Peter-Paul Knops-Gerrits

Raman spectroscopy on zeolites

Abstract The zeolite Raman literature is reviewed, with an emphasis on zeolite structure and synthesis, adsorption and metal complex formation in zeolites

Oxidation catalysis with semi-inorganic zeolite-based Mn catalysts

Abstract The chelation of zeolite-exchanged Mn 2+ by N-containing ligands gives rise to a whole class of heterogeneous liquid phase oxidation catalysts. Bi-, tri- or tetradentate ligands can be used. A high degree of metal complexation is required to avoid side reactions due to the presence of zeolite-coordinated manganous ions. Applied physico-chemical techniques include IR, ESR and electronic spectroscopy. Oxidation-resistant chelands, e.g. with aromatic pyridine groups, are employed to ensure long-term catalyst stability. Use of hydrogen peroxide is most successful in combination with 2,2′-bipyridine (bpy) or 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn); with both systems double bond oxidation proceeds with high selectivity. Olefin oxidations with other oxidants, e.g. tert -butylhydroperoxide ( t BuOOH) or iodosylbenzene, are less selective or slower. Alkane oxidation with t BuOOH is possible with various tetradentate diimine ligands. A principal effect of the zeolite matrix is that formation of Mn clusters is impeded in comparison with solution chemistry. Other effects of the zeolite matrix include modulation of the acid strength and suppression of side reactions, such as allylic oxidation of olefins or formation of isomerized epoxides.

Membrane occluded catalysts: a higher order mimic with improved performance

Abstract A general method to immobilise homogeneous catalysts and to improve the performance of heterogeneous catalysts is discussed. The method consists in embedding the catalysts in hydrophobic PDMS (polydimethylsiloxane)-membranes. Inspired on a complete structural mimic of cytochrome P-450 up to the level of the membrane, this technique gives superior properties to the membrane resident catalyst. The scope and limitations of this method are discussed by two examples of heterogeneous catalysts, i.e., FePc-Y (iron phthalocyanine zeolite Y) and [Mn(bpy) 2 ] 2+ -Y (manganese bis(bipyridyl) zeolite Y), and three examples of homogeneous complexes, i.e., FePc, Ru-binap ([2,2′-bis(diphenylphosphino-1,1′-binaphtyl]chloro( p -cymene)-ruthenium chloride) and the Jacobsen catalyst ( N,N′ -bis(3,5-di- tert -butylsalicylidene)-1,2-cyclohexane-diamine manganese chloride). Due to changed sorption in the zeolites, catalyst activity is enhanced and deactivation is suppressed. Furthermore, the membrane incorporation makes the use of a solvent redundant. For homogenous complexes, this procedure represents a general method for heterogenisation. Moreover, the technique opens new ways in the field of oxidation chemistry, where solvents are necessary to homogenise reagents which usually differ in polarity.

Novel catalytic membranes for selective reactions

A survey is given of the potentials of a new kind of catalytic membranes consisting of a catalyst that is immobilised in a dense polymer matrix. When homogeneous, catalytically active complexes are occluded, these membranes constitute a new way of heterogenation. In the case of solid state catalysts, these composite membranes can improve the activity of the catalyst by changing sorption or by allowing experimental set-ups in which solvents become redundant.

Oxidation catalysis with well-characterised vanadyl bis-bipyridine complexes encapsulated in NaY zeolite

Vanadyl exchanged faujasite (VO2+-NaY) allows bipyridine complexation, giving a heterogeneous“ship-in-a-bottle” catalyst denoted as [VO(bpy)2]2+-NaY. The [VO(bpy)2]2+ complexes associated with the zeolite are characterised with FT-Raman, FT-IR, XPS, and diffuse reflectance spectroscopy (DRS), as well as with electron paramagnetic resonance (EPR). It is established that the cationic complexes are intrazeolitic and homogeneously distributed across the zeolite crystals and both the zeolite and the neutral bipyridine ligands stabilise VIV. The catalytic oxidation of cyclohexane and cyclohexene with different peroxides or mono-oxygen atom donors in presence of several solvents is described. Good epoxide selectivity results from the complexation by bipyridine thus favoring the heterolytic over the homolytic decomposition pathway of V-peroxo-intermediates.

Alkane oxidation by dinuclear iron complexes in hexagonal mesoporous solids

Biomimetic oxygen activation on binuclear iron active sites occluded within the voids of mesoporous oxides such as HMS (hexagonal mesoporous silica) is an interesting new field of research. As guests, binuclear iron(III) complexes were chosen. Heptapodate N,N,N′,N′-tetrakis(2-R methyl)-2-hydroxy-1,3-diaminopropane ligands in which R is either a pyridyl or a benzimidazole group allow metal coordination as in methane mono-oxygenase active sites. The physicochemical characterization with SEM, TGA and sorption analysis shows the interactions of the complexes with the support. IR, Raman and diffuse reflectance-UV-Vis-NIR spectroscopy is used to monitor interaction of oxygen (16O2 and 18O2) or peroxides with the supported active sites. The cyclohexane, cyclohexylhydroperoxide and isobutane oxidation is described and the mono- and binuclear mechanisms that are occurring in nucleophilic and electrophilic activation pathways are discussed.

Mono- and Binuclear Iron Complexes in Zeolites and Mesoporous Oxides as Biomimetic Alkane Oxidation Catalysts

Many enzymes [1–10] successfully catalyse the oxidation of alkanes, even the most inert ones such as methane. The precise architecture of their active site allows such catalytic properties. Such an active site contains a dinuclear iron core, in which μ2-O or μ2-OH bridging is observed. The environment in which the active site is embedded e.g. the protecting protein matrix, adds to the substrate specificity of the enzyme. Apart from the dinuclear iron active site, proximal functions such as catalytic initiators e.g. tyrosyl-radicals, help to establish long range, proton coupled electron transfer chains. Furthermore the interactions between enzymatic subunits add to the complexity of these systems.