Xianhui Bu, PhD, California State University (Long Beach)
The first part of this research deals with single-metal indium metal-organic frameworks and systematization of structural building blocks based on indium. In this part of our research that is focused on the synthesis of indium-based metal-organic frameworks (MOF) by systematically varying ligands and solvents, we have discovered the indium dimer and two indium super-trimers, novel structural building blocks previously unknown in porous materials. This discovery offers new insight into rich synthetic and structural chemistry of indium metal-organic framework materials and has led to our systematization of six indium-based framework building units into two analogous series (i.e., the monomer series vs. the trimer series). These framework building units, either alone or in combination with others, are capable of generating different novel 3D framework materials in response to simple changes in synthetic conditions. The tendency of these Indium-MOFs to form chiral porous frameworks is worth noting and adds a new dimension to their possible applications.
The second part of our research involves the synthesis of zeolite-type metal-organic frameworks encapsulating immobilized transition metal clusters. This part of the research expands on the first part by introducing a heterometal into the synthesis. In this part of our research that is focused on the synthesis of indium-based metal-organic frameworks modified by a heterometal ion or cluster (such as dimer or trimer), we have synthesized three series of MOFs (CPM-16, -17, and -26). We demonstrate the general feasibility to construct zeolite-like frameworks from polyfunctional ligands. For the BTC ligand (H3BTC = benzenetricarboxylic acid) with three geometrically equivalent -COO groups, a key aspect of our strategy is to devise ways to enable -COO groups to bond unsymmetrically to both framework metal nodes (indium in this work) and metal clusters formed from divalent metal ions such as Zn and Co. In this work, this kind of "symmetry-breaking" mode is achieved by employing the heterometallic system that helps to generate various metal ions and clusters with complementary coordination chemistry. This work demonstrates the great promise of different polyfunctional ligands as well as other heterometallic systems. The need for further exploring this synthetic strategy comes from the potential of this method for the creation of a new generation of zeolite-like materials whose frameworks are decorated with metal centers that may be useful for applications such as catalysis or adsorption. In addition, this method may provide a path for the pore space partition and optimization through immobilized metal ions or clusters within channels or cages of zeolite-like nets. The possibility of using metal-ligand coordination interactions to template the framework formation and to control its topology, as demonstrated by this work, may have a broad impact in the synthetic design of porous frameworks.
The research supported by this PRF grant has had significant impact on the PI's project development and on the career of students involved. It is with this PRF support, the PI was able to start experimenting with the effect of heterometal in the synthesis of MOFs and extended the application of urothermal synthesis to the heterometallic system. One graduate student, Chengtsung Chou finished his MS thesis (only MS degree is offered in our graduate program) through participating in this PRF-supported project. An undergraduate student, Addis Fuhr, was recently accepted to the NIH RISE program, which provides partial financial support for his continued research involving this PRF project.