Design and synthesis of heterogeneous catalysts and catalyst supports derived from thermolytic molecular precursors

This dissertation describes the use of the thermolytic molecular precursor (TMP) method for the synthesis of new heterogeneous catalysts and catalyst supports. A number of such precursors, which include transition metal tris(tert-butoxy)siloxy complexes of the type LnM[OxSi(OR)4-x]m (where L = alkoxide or amine; R = tBu or Si(OtBu)3; M = Ti, Ta, Zr, or Co), have been prepared and studied. These molecular precursors can be thermolyzed under mild conditions (non-polar solvents, <200°C) to yield high surface area MOx/SiO2 xcrogels upon elimination of isobutylene and H2O. They can also be grafted onto the surface of mesoporous supports (e.g., SBA-15) to yield site-isolated metal active sites for high-performance oxidation catalysts.; A new dimeric complex [(tBuO)2Ti{lcub}mu-O 2Si[OSi(OtBu)3]2{rcub}] 2 was used as a precursor to titanium-silica materials through solution thermolysis (yielding TiO2•3SiO2) and by grafting the complex onto the surface of SBA-15 (yielding Ti2SBA15). The resulting catalysts were found to be active and highly selective in the epoxidation of cyclohexene, yielding up to 71% of cyclohexene oxide based on oxidant (cumene hydroperoxide). Additionally, the molecular precursor (iPrO) 2Ta[OSi(OtBu)3]3 was used to prepare tantalum-silica materials via solution thermolyses (yielding Ta2O5•6SiO2 and Ta2O 5•18SiO2) or by grafting the complex onto surface of SBA-15 silica (yielding TaSBA15). It was demonstrated these materials are active and selective catalysts for cyclohexene epoxidation with aqueous H 2O2.; Surface Si-OH/Ti-OH sites of the TiSBA15 catalysts were surface-modified with a series trialkylsilanes, -OSiMe2(R) (where R = Me, nBu, or nOc). By in-situ FTIR and DRUV-vis spectroscopies, it was determined that active sites with capped titanol centers, (SiOsurface)3Ti(OSiR3), likely undergo Ti-OOH formation upon addition of H2O2, in a manner analogous to that for active sites of the type (SiOsurface )3TiOH. Compared to the unmodified catalysts, however, the surface-modified catalysts are more active in the oxidation of cyclohexene with H2O2, and exhibit a significantly higher selectivity for cyclohexene oxide formation (vs. allylic oxidation products). Based on the selectivities for cyclohexene epoxidation using catalysts with capped and uncapped titanol active sites, it is hypothesized that the presence of active sites with a (SiOsurface)3Ti(OSiR3) structure yield more selective catalysts.; Catalyst supports with interesting surface properties were synthesized by cothermolyses of Zr[OSi(OtBu)3] 4 in the presence of (EtO)3Si(CH2)nSi(OEt) 3 (n = 1, 2) or (EtO)3Si(C6H4) nSi(OEt)3 (n = 1, 2; C6H4 = 1,4-phenylene) monomers to afford high surface area xerogels. The resulting materials possessed a high concentration of surface OH groups (2--5 sites nm-2 ) and were hydrophobic (by AFM surface adhesion measurements).; Novel heterogeneous catalysts for alkylaromatic oxidation were prepared by grafting a new molecular precursor, (4,4'-di- tBy-bipy)Co[OSi(OtBu) 3]2, onto the surface of SBA-15. The resulting CoSBA15 catalysts demonstrated good activity (rate = 582 TON h-1) and selectivity (83%) for ethylbenzene oxidation with tert-butyl hydroperoxide. It was determined that the reaction mechanism initiates by abstraction of benzylic hydrogen atoms by radical species. The significance of benzylic hydrogen abstraction in the rate determining step is supported by the kinetic isotope effects for toluene and ethylbenzene (kH/ kD ≈ 16 and 9, respectively).