Preparation and pervaporation performance of vinyl-functionalized silica membranes

Template-Free Synthesis of High Dehydration Performance CHA Zeolite Membranes with Increased Si/Al Ratio Using SSZ-13 Seeds

Pervaporation is an energy-efficient alternative to conventional distillation for water/alcohol separations. In this work, a novel CHA zeolite membrane with an increased Si/Al ratio was synthesized in the absence of organic templates for the first time. Nanosized high-silica zeolite (SSZ-13) seeds were used for the secondary growth of the membrane. The separation performance of membranes in different alcohol-aqueous mixtures was measured. The effects of water content in the feed and the temperature on the separation performance using pervaporation and vapor permeation were also studied. The best membrane showed a water/ethanol separation factor above 100,000 and a total flux of 1.2 kg/(m2 h) at 348 K in a 10 wt.% water-ethanol mixed solution. A membrane with high performance and an increased Si/Al ratio is promising for the application of alcohol dehydration.

Microporous framework membranes for precise molecule/ion separations

Microporous framework membranes such as metal-organic framework (MOF) membranes and covalent organic framework (COF) membranes are constructed by the controlled growth of small building blocks with large porosity and permanent well-defined micropore structures, which can overcome the ubiquitous tradeoff between membrane permeability and selectivity; they hold great promise for the enormous challenging separations in energy and environment fields. Therefore, microporous framework membranes are endowed with great expectations as next-generation membranes, and have evolved into a booming research field. Numerous novel membrane materials, versatile manipulation strategies of membrane structures, and fascinating applications have erupted in the last five years. First, this review summarizes and categorizes the microporous framework membranes with pore sizes lower than 2 nm based on their chemistry: inorganic microporous framework membranes, organic-inorganic microporous framework membranes, and organic microporous framework membranes, where the chemistry, fabrications, and differences among these membranes have been highlighted. Special attention is paid to the membrane structures and their corresponding modifications, including pore architecture, intercrystalline grain boundary, as well as their diverse control strategies. Then, the separation mechanisms of membranes are covered, such as diffusion-selectivity separation, adsorption-selectivity separation, and synergetic adsorption-diffusion-selectivity separation. Meanwhile, intricate membrane design to realize synergistic separation and some emerging mechanisms are highlighted. Finally, the applications of microporous framework membranes for precise gas separation, liquid molecule separation, and ion sieving are summarized. The remaining challenges and future perspectives in this field are discussed. This timely review may provide genuine guidance on the manipulation of membrane structures and inspire creative designs of novel membranes, promoting the sustainable development and steadily increasing prosperity of this field.

Progress on Silica Pervaporation Membranes in Solvent Dehydration and Solvent Recovery Processes

Separation processes aimed at recovering the solvent from effluent streams offer a means for establishing a circular economy. Conventional technologies such as distillation are energy-intensive, inefficient and suffer from high operating and maintenance costs. Pervaporation based membrane separation overcomes these challenges and in conjunction with the utilization of inorganic membranes derived from non-toxic, sufficiently abundant and hence expendable, silica, allows for high operating temperatures and enhanced chemical and structural integrity. Membrane-based separation is predicted to dominate the industry in the coming decades, as the process is being understood at a deeper level, leading to the fabrication of tailored membranes for niche applications. The current review aims to compile and present the extensive and often dispersive scientific investigations to the reader and highlight the current scenario as well as the limitations suffered by this mature field. In addition, viable alternative to the conventional methodologies, as well as other rival materials in existence to achieve membrane-based pervaporation are highlighted.

Mesoporous Silica Membranes Silylated by Fluorinated and Non-Fluorinated Alkylsilanes for the Separation of Methyl Tert-Butyl Ether from Water

It is of great significance to separate hazardous methyl tert-butyl ether (MTBE) from water in terms of environmental protection and human health. In the present work, α-Al₂O₃-suppotred silica membranes were prepared by the sol-gel and dip-coating technique. Two fluorinated alkylsilanes (1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) and trifluoropropyltriethoxysilane (TFPTES)) and two non-fluorinated alkylsilanes (octyltriethoxysilane (OTES) and propyltriethoxysilane (PTES)) were adopted to silylate the silica membrane by the post-grafting method which is used for the separation of MTBE from water by pervaporation. The results show that silylation enhances the hydrophobicity of silica membranes. The silylated silica membranes are selective towards MTBE, and the MTBE/water separation factor varies with grafting agents in the order: PFOTES > TFPTES > OTES > PTES. Membranes silylated with fluorinated carbon chains seem to be more selective towards MTBE than those with non-fluorinated carbon chains. The total flux is proportional to the pore volume of silica membranes, which depends on grafting agents in the order: PTES > PFOTES > OTES > TFPTES. Considering both total flux and selectivity, the PFOTES-SiO₂ membrane is most effective in separation, with a MTBE/water separation factor of 24.6 and a total flux of 0.35 kg m^-2 h^-1 under a MTBE concentration of 3.0% and a feed temperature of 30 °C.

Gas Separation Silica Membranes Prepared by Chemical Vapor Deposition of Methyl-Substituted Silanes

The effect on the gas permeance properties and structural morphology of the presence of methyl functional groups in a silica membrane was studied. Membranes were synthesized via chemical vapor deposition (CVD) at 650 °C and atmospheric pressure using three silicon compounds with differing numbers of methyl- and methoxy-functional groups: tetramethyl orthosilicate (TMOS), methyltrimethoxysilane (MTMOS), and dimethyldimethoxysilane (DMDMOS). The residence time of the silica precursors in the CVD process was adjusted for each precursor and optimized in terms of gas permeance and ideal gas selectivity criteria. Final H₂ permeances at 600 °C for the TMOS-, MTMOS-, and DMDMOS-derived membranes were respectively 1.7 × 10^-7, 2.4 × 10^-7, and 4.4 × 10^-8 mol∙m^-2∙s^-1∙Pa^-1 and H₂/N₂ selectivities were 990, 740, and 410. The presence of methyl groups in the membranes fabricated with the MTMOS and DMDMOS precursors was confirmed via Fourier-transform infrared (FTIR) spectroscopy. From FTIR analysis, an increasing methyl signal in the silica structure was correlated with both an improvement in the hydrothermal stability and an increase in the apparent activation energy for hydrogen permeation. In addition, the permeation mechanism for several gas species (He, H₂, Ne, CO₂, N₂, and CH₄) was determined by fitting the gas permeance temperature dependence to one of three models: solid state, gas-translational, or surface diffusion.