Enhancing the Catalytic Activity of Site-Isolated Heterogeneous Transition Metal Expoxidation Catalysts Prepared via the Thermolytic Molecular Precursor Method
- Author(s): Cordeiro, Paul Joseph
- Advisor(s): Tilley, T. Don
- Newman, John
- et al.
This dissertation describes the application of the thermolytic molecular precursor (TMP) method for the synthesis of new heterogeneous catalysts and the performance of these catalysts in the epoxidation of olefins with simple peroxides. The precursors used in this method include complexes of the form LnM[OE(X)3]m, where Ln = OiPr, E = Si or Ge, X = iPr or OtBu, and M = Ti(IV) or Ta(V). These precursors were employed to introduce site-isolated metal centers onto the surface of mesoporous silica (SBA-15), and represent excellent models for the supported metal sites generated. The precursors and materials were characterized using a variety of analytical techniques, and structural comparisons are presented. Surface modification has been used to introduce new functionalities designed to enhance the catalytic activity of two types of supported catalysts, and the effect of the new functionalities was explored. Additionally, a new precursor containing some of these new functionalities has been synthesized and employed to generate new catalytic materials.
Site-isolated Ta(V) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. After treatment in oxygen, the resulting Si–OH and Ta–OH sites of TaSBA15-O2 were modified with a series of trimethyl group 14 species, Me3E–, by treatment with Me3E–NMe2 (E = Si, Ge, Sn) reagents. The surface modified catalysts, (Me3E)capTaSBA15, exhibit a significantly increased rate of cyclohexene epoxidation with H2O2 as an oxidant, and a decreased amount of allylic oxidation products with respect to the unmodified material, TaSBA15-O2. The rate of non-productive H2O2 decomposition, as monitored via 1H NMR spectroscopy, significantly decreased after the surface modification. The structure of the TaSBA15 catalysts and potential Ta(V) epoxidation intermediates (formed upon treatment of Ta(V) materials with H2O2) were probed using UV-visible absorbance and diffuse-reflectance UV-visible spectroscopy. A Ta(V)(η2-O2) intermediate species is proposed for the TaSBA15-O2, (Me3Si)capTaSBA15, and (Me3Ge)capTaSBA15 catalysts, while intermediate species for the (Me3Sn)capTaSBA15 catalysts could not be characterized.
The thermolytic molecular precursor method was used to introduce site-isolated Ti(IV) centers onto the surface of a mesoporous SBA-15 support. Prior to thermal treatment to generate Ti–OH sites, residual silanol groups were capped via reaction with Me2N–SiMe3 to give TiMecapSBA15. After low-temperature treatment in oxygen, the resulting Ti–OH sites of TiMecapSBA15-O2 were modified by reaction with a series of protic reagents: phenol, pentafluorophenol, acetic acid, and trifluoroacetic acid. The structure of the resulting TiSBA15 catalysts and the Ti(IV) epoxidation intermediates (formed upon treatment of Ti(IV) materials with tert-butyl hydroperoxide or H2O2) were probed using diffuse-reflectance UV-visible spectroscopy and infrared spectroscopy. A titanium-hydroperoxo species similar to that found in TS-1 is proposed for all catalysts. Samples modified with phenol and pentafluorophenol exhibited conversions of 1-octene that are 20 to 50% higher than those for TiMecapSBA15-O2, without a significant drop in selectivity for the epoxide product, 1,2-epoxyoctane, when TBHP was used as the oxidant. With aqueous H2O2 as the oxidant, the phenol-treated materials exhibited 1-octene conversions that are 15 to 50% greater than those observed for TiMecapSBA15-O2, and an increased selectivity for 1,2-epoxyoctane of 10 to 30%. Additionally, the efficiency of H2O2 usage, as monitored via 1H NMR spectroscopy, increased by a factor of two to three for catalysts modified with phenol and pentafluorophenol, with respect to the efficiency observed over TiMecapSBA15-O2. Catalysts modified with acetic acid and trifluoroacetic acid displayed decreased catalytic turnover numbers and epoxide selectivities when TBHP was used as the oxidant, but exhibited catalytic turnover numbers and epoxide selectivities similar to TiMecapSBA15-O2 when H2O2 was used as the oxidant. After treatment of TiMecapSBA15-O2 with acetic acid, the H2O2 efficiency decreased by a factor of two for the epoxidation of 1-octene with H2O2.
The complex Ti[OGe(iPr)3]4 (1) was prepared via the reaction of Ti(OiPr)4 with (iPr)3GeOH, and is a useful structural and spectroscopic model for titanium-germanium species dispersed on silica. This precursor was used to introduce site-isolated Ti(IV) centers onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. The supported materials, TiGe4SBA15, are active catalysts for the epoxidation of cyclic and terminal olefins with alkyl hydroperoxides under anhydrous conditions. Compared to catalysts synthesized from siloxide-only precursors (TiSi4SBA15), the new catalysts (TiGe4SBA15) displayed two to three times higher catalytic turnover numbers during identical cyclohexene and 1-octene epoxidation reactions, while simultaneously maintaining higher selectivity. The new materials did not significantly leach under these conditions, demonstrating the robustness of this new complex as a molecular precursor to supported titanium(IV) epoxidation catalysts.