Crystal-to-Cocrystal Transformation as a Novel Approach for the Removal of Aromatic Sulfur Compounds from Fuels

Refractory aromatic sulfur compounds present in gasoline, diesel, and jet fuels cause serious environmental problems and must be removed to minimize the emissions of SO_x, according to environmental regulations worldwide. Herein, we present a novel approach for the removal of organosulfur compounds from model liquid fuel solutions employing a nonporous host crystalline material (A1) that undergoes transformation into cocrystals with aromatic sulfur compounds like benzothiophene (BT) and dibenzothiophene (DBT) (i.e., A1⊃BT and A1⊃DBT). In experiments using a single component in cyclohexane solutions, crystalline A1 reached a quantitative uptake of DBT after 12 h at 25 °C with equimolar or slightly increased molar ratios of DBT over A1 (e.g., 1.45 ± 0.05 mmol of S/g of A1, which is equivalent to 267.3 ± 9.3 mg of DBT/g of A1 at C₀ = 3000 ppmwS). The crystalline material A1 can be recycled from cocrystal A1⊃DBT by a two-step process: (i) the generation of the solid A1⊃toluene upon extraction of DBT in toluene and (ii) the subsequent thermal treatment at 150 °C for desolvation to recover microcrystalline A1; after four recycling processes, the uptake capacity of A1 is maintained at 97.5%. In competitive experiments using a five-component equimolar cyclohexane solution containing BT, DBT, 4,6-dimethyldibenzothiophene (DMDBT), fluorene (FLUO), and naphthalene (NAPH), the uptake efficiency of A1 after 12 h at 25 °C followed the trend: DBT > FLUO > NAPH > BT > DMDBT. The selective uptake of DBT in these model fuel solutions is explained by matching of the π-electron rich guest with π-electron deficient diamine linkers and concomitant CH−π interactions within the enclosed cavities of the double-tweezer B←N host A1, as exhibited in the X-ray crystal structure. Size fitting is important for guest selectivity because attempts to isolate cocrystals with DMDBT were unsuccessful. Crystal-to-cocrystal is a solution mediated phase transformation that competes favorably in the removal of DBT with several examples of high-surface nano/microporous materials and composites used in batch adsorption experiments, and A1 can be recycled without compromising efficiency. Thus, crystal-to-cocrystal transformation emerges as a remarkable methodology for the quantitative removal of aromatic organosulfur compounds from liquid fuels because it can be extended to many other aromatic fuel contaminants by using the appropriate host molecule to cocrystallize with it.