Kinetics Quantitative Structure Reactivity Correlations

The rate constants within each reaction family were described in terms of a reaction family-specific Arrhenius A factor and the Polanyi relationship parameters that related the activation energy to the enthalpy change of reaction, as shown in Eq. 1. 

The Polanyi relationship and the Arrhenius e qiression can be combined to represent the rate constant, k ij, where i denotes the reaction family and j denotes the specific reaction in the family, and is shown in Eq. 4  

The acidity of the catalyst was captured by a single parameter stabilization = signifying the relative stabilization of the H+ ion as compared to other carbenium ions. Since the reactions on the acid site are rate controlling, this was a useful way to capture the catalyst property (acidity) in the rate constant formalism, as shown in Eq. 5. 

In short, each reaction family could be described with a maximiun of three parameters (A, Eo, a). Procurement of a rate constant from these parameters required only an estimate of the enthalpy change of reaction for each elementary step. In principle, this enthalpy change of reaction amoimted to the simple calculation of the difference between the heats of formation of the products and reactants. However, since many model species, particularly the ionic intermediates and olefins, were without experimental values, a computational chemistry package, MOPAC, ° was used to estimate the heat of formations on the fly . Ihe organization of the rate constants into quantitative structure-reactivity correlations (QSRC) reduced the number of model parameters greatly Ifom O(IO ) to 0(10). 

A Ci6 hydrocracking model with 465 species and 1503 reactions was built automatically in only 14 CPU seconds on a Pentium Pro 200 PC. The corresponding plug flow reactor (PFR) model with molar expansion was then 

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During the past 20 years, academic and industrial researchers developed composition-based kinetic models with hundreds or even thousands of lumps and pure compounds. The QSRC (quantitative stracture-reactivity correlation) and LFER (linear free energy relationship) lumping techniques are discussed in Chapter 20 by Professor Klein and Chapter 9 by Professor Mochida. The structure-oriented liunping (SOL) approach of Quann and Jaffe yields models rigorous enough for use in closed-loop real-time optimizers (CLRTO), which automatically adjust setpoints for commercial process units several times each day. ... 

Thermal reactivity of a-carbonyl- and a-thiocarbonyl-stabilized methylenet-riphenylphosphoranes has been investigated. The kinetic data provided valuable support and insight into the mechanism of pyrolysis and a quantitative basis for structure/molecular reactivity correlations. The pyrolysis reactions of triphenylphos-phonium ylides stabilized by sulfone and C=X (X=0, S) groups represent alternative and environmentally friendly routes to the synthesis of novel alkene and alkyne compounds. 

A quantitative description of relations between structural parameters of solid catalysts or substrates on one side and reaction rates or adsorption equilibria on the other side, even if valid only in limited areas, may form an important step in the development of a general theory of catalysis. Some years ago, Boudart 1) noticed that such correlations in heterogeneous catalysis can be divided into two broad classes. In the first type a series of catalysts is tested by means of a standard reaction and some kinetic parameter is related to a property of the solid catalyst. In correlations of the second type, the reactivity of a series of compounds is studied on a single catalyst and some kinetic parameter is related to a property of the reacting molecules. Boudart pointed out that correlations of the first type are more frequent in the literature than those of the second type. He also presented some examples of both types. Correlations between the substrate structure and its reactivity were qualitative or semiquantitative. 

The reactivity of vinylojy and allylo q monomers derived from polyols and simple carbohydrates was quantitatively evaluated in free-radical UV-initiated copolymerization with acceptor monomers such as DEF or DEM. Kinetic data confirmed the lower reactivity of allylethers compared with vinylo>qr analologs as described in literature. Interestingly, allyl ribosides exhibited the highest reactivity in the allylojgr series combined with the highest final conversion levels. A correlation between the donor monomer structure described by Hansen parameter 8h, corresponding to H-bonding interactions, and the initial polymerization rate has been established. 

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