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Facile synthesis of KFI-type zeolite and its application to selective catalytic reduction of NOx with NH3 Jonghyun Kim, Sung J Cho, and Do H Kim ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00697 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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Facile synthesis of KFI-type zeolite and its application to selective catalytic reduction of NOx with NH3 Jonghyun Kim,a Sung J. Chob and Do H. Kim*a

a.

School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul

National University, Seoul 08826, Republic of Korea. b.

Department of Chemical Engineering, Chonnam National University, Yongbong 77, Bukgu,

Gwangju 61186, Republic of Korea.

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Abstract

The small-pore KFI-type zeolite was synthesized via the hydrothermal conversion of zeolite Y with Na+ and K+ ions without using organic structure-directing agent (OSDA). The effects of alkali metal ions and hydroxide ion on the crystallization of KFI were individually investigated by introducing both hydroxide and nitrate salts into synthesis media. The zeolite KFI synthesized at the optimized condition was copper ion-exchanged, and then applied for selective catalytic reduction of NOx with NH3 (NH3-SCR) reaction. The Cu-KFI was found to exclusively contain isolated copper ions up to very high loading (Cu/Al2=75%) based on the Cu K-edge extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), ultra violet visible near infrared spectroscopy (UV-vis-NIR) and temperature programed reduction by H2 (H2-TPR) analyses. Interestingly, Cu-SSZ-13 and Cu-KFI have almost identical activation energy for NH3-SCR reaction as well as the reduction temperature of “hydrated” copper ions by H2, indicating the presence of similar active sites on both catalysts. After severe hydrothermal aging at 800 °C for 16 h, Cu-KFI substantially maintained structural integrity, which is remarkably stable compared with CuChabazite synthesized from organic-free media. The NH3-SCR activity test under realistic condition showed that Cu-KFI with high Cu loading has comparable activity to Cu-SSZ-13 even after hydrothermal aging. All combined results evidently confirm that zeolite-based catalyst prepared from organic-free condition can also exhibit excellent hydrothermal stability, which is attributed to the high crystallinity of zeolite and the presence of only isolated copper ions.

Keywords: Zeolite KFI, organic-free synthesis, NH3-SCR, Cu-SSZ-13, hydrothermal aging

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1. Introduction Zeolite KFI, the first synthetic zeolite which does not exist in nature, has small pore (0.39 nm) with three-dimensional window system containing secondary building units of lta, pau and d6r.1 It has been known that KFI-based materials show high activity for the selective synthesis of dimethylamine, and the separation of CO2/N2 or CO2/CH4 mixture gases.1-3 Zeolite KFI was firstly synthesized by Barrer in 1948, who denoted this zeolite as species P and Q.4 Following the discovery by Barrer, there have been a few methods to synthesize the zeolite KFI, and among these, the one using Sr+ and K+ ions as structure-directing agents have been applied for the catalytic application due to the easy extraction of counter-cation.2, 5 However, this method has some drawbacks, one of which is the formation of impurity due to the sensitive synthesis condition.6-7 Hence, the use of organic agent (i.e. 18-crown-6) is crucial to prevent the formation of by-products.3, 8 Selective catalytic reduction of NOx with NH3 (NH3-SCR) is a well-established technology for reducing NOx from diesel engine exhaust. In the last decades, copper-exchanged zeolites such as Cu-ZSM-5 (MFI) and Cu-beta (BEA) have been extensively studied for NH3-SCR due to their wide operating temperature and high N2 selectivity.9-10 One of the most critical problems in the conventional catalysts was severe loss of their activity after hydrothermal aging due to the dealumination of zeolite and the subsequent formation of inactive species. Recently, Cu-SSZ-13 (CHA) was reported to show exceptional hydrothermal stability as well as remarkable activity for NH3-SCR.11-12 The presence of small-pore in Cu-SSZ-13, which hinders the extraction of Al(OH)3 species from zeolite cage, was suggested as a reason for the high stability.13 Such fact has brought great attention to other types of small pore zeolite such

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as SSZ-16 (AFX),13 SSZ-39 (AEI),14 and LTA,15 which also exhibit great hydrothermal stability. In general, organic structure-directing agent (OSDA) is essentially required to synthesize the hydrothermally stable small-pore zeolites. Although the use of OSDA is effective for the synthesis of target zeolite because of its strong structure-directing effect, organic-free synthesis is more favorable from an economic and an environmental point of view.16-18 Recently, Ji and co-workers reported that low-silica Chabazite (CHA) synthesized from inorganic media showed long-term stability for methanol-to-olefins (MTO) reaction after steaming treatment, which provided a possibility to use the inorganic based small-pore zeolites.19 In the following study, they showed that RHO- and KFI-type zeolite can also be applied for MTO reaction.20 In the case of NH3-SCR reaction, Cu-Chabazite exhibited comparable activity to Cu-SSZ-13, while it did not resist the hydrothermal aging at 700 °C.21 Although Cu-Chabazite with low Si/Al2 ratio shows poor hydrothermal stability, there are various types of zeolite with small pore that have not been explored for NH3-SCR reaction to our knowledge. The structure of CHA is constructed with double six membered ring, and the large cage, cha, can be generated in which the plane of d6r is pointed toward the cage, allowing the movement of Cu ion, which is unique characteristics for small pore zeolite with potential for SCR reaction. The Cu ion in CHA structure can be stabilized inside of d6r at high temperature. Similarly, the composite building unit of d6r can be found in many zeolite topologies. The zeolite with such a corresponding d6r composite building unit and high Si/Al2 ratio such as AEI, ERI, AFX and SFW shows the promising potential for SCR reaction.14, 22-23 KFI also contains the same composite building unit, d6r and the resulting large cage are pau and lta, which are similar to cha in CHA zeolite. Since these small-pore 5


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zeolites are basically composed of d6r, they are expected to show similar catalytic properties, while different cage type of the zeolites can result in distinct characteristics. However, few studies have focused on the comparison of the copper state or catalytic activity depending on the framework type of zeolite. The nature of active sites in Cu-SSZ-13 for NH3-SCR reaction has been extensively studied since all framework sites of SSZ-13 are structurally identical, which makes the characterization of the catalyst easier. Fickel et al. firstly revealed that Cu2+ ions exclusively exist at the face of double six-membered ring in the dehydrated Cu-SSZ-13 based on Rietveld refinement analysis.12 In later studies, the additional site for dehydrated Cu ions near 8membered ring (i.e. [CuOH]+) was also identified on the basis of the extensive characterizations such as temperature programed reduction by H2 (H2-TPR), extended X-ray absorption fine structure (EXAFS), diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and Rietveld/maximum entropy analyses.24-27 At an ambient condition, all copper ions in Cu-SSZ-13 exist in hydrated form with high mobility as identified by X-ray absorption near edge structure (XANES) and electron paramagnetic resonance (EPR) analyses.28-29 The coordinating H2O of hydrated copper ion effectively reduces the interaction between copper ion and zeolite framework, resulting in the significantly lower reduction temperature of hydrated copper ion than bare copper ion during H2-TPR analysis.30 According to recent studies, the presence of [Cu(NH3)x]2+ and [Cu(H2O)x]2+ complexes in Cu-SSZ-13 catalyst during NH3-SCR reaction was suggested based on operando X-ray absorption spectroscopy (XAS) and ab initio calculation results.31-32 Herein, we report a novel method for facile synthesis of zeolite KFI in the presence of Na+ and K+ ions via the interzeolite transformation of zeolite Y (CBV712, Si/Al2=12) without using OSDA, which enables us to synthesize highly crystalline KFI without forming any 6


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impurity. Several characterizations of Cu-KFI catalyst including EXAFS, XANES, UV-visNIR and H2-TPR are primarily performed, while focusing on the identification of the state of hydrated copper ions in Cu-KFI. The kinetic study for NH3-SCR reaction over Cu-KFI and Cu-SSZ-13 catalysts is examined to find out the effects of copper loading and zeolite framework on the activity. The remarkable hydrothermal stability of zeolite KFI is confirmed by the analyses of X-ray diffraction and N2 adsorption-desorption, and NH3-SCR activity of hydrothermally aged Cu-KFI catalyst.

2. Experimental 2.1. Catalyst preparation Commercial zeolite Y with different Si/Al2 ratios (5.1, 5.2 and 12 for CBV100, CBV500 and CBV712, respectively) were purchased from Zeolyst. Before the synthesis, CBV500 and CBV712 were calcined at 550 ºC for 2 h to decompose ammonium ion. In a typical synthesis (i.e. standard condition), 1.72 g of NaNO3 (Samchun Chemical), 5.11 g of KNO3 (Samchun Chemical), and 4.15 g of 1 M NaOH (Sigma-Aldrich) solution were added to 11.1 g of deionized water in 40 ml Teflon cup. After mixing to dissolve all salts for 1 h, 0.50 g of calcined zeolite Y with the Si/Al2 of 12 (CBV712) was added and stirred for about 1 min. The resulting mixture was transferred to a stainless steel autoclave and kept at 140 ºC for 3 days in a static oven. Then, the products were filtrated, washed with deionized water, and dried at 105 ºC. The batch composition of the standard condition was as follows: 1 SiO2 : 0.083 Al2O3 : 3.3 Na+ : 7 K+ : 0.56 OH– : 117 H2O. In order to investigate the effect of the composition on the zeolite formation, the concentrations of Na+, K+ and OH– ions were varied, while keeping the same amounts of SiO2, Al2O3 and H2O. 7


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Zeolite SSZ-1328, 33 and low-silica Chabazite34 were synthesized by well-known method that can be found elsewhere. For the synthesis of SSZ-13, 5 g of sodium silicate solution (26.5% SiO2, Sigma-Aldrich), 4.16 g of 1 M NaOH solution, and 6.4 g of deionized water were stirred. Then, 2.1 g of N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH, ZeoGen 2825, Sachem) followed by 0.5 g of zeolite Y with the Si/Al2 of 5.2 (CBV500) were added and mixed. The mixture was transferred to a stainless steel autoclave and heated at 140 ºC for 5 days in a static oven. Prior to the preparation of Chabazite, zeolite Y with the Si/Al2 of 5.1 (CBV100) was ion-exchanged in 0.5 M NH4NO3 solution and repeated twice, and then calcined at 550 °C for 2 h. For the synthesis of Chabazite, 5.83 g of potassium hydroxide (90%, Sigma-Aldrich) and 93.8 g of deionized water were dissolved. Then, 10 g of HCBV100 was added to the solution, and mixed for 1 min. The mixture in a borosilicate glass bottle was heated in a water bath at 95 ºC for 4 days. The resulting solid was obtained by filtration, washing with deionized water, and then drying at 105 ºC. As-synthesized KFI was ion-exchanged three times in 1 M NH4NO3 solution at 80 ºC for 24 h. Resulting NH4-exchanged KFI (NH4-KFI) was filtered, washed with deionized water and dried at 105 ºC overnight. The Cu-KFI catalysts with various copper loadings were prepared by (repeated) wet ion-exchange of NH4-KFI in 0.016–0.08 M Cu(NO3)2 solution at 25 °C. For the preparation of Cu-KFI with highest copper loading, the ion-exchange was carried out in 0.08 M Cu(NO3)2 solution at 80 °C. After filtration and extensive washing with deionized water, Cu-KFI was calcined at 550 ºC for 2 h in static air condition. The hydrothermal aging of selected Cu-KFI catalysts was conducted at 800 ºC for 16 h in the presence of 10% H2O and 15% O2. The calcined and hydrothermally aged Cu-SSZ-13 and Cu-Chabazite catalysts were also prepared following the similar procedures. In this paper,

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copper-exchanged zeolite catalysts are denoted as Cu(x)-KFI or Cu(x)-SSZ-13, where x represents the Cu/Al2 molar ratio (%).

2.2. Characterization Nitrogen adsorption-desorption isotherms were measured at −196 °C using ASAP 2010 (Micrometrics Instrument Co.). Before the measurement, each sample was degassed at 300 ºC for at least 12 h under vacuum condition. Specific surface area of samples was calculated using BET method. The chemical compositions of samples were analyzed with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using Optima-4300 DV (Perkin Elmer). X-ray diffraction (XRD) patterns were taken with powder X-ray diffractometer (Smartlab, Rigaku) operated at 40 kV and 30 mA. The crystallinity of KFI was calculated by the sum of XRD peak intensity at 9.54, 20.25, 21.36, 23.42, 27.94, 29.58 and 31.88 ° (2 theta). The relative crystallinity of KFI is defined as the crystallinity of KFI prepared from each condition divided by that from the “standard condition”. The morphology, particle size, and chemical composition of samples were identified by using field emission scanning electron microscopy (FE-SEM) with energy-dispersive X-ray spectroscopy (MERLIN Compact, ZEISS). 29

Si and

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Al NMR spectra were measured on Bruker Avance II operating at 500 MHz

using a 4 mm MAS probe. For

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Si NMR measurement, spinning frequency of 5 kHz,

spectral window of 60 kHz, complex points of 4754, and pulse delay of 4.0 µs were utilized to acquire 512 time-averaged scans. For

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Al NMR measurement, spinning frequency of 10

kHz, spectral window of 65 kHz, complex points of 6572, and pulse delay of 2.0 µs were utilized to acquire 512 time-averaged scans. Time domain free induction decays were 9


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apodized with exponential functions corresponding to 100 Hz of Lorentzian line broadening prior to Fourier transformation. Temperature programed reduction by hydrogen (H2-TPR) of catalysts was performed on a chemisorption analyzer (BEL-CAT B, BEL Japan Inc.). For the measurement, about 0.03 g of the sample loaded on quartz wool in a U-shaped quartz tube was exposed to 5% H2/Ar flow with increasing temperature at 10 °C/min. The consumption of hydrogen was monitored with a thermal conductivity detector (TCD), which was stabilized for 60 min in 5% H2/Ar flow before the measurement. Ultra violet visible near infrared spectroscopy (UV-vis-NIR) spectra in a diffuse reflectance mode of the NH4- and Cu-KFI samples were recorded using Shimadzu UV-3600 plus spectrometer at ambient condition. Before the sample measurement, baseline spectrum was recorded using BaSO4 as a reference material. Cu K-edge X-ray absorption fine structure (XAFS) spectroscopy was acquired at Pohang Accelerator Laboratory (7D-XAFS beamline in PLS-II) using Si(111) crystal as monochromator where the beam energy and ring current were 2.5 GeV and 300 mA, respectively. Energy calibration was carried out with Cu foil (E0 = 8993 eV). Ionization chamber for incident and transmitted beam were purged with 1 atm of He and N2, respectively. The step and duration time for X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were 0.4 eV and 2 sec, and 0.30 nm−1 and 3 sec, respectively. The obtained EXAFS data were analyzed by ATHENA and ARTEMIS following the literature.35 For XAFS analysis, Artemis implemented in in Demeter program package (0.9.25) was utilized after the data processing using Athena. The background removal was performed to extract XAFS signal using AUTOBK program for Rbkg=0.1 nm and subsequently the corresponding XAFS data in k space was Fourier transformed with the Kaiser-Bessel window function, 10 nm−1 after k3 weighting to amplify 10


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the high k information. The range for Fourier transformation for Cu containing samples, ∆k were 30–130 nm−1. The phase shifts and amplitude functions of the reference was generated using Feff6L. The number of independent point of the data for the curve fit, Nidp determined from Nyquist theorem was always larger than the number of variable, providing the sufficient degree of freedom, Nvar. The scattering path from the possible model structure was obtained from the Feff calculation. The zeolite structure of Si/Al2~8 containing Cu ion was subject to lattice energy minimization calculations using the General Utility Lattice Program (GULP) (version 4.3). All atomic coordinates and cell parameters were optimized to zero force using the BroydenFletcher-Goldfarb-Shanno (BFGS) minimization method. Calculations were performed at constant pressure using a zeolite shell model as the potential model in which a Buckingham function was used to describe the short-range interactions and a three-body (bond bending) term was included to accurately model O-T-O angles. A shell model was also used to simulate the polarizability of the oxygen atoms. The Newton-Raphson optimizer was employed during energy minimization, with maximum function and gradient tolerances of 0.0001 and 0.001 eV Å−1, respectively; the symmetry was constrained and the cell parameters varied. A gradient-norm convergence criterion of 0.001 eV Å−1 was used for all optimizations. All calculations were performed on a Dell XPS 8700 desktop computer with 8 cores based on the Intel Core i7-4770 CPU processor (3.4 GHz) with 24 GB RAM. In order to estimate the relative stability, the lattice energy of the zeolite was compared with respect to that of quartz, pure SiO2.

2.3. Catalytic activity test 11


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The kinetic experiment for NH3-SCR reaction was performed with a fixed-bed quartz reactor (inner diameter=1/2 inch). Prior to the reaction, each catalyst was calcined at 550 ºC for 2 h followed by pelletizing and sieving to 180–250 µm. To minimize the effect of reaction exotherm, the catalyst was diluted with α-Al2O3 bead. Reaction feed gas containing 500 ppm NO, 500 ppm NH3, 8% O2 and balanced with N2 was introduced to 0.012–0.02 g of catalyst and 0.3 g of α-Al2O3 bead (GHSV=300,000–500,000 h–1). Fourier transform infrared (FT-IR) spectrometer (Nicolet 6700, Thermo scientific) with a 2 m gas cell heated at 120 ºC was used to measure the concentration of nitrogen oxides. The kinetic data was collected in the NOx conversion below 20% after the reaction reached steady-state. The NO conversion was calculated according to the following equation: NO conversion X =

NO − NO × 100 % NO

where NO and NO are inlet and outlet NO concentration, respectively. The turnover frequency (TOF) of NH3-SCR reaction was calculated based on the following equation: TOF =

n ∙ X M& W#$ ∙ 63.546 g/mol

where n, X , W#$ and M& represent the molar flow rate of NO (mol/s), the NO conversion, the weight of loaded catalyst (g) and the weight percent of copper in a catalyst (%) obtained from ICP-AES analysis, respectively. For the light-off test of NH3-SCR reaction under realistic condition, H2O was added by using syringe pump (KDS 100, kd Scientific). All stainless steel gas lines were heated at 120 °C to prevent the condensation of H2O. Reaction feed gas containing 500 ppm NO, 500 ppm NH3, 8% O2, 5% H2O and balanced with N2 was introduced to 0.05 g of catalyst without α12


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Al2O3 bead (GHSV=120,000 h–1). Other conditions of reaction test were same as the kinetic test as described above.

3. Results and discussion 3.1. Interzeolite transformation of FAU-type zeolite The synthesis of zeolite in inorganic media is known to be affected sensitively by several factors such as Si/Al ratio, OH−/Si ratio, temperature and so on.16, 36 To identify the role of alkali metal ion and hydroxide ion individually, both nitrate and hydroxide salts were used as alkali metal sources for hydrothermal conversion of zeolite Y. The hydroxide salt (i.e. NaOH) was used to control the concentration of OH− ion whereas the concentration of Na+ and K+ was adjusted by using nitrate salts (i.e. NaNO3 and KNO3). First, the effects of Na+ and K+ ions on the interzeolite transformation of zeolite Y were examined while fixing other compositions. Figure 1 displays the main zeolite phase synthesized as a function of the amount of Na+ and K+ at 140 °C for 3 days. In the K+ rich region, zeolite KFI was produced and its relative crystallinity (number in Figure 1) gradually increased as the concentration of Na+ and K+ ions increased, indicating that both ions had the promoting effect on the formation of KFI. Note that CHA-type zeolite was formed in Na+ rich region, which is beyond the scope of the current study. A pure and highly crystalline KFI phase can be obtained within 3 days at 140 °C with the high concentration of Na+ and K+: Al/Si=0.083, Na+/Si=3.3, K+/Si=7, OH−/Si=0.56 and H2O/Si=117, which is denoted as “standard condition”. In other K+ rich conditions where KFI zeolite was produced (Na+/Si=0−3.3 and K+/Si=3−7), the absence of any impurity phase (i.e. other types of zeolite) was confirmed with XRD (data not shown). Since KFI phase can be formed even in the absence of Na+ ion, 13


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it appears that K+ ion is more essential than Na+ ion for the nucleation of KFI phase as a structure-directing agent. One of the important factors in the organic-free synthesis of a zeolite is the amount of OH−, a mineralizing agent.36 From the “standard condition”, OH−/Si ratio was varied from 0.33 to 1.67 while the other compositions were fixed as shown in Figure 2. At low OH− concentration (OH−/Si=0.45), the crystallization rate of zeolite KFI decreased, resulting in the lower crystallinity of KFI phase after crystallization for 3 days (data not shown). After the synthesis for 5 days at this condition, fully crystalline KFI zeolite without forming amorphous phase was produced. When the alkalinity went even lower (OH−/Si=0.33), any crystalline phase was not obtained until 7 days. Such lower crystallization rate with less amount of OH− may be explained by the role of OH−, which facilitates the breakage and the recombination of T–O–T bonds (T: Si and Al).36-37 At high OH− concentration with OH−/Si ratio of 0.83 and 1.66, other types of zeolites such as LTL and CHA were produced, indicating that the thermodynamics of zeolite crystallization was greatly influenced by alkalinity. Similar effect of OH− ion was observed by Goel et al. who synthesized CHA-type zeolite by applying hydrothermal conversion of FAU-type zeolite in the absence of OSDA.17 In our case, the use of nitrate salts with appropriate amount of OH− was also crucial to obtain KFI phase. When the synthesis was conducted by using only KOH as an alkali metal ion source at OH−/Si=0.67, which was same as the standard condition, any crystalline phase was not observed with XRD (Figure S1). At higher KOH concentration to enhance the crystallization rate, only CHA-type zeolite was formed. All combined results demonstrate the important role of nitrate salts, which enhance the crystallization rate of zeolite KFI without altering the phase equilibrium.

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It is interesting to note that the main products (KFI, LTL and CHA) from interzeolite transformation of zeolite Y contain double six-membered ring (d6r). The synergistic role of common composite building unit (e.g. d6r) in the synthesis of target zeolite has also been observed from the seed-assisted synthesis of zeolite.38 It is likely that some d6r from zeolite Y remain stable in the early stage of the synthesis, which may lead to the nucleation of zeolite containing d6r. When other Si and Al sources were applied instead of zeolite Y, other products such as MER, LTL and amorphous phase were obtained, which also confirmed the importance of using zeolite Y (Figure S2). The XRD pattern of as-synthesized KFI from “standard condition” clearly shows that all peaks are well matched with reference KFI peaks without forming any impurity phase (Figure S3). The morphology of the KFI is cubic as revealed with FE-SEM (Figure S4).3, 6 The crystal size of KFI in this study (2–12 µm) is larger than that of KFI prepared from Sr, K system (~2 µm).3 The growth of KFI structure exhibits typical S-shaped curve with the complete crystallization time of 48 h (Figure S5), which is significantly shorter than previous method (120 h).8 Solid state

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Al NMR spectra show only one peak at around 59 ppm

corresponding to tetrahedral aluminum, representing that all aluminum constitute the framework of KFI (Figure S6). The

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Si NMR spectra show three peaks at −100, −105 and

−110 ppm, corresponding to Si(–OSi)4-n(–OAl)n with n=2, 1 and 0, respectively (Figure S7). The Si/Al2 ratios of KFI obtained from SEM-EDS and

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Si-NMR analysis are 7.5±0.2 and

7.2, respectively. The resemblance of two Si/Al2 ratios also indicates that the majority of aluminum exist in the zeolite framework.

3.2. Textural property of Cu-KFI catalysts 15


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Zeolite KFI prepared at the standard condition was ion-exchanged three times in 1M NH4NO3 solution at 80 °C to completely extract the alkali metal ions. The NH4-KFI was ionexchanged in Cu(NO3)2 solution under the various condition and calcined at 550 °C for 2 h. Negligible amount of Na+ ( Cu(55)-KFI. The N2O formation was very low (80%) over the wide temperature range (250–450 °C), which is comparable to aged Cu(50)-SSZ-13. Such high activity of aged Cu(75)-KFI reflect that the optimum copper loading of Cu-KFI is higher than Cu-SSZ-13 in terms of NH3-SCR activity. The N2O formation of aged catalyst slightly increased in the high temperature region for all catalysts (Figure S21). Although the activity of aged Cu-KFI catalyst is somewhat lower than that of aged Cu-SSZ-13 catalyst, it should be noted that the Cu-KFI catalyst, which can be prepared without using OSDA, is advantageous from an economic and environmental point of view. We also note that Cu-KFI is much more stable than Cu-Chabazite (Si/Al2~4.2) prepared by organic-free method, which completely lost its crystallinity after hydrothermal aging according to the previous study21 and our result (Figure S22), presumably due to higher Si/Al2 ratio and crystallinity of KFI than Chabazite.

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4. Conclusion The highly crystalline KFI-type zeolite was synthesized via the interzeolite transformation of zeolite Y without using organic structure-directing agent. The use of NaNO3 and KNO3 is found to be essential to obtain KFI phase, because it facilitates to easily control the concentration of Na+ and K+ while maintaining the pH. The synthesis period of KFI at “standard condition” is 2 days, which is much shorter than that of previous system using Sr and K (5 days). The presence of only isolated copper ions in Cu-KFI catalyst with Cu/Al2 ratio=17−75% is identified with XANES, EXAFS, vis-NIR and H2-TPR analysis. The CuKFI catalyst with Cu/Al2≤55%, in which only isolated Cu2+ ion is present, exhibits similar activation energy with Cu-SSZ-13 catalyst for NH3-SCR reaction. The Cu2+ ions exist as hydrated form at ambient condition, which is reduced by H2 at identical temperature over all Cu-KFI and Cu-SSZ-13 catalysts irrespective of copper content due to the weak interaction between copper ion and zeolite framework. The kinetic study for NH3-SCR reaction gives the lower pre-exponential factor of Cu-KFI than Cu-SSZ-13, which can be explained by the lower collision frequency of Cu-KFI resulting from larger cage size although the energetic state of the active site seems to be identical for both catalysts. After severe hydrothermal aging at 800 °C for 16 h, XRD and N2 physisorption results indicate that the framework structure of Cu-KFI is largely preserved. The NH3-SCR activity of Cu-KFI with highest copper loading (i.e. Cu(75)-KFI) under realistic condition is comparable to Cu-SSZ-13 even after hydrothermal aging. Based on the results, it is firstly shown that copper ion-exchanged zeolite with relatively low Si/Al2 ratio, which is prepared from organic free-media, can exhibit excellent hydrothermal stability and SCR activity.

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ASSOCIATED CONTENT Supporting Information. The results of zeolite synthesis, catalysts characterization and reaction test. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. The authors have applied for a patent based on the reaction reported in this paper. ACKNOWLEDGMENT This research was supported by the R&D Center for reduction of Non-CO2 Greenhouse gases (0458-20160034) funded by Korea Ministry of Environment (MOE) as Global Top Environment R&D Program. We thank Pohang Accelerator Laboratory (PAL) for XAFS experiment at the 7D beamline. We acknowledge Sachem, Inc. for kindly providing Zeogen 2825 (TMAdaOH) used to synthesize SSZ-13. This research was partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF2016R1A5A1009592).

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Figure 1 Effect of Na+/Si and K+/Si ratio on the interzeolite transformation of zeolite Y. The numbers in the figure represent the relative crystallinity of as-synthesized zeolite KFI. The gel composition is as follows: 1 SiO2 : 0.83 Al2O3 : x Na+ : y K+ : 0.56 OH− : 117 H2O.

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Figure 2 XRD patterns of products from the interzeolite transformation of zeolite Y with different OH−/Si ratio. The gel composition is as follows: 1 SiO2 : 0.83 Al2O3 : 3.3 Na+ : 7 K+ : x OH− : 117 H2O.

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Figure 3 Normalized

29

Si-NMR spectra of dried (a) and calcined (b) Cu-KFI catalysts

measured at ambient condition.

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Figure 4 XANES spectra of Cu-KFI catalysts with Cu/Al2 ratio of 17, 30, 38, 55 and 75%, Cu-SSZ-13, Cu2O and CuO measured at ambient condition.

Figure 5 Vis-NIR spectra of Cu-KFI catalysts with Cu/Al2 ratio of 17, 30, 38, 55 and 75% measured at ambient condition.

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Figure 6 H2-TPR spectra of dehydrated (a) and hydrated (b) Cu-KFI catalysts with the Cu/Al2 ratio of 17, 30, 38, 55 and 75%.

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Figure 7 Arrhenius plot of Cu-KFI catalysts with the Cu/Al2 ratio of 17, 30, 38, 55 and 75%, and Cu-SSZ-13 catalyst.

Figure 8 First derivative of Cu K-edge XANES spectra of Cu(55)-KFI and Cu(75)-KFI catalysts.

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Figure 9 H2-TPR spectra of hydrothermally aged Cu(55)- and Cu(75)-KFI catalysts.

Figure 10 NH3-SCR activity of fresh and hydrothermally aged Cu(55)-KFI, Cu(75)-KFI and Cu(50)-SSZ-13 catalysts. Feed composition: 500 ppm NO, 500 ppm NH3, 8% O2, 5% H2O balanced with N2 (GHSV=120,000 h−1). 37


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Table 1. Cu/Al2 ratios, BET surface areas, pore volumes and (Si/Al2)f ratios of Cu-KFI and Cu-SSZ-13 catalysts.

Cu/Al2 ratio (%)

BET surface area of fresh catalysts (m2/g)

Pore volume of fresh catalysts (cm3/g)

BET surface area of aged catalysts (m2/g)

Pore volume of fresh catalysts (cm3/g)

(Si/Al2)f ratio of dried catalysts

(Si/Al2)f ratio of calcined catalysts

Cu(17)-KFI

17

619

0.22

–

–

7.2

10.5

Cu(30)-KFI

30

619

0.22

–

–

8.1

10.1

Cu(38)-KFI

38

584

0.21

–

–

8.1

9.8

Cu(55)-KFI

55

593

0.21

543

0.20

8.2

9.9

Cu(75)-KFI

75

604

0.23

496

0.18

7.9

8.4

Cu(50)-SSZ-13

50

659

0.24

569

0.21

–

10.4

Table 2. EXAFS fitting parameters for Cu-KFI and Cu-SSZ-13 catalysts. Sample name

Pair

CN

R(Å)

σ2(Å2)

∆E(eV)

R factor

Cu(17)-KFI (fresh)

Cu-O

3.5±0.3

1.94±0.01

0.0045±0.0009

-2.0±1.1

0.0027

Cu(30)-KFI (fresh)

Cu-O

3.7±0.5

1.96±0.01

0.0049±0.0012

-2.4±1.5

0.0053

Cu(37)-KFI (fresh)

Cu-O

3.7±0.4

1.96±0.01

0.0050±0.0010

-2.5±1.3

0.0036

Cu(55)-KFI (fresh)

Cu-O

3.7±0.4

1.96±0.01

0.0048±0.0011

-2.7±1.3

0.0024

Cu(75)-KFI (fresh)

Cu-O

3.8±0.2

1.95±0.01

0.0049±0.0006

-3.5±0.8

0.0011

Cu(50)-SSZ-13 (fresh)

Cu-O

3.5±0.4

1.96±0.01

0.0045±0.0010

-2.9±1.3

0.0037

Cu(55)-KFI (aged)

Cu-O

3.4±0.4

1.95±0.01

0.0047±0.0012

-1.5±1.6

0.0046

Cu(75)-KFI (aged)

Cu-O

3.7±0.3

1.95±0.01

0.0053±0.0009

-2.8±1.1

0.0022

Cu(50)-SSZ-13 (aged)

Cu-O

3.3±0.4

1.96±0.01

0.0042±0.0010

-1.7±1.3

0.0039

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Table of Contents Graphic

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