CA1124204A - Enrichment of carbon-13 by multiphoton dissociation of cf.sub.3cocf.sub.3 - Google Patents

Abstract

TITLE

INVENTORS

Peter A. Hackett Clive Willis Michel Gauthier ABSTRACT OF DISCLOSURE

A method of producing carbon-l 3 by multiphoton decomposition of gaseous CF3COCF3 wherein the gas is irra-diated at a pressure below 10 Torr with a laser beam at a minimum fluence of 3 joules per sq. cm. and a frequency selected such as to optimize the excitation of one of the specified species relative to the other, and then passed through a chemical separation stage to obtain a 13co or a 13C2F6 product or both.

Description

~lZ421;)4 ~ This invention relates to a method for isotopic enrichment by laser radiation and more particularly to a method of producing carbon-13 by multiphoton decomposition o~
CF3COCF3.
The use of carbon-13 as an industrial compound is not widespread at the present time but is expected to increase. Extensive application of carbon-13 to medical screening programs is anticipated. The present method of production is by low temperature distill~ation of carbon monoxide and this process is expensive, provides small throughput, needs large inventory of starting materials, and must be symbiotic with a liquid air plant.
A m~thod of isotopic separation by photopredis~
sociation is described in United States Patent No.3,983,020 issued Sept. 28, 1976 to C.B. Moore and E.S. Ycung.
A photochemical method for carbon isotopic enrichment is disclosed in United States Patent No. 4,120,767 issued October 8, 1978 to S.N. Bittenson and P. Houston.
This patent describes a process using, as a starting material a gaseous mixture of at least two isotopic CF3I species and irradiating this mixture in a reaction zone with laser radiation at a pressure below about 10 Torr to cause relative enrichment of one of the s~ecies. The enriched residual substrate is 13CF3I. This process is effective but yields are low and therefore production costs are high.
In drawings which illustrate an embodiment of the invention, Figure 1 is a flow diagram of the process, Figure 2 is a composite graph showing pressure 30 VS irradiation intensity and alpha-value.' Referring to figure 1, a beam of photons ~rom a laser 10 (preferably a CO2 laser~ is directed into a ~.2~2V4 reaction cha~ber ll containin~ gaseous CF3COCF3. ~his material occurs as a mixture of many isotopic species e.g.
CF312COCF3 and CF313CoCF3. The frequency v of the laser light is chosen to optimize the excitation of one of the specified species relative to the other species. The reactions that take place are as follows:

CF3COcF3 ~ 3 3 CF3C~ + CF3 ~ 13C16O + 13C18O ~ 12C

3C2F6 ~ 12C180 + 12C160 The enriched carbon-13 products that are obtained are carbon monoxide (13Co~ and hexafluorethane (13C2F6). It has been found that this method of enrichrnent of carbon-13 in the products of multiphoton dissociation of CF3COCF3 compounds results in high yields and reduced costs of carbon-13 production. A product enriched in oxygen-18 i.e. 13C18O and 2C18O may also be obtained as a by-product.
After irradiation the gas mixtur~ is passed to further stages for chemical separation of the products.
Stripper 13 separates the enriched products C2F6 and CO with residual CF3COCF3 going to waste. The products are passed to oxidizer 13`where the CO is changed to CO2 followed by C2 absorber 14 such that a carbon-13 enriched product is obtained as a stable carbonate (e.g. Bal3CO3). The residual C2F6 is passed to purification stage 15 from which an enriched carbon-13 product i.e. 13C2F6 is obtained. The Bal3CO3 product also includes a component Bal3C18O3 from which carbon-18 may be obtained.
The irradiation system used would preferably involve focussed or parallel laser beams providing a minimum fluence of 3 joules per sq. c.m (J cm 2). Th~ laser frequency ~1242~4 ~or the process is about 95Q w~venu~be'rs. The 'irradiation takes place at a pressure below 10 Torr. It has been found that effective 'action will take place over a wide temperature range i.e. between -80C and ~ 100C, with good results being obtained at room temperatures i.e. 20C.
Figure 2 is a composite graph showing experi-mental results of decomposition of CF3COCF3 by means of focussed laser beam radiation. The left side of the graph relates pressure to a parameter Vd (Fractional Decomposition of Irradiated Volume) which is in fact a decomposition effecti-veness factor. The upper curve gives results for the C-13 component and the lower curve for the C-12 component. The third curve relates pressure to alpha-value which is defined as follows:
= 13C~12C ratio in ~roducts 3c/12c ratio in starting material Referring again to figure 1, the depleted CF3COCF3 substrate, instead of going to waste, may have its oxygen isotope ratio re-established at natural abundance in equilibration stage 16 with water and is recycled as feedstock (as shown by dotted line 17).

Claims (4)

Claims:

1. A method of producing carbon-13 by multiphoton decomposition of CF3COCF3, said material containing carbon-12 and carbon-13 isotopic species, comprising:
a) irradiating gaseous CF3COCF3 is a reaction chamber at a pressure below 10 Torr with a laser beam at a frequency of about 950 wavenumbers and providing a minimum fluence of 3 joules per sq. cm. such as to optimize the excitation of one of the specified species relative to the other causing the following reactions to take place:

and b) passing the material through a chemical processing stage to obtain a 13CO or a 13C2F6 product or both.

2. A method as in claim 1 wherein the irradiating step is carried out at a temperature near 20°C.

3. A method as in claim 1 wherein the chemical processing stage also produces a product enriched in oxygen-18.

4. A method as in claim 3 wherein the depleted CF3COCF3 sukstrate has its oxygen isctope ratio re-established at natural abundance by equilibration with water and is recycled as feedstock.