Ammonia adsorption–desorption over the strong solid acid catalyst H3PW12O40 and its Cs+ and NH4+ salts Comparison with sulfated zirconia
Abstract
Ammonia thermodesorption using a thermal conductivity
detector does not allow an unambiguous study of the acidic
properties of strong solid acids such as heteropolycompounds
and sulfated zirconia. As a matter of fact, these strong
acids release compounds other than NH
3
during TPD
experiments which obviously involves side-reactions.
In the present work, in order to circumvent the preceding
problems, NH
3
adsorption and TPD experiments were
performed using a dual mass spectrometry (MS)–TCD
detection system. The equipment was applied to the
characterization of the acidic properties of porous
heteropolycompounds.
Cs
x
H
3-x
PW
12
O
40
and
(NH
4
)
2.8
H
0.2
PW
12
O
40
, by comparison with the parent acid
H
3
PW
12
O
40
and another
strong solid acid, sulfated zirconia.
It was observed that gaseous NH
3
reacted and
formed ionic species, leading to
(NH
4
)
3
PW
12
O
40
in
the case of heteropolyacids (HPA) and probably
(NH
4
)HSO
4
in the case of sulfated
zirconia. Consequently, based on the amount of
NH
3
irreversibly adsorbed at 373 K the number of
acid sites has been deduced. For all HPA samples, TPD
profiles of NH
3
appeared quite similar suggesting
that the TPD curves were more connected to the stability of
these ammonium salts rather than to the acid strength of the
solid.
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