Atmospheric-Pressure Plasma-Induced In Situ Polymerization of Liquid Silsesquioxane Monomer for the Synthesis of Polysilsesquioxane Nanocomposite Membranes with Sub-Nanometer Pores for Molecular Separation

Plasma-induced polymerization of liquid silsesquioxane monomer (1,2-bis(triethoxysilyl)ethane, BTESE) at the gas–liquid interface was achieved at room temperature by employing an atmospheric-pressure plasma jet. The polymerization of the liquid monomer under exposure to atmospheric-pressure plasma resulted in the formation of a thin polysilsesquioxane layer at the immediate surface of the porous support. The confined polymerization at the plasma-liquid interface was demonstrated to be beneficial for preparing nanocomposite membranes with a thin selective layer on top of the porous support. The BTESE-derived polysilsesquioxane membrane exhibited high selectivity for gas separation (H2/CH4 = 195) based on the molecular sieving mechanism

Fluorine Doping of Microporous Organosilica Membranes for Pore Size Control and Enhanced Hydrophobic Properties

Fluorine-doped organosilica membranes for gas and pervaporation (PV) separation were fabricated using a sol–gel method. NH₄F and bis­(triethoxysilyl)­methane (BTESM) were selected as the dopant and Si precursor, respectively, for the fabrication of fluorine-doped organosilica membranes. Doping with fluorine was evaluated for its effect on the physicochemical properties of organosilica (hydrophobicity/hydrophilicity and network size). Fluorine doping dramatically eliminated the formation of Si–OH groups in the sol, so that the condensation of Si–OH groups during the calcination process was suppressed. It is possible that fluorine doping enlarged the network pore sizes in organosilica, because the F-BTESM (F/Si = 1/9) membrane showed superior He and H₂ permeance with a low H₂/N₂ permeance ratio that corresponded to the network pore size by comparison with an undoped BTESM membrane. The F-BTESM (F/Si = 1/9) membranes clearly showed a high level of C₃H₆ permeance (>3.0 × 10^–7 mol m^–2 s^–1 Pa^–1) with a high C₃H₆/SF₆ permeance ratio (∼250), which suggests that the network pore size of F-BTESM is suitable for the separation of large molecules such as hydrocarbon gases (C3/C4, C4 isomer, etc.). Organosilica membranes both with and without fluorine doping showed stable PV performance because of the fact that H₂O permeance and each permeance ratio under different separation systems was approximately constant over 10 h at 70 °C. Fluorine doping enhanced the hydrophobic nature of the organosilica, which was confirmed by the H₂O adsorption and PV properties

Tailoring a Thermally Stable Amorphous SiOC Structure for the Separation of Large Molecules: The Effect of Calcination Temperature on SiOC Structures and Gas Permeation Properties

A SiOC membrane with high oxidative stability for gas separation was tailored by utilizing vinyltrimethoxysilane, triethoxysilane, and 1,1,3,3-tetramethyldisiloxane as Si precursors. Amorphous SiOC networks were formed via the condensation of Si–OH groups, the hydrosilylation of Si–H and Si–CHCH₂ groups, and a crosslinking reaction of Si–CH₃ groups, respectively. The crosslinking of Si–CH₃ groups at temperatures ranging from 600 to 700 °C under a N₂ atmosphere was quite effective in constructing a Si–CH₂–Si unit without the formation of mesopores, which was confirmed by the results of N₂ adsorption and by the gas permeation properties. The network pore size of the SiOC membrane calcined at 700 °C under N₂ showed high oxidative stability at 500 °C and was appropriate for the separation of large molecules (H₂/CF₄ selectivity: 640, H₂/SF₆: 2900, N₂/CF₄: 98). A SiOC membrane calcined at 800 °C showed H₂/N₂ selectivity of 62, which was approximately 10 times higher than that calcined at 700 °C because the SiOC networks were densified by the cleavage and redistribution reactions of Si–C and Si–O groups

Nanogradient Hydrophilic/Hydrophobic Organosilica Membranes Developed by Atmospheric-Pressure Plasma to Enhance Pervaporation Performance

Organosilica membranes are a promising candidate for pervaporation dehydration owing to their tunable molecular sieving characteristics and excellent hydrothermal stability. Herein, we report a facile modification using an atmospheric-pressure water vapor plasma to enhance the pervaporation performance of organosilica membranes. The surface of methyl-terminated organosilica membranes was treated by water vapor plasma to develop an ultrathin separation active layer suitable for pervaporation dehydration. The surface hydrophilicity was increased by water vapor plasma due to oxidative decomposition of methyl groups to form silanol groups. The plasma-modified layer had a thickness of several nanometers and had a silica-like structure due to the condensation of silanol groups. The plasma-modified organosilica membranes exhibited an improved molecular sieving property owing to the formation of highly cross-linked siloxane networks with a pore size of approximately 0.4 nm. The membranes also exhibited an excellent permselectivity in the dehydration of alcohols due to the nanometer-thick separation active layer with controlled pore size and increased hydrophilicity. The plasma-modified membranes showed high H2O permeance exceeding 10–6 mol m–2 s–1 Pa–1 with permeance ratios for H2O/EtOH and H2O/IPA of 517–3050 and >10 000, respectively, in the dehydration of 90 wt % aqueous solutions at 50 °C, which is among the highest permselectivities for silica-based membranes. Furthermore, the plasma-modified membranes displayed highly efficient dehydration performance for a H2O/MeOH mixture. The H2O permeance and H2O/MeOH permeance ratio in the dehydration of a 90 wt % MeOH aqueous solution at 50 °C were (2.3–3.0) × 10–6 mol m–2 s–1 Pa–1 and 31–143, respectively, which exceeded the permeance-selectivity trade-off of conventional membranes including polymeric, silica-based, and zeolite membranes. The results indicate that the proposed plasma-assisted approach can enhance the pervaporation performance of organosilica membranes via the modification under atmospheric pressure and at room temperature

Effects of Calcination Condition on the Network Structure of Triethoxysilane (TRIES) and How Si–H Groups Influence Hydrophobicity Under Hydrothermal Conditions

Network size control was evaluated for microporous membranes derived from triethoxysilane (TRIES) that contains highly reactive Si–H groups. It was possible to control the concentration of the Si–H groups via the conditions of calcination (temperature, atmosphere). Si–H groups remained within their network structure when the TRIES membrane was calcined at 350 °C under a N2 atmosphere, and had a loose network structure (H2 permeance: 5.40 × 10–7 mol m–2 s–1 Pa–1, H2/CH4 selectivity: 36). When calcination at high temperatures converted the Si–H groups to Si–O–Si groups, the TRIES membrane showed a high level of separation performance (H2 permeance: 2.34 × 10–7 mol m–2 s–1 Pa–1, H2/CH4 selectivity: 590) due to a densification of the network structure. Compared with conventional microporous silica membranes, a TRIES membrane with Si–H groups showed hydrophobic properties, but water vapor was adsorbed and/or capillary-condensed in the microporous structure, and permeation blocking for He molecules was observed at temperatures below 150 °C in the presence of saturated water vapor at 25 °C. Hydrophobic Si–H groups improved the hydrothermal stability at 300 °C, but depending on the partial pressure of the steam, the reaction between Si–H groups and water vapor degraded the hydrothermal stability of the TRIES membranes

Tailoring the Separation Behavior of Polymer-Supported Organosilica Layered-Hybrid Membranes via Facile Post-Treatment Using HCl and HN₃ Vapors

A promising layered-hybrid membrane consisting of a microporous organosilica active layer deposited onto a porous polymer support was prepared via a facile sol–gel spin-coating process. Subsequently, the pore sizes and structures of the organosilica top layers on the membrane surface were tuned at mild temperature combined with vapor treatment from either hydrochloric acid (HVT) or ammonia (AVT), thereby tailoring the desalination performance of the membranes during reverse osmosis (RO) processing. The effects of HVT and AVT on the pore size, structure, and morphology of organosilica layers and on the separation performances of membranes were investigated in detail. We confirmed that both HVT and AVT processes accelerated the condensation of silanol (SiOH) in the organosilica layer, which led to dense silica networks. The layered-hybrid membranes after HVT showed an improved salt rejection and reduced water flux, while membranes after AVT exhibited a decrease in both salt rejection and water permeability. We found that HVT gave rise to smoother and denser organosilica layers, while AVT produced large voids and formed pinholes due to Ostwald ripening. These conclusions were supported by a comparative analysis of the results obtained via FTIR, TG-MS, SPM, and RO desalination

Physicochemical Treatments of Graphene Oxide to Improve Water Vapor/Gas Separation Performance of Supported Laminar Membranes: Sonication and H₂O₂ Oxidation

We investigated the previously unexplored domain of water vapor/gas separation using graphene oxide (GO) membranes, expecting future applications, including gas dehumidifiers and superior humidity controllers. While the importance of manipulation of GO nanosheet size and surface chemistry in traditional water purification and gas separation has been acknowledged, their potential impact on water vapor/gas separation remained unexplored until now. We applied sonication and hydrogen peroxide treatments to GO water dispersions and systematically evaluated the size and surface chemistry of each GO nanosheet. Both treatments reduced the GO nanosheet size to shorten the diffusion length, which improved water permeance. In addition, hydrogen peroxide treatment improved the hydrophilicity of the nanosheet. Our novel findings demonstrate that optimization of GO nanosheet size and the increase in their hydrophilicity via hydrogen peroxide treatments for 5 h significantly enhance water permeance, leading to a remarkable water vapor permeance of 4.6 × 10–6 mol/(m2 s Pa) at 80 °C, a 3.1-fold improvement over original GO membranes, while maintaining a water vapor/nitrogen permeance ratio exceeding 10,000. These results not only provide important insights into the nature of water vapor/gas separation but also suggest innovative methods for optimizing the GO membrane structure

Metal-Induced Aminosilica Rigidity Improves Highly Permeable Microporous Membranes via Different Types of Pendant Precursors

In this study, nickel-doped aminosilica membranes containing pendant groups were prepared with 3-aminopropyltriethoxysilane (APTES), trimethoxy[3-(methylamino)propyl]silane (MAPTS), 3 N,N-dimethyl aminopropyltrimethoxysilane (DAPTMS), N-[3-(trimethoxysilylpropyl]ethylene diamine (TMSPED), and 1-[3-(trimethoxysilyl)propyl] urea (TMSPU). Differences in the structures of terminal amine ligands significantly contributed to the formation of a coordinated structural assembly. Ultraviolet–visible spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and N2 adsorption isotherms revealed that short and rigid pendant amino groups successfully coordinated with nickel to produce subnanopores in the membranes, while an ion-exchange interaction was suggested for longer and sterically hindered aminosilica precursors. Moreover, the basicity of amine precursors affected the affinity of ligands for the development of a coordinated network. A pristine aminosilica membrane showed low levels of H2 permeance that range from 0.1 to 0.5 × 10–6 mol m–2 s–1 Pa–1 with a H2/N2 permeance ratio that ranges from 15 to 100. On the contrary, nickel coordination increased the H2 permeance to 0.1–3.0 × 10–6 mol m–2 s–1 Pa–1 with H2/N2 permeance ratios that range from 10 to 68, which indicates the formation of a microporous structure and enlargement of pore sizes. The strong level of coordination affinity between nickel ions and amine groups induced rearrangement of the flexible pendant chain into a more rigid structure

CO₂ Permeation through Hybrid Organosilica Membranes in the Presence of Water Vapor

Hybrid organosilica membranes have become attractive for industrial applications because of high performance and long-term stability. This work investigated the influence of water vapor on CO2 gas permeation through the hybrid membranes. Two types of organoalkoxysilanes, bis­(triethoxysilyl)­ethane (BTESE) and bis­(triethoxysilyl)octane (BTESO), were used as precursors to prepare membranes via the sol–gel method. The two membranes showed distinct properties of porosity and water affinity because of the differences in the bridging methylene numbers between the two Si atoms. Under dry conditions, the BTESE and BTESO membranes showed CO2 permeances as high as 7.66 × 10–7 and 6.63 × 10–7 mol m–2 s–1 Pa–1 with CO2/N2 selectivities of 36.1 and 12.6 at 40 °C, respectively. In the presence of water vapor, CO2 permeance was decreased for both membranes, but the effect of water vapor on CO2 permeation was slighter for BTESO membranes than it was for BTESE membranes because of more hydrophobicity and denser structures with a longer linking-bridge group. The hybrid organosilica membranes both showed good reproducibility and stability in water vapor