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Managing Editor  | June 2017

Researchers finding greener ways to make plastic precursors


Researchers at the University of Illinois are developing more environmentally-friendly catalysts for the production of plastic precursors based on knowledge of the properties of specific metals and how those metals react with hydrogen peroxide, according to a report on the school website.

 

illinois_600_01

Illinois professor David Flaherty, right, and graduate student Daniel Bregante are
working on a greener way to produce plastic and resin precursors that are
derived from fossil fuels. (Joyce Seay-Knoblauch)

 

The article explained that many plastics are created from olefins, molecules that are derived from fossil fuels. The olefins are oxidized with chemicals to alter their chemical bonds and create monomers, which link onto other monomers and form chains.

 

As one of the researchers pointed out, this process also creates byproducts such as chlorine and carbon dioxide that can be corrosive or damaging to the environment. With this in mind, the researchers set out to find a greener process for manufacturing plastics.

 

The researchers studied how transition metals affected the reactions and also explored the use of hydrogen peroxide in the process because its only waste product is water.

 

“To form the critical monomers,” the article explained, “olefins and oxidizers pass through tiny, rigid spongelike structures called zeolites. These zeolites contain metal ions in the pore spaces that act as catalysts to push the chemical reaction toward the plastic-producing pathway.”

 

While this process is not new, having been used for decades, the researchers have been able to describe which metals need to be used in order to avoid the decomposition of hydrogen peroxide and to form monomers.

 

The researchers will move on to studying how the size of the zeolites may affect the reactions and are hopeful that this work will lead to a wider adoption of this greener process within the plastics industry.

 

The work was recently published in Journal of the American Chemical Society. The abstract stated:

 

“Group IV and V framework-substituted zeolites have been used for olefin epoxidation reactions for decades, yet the underlying properties that determine the selectivities and turnover rates of these catalysts have not yet been elucidated.

 

“Here, a combination of kinetic, thermodynamic, and in situ spectroscopic measurements show that when group IV (i.e., Ti, Zr, and Hf) or V (i.e., Nb and Ta) transition metals are substituted into zeolite *BEA, the metals that form stronger Lewis acids give greater selectivities and rates for the desired epoxidation pathway and present smaller enthalpic barriers for both epoxidation and H2O2 decomposition reactions.

 

“In situ UV–vis spectroscopy shows that these group IV and V materials activate H2O2 to form pools of hydroperoxide, peroxide, and superoxide intermediates. Time-resolved UV–vis measurements and the isomeric distributions of Z-stilbene epoxidation products demonstrate that the active species for epoxidations on group IV and V transition metals are only M-OOH/-(O2)2– and M-(O2) species, respectively.

 

“Mechanistic interpretations of kinetic data suggest that these group IV and V materials catalyze cyclohexene epoxidation and H2O2 decomposition through largely identical Eley–Rideal mechanisms that involve the irreversible activation of coordinated H2O2followed by reaction with an olefin or H2O2.

 

“Epoxidation rates and selectivities vary over five- and two-orders of magnitude, respectively, among these catalysts and depend exponentially on the energy for ligand-to-metal charge transfer (LMCT) and the functional Lewis acid strength of the metal centers.

 

“Together, these observations show that more electrophilic active-oxygen species (i.e., lower-energy LMCT) are more reactive and selective for epoxidations of electron-rich olefins and explain why Ti-based catalysts have been identified as the most active among early transition metals for these reactions.

 

“Further, H2O2 decomposition (the undesirable reaction pathway) possesses a weaker dependence on Lewis acidity than epoxidation, which suggests that the design of catalysts with increased Lewis acid strength will simultaneously increase the reactivity and selectivity of olefin epoxidation.” 

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