Dehalogenation of halogenated aromatic compounds

Various halogenated aromatic compounds, particularly polychlorinated biphls, and the like, are dehalogenated by reaction with alkali metal in the presence of liquid hydrosiloxane. A selected solvent or diluent may be present. The alkali metal halide reaction product may be removed by washing. It has been found substantially complete dehalogenation is achieved readily at ambient temperatures when the hydrosiloxane is present. The dehalogenated aromatic moiety normally is recovered intact. This method has been found very effective in the destruction and removal of PCB contaminants.

This invention deals with the removal of halogen atoms from halogenated 
aromatic materials especially the aromatics based on benzeme, biphenyl, 
other polyphenyls, naphthalene, anthracene and the like. The halogen 
substituents are reacted with alkali metal in the presence of a 
hydrosiloxane. The hydrosiloxane has been found to facilitate the reaction 
so that it will proceed to completion at room temperature in relatively 
short times. 
BACKGROUND AND PRIOR ART 
Polychlorinated biphenyls (PCBs) are organic chemicals that were produced 
on a large scale in the period 1930-1980. Approximately 600,000 tons of 
the material were manufactured and were used in a wide variety of 
applications. However, the most important use for the material was as an 
insulator in electrical transformers and capacitors. PCBs were ideally 
suited to this role because of their chemical inertness and lack of 
flammability. 
In the mid 1970s concerns began to be expressed about the detrimental 
effects of PCBs on health and the environments. While these concerns were 
not substantiated to a large extent, there still existed a very strong 
public pressure to eliminate the use of PCBs. Accordingly, large scale 
manufacture was halted and attempts began to be made to eliminate PCBs 
from the environment. However, the very properties that made PCBs 
desirable in the first place--chemical inertness and lack of 
flammability--made their destruction extremely difficult. 
The most common method for disposal is to dilute PCBs with combustible 
organic materials and to incinerate them at extremely high temperatures 
(1100.degree. C.). 
This method has the significant drawback that incomplete combustion can 
lead to polychlorinated dibenzofurans which are known to be extremely 
toxic. 
Many other methods have been developed most of which are based on the use 
of alkali metals (or their hydroxides), especially dispersions of sodium 
metal. A typical process using sodium is described in U.S. Pat. No. 
4,340,471, Jul. 10, 1982. PCB-contaminated silicone-based or hydrocarbon 
oils have been treated with hydrocarbon dispersions of sodium (U.S. Pat. 
No. 4,379,746, Apr. 12, 1983). Sodium metal also has been used in the 
presence of an electron carrier (e.g., benzophenone, alkylbiphenyl) and an 
aprotic complexing solvent (e.g. tetrahydrofuran, dimethylformamide) in 
U.S. Pat. No. 4,377,471, Mar. 22, 1983. Japanese Patent No. 49082570 
mentions the use of isopropanol with sodium and removes excess sodium with 
methanol. Carbon dioxide gas and water have been used to remove excess 
sodium (U.S. Pat. No. 4,416,767, Nov. 22, 1983). 
Dehalogenation also has been carried out with alkali metal aromatic radical 
anion reagents e.g. sodium naphthalide, lithium anthracide--see U.S. Pat. 
No. 4,284,516, Aug. 18, 1981. This type of reagent has been used in the 
presence of ether-type solvents (U.S. Pat. No. 4,326,090, Apr. 20, 1982). 
The reaction may be quenched using carbon dioxide (U.S. Pat. No. 
4,447,667, May 8, 1984). 
Another type of dehalogenation has involved the use of hydrogen gas under 
pressure in the presence of a catalyst: the process requires elaborate 
equipment and is sensitive to impurities (U.S. Pat. No. 4,623,448, Nov. 
18, 1986). Still another type of process has involved reaction with sulfur 
at high temperatures (U.S. Pat. No. 4,581,442, Apr. 8, 1986). 
The PCB--contaminated silicone oils mentioned in prior art such as U.S. 
Pat. No. 4,379,746 are transformer oils, heat transfer fluids or 
lubricants based on polysilanes, and are distinct from hydrosiloxanes. 
It would be desirable to provide such a dehalogenation process that would 
be more effective at room temperature, use relatively inexpensive reagents 
and equipment, and be relatively insensitive to impurities. 
SUMMARY OF THE INVENTION 
It has been found that hydrosiloxanes enhance the dehalogenation of 
halogenated aromatic materials when using alkali metal reactant. 
The invention includes a process for dehalogenating aromatic halogenated 
compounds, comprising: reacting an alkali metal with halogenated aromatic 
material in the presence of a liquid hydrosiloxane, until substantially 
all of the halogen has reacted, leaving the aromatic moiety in 
non-halogenated form. Preferably a non-halogenated non-aqueous polar 
solvent or diluent is present during the reaction. The excess alkali metal 
can be reacted with added termination agent, and excess hydrosiloxane can 
be precipitated and the solids separated. 
The invention includes a reagent mixture for dehalogenating halogenated 
aromatic material, comprising an alkali metal and a hydrosiloxane. The 
invention further includes a kit for dehalogenating halogenated aromatic 
material comprising: a container containing alkali metal, a container 
containing liquid hydrosiloxane, with the proviso that one container may 
contain both. 
DETAILED DESCRIPTION 
The starting material to be dehalogenated may be any halogenated aromatic 
compounds or mixtures containing such compounds. For example, the 
compounds may include halogenated benzenes, halogenated polyphenyls, and 
halogenated polynuclear aromatics. In most cases the compounds will be 
polychlorinated biphenyls alone or as mixtures with various oils such as 
hydrocarbons or silicone-based oils e.g. transformer oils, ballast oils, 
heat transfer fluids, or lubricants. Some chlorinated aromatic pesticides 
also may be treated. 
The alkali metals suitably are lithium, sodium or potassium, with sodium 
being the most economical and most widely used. It is preferable to add 
the alkali metal in excess of the stoichiometric amount based on the 
halogen present, most preferably about a fivefold excess. The alkali metal 
may be added to the starting material as a suspension in a suitable inert 
liquid or alone. Preferably Li is added as a powder, K as small pieces, 
and Na as small pieces, shot or dispersion in paraffin, light oil or 
mineral spirits. 
The hydrosiloxane should be a liquid miscible with the starting material. 
Preferably the hydrosiloxane will be a polyorganohydrosiloxane of 
relatively low molecular weight. Most preferred polyhydrosiloxanes are 
those of the formula 
##STR1## 
where R=lower alkyl of 1 to 4 carbon atoms and n=3-50. 
Polymethylhydrosiloxanes of molecular weight about 1500-3000 are liquids of 
low viscosity and have been found very suitable. Normally the amount of 
polyhydrosiloxane added will be an excess (stoichiometric excess of 
available hydrogens from the polyhydrosiloxane relative to the chlorine 
sites) preferably at least about 20-fold excess. The polyhydrosiloxane 
should be present at the start of the reaction. If some water is present 
in the starting material, the polyhydrosiloxane may be added as a drying 
agent prior to addition of the alkali metal. 
If desired, a non-halogenated, non-aqueous polar solvent or diluent may be 
present during the reaction. Such solvents or diluents are used to adjust 
the viscosity and facilitate contact of the alkali metal with the 
halogenated compounds. Suitable solvents or diluents include 
tetrahydrofuran, dioxane, dimethylformamide, dimethylsulfoxide, ethers 
such as ethyleneglycoldimethylether and diglyme, and mixtures thereof. 
In the presence of the hydrosiloxane, the reaction will proceed readily at 
ambient temperatures and usually will be complete in about 10 to 24 hours. 
Slightly elevated temperatures (below the boiling point of solvents 
present) will shorten this time, but are not necessary. 
When the dehalogenation reaction is substantially complete, a termination 
agent normally is added to destroy any excess alkali metal. Suitable 
termination agents include water, alkanols, glycols, phenols especially 
polyhydric phenols, carbon dioxide (gas or solid) and mixtures thereof. If 
desired, these agents can form a separate phase from the dehalogenated 
material if necessary with an immiscible organic species (such as liquid 
alkanes (pentane, hexane), petroleum ethers etc.) and can be separated. 
Some of these agents, particularly aqueous media, also serve as 
extractants to remove the alkali metal halide reaction product. The 
aqueous media form a separate phase which is readily removed, and since it 
contains no hazardous materials is suitable for disposal. 
Excess hydrosiloxane can be precipitated and removed as a solid residue. 
Normally the termination agent also will precipitate excess hydrosiloxane. 
Preferred precipitants are alkanols (1-4C) and water. The precipitate may 
be separated by settling, filtration or centrifugation. 
The polar solvent or diluent may be recovered from the dehalogenated 
material and recycled e.g. by distillation, membrane separation, 
preferential extraction etc. The residual organic material may be reused 
or safely incinerated. 
A mixture of the hydrosiloxane and the alkali metal has been found to be 
quite stable (no loss of activity) if kept moisture free. This mixture 
constitutes a useful dehalogenation reagent which may used in various 
syntheses. 
A kit which includes the liquid hydrosiloxane and alkali metal in the same 
or separate containers, is very useful for field decontaminations. The kit 
may also comprise a container containing the termination agent, a 
container containing the solvent or diluent and/or a container containing 
an aqueous medium for extraction of halide salt.

The following examples are illustrative and typical of the many tests which 
have been carried out. 
EXAMPLE 1 
A 100 mL three-necked round bottom flask equipped with a water cooled 
condenser, a septum, a gas inlet and a magnetic stirring bar was charged 
with a suspension of lithium powder (500 mg, 72.5 mmoles) in 25 mL of dry 
tetrahydrofuran which was stirred under an inert atmosphere of nitrogen or 
argon for 10 minutes. A solution of p-chlorobiphenyl (3.76 g., 20 mmoles) 
in 5 mL of dry tetrahydrofuran was added via syringe to the aforementioned 
suspension and the resulting mixture was then stirred for an additional 5 
minutes. Polymethylhydrosiloxane of M.W. 2270 (5 g.) was added slowly, 
inducing an exothermic reaction. Upon completion of the addition the 
reaction mixture was stirred at ambient temperature for 16 hours to afford 
a homogeneous yellow coloured solution. The reaction mixture was cooled in 
an ice bath while methanol (ca 15 mL) was added dropwise. After addition 
of the alcohol the mixture was stirred in the ice bath for 3 hours to 
ensure the total destruction of the excess metal. 
For analysis the resulting suspension was then concentrated under vacuum 
with the aid of a rotary evaporator (bath temperature: 40.degree. C.) and 
the off-white solid thus obtained was partitioned between hexanes and 
water (ca. 100 mL), filtered, and the layers decanted in a separatory 
funnel. The aqueous phase was extracted with two 50 mL. portions of 
hexanes, and discarded. The combined organic extracts were dried over 
anhydrous magnesium sulfate and an aliquot was analyzed by gas 
chromatography (GC). This analysis indicated the complete conversion of 
the starting material into one new product, shown by comparison of its GC 
retention time with that of an authentic sample, gas chromatography-mass 
spectrometry (GC-MS) and its mixed melting point, to be biphenyl. The 
recovered yield of the latter product after removal of the solvent under 
reduced pressure and drying, was &gt;99%. 
Complex mixtures of polychlorinated biphenyls were purchased under the 
trademark Arochlor.TM. and samples were treated as follows. 
EXAMPLE 2 
Method A 
A sample of Arochlor.TM. 1242 (100 mg., believed to contain about 42% by wt 
chlorine) was dechlorinated by treatment with 250 mg. of lithium powder 
and 3 g. of polymethylhydrosiloxane in the manner of Example 1. After 
workup as described above, using doubly deionized water, the aqueous phase 
was acidified with concentrated nitric acid and analyzed for Cl.sup.- ion 
by silver nitrate titration to a potentiometric endpoint. This analysis 
indicated that 100% of the chlorine from the PCB sample, corresponding to 
42% of the total weight of the Arochlor.TM. 1242 was now in the aqueous 
phase. The organic phase, after the addition of a measured amount of 
decane as a standard for quantitation, was submitted to GC and GC-MS 
analyses, which indicated the complete disappearance of the original PCB 
components, the absence of any newly formed chlorine containing materials 
and the formation of biphenyl as the main product, accompanied by minor 
amounts (&lt;10% of the total weight) of higher molecular weight oligomers 
(terphenyl, tetraphenyl). The total recovery of the organic material was 
better than 99%. 
Method B 
A 100 mg sample of Arochlor.TM. 1242 was dechlorinated as described in 
Example 2, Method A, but using sodium (500 mg) as the metal. After workup 
as described above, analysis of organic phase indicated a level of 
dechlorinated comparable to that achieved with lithium metal. 
Method C 
A 100 mg sample of Arochlor.TM. 1242 was dechlorinated as described in 
Example 2, Method A, but using sodium as the metal and dry toluene as the 
solvent. After the usual workup, analysis of the aqueous phase indicated 
the recovery of 90% of the chlorine originally present in the PCB sample 
as Cl.sup.- ion, while the organic phase contained no detectable levels of 
chlorinated materials. Up to 5% of the recovered organic material 
consisted of mixed biphenyl-solvent coupling products. 
Method D 
A 100 mg sample of Arochlor.TM. 1242 was dechlorinated as described in 
Example 2, Method A, but employing a preformed suspension of lithium metal 
in polymethylhydrosiloxane which had been stored for 1 week prior to being 
used. The concentration of this suspension was comparable to that of the 
final reaction mixture described in Method A. The level of dechlorination 
attained was identical to that achieved by Method A. 
Example 3 
A 100 mg sample of Arochlor.TM. 1254 (believed to contain about 54% by wt. 
chlorine) was dechlorinated as described in Example 2, Method A. After the 
usual workup, analysis indicated the complete absence of any chlorinated 
materials in the organic phase, which contained similar proportions of the 
same products obtained upon dechlorination of Arochlor.TM. 1242. 
Control dehalogenations with alkali metal as sole reactant were not 
complete at ambient temperatures even after several days (the products 
remained environmentally unacceptable). Test carried out in the presence 
of the solvent tetrahydrofuran (but in the absence of hydrosiloxane) were 
unsatisfactory since conversions were incomplete. 
According to this invention significantly improved dehalogenations are 
achieved at ambient temperatures. This method using hydrosiloxanes leads 
to the destruction of PCB's to the point where they can no longer be 
detected by GC.