Competition Adsorption, Equilibrium, Kinetic, and Thermodynamic

Article pubs.acs.org/jced

Competition Adsorption, Equilibrium, Kinetic, and Thermodynamic Studied over La(III)-loaded Active Carbons for Dibenzothiophene Removal Jianhong Wang, Huipeng Liu, Hao Yang,* Congzhen Qiao, and Qian Li College of Chemistry and Chemical Engineering, Institute of Fine Chemistry and Engineering, Henan University, Kaifeng 475004, China ABSTRACT: La(III)/AC adsorbents were prepared by ultrasonic impregnation. The optimal preparation conditions of La(III)/AC were studied by calcination temperatures and La-loaded amount. Competition adsorption desulfurization from aromatics and adsorption mechanism were investigated onto La(III)/AC. Benzene, methylbenzene and naphthalene were selected to explore adsorption mechanism on dibenzothiophene (DBT). The results showed that La(III)/AC with calcination temperature for 803 K and La-loaded amount for 2 wt % possessed more effective performance of desulfurization. La(III) helped to enhance adsorptive capacity and selectivity, the effective degree of the aromatics over La(III)/AC for DBT removal decreased in the order naphthalene, methylbenzene and benzene. It was founded that the adsorption mechanism was mostly like to be π complex. Adsorption isotherms and kinetics were investigated, and the parameters were obtained. Results showed that adsorption isotherms could be well fitted by Langmuir model and maximum adsorption capacities increased with the temperature increasing. Adsorption kinetics could be represented by pseudo-second-order model, which suggested that chemical reaction along with electronic transfer and share seemed significant in the adsorption rate-controlling step. Adsorption process was spontaneous and exothermic.

1. INTRODUCTION Sulfur compounds in transportation fuels can cause air pollution and acid rain because they are converted into SOx species during combustion.1 Because of more rigorous regulations for sulfur contents (≤10 μg·L−1) in transportation fuels, desulfurization has become a hot topic and focus for fuels.2 The enhanced interests in ultradeep desulfurization are ascribed to the great need for making ultralow sulfur fuels to avoid pollution of acid rain, which essentially requires zerosulfur fuels.3 Therefore, production of clean fuels has become imminent for environmental and energy aspects.4 Conventional hydrodesulfurization (HDS) can efficiently remove thiols, sulfides, and disulfides, but less efficiently remove thiophene (T) and its derivatives due to very high hydrogen5,6 to produce clean fuel to meet more stringent regulations based on conventional approaches for HDS of diesel fuels. If the sulfur level reduces to 0.1 ppmwS, the volume of catalyst bed will have to be increased seven times as that of the current HDS catalyst bed; however, the reductive desulfurization process requires higher operation temperature and pressure for kinetics and bigger reactor sizes, all of these involve an unavoidable and significant capital investment, which are more expensive than before.7−10 Furthermore, reductive desulfurization process can cause a significant reduction of the octane number due to the nonselective hydrogenation of olefins and aromatics and consume more hydrogen.11 Hence, © XXXX American Chemical Society

several new desulfurization technologies, such as adsorption, extraction, oxidation, and bioprocess, have been explored for producing clean fuel.12−15 Among them, adsorption desulfurization (ADS) is superior to others, because ADS can be accomplished at mild environmental conditions.16 An efficient adsorbent is vital to ADS. Many porous materials have been studied as adsorbents such as molecular sieve, activated carbons (ACs), metallic oxide (magnetic alumina, zinc oxide, etc.), and other materials.17−21 Among the porous materials, ACs are universally known adsorbents because of large specific surface area, high density of surface functional groups (lactonic, carboxyl, and phenolic hydroxyl groups, etc.) and abundant porous structure.22 According to reports, ACs have adsorption properties for organ sulfur compound removal due to importance of structural and chemical heterogeneity of ACs. Furthermore, the structural heterogeneity (physical adsorption) plays a crucial role on adsorption capacity over AC, and chemical heterogeneity(chemical adsorption) decides the selectivity of AC.23 Therefore, chemical modification of ACs to improve selectivity is a meaningful research direction. Al-Ghouti20 showed that Mn-AC had a better adsorption value than AC, and instructed that AC was mainly dependent on Received: June 1, 2016 Accepted: August 17, 2016

A

DOI: 10.1021/acs.jced.6b00431 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

was measured by AXIS ULTRA from Kratos England to confirm the valence state of La on La(III)/AC. 2.3. Adsorption Experiment. 2.3.1. Preparation of Model Fuels. The initial model oils were prepared at first. DBT, hardly removed by HDS, was selected as sulfur source in model oil. n-Octane was used as solvent for its similar property to commercial fuels. Different initial sulfur concentration of model oils with 6.25, 7.81, 9.38, 12.50, 15.63, 18.75, and 21.88 mmol·L−1 were prepared. To explore the effects of competition components on competitive ADS of model oil over La(III)/ AC, several kinds of model oils with 9.38 mmol·L−1 were made by adding 5 wt % (quality percentage) of naphthalene, benzene, and methylbenzene, respectively. In this paper, the initial sulfur concentration was tested in each experiment. The analytical precision reached ±0.01 mmol·L−1. The volume of competition components was calculated by eq 1

hydrophobic−hydrophobic interactions, Mn/AC adsorption controlling mechanism was π-complex that d-orbital in Mn4+ and S atoms combines. Nunthaprechachan4 had found that sewage sludge-derived activated carbons (S-ACs) by KOH activation exhibited the highest adsorption capacity and pointed out that hydrogen bonding interactions was mainly adsorptive mechanism. High adsorption capacity and selectivity of ADS were being hot topics. In this work preparation conditions (calcination temperatures, La-loaded amount) for La(III)/AC were confirmed. Adsorption selectivity of adsorbents (La(III)/AC) was investigated by comparing to HNO3/AC in model oil with competition components (benzene, toluene, and naphthalene). Adsorption mechanism was discussed by comparison adsorption T between DBT onto La(III)/AC. Meanwhile, adsorption isotherms, kinetics and thermodynamics of adsorption desulfurization process were described by static adsorption test.

2. EXPERIMENTAL SECTION 2.1. Materials. Coconut-shell-based active carbon (AC) (20−40 mesh) was purchased from KeXing Factory China. nOctane (98%) and La(NO3)3·6H2O were obtained from Institute of Guangfu Fine Chemical Research China. Nitric acid, dibenzothiophene (DBT, 98%), naphthalene (AR), and thiophene (98%) were gained from Kermel Co. Ltd., China. Benzene (AR) was supplied by Fuyu Fine Chemical Co. Ltd., China. Methylbenzene (≥99.5%) was purchased from Haohua Chemical Reagent Co. Ltd., China. The calibration table samples of sulfur (sulfur concentration: 100, 200, and 300 ppmwS) were obtained from Jiangsu Jiangfen Electro analytical Instrument Co. Ltd., China. 2.2. Preparation of Adsorbents. Impregnation was adopted to prepare adsorbent. AC was boiled in distilled water for 2 h, and then dried at 383 K for 12 h. Then, 50 g AC after washing was pretreated in HNO3 (30 wt %) at 298 K for 4 h with magnetic stirring, subsequently extensively washed to neutral. After pretreatment, the AC was marked as HNO3/AC. Adsorbents were prepared by impregnating HNO3/AC in La(NO3)3 solution in water bath oscillator (T = 308 K, r = 90 rpm) for 12 h followed by ultrasonic treatment for 0.5 h, and then dried at 383 K for 24 h. Finally, the samples were calcined for 4 h at different temperatures with a heating rate of 3 K· min−1 in N2 atmosphere. After impregnation, the samples were marked as La(III)/AC. 2.2.1. Characterization. Nitrogen adsorption isotherms were carried out on Automated Physics and Chemisorption Analyzers (Autosorb-iQ-MP-C) from Quantachrome American. Before measurement, all adsorbents were outgased at 573 K for 5 h. Approximately 0.040 to 0.045 g of each adsorbent was used for these analyses. The specific surface area (SBET) was obtained by the Brunauer−Emmett−Teller (BET) method, the total pore volume (Vt) was calculated from the last point on the isotherm. To calculate narrow micropores (Vnmicro) with methylbenzene > benzene. AC treated by HNO3 could increase the oxygen-containing groups on the carbon surface. Previous studies pointed that HNO3/AC enhanced its ADS performance by increasing the amount of oxygen-containing groups on the AC surface, which E



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To sum up, π electron density played a crucial impact on removal DBT over HNO3/AC and La/AC. Adsorptive mechanism of La(III)/AC was most likely π complex. 3.5. Equilibrium Isothermal Adsorption. Adsorption equilibrium isotherm was investigated by using model oil (6.25, 7.81, 9.38, 12.50, 15.63, 18.75, and 21.88 mmol·L−1 DBT, respectively) with a contact time of 4 h at 293, 303, and 313 K. Results were described in Figure 13. Initially, qe increased

suggested that polar−polar interaction,34 acid−base36 interaction, or π−H interaction (one of weak hydrogen bonds) played the leading role.21 Adsorption competitiveness closely relates to π electron density of competitive components, and the sequence of π electron density increases by the order of benzene, methylbenzene, and naphthalene.21 Experiment datum showed that the order of adsorption competitiveness: naphthalene > methylbenzene > benzene. This meant that π− H interaction played a significant role on selectivity removal of DBT over HNO3/AC. La(III)/AC showed better selectivity comparison with HNO3/AC, and their orders of adsorption competitiveness were similar. Results indicated that adsorptive selectivity over La(III)/AC had direct relation with π electron density, and the mechanisms of La(III)/AC might comply with the π complex.37 However, La(III)/AC showed better adsorption selectivity than HNO3/AC which was attributed to different adsorption mechanisms onto La(III)/AC. It was reported that adsorption mechanisms might be direct metal-S (M-S) interaction between adsorbed DBT molecule and adsorbents.38 In order to obtain exact adsorptive mechanism, further experiments were carried out to explore them. Adsorption removal of T and T with naphthalene was attempted at same experiments conditions with above. Results were described in Figure 12. It was reported that DBT and T were aromatic

Figure 13. Equilibrium isothermal adsorption of DBT onto La(III)/ AC (0.2 g 2 wt % adsorbent, voil, 20 mL, initial concentrations: 6.25, 7.81, 9.38, 12.50, 15.63, 18.75, and 21.88 mmol·L−1, adsorption time 4 h).

sharply with the increase of the equilibrium concentration of DBT in the solution (ce, (mmol·L−1)), subsequently changed slightly. Results indicated that most of the DBT may be removed by La(III)/AC for lower initial concentration of DBT. Langmuir isotherm model40 has been widely used to describe adsorption isotherms. It has produced good agreement with a wide variety of experimental datum, when the adsorbents adsorption is quite uniform and monolayer adsorption. In general, Langmuir isotherm model can be expressed as eq 3 KLceqmax qe = 1 + KLce (3)

Figure 12. Effect of naphthalene on the sulfur adsorption (5 wt % naphthalene, voil, 10 mL; c0:9.38 mmol·L−1; adsorbent, 2 wt % La(III)/ AC, 0.1 g; adsorption temperature, 313 K; adsorption time, 4 h).

At the adsorption equilibrium, the value of qmax (mmol·L−1) is the theoretical maximum adsorption capacity; KL is Langmuir equilibrium constant. The Langmuir equation could be represented in the linear form ce c 1 = e + qe qmax KLqmax (4)

sulfur-containing compounds with three and one aromatic rings, respectively. Naphthalene possesses two aromatic rings without sulfur atom. For same experiment conditions and adsorbents (2 wt % La/AC), if the adsorptive mechanisms of La(III)/AC was direct metal−S (M−S) interaction between adsorbed DBT molecule and adsorbents, the decreases of DBT removal should be approximate to the result of T before and after the addition of naphthalene. Interestingly, naphthalene adsorption caused decrease of sulfur removal by 55.84% and 68.79% as to DBT and T, respectively. The results indicated that the adsorptive mechanism of La/AC was most likely π complex. It should be noted that DBT removal was always much higher than that of T. It was reported that molecule diameters of DBT and T were 0.65 and 0.53 nm, respectively. Recent research showed that micropores with sizes around molecule diameter of adsorbate and the stronger electronic density was beneficial for the adsorption from oil.39 According to Table 2 and Figure 8, proportion of pores (around 0.53 nm) was less than that of 0.65 nm, and electron density of DBT was stronger than that of thiophene.39

Figure 14 showed the linear fitting results by Langmuir isotherm model. Results and correlation coefficient (R2) were shown in Table 4. The correlation coefficient (R2) at different temperatures exceeded 0.99, which indicated that Langmuir isotherm model was suitable to fit the experiment data. According to Figure 14, we found that the slope of the fitting line decreased slightly with temperature increasing, which indicated that qmax enhanced slightly with the increasing in temperature. The similar conclusion was observed by Song.5 They found that, qmax increased with the increasing of temperature, which suggested that the affinity between adsorbent and adsorbate increased with the temperature increasing. 3.6. Adsorption Kinetics. 3.6.1. Pseudo-First-Order and Pseudo-Second-Order Models. It is understandable that the F



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Figure 14. Linear fitting of Langmuir model by mathematics treatment at different temperatures (293, 303, and 313 K).

Figure 15. Nonlinear fitting according to PFO and PSO models (voil, 20 mL; c0, 9.38 mmol·L−1; adsorbents, 0.2 g; 2.0 wt % La(III)/AC; adsorption temperature, 323 K).

Table 4. Parameters of Langmuir Adsorption Isotherm T/K

qmax/mmol·g−1

KL/L·mmol−1

R2

293 303 313

0.742 0.801 0.966

1.040 0.714 0.450

0.997 0.995 0.993

the adsorption kinetics. The value of qe calculated from the PSO model was 0.532 mmol·g−1 and was very close to experimental value (0.514 mmol·g−1, adsorption time for 24 h). Kinetics simulation results implied that chemical reaction (involving electronic transfer and share) seemed significant in the rate-controlling step.41 3.7. Thermodynamics. Adsorption Thermodynamics5 was investigated in this paper, because it helped to analyze the ultimate of adsorption process, and could explain the characteristics and regularity of adsorption DBT onto La(III)/AC. The Gibbs free energy (ΔG, kJ·mol−1) equation can be expressed by the Langmuir equilibrium constant KL in the form for eq 8

study of adsorption kinetics in adsorption DBT process is significant as it provides valuable insights into the reaction pathways and into the mechanism of adsorption reaction. In addition, kinetics describes the solute uptake rate which in turn controls the residence time of adsorbate uptake at the solid− solution interface.41 Therefore, to get a better understanding of the adsorption process, pseudo-first-order (PFO) and pseudosecond-order (PSO)41,42 were applied to fit the adsorption kinetics, which were frequently used as the adsorption kinetic models. The PFO and PSO models expression were given as eqs 5 and 6 qt = qe(1 − e−k1t )

qt =

ΔG = −RT ln KL

The value of ΔG can be calculated by the KL shown in Table 3, the values of ΔG are −16.922, −16.553, and −15.898 kJ· mol−1 at 293, 303, and 313 K, respectively. These results indicated that the adsorption of DBT onto La(III)/AC was a spontaneous process. Considering ΔG = ΔH − TΔS, eq 8 can be rearranged

(5)

k 2tqe2 1 + k 2qet

ΔH ΔS + (9) RT R −1 where ΔH (kJ·mol ) is the adsorption enthalpy and ΔS (J· mol−1·K−1) is the entropy of adsorption. The relationship between ln KL and 1/T (10−3 K−1) had been plotted in Figure 16. According to the data shown in Table 4, and then, the values of ΔH and ΔS were calculated by the slope and intercept of the line in Figure 16. The values of ΔH and ΔS were −31.890 kJ·mol−1 and −50.930 J·mol−1·K−1, respectively. The results of ΔH and ΔS indicated the adsorption of DBT onto La(III)/AC was an exothermic process and a decrease in the degree of freedom of the adsorbed species.

(6)

ln KL = −

where k1 (min−1) and k2 (g·mmol−1·min−1) were the rate constant of PFO and PSO pseudo-first-order and pseudosecond-order, respectively. The root-mean-square error (RMSE) was used to estimate the fitness of kinetics models, RMSE was defined as ⎡1 RMSE = ⎢ ⎢⎣ n

n i=1

⎤1/2

2⎥

∑ (yi − ycal )

⎥⎦

(8)

(7)

where n is the number of the experimental points, yi is the experimental data of adsorptive account; and ycal is the adsorptive account calculated by eqs 5 and 6. In the following, two adsorption kinetics models including PFO and PSO models were evaluated to fit the kinetics of the adsorption process on La(III)/AC. The results were shown in Figure 15 and Table 5 (model parameters), the PSO adsorption kinetics model gave very good fits to the experimental data. The correlation coefficient R2 (Table 5) of PSO model was 0.998 and higher than that of PFO model(0.978), meanwhile the Root Mean Square Error RMSE (Table 5) of PSO model was 0.006 and more close to 0 than that of PFO model(0.020), which showed that PSO model was more suitable to describe

4. CONLUSIONS The optimum prepared conditions (calcinations temperature, 803 K; amount of loading, 2 wt %) were confirmed by impregnation. Competition adsorption effects over La(III)/AC for DBT removal were in the order of naphthalene > methylbenzene > benzene, which indicated that electron density played a significance role on adsorption DBT over La(III)/AC and HNO3/AC. Adsorption mechanism of La(III)/ AC was likely to be π-complex, which was based on G



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Table 5. Parameters of Adsorption Kinetic Models pseudo-second-order qe(exp)/mmol·g

−1

qe(cal)/mmol·g

0.514

−1

0.461

pseudo-second-order

k1/min−1

R2

RMSE

0.036

0.978

0.02

experiments about adsorption thiophene and DBT over 2 wt % La(III)/AC. The equilibrium isotherm indicated that the adsorption can be represented by the Langmuir model, qmax increased with an increase of temperature, KL decreased with temperature increasing. Kinetics studies indicated that the adsorption process can be well represented by pseudo-secondorder. Results demonstrated that chemical reaction involving electronic transfer and share seems significant in the ratecontrolling step. Thermodynamics indicated that the adsorption process onto La(III)/AC was spontaneous and exothermic.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

0.532

−1

k2/(g·mmol−1·min−1)

R2

RMSE

0.0845

0.998

0.006

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Figure 16. Plot of ln KL versus 1/T.

■

qe(cal)/mmol·g

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Competition Adsorption, Equilibrium, Kinetic, and Thermodynamic

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