The procedure of catalysis — where a substance accelerates a chemical reaction — is vital for the creation of numerous chemicals that we encounter in our daily lives. However, despite the prevalence of these catalytic processes, scientists frequently lack a comprehensive grasp of how they operate precisely.
A recent examination by scholars at MIT has demonstrated that a significant industrial synthesis method, the creation of vinyl acetate, necessitates a catalyst to exist in two distinct forms, which alternate back and forth as the chemical reaction progresses.
It had previously been believed that only one of the two forms was essential. The new results have been published today in the journal Science, in a paper authored by MIT graduate students Deiaa Harraz and Kunal Lodaya, along with Bryan Tang PhD ’23, and MIT professor of chemistry and chemical engineering Yogesh Surendranath.
There exist two primary categories of catalysts: homogeneous catalysts, which are made up of dissolved molecules, and heterogeneous catalysts, which consist of solid materials that present surfaces for the chemical reaction. “For an extensive period,” Surendranath remarks, “a prevailing perception has been that catalysis occurs either on these surfaces or on these soluble molecules.” However, recent research indicates that in the case of vinyl acetate — a vital substance in various polymer products like the rubber used in your shoe soles — there exists an interaction between both categories of catalysis.
The synthesis process of vinyl acetate has been a large-scale industrial reaction since the 1960s and has undergone extensive research and enhancements over the years to maximize efficiency. According to the researchers, this evolution has largely been achieved through trial-and-error, without a thorough comprehension of the fundamental mechanisms involved. The reaction leading to vinyl acetate necessitates a catalyst to activate the oxygen molecules, which are part of the reaction’s ingredients, along with acetic acid and ethylene. The researchers discovered that the catalyst form that proved most effective for one aspect of the process was not optimal for the other. It turns out that the molecular version of the catalyst engages in crucial chemistry with ethylene and acetic acid, while the surface carries out the activation of oxygen. Their findings revealed that the fundamental process involved in converting between the two catalyst forms is akin to corrosion, resembling the rusting process. “Interestingly, in rusting, you actually pass through a soluble molecular species at some point in the sequence,” Surendranath states. The team applied techniques typically used in corrosion research to investigate the process. They utilized electrochemical tools to analyze the reaction, even though the overall process does not require an external electricity supply. Through potential measurements, the researchers established that the corrosion of the palladium catalyst material into soluble palladium ions is propelled by an electrochemical interaction with the oxygen, converting it into water. Lodaya mentions, “Corrosion is one of the oldest subjects in electrochemistry,” adding that “applying corrosion science to comprehend catalysis is much more recent and was crucial to our discoveries.” By correlating measurements of catalyst corrosion with other assessments of the occurring chemical reaction, the researchers suggested that the corrosion rate was limiting the overall reaction. “That’s the bottleneck controlling the rate of the entire process,” surmises Surendranath. The interaction between the two types of catalysis operates effectively and selectively “because it harnesses the strengths of a material surface and a molecule,” states Surendranath. This discovery implies that when innovating new catalysts, rather than concentrating solely on solid materials or soluble molecules, researchers should also consider how the interplay of both might unveil novel methods.
Recognizing that “catalysts can transition between molecule and material and revert, alongside the role of electrochemistry in these transformations, is a concept we are eager to further explore,” Lodaya expresses. Harraz concludes: “With this newfound understanding that both types of catalysis may have a role, what other catalytic processes exist that actually engage both? Perhaps many of them are primed for improvements that could take advantage of this insight.” This work is “enlightening, a topic worthy of undergraduate education,” states Christophe Coperet, a professor of inorganic chemistry at ETH Zurich, who did not participate in the research. “The work promotes innovative perspectives … [It] is remarkable in that it not only reconciles homogeneous and heterogeneous catalysis, but depicts these intricate processes as half-reactions, where electron transfers can circulate between distinct entities.” The research received partial support from the National Science Foundation as a Phase I Center for Chemical Innovation; the Center for Interfacial Ionics; and the Gordon and Betty Moore Foundation.