An alternative approach to controlling selectivity is the spatial confinement of catalysts within supramolecular host-guest structures to control the conformational arrangement of substrates, product, and catalysts. The encapsulation of Rh I based hydroformylation catalysts within a Zn IItetraphenylporphyrin host (Zn-TPP) represents a pioneering example. The host was assembled via coordination of three Zn-TPP units to a tris(m-pyridyl)phosphine, which then binds the Rh I . In the hydroformylation of 1-octene, selectivity was inverted towards the branched aldehyde using the encapsulated system, in contrast to the linear selectivity observed for (Rh(acac)(CO)2). Several reviews have since been written on supramolecular host-guest catalysis that cover both effects on activity and selectivity. Spatially localizing a catalyst on the surface of a silica support is another important spatial control method that can be used to address compatibility issues. Although the general perception is that immobilizing the catalyst onto a surface reduces activity due to hindered mass transport, the activity loss can be compensated with appropriate system modifications and optimization. Following the work of Brookhart and Goldman in homogeneous Ir alkane (de)hydrogenation, Huang and co-workers reported the use of various γ-Al2O3 supported Ir complexes (Ir@γ-Al2O3) for polyolefin degradation (FIG. ). In this example, Ir@γ-Al2O3 was used in concert with an alkane metathesis catalyst (Re2O7/Al2O3) and a light alkane to degrade polyethylene. The separation of molecular Ir complexes from the Re alkane metathesis catalyst circumvents any unwanted catalyst-catalyst interaction. In addition, this system proved effective even when commercial polyethylene products such as plastic bottles and food packaging were employed.