Oxidative Coupling of Light Alkanes to Liquid Fuels Using Isobutane as an Oxygen Carrier and the Alkane Structure–Reactivity Relationship

Abstract

A process scheme for the liquid-phase oxidative coupling of light alkanes (C 3 −C 7 ) to transportation fuels using isobutane as the oxygen carrier is devised. The process consists of (1) liquid-phase oxidation of isobutane to tert-butyl hydroperoxide (TBHP), with tert-butyl alcohol (TBA) coproduct; (2) converting TBHP and TBA over an acid catalyst to di-tert-butyl peroxide (DTBP); (3) using DTBP to generate alkyl radicals from the feed alkane, which are coupled to form heavier alkanes, with TBA as coproduct; and optionally (4) regeneration of isobutane from TBA via dehydration−hydrogenation. Light alkane coupling to heavier alkanes using DTBP, a critical step in this scheme, is experimentally demonstrated. Prototype jet and diesel products derived from npentane coupling show excellent low-temperature properties, reasonable cetane numbers, satisfactory oxidation stability, and attractive density, all consistent with the branched products as a result of alkyl radical coupling. The relative reactivity of primary (1°), secondary (2°), and tertiary (3°) C−H bonds in alkanes toward hydrogen atom abstraction (HAA) by tert-butoxy radical has been determined: 1°(1) < 2°(9) < 3°(29) at 150 °C. Relative reactivity of a feed can be estimated based on the number and the relative reactivity of each type of C−H bond in the feed alkane or alkane mixtures. The coupling efficiency, defined as the rate ratio of HAA vs β-scission of tert-butoxy radical (k a /k b ), is found to correlate linearly with the reactivity estimated from the structure of the feed alkane. With this correlation, the coupling efficiency for a given alkane or alkane mixture can be estimated from the structure of the alkane(s), which will provide guidance to optimize the alkane coupling reaction for eventual large-scale applications.