ABSTRACT
In a supported ionic liquid phase (SILP) catalyst system, an ionic liquid (IL) film is immobilized on a high-surface area porous solid and a homogeneous catalyst is dissolved in this supported IL layer, thereby combining the attractive features of homogeneous catalysts with the benefits of heterogeneous catalysts. In this review reliable strategies for the immobilization of molecular catalysts in SILPs are surveyed. In the first part, general aspects concerning the application of SILP catalysts are presented, focusing on the type of catalyst, support, ionic liquid and reaction conditions. Secondly, organic reactions in which SILP technology is applied to improve the performance of homogeneous transition-metal catalysts are presented: hydroformylation, metathesis reactions, carbonylation, hydrogenation, hydroamination, coupling reactions and asymmetric reactions.
ABSTRACT
The palladium-catalyzed hydrogenolysis of aromatic ketones to alkylbenzenes was studied in mixtures of ionic liquids to explore the promotional effect of these reaction media. Choline-based ionic liquids displayed complete miscibility with the aromatic ketone substrate at reaction temperature and a clear phase separation of the derived alkylbenzene product at room temperature. Selected ionic liquids were then assessed as reaction media in the hydrogenolysis of aromatic ketones over palladium catalysts. A binary mixture of choline and betainium bis(trifluoromethylsulfonyl)imide ionic liquids resulted in the highest conversion and selectivity values in the hydrogenolysis of acetophenone. At the end of the reaction, the immiscible alkylbenzene separates from the ionic liquid mixture and the pure product phase can be isolated by simple decantation. After optimization of the reaction conditions, high yields (>90 %) of alkylbenzene were obtained in all cases. The catalyst and the ionic liquid could be used at least three times without any loss of activity or selectivity.
Subject(s)
Betaine/analogs & derivatives , Choline/chemistry , Complex Mixtures/chemistry , Hydrogen/chemistry , Ionic Liquids/chemistry , Ketones/chemistry , Benzene/chemistry , Betaine/chemistry , Catalysis , Hydrogen-Ion Concentration , Palladium/chemistryABSTRACT
Reliable strategies are presented for the immobilization of molecular catalysts for selective oxidation in the liquid phase. Besides classical strategies such as ion exchange or covalent anchoring, new approaches are emerging, e.g. based on supported ionic-liquid phases or on incorporation of the active centre in a coordination polymer or a metal-organic framework.
ABSTRACT
Pretreatment of molybdate-exchanged layered double hydroxides in polyalcohols such as ethylene glycol affords heterogeneous catalysts showing largely improved oxidant efficiency compared to the unmodified materials.
Subject(s)
Glycols/chemistry , Hydrogen Peroxide/chemistry , Molybdenum/chemistry , Singlet Oxygen/chemistry , CatalysisABSTRACT
The catalytic disproportionation of hydrogen peroxide into singlet molecular oxygen was studied using the combined action of lanthanum(III) and molybdenum(VI). A synergistic effect was observed between both metals, resulting in a strong acceleration of the H2O2 disproportionation. An optimum in the catalytic activity was found at La/Mo and La/NaOH molar ratios of 4/1 and 1/3, respectively.
ABSTRACT
The convergent synthesis of a series of porphyrin-functionalized pyrimidine dendrimers has been accomplished by a procedure involving the nucleophilic aromatic substitution (NAS) as a key reaction step. The resulting dendritic porphyrin catalysts show high activity in the light-induced generation of singlet oxygen ((1)O2) from ground-state oxygen. These materials are synthetically useful photosensitizers for the oxidation of various olefinic compounds to the corresponding allylic hydroperoxides. Catalytic activities and regio- and stereoselectivities of the dendritic photosensitizers are comparable to those observed for mononuclear porphyrin catalysts. Recycling of the dendrimer-enlarged homogeneous photocatalysts was possible by solvent-resistant nanofiltration (SRNF) by using an oxidatively stable membrane consisting of a polysiloxane polymer and ultrastable Y zeolite as inorganic filler. Moreover, this membrane technology provides a safe way to isolate the hydroperoxide products under very mild conditions. The membrane showed high retention for the macromolecular catalysts, even in chlorinated solvents, but some oxidative degradation of the porphyrin units of the dendrimer was observed over multiple catalytic runs.
Subject(s)
Membranes, Artificial , Porphyrins/chemistry , Pyrimidines/chemical synthesis , Catalysis , Filtration , Molecular Structure , Nanotechnology , PhotochemistryABSTRACT
Lanthanum(III)-exchanged zeolites Beta and USY are active and selective catalysts for the generation of singlet oxygen from H2O2 showing superior activity and oxidant efficiency compared to unsupported La-catalysts, e.g. La(OH)3.
Subject(s)
Hydrogen Peroxide/chemistry , Lanthanum/chemistry , Singlet Oxygen/chemistry , Zeolites/chemistry , CatalysisABSTRACT
[reaction: see text] Electrophilic activation of hydrogen peroxide can be achieved in acidic alcohol solvents without the need for a metal catalyst. This concept is illustrated by the epoxidation of alkenes with H(2)O(2) employing phenol as a solvent. It is proposed that intermolecular hydrogen bonding between H(2)O(2) and phenol activates H(2)O(2) for oxygen-atom transfer. In this interaction, the role of phenol is purely catalytic.
