CHA Zeolite Research Articles

Mechanism for Cu-enhanced hydrothermal stability of Cu–CHA for NH3-SCR

The presence of Cu delays dealumination of CHA zeolites when exposed to water at high temperatures.

Efficient nitric oxide capture and reduction on Ni-loaded CHA zeolites

As a prominent contributor to air pollution, nitric oxide (NO) has emerged as a critical agent causing detrimental environmental and health ramifications. To mitigate emissions and facilitate downstream utilization, adsorption-based techniques offer a compelling approach for direct NO capture from both stationary and mobile sources. In this study, a comprehensive exploration of NO capture under oxygen-lean and oxygen-rich conditions was conducted, employing Ni ion-exchanged chabazite (CHA-type) zeolites as the adsorbents. Remarkably, Ni/Na-CHA zeolites, with Ni loadings ranging from 3 to 4 wt%, demonstrate remarkable dynamic uptake capacities and exhibit exceptional NO capture efficiencies (NO-to-Ni ratio) for both oxygen-lean (0.17–0.31 mmol/g, 0.32–0.43 of NO/Ni) and oxygen-rich (1.64–1.18 mmol/g) under ambient conditions. An NH3 reduction methodology was designed for the regeneration of absorbents at a relatively low temperature of 673 K. Comprehensive insights into the NOx adsorption mechanism were obtained through temperature-programmed desorption experiments, in situ Fourier transform infrared spectroscopy, and density functional theory calculations. It is unveiled that NO and NO2 exhibit propensity to coordinate with Ni2+via N-terminal or O-terminal, yielding thermally stable complexes and metastable species, respectively, while the low-temperature desorption substances are generated in close proximity to Na+. This study not only offers micro-level perspectives but imparts crucial insights for the advancement of capture and reduction technologies utilizing precious-metal-free materials.

Synthesis and Catalytic Applications of Advanced Sn- and Zr-Zeolites Materials.

The incorporation of isolated Sn (IV) and Zr (IV) ions into silica frameworks is attracting widespread attention, which exhibits remarkable catalytic performance (conversion, selectivity, and stability) in a broad range of reactions, especially in the field of biomass catalytic conversion. As a representative example, the conversion route of carbohydrates into valuable platform and commodity chemicals such as lactic acid and alkyl lactates, has already been established. The zeotype materials also possess water-tolerant ability and are capable to be served as promising heterogeneous catalysts for aqueous reactions. Therefore, dozens of Sn- and Zr-containing silica materials with various channel systems have been prepared successfully in the past decades, containing 8 membered rings (MR) small pore CHA zeolite, 10-MR medium pore zeolites (FER, MCM-56, MEL, MFI, MWW), 12-MR large pore zeolites (Beta, BEC, FAU, MOR, MSE, MTW), and 14-MR extra-large pore UTL zeolite. This review about Sn- and Zr-containing metallosilicate materials focuses on their synthesis strategy, catalytic applications for diverse reactions, and the effect of zeolite characteristics on their catalytic performances.

Conversion of NOx over Aluminosilicate Cu-CHA Zeolite Catalysts Synthesized Free of Organic Structure-Directing Agents

Cu-CHA zeolites have proven to be effective for NOx reduction, but a drawback in using CHA zeolites is the cost associated with using expensive organic structure-directing agents. To overcome this drawback, we are reporting here the synthesis of Cu-CHA zeolite catalysts in both their NH4-form as well as K-form that do not require the use of organic structure-directing agents. After comprehensive characterization by XRF, XRD, 27Al NMR spectroscopy, FE-SEM, SEM/EDS, N2-adsorption/desorption, NH3-TPD, H2-TPR, and XPS, the zeolite catalysts were tested for NOx conversion by NH3-selective catalytic reduction (NH3-SCR). Cu-NH4-CHA zeolite catalysts exhibited remarkable activity and thermal stability over a wide temperature window, outperforming their counterpart K-forms. Among the NH4-forms of CHA zeolite catalysts, the 0.1 M Cu-NH4-CHA showed the best catalytic performance, achieving 50% NOx conversion at a temperature as low as 192 °C, and reaching full conversion of NOx at 261 °C. These Cu-based CHA zeolite catalysts are promising thanks to their environmentally friendly synthesis and offer the opportunity of maximizing DeNOx strategies in applications for NOx pollution abatement.

Construction of Sunflower-like Superstructure of CHA Zeolite through Oriented Attachment for Superior CO2 Separation Performance via Thermodynamic–Kinetic Synergistic Adsorption

Construction of Sunflower-like Superstructure of CHA Zeolite through Oriented Attachment for Superior CO₂ Separation Performance via Thermodynamic–Kinetic Synergistic Adsorption

Analysis and Augmentation of Guest–Host Interaction Energy Models as CHA and AEI Zeolite Crystallization Phase Predictors

Analysis and Augmentation of Guest–Host Interaction Energy Models as CHA and AEI Zeolite Crystallization Phase Predictors

CO2 Adsorption Behavior of Nanosized CHA Zeolites Synthesized in the Presence of Barium or Calcium Cations

AbstractAlkaline‐earth metal cations impact the rate of crystallization and the adsorption of N2 and CO2 on nanosized chabazite zeolites. In this study, either calcium or barium is added to precursor mixtures containing alkali‐metal cations (Na+, K+, Cs+) to prepare chabazite (labeled Ca‐CHA and Ba‐CHA), and are compared to a reference sample (Ref‐CHA) synthesized using exclusively Na+, K+, and Cs+. Partial substitution of the Na+ and the pore‐blocking Cs+ extra‐framework cations is observed for Ca‐CHA depending on the molar ratio of K2O used in the synthesis, while a change to the amount of Na+ cations only is observed for Ba‐CHA. The type of alkaline‐earth metal cation affects the crystallization rate; slower in the presence of Ca2+ (10 h to full crystallinity) and similar rates in the presence of Ba2+ (4 h to full crystallinity); the crystallite size and morphology remained similar. The presence of Ca2+ or Ba2+ extra‐framework cations leads to N2 uptake values of 290 and 169 mmol g−1 (−196 °C, 100 kPa), respectively, while at low CO2 pressure (<1 kPa, 25 °C), the physisorbed CO2 capacity for Ref‐CHA, Ca‐CHA, and Ba‐CHA zeolites is 0.63, 0.66, and 0.59 mmol g−1, respectively. Interestingly, an opposite effect is observed for the amount of chemisorbed CO2 species.

Large and Highly Selective and Permeable CHA Zeolite Membranes

Large and Highly Selective and Permeable CHA Zeolite Membranes

A novel Zn-doped CHA zeolite coupled CDs for photocatalytic nitrogen fixation

The construction of zeolite/semiconductor was proved to be one of the effective ways to improve the photocatalytic performance. In this paper, CDs@Zn-CHA/CN was prepared by hydrothermal method, and a synergistic interaction was formed between CDs@Zn-CHA and CN to enhance the photocatalytic nitrogen fixation activity. The photoelectric properties characterization and XPS and in situ IR results indicated that CDs, Zn-CHA and CN formed a charge transfer interface, which improved the separation of photogenerated carriers and greatly enhanced the nitrogen fixation efficiency. Nitrogen is adsorbed by Zn-CHA with microporous structure and activated by contact with Zn sites on CHA. CN has a good visible light response and provides photogenerated carriers under visible light irradiation. CDs act on the interfacial charge transfer between Zn-CHA and CN, promoting the transfer of photogenerated electrons to the nitrogen adsorbed by Zn-CHA and accelerating the dissociation of NN, thus increasing the yield of NH3. Compared with CDs@Zn-CHA and CN, 10% CDs@Zn-CHA/CN achieved 5.82 and 2.99 times of nitrogen fixation yield, respectively. Furthermore, the higher hydrophobicity of CDs@Zn-CHA/CN was found by contact angle measurement, which provided more abundant three-phase contact points for the reaction process. This study provides new ideas and directions for the design of photocatalyst.

Investigation of copper sites in CHA zeolite using 3D electron diffraction

Investigation of copper sites in CHA zeolite using 3D electron diffraction

Crystallographic determination of Al or Ga position in small-pore CHA zeolite combined with DFT simulation

Crystallographic determination of Al or Ga position in small-pore CHA zeolite combined with DFT simulation

Exploring the framework of small pore zeolites for passive NOx adsorption

The use of a passive NOx adsorber (PNA) for NOx storage during the cold-start period is crucial for eliminating NOx emissions from the diesel exhaust. Palladium (Pd)-loaded zeolites have garnered significant attention owing to their high potential as PNA. In this study, small-pore zeolites with different frameworks were synthesized, and their structural characteristics and passive NOx adsorption abilities were investigated. Five zeolites, with CHA, AEI, AFX (low- and high- Si/Al ratio), and LEV framework structures, were synthesized by the hydrothermal conversion of faujasite-type (FAU) zeolite. Pd (1 wt%) was impregnated in combination with hydrothermal treatment. NOx adsorption and desorption behaviors were studied based on breakthrough NO adsorption and subsequent temperature-programmed desorption (TPD) experiments. Pd-loaded CHA, AEI, and LEV zeolites and high-silica AFX zeolites adsorbed approximately 93.88, 107.93, 97.57 and 106.53 μmol-NO g−1, respectively. On the other hand, the incorporation of Pd into AFX zeolite with a low Si/Al ratio led to significant hydrothermal damage and resulted in lower levels of adsorption and desorption capabilities. This demonstrates that hydrothermally stable small-pore zeolites on Pd-incorporation have great potential for passive NOx adsorption. The zeolite with different framework exhibited the different temperature range of NOx release. CHA, AEI, and high-silica AFX zeolites desorbed NOx in the low-temperature region (T < 250 °C), whereas the LEV zeolite showed a relatively high NOx desorption amount in the middle-temperature region (250 °C < T < 350 °C).

Dynamic adsorption of CO2 by CHA zeolites – Size matters

The performance of K-CHA nanozeolite composed of discrete nanoparticles prepared by an organic template-free direct synthesis is compared to that of conventional micron-sized K-CHA with an order of magnitude difference in particle size and identical chemical composition (Si/Al ratio and cations) for the separation of dilute CO2 from N2. Breakthrough curve analysis of the zeolites meshed to identical sizes demonstrates the superior performance of nanozeolites for gas separation due to the significantly faster diffusion of CO2 molecules through the bed composed of meshed nanozeolite particles evidenced by the enhanced mass transfer, more rapid establishment of equilibrium, and faster desorption of CO2. Modelling of the CO2 breakthrough curves indicates enhanced mass transfer properties of the CHA nanozeolite due to the higher linear driving force parameter values. Variation of the modelling parameters and back-calculation of the diffusion coefficients indicates that axial diffusion contributes significantly to the mass transfer behaviour, yet also reveals significantly lower micropore diffusion resistance for the nano-sized K-CHA. These results demonstrate for rapid kinetically controlled sorption-based separations, that by reducing the size of the discrete crystallites of the meshed adsorbent particle by an order of magnitude, significant improvements in the utilization of the bed can be achieved due to the shortening of the mass-transfer zone, improving the separation performance.

Low‐silica Cu‐CHA Zeolite Enriched with Al Pairs Transcribed from Silicoaluminophosphate Seed: Synthesis and Ammonia Selective Catalytic Reduction Performance

AbstractCu‐exchanged low‐silica CHA zeolites (Si/Al≤4) synthesized without organic templates are promising candidate catalysts for ammonia selective catalytic reduction of nitrogen oxides (NH3‐SCR), but their practical application is restricted due to the low hydrothermal stability. Here, inspired by the transcription from duplex DNA to RNA, we synthesized Al pairs enriched low‐silica CHA zeolite (CHA‐SPAEI, Si/Al=3.7) by using silicoaluminophosphate (SAPO) featured by strict alternation of −Al−O−P(Si)−O−Al−O− tetrahedra as seed. The proportion of Al pairs in CHA−SPAEI is 78 %, which is much higher than that in the conventional low‐silica CHA (CHA‐LS, 52 %). After hydrothermal ageing at 800 °C for 6 h, Cu‐exchanged CHA‐SPAEI shows NO conversion above 90 % within 225–500 °C under a gas hourly space velocity of 200,000 h−1, which is much better than that of Cu‐exchanged CHA‐LS. The spatial close proximity of Al pairs in CHA‐SPAEI is confirmed by the 27Al double‐quantum single‐quantum two‐dimensional NMR analyses. The strict −P(Si)−O−Al−O−P(Si)−O‐ sequence in the fragments from the dissolution of SAPO seed promotes the Al pairs with the −Al−O−Si−O−Al−O− sequence via a transcription process. The utilization of aluminophosphate‐based zeolites as seeds opens up a new avenue for the regulation of the Al distribution in zeolites.

Cu-Speciation in Dehydrated CHA Zeolites Studied by H2-TPR and In Situ EPR

Cu-Speciation in Dehydrated CHA Zeolites Studied by H₂-TPR and In Situ EPR

Low-silica Cu-CHA Zeolite Enriched with Al Pairs Transcribed from Silicoaluminophosphate Seed: Synthesis and Ammonia Selective Catalytic Reduction Performance.

Cu-exchanged low-silica CHA zeolites (Si/Al ≤ 4) synthesized without organic templates are promising candidate catalysts for ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR), but their practical application is restricted due to the low hydrothermal stability. Here, inspired by the transcription from duplex DNA to RNA, we synthesized Al pairs enriched low-silica CHA zeolite (CHA-SPAEI, Si/Al = 3.7) by using silicoaluminophosphate (SAPO) featured by strict alternation of -Al-O-P(Si)-O-Al-O- tetrahedra as seed. The proportion of Al pairs in CHA-SPAEI is 78%, which is much higher than that in the conventional low-silica CHA (CHA-LS, 52%). After hydrothermal ageing at 800 °C for 6 h, Cu-exchanged CHA-SPAEI shows NO conversion above 90% within 225-500 °C under a gas hourly space velocity of 200,000 h-1, which is much better than that of Cu-exchanged CHA-LS. The spatial close proximity of Al pairs in CHA-SPAEI is confirmed by the 27Al double-quantum single-quantum two-dimensional NMR analyses. The strict -P(Si)-O-Al-O-P(Si)-O- sequence in the fragments from the dissolution of SAPO seed promotes the Al pairs with the -Al-O-Si-O-Al-O- sequence via a transcription process. The utilization of aluminophosphate-based zeolites as seeds opens up a new avenue for the regulation of the Al distribution in zeolites.

Understanding of low- and high-temperature DeNOx efficiency for NH3-SCR via comparison on Cu modified CHA and AFX zeolites

Pursuing simultaneously with excellent low- and high-temperature (wide operation temperature window) NOx reduction efficiency (LT-/HT-NRE) is greatly appealing for the NH3-SCR applications. We herein reveal the determinant of operation temperature window by comparison of Cu-SSZ-13(CHA) and Cu-SSZ-16(AFX) catalysts. And the outstanding HT-NRE performance is obtained over the Cu0.091-SSZ-164.2 (NOx conversion > 95% up to 670 °C), which makes this catalyst has great potential for applications for HT deNOx. The combined experimental (in-situ DRIFTS) and theoretical studies [DFT, ab initio molecular dynamics &ab initio thermodynamics (AIMD/AIT)] shed deeper mechanistic insights: i) the stronger resistance of Cu speciation ([CuOH]+ → CuOx) and lower HT NH3-overoxidation activity caused by the weak thermodynamic stability of the adsorbed NH3 over Cu active site constitute two major factors leading to the superior HT-NRE of Cu0.091-SSZ-164.2; ii) while the limited internal diffusions of [Cu(NH3)2] to generate the key intermediate of [(NH3)2Cu-O2-Cu(NH3)2] causes its slightly lower LT-NRE than that of Cu0.101-SSZ-135.1. Generally, present work clarifies the NRE performance of Cu-SSZ-16 at both LT and HT, which would progress highly-efficient catalyst development.

Low-cost and highly durable kaolin supported CHA zeolite composite membrane for efficient removal of heavy metals from aqueous solution

Low-cost and highly durable kaolin supported CHA zeolite composite membrane for efficient removal of heavy metals from aqueous solution

Interzeolite transformation from H+-type FAU zeolite into pure CHA zeolite via OSDA/seeds-free and steam-assisted conversion process

Pure CHA zeolite was transformed from FAU zeolite via organic structure-directing agent (OSDA)/seeds-free steam-assisted conversion (SAC). CHA zeolite was transformed using H+-type FAU zeolite with a low Si/Al ratio (Si/Al = 2.8) and a small amount of potassium hydroxide (KOH) and H2O. The Si/Al ratio of the CHA zeolite of the final product was the same as that of FAU-type zeolite as raw material because OH− ions derived from KOH reacted with H+ ions in FAU zeolite. Then, Si constituting FAU zeolite was little dissolved in a small amount of the basic solvent during the interzeolite transformation process. In general, in the interzeolite transformation process, the Si/Al ratio of the final product is lower than that of zeolite as raw material due to the dissolution of Si in the framework in the basic solvent. However, the interzeolite transformation process in this study can reduce the dissolving Si ratio using H+-type FAU zeolite. In addition, the PHI framework with low CO2 adsorption capacity arose with CHA zeolite transformed from Na+-type FAU zeolite via OSDA-free interzeolite SAC in a previous study. However, in this study, the PHI framework did not arise in OSDA-free steam-assisted interzeolite transformation process due to the use of H+-type FAU zeolite. Furthermore, the second particle morphology of CHA zeolite transformed from H+-type FAU zeolite was spherical, which has not been seen in CHA zeolite particles synthesized in previous reports. Finally, the CO2 adsorption capacity and performance of FAU and CHA zeolites were measured. CHA zeolite showed higher CO2 adsorption capacity than H+-type FAU zeolite as the starting material and high CO2 loadings at low CO2 equilibrium pressure. This study investigated that the synthesis condition, chemical properties, and CO2 adsorption performance of CHA zeolite transformed from H+-type FAU zeolite via the OSDA/seeds-free steam-assisted interzeolite transformation process.

Efficient nitrous oxide capture by cationic forms of FAU and CHA zeolites

Nitrous oxide is a strong greenhouse gas contributing over 300 times global warming potential higher than carbon dioxide. The capture of nitrous oxide from industrial emission source represents an efficient and promising strategy for its further treatment or recycling, which receives less attention in the past decades. Herein, a comprehensive study on nitrous oxide capture by alkali and alkaline-earth metal ions (Li, Na, K, Mg, Ca and Ba) exchanged FAU and CHA zeolites was conducted, with focuses on the underlying adsorption mechanism and the structure-performance relationship. Under ambient conditions, Ba-FAU and Na-CHA show remarkable nitrous oxide dynamic uptakes of 2.25 and 2.41 mmol/g, respectively, at high concentration of 50000 ppm, and Ba-FAU shows good dynamic uptake of 0.35 mmol/g at extremely low concentration of 1000 ppm. The dynamic nitrous oxide uptakes by zeolites at high concentrations are related to the surface area and the adsorption strength controlled by local electrostatic field, while the diffusion resistance shows serious negative impacts at low concentrations. The intrinsic stability of zeolite and the perfect recyclability make Ba-FAU an eligible industrial adsorbent for nitrous oxide capture. In situ Fourier transform infrared spectroscopy and Grand Canonical Monte Carlo simulations reveal that nitrous oxide molecules prefer to anchor to extraframework cations via terminal-oxygen atoms and meanwhile stabilized by framework oxygen through electrostatic interaction via central nitrogen atoms. The explicit structure-performance relationship and the underlying mechanism are very important for the understanding of nitrous oxide capture by zeolites and most helpful for the rational design of robust adsorbents.