Hydrogenated Terphenyl: Production, Applications, and Health Concerns

Introduction

Hydrogenated terphenyl is a complex mixture of various isomers of ortho-, meta-, and para-terphenyls in various stages of hydrogenation. Five such stages exist for each of the three isomers. Commercial available mixtures are approximately 40% hydrogenated mixtures of all three isomers. Hydrogenated terphenyl is a substance that is very persistent and very bioaccumulative. It is, therefore, considered by the European Chemicals Agency as a substance of very high concern. [1]

Reactor's coolant

Under simulated reactor conditions, irradiated hydrogenated terphenyl has characteristics that are, in general, very similar to those of irradiated Santowax OM, and it appears suitable for use as a reactor coolant at temperatures up to at least 375°C. The main advantages of hydrogenated terphenyl are lower makeup cost, availability in greater quantities, and complete fluidity when cold. The main disadvantages are a lower autoignition temperature, and greater formation of volatile products leading to higher vapor pressures or increased coolant consumption if the volatiles are continuously removed. [2]

The commercial, hydrogenated terphenyl mixture is of interest as a low-cost, low-melting organic coolant for reactors and has been irradiated at 350°C with 1.5-MeV electrons. It is in use as the initial coolant charge for the WR-1 reactor which has recently gone into operation. Decomposition proceeded at half the rate per unit dose observed previously during reactor irradiations where 62% of the absorbed energy was due to fast neutrons. This indicates a dependence of hydroterphenyl radiolysis on Linear Energy Transfer, whereby recoil protons produce 3.2 ± 0.6 times as much decomposition as electrons. Some differences between the physical properties of electron-irradiated material and the properties of reactor-irradiated material were noted. [3]

Hydrogenation of Terphenyl

Hydrogenated terphenyl is produced by the hydrogenation of terphenyl. Noble metal catalysts on mesoporous SiO2 and modified carbon supports were found to enhance the activities of terphenyl (TPh) hydrogenation without side reactions, such as cracking, hydrogenolysis, ring opening and/or coke formation. The noble metal catalysts could be used for a reversible hydrogen storage system. Five percent Pt/SiO2 catalyst was highly active in TCH dehydrogenation without stirring, due to an easier diffusion of organic molecules to the small catalyst particles during dehydrogenation. [4]

Terphenyls are produced commercially as ortho, meta, and para isomers. They are produced as the pure compounds. These terphenyls are blended and partially hydrogenated to form principal components. Commercial available hydrogenated terphenyls are approximately 40% hydrogenated mixtures of ortho-, meta-, and para-terphenyls in various stages of hydrogenation, which are clear, yellow oils. [5]

The partly hydrogenated terphenyl mixture HB-40 is produced commercially by the catalytic hydrogenation of Santowax® to about 40% of maximum theoretical hydrogen uptake. Santowax has the approximate composition: 7 wt% ortho-terphenyl, 50 wt% meta-terphenyl,23 wt% para-terphenyl, and 20 wt% triphenylene plus higher polyphenyls. [1]

Health Effects         

In the advisory report such a recommendation is made for hydrogenatedterphenyl. During use in a nuclear reactor, hydrogenated terphenylbecomes irradiated and conversion of the terphenyl to other hydrocarbonsand high polymers occurs to some extent. Because differences intoxicity have been observed for irradiated and nonirradiated hydrogenatedterphenyl, and the composition of irradiated hydrogenated terphenyldepends on the manner of operation of the nuclear reactor, this evaluationis restricted to the toxicity of nonirradiated hydrogenated terphenyl.

The main effects of hydrogenated terphenyl reported in humans are skin irritation, headaches and sore throats after accidental exposure. Limited human data are available on the association between exposure to hydrogenated terphenyl at different exposure levels and adverse health effects. Repeated exposure studies in animals revealed increase in organ weights (e.g., the liver), and bodyweight loss after exposure to hydrogenated terphenyl. Hydrogenated terphenyl is rapidly absorbed through the lungs and gastrointestinal tract. Although a limited number of studies indicate that hydrogenated terphenyl is absorbed via the skin, data on skin absorption rates are missing. [6] 

References:

[1] https://www.gezondheidsraad.nl/binaries/gezondheidsraad/documenten/adviezen/2020/06/15/gehydrogeneerd-terfenyl/Advisory-report-Hydrogenated-Terphenyl.pdf

[2] Tomlinson, M., Smee, J. L., Winters, E. B., & Arneson, M. C. (1966). Reactor Organic Coolants: I. Characteristics of Irradiated Hydrogenated Terphenyls. Nuclear Science and Engineering, 26(4), 547-558.

[3] Tomlinson, M., Tymko, R. R., & Wuschke, D. (1967). Reactor Organic Coolants II: Electron-Irradiation Studies of Hydrogenated Terphenyls. Nuclear Science and Engineering, 30(1), 14-19.

[4] Sung, J. S., Choo, K. Y., Kim, T. H., Tarasov, A. L., Tkachenko, O. P., & Kustov, L. M. (2008). A new hydrogen storage system based on efficient reversible catalytic hydrogenation/dehydrogenation of terphenyl. International journal of hydrogen energy, 33(11), 2721-2728.

[5] Farr CH, Nair RS, Daly IW, Terrill JB, Johannsen FR. Subchronicinhalation and oral toxicity of hydrogenated terphenyls in rats. FundamAppl Toxicol 1989; 13(3): 558-67.

[6] Johnson D. Monsanto Company, St. Louis, Missouri 63166, USA. 21-Day Dermal Toxicity Study in Rabbits. Available for consultation at The Health Council of The Netherlands: Unpublished report, 1981;1R-79-356.

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