Evolution of the active species of homogeneous Ru hydrodeoxygenation catalysts in ionic liquids.

This work establishes structure-property relationships in Ru-based catalytic systems for selective hydrodeoxygenation of ketones to alkenes by combining extensive catalytic testing, in situ X-ray absorption spectroscopy (XAS) under high pressures and temperatures and ex situ XAS structural characterization supported by density functional theory (DFT) calculations. Catalytic tests revealed the difference in hydrogenation selectivity for ketones (exemplified by acetone) or alkenes (exemplified by propene) upon changing the reaction conditions, more specifically in the presence of CO during a pretreatment step. XAS data demonstrated the evolution of the local ruthenium structure with different amounts of Cl/Br and CO ligands. In addition, in the absence of CO, the catalyst was reduced to Ru0, and this was associated with a significant decrease of the selectivity for ketone hydrogenation. For the Ru-bromide carbonyl complex, selectivity towards acetone hydrogenation over propene hydrogenation was explained on the basis of different relative energies of the first intermediate states of each reaction. These results give a complete understanding of the evolution of the Ru species, used for the catalytic valorization of biobased polyols to olefins in ionic liquids, identifying the undesired deactivation routes as well as possibilities for reactivation.

Acid zeolites as alcohol racemization catalysts: screening and application in biphasic dynamic kinetic resolution.

Acid zeolites were screened as heterogeneous catalysts for racemization of benzylic alcohols. The most promising zeolites appeared to be H-Beta zeolites, for which the optimal reaction conditions were studied in further detail. The zeolite performance was compared to that of homogeneous acids and acid resins under similar reaction conditions. In a second part of the research, H-Beta zeolites were applied in dynamic kinetic resolution (DKR) of 1-phenylethanol, which was conducted by means of a two-phase approach and which resulted in yields smoothly crossing the 50% border up to 90%, with an enantiomeric excess of >99%. To explore the applicability of this biphasic methodology, several other substrates were examined in the standard racemization reaction and in the biphasic dynamic kinetic resolution.

Use of WO(4)(2-) on layered double hydroxides for mild oxidative bromination and bromide-assisted epoxidation with H(2)O(2).

Tungstate, exchanged on a (Ni,Al) layered double hydroxide, is applied as a heterogeneous catalyst in the oxidation of bromide with H(2)O(2) and the ensuing electrophilic bromination of olefins. The high halogenation activity of the catalyst in essentially neutral conditions mimicks the activity of V-bromoperoxidase enzymes. In water, aromatic and aliphatic olefins are selectively converted to bromohydrins; in methanol, methoxybromides are produced. In appropriate solvent conditions, the bromohydroxylation of geminally di-, tri-, and tetrasubstituted olefins proceeds via dehydrobromination to the epoxide. Evidence for this mechanism is provided by kinetic and labeling experiments. This one-pot alternative for the two-step halohydrin epoxidation process is enabled by the mild pH conditions; bromide is effective in substoichiometric, catalytic amounts. All new catalytic procedures are characterized by a high oxidative stability of the catalyst, high productivity of the catalyst on weight basis, high W turnover frequencies in ambient conditions (up to 50 mol of product per W per h), and high chemo-, regio-, and stereoselectivities.

Kinetic and spectroscopic study of 1O2 generation from H2O2 catalyzed by LDH-MoO4(2-) (LDH=layered double hydroxide).

Layered double hydroxides (LDHs), exchanged with molybdate, decompose H2O2 to form one molecule of singlet-state dioxygen (1O2) from two molecules of H2O2. The dependence of the kinetics of H2O2 decomposition on Mo and H2O2 concentrations and on temperature has been related to structural characteristics of the material (X-ray diffraction (XRD), scanning electron microscopy (SEM), IR spectroscopy, N2 adsorption, thermogravimetry) and to molybdate speciation as revealed by in-situ studies in the presence of peroxide (FT Raman, diffuse reflectance UV/visible spectroscopy). The H2O2 decomposition rate is linearly correlated with the amount of LDH-exchanged molybdate, except when a considerable fraction of the molybdate occupies less accessible interlayer positions. A maximum in the H2O2 decomposition rate as the H2O2 concentration is increased is due to the successive formation of mono-, di-, tri-, and tetraperoxomolybdates. This behavior was modeled successfully by using the equilibrium constants for formation of the Mo-peroxo complexes, and the rate constants for decay of the peroxomolybdates with 1O2 liberation. Time-resolved diffuse reflectance and Raman observations of the various MoO4(2-)-peroxide adducts are in line with the proposed kinetic scheme. Of all the Mo-peroxo species on the LDH, the triperoxomolybdate has the highest rate for decay to 1O2. Comparison with the kinetics of dissolved molybdate shows that the monomolecular decay of all peroxomolybdate species proceeds much faster at the LDH surface than in solution. Consequently, maximal rates per Mo atom are at least twice as high for the heterogeneous LDH catalyst as for the homogeneous systems.