Extraframework Cation Locations in Cu-UZM-35 NH3-SCR Catalyst

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

Extraframework Cation Locations in Cu-UZM-35 NH-SCR Catalyst Nak Ho Ahn, Jiho Shin, Taekyung Ryu, and Suk Bong Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09764 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 15, 2018

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The Journal of Physical Chemistry

1 Extraframework Cation Locations in Cu-UZM-35 NH3-SCR Catalyst

Nak Ho Ahn, Jiho Shin, Taekyung Ryu and Suk Bong Hong*

Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 37673, Korea

ABSTRACT We have recently demonstrated that divalent copper ions exchanged into the large-pore zeolite UZM-35 with MSE topology (Cu-UZM-35) when hydrothermally aged at 750 °C exhibit a considerably wider operating temperature window for NH3-SCR than those into the small-pore zeolite SSZ-13 with CHA topology (Cu-SSZ-13), the current commercial catalyst for this reaction. Here we report the physical states and locations of copper ions in the fresh and 750 °C-aged forms of Cu-UZM-35 determined using quantitative EPR measurements and synchrotron powder X-ray diffraction and Rietveld analyses. Among the four different cation sites (Cu1 – Cu4) identified in fresh Cu-UZM-35, sites Cu3 and Cu4 are easily accessible via 12-ring and 10-ring channels, respectively, allowing their copper ions to serve as main active sites for NH3-SCR. It was also found that while the Cu2+ ions at site Cu3 remain unchanged even after hydrothermal aging at 750 °C, the [Cu(OH)]+ ions at site Cu4 are transformed into CuOx and/or CuAl2O4 species. This led us to conclude that the Cu2+ ions at the former site are mainly responsible for the wide operating temperature window of Cu-UZM35.

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2 INTRODUCTION

The interest in improving air-quality has led to an ever-intensifying social demand for environmentally benign technologies, as well as for tighter emission standards.1 Nitrogen oxides (NOx) in the diesel engine exhaust are major air pollutants, which contribute to environmentally hazardous issues, such as acid rain and photochemical smog, and continuously endanger human health.2 The improvement of commercially applied lean-NOx abatement by selective catalytic reduction of NOx with urea and thus ammonia (NH3-SCR) over copper and iron ion-exchanged zeolites is a current challenge towards the widespread proliferation of fuelefficient diesel engines.3-6 Despite the discovery of Cu2+-exchanged ZSM-5 (framework type MFI) as an efficient deNOx catalyst by Iwamoto et al. in 1986,7 its commercial success was severely limited by the poor hydrothermal stability of the catalyst to the frequent temperature spikes above 700 °C during diesel particulate filter regeneration. Since then, transition metal-exchanged zeolites with different framework topologies have been extensively studied over the last three decades to find an efficient and simultaneously hydrothermally stable deNOx catalyst.3-6 A remarkable breakthrough was provided by the application of Cu-SSZ-13 (CHA) and Cu-LTA, which have proved to exhibit superior hydrothermal stability, together with good catalytic activity, even after being hydrothermally aged at 750 °C and higher.8-10 In the latter case, the electrostatic interactions between one divalent cation and two single negative net charges in single 6-rings (s6rs) have been suggested to suppress dealumination and copper ion migration.10 The accurate characterization of the physical states and locations of copper ions in a wide variety of zeolites with different pore structures is of major importance in the design and development of new NH3-SCR catalysts with better performance and hydrothermal stability. Until now, however, the majority of research related to the hydrothermal stability of copper


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3 ion-exchanged zeolites has been focused on the copper ions located in or on the planar 6- or 8rings of CHA and LTA structures.10,11 The large-pore zeolite MCM-68 (MSE) contains a threedimensional pore system consisting of one straight 12-ring (6.4 × 6.8 Å) channel and two twisted 10-ring (5.2 × 5.8 and 5.2 × 5.2 Å) channels (Figure 1).12,13 Consequently, the MSE structure possesses two different cages, the 16-hedral ([46610]) t-mse-1* and 24-hedral ([465866104]) t-mse-2* cages. UZM-35, one of the MSE-type zeolites, is characterized by a similar Si/Al ratio (ca. 9) to that of MCM-68, whereas its synthesis includes the use of a rather simple organic structure-directing agent (dimethyldipropylammonium hydroxide) in the presence of both Na+ and K+ ions.14 We have recently reported the detailed synthesis conditions of pure UZM-35 and the catalytic properties of its copper-exchanged form for NH3-SCR.15 An interesting observation was that Cu-UZM-35 exhibited an unexpectedly wider operating temperature window than that of Cu-SSZ-13, although Cu-SSZ-13 outperformed CuUZM-35 with similar Cu and Al contents after hydrothermal aging at 750 °C, unlike the case of their fresh forms which show similar deNOx activities.15 To understand the precise reason for this, we have investigated changes in the local environment of copper ion sites in Cu-UZM35 caused by hydrothermal aging at 750 °C, using IR and EPR spectroscopies and synchrotron powder X-ray diffraction, and Rietveld analyses.

EXPERIMENTAL SECTION

Catalyst Preparation. A Cu-UZM-35 zeolite with Si/Al = 8.9 and Cu/Al = 0.38 (corresponding to a Cu2+ exchange level of 76%) was prepared according to the procedures described in our recent work15 and activated at 500 °C for 5 h. For comparison, Cu-SSZ-13 zeolite with Si/Al = 14 and Cu/Al = 0.49 was synthesized following the method given elsewhere.16 Both zeolite catalysts, the copper contents (3.3 and 2.9 wt%, respectively) of



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4 which are similar to each other, were hydrothermally aged under flowing air (1,000 mL min-1) containing 10% H2O at 750 °C for 24 h. General Characterization. Powder X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert diffractometer (Cu Kα radiation) with an X’Celerator detector. Elemental analysis for Al, Si, and Cu was performed by the Analytical Laboratory of the Pohang Institute of Metal Industry Advancement. The 27Al MAS NMR were measured at a 27Al frequency of 78.156 MHz using a pulse length of 1.8 μs and a recycle delay of 0.5 s. Approximately 3000 pulse transients were accumulated, and the

27Al

chemical shifts are reported relative to an

Al(H2O)63+ solution. To more clearly examine changes of the 27Al resonance intensity caused by hydrothermal aging, the amount of copper-exchanged zeolites used in

27Al

MAS NMR

measurements was kept constant. IR spectra in the 650 - 4000 cm-1 region were measured on a Nicolet 6700 FT-IR spectrometer equipped with an MCT detector and a DRIFT cell (Pike) with ZnSe windows accumulating 64 scans at a spectral resolution of 4 cm-1. Before measurements, the powder samples (~10 mg) were mounted in a ceramic holder, preheated from 25 to 500 °C at a ramp rate of 10 °C min-1 with 21% flowing O2 in He, and held at 500 °C for 1 h. Then, the spectra were measured at 300 °C under the same flow conditions. The total flow rate was maintained at 100 ml min-1 using mass flow controllers and a home-built cold-trap system. Background spectrum was recorded in flowing He and subtracted from the sample spectrum for each measurement, except in the OH region. Then, the difference DRIFT spectra of adsorbed NH3 were obtained after exposure of the sample to 500 ppm NH3 at room temperature in He flow for 1 h followed by purging with He at the same temperature for 30 min. EPR spectra were recorded on a Bruker EMXplus-9.5/2.7 spectrometer operating at the X-band (~ 9.4 kHz) with 100-kHz field modulation. Before each EPR analysis, powder samples (~30 mg) were placed in a quartz tube, and if necessary, they were pretreated from room


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5 temperature to 300 °C at a ramp rate of 5 °C min-1 under dry pure O2 flow to prevent possible reduction of divalent copper ions due to the vacuum treatment and then were held for 1 h at the same temperature.11,17 The flow was changed to dry He for 2 min at room temperature to remove paramagnetic O2 gas in the tube. After sealing the tube, EPR measurements were carried out in the field region 2000 - 4000 G with a sweep time of 10.24 s at -150 °C to immobilize Cu2+ ions. The cryogenic temperature was indispensable, because severe line broadening from antiferromagnetic interactions between highly mobile copper ions could induce signal loss in the EPR spectrum when measured at room temperature. The copper ion contents in Cu-UZM-35 zeolites were quantified by comparing the intensities of the spectra (found as the double integral) with those of EPR calibration spectra obtained from solid solutions of CuSO4 diluted in K2SO4. For all samples, such quantifications were repeated three times in order to ensure reliability of experiments. As a result, copper ions content determined by EPR signal were in good agreement with those obtained from XANES and elemental analyses with an error of less than 10%. Structural Analysis. Both fresh and aged Cu-UZM-35 zeolites were loaded in a 0.7 mm quartz glass capillary, pretreated by heating from room temperature to 350 °C at a ramp rate of 10 °C min-1 under an O2 atmosphere, held at the same temperature for 2 h, and evacuated under a vacuum of 10-4 Torr for another 1 h. The capillary was then sealed and analyzed at room temperature on the 5A beamline at the Pohang Acceleration Laboratory (PAL; Pohang, Korea) using monochromated X-rays (λ = 0.6926 or 1.0716 Å). The synchrotron powder XRD data were obtained with a step size of 0.01° or 0.015° for a scan time of 1 s per step over the 2θ range 2-45° or 2-75°. The initial framework structure model with space group P42/mnm (no. 136) was obtained from the Structure Commission of the International Zeolite Association13 and then preliminary optimized using the Sanders-Leslie-Catlow (SLC) potential18 in the GULP program.19 The structural refinements were performed by Rietveld method using the



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6 GSAS suite of programs and EXPGUI graphical interface.20-22 The framework T-O and O-O distances were soft constrained to 1.61 Å (σ = 0.03 Å) and 2.65 Å (σ = 0.08 Å), respectively. The background was refined using the Chebyschev polynomial function, and the pseudo-Voigt profile function was used to fit the observed peaks.23 The copper ion positions were derived from Fourier difference maps. Difference Fourier peaks located at reasonable distances from the framework atoms were investigated as possible sites by refinement of occupancy and position. The total copper ion contents (3.5 and 2.2 per unit cell (uc), respectively) in fresh and aged Cu-UZM-35 zeolites were soft-restrained based on the values calculated by multiplying the overall copper content (3.3 Cu wt%) in the fresh sample determined using elemental analysis by the percentages (81 and 50%, respectively) of copper ions in both samples, which were calculated from their K-edge XANES spectra using linear combination fitting (Table S1).15 In the final stage of the refinements, the weight of the soft-restraint was reduced, which did not lead to any significant changes in the interatomic distances. The isotropic displacement factors were refined by grouping the framework tetrahedral atoms, the framework oxygen atoms, and the copper ions, respectively. The isotropic atomic displacement parameters of copper ions in Cu-UZM-35 were fixed to 0.025 Å2 for the convergence. The convergence was achieved by simultaneously refining profile parameters, scale factor, lattice constants, 2θ zero, the atomic positional, thermal displacement parameters, and occupancy factors for the copper ions.

RESULTS AND DISCUSSION

Comparison of the powder XRD pattern of fresh Cu-UZM-35 with that of the aged one reveals that no noticeable decrease in the crystallinity was caused by hydrothermal aging at 750 °C (Supporting Information Figure S1). Also, the XRD pattern of aged Cu-UZM-35


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7 showed no X-ray reflections assignable to oxidic copper species such as CuO, Cu2O, and CuAl2O4.24,25 However, our recent XANES study has shown that about 60% of Cu2+ ions in fresh Cu-UZM-35 remain intact after hydrothermal aging, which is accompanied by a decrease in N2 BET surface area (from 460 to 370 m2 g-1).15 The latter decrease could be a result of dealumination, although its extent is not so large as evidenced by 27Al MAS NMR spectroscopy (Figure S2). It also appears that a non-negligible amount of oxidic copper species is present in aged UZM-35, whereas their particle size is not large enough to be detected by powder XRD. There is a general consensus on the existence of two types of divalent copper ions, i.e., Cu2+ and [Cu(OH)]+, in copper ion-exchanged zeolites, especially in Cu-SSZ-13. It has recently been shown that the ratio of these two copper species (Cu2+/[Cu(OH)]+) is strongly affected by the Si/Al and Cu/Al ratios of Cu2+-exchanged zeolites: the Cu2+ ion becomes more dominant with decreasing Si/Al and Cu/Al ratios.26 To elucidate their existence in Cu-UZM-35, we first obtained the DRIFT spectra in the OH region of Cu-UZM-35 before and after hydrothermal aging, as well as of H-UZSM-35 for comparison. Five OH bands around 3780, 3740, 3730, 3650, and 3600 cm-1 were resolved in the spectrum of H-UZM-35 (Figure S3). The band at 3780 cm-1 is assignable to OH groups attached to the tricoordinated Al atom partially connected to the zeolite framework,27 which completely disappears after Cu2+ exchange. Also, the bands around 3740 and 3730 cm-1 are assigned to terminal and either germinal or vicinal Si-OH groups, respectively.28 The DRIFT spectrum of aged Cu-UZM-35 shows the reversal in intensity of these two bands, suggesting the additional formation of germinal or vicinal Si-OH groups that could be caused by the dealumination during hydrothermal aging at 750 °C. The intensity of the band at 3600 cm-1 due to bridging Si-OH-Al groups (Brønsted acid sites) was found to decrease after Cu2+ ion exchange and to almost disappear after hydrothermal aging. According to previous studies on Cu-SSZ-13, on the other hand, it is well known that the OH band appearing at 3650



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8 cm-1 in the IR spectrum of Cu-SSZ-13 corresponds to Cu[(OH)]+ ions,17,29 because of the negligible formation of extraframework Al. As shown in Figure S3, however, the band at 3650 cm-1 is rather stronger in intensity for H-UZM-35 than for fresh Cu-UZM-35. This did not allow us to ascertain the presence of Cu[(OH)]+ ions in Cu-UZM-35 using DRIFT spectroscopy. Next, we used EPR spectroscopy to check whether the Cu[(OH)]+ ion is really present in fresh and aged Cu-UZM-35 zeolites and, if so, to quantify its amount. In the fully hydrated state, the Cu2+ and Cu[(OH)]+ ions are expected to exist as [Cu(H2O)6]2+ and [Cu(OH)(H2O)5]+, respectively.29 Although both states are EPR-active and thus possible to be quantified, their differentiation is almost unattainable owing to very similar spin Hamiltonian parameters.30 When fully dehydrated, hydrated [Cu(H2O)6]2+ ions are changed to naked Cu2+ ions which are still EPR-active. By contrast, hydrated [Cu(OH)(H2O)5]+ ions are transformed to naked Cu[(OH)]+ or dimeric [Cu-O-Cu]2+ species, via consecutive reactions, which are both EPRsilent due to the pseudo-Jahn-Teller effect and signal broadening from antiferromagnetic coupling, respectively. Also, regardless of the dehydration, copper oxide (CuOx) species, as well as CuAl2O4, are EPR-silent because of the strong antiferromagnetic coupling effect.29-31 Thus, since the signal corresponding to only naked Cu2+ ions is observable in the dehydrated samples, two different forms of divalent copper ions can be identified and quantified by comparing the EPR spectra of hydrated and dehydrated samples. Figure 2 shows the EPR spectra at -150 °C of fresh and aged Cu-UZM-35 zeolites in both hydrated and dehydrated states. The spectra of their hydrated form are characterized by one signal with g|| = 2.385, A|| = 138 G and g|| = 2.377, A|| = 141 G, respectively. These g|| and A|| values are in good agreement with those observed for the copper ions in the hydrated form of various other zeolites, including Y (FAU), beta (*BEA), mordenite (MOR), and SSZ-13.30 The EPR spectra of fresh and aged Cu-UZM-35 in the hydrated state also exhibit one signal with g|| = 2.335, A|| = 149 G and g|| = 2.324, A|| = 155 G, respectively. After dehydration, the g|| and


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9 A|| parameters of both zeolites were found to somewhat decrease and increase, respectively. A similar trend was also observed for the other copper ion-exchanged zeolites and has been explained by the higher coordination environment around copper ions, as well as by the ligand donor profile of water molecules different from that of zeolite framework atoms.30,31 As shown in Figure 2, in addition, dehydration accompanies notable differences in the signal shape, width, and intensity. This implies that there are considerable changes in the local environment around Cu2+ ions in Cu-UZM-35, although the details are beyond the scope of our study. Table 1 lists the numbers of Cu2+ and Cu[(OH)]+ ions per uc in fresh and aged Cu-UZM35 zeolites determined from a comparison of the intensities of their double integrated EPR spectra (Figure S4) with those obtained from the known amounts of CuSO4. A considerable decrease in intensity of the double integrated spectra when dehydrated clearly shows the existence of Cu[(OH)]+ in both zeolites. It is worth noting that the numbers of total copper ions in fresh and aged UZSM-35 zeolites quantified by EPR match well with those determined by a combination of elemental and XANES analyses. As expected, in addition, the EPR signal intensity significantly decreases after hydrothermal aging at 750 °C (Figures 2 and S4). Therefore, it is clear that the Cu2+ and/or Cu[(OH)]+ ions in Cu-UZSM-35 are converted to EPR-silent CuOx species. We should note here that the intensities of the double integrated spectra of fresh and aged Cu-UZM-35 zeolites in the dehydrated states are essentially identical with each other. This means that while EPR-active Cu2+ ions are still intact during hydrothermal aging, Cu[(OH)]+ ions are the main species converted to EPR-silent CuOx. This is not unexpected because the Cu[(OH)]+ ions have been known to interact more weakly with the zeolite framework than Cu2+ ions.4 The EPR characterization results in Table 1 reveal that less than 20% of Cu[(OH)]+ ions in fresh Cu-UZM-35 remain unchanged after hydrothermal aging at 750 °C.



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10 On the basis of the numbers of Cu2+ and Cu[(OH)]+ ions per uc in Table 1, we attempted to determine the locations of these two copper species in the dehydrated form of fresh and aged Cu-UZM-35 zeolites using Rietveld analysis of synchrotron powder XRD data. For the Rietveld refinement, the framework atomic positions reported for MCM-68 (MSE) were taken as a starting model.13 The crystallographic data for both zeolites, together with the agreement factors for the Rietveld analyses, are listed in Table 2. Final Rwp, Rp, and RF2 values were in the acceptable range i.e., less than 5%, confirming the proposed structure model. The Rietveld plots for fresh and aged Cu-UZM-35 zeolites in Figure 3 provide a good match between observed and simulated patterns. The final refined atomic coordinates, fractional occupancy, and thermal displacement parameters are listed in Tables S1 and S2. The average T-O bond lengths (1.607 and 1.608 Å) and average O-T-O (109.4 and 109.4°) and T-O-T angles (151.0 and 150.0°) obtained are in good agreement with those expected for zeolitic materials (Tables S3 and S4). Figure 4 shows the refined copper ion positions in the dehydrated from of fresh Cu-UZM35. Four different copper ion sites labelled sites Cu1-Cu4 were found in this large-pore zeolite. Interestingly, all of these four cation sites are not located within the 10- and 12-ring windows but near the considerably smaller 6-rings in Cu-UZM-35, which can also be supported by the T-O-T vibrational region in DRIFT spectra perturbed by NH3 adsorption. In the case of CuSSZ-13, its DRIFT spectrum when perturbed by NH3 has been repeatedly reported to exhibit two T-O-T vibrational bands at 900 and 950 cm-1, assignable to the Cu2+ and Cu[(OH)]+ ions located at double 6-rings and within the 8-ring windows of cha cages, respectively.29,32 As shown in Figure S5, however, the DRIFT spectrum of fresh Cu-UZM-35 is characterized by only one broad band around 920 cm-1, although its copper content (Si/Al = 8.9 and Cu/Al = 0.38) is rather large compared to the Cu-SSZ-13 catalyst with Si/Al = 12 and Cu/Al = 0.28 studied by Song et al.29


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11 As shown in Figure 4, sites Cu1 and Cu2 are surrounded by the most and least severely puckered 6-rings among those of the t-mse-1* cage, respectively. We note that these two 6rings are also part of the large t-mse-2* and much smaller mtw cages, respectively. The Cu-O distances (2.4 - 2.5 Å) of copper ions at site Cu1 were determined to be rather longer than the distances (2.1 - 2.3 Å) of Cu2+ ions in other zeolites with different structures (Table 3),10,33,34 revealing their weak interactions with the UZM-35 framework oxygen atoms. However, because of their innermost position in the tortuous 10-ring channels, NH3-SCR reactants cannot have good access to the cations at site Cu1. This should be more apparent to the site Cu2 located within the t-mse-1* cage consisting of 4- and 6-rings only. As shown in Figure 4, site Cu3 is also surrounded by the puckered 6-rings of the t-mse-1* cage, but these are less puckered than those associated with site Cu1. An important point is that unlike sites Cu1 and Cu2, this site is in proximity to 12-ring channels. Thus, the copper cations at site Cu3 could serve as active sites for catalysis. A similar conclusion can be drawn for the site Cu4 located at the relatively planar 6-rings of the t-mse-1* cage, because its cations are easily accessible via tortuous 10ring channels. Figure 5 shows the refined copper ion positions in the dehydrated form of aged Cu-UZM35. Unlike the case of fresh Cu-UZM-35, there are three different cation sites labelled sites Cu1ʹ - Cu3ʹ. Inspection of the Fourier difference maps indicates no residual electron density at site Cu4 in fresh Cu-UZM-35, implying that the copper ions at this site have either been migrated to other sites or most likely converted to CuOx species during hydrothermal aging at 750 °C. Since the DRIFT spectrum of aged Cu-UZM-35 exhibits only one broad band around 920 cm-1, like that of fresh Cu-UZM-35 (Figure S5), no copper ions appears to be present within its 10- and 12-ring windows. Comparison of the structure of aged Cu-UZM-35 with that of fresh Cu-UZM-35 reveals that the copper ions at sites Cu1ʹ and Cu2ʹ are somewhat shifted toward and away from the t-mse-1* cage center, respectively, but without changes in their



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12 coordination environment. On the other hand, the copper ions at site Cu3 in fresh Cu-UZM-35 are strongly coordinated with two framework oxygen (O14) atoms with a bond length of 2.09 Å (Table 3). As shown in Figure 5, however, the cations at site Cu3ʹ in aged Cu-UZM-35 are shifted toward the center of the 6-rings with an intermediate degree of puckering in the t-mse1* cage. Compared to those at site Cu3 in fresh Cu-UZM-35, as a result, they have two additional strong coordination bonds with the framework oxygen (O5) atoms with a bond length of 2.21 Å (Table 3). This led us to conclude that the copper ions at site Cu3ʹ are thermodynamically more stable than those at site Cu3. Table 4 lists the numbers per uc of copper ions at various sites in fresh and aged Cu-UZM35 zeolites. No noticeable differences in the number per uc of copper ions were found between Cu1/Cu1ʹ and Cu3/Cu3ʹ site pairs. However, there is a notable decrease (0.6 per uc) in copper ion number for the Cu2/Cu2ʹ site pair. This is particularly true for the cations at site Cu4, because no such site exists in aged Cu-UZM-35. Therefore, it is clear that unlike the copper ions at sites Cu1 and Cu3, the majority, if not all, of the cations at sites Cu2 and Cu4 are not stable enough to remain intact during hydrothermal aging at 750 °C. The quantitative EPR results in Table 1 reveal that while no detectable decrease in number per uc of Cu2+ ions in fresh Cu-UZM-35 is caused by hydrothermal aging, over 80% of Cu[(OH)]+ ions in the same zeolite has disappeared and has been mainly converted to CuOx species. When combining this with the numbers of copper ions at various sites in Table 4, we can conclude that site Cu4 in fresh Cu-UZM-35 is predominantly occupied by Cu[(OH)]+ ions. Also, ca. 40% of the copper ions residing at site Cu2 appear to exist as Cu[(OH)]+ ions. Because this OH group-containing cation is less stable than Cu2+, it is not difficult to expect a notable decrease in its amount after hydrothermal aging at 750 °C. However, further study is necessary to understand the reason for the unexpectedly high preference of Cu[(OH)]+ ions for site Cu4 in Cu-UZM-35.


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13 The results presented thus far provides two useful insights towards understanding the nature of active sites in Cu-UZM-35. First, as well known in the case of Cu-SSZ-13,4,29 the Cu2+ ions are the main active sites in aged Cu-UZM-35 for NH3-SCR. In other words, the Cu[(OH)]+ ions may be mainly responsible for the high low-temperature deNOx activity of fresh Cu-UZM-35 (Figure S6). Second, the operating temperature window of aged Cu-UZM35 for NH3-SCR, which is considerably wider than that of aged Cu-SSZ-13 cannot be rationalized without considering notable differences in the local environment of Cu2+ ions in these two catalysts with different zeolite support structures. This suggests that the local environment of these ‘isolated’ divalent cations and thus the pore topology of zeolite supports is a critical factor governing the width of the operating temperature window of copper ionexchanged zeolite catalysts for NH3-SCR. Finally, it is interesting to note that while no significant decrease in high-temperature (> 400 oC) NH3 oxidation activity due to hydrothermal aging at 750 oC is observed for Cu-SSZ-13, the opposite holds for Cu-UZM-35. As shown in Figure S7, the NH3 oxidation activity of aged Cu-UZM-35 is still quite low even at 500 oC, unlike that of its fresh form. The fact that differences in the CuOx content between aged CuSSZ-13 and Cu-UZM-35, as well as between their fresh form, are not so large (Table S1), strongly suggests that the contribution of CuOx species to high-temperature NH3 oxidation is considerably lower for the latter catalyst than for the former one. This led us to speculate that the CuOx within aged Cu-UZM-35 may be relatively less accessible to NH3 molecules compared to those within aged Cu-SSZ-13.



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14 CONCLUSIONS

The physical states and locations of copper ions in Cu-UZM-35 with Si/Al = 8.9 and Cu/Al = 0.38 before and after hydrothermal aging at 750 °C have been investigated by using IR and EPR spectroscopies and synchrotron powder X-ray diffraction and Rietveld analyses. It was found that all the copper ions are positioned near the 6-rings in Cu-UZM-35 but not within the 10- and 12-ring windows. There are four different cation positions (Cu1-Cu4) in fresh Cu-UZM-35, and the Cu2+ and [Cu(OH)]+ ions located mainly at sites Cu3 and Cu4 accessible through 12-ring and 10-ring channels, respectively, have been identified as active sites for NH3-SCR. Interestingly, the [Cu(OH)]+ ions at site Cu4 are mainly converted to oxidic copper species during hydrothermal aging at 750 °C, which may be responsible for a notable decrease in low-temperature deNOx activity. By contrast, the Cu2+ ions at site Cu3 remain intact after hydrothermal aging, clearly showing that they are the main active species in aged CuUZM-35 and play a key role in maintaining the wide operating temperature window of aged Cu-UZM-35. The overall results of our study suggest that the isolated Cu2+ ions can be stabilized even on the puckered single 6-rings that are one of major building units of wellknown zeolites such as ZSM-5 and beta.

ASSOCIATED CONTENT

Supporting Information. Powder XRD patterns and 27Al MAS NMR, DRIFT IR, and double integration EPR spectra of Cu-UZM-35 zeolites and Rietveld plots, refined atomic coordinates, site occupancies, thermal parameters, and selected bond lengths and angles for their crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.


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15 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Nak Ho Ahn: 0000-0002-6015-0137 Jiho Shin: 0000-0002-0279-4006 Taekyung Ryu: 0000-0001-5242-9923 Suk Bong Hong: 0000-0002-2855-1600 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support from the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea. We also thank Prof.

H.

I.

Lee

(KNU)

for

helpful

discussion,

Sachem

for

providing

dimethyldipropylammonium hydroxide, an organic structure-directing agent for UZM-35 synthesis, and PAL (5A, H. H. Lee) for synchrotron diffraction beam time. PAL is supported by MSIP and POSTECH.



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16 REFERENCES (1) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E. Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts. Catal. Rev. 2004, 46, 163-245. (2) Bosch, H.; Janssen, F. Catalytic Reduction of Nitrogen Oxides. Catal. Today 1988, 2, 369-532. (3) Chen, H.-Y. In Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts; Nova, I., Tronconi, E., Eds.; Springer: New York, 2014; pp 123-147. (4) Beale, A. M.; Gao, F.; Lezcano-Gonzalez, I.; Peden, C. H. F.; Szanyi, J. Recent Advances in Automotive Catalysis for NOx Emission Control by Small-Pore Microporous Materials. Chem. Soc. Rev. 2015, 44, 7371-7405. (5) Xin, Y.; Li, Q.; Zhang, Z. Zeolitic Materials for DeNOx Selective Catalytic Reduction. ChemCatChem, 2018, 10, 29-41. (6) Dusselier, M.; Davis, M. E. Small-Pore Zeolites: Synthesis and Catalysis. Chem. Rev. 2018, 118, 5265-6329. (7) Iwamoto, M.; Furukawa, H.; Mine, Y.; Uemura, F.; Mikuriya, S.; Kagawa, S. Copper (II) Ion-Exchanged ZSM-5 Zeolites as Highly Active Catalysts for Direct and Continuous Decomposition of Nitrogen Monoxide. J. Chem. Soc., Chem. Commun. 1986, 16, 12721273. (8) Andersen, J.; Bailie, J. E.; Casci, J. L.; Chen, H. Y.; Fedeyko, J. M.; Foo, R. K. S.; Rajaram, R. R. Patent WO/2008/132452, 2008. (9) Bull, I.; Xue, W. M.; Burk, P.; Boorse, R. S.; Jaglowski, W. M.; Koermer, G. S.; Moini, A.; Patchett, J. A.; Dettling, J. C.; Caudle, M. T. U.S. Patent 7,601,662 B2, 2009. (10) Ryu, T.; Ahn, N. H.; Seo, S.; Cho, J.; Kim, H.; Jo, D.; Park, G. T.; Kim, P. S.; Kim, C. H.; Bruce, E. L.; Wright, P. A.; Nam, I.-S.; Hong, S. B. Fully Copper-Exchanged High‐Silica LTA Zeolites as Unrivaled Hydrothermally Stable NH3-SCR Catalysts. Angew. Chem. Int. Ed. 2017, 56, 3256-3260. (11) Andersen, C. W.; Bremholm, M.; Vennestrøm, P. N. R.; Blichfeld, A. B.; Lundegaard, L. F.; Iversen, B. B. Location of Cu2+ in CHA Zeolite Investigated by X-ray Diffraction Using the Rietveld/Maximum Entropy Method. IUCrJ. 2014, 1, 382-386. (12) Calabro, D. C.; Cheng, J. C.; Crane, R. A.; Kresge, C. T.; Dhingra, S. S.; Steckel, M. A.; Stern, D. L.; Weston, S. C. U.S. Patent 6,049,018, 2000.


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17 (13) Baerlocher, Ch.; McCusker, L. B.; Database of Zeolite Structures, http://www.izastructure.org/databases/ (accessed November 22, 2018). (14) Moscoso, J. G.; Jan, D. Y. U.S. Patent 7,922,997, 2011. (15) Lee, J. H.; Kim, Y. J; Ryu, T.; Kim, P. S.; Kim, C. H; Hong, S. B. Synthesis of Zeolite UZM-35 and Catalytic Properties of Copper-exchanged UZM-35 for Ammonia Selective Catalytic Reduction. Appl. Catal. B 2017, 200, 428-438. (16) Kim, Y. J.; Lee, J. K.; Min, K. M.; Hong, S. B.; Nam, I.-S.; Cho, B. K. Hydrothermal stability of CuSSZ13 for reducing NOx by NH3. J. Catal. 2014, 311, 447-457. (17) Giordanino, F.; Vennestrøm, P. N. R.; Lundegaard, L. F.; Stappen, F. N.; Mossin, S.; Beato, P.; Bordiga, S.; Lamberti, C. Characterization of Cu-Exchanged SSZ-13: A Comparative FTIR, UV-Vis, and EPR Study with Cu-ZSM-5 and Cu-β with Similar Si/Al and Cu/Al Ratios. Dalton Trans. 2013, 42, 12741-12761. (18) Schröder, K.-P.; Sauer, J.; Leslie, M.; Catlow, C. R. A.; Thomas, J. M. Bridging Hydroxyl Groups in Zeolitic Catalysts: A Computer Simulation of Their Structure, Vibrational Properties and Acidity in Protonated Faujasites (H-Y Zeolites). Chem. Phys. Lett. 1992, 188, 320-325. (19) Gale, J. D.; Rohl, A. L. The General Utility Lattice Program (GULP). Mol. Simul. 2003, 29, 291-341. (20) Rietveld, H. A Profile Refinement Method for Nuclear and magnetic Structures J. Appl. Crystallogr. 1969, 2, 65-71. (21) Larson, A. C.; Dreele, R. B. V. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report; LAUR, 2000; pp 86-748. (22) Toby, B. H. EXPGUI, a Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210-213. (23) Hastings, J. B.; Thomlinson, W.; Cox, D. E. Synchrotron X-Ray Powder Diffraction. J. Appl. Crystallogr. 1984, 17, 85-95. (24) Hu, C.-Y.; Shin, K.; Leckie, J. O. Formation of Copper Aluminate Spinel and Cuprous Aluminate Delafossite to Thermally Stabilize Simulated Copper-Laden Sludge. J. Hazard. Mater. 2010, 181, 399-404. (25) Dalconi, M. C.; Cruciani, G.; Alberti, A.; Ciambelli, P. Over-Loaded Cu-ZSM-5 upon Heating Treatment: A Time Resolved X-Ray Diffraction Study. Microporous Mesoporous Mater. 2006, 94, 139-147.



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18 (26) Paolucci, C.; Parekh, A. A.; Khurana, I.; Di Iorio, J. R.; Li, H.; Albarracin Caballero, J. D.; Shih, A.; Anggara, T.; Delgass, W. N.; Miller, J. T., et al. Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites. J. Am. Chem. Soc. 2016, 138, 6028-6048. (27) Vimont, A.; Thibault-Starzyk, F.: Lavalley, J. C. Infrared Spectroscopic Study of the Acidobasic Properties of Beta Zeolite. J. Phys. Chem. B 2000, 104, 286-291. (28) Lercher, J. A.; Jentys, A. Infrared and Raman Spectroscopy for Characterizing Zeolites. Stud. Surf. Sci. Catal. 2007, 168, 435-476. (29) Song, J.; Wang, Y.; Walter, E. D.; Washton, N. M.; Mei, D.; Kovarik, L.; Engelhard, M. H.; Prodinger, S.; Wang, Y.; Peden, C. H. F.; Gao, F. Toward Rational Design of Cu/SSZ-13 Selective Catalytic Reduction Catalysts: Implications from Atomic-Level Understanding of Hydrothermal Stability. ACS Catal. 2017, 7, 8214-8227. (30) Godiksen, A.; Vennestrøm, P. N. R.; Rasmussen, S. B.; Mossin, S. Identification and Quantification of Copper Sites in Zeolites by Electron Paramagnetic Resonance Spectroscopy. Top. Catal. 2017, 60, 13-29. (31) Godiksen, A.; Stappen, F. N.; Vennestrøm, P. N. R.; Giordanino, F.; Rasmussen, S. B.; Lundegaard, L. F.; Mossin, S. Coordination Environment of Copper Sites in Cu-CHA Zeolite Investigated by Electron Paramagnetic Resonance. J. Phys. Chem. C 2014, 118, 23126-23138. (32) Luo, J. Y.; Gao, F.; Kamasamudram, K.; Currier, N. W.; Peden, C. H. F.; Yezerets, A. New Insights into Cu/SSZ-13 SCR Catalyst Acidity. Part I: Nature of Acidic Sites Probed by NH3 Titration. J. Catal. 2017, 348, 291-299. (33) Deka, U.; Juhin, A.; Eilertsen, E. A.; Emerich, H.; Green, M. A.; Korhonen, S. T.; Weckhuysen, B. M.; Beale, A. M. Confirmation of Isolated Cu2+ Ions in SSZ-13 Zeolite as Active Sites in NH3-Selective Catalytic Reduction. J. Phys. Chem. C 2012, 116, 48094818. (34) Attfield, M. P.; Weigel, S. S.; Cheetham, A. K. On the Nature of Nonframework Cations in a Zeolitic DeNOx Catalyst: Cu-Mordenite. J. Catal. 1997, 170, 227-235.


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19 Table 1. Amounts of Cu2+ and [Cu(OH)]+ Ions and Their Ratios in Fresh and Aged Cu-UZM-35 Zeolites. catalyst

copper ions/uca copper ions/hydrated ucb Cu2+ ions/dehydrated ucb [Cu(OH)]+ ions/ucc [Cu(OH)]+/Cu2+ (%)

fresh Cu-UZM-35

3.5

3.5

aged Cu-UZM-35

2.2

2.3

aDetermined

2.1

1.4

67

2.1

0.2

10

bDetermined

by a combination of elemental and XANES analyses. by EPR analysis. between the EPR-active copper ions in the hydrated state and those in the dehydrated state.


cDetermined

from the difference


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20 Table 2. Data Collection and Crystallographic Data for the Dehydrated Form of Fresh and Aged Cu-UZM-35 Zeolites catalyst unit cell composition symmetry space group a (Å) b (Å) c (Å) unit cell volume (Å3) Rwp (%) Rp (%) RF2 (%)

fresh Cu-UZM-35 |Cu3.5|[Si112O224]

aged Cu-UZM-35 |Cu2.2|[Si112O224] tetragonal P42/mnm

18.28954(21) 18.28954(21) 20.2638(4) 6778.38(18) 4.74 3.31 3.80

18.25853(20) 18.25853(20) 20.2694(4) 6757.27(17) 3.79 2.72 3.11


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21 Table 3. Selected Cu-O Bond Lengths for the Dehydrated Forms of Fresh and Aged CuUZM-35 Zeolites fresh, Cu-UZM-35 aged, Cu-UZM-35 bond length (Å) bond length (Å) a Cu1-O1 2.44(4) ×2 Cu1ʹ -O12 2.40(5) ×2 Cu1-O19 2.49(5) ×4 Cu1ʹ -O19 2.54(5) ×4 Cu2-O6 2.17(4) ×1 Cu2ʹ -O6 2.30(6) ×1 Cu2-O19 2.64(2) ×2 Cu2ʹ -O6 2.53(6) ×1 Cu3-O5 2.54(3) ×2 Cu2ʹ -O19 2.49(3) ×2 Cu3-O14 2.09(2) ×2 Cu3ʹ -O5 2.21(3) ×2 Cu3-O1 2.64(5) ×1 Cu3ʹ -O11 2.52(6) ×1 Cu4-O4 2.45(7) ×1 Cu3ʹ -O14 2.15(4) ×2 Cu4-O9 2.11(7) ×1 Cu4-O17 2.40(3) ×2 a Multiplication number indicates the number of corresponding bonds between Cu and O atoms.



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22 Table 4. Site Occupancies, Multiplicities, and Number per Unit Cell for the Copper Ion Sites in the Dehydrated Form of Fresh and Aged Cu-UZM-35 Zeolites fresh Cu-UZM-35 site(s)

occupancy multiplicity

aged Cu-UZM-35

no./uca,b

occupancy multiplicity

no./uca,b

Cu1/Cu1ʹ

0.075(8)

4f

0.3

0.079(8)

4f

0.3

Cu2/Cu2ʹ

0.175(5)

8j

1.4

0.104(6)

8j

0.8

Cu3/Cu3ʹ

0.138(6)

8i

1.1

0.127(5)

8i

1.0

Cu4

0.085(5)

8j

0.7

0

8j

0.0

Calculated as the multiplication of site occupancy and multiplicity. The total numbers of copper ions in fresh and aged Cu-UZM-35 zeolites are 3.5 and 2.1 per unit cell, respectively. a

b


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23 FIGURE CAPTIONS Figure 1. MSE structure and its two unique cages, the 16-hedral ([46610]) t-mse-1* and 24hedral ([465866104]) t-mse-2*. Figure 2. EPR spectra at -150 °C of fresh (black, blue) and aged (red, green) Cu-UZM-35 zeolites. The former and latter colors in parentheses indicate the hydrated and dehydrated states, respectively. The inset shows a magnification of the hyperfine region of the EPR spectra. Figure 3. Rietveld plots for (a) fresh and (b) aged Cu-UZM-35 zeolites: observed data (black), calculated fit (red) and difference plot (blue). The tick marks represent the positions of allowed reflections. Figure 4. Four different copper ion sites labelled as Cu1-Cu4 found in (a) fresh Cu-UZM-35 and its (b) t-mse-1*, (c) t-mse-2*, and (d) mtw cages. Color code: yellow, Si; red, O; blue, Cu. Figure 5. Three different copper ion sites labelled as Cu1ʹ-Cu3ʹ found in (a) aged Cu-UZM35 and its (b) t-mse-1* cage. Color code: yellow, Si; red, O; blue, Cu.



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Figure 1


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Figure 2



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Figure 3


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Figure 4



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Figure 5


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29 TOC Graphic of Ahn et al., “Extraframework Cation Locations in Cu-UZM-35 NH3-SCR Catalyst”


Extraframework Cation Locations in Cu-UZM-35 NH3-SCR Catalyst

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