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

Abstract

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 C 0 = 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.

Molecular Dynamics Studies of Aromatic Guests in Three Isostructural Inclusion Compounds with Molecular Boron–Nitrogen Hosts

“…16−20 The strong covalent character, directionality, and reversible nature of the B←N bond make this interaction suitable also for diverse applications in supramolecular chemistry, particularly the self-assembly of finite and infinite aggregates. 21−28 A series of recent investigations have shown that crystalline B ← N assemblies formed from triorganoboranes or organoboronic esters and pyridyl-donor ligands are suitable for selective molecular recognition of aromatics, 29 the separation of petrochemicals, 30,31 the removal of harmful polycyclic aromatic hydrocarbons (PAHs), 32 as well as the generation of luminescent 8,9 and semiconductor materials. 33 Moreover, in the presence of alkenyl groups, [2+2] photocycloaddition reactions can be achieved, giving cyclobutanes in stereochemically pure form.…”

“…Organoboranes and organoboronic esters are versatile Lewis acids that readily coordinate to nitrogen-containing Lewis bases, which can be used to prepare inclusion complexes. The generally good stability at ambient conditions (air/water) and interesting physical and chemical properties has stimulated intense activity in the investigation of molecular boron←nitrogen (B←N) compounds. − The strong covalent character, directionality, and reversible nature of the B←N bond make this interaction suitable also for diverse applications in supramolecular chemistry, particularly the self-assembly of finite and infinite aggregates. − A series of recent investigations have shown that crystalline B ← N assemblies formed from triorganoboranes or organoboronic esters and pyridyl-donor ligands are suitable for selective molecular recognition of aromatics, the separation of petrochemicals, , the removal of harmful polycyclic aromatic hydrocarbons (PAHs), as well as the generation of luminescent , and semiconductor materials . Moreover, in the presence of alkenyl groups, [2+2] photocycloaddition reactions can be achieved, giving cyclobutanes in stereochemically pure form. , The common feature in these applications is the presence of cavities or channels in the crystalline solid matrix formed by the B←N assemblies…”

Confinement and Separation of Benzene from an Azeotropic Mixture Using a Chlorinated B←N Adduct

“…Our group and others have employed boronic ester coordination with pyridines (B←N) to generate H-shaped and T-shaped adducts. The adducts have enabled the confinement and separation of petrochemicals, and the design of electronic and dynamic materials . Our design has exploited the generation of electron-deficient surfaces resulting from coordinated pyridyl linkers to boronic esters and aided by additional noncovalent interactions (e.g., [C–H···F]) with 2,4-difluorophenylboronic acid ( F-ba ) .…”

A molecular T-pentomino for separating BTEX hydrocarbons

“…Inspired by work of Höp , 14,15 the rigid and conjugated structure of DBP-DEPN would exhibit a capacity to allow for inclusion and capture of aromatic-rich hydrocarbons upon crystallization. [16][17][18][19] Molecular T-pentomino structure and packing DBP-DEPN was generated through condensation of DEPN, phenylboronic acid, and catechol in dichloromethane. DBP-DEPN crystallizes from CH 2 Cl 2 as thin yellow blades in the monoclinic space P2 1 /c (Fig.…”