Chemistry Reference
In-Depth Information
the critical issues that should be considered when selecting possible applications of
cavitating conditions for the intensification of chemical processing:
1.
In the case of homogeneous reactions, only those which proceed via radical or
radical-ion intermediates are usually sensitive to the cavitational effect. This means
that cavitation can intensify reactions proceeding through radicals, whereas ionic
reactions are not likely to be modified.
2.
In the case of heterogeneous reactions, those proceeding through ionic intermediates
can also be stimulated by the mechanical effects of cavitation. However, in this case
appropriate balance needs to be maintained between the positive effects of cavitation
and the associated costs of operation, as sometimes the same effects might be
achieved by using improved agitation conditions.
3.
In the case of heterogeneous reactions, where mixed mechanisms (radical and ionic)
exist, cavitation can improve both. In this case, if the two mechanisms lead to the
same product, an overall increase in the rate of reaction will be obtained, but if the two
mechanisms lead to different product distributions, cavitational switching might
occur due to enhancement in the radical pathway and the nature of the reaction
products will be changed due to the cavitational effects.
In general, the different benefits that can occur due to the use of cavitational reactors in
chemical processing applications (one illustrative example has been cited in each case for
better understanding) are:
1. Reaction Time Reduction: Ultrasound can be used to substantially enhance the rates
of chemical reactions. For example, Javed et al. [62] have shown that the Diels-Alder
cyclization reaction gives 77.9% yield in 35 hours through conventional means but
97.3% in only 3.5 hours with the application of ultrasound.
2. Increase in the Reaction Yield: The conventional method of epoxidation of long-
chain unsaturated fatty esters results in a yield of 48% in 2 hours' reaction time,
whereas application of ultrasound (20 kHz frequency of irradiation) increases the
yield to 92%, and importantly in just 15 minutes of reaction time [63].
3. Possible Switching of the Reaction Pathways Resulting in Increased Selectivity:In
some cases, use of ultrasound switches the reaction pathway, thereby totally changing
the product distribution. For example, with conventional reactions, stirring benzyl
bromide and toluene in the presence of KCN and Al
2
O
3
results in benzylation
(Friedel-Crafts reaction), whereas sonication of the same components under identical
conditions yields benzyl cyanide. This is due to the fact that sonication impregnates
cyanide ions on Al
2
O
3
, thereby masking the acidic sites, and hence prevents the
reaction from proceeding down the Friedel-Crafts route [64].
4. Use of Less Forcing Conditions (Temperature and Pressure): Van Lersel et al. [65]
have described a new route for the chlorination of methane using ultrasound
irradiation, which allows for an intrinsically safe process at ambient pressure and
temperature. Currently, the industrial chlorination of methane is performed as a
noncatalytic, thermally initiated gas-phase reaction, and this route requires elevated
pressures and temperatures in order to obtain a significant yield.
5. Use of Green Solvents and Less Hazardous Reactants: Puri et al. [66] have described
a copper perchlorate-catalysed, highly efficient, one-pot, green protocol for the
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