Energy & Fuels 2006, 20, 909-914
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Effects of Nitrogen Compounds and Polyaromatic Hydrocarbons on Desulfurization of Liquid Fuels by Adsorption via π-Complexation with Cu(I)Y Zeolite Ambalavanan Jayaraman, Frances H. Yang, and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136 ReceiVed September 23, 2005. ReVised Manuscript ReceiVed February 14, 2006
Organonitrogen compounds are present in commercial fuels after the hydrodenitrogenation process in quantities up to a few hundred ppmw N. Polyaromatic hydrocarbons (PAH) are present in significant quantities (1.4-11 wt %) in transportation fuels depending on both the region and season. In this work, the effect of organonitrogen (quinoline, carbazole) and PAH (naphthalene, fluorene, phenanthrene) compounds on desulfurization by π-complexation with Cu(I)Y zeolite was studied and found to be moderate. The high selectivity for sulfur was not affected because a very low-sulfur fuel ( quinoline > phenanthrene > fluorene ≈ naphthalene. Ab initio molecular orbital calculations were also performed to provide an understanding of the inhibition effect. The observed order of inhibition was in good agreement with that predicted by molecular orbital calculation results.
Introduction Desulfurization of liquid fuels is of primary interest to refiners worldwide due to stricter environmental regulations. It is also of crucial importance for fuel cell applications. The EPA Tier II regulations require the sulfur level in diesel to be less than 15 ppmw by June 2006 from the current levels of 350-500 ppmw and sulfur level in gasoline to be 30 ppmw by 2006.1,2 Similar regulations for jet fuels are expected in the near future.3 Jet fuel is also considered as a major source of hydrogen for future fuel cell applications.4 Fuel cells will require deepdesulfurized fuels with sulfur concentrations less than 0.1 ppmw.5 Sulfur removal in refineries is achieved by a hydrodesulfurization (HDS) process using Co-Mo/Al2O3 or Ni-Mo/ Al2O3 catalyst.6 After HDS, mainly refractory sulfur compounds such as methyl-substituted dibenzothiophenes remain in diesel fuel, making deep-desulfurization by HDS difficult.7 * Corresponding author. Phone: 734-936-0771. Fax: 734-763-0459. E-mail:
[email protected]. (1) Avidan, A.; Klein, B.; Ragsdale, R. Improved planning can optimize solutions to produce clean fuels. Hydrocarbon Process. 2001, 80, 47-53. (2) Knudsen, K. G.; Cooper, B. H.; Topsoe, H. Catalyst and process technologies for ultra low sulfur-diesel. Appl. Catal., A 1999, 189, 205215. (3) Link, D. D.; Baltrus, J. P.; Rothenberger, K. S.; Zandhuis, P.; Minus, D. K.; Striebich, R. C. Class- and Structure-Specific Separation, Analysis, and Identification Techniques for the Characterization of the Sulfur Components of JP-8 Aviation Fuel. Energy Fuels 2003, 17, 1292-1302. (4) Piwetz, M. M.; Larsen, J. S.; Christensen, T. S. Hydrodesulfurization and Prereforming of Logistic Fuels for Use in Fuel Cell Applications. Presented at the 1996 Fuel Cell Seminar, Orlando, FL, Nov 17-20, 1996; pp 780-783. (5) Babich, I. V.; Moulijn, J. A. Science and technology of novel processes for deep desulfurization of oil refinery streams: a review. Fuel 2003, 82, 607-631. (6) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (7) Ho, T. C. Deep HDS of diesel fuel: chemistry and catalysis. Catal. Today 2004, 98, 3-18.
There is an increasing need for removal of nitrogen compounds from liquid fuels to lower the emission of nitrogen oxides from combustion processes.8-10 Crude oil contains organic nitrogen compounds, and the nitrogen content strongly increases as we get to heavy oil fractions.11-13 Two classes of organic nitrogen compounds are present in crude oil, namely, heterocycles (five-membered pyrroles (neutral) and six-membered pyridines (basic)) and nonheterocycles (aliphatic amines and anilines). Commercial diesel is a blend produced from straight run distillates and cracked heavier feedstocks. Most of the nitrogen in heavier petroleum fractions is present as polyaromatic heterocycles (PAH; quinolines, acridines, indoles, carbazoles, and benzocarbazoles).12-15 Denitrogenation and desulfurization is carried out simultaneously during catalytic hydrotreating. However, the reactivities of organonitrogen compounds for hydrodenitrogenation process (HDN) are sig(8) Ho, T. C. Hydrodenitrogenation Catalysis. Catal. ReV.-Sci. Eng. 1988, 30, 117-160. (9) Wiwel, P.; Knudsen, K.; Zeuthen, P.; Whitehurst, D. D. Assessing Compositional Change of Nitrogen Compounds during Hydrotreating of Typical Diesel Range Gas Oils Using a Novel Preconcentration Technique Coupled with Gas Chromatography and Atomic Emission Detection. Ind. Eng. Chem. Res. 2000, 39, 533-540. (10) Zeuthen, P.; Knudsen, K. G.; Whitehurst, D. D. Organic nitrogen compounds in gas oil blend, their hydrotreated products and the importance to hydrotreatment. Catal. Today 2001, 65, 307. (11) Minderhoud, J. K.; Van Veen, J. A. R. First-stage hydrocracking: Process and catalytic aspects. Fuel Process. Technol. 1993, 35, 87. (12) Boduszynski, M. M. Characterization of “Heavy” Crude Components. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1985, 365-382. (13) Boduszynski, M. M. Composition of Heavy Petroleums. I. Molecular Weight, Hydrogen Deficiency, and Heteroatom Concentration as a Function of Atmospheric Equivalent Boiling Point up to 1400 °F (760 °C). Energy Fuels 1987, 1, 2-11. (14) Snyder, L. R. The Nitrogen and Oxygen Compounds in Petroleum. Prepr. Pap.-Am. Chem. Soc., DiV. Pet. Chem. 1970, 15, C44-C62. (15) McKay, J. F. K.; Weber, J. M.; Latham, D. R. Characterization of Nitrogen Bases in High-Boiling Petroleum Distillates. Anal. Chem. 1976, 48, 891.
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nificantly lower than that of corresponding organosulfur compounds for HDS.8-10,16,17 Carbazoles and quinolines have the lowest reactivity for HDN among the neutral and basic nitrogen compounds, respectively.18 Hence, they are among the major organonitrogen compounds left in the fuel after HDN with total nitrogen content greater than 70 ppmw N.9 The past few years have seen an increasing interest in selective desulfurization by adsorption at ambient temperature and pressure,19-21 and also reactive adsorption of sulfur/sulfur compounds by chemisorption at higher temperatures is being considered.22-25 Recently, a class of sorbents that form bonds through π-complexation and are stronger than van der Waals interactions but weaker than conventional chemical bonds have been used to selectively remove organosulfur and organonitrogen molecules from commercial fuels.26-28 In these sorbents, transition metal ions such as Ag+, Cu+, Ni2+, and Zn2+ were introduced as charge balancing cations into the zeolite framework through ion-exchange techniques, and they interacted with the organosulfur molecules. These materials are capable of producing liquid fuels with total sulfur concentrations below 1 ppmw and a total nitrogen concentration below 0.1 ppmw.26-35 Adsorption of organonitrogen compounds is stronger than adsorption of organosulfur compounds on Cu(I)Y zeolite.28,36 (16) Gates, B. C.; Topsoe, H. Reactivities in deep catalytic hydrodesulfurization: challenges, opportunities, and the importance of 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene. Polyhedron 1997, 3213-3217. (17) Whitehurst, D. D.; Isoda, T.; Mochida, I. Present state of the art and future challenges in the hydrodesulfurization of polyaromatic sulfur compounds. AdV. Catal. 1998, 42, 345-471. (18) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 5. Denitrogenation Reactivity of Basic and Neutral Nitrogen Compounds. Ind. Eng. Chem. Res. 2001, 40, 4919-4924. (19) Haji, S.; Erkey, C. Removal of Dibenzothiophene from Model Diesel by Adsorption on Carbon Aerogels for Fuel Cell Applications. Ind. Eng. Chem. Res. 2003, 42, 6933. (20) McKinley, S. G.; Angelici, R. J. Deep desulfurization by selective adsorption of dibenzothiophenes on Ag+/SBA-15 and Ag+/SiO2. Chem. Commun. 2003, 2620. (21) Ng, F. T. T.; Rahman, A.; Ohasi, T.; Jiang, M. A study of the adsorption of thiophenic sulfur compounds using flow calorimetry. Appl. Catal., B 2005, 56, 127. (22) Khare, G. P. Desulfurization process and novel bimetallic sorbent systems for the same. U.S. Patent 6,274,533, August 14, 2001. (23) Khare, G. P. Process for the production of a sulfur sorbent. U.S. Patent 6,184,176, February 6, 2001. (24) Velu, S.; Ma, X.; Song, C. Selective adsorption for removing sulfur from jet fuel over zeolite-based adsorbents. Ind. Eng. Chem. Res. 2003, 42, 5293-5304. (25) Ma, X.; Velu, S.; Kim, J. H.; Song, C. 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. (26) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Desulfurization of transportation fuels with zeolites under ambient conditions. Science 2003, 301, 79. (27) Yang, R. T.; Takahashi, A.; Yang, F. H.; Hernandez-Maldonado, A. J. Selective sorbents for desulfurization of liquid fuels. U.S. and foreign patent applications filed, 2002. (28) Hernandez-Maldonado, A. J.; Yang, R. T. Denitrogenation of transportation fuels by zeolites at ambient temperature and pressure. Angew. Chem., Int. Ed. 2004, 43, 1004. (29) Hernandez-Maldonado, A. J.; Yang, R. T. Desulfurization of diesel fuels by adsorption via π-complexation with vapor phase exchanged Cu(I)-Y zeolites. J. Am. Chem. Soc. 2004, 126, 992. (30) Hernandez-Maldonado, A. J.; Yang, R. T. New sorbents for desulfurization of diesel fuels via π-complexation. AIChE J. 2004, 50, 791. (31) Hernandez-Maldonado, A. J.; Yang, R. T. Desulfurization of Diesel Fuels via π-Complexation with Nickel(II)-Exchanged X- and Y-Zeolites. Ind. Eng. Chem. Res. 2004, 43, 1081. (32) Hernandez-Maldonado, A. J.; Stamatis, S. D.; Yang, R. T.; He, A. Z.; Cannella, W. New Sorbents for Desulfurization of Diesel Fuels via π-Complexation: Layered Beds and Regeneration. Ind. Eng. Chem. Res. 2004, 43, 769.
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Hence, the presence of nitrogen compounds in fuel reduces the sulfur removal capability of Cu(I)Y and other π-complexation sorbents. Gasoline and diesel are complex mixtures of numerous organic compounds (paraffins, olefins, naphthenes, aromatics). Typically, diesel is heavier with compounds having carbon numbers between 10 and 22, whereas gasoline contains compounds with carbon numbers between 4 and 12. Additives are added to fuels (gasoline and diesel) to improve fuel properties and performance. Antioxidants (aromatic amines and hindered phenols) are added to prevent peroxides and gum formation on exposure to air, and basic stabilizers are added to fuels to improve their stability. Oxygenates (ethanol, MTBE, etc.) and cetane improvers (aromatic nitrates) are added to gasoline and diesel, respectively. Oxygenates are added to improve octane rating and also to meet the federal regulations for air quality standards.37,38 The EPA standard for oxygenates in all U.S. gasoline is minimum 2 wt % oxygen with a maximum limit of 3.7 wt % oxygen for ethanol and 2.7 wt % oxygen for ethers.37,39 Oxygenated gasoline also could dissolve up to 600 ppmw moisture compared to 150 ppmw for conventional gasoline depending on its aromatic content. Fuel generally accumulates moisture on storage. PAHs are present in fuel and make up a part of the aromatic content. PAHs are known to increase harmful emissions from transportation vehicles. The EU has set a maximum limit of 11 wt % for PAHs in the EURO 4 vehicle standard effective 2005. The Worldwide Fuel Charter developed by automakers and engine manufacturers has set a limit of 2 wt % of PAHs in advanced markets such as the United States. In California, the California Air Resources Board requires that the PAHs in on-road diesel be less than 1.4 wt % (4 wt % for small refiners). In Canada, diesel typically has PAHs between 2.7 and 18%, depending on the region and summer or winter fuel.40 The presence of oxygenates, antioxidants, cetane improvers, moisture, PAHs, and organonitrogen compounds in commercial transportation fuels has an impact on desulfurization by π-complexation. π-Complexation sorbents such as Cu(I)Y also adsorb these compounds apart from the regular aromatics and organosulfur compounds. Hence, a systematic study of the effect of these compounds is needed to completely understand desulfurization of commercial fuels by π-complexation. In this work, the effect of organonitrogen compounds and PAHs on desulfurization is studied using liquid model fuels. The breakthrough (33) Hernandez-Maldonado, A. J.; Yang, R. T.; Cannella, W. Desulfurization of Commercial Jet Fuels by Adsorption via π-Complexation with Vapor Phase Ion Exchanged Cu(I)-Y Zeolites. Ind. Eng. Chem. Res. 2004, 43, 6142. (34) Hernandez-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. (35) Hernandez-Maldonado, A. J.; Yang, F. H.; Qi, G. S.; Yang, R. T. Desulfurization of commercial fuels by π-complexation: Monolayer CuCl/ γ-Al2O3. Appl. Catal., B 2005, 56, 111. (36) Yang, F. H.; Hernandez-Maldonado, A. J.; Yang, R. T. Selective adsorption of organosulfur compounds from transportation fuels by π-complexation. Sep. Sci. Technol. 2004, 39, 1717-1732. (37) Motor Gasoline Technical Review. http://www.chevron.com/products/ prodserv/fuels/bulletin/motorgas/downloads/Motor_Fuels_Tch_Rvw_complete.pdf. Chevron Products Company: San Ramon, CA, 2003. (38) Diesel Fuels Technical Review. http://www.chevron.com/products/ prodserv/fuels/bulletin/diesel/Diesel%20Fuel%20Rev.pdf. Chevron Products Company: San Ramon, CA, 1998. (39) Li, Y.; Yang, F. H.; Qi, G.; Yang, R. T. Effects of Oxygenates and Moisture on Adsorptive Desulfurization of Liquid Fuels with Cu(I)Y Zeolite. Catal. Today 2006, in press. (40) The Green Lane, Environment Canada’s World Wide Web site. http://www.ec.gc.ca/energ/fuels/reports/fullreport/fullReport_p4_e.cfm (accessed 2003).
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behavior of a model fuel containing various organosulfur species is presented. Also, a molecular orbital study is undertaken to understand the bonding of these molecules with CuY zeolite and to assess their relative inhibition effect for desulfurization. Previously, moisture and oxygenates such as ethanol and MTBE have been shown to adsorb more strongly on Cu(I)Y zeolites and are detrimental to desulfurization by adsorption. The complete results can be found elsewhere.39 Theoretical Section Ab Initio Molecular Orbital Computational Details. Molecular orbital (MO) studies on the π-complexation bonding for thiophene and other related adsorbates on sorbent surfaces and zeolites were investigated recently.36 In this work, similar MO studies were extended to two organonitrogen compounds (quinoline and carbazole) and also to three polyaromatic hydrocarbons (naphthalene, fluorene, and phenanthrene). The Gaussian 98 package41 and Cerius2 molecular modeling software42 were used for all MO calculations. Geometry optimizations were performed at the Hartree-Fock (HF) level first, and then binding energies were performed at density functional theory (DFT) level using effective core potentials (ECPs). 43-46 Density Functional Theory. DFT is an efficient tool for studying molecular properties of transition metal compounds. It can provide an accurate description of the metal-ligand interactions47 at an affordable computational cost. In this work, a hybrid method consisting of HF and DFT, known as the B3LYP approach, is used. The B3LYP method can provide reliable geometric, thermodynamic, and spectroscopic parameters for metal-ligand interactions, ranging from covalent bonds to weak noncovalent interactions.48-50 It is the combination of HF and Becke exchange51 with the Lee-Yang-Parr (LYP) correlation potential.52 Effective Core Potentials. The LanL2DZ basis set53 is a double-ζ basis set containing ECP representations of electrons near the nuclei for post-third-row atoms. ECP is simply a group of potential functions that replace the inner shell electrons and orbitals, which are normally assumed to have minor effects on (41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (42) Cerius2, version 4.6; Accelrys: San Diego, CA. (43) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potential for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270-283. (44) Wadt, W. R.; Hay, P. J. Ab Initio Effective Core Potential for Molecular Calculations: Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 82, 284-298. (45) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potential for Molecular Calculations: Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82, 299-310. (46) Gordon, M. S.; Cundari, T. R. Effective Core Potential Studies of Transition Metal Bonding, Structure and Reactivity. Coord. Chem. ReV. 1996, 147, 87-115. (47) Ziegler, T. Approximate Density Functional Theory as a Practical Tool in Molecular Energetics and Dynamics. Chem. ReV. 1991, 91, 651667. (48) Barone, V.; Adamo, C. Validation of Hybrid Density Functional Hartree-Fock Approaches for the Study of Homogeneous Catalysis. J. Phys. Chem. 1996, 100, 2094-2099.
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the formation of chemical bonds. Calculations of the valence electrons using ECP can be carried out at a fraction of the computational cost that is required for an all-electron calculation, while the overall quality of the computation does not differ significantly from the all-electron calculations.43,44 The reliability of this basis set has been confirmed by the accuracy of calculation results as compared with experimental data. Therefore, the LanL2DZ basis set was employed for all calculations (i.e., geometry optimization, frequency, and energy analysis). Geometry Optimization and Bond Energy Calculations. Frequency analysis was used to verify that all geometryoptimized structures were true minima on the potential energy surface. The optimized structures were then used for bond energy calculations according to the following expression:
Eads ) Eadsorbate + Eadsorbent - Eadsorbent-adsorbate
(1)
where Eadsorbate is energy of free adsorbate, Eadsorbent is energy of free adsorbent, and Eadsorbent-adsorbate is energy of the adsorbate/ adsorbent system. A higher value of Eads corresponds to a stronger adsorption. Models for Cu-Zeolite (CuZ). The Cu zeolite model selected for this study is similar to the ones used in our previous work36 with the molecular formula of (HO)3Si-O-Al(OH)3, and the cation Cu+ sits 2.14 Å above the bridging oxygen between Si and Al. This is a good cluster model representing the chemistry of a univalent cation bonded on site II (SII) of the faujasite framework (Z). Once the optimized structure of CuZ is obtained at the HF/LanL2DZ level, then an adsorbate molecule is added onto the Cu of zeolite model, and the resulting structure is further optimized at the HF/LanL2DZ level. Experimental Details Materials. n-Octane (Aldrich, anhydrous 99+%), benzene (Fluka, thiophene-free 99% (GC)), thiophene (Sigma, 99%), benzothiophene (ACROS Organics, 97%), dibenzothiophene (ACROS Organics, 99%), carbazole (Fluka, 96% (GC)), quinoline (Sigma, ∼98%), fluorene (Fluka, 99% (HPLC)), naphthalene (Fischer Scientific, reagent grade crystal), phenanthrene (Aldrich, 99.5+%), cupric nitrate trihydrate (Fluka, >98%), and Type Y molecular sieve (Strem Chemicals, sodium ion powder) and helium (Cryogenic gases, UHP grade) were used as obtained. Sorbent Preparation. Cu(I)Y liquid-phase ion exchange (LPIE) zeolites were prepared by liquid-phase ion exchange of Na-Y zeolites to get Cu(II)Y zeolites, which were then autoreduced to Cu(I)Y zeolite using He at 450 °C for 18 h. The ion exchange was carried out with 0.5 M cupric nitrate solution for 48 h at room temperature using five times the cation exchange capacity (CEC) of salt solution to get ∼70% exchange. After exchange, the zeolites were washed with deionized water and dried at 100 °C for 24 h.54 Cu(I)Y prepared via vapor-phase ion exchange (VPIE) contained nearly twice the amount of exchanged Cu+ cations as compared (49) Halthausen, M. C.; Heinemann, C.; Cornhl, H. H.; Koch, W.; Schwarz, H. The Performance of Density-Functional/Hartree-Fock Hybrid Methods: Catalytic Transition-Metal Methyl Complexes MCH3 + (M ) Sc-Cu, La, Hf-Au). J. Chem. Phys. 1995, 102, 4931-4941. (50) Ricca, A.; Bauschlicher, C. W. Successive Binding Energies of Fe(CO)5+. J. Phys. Chem. 1994, 98, 12899-12903. (51) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. ReV. A 1988, 38, 3098-3100. (52) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. ReV. B 1988, 37, 785-789. (53) Russo, T. V.; Martin, R. L.; Hay, P. J. Effective Core Potentials for DFT Calculations. J. Phys. Chem. 1995, 99, 17085-17087. (54) Hernandez-Maldonado, A. J.; Yang, R. T. Desulfurization of Liquid Fuels by Adsorption via π-Complexation with Cu(I)-Y and Ag-Y Zeolites. Ind. Eng. Chem. Res. 2003, 42, 123-129.
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Figure 2. Optimized chemical structures of PAHs: (a) naphthalene, (b) fluorene, and (c) phenanthrene. Figure 1. Optimized chemical structures of (a) dibenzothiophene and nitrogen compounds, (b) quinoline, and (c) carbazole. The heterocyclic S or N atom is indicated by the atom with a shade/size slightly different from the carbon atom: S in (a) and N in (b) and (c). Table 1. Composition of Various Model Fuels Prepared in n-Octane fuel name model fuel No. 1 model fuel No. 2 model fuel No. 3
benzene (wt %)
thiophene (T) (ppmw S)
benzothiophene (BT) (ppmw S)
dibenzothiophene (DBT) (ppmw S)
-
114
-
-
20
50
100
150
20
-
-
100
with the Cu(I)Y (LPIE) from liquid-phase ion exchange (i.e., 50 cations per unit cell versus 25 per unit cell).29 Consequently, the desulfurization capacity of Cu(I)Y (VPIE) was substantially higher than that of Cu(I)Y (LPIE).29 However, it was more time-consuming to prepare Cu(I)Y (VPIE); thus, only Cu(I)Y (LPIE) was employed in this work. Model Fuel. Three different model fuels were prepared, each with a different S concentration and different sulfur compound. Table 1 lists the composition of various model fuels used in this study. Model fuel No. 1 has no aromatics. Model fuel No. 2 has three different S compounds (thiophene, benzothiophene, and dibenzothiophene) and was used to study the effect of concentration and breakthrough order. Model fuel No. 3 was used to study the effect of organonitrogen and PAHs on desulfurization. Carbazole and quinoline were chosen as two model organonitrogen compounds due to their significant presence in fuels after HDN and similarity in structure between carbazole and DBT. Figure 1 shows the chemical structure of DBT and organonitrogen compounds used in this study. Naphthalene, fluorene, and phenanthrene are the model PAHs used in this study due to similarity in chemical structure to quinoline and carbazole as seen from Figures 1 and 2. Carbazole is only partially soluble in organic solvents.55 Hence, the concentration of nitrogen compounds was fixed at 1.6 mmol/L (30 ppmw N ≈ typical concentration of carbazoles in fuel) to study their effect on desulfurization. Similar concentrations of PAHs (1.6 mmol/L ≈ 275 ppmw naphthalene ≈ 356 ppmw fluorene ≈ 382 ppmw phenanthrene) were used to compare them against nitrogen compounds. Quinoline (100 ppmw N) and naphthalene (2 wt %) are typical upper limit values of nitrogen compounds and PAHs in fuel and were used in this work.9,40 Fixed-Bed Adsorption. The breakthrough experiments were performed in fixed-bed mode with flow in the downward direction (55) Takahashi, A.; Yang, R. T.; Munson, C. L.; Chinn, D. Cu(I)-Y Zeolite as a Superior Adsorbent for Diene/Olefin Separation. Langmuir 2001, 17, 8405.
Figure 3. Breakthrough of thiophene on Cu(I)Y (LPIE) sorbent at 44.8 h-1 space velocity, with model fuel No. 1 containing 300 ppmw thiophene (∼114 ppmw S) in n-octane.
using a custom-made quartz adsorber with a glass frit supporting the adsorbent. The sorbent was activated (auto-reduction) in situ and then flushed with n-octane. After octane flush the model fuel was allowed to contact the bed and the S concentration in the effluent fuel was monitored as a function of time. The samples were analyzed for sulfur in a Shimadzu GC-17A v3 unit equipped with flame photometric detector. Complete details of the GC operating conditions and calibration are provided elsewhere.34 The liquid hourly space velocity was a critical parameter in fixed-bed desulfurization and was controlled using helium pressure, which was used to overcome the pressure drop in the bed.39 Sulfur breakthrough curves were generated by plotting the transient total sulfur concentration in the effluent normalized by the feed total sulfur concentration against the cumulative fuel effluent volume that was normalized by total bed weight. The breakthrough loading was obtained at the point where the outlet fuel sulfur content was less than approximately 1 ppmw, and the saturation loading was obtained when the outlet sulfur reached the feed value.
Results and Discussion Model Fuel No. 1. Figure 3 shows the sulfur breakthrough on the Cu(I)Y (LPIE) sorbent with model fuel No. 1 containing 300 ppmw thiophene (∼114 ppmw S) in n-octane at a space velocity of 44.8 h-1. The breakthrough point is 155 cm3/g, which is comparable with 105 cm3/g obtained earlier for 500 ppmw thiophene in n-octane at a space velocity of ∼15 h-1.54 The breakthrough adsorption amount of sulfur is 0.4 mmol/g. Lower
Nitrogen and PAHs on Desulfurization of Liquid Fuels
Figure 4. Breakthrough of various sulfur compounds on Cu(I)Y (LPIE) sorbent at 1.8 h-1 space velocity, with model fuel No. 2 containing 300 ppmw total sulfur (50 ppmw S, thiophene; 100 ppmw S, benzothiophene; 150 ppmw S, dibenzothiophene); 20 wt % benzene in n-octane.
LHSV would have given higher breakthrough loadings. Assuming at least 50% of Cu2+ ions are reduced to Cu+ by autoreduction, then Cu+ concentration in the zeolite would be approximately 0.75 mmol/g.55 Then at breakthrough not all of the Cu+ ions are used for thiophene adsorption (i.e., π-complexation). The total adsorption amount is 1.25 mmol/g when the effluent concentration reaches ∼35 ppmw S. This implies that, at total saturation, more than one thiophene molecule would be adsorbed per Cu+ ion in the zeolite similar to previous results obtained for model fuel. Model Fuel No. 2. Breakthrough of various sulfur compounds on Cu(I)Y (LPIE) sorbent is shown in Figure 4. Sulfur starts breaking through at 11 cm3/g with a breakthrough loading of 0.06 mmol/g and 90% saturation loading of 0.4 mmol/g. From Figure 4, we could see that dibenzothiophene (11 cm3/g ≈ 0.032 mmol/g) starts breaking through first followed by thiophene (14 cm3/g ≈ 0.013 mmol/g) and finally benzothiophene (24 cm3/g ≈ 0.047 mmol/g). The adsorptive selectivity of various sulfur compounds on Cu(I)Y zeolite follows:36 dibenzothiophene > benzothiophene > thiophene. Hence, we would expect breakthrough order on Cu(I)Y to be similar when the system is not diffusion-limited. Desulfurization by adsorption, however, is diffusion-limited with LHSV playing a crucial role, and also the concentration of various S compounds in model fuel No. 2 is not the same (dibenzothiophene is 150 ppmw S whereas that of thiophene and benzothiophene is 50 ppmw S and 100 ppmw S, respectively). Concentration and diffusion play a limiting role here, and hence the breakthrough order is mixed up. Model Fuel No. 3. Figure 5 shows the sulfur breakthrough for model fuel No. 3 on Cu(I)Y zeolite. Sulfur breakthrough occurs at 20 cm3/g with a breakthrough loading of 0.044 mmol/g and saturation loading of 0.096 mmol/g. Dibenzothiophene breakthrough loading is higher for model fuel No. 3 compared with that of model fuel No. 2 since DBT concentration is lower and also only competes with benzene for adsorption. Organonitrogen Compounds. Figure 5 shows the effect of organonitrogen compounds on dibenzothiophene breakthrough. It can be seen that 30 ppmw N of nitrogen compounds has a moderate effect on desulfurization with DBT breakthrough at 13 and 11 cm3/g for quinoline and carbazole, respectively. Table 2 shows the effect of nitrogen compounds on breakthrough and saturation loading. This shows that both quinoline and carbazole adsorb strongly compared to DBT. When a mixture of 30 ppmw
Energy & Fuels, Vol. 20, No. 3, 2006 913
Figure 5. Effect of various nitrogen compounds on the desulfurization of model fuel No. 3 containing 100 ppmw S of DBT, 20 wt % benzene in n-octane, on Cu(I)Y (LPIE) sorbent at 2.6 h-1 space velocity. Table 2. Breakthrough and Saturation Sulfur Loadings for 100 ppmw S DBT in 20 wt % Benzene and 80 wt % n-Octane on Cu(I)Y (LPIE) Sorbent
additive -c
breakthrough loading (mmol/g)a,b
saturation loading (mmol/g)a
0.044
0.096
Nitrogen Compounds 30 ppmw N quinoline 0.028 30 ppmw N carbazole 0.025 30 ppmw N quinoline + 0.014 30 ppmw N carbazole 100 ppmw N quinoline 0.008 Polyaromatic Hydrocarbons 275 ppmw naphthalened 0.035 356 ppmw fluorened 0.032 382 ppmw phenanthrened 0.028 2 wt % naphthalene 0.012
0.074 0.066 0.044 0.028 0.066 0.062 0.053 0.028
a Loading amounts normalized by total bed weight. b Breakthrough was based on effluent showing 1 ppmw total sulfur. c 100 ppmw S DBT, 20 wt % benzene, and 80 wt % n-octane. d Concentration of PAH additive is 1.6 mmol/L ) 30 ppmw N for nitrogen compounds.
N quinoline and 30 ppmw N carbazole is used as additive, the decrease in sulfur breakthrough loading is not additive, whereas the change in saturation loading is additive. As the concentration of quinoline is increased from 30 to 100 ppmw N, the DBT breakthrough occurs at 4 cm3/g, which is 20% of the base case (no additive). The presence of organonitrogen compounds in a commercial fuel would significantly reduce the desulfurization capacity of Cu(I)Y. Ni-based adsorbent was shown to have adsorptive selectivity in the order of indole > quinoline > dibenzothiophene > naphthalene.56 Similarly, Cu(I)Y has the adsorptive selectivity in the order of carbazole > quinoline. Cu(I)Y shows selectivity toward carbazoles due to the presence of an extra aromatic ring and also due to the presence of nitrogen as a neutral group. Hence, Cu(I)Y could be used for denitrogenation to remove carbazoles and alkyl carbazoles before ultradeep HDS.7 PAHs. Figure 6 shows the effect of various PAHs on dibenzothiophene breakthrough. It can be seen that, with 1.6 mmol/L of naphthalene (275 ppmw) or fluorene (356 ppmw) (56) Kim, J. H.; Ma, X.; Song, C. Ultra-deep desulfurization of diesel fuel over different adsorbents: A fundamental study of adsorption selectivity and mechanism. 19th North American Catalysis Society Meeting, Philadelphia, PA, May 2005.
914 Energy & Fuels, Vol. 20, No. 3, 2006
Jayaraman et al. Table 3. Energy of Adsorption in kcal/mol for Different Adsorbates ∆E on CuZ ∆E on CuCl
adsorbate benzene36 thiophene36 fluorene naphthalene phenanthrene benzothiophene36 dibenzothiophene36 4,6-dimethyldibenzothiophene36 quinoline carbazole
Figure 6. Effect of various polyaromatic hydrocarbons (1.6 mmol/L of PAH) on the desulfurization of model fuel No. 3 containing 100 ppmw S of DBT, 20 wt % benzene in n-octane, on Cu(I)Y (LPIE) sorbent at 2.6 h-1 space velocity.
as additive to model fuel No. 3, there is only a slight effect on desulfurization with DBT breakthrough at 16 and 15 cm3/g, respectively. With phenanthrene as additive, the effect is more pronounced with sulfur breakthrough at 13 cm3/g. The PAH concentration is increased to 2 wt % naphthalene similar to those found in commercial fuels in the United States. The effect on DBT breakthrough is severe, with breakthrough at 6 cm3/g and reducing total sulfur adsorption capacity to 30% of base case (no additive). Naphthalene breakthrough occurs at approximately 4 cm3/g as followed from the naphthalene concentration front, and the DBT breakthrough follows closely. The adsorptive selectivity of various PAHs on Cu(I)Y follows: phenanthrene > fluorene > naphthalene. It is also clear by comparing Figures 5 and 6 that the inhibition effect of quinoline is stronger than that by phenanthrene. The decrease in the breakthrough sulfur capacity is nearly the same by 30 ppmw N in quinoline as that by 275 ppmw N in phenanthrene. From Table 2 we can see that, for a commercial U.S. fuel, which would have approximately 100 ppmw N organonitrogen and 2 wt % PAH compounds, the desulfurization capacity of Cu(I)Y zeolite would be affected significantly by the presence of these compounds. The inhibition ranking for commercial fuel would then follow: organonitrogen compounds > PAHs. Selectivity for Sulfur Molecules. As described in detail previously,39 the selectivity factor or separation factor (R) can be estimated from the adsorption bond energies. For the selectivity of thiophene over benzene, R > 5-30, which results in the high selectivity for sulfur over the fuel.39 Thus, the effluent prior to the breakthrough point contained quinoline. Like S in the thiophenic compounds, N in the organonitrogen compounds also participates in the aromaticity of the rings, thus further increasing the π-complexation. On the basis of the results shown in Table 3, the degree of inhibition for desulfurization capacity of CuY by different compounds should follow the order: carbazole > quinoline > phenanthrene > fluorene ≈ naphthalene. Carbazole forms the strongest bonds with CuY. Thus, it exhibits the strongest inhibition for desulfurization. The order of inhibition observed from experiment agrees remarkably well with the order predicted from the molecular orbital results. The heat of adsorption from solution is actually lower than the bond energy because of the energy of dissolution (which needs to be subtracted).21 Thus, relative solubilities also play an role in determining the relative inhibition effects. From the results above, the relative solubilities are probably similar. Conclusion Organonitrogen compounds and PAHs are found to be disadvantageous to dibenzothiophene removal by π-complexation using Cu(I)Y, because the sulfur capacity is decreased. The relative inhibition effects of nitrogen compounds and PAHs were found to follow the order: carbazole > quinoline > phenanthrene > fluorene ≈ naphthalene. The order is in agreement with that predicted by the relative bond energies obtained from molecular orbital calculations. The high selectivity for sulfur compounds by Cu(I)Y for desulfurization is, however, not affected by these compounds. To achieve good sulfur capacities for commercial fuels during desulfurization, it is imperative that the fuel contains fewer organonitrogen compounds and PAHs apart from minimal oxygenates and moisture. Acknowledgment. The support from the International Copper Association (ICA, 260 Madison Avenue, New York, NY 10016) and NSF CTS-0455176 is gratefully acknowledged. EF050308H
