TiO2-Al2O3 adsorbents on the adsorption desulfurization of diesel

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 43, Issue 8, Aug 2015 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2015, 43(8), 990997

RESEARCH PAPER

Effect of B2O3 modified Ag/TiO2-Al2O3 adsorbents on the adsorption desulfurization of diesel LI Li-da1,2, XU Cheng-zhi1,2, ZHENG Mei-qin1,2, CHEN Xiao-hui1,2,* 1

National Engineering Research Center of Chemical Fertilizer Catalysts, Fuzhou 350002, China;

2

School of Chemical Engineering, Fuzhou University, Fuzhou 350116, China

Abstract:

Ag/TiO2-B2O3-Al2O3 adsorbents with different B2O3 loadings (e.g. 5%–20% (w)) were prepared by impregnation. Static

adsorption tests were carried out by contacting the adsorbents with commercial diesel containing 245.36 mg(S)/L sulfur at ambient conditions to investigate the adsorption desulfurization activity. The results show that Ag/TiO2-Al2O3 modified by B2O3 exhibits a great enhancement for the adsorption desulfurization activity. When B2O3 (15% (w)) was loaded, 2%Ag/4%TiO2-15%B2O3-Al2O3 (w) could achieve the best desulfurization activity with a saturation capacity of 2.36 mg(S)/g adsorbent. This is a significant achievement regarding the desulfurization efficiency, especially for commercial diesel without pretreatment. The effects of B2O3 on the textural properties, crystal structure and surface acidity properties of the adsorbent were studied by N2-physisorption, O2-chemisorption, X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH3-TPD), Fourier transform infrared spectrometer (FT-IR spectra) and 11B nuclear magnetic resonance (11B-NMR) techniques. Correlating the characterizations with the desulfurization activity, it is found that the adsorption desulfurization activity is well related with the amounts of weak acid sites on the adsorbents. B2O3 modification induces larger amounts of BO4 species and improves the surface weak acidity, resulting in a higher adsorption desulfurization activity. Key words: B2O3; silver-based adsorbent; diesel; adsorption desulfurization; weak acid sites

Diesel has been widely used for its high energy density, ready availability and easy for storage, but the sulfur compounds in it will poison the catalysts in both automobile and fuel cell, and pollute the air when it was burnt to be toxic SOx[1,2]. Removal of sulfur from diesel has been mandated by governments around the world, the industrial and dominant desulfurization process is hydrodesulfurization (HDS). However, it operates at high temperature (300–400°C) and high hydrogen pressure (3–6 MPa) to convert organosulfur compounds to hydrogen sulfide (H2S) over CoMo or NiMo catalysts, and suffers from significant operating costs and investment[3,4]. Adsorptive desulfurization is considered to be one of the most promising techniques[5] for selectively adsorbing sulfur compounds from liquid fuel at mild conditions (<100°C, ambient pressure, no hydrogen requirement)[3]. Different kinds of active metals were employed to prepare the adsorbents for adsorption desulfurization, such as Ni[6–8], Ce[9–11], Cu[12–15], Ag[16–19] and Pd[20]. It was found that Ag and Pd exhibited the highest adsorption desulfurization activity,

and Ag was more widely studied for its relatively low cost compared with Pd. Nair et al[18] used Ag(4% (w))/TiO2 adsorbent prepared by impregnation and obtained a saturation sulfur capacity of 2.9 mg(S)/g for JP-8 fuel containing 630 ×10-6(w) sulfur at a LHSV~2 h–1 in a fixed-bed. However, the sulfur compounds in diesel were much more difficult to adsorb than model fuel, gasoline and jet fuel, because the major sulfur compounds in diesel are the alkyl dibenzothiophenes (DBTs) with one or two alkyl groups at 4or/and 6-positions with obvious steric hindrance[5]. In addition, as for commercial diesel, the aromatics, oxygen-containing fuel additives, nitrogen compounds, and moisture present in it have a significantly strong inhibiting effect on adsorption desulfurization[21]. Generally, the saturation sulfur capacity for commercial diesel containing less than 250×10-6(w) sulfur is confined to be only 1.1 mg(S)/g adsorbent[14]. Therefore, it’s really a big challenge and of great essential to find new adsorbents of higher sulfur capacity to improve the adsorption desulfurization activity for commercial diesel.

Received: 16-Apr-2015; Revised: 25-Jun-2015. Foundation Item: Supported by the National Natural Science Foundation of China (21376055). *Corresponding author: Tel: +86-591-83731237-8607; E-mail: [email protected]. Copyright  2015, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

LI Li-da et al / Journal of Fuel Chemistry and Technology, 2015, 43(8): 990997

Various methods of catalyst modification were applied to improve the catalytic performance and effects of the additives were also elucidated. Many researchers suggested that the adsorption desulfurization activity was mainly attributed to large specific area, high metal dispersion and more surface acid sites on the adsorbents[7,11,22,23]. TiO2 and B2O3 additives have been reported to be effective to improve the surface area, metal dispersion and the surface acidity, respectively[24–28]. Previously, we found that 2%Ag/Al2O3 exhibited an excellent performance in removing sulfur from commercial diesel and obtained a much higher saturation capacity by TiO2 modification to improve the dispersion of Ag. In this work, we intend to further improve the desulfurization activity by B2O3 modification. Thus, a series of Ag/TiO2-B2O3-Al2O3 adsorbents with different B2O3 loadings (e.g. 5%–20% (w)) were prepared by impregnation. The adsorption desulfurization activities of the adsorbents were investigated in detail through static adsorption tests at ambient conditions. The effects of B2O3 were studied by X-ray diffraction (XRD), N2-physisorption, O2-chemisorption, temperature-programmed desorption of ammonia (NH3-TPD), Fourier transform infrared spectrometer (FT-IR spectra) and 11B nuclear magnetic resonance (11B NMR) techniques.

1 1.1

Experimental Adsorbent preparation

Parent γ-alumina (white powder, SBET=239 m2/g, total volume of pores = 0.35 cm3/g, average radius of the pores =5.88 nm) was prepared via calcination of commercial pseudoboehmite (Aluminum Corporation of China Limited) in a muffle furnace at 550°C for 4 h. Ag/TiO2-B2O3-Al2O3 adsorbents were prepared by successive impregnation. An initial impregnation step was performed with aqueous solution of H3BO3 (Sinopharm Chemical Reagent Co., Ltd, AR) and γ-alumina powder. The slurries were then dried at 110°C for 2 h and followed by calcination in a muffle furnace at 550°C for 2 h to obtain 5%, 10%, 15% and 20% B2O3-Al2O3, respectively. Subsequently, the solids were impregnated in anhydrous ethanol solution of titaium (IV) tetrabutaoxide (Tianjin Fuchen Chemical Reagent Factory, AR), dried at 110°C for 2 h and followed by calcination in a muffle furnace at 550°C for 2 h to obtain TiO2-B2O3-Al2O3. Finally the solids were impregnated in aqueous solution of AgNO3 (Shanghai Colloid Chemical Plant, AR) , dried at 110°C for 2 h and followed by calcination in a muffle furnace at 450°C for 2 h to obtain Ag/TiO2-B2O3-Al2O3. The loadings of Ag and TiO2 were 2% and 4%, respectively. 1.2

Characterizations of the adsorbents

1.2.1

XRD analysis

XRD patterns of the samples were recorded by a PANalytical X’pert Pro diffractometer (Netherlands) equipped with Co-Kα (λ = 0.1789 nm) radiation. The samples were operated at 40 kV and 30 mA for 2θ angles ranging from 10° to 90° and scanned at 0.2088°/s with a step size of 0.0167°. The diffraction pattern was identified by comparing with Joint Committee of Powder Diffraction Standards (JCPDS) cards. 1.2.2

N2 physisorption analysis

The BET surface area and pore volume were measured by American Micrometrics ASAP 2020 instrument using nitrogen adsorption-desorption at 77 K. 1.2.3

O2 chemisorption

Oxygen pulse adsorption was carried out in a Micromeritics AutoChem II 2920 equipment to measure the silver dispersion on the adsorbents. The samples were pretreated at 150°C for 30 min in vacuum (3.99×10−11 kPa) followed by reduction at 400°C for 1 h in hydrogen (atmosphere pressure) in a sample cell. Then the cell was evacuated again to remove any physically adsorbed H2. It was assumed that all silver on the surface was reduced metallic phase. Selective oxygen chemisorption was carried out at 170°C and the oxygen uptake was calculated at p/p0=0 by extrapolating the isotherm. Silver dispersion (D) and active metal surface area (s) and average crystallite size (d) were calculated from oxygen uptake[18]. 1.2.4

NH3-TPD

NH3 temperature-programmed desorption (NH3-TPD) was carried out using a Micromeritics AutoChem I 2910 instrument equipped with a TCD detector. The samples (≈100 mg) were pretreated in He with flow rate of 30 mL/min at 200°C. Then NH3 was introduced for 1 h after the samples was cooled to 50°C. Subsequently, the samples were purged by high purity helium gas for 1 h to remove physically adsorbed NH3. After that, it was heated from room temperature to 700°C at 10°C/min. The amount of desorbed NH3 was monitored by TCD. 1.2.5

FT-IR

FT-IR spectra were recorded on a Nicolet 6700 FTIR spectrometer in the range of 400–4000 cm–1 with KBr as the reference.

LI Li-da et al / Journal of Fuel Chemistry and Technology, 2015, 43(8): 990997

1.2.6

NMR spectroscopy

11

B MAS NMR spectra were recorded at 160.41 MHz on a Bruker AVANCE Ⅲ 500 multinuclear spectrometer, while a MAS spinner was rotated at a rate of 10 kHz. The rotors have a diameter of 4 mm. Repetition time of 2 s were allowed between 2.5 μs pulses. Up to 128 scans at 10 kHz were carried out in order to attain the real bandwidth of the spectra. 1.3

Adsorption experiments

Static adsorption tests were carried out by contacting the adsorbents with commercial diesel containing 245.36 mg(S)/L sulfur to investigate the adsorption desulfurization activity. All desulfurization experiments were performed in a test flask by mechanically mixing the adsorbents and diesel (the ratio of diesel and adsorbents is 30 mL: 1g) and stirring for 2 h under ambient conditions. Sulfur content was determined by TS 3000 total sulfur analyzer (Jiangsu Jiangfen Electroanalysis Instrument Limited). The instrument was calibrated by SH/T 0689-2000 and ASTMD5453-05 sulfur standards. And the detection limit was 200 μg(S)/L. The adsorption desulfurization activity was measured by saturation capacity (q, mg(S)/g adsorbent) calculated by the following expressions:

(  e )  V q 0 1000 m Where, ρ0 was the initial concentration of sulfur in diesel (mg(S)/L); ρe was the adsorption equilibrium concentration of sulfur in diesel (mg(S)/L), V was the volume of diesel (mL), m was the weight of the adsorbent (g).

2

and slightly decreases with a further increase B2O3 loading up to 20%. 2%Ag/4%TiO2-15%B2O3-Al2O3 (w) exhibits the highest saturation capacity of 2.36 mg(S)/g adsorbent, which has increased saturation sulfur capacity by 12.2% than the unmodified 2%Ag/4%TiO2-Al2O3 adsorbent. The 2%Ag/4%TiO2-15%B2O3-Al2O3 adsorbent has obtained a significant high saturation sulfur capacity of 2.36 mg(S)/g adsorbent for commercial diesel containing 245.36 mg(S)/L sulfur at ambient conditions. The effects of B2O3 modification on the textural properties, crystal structure and surface acidity properties of the adsorbent has been studied by N2-physisorption, O2-chemisorption, X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH3-TPD), Fourier transform infrared spectrometer (FT-IR spectra) and 11 B nuclear magnetic resonance (11B NMR) techniques. Correlating the characterizations with the desulfurization activity of the adsorbents, we can find the relationship between the physical chemical properties and the desulfurization activity of the adsorbent to elucidate the effect of the B2O3 and provide instructions for our further work. 2.2 Structural and textural characterization of the adsorbents The isotherms of 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3 are shown in Figure 2. The adsorption-desorption curves of the samples are nearly the same with a hysteresis in the relative pressure range of 0.4–1.0, and they are all attributed to be type IV isotherms and type H4 loops[29]. It suggests that the B2O3 modification does not change the original narrow slit-like mesoporous structure[29] of 2%Ag/4%TiO2-Al2O3.

Results and discussion

2.1 Adsorption desulfurization performance of the adsorbents The adsorption desulfurization activity was generally measured by saturation sulfur capacity. The saturation sulfur capacities of 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3 are shown in Figure 1. A saturation capacity as high as 2.11 mg(S)/g adsorbent is obtained for 2%Ag/4%TiO2-Al2O3, and B2O3 modification results in an obvious enhancement for the adsorption desulfurization activity. As for the samples of 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3, the saturation capacity follows the sequence: 2%Ag/4%TiO2-15%B2O3-Al2O3 > 2%Ag/4%TiO2-20%B2O3-Al2O3 >2%Ag/4%TiO2-10%B2O3-A l2O3 >2%Ag/4%TiO2-5%B2O3-Al2O3. Thus the adsorption desulfurization activity increases with the increase of the B2O3 loading and reaches a maximum point at B2O3 loading of 15%,

Fig.1

Saturation sulfur capacities of 2%Ag/ 4%TiO2-Al2O3

modified by different loadings of B2O3 determined by commercial diesel of 245.36 mg(S)/L sulfur a: 2%Ag/4%TiO2 -Al2O3; b: 2%Ag/4%TiO2-5% B2O3-Al2O3; c: 2%Ag/4%TiO2-10%B2O3-Al2O3; d: 2%Ag/4%TiO2-15% B2O3-Al2O3; e: 2%Ag/4%TiO2-20% B2O3-Al2O3

LI Li-da et al / Journal of Fuel Chemistry and Technology, 2015, 43(8): 990997 Table 1

Textural properties for Ag/Al2O3 modified by different loadings of B2O3

Sample

SBETa 2

–1

O2 uptakec

vpb 3

–1

–1

SAgd 2

–1

DAge

dAgf

/(m ·g )

/(cm ·g )

/(μmol·g )

/(m ·g )

/%

/nm

2%Ag/4%TiO2 -Al2O3

228.73

0.35

42.1

4.41

45.5

2.59

2%Ag/4%TiO2-5%B2O3-Al2O3

214.84

0.32

28.1

2.94

30.3

3.89

2%Ag/4%TiO2-10%B2O3-Al2O3

212.53

0.31

26.8

2.80

28.9

4.08

2%Ag/4%TiO2-15%B2O3-Al2O3

209.58

0.30

26.5

2.77

28.6

4.12

2%Ag/4%TiO2-20%B2O3-Al2O3

198.46

0.28

21.3

2.23

23.0

5.12

a: SBET is the BET surface area of the adsorbent measured by N2 physical adsorption; b: vp is the total pore volume of the adsorbent measured by N2 physical adsorption; c: O2 uptake is the O2 consumption of the adsorbent measured by selective oxygen chemisorption at 170°C; d: SAg is the active metal surface area of Ag on the adsorbent calculated by the equation: SAg =2×(O2 uptake)×cross-sectional area occupied by each surface Ag atom /166, assuming the cross-sectional area=8.6960 Å2/Ag atom; e: DAg is the dispersion of Ag metal particles: 2×(O2 uptake)×108/(100×Ag loading); f: dAg is the average crystallite size of Ag: 100×Ag loading×the shape factor/( SAg×density of Ag), assuming the shape factor=6 for the assumed spherical shape, density of Ag=10.5 g/mL

Fig.2

N2 adsorption-desorption isotherms of 2%Ag/ 4%TiO2-Al2O3 modified by different loadings of B2O3 a: 2%Ag/4%TiO2 -Al2O3; b: 2%Ag/4%TiO2-5% B2O3-Al2O3;

c: 2%Ag/4%TiO2-10%B2O3-Al2O3; d: 2%Ag/4%TiO2-15% B2O3-Al2O3; e: 2%Ag/4%TiO2-20% B2O3-Al2O3

N2-physisorption and O2-chemisorption were used to determine surface characteristics and Ag dispersion. BET surface area, total pore volume and Ag dispersion of 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3 are listed in Table 1. These morphological parameters all gradually decrease with the increasing loading of B2O3. This is in agreement with other workers[27,30,31], who suggested that B2O3 particles gradually aggregate and block the pores of alumina during calcinations with the increase of boron loading, yielding a low surface area and metal dispersion. Correlating these data with the adsorption desulfurization activity, it is seen that the BET surface area and Ag dispersion are not the dominant reason for determining the desulfurization activity for 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3 adsorbents. The XRD patterns of 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3 adsorbents are displayed in Figure 3. Two sets of diffraction patterns are found for all the

Fig.3

XRD patterns of 2%Ag/ 4%TiO2-Al2O3 modified by different loadings of B2O3

a: 2%Ag/4%TiO2 -Al2O3; b: 2%Ag/4%TiO2-5% B2O3-Al2O3; c: 2%Ag/4%TiO2-10%B2O3-Al2O3; d: 2%Ag/4%TiO2-15% B2O3-Al2O3; e: 2%Ag/4%TiO2-20% B2O3-Al2O3

samples: those marked with “#” are indexed to cubic Al2O3 (JCPDS file no. 00-001-1303), and those labeled with “*” are ascribed to anatase, syn (JCPDS file no. 00-083-2243). There are no obvious diffraction peaks of crystalline Ag or B2O3 for all the samples, suggesting that the Ag or B2O3 species might be highly dispersed or present in an amorphous state on the surface. This is consistent with the suggestion in the literature that 2%Ag was well dispersed on the surface of alumina[22] and B2O3 was hard to crystallize and remained amorphous even after calcination at 977°C for boric acid[32]. Besides, it is found that the diffraction peaks of anatase gradually appear and the intensity increase with the increasing loading of B2O3. It was probably because of a progressive clogging of pores with increasing B2O3 loading, the decrease in pore volume and surface area and less highly dispersion of TiO2 on the support. This is verified by the results of the morphological parameters of 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3 adsorbents listed in Table 1.

LI Li-da et al / Journal of Fuel Chemistry and Technology, 2015, 43(8): 990997

Fig.4 NH3-TPD profiles of 2%Ag/ 4%TiO2-Al2O3 modified by different loadings of B2O3

2.3

Fig.5

FT-IR spectra of Ag/TiO2-Al2O3 modified by different loadings of B2O3

a: 2%Ag/4%TiO2 -Al2O3; b: 2%Ag/4%TiO2-5% B2O3-Al2O3;

a: 2%Ag/4%TiO2 -Al2O3; b: 2%Ag/4%TiO2-5%B2O3-Al2O3;

c: 2%Ag/4%TiO2-10%B2O3-Al2O3; d: 2%Ag/4%TiO2-15% B2O3-Al2O3;

c: 2%Ag/4%TiO2-10%B2O3-Al2O3; d: 2%Ag/4%TiO2-15%B2O3-Al2O3;

e: 2%Ag/4%TiO2-20% B2O3-Al2O3

e: 2%Ag/4%TiO2-20%B2O3-Al2O3

Surface acid properties of the adsorbents

Surface acid sites on the adsorbents are reported to be responsible for the adsorption desulfurization performance[7,11,22,23]. Hence, NH3-TPD was used to investigate the surface acidity of the 2%Ag/4%TiO2-Al2O3 adsorbents modified by different loadings of B2O3. The NH3-TPD profiles of these samples are shown in Figure 4. All samples have three NH3-desorption peaks at around 150, 350 and 550°C, which are generally assigned to weak, medium and strong acid sites, respectively. It is found that B2O3 modification yields the adsorbents with higher total acidity, and variations in the B2O3 loading have a significant effect on the total acidity and on the acid site strength distribution. Especially, as for samples of 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3, there is an obvious sequence for the amounts of weak acid sites: 2%Ag/4%TiO2-15%B2O3-Al2O3 > 2%Ag/4%TiO2-20%B2O3 -Al2O3>2%Ag/4%TiO2-10%B2O3-Al2O3 > 2%Ag/4%TiO2-5% B2O3-Al2O3. It is completely consistent with the sequence of desulfurization activity for 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3 adsorbents, indicating that weak acid sites are the active sites on the adsorbents. Surface weak acid sites on the adsorbents have been verified to be responsible for the enhancement for the adsorption desulfurization activity for 2%Ag/4%TiO2-Al2O3 adsorbents modified by B2O3. It was reported that the increasing surface acidity for Al2O3 modified by B2O3 was generally attributed to the formation of Al–O–B bridges or tetrahedral oxygen-coordinated borate species (BO4)[26,33–35]. Here, FT-IR and 11B-NMR were used to identify the effects of B2O3 on the surface weak acidity. The FT-IR spectra of Ag/TiO2-Al2O3 modified by different loadings of B2O3 are shown in Figure 5. Background spectra

were subtracted for all sample spectra. The regions of 3000–4000 and 500–1800 cm-1 are termed as the hydroxyl stretching vibration region and Al–O, B–O stretching vibration region, respectively. The peak around 3755 cm–1 is the hydroxyl stretching for the surface adsorbed H2O and 3450 cm–1 represents the hydroxyl stretching for Al–OH. The peak at 1630 cm–1 represents the hydroxyl blending for Al–OH. The peak around 1460 cm–1 and 1350 cm–1 is the BO3 stretching and BO4 stretching, respectively[26]. The peak around 1050 cm-1 represents the in-plane bending of BO4[26]. The broad bands around 800 cm–1 and 580 cm–1 stands for the stretching of Al–O. Figure 5 shows that when B2O3 were loaded, there was no peak shift or any change for the peak intensity of Al2O3, indicating that the boron atom was not accessible to the framework to form the Al–O–B bridge but existed on the surface of alumina. As for 2%Ag/4%TiO2-Al2O3 modified by different loadings of B2O3, the intensity of BO3 and BO4 stretching peaks increased with the increasing loading of B2O3, but it was difficult to determine the ratio of BO4/BO3. However, the adsorbing peaks at 1050 cm–1 representing the in-plane bending of BO4 gradually appeared and its intensity increased with the increasing loading of B2O3, indicating that the BO4/BO3 ratio increased with the increasing loading of B2O3. To further clearly identify the ratio of BO4 to BO3 for Ag/TiO2-Al2O3 adsorbents modified by different loadings of B2O3 and elucidate the effects of modification B2O3 on the enhancement of the surface weak acidity, 11B NMR spectroscopy was employed and shown in Figure 6. The narrow single sharp signal at 1 was attributed to the tetrahedral boron (BO4) units for the highly symmetrical arrangement of the four oxygen in BO4 framework, while trigonal boron (BO3) produced a characteristic quadrupolar doublet pattern at 10 and 15 due to its high quadrupolar

LI Li-da et al / Journal of Fuel Chemistry and Technology, 2015, 43(8): 990997

interaction[32]. The BO4/BO3 ratio increased with the increase loading of B2O3, and this was in agreement with the FT-IR results above. Moreover, it was observed that when the modification B2O3 was below 15%, the intensity of BO4 increased dramatically, but only achieved a slight increase when the modification B2O3 was further increased to be 20%. Correlating the FT-IR, 11B-NMR results with NH3-TPD results, it was found that when the modification of B2O3 was below 15%, both the intensity of BO4 and the ratio of BO4 to BO3 increased with the increase of boron loading, and the corresponding increase of surface weak acidity was achieved, indicating that it is probably the BO4 species resulted in a larger amounts of weak acid sites. A similar effect of larger content of BO4 resulting in more acid sites has been reported by many researchers[34,35]. When the B2O3 loading was further increased to be 20%, the BO4/BO3 ratio slightly increased, but the surface weak acidity decreased, it might not be

Fig.6

11

contradictory with the conclusion that BO4 induced more weak acid sites. It was probably because that when the B2O3 loading was 20%, the surface B2O3 came to being multiple-layered, and the bottomed layer BO4 species could be covered by the other species on the upper layer. It can also be inferred from the data in Table 1 that 2%Ag/4%TiO2-20%B2O3-Al2O3 (w) has obtained a relatively remarkable decrease in the surface area of the adsorbent. Moreover, it was reported that only BO3 species existed for B2O3 at normal atmosphere[36], and the BO3 species were saturated, less defected and non-acid for B2O3 obtained from the decomposing of boric acid at normal atmosphere. The obtained coordinately unsaturated acid BO4 species were probably formed by the B occupying the Al vacant sites on the surface of Al2O3. Therefore, the increase of surface weak acidity for the adsorbents modified by B2O3 was attributed to inducing more surface BO4 species.

B NMR spectra of Ag/TiO2-Al2O3 modified by different loadings of B2O3

a: 2%Ag/4%TiO2-5%B2O3-Al2O3; b: 2%Ag/4%TiO2-10%B2O3-Al2O3; c: 2%Ag/4%TiO2-15%B2O3-Al2O3; d: 2%Ag/4%TiO2-20%B2O3-Al2O3

3

Conclusions

A series of Ag/TiO2-B2O3-Al2O3 adsorbents with different loadings of B2O3 (e.g. 5%–20% (w)) prepared by impregnation contacting with commercial diesel containing 245.36 mg(S)/L sulfur were carried out by static adsorption. The results show that Ag/TiO2-Al2O3 modified by B2O3

exhibited a great enhancement for the adsorption desulfurization activity and 2%Ag/4%TiO2-15%B2O3-Al2O3 (w) achieved the highest saturation capacity of 2.36 mg(S)/g adsorbent. Correlating the characterizations with the results of desulfurization activity, it was found that B2O3 species present in an amorphous state on the surface of alumina. The effect of B2O3 modification is mainly because of a significant

LI Li-da et al / Journal of Fuel Chemistry and Technology, 2015, 43(8): 990997

enhancement for the amounts of weak acid sites generated from more BO4 species on surface of the adsorbent.

Adsorbents for selective adsorption of thiophene out of hydrocarbon. Ind Eng Chem Res, 2006, 45(18): 6169–6174. [14] Shao X-C, Duan L-H, Wu Y-Y, Qin Y-C, Yu W-G, Wang Y, Li

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