Chemistry Reference
In-Depth Information
Fig. 7.7
Direct route to organically
modified micelle-templated silicas.
of the catalyst (e.g. shape selectivity or enantioselec-
tive reactions, where inclusion phenomena may play
a key role in transition state determination), the
small amount of external surface sites may need to
be blocked, leaving the only active sites inside the
pores. This can be done by using an appropriate
poison of large dimensions to block selectively the
external sites while causing minimal disruption to
in-pore sites. Poisons such as triphenylamine [45,46]
have been used to block external acid sites, whereas
large silanes such as Ph
2
Si(OR)
2
have been used suc-
cessfully to cap external silanols [59,70], which are
often a source of some catalytic activity.
In summary, there exists a range of methods for
preparing catalysts under controlled conditions that
display a range of excellent properties, including
structural regularity, high surface area and chemical
and thermal stability, the synthesis of which can be
modified by choice of templating agent and synthe-
sis conditions and can be modified subsequently
by a range of techniques. Further sections of this
chapter will concentrate on the catalytic applications
of these materials.
H-MCM-41
ROH +
RO
O
O
Fig. 7.
8
Protection of alcohols as tetrahydropyranyl ethers.
tion strategy in synthetic chemistry that can be
carried out with the H
+
form of MCM-41 [71].
The catalyst H-MCM-41 has been found to be a
good catalyst for the acylation of 2-methoxy-
naphthalene (Fig. 7.9), which is of interest in the
synthesis of intermediates for Naproxen, an anti-
inflammatory [72]. The catalyst is capable of the
acylation of this activated substrate in reasonable
conversion and good selectivity towards the 1-acyl
isomer. The acylating agent used is acetic anhydride,
making the process potentially green, with the acetic
acid by-product potentially recyclable. The Zn-
exchanged MCM-41 also is a reasonably active cat-
alyst for the same reaction, but using acetyl chloride
as acylating agent. The reaction in this case is some-
what less selective. The H-MCM-41 catalyst also has
shown activity in the related Fries rearrangement
[73].
In short, the activity of the acid forms of MCM-41
(and probably its other mesoporous analogues) is
substantially lower than the very high levels of
acidity displayed by the zeolites. Thus, few real
opportunities have arisen where these materials
have shown promise as direct analogues of zeolites.
Nonetheless, they have shown, and continue to
show, great promise as catalyst supports and their
heteroatom analogues (especially Ti) also display
interesting behaviour in a range of applications.
The Na form of MCM-41 has been shown to have
some activity for the Knoevenagel condensation
[74,75]. This material, being slightly acidic due to the
silanols in the walls but also containing mildly basic
3 Catalytic Applications
3.1 Fundamental activity of micelle-
templated silicas and aluminosilicas
Because the zeolites display exceptional acidic prop-
erties, especially when containing a small quantity of
Al in their framework, it is not surprising that early
attempts to utilise their larger pore congeners
focused on similar acidic processes. Somewhat dis-
appointingly, the MCMs and related compounds are
only mildly acidic. Thus, with a few exceptions, the
hydrocarbon cracking reactions typical of zeolites are
beyond the scope of these materials.
More typical of the reactivity is the formation of
tetrahydropyranyl ethers (Fig. 7.8)—a useful protec-
