Abstract
Electron spin resonance spectroscopy was used to characterize paramagnetic Rh species formed in RhCa-L and RhCa-M channel zeolites during ethylene dimerization. Activation of RhCa-M in the range 560-580 K generates species M1, assigned to Rh(II) coordinated to four oxygens of a twisted eight-membered ring in the zeolite structure. Similar activation of RhCa-L generates very weak signals, indicating that only very small amounts of Rh(II) are formed after activation. Adsorption of ethylene onto RhCa-M at 77 K followed by warming to 195 and 296 K generates paramagnetic Rh(II) species E2, which was observed previously in the cage zeolite RhCa-X. Two paramagnetic Rh(II) species, E1 and E2, are formed in RhCa-L. In contrast to the cage zeolite RhCa-X, where E2 has axial symmetry, species E2 in RhCa-L and RhCa-M has rhombic symmetry, resulting from the lower symmetry of the cation sites of these zeolite structures.
Published Version
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