Single-Event MicroKinetics of Aromatics Hydrogenation on a Pt catalyst

A fundamental single-event microkinetic (SEMK) model for the hydrogenation of aromatic components on a Pt catalyst has been developed. It is based on the Horiuti-Polanyi mechanism considering atomic hydrogen addition steps to the (partially hydrogenated) aromatic species on the catalyst surface. The reaction network used accounts for the position at which the hydrogen atoms are added to the ring. The results from quantum chemical calculations allowed assuming that the kinetic parameters only depend on the degree of saturation of the nearest neighbour carbon atoms of the carbon atom involved in the hydrogen atom addition. The total number of 18 model parameters was reduced to 7 by calculation of the preexponential factors and by accounting for thermodynamic constraints. The model has been regressed to an experimental benzene hydrogenation data set consisting of 43 points measured at temperatures in the range from 423 K to 498 K, benzene inlet partial pressures in the range from 10 kPa to 60 kPa and hydrogen inlet partial pressures from 100 kPa to 600 kPa. The simultaneous estimation of all parameters results in an F-value of 5 103. The coverage dependent chemisorption enthalpy of hydrogen amounts to -60.0 ± 11.3 kJ/mol and corresponds with a surface coverage of 25%. A value of -64.4 ± 6.4 kJ/mol for the chemisorption enthalpy of benzene is estimated. Under typical experimental conditions the surface coverages of the partially hydrogenated species range from 1.2% to 14.7%. The total surface coverage by hydrocarbon species amounts to 60%, without a single dominant surface intermediate. The remaining 15 % of the surface is free. INTRODUCTION The hydrogenation of benzene is studied as part of the hydrocracking process. The hydrogenation or saturation of aromatic components is of increasing interest due to the more stringent environmental legislation. Moreover, the removal of aromatic components is benificial for a diesel’s quality as the cetane number increases with decreasing aromatic content [1]. The kinetic modeling of aromatic hydrogenation reactions based on steady-state data without in situ characterization of the surface cannot make use of direct information concerning the surface intermediates. This has lead to a variety of possible kinetic models presented for the hydrogenation of aromatic model components. However, recently a reaction path analysis for the catalyzed benzene hydrogenation based on quantum chemical calculations [2] has lead to the construction of more fundamental kinetic models [4][5]. Originally, the SEMK methodology has been applied to thermal [6] and acid catalyzed reactions [7][8]. Recently the methodology has been extended to metal catalyzed reactions using Fischer Tropsch synthesis as example reaction [9]. The current work demonstrates the adequacy of the SEMK methodology for the hydrogenation of aromatic components on a noble metal such as Pt. The model is being validated for benzene hydrogenation and is ready for validation with any other monoaromatic component. PROCEDURES The experiments were performed in a gas phase continuous stirred tank reactor at temperatures in the range of 423–498 K, H2 inlet partial pressures of 100–600 kPa and benzene inlet partial pressures of 10–60 kPa. The total pressure was in the range 1-3 MPa, while space times from 22.5 to 69.2 kg s mol -1 have been used. At these conditions intrinsic kinetics are measured since the maximum Weisz moduli obtained amounted to 10 -2 . A Levenberg-Marquardt algorithm was used for parameter estimation. An objective function defined as the sum of squared residuals between the observed and calculated outlet cyclohexane flow rates, is minimized. The minimization occurs through adjustment of the model parameter vector b which approximates the real parameter vector β at the minimal value of the objective function: min ) F̂ F ( SSQ b cyclohex nob j cyclohex →  − =∑ = 2