Insights on the Molecular Mechanisms of Hydrogen Adsorption in

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Insights on the Molecular Mechanisms of Hydrogen Adsorption in Zeolites Kathryn Suzanne Deeg, Juan Jose Gutierrez-Sevillano, Rocío BuenoPérez, Jose B. Parra, Conchi O. Ania, Manuel Doblare, and Sofia Calero J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4037233 • Publication Date (Web): 19 Jun 2013 Downloaded from http://pubs.acs.org on June 24, 2013

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The Journal of Physical Chemistry

50x50mm (150 x 150 DPI)

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Insights on the Molecular Mechanisms of Hydrogen Adsorption in Zeolites

Kathryn S. Deeg1, Juan José Gutiérrez-Sevillano1, Rocío Bueno-Pérez1, José B. Parra2, Conchi O. Ania2, Manuel Doblaré3, Sofía Calero1*

1

Department of Physical, Chemical, and Natural Systems. University Pablo de Olavide. Ctra. Utrera km. 1. 41013 Seville, Spain. 2

3

CSIC). P.O. 73, 33080 Oviedo, Spain.

Abengoa Research, Campus Palmas Altas, Energía Solar, 1. (Palmas Altas) 41014 Seville, Spain.

*

E-mail: [email protected]; Phone: +34 954 977 594

1 ACS Paragon Plus Environment

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Abstract Physisorption on microporous materials has emerged as a promising strategy to store hydrogen for use as an energy carrier. Here, we investigate adsorption of hydrogen in allsilica zeolites ITQ-29 and MFI at low temperatures using molecular simulations. Out of five existing models for hydrogen and its interactions with the zeolite, we determine that a model with hydrogen as a single uncharged Lennard-Jones center best reproduces experimental adsorption isotherms. We present a new set of Lennard-Jones parameters for this model and find that small variations in the parameters have a large impact on computed hydrogen adsorption at 77 K. Preferential adsorption sites of hydrogen in the two zeolite structures are analyzed. We investigate the effect of incorporating quantum corrections via use of a Feynman-Hibbs effective interaction potential and determine that quantum corrections are important at 25 K.

Keywords Monte Carlo simulation, Feynman-Hibbs effective interaction potential, ITQ-29, MFI, hydrogen storage



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1. Introduction

Efficient storage of hydrogen remains a major challenge in its widespread use as an energy carrier.1-5 A promising strategy for safe and efficient storage is molecular adsorption of hydrogen on microporous materials, with many recent investigations on zeolites, metalorganic frameworks (MOFs), and carbon-based structures.6-10 Such materials exhibit fast, reversible hydrogen uptake and could be used to create simple and safe storage systems.10,11 One of the most prominent and interesting class of materials studied is zeolites. Maximum hydrogen adsorption capacities in zeolites have been calculated at around 2.9 wt.%12 and observed in the range of 1.2 to 2.2 wt.%13-15 at 77 K and moderate pressure – conditions under which physical sorbents demonstrate appreciable hydrogen loading. Although still well below practical targets for mobile applications16, these capacities are significant and deserving of further study. Furthermore, zeolites are advantageous over other sorbent materials due to their low cost, high thermal stability and bulk density, adjustable composition, and possession of highly polarizing sites caused by nonframework cations.12,17,18 Moreover, zeolites’ crystallinity and the large variety of available, wellknown structures and compositions can make them model systems in which to carry out fundamental investigations on the interaction of hydrogen with storage materials. These factors have encouraged many studies19-21 on zeolites, with the aim of improving storage capacity and increasing operational temperature. Both experimental and simulation studies have demonstrated that hydrogen adsorption capacity in zeolites depends on the framework’s topology, composition, micropore volume, specific surface area, and channel diameter, as well as the type of nonframework cations.12,13,22-26 Adsorption capacity is


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maximized in structures that exhibit a proper balance between these factors.15,27-30 From these studies it is clear that designing and improving zeolites as storage materials require a fundamental understanding of the adsorption behavior of hydrogen in adsorbents. Of experimental studies reported so far, most focus on the storage capacity of highly porous materials at room temperature and at or above atmospheric pressures. Very few experimental studies have focused on low temperature adsorption behavior, sites available for adsorption, and the nature of the adsorption of molecular hydrogen on zeolites. In light of these facts, we present here a low-temperature study of hydrogen adsorption in all-silica zeolites ITQ-29 and MFI using molecular simulations. We have assessed five different models for hydrogen and its interactions with the zeolite, and we present adjusted parameters for one of the models. We discuss the effect of topology on hydrogen adsorption, identify preferential adsorption sites, and discuss heats of adsorption. At the cryogenic temperatures of our study, it has been shown that the quantum behavior of hydrogen is non-negligible in the nanoscale confinement of zeolite pores and other nanostructured materials.31-33 We therefore examine the effect of incorporating quantum corrections via use of an effective potential derived from the Feynman-Hibbs variational approach34, one method that has been employed31,32,35-38 to model hydrogen’s quantum behavior in porous materials. In this approach, an effective interaction potential is obtained from the Feynman-Hibbs variational estimate of the quantum partition function of an assembly of particles with positions described by Gaussian distributions. We discuss the conditions under which incorporation of quantum effects is necessary.



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2. Simulation Methods and Models

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Figure 1. Schematic pictures showing the pore space of the zeolite structures studied: allsilica a) ITQ-29 and b) MFI. Each image displays 2×2×2 unit cells.



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High-silica MFI undergoes a reversible phase transition at around 340 K from a monoclinic to an orthorhombic structure.41 W f m w h h p g

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Table 1. Lennard-Jones parameters of all hydrogen-zeolite models assessed and the parameters presented in this worka H2 – H2

H2 – Ozeo

H2 – Sizeo

ɛ/kB (K)

σ (Å)

ɛ/kB (K)

σ (Å)

ɛ/kB (K)

σ (Å)

M1

38.70

2.782

56.67

3.03

30.23

3.42

M2

36.733

2.958

61.162

2.8330

26.163

1.8175

M3

34.02

2.96

51.233

2.62

–

–

M4

36.5

2.82

40.0

3.83

–

–

Same as M4 M5 This Same as M2 66.055 2.890 28.256 1.854 work a Note that the mixed parameters in model M1 were calculated using the mixing rules specified by Rahmati et al. The M2 ɛ/kB parameters were converted from eV.

Table 2. Partial charges in the charged hydrogen-zeolite models H2 center of mass -0.9658

H atoms in H2 +0.4829

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Same as M4

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Ozeo

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

h M

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M5.

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M5

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gh . All-silica MFI and ITQ-29 (pure-silica LTA structure) were kindly supplied by

ITQ (CSIC) and both correspond to a pure porous crystalline silicon dioxide (Si/


≈ ∞).

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U

p

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99.99996%) w

p

h

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M á

.

3. Results and Discussion We assessed the existing hydrogen/zeolite models by computing hydrogen adsorption isotherms in ITQ-29 and monoclinic MFI at 77 K and comparing to experimental data (Figure 2).

Figure 2. Hydrogen adsorption isotherms at 77 K in all-silica a) ITQ-29 and b) MFI. The figures provide a comparison of the adsorption calculated using models M1 (squares), M2 (triangles), M3 (diamonds), M4 (×), and M5 (+) with our experimental data (empty circles) 10 ACS Paragon Plus Environment


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as well as the adsorption calculated using the parameters presented in this work (filled circles). The insets show loading at low pressures, with the same axes units as in the main graphs.

Models M1 and M3 over- and underestimate adsorption, respectively. The relative difference between computed and experimental adsorption is greater at lower pressures. The two charged models M4 and M5 overestimate adsorption and reproduce poorly the shapes of the isotherms, conceivably due to the inclusion of charges or inadequacy of the model in describing electrostatic interactions. Model M2 best reproduces experimental adsorption values and shapes of the isotherms, indicating that modeling hydrogen simply as a single Lennard-Jones center can adequately describe hydrogen’s interactions. However, since this model gives adsorption values that are slightly low at higher pressures, we adjusted the hydrogen-zeolite Lennard-Jones parameters in order to better reproduce the experimental data. Model M2 does not incorporate quantum effects, but studies have shown that at cryogenic temperatures, the quantum behavior of hydrogen is non-negligible.31-33 Thus before adjusting the parameters, we first examined the effect of incorporating quantum effects into the M2 model via the Feynman-Hibbs potential by calculating the saturation capacity of ITQ-29 at 25 K. The experimental saturation capacity is 13.4 mol/kg. M2 gives a saturation capacity of 19.4 mol/kg. However, incorporating the Feynman-Hibbs potential into M2 results in the more accurate value of 14.4 mol/kg. We therefore decided to use the


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Feynman-Hibbs potential while adjusting the M2 parameters. The Feynman-Hibbs potential was used in all subsequent calculations, unless otherwise noted. We adjusted the hydrogen-zeolite Lennard-Jones parameters of model M2 with respect to our experimental hydrogen adsorption isotherm in ITQ-29 at 77 K. The new parameters are given in Table 1. Very small changes in the parameters were enough to correct the adsorption, suggesting that hydrogen-zeolite dispersive interactions dominate adsorption behavior at low temperatures. Figure 2a shows the isotherm obtained with these adjusted parameters. As a further validation of the adjusted parameters, the saturation capacity of ITQ-29 at 25 K computed using the adjusted parameters is 14.5 kg/mol, in good agreement with the experimental value. Figure 2b compares to experimental data the adsorption isotherm of hydrogen in MFI obtained using the adjusted parameters. Although adsorption results are improved over the original model, the computed adsorption deviates slightly from the experimental isotherm. This demonstrates that the structure of the zeolite – cages in ITQ-29 versus intersecting channels in MFI – impacts the adsorption behavior of small molecules such as hydrogen. We further validated the adjusted parameters by reproducing our experimental isotherms in both structures at different temperatures (Figure 3). The good agreement with experiment shows that the parameters are transferable to MFI and to different temperatures. Unfortunately there are no available experimental data for these all-silica MFI and LTA structures, apart from the experiments carried out in this work, with which to validate our results. Figure 3 also shows that at 77 K and at pressures above 25 kPa, the loading of hydrogen is higher in ITQ-29 than in MFI.



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Figure 3. Comparison of computed and experimental isotherms (filled and empty symbols, respectively) obtained in ITQ-29 at 77 K (circles) and 120 K (squares) and in MFI at 77 K (triangles) and 90 K (diamonds). The insets show loading at low pressures, with the same axes units as in the main graph.

To investigate in greater depth the effect of zeolite topology on hydrogen adsorption, we computed isosteric heats of adsorption at different temperatures in both structures (Figure 4). For comparison, the figure includes data for both the monoclinic and orthorhombic forms of MFI; up to 300 K the results for the monoclinic structure should be more realistic, while at 350 K and above the results for the orthorhombic structure should be more realistic.


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Figure 4. Isosteric heats of adsorption of hydrogen in ITQ-29 – with unblocked (circles) and blocked (diamonds) sodalite cages; monoclinic MFI (up-pointing triangles); and orthorhombic MFI (down-triangles). Heats of adsorption were computed with and without the Feynman-Hibbs potential (solid and empty symbols, respectively).

The heats of adsorption in ITQ-29 and MFI zeolites have different temperature dependence patterns in the 10 K–100 K range (Figure 4), demonstrating that zeolite structure and the size of the pores affect adsorption especially at low temperatures. In particular, adsorption in the sodalite cages of ITQ-29 causes the unusual decrease in heat of adsorption up to 200 K. At very low temperatures, the adsorption sites for hydrogen are limited to the sodalite cages. Increased temperature leads to higher mobility for hydrogen molecules (see Figure S1 of the Supporting Information). This reduces the affinity of the molecule for the sodalite cages, thereby decreasing the heat of adsorption. To confirm that the sodalite cages are the cause of this pattern, we computed heats of adsorption as a function of temperature for ITQ-29 with the sodalite cages blocked. Hard



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spheres placed in the sodalite cages prevented adsorption of hydrogen in these regions. As shown in Figure 4, blocking the cages eliminates the unusual pattern observed in the nonblocked ITQ-29. When the sodalite cages are blocked, at all temperatures hydrogen adsorbs only outside of the cages, so heat of adsorption does not decrease in the 10 K–200 K range. Radial distribution functions of hydrogen-oxygen distances in blocked and unblocked ITQ29 further corroborate the existence of different adsorption sites – namely, the sodalite cages and outside the sodalite cages (Figure S1 of the Supporting Information). Further confirmation of the effect of the sodalite cages is provided by average occupation density profiles in ITQ-29 with and without blocked sodalite cages (Figure 5). At 10 K – the lowest temperature analyzed – hydrogen adsorbs preferentially in the sodalite cages. At higher temperatures, the sodalite cages fill, and hydrogen adsorbs outside the sodalite cages as well. By comparing the heats of adsorption in ITQ-29 with and without blocked sodalite cages, we see that in the case of the ITQ-29 structure, the availability of small pockets for hydrogen adsorption increases the heat of adsorption, especially at low temperatures.


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Figure 5. Average occupational density profiles of hydrogen in ITQ-29 with unblocked and blocked sodalite cages (left and right columns, respectively) at 10 K, 50 K, and 100 K (top, middle, and bottom rows). The profiles are projected on the x-y plane. The relationship between color and occupational density is shown by the bar located on the right side of the figure. The same color gradient (from black to red) is employed in each occupational profile. There is one molecule in the simulation box in each calculation.



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We computed isosteric heats of adsorption also to further examine the effect of incorporating quantum effects via the Feynman-Hibbs potential (Figure 4). For both structures, use of the Feynman-Hibbs potential decreases heats of adsorption, with this effect being greater at lower temperatures and nearly negligible at higher temperatures. This explains the sensitivity of the calculated saturation capacity of ITQ-29 at 25 K to introduction of the Feynman-Hibbs potential. The difference between heats of adsorption calculated with and without the Feynman-Hibbs potential is greater in MFI than in ITQ-29, suggesting that quantum effects are greater in MFI. Although introduction of the Feynman-Hibbs potential has a large influence on heats of adsorption in MFI, the preferential sites are not affected. Average occupational density profiles computed with the Feynman-Hibbs potential, displayed in Figure 6, show that at low temperatures, the preferential sites are in the sinusoidal channels, whereas at higher temperatures, hydrogen adsorbs throughout both the longitudinal and sinusoidal channels. Occupational profiles computed without the Feynman-Hibbs potential, compiled in Figure S2 of the Supporting Information, show a similar pattern of adsorption sites.


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Figure 6. Average occupational density profiles of hydrogen in MFI at 10 K (top left), 50 K (top right), 100 K (bottom left), and 300 K (bottom right). The profiles are projected on the x-y plane. The relationship between color and occupational density is shown by the bar located on the right side of the figure. The same color gradient (from black to red) is employed in each occupational profile. There is one molecule in the simulation box in each calculation.

Figure 7 compares computed adsorption isotherms of hydrogen in ITQ-29 at 25 K to our experimental isotherm, the first reported at this temperature. It is interesting to note that simulations using our adjusted hydrogen-zeolite parameters estimate the saturation capacity



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well, but overestimate adsorption at low pressures. The original M2 parameters give similar loading results at higher pressures but show better agreement with experiment at lower pressures. These hydrogen-zeolite parameters were obtained via Lorentz-Berthelot mixing rules26,47, and the zeolite atom self Lennard-Jones parameters used were based on fitting to experimental data at 298 K.48 In contrast, the parameters presented in this work were fitted to experimental data at 77 K, where the nature of hydrogen’s interactions is different from that at room temperature. Thus it follows that the original M2 parameters give more accurate results at low pressures, where hydrogen loading is lower.

Figure 7. Comparison of computed and experimental adsorption (unconnected and connected symbols, respectively) in ITQ-29 at 25 K. Isotherms were computed using the adjusted and the original M2 parameters (circles and squares, respectively) with and without the Feynman-Hibbs potential (filled and empty symbols, respectively).

As seen in Figure 7, for both sets of parameters, use of the Feynman-Hibbs potential lowers adsorption, as found in previous studies.31,37,49,50 The resulting improvement in agreement


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with experiment demonstrates the importance of modeling quantum effects at 25 K in this structure. We further compared the adjusted and original parameters by computing heats of adsorption as a function of temperature using both sets of parameters (Figure 8). For both structures, heats of adsorption computed using the adjusted parameters are greater than those computed using the original parameters, with this difference being greater at lower temperatures. Based on the performance of both sets of parameters in reproducing experimental isotherms (Figures 2, 7), we conjecture that at temperatures of 77 K and above, the adjusted parameters result in more accurate values of heats of adsorption, while the original parameters perform better at very low temperatures.

Figure 8. Isosteric heats of adsorption of hydrogen in ITQ-29 (circles, +), monoclinic MFI (up-pointing triangles, ×), and orthorhombic MFI (down-triangles, ), calculated using the adjusted (filled symbols) and original (+, ×, ) M2 parameters.



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Insights on the Molecular Mechanisms of Hydrogen Adsorption in

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