kilomentor

Introduction

Industrially important chemical transformation are usually conducted in solution; however, these processes often lead to partial loss of the solvent into the atmosphere. The equipment used conventionally in batch chemical processing is typically not adequate to prevent troublesome emissions of the most volatile solvent vapours. Additionally, the contaminated waste solvents are commonly sent for destruction rather than being recycled, increasing the likelihood of cumulatively damaging emissions. Chemicals in the atmosphere are sometimes serious pollutants and there is a need to reduce these leakages. Today many of the most common volatile solvents used in chemical process development are under a cloud. It has been proposed that solvents be more efficiently recovered and recycled but this is often discouragingly expensive. One suggestion has been to use dissolution media with much higher boiling points than the most popular solvents, because their vapour pressures are lower, but distillation of such solvents consumes more energy. Another suggestion has been to use special designed substances with solvent-like dissolution properties that can be reversibly chemically dissociated into more volatile fragmented products while they are removed from a reactor and which then can be recombined to regenerate the solvent-like substance. An example is the adduct between sulfur dioxide and perylene (1,3-penadiene) which is a liquid. Another example is the combination of sulfur dioxide, formaldehyde that together wit water produce hydroxymethylsulfonic acid. Often the recommendation is made that the industrial process be reengineered to use a more environmentally friendly solvent even if the relative volatility of the solvent is not reduced. What has not been recommended up until now is some means to use more volatile, lower-boiling solvents which can be easily disltilled with a low energy requirement so that the increased risk of pollution is avoided.

Kilomentor [Dr. Clarke Slemon] has filed a US patent application US61/069,688 to address this problem. The key to this new technology is a means of reducing the vapour pressure of the volatile solvent when it is not in use as a reaction solvent. The central claim of the application is the combination of a recycled, constrained solvent with a substantially less volatile complexing agent in a closed storage vessel. When the volatile solvent material is complexed, its vapour pressure is conveniently low for easy storage. When it is needed as a reaction solvent, heating the complex dissociates it and the solvent can be distilled into the reactor. When the solvent is no longer needed in the reactor, it can be distilled back into the reservoir where it recombines with the complexing agent.

Solvents that form such useful complexes are called constrained solvents by Kilomentor. The combination of a constrained solvent with a complexing agent is not new. Solvates of ammonia, for example, calcium chloride mono ammoniate or zinc chloride diammoniate have been long known. An intelligent and experienced reader, once presented with the inventive combination of a low boiling readily gaseous solvent, a complexing agent and a confining element in the context of the problem to be solved might easily assemble the useful combinations. The invention is the combination to address the opportunity (problem).

Particular pairs of a constrained solvent and complexing agent are: dinitrogen tetroxide and 1,4-dioxane; dinitrogen tetroxide and dimethylsulfoxide; sulfur dioxide and potassium bromide; sulfur dioxide and sodium iodide, or ammonia and calcium nitrate.

There are advantages besides ecological ones to working with a solvent under conditions where it is a constrained solvent. Some solvents are too volatile, explosive with air, flammable, poisonous, smelly or irritating to be used in regular processes where significant leakage into the atmosphere might more likely occur. Carbon disulfide for example is an excellent solvent with a well known unique combination of properties, but because of it flammability and low flash point it is unacceptable for process chemistry. Carbon disulfide (B.P. 46 C) if it could be put in combination with an appropriate complexing agent might be useful.

A possible advantage of this technology might be that purification by distillation would be inexpensive because the heat of vaporization for such liquids is low and because the boiling point is above 0 C the first stage of cooling can be a brine chiller. Residual uncondensed gas might be delivered below the surface of the non-volatile storage solvent.

This solvent could be stored in pressure resistant tanks mixed with a low vapour pressure environmentally more benign liquid that has a rapidly increasing solubility for the low boiling solvent with increasing pressure and which had a negatively deviating Raoult’s law vapour pressure.

By proper choice of reagents, co-reactants and catalysts, the very volatile solvent can be set up so that it can be distilled away from all the other components of the reaction mixture in such a state of purity that it would be usable, if not as a replacement for commercial grade solvent in every use, at least for a subsequent batch of the same product.

When the stored volatile solvent was needed in a repeat of the process step, it could be distilled out of its reservoir and condensed into the sealed preloaded reactor. The same cooling that is required to reach the reaction temperature is used to condense and retain the solvent. What is sacrificed is the ability to run reactions at 50 C and above. For these reactions the ecologically benign choice would have to be made from other alternatives.

Reactions that substantially proceed at ambient temperature but using the present technology are driven to completion by raising the temperature, could be driven to completion by concentrating the reaction mixture by starting the removal of the highly volatile solvent. This would dramatically increase the rate for reactions with a molecularity of two or higher. This includes most reactions that use a particular chemical reagent but does not include intramolecular rearrangements, hydrolyses or solvolyses.

The table shows solvents that might be used if a satisfactory complexing agent could be found.

Chemical Name	Boiling point at 760 torr.
methylene chloride	40.0
pentane	36.0
diethyl ether	34.6
tetramethylsilane	26.5
carbon disulfide	46
dibromodifluoromethane	24.5
2-chloropropene	22.7
dinitrogen tetroxide	21.3
3-methyl-1-butene	20
1,1-dimethylcyclopropane	20
hydrogen fluoride	19.4
ethylamine	16.6
vinylbromide	15.8
nitrylchloride (NO2Cl)	15-17
cyanogen chloride	12.7
boron trichloride	12.5
ethyl chloride	12.3
methyl vinyl ether	12.0
2- fluorobutadiene	12.0
ethyl methyl ether	10.8
dichlorofluoromethane	9
nitrosyl chloride	–5.5
trifluoromethylamine	0- -0.5
butane
trifluoromethylsulfenylchloride	-0.7
perfluorotrimethyl amine	-7 to –6
dimethyloxonium chloride	-2
allyl fluoride	-3
butadiene	-4.4
methylamine	-6.3
trifluoromethylamine	-6.7
isobutene	-6.9
ethoxytrifluorosilane	-7
dimethylamine	-7.4
sulfur dioxide	-10
iodotrifluoromethane	-22.5

Low boiling liquids that are already frequently used solvents are marked in bold in the table. The four highest boiling liquids are common organic solvents the first, second and third of which are typically avoided in process chemistry. Fluorine containing choices desirably handled in this confined manner because they may be ozone depleting substances: dibromodifluoromethane, dichlorofluoromethane, dibromodifluoromethane.

Two of these possibilities are not new to consideration as solvents, dinitrogen tetroxide and sulfur dioxide are two volatile dipolar aprotic solvents. In fact, I have on my book shelf a thin volume called Chemistry in Non-Aqueous Solvents, by Harry H. Sisler, Reinhold Publishing Company, 1961. In the pertinent chapters there is no discussion of possibility that these solvents could be easily recycled. This is the aspect, which provides the new perspective.

Dintrogen Tetroxide

Dinitrogen tetroxide melts at –12.C and its normal boiling point is 21.3 C thus its liquid range is convenient for its use as a solvent. The liquid may be readily supercooled and has been cooled as low as –110 C without it crystallizing. Its critical temperature is 158.2 C and its critical pressure is 100.0 atm. The density of dinitrogen tetroxide is 1.49 g/cc. at 0 C. The electrical conductance of liquid ditrogen tetroxide is very low. The specific conductance at 17 C is 2.36 X10-13. There is an equilibrium between ditrogen tetroxide and two molecules of the paramagnetic nitrogen dioxide. Although there is very little of the monomeric triatomic compound in the liquid at the boiling point there is about 16% in the gas phase.

A possible key to how dinitrogen tetroxide could be recycled is that it forms several fairly stable solvates with higher boiling liquids which comprise a large percentage of dinitrogen tetroxide. With p-dioxane dinitrogen tetroxide forms a 1:1 salt of melting point +45.2 C and a less stable one with 1,3-dioxane of mp 2 C. These complexes contain 60.7% dinitrogen tetroxide by weight. Thus to generate 100 ml of solvent it would only be necessary to decompose 250 gm of complex. The solid complex is heated to melting and the solvent distilled away from the dioxane residue. When the reaction in which it was mediating is complete the dinitrogen tetroxide can be distilled and condensed back into the dioxane solution where it reacts and returns to the solid state. The flask of solid is secure in the refrigerator in a stoppered flask. To se it as a solvent it needs to be appreciated that primary alcohols, amines, alkenes, and amides all react with dinitrogen tetroxide.

Sulfur Dioxide

Sulfor dioxide is widely used in the petrochemical industry as a solvent because of its ability to discriminate between function group classes, dissolving alkenes and aromatic hydrocarbons while having little solubility for saturated hydrocarbons. Means for recovering sulfur dioxide on scale are therefore most likely well developed by our engineering colleagues.

The boiling point of sulfur dioxide is –10.02 C and its freezing point is –75.46 C. Its density at –10 C is 1.46 g/cc. Sulfur dioxide displays some useful solvent properties for metathesis reactions and is a good solvent for Friedel Craft reactions in part because AlCl₃ dissolves readily in it.

The characteristic with which we are particularly focussed here however is the possibility that the sulfur dioxide could be trapped as a reversible adduct for storage. Looking at the data available in the Sisler book one can see that the potassium bromide solvate might be a good choice. The solvate combines 4 equivalents of sulfur dioxide with one formula weight of potassium bromide working out to a sulfur dioxide content of about 68%. Thus to prepare 100 ml of liquid sulfur dioxide would require 215 gm of the solvate. At –1 C the vapour pressure over this solid is already 1 atmosphere so it would need to be refrigerated strongly to keep it confined as the complex. More stable compounds however contain a smaller weight percent of sulfur dioxide. Aluminum chloride forms a disolvate with sulfur dioxide. As with the dinitrogen tetroxide case, the gas can be distilled away from the reservoir solid and condensed as liquid in the reactor and then distilled back into mixture r in the reservoir when the solvent was no longer needed.

Any communications regarding the commercial development of this idea whichis covered by a provisional US patent application should be addressed to kilomentor@sympatico.ca

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