Cyclotetrabenzoin Acetate: A Macrocyclic Porous Molecular Crystal for CO₂ Separations by Pressure Swing Adsorption**

Abstract

A porous molecular crystal (PMC) assembled by close-packing of macrocyclic cyclotetrabenzoin acetate is an efficient adsorbent for selective CO 2 capture. The 7.1´7.1 Å square pore of PMC and its ester C=O group play important roles in improving its affinity for CO 2 molecules. Thermodynamically, the benzene walls of macrocycle strongly promote CO 2 adsorption via [p•••p] interactions at low pressure. In addition, the polar carbonyl groups pointing inward the square channels reduce the size of aperture to a 5.0´5.0 Å square, which offers kinetic selectivity for CO 2 capture. The PMC features water tolerance and high structural stability under vacuum and various gas adsorption conditions, which are rare among intrinsically porous organic molecules. In mixed-gas breakthrough experiments, it exhibits efficient CO 2 /N 2 and CO 2 /CH 4 separations under kinetic flow conditions. Most importantly, the moderate adsorbate-adsorbent interaction allows the PMC to be readily regenerated, and therefore applied to pressure swing adsorption (PSA) processes. The eluted N 2 and CH 4 are obtained with over 99.9% and 99.8% purity, respectively, and the separation performance is stable for 30 cycles. Coupled with its easy synthesis, these properties make cyclotetrabenzoin acetate a promising adsorbent for CO 2 separations from flue and natural gases. File list (2) download file view on ChemRxiv CO2 Separation-020421.pdf (1.34 MiB) download file view on ChemRxiv CO2 Separation-ESI-020421.pdf (1.56 MiB)

Cyclotetrabenzoin Acetate: A Macrocyclic Porous Molecular Crystal for CO2 Separations by Pressure Swing Adsorption**

“…Tw oc o-crystal structures of cyclotetrabenzoin acetate were obtained with CO 2 . [27] Thet wo structures both crystallized in the P4 2 1 c space group and are very similar in the arrangement of molecules of 1.H owever, they differ in their CO 2 occupancya nd orientation. In the first, low-occupancys tructure (Figure 3A), two distinct positions of CO 2 guests are visible.T he total CO 2 occupancy over both positions is 12 %, allowing the formulation of the structure as 1•0.12 CO 2 .O ne CO 2 molecule is included into the central cavity (occupancy 7%), where it is stabilized by the [p•••p]s tacking interactions of its C=Ob onds with the benzene "walls" of 1.These interactions are characterized by the idealized 3.41 distances between the CO 2 scarbon atom and the benzene ring planes.The other unique CO 2 molecule (occupancy5%) resides above the cavity of 1,e stablishing ashort (3.17 )contacts with aromatic carbon atoms in 1.…”

Efficient CO2/CO Separation by Pressure Swing Adsorption Using an Intrinsically Nanoporous Molecular Crystal

“…In the crystal structure, the linearly aligned 7.1 × 7.1 Å 2 square channels of 1 are lined with the polar carbonyl groups (from adjacent macrocycles) pointing inward in the square pores and creating 5.0 × 5.0 Å square apertures, which endow these materials with kinetic selectivity for CO 2 adsorption. These pores additionally appear quite rigid, as evidenced by minimal (<0.4%) changes in the unit cell parameters upon CO 2 inclusion . This environment results in a high affinity of 1 to the quadrupoles of CO 2 molecules, while the benzene walls of 1 can interact with CO 2 molecules via π···π stacking.…”

“…To date, the ability of intrinsic PMCs to separate CO 2 and CO has not been studied. While many PMC materials have been proposed and tested as adsorbents in gas separations, their use in PSA separation is rare on account of their moisture sensitivity and inability to sustain repeated adsorption/desorption cycles.…”

“…We previously demonstrated that an intrinsic PMC material based on cyclotetrabenzoin acetate ( 1 in Figure ) is an efficient PSA adsorbent for the separation of CO 2 from nitrogen (N 2 ) and methane (CH 4 ) gases . In the crystal structure, the linearly aligned 7.1 × 7.1 Å 2 square channels of 1 are lined with the polar carbonyl groups (from adjacent macrocycles) pointing inward in the square pores and creating 5.0 × 5.0 Å square apertures, which endow these materials with kinetic selectivity for CO 2 adsorption.…”

Water adsorption in porous organic crystals of adamantane-bearing macrocycles

“…[11][12][13][14] Porous organic macrocycles have advantages in terms of handling and processability given their solubility in organic media, although crystalline solids composed of macrocycles, which are held together by weak noncovalent interactions between component units, are less stable than infinite solids of MOFs and COFs. [15][16][17][18] A number of inclusion crystals of macrocyclic compounds (e.g., resorcin [4]arenes, pyrogallol [4]arenes, and cyclotribenzoins) that can capture water molecules or clusters have been prepared by solution crystallization, [19][20][21][22] and crystal structures of single and oligomeric water clusters have been determined by X-ray analysis. However, there are only a handful of reports on water uptake by porous organic macrocycles.…”