Kinetic and Thermodynamic Studies on Raney Ni/Al2O3 Adsorption for Deep Desulfur

Cao Yongzheng; Luo Guohua,2; Zhang Lanxi; Xu Xin,2;Jin Haibo,2; Dong Sen; Guo Xuehua

(1. College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617;2. Beijing Key Laboratory of Fuels Cleaning and Adνanced Catalytic Emission Reduction Technology, Beijing 102617;3. Coal Chemical R&D Center of Kailuan Group, Tangshan 063611;4. Hebei Proνincial Technology Innoνation Center of Coal-based Materials and Chemicals, Tangshan 063018)

Abstract: How to remove trace amount of sulfur in benzene is of signi ficance, and deep desulfurization by Ni-containing absorbents is an efficient and promising method. In this paper, The Raney Ni/Al2O3 adsorbent was prepared and its kinetic and thermodynamic characteristics on adsorptive desulfurization of benzene were studied. The results showed that Raney Ni/Al2O3 adsorbent exhibited good adsorption performance. The equilibrium isotherms indicated that the adsorption of thiophene over the Raney Ni/Al2O3 adsorbent complied with the Freundlich model. The results of adsorption kinetics studies showed that the pseudo-second-order kinetics equation was more advantageous than the pseudo- first-order kinetics equation in describing the adsorption kinetics of thiophene on Raney Ni/Al2O3. The rate constant value (kp) was positively correlated with the temperature whereas the adsorption mass transfer process was mainly determined by the intraparticle and film diffusion. The adsorption thermodynamic analysis proved that ΔG0 ＜ 0, ΔH0 ＞ 0, and ΔS0 ＞ 0, implying that the adsorption was a spontaneous, entropy-increasing and endothermic process.

Key words: Raney Ni/Al2O3; desulfurization; adsorption kinetics; adsorption thermodynamic

1 Introduction

Benzene is one of the most important basic materials for chemical industry. However, some applicatons,especially those processes involving noble-metal catalysts, are mainly dependant on the sul fides amount so that deep desulfurization of benzene is of signi ficance[1].For instance, a trace amount of sulfur (～10 ng/g) will make Ru, the active component for benzene selective hydrogenation, inactive. Conventional desulfurization methods including hydrodesulfurization[2-3], extractive[4]and oxidative[5]desulfurization processes may be inadequate to remove trace amount of sulfur contained in benzene. However, adsorptive desulfurization is more advantageous due to its mild operating conditions and great potential for industrial applications[6-7]. Among those absorbents, the Ni-containing materials are more attractive due to their high efficiency in deep adsorptive desulfurization.

Kinetic and thermodynamic studies on adsorbents are particularly important to adsorption behavior and mechanism researches. The adsorption mechanism can be deduced by fitting in the data with the kinetic model and then the adsorption principles are acquired according to the adsorption thermodynamic studies. However,few studies on kinetics and thermodynamics of the Nicontaining adsorbents have been proposed. In this paper,the Raney Ni/Al2O3adsorbent was initially prepared via formation, drying and calcination. Subsequently,the benzene containing 10—100 mg/L of thiophene served as the reactants, and the adsorption behavior and mechanism of the desulfurization process on this absorbent were studied. Then the adsorption kinetics process was acquired according to the pseudo- first-order and the pseudo-second-order models. The adsorption thermodynamic parameters, especially ΔG0, ΔH0, and ΔS0, were also calculated to lay a theoretical foundation for the application of Raney Ni/Al2O3adsorbent in deep desulfurization process.

2 Experimental

2.1 Reagents and instruments

2.2 Preparation of Raney Ni/Al2O3 adsorbent

The Raney Ni/Al2O3adsorbent was prepared by the following procedures. Nickel-aluminum alloy powder(with a mass ratio of Ni/Al = 48/52, and a particle size＜ 75 μm) and boehmite (P-DF-03, with a pore volume≥0.5 mL/g, produced by the China Aluminum Corporation Shandong Branch) were mixed adequately and then a proper amount of 15% HNO3solution was added. The mixture after kneading was extruded to form extrudates, 2.0 mm in diameter and 1.0 cm in length,which were then dried at 120 °C for 2 h. The dried extrudtes were calcinated at 860 °C for 4 h to prepare the precursor. The precursor was leached at 70 °C in a proper amount of 15% NaOH solution for 4 h to obtain the Raney Ni/Al2O3adsorbent. Finally, the adsorbent was washed with deionized water for several times to remove residual alkali and was then preserved in water.

2.3 Characterization of Raney Ni/Al2O3

The powder X-ray diffraction (XRD) patterns of Raney Ni/Al2O3adsorbent were obtained by XRD (SHIMADZU XRD-7000) using CuKα radiation operated at 40 kV and 20 mA. The patterns were taken over at a 2θrange of 10° — 90° with a scanning rate of 4(°)/min. Furthermore,the Raney Ni/Al2O3adsorbent was studied through the temperature-programmed reduction (H2-TPR)experiments in a 10% H2/Ar stream and was heated from 40 °C to 1000 °C at a heating rate of 10 °C/min.

2.4 Isothermal adsorption experiment

The Raney Ni/Al2O3adsorbent was dried at 353 K in N2and then it was applied in removing the sulfur compounds from benzene. 2 g of Raney Ni/Al2O3and 60 mL of benzene with a certain concentration of thiophene(ranging from 10 mg/L to 85 mg/L) were added into a micro high-pressure reactor. The absorption process was performed under magnetic stirring at a rate of 400 r/min for 3 h with adsorption temperature controlled at 383 K, 393 K, 403 K, 413 K, and 423 K, respectively.After the adsorption process reached an equilibrium,the concentration of thiophene in the benzene samples was quantified by the gas chromatography-flame photometric detection (GC-2010, FPD) analysis. The concentration of thiophene was calculated by a standard calibration curve and the amount of adsorbed thiophene per gram of adsorbent at equilibrium was calculated by Eq. (1)[8].

in whichqeis the amount of thiophene adsorbed per unit mass of adsorbent at equilibrium (mg/g),C0andCeare the respective initial and equilibrium concentrations of thiophene (mg/L) in liquid phase,Vis the volume of the solution (L), andmis the weight of the dried adsorbent (g).

2.5 Study on adsorption kinetics

In this section, 2 g of Raney Ni/Al2O3and 60 mL of benzene containing 10 mg/L of thiophene were added to a micro high-pressure reactor. The adsorption experiments were performed under magnetic stirring at a rate of 400 r/min with the adsorption temperature maintained at 393 K, 403 K, 413 K, and 423 K, respectively. The adsorption rate was assessed after 20 min, 40 min, 60 min, 70 min,80 min, 90 min, and 100 min, respectively. The amount of remaining thiophene was quanti fied by GC-2010 analysis.The amount of thiophene adsorbed at any given time was calculated by Eq. (2)[8].

in whichCtis the concentration of thiophene (mg/L) in the product at a specified time (min). The data at each temperature were applied to the pseudo- first-order model,the pseudo-second-order model, and the intraparticle diffusion model. Table 1 summarizes the kinetic equations used in this study.

Table 1 Model of adsorption kinetics

2.6 Thermodynamic study

The thermodynamic parameters, the Gibbs energy change (ΔG0), the enthalpy change (ΔH0), and the entropy change (ΔS0) were calculated by the Van’t Hoffequation. Table 2 summarizes the equations used for parameter estimation.

My grandfather left Korea to live with us in New York when he was almost eighty years old. My parents fixed1 up the attic2（） so that he had his own room. He wore traditional Korean clothes: shiny vests（） with gold buttons, and puffy（） pants that made his legs look fat even though he was really very skinny. He chewed on small dried fish snacks（，） that smelled up everything. He coughed a lot.

Table 2 Correlation formula of adsorption thermodynamic

3 Results and Discussion

3.1 XRD and H2-TPR characterization

3.1.1 XRD patterns of precursor and Raney Ni/Al2O3

As the active component of adsorbent, Ni plays an important role in the performance of Raney Ni/Al2O3adsorbent so that the XRD patterns of precursor (pattern A) and Raney Ni/Al2O3(pattern B) are shown in Figure 1. Pattern A exhibits the peaks attributed to crystalline AlNi (2θ= 31.32°, 45.11°) and Ni2Al3(2θ= 17.99°,25.39°, 45.11°). After the precursor was activated by the sodium hydroxide solution, the peak intensities of AlNi and Ni2Al3were dramatically weakened and the dispersive diffraction peak at 45° attributed to skeletal nickel was generated. This phenomenon denoted that the leaching process decreased the amount of the AlNi and Ni2Al3and simultaneously generated amorphous skeletal nickel.

Figure 1 The XRD patterns of precursor (A) and Raney Ni/Al2O3 (B)

3.1.2 H2-TPR pattern of Raney Ni/Al2O3

The H2-TPR pattern of Raney Ni/Al2O3shows three hydrogen consumption peaks (Figure 2) at 408 K, 773 K,and 913 K, respectively. The peak at 408 K is ascribed to skeletal nickel belonging to the active component of adsorbent. Besides, the other two peaks at 773 K and 913 K are assigned to nickel oxides with different reduction properties. This fact is attributed to the different interactions between NixOyand Al2O3in Raney Ni/

Figure 2 H2-TPR pattern of Raney Ni/Al2O3

3.2 Adsorption isotherm and equilibrium analysis

The equilibrium isothermal adsorption study was performed under conditions covering a thiophene concentration in reactants equating to 20 mg/L, 45 mg/L,65 mg/L, and 85 mg/L, respectively. The absorption experiments were performed for 3 h at 403 K, 413 K,and 423 K, respectively. The corresponding results are summarized in Table 3 and Figure 3. Table 3 shows the adsorption behavior of thiophene over Raney Ni/Al2O3. It is obvious thatqegradually decreases with a decreasingCe. However, the effects of increasing experimental temperature onqecan be neglected (Δqe≈ 0.2, Figure 3).

Table 3 Equilibrium isothermal adsorption of thiophene in benzene over Raney Ni/Al2O3 at different temperatures

Figure 3 Adsorption isotherms of thiophene over Raney Ni/Al2O3 at different temperatures

The adsorption equilibrium, which was fitted to the Langmuir and Freundlich adsorption isotherm models,was applied to reveal the mechanisms of the adsorption process. The calculated parameters, including the regression coefficient (R2), are summarized in Table 4.

Eq. (3) and Eq. (4) exhibit the clear linear correlations betweenCe/qeandC,logqeand logCe, respectively.

The results, including the corresponding regression coefficients (R2), obtained from different adsorption temperatures are shown in Figure 4 and Table 4. TheR2at different temperatures are all near 0.99, indicating that experimental data are consistent with the Freundlich adsorptive isothermal model. For the Freundlich adsorption isotherm, the maximum adsorption capacity(KF) at 403 K, 413 K, and 423 K is 17.18 mg/g, 14.03 mg/g, and 7.17 mg/g, respectively. TheKFdecreases with an increasing temperature, implying a positive correlation between the adsorption capacity and adsorption temperature. Separation factors (0 ＜ 1/n＜ 1, especially those from 0.842 to 0.718) of the data are related to the adsorption intensity or surface uniformity of Raney Ni/Al2O3. The slope of the 1/nline decreases slightly with an increasing adsorption temperature (Figure 4).This demonstrates that the affinity between Raney Ni/Al2O3and thiophene molecules was strengthened by the increasing adsorption temperature[12].

Table 4 Langmuir and Freundlich isotherm constants for the adsorption of thiophene over Raney Ni/Al2O3

Figure 4 Langmuir (A) and Freundlich (B) adsorption model fitting curves for thiophene adsorbed by the Raney Ni/Al2O3

3.3 Adsorption kinetics analysis

3.3.1 Effect of temperature on the adsorption capacity

Figure 5 shows the effects of different temperature on the adsorption capacity of the adsorbent. It is shown in Figure 6(A) that the adsorption rate and the equilibrium adsorption capacity increase with a rising adsorption temperature.

The fitting curves of the pseudo-first-order model and the pseudo-second-order model are shown in Figure 5(B) and Figure 5(C), respectively, with the fitting curve parameters listed in Table 5. TheR2value of the pseudosecond-order model is close to 1, which is higher than that of the pseudo- first-order model. It can be seen from Table 5 that the experimental value of the equilibrium adsorption amount (qe,exp) is similar to the calculated value of the pseudo-second-order model (qe,cal). This fact means that the pseudo-second-order model is more advantageous in describing the adsorption kinetics of thiophene on Raney Ni/Al2O3than the pseudo- first-order model according to current experimental results.

Thet0.5-qtplots are employed to investigate the ratelimiting step of the adsorption process in accordance with the intraparticle diffusion model, with the corresponding results shown in Figure 5(D). In general, the intraparticle diffusion model in the adsorption process illustrates three straight-line sections with different slopes. This fact means the presence of three individual steps during the adsorption process including the film diffusion, the intraparticle diffusion, and the adsorption reaction. For an intraparticle diffusion-controlled adsorption, thet0.5-qtplot should be a straight line with a positive intercept and it should pass through the origin. However, the intraparticle diffusion patterns fail to pass through the origin and they are not straight lines according to Figure 5(D).This fact means that the intraparticle diffusion has little effect on the adsorption process so that it is not the ratecontrolling step. It suggests that intraparticle diffusion in combination with film diffusion is the rate-limiting step of the adsorption process[6].

Figure 5 Adsorption isotherm diagram and adsorption kinetics diagram of thiophene on the Raney Ni/Al2O3 at various adsorption temperatures ( 2 g of adsorbent, 60 mL of benzene with 10 mg/L of thiophene concentration)

Table 5 Rate constants of the pseudo- first-order model and the pseudo-second-order model for thiophene adsorption over Raney Ni/Al2O3 at different temperatures

3.3.2 Effect of initial concentration on adsorption capacity

The effect of initial thiophene concentration on adsorption capacity was investigated by varying thiophene concentration from 10 mg/L to 85 mg/L, with the results shown in Figure 6. It can be seen from Figure 6(A) that the adsorption equilibrium is available in 60 min and the equilibrium adsorption capacity increases with a rising initial concentration of thiophene.

Figure 6 Adsorption isotherm diagram and adsorption kinetics diagram of thiophene on Raney Ni/Al2O3 at different thiophene concentrations (2 g of adsorbent, 60 mL of benzene with different thiophene concentrations, T=423 K)

The fitting curves of the pseudo-first-order and the pseudo-second-order models are shown in Figure 6(B) and Figure 6(C), respectively, and the fitting curve parameters are listed in Table 6. TheR2value of the pseudo-second-order model is close to 1 and it is higher than that of the pseudo-first-order model (Table 6). Besides, the experimental value of the equilibrium adsorption amount (qe,exp) is similar to the calculated value of the pseudo-second-order model (qe,cal). This result means that the pseudo-second-order model is more advantageous in describing the adsorption kinetics of thiophene on Raney Ni/Al2O3than the pseudo- first-order model according to current experimental results.

Table 6 Rate constants of the pseudo- first-order model and the pseudo-second-order model for thiophene adsorption with different initial concentrations of thiophene

3.4 Adsorption thermodynamics analysis

The thermodynamic parameters including ΔH0, ΔS0, lnKeand 1/Tare calculated by the Van’t Hoff equation so as to determine their respective values, and the lnKe-1/Tplot is shown in Figure 7. The values of ΔH0and ΔS0are calculated by slope and intercept and the value of ΔG0can be acquired subsequently. The values ofKe, ΔH0, ΔS0, and ΔG0are presented in Table 7.

Figure 7 The lnKe-1/T plot of calculating Ke

Table 7 Adsorption thermodynamic parameters of thiophene over Raney Ni/Al2O3

The absorption is an spontaneous, endothermic and irreversible process since ΔG0is negative and the values of ΔH0and ΔS0are equal to 111.548 kJ/mol and 316.760 J/(molK), respectively. The absolute value of ΔG0increases from 9.771 kJ/mol to 22.442 kJ/mol with the adsorption temperature increasing from 383 K to 423 K. That means higher temperature is conducive to the adsorption process. Generally, ΔG0of physical adsorption ranges from -20 kJ/mol to 0 kJ/mol, but for chemical adsorption, it is -400—-80 kJ/mol[13]. This fact means that the adsorption of thiophene on Raney Ni/Al2O3is a physicochemical process since ΔG0ranges from-9.771 kJ/mol to -22.442 kJ/mol (Table 7).

4 Conclusions

The Raney Ni/Al2O3adsorbent with good desulfurization performance is proposed in this paper. Experimental results indicate that the Freundlich adsorption isotherm model is adequate to describe the adsorption process.The results from adsorption kinetics study shows that the pseudo-second-order kinetics equation is more advantageous to describe the adsorption kinetics of thiophene on Raney Ni/Al2O3. In addition, the intraparticle diffusion in combination with film diffusion is the ratelimiting step of the adsorption process. The results from adsorption thermodynamic analysis can prove that the adsorption is an entropy-increasing, spontaneous and endothermic process, since ΔS0＞ 0, ΔG0＜ 0, and ΔH0＞ 0.

Acknowledgement:This work was financially supported by the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT & TCD 20130325) and the Project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges under Beijing Municipality (IDHT20180508).