Benzene synthesis for radiocarbon dating and study of the catalyst used for acetylen trimerization

• Published in: The International journal of applied radiation and isotopes 1978, Vol.29 (7), p.453-459
• Main Authors: Belluomini, Giorgio, Delfino, Alessandro, Manfra, Luigia, Petrone, Vincenzo
• Format: Article
• Language: English
• ISSN: 0020-708X
• DOI: 10.1016/0020-708X(78)90083-2

The succession of steps undergone by the sample in the benzene synthesis line for radiocarbon dating from its original state to benzene through the intermediate formation of carbon dioxide in the Vycor tube, lithium carbide with molten lithium metal heated to at least 700°C and in absence of air, acetylene by its hydrolysis and its conversion to benzene on the catalyst, is schematically described. The optimum conditions for the above mentioned operations are defined and secondary reactions, responsible for reduced yields in the various steps, are described. Particularly, a silica-alumina catalyst activated by K 2CrO 4 employed for the conversion of acetylene to benzene is studied. Sixty grams of catalyst, dehydrated at a temperature of about 400°C for 1 hr and in absence of air, proved to be sufficient to convert in benzene, in 90 min time, 98–99% of a volume of acetylene averaging 4.51 at P 0 T 0. The benzene is extracted under vacuum at 150°C for 2 hr and its purity checked by gas chromatograph analysis is about 99.0%. The efficiency of the catalyst is maximum when it is activated by Cr(VI) and it is sufficient to expose the poisoned catalyst to an air flow at a temperature of 550–600°C during 60 min to bring it to same efficiency as the fresh catalyst. No cross contamination effects are present when the regenerated catalyst is used since the values of dates are in the range of 3 σ. The reducing action of acetylene on the catalyst promotes the reduction of Cr(VI) to Cr(IV) and Cr(III) with remarkable loss of its efficiency and is responsible for reduced yield of benzene in conversion reaction with formation of by-products which are a mixture of aliphatic and aromatic polymers, the last containing a carbonyl group and an aliphatic chain. The total amount of by-products is about 5% of acetylene used. The order of reaction for conversion of acetylene to benzene, including by-products in respect of the acetylene is 1.5 in the range 20–35 mmole/1 and in the temperature range of 0–55°C. The value of the energy of activation is −4.9 ± 0.3 kcal/mole.