(181o) Natural Gas Desulfurization Using Titania and Ceria Nanotubes/Zeolitic Imidazolate Framework-8 Nanocomposites Suspended in Water

Natural gas is widely used for power generation and other industrial activities. Its consumption is growing rapidly with a projection that 220 trillion cubic feet (tcf) will be consumed in 2050 relative to about 130 tcf in 2020 [1]. Most natural gas reserves contain significant levels of hydrogen sulfide (H₂S), which represents a significant hazard during the processing of sour natural gas and other H₂S-containing gases. This lethal gas originates naturally in the gas formations from the anaerobic degradation of sulfate minerals and biomass by sulfate-reducing bacteria (SRB) as well as the thermochemical decomposition of sulfur compounds catalyzed by anhydrites [2,3]. H₂S, recognized for its high toxicity and flammability, has a distinct odor reminiscent of rotten eggs; however, at concentrations above 100 ppm, the human ability to smell H₂S is completely lost [4], increasing the risk of exposure. The exposure to H₂S carries serious health and safety risks not only at high concentrations but also at low concentrations upon prolonged exposure, which can potentially result in severe chronic health problems, leading to death in some cases. Accordingly, the Occupational Health and Safety Administration (OSHA) sets the maximum exposure time to 50 ppm H₂S, for instance, as 10 min.

Beside its toxicity, H₂S becomes explosive when mixed with air within the concentration of 4.3–45%. Additionally, the acidic nature of H₂S makes it highly corrosive to handling and processing metallic equipment. For instance, the interaction of H₂S with steel yields free hydrogen ions and iron sulfide scales, leading to the degradation of metallic structures via mechanisms such as hydrogen embrittlement, sulfide cracking, and pitting corrosion. These processes collectively curtail the operational lifespan of natural gas handling and processing equipment [4,5]

Currently, the most widely utilized process for H₂S removal from sour natural gas is amine absorption. However, this process is energy intensive, corrosive, and utilizes organic solvents, highlighting the urgent need for developing noncorrosive, organic solvent-free and less energy consuming technologies. Zeolitic imidazolate frameworks (ZIFs) have recently emerged as attractive nanomaterials for various applications. ZIFs belong to the category of metal organic frameworks (MOFs). They are formed by the coordination of zinc (Zn²⁺) or cobalt (Co²⁺) with imidazole ligands, creating a three-dimensional framework [6,7]. Their attractiveness stems from their adjustable nature, thermal stability, high specific surface area, structural adaptability, and high porosity [6–8]. Thus, they find applications across various domains such as catalysis, gas detection, and wastewater treatment. Specifically, ZIF-8 stands out among these materials due to its ease of fabrication, high production yield, robustness, hydrophobicity, and oleophilicity. However, ZIFs (including ZIF-8) usually suffer from water instability. However, it has been reported that the modification of ZIFs has the potential to significantly boost their water stability [6]. Accordingly, we modify ZIF-8 with two novel nanomaterials, titania nanotubes (TNTs) and ceria nanotubes (CeNTs). The synthesis of TNTs/ZIF-8 (abbreviated as TNTs@ZIF-8) and CeNTs/ZIF-8 (abbreviated as CeNTs@ZIF-8) nanocomposites has not been reported so far in the published literature to the best of our knowledge, highlighting the novelty of this study.

The synthesis protocol of TNTs@ZIF-8 is as follows. First, TNTs were synthesized by adding 1 g TiO₂ (anatase) to NaOH aqueous solution (10 M, 50 mL) and the mixture was, then, stirred for 1 h, followed by 1 h sonication. The resulting suspension was, then, transferred to a Teflon-lined autoclave reactor and incubated at 150 °C for 48 h. The formed product was washed with distilled water and 0.1 M HCl until the filtrate pH reached 2 (acid-washing), and then it was aged at room temperature in a dilute HCl solution having pH 2 for 8 h. The produced nanomaterial was then filtered, washed with distilled water and ethanol, respectively. Then, the produced white titania nanotubes nanomaterial was fully dried. Following the synthesis of TNTs, the TNTs@ZIF-8 nanocomposite was prepared using the hydrothermal method where a certain amount of the produced TNTs was dispersed in a zinc aqueous solution before the addition of the organic linker (i.e., 2-methyl imidazole). The mixture was allowed to react at ambient conditions for 1 h under stirring, followed by 1 h aging. The produced TNTs@ZIF-8 nanocomposite was, then, recovered and purified.

In the synthesis of CeNTs, 5 mL of 0.8 M Ce(NO₃)₃ aqueous solution was added to 75 mL of 6.4 M NaOH aqueous solution and stirred for 30 min. Then, the mixture was transferred to a Teflon-lined autoclave reactor and treated hydrothermally at 100 °C for 24 h. The obtained precipitates were washed using distilled water and ethanol alternatively for 3 times each and then dried. Subsequently, 0.26 g of the dried material was dispersed in 80 mL distilled water, followed by the addition of 3 mmol Ce(NO₃)₃. Then, the mixture was kept at 100 °C for 2 h. Afterwards, the obtained CeNTs were extensively washed using distilled water and dried. Finally, the CeNTs@ZIF-8 nanocomposite was synthesized using the hydrothermal method and purified as outlined in the synthesis of the TNTs@ZIF-8 nanocomposite stated above. The synthesized TNTs@ZIF-8 and CeNTs@ZIF-8 nanocomposites were characterized using BET surface area, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). These characterizations confirmed the successful synthesis of the TNTs@ZIF-8 and CeNTs@ZIF-8 nanocomposites.

After the successful synthesis of the TNTs@ZIF-8 and CeNTs@ZIF-8 nanocomposites, their performance in the capture of H₂S from sour natural gas was evaluated. To the best of our knowledge, this is the first study reporting the application of these nanocomposites for the desulfurization of sour natural gas or any other applications. In the natural gas desulfurization experiments, the TNTs@ZIF-8 and CeNTs@ZIF-8 nanocomposites were suspended (separately) in water via stirring while bubbling the sour natural gas into the suspensions. The use of this stirred tank process for the desulfurization of sour natural gas is another novel aspect of this study. Figure 1 displays the breakthrough curves of the sour natural gas desulfurization using the TNTs@ZIF-8 and CeNTs@ZIF-8 nanocomposites suspended in a continuously stirred tank reactor (CSTR). The results revealed that the TNTs@ZIF-8 nanocomposite could completely eliminate H₂S from the influent sour natural gas stream up to 390 min (i.e., the breakthrough time). After this breakthrough time, the H₂S concentration in the gas effluent gradually increased until it reached the same concentration as in the influent gas stream, implying that the TNTs@ZIF-8 nanocomposite became fully saturated with the captured H₂S. The time at which the saturation was attained is 475 min. On the other hand, the breakthrough and the saturation times obtained using the CeNTs@ZIF-8 nanocomposite were 376 and 675 min, respectively.

Overall, the TNTs@ZIF-8 and CeNTs/ZIF-8 nanocomposites are promising nanomaterials for the desulfurization of sour natural gas, and potentially other industrial applications. In addition to being organic-free and mild process (the natural gas desulfurization was conducted at ambient conditions), the replacement of packed bed typically used in industrial adsorption and catalytic processes with catalysts/adsorbents submerged in water could eliminate some technical challenges such as gas channeling, high pressure drop, among others problems. Nonetheless, further studies are still needed to explore the scaling-up and economic aspects of the synthesized TNTs@ZIF-8 and CeNTs/ZIF-8 nanocomposites and the proposed CSTR-based natural gas desulfurization process.

References

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