UTSA-74: A MOF-74 Isomer with Two Accessible Binding Sites per Metal Center for Highly Selective Gas Separation

A new
metal–organic framework Zn2(H2O)­(dobdc)·0.5­(H2O) (UTSA-74, H4dobdc
= 2,5-dioxido-1,4-benzenedicarboxylic acid), Zn-MOF-74/CPO-27-Zn isomer,
has been synthesized and structurally characterized. It has a novel
four coordinated fgl topology with one-dimensional channels
of about 8.0 Å. Unlike metal sites in the well-established MOF-74
with a rod-packing structure in which each of them is in a five coordinate
square pyramidal coordination geometry, there are two different Zn2+ sites within the binuclear secondary building units in UTSA-74 in which one of them (Zn1) is in a tetrahedral while
another (Zn2) in an octahedral coordination geometry. After activation,
the two axial water molecules on Zn2 sites can be removed, generating UTSA-74a with two accessible gas binding sites per Zn2 ion.
Accordingly, UTSA-74a takes up a moderately high and
comparable amount of acetylene (145 cm3/cm3)
to Zn-MOF-74. Interestingly, the accessible Zn2+ sites
in UTSA-74a are bridged by carbon dioxide molecules instead
of being terminally bound in Zn-MOF-74, so UTSA-74a adsorbs
a much smaller amount of carbon dioxide (90 cm3/cm3) than Zn-MOF-74 (146 cm3/cm3) at room
temperature and 1 bar, leading to a superior MOF material for highly
selective C2H2/CO2 separation. X-ray
crystal structures, gas sorption isotherms, molecular modeling, and
simulated and experimental breakthroughs comprehensively support this
result.

一种新型金属有机框架（metal–organic framework, MOF）Zn₂(H₂O)­(dobdc)·0.5­(H₂O)（命名为UTSA-74，其中H₄dobdc为2,5-二氧代-1,4-苯二甲酸（2,5-dioxido-1,4-benzenedicarboxylic acid）），即Zn-MOF-74/CPO-27-Zn的同分异构体，已被成功合成并完成结构表征。该材料具有独特的四配位fgl拓扑结构，拥有直径约8.0 Å的一维孔道。与经典的棒状堆积型MOF-74不同，后者的每个金属位点均为五配位四方锥配位构型；UTSA-74的双核次级结构单元中存在两种不同的Zn²+位点，其中Zn1位点为四面体配位，Zn2位点则为八面体配位。活化处理后，Zn2位点上的两个轴向水分子可被脱除，得到UTSA-74a，此时每个Zn²+离子均带有两个可供气体结合的活性位点。据此，UTSA-74a对乙炔的吸附量可达145 cm³/cm³，与Zn-MOF-74的吸附量相当，处于较高水平。值得注意的是，UTSA-74a中可接触的Zn²+位点与二氧化碳分子形成桥连配位模式，而非如Zn-MOF-74那般采用端基配位；因此在室温、1 bar条件下，UTSA-74a对二氧化碳的吸附量仅为90 cm³/cm³，远低于Zn-MOF-74的146 cm³/cm³，使其成为一种可实现高选择性C₂H₂/CO₂分离的优异MOF材料。通过X射线晶体衍射结构、气体吸附等温线、分子模拟以及模拟与实验穿透曲线等手段，该结论得到了全面验证。