New composite aerogel-like adsorbents for thiophene based on π-complexation
Graphical abstract
Introduction
Hydrocarbon fuels are suitable for fuel cell because of its high energy density, ready availability, safety and ease of storage. However, the sulfur-containing compounds in hydrocarbon fuels are easily poisonous to the catalysts on the electrode of the fuel cell. Therefore the sulfur concentration in the liquid hydrocarbon fuels must be reduced less than 0.1 ppm [1], [2], [3]. Although being the most common desulfurization method, hydrodesulfurization(HDS) is unable to be applied in the fuel cell devices for which must be operated at high temperature and pressure, and high hydrogen consumption. In addition, HDS is effective for the removal of thiol, sulfides and disulfides [4], [5] while it is difficult to remove the thiophene and thiophene derivatives from hydrocarbon fuels by HDS because these sulfur-containing compounds have a conjugation structure between the lone pairs on S atom and the π-electrons on aromatic ring which lead to the difficult hydrogenolysis of them. So the removal of thiophene and thiophene derivatives from hydrocarbon fuels has raised an increasing concern. A large number of desulfurization methods have been already explored so as to more efficiently remove thiophene and thiophene derivatives [6], [7], [8], [9]. Among these, adsorption desulfurization, with low energy consumption, low cost and environment-friendly, was referred to as the most promising desulfurization method [10], [11], [12].
Today, particular attention has been attached to selective adsorption desulfurization via π-complexation. The π-complexation usually forms between d-block transition metals (eg. Cu+,Ag+,Ni2+) and the chemicals with π-electrons (eg. olefins, aromatics) [13], in which the σ-bonding can be formed between the metal’s s-orbital and the π-molecular orbitals of these chemicals, at the same time, metal’s d-orbitals can feed back electrons to the antibonding π-orbital(π∗) of these chemicals [14]. The strength of π-complexation is stronger than that of van der Waals interaction which leads to the high selectivity, while at the same time it is also weak enough to be broken by the mild thermal and chemical treatment to be beneficial to the regeneration of adsorbents, provides the opportunity for selectivity adsorption of thiophene and thiophene derivatives from fuels. Up to now, a large number of desulfurization adsorbents based on π-complexation have been explored. Among them, more attention have been focus on developing mesoporous material [15], microporous material [4], porous organic polymers(POPs) [16] and metal-organic frameworks (MOFs) [17]. Yang and coworkers [18] have prepared zeolites Cu(I)Y through the autoreduction of Cu(II)Y and Ag(I)Y by the ion exchange of NaY zeolite, which were used as adsorbents for desulfurization of commercial gasoline and diesel oils. They have found that the Cu+ or Ag+ ions containing zeolites could adsorb thiophene preferentially over benzene and that Cu+ is stronger than Ag+ in bonding with thiophene, implying a higher adsorption capacity of thiophene on Cu(I)Y. Hu and coworkers [16] have loaded various transition metals species including PdCl2, Ag+, Fe3+, Ni2+, Zn2+, Cu2+ and Mg2+ into hierarchical porous polymer poly-methylbenzene (PMB) by impregnation method to fabricate a series of efficient desulfurization adsorbents, which were applied in the adsorption of thiophene(TP), benzothiophene(BT) and dibenzothiophene(DBT) from model oils. The PdCl2/PMB-40 adsorbent showed a most promising desulfurization performance with the DBT adsorption capacity as high as 25.97 mgS/g. Tran and coworkers [19] have impregnated silver nitrate on mesoporous material MCM-41 and mesoporous silica nanoparticles (MSN) whose adsorption capacities for alkylated benzothiophenes in JP-8 were 24.5 mgS/g and 32.6 mgS/g, respectively.
Aerogel is a kind of three-dimensional porous network of solid materials formed by the mutual aggregate of nanoscale particles. It appears to the very efficient catalytic supports and adsorbents because of its large surface area (500–1200 m2/g), high porosity (80–98%) and the nanoscale pore diameter [20], [21]. During the preparation of π-complexing aerogel-like adsorbents, the transition metals ions with π-complexing ability could be introduced into their skeleton particles, whose content could be adjusted in a wide range. Owing to their nanoscale skeleton particles and large surface area, the introduced transition metals ions could be fully exposed and dispersed. In addition, the high porosity and mesoporous structure of aerogel-like materials could also significantly reduce the diffusion resistance of macro-molecules during the adsorption process. Therefore, π-complexing aerogel-like adsorbents would be anticipated to exhibit excellent adsorption performance for the thiophene and thiophene derivatives. The Cu+, Ag+ and Ni2+ cations were commonly used in π-complexing adsorbents reported. However, there are fewer papers refer to aerogel-like desulfurization adsorbents containing Cu+, Ag+ and Ni2+ cations. Our group have ever prepared a series of NiO-SiO2 composite aerogels-like adsorbents with different molar ratio of Si/Ni which have been applied in the adsorption of thiophene(TP) and benzothiophene(BT) from model gasoline. The results showed that the NiO-SiO2 composite aerogels-like adsorbents exhibit good adsorption desulfurization performance [22]. In this work, the Cu2O-SiO2-50, Ag2O-SiO2-50 and NiO-SiO2-23 composite aerogels-like materials were synthesized by sol-gel method followed by ambient pressure drying technology. And their adsorption performance and mechanism for thiophene from model gasoline were also investigated by equilibrium and breakthrough adsorption experiments, respectively.
Section snippets
CuO-SiO2. and Cu2O-SiO2 composite aerogel-like materials
8 mL of tetraethoxysilane (TEOS) was mixed with water (3 mL) and ethanol (10 mL) in glass vials under stirring. The pH value of mixture was adjusted to 3.5 by 1 mol/L HCl solution. After 20 min, 0.133 g cupric acetate monohydrate (Cu(OAc)2H2O) was added into the above mixture, continuously stirring for 60 min to form the sol. Subsequently, 0.8 mol/L aqueous ammonia was added drop-wise to adjust the pH to 6.5 and then alcogel were obtained after standing about 15 min. The obtained gel was immersed in 50
Characterization of adsorbents
The XRD patterns of CuO-SiO2-50, NiO-SiO2-23 and Ag2O-SiO2-50 composite aerogels-like adsorbents were displayed in Fig. 1. It can be observed that all the samples only have a wide peaks at 2θ of 20–30°, which could be attributed to the amorphous SiO2 [23]. However, the characteristic diffraction peaks of CuO, NiO and Ag2O were not observed, indicating that the CuO, NiO and Ag2O were homogeneously dispersed in silica matrix or a tiny crystal particles which were too small to be detected, in
Conclusion
The Cu2O-SiO2-50, NiO-SiO2-23 and Ag2O-SiO2-50 composite aerogels-like adsorbents were prepared by sol-gel synthesis followed by ambient pressure drying technology. Their adsorption performance and mechanism for thiophene from model gasoline were also investigated by equilibrium and breakthrough adsorption experiments, respectively. And the adsorption equilibrium data of thiophene over the three samples were interpreted by Langmuir, Freundlich and Temkin isotherm models. The results show that
Acknowledgement
We are grateful to the financial supports from Zhejiang Provincial Science Foundation of China (grant no. LY17B070007).
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