Prabir K. Dutta and Subhrakanti Chahraborty
Department of Chemistry, The Ohio State University, Columbus,
OH 43210, USA
2.1 INTRODUCTION
Zeolites are crystalline microporous aluminosilicates with primary building units of TO4 (T = Al, Si) tetrahedra, connected by T—O—T bonds and forming a topology with cages and channels of molecular dimensions [1,2]. Due to the presence of aluminum in the framework, the charge imbalance is balanced by extra framework cations, which are readily ion-exchangeable. About 140 zeolite frameworks are known [3], though many more are possible, and synthesis of new zeolites is a very active area of research [4]. Thus, not only do zeolites provide a very diverse set of topologies, but even within the same framework, the ion-exchangeable cations and the Si/Al ratio can be varied. These structural features give rise to unique ion-exchange, separation and catalysis properties, and zeolites are important technological materials that find extensive use in environmental, consumer, chemical, and petrochemical industries [2].
In recent years, the physics and chemistry of photo-induced processes in zeolites have been an area of active research, especially the constraining role of the zeolite and intrazeolitic electric fields that influence excited state properties. Besides, zeolites can stabilize metastable species, providing photochemical routes that are typically unfavorable in other media. There are several reviews of light activated processes in zeolites [5-7].
In the present article, we discuss several photoactivated processes using zeolites. These studies are relevant in materials science, energy, and environmental applications. Our focus has been on the most recent literature since our last review was written [7]. Figure 2.1 shows the frameworks of six different zeolite structures that are commonly studied as photochemical hosts. These include zeolite MOR (Figure 2.1a and b, intersecting 8- and 12-membered rings
6.5
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x 2 and 2.6 x 5.7 A2), FER (Figure 2.1c and d, 10-member ring intersecting 8-member
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(e)
FIGURE 2.1 Stick and 3D models of zeolite framework types MOR(a, b, intersecting 8- and 12-membered rings 6.5 x 7.0 A2 and 2.6 x 5.7 A2), FER(c, d, 10-member ring intersecting 8-member ring 4.2 x 5.4 A2 and 3.5 x 4.8 A2), LTL(e, f, 1-dimesional 7.1 A 12-membered ring), LTA(g, h, 3-dimensional 8-ring 4.1 A pore opening), MFI(i, j, 3-dimensional intersecting 10- member ring 5.3 x 5.6 A2 and 5.1 x 5.5 A2), FAU(k, l, 3-dimensional 12-ring 7.4 A pore opening).
(k)
FIGURE 2.1 Continued.
ring 4.2 x5.4 A2 and 2.5 x4.8 A2), LTL (Figure 2.1e and f, 1-dimensional 7.1 A 12-membered ring), LTA (Figure 2.1g and h, 3-dimensional 8-ring 4.1 A pore opening), MFI (Figure 2.1i and j, 3-dimensional intersecting 10-member ring 5.3 x 5.6 A2 and 5.1 x 5.5 A2), FAU (Figure 2.1k and l, 3-dimensional 12-ring 7.4 A pore opening).
