range, as indicated for samples of Fig. 4(d) and
Fig. 5(b)) known for the industrial ones. The meaning
of A is that of a Si/Al weight concentration ratio that
should ideally be measured at the nanoscale level on
all the explored points of an ideally homogeneous
specimen having this composition. B is simply the
arithmetic average of the experimentally measured
values of the Si/Al weight concentration ratio on all
the analysed points for each specimen. It is evident
that extreme (inÆnite) values of the Si/Al ratio sometimes
recorded (as those shown in Fig. 3(d) and
Fig. 4(a)) could not be mathematically taken into
consideration for the calculation of B.
Impurities were detected in many of the investigated
specimens, both in laboratory and in industrial
ones, as shown in Table 2. In some samples, these
were present in relatively high quantities. The impurities
were very inhomogeneously distributed among
the analysed points. Nano-size domains contained
elements other than Si and Al, whereas other ones
were free from impurities. This left little hope to
quantify impurity concentration, either chemically
or through EDS, because this made it impossible to
relate the corresponding values to the non-uniformity
of the Si±Al ratio.
It is striking that particles composed of pure silica
were found in the industrial samples and that parts of
the laboratory samples were very rich in silica. Most of
the silica±aluminas are extremely heterogeneous at
the scale of our measurements. Exceptions are the
laboratory samples of Fig. 3(c) and Fig. 4(c). Nevertheless,
relative Øuctuations of10% to20% around
the average values were observed for these samples
(5<Si/Al<7 for the former case and 2.5<Si/Al<2.7 for
the latter, when neglecting a few more diverging
points).
4. Discussion
The heterogeneity of composition of all silica±
aluminas at the nanometer level is disappointingly
high. This is true even for samples prepared in small
batches under intense agitation and in well-controlled
conditions. These samples are possibly slightly more
homogeneous than the industrial ones, but it is very
doubtful that this would be sufficient for ensuring a
satisfactory control of the uniformity of acidity. The
conclusion is that all the investigated samples contain
practically all the catalytic sites detected in the measurements
reported in the introduction [1±6]. Only the
proportion of these sites is different. The presence of
inhomogeneously distributed impurities could possibly
enhance this effect. The industrial catalysts generally contain less impurities than the laboratory prepared ones. As indicated in the introduction, the ammonia TPD
measurements made on some of these samples did not
allow any clear correlation with the degree of inhomogeneity.
They just show the general trends widely
described in literature, or which can be deduced from
the results of other more elaborate techniques used for
the identification and quantification of acid sites. This
is understandable, because four main parameters are
varying in our set of just 20 samples: overall composition,
heterogeneity, and nature and amount of impurities.
A statistical study should involve sets of about
10 samples with the same overall Al, Si and impurity
content, to be significant.
There is obviously a need for more elaborate
approaches to preparation of silica±aluminas with
uniform acidity. In this we agree with an excellent
article published recently, which addressed the problem
in a very broad way [17]. Very few investigators
searched for rationally designed methods to control
the hydrolysis of the starting alkoxides of silicon and
aluminium so that both react simultaneously in the
right proportions. In principle, this could be achieved
by selecting carefully the nature of the alkoxy groups
of silicon and aluminiumto ``tune'' their reactivity and
by controlling all the reaction parameters (solvent
used, temperature, rate of water addition, agitation
etc.). On the other hand, the extremely large number of
studies devoted to the sol±gel preparation of silica±
aluminas may have nearly exhausted the possibilities
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