Water Vapor Single-Gas Selectivity via Flexibility of Three Potential Materials for Autonomous Indoor Humidity Control

Two new isostructural
porous supramolecular materials {[Cu2(amp)4Cl]­[M­(C2O4)3]­·6H2O}n (amp = 2-aminomethylpyridine), designated
as II and III for M = Cr­(III) and M = Fe­(III)
respectively, have been
synthesized by a self-assembly process of two ionic complexes [Cu2(amp)4Cl]3+ and [M­(C2O4)3]3–, M = Fe­(III) and Cr­(III).
They build heterometallic hydrogen-bonded-, oxalato- and chlorido-bridged
zigzag chains with interchain hydrogen bonds and π–π
interactions. This results in supramolecular potentially porous architectures
exhibiting large channels filled with hexameric water clusters. Their
activated phases, II′ and III′, can readsorb the water molecules to regenerate the initial materials
which are stable for many water adsorption and desorption cycles like
that of their homologous catena-{[(Co­(amp)3]­[Cr­(C2O4)3]­·6H2O} (I′). The three materials I′, II′, and III′ exhibit water adsorption
and desorption isotherms having a sigmoidal shape and resulting in
the combination of a type I­(b) profile followed by an S-shaped type
V isotherm with hysteresis. At 20 °C, the steep water sorption
of the S-shaped isotherm occurs at 0.1P/P0 for II′ and III′. This water sorption behavior is quite different from the related
compound I′, where a gate opening and closure
process is involved, giving a type V isotherm with a pronounced H1-type
hysteresis loop with parallel and steep adsorption (at 0.25P/P0) and desorption branches
(at 0.17P/P0). The water
adsorption capacities of the three materials I′, II′, and III′ are 17, 12,
and 11 wt %, respectively. Temperature does not have a great effect
on their water sorption properties, and they all exclude N2 and CO2 gases in the low pressure range. Compounds II′ and III′ are classified among
the materials for which the dehumidification and the humidification
trigger points are the same or too close (10% relative humidity (r.H.)
in their case), while I′ shows a good potential
to be used for automatic indoor control in the range of 15–25%
r.H. recommended for many activities. All the differences observed
in the water sorption properties of the three materials are related
to (i) the type of water cluster which is built in each material,
(ii) the strength of the hydrogen bonds within each water cluster
and between the clusters and its host, and (iii) the strength of the
intrahost interactions which keep the pores of the material closed.

本研究通过两种离子配合物[Cu₂(amp)₄Cl]³⁺与[M(C₂O₄)₃]³⁻（M分别为Fe(III)和Cr(III)）的自组装过程，成功合成了两例同构多孔超分子材料（porous supramolecular materials），其通式为{[Cu₂(amp)₄Cl][M(C₂O₄)₃]·6H₂O}ₙ，其中amp为2-氨基甲基吡啶，当M为Cr(III)与Fe(III)时，该材料分别记为II与III。该类材料构筑了由链间氢键与π-π相互作用（π–π interactions）稳定的异金属氢键桥联、草酸根桥联及氯离子桥联锯齿链（zigzag chains）。由此形成的超分子多孔骨架中存在大尺寸孔道，孔道内填充有六聚体水团簇（hexameric water clusters）。其活化相II'与III'可重新吸附水分子以再生初始骨架，该类材料与同系物catena-{[(Co(amp)₃][Cr(C₂O₄)₃]·6H₂O}（记为I'）一样，可稳定经受多次水吸附-脱附循环。三种材料I'、II'与III'的水吸附-脱附等温线均呈S形，表现为I(b)型等温线轮廓（type I(b) profile）与S型V型等温线的叠加特征，并伴随滞后现象（hysteresis）。在20℃条件下，II'与III'的S型等温线出现陡峭吸附阶段的相对压力为0.1P/P₀。该吸附行为与同系物I'存在显著差异：I'的吸附过程涉及“开门-关门”机制，其等温线为V型，且呈现特征明显的H1型滞后环（H1-type hysteresis loop），吸附支与脱附支均陡峭且平行，吸附拐点位于0.25P/P₀，脱附拐点位于0.17P/P₀。三种材料I'、II'与III'的水吸附容量分别为17 wt%、12 wt%与11 wt%。温度对三者的水吸附性能影响较小，且在低压范围内均不吸附氮气与二氧化碳气体。II'与III'属于除湿与加湿触发点相同或极为接近的材料（其触发点相对湿度（relative humidity, r.H.）为10%），而I'则具备良好的应用潜力，可用于多数活动推荐的15%~25%相对湿度区间内的室内自动控湿。三种材料水吸附性能的差异主要源于三方面：(i) 不同材料内部形成的水团簇类型；(ii) 水团簇内部以及水团簇与主体骨架之间的氢键强度；(iii) 维持材料孔道闭合的主体骨架内相互作用强度。