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

• Published in: Industrial & engineering chemistry research 2020-12, Vol.59 (50), p.21630-21641
• Main Authors: Wang, Kun, Mitchell, Jonathan E, Ho, Suzzy C, Walker, Elizabeth L
• Format: Article
• Language: English
• Subjects: Applied Chemistry
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/acs.iecr.0c05244

A process scheme for the liquid-phase oxidative coupling of light alkanes (C3–C7) 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 n-pentane 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.