Hendrikus C. L. Abbenhuis

Advances in homogeneous and heterogeneous catalysis with metal-containing silsesquioxanes

Metal-containing silsesquioxane derivatives provide new catalysts with both homogeneous and heterogeneous applicability. The steric and electronic properties of silsesquioxane silanolate ligands render metal centers more Lewis acidic than conventional alkoxide or siloxide ligands do. This concept has been exploited in newly developed catalysts for alkene metathesis, polymerization, epoxidation, and Diels-Alder reactions of enones. Other applications are envisioned in the near future.

Solid-phase immobilization of a new epoxidation catalyst

The heterogenization of a titanium(IV) silsesquioxane complex in an MCM-41 molecular sieve, by tailoring the polarity of MCM-41, results in self-assembled materials (see the space-filling model on the right) that are active, truly heterogeneous, and recyclable catalysts for liquid-phase alkene epoxidation.

Heterogenization of Metallocene Catalysts for Alkene Polymerization

Beyond mere convenience: In large-scale industrial processes for alkene polymerization the essentially homogeneous metallocene/activator catalyst systems are heterogenized (example shown) to improve polymer properties. New mesoporous and organic catalyst supports and their application in this field of catalysis are described.

Titanium silsesquioxanes grafted on three-dimensionally netted polysiloxanes: catalytic ensembles for epoxidation of alkenes with aqueous hydrogen peroxide

Catalytic ensembles suitable for alkene epoxidation with aqueous hydrogen peroxide are now accessible. They are made by grafting robust titanium silsesquioxane sites onto dimethylsiloxane polymers (see picture) followed by cross-linking to three-dimensionally netted polymers.

Super-microporous organosilicas synthesized from well-defined nanobuilding units

Super-microporous organosilica with bridging ethylene and pendant vinyl groups has been synthesized by assembling predefined nanobuilding block polyhedral oligomeric silsesquioxanes (POSS) with nonionic surfactant Brij-76 as the template. The material shows wormhole-like super-micropores with uniform size of 1.9 nm, high BET surface area of 872 m2 g–1 and pore volume of 0.52 cm3 g–1. IR and NMR results show that the bridging ethylene, the pendant vinyl groups and the double-4-membered ring structure were successfully transferred from the building blocks to the super-microporous organosilica material. The material shows high hydrothermal stability and can further react with Br2. The advantage of the present approach lies in that the relative contents and proximity of the different organic functionalities in the final material can be well controlled through the starting nanobuilding blocks.

X-ray photoelectron spectroscopy study on the chemistry involved in tin oxide film growth during chemical vapor deposition processes

The chemistry of atmospheric pressure chemical vapor deposition (APCVD) processes is believed to be complex, and detailed reports on reaction mechanisms are scarce. Here, the authors investigated the reaction mechanism of monobutyl tinchloride (MBTC) and water during SnO2 thin film growth using x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). XPS results indicate an acid–base hydrolysis reaction mechanism, which is tested with multilayer experiments, demonstrating self-terminating growth. In-house developed TEM wafers are used to visualize nucleation during these multilayer experiments, and results are compared with TEM results of APCVD samples. Results show almost identical nucleation behavior implying that their growth mechanism is identical. Our experiments suggest that in APCVD, when using MBTC and water, SnO2 film growth occurs via a heterolytic bond splitting of the Sn-Cl bonds without the need to invoke gas-phase radical or coordination chemistry of the MBTC precursor.

Synthesis and characterisation of microporous bimetallic Fe-Cr-Si-O materials derived from silsesquioxane precursors

Calcination of silsesquioxane mixtures of (c-C5H9)7Si7O9(OH)3, 1, (c-C5H9)7Si7O12Fe(CH3)2N(CH2)2N(CH3)2, 2, and (c-C5H9)7Si7O9(OSiMe3)O2CrO2, 3, led to microporous amorphous bimetallic Fe–Cr–Si–O materials with different Fe∶Cr ratios. A set of complementary characterisation techniques including N2 physisorption, XRD, XPS, RS, IR, HRTEM and Mossbauer spectroscopy were used to follow the variation of the textural properties, metal oxide dispersion and speciation with metal content. Fe–Cr–Si–O materials possess high surface areas and uniformly controlled micropores with an average pore size diameter of around 6–7 A. Metal oxide speciation of calcined silsequioxane mixtures appears to be significantly different from that observed for these metals in the individually calcined metal silsesquioxanes. The iron oxide and monochromates are the predominant species in the calcined precursors 2 and 3 while very small particles (2–4 nm) of bimetallic mixed oxides are the major species in the Fe–Cr–Si–O materials. This suggests that the metal oxide species are highly interdispersed and can come into close contact with each other during the calcination procedure thus favoring the formation of the bimetallic mixed oxide phase. In contrast, a silica reference material containing 7% Fe and 3% Cr prepared via the impregnation method showed only chromate species and large particles (10–30 nm) of iron oxide. This suggests that metallasilsesquioxane mixtures may be used as versatile precursors for the preparation of silica-based catalysts containing very small and well dispersed particles of mixed metal oxides.

Festphasen‐Immobilisierung eines neuen Epoxidierungskatalysators

Die Heterogenisierung eines Titan(IV)-Silsesquioxankomplexes in einem MCM-41-Molekularsieb durch Einstellen der Polaritat des MCM-41-Netzwerkes fuhrt zu selbstorganisierten Materialverbanden (Kalottenmodell rechts), die sich als aktive, heterogene und somit wiederverwendbare Katalysatoren fur die Alken-Oxidation in flussiger Phase eignen.