Light olefins, such as ethene, propene and butenes, are chemical compounds that are vital in the production of most consumer products sold today. They are most commonly produced by the process of cracking of naphtha at 850°C. Over 400 million tonnes of light olefins are produced by this method each year, consuming an enormous amount of energy globally and meanwhile placing significant impacts on the environment. Developing a more sustainable route to produce these chemicals through the use of renewable resources is a vital task.
One such method of producing light olefins sustainably is to harness the vast amounts of plant biomass that is produced for over 200 billion tonnes each year. The compound γ-valerolactone (GVL) can be produced from agricultural waste, so is a sustainable resource, and can then be used to create butenes.
In order for GVL to be converted into butenes, the C-O and C-C bonds within it must be broken, in a process called decarboxylation (the removal of the carboxyl group from the molecule). Dr Longfei Lin and the team led by Dr Sihai Yang at the University of Manchester used the TOSCA beamline at ISIS to study a catalyst for this reaction and to observe its effectiveness of breaking the bonds. The catalyst that was studied was a zeolite known as NbAlS-1. Zeolites are formed naturally when alkaline groundwater reacts with volcanic rocks. They are porous materials consisting of AlO45- and SiO44- units with metal ions integrated within the framework. Due to their porous nature, zeolitic materials have a very high surface area, making them ideal catalysts for chemical reactions. In this experiment, there were Niobium (Nb) species incorporated within the zeolite catalyst alongside the Al and Si species. For more information on zeolites and other porous frameworks read our feature article.
Dr Lin and co-workers used TOSCA to carry out inelastic neutron scattering (INS) on the zeolite catalyst. INS measures the change in the energy of the neutron as it scatters from a sample. The initial data collected indicated that the catalyst exhibits extremely high activity at the active sites and thus conversion of GVL to butenes can occur at lower temperatures than its precedents.
Alongside GVL, methanol can also be obtained from biomass. Methanol can be used to create propene, a chemical where the demand vastly outweighs the supply. Finding a method to efficiently convert methanol to propene would be not only economically advantageous, but also greatly environmentally beneficial. During the conversion from methanol, ethene is also created as a less-desired by-product.
Dr Lin's aim was to improve the ratio of propene to ethene produced during this process, using new zeolites he and co-workers had developed that contain species of Tantalum (Ta). Unlike when converting GVL to butenes, where C-C bonds had to be broken, the process by which methanol is converted into propene involves the formation of C-C bonds to achieve C=C bonds. The process of methanol (an alcohol) being converted to propene (an alkene) is a dehydration reaction performed in the presence of an acid, in this case using Brønsted acid sites on the catalyst.
Dr Lin used the both TOSCA and MAPS beamlines to study the zeolite catalyst using INS. These zeolites were found to exhibit a simultaneously high propene selectivity, propene to ethene ratio, and catalytic stability, which sets a new benchmark for methanol-to-olefin reactions.
Samples of the Ta-containing catalyst were found to show significantly enhanced catalytic performance as well as high propene selectivity. In the study, published in Nature Communications, in collaboration with Professor Buxing Han from Chinese Academy of Science, Dr Lin attributes this to the fact that in zeolite catalysts, the different sites favour different reaction pathways: the strong acidic sites favour the production of ethene, and the weaker acidic sites favour propene. By adding the Ta species to the zeolite catalyst, Dr Lin and his team were able to moderate the sites on the catalyst, shifting the balance towards the desired propene product.
The full papers can be found at DOI: 10.1038/s41563-019-0562-6 and DOI: 10.1038/s41467-021-21062-1