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 catalysti.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

Combined in situ 29Si NMR and small-angle X-ray scattering study of precursors in MFI zeolite formation from silicic acid in TPAOH solutions

Silicic acid powder was dissolved and polymd. in a concd. aq. tetrapropylammonium (TPA) hydroxide soln. at room temp. Two complementary techniques were employed to follow this process leading to silicalite-1 zeolite upon heating. The formation of small silicates and specific oligomers involved in the assembly of silicalite-1 nanoprecursors was investigated using 29Si NMR. Small-angle X-ray scattering (SAXS) was used to follow processes at a colloidal level. Dissoln. and polymn. of silicic acid could then be related to events occurring at both mol. and colloidal scales. The appearance of very well-defined colloidal particles was linked to a specific intermediate already obsd. in systems using an org. and monomeric silica source. In situ time-resolved ultra-small-angle X-ray scattering (USAXS) using synchrotron radiation showed a linear growth of the av. crystal diam., which was slower than of that encountered in Na+ contg. synthesis mixts. Using the results presented here, we propose a mechanism describing the TPA-mediated self-assembly of silicalite-1 from silicic acid powder as silica source. This model is in agreement with rising evidence of a common mechanism involving nanoblock aggregation for org. mediated crystn. of high-silica zeolites