Periodic mesoporous organosilicas (PMOs) are a versatile inorganic/organic hybrid with well-defined nanoporous structures and high functionalities of the organosilica framework.(1,2) PMOs are synthesized by surfactant-directed self-assembly of organic-bridged alkoxysilane precursors, generally represented as R[Si(OR')3]n (n ≥ 2) (Fig.2). Various organic species ranging from hydrocarbons and heteroaromatics to metal complexes have been introduced into the pore walls of PMOs as a R bridging group.(3,4) The density, distribution, and molecular-scale ordering of the bridging groups within pore walls can be controlled by the molecular design of the precursors,(5-8) optimization of the synthesis conditions, and appropriate selection of the co-condensation technique. Expansion of the variation of organic bridges has broadened the potential applications of PMOs to not only catalysis and adsorption but also optical(9,10) and electronic devices(11).
INAGAKI Senior Fellow Laboratory
Development of new catalysts for efficient conversion of abundant small molecules such as CO2, H2O, and N2 into fuel and resources is one of the most important challenges we should overcome in the near future in order to free ourselves of the oil dependency. However, it is generally difficult to convert those small molecules at mild conditions because they are chemically stable molecules. In this project, we will attempt to mimic the essences of bio-functions of enzymes and photosynthesis, which convert such small molecules at ambient conditions, using highly ordered organic-based nanoporous materials as a scaffold (Fig.1). This study aims to understand the nature of the bio-catalysis and to obtain new insights to design catalysts for efficient conversion of small molecules to useful resources.
Fig. 1 Chemical conversion of small molecules by photosynthesis and enzymes
- 1. Synthesis of mesoporous materials
Fig. 2 Surfactant-directed self-assembly of periodic mesoporous organosilica
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- 2. Artificial photosynthesis
Natural photosynthesis shows the efficient photocatalysis of CO2 reduction to form carbohydrates. One of the key components of natural photosynthesis is a light-harvesting antenna, which absorbs sunlight and funnels the captured energy to a reaction center by fluorescence resonance energy transfer (FRET) with a quantum efficiency of almost unity. We found unique light-harvesting antenna properties of periodic mesoporous organosilicas (PMOs) with organic-inorganic hybrid frameworks in a supramolecular architecture of organic moieties covalently fixed within a siloxane network.(12-14) PMOs funnel the light energy absorbed by approximately 125 organic moieties (biphenyl groups) in the framework into a single dye molecule doped in the mesochannels by FRET, with almost 100% quantum efficiency.(15) We have constructed PMO-based photocatalysis systems(16) for CO2 reduction (Fig. 3),(17,18) and H2 (19) and O2 (20) evolution from water. We are now aiming to construct a sacrificial agent-free molecular photocatalysis system by combining photo-reduction and water oxidation catalysts using a PMO platform.
Fig. 3 Light-harvesting PMO photocatalyst for CO2 reduction
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- 3. Molecular-based heterogeneous catalysis
Synthesis of a solid chelating ligand for the formation of efficient heterogeneous catalysts is highly desired in the fields of organic transformation and solar energy conversion. We succeeded in surfactant-mediated synthesis of a novel periodic mesoporous organosilica (PMO) containing 2-phenylpyridine (PPy) (21) and 2,2’-bipyridine (BPy) (22) ligands within the frameworks (named as PPy-PMO and BPy-PMO, respectively) (Fig.4a).
BPy-PMO is a unique solid chelating ligand because metal complexes can be formed directly on the pore surface, while conventionally they are usually grafted on the surface using organic linkers. BPy-PMO showed excellent solid ligand properties for heterogeneous Ir-catalyzed direct C−H borylation of arenes, resulting in superior activity, durability, and recyclability to the homogeneous analogous Ir catalyst (Fig. 4b).(22,23) An efficient photocatalytic hydrogen evolution system was also constructed by integration of a Ru-complex as a photosensitizer and platinum as a catalyst on the pore surface of BPy-PMO without any electron relay molecules.(22) These results demonstrate the great potential of BPy-PMO as a solid chelating ligand and a useful integration platform for construction of efficient molecular-based heterogeneous catalysis systems.
Fig. 4 Structural models of BPy-PMO (a) and Ir-complexes formed on the pore surface of BPy-PMO (b).
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