Investigation of Suitable Templates for One-Pot-Synthesized Cu-SAPO-34 in NOx Abatement from Diesel Vehicle Exhaust

Zeolite Properties, Methods of Synthesis, and Selected Applications

Zeolites, a group of minerals with unique properties, have been known for more than 250 years. However, it was the development of methods for hydrothermal synthesis of zeolites and their large-scale industrial applications (oil processing, agriculture, production of detergents and building materials, water treatment processes, etc.) that made them one of the most important materials of the 20th century, with great practical and research significance. The orderly, homogeneous crystalline and porous structure of zeolites, their susceptibility to various modifications, and their useful physicochemical properties contribute to the continuous expansion of their practical applications in both large-volume processes (ion exchange, adsorption, separation of mixture components, catalysis) and specialized ones (sensors). The following review of the knowledge available in the literature on zeolites aims to present the most important information on the properties, synthesis methods, and selected applications of this group of aluminosilicates. Special attention is given to the use of zeolites in agriculture and environmental protection.

Boosting the catalytic performance of Cu-SAPO-34 in NOx removal via hydrothermal treatment

Phosphate ions promoted Cu-SAPO-34 (P-Cu-SAPO-34) were prepared using bulk CuO particles as Cu2+ precursor by a solid-state ion exchange technique for the selective catalytic reduction of NOx with NH3 (NH3-SCR). The effects of high temperature (H-T) hydrothermal aging on the NOx removal (de-NOx) performance of Cu-SAPO-34 with and without phosphate ions were systematically investigated at atomic level. The results displayed that both Cu-SAPO-34 and P-Cu-SAPO-34 presented relatively poor NOx removal activity with a low conversion (< 30%) at 250-500°C. However, after H-T hydrothermal treatment (800°C for 10 hr at 10% H2O), these two samples showed significantly satisfied NOx elimination performance with a quite high conversion (70%-90%) at 250-500°C. Additionally, phosphate ions decoration can further enhance the catalytic performance of Cu-SAPO-34 after hydrothermal treatment (Cu-SAPO-34H). The textural properties, morphologies, structural feature, acidity, redox characteristic, and surface-active species of the fresh and hydrothermally aged samples were analyzed using various characterization methods. The systematical characterization results revealed that increases of 28% of the isolated Cu2+ active species (Cu2+-2Z, Cu (OH)+-Z) mainly from bulk CuO and 50% of the Brønsted acid sites, the high dispersion of isolated Cu2+ active component as well as the Brønsted acid sites were mainly responsible for the accepted catalytic activity of these two hydrothermally aged samples, especially for P-Cu-SAPO-34H.

Improvement of Cu-SAPO-34 hydrothermal stability by tuning P/Al ratio for selective catalytic reduction of NO by NH3

Cu-SAPO-34 is a promising catalyst for abatement of NO via selective catalytic reduction with NH₃ (NH₃-SCR), but its hydrothermal stability needs to be enhanced. In this work, the Cu-SAPO-34 catalysts with different P/Al ratios of 0.8, 1.0 and 1.2 were prepared, and the temperature window with NO conversion >90% (T₉₀) for all catalysts were similar (160-570 °C). The T₉₀ of Cu-SAPO-34 with P/Al of 0.8 dramatically decreased (220-470 °C) after hydrothermal treatment, and interestingly, the catalysts with high P/Al ratios (1.0 and 1.2) remained high activity. The T₉₀ of the aged catalysts with P/Al of 1.2 was 155-525 °C. The characterizations showed that the increase of P/Al ratio not only enhanced the crystallinity but also enlarged the grain size of catalysts, which were conducive to the zeolite framework stability. Moreover, the Cu-SAPO-34 with large grain size facilitated the conversion of CuO to isolated Cu²⁺ ions as well as inhibited the aggregation of Cu species. Furthermore, the large grain sized catalysts provided more acid sites, and thus, the catalysts presented excellent hydrothermal stability. In situ DRIFTS analysis confirmed the existence of both Langmuir-Hinshelwood and Eley-Rideal pathway over the catalyst with a P/Al ratio of 1.2. This work provided a facile method to promote the hydrothermal stability of Cu-based zeolite catalysts.

Economical and Sustainable Synthesis of Small-Pore Chabazite Catalysts for NOx Abatement by Recycling Organic Structure-Directing Agents

The application of small-pore chabazite-type SSZ-13 zeolites, key materials for the reduction of nitrogen oxides (NO_x) in automotive exhausts and the selective conversion of methane, is limited by the use of expensive N,N,N-trimethyl-1-ammonium adamantine hydroxide (TMAdaOH) as an organic structure-directing agent (OSDA) during hydrothermal synthesis. Here, we report an economical and sustainable route for SSZ-13 synthesis by recycling and reusing the OSDA-containing waste liquids. The TMAdaOH concentration in waste liquids, determined by a bromocresol green colorimetric method, was found to be a key factor for SSZ-13 crystallization. The SSZ-13 zeolite synthesized under optimized conditions demonstrates similar physicochemical properties (surface area, porosity, crystallinity, Si/Al ratio, etc.) as that of the conventional synthetic approach. We then used the waste liquid-derived SSZ-13 as the parent zeolite to synthesize Cu ion-exchanged SSZ-13 (i.e., Cu-SSZ-13) for ammonia-mediated selective catalytic reduction of NO_x (NH₃-SCR) and observed a higher activity as well as better hydrothermal stability than Cu-SSZ-13 by conventional synthesis. In situ infrared and ultraviolet-visible spectroscopy investigations revealed that the superior NH₃-SCR performance of waste liquid-derived Cu-SSZ-13 results from a higher density of Cu²⁺ sites coordinated to paired Al centers on the zeolite framework. The technoeconomic analysis highlights that recycling OSDA-containing waste liquids could reduce the raw material cost of SSZ-13 synthesis by 49.4% (mainly because of the higher utilization efficiency of TMAdaOH) and, meanwhile, the discharging of wastewater by 45.7%.

SO2-Tolerant Catalytic Reduction of NOx via Tailoring Electron Transfer between Surface Iron Sulfate and Subsurface Ceria

Currently, SO₂-induced catalyst deactivation from the sulfation of active sites turns to be an intractable issue for selective catalytic reduction (SCR) of NO_x with NH₃ at low temperatures. Herein, SO₂-tolerant NO_x reduction has been originally demonstrated via tailoring the electron transfer between surface iron sulfate and subsurface ceria. Engineered from the atomic layer deposition followed by the pre-sulfation method, the structure of surface iron sulfate and subsurface ceria was successfully constructed on CeO₂/TiO₂ catalysts, which delivered improved SO₂ resistance for NO_x reduction at 250 °C. It was demonstrated that the surface iron sulfate inhibited the sulfation of subsurface Ce species, while the electron transfer from the surface Fe species to the subsurface Ce species was well retained. Such an innovative structure of surface iron sulfate and subsurface ceria notably improved the reactivity of NH_x species, thus endowing the catalysts with a high NO_x reaction efficiency in the presence of SO₂. This work unraveled the specific structure effect of surface iron sulfate and subsurface ceria on SO₂-toleant NO_x reduction and supplied a new point to design SO₂-tolerant catalysts by modulating the unique electron transfer between surface sulfate species and subsurface oxides.

Understanding the direct relationship between various structure-directing agents and low-temperature hydrothermal durability over Cu-SAPO-34 during the NH3-SCR reaction

Hydrolysis of Si–O(H)–Al bonds and the loss of active Cu(OH)+ species jointly contribute towards the deactivation of Cu-SAPO-34 under a moist environment at low temperature (<100 °C).

Synthesis and Applications of SAPO-34 Molecular Sieves

Silicoaluminophosphate zeolite (SAPO-34) has been attracting increasing attention due to its excellent form selection and controllability in the chemical industry, as well as being one of the best industrial catalysts for methanol-to-olefin (MTO) reaction conversion. However, as a microporous molecular sieve, SAPO-34 easily generates carbon deposition and rapidly becomes inactivated. Therefore, it is necessary to reduce the crystal size of the zeolite or to introduce secondary macropores into the zeolite crystal to form a hierarchical structure in order to improve the catalytic effect. In this review, the synthesis methods of conventional SAPO-34 molecular sieves, hierarchical SAPO-34 molecular sieves and nanosized SAPO-34 molecular sieves are introduced, and the properties of the synthesized SAPO-34 molecular sieves are described, including the phase, morphology, pore structure, acid source, and catalytic performance, in particular with respect to the synthesis of hierarchical SAPO-34 molecular sieves. We hope that the review can provide guidance to the preparation of the SAPO-34 catalysts, and stimulate the future development of high-performance hierarchical SAPO-34 catalysts to meet the growing demands of the material and chemical industries.

Controlled synthesis of Cu-based SAPO-18/34 intergrowth zeolites for selective catalytic reduction of NOx by ammonia

Cu-based SAPO-18, SAPO-18/34 intergrowth and SAPO-34 zeolites were applied for the selective catalytic reduction of NO_x by ammonia (NH₃-SCR) catalysts. Comprehensive characterization results revealed that the SAPO-18/34 with higher amount and strength of acid sites could facilitate the generation of more isolated copper ions (Cu²⁺ and Cu⁺) and suppress the formation of CuO_x, which might account for the fact that intergrowth crystal structure of Cu-SAPO-18/34 exhibited higher fresh NH₃-SCR activity, more robust hydrothermal durability and better SO₂-resistance ability than that Cu-SAPO-18 and Cu-SAPO-34. In situ DRIFTS results provided the formation of reaction intermediates, such as -NH₂, NH₄⁺, NO₃^-, NO₂^-, etc. Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction mechanisms were both involved in Cu-based SAPO-18 and SAPO-18/34 intergrowth zeolites, but the L-H mechanism dominated the NH₃-SCR reaction, in addition, Cu-SAPO-34 only followed "L-H" mechanism.

One-pot hydrothermal synthesis of dual metal incorporated CuCe-SAPO-34 zeolite for enhancing ammonia selective catalytic reduction

A series of dual metal incorporated CuCex-SAPO-34(x = 0-0.04) samples were synthesized using one-pot hydrothermal method with diethylamine as organic structure-directing agent for selective catalytic reduction of NO_x by NH₃. The catalytic properties were elucidated in detail with physicochemical properties being analyzed using various instruments. All the catalysts exhibited typical SAPO-34 crystal structures with high specific surface areas. With the dual-metal incorporation, the surface acidity and amount of isolated Cu²⁺, which may be active sites for NH₃-SCR, were significantly enhanced. However, excessive Ce restrained the formation of isolated Cu²⁺ due to its occupation of cationic sites. Therefore, the 0.05CuCe0.02-SAPO-34 exhibited high NO conversion (≥80%) at 168°C-500°C. Furthermore, the NH₃-SCR mechanism over different catalysts was investigated in-situ DRIFTS experiments. For the 0.05Cu-SAPO-34, the adsorbed NH₃ species react with gaseous NO and following the E-R mechanism throughout the reaction temperature range. Meanwhile, adsorbed NO₂ was detected and reacted with the adsorbed NH₃ species according to the L-H mechanism in low-temperature region. In contrast, the NH₃-SCR reaction over the 0.05CuCe0.02-SAPO-34 primarily followed the E-R mechanism throughout the temperature range. The L-H mechanism was cut off due to the loss of the adsorption ability of nitrous species at high temperatures., resulting in NO conversion decreasing.

Alkali-Resistant NOx Reduction over SCR Catalysts via Boosting NH3 Adsorption Rates by In Situ Constructing the Sacrificed Sites

Currently, improving the alkali resistance of vanadium-based catalysts still remains as an intractable issue for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). It is generally believed that the decrease in adsorbed NH_x species deriving from the declined acidic sites is the chief culprit for the deactivation of alkali-poisoned catalysts. Herein, alkali-resistant NO_x reduction over SCR catalysts via boosting NH₃ adsorption rates was originally demonstrated by in situ constructing the sacrificed sites. It is interesting that the adsorbed NH_x species largely decrease while the NH₃ adsorption rate is well kept over the V₂O₅/CeO₂ catalyst by in situ constructing the sacrificed sites. The SCR activity could be maintained after alkali poisoning because in situ constructed SO₄^2- groups would prefer to be combined with K⁺ so that the specific V═O species can endow K-poisoned V₂O₅/CeO₂ with high adsorption rate of NH₃ and high reactivity of NH_x species. This work provides a new viewpoint that NH₃ adsorption rate plays more decisive roles in the performance of alkali-poisoned catalysts than the amount of NH₃ adsorption and enlightens an alternative strategy to improve the alkali-resistance of catalysts, which is significant to both the academic and industrial fields.