1 Introduction
The use of diesel engines not only promotes the development and progress of society, but also brings about environmental pollution problems, in which nitrogen oxides are important pollutants. The treatment technology of nitrogen oxides in diesel vehicle exhaust has been developing continuously, from the initial use of precious metal three-way catalyst, to metal oxide catalyst (V
2O
5-WO
3/TiO
2), and then to the present zeolite-based metal catalyst, which has gradually overcome the defects of high cost, low activity under oxygen-poor conditions, and poor hydrothermal stability
[1]. Cu-zeolite catalysts have been developed from Cu-ZSM-5, Cu-beta to Cu-SSZ-13, which have higher activity and selectivity, and are not easily inhibited by side reactions or deactivated by thermal degradation
[2⇓~4][5][6⇓⇓⇓⇓⇓~12].
SSZ-13 zeolite was first synthesized in 1985 by using N, N-trimethyl-1-adamantane ammonium hydroxide (TMADaOH) as a template. It is isostructural with natural chabazite, but its Si/Al ratio (Si/Al ≥ 5) is higher than that of chabazite (Si/Al < 2.5)
[13]. Its structure is the same as that of chabazite, which belongs to the trigonal system with the space group of
and CHA type. The framework can be regarded as a framework structure formed by the stacking of double six-membered ring layers connected by four-membered rings in the order of ABCA, in which the regularly arranged cages are interconnected by eight-membered ring windows (8 R). Because the asymmetric unit in the structure contains only one central atom and four non-equivalent oxygen atoms, it has high symmetry.
In 2010, Cu-SSZ-13 was commercialized as a catalyst for diesel vehicle exhaust after-treatment system
[14]. A large number of research results confirm that the excellent catalytic performance of Cu-SSZ-13 in NH
3-SCR reaction benefits from its special structure and high hydrothermal stability. The high Si/Al ratio endows the framework with good hydrothermal stability, and the appropriate pore size (3.88 Å × 3.88 Å) can greatly reduce the "poisoning" of the catalyst or the destruction of the framework caused by oxidation when hydrocarbons enter the cage. At the same time, the interaction between copper ions and the framework also makes Cu-SSZ-13 have excellent catalytic stability
[6⇓⇓⇓⇓⇓~12].
It is found that copper ions in the chabazite cage (CHA cage) of SSZ-13 not only exist in multiple positions and different valence States (Figure 1), but also the dynamic changes of their positions, valence States, ligand types and quantities run through the whole activation and reaction process
[9,15⇓ ~17].
The location of copper ion in the zeolite framework is restricted by the position of protonic acid sites, while the number and location of acid sites depend on the silica to alumina ratio and aluminum distribution of the zeolite framework composition, and the important factor affecting the silica to alumina ratio and aluminum distribution is the template used in the synthesis
[18⇓~20].
Since its first use, TMADaOH has remained the template of choice for the synthesis of SSZ-13 zeolite. However, for a long time, the price of commercial TMADaOH is high, and the cost of Cu-SSZ-13 catalyst for diesel engine exhaust purification prepared with TMADaOH as template is high, which restricts its application scope. Finding alternatives to TMADaOH to reduce its production cost once became a hot spot in the field of zeolite synthesis. At present, a variety of organic compounds have been found to be able to replace TMADaOH to synthesize SSZ-13 zeolite, such as benzyltrimethylammonium hydroxide (BTMAOH), tetraethylammonium hydroxide (TEAOH), Choline Chloride, N, N-dimethyl-N-ethyl-cyclohexylammonium bromide (DMCHABr) and Cu-tetraethylenepentamine complex (Cu-TEPA)
[11,21⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~38]. All of them can lead to the synthesis of products with a close range of silica-alumina ratio and X-ray diffraction (XRD) patterns consistent with TMADaOH-SSZ-13. SSZ-13 was also successfully synthesized using a combination of two of the above templating agents
[20,34,39⇓~41].
In recent years, the comparison of catalytic properties of several types of zeolites for NH
3-SCR, the synthetic methods of SSZ-13 zeolite, the site occupancy and stability of Cu-SSZ-13 catalyst and their relationship with the distribution of aluminum in the zeolite framework, and the mechanism of NH
3-SCR reaction have been reviewed
[8,10][17,42,43]. Among them, the synthesis methods of SSZ-13 zeolite with different templates have also been summarized. In this paper, the effect of NH
3-SCR reaction on the content, location and migration of copper ions on the catalyst, the effect of acid sites on the properties of copper ions,And determine that quantitative relationship between the catalytic performance of the excellent NH
3-SCR and the copper content, the aluminum content in the zeolite framework and the aluminum distribution. The structure and composition of Cu-SSZ-13 prepared by different templates or their combinations were summarized, and the catalytic performance of Cu-SSZ-13 for NH
3-SCR was compared, and the performance differences and reasons were analyzed.
2 Selective Catalytic Reduction of Cu-SSZ-13 and NOx
2.1 Properties and Functions of Copper Ions in Cu-SSZ-13
In the process of NH
3-SCR catalyzed by copper ion-loaded zeolite, copper ion is the active center of NH
3-SCR reaction, which determines the catalytic performance of Cu-zeolite
[42⇓⇓⇓⇓~47]. Copper modified ZSM-5 and Beta zeolites also showed good catalytic activity for NH
3-SCR reaction. However, the hydrothermal stability of copper ions on these two zeolites was worse than that on Cu-SSZ-13
[2⇓⇓~5]. Obviously, there is a special interaction between copper ions and SSZ-13 zeolite. The results show that the site of copper in SSZ-13, the transition between Cu
2+ and Cu
+ valence, and their dynamic changes with reaction conditions and coordination environment are directly or indirectly related to the reaction performance of the catalyst
[15⇓~17,42⇓⇓⇓~46].
2.1.1 Site of copper ion in Cu-SSZ-13.
The results of the study on the relationship between the copper ion loading and the NH
3-SCR catalytic performance of Cu-SSZ-13 show that low copper ion loading is not conducive to the conversion of NO
x; If the loading is too high, N
2O will occur, which will reduce the selectivity of the target product N
2[1]. More detailed studies have shown that copper ions can exist in different positions on SSZ-13 zeolite, forming active centers with different reaction ability and stability, and the difference of loading amount causes the difference of the number of copper ions in different positions, which affects the catalytic results
[52].
For ion-exchanged SSZ-13 zeolite, there are four possible sites for cations (see Fig. 1): (1) at the center of the double six-membered ring (A-site), (2) near the six-membered ring surface in the CHA cage cavity (B-site), (3) on the cubic rotation axis in the CHA cage cavity (C-site), and (4) at or near the window plane of the eight-membered ring (D-site). In dehydrated samples, small cations tend to be located at A and B sites. Under hydration conditions, cations tend to reside in the middle of the CHA cage, maximally coordinated to water. Cations with large radii (e.g., Cs
+), tend to occupy the D site
[15]. Early studies, through EXAFS and other techniques, basically ruled out the possibility of copper ions in the double six-membered ring, and considered that copper ions were in the remaining three positions
[10]. Later studies mainly tended to limit the location of copper ions to two places: near the six-membered ring and near the eight-membered ring (Figure 2)
[53]. However, recent studies have shown that naked divalent cations (such as :Co
2+) can exist in the A-site in Figure 1, but cannot be contacted by guest molecules. At this time, there are two aluminum atoms on the double six-membered ring to balance its charge, and the two aluminum atoms are located on different six-membered rings, facing different cages, showing the properties of a single acid center
[54].
The protonic acid site is an important factor affecting the landing of copper ions. If the protonic acid site (B acid site) formed by the framework aluminum atoms on the zeolite is represented as Z, when the aluminum on the six-membered ring exists in the form of "aluminum pair",The framework forms a pair of acid centers (both B acid centers are on the six-membered ring), which is denoted as Z
2, and the copper ion interacts with it near the six-membered ring to form Z
2Cu(Ⅱ) (see Figure 3A, B). When the aluminum on the six-membered ring exists in the form of isolated aluminum, there is only one B acid center, which can also be regarded as located on the common edge of the six-membered ring and the eight-membered ring, and it is marked as Z. At this time, the copper ion forms ZCu (II) OH near the eight-membered ring (see Figure 3C). The spatial density and relative ratio of the two copper species are related to the number of paired Bronsted acid sites in the zeolite framework and the total copper loading. For the dehydrated sample, at high acid density (more acid sites in zeolite framework) and low Cu/Al ratio (less loaded copper), the copper species mainly exist in the form of Z
2Cu(Ⅱ), which is located near the six-membered ring, while at low acid density and high Cu/Al ratio, the copper species mainly exist in the form of Z [CuII (OH)], which is located near the eight-membered ring. From the point of view of catalytic performance, the Z
2Cu(Ⅱ) near the six-membered ring is not easy to deactivate during catalysis, and has stronger hydrothermal stability and resistance to sulfur poisoning
[42⇓⇓⇓~46,55,56].
In many reviews, when summarizing the landing of copper ions, it is considered that copper ions are preferentially loaded near the six-membered ring, and then begin to occupy the position near the eight-membered ring after saturation
[9,10]. It is worth noting that when Cu-SSZ-13 is completely dehydrated, the acid center has a strong interaction with the copper ion because there is no hindrance from the H
2O ligand, which can fix the copper ion near the six-membered ring or eight-membered ring, and the copper ion preferentially occupies the more stable six-membered ring. However, in the presence of water, the solvation of the H
2O weakens the electrostatic interaction between the acid sites and the copper ion, and there is no significant difference in the interaction between the single acid site and the paired acid sites, and the copper ion begins to occupy the eight-membered ring before the six-membered ring site is saturated. With the increase of temperature and the removal of coordination H
2O, the Z [Cu (Ⅱ) OH] coordinated with a single acid center will migrate from an eight-membered ring to a six-membered ring and react with two protonic acids to form a more stable Z
2Cu(Ⅱ). For example, some studies have found that in the dehydration process, copper ions begin to occupy the positions near the eight-membered ring before the positions near the six-membered ring are saturated. Martini et Al. Studied a sample with Si/Al = 14 and low Cu/Al = 0.1. Theoretically, 90% of the samples should be Z
2Cu(Ⅱ), but the results show that Z
2Cu(Ⅱ) only accounts for 18% of all copper species
[45,46,50]. And Z
2Cu(Ⅱ) and Z [Cu (Ⅱ) OH] are formed simultaneously during dehydration
[50]. Luo et al. Also showed that under mild hydrothermal conditions, the results of NH
3-TPD detection proved that Z [Cu (Ⅱ) OH] migrated to the six-membered ring and converted to a Z
2Cu(Ⅱ), from a single coordination with zeolite to a more stable double coordination
[46].
In the hydrothermal treatment of Cu-SSZ-13 at different temperatures, Gao et al. Found that the conversion of Z [Cu (Ⅱ) OH] to Z
2Cu(Ⅱ) occurred after hydrothermal treatment at high temperature (more than 550 ℃) for the sample treated simply (only for 1 H at 250 ℃ in helium)
[45]. This indicates that when accurately quantifying the amount of the two copper species in the catalyst, attention should be paid to whether the dehydration treatment of the sample is sufficient and whether the conversion of Z [Cu (Ⅱ) OH] to Z
2Cu(Ⅱ) is complete.
2.1.2 Existing form, valence state and transformation of copper species in activation process
Before activation, the copper ion initially loaded on the zeolite exists in the form of hydrate, and the number and position of ligand water molecules are different according to the number of acid sites around it. Two stable copper ion hydrates, Z
2[CuII(H
2O)
4](H
2O)
2 and Z[CuII(OH)(H
2O)
3](H
2O)
3, are located in the vicinity of paired acid sites and single acid sites, respectively
[16].
Although Z
2[CuII(H
2O)
4](H
2O)
2 and Z[CuII(OH)(H
2O)
3](H
2O)
3 interact with acid sites, the solvation of water makes these copper ion hydrates have good mobility. The research results of Peden et al. In 2014 showed that the hydrated copper species had almost no mobility at -150 ℃, but showed high mobility from room temperature to 200 ℃
[48].
With the increase of temperature (> 250 ℃) during activation, copper ions lose coordination water accompanied by the change of valence, forming Cu
2+, Cu
+ Paired acid sites and single acid site sites form Cu
2+, [CuOH]
+, respectively. Cu
+ is formed by "self-reduction" after the loss of ligand from [CuOH]
+ upon activation in vacuum or inert gas atmosphere, preferentially occupying the six-membered ring in the presence of paired acid centers.
[50]According to the number of paired acid sites in the framework of Cu-SSZ-13, the loading of copper ions and the activation atmosphere, the proportion of Cu
2+, Cu
+, [CuOH]
+ and other species formed after activation will be significantly different, and the change of catalytic atmosphere will also lead to the mutual transformation and position migration of these copper species
[1,17,50,51,56].
2.1.3 Different Valence States of Copper Species and Transformation between Existing Forms in Low Temperature SCR Reaction
When the reaction catalyzed by Cu-SSZ-13 is carried out at low temperature (< 300 ° C), the copper ion is simultaneously bound by the acid sites and solvated by NH
3 and H
2O
[17,42⇓⇓~45,49⇓~51]. Because the solvation of NH
3 is stronger at this temperature, NH
3 gradually becomes the ligand of copper ion.Z[CuⅡ(OH)(NH
3)
3], Z[CuⅠ(NH
3)
2], Z
2[CuⅡ-(NH
3)
4], [ZNH
4]Z[CuⅠ(NH
3)
2] are formed, and Cu is released from the interaction with the acid site, at which time the copper species has a high mobility
[18][16,44,45,48]. The H
2O molecules in the system increase the mobility of copper species, so that there is almost no difference in the effect of paired acid sites and single acid sites on copper ions, and the catalytic activity of copper at the two sites is similar at low temperature SCR
[44]. This makes Cu-SSZ-13 different from the traditional single-point zeolite catalyst (that is, when catalyzing at low temperature, the active center does not act independently, and the reaction rate is related to the space density of the active center); It is also different from homogeneous catalysts for reactions in solution (the movement of active centers is not unlimited), but migrates within a certain range
[19,48,49].
Peden et al. Found in 2014 that at low copper loading, the SCR reaction rate is quadratically related to the loaded copper content, and proposed that the active center at this time may contain two coppers
[48]. In 2017, Paolucci et al. published the low temperature NH
3-SCR mechanism of Cu-SSZ-13 in Science, and used the reduction-oxidation cycle to describe the change of copper species in the catalytic NH
3-SCR reaction (Fig. 4): under low temperature conditions (<300℃),Z[CuⅡ(OH)(NH
3)
3] (Fig. 4A) and Z
2[CuⅡ(NH
3)
4] (Fig. 4B) react with NO, and copper is reduced to monovalent to form CuⅠ(NH
3)
2, which is a reduction half-cycle
[49]; Because the redox barrier is too high for oxygen to directly oxidize the CuⅠ(NH
3)
2, the solvation of the NH
3 makes the CuⅠ(NH
3)
2 highly mobile, migrating in the cage, sometimes even across the eight-membered ring.When two copper species collide, they can form dimers to activate oxygen molecules, thus oxidizing Cu
+ to Cu
2+, which becomes Z
2CuⅡ, Z [CuⅡOH] and coordinates with NH
3 under the action of acid sites, completing the oxidation half-cycle (Figure 4A). The mobility of the copper species at the two positions is similar, and the copper ion can exist as a simple catalytic active center.
It can be seen that there is almost no difference in the low temperature reaction performance of copper at different sites in low temperature SCR, and it is the space density of copper rather than the initial site that determines the reaction performance. Especially in the presence of H
2O, the effect of acid sites on copper ions is further weakened, and there is no significant difference between the effect of single acid site and paired acid sites on copper species
[44].
2.1.4 Different Valence States of Copper Species and Transformation between Existing Forms in High Temperature SCR Reaction
At high temperature (> 350 ℃), copper ion loses its ligand, and the effect of acid center on it is enhanced, which makes it difficult to be solvated by NH
3, and its position is relatively fixed
[19,48,49].
During the high temperature SCR reaction, the molecular motion is intensified, and the copper ion has a tendency to break away from its original position (six-membered ring or eight-membered ring window) and aggregate in the CHA cage to form an oxide. On the other hand, the action of the acid site restricts the movement of the copper ion to a certain extent. Especially when there is a paired acid center on the nearby six-membered ring, the force on the copper ion is strong, the copper ion is more stable at this position, and the tendency to aggregate into the cage is lower
[43,45,46]. At the position of a single acid site, the interaction between the copper ion and the acid site is weak, and the tendency to return to the original position is relatively small. This labile copper ion tends to irreversibly aggregate in the cage, forming higher aggregation oxide clusters that eventually lead to deactivation
[10,26,45,57].
2.2 Distribution of Al in Cu-SSZ-13 and its effect on copper species
The distribution of Al in Cu-SSZ-13 determines the number of paired acid sites and also determines the ratio of strong acid to weak acid. The reaction conditions of NH3-SCR require Cu-SSZ-13 as a catalyst to have excellent hydrothermal stability at high temperature. There are two main reasons for the deactivation of the catalyst: the aggregation of copper species and the structural destruction caused by the dealumination of the zeolite framework under high temperature hydrothermal conditions.
2.2.1 Cu-SSZ-13 aluminum distribution forms as many "aluminum pairs" as possible and forms the contradiction of "strong aluminum pairs".
From the introduction of 2.1, it is not difficult to find that the paired acid sites play a decisive role in the stability of copper ions, and the formation of acid sites is attributed to the framework aluminum, that is, the "aluminum pair" on the six-membered ring of SSZ-13 zeolite structure can produce paired acid sites.
If the goal is to form more aluminum pairs, when there is a six-membered ring containing aluminum pairs in each CHA cage of SSZ-13, the calculation shows that Si/Al = 5. On the other hand, the zeolite forms strong acid sites to strengthen the interaction with copper ions and ensure the high hydrothermal stability of the zeolite, which requires the silica-alumina ratio of SSZ-13 to be as high as possible. When strong acid sites are produced on the framework of CHA-type zeolite, that is, there are at least two silicon-oxygen tetrahedra between two adjacent aluminums, according to the calculation of Barthomeuf et Al., when the aluminum distributed in the form of -Al-O-Si-O-Si-O-Al- occurs to the maximum extent considering the Lowenstein rule (see fig. 3A, such aluminum pairs are hereinafter referred to as "strong aluminum pairs"), Si/Al = 6.87
[58].
Comparing the two values, it can be found that when Si/Al = 5, the aluminum distribution of -Al-O-Si-O-Al- must appear, and the acid centers produced at this time must contain weak acid centers (see Figure 3B, such aluminum pairs are referred to as "weak aluminum pairs" hereinafter). Although this weak aluminum pair can produce paired acid centers, the copper ion interacting with the weak aluminum pair is not as stable as the strong aluminum pair
[20,50]. In addition, the skeleton stability of weak aluminum pair is not as good as that of strong aluminum pair at high temperature, and it is easy to dealuminate under the action of water vapor. In the hydrothermal treatment of Cu-SSZ-13 at different temperatures, Gao et al. Found that the amount of Cu
2+ was basically unchanged after hydrothermal treatment above 750 ℃, and considered that these Cu
2+ existed near the six-membered ring containing aluminum pairs. However, the decrease of Cu
2+ content in hydrothermal treatment between 700 ℃ and 750 ℃ is due to the fact that this part of Cu
2+ is stabilized by aluminum on different six-membered rings, which coalesces to form CuO
x in this temperature range. However, based on the analysis of the latest research results, the stable Cu
2+ after hydrothermal treatment above 750 ℃ may be located in the six-membered ring containing strong aluminum pairs, and the reduced Cu
2+ from 700 ℃ to 750 ℃ may be located in the six-membered ring containing weak aluminum pairs, which can also be supported by DFT calculation results
[45][16,59].
However, upon ion exchange, the copper ion preferentially occupies the acid site at the weak aluminum pair position. Experiments show that there is more CuII on the -Al-O-Si-O-Al- site when the loading is low. In the study of Gao et Al., electron paramagnetic resonance spectroscopy (EPR) was used to characterize the environment of the aluminum pair on the Cu-SSZ-13 prepared with TMADa
+ as the template. The results show that all the copper ions in the Cu-SSZ-13 with Si/Al = 6 and Cu/Al = 0.032 are located at the -Al-O-Si-O-Al- site
[48].
This means that if there is an -Al-O-Si-O-Al- structure in the skeleton, the copper ion will inevitably fall on this position. In order to reduce this weak aluminum pair, the ratio of silicon to aluminum should be increased appropriately, which is contradictory to the need to form as many aluminum pairs as possible to stabilize more copper ions. Moreover, this weak aluminum pair occurs even when the silica-to-aluminum ratio is high. For example, Godiksen et Al. Showed that copper at the -Al-O-Si-O-Al- and -Al-O-Si-O-Si-O-Al- sites in TMADa-Cu-SSZ-13 with Si/Al = 15 and Cu/Al = 0.09 accounted for 72% and 20% of the total copper content, respectively
[60].
The ratio of silicon to aluminum in the actual catalyst is a balance point selected after considering the stability of copper ions, the stability of the framework and the catalytic activity. The Si/Al ratio of BASF's first generation Cu-SSZ-13 catalyst is 17.5, and by 2017, the Si/Al ratio of the commercial catalyst has been reduced to 9.5
[46,55,61]. Therefore, it is a challenge for the synthesis of SSZ-13 zeolite to make all the framework aluminum of SSZ-13 zeolite exist in the form of strong aluminum pairs based on the copper content required for SCR reaction.
2.2.2 Contradiction between host-guest charge matching and ideal framework aluminum distribution in SSZ-13 zeolite composition
When TMADa
+ is used as a template to synthesize SSZ-13, the feeding ratio of Na
+ will affect the content of aluminum pairs in the skeleton of the product. When only TMADa
+ was used, the synthesized SSZ-13 did not contain the aluminum pair. The density of paired aluminum reaches a maximum when Na
+/TMADa
+=1 (see Figure 5C)
[62,63].
It is the host-guest charge matching problem of SSZ-13 composition that leads to this phenomenon. The results of crystal structure analysis show that each chabazite cage of SSZ-13 zeolite can only accommodate one TMADa
+ due to the large volume of TMADa
+.If an aluminum pair is formed on the six-membered ring of the cage, another cation must be present in the cage to balance the negative charge generated by the aluminum pair on the six-membered ring, and the good synergistic effect of TMADa
+ and Na
+ in the chabazite cage is beneficial to the formation of the aluminum pair on the six-member ring (Fig. 5B)
[64]. Therefore, when the Na
+ and TMADa
+ in the system appear in pairs, that is, Na
+/TMADa
+=1, the aluminum in the system tends to exist in the form of aluminum pairs, and the density of paired aluminum reaches the maximum
[62].
In the actual zeolite structure, the framework aluminum can not exist in the form of aluminum pairs, which is due to the unbalanced distribution of framework aluminum atoms and cations in the cage from the host-guest charge matching. (1) When Si/Al > 5, the content of aluminum atoms can not meet the requirement that there is an aluminum pair distributed on each CHA cage, and there must be only one aluminum atom on some cage frameworks in the whole zeolite crystal.When the framework generates only one negative charge, that is, aluminum exists in the form of isolated aluminum (see Figure 5A), there should be only one TMADa
+ in the cage according to the principle of charge matching between the framework host and the guest molecule outside the framework. Even if there are both TMADa
+ and Na
+ cations in the cage, they will also satisfy the charge balance through partial self-consistency, or the formation of framework defects
[61]. (2) When Si/Al < 5, the distribution of cations in the CHA cage is also inconsistent. In some cages, there are at least three aluminum atoms on the cage skeleton, and their negative charges can only be balanced by the Na
+. In this case, there can only be three Na
+,TMADa
+ in the cage (because the volume of the TMADa
+ is too large), and the Na
+ itself has no advantage in forming aluminum pairs. It can be seen that due to the unbalanced distribution of cations in the cage, it is impossible to achieve that all aluminum in the synthesis product exists in the form of aluminum pairs on the framework, and it is inevitable that isolated aluminum atoms exist in the zeolite framework.
The effect of isolated aluminum on supported copper ions is similar to that of Cu-Beta and Cu-ZSM-5. During the NH
3-SCR reaction, copper ions tend to agglomerate in the cage to form oxides, which eventually leads to the deactivation of the catalyst
[2⇓⇓~5,18]. After ion exchange, the copper in the dehydrated sample is preferentially loaded on the paired acid sites, so as long as the loading amount is properly controlled, even if there is isolated aluminum in the skeleton, the production of copper ions interacting with single acid sites can be avoided.
2.2.3 Theoretical optimum copper content of Cu-SSZ-13 under low temperature SCR reaction
For the low-temperature SCR reaction, on the one hand, considering the need to form dimers in the oxidation half-cycle, the higher mobility of Z[CuⅡ(OH)(NH
3)
3] on the isolated aluminum site after reduction to CuⅠ(NH
3)
2 is more conducive to the formation of dimers
[44]; On the other hand, although the deactivation caused by the aggregation of copper ions to form oxide clusters is not obvious when Cu-SSZ-13 is used as catalyst at low temperature, a small amount of copper ions will still aggregate after a long time of reaction, and aluminum has an advantage over the stability of nearby copper ions
[45]. Therefore, the two copper species of the dimer formed by the low temperature SCR reaction are provided by the isolated aluminum and the aluminum pair, respectively, which is more favorable for the low temperature SCR reaction. The theoretical optimum copper content of Cu-SSZ-13 at a given silica-alumina ratio can be calculated: for example, for the latest generation of Cu-SSZ-13 catalyst of BASF, Si/Al = 9.5, the theoretical value of aluminum content is about 28%, and the optimum Cu/Al = 0.21 for low temperature SCR reaction
[46]; For Cu-SSZ-13 with Si/Al = 5.0, which was commonly used in earlier studies, the aluminum content is about 44%, corresponding to a theoretical optimum Cu/Al = 0.33.
2.2.4 Factors affecting aluminum distribution in SSZ-13
The formation and amount of two kinds of aluminum pairs and isolated aluminum in the framework of zeolite SSZ-13 are related to many factors, such as the ratio of silicon to aluminum in zeolite, the content of Na+ added in the synthesis, and the preparation method.
2.2.4.1 Quantitative relationship between Si/Al ratio and Al pair
The silica-alumina ratio of the zeolite determines the total amount of aluminum in the framework, the respective amounts of aluminum pairs and isolated aluminum, and the ratio of the two aluminum pairs. At present, many studies have calculated the ratio of the corresponding aluminum pair to the isolated aluminum on the zeolite framework at a given silica-alumina ratio based on the L'Löwenstein rule and the random distribution of aluminum in the framework
[16,62,63,65]. For example, Schneider et al. calculated the theoretical values of aluminum pair content at different silicon-aluminum ratios and the ratio of two copper species at different copper contents under these two premises, and then used TMADa
+ as a template to synthesize a series of products with different silicon-aluminum ratios and copper-aluminum ratios for characterization, and the results were consistent with the theoretical values, as shown in Figure 6
[16].
Only a few studies have quantitatively analyzed the different aluminum pairs on SSZ-13 used in the experiment: Godiksen et Al., in the study on the reaction performance difference of different copper sites in the NH
3-SCR reaction of Cu-SSZ-13, synthesized SSZ-13 with Si/Al = 15 by using TMADa
+ as a template, and controlled the copper content to make the copper ions on the aluminum pair sites close to saturation (Cu/Al = 0.09 at this time). The ERP characterization of the sample found that the copper ion was located on two different aluminum pairs. The further quantification results show that the copper in the -Al-O-Si-O-Al- and -Al-O-Si-O-Si-O-Al- sites account for 72% and 20% of the total copper content, respectively
[23]. However, there is a lack of systematic research on the change of aluminum content under different conditions of silicon to aluminum ratio.
2.2.4.2 The first method to control the formation of aluminum pairs: "matching" between organic templates and inorganic cations
The content of Na
+ is particularly important for the control of the ratio of aluminum pairs in the synthesis of SSZ-13 using amorphous silica-alumina source as raw material and TMADa
+ as template
[62,63]. When the Na
+ is not used in the precursor solution of the synthetic zeolite, only a single aluminum atom exists on the six-membered ring. When sodium ion is added, the density of aluminum pair increases linearly with the increase of Na
+/TMADa
+. When Na
+/TMADa
+=1, the density of paired aluminum reaches a peak value, which is close to the theoretical value. When the Na
+/TMADa
+(1~2) continues to rise, the density of paired aluminum decreases, and when the Na
+/TMADa
+ is greater than 2, the CHA structure can no longer exist stably, and the MOR zeolite is formed. By contrast, when TMADa
+ and K
+ act together, TMADa
+ and K
+ compete with each other in the CHA cage due to their larger ionic radius.Resulting in only one TMADa
+ or three K
+ in the cage, and almost no aluminum pair in the as-synthesized CHA zeolite
[63].
SSZ-13 can be synthesized by a variety of organic templates, each of which has different properties such as size, shape and electron cloud distribution, and has different ability to match with the Na+“ in the same cage, so the number of SSZ-13 aluminum pairs synthesized may be different. This may be an important condition for the synthesis of SSZ-13 with high aluminum content.
2.2.4.3 The second method to control the formation of aluminum pairs: the effect of raw materials on the distribution of aluminum
SSZ-13 with different aluminum content can be synthesized by using different amorphous aluminum sources. Iorio et Al. Used different aluminum sources to synthesize SSZ-13 under the conditions of equimolar Na
+ and TMADa
+, and the aluminum pair content of the samples obtained was also different. Among them, the aluminum pair content synthesized with aluminum isopropoxide as the aluminum source was the highest, and the sample with Si/Al = 15 contained about 40% of aluminum pair, which was nearly twice the theoretical value (18%) under this Si/Al ratio
[63][16].
The number of aluminum pairs also changes when SSZ-13 is synthesized by the crystal transformation method. According to the research of Paolucci et Al., the aluminum distribution of low-silicon (Si/Al < 7) SSZ-13 synthesized by USY conversion method with TMADaOH as template is not "uniform", and the number of aluminum pairs is reduced accordingly
[16]. However, the reason for this "inhomogeneity" may not only come from the crystal transformation method, but also from the matching of template cation and framework charge when the silica-alumina ratio of zeolite is too low. As mentioned at the beginning of this section, the distribution of cations in the cage is not consistent at this time, which can be verified by the thermogravimetric curve results of TMADa-SSZ-13 with Si/Al = 4.7 in Guo et Al
[66]. Sano et al. Also found that the Na
+ content of SSZ-13 synthesized by amorphous silica-alumina source and crystal transformation method was different. For the sample with Si/Al ≈ 16, the Na/Al ratios of SSZ-13 synthesized by the two methods are 0.35 and 0.28, respectively, but the reasons are not analyzed
[25]. This can also be proved by the study of Devos et Al.: they used TMADaOH as a template to synthesize SSZ-13 with a similar silica-alumina ratio in a system without Na
+ using an amorphous silica-alumina source and USY, respectively. The former (Si/Al = 28) did not contain any aluminum pair as described above
[67]. The latter (crystallization time is very short, much shorter than that of other crystal transformation methods, Si/Al = 37) has a high Co/Al ratio of 0.24 after Co
2+ exchange. In the study of Sklenak et Al., TMADa
+ was used as template to prepare Cu-SSZ-13 with Si/Al = 12 by crystal transformation method, and the
29Si MAS NMR showed that there was no weak aluminum pair in the framework
[54]. It is quite different from the results of Godiksen et Al. (TMADa
+ as template, aluminum isopropoxide as aluminum source, Si/Al = 15) that the copper in the -Al-O-Si-O-Al- and -Al-O-Si-O-Si-O-Al- sites accounts for 72% and 20% of the total copper content, respectively
[60].
2.2.5 Characterization of aluminum pair in Cu-SSZ-13.
For SSZ-13 not loaded with copper ions, the number of aluminum pairs in the framework is often quantitatively analyzed by Co
2+ ion titration
[63,68]. In the
29Si MAS NMR, the Si (2Al, 2Si) atom is generated from -Al-O-Si-O-Al- and has a specific chemical shift (-92 to -100 ppm), which can be used to determine the content of -Al-O-Si-O-Al- in the zeolite framework, and the ratio of strong and weak aluminum pairs can be obtained by combining these two methods
[69⇓~71][72,73].
However, it has recently been pointed out that the Co
2+ in the dehydrated sample after ion exchange not only exists in the aluminum pair on the same six-membered ring (for example, -Al-O-Si-O-Si-O-Al- on the eight-membered ring), but also can be exchanged with the bare Co
2+),Co
2+. The result of ion exchange can only explain its divalent cation exchange ability
[54,67]. At this time, it is necessary to combine the results of
27Al(3Q)MAS NMR and DFT calculation of chemical shifts for further analysis
[74,75]. In addition, the results of infrared and ultraviolet-visible spectroscopy can also determine the environment of the Co
2+, so that the distribution of aluminum atom sites in the framework can be accurately obtained by combining a variety of methods
[54].
For Cu-SSZ-13, the distribution of aluminum in the framework can be analyzed by studying copper ions: Z
2Cu and ZCuOH can be directly distinguished by infrared spectroscopy and ultraviolet-visible spectroscopy, or indirectly distinguished by H
2-TPR based on the different reduction temperatures of the two copper species, and then related to the distribution of aluminum atoms in the framework to obtain the proportion of aluminum pairs and isolated aluminum atoms
[16,19,45]. EPR can judge the environment of copper ions, determine the ratio of two aluminum pairs on the skeleton, and combine them to obtain the ratio of two aluminum pairs
[48].
2.3 Possible location of acid sites in Cu-SSZ-13
As mentioned at the beginning of this article, there is only one independent T atom and four oxygen atoms attached to the T atom in the CHA structure, and theoretically, the framework aluminum can produce four acid sites (see Figure 7)
[76]. Although many experimental data of infrared spectroscopy clearly show that SSZ-13 has two different Brønsted acid sites, the exact location of the acid sites has been debated by researchers
[77⇓⇓~80].
Smith et al. Studied D-SSZ-13(D
2O, deuterium oxide) by neutron diffraction. During the structure refinement, it was found that there were two kinds of acid sites in the structure, at the O (1) and O (2) sites, and the deuteron occupancy was 4% and 8.3%, respectively. The deuteron at O (1) makes an angle of 19 ° with Si-O-Al and points to the center of the CHA cage, while the deuteron at O (2) makes an angle of 45 ° with the six-membered ring plane and points to the center of the six-membered ring. They assigned the high frequency (HF:3603 cm
-1) and low frequency (LF:3579 cm
-1) bands in the infrared spectrum to O (1) and O (2), respectively
[77]. Gale et al. Found that O (1) and O (3) are the most likely proton binding sites through first-principle molecular dynamics and other theoretical calculations, and gave the theoretical calculation value of the Mulliken charge of the most stable acid site of SSZ-13
[79].
In 2005, Brodiga et al. Used CO as a molecular probe to study the acidity distribution of SSZ-13 by Fourier transform infrared spectroscopy, and found that for CO, the acid sites in the high and low frequency bands have the same acidity, but the low frequency component only interacts with CO at higher pressure
[80]. Instead of being limited to a two-site distribution, they included four sites in the acid profile. They assigned the LF band to O (3) — the only proton not directly exposed on the oxygen atom in the eight-membered ring. The HF band is assigned to the protons on the oxygen atoms on the three eight-membered ring windows of O (1), O (2), O (4). This is also consistent with the experimental results that HF band is consumed faster and LF starts to react with CO under high pressure, because the protons at O (1), O (2) and O (4) (HF) are exposed to two CHA cages at the same time, which is easier to contact the probe molecule, while the protons at O (3) (LF) are exposed to only one cage, which is more difficult for the probe molecule to contact the acid site. Brodiga et al. 'S analysis is worth considering, for example, they believe that the number of acid centers at O (1) and O (2) positions should be the same considering symmetry
[17].
At present, although a series of studies on the more detailed catalytic sites of Cu-SSZ-13, the location of specific acid sites, have been carried out, the precise quantitative analysis and the differences in the NH3-SCR process need to be further explored.
3 Preparation of Cu-SSZ-13 with Different Templates and Its NH3-SCR Properties
At present, the research on the catalytic performance and influencing factors of Cu-SSZ-13 is mainly based on the Cu-SSZ-13 prepared by using TMADaOH as a template, and this part will describe the preparation of Cu-SSZ-13 by using other templates except TMADaOH and its catalytic performance. For the study of NH3-SCR performance, the current focus is on the Standard SCR process, that is, the only nitrogen oxide in the reactant is NO. Therefore, we judge the NH3-SCR activity of the catalyst at a specific temperature and gas flow rate by the conversion of NO.
3.1 Preparation and NH3-SCR of Cu-SSZ-13 with Benzyltrimethylammonium Hydroxide as Template
BTMAOH was the first template used to replace TMADaOH for the synthesis of SSZ-13, and the molar ratio of reactants was 20SiO
2:Al
2O
3:(1.0~4.0)Na
2O:(0.02~8.0)BTMAOH:(20~100)H
2O
[21]. Sano et Al. Conducted a series of experiments on the synthesis of SSZ-13 using BTMAOH as a template: BTMAOH was used as a template to synthesize SSZ-13 by the crystal transformation method (Y zeolite as the silica-alumina source, product Si/Al = 5.2 ~ 21.5) and the stepwise gel method (product Si/Al = 15 ~ 40)
[22⇓~24]. Compared with TMADaOH, BTMAOH does not have a good ability to dissolve the starting FAU zeolite into locally ordered aluminosilicate species and convert them into CHA zeolite, but when Si/Al is greater than 10, the SSZ-13 synthesized by BTMAOH is more resistant to acid treatment
[23,25].
They subsequently studied the cation content in the two samples with high silica-alumina ratio (Si/Al ≈ 16), and found that the sodium ion content of SSZ-13 synthesized by TMADaOH was much higher: the Na/Al of SSZ-13 synthesized by FAU as silica-alumina source and amorphous silica-alumina source was 0.28 and 0.35, respectively; The Na/Al ratio of SSZ-13 synthesized with BTMAOH as template is only 0.11. The position of copper ions loaded on the two samples was verified by XRD structure refinement, and it was found that the copper ions were near the six-membered ring after dehydration, which was consistent with the Cu-SSZ-13 synthesized using TMADaOH as the template.Therefore, they considered that there was no significant difference in the distribution of framework aluminum atoms between SSZ-13 synthesized by the two templates, and the difference in the content of Na
+ led to the different number of defects formed at the corresponding positions of zeolites, so TMADaOH-SSZ-13 was less resistant to acid treatment than BTMAOH-SSZ-13
[25].
Although the above research focuses on the acid treatment resistance, the data also reflect an important feature of the synthesis of SSZ-13 from TMADaOH: considering the host-guest charge balance, the difference in the content of Na+ will eventually lead to the formation of framework aluminum atoms or defects at the corresponding positions in SSZ-13 to balance its charge. At this high Si/Al ratio, only a very small number of aluminum pairs can be formed (at Si/Al = 16, on average, about two aluminum atoms per unit cell in three CHA cages), and most of the cages are isolated aluminum. The results of thermogravimetry and energy dispersive X-ray spectroscopy (EDX) showed that there were organic cations in each cage, and the content of Na+ in TMADaOH-SSZ-13 was much higher than that in BTMAOH-SSZ-13. Considering the limited aluminum content, most of the Na+ in TMADaOH-SSZ-13 is not to form aluminum pairs, but to form defects. Even so, a considerable number of Na+ and TMADa+ coexist in one cage. It can be seen that the ability of "coexistence" of TMADa+ and Na+ in the same cage is very strong, which also reflects from the side that at lower Si/Al ratio (such as Si/Al = 10, when the aluminum content is sufficient), more aluminum pairs can be formed when TMADa+ is used as a template agent to synthesize SSZ-13.
There are only a few literatures investigating SSZ-13 with different Si/Al ratios synthesized by BTMAOH-directed synthesis and its NH
3-SCR performance at different copper ion loadings
[24,26,27]. Comparing the NH
3-SCR performance of BTMAOH-Cu-SSZ-13 and TMADaOH-Cu-SSZ-13, the results showed that the catalysts without hydrothermal aging showed 100% NO conversion under similar conditions. However, after hydrothermal aging at 850 ℃, the performance of the catalyst decreased seriously. The maximum NO
x conversion of BTMAOH-Cu-SSZ-13 catalyst is about 65% at 350 ° C, which is much lower than the 90% conversion of TMADaOH-SSZ-13 in the range of 250 to 500 ° C
[26].
Electron spin resonance (ESR) and X-ray absorption near edge structure (XANES) were used to study the state change of copper ions during hydrothermal aging. The results showed that there were two kinds of sites for copper ions on SSZ-13 zeolite: double six-membered ring site and CHA-cage eight-membered ring site
[26]. After aging at 850 ° C, most of the copper ions at the above two sites disappeared, and new copper species appeared, namely CuO
2, Cu
2O, Cu/Al
2O
3, accompanied by the destruction of the zeolite framework
[26].
First of all, considering the ratio of silicon to aluminum and the ratio of copper to aluminum of the samples, the content of copper ions in most samples has far exceeded the amount of framework aluminum pairs in zeolites, so there are a large number of copper ions near the eight-membered ring interacting with single acid centers (formed by isolated aluminum) in these BTMAOH-Cu-SSZ-13. The difference in hydrothermal stability between BTMAOH-Cu-SSZ-13 and TMADaOH-Cu-SSZ-13 reflects that there may be fewer paired acid sites (aluminum pairs) formed on the framework, and more copper ions near the eight-membered ring interact with a single acid site, which is unstable and easy to aggregate into the cage to form oxide clusters under hydrothermal conditions, resulting in deactivation. And only when most of the copper ions in Cu-SSZ-13 interact with a single acid site, such a serious deactivation phenomenon will occur. Although BTMAOH is cheaper than TMADaOH, there are few reports on this aspect due to its poor catalytic NH3-SCR performance.
3.2 Cu-tetraethylenepentamine (Cu-TEPA) -directed synthesis of Cu-SSZ-13 and its NH3-SCR performance
In 2011, Xiao Fengshou et al. Of Zhejiang University reported a special template of Cu-TEPA, which was used to directly prepare Cu-SSZ-13 by "one-pot method". The typical raw material composition for the synthesis was 14.8Na
2O:3.0Al
2O
3:600H
2O:30SiO
2:4Cu-TEPA
[28]. The Cu-TEPA size dimension is 7.28 × 9.2222 Å, and each Cu-TEPA carries two positive charges. Obviously, Cu-TEPA is not only a low-cost organic structure directing agent, but also can introduce copper ions into the zeolite, so that the product has higher Cu loading and good Cu dispersion compared with Cu-SSZ-13 prepared by traditional ion exchange. However, the Si/Al ratio of TEPA-Cu-SSZ-13 is usually only 2.4 to 7.5
[11,28].
According to the host-guest charge matching principle, if one Cu-TEPA molecule in each cage has two positive charges, which has been calculated in Section 2.2.1, the framework Si/Al = 5, which is close to the actual synthetic value. Under the constraint of host-guest charge matching rule, the high charge density of Cu-TEPA results in the low silica-alumina ratio of Cu-SSZ-13.
The position and proportion of copper in the structure of Cu-SSZ-13 were obtained after Rietveld refinement of the XRD data of Cu-SSZ-13 structure by Maud software
[29]. The results show that there are three kinds of sites for isolated copper ions in the SSZ-13 structure. These isolated copper ions (Cu1, Cu2 and Cu3) occupy the corresponding sites D, B and A, respectively (Fig. 1). The content of Cu2 is the highest, and the order of the three contents is as follows: Cu2 (1. 81%) > Cu1 (0. 98%) > Cu3 (0. 46%).
The results of NH
3-SCR reaction (Fig. 8) show that without aging treatment, TEPA-Cu-SSZ-13 with various copper contents can achieve 100% conversion of NO
x in the range of 150 ~ 460 ℃, among which the sample with 2.42 wt% Cu content has the widest conversion range (150 ~ 500 ℃); However, after hydrothermal treatment at 750 ℃, the range of 100% NO conversion was reduced to 170 ~ 430 ℃. Recent studies have shown that the activity of TEPA-Cu-SSZ-13 for NO reduction decreased to a certain extent after hydrothermal treatment at 750 ℃ for 32 H, and the specific surface area of the catalyst also changed: in the untreated TEPA-Cu-SSZ-13, the specific surface area was 439 m
2/g, and after hydrothermal treatment, it was 242 m
2/g
[30]. The large decrease of specific surface area indicates that hydrothermal aging treatment not only causes copper ions to aggregate into CuO
x and block the pores, thus reducing the specific surface area, but also may cause dealumination of the zeolite framework, destroying the integrity of the zeolite framework and producing aluminum atoms outside the framework. The reason for this inactivation is similar to that of BTMAOH-Cu-SSZ-13 in 2.1, which is made more obvious by the high copper loading of TEPA-Cu-SSZ-13: when the copper ion on the six-membered ring reaches saturation, the copper loading continues to increase, and a large number of copper ions interact with a single acid site. Under hydrothermal conditions, Cu-SSZ-13 is deactivated due to the aggregation of unstable copper ions, which is consistent with the position and proportion of copper ions in the CHA structure obtained by Rietveld refinement.
3.3 Preparation of Cu-SSZ-13 with Choline Chloride as Template and Its NH3-SCR Performance
Choline chloride (CC), as a cheap and non-toxic chemical, was successfully used as a template for the synthesis of SSZ-13 zeolite for the first time in 2014. The molar ratio of the reactants used was 15Na
2O:1.0Al
2O
3:40SiO
2:5.5CC:292.6H
2O.SSZ-13 zeolite with Si/Al ratio of 6. 5 was obtained by crystallization at 140 ℃ for 5 d. The effects of different feed compositions and crystallization conditions on the synthesis were reported in the subsequent study. Finally, the Si/Al ratio of the product was increased to 8. 1 by increasing the silicon content in the initial condensation
[31][32].
The characterization results of TG-DSC and FTIR spectra, as well as the theoretical calculation of the molecular model of GAR2PED, all proved that there were two choline chloride molecules in each CHA cage (the molecular size of dimeric choline chloride was 7.21 × 10.7272 Å) and there was an interaction between the two choline chloride molecules (fig. 9)
[32,33].
According to the previous analysis in this paper, considering the problem of host-guest charge balance, when two cations appear in a CHA cage, two aluminum atoms on each cage framework need to balance their charges, as calculated in Section 2.2.1. At this time, the Si/Al ratio of the zeolite framework is 5, and the initial Si/Al ratio is 6.5, which is close to the theoretical value. However, Si/Al > 5 indicates that Cl- coexist in the cage or the zeolite framework forms defects to achieve charge balance. If the Si/Al ratio is further increased, too much Cl- in the cage may affect the directing effect of choline cation, or too many defects are formed to make SSZ-13 unstable, which ultimately leads to the failure to synthesize CC-SSZ-13 with high Si/Al ratio. In addition, from the molecular model, the two framework aluminum atoms used to balance the charge do not tend to occupy the same six-membered ring.
The newly prepared CC-Cu-SSZ-13 catalyst has a high NO conversion rate for NH
3-SCR reaction. When the Si/Al ratio is 6.3, the conversion rate is close to 100% in the range of 150 ~ 350 ℃ (Fig. 10a), and when the Si/Al ratio rises to 8.1, the conversion rate can even remain close to 100% in the range of 150 ~ 450 ℃ (Fig. 10B). Compared with Cu-SSZ-13 synthesized by other templates, its NH
3-SCR reaction performance is close
[31][32]. Although it is gratifying that this green and cheap template can be used to synthesize SSZ-13, there is still a lack of research on the use of this catalyst in NH
3-SCR after hydrothermal aging due to the low silica-alumina ratio of the synthetic product. Even if the silicon and aluminum of CC-Cu-SSZ-13 are low, the actual ratio of paired acid centers may not be satisfactory, as the molecular model of dimeric choline chloride mentioned in the previous paragraph has provided us with some basis for judgment. Considering the existence of paired weak acid sites, the copper ion at this position is not as stable as paired strong acid sites. If the proportion of weak acid sites in paired acid sites is large, it will also be deactivated under high temperature reaction conditions. The increase of Si/Al ratio from 6.3 to 8.1 at 350 ~ 450 ℃ seems to be consistent with this speculation.
3.4 Preparation of Cu-SSZ-13 with TEAOH as template and its NH3-SCR performance
The synthesis route of SSZ-13 using TEAOH as template was first realized by interzeolite conversion using ultrastable Y zeolite (USY) as raw material
[34]. It is found that SSZ-13 can be obtained in a relatively narrow phase region when USY is used as both silicon and aluminum sources. A typical synthesis reactant composition is 4.2NaOH:21SiO
2:Al
2O
3:4.2TEAOH:105H
2O, and the product is pure phase SSZ-13 after hydrothermal crystallization at 140 ℃ or 160 ℃ for 3 d. The idea of this synthesis route is that the FAU structure of USY zeolite as the raw material contains the same unit (double six-membered ring) as the CHA structure of SSZ-13 zeolite, and the silica-alumina ratio of the two zeolites is close (Si/Al = 10.5).
The synthesized TEAOH-SSZ-13 has a Si/Al ratio of 8. 5, a crystal size of 0. 1 ~ 0.2 μm, and a TEA
+ in each CHA cage
[34]. The highest value of silica-alumina ratio of SSZ-13 synthesized by this method can only reach 9
[36].
Also combined with the previous analysis in this paper, considering the problem of host-guest charge matching, the Si/Al ratios of SSZ-13 containing one and two aluminum on each CHA cage skeleton are 5 and 11, respectively. SSZ-13 overall Si/Al = 7 when CHA cages containing two aluminum and one aluminum are present in a one-to-one ratio. Si/Al = 8.5 indicates that the number of CHA cages containing one aluminum is dominant, and considering that aluminum in CHA cages containing two aluminum may not exist in the form of aluminum pairs, it is speculated that most of the aluminum in TEAOH-SSZ-13 exists in the form of single aluminum on the zeolite framework.
Cu-SSZ-13 with 2. 2 wt% copper ion was obtained by ion exchange of TEAOH-Cu-SSZ-13, and the NH
3-SCR reaction was tested in the temperature range of 170 ~ 550 ℃. The results show that the NO conversion rate of Cu-SSZ-13 without hydrothermal aging treatment can reach more than 90% at 250 ~ 550 ℃, and the highest can reach 95% (Fig. 11 TEAOH-Cu-SSZ-13)
[34]. After thermal aging at 750 ℃ for 13 H, the catalytic activity of Cu-SSZ-13 decreased under the same conditions mentioned above, but it still maintained a conversion rate of more than 80% at 300 ~ 500 ℃ (Fig. 11 TEAOH-Cu-SSZ-13-HT-750).
It seems that TEAOH-Cu-SSZ-13 has a catalytic activity similar to that of TMADaOH-Cu-SSZ-13. However, the space velocity of SCR reaction of TMADaOH-SSZ-13 is 600 000 mL·h-1·g-1, while the space velocity of the above SCR reaction is only 450 000 mL·h-1·g-1. Therefore, it is difficult to assert that the catalytic activity of Cu-SSZ-13 prepared with TEAOH as the template is comparable to that of TMADaOH-Cu-SSZ-13. However, from the performance of hydrothermal SCR reaction, the performance of TEAOH-Cu-SSZ-13 shows that at least a considerable part of the copper ions are located near the six-membered ring with paired acid centers. However, considering the particularity of the crystal transformation method, it is difficult to judge whether the template or the USY crystal transformation method has a greater effect on the production of paired acid sites. Unfortunately, there are few studies on the copper ion in the SCR reaction of TEAOH-Cu-SSZ-13, and it is impossible to understand the proportion of copper on different acid sites, and the catalytic activity under more stringent reaction conditions is also unknown.
3.5 Preparation of Cu-SSZ-13 using DMCHABr as template and its NH3-SCR performance
The synthesis of SSZ-13 by Xiao et al. Using N, N, N-dimethylethylcyclohexylammonium bromide (DMCHABr) as a template can be achieved via a solvent-free route. One typical reactant composition is 28.6SiO
2:Al
2O
3:5.15Na
2O:3.43DMCHABr
[37]. SSZ-13 with high crystallinity and Si/Al ratio of 6. 9-10. 5 can be obtained by grinding the raw material mixture and heating at 180 ℃ for 5 d. In the solvent-free route, only a small amount of silicon is dissolved into the liquid phase. Therefore, the utilization rate of silica in this method can be as high as 95.7%, which is much higher than that in the conventional hydrothermal synthesis method (52.6%).
With the development of crystal transformation method and solvent-free method for zeolite synthesis, they successfully used zeolite Y (Si/Al = 10.8) as silicon source and aluminum source to prepare DMCHABr-Cu-SSZ-13 by crystal transformation method and seed assisted guidance in anhydrous system in 2017. A typical reactant composition is 3.02DMCHABr:21.6SiO
2:Al
2O
3:2.81Na
2O and 2 wt% seed
[38]. SSZ-13 with high crystallinity can be obtained by grinding the raw material mixture for 5 ~ 7 min and crystallizing at 150 ~ 180 ℃ for 1 ~ 3 d. The Si/Al ratio of SSZ-13 synthesized by this method is 6.2 to 11.4. Compared with the conventional silica-alumina source synthesis system, this method has little effect on the properties of the product, but greatly shortens the synthesis time.
The DMCHABr-SSZ-13 prepared by the two methods were loaded with 2.0 wt% copper ions for SCR performance test (Fig. 12 DMCHABr-Cu-SSZ-13-1 and DMCHABr- Cu-SSZ-13-2 represent the conventional silica-alumina source and the DMCHABr-Cu-SSZ-13 prepared by the crystal transformation method), and the results show that the NO conversion rate of the sample can be maintained at more than 90% in the range of 200 ~ 400 ℃. At 500 ° C, the NO conversion of the sample decreased rapidly to less than 80%.
By analyzing the SCR reaction results in the range of 350 ~ 550 ℃ (Fig. 12), it can be found that the DMCHABr-Cu-SSZ-13 sample has a lower NO conversion at high temperature compared with TMADa-Cu-SSZ-13. At high temperature, the decrease of NO conversion is caused by the aggregation of unstable copper ions, and the copper ions on the paired acid sites of Cu-SSZ-13 are more stable than those on the single acid sites, which indicates that the copper ions on the single acid sites of DMCHABr-Cu-SSZ-13 are more, that is, the aluminum pairs on SSZ-13 synthesized by DMCHABr are less.
Compared with the reaction results of the two DMCHABr-Cu-SSZ-13, the DMCHABr-Cu-SSZ-13 prepared by the crystal transformation method performs worse at high temperature. Their Si/Al ratio, specific surface area and crystallinity are very close, and the copper content loaded is also the same. The only possibility is that the Cu-SSZ-13 aluminum prepared by the crystal transformation method is different. The solvent-free system used in this crystal transformation method may limit the diffusion of materials, which may not be conducive to the formation of aluminum pairs.
4 Cu-SSZ-13 prepared by several co-template methods and its NH3-SCR properties
The SSZ-13 zeolites prepared by different template-directed synthesis have different framework silicon to aluminum ratio, aluminum atom distribution, grain size and morphology, and the catalytic effect of Cu-SSZ-13 zeolites prepared by different template-directed synthesis is also different. At present, the best catalytic effect is still the Cu-SSZ-13 catalyst prepared by TMADaOH-directed synthesis. In order to reduce the cost of the catalyst and ensure that the Cu-SSZ-13 catalyst has good catalytic performance, some researchers used the dual-template method to prepare Cu-SSZ-13. In this section, the preparation of SSZ-13 by co-template method and the characterization of its catalytic performance are reviewed, and the template combinations that have successfully synthesized SSZ-13 but have not been studied for catalytic performance are not described in this paper.
4.1 Cu-SSZ-13 prepared by TMADaOH and Cu-TEPA as co-template and its catalytic effect on NH3-SCR
In 2013, Mart Martínez-Franco et al. Successfully prepared Cu-SSZ-13 in one step for the first time by using TMADaOH and Cu-TEPA co-template method
[39]. The method can avoid ion exchange and improve the Si/Al ratio (10.6-14.2) of SSZ-13 generated by Cu-TEPA guidance by changing the Si-Al ratio and copper content of the product of Cu/ (Al + Si) and Si/Al ratio modulation in the initial gel, and successfully control the loading amount of copper ions. The results of XRD, pore properties and NH
3-SCR also show that the hydrothermal stability is good. Further study on the catalytic performance of Cu-SSZ-13 prepared by the co-template method for NH
3-SCR showed that the catalyst with Si/Al ratio of 15 had higher conversion and better hydrothermal stability (800 ℃, 850 ℃) among the samples with different Si/Al ratios (Si/Al = 6, 15, 30)
[20]. The catalytic performance of Cu-SSZ-13 with Si/Al ratio of 6 and 15 before aging was very similar, and the conversion rate was close to 100% at 250 ~ 550 ℃. After steam treatment at 800 ℃, the conversion rate of Cu-SSZ-13 with Si/Al ratio of 6 and 15 was slightly decreased, but still close to 95% at 250 ~ 550 ℃. After steam treatment at 850 ℃, the conversion of NO to N
2 was still close to 90% in the temperature range of 250 ~ 550 ℃ for Cu-SSZ-13 with Si/Al = 15, but the reactivity of Cu-SSZ-13 with Si/Al = 6 was significantly reduced. However, Cu-SSZ-13 with Si/Al ratio of 30 shows a significant difference from the former two under these three conditions.
This co-templating method not only reduces the production cost by partially replacing TMADaOH with Cu-TEPA, but also ensures the catalytic activity and good hydrothermal stability of the catalyst, which has a good commercial prospect. However, a series of important issues, such as whether more paired acid sites are formed under this co-template condition, and how the two template molecules and Na+ distribute to form paired acid sites in the skeleton, have not been discussed in the above studies and need to be further explored.
4.2 Cu-SSZ-13 prepared by TEAOH and Cu-TEPA as co-template and its catalytic effect on NH3-SCR;
The preparation of Cu-SSZ-13 by co-template method without TMADaOH has also been reported. Martin et al. Studied the direct preparation of Cu-SSZ-13 by interzeolite conversion using USY. In this method, Cu-TEPA and TEAOH were used as templates, and the crystallization was carried out at 160 ℃ for 3 d. The typical formula was 4.2NaOH:21SiO
2:Al
2O
3:4.2TEAOH:1.06 Cu-TEPA:105H
2O
[34]. The obtained Cu-SSZ-13 has a copper ion content of 4. 1 wt% and a Si/Al ratio of 10. 3, which is slightly higher than that when only TEAOH or Cu-TEPA is used as a template (the highest Si/Al ratio is 9 and 7. 5, respectively), but the specific reasons are not explored. UV-Vis spectra showed that Cu-TEPA was perfectly present in the CHA cage of SSZ-13 after crystallization, indicating that Cu-TEPA played a guiding role in the crystallization process. The conversion rate of TEPA-TEAOH-Cu-SSZ-13 to NO can exceed 90% at 250 ~ 450 ℃ (see Fig. 11), which is slightly lower than that of TEAOH-SSZ-13 exchanged by Cu ions. But when the temperature is higher than 450 ° C, the conversion to NO decreases very fast, decreasing to below 80% at 550 ° C (Fig. 11). We speculate that there may be two reasons for this difference. One is that Cu-TEPA as a template affects the location of copper ions in SSZ-13 zeolite, thus affecting the catalytic activity. The other is that the content of copper ions in Cu-SSZ-13-TEPA-TEAOH (4. 1 wt%) is significantly higher than that in Cu-SSZ-13-TEAOH (2. 2 wt%), and more copper ions are located on the single acid center of the eight-membered ring, that is, more unstable copper ions. Therefore, it is easier to form aggregated oxides at high temperatures, resulting in a decrease in catalytic activity. Cu-SSZ-13 was prepared by TEAOH and Cu-TEPA co-template method in one step with two cheaper templates, which greatly reduced the cost, but as a NH
3-SCR catalyst, its hydrothermal stability still needs to be further improved.
4.3 Preparation of multi-metal ion Cu-SSZ-13 and its catalytic effect on NH3-SCR
Inspired by multi-metal supported catalysts, some researchers have recently successfully prepared Cu/Ni-SSZ-13 using Ni-DETA and Cu-TEPA as co-templates. Cu/Ni has the greatest influence on the catalytic effect of the product, and Cu/Ni = 5.33 has the best effect in the experiment; compared with Cu/Ni-SSZ-13, Cu/Ni-SSZ-13 has higher conversion rate at high temperature, and the activity and conversion rate do not decrease at low temperature because too many Cu sites are occupied by Ni
[40]. Although the Si/Al ratio of the product is very low (about 2.4), the prepared Cu/Ni-SSZ-13 can maintain a high conversion rate at high temperature in the SCR reaction, which shows that Ni plays an important role in it. The introduction of Ni inhibits the formation of copper oxide at high temperature, which makes it have better activity at high temperature, but the specific mechanism is not clear.
Chen Biaohua et al. Conducted a similar study in 2020, and they prepared Cu/Zn-SSZ-13 by one-pot method
[41]. Cu/Zn-TEPA was used as a template, and metal ions were directly introduced at the same time to obtain Cu/Zn-SSZ-13 with Si/Al = 3.6, Cu/Al = 0.12, and Zn/Al = 0.43. Compared with the Cu-SSZ-13 prepared by the ion exchange method using TMADaOH as the template, the one-pot Cu/Zn-SSZ-13 still showed good NH
3-SCR catalytic activity and hydrothermal stability despite the very low silica-alumina ratio of the catalyst framework (see Fig. 13). Compared with the introduction of Zn by ion exchange after the synthesis of SSZ-13, the introduction of Zn by one-pot method will not cause the competition between Zn
2+ and Cu
2+, so that the Zn
2+ will replace a part of the Cu
2+ sites to form Zn
2+ or ZnO, which will interact with acid sites.These two species are not conducive to the SCR reaction, and these replaced copper species lack the interaction with the acidic sites of the framework and become unstable, which is easy to form CuO in the SCR reaction and affect the catalytic performance. When Zn is introduced in one pot, copper species interact with zinc species to form [Zn-O-Zn] -Z, [Cu-O-Zn] -Z and [Zn-OH] -Z, which are more stable compared with single copper species (Z represents an acid site). The latter two species play an important role in preventing the collapse of the CHA framework under the attack of water vapor under hydrothermal conditions, and can also play a role in stabilizing copper ions, and by dispersing copper ions, they can not aggregate in large quantities under this condition, resulting in the deactivation of blocked pores. Considering the space density of copper, the high conversion of the newly prepared catalyst is reasonable, but it is worth noting that it still maintains high hydrothermal stability at such a low silica-alumina ratio. Although there is a lack of further characterization to verify the existence of [Zn-O-Zn] -Z, [Cu-O-Zn] -Z and [Zn-OH] -Z, and the essential reason for the excellent hydrothermal stability needs to be further discussed, this method provides a valuable idea and a relatively reasonable explanation for the stabilization of copper ions in Cu-SSZ-13 by co-template method, which is of great reference value.
5 Template-free synthesis of CHA zeolite (SSZ-13 with low Si/Al)
Natural chabazite, discovered in 1772, is composed of aluminosilicate with Si/Al less than 2.5 and can be synthesized without organic template
[81]. SSZ-13 can be regarded as chabazite with Si/Al higher than 5.0. At present, the silica-alumina ratio of CHA-type zeolite synthesized without organic template is obviously low. High-silica chabazite is usually synthesized by adding seeds in the template-free system, in which a typical crystallization composition is 20SiO
2:Al
2O
3:8Na
2O:1.8K
2O:2000H
2O, and the Si/Al ratio of chabazite is 3. 3
[83,84]. Recently, considering that LEV-type zeolite and CHA-type zeolite have similar structural units, LEV-type zeolite has been successfully converted into chabazite by crystal transformation without adding a template
[83]. The mixture of hydrothermal conversion is Si/Al=6~22,NaOH/SiO
2=3,H
2O/SiO
2=80, and the mixture is crystallized at 90 ~ 200 ℃ for 1.5 ~ 24 H. The Si/Al ratio of chabazite is 4.9 and the yield is only 20%.
However, it is known that the Si/Al ratio of CHA-type catalyst for NH3-SCR reaction is at least more than 6 by understanding various Cu-SSZ-13 catalysts, so there is still a long way to go for the synthesis of high silica-alumina ratio chabazite without template.
6 Conclusion and outlook
In the discussion of the properties of Cu in the catalyst and the catalytic mechanism, we found that the difference of copper species in different positions essentially comes from the acid sites with which they interact. The copper species controlled by paired acid sites and single acid site show different activity and stability during activation, low temperature reaction and high temperature reaction. In this series of dynamic changes, we believe that the paired acid sites play the most important role in stabilizing copper ions at high temperatures, which ensures the excellent hydrothermal stability of Cu-SSZ-13, which is closely related to the Al distribution on the zeolite structure.
In order to make the ratio of strong aluminum pair which is beneficial to the stability of copper ions as large as possible, many factors need to be considered: the range of silica-alumina ratio, the charge matching of template, the influence of raw materials, etc. The control of framework aluminum distribution on zeolite is the focus of zeolite synthesis and catalytic activity center control.Chen Tiehong et Al. Initially proposed a formula for calculating the ratio of aluminum in zeolite at two asymmetric tetrahedral sites based on the peaks observed in the
29Si MAS NMR spectrum and the chemical analysis of Si/Al ratio
[85]. Recently, IFR zeolites with controlled Al distribution have been successfully synthesized at different tetrahedral sites (T atom sites)
[86]. For CHA zeolite with only one T site, the distribution of Al is also one of the important research directions in the future.
The following conclusions can be drawn from the summary of different templates as well as the co-template method Cu-SSZ-13: (1) In addition to TMADaOH, many organic compounds such as BTMAOH, Cu-TEPA, choline chloride, TEAOH, DMCHABr have been confirmed to be able to act as structure-directing agents to direct the formation of SSZ-13 zeolite, and the Si/Al of the SSZ-13 samples synthesized by these organic structure-directing agents can meet the requirements of catalytic NH3-SCR reaction. Cu-SSZ-13 prepared by these structure-directing agents can be comparable to TMADa+-Cu-SSZ-13 in a wide temperature range in the NH3-SCR reaction. However, after hydrothermal aging, the conversion rate of NO decreased sharply. (2) The copper ions in SSZ-13 will form CuOx and coalesce, which will destroy the cage and pore of zeolite and lead to the collapse of CHA structure. Affected by the acidity of the skeleton, the stability of copper ions at different positions is significantly different. When TMADaOH was used as template and Na+ coexisted, the number of aluminum pairs on SSZ-13 was the largest, and the proportion of copper ions acting on paired strong acid centers was the largest, so that copper ions could exist stably near the six-membered ring, while the ability of other templates to form aluminum pairs was weak. The ability of organic cations and Na+ to "match" in the same CHA cage is one of the possible reasons for the difference in the number of aluminum pairs. (3) Some progress has been made in the synthesis of SSZ-13 by using TMADaOH as a mixed template. First, the amount of TMADaOH can be significantly reduced by using a mixed template. Secondly, the Cu-SSZ-13 catalysts prepared by some mixed templates have outstanding performance in NH3-SCR reaction. For example, the Cu-SSZ-13 catalyst prepared by Cu-TEPA and TMADaOH mixed templates not only omits the step of copper ion exchange, but also has good NH3-SCR reaction performance after hydrothermal aging treatment at 850 ℃, which is an excellent combination at present.
After summarizing the differences in the preparation of Cu-SZZ-13 directed by different organic structure-directing agents, we made it clear that the energy of the zeolite-structure-directing agent complex determines the proportion of Al at different T atom sites. It is a feasible method to find a cheap structure-directing agent or a combination of several organic/inorganic structure-directing agents to increase the proportion of Al in the structure of SSZ-13. In addition, the crystal transformation method also provides a new possibility to control the distribution of Al atoms.
In this review, the hydrothermal stability of Cu-SSZ-13 has been emphasized and focused on, but in fact, the importance of Cu-SSZ-13 for low temperature NH
3-SCR(<250℃) catalytic performance has been gradually recognized. In the temperature range of 250 ~ 350 ℃, the catalytic activity of the catalyst is slightly lower than that at 250 ℃ due to the simultaneous existence of high and low temperature SCR catalytic mechanisms due to the gradual desorption of solvating NH
3, which leads to the decrease of copper ion mobility, but the catalyst still performs well in this temperature range with the increase of temperature
[87]. However, when the diesel engine starts and runs at low load, the temperature of the exhaust gas is too low (< 250 ℃), and the conversion rate of the catalyst will decrease significantly. This is unavoidable when diesel vehicles are used, so reducing nitrogen oxide emissions at low temperatures remains a challenge for the future. Based on this, a seminar on future exhaust treatment solutions in the United States in 2012 proposed the so-called "150 ℃ Challenge": to improve existing technologies or develop new technologies to maintain good catalytic activity at significantly lower temperatures to meet increasingly stringent emission regulations
[88]. In the field of PNA(passive NO
xabsorber), some basic knowledge of Cu-SSZ-13 catalyst has promoted the research and development of this new technology. In the field of SCR, the "150 ℃ Challenge" has shown some hope, but it still needs further efforts from researchers.
PDF
