(300f) Kinetics of Reaction Networks with Multiple Overall Reactions

Most industrial chemical processes involve multiple overall reactions (ORs), their chemistry being described in terms of a common molecular mechanism comprising a set of elementary reaction steps. In fact, innumerable ORs can be written for a specified set of reactants and products. How does, then, one determine which ORs are germane? Which of these ORs are really occurring, and which are superfluous? Does one need a rate expression for each of the chosen ORs? How are these rate expressions derived in terms of the kinetics of the elementary steps? These questions are of increasing significance with the improving capability of predicting the kinetics of the elementary steps from first principles. We discuss a comprehensive approach for addressing these questions based on our Reaction Route (RR) Graph approach for representing molecular mechanisms, along with stoichiometric considerations of the reaction system. The latter provides an independent number of ORs, along with the independent number of pathways in the RR Graph. The RR Graph is a quantitative network, much like electric circuits, that is consistent with Kirchhoff's network laws. The kinetics of the independent ORs can, hence, be determined in analogy to electric circuits, i.e., in an Ohm's law form of QSS rate for the reaction network. The flux analysis of the RR graph allows identification of the dominant pathways in an intuitive manner. We further show how the LHHW methodology, combined with the concept of intermediate reaction might be utilized to obtain the step resistances involved. For illustration, we utilize the example of zeolite catalyzed N₂O decomposition reaction in presence of NO with DFT-predicted step kinetics.