Molecular engineering of mesoporous molecular sieves for the catalytic oxidtion of Volatile organic compounds

Field: Chemical Engineering (Physical Sciences)

In 1992 scientists at Mobil Oil Research and Development announced the direct synthesis of high surface area mesoporous molecular sieves (denoted M41S) using surfactants around which silicate ions from solution self assemble. The diameter of these pores can be tailored with in the range of 1.5 to 10 nm by varying the synthesis
conditions. Their framework composition can be easily modified by incorporating or substituting transition metals such as titanium and vanadium, for silicons during the synthesis to induce the desired catalytic properties. This technique is referred to as isomorphous substitution throughout this work. Moreover the presence of silanol groups on the surface of these mesoporous materials allows for the bonding (grafting) of organic and inorganic species. This technique is referred to as molecular designed
dispersion throughout this work. These mesoporous materials are very promising as catalysts and catalyst supports since they are capable of transforming much larger or bulky molecules than their microporous counterparts. Consequently these materials can be successfully applied to the oxidation of bulky organic molecules which have now been recognized as a serious health and environmental issue. Our studies show that hexagonal mesoporous materials, denoted HMS, with high surface area, large mesopore volume, narrow pore size distribution and high textural porosity can be synthesized with the following optimized stoichiometry. 1Si(0C2H5)4: 0.263CH3(CH2)11 NH2: ICH3CH2OH: l(CH3)2CHOH: 25H2O. It is possible, by simply varying the sol composition during synthesis, to produce HMS materials with a wide array of physical
properties. For example we can control the mesopore volume, radius and textural porosity by varying the alcohol/water volumetric ratio. Textural mesoporosity is a highly desired physical property as it facilitates access of the framework-confined mesopores, especially by large or bulky substrates. The formation mechanism of HMS was investigated, and can be explained in terms of the sol-gel reaction kinetics and micellization thermodynamics. Upon the addition of TEOS to the surfactant solution a wormhole like
HMS mesostructure is formed via H-bonding interactions between the amine head-group and the silica precursors. The molecular designed dispersion technique was optimized, and proved to be a very good technique by which the desired catalytic properties can be imparted to a silica support with the desired physical properties. This is achieved by first adsorbing the titanium or vanadium complex onto the support and then thermally decomposing the complex to yield the supported metal oxide catalyst. The resulting
metal oxide species are accessible to the substrate molecules as all sites are located on the surface of the mesopore walls. The surface area, mesopore volume, mesopore diameter, pore size distribution, total volume and textural porosity, of the parent are all preserved by this method. The temperature programmed oxidation of TiO(acac)2 and VO(acac)2-HMS complexes was performed and showed that the metal complexes adsorb onto the HMS surface by a combination
of hydrogen bonding and ligand exchange. A linear relationship between the amount of metal oxide acac complex and the final metal concentration of the final product is also observed. This is a very desirable property and will allow for the design of the Ti- and V-HMS catalysts with the desired metal content. The isomorphous substitution technique was optimized and used to synthesize HMS with the desired catalytic properties. However, the addition of the titanium and vanadium sources to the synthesis gel was sometimes observed to significantly change the
physical properties of the Ti- or V-HMS. The concentration of titanium in the final Ti-HMS was typically greater than the initial synthesis mixtures. This is due to the difference in hydrolysis rates of the TEOS (silica source) and titanium isopropoxide (titanium source). The titanium isopropoxide hydrolyzes much more rapidly than the TEOS so may tend to form -Ti-O-Ti- bonds instead of the desired -Ti-O-Si- bonds which are then incorporated into the framework of the Ti-HMS. It is therefore very important to ensure that the titanium source is
evenly distributed within the synthesis gel. Decreasing the Si/Ti molar ratio during generally resulted in a greater proportion of five and six-coordinated titanium species while increasing the Si/Ti ratio resulted in a greater proportion of four-coordinated titanium species. The concentration of vanadium in the final V-HMS was frequently less than the initial synthesis mixture. This is because the vanadium source hydrolyzes more slowly than the silicon source and is therefore not completely incorporated into the silica framework. It is therefore very important to ensue that the vanadium and silicon are evenly distributed within the synthesis mixture. When high Si/V molar ratios were used during synthesis vanadium sites with an isolated tetrahedral coordination were favored. When lower Si/V molar ratios were used during synthesis vanadium sites of the pseudo tetrahedral coordination were favored. Having synthesized high quality Ti-HMS and V-HMS via the molecular designed dispersion and isomorphous substitution methods the UV-vis and pyridine-TPD results were then compared to determine the
location of the active metal species within the silica framework of the HMS. This led us to the conclusion that molecular designed dispersion results in metal sites located on the surface of the HMS, while isomorphous substitution results in metal sites which are distributed randomly throughout the silica matrix of the material. The conversion of toluene over Ti-HMS results in total oxidation and the only detectable products were CO, CO2 and H2O. The greatest conversion was observed at 550°C. The conversion of toluene by Ti-HMS, synthesized via both methods, is greatly improved when the titanium content and textural porosity are increased. It was shown that increasing the textural porosity reduces the auto ignition temperature from 450 to 400°C. Increasing the titanium content was found to increase the toluene conversion by increasing the number of available acid sites where oxidation can take
place. The conversion of toluene over V-HMS, synthesized via isomorphous substituion, resulted in only partial oxidation and carbon oxides, benzene, benzaldehyde and water were produced. The greatest conversion was observed at 550°C. The conversion was greatly improved when the vanadium content of the catalyst was increased. However increasing VTEX/VMESO ratio above 0.3 had no discernable effect on the
toluene conversion. The conversion of toluene by V-HMS, synthesized by molecular designed dispersion resulted in the total oxidation of toluene to carbon oxides and water. The activity was greatly improved when the VTEX/VMESO ratio was increased from 0.1 to 0.6. This was expected as this increased textural porosity should minimize any diffusion limitations. The different behavior of the V-HMS catalysts was explained in terms of the location and the number of vanadium active species on the surface of the HMS support. Activation energies obtained for the various catalysts supports this view.