Dehydration of C₅−C₁₂ Linear 1-Alcohols over η-Alumina to Fuel Ethers

Abstract

Naphtha from low temperature Fischer−Tropsch (LTFT) synthesis is rich in n-paraffins, but it also contains alcohols and olefins. These alcohols and olefins can be converted into linear fuel ethers and olefin dimers to improve the overall yield and quality of distillate from LTFT refining. The reaction network was studied for the conversion of 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1-decanol, and 1-dodecanol over an η-alumina catalyst in a fixed bed flow reactor at 250−350 °C, 0−4 MPa, and WHSV of 1−4 h−1. The main products were the corresponding linear ethers and linear α-olefins. The highest ether yield (54%) was obtained at 300 °C, 1 MPa, and WHSV of 1 h−1 (unoptimized conditions). The main side-products were aldehydes and olefin dimers. Dehydration occurred predominantly on Lewis acid sites, with acid-catalyzed side-reactions taking place over Brønsted acid sites and dehydrogenation taking place over basic and/or redox sites. Dehydration to produce 2-olefins was cis-selective and occurred mainly over Lewis acid sites by dehydration−hydration−dehydration. GC-FID response factors for di-n-butyl ether and heavier linear ethers were determined experimentally. The GC-FID response factors were expressed in terms of the equivalent carbon number (ECN) concept as an ECN of −0.6 ± 0.1 for the linear ether oxygen atom, and this value could be mechanistically justified (literature ECN is −1.0).