4 research outputs found
A QSPR Investigation of Thermal Stability of [Al(CH<sub>3</sub>)O]<sub><i>n</i></sub> Oligomers in Methylaluminoxane Solution: The Identification of a Geometry-Based Descriptor
Fifty-six methylaluminoxane (MAO) cage structures with
the general formula [AlMeO]<sub><i>n</i></sub>, 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 <sup>27</sup>Al 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 (Ī<i>G</i><sub>r</sub>), leading to the identification of a relevant
descriptor based on the calculation of the distortion of aluminum
sites in the [AlMeO]<sub><i>n</i></sub> structures. The
identified descriptor was thus applied to predict Ī<i>G</i><sub>r</sub> for [AlMeO]<sub><i>n</i></sub> structures
with <i>n</i> ranging from 6 to 33. The study of the evolution
of Ī<i>G</i><sub>r</sub> as a function of temperature
and size (<i>n</i>) reveals that there is a window of stable
sizes for [AlMO]<sub><i>n</i></sub> 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]<sub><i>n</i></sub> structures
with <i>n</i> greater than 18 become destabilized due to
entropic effects
Thermochemistry of 1āMethylnaphthalene 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 Bensonās
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
Bensonās 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 Bensonās
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 [(Ī·<sup>5</sup>-C<sub>5</sub>H<sub>4</sub>CMe<sub>2</sub>C<sub>6</sub>H<sub>5</sub>)]ĀTiCl<sub>3</sub>/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
BrĆønstedāEvansāPolanyi relationship between the
energy barriers and reaction energies