Organic–Inorganic Hybrid Silica Membranes with Controlled Silica Network Size for Propylene/Propane Separation

Pore size engineering of cost-effective all-nanoporous multilayer membranes for propane/propylene separation

In this study, a new multi-layer hybrid nanocomposite membrane named MFI/GO/ZIF-8 has been synthesized. This membrane combines three nanoporous materials with different morphologies in one membrane without using polymer materials. This allows access to a previously accessible region of very high permeability and selectivity properties. In addition to introducing a new and efficient MFI/GO/ZIF-8 membrane in this work, controlling the pore size of the zeolite layer has been investigated to increase the selectivity and permeability of propylene. The membrane was made using a solvent-free hydrothermal method and a layer-by-layer deposition method. To control the pore size of the MFI layer, a two-step synthesis strategy has been implemented. In the first step, three key parameters, including crystallization time, NaOH concentration and aging time of initial suspension, are controlled. In the second step, the effect of three additional parameters including hydrothermal time, hydrothermal temperature and NH4F concentration has been investigated. The results show that the optimal pore size has decreased from 177.8 nm to 120.53 nm (i.e., 32.2%). The MFI/GO/ZIF-8 membrane with fine-tuned crystal size in the zeolite layer was subjected to detailed tests for propylene selectivity and permeability. The structural characteristics of the membrane were also performed using FT-IR, XRD, FESEM and EDS techniques. The results show that the synergistic interaction between the three layers in the nanocomposite membrane significantly improves the selectivity and permeability of propylene. The permeability and selectivity of propylene increased from 50 to 60 GPU and from 136 to 177, respectively, before and after precise crystal size control. MFI/GO/ZIF-8 membrane by controlling the pore size of the zeolite layer shows a significant increase of 23.1% in selectivity and 16.7% in propylene permeability compared to the initial state. Also, due to the precise synthesis method, the absence of solvent and the use of cheap support, the prepared membrane is considered an environmentally friendly and low-cost membrane. This study emphasizes the potential of increasing the selectivity and permeability of propylene in the MFI/GO/ZIF-8 hybrid membrane by controlling the crystal size of the zeolite layer.

A Brief Overview of the Microstructural Engineering of Inorganic-Organic Composite Membranes Derived from Organic Chelating Ligands

This review presents a concise conceptual overview of membranes derived from organic chelating ligands as studied in several works. The authors' approach is from the viewpoint of the classification of membranes by matrix composition. The first part presents composite matrix membranes as a key class of membranes and makes a case for the importance of organic chelating ligands in the formation of inorganic-organic composites. Organic chelating ligands, categorized into network-modifying and network-forming types, are explored in detail in the second part. Four key structural elements, of which organic chelating ligands (as organic modifiers) are one and which also include siloxane networks, transition-metal oxide networks and the polymerization/crosslinking of organic modifiers, form the building blocks of organic chelating ligand-derived inorganic-organic composites. Three and four parts explore microstructural engineering in membranes derived from network-modifying and network-forming ligands, respectively. The final part reviews robust carbon-ceramic composite membranes as important derivatives of inorganic-organic hybrid polymers for selective gas separation under hydrothermal conditions when the proper organic chelating ligand and crosslinking conditions are chosen. This review can serve as inspiration for taking advantage of the wide range of possibilities presented by organic chelating ligands.

Recent Advances in Catalysts and Membranes for MCH Dehydrogenation: A Mini Review

Methylcyclohexane (MCH), one of the liquid organic hydrogen carriers (LOHCs), offers a convenient way to store, transport, and supply hydrogen. Some features of MCH such as its liquid state at ambient temperature and pressure, large hydrogen storage capacity, its well-known catalytic endothermic dehydrogenation reaction and ease at which its dehydrogenated counterpart (toluene) can be hydrogenated back to MCH and make it one of the serious contenders for the development of hydrogen storage and transportation system of the future. In addition to advances on catalysts for MCH dehydrogenation and inorganic membrane for selective and efficient separation of hydrogen, there are increasing research interests on catalytic membrane reactors (CMR) that combine a catalyst and hydrogen separation membrane together in a compact system for improved efficiency because of the shift of the equilibrium dehydrogenation reaction forwarded by the continuous removal of hydrogen from the reaction mixture. Development of efficient CMRs can serve as an important step toward commercially viable hydrogen production systems. The recently demonstrated commercial MCH-TOL based hydrogen storage plant, international transportation network and compact hydrogen producing plants by Chiyoda and some other companies serves as initial successful steps toward the development of full-fledged operation of manufacturing, transportation and storage of zero carbon emission hydrogen in the future. There have been initiatives by industries in the development of compact on-board dehydrogenation plants to fuel hydrogen-powered locomotives. This review mainly focuses on recent advances in different technical aspects of catalytic dehydrogenation of MCH and some significant achievements in the commercial development of MCH-TOL based hydrogen storage, transportation and supply systems, along with the challenges and future prospects.

Recent Progress in a Membrane-Based Technique for Propylene/Propane Separation

The similar physico-chemical properties of propylene and propane molecules have made the separation process of propylene/propane challenging. Membrane separation techniques show substantial prospects in propylene/propane separation due to their low energy consumption and investment costs, and they have been proposed to replace or to be combined with the conventional cryogenic distillation process. Over the past decade, organosilica membranes have attracted considerable attention due to their significant features, such as their good molecular sieving properties and high hydrothermal stability. In the present review, holistic insight is provided to summarize the recent progress in propylene/propane separation using polymeric, inorganic, and hybrid membranes, and a particular inspection of organosilica membranes is conducted. The importance of the pore subnano-environment of organosilica membranes is highlighted, and future directions and perspectives for propylene/propane separation are also provided.

A Bibliometric Survey of Paraffin/Olefin Separation Using Membranes

Bibliometric studies allow to collect, organize and process information that can be used to guide the development of research and innovation and to provide basis for decision-making. Paraffin/olefin separations constitute an important industrial issue because cryogenic separation methods are frequently needed in industrial sites and are very expensive. As a consequence, the use of membrane separation processes has been extensively encouraged and has become an attractive alternative for commercial separation processes, as this may lead to reduction of production costs, equipment size, energy consumption and waste generation. For these reasons, a bibliometric survey of paraffin/olefin membrane separation processes is carried out in the present study in order to evaluate the maturity of the technology for this specific application. Although different studies have proposed the use of distinct alternatives for olefin/paraffin separations, the present work makes clear that consensus has yet to be reached among researchers and technicians regarding the specific membranes and operation conditions that will make these processes scalable for large-scale commercial applications.

Fabrication and Evaluation of Trimethylmethoxysilane (TMMOS)-Derived Membranes for Gas Separation

Gas separation membranes were fabricated with varying trimethylmethoxysilane(TMMOS)/tetraethoxy orthosilicate (TEOS) ratios by a chemical vapor deposition (CVD) method at650 °C and atmospheric pressure. The membrane had a high H₂ permeance of 8.3 × 10^-7 mol m^-2 s^-1Pa^-1 with H2/CH4 selectivity of 140 and H₂/C₂H₆ selectivity of 180 at 300 °C. Fourier transforminfrared (FTIR) measurements indicated existence of methyl groups at high preparationtemperature (650 °C), which led to a higher hydrothermal stability of the TMMOS-derivedmembranes than of a pure TEOS-derived membrane. Temperature-dependence measurements ofthe permeance of various gas species were used to establish a permeation mechanism. It was foundthat smaller species (He, H2, and Ne) followed a solid-state diffusion model while larger species (N₂,CO₂, and CH₄) followed a gas translational diffusion model.

Organosilica-Based Membranes in Gas and Liquid-Phase Separation

Organosilica membranes are a type of novel materials derived from organoalkoxysilane precursors. These membranes have tunable networks, functional properties and excellent hydrothermal stability that allow them to maintain high levels of separation performance for extend periods of time in either a gas-phase with steam or a liquid-phase under high temperature. These attributes make them outperform pure silica membranes. In this review, types of precursors, preparation method, and synthesis factors for the construction of organosilica membranes are covered. The effects that these factors exert on characteristics and performance of these membranes are also discussed. The incorporation of metals, alkoxysilanes, or other functional materials into organosilica membranes is an effective and simple way to improve their hydrothermal stability and achieve preferable chemical properties. These hybrid organosilica membranes have demonstrated effective performance in gas and liquid-phase separation.

Mixed matrix membranes for hydrocarbons separation and recovery: a critical review

The separation and purification of light hydrocarbons are significant challenges in the petrochemical and chemical industries. Because of the growing demand for light hydrocarbons and the environmental and economic issues of traditional separation technologies, much effort has been devoted to developing highly efficient separation techniques. Accordingly, polymeric membranes have gained increasing attention because of their low costs and energy requirements compared with other technologies; however, their industrial exploitation is often hampered because of the trade-off between selectivity and permeability. In this regard, high-performance mixed matrix membranes (MMMs) are prepared by embedding various organic and/or inorganic fillers into polymeric materials. MMMs exhibit the advantageous and disadvantageous properties of both polymer and filler materials. In this review, the influence of filler on polymer chain packing and membrane sieving properties are discussed. Furthermore, the influential parameters affecting MMMs affinity toward hydrocarbons separation are addressed. Selection criteria for a suitable combination of polymer and filler are discussed. Moreover, the challenges arising from polymer/filler interactions are analyzed to allow for the successful implementation of this promising class of membranes.

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations

As primary feedstocks in the petrochemical industry, light olefins such as ethylene and propylene are mainly obtained from steam cracking of naphtha and short chain alkanes (ethane and propane). Due to their similar physical properties, the separations of olefins and paraffins-pivotal processes to meet the olefin purity requirement of downstream processing-are typically performed by highly energy-intensive cryogenic distillation at low temperatures and high pressures. To reduce the energy input and save costs, adsorptive olefin/paraffin separations have been proposed as promising techniques to complement or even replace cryogenic distillation, and growing efforts have been devoted to developing advanced adsorbents to fulfill this challenging task. In this Review, a holistic view of olefin/paraffin separations is first provided by summarizing how different processes have been established to leverage the differences between olefins and paraffins for effective separations. Subsequently, recent advances in the development of porous materials for adsorptive olefin/paraffin separations are highlighted with an emphasis on different separation mechanisms. Last, a perspective on possible directions to push the limit of the research in this field is presented.

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.

Tuning the pore sizes of novel silica membranes for improved gas permeation properties via an in situ reaction between NH3 and Si–H groups

The tuning of pore sizes in triethoxysilane (TRIES)-derived membranes via an in situ reaction between NH₃ and Si–H groups at high temperatures.