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Supercrtitical CO2 as an alternate
industrial solventby
Nauman Mithani (#301016320)Chem 372
2008-02-01
"Conducting oxidation reactions using CO2 as the solvent is a
promising strategy for creation of 'greener' " [ref. P], economical
chemical processes, since CO2 and water are, perhaps, the only two
solvents that may be employed (at the industrial OR mass production
level) in oxidation reactions without yielding any solvent by-products.
Although fine chemical syntheses produce more waste on a per-
mass-of-product basis, the volumes for commodity chemical
production are so high, even small improvements would have a
substantial impact on waste released (to the environment)" [ref. P.3]. In
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the production of such products by the chemical industry,
"oxidation"-type reactions are common and often the prime reactions
[ref. P.1], e.g. the large-scale syntheses of substances such as
, ,adipic acid phenol
, teraphthalic acid caprolactam.
These examples are also the objects of the larger endeavour to modify
or tweak the production process so as to have a lesser impact on the
environment. Commodity producers have also been deemed to be
potentially more receptive to process revisions with respect to (at least)
alternate solvents [ref. P.4]. Use of CO2 as an alternate solvent is one
such modification that is part of this endeavour.
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A "benign yet feeble solvent in the supercritical stage, CO2... is
naturally abundant, and is relatively inexpensive" [ref. P], its most
outstanding advantages or critical features, in its use in oxidation
processes, is that is does not ignite (non-flammable) nor does it
oxidise, it is miscible with oxygen thereby reducing transport
resistance, which has the practical effect of lower reaction times.
log (mass transfer coefficient ratios) vs. CO2 fraction in total feed
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>> The CO2 proposition was experimentally tested in the
production of H2O2 (hydrogen peroxide) by way of oxidation of
tetrahydroanthroquinones in CO2; organic solvents were, thus, not
used. Overall, a five-fold rise in the rate was observed; moreover,
there were no solvent by-products (in other words, no waste) [refP, P.
5].
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Nought for nought_____________________________________________________
1. CO2 is not a particularly strong solvent [ref. P], and so high
pressures are often required in order to bring about a single phase,
even with non-polar substrates [ref. P]. Naturally, there are limits to
the concentrations one can attain at economically tractable
pressures [ref. P]. The downside or price one has to pay (for) is the
necessity of elevated operating pressures and dilution of
reactants [ref. P].
pressure (bar) vs. (noxygen+ nhexane)/ntotal
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2. It is a distinct possibility that the catalyst would have to be
chemically altered so as to allow for dissolution in a mixture that is
primarily CO2 [ref. P]. As mentioned earlier, since it is a weak
solvent, it may not allow for the dissolution of the same amount of a
traditional homogenous oxidation catalyst [ref. P].
The common work-around is functionalisation with fluorinated
ponytails [ref. P], which significantly enhances solubility in CO2
[ref. P.7]. However, the cost, for the additional chemistry required
plus a gain in molecular mass due to additional ligands, is the
principle drawback.
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An oft overlooked advantage..._____________________________________________________
... of the solvent characteristics of CO2 is that there is little
leaching of metals from the catalyst. In fact, in the Pd (lead) and Pt
(platinum) catalysed oxidation of alcohols, leaching of the metal was
notobserved [ref. P.8]; it was, however, observed when water was
employed as solvent.
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CASE STUDIES_____________________________________________________
1. Oxidation ofpropylene to propylene
oxide :
World-wide production ofpropylene oxide measures at
approx. 4 million tonnes [ref. P.19]. It is produced via the
chlorohydrin or theperoxidation process.
Chlorohydrin option: more than 2 kg of waste salt is
generated per kg ofpropylene oxide, which in turn leads to volumes
of waste water approx. 40 times that of thepropylene oxide produced.
Peroxidation option: The ethyl benzene oxidation step alone
generates 20% waste. Numerous further steps have to be carried out
for the production and the purification ofpropylene oxide, each
generates its own waste stream (and is rather energy intensive as well).
In spite of several favourable features such as H2O2 being an
"environmentally benign oxidant" [ref. P] and that the only by-product
is water, it is far from the ideal option for this additional reason of
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being un-economical in terms of cost [ref. P.23].
The idealoption: direct oxidation ofpropylene topropylene
oxide by reaction with O2 (g). This is not viable due to the presence of
allylic hydrogens ofpropylene, which make the process highly
combustible, thus un-safe [ref. P.20, .21].
Enter CO2: it was tested as the sole solvent in the creation of
propylene oxide from O2 (g), H2 (g) andpropylene with a Pd
(palladium) catalyst [ref. P.27]. For this particular case, CO2 is unique
in the sense that it can solubilise large volumes of gases, it is " immune
to oxidative degradation" [ref. P], creates an environment in which
oxygen and hydrogen do not ignite/explode, and is miscible with
propylene andpropylene oxide. Water-methanol-CO2 and water-
methanol-nitrogen solvent mixtures were also tested in bringing about
the same operating pressures as the sole-CO2 solvent.
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"As shown in Table 1, the use of CO2 as the sole solvent allows for high
selectivity to propylene oxide at reasonable conversions" [ref. P]. Other
"...results are consistent with observations that methanol is a major
contributor to by-product formations": methyl methanoate, 1-
methoxypropan-2-ol and 2-methoxypropan-1-ol [ref. P].
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2. Aerosolisation assisted by supercritical CO2 : [ref. S]
Supercritical CO2-aided techniques allowing for
aerosolisation of hydrophilic water-soluble as well as lipophilic
substances, are under research. A process that has come from the
research enables the aerosolisation of any water-soluble substance;
this is potentially suited for:
2.1: "nebulisation of selected proteins for pulmonary drug delivery.
2.2: "Nebulisation of inexpensive metallic salts for the deposition of thin
films and coatings... . Metal and mixed-metal coatings formed using
this method are fine-grained, uniform and durable.
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2.3: "Dehydration of the aerosol droplets produces nano-particle
powders, suitable for pharmaceutical use...
OR
2.4: "...pyrolytic spray coating processes."
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THE LAST WORD: Further avenues for implementation:_____________________________________________________
One such avenue is the design of new reactors with dedicated
support for the deployment of CO2. E.g., in the "Dupont" process for
the oxidation of butane to maleic anhydride, the "use of liquid CO2
could allow for higher oxygen concentration and hence more efficient
operation" [ref. P, P.41].
fin__________________________________________________________________________________________________________________________
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REFERENCES
___________________________________
[ref. P] Beckman, E. J.. "Oxidation Reactions in Carbon dioxide: Academic
Exercise or Future Green Processes?." Environmental Science and
Technology 37 (2003): 5289-5296.
[ref. P.1] Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of Organic
Compounds; Academic Press: New York, 1981.
[ref. P.3] Sheldon, R. A. Pure Appl. Chem 2000, 72, 1233.
[ref. P.4] Blaser, H.-U.; Struder, M. Green Chem. 2003, 5, 112.2000.
[ref. P.5] Hancu, D.; Beckman, E. J. Ind. Eng. Chem. Res. 2000, 39, 2843.
[ref. P.7] Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev. 1999, 99, 475.
[ref. P.8] Birnbaum, E. R.; Le Lacheur, R. M.; Horton, A. C.; Tumas, W. ,
J. Mol. Catal. A: Chem. 1999, 139, 11.
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[ref. P.20] Monnier, J. R. Appl. Catal. A: Gen. 2001, 221, 73.
[ref. P.21] Lu, G.; Zuo, X. Catal. Lett. 1999, 58, 67 (b) Murata, K.; Kiyozumi,
Y. Chem. Commun. 2001, 1356.
[ref. P.23] McCoy, M. Chem. Eng. News 2001, 79 (43), 19.
[ref. P.27] Danciu, T.; Beckman, E. J.; Hancu, D.; Cochran, R.; Grey, R.;
Hajnik, D.; Jewson, J. Angew. Chem., Int. Ed. 2003,.
[ref. P.41] Emig, G.; Liauw, M. A. Top. Catal. 2002, 21, 11.
___________
[ref. S] Sievers, R., Karst, U., Milewski, P., Sellers, S., Miles B., Schaefer J.,
Stoldt, C. and Xu, C.. "Formation of Aqueous Small Droplet Aerosols
Assisted by Supercritical carbon dioxide." Aerosol Science and
Technology 30.1 (1999): 3-15.