Design, Synthesis and Characterization of Ti-Containing Ordered Mesoporous Materials for Epoxidation of Vernonia Oil
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Date
2020-06-18
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Addis Ababa University
Abstract
This study focusses on obtaining heterogeneous Ti-containing catalysts through the design
and optimization of pure silica SBA-15 and hybrid organic-inorganic PMO mesostructured
materials in which the pore size, the particle morphology, and the surface chemistry are
controlled to enhance the accessibility of the active sites and to avoid diffusion problems
of substrates and products. The synthesis conditions of pure silica SBA-15 and hybrid
organosilica PMO materials with different particle morphologies and pore sizes have been
optimized. Ti has been incorporated in the framework/on the channel of both types of
ordered mesoporous materials by using either co-condensation or grafting methods. To do
this, the optimum SBA-15 and PMO materials, which have different pore diameters have
been selected for Ti-grafting. A strategy has been designed to incorporate Ti in the
framework of the mesoporous material during the support synthesis step (cocondensation).
A detailed study has been undertaken to analyze the use of different
surfactants, different Ti-source precursors, additives and different Ti/Si ratios of the
synthesis gel to obtain Ti-incorporation in the Td structure and thereby avoiding or
minimizing the agglomeration of TiO2 species. The prepared samples were characterized
by X-ray diffraction (XRD), Thermogravimetric Analysis (TGA), N2 adsorptiondesorption
isotherms, Scanning Electron Microscopy (SEM), Transmission Electron
Microscopy (TEM), Inductively Coupled Plasma Optical Emission Spectroscopy (ICPiv
OES) and Diffuse Reflectance UV-vis Spectroscopy (DRS UV-vis). The following Ticontaining
SBA-15 and PMO with different particle morphologies and pore sizes with
isolated Ti (IV) species have been synthesized:
- Ti-containing SBA-15 and PMO materials with hexagonal plate-like and fiber-like
particle morphology using Pluronic P104 and P123 surfactants;
- Ti-containing SBA-15 materials with hexagonal pore structure and large pore
diameters (> 10 nm) using TIPB micelle expander;
- Ti-containing PMO materials with moderately ordered hexagonal pore structure
and large pore diameters (> 8.5 nm) using TIPB micelle expanders;
- Ti-grafted SBA-15 and PMO materials with highly populated isolated Td Ti (IV)
species and without formation of anatase TiO2 species.
The catalytic activities of selected catalysts were tested for the epoxidation of cyclohexene
using TBHP (5.5 M in decane solution) as an oxidant. The catalysts which give good
conversion and epoxide selectivity in the epoxidation of cyclohexene were selected for
epoxidation of vernonia oil. Solvent-free epoxidation of vernonia oil with anhydrous TBHP
(5.5 M TBHP in decane solution) in the presence of selected Ti-containing SBA-15 and
PMO catalysts was investigated. Ti-grafted materials show better catalytic activity than
their corresponding Ti-containing materials, synthesized by the co-condensation method.
Ti-containing SBA-15 materials are better in conversion when compared to PMO type of
materials. Among them, Ti-grafted SBA-15 with large pore diameter (TiG-ESBA-L) gives
the highest conversion (71%) with 80% of epoxide selectivity. Ti-containing PMO
materials are better in epoxide selectivity in epoxidation of vernonia oil (achieving more
than 90% epoxide selectivity) when compared to their corresponding Ti-SBA-15 materials.
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Keywords
Design, Synthesis and Characterization, Ti-Containing Ordered, Mesoporous Materials, Epoxidation, Vernonia Oil