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Reports: UR1050635-UR10: Synthesis of Crystalline Porous Materials with Functional Open Metal Sites

Xianhui Bu, PhD, California State University (Long Beach)

In this funding period, the PI’s group developed a new strategy for synthesizing cluster-based lithium metal-organic frameworks (Li-MOFs) by using charge-complementary ligands. Our continuing interest in Li-MOFs is in part sustained by the desire to mimic certain features of MOF-74-Mg (the best porous material for carbon capture) such as its low molecular mass. In this study, we seek to develop a strategy to address the twin goals of the strengthening the cluster-ligand linkages while still being able to take advantage of rich chemistry of lithium-aryloxide-type clusters.

A specific strategy is to explore the Li-OPy system under the conditions involving competing neutral ligands, because such study can lay the groundwork for developing stable porous Li-MOFs with open lithium sites. Here we present two new Li-MOFs synthesized in this study.

CPM-41 (Li2(OPy)2(DMF)2) was synthesized by reacting t-BuOLi and 4-hydroxypyridine in a mixture of DMF and THF at 120 ºC. It crystallizes in a chiral monoclinic P21 space group and has a layered structure constructed by crosslinking Li2 dimers with -OPy ligand. For each lithium in CPM-41, the remaining fourth coordination site is completed by a terminal DMF solvent molecule, which explains the 2-D covalent nature of CPM-41. The structural feature of CPM-41 mimics the square net formed by Cu or Zn paddlewheels and linear dicarboxylate ligands. As is the case with transition-metal paddlewheel units, there is clearly a possibility to further build Li2-based sheets into 3-D pillared structure by introducing pillar ligands.

Indeed, when dioxane, which can be used as both a co-solvent and a linker, was introduced into the synthesis, CPM-42 with formula of [Li2(OPy)2(diox)]•(diox) was obtained. In CPM-42, dioxane replaces DMF in CPM-41 and acts as a pillar to connect adjacent layers through Li-O coordination bonds. As a result, the 2-D structure in CPM-41 is transformed into a 3-D coordination framework, which again mimics the pillared layered structure found previously with transition metal MOFs. When viewed down the a axis, an uncoordinated dioxane molecule is seen at the center of the channel. The PLATON calculation indicates that about 38.6% of the crystal volume of CPM-42 is accessible to extra-framework guest molecules. Pillared layered structures based on transition metals have been extensively studied because of their flexibility, however, this is the first time such structure type is achieved in Li-MOFs, which exhibit quite different chemistry compared to transition metals.

The thermogravimetric analysis of CPM-42 shows that there is an initial sharp drop below 140 ºC, which reflects the loss of dioxane molecules. There is no obvious weight loss until ~500 ºC, beyond which the framework is expected to disintegrate. Because of its 3-D open-framework nature, CPM-42 was selected for gas sorption study. Different sample activation procedures were attempted and the best results were obtained from samples soaked in acetonitrile for three days prior to the measurements. The hydrogen adsorption shows that the sample has a gravimetric uptake of 59.9 cm3 g-1 (or 0.53 wt%) and a volumetric uptake of 4.89 g L-1 (calculated crystal density: 0.922 g cm-3) at 77K and 1 atm, which is among the best hydrogen uptake values for reported lithium-organic frameworks under similar conditions. The CO2 and C2H2 adsorptions at 273 K and 298 K were also investigated. At 1 atm, CPM-42 has a CO2 uptake of 0.54 mmol g-1 at 273 K and 0.29 mmol g-1 at 298 K. The corresponding value for C2H2 uptake is 0.76 mmol g-1 at 273 K and 0.28 mmol g-1 at 298 K, respectively.

In summary, two new Li-MOFs are presented here. CPM-41 contains Li2 dimers linked into a neutral layer by deprotonated mononegative 4-hydroxypyridine and terminated in the third dimension by the solvent DMF molecule. CPM-42 can be conceptually visualized as pillared CPM-41 with the solvent DMF molecules in CPM-41 being replaced by pillaring ligands to create an extended network in the third dimension. These two Li-MOFs bear significant resemblance to those based on transition-metal paddlewheel units. This work demonstrates the broad applicability of the charge-complementary-ligand (CCL) based synthetic strategy first introduced in the synthesis ZIF-like structures. In contrast with the earlier study with monomeric lithium nodes and N-donor ligand pairs, the work reported here is the first example involving Li2 dimeric clusters. The successful transitioning of the CCL strategy away from pure N-donor-based ligand pairs into O-donor and mixed N,O-donor based ligand pairs is also worth noting because it opens up new opportunities in the design of novel Li-MOFs.

The research supported by this PRF grant made a significant impact on the PI's project development and on the careers of students involved. One graduate MS thesis (only MS degree is offered on campus) was completed during this reporting period. Two additional graduate students are making excellent progress towards their MS theses. We thank the ACS-PRF program for supporting our undergraduate and graduate students by supporting their research at the PI’s group through this grant.