Preparation of Ni-MOF-74/SBS mixed matrix membranes and its application of CH4/N2 separation

Naphthalene Tetrazole-Based Nickel Metal-Organic Framework as a Filler of Polycarbonate Membranes to Improve CO2 and H2 Separation

A tetrazole-naphthalene linker was used to prepare a nickel MOF (metal-organic framework) (NiNDTz) with interesting properties: a specific surface area SBET of 320 m2g-1 (SLangmuir 436 m2g-1), high thermal stability (Tdonset = 300 °C), and CO2 uptake of 1.85 mmolg-1, attributed to the tetrazole groups to be used as fillers in gas separation membranes. The role of these groups was crucial in the mechanical properties of mixed membranes prepared using polycarbonate as a polymer matrix, providing a very homogeneous filler distribution and also in the gas separation properties since a simultaneous increase in permeability and selectivity was achieved, especially in the hybrid membrane containing 20% filler (PC@NiNDTz-20%). This membrane exhibited an excellent balance between permeability (P) and selectivity (α) with an increase in the permeability of CO2 and H2, 177 and 185%, respectively, and improvements in the selectivity of these gases against greenhouse gases such as methane and ethylene (between 15 and 28% improvement). These results make this membrane competitive to deal with separations in which these gases are involved, and are of special interest for the H2/CH4 separation since it clearly improves the performance of pure PC and no better PC-based membranes have been reported in the literature for this separation.

Non-CO2 greenhouse gas separation using advanced porous materials

Global warming has become a growing concern over decades, prompting numerous research endeavours to reduce the carbon dioxide (CO2) emission, the major greenhouse gas (GHG). However, the contribution of other non-CO2 GHGs including methane (CH4), nitrous oxide (N2O), fluorocarbons, perfluorinated gases, etc. should not be overlooked, due to their high global warming potential and environmental hazards. In order to reduce the emission of non-CO2 GHGs, advanced separation technologies with high efficiency and low energy consumption such as adsorptive separation or membrane separation are highly desirable. Advanced porous materials (APMs) including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), porous organic polymers (POPs), etc. have been developed to boost the adsorptive and membrane separation, due to their tunable pore structure and surface functionality. This review summarizes the progress of APM adsorbents and membranes for non-CO2 GHG separation. The material design and fabrication strategies, along with the molecular-level separation mechanisms are discussed. Besides, the state-of-the-art separation performance and challenges of various APM materials towards each type of non-CO2 GHG are analyzed, offering insightful guidance for future research. Moreover, practical industrial challenges and opportunities from the aspect of engineering are also discussed, to facilitate the industrial implementation of APMs for non-CO2 GHG separation.

Selective Photonic Gasification of Strained Oxygen Clusters on Graphene for Tuning Pore Size in the Å Regime

Controlling the size of single-digit pores, such as those in graphene, with an Å resolution has been challenging due to the limited understanding of pore evolution at the atomic scale. The controlled oxidation of graphene has led to Å-scale pores; however, obtaining a fine control over pore evolution from the pore precursor (i.e., the oxygen cluster) is very attractive. Herein, we introduce a novel "control knob" for gasifying clusters to form pores. We show that the cluster evolves into a core/shell structure composed of an epoxy group surrounding an ether core in a bid to reduce the lattice strain at the cluster core. We then selectively gasified the strained core by exposing it to 3.2 eV of light at room temperature. This allowed for pore formation with improved control compared to thermal gasification. This is because, for the latter, cluster-cluster coalescence via thermally promoted epoxy diffusion cannot be ruled out. Using the oxidation temperature as a control knob, we were able to systematically increase the pore density while maintaining a narrow size distribution. This allowed us to increase H2 permeance as well as H2 selectivity. We further show that these pores could differentiate CH4 from N2, which is considered to be a challenging separation. Dedicated molecular dynamics simulations and potential of mean force calculations revealed that the free energy barrier for CH4 translocation through the pores was lower than that for N2. Overall, this study will inspire research on the controlled manipulation of clusters for improved precision in incorporating Å-scale pores in graphene.

ZIF-62 glass foam self-supported membranes to address CH4/N2 separations

Membranes with ultrahigh permeance and practical selectivity could greatly decrease the cost of difficult industrial gas separations, such as CH₄/N₂ separation. Advanced membranes made from porous materials, such as metal-organic frameworks, can achieve a good gas separation performance, although they are typically formed on support layers or mixed with polymeric matrices, placing limitations on gas permeance. Here an amorphous glass foam, a_gfZIF-62, wherein a, g and f denote amorphous, glass and foam, respectively, was synthesized by a polymer-thermal-decomposition-assisted melting strategy, starting from a crystalline zeolitic imidazolate framework, ZIF-62. The thermal decomposition of incorporated low-molecular-weight polyethyleneimine evolves CO₂, NH₃ and H₂O gases, creating a large number and variety of pores. This greatly increases pore interconnectivity but maintains the crystalline ZIF-62 ultramicropores, allowing ultrahigh gas permeance and good selectivity. A self-supported circular a_gfZIF-62 with a thickness of 200-330 µm and area of 8.55 cm² was used for membrane separation. The membranes perform well, showing a CH₄ permeance of 30,000-50,000 gas permeance units, approximately two orders of magnitude higher than that of other reported membranes, with good CH₄/N₂ selectivity (4-6).

Transforming 3D CAU-10-H into 2D Materials with High Base Stability for Membrane Separation

Two-dimensional (2D) nanomaterials have received a significant research attention owing to their unique chemical and physical properties. These materials not only provide the chemically active sites and exposed surface atoms, but also display the porous nature suitable for their use as membranes for gas separation. In this study, 3D CAU-10-H has been transformed into a novel alkali stabilized 2D CACl-10 (180). Though CACl-10 (180) is similar to AlOOH, it is a novel 2D nanomaterial synthesized by using 4-chloroisophthalic acid and aluminum nitrate nonahydrate, with thermal decomposition at 300 °C. Further, CACl-10 (180) is noted to retain its framework structure in strong alkali solutions, attributed to the alkali-resistant aluminum hydroxide. At the same time, it has been demonstrated that 3D CAU-10-H can also transform into 3D CACl-10 (140) and 3D CACl-10 (130), and the halogen atoms of the ligands (-Cl) affect the alkali stability of the materials. Subsequently, the PVAm-CACl-10 (180)/MPSf mixed matrix membranes were prepared and applied for CH₄ /N₂ separation. The developed membrane exhibits the CH₄ permeance of 1647.99 GPU with a CH₄ /N₂ selectivity of 3.1. As a result, 2D CACl-10 (180), with a strong alkali stability and an acceptable CH₄ /N₂ membrane separation performance, represents a high potential of application in the membrane separation process.

An overview on trace CO2 removal by advanced physisorbent materials

This review paper focuses on various gas processing technologies and materials that efficiently capture trace levels of carbon dioxide (CO₂). Fundamental separation mechanisms such as absorption, adsorption, and distillation technology are presented. Liquid amine-based carbon capture (C-capture) technologies have been in existence for over half a century, however, liquid amine capture relies upon chemical reactions and is energy-intensive. Liquid amines are thus not economically viable for broad deployment and offer little room for innovation. Innovative C-capture technologies must improve both the environmental footprint and cost-effectiveness. As a promising alternative, physisorbents have many advantages including considerably lower regeneration energy. Generally, existing classes of physisorbent materials, such as metal-organic frameworks (MOFs) and zeolites are selective toward C-capture. However, their selectivity is currently not high enough to remove trace levels (e.g., ~1%) of CO₂ from various natural gas process streams. This review summarizes the current advancements in physisorbent materials for CO₂ capture. Here, key performance parameters needed to select the most suitable candidate are highlighted. Furthermore, this review discusses the scope for the development of better performing CO₂ selective physisorbents from both environmental and economic perspectives. In addition, hybrid ultra microporous materials (HUMs), characterized mainly by ultra-micro pores (<0.7 nm), are discussed in reference to C-capture. Various characteristics of HUMs result in high selectivity and applicability in difficult separations such as the gas sweetening and C-capture from complex humid mixed gas streams.