Photoelectron Transfer in Zeolite Cages and Its Relevance to Solar Energy Conversion

The cages and channels of aluminosilicate zeolites provide a novel environment for molecular and nanoparticle assembly for photochemical reactions. In their dehydrated forms, zeolites can be active participants in reactions with photoexcited entrapped molecules as electron donors and acceptors. The charge-separated species thus formed are stabilized for hours. With hydrated zeolites, the encapsulation and the restricted mobility can result in long-lived chargeseparated species. In order to exploit intrazeolitic photoelectron transfer, the role of structural defects, steric effects, electrostatic polarizing fields, and extraframework cations in formation and stabilization of charge-separated species needs to be better elucidated. Such efforts will be facilitated with better control of synthesis of molecular and nanoparticle assemblies within the zeolite, rather than the random distribution mostly practiced to date. Artificial photosynthetic assemblies within zeolites aimed toward practical photolytic water splitting have potential because of varied ways of charge transport, including via the framework or molecules, as well as the synthesis of zeolite membranes that can propagate light, cations, and electrons over macroscopic distances. Assembly of catalysts capable of multielectron/hole processes within and at zeolite interfaces needs to be coupled with photochemical systems. Better integration strategies for combining efficient light collection, directed charge separation/propagation, and catalysis are necessary for practical impact. S chemistry is a potentially new way of generating fuels and chemicals without the environmental and geopolitical hazards currently facing society. However, these chemistries, which in many cases involve redox reactions, for example, formation of H2 from water, are complex, and suitable architectures will be necessary.1-3 Nature provides suitable models of such architecture, and biomimetic approaches are being actively researched. Photosynthesis, which makes possible life on earth, involves collection, conversion, and storage of solar energy as chemical energy. Mediated by an enzyme, photosystem II uses light energy to make oxygen, protons, and electrons from water. The electrons are used by photosystem I along with light to reduce nicotinamide adenine dinucleotide phosphate, whereas the protons generate a transmembrane electrochemical potential that drives ATP synthesis. The architecture necessary to accomplish this is a sophisticated assembly of pigments, enzymes, and proteins in a membrane scaffold. For a practical artificial photosynthesis system, the efficiencies of the solar to chemicals process have to exceed 10%. The successful assembly of superstructures necessary to accomplish these goals requires both fundamental and technological breakthroughs. Microheterogeneous systems such as vesicles, clays, mesoporous materials, and zeolites are being actively studied as host systems for assembly of photoactive units. In this Perspective, we focus on zeolites, with primary emphasis on light-driven electron transfer and photocatalysis for H2 formation from water, the fundamental ingredients for solar energy conversion. Brief Primer on Zeolites. Zeolites are microporous, crystalline aluminosilicates with the framework made up of T—O—T (T = Si, Al) bonds and enclosed cages and channels of molecular dimensions. Figure 1a-d shows the topology of several zeolites, including zeolites L, Y, ZSM-5, and the titanosilicate ETS-4, respectively (along with a 3-D perspective); these frameworks have been the most examined for studies relevant to this Perspective. Zeolite synthesis typically takes place in an aqueous medium, and over 140 frameworks are known. Because the aluminosilicate framework carries a negative charge, extraframework cations are present as charge-balancing units within the porous framework. Neutral molecules are typically introduced into the empty zeolite after removal of intrazeolitic water, whereas charged cations can be introduced via ion exchange in aqueous Received: November 4, 2010 Accepted: February 4, 2011