High-silica CHA Research Articles

Improved Synthesis of Hollow Fiber SSZ-13 Zeolite Membranes for High-Pressure CO2/CH4 Separation.

High-silica CHA zeolite membranes are highly desired for natural gas upgrading because of their separation performance in combination with superior mechanical and chemical stability. However, the narrow synthesis condition range significantly constrains scale-up preparation. Herein, we propose a facile interzeolite conversion approach using the FAU zeolite to prepare SSZ-13 zeolite seeds, featuring a shorter induction and a longer crystallization period of the membrane synthesis on hollow fiber substrates. The membrane thickness was constant at ~3 μm over a wide span of synthesis time (24-96 h), while the selectivity (separation efficiency) was easily improved by extending the synthesis time without compromising permeance (throughput). At 0.2 MPa feed pressure and 303 K, the membranes showed an average CO2 permeance of (5.2±0.5)×10-7 mol m-2 s-1 Pa-1 (1530 GPU), with an average CO2/CH4 mixture selectivity of 143±7. Minimal defects ensure a high selectivity of 126 with a CO2 permeation flux of 0.4 mol m-2 s-1 at 6.1 MPa feed pressure, far surpassing requirements for industrial applications. The feasibility for successful scale-up of our approach was further demonstrated by the batch synthesis of 40 cm-long hollow fiber SSZ-13 zeolite membranes exhibiting CO2/CH4 mixture selectivity up to 400 (0.2 MPa feed pressure and 303 K) without using sweep gas.

Directly synthesized high-silica CHA zeolite for efficient CO2/N2 separation

Carbon dioxide capture is an effective method to mitigate climate change. Highly CO2-selective and highly moisture-resistant absorbent is essential for energy-efficient carbon capture by absorption. Herein, a highly CO2-selective and high-silica CHA zeolite in a pure lithium form (Li-SSZ-13) was synthesized using a direct synthesis method for the first time. The effects of gel n(SiO2)/n(Al2O3) ratio, gel n(SiO2)/n(Li2O) ratio and synthesis time on the crystallinity, yield, micropore volume and CO2 loading of the crystals were systematically investigated. The adsorption isotherms of CO2 and N2 were measured for directly synthesized Li-SSZ-13, conventional Na-SSZ-13 and ion-exchanged Li-Na-SSZ-13 zeolites. The dynamic breakthrough adsorption properties were measured for Li-SSZ-13 in comparison with Na-SSZ-13 and Li-Na-SSZ-13. The obtained Li-SSZ-13 exhibited a high CO2 adsorption capacity of 4.48 mmol/g and a high CO2/N2 IAST selectivity of 310 at 100 kPa and 273 K, which were 45.0% and 441% higher than Na-SSZ-13 zeolite, 24.8% and 179% higher than Li-Na-SSZ-13 zeolite, respectively. It suggests that directly synthesized Li-SSZ-13 zeolite has great potential for efficient CO2 capture from flue gas.

Tungsten-doped high-silica CHA zeolite membranes with improved hydrophobicity for CO2 separation

High-silica CHA (SSZ-13) zeolites membranes, with pore size between the molecular size of CO2 and N2 or CH4, are promising for some industrial CO2 separation process. Considering water is typically present in these industrial gas feed and the negative impact of moisture on the membranes’ gas permeation, it is highly desirable to increase the surface hydrophobicity of membranes to reduce the adsorption of water. In this paper, a W-doping strategy was proposed to increase the hydrophobicity of SSZ-13 membranes by eliminating the silanols on the surface, and the W-doped high-silica CHA (W-SSZ-13) zeolite membranes were prepared for the first time. Characterizations such as SEM, XRD, and XPS confirmed the successful incorporation of W into CHA membrane. The incorporation of W reduced the pore mouth size, and the W-SSZ-13 zeolite membranes showed improved CO2/N2 and CO2/CH4 separation performance. After W-doping, the maximum separation factors (αmax) for the CO2/N2 mixture with equimolar composition under dry conditions increased from 8.8 to 16.9, and for the equimolar CO2/CH4 mixture, the value increased from 105.9 to 176. The W-SSZ-13 membranes also showed better separation performance under wet conditions. For both systems, the W-SSZ-13 membrane showed higher αmax and less CO2 permeance reductions, indicating a more resistance in wet conditions.

A machine learning approach for dynamical modelling of Al distributions in zeolites via23Na/27Al solid-state NMR.

One of the main limitations in supporting experimental characterization of Al siting/pairing via modelling is the high computational cost of ab initio calculations. For this reason, most works rely on static or very short dynamical simulations, considering limited Al pairing/siting combinations. As a result, comparison with experiment suffers from a large degree of uncertainty. To alleviate this limitation we have developed neural network potentials (NNPs) which can dynamically sample across broad configurational and chemical spaces of sodium-form aluminosilicate zeolites, preserving the level of accuracy of the ab initio (dispersion-corrected metaGGA) training set. By exploring a wide range of Al/Na arrangements and a combination of experimentally relevant Si/Al ratios, we found that the 23Na NMR spectra of dehydrated high-silica CHA zeolite offer an opportunity to assess the distribution and pairing of Al atoms. We observed that the 23Na chemical shift is sensitive not only to the location of sodium in 6- and 8MRs, but also to the Al-Sin-Al sequence length. Furthermore, neglect of thermal and dynamical contributions was found to lead to errors of several ppm, and has a profound influence on the shape of the spectra and the dipolar coupling constants, thus necessitating the long-term dynamical simulations made feasible by NNPs. Finally, we obtained a predictive regression model for the 23Na chemical shift in CHA (Si/Al = 35, 17, 11) that circumvents the need for expensive NMR density functional calculations and can be easily extended to other zeolite frameworks. By combining NNPs and regression methods, we can expedite the simulations of NMR properties and capture the effect of dynamics on the spectra, which is often overlooked in computational studies despite its clear manifestation in experimental setups.

Structural analysis of Cu/Zeolite with controlled Si/Al ratio and the resulting thermal stability

Cu-loaded zeolite (Cu/Zeolite or Cu/CHA) has been prepared from USY (SiO2/Al2O3 =30) through hydrothermal conversion using trimethyladamantylammonium ion (TMAda) as an organic structure directing agent. Simple control over the concentration of mineralizing agent in the hydrothermal synthesis from 0.2 to 0.6 with respect to SiO2, changed the Si/Al ratio in the resulting zeolite, CHA with a decrease of overall yield. The result of 29Si NMR and 27Na spectrum of the synthesis gel and mother liquor suggested the zeolite formation through hydrothermal conversion did not occur when the monomeric Si species was increased significantly while TMAdaOH can lead to the formation of CHA similar to the synthesis of conventional silica source. Still, the textural properties and crystallographic structure of the obtained CHA have remained the same but the fraction of the strong acidity was increased significantly with the increase of the Si/Al ratio. High silica CHA shows superior hydrothermal stability in selective catalytic reduction of NOx, consistent with the result of lattice optimization using General Utility Lattice program (GULP) with the core-shell potential. Further, the results of the molecular dynamics simulation using Car-Parrinello method indicated the rapid migration of proton in CHA with a low Si/Al ratio while monovalent alkali metal ions stabilized near the double six membered rings, implying proton hopping can affect the stability of the zeolite.

Berichtigung: High‐Silica CHA Zeolite Membrane with Ultra‐High Selectivity and Irradiation Stability for Krypton/Xenon Separation

In the Introduction section of this Research Article, the description on the membrane geometry was incorrect. The first sentence of the fourth paragraph in this section, “Herein we prepared high-silica CHA zeolite membranes on ceramic hollow fiber substrates for Kr/Xe separation.” should be revised to read “Herein we prepared high-silica CHA zeolite membranes on ceramic disk substrates for Kr/Xe separation.” This change does not affect the interpretation of the data and the conclusions, but guarantees a consistent description with other sections.

Syngas to light olefins over ZnAlOx and high-silica CHA prepared by boron-assisted hydrothermal synthesis

Transformation of syngas into light olefins (STO) over bifunctional oxide and zeolite (OX-ZEO) composite catalysts requires rational design of zeolites with moderate to low acid density and strength. As a promising zeolite component in OX-ZEO towards olefins production, the synthesis of CHA with desirable acidity remains to be a challenging task. Herein, high-silica CHA zeolite with Si/Al ranges from to 20 to ∞ were prepared via a boron-assisted hydrothermal synthesis method. The influence of Si/Al ratio on the acidity and textural properties of boron-containing CHA (BAl-CHA) was systematically studied. The syngas to olefin performance over ZnAlOx/BAl-CHA was explored. The roles of acidity, framework boron and reaction parameters in determining the olefin selectivity as well as catalyst stability were revealed. The 85.1% selectivity of light olefins was achieved over the BAl-CHA catalyst with Si/Al ratio over 300. The boron played a vital role in directing the formation of CHA structure (Si/Al > 100) and reduced the strong acid sites. The H2/CO ratio, reaction temperature, total pressure and proximity of the two components also played crucial role in determining the light olefins selectivity. A DME/methanol involved aromatic-cycle is proposed to play a predominant role in the olefin production. The residual CO and H2 are likely involved in the aromatic-cycle, and then affect the product distribution, with CO promoting the ethene selectivity while H2 enhancing the alkane selectivity.

Study on the Synthesis of Chabazite Zeolites via Interzeolite Conversion of Faujasites.

The interzeolite conversion of faujasite (FAU-type) zeolites to chabazite (CHA-type) zeolite in the presence of N,N,N-trimethyladamantammonium and N,N,N-dimethylethylcyclohexylammonium cations was investigated over a large compositional range by carefully controlling the reaction mixture compositions. Highly crystalline CHA zeolites were also obtained by the transformation of several zeolite types including EMT, LTL, LEV, RTH, and MFI frameworks. The formation of CHA zeolite from FAU zeolite precursors was substantially faster than that from zeolite L with a similar composition. High-silica CHA zeolites were also produced successfully using a mixture of TMAda with a number of less expensive organic structure-directing agents. The CHA zeolite materials have been synthesized with high crystallinity and with a Si/Al ratio ranging from 5 to 140. Our data support the importance of structural similarity between the zeolite precursors, nucleation/crystallization processes, and the zeolite product in the interzeolite conversion compared to conventional amorphous aluminosilicate gels. Our synthetic methods could be used to prepare other 8-membered ring zeolites such as AEI and AFX frameworks, potential candidates for selective catalytic reduction of NOx, light olefin production, and CO2 abatement.

SSZ-13 zeolite membranes on four-channel α-Al2O3 hollow fibers for CO2 separation

High-silica CHA (SSZ-13) zeolite membranes exhibit great potential for CO2 separation. However, the practical application is significantly hindered by the high investment cost. Here, we synthesized SSZ-13 zeolite membranes on four-channel hollow fibers with pore size of 150 nm, 370 nm and 600 nm, respectively. The CO2/CH4 selectivity of 127 ± 11 as well as CO2 permeance of (3.1 ± 0.6) × 10-7 mol m−2 s−1 Pa−1 was achieved on the membranes synthesized on the hollow fibers with 150 nm pore size at 433 K for 96 h. The single gas permeation was quantitively assigned to evaluate surface diffusion, Knudsen diffusion and viscous flow through the as-synthesized membranes. The permeation of small molecules (e.g., CO2 and N2) is mainly contributed by the zeolitic pores, however, the permeation of relatively big molecule (e.g., CH4) is more sensitive to the non-zeolitic pores. For the membranes with αCO2/CH4 of 130 and 48, the non-zeolitic pores contributed 24.7% and 69% of CH4 permeance, respectively.

High-Silica CHA Zeolite Membrane with Ultra-High Selectivity and Irradiation Stability for Krypton/Xenon Separation.

Capture and storage of the long‐lived 85Kr is an efficient approach to mitigate the emission of volatile radionuclides from the spent nuclear fuel reprocessing facilities. However, it is challenging to separate krypton (Kr) from xenon (Xe) because of the chemical inertness and similar physical properties. Herein we prepared high‐silica CHA zeolite membranes with ultra‐high selectivity and irradiation stability for Kr/Xe separation. The suitable aperture size and rigid framework endures the membrane a strong size‐exclusion effect. The ultrahigh selectivity of 51–152 together with the Kr permeance of 0.7–1.3×10−8 mol m−2 s−1 Pa−1 of high‐silica CHA zeolite membranes far surpass the state‐of‐the‐art polymeric membranes. The membrane is among the most stable polycrystalline membranes for separation of humid Kr/Xe mixtures. Together with the excellent irradiation stability, high‐silica CHA zeolite membranes pave the way to separate radioactive Kr from Xe for a notable reduction of the volatile nuclear waste storage volume.

Importance of controllable Al sites in CHA framework by crystallization pathways for NH3-SCR reaction

As a commerical catalyst of high silica CHA zeolite for NH3-SCR reaction, controllable Al sites in CHA framework for improving catalytic performance are rarely reported. In this work, two organic templates (TMAda+ and DMCHA+) were employed to prepare aluminosilicate CHA zeolite with similar Si/Al ratios, donated as CHA-T and CHA-D. Detailed characterizations demonstrate that two completely distinguishable crystallization pathways can be identified as the preferential formation of smaller D6R units for CHA-D with DMCHA+ and larger 8MRs for CHA-T with TMAda+, resulting in spatially closer Al sites in CHA-D than CHA-T. As an interpretation, CHA-D with more paired Al atoms in 6MR generating more hydrothermally stable Cu2+-2Z sites contributes to higher activity than CHA-T at 400−550 °C. These results suggest that the crystallization pathways directed by rational choice of organic templates might be responsible for the controllable Al sites in the framework, which is important for designing efficient NH3-SCR catalysts.

Defect-free high-silica CHA zeolite membranes with high selectivity for light gas separation

A systematic approach is described for the fabrication of defect-free high-silica zeolite membranes with CHA (SSZ-13) topology. Home-made hydrothermally-synthesized CHA seeds were coated on porous α-alumina substrates with a pore diameter of 80 nm and by means of a further hydrothermal treatment a zeolite membrane layer was formed. In order to obtain a thin and defect-free zeolite layer, the influence on the final microstructure of seed concentration during coating, coating method (rubbing, dip- or spin-coating) and crystal growth time was investigated. The template removal procedure was optimized to avoid the formation of cracks or defects. For an optimal thermal treatment, using a step-wise temperature increase to 500 °C, the membranes exhibit CO2/CH4 permselectivities of 25–30 with CO2 permeances of around 2 x 10−7 mol m−2 s−1 Pa−1 at 22 °C and 2 bar of pressure difference. O2 plasma pre-treatment prior to template removal increased the CO2/CH4 permselectivity to 176, while maintaining the same CO2 permeance values when no pre-treatment was used. The SF6 permeances, both at low (22 °C) and high (200 °C) temperatures, were below the detection limit (2 x 10−10 mol m−2 s−1 Pa−1), which in return results in very high N2/SF6 permselectivities of more than 700.

Diffusion of methane in high-silica CHA zeolite

Diffusivity of methane in CHA-type zeolite (Si/Al = 160) was investigated by uptake rate method using a volumetric adsorption setup. The methane diffusivities were measured over a range of temperature (298-393 K) and pressure (0-0.9 bar) to study the effect of temperature and pressure on the diffusion coefficients. The results showed that the diffusivities decrease with pressure while they increase with increase in temperature. Furthermore, activation energy of diffusion decreases with pressure. The limiting values of activation energy and diffusivity of methane (at 298 K) at p = 0 (zero loading) were estimated to be 18.0 kJ/mol and ~3.2 × 10−13 m2/s, respectively. The results were compared with the diffusivities of CH4 in 12-, 10- and 8-MR zeolites.

Improved methanol-to-olefin reaction selectivity and catalyst life by CeO2 coating of ferrierite zeolite

The catalytic performance of the methanol-to-olefin (MTO) reaction over ferrierite zeolite was improved greatly by introduction of a CeO2 coating that selectively masked the acid sites on the external surface of the catalyst. In the absence of this coating, significant deterioration of the catalytic performance was observed owing to coke formation on the external surface and resultant blockage of the portals. Selective coating of CeO2 on the external surface was investigated using transmission electron microscopy, temperature-programmed desorption of NH3, N2 adsorption–desorption isotherms, and xenon adsorption measurements to reveal the structure–activity relationship. The resulting catalytic performance of CeO2-coated ferrierite zeolite in the MTO reaction was similar to that of MFI or high-silica CHA zeolite. The observed preferential formation of C4,= and C6,=, ∼90% free of aromatics was obtained because of the negligible deposition of heavy coke. This work shows that selective CeO2 coating on ferrierite zeolite was effective during catalyst regeneration and hydrothermal aging, leading to extended catalyst life and improved selectivity for catalytic applications.

Efficient and rapid transformation of high silica CHA zeolite from FAU zeolite in the absence of water

High quality CHA zeolite catalysts were efficiently synthesized by fast transformation of FAU zeolite in the absence of water.

Atom-economical synthesis of a high silica CHA zeolite using a solvent-free route.

A high silica CHA zeolite is successfully synthesized in the presence of a small amount of N,N,N-dimethylethylcyclohexylammonium bromide under solvent-free conditions. Catalytic tests for the selective catalytic reduction of NOx with NH3 (NH3-SCR) and methanol-to-olefins (MTO) show that the sample from the solvent-free route exhibits comparable catalytic properties to that from the conventional route.

Preparation of steam-stable high-silica CHA (SSZ-13) membranes for CO2/CH4 and C2H4/C2H6 separation

High-quality SSZ-13 membranes were synthesized using the combinative structure-directing agents of N,N,N, trimethtyl-1-adamantammonium hydroxide and tetraethylammonium hydroxide by one hydrothermal treatment step. The two organics took a “cooperative structural-directing” role on SSZ-13 crystallization. The best membrane had a CO2 permeance of 2.0×10−7mol/(m2sPa) with a CO2/CH4 selectivity of 300 and a C2H4 permeance of 2.9×10−9mol/(m2sPa) with a C2H4/C2H6 selectivity of 11, at 303K and 0.2MPa feed pressure for these equimolar binary mixtures, respectively. SSZ-13 membranes also displayed high performances for CO2/CH4 and C2H4/C2H6 separations at higher feed pressures of 0.8–1MPa. These SSZ-13 membranes were hydrothermally stable at steam for at least 4 days at 378K and acid-resistance at pH=1.

Acid stability evaluation of CHA-type zeolites synthesized by interzeolite conversion of FAU-type zeolite and their membrane application for dehydration of acetic acid aqueous solution

High-silica CHA-type zeolites with various Si/Al ratios were prepared by interzeolite conversion of FAU-type zeolite using the benzyltrimethylammonium (BTMA+) cation as a structure-directing agent (SDA), and the acid-resistance (structure stability and composition stability) of these materials was compared with SSZ-13 zeolite synthesized using the N,N,N-trimethyl-1-adamantammonium (TMAda+) cation as a conventional SDA. Si/Al ratios of zeolite ranging from 5 to 10 produced no difference in the 90vol% acetic acid stability between the high-silica CHA synthesized with BTMA+ and the SSZ-13 synthesized with TMAda+. In the case of Si/Al ratio >10, considerable differences were observed in the composition stability. To elaborate, an increase in the Si/Al ratio was observed for the SSZ-13 after acetic acid treatment, whereas no differences in the Si/Al ratios prior and subsequent to the treatment were observed for the CHA synthesized with BTMA+. These findings strongly indicate a high acetic acid stability of the CHA-type zeolite synthesized by the interzeolite conversion method. Their high acid stability towards several common mineral acids, such as HCl, H2SO4, and HNO3, was also confirmed. The CHA-type zeolite membrane was fabricated on a porous α-alumina tube, and the membrane performance was measured in a feed mixture of water/acetic acid (50/50wt%) at 75°C. The membrane demonstrated a high separation factor, α(H2O/CH3COOH), of ca. 2500 with a permeate flux of ca. 8kgm−2h−1.

Single parameter synthesis of high silica CHA zeolites from fluoride media

A series of high silica CHA zeolites with a true single parameter variation has been synthesized, where only the Si/Al ratio changes, using N,N,N-1-trimethyladamantammonium hydroxide as the structure directing agent in fluoride media. The materials have been synthesized with high crystallinity, and with a Si/Al ratio ranging from 20 to infinity, i.e. pure SiO2. Powder XRD shows that the unit cell parameters increase with increasing amount of Al incorporated into the framework, while BET and SEM shows that the specific surface area and the particle size is fairly independent of the Si/Al ratio. We also present an explanation for the earlier reported disagreement in Si/Al ratio determined by elemental analysis and 29Si MAS NMR.

Synthesis of high-silica CHA type zeolite by interzeolite conversion of FAU type zeolite in the presence of seed crystals

The influence of seed crystals on the interzeolite conversion of FAU type zeolite into CHA type zeolite was investigated in the presence of benzyltrimethylammonium hydroxide as a structure-directing agent under various hydrothermal synthesis conditions. Pure and highly crystalline CHA type zeolites with a wide range of Si/Al ratios were obtained in a shorter crystallization time as compared with those obtained without seed crystals. Furthermore, we achieved the first successful synthesis of high-silica CHA type zeolite in the absence of Na + cations by increasing the seed content. The protonated CHA type zeolite with a Si/Al ratio of ca. 15 yielded the highest propylene yield of ca. 48 C-% in ethanol conversion into light olefins.