Carbon dioxide | Chem 372

8/14/2019 Carbon dioxide | Chem 372

1/15

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


2/15

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.


3/15

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


4/15

>> 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].


5/15

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


6/15

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.


7/15

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.


8/15

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


9/15

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.


10/15

"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].


11/15

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.


12/15

2.3: "Dehydration of the aerosol droplets produces nano-particle

powders, suitable for pharmaceutical use...

OR

2.4: "...pyrolytic spray coating processes."


13/15

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__________________________________________________________________________________________________________________________


14/15

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.


15/15

[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.

Carbon dioxide | Chem 372

Documents

Transcript of Carbon dioxide | Chem 372