A QSPR Investigation of Thermal Stability of [Al(CH₃)O]_<i>n</i> Oligomers in Methylaluminoxane Solution: The Identification of a Geometry-Based Descriptor

Fifty-six methylaluminoxane (MAO) cage structures with the general formula [AlMeO]_<i>n</i>, where <i>n</i> ranges from 6 to 12, have been optimized using density functional theory calculations in order to identify relevant chemical descriptors to reveal the thermodynamic stability of MAO. First, NMR properties were calculated for the most stable optimized structures, showing a good agreement with experimental results and revealing a relationship between the calculated ²⁷Al NMR shifts and local geometry of the aluminum atoms. Then, different electronic and geometric descriptors of optimized structures have been calculated and compared via a QSPR approach to various thermodynamic functions: internal energy, enthalpy, and Gibbs free enthalpy (Δ<i>G</i>_r), leading to the identification of a relevant descriptor based on the calculation of the distortion of aluminum sites in the [AlMeO]_<i>n</i> structures. The identified descriptor was thus applied to predict Δ<i>G</i>_r for [AlMeO]_<i>n</i> structures with <i>n</i> ranging from 6 to 33. The study of the evolution of Δ<i>G</i>_r as a function of temperature and size (<i>n</i>) reveals that there is a window of stable sizes for [AlMO]_<i>n</i> depending on the temperature, which is between <i>n</i> = 12 and <i>n</i> = 24. Low temperatures disfavors smaller (<i>n</i> < 12) sized oligomers due to strong distortion of aluminum sites, while at high temperatures [AlMO]_<i>n</i> structures with <i>n</i> greater than 18 become destabilized due to entropic effects

Thermochemistry of 1‑Methylnaphthalene Hydroconversion: Comparison of Group Contribution and ab Initio Models

As a necessary step in the development of microkinetic models for the hydroconversion of heavy hydrocarbon fractions, we report an assessment of various Density Functional Theory (DFT) models for the calculation of molecular thermochemical properties in comparison with Benson’s group contribution method for reactants, intermediates, and products involved in the hydrogenation of 1-methylnaphtalene. The association of the G4 level with homodesmotic decomposition schemes (HI-G4-iso method) has significantly improved the accuracy of the calculated thermodynamic properties when the resemblance of reactants and products is taken into account. Although smaller deviations are observed for Benson’s GA method, some limitations appear when position isomers are included. This gap can be fulfilled with homodesmotic/DFT models, whose deviations are not so far from those obtained with Benson’s GA method

DFT Study on the Impact of the Methylaluminoxane Cocatalyst in Ethylene Oligomerization Using a Titanium-Based Catalyst

A computational study within the framework of density functional theory is presented on the oligomerization of ethylene to yield 1-hexene using [(η⁵-C₅H₄CMe₂C₆H₅)]­TiCl₃/MAO] catalyst. This study explicitly takes into account a methylaluminoxane (MAO) cocatalyst model, where the MAO cluster has become an anionic species after having abstracted one chloride anion, yielding a cationic activated catalyst. Hence, the reaction profile was calculated using the zwitterionic system, and the potential energy surface has been compared to the cationic catalytic system. Modest differences were found between the two free energy profiles. However, we show for the first time that the use of a realistic zwitterionic model is required to obtain a Brønsted–Evans–Polanyi relationship between the energy barriers and reaction energies