Stefan Ernst

Cooperative acid-base effects with functionalized mesoporous silica nanoparticles: applications in carbon-carbon bond-formation reactions.

Acid-base bifunctional mesoporous silica nanoparticles (MSN) were prepared by a one-step synthesis by co-condensation of tetraethoxysilane (TEOS) and silanes possessing amino and/or sulfonic acid groups. Both the functionality and morphology of the particles can be controlled. The grafted functional groups were characterized by using solid-state (29)Si and (13)C cross-polarization/magic angle spinning (CP/MAS) NMR spectroscopy, thermal analysis, and elemental analysis, whereas the structural and the morphological features of the materials were evaluated by using XRD and N(2) adsorption-desorption analyses, and SEM imaging. The catalytic activities of the mono- and bifunctional mesoporous hybrid materials were evaluated in carbon-carbon coupling reactions like the nitroaldol reaction and the one-pot deacetalization-nitroaldol and deacetalization-aldol reactions. Among all the catalysts evaluated, the bifunctional sample containing amine and sulfonic acid groups (MSN-NNH(2)-SO(3)H) showed excellent catalytic activity, whereas the homogeneous catalysts were unable to initiate the reaction due to their mutual neutralization in solution. Therefore a cooperative acid-base activation is envisaged for the carbon-carbon coupling reactions.

A novel family of solid basic catalysts obtained by nitridation of crystalline microporous aluminosilicates and aluminophosphates

Abstract Novel basic catalysts are obtained by ammonia treatment of crystalline, microporous aluminosilicates (zeolites) and aluminophosphates at temperatures above 800°C. The resulting materials are active catalysts in the Knoevenagel condensation of benzaldehyde with malononitrile, presumably due to the presence of nitrogen-containing species bound to the crystalline framework. While nitridation of zeolite NaY at temperatures around or below 800°C does not result in an increase of the catalytic activity, ammonia treatment at 850–875°C produces a significantly more active material. Further typical experimental results are presented which suggest that the activity gain seems to depend not only on the temperature of ammonia treatment but also on the structure and the chemical composition of the parent material. The novel microporous catalysts with their reasonable base strength offer the principle possibility to perform base catalyzed reactions in a shape selective manner.

Activity and selectivity of zeolites MCM-22 and MCM-58 in the alkylation of toluene with propylene

The influence of the pore architecture of zeolites MCM-22 and MCM-58 on their catalytic activity and selectivity in the gas-phase alkylation of toluene with propylene has been investigated. The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-membered-ring pores in the structure of this zeolite, no enhanced selectivity to p-cymene is observed. Moreover, also n-propyltoluenes, which are usually formed over three-dimensional 10-membered-ring zeolites (e.g., ZSM-5 and ZSM-11) via bimolecular transalkylation/isomerization reactions of cymenes with toluene, are only formed to a negligible extent. This has been rationalized by the assumption that most of the reactions occurring in this system take place on acid sites at or close to the external surface. This is supported by the FTIR-spectroscopic observation that p-cymene can reach only about one half of the bridging OH groups, while the smaller d3-acetonitrile reaches/covers virtually all remaining Bronsted and Lewis acid sites. Over zeolite MCM-58, relatively high selectivities to n-propyltoluenes were observed, which has been ascribed to the peculiar undulating 12-membered-ring channel system of this zeolite. It is proposed that the 1,2-p-ditolylpropane-type transition state required for the formation of n-propyltoluenes is preferentially located in positions similar to those which are occupied by the N-benzyl-1,4-diazabicyclo[2.2.2]octane cations used as templates for the synthesis of zeolite MCM-58.

Hydrogenation of olefins over hydrido chlorocarbonyl tris-(triphenylphosphine) ruthenium(II) complex immobilized on functionalized MCM-41 and SBA-15

Abstract Hydrido chlorocarbonyl tris-(triphenylphosphine) ruthenium(II) complex [RuHCl(CO)(PPh 3 ) 3 ] has been immobilized inside the pores of amine functionalized MCM-41 and SBA-15 materials. These grafted complexes were characterized by XRD, FTIR, N 2 adsorption measurements, XPS and tested as catalyst for the hydrogenation of olefins. Immobilization and pore size has marked effects on the catalytic activity in the hydrogenation of olefins and the complex immobilized on functionalized SBA-15 was found to be catalytically more active than the one immobilized on MCM-41.

Oxidation of adamantane by urea hydroperoxide using vanadium complex anchored onto functionalized Si-MCM-41

Abstract A tetradentate complex 3-[ N , N ′-bis-3-(salicylidenamino)ethyltriamine] vanadium(IV), VO(Salten) was grafted onto MCM-41 through a covalently linked organic moiety. This covalently anchored complex was characterized by FTIR, X-ray diffraction, thermogravimetric and differential thermogravimetric (TG–DTA) and BET measurements. The structure of the support was retained after anchoring the complex. Based on the spectroscopic evidences it was envisaged that the complex retains its structure inside the channels and it hangs like a pendant inside MCM-41. This anchored complex allowed the catalytic oxidation of adamantane using urea hydroperoxide (UHP) as the oxidizing agent at 60xa0°C and atmospheric pressure. The reaction conditions have been optimized towards maximum conversion by varying different parameters. No leaching of the complex was observed after the reaction.

Liquid-phase hydrogenation of 1,5-cyclooctadiene on zeolite-encapsulated noble metal–salen complexes

Abstract The preparation of palladium and rhodium complexes of the Schiff base salen ( N , N ′-bis-(salicylidene)-ethylenediamine) in the intracrystalline cavities of zeolites with the FAU and the EMT framework topology is reported. The host/guest compounds obtained are active catalysts for the selective hydrogenation of 1,5-cyclooctadiene in the liquid phase at 60°C and under a hydrogen pressure of 1.5xa0MPa. In addition, the immobilization of palladium complexes of the chiral salen-type ligand R , R - N , N ′-bis-(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediamine in zeolites FAU and EMT is reported for the first time. These materials too are active catalysts for the selective hydrogenation of 1,5-cyclooctadiene. However, their ability to catalyse enantioselective hydrogenations remains to be demonstrated.

The enhanced catalytic activity and stability of oxodiperoxomolybdenum-modified mesoporous organosilicas in selective epoxidation reactions

Complexes of the type (L–L)MoO(O2)2 heterogenized on phenylene-bridged mesoporous organosilicas show a 10-fold increase in catalytic activity and a high stability in liquid phase epoxidation reactions, with H2O2 as the oxidant, compared to the corresponding MCM-41-derived systems.

Covalent Immobilization of Imidazolium Cations Inside a Silica Support: Palladium-Catalyzed Olefin Hydrogenation

Mesoporous organosilica materials with different contents of bistrialkoxysilyl imidazolium salts in the framework were synthesized by a one‐step synthesis. Textural characterization of the materials confirmed that the morphology and surface properties of the imidazolium‐bridged organosilicas depended critically on the amount of organic groups in the framework, whereas solid‐state NMR characterization showed that the imidazolium fragments were integrated covalently into the framework. Further reaction of these materials with Pd(OAc)2, followed by reduction with NaBH4 yielded palladium nanoparticles stabilized in the mesoporous organosilicas. The stabilizing effect of the imidazolium cations and the mesostructure contributed to the high activity, selectivity, and stability of the palladium nanoparticles and allowed olefin hydrogenation under mild reaction conditions.

Adsorption and separation of amino acids from aqueous solutions on zeolites

The adsorption of various amino acids on zeolites with different structures was studied with regard to dependence of the pH value of the solution and the aluminum content of the zeolite in order to design tailor-made adsorbents for amino acid separations.

A Covalently Supported Pyrimidinylphosphane Palladacycle as a Heterogenized Catalyst for the Suzuki–Miyaura Cross Coupling

The amino moiety of an aminopyrimidinyl phosphane allows rapid functionalization of the ligand with a silylated side chain containing a urea linker for catalyst heterogenization. The urea group causes the resulting ligand to undergo spontaneous Cuf8ffH activation at the pyrimidinyl site when reacted with (C6H5CN)2PdCl2 in CH2Cl2. Grafting of the resulting zwitterionic palladacycle complex onto siliceous supports leads to highly active hetereogeneous catalysts for the Suzuki–Miyaura coupling. Leaching tests proved that the catalysts obtained this way are truly heterogeneous.