DFT Study of the Isomerization of Hexyl Species Involved in the Acid-Catalyzed Conversion of 2-Methyl-Pentene-2

Abstract

Quantum-chemical calculations were conducted on the basis of density-functional theory to study reactions of hexyl species involved in the acid-catalyzed isomerization of 2-methyl-pentene-2. The production of 4-methyl-pentene-2 and 3-methyl-pentene-2 involves 1,2-migrations of hydrogen atoms and methyl groups whose activation energies are lower than 30 kJ/mol for gaseous carbenium ions. The activation energy for branching rearrangements of gaseous hexyl cations to form 2,3-dimethyl-butene-2 is 94 kJ/mol. Transformations of hexyl species were studied in the presence of gaseous water and an aluminosilicate site to simulate reactions on acidic oxides. In the presence of these oxygenated (conjugate) bases, the cationic center in the carbenium ions bonds with oxygen to form alkoxonium ions and alkoxy species, respectively. The relative energies of these species are fairly insensitive to their secondary or tertiary nature. Reactive intermediates of the same order are stabilized more than the corresponding transition states upon interaction with an oxygenated base, thus leading to an increase in the activation energies of isomerization reactions. Transition states have greater separation of electronic charge than the corresponding alkoxonium ions and alkoxy species. The transition state for branching rearrangement requires the greatest separation of electronic charge in the aluminosilicate cluster; the transition state for methyl migration requires the second greatest separation of electronic charge; transition states for hydride shifts require a smaller separation of electronic charge; and transition states for the protonation of alkenes to form alkoxy species require the least separation of electronic charge in the aluminosilicate cluster. These observations imply the existence of a correlation between the positive charge localized in the hydrocarbon fragment of a transition state and the sensitivity of the corresponding reaction pathway to changes in the acidity of the catalyst. Lastly, activation energies for alkene isomerization reactions over aluminosilicates are determined by the energies of transition states with respect to the gaseous reactants plus the acid site and not by the relative stabilities of the alkoxy intermediates in the reaction scheme.