Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by

Dec 28, 2009 - J. Phys. Chem. C , 2010, 114 (3), pp 1633–1640. DOI: 10.1021/jp9105025 ... Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16...
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J. Phys. Chem. C 2010, 114, 1633–1640

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Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD Dustin W. Fickel and Raul F. Lobo* Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: NoVember 3, 2009; ReVised Manuscript ReceiVed: December 2, 2009

Nitrogen oxides (NOx) are a major atmospheric pollutant produced through the combustion of fossil fuels in internal combustion engines. Copper-exchanged zeolites are promising as selective catalytic reduction catalysts for the direct conversion of NO into N2 and O2, and recent reports have shown the enhanced performance of Cu-CHA catalysts over other zeolite frameworks in the NO decomposition of exhaust gas streams. In the present study, Rietveld refinement of variable-temperature XRD synchrotron data obtained for Cu-SSZ-13 and Cu-SSZ-16 is used to investigate the location of copper cations in the zeolite pores and the effect of temperature on these sites and on framework stability. The XRD patterns show that the thermal stability of SSZ-13 is increased significantly when copper is exchanged into the framework compared with the acid form of the zeolite, H-SSZ-13. Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16. From the refined diffraction patterns, the atomic positions of atoms, copper locations and occupancies, and thermal displacement parameters were determined as a function of temperature for both zeolites. Copper is found in the cages coordinated to three oxygen atoms of the six-membered rings. 1. Introduction Nitrogen oxides (NOx) are a major atmospheric pollutant produced through the combustion of fossil fuels. Many negative environmental effects result from having nitrogen oxides in the atmosphere, including the greenhouse effect, acid rain, and ozone depletion.1 The use of copper-exchanged zeolites as selective catalytic reduction (SCR) catalysts for the direct conversion of NO into N2 and O2 has been studied in the past.2-6 Much research has been done on Cu-ZSM-5 catalysts, since its discovery in 1986 by Iwamoto et al.,3 and considerable advancements have been made in the understanding of how this system is superior to other copper-exchanged zeolites7-11 and in the understanding of the active sites in Cu-ZSM-512-16 and the mechanism of NO decomposition.17-19 Several authors have carried out Rietveld studies of Cu-ZSM-5 to identify the active sites and effect of temperature on these catalysts.20,21 Cu-ZSM5, however, is limited by its relatively low activity in the presence of water vapor and by dealumination at high temperatures, resulting in a loss of activity. Along with these problems, ZSM-5 is susceptible to adsorbing hydrocarbons at low temperatures that generate heat as the temperature is raised, which can be damaging to the zeolite structure.22 This is particularly problematic in vehicle emissions, where significant quantities of hydrocarbons can be adsorbed on the catalyst during cold start.22 Other copper zeolite systems have been recently discovered as SCR catalysts with improved activity and selectivity in the decomposition of NO. One such system discovered by Zones et al. mentions the use of the small-pore zeolite SSZ-62 (CHA) for the reduction of NO.23 Others have reported data, showing the enhanced performance of metal-exchanged small-pore zeolites and Cu-Chabazite (Cu-CHA) in the NO decomposition of exhaust gas streams.22,24 These catalysts have higher SCR activity at low temperatures and improved hydrothermal stability over existing copper-zeolites.24 Reaction experiments with CuCHA show NO conversion to be 90-100% over a temperature * To whom correspondence should be addressed. Phone: 302-831-1261. Fax: 302-831-2085. E-mail: [email protected].

Figure 1. Zeolite framework structures of (a) SSZ-13 (CHA) and (b) SSZ-16 (AFX).

range of 250-450 °C, and even after hydrothermally aging the zeolite at 800 °C, conversion is still in excess of 80%.24 Zeolite SSZ-13 (CHA) was first synthesized by Zones and Chevron in 1985.25 A small-pore zeolite, SSZ-13 has a tetrahedral framework composed of double six-membered rings (D-6R) in an AABBCC sequence, units which connect to form a cavity with eight-membered ring pore windows (Figure 1a). A related small-pore zeolite of interested is SSZ-16 (AFX). SSZ16 is crystallized in the presence of a diquaternary ammonium compound26 and is also a member of the ABC... type of zeolites having a sequence AABBCCBB...27 SSZ-16 is isotypic with the aluminophosphate SAPO-56 (AFX), and the pore structure of this zeolite is formed by two types of cages: the gmelinite cage and the large cage of ALPO4 (AFT), as shown in Figure 1b.27,28 Rietveld structure refinement studies of X-ray diffraction (XRD) patterns for NH4-CHA have shown that the ion-exchange sites are located just outside the six-membered rings on the top and bottom of the eight-membered ring pore cage.29 Previous structural refinements of CuCHA have also found this to be the same position in which exchanged copper is located.30 Studies using electron spin resonance (ESR) on Cu-SAPO-34

10.1021/jp9105025  2010 American Chemical Society Published on Web 12/28/2009

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Fickel and Lobo

(isotypic with CHA) have also found copper at the same position in this related material.31 The preferred location of copper cations within these zeolite frameworks is important in understanding the oxidation state, coordination geometry, and activity of the copper cations and thus helps in the explanation of the mechanism by which these zeolites act as SCR catalysts. Here, Rietveld refinements of variable-temperature XRD synchrotron data obtained for NH4-SSZ-13, Cu-SSZ-13, and Cu-SSZ-16 are used to investigate the location of copper cations and the effect of temperature on these sites and to measure relative framework stability.

TABLE 1: Experimental Parameters for the Collection of Variable-Temperature Synchrotron XRD Patterns (Collected on the Brookhaven NSLS x7b Beamline)

2. Experimental Methods 2.1. Structure-Directing Agent Synthesis. The structuredirecting agent (SDA) used in the synthesis of SSZ-13 was N,N,N-trimethyl-1-adamantanamine iodide (TMAAI).25 TMAAI was synthesized by adding 10 g of 1-adamantanamine (97%, Sigma-Aldrich) to 24.8 g of methanol (Fisher Scientific) and stirring until the solid is dissolved. Next, 29 g of tributylamine (98.5%, Sigma-Aldrich) was added to the solution and stirred for 15 min. The solution was then placed in an ice bath, and 28.4 g of methyl iodide (99.5% Sigma-Aldrich) was added dropwise into the solution. This solution was stirred for 5 days at RT. After the addition of 100 mL of diethyl ether (Fisher Scientific) to precipitate the product, the solution was further stirred for 30 min. The product was then vacuum-filtered with more diethyl ether and dried at room temperature overnight. For the synthesis of SSZ-16, the SDA 1,4-diazabicyclo[2.2.2]octane-C4-diquat dibromide was used.32 The synthesis was carried out by mixing together 20 g of 1,4-diazabicyclo[2.2.2]octane (DABCO) (98%, Sigma-Aldrich) with 19.77 g of methanol (Fisher Scientific) until the DABCO was dissolved. A separate solution was made of 12.83 g of 1,4-dibromobutane (99%, Sigma-Aldrich) and 6.6 g of methanol (Fisher Scientific), which was allowed to stir for 15 min. Next, the DABCO solution, while still stirring, was placed in an ice bath and the 1,4-dibromobutane solution was added dropwise to ensure product selectivity. The combined solution was stirred for 2 h. A white precipitate begins to form after approximately 1 h. After 2 h of stirring, 100 mL of diethyl ether (Fisher Scientific) was added to the solution and allowed to stir for 20 min to precipitate more product. The product was then vacuum-filtered with additional diethyl ether and dried at room temperature overnight. 2.2. Zeolite Synthesis. SSZ-13 was synthesized using a procedure similar to that reported by Zones.33 First, 5 g of sodium silicate (Sigma Aldrich) and 0.16 g of NaOH (Fisher Scientific) were added to 12 g of water. The resulting solution was stirred at room temperature for 15 min; then, 0.5 g of NH4-Y (Zeolyst CBV100) was added to the solution and stirred for 30 min. Next, 0.8 g of N,N,N-trimethyl-1-adamantanamine iodide was added to the solution and stirred for another 30 min. The resulting solution was then transferred into Teflon-lined autoclaves and heated at a temperature of 140 °C under rotation for 6 days. The product was recovered by vacuum filtration, washed with deionized water, and dried at room temperature. The asmade product was then calcined in air at 550 °C for 8 h. SSZ-16 was synthesized using a procedure similar to that published in U.S. Patent 5,194,235.26 Sodium silicate (5 g) (Sigma Aldrich) and 0.15 g of NaOH (Fisher Scientific) were added to 11.69 g of water. This solution was stirred at room temperature for 15 min; then, 0.5 g of Na-Y (Zeolyst CBV100) was added to the solution and stirred for 30 min. Next, 1.22 g of 1,4-diazabicyclo[2.2.2]octane-C4-diquat dibromide was added to the solution and stirred for another 30 min. The resulting solution was then transferred into Teflon-lined autoclaves (Parr)

and heated at a temperature of 150 °C under rotation for 6 days. The product was recovered by vacuum filtration, washed with deionized water, and dried at room temperature. The as-made product was first calcined in N2 at 550 °C for 8 h and then in air at 550 °C for 8 h. After calcination, all zeolite samples were ion-exchanged in a 0.1 M solution of NH3NO3 (Fisher Scientific) at 80 °C for 8 h and dried in air at room temperature. 2.3. Copper Ion Exchange. A 0.5 L solution of 0.1 M Cu(II)SO4 was made by adding 7.98 g of copper(II) sulfate (Sigma-Aldrich) to 0.5 L of water. The pH of the solution was then adjusted to 3.5 by the addition of nitric acid (Fisher Scientific). NH4-SSZ-13 (0.91 g) was then added to the CuSO4 solution. Another CuSO4 solution was prepared into which 0.58 g of NH4-SSZ-16 was added. These solutions were stirred in an oil bath at 80 °C for 1 h. Solutions were then vacuumfiltered with deionized water, and the resulting Cu-zeolite products were dried at room temperature. 2.4. Characterization. Powder X-ray diffraction (XRD) data were collected on a Philips X’pert diffractometer using a Cu KR source. The patterns were obtained from 5 to 50° 2θ using a step size of 0.02° 2θ and 2 s per step. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDAX) chemical analysis were obtained on a JEOL JSM7400F microscope. Adsorption isotherms were collected on

parameter

value

wavelength (λ) polarizability scanned region (2θ) initial temperature (°C) final temperature (°C) heating rate (°C/h) scan time (min)

0.3184 0.93 0.033-33.033 25 900 218 11.83

Figure 2. SEM images of (a) SSZ-13 and (b) SSZ-16.

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Figure 3. Stacked plots of variable-temperature synchrotron XRD patterns (low-2θ) for (a) NH4-SSZ-13 and (b) Cu-SSZ-13.

a Micromeritics ASAP 2010 at 77 K using N2 as the adsorbate. Chemical analyses were preformed by Galbraith Laboratories. Beamline x7b at the Brookhaven National Syncrotron Light Source (NSLS) was used to measure the variable-temperature XRD patterns. Zeolite samples were packed into 0.7 mm quartz capillaries, which were then wrapped with a heating coil and loaded into an in situ flow cell, through which either He or a 5% O2/He mixture was flown. The addition of water to the gas stream was considered but was unavailable at beamline x7b. The beamline is equipped with a Mar345 full imaging plate detector. Table 1 shows the beamline parameters along with the heating rate and scan range used during the measurements. 3. Results and Discussion 3.1. Cu-SSZ-13. The chemical analyses of the as-synthesized SSZ-13 crystals show that they have a silica-to-alumina (SiO2/ Al2O3) ratio of 12. The particles are agglomerates made up of smaller individual cube-shaped crystals measuring approximately 1 µm in size (see Figure 2a). After copper ion exchange, the SSZ-13 crystals maintain their previous morphology, and chemical analysis reveals a total of 4.39 wt % Cu or equivalently a molar ratio of Cu/(Al + Si) of 0.05 and Cu/Al of 0.35. The thermal stability of Cu-SSZ-13 and NH4-SSZ-13 can be compared using variable-temperature XRD. Figure 3a shows a

stacked temperature plot of the XRD patterns for NH4-SSZ-13. At about 600 °C, the peaks in the pattern begin to broaden and decrease in intensity, until they eventually disappear entirely as the zeolite is further heated, indicating a deterioration and collapse of the zeolite framework. Figure 3b displays a stacked temperature plot of the XRD patterns attained for Cu-SSZ-13. Collapse of the diffraction pattern for this sample is not observed until the catalyst reaches approximately 800 °C. A comparison of the thermal stability can be made by plotting the intensity of the [100] reflection as a function of temperature for the two samples (Figure 4). The intensities are shown on a relative scale based upon the XRD pattern that gives the most intense peak [100]. As evidenced by Figure 4, the Cu-SSZ-13 framework can withstand temperatures up to 200 °C higher than that of NH4-SSZ-13, making it a much more thermally stable form of the zeolite. This increased thermal stability is a positive attribute of Cu-SSZ-13 as a SCR catalyst for use in automotive pollution abatement systems. Rietveld refinement of the Cu-SSZ-13 patterns was performed using the GSAS package34 along with the EXPGUI graphical interface.35 Each XRD pattern was analyzed independently during the refinement process. Starting atomic coordinates for the structural models were taken from the International Zeolite Association (IZA) website for the CHA framework.36 A

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Fickel and Lobo parameters (Uiso) was carried out while using soft constraints on the tetrahedral bond lengths (T-O). This length was set to 1.59 Å with a standard deviation of 0.015 Å. The soft constraints were initially used with a weight of 50; however, this weighting was progressively reduced to zero as the refinement converged. All tetrahedral positions were modeled as silicon atoms. Substitution of aluminum atoms in for silicon along with an adjustment of the fractional occupancy had very little impact on the overall refinement of the patterns and lead to little change to the atomic positions, bond lengths, and bond angles. Table 2 shows a summary of the agreement factors for the Cu-SSZ-13 XRD pattern taken at 435 °C. The agreement factors for all other refinements were nearly identical to these results. The low wRp value indicates an excellent agreement to the experimental data. This can also be observed in Figure 5, where the observed and refined patterns are plotted together along with a difference curve for the Cu-SSZ-13 XRD pattern taken at 435 °C. The final atomic positions, Cu location and fractional occupancy, and thermal displacement parameters are presented in Table 3 for the Cu-SSZ-13 XRD pattern taken at 435 °C (summaries of the other refined patterns can be found in the Supporting Information). Table 4 lists the calculated bond lengths and angles for this particular refinement. An average T-O bond length of 1.605 Å is in very good agreement with an expected bond length of 1.610 Å, which is based on the composition of the zeolite (Si/Al) and the bond lengths of Si-O and Al-O bonds (1.59 and 1.73 Å, respectively). An average bond angle of 109.4° is also consistent with tetrahedral symmetry. Our refinements are in good agreement with others,29-31 where the Cu cations are shown to be coordinated to three oxygen atoms just outside the plane of the six-membered rings, as shown

Figure 4. Relative thermal stability of NH4-SSZ-13 and Cu-SSZ-13 based on the relative intensity of the [100] reflection.

TABLE 2: Refinement Agreement Factors for Cu-SSZ-13 and Cu-SSZ-16 Patterns Collected at 435°C chi2 (χ2) wRp Rp refinement region (2θ) d minimum (Å) no. reflections

Cu-SSZ-13

Cu-SSZ-16

0.98 0.018 0.014 2.3-20.36 0.9 904

1.06 0.017 0.014 2.9-20.369 0.9 874

hexagonal unit cell with the space group R3jm was used along with the initial unit cell dimensions of a ) 13.675 Å and c ) 14.767 Å. The background profiles were first edited manually and further optimized using a five-coefficient linear background model. The profile of the diffraction peaks was modeled using a pseudo-Voigt function. The refinement of atomic coordinates, site occupancies for Cu atoms, and thermal displacement

Figure 5. Observed (crosses) and calculated (line) patterns with the relative difference curve (bottom line) of the refinements of Cu-SSZ-13 patterns (T ) 435 °C).

TABLE 3: Refined Atomic Positions, Occupancies, and Thermal Displacement Parameters for the Cu-SSZ-13 XRD Pattern Measured at 435°Ca

a

site

x

y

Z

multiplicity

occupancy

Uiso

T (Si/Al) O1 O2 O3 O4 Cu

-0.00041 0.905219 0.974521 0.121600 0 0

0.229285 0.094778 0.307923 0.243102 0.265465 0

0.102189 0.111781 0.1667 0.127536 0 0.122063

36 18 18 18 18 6

1 1 1 1 1 0.3597

0.02222 0.03057 0.03057 0.03057 0.03057 0.02474

Space group: R3m j . Refined unit cell dimensions: a ) b ) 13.461 Å, c ) 15.044 Å, V ) 2725.86 Å3.

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TABLE 4: Tetrahedral Bond Lengths and Angles for the Refinement of Cu-SSZ-13 XRD Patterns Taken at 435°C vector

length (Å)

angle

degrees

T-O1 T-O2 T-O3 T-O4 T-O (avg) Cu-O1

1.617(1) 1.591(9) 1.604(5) 1.611(7) 2.213(6)

O1-T-O2 O1-T-O3 O1-T-O4 O2-T-O3 O2-T-O4 O3-T-O4 O-T-O (avg) T-O1-T T-O2-T T-O3-T T-O4-T T-O-T (avg)

112.5(5) 107.0(4) 107.9(4) 108.7(8) 110.4(7) 110.2(4) 144.6(5) 153.8(6) 149.9(6) 144.9(6)

Figure 7. Cu-O bond length and Cu fractional occupancy as a function of temperature for Cu-SSZ-13.

in Figure 6. Due to the composition of the zeolite, there is, on average, one aluminum atom in every six-membered ring, so the position of the copper is likely to be slightly off-axis due to the pull of this negatively charged tetrahedral position. We investigated the possibility of an off-center position, but the Cu cations always converged to x ) y ) 0. As the zeolite is initially heated, the “z” coordinate for copper decreases and the copper cation moves closer to the six-membered ring. As shown in Figure 7, movement of the copper cation is made apparent through a decrease in the Cu-O bond length (2.33-2.21 Å). The Cu-O bond length levels off at about 450 °C before it begins to increase slightly again at 800 °C, possibly due to framework decomposition. The point at which the Cu-O bond length levels off is also in good agreement with the point in Figure 4, where the intensity of the [100] peak for Cu-SSZ-13 reaches a maximum and plateaus. The decrease in the Cu-O bond length can be attributed to the copper being initially partially hydrated, and as the temperature is increased, the water is removed from the zeolite and the copper cations move closer to the six-membered rings due to the electronegativity of the oxygen atoms. In our experimental setup, time is needed for Figure 8. Unit cell volume, a, and c unit cell dimensions for CuSSZ-13 as a function of temperature.

Figure 6. Location of copper ions in Cu-SSZ-13: (a) side view and (b) entire cage. (c) Locations of copper ions (Cu1 and Cu2) in CuSSZ-16.

the water to desorb due to the chromatographic effect of flow through a small well-packed capillary cell, so the exact point at which all the water is removed from the system is not known precisely. The length of a typical copper oxide (Cu(II)O) bond based on ionic radii is 2.04 Å; our calculated bond length is slightly larger than this, revealing perhaps a weaker electrostatic attraction. Other groups have noted a shorter observed Cu-O bond length of about 2.0-2.1 Å.30,31 The discrepancy with our calculated values is perhaps due to the higher silica-to-alumina ratio of our samples and thus a slightly lower electrostatic attraction. It is unlikely that the copper ions in our samples are hydrated because the results our refinements show no additional positions of missing electron density above the ions and due to the high temperatures to which our samples are heated. Figure 7 also shows that the fraction of copper located at this position increases until leveling off near 600 °C. This is most likely due to the mobility of copper cations at lower temperatures, but as the water is desorbed from the zeolite, the copper cations coordinate more firmly to the framework oxygen atoms. The fraction of copper determined through the refinements corresponds to an approximate molar composition (Cu/ Si) of 0.06 or 5.94 wt % Cu, which correlates well with the experimental composition (Cu/(Si + Al)) of 0.05 found earlier.

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Figure 9. Cu-SSZ-16 (a) stacked temperature plot of XRD patterns and (b) thermal stability based on relative intensity of the [101] reflection.

The thermal displacement parameters for all atoms, especially Cu, increase as the temperature is increased (see the Supporting Information). This is expected, but also brings up the possibility that the copper atoms are mobile and transitioning between identical sites in the framework. Refinements of the XRD patterns also show a contraction of the unit cell during heating (Figure 8). A decrease in the “a” unit cell dimension appears to be the reason for a 1.2% loss of volume; thus, the pores in the zeolite become slightly narrower at high temperatures. This negative thermal expansion coefficient has been reported before for CHA zeolites.37 UV/vis diffuse reflectance experiments (not shown) of Cu-SSZ-13 samples heated in air as a function of temperature give no evidence of self-reduction of Cu2+ ions in the zeolite, as has been reported for Cu-ZSM-5.18,20,38,39

Fickel and Lobo 3.2. Cu-SSZ-16. The chemical composition analyses of the synthesized SSZ-16 crystals show that they have a silica-toalumina (SiO2/Al2O3) ratio of 9. The crystals measure approximately 1 µm in size (see Figure 2b). After copper ion exchange, the SSZ-16 crystals maintain their previous morphology, and chemical analysis of the crystals indicate a 5.65 wt % of Cu loading. Compositional analysis also shows the that CuSSZ-16 has an extra-framework cation-to-T atom ratio composition (Cu/(Si + Al)) of 0.08 and a Cu/Al ratio equal to 0.449. Figure 9a shows a stacked temperature plot of the synchrotron XRD patterns for Cu-SSZ-16. The thermal stability of Cu-SSZ16 is lower than that of Cu-SSZ-13. This is conveyed in Figure 9b where peak broadening and intensity loss start at approximately 670 °C, at which point, the framework rapidly breaks down. Rietveld refinement of the Cu-SSZ-16 synchrotron XRD patterns was accomplished using the same procedure as that of Cu-SSZ-13. The AFX framework with a hexagonal unit cell and the space group P63/mmc was used along with the initial unit cell dimensions of a ) 13.674 Å and c ) 19.695 Å.36 Table 2 shows a summary of the refinement agreement factors for the Cu-SSZ-16 XRD pattern taken at 435 °C. Figure 10 shows the observed and refined patterns for the refined Cu-SSZ-16 XRD pattern taken at 435 °C plotted along with the difference curve. As with the refined patterns of Cu-SSZ-13, the low wRp values for the Cu-SSZ-16 patterns along with the low intensity and relative flatness of the difference curve are indicative of a good agreement. The final results of the refinement of the framework atomic positions, Cu locations and fractional occupancies, and thermal displacement parameters are presented in Table 5 for the Cu-SSZ-16 XRD pattern taken at 435 °C (summaries of the other refined patterns at different temperatures can be found in the Supporting Information). Table 6 lists the calculated bond lengths and angles for this particular refinement. The AFX framework has two different T sites in the asymmetric unit and, as shown in Figure 1b, also two different types of cages. The bond length of the T2-O bond is slightly longer than that of the T1-O bond, 1.614 and 1.605 Å, respectively. The reason for this difference might be that there is a slight nonuniform distribution of aluminum between the two T sites with more aluminum on average being located at site T2. Both of these refined T-O bond lengths coincide

Figure 10. Observed (crosses) and calculated (line) diffraction patterns with the relative difference curve (bottom line) of the refinements of Cu-SSZ-16 patterns taken at 435 °C.

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TABLE 5: Refined Atomic Positions, Occupancies, and Thermal Displacement Parameters for the Cu-SSZ-16 XRD Pattern Taken at 435°Ca

a

site

X

y

Z

multiplicity

occupancy

Uiso

T1 (Si/Al) T2 (Si/Al) O1 O2 O3 O4 O5 O6 O7 Cu1 Cu2

0.000544 0.334434 0.090488 0.872622 0 0.022837 0.234848 0.342645 0.451324 0 0.333333

0.226435 0.43596 0.180977 0.1127379 0.259347 0.328567 0.469801 0.402375 0.548677 0 0.666667

0.077151 0.17329 0.087131 0.089412 0 0.126569 0.163379 0.25 0.153947 0.090906 0.15515

24 24 12 12 12 24 12 12 12 4 4

1 1 1 1 1 1 1 1 1 0.3221 0.4915

0.02242 0.02242 0.026 0.026 0.026 0.026 0.026 0.026 0.026 0.02186 0.02186

Space group: P63/mmc. Refined unit cell dimensions: a ) b ) 13.481 Å, c ) 20.123 Å, V ) 3657.20 Å3.

TABLE 6: Tetrahedral Bond Lengths and Angles for the Cu-SSZ-16 XRD Spectrum Taken at 435°C vector

length (Å)

angle

degrees

T1-O1 T1-O2 T1-O3 T1-O4 T1-O (avg) T2-O4 T2-O5 T2-O6 T2-O7 T2-O (avg) Cu1-O1 Cu2-O5

1.621(4) 1.585(9) 1.615(7) 1.600(3) 1.602(9) 1.631(7) 1.627(9) 1.596(6) 2.114(3) 2.304(4)

O1-T1-O2 O1-T1-O3 O1-T1-O4 O2-T1-O3 O2-T1-O4 O3-T1-O4 O-T1-O (avg) O4-T2-O5 O4-T2-O6 O4-T2-O7 O5-T2-O6 O5-T2-O7 O6-T2-O7 O-T2-O (avg) T1-O1-T1 T1-O2-T1 T1-O3-T1 T1-O4-T1 T1-O-T1 (avg) T1-O4-T2 T2-O5-T2 T2-O6-T2 T2-O7-T2 T2-O-T2 (avg)

111.4(8) 109.2(5) 113.2(0) 103.2(0) 106.5(6) 112.6(4) 108.6(1) 108.9(2) 113.2(2) 111.9(3) 105.2(2) 108.8(1) 139.0(3) 149.4(6) 147.8(5) 147.8(5) 153.6(7) 148.5(1) 142.9(6) 151.5(5)

well with the predicted bond length of 1.613 Å obtained from the composition of the zeolite. Along with these two different T sites, our refinements show that two different Cu positions are present within the AFX framework. Each of these Cu positions is coupled with one of the different six-membered rings. Cu1 is located within the gmelinite cage just outside the six-membered rings, whereas Cu2 is positioned in the larger cage, as shown in Figure 6c. Figure 11 tracks the Cu-O bond lengths for both sites as a function of temperature. The Cu1-O bond length decreases until it reaches a minimum of 2.10 Å at 478 °C before increasing slightly and leveling off at 2.15 Å. The Cu2-O bond is longer than the Cu1-O bond. Initially, the Cu2-O length decreases but then levels off and oscillates around a median value of 2.27 Å. This difference in Cu-O bond lengths between the two sites is not entirely understood, but perhaps it is due to an uneven distribution of aluminum within the framework. Again, as was the case for Cu-SSZ-13, the copper cations move closer to the six-membered rings in both cages of Cu-SSZ-16 as the temperature increases before reaching an equilibrium position, as can be seen through a plot of the “z” coordinate of the Cu ions in the Supporting Information. Figure 12 shows that the distribution of copper between the two sites is not uniform. A larger fraction of copper is found in

Figure 11. Bond lengths of Cu1-O and Cu2-O for Cu-SSZ-16 as a function of temperature.

Figure 12. Cu1 and Cu2 fractional occupancies in Cu-SSZ-16 as a function of temperature.

the Cu2 location. This is consistent with the observation of more aluminum located near the T2 position, so more copper will be positioned at the Cu2 location for charge balance. The refined Cu-SSZ-16 data give a molar ratio of copper in the sample (Cu/ Si) of 0.068 along with 6.69 wt % Cu, which both match well with the experimental compositions presented earlier. 4. Conclusion The Rietveld refinement of variable-temperature synchrotron XRD patterns for two copper-exchanged small-pore zeolites, Cu-SSZ-13 and Cu-SSZ-16, has been performed. The XRD patterns show that the thermal stability of SSZ-13 is increased significantly when copper is exchanged into the sample compared with the acid form of the zeolite, NH4/H+-SSZ-13. CuSSZ-13 is also found to be more thermally stable than Cu-SSZ16. From the refined patterns, the atomic positions of atoms, copper locations and occupancies, and thermal displacement parameters were determined as a function of temperature for both zeolites. The location of copper within SSZ-13 is consistent

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with previous findings and is located within the cage just outside the six-membered rings of the zeolite.29-31 In the procedure used by Bull et al., prior to the deNOx reaction experiments, their CuCHA samples are initially calcined in air at 640 °C.24 This initial calcination is likely done to “activate” the zeolite for the deNOx experiments by moving the copper ions to places of higher activity within the zeolite. From our refinements, the significance of this initial heating is a shortening of the Cu-O bond length. Contrary to Cu-SSZ-13, the refinements of CuSSZ-16 indicate the presence of two different copper ion sites, each associated with one of the distinctly different six-membered rings within the framework. Our findings suggest that, because SSZ-13 has been shown to be highly active for deNOx reactions, the related small-pore zeolite, SSZ-16, will show similar activities. Acknowledgment. This research was supported by the U.S. Department of Energy under Grant No. DE-FG02-07ER15921. We thank Dr. Jonathan Hanson and Michael Estrella at Brookhaven National Laboratory’s National Synchrotron Light Source for their help with the XRD data collection and analysis. Note Added after ASAP Publication. One of the author names (J.M.F.) has been removed. This manuscript was published on the web on December 28, 2009. It was reposted on January 21, 2010. Supporting Information Available: Summaries for all refined synchrotron XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Garin, F. Appl. Catal., A 2001, 222, 183–219. (2) Iwamoto, M.; Yokoo, S.; Sakai, K.; Kagawa, S. J. Chem. Soc., Faraday Trans. 1981, 77, 1629–1638. (3) Iwamoto, M.; Furukawa, H.; Mine, Y.; Uemura, F.; Mikuriya, S. I.; Kagawa, S. J. Chem. Soc., Chem. Commun. 1986, 1272–1273. (4) Iwamoto, M.; Yahiro, H.; Mine, Y.; Kagawa, S. Chem. Lett. 1989, 213–216. (5) Iwamoto, M.; Yahiro, H.; Kutsuno, T.; Bunyu, S.; Kagawa, S. Bull. Chem. Soc. Jpn. 1989, 62, 583–584. (6) Iwamoto, M.; Yahiro, H.; Shundo, S.; Yoshihiro, Y.; Mizuno, N. Appl. Catal., A 1991, 69, L15–L19. (7) Iwamoto, M.; Yahiro, H.; Tanda, K.; Mizuno, N.; Mine, Y.; Kagawa, S. J. Phys. Chem. 1991, 95, 3727–3730. (8) Li, Y. J.; Hall, W. K. J. Catal. 1991, 129, 202–215.

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