The preparation of cocrystals from active pharmaceutical ingredients (APIs) and biologically relevant coformers offers the opportunity of obtaining compounds with more desirable physicochemical and biological properties. This work focuses on theophylline–­trimesic acid, caffeine–­isophthalic acid, and caffeine–­trimesic acid cocrystals. All the cocrystals were produced via slow evaporation and were characterized using Fourier transform infrared, differential scanning calorimetry, thermogravimetric analysis, and single-crystal X-ray diffraction. Structural characterization revealed that interactions such as CO···H, N···H···O, π–π, and C–H···π between the APIs and coformers significantly contribute to crystal packing. Density functional theory studies further revealed the electronic properties of cocrystals, as well as the functional groups that enhance their solubility. Drug activity through the weak groove-binding mode was realized through docking studies of the cocrystals with the DNA structure (Protein Data Bank identifier 1ZEW). Similarly, major interactions, including hydrogen bonding and π-π bonding, were observed between cocrystals and 4HL2, a New Delhi metallo-β-lactamase-1 produced by resistant clinical strains of K. pneumoniae. Biological studies revealed cocrystals with antimicrobial properties, particularly against clinically relevant gram-negative bacterial pathogens (Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa). So, these compounds represent a novel promising group of anti-infective agents.

Desulfurization of fuel oils is an essential process employed in petroleum refineries to reduce the sulfur content to levels mandated for environmental protection. Hydrodesulfurization (HDS), which is currently being employed, is limited in treating refractory organosulfur compounds and only reduces the sulfur content in fuels to a range of 200-500 ppmS. In this chapter, several scientific and technological advances reported in the literature for the desulfurization of fuels are reviewed and discussed. Amongst these techniques, oxidative desulfurization (ODS) and adsorptive desulfurization (ADS) are proposed as additional steps to complement HDS in meeting the mandated ultra-low sulfur levels (10 ppmS). In the ODS technique, refractory organosulfur compounds are oxidized to organosulfones, followed by solvent extraction or adsorption of the organosulfones. The chemistry involved in the development and fabrication of sulfur/sulfone responsive adsorbents is also discussed. The use of molecular imprinted polymers (MIPs) and coordination polymers (CPs) for the selective adsorption of organosulfone compounds (in ODS) and/or organosulfur (in ADS) offers various properties such as imprinting effect, hydrogen bonding, π-π interactions, van der Waals forces, π-complexation, and electrostatic interactions. CPs, in particular metal organic frameworks (MOFs), have been reported to possess suitable features to overcome most of these challenges associated with adsorptive ultra-deep desulfurization when design strategies to achieve good selectivity are strictly followed. Matching the sizes of the cavities to the critical dimensions of the sulfur containing compounds (SCCs), using suitable metal centres which allow for coordinative interaction with the SCCs and using linkers with suitable functionality as to enhance specific interaction (dispersion forces) with the SCCs were considered to be pivotal features to prioritize. The prospects for the use of MIPs and CPs for future industrial applications in desulfurization are envisaged.

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Reliance on crude oil remains high while the transition to green and renewable sources of fuel is still slow. Developing and strengthening strategies for reducing sulfur emissions from crude oil is therefore imperative and makes it possible to sustainably meet stringent regulatory sulfur level legislations in end-user liquid fuels (mostly less than 10 ppm). The burden of achieving these ultra-low sulfur levels has been passed to fuel refiners who are battling to achieve ultra-deep desulfurization through conventional hydroprocessing technologies. Removal of refractory sulfur-containing compounds has been cited as the main challenge due to several limitations with the current hydroprocessing catalysts. The inhibitory effects of nitrogen-containing compounds (especially the basic ones) is one of the major concerns. Several advances have been made to develop better strategies for achieving ultra-deep desulfurization and these include: improving hydroprocessing infrastructure, improving hydroprocessing catalysts, having additional steps for removing refractory sulfur-containing compounds and improving the quality of feedstocks. Herein, we provide perspectives that emphasize the importance of further developing hydroprocessing catalysts and pre-treating feedstocks to remove nitrogen-containing compounds prior to hydroprocessing as promising strategies for sustainably achieving ultra-deep hydroprocessing.

A series of oxidovanadium(iv) complexes based on 2-(2′-hydroxyphenyl)imidazole (HPIMH), with substituent groups of different electronegativities on the phenolic para position (HPIMX; X = –H, –Br, –OMe and –NO2), were synthesized and characterized.

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Silica-supported vanadium oxides (V x O y -silica 600 °C) and polymer nanofiber [2-(2′-hydroxy-5′-ethenylphenyl)­imidazole (PIMv) and styrene (ST) copolymer]-supported oxidovanadium­(IV) ([VIVO-p­(PIMv-co-ST)]) were synthesized and used as catalysts for the oxidation of refractory organosulfur compounds in fuels in a continuous flow system. Conversion of dibenzothiophene (DBT) to dibenzothiophene sulfone (DBTO2) increased as the flow rate decreased, reaching 100% at flow rates of 0.1 and 0.2 mL/h for (V x O y -silica 600 °C) and [VIVO-p­(PIMv-co-ST)], respectively. This was attributed to improved contact time between the catalyst and substrate, which allowed further oxidation to take place. However, the catalytic activity of V x O y -silica 600 °C dropped by 33% after the first oxidation cycle at a flow rate of 0.1 mL/h at 60 °C, unlike [VIVO-p­(PIMv-co-ST)], which maintained its activity at 100% after three cycles. Optimized conditions were employed in the oxidation of a hydrotreated fuel sample (Sasol diesel 500) followed by extraction of the resulting sulfones using acetonitrile. Analyses of the fuel samples using two dimensional gas chomatography coupled to sulfur cheminiluscence and time-of-flight mass spectrometer detectors (GC × GC-SCD and GC × GC-HRT) confirmed the oxidation of organosulfur compounds and removal of the resulting sulfones. This study revealed that [VIVO-p­(PIMv-co-ST)] was a more robust and more efficient catalyst for the oxidation of organosulfur compounds compared to (V x O y -silica 600 °C). Monitoring the 51V electron paramagnetic resonance signal from the catalysts upon adding oxidant and then substrate showed that the catalysis is of redox nature, involving the V4+ sites.