Metal–Organic Frameworks for CO₂ Separation from Flue and Biogas Mixtures

Abstract

Great efforts have been made toward the separation of CO 2 from flue gas and biogas to mitigate environmental pollution and the demand for renewable fuels, respectively. Nonthermal-based separations, such as adsorption-based or membrane-separation technology employing porous materials, are considered to be more promising than traditional cryogenic and absorption-based systems. Due to several advantages of metal-organic frameworks (MOFs) over other conventional porous materials, reports on flue and biogas separation by MOFs are burgeoning (423 for adsorption and 56 for membrane-based separations until June 2021) and demand urgent summarization. This review presents a bird's eye view on such separations while organizing the developed strategies and considering several performance parameters, such as tradeoff between sorption capacity and separation selectivity in adsorption-based systems and permeability versus separation selectivity in membrane-based systems. In addition, the mechanisms involving such separations at the molecular level are presented. A critical discussion section offers more crucial insights into these materials from industrial deployment viewpoints. Finally, future recommendations are made for further developments of MOF materials as flue and biogas separators and thus toward solving both the challenging universal problems of global warming and energy scarcity simultaneously.

Largely Entangled Diamondoid Framework with High-Density Urea and Divergent Metal Nodes for Selective Scavenging of CO2 and Molecular Dimension-Mediated Size-Exclusive H-Bond Donor Catalysis

“…The pore wall decoration in this framework with polar functional groups (−N�N− and −NHCONH−) and the presence of ample π-electron cloud motivated us to look into its gas adsorption capacity. 10 Prior to gas uptake studies, the N 2 adsorption isotherm of 17a was recorded at 77 K up to a relative pressure of 1 (p/p 0 = 1). The adsorption profile exhibited a type II isotherm with hysteresis (Figure S7a), accounting for a saturated nitrogen uptake of 83.92 cm 3 /g (3.75 mmol/g) that corresponds to the Brunauer−Emmett− Teller (BET) surface area of 189 m 2 /g.…”

“…The pore wall decoration in this framework with polar functional groups (−NN− and −NHCONH−) and the presence of ample π-electron cloud motivated us to look into its gas adsorption capacity . Prior to gas uptake studies, the N 2 adsorption isotherm of 17a was recorded at 77 K up to a relative pressure of 1 ( p / p 0 = 1).…”

“…Precisely, use of porous adsorbents is ideal for CO 2 capture through physisorption, as the cost of regeneration will be lower. In this milieu, metal–organic frameworks (MOFs) received momentous attention for a vast range of potential applications because of their three-dimensional (3D) spanned architectures with assorted topology, regular and adjustable pores, potential to tailorability, and diversity of metal centers as well as functional groups. − In particular, coherent pore surface engineering endorses multifarious opportunities to systematically fine-tune their voids and control pore performances to address environmental remediation and sustainable catalysis. The candid methodology for CO 2 capture in MOFs is to introduce ample heteroatoms inside their pores that affect the stronger interaction of the CO 2 quadrupole with the localized dipoles of duly grafted task-specific sites and promotes improved adsorption and separation capabilities. − However, strategic incorporation of functional organic groups in MOF pores is challenging as they either do not precisely line through the channels or considerably block the void space, thereby displaying a trivial effect on the CO 2 sorption performance. ,, Given effective CO 2 separation needs optimized pores because of similar kinetic diameters of CO 2 (0.33 nm) and N 2 (0.36 nm), the interpenetration-induced fabrication of narrow channels in MOFs is highly desirable. − …”

Enhanced Adsorption Selectivity of Carbon Dioxide and Ethane on Porous Metal–Organic Framework Functionalized by a Sulfur-Rich Heterocycle

“…According to a very recent review, these values are among the top 15% of ca. 400 reported selectivity data for MOFs [ 50 ]. It must be mentioned that the majority of those top 15% of highly selective MOFs contain either strong Lewis acidic adsorption centers (unsaturated metal sites) or strong Lewis basic adsorption centers (lone electron pairs on nitrogen atoms) which improves their selective adsorption of CO 2 due to strong chemical interactions with those centers.…”

Solid-State Proton Conduction Driven by Coordinated Water Molecules in Metal–Organic Frameworks and Coordination Polymers

“…Thus, progress in SSPCs with high proton conductivity, especially at low temperatures, is essential for the implementation and realization of hydrogen-based technologies in PEMFCs. , Because of several disadvantages (high cost due to perfluorination, amorphous nature) of state-of-the-art materials such as Nafion and Flemion, researchers are very much interested in developing alternative solid-state proton-conducting materials. In this regard, porous metal–organic frameworks (MOFs) and coordination polymers (CPs) have evolved as emerging classes of new crystalline proton conductors due to their structural diversity and tunability. − It is really inspirational that many of them exhibit an ultra-high super-protonic conductivity to a level of 10 –2 S cm –1 , and for a few, the conductivity even soars to 10 –1 S cm –1 or beyond. , Such a superior conductivity could be originated by tactful grafting of various proton sources where an extended continuous hydrogen-bonding network could be formed, in which both conducting medium and protic guests play crucial roles. , The highly crystalline nature of MOFs and CPs not only assists to understand the plausible proton transport mechanism but also is very helpful to interpret the structure–property relationship, thus driving further developments in the field. The proton sources that are incorporated into/​installed onto the proton-conducting MOFs/CPs are of two types: ( i ) intrinsic and ( ii ) extrinsic . ,, For the first case, uncoordinated acidic functional groups (−SO 3 H, −PO 3 H 2 , and −COOH) are flanked from the framework’s backbone toward the pore channels as an integral part of the backbone and act as proton sources or, due to charge neutrality, protic counterions (H 3 O + , Me 2 NH 2 + , NH 4 + , H 2 PO 4 – , etc.)…”