SIMILARITIES AND DIFFERENCES IN ACTIVATION OF SMALL ALKANES BY LIQUIDAND SOLID STRONG ACIDS - AN NMR, MS, AND UV SPECTROSCOPIC STUDY

The regioselectivity of H/D exchange occurring between isobutane and s trong solid acids such as D2O exchanged sulfated zirconia and acidic z eolites has been reinvestigated with increasing contact times on D2O-e xchanged USY zeolite and the results compared with those obtained by u sing 98% D2SO4, As expected, during the initial stage, both on the sol id and in the liquid acid only the primary hydrons of isobutane were e xchanged for deuterons. However, with aging catalysts, deuterium is sl owly introduced also in the tertiary position of isobutane, by a mecha nism related to the formation of polyenylic ions which could be monito red by UV spectroscopy. When the catalyst is repeatedly replaced by fr esh material, isobutane can be completely selectively deuterated on th e primary position with both acids, The MS and NMR spectra show howeve r that the isotopologs distribution in partially deuterated isobutane is very different when D2SO4 is used in comparison with the solid acid . (C) 1997 Academic Press.