Engineered solid catalysis for synthesis of fine chemicals in multiphasic reactant systems

Vos, Dirk De; Sels, Bert F.; Rhijn, Wim M. Van; Jacobs, Pierre

doi:10.1016/s0167-2991(00)80950-7

Cited by 7 publications

(1 citation statement)

References 29 publications

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“…Additionally, WO 4 2--LDH bears other similarities to VB-POs. 28,29 For instance, in the absence of organic molecules that can readily trap the halonium equivalents, the 'Br + ' reacts with H 2 O 2 to produce 1 O 2 via the Kasha-Khan reaction. Singlet dioxygen has been detected in similar conditions in reactions with VBPO enzymes.…”

Section: Methodsmentioning

confidence: 99%

Peroxide Reactions with Anion-exchanged Layered Double Hydroxide Catalysts

Vos¹,

Wahlen²,

Sels³

et al. 2002

Synlett

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Layered double hydroxides (LDHs) are inorganic anion exchangers with basic surface properties. Combination of these LDHs with d 0 oxy-anions leads to a class of oxidatively stable and leak-free heterogeneous oxidation catalysts. Using peroxides as the oxidant, WO 4 2--LDH and V 10 O 28 6--LDH catalyze epoxidations; WO 4 2--LDH effects bromide oxidation and bromide-assisted epoxidation; MoO 4 2--LDH is an inorganic generator of 1 O 2 from H 2 O 2 . The properties of these LDH catalysts can be finely adjusted in order to perform synthetically useful reactions. IntroductionMineral cation exchangers are abundantly found in nature, and comprise a broad variety of structures, such as clays, hydroxyapatite and zeolites. In contrast anion-exchanging minerals are rather rare, and in many cases their structure is related to that of the mineral hydrotalcite. 1 Hydrotalcite consists of brucite-type layers of Mg(OH) 2 in which part of the octahedral Mg 2+ is isomorphously substituted with Al 3+ (Figure 1). This generates an excess positive charge on the brucite layers, and in hydrotalcite this charge is compensated by carbonate anions intercalated in between the brucite layers. By simple co-precipitation of divalent and trivalent metal salts at a pH between 7 and 11, synthetic hydrotalcite analogues can be prepared, and the natural and synthetic materials are generally designed as layered double hydroxides (LDHs) or anionic clays. 2 Ions that are commonly introduced in the octahedral positions are Mg 2+ , Co 2+ , Cu 2+ , Ni 2+ , Zn 2+ , Al 3+ , Cr 3+ , Mn 3+ and Fe 3+ . Catalysis chemists frequently use these layered double hydroxides as precursors to oxide catalysts. Indeed, due to the homogeneous distribution of the cations in the LDH structure, high temperature decomposition of LDHs leads to mixed oxides or mixed oxide solutions. Nevertheless, LDHs can also be useful for catalytic applications in the non-calcined form, by virtue of their anion exchange capacity. 3 Carbonate itself has a very high affinity for the LDH structure, but if synthetic LDHs are prepared with nitrate or chloride anions, these monovalent charge-compensating anions can be partially or fully displaced by catalytically active anions. 4,5 Our involvement in the application of layered double hydroxides started around 1994, at a moment when the potential of heterogeneous Ti catalysts for selective oxygenation became fully clear. 6 While the TS-1 zeolite and related materials such as Ti-Beta zeolite are truly heterogeneous catalysts for liquid phase oxidations, homogeneous oxidation catalysis knows countless examples of useful compounds based on V, Cr, Mn, Fe, Mo, Ru, Pd, W, Re, Os or Pt. 7 Attempts by other laboratories and ourselves to create redox active solids by doping of zeolite frameworks with V or Cr proved unsuccessful, due to the lability of the association of these metals with the framework. 8 At the same time, the value of anionic d 0 W-complexes in biphasic epoxidation became clear. 9,10 Therefore, our initial challenge was to create new, trul...

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Section: Methodsmentioning

confidence: 99%