Copper-exchanged SUZ-4 zeolite catalysts for selective catalytic reduction of NO_x

Abstract

Copper-based SSZ-13 (Cu-SSZ-13, CHA topology) zeolite catalysts have been commercialized towards the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR), but the applications of Cu-SSZ-13 catalysts are still limited by the great challenge of high-cost organic templates. To this end, zeolite catalysts with other topologies have been attempted to substitute SSZ-13 to achieve the target of low cost and high performance. Herein, a series of SUZ-4 zeolites (SZR topology), structurally related to the FER topology, are successfully synthesized by using tetraethylammonium hydroxide (TEAOH) as the organic template. Compared to SSZ-13 zeolites synthesized by the addition of N,N,N-trimethyl-1-adamantylammonium hydroxide (TMAdaOH), the obtained SUZ-4 zeolites show higher economic benefits. Moreover, copper-exchanged SUZ-4 (Cu-SUZ-4) zeolites exhibit comparable NH₃-SCR performance to commercial Cu-SSZ-13. Particularly, the NO conversion of the Cu-SUZ-4-2 zeolite with optimal Cu loading is above 90% in the temperature range of 250–550 °C at high gaseous hourly space velocity (200 000 h⁻¹). After hydrothermal ageing (HTA), the NO conversion of Cu-SUZ-4-2-HTA is close to 90% in the temperature range of 300–550 °C, indicating its high hydrothermal stability. The present work provides an alternative catalyst that can potentially substitute SSZ-13 with high NH₃-SCR catalytic properties and low cost.

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Introduction

Nitrogen oxides (NO_x), mainly from diesel vehicle exhaust fumes, are harmful to the environment and human health. Zeolite-based ammonia selective catalytic reduction (NH₃-SCR) technology has been recognized as the most effective method to eliminate NO_x.^1–5 Particularly, the Cu-SSZ-13 zeolite has been commercialized and employed in diesel vehicles due to its good hydrothermal stability and catalytic activity.^6–11 However, it is faced with the challenge of high-cost organic template of N,N,N-trimethyl-1-adamantylammonium hydroxide (TMAdaOH).¹²

To deal with the above challenge, various low-cost organic templates have been proposed to synthesize SSZ-13 zeolites. For example, benzyl trimethylammonium hydroxide (TMBAOH),¹³ tetraethylammonium hydroxide (TEAOH),¹⁴ and choline chloride (CC)¹⁵ were used as the organic templates for the preparation of SSZ-13 zeolites. However, their further applications were limited by their biotoxicity (TMBAOH), the narrow synthetic phase region (TEAOH), poor repeatability and low yields (CC).

Besides, developing high-efficiency and cost-effective zeolites with other topologies to substitute SSZ-13 is another feasible strategy. For instance, Xu et al. reported that the Cu-RTH (RTH topology) zeolite, synthesized by using complex 2,6-dimethyl-N-methylpyridine as the organic template, was applied in the NH₃-SCR reaction.¹⁶ However, the temperature window of NO conversion above 90% (T₉₀) for Cu-RTH was only at 200–400 °C, which was narrower than that of the commercial Cu-SSZ-13 zeolite (225–500 °C). The Cu-ZJM-7 zeolite with KFI topology, synthesized by using inorganic templates (K⁺ and Na⁺), exhibited a T₉₀ temperature window of 200–450 °C, but the long crystallization time (15 days) was a great obstacle for its large-scale production.¹⁷ In addition, the high-silica Cu-LTA (LTA topology) zeolite, synthesized by the addition of F⁻, displayed higher hydrothermal stability than the Cu-SSZ-13 zeolite; however, the harmful nature of F⁻ limited its practical applications.¹⁸

It is known that Cu-FER (FER topology) zeolites have been applied in NH₃-SCR reactions.^6,19,20 The SUZ-4 zeolite (SZR topology), first synthesized by using TEAOH as the organic template in 1992, possesses ten-member ring windows composed of d6r, mtt and mso building units, and is structurally related to the FER topology.^21–23 Thus, Cu-SUZ-4 could be considered as a potential NH₃-SCR catalyst. However, because of the lack of optimal regulation in synthesis conditions and compositions,²⁴ Cu-SUZ-4 zeolites exhibited inferior NH₃-SCR performance, and were rarely investigated.

In this work, a series of SUZ-4 zeolites with different Si/Al ratios were successfully synthesized by using TEAOH as the organic template. Among all the synthesized SUZ-4 zeolites, the sample with an initial Si/Al ratio of 11 exhibited the highest crystallinity. Crystallization process studies indicated that the SUZ-4 zeolites achieved a complete crystallization at 84 h. In addition, three Cu-SUZ-4 samples with different copper loadings were prepared via the ion-exchange method. The results of catalytic experiments demonstrated that the Cu-SUZ-4-2 zeolite with optimal 1.89 wt% Cu loading exhibited comparable NH₃-SCR performance to the commercial Cu-SSZ-13 zeolite, whose T₉₀ window was in the range of 250–550 °C. Experimental characterization revealed that more isolated Cu²⁺ ions and less CuO_x accounted for the better catalytic activity and hydrothermal stability of the Cu-SUZ-4-2 zeolite. The effects of Si/Al ratios and Cu loadings, crystallization process and the forms of Cu species for Cu-SUZ-4 were systematically investigated. Importantly, the developed Cu-SUZ-4 zeolites exhibited higher catalytic performance than that reported in the previous work. This was because of the very high Cu content possessed by the sample in the previous study (up to 3.5 wt%), leading to the formation of more inactive CuO_x. The present work demonstrates that the copper-exchanged SUZ-4 zeolite might be promising as an attractive substitute for Cu-SSZ-13 in the future.

Results and discussion

Crystallization process of the SUZ-4 zeolite

X-Ray diffraction (XRD) patterns of the simulated SUZ-4 zeolite and samples with different initial Si/Al ratios ranging from 7 to 25 are shown in Fig. S1 (ESI†). Pure-phase SUZ-4 zeolites can be successfully synthesized in the Si/Al ratio range of 8–20, showing the typical characteristic peaks of SZR at 2θ = 7.8°, 11.79°, 15.5°, 19.38°, 22.18°, 22.51°, 25.03°, 25.56°, 28.45° and so on.^25,26 Among all the as-synthesized samples, the SUZ-4 zeolite with an Si/Al ratio of 11 exhibits the highest crystallinity. Lower Si/Al ratios lead to the formation of a dense phase, while higher Si/Al ratios result in the incomplete crystallization of the SUZ-4 zeolite. The final Si/Al ratios and yields of products are given in Table S1. It can be observed that the final Si/Al ratio of the product is lower than the initial Si/Al ratio of the gel as Al species preferentially incorporate into the zeolite framework in reference to Si species.²⁷ In addition, the yield of the product gradually decreases as the initial Si/Al ratio of the gel increases. The zeolite with a final Si/Al ratio of ∼6 was selected for further study.

The crystallization process of the SUZ-4 zeolite was investigated and the corresponding results are shown in Fig. 1. As seen in Fig. 1a, at the beginning of crystallization, amorphous species exist in the reaction system. As the crystallization time prolongs to 36 h, the characteristic peaks of SZR appear, indicating the formation of the SUZ-4 zeolite. Until the crystallization time reaches 84 h, the intensities of diffraction peaks become the highest, suggesting a completed crystallization of SUZ-4. The crystallization curve provided in Fig. 1b further confirms that the SUZ-4 zeolite achieves complete crystallization at 84 h.

Fig. 1 (a) Time-dependent XRD patterns of SUZ-4 obtained at various crystallization times and (b) the crystallization curve of SUZ-4.

Scanning electron microscopy (SEM) characterization was performed to investigate the morphology change of the SUZ-4 zeolite during the crystallization process. As shown in Fig. 2, amorphous species is predominant at the beginning of crystallization. A few needle-like species appear (inside the red circle) when the crystallization time is prolonged to 24 h. When the crystallization time is increased further, needle-like species are formed accompanied by amorphous raw materials. The product appears completely in the needle form at 84 h. In other words, amorphous gel particles are first formed and merged, followed by stacking and coalescing nucleation and growth of needle-like crystals. Then, needle-like crystals are generated via ripening processes of coalescent particles. Finally, the amorphous gel particles completely transform to a crystalline zeolite product. The growth kinetics is in fact complicated which needs to be further investigated in the future. These results are consistent with the result of XRD.

Fig. 2 SEM images of the SUZ-4 zeolite at different crystallization times.

Characterization of Cu-SUZ-4 zeolites

Cu-SUZ-4 zeolites with an Si/Al ratio of ∼6 and different Cu contents were prepared via an ion-exchange method. The Cu content is determined as 1.43 wt%, 1.89 wt%, and 2.21 wt% for Cu-SUZ-4-1, Cu-SUZ-4-2, and Cu-SUZ-4-3, respectively (Table S2, ESI†). In addition, the Cu/Al ratio increases as the Cu content increases (Table S2, ESI†). The Brunauer Emmett Teller (BET) surface area and microporous volume of Cu-SUZ-4 zeolites slightly decrease compared to the initial SUZ-4 zeolite due to the incorporation of Cu species. The N₂ adsorption–desorption curves (Fig. S2, ESI†) demonstrate that all samples exhibit characteristic type I isotherms, indicating their microporous structure.^28,29

Ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) were obtained to investigate the change of Cu species for Cu-SUZ-4 zeolites. As shown in Fig. S3 (ESI†), each sample displays two bands. The bands at 220 nm and 600–800 nm are assigned to isolated Cu²⁺ ions and the d–d transition of Cu²⁺ ions in CuO_x particles, respecitvely.^30,31 Note that Cu-SUZ-4-2 exhibits higher intensity of Cu²⁺ peak than Cu-SUZ-4-1, suggesting the presence of more Cu active sites. Compared to Cu-SUZ-4-3, the intensity of the CuO_x peak in Cu-SUZ-4-2 is weaker. This is largely due to the excessive Cu loading in Cu-SUZ-4-3 leading to the formation of inactive CuO_x particles.

Fig. 3 shows the Cu 2p X-ray photoelectron spectroscopy (XPS) spectra of Cu-SUZ-4 zeolites. The Cu 2p_1/2 peak is assigned to isolated Cu²⁺ ions. The Cu 2p_3/2 peak could be deconvoluted into two peaks: the peak at 933.6 eV is attributed to CuO_x species while the peak at 936.2 eV is attributed to Cu²⁺ ions.^29,32 Note that Cu-SUZ-4-3 possesses more amount of CuO_x than Cu-SUZ-4-1 and Cu-SUZ-4-2. This indicates that excessive Cu content could result in the aggregation of Cu ions. In addition, Cu-SUZ-4-2 exhibits higher intensity of the Cu²⁺ peak than Cu-SUZ-4-1, implying the presence of more Cu active sites. Transmission electron microscopy (TEM) characterization was conducted to further investigate the particle size of CuO_x (Fig. S4, ESI†). It can be observed that the amount of CuO_x increases accompanied by the enlargement of size as the Cu content increases. The particle size of CuO_x is measured to be 1–2 nm, 3–4 nm and 6–7 nm for Cu-SUZ-4-1, Cu-SUZ-4-2 and Cu-SUZ-4-3, respectively, which could directly affect the catalytic performance of zeolites.

Fig. 3 XPS spectra of Cu-SUZ-4 samples.

Hydrogen temperature-programmed reduction (H₂-TPR) experiments were carried out to investigate the distribution of Cu active sites in Cu-SUZ-4 zeolites. As shown in Fig. 4, three peaks appear for each sample. It is clear that two Cu active sites exist in the Cu-SSZ-13 zeolite where the peak at lower temperature is attributed to Cu²⁺ ions next to eight-member rings (8MRs) while the peak at higher temperature is attributed to Cu²⁺ ions close to 6MRs.^33–35 Therefore, we deem that the peak at 287 °C is assigned to Cu²⁺ ions next to 10MRs, which is due to the large moving space that can result in a weak interaction between Cu²⁺ ions and the framework. The peak at 402 °C is assigned to Cu²⁺ ions close to 6MRs due to their strong interaction with the framework, while the peak at 350 °C is attributed to the reduction peak of CuO_x. Note that Cu-SUZ-4-2 and Cu-SUZ-4-3 possess more amount of Cu²⁺ ions close to 6MRs than Cu-SUZ-4-1. The intensity of the peak at 350 °C in Cu-SUZ-4-3 is higher than that in Cu-SUZ-4-1 and Cu-SUZ-4-2, suggesting more CuO_x in Cu-SUZ-4-3. The changes of the distributions of Cu species with the increasing Cu content in Cu-SUZ-4 are consistent with those observed in Cu-SSZ-13.^36,37 This result also agrees with the results of UV-vis DRS and XPS.

Fig. 4 H₂-TPR profiles of Cu-SUZ-4 samples.

Ammonia temperature-programmed desorption (NH₃-TPD) analyses were performed to investigate the acidity of Cu-SUZ-4 zeolites. As shown in Fig. S5 (ESI†), the peaks centered at 180 °C and 280 °C are attributed to NH₃ molecules adsorbed on the weak acid sites and Cu²⁺ sites, respectively,^38–40 while the peak at 485 °C is assigned to NH₃ molecules adsorbed on the strong Brønsted acid sites.^41,42 Cu-SUZ-4-2 and Cu-SUZ-4-3 exhibit lower intensities of the peak at 180 °C than Cu-SUZ-4-1, which indicates that the introduction of Cu²⁺ ions can fill the defect sites of the SUZ-4 zeolite. The existence of CuO_x may lead to the shift of the peak related to the strong Brønsted acid sites to a higher temperature.⁴¹ Considering the similar Si/Al ratio in each Cu-SUZ-4 zeolite, the shift of the third peak in Cu-SUZ-4-3 might be caused by the existence of more CuO_x.

Catalytic activity and hydrothermal stability

In general, Cu²⁺ ions can act as the active sites for the NH₃-SCR reaction and prevent the dealumination of zeolites during hydrothermal ageing.^43,44 However, CuO_x species show poor NH₃-SCR activity and cause the formation of undesired N₂O at high temperatures.⁴⁵ The migration of CuO_x during hydrothermal ageing can even destroy the zeolite structure.⁴⁶

Fig. 5a shows the NH₃-SCR performance of fresh Cu-SUZ-4 zeolites. Cu-SUZ-4-2 and Cu-SUZ-4-3 exhibit ∼10% higher NO conversion than Cu-SUZ-4-1 in the temperature range of 200–350 °C at a gaseous hourly space velocity (GHSV) of 200 000 h⁻¹. Especially, the T₉₀ windows of Cu-SUZ-4-2 and Cu-SUZ-4-3 (250–550 °C) are wider than that of Cu-SUZ-4-1 (350–550 °C). At a higher GHSV of 400 000 h⁻¹ (Fig. S6, ESI†), Cu-SUZ-4-2 exhibits remarkable NH₃-SCR performance among all samples, showing a wider T₉₀ window (300–550 °C) than Cu-SUZ-4-1 (350–550 °C) and Cu-SUZ-4-3 (300–500 °C). Note that the NO conversion of each sample shows a trend of first increasing and then decreasing as the reaction temperature increases, which is due to the occurrence of NH₃ oxidation and the decrease in NH₃ adsorption at high temperatures.^47,48 In addition, each sample exhibits low N₂O production (less than 5 ppm) throughout the whole temperature range (Fig. 5b), indicating their high N₂ selectivity. The cycling stability test of Cu-SUZ-4-2 further demonstrates that Cu-SUZ-4-2 exhibits excellent cycling stability after continuous reactions for three times (Fig. S7, ESI†). In brief, more Cu active sites and less CuO_x content account for the best catalytic performance of Cu-SUZ-4-2. The catalytic performance of Cu-SUZ-4-2 is further compared with those of other similar zeolite catalysts reported in the literature (Table S3, ESI†).^24,49–51 Cu-SUZ-4-2 exhibits similar or even superior catalytic activity to other zeolite catalysts.

Fig. 5 (a) and (c) NO conversion and (b) and (d) N₂O formation as a function of temperature for Cu-SUZ-4 samples before and after hydrothermal ageing. Reaction conditions: 500 ppm NO, 500 ppm NH₃, 5% O₂, and 5% H₂O, balanced with N₂ at a GHSV of 200 000 h⁻¹.

After hydrothermal ageing (HTA), the catalytic activities of all aged samples decline compared to that of fresh counterparts (Fig. 5c). The T₉₀ window of Cu-SUZ-4-2-HTA can be maintained at 350–550 °C, which is wider than that of Cu-SUZ-4-1-HTA (no T₉₀ window) and Cu-SUZ-4-3-HTA (400–550 °C), implying its higher hydrothermal stability. Moreover, the N₂O contents of all aged samples increase at high temperatures (>350 °C) (Fig. 5d). This is due to the generation of more CuO_x species during hydrothermal ageing, which resulted in side reactions to yield more N₂O. Cu-SUZ-4-3-HTA possesses a higher amount of N₂O than Cu-SUZ-4-1-HTA and Cu-SUZ-4-2-HTA, suggesting the formation of more CuO_x in Cu-SUZ-4-3 during hydrothermal ageing. Interestingly, the N₂O content in each aged sample remains at a very low level (less than 10 ppm), showing excellent N₂ selectivity of Cu-SUZ-4 zeolites even after hydrothermal ageing. Furthermore, compared to commercial Cu-SSZ-13 (Si/Al = 10, Cu loading = 2.5 wt%, Fig. S8, ESI†), Cu-SUZ-4-2 exhibits superior catalytic activity at high temperatures (≥350 °C) but inferior catalytic activity at low temperatures (≤350 °C). In short, the Cu-SUZ-4-2 zeolite displays comparable NH₃-SCR performance to the commercial Cu-SSZ-13 zeolite, which suggests that the Cu-SUZ-4 zeolite might act as an attractive substitute for Cu-SSZ-13 in the future.

As indicated by the results of XRD patterns (Fig. S9, ESI†), a typical SZR structure is reserved and no CuO diffraction peaks are observed in each Cu sample after hydrothermal treatment, suggesting that the generated CuO particles are relatively small and dispersed. ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS NMR) spectra were obtained to investigate the hydrothermal stability of Cu-SUZ-4 zeolites (Fig. 6). The signal at 56 ppm is assigned to the tetrahedral-coordination framework Al.^46,52 All fresh samples exhibit similar intensities to framework Al, which suggests that the incorporation of Cu²⁺ ions does not destroy the zeolite structure. However, the intensity of framework Al in each sample decreases after hydrothermal ageing, implying the occurrence of dealumination. Note that the intensity of framework Al in Cu-SUZ-4-2-HTA is higher than that in Cu-SUZ-4-1-HTA and Cu-SUZ-4-3-HTA, indicating its higher hydrothermal stability. This is because the low Cu²⁺ content in Cu-SUZ-4-1 cannot stabilize the zeolite framework while more CuO_x formed in Cu-SUZ-4-3 leads to a decrease in the hydrothermal stability.

Fig. 6 ²⁷Al MAS NMR spectra of fresh and aged Cu-SUZ-4 zeolites.

Conclusions

In summary, a series of Cu-SUZ-4 zeolites were prepared by using TEAOH as the organic template, showing higher economic benefits over the Cu-SSZ-13 zeolite synthesized by the addition of TMAdaOH. Particularly, Cu-SUZ-4-2 with a final Si/Al ratio of ∼6 and an optimal Cu loading of 1.89 wt% exhibits the best NH₃-SCR catalytic performance among all samples, showing comparable catalytic properties to commercial Cu-SSZ-13. Excessive Cu content in SUZ-4 can result in the formation of CuO_x, leading to the decline of catalytic activity and hydrothermal stability, while Cu-SUZ-4 with low Cu loading cannot provide sufficient Cu²⁺ active sites for NH₃-SCR reactions. This work demonstrates that the copper-exchanged SUZ-4 zeolite may act as a promising substitute for Cu-SSZ-13.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant 22288101, 21920102005 and 21835002), the National Key Research and Development Program of China (Grant 2021YFA1501202, 2022YFA1503600), and the 111 Project (B17020) for supporting this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qm00813d

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