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Assessing the acid strength of solid acid catalysts with the use of linear free energy relationship: H/D exchange with substituted benzene derivatives

2007; Elsevier BV; Volume: 248; Issue: 2 Linguagem: Inglês

10.1016/j.jcat.2007.03.012

1090-2694

Vinicius Ottonio O. Gonçalves, Rogerio Gomes Rodrigues, R LORENCATO, Cláudio J. A. Mota,

Advanced NMR Techniques and Applications

Abstract The acid strength of solid acid materials can be assessed using the linear free energy relationship, applying the Hammett–Brown equation ( log k X / k H = ρ σ + ) for the H/D exchange with substituted benzenes. The magnitude of the ρ varies for different solid acid materials (Amberlyst-15, zeolite HUSY, K-10 montmorillonite, and niobic acid) and is associated with the degree of proton transfer in the transition state. The results were compared with the ρ values obtained for H/D exchange with sulfuric acid solutions, indicating that neither solid acid is stronger than 98% sulfuric acid solution. Calculations also revealed that H/D exchange is mostly concerted, with little degree of charge development in the aromatic ring in the transition state.