Chemistry Reference
In-Depth Information
that is Brønsted or Lewis, which will be detailed at the appropriate places of this chapter.
Carbon monoxide, on the other hand, is a very sensitive probe molecule in terms of peak
shift depending on the strength of the acidic surface site. However, since CO is a
very weak basic molecule, it is less sensitive regarding weak acidic sites.
Alternatively, temperature programmed desorption (TPD) of ammonia gives a com-
prehensive figure about the distribution of acid strength of surface sites.
The principle of this method is, in brief, as follows. The respective solid sample is
deposited in a suitable flow reactor, which can be controlled and heated and which is
connected with an FTIR-cell. On the precalcined sample (usually between 523 and 573 K)
the first dry ammonia is adsorbed at e.g. 373 K. After saturation with ammonia the sample
is then flashed by an inert gas stream passing through the connected FTIR cell until
ammonia that is no longer desorbed can be detected. Now the reactor will be subjected
to a programmed temperature raise and the desorbed ammonia is followed as a function of
temperature. Thus a concentration versus temperature profile will be obtained (see for
example Figure 3.1) that gives a reflection of the acid site distribution in which the strength
of sites is indicated by the temperature (the higher the temperature the stronger the sites)
and the concentration of sites of a certain strength is proportional to the intensity of the IR
signal. By absorbing the desorbed ammonia by an aqueous acid, the overall amount of
desorbed ammonia can be exactly determined that corresponds to the square below the
desorption graph. Based on this one may go to determine different ranges of acidic
strengths. This might be acceptable as long as different solids that are similar in nature
(e.g. metal fluorides) are compared and as long as the same equipment is used. However,
since these profiles depend on several parameters (surface morphology, design of the
desorption cell, heating rate, detection system etc.) such data would not really allow any
serious comparison of different samples measured in different labs. Hence, although both
of the techniques mentioned above are very often used - and are also used by the authors -
it seems less useful to apply real quantification because of the problems briefly mentioned.
If not otherwise stated the different catalytic parameters used in this chapter have the
following meaning:
Conversion defines the part of converted compound A (n
0
A
n
A
) in relation to the
starting concentration (n
0
A
); X
¼
(n
0
A
n
A
)/n
0
A
Selectivity defines the part of a desired product P among all the products formed:
S
P
¼
nP
v
educt
ð
Þ=
n
P
n
0educt
n
educt
ð
Þ
Yield defines the part of the desired product in the reaction mixture, Y
P
¼
S
P
X
A
3.2 High Surface Area Aluminium Fluoride as Catalyst
The very high Lewis acidity of HS-AlF
3
causes it to react easily with most electron pair
donating solvents such as alcohols or aqueous ones and many reactants whereby the acidic
sites of HS-AlF
3
become blocked. Therefore, if its high Lewis acidity is of interest for
catalytic reactions it can be used at best in fluoro-organic reactions although a few others
have also been reported. Seven reactions of fluoro-organic compounds have been success-
fully catalysed by HS-AlF
3
(Equations (3.1) to (3.7)) [3], the use of the reactions in
