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Ind. Eng. Chem. Res. 2004, 43, 2368-2375
On the Nature of Nitrogen-Containing Carbonaceous Deposits on Coked Fluid Catalytic Cracking Catalysts J.-O. Barth, A. Jentys,* and J. A. Lercher Technische Universita¨ t Mu¨ nchen, Lehrstuhl II fu¨ r Technische Chemie, 85747 Garching bei Mu¨ nchen, Germany
The regeneration of FCC catalysts leads to significant NOx emissions. Hence, the identification of surface deposits and reaction intermediates is important for understanding the mechanisms by which nitrogen-containing species are converted into NOx or N2. Characterization of the feed and of carbonaceous deposits on spent FCC catalysts by IR and NMR spectroscopy as well as by (MA)LDI-TOF mass spectrometry showed that polyaromatic pyrrole derivatives (alkylcarbazoles, alkylbenzocarbazoles, alkylindoles) are the main source of nitrogen in the feed of FCC units. Consequently, (poly-)aromatic compounds (m/z ) 350-850) such as carbazole and quinoline derivatives are the main nitrogen-containing components in the coke. During oxidative regeneration, these pregraphitic species are converted into smaller aromatic compounds similar to the nitrogen molecules originally present in the feedstock. With increasing temperature, nitrogen-containing coke species accumulate on the catalyst surface and can only be removed by oxidative regeneration at 550-700 °C. 1. Introduction Fluid catalytic cracking (FCC) is a key process in modern refineries.1 Worldwide, approximately 350 FCC units are operated in petrochemical complexes, converting vacuum gas oil and high-boiling residues into lighter fuel products and chemical feedstocks. Because of its central function, a range of technological improvements have been implemented to increase the economic advantages of FCC units.2 In addition to subtle improvements in process design and operation, new catalysts and additives are continuously being developed to improve the product output.3 However, investment in technologies significantly reducing the emissions of environmental pollutants is also enforced by increasingly stringent legislation, especially for NO, SOx, CO, and CO2 emissions.1 Note in this context that approximately 2000 t/year of NOx is released from a typical refinery and that FCC regenerator flue gases contribute approximately 50% of the total NOx emissions. Consequently, special attention has been directed toward reducing emissions of the largest single source of NOx in the refinery. The concentrations of NOx emissions from regenerator flue gases vary between 50 and 500 ppm depending on the feed, the operating conditions, and the amount of CO combustion promoter (Pt-based additive) used.4,5 In the FCC process, nitrogen-containing species in the feedstock are cracked in the riser reactor to lighter molecules, while approximately 40% of the nitrogen is deposited in the coke on the spent catalyst. The aromatic feedstock composition and, in particular, the basic nitrogen-containing molecules enhance the coke yield.6 During oxidative regeneration, approximately 90% of the coke-bound nitrogen is converted to molecular nitrogen (N2), and the rest is released in the form of NOx.4,7 The main source of nitrogen leading to NOx formation is the FCC feedstock (“fuel NOx”), whereas only minor amounts (500 °C in the FCC riser reactor. Their presence is also supported by the IR and NMR spectra discussed above. At this point, it should be mentioned that odd masses are characteristic of hydrocarbons with one or a higher odd number of nitrogen atoms. The sequence of peaks with mass differences of 14 (addition of one methylene group) and 11 or 50 (extension of the aromatic ring system by one or four C atoms), e.g., at 427, 477, 487, and 501 amu, is remarkable, because such intense peaks at odd masses are typical for aromatic, pregraphitic macromolecules with nitrogen atoms. The signals can be assigned to five- and six-ring nitrogen-containing heteroatomic species (alkylbenzocarbazole, quinoline derivatives), similar to that naturally present in crude
oil, atmospheric residues, and kerogens (feedstock of the FCC unit, cf. Figure 2).18,21 Other possible assignments could be amino or cyano PAH derivatives.22 However, after the cracking reaction in the riser reactor, such functional groups should not be present in the carbonaceous deposits. In addition to the molecular species discussed above, lower-intensity peaks at associated masses that vary from the main peaks by ∆m/e ) (1 were observed. The distribution of masses is probably caused by multiple laser-induced addition and/or elimination of hydrogen radicals to polycyclic (alkylated) hydrocarbons (PAHs). LDI is known to induce extensive and uncontrolled gasphase reactions. In complex matrixes, such as PAHs in carbonaceous deposits on deactivated FCC catalysts, hydrogenation and dehydrogenation reactions will take place during the laser ablation and photoionization processes. These reactions will produce variable amounts of various poly(de)hydrogenated aromatic hydrocarbons.14,23,24 Moreover, ionization of coke molecules by the addition of cations (e.g., Na+, ∆m/e ) +23) cannot be ruled out completely, as such ions are present in the original catalyst and might be still present after demineralization of the catalyst.14 As pregraphitic polycyclic aromatic carbonaceous deposits are UV chromophores, thus adsorbing intensively at the wavelength (337 nm) of the N2 laser used, it is concluded that ionization in the LD-TOF-MS experiments described above occurs predominantly by multiphoton ionization, energy pooling, and charge-transfer processes of these species.12,13,15,16 The high absorption in the UV region is linked to the conjugated π systems of these compounds.12,13,15,16 In line with this reasoning, it was observed that, for the analysis of carbonaceous deposits on spent FCC catalysts, addition of organic matrix molecules was not necessary, because the analyte itself had matrix properties. MALDI-TOF-MS experiments with 2,5-dihydroxybenzoic acid (DHB) as the organic matrix showed, in principle, the same mass pattern in the range of m/e )150-400 as the experiments without the matrix. Compared to the LD-TOF-MS experiments, odd masses occurred in the region of m/z ) 400-800 with significantly lower intensities, suggesting that the MALDI matrix (acid) protonates selected (basic) coke molecules in the course of the matrix-assisted laser-desorption process. However, under such MALDI conditions, a clear
2372 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004
Figure 5. LD-TOF-MS of carbonaceous deposits on a coked FCC catalyst (inorganic material dissolved in 40% HF prior to measurement and washed with H2O). Selected peaks have been assigned to typical constitutional isomers with the appropriate molecular masses.
assignment of signals related to nitrogen-containing coke compounds is more difficult, because such protonated species (with even masses) now occur at the same value of m/e as many other nonheteroatomic coke compounds. The present measurements show that a significant fraction of nitrogen from the FCC feedstock is trapped in relatively large polyaromatic molecules on the spent FCC catalyst. Because of their high molecular masses and the resulting large minimum diameters, such coke species must be present in the mesopores of the FCC catalyst particles. This is in agreement with the XPS measurements reported by Quian et al.20 showing that nitrogen-containing coke is preferably deposited on the outer rim of these particles. It has to be pointed out that FCC catalysts are usually microspheres with diameters of about 60 µm, which are formed of binder materials that constitute a macro- and mesoporous matrix for the incorporation of the particles of the actual microporous US-Y zeolite cracking catalyst. XPS measurements cannot penetrate deeply into the inner part of the microporous zeolites; therefore, they show the composition of coke compounds trapped in the macro- and
mesoporous voids of the microspheres. However, as deduced from the absence of the band of the bridging hydroxyl groups in the IR spectra of the deactivated zeolites, a portion of the nitrogen coke species will also be adsorbed on Brønsted acid sites of the microporous zeolite US-Y crystallites. This observation is confirmed by species with odd molecular masses characterized by peaks with lower intensity in the range of m/e ) 150350. 3.3. Transformation of Nitrogen-Containing Coke Species during the FCC Regeneration Process. To follow the molecular transformation of the carbonaceous deposits during oxidative regeneration, deactivated FCC catalyst samples from three different commercial FCC units were regenerated with 2% O2 in a stream of He, and the materials were analyzed at different stages of regeneration (450, 550, and 700 °C). The concentrations of nitrogen, carbon, and hydrogen on the catalysts were determined by elemental analysis, and the deposits were investigated by LD-TOF-MS. At temperatures above 550 °C, the nitrogen contents of the deactivated catalysts decreased significantly from initially 125-230 to ∼30 ppm (cf. Figure 6). Figure 7
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2373
Figure 6. Concentration of nitrogen in coked FCC catalysts during temperature-programmed oxidation (2% O2 in He, total flow ) 100 mL/min, ramp ) 10 °C/min).
Figure 7. N/C ratio and carbon concentration of a coked (spent) FCC catalyst during temperature-programmed oxidation (2% O2 in He).
shows for sample 3 that the N/C ratio increased while the amount of carbon on the catalyst decreased with increasing temperature of regeneration. The same observation was also made for the other coked samples. This agrees well with the work of Yaluris et al., who reported that carbon- and nitrogen-containing species are burned sequentially during temperature-programmed oxidation.25 The LD-TOF mass spectra (Figures 8 and 9) of carbonaceous deposits show that, with increasing temperature, the intensities of peaks for odd masses in the range of m/e ) 350-800 decrease. Interestingly, at 550 °C, new species with odd masses between 150 and 300 amu were observed (Figure 9). Given the finding that the relative concentration of nitrogen on the catalyst increased with oxidation temperature, we conclude that the aromatic ring system of the large pregraphitic nitrogen-containing polycyclic hydrocarbons was partly oxidized. This leads to lighter products, such as carbazole- and quinoline-type molecules. As can be seen from Figures 8 and 9, these species accumulate on the catalyst surface before being completely oxidized at T ) 700 °C (hardly any coke species are detectable on the samples by LD-TOF-MS). The mass distribution of the coke species at 550 °C is comparable to that reported for electron-ionization mass spectra of nitrogen-containing coke compounds liberated from FCC catalysts by supercritical fluid extraction (SFE).20 However, the relatively low molecular weights (m/e < 400) observed after SFE suggest that leachable coke molecules are mostly nitrogen-containing polyaromatic molecules of three to five rings with short alkyl substitutions.20 In contrast, LD-TOF-MS detects significantly larger com-
Figure 8. Results of LD-TOF-MS of deposits on a coked FCC catalyst recorded during oxidative regeneration.
pounds, which cannot be leached from the FCC catalyst microspheres. Consequently, LD-TOF-MS allows a better analysis of (pregraphitic) polycyclic aromatic coke species entrapped in the meso- and macropores of the catalyst particles, providing a more detailed insight into the coke chemistry occurring during the cracking and regeneration process. Interestingly, at temperatures above 550 °C, FCC coke consists mainly of species, such as isomers of alkyl(benzo)carbazoles, alkylindols, alkyltetrahydroquinolines, etc., (e.g., for carbazole, m/z ) 167.2), that are also the dominating nitrogen compounds in the feedstock of the FCC unit (cf. Figure 2). Their oxidation leads to the
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Acknowledgment Financial support of the European Union (Project G1RD-CT99-0065-“DENOXPRO”) is gratefully acknowledged. Special thanks go to Prof. I. A. Vasalos and Dr. E. Efthimiadis (Chemical Process Engineering Research Institute, CPERI, Thessaloniki, Greece) for elemental analysis of the coked catalysts. The help of Mr. H. Krause and Prof. J. Buchner (Lehrstuhl fu¨r Biotechnologie, Institut fu¨r Organische Chemie und Biochemie, TU Munich, Munich, Germany) with MALDI-TOF-MS measurements is gratefully acknowledged. Literature Cited
Figure 9. LD-TOF-MS of deposits on a coked FCC catalyst recorded during oxidative regeneration. With increasing temperature, odd masses characteristic of nitrogen-containing coke species are determined in the range of m/e ) 150-350.
formation of N2, NO, and nitrogen intermediates such as NH3 and HCN at temperatures above 550 °C.26 It is speculated that the heteroatomic PAHs are oxidized in the final stage (highest temperature) of the regeneration, when the FCC catalyst is present in the diluted phase of the regenerator (upper part near the cyclones, T ) 720-760 °C). As the spent catalyst enters the regenerator in the lower part of the unit (dense phase close to the air grid, T ) 550-680 °C), polycyclic aromatic hydrocarbons that do not contain nitrogen will be removed first, whereas the nitrogen-containing species will accumulate on the surface and be removed in the region of higher temperatures.26 4. Conclusions Polyaromatic pyrrole derivatives (alkylcarbazoles, alkylbenzocarbazoles, alkylindoles) are the main source of nitrogen in the feedstock of FCC units. These molecules dominate over six-ring nitrogen species (pyridine derivatives) such as alkylquinolines and alkyltetrahydroquinolines. After cracking is achieved in the riser reactor, a fraction is deposited as nitrogen-containing species in the coke on the spent catalyst. Polycyclic aromatic compounds such as carbazole and quinoline derivatives were identified by IR, 13C MAS NMR, and (MA)LDI-TOF mass spectroscopies as the main nitrogencontaining components of the coke. The majority of these species have relatively high molecular masses (m/e ) 350-850) and are probably trapped in the meso-/ macropores of the FCC catalyst microspheres. During oxidative regeneration, these large, pregraphitic-type compounds are converted into smaller carbazole- and quinoline-type molecules. With increasing oxidation temperature, smaller nitrogen-containing coke species accumulate on the catalyst surface. These species are oxidized only after most of the nonheteroatomic hydrocarbons have been burned off. Such pyrrole and pyridine derivatives are probably trapped during that stage of regeneration on the strong Brønsted acid sites of the cracking catalyst. Above 550 °C, these molecules are converted into N2, NO, and reduced nitrogen-containing intermediates (such as NH3 and HCN). This occurs in the diluted phase of the FCC regenerator, where the temperatures are the highest (720-760 °C).
(1) Harding, R. H.; Peters, A. W.; Nee, J. R. D. New developments in FCC catalyst technology. Appl. Catal. A: Gen. 2001, 221, 389. (2) Mann, R. Fluid catalytic cracking: Some recent developments in catalyst particle design and unit hardware. Catal. Today 1993, 18, 509. (3) Cheng, W. C.; Kim, G.; Peters, A. W.; Zhao, X.; Rajagopalan, K.; Ziebarth, M. S.; Pereira, C. J. Environmental Fluid Catalytic Cracking Technology. Catal. Rev.-Sci. Eng. 1998, 40, 39. (4) Zhao, X.; Peters, A. W.; Weatherbee, G. W. Nitrogen chemistry and NOx control in a FCC regenerator. Ind. Eng. Chem. Res. 1997, 36, 4535. (5) Peters, A. W.; Yaluris, G.; Weatherbee, G. W.; Zhao, X. Origin and control of NOx in the FCCU regenerator. Fluid Cracking Catal. 1998, 259. (6) Harding, R. H.; Zhao, X.; Qian, K.; Rajagopalan, K.; Cheng, W. C. Fluid Catalytic Cracking Selectivities of Gas Oil Boiling Point and Hydrocarbon Fractions. Ind. Eng. Chem. Res. 1996, 35, 2561. (7) Dishman, K. L.; Doolin, P. K.; Tullock, L. D. NOx Emissions in Fluid Catalytic Cracking Regeneration. Ind. Eng. Chem. Res. 1998, 37, 4631. (8) Yaluris, G.; Peters, A. W. Realistic evaluation of the performance of FCCU regenerator additives in the laboratory. In Proceedings of the 2nd International Topical Conference on Refining Processes, AIChE Spring National Meeting; American Institute of Chemical Engineers (AIChE): New York, 1999; p 1999. (9) Feller, A.; Zuazo, I.; Guzman, A.; Barth, J.-O.; Lercher, J. A. Common mechanistic aspects of liquid and solid acid-catalyzed alkylation of isobutane with n-butene. J. Catal. 2003, 216, 313. (10) Feller, A.; Barth, J.-O.; Guzman, A.; Zuazo, I.; Lercher, J. A. Deactivation pathways in zeolite-catalyzed isobutane/butene alkylation. J. Catal. 2003, 220, 192. (11) Barth, J.-O.; Jentys, A.; Lercher, J. A. Towards understanding the genesis and removal of NOx in FCC regenerators. In Proceedings of the 17th World Petroleum Congress; Institute of Petroleum (IP): London, 2003; Vol. 3, p 445. (12) Hanton, S. D. Mass Spectrometry of Polymers and Polymer Surfaces. Chem. Rev. 2001, 101, 527. (13) Hillenkamp, F.; Karas, M. Matrix-assisted laser desorption/ionisation, an experience. Int. J. Mass Spectrom. 2000, 200, 71. (14) Zenobi, R.; Knochenmuss, R. Ion formation in MALDI mass spectrometry. Mass Spectrom. Rev. 1998, 17, 337. (15) Karas, M.; Bachmann, D.; Bahr, U.; Hillenkamp, F. Matrix-assisted ultraviolet laser desorption of nonvolatile compounds. Int. J. Mass Spectrom. Ion Processes 1987, 78, 53. (16) Macha, S. F.; McCarley, T. D.; Limbach, P. A. Influence of ionization energy on charge-transfer ionization in matrix-assisted laser desorption/ionization mass spectrometry. Anal. Chim. Acta 1999, 397, 235. (17) Guisnet, M. Carbonaceous Deposits. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds. VCH: Weinheim, Germany, 1997; Vol. 2, p 626. (18) Zhan, Q.; Zenobi, R.; Buseck, P. R.; Teerman, S. Analysis of Polycyclic Aromatic Hydrocarbons in Kerogens Using Two-Step Laser Mass Spectrometry. Energy Fuels 1997, 11, 144. (19) Snape, C. E.; McGhee, B. J.; Andresen, J. M.; Hughes, R.; Koon, C. L.; Hutchings, G. Characterization of coke from FCC refinery catalysts by quantitative solid state 13C NMR. Appl. Catal. A: Gen. 1995, 129, 125.
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2375 (20) Qian, K.; Tomczak, D. C.; Rakiewicz, E. F.; Harding, R. H.; Yaluris, G.; Cheng, W. C.; Zhao, X.; Peters, A. W. Coke Formation in the Fluid Catalytic Cracking Process by Combined Analytical Techniques. Energy Fuels 1997, 11, 596. (21) Sutton, D. L. Crude-Oil and Shale-Oil. Anal. Chem. 1993, 65, 175. (22) Clemett, S. J.; Maechling, C. R.; Zare, R. N.; Swan, P. D.; Walker, R. M. Identification of complex aromatic molecules in individual interplanetary dust particles. Science 1993, 262, 721. (23) Haefliger, O. P.; Bucheli, T. D.; Zenobi, R. Comment on “Real-Time Characterization of the Organic Composition and Size of Individual Diesel Engine Smoke Particles”. Environ. Sci. Technol. 1999, 33, 3932. (24) Zenobi, R. In situ analysis of surfaces and mixtures by laser desorption mass spectrometry. Int. J. Mass Spectrom. Ion Processes 1995, 145, 51.
(25) Yaluris, G.; Peters, A. W. Studying the chemistry of the FCCU regenerator in the laboratory under realistic conditions. In Designing Transportation Fuels for a Cleaner Environment; Reynolds, J. G., Khan, M. R., Eds.; Taylor & Francis: Philadelphia, PA, 1999; p 151. (26) Barth, J.-O.; Jentys, A.; Lercher, J. A. Elementary reactions and intermediate species formed during the oxidative regeneration of spent FCC catalysts. Ind. Eng. Chem. Res., in press.
Received for review October 2, 2003 Revised manuscript received February 27, 2004 Accepted March 10, 2004 IE034163I
