Transmission ultraviolet–visible–near-infrared spectroscopic investigations of the formation of carbocations in zeolites. Re-examination of propene/CoNaY and extension to trans-butene/CoNaY, propene/H-mordenite and cyclopropane/H-mordenite

Abstract

The applicability of the u.v.–vis.–n.i.r. transmission technique in surface studies has been explored using the sorption of propene, trans-butene and cyclopropane in activated zeolites CoNaY and HM. On CoNaY, after dehydration, near-tetrahedral Co²⁺ complexes remain, which are approached by the hydrocarbons without change of coordination number or symmetry. In HM at room temperature and in CoNaY at slightly elevated temperatures the appearance of new bands proves that allylic and dienylic carbenium ions are formed in an isomerization–oligomerization reaction. Mechanisms for the formation of these carbocations via Lewis or Brönsted acid sites are discussed.