CATALYSIS UNDER TRANSIENT CONDITIONS Catalytic

In situ spectroscopic measurements of a catalytic system provide a considerable opportunity to determine the chemical species present under reactive conditions. FTIR and NMR have been the two most frequently used in situ spectroscopic methods (see Chapters 2 and 3). They have been successfully used to identify labile, non-isolatable transient species believed to be involved in the catalytic product formation. Furthermore, efforts have been made to use this information in order to obtain more detailed kinetics, by decoupling induction, product formation, and deactivation. Thus, in situ spectroscopic techniques have the potential for considerably advancing mechanistic studies in homogeneous catalysis. 

Figure 8.31. Comparison between experimental and calculated according to eq. 8.117 data for three-phase catalytic hydrogenation in a fixed bed reactor (E. Toukoniitty, P. MSki-Arvela, A. Kalantar Neyestanaki, T. Salmi, D. Yu. Murzin, Continuous hydrogenation of l-phenyl-1,2 - propanedione under transient and steady-state conditions, regioselectivity, enantioselectivity and catalyst deactivation, Applied Catalysis A General, 235 (2002) 125).

With the inclusion of more sophisticated techniques such as those mentioned above and others, NMR is an excellent quantitative tool for structural catalyst characterization. On the other hand, the question of how this information can be used to understand catalytic mechanisms and to design more potent catalysts often remains unresolved by these studies. Perhaps this is so because to date most NMR applications have sought to correlate catalytic activities with specific structural features present either in the catalyst or on its surface under room temperature conditions. In future studies there should be increasing emphasis on catalyst characterization under operation conditions in situ, including the search for transient adsorbates and reactive intermediates. In fact, such studies are now emerging in other fields of catalysis [90-92]. 

---