Site‐Isolated Heterogeneous Catalysts

Abstract

IntroductionNumerous supported metal catalysts have been utilized for a variety of chemical processes, and their crucial advantages are (i) easy separation of the catalyst and reaction medium, (ii) high durability of the catalyst, and (iii) unique catalytic performance from that of homogeneous analogs. The structures of homogeneous catalysts have been thoroughly studied in organometallic and coordination chemistry, wherein catalytic behavior can be regulated by tuning the ligands of homogeneous metal complexes. In contrast, the heterogeneity and complexity of support surfaces often hinder full understanding of relationships between catalytically active structures and reactivity for heterogeneous catalysts. Regulating catalytic performance at the molecular level therefore remains more difficult for heterogeneous supported metal catalysts compared with their homogenous analogs.To construct well-controlled metal coordination structures for selective catalysis, attachment techniques for metal complexes have been developed, and various homogeneous precursors have been chemically attached to support surfaces in a controllable manner [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. There are several types of attachment techniques: coordination of metal-complex precursors to immobilized ligands on organic polymers [17][18][19][20][21], coordination of metal-complex precursors to functional ligands bound to oxide surfaces [2,4,[22][23][24][25], intercalation into clay materials [26], ion exchange into porous materials such as zeolites and mesoporous silicas [27], and direct attachment of metal-complex precursors onto oxide surfaces [3,6,11,28,29]. The selective immobilization of a homogeneous metal complex can isolate each metal site on a support surface, resulting in the formation of a well-defined single-site catalyst. The interaction between a supported metal complex and support surface often produces a unique metal coordination structure, different from that of a homogeneous counterpart. Consequently, unique catalytic performance can be obtained on the surface.In this chapter, several recent examples of site-isolated heterogeneous catalysts on solid support surfaces are highlighted. The coordination structures of siteisolated supported metal complexes, their unique catalytic performance, and their Heterogeneous Catalysts for Clean Technology: Spectroscopy, Design, and Monitoring, First Edition. Edited

Catalysis by Supported Single Metal Atoms

“…The organometallic complexes possess well-defined structures, and they can be anchored to the support surfaces by ligands. A typical example is to graft organometallic complexes on a silica surface. − The active sites of SIHCs may contain many metal atoms and the metals do not have to be atomically dispersed. The unique feature of a SIHC is that the active center is protected by ligands, and all the active centers behave in the same manner.…”

“…Generally speaking, SSHC can usually be realized with ordered microporous/mesoporous supports such as zeolites, whereas functionalization and immobilization of functional groups are critical to developing SIHCs. − SACs in general are not SSHCs because the interactions of isolated metal atoms with the support strongly depend on the heterogeneous nature of the surface structures and defects of the support materials. When single metal atoms are incorporated into well-defined and highly ordered mesoporous structures or when they only interact with structurally well-defined functional groups on the support surfaces, they can be considered as SSHCs.…”