Ethane-to-Ethylene Conversion on Cobalt Molybdate Catalyst Using Carbon Dioxide as a Soft Oxidant
by Jayson Pinals
mentor: Prashant Deshlahra, Chemical and Biological Engineering; funding source: Provost’s Office
pinalsjayson_37143_2246789_Cobalt-Molybdate-CatalystI am very excited to be sharing my research, which is based on understanding environmentally friendly alternatives to energy production. This includes the use of the specific alkane, ethane, instead of traditional oils as a source of energy. Ethane is very important for the production of ethylene, which can be used to make many different chemicals and fuels. Currently, oxygen is reacted with ethane to produce ethylene and water (Scheme 1). The reaction that my research focused on was the reaction between ethane and carbon dioxide instead of oxygen to create ethylene, carbon monoxide, and water (Scheme 2).
There are important benefits of using the second reaction scheme (Scheme 2). The first benefit is that more of the desired product (ethylene) will be created and ethane won’t be converted into undesired products (like carbon monoxide and carbon dioxide) in total and partial combustion. The next benefit is that it reduces carbon dioxide emissions because the right pathway uses up carbon dioxide. Carbon dioxide is a greenhouse gas and contributes to climate change. Therefore, using the right pathway instead of the left is a clear benefit for our planet. So why hasn’t it been put to use in the industry? One of the challenges of this process is the energy required for dehydrogenation, which was the main focus of my research for the summer.
A transition state is a specific molecular configuration with the highest potential energy between reactants and products. The activation energy is the minimum energy required to complete a reaction from reactants to products. How could the rate of the reaction be increased to create more of the desired product? More money could be spent on energy to use for production, but this would be expensive and a waste of energy. Instead, a catalyst could be added to the reaction to lower the activation energy. A catalyst lowers the activation energy for a reaction by either orienting the reacting particles in such a way that successful collisions are more likely or reacting with the reactants to form an intermediate that requires lower energy to form the product. The catalyst being explored for my research is cobalt molybdate.
The first mechanism that was explored was a pathway for the process of activating carbon dioxide, which is an important step in the overall ethane to ethylene reaction. The second mechanism is a pathway for activating carbon dioxide that is very similar to the first. The difference between the two is the location of the hydrogen and carbon dioxide on the surface, and which oxygen the hydrogen bonds to. In this case, the hydrogen bonds directly to the free oxygen.
The energy of each state was found using VASP, which is a package for performing density functional theory (DFT) calculations. Linux was used to edit and submit VASP calculations in the Tufts HPC Research Cluster and Stampede2 Supercomputer was used to run the VASP calculations. The energy graph shows the electronic energy of the 6 states in order. On this graph, a state with lower electronic energy is more stable. To go from the physisorbed to the CO 2 state, the system first needs about 30 kJ of energy per mole of reactants to get to the adsorption transition state. The second transition state is much higher in energy.
The energy graph also compares the two pathways. The COOH_v2 result is more stable than that for COOH. There’s about a 40% decrease in energy change from the first pathway final structure to the second. More interestingly, the transition state is also more stable. There is an 86% decrease in energy change from the first pathway transition state to the second. This means that not only is the second version of the final state more stable, but the second pathway also requires less energy to reach the second transition state.
The energy graph of higher hydrogen coverage shows the energies of the second pathway but with added hydrogen coverage on the surface. Each of the colors shows a different location of one additional Hydrogen on the surface. This is meant to help understand what happens when there are two hydrogens on the surface. These results will be used to model and calculate the electronic energies of the final steps of the overall ethane dehydrogenation reaction.
This is really fascinating work, and catalysis with environmental applications is a great subfield. I’d love to get a better understanding of why this eliminates byproducts; my thought is that by lowering the activation energy of the process which forms activated carbon dioxide, you make less favorable the other pathways, which form different (maybe radical if the energy is high enough?) reactive species.
Thank you for your interest in my research Mitchell. I’m glad that you also find this field fascinating.
You are correct in that reaction pathways with lower activation energies are more favorable than those that require higher energy input. The more favorable processes are expected to be more prevalent in experiments with this conversion reaction for this specific catalyst.
The current pathway of ethylene production shown in Scheme 1 suffers from coke formation and is somewhat energetically intensive. Oxygen may also react with ethane or ethylene in total or partial combustion reactions. These reactions will decrease the overall carbon atom efficiency and lead to the formation of CO and CO2. Therefore, the chosen catalyst had to be stable against coking and selective to ethylene. Cobalt molybdate was chosen because it was promising in experimental studies.