Chemistry Reference
In-Depth Information
tially insoluble and are probably the leading candi-
dates for potentially useful true solid HPA catalysts.
The Cs
2.5
H
0.5
PW
12
O
40
catalyst is formed via the addi-
tion of Cs
2
CO
3
to aqueous solutions of H
3
PW
12
O
40
.
Using this approach, ultrafine crystallites in the acid
form (H
3
PW
12
O
40
) are deposited epitaxially on the
surface of Cs
3
PW
12
O
40
crystallites. Upon gentle
heating, H
+
and Cs
+
migrate in the solid state from a
nearly uniform solid solution in which the protons
distribute randomly throughout the entire bulk,
the final composition is Cs
2.5
H
0.5
PW
12
O
40
(when the
Cs/H ratio is set to 2.5-0.5). This was confirmed
using x-ray diffraction (XRD) and
31
P-NMR. The
Cs
2.5
H
0.5
PW
12
O
40
catalyst has mesopores as well as
micropores, which are interparticle voids of the crys-
tallites. The high catalytic activity of Cs
2.5
H
0.5
PW
12
O
40
reported for solid-liquid reaction systems is attrib-
uted to the strength and number of accessible acid
sites and the mesoporous structure that allows rapid
transport of reactants and products within the HPA.
As synthesised, the surface area (m
2
g
-1
) and crys-
tallite size (Å) within the agglomerated powders of
H
3
PW
12
O
40
and Cs
2.5
H
0.5
PW
12
O
40
are 5 and 135, and
2264 and 69, respectively. Thus, the Cs salts are com-
prised of aggregates of extremely small crystallites
linked together, which in turn build in mesoporos-
ity and microporosity (hence the much higher
surface area). Some of the highest surface areas are
obtained at a Cs/H ratio of 2.5-0.5. The small crys-
tallite size of the Cs
2.5
H
0.5
PW
12
O
40
catalyst was con-
firmed also using scanning electron microscopy. This
was in stark contrast to H
3
PW
12
O
40
, which under the
resolution observed had a smooth non-porous
appearance. Under these conditions, most of the HPA
is crystallised from an aqueous solution, which
reflects the lower solubility of the Cs
2.5
species (Fig.
6.4).
The high catalytic activity of the solid acid catalyst
Cs
2.5
H
0.5
PW
12
O
40
arises in part by the ease of access
to the acid sites within the microstructure. The role
of the Cs is to help develop a microstructure that,
due to its nucleation and growth, has a large amount
of porosity within the particulates. These acid sites
are highly acidic and catalyse a range of reactions. In
contrast, H
3
PW
12
O
40
, which is also extremely acidic,
actually has a higher number of acid sites but only a
small proportion of these are available for reactivity.
A clear challenge would be to develop a very high
surface area form of H
3
PW
12
O
40
. One prerequisite for
at least some reactions has to be the need for insol-
ubility, creating a truly active solid acid catalyst.
The high activity of Cs
2.5
H
0.5
PW
12
O
40
has been
realised in a number of reactions. The heteropoly-
acid was more active by two orders of magnitude
than aluminosilicate, sulfated zirconia, H-ZSM-5 and
sulfuric acid for the decomposition of cyclohexyl
acetate and the alkylation of 1,3,5-trimethyl
benzene.
Supported HPA has been an active area of interest
as a means of increasing the surface area, e.g.
H
3
PW
12
O
40
[34]. In general, at low loadings HPAs
strongly interact with the supports, reducing some of
the catalytic properties, whereas the bulk properties
prevail at higher loadings. The most frequently used
Fig. 6.3
Heteropolyacid.
Fig. 6.4
Formation of 'porous'
heteropolyacids: initial cubic crystallite
of C3 (left) and conversion of
adsorbed H
3
PW
12
O
40
to the solid of
Cs
2.5
H
0.5
PW
12
O
40
(right).
