As observed during a-pinene isomerisation, the lowest
loaded catalysts (o6.2 wt%) proved largely ineffective with
no methanol condensation observed. This again correlates
with acidity measurements which revealed that loadings below
6.2 wt% only possess weak acid sites and suggests that while
structurally the Keggin unit is intact, their acidity is decreased
by site isolation. Fig. 11a and b show that as the HPW loading
is increased the light off temperature steadily decreases, reach-
ing a limiting value of B169 1C for the highest loading
catalysts. All supported catalysts Z 6.2 wt% HPW exhibited
greater activity and lower T50% for methanol conversion than
the unsupported HPW sample. These results correlate well
with (steady state) isothermal GC results which were used to
calculate reaction selectivity. At maximum conversion levels
above 175 1C activity reached 33 mmol L catꢀ1 sꢀ1 over the
0.25 ml catalyst bed. All active catalysts exhibited >99%
selectivity towards DME when reactions were performed
o250 1C. At higher temperatures an increase in methanol to
gasoline (MTG) type chemistry27 was observed at the expense
of DME formation, with hydrocarbon products ethane and/or
ethene resulting from breakdown of MeOH and/or DME
observed.
B0.14 ML at which point it is possible to form tetramers
with a pocket for crystalline water as illustrated in Scheme 3.
Conclusions
A series of SiO2 supported H3PW12O40 catalysts have been
prepared and fully characterised. Structurally we find that the
Keggin unit remains intact on immobilisation and the acid site
strength is identical for all loadings >6.2 wt%. During the
evolution of the monolayer inequivalent W environments can
be observed by XPS which are in a 3 : 1 ratio. These are
attributed to pertubation of Keggin W atoms which are in
direct contact with the SiO2 surface. NH3 calorimetry mea-
surements reveal that the acid strength is invariant of loading
Z 6.2 wt% HPW and is comparable to the strength of bulk
HPW species. Structural characterisation using EXAFS con-
firms that the Keggin unit is intact for all loadings, suggesting
the decrease in acid strength o6.2 wt% HPW is attributable to
the presence of highly dispersed clusters which are unable to
form crystalline water between adjacent Keggin units. We
propose that there is a threshold HPW coverage of B0.14
ML at which point it is possible to form tetramers with a
pocket for crystalline water. The observed trends in catalyst
activity are consistent with the acid strength measurements.
For reactions involving non-polar hydrocarbon substrates
optimum catalyst activity is observed when the interfacial
species and thus accessible tungstate species is maximised. In
contrast the activity of supported HPW catalysts in polar
reactions, which can take advantage of the pseudo-liquid
phase, scales with loading.
In contrast to a-pinene isomerisation, MeOH conversion
did not pass through a maximum between 26 and 43.5 wt%,
instead it reached a plateau at high loadings. This may reflect
differences in the polarities of these processes: methanol con-
densation involves polar reactants and products and can thus
occur in the pseudo-liquid bulk in addition to surface acid
sites. Methanol conversion therefore scales with total HPA
content, whereas a-pinene isomerisation scales with the com-
pletion of the monolayer dispersed HPW, which itself passes
through a maximum at 26–43.5 wt%.
There have been previous reports of reduced acid strength in
low loaded HPW/SiO2 samples which were ascribed to strong
interaction with the support.28 Clearly this study demonstrates
that loss of acid strength for HPW loadings o6.2 wt% is not
due to decomposition of the Keggin structure, rather that the
high dispersion results in an inability to form crystalline H2O
between adjacent Keggin units. Given that we estimate the
monolayer saturation occurs at B44 wt% HPW, we
thus propose that there is a threshold HPW coverage of
Acknowledgements
Financial support by the UK Engineering and Physical
Sciences Research Council under grants GR/M20877/01 and
GR/R39436/01, and from BP Chemicals is gratefully acknowl-
edged.
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Scheme 3 Evolution of dispersed HPW clusters on SiO2 support.
ꢁc
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