Ind. Eng. Chem. Res. 2006, 45, 2845-2849
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Feasibility Study on the Integration of Adsorption/Bioregeneration of π-Complexation Adsorbent for Desulfurization Wangliang Li,†,‡ Jianmin Xing,† Xiaochao Xiong,† Jiexun Huang,†,‡ and Huizhou Liu*,† Laboratory of Separation Science and Engineering, State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
A novel integration study on the feasibility of the use of a combined physical-biological procedure was reported for desulfurization of fuels. During this process, sulfur compounds are removed through adsorption and then biodegraded in a subsequent stage with Pseudomonas delafieldii R-8 strain, with the regeneration of the adsorbent. To select suitable π-complexation adsorbents for desulfurization, dibenzothiophene (DBT) was used as a model compound to study the adsorption capacity of these adsorbents. Different metal ions, namely Co2+, Ni2+, Ce3+, and Cu+ were used to prepare desulfurization adsorbents using the ion exchange method. The reduction behavior of Cu(II)-Y was studied by using temperature programmed reduction (TPR) with reducing gas (10% H2 and 90% N2, v/v). The acidity of the adsorbents was characterized by pyridine adsorption thermogravimetric-differential thermal analysis (TG-DTA) and Fourier transform infrared (FTIR). π-Complexation adsorbent Cu(I)-Y was successfully bioregenerated. The adsorption capacity of the regenerated adsorbent is 85% that of the fresh one after being regenerated with Pseudomonas delafieldii R-8, washed with n-octane, dried at 100°C for 24 h, and reduced in H2 and N2. After bioregeneration, the adsorbent can be reused. 1. Introduction Sulfur in transportation fuels remains a major source of air pollution. Ultradeep removal of sulfur from transportation fuels has become very imperative for the petroleum refining industry due to increasing stringent environmental regulations. The removal of sulfur compounds from fuels is carried out industrially via catalytic hydrodesulfurization (HDS). The HDS process is highly efficient in removing thiols, sulfides, and disulfides but is less effective for dibenzothiophenes and dibenzothiophene derivatives.1 Recently, to produce ultralow sulfur fuels, some non-HDS-based desulfurization technologies such as adsorptive desulfurization,2,3 extraction with ionic liquids,4 biodesulfurization,5,6 and charge transfer (CT) complex formation7 were thoroughly studied in the desulfurization of fuels. Desulfurization by adsorption is a method to remove sulfur compounds with modified metal oxides, molecular sieves, and activated carbons as adsorbents under ambient conditions.8 Selective adsorption of dibenzothiophenes on adsorbents might be the most economical method for the removal of organosulfur compounds. During the last few decades there has been an enormous amount of research conducted regarding adsorption desulfurization. The sulfur compounds can be removed from commercial fuels either via π-complexation,9-13 van der Waals,8 electrostatic interactions, or chemisorption.2,7,14 Desulfurization by adsorption faces two challenges: (i) to develop adsorbents with a high capacity and (ii) to find adsorbents with high selectivity toward sulfur compounds over other aromatic and olefinic compounds present in the fuels. The ab initio molecular orbital (MO) calculations showed that the π-complexation bonds for thiophene are stronger than those with benzene.12 Compared with aromatic and olefinic compounds, sulfur compounds * To whom correspondence should be addressed. Tel.:+86-1062555005. Fax: +86-10-62554264. E-mail address: hzliu@ home.ipe.ac.cn. † Institute of Process Engineering, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
can be selectively removed by π-complexation adsorbents. π-Complexation adsorbents were thoroughly studied in a number of fields for separation and purification such as desulfurization and denitrogenation and olefin/paraffin, diene/olefin, and aromatic/aliphatic separations.15-22 Weitkamp et al. reported that thiophene adsorbed more selectively than benzene on ZSM-5 zeolite.23 Organic sulfur compounds with more than one ring will be sterically hindered or excluded. Zeolites with larger pores as well as larger volumes will be more desirable than ZSM-5 as selective adsorbents. As reported in the literature, modified NaY, 13X, and mesoporous SBA-15 with transition metal ions Cu+ and Ag+ were used as π-complexation adsorbents for desulfurization.9,10,27 The adsorption capacity of sulfur compounds on adsorbents depends on the properties of the adsorbents. The mechanism of heterocyclic aromatic compounds adsorption on adsorbents has not been thoroughly studied in the literature. Jeevanandam24 prepared modified adsorbents based on nanocrystalline Al2O3, tested for adsorption of thiophenes, and found that modified adsorbents performed well toward thiophene adsorption compared with unmodified ones. The Lewis acid sites (silver) present on the surface of Ag-AP-Al2O3 act as adsorption centers for the thiophene molecule. Larrubia25 reported that the adsorption process of benzothiophene, dibenzothiophene, 4,6-dimethyldibenzothiophene, dibenzofuran, indole and carbazole on alumina, zirconia, and magnesia was due to adsorption on Lewis sites or on acid-base pairs. Biodesulfurization (BDS) can selectively remove sulfur from condensed thiophenes such as benzothiophene and dibenzothiophene (DBT) without losing the heat value.4,26 One of the advantages of BDS is to remove sulfur compounds from fuels under ambient conditions. The efficiency of the BDS process largely depends on a sufficient oil/water (O/W) contact, because the reactions proceed mainly at the interface.28 During the BDS process, the fuel desulfurized can be separated by the methods of centrifugation and de-emulsification, but the direct BDS
10.1021/ie051125l CCC: $33.50 © 2006 American Chemical Society Published on Web 03/16/2006
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process also has some disadvantages such as a low ratio of the oil/aqueous phase and low reactivity of microbial strains. In this paper, several π-complexation adsorbents were prepared by liquid phase ion-exchange methods. Pyridine adsorption thermogravimetric-differential thermal analysis (TGDTA) and Fourier transform infrared (FT-IR) were used to characterize the acidity of the adsorbents, and fluorescence was used to study the ability of different metal ions to form π-complexation with DBT. A novel process to remove sulfur compounds from fuels was developed by coupling adsorption and biodesulfurization. Sulfur compound, DBT, was first adsorbed on π-complexation desulfurization adsorbents, and then, the desorption of the adsorbed DBT is done in an oil phase and promoted by microbial conversion using a desulfurization strain. The desorbed DBT was converted to 2-HBP and sulfate, which was released into the water phase. During this process, the sketch of sulfur compounds and other aromatics adsorbed on the adsorbents has not been broken; therefore, sulfur compounds can be removed from fuels without losing the heat value in a real sense. At the same time, the adsorbent was regenerated. The amount of oil phase needed in the bioregeneration process is much less than that in the direct BDS process. Therefore, the cost of the separation of fuels from emulsion is much lower in the bioregeneration process. Furthermore, it is much easier to produce sulfur-free and ultralow sulfur by the method of first removing sulfur compounds by adsorption and then using bioregeneration than by the direct BDS method. A Pseudomonas delafieldii R-8 strain was used in our experiments. The adsorption capacity toward DBT of the regenerated adsorbent was investigated.29 2. Experimental Section 2.1. Materials. NaY zeolite was obtained from the Catalyst Plant of Qilu Petrochemical Company CNPC. DBT (purity, 99%) was purchased from Acros Organics, USA. 2-Hydroxybiphenyl (2-HBP) was purchased from Tokyo Chemical Industry, Ltd. (TCI), Japan. n-Octane was purchased from Shanghai Reagents Co., China National Pharmaceutical Group Corporation. Methanol was HPLC grade; sodium chloride and ethanol were analytical reagent grade. 2.2. Adsorbents. The starting adsorbent material was NaY zeolite. HY zeolite was obtained after ion-exchanging NaY with NH4NO3 at room temperature for 24 h and calcination of NH4-Y at 500 °C for 4 h in the air. All the adsorbents were prepared by liquid phase ion-exchange techniques. Co-, Ni-, Ce(III)-, and Cu(II)-Y were prepared by the ion-exchange of NaY with Co(NO3)2, NiCl2, Ce(NO3)3, and Cu(NO3)2 aqueous solutions (0.5 mol‚L-1) for 24 h at room temperature. The adsorbent was recovered by filtration and washed with deionized water, followed by drying at 100 °C for 24 h and calcinating at 500°C for 3 h. Cu(I)-Y was prepared by reduction of Cu(II)-Y to Cu(I)-Y in H2 and N2 (10:90, v/v) at 190 °C for 3 h. The amounts of transition metal in the ion-exchange solution were equivalent to a 5-10-fold cationexchange capacity. The ion-exchange process was operated at room temperature. 2.3. Characterization. The composition of the adsorbents was determined by the inductively coupled plasma (ICP) technique on Optima 5300 DV (Perkin-Elmer Inc.) after dissolution in a nitric acid-H2O2 mixture. The temperature programmed reduction (TPR) measurement was carried out in a system supplied by Tianjin Xianquan Apparatus Co. Ltd, China (TP 5000). During the TPR, the consumption of hydrogen was detected using a thermal conduc-
tion detector (TCD). An adsorbent sample of 200 mg was used for the measurement. The reduction was conducted in the atmosphere of an H2 and N2 mixture (volume ratio, 10:90) at a flow rate of 60 mL‚min-1. The temperature was programmed to increase from room temperature to 450 °C at a heating rate of 10 °C‚min-1. The nature and strength of the acidity of the adsorbents were determined by pyridine adsorption. The sample was heated to 400 °C in flow of inert gas (N2) for 2 h. It was cooled to 100 °C, and pyridine was adsorbed on the sample. The physisorbed pyridine was removed by flushing the adsorbents with N2 for 30 min at 100 °C. The spectra were recorded after maintaining the temperature for 30 min. Thermogravimetric-differential thermal analysis was carried out in SP-4320 (Shanghai Precision Scientific Instruments Co. Ltd.) thermal balance. About 20 mg of adsorbents was loaded, and the N2 flow used was 50 mL‚min-1. The heating rate was 10 °C‚min-1, and the final temperature was 700 °C. The number of weak acid sites, midstrong acid sites, and strong acid sites were determined by the amount of pyridine desorbed from the adsorbent at temperatures Cu2+ ) Ni2+ > Co2+ > Ce3+. 3.2. TPR Study of Adsorbent Cu(II)-Y. The reduction of Cu(II)-Y was studied by TPR with reducing gas (10% H2 and 90% N2). As shown in Figure 1, the reduction took place in two steps with peak temperatures at 190 and 240 °C. The first peak corresponded to the reduction of Cu2+ to Cu+, and the second was for Cu+ to Cu0. Thus, with the given reducing gas, the optimum temperature for reduction of Cu(II)-Y was approximately 170-200 °C. The large difference in intensities of the two peaks was likely caused by incomplete conversion of Cu2+ to Cu+ because the reduction process is affected by the location and the amount of Cu(II) ions exchanged in the Y zeolite. To ensure the complete reduction of Cu2+ to Cu+, Cu(II)-Y should be left in the reducing gas for 3 h. Cu+ can be changed into Cu0 at a higher reducing temperature which can also decrease the adsorption capacity. The adsorbent Cu(I)-Y was prepared by reducing Cu(II)-Y at 190 °C for 3 h. 3.3. Acidity Characterization of Adsorbents. FT-IR of adsorbed pyridine was conducted to elucidate the nature and relative amounts of Brønsted and Lewis acid sites in the adsorbents. Generally, bands around 1540-1548 and 14451460 cm-1 are characteristic of Brønsted (Py-H+) and Lewis (L-Py) acid sites, respectively.30 Moreover, bands of hydrogenbonded pyridine (HB-Py) are expected in the range of 14401447 and 1580-1600 cm-1 and bands of physically adsorbed pyridine (Ph-Py) are expected at 1439 and 1580 cm-1. However, the thermal stability of the adsorbed pyridine species differs, increasing in the order Ph-Py < HB-Py < L-Py, Py-H+. Accordingly, Lewis-bonded pyridine can be properly determined from a record of the infrared spectra at sufficiently high temperatures, to accomplish complete desorption of HB-Py.31 From Figure 2, it can be seen that most of the acidity of the adsorbents modified by transition metal ions (Ce-Y, Ni-Y, Cu(I)-Y, Co-Y) is Lewis acidity. The Brønsted acidity of adsorbent HY is much stronger than that of other adsorbents. It can be concluded that the transition metal ions provide the Lewis acid centers. The acidity of these adsorbents determined by pyridine adsorption is shown in Table 2. It can be seen that metal ions have significant effects on the acidity of these adsorbents, especially on the midstrong and strong acidity. The number of total acid sites decreases with this sequence: HY > Cu(I)-Y > Co-Y > Ni-Y > Ce(III)-Y. 3.4. Adsorption Process. DBT was used as a model compound to study the adsorption capacity of the adsorbents. Adsorption capacities for sulfur removal depend on the preparation method of the adsorbents and transition metal ions. Modified adsorbents based on NaY zeolite with incorporated soft Lewis acid sites were prepared and tested for adsorption of DBT. As shown in Figure 3, based on equilibrium adsorption
Figure 1. TPR spectra of Cu(II)-Y with reducing gas (10% H2 and 90% N2) at a heating rate of 10 °C‚min-1.
Figure 2. FT-IR spectra of adsorbed pyridine on modified adsorbents. Table 2. Acidity of Adsorbents Determined by Pyridine Adsorption acid sites (mmol‚g-1) adsorbent
weak
midstrong
strong
total
Cu(I)-Y Co-Y Ni-Y Ce(III)-Y H-Y
0.70 0.75 0.70 0.67 0.75
1.06 0.96 1.13 0.85 0.93
0.69 0.65 0.50 0.62 0.79
2.45 2.36 2.33 2.14 2.47
data, it was found that the sorbent capacities for sulfur removal decreased with this sequence: Cu(I)-Y > Co-Y > Ni-Y > H-Y > Ce-Y. The adsorption capacities of DBT on the adsorbents follow the same sequence as Lewis acidities of the adsorbents. From above acidity characterization, it can be seen that the Lewis acid sites provided by transition metal ions and Al3+ in the framework of NaY zeolite act as adsorption centers for DBT. 3.5. Bioregeneration of Cu(I)-Y. Cu(I)-Y has a large adsorption capacity for thiophenes.12 During the adsorption process, DBT can be adsorbed on the surface of adsorbent quickly. Bioregeneration of the spent adsorbents takes place as a result of interaction between microorganisms and molecules of the DBT adsorbed. The porous structure of the adsorbent is one factor that largely determines the rate and extent of biological regeneration. During the regeneration process, a certain amount of oil phase such as n-octane was added into the regeneration system. Most of the DBT molecules adsorbed
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4. Conclusion
Figure 3. Adsorption capacity of different adsorbents.
A series of π-complexation adsorbents based on NaY zeolite were prepared by the liquid phase ion-exchange method. The adsorbent Cu(I)-Y was prepared by reducing Cu(II)-Y in reducing gas (10% H2 and 90% N2 ) at 190 °C for 3 h. Dibenzothiophene was chosen as a model compound. The effects of different transition metals on the adsorption capacity of DBT were studied. Among these adsorbents, Cu(I)-Y has the largest adsorption capacity. The acidity of the adsorbents was characterized by pyridine adsorption TG-DTA and FT-IR. The Lewis acid sites provided by transition metal ions and Al3+ in the framework of the NaY zeolite act as adsorption centers for DBT. During the process of bioregeneration, the amount of DBT first increases and then decreases within 24 h. The amount of 2-HBP increases rapidly in the first 6 h and then increases slowly. The adsorption capacity of the regenerated adsorbent is 85% of that of the fresh one after being desorbed with Pseudomonas delafieldii R-8, washed with n-octane, dried at 100 °C for 24 h, and reduced in H2 and N2. π-Complexation adsorbent Cu(I)-Y can be successfully regenerated by a biological method. Acknowledgment This work was supported financially by the National Natural Science Foundation of China (Grant No. 30370046), State Major Basic Research Development Program of China (Grant No. G2000048004), and the National High Technology Research and Development Program of China (Grant No. 2002AA213041). Literature Cited
Figure 4. Effects of time on the desorption of adsorbents Cu(I)-Y.
can be desorbed from the surface of the adsorbent by contacting with the oil phase. Oil, water, and biocatalysts can be fully mixed by agitation and emulsified into microdroplets at the oil/ water interface. DBT molecules can be converted into 2-HBP and sulfate by microbial cells at the oil/water interface. These can reduce the DBT concentration in oil phase and promote DBT desorbing from the surface of the adsorbent. Therefore, the adsorbent has been regenerated. The relationship between the amounts of DBT and 2-HBP and the reaction time is shown in Figure 4. The amount of DBT first increases and then decreases within 24 h. The amount of 2-HBP increases rapidly in the first 6 h and then increases slowly. 3.6. Assessment of Bioregenerated Cu(I)-Y Adsorption Capacity. For Cu(I)-Y, Cu+ species are highly unstable in the presence of water. Cuprous ions are known to disproportionate as follows: 2Cu+ T Cu2+ + Cu0.32 During the process of bioregeneration, the spent Cu(I)-Y adsorbents were added into the Pseudomonas delafieldii R-8 suspension. The adsorbents were contacted with water and exposed to air; as a result, Cu+ was disproportionated into Cu2+ and Cu. To assess the adsorption capacity of the bioregenerated adsorbents, the bioregenerated adsorbents should be reduced in H2 and N2 (5:95 v/v). The adsorption capacity was tested with 15 mmol‚L-1 DBT when the ratio of oil to adsorbent was 100 mL‚g-1. The adsorption capacity of the regenerated adsorbent was 85% of that of the fresh one after being desorbed with Pseudomonas delafieldii R-8, washed with n-octane, dried at 100 °C for 24 h, and reduced in H2 and N2.
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ReceiVed for reView October 7, 2005 ReVised manuscript receiVed February 19, 2006 Accepted February 21, 2006 IE051125L
