C O MMU N I C A T I O N S
pores less than 1.9 nm, diffusion effects may determine the rate.
Second, the local concentration or orientation of methanol may play
a role, although we have not as yet clarified the state of methanol
on the M41 surface. Previous work has described the importance
of the radius of curvature in determining the adsorption properties
of molecular sieves.10 The third possible explanation is that an
assembly of weak acid sites may act as an effective/strong acid
site. A reasonable hypothesis is that all of the surface hydroxyl
groups on the pore wall point to the center of each pore, and thus
they could work as a group. Such an assembly might induce very
active acid catalysis despite the low acidity of each OH.
In a separate experiment, we employed pentanal as a substrate
for acetalization on the M41 samples and confirmed again that the
best pore diameter for catalysis was 1.9 nm. This indicates that the
catalytic activity is not dependent on the molecular size of the
substrate but on the pore diameter of catalyst.
Figure 1. Dependence of catalytic activity for the acetalization of
cyclohexanone on the pore diameter of silica MCM-41. The catalyst was
evacuated at 333 K for 1 h before use in the catalytic run. Experimental
conditions: cyclohexanone 2.0 mmol, methanol 5.0 mL, catalyst 30 mg,
reaction temp 298 K.
Many previous studies have reported on the preparation and the
use of mesoporous materials, but few studies have found charac-
teristic phenomena deriving from regular mesopores. Our discovery
in this research sheds light on a new effect of nanometer-sized
spaces on the activation of the substrates in pores, although it is
clear that this work raises many questions. The specific effects of
nanospaces have also been reported in the fields of molecular
recognition11 and organic zeolites.
Next, we studied the effect of aluminum ion concentration in
the samples, which derives from impurities in the colloidal silica.
The Si/Al ratios of the M41 samples were all 380-670. The Si/Al
ratio of silica gel prepared from the colloidal silica was 400, which
was the same as that of the M41 samples. This shows the
importance of the pore structure of M41 samples in producing
catalytic activity. In addition, the aluminum ion concentration was
varied widely from Si/Al ) 400 to 2400 or to 48 by acid washing
or template ion exchange treatment,8 and we investigated the
catalytic activity of the resulting M41 samples (Table 1, entries 3
and 4). As can be seen, the catalytic activity of these M41 samples
is almost the same as that of the parent M41, which clearly
demonstrates that aluminum content is not a significant factor
determining catalytic activity.
12
Acknowledgment. This work was supported by Grant-in-Aids
for scientific research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
Supporting Information Available: Temperature dependence of
the catalytic activity (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
We studied the dependence of catalytic activity on the pore size
of the M41 samples, as shown in Figure 1. Surprisingly, catalytic
activity was strongly dependent on the pore diameter of the catalyst
and was maximized at around 1.9 nm. Smaller or larger pores were
not suitable for the catalytic acetalization of cyclohexanone. The
catalytic activity of M41 with a pore diameter of 4.0 nm was almost
zero. The M41 samples were repeatedly prepared and used in the
catalytic reaction, and the catalytic activity values all fell on one
correlation line within experimental error, as shown in Figure 1.
Three kinds of shape selective catalysis are widely known:9
reactant, product, or intermediate selectivity. The underlying concept
is that one of the compounds cannot pass through or be produced
in the narrow pores and therefore the catalytic selectivity is
controlled by the pore diameter. In the present reaction, the diameter
of cyclohexanone is 0.75 nm (the distance between oxygen and
most distant hydrogen is 0.51 nm; the van der Waals radius of
oxygen is 0.14, and that of hydrogen is 0.10), and that of the product
should be 0.96 nm, while the most appropriate pore diameter for
the catalysis is approximately 1.9 nm. The phenomenon cannot be
accounted for by existing concepts. Why does such a correlation
exist between pore diameter and catalytic activity? We first checked
the aluminum content of the different samples, but there was no
evidence of correlation between catalytic activity and aluminum
content, as was already discussed in the previous section.
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2
At present, we can suggest a few possible explanations. First,
some synergistic effect may be involved at larger pores, but for
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