Zachary Pribyl and Kara Stowers, Department of Chemistry and Biochemistry
In recent years there has been a growing interest to move away from petroleum feedstocks and instead use renewable fuel sources. As this interest has increased and biofuel has been used in greater volume, glycerol (a byproduct of biofuel production) has become over-produced in the world market because it has limited commercial use as a non-edible oxygenated hydrocarbon. The ability to utilize this by-product in commercial bulk chemicals would correspondingly increase the demand for biofuel, promoting a greater shift toward renewable sources.
Reduction of glycerol to alcohols and polyols and oxidation to ketones and acids has been demonstrated. Despite all of the glycerol products being commercially useful, none of these processes are efficient or selective enough to make them commercially viable. The purpose of this project was to advance knowledge of the underlying mechanism of glycerol conversion (both oxidation and reduction) in order to improve the reaction processes.
This project had two primary steps. First, the design and construction of an Ultra-High Vacuum (UHV) system capable of performing the experiments necessary for mechanistic studies of glycerol conversion. Second, performing controlled glycerol reactions on a single crystal surface to collect data necessary to improve catalytic performance.
The UHV system began as a standard vacuum chamber (Figure 1), at the inception of this project it could hold a rough vacuum of 1×10-7 torr with a Hiden Quadrouple Mass Spectrometer as the only configured peripheral. For the project to proceed the chamber had to hold vacuum of at least 3×10-10 torr. To improve the vacuum, another turbo pump was added. It was discovered that the ion pumps (which use high voltage to ionize and trap gas molecules in the chamber) were dirty and non-functional. A frame was designed to lift the top half of the chamber allowing us to reach in and remove the old ion pumps and replace them with new ones.
In addition to obtaining UHV, the following components needed to be designed, built, and tested for functionality:
A sample holder that electrically isolates the sample and allow controlled heating and temperature measurement. We took the current sample holder and re-built it to ensure that all electrical connections were made properly. We connected a power supply to a Eurotherm control box in order to regulate the temperature. Three dosing lines equipped with a reagent inlet valve and a pressure gauge connect to the chamber through precision leak valves and were to be placed under vacuum through the manifold. An electron sputter gun is used for cleaning. This electron beam bombards the sample in the presence of argon in order to clean the sample after experiments. A lecture bottle of argon was connected to the chamber via leak valve. An X-Ray Photoelectron Spectroscopy (XPS) apparatus collects data about the composition and oxidation state of the sample. We have installed an x-ray source and hemispherical analyzer that connects to the computer for spectral data. The apparatus has been calibrated to a pure copper single crystal.
The project is currently ongoing. At this stage in the project the most tangible results are those of the chamber. The increased complexity of the system can be seen in Figure 2.
The chamber currently holds an average vacuum of 3×10-10 torr and can reach even lower when cooled during experiments. Test runs of the sample holder’s electrical heating show that the system can heat to a specific set-point but more work will have to be done to determine the error on the measurement and the linearity of the ramp. The dosing lines are installed and testing with liquid nitrogen indicates that they function as expected. The sputter gun and argon have been used to clean the sample from ambient oxidation from when it was outside the chamber. XPS spectral data confirms that the carbon and oxygen due to ambient contamination have been removed. The latter test confirms that the XPS is functioning properly and is able to obtain expected data.
Although no chemical data has been taken at this time, the complex testing apparatus necessary to obtain the data has been designed, built, and tested and is a few minor modifications and calibrations away from being the key instrument to fulfill the second half of this project. The equipment needed is quite complex and took much longer than anticipated to assemble and calibrate to have it suitable for the application.
With the growing interest in glycerol as a feedstock, the mechanism of oxidation and reduction of the molecule is of the utmost importance. The conditions necessary for the mechanist studies are difficult to obtain. However, now that they have been obtained, chemical testing can proceed.