Extraction of benzothiazole and thiophene from their mixtures with n-heptane using tetrahexylammonium bromide-based deep eutectic solvents as extractive denitrogenation and desulfurization agents
Graphical abstract
Introduction
In order to prevent environmental pollution and due to stringent regulations, there is a burgeoning need to reduce NOx and SOx emissions [1,2]. This can be achieved by removing nitrogen- and sulfur-containing (aromatic) compounds from fuels. High aromatic content in fuels has been discovered to lower the fuel quality; hence, it is essential for the aromatic compounds to be separated from the aliphatic hydrocarbons [3]. The need for denitrogenation has also been spurred by the inhibition of the hydrodesulfurization process by the presence of nitrogen-containing compounds, which poison the catalysts [[4], [5], [6], [7], [8], [9]]. The limit on the nitrogen content in diesel fuel has been regulated from 70 ppm to <0.10 ppm in the year 2011 [10]. Likewise, the sulfur content in fuels in most countries has been set to as low as 10 ppm on an annual average basis beginning from January 2017 and 80 ppm on a per-gallon basis [11,12].
The most common method of desulfurization/denitrogenation is hydrotreatment, which comprises of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). For both processes, intense operating conditions and large amounts of hydrogen (300 atm H2 at 600 K, large reactor size, valving difficulty) are required in order to achieve higher efficiency [13,14]. Asides these demerits, hydrotreatment is not potent for the extraction of refractory aromatic nitrogen and sulfur-containing compounds. Considering HDN, basic nitrogen-containing compounds (like quinoline and pyridine) cannot be extracted alongside non-basic nitrogen-containing compounds (like pyrrole and indole). Basic nitrogen-containing compounds possess a lone pair of electrons, which is unassociated with the aromatic compound. Non-basic nitrogen-containing compounds, on the other hand, have the lone pair of electrons as a constituent of the п-bond [8,[15], [16], [17]]. Although HDS is effective in extracting sulfides, disulfides and thiols, it is less potent for the extraction of thiophene and its derivatives [18,19]. Due to these limitations associated with hydrotreatment, easier and cost-effective methods, which do not require hydrogen, are often investigated.
One of the techniques that does not require hydrogen is the acid adsorption method. Acidic adsorbents, which usually contain transition metal ions, have been found to effectively extract the basic nitrogen-containing compounds from fuel. However, their minimal affinity for the non-basic nitrogen-containing compounds have made them a poor option for denitrogenation. Contrary to acidic adsorbents, aprotic solvents (e.g. tetrahydrofuran (THF), dimethylformamide (DMF), dimethylsulfoxide (DMSO) and hexamethyl phosphoric triamide (HMPT)) have high affinity for non-basic nitrogen-containing compounds, but are not powerful in the extraction of basic nitrogen-containing compounds [[20], [21], [22], [23]]. Also, during an extractive denitrogenation process using aprotic solvents, fuels are usually wasted by co-extraction. This is attributed to the low selectivity for the nitrogen-containing compounds in the fuels. In other to combat this, more selective Lewis acid-base reactions between the nitrogen-containing compounds and the extractive agents are carried out, but this technique is accompanied by a more difficult and expensive regeneration process of the solvent [8].
Other methods of denitrogenation and desulfurization include oxidative denitrogenation/desulfurization, biodenitrogenation/desulfurization, and extractive denitrogenation/desulfurization using novel solvents, which are still undergoing lots of research [[24], [25], [26]]. Due to easier operating conditions and un-altering of the fuel properties, extractive denitrogenation/desulfurization using novel solvents is the most preferable amongst other techniques. For example, ionic liquids (ILs) have been reported extensively in literature as extracting agents for the extraction of nitrogen- and sulfur-containing compounds from fuels [19,25,[27], [28], [29], [30], [31], [32], [33], [34], [35]].
Although ILs can successfully extract nitrogen- and sulfur-containing compounds from fuels, the required large cost of purification, the complex synthesis and the toxicity of some ILs have generated concerns. These concerns have brought about an increase in attention for alternative solvents, including deep eutectic solvents (DESs).
The amount of research on denitrogenation and desulfurization using DESs is still limited. Ali et al. [8] were able to successfully use choline chloride and phenylacetic-based DESs for the simultaneous extraction of both basic and non-basic nitrogen-containing compounds from n-heptane, a challenge which other methods of denitrogenation have not been able to overcome so far. This was the first report in literature in which a DES was used for an extractive denitrogenation process. In another recent work, the removal of pyrrole, pyridine, indoline and quinoline from n-hexadecane using tetrabutylammonium bromide based DESs was successfully carried out [36].
With regards to extractive desulfurization using DESs, Warrag et al. [37] carried out the extraction of thiophene from n-hexane and n-octane using tetrahexylammonium-based DESs, which proved to be potent for the extraction of sulfur-containing compounds from fuels and could also be cost-effectively regenerated after extraction. Few other studies on the removal of thiophene and its derivatives from fuels using DESs have also been reported in literature [[37], [38], [39], [40]].
Still, the extraction of a sulfur- and nitrogen-containing compound from fuels using DESs has never been investigated. Therefore, this liquid-liquid extraction technique was studied for the first time in this work. Specifically, we will extract benzothiazole (which contains both nitrogen and sulfur) and thiophene (which only contains sulfur) from n-heptane using two tetrahexylammonium bromide-based DESs. These DESs were selected based on their previous successful performance in extraction of thiophene [37,38] and its derivatives [40] from different aliphatic hydrocarbons, and because they have been successfully regenerated before by using vacuum evaporation [37]. Because the DESs are actually mixtures themselves, we will measure pseudo-ternary liquid-liquid equilibrium (LLE) data. The assumption to treat both DESs as single species is only correct in case the ratio between the constituents is not changed, which means that the DESs stay intact in one phase only (no losses to the raffinate phase), which has to be experimentally verified. All obtained LLE data will be correlated using the non-random two-liquid (NRTL) model and used to evaluate the feasibility of both DESs as extractive denitrogenation/desulfurization agents.
Section snippets
Materials
The chemicals used for the determination of the LLE data were used without further purification. Their various sources, purity and CAS number are included in Table 1.
DES preparation
The 2 DESs were prepared at a temperature of 308 K. DES 1 and DES 2 consisted of tetrahexylammonium bromide (acting as hydrogen bond acceptor, HBA) and either ethylene glycol or glycerol (serving as hydrogen bond donor, HBD), respectively. The molar ratio between the HBD and the HBA was 2:1 in both cases, as this ratio for both
Solubility data
The solubility of benzothiazole in DES1, DES2, ethylene glycol, glycerol and n-heptane was determined using the cloud point method, while the solubility of n-heptane in both DESs was measured using the equilibrium cell method. The solubility data of thiophene in different solvents had been determined in previous work [37], but have been added for comparative reasons in Table 4 as well.
The solubility of benzothiazole can be observed to be greater than that of thiophene for all the different
Conclusions
This work was able to show the extractive denitrogenation and desulfurization using tetrahexylammonium based DESs. DES 1 consists of tetrahexylammonium bromide: ethylene glycol in molar ratio 1:2 with ethylene glycol being replaced with glycerol in DES 2. Initially, the binary solubilities of benzothiazole and thiophene in n-heptane, DES 1, DES 2, ethylene glycol and glycerol were determined. Afterwards, the LLE data for the four ternary systems {n-heptane + benzothiazole/thiophene + DES 1/DES
Funding sources
Financial support was gotten from the Petroleum Institute (startup fund M. C. Kroon) ESP 16002.
Acknowledgements
The authors would like to thank Dr. Inas AlNashef for his help in providing some of the chemicals.
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