Chemists and chemical engineers seek to utilize zeolites 'under control' in heterogeneously catalyzed reactions. By choosing specific zeolite structures, channel and cage geometry can be varied, which can strongly influence the activity and selectivity in chemical transformations. Brønsted and Lewis acid sites initiate catalytic reactions via proton transfer or hydride abstraction, and negatively charged framework oxygen can stabilize formed hydrocarbon cations. These multiple active centers of zeolites can work together to promote a desired reaction process. By ion-exchange of parent zeolites, the formation of Brønsted acid sites follows the Hirschler-Plank mechanism upon thermal dehydration of these zeolites at temperatures higher than 473 K. Increasing the dehydration temperatures, the concentration of Brønsted acid sites is decreasing. At the same time, highly charged metal cations are formed in this process acting as Lewis acid sites, and bridging OH groups are deprotonated to generate basic sites. The strength of these Lewis acid sites depends on the ratio between the charge of the metal cation and its size. These active sites show different performance at the early stages of acid catalyzed reactions. In-situ MAS NMR investigations indicated that Brønsted acid sites prefer to protonate aromatic rings of alkyl benzene with low activation energies. Lewis acid sites are selectively activating the ethyl group of ethylbenzene by hydride abstraction with high activation energies. Based on this knowledge, we tuned acidic properties of zeolites X and Y, which can switch on/off processes for desired reactions.
|Number of pages||1|
|Publication status||Published - 2011|
|Event||Chemeca (39th : 2011) - Sydney, Australia|
Duration: 18 Sep 2011 → 21 Sep 2011
|Conference||Chemeca (39th : 2011)|
|Period||18/09/11 → 21/09/11|