered distribution of Si/Al atoms in the framework. The chain rotational angles  given is defined in Figure 1.

■RESULTS AND DISCUSSION The unit cell volume changes observed in Ca-NAT under pressure reveal a two-step process where above 1 GPa a volume increase by about 2% takes place. This is followed by another volume increase of almost 4% at 2 GPa. During the first PIH Ca8Al16Si24O80 · 24H2O takes up 8 H2O molecules while maintaining its monoclinic symmetry (space group Cc) and at 1.32 GPa a first super-hydrated phase (Ca-NAT-1), Ca8Al16Si24O80 ·32H2O, is characterized by Rietveld refinement. At 2.65 GPa a second superhydrated phase (Ca-NAT-2), Ca8Al16Si24O80·40H2O is found which incorporated 8 more molecules of H2O per formula unit. In earlier work1, we reported on high-pressure diffraction experiments on a natural sample of scolecite from Nasik, India with a composition of Ca8.1Al15.7Si24O80· 24H2O which were hampered by peak broadening and rapid loss of crystallinity. While we noticed an expansion of the unit cell volume near 2 GPa the experiments were not conclusive and the data could not be used for Rietveld refinement. Our current experiments using a synthetic sample made as outlined above allows the unequivocal confirmation of PIH in scolecite and establishes a two-step process.

A similar PIH sequence is found in Sr-NAT (Sr8Al16Si24O80·24H2O): above 1 GPa, a first PIH results in the formation of Sr-NAT-1 (Sr8Al16Si24O80 ·32H2O), with a ~2.5% larger unit cell volume and near 3GPa Sr-NAT-2, (Sr8Al16Si24O80·40H2O) is found, albeit with a smaller unit cell volume than super-hydrated Sr-NAT-1. Both Sr-NAT1 and Sr-NAT-2 have triclinic symmetry in contrast to monoclinic symmetry observed in Sr-NAT. The EFC-H2O topologies of the fully super-hydrated CaNAT-2 and Sr-NAT-2 are similar as shown in Figure 2, however, the EFC and H2O locations are flipped with respect to a mirror plane along the minor axis of the pore. In the partially super-hydrated Ca-NAT-1 Ca2+ is coordinated more symmetrically by H2O than Sr2+ is in SrNAT-1. DFT calculations might shed light on the energetic reasons for these differences. In orthorhombic Cd-NAT, Cd8Al16Si24O80 · 16H2O, one observes a single step PIH to a monoclinic Cd-NAT-1 phase with composition Cd8Al16Si24O80·32H2O. The dehydration after pressure release is quite sluggish: Cd-NAT-1 initially remains in its monoclinic form with a ~4 % larger unit cell volume and reverts back to its original orthorhombic form after 5 days. The ambient structure and H2O content of



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Cd-NAT is of the same type as found in Li- and Na-NAT with two EFC sites half occupied. The super-hydrated CdNAT-1 above 1 GPa phase has the same H2O content as found in Li-Nat and Na-NAT, however, the major and minor axes of the pores are inverted. Axis inversion has up to now only been found during PIH of K-NAT, K16Al16Si24O80·16H2O, which changes its EFC-H2O topology at higher pressures from one where the EFC is closest to the aluminosilicate framework at 1.6 GPa to one at 2.5 GPa where H2O is located near the framework. Such reconstructive EFC-H2O rearrangements are accompanied

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by inversion of the major and minor axes. The PIH sequence in Pb-NAT is the most complex: Pb-NAT, Pb8Al16Si24O80 · 16H2O, which like Cd-NAT has a cationdisordered structure and composition resembling Li- and Na-NAT, inserts H2O near 0.5, 1 and above 2.5 GPa to form the super-hydrated phases Pb-NAT-1, Pb8Al16Si24O80·24H2O, Pb-NAT-2, Pb8Al16Si24O80·32H2O, and fully-super-hydrated Pb-NAT-3 with a composition of Pb8Al16Si24O80 · 40H2O. The PIH of Pb-NAT to Pb- NAT-1 resembles the observed first PIH in Na-NAT to a para-natrolite related phase3, maintaining the cation-disorder and symmetry but with a

Table 1. Changes in the refined cell parameters and volume for Ca-NAT as a function of hydrostatic pressure mediated by pure water. Pressure (GPa)

Space Group

a (Å )

b (Å )

c (Å )

α (Å )

β (Å )

γ (Å )

Volume (O80f) (Å 3)

H2O per unit cell

ambient

Cc

6.5243(2)

18.9870(8)

9.8406(5)

90

109.651(3)

90

2296.0(1)

24

0.54

Cc

6.5072(1)

18.9403(5)

9.8262(4)

90

109.630(2)

90

2281.3(1)

24

0.78

Cc

65020(2)

18.935(1)

9.8282(6)

90

109.603(3)

90

2279.7(1)

24

1.02

Cc

6.4927(1)

18.9402(7)

9.8483(4)

90

109.567(2)

90

2282.2(1)

24

1.32

Cc

6.4740(2)

19.112(1)

9.9291(5)

90

109.092(3)

90

2321.8(1)

32

2.03

Cc

6.4802(3)

19.438(2)

9.861(2)

90

109.477(5)

90

2342.0(2)

32

C1

19.312(1)

19.460(1)

6.4176(7)

90.254(9)

90.372(6)

90.207(5)

2411.7(3)

40

2.65

C1

19.416(2)

19.275(1)

6.4138(5)

89.860(5)

90.192(8)

89.835(7)

2400.3(4)

40

3.83

C1

19.296(1)

19.053(2)

6.4148(6)

89.696(8)

89.788(8)

90.242(6)

2358.3(5)

40

4.09

C1

19.276(1)

19.032(2)

6.4088(6)

89.63(1)

89.865(9)

90.107(6)

2351.1(6)

40

4.85

C1

19.294(2)

18.994(3)

6.4011(9)

89.72(1)

89.78(1)

89.95(1)

2340.2(8)

40

release

Cc

6.5466(2)

18.8970(7)

9.8341(4)

90

109.476(3)

90

2293.9(1)

24

Table 2. Changes in the refined cell parameters and volume for Sr-NAT as a function of hydrostatic pressure mediated by pure water. Pressure (GPa)

Space Group

a (Å )

b (Å )

c (Å )

α (Å )

β (Å )

γ (Å )


ambient

Cc

6.5407(3)

19.237(1)

10.0154(6)

90

109.470(3)

90

2376.2(2)

H2O per unit cell 24

0.27

Cc

6.5170(2)

19.2622(5)

10.1123(3)

90

109.498(1)

90

2393.2(1)

24

0.91

C1

19.1161(5)

19.5742(5)

6.5076(2)

88.876(1)

89.529(2)

89.941(2)

2434.5(2)

32

0.97

C1

19.1126(5)

19.5727(4)

6.5073(2)

88.866(1)

89.529(2)

89.948(2)

2433.7(1)

32

1.09

C1

19.0996(4)

19.5602(4)

6.5043(1)

88.846(1)

89.520(1)

89.953(1)

2429.4(1)

32

2.15

C1

19.0410(6)

19.4771(8)

6.4822(3)

88.769(2)

89.548(2)

89.887(3)

2403.4(2)

32

2.77

C1

19.019(1)

19.4741(8)

6.4817(4)

88.629(3)

89.373(4)

90.200(3)

2399.8(2)

32

C1

19.1835(6)

19.5397(9)

6.4687(3)

88.546(3)

89.753(3)

90.045(3)

2423.9(3)

40

2.82

C1

19.2042(9)

19.5314(8)

6.4615(4)

88.552(4)

89.868(3)

89.911(4)

2422.8(3)

40

3.30

C1

19.163(1)

19.4923(9)

6.4432(4)

88.521(4)

89.896(5)

89.921(4)

2406.0(3)

40

3.75

C1

19.1300(9)

19.4730(9)

6.4399(4)

88.485(4)

89.764(4)

89.880(4)

2398.1(3)

40

4.68

C1

19.0724(9)

19.432(1)

6.4330(4)

88.823(5)

89.580(4)

90.122(4)

2383.6(3)

40

release

Cc

6.5391(2)

19.2372(5)

10.0133(3)

90

109.447(1)

2375.5(1)

24


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Figure 3. Polyhedral representations of the (a) Pb- and (b) Cd-NAT showing the structural and chemical changes. Black, purple, and red circles indicate Pb2+, Cd2+, and water oxygen atoms. Tetrahedra illustrate an ordered distribution of Si/Al atoms in the framework. The legend of the crosses are proportional to the lengths of the major and minor axes of the elliptical pores and the rotational angles as defined in Figure 1 are given.

concomitant inversion of the major and minor axes of the pore. The second stage H2O insertion transforming PbNAT-1 to PbNAT-2 resembles the PIH observed to fully super-hydrated Na-NAT while maintaining both space group and the orientation of the major and minor axis of the pores. Above 2.5 GPa, a third PIH to a new orthorhombic Pb-NAT-3 phase with 40 H2O takes place

a)

together with a second inversion of the axes reverting the major and minor axes back to the orientation they had originally in Pb-NAT. Figure 3 summarizes the PIH processes occurring in Pb- and Cd-NAT. Figure 1 depicts the possible sites observed during PIH of natrolites with divalent EFC: One of the EFC sites M1 or M2 in Sr- and CaNAT is occupied fully whereas in Cd- and Pb-NAT these

b)

Figure 4. Pressure dependence (a) of the unit-cell volume expansion.), and (b) H2O content per formula unit of mono- and divalent-EFC in natrolites as a function of the ionic potential of the EFC. The connecting lines are to guide the eyes.



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EFC are disordered between both sites. In the case of the fully super-hydrated materials the extra H2O is accommodated on an EFC site. We also show the various H2O molecule arrangements in ambient and superhydrated materials. In natrolites containing monovalent EFC we observe two different sequences of PIH: in Na-NAT and Ag-NAT, first the paranatrolite-type phase3 with 24 and then the full super-hydrated phase with 32 H2O/unit cell are observed. In Li-NAT, Rb-NAT, K-NAT and Ag-NAT only one super-

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hydrated phase with 32 H2O molecule per unit cell has been found. Cs-NAT does not undergo PIH. Natrolites can also be distinguished on the basis of their EFC-H2O arrangement within the pores: in the type-I natrolites Li-, Ag- and Na-NAT the H2O molecules within the pores are located in proximity to the aluminosilicate framework and the EFC near the center of the pores, whereas in the typeII natrolites, K-NAT below 2 GPa, Rb-NAT and Cs-NAT the EFC-H2O positions are inverted and H2O molecules are found near the center of the pores and the EFC closer to

Table 3. Changes in the refined cell parameters and volume for Pb-NAT as a function of hydrostatic pressure mediated by pure water. Pressure (GPa)

Space Group

a (Å )

ambient

Fdd2

18.8613(3)

0.28

Fdd2

18.9298(5)

0.48

Fdd2

18.9944(7)

Cc

6.5297(1)

0.66

Cc

0.77

Cc

0.98

b (Å )

c (Å )

α (Å )

β (Å )

γ (Å )


H2O per unit cell

19.2992(3)

6.5385(1)

90

90

90

2380.07(8)

16

19.3009(4)

6.5261(2)

90

90

90

2384.4(2)

16

19.3263(7)

6.5261(2)

90

90

90

2395.7(2)

16

19.5011(4)

10.1234(2)

90

109.137(1)

90

2435.66(5)

24

6.5237(2)

19.4883(4)

10.1240(2)

90

109.289(1)

90

2429.72(6)

24

6.5182(1)

19.4706(3)

10.1159(2)

90

109.3247(8)

90

2423.00(4)

24

Cc

6.5101(1)

19.4464(5)

10.1059(2)

90

109.336(1)

90

2414.42(6)

24

Cc

6.4997(2)

19.2181(5)

10.3542(3)

90

108.823(1)

90

2448.40(8)

32

1.18

Cc

6.5069(2)

19.2284(4)

10.3643(2)

90

108.842(1)

90

2454.5(1)

32

1.57

Cc

6.4988(2)

19.2225(4)

10.3621(3)

90

108.825(2)

90

2450.46(7)

32

1.69

Cc

6.4932(2)

19.2232(4)

10.3576(3)

90

108.800(2)

90

2447.74(8)

32

2.68

Fdd2

19.2531(9)

19.652(1)

6.4767(3)

90

90

90

2450.6(3)

40

2.86

Fdd2

19.2386(4)

19.6446(3)

6.4742(1)

90

90

90

2446.8(1)

40

3.06

Fdd2

19.2252(4)

19.6316(4)

6.4728(2)

90

90

90

2443.0(1)

40

4.58

Fdd2

19.1357(6)

19.5063(6)

6.4701(2)

90

90

90

2415.1(2)

40

release

Fdd2

18.884(1)

19.315(1)

6.5368(4)

90

90

90

2384.2(4)

16

Table 4. Changes in the refined cell parameters and volume for Cd-NAT as a function of hydrostatic pressure mediated by pure water. Pressure (GPa)

Space Group

a (Å )

b (Å )

c (Å )

α (Å )

β (Å )

γ (Å )


H2O per unit cell

ambient

Fdd2

18.4366(4)

18.8334(4)

6.5235(1)

90

90

0.23

Fdd2

18.4194(3)

18.8122(3)

6.5188(1)

90

90

90

2265.1(1)

16

90

2258.81(9)

16

0.42

Fdd2

18.4063(3)

18.7963(3)

6.5150(1)

90

90

0.63

Fdd2

18.4010(3)

18.7862(3)

6.5108(1)

90

90

90

2253.98(9)

16

90

2250.69(9)

16

0.89

Fdd2

18.4141(3)

18.7976(3)

6.5028(1)

90

1.04

Fdd2

18.4565(6)

18.8484(6)

6.4947(2)

90

90

90

2250.87(9)

16

90

90

2259.4(2)

16

Cc

6.5553(5)

19.540(2)

10.260(1)

90

110.55(1)

90

2461.1(2)

32

1.68

Cc

6.5323(5)

19.556(2)

10.2234(7)

90

109.938(3)

90

2455.4(2)

32

2.71

Cc

6.4717(5)

19.531(3)

10.183(2)

90

109.532(9)

90

2426.2(3)

32

3.38

Cc

6.5028(15)

19.454(4)

10.1017(29)

90

109.10(1)

90

2415.0(7)

32

release

Cc

6.6147(4)

19.049(2)

9.8517(9)

90

108.601(7)

90

2353.0(2)

32

Fdd2

18.4809(6)

18.8961(6)

6.5224(2)

90

90

90

2277.7(2)

16


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the framework. Natrolites containing divalent EFC are observed to be of type-I. Kremleva’s DFT calculations17 revealed that in type-I natrolites the EFC-H2O interaction are stronger than in type-II natrolites. Due to their higher positive charge the EFC-H2O interaction energies in natrolites with divalent EFC are expected to be larger than the ones observed in natrolites with monovalent EFC and therefore it is reasonable that they adopt the type-I topology with H2O molecules being in close proximity to the framework. The natrolites with divalent EFC investigated so far can be grouped into two subgroups with respect to the EFC sites occupied: Ca-NAT and Sr-NAT with 24 H2O crystallize in a structure that resembles the para-natrolite structure of Na-NAT where one EFC is removed to account for the charge. The location of the EFC in the Pb-NAT and Cd-NAT resemble that of ambient NaNAT as these divalent EFC are disordered over the two cation sites. The observed complex structural behavior of natrolite containing mono- and divalent EFC is highly dependent on both charge and size of the EFC. Plotting the unit cell volume expansion during PIH in % as a function of the ionic potential reveals a clear separation into natrolites containing mono- and divalent EFC (Figure 4a). With the exception of Li-NAT the amount of volume expansion of natrolites containing mono- and divalent EFC is found in a similar range between 1% and 8%. In Figure 4b we show a phase diagram of all super-hydrated natrolite phases as a function of the ionic potential. This phase diagram reveals that having a large ionic potential is a necessary but not sufficient condition to obtain super-hydrated materials with 40 H2O located in the zeolite. This tradeoff between size and charge related effects at high pressures calls for DFT calculations.

■CONCLUSION In summary our systematic structural work on natrolites with divalent EFC reveals that in the case of Sr-, Ca- and Pb-NAT they can be super-hydrated under pressure in the presence of water to materials with a higher H2O content (40 H2O) than those with monovalent EFC (32 H2O) as excess water can now be accommodated in sites no longer needed for charge compensation. Cd-NAT can only be super-hydrated to compounds containing 32 H2O despite having the EFC with the highest ionic potential. We present a phase diagram of natrolites under PIH conditions which provides evidence for the importance of both EFC size and charge. Combined and systematic experimental and theoretical studies of the high pressure insertion chemistry of natrolites is an important step towards the goal to predict ‘in-silico‘ pressure-induced insertion of many chemical species (i.e. H2O, CO2, CO, noble gases) into microporous materials.

■ Supporting Information Changes of the synchrotron X-ray powder diffraction patterns and normalized unit-cell volume of NAT with divalent EFCs. Final refined atomic coordinates for the NAT with divalent EFCs.

* email: [email protected]. The Authors declare no competing financial interest.

This work was supported by the Global Research Lab Program of the Korean Ministry of Science, ICT and Planning (MSIP). Experiments using x-ay synchrotron radiation were supported by the Pohang Accelerator Laboratory in Korea through the abroad beam time program of Synchrotron Radiation Facility Project under the MSIP and have been performed under the approval of the SSRL at Stanford University.

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Ca8Al16Si24O80·24H2O, a Ca2+ containing natrolite undergoes a two-step pressure-induced hydration up to 2.65 GPa to super-hydrated materials with 32 H2O and 40 H2O per unit-cell accompanied by a ca. 6% unit cell volume expansion. Other divalent cation exchanged natrolites containing Sr2+, Pb2+ and Cd2+ reveal complex structural changes with pressure in the presence of water which depend on charge and size of the cation.

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Two-Step Pressure-Induced Superhydration in Small Pore Natrolite

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