On October 21, 2022, Professor Zhiping Zhao's team from the School of Chemistry and Chemical Engineering of Beijing Institute of Technology published the paper High-flexible and superhydrophobic MOF nanosheet membrane for ultrafast alcohol-water separation inScience, which proposed a new strategy of precisely constructing MOF nanosheet membranes by surface crystal-induced growth method by encapsulating crystalline species on polymer substrates. By realizing the hierarchical construction of highly flexible superhydrophobic MOF membranes on the surface of polymer substrate, they resolved the crystal structure of nanosheets and their internal molecular transport channels, revealed the synergistic mechanism between polymer and nanosheet layers in the separation process, and broke through the bottleneck of flexible MOF membrane preparation, which provided theoretical basis and technical support for the large-scale preparation and application.

The first authors of this work are LiHao Xu and ShenHui Li, PhD students in the School of Chemistry and Chemical Engineering, Beijing University of Technology, and the corresponding authors are Zhiping Zhao and YingNan Feng, with Beijing University of Technology as the first author and sole corresponding author.

01

Background

The separation process is the most concentrated part of the chemical industry in terms of energy consumption, investment and cost, accounting for 40-70% of investment and cost, and more than 10% of the world's energy consumption, and is also an indispensable part of the energy, environmental, food and biomedical fields. The pervaporation membranes separation technology has great potential in industrial separation applications, which can save 30-60% of energy. It is not only a key technology to support sustainable development, but also plays a pivotal role in the process of achieving China’s carbon peaking and carbon neutrality goals. Overcoming the permeability-selectivity trade-off and developing high- performance separation membranes is the unremitting pursuit of scientists in the field of membrane science and technology.

In recent years, substrate-supported heteroepitaxial metal-organic framework (MOF) membranes have shown great potential for separation applications. Current methods mostly prepare MOF membranes on rigid inorganic substrates. To break the technical bottleneck of difficult membrane scale-up preparation and poor flexibility in membrane component processing and fabrication, Prof. Zhiping Zhao's team has prepared a highly flexible metal-organic framework nanosheet (MOF-NS) membranes, starting from breaking through the technical bottlenecks.

02

Fabrication and structure analysis of MOF nanosheet membranes

To solve the surface interface bonding problem between the MOF layer and the polymer substrate, the research team co-blended ZIF-8 crystals into the polymer casting solution and used the Nonsolvent Induce Phase Separation (NIPS) method to ingeniously prepare polyvinylidene fluoride (SEEDS/PVDF) membranes with bud-like crystals embedded in the polymer substrate. The bud-like seed embedded in the polymer substrate not only becomes the anchor point connecting the MOF nanosheet with the polymer, but also lays the foundation for the growth of the nanosheet with its unique petal-like structure. On this basis, a complete honeycomb-like MOF nanosheet membranes (MOF-NS/PVDF) was fabricated by inducing MOF-limited growth. The crystal structure of the MOF nanosheets and their internal molecular transport channels were resolved by X-ray diffraction (XRD) and Monte Carlo molecular simulation methods, and their topology is based on [Zn2(MeIm)4]n with a thickness of 0.525 nm as a lattice-like plane containing 0.435 nm subnanometer interlayer channels, indicating the lattice distortion of ZIF-8 during membrane formation.

Fig. 1. Preparation protocols and structures of the MOF-NS/PVDF membrane. (A) Surface morphologies of the membranes (MOF-NS/PVDF membrane prepared from SEEDS/PVDF substrate after 1 h, 3 h and 6 h growth, respectively). (B) Schematic of the membrane fabrication process for SEEDS/PVDF and MOF-NS/PVDF. (C) XRD patterns of the pristine PVDF, SEEDS/PVDF, MOF-NS/PVDF, and simulated [Zn2(MeIm)4]n membranes.

Fig. 2. Molecular transport channels of the MOF-NS membrane. (A) One-dimensional channel and pore diameter of Zn2(MeIm)4. (B) HR-TEM images of MOF-NS nanosheets. (C) Lamellar structure and interlayer channels of MOF-NS.

03

MOF nanosheet shows excellent flexibility

The reversible deformations (i.e. flip, twist, and sway of the MOF nanosheet) was first observed by adjusting the electron beam bombardment density in the observation area under the electron microscope, and the thickness of the nanosheet was about 13 nm. MOF nanosheets exhibit a good lattice structure different from ZIF-8 under transmission electron microscopy. The lamellar structure of honeycomb-like MOF nanosheets and their internal continuous channels enable them to show ultra-high permeability during the permeation vaporization process.

Fig. 3. Highly flexible structure of MOF-NS/PVDF membrane. (A) SEM images (including flip, twist, and sway) of morphology variations of MOF-NS/PVDF under electron beam bombardment. (B) Illustration of flexible reversible dynamic deformation of MOF-NS/PVDF membrane. (C) Bending resistance test of MOF-NS/PVDF. (D) Surface and cross-section morphologies of MOF-NS/PVDF after bending resistance test.

04

MOF nanosheet membrane defects modification

The MOF nanosheet membrane (MOF-NS/PVDF) was modified by titration coating with polydimethylsiloxane (PDMS) solution to form a PDMS coating with a honeycomb structure, which not only repaired the molecular scale defects between MOF nanosheets, but also realized the transformation of membrane surface properties from superhydrophilic to superhydrophobic (with a water contact angle of 158.3°), and constructed a bifunctional membrane (PDMS/MOF-NS/PVDF) with both superhydrophobic surface properties and MOF-NS fast molecular diffusion channels inside the membrane. The bifunctional membrane (PDMS/MOF-NS/PVDF) with superhydrophobic surface properties and fast molecular diffusion channels of MOF-NS in the membrane was constructed.

Fig. 4. Titration coating protocols, structures, and surface characteristics of the PDMS-modified MOF-NS/PVDF membrane. (A) Schematic of titration coating. (B) Changes of surface morphologies before and after titration coating modification.

05

MOF nanosheet membrane separation mechanism and performance

PDMS/MOF-NS/PVDF composite membrane permeation vapor separation tests and molecular simulations reveal the synergistic mechanism of PDMS and MOF nanosheet layers in the ethanol-water separation process. Firstly, the pro-organic PDMS layer prevents water molecules from dissolving and permeating while alcohol molecules are preferentially dissolved and permeated; the dimethylimidazole in the lamellar structure of MOF nanosheets selectively adsorbs alcohol molecules permeating PDMS, forming a secondary selection to improve the separation factor, while its internal continuous pore structure becomes a fast channel for molecular transfer, reducing the transportation resistance. In addition, the honeycomb-like structure of the membrane surface increases the effective contact area with the feed and promotes the permeate flux enhancement. During the separation process, the sub-nanoscale channels exhibit molecular sieving and retention of larger molecules of butanol. The PDMS-MOF nanosheet composite layer constructed on the polymer substrate not only enhanced the molecular mass transfer within the membrane, but also effectively promoted the turbulence near the membrane surface, mitigating the concentration and temperature polarization phenomena in the permeate vaporization process, and thus significantly improved the separation performance of the composite membrane, with permeate flux and separation factor 13.6 and 1.2 times higher than those of PDMS/PVDF membranes prepared by conventional methods, respectively.

Fig. 5. Pervaporation performances and the morphology effect on flow behavior of feed. (A) Pervaporation performance for separating 5 wt% ethanol-water solutions at 40°C.（B）Comparison of separation performance. (C) Long-term separation performance of PDMS/MOF-NS/PVDF-15s. (D and E) Flow field around the membrane surface.

P.S.: This research was supported by the Key Project of National Natural Science Foundation of China, the National Key Research and Development Program of China, and the Young Faculty Academic Initiation Program of Beijing Institute of Technology.

About the authors

Zhiping Zhao

Professor of School of Chemistry and Chemical Engineering, Beijing Institute of Technology

He is currently the Director of Institute of Chemical Engineering, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, responsible professor of Chemical Engineering and Technology, responsible professor of National First-class Undergraduate Program, and head of Advanced Functional Membranes and Membrane Processes Team. He is also the chairman of the Beijing Membrane Society, the editorial board member of Advanced Membranes and Membrane Science and Technology. He was awarded the title of "National Excellent Teaching Team of Petroleum and Chemical Education" (2021) As the leader, and the "Second Prize of National Excellent Teaching Achievement of Petroleum and Chemical Education" (2022) as the presenter. He has been engaged in the design and preparation of new functional membrane materials, biomass conversion and separation processes, new membrane reactor design and development, and related application demonstrations, with biomass and volatile organic compounds (VOCs) as the research objects and efficient conversion and separation of biomass as well as efficient treatment of water and gas as the goal. As the project leader, he has presided over 10 projects of the National Natural Science Foundation of China, National Key Research and Development Program, and provincial and ministerial projects, etc. He has published inScience,AIChE J,J. Membr. Sci.,Chem. Eng. J.,Chem. Eng. Sci.,Ind. & Eng. Chem. Res.,Sep. & Purif. Tech.,DesalinationandJ. Mater. Chem. A, etc. He has published more than 80 SCI-indexed academic papers and granted more than 10 invention patents in international top journals.

Yingnan Feng

Associate Researcher of School of Chemistry and Chemical Engineering, Beijing Institute of Technology

He graduated from National University of Singapore (NUS) in 2019 under the supervision of Prof. Chung Tai-Shung, and then continued his postdoctoral research at NUS before joining the Advanced Functional Membranes and Membrane Processes team in the School of Chemistry and Chemical Engineering of BIT in 2021. His research work focuses on the preparation of high-performance organic membranes, including the investigation of membrane formation mechanism, membrane structure regulation and surface property modulation, and the construction of super-impregnated organic membrane surfaces. He has been involved in a number of key projects of the National Science Foundation of Singapore and collaborative projects with companies. He has published more than 10 SCI papers in top international journals such asScience,J. Membr. Sci.,Chem. Eng. Sci.,Ind. & Eng. Chem. Res.,Sep. & Purif. Tech.andDesalination, and edited an English monograph on hollow fiber membranes.

Lihao Xu

He has been pursuing his PhD in Chemical Engineering and Technology at BIT since September 2018 under the supervision of Prof. Zhiping Zhao, and received his PhD in Engineering in September 2022, and stayed in the university as a postdoctoral researcher after graduation. Mainly engaged in the research of membrane material design and preparation, he has participated in the key projects of National Natural Science Foundation of China and National Key Research and Development Program topics, and has published 5 SCI-indexed papers on journals including inScience,J. Membr. Sci.andJ. Mater. Chem. Aas the first author, participated in 10 SCI-indexed papers, and obtained 5 authorized invention patents.

Shenhui Li

He started his M.S. degree in chemical engineering at Beijing Institute of Technology under the supervision of Prof. Zhiping Zhao in September 2018 and has been pursuing his Ph.D. degree in chemical engineering and technology at BIT since September 2021. His research interests include membrane material design and molecular simulation of membrane separation mechanism. He has participated in the key projects of National Natural Science Foundation of China and National Key Research and Development Program topics, and has co-authored 16 SCI-indexed papers.