Membrane separation, particularly using CO2-permselective membranes, presents an energy-efficient alternative for CO2 removal from natural gas. However, the limited performance of commercial membranes restricts its market share. Zeolite membranes, especially all-silica 8-member-ring (8MR) zeolite membranes, offer the potential for superior flux and selectivity due to their uniform micropores. Their robust mechanical strength, thermochemical, and hydrothermal stability ensure longevity under harsh conditions. A significant challenge, however, is the formation of non-selective macropores during the high-temperature calcination needed for template removal. This detemplation process, typically requiring temperatures of 550–700 °C, often leads to the creation of defects that compromise the membrane's selectivity. The mismatch in thermal expansion between the zeolite framework and the ceramic support is a major contributor to this issue. While techniques like rapid thermal processing and ozonization have been explored to address this, they have limitations, especially with 8MR zeolites, where diffusion of reactants and byproducts is significantly slower. The need for a more efficient and cost-effective detemplation method for ultra-selective 8MR DD3R zeolite membranes remains a challenge. This research introduces a template-modulated crystal transition (TMCT) method aimed at overcoming these limitations to produce high-performance DD3R zeolite membranes for CO2/CH4 separation. The TMCT method aims to synchronize template decomposition with zeolite framework relaxation, thereby reducing the formation of non-selective macropores. This is achieved through a brief, high-temperature treatment that converts the organic template into tightly bound carbon species, followed by a moderate calcination to remove these residual carbons.

The use of zeolite membranes for gas separation, particularly CO2/CH4 separation, has been extensively studied. All-silica 8MR zeolite membranes like those based on DD3R structure show promise due to their uniform micropores and high potential selectivity. However, a major obstacle is the formation of non-selective macropores during the high-temperature calcination required to remove the organic template used in zeolite synthesis. Previous efforts to mitigate this have included rapid thermal processing to strengthen grain bonding (Tsapatsis et al.), and lower-temperature ozonization (Kapteijn et al.). These approaches, however, have limitations. Rapid thermal processing has been effectively applied to larger-pore 10MR zeolites, but is less effective for smaller-pore 8MR systems. Similarly, the slow diffusion rate of oxygen/ozone in 8MR zeolites necessitates lengthy treatment times, making ozonization less practical for large-scale production. The current research aims to circumvent these limitations by developing a novel template removal strategy.

The study employed a template-modulated crystal transition (TMCT) method for fabricating ultra-selective DD3R zeolite membranes. DD3R zeolite membranes were synthesized on α-Al2O3 hollow fiber supports using a secondary growth method. The TMCT process involves a short (one-minute) instantaneous overheating of the as-synthesized membranes at 700 °C. This step converts the organic template (1-adamantanamine) into sp2 carbon species, creating space for framework relaxation and reducing stress during subsequent template removal. The residual carbon species are then removed through a moderate calcination step at 550 °C. This approach was compared to conventional calcination (CC) methods at 550 °C and 700 °C. The membrane morphology and pore structure were characterized using field emission scanning electron microscopy (FE-SEM), confocal laser scanning microscopy (CLSM), and nitrogen adsorption analysis (BET). The chemical state of the template and its conversion products during the TMCT treatment were investigated using 13C solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS). High-resolution synchrotron single-crystal X-ray diffraction was used to study the zeolite crystal structure before and after TMCT treatment. In-situ high-temperature X-ray diffraction was performed to track the thermal expansion behavior of the zeolite and its dependence on the TMCT treatment. Finally, the gas separation performance of the membranes was evaluated by measuring CO2 and CH4 permeance and selectivity under various conditions, including high-pressure conditions up to 31 bar. Specific details on the preparation of the α-Al2O3 supports, DD3R zeolites, and single crystals, as well as gas separation testing procedures are provided in the supplementary information. Quantitative analysis of permeance through zeolitic pores and defects was also conducted using SF6 as a probe molecule.

Conventional calcination methods produced membranes with significant non-selective macro-pores, evidenced by CLSM imaging. The TMCT method, however, resulted in defect-free membranes. The one-minute 700 °C overheating step in the TMCT process converted the 1-adamantanamine template into sp2 carbon species, confirmed by NMR and XPS analysis. Single-crystal X-ray diffraction revealed a change in zeolite symmetry from R-3m to R-3 after the TMCT treatment, indicating framework relaxation. This structural change correlates with a ~0.45% increase in unit cell volume. In-situ XRD showed that TMCT membranes exhibit significantly reduced thermal expansion compared to untreated membranes, attributed to the increased framework flexibility after template conversion. The TMCT-treated membranes showed significantly enhanced CO2/CH4 separation performance, with CO2/CH4 selectivity ranging from 157 to 1172 and CO2 permeance ranging from (890-1540) × 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹. Even at high feed pressures of up to 31 bar, the CO2 flux reached 3.6 Nm³ m⁻² h⁻¹ with a mixture selectivity of 43. Large-scale production yielded 17 membranes (total area 405 cm²) with an average CO2/CH4 selectivity of 422. The study showed that CO2 permeance increased with TMCT temperature up to 700 °C, but further increasing the temperature to 800 °C led to a decrease in selectivity. The optimal TMCT time was found to be less than 8 min. The membrane performance significantly outperforms that of commercially available polymeric membranes and other zeolite membranes reported in the literature. Analysis under high pressure showed that zeolitic pores contributed 99.99% of CO2 permeance, indicating high-quality membranes. The slight decrease in CO2 permeance at high pressures was attributed to reduced CO2 diffusivity, while CH4 permeance increased due to decreased energy barriers for diffusion at higher loading.

The findings demonstrate the effectiveness of the TMCT approach in controlling zeolite framework flexibility to produce high-performance CO2/CH4 separation membranes. The unprecedented selectivity and permeance achieved significantly surpasses the performance of currently available polymeric membranes and establish a new upper bound for this separation technology. The success of the TMCT method is attributed to the synchronized template decomposition and framework relaxation, effectively mitigating the formation of non-selective macropores. This innovative technique offers a significant advancement in zeolite membrane fabrication, addressing a major bottleneck in scaling up zeolite membrane technology for industrial applications. The ability to maintain high performance even at high feed pressures makes these membranes particularly suitable for natural gas upgrading. The results open possibilities to optimize the synthesis process and explore other zeolite systems using this TMCT approach. This study clearly demonstrates the potential of zeolite membranes as a competitive and energy-efficient technology for CO2 separation.

This study successfully developed a template-modulated crystal transition (TMCT) approach to fabricate ultra-selective DD3R zeolite membranes for CO2/CH4 separation. By carefully controlling the zeolite framework flexibility through a rapid template decomposition strategy, the researchers achieved unprecedented CO2/CH4 selectivity (up to 1172) and CO2 permeance (up to 1540 × 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹). The superior performance surpasses existing commercial membranes and establishes a new upper bound for this separation technology. The scalability of the method was demonstrated through the successful synthesis of 17 large-area membranes. Future research could explore the application of this TMCT method to other zeolite systems and investigate the long-term stability and durability of the fabricated membranes under industrial operating conditions.

While the TMCT method showed significant improvement in membrane performance, there is limited discussion on the long-term stability of these membranes under continuous high-pressure operation. The study primarily focuses on CO2/CH4 mixtures, and further investigation could be done to assess performance with other gas mixtures relevant to industrial applications. The current study focused on the use of the α-Al2O3 hollow fiber supports. Further investigation into other support materials could optimize the membrane’s performance and scalability.