N2-selective adsorbents and membranes for natural gas purification

Enhanced CH4/N2 Separation Efficiency of UiO-66-Br2 through Hybridization with Mesoporous Silica

Efficient separation of CH4 from N2 is essential for the purification of methane from nitrogen. In order to address this problem, composite materials consisting of rod-shaped SBA-15-based UiO-66-Br2 were synthesized for the purpose of separating a CH4/N2 mixture. The materials were characterized via PXRD, N2 adsorption-desorption, SEM, TEM, FT-IR, and TGA. The adsorption isotherms of CH4 and N2 under standard pressure conditions for the composites were determined and subsequently compared. The study revealed that the composites were formed through the growth of MOF nanocrystals on the surfaces of the SBA-15 matrix. The enhancements in surface area and adsorption capacity of hybrid materials were attributed to the structural modifications resulting from the interactions between surface silanol groups and metal centers. The selectivity of the composites towards a gas mixture of CH4 and N2 was assessed utilizing the Langmuir adsorption equation. The results of the analysis revealed that the U6B2S5/SBA-15 sample exhibited the greatest selectivity for CH4/N2 adsorption compared to the other samples, with an adsorption selectivity parameter (S) of 20.06. Additional research is necessary to enhance the enrichment of methane from CH4/N2 mixtures using SBA-15-based metal-organic framework materials.

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.

Screening of Alkali Metal-Exchanged Zeolites for Nitrogen/Methane Separation

Methane (CH4) is the primary component of natural gas and must be purified to a certain level before it can be used as pipeline gas or liquified natural gas (LNG). In particular, nitrogen (N2), a common contaminant in natural gas needs to be rejected to increase the heating value of the gas and meet the LNG product specifications. The development of energy-efficient N2 removal technologies is hampered by N2's inertness and its resemblance to CH4 in terms of kinetic size and polarizability. N2-selective materials are so rare. Here, for the first time, we screened 1425 alkali metal cation exchange zeolites to identify the candidates with the best potential for the separation of N2 from CH4. We discovered a few extraordinary zeolite frameworks capable of achieving equilibrium selectivity toward N2. Particularly, Li+-RRO-3 zeolite with a specific two-dimensional structure demonstrated a selective N2 adsorption capacity of 2.94 mmol/g at 283 K and 1 bar, outperforming the capacity of all known zeolites. Through an ab initio density functional theory study, we found that the five-membered ring of the RRO framework is the most stable cationic site for Li+, and this Li+ can interact with multiple N2 molecules but only one CH4, revealing the mechanism for the high capacity and selectivity of N2. This work suggests promising adsorbents to enable N2 rejection from CH4 in the gas industry without going for energy-intensive cryogenic distillations.