Two-step adsorption on jungle-gym-type porous coordination polymers: Dependence on hydrogen-bonding capability of adsorbates, ligand-substituent effect, and temperature

摘要

A preliminary study of isopropanol (IPA) adsorption/desorption isotherms on a jungle-gym-type porous coordination polymer, [Zn_2(bdc) _2(dabco)]_n (1, H_2bdc = 1,4-benzenedicarboxylic acid, dabco =1,4-diazabicyclo[2.2.2]octane), showed unambiguous two-step profiles via a highly shrunk intermediate framework. The results of adsorption measurements on 1, using probing gas molecules of alcohol (MeOH and EtOH) for the size effect and Me_2CO for the influence of hydrogen bonding, show that alcohol adsorption isotherms are gradual two-step profiles, whereas the Me_2CO isotherm is a typical type-I isotherm, indicating that a two-step adsorption/desorption is involved with hydrogen bonds. To further clarify these characteristic adsorption/desorption behaviors, selecting nitroterephthalate (bdc-NO_2), bromoterephthalate (bdc-Br), and 2,5-dichloroterephthalate (bdc-Cl_2) as substituted dicarboxylate ligands, isomorphous jungle-gym-type porous coordination polymers, {[Zn _2(bdc-NO_2)_2(dabco)]?solvents}_n (2 solvents), {[Zn_2(bdc-Br)_2(dabco)]?solvents} _n (3 solvents), and {[Zn_2(bdc-Cl_2) _2(dabco)]?solvents}_n (4 solvents), were synthesized and characterized by single-crystal X-ray analyses. Thermal gravimetry, X-ray powder diffraction, and N_2 adsorption at 77 K measurements reveal that [Zn_2(bdc-NO_2)_2(dabco)]_n (2), [Zn_2(bdc-Br)_2(dabco)]_n (3), and [Zn _2(bdc-Cl_2)_2(dabco)]_n (4) maintain their frameworks without guest molecules with Brunauer-Emmett-Teller (BET) surface areas of 1568 (2), 1292 (3), and 1216 (4) m~2 g~(-1). As found in results of MeOH, EtOH, IPA, and Me_2CO adsorption/desorption on 2-4, only MeOH adsorption on 2 shows an obvious two-step profile. Considering the substituent effects and adsorbate sizes, the hydrogen bonds, which are triggers for two-step adsorption, are formed between adsorbates and carboxylate groups at the corners in the pores, inducing wide pores to become narrow pores. Interestingly, such a two-step MeOH adsorption on 2 depends on the temperature, attributed to the small free-energy difference (ΔF_(host)) between the two guest-free forms, wide and narrow pores.