Surface-gel-conversion (SGC) synthesis was shown in 2020/2021 to produce crack-free, roughly 500-nm-thick pure-silica MFI (silicalite-1) zeolite membranes on porous alumina that separate hexane isomers with an exceptionally high n-hexane permeance of 24.5×10−724.5\times10^{-7} mol m−2^{-2} s−1^{-1} Pa−1^{-1} and separation factors of 13.3-22.6 of n-hexane over its isomers. The authors left three issues explicitly open: (i) the intrinsic separation factors, notably n-hexane over 2,2-dimethylbutane, while good, 'can be further improved'; (ii) under multicomponent mixed-vapor feeds the MFI crystal lattice expands, causing a transient decline of n-hexane permeance and inferred permeation of the bulkier isomers, so the mechanism of this expansion, the operating conditions, membrane designs, or post-synthesis treatments that prevent or mitigate it are unknown; and (iii) translation of the laboratory SGC route to large-area, industrially deployable membranes (uniform deposition of the tuned viscous gel on industrial supports, long-term mechanical and chemical stability) is unaddressed.

Background

MFI designates the zeolite framework type of ZSM-5 and its all-silica form silicalite-1, a crystalline molecular sieve with intersecting straight and sinusoidal channels bounded by 10-membered-ring oxygen windows about 0.55 nm across. Zeolite membranes are polycrystalline films of such frameworks grown on porous ceramic supports; they separate molecules by molecular sieving and adsorption-diffusion differences, and their throughput (permeance) scales inversely with film thickness, which motivates pushing thickness below one micrometer without introducing cracks or pinholes. Hexane isomers are the C6 alkanes: linear n-hexane (nHEX), the mono-branched 2- and 3-methylpentane, and the di-branched 2,2- and 2,3-dimethylbutane (22DMB and 23DMB), whose kinetic diameters grow from about 0.43 nm (nHEX) to about 0.62 nm (22DMB). Separating linear and mono-branched from di-branched isomers raises the research octane number of gasoline-range streams, a large-scale petrochemical need. Permeation of hexane isomers through MFI membranes has been studied experimentally and with Maxwell-Stefan diffusion models since around 2000 (Matsufuji et al.; Krishna and Paschek): nHEX fits the channels and diffuses fast, whereas 22DMB is essentially excluded, so an ideal MFI membrane should reject it almost completely. A long-standing complication is framework flexibility: silicalite-1 undergoes adsorption-induced structural changes and unit-cell (lattice) expansion upon hydrocarbon uptake, documented by X-ray diffraction and molecular simulation (Vlugt and Schenk), and transient mixture effects in MFI membrane permeation have been modeled for polar organics (Yu, Falconer, Noble, and Krishna). On the synthesis side, ultrathin oriented MFI films were achieved by secondary growth of nanosheet seed layers (Agrawal et al.) and from directly synthesized or anisotropically etched nanosheets (Jeon et al.; Liu et al.), reaching sub-100-nm films demonstrated mainly for xylene and light-gas separations. The source work added the surface-gel-conversion (SGC) route: a viscous gel of deliberately reduced wettability is coated on a seeded alumina support so it stays on the surface rather than penetrating the pores, and is then crystallized with minimal water steam into a roughly 500-nm, visibly crack-free silicalite-1 film. The as-made membranes showed the nHEX permeance of 24.5×10−724.5\times10^{-7} mol m−2^{-2} s−1^{-1} Pa−1^{-1} cited above and single-component nHEX/isomer separation factors of 13.3-22.6 at 100 °C and 1.2 bar feed pressure, but during testing with a multicomponent mixed-vapor feed (a five-component n-hexane:2-methylpentane:3-methylpentane:2,3-dimethylbutane:2,2-dimethylbutane mixture at 100 °C and 1.2 bar) the crystal lattice was observed to expand, nHEX permeance transiently declined, and the bulkier isomers appeared to permeate as well, so the selectivity actually realized under realistic mixed feeds is limited by a sorbate-induced framework effect rather than by the equilibrium pore size alone.

Problem Statement

As left open by Rong et al. (Sci. China Mater. 64 (2021) 374-382) for their ~500-nm surface-gel-conversion silicalite-1 membranes (single-component n-hexane (nHEX) permeance 24.5×10−724.5\times10^{-7} mol m−2^{-2} s−1^{-1} Pa−1^{-1} and nHEX/isomer separation factors 13.3-22.6, both measured at 100 °C and 1.2 bar feed pressure): (1) raise the intrinsic single-component (ideal) separation factors for n-hexane over its isomers — in particular over 2,2-dimethylbutane — beyond these values under the same single-component measurement conditions while keeping the membrane submicron-thick and high-flux (its single-component nHEX permeance no lower than the baseline); (2) determine the mechanism by which multicomponent mixed-vapor C6 feeds expand the MFI crystal lattice and transiently depress n-hexane permeance while admitting the bulkier isomers, and establish operating conditions, membrane design changes (e.g., thickness, orientation, support coupling), or post-synthesis treatments that prevent or mitigate this expansion so that mixed-vapor permeance and selectivity are stable rather than transiently degrading; and (3) demonstrate the surface-gel-conversion route on large-area industrial supports (uniform deposition of the tuned viscous gel, long-term mechanical and chemical durability in realistic streams). All three components are mandatory, so a complete resolution must address each of (1), (2), and (3), while each component is separately assessable so that partial progress on one can be credited; together they share one objective — reliable, shape-selective separation of hexane isomers by submicron MFI zeolite membranes, in which the actually delivered mixed-feed selectivity, not just the single-component ideal, approaches the molecular-sieving limit of the framework.

References

  1. Huazhen Rong, Gaimei Wang, Jiaxu Yan, Xiaoqin Zou, Guangshan Zhu. Surface-gel-conversion synthesis of submicron-thick MFI zeolite membranes to expedite shape-selective separation of hexane isomers. Science China Materials 64 (2021) 374-382. DOI: 10.1007/s40843-020-1400-y. https://doi.org/10.1007/s40843-020-1400-y
  2. Takaaki Matsufuji, Kensuke Watanabe, Norikazu Nishiyama, Yasuyuki Egashira, Masahiko Matsukata, Korekazu Ueyama. Permeation of Hexane Isomers through an MFI Membrane. Industrial & Engineering Chemistry Research 39 (2000) 2434-2438. DOI: 10.1021/ie000026z. https://doi.org/10.1021/ie000026z
  3. Rajamani Krishna, Dietmar Paschek. Permeation of Hexane Isomers across ZSM-5 Zeolite Membranes. Industrial & Engineering Chemistry Research 39 (2000) 2618-2622. DOI: 10.1021/ie990912d. https://doi.org/10.1021/ie990912d
  4. Thijs J. H. Vlugt, Merijn Schenk. Influence of Framework Flexibility on the Adsorption Properties of Hydrocarbons in the Zeolite Silicalite. The Journal of Physical Chemistry B 106 (2002) 12757-12763. DOI: 10.1021/jp0263931. https://doi.org/10.1021/jp0263931
  5. M. Yu, J. Falconer, R. Noble, R. Krishna. Modeling transient permeation of polar organic mixtures through a MFI zeolite membrane using the Maxwell-Stefan equations. Journal of Membrane Science 293 (2007) 167-173. DOI: 10.1016/j.memsci.2007.02.015. https://doi.org/10.1016/j.memsci.2007.02.015
  6. Kumar Varoon Agrawal, Berna Topuz, Tung Cao Thanh Pham, Thanh Huu Nguyen, Nicole Sauer, Neel Rangnekar, Han Zhang, Katabathini Narasimharao, Sulaiman Nasir Basahel, Lorraine F. Francis, Christopher W. Macosko, Shaeel Al-Thabaiti, Michael Tsapatsis, Kyung Byung Yoon. Oriented MFI Membranes by Gel-Less Secondary Growth of Sub-100 nm MFI-Nanosheet Seed Layers. Advanced Materials 27 (2015) 3243-3249. DOI: 10.1002/adma.201405893. https://doi.org/10.1002/adma.201405893
  7. Mi Young Jeon, Donghun Kim, Prashant Kumar, Pyung Soo Lee, Neel Rangnekar, Peng Bai, Meera Shete, Bahman Elyassi, Han Seung Lee, Katabathini Narasimharao, Sulaiman Nasir Basahel, Shaeel Al-Thabaiti, Wenqian Xu, Hong Je Cho, Evgenii O. Fetisov, Raghuram Thyagarajan, Robert F. DeJaco, Wei Fan, K. Andre Mkhoyan, J. Ilja Siepmann, Michael Tsapatsis. Ultra-selective high-flux membranes from directly synthesized zeolite nanosheets. Nature 543 (2017) 690-694. DOI: 10.1038/nature21421. https://doi.org/10.1038/nature21421
  8. Yi Liu, Weili Qiang, Taotao Ji, Mu Zhang, Mingrun Li, Jinming Lu, Yi Liu. Uniform hierarchical MFI nanosheets prepared via anisotropic etching for solution-based sub-100-nm-thick oriented MFI layer fabrication. Science Advances 6 (2020) eaay5993. DOI: 10.1126/sciadv.aay5993. https://doi.org/10.1126/sciadv.aay5993
  9. Qiang Ma, Jiajia Li, Yanshuo Li, Jungkyu Choi. Recent progress and issues facing zeolite and metal-organic framework membranes: From membrane synthesis to applications. Journal of Membrane Science 690 (2024) 122201. DOI: 10.1016/j.memsci.2023.122201. https://doi.org/10.1016/j.memsci.2023.122201
  10. Kunlin Yu, Hong Xiao, Jiayu Ding, Siwei Yan, Kebin Chi, Qihan Gong, Guangren Qu, Shipeng Sun, Rongfei Zhou, Weihong Xing. Efficient separation of linear and mono-branched hexane isomers using large-area silicalite-1 membranes. Separation and Purification Technology 354 (2025) 129115. DOI: 10.1016/j.seppur.2024.129115. https://doi.org/10.1016/j.seppur.2024.129115
  11. Hao Yang, Haoyuan Zhang, Zhengxuan Ji, Dongchen Shi, Chunqing Ji, Kexin Yu, Chengjun Kang, Dan Zhao. Isomeric Separation of C6 Alkanes via Organic Solvent Reverse Osmosis with Covalent Organic Framework Membranes. Journal of the American Chemical Society 147 (2025) 14077-14082. DOI: 10.1021/jacs.5c04063. https://doi.org/10.1021/jacs.5c04063
  12. Motomu Sakai, Yuna Koshiishi, Atsuto Okada, Ko Kawanishi, Ko Masuda, Masahiko Matsukata. A Review of Strategies for Developing Zeolite Membranes: From a Microstructure to a Membrane Module. Crystal Growth & Design 25 (2025) 5584-5620. DOI: 10.1021/acs.cgd.5c00457. https://doi.org/10.1021/acs.cgd.5c00457
  13. Tian Gui, Xiaowei Wu, Yuqin Li, Jianggui Tan, Xiaoyan Wang, Fei Zhang, Xiangshu Chen, Hidetoshi Kita. Reproducible scale-up of high-selectivity 80-cm-long NaA zeolite membranes via green gel-less conversion method for dehydration of ethanol. Materials & Design 257 (2025) 114524. DOI: 10.1016/j.matdes.2025.114524. https://doi.org/10.1016/j.matdes.2025.114524