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A. Kukovecz et al. / Journal of Molecular Structure 565±566 (2001) 121±124
1
22
aqueous HPWA solution. The resulting gels were
dried, calcined at 873 K and labelled PW(0), PW(5)
and PW(20), respectively. Details of the synthetic
procedure are given elsewhere [7]. A silica±alumina
composite with Si/Al ratio of 20 was prepared by the
subtracting the spectrum of the clean, activated
catalyst wafer from the actual measurement.
In order to be able to use vibrational spectro-
scopy for monitoring catalytic reactions, one ®rst
has to ®nd molecular parts that make the spectra
of the reactants and products different from each
other. It is visible in Scheme 1 (Diels±Alder type
reaction between 1,3-cyclohexadiene and 2-
propenal. I: 1,3-cyclohexadiene, II: 2-propenal,
III.a: endo bicyclo(2.2.2)-5-octene-2-carboxyalde-
hide, III.b: exo bicyclo(2.2.2)-5-octene-2-carbox-
yaldehide) that in our Diels±Alder reaction this
is no easy task, as the reactants and the products
possess very similar bonds and groups. Our experi-
mentalconditionsandinstrumentationareclearlyinade-
quate to distinguish between endo and exo products,
therefore no attempt shall be made to do so.
co-hydrolysis of TEOS and Al(i-OC H7)3 in the
3
presence of a non-ionic detergent dried, calcined at
673 K and labelled Al(20).
Catalytic measurements were performed by placing
2
a self-supported catalyst wafer (ca. 10 mg/cm ) into a
heatable cell allowing in situ IR measurements. The
catalyst was outgassed for 1 h at 673 K and then the
2:1 mixture of 1,3-cyclohexadiene and 2-propenal
was introduced into the cell at room temperature.
Both the gas phase and the catalyst surface was
monitored at regular intervals by recording their IR
2
1
spectrum (32 scans, 2 cm resolution) on a Mattson
Genesis FT-IR 1 instrument.
The key difference between reactants and products
is that the carbonyl group is conjugated with the vinyl
CH yCH± in 2-propenal, and this conjugation no
2
3. Results and discussion
longer exists in the product molecules. On this basis,
2
1
we can expect a 20±30 cm increase in the frequency
of n(CyO) as the reaction propagates. It should be
noted that the conjugation of the CyC double bonds
in 1,3-cyclohexadiene also disappears in the product.
However, the frequency shifts of the n(CyC) and
n(H±Cy) bands corresponding to this structural
change are smaller and, especially in the adsorbed
state, hard to identify, therefore in this paper the
shift of the n(CyO) band shall be used as a measure
of the propagation of the reaction.
To make sure that this shift really measures a bimo-
lecular catalytic reaction, let us ®rst examine Parts I
and II of Fig. 1 describing two critical blank tests. It is
easily noticeable that without a reaction partner 2-
propenal is not transformed even on an acidic surface
and that even if both partners are present they are not
able to react on a neutral surface.
The acidity of the samples was determined in a
previous study [7] using the pyridine adsorption tech-
nique. It is enough to note here that sample PW(0) has
a practically neutral surface, samples PW(5) and
PW(20) possess considerable Lewis but no Broensted
acidity and sample Al(20) has both Lewis and
Broensted acidic centres in commensurable amounts.
Note that the PW composites were deliberately
deprived of their Broensted acidity by using a high
calcination temperature. In the literature, it is
accepted [8] that at the applied 873 K temperature
phosphotungstenic acid suffers partial decomposition
and therefore, the active catalytic centres are likely to
be various oxide species and not Keggin-type acid
molecules.
Examination of the gas phase spectra revealed no
changes in either series, as only the additive spectrum
of 1,3-cyclohexadiene and 2-propenal was visible.
This phenomenon is most likely caused by the strong
adsorption of both the reactants and the products on
the catalyst surface. Although these species are prob-
ably removable by elevating the temperature of the
cell, such experiments were not performed to avoid
the oligomerisation of 2-propenal. For this reason,
only adsorbed phase spectra will be discussed further
herein. All presented spectra were derived by
Scheme 1.
Kukovecz
