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Back | Show only these items: | Characterization of adsorbed xenon in Zeolite-A, X, and Y by high-pressure 129Xe NMR spectroscopy

1
Characterization of adsorbed xenon in Zeolite-A, X, and Y by high-pressure ¹²⁹Xe NMR spectroscopy

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The local structure of confined xenon in A-, X- and Y-type zeolites (molecular sieve 5A, 13X and zeolite NaY) was investigated by in situ high-pressure ¹²⁹Xe NMR spectroscopy.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

Xenon confined in dehydrated samples gives a resonance peak in the chemical shift range from 50 to 100 ppm at 0.1 MPa, and the chemical shift values increase as pressure increases.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

The pressure dependence of ¹²⁹Xe chemical shift on xenon confined in the micropores can be approximately described by the Langmuir adsorption model, resulting in the parameters being able to describe pore diameter, adsorption ability of the pore, and the local structure of the confined xenon atoms.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

Using these parameters, the pore structure, the mobility and the cluster size of xenon in the micropore are discussed for A- and X-type zeolites.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

Furthermore, the effect of pre-adsorbed water and the binder for pellet preparation on the pore structure is also examined.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

In the Y-zeolite, a chemical exchange of xenon between the outside and inside of the micropores was observed above 0.75 MPa.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

Spectral simulation assuming a simple two-site exchange model revealed the exchange rate constant and the population ratio of xenon in the exchange between the outside and the inside the micropores.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

The exchange of xenon is optimum around the supercritical point of the bulk xenon (∼6 MPa), and decreases above 6 MPa, and this originates from the cooperative phenomenon in the supercritical fluid.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Introduction

Zeolites are crystalline porous aluminosilicates including alkali and/or alkaline earth metals, with the chemical composition of M_x/n·[(AlO₂)_x(SiO₂)_y]·wH₂O. These compounds exhibit interesting properties such as molecular sieve ability and catalysis.¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

These properties can be modified by chemical treatment such as ion exchange and/or de-alumination.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Many researchers have investigated these materials at the scientific and engineering points of view,^2–4 and it has been found that the adsorption behavior, reactivity, and molecular sieve ability of zeolites are closely related to the framework, porous structure, and intermolecular interactions between the pore wall and the adsorbed molecule, as well as between the adsorbed molecules.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

In addition, adsorbed water present in the micropores and the formation of macro- and mesopores due to the aggregation of fine particles of zeolites, will also affect the adsorption behavior of the zeolites.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

The adsorbed water decreases the volume of the micropore to less than 0.5 nm, which is a so-called “ultra-micropore”, and screens the guest molecules from the pore wall, affecting the functional properties of zeolites such as catalysis, the molecular sieve and reactivity.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

Furthermore, the adsorbent products consist of fine particles of synthetic zeolite with a μm diameter order, which are bound by binder agents such as clay, phosphates, and glass with a low melting point to prepare the pellets and/or beads for easy handling.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The addition of binder agents to form pellets leads to extra pores of 1–50 nm.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

These meso- and macropores increases in the amount of adsorbate at the large value of the relative pressure and play an important role in the process of the diffusion of the adsorbates such as molecular diffusion and Knudsen diffusion.⁶⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

¹²⁹Xe NMR is an important tool in the study of ultra-micro, micro-, meso-, and macropores.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

Xenon-129 NMR spectroscopy has been developed to study the structure and topology of internal voids, adsorption sites, and intermolecular interactions in porous materials, since the potential of xenon-129 as a probe for the inner space of materials was proposed by Ito and Frassard⁵ in 1980.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

Recently, this smart methodology has been widely used in the characterization of porous materials as a modern and sophisticated technique of porosimetry,^5–9 and the local structure of materials such as zeolites,^5,10–15 glass,¹⁶ fullerenes,¹⁷ carbon nanotubes,¹⁸ polymers,^19–22 and clathrate compounds,^23–26 liquid crystals,^27–29 some organic solvents,^9,30 and proteins in solution^30–32 have been investigated.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

In the past decade, hyperpolarized (HP) xenon NMR techniques have been developed to enhance the sensitivity of ¹²⁹Xe NMR.^33,34

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

However, to obtain information from ultramicro- and/or meso- and macropores using xenon as a probe, it is necessary to confine xenon in ultra-micropores of the same size as xenon, and/or to slow down the exchange between xenon adsorbed in meso- and macropores and the bulk.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met3

It is difficult to achieve this situation under ambient conditions, and high-pressure conditions offer a solution.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met3

We recently described a conventional and newly designed in situ variable pressure NMR probe, allowing studies with pressures up to 20 MPa,²² and it has been applied in the study of xenon–wall interactions in the free volume of polymers,³⁵ the intermolecular interaction of xenon confined in 1D-nanochannels,^36,37 and the supercritical phenomenon of xenon in mesoporous silica.³⁸

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

Furthermore, Brunner et al. have reported the pressure dependence of ¹²⁹Xe chemical shift in the supercritical state up to a pressure of about 70 MPa using a high-pressure cell made from a sapphire tube.³⁹

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

This high pressure condition is expected to offer new insights into xenon–wall and xenon–xenon interactions for xenon confined in nanospaces, as well as the dynamic behavior of xenon in nanospaces.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

In this study, we applied the high-pressure ¹²⁹Xe NMR technique to some typical zeolites: molecular sieve 5A, 13X, and zeolite NaY.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Molecular sieve 5A has a type-A framework (LTA) with the chemical composition of (Ca_4.8, Na_2.4)[(AlO₂)₁₂(SiO₂)₁₂] ·wH₂O, with the unit cell having a lattice symmetry of Pm3m, a lattice constant of a₀ = 1.24 ± 0.001 nm, and a maximum diameter of 1.14 nm for the super cage which is linked each other by the window with a diameter of about 0.5 nm.^40–42

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

In contrast, molecular sieve 13X and zeolite NaY have a faujasite X- and Y- type framework (FAU) with the chemical composition of Na_x[(AlO₂)_x(SiO₂)_192-x]·wH₂O, respectively, with the unit cell having a lattice symmetry of Fd3m, a lattice constant of a₀ = 2.49 ± 0.01 nm and a maximum diameter of 1.37 nm for the super cage which is linked each other by the window with a diameter of 0.74 nm.^42,43

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

We have investigated the adsorption behavior of xenon on these zeolites using high-pressure ¹²⁹Xe NMR spectroscopy to clarify the influence of the framework, the adsorbed water and the binder agents on the adsorption behavior of xenon under high-pressure conditions.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Experimental

Chemicals

Powdered samples of molecular sieve 5A, 13X, and zeolites NaY were purchased from Aldrich Co. Ltd. Pellets of molecular sieves were purchased from Nakarai Co. Ltd. Sample dehydration was achieved by heating at 400 °C under reduced pressure (P < 2 × 10⁻² Torr) for 3–24 h after roughly drying the specimens at ambient temperature under reduced pressure for 12 h.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

All the specimens were characterized by thermogravimetric analysis (TG), nitrogen adsorption isotherm, ²⁹Si MAS NMR spectrum, and powder X-ray diffraction (XRD) to confirm the amount of adsorbed water, the pore volume and the specific surfaces, and the ratio of SiO₂ to AlO₂ (Si/Al) in the framework.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

Thermogravimetric analysis (TG)

TG analysis was carried out using SEIKO I SSC-5020M II TG/DTA 200-type thermal analysis equipment.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

TG diagrams were recorded at a heating rate of 10 K min⁻¹ up to 600 °C under nitrogen gas flow at a rate of 50 ml min⁻¹.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

α-Alumina, α-Al₂O₃, was used as a reference sample.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

N₂ adsorption isotherm measurements

Nitrogen adsorption isotherms were measured using Shimazu Gemini 2375 automatic specific surface measuring apparatus in the relative pressure range P/P₀ from 0.001 to 0.95.

²⁹Si MAS NMR measurements

²⁹Si magic-angle spinning (MAS) NMR spectra were measured using a Bruker DSX-200 spectrometer with a Larmor frequency of 39.7 MHz.

Free-induction decay (FID) signals were recorded in a single pulse experiment using a pulse delay of 80 s, and π/2-pulse of 8 μs, under 4 kHz of MAS rate at room temperature.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

The chemical shift was referenced by tetramethylsilane (TMS) as an external standard.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Powder X-ray diffraction (XRD) measurements

XRD measurements were carried out using a Rigaku Rotaflex Rint 2000 diffractometer on HP 9000 S712/60 system from 3° to 60° in 2θ.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

The scan rate was 1.00° min⁻¹, and the increment step was 0.01°.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

High-pressure ¹²⁹Xe NMR measurements

High-pressure ¹²⁹Xe NMR measurements were carried out using a Bruker MSL-200 spectrometer with a Larmor frequency of 55.6 MHz, in which a home-built in situ pressure variable NMR probe²² was installed.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The powdered or pellet samples were packed into a glass tube of 5 mmϕ, with both ends capped with glass wool, and the tube and samples were placed in a pressure-resistant NMR cell made from ZrO₂.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The prepared NMR cell was then mounted onto the high-pressure NMR probe.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The FID signals were recorded in a single-pulse experiment with a pulse delay of 5 s, π/2-pulse of 3 μs and accumulation of 8-8192, depending on the signal-to-noise ratio, over a pressure range of 0.01 to 10 MPa at 298 K. ¹²⁹Xe chemical shift was referenced by a signal from xenon gas at zero density.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Temperature and pressure were controlled within the experimental error of ±0.5 K and 10% in MPa, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Results

Characterization of samples using TG, N₂ isotherm, and ²⁹Si MAS NMR

Thermogravimetric (TG) measurement of pre-heated samples up to 600 °C suggests that water adsorbed in the samples was removed by heating above 400 °C.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

The amount of adsorbed water, defined by the ratio of the weight of water to the weight of the dehydrated zeolites, was determined from the weight loss as follows; 41% for molecular sieve 5A, 100% for molecular sieve 13X and 163% for Zeolite NaY.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

After dehydration of the samples by heating at 400 °C under reduced pressure, the N₂ adsorption isotherm was measured to characterize the amount of nitrogen at the maximum adsorption (W_sat), the specific surface area (S_total), the pore volume (V_pore), the external surface area (S_ex), and the internal specific surface area (S_in).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

These samples give the typical I-type adsorption isotherm according to the IUPAC classification.^44,45

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

The pore volume, external and internal surface areas are given by t-plot analysis.⁴⁶

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

The parameters determined are listed in Table 1.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

²⁹Si magic angle spinning (MAS) NMR spectrum gives the atomic ratio of Si to Al, Si/Al for each sample as listed in Table .1^47–50

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

According to the results of TG and ²⁹Si MAS NMR, we can determine the chemical composition of each sample: (Ca_4.8, Na_2.4)[(AlO₂)₁₂(SiO₂)₁₂]·(∼39)H₂O for molecular sieve 5A, Na₈₄[(AlO₂)₈₄(SiO₂)₁₀₈]·(∼750)H₂O for molecular sieve 13X, and Na₅₂[(AlO₂)₅₂(SiO₂)₁₄₀]·(∼1200)H₂O for Zeolite NaY.

¹²⁹Xe NMR spectrum for dehydrated powder samples

Figs. 1(a)–(c) show the pressure dependence of the ¹²⁹Xe NMR spectrum in dehydrated powder samples of molecular sieve 5A, 13X and zeolite NaY, respectively.

The pressure dependence of the chemical shift values for each resonance line is shown in Fig. 2.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

In each sample, the spectrum at 0.1 MPa mainly consists of two dominant peaks; one near 0 ppm and other at the chemical shift range from 50 to 100 ppm.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The resonance line near 0 ppm comes from bulk free xenon gas coexisting on the powder sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

The chemical shift of this resonance line agreed with that for pure bulk xenon gas within the experimental error, suggesting that the observed peak for bulk free xenon is not affected by the chemical exchange with xenon adsorbed in the micropores.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

On the other hand, the peak appearing in the lower field corresponds to xenon adsorbed in the micropore, and shifts toward the lower field side as pressure increases.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

The δ-value, which is extrapolated to zero pressure using the data below 0.1 MPa, gives 90 ppm for molecular sieve 5A, 65 ppm for 13X and 52 ppm for zeolite NaY, and increases rapidly as pressure increases to 1 MPa.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

For molecular sieve 5A and 13X, it reaches 260 ppm and 230 ppm at 6 MPa, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

In addition to these two peaks, the extra peak appears at the chemical shift range 20–30 ppm higher than that of bulk-free xenon above 5 MPa.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

This peak originates from partial components of xenon contributing to the chemical exchange between the inside and outside of the micropore.^11,51

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

In contrast, in zeolite NaY, the resonance line shifts again toward the higher field above 1 MPa with the line broadening, and the δ-value for xenon in the micropore shows the minimum level of 110 ppm at 6 MPa.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

Above 6 MPa, the resonance line shifts again to the low field with narrowing of the lines.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

Furthermore, under 4 MPa pressure, the resonance line at 306 K (FWHM ∼70 ppm) becomes narrower than that at 298 K (FWHM ∼100 ppm) as shown in Fig. 3.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

This anomalous pressure dependence is caused by the influence of the rapid chemical exchange of xenon between the inside and outside of the micropores.

¹²⁹Xe NMR spectrum for hydrated powder samples

Figs. 4(a) and 4(b) show the pressure dependence on the ¹²⁹Xe NMR spectrum for the powder sample of molecular sieve 5A and 13X before dehydration.

The chemical shift for xenon in the micropore entirely shifts the lower field compared to the dehydrated samples.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

Fig. 5 shows the pressure dependence of the chemical shift value for each sample, in which the δ-value extrapolating to zero pressure is evaluated as 127 ppm for molecular sieve 5A, and 160 ppm for 13X.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

These values are 37 ppm and 95 ppm greater than for each dehydrated sample.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

The chemical shift value increases 100 ppm for 5A and 35 ppm for 13X as pressure increases up to 6 MPa.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

In these samples, the extra broad peak, which appears at 20–30 ppm higher than the peak of free xenon in the dehydrated samples above 5 MPa (See Fig. 1(a) and 1(b)), was not observed.

¹²⁹Xe NMR spectrum for pellet samples

Figs. 6(a)–(d) show the pressure dependence of ¹²⁹Xe NMR spectra of xenon adsorbed in the pellet samples of molecular sieve 5A and 13X.

In molecular sieve 5A, the pressure dependence of ¹²⁹Xe NMR spectra is similar in both the hydrated (Fig. 6(a)) and dehydrated (Fig. 6(b)) pellet samples.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

The extra resonance peak, which appears above 4 MPa in the dehydrated powder sample in Fig. 1(a), was also observed in the pellet dehydrated sample.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

It is considered that the existence of some additional porous structure such as micro-, meso- and macropores, which is caused by binder agents, is not clear in molecular sieve 5A.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

On the other hand, in molecular sieve 13X, the resonance peak from xenon in the micropore is split into two peaks in the pellet sample.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

It is considered that there are two types of micropores in the pellet samples.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

The intensity of these peaks depends on the presence of adsorbed water.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

In the partially hydrated sample (Fig. 6(c)), the peak in the higher field is dominant, whereas in the dehydrated sample (Fig. 6(d)), the intensity of these peaks is reversed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

This suggests that micropores corresponding to these two peaks have different affinity for water adsorption, that is, the micropore with a resonance peak in the lowest field can adsorb water easier than that with a resonance peak in the higher field.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

As pressure increases, the line width of both peaks broadens, but these peaks did not coalesce.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

Discussion

Pressure dependence of ¹²⁹Xe chemical shift

The pressure dependence of the ¹²⁹Xe chemical shift of xenon confined to micropores is similar to the xenon adsorption isotherm classified in type-I by IUPAC,^44,45 suggesting that the adsorption of xenon is accelerated in low pressure regions due to the deep potential, which is emphasized by the micropore field.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

When the contribution of local magnetic fields in the pore to the chemical shift is quite small and there are no strong adsorption sites for xenon, the ¹²⁹Xe chemical shift for xenon adsorbed in the pore is described byδ = δ_S + δ_Xe + δ_Ewhere δ_S is the interaction between xenon and the wall of the pore, and δ_Xe is the contribution of Xe–Xe interaction.^10,52

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The second term of the right hand side of eqn. (1) is assumed to be proportional to the density of xenon in the pore, and is approximated to be δ_Xe = δ_Xe–Xeρ.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Therefore, the plateau observed on the pressure dependence of ¹²⁹Xe chemical shift in high-pressure regions indicates that the adsorption of xenon in the micropores of molecular sieve 5A and 13X has reached saturation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

The third term is contribution of local electric field caused by charge center such as Ca²⁺.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

This term will contribute the chemical shift in molecular sieve 5A containing Ca²⁺.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Although the adsorption amount of xenon in molecular sieve 5A and 13X in high-pressure regions up to 10 MPa enables the discussion of the density dependence of the chemical shift, the xenon adsorption isotherm in these systems is unknown.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

The appropriate assumption of the adsorption of xenon in high-pressure regions leads to the pressure dependence of xenon density in micropores.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

In this study, this is assumed by the Langmuir equation, because the physisorption due to micropore filling (type-I) can be qualitatively approximated by the Langmuir equation.^53,54

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

The surface coverage of xenon in micropores, θ, is defined by the density ratio at equilibrium pressure to the density when the adsorption is saturated, ρ(P)/ρ(∞).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

In Langmuir-type adsorption behavior, the coverage is given bywhere K is the ratio of the rate constant for adsorption to desorption.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

Furthermore, the change in the chemical shift value between 0 MPa and infinite pressure is represented by Δδ_Xe–Xe = δ(∞) − δ_S.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

Using these relationships, the ¹²⁹Xe chemical shift at pressure, P, is represented by

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

We can discuss the adsorption behavior of xenon using the parameters, δ_S, Δδ_Xe–Xe, and K, obtained from the pressure dependence of ¹²⁹Xe chemical shift value.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

In particular, parameters such as Δδ_Xe–Xe and K, which relate to xenon–xenon interaction in the adsorption process of xenon, are given as high-pressure ¹²⁹Xe NMR measurements, in addition to δ_S which can be determined from conventional ¹²⁹Xe NMR measurements covering only the low-equilibrium pressure range.^10–15

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

The analysis of the pressure dependence of ¹²⁹Xe chemical shift using eqn. (3) leads to three kinds of parameters for each sample as listed in Table 2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

In NaY zeolite, the analysis was carried out using chemical shift data below 0.75 MPa.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

Pore size from δ_S

δ_S reflects the interaction between xenon and pore walls.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

Fraissard et al. assumed that δ_S is given by the average value between the chemical shifts of xenon on the wall, δ_a, and in free space, δ_V, for de-aluminated zeolites and zeolites containing only Na⁺ as a cation as follows:⁵⁵where δ_V depends on the volume of free space in the pores.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

On the basis of this assumption,⁵⁶ there is a relation between δ_S and the mean free path, l, of xenon in the pore as represented byThis equation suggests a linear relation between the reciprocal value of δ_S and the pore size.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

Assuming the shape of the pore as a sphere and cylinder, the mean free path of xenon is given by l = (D_S − D_Xe)/2 and l = (D_C − D_Xe), respectively, using the pore diameter, D_S and D_C for spherical and cylindrical pores and the van der Waals diameter of xenon, D_Xe.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

According to these equations, the following relations are derived between δ_S and D_C and/or D_S:⁵⁶

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

These equations have previously been proposed to explain the relation between the chemical shift and the free space in micropores.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

100

Although it has recently been attempted to elucidate the chemical shift of xenon in the pores using ab initio MO calculations on the basis of xenon–wall as well as xenon–xenon interactions,⁵⁷ it is convenient to describe the relation between δ_s and the pore size of zeolites.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

101

Using these equations, we can evaluate the pore diameter of molecular sieve 5A and 13X for hydrated and dehydrated samples, and dehydrated zeolite NaY as listed in Table 2.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

102

In molecular sieve 5A, δ_E also contributes to δ at zero pressure.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

103

Although the effect of δ_E on δ will leads to overestimation of δ_S, δ at zero pressure, which corresponds to δ_S + δ_E, enables us to roughly estimate the pore diameter.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

104

The pore diameter evaluated from the crystal structure is 1.14 nm for molecular sieve 5A and 1.37 nm for molecular sieve 13X and zeolite NaY.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

105

Comparing the pore size from δ_S assuming the spherical and cylindrical shape to that one, the spherical and cylindrical pore model seems to be valid for molecular sieve 5A and zeolite NaY, respectively.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

106

For molecular sieve 13X, the pore size from the crystal structure is the intermediate value of the spherical and cylindrical pore model.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

107

This aspect may concern the mobility of adsorbed xenon in the pore.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

108

In molecular sieve 5A, xenon is strongly adsorbed in the super cage, resulting in the long residence time of xenon in the cage.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

109

Consequently, the adsorbed xenon assumes a spherical shape in the occupying space.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

110

As the rate of translational motion of xenon increases in the pore, the residence time of the adsorbed xenon in the super cage decreases and xenon translating from the super cage to others assumes a cylindrical shape in the occupying space.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

111

The effective shape of the pore describing δ_S implies that the mobility of xenon decreases in the order; NaY > 13X > 5A.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

Influence of adsorbed water on the xenon adsorption

112

The analysis of the pressure dependence of ¹²⁹Xe chemical shift in the hydrated sample for molecular sieves gives the parameters, δ_S, Δδ_Xe–Xe, and K as listed in Table 2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

113

In fact, the influence of the adsorbed water on the xenon chemical shift was already examined in NaY.⁶⁶

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

114

In NaY, both δ_wall and δ₁, which correspond to δ_S and δ_Xe in this paper, were affected by the amount of water.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

115

The effect of the presence of water on the xenon environments is different between the adsorption and desorption processes of water: Adsorption of water leads to inhomogeneous distribution of xenon environments, whereas desorption leads to uniform one.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

116

In molecular sieve 5A and 13X, xenon environment seems to be uniform because of the single and symmetric peak for confined xenon (See Fig. 4).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

117

This implies that the samples used in this study are in the partially dehydrated condition.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

118

The δ_S values in the hydrated samples are larger than the correspondence values for the dehydrated samples.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

119

Since the reciprocal value of δ_S is proportional to the pore size, which depends on the pore model, this aspect suggests that the pore size felt by adsorbed xenon decreases due to water adsorption in the hydrated samples.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

120

In molecular sieve 5A, the decreased pore diameter for the spherical pore shape is 0.30 nm, which corresponds to a 0.5 monolayer of water with 0.27 nm cross-section based on the van der Waals diameter of O- and H-atoms.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

121

On the other hand, in molecular sieve 13X, the decreased pore diameter is 0.95 nm for the spherical pore and 0.45 nm for the cylindrical pore and the average of these values is 0.70 nm, suggesting that about 1.3 molecular layers of water are formed on the wall, that is, the amount of adsorbed water in 13X is 2.6 times greater than that in 5A.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

122

This is consistent with the amount of water adsorbed in the pretreated samples determined by TG measurements, in which the amount of adsorbed water in 13X is 2.3 times greater than that in 5A; 41 wt.% for 5A and 100 wt.% for 13X.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

123

The δ_wall value in NaY changes from 58 ppm for completely dehydrated sample to 158 ppm for the sample containing 85% of the saturated water content.⁶⁶

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

124

This change is similar to that in molecular sieve 13X.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

125

The Δδ_Xe–Xe value is related to the local density of xenon in the pore when the micropore is saturated by xenon.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

126

In addition, at low density where the additivity of the ¹²⁹Xe chemical shift value is valid for the amount of xenon occupying the nearest neighbors, this value will approximately reflect the number of surrounding xenon atoms for a xenon atom.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

127

According to the discussion on the additivity of the ¹²⁹Xe chemical shift to gaseous xenon adsorbed in the α-cage of zeolite A by Jameson et al.,⁵⁸ the additivity is valid for the cluster sizes up to Xe₆ and the shift value increases to about 22 ppm on average by the addition of one xenon atom.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

128

For Xe₇ and Xe₈ clusters, the chemical shift increases by about 45 ppm and 44 ppm when a xenon atom is added to the cluster, because the increase in cluster size leads to a large contribution of the chemical shielding at a shorter interatomic distance.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

129

On the basis of this discussion, the Δδ_Xe–Xe value for a dehydrated sample for 5A and 13X suggests the formation of a Xe_n cluster with an average size of n = 7, whereas for the hydrated sample, the decrease in Δδ_Xe–Xe value leads to the average size of cluster Xe₆ for 5A and of Xe₃ for 13X.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

130

In addition, the hydration also decreases in K value, implying that the hydrophobic interaction between xenon and the wall consisting of SiO₂ and/or AlO₂ is more efficient than the xenon–water interaction.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

Pore structure in pellet samples

131

Figs. 6(c) and 6(d) suggest that there is a space larger than the original micropore of the zeolites in molecular sieve 13X.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

132

This may be the effect of the binder agents used in preparing the pellets, that is, relatively large micropores and/or mesopores are formed in the interspace between microcrystallites.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

133

This effect is clear in molecular sieve 13X.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

134

Above 0.1 MPa, the extra resonance line appears at a chemical shift 15 ppm smaller than the chemical shift corresponding to xenon in the original micropore.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

135

Since the potential well for adsorption due to the pores is inversely proportional to the pore size,^54,59 the larger the micropore diameter, the smaller the chemical shift value.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

136

Therefore, the extra peak will correspond to xenon adsorbed in the micropores and/or mesopores formed by the process of fine particle binding.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

137

In addition, the broadening of the line width of the peak for xenon in larger sized pores than the original, indicates the exchange of xenon between these pores, but these peaks did not coalesce.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

138

This implies that the exchange of xenon between these sites is very slow compared with the frequency separation of about 800 Hz (∼15 ppm).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

139

For xenon adsorbed in the extra pores, the spectrum depends on the hydration condition of the sample.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

140

In the hydrated sample, the intensity of the peak from xenon in the original micropore is smaller than that in the extra-large pores.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

141

In contrast, in the dehydrated sample, the intensities of these peaks are reversed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

142

This aspect suggests that the hydration water is adsorbed in the original micropore formed by the zeolite frameworks more efficiently than the extra-large pores formed in the intercrystallites.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

143

This is caused by the deeper potential depth in the original micropore of 13X framework formed by the pore wall.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

144

For molecular sieve 5A, this effect cannot be observed.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

Chemical exchange of xenon in NaY zeolite

145

In NaY, the pressure dependence of the resonance line with line broadening is related to the chemical exchange of xenon between the outside and inside of pores.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

146

The change in the chemical shift at pressures above 1 MPa in Fig. 2 is quite different from those for molecular sieve 5A and 13X.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

147

As an initial approximation, the pressure dependence of the chemical shift in pressure regions from 0.02 to 0.75 MPa is analyzed by the Langmuir adsorption model, because in this region, the pressure dependence is similar to those for the other two samples.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

148

The analysis leads to the following parameters: δ_S = 52 ppm, Δδ_Xe–Xe = 121 ppm, and K = 6.0 MPa⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

149

In the pressure region higher than 0.75 MPa, the Langmuir model cannot explain the pressure dependence of the chemical shift.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

150

In contrast, part of the xenon gas co-existing with adsorbed xenon in the pore gives rise to a resonance line which coincides with the resonance for pure bulk xenon gas under the same conditions, implying that the effect of the chemical exchange of xenon is rarely affected in the observed part as free xenon gas in the spectrum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

151

When the chemical exchange of xenon between the outside and inside of the pore occurs with the exchange rate faster than the frequency separation between two resonance peaks in relation to the exchange, a resultant resonance peak appears at a frequency given by the weighted average of these two resonance frequencies.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

152

The line width of the averaged resonance line is efficiently affected by the exchange rate and the population of xenon in both sites.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

153

Thus, the temperature dependence of the spectrum in Fig. 3 strongly suggests the existence of the chemical exchange of xenon.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

154

Using the simple two-site exchange model,⁶⁰ we then analyzed the pressure dependence of the spectrum to evaluate the exchange rate for xenon between the outside and inside of the pores.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

155

Two sites for xenon, bulk-xenon gas and adsorbed xenon in the pore, are denoted by A and B, respectively, where the population of each site is p_A and p_B and the exchange rate constant from A to B site is k_A and that from B to A site is k_B.