Benchmark C₂H₂/CO₂Separation in an Ultra‐Microporous Metal–Organic Framework via Copper(I)‐Alkynyl Chemistry

Abstract

Separation of acetylene from carbon dioxide remains a daunting challenge because of their very similar molecular sizes and physical properties. We herein report the first example of using copper(I)‐alkynyl chemistry within an ultra‐microporous MOF (CuI@UiO‐66‐(COOH)2) to achieve ultrahigh C2H2/CO2 separation selectivity. The anchored CuI ions on the pore surfaces can specifically and strongly interact with C2H2 molecule through copper(I)‐alkynyl π‐complexation and thus rapidly adsorb large amount of C2H2 at low‐pressure region, while effectively reduce CO2 uptake due to the small pore sizes. This material thus exhibits the record high C2H2/CO2 selectivity of 185 at ambient conditions, significantly higher than the previous benchmark ZJU‐74a (36.5) and ATC‐Cu (53.6). Theoretical calculations reveal that the unique π‐complexation between CuI and C2H2 mainly contributes to the ultra‐strong C2H2 binding affinity and record selectivity. The exceptional separation performance was evidenced by breakthrough experiments for C2H2/CO2 gas mixtures. This work suggests a new perspective to functionalizing MOFs with copper(I)‐alkynyl chemistry for highly selective separation of C2H2 over CO2.

Dense Packing of Acetylene in a Stable and Low‐Cost Metal–Organic Framework for Efficient C2H2/CO2 Separation

“…This material also shows extremely high and long-term water and acid-base stability.O ur breakthrough experiments confirmed its highly efficient separation performance for 50:50 (v/v) C 2 H 2 /CO 2 mixtures,a ffording both large C 2 H 2 uptake (3.3 mmol g À1 )a nd high separation factor (3.4). Both values outperform those of most benchmark MOFs reported, such as Cu I @UiO-66-(COOH) 2 (2.89 mmol g À1 and 3.4), [15] NKMOF-1-Ni (2.48 mmol g À1 and 2.6), [16] FJU-90 (1.87 mmol g À1 and 2.1), [8] and FeNi-M'MOF (2.98 mmol g À1 and 1.7). [19] Thecombined features of excellent separation performance,high chemical stability,low cost, and easy scalability endow CAU-10-H with higher technological readiness levels.…”

“…[19] CAU-10-H thus exhibits excellent C 2 H 2 /CO 2 separation performance with both large C 2 H 2 uptake (3.3 mmol g À1 )a nd high separation factor (3.4). Both values are among the highest in the indicated benchmark materials (Figure 3eand Table S2), higher than those of NKMOF-1-Ni (2.48 mmol g À1 and 2.6), [16] Cu I @UiO-66-(COOH) 2 (2.89 mmol g À1 and 3.4), [15] FJU-90 (1.87 mmol g À1 and 2.1), [8] and FeNi-M'MOF (2.98 mmol g À1 and 1.7). [19] The breakthrough experiments for aw et gas mixture under 60 % RH reveal an early unchanged separation factor of 3.3, maintaining its separation performance under humid conditions (Figure S8).…”

“…Single-component adsorption isotherms of CAU-10-H for C 2 H 2 and CO 2 were measured at 296 Kand 273 K(Figure 2b and S3), respectively.C AU-10-H exhibits an obviously preferential adsorption of C 2 H 2 over CO 2 ,with amuch higher C 2 H 2 uptake of 89.8 cm 3 g À1 over CO 2 (60.0 cm 3 g À1 )a t2 96 K and 1bar.A tapartial pressure of 0.5 bar,w hich is an indicator of the C 2 H 2 capture ability of adsorbents from a5 0:50 gas mixture,C AU-10-H features ah igh C 2 H 2 uptake of 76.5 cm 3 g À1 at 296 K, outperforming that of JCM-1( 63.5 cm 3 g À1 ), [18] NKMOF-1-Ni (55.5 cm 3 g À1 ), [16]] and Cu I @UiO-66-(COOH) 2 (43.4 cm 3 g À1 ). [15] Considering the relatively low surface area and theoretical pore volume,wenote that the density of adsorbed C 2 H 2 inside CAU-10-H was calculated to be extremely high of 392 gL À1 at 296 Ka nd 1bar,w hich is 335 times larger than the density of gaseous C 2 H 2 (1.17 gL À1 )a nd even comparable to the liquid C 2 H 2 density of 393 gL À1 at the same conditions.T his value is among the highest reported so far,a nd surpasses that of SIFSIX-1-Cu (387 gL À1 ) [26] and other promising MOFs including NKMOF-1-Ni (370 gL À1 ), [16] UTSA-74 (343 gL À1 ), [9] JCM-1 (334 gL À1 ), [18] and FJU-90 (256 gL À1 ), [8] indicating ahighly efficient packing of C 2 H 2 molecules inside CAU-10-H (Figure 2c). Such dense packing of C 2 H 2 molecules can be partially explained by the experimental isosteric heat of adsorption (Q st ), calculated based on adsorption isotherms.…”

“…[8][9][10][11][12][13][14] Themost popular strategy so far is to immobilize strong functional sites (e.g., open metal sites) into ultra-microporous MOFs to maximize C 2 H 2 binding affinity and thus improve C 2 H 2 /CO 2 selectivity to be record high. [15][16][17][18][19][20][21][22][23][24][25] However,s uch ultra-strong interactions with C 2 H 2 typically induce high regeneration energy and concurrently small pores prone to result in relatively low uptake capacities, as exemplified by Cu I @UiO-66-(COOH) 2 and NKMOF-1-Ni. [15,16] To alleviate this issue,abetter design is to construct suitable pore confinement without strong binding sites in ultra-microporous MOFs,e nabling the efficient packing of targeted gas molecules inside the pores (Scheme 1).…”

“…[15][16][17][18][19][20][21][22][23][24][25] However,s uch ultra-strong interactions with C 2 H 2 typically induce high regeneration energy and concurrently small pores prone to result in relatively low uptake capacities, as exemplified by Cu I @UiO-66-(COOH) 2 and NKMOF-1-Ni. [15,16] To alleviate this issue,abetter design is to construct suitable pore confinement without strong binding sites in ultra-microporous MOFs,e nabling the efficient packing of targeted gas molecules inside the pores (Scheme 1). Thus,the small pores optimize size-sieving effects,and dense packing of the preferred gas molecules can fully make use of pore spaces to notably improve gas uptake density through low adsorption enthalpy.This strategy has been well exemplified in the MOFs of SIFSIX-1-Cu and UTSA-16 for C 2 H 2 or CO 2 capture and separation.…”