A consistent data base of reaction pathways, kinetics, and mechanisms for catalytic hydrogenation of one-, two-, three-, and four-fused aromatic ring compounds allowed for correlation of their Langmuir-Hinshelwood-Hougen-Watson (LHHW) rate law parameters with molecular structure. A total of 68 hydrogenation and dehydrogenation rate law parameters for 28 aromatic and hydroaromatic compounds were summarized into 7 parameters for quantitative structure/reactivity correlations (QS/RC) that characterized the associated set of series of homologous reactions, i.e. reaction families. Evaluation of the 28 LHHW adsorption constants was accomplished by imposing a correlation betwen the adsorption constant and the number of aromatic rings and the number of saturated carbons. Surface reaction rate constants correlated with the enthalpy of hydrogenation and the highest bond order in the aromatic ring being saturated. Semiempirical molecular orbital calculations provided acceptable estimates of the enthalpy of reaction, which, via compensation, provided estimates of the entropy of reaction, and thus equilibrium constants. The overall parity between measured parameters and those predicted by the 10 QS/RC parameters was very good, and allowed for 88% reduction in the number of parameters needed to model the saturation kinetics of polynuclear aromatic hydrocarbons.
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Table 3 summarizes calculation errors and model likelihood information associated with the replacement of individually fitted hydrogenation rate, equilibrium, and adsorption parameters with the corresponding QS/RCs. Clearly, the total sum-of-squares error on concentrations (E) increases as the number of adjustable parameters is reduced with the introduction of QS/RCs. The likelihood of the model, however, as expressed by the probability based on 104E, generally increases with the introductions of QS/RCs. Of all the possible combinations, only two achieve probability greater than 90%, and only one greater than 95%. In this latter case, eq. (llc) is used for the calculation of adsorption constants, eq. (16b) for the rate constants, and eq. (21b) for the equilibrium constants. We may therefore single out this combination as optimal: lnKi = 1.04 + 0.654NAR + 0.0964Nsc (llc) Acknowledgement--The authors acknowledge the financial support of Mobil Research and Development Corporation (Paulsboro Research Laboratory), and the State of Delaware, as authorized by the State Budget Act of Fiscal Years 1990-1992.
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