Low temperature oxidation of alkenes on cationic forms of zeolites : a spectroscopic study
Zeolites have been used as catalysts in a number of industrial processes for the last
half century. The photooxidation of hydrocarbons on cationic forms of zeolites is one area
of research that has recently attracted growing interest. In this work the selective oxidation
of I-butene on cationic forms of zeolites was achieved in the presence and absence of light,
at temperatures ranging from 30-IOO°C. The latter was performed successfully both in a
static and continuous tlow system, monitored by Fourier transform infrared spectroscopy
and diffuse reflectance infrared spectroscopy respectively. Of the samples tested the most
informative results were obtained from experiments employing Y zeolites, which
possessed pores and cages of adequate size to allow for reaction to take place, with
minimal amounts of acid sites thereby preventing the occurrence of competing
polymerisation reactions. By utilising UV diffuse reflectance it was possible to observe the
charge-transfer state of I-butene and oxygen on the more reactive materials.
The products of I-butene oxidation were then employed in a series of adsorptiondesorption
experiments on cation-exchanged forms of zeolite Y. This study showed that
the removal of such products intact from a zeolite suhstrate was difficult, with complete
desorption from some samples only at 350°C, and in some cases the procedure was
complicated by decomposition and transformation of the sorbates.
An investigation into the structure of SrY and its relationship with adsorbed
ethylene was performed utilising X-ray diffraction (XRD) with synchrotron radiation. A
study with similar objectives was also carried out via a computational approach, which
utilised NaY and I-butene as the host and guest respectively. The findings from both
analyses suggested that the extra-framework cations and guest molecules were in close
proximity of one another, with the XRD study indicating the possibility of cation migration
from the smaller cages to the supercages upon adsorption of ethylene.
Overall, this research has shown that the partial oxidation of short-chain
hydrocarbons was possible without the addition of sensitisers and in mild conditions; the
electrostatic fields generated by the extra-framework cations were sufficient for a reaction
to take place, and for divalent cations this occurred even in the absence of a light source.
The removal of oxidation products was more problematical. Consequently, the
concentration of cations present in the zeolite proved to be a significant factor in both the
zeolite's reactivity and the retention of the products formed.