Propylene oxide is a commodity chemical often used as a precursor to commercially relevant polyurethanes. Unfortunately, current methods of producing it on an industrial scale are generally not profitable or environmentally friendly. New research, reported in a paper in this week's edition of Science, describes how it's possible to use clusters of only three silver atoms to effectively catalyze the oxidation of propylene to propylene oxide.
The catalyst is composed of groups of three silver atoms (Ag3) supported by an amorphous alumina film. Reactions were carried out by flowing a dilute gas mixture of propylene and oxygen in helium over and through the alumina support. Analysis of the reaction effluent found that propylene oxide, acrolein, and carbon dioxide were the primary products—the latter two are undesired byproducts. As the temperature increased, the selectivity (the ratio between the amounts of two products produced in a reaction) for propylene oxide over acrolein increased. However, above 60°C a combustion reaction began that resulted in less acrolein and more propylene oxide, but also a lot more carbon dioxide.
Typically, small metals clusters see their catalytic activity degrade as the temperature increases. That's because they merge into medium-sized blobs—a process known as sintering. The research team found that their silver trimers began sintering around 110°C and, by 200°C, had morphed into a stable population of 3.5 nm sized silver clumps. Not wasting a chance to do science, the team also examined how well these nanoparticle clumps could work as a catalyst for the reaction of interest.
The nanoparticle aggregates had a performance "strikingly different" from their trimer counterparts under identical reaction conditions. The nanoparticles showed a superior selectivity for propylene oxide up to 120°C, and maintained a reactivity per surface atom that rivaled the silver trimers. The paper reports that one propylene oxide molecule was formed per surface silver atom every second—a rate two orders of magnitude higher than any other silver catalyst. The nanoparticles were also found to be stable over the 4-hour reaction period, and didn't experience any significant deactivation due to a build-up of carbon.
To understand the excellent reactive properties of this type of silver catalyst, the team carried out a series of calculations using quantum density functional theory. They determined that the reaction conditions produced Ag3O clusters. These have an open-shell electronic structure that is highly reactive with the double bond present in propylene—a match made in chemical heaven. Similar calculations on the larger nanoparticles of silver revealed analogous reactive surface areas that would preferentially react with the double bond in propylene.
Based on their studies of the detailed chemical reaction pathways, the authors propose that the addition of water to the feed stream would further enhance the selectivity towards propylene oxide and away from the production of acrolein. The reason is simple reaction dynamics: water is a product of this secondary pathway, and having more of the product around makes that reaction less likely to proceed.
Of course, as with nearly all new breakthroughs in this area, the trick now is to see if the process can be scaled up from bench to production levels and while retaining its better properties. Ensuring it does should keep a chemical engineer off the streets for a semester or two.
Listing image by Science/AAAS