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Mar 21, 2018 - (35) Perez, C. A. C.; de Resende, N. S.; Salim, V. M. M.; Schmal, M. Water Interaction in Faujasite Probed by in Situ X-ray Powder. Dif...
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Structure and Thermal Stability of (H2O)4 Tetrahedron and (H2O)6 Hexagon Adsorbed on NaY Zeolite Studied by Synchrotron-Based Time-Resolved X‑ray Diffraction Maocong Hu,† Jonathan C. Hanson,‡ and Xianqin Wang*,† †

Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102, United States ‡ Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: Water adsorption and desorption features play an important role in determining the adsorptive and catalytic properties of zeolites. In this study, the extraframework structure (water and cations) in NaY faujasite zeolite was studied by synchrotron-based in situ time-resolved X-ray diffraction (TR-XRD) in combination with the Rietveld refinement method. It was observed that Na cations locating at different sites migrated between each other during the dehydration process. Water molecules at three separate sites showed interesting geometries: (H2O)4 tetrahedron W1, (H2O)6 ice-like hexagonal W2, and disordered triangle W3. Interactions between each form of water and Na cations were deeply investigated. The stability of the waters followed the sequence of W1 > W3 > W2. This work provides insights into both framework structure and cation positions of model zeolite under actual working conditions, which helps fully understand the properties of zeolites.

1. INTRODUCTION Zeolites possess unique features as gas adsorbents, gas separators, chemical sensors, and catalysts.1−4 The positions of the extraframework cations in cation-exchanged zeolitic materials play a critical role for their adsorption, separation, dielectric, and catalytic properties.5,6 The positions of the cations have been found to be closely dependent on the temperature and guest polar molecules.7,8 Water is an important guest molecule in the pore system of natural and synthetic zeolites. Both adsorptive and catalytic properties of zeolites are strongly determined by their water content. Therefore, the study of the adsorption and desorption of water on different types of zeolite has been scrutinized.9−11 Several factors have been reported to play important roles in the water−host interaction, such as polar interactions, steric hindrance, and sample preparation methods. Calorimetric measurements of the water adsorption process have been shown to occur by a three-step mechanism: adsorption on cations, formation of a monolayer on the pore walls, and multiplayer adsorption in the cavities.12 The X-ray diffraction technique is a common tool to investigate the cation distribution and the interaction between host, cations, and guest molecules. Rietveld refinement in combination with difference electron density analysis is a useful approach to this problem.12 Commonly, such studies are made at room temperature from predehydrated and sealed samples. However, with this method only a limited set of snapshots is obtained, and cooling the samples to room temperature may obscure the true dehydration process.13 In order to fully © XXXX American Chemical Society

understand physical properties of the material, it is necessary to determine both the framework structure and the cation positions under actual working conditions (i.e., in situ measurement) of thermal dehydration. Faujasite type (FAU) zeolites have been widely used as catalysts and for gas separation owing to their acidity and selective adsorption properties, respectively. The purpose of this work is to investigate the water (behavior) in a NaY faujasite zeolite with a broad range of water content by the in situ time-resolved XRD method. The cation and water positions were determined at a series of temperatures during the thermal dehydration. The structure of FAU zeolite Y was adopted from the widely accepted model.14,15 The cation positions employed the nomenclature of Smith16 and are normally recognized to locate at some specific sites, such as SI, SI′, SII, SII′, and SIII sites.15

2. EXPERIMENTAL AND DATA REFINEMENT NaY FAU zeolite with a Si/Al ∼ 2.6 was obtained from Zeolyst International Co. (CBV 100 CY 1.6). The dehydration process and time-resolved X-ray diffraction (TR-XRD) experiments were carried out at the beamline X7B at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. A small amount of sample (∼0.005 g) was heated Received: Revised: Accepted: Published: A

January 30, 2018 March 16, 2018 March 21, 2018 March 21, 2018 DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. TR-XRD patterns of temperature-programmed thermal dehydration of NaY (temperature increase rate (β) = 3 °C/min). Inset: intensities of (111) diffraction line as a function of temperature.

Table 1. Fractional Coordinates, Occupancies, Isothermal Parameters (Å2), and Unit Cell Parameters (Å) Refined from the Data Collected at 25 °Ca T O1 O2 O3 O4 NaI NaII NaI′ W1 W2 W3 NaIII′ NaII′ a

X

Y

Z

occup

unio

mul

−0.05341(8) 0 −0.00177(16) 0.17498(16) 0.17825(21) 0 0.24186(31) 0.06877(26) 0.8310(4) 0.5772(6) 0.3021(11) 0.4157(5) 0.8310(4)

0.03570(8) −0.10798(16) −0.00177(16) 0.17498(16) 0.17825(21) 0 0.00814(31) 0.06877(26) −0.0811(4) −0.0372(5) 0.3020(11) 0.4157(5) −0.0811(4)

0.12494(10) 0.10798(16) 0.14191(23) −0.02855(24) 0.32111(23) 0 0.00814(31) 0.06877(26) −0.0811(4) −0.0372(5) 0.2668(24) 0.1991(7) −0.0811(4)

1 1 1 1 1 0 0.448(9) 0.636(10) 1 1 0.270(11) 0.204(7) 0

0.0136(4) 0.0199(7) 0.0199(7) 0.0199(7) 0.0199(7) 0.0358(33) 0.0358(33) 0.0358(33) 0.217(5) 0.435(9) 0.326(7) 0.0358(33) 0.217(5)

192 96 96 96 96 16 32 32 32 96 96 96 32

The agreement factors were χ2 = ∑w(Io − Ic)2/(Nobs − Nparm) = 0.57, R(F2) = 6.02%.

from 25 to 350 °C while being evacuated with mechanical pump (pressure ∼ 1 mTorr). The detail experimental setup has been described elsewhere.17−19 The powder rings were integrated with the FIT2D code. The FIT2D parameters for the integration of the data were obtained from a standard LaB6 crystal compound. A two-dimensional (2D) chart document was obtained with a Fortran CHITOUXDS code, and the timeresolved XRD pattern files were plotted with an IDL code. Rietveld refinements were performed with the program GSAS with the EXPGUI graphical interface. Atomic coordinates for the framework of dehydrated Na−Y obtained from previous refinements were used as the starting model. The T−O distances and angles were constrained. The background curve was fitted with a Chebyshev polynomial with 30

coefficients. The diffraction peak shape was modeled with a pseudo-Voigt function with three Gaussian and two Lorentzian line-broadening terms. One scale factor and the unit-cell parameters were allowed to vary for all patterns. Occupancy factors and isotropic displacement coefficients of the extraframework species and the thermal parameters were refined simultaneously. However, the thermal parameters were constrained for all the atoms with the same structural role. For instance, the thermal parameters for the framework oxygen atoms were constrained to be the same and as were the thermal parameters for all the extraframework cations. The thermal parameters for water molecules were constrained with a relative ratio, which was chosen based on the relative stability of the different molecules obtained from preliminary refineB

DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research ments. A special treatment was required for water in sodalite cage (W1) and Na(II′) which are nearly overlapping. The atom at this position was refined as an oxygen, but part of its electrons were allocated to site II′ so that the unit cell contents of sodium could remain constant. Effectively this resulted in Na in site II′ for a narrow temperature range as will be seen in the Discussion section. The pictures were generated with the Materials Studio program from Accelyrs. Refinements were preformed every 8 deg up to 145 °C where the structure changes became small. Then five more refinements were performed with larger temperature intervals up to 341 °C.

3. RESULTS AND DISCUSSION A typical TR-XRD pattern for NaY FAU zeolite dehydration process is shown in Figure 1. The relative intensities of peaks in XRD patterns changed during the dehydration process; this feature of zeolite results from the changes of extraframework water molecules and cations.15 The inset in Figure 1 presents the intensity of peak (111) as a function of temperature. The XRD intensities increased with the removal of water, reached a maximum at the temperature of 100 °C, and then decreased slightly with the increase of temperature owing to the beam intensity decay (also supported by the expanded 111 peak area in Figure S1). Details of the Rietveld refinements and the full list of refined parameters at 25 °C are listed in Table 1. The fitted pattern, experimental pattern, and their difference at 25 °C are presented in Figure 2 and indicate that a good fit was obtained.

Figure 3. Water molecules and Na positions in unit cell of zeolite including a supercage, a sodalite cage, and a double 6-ring. The coordinates are from the Rietveld refinement of the data at room temperature. The Na cations and water at different sites are represented in different colors.

several disordered positions can be occupied. Each type of Na cation and water molecule is represented with different colors. The water molecules at three separate sites interacted with framework differently and showed interesting geometries: tetrahedral water cluster (H2O)4 in sodalite cage (the geometry is close to that observed in the sodalite cage of NaX20 and CsY15), hexagonal ice-like water cluster (H2O)6 in the 12-ring between supercages, and a 3-fold disordered atom above NaII in the supercage, named as W1, W2, and W3, respectively. During the dehydration process, the coordinates and occupancies of the atoms changed, and thus the geometries and bond distances for each type of water and Na cations varied. In order to better understand the water behavior during dehydration, each type of water is shown separately in subsequent figures. Figure 4 shows the (H2O)6 hexagons in 12-rings of the supercage at temperatures of 25, 68, and 102 °C viewed from three different directions (top, tilted, and side directions). The W2−W2 and W2−O4(framework) distances are listed in Table 2. At 25 °C, the W2 molecules were fully occupied and had no interaction with Na cations closer than 3.45 Å (with NaIII′) and were fully stabilized by hydrogen bonding with each other and the zeolite oxygen atoms. By comparison of the pictures viewed from top at three different temperatures, the diameter of 6-ring is seen to increase with the increase of temperature. The pictures viewed from side directions showed that the 6-ring was distorted from flat surface to a chairlike geometry with the temperature increase. At room temperature, there was a strongly cooperative ring of water hydrogen bonds that became weaker when the occupancy dropped, and the cooperative interaction broke down. When fully occupied, there was always an adjacent oxygen atom to stabilize the hydrogen bond between W2 molecules. The room temperature ring was planar rather than the relaxed chair structure because the combined zeolite−W2 and W2−W2 interactions defined the geometry.

Figure 2. Observed, calculated, and difference powder diffraction profiles after Rietveld refinement of the data collected at 25 °C.

Na cations were found at positions of I, I′, II, II′, and III′ sites and are named as NaI, NaI′, NaII, NaII′, and NaIII′, respectively. Water molecules were observed at three different locations. The refinements were successful based on the fact that both R(F2) and χ2 parameters were very small (the values of χ2 for all the refinements were less than 1.0). The highest R(F2) factor was 6.02% for the data at 25 °C where there are disordered water molecules which cannot be modeled. In order to understand the interaction among water molecules, Na cations, and the framework, a picture was generated by plotting the extraframework atoms from Rietveld refinement in faujasite structure with the Accelyrs program. Figure 3 shows a part of the unit cell of zeolite including a supercage, a sodalite cage, and a double 6-ring. The atoms plotted in Figure 3 are from the data at room temperature refinement. As can be seen from Table 1, the extraframework atoms are not fully occupied, and in some cases only one of C

DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. (H2O)6 hexagon in 12-rings at temperatures of 25, 68, and 102 °C viewed from three different directions (top, tilted, and side directions). The coordinates are from the Rietveld refinement of the data at room temperature. The Na cations and water at different sites are represented in different colors.

Figure 5. A sodalite cage, a double 6-ring with W1 water and NaI, NaI′, and/or NaII′ cations. The coordinates are from the Rietveld refinement of the data at room temperature. The Na cations and water at different sites are represented in different colors.

Table 2. Selected Atom Distances (Å) for Hexahydrate in Zeolite 12-Ring along with Occupancies of W2 and NaIII′ at Different Temperatures

Table 3. Selected Atom Distances (Å) for Tetrahedral Water in Sodalite Cage along with the Occupancies of W1, NaI, NaI′, NaII, and NaII′ at Different Temperatures

atom distance at different temperatures (Å) bond

25 °C

51 °C

68 °C

102 °C

W2−W2 W2−O4 W2- NaIII′ NaIII′ occup W2 occup

2.31 (1) 2.78 (1) 3.45 (2) 0.20 1.00

2.36 (2) 2.78 (1) 3.22 (3) 0.09 0.66

2.50 (6) 2.86 (4) 2.88 (10) 0.02 0.31

3.36 (15) 2.09 (13)

atom distance at different temperatures (Å)

0.00 0.08

The distance between W2 increased from 2.31 to 2.50 Å at 68 °C and finally 3.3 Å at 102 °C where W2 was almost gone, indicating that the strength of the hydrogen bond between W2 molecules became weaker with the temperature increase and lower fractional occupancy. The hydrogen-bonding interaction between W2 molecule and framework atom O4 stabilized the hexagonal water ring. At 25 °C, this hydrogen bond (2.78 Å) was close to the expected value of 2.82 Å. Compared to W2− W2 interaction, the interaction between W2 and framework at 25 °C was weaker. The same is true at 68 °C. However, at 102 °C where W2 occupancy was only 0.03, the W2−W2 distance became much larger than W2−O4, indicating the interaction between W2 and O4 stabilized W2. W2 was completely gone when the temperature reached 119 °C. Figure 5 shows a sodalite cage and a double 6-ring with W1 water, NaI, NaI′, and/or NaII′ cations. The selected bond distances related to this part of framework at several different temperatures are listed in Table 3. Water in sodalite cage was stabilized by hydrogen bonding and interactions with NaI′ and/ or NaII′ cations. At room temperature, the water interacted strongly with NaI′ cations; the distance between NaI′−W1, 2.51 Å, was close to the distance of Na−Ow in sodium trioxodinitrate(II) hydrate.21 W1−W1 hydrogen bonding must be very weak at 25 °C since its distance was 3.07 Å (W1−O3 is 2.96 Å). With temperature increase and water occupancy decrease, the hydrogen bonding became stronger, while the

bond

25 °C

W1−W1 W1−NaI′ W1−NaII W1−NaII′ NaI−NaI′ NaI′−O3 W1 occup NaI occup NaI′ occup NaII occup NaII′ occup

3.07 (2) 2.51 (1) 3.12 (2) 2.94 (1) 2.41 (1) 1.00 0.00 0.64 0.45 0

68 °C 2.46 2.59 3.24 2.46 2.69 2.36 0.62 0.09 0.56 0.85 0.16

(3) (1) (2) (3) (2) (1)

145 °C 2.10 (6) 2.81 (1) 3.93 (3) 2.40 (1) 2.29 (1) 0.21 0.29 0.57 0.98 0

interaction with NaI′ cation became weaker. The removal of water from the sodalite cage made the room for NaII′ atoms. When there was NaII′ in sodalite cage, W1 would strongly interact with NaII′ based on the fact that the distance of W1− NaII′ at 68 °C was 2.46 Å, close to the Na−Ow distance in hydrate also.21 No NaII′ was observed at 145 °C when W1 occupancy was just 0.21 (1.5 waters per sodalite cage), indicating that the existence of NaII′ depends on the strong interaction with W1 water. When there was not enough water to stabilize it, it migrated to other positions, such as NaII and/ or NaI. All the atoms including water and sodium cations were stabilized by the equilibrium of several forces from different directions. For instance, at 25 °C NaI′ interacted with W1 water (because the water was fully occupied) and framework oxygen O3. At this temperature, the interaction with W1 was comparatively strong; its position shifted closer to the center of sodalite cage or further away from double six rings (NaI′−NaI distance is 2.94 Å). However, with water removal, the force from the interaction with W1 decreased, and the force from the D

DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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coexistence of NaIII′ and W3 during dehydration process supports this NaIII′−W3 interaction. A similar type of bridge (NaII*−Ow−NaIII) was reported by Kirschhock et al. in a NaY sample. Their ex situ sample was prepared by sealing the sample after dehydration in a helium flow at 250 °C for an hour.22 Their bridging model was based on NaII displacement away from the zeolite and given label NaII*. Thus, their bridge between site II* and site III was shifted relative to the model obtained from our data. The difference could arise because of the different conditions. The fact that they did not see any other water molecules in their measurements was not surprising because their ex situ preparation was at much higher temperature than the lowtemperature portion of our in situ measurements where we observed the (H2O)4 tetrahedron and (H2O)6 hexagon. The dynamics of water and NaII and NaIII′ cations followed the similar roles to those in sodalite cage. NaII moved toward the center of supercage because of the strong interaction with water at 25 °C. With heating, W3 water moved away from the sodium cations (NaII and NaIII′); the relative attractive force from the interaction with zeolite framework atom O2 increased and NaII moved closer to the framework. The numbers of Na cations and water molecules per unit cell as a function of temperature are presented in Figures 7a and 7b, respectively. In addition, the relative number of atoms for each type of Na cation changed in correlation with the changes in relative numbers of water molecules. The total number of Na cations per unit cell was near 54 and satisfied the charge balance for a faujasite zeolite with Si/Al ratio of 2.6, indicating

interaction with O3 pulled NaI′ atom closer to O3, hence, closer to NaI. The decreasing distance of NaI′−NaI with the removal of water as a function of temperature is consistent with this observation. The geometry of W3 and its interaction with NaIII′ and NaII is displayed in Figure 6. The left panel shows the structure

Figure 6. Part of framework, W3, NaII, and NaIII′. The coordinates are from the Rietveld refinement of the data at room temperature. Left panel: the three disordered W3 molecules are only partially occupied. Right panel: the proposed NaII−W3−NaIII′ bridge structure.

generated from the NaII, disordered W3, and NaIII′. The distances between atoms at temperatures of 25 and 68 °C are listed in Table 4. The W3−W3 distance at room temperature, Table 4. Selected Atom Distances (Å) for NaII−W3−NaIII′ Water Bridge along with Occupancies of W3 and NaIII′ at Different Temperatures atom distance at different temperatures (Å) bond NaII−W3 NaII−O2 NaII−NaIII′ NaIII′−O2 NaIII′−O1 NaIII′−W3 NaIII′−W3 NaIII′−NaIII′ W3−W3 NaIII′ occup W3 occup

25 °C 2.19 2.44 3.39 3.21 3.12 2.01 3.14 2.84 1.23 0.20 0.27

(3) (1) (2) (2) (1) (6) (5) (4) (11)

51 °C 2.43 2.35 3.62 3.18 2.99 1.58 3.84 2.69 2.40 0.09 0.16

(7) (1) (4) (3) (3) (7) (8) (7) (12)

1.23 Å, was too short, and consequently, only one W3 molecule could exist in the vicinity of a NaII, requiring that the occupancy of W3 could not exceed 0.33. The fact that the occupancy for W3 at 25 °C was only 0.27 from the refinement supported the partial occupancy disordered model. The distance between W3−NaIII′ (2.01 Å) was also too short when both atoms were on the same side of NaII from top view of the structure (top part in left panel in Figure 6). Thus, W3 and NaIII′ cannot simultaneously exist on the same side of NaII. When one of the other disordered W3 sites was used as bridging water, bridging oxygen distances were 2.19 and 3.14 Å at 25 °C as shown in the right panel of Figure 6. Thus, a NaII− W3−NaIII′ bridge is possible. As it will be discussed later, the

Figure 7. Populations of extraframework cations and water molecules per unit cell obtained from the Rietveld refinement. (a) NaI, NaI′, NaII, NaII′, NaIII′, and total amount of Na cations per unit cell as a function of temperature. (b) W1, W2, W3, and total amount of water per unit cell as a function of temperature. E

DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research that the refinement results were very reliable. The trend of the amount of NaI increase was closely related to the trend of the amount of NaI′ decrease, indicating that NaI′ migrated to I position along the 3-fold axis during the removal of water, as it was discussed above. Compared to dehydrated sample, the amount of NaI′ increased in hydrated zeolite because of the interaction with water in sodalite cage. The NaI′ was stabilized in the sodalite cage by interactions with water. The decrease amount of NaIII′ was nearly equal to the amount of NaII′ and NaII increase, suggesting that NaIII′ cations migrated to II and II′ positions with the interaction with water. Interestingly, NaII′ behaved differently from other cations, since the amount increased initially and then decreased until it completely disappeared. NaII′ occupied nearly the same site as W1, and its increase was derived from the assumption that the total Na content per unit cell must be constant. At low temperature, there were NaI′ cations in the sodalite cage which stabilized the W3 cage, and there was no room for NaII′ cations in the sodalite. However, after some NaI′ cations migrated back to NaI position, and some water was also removed, NaII′ started to appear. The existence of NaII′ depended on the interaction with water. Therefore, the amount of NaII′ decreased with the removal of water. NaIII′ decreased dramatically and disappeared when temperature reached 80 °C. The fact that NaIII′ and W3 disappeared at the same time (Figure 7b) and the amount of NaIII′ was much less than that of W3 indicated that the existence of NaIII′ depended on the weak interaction with W3. It has been reported that the water adsorption capacity of NaY zeolite (Si/Al ∼ 2.6) was 7.54 nm3 per unit cell at room temperature, corresponding to about 260 water molecules per unit cell.23 This amount of water (260) includes three different kinds of water molecules: water on Na cations, water monolayer on the walls of the supercage by hydrogen bonds between water and oxygen atoms of the framework, and the water in cavity of supercage. Two types of water adsorption were observed in this work: water on cations (W1 and W3) and monolayer on the walls (W2). The third type of atom is too disordered to be detected in the difference maps. Based on the assignments of atoms as described above, the maximum amount of water (W1(96) + W2(32) + W3(32)) is 160 atoms. At room temperature, the total number we found from the refinement is 154 per unit cell (Figure 7b). This again proved our assignment of atoms was reliable. The removal rates based on the slope of the water amount as a function of temperature for W1, W2, and W3 followed the sequence of W2 > W3 > W1, indicating that the interaction strength was stronger for W1, while it was much weaker for W2. The results correlated well with the previous findings.23 Because of the strong interaction with cations, there was still one W1 molecule in sodalite cage (4 per unit cell) even at 350 °C, the highest operating temperature in this work. Another probe of the structural changes of the zeolite during the dehydration process is the unit cell parameter as a function of temperature as shown in Figure 8a along with the total amount of bound water. Three clearly different slopes are seen with the separation points at around 85 and 188 °C. By comparison with the bound water curve, it is interesting to see that the cell expansion was closely related to water removal rate. At the temperature below 85 °C, the water removal was rapid and a big thermal expansion coefficient of 1.43 × 10−3 Å/°C was observed, while a small thermal expansion coefficient of 2.97 × 10−4 Å/°C was obtained when the temperature is above

Figure 8. (a) Total water amount per unit cell and cell dimensions as a function of temperature during dehydration process and the cell dimensions during cooling down after dehydration (dashed line). (b) Cell dimensions obtained from heating and cooling the dehydrated NaY faujasite; the temperature program was also presented in the figure.

85 °C where the water removal was much slower. The NaY studied here and CsY15 expand during the dehydration process. In this work, the unit cell expansion during the dehydration was due to the breaking of the strong attractive interaction among water, cation, and zeolite framework. This attractive interaction stabilized smaller unit cell. However, the unit cell volume of zeolite rho,24 zeolite A,25 Laumontite,26 and Edingtonite27 was found to decrease during dehydration process. In this study, the forces between extraframework atoms and the zeolite are repulsive and removal of water molecules lead to a shrinking of the volume of unit cell. Upon further heating above 188 °C, when the bound water was nearly zero, a small negative expansion (thermal expansion coefficient: −6.18 × 10−5 Å/°C) was observed. The dehydrated sample was cooled down to 25 °C, and the unit cell was found to be 24.784 Å0.005 Å larger than the unit cell at 350 °C (24.779 Å). The slope of unit cell after 188 °C matched the slope obtained from cooling process (dashed line in Figure 8a). In order to confirm this negative expansion, a heating and cooling process was carried out on the dehydrated NaY sample (Figure 8b). The unit cell volume decreased during the heating process and increased during the cooling process although the volume did not return to the starting volume. An intrinsic negative thermal expansion of this type has also been reported for NaX and other kinds of zeolites.28,29 A similar expansion property was also observed with the faujasite zeolite exchanged by Ba2+.30 F

DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research In addition to the NaII−W3−NaIII′ bridge,22 two other interesting water cluster geometries were observed in this study. These two types of water were seen more clearly without framework of zeolite (Figure 9). Tetrahedral water clusters in

of the water molecules and location of Na cations in the zeolite framework were determined as a function of temperature. It was found that the location of the Na cations depended on the position and amount of the water molecules. In the model generated from Rietveld refinement, three different kinds of water interactions were observed in NaY below 80 °C: tetrahedral water cluster (H2O)4 in sodalite cage, hexagonal ice-like water cluster (H2O)6 in supercage near the 12-ring, and disordered water above NaII, named as W1, W2, and W3, respectively. The W1 strongly interacts with NaI′ and/or NaII′ cations, while W2 is stabilized by the interaction with framework and the W2−W2 hydrogen bonding. W3 water is found to interact with NaII and NaIII′ cations. A bridge NaII− W3−NaIII′ is observed, and the amount of NaIII′ cation depends on the amount W3 water. The stability of the waters followed the sequence of W1 > W3 > W2. The positions of the water molecules strongly depended on the amount of water in the zeolite. The unit cell expansion was closely related to the amount of water in zeolite.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b00483. Expanded 111 peak area of TR-XRD patterns (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 973 596 5707. Fax: +1 973 596 8436 (X.W.).

Figure 9. Different types of water without framework of zeolite.

ORCID

Maocong Hu: 0000-0002-2726-5979 Xianqin Wang: 0000-0003-1056-7214

sodalite cage has been observed in other sodalite cages (NaY, NaX, CsY, and LTA), but herringbone stack of hexagon rings is very unique. By stacking of 6-ring together, a nanotube-like feature was formed. There were actually four interpenetrating herringbone stacks of 6-rings. Viewed from different direction, the stack of hexagon rings appears as a zigzag structure with NaIII′ between two rings (bottom of Figure 9). However, the interaction between hexagon rings with NaIII′ is very weak because the water to NaIII′ distance is quite long (3.45 Å). Water structure in zeolites attracted general attention as a result of transformation of these structures under high pressure. A confined water wire was reported by Lee et al. in a natrolite under high pressure.31−34 The water interaction with faujasite was recently reported.35 Our results are consistent with their work but provide more atomic level analysis with the timeresolved technique. Water nanowire was found in the channel of natrolite. The formation of interesting features for water in our NaY zeolite study is believed to depend on the structures of zeolites and water concentration and temperature. Because of the symmetry of sodalite cage and shape of interconnecting 12ring between the supercages in the faujasite zeolite, pyramidlike water cluster (H2O)4 in sodalite cage and hexagonal ice-like water cluster (H2O)6 in supercage were formed, respectively. We have also observed the tetrahedral water and hexagonal icelike water stack in BaY faujasite zeolite saturated with water.19

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This contribution was identified by Prof. Yun Hang Hu (Michigan Technological University) as the Best Presentation in the “ENFL: Carbon Management: Advances in Carbon Efficiency, Capture, Conversion, Utilization & Storage” session of the 2017 ACS Fall National Meeting in Washington, DC. The research was carried out at the Brookhaven National Laboratory. J.C.H. acknowledges Kenny Ståhl DTU Lyngby, Denmark, for introducing him to study of thermal dehydration of zeolites. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

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REFERENCES

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4. CONCLUSIONS The synchrotron-based time-resolved X-ray diffraction (TRXRD) technique was used in this work to study the dehydration process of NaY faujasite zeolites. The behavior G

DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.iecr.8b00483 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX