Perspectives on strategies for improving ultra-deep desulfurization of liquid fuels through hydrotreatment: Catalyst improvement and feedstock pre-treatment

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

Synthesis and crystal structures of zinc(II) coordination polymers of trimethylenedipyridine (tmdp), 4-nitrobenzoic (Hnba) and 4-biphenylcarboxylic acid (Hbiphen) for adsorptive removal of methyl orange from aqueous solution

Two novel Zn(II) coordination polymers (CPs), [Zn(nba)2(tmdp)]n (1) and [Zn(biphen)2(tmdp)]n (2), were synthesised by reacting Zn(NO3)2·6H2O and 4,4′-trimethylenedipyridine (tmdp) with corresponding carboxylates: 4-nitrobenzoic (Hnba) and 4-biphenylcarboxylic acid (Hbiphen). Their structures were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. Compounds 1 and 2 are one-dimensional CPs with the zinc(II) carboxylate units bridged through the N-donor spacer ligand. The zinc (II) atom adopts a tetrahedral arrangement in 1 and 2 coordinated by two nitrogen atoms from two tmdp ligand molecules and two deprotonated oxygen atoms from two carboxylate ligand molecules. The adsorption capacities of MO in this study was found to be 546.31 mg/g and 22.67 mg/g for 1 and 2, respectively. DFT studies confirmed that adsorption is primarily due to π-π stacking and electrostatic interactions between MO and 1. It is noteworthy that binding energy (BE) values for 1 (-74.14 KJ/mol) and 2 (-61.11 KJ/mol) correlate reasonably well with the observed adsorption capacities of MO. The study demonstrated that 1 has higher adsorption efficiency in comparison to 2 and could be an effective and easily reusable adsorbent for the removal of MO from wastewater

Coordination polymers for denitrogenation of fuel oils

In this thesis, we present adsorptive removal of nitrogen-containing compounds from fuel oil as an alternative to complement conventional hydrotreatment to obtain ultra-low sulfur and nitrogen levels. This is in cognizance of the challenges nitrogen-containing compounds pose to the hydrotreatment process, particularly their inhibition and/or poisoning of the catalysts used in the process, of which basic nitrogen-containing compounds are the major culprits. Selectivity is the biggest challenge for adsorptive removal of nitrogen-containing compounds. We explore reticular synthesis of metal organic frameworks and the use of coordinatively unsaturated metal sites in 1-dimensional coordination polymers to achieve good selectivity for nitrogen-containing compounds. In the first part of the thesis, reticular synthesis of metal organic frameworks to control the size of the cavity, and strategically use the linkers and metal centres was envisaged. In this work we explored variation of the metal centres in the secondary building units (SBUs of the MOFs as the first step to the testing and implementation of the design strategies. Carbazole, representing carbazoles which the major compounds that remain in hydrotreated fuel, was the target compound. Four MOFs of zinc (Zn-CDC-bpe), copper (Cu-CDC-bpe), nickel (Ni-CDC-bpe) and cobalt (Co-CDC-bpe) based on the formation of a dinuclear metal paddlewheel SBUs with the ligand 9H-Carbazole-3,6-dicarboxylic acid (H2CDC) and occupation of the axial positions of the paddlewheel by 1,2-Bis(4-pyridyl)ethane (bpe) to form porous networks were synthesized. A fifth MOF containing only CDC which forms a [Zn4O(O2C-R)5(O2HC-R)] SBU was also synthesized (Zn-CDC). The ligand H2CDC was inspired by the possibility of improving selectivity for carbazole via π–π interactions through the more preferred parallel-offset stacking as well as the possibility for further substitution of the carbazole N-H to add groups that improve selectivity. The sizes of the MOF cavities can then be controlled by choosing different lengths of ligands analogous to 1,2-Bis(4-pyridyl)ethane (bpe), e.g. 4,4’-bipyridine and pyrazine. All the MOFs showed good selectivity of carbazole. The Zn-CDC MOF also had good selectivity for the basic nitrogen-containing compounds tested: quinoline, isoquinoline, quinaldine and 1-naphthylamine. Its uptake of carbazole was also slightly higher. This was attributed to the presence of an unsaturated Zn site in the SBU. Adsorption in all the MOFs was primarily due to physisorption. It was concluded that the role of the metal centre does not play a significant role in the adsorption of carbazole besides providing a template for reticular synthesis.Thesis (PhD) -- Faculty of Science, School of Biomolecular and Chemical Sciences, 202

Coordination polymers for denitrogenation of fuel oils

In this thesis, we present adsorptive removal of nitrogen-containing compounds from fuel oil as an alternative to complement conventional hydrotreatment to obtain ultra-low sulfur and nitrogen levels. This is in cognizance of the challenges nitrogen-containing compounds pose to the hydrotreatment process, particularly their inhibition and/or poisoning of the catalysts used in the process, of which basic nitrogen-containing compounds are the major culprits. Selectivity is the biggest challenge for adsorptive removal of nitrogen-containing compounds. We explore reticular synthesis of metal organic frameworks and the use of coordinatively unsaturated metal sites in 1-dimensional coordination polymers to achieve good selectivity for nitrogen-containing compounds. In the first part of the thesis, reticular synthesis of metal organic frameworks to control the size of the cavity, and strategically use the linkers and metal centres was envisaged. In this work we explored variation of the metal centres in the secondary building units (SBUs of the MOFs as the first step to the testing and implementation of the design strategies. Carbazole, representing carbazoles which the major compounds that remain in hydrotreated fuel, was the target compound. Four MOFs of zinc (Zn-CDC-bpe), copper (Cu-CDC-bpe), nickel (Ni-CDC-bpe) and cobalt (Co-CDC-bpe) based on the formation of a dinuclear metal paddlewheel SBUs with the ligand 9H-Carbazole-3,6-dicarboxylic acid (H2CDC) and occupation of the axial positions of the paddlewheel by 1,2-Bis(4-pyridyl)ethane (bpe) to form porous networks were synthesized. A fifth MOF containing only CDC which forms a [Zn4O(O2C-R)5(O2HC-R)] SBU was also synthesized (Zn-CDC). The ligand H2CDC was inspired by the possibility of improving selectivity for carbazole via π–π interactions through the more preferred parallel-offset stacking as well as the possibility for further substitution of the carbazole N-H to add groups that improve selectivity. The sizes of the MOF cavities can then be controlled by choosing different lengths of ligands analogous to 1,2-Bis(4-pyridyl)ethane (bpe), e.g. 4,4’-bipyridine and pyrazine. All the MOFs showed good selectivity of carbazole. The Zn-CDC MOF also had good selectivity for the basic nitrogen-containing compounds tested: quinoline, isoquinoline, quinaldine and 1-naphthylamine. Its uptake of carbazole was also slightly higher. This was attributed to the presence of an unsaturated Zn site in the SBU. Adsorption in all the MOFs was primarily due to physisorption. It was concluded that the role of the metal centre does not play a significant role in the adsorption of carbazole besides providing a template for reticular synthesis.Thesis (PhD) -- Faculty of Science, School of Biomolecular and Chemical Sciences, 202

Synthesis, structural and DFT investigation of Zn(nba) 2 (meim) 2 for adsorptive removal of eosin yellow dye from aqueous solution

A novel Zn(II) mixed‐ligand complex, Zn(nba)2(meim)2 (1), synthesized from Zn(NO3)2.6H2O, nitrobenzoic acid (Hnba) and 1‐methylimidazole (meim) is reported. The complex was characterized by elemental analysis, FT‐IR, powder and single crystal X‐ray crystallography and TGA/DSC. 1 exhibits a tetrahedral geometry for Zn(II), which is coordinated to two carboxylate oxygen atoms from two nba anions and two imidazole nitrogen atoms from two meim molecules. Thermal analysis shows the stability of 1 up to 260 °C. The adsorption of eosin yellow (EY) dye on 1 was investigated. The adsorption capacity of 1 for EY amounted to 65.32 mg/g, fitting best into Langmuir isotherm and pseudo second order kinetic models. From DFT studies, it was determined that adsorption is predominantly due to electrostatic, hydrogen bonding and π‐π interactions

A zinc-based coordination polymer as adsorbent for removal of trichlorophenol from aqueous solution : synthesis, sorption and DFT studies

[Zn(hba)2(tmdp)]n (1), a Zn-coordination polymer (CP), prepared by the reaction of Zn(NO3)2•6H2O, 4-hydroxybenzoic acid (Hhba) and 4,4’- trimethylenedipyridine (tmdp) was reported. The compound was characterised, using CHN, single crystal and powder X-ray diffraction analysis, FT-IR, and TGA techniques. It exhibits a square pyramidal geometry, with the zinc (II) atom coordinated to two nitrogen atoms from two tmdp ligand molecules and to three oxygen atoms from two hba molecules. The zinc (II) carboxylate units are bridged through the N-donor spacer ligand, thereby giving rise to a one-dimensional CP. PXRD analysis confirmed the purity of the bulk of (1). Compound (1) presented an adsorption capacity of 207.8 mg/g for the removal of 2,4,6-trichlorophenol (TCP) from aqueous solution. The adsorption mechanism is governed by π-π stacking and electrostatic interactions, as obtained from DFT studies. The feasibility and exothermic nature of the adsorption process is indicated by the negative binding energy obtained