Paddlewheel SBU based Zn MOFs: Syntheses, structural diversity, and CO 2 adsorption properties

Abstract

Four Zn metal-organic frameworks (MOFs), [[Zn 2 (2,6-ndc) 2 (2-Pn)]·DMF] n (1), [[Zn 2 (cca) 2 (2-Pn)]·DMF] n (2), [[Zn 2 (thdc) 2 (2-Pn)]·3DMF] n (3), and [[Zn 2 (1,4-ndc) 2 (2-Pn)]·1.5DMF] n (4), were synthesized from zinc nitrate and N,N'-bis(pyridin-2-yl)benzene-1,4-diamine (2-Pn) with naphthalene-2,6-dicarboxylic acid (2,6-H 2 ndc), 4-carboxycinnamic acid (H 2 cca), 2,5-thiophenedicarboxylic acid (H 2 thdc), and naphthalene-1,4-dicarboxylic acid (1,4-H 2 ndc), respectively. MOFs 1-4 were all constructed from similar dinuclear paddlewheel [Zn 2 (COO) 4 ] clusters and resulted in the formation of three kinds of uninodal 6-connected non-interpenetrated frameworks. MOFs 1 and 2 suit a topologic 4 8 ·6 7 -net with 17.6% and 16.8% extra-framework voids, respectively, 3 adopts a pillared-layer open framework of 4 8 ·6 6 ·8-topology with sufficient free voids of 39.9%, and 4 features a pcu-type pillared-layer framework of 4 12 ·6 3 -topology with sufficient free voids of 30.9%. CO 2 sorption studies exhibited typical reversible type I isotherms with CO 2 uptakes of 55.1, 84.6, and 64.3 cm 3 g -1 at 195 K and P/P 0 =1 for the activated materials 1', 2', and 4', respectively. The coverage-dependent isosteric heat of CO 2 adsorption (Q st ) gave commonly decreased Q st traces with increasing CO 2 uptake for all the three materials and showed an adsorption enthalpy of 32.5 kJ mol -1 for 10, 38.3 kJ mol -1 for 2', and 23.5 kJ mol -1 for 4' at zero coverage.