In a new paper published in Nature Chemistry, researchers at the University of Pennsylvania report a new method to convert hydrocarbons to alkenes, which are building blocks to polymers, natural products and chemical reagents ubiquitous in industry. According to the researchers, they get 100 percent selectivity, transforming the hydrocarbons towards a more desirable product with zero emissions of carbon dioxide.

Another benefit of this new energy-efficient and environmentally friendly process is that it uses inexpensive materials.

“We have discovered a unique way,” said Daniel Mindiola, a Presidential Professor in the Department of Chemistry in the School of Arts & Sciences at Penn, “to grab a hydrocarbon, a molecule that is usually only looked at as a source of energy by burning it, and convert it to a more value-added material that is a building block to polymers and numerous reagents of immense value to the chemical industry. Notably, we found we can do this using cheap, Earth-abundant elements such as titanium.”

Generally, these reactions can be done with very expensive, rare metals. The researchers’ system not only applies a new strategy to remove hydrogens selectively from an alkane but uses a base metal such as titanium to accomplish it. Although the researchers have yet to do this in high yields and turnovers, Mindiola said, their study provides “a new vista for how a cheap metal such as titanium can accomplish what precious metals such as rhodium, osmium and iridium are known to do.” 

The research was done by Mindiola; Penn alumnus Douglas Solowey; Takashi Kurogi, visiting postdoctoral fellow from the Japanese Society for the Promotion of Science; Patrick Carroll, director of Penn’s X-Ray Crystallography Facility; and Staff Crystallographer Brian Manor. Mu-Hyun Baik and postdoc Manoj Mane of the Korea Advanced Institute of Science & Technology also contributed to the study.

Their method uses an extremely reactive titanium-based molecule that can pull apart the carbon-hydrogen bond of these alkanes. The trick, Mindiola said, was to constrain the geometry of the titanium center to one it did not like in order to render it a reactive system. The researchers also protect the metal center with a large ligand to allow only the least hindered carbon-hydrogen bond to come close and thus be activated.  This allows them to selectively pull apart the less reactive but more important carbon-hydrogen bond. 

Mindiola said that, when they first started this project in 2003, they were trying to make polymerization catalysts but instead discovered this more interesting reaction.

“What is truly remarkable to me,” he said, “is that this project was purely fundamental when we started and now has been become a very applicable project relevant to energy, the petrochemical industry, and it is still very heavy organometallic-wise.”          

Mindiola believes that all petrochemical industries working with natural gas and volatile hydrocarbons will be interested in this research because it offers a new way on how to transform these fuels into more value-added chemicals, selectively and mildly. 

“They want to stop burning these abundant resources and instead convert them to more valuable raw materials,” he said. “A huge amount of energy is lost in a very important industrial process such as steam cracking. In this process, the petrochemical industry uses a lot of energy to break hydrocarbons apart and make more useful hydrocarbons, which ultimately results in energy being consumed but also in the formation of carbon dioxide.” 

This process also involves the formation of many different olefins, so separation is required. The researchers’ new system, although not very active catalytically, provides a unique pathway to saving energy using cheap materials and obtaining exclusive selectively with no carbon dioxide emissions.

The next step, Mindiola said, is to improve the system by increasing the number of turnovers. The systems are well understood, so scientists know exactly how to do this because they know which specific step is hurting and slowing down the cycle. With help from the Baik group at KAIST, they have very important clues on how to improve the system by several orders of magnitude. 

“For me,” Mindiola said, “the most exciting aspect is that we trouble-shot the system year by year by understanding each step of the cycle. By learning it, we could fix problems along the way and will soon improve it even more. It is so much fun to see how research in this area has  slowly but progressively come together over the years.”

This research was supported by National Science Foundation grants CHE30848248 and CHE31152123.