Small-pore Zeolites Research Articles

Mixed matrix membranes incorporated with small pore zeolite UZM-5 for enhanced CO2/CH4 separation

Mixed matrix membranes (MMMs), which is polymeric matrix incorporated with inorganic filler materials, appear to be potential candidate for CO2 separation. Current work focuses on fabrication of MMMs incorporated with varying loading of small pore zeolite UZM-5 and modified UZM-5 for CO2/CH4 separation. Zeolite UZM-5 was firstly synthesized and then modified by using 3-aminopropyltriethoxysilane (APTES). MMMs was fabricated by incorporating zeolite UZM-5 and modified UZM-5 (NH2-UZM-5) into polysulfone (PSF) matrix. Both modified and unmodified zeolite UZM-5 were characterized via Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Field Emission Scanning Electron Microscope (FESEM) and Brunauer-Emmett-Teller (BET) analysis. The fabricated membranes were characterized using FTIR, XRD, TGA and FESEM analysis. The successful synthesis of zeolite UZM-5 was proven by XRD analysis. FESEM analysis of the membranes revealed that the successful the MMMs were successfully fabricated with even distribution of zeolites in the polymer matrix. In CO2 and CH4 gas permeation and separation study for the membranes, the highest CO2/CH4 ideal selectivity of 33.1 was obtained by the MMMs with 1 % NH2-UZM-5, which was a significant enhancement of 212 % in CO2/CH4 ideal selectivity as compared to the pristine PSF membrane.

A low-temperature regenerable catalyst for stable and selective propylene production from ethylene using Ni and P-supported SSZ-13 zeolite

Due to the increasing demand for propylene, highly selective propylene production from ethylene using small pore zeolite has received significant attention. However, rapid catalyst deactivation due to coke deposition on the catalyst impedes effective propylene production in industry. Therefore, it is essential to develop a low-temperature regeneration ethylene-to-propylene (ETP) catalyst with stable catalytic activity. To achieve such a catalyst, various hydrocracking metal species and phosphorus-supported SSZ-13 were prepared and evaluated using a one-pass ETP and ETP-Regen test. The optimized catalyst NiO(1.0)-P(0.6)-SSZ-13 exhibited a constant, excellent propylene yield of over 70 wt% with 90 wt% of propylene selectivity over ETP reaction and consecutive ETP-Regen cycles. The unprecedented catalytic performance of NiO(1.0)-P(0.6)-SSZ-13 was ascribed to the synergistic effect of NiO and P components, where nickel is responsible for catalyst recovery through effective coke removal, while phosphorus plays an essential role in size-selective diffusion.

Achieving High Dispersion of Pd in Small-Pore Zeolite SSZ-13: A High-Efficiency Low-Temperature NOx Adsorber.

Passive NO x adsorber (PNA) materials are primarily considered for reducing nitrogen oxide emissions during the low-temperature cold start of a motor vehicle. Pd/SSZ-13 has attracted considerable attention because of its outstanding hydrothermal stability and sulfur resistance. Optimizing the dispersion of precious metal Pd in Pd/SSZ-13 is crucial for enhancing PNA performance and nitrogen oxide adsorption capability. In this study, we prepared Pd/SSZ-13 using different methods and evaluated their influence on the NO x adsorption capability. The characterization results show that the dispersion of precious metal Pd in the Pd/SSZ-13 catalyst prepared by the quantitative ion-exchange method is as high as 92.13%, and the loading amount is as high as 98.93%. Pd predominantly exists as Pd2+, achieving near-total loading and further improving the catalyst's NO x adsorption capacity. This study offers innovative approaches and methods for applying Pd/SSZ-13 as a PNA material, serving as a reference for its further optimization and performance enhancement. Continued research into the preparation and adsorption performance of Pd/SSZ-13 materials could offer solutions to reduce motor vehicle nitrogen oxide emissions.

Exploring the limit of Cu-based small-pore zeolite framework collapse during hydrothermal aging in NOx abatement from diesel vehicle exhaust

Cu-SSZ-13 small-pore zeolites have been commercialized for NOx abatement in diesel engines, while they still suffer from hydrothermal deactivation. To identify the changes occurring in catalysts during the whole hydrothermal aging (HTA) process, the selective catalytic reduction by ammonia (NH3-SCR) performance of hydrothermally aged catalysts was investigated until total deactivation occurred. Through precisely identifying the alterations in the zeolite framework and active Cu sites during HTA, three stages were distinguished under progressively more severe HTA conditions, which were dealumination/transformation of active Cu species, CuOx accumulation, and structural collapse. These three stages were found to cause a slight decrease, serious attenuation, and total deactivation of the deNOx efficiency, which stemmed from the loss of Bronsted acid sites, reduction in the number of active Cu ion sites and structural collapse, respectively. This work also depicted simulated deactivation curves of Cu-SSZ-13 with various compositions during the HTA process, which helps in understanding the hydrothermal aging limits of catalysts with various Si/Al ratios and Cu loadings.

Efficient Synthesis of All-silica DDR Zeolite

Zeolite DDR is a small pore zeolite with high application potential in catalysis, adsorption and membrane separation. However, its application is greatly limited by lengthy synthesis time. Various approaches, such as fast heating, reactive raw silica source, special mineralizing agent, seeding, and high synthesis temperature were combined to accelerate crystallization kinetics. The obtained DDR samples were systematically characterized by XRD, SEM, NMR and nitrogen adsorption-desorption. The fastest synthesis was realized by combining microwave radiation, and active (NH4)2SiF6 as silica source, in which high quality DDR zeolite with high yield was obtained in only 5 min. Moreover, replacing toxic (NH4)2SiF6 with safe silica sol led to improved process environment-friendliness and longer synthesis time of 10 min. Finally, replacing expensive microwave heating with economical oil-bath heating resulted into efficient synthesis of zeolite DDR in only 15 min, which would facilitate its commercial applications in adsorption, catalysis and membrane separation.

Characterization and synthesis of high permeance SSZ-13 membranes to separate CO2 from CH4 for biogas upgrading

Small-pore zeolites, such as CHA (0.38 nm), have pores that are comparable to the size of CH4 but larger than the size of CO2. As a result of the combination of diffusion and adsorption, these membranes are projected to have strong CO2/CH4 selectivity and CO2 permeance. During this work, SSZ-13 CHA membranes were synthesized, evaluated, and characterized by X-ray Diffraction, Field-Emission Scanning Electron Microscopy, Energy-Dispersive X-ray Spectroscopy, Fourier Transform Infrared Spectroscopy, Solid-State Nuclear Magnetic Resonance, and Thermogravimetric Analysis. Synthesized membranes on the inner surface of single channel alumina supports with a pore size of 200 nm and a length of 105 mm and an active membrane area of ∼17 cm2 showed an ideal permselectivity for CO2/CH4 of 122 (in the single gas permeation measurement) and a CO2 permeance of 36.1 × 10−7 [mol/(m2 s Pa)] and CO2/CH4 selectivity of 173 and a CO2 permeance of 6.5 × 10−7 [mol/(m2 s Pa)] for equimolar CO2 − CH4 mixture. According to the results, the membranes are capable of separating CO2 from natural gas and upgrading biogas for industrial purposes.

Investigating the Sole Olefin-Based Cycle in Small-Cage MCM-35-Catalyzed Methanol-to-Olefins Reactions.

Small-pore zeolites catalyze the methanol-to-olefins (MTO) reaction via a dual-cycle mechanism, encompassing both olefin- and aromatic-based cycles. Zeolite topology is crucial in determining both the catalytic pathway and the product selectivity of the MTO reaction. Herein, we investigate the mechanistic influence of MCM-35 zeolite on the MTO process. The structural properties of the as-synthesized MCM-35 catalyst, including its confined cages (6.19 Å), were characterized, confirming them as the catalytic centers. Then, the MTO reactions were systematically performed and investigated over a MCM-35 catalyst. Feeding pure methanol to the reactor yielded minimal MTO activity despite the formation of some aromatic species within the zeolite. The results suggest that the aromatic-based cycle is entirely suppressed in MCM-35, preventing the simultaneous occurrence of the olefin-based cycle. However, cofeeding a small amount of propene in methanol can obviously enhance the methanol conversion under the same studied reaction conditions. Thus, the exclusive operation of the olefin-based cycle in the MTO reaction, independent of the aromatic-based cycle, was demonstrated in MCM-35 zeolite.

Optimization of isolated copper species on the NH3-SCR performance over Cu/SSZ-39 modified by ammonia water

To further improve the activity of Selective Catalytic Reduction of nitrogen oxide (NOx) with Ammonia (NH3-SCR) and the hydrothermal stability at 900 °C on Cu-based small-pore zeolite catalyst, the interaction between ammonia water and Cu2+ is used to regulate the interaction between Cu2+ and Si-O-Al structures of the zeolite, and affect the migration of Cu2+ in the channels of zeolite. The characterization results show that the microchemical environment of isolated Cu2+ species is optimized, and more isolated Cu2+ species can stably exist over the surface of Cu/SSZ-39, exhibiting a strong redox property. Besides, the ability of acid sites to adsorb and activate NH3 is also enhanced, and the amount of H2O adsorbed in the pores of zeolite is reduced due to the stable surface isolated Cu2+ species, which is conducive to the stability of the catalyst after hydrothermal aging at 900 °C. When the mole ratio of ammonia and copper is 3, the NH3-SCR performance and the hydrothermal stability at 900 °C are both optimized on the modified catalyst. The temperature window that the NOx conversion is higher than 90% (ΔT90) on the unmodified catalyst is 200–475 °C, but the ΔT90 on the modified catalyst is broadened to 178–544 °C. More importantly, after hydrothermal aging at 900 °C, the ΔT90 on the modified catalyst is shifted to the higher temperature by 48 °C, with the maximum NOx conversion of 96%. This work would provide a new strategy for further optimizing Cu/SSZ-39 catalyst for deNOx emitted from diesel engines to meet higher emission standards in future.

High-silica KFI zeolite: highly efficient synthesis and catalysis in methanol amination reaction

The hydrogen form of low-silica KFI zeolite, with a Si/Al ratio of 3.2 (referred to as H-KFI-3.2), has been shown to possess significant selectivity for monomethylamine (MMA) and dimethylamine (DMA) in methanol amination reactions. However, its industrial viability for MMA and DMA synthesis has been hindered by its relatively low methanol (MeOH) conversion and yield of MMA plus DMA. In this study, we synthesize high-silica KFI zeolite with an elevated framework Si/Al ratio of 5.4 (designated as KFI-5.4) using a novel K+ and 18-crown-6 complex [referred to as (K+)CCH, with a ratio of 18-crown-6 to K+ of 2.85] as an organic structure-directing agent. Control experiments have demonstrated that the presence of both K+ and OH- ions is essential for the formation of KFI-5.4 zeolite. The hydrogen form of KFI-5.4 (H-KFI-5.4) exhibits significantly enhanced MeOH conversion (95.2%) and yield (75.1%) of MMA plus DMA compared to the low-silica H-KFI-3.2 under similar reaction conditions. This represents the highest level of catalytic performance among reported small-pore zeolites to date. Furthermore, the yield and selectivity of MMA plus DMA can be further improved by modifying KFI-5.4 with an appropriate loading of Na+, which suppresses the formation of the by-product dimethyl ether. This study introduces a new high-silica KFI zeolite catalyst with exceptional MeOH conversion and yield for the production of MMA plus DMA.

CO2 Desorbs Water from K-MER Zeolite under Equilibrium Control.

Competitive adsorption by water in zeolites is so strongly prevalent that established gravimetric techniques for quantification have assumed that humid CO2 has no effect on preadsorbed water at the same relative humidity. Here, we demonstrate sites in small-pore zeolite K-MER, in which CO2 adsorption causes 20% of preabsorbed water to desorb under equilibrium control at 30 °C and 5% relative humidity. Diffuse reflectance IR spectroscopic data demonstrate that dimeric water species that are coordinated to cationic sites in K-MER zeolite are selectively displaced by CO2 under these humid conditions. Though Cs-RHO contains more weakly bound water than K-MER, we observe a lack of dimeric water species and no evidence of CO2 outcompeting water in Cs-RHO. We conclude that the desorption of water by CO2 in K-MER is driven by a highly desired site for CO2 adsorption as opposed to an intrinsically weak binding of water to the zeolite. Our demonstration that CO2 can outcompete water in a zeolite under wet conditions introduces new opportunities for the design of selective sites for humid CO2 adsorption and stresses the importance of independently characterizing adsorbed water and CO2 in these systems.

Generating silicon-rich AFX zeolite using rigid diquaternary alkylammonium structure directing agent towards stable selective catalytic reduction of NOx by NH3

New synthetic protocols to manipulate Si/Al ratios of small pore zeolites hold promise to find better selective NOx reduction catalyst using NH3 as reducing agent (NH3-SCR). AFX zeolite as NH3-SCR catalyst is currently impeded by the insufficient hydrothermal stability owing to the low Si/Al ratio. In this contribution, we report the synthesis of AFX zeolite using N,N-(1,4-phenylenebis(methylene))bis(N,N,N-triethylammonium) as organic structure directing agent (OSDA) to increase the framework Si/Al ratio to 7.8, giving rise to a marked enhancement of hydrothermal stability to withstand hydrothermal aging at 850 °C for up to 10 h. The increase in Si/Al ratio for AFX zeolite minimizes catalytic cycle driven by Brønsted acid site and broadens high temperature operation window (150–515 °C) for Cu-exchanged catalyst. Moreover, the hydrothermally aged Cu-AFX catalyst exhibits comparable catalytic activity to that of the fresh catalyst owing to Cu predominant distribution close to the 6-MR.

Investigation into the shape selectivity of zeolites for conversion of polyolefin plastic pyrolysis oil model compound

The relationship between the structure of zeolites and their performance in 1-octene conversion, a model compound in polyolefin plastic pyrolysis oil, has been elucidated. Zeolite topology determines reaction pathways being possible to direct the reaction to the formation of light olefins or aromatics. Small pore zeolites (≤ 6.1 Å), facilitate octene-cracking reactions. For example, MOR produces 98% of small olefins (54% C5). Large pore size zeolites catalyze aromatizations to BTX, also producing paraffins and hydrogen. Zeolites with pore sizes around 6.1 Å produce BTX with reduced coke formation. Thus, MFI produces 32% of C2-C5 olefins, 27% paraffins, and 23% of BTX. The olefins, paraffins, and BTX are found in an alkylation/dealkylation equilibrium conditioned by the catalyst’s pore size: the larger the pore size, the longer and more branched the products are. In addition, 1-octene isomers are dominant products at low contact times (∼0.045 s) and BTX at long contact times (∼4.5 s).

Synthesis of stable ECR-18 zeolite and its catalytic properties in methanol amination

ECR-18 is the synthetic analog of paulingite (framework type PAU), a naturally-occurring cage-based, small-pore zeolite. Because of its high framework Al content (Si/Al ∼ 3), this third generation of the RHO family of embedded isoreticular zeolites is structurally unstable, thereby hampering any catalytic application. Here we present the synthesis of ECR-18 with Si/Al ratios up to 4.4 using N-methyltropinium as an organic structure-directing agent and Na+ and/or K+ as inorganic structure-directing agents. The proton form of ECR-18 with Si/Al = 4.4 was found to retain the structural integrity. The catalytic properties of this ECR-18 for methylamines synthesis from ammonia and methanol have been compared with those of H-rho and H-PST-29, the two most selective catalysts for this reaction. Despite the similarity in acidity, H-ECR-18 showed a low sum yield of monomethylamine and dimethylamine compared to H-PST-29, mainly due to the lower ratio of scaffold (i.e., lta, pau, d8r, and t-plg) cages to embedded (i.e., t-oto, t-gsm, and t-phi) cages per unit cell of the former zeolite catalyst.

Chloromethane-to-olefin using metal oxide-impregnated small-pore SSZ-13 zeolites with controlled acidity, reactant affinity, and reaction pathway

Chloromethane-to-olefin using metal oxide-impregnated small-pore SSZ-13 zeolites with controlled acidity, reactant affinity, and reaction pathway

L-lysine-assisted synthesis of gismondine and chabazite nanozeolites for direct air capture of CO2

The search for more efficient and selective solid adsorbents towards carbon capture, particularly in trace CO2 concentration, remains a significant challenge. Here, we present the synthesis of nanocrystalline sodium gismondine (Na-GIS) with Si/Al = 2.5 and potassium chabazite (K-CHA) zeolites with Si/Al = 2.0 using L-lysine as a crystal growth inhibitor. Na-GIS nanozeolites were found to show a CO2 uptake as high as 0.70 mmol g−1 under direct air capture (DAC) conditions (i.e., 298 K and 400 ppm CO2), about 18 times greater than that of microcrystalline Na-GIS, together with high adsorbent stability. Such behavior was also observed for Na-CHA nanozeolites, although the extent of increase (0.47 vs 0.13 mmol g−1) was smaller compared to the microcrystalline counterpart. This study demonstrates that the introduction of nanocrystallinity into small-pore zeolites without losing intrinsic microporosity is a viable route to the development of more efficient zeolite-based DAC adsorbents.

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).

Promoted NH3-SCR activity and hydrothermal stability of Cu-SSZ-50 catalyst synthesized by one-pot method

Nitrogen oxide (NOx) is one of the most critical contaminants in the air, and the control of NOx emission from diesel vehicles is very important. Cu-based small-pore zeolites have already been applied for NOx abatement on diesel vehicles. Among the small-pore zeolites, Cu-SSZ-50 catalysts with good NH3-SCR catalytic activity were believed to have potential for application. In this study, a one-pot synthesis method for Cu-SSZ-50 catalysts was developed for the first time, using the co-templates of Cu-TEPA and 2,6-dimethyl-N-methylpyridinium hydroxide. In this synthesis method, Cu-SSZ-50 with various Cu contents can be obtained by adjusting the amount of Cu-TEPA without the need for a further after-treatment process. The addition of Cu-TEPA affected the framework atoms and Cu species, and a lower Si/Al ratio and more SCR active Cu species were obtained. The synthesized catalyst with a Cu/Al ratio of 0.40 exhibited over 90% NOx conversion between 200 °C and 450 °C for the selective catalytic reduction of NOx with NH3 (NH3-SCR). Meanwhile, over 80% NOx conversion could be obtained from 250 °C to 450 °C after hydrothermal aging at 750 °C for 16 h. In addition, both L-H and E-R mechanisms were proven to exist for the one-pot-synthesized Cu-SSZ-50 by in situ DRIFTS experiments. The simple synthesis procedure, excellent catalytic activity and hydrothermal stability brighten the prospects for the application of Cu-SSZ-50.

Highly Cooperative CO2 Adsorption via a Cation Crowding Mechanism on a Cesium-Exchanged Phillipsite Zeolite.

An understanding of the CO2 adsorption mechanisms on small-pore zeolites is of practical importance in the development of more efficient adsorbents for the separation of CO2 from N2 or CH4 . Here we report that the CO2 isotherms at 25-75 °C on cesium-exchanged phillipsite zeolite with a Si/Al ratio of 2.5 (Cs-PHI-2.5) are characterized by a rectilinear step shape: limited uptake at low CO2 pressure (PCO2 ) is followed by highly cooperative uptake at a critical pressure, above which adsorption rapidly approaches capacity (2.0 mmol g-1 ). Structural analysis reveals that this isotherm behavior is attributed to the high concentration and large size of Cs+ ions in dehydrated Cs-PHI-2.5. This results in Cs+ cation crowding and subsequent dispersal at a critical loading of CO2 , which allows the PHI framework to relax to its wide pore form and enables its pores to fill with CO2 over a very narrow range of PCO2 . Such a highly cooperative phenomenon has not been observed for other zeolites.

Metal cation-exchanged LTA zeolites for CO2/N2 and CO2/CH4 separation: The roles of gas-framework and gas-cation interactions

Selective CO2 adsorption for efficient carbon capture from flue gas (CO2/N2 - 15/85, v/v) and biogas (CO2/CH4 - 50/50, v/v) is important for achieving global energy and climate goals but remains a challenge due to the lack of effective adsorbents. To address this issue, we attempted to develop zeolite adsorbents by systematically investigating the effect of different extra-framework metal cations (i.e., Na+, Ca2+, Mn2+, and Ce3+-exchanged in LTA zeolites) on selective CO2 adsorption from CO2/N2 and CO2/CH4. Analyzing the isosteric heat of adsorption results showed that CO2 adsorption at moderate pressure (e.g., 15 and 50 kPa relevant to the compositions of flue gas and biogas, respectively) is governed by the gas-framework interaction. On the other hand, the adsorption of N2 and CH4 was found to be dominated by the gas-cation interaction. Therefore, we concluded that metal cations with a small charge-to-size ratio are beneficial for selective CO2 adsorption from flue gas and biogas because they tend to induce strong CO2-framework interaction (due to the enhanced charge induction to zeolite framework O) and weak N2 or CH4-cation interaction (due to the weakened gas-cation electrostatic interaction). Specifically, LTA zeolites in the form of Na+, with the smallest charge-to-size ratio of cation in this study, exhibit the highest CO2 uptake and separation factor of CO2/N2 and CO2/CH4, as demonstrated by both static single-component adsorption and dynamic binary adsorption results. In addition, the potential of metal cation-exchanged LTA zeolites for VSA and PSA processes was evaluated. Our study provides valuable insights for designing small-pore zeolites as adsorbents for carbon capture.