Low‐Concentration C₂H₆ Capture Enabled by Size Matching in the Ultramicropore

Abstract

Low-concentration ethane capture is crucial for environmental protection and natural gas purification. The ideal physisorbent with strong C 2 H 6 interaction and large C 2 H 6 uptake at low-concentration level has rarely been reported, due to the large pK a value and small quadrupole moment of C 2 H 6 . Herein, we demonstrate the perfectly size matching between the ultramicropore (pore size of 4.6 Å) and ethane (kinetic diameter of 4.4 Å) in a nickel pyridine-4carboxylate metal-organic framework (IISERP-MOF2), which enables the record-breaking performance for low concentration C 2 H 6 capture. IISERP-MOF2 exhibits the large C 2 H 6 adsorption enthalpy of 56.7 kJ/mol, and record-high C 2 H 6 uptake at low pressure of 0.01-0.1 bar and 298 K (1.8 mmol/g at 0.01 bar). Molecule simulations and C 2 H 6loading crystal structure analysis revealed that the maximized interaction sites in IISERP-MOF2 with ethane molecule originates the strong C 2 H 6 adsorption. The dynamic breakthrough experiments for gas mixtures of C 2 H 6 /N 2 (1/999, v/v) and C 2 H 6 /CH 4 (5/95, v/v) proved the excellent low-concentration C 2 H 6 capture performance.Ethane (C 2 H 6 ) has been proved harmful to the atmosphere and human health, as suggested by the American Conference of Governmental Industrial Hygienists (threshold limit of 1000 ppm). [1] The total global ethane emissions to the air are estimated at 10-13 teragrams (1 Tg = 10 12 g) per year. [2] The main source of ethane in the air comes from the inevitable release from production, processing, transmission, and use of natural gas (mainly CH 4 , 5-10 % for C 2 H 6 ). [1,3] Low concentration C 2 H 6 capture from air and natural gas is vital for controlling the C 2 H 6 level, however rarely investigated. Compared to traditional distillation technology with large energy input, physisorption based separation process could be a better alternative to address this issue owing to its fast sorption kinetics and low regeneration energy.Relatively chemical inertness of ethane (small quadrupole moment and large pK a : 0.65 × 10 À 26 esu cm 2 and 50) make it only strongly adsorbed in porous material by effective superposition of weak Van der Waals and hydrogen bonding interaction, in which pore size is a very important factor. [4] The exquisite pore structure control (pore size and pore chemistry) endows the new type of porous material (metal-organic framework/MOF, [5] also noted as metal-organic material/MOM [6] or porous coordination polymer/PCP) [7] with the significant power to manipulate these weak interaction sites inside to enhance C 2 H 6 adsorption. [8] For MOF structure with much larger pore size (c. a. > 7 Å, Figure 1a) than kinetic diameter of ethane molecule [a

A Zn(ii) pillared-layer ultramicroporous metal–organic framework with matching molecular pockets for C2H2/CO2 separation

“…For C 2 H 2 and CO 2 with very similar properties, the distribution of matching sites in the cavity is the key to separation, and this phenomenon is also common in other ultramicroporous materials. 34–36…”

Three Polyhedron-Based Metal–Organic Frameworks Exhibiting Excellent Acetylene Selective Adsorption

“…High-purity acetylene (C 2 H 2 ) is a basic raw material for the production of various chemicals and polymers, such as acrylic acid, butylene glycol, polyvinyl chloride, acrylic acid derivatives, etc. − However, it is principally produced by steam cracking of hydrocarbons or the partial combustion methane methods and is inevitably mixed with ethylene (C 2 H 4 ) or ethane (C 2 H 6 ). − Based on the structure of the carbon–carbon bonds, their molecular sizes, boiling points, and electronegativity/shape are very similar − (Table S1). Therefore, the separation of C 2 H 2 from C 2 H 6 /C 2 H 4 /C 2 H 2 mixtures is a crucial and challenging task .…”

General pore features for one-step C2H4 production from a C2 hydrocarbon mixture

“…26–28 To achieve reverse adsorption, the C 2 H 6 –host interaction should be enhanced. Despite the individual van der Waals C–H⋯C interaction and C–H⋯O/N hydrogen bonds of C 2 H 6 usually being weaker than those of C 2 H 4 with stronger acidity, C 2 H 6 can be adsorbed selectively via multiple binding sites, which leads to its strong binding affinity (such as the extraordinarily high C 2 H 6 adsorption enthalpy ( Q st ) values of MAF-49 29 and IISERP-MOF2 30 ) due to its larger molecule size and more H atoms compared with C 2 H 4 . Taking the above into consideration, we came up with general rules for C 2 H 4 separation from a C 2 H 2 /C 2 H 4 /C 2 H 6 mixture (Fig.…”