N-CNT/ZIF-8 nano-adsorbent for adsorptive desulfurization of the liquid streams: Experimental and

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Highlights

  • N-doped CNT/ZIF-8 hybrid was prepared by hydrothermal method and its performance was evaluated in desulfurization process.

  • NCNT/ZIF-8 exhibited the DBT adsorption capacity of 81.2 mg/g compared to the pure ZIF-8 as 69.1 mg/g.

  • Regenerated NCNT/ZIF-8 showed a good performance after 3 cycles of adsorption-desorption.

Abstract

With the aim of providing clean fuels through the desulfurization, the nanocomposite of Nitrogen-doped carbon nanotube/ZiF-8 (NCNT/ZIF-8) was prepared via the hydrothermal method. The nanocomposite sorbents were evaluated in the adsorption of dibenzothiophene (DBT) from the liquid phase. In this regard, the microstructure, morphology, particle size, thermal stability and physicochemical properties of nanoadsorbents were determined by using different characterization techniques. Influences of various parameters on the adsorption capacity of the nanoadsorbents were investigated; such as the adsorbent type, adsorption time, temperature and the initial DBT concentration. The as-prepared nanoadsorbents, ZiF-8 and NCNT/ZIF-8, possessed a high surface area of 1123.9 and 1207.4 m2/g, respectively. The NCNT/ZIF-8 exhibited the DBT adsorption capacity of 81.2 mg/g which is much higher than that of the pure ZIF-8 (69.1 mg/g). This excellent performance can be ascribed to the synergistic effects of both NCNT and ZiF-8, the presence of basic groups in the NCNT structure, and the high surface area and pore volume. Moreover, the DBT equilibrium adsorption was investigated by using Langmuir, Freundlich models. The Langmuir isotherm indicated a good agreement with experimental results. In addition, adsorption kinetics were studied and it was revealed that the pseudo-second-order model exhibited a favorable fitting with the experimental data. Finally, the regenerated NCNT/ZIF-8 provided a good performance even after 3 adsorption-desorption cycles and its adsorption capacity was almost unchanged.

Introduction

Desulfurization of the liquid phase has recently achieved more attention due to the increase in the sulfur content of the oil. Sulfur compounds are considered as the primary sources of toxic emissions due to their contributions to the air pollution, acid rain, and destruction of the ecological environment and industrial devices. So, environmental regulations have restricted the sulfur level of fuels to low-to-zero level to minimize the air pollution [[1], [2], [3], [4], [5], [6], [7]].

The hydrodesulfurization (HDS) method which is commonly used in desulfurization processes is applied at extreme operating conditions, high temperature and pressure, for reducing the sulfur content of various liquid streams. However, the HDS method is accompanied with two main disadvantages: first, it cannot decrease the sulfur content to less than 20 ppm; second, it has a low selectivity toward removing the heavy aromatic sulfur compounds such as benzothiophenes (BTs) and dibenzothiophenes (DBTs) which occur in liquid fuels (gasoline or diesel) [8,9]. As an environment-friendly and low-cost method, the adsorptive desulfurization (ADS) of liquids on a solid surface has attracted much notice due to the low cost, high desulfurization capacity, simplicity, selectivity, and the good market outlook. The ADS adsorbents can remove sulfur compounds of fuels via the selective adsorption, and they can reduce the sulfur content in oils to less than 1.0 ppm under mild conditions. So, in recent years, numerous investigations have focused on ADS adsorbents with a good performance [[10], [11], [12]]. Various adsorbents can be applied in the desulfurization of gas and liquid fuels, including metal and carbon based materials, zeolites, metal oxide, and molecular sieves [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. Actually, the main goal is development of adsorbents that are highly selective to adsorb thiophenic and aromatics compounds [32].

Metal-organic frameworks (MOFs) are new crystalline inorganic-organic porous materials with a higher porosity and specific surface area than conventional inorganic porous materials. They are also considered as the promising candidates in the scope of sulfur component adsorption due to tunable pore size distribution and chemical functionality via metal ions and organic ligands [[33], [34], [35], [36], [37]]. Although MOFs possess a large porosity, all their porosity is not accessible during the adsorption processes. So, the MOF is combined with other materials such as metal or carbon materials which are designated as a new category of hybrid materials. Various MOF hybrids have been reported for desulfurization of gas and liquid fuels, such as the GO-modified bimetallic organic framework Ni-MOF-199 [38], silver-doped Cu-BTC metal-organic framework [39], Cu-BTC@TiO2 microspheres [40], HKUST-1 and HKUST-1/graphene oxide [41] and functionalized UiO-66(Zr) [42].

Du et al. synthesized MOF-199/attapulgite (APT) composite by the solvothermal method, and its performance was examined in the dimethyl disulfide (DMDS) adsorption under dynamic experiments. Results showed that adding attapulgite to the MOF structure leads to a mesoporous structure and improves the DMDS removal at 30 °C [43]. Ni2P/SBA-15 adsorbents were produced by employing the nickel phosphide as a Ni source via the deposition-precipitation method. Ni precursor was uniformly dispersed on the SBA-15 and applied in capturing sulfur compounds of the model diesel fuel. The total sulfur breakthrough of Ni2P/SBA-15 was one time higher than the SBA-15 due to the small particle size of Ni2P crystallites [44]. In another study, the Zr based MOF was functionalized with various groups and exploited in removing mercaptans from gas and liquid mixture [45].

In addition, MOF/AC composites have also been used in the adsorption of sulfur compounds such as H2S and CH3SCH3 by Fan et al. [46]. They reported that MOF/AC composites with 2 % active carbon had the highest sulfur capacity of 8.46 and 8.53 % for hydrogen sulfide and mercaptane, respectively. The desulfurization performance of MOF/AC composites was improved by 51 and 41 % compared to that of pristine MOF-199 in the H2S and CH3SCH3 adsorption, respectively. This good performance was attributed to the micropores and presence of the active copper sites. Wang et al. reported using the sodium dodecyl sulfate (SDS) as the template in synthesis of the porous ZIF-8 (containing mesopores and macropores) could modify the surface area of the ZIF-8 for a better mercaptan adsorption. The hierarchically porous ZIF-8 with 6.0 g SDS exhibited a superior capacity in the mercaptan adsorption from gasoline and it had an excellent regeneration ability due to the thermal stability at 773 K [47]. Saeedirad [48] synthesized hybrids of ZIF-8 in combination with MCM-41, SBA-15, and UVM-7 and evaluated the sulfur-removal performance (H2S and mercaptane) at 30–50 °C from gas stream under dynamic tests. Results clarified that the UVM-7@ZIF-8 had the best performance with breakthrough capacities of 6.12 and 1.47 gS/g adsorbent for removal of H2S and mercaptane, respectively. This good performance was attributed to the dual-pore size distribution in the hybrid UVM-7@ZIF-8 and also presence of active metal sites (Zn) in the ZIF-8.

Zeolitic imidazolate frameworks (ZIFs) and specially ZIF-8 with a high surface area [49] possesses a superior thermal stability and resistance to water, so, it can be well engineered toward adsorption of various pollutants [50]. Carbon nanomaterials, such as the CNT, have unique characteristics such as a high surface area, high chemical and thermal endurances, which cause them to be significant candidates for pollutant removal [51]. In this work, for the first time, we report the synthesis of ZIF-8 which was then modified through incorporating the nitrogen doped CNT as novel nano adsorbent hybrid (NCNT/ZIF-8). Actually, the synergistic effects arising from the favorable functional groups of the NCNT and porosity of the ZIF-8 can bring about a significant performance in the DBT removal. These novel and tuned nanosorbents were then used in the adsorptive desulfurization of the DBT from the liquid stream. Finally, nano-adsorbents were investigated by thermodynamics and kinetics models. The introduction NCNT and ZiF-8 could establish interactions between ZIF-8 crystals and NCNTs and accordingly a highly stable nanocomposite was prepared. As a result of synthesizing the engineered NCNT/ZIF-8 nanosorbent with optimized porosity and surface chemistry, a much higher adsorption capacity was achieved compared to the pure ZIF-8.

Section snippets

Materials

The starting materials used in the synthesis of the nanocomposites include Zn(NO3)2.4H2O (98 %), 2-methylimidazole (98 %), N-butylamine (99 %) which were supplied by Merck Chemical Company. N-CNT was provided from the Research Institute of Petroleum Industry (RIPI)). Distilled water was used throughout the syntheses.

Synthesis of the NCNT/ZIF-8

Initially, the ZIF-8 was synthesized using the hydrothermal method [48]. Moreover, the NCNT was prepared by the chemical vapor deposition (CVD) method by using solid precursors [52

Characterization

The XRD patterns of hybrid adsorbents are represented in Fig. 3 to survey the crystalline structure and arrangement of atoms. The XRD pattern of the NCNT/ZIF-8 hybrid resembles that of the pure ZIF-8 [53], which indicates that the NCNT incorporation did not devastate the formation of the ZIF-8 crystalline structure. Moreover, the well-defined framework units existed in the synthesized hybrid nanoadsorbents. The presence of strong peaks implies the good crystallization of the adsorbents and no

The DBT adsorption at different concentrations

Fig. 9 exhibits the effect of the initial DBT concentration (100–1000 ppm) on the desulfurization performance of the prepared nanosorbents. For the DBT concentration of 100 ppm, the adsorption capacity of the ZIf-8 and NCNT/ZIF-8 was 69.1 and 81.2 mg/g, respectively. It was observed that along with increasing the initial DBT concentration, the removal rate was decreased. It is attributed to the saturation of nanoadsorbents by the DBT at high concentrations. A nanoadsorbent must have a high

Conclusion

The ZiF-8 and NCNT/ZIF-8 nanoadsorbents were successfully synthesized by the hydrothermal method and used for removal of the dibenzothiophene (DBT) from the liquid phase. The effects of various parameters on the adsorption performance of the nanoadsorbents were pursued, such as the adsorbent type, adsorption time, adsorbate concentration, and temperature. The modeling of adsorption kinetics and thermodynamics was successfully performed. The introduction of the NCNT into the ZiF-8 led to

CRediT authorship contribution statement

Maryam daraee: Conceptualization, Methodology, Investigation, Writing - original draft. Raheleh Saeedirad: Conceptualization, Methodology, Investigation, Writing - original draft. Ebrahim Ghasemy: Conceptualization, Investigation, Writing - review & editing. Alimorad Rashidi: Conceptualization, Methodology, Investigation, Supervision.

Declaration of Competing Interest

The authors declare that they have no conflict of interest.

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