Porous 3D and 2D frameworks differentiation induced by metal doping: Structure flexibility, capture behaviors, post-synthetic metal exchange and amorphous electrocatalyst transformation

Abstract

The obtaining of high porosity and understanding the dynamic pore responses to various inclusion guests represents one main focus in the functionalization of metal-organic frameworks (MOFs). A new solvothermally generated MOF of {(Me2NH2)[Ni3(μ3OH)(tzba)3(H2O)3]}·8DMF (Ni3−MOF, H2tzba = 4-(1H-tetrazol-5-yl)benzoic acid) processes negative charged, new triangular [Ni3(μ3OH)(COO)3(tz)3(H2O)3] node based, MIL-88-topology resembled framework, with high void fraction of 73.3%. While the introduction of equivalent Mn2+ will greatly change the coordination assembly, leading to 2D (3,6)-grid layer of [Ni1.4Mn1.6(tzba)3(H2O)6]·12DMF (NiMn-MOF) as the first example of synergistical construction of rare [M2(tz)3(H2O)3] and [M(COO)3] nodes. The 2D layers parallelly stacked in ABAB fashion through interlayer hydrogen bonding, and generated a neutral framework with high void fraction of 72.9% and 1D mesoporous hexagonal channels with diameter of 24.0 Å. The Ni3−MOF is highly flexible and tends to close its pores after full guest removing, yet its gas accessible porosity is sensitive to residual guest in the pores and the porosity are fully reserved in organic solution phase to capture Methylene Blue and iodine. The inherent strong structure dynamic of Ni3−MOF facilitates post-synthetic metal exchange of the Ni2+ by Mn2+, Co2+, Cu2+ and Cd2+, as well as the in-situ formation of amorphous Ni(OH)2 electrocatalyst for dual water splitting with full recovery of costly organic ligands. For the NiMn-MOF, large gas accessible porosity with Langmuir surface area of 1052 m2 g−1, as well as guest dependent shrink-swell behavior was obtained. The NiMn-MOF is found to be an effective porous adsorbent to capture the iodine in either solution or gaseous phase, with uptake up to 2.4 mol iodine per mol of MOFs and capture of iodine with concentration lower to 5.0 × 10−5 mol/L.