NiO Aerogel Sorbents for Desulfurization by π-Complexation

Article pubs.acs.org/IECR

New SiO2−NiO Aerogel Sorbents for Desulfurization by π‑Complexation: Influence of Molar Ratio of Si/Ni Jing Chen, Bo Zhang,* Guangw Miao, and Jun Men College of Chemical Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou 310014, China ABSTRACT: SiO2−NiO aerogels with different molar ratios of Si/Ni (n(Si/Ni)) were prepared by a sol−gel method and atmospheric pressure drying technology. The morphology and structure of SiO2−NiO aerogels were characterized by XRD, BET, XRF, and SEM methods. The equilibrium adsorption performance of thiophene or benzothiophene on the SiO2− NiO aerogels were studied at three different adsorption temperatures (298, 313, 333 K). The adsorption equilibrium data of thiophene on SiO2−NiO aerogels were more in line with the Temkin model at the lower temperature, while they were well fitted with both Langmuir and Temkin isotherms at the higher temperature. The maximum saturation adsorption capacities obtained by the Langmuir isotherms at different temperature decreased in the order qm(298 K) > qm(313 K) ≈ qm(333 K). The maximum saturation adsorption capacities of thiophene or benzothiophene on SiO2−NiO aerogels with different n(Si/ Ni) values at 298 K decreased in the order SiO2−NiO-23 > SiO2−NiO-74 > SiO2−NiO-6. The maximum saturation adsorption capacities of thiophene and benzothiophene on SiO2−NiO-23 aerogels are 6.4564 and 2.1316 mgS/g, respectively. Both the amount of adsorption active sites and the textural structure of aerogels could significantly affect the adsorption performance. The higher the nickel content, specific surface area, pore volume, and larger pore size of the aerogel, the larger the adsorption capacity. The absolute value of adsorption heat of thiophene on SiO2−NiO aerogels increased gradually with the increase of n(Si/Ni) (the reduction of the Ni content).

1. INTRODUCTION Deep removal of sulfur from fuels has received more and more attention in research and development worldwide, not only because of health and environmental considerations but also due to the great need for producing ultralow-sulfur fuels.1,2 The new U.S. Environmental Protection Agency sulfur standards require that the sulfur contents in gasoline and diesel fuels be reduced to less than 30 and 15 ppm, respectively.3 The organosulfur compounds in liquid hydrocarbon fuels are poisonous to the catalysts used in fuel processors; sulfur content in the fuels must be reduced to less than 1 ppm for fuel cell application.4 Traditionally, the hydrodesulfurization (HDS) process is highly efficient in removing thiols and sulfides in the fuels. However, the aromatic ring in thiophene and its derivatives make their C−S bonds quite stable, and for those molecules the hydrodesulfurization reactions are much more difficult than for other sulfur-containing molecules such as thiols, sulfides, and disulfides.5 Among various deep desulfurization technologies, adsorption desulfurization with high selectivity, low operation cost, and mild conditions has been widely studied. Among those, π-complexation adsorption, whose selectivity is higher than that of physical adsorption because the strength of π-complexation bonds is stronger than that of van der Waals interactions, while at the same time they are also weak enough to be broken by moderate changes in temperature or pressure, provides the opportunity for selective adsorption of sulfur compounds from fuels.2,6 Hence, π© XXXX American Chemical Society

complexation desulfurization is considered to be one of the most promising ultradeep desulfurization methods. Transition metal ions such as Cu2+, Ag+, Ni2+, and Co2+ could adsorb thiophene and its derivative molecules through πcomplexation, in which the usual σ bonds can be formed between cations’ empty s-orbitals and the thiophenic aromatic rings; in addition, their d-orbitals can feed back electrons to the antibonding π-orbitals (π*) of the aromatic rings.7 π-Complexation adsorbents loading the transition metal ions onto the porous materials showed higher adsorption performance for the removal of thiophene and its derivatives from fuels. Zeolites such as 13X,8 ZSM-5,9 and HY10,11 have been widely studied in the desulfurization of transportation fuels due to their high ion exchange and size-selective adsorption capacities as well as thermal and mechanical stabilities.12 Yang and coworkers13−15 reported that the ion-exchanged Y zeolites Cu(I)Y, AgY, and NiY are selective for the removal of sulfur compounds in fuels by π-complexation of sulfur compounds due to the transition metal ions in the zeolites. Campared to Y zeolites containing other cations, Cu(I)Y zeolite exhibited higher selectivities and adsorption capacity. Cu(I)Y could adsorb 0.55 mmol/g for thiophene, 0.83 mmol/g for benzothiophene, and 1.0 mmol/g for dibenzothiophene.15 Received: January 18, 2016 Revised: April 8, 2016 Accepted: April 13, 2016

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DOI: 10.1021/acs.iecr.6b00211 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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work, SiO2−NiO aerogels with different n(Si/Ni) values were prepared by a sol−gel method and atmospheric pressure drying technology. The morphology and structure of SiO2−NiO aerogels were characterized by X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET) analysis, X-ray fluorescence (XRF), and scanning electron microscopy (SEM). The equilibrium adsorption performances of thiophene and benzothiophene on the SiO2−NiO aerogels were investigated at three different temperatures. The results were fitted by three kinds of adsorption isotherm models (Langmuir, Freundlich, and Temkin models). Meanwhile, combined with the Langmuir model data and the Clausius−Clapeyron equation, the heats of adsorption of thiophene on SiO2−NiO aerogels with different n(Si/Ni) values were estimated. The influence of the molar ratio of Si/Ni on the equilibrium adsorption performance of thiophene and benzothiophene over the SiO2−NiO aerogels was studied.

Song and co-workers16−18 reported that ion-exchanged Y zeolites (Cu, Ni, Zn, Pd, and Ce) are effective for the adsorption of sulfur. Xiao-Lin Tang and Li Shi19 demonstrated that cuprous ions supported on HY−Al2O3 show a high sulfur capacity of 10 mg of sulfur/g of sorbent under the best preparation conditions. Wang and Yang20 also showed that metal halides supported on activated carbon (AC), such as CuCl/AC or PdCl2/AC, were effective in the desulfurization of JP-5 jet fuel. Sung Hwa Jhung and his co-workers21 also prepared CuCl-supported carbon adsorbents through a facile and mild reduction of CuCl2 to improve the desulfurization capability of the porous activated carbon. The CuCl-supported AC adsorbents showed a significantly improved maximum adsorption capacity and the potential capability to adsorb refractory sulfur-containing compounds from liquid fuels. But π-complexation adsorbents based on the zeolites and activated carbon presented lower adsorption capacities for macromolecular thiophene derivatives because most pores of zeolites and activated carbon are microporous. The monolayer CuCl/γAl2O3 sorbent was studied for desulfurization of a commercial jet fuel (364.3 ppmw S) and a commercial diesel (140 ppmw S) by Ralph T. Yang and his co-workers.22 The results showed that the CuCl/γ-Al2O3 adsorbent was capable of removing 6.4 and 11.2 mg of sulfur/g of jet fuel at breakthrough (at qm(313 K) ≈ qm(333 K). The Langmuir isotherm constant KL and the Temkin isotherm constant KT increased, while the Temkin isotherm constant B decreased with an increase of temperature. But KL, KT, and B values at 313 and 333 K were similar, respectively. The constants KL, KT, and B were relevant to the interaction force between adsorbate and adsorbent. The π-complexation bond can be formed between transition metal ions Ni2+ with the electronic configuration of 3d84s0 and the thiophenic aromatic rings. Therefore, thiophene in model fuels could be adsorbed on SiO2−NiO aerogels through π-complexation interaction. The π-complexation bond is stronger than van der Waals interactions but weaker compared with typical covalent bonds.33 Therefore, π-complexation adsorption has higher selectivity and easier desorption compared to physical adsorption and chemical adsorption, respectively. In general, raising the temperature or decreasing the pressure can break a π-complexation bond. Thus, in this experiment, desorption phenomenon could occur with an increase of temperature, resulting in the lower adsorption capacity at high temperatures. The results indicated that thiophene can be easily adsorbed on the composite SiO2−NiO aerogels at room temperature and desorbed provided the temperature was properly raised. Though the adsorption of thiophene on SiO2−NiO aerogels is more fitted to the Temkin model at 298 K, the maximum saturated adsorption capacity can only be calculated by the Langmuir isotherm model. Therefore, the adsorption data of thiophene on SiO2−NiO aerogels with different n(Si/Ni) values obtained at 298 K were fitted by the Langmuir isotherm model and the adsorption isotherms are shown in Figure 5.

Figure 3. SEM images of SiO2−NiO aerogel samples. (a) SiO2−NiO6; (b) SiO2−NiO-74.

three-dimensional structure formed by accumulation of nanoscale particles and their particle sizes are similar (about 50−70 nm). SiO2 −NiO-74 aerogel presents a looser particle accumulation, i.e. higher porosity, than that of SiO2−NiO-6 aerogel, consistent with the results of the BET analysis. The nickel content has no obvious effect on the matrix particle size of the composite aerogels, but has a significant impact on the porosity and surface area of the aerogels. 3.2. Adsorption Isotherms. Adsorption isotherm models are commonly used to describe the adsorption and its mechanisms. Here, three isotherm modelsLangmuir, Freundlich, and Temkinwere applied to analyze the above equilibrium experimental data of the sorption of thiophene or benzothiophene on SiO2−NiO aerogel at 298, 313, and 333 K. The Langmuir isotherm has been successfully used to characterize the monolayer adsorption process, and the adsorption sites on the surface of the adsorbent have energy uniformity. There is no interaction between adsorbed molecules. Langmuir isotherm adsorption is an ideal and can be represented by the following equation: Ce C 1 = + e qe KLqm qm

(4)

(2)

where qe is the equilibrium adsorption capacity (mg/g), Ce is the equilibrium adsorption concentration (mg/L), qm (mg/g) is the maximum adsorption capacity corresponding to the complete monolayer coverage, and KL (L/mg) is the Langmuir constant which is related to the adsorption energy and can be obtained from the plot of Ce/qe versus Ce. The adsorption which deviated from Langmuir type was called nonideal adsorption. There are many reasons for deviation such as the nonuniformity of the surface, the interaction between the molecules, and multilayer adsorption. For these reasons, several experienced adsorption isotherms were established and the most influential are the Freundlich isotherm and the Temkin isotherm. The Freundlich isotherm (Freundlich, 1906) is derived by assuming a heterogeneous surface with a nonuniform distribution of the heat of sorption over the surface. It can be linearly expressed as follows: 1 log(qe) = log KF + log(Ce) (3) n where KF and n are the Freundlich parameters related to adsorption capacity and adsorption intensity, respectively. If the value of 1/n is lower than 1, it indicates a normal Langmuir isotherm; otherwise, it is indicative of cooperative adsorption. The Freundlich constants can be obtained from the plot of log qe versus log Ce. D

DOI: 10.1021/acs.iecr.6b00211 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Langmuir adsorption isotherms of thiophene on SiO2−NiO aerogels with different n(Si/Ni) values at 298 K.

According to Table 3, the maximum saturation adsorption capacities of SiO2−NiO aerogels with different n(Si/Ni) values Table 3. Langmuir Adsorption Constants of Thiophene on SiO2−NiO Aerogels with n(Si/Ni) at 298 K SiO2−NiO aerogels content of Ni (mol %)XRF qm (mg/g) KL (L/mg) R2

Table 2. Constants in Langmuir, Freundlich, and Temkin Isotherm Equations for the Adsorption of Thiophene on SiO2−NiO-23 Aerogel temperature constant

298 K

313 K

333 K

Langmuir

qm (mg/g) KL (L/mg) R2 KF 1/n R2 KT (L/mg) B R2

6.4562 0.0008 0.9593 0.0168 0.7511 0.9755 0.0098 1.2128 0.9936

2.9954 0.0022 0.9583 0.0568 0.5250 0.9057 0.0178 0.7147 0.9487

2.8751 0.0020 0.9849 0.1286 0.3918 0.9734 0.0174 0.6753 0.9880

Freundlich

Temkin

SiO2−NiO-23

SiO2−NiO-74

12.75 1.65 0.0018 0.9767

3.07 6.46 0.0008 0.9593

0.84 6.05 0.0007 0.9558

calculated by the Langmuir adsorption isotherm decreased in the order SiO2−NiO-23 > SiO2−NiO-74 > SiO2−NiO-6. SiO2−NiO-23 aerogel has the maximum saturation adsorption capacity. Hence, a too high or too low Ni content is not beneficial for the adsorption of thiophene on SiO2−NiO aerogels. Theoretically, the more Ni content, the more adsorption active sites, and the larger the amount of thiophene adsorbed on SiO2−NiO aerogels by π-complexation. The data in Table 3 show that the nickel content of SiO2−NiO-6 aerogel is 4 and 15 times that in SiO2−NiO-23 and SiO2−NiO-74 aerogels, respectively. Hence, the number of adsorption active sites of SiO2−NiO-6 aerogel is far more than that of SiO2− NiO-23 and SiO2−NiO-74 aerogel. Besides the adsorption active site amount, the textural structure of aerogels could also significantly affect the adsorption performance. The bigger the specific surface area, the more the number of active sites exposed, and the better the adsorption performance. The larger pore volume could lead to the smaller internal diffusion resistance of the adsorbate in the aerogel. It can be seen in Table 1 that the specific surface area and the pore volume of SiO2−NiO-6 aerogel are only 275.7 m2/g and 0.18 cm3/g, respectively, which are 37.5% those of SiO2−NiO-23 aerogel. Although SiO2−NiO-6 aerogel has more adsorption active sites, the smaller specific surface area may make most of the active sites be surrounded in the skeleton particles of aerogel, resulting in that the number of active sites fully exposed is rare. The lower pore volume of SiO2 −NiO-6 aerogel also significantly enhances the internal diffusion resistance of adsorbate in the aerogel, so the lower specific surface area and pore volume of SiO2−NiO-6 aerogel lead to its maximum adsorption capacity being far less than that of the SiO2−NiO-23 aerogel. Although the nickel amount in SiO2−NiO-74 aerogel is

Figure 4. Langmuir (a), Freundlich (b), and Temkin (c) adsorption isotherms of thiophene on SiO2−NiO-23 at different temperatures.

isotherm model

SiO2−NiO-6

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DOI: 10.1021/acs.iecr.6b00211 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research only 33.3% that of SiO2−NiO-23 aerogel, its specific surface area, pore volume, and pore diameter are higher than those of SiO2−NiO-23 aerogel, resulting in a more full exposure of active sites and a reduction in the internal diffusion resistance of adsorbate, so that the maximum adsorption capacity is only slightly lower than that of SiO2−NiO-23 aerogel. Therefore, only when the nickel content, specific surface area, pore volume, and pore size of the aerogel are higher, is the adsorption capacity larger. The Langmuir adsorption isotherms of benzothiophene on SiO2−NiO aerogels with three different n(Si/Ni) values at 298 K are shown in Figure 6.

Figure 7. Maximum saturated adsorption capacity of thiophene and benzothiophene on SiO2−NiO aerogels with different n(Si/Ni) values.

thiophene on SiO2−NiO aerogels with different n(Si/Ni) values were estimated and are shown in Table 5. The Table 5. Adsorption Heat of Thiophene on SiO2−NiO Aerogels with Different n(Si/Ni) Values SiO2−NiO aerogel ΔH (kJ/mol)

Benzothiophene also is adsorbed on SiO2−NiO aerogels by π-complexation interaction. The effect of n(Si/Ni) of SiO2− NiO aerogels on adsorption results of benzothiophene is similar to those on thiophene. As shown in Table 4, the maximum Table 4. Langmuir Adsorption Constants of Benzothiophene on SiO2−NiO Aerogels with Different n(Si/Ni) Values at 298 K SiO2−NiO-6

SiO2−NiO-23

SiO2−NiO-74

1.3944 0.0008 0.9435

2.1316 0.0006 0.9684

1.9823 0.0005 0.9783

SiO2−NiO-74

−28.89

−35.87

−41.19

4. CONCLUSION SiO2−NiO aerogels with different n(Si/Ni) values were prepared by a sol−gel method and atmospheric pressure drying technology. The equilibrium adsorption performances of thiophene on the SiO2−NiO aerogels with different n(Si/Ni) values were studied at three different temperatures (298, 313, 333 K). The results were fitted by three kinds of adsorption isotherm models (Langmuir, Freundlich, and Temkin). At low temperature, the adsorption equilibrium data of thiophene on SiO2−NiO aerogels were more in line with the Temkin model, while at higher temperatures, they agreed well with both the Langmuir and Temkin isotherms. The maximum saturated adsorption capacity obtained by the Langmuir isotherm at different temperatures decreased in the order qm(298 K) > qm(313 K) ≈ qm(333 K). The maximum saturation adsorption capacities of thiophene or benzothiophene on SiO2−NiO aerogels with different n(Si/Ni) values decreased in the order SiO2−NiO-23 > SiO2−NiO-74 > SiO2−NiO-6. The maximum adsorption capacity of benzothiophene is lower than that of thiophene. Both the adsorption active site amount and the textural structure of aerogels could significantly affect the adsorption performance. The higher the nickel content, specific surface area, pore volume, and larger pore size of the aerogel, the larger the adsorption capacity. The absolute value of the

SiO2−NiO aerogels constant

SiO2−NiO-23

adsorption heat ΔH is a negative value, indicating that the adsorption process is exothermic. With the increase of n(Si)/ n(Ni) (the reduction of the Ni content), the absolute value of the adsorption heat increases gradually, indicating an enhance of interaction strength between the adsorption active sites and thiophene. The lower the nickel content, the higher the specific surface area, pore volume, and pore diameter, leading to a higher dispersion degree of adsorption active sites in SiO2− NiO aerogels. Therefore, the π-complexation interaction between highly dispersed Ni2+ in the SiO2−NiO aerogels with thiophene is strong.

Figure 6. Langmuir adsorption isotherms of benzothiophene on SiO2−NiO aerogels with different n(Si/Ni) values at 298 K.

qm (mg/g) KL (L/mg) R2

SiO2−NiO-6

saturation adsorption capacities of benzothiophene on SiO2− NiO aerogels with different n(Si/Ni) values decreased in the order SiO2−NiO-23 > SiO2−NiO-74 > SiO2−NiO-6. A too high or too low Ni content is not beneficial for the adsorption of benzothiophene on SiO2−NiO aerogels. The reason is similar to that for the adsorption of thiophene. But Figure 7 shows that, for the same adsorbent, the maximum adsorption capacity of benzothiophene is lower than that of thiophene, which is opposite to the results reported in the literature. The reason needs to be further studied. 3.3. Adsorption Heat. The heat of adsorption directly reflects the interaction strength between the adsorbate and adsorbent.The greater the adsorption heat, the stronger the interaction between adsorbate and adsorbent. Therefore, combined with the Langmuir adsorption isotherm data of thiophene on SiO2−NiO aerogels at 298 and 313 K and the Clausius−Clapeyron equation, the heats of adsorption of F

DOI: 10.1021/acs.iecr.6b00211 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research adsorption heat of thiophene on SiO2−NiO aerogels increased gradually with the increase of n(Si)/n(Ni) (the reduction of the Ni content). The higher dispersion degree of Ni2+ in the SiO2− NiO aerogels led to the stronger π-complexation interaction between Ni2+ and thiophene.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 571 88320417. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Prof. Zheng Yifan of the College of Chemical Engineering, who provided the XRD measurement of the SiO2−NiO aerogel samples.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b00211 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX