N. Essayem et al., Activation of small alkanes by heteropolyacids, a H/D exchange study: The key role of hydration water, J CATALYSIS, 183(2), 1999, pp. 292-299
The regiospecificity observed for the solid acid-catalyzed H/D exchange at
473 K between isobutane and the D2O-exchanged H3PW12O40 heteropolyacid of t
he Keggin type and its Cs1.9H1.1 PW12O40 Salt corresponds to an exchange so
lely with the hydrons present in the cr position to the branched carbon. Th
is is comparable to that observed with D2O-exchanged strong acids such as s
ulfated zirconia, acid zeolites, and liquid 100% D2SO4. However, in exchang
es with liquid superacids all the hydrons of the isobutane molecule are exc
hanged, This indicates that the heteropolyacid and its Cs salt are not supe
racids, 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 anhyd
rous forms of the heteropolyacids (HPAs): H3PW12O40 and Cs1.9H1.1PW12O40. M
oreover, if some D2O molecules were still present after incomplete outgassi
ng, then all of the deuterons [both from the anhydrous HPA (D+ counterions)
and from crystallization D2O] would undergo exchange with the hydrons of i
sobutane 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 conf
irm the presence of H(H2O)(n)(+) clusters such as H3O+ and/or H5O2+ in thes
e catalysts, their concentration dependent on the hydration level. Heteropo
lyacids were shown to recover their protonic acidity by rehydration of the
anhydride form in flowing N-2 + D2O at 473 K, yielding completely deuterate
d HPA. The anhydrous forms of both samples were very active for the isomeri
zation of n-butane to isobutane at 473 K, but their catalytic activity decr
eased when small amounts of water were added. This indicates that n-butane
isomerization requires strong acid sites, while H/D exchange of isobutane d
epends on much weaker acid sites. (C) 1999 Academic Press.