Occurrence of Uncommon Infinite Chains Consisting of Edge-Sharing Octahedra in a Porous Metal Organic Framework-Type Aluminum Pyromellitate Al₄(OH)₈[C₁₀O₈H₂] (MIL-120): Synthesis, Structure, and Gas Sorption Properties

Abstract

A new metal organic framework (MOF)-type aluminum pyromellitate or Al 4 (OH) 8 [C 10 O 8 H 2 ]·4.8−5H 2 O (called MIL-120) has been hydrothermally synthesized at 210 °C for 24 h. Its structure was analyzed by single-crystal microdiffraction using the synchrotron radiation beamline at ID13 station (ESRF, Grenoble). It consists of infinite chains of aluminum centers in octahedral coordination connected to each other through the pyromellitate linker. The structural feature is the existence of the AlO 2 (OH) 4 octahedra linked to each other via a common edge consisting of two μ 2 -hydroxo groups, along the [102] direction. The different positions of the common edge in the two distinct Al crystallographical sites induce a cis−trans connection mode of the octahedral units, and zigzag chains are generated. All the carboxylate groups of the pyromellitate molecules are perpendicularly connected to the inorganic chains, and it results in the formation of channels running along the c axis (5.4 × 4.7 Å), in which water molecules are encapsulated. 27 Al solid state NMR revealed two well resolved signals with two distinct quadrupolar coupling constants ( C Q = 8.07 and 4.77 MHz) related to two Al sites. Brunnauer-Emmett-Teller (BET) surface area is 308(4) m 2 g −1 (Langmuir: 432(1) m 2 g −1 ). H 2 adsorption is 1.3 wt % at 77 K and 3 MPa. CH 4 or CO 2 adsorptions are 1.8 mmol g −1 and 4.8 mmol g −1, respectively, at 303 K (under 1 MPa).