Matteo Guidotti

Epoxidation on titanium-containing silicates: do structural features really affect the catalytic performance?

Five titanium-containing silicates, with different structural features, were compared: Ti–MCM-41, ordered titanium-grafted mesoporous silica, Ti–SiO2 Davison, nonordered titanium-grafted porous silica, Ti–SiO2 Aerosil, nonporous pyrogenic titanium-grafted silica, MST, nonordered in-framework mesoporous material, and TiO2–SiO2 Grace, commercial amorphous porous mixed oxide. They were tested in the liquid-phase epoxidation reaction on six unsaturated cyclic terpenes. Good performances were obtained on the commercial mixed oxide and also on the three grafted silicates. The in-framework MST showed the worst activity results. Under these conditions, the porosity features do not affect the catalytic performances noticeably and the use of an ordered mesoporous material is not strictly required. Likewise, a very high surface area is not mandatory in order to have an efficient titanium-grafted catalyst in the epoxidation of these substrates.

The use of H2O2 over titanium-grafted mesoporous silica catalysts: a step further towards sustainable epoxidation

The epoxidation of cyclohexene with aqueous hydrogen peroxide over mesostructured Ti(Cp)2Cl2-grafted silica catalysts is described for the first time. Three kinds of Ti-containing systems with different textural properties have been employed: Ti/SiO2, Ti/MCM-41 and Ti/MCM-48. A minimal local H2O2 concentration in the surroundings of Ti sites, obtained by a controlled dropwise addition of aqueous H2O2 (4.17 mmol H2O2 h−1 gcat−1), is crucial for an effective and highly selective epoxidation. Excellent selectivity (>98%) in cyclohexene epoxide is obtained at the end of the slow H2O2 addition in acetonitrile. Higher yields in epoxide are found over Ti/MCM-48 than over Ti/MCM-41 or Ti/SiO2 thanks to better isolation, dispersion and stability of Ti(IV) sites.

Heterogeneous catalytic epoxidation of fatty acid methyl esters on titanium-grafted silicas

Three different titanium-grafted silicates, with different morphological features were compared in the epoxidation, using tert-butylhydroperoxide as oxidant, of methyl oleate, methyl elaidate and of a mixture of methyl esters, obtained from high-oleic sunflower oil. Ti-MCM-41, a well-ordered mesoporous catalyst, showed very high conversions and excellent selectivity on all the substrates. The fatty acid methyl esters derived from renewable raw material were epoxidised selectively with very high yields in a reaction medium completely free from organic acidic compounds. The influence of the pore order of the catalysts on the catalytic performance was also examined. The interactions between the organic substrate and the ordered array of mesopores in Ti-MCM-41 were evidenced.

Design and use of nanostructured single-site heterogeneous catalysts for the selective transformation of fine chemicals.

Nanostructured single-site heterogeneous catalysts possess the advantages of classical solid catalysts, in terms of easy recovery and recycling, together with a defined tailored chemical and steric environment around the catalytically active metal site. The use of inorganic oxide supports with selected shape and porosity at a nanometric level may have a relevant impact on the regio- and stereochemistry of the catalytic reaction. Analogously, by choosing the optimal preparation techniques to obtain spatially isolated and well-characterised active sites, it is possible to achieve performances that are comparable to (or, in the most favourable cases, better than) those obtained with homogeneous systems. Such catalysts are therefore particularly suitable for the transformation of highly-functionalised fine chemicals and some relevant examples where high chemo-, regio- and stereoselectivity are crucial will be described.

Preparation and characterisation of mesoporous silica–alumina and silica–titania with a narrow pore size distribution

Abstract Amorphous mesoporous materials with a different degree of order in the arrangement of pores are outlined. Particularly, the synthesis of a class of mesoporous silica–alumina (MSA) materials with narrow pore size distribution and a disordered arrangement of pores is reported and discussed. Likewise, the preparation of titanium-containing ordered mesoporous silicates (Ti-MCM-41) and disordered mesoporous silica–titania (MST) are also described in detail. The structural properties of the solids are compared by means of X-ray diffraction and UV-Vis diffuse reflectance spectroscopy. The nitrogen adsorption–desorption measurements were performed and the textural properties are evaluated by the BET, DFT, BJH and t -plot methods. The high specific surface area and pore volume, as well as the acidity, make MSA solids interesting catalysts in several petrochemical transformations, i.e. oligomerisation, alkylation, hydroisomerisation, rearrangement reactions. Besides, thanks to the width of the mesopores of such solids, the catalytic activity of titanium-containing silicates may have a potential application in the epoxidation of bulky unsaturated fine chemical substrates.

Epoxidation of methyl oleate with hydrogen peroxide. The use of Ti-containing silica solids as efficient heterogeneous catalysts

Titanium-silica catalysts, obtained by grafting titanocene dichloride onto mesoporous silica (MCM-41 and MCM-48) and onto nanosized pyrogenic silica, were tested in the epoxidation of methyl oleate using aqueous hydrogen peroxide. All titanium-containing materials, combined with the slow addition of hydrogen peroxide, showed good catalytic activity and comparable behaviour in terms of conversion, selectivity and yield of methyl epoxystearate, notwithstanding the morphology and the texture of the silica support. By optimizing the reaction conditions (especially, the catalyst to substrate ratio), high yields up to 91% in epoxide were obtained for Ti/MCM-41 with a high stereoselectivity (80%) for the cis-epoxide. The formation of the cis/trans-epoxide and the nature of the catalytic reaction pathway are also discussed.

Titanosilsesquioxane Anchored on Mesoporous Silicas: A Novel Approach for the Preparation of Heterogeneous Catalysts for Selective Oxidations

Polyhedral oligomeric silsesquioxanes (POSS) are a class of condensed three-dimensional organosiliceous compounds with cage frameworks that have different degrees of symmetry. These molecular materials have a general formula R8Si8O12 [1–3] and display silicon atoms bound to one-and-ahalf oxygen atoms (sesqui), the remaining valence being saturated by organic entities (R). Numerous metals have been incorporated successfully into POSS cages with the aim to obtain either specific homogeneous catalysts, or models for active sites of heterogeneous catalysts. For example, POSS cages with titanium(IV) complexes proved to be highly active in the epoxidation of olefins. Special attention was recently devoted to the heterogenisation of Ti–POSS compounds onto insoluble and easily recoverable supports. Interesting examples encompass the incorporation of Ti–POSS into mesoporous MCM-41, sol–gel matrices, polysiloxanes and the preparation of organic–inorganic hybrid materials and coatings based on polystyrene polymers containing Ti–POSS moieties. However, the literature on the direct anchoring, through covalent bonding of preformed Ti–POSS cages onto silica supports is very poor. The stabilisation of Ti–POSS compounds onto silica surfaces through covalent bonds is a relevant issue for obtaining heterogeneous catalysts with isolated active centres, because it may avoid metal leaching from the catalysts. This work is focused on the preparation of Ti–POSSbased heterogeneous catalysts, by anchoring of a functional titanium-containing silsesquioxane on the surface of an ordered mesoporous silica (SBA-15) and of a non-ordered silica (SiO2-Dav). An innovative bifunctional POSS, bearing in the structure both a Ti metal centre and a triethoxy group for grafting on the silica surfaces, was specifically synthesised for this purpose and named Ti–POSS–TSIPI (Scheme 1, 2). Ti–POSS–TSIPI was prepared by an equimolar reaction between Ti–NH2POSS (Scheme 1, 1), the synthesis of which was reported by some of us, and 3-isocyanatopropyl triethoxysilane (TSIPI) under basic conditions. The product of the reaction was monitored by both IR (Figure 1) and H NMR spectroscopy (see Experimental

Niobium(V) Saponite Clay for the Catalytic Oxidative Abatement of Chemical Warfare Agents

A Nb(V)-containing saponite clay was designed to selectively transform toxic organosulfur chemical warfare agents (CWAs) under extremely mild conditions into nontoxic products with reduced environmental impact. Thanks to the insertion of Nb(V) sites within the saponite framework, a bifunctional catalyst with strong oxidizing and acid properties was obtained. Remarkable activity and high selectivity were observed for the oxidative abatement of (2-chloroethyl)ethyl sulfide (CEES), a simulant of sulfur mustard, at room temperature with aqueous hydrogen peroxide. This performance was significantly better compared to a conventional commercial decontamination powder.

Highly efficient production of 2,3,5-trimethyl-1,4-benzoquinone using aqueous H2O2 and grafted Ti(iv)/SiO2 catalyst

The oxidation of 2,3,6-trimethylphenol with aqueous H2O2 over titanium(IV) grafted on commercial mesoporous silica produces 2,3,5-trimethyl-1,4-benzoquinone, with nearly quantitative yield.

Diastereoselective epoxidation of hydroxy-containing unsaturated terpenes on heterogeneous titanium-catalyst

The diastereoselective epoxidation of substrates of interest as fine chemicals is here reported and the role of the OH-function in this reaction was studied particularly on non-allylic unsaturated terpenes. A heterogeneous system obtained grafting a titanocene precursor onto a commercial silica was used as catalyst.