Abstract: Ti(IV) and Nb(V) single site catalysts have attracted much recent attention due to their ability to catalyze selective oxidations with the green oxidant hydrogen peroxide. However, their catalytic performance differs significantly in terms of activity, chemo- and regioselectivity [1-3]. In this lecture, we present our recent works directed toward understanding the mechanisms of H2O2 activation over Ti(IV) and Nb(V). Ti- and Nb-substituted tungstates of the Lindqvist structure [MW5O18]n- (MW5, M = Ti and Nb) mimic well the catalytic performance of heterogeneous M-silicates. These compounds have been employed as tractable molecular models to gain insights into the structure and reactivity of Ti and Nb peroxo species using UV-vis, FT-IR, Raman, 93Nb, 17O and 183W NMR spectroscopic techniques and DFT calculations [4,5]. For both metals, peroxo form ‘HMO2’ is more stable than hydroperoxo form ‘MOOH’, but ‘MOOH’ is more reactive than ‘HMO2’ toward epoxidation of alkenes. The superior heterolytic pathway selectivity observed for Nb catalysts can be explained by a combination of two main factors: 1) a lower energy barrier for the oxygen atom transfer from ‘NbOOH’ to the C=C bond leading to epoxide and 2) a higher energy cost of homolytic O–O bond breaking in the ‘NbOOH’ intermediate. The size of M and the flexibility of its coordination environment affect the oxygen transfer mechanism. For a rigid Ti(IV) environment, β-oxygen transfer from Ti(h1-OOH) is favored over α-oxygen transfer. The larger size of Nb(V) enables formation of a 7-coordinated Nb(h2-OOH) species for which α-oxygen transfer is energetically preferable over β-oxygen transfer. The different oxygen transfer mechanisms may account for different regioselectivities observed in heterogeneous Ti- and Nb-catalyzed epoxidations.

Cite: Kholdeeva O.A.
Mechanisms of Hydrogen Peroxide Activation over Ti(IV) and Nb(V) Single Sites
XI International Conference Mechanisms of Catalytic Reactions 07-11 Oct 2019