Petroleum-like fuels with substantially enriched branched iso-paraffins and benzenes via boehmite-assisted pyrolysis of oil shale

Abstract

Oil shale (OS) is a potential alternative to substitute/supplement traditional fossil fuels, where catalyst-assisted OS pyrolysis has become a subject of general concern. While great progresses have been realized in this field, the pursuit of high-performance catalysts for a niche OS utilization still remains a formidable challenge, because most of the catalysts face various problems, including, high cost, unsatisfactory performances, etc. Based on a two-stage flash pyrolysis, we herein report our efforts about this issue by using boehmite, a low-cost industrially relevant material of an easy availability, as catalyst. We show that boehmite could arouse 17.0 % decreases in activation energy of OS pyrolysis. When the pyrolysis is executed at the 1st stage (450 °C), the content of hydrocarbons and short-chain aliphatic hydrocarbons (≤C13), which are desired for petroleum-like fuels (PLFs), increases evidently by 56.73 % and 158.5 %, respectively, while that of heteroatomic compounds and long-chain aliphatic hydrocarbons (≥C20), which are undesired for PLFs, decreases distinctly by 60.09 % and 41.6 %, respectively, indicating an evident improvement in the quality of the pyrolysates. Fascinatingly, the content of branched iso-paraffins, especially, monomethyl-, dimethyl- and trimethyl-branched paraffins, which are either efficient octane number boosters, important tectons for functional surfactants, or significant component for high-quality fuels of excellent low-temperature performances, increases substantially by ca. 180.2 % from ca. 16.02 % to 44.89 %. For the 2nd stage at 585 °C, besides a distinctly improved quality of the pyrolysates, the content of benzene, toluene, xylene and trimethylbenzene, which are value-added chemicals broadly employed in numerous industrial fields of paramount importance, is enriched distinctly by 94 %, 65.2 %, 13 %, and 19 %, respectively. We find that the concentration of the Brönsted acid sites (BASs) of the employed boehmite, which contain 94 % medium and strong BASs, is substantially lower than that of Lewis acid sites (LASs), which contain merely 54.91 % medium and strong LASs, such that the competitive-collaborative actions of BASs and LASs confer boehmite with outstanding catalytic reactivity.