Preparation of High Performance Adsorbents by Functionalizing

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SEPARATIONS Preparation of High Performance Adsorbents by Functionalizing Mesostructured Silica Spheres for Selective Adsorption of Organosulfur Compounds Liming Yang, Yujun Wang, Dan Huang, Guangsheng Luo,* and Youyuan Dai The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China

Micrometer-sized silica spheres with uniform spherical morphology and highly ordered mesostructures were synthesized by a temperature-induced and polyethylene glycol (PEG)-assisted assembly method. A modified and convenient incipient wetness impregnation method was then presented, and high performance adsorbent with silver nitrate (AgNO3) supported on micrometer-sized mesoporous silica spheres was prepared. The characteristics of adsorbing benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-Me2DBT) from n-octane solution on the adsorbent were studied, and the equilibrium absorption data were described with the Freundlich equation and the Langmuir equation separately. The experiment of adsorptive desulfurization was also carried out in fixed-bed, and the results showed that the adsorption capacity reached as high as 20.5 mg S/g of adsorbent for DBT. Introduction The deep desulfurization of transportation fuels is becoming a more and more important issue in recent years because of increasingly stringent environmental regulations around the world1 and rapid development of fuel cells.2-8 Desulfurization by selective adsorption at an ambient temperature without H2 is a prospective process, and the synthesis of special adsorbents with high performance is a significant and challenging work. During the past decade, all of the commercially available adsorbents, such as zeolites, activated carbon, and activated alumina, et al., have been studied to be used in liquid fuel desulfurization by selective adsorption. Recently, scientists have showed that Y-zeolites exchanged with metal ions (such as Ag+, Cu+, Ni2+, et al.) are excellent adsorbents based on π-complexation for desulfurization of simulated and commercial transportation fuels, which are superior to all previously reported adsorbents in this application.2-22 Because the pore dimensions of Y zeolites are comparative with dibenzothiophene (DBT) and its homologues, such as 4,6-dimethyldibenzothiophene (4,6-Me2DBT), dibenzothiophenes will be sterically hindered when they transfer into the micropores of zeolite. Therefore, it is highly required to synthesize adsorbents with larger pore size. Mesoporous silica with high surface areas, large pore volumes, controlled nanostructures, and macroscopic morphologies can serve as the high performance support for the adsorbent.23-26 McKinley et al.25 have reported the use of solidphase extractants (SPE) consisting of AgBF4 adsorbed on mesoporous SBA-15 for the removal of DBT from n-octane solution feedstock. In their experiments, the maximum adsorption capacity was 6.9 mg S/g adsorbent. In our previous research,26 SO3Ag-SBA-15 via ion exchange of SO3H-SBA15 with Ag+ has already been used to remove DBT from n-octane solution. The adsorption capacity reached 17.0 mg S/g of SO3Ag-SBA-15 in a single equilibrium experiment. * To whom correspondence should be addressed. Tel.: +861062783870. Fax: +861062770304. E-mail: [email protected].

Herein, we synthesized micrometer-sized silica spheres with uniform spherical morphology and highly ordered mesostructures,27 which can be easily packed into the existing adsorption column. A modified and convenient incipient wetness impregnation method was then developed to prepared high performance adsorbent with AgNO3 supported on micrometer-sized mesoporous silica spheres. Finally, the equilibrium adsorption characteristics and the fixed-bed adsorption performance of the adsorbent adsorbing sulfur compounds from n-octane or the mixture of n-octane and 2-methylnaphthalen (2-MNP) were studied. Experimental Section Materials. The chemicals used for the synthesis were as follows: F127 (EO106PO70EO106) was purchased from Sigma, tetraethyl orthosilicate (TEOS) was purchased from Xilong Chemical Reagent Corp., and benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6Me2DBT) were purchased from Acros. Other reagents were obtained from Beijing Chemical Reagent Corp. All of the chemicals were analytical reagents, and they were used as received without further purification. Synthesis. Mesoporous silica spheres were prepared with F127 as the template. In a typical synthesis, 2.0 g of F127 and 1.0 g of polyethylene glycol (PEG 20 000) were dissolved under stirring in 100 g of a 0.1 M HCl aqueous solution, and then 9.5 g of TEOS was added. The resultant mixture was stirred for 16 h at room temperature to obtain a clear solution. Next, the solution was transferred into an autoclave and treated at 100 °C for 24 h. Finally, the solid product was filtered, washed with deionized water and ethanol, air-dried at 80 °C overnight, and then calcined at 550 °C for 6 h. Adsorbent with AgNO3 supported on micrometer-sized mesoporous silica spheres was prepared according to the following procedures: typically, 1.0 g of silica spheres was added into 15 g of 15 wt % aqueous AgNO3 solutions. The resultant

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mixture was stirred for 5 min at room temperature. Next, the solid product was filtered by vacuum and air-dried at 80 °C overnight. Sample Characterization. Scanning electron microscopy (SEM) observations were performed on a Hitachi S-450 microscope operating at 20 kV. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Quantachrome Autosorb-1-C Chemisorption-Physisorption Analyzer after the samples were outgassed for 45 min at 150 °C. The BET surface area was calculated from the adsorption branches in the relative pressure range of 0.05-0.25, and the total pore volume was evaluated at a relative pressure of about 0.99. The pore diameter and the pore size distribution were calculated from the desorption branches using the Barrett-Joyner-Halenda (BJH) method. The loaded amount of Ag on the silica spheres was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) on a Perkin-Elmer Optima 3300RL. Adsorption Experiments. The adsorbent (denoted as AgNO3) was used to remove BT, DBT, or 4,6-Me2DBT from n-octane solution typically by adding 100 mg of AgNO3 to 5 cm3 of n-octane solution containing about 500 ppmw of sulfur as either BT, DBT, or 4,6-Me2DBT. This mixture was stirred for 30 min at room temperature, and then centrifugated to separate the adsorbents from the oil. The amount of BT, DBT, or 4,6-Me2DBT in the n-octane phase was determined by gas chromatography (GC) methods using an HP 6890 with a flame ionic detector, whose detection limit is 0.2 ppmw S as DBT. The weight ratio of AgNO3 and n-octane solution was changed to obtain various equilibrium solution concentrations. The fixed-bed adsorption desulfurization experiments were carried out via pumping the model diesel oil through a stainless steel column (4.6 × 250), which contained 1.64 g of adsorbent at a flow rate of 0.5 cm3/min. The adsorbent was used without activation. The AgNO3 column was regenerated by passing diethyl ether (Et2O) through it at a flow rate of 0.5 cm3/min according to the literature.25 The sulfur content of the effluent was analyzed as above. Results and Discussion Synthesis of Mesoporous Silica Spheres. For the classical synthesis of SBA-16, concentrated HCl (i.e., 2 M) is used to catalyze TEOS to hydrolyze rapidly. Phase separation usually happened during the hydrolysis process, and the irregular particles formed as a result. In the current method, a stable silica sol was obtained after the hydrolysis process by choosing a proper pH (approximately 1.0). In the subsequent hydrothermal crystallization process, colloidal particles collide with each other by the activated Brownian motion, and then accumulate and condense to form micrometer-sized particles. PEG is a water-soluble hydrogen-bonding polymer whose water solubility decreases with the increasing temperature, aggregates to form micellar structures, and even separates out from aqueous solution at cloud point.28 The structure transition of PEG will control the primary particle aggregation, and the appropriate amount of PEG can facilitate the formation of silica spheres by the hydrogen bond between their chains and colloidal particles. The SEM image of silica spheres is shown in Figure 1. The nitrogen adsorption-desorption isotherm of silica spheres and their pore size distribution (inset) are shown in Figure 2. The type IV curve with hysteresis shows that the silica spheres have typical mesostructure. The pore size distribution calculated by the BJH method using desorption branch shows a sharp peak

Figure 1. SEM image of silica spheres.

Figure 2. Nitrogen adsorption (b) and desorption (O) isotherm of silica spheres and their pore size distribution (inset). Table 1. Physicochemical Properties of Silica Spheres before (MSS) and after (AgNO3) Impregnation MSS AgNO3

SBET (m2/g)

Vtotal (mL/g)

DBJH (nm)

DA (nm)

1078 652

0.92 0.59

3.77 3.72

3.40 3.62

centered at ca. 3.7 nm. The physicochemical parameters of silica spheres are listed in Table 1. Preparation of Adsorbent with AgNO3 Supported on Mesoporous Silica. Adsorption capacities of silica spheres impregnated in various concentration of AgNO3 for DBT are shown in Figure 3. The concentration of aqueous AgNO3

Figure 3. Adsorption capacities of silica spheres impregnated in various concentrations of AgNO3 (1.0 g of silica sphere was added into 15 g of aqueous AgNO3 solutions) for DBT.

solution in which mesoporous silica is impregnated has a prominent effect on the adsorption performance of the resultant adsorbent. The adsorption capacity of the resultant adsorbent first increased and then decreased with the increase of solution concentration. When the concentration of AgNO3 solution was 15 wt %, the adsorption capacity of the adsorbent reached the maximum of 15.3 mg S/g adsorbent. For this case, the loaded amount of Ag on the silica spheres was about 6.8 wt %.

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Figure 4. Scheme of AgNO3 deposition on the inner surface of mesopores with solvent volatilization. Figure 6. Pore size distributions of functionalized mesoporous silica spheres impregnated with silver nitrate (AgNO3) and their blank samples (MSS). Table 2. Adsorption Capacities of Adsorbents Prepared by Various Methods loaded amount of adsorption capacity Ag (100 wt %) (mg S/g adsorbent)

method

Figure 5. TEM images of SBA-15 mesoporous silica with AgNO3 supported by the traditional incipient wetness impregnation method.25

In fact, functionalized porous silica with AgNO3 has been investigated widely as π-complexation adsorbents for selective adsorption and chromatography.29-32 The traditional incipient wetness impregnation method usually adopts diluted ethanol solution of AgNO3, and solvent volatilization is carried out after impregnation. With the solvent volatilization, the solution concentration increases. As a result, the target compound is gradient distribution on the support surface (scheme is seen in Figure 4), and even agglomerate target compound separates out from the solution (seen in Figure 5). Yang and co-workers29-31 improved the incipient wetness impregnation technique by using a volume of AgNO3 solution equal to the total pore volume of support, and they prepared the adsorbents with AgNO3 supported on SiO2, which showed higher capacity and selectivity than those synthesized previously in the research of olefin-paraffin separations by adsorption.29,30 However, this method hardly applies to mesoporous silica support with small particle size, because a volume of AgNO3 solution equal to 3-4-fold total pore volume of support is required to wet all of the particles uniformly. The modified impregnation method presented here can avoid the gradient distribution because only a liquid film of AgNO3 solution was retained on the inner and outer surfaces of the support after filtration by vacuum. The pore size distributions and physicochemical parameters of functionalized mesoporous silica spheres impregnated with silver nitrate (AgNO3) and their blank samples (MSS) are given in Figure 6 and Table 1 separately. The surface area, pore volume, and pore size decrease, and the pore size remains uniform after AgNO3 loading. These results indicate that AgNO3 is uniformly supported on mesoporous silica spheres in a thin layer form. The adsorption capacities of adsorbents prepared by various incipient wetness impregnation methods separately are listed in Table 2, which indicates that the adsorbent prepared by the modified impregnation method shows the maximum adsorption capacity despite having the minimum loaded amount of Ag. Adsorption Characteristics. The characteristics of adsorbing benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-Me2DBT) from n-octane solution on the adsorbent were studied, and the equilibrium absorption

impregnation with AgBF425 impregnation with AgNO3a impregnation with AgNO3b impregnation with AgNO3b modified impregnation with AgNO3

15.7 16.4 15.6 6.8 6.8

6.9 2.5 10.8 9.0 15.3

a Prepared by using ethanol solution of AgNO according to the literature 3 method.25 b Prepared by using aqueous solution of AgNO3 according to 31 the literature method, but a volume of AgNO3 solution equal to 3-4-fold total pore volume of support is used for impregnation.

Figure 7. Equilibrium sorption isotherms for various sulfur compounds onto functionalized silica spheres (AgNO3) at 25 °C.

isotherms are plotted in Figure 7. The adsorbed amount of different sulfur compounds on the adsorbent increases as BT< 4,6-Me2DBT < DBT under the same experiment conditions, because of the relative π-complexation strength as BT < DBT and the steric hindrance effect as 4,6-Me2DBT > DBT. The equilibrium absorption data were described with the Langmuir equation (eq 1) and the Freundlich equation (eq 2) separately,

( )

Ce 1 b ) + C qe K L KL e

(1)

1 ln qe ) ln KF + ln Ce n

(2)

where Ce is the equilibrium solution concentration, qe is the amount of BT, DBT, or 4,6-Me2DBT adsorbed at the equilibrium, and constants KL, b, KF, and n are the characteristic constants. The adsorption isotherms of DBT and 4,6-Me2DBT are fitted well with the Langmuir equation, and the isotherms of BT and

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Table 3. Parameters Obtained from Langmuir and Freundlich Equationsa sulfur compound BT DBT 4,6-Me2DBT

Langmuir KL (dm3/g) b (dm3/mmol) 0.012 0.057 0.028

0.019 0.081 0.041

R2 0.961 0.998 0.997

Freundlich KF (mmol/g) n 0.025 0.121 0.102

R2

1.56 0.997 2.43 0.978 2.67 0.996

a K and n are Freundlich constants. The constants K and b are F L characteristic of the Langmuir equation. R2 is the coefficient of correlation.

Figure 9. Desorption curves of DBT and 2-MNP with Et2O as eluant. C0 and Cef are DBT concentrations separately in model diesel oil (sulfur content is 550 ppmw S) and effluent.

Figure 8. Sulfur removal efficiency in model diesel oil on AgNO3 in fixedbed.

4,6-Me2DBT are fitted well with the Freundlich equation. All of the isotherms are fitted with the Langmuir equation or the Freundlich equation to varying degrees, indicating that the adsorption is of monomolecular layer (Table 3). Fixed-Bed Adsorption Experiment. The fixed-bed adsorption desulfurization experiments were carried out in a stainless steel column containing 1.64 g of adsorbents using model diesel oil. The simulated diesel fuels were allowed to contact the fixedbed at room temperature and conditions without activation of adsorbents. Breakthrough occurred at about 57.4 cm3 of effluent pure n-octane for 1.0 g of adsorbent, and the adsorption capacity reached as high as 20.5 mg S/g of adsorbent before breakthrough when the model diesel oil was n-octane solution of DBT (sulfur content is 503 ppmw S). However, when the model diesel oil contained 5 wt % of 2-methylnaphthalene in n-octane solution of DBT (sulfur content is 550 ppmw S), breakthrough occurred at about 12.5 cm3 of effluent for 1.0 g of adsorbent, and the adsorption capacity reached only 5.0 mg S/g of adsorbent before breakthrough, due to the competitive adsorption of DBT and 2-methylnaphthalene (2-MNP). The sulfur breakthrough curves are shown in Figure 8. The breakthrough curves possess very precipitous slopes, indicating that the mass transfer of DBT in mesoporous silica spheres is very fast and adsorption equilibrium can be reached rapidly with AgNO3 as the adsorbent. The AgNO3 column was regenerated by passing Et2O through it at a flow rate of 0.5 cm3/min according to the literature.25 The desorption curves of DBT and 2-MNP are shown in Figure 9, which indicates that DBT and 2-MNP are easily desorbed and can be recovered in concentrated form. Conclusion Micrometer-sized silica spheres with uniform spherical morphology and highly ordered mesostructures were synthesized by a temperature-induced and PEG-assisted assembly method. A modified and convenient incipient wetness impregnation method was presented by which AgNO3 is uniformly supported on mesoporous silica spheres in a thin layer form. When the concentration of AgNO3 solution is 15 wt %, the adsorption

capacity of the adsorbent reaches the maximum of 15.3 mg S/g adsorbent. The adsorbed amount of different sulfur compounds on the adsorbent increases as BT < 4,6-Me2DBT < DBT under the same experiment condition. The adsorption is of monomolecular layer, and the equilibrium absorption data are fitted well with the Langmuir equation or the Freundlich equation. Fixedbed adsorption experiments show that the adsorption capacity reached as high as 20.5 mg S/g of adsorbent before breakthrough when the model diesel oil was n-octane solution of DBT. The adsorption capacity declines as a result of the competitive adsorption of aromatics. The AgNO3 column was regenerated easily with Et2O as eluant, and DBT and 2-MNP can be concentrated. Acknowledgment We gratefully acknowledge financial support for this work from the National Natural Science Foundation of China (20476050, 20490200, 20525622). Literature Cited (1) Parkinson, G. Diesel desulfurization puts refiners in a quandary. Chem. Eng. 2001, 108, 37. (2) Song, C. Fuel processing for low-temperature and high-temperature fuel cells; challenges, and opportunities for sustainable development in the 21st century. Catal. Today 2002, 77, 17-49. (3) Ma, X.; Sun, L.; Song, C. A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultra-clean fuels and for fuel cell applications. Catal. Today 2002, 77, 107-116. (4) Song, C.; Ma, X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl. Catal., B: EnViron. 2003, 41, 207-238. (5) Velu, S.; Song, C. S.; Engelhard, M. H.; Chin, Y. H. Adsorptive removal of organic sulfur compounds from jet fuel over K-exchanged NiY zeolites prepared by impregnation and ion exchange. Ind. Eng. Chem. Res. 2005, 44, 5740-5749. (6) Ma, X.; Sprague, M.; Song, C. Deep desulfurization of gasoline by selective adsorption over nickel-based adsorbent for fuel cell applications. Ind. Eng. Chem. Res. 2005, 44, 5768-5775. (7) Velu, S.; Ma, X.; Song, C. S.; Namazian, M.; Sethuraman, S.; Venkataraman, G. Desulfurization of JP-8 jet fuel by selective adsorption over a Ni-based adsorbent for micro solid oxide fuel cells. Energy Fuels 2005, 19, 1116-1125. (8) Ma, X. L.; Velu, S.; Kim, J. H.; Song, C. S. Deep desulfurization of gasoline by selective adsorption over solid adsorbents and impact of analytical methods on ppm-level sulfur quantification for fuel cell applications. Appl. Catal., B: EnViron. 2005, 56, 137-147. (9) Yang, R. T.; Takahashi, A.; Yang, F. H. New sorbents for desulfurization of liquid fuels by π-complexation. Ind. Eng. Chem. Res. 2001, 40, 6236-6239. (10) Takahashi, A.; Yang, F. H.; Yang, R. T. New sorbents for desulfurization by π-complexation: thiophene/benzene adsorption. Ind. Eng. Chem. Res. 2002, 41, 2487-2496.

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ReceiVed for reView September 6, 2006 ReVised manuscript receiVed October 6, 2006 Accepted October 10, 2006 IE061174D