Hcl Erik Abbenhuis

Modeling acidic sites in zeolites and aluminosilicates by aluminosilsesquioxanes

Protolysis of alkylaluminum compounds with silsesquioxanes is an efficient procedure to synthesize both Lewis and Bronsted acidic aluminosilsesquioxanes. Treatment of AlEt3 with (c-C5H9)(7)Si7O9(OH)(3) and (c-C5H9)(7)Si7O9(OSiMe3)(OH)(2) gives the corresponding Lewis acidic aluminosilsesquioxanes, {[ (c-C5H9)(7)Si7O12]Al}(n) (1) and [ (c-C5H9)(7)Si7O11(OSiMe3)]AlEt . NEt3 (2). By allowing AlEt3 to react with two equivalents of (C-C5H9)(7)Si7O9(OSiMe3)(OH)(2), the Bronsted acidic aluminosilsesquioxane [(c-C5H9)(7)Si7O11(OSiMe3)]Al[(c-C5H9)(7)Si7O10(OSiMe3)(OH)] (3) is selectively formed. H-1 NMR and IR spectroscopy and density functional theory (DFT) calculations show that complex 3 contains a strong intramolecular hydrogen bond. Although the high strength of this hydrogen bond reduces the Bronsted acidity of 3 substantially, 3 can easily be deprotonated by amines to yield the corresponding ammonium salts {[(cC(3)H(9))(7)Si7O11(OSiMe3)](2)Al}-X+ (X = Et3NH 4a, PhN(H)Me-2 4b, C5H5NH 4c). The X-ray crystal structure of 4a demonstrates that the ammonium cation is bonded to the aluminosilsesquioxane anion by a hydrogen bond. The corresponding lithium salt {[(c-C5H9)(7)Si7O11(OSiMe3)](2)Al}(-){Li . 2THF}(+) (5) could best be prepared by protolysis of (c-C5W9)(7)Si7O9(OSiMe3)(OH)(2) with half an equivalent of LiAlH4.

MCM-41 heterogenised titanium silsesquioxane epoxidation catalysts: a spectroscopic investigation of the adsorption characteristics

This paper describes a spectroscopic study of the heterogenisation of a novel liquid-phase epoxidation catalyst, a titanium(IV) silsesquioxane complex. Its immobilisation is performed exclusively via a straightforward adsorption of the homogeneous catalyst, i.e. the metal complex, in the pores of an MCM-41 host material. Applying all-silica MCM-41 hosts, stable and heterogeneous liquid-phase epoxidation catalysts are obtained. These highly active catalytic materials were extensively characterised using diffuse reflectance UV-Vis, XPS and Raman spectroscopy. With these techniques the strong adsorption of the intact catalytic complex within an all-silica MCM-41 host is demonstrated. A spectroscopic investigation on the ancillary cyclohexyl ligands of the silsesquioxane complex reveals strong interactions upon adsorption inside the MCM-41 pore. The interaction of these cyclohexyl ligands with the MCM-41 pore wall, as shown by Raman spectroscopy, reveals a constrained configuration of these ligands. Moreover, a hardly affected cyclopentadienyl ligand on the titanium site upon adsorption suggests a preferable orientation in which the catalytic active titanium site is pointing inside the MCM-41 pore. As such, the accessibility of the titanium site for substrate and oxidant in heterogeneous epoxidation catalysis is guaranteed, in agreement with the high catalytic activity found for these heterogeneous catalysts.

Role of Intermolecular Interactions in Oxygen Transfer Catalyzed by Silsesquioxane Trisilanolate Vanadium(V)

The polyhedral oligomeric silsesquioxane trisilanolate vanadium(V) complex 1 efficiently catalyzes oxidations at room temperature using cumyl hydroperoxide as the terminal oxidant. Sulfoxidations and N-oxidations have been carried out yielding the corresponding products in good yields. The addition of a Lewis base as a coligand can markedly affect reactivity, stability, and chemo- and stereo-selectivity. A proposal for the intermolecular activation, using Gutmann analysis supported by MP2/TZVP calculations, is presented.

Chromium silsesquioxane based synthesis and characterization of a microporous Cr–Si–O material

The controlled calcination of the chromium containing polyhedral oligosilsesquioxane (c-C5H9)7Si7O9- (OSiMe3)O2CrO2, 3b, monitored by MS analysis of the evolved gases and thermogravimetry measurements, led to the formation of a microporous Cr–Si–O amorphous mixed oxide containing 10.2 wt.% of chromium. The textural properties of the material as well as the speciation and dispersion of the chromium oxide on the silica surface were investigated by the use of nitrogen physical adsorption, XRD, XPS as well as DRS, RS and IR techniques. The nitrogen physical adsorption indicates a high surface area, a rather large pore volume and a very narrow pore size distribution around 6 A diameter. The spectroscopic analysis of the material suggests a rather high dispersion of the chromium oxide species on the silica surface mainly as monochromate and to a certain extent as dichromate and clusters of Cr2O3. The material was briefly tested in the ammonia oxidation reaction and was found to have a catalytic activity, which was slightly higher than that of a chromium oxide on a silica reference catalyst.

NMR study of the role of isopropylsulfates in the two-step "conjunct oligomerization" of propylene and isopentane-propylene alkylation catalyzed by 95% sulfuric acid

Abstract“Conjunct oligomerization” of propylene or the isopentane–propylene alkylation catalyzed by an excess of 95% sulfuric acid was performed in two consecutive steps. First di-isopropylsulfate was prepared by interaction of sulfuric acid with propylene. The ester was then either decomposed at room temperature in the presence of the 5–10 molar excess of 95% acid or was used in the acid-catalyzed alkylation of isopentane. In situ1H and 13C NMR study of the reaction mixture of “conjunct oligomerization” indicated that the diester participates in two equilibria with sulfuric acid. The first one transforms the diester into a monoester. The second equilibrium corresponds to protonation of the monoester with an excess of sulfuric acid. This converts a minor fraction of the mono-alkylsulfate into isopropyl carbenium ions that are only weakly solvated with sulfuric acid: C3H7HSO4 + H2SO4 ⇄ C3H7+ H2SO4 + HSO4−. The subsequent reactions of alkyl carbenium ions with the non-protonated alkylsulfate result in final products of “conjunct oligomerization” while in the presence in the reaction mixture of isopentane, alkylation with the predominant formation of C8 branched paraffins takes place. A very low yield of propane indicates a minor role of hydride transfer in alkylation. Another unexpected result is the absence in both reaction mixtures of propylene. These findings are in contradiction with the classical mechanism of isoparaffin–olefin alkylation by Schmerling. Therefore, an alternative mechanism of this reaction is suggested via a direct alkylation of isopentane with the mono-alkylsulfate.

Synthesis and structural characterisation of platinum silasesquioxane complexes

The reaction of partially condensed silasesquioxanes of the type [R17Si7O9(OH)2(OR2)] (R1 = c-C5H9, c-C6H11; R2 = H, SiMe3) with either [Pt(dppe)(CO3)] or [Pt(dppe)Cl2]/Ag2O leads to the formation of [R17Si7O9(OR2)O2Pt(dppe)], crystallographically characterised for R1 = c-C5H9; R2 = SiMe3, providing unprecedented routes to late transition metal silasesquioxane complexes.

Incompletely condensed silasesquioxanes as models for zeolite defect sites: an FTIR and density functional study

FTIR hydroxyl vibration bands in highly dealuminated zeolites could be unambiguously assigned to well defined hydroxyl clusters present at defect sites by the use of density functional calculations on incompletely condensed silasesquioxane complexes which, in addition, provided useful spectroscopic references.