It’s tough to get a detailed account of what’s going on inside catalytic chemical reactors while those workhorse pieces of equipment are running. If researchers could peer inside and monitor—at a microscopic level and in real time—the chemical reactions under way, they would gather a treasure trove of useful information. Engineers could then customize reactor geometry and dimensions and tailor the catalyst distribution to maximize energy efficiency, product output, and chemical selectivity.
That type of custom reactor engineering may be close at hand, thanks to a study conducted by researchers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory.
The team designed a miniature reactor whose interior can be probed microscopically with infrared and X-ray beams. They used it to interrogate a multistep chemical reaction with extreme spatial resolution. The group pinpointed to within 15 μm the regions inside the reactor in which a flowing starting material was transformed to an initial product and then a final product. They correlated that information with the microscopic location, concentration, and chemical state of catalytic nanoparticles (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja412740p).
For demonstration purposes, the Berkeley team chose a two-step reaction: the catalytic rearrangement of propargyl vinyl ether to an allenic aldehyde and the reaction of that product to form a dihydropyran. The team, which includes Elad Gross, F. Dean Toste, and Gabor A. Somorjai, selected that reaction because each compound has a unique IR spectral signature. They admitted the starting material to a flow reactor loaded with a catalyst, 2-nm gold clusters.
By probing the reactants and products with an IR beam and the catalyst with an X-ray beam, the team uncovered a host of reaction details that are normally hidden. For example, they found that by adjusting the reactant flow rate and time spent in the reactor, they could substantially alter product yield and selectivity—keys to understanding reaction kinetics and mechanisms.
They also found that nearly all of the chemistry takes place near the reactor inlet—the initial 10% of the reactor volume. On the basis of that unexpected finding, the team reduced the quantity of catalyst to one-tenth the initial value and confined it to a 2-mm region at the reactor inlet without sacrificing catalytic reactivity and selectivity, Gross says. That type of finding could mean big savings for commercial processes.
University of Wisconsin, Milwaukee, physicist Carol Hirschmugl, a synchrotron microscopy specialist, describes this novel combination of microspectroscopies as “an exciting advance.” The technique provides the opportunity to detect intermediates for complex reactions that until now have been impossible to capture in situ, she notes.