Reaction of Pentenes, Hexenes, and Heptenes with Phosphoric Acid

Abstract

A temperature swing absorption process using phosphoric acid was proposed for the removal of olefins from cracked naphtha. Evaluation of this required information on the reaction of naphtha range olefins with phosphoric acid. Reactive absorption of olefins at 50 °C in 85% phosphoric acid (H3PO4) was studied. The olefins evaluated were 1-pentene, 2-methyl-1-butene, 2-methyl-2-butene, cyclopentene, 1-hexene, 2-methyl-1-pentene, cyclohexene, 1-heptene, 2-methyl-1-hexene, and 4-methylcyclohexene. All of the olefins formed alkyl phosphoric acid esters, with Cn olefins forming Cn-, C2n-, and sometimes C3n-alkyl phosphoric acid esters. Branching in the olefin affected alkyl phosphoric acid ester stability; ester bonds to the α-carbon were more stable. The acyclic C5-alkyl phosphoric acid esters were more water soluble than oil soluble, and as the chain length of the alkyl group increased, oil solubility increased. Cyclohexyl and methyl cyclohexyl phosphoric acid esters formed stable water–oil emulsions. Reaction rates were low, but double bond isomerization took place and proceeded stepwise, with alkyl phosphoric acid esters being intermediate products rather than reaction intermediates. Although it was found that temperature swing absorption using phosphoric acid would not result in a viable process, the study provided several insights about the intermediate products and phase behavior of phosphoric acid catalyzed olefin conversion.