A Comprehensive Review on the production of Polyoxymethylene dimethyl ethers as alternative synthetic fuel: From conventional indirect methodologies to sustainable direct routes

Abstract

Global warming and climate change have led to the development of technologies for reducing and recycling CO₂ emissions and the establishment of environmentally friendly fuel systems. Polyoxymethylene dimethyl ethers (PODE_n), a highly promising renewable oxygenated synthetic fuel, can efficiently improve engine combustion performance and significantly reduce exhaust emissions as diesel blending components or substitutes. In previous, the synthesis of PODE_n mainly used indirect methods, also known as two-step synthesis, which include the synthesis of methylal and formaldehyde from methanol, followed by acetalization reaction of methylal/methanol with formaldehyde catalyzed by acid catalysts to yield the PODE_n. Recently, some emerging methods involve the use of bifunctional catalysts and tandem catalytic technology can achieve one-step production of PODEn. The technological development of CO₂ hydrogenation/reduction to methanol and directly coupling to synthesis PODEn has made the production of PODEn cleaner and more sustainable. In this review, firstly, indirect reaction route and diversified direct pathways are summarized and compared in terms of reactant sources, process flow, and energy evaluation. Furthermore, centered around catalytic reactions, a specific discussion is made on the latest progress in the chemical reactions, catalytic activity, structure-activity relationships, and reaction mechanisms of different routes for synthesizing PODEn. The systematic analysis of the research progress, existing challenges, and future trends of PODE_n will provide possible directions for future research, especially in catalyst design, and provide new perspectives and insights for the industrial production of PODE_n.