An effective strategy to address increasing greenhouse gas emissions and combat climate change is the capture and reuse of carbon dioxide (CO₂). The reaction of CO₂ and hydrogen (H₂) can produce industrially useful chemicals, such as methanol and carbon monoxide, and synthetic fuels. However, in traditional reactors these chemical processes are limited by thermodynamic constraints and slow reaction rates, resulting in a low CO₂ conversion. This is because these reactors reach equilibrium before all the reactants are converted into desired products. To increase reaction speed, higher temperatures or pressures are required, but this increases energy consumption, posing a major challenge for CO₂ utilization technologies.

Membrane reactors are a promising solution to this problem. These reactors use hydrophilic membranes to selectively remove water (H₂O), a common byproduct in CO₂ conversion reactions while retaining the reactants. This shifts the reaction equilibrium towards the products, improving CO₂ conversion and yields. Zeolitic membranes, such as those based on ZSM-5, are attractive candidates for membrane reactors. These materials can selectively adsorb water molecules within their micropores, preventing permeation of reactants like H₂. However, at high temperatures and low H₂O concentrations, which are typical in real-world operating conditions, their performance gets degraded due to poor pore blocking, allowing H₂ to permeate along with H₂O molecules. This in turn limits the practical applicability of membrane reactors.

To address this issue, a research team led by Associate Professor HIROTA Yuichiro Hirota from the Department of Life Science and Applied Chemistry at Nagoya Institute of Technology in Japan recently developed new ZSM-5 membranes modified by a silsesquioxane framework containing ionic liquids (SQILs). "In previous studies, ionic liquid coating has been shown to improve the H₂O capture capacity of metal-organic framework membranes," says Dr. HIROTA. "Building on this result, we designed SQIL-modified ZSM-5 membranes that maintain high H₂O permselectivity even under dilute H₂O concentrations and high temperatures. We also analyzed how hydrophilicity of the SQIL and ZSM layers contributes to this performance."

The study, which included contributions from Assistant Professor SAKAI Motomu from Waseda University, Japan, was made available online on August 18, 2025, and will be published in Volume 735 of the Journal of Membrane Science on November 01, 2025.

In their study, the team fabricated two SQILs from polymerized 1-methyl-3-(1-triethoxysilylpropyl)imidazolium (Sipmim) cations: one containing trifluoromethanesulfonate (OTf^-) anions, forming polySipmimOTf, and the other containing bis(trifluoromethylsulfonyl)imide (Tf₂N^-) anions, forming polySipmimTf₂N. In the tests conducted, polySipmimOTf was found to be more hydrophilic of the two. 

Furthermore, the team prepared two ZSM-5 membranes: a hydrophilic sodium cation (Na⁺)-type (NaZ-5) and a less hydrophilic hydrogen cation (H⁺)-type (HZ-5). The surfaces of these membranes were then modified with SQILs and tested for H₂ gas permeation and H₂O permselectivity at different concentrations at a temperature of 473 K (200 ℃).

The results showed that SQIL modification significantly reduced H₂ permeability and enhanced H₂O permselectivity compared to that of unmodified ZSM-5 membranes. Moreover, membranes modified with polySipmimOTf performed better compared to those modified with polySipmimTf₂N. Furthermore, polySipmimOTf-modified NaZ-5 (OTf^-/NaZ-5) showed significantly better H₂O permselectivity, outperforming the Otf^-/HZ-5 membranes.

Importantly, the SQIL-modified ZSM-5 membranes retained low H₂ permeation even at dilute H₂O concentrations while maintaining high H₂O permeation at elevated concentrations. The team attributed this finding to a synergistic effect arising from combining hydrophilic SQIL and ZSM layers. As Dr. HIROTA explains, "The superior dehydration of the SQIL-modified membranes arises from three factors: the low H₂ solubility and high hydrophilicity of SQILs, which minimize H₂ permeation at low H₂O concentrations; the selective adsorption and capillary condensation of H₂O in ZSM-5 micropores; and the shift in H₂O sorption from Langmuir-type to Henry-type owing to SQIL modification."

"The effective conversion of CO₂ into chemicals and fuels is essential for achieving carbon neutrality and a circular economy," adds Dr. HIROTA. "Our SQIL-modified membranes can help improve CO₂ conversion efficiency in membrane reactors and could pave the way for their broader adoption."

Indeed, these innovative membranes could open doors to more efficient and practical membrane reactors for CO₂ conversion, potentially contributing to reduced carbon emissions and energy conservation in chemical industries and mitigating the effects of global warming.

Reference

Title of original paper	Silsesquioxane framework containing ionic liquid−modified NaZSM-5 membrane for H2O/H2 separation at high temperature
Journal	Journal of Membrane Science
DOI	10.1016/j.memsci.2025.124568
Latest Article Publication Date	1 November 2025

About Professor SHIBATA Norio

Dr. HIROTA Yuichiro obtained his Master's and PhD degrees in Engineering from Osaka University in 2009 and 2012, respectively and joined Nagoya Institute of Technology as an Associate Professor in 2020. His research focuses on nanostructured materials, such as nanoporous inorganic and organic/inorganic hybrid materials and nano-scale reactors. In addition, Dr. HIROTA investigates membrane separation and catalytic reaction processes using such compounds. He has published over 70 research papers on these topics.

Funding information

This study was supported by the Kazuchika Okura Memorial Foundation and Tokai Foundation for Technology.

Contact

Associate Professor HIROTA Yuichiro
Website : https://pure.nitech.ac.jp/en/persons/yuichiro-hirota
E-mail : hirota.yuichiro [at] nitech.ac.jp

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