CHA-type Zeolite Research Articles

A Computational Mechanistic Study on Copper Autoreduction in Cu-CHA Zeolite Catalysts.

The activation of Cu-zeolite catalysts is accompanied by an autoreduction reaction, in which a part of Cu(II) species is spontaneously reduced to Cu(I) species. The stoichiometry of autoreduction in which the release of one O2 is accompanied by the reduction of four Cu(II) to Cu(I) has been proposed, but the detailed mechanism of this autoreduction remains unclear. In this work, we used DFT calculations to study the autoreduction mechanism in Cu-CHA zeolites. The two reduction mechanisms of [CuOH]+ to Cu+ in CHA-type zeolite were systematically studied. In Mechanism I, two [CuOH]+ react via dehydration to form [Cu-O-Cu]2+, and the further reaction of two [Cu-O-Cu]2+ to produce O2 is the most critical step, which requires four charge-compensating framework Al in close proximity. In Mechanism II, the production of O2 occurs via [CuO]+ intermediates, and the generation of possible [CuO]+ is the most critical step. The exploration of autoreduction reactions in a variety of Cu-CHA models with different Al sittings shows that the O-O distances between two intermediate precursors, i.e., two [Cu-O-Cu]2+ in Mechanism I, or two [CuO]+ in Mechanism II, are key factors determining the activation barriers of O2 production during autoreduction.

Superparamagnetically modified A-type, X-type and CHA-type zeolites with silica-coated Fe3O4 and CoFe2O4 nanoparticles for removal of Sr2+ and Cs+ from radioactively contaminated water

Superparamagnetically modified A-type, X-type and CHA-type zeolites with silica-coated Fe3O4 and CoFe2O4 nanoparticles for removal of Sr2+ and Cs+ from radioactively contaminated water

Nickel silicate CHA-type zeolite prepared by interzeolite transformation and its catalytic activity in dry reforming of methane

We newly synthesized the nickel silicate CHA-type zeolitic material (Ni-CHA) through the interzeolite transformation of the parent nickel silicate MWW-type zeolite (Ni-MWW) using the structure-directing ability of N,N,N-trimethyl-1-adamantammonium hydroxide. With increasing crystallization time under hydrothermal conditions, the parent Ni-MWW was decomposed into an X-ray amorphized nickel silicate, which was then consecutively reassembled into Ni-CHA. Furthermore, as the crystallization progressed, the framework Ni species gradually transformed from intermediate Ni to isolated Ni species, achieving a more thermodynamically stable chemical state. The metallic Ni clusters can be formed by exsolving from the CHA framework during the high-temperature reduction treatment, which were mainly distributed on the external surface. A series of in-situ reduced Ni-CHA-x catalysts obtained after different crystallization times (x = 0–72 h) were applied to the dry reforming of methane to evaluate their catalytic activity corresponding to the differences in crystallinity and chemical state. As a result, the interzeolite transformed Ni-CHA-72 h catalyst exhibited the highest catalytic performance even compared to the Ni-impregnated commercial SSZ-13.

Formation of paired Ga sites in CHA-type zeolite frameworks via a transcription-induced method.

Paired Ga sites represented by the Ga-O-Si-O-Ga sequence were firstly formed intentionally in CHA-type zeolite frameworks via the transcription of pre-formed paired Ga species in a Ga-rich amorphous silica-gallia under seed-assisted hydrothermal conditions. Such paired Ga sites behaved as ion-exchange sites for capturing divalent cation, Co2+.

Fast Synthesis of CHA-type Zeolite Using Dealuminated Amorphous Aluminosilicates with High Reactivity

Abstract Use of dealuminated amorphous aluminosilicates (deAl-Amor) achieved faster synthesis of CHA-type zeolites compared with using conventional amorphous aluminosilicates (Amor). The deAl-Amor also expanded the synthesis window of reactant compositions. Moreover, there was a large difference in morphologies of solid products obtained during the crystallization process between deAl-Amor and Amor, suggesting the time gap in supplying aluminosilicate species contributing to the CHA crystal growth. These results indicate the high potential of deAl-Amor as starting materials for zeolite synthesis.

Thermal Adsorption Spectroscopy of Hydrogen Isotopes from CHA-Type Zeolites

We examine the hydrogen adsorption behavior of chabazite (CHA) and Linde Type A (LTA) zeolites, both of which possess an eight-membered ring (Si8O8), as the temperature was increased from 77 K (thermal adsorption spectroscopy). The CHA-type zeolites started to uptake hydrogen at around 200 K, whereas no other adsorbent showed any hydrogen uptake. The hydrogen adsorption temperature of CHA was similar to that observed by thermal desorption spectroscopy. A high D2/H2 separation ability at relatively high temperature in the CHA-type zeolites may be expected in the intrinsic adsorption property.

Fabrication of hierarchically porous CHA-type zeolite composites derived from industrial solid waste for cesium removal

Fabrication of hierarchically porous CHA-type zeolite composites derived from industrial solid waste for cesium removal

Revealing Main Reaction Paths to Olefins and Aromatics in Methanol-to-Hydrocarbons over H-ZSM-5 by Isotope Labeling

The nature of hydrocarbon pool (HCP) intermediates in the methanol-to-hydrocarbons (MTH) process has been thoroughly investigated, especially for BEA- and CHA-type zeolite catalysts like H-β and H-SAPO-34. Herein, we further reveal the dynamic mechanistic details of the MTH process over the H-ZSM-5 catalyst at 400 °C, based on the dual-cycle mechanism and HCP in this medium-pore zeolite. Application of switching sequences of 13C-labeled and unlabeled methanol pulses over a model H-ZSM-5 catalyst combined with on-line MS analysis and a recently reported technique called “fast scanning-pulse GC analysis” provides a direct and quantitative insight into the MTH reactions under quasi-steady-state conditions. The transient product responses showed the almost instant formation of hydrocarbons upon a small pulse of methanol, followed by secondary formation of light aromatics via HCP decomposition and olefin alkylation–dealkylation, especially in a long catalyst bed when methanol is quickly consumed in the initial reaction zone in the catalyst bed. The isotopic analysis of typical aliphatic C3+ product responses after switching 13C-methanol pulses to the unlabeled methanol pulses showed a fast isotope scrambling in the formation of C3+ species. MS analysis of the light aromatics indicates a complete consecutive but slower isotope incorporation process of 12C into 13C-aromatics. Results provide direct experimental confirmation of the kinetically preferred olefin-based cycle over the aromatic-based cycle. The sequential isotopic incorporation strongly suggests that the paring reaction pathway through aromatic ring contraction and re-expansion steps is operative. In the appearance of aromatics upon pulsing methanol over larger catalyst beds, four processes are directly discerned, involving the displacement of adsorbed species by formed water, isotope incorporation yielding directly labeled and unlabeled products through the paring mechanism and direct aromatization, and HCP conversion through secondary reactions.

High Efficient CO2 Separation at High Pressure by Grain-Boundary-Controlled CHA Zeolite Membrane Investigated by Non-Equilibrium Molecular Dynamics

The CO2 permeability and selectivity of CHA-type zeolite membranes in the separation of a CO2/CH4 mixture gas at high pressure were evaluated using non-equilibrium molecular dynamics (NEMD). It was found that in a perfectly crystalline, defect-free CHA membrane, the adsorption of CH4, which diffuses slowly in the pores, hinders CO2 permeation. Therefore, an increase in the amount of CH4 adsorbed at high pressure decreases the CO2 permeability and significantly reduces the CO2 selectivity of the CHA membrane. CHA membranes with grain boundaries parallel to the permeation direction were found to show higher CO2 selectivity than perfectly crystalline CHA membranes at high pressure, as the blocking effect of CH4 on CO2 permeation occurring within the grain boundary is not significant. This paper is the first to show that the CO2 permeability of CHA membranes with controlled grain boundaries can exceed the intrinsic performance of fully crystalline zeolite membranes at high pressure.

Estimation of CO2 Separation Performances through CHA-Type Zeolite Membranes Using Molecular Simulation.

Chabazite (CHA)-type zeolite membranes are a potential material for CO2 separations because of their small pore aperture, large pore volume, and low aluminum content. In this study, the permeation and separation properties were evaluated using a molecular simulation technique with a focus on improving the CO2 separation performance. The adsorption isotherms of CO2 and CH4 on CHA-type zeolite with Si/Al = 18.2 were predicted by grand canonical Monte Carlo, and the diffusivities in zeolite micropores were simulated by molecular dynamics. The CO2 separation performance of the CHA-type zeolite membrane was estimated by a Maxwell-Stefan equation, accounting for mass transfer through the support tube. The results indicated that the permeances of CO2 and CH4 were influenced mainly by the porosity of the support, with the CO2 permeance reduced due to preferential adsorption with increasing pressure drop. In contrast, it was important for estimation of the CH4 permeance to predict the amounts of adsorbed CH4. Using molecular simulation and the Maxwell-Stefan equation is shown to be a useful technique for estimating the permeation properties of zeolite membranes, although some problems such as predicting accurate adsorption terms remain.

Impacts of framework Al distribution and acidic properties of Cu-exchanged CHA-type zeolite on catalytic conversion of methane into methanol followed by lower hydrocarbons

This work highlighted the significance of the Al distribution in the CHA-type zeolite in the formation of active Cu species for the oxidative conversion of CH4; isolated Al atoms in the zeolite framework are found to be advantageous for producing CH3OH.

Direct synthesis of nanosized CHA zeolite free of organic template by a combination of cations as structure directing agents

There is an increasing demand for porous materials able to separate the components of natural gas, mainly CH4 and CO2. Flexible small pore zeolites are interesting candidates due to their high sorption capacity and ability to selectively separate molecules of similar sizes. This selectivity comes from their small pores, large cavities, and pore shapes which can be controlled by the introduction of different extra-framework cations such as Na+, K+, Cs+. Furthermore, the extra-framework cations selectively provide access to the pores and cages of the zeolite depending on the nature of the molecules present. The strategy to prepare CHA-type zeolites across a range of particle sizes, from micron-sized dimensions to nanocrystals with a size of 50 nm, was investigated. The quantities of extra-framework cations and the hydrothermal conditions were optimised in order to avoid the formation of secondary structural phases and selectively target the CHA phase. The CHA-type zeolites were characterised by electron microscopy, elemental analysis, MAS NMR and FTIR spectroscopy, and PXRD. The Si/Al ratio of the obtained CHA zeolite was between 1.7 and 2.6 which is ideal for gas separation applications. The nanosized as-synthesised CHA zeolite (Si/Al = 2.4) demonstrates a high CO2 storage capacity (4.6 mmol/g) making it an ideal candidate for gas separation applications.

Hydrothermal Modification of Chabazite for the Fixation of Cesium Ions

During the accident at the Fukushima Dai-ichi Nuclear Power Plant at March 2011, radioactive cesium (137Cs), which has a long half-life of about 30 years, was released into the environment around the plant. Therefore, methods for the recovery and fixation of 137Cs are required. The CHA-type zeolite, chabazite, has high Cs+ selectivity; Cs+ ions have a diameter of approximately 0.36 nm and can thus enter into and be adsorbed by chabazite, which contains pores with diameters of 0.38 nm. However, some Cs+ ions in the large pores may be released by cation exchange in solutions with a high cation concentration. The ANA-type zeolite (ANA) has pores with 0.28 nm diameters, which are smaller than the diameter of Cs+ ions. ANA is synthesized by an alkaline hydrothermal treatment of natural zeolites such as clinoptilolite, mordenite, and chabazite. This hydrothermal modification may be useful for the fixation of Cs+. In the present study, the modification of chabazite exchanged with Cs+ to produce ANA by hydrothermal treatment was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), and the solution stability of Cs+ in the ANA was also evaluated using inductively coupled plasma-mass spectrometry (ICP-MS). Modification of chabazite exchanged with Cs+ to produce single-phase ANA was confirmed in a 1.0 mol/L NaOH solution held at 200 °C for 3 to 144 h. The intensity of XRD peaks corresponding to (211) and (220) planes decreased with increasing Cs+ concentration. SEM observations showed that the ANA crystals had a trapezohedral shape and a size of about 5 µm. EDX analysis of ANA crystals confirmed the presence of Cs+. The amount of Cs+ eluted from ANA in a 0.6 mol/L NaCl solution was less than 1.7%.

New CHA-Type aluminoborosilicates as efficient catalysts for MTO and NH3-SCR of NOx reactions

The small-pore CHA-type zeolites have received intensive attentions due to their outstanding performance in both of the methanol-to-olefin (MTO) process and ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR of NOx) reactions. However, the costly synthesis of CHA-type aluminosilicate is a great issue for the practical application. In this study, a novel and cheap organic structure-directing agent of N,N,N-trimethylisobutylammonium hydroxide was developed and used to synthesize CHA-type zeolites (name as ECNU-25) with boric acid as the supporting agent. The Si/Al ratio in the synthetic gel could be tailored in a wide range of 50 - ∞. [Al,B]-ECNU-25 zeolite thus synthesized served as an effective catalyst for the MTO reaction. After Cu2+-exchange, the Cu-[Al,B]-ECNU-25 was also highly active in the NH3-SCR of NOx.

Understanding the superior NH3-SCR activity of CHA zeolite synthesized via template-free interzeolite transformation

A highly active NH3-SCR catalyst (Cu-CHAUSY) derived from CHA-type zeolite (CHAUSY) that is synthesized via template-free interzeolite transformation from ultra-stable Y (USY) with specific silicon and aluminum configurations.

Grain Boundary Modelling of Zeolite Membranes for Higher CO<sub>2</sub> Selectivity

Non-equilibrium molecular dynamics was performed to investigate the influence of the grain boundary region in CHA-type zeolite membranes on the permeability and selectivity of a CO2/CH4 gas mixture. It was confirmed that under the investigated gas pressure, the grain boundary was preferentially occupied by CO2, resulting in an increase in the CO2 selectivity and flux. The results indicated that a CHA membrane with a fine controlled grain boundary structure could exhibit increased selectivity and permeability for condensable gas species compared to perfect crystalline CHA membranes.

In situ crystallization of CHA / FAU zeolites on volcanic stone in the absence of organic agents

Japan has over 100 active volcanoes, which accounts for approximately 10% in the world. This paper discloses a novel way of utilizing volcanic stone. Microporous zeolites were crystallized on the macroporous volcanic stone by in situ crystallization method which is one-step soft hydrothermal treatment in NaOH solution with no additives. The volcanic stone has two roles in this method. One role is sources of SiO 2 , Al 2 O 3 , and alkali cations for crystallizing zeolites, and another role is a macroporous support of the zeolites. Each type of zeolite, CHA (0.38 nm pore) and FAU (0.74 nm pore), was mainly crystallized on the whole surface of the volcanic stone, which was controlled by adjusting concentration of NaOH solution. All the composite samples showed higher specific surface area than the raw volcanic stone. The highest value achieved in FAU zeolite sample was 439 m 2 /g which is as high as a commercial powdery zeolite. The adsorption abilities of the composite samples for ammonium ion measured by batch experiments were higher than the commercial powdery zeolite. • Zeolite-stone composites were prepared by in situ crystallization of volcanic stone. • Zeolites were precipitated without collapsing macroporous structure of the stone. • CHA-type zeolite was successfully crystallized in the absence of organic agents. • The highest surface area was 439 m 2 /g, as high as a commercial zeolite powder. • The samples showed higher adsorption abilities for NH 4 + than the commercial zeolite.

Supported Nickel Zeolite Catalyst for Oxidative Conversion of Methane: Effect of Heteroatoms in the Zeolite Framework on Its Physicochemical and Catalytic Properties

Abstract Supported nickel CHA-type zeolites with different framework compositions including borosilicate, gallosilicate and aluminosilicate were synthesized. The effect of heteroatoms in the framework on the state of the Ni species introduced was investigated. The size and dispersibility of the Ni species differed greatly depending on the heteroatom in the zeolite framework, and the use of the aluminosilicate-type framework led to the formation of a highly dispersed and nano-sized Ni species. Thus, prepared Supported Ni CHA-type aluminosilicate exhibited the highest catalytic performance in oxidative conversion of CH4.

NH3-SCR catalysts for heavy-duty diesel vehicles: Preparation of CHA-type zeolites with low-cost templates

Computer-assistance allows selecting the most adequate low-cost organic structure directing agents (OSDAs) for the crystallization of Al-rich CHA-type zeolites. The host-guest stabilization energies of tetraethylammonium (TEA), methyltriethylammonium (MTEA) and dimethyldiethylammonium (DMDEA), in combination with Na, were first theoretically evaluated. This “ab-initio” analysis reveals that two TEA show a serious steric hindrance in a cha cavity, whereas two MTEA would present excellent host-guest confinements. The synthesis of Al-rich CHA-type zeolites has been accomplished using TEA and MTEA. Electron diffraction and high-resolution transmission electron microscopy reveal large CHA-domains with narrow faulted GME-domains in the CHA-type material synthesized with TEA, confirming the better OSDA-directing roles of MTEA cations towards the cha cavity, in good agreement with DFT calculations. Cu-exchanged Al-rich CHA-type samples achieved with MTEA and TEA show excellent catalytic activity and hydrothermal stability for the selective catalytic reduction (SCR) of NOx with ammonia under conditions relevant for future heavy duty diesel conditions.

Transformation synthesis of SSZ-13 zeolite from ZSM-35 zeolite

Interzeolite conversion as a promising alternative strategy for zeolite synthesis has received extensive attention. It is of great significance to understand the potential rules of conversion between zeolites with different topologies for effective regulation of zeolite synthesis. Hydrothermal conversion of ZSM-35 (FER-type) zeolite containing the mor composite building units into SSZ-13 zeolite (CHA-type) using N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as template was performed for the first time. The effects of TMAdaOH/SiO2 ratio, Na2O/SiO2 ratio, the additional starting zeolite and crystallization time on the interzeolite conversion of ZSM-35 into SSZ-13 were investigated. The interzeolite conversion mechanism concerning the synthesis of SSZ-13 from ZSM-35 zeolite was proposed and verified by DFT calculation. The results of DFT calculations suggested that ZSM-35 zeolite with mor composite building unit had the potential to decompose into 6-Membered Rings, and further transform into CHA-type zeolite containing d6r composite building unit. Therefore, zeolites containing mor structure have the potential to be converted into zeolites containing d6r structure.