Water Confinement in Faujasite Cages: A Deuteron NMR Investigation

Article pubs.acs.org/JPCA

Water Confinement in Faujasite Cages: A Deuteron NMR Investigation in a Wide Temperature Range. 1. Low Temperature Spectra A. M. Szymocha,† A. Birczyński,‡ Z. T. Lalowicz,*,‡ G. Stoch,‡ M. Krzystyniak,§,∥ and K. Góra-Marek⊥ †

University of Agriculture, 31-120 Kraków, Poland H. Niewodniczański Institute of Nuclear Physics PAS, ul. Radzikowskiego 152, 31-342 Kraków, Poland § School of Science and Technology, The Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, U.K. ∥ ISIS Spallation Source, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, U.K. ⊥ Faculty of Chemistry, Jagellonian University, 30-060 Kraków, Poland ‡

ABSTRACT: Deuteron NMR spectra were measured for D2O confined in NaX, NaY, and DY faujasites with various D2O loadings at temperatures ranging from T = 70 K to T = 200 K with the aim to study the molecular mobility of confined water as a function of Si/Al ratio and loading. The recorded spectra were fitted with linear combinations of representative spectral components. At low loading, with the number of water molecules per unit cell close to the abundance of sodium cations, a component related to π-jumps of water deuterons about the 2-fold symmetry axis dominated. For loadings at levels 3 times and 5 times higher than the initial loading level, Pake dublets due to rigid water deuterons dominated the recorded spectra. A set of the quadrupole coupling constant values of localized water deuterons was derived from the analysis of the Pake dublets. Their values were attributed to deuteron positions corresponding to the locations at oxygen atoms in the faujasite framework and locations within hydrogen-bonded water clusters inside faujasite cages. The contributions of the different spectral components were observed to change with increasing temperature according to the Arrhenius law with a characteristic dynamic crossover point at T = 165 K. Below T = 165 K a spectral component was observed whose contribution changed with temperature, yielding the activation energy of about 2 kJ/mol, characteristic for jumps between inversion-related water positions in clusters.

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INTRODUCTION Studies of molecular mobility are among classical applications of NMR spectroscopy, owing to the fact that the molecular motion shapes the NMR spectra, making the nuclear spin interactions time dependent, and introducing different degrees of spectral averaging within the constraints imposed by the transformation properties of the respective nuclear spin Hamiltonians. Equipped with this basic knowledge an NMR experimentalist is faced with a problem of choosing the most appropriate nuclear spin probes for the investigation of given system properties. Small values of the coupling constant, and multispin character of dipole−dipole interaction characterizing the spectra of I = 1/2 spin systems, render the theoretical analysis of motional modes resulting from a given effective interaction potential unattractive. Conversely, the choice of deuterons as spin probes characterized by a much stronger quadrupole interaction with suitable oneparticle transformation properties, guarantees more success. By designing the experimental protocol to consist of simultaneous measurements of deuteron relaxation and spectra over a wide range of temperatures, one can study reorientation rates and details of motional dynamics in a great detail. In such an experimental protocol two major factors need to be taken into account to enable a successful theoretical interpretation of the 2H NMR experimental results for a given system: temperature © 2014 American Chemical Society

dependence of the motional dynamics and structure dependence of quadrupole coupling constants. Different scenarios are possible as far as the temperature dependence of the motional modes shaping 2H NMR spectra is concerned. In the simplest case the motion can be characterized by a single correlation time that follows the Arrhenius law. Situations where two motions about a given axis may simultaneously contribute to the line narrowing are also considered, and the usual approach is to treat them sequentially.1 In many cases, however, more complex three-dimensional motions composed of uncorrelated rotations or oscillations about noncoincident axes need to be taken into account when the 2H NMR spectra are simulated.2 The quadrupole interaction depends on the value of the electric field gradient (efg) at the position of a deuteron. Its value reflects differences in binding and may be different for different locations of deuterons.3,4 The quadrupole coupling constant for D2O depends on the state of matter and equals 318.6 kHz in gas phase, about 230 kHz in liquid phase, 216−225 kHz in solids (ref 5 and references therein). It is equal to 250 kHz for hydrogenReceived: March 21, 2014 Revised: July 2, 2014 Published: July 2, 2014 5359

dx.doi.org/10.1021/jp504648s | J. Phys. Chem. A 2014, 118, 5359−5370

The Journal of Physical Chemistry A

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bonded (HB) molecules in crystalline hydrates,6,7 and 225 kHz in ice.8 The quadrupole coupling constant for deuterons on Si− ODAl sites amounts to 236 kHz.9 A semiempirical relation between the quadrupole constant and the lengths of the hydrogen bond was given by Chiba.7 The 2H NMR spectroscopy may become a method of choice for the investigation of molecular dynamics (MD) in a plethora of molecular and solid state systems employing the properties of the quadrupole interaction outlined above in a carefully chosen experimental protocol. The 2H NMR has been successfully applied over the past decade for studies of dynamical processes in a variety of solid state systems, e.g., polymers,10 crystalline amino acids,11 and liquid crystals and lipids.12 The 2H NMR techniques most frequently chosen are 1D spectroscopy, quadrupole echo (QE), spin alignment echo, and 2D exchange spectroscopies.13 The analysis of experimental data acquired with the help of these techniques provides a means for measuring molecular jump rates over 10 orders of magnitude. The sensitivities of 1D and QE spectroscopies are restricted to relatively fast motions with correlation times shorter than about 10−3 s. The deuteron spin alignment and 2D exchange spectroscopy methods extend this range down to very slow motions.14 In the present study we focus our attention on D2O in NaX and NaY zeolites. Their framework consists of cubo-octahedral sodalite cages made up of eight 6-membered and 4-membered rings bonded with hexagonal prisms, resulting in the same structure as natural zeolite, the faujasite. The structure is characterized by the existence of large ellipsoidal supercages with diameters of about 1.18 nm between cubo-octahedra connected with four other supercages by 12-ring windows with free apertures of about 0.8 nm. The compositions of typical zeolites N a X a n d Na Y a r e N a 8 6 [ ( S i O 2 ) 1 0 6 ( A l O 2 ) 8 6 ] a n d Na56[(SiO2)136(AlO2)56], respectively. The ratio of Si/Al amounts to 2.4 for zeolite NaY and 1.3 for NaX. Due to the higher density of AlO4− tetrahedra in zeolite NaX the framework has bigger negative charge and the framework oxygen atoms are also more negative. The location of cations in zeolites NaX and NaY depends on the cation charge, their degree of hydration and the presence of adsorbed molecules.15 In zeolites NaX and NaY the Na+ cations are situated inside hexagonal prisms (sites SI), inside cubo-octahedra (sites SI′ and SII′) and inside supercages (sites SII and SIII).16 The unit cell of the NaA zeolite contains two kinds of cavities: the α-cages with free diameter of about 1 nm, connected through octagonal windows about 0.4 nm wide, and β-cages with a free diameter of about 0.6 nm are located between α-cages.17 Numerous studies, mostly MD simulations, were devoted to water dynamics in NaA zeolite (refs 17−20 and references therein). The energy of the water−sodium cation interaction in the NaA zeolite of about 80 kJ/mol was reported.18 However, when HB water molecules were considered inside the NaA cages, activation energy of the order of 10 kJ/mol was observed. Hydrogen bonds formed with zeolite framework oxygen atoms are weaker.19 The activation energy of about 7.5 kJ/mol was obtained when a flip motion of water molecules around the 2-fold axis (bisector) was considered.17 These different activation energy values indicate that water confined in zeolites should be considered as a very heterogeneous system. MD studies for various zeolites provide a picture of water mobility as a function of temperature. At temperatures higher than about 205−220 K the behavior is fragile liquid-like, for about 150−180 K < T < 205−220 K it is strong liquid-like, whereas for T < 150−180 K it is solid-like,19 with an open question whether amorphous or

crystalline for the latter phase. Dynamic crossover phenomena were found in all considered cases in spite of the size of clusters and the degree of hydrogen bonding. Although the activation energy of about 11.1 kJ/mol between dynamic crossovers is compatible with the strength of the hydrogen bonding, the activation energy of 1.1 kJ/mol, obtained at lower temperatures, is difficult to interpret.19 The above MD studies constituted a major source of motivation for the present work, which is the result of an extensive 2H NMR study of confined water dynamics in faujasites in a wide range of temperatures. The work is divided into two parts. Part 1 reports on results from measurements of 2H NMR spectra in the range 70−210 K for samples of NaX and NaY zeolites loaded at 100%, 300%, and 500% (with the loading level defined with respect to the abundance of sodium cations in the respective unit cell). The measured spectra are reported to be composed of a number of components specific for motional modes of given well-defined symmetries with the composition depending on zeolite loading and temperature. This dependency allows us to characterize molecular mobility related to the water− zeolite framework interactions and water−water HB network. Moreover, from the analysis of the spectra quadrupole coupling constant values for localized deuterons are derived. The obtained values are shown to be characteristic for specific sites corresponding to the location of zeolite framework oxygen atoms and within water clusters confined in the zeolite. Part 221 of this work is devoted to the analysis of the results obtained from the measurement of 2H NMR spectra and the deuteron spin− lattice relaxation in the temperature range from 220 to 310 K. The main result of the second part of the work is the proposition that the so-called high temperature fragile-to-strong transition is an underlying mechanism for the observed temperature-induced changes in molecular mobility of water confined in NaX and NaY zeolite cages. By itself, this work demonstrates the potential and applicability of a coherent approach consisting of a simultaneous measurement of the 2H NMR spectra and the deuteron spin− lattice relaxation in a wide range of temperatures.

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THEORETICAL PRINCIPLES: SPECTRA The total Hamiltonian for a system of N deuterons with individual nuclear spins I(i) = 1 (i = 1, ..., N) in the presence of a large magnetic field B⃗ 0 contains Zeeman, quadrupole, and dipolar terms. The Zeeman Hamiltonian in the laboratory reference frame with B⃗ 0 defining the z axis of the frame can be written in the following form: N

/z = −γDℏB0 ∑ Iz(i)

(1)

i=1

where γD is the deuteron spin gyromagnetic ratio and is the zcomponent of the nuclear spin operator I(i). In further considerations, we may restrict ourselves to the secular part of the quadrupole Hamiltonian with the asymmetry parameter η = 0 (based on experimental evidence) for a set of deuteron spins:22 I(i) z

/ ′Q =

1 hCQ 8

N

∑ (3 cos2 θi − 1)[3(Iz(i))2 − I(I + 1)] i=1

(2)

where θi is the angle between direction of the axially symmetric efg of the ith deuteron (e.g., O−D bond direction in D2O) and the magnetic field B⃗ 0. The quadrupole coupling constant of a deuteron with the electric quadrupole moment eQ, placed in an 5360



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axially symmetric efg with the strength eq, equals CQ = e2qQ/h. The quadrupole Hamiltonian (eq 2) is diagonal in the simple product spin representation and the spectra can be calculated analytically. NMR spectrum of a single immobile deuteron (with efg making an angle θ with B⃗ 0) consists of a pair of lines at frequencies ν given by 3 ν(θ ) = ± CQ (3 cos2 θ − 1) 8

Introduction of the dipolar interaction or rotational tunnelling requires diagonalization of the total Hamiltonian matrix, leading to the eigenvalues and eigenvectors providing the basis for the calculation of spectra. Solutions were reported in literature for spin systems consisting of three (methyl group symmetry)25 and four (ammonium ion or methane of tetrahedral symmetry) deuterons.26 Generally, NMR spectra are sensitive to molecular reorientations on the NMR time scale,13,24 which is reflected in the condition for the narrowing of spectra expressed as στc ∼ 1, where σ is the width of a given spectrum and τc is the temperature-dependent correlation time. For a typical width of a deuteron spectrum, σ = 135 kHz for CQ = 180 kHz, the condition στc ∼ 1 leads to the correlation time τc = 10−6 s. For τc ≫ 10−6 s deuterons are considered to be immobile and the Pake doublet is observed (Figure 1a). The doublet separation equals (3/4)CQ, which can be used to provide the value of the quadrupole coupling constant. Narrowed spectra result for τc ≪ 10−6 s. Deuteron NMR spectra are inhomogeneous27 and consist of the doublets covering the whole spectral range. The doublets therefore undergo motional narrowing sequentially at somewhat different temperatures for differently oriented deuterons. For example, deuteron NMR spectra of polycrystalline ND4VO3 measured between 60 and 83 K were found to be sensitive to correlation frequencies in the range 210 to 1 kHz (corresponding to correlation times in the range 7.6 × 10−7 s ≤ τc ≤ 1.6 × 10−4 s).28 This result shows that the range of molecular mobilities, where the spectra can be considered to be in the intermediate narrowing regime, is quite broad.29,30 Representative deuteron spectra, obtained from first-principles calculation for the intermediate narrowing regime, used for fitting experimental spectra,28 were calculated31 using density matrix formalism with the Liouville−von Neumann equation,32 where quantum evolution of the spin system is governed by the quadrupole Hamiltonian and reorientation is described by a respective exchange matrix. Such an approach allows us to dissect the observed spectra into contributions due to motional modes of given well-defined symmetries. Evolution of 2H NMR spectra of ND4+ performing 120° jumps about C3 symmetry axis leads to central doublet combined with a background signal coming from a rigid deuteron doublet (Figure 2 in ref 31). Fast rotation of deuterons with γ = 70.53° with respect to the axis, describing for instance the CD3 group, leads (eq 4) to the averaged doublet with the separation reduced by 1/3 (Figure 1c). The reduction factor equals 1/2 (eq 4), when the rotation axis is perpendicular to efg, e.g., to the plane of D2O molecule. Not only the coupling constant but also the value of the asymmetry parameter can be obtained from the analysis of the deuteron spectra in the regime of motional averaging. For example, the exchange of deuteron positions about the axis bisecting the tetrahedral angle (2-fold symmetry axis of the water molecule), called also π-jumps, leads to the spectrum shown in Figure 1b, which was obtained for the asymmetry parameter η = 1.33 The spectrum was calculated on increasing rate of π-jumps of ND4+ ion about the 2-fold axis (Figure 5 in ref 31). Exchange of deuteron positions within a set of orientations related by the tetrahedral symmetry (as for ammoniun ion, or as it will be proposed in the following, within a cluster of HB water molecules) leads to a perfectly narrow line, as the quadrupole Hamiltonian averages out to zero analytically. Also, fully isotropic reorientations lead to a narrow line characterized by a width, resulting from the intermolecular deuteron spin orientations (Figure 1e). Fast but restricted in-space exchange of deuteron

(3)

From eq 3, after integration over space, a powder spectrum results,23 which has a shape of Pake doublet with the separations Δ = (3/4)CQ between the horns at θ = 90° and 2Δ = (3/2)CQ between the edges at θ = 0° (Figure 1a).

Figure 1. Typical theoretical deuteron NMR spectra for differential models: (a) immobile deuterons; (b) 2-fold exchange; (c) 3-fold rotation (e.g., CD3 structure); (d) partially effective isotropic reorientation; (e) effective isotropic reorientation. Effects of Gaussian broadening, required when experimental spectra are fit, are shown in red.

The spectral width is reduced due to fast uniaxial reorientation of a deuteron and addopts the following form:24 3 ⟨ν⟩av = ± CQ ⟨3 cos2θ − 1⟩av 8 ⎛3 3 1⎞ = ± CQ (3 cos2 θ′ − 1)⎜ cos2 γ − ⎟ ⎝ (4) 8 2 2⎠ where θ′ is the angle between magnetic field and rotation axis and γ is the angle between rotation axis and the efg direction. Integration over the angle θ′ provides powder spectrum for a given γ, with an effective quadrupole coupling constant Ceff Q = (1/ 2)CQ(3 cos2 γ − 1). 5361



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position provides a Gaussian shape of spectra characterized by a bigger width (Figure 1d).

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G(ν) =

EXPERIMENTAL SECTION

⎡ (ν − ν )2 ⎤ 1 0 ⎥ ⎢− exp 2σ 2 ⎦ ⎣ σ(2π )1/2

(5)

with the full width at half-maximum (fwhm) equal to 2(2 ln 2)1/2σ ≈ 2.355σ. The FID and QE sequences were applied according to the complexity of the recorded NMR signal. The signals recorded for the zeolite samples exhibited a dominating narrow line in the high temperature regime; thus FID was observed and processed. Broad components at lower temperatures forced the use of the QE method. In what follows, the temperature TS will be used to indicate the limit of the applicability of the FID method for the measurement of the relaxation rate and narrow spectra. A modification of the subtraction technique introduced by Speier was used36 to eliminate spurious ringing observed at low temperatures from the NMR probe as well as artifacts at the beginning of the signal appearing at high temperatures. For eliminating spectral baseline distortion of Fourier-transformed FID signals, an extension37 of Heuer’s method38 dedicated to wide 2H NMR spectra was used. The nature of the quadrupole interaction allows us to expect symmetric spectra. Thus, the zeroing of the “imaginary” signal in the time domain was possible. An acceptable S/N ratio in the spectra was achieved by signal averaging over a several thousands of accumulations depending on loading. The repetition time was kept at least 3 times longer than the longest time constant observed in relaxation. From the experimental point of view a compromise had to be made to choose the most suitable signal acquisition protocol for the recording of the deuteron spectra, based on the estimates of the values of the deuteron spin−lattice relaxation time constants at various temperatures. The measurements were performed in a wide range of temperatures from T = 70 K up to the room temperature. Broad spectra were measured up to a temperature where overall narrowing becomes significant. Attempts were made to record spectra below 70 K, but that brought no significant change in the spectral shapes recorded and the long deuteron spin−lattice relaxation time constants at these very low temperatures, made measurements very time-consuming. From the preliminary measurements the following compromise was reached for the acquisition protocol of the deuteron spectra at low temperatures. The QE sequence was applied up to a temperature where the wide components of the recorded deuteron spectra disappear. It proved very difficult from the experimental point of view to continue applying the QE sequence in the intermediate range, close to temperature where a narrow line component appears in the recorded spectra. Conversely, when going down with temperature, the lowest temperature TS was reached where narrow spectral components became too weak and FID technique could not provide the good quality spectra.

Zeolites NaX (supplied by Sigma-Aldrich) and NaY (purchased from Linde company) were activated in situ in an NMR cell. First, samples were evacuated at room temperature for 30 min, then the temperature was raised with the rate 5 K/min up to 700 K and kept at this temperature at vacuum for 1 h. The doses of D2O were sorbed in zeolites NaX and NaY up to 100%, 300%, and 500%, as expressed in the units of the total coverage of Na+ ions. The samples were sealed in 24 mm long glass tubes with the outside diameter 5 mm. The sample NaNH4Y zeolite was kept at 450 °C under vacuum (10−6 Torr) for 4 h to decompose ammonium ions and produce protonic form HY. IR spectra measured after this procedure were free of all bands attributable to ammonium ions. Deuteration of zeolite HY was achieved by isotopic exchange with D2O at 120 °C. The sample HY was contacted with D2O vapor and kept at the same temperature for 2 h (D2O pressure was ca. 20 Torr). Then water vapor was removed by evacuation at 120 °C for 1 h. The D2O vapor treatment followed by evacuation was repeated 5 times. To achieve a total degree of isotopic exchange, the last D2O treatment at 120 °C was done overnight and was followed by evacuation at 400 °C. Next, the sample was cooled to 100 °C. The sorption of a measured dose of water was carried out at 100 °C, after 5 min contact time the sample was cooled to the room temperature. To ensure the completeness of water adsorption, the tube was placed in liquid nitrogen and sealed. The abundance of deuterons in DY, equal to the abundance of sodium cations at the given Si/Al ratio, defines formally the loading. The NMR experiments were carried out over a range of temperatures regulated by the Oxford Instruments CT503 temperature controller to the accuracy of ±0.1 K. The static magnetic field 7.04 T was created by the superconducting magnet by Magnex, and the 2H resonance frequency was equal to 46 MHz. The NMR probe was mounted inside the Oxford Instruments CF1200 continuous flow cryostat. Pulse formation and data acquisition were provided by a Tecmag Apollo 500 NMR console. The dwell time was set to 2 μs. The π/2 pulse equal to 3 μs assured the uniform excitation34 for our 200 kHz spectra. NMR spectra were obtained by a Fourier transformed free induction decay (FID) or quadrupole echo (QE) signals. For the FID and QE spectra the sequences (π/2)x−t and (π/2)x−τ−(π/ 2)y−t were used with separation time τ of the order of 50 μs in the QE sequence. The pulse separation time τ was adjusted by means of the home-designed code for Tecmag console to optimize QE signal intensity for each temperature. The phase cycling sequence was applied for the sake of the FID signal cancellation in the overall signal after the quadrupole echo sequence. Representative deuteron NMR spectra were calculated from first-principles for different motional modes characterized by well-defined symmetries, and their linear combinations were fitted to observed deuteron NMR spectra to characterize the motion of confined water deuterons (for details of the method, see ref 35). The fitting procedure involved adjusting the value of the quadrupole coupling constant and the width of the Gaussian broadening for all necessary components. The Gaussian component was defined as

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LOW TEMPERATURE SPECTRA 100% Loading. The deuteron spectra of heavy water confined in the NaX and NaY zeolites at 100% loading level are shown in Figures 2 and 3. The spectra are practically identical at 70 K. The numerical decomposition of the recorded spectra yielded five distinct doublets with the effective quadrupole coupling constant values equal to (with the relative contributions given in brackets) 90 kHz (3%), 150 kHz (5%), 185 kHz (16%), 220 kHz (33%), and 260 kHz (20%). The pagoda component related to D2O molecules performing π-jumps contributes at the level of 23% of the entire spectral intensity. The value of the 5362



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effective quadrupole coupling constant for this component fitted in the process of numerical decomposition is Ceff Q = 260 kHz. The pagoda component dominates in the spectra already at 125 K as Pake’s contributions significantly diminish to about 40% and 15% for NaX and NaY, respectively. A Gaussian component appeared above 125 K, and its contribution was observed to increase at the expense of the contribution of the pagoda component. The temperature dependence of the contributions of the Gaussian spectral components was observed to obey the Arrhenius law with values of the activation energy of about 4 and 6 kJ/mol for NaX and NaY, respectively. The obtained values of the activation energy indicate that the transition from π-jumps into reorientation depends on the bonding of water molecules to Na+ cations, which is most likely stronger in case of the NaY zeolite. Powder X-ray diffraction study of the water adsorbed in faujasites at low loading indicates strongly preferred location of water molecules between two sodium cations.39 The activation energies, derived from the temperature dependence of the intensities of the pagoda components and the sums of observed Pake doublet components in the temperature range up to about 125 K, are 1.6 ± 0.4 kJ/mol for water confined in both NaX and NaY zeolites. The narrow doublet with Ceff Q = 90 kHz detected at 70 K, having in mind an expectable abundance, can only be attributed to deuterons of silanol groups rotating about the 3-fold symmetry axis with the static value of the effective quadrupole coupling constant, Ceff Q = 270 kHz. Additional structural information from zeolite studies present in the literature is required to assign the values of the effective quadrupole coupling constants, obtained from the analysis of the deuteron spectra, to specific locations of heavy water molecules within the framework of the zeolite. The first piece of this vital information is provided by the work of Hunger40 discussing the case of 72 D2O molecules per unit cell in NaX which is close to our 100% loading. Two different groups of water molecules located at two distinct crystallographic positions are postulated in this work from the interpretation of DRIFT and low temperature neutron diffraction studies. The group of water molecules, located at positions labeled D2O (9) has the highest abundance. Water molecules in this group are arranged in a form of a cyclic chain of molecules, the flat hexamer, at 12-rings in vicinity of SIII cations. Water deuterons are HB either to O1 framework oxygen atoms or to oxygen atoms in neighboring water molecules. Thus, the local potential of all molecules in the hexamer has a 2-fold symmetry. The second most abundant group of water molecules consists of molecules located at the position D2O (4) and involved in only one hydrogen bond to O4 framework oxygen atoms, with possible location in the vicinity of sodium SII.40 Clearly, these two crystallographically distinct groups of water molecules confined in the zeolite framework map onto the two distinct spectral components observed in deuteron spectra at 100% loading level. The spectral component, characterized by the effective coupling constant Ceff Q = 220 kHz, with the highest contribution to the total recorded spectral intensity may be attributed to water molecules in the flat hexamers. As described above, the local potential of all molecules in the hexamers has a 2fold symmetry. This feature, inferred purely from the crystallographic study, nicely agrees with information obtained here from the deuteron NMR at the same water loading level. Both pieces of information indicate that, for the group of heavy water molecules contributing the most to the total spectral intensity, only π-jumps about the 2-fold symmetry axes are possible.

Figure 2. Deuteron spectra of water confined in NaX zeolite at 100% D2O loading level. Spectral components resulting from the numerical decomposition of the total spectra are shown.

Figure 3. Deuteron spectra of water confined in NaY zeolite at 100% D2O loading level. Spectral components resulting from the numerical decomposition of the total spectra are shown.

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from the numerical decomposition of the total spectrum are shown in Figure 5. Similarly as in the case of the NaX zeolite, the

Consequently, water molecules at the position D2O (4), involved in only one hydrogen bond to O4 framework oxygen atoms, with possible location in the vicinity of sodium SII, are identified from the assignment of the deuteron NMR spectra to contribute to the doublets with Ceff Q = 150 kHz and 185 kHz for the bonded water deuterons, and Ceff Q = 260 kHz for the not bonded ones. 300% Loading. Numerical decomposition yields in this case the four doublets and a Gaussian component for NaX (Figure 4).

Figure 5. Deuteron spectra of water confined in NaY zeolite at 300% D2O loading level. Doublets are marked for discussion in the text.

numerical decomposition yields in this case four doublets and a Gaussian component. The first doublet may be attributed to the 3-fold rotation of deuterons. The contribution of the Gaussian spectral component to the total spectral intensity changes with temperature and the changes can be very well accounted for by the Arrhenius type of activation curve with the value of activation energy Ea = 4.5 kJ/mol (Figure 6). The Pake doublets, denoted as spectral component numbers 2−4, are fitted with the value of the effective quadrupole coupling constants Ceff Q (2) = 140 kHz, eff Ceff (3) = 215 kHz, and C (4) = 247 kHz. Interestingly, doublet Q Q no. 4 is observed only up to 182 K. The sum of Pake’s

Figure 4. Deuteron spectra of water confined in NaX zeolite at 300% D2O loading level. Doublets are marked for discussion in the text.

For the first doublet a value of the effective quadrupole coupling constant Ceff Q = 80 kHz and 7% contribution are obtained. With the aid of this value one can assign water deuterons contributing to the NMR signal present in the form of the first doublet to deuterons rotating about a 3-fold symmetry axis. The contribution of these deuterons to the total spectral intensity increases with temperature which may indicate that the reorientation about a 3-fold symmetry axis is restricted to single bonded water molecules, e.g., with one water deuteron pointing out in the water clusters. The numerical decomposition provides also more data: Ceff Q = eff 150 kHz for the second doublet, and Ceff Q = 200 kHz and CQ = 250 kHz for the third and the fourth doublets, respectively. The contributions of these doublets to the total spectral intensity, obtained from fitting the spectra recorded at 90 K, are 13%, 47%, and 30%, respectively. The contribution of the Gaussian component increases with temperature. The assumption of the Arrhenius type of temperature dependence yields values of the activation energy of Ea = 5.8 kJ/mol and Ea = 2 kJ/mol, for the Gaussian components and the third and the fourth doublets, respectively. Deuteron spectra of water confined in NaY zeolite at 300% D2O loading level together with spectral components resulting

Figure 6. Temperature dependence of the contributions of individual spectral components for NaY with 300% loading of D2O: labels 2, 3, and 4 refer to Pake doublets in Figure 5, and labels 5 and 6 to pagoda and Gaussian components, respectively. 5364



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components in both zeolites completes the picture. At higher temperatures, the contribution of the Gaussian spectral component is smaller, but contributions of Pake doublets are bigger for the NaY as compared to the NaX zeolite. This indicates more stable clusters being more strongly bonded to sodium cations that are more charged in the NaY case. 500% Loading. The numerical decomposition yields in this case four doublets and a Gaussian component for NaX (Figure 7). For three Pake doublets the numerical decomposition

contributions appears temperature independent up to about 143 K. However, a closer inspection of values of the relative contributions of the separate components to the total recorded spectral intensity, shown in Figure 6, reveals a transfer of intensity from component no. 4 (Ea = 1.9 kJ/mol) to component no. 3 (Ea = 0.7 kJ/mol). Similar values of activation energy were obtained from rotational relaxation constants of water in silicalite and NaX zeolite.19 On the basis of these results, a physical picture may be proposed in which some heavy water molecules present in water clusters that have deuterons pointing out of the clusters, rearrange into the inner positions in the clusters. The temperature dependence of the pagoda spectral component and the Gaussian component, labeled with nos. 5 and 6 in Figure 6, are characterized by activation energy values of 1 and 4.5 kJ/ mol, respectively. An interpretation may be put forward, where external molecules in water clusters localized at sodium cations and contributing to the pagoda spectral component, undergo motional narrowing and contribute to the increasing temperature intensity of the Gaussian component above 125 K. Crucially, from the point of view of the whole picture of water confinement in NaY zeolite, thermally activated processes are observed in NaY with 300% loading of D2O above 165 K. As shown in Figure 6, the intensity of spectral component no. 2 is increasing (with corresponding activation energy Ea = 15.6 kJ/ mol) and that of component no. 3 is decreasing with temperature (Ea = 5.8 kJ/mol). Spectral component no. 2 may be attributed to water molecules with deuterons HB to O4 oxygen atoms of the zeolite framework. Spectral component no. 3 may be attributed to molecules inside clusters. This component decreases above 165 K. The physical picture emerging from these observations is that water molecules leaving the cluster move to positions at O4 oxygen atoms, as evidenced by the increasing contribution of spectral component no. 2 (Figure 6). Experimentally obtained activation energies may be attributed to specific locations of heavy water deuterons. The activation energy value of Ea = 5.8 kJ/mol for water molecules attributed to spectral component no. 3 is close to the value of 4.5 kJ/mol calculated as an average bond energy within a network of HB water molecules.41 However, another estimate of the activation energy value of Ea = 10 kJ/mol was obtained from the consideration of a thermally activated process of breaking of the HB network of water molecules.19 The bond strength of deuterons to the O4 zeolite framework oxygen atoms is higher than the strength of deuteron bonding to water oxygen atoms as obtained from the spectroscopic results presented in this work (Ea = 15.6 kJ/mol). Thus, in light of the above results, the assignment of the activation energies to distinct structural features in the water−zeolite system seems a complicated task. This difficulty may partially arise from the fact that the dynamic transfer of water deuterons from one location in the water−zeolite structure to another may be also indirect. Moreover, a change in water mobility at some locations in the clusters may indicate a phenomenon resembling a phase transition related to breaking of tight assembling of crystallinelike network of water clusters. On the whole, the deuteron spectroscopic data shown above paint a picture of heavy water confined in the NaX and NaY zeolites at the 300% loading level in which the role of sodium cations as primary adsorption centers postulated in the literature is confirmed again. Moreover, NMR data presented here are consistent with a model assuming that water molecules located at positions in the NaY zeolite close to the sodium cations are subject to a local effective 2-fold symmetry potential and perform π-jumps. The analysis of the behavior of the Gaussian spectral

Figure 7. Deuteron spectra of water confined in NaX zeolite at 500% D2O loading level. Spectral components resulting from the numerical decomposition of the total spectra are shown.

procedure of the entire recorded spectrum yielded values of effective quadrupole coupling constants: Ceff Q = 160 kHz (12%), eff Ceff Q = 210 kHz (52%), and CQ = 255 kHz (23%). The Gaussian component shows systematic narrowing with temperature. The temperature dependence of the relative contribution of this spectral component to the entire spectral intensity fits well within the Arrhenius-type activation curve and yields the value of the activation energy Ea = 2.2 kJ/mol. The narrow doublet with Ceff Q = 90 kHz contributes 5% up to 150 K. Its contribution increases stepwise at about 166 K to 17%. This feature can be attributed to the onset of the 3-fold rotation motional mode of external deuterons in heavy water clusters. Among the other Pake doublets, only that with Ceff Q = 210 kHz shows the temperature dependence above 150 K. Its contribution decreases with temperature in the temperature range in the vicinity of 190 K. The temperature dependence of the relative contribution of this component is very well approximated by the Arrhenius-type activation curve with the value of the activation energy Ea = 2.2 kJ/mol. Above 190 K there is a stepwise decrease in the relative contribution of this spectral component to about 19%. One may attribute the spectral component with the value of the effective quadrupole coupling 5365



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constant Ceff Q = 210 kHz to deuterons of water molecules inside clusters. These deuterons are involved in mutual, donor− acceptor hydrogen bonding within a structure similar to that of the water tetramer with local tetrahedral symmetry.42 The observation that the contribution of the Gaussian component increases with temperature can be explained within the following model. Temperature-activated internal vibrations lead to hydrogen bond breaking upon which the only motional mode contributing to the Gaussian spectral component is related to restricted reorientations of O−D axes. Moreover, similar values of the activation energy obtained from the numerical decomposition of deuteron spectra recorded for water confined in the zeolites at 500% loading indicate exchange between the deuteron sites contributing to the Gaussian and the Pake doublet characterized by Ceff Q = 210 kHz. At 200 K two additional narrow Gaussian components appeared in the intermediate temperature range up to TS = 217.4 K, above which only narrow lines were observed. From this temperature point onward Gaussian spectral components start to dominate the recorded spectra. Numerical decomposition of spectral shapes of the deuteron NMR lines in this intermediate temperature range proves very difficult if not impossible, most likely due to the existence of distributions of correlation times related in a not straightforward manner to different motional modes of deuterons. However, more light can be shed on the MD of water in this interesting temperature regime by means of extensive deuteron relaxation measurements that will be the subject of the second paper in this sequel (part 2).21 Deuteron NMR spectra recorded for NaY at such high loading appear simple and behave in a particularly regular manner as a function of temperature (Figure 8). The numerical decom-

position in this case yields only two Pake doublets with Ceff Q = 220 kHz and Ceff = 255 kHz, with contributions 55 ± 2% and 42 ± Q 3%, respectively. The contributions of both spectral components show no temperature dependence up to 200 K, in spite of proximity to the characteristic temperature TS. The contribution of the doublet with Ceff Q = 255 kHz is twice of that for NaX. The following physical interpretation can be put forward to explain the observed behavior of individual spectral components of deuteron NMR lines observed as a function of temperature for the system of water confined in NaY zeolite at 500% D2O loading level. Water molecules located at positions corresponding to the location at sodium cations in the zeolite framework, performing π-jumps at low loading, are all rigid in both zeolites at high water loading level. Deuterons HB to framework oxygen atoms, characterized by the value of the effective quadrupole coupling constant CQeff = 160 kHz, which lead to visible spectral components in NaX, do not lead to the formation of a detectable spectral component for the case of NaY. The most likely reason for such behavior is the existence of a network of water molecules forming very strong hydrogen bonds to zeolite framework oxygen atoms in NaX. The lack of temperature dependence of the spectral components recorded for the NaY zeolite at the highest loading level strongly suggests that the network of confined water molecules in NaY exhibits a high degree of structural stability characterized by nearly perfect tetrahedral symmetry of water clusters. Thus, for the case of water confined in NaY at this high loading level, one may exclude sufficiently frequent reorientations of water deuterons along the O−D axes. Moreover, jumps about 2-fold pseudosymmetry axis (defined as R in Figure 9), lead to exchanging the local water tetrahedron

Figure 9. Tetrahedral symmetry of water molecule.

between two inversion-related positions and thus do not change the value of the effective quadruploar coupling constant, a feature fully consistent with experimental observation for deuteron spectra of water at the highest loading level in the NaY zeolite. The scenario described above is similar to the situation already observed in deuteron NMR spectra and relaxation for the case of deuterated ammonium ions.43 Spectra for 500% loading of D2O in DY complement the view presented so far of confined water in faujasites (Figure 10). The numerical decomposition yielded in this case three Pake doublets with the effective quadrupole coupling constants of Ceff Q = 200 eff kHz (23%), Ceff Q = 220 kHz (34%), and CQ = 250 kHz (39%). Their relative contributions given in brackets refer to results at 90 K. These values decrease only slightly on increasing temperature, for the benefit of the Gaussian components increasing from 4% at 90 K to 13% and 18% at 130 and 170 K, respectively. A picture of stable cluster structure results. The two inner doublets contribute about 50% to the spectra above 100 K and represent molecules inside clusters, whereas those with Ceff Q = 250 kHz refer to

Figure 8. Deuteron spectra of water confined in NaY zeolite at 500% D2O loading level. Spectral components resulting from the numerical decomposition of the total spectra are shown. 5366



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an instructive example here. In this work, molecular groups (CD3, OD) with different mobilities were considered for methanol molecules located at two positions in the zeolite framework. The mean activation energies and the widths of their distributions were derived. The analysis of the deuteron relaxation data in this case leads to the conclusion that different correlation time distributions, covering a wide range of temperature, contribute to all observed relaxation rates. The main conclusion from the deuteron NMR relaxation work described above is that the distinct spin−lattice relaxation time constants do not map unambiguously onto different distributions of correlation times and thus do not provide any direct information on molecular mobility in complex systems, like the water confined in zeolites. This conclusion is further reinforced by the observation that there is a weak, if any, temperature dependence of three observed relaxation time constants for the NaX sample with 500% loading (Figure 11). The relative weights of the distinct exponential components may, however, provide some more insight. The weight of the exponential component with the shortest relaxation time constant decreases with temperature linearly from about 70% to about 40%. The weight of the component with intermediate relaxation time constant increases from about 15% up to about 40%, whereas the weight of the component characterized by the longest relaxation time constant is practically constant at 20%. Despite the difficulty in the interpretation of the spin−lattice relaxation data due to the presence of the distributions of correlation times, described above, the analysis of the recorded deuteron spectra provides some additional useful details. The first distinct feature observed in the spectra is that Pake doublets dominate in the whole temperature range (Figure 7). The deuterons contributing to the Pake spectral components most likely relax exponentially with the longest time constant as their correlation times may be found in the long correlation time tail of the distribution. Mobile water molecules, for which Gaussian peak shapes are observed, contribute 31% and 9% at 200 and 100 K to the spectral intensity, respectively, and relax most likely with the fastest time constant. A more detailed picture of molecular mobility of heavy water confined in zeolites can only be provided on the grounds of the detailed analysis of the correlation time distributions derived from deuteron relaxation data, for instance using the method recently developed by Stoch et al.44 Effects related to the distribution of the correlation time are visible in the deuteron spectra. The Pake doublet observed at low temperature indicates a correlation time τc longer than 10−6 s. Intermediate spectra (τc ∼ 10−6 s) are too weak to be visible as a lot of the spectral intensity is moved out of the spectral window. Therefore, only narrow components (τc ≪ 10−6 s) are observed when TS is approached.

Figure 10. Deuteron spectra of water confined in DY zeolite at 500% D2O loading level. Spectral components resulting from the numerical decomposition of the total spectra are shown.

deuterons pointing out. The hexamers may not be formed in the absence of sodium cations. It may enable some unbound water molecules to commute between supercages and sodalite cages, resulting in the narrow line. A wider Gaussian component indicates more restricted mobility of some water molecules. The temperature dependence of the spin−lattice relaxation rate for NaX at 500% water loading is shown in Figure 11. We

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CONCLUDING REMARKS Low temperature spectra of water confined in NaX and NaY zeolites at various water loading levels exhibit numerous components which, as the temperature increases, overlap and narrow to a different extent. Moreover, a significant proportion of the total spectral intensity is observed to be transferred from the central spectral position into broad sidebands far away in the frequency domain. From the point of view of a local structure of a microheterogenous system, a water melting process is observed and the temperature evolution of NMR spectral components needs to be related to a complex pattern of motional modes. Within this complex process, steps attributed to liquid-to-liquid

Figure 11. Temperature dependence of the spin−lattice relaxation rate for NaX with 500% loading of D2O.

discuss here the low temperature range below TS, leaving the range above to part 2.21 Magnetization recovery is strongly nonexponential and can be fitted by three exponential components with time constants on the order of 0.01, 0.1, and 1 s, respectively. Such a result is common for molecular systems with a broad distribution of correlation times. Deuteron relaxation data analysis for methanol-d4 molecules in the NaX zeolite44 provides 5367



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NaY at higher temperature, indicating again the role of water bonding to sodium cations. Similar conclusions can be drawn for 500% loading (Figure 7 and 8). A Gaussian component and narrow doublets are present for NaX. Clusters are significantly more stable in NaY. Two Pake doublets with temperature independent contributions exist up to 190 K. Two quadrupole coupling constants, 220 kHz and 255 kHz, can be attributed to HB deuterons inside clusters and at external positions, respectively. Despite the undisputed role in the water binding played by sodium cations in the zeolite framework, there are other spectral features observed in deuteron spectra of water−zeolite systems that indicate the importance of water HB to framework oxygen atoms. Namely, at 100% loading π-jumps persist, still contributing to observed spectra at high temperatures for NaX (Figure 2). The higher potential for 2-fold jumps in hexamers comes thus most likely from the bonding to O1 oxygens.40 Pake doublets with Ceff Q = 140−150 kHz are present in the spectra of both NaX and NaY at 300% loading, but at 500% loading for NaX only. That spectral component is thus attributed to deuterons that are HB to O4 framework oxygens, more strongly in NaX. The effective quadrupole coupling constant results from a vector sum of internal and external efg at the deuteron. The 260 kHz value, considered to be the intramolecular reference value, obtained for the pagoda spectral component, is only slightly smaller for external deuterons in water clusters. Such a reduction of the value of the quadrupolar coupling constant may be related to some limited motions like torsional oscillations. DFT work is in progress to confirm the physical picture described above. The structure of the water clusters in both zeolites remains stable up to about 165 K. Transfer between spectral components appears at higher temperatures, followed by effects of melting and related motional narrowing of spectra. Very low activation energies, close to 1 kJ/mol, were obtained from the analysis of temperature behavior of spectral components below 125 K at all loadings. The shape of Pake doublets does not change in that temperature range for NaY with 500% loading even up to 190 K. For the case of water confined in NaY at 500% loading level, a mechanism of jumps about the 2-fold pseudosymmetry axis that do not change the value of the effective quadruploar coupling constant has been proposed, similarly as in the case of deuterated ammonium ions.43 This mechanism, proposed at the level of interpretation of experimental NMR data, requires further scrutiny and needs to be contrasted with other theoretical and experimental work on water clusters formed in confined geometry. Water molecules (Figure 9) form clusters reflecting the tetrahedral symmetry of individual water moelcules. Many cluster structures were considered in theoretical studies,42 but most of experimental evidence was given for a water tetramer (Figure 12) as the basic unit of S4 point symmetry.51,52 Directions of O−D bonds are parallel for molecules positioned on diagonals of the tetramer. Rotations by 180° about axis R (Figure 9) exchange their orientations. Directions of O−D before and after such jumps have the same orientation in the laboratory reference frame and the quadrupole nuclear spin interaction remains unaffected. States before and after such jumps are related also by inversion. Such flip jumps could propagate across water clusters. Such simultaneous flipping of all deuterons in clusters, somewhat analogous to the ammonia inversion, was shown on the basis of group theory considerations to exhibit the symmetry of a permutation-inversion operation (R2π, σh) (Figure 7 of ref 51), which fulfills symmetry

transition at 225 K, fragile to strong dynamics crossover and crystallization below 160 K are recognized.45 The detailed analysis of the deuteron NMR spectra for water confined in the NaX and NaY zeolites, presented in this work, indicates that the presence of sodium cations in zeolite framework is particulary important for observed mobility of water molecules and stability of water clusters. Our results confirm some conclusions of Oehme et al.46 published a long time ago. Proton NMR spectra had been measured in hydrated zeolites in the range between 77 and 420 K. Careful analysis of the spectra and comparison with those obtained for hydrosodalites led to pointing out the stabilizing influence of sodium ions on the mobility of water molecules. The role of sodium cations is particularly evident at 100% loading, where all water molecules are adsorbed, however, at different locations. Sodium cations are distributed among the different crystallographic positions to maximize their interaction with the framework oxygen atoms as well as to minimize the cation−cation electrostatic repulsion. The electrical charge on sodium cations is partially neutralized by framework oxygen atoms. The extent of neutralization is smaller in NaY and the electrical charge of Na+ is higher.47 Basically, Na+ cations have a strong affinity for the SI position in the hexagonal prisms and SII site, because such an arrangement minimizes the electrostatic repulsion between cations. However, where there are more than 48 cations per unit cell, i.e., 86 Na+ in zeolite NaX, the occupation of site SI diminishes in favor of site SI′, which additionally appears as the more favorable of the two up to 64 cations per unit cell. In the end, the sites SII and SI′ are completely occupied (i.e., close to their maximum number of 32 each); thus the rest of Na+ is distributed among sites SIII or SIII′.40,48 Alternatively, in zeolite NaY site SII is nearly always the preferred one, because that position minimizes cation−cation repulsion. It is nearly always fully populated, close to its maximum of 32 from the total number of 56 for Si/Al = 2.4. The positions SI and SI′ are less preferred; however, the rest of the cations, namely 24, are distributed between them. Site SIII is always hardly populated. Many factors can perturb this situation, such as the presence of adsorbed molecules. Their presence tends then to attract cations toward the supercage, even more strongly when the adsorbed molecules are polar.49,50 Small polar water molecules can access the sodalite cage and affect the Na+ cations located there. The main conclusion is that all water molecules experience 2-fold potential at 100% loading in NaY and NaX zeolites. Reorientation of water molecules in NaY was characterized with a higher activation energy than in NaX as a result of stronger interaction with cations there. Water molecules were found both in supercages and in sodalite cages. As the 12-ring aperture is reduced due to formation of water hexamers, one may expect that, on increasing water loading, water clusters will build up mainly in supercages. The role of bonding to sodium cations is significant also for 300% loading. The pagoda spectral component is seen at 300% loading for NaY (Figure 5) but is absent in NaX (Figure 4), a Gaussian component appears instead. Molecules at sodium cations represent a first adsorption layer. Its stability under perturbation by other molecules at higher loading depends on coupling to sodium cations, which is stronger in NaY. The structure of clusters building up at 300% loading can be seen as more stable in NaY at higher temperature as the contribution of Pake components is significantly bigger in NaY than in NaX (Figure 5 and 4). A simple physical picture may be derived from the above observation that breaking up of water clusters happens in 5368



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REFERENCES

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Figure 12. Water tetramer,42 available from the Cambridge Cluster Database.

requirements for the quadrupole interaction-invariant motion proposed above. A separate question arising in the context of the quadrupole interaction-invariant motion of confined water deuterons discussed above is the possible role of deuteron tunnelling in such a motional mode. Simultaneous flipping of all deuterons in heavy water clusters was for instance analyzed to explain the doublet structure in vibration rotation-tunnelling spectra recorded near 2.04 GHz for tetramer (D2O)441 and octamer (D2O)8.53 Another signature of incoherent tunnelling, which may be the case for heavy water confined in zeolites, is the observation of activation energies that are lower than expected for classical motion on the ground of thermally activated MD. A more quantitative analysis along the lines indicated by Horsewill et al.54 cannot be, however, undertaken here due to complexity of the water cluster structure in zeolites under investigation and unknown key parameters of molecular interaction. Thus, the picture in which incoherent tunnelling is present in the case of inversion jumps of water molecules in clusters confined in the zeolite structure needs further corroboration based on ab initio simulation and more experimental evidence. By itself, the experimental work presented here demonstrates the clear potential of deuteron NMR spectroscopy aided by relaxometry for the investigation of water confinement in microheterogenous systems like NaX and NaY zeolites at low temperature. To fully explore the potential of the deuteron NMR, the method needs to be combined with modern ab initio methodology like DFT and MD simulation and other experimental methods.

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AUTHOR INFORMATION

Corresponding Author

*Z. T. Lalowicz. E-mail: [email protected]. Phone: +48 12 6628259. Fax: +48 12 6628458. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Preliminary results, obtained during 2006−2009, were funded by the Ministry of Science and Higher Education, Poland, under grant No. 202 08931/0621. Further work was supported by the National Science Centre, Poland, under Grant No. N202 12 17939 during 2010−2014. 5369



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Water Confinement in Faujasite Cages: A Deuteron NMR Investigation

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