Abstract
Mesoporous metal oxides have been a growing part of heterogenous catalysis, with many different synthesis methods and applications. However, with the increased need for chemistry that remedies the planet, the conventional approach of carrying out individual experiments is simultaneously tedious and time-consuming. The introduction of high-throughput experimentation, as well as the design of experiments into catalysis, is an excellent way of finding new mesoporous metal oxide catalysts that will be used in reactions to better the state of the planet. These two techniques are used to screen and optimize many experiments over a short period, thus allowing for the screening of many different catalysts. This study investigated incorporating these techniques, high-throughput experimentation, and design of experiments into heterogeneous catalysis in mesoporous metal oxides.
Model dye oxidation reactions were conducted to test the efficiency of high-throughput experimentation with solid catalysts while testing the synthesized mesoporous metal oxides. Mesoporous manganese oxide catalysts were synthesized via the soft-templating method. A mixture of the surfactant, precursor salt, and butanol was stirred until a solution was dried at 120 ºC, washed and dried at 60 ºC, then calcined. The catalysts underwent calcination cycles from 150 ºC to 450 ºC, resulting in 4 different manganese oxide catalysts. Morin, Rhodamine B, and Methylene Blue were oxidized in 96-well plates, with four catalysts being tested on each dye simultaneously. These reactions were monitored in a plate reader over 3 hours. To prevent the solid catalyst from blocking the light from passing through the plate; each of the catalysts was dispersed in the solvent (bicarbonate buffer) by sonication and then dispensed into the wells using a multichannel pipette. A kinetic study was carried out as the oxidation reactions followed pseudo-first-order kinetics. A catalyst concentration variation study was conducted first to determine the optimum concentrations for the catalysts for each dye. From then on, substrate (dye) variation was done, and then oxidizing agent (hydrogen peroxide) variation studies were carried out. The optimum catalysts for morin, Rhodamine B, and methylene blue, respectively, were mesoporous manganese calcined at 150 ºC, 250 ºC, and the one calcined at 450 ºC. Over 500 reactions were carried out in a combined time of under 20 hours. This study proved that high-throughput experimentation can be incorporated into
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heterogeneous catalysis and that soft-template synthesized mesoporous metal oxides can produce some easily tunable catalysts.
The design of experiments screening technique was carried out in the selective Oppenauer oxidation of benzyl alcohol to benzaldehyde in the presence of acetone, which acted as a hydrogen acceptor. Mesoporous zirconia-based catalysts were used to catalyze these reactions This study tested for the efficiency of the screening technique and the soft-template synthesized catalysts. The studied catalysts were pure mesoporous zirconia, cerium-doped mesoporous zirconia, and manganese-doped mesoporous zirconia. Unlike the mesoporous manganese catalysts, the zirconia-based catalysts had no washing step and no calcination cycles. After the solvent was evaporated at 120 ºC, the glass-like products were calcined directly at 350 ºC. A Plackett-Burman Design was set up for each catalyst, studying five factors. There were 12 runs in the Plackett-Burman Design, leading to 36 runs overall. These reactions were carried out in a Radley’s carousel set up for ease with time. From the design of experiments, it was found that for the best product yield, all the reaction parameters need to be at their best setting, which could be either the low or high setting. This showed that one factor set incorrectly can negatively impact the response outcome. The success of the runs was measured by the highest amount obtained from reaction conversion multiplied by benzaldehyde selectivity to give the overall benzaldehyde yield. The highest benzaldehyde yield was from the runs catalyzed by the pure mesoporous zirconia catalyst. This study showed that the design of experiments can be used as a screening technique in heterogeneous catalysis. It can also save time by running several reactions simultaneously and studying several factors instead of one factor at a time.
The studies that were carried out showed that the aforementioned screening techniques can help find the optimum catalysts for different reactions and figure out the ideal conditions for these reactions. It was also found that mesoporous metal oxides are very efficient catalysts that are cost-effective and not harmful to the environment.