Huang Huan; Salissou M. Nour; Yi Dezhi; Meng Xuan; Shi Li
(State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237)
Study on Reactive Adsorption Desulfurization of Model Gasoline on Ni/ZnO-HY Adsorbent
Huang Huan; Salissou M. Nour; Yi Dezhi; Meng Xuan; Shi Li
(State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237)
The reactive adsorption desulfurization of model gasoline was carried out on Ni/ZnO-HY adsorbent. The Ni/ ZnO-HY adsorbent was characterized by N2adsorption-desorption test (BET), X-ray diffractometry (XRD), and temperature-programmed reduction (TPR) analysis. The test results have demonstrated that HY-zeolite is a feasible support for Ni/ ZnO components used in reactive adsorption desulfurization. The results of XRD and TPR analyses showed that most of nickel element was present as Ni2+species with only a small part existing as Ni3+species, and the Ni2+species had interactions with HY-zeolite. Under the conditions of this study, which specified a 50% ratio of HY-zeolite in the adsorbent, a Zn/ Ni molar ratio of 10, and a reduction temperature of 400 ℃, the Ni/ZnO-HY adsorbent showed the best desulfurization performance. The sulfur capacity of Ni/ZnO-HY adsorbent could be recovered to 92.19% of the fresh one after being subjected to regeneration at 500 ℃, and could be maintained at 82.17% of the fresh one after 5 regeneration cycles.
desulfurization; reactive adsorption; S-Zorb; Ni/ZnO; HY-zeolite
The environmental regulatory agencies are continuing to promulgate more stringent standards and requiring more improvements in the quality of transportation fuels in order to reduce the sulfur emissions from motor vehicles[1]. Sulfur is a naturally occurring compound in transportation fuels. When fuel is burned, the sulfur combines with oxygen (SOx) to create emissions that have negative environmental and health effects. These emissions contribute to air pollution, cause acid rain and decrease the lifetime of any operating system and units, in particular those catalytic converters, by deactivating the metal catalyst as well as corroding of internal combustion engine components[2]. Therefore, sulfur removal from transportation fuels, in particular from gasoline and diesel, has become very important in the petroleum re fining industry. In a typical refinery gasoline pool, over 90% of the sulfur species originates from the fluid catalytic cracking (FCC) units. The main sulfur components encountered in FCC gasoline include thiols, sul fides, thiophenes, alkylthiophenes, tetrahydrothiophene, thiophenols and benzothiophenes[3-4]. In term of technological measures, the current conventional hydrodesulfurization (HDS) process is not suitable for the removal of thiophenic compounds partially because of its high hydrogen consumption and high temperature reactors which lead to a maximum cost and significant reduction of gasoline octane number due to olefin saturation[5]. For example, the presence of 100 ppm of thiophene in a simulated gasoline mixture caused the volume fraction of H2in the effluent from the fuel processing unit to drop from 60% to 40% over 25 hours of reaction duration[6]. Consequently, it is difficult to achieve such stringent conditions efficiently for obtaining an ultra clean gasoline.
Several non-HDS-based desulfurization technologies such as adsorptive desulfurization[7-8], oxidative desulfurization[9-10], extraction using ionic liquids[11-12], biocatalytic treatment[13], etc., have been proposed recently for the desulfurization of liquid fuels. Among them, the reactive adsorption desulfurization (RADS) is considered to be one of the most promising approaches for deep desulfur-ization because it combines the advantages of both the catalytic HDS and adsorption[14-15]. Among the adsorbents used in the RADS process, Ni/ZnO was found to be the effective adsorbent combination for removing sulfur species. Ni functions as hydrodesulfurization sites, while ZnO takes up the resulting H2S which is converted into ZnS simultaneously[16]. Tawara, et al.[14]first reported that Ni/ZnO could be used as an “adsorptive catalyst” for removing sulfur in kerosene to less than 0.1 wppm. After that, several papers[2,17-19]have been published on fuel desulfurization by Ni/ZnO adsorbents based on the mechanism of RADS, while confirming that Ni/ZnO adsorbent showed quite good performance in RADS. Y zeolite is one of the most popular catalyst/adsorbent supports in the world and is also used in adsorptive desulfurization extensively. Yang and coworkers[7]reported that thiophenes, including 4,6-R2DBT, can be removed from petroleum feedstocks by adsorption on Y zeolites containing ionexchanged Cu+and Ag+ions. Song and coworkers[20]also researched on the selective adsorption for removing sulfur compounds from transportation fuels over transition metal ion exchanged Y zeolite adsorbents and achieved a high sulfur capacity. Both Ni/ZnO and Y zeolite showed good adsorptive desulfurization performance, but few efforts have been made to study the RADS performance over adsorbents composed of Ni/ZnO supported on Y zeolites.
In the present study, a series of Ni/ZnO-HY adsorbents were prepared and characterized by N2-adsorption analysis (BET), X-ray diffractometry (XRD), thermogravimetric analysis (TGA) and temperature-programmed reduction (TPR) technique. To evaluate the RADS activity of the Ni/ZnO adsorbents, a series of RADS experiments were performed using a fixed-bed reactor. The main objective of this work is to study the efficiency of Ni/ZnOHY adsorbents for RADS and to link the result to their surface chemistry and structural characteristics.
2.1 Materials
The HY zeolite used in this study was purchased from the Wenzhou Catalyst Co., Ltd. Zinc oxide (ZnO), nickel oxide (Ni2O3) andn-octane (AR grade) were obtained from the Sinopharm Chemical Reagent Co., Ltd. Thiophene was obtained from Aldrich. In this study, a model gasoline was prepared by adding thiophene to n-octane with the sulfur concentration equating to 2 000 μg/g.
2.2 Adsorbent preparation
The Ni/ZnO-HY adsorbent was prepared by the kneading method. In compliance with the adsorption ratio, a specified amount of HY-zeolite (50%) was ground in a mortar with powdered nickel oxide (Ni2O3) and zinc oxide (ZnO). After grinding of the mixture, nitric acid was added to the mixture to form an adhesive paste. The slurry is approximately extruded in 1-mm-diameter extrudates, followed by drying in the oven at 120 ℃for 12 h. Finally the resulting materiel is calcined at 600 ℃for 1 h in air in a muf fle furnace. The pellets were crushed and sieved to 20—40 mesh before use.
2.3 Catalyst characterization
2.3.1 BET surface area and pore size measurements
The N2adsorption and desorption isotherms were measured at -196 ℃in a Micromeritics ASAP 2010 instrument. Samples were preliminarily heated and outgassed overnight at 120 ℃under a constant vacuum pressure of 10-5torr. The pore size distribution and the speci fic surface area were respectively calculated using the Barret-Joyner-Halenda (BJH) equation and the Brunauer, Emmett, and Teller (BET) equation.
2.3.2 X-ray diffraction
The X-ray diffraction (XRD) patterns were acquired on a D/Max 2550VB/PC type diffractometer system (Rigaku, Japan) equipped with a curved graphite monochromator using Cu-Kα radiation (λ= 1.540 56?). The system was operated at 40 kV and 100 mA with a scan rate of 12 (°)/min. The powder X-ray patterns were collected within a 2θrange of 10°—80°.
2.3.3 Temperature programmed reduction
TPR experiments were performed to determine the reducibility of the surface oxides. Prior to the TPR experiments, the samples were pretreated in a He flow up to 250 ℃and kept at that temperature for 1 h to remove the adsorbed water and other contaminants followed by cooling down to 25 ℃. The reducing gas containing 5% of H2balanced with Ar mixture was passed over the samples at a flow rate of 30 mL/min and a heating rate of 10 ℃/min up to 800 ℃and was then kept at that temperature for 20 min.
2.3.4 Adsorption activity test
The RADS experiments were performed in a fixed-bed reactor at a reaction temperature (T) of 370 ℃, a reaction pressure (P) of 1 MPa, a WHSV (weight hourly space velocity) of 4 h-1, and a H2/oil volume ratio of 400. About 1.057 4 g of the adsorbent was put in a stainless steel column, 6 mm in bed diameter and 250 mm in length. The packed column was placed in a multi-channel convection oven designed in our laboratory for performing the adsorption experiments. In order to ensure that the Ni element in the Ni/ZnO-HY adsorbent is in the reduced form, the adsorbent bed was pretreated with H2gas at a flow rate of 40 ml/min under a pressure of 0.5 MPa at 400 ℃, and was then kept at this temperature for about 1 h. The outflow products were collected in a cryogenic trap with ice water bath and subjected to analysis regularly. The sulfur content of the feedstocks and products were determined by an Antek 9 000 S total sulfur analyzer. The sulfur removal (x) and sulfur capacity (a) were calculated according to:
whereC0is the initial mass fraction of sulfur, g/g,Cis the final mass fraction of sulfur, g/g,Sis the WHSV (h-1) andtis the liquid flow time (h).
3.1 Nitrogen adsorption-desorption test
N2adsorption-desorption test was performed to better investigate the physical properties of Ni/ZnO-HY adsorbents. Nitrogen adsorption-desorption isotherms in Figure 1 show that there was no significant change in the morphology of the three samples. All curves presented the same shape and resembled the reversible type I adsorption/desorption isotherms. Two steep rises were observed in the isotherm, the first one atp/p0<0.01 was typical for microporous solid, the second one atp/p0>0.95 exhibited that macroporous structure also existed in the adsorbent[21]. This characteristic was also confirmed by pore size distribution curves of adsorbents shown in Figure 2. According to BJH analysis of the desorption branch of isotherms, the ratio of macropores decreased while the ratio of micropores increased with increase of the proportion of HY-zeolite, which indicated that macroporous structure was mainly formed by NiO and ZnO. Table 1 summarizes the main textural characteristics of adsorbents with different proportions of HY-zeolite (70%, 50% and 30%). With an increasing ratio of the HY-zeolite, the BET surface area, the total pore volume, and the micropore volume also increased, while the average pore size decreased. This behavior can be attributed to the fact that zeolite pores can be blocked when the Zn and Ni ions are in large amount at sites inside the cage which can reduce also the total pore volume and the total surface area.
Figure 1 Nitrogen adsorption-desorption isotherms at -196 ℃for different HY-zeolite ratios (Zn/Ni mole ratio = 10)
Figure 2 Pore size distribution curves of samples with different HY-zeolite proportions (Zn/Ni mole ratio = 10)
Table 1 Nitrogen adsorption isotherm parameters of samples with different proportions of HY-zeolite (Zn/Ni mole ratio = 10)
3.2 Characterization of adsorbents XRD patterns
Figure 3 exhibits the evolution of the main crystalline phases of Ni/ZnO-HY adsorbents before and after adsorption desulfurization reaction. It can be seen that the characteristic diffraction peaks of NiO cubic (at 2θ=37.240°, 43.286°, 62.880°, and 75.430°) completely disappeared after the catalyst was reduced to form new Ni cubic phases (at 2θ=39.100°, 44.520°, and 51.846°). After reaction, the intensities of ZnO characteristic diffraction peaks and those of Ni all decreased. In addition, new hexagonal crystalline phases of ZnS were formed with the diffraction peaks identified at 2θ=28.633°. It is well know that Ni0is very sensitive to sulfur, and can be converted to Ni3S2in the presence of sulfur compounds, while ZnO can react on H2S efficiently and is inactive to organic sulfur compounds like thiophene[22]. It can be seen from our XRD patterns that most of the ZnO is sulfidized while no Ni3S2but Ni0is found, indicating that the catalyst effect on thiophene desulfurization may synergistically occur over Ni/ZnO-HY adsorbent. The XRD patterns of the adsorbent after desulfurization reaction confirm that Ni may function as hydrodesulfurization sites, while ZnO plays a crucial role by taking up S from H2S and converting it to ZnS in the desulfurization process, which is in accordance with the RADS mechanism reported by former researchers[2,20].
Figure 3 XRD patterns of ZnO/NiO adsorbents before and after desulfurization reaction
3.3 Temperature programmed reduction (TPR) characterization
The TPR profile of Ni/ZnO-HY adsorbent (with Zn/Ni molar ratio equating to 10, and HY zeolite amount equating to 50%) is presented in Figure 4. Three hydrogen consumption peaks are observed in the figure. The first micro peak withTmaxat about 275 ℃ can be attributed to the reduction of Ni2O3according to the literature data[23], evidencing the presence of a small amount of Ni2O3in the Ni/ZnO-HY adsorbent. Normally, the NiO species without interaction with the support can be reduced atTmaxof 360—370 ℃[24], but the interaction of NiO with the support may decrease its reducibility[25]. Therefore, the second peak withTmaxat about 405 ℃ can be assigned to the reduction of NiO which has interactions with the HY zeolite. The third broader peak withTmaxat about 620 ℃can be ascribed to the reduction of a part of ZnO species located on the solid surface[26-27].
Figure 4 TPR profile of Ni/ZnO-HY adsorbent (Zn/Ni mole ratio = 10, HY zeolite proportion = 50%)
3.4 Effect of HY-zeolite proportion on the desulfurization performance
The results of RADS of model gasoline over Ni/ZnO-HY adsorbents with different HY-zeolite proportions operating under identical experimental conditions are shown in Figure 5. It can be seen from Figure 5 that the desulfurization performance of all samples plunged rapidly at the beginning of the experiment, and stabilized when the volume of treated model gasoline increased to about 8 mL.It should be noted that a thiophene concentration (2 000 μg/g) which was much higher than that found in the FCC gasoline (typically in the range of 500—800 μg/g) of ordinary refineries was used for these experiments. The desulfurization capacity of the adsorbents first increased with an increasing proportion of HY-zeolite and reached a maximum when the proportion of HY-zeolite was 50%, and the desulfurization capacity sharply dropped when the proportion of HY-zeolite increased to 60% and 70%, respectively. As it has been studied in the nitrogen adsorption test, the BET surface area, the total pore volume, and the micropore volume also increased with an increasing HY-zeolite quantity in the adsorbent. However, the desulfurization activity did not follow this trend, which indicated that a proper proportion of HY-zeolite could provide a necessary surface area to support the active components of RADS adsorbent and could make the active components NiO and ZnO better dispersed. But if the proportion of HY-zeolite in the adsorbent is too high, the decrease of the amount of active components may influence the desulfurization capacity.
Figure 5 Breakthrough curves on Ni/ZnO-HY at different HY-Zeolite proportions (Zn/Ni mole ratio = 10)
3.5 Effect of Zn/Ni mole ratio on the desulfurization performance
Figure 6 shows the breakthrough curves on Ni/ZnO-HY adsorbents at different Zn/Ni molar ratios. It can be seen from Figure 6 that the evolution of Zn/Ni molar ratios and the desulfurization performance of adsorbents at first increased with an increasing Zn/Ni molar ratio and then decreased afterwards. It can be clearly seen that at a Zn/Ni molar ratio of 10, the desulfurization rate was the highest and the adsorbent’s sulfur capacity reached 12.97 mg of S on one gram of adsorbent. The effect of Zn/Ni molar ratio on the crystal phase evolution was investigated and the obtained XRD patterns with their relative intensities versus the diffraction angle are shown in Figure 7.
Figure 6 Breakthrough curves on Ni/ZnO-HY at different Zn/Ni molar ratios (at HY zeolite proportion = 50%)
Figure 7 X-ray diffraction patterns of Ni/ZnO-HY adsorbent at different Zn/Ni molar ratio (HY zeolite proportion = 50%)
The characteristic diffraction peaks can be identified for ZnO at 2θ=31.741°, 34.400°, 36.240°, 47.520°, 56.560°, 62.840°, 67.940°, and 69.060°, respectively, and the diffraction peaks for NiO can be found at 2θ=37.220°, 43.260°, 75.380°, and 79.380°, respectively, with their respective phase reflections identified at (100), (002), (101), (102), (110), (103), (112), (201) and (111), (200), (311), (222), respectively. The presence of these reflections confirms the polycrystalline nature of the sample, in which the ZnO is hexagonal and NiO is cubic in crystalline structure. As it has been illustrated, the peak intensity of ZnO becomes sharper with an increasing moles of Zn(coupled with a decrease of moles of Ni), indicating to an increase in the crystallinity of ZnO. However, it leads to the appearance of new ZnO phases (at 2θ=66.359° and 76.959° with Zn/Ni molar ratio=1). When the Zn/Ni molar ratio was 10, the NiO peaks tended to completely disappear which indicated that NiO crystal particles formed a mono-dispersion state on the support at this Zn/Ni ratio. In comparison with the breakthrough curves, it can be found that the dispersion of NiO crystal particles contributes to the desulfurization activity of Ni/ZnO-HY adsorbent. In addition, the stabilization of the layered structure with increase in the moles of Zn may also be attributed to the lower electronegativity of Zn (1, 65) than that of Ni (1, 91) which makes the —OH groups more strongly bound to Zn[28]. The MDI Jade software was used to fit selected peaks of the XRD pattern to determine the peak positions and characterize the crystal lattices. It showed that the FOM (Figure of Merit) of different ZnO upon increasing Zn/Ni molar ratio are (3; 1.6; 0.8; 0.7; 1). The smaller the FOM is, the more likely the hit would be. Therefore we conclude that the adsorbent at a Zn/Ni molar ratio of 10 showed a highest stability at the relevant proportion of ZnO and NiO, which also contributes to the desulfurization activity of Ni/ZnO-HY adsorbent.
3.6 Effect of reduction temperature on the desulfurization performance
Sufficient hydrogen reduction pretreatment can greatly improve the adsorptive desulfurization abilities of nickelbased adsorbents, while insufficient reduction pretreatment can inhibit the desulfurization performance of Ni-Y zeolite adsorbents[29]. Therefore, the temperature of hydrogen treatment should be investigated to achieve the best desulfurization ability. The effect of reduction temperatures on the desulfurization activity is presented in Figure 8, which indicates that the reduction temperature at 400 ℃exhibited the best desulfurization performance. As it has been mentioned before, in Ni/ZnO adsorbents, Ni may function as hydrodesulfurization sites, while ZnO plays a crucial role by taking up S from H2S and converting it to ZnS during the RADS process, and it is better to reduce all the nickel species to Ni0and keep the zinc species in ZnO phase simultaneously. By taking into account the TPR results, the reduction temperature at 350 ℃is not sufficient to reduce Ni2+to Ni0. Tang, et al.[29]reported that insufficient reduction may produce Ni+, which can stably exist in the Y-typed zeolite due to its special textural characterization, and the desulfurization capacity is inhibited when Ni+cations exist. According to the TPR profiles, reduction at a temperature range of between 450 ℃and 500 ℃may reduce a part of ZnO to Zn, which will also reduce the desulfurization capacity of the adsorbent.
Figure 8 Breakthrough curves on Ni/ZnO-HY at different reduction temperatures (at Zn/Ni mole ratio = 10, and HY zeolite proportion = 50%)
3.7 Regeneration performance
Desulfurization adsorbents are required to be regenerable for multiple cycles of operation to reduce the operating cost and the adsorbent material cost. The regeneration tests of Ni/ZnO-HY adsorbents were carried out at various regeneration temperatures and after multiple regeneration cycles at an air flowrate of 100 mL/min for 2 h to evaluate the regenerability of adsorbents. Effects of the regeneration temperatures and regeneration circulating times are shown in Figure 9. It can be seen from Figure 9 that the regeneration temperature has a significant effect on the performance of the regenerated adsorbents. The adsorbents at a regeneration temperature of 500 ℃showed the best performance among the samples tested and could recovered 92.19% of the initial sulfur capacity of the fresh Ni/ZnO-HY adsorbent. After 5 regeneration cycles, the sulfur capacity of the adsorbents could also be recovered to 82.17% of the initial sulfur capacity of the fresh one. The results indicated that Ni/ZnO-HY adsorbent is highly regenerable for operating at multiple cycles.
Figure 9 Effect of regeneration temperature and regeneration cycles of Ni/ZnO-HY at a Zn/Ni molar ratio of 10, and HY zeolite proportion of 50%
HY-zeolite is proved to be a feasible support for Ni/ZnO components used in reactive adsorption desulfurization. A 50% proportion of HY-zeolite in the adsorbent can provide necessary surface area and pore structure to support the active components of RADS adsorbent and achieve the best desulfurization performance. Most of nickel element was present as Ni2+species with a small part existing as Ni3+species according to the results of XRD and TPR analyses. NiO crystal particles can form a mono-dispersion state on the support when the Zn/Ni molar ratio is 10, and the dispersion of NiO crystal particles contributes to the desulfurization activity of Ni/ZnO-HY adsorbent. Hydrogen reduction pretreatment at a temperature of 400 ℃for 1 h is in favor of the desulfurization activity on Ni/ZnO-HY adsorbent. The Ni/ZnO-HY adsorbent is highly regenerable for multiple-cycle applications, and the best regeneration condition is specified at 500 ℃for 2 h.
Acknowledgment:This work is financially supported by the National Natural Science Foundation of China (No. 21276086) and the Opening Project of State Key Laboratory of Chemical Engineering of East China University of Science and Technology (No. SKL-ChE-11C04).
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Recieved date: 2013-05-03; Accepted date: 2013-05-25.
Prof. Shi Li, Telephone: +86-21-64252274; E-mail: yyshi@ecust.edu.cn.