High-silica Zeolites Research Articles

Direct synthesis of low-silica ZSM-48 zeolite via seed-assisted hydrothermal synthesis with 1,6-hexanediamine as template

ZSM-48 zeolite is characteristic of 10-member ring (10-MR) one-dimensional tubular channel structure and finds extensive applications in catalytic hydroisomerization reactions. Here we developed an efficient synthetic strategy for low-silica ZSM-48 zeolite with a SiO2/Al2O3 ratio at 30∼176 via a seed-assisted hydrothermal synthesis method, overcoming the limit that 1,6-hexanediamine (HDA) template can only be used to synthesize high-silica (i.e., SiO2/Al2O3 ratio >200) zeolites. A two-step crystallization procedure coupled with zeolite seed-assisted synthesis strategy was enrolled to realize well crystallized ZSM-48 zeolites, in which the zeolite nuclei are sufficiently produced at the nucleation temperature at 100 °C for 24 h with the aid of zeolite seeds, and the zeolite growth is proceeded at the crystallization temperature at 160 °C for 48 h. The morphology and particle size of low-silica ZSM-48 have been manipulated by adjusting the synthetic parameters, such as the alkalinity and silicon source of aluminosilicate gel for zeolite, the crystallization manner (static or dynamic crystallization), as well as the presence of additive (i.e., sodium chloride). The as-synthesized ZSM-48 zeolite exhibits a significantly high acid content of 0.194 mmol/g, highlighting its potential as an excellent acidic support for hydroisomerization catalysts. The cost-effective and environmentally friendly synthesis strategy, which is anticipated to expand the application of alkylamine organic templates in the synthesis of low-silica ZSM-48 zeolite.

Enhancing the Acidity Window of Zeolites by Low-Temperature Template Oxidation with Ozone.

Revisiting the impact of the first and often deemed trivial postsynthetic step, i.e., a high-temperature oxidative calcination to remove organic templates, increases our understanding of thermal acid site evolution and Al distributions. An unprecedented degree of control over the acidity of high-silica zeolites (SSZ-13) was achieved by using a low-temperature ozonation approach. Fourier transform infrared spectroscopy of adsorbed probe molecules and solid-state NMR spectroscopy reveal the complexity of the thermal evolution of acid sites. Low-temperature activated (ozonated) zeolites maintain the original Brønsted acidity content and high defect content and have virtually no Lewis acidity. They also preserve the "as-made" Al distribution after crystallization and show a clear link between synthesis conditions and divalent cation capacity, as measured with aqueous cobalt ion uptake. The synthesis protocol is found to be the main contributor to Al proximity, yielding record high exchange capacity when ozonated. After conventional calcination at 500-600 °C, however, the presence of water leads to the gradual depletion of Brønsted acid sites, in particular, in small crystals. This work indicates that low-temperature ozonation followed by thermal activation at different temperatures can be used as a novel tool for tuning the amount and nature of acid sites, providing insights into the activity of zeolites in acid-catalyzed reactions, such as CO2 hydrogenation to dimethyl ether, and thereby expanding the possibilities of rational acidity tuning.

Competitive Adsorption of 4-Hydroxybenzaldehyde and Toluene onto High Silica Zeolites

To evaluate the ability of zeolites to remove natural low molecular organic substances, p-hydroxybenzaldehyde (p-HBA), a phenolic compound derived from lignin, was chosen as a representative of naturally occurring dissolved organic substances. Two distinct high-silica zeolite materials, namely ZSM-5 and Y, were utilised for the study, and the adsorption process was investigated under a wide range of conditions. It has been observed that p-HBA is adsorbed by both zeolites, and the pH significantly impacts the adsorption of p-HBA, particularly within the low concentration range, while exerting minimal influence on the saturation capacity. For both zeolites, various isotherm models were assessed to accurately describe the adsorption data obtained from aqueous solutions of p-HBA. In addition, to comprehend the selectivity of the adsorbents towards natural organic substances and xenobiotics, the competitive adsorption of mixtures of p-HBA and toluene (TOL) was investigated. The zeolite’s saturation capacity for p-HBA diminishes with increasing contaminant concentration. Conversely, the adsorption of toluene remains minimally affected by p-HBA, and it has been demonstrated that toluene can displace adsorbed p-HBA from the zeolites’ sites. This finding has been confirmed by diffractometric study that indicates that TOL and p-HBA occupy “the same” adsorption sites. Furthermore, Rietveld refinements reveal the formation of p-HBA complexes interacting with the framework and stabilising the guest structures within the zeolite porosity. The results obtained are important for the selection of proper adsorbent for the removal of hydrocarbons in environmental application (natural waters).

Investigation of alite hydration in the presence of low- and high-silica zeolites

Synthetic zeolites that are obtained from the hydrothermal reaction of NaOH with fly ash have a well-defined pore structure and pozzolanic properties. This work examines the effect of the low-silica (Na-A) and high-silica (ZSM-5) zeolite structure on the kinetics of alite hydration and mineralogical characteristics of the reaction products. The addition of the Na-A and ZSM-5 zeolites reduced the rate of heat release during the hydration reaction, which is typical for commonly used pozzolanic additives. The ZSM-5 zeolite had a much higher Si/Al ratio relative to the Na-A zeolite, which translated into a higher pozzolanic activity and resulted in greater stability of the hydration reaction kinetics. The addition of zeolites caused the heat of hydration to decrease compared to the reference pastes (the alite reference paste—206 J/g): Na-A 15%—148.9 J/g; ZSM-5 15%—149.9 J/g. The concentration of calcium ions in the alite solution and the content of active silica were identified as the most important parameters affecting the kinetic, morphological, and structural characteristics of the C–S–H phase. The addition of zeolites increased the amount of C–S–H gel phases formed. At the same time, the proportion of portlandite in the hydration products of the alite–zeolite systems decreased. Pozzolans reduced the ratio of CaO/SiO2 in the C–S–H phase, which for pastes with the addition of the Na-A ranged from 1.61 to 1.37, and for the ZSM-5 from 1.91 to 0.47. This ratio decreased with the dosage and the age of alite paste. In summary, this research has shown that synthetic zeolites are a viable pozzolanic additives.

STUDYING THE INFLUENCE OF ALKALINE TREATMENT AND MODIFICATION OF ZEOLITE ON ITS PHYSICAL-CHEMICAL AND CATALYTIC PROPERTIES IN THE PROCESS OF PROPANE CONVERSION TO OLEFIN HYDROCARBONS

In this work, the effect of alkaline treatment of ZSM-5 zeolite with a modulus of 100 on its physicochemical properties and activity in the process of converting propane into olefin hydrocarbons was investigated. The growing demand for lower olefins is driving the search for new ways to improve existing technologies and develop new, more efficient catalysts. Catalytic dehydrogenation of light alkanes is an alternative to the petrochemical method for producing lower olefins from cheap and available gas and oil and gas feedstocks. At the same time, dehydrogenation reactions of light alkanes have a number of features and limitations that determine the range of possible catalysts. Zeolites are a class of supports that, due to their large surface area, nanometer-sized pores, and good thermal stability, can be used to prepare catalysts for the dehydrogenation of C2−C4 alkanes. An important feature of crystalline high-silica zeolites is the ability to change their acidic properties under the influence of various pre-treatments, which opens up great opportunities for their regulation and, accordingly, the selection of the most effective catalyst for a particular chemical process. It has been shown that alkaline treatment of zeolite leads to the formation of an additional amount of mesopores, which, in turn, affects its catalytic properties in the process of converting propane into olefinic hydrocarbons. The dependence of the activity and selectivity of the zeolite catalyst on the concentration of the alkali solution used for its treatment has been established. It was found that treatment of the zeolite catalyst with a 1.0 M NaOH solution leads to an increase in its selectivity with respect to the formation of C2–C4 olefins from propane, which at a process temperature of 650 °C reaches 32.0% with a propane conversion of 81%. It has been shown that additional promotion of desilicated zeolite with magnesium or manganese increases its activity with respect to the formation of olefinic hydrocarbons from propane. For citation: Vosmerikov A.A., Vosmerikova L.N., Vosmerikov A.V. Studying the influence of alkaline treatment and modification of zeolite on its physical-chemical and catalytic properties in the process of propane conversion to olefin hydrocarbons. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 8. P. 50-58. DOI: 10.6060/ivkkt.20246708.11t.

Preparation of MAZ-Type Zeolite with High Silica.

The Si/Al molar ratio of MAZ aluminosilicate zeolite prepared by the direct hydrothermal method is generally less than five, thus giving rise to poor thermal and hydrothermal stability for this low-silica zeolite. With the purpose of enhancing the Si/Al molar ratio of MAZ zeolite, post-synthesized methods including acetic acid treatment and steaming treatment, as well as interzeolite transformation from FAU zeolite, were employed to prepare MAZ zeolite with high silica. It was found that steaming treatment was more effective in increasing the Si/Al molar ratio in comparison with acetic acid treatment, affording a maximum Si/Al molar ratio of 16.9 along with a preserved crystallinity of approximately 75%. Additionally, high-silica MAZ zeolite with a Si/Al molar ratio of up to 7.3 was also capable of being directly hydrothermally synthesized using interzeolite transformation from FAU zeolite.

Strong Swelling and Symmetrization in Siliceous Zeolites due to Hydrogen Insertion at High Pressure.

Hydrogen and helium saturate the 1D pore systems of the high-silica (Si/Al>30) zeolites Theta-One (TON), and Mobile-Twelve (MTW) at high pressure based on x-ray diffraction, Raman spectroscopy and Monte Carlo simulations. In TON, a strong 22 % volume increase occurs above 5 GPa with a transition from the collapsed P21 to a symmetrical, swelled Cmc21 form linked to an increase in H2 content from 12 H2/unit cell in the pores to 35 H2/unit cell in the pores and in the framework of the material. No transition and continuous collapse of TON is observed in helium indicating that the mechanism of H2 insertion is distinct from other fluids. The insertion of hydrogen in the larger pores of MTW results in a strong 11 % volume increase at 4.3 GPa with partial symmetrization followed by a second volume increase of 4.5 % at 7.5 GPa, corresponding to increases in hydrogen content from 43 to 67 and then to 93 H2/unit cell. Flexible 1D siliceous zeolites have a very high H2 capacity (1.5 and 1.7 H2/SiO2 unit for TON and MTW, respectively) due to H2 insertion in the pores and the framework, in contrast to other atoms and molecules, thereby providing a mechanism for strong swelling.

Removal and complete combustion of 1,4-dioxane in water by a two-step reaction combining adsorption and catalytic combustion

Catalytic wet air oxidation is not so effective to decompose 1,4-dioxane in water, while there have been several studies on this reaction. As an alternative, we propose a two-step purification system for removal and complete combustion of 1,4-dioxane, where the first step is removing 1,4-dioxane from water by adsorption, and in the second step, the adsorbent is heated to release 1,4-dioxane, and the released 1,4-dioxane is completely combusted by gas-phase catalytic reaction. We had searched for adsorbents and combustion catalysts suitable for the system and found a high-silica MFI zeolite and Pt/SiO2, respectively. The high-silica MFI zeolite adsorbed up to 121 mg g−1 of 1,4-dioxane, and 1.1 mmol L−1 of 1,4-dioxane in water was almost completely removed by 10 g L−1 dose. The zeolite was reusable for adsorption-desorption cycles at least five times without any loss of the performance. Pt/SiO2 was highly active for the complete combustion of 1,4-dioxane even at 200 °C. By using the high-silica MFI zeolite and Pt/SiO2, we conducted the release of 1,4-dioxane from the zeolite by heating and the gas-phase catalytic combustion of 1,4-dioxane using two reactors connected in series, which were filled with the zeolite and Pt/SiO2, respectively, and demonstrated that all 1,4-dioxane was released by 300 °C and was completely combusted in the upstream and downstream reactors, respectively.

The role of defects in high-silica zeolite hydrolysis and framework healing

The stability of high silica zeolites under standard laboratory and mild steaming conditions is understood to be due to the lack of hydrophilic Al-O-Si moieties, which can be targeted by water via hydrolysis reactions with relatively low barriers. However, in hydrophobic high-silica and siliceous frameworks, the specific interactions between water and the internal framework sites are incompletely understood. In particular, the behaviour of common internal defects, including partial hydrolysis species and silanol nests are not established, despite their expected role in accelerating the decomposition of the framework. In this work, we utilise machine learning potentials combined with density functional calculations to rigorously sample the hydrolysis processes in siliceous zeolites with topologies CHA and MFI under low water conditions and quantify the effect of defects. Internal silanol defect sites are found to accelerate zeolite decomposition primarily by bypassing the initial, high-barrier hydrolysis step. Subsequent steps proceed with lower reaction barriers. However, all reaction steps are found to be highly activated, and unlikely to occur rapidly at room temperature under conditions of low internal water concentration. Exchange-healing routes, which reverse hydrolysis while incorporating oxygen from water molecules are found to be competitive along the entire hydrolysis pathway in CHA, providing an additional source of stabilization against hydrolytic decomposition.

Mechanism of the Low-Temperature Organic Removal from Imidazolium-Containing Zeolites by Ozone Treatment: Fluoride Retention in Double-4-Rings.

For zeolites synthesized using imidazolium cations, the organic matter can be extracted at very low temperatures (100 °C) using ozone. This is possible for zeolites with 12-ring or larger pores but requires higher temperatures in medium-pore zeolites. The first chemical events in this process occur fast, even at room temperature, and imply the loss of aromaticity likely by the formation of an adduct between ozone and the imidazole ring through carbons C4 and C5. Subsequent rupture of the imidazole ring provides smaller and more flexible fragments that can desorb more readily. This process has been studied experimentally, mainly through infrared spectroscopy, and theoretically by density functional theory. Amazingly, fluoride anions occluded in the small double-four-ring units (d4r) during the synthesis remain inside the cage throughout the whole process when the temperature is not too high (≤150 °C). However, fluoride in larger cages in MFI ends up bonded to silicon in penta or hexacoordinated units, likely out of the cages, after ozone treatment at 150 °C. For several germanosilicate zeolites, the process allows their subsequent degermanation to yield stable high-silica zeolites. Quaternary ammonium cations require harsher conditions that eventually also extract fluoride from zeolite cages, including the d4r unit.

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.

Bimetallic PtZn nanoparticles anchored in high-silica SSZ-13 zeolite for efficient propane dehydrogenation

Direct dehydrogenation of propane to propylene (PDH) is a promising route to meet the ever-growing global propylene demand, but the industrial Pt-based catalysts usually suffer from serious deactivation derived from coke deposition and sintering under harsh reaction conditions. In this work, PtZn bimetallic nanoparticles encapsulated into high-silica SSZ-13 zeolites (PtZn@SSZ-13) were synthesized via a facile ligand-protected interzeolite transformation strategy. In a PDH reaction, the synthesized 0.5Pt3Zn@SSZ-13 catalyst exhibits superior catalytic performance with a propane conversion of 23.7% and a propylene selectivity of 96.8%, which is significantly better than that of PtZn supported on high-silica SSZ-13 (0.5Pt3Zn/SSZ-13). Particularly, the high catalytic activity of this catalyst can be kept during three successive oxidation–reduction cycles. Detailed characterizations including HAADF-STEM, XPS, UV–Vis, H2-TPR reveal that the remarkable improvement of PDH performance is attributed to the PtZn synergistic effect as well as the confinement of SSZ-13 zeolites. The DFT calculations and microkinetic simulations also evidenced that the Pt particles anchored on the Zn-doped SSZ-13 zeolite pore framework possessed obviously higher propane dehydrogenation activity, whereas the anti-coking ability of 0.5Pt3Zn@SSZ-13 was also stronger than Pt (111) surface, demonstrating the superior performance of 0.5Pt3Zn@SSZ-13.

Tackling the challenge of efficient VOCs removal across varied molecular sizes over zeolite adsorbent under humid conditions

Efficient removal of volatile organic compounds (VOCs) with different molecular sizes over a single zeolite remains a great challenge, especially under humid conditions. Here, we introduced an innovative base leaching strategy to construct hierarchical pores within high-silica MFI zeolite on premise of maintaining a similar high micropore volume. The key points were the introduction of Al3+ and B3+ as pore directing agents into the base solution and meticulous regulation of the acid washing process. Because of the introduction of mesopores, the adsorption capacities for m-xylene increased by 16.1 times compared to that on conventional MFI adsorbent. Meanwhile, it exhibited comparable adsorption capacities for smaller VOCs, including dichloromethane and toluene. The adsorption capacities for ethanol showed a notable increase, accompanied by a decrease in the desorption temperature. This was attributed to the generation of additional silanol adsorption sites, as confirmed by 1H MAS NMR and in situ FT-IR spectra. Moreover, the optimal adsorbent presented good regeneration performance throughout the four dynamic adsorption–desorption cycling tests for all VOCs.

Properties of multiple Lewis acid sites in alkali metal-exchanged chabazites probed by CO adsorption

Carbon monoxide adsorption on alkali-metal exchanged chabazites (M-CHA, where M = Li, Na, K) was investigated across various Si/Al ratios. The study reveals significant insights into the adsorption behavior, including the persistence of cationic preferences with decreasing Si/Al ratios and the existence of multiple-center interactions involving alkali-metal cations and CO. Results show that for high-silica M-CHA zeolites, CO adsorption is effectively described by single and dual adsorption site models, with cation preferences varying by type. In low-silica zeolites, cation positions are primarily influenced by the aluminum distribution and Coulombic interactions. However, the propensity for single-site cation positions (Si/Al→∞) is preserved to a certain degree. The most noticeable example is the small difference between SIII’ occupancies (cations in 8-membered ring windows) in Na-CHA-2 and K-CHA-2 (0.80 vs. 0.85) that strongly influences the rate of diffusion of CO in the M-CHA-2 samples. While FT-IR spectra of high-silica zeolites can be accurately described using cation site stabilities, interaction energies, and CO stretching frequencies, predicting spectra of low-silica chabazites requires a statistical approach and/or molecular dynamics simulations at the DFT level. The findings demonstrate that the dynamical behavior of adsorbates changes dramatically between different alkali metal-exchanged chabazites, highlighting the complex nature of CO adsorption at multiple Lewis acid sites.

Monte Carlo simulation of ammonia adsorption in high-silica zeolites for refrigeration applications

Climate change and the continual rise in cooling demand means more efficient and environmentally friendly refrigeration technologies are required more than ever. One attractive route to reducing future demand is to improve adsorption refrigeration technologies based on natural refrigerants such as ammonia. The choice of ammonia adsorbent plays an important role in achieving improved refrigeration efficiency and suitable operating conditions. This paper reports a detailed study on the suitability of zeolites as an adsorbent of ammonia in refrigeration applications. Systematic Monte Carlo simulations were conducted to study ammonia adsorption in five high-silica zeolites with a wide range of pores sizes and porosities. Simulations were carried out at temperatures between -50 and 50°C and pressures up to 4.0 bar. It is found that zeolites, in particular the ones with large porosities, could be very good ammonia adsorbents for adsorption refrigeration applications, since their use allows for large refrigeration capacities and tuneable operating conditions with good coefficients of performance (COP).

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.

Modulation of Al Distribution in High-Silica ZSM-5 Zeolites for Enhancing Catalytic Performance.

The spatial distribution of framework Al (AlF) has been one of the important factors that affect the catalytic properties of zeolites in diverse chemical reactions; however, the synthesis of high-silica zeolites with special AlF distribution remains a challenge. In this study, we successfully synthesized high-silica ZSM-5 zeolites with a unique AlF distribution by employing pentaerythritol (PET) as an additive in the presence of a few tetrapropylammonium hydroxide (TPAOH). The results demonstrated that the introduction of PET led to a higher proportion of Al atoms located at the sinusoidal and/or straight channels. It was observed that the addition of PET prevented the interaction between TPA+ and tetrahedral [AlO4]- during the crystallization process, resulting in enhanced availability of TPA species in the form of ion-paired TPA+. This effect leads to AlF atoms dominantly distributed away from the intersection and located in narrow channels, where acidic sites more effectively inhibit hydrogen transfer and coke formation. In the reaction of dimethyl ether (DME) to olefins, the catalyst with a unique Al distribution exhibited a significant prolonged catalytic lifetime, surpassing traditional TPA-ZSM-5 by more than 2-fold and maintaining DME conversion above 90% for a maximum of 148 h. The results of multiple pulse experiments also showed that these PET-assisted ZSM-5 zeolites significantly enhanced the selectivity of propene and butene. This approach provides an effective strategy to regulate AlF distribution in high-silica ZSM-5 catalysts with the assistance of neutral alcohol. It holds great potential for application in the synthesis of other high-silica zeolites, thereby enriching the diversity of zeolite catalysis.

Template-Free Synthesis of High Dehydration Performance CHA Zeolite Membranes with Increased Si/Al Ratio Using SSZ-13 Seeds.

Pervaporation is an energy-efficient alternative to conventional distillation for water/alcohol separations. In this work, a novel CHA zeolite membrane with an increased Si/Al ratio was synthesized in the absence of organic templates for the first time. Nanosized high-silica zeolite (SSZ-13) seeds were used for the secondary growth of the membrane. The separation performance of membranes in different alcohol-aqueous mixtures was measured. The effects of water content in the feed and the temperature on the separation performance using pervaporation and vapor permeation were also studied. The best membrane showed a water/ethanol separation factor above 100,000 and a total flux of 1.2 kg/(m2 h) at 348 K in a 10 wt.% water-ethanol mixed solution. A membrane with high performance and an increased Si/Al ratio is promising for the application of alcohol dehydration.

SSZ-13 membranes on novel silica carbide monoliths for efficient CO2 separation

The usage of normal alumina supports brings the uncertain and undesirable aluminum transformation to the high-silica zeolite membranes to reduce Si/Al ratio in framework. In this current study, a novel aluminum-free silica carbide (SiC) monolithic support is used for the synthesis of SSZ-13 membranes with enhanced Si/Al ratio for the first time. Moreover, this high-porosity and monolithic SiC support enables membrane to increase gas permeance and surface-to-volume ratio. High-quality SSZ-13 membranes were prepared after rational modification of synthesis parameters such as synthesis temperature and time and support material. The effects of test temperature, pressure drop, feed flow rate and humidity on separation performance of the monolithic membranes were studied. The formation mechanism of SSZ-13 membrane was also investigated. The Si/Al ratio and CO2 permeance of SSZ-13 membranes on SiC monolith were 100% and 60% higher than that of membrane on alumina monolith, respectively. This monolithic membrane had comparable separation performance but about 2.3 times higher surface-to-volume ratios to the reported tubular zeolite membranes. This solution could be suitable to prepare other high-silica and all-silica zeolite membranes.

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.