Behaviour of Mesoporous Silica (MCM-41) Supported Catalysts in Degradation Reactions
Guthrie, Colin Peter
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A research project was carried out to investigate several aspects of Mobil’s composition of matter #41 (MCM-41) pertaining to its potential application in water treatment technologies. The goal of the work was to lead to a recommendation of whether further work investigating this particular, potential application is warranted at the present time. A dissolution experiment indicated that Pd/MCM-41 is more stable in contact with deionized water at 25±0.1 degrees Celsius than purely siliceous MCM-41. Stability increases with increasing mass percent loading of Pd in the Pd/MCM-41 material. The increased stability was attributed to a reduction in Pd/MCM-41 surface areas relative to the parent MCM-41 material. The reduction in surface area is likely the result of partial or complete blocking of the MCM-41 mesopores by Pd centres. Ni/MCM-41 was less stable in contact with deionized water than MCM-41 and Pd/MCM-41. Both Ni and Pd impregnated MCM-41 exhibited enhanced stability relative to purely siliceous MCM-41 in 0.01 M NaCl solution. The stability enhancement was more pronounced for Pd/MCM-41. A long term dissolution experiment showed that MCM-41 retained its characteristic hexagonal mesopore structure and high surface area after 1,174 days of contact with deionized water. The ability of Pd/MCM-41 to absorb hydrogen was investigated in a series of experiments in pressure cells at 25±0.1 degrees Celsius and approximately 101.3 kPa. The Pd in Pd/MCM-41 was able to absorb 0.85±0.18 moles of hydrogen per mole of Pd present. This was a higher level of absorption than found in most other, published Pd-hydrogen investigations. It is proposed that enhanced uptake of hydrogen by Pd/MCM-41 may be the result of a higher proportion of surface and subsurface sites in the samples relative to other supported Pd materials. Batch and column trichloroethylene (TCE) degradation experiments indicated that Pd/MCM-41 has substantial longevity while degrading at least 91% of inflow TCE concentrations in hydrogen-saturated deionized water over 5,036 pore volumes. The average inflow TCE concentration was 4.94E-2 ± 4.87E-3 mmol/l with maximum and minimum concentrations of 7.24E-2 mmol/l and 2.63E-2 mmol/l, respectively. The dominant breakdown product of TCE degradation in the presence of Pd/MCM-41 was ethane. Minimal concentrations of intermediate degradation products were detected, if at all. This result suggests that TCE completely degrades to ethane before desorbing from the Pd/MCM-41 surface. It was shown that Pd/MCM-41 was more effective and had better longevity at treating inflow TCE compared to a lower cost substitute, Pd/sand. Unlike Pd/MCM-41, Ni/MCM-41 did not induce degradation of TCE. Column experiments using Ni-Pd/MCM-41 materials indicated that while the material does induce degradation of TCE, the Pd cannot be substituted for Ni on a 1:1 basis while still obtaining similar degradation results as Pd/MCM-41. Pd/MCM-41 was not able to substantially reduce initial concentrations of hexamethylphosphoramide (HMPA) in a batch experiment. A small reduction in the initial HMPA concentration was attributed to adsorption onto the Pd/MCM-41 surfaces rather than degradation of the compound. Several of the above-mentioned results suggest that MCM-41 and Pd/MCM-41 show great potential for being incorporated into water treatment technologies. Both the long term stability of the material in contact with deionized water and the longevity of a Pd/MCM-41 column in treating inflow TCE are very promising results with respect to applications in water treatment technologies. It is recommended that future work be pursued with this goal in mind.
Cite this work
Colin Peter Guthrie (2013). Behaviour of Mesoporous Silica (MCM-41) Supported Catalysts in Degradation Reactions. UWSpace. http://hdl.handle.net/10012/7181