Tokyo, Japan – Researchers from Tokyo Metropolitan University have successfully traced the mechanism behind how an industrially important "superbase" catalyst is synthesized in a faster, microwave-assisted reaction. They took measurements using X-rays while the reaction occurred, uncovering how small precursor molecules were formed first before they clustered to create the final product. Their insights promise finer control over a promising technology for speeding up chemical synthesis in industry.
Polyoxometalates are industrially important compounds used to catalyze chemical reactions, from the synthesis of drugs to the fixation of carbon dioxide. Their ability to speed up reactions is largely down to their intricate molecular structure, a jigsaw of metal oxide ions assembled into isolated, cage-like clusters. One of these, a hexaniobate ion (six niobium atoms per cluster), is known to be a superbase, compounds with an especially strong affinity to protons which are invaluable in organic catalysis.
Recently, a process has been introduced to produce hexaniobate rapidly and at high purity using microwaves. Conventional methods require more than 24 hours at elevated temperature; with the help of microwaves, reaction times can be kept to within ten minutes. While this is good news for industry, the mechanism is not well understood.
A team led by Professor Seiji Yamazoe from Tokyo Metropolitan University has now used X-ray absorption measurements to uncover the sequence of events which take place when hexaniobate clusters are formed in such a short period of time. Their measurements were "in-situ," taken inside a reacting sample as it takes place, and fast, letting them distinguish individual processes before the final product is made. Specifically, they focused on X-ray absorption fine structure (XAFS), which can reveal changes in molecular structure.
Starting with a solution of niobium oxide, their measurements over time revealed how mononuclear niobium species (those containing only one niobium atom) were formed first at low temperature as the sample was heated. When the sample hit around 160 degrees Celsius, these species began to cluster, forming a precursor from which the final hexaniobate product was able to rapidly form.
The value of a shorter reaction time is not only efficiency, but a better product. Hexaniobate anions are produced as a complex with a cation, tetrabutyl ammonium (TBA), which can break down over time and bind with the cluster, effectively polluting the sites on the anion where reactions can occur. The team's insights into the process promise tighter control over the process, a purer product, and better performance for an industrially important catalyst.
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO, No. P14004), the Japan Science and Technology Agency (JST) ACT-X Program (No. JPMJAX23D7), Japan Society for the Promotion of Science (JSPS) KAKENHI Grants (Nos. 22K14543, 23K18546, 24K01259, 24K17562, and 24H02217) and a Grant-in-Aid in Transformative Research Areas (A) JP24A202 Integrated Science of Synthesis by Chemical Structure Reprogramming), the Tokyo Human Resources Fund for City Diplomacy, the Tokyo Metropolitan University Research Fund for Young Scientists, and a Tokyo Metropolitan Government Advanced Research Grant (R3–1).
