Chemistry Reference
In-Depth Information
Si/Al ratio were noted, it appeared that these effects
were minor in comparison. An improvement in
activity and reduction in coking was ascribed to the
ease of product removal from the smaller particle size
zeolite, freeing up acid sites for further reactivity
and physically clearing pore channels more readily
to allow easier diffusion of reactants and products.
Further optimisation is needed, however, in these
kinds of systems due to deactivation. It was pointed
out that both understanding these kinds of interac-
tions and tailoring the zeolite catalyst are needed to
prevent these kinds of deactivation pathways.
The use of zeolites for the synthesis of fine chemi-
cals is discussed also by Hoelderich & Heinz; they
describe the research and development of zeolite
catalysis in the 1980s and 1990s [16]. The reactions
described include olefin oligomerisation, dewaxing,
alkylation of arenes, the
para
-directed isomerisation
of xylenes and the disproportionation or alkylation
of monosubstituted arenes. From an environmental
standpoint they also describe some shortcomings of
some of the present-day catalysts. The Beckmann
rearrangement—cyclohexanone oxime rearranged
to caprolactam (an important precursor to Nylon
6)—is carried out in sulfuric acid. Large amounts of
ammonium sulfate are used due to neutralisation of
the oleum (sulfuric acid) used in the oximation. The
rearrangement is considered to be an environmen-
tally unacceptable drawback. There are several
studies on the rearrangement of the cyclohexanone
oxime to caprolactam over zeolite catalysts. This new
route has a number of advantages compared with
the conventional method. It is energetically and
economically favourable as well as environmentally
friendly.
Corma & Garcia have reviewed the use of zeolites
[22]. They have categorised zeolites as solid acid
catalysts based upon reactions with low, medium
and high acid strength requirements. The vapour-
phase Beckman rearrangement to caprolactam was
described using high-silica ZSM-5. The best catalyst
for the process was a solid containing indium in
the framework, which corresponds to the lowest
acid strength. Acetal formation and hydrolysis was
described as a reaction that requires medium-
strength acid sites. The formation of acetals by the
reaction of carbonyl groups and alcohols or diols has
been catalysed by acid zeolites. It was suggested also
that the skeletal isomerisation of
n
-alkanes occurs at
the bridging Si-(OH)-Al acid sites having one Al in
Fig. 6.2
Zeolite structure.
to achieve acceptable rates (overcoming diffusion
problems). The use of colloidal-size nanocrystalline
zeolites is expanding. Reactions that would be of par-
ticular interest would be acylation using acetic acid
as the acylating agent, nitration with nitric acid and
anti-Markovnikov additions. Rhodia has established
the first industrial applications of zeolites for the acy-
lation of anisole and veratrole [21]. The drive here
is to replace stoichiometric agents such as aluminium
trichloride, or more hazardous material such as
HF. Activated aromatics (e.g. anisole) with acetic
anhydride over zeolites have been demonstrated,
although more developments are needed in the
acylation with acetic acid or other non-activated
reactants.
Botella
et al
. recently have described the acylation
of toluene with acetic anhydride over beta zeolites
[21]. At 150°C with an arene/anhydride molar ratio
of 10-20, high yields of 4-methylacetophenone were
obtained, with selectivity close to 100%. Catalyst
decay was ascribed to adsorption of the product to
acid sites, which subsequently undergo further reac-
tion to strongly adsorbed products that block acid
sites for further reactivity. The effects of Si/Al ratio,
variations of the zeolite hydrophilic/hydrophobic
character and particle size were investigated. Of
these three variables it appears that the particle size
had the most pronounced effect. The smaller parti-
cle size of the H-beta zeolite (ca. 0.05 mm compared
with ca. 2 mm) showed the highest activity: ca. 83%
conversion with 100% selectivity (compared with
ca. 50% conversion). Although some effects of the
