Aromatization-Driven Upgrading of Diesel in Supercritical Water: Water-Mediated Hydrogen Transfer

Abstract

The ongoing electrification of transportation and the rapid growth of renewable energy have stimulated increasing interest in the development of upgrading strategies for surplus diesel. Supercritical water (SCW) has emerged as a promising medium for hydrocarbon upgrading, capable of promoting cracking while simultaneously suppressing coke formation, yet its applicability to middle distillates such as diesel insufficiently explored. In this work, we systematically investigate diesel conversion in SCW across a parameter space spanning 440–470 °C, water-to-oil ratios of 0:1–4:1, water densities of 0.1–0.2 g/cm3, and residence times of 15–60 min, complemented by model-compound experiments, lumped kinetic modeling, and deuterium oxide (D2O) isotopic tracing. The results demonstrate that the conversion of diesel in SCW involves both cracking and condensation reactions. A four-lump kinetic model identifies the cyclization–dehydrogenation sequence as the dominant reaction pathway. Deuterium tracing coupled with spectroscopic analysis provides evidence that SCW participates in hydrogen transfer predominantly as a hydrogen-transfer medium, stabilizing cracking fragments and capping radical intermediates without introducing oxygen-containing functionalities into the hydrocarbon products. Collectively, these findings reveal the aromatization tendency of diesel under SCW conditions and provide a mechanistic basis for the development of SCW-based upgrading strategies for middle-distillate feedstocks.