Resolving the Mechanism for H2O2 Decomposition over Zr(IV)-Substituted Lindqvist Tungstate: Evidence of Singlet Oxygen Intermediacy

Abstract

The decomposition of hydrogen peroxide (H₂O₂) is the main undesired side reaction in catalytic oxidation processes of industrial interest that make use of H₂O₂ as a terminal oxidant, such as the epoxidation of alkenes. However, the mechanism responsible for this reaction is still poorly understood, thus hindering the development of design rules to maximize the efficiency of catalytic oxidations in terms of product selectivity and oxidant utilization efficiency. Here, we thoroughly investigated the H₂O₂ decomposition mechanism using a Zr-monosubstituted dimeric Lindqvist tungstate, (Bu₄N)₆[{W₅O₁₈Zr(μ-OH)}₂] ({ZrW₅}₂), which revealed high activity for this reaction in acetonitrile. The mechanism of the {ZrW₅}₂-catalyzed H₂O₂ degradation in the absence of an organic substrate was investigated using kinetic, spectroscopic, and computational tools. The reaction is first order in the Zr catalyst and shows saturation behavior with increasing H₂O₂ concentration. The apparent activation energy is 11.5 kcal·mol^-1, which is significantly lower than the values previously found for Ti- and Nb-substituted Lindqvist tungstates (14.6 and 16.7 kcal·mol^-1, respectively). EPR spectroscopic studies indicated the formation of superoxide radicals, while EPR with a specific singlet oxygen trap, 2,2,6,6-tetramethylpiperidone (4-oxo-TEMP), revealed the generation of ¹O₂. The interaction of test substrates, α-terpinene and tetramethylethylene, with H₂O₂ in the presence of {ZrW₅}₂ corroborated the formation of products typical of the oxidation processes that engage ¹O₂ (endoperoxide ascaridole and 2,3-dimethyl-3-butene-2-hydroperoxide, respectively). While radical scavengers ^tBuOH and p-benzoquinone produced no effect on the peroxide product yield, the addition of 4-oxo-TEMP significantly reduced it. After optimization of the reaction conditions, a 90% yield of ascaridole was attained. DFT calculations provided an atomistic description of the H₂O₂ decomposition mechanism by Zr-substituted Lindqvist tungstate catalysts. Calculations showed that the reaction proceeds through a Zr-trioxidane [Zr-η²-OO(OH)] key intermediate, whose formation is the rate-determining step. The Zr-substituted POM activates heterolytically a first H₂O₂ molecule to generate a Zr-peroxo species, which attacks nucleophilically to a second H₂O₂, causing its heterolytic O-O cleavage to yield the Zr-trioxidane complex. In agreement with spectroscopic and kinetic studies, the lowest-energy pathway involves dimeric Zr species and an inner-sphere mechanism. Still, we also found monomeric inner- and outer-sphere pathways that are close in energy and could coexist with the dimeric one. The highly reactive Zr-trioxidane intermediate can evolve heterolytically to release singlet oxygen and also decompose homolytically, producing superoxide as the predominant radical species. For H₂O₂ decomposition by Ti- and Nb-substituted POMs, we also propose the formation of the TM-trioxidane key intermediate, finding good agreement with the observed trends in apparent activation energies.