Adsorptive Desulfurization of Propylmercaptan and Dimethyl Sulfide by CuBr2Modified Bentonite

(The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237)

Adsorptive Desulfurization of Propylmercaptan and Dimethyl Sulfide by CuBr2Modified Bentonite

Cui Yuanyuan; Lu Yannan; Yi Dezhi; Shi Li; Meng Xuan

(The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237)

Adsorptive desulfurization for removing propylmercaptan (PM) and dimethyl sul fide (DMS) over CuBr2modified bentonite was investigated under ambient conditions in this work. A saturated sulfur capacity as high as 196 mg of S per gram of adsorbent was demonstrated. The in fluence of loading amount of Cu (II) and calcination temperature on adsorptive desulfurization was investigated. Test results revealed that the optimum loading amount of Cu (II) was 15%, and the calcination temperature was 150 ℃. The pyridine-FTIR spectroscopy showed that a certain amount of Lewis acid could contribute to the increase of adsorption capacity. Spectral shifts of the ν(C-S) and ν(Cu-S) vibrations were detected from the Raman spectra of the Cu (II) complex which was a reaction product of CuBr2with DMS. According to the hybrid orbital theory and the complex adsorption reaction, the desulfurization of PM and DMS over the CuBr2modi fied bentonite is ascribed to the formation of S-M (σ) bonds.

desulfurization; bentonite; propylmercaptan; dimethyl sul fide; mechanism

1 Introduction

Sulfides often exist as the main pollutants in liquid fuels and natural gas and its oxidation can bring about the formation of tropospheric sulfur dioxide (SO2), which would cause acid rains[1]. The presence of sulfur compounds will reduce the purity of petrochemical products, deteriorate the service performance, and severely poison the noble metal catalysts used in subsequent processes[2-4]. The deep desulfurization of gasoline and diesel is becoming more and more difficult, since the sulfur content of crude oil is becoming higher and the permitted sulfur limits in oil product are becoming stricter. Therefore, it is a worldwide urgent challenge to produce increasingly cleaner fuels[5-11]. Hydrodesulfurization (HDS) process is a conventional method to remove sulfur compounds using Co-Mo/ Al2O3or Ni-Mo/Al2O3catalysts at high temperature (300—340 ℃) and high pressure (2.0—10.0 MPa of H2). However, the hydrogenation of olefins would take place simultaneously during the HDS process, which will reduce the octane number of gasoline[7,9,12-15]. Compared with HDS, adsorptive desulfurization is a more economical method[14]and has attracted researchers’ interest extensively.

Bentonite is a relatively economical material used for adsorptive desulfurization. Its partial-amorphous nature provides mesopores with a wide range of pore sizes and some peculiar physical and chemical properties (i.e., large specific surface area and satisfactory adsorptive affinity for organic and inorganic ions) and has been attracting more and more attention as effective separating agents or adsorbents[4,7-10,14,16].

The primary objective of this study is to investigate the adsorptive desulfurization efficiency of the Cu (II)-modified bentonite for removing propylmercaptan (PM) and dimethyl sulfide (DMS). The adsorbents, raw bentonite and Cu (II)-modified bentonite, are characterized by X-ray diffractometry (XRD), thermogravimetric analysis (TGA) and pyridine-FTIR spectroscopy. Raman spectroscopy is used to characterize the Cu (II) complex. Finally, combined with the hybrid orbital theory and complex adsorption reaction, the adsorption mechanism is discussed briefly.

2 Experimental

2.1 Adsorbents and feedstocks

In this work, the raw bentonite was bought from theHangzhou Yongsheng Catalyst Co., Ltd, Zhejiang province, China. It was used directly without further treatment. The composition analysis of the raw bentonite is shown in Table 1.

Table 1 The composition of the raw bentonite

The chemical reagents (CH3COO)2Cu, Cu(NO3)2, CuSO4, CuBr and CuBr2were purchased from the Sinopharm Chemical Reagent Co., Ltd. PM and DMS were purchased from the Shanghai Chemical Reagent Co., Ltd. The adsorbents in this work were prepared by means of the kneading method. The raw bentonite was mixed with (CH3COO)2Cu, Cu(NO3)2, CuSO4, CuBr and CuBr2for 0.5 h, respectively. Dilute nitric acid used as a liquid binder was added into the mixture to make the slurry. Pellets were formulated by an extruder with an outer diameter of 1 mm. All of the adsorbents were dried at 120 ℃ overnight and calcined in a muffle furnace at 150 ℃ for 4 h in air. The mass fraction of Cu (II) species in all adsorbents was 15%. In the follow-up experiments, the CuBr2modified bentonite with different Cu (II) contents and at different calcination temperature were prepared in the similar way.

The model oil for adsorptive desulfurization was prepared by adding PM and DMS into n-hexane simultaneously, and the sulfur content of PM and DMS in the mixture was 2 000 μg/g each.

2.2 Characterization of adsorbents

2.2.1 X-ray diffraction

The crystal structure of powder adsorbents was characterized by X-ray diffraction (XRD) method, using a Siemens D-500 X-ray diffractometer equipped with Ni-filtered CuKα radiation (40 kV, 100 mA). The 2θ scanning angle range was 10°—70° with each step of 0.02(°)/s.

2.2.2 Thermogravimetric analysis

Thermogravimetric analysis (TGA) was carried out using a STA 449F3 thermal analyzer, made by Netzsch, Germany. The instrument was heated at a heating rate of 10 ℃ /min to 700 ℃ in air with an air flow rate of 100 mL/min.

2.2.3 Acidity characterization

The amount of acids, the acid density, and the acid variety were measured via the Fourier transform infrared (FT-IR) spectroscopy (Magna-IR550, Nicolet Company), using pyridine as the probe molecule.

2.2.4 Raman spectra

The Raman spectroscopic studies were conducted on a Renishaw System 100 Raman spectrometer. The laser power was 3 mW at the sample position. The Raman scattered light was detected perpendicular to the laser beam with a Peltiercooled CCD detector, and the spectral resolution for all measurements was 1 cm-1.

2.3 Adsorption experiments

2.3.1 Dynamic tests

The adsorptive desulfurization capacity of the adsorbents for PM and DMS was measured using dynamic tests on a fixed-bed reactor under ambient conditions in a custommade quartz tube (with an internal diameter of 9 mm, a length of 500 mm and a bed volume of 1.25 cm3). The weight hourly space velocity (WHSV) of the model oil was 5 h-1. Reaction products were sampled every halfhour and analyzed with a HP5890 gas chromatograph equipped with FID and GC-MS (type GC6890-MS 5973N, made by the Agilent Co.).

where C0is the initial molar concentration of sulfur (mol/L), C is the final molar concentration of sulfur (mol/L).2.3.2 Static tests

The saturated desulfurization capacity of the adsorbents for DMS and PM was evaluated by static tests at ambient temperature. 0.1 g of adsorbent and 10 g of model oil were put into an airtight container to enter into reaction for 24 hours. The concentration of sulfur was analyzed bya gas chromatograph.

Sulfur capacity (mg S/g of adsorbent) = 1 000×(C0- C)VM/m where C0is the initial molar concentration of sulfur (mol/L), C is the final molar concentration of sulfur (mol/L), V is the volume of solution (L), M is the molar mass of sulfur (g/mol), and m is the mass of adsorbent (g).

2.3.3 Preparation of the Cu (II) complex

The Cu (II) complex was prepared by static complex adsorption of CuBr2and an excess of DMS in an airtight container for 24 hours. The deposit formed during the reaction was filtered and extensively washed with n-hexane subsequently. The product was obtained after drying at room temperature.

3 Results and Discussion

3.1 Desulfurization performance of the modified bentonite

The desulfurization performance of the (CH3COO)2Cu, Cu(NO3)2, CuSO4, CuBr and CuBr2modified bentonite (BE for short) were evaluated via dynamic tests at room temperature. Breakthrough curves for PM and DMS are shown in Figure 1 and Figure 2, respectively.

Figure 1 Breakthrough curves for PM adsorption on modified bentonite calcined at 150 ℃

It can be seen from Figure 1 that all of the modified bentonite samples could absorb more PM than the raw bentonite. Among them, the CuBr2, Cu(NO3)2, and CuSO4modified bentonite samples showed more excellent PM removal performance, which could achieve a complete elimination of PM for 5 h. Figure 2 shows that except the (CH3COO)2Cu modified bentonite sample, the other samples had a better DMS removal performance than the raw bentonite. The CuBr2modified bentonite showed the best DMS desulfurization performance, which could maintain a complete elimination of DMS for 4 h. As a result, the CuBr2modified bentonite had a better desulfurization performance for both PM and DMS compared with other samples. Therefore, CuBr2was selected as the active component.

Figure 2 Breakthrough curves for DMS adsorption on modified bentonite calcined at 150 ℃

3.2 Effects of the amount of Cu (II) loading on adsorption of PM and DMS

Figure 3 Breakthrough curves for PM adsorption on CuBr2modified bentonite with different contents of Cu (II) calcined at 150 ℃

Figure 4 Breakthrough curves for DMS adsorption on CuBr2modified bentonite with different contents of Cu (II) calcined at 150 ℃

A correlation between desulfurization performance and contents of Cu (II) on bentonite had been tested. The results presented in Figure 3 and Figure 4 denote the breakthrough curves for PM and DMS. It can be seen from the curves shown in Figure 3 and Figure 4 that the bentonite loaded with CuBr2could adsorb more sulfur than the raw bentonite. The desulfurization performance increased gradually with an increasing amount of Cu (II) loading. When the amount of Cu (II) loading was 15%, the sulfur adsorption capacity reached a maximum for both PM and DMS. If the Cu (II) loading increased to 20%, as presented in Figure 3, there was no significant change in the desulfurization efficiency compared with the case using the 15% of Cu (II) loading. The results showed that the CuBr2modified bentonite with 15% of Cu (II) loading could be the proper adsorbent for removal of PM and DMS from the model oil.

The X-ray diffraction analysis in Figure 5 was carried out to identify the mineralogical structure of the raw bentonite and the CuBr2modified bentonite adsorbents with different contents of Cu (II) species. The XRD patterns of the bentonite adsorbents loaded with CuBr2showed the characteristic reflections for CuBr2at 2θ = 14.42°, 24.64°, 29.08°, 29.44°, 35.94°, 46.34°, 47.24° and 60.32° corresponding to the planesof cubic CuBr2crystal, respectively. Figure 5 confirms that the strength of the peaks of CuBr2increased with an increasing Cu (II) content, and the crystallinity of the modi fied bentonite adsorbents slightly decreased. These facts suggested that CuBr2on the bentonite existed as an amorphous material with a low Cu (II) content of less than 5%. With the increase of Cu (II) content, CuBr2on the bentonite existed as a crystalline solid. Excessive CuBr2content would block the pores of the bentonite adsorbents, which could affect the adsorptive desulfurization efficiency. To investigate the type and number of surface acidic sites of the adsorbents, FT-IR spectra for the adsorption of pyridine at 200 ℃and 450 ℃ were obtained as shown in Figure 6 and Figure 7. It can be seen from Figure 6 thatthe spectra presented the adsorption band at 1 450 cm-1, which was attributed to the ν(C-C) vibration of pyridine adsorbed at the Lewis acid sites[14,17]in the Cu (II)-bentonite. When the temperature rose up to 450 ℃, as shown in Figure 7, no peak could be found. Therefore, it could be concluded that the Lewis acid sites would be increased by the addition of CuBr2on bentonite, and a certain amount of weak Lewis acid sites could contribute to the adsorption of sulfur compounds.

Figure 5 X-ray diffraction patterns of CuBr2modified bentonite with different Cu (II) contents calcined at 150 ℃

Figure 6 FT-IR spectra at 200 ℃ for Cu (II)-bentonite (containing 15 % of Cu (II) calcined at 150 ℃) and bentonite

Figure 7 FT-IR spectra at 450 ℃ for Cu (II)-bentonite (containing 15 % of Cu (II) calcined at 150 ℃) and bentonite

3.3 Effects of calcination temperature on adsorption of PM and DMS

四川省南充市仪陇县作为中国千千万万乡村中的一员，其打造的现代农业生产经营模式，在一定程度上带动着当地区域产业的快速发展。柑橘示范园区规划采用“大园小镇”模式，即乡村旅游开发中所提出的一种创新休闲农业的模式，也是乡村发展的一种创新模式，使得产业振兴不再拘泥于以往的固有模式，跳出陈旧“圈子”,打开振兴新思路。

A correlation between the desulfurization performance and the calcination temperature of the CuBr2modified bentonite has been tested. As shown in Figure 8, the sulfur adsorption capacity decreased with an increase of the calcination temperature.

Figure 8 Effects of calcination temperature on desulfurization performance of Cu (II)-bentonite (containing 15 % of Cu (II) species)

Figure 9 X-ray diffraction patterns of CuBr2modified bentonite at different calcination temperatures

The modified bentonite samples calcined at different temperatures were characterized by X-ray diffraction, as shown in Figure 9. The samples calcined at 150 ℃showed the presence of the CuBr2phase. The intensity of the peaks corresponding to CuBr2decreased with an increasing calcination temperature. When the sample was calcined at 250 ℃, the XRD peaks for CuBr were noticed at 2θ=27.08°, 44.98° and 53.30°, respectively, which were attributed to the planes (111), (220) and (311), respectively. The intensity of these characteristic peaks decreased or even disappeared when the calcination temperatures went up to 350 ℃ and 450 ℃. In addition, new crystallite phases of CuO were observed and the most intense signals were located at 2θ of 35.48°, 38.66° and 48.76°, respectively. This suggests that most of CuBr species have been transformed to CuO. In order to get more structural information, the thermogravimetric analysis method under air flow was conducted to further evaluate the effect of calcination temperature on the desulfurization performance. The differential scanning calorimetry-thermogravimetric analysis (DSCTGA) curves for raw bentonite and the CuBr2modified bentonite adsorbents, which were dried at 120 ℃ for 24 h, are shown in Figure 10 and Figure 11, respectively. In the curve of raw bentonite, no single remarkable peaks in TG and endothermic or exothermic curves were found. This phenomenon indicated that the framework of the bentonite was stable in this temperature range and most of the adsorbed water would be evaporated during the pretreatment of the sample at 120 ℃. Being different from the raw bentonite, a considerable weight loss and two endothermic peaks were observed for the CuBr2modi-fied bentonite, as it can be seen in Figure 11. Combined with XRD characterization, the weight loss peaks formed in the modi fied bentonite were typically attributed to the stepwise decomposition of CuBr2·xH2O, denoting that the peak in the range of 50—150 ℃ was caused by the evaporation of water adsorbed in the bentonite and the dehydration of crystalline water in CuBr2·xH2O, while the peak in the range of 200—400 ℃ was resulted from the transformation of CuBr2to CuBr and CuO.

Figure 10 TGA-DSC curves of the raw bentonite obtained under air flow

Figure 11 TGA-DSC curves of the CuBr2modified bentonite adsorbent obtained under air flow

4 Raman Characterization of the Cu (II) Complex

With the purpose of discovering the reaction mechanism of DMS and Cu (II), the Cu (II) complex was prepared thereby. The Raman spectra of the products are presented in Figure 12.

Figure 12 Raman spectra of CuBr2and the Cu (II) complex: (a) CuBr2; (b) Cu (II) complex

Cu (II) atoms, with extranuclear electrons configuration (1s22s22p63s23p63d94s0), can form the usual σ bonds using the vacant s-orbitals and p-orbitals[14]. As a result, the usual S-M (σ) bonds could be obtained if sulfur atoms of DMS could provide lone pair electrons to Cu (II)[14,18]. As shown in Figure 12, the distinct peaks of Cu-S stretching vibrations at 300 cm-1and C-S vibration at 700 cm-1were detected in the Cu (II) complex, which suggested that DMS could bind to the Cu (II) species without breaking up its C-S bonds in its molecule[19,20].

5 Conclusions

The XRD and TGA results have synergistically demonstrated that the bentonites loaded with CuBr2are excellent adsorbents for the removal of PM and DMS from liquid fuels. Moreover the bentonite, which was loaded with 15% of Cu (II) and baked at 150 ℃, exhibited a sulfur adsorption capacity of about 200 mg-S/g of adsorbent during the desulfurization of model oil containing about 2 000 μg/g of PM and 2 000 μg/g of DMS. The FT-IR analyses indicated that a certain amount of weak Lewis acid sites could contribute to the adsorption of sulfur compounds. The characteristics of the Cu-S and C-S stretching vibrations were simultaneously identified in the Raman spectra of the Cu (II) complex. On the basis of complex adsorption reaction and hybrid orbital theory, the adsorption of DMS on the CuBr2modified bentonite occurred via the formation of S-M (σ) bonds. More studies such as the performance of the CuBr2modified bentonite prepared by different methods or the reaction of CuBr2with bentonite excavated from different regions will be needed in order to fully understand the effect of copper loading and the combination mechanism of Cu (II) species with sulfur.

Acknowledgments: This work is financially supported by the National Natural Science Foundation of China (No. 21276086).

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date: 2014-08-22; Accepted date: 2015-01-09.

Dr. Meng Xuan, Telephone: +86-21-64252274; E-mail: mengxuan@ecust.edu.cn.