Activation of small alkanes by heteropolyacids, a H/D exchange study: The key role of hydration water

The regiospecificity observed for the solid acid-catalyzed H/D exchange at 473 K between isobutane and the D2O-exchanged H3PW12O40 heteropolyacid of the Keggin type and its Cs1.9H1.1 PW12O40 Salt corresponds to an exchange solely with the hydrons present in the cr position to the branched carbon. This is comparable to that observed with D2O-exchanged strong acids such as sulfated zirconia, acid zeolites, and liquid 100% D2SO4. However, in exchanges with liquid superacids all the hydrons of the isobutane molecule are exchanged, This indicates that the heteropolyacid and its Cs salt are not superacids, The number of exchangeable protons of the isobutane at 473 K, after outgassing the two deuterated solid samples under dry nitrogen flow at 573 and 523 K, respectively, was very close to the proton content of the anhydrous forms of the heteropolyacids (HPAs): H3PW12O40 and Cs1.9H1.1PW12O40. Moreover, if some D2O molecules were still present after incomplete outgassing, then all of the deuterons [both from the anhydrous HPA (D+ counterions) and from crystallization D2O] would undergo exchange with the hydrons of isobutane at 473 K. This shows that such a H/D exchange reaction is fast at this temperature and does not require strong acid sites, Infrared data confirm the presence of H(H2O)(n)(+) clusters such as H3O+ and/or H5O2+ in these catalysts, their concentration dependent on the hydration level. Heteropolyacids were shown to recover their protonic acidity by rehydration of the anhydride form in flowing N-2 + D2O at 473 K, yielding completely deuterated HPA. The anhydrous forms of both samples were very active for the isomerization of n-butane to isobutane at 473 K, but their catalytic activity decreased when small amounts of water were added. This indicates that n-butane isomerization requires strong acid sites, while H/D exchange of isobutane depends on much weaker acid sites. (C) 1999 Academic Press.