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Energy & Fuels 2007, 21, 250-255
Deep Desulfurization by the Adsorption Process of Fluidized Catalytic Cracking (FCC) Diesel over Mesoporous Al-MCM-41 Materials B. S. Liu,*,† D. F. Xu,† J. X. Chu,† W. Liu,‡ and C. T. Au‡ Department of Chemistry, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, and Department of Chemistry and Centre for Surface Analysis and Research, Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong, People’s Republic of China ReceiVed June 1, 2006. ReVised Manuscript ReceiVed September 27, 2006
Al-MCM-41 adsorbents with different SiO2/Al2O3 molar ratios (100, 50, and 30) were synthesized and used for desulfurization of a commercial diesel fuel (sulfur content ) 1786 ppmw) at 373 K. The adsorbents were characterized by means of X-ray diffraction, N2 adsorption isotherm, Fourier transform infrared, and high-resolution transmission electron microscopy techniques. The adsorption capacity for sulfur-containing compounds was found to follow the order: Al-MCM-41(50) > Al-MCM-41(30) > Al-MCM-41(100) (the number in parentheses is the SiO2/Al2O3 ratio hereafter). The efficiency of sulfur removal is 95% over AlMCM-41(50) at the initial state and still above 75% after a cumulative effluent volume of 17 mL. The effect of the adsorption temperature on the adsorption capacity of Al-MCM-41(100) was investigated. The results indicate that high temperature is disadvantageous for desulfurization. After the regeneration of a spent AlMCM-41(100) sample (calcined at 823 K for 6 h), there is good recovery of the adsorption capacity. In addition, with the presence of Cu+ ions in Al-MCM-41, there is enhancement in the adsorption capacity and sulfur removal efficiency at 373 K and the effect can be related to the formation of π complexation between sulfurcontaining compounds and Cu+ ions.
Introduction Diesel is a major transportation fuel, and its combustion emits SOx. Because of the worldwide mandates on pollution control, the removal of organic sulfur compounds in transportation fuels has become an important issue. According to the U.S. Environmental Protection Agency (EPA) Tier II regulations, the level of sulfur in diesel has to be limited to 15 ppmw by June 2006; the previously permitted level was 500 ppmw.1 The current practice for the removal of sulfur-containing compounds from liquid fuels is via the hydrodesulfurization (HDS) technique where Co-Mo/Al2O3 or Ni-Mo/Al2O3 is used as the catalyst.2 The technique is highly effective in removing thiols, sulfides, and disulfides but less effective for the removal of thiophene derivatives, which constitute as much as 85% of the sulfur content in diesel fuel. Furthermore, the result of a kinetic study suggested that, to bring the sulfur level from “500 ppmw” to “below 15 ppmw” using HDS technology, the size of the reactor has to be enlarged by 3 times.3 Recently, deep desulfurization by means of an adsorption technique has been considered as an alternative. In 2001, the Phillips Petroleum Company developed an S-Zorb process for the production of low-sulfur * To whom correspondence should be addressed. Telephone: 86-2227892471. Fax: 86-22-87892946. E-mail:
[email protected]. † Tianjin University. ‡ Hong Kong Baptist University. (1) Avidan, A.; Klevn, B.; Ragsdale, R. Improved planning can optimize solutions to produce clean fuels. Hydrocarbon Process. 2001, 80, 47. (2) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979; pp 390-426. (3) Ma, X. L.; Sakanishi, K.; Mochida, I. Hydrodesulfurization reactivities of various sulfur compounds in diesel fuel. Ind. Eng. Chem. Res. 1994, 33, 218-222.
gasoline by the reaction-adsorption method;4-7 employing a fluidized-bed reactor operating in the 650-775 K and 7.121.2 kg/cm2 range, the company produced “S-Zorb” gasoline of 10 ppmw sulfur over a solid adsorbent. Other adsorption processes without using H2 gas were also probed for desulfurization, and adsorbents such as metal-supported oxide,8 metal ion-exchanged microporous zeolites,9-11 as well as various mesoporous materials12 have been reported. Ma et al. studied the desulfurization of gasoline over a Ni-based adsorbent and reported a breakthrough adsorption capacity of 0.37 mg/g at room temperature (RT). They also confirmed that, at 473 K, the capacity for sulfur adsorption increased by 38%.8 Yang and (4) Khare, G. P. Dusulfurization Process and Novel Bimetallic Sorbent Systems for Same. U.S. Patent 6,274,533, Aug 14, 2001. (5) Khare, G. P. Process for the Production of a Sulfur Sorbent. U.S. Patent 6,184,176, Feb 6, 2001. (6) Gislason, J. Phillips sulfur-removal process nears commercialization. Oil Gas J. 2001, 99 (47), 74-76. (7) Babich, I. V.; Mouljin, J. A. Science and technology of novel process for deep dusulfurization of oil refinery streams: A review. Fuel 2003, 82, 607. (8) Ma, X. L.; 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 2005, 56, 137-147. (9) Yang, R. T.; Herna´ndez-Maldonado, A. J.; Yang, F. H. Desulfurization of transportation fuels with zeolites under ambient conditions. Science 2003, 301, 79. (10) Herna´ndez-Maldonado, A. J.; Yang, R. T. Desulfurization of diesel fuel by adsorption via π-complexation with vapor-phase exchanged Cu(I)-Y zeolites. J. Am. Chem. Soc. 2004, 126, 992. (11) Herna´ndez-Maldonado, A. J.; Yang, F. H.; Qi, G. S.; Yang, R. T. Desulfurization of transportation fuels by π-complexation sorbents: Cu(I)-, Ni(II)-, and Zn(II)-zeolites. Appl. Catal., B 2005, 56, 111-126. (12) Tian, F. P.; Jiang, Z. X.; Liang, C. H.; Li, Y.; Cai, T. X.; Li, C. Deep desulfurization of gasoline by adsorption on mesoporous MCM-41. Chin. J. Catal. 2005, 26, 628.
10.1021/ef060249n CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2006
Adsorption Process of FCC Diesel oVer Al-MCM-41
co-workers developed a series of adsorbents based on molecular sieves such as NaY and 13X modified by Cu+, Ag+, and Ni2+ ions for gasoline desulfurization. With the use of CuI-Y (generated by means of vapor-phase ion exchange), the sulfur level below 0.2 ppmw could be reached and the handling capacity was almost 38 cm3 of fuel/g of adsorbent.13 Recently, Tian et al.12 proposed the use of Al-MCM-41 as a desulfurization agent in view of its large pore volume and high specific surface area. The results showed that the introduction of Al into the mesoporous framework of MCM-41 can enhance sulfur removal from gasoline. Lately, we reported that, with the introduction of copper ions, the adsorption capacity of SiMCM-41 was enhanced and there was a distinct increase in selectivity toward the removal of sulfur-containing compounds such as thiophene derivatives in commercial fluidized catalytic cracking (FCC) fuel.14 In this investigation, several Al-MCM-41 adsorbents with different SiO2/Al2O3 ratios were prepared and tested for the removal of sulfur from commercial FCC diesel fuel. The effects of the adsorption temperature and adsorbent regeneration on the adsorption capacity was also examined. Experimental Section Preparation of the Adsorbent. Al-MCM-41 adsorbents of varying contents of aluminum were synthesized according to the following procedure. First, hexadecyltrimethyl ammonium bromide (CTMAB) was dissolved in 40 mL of deionized water at 323 K to yield a 15 wt % template solution. The sodium aluminate solutions of different Al contents were prepared by dissolving 0.59, 1.08, and 1.345 g of Al(NO3)3‚9H2O in NaOH solution at RT, respectively. The aluminate and tetraethoxysilane (TEOS, as a silica resource) solutions were added dropwise into the CTMAB solution under constant stirring, and the pH was regulated with NaOH solution (4 mol/L) and kept within the 11-12 range. The final cogel was stirred for another hour and transferred to an autoclave for hydrothermal synthesis at 413 K. The synthesis time for Al-MCM41(100) was 30 h (as suggested by Corma et al.),15 whereas the synthesis time for Al-MCM-41(50) and Al-MCM-41(30) was 48 h. Then, the solid product was filtered out, rinsed with deionized water, dried in air at 353 K, and finally calcined (for template removal) at 813 K for 7 h. The Cu2+-Al-MCM-41(100) adsorbent was prepared by means of ion exchange, having Al-MCM-41(100) immersed in a CuCl2 aqueous solution (0.5 M) at RT for 48 h. The solid was filtered out, washed with deionized water before being dried (at 353 K), calcined at 813 K for 2 h. Before being used for adsorption desulfurization, the Cu2+-Al-MCM-41(100) adsorbent was reduced to Cu+-Al-MCM-41(100) in hydrogen at 463 K for 2 h.16 Characterization of the Adsorbent. A powder X-ray diffraction (XRD) investigation was performed on a Rigaku automatic diffractometer (Rigaku D-MAX) with monochromatized Cu KR radiation (λ ) 0.154 06 nm, 40 kV, 50 mA). The adsorption isotherm of N2 and pore-size distribution of adsorbents were examined by means of a NOVA-1200 instrument. Fourier transform infrared (FT-IR) spectra of Al-MCM-41 adsorbents were acquired on a BIO-RAD FTS 3000 spectrophotometer. The fine structure of Al-MCM-41(50) was characterized by high-resolution transmis(13) Herna´ndez-Maldonado, A. J.; Yang, R. T. Desulfurization of transportation fuels. Catal. ReV. 2004, 48, 111-150. (14) Liu, B. S.; Xu, D. F.; Wu, Z. X. Preparation of a Cu-MCM-41 adsorbent and its desulfurization performance for diesel fuel. Chin. J. Catal. 2006, 27, 372. (15) Corma, A.; Kan, Q.; Nararro, M.; Perez-Pariente, J.; Ray, F. Synthesis of MCM-41 with different pore diameters without addition of auxiliary organics. Chem. Mater. 1997, 9, 2499. (16) Li, W.; Xing, J.; Xiong, X.; Huang, J.; Liu, H. Feasibility study on the integration of adsorption/bioregeneration of π-complexation adsorbent for desulfurization. Ind. Eng. Chem. Res. 2006, 45, 2845-2849.
Energy & Fuels, Vol. 21, No. 1, 2007 251 Table 1. Property of FCC Diesel items
results
initial boiling point/10% point (°C) 30% point/50% point (°C) 70% point/90% point (°C) 95% point/final boiling point (°C) density (at 20 °C) (g/mL) sulfur content (ppmw) acidity (mg of KOH/100 mL) bromine value (G-Br/100 g) kinematic viscosity (at 20 °C) (mm2/s) aromatics (%) cetane number
174/224 246/265 295/337 358/359 0.857 1786 0.81 1.24 4.076 43.2 41.8
sion electron microscopy (HRTEM) equipped with a field emission gun. The TEM images were captured with a Tecnai G2 F 20 electron microscope operating at 200 kV. The sample was prepared according to the approach described elsewhere.17 The Lewis and Brønsted acidity of the adsorbents was characterized by temperatureprogrammed desorption of ammonia (TPDA). The samples were purged at 773 K in a synthetic air stream (50 mL/min) for 1 h before being cooled down to RT in a N2 stream (50 mL/min). The sample was then exposed to NH3 (30 mL/min) until saturation and subsequently purged for another 1 h with flowing N2 (50 mL/min) for the removal of excess and/or physically adsorbed NH3. The TPD spectra were obtained by heating the samples from RT to 873 K at a rate of 10 K/min under a N2 flow. The desorbed NH3 was monitored with a thermal conductivity detector (TCD). Thermogravimetric (TG) analysis was carried out by means of a Shimadzu TAC-50 thermogravimetric analyzer. The study was done in an air flow of 15 mL/min and at a heating rate of 10 K/min. Before the TG analysis, the sample was heated at 353 K for 12 h for the removal of low boiling point species in diesel fuel. FCC Diesel Desulfurization. A custom-made vertical quartz tube with an internal diameter of 10 mm and a length of 120 mm was used to perform the experiment of desulfurization. The apparatus consisted of a feed tank, a knob to control the liquid flow, and a system to control the temperature. The adsorbent was loaded inside the quartz tube and outgassed at 473 K under vacuum (∼10 kPa) for 2 h. The remaining adsorbed species were removed by introducing the diesel fuel to the adsorbent. After the adsorption bed was filled by diesel fuel (from Tianjin Petrochemical Company, China; the property of which is listed in Table 1), the airtight system was opened to air. Then, the diesel was introduced into the tube at a flow rate of 4.5 mL/h. The effluent from the adsorbent bed was collected periodically at intervals of 10-60 min. The sulfur content in the diesel before and after desulfurization was analyzed by means of a KLT-1 microcoulometer. At first, the diesel sample was combusted in a custom-made furnace for cracking at 1133 K at an air flow of 380 mL/min and the emitted SO2 was collected in an electrolytic cell with I3- solution (pH 8). The sulfur analyzer was calibrated against diesel samples of known sulfur contents (from 50 to 2000 ppmw). The sulfur removal efficiency of each adsorbent was calculated according to the following equation:
( )
sulfur removal ) 1 -
ci × 100% c0
where ci is the sulfur concentration (ppmw) of the effluent after desulfurization and c0 is the initial sulfur concentration in the commercial diesel. (17) Liu, B. S.; Tang, D. C.; Au, C. T. Fabrication of analcime zeolite fibers by hydrothermal synthesis. Microporous Mesoporous Mater. 2005, 86, 106-111.
252 Energy & Fuels, Vol. 21, No. 1, 2007
Figure 1. XRD patterns of (a) Al-MCM-41(100), (b) Al-MCM-41(50), and (c) Al-MCM-41(30).
Liu et al.
Figure 3. Pore-size distribution of (2) Al-MCM-41(30), (b) AlMCM-41(50), and (9) Al-MCM-41(100). Table 2. Properties of Adsorbents
sample
SiO2/ Al2O3 ratio
specific surface area (m2/g)
pore volume (mL/g)
pore diameter (nm)
Al-MCM-41 Al-MCM-41 Al-MCM-41 Al-MCM-41 (Reg.1)a Al-MCM-41 (Reg.2)a Cu-Al-MCM-41
100 50 30 100 100 100
310.94 604.90 687.98 81.02 78.11 115.73
0.46 0.68 0.67 0.21 0.20 0.21
2.7 3.2 3.0 6.1 6.2 3.5
a Reg.1 and Reg.2 denote that the sample has been regenerated 1 and 2 times, respectively.
Figure 2. N2 adsorption isotherms of (a) Al-MCM-41(30), (b) AlMCM-41(50), and (c) Al-MCM-41(100). (2, b, and 9 and 4, O, and 0 denote adsorption and desorption, respectively).
Results and Discussion Structure and Physical Properties of Al-MCM-41. According to what has been reported in the literature,18-20 a XRD pattern with a strong (100) peak at 2.69° and two smaller peaks between 4 and 6° corresponding to the (110) and (200) reflections is typical of a long-range order structure. The XRD patterns of Al-MCM-41 with different SiO2/Al2O3 ratios are shown in Figure 1. The Al-MCM-41(100) sample shows no signal at 2θ ) 2°, indicating the absence of a crystalline structure (Figure 1a). According to the XRD patterns of Al-MCM-41(50) and Al-MCM-41(30), with an increase in the Al content, the intensity of the (100) peak decreases significantly and the (110) and (200) peaks overlap to give a single broad band, similar to that reported by Luan et al.18 The adsorption isotherms of Al-MCM-41 samples are shown in Figure 2. According to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, the adsorption isotherms exhibit a typical type-IV feature, evidence of the presence of uniform mesopores. The sudden N2 adsorption within the narrow p/p0 range (0.3-0.4) over Al-MCM-41(30) and Al-MCM-41(50) reflects the capillary condensation of N2 (18) Luan, Z. H.; Cheng, C. F.; Zhou, W. Z.; Klinowski, J. Mesopore molecular sieve MCM-41 containing framwork aluminum. J. Phys. Chem. 1995, 99, 1018-1024. (19) Selvam, P.; Bhatia, S. K.; Sonwane, C. G. Recent advances on processing and characterization of periodic mesoporous MCM-41 silicate molecular sieves. Ind. Eng. Chem. Res. 2001, 40, 3237-3261. (20) Gru¨n, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Novel pathways for the preparation of mesoporous MCM-41 materials: Control of porosity and morphology. Microporous Mesoporous Mater. 1999, 27, 207-216.
Figure 4. FT-IR spectra of (a) Si-MCM-41, (b) Al-MCM-41(100), (c) Al-MCM-41(50), and (d) Al-MCM-41(30).
molecules in uniform mesopores; a similar conclusion can be drawn based on the narrow and sharp pore-size distribution curves of the two adsorbents (Figure 3). This is in agreement with the observation of Selvam et al. and Gru¨n et al.19,20 As for Al-MCM-41(100), the pore volume, N2 adsorption capacity, and uniform mesoporosity are significantly low (Figures 2c and 3) because of the lack of an intact crystalline structure (Figure 1a). Summarized in Table 2 are the specific surface areas, pore volumes, average pore diameters, and SiO2/Al2O3 ratios of the three Al-MCM-41 adsorbents. It can be seen that the AlMCM-41(30) and Al-MCM-41(50) samples are similar in pore volume and pore diameter but the former is larger in specific surface area. Among the three absorbents, Al-MCM-41(100) is the lowest in specific surface area, pore volume, and pore diameter. After the regeneration of a spent Al-MCM-41(100) sample by means of calcination at 823 K for 6 h, there is a significant decline in specific surface area and pore volume but a marked increase in pore diameter, indicative of the partial collapse of the MCM-41 framework. The FT-IR absorption spectra of Al-MCM-41 samples are depicted in Figure 4. The intense bands of the Si(Al)-O
Adsorption Process of FCC Diesel oVer Al-MCM-41
Energy & Fuels, Vol. 21, No. 1, 2007 253
Figure 5. HRTEM image and SAED (inset) of Al-MCM-41(50).
cm-1
stretching and bending vibration within the 400-1300 range had been observed over aluminosilicate by Gnatyuk et al.21 In the studies of Kosslick et al.,22 the absorption bands at ca. 1095, 805, and 450-470 cm-1 were assigned to the antisymmetric and symmetric stretching vibrations of T-O-T framework units (T ) Si, Al, Ga, and Fe) and the deformation modes of TO4 tetrahedra, respectively. A Si-MCM-41 sample shows a band at ca. 1093.6 cm-1 (Figure 4a), close enough to the 1095 cm-1 position reported by Kosslick et al. In comparison to the antisymmetric T-O-T vibration band of pure Si-MCM41, those of Al-MCM-41(100), Al-MCM-41(50), and AlMCM-41(30) have shifted (from 1093.6 cm-1) to 1087.8 cm-1 (Figure 4b), 1083.9 cm-1 (Figure 4c), and 1078.2 cm-1 (Figure 4d), respectively. The shift becomes the most obvious at a SiO2/ Al2O3 ratio of 30. It was reported that such a shift is a result of a change in the radius of the metal ions as well as a change in the extent of Al substitution.22 The HRTEM image of AlMCM-41(50) shown in Figure 5 is with the pore channel viewed perpendicularly to the pore axis. The width of the dark strips is approximately 2.3 nm, not far away from the pore diameter (ca. 3.0 nm) of Figure 3. The image of the selected area electron diffraction (SAED) is displayed in the inset; the presence of two bright spots on the diffraction circle is an indication of the formation of a single-channel structure. The distance between the two spots was estimated to be 2.14 nm, lower than the d100 spacing (3.6 nm) obtained on the basis of XRD analysis. Efficiency of Sulfur Removal. Figure 6 shows the efficiency of sulfur removal. The original content of sulfur-containing compounds inside the commercial diesel fuel was 1786 ppmw. Sulfur removal over Al-MCM-41(100) was 85% initially and dropped quickly to 30% within 1 h. The Al-MCM-41(30) adsorbent showed an initial sulfur removal of 79%, and at a removal of 30%, the effluent volume doubled that of Al-MCM41(100) at equal sulfur removal, demonstrating a relatively high adsorption capacity of the Al-MCM-41(30). The TPDA spectra (Figure 7) showed that there is no significant NH3-desorption peak within the temperature range of ∼573-873 K (attributable to Brønsted acidic sites)23 over the Na-Al-MCM-41 adsorbents. In other words, the absence of protons in Na-Al-MCM41 prohibits the adsorption of NH3 on Brønsted acidic sites. As shown in Figure 7, the amount of ammonia desorption over (21) Gnatyuk, I.; Puchkovskaya, G.; Yaroshchuk, O.; Goltsov, Y.; Matkovskaya, L. Spectroscopic study of liquid crystals in confined volume. J. Mol. Struct. 1999, 511-512, 189-197. (22) Kosslick, H.; Lischke, G.; Walther, G. Storek, W.; Martin, A.; Fricke, R. Physico-chemical and catalytic properties of Al-, Ga- and Fe-substituted mesoporous materials related to MCM-41. Microporous Mesoporous Mater. 1997, 9, 13-33. (23) Kosslick, H.; Lischke, G.; Parlitz, B.; Storek, W.; Fricke, R. Acidity and active sites of Al-MCM-41. Appl. Catal., A 1999, 184, 49-60.
Figure 6. Removal efficiency of sulfur in FCC diesel over Al-MCM41 with different SiO2/Al2O3 ratios at 373 K (weight of adsorbent ) 0.4 g, and flow rate of diesel ) 4.5 mL/h).
Figure 7. TPDA profiles of (a) Al-MCM-41(50), (b) Al-MCM-41(30), (c) Al-MCM-41(100).
Al-MCM-41(30) is slightly higher than that over Al-MCM41(100), indicating that Al-MCM-41(30) contained more Lewis active sites. Hence, a strong interaction of Lewis acidic sites with sulfur-containing compounds in diesel fuel resulted in the enhancement of the adsorption capacity over Al-MCM-41(30). Tian et al.12 also reported that, with the increase of the Al content in the MCM-41, there is a significant rise in Lewis acidity and the adsorption capacity of the adsorbent increased. They reported that a Al-MCM-41 (Si/Al ) 30) adsorbent showed an initial sulfur removal of 70.1% and sulfur removal dropped to 30% after a cumulative effluent volume of 10 mL, slightly lower than that observed by us over the Al-MCM-41(30) adsorbent, possibly because of the different nature of the sulfur-containing compounds in the fuels. Under similar conditions, the initial sulfur removal was 95% over Al-MCM-41(50) and sulfur removal was still above 75% at a cumulative effluent volume of 17 mL. It is impossible to calculate the breakthrough capacity of the Al-MCM-41(50) adsorbent because of the high sulfur content in the diesel fuel. In our experiment, 100 ppmw was set as the “breakthrough” point to measure the adsorption capacity. The adsorption amount was obtained from the equation reported by Herna´ndez-Maldonado et al.11 The Al-MCM-41(50) adsorbent is capable of removing 0.25 mmol of S/g, which is slightly higher than the breakthrough capacity of CuIY(LPIE-RT) but lower than the saturation capacity of the same adsorbent. Kosslick et al.23 reported that the acidity in AlMCM-41 increased with a rise in the Al content that maximized at SiO2/Al2O3 ) 13.7 before declining because of structural deterioration. The XRD pattern (Figure 1) of adsorbents has
254 Energy & Fuels, Vol. 21, No. 1, 2007
Figure 8. Effect of the temperature and copper loading on desulfurization of FCC diesel over Al-MCM-41(100). Curve b denotes sulfur removal over Cu+-Al-MCM-41(100) (weight of adsorbent ) 1 g).
Liu et al.
Figure 10. TG/DTG profiles of spent Al-MCM-41(100) adsorbent.
Figure 11. Removal efficiency of sulfur in FCC diesel over fresh and regenerated Al-MCM-41(100) at 373 K (weight of adsorbent ) 1 g). Figure 9. Pore-size distribution (inset) and N2 adsorption isotherms of spent Al-MCM-41(30).
confirmed that Al-MCM-41(50) exhibits an intact crystal structure of order hexagonal arrangement. That is to say, the Al-MCM-41(50) adsorbent contains more Lewis acidic sites than Al-MCM-41(30), which is confirmed by TPDA investigation (Figure 7), despite the fact that the SiO2/Al2O3 ratio of AlMCM-41(50) is somewhat different from that reported by Kosslick et al.23 The result indicates that the desulfurization performance of Al-MCM-41 adsorbents to a large extent depends upon the amount of Lewis acidic sites as well as its crystallinity: too low or too high Al content in Al-MCM-41 would result in the destruction of the crystalline structure (Figure 1) and a decrease of Lewis acidic sites. Thus, the Al-MCM41(50) adsorbent being high in adsorption capacity is apt for sulfur removal. Investigation of Adsorption Parameters. Keeping the flow rate (4.5 mL/h) of feed constant, the desulfurization performance of Al-MCM-41(100) was investigated at 298, 373, and 473 K, respectively, for temperature dependence. As shown in Figure 8, the temperature has a significant effect on the adsorption performance. At 298 K, the efficiency of sulfur removal is above 34% at an effluent volume of 40 mL. At 373 and 473 K, the adsorbent is saturated at around 26 mL (Figure 8c) and 18 mL (Figure 8d), respectively. The results indicate that a rise in temperature is disadvantageous for sulfur removal. This is because the adsorption of sulfur-containing compounds (such as thiophene and its derivatives) on Al-MCM-41(100) is physical. Besides the adsorption temperature, the adsorption capacity of Al-MCM-41(100) is dependent upon the molecular
weight of adsorbates as well as the nature of the interaction among them. Because of the competitive adsorption of heavy aromatics and alkenes, the adsorption capacity and selectivity of thiophene and its derivatives on Al-MCM-41(100) is low (9 in Figure 6). If Cu+ ions were present in Al-MCM-41(100), there was enhancement in the adsorption capacity: at 373 K, the sulfur removal over Cu+-Al-MCM-41(100) at an effluent volume of 10 mL was 62% (Figure 8b), whereas at an effluent volume of 5 mL, the sulfur removal over Al-MCM41(100) was 51% (Figure 8c). According to Yang et al.,13 the interaction of Cu+ with thiophene derivatives in diesel can be related to a π-complexation mechanism. On the basis of such a mechanism, we demonstrated recently that the adsorption capacity of thiophene on Cu+-Si-MCM-41 could be enhanced by raising the temperature.14 Figure 9 shows the adsorption isotherms and the pore-size distribution of a spent Al-MCM-41(30) adsorbent. It can be seen that the mesopores of the spent adsorbent were nearly completely filled with stuffs and the adsorption isotherms showed a typical type-II feature. We attempted to regenerate a spent Al-MCM-41(100) adsorbent via calcination at 823 K for 6 h (for the removal of the adsorbed sulfur-containing compounds). According to the TG curve of spent Al-MCM-41(100) (Figure 10), there is a large weight loss peak at about 473 K attributable to the oxidation of adsorbed hydrocarbon. As revealed in the differential thermogravimetry (DTG) curve, there is further weight loss up to a temperature of 923 K. It was reported that the surface area of Al-MCM-41 decreases
Adsorption Process of FCC Diesel oVer Al-MCM-41
Figure 12. N2 adsorption isotherms of (a) fresh Al-MCM-41(100) and (b and c) regenerated Al-MCM-41(100) (Reg.1 and Reg.2 denote that the adsorbent has been regenerated 1 and 2 times, respectively).
continuously above a calcination temperature of 823 K.24 The decrease in surface area and total pore volume is disadvantageous for adsorption capacity. With due consideration to both the thermal stability of Al-MCM-41 and removal efficiency of adsorbed compounds, we adopted 823 K as the temperature for adsorbent regeneration. The efficiency of desulfurization observed over the fresh and regenerated samples at 373 K are shown in Figure 11. In comparison to a fresh sample, a regenerated sample is only slightly lower in the capacity of adsorbing sulfur-containing compounds. Considering the high cost of Al-MCM-41(100) synthesis, the recycling of a spent adsorbent has economic implications, despite the fact that the temperature adopted for regeneration is relatively high. The slight reduction of the adsorption capacity is attributed to the decline in pore volume as well as in specific surface area. The data listed in Table 2 reveal that the specific surface area (8178 m2/g) of regenerated adsorbents was significantly lower than (24) Chen, L. Y.; Jaenicke, S.; Chuah, G. K. Thermal and hydrothermal stability of framework-substituted MCM-41 mesoporous materials. Microporous Mesoporous Mater. 1997, 12, 323-330.
Energy & Fuels, Vol. 21, No. 1, 2007 255
Figure 13. Pore-size distribution of fresh and regenerated Al-MCM41(100).
that (310.9 m2/g) of fresh Al-MCM-41(100); the pore volume of the adsorbent reduced from 0.46 to ∼0.2 mL/g, and the pore diameter enlarged from 2.7 to ∼6.2 nm. The N2 adsorption isotherms (Figure 12) and pore-size distribution of adsorbents obtained before and after regeneration differ significantly (Figure 13). As shown in Figure 13, there was significant enlargement in the pore size during the regeneration process, indicative of the partial collapse of the adsorbent structure. Conclusion Al-MCM-41(50) adsorbent with high specific surface area and pore volume was generated for deep desulfurization of FCC diesel fuel. The experimental results showed that the adsorbent exhibits excellent adsorption capacity. The efficiency of sulfur removal is 95% initially and still above 75% at a cumulative effluent volume of 17 mL. Acknowledgment. We thank the Center for Surface Analysis and Research, HKBU, for BET, TPDA, and XRD investigation and the Analytical Center, TJU, for FT-IR, TG, and TEM analyses. EF060249N
