Potential and Limitations of MCM-41 in Dechlorination Reactions
Guthrie, Colin Peter
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The purpose of this thesis was to conduct preliminary research into the feasibility of using MCM-41 as a catalyst support material in the treatment of organochloride contaminated water. Specifically, the stability of MCM-41 in water and its efficiency as a Pd metal catalyst support in the degradation of trichloroethylene (TCE) was examined. MCM-41 is a mesoporous siliceous material that was developed by scientists with the Mobile Corporation in 1992. Since its development, MCM-41 has been the subject of a great deal of research into its potential application in catalytic sciences. The material possesses two especially notable characteristics. First, the diameter of its pores can be adjusted between 2 and 10 nm depending on the reagents and procedure used in its synthesis. Second, MCM-41 has an exceptionally high surface area, often in excess of 1 000 m2/g in well-formed samples. Other researchers have succeeded in grafting a variety of different catalytic materials to the surfaces and pores of MCM-41 and reported dehalogenation reactions proceeding in the presence of hydrogen. Thus, MCM-41 shows promise in treating a variety of chlorinated volatile organic compounds (cVOCs), such as chlorinated benzenes, trichloroethylene (TCE), perchloroethylene (PCE) and some polychlorinated biphenyls (PCBs). Preliminary stages of this research were devoted to synthesising a well-formed sample of MCM-41. The method of Mansour et al. (2002) was found to be a reliable and repeatable procedure, producing samples with characteristic hexagonal crystallinity and high surface areas. Crystallinity of all materials was characterized by small angle X-ray powder diffraction (XRD). Samples of MCM-41 prepared for this research exhibited a minimum of three distinct peaks in their XRD traces. These peaks are labelled 100, 110, and 200 according to a hexagonal unit cell. The 100 peak indicates that the sample is mesoporous. The 100, 110, and 200 peaks together indicate a hexagonal arrangement of the mesopores. An additional peak, labelled 210, was also observed in materials prepared for this research, reflecting a high degree of crystallinity. The position of the 100 peak was used to calculate the unit cell parameter - “a” - of the samples according to Bragg’s Law. The value of the unit cell parameter corresponds to the centre to centre distance of the material’s pores and thus the relative diameter of the pores themselves. The unit cell parameter of samples prepared for this research ranged from 4.6 nm to 5.3 nm with an average value of 4.8 nm. Surface areas of prepared samples were determined by BET nitrogen adsorption analysis and ranged from 1 052 to 1 571 m2/g with an average value of 1 304 m2/g. Field emission scanning electron microscope (SEM) images of a representative sample of MCM-41 revealed a particle morphology referred to as ‘wormy MCM-41’ by other researchers. A sample of aluminum-substituted MCM-41 (Al-MCM-41) was also synthesized. The crystallinity of Al-MCM-41 was characterized by small angle XRD. The XRD trace of the material showed only one distinct peak centred at 2.1 degrees 2θ. The 110 and 200 peaks seen in MCM-41 were replaced by a shoulder on the right hand side of the 100 peak. The shape of this trace is typical of Al-MCM-41 prepared by other researchers and is indicative of the lower structural quality of the material, i.e. a less-ordered atomic arrangement in Al-MCM-41 compared to that of regular MCM-41. The unit cell parameter of the Al-MCM-41 sample was 4.9 nm. The surface area of the sample was determined through BET nitrogen adsorption analysis and found to be 1 304 m2/g. Attempts were made to synthesize an MCM-41 sample with enlarged pores. Difficulties were encountered in the procedure, specifically with regards to maintaining high pressures during the crystallization stage. Higher temperatures used during these procedures caused failure of the O-ring used in sealing the autoclave, allowing water to be lost from the reaction gel. Samples generated in these attempts were amorphous in character and were subsequently discarded. A solubility study involving MCM-41 was undertaken to determine the stability of the material in water at ambient temperature and pressure. The experiment included several different solid/water ratios for the dissolution experiments: 1/200, 1/100, 1/75, 1/25. Results indicated that MCM-41 is metastable at ambient temperatures and more soluble than amorphous silica in water. The maximum silica concentration observed during the experiment was used to calculate a minimum Gibbs free energy of formation for MCM-41 of - 819.5 kJ/mol. The higher free energy value compared to quartz (- 856.288 kJ/mol) is indicative of the metastability of the material in water. Supersaturation with respect to amorphous silica was observed in samples prepared with relatively high concentrations of MCM-41. A subsequent decrease in dissolved silica concentration with time in these samples represented precipitation of amorphous silica, driving the concentration downward towards saturation with respect to this phase (120 ppm). The equilibrium concentration of 120 ppm recorded in these samples represented 4.8 mg out of 200, 400, 500, and 1 600 mg of initial MCM-41 dissolving into solution in the solid/liquid ratios of 1/200, 1/100, 175, and 1/25, respectively. Supersaturation with respect to amorphous silica did not occur in experiments with very low solid/water ratios. It also did not occur in higher solid/water experiments from which the SiO2 saturated supernatant was decanted and replaced with fresh deionized water after two weeks of reaction. The difference in dissolution behaviour is believed to result from deposition of a protective layer of amorphous silica from solution onto the MCM-41 surfaces, which reduces their dissolution rate. Thus, supersaturation with respect to amorphous silica is only manifested at early time and only when relatively large amounts of fresh MCM-41 are added to water. The solubility experiment was repeated using samples of Al-MCM-41 to determine the effect of Al substitution on the stability of the MCM-41. Dissolution curves for the Al-MCM-41 samples revealed behaviour that was analogous to that of the silica-based MCM-41 at similar solid/water ratios. Substitution of Al into the structure of MCM-41 appeared to have no positive or negative effects on the stability of the material in water. Solid MCM-41 material was recovered on days 28 and 79 of the solubility experiment and dried under vacuum. Solid material was also recovered from the Al-MCM-41 solubility experiment on day 79. These recovered samples were characterized by XRD and BET nitrogen adsorption analysis. An increase in background noise in the XRD plot of MCM-41 from the fresh to the 79 d sample indicated an increased proportion of an amorphous phase in the sample. The XRD plot of the 79 d sample of Al-MCM-41 also showed increased background noise corresponding to an increased proportion of an amorphous phase. The increased amorphous phase would have resulted from the continuous dissolution of the crystalline MCM-41 and reprecipitation as amorphous silica in the samples. BET surface area analysis of recovered MCM-41 compared to the freshly prepared material showed no significant change in surface area after 28 and 79 days in water. Analysis of the 79 d Al-MCM-41 indicated a 10% decrease in surface area relative to the as-prepared material. A set of SEM images were taken of the day 28 and 79 MCM-41 samples and compared to a sample of freshly prepared material. No substantial change in morphology was observed in the day 28 sample when compared to the fresh material. Some change was noted in the day 79 sample particle morphology, with worm-like structures appearing to be better developed than in the as-synthesized material. A series of palladized MCM-41 (Pd/MCM-41) samples with varying mass percent loadings of Pd was prepared to investigate the dehalogenation efficiency of Pd/MCM-41 in contact with TCE. TCE degradation was investigated in batch experiments. MCM-41 samples were prepared with calculated Pd loadings of 0.1, 1, and 5 mass %. The actual palladium content of the materials was determined using an EDAX-equipped SEM. The success of the loading technique was better at lower mass loadings of Pd, i.e. there was a greater deviation of actual Pd content from targeted or calculated contents at higher loadings of Pd. It was found that a procedure designed to yield 1% by mass Pd/MCM-41 produced an average loading of 0.95% Pd by mass. A procedure designed to produce a 5% Pd/MCM-41 sample resulted in an average loading of 2.6 mass %. These deviations were attributed to error inherent in the EDAX analysis and reduced effectiveness of the loading technique at higher Pd concentrations. All batch experiment reaction bottles were prepared with solid/liquid ratios of 1/800. The various Pd/MCM-41 samples induced rapid dehalogenation reactions, with the maximum extent of TCE degradation occurring before the first sample was taken at 7 to 12 min and within 35 min in the case of 0.1% Pd/MCM-41. The 0.1% Pd/MCM-41 sample degraded 70% of total TCE in solution with an estimated degradation half-life of 14 min. The 1% Pd/MCM-41 sample degraded 92% of total TCE in solution with an estimated half life of between 3 and 6 min. The 5% Pd/MCM-41 sample degraded only 22% of total TCE in solution; degradation half-life could not be determined. The seemingly paradoxical result of lower degradation efficiency at higher Pd loadings is proposed to result from absorption of hydrogen from solution by Pd, which is unreactive relative to the dissolved hydrogen in solution. Production of reaction intermediates and daughter products was also lower in the 1% by mass Pd/MCM-41 experiment compared to the 0.1 and 5% by mass Pd/MCM-41. Analysis of degradation products results from the experiments indicated that TCE degrades to ethane in the presence of Pd/MCM-41 with relatively low concentrations of chlorinated daughter products resulting from a random desorption process. A batch experiment using pure silica MCM-41 was also conducted to determine if there was adsorption of TCE to the support material itself. A lack of change in TCE concentration between the control sample and the MCM-41 sample during the experiment indicated no significant adsorption of TCE onto MCM-41. The conclusion of this research is that although MCM-41 is relatively unstable in water, its high TCE degradation efficiency shows promise for its application in developing water treatment technologies. However, more research needs to be conducted to fully determine the potential use of MCM-41 in water treatment and to investigate ways to improve its long-term stability in water.