Fluorocarbon-Functionalized Superhydrophobic Metal-Organic Framework: Enhanced CO2 Uptake via Photoinduced Postsynthetic Modification.

Abstract

The design and synthesis of porous materials for selective capture of CO2 in the presence of water vapor is of paramount importance in the context of practical separation of CO2 from the flue gas stream. Here, we report the synthesis and structural characterization of a photoresponsive fluorinated MOF {[Cd(bpee)(hfbba)]·EtOH}n (1) constructed by using 4,4'-(hexafluoroisopropylidene)bis(benzoic acid) (hfbba), Cd(NO3)2, and 1,2-bis(4-pyridyl)ethylene (bpee) as building units. Due to the presence of the fluoroalkyl -CF3 functionality, compound 1 exhibits superhydrophobicity, which is validated by both water vapor adsorption and contact angle measurements (152°). The parallel arrangement of the bpee linkers makes compound 1 a photoresponsive material that transforms to {[Cd2(rctt-tpcb)(hfbba)2]·2EtOH}n (rctt-tpcb = regio cis,trans,trans-tetrakis(4-pyridyl)cyclobutane; 1IR) after a [2 + 2] cycloaddition reaction. The photomodified framework 1IR exhibits increased uptake of CO2 in comparison to 1 under ambient conditions due to alteration of the pore surface that leads to additional weak electron donor-acceptor interactions with the -CF3 groups, as examined through periodic density functional theory calculations. The enhanced uptake is also aided by an expansion of the pore window, which contributes to increasing the rotational entropy of CO2, as demonstrated through force field based free energy calculations.