Ion Exchange of One-Pot Synthesized Cu-SAPO-44 with NH₄NO₃ to Promote Cu Dispersion and Activity for Selective Catalytic Reduction of NO_x with NH₃

Abstract

Cu-containing CHA type (Cu-CHA) zeolites have been widely investigated owing to their excellent low-temperature activity and high hydrothermal stability in selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Herein, a series of Cu-SAPO-44 zeolites were prepared by one-pot method with dual-amine templates and the subsequent ion exchange (IE) with NH₄NO₃. The effect of NH₄NO₃ treatment on Cu species was investigated by X-ray powder diffraction (XRD), N₂ adsorption-desorption isotherm, inductively coupled plasma (ICP); field-emission scanning electron microscope (FE-SEM), high-resolution transmission electron microscope (HRTEM), X-ray absorption fine structure (XAFS), and H₂-temperature-programmed reduction (H₂-TPR). The results indicated that—besides the main SAPO-44 structure—the CuO phase was detected by XRD in original samples. After IE with NH₄NO₃, the Cu contents decreased greatly from ICP analysis. The removal of CuO agglomerations and the presence of highly dispersed CuO nanoparticles (~2.36 nm) were confirmed by SEM, TEM and H₂-TPR. Furthermore, a significant increase in the proportion of isolated Cu²⁺ was derived from XAFS. As a result, the activity at higher temperature (≥350 °C) was improved a lot.

1. Introduction

Aqueous ion exchange (IE) is the most widely used method in preparation of small pore Cu-chabazite (Cu-CHA) zeolites [1,2,3]. Multiple IE procedures are time consuming; therefore, one-pot synthesis methods were hence developed [4,5,6,7,8,9]. For instance, Ren et al. pioneered the synthesis of Cu-SSZ-13 using Cu²⁺-tetraethylenepentamine (Cu-TEPA) complex as structure-directing agent (SDA), with Cu loading in the range 0–10 wt.% [10]. Soon after, Cu-SAPO-34 zeolites with controllable Cu-loadings were synthesized by one-pot method using Cu-TEPA and low-cost SDAs [6,11,12,13]. More recently, another Cu-CHA zeolite, Cu-SAPO-44, was also reported [14]. In contrast to IE method, one-pot method could directly introduce Cu²⁺ ions into zeolites frameworks, and thus achieve high Cu loading and high dispersion of Cu species, leading to excellent NH₃-SCR activity [15].

The downside, however, is that one-pot synthesis method tend to introduce excessive Cu species into CHA zeolites, which is detrimental to the hydrothermal stability of Cu-CHA catalysts [5]. IE with NH₄NO₃ was discovered as an efficient approach to achieve moderate Cu-loadings in one-pot synthesis of Cu-CHA zeolites by removing the excessive Cu species [10,16,17]. Xie et al. reported that one-time IE with NH₄NO₃ was completely sufficient to eliminate excess Cu species from the SSZ-13 structure and re-disperse the remaining Cu²⁺ ions [15]. Guo et al. demonstrated that Cu²⁺ ions migrated from large cages to more stable locations in six-membered rings of CHA structure after IE with NH₄NO₃ [17]. Nevertheless, there still lacks direct observation on the structural change of Cu species from one-pot synthesized Cu-CHA zeolites.

Herein, a series of Cu-SAPO-44 zeolites were fabricated through one-pot method with the dual-amine templates (denoted as Cu_x-SAPO-44) followed with ion exchange with NH₄NO₃ solution (denoted as Cu_x-SAPO-44-IE). The effects of IE with NH₄NO₃ on the textural and physical-chemical properties of Cu-SAPO-44, as well as the nature and location of active Cu species, were investigated. Interestingly, IE with NH₄NO₃ removed excessive CuO agglomerations, leading to a high proportion of isolated Cu²⁺ ions responsible for the high SCR activity.

2. Results and Discussion

2.1. XRD

Figure 1 displays XRD patterns of Cu-SAPO-44 zeolites before and after IE with NH₄NO₃. The diffraction peaks for all samples are in accordance with those of pure SAPO-44 reported in our previous work [18], confirming the CHA structure [14,19]. However, the CuO phase was present in Cu_x-SAPO-44. Interestingly, the Cu_x-SAPO-44-IE samples show only diffraction patterns of SAPO-44, suggesting that IE with NH₄NO₃ can remove the excess CuO species and simultaneously retain the SAPO-44 structure.

2.2. ICP

The compositions of all samples were analyzed by ICP and listed in

Table 1. ICP results for Cu_x-SAPO-44 and Cu_x-SAPO-44-IE.

Sample	Element Content from ICP (wt.%)
	Cu	Si	Al	Si/Al	P
Cu4.3-SAPO-44	4.3	5.7	17.2	0.32	16.5
Cu5.6-SAPO-44	5.6	6.4	16.7	0.37	13.6
Cu7.6-SAPO-44	7.6	5.1	17.6	0.28	15.1
Cu1.0-SAPO-44-IE	1.0	6.1	18.8	0.31	18.2
Cu1.6-SAPO-44-IE	1.6	7.1	18.7	0.37	14.0
Cu2.2-SAPO-44-IE	2.2	4.8	20.2	0.23	18.4

. Although the Si/Al ratios do not change much after NH₄NO₃ treatment, the Cu contents decrease evidently after IE with NH₄NO₃, in accordance with above XRD results.

2.3. Catalytic Activity

SCR performances of Cu_x-SAPO-44 and Cu_x-SAPO-44-IE are shown in Figure 2. The Cu_x-SAPO-44 samples show 90% NO_x conversion between 200 and 350 °C as well as high N₂ selectivity. However, NO_x conversions decrease rapidly when the reaction temperatures are above 300 °C, due to the over-oxidation of NH₃ in the presence of CuO [15,20,21]. Notably, Cu_x-SAPO-44-IE samples exhibit improved NO_x conversion at high temperatures, reaching above 90% at 200–450 °C with nearly 100% N₂ selectivity. This is consistent with the fact that the excess CuO species promote the oxidation of NH₃ [10,15,17]. To gain more information about the beneficial role of IE, further characterization was performed taking the control sample (Cu_5.6-SAPO-44) and the best sample (Cu_1.6-SAPO-44-IE) as examples.

Based on the practical consideration, NH₃-SCR performance of the Cu_1.6-SAPO-44-IE catalyst was also evaluated as 5 vol.% H₂O contained in feeding gas under the GHSV of 100,000 h⁻¹, and the result was shown in Figure 3. Unsurprisingly, a decrease in NO_x conversion was observed at low temperatures (<250 °C), which could be ascribed to the competitive adsorption of H₂O with NH₃. In contrast, NO_x conversion at high temperatures (>400 °C) was promoted, most likely due to the inhibition effect of H₂O on the unselective catalytic oxidation of NH₃ [22].

2.4. N₂ Adsorption/Desorption

The N₂ adsorption/desorption isotherms of Cu_5.6-SAPO-44 and Cu_1.6-SAPO-44-IE exhibit the typical microporosity (Figure 4), similar to the pure SAPO-44 [18]. As a result, their BET surface areas are comparable. Furthermore, the pore volume of Cu_1.6-SAPO-44-IE slightly increased in comparison with Cu_5.6-SAPO-44 (

Table 2. Textural properties and XAFS fitting results for Cu_5.6-SAPO-44 and Cu_1.6-SAPO-44-IE.

Sample	BET Surface Areas (m2/g)	Pore Volume (cm3/g)	Cu2+/CuO Data from LCF of XANES Spectra (wt.%/wt.%)
Cu5.6-SAPO-44	338.8	0.15	22.4/77.6
Cu1.6-SAPO-44-IE	363.7	0.17	69.7/30.3

), suggesting the CuO species that block the pores in CHA zeolite have been partially removed, as detected by ICP (Table 1).

2.5. Electron Micrograph

SEM and TEM were conducted for Cu_5.6-SAPO-44 and Cu_1.6-SAPO-44-IE (Figure 5). It is notable that both Cu_5.6-SAPO-44 (Figure 5a,b) and Cu_1.6-SAPO-44-IE (Figure 5d,e) show the cubic-like rhombohedral morphology, similar to those reported in literature [19]. According to EDS elemental mappings, some large Cu-containing agglomerations are observed in Cu_5.6-SAPO-44 that can be ascribed to the aggregated CuO nanoparticles (Figure 5c), owing to its high Cu content. As reported by He et al., the excess Cu species in Cu-SSZ-13 zeolites could be effectively removed by ion exchange using NH₄NO₃ solution [15]. Accordingly, large CuO particles can be scoured into CuO nanocrystals and re-dispersed as Cu²⁺ ions during this ion exchange process. In this work, after IE with NH₄NO₃, highly dispersed Cu-containing species were observed over the entire zeolite as for Cu_1.6-SAPO-44-IE (Figure 5f), which can be attributed to the remaining CuO nanoparticles and Cu²⁺ ions at the framework ion-exchange sites. In order to clarify the highly dispersed Cu-containing species after IE with NH₄NO₃, Cu_1.6-SAPO-44-IE was further characterized by TEM (Figure 5g–i). The well-dispersed nanocrystals were observed in Cu_1.6-SAPO-44-IE (Figure 5g), which were proved to be CuO nanoparticles by identifying the characteristic spacings of 2.27 and 1.87 Å for the (2 0 0) and the (-2 0 2) lattice planes of monoclinic CuO (Figure 5h). A particle count taken from many TEM images, obtained from different regions of the sample, confirmed the presence of monodispersed CuO nanoparticles with a mean diameter of 2.36 nm anchored on the surface of Cu_1.6-SAPO-44-IE (Figure 5i). Combined with ICP data, the results confirm that IE with NH₄NO₃ can not only remove the excessive Cu species (in the form of CuO agglomerations) but also improve the dispersion of the remaining CuO nanoparticles. This result is consistent with the observation from other researchers that CuO aggregates decrease the SCR performance at high temperature range as a result of parasitic NH₃ oxidation [15,23].

2.6. H₂-TPR

H₂-TPR was also performed on Cu_5.6-SAPO-44 and Cu_1.6-SAPO-44-IE, and the results were shown in Figure 6. For Cu_5.6-SAPO-44 (a), the obvious reduction peak centered at ~300 °C could be ascribed to the reduction of CuO (>3 nm) to Cu⁰ [24]. However, this peak disappeared in Cu_1.6-SAPO-44-IE (b), and a broad reduction peak ranging from 160 to 750 °C emerged, suggesting the removal of large agglomerated CuO particles and the existence of highly dispersed Cu²⁺ and CuO microcrystals after NH₄NO₃ treatment, in accordance with the results of XRD and SEM.

2.7. XAFS

Individual Cu species were studied by XAFS in combination with linear combination fitting. Figure 7a shows the X-ray absorption near edge structure (XANES) spectra of Cu K-edge for Cu_5.6-SAPO-44, Cu_1.6-SAPO-44-IE, and reference samples. The XANES spectrum of Cu_5.6-SAPO-44 resembled that of CuO, while that for Cu_1.6-SAPO-44-IE is similar to that of CuSO₄. The corresponding Fourier-transformed k³-weighted Cu K-edge patterns give more direct evidence (Figure 7b). In the case of Cu_5.6-SAPO-44, two characteristic peaks at approximately 2.6 and 3.0 Å were detected, which are attributed to the neighboring Cu atoms (Cu-O-Cu) in CuO [25]. For Cu_1.6-SAPO-44-IE, the peak at ~1.5 Å assigned to the Cu-O scatterings is similar with that of CuSO₄, confirming the predominance of isolated mononuclear Cu²⁺ species [26]. In addition, weak peaks at ~2.5 Å for Cu_1.6-SAPO-44-IE were distinguished, indicating the coexistence of few CuO nanoparticles. The relative amount of Cu²⁺ and CuO could be obtained from the intense analysis of XANES spectra using linear combination fitting (LCF) [27]. For Cu_5.6-SAPO-44, the majority of Cu species are CuO nanoparticles, while the Cu²⁺ ions are dominant in Cu_1.6-SAPO-44-IE (Figure 8). Thus, the isolated Cu²⁺ proportion was significantly improved via IE.

Consequently, the enhanced high-temperatures activity is attributed to the removal of CuO agglomerations and the relocation of the Cu²⁺ ions for Cu_x-SAPO-44-IE [10,17]. However, Cu_1.6-SAPO-44-IE still exhibited lower activity at high temperature (350–550 °C) compared with the traditional ion-exchanged counterpart with a comparative Cu content that contains only isolated Cu²⁺ ions [28].

3. Materials and Methods

3.1. Catalyst Preparation

In preparation of Cu-SAPO-44, the gel has a molar ratio of 0.14–0.22 Cu:0.44 TEPA:0.6 SiO₂:0.8 Al₂O₃:1 P₂O₅:40 H₂O:2 N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHD). CuSO₄ solution (20 wt.%) and TEPA were mixed under vigorous stirring at room temperature to get a homogeneous sol After that, phosphoric acid (H₃PO₄, 85 wt.% aqueous solution), pseudo-boehmite (Al₂O₃, 78 wt.%), colloidal silica sol (30 wt.% suspension in water), TMHD template and deionized water were added to the Cu-TEPA complex solution under vigorous stirring. Then, the resulting viscous gel was aged with stirring at room temperature over 12 h, and further hydrothermally heated statically in a Teflon-lined steel autoclave at 200 °C for 96 h. The product was obtained by centrifugal, washed several times with deionized water, and dried at 100 °C for 12 h. Cu_x-SAPO-44 zeolites were obtained by calcination at 550 °C for 6 h in air to remove the structure-directing agent, where “x” represents the Cu content in the catalyst determined by ICP analysis.

In this study, the as-prepared Cu-CHA zeolite samples, have high Cu contents of 4.3, 5.6, and 7.6 wt.% respectively. Then unroasted Cu_x-SAPO-44 (1 g) zeolites were conducted ion exchange with 1.0 M NH₄NO₃ solution at 80 °C for 8 h to remove the abundant Cu, and dried Cu_x-SAPO-44-IE (0.9 g) can be obtained. Finally, the dried samples were calcined at 550 °C for 6 h [17] to remove structure-directing agent, and obtained Cu_x-SAPO-44-IE, samples with Cu contents of 1.0, 1.6, and 2.2 wt.% (corresponding original x = 4.3, 5.6, and 7.6 wt.%, respectively).

3.2. Catalyst Characterizations

X-ray diffraction (XRD) was used to obtained the information about the crystalline structure of the catalysts, and equipped with Cu Kα radiation (λ = 1.5418 Å). The BET surface area and pore size distribution of Cu_5.6-SAPO-44 and Cu_1.6-SAPO-44-IE were determined using BET measurements (Micromeritics ASAP 2020, Norcross, GA, USA) after dehydration of the catalysts at 300 °C for 9 h under vacuum. ICP-atomic emission spectrometry (ICP-AES) was carried out to determine the elemental contents of samples by using a PerkinElmer Optima 2100DV (Waltham, MA, USA). FE-SEM equipped with energy dispersive spectroscopy (EDS) was performed on a Hitachi SU-70 microscope (Tokyo, Japan). HRTEM was conducted on a JEOL JEM-2010 (Tokyo, Japan) and a FEI Tecnai G2 F20 transmission electron microscope operating at 200 kV. H₂-TPR experiments were performed on a chemisorption analyzer (XianQuan, TP-5000, TianJin, China). The samples (50 mg) were pretreated with pure O₂ flow at 500 °C for 30 min, and cooled down to the room temperature in the presence of O₂. Subsequently, the samples were reduced under a flow of 5 vol.% H₂/N₂ (50 mL/min) and then was heated to 800 °C with the rate of 10 °C/min. XAFS spectra were measured for the Cu K-edge at 1W1B beamline of Beijing synchrotron radiation facility (BSRF, Beijing, China) in the transmission mode and fluorescence mode at room temperature. XAFS raw data were analyzed using IFEFFIT software package [29].

3.3. Catalyst Activity Measurements

The steady state NH₃-SCR activity tests of Cu-SAPO-44 catalysts (~120 mg, 40–60 meshes) were performed at atmospheric pressure in a fixed-bed quartz tube reactor (6.0 mm i.d.). The gas consisting of 500 ppm NO, 500 ppm NH₃, 5.3 vol. % O₂, 5 vol. % H₂O (when used) and balance, He. The total gas flow rate was 300 mL/min, corresponding to a gas hourly space velocity (GHSV) of 100,000 h⁻¹. The measurement was carried out in the temperature range 100–550 °C. The NO and NO₂ concentrations of the reactor inlet/outlet were monitored by a chemiluminiscence NO_x analyzer (42i-HL, Thermo, Waltham, MA, USA). In addition, N₂O and NH₃ were detected by quadrupole mass spectrometer (MS, OmniStar 200, Balzers, Switzerland) with a m/z of 44 for N₂O, and 17 for NH₃. During the test, each temperature point is stable for at least 30 min. The catalytic activity and N₂ selectivity were calculated according to the following equations:

$${\mathrm{NO}}_x \mathrm{conversion} (\%) = \frac{{{[\mathrm{NO}}_x]}_{\mathrm{in}}-{{[\mathrm{NO}}_x]}_{\mathrm{out}}}{{{[\mathrm{NO}}_x]}_{\mathrm{in}}}\times 100\%,$$

$${\mathrm{N}}_2 \mathrm{selectivity}(\%) = \frac{{{[\mathrm{NO}}_x]}_{\mathrm{in}} + {\left[{\mathrm{NH}}_3\right]}_{\mathrm{in}}-{{[\mathrm{NO}}_x]}_{\mathrm{out}} - {\left[{\mathrm{NH}}_3\right]}_{\mathrm{out}}{ - 2[\mathrm{N}}_2\mathrm{O}]}{{{[\mathrm{NO}}_x]}_{\mathrm{in}} + {\left[{\mathrm{NH}}_3\right]}_{\mathrm{in}}-{{[\mathrm{NO}}_x]}_{\mathrm{out}}-{\left[{\mathrm{NH}}_3\right]}_{\mathrm{out}}} \times 100\%.$$

4. Conclusions

Cu-SAPO-44 zeolites were synthesized using a one-pot approach with dual-amine templates. Furthermore—although the total content of Cu was decreased—the CuO dispersion and isolated Cu²⁺ proportion were significantly improved via the subsequent IE with NH₄NO₃. The SCR activity was thus promoted at higher temperature. Cu_1.6-SAPO-44-IE shows >90% NO_x conversion in the temperature range 200–500 °C.