Compensation between activation entropy and enthalpy in reactions of aromatic hydrocarbons catalyzed by solid acids

Abstract

Reaction rates of toluene disproportionation (A) and cumene cracking (B) normalized by the number of Brønsted acid sites were analyzed on aluminosilicates. The activation entropy showed linear and compensatory relationship against the activation enthalpy. The slope of entropy-enthalpy plot was in the order of (B) > (A) > small alkane cracking, whereas the intercept on the horizontal axis was in the order of propane and isobutane cracking > linear C4-8 alkanes and iso-pentane cracking > (A) > (B). The former is consistent with the bulkiness of reactant, while the latter is consistent with intrinsic difficulty of formation of intermediate cations.

Controlled Deposition of Silica on Titania-Silica to Alter the Active Site Surroundings on Epoxidation Catalysts

“…Lower apparent activation enthalpies reflect stronger adsorption of the reactant, which leads to lower apparent activation entropies, as the reactant loses more degrees of freedom. 40,41 This is illustrated in Figure 5c for Ti-SiO 2 and 2cPO catalysts.…”

“…Apparent activation entropies ranged from −240 to −165 J/(mol K), which must be due to loss of limonene translational entropy and changes to solvent mobility near the active site. , Figure b shows a strong compensation effect between low apparent activation enthalpies and more negative apparent activation entropies. Lower apparent activation enthalpies reflect stronger adsorption of the reactant, which leads to lower apparent activation entropies, as the reactant loses more degrees of freedom. , This is illustrated in Figure c for Ti-SiO 2 and 2cPO catalysts.…”

“…Given that the surfaces of all of these materials are hydroxylated silica, it suggests something akin to steric confinement due to the microporous SiO 2 layer deposited around the active sites. Confinement effects have thus far been exclusively observed for materials with an extended pore structure. ,, We are unable to directly assess the size of any putative cavity at the active site, and thus we cannot claim a conventional confinement effect. However, these experiments do show that adsorbate stabilization can also arise from the engineering of the local oxide environment.…”

“…Confinement effects have thus far been exclusively observed for materials with an extended pore structure. 6,41,44 We are unable to directly assess the size of any putative cavity at the active site, and thus we cannot claim a conventional confinement effect. However, these experiments do show that adsorbate stabilization can also arise from the engineering of the local oxide environment.…”

Beyond Disorder: A New Perspective on Entropy in Chemistry

“…The rearrangement of atoms and the associated changes in enthalpy and entropy contribute to shifts in the Gibbs free energy of the system. The interplay of entropy, enthalpy and Gibbs free energy illustrates the intricate thermodynamics of chemical reactions, where disorder, energy changes and the overall feasibility of the process are closely intertwined [6,7].…”